ALBERT

Description: Tectonic pseudotachylytes are thought to be unique to certain water-deficient seismogenic environments and their presence is considered to be rare in the geological record. Here, we present field and experimental evidence that frictional melting can occur in hydrothermal fluid-rich faults hosted in the continental crust. Pseudotachylytes were found in the 〉40 km-long Bolfín Fault Zone of the Atacama Fault System, within two ca. 1 m-thick (ultra)cataclastic strands hosted in a damage-zone made of chlorite-epidote-rich hydrothermally altered tonalite. This alteration state indicates that hydrothermal fluids were active during the fault development. Pseudotachylytes, characterized by presenting amygdales, cut and are cut by chlorite-, epidote- and calcite-bearing veins. In turn, crosscutting relationship with the hydrothermal veins indicates pseudotachylytes were formed during this period of fluid activity. Rotary shear experiments conducted on bare surfaces of hydrothermally altered rocks at seismic slip velocities (3 m s-1) resulted in the production of vesiculated pseudotachylytes both at dry and water-pressurized conditions, with melt lubrication as the primary mechanism for fault dynamic weakening. The presented evidence challenges the common hypothesis that pseudotachylytes are limited to fluid-deficient environments, and gives insights into the ancient seismic activity of the system. Both field observations and experimental evidence, indicate that pseudotachylytes may easily be produced in hydrothermal environments, and could be a common co-seismic fault product. Consequently, melt lubrication could be considered one of the most efficient seismic dynamic weakening mechanisms in crystalline basement rocks of the continental crust.

Description: The authors would like to acknowledge the support of ERC CoG No 614705 NOFEAR. R. Gomila has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska- Curie grant agreement No 896346 – FRICTION.

Description: Published

Description: e2021GC009743

Description: 3T. Fisica dei terremoti e Sorgente Sismica

Description: JCR Journal

Description: Anhydrite-rich layers within foreland fold and thrust belts are frequently observed to be the weakest horizon of the sequence. Characterizing the microstructure of anhydrite is therefore important for interpreting the larger-scale deformation history of these rocks. Two microstructures from naturally deformed, fine-grained (〈15 µm mean grain size) anhydrite samples from the Triassic Evaporites of the Umbria–Marche Apennines, Italy were analysed using electron backscatter diffraction (EBSD). Microstructural observations, misorientation analysis and crystallographic preferred orientation (CPO) determination were carried out on these samples. Both samples have a CPO characterized by alignment of 〈001〉 and distribution of 〈100〉 and 〈010〉 on a great circle normal to this. This anhydrite 〈001〉 ‘fibre texture’ has not been described before. Microstructure A is characterized by a moderate to weak CPO and a weak shape preferred orientation at 55° to 70° from the trace of the 〈001〉 maximum. Low-angle boundaries are revealed by misorientation analysis. A change in grain size from c. 10 to c. 7 µm corresponds to reduction in strength of CPO and reduction in the number of low-angle grain boundaries. Microstructure B is characterized by a very strong CPO. The orientation of the CPO changes between different microstructural domains. The 〈001〉 maximum is always perpendicular to the trace of a strong grain elongation and high-angle grain boundaries have misorientations close to 〈001〉, suggesting that the CPO is geometrically controlled: anhydrite grains are platy with 〈001〉 short axes. The origin of the CPO is therefore unclear but it need not relate to dislocation creep deformation. Whether or not CPO relates to dislocation creep, both samples have a high number of lower-angle grain boundaries and internal grain distortions with 〈010〉 and 〈001〉 misorientation axes. These are indicative of dislocation activity and the data are best explained by slip on either (100)[010] (dominant) and (001)[100] or a combination of these. Neither of these slip systems has been recognized before. Both microstructures are interpreted to have undergone dynamic recrystallization, and the weakening of the CPO with decreasing grain size in microstructure A is suggested to be indicative of a grain-boundary sliding mechanism becoming active. Comparison with experimental data shows that creep mechanisms involving dislocations at the observed grain sizes require the differential stress magnitudes driving deformation to be greater than c. 100 MPa.

