Detachment faulting has been hypothesized as the main process of tectonic spreading in mid-ocean ridges. The ongoing faulting leads to exhumation of oceanic core complexes (OCC) through large-scale normal faults, exposing heterogeneous sectors of the mylonitic lower crust, locally interlayered with pristine upper-mantle rocks. However, the mechanisms involved in this process – and the interplay between magmatism, deformation and fluid-rock interaction – are still debatable. To address these issues, we performed a quantitative microstructural analysis and thermodynamic modelling on mafic shear zones that occur in the lower section (≥ 600 meters below sea-floor) of Site U1473A (Atlantis Bank OCC, SW Indian Ridge), the target of IODP Expedition 360, to constrain deformation conditions and strain localization mechanisms during detachment faulting. The gabbroic shear zones consist of large (up to 5 mm in size) porphyroclasts of clinopyroxene, orthopyroxene, plagioclase and olivine embedded in a fine-grained (≤ 30 µm), polyphase matrix composed of plagioclase, clinopyroxene, orthopyroxene, amphibole, ilmenite, magnetite and olivine. Plagioclase-rich layers (~ 80 µm) are in abrupt contact with the fine-grained mixture, which define the mylonitic foliation. The porphyroclasts have undulose extinction, subgrains and are surrounded by fine-grained recrystallized grains (core-mantle structure) showing internal lattice distortion. Microfractures are common in orthopyroxene porphyroclasts. Amphibole replaces clinopyroxene and orthopyroxene porphyroclasts at their margins and fills cleavage planes. The plagioclase-rich layers show undulose extinction and subgrain boundaries in the larger grains within the layers. Mechanical twin lamellae occur in some grains regardless of grain size. Plagioclase grains show a weak shape preferred orientation with their long axes parallel to the main planar fabric of the shear zone. The grains in the polyphase matrix are mostly strain free. EBSD data in clinopyroxene clasts indicate activation of (010)[001] slip system and twinning along (001)[100]. Plagioclase-rich layers deforms by slip along the (010)[100] system. The polyphase matrix has a very weak but non-random CPO pattern. #Mg and Al content in the recrystallized clinopyroxene and orthopyroxene grains are lower compared to the porphyroclasts. Plagioclase has similar An content in both porphyroclasts and recrystallized grains. Amphibole has low concentrations of Cl and high content of F. The content of #Mg, Al and Si is similar in amphibole grains replacing pyroxene and in the polyphase matrix. Thermodynamic modelling indicates that the gabbroic shear zones formed at 820-870 °C and 2.0-2.8 kbar. Our results suggest that deformation in the porphyroclasts was accommodated by combined mechanical fragmentation and intracrystalline plasticity, which resulted in fractured grains of orthopyroxene, and clasts rimmed by recrystallized neoblasts. Plagioclase-rich layers formed mainly through dislocation creep. Phase mixing and weak CPO in the polyphase matrix point to oriented-growth during diffusion-assisted grain boundary sliding, mainly in the presence of melt, as evidenced by amphibole formed at the expense of pyroxene. Magmatic fluids are the possible source of reactant amphibole. Such mechanisms effectively resulted in strain localization in fine-grained, polyphase shear zones that contributed to the weakening of the ocean crust during detachment faulting and subsequent exhumation of the Atlantis Bank OCC.

How to cite: Taufner, R., Viegas, G., Faleiros, F., Castellan, P., and Silva, R.: Microstructures reveal brittle and viscous flow during exhumation of the high-temperature lower oceanic crust from Site U1473A, Atlantis Bank, Southwest Indian Ridge (IODP Expedition 360), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1905, https://doi.org/10.5194/egusphere-egu21-1905, 2021.

Gabbros are the main component of the oceanic crust and represent ~2/3 of the total magmatic crustal thickness. At the interface between magmatic, tectonic and hydrothermal processes, gabbros from slow spreading ridges may have a complex mineralogy and microstructural evolution. This includes structures that vary from purely magmatic fabrics, with layering and magmatic alignment of minerals, to rocks deformed from subsolidus temperatures to the lower-T brittle-ductile conditions. Such a variation is normally accompanied with changes in mineralogy, microstructures and crystallographic preferred orientations (CPO) of the main phases of these rocks, which in turn a!ect their seismic properties. Here we present a database of the CPO-derived seismic properties of 70 samples collected during the IODP Expedition 360 (site U1473). The dominant phases are plagioclase and clinopyronexe, with variable contents of olivine, enstatite, magnetite, ilmenite, chlorite and amphibole.  Velocities of compressional and shear waves decrease drastically with increasing of plagioclase content, increase strongly with increasing of ilmenite content, but increase only slightly with clinopyroxene, while variations in olivine and enstatite content seem to be less important. Maximum velocities can be either parallel to the strongest concentration of (010) poles of plagioclase or olivine/clinopyroxene [001], depending on the proportions between these phases. Anisotropy of P waves vary from ~2% in the more isotropic gabbros with weak magmatic fabric to a maximum of ~9% in more mylonitic terms. A similar effect is observed for the S-waves. Destructive interference between plagioclase CPO vs. clinopyroxene/olivine reducing anisotropy observed in some samples. This is because the maximum Vp in a foliated gabbro is parallel to the maximum concentration of poles to (010), and perpendicular to olivine and clinopyroxene. As the lineation in our gabbros is generally marked by olivine and clinopyroxene [001] (instead of the fast direction [100]), this possibly cause anisotropy reduction. When present in the more mylonitized gabbros, amphibole has strong CPOs and help to increase the general anisotropy of P and S waves, but the increase is not drastic. An increase of Vp and Vs anisotropy is also observed with stronger plagioclase CPOs, which is not observed in the case of clinopyroxene. The elastic constants calculated from these aggregates will be used as input for more physically robust calculations using differential effective medium approaches to better understand the e!ect of melt inclusions in these rocks by the time of their deformation in the lower crust.

How to cite: Morales, L. F. G., Allard, M., and Ildefonse, B.: A database of gabbro seismic properties from an ultraslow spreading ridge (IODP Hole U1473A, Southwest Indian Ridge), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14607, https://doi.org/10.5194/egusphere-egu21-14607, 2021.

Feedback between deformation mechanisms, fluid-rock interactions and porosity evolution in fault zone rocks has a crucial impact on their bulk rheology. Porosity formation within mid-crustal fault rocks (typically mylonites) can facilitate fluid flow, formation of mineral and geothermal resources, and can promote strain localization. On the contrary, porosity reduction in rocks from a brittle fault core (typically cataclasites) can cause elevated pore fluid pressures, and consequently influence the recurrence time of earthquakes.

We characterized the porosity distribution within the New Zealand’s Alpine Fault zone in cataclasite samples recovered during the first phase of the Deep Fault Drilling Project and outcropping mylonitic rocks collected at Stoney Creek, New Zealand. Synchrotron X-ray microtomography-derived analyses of open pore spaces show total microscale porosity values in the range of 0.1-0.24% within the cataclasites and up to 0.44% in the mylonites. Synchrotron nanotomography datasets reveal additional 0.03 to 0.19% pore volumes within the mylonites. In all samples, pores are very small, not connected, with mainly non-spherical, elongated, flat shapes and show subtle bipolar orientation. Scanning and transmission electron microscopy reveal the samples’ microstructural organization, where nanoscale pores ornament grain boundaries of the constituent minerals. Pores are mostly associated with (often newly formed) clay minerals in the cataclasite samples, suggesting the orientation of clays controls the shape and orientation of the associated pores. In the mylonitic samples, pores are sub-parallel to the foliation, and often associated with C’-type shear bands, indicating formation during creep cavitation.

Our observations imply that porosity within the Alpine Fault core was reduced due to pressure solution processes and the associated mineral precipitation. Simultaneously propagation of fluids triggered by cavity formation in the ductile regime is likely to cause further mineral precipitation in fluid filled pores within the fault zone. Such precipitation can affect the mechanical behavior of the Alpine Fault by decreasing the already critically low total porosity of the fault core, causing elevated pore fluid pressures, and/or introducing weak mineral phases, and thus lowering the overall fault frictional strength. We conclude that the current state of porosity in the Alpine Fault zone is likely to play a key role in the initiation of the next fault rupture.

How to cite: Kirilova, M., Toy, V., Sauer, K., Renard, F., Gessner, K., Wirth, R., Xiao, X., Matsumura, R., Prior, D., Cappuccio, F., and Morrison, S.: Porosity evolution within the active Alpine Fault zone, New Zealand. Implications for fault zone rheology., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9399, https://doi.org/10.5194/egusphere-egu21-9399, 2021.

Fluid pressurization of critically stressed sheared zones can trigger slip mechanisms at the origin of many geological rupture processes such as earthquakes and landslides. It is now well assumed that the reduction of effective stress induced by fluid pressurization can lead to the reactivation of shear zones. However, the micromechanisms that govern this reactivation remain poorly understood. By using discrete element modeling, we simulate pore-pressure-step creep test experiments on a sheared granular layer at a sub-critical stress state in order to investigate the micromechanical processes at stake during fluid induced reactivation. The simulated responses are consistent with both laboratory and in situ experiments, confirming the scale independent nature of fluid induced slip. The progressive increase of pore pressure promotes slow steady slip at sub-critical stress states and fast accelerated dynamic slip once the critical strength is overcome. The analyses of both global and local quantities show that these two emergent slip behaviors correlate to characteristic deformation modes: diffuse deformation for slow slip and highly localized deformation for fast slip. Our results suggest that, besides the control of the fabric of shear zones on their emergent slip behavior, failure is associated to grain rotations resulting from unlocking of interparticle contacts mostly located within the shear band, which, as a consequence, acts as a roller bearing for the surrounding bulk.

How to cite: Nguyen, H. N. G., Scholtès, L., Guglielmi, Y., Donzé, F. V., Ouraga, Z., and Souley, M.: Grain scale investigation of shear reactivation by fluid pressurization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7855, https://doi.org/10.5194/egusphere-egu21-7855, 2021.

As sandstone reservoirs are depleted, the pore pressure reduction can sometimes result in pore collapse and the formation of compaction bands. These are localised features which can significantly reduce the bulk permeability of the reservoir and are therefore problematic in the oil, water, geothermal, and CO₂ sequestration industries. However, the influence that grain size, grain shape and sorting have on compaction band formation in sandstone is still poorly understood, due to the fact that finding natural sandstones with specific properties is challenging. Consequently, a method of forming synthetic sandstones has been developed, in order to produce a suite of sandstone specimens with controlled grain size and porosity characteristics. During production of the synthetic sandstones, amorphous quartz cement and sodium chloride are precipitated between sand grains as a product of the reaction between sodium silicate and hydrochloric acid. The salt can then be dissolved, resulting in synthetic sandstones that have very comparable physical properties to their natural counterparts. In this study, triaxial experiments were performed on synthetic sandstone cores with four different grain size ranges of 250-300, 425-500, 600-710 and 850-1000 microns, at three different starting porosities of 27%, 32% and 37%. The samples were each axially loaded from a point along their hydrostat corresponding to 85% of their hydrostatic yield point, P*, values. These conditions mean that failure will occur within the shear-enhanced compaction regime so as to try and produce localised compaction structures. All samples were taken to 5% axial strain. The microstructural results indicate that localisation of deformation within the samples did occur and was favoured in the low starting porosity, small grain size samples. Localisation of deformation was most easily recognised by grain size reduction through grain crushing. This was weakly correlated to a change in porosity but recognition of the localisation of deformation was difficult to make using variations in porosity alone. Porosity reduction was not necessarily associated with a reduction in grain size. With increasing grain size and starting porosity, the deformation becomes more distributed in the samples with the highest starting porosity samples (37%) exhibiting more widely distributed grain crushing which was less intense overall. The results indicate a significant grain size and starting porosity influence on localisation, but also that compaction can occur by two mechanisms; one involving mostly grain rearrangement and the other primarily by grain fracturing. Consequently, the localisation of deformation is most evident in grain size reduction and is only weakly shown by porosity reduction.

How to cite: Rice-Birchall, E., Faulkner, D., and Bedford, J.: The effect of grain size and porosity on compaction localisation in high-porosity sandstones, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11020, https://doi.org/10.5194/egusphere-egu21-11020, 2021.

The cracking phenomenon of the brittle rock and rock-like materials (concrete, gypsum) have been widely researched. Such long-standing intensive research requirement is due to the fact that crack initiation, propagation and coalescence are some of the most important parameters for evaluating the rock failure behavior and strength properties. Especially defining the crack initiation stress is a fundamental part of crack propagation that leads to the rock material's final failure. However, due to the nature of rocks, they may have complex inherit structures containing various gaps and void with different sizes and numbers. Rocks mostly tend to have circular and ellipsoidal voids as a result of long and complex geological processes. Owing to this limitation, it is always hard to understand and assess the crack initiation stress comprehensively. Especially for a couple of decades, with the help of developing computer science and technology, numerical models were used on this subject. In this study, various two-dimensional numerical rock models created using Distinct Element Method (DEM) based Particle Flow Code (PFC) were used to understand the effect of different gap geometries over crack initiation stress values of rock materials under uniaxial loading conditions. A base numerical model was calibrated using laboratory test results belonging to basalt rocks. In order to calibrate the numerical model, uniaxial, conventional triaxial and in-direct tensile test results were used. A flat-jointed contact model was chosen to create bonded material during the calibration process. Seven different numerical models were used to investigate the gap geometry effect on crack initiation stress under uniaxial conditions. The base model has a circular gap with 5.40 mm diameter. The other models created to understand the effect of geometry on crack initiation stress have different ellipsoidal geometry depending on the initial circular gap, 1.5 (8.10 mm), 2.5 (13.50 mm) and 3.5 (18.20 mm) times the diameter in the vertical and horizontal direction, respectively. The results of numerical models reveal that the crack initiation stress value decreases with the increase of the gap's vertical length while the width of gaps remains constant. Based on numerical models' results, the crack initiation stress value decreases with the increase of the gap's vertical length while the diameter of gaps remains constant.

How to cite: Zengin, E. and Erguler, Z. A.: Effect of Gap Geometries on the Crack Initiation Stress of Synthetic Rock Material, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9409, https://doi.org/10.5194/egusphere-egu21-9409, 2021.

Strain localization occurs throughout the crust, in both the brittle and viscous regimes. The causes of strain localization remain under discussion. However, realistic rock records indicate that variations of material properties (e.g. active deformation mechanisms, crystallographic orientation, phase distribution, grain shapes, etc.) are likely to be the dominant factor for weakening. Determining the cause(s) of localization requires investigation of the earliest stages of strain concentration in different P-T conditions. Our study focuses on two rocks that experienced low macroscale strain at amphibolite and/or granulite facies conditions yet exhibit localization on the millimeter and smaller scale. We combine optical and electron beam petrography with chemical mapping and electron backscatter diffraction to characterize these rheologically important domains. Morphologically, these localized zones appear to mechanically link rheologically weak phases or domains. These “bridge” zones typically comprise a band of relatively fine grains with weak crystallographic preferred orientation. The major element compositions of like phases inside and outside the bridge zone are similar, but the modal mineralogy and trace elements vary somewhat. Bridge zones result from not only in-situ grain size reduction (due to, for example, nucleation, recrystallization, or cataclasis), but also chemical processes resulting in phase mixing or element mobility on a short spatial scale. Their spatial distribution suggests that the small modal fraction of microstructural change represented by the bridge zones can lead to a high degree of bulk weakening.

How to cite: Feng, H., Gerbi, C., and Johnson, S.: Rheological Bridge Zone: Initialization of Localization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7917, https://doi.org/10.5194/egusphere-egu21-7917, 2021.

C' shear bands are common structures in ductile shear zones but their development is poorly understood. They occur in rocks with a high mechanical strength contrast so we used numerical models of viscoplastic deformation to study the effect of the proportion of weak phase and the phase strength contrast on C' shear band development. We employed simple shear to a finite strain of 18 in 900 steps and recorded the microstructure, stress and strain distribution at each step. We found that C' shear bands form in models with ≥5% weak phase when there is a moderate or high phase strength contrast, and they occur in all models with weak phase proportions ≥15%. Contrary to previous research, we find that C' shear bands form when layers of weak phase parallel to the shear zone boundary rotate forwards. This occurs due to mechanical instabilities that are a result of heterogeneous distributions of stress and strain rate. C' shear bands form on planes of low strain rate and stress, not in sites of maximum strain rate as has previously been suggested. C' shear bands are ephemeral and they either rotate backwards to the C plane once they are inactive or rotate into the field of shortening and thicken to form X- and triangle- shaped structures.

How to cite: Finch, M., Bons, P., Steinbach, F., Griera, A., Llorens, M.-G., Gomez-Rivas, E., Ran, H., and de Riese, T.: The ephemeral development of C' shear bands, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-752, https://doi.org/10.5194/egusphere-egu21-752, 2021.

The coupling between fluid flow and solid deformation plays important roles in earth dynamics at different timescales and length-scales. Related processes include, magma migration and focusing in the Mid-Ocean Ridges, fluid migration after slab dehydration in the subduction zone, channelized fluid flow observed as seismic chimney in the continental margin, as so on. Here we study how localized fluid channels can develop through asymmetric compaction and decompaction processes of the solid matrix by solving coupled two-phase equations with viscoplastic rheology. Previous studies produced fluid channels with decompaction weakening, while negative effective pressure (P_t-P_f) is inevitable due to the simplified rheology formulation. We develop a viscoplastic rheology formulation that considers the effects of shear stress and plastic failure on the volumetric deformation, consistent with experimental data.

Our model results show that this new rheology can produce channelized fluid flow without negative effective pressure in the model. Our numerical results also clarified that it is the flow instability of the coupled two-phase system that cause the formation of fluid channels. The ratio between shear viscosity and bulk viscosity determines how fast the flow instability develops and manifests. The geometry of the Reservoir, on the other hand, can affect where the channels form. We further study the effects of different background and reservoir porosity, different rock layer, permeability exponents, decompaction weakening factor, and so on. These results provide a better understanding of the two-phase system and its potential applications in geological environments.   

How to cite: Wang, L. H., Yarushina, V., and Podladchikov, Y.: Modelling coupled fluid flow and solid deformation with the viscoplastic rheology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12021, https://doi.org/10.5194/egusphere-egu21-12021, 2021.

How to cite: Rast, M. and Ruh, J.: Numerical modelling of quartz-biotite aggregates: Insights on strain weakening and two-phase flow laws, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10167, https://doi.org/10.5194/egusphere-egu21-10167, 2021.

Ice 1h shows a strong viscoplastic anisotropy, as the resistance to activate dislocation glide on basal planes is at least one order of magnitude smaller than on the other slip planes. During flow the viscoplastic anisotropy leads to the development of a crystallographic preferred orientation (CPO). The anisotropic behaviour of flowing ice can lead to strain localisation. Only when the ice is layered (e.g. due to cloudy bands) it may be possible to identify localisation structures, as ice otherwise has no readily recognisable strain markers.

We use the Viscoplastic Full-Field Transform (VPFFT; Lebensohn and Rollett, 2020) crystal plasticity code coupled with the modelling platform ELLE (http://www.elle.ws; Piazolo et al., 2019) to simulate the deformation of intrinsically anisotropic ice 1h with an initial single maximum CPO in dextral simple shear up to very high strains. The VPFFT-approach simulates deformation by dislocation glide, taking into account the different available slip systems and their critical resolved shear stresses. We use an anisotropy similar to that of ice 1h, systematically vary the orientation of the initial CPO, and use passive markers/layers to visualise deformation structures.

The localisation behaviour strongly depends on the initial CPO, but reaches a consistent steady state after very high shear strains of about 30. The fabric and stress evolution reach a steady-state situation as well. The orientation of the CPO controls the style of deformation, which varies from (1) synthetic shear zones with a stable shear-direction parallel orientation and that widen with ongoing strain to unstable, (2) rotating antithetic shear bands, (3) initial formation of antithetic shear bands and subsequent development of synthetic shear bands and (4) distributed localisation. Furthermore, evolving visual structures depend on the presence and orientation of a visual layering in the material. However, at very high strains, the material is almost always strongly mixed and any original layering would be destroyed.

Our results highlight the challenge to identify strain localisation in ice, yet they can help the ice community to identify and interpret deformation structures in large ice masses (e.g. the Greenland ice sheet). As strain localisation in anisotropic materials behaves scale independent (de Riese et al., 2019), large-scale equivalents may occur of the observed small-scale structures (Jansen et al., 2016).

References:

de Riese, T., Evans, L., Gomez-Rivas, E., Griera, A., Lebensohn, R.A., Llorens, M.G., Ran, H., Sachau, T., Weikusat, I., Bons, P.D. 2019. Shear localisation in anisotropic, non-linear viscous materials that develop a CPO: A numerical study. Journal of Structural Geology, 124, 81-90.

Jansen, D., Llorens, M.-G, Westhoff, J., Steinbach, F., Kipfstuhl, S., Bons, P.D., Griera, A., Weikusat, I. 2016. Small-scale disturbances in the stratigraphy of the NEEM ice core: observations and numerical model simulations. The Cryosphere 10, 359-370.

Lebensohn, R.A., Rollett, A.D. 2020. Spectral methods for full-field micromechanical modelling of polycrystalline materials. Computational Materials Science, 173, 109336.

Piazolo, S., Bons, P.D., Griera, A., Llorens, M.G., Gomez-Rivas, E., Koehn, D., ... Jessell, M.W. 2019. A review of numerical modelling of the dynamics of microstructural development in rocks and ice: Past, present and future. Journal of Structural Geology, 125, 111-123.

How to cite: de Riese, T., Bons, P. D., Gomez-Rivas, E., Griera, A., Llorens, M.-G., and Weikusat, I.: Deformation structures resulting from anisotropy during high-strain deformation of ice 1h with initial CPO, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10209, https://doi.org/10.5194/egusphere-egu21-10209, 2021.

How to cite: Richards, D., Pegler, S., Piazolo, S., and Harlen, O.: Ice fabrics in natural flows: moving away from pure and simple shear, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12078, https://doi.org/10.5194/egusphere-egu21-12078, 2021.

Adhesive and repulsive, nm-range surface forces acting between mineral grains control colloidal stability and mineral aggregation but less is known about how these forces are affected by surface reactivity and to what extent these grain-scale forces can influence various deformation processes in rocks. In this experimental work, we explore and quantify the surface forces acting between dynamic mineral surfaces that undergo recrystallization or are chemically reactive in contact with water or aqueous salt solutions. Our experimental setup consists of the surface forces apparatus (SFA) coupled with the multiple beam interferometry (MBI). This setup can excellently reproduce a typical grain contact geometry with nanometer-thin water films confined between contacting mineral grains over relatively large, micron-sized contact areas. Owing to the use of MBI, both surface growth or dissolution processes can be monitored during force measurements in real-time. As such, SFA can provide information about the links between surface reactivity and adhesive or repulsive surface forces. Using the examples of force measurements between recrystallizing or chemically reactive mineral surfaces such as carbonates, hydroxides, and silicates, we comment on the relationship between the measured surface forces and surface reactivity. We link our findings with the observed changes in mineral phases, surface topographies, or surface roughness. We also comment on how the micron-scale confinement in the SFA affects the growth and dissolution processes in contrast to less confined regions. The magnitude of the forces associated with dynamic mineral surfaces and the potential significance of these forces to macroscopic deformation processes and cohesion in rocks are discussed.

How to cite: Dziadkowiec, J., Zareeipolgardani, B., Cheng, H.-W., Dysthe, D. K., Røyne, A., and Valtiner, M.: Forces between reactive surfaces, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-362, https://doi.org/10.5194/egusphere-egu21-362, 2021.

The initiation and propagation of fractures in floating regions of Antarctica has the potential to destabilize large regions of the ice sheet, leading to significant sea-level rise. While observations have shown rapid, localized deformation and damage in the margins of fast-flowing glaciers, there remain gaps in our understanding of how rapid deformation affects the creep and toughness of ice. Here we derive a model for dynamic recrystallization in ice and other rocks that includes a novel representation of migration recrystallization, which is absent from existing models but is likely to be dominant in warm areas undergoing rapid deformation within the ice sheet. We show that, in regions of elevated strain rate, grain sizes in ice may be larger than expected (~15 mm) due to migration recrystallization, a significant deviation from solid earth studies which find fine-grained rock in shear zones. This may imply that ice in shear margins deforms primarily by dislocation creep, suggesting a flow-law exponent of n=4 in these regions. Further, we find from existing models that this increase in grain size results in a decrease in tensile strength of ice by ~75% in the margins of glaciers. Thus, we expect that this increase in grain size makes the margins of fast-flowing glaciers less viscous and more vulnerable to fracture than we may suppose from standard model parameters.

How to cite: Ranganathan, M., Minchew, B., Meyer, C., and Pec, M.: Recrystallization of ice enhances the creep and vulnerability to fracture of ice shelves , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3266, https://doi.org/10.5194/egusphere-egu21-3266, 2021.

Approximately one third of the worlds known impact structures are formed in carbonate-bearing target rocks. However, the response of their main constituent mineral, calcite, upon shock loading and unloading is still not well understood. Mechanical twins in calcite are described from natural impactites and shock experiments, yet, reliable indicators to distinguish these shock effects from the very common calcite twins generated in tectonites are missing. Here, we present scanning electron microscopic investigations of twinned calcite within calcite cemented brecciated gneisses from the Ries impact structure.

Calcite cemented brecciated gneisses occur at several outcrops of the Ries impact structure as well as in samples recovered from depth as low as 977 m in the research drilling 1973. At Maihingen, the polymict impact breccias contain shocked gneiss fragments and various generations of calcite in veins. The occurrence of rhombohedral PDFs in quartz from the gneiss fragments indicates shock conditions of >10 GPa. Coarse calcite grains, representing an early generation of calcite in the veins, show exceptionally fine-lamellar twins, indicating high stress and strain rates. The calcite twins show widths of < 0.5 µm, a high density of up to a few hundred lamellae per mm, and appear to crosscut each other, which has been suggested as a criterion for shock-induced twinning. Furthermore, a high density of sets of planar features occur associated and parallel to these twins, but along which no twin domains were resolved in the scanning electron microscope. Twin systems detected by EBSD measurements include e-twins, common also in calcite from tectonites, and another more rarely occurring twin system, characterized by a rotation axes parallel to <-2110> and a rotation angle of ca. 35°. A second generation of calcite without twins is represented by elongate palisade calcite, fine-grained aggregates and rims forming sutured grain boundaries surrounding twinned coarse calcite grains. EDS measurements show that these calcite grains contain up to 2.5 % Fe as well as traces of Mn, Mg, Si, Na and Al. In contrast, the coarse twinned calcite is almost pure CaCO₃. Whereas the fine-grained aggregates and sutured grain boundaries indicate recrystallization, the palisade grains indicate precipitation from the pore fluid.

The twinned coarse calcite grains within the polymict impact breccias are interpreted to be shock induced. As coarse calcitic sedimentary target rocks are not known from the Ries area, they can either represent pre-shock calcite veins within the gneisses or possibly marbles that were brecciated together with shocked gneisses during impact cratering. The second generation of calcite represents post-shock recrystallization and precipitation from a fluid.

How to cite: Seybold, L., Trepmann, C. A., Hölzl, S., and Kaliwoda, M.: Twinned calcite within polymict impact breccias from the Nördlinger Ries impact structure, Germany – shock effects and post-shock annealing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15559, https://doi.org/10.5194/egusphere-egu21-15559, 2021.

The strength of experimentally deformed natural and synthetic quartz is strongly affected by the intracrystalline water content. Water–related defects weaken quartz by either decreasing the resistance to dislocation motion (Peierls stress) or by enhancing the nucleation of dislocations, during what is commonly referred to as hydrolytic weakening. However, hydrolytic weakening has been observed predominantly in synthetic quartz grains, with water contents higher than 20–30 wt ppm H₂O and at high-homologous temperatures, for which the activation of dislocation climb and recovery processes is enhanced.

In the low-temperature plasticity (LTP) regime, at low-homologous temperatures and high stress conditions, quartz plasticity is mainly controlled by dislocation glide. At these conditions, the possible effect of intracrystalline water on quartz strength is still a matter of debate.

In order to analyse the effects of intracrystalline water content on the plastic yield and hardness of quartz in the LTP regime, natural samples from recrystallized quartz domains of a granulite-facies migmatitic gneiss, presenting different water contents and microstructures, have been investigated through a series of spherical and Berkovich nanoindentation tests at room conditions. Nanoindentation tests have been integrated with measurements of intracrystalline water contents of the indented grains with secondary ion-mass spectrometry (SIMS), and with electron backscatter diffraction (EBSD) measurements of the crystallographic orientation of the indented grains.

Water content of indented quartz grains ranges between 2 and 104 wt ppm H₂O. Samples and related nanoindentation tests were thus classified as either “dry” (DQ, for water contents < 20 wt ppm H₂O) or “wet” (WQ, for water content > 20 wt ppm H₂O). Spherical nanoindentation tests revealed comparable yield stresses (ranging between 3.5 and 8.8 GPa, depending on the crystal orientation) for DQ and WQ grains. In addition, significant strain hardening was observed in both DQ and WQ grains. Berkovich nanoindentation tests also resulted in comparable hardness (ranging from 8.0 to 13.5 GPa) in both DQ and WQ grains. The hardness also increases with indentation depth, which is consistent with the “size-effect” on mineral strength during LTP.

These results suggest that, for the investigated range of water contents, the yield strength and flow stress of quartz in the LTP regime is not affected by the intracrystalline water content of the indented grain. Both the dry and wet quartz experienced significant crystal plastic deformation prior to the nanoindentation tests, as evidenced by the occurrence of undulatory extinction, misorientation bands, subgrains, and recrystallized grains. This pre-indentation strain history may have had a major role in generating the dislocation density, which then controlled the yield stresses during low-temperature plasticity in our experiments. Hence, inherited strain history, crystallographic orientation, and grain size may play a more important role than water in controlling the strength of the continental crust at the brittle–ductile transition, where LTP is dominant and quartz is the most abundant phase.

How to cite: Menegon, L., Ceccato, A., and Hansen, L. N.: Strength of dry and wet quartz in the low–temperature plasticity regime: insights from nanoindentation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14625, https://doi.org/10.5194/egusphere-egu21-14625, 2021.

Hydrolytic weakening has been suggested as a major process facilitating strain localisation, consistent with many studies that have found a positive correlation between water content and intensity of deformation. We examine the role of water in an unusually thick shear zone: the 1 km-thick ultramylonitic layer of the El Pichao shear zone, NW Argentina. We used Fourier Transform Infrared Spectroscopy to measure water content in quartz and feldspar, comparing ultramylonitic rocks to mylonites and weakly-deformed rocks. We found that quartz and feldspar in ultramylonites contained half the amount of water of weakly-deformed rocks, contrary to findings in previous studies. We propose that the kilometre-thick ultramylonite formed in three stages: (1) localised deformation and recrystallisation caused release of intracrystalline water to grain boundaries, which promoted grain-boundary sliding, forming the ultramylonite, (2) high pressure in the shear zone continuously expelled intercrystalline water to the surroundings, drying the boundaries and leading to strain hardening, (3) water migrated to neighbouring, less-deformed rocks causing hydrolytic weakening, repeating the cycle, and widening the ultramylonite.

How to cite: Finch, M., Weinberg, R., and Hunter, N.: Water loss and the origin of thick ultramylonites, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3456, https://doi.org/10.5194/egusphere-egu21-3456, 2021.

Partial melting of metapelites at high-P and high-T conditions typical of lower crustal levels is a well-known phenomenon. Its role in strain localization ­– both at micro- and regional scale – and subsequent rheological weakening of rocks have been widely investigated. Previous researchers have also explored the influence of such melt networks on the variation of the dominant phases' active deformation processes – especially quartz – over the entire range of tectono-thermal evolution of the rock. However, mechanisms driving the deformation of the phases crystallized in-situ from the melt has so far been largely overlooked.

In this work, we focus on the deformation behavior of the in-situ crystallized phases with increasing shear strain. In that pursuit, we took a quartz-muscovite mixture (dry) that was initially cold pressed at 200 MPa, followed by hot pressing at 160 MPa and 580 °C to obtain an analogue of pelite. The cylindrical sample was then experimentally deformed in a Patterson-type apparatus under a finite shear strain (γ) of 15.0 at 750 °C, a confining pressure of 300 MPa, and constant shear strain rate ( 3 × 10^-4s^-1). Subsequently, a longitudinal axial section was cut and was examined using electron backscattered diffraction (EBSD).

The initial minerals, quartz (Qtz) and muscovite (Ms), underwent deformation and reacted to produce K-Feldspar (Kfs), Mullite (Mul), Cordierite (Crd), Ilmenite (Ilm), and Biotite (Bt). The Qtz grains show limited evidence of dynamic recrystallization. Ms, on the other hand, exhibit strong crystal preferred orientations (CPO). The J-Indices of both Qtz and Ms increase with shear strain (from the center to the edge of the cylinder). Among the reaction products, Kfs (maximum in volume) show weak CPO throughout, similar to Qtz. The maxima of [001] plot near parallel to the shear direction in the pole figures for all values of γ. The rest of the phases show strong CPOs. The J-Index of Crd and Mul increase with shear strain, whereas that of Ms and Kfs increase till γ = 7 and fall at higher strains. Neighbor-pair misorientation axes for Crd, Ilm, and Kfs, corresponding to the high-angle boundaries (HAGBs), are randomly oriented, implying ‘rigid grain rotation,’ which could also be responsible for the lower [001] pole figure intensities. Overall, with increasing shear strain, the number of HAGBs decreases. The corresponding misorientation axes exhibit stronger preferred alignment, probably signifying restricted rotation with progressive melt crystallization. Although the area-equivalent diameters for all the melt-crystallized phases are nearly close (RMS: 1.5 – 2 µm), the Kfs CPOs are considerably weaker than the rest. This possibly affirms the dominance of fluid/melt in triggering diffusion creep and grain boundary sliding over grain size.

How to cite: Dutta, D., Misra, S., and Mainprice, D.: Understanding deformation behavior of lower crustal rocks from experimentally deformed metapelitic aggregate: an EBSD-based approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14101, https://doi.org/10.5194/egusphere-egu21-14101, 2021.

The geological properties of the subduction interface, such as stable metamorphic assemblages and the rheology of shear zone rocks, change with depth. Studies based on seismic and geodetic observations suggest that these changes can be accompanied by differences in seismic styles. In this realm, slow slip events (SSEs) and related tremor signals, grouped as episodic tremor and slip (ETS) events, have been detected down-dip of the subduction megathrust seismogenic zone. A wide range of mechanisms, some invoking rheological heterogeneity, has been proposed to explain ETS occurrence. Given that ETS events accommodate most of the plate interface displacement in a depth range below the seismogenic zone, it is of great interest to understand the rheology of the rock lithologies that are likely to host ETS along the deep subduction interface.

Here, we present data from an exhumed subduction complex in Ishigaki Island, Ryukyu Arc. In particular, we analyse the Triassic high pressure-low temperature Tomuru metamorphic rocks, which comprise blueschist and greenschist facies metabasites that underwent subduction-related deformation. These rocks offer an important natural laboratory in which to study the characteristics of blueschist deformation structures to infer rheology and, in particular, the role played by heterogeneities in an environment comparable to modern ETS down-dip of the seismogenic zone.

Through multiscale and multidisciplinary, field- and laboratory-based studies, including quantitative microstructural and image analyses, we focus on two main topics. Firstly, we aim to understand blueschist rheology, by documenting the deformation mechanisms active in blueschist rocks through electron backscatter diffraction (EBSD), in order to quantify intracrystalline deformation and lattice preferred orientation (LPO) development. Secondly, we study the effect of grain size on blueschist foliation development and, ultimately, on blueschist deformation.  Through these analyses, we hope to constrain both subduction interface strength and dominant mineral- scale deformation mechanisms at blueschist conditions.

How to cite: De Caroli, S., Fagereng, A., Ujiie, K., and Meneghini, F.: Types of heterogeneities and deformation mechanisms in blueschist rocks: an example from an exhumed subduction complex in Ishigaki Island, Ryukyu Arc, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10990, https://doi.org/10.5194/egusphere-egu21-10990, 2021.

Blueschists are a major constituent rock type along the subduction zone interface and therefore critical to our understanding of subduction zone dynamics. Previous experimental work on natural blueschists focus on either seismic anisotropy or on the process of eclogization of a blueschist aggregate. We have begun an experimental investigation to constrain the rheology and mechanical anisotropy of a naturally foliated blueschist from the Condrey Mountain Window, CA, USA. General shear experiments were performed in a Griggs apparatus using cores of the natural blueschist at 700°C, 1 GPa, and a shear strain rate of ~10^-5 s^-1. The starting material consists of ~55% glaucophane, ~40% epidote, ~5% titanite, and <5% quartz where both glaucophane and epidote have strong crystallographic fabrics and shape-preferred orientations that define the foliation. Three types of experiments were performed: 1) with the foliation parallel to the shear plane, 2) with the foliation parallel to the sigma1 direction, and 3) where the starting material was crushed into a powder representing no foliation. Both of the foliated experiments achieve similar peak shear stresses of ~250 MPa; however, the sample with the foliation parallel to the shear plane shows strain weakening while the sample with the foliation parallel to the sigma1 direction shows no strain weakening. We also observe several stress drops of ~20-30 MPa in the sample with the foliation parallel to the sigma1 direction prior to peak stress conditions. Microstructures from both of the foliated samples show evidence for brittle deformation processes, while kinking is also commonly observed in glaucophane. The sample with no foliation has a lower shear stress of ~130 MPa and shows no evidence for brittle deformation processes but rather shows development of a S-C-C’ mylonitic fabric. Additional experiments will be performed at different temperatures and strain rate conditions. A detailed microstructural analysis will accompany the mechanical results.

How to cite: Tokle, L., Hufford, L., Morales, L., and Behr, W.: The rheology and mechanical anisotropy of a foliated blueschist, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10686, https://doi.org/10.5194/egusphere-egu21-10686, 2021.

Amphibole’s ubiquitous occurrence in the lower crust and subduction zones together with its anisotropic elastic and rheological properties makes its texture evolution essential for assessing the past and current tectonic regimes. Amphibole often display a typical crystallographic preferred orientation (CPO) where the crystals [001] axes align with lineation and the [100] axes align with the normal to the foliation plane. However, this common CPO was attributed to numerous different deformation mechanisms, such as rigid body rotation, dislocation creep, or dissolution precipitation, and there yet to be found a distinct relation between amphibole CPO attributes and the prevailing deformation mechanism. Here, we present a microstructural analysis using electron backscatter diffraction (EBSD) of a highly deformed amphibolite from the metamorphic sole of Mamonia complex in Cyprus in order to investigate texture evolution in amphibole-rich samples. Samples from two localities ~40 km from each other were analyzed: ‘Agia Varvara’ (AV), and ‘Bath of Aphrodite’ (BOA). The two amphibolites show well-foliated microstructure, comprised mainly of hornblende (50-70%), and plagioclase (20-30%) grains under similar calculated P-T conditions of ~600 °C and 6 kbar. Despite the similar compositions and conditions, there are significant differences in the overall texture between the two samples. Samples from AV show strongly clustered amphibole CPO, with the [001] axis forming a strong point maximum parallel to the lineation (X-axis) and the [100] axis aligned perpendicular to foliation (Z-axis). In addition, amphiboles are aligned with the lineation with relatively curved boundaries and moderate aspect ratio (~2). For samples from BOA, amphiboles grains show two distinct CPO types: axial [001], where the [001] is aligned parallel to the shear direction while [100] and [010] oriented along the Y-Z plane, and orthorhombic, where the [001] and [100] are aligned with the lineation and normal to foliation, respectively. In addition, amphibole are tabular-shaped, elongated grains with distinctively straight boundaries and high aspect ratio of ~3.5. Comparison between the AV and BOA grains with average misorientation spread of >1° shows higher fraction for AV (35%) than BOA (13%). We interpret the textural and microstructural analysis of the amphibolites to reflect different deformation mechanisms for AV and BOA. The lack of compositional zoning within hornblende grains suggests no significant deformation by dissolution precipitation for both AV or BOA. For AV, the strong CPO, curved grains boundaries, and high ratio of grains with intragrain misorientations suggest deformation through dislocation creep. Differently, in BOA, the observations of tabular-shaped amphibole grains, the low amount of intra-grain misorientations, along with shape and crystal orientations that vary together with [001] as the rotation angle suggest deformation by rigid body rotation.

How to cite: Topaz, A., Boneh, Y., and Golan, T.: Texture Evolution of Amphiboles - a Case Study from the Mamonia Complex, Cyprus, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11935, https://doi.org/10.5194/egusphere-egu21-11935, 2021.

Pseudotachylyte-bearing amphibole-rich gneisses with concordant quartz-rich layers from the base of the Silvretta nappe, Austria, are analyzed by polarized light microscopy, scanning electron microscopy and electron back scattered diffraction. Amphibole grains show microfractures, undulatory extinction, deformation lamellae, kink bands, mechanical twins and locally recrystallized grains restricted to sites of high strain, e.g. along microshear zones and twin boundaries. The twins are characterized by a twin plane parallel to (-101), a rotation axis parallel to [101] and a misorientation angle of 178°. The (-101) amphibole twins document the high differential stresses during crystal plasticity coeval with pseudotachylyte formation, given their high critical resolved shear stress of 200 MPa. Directly at the contact to twinned amphibole within the gneisses, quartz grains commonly show subbasal deformation lamellae, short-wavelength undulatory extinction and cleavage cracks mostly parallel to {10-11} rhombohedral planes that are decorated by recrystallized grains with a diameter of < 10 µm. The small recrystallized grains show a crystallographic preferred orientation (CPO) that is controlled by the orientation of the host grains. This quartz microstructure consistently indicates high-stress crystal plasticity of quartz concurrent with high-stress crystal plasticity of amphibole and pseudotachylyte formation.

Quartz-rich layers (>90% quartz) concordant to the foliation of the gneisses commonly show evidence of dynamic recrystallization in the regime of dislocation creep. The recrystallized grain microstructure is mostly homogenous without a gradient towards the lithological contact to the amphibole-rich gneisses. Locally, however, a gradient of decreasing strain towards the contact can be observed as indicated by a decreasing number of recrystallized grains. Close to the contact, quartz grains are coarse with long axes of a few mm. A core-and-mantle structure, where recrystallized grains surround a few hundred µm wide and mm-long porphyroclasts, is occurring in transition towards an almost completely recrystallized microstructure. The recrystallized grains show a CPO indicating rhombohedral <a> dislocation glide. Recrystallized grains are isometric and subgrains in porphyroclasts are of similar shape and size, indicating dynamic subgrain rotation recrystallization. Stresses on the order of hundred MPa are suggested by the diameter of recrystallized grains of in average about 10 µm. Locally, the recrystallized quartz aggregate is affected by subsequent low-temperature plasticity, as indicated by shear fractures offsetting the recrystallized microstructure. The missing or decreasing strain gradients of dislocation creep within the quartz-rich layers towards the amphibole-rich gneisses indicate that dislocation creep in the quartz-rich layers cannot be responsible for transferring high stresses required for high-stress crystal-plasticity of quartz and amphibole as well as pseudotachylyte-formation and suggest that dislocation creep of quartz represents an independent earlier stage of deformation.

How to cite: Brückner, L. M. and Trepmann, C. A.: High-stress crystal plasticity of amphibole and quartz in pseudotachylyte-bearing gneisses and dislocation creep in concordant quartz-rich layers from the Silvretta basal thrust (Austria), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12346, https://doi.org/10.5194/egusphere-egu21-12346, 2021.

Deformation of natural mafic rocks by viscous deformation mechanisms can occur even at low temperature conditions. In such instances, crystal plastic mechanisms are not operative, as their activity is restricted to very high temperatures for amphiboles, pyroxenes, and plagioclase. Instead, simultaneous mineral reactions may facilitate deformation at low temperature conditions. The gabbro from the Lyngen Magmatic Complex (LMC) constitutes a good example of such processes, because it has experienced deformation at low temperatures of greenschist to lower amphibolite-facies conditions, and the rock has been transformed from gabbro to greenschist. This study focuses on detailed analysis of deformation processes, metamorphic reactions and fabric development in the LMC gabbro. Most samples are overprinted by epidote amphibolite and greenschist-facies mineral assemblages. Preliminary observations distinguish two different types of amphiboles, which have been interpreted as different generations. The predominant type defines the stretching lineation and shows long prismatic habits whereas the less abundant type crystallized in a sub- to anhedral manner. The metamorphic conditions of growth for each amphibole type is yet not well constrained. However, we initially interpret the former to grow during epidote amphibolite- or greenschist facies-conditions, whereas the latter could represent relict grains from the original magmatic assemblage or products generated at amphibolite- or epidote amphibolite-facies conditions. Further analysis will determine the orientation, geochemistry and metamorphic conditions during growth for both amphibole types. A recent model proposed for eclogites suggests that simultaneous mineral growth and deformation can result in new products growing in a preferred direction. Such preferential growth can generate a shape preferred orientation parallel to the lineation, which results in the formation of crystal preferred orientations (CPO). We aim to test if similar microstructural observations can be translated to the amphiboles of the LMC gabbro. In such case, amphibole CPO’s would not be the product of crystal plasticity but of preferential growth. The large scale deformation of the LMC emphasizes the relevance of these results, as it would demonstrate that the interaction between mineral reactions and deformation can play a major role on regional deformation of large mafic bodies, such as the ocean floor.

How to cite: Galindos Alfarache, M., Stünitz, H., Konopásek, J., and Lee, A.: Amphibole CPO in retrograded gabbro from the Lyngen Magmatic Complex (Northern Scandinavian Caledonides, Norway)., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1303, https://doi.org/10.5194/egusphere-egu21-1303, 2021.

Mafic rocks consist of strong minerals (e.g. clinopyroxene, plagioclase) that can only be deformed by crystal plastic mechanisms at high temperatures (>800°C). Yet, mafic rocks do show extensive deformation by non-brittle mechanisms when they have only reached lower temperatures (~650°C). In many of such cases, the deformation is accommodated by an interaction of deformation with simultaneous mineral reactions. Here we show that dissolution-precipitation creep plays a major role in deformation of gabbro lenses at mid and upper amphibolite facies conditions.

The Kågen gabbro in the North Norwegian Caledonides intruded the Vaddas Nappe at 439 Ma at pressures of 7-9 kbar, temperatures of 650-900°C, and depths of ∼26-34 km. The Kågen gabbro on south Arnøya is comprised of undeformed gabbro lenses with sheared margins wrapping around them. This contribution analyses the evolution of the microstructures and metamorphism from the low strain gabbro lenses to high strain mylonites at margins of the lenses. Microstructural and textural data indicate that dissolution-precipitation creep is the dominant deformation mechanism, where dissolution of the gabbro took place in reacting phases of clinopyroxene and plagioclase, and precipitation took place in the form of new minerals: new plagioclase and clinopyroxene, amphibole, and garnet. Amphibole shows a strong CPO that is primarily controlled by its preferential growth in the extension direction. Synchronous deformation and mineral reactions of clinopyroxene suggests mafic rocks can become mechanically weak during the general transformation weakening process, i.e. the interaction of mineral reaction and deformation by diffusion creep. The weakening is directly connected to a fluid-assisted transformation process that facilitates diffusion creep deformation of strong minerals at far lower stresses and temperatures than dislocation creep. Initially strong lithologies can become weak, provided that reactions can proceed during deformation, the transformation process itself is an important weakening mechanism in mafic (and other) rocks, facilitating deformation at low differential stresses.

How to cite: Lee, A., Stünitz, H., Soret, M., Battisti, M. A., and Konopásek, J.: Creeping gabbro: dissolution-precipitation creep facilitating deformation in mafic rocks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12360, https://doi.org/10.5194/egusphere-egu21-12360, 2021.

　　Microstructural analysis is essential for estimating the deformation conditions of plastically deformed rocks. In this study, we analyze the microstructures of carbonate mylonites and deformation conditions in natural shear zone to reconstruct tectonics. Carbonate mylonites originated from late Carboniferous Tateishi Formation and mylonitized in middle Cretaceous by the strike-slip motion of Shajigami shear zone in the eastern margin of the Abukuma Mountain, Northeastern Japan.
　　Microstructural analysis was carried out by optical microscope and electron backscattered diffraction (EBSD) mapping to determine grain size, aspect ratio, shape preferred orientation (SPO) and crystallographic preferred orientation (CPO) of calcite aggregates.
　　Pervasive deformation twins and dynamically recrystallized grains are observed. Although most porphyroclasts show symmetric structure, some show asymmetric structure that indicates dextral shear sense. Mean dynamically recrystallized grain size is 16-67 µm, and it decreases close to the shear zone. CPOs show that c-axes concentrate normal to the shear plane or slightly rotate to the shear sense. The strong CPOs suggest that the dominant deformation mechanism is dislocation creep. SPOs show the foliation which is slightly oblique or almost parallel to the shear plane. However, we observed the SPOs parallel to the shear plane at the location 150 m away from the shear zone.  The 3D dynamically recrystallized grain shapes are between plane-strain ellipsoid and oblate ellipsoid. The grain shapes tend to be relatively polygonal close to the shear zone, while more elongated further away from the shear zone. The distribution of the carbonate mylonite originated from same Tateishi Formation is known to be about 5 km apart from the Shajigami shear zone (Tateishi location). However, based on many aspects of differences in microstructures among both locations such as SPOs of recrystallized grains, we infer that the deformation of Shajigami shear zone was not related to one at Tateishi location. The pervasive dynamic recrystallization suggests that the deformation temperature was at least 200°C. Observed type Ⅱ and type Ⅲ twin morphologies (Burkhard, 1993) of calcite grains suggest deformation temperature below 300°C. 
　　These results indicate that the deformation of the Shajigami shear zone was in the range from 200 to 300℃ and deformation was stronger near the shear zone. In addition, the polygonal grain shape close to the shear zone suggests that the deformation temperature is higher close to the shear zone. Furthermore, SPOs show that pure shear component is larger than simple shear component in terms of SPOs that almost parallel to the shear plane away from the shear zone. This study including several additional results will provide the microstructural development of carbonate mylonites in natural strike-slip shear zones deformed near the brittle-ductile condition of the upper crust.

How to cite: Yokoyama, H., Muto, J., and Nagahama, H.: Microstructures and deformation temperature of carbonate mylonites in Shajigami shear zone at eastern margin of Abukuma Mountains, Northeastern Japan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14195, https://doi.org/10.5194/egusphere-egu21-14195, 2021.

The Calamita Schists in the aurole of the Late Miocene Porto Azzurro pluton underwent partial melting and HT metamorphism at P < 0.2 – 0.3 GPa and T > 650 – 700 °C, coeval with regional deformation. Deformation produced a network of shear zones that evolved from melt-present conditions to the brittle-ductile transition. Shearing at high temperature in the presence of melt allowed deformation to remain relatively distributed in wide high-strain zones. As the thermal pulse associated with the intrusion progressively faded away, deformation localized into anastomosing, mylonitic greenschist-facies shear zones surrounding lozenges of high-grade migmatitic schist. Mylonitic shear zones formed at low-angle with respect to the well-established high grade foliation preserved as a relic, oblique foliation. We show that such an extreme strain localization was determined by strain hardening of the no longer melt-bearing quartz-feldspar schist, localized embrittlement on precursory shear bands, and fluid-enhanced reaction softening that caused the breakdown of Al-silicates and the development of phyllosilicate-rich mylonitic bands. Consequently, tectonic structures with different orientation developed under the same kinematic regime, as a result of the changing physical and mechanical properties of the cooling rock volume.

How to cite: Papeschi, S., Musumeci, G., Bartoli, O., Cesare, B., Massonne, H.-J., and Mazzarini, F.: Evolution of melt-bearing shear zones during cooling within an upper crustal aureole: the Calamita Schists (Island of Elba, Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14906, https://doi.org/10.5194/egusphere-egu21-14906, 2021.

The NW Iberian Massif represents a segment of the Variscan Belt, where several allochthonous complexes crop out: : Cabo Ortegal, Ordenes and Malpica-Tuy, in Spain, and Bragança and Morais in Portugal. These allochthonous complexes comprise allochthonous units, overthrusting parautochthonous and autochthonous units. The suture zone of the Variscan orogeny in the NW Iberia preserves the testimony of the collisional dynamics between Gondwana and Laurussia during the Carboniferous. The stacking of allochthonous units into an accretion wedge, and their subsequent incorporation by thrusts into the continental margin of Gondwana, resulted in polyphasic tectonothermal evolution. Different units record valuable information about the deformation mechanisms, rheological behaviour and the configuration of plates during the Palaeozoic.

The kinematic and deformational evolution of major tectonic boundaries of the Variscan Allochthonous units, as well as their mutual relationship in Iberia is critical, in order to constrain their regional meaning and correlation with similar units along the European Variscan Belt. In shear-zones, plastic deformation of polycrystalline aggregates result into microstructural and textural fingerprints that need to be interpreted. Quantitative analyses of fabrics has been crucial in untangling complex tectonothermal evolutions. In this case neutron diffraction experiments have been conducted in transmission mode in the Institute Laue-Langevin (ILL) (France), to characterize mylonites from the basal shear zone of the Lower Allochthon in Morais Complex. Two different experimental sets have been tested in D1B and D20 beamlines, comparing textural standards and new vanadium sample holders in order to optimize the procedure. Diffraction data were refined with Rietveld software MAUD to obtain quantitative texture information and orientation distribution functions (ODF) for main phases. Afterward, pole figures of relevant planes were interpreted in terms of slip-system activity to understand deformation conditions. Overall, microstructural data and fabric analysis points to a top-to-the SE shearing with a pure-shear component in the mylonitic flow.

Keywords: Shear zones, texture analysis, neutron diffraction, Rietveld method, Variscan orogeny, Morais complex.

How to cite: Malecki, J., Gómez Barreiro, J., Durán Oreja, M., Martínez Catalán, J. R., Tettamanti, M., Barrios Sánchez, S., Sánchez Migallón, J. M., and Puente Orench, I.: Basal Shear-Zone of the Lower allochthon in the Morais complex (Portugal): microstructural and neutron diffraction constraints., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9986, https://doi.org/10.5194/egusphere-egu21-9986, 2021.

Neoproterozoic rocks from the Kingston Peak Formation (KPF) in the Valjean Hills (USA) expose a succession of diamictites associated with major glacial events taking place during the Cryogenian, approximately 700 million years ago. Within any glacial period, diamictites are widespread and in addition, their mechanics of deposition are highly variable. Some are massive in appearance at outcrop or in hand specimen, and apparently lacking any information that allows their mode of emplacement to be elucidated. Yet the correct interpretation for deep time successions in this area is especially important, since it is debated whether the diamictites have a tectonically driven, gravitational (Mrofka & Kennedy, 2011) or direct (sub)glacial origin (Le Heron et al. 2016).

In this contribution we determine the origin of the diamictites based on its internal microfabric and associated microstructures. We base our method on the technique of Philips et al. (2011) for Quaternary sediments, by mapping the apparent longest axes of skeleton grains (ranging from fine-grained sand to fine-grained pebbles) in oriented thin sections and reconstructing their fabric in a 3D space, we could identify a bimodal signal in the orientation of the longest axes. Contrary to gravitational deposition, clasts in subglacial diamictites tend to align themselves to a stress field, induced by the movement of the glacier. Macroscopic observations (Fig. 1A), microtexture- and structures (Fig. 1B) as well as the reconstructed microfabric domains (Fig. 1C) suggests a subglacial origin. These circumstances suggest temperate glacial conditions with wet based ice sheets during the deposition of the KPF. Moreover, the quantitative data allow confident flow directions to be extracted from seemingly chaotic diamictites.

Figure 1: (A) Valjean Hills Diamictite (label is 5x5 cm), (B) Rotational structure around bigger skeleton grain, (C) traced long axes of clasts (white lines) and interpreted  microfabric domains (blue, orange)

References:

Le Heron, D.P., Tofaif, S., Vandyk, T. & Ali, D.O. (2017): https://doi.org/10.1130/G38460.1

Mrofka, D., Kennedy, M., (2011):  https://doi.org/10.1144/M36.40

Phillips, E. et al., (2011): https://doi.org/10.1016/j.quascirev.2011.04.024

How to cite: Kettler, C., Pichler, K., Smirzka, D., Vandyk, T., and Le Heron, D.: 3D-Microfabric reconstruction of Neoproterozoic diamictites from the Valjean Hills, California (USA), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13064, https://doi.org/10.5194/egusphere-egu21-13064, 2021.

Abstracts:

Graphitic carbon-bearing rocks can occur in low- to high-grade metamorphic units. In low-grade matamorphic rocks, graphitic carbon is often associated with brittle fault gouge whereas in middle- to high-grade metamorphic rocks, graphitic carbon commonly occurs in marble, schist or paragneiss. Previous studies showed that carbonaceous material gradually ordered from the amorphous stage, e.g. graphitization, is mainly controlled by increasing thermal metamorphism and has a good correlation with the metamorphic temperature. Besides, this ordered process is irreversible and the resulting structure is not affected by late metamorphism. Subsequently, the degree of graphitization is believed to be a reliable indicator of peak temperature conditions in the metamorphic rock. In this contribution, based on detailed field observations, the variably deformed and metamorphosed graphitic gneisses to phyllites, located within the footwall and hanging-walls unit of the Cenozoic Ailaoshan-Red River strike-slip shear zone are studied. According to lithological features and temperature determined by Raman spectra of carbonaceous material, these graphitic rocks and deformation fabrics are divided into three types. Type I is represented by medium–grade metamorphism and strongly deformed rocks with an average temperature of 509 °C and a maximum temperature of 604 °C. Type II is affected by low-grade metamorphism and deformed rocks with an average temperature of 420 °C. Type III is affected by lower–grade metamorphism and occurs in weakly deformed/undeformed rocks with an average temperature of 350 °C. Slip–localized micro–shear zone and laterally continuous or discontinuous slip planes constituted by graphitic carbon aggregates are developed in Types I and II. The electron back–scattered diffraction (EBSD) lattice preferred orientation (LPO) patterns of graphitic carbon grains were firstly observed in comparison with LPO patterns of quartz and switch from basal <a>, rhomb <a> to prism <a> slip systems, which indicate increasing deformation temperatures. According to the graphitic slip–planes, micro–shear zones and mylonitic foliation constituted by graphitic carbon minerals, we also propose that the development of fine–grained amorphous carbon plays an important role in rheological weakening of the whole rock during progressive ductile shearing.

Key Words: graphitic carbon, strain localization, graphitic thermometry, slip–localized micro–shear zone, rheological weakening

How to cite: Lyu, M. and Cao, S.: Strain localization mechanism of graphitic carbon-bearing rocks: Constraints from microstructure, texture and graphite geothermometry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-808, https://doi.org/10.5194/egusphere-egu21-808, 2021.

Naturally fractured reservoirs represent one of the most challenging resource in the oil and gas industry. The understanding based on centimeter scale observations is upscaled and modeled at 100-meter scale.

In this paper, we will illustrate with case study examples of conceptual fracture model elaborated using static and dynamic data, the disconnect between the scale of observation and the scale of modelling. We will also discuss the potential disconnect between the detail of fundamental, but necessary, research work in universities against the coarse resolution of the models built in the oil industry, and how we can benefit of the differences in scales and approaches.

The appraisal and development of fractured reservoirs offer challenges due to the variations in reservoir quality and natural fracture distribution. Typically, the presence of open, connected fractures is one of the key elements to achieve a successful development. Fracture modelling studies are carried out routinely to support both appraisal and development strategies of these fractured reservoirs.

Overall fracture modelling workflow consists first of a fracture characterization phase concentrating on the understanding of the deformation history and the evaluation of the nature, type and distribution of the fractures; secondly of a fracture modelling part where fracture properties for the dynamic simulation are generated and calibrated against dynamic data. The pillar of the studies is the creation of 3D conceptual fracture diagrams/concepts which summarize both the understanding and the uncertainty of the fracture network of interest. These conceptual diagrams rely on detailed observations at the scale of the wellbore using core and borehole image data which are on contrasting scale compare to the 10’s of meters to 100’s of meter scale of the grid cells of the dynamic models used for the production history match and forecast. These contrasting scales will be the thread of the presentation.

How to cite: Richard, P. and Bazalgette, L.: Scale discrepancy paradox between observation and modelling in fractured reservoir models in oil and gas industry., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14729, https://doi.org/10.5194/egusphere-egu21-14729, 2021.

It is well known that fracture networks play an important role in fluid circulation in crystalline rock mass. Given that crystalline basements have a negligible primary porosity (porosity of the rock matrix) in comparison to their secondary porosity (porosity due to fractures), fracture characterization generally constitute the most important parameter for the determination of the hydraulic characteristics of the rock mass. Fracture characterization may involve fracture samples from different surveying sources such as outcrops, tunnels and boreholes. For a matter of building a conceptual model, for a study area, the geologist compartmentalizes the study area into several structural homogeneous sub-areas. Those homogeneous sub-areas are called structural domains and how fracture samples are grouped in the same structural domain is the question treated in this presentation.

From field investigations to grouping fracture samples into structural domains, geologists have used methods that are mainly based on the geologist experience and use major structural elements such as faults as domain boundaries. In the case of total absence or limited presence of major structural elements, grouping fracture samples into structural domains becomes complicated. Therefore, several statistical methods which use fracture characteristics have been developed to assist the geologist for that matter. Those methods can be classified into two approaches, which have been introduced by Miller (1983) and Mahtab and Yegulalp (1984). Miller’s approach consists of grouping fracture samples which are totally homogeneous with regard to the fracture characteristic(s) of interest, especially fracture orientation. On the other hand, Mahtab and Yegulalp’s approach consists of grouping fracture samples which share a similar fracture set. While, Miller’s approach got a lot attention, especially in the engineering fields, Mahtab and Yegulalp’s method has the advantage of allowing taking into consideration the blind zones of fracture samples as in practice a fracture sample can hardly be constituted by all the fracture sets of its belonging structural domain. However, Mahtab and Yegulalps’s method ignore fracture characteristics such as fracture spacing, aperture and persistence which are important for fluid circulation in the rock mass.

This presentation proposes a new method that improves Mahtab and Yegulalp’s method by including fracture characteristics such as aperture, persistence and fracture spacing in addition to the fracture orientation considered in the original method. The field investigations took place in the Greenville geological province of the Canadian shield, in Lanaudière region, in Quebec; where fractures were sampled from 30 outcrops and four boreholes. The new method adds a higher level of confidence with regard to the similarity of samples within a structural domain. As a result of the new method, each structural domain has a unique combination of fracture set(s) characteristics which characterize its fracture network. The structural domain compartmentalization impact on the hydrogeological behavior of water flow within the rock mass constitutes the topic of an ongoing research project.

How to cite: Abi, A., Walter, J., Saeidi, A., and Chesnaux, R.: A similarity test for the compartementalization of crystalline rocks into structural domains, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9979, https://doi.org/10.5194/egusphere-egu21-9979, 2021.

Adequately characterizing the properties of a fracture network is the first step in accurately modelling its behavior, be it mechanically or hydraulically. Characterizing fracture networks requires determining fracture frequency, orientation, connectivity, and fracture properties. This becomes particularly challenging in subsurface systems, where hard data on fracture networks comes mainly from boreholes, that are samples of very limited volume in relation to the fracture network. Because of this scale relationship between sample dimension and the dimension of natural fracture networks, boreholes capture a very partial picture of the fracture network. This is particularly relevant when attempting to estimate fracture frequency and network connectivity from borehole data. Corrections are normally used to account for sampling bias related to fracture size and orientation. Whereas these corrections are valid for the sample itself, the topology and heterogeneity of fracture networks means that measurements obtained in any given borehole are not necessarily representative of the broader fracture network.

To determine how “wrong” single-borehole analyses can be, we have conducted experiments on synthetic datasets to quantify how representative borehole samples are of entire fracture networks. Results show that properties that have an impact on the anisotropy of the fracture network (orientation, number of fracture sets) can be accurately resolved even in low data-density scenarios. On the contrary, accurately determining fracture frequency (which also impacts connectivity) for the entire fracture network is strongly dependent on the ratio between fracture frequency and the sampled volume. Measurements of fracture frequency in individual boreholes indicate that it frequency easily be overestimated or underestimated by a factor of 2 relative to the real network’s fracture frequency. The application of sampling bias corrections has a limited impact on reducing this error.

Based on the results from our experiments, we present methods to assess how representative of a fracture network a single borehole is. Representativity can be translated into uncertainty in fracture frequency, a metric that can be used in fracture modelling.

How to cite: Nazari Vanani, F. and Fernandez, O.: Now you see me... : Impact of sample representativity in fracture network characterization., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16341, https://doi.org/10.5194/egusphere-egu21-16341, 2021.

Little research has been conducted to clarify the mechanism, extent, or factors involved in the fracturing of rocks exposed along the Niagara Escarpment in Ontario. Of particular interest are the effects of fluctuating temperatures during region’s cold winters which may be a contributor to the formation and expansion of fractures within these rocks. The results of a preliminary field-based study of temperature changes in fractured sedimentary rocks exposed at several sites along the Escarpment in Hamilton, Ontario are reported here. The objectives of the study were to examine the characteristics of operant thermal processes and to determine the effectiveness of mechanisms such as freeze-thaw and thermal stress in contributing to fracture formation and development. Fractured dolostone units were identified at three field sites along the escarpment that varied in their aspect, vegetation, and proximity to water. At each site, temperature probes were affixed to the exposed rock surface and inserted into a nearby fracture. Temperature measurements were taken at one-minute intervals throughout the winter of 2020-21.  In-situ field measurements of thermal changes within fractured dolostones on the escarpment were supplemented with recordings of rock surface and interior temperatures taken from unfractured dolostone blocks placed in a ‘controlled’ outdoor environment throughout the winter.  Initial results from the escarpment probes in the early winter show frequent, rapid shifts from warm to sub-zero temperatures and indicate that changes in temperature recorded at the rock surface closely follow diurnal atmospheric oscillations in both magnitude and timing.  However, temperature changes recorded within fractures are less extreme and show a temporal lag. Temperature fluctuations recorded at the field site with the highest degree of exposure, a southeasterly aspect, and little vegetation cover, are significantly higher and show larger thermal responses within fractures. Temperature fluctuations recorded from unfractured blocks in the ‘controlled’ outdoor environment show similar diurnal trends to those recorded on the escarpment but with reduced differential between temperatures at the block surface and interior. Together, these data indicate that temperature fluctuations sufficient to generate freeze-thaw cycles are abundant during the early winter months, temperature variability within fractures does occur, and slope aspect and exposure plays an important role in the determining the magnitude of diurnal temperature fluctuations experienced by surface rocks on the escarpment. The role of thermal stress in fracture development, created by rapid and substantial thermal changes, has yet to be determined.  

How to cite: Gage, H., Eyles, C., and Lee, R.: Thermal controls on the development of fractures in dolostones of the Niagara Escarpment, Hamilton, Ontario, Canada, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9140, https://doi.org/10.5194/egusphere-egu21-9140, 2021.

The Niagara Escarpment is a geological feature comprised of highly fractured Ordovician and Silurian shales and carbonates stretching through southern Ontario and parts of the north-eastern United States. Differential erosion of the shale and carbonate strata has generated a steep cliff face bisecting the city of Hamilton, Ontario. Fractures occur throughout the cliff face and result in the formation of loose blocks of rock that are subject to erosion through rockfalls. This presents structural stability issues and an associated geohazard, which is of particular concern due to the proximity of the escarpment to city infrastructure. Previous work has alluded towards the role of geologic fractures in controlling erosion and stability of the Niagara Escarpment, but the causal mechanisms and extent to which these processes operate remains unclear. As such, the aim of this study is to quantify and analyse fracture networks using a combined field and numerical modelling-based approach to understand the distribution and nature of fractures throughout the escarpment, their connectivity, fluid flow properties, and relationship to structural stability. The location, orientation, and aperture of fractures were systematically quantified and documented around Hamilton. Data were plotted and analysed using the software Orient to identify clusters representative of fracture sets and to calculate average fracture set orientations and the respective confidence intervals. Three primary sets of geological fractures were identified including: 1) a near-vertical bedding confined set oriented north-south, 2) a near-vertical bedding confined set oriented east-west and 3) sedimentary bedding planes which have facilitated fracture migration and controlled resultant fracture geometry. Discrete fracture network modelling of these fracture sets in MOVE highlights their high degree of connectivity and indicates that the distribution and nature of these discontinuities are predominant controls on the locations and sizes of rock fragments generated on the cliff face resulting in rockfalls. Moreover, fracture-controlled porosity is a significant contributor to fluid flow throughout the escarpment. We conclude that geologic fractures present a first-order control on the stability of the Niagara Escarpment near Hamilton.

How to cite: Formenti, S., Peace, A., Waldron, J., Eyles, C., and Lee, R.: The influence of fracture networks on stability and geohazards of the Niagara Escarpment in southern Ontario, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9094, https://doi.org/10.5194/egusphere-egu21-9094, 2021.

The Bougouni region is located in the southern Mali, 170 km south-east of Bamako. It is part of the northern edge of the Leo-Man [UdMO1] shield (southern part of the West-African Craton). It is known for its swarm of pegmatites, most investigated as a lithium resource (spodumene pegmatites) and its gold potential. It is made up of metavolcanosedimentary rocks and plutonic complexes of the Birimian (Paleoproterozoic).

The objectives of this study are: 1) to make a review of the main regional structural features, 2) to define the structural control of the pegmatite emplacement. To reach these aims a structural analysis was carried out using the information collected during our field campaigns (2018 and 2019) and data from previous works.

Metavolcanosedimentary rocks had undergone a low grade metamorphism degree. Schistosities in these rocks (n=156 measurements) are distributed along 2 main directions which indicate two deformation phases. The first one, D1, oriented NNW-SSE to N-S. It is ductile, responsible for low grade regional metamorphism (Baratoux et al., 2011) and crustal thickening of the volcano-sedimentary rocks (Wane et al., 2018). It is linked to a compression event oriented E-W to ESE-WNW (Baratoux et al., 2011; Wane et al., 2018). The second one, D2, oriented NNE-SSW to SE-NW, is a ductile-brittle transpressive deformation. It affects metavolcano-sedimentary rocks. It would be contemporaneous with the location of most birimian granitoids (Baratoux et al., 2011; Wane et al., 2018) and it would be responsible for the gold mineralization (McFarlane et al., 2011). The third one, oriented E-W is marked by fracture cleavage and extensional cracks sometimes sigmoidal and generally filled by quartz. It has a brittle-ductile character. The faults have been observed in the field but weren’t measured. Feybesse et al. (2006) showed that they intersect all the lithologies without having any direct link with the different deformation phases.

Pegmatites are hosted in both metavolcanosedimentary and granitoids. Lithium bearing pegmatites are characterized by an assemblage containing spodumene (15-65%; main lithium-host mineral), albite (10-55%), microcline (1-10%), quartz (25-50%) and muscovite (2-10%). The accessory minerals are: apatite; garnet; columbo-tantalite, tourmaline, beryl and rutile. Lithium pegmatites are distributed in three directions (n=209): NNE-SSW (minor, ≃10 %?), ENE-WSW to E-W and ESE-WNW to SE-NW. Most of them are characterized by a steep dip (≃90°). The dyke-host unit contacts are generally sharp and brittle. These results suggest that the emplacement of the lithium bearing pegmatites of Bougouni even of all the Birimian pegmatites (Example of Issia pegmatite, côte d'ivoire Allou, 2005) could be related to the brittle deformation phase D3. This phase is also thought to be linked with the gold mineralization (McFarlane et al., 2011).

Ref: Allou, 2005, thèse à l’université de Chicoutimi ; Baratoux et al., 2011, Precamb. Res. 191, 18–45 ; Feybesse et al., 2006, Carte et Notice explicative de la Carte du Birimien du Mali ; McFarlane et al., 2011, Econ. Geol. 106, 727-750; Wane et al., 2018, Precamb. Res. 305, 444-478

Key words: pegmatite, mineralization, lithium, spodumene, granitoid, Leo-Man shiel, Mali, West African Craton

How to cite: Sanogo, S., Durand, C., Dubois, M., and Wane, O.: Structural constraints of the Birimian lithium pegmatites of Bougouni (Southern Mali, Leo-Man shield), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6752, https://doi.org/10.5194/egusphere-egu21-6752, 2021.

Faults and fractures play a key role in the permeability of the upper crust. Since anticlines represent very common structural traps for fluids, geometrical (i.e., orientation, length distribution) and topological (i.e., cross-cutting and abutting relationships, intensity) characterization of their fracture network is crucial to assess the migration and accumulation of fluids for CO₂ sequestration or hydrocarbon exploitation purposes. For this reason, many previous studies focused on anticlines worldwide, and in particular on the Zagros fold-and-thrust belt where they represent the outcropping analogs of oil fields in SW Iran.

The Zagros fold-and-thrust belt involve sediments of the pre-collisional Arabian plate passive margin, arranged in folds elongated in a NW-SE direction and tectonic transport toward SW. The belt is dissected by N-S dextral strike slip transfer faults reactivating former rift-related normal faults. Most of the studies on fracturing in the Zagros belt are based on fracture orientation data collected mainly in the field, or alternatively coming from satellite images, and deal with the origin of fracture sets (fold-related or not). Although two of the classical fold-related sets, oriented roughly parallel and perpendicular to fold axis (i.e., NW-SE and NE-SW striking respectively) can be generally recognized everywhere in the belt, other fracture orientation (e.g., N-S and E-W striking) are locally predominant and there is still no consensus on the nature of all fracture sets. For example, the role of the strike-slip reactivation of N-S and E-W striking inherited faults on fracture set distribution is still not clear.

In this study we leverage on high quality Bing Maps satellite images of the Zagros anticlines and on scanlines performed in the field to provide a multiscale investigation of geometry and topology of the fracture network affecting three anticlines, namely Sim, Kuh-e-Asmari, and Kuh-e-Sarbalesh. The three anticlines have similar dimensions and are variably affected by ~N-S striking dextral strike slip tectonic lineaments. In particular, Kuh-e-Asmari and Sim anticlines are located ~10km far from the Izeh and Sabz-Pushan faults respectively, whilst the Kuh-e-Sarbalesh anticline is characterized by an evident drag in map view against the Kazerun fault.

We manually interpreted the fracture network on satellite images at different scales (1:100 to 1:100.000), producing fracture maps with resolution ranging from 10m to 1km. Each fracture map was then analyzed using the NetworkGT plugin in QGIS. In particular, we were able to identify fracture sets, their spatial distribution and, were possible, the topology of the fracture network. In this framework, scanlines performed in the field represent punctual observations at furtherly higher resolution (~1 cm). Following the same procedure for the three anticlines enables us to test the role of N-S faults on fracture set distribution at various scales.

With such a multiscale approach we provide a “big picture” that can help to shed light on the nature and distribution of the various fracture sets in the anticlines of the Zagros belt. Moreover, fracture sets identified at different scales in this study can be used to better interpret previous and future fracture data collected in the field.

How to cite: Mercuri, M., Carminati, E., Aldega, L., and Trippetta, F.: Geometry and topology of fractures and faults affecting anticlines in the Zagros fold-and-thrust belt: a multiscale approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7774, https://doi.org/10.5194/egusphere-egu21-7774, 2021.

The Esla Nappe is located in the foreland and thrust belt of the Variscan Orogen (Cantabrian Zone, NW Iberia). It is formed by a near-complete Palaeozoic sedimentary succession. With a displacement of around 19 km, the nappe was emplaced along a thin (<2–3 m) basal shear zone (ENSZ) at a minimum depth of 4 km during the Moscovian (ca. 312 Ma). Fault-rock assemblages record a variety of alternating deformation mechanisms and processes, including cataclastic flow, pressure solution and hydrofracturing and vein precipitation.

Following emplacement, the ENSZ was breached by clastic dykes and sills injected within re-opened previous anisotropies such as bedding planes, thrust surfaces, joints and stylolites. Together, they constitute an interconnected network of quartz sand-rich lithosomes that reach structural heights in excess of 20 m above the ENSZ. The orientation of the dykes suggests that the injection process took place under low differential stress conditions in the hangingwall and near-lithostatic fluid pressure conditions in the footwall. The injected slurry consisted of overpressured pore fluid, quartz-sand grains derived from the footwall and entrained host-rock fragments. The temperature of the fluids estimated from the clumped isotope composition of calcite cements is 71–86 °C, with an average of 80 ± 4 °C. The calcite isotopic composition (δ¹³C = -0.15, δ¹⁸O = -9.53, both VPDB) is well within the typical values of the host Láncara Fm., which suggests that the fluids achieved equilibrium with the host prior to calcite precipitation. Using this calculated temperature and depth estimates for the base of the Esla Nappe, the geothermal gradient during deformation is estimated to be in the order of 16–24 °C/km, a relatively low value.

Flow conditions within the injections have been inferred from properties such as the particle drag coefficient, morphology, diameter and concentration, and the fluid density and viscosity, necessary for the calculation of the terminal fall velocity of the particle array. Thin injections formed of pure quartz, with a thickness <1 cm, are consistent with flow velocities of 0.01–0.35 m/s and a laminar flow (Reynolds number (Re) <800). Thicker pure quartz injections (<10 cm), on the other hand, required faster flow velocities (0.35 m/s) and transitional to turbulent flows (800 < Re < 8000). The thicker injections (≈1 m) that entrained larger host-derived fragments would require transitional to turbulent flows (1200 < Re < 1.2×10⁴) at fast velocities (0.35 m/s).

The estimated geothermal gradient is consistent with the lower estimations for current foreland basins, and very similar to ocean trenches. The velocities and Reynolds numbers derived for the Esla Nappe are larger than usually estimated for deep seated injections without hydraulic connection with the surface, where the vertical pressure gradient driving them is limited. In those cases, laminar flow conditions are usually invoked, but our results suggest that turbulent flow is possible in the thicker injections. Nonetheless, the values are lower than those reported for shallow injections in connection with the surface.

How to cite: de Paz Álvarez, M. I., Llana-Fúnez, S., Bernasconi, S. M., Alonso, J. L., and Stoll, H. M.: Temperature and flow conditions of quartz-sand injections at the base of the Esla Nappe (Cantabrian Zone, NW Iberia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8415, https://doi.org/10.5194/egusphere-egu21-8415, 2021.

Transfer faults are generally identified as transversely oriented discrete faults linking normal fault segments in extensional tectonic settings.  The presence of the transfer faults in fault networks provides displacement transfer between the normal faults. The role and tectonic significance of transfer faults in overall extensional deformation of the upper crust is however not known very well. Micropolar theory extended by J-2 plasticity facilitates evaluation of a deforming medium in which cataclastic flow takes place with respect to each component of deformation. In this study, a series of experiments based on the Micropolar theory are performed, using fault-slip patterns, to better understand interplay among dip angle of normal and transfer faults connecting to each other, angle of linkage, and extensional direction. Synthetic linkage cases are created systematically considering various orientation of both faults sharing common stretching direction.
Our findings reveal that in orthogonal and oblique linkage cases, 3D strain field is mostly observed; a few cases exhibit plane strain. All cases are subjected to simple shearing. In cases of orthogonal linkage, extensional direction is predominantly oblique to the strike of the normal faults. Many of these cases have no block rotation (microrotation) independent from macrorotation. No particular relationship between changing dip amount of faults and direction of extension is observed. In cases of oblique linkage, (sub)orthogonal direction of extension appear in nearly half of experiments, especially those including normal faults dipping less than 60˚. The frequency of non-zero microrotation is seen apparently more than that in orthogonal linkage cases.
The study represents that structural togetherness of the transfer and normal faults essentially can accommodate complete micropolar strain in a region. This further suggests that not only the normal faults but the transfer faults should also be considered as major primary structural elements in extending domains.

How to cite: Tokay, B. and Bozkurt, E.: Strain Analysis of Transfer Faults in Extending Regions: Inferences from Micropolar Theory, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14589, https://doi.org/10.5194/egusphere-egu21-14589, 2021.

Fluid mixing is one of the important ore-forming processes for hydrothermal mineralization. A common hypothesis envisages hot metal-bearing fluids entering rocks that contain brines derived from paleo-seawater. Upwelling of pressurized hydrothermal fluids may occur along permeable faults or fracture systems along fluid pressure gradients. When the upwelling- and pore fluids mix, the conditions for ore-precipitation are met for instance by varying pH, temperature, or redox-conditions. This would be reflected best by changes in the saturation states of the respective mineral phases.

However, how does this process work in detail? How are the two fluids moving, where are they are meeting, how are they mixing and what minerals are precipitating? In order to study such a system, we link a transport model in the software ELLE with the IPHREEQC library of the USGS. We investigate the case of two fluids, a pore fluid representing paleo-seawater and a hot metal-rich fluid that enters the system at high fluid overpressure. As an initial condition, the pore fluid is equilibrated with different mineral phases reflecting different lithologies with varying permeabilities. Temperature as well as multi-components in the system are advecting along pressure gradients as a function of the local Darcy velocity that is calculated as the influx of a compressible fluid initially entering the system and subsequently leading to a flux through the system. In addition, the different species are diffusing as a function of the timescales set by the pressure diffusion into the system. IPHREEQC then calculates the fluid properties and mineral saturation states for every node in the system.

We show that a reactive wave develops between the two fluids, pore matrix, and infiltrating fluid. The incoming fluid pushes the pore fluid out of the system and the mixing process is mainly governed by diffusion. Depending on the time scales involved, minerals will preferably precipitate between the fluids in relatively “quiet” domains where the reactive zones prevail for an extended time. An example are fault walls where the reactive zone is stable for a longer time, whereas it is moving relatively fast along the faults. We discuss the implications of our observations with respect to low temperature ore deposits, present a first model of reactive domain development in a two-fluid system, and compare the results obtained by utilizing different thermodynamic databases.

How to cite: Koehn, D., Mullen, G., Boyce, A., Ulrich, K., and Toussaint, R.: Fluid mixing and mineralization along faults – the role of stable and traveling reactive zones, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16293, https://doi.org/10.5194/egusphere-egu21-16293, 2021.

Understanding the impact of fault zones on reservoir trap properties is a major challenge for a variety of geological ressources applications. Fault zones in cohesive rocks are complex structures, composed of 3 components: rock matrix, damage zone fractures and fault core rock. Despite the diversity of existing methods to estimate fault zone permeability/drain properties, up to date none of them integrate simultaneously the 3 components of fracture, fault core and matrix permeability, neither their evolution with time. We present a ternary plot that characterizes the fault zones permeability as well as their drainage properties. The ternary plot aims at (i) characterizing the fault zone permeability between the three vertices of matrix, fractures and fault core permeability ; and at (ii) defining the drain properties among 4 possible hydraulic system: (I) good horizontal and vertical, fault-perpendicular and -parallel; (II) moderate parallel fluid pathway; (III) good parallel fault-core and (IV) good parallel fractures. The ternary plot method is valid for 3 and 2 components fault zones. The application to the Castellas Fault case study show the simplicity and efficiency of the plot for studying underground and/or fossil, simple or polyphase faults in reservoirs with complete or limited permeability data.

How to cite: Aubert, I., Lamarche, J., and Leonide, P.: Partitioned Permeability Diagram: an innovative way to estimate Fault Zones hydraulics., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12238, https://doi.org/10.5194/egusphere-egu21-12238, 2021.

Faults, joints and stylolites are ubiquitous features in fold-and-thrust belts commonly used to reconstruct the past fluid flow (or plumbing system) at the scale of folded reservoir/basins. Through the textural and geochemical study of the minerals that fills the fractures, it is possible to understand the history of fluid flow in an orogen, requiring a good knowledge of the burial history and/or of the past thermal gradient. In most of the case, the latter derives from the former, itself often argued over, limiting the interpretations of past fluid temperatures. We present the results of a multi-proxy study that combines novel development in both structural analysis of a fracture-stylolite network and isotopic characterization of calcite vein cements/fault coating. Together with new paleopiezometric and radiometric constraints on burial evolution and deformation timing, these results provide a first-order picture of the regional fluid systems and pathways that were present during the main stages of contraction in the Tuscan Nappe and Umbria-Marche Apennine Ridge (Northern Apennines). We reconstruct four steps of deformation at the scale of the belt: burial-related stylolitization, Apenninic-related layer-parallel shortening with a contraction trending NE-SW, local extension related to folding and late stage fold tightening under a contraction still striking NE-SW. We combine the paleopiezometric inversion of the roughness of sedimentary stylolites - that provides a temperature-free constraint on the range of burial depth of strata prior to layer-parallel shortening -, with burial models and U-Pb absolute dating of fault coatings in order to determine the timing of development of mesostructures. In the western part of the ridge, layer-parallel shortening started in Langhian time (~15 Ma), then folding started at Tortonian time (~8 Ma), late stage fold tightening started by the early Pliocene (~5 Ma) and likely lasted until recent/modern extension occurred (~3 Ma onward). The textural and geochemical (δ¹⁸O, δ¹³C, ∆₄₇CO₂ and ⁸⁷Sr/⁸⁶Sr) study of calcite vein cements and fault coatings reveals that most of the fluids involved in the belt during deformation are basinal brines evolved from various degree of fluid rock interactions between pristine marine fluids (δ¹⁸O_fluids = 0‰ SMOW) and surrounding limestones (δ¹⁸O_fluids = 10‰ SMOW). The precipitation temperatures (35°C to 75°C) appear consistent with the burial history unraveled by sedimentary stylolite roughness paleopiezometry (600 m to 1500m in the range) and geothermal gradient (23°C/km). However, the western edge of the ridge recorded isotopically depleted past fluids of which corresponding precipitation temperature (100°C to 130°C) are inconsistent with local burial history (1500m). We interpret then pulses of eastward migration of hydrothermal fluids (>140°C), driven by the tectonic contraction and by the difference in structural style of the subsurface between the eastern Tuscan Nappe and the Umbria-Marche Apennine Ridge. Allowed by an unprecedented combination of paleopiezometry and isotopic geochemistry, this fluid flow model illustrates how the larger scale structures control the fluid system at the scale of the range.

How to cite: Beaudoin, N., Labeur, A., Lacombe, O., Koehn, D., Billi, A., Hoareau, G., Boyce, A., John, C., Marchegiano, M., Roberts, N., Millar, I., Claverie, F., Pecheyran, C., and Callot, J.-P.: Long-term orogenic-scale paleofluid system across the Tuscan Nappe – Umbria-Marche Apennine Ridge (northern Apennines, Italy) as revealed by mesostructural and isotopic analyses of stylolite-vein networks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-561, https://doi.org/10.5194/egusphere-egu21-561, 2021.

The isotopic carbon and oxygen isotope composition of carbonates (δ¹³C and δ¹⁸O), determined by temperature and the relative abundances of stable isotopes of both elements in water at the time the carbonate is precipitated, can be modified subsequently during geological processes that involve the recrystallization of carbonate. Temperature changes mostly affect δ¹⁸O, while additional sources of carbon have a greater impact on δ¹³C. Amongst the various processes that may alter the original isotopic signature of carbonate rocks are deformation processes, which can lead the dissolution and reprecipitation of carbonates during deformation, or the involvement of fluids of various origin during younger tectonic events.

Here, we present the results of isotope analysis in fault rocks from two distinct faults in the Cantabrian Zone (CZ) in northern Spain. It represents the foreland fold and thrust belt of the Variscan orogen in Iberia and is characterized by numerous and large thrust sheets that were emplaced during the Carboniferous. Subsequent rifting episodes in the Mesozoic and more recently Alpine North-South convergence produced the overprinting of some of the earlier Variscan structures. In all cases, brittle processes produced often similar-looking rocks as the fracturing occurred under upper crustal conditions, relatively close to the surface. Fluids involved during deformation on both cycles are likely to differ, so to evaluate alternative tools to distinguish the different cycles of fracturing in carbonates, a stable isotope analysis on carbon and oxygen was undertaken in two well-known structures in the region: the Somiedo nappe and the Ventaniella fault.

The Somiedo nappe is one of the largest thrust sheets in the Cantabrian Zone, with an estimated offset close to 20 km. The base of the thrust sheet is characterized by well-developed cataclasites and ultracataclasites that formed on Cambrian fine-grained dolostones. It has relatively minor vein activity associated, although the dolostones have been partially recrystallized. The Ventaniella fault is a dextral strike-slip structure cutting obliquely the Cantabrian Mountains. It runs for tens of kilometres inland and has an estimated offset of approximately 5 km. The fault zone in the studied area is characterized by the fracturing and dextral offset of Carboniferous micritic limestones and, more importantly, a relatively strong vein activity that formed a distributed network of calcite veins.

Cataclasite matrix and fragments, and associated veins were sampled for isotope analysis in the two fault zones. In both cases, the matrix has a signature which is intermediate between the undeformed rock and that of the veins. The fragments have a signature which is indistinguishable from the matrix, suggesting the reworking of the fault rock. The veins have a distinct pattern in both faults, but different from each other. Those related to the Ventaniella fault are mostly hydrothermal, with limited range in δ¹⁸O and δ¹³C, while the veins from the base of the Somiedo nappe have a larger range of δ¹³C, but limited δ¹⁸O variation.

How to cite: Llana-Funez, S., de Paz-Álvarez, M. I., Lopez-Sanchez, M. A., Bernasconi, S. M., Alonso, J. L., Berrezueta, E., Méndez, A., and Stoll, H.: Carbon-oxygen isotope analysis of fault rocks in carbonates from different orogenic cycles in the Cantabrian Zone (N Spain): similarities and differences, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7335, https://doi.org/10.5194/egusphere-egu21-7335, 2021.

In the eastern Paris Basin, the Oxfordian (Upper Jurassic) and Bathonian to Bajocian (Middle Jurassic) carbonate platforms have been intensively cemented, despite a relatively low burial history (< 1000 m). These limestones units are separated by a 150 m thick succession of Callovian-Oxfordian tight clay-rich rocks that are currently investigated by the French national radioactive waste management agency (Andra).

Most of the initial porosity in the Middle and Upper Jurassic limestones is now cemented by successive stages of calcite, which were thoroughly characterized both petrographically and geochemically over the last fifteen years (Buschaert et al., 2004; Vincent et al., 2007; Brigaud et al., 2009; André et al., 2010; Carpentier et al., 2014). However, despite such research efforts, the timing and temperature of the fluids involved in the cementation of these carbonate rocks are still debated.    

Here, we complement these efforts by coupling ∆₄₇ temperatures and U-Pb ages on calcite cement filling tectonic microfractures, as well as the intergranular pore space and vugs.

Our findings indicates that the Middle Jurassic limestones were largely cemented during the Late Jurassic / Early Cretaceous period, with new LA-ICP-MS U-Pb ages (Brigaud et al., 2020) in agreement with previously published Isotope Dilution-TIMS U-Pb age of 147.8 ± 3.8 Ma (Pisapia et al., 2017). This event is believed to be associated to the Bay of Biscay rifting. A second and more discrete crystallization event occurred during the Late Eocene / Oligocene period, related to the European Cenozoic Rift System (ECRIS).

The Upper Jurassic limestones were by contrast affected by a broader range of successive deformation events spanning the Late Mesozoic / Cenozoic period. New LA-ICP-MS U-Pb ages acquired in ca. 200 µm-thick fractures show that calcite crystallized during three successive periods corresponding respectively to the Pyrenean compression, the ECRIS extension and the Alpine compression.

Our study highlights tectonic stress propagation across hundreds of kilometers, from the rifting or collisional areas toward the cementation area of carbonate rocks. Thanks to the direct radiometric dating and clumped isotope thermometry of calcite cements in microfractures, a refined paragenetic sequence is proposed with emphasis on the genetic link between large-scale deformation and calcite precipitation.

References :

Buschaert et al., 2004. Applied Geochemistry 19, 1201 – 1215. Vincent et al., 2007. Sedimentary Geology 197, 267 – 289. Brigaud et al., 2009. Sedimentary Geology 222, 161 – 180. André et al., 2010. Tectonophysics 490, 214 – 228. Carpentier et al., 2014. Marine and Petroleum Geology 53, 44 – 70. Pisapia et al., 2017. Journal of the Geological Society of London 175, 60 – 70. Brigaud et al., 2020. Geology 48, 851 – 856.

How to cite: Blaise, T., Brigaud, B., Carpentier, C., and Mangenot, X.: Relationships between intraplate deformations and the cementation of Jurassic carbonates in the eastern Paris Basin revealed by calcite U-Pb geochronology and ∆47 thermometry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7400, https://doi.org/10.5194/egusphere-egu21-7400, 2021.

In the last decade, important improvements in dating methods have been made and make it possible to go into the details of fault gouge formation and evolution. Common minerals like calcite and hematite can now bring detailed information on timing of fault development and fluid-rock interaction. We applied those novel techniques to a tectonically well constrains alpine context, though still lacking key chronological constrains. The targeted fault zone is the Penninic Frontal Thrust (PFT) of SW Alps, which is a major tectonic boundary that juxtaposed the metamorphic internal Alps over the unmetamorphosed external Alps, primarily as a thrust during the Oligocene (Simon-Labric et al., 2009). The PFT was later reactivated as an extensional detachment in the Mio-Pliocene, though the age of this reactivation remained unconstrained. Sue and Tricart (2003) showed that ongoing extensional seismic activity along the PFT, corresponding to the High-Durance Fault System (HDFS), is characterized at the surface, by an extensional fault network. In this context, the HDFS corresponds to extensional reactivation of the PFT as a consequence of Pelvoux external crystalline massif exhumation.

In this study, we coupled field tectonic, in-situ calcite U-Pb and hematite (U-Th-Sm)/He dating to stable and clumped isotope analysis to infer the HDFS activation age and to investigate the related fluid circulations. Isotopic signature (δ¹³C and δ¹⁸O) of compressional veins, en-echelon extensional veins and cataclasite fault gouge have been determined (Bilau et al., 2020).

This study allows pinpointing the evolution of deformation and fluid-rock interaction in the PFT footwall during its progressive extensional exhumation. The older U-Pb ages obtained on the cement of the gouge fault range between 5 to 3.5 Ma and taking into consideration uplift rate, comparison to currently seismicity depth and calcite brittle/ductile transition temperature, calcite crystallization may have occurred between 5 to 2 km. The hematite crystallization appears at shallower levels in the latest stages of the fault displacement at 3-1 km depth. A transition in the nature of fluids, controlling the redox state, can be highlighted here. This transition occurs between the calcite and hematite forming events at 2-3 km depth, which is probably related to a significant influx of meteoric fluids into the drainage of the fault system.

How to cite: Bilau, A., Rolland, Y., Schwartz, S., Godeau, N., Guihou, A., Deschamps, P., Gautheron, C., Pinna-Jamme, R., Brigaud, B., Mangenot, X., Noret, A., Melleton, J., and Dumont, T.: Timing of fault gouge formation and fluid-rock interaction during tectonic inversion of the Penninic Frontal Thrust (SW Alps), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7161, https://doi.org/10.5194/egusphere-egu21-7161, 2021.

The timing and duration of fold-related deformation - including layer-parallel shortening (LPS) – related to fold growth, are difficult to estimate because of a lack of data in most natural cases. We propose an original combination of direct and indirect dating methods to reconstruct the burial-deformation history of the Mesozoic carbonates that crop out in the Cingoli Anticline in the Umbria-Marche Apennine Ridge.

The Cingoli anticline displays an arcuate shape in map view, trending N140 in its northern part and N160 in its southern part). We first study the fracture-stylolite network to characterize the successive stages of deformation. Several sets of mesostructures were discriminated according to their orientation and relative chronology:

• (i) N-S trending vertical joints (after unfolding), likely related to foreland flexure/forebulge development;
• (ii) N045 trending vertical, bedding and fold-axis perpendicular joints/veins, associated with early folding stylolites with N045-oriented peaks and reverse faulting associated with a N045 σ₁ (after unfolding), reflecting LPS;
• (iii) bedding-perpendicular and fold axis -parallel joints/veins, e., trending N140 in the northern part and N160 in the southern part of the anticline, reflecting outer-arc extension associated to strata curvature at fold hinge, the variation about 20° in orientation between the northern and southern parts of the fold being consistent with the arcuate shape of the anticline;
• (iv) tectonic stylolites with horizontal peaks striking N045, along with conjugate vertical strike-slip faults, associated with a horizontal N045 contraction affecting the strata after the fold was locked, corresponding to the late stage of fold tightening (LSFT).

These results suggest that the Cingoli anticline developed under a continuous N045 contraction and that its arcuate shape is likely primary and was achieved in relation to the reactivation of an N-S normal fault inherited from the Tethyan rifting, without any vertical-axis rotation of the fold axis.

We further reconstructed burial curves considering sedimentary formation thicknesses, corrected from physical and chemical compaction. We also quantified the vertical stress experienced by sedimentary stylolites from a roughness inversion technique, allowing derivation of the maximum depth experienced by the strata prior to contraction (using bedding-parallel sedimentary stylolites) and during exhumation (using horizontal sedimentary stylolites related to a post-folding compaction). Oxygen and carbon isotope ratios measured in tectonic vein cements point towards a locally-sourced fluid system with limited vertical migration at the scale of the carbonate core, enabling the use of the absolute temperatures obtained from CO₂ clumped isotope (D₄₇) to reconstruct the depth during layer-parallel shortening and folding. The comparison of reconstructed depth at which each deformation phase occurred with the burial curve provides absolute timing for the development and exhumation of the Cingoli Anticline. Together with U-Pb ages of calcite vein cements and fault coatings from the nearby San Vicino Anticline, located west of the Cingoli Anticline, our data suggest that contraction started by ~8 Ma (LPS) and ended by ~3 Ma (LSFT), and that the growth of the Cingoli anticline lasted from ~5.5 to 4 Ma.  

How to cite: Labeur, A., Beaudoin, N. E., Lacombe, O., Hoareau, G., Petraccini, L., Emmanuel, L., Daëron, M., and Callot, J.-P.: Burial-deformation history of an arcuate fold unraveled by fracture analysis, stylolite paleopiezometry and vein cement geochemistry: a case study in the Cingoli Anticline (Umbria-Marches, northern Apennines), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12742, https://doi.org/10.5194/egusphere-egu21-12742, 2021.

Basement/cover interfaces are important transfer zones for hydrothermal fluids responsible for ore deposition, such as U and Pb-Zn deposits. Unconformities are peculiarly mixing zone where basement-derived fluids encounter sedimentary- and/or meteoric-derived fluids; leading to precipitation of these ores. Fluids are channelized by permeability contrast, i.e. impermeable barriers, until being trapped in porous units, i.e. intrinsic porosity and/or secondary porosity (dissolution and karstification process). In this configuration fracturing channelize the fluid flow by breaking impermeable barriers allowing external fluids to enter and react with the rocks (precipitation and/or dissolution). In this way, structural studies are crucial to highlight the fracture network and the potential of geological units to be good reservoirs.

In France, many occurrences of sediment-hosted deposits are known in Mesozoic basins (i.e. Aquitaine and Paris Basin) especially above the Variscan basement (Morvan district, SW Massif Central district, Poitou High district). The Vendée coast deposit (South Armorican Massif, France) is known for two Pb-Zn(-Ag) occurrences located in Liassic sediments overlying the Variscan basement. Previous works show that, during the Upper Jurassic extensional event (NNE-SSW horizontal stretching), the ore deposition results from the mixing of two different fluids: (1) low temperature brines following a horizontal path from evaporite to basin borders within Liassic sediments along the unconformity, (2) a high temperature and low salinity fluid rising up through the basement from several kilometres depth by a probable vertical pathway.

However, the permeability architecture leading to such mixing remains poorly constrained. The Vendée ore deposits present favourable outcrop conditions to study the structural control of the fluid plumbing system along the basement/cover unconformity. Structural studies assisted by drone imagery coupled with the characterization of the alteration-mineralization pattern show that:

(1) Horizontal path for basin brines is controlled by the impermeable barrier of the Toarcien layer overlying Liassic hosting karsts.

(2) Vertical path of basement-derived fluids is enhanced by new faults and inherited fractures, respectively generated and reopened by the Jurassic extension.

(3) Relative abundance of faults and veins in the Liassic sedimentary cover and the basement is consistent with a mechanical decoupling in a context of fluid overpressure.

How to cite: Bouat, L., Strzerzynski, P., Mourgues, R., and Branquet, Y.: Horizontal and vertical fluid flows as a key control of ore deposition at the basement/cover unconformity: insight from drone imagery of the Vendée coast, France, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15409, https://doi.org/10.5194/egusphere-egu21-15409, 2021.

Archean greenstone belts gained prominence for its gold mineralization. Gold bearing vein infillings within fracture systems are significant for its economic utility. Fracture formation is often associated with reactivation of the pre-existing host rock fabric under a compatible stress field. Upper crustal fluids are mostly channelized through these fracture systems under variable fluid pressure conditions generating a widespread network of veins. A wide range of vein infilled crosscutting fractures of variable thicknesses, are investigated from the gold-bearing massive metabasalts (supracrustals) of the Chitradurga Schist Belt (CSB) adjacent to the Chitradurga Shear Zone (CSZ), Western Dharwar Craton, southern India. Anisotropy of magnetic susceptibility (AMS) studies are adopted for determining the internal anisotropy of the apparently massive metabasalt hosts. The study involves tensile strength determination of the metabasalts, deciphering the paleostress condition using fault-slip analysis and propensity of fracture/fault reactivation under the prevailing stress field. Parameters like stress ratio (ϕ) and driving pressure ratio (R´) are evaluated for understanding the conditions of fluid induced fracture opening/reactivation. Change in the opening angle (µ) of fractures with fluid pressure (P_f) variation, ϕ and R´ variations with the range of fracture orientations are also ventured upon.

            We conclude ~NW-SE oriented (mean 337°/69° NE) magnetic fabric in the metabasalts are a product of regional D1/D2 deformation on an account of NE–SW shortening. This was followed by the D3 deformation with NW–SE to E–W shortening that led to the sinistral movement along CSZ. Thus, prominent fracture orientations representing riedel shear components were formed as a consequence of this sinistral shearing. Under compatible fluid pressure conditions, all such cohesionless pre-existing pathways were reactivated. Schematic models help to understand the mechanism of vein emplacement under episodic fluid pressure fluctuations from high to low P_f at shallow crustal depth (~2.4 km). With respect to the prevailing stress field, fracture orientations coinciding with the host rock fabric show higher values of slip/dilatation tendencies justifying maximum vein thickness along this orientation. Multiple methods were integrated to develop a better understanding of the fracture networking system, channelizing fluids and assisting gold mineralization in the greenstone belt.

How to cite: Mondal, T. K. and Bhowmick, S.: Role of pre-existing fabric in abetting fracture formation, fluid flow and vein emplacement in the metavolcanics: a domain for shallow crustal gold mineralization in the Archean greenstone belt, India, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1943, https://doi.org/10.5194/egusphere-egu21-1943, 2021.

The Picos de Europa Region constitutes one of the outermost areas of the Cantabrian Zone, the foreland and thrust belt of the Variscan orogen in NW Iberia. It constitutes a thrust imbricate formed of Carboniferous limestones that was emplaced towards the S-SW during the latest Pennsylvanian. During the Permian and throughout the Mesozoic, the area was subjected to extension, as attested by the scarce remnants of contemporary sedimentary successions. During the N-S Cenozoic Alpine convergence between Iberia and Eurasia, the Picos the Europa Massif was deformed under shallow crustal conditions through the reactivation of previous structures.

Zn-Pb ores, in the form of sphalerite and galena, are abundant in the central and eastern sections of the Picos de Europa Massif, where they formed as Mississippi Valley-type deposits. Although a direct dating of the minerals has not been performed to date, indirect attempts have been made based on field observations and paleomagnetic studies that have resulted in a broad span of age estimations comprised between Permian and Cenozoic times. Our ongoing research includes the study of Pb isotopes within galena samples in several localities in the Picos de Europa. The measured Pb isotopic ratios (206Pb/204Pb = 18.604–18.771, and 207Pb/204Pb = 15.686–15.707) are comparable to those of other Mississippi-Valley-type and Sedex-type ore deposits situated further east in the Basque-Cantabrian Basin. This basin was formed throughout the Mesozoic as an extensional basin, and the associated ores have been dated through ore-typology (syn-sedimentary Sedex-type deposits), metallogenic data, and other geological criteria. The similarity of the isotopic ratios in these deposits and our samples from the Picos de Europa Massif suggests a similar ore formation age, around the Lower Cretaceous, based on the interpretation of a comparable Pb crustal source.

The ores from Picos de Europa are largely associated with kilometre-scale faults that have acted simultaneously as fluid conduits and zones of preferential mineralisation. Many of the studied localities display significant deformation of the ore deposits, suggesting subsequent fault reactivation events following precipitation. Thus, the age of the deposits is useful for determining the relative timing of fault reactivation. There are two main mineralised fault systems: faults trending W-E with a variable dip, and a subvertical NW-SE-trending set. Faults from the first system were originally developed as Variscan thrusts and in some cases reactivated as normal and/or, subsequently, reverse faults during the Alpine orogenic cycle (e.g. the Cabuérniga Fault System). In contrast, the age and kinematics of the second fault system are more debated. Zn-Pb deposits from the Ándara and Liordes mining districts constitute illustrative examples of ore precipitation and subsequent brittle deformation along the San Carlos N118E-trending subvertical fault and the Liordes N117E-trending high-angle fault. While the San Carlos Fault accommodated an oblique but mainly dextral strike-slip displacement during ore deformation, the Liordes Fault acted as a dextral oblique fault with a larger reverse component, likely as a result of its slightly different dip angle. The last activity on these structures post-dates the Lower Cretaceous, suggesting a clear linkage with the Alpine orogeny.

How to cite: Flórez-Rodríguez, A. G., García-Sansegundo, J., and Martín-Izard, A.: Zn-Pb deposits as a clue for recognising reactivated structures in the Picos de Europa area (NW Spain), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13558, https://doi.org/10.5194/egusphere-egu21-13558, 2021.

Detailed knowledge on the temporal and spatial distribution of faults and fractures not only reveals the geodynamic and tectonic evolution of the lithosphere. It is also of increasing importance with regard to economic, social, and environmental challenges such as nuclear waste disposal, gas storage, geothermal energy, natural hazards, and mineral resource exploration. In this context reliable data on both timing and kinematics of deformation and their regional impact on faulting and fracture formation provide crucial information to evaluate exploration, storage, and production risks, which in turn stresses the need for comprehensive data on paleostress fields and their influence on deformation, fault reactivation, fluid activity, and hydrothermal mineralization.

In this study we present a first comprehensive approach to compile and visualize information on the crustal paleostress field of Central Europe with a focus on northern Bavaria and adjacent areas. The compilation includes published structural data from kinematic paleostress analyses (e.g. fault-slip analysis, tectonic stylolites) and geo- and thermochronological ages of fracture mineralization and fault activity, respectively. The present compilation comprises structural records from more than 40 studies and age information from more than 100 geo-thermochronological studies. All structural data are categorized according to its tectonic stress regime and quality-ranked for reliability and comparability. The consequent linkage of structural data with thermochronological data wherever possible allows to correlate local paleostress fields and deformation patterns with regional to global tectonic events. As one result, the “Paleostress Chart for Northern Bavaria and adjacent Areas” visualizes the temporal and spatial evolution of several regions in Central Europe together with known tectonic phases, sedimentary unconformities and the plate kinematic framework since the Carboniferous.

This compilation may therefore help to better understand the timing and the spatio-temporal evolution of crustal stress patterns for tectonic events across Central Europe in the context of plate tectonics. 

We aim to supplement and improve existing paleostress models on both, regional, and temporal scale by compiling published and original data. In the long term the database is intended as a continuing compilation where data from all across Central Europe are supposed to be included and refined subsequently.

How to cite: Duschl, F., Stephan, T., Köhler, S., Köhn, D., Stollhofen, H., and Drews, M.: Compiling and correlating paleostress fields across Central Europe - A paleostress chart for northern Bavaria and adjacent areas, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10098, https://doi.org/10.5194/egusphere-egu21-10098, 2021.

Here, I use seismological observations (~70 broadband stations at distances between 100 and 400 km from the source) to characterize the rupture properties of the Petrinja mainshock (Mw 6.4). First, I perform a spectral analysis of the P-waves to compute the corner frequency. In order to remove the wave propagation effects and isolate the source properties, I use the largest foreshocks and aftershocks (Mw>4) as empirical Green’s functions (EGFs). Assuming a Brune’s source model, the obtained stress drop is ~20 MPa. This rather large value is in agreement with the short rupture length of ~8 km inferred by InSAR data (Ganas et al. 2021). In addition, the weak azimuthal variations of the corner frequencies indicates a bilateral rupture, that is a rupture nucleating close to the fault center. Second, I compute the apparent source time functions (i.e. the source time functions “seen” from any station) using an EGF deconvolution approach. The results indicate an average rupture duration of 5-6 s with weak azimuthal variation of the apparent rupture duration, in agreement with the spectral analysis. Finally, I perform a Bayesian inversion of the apparent source function, in order to obtain a kinematic model of the rupture propagation (slip distribution, rupture velocity). The preliminary results reveal a slow velocity of the rupture propagation. Such a slow rupture velocity associated with a large stress drop has been observed on other faults with slow slip rates (e.g. Causse et al. 2017). This work provides insight on the rupture process of this major event on a poorly documented fault. I am fully open for collaborations to further develop and enrich this study.

References
Causse, M., G. Cultrera, L. Moreau, A. Herrero, E. Schiapappietra and F. Courboulex. Bayesian rupture imaging in a complex medium. The 29 May 2012 Emilia, Northern Italy, earthquake (2017), Geophysical Research Letters, DOI : 10.1002/2017GL074698.
Ganas, A., Elias, P., Valkaniotis, S., Tsironi, V., Karasante, I., Briole, P., 2021, Petrinja earthquake moved crust 10 feet, Temblor, http://doi.org/10.32858/temblor.156

How to cite: Causse, M.: Rupture analysis of the 2020 Petrinja earthquake based on seismological observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16577, https://doi.org/10.5194/egusphere-egu21-16577, 2021.

On December 29, 2020, at 11:19 UTC, a strong (M6.4), shallow earthquake occurred in the central region of Croatia. The epicentre was located near the town of Petrinja, about 40 km to the south of the capital, Zagreb. Here we present a preliminary analysis of the geodetic data (differential InSAR & GNSS) and preliminary estimates of the slip that occurred on the fault during the earthquake and subsequent aftershocks. We picked InSAR data to invert for the seismic fault assuming linear rheology and Okada-type dislocation (rectangular) source with non-uniform slip. The interferograms show an asymmetric, four‐lobed pattern, centered on a NW‐SE oriented discontinuity that is in agreement with the right-lateral plane of the moment tensor solutions for the mainshock. We found that the Petrijna earthquake ruptured a segment of a strike-slip fault zone that is shorter (8 km) than average and with larger slip (~ 3 m). All parameters of the seismic fault are well constrained by InSAR modeling due to the full azimuthal coverage with both ascending and descending data of good quality. The fit to the fringes is better with a steep dip angle (76°) than with a purely vertical fault. The upper edge of the modeled fault is at a depth of ~1 km, this means that the slip drop from 3 to 0 m in the uppermost kilometer and our geodetic analysis cannot assess whether the fault reached the surface in some sections of the fault, however should this be the case, we expect ruptures at the surface in the range of 0.1 to 0 m for consistency with our model and the structure of the fringes pattern. In particular, preliminary modelling results with distributed fault-slip show that the slip reached a peak of more than 2.5 m at a depth of about 2 km. We also found that, differently from what reported in the European database of seismogenic sources (EDSF), the seismic fault dips westward instead of eastward. Additionally, the 2020 rupture and the InSAR mapped trace do not match the EDSF composite seismogenic fault surface trace. Kinematic analysis of GNSS waveforms at station BJEL (about 70-km east of the epicentre) revealed that horizontal ground motion reached 7-cm (peak-to-peak). The InSAR data revealed a 7 km of right-lateral afterslip on the NW-edge of the rupture, and 5 km to the south of the main fault rupture. In particular, the afterslip data on the NW edge of the rupture document the curved shape of the post-seismic deformation, that highlights the non-planarity of faults in nature and possibly indicating the existence of a ramp structure connecting to the neighboring segment towards north.

How to cite: Ganas, A., Valkaniotis, S., Elias, P., Tsironi, V., Karasante, I., Briole, P., Sansosti, E., and De Novellis, V.: Ground deformation related to slip and afterslip of the 29 December 2020 Mw 6.4 Petrijna earthquake (Croatia) imaged by InSAR, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16576, https://doi.org/10.5194/egusphere-egu21-16576, 2021.

The earthquake with magnitude ML=6.2 that occurred on 29th December 2020 has caused significant material damage to objects and infrastructure in the towns of Petrinja, Sisak,Glina and the surrounding area. According to the satellite interferometry data, the coseismic and postseismic deformation area covers around 500 square kilometers. The existing geodetic benchmarks have been set in the affected towns, and their coordinates have been determined based on previous GPS campaigns. The GPS network was set up and adjusted at the State Geodetic Administration's request for geodetic monitoring of infrastructure and cadastral projects. These points are not primarily intended for high accuracy measurements at the level of a few millimeters, so their accuracy and the absolute shift concerning geodynamic processes in the region should be taken into account. Nevertheless, the data obtained by their observation after the earthquake can provide valuable information about the horizontal and vertical displacements with a certain level of confidence. The field survey has detected disappearance of a large number of benchmarks and some valuable information has been lost. Still, 58 points were found and observed and it has been concluded that 52 points are reliable and can be used for future research. Because the network of benchmarks is not developed in rural areas, there is a gap in the distribution of benchmarks in affected area. Therefore, the additional data was collected using the benchmarks established for the engineering and cadastral projects and studies. From a total of 67 points that have been found and observed, 42 points will be used. Along with the data collected in urban areas, there will be a total of 94 benchmarks. The accuracy of the geodetic benchmark measurements is at the centimeter level, while the values of deformation are at the level of a few decimeters. Therefore, the obtained data can be used to better assess the displacement recorded during the 29 December 2020 event. In the future, field research will focus on finding additional benchmarks to reach a better spatial distribution.

How to cite: Kordić, B., Vukovski, M., Budić, M., Špelić, M., Barbača, J., Belić, N., Brčić, V., Filjak, R., Kurečić, T., Palenik, D., Bočić, N., Atanackov, J., Bavec, M., Brajkovič, R., Celarc, B., Novak, A., Novak, M., Jamšek Rupnik, P., Amoroso, S., Civico, R., Pucci, S., Ricci, T., Bonico, P., Iezzi, F., Pace, B., Testa, A., Benedetti, L., Henriquet, M., Moulin, A., Baize, S., Métois, M., and Markusic, S.: Geodetic benchmark displacement measurements following the 2020 Petrinja earthquake in Croatia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16588, https://doi.org/10.5194/egusphere-egu21-16588, 2021.

On December 29th, 2020, a strong Mw 6.4 earthquake hit central Croatia. The epicenter was located approximately 3 km southwest of Petrinja, and the intensity was estimated to VIII-IX EMS. The earthquake led to significant environmental effects related to earthquake magnitude, focal depth, and geological and geotechnical properties of the affected area.
The Croatian Geological Survey (HGI-CGS) conducted extensive geological and geodetic surveys starting a few hours following the main shock to measure the earthquake’s effects,
including those on infrastructures. Ten geologists from the Department of Geology carried out surveys from Decmber 31st, 2020 to January 7th, 2021 along the potential seismogenic source (inferred from geological maps and InSAR data) and in the wider epicentral area that suffered significant damage (e.g., Glina and Sisak).
During a second phase, researchers from the University of Zagreb (PMF UniZG), Slovenia (GeoZS), Italy (INGV, ISPRA, U. Chieti) and France (CEREGE, IRSN) were mobilized to complete the observations. The collaboration with these geologists allowed to deepen the investigations and to bring further detail to quantify the effects. The surveys were then compiled based on data formats used by the European Community, namely those of the INGV EMERGEO team (Villani et al., 2017; for environmental effects including surface ruptures and liquefaction) and those of the SURE group (Baize et al., 2019 for surface ruptures).
These observations revealed that the earthquake triggered a discontinuous, few km-long surface rupture with a maximum displacement of about 20 cm, which is consistent with the lower average of observations made on similar events (Wells and Coppersmith, 1994). Liquefaction spread over several tens of square kilometers mostly in river plains, the most distant being about 20 km from the epicenter (to be confirmed!). Other observed effects include lateral spreading, landslides, groundwater regime changes, rockfalls, and various infrastructure damage.
The compilation of the acquired dataset into a unified database, consistent with database of other historical and recent events, is essential for establishing reliable empirical relations between geological effects and physical characteristics of earthquakes (magnitude, depth). This forms the basis for seismic hazard assessments, whether for “surface rupture”, “liquefaction”, or “ground-shaking” potential.

How to cite: Vukovski, M., Budić, M., Špelić, M., Barbača, J., Belić, N., Brčić, V., Filjak, R., Korbar, T., Kordić, B., Kurečić, T., Palenik, D., Bočić, N., Atanackov, J., Bavec, M., Brajkovič, R., Celarc, B., Novak, A., Novak, M., Jamšek Rupnik, P., Amoroso, S., Cinti, F. R., Civico, R., Pantosti, D., Pucci, S., Ricci, T., Boncio, P., Lezzi, F., Pace, B., Testa, A., Blumetti, A. M., Di Manna, P., Benedetti, L., Hnriquet, M., Moulin, A., and Baize, S.: A database of the environmental effects associated to the December 29th, 2020 Mw 6.4 Petrinja earthquake (Croatia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16574, https://doi.org/10.5194/egusphere-egu21-16574, 2021.

The 29 December 2020, Mw 6.4 Petrinja earthquake nucleated at a depth of ~10 km in the Sisak-Moslavina County in northern Croatia, ~6 km WSW of the Petrinja town. Focal mechanisms, aftershocks distribution, and preliminary Sentinel-1 InSAR interferogram suggest that the NW-SE right-lateral strike-slip Pokupsko-Petrinja fault was the source of this event.
The Croatian Geological Survey, joined by a European team of earthquake geologists from France, Slovenia and Italy, performed a prompt systematic survey of the area to map the surface effects of the earthquake. The field survey was guided by geological maps, preliminary morphotectonic mapping based on 1:5,000 topographical maps and InSAR interferogram. Locally, field mapping was aided by drone survey.
We mapped unambiguous evidence of surface faulting at several sites between Župić to the NW and Hrastovica to the SE, in the central part of the Pokupsko-Petrinja fault, for a total length of ~6.5 km. This is probably a minimum length since several portions of the fault have not been explored yet, and in part crossing forbidden uncleared minefields. Surface faulting was observed on anthropic features (roads, walls) and on Quaternary sediments (soft colluvium and alluvium) and Miocene bedrock (calcarenites). The observed ruptures strike mostly NW-SE, with evidences of strike-slip right-lateral displacement and zones of extension (opening) or contraction (small pressure ridges, moletracks) at
local bends of the rupture trace. Those ruptures are interpreted as evidences of coseismic surface faulting (primary effects) as they affect the morphology independently from the slope direction. Ground failures due to gravitational sliding and liquefaction occurrences were also observed, mapped and interpreted as secondary effects (see Amoroso et al., and Vukovski et al., this session). SE of Križ, the rupture broke a water pipeline with a right-lateral offset of several centimetres. Measured right-lateral net displacement varies from a few centimetres up to ~35 cm. A portion of the maximum measured displacement could be due to afterlisp, as it was mapped several days after the main shock. Hybrid surface ruptures (shear plus opening and liquefaction), striking SW-NE, with cm-size left-lateral strike-slip offsets were mapped on the northern side of the Petrinja town, ~3 km NE of the main fault.
Overall, the rupture zone appears discontinuous. Several factors might be inferred to explain this pattern such as incomplete mapping of the rupture, inherited structural discontinuities within the Pokupsko-Petrinja fault system, or specific mechanical properties of the Neogene-Quaternary strata

How to cite: Boncio, P., Amoroso, S., Atanackov, J., Baize, S., Barbača, J., Bavec, M., Belić, N., Benedetti, L., Brajkovič, R., Brčić, V., Budić, M., Caciagli, M., Celarc, B., Civico, R., Cinti, F. R., De Martini, P. M., Filjak, R., Henriquet, M., Kordić, B., Iezzi, F., Minarelli, L., Moulin, A., Nappi, R., Novak, A., Novak, M., Pace, B., Palenik, D., Pantosti, D., Pucci, S., Jamšek Rupnik, P., Špelić, M., Testa, A., Valkaniotis, S., and Vukovski, M.: Surface faulting during the 29 December 2020 Mw 6.4 Petrinja earthquake (Croatia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16575, https://doi.org/10.5194/egusphere-egu21-16575, 2021.

Devastating M6.2 earthquake (1) hit Petrinja epicentral area (2) on 2020-12-29. M5.0 foreshock on 2020-12-28 (1) caused moderate damage on buildings and forced many inhabitants to move out form their homes. Thus, the foreshock was a kind of lucky event that saved many human lives.

Considering the shallow focal depth (1) and QMTS that show clear strike-slip focal mechanisms (3, 4), surface failures were expected after the mainshock. Immediate reports in media allowed quick online research of surface failures indicating that linear infrastructure damages appear along ~30 km long portion of sinistral NE-SW striking Sisak-Petrinja-Glina-Topusko Fault. Quick field inspection revealed that fresh fault planes in the bedrock appear mostly along longitudinal NW-SE striking (Dinaric strike) Pokupsko-Kostajnica-Banja Luka Fault, and show clear dextral co-seismic stike-slip displacements. The map view time-lapse animation of the seismic sequence (5) revealed that ~20 km long portion of the Pokupsko Fault was (re)activated. The two subvertical  mutually perpendicular faults intersect near the epicenters. The historically important Pokupsko earthquake occured in the vicinity (6), and was used by a famous Croatian geophysicist Andrija Mohorovičić to discover the MOHO discontinuity.

The fault system is textbook example of major failure in the upper crust along the pre-existing fault net (7) at the critical moment of centennial release of generally north-south oriented compressional strain that is accumulating in the crust because of continuous northward movement of the Adriatic microplate (Adria). Up to 10 mm/yr Adria GPS velocities measured in the Adriatic foreland are mostly accommodating along major External Dinarides active faults, since the Internal Dinarides GPS velocities are only 1-2 mm/yr, while the velocities in the Pannonian basin are near zero (8). The dextral Pokupsko-Banja Luka Fault could be one of the main inherited active faults between the crustal segments of the Adria, while sinistral Petrinja fault could represent reactivated Mesozoic transform fault bordering the crustal fragments (9) of once greater Adria (10).

• (1) https://www.pmf.unizg.hr/geof/seizmoloska_sluzba, Accessed: 2020-12-29 11:50 UTC
• (2) Stanko D, Markušić S, Korbar T, Ivančić J. (2020): Estimation of the High-Frequency Attenuation Parameter Kappa for the Zagreb (Croatia) Seismic Stations. Applied Sciences. 10(24):8974.
• (3) https://www.emsc-csem.org/#2, Accessed: 2020-12-28 05:28:07 UTC
• (4) https://www.emsc-csem.org/#2, Accessed: 2020-12-29 11:35 UTC
• (5) https://www.pmf.unizg.hr/geof/seizmoloska_sluzba, Accessed: 2021-01-03 07:50 UTC
• (6) Herak, D and Herak, M. (2010): The Kupa Valley (Croatia) earthquake of 8 October 1909 – 100 years later. Seismological research letters, 81, 30-36.
• (7) Pikija, M. (1987): Osnovna geološka karta SFRJ, 1: 100 000: List Sisak, L 33-93. hgi-cgs.hr
• (8) Battaglia, M., Murray, M.H., Serpelloni, E. and Bürgmann, R. (2004). The Adriatic region: An independent microplate within the Africa-Eurasia collision zone. Geophysical Research Letters, 31, 1–4.
• (9) Korbar (2009): Orogenic evolution of the External Dinarides in the NE Adriatic region: a model constrained by tectonostratigraphy of Upper Cretaceous to Paleogene carbonates. Earth Science Reviews, 96/4, 296-312.
• (10) van Hinsbergen, D.J.J., Torsvik, T.H., Schmid, S.M., Maţenco, L.C., Maffione, M., Vissers, R.L.M., Gürer, D., Spakman, W. (2020): Orogenic architecture of the Mediterranean region and kinematic reconstruction of its tectonic evolution since the Triassic. Gondwana Research, 81, 79-229.

How to cite: Korbar, T. and Markušić, S.: Petrinja M6.2 earthquake (Croatia) on 29/12/2020 occurred on the intersection of the two regional active faults at the transition between Dinarides and Pannonian basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9434, https://doi.org/10.5194/egusphere-egu21-9434, 2021.

We provide here a first-hand description of the coseismic surface effects caused by the Mw 6.4 Petrinja earthquake that hit central Croatia on 29 December 2020. This was one of the strongest seismic events that occurred in Croatia in the last two centuries. Field surveys in the epicentral area allowed us to observe and map primary coseismic effects, including geometry and kinematics of surface faulting, as well as secondary effects, such as liquefaction, sinkholes and landslides. The resulting dataset consists of homogeneous georeferenced records identifying 222 observation points, each of which contains a minimum of 5 to a maximum of 14 numeric and string fields of relevant information. The earthquake caused surface faulting defining a typical ‘conjugate’ fault pattern characterized by Y and X shears, tension cracks (T fractures), and compression structures (P shears) within a ca. 10 km wide, right-lateral strike-slip fault zone (i.e. the Petrinja Fault Zone, PFZ). We believe that the results of the field survey provide fundamental information to improve the interpretation of seismological, GPS and InSAR data of this earthquake. Moreover, the data related to the surface faulting may impact future studies focused on earthquake processes in active strike-slip settings, integrating the estimates of slip amount and distribution in assessing the hazard associated with capable transcurrent faults.

How to cite: Tondi, E., Blumetti, A. M., Čičak, M., Di Manna, P., Đuroković, Z., Galli, P., Invernizzi, C., Mazzoli, S., Piccardi, L., Valentini, G., Vittori, E., and Volatili, T.: Coseismic surface "conjugate" faulting of the 29 December 2020., MW 6.4, Petrinja earthquake (Sisak-Moslavina, Croatia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16598, https://doi.org/10.5194/egusphere-egu21-16598, 2021.

Archaeological excavations of the Roman city of Siscia (Sisak, Croatia) found walls of the city, up to 2 m thick, toppled in the moat. Brick masonry wall segments were found in various orientations: tilted, rotated, twisted, toppled, overturned. Foundations display features of twisting and shearing. There are additional shearing planes within the fallen walls, which allowed the segments to extend during collapse. Much of construction material was robbed in later centuries, so original dimensions are estimates only. Subsoil is alluvial sandy clay. We suggest that a major earthquake damaged the city wall of Siscia. Excitated by site effects of loose soil, high peak ground acceleration caused the wall to be sheared off from its foundation, landing it ultimately in the adjacent moat. Rebuilding of the city wall in the late antique period suggests that the first wall collapsed between the beginning of the 3rd and the middle of the 4th century. This earthquake between ~200 AD and ~350 AD is missing from historical catalogues. Both the Antique and the modern earthquakes were of intensity IX. The St. Quirinus site at Siscia is 12 km from the fault which caused the destruction in Petrinja on 29 December 2020, mere 3 km from the fault. We suggest that the Antique earthquake was stronger than the M 6.2 modern event.

How to cite: Kázmér, M. and Škrgulja, R.: A stronger predecessor to the M 6.2 Petrinja, Croatia earthquake in Antiquity – archaeoseismology of the 4th century Siscia event, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16572, https://doi.org/10.5194/egusphere-egu21-16572, 2021.

The Petrinja-Pokupsko fault-system is a NW-SE right-lateral fault system that ruptured during the 29 December 2020 Mw 6.4 earthquake (~40km south-east of Zagreb, Croatia). Field analysis revealed opening of cracks and offsets of several centimeters (3 to 40 cm) along a ~20 km long fault zone extending from the Kupa river (in the northwest) to the Petrinjčica river (in the southeast). Optical image correlation based on WorldView satellite images has been used to document the first-order near-field rupture signal. The pre-event image was acquired on 7th December 2017, and the post-event image on 15th January 2021. The first results indicate a right-lateral displacement of ≈75 cm with a small (<10 cm) extensional dip-slip component localized on the Petrinja fault. Using 1:5,000 topographic maps, a WorldView-derived DEM (1 m), and field observations, we identified and quantified cumulative dextral offsets along the central and southern section of the fault (south of Župić). Right-lateral offsets range from 5 to 200 m near Križ and Cepeliš (central sector). Diverted streams also extend southeast of the Petrinjčica river, where no surface ruptures have currently been reported to date. To the northwest, perched valleys, wind gaps, and karst features all testify to ongoing uplift across NW-SE-trending anticlines. It is unclear if the primary component of faulting changes from strike-slip (in the SE) to reverse (in the NW), or if these folds merely record a transpressive component across the fault. The activity of this fault system is poorly known. The region experienced a magnitude Mw 5.8 in 1909, ~30 km northwest of Petrinja, which may have been associated with the Petrinja-Pokupsko fault system. The recent 29 December 2020 earthquake confirms the seismic potential of this fault system to generate Mw>6 earthquakes. Since the fault extends farther NW and SE, from the Vukomeričke Gorice hills to Mount Kozara (Bosnia), for a total length of about 100 km, it could generate potentially larger events. It is also noteworthy that the 2020 Petrinja event occurred only 9 months after the Zagreb March 2020 (Mw 5.3) earthquake. This event occurred on an ENE-WSW-trending thrust fault, broadly orthogonal to the right-lateral Petrinja-Pokupsko fault system, ~45 km north of Petrinja, and raises the prospect of potential interplay between strike-slip and thrust faults in moderate strain-rate intra-plate settings. To address this problem, future works will aim at constraining the geometry of this fault network and its seismogenic potential.

How to cite: Henriquet, M., Moulin, A., Vukovski, M., Kordić, B., Budić, M., Hollingsworth, J., Gold, R., Baize, S., and Benedetti, L.: Comparison between the coseismic surface displacement during the 29 December 2020 Mw 6.4 Petrinja earthquake (Croatia) from optical image correlation and long-term geomorphological observations of cumulative displacements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16590, https://doi.org/10.5194/egusphere-egu21-16590, 2021.