Description: Hyaloclastites commonly form high-quality reservoir rocks in volcanic geothermal provinces. Here, we investigated the effects of confinement due to burial following prolonged accumulation of eruptive products on the physical and mechanical evolution of surficial and subsurface (depths of 70 m, 556 m, and 732 m) hyaloclastites from Krafla volcano, Iceland. Upon loading in a hydrostatic cell, the porosity and permeability of the surficial hyaloclastite decreased linearly with mean effective stress, as pores and cracks closed due to elastic (recoverable) compaction up to 22-24 MPa (equivalent to ~1.3 km depth in the reservoir). Beyond this mean effective stress, denoted as P∗, we observed accelerated porosity and permeability reduction with increasing confinement, as the rock underwent permanent inelastic compaction. In comparison, the porosity and permeability of the subsurface core samples were less sensitive to mean effective stress, decreasing linearly with increasing confinement as the samples compacted elastically within the conditions tested (to 40 MPa). Although the surficial material underwent permanent, destructive compaction, it maintained higher porosity and permeability than the subsurface hyaloclastites throughout the experiments. We constrained the evolution of yield curves of the hyaloclastites, subjected to different effective mean stresses in a triaxial press. Surficial hyaloclastites underwent a brittle-ductile transition at an effective mean stress of ~10.5 MPa, and peak strength (differential stress) reached 13 MPa. When loaded to effective mean stresses of 33 and 40 MPa, the rocks compacted, producing new yield curves with a brittle-ductile transition at ~12.5 and ~19 MPa, respectively, but showed limited strength increase. In comparison, the subsurface samples were found to be much stronger, displaying higher strengths and brittle-ductile transitions at higher effective mean stresses (i.e., 37.5 MPa for 70 m sample, 〉75 MPa for 556 m, and 68.5 MPa for 732 m) that correspond to their lower porosities and permeabilities. Thus, we conclude that compaction upon burial alone is insufficient to explain the physical and mechanical properties of the subsurface hyaloclastites present in the reservoir at Krafla volcano. Mineralogical alteration, quantified using SEM-EDS, is invoked to explain the further reduction of porosity and increase in strength of the hyaloclastite in the active geothermal system at Krafla.

Description: Porous rock deformation has important implications for fluid flow in a range of crustal settings as compaction can increase fluid pressure and alter permeability. The onset of inelastic strain for porous materials is typically defined by a yield curve plotted in differential stress (Q) versus effective mean stress (P) space. Empirical studies have shown that these curves are broadly elliptical in shape. Here conventional triaxial experiments are first performed to document (a) the yield curve of porous bassanite (porosity ≈ 27–28%), a material formed from the dehydration of gypsum, and (b) the postyield behavior, assuming that P and Q track along the yield surface as inelastic deformation accumulates. The data reveal that after initial yield, the yield surface cannot be perfectly elliptical and must evolve significantly as inelastic strain is accumulated. To investigate this further, a novel stress-probing methodology is developed to map precisely the yield curve shape and subsequent evolution for a single sample. These measurements confirm that the high-pressure side of the curve is partly composed of a near-vertical limb. Yield curve evolution is shown to be dependent on the nature of the loading path. Bassanite compacted under differential stress develops a heterogeneous microstructure and has a yield curve with a peak that is almost double that of an equal porosity sample that has been compacted hydrostatically. The dramatic effect of different loading histories on the strength of porous bassanite highlights the importance of understanding the associated microstructural controls on the nature of inelastic deformation in porous rock. ©2018. The Authors.

Description: Naturally and experimentally deformed gouges from sliding surfaces within the Galera Fault Zone were analyzed using scanning and transmission electron microscopy (SEM, TEM) to identify changes in the fault rocks as a consequence of ongoing deformation. The two gouges studied have a particular mineral association that includes planar (mainly smectite and illite) and fibrous clay minerals (sepiolite and palygorskite). Microstructural findings include a radical difference in grain alignment between the two gouges, a phenomenon that strongly influences gouge permeability. Smectite crystals are aligned on the same orientation and show a great number of layer terminations and delamination on the basal planes that contribute to a distributed mode of deformation in the gouge. In contrast, the sepiolite-rich gouge exhibits a grid-like microfabric that results in localized deformation limited to small areas where the needle-like crystals are bent and broken producing “feather-like” structures, without the presence of lattice distortions. Meanwhile, significant chemical results include: (1) Al content identified in sepiolite fibers through analytical electron microscopy (AEM), together with variability in the (110) d-spacing of sepiolite across single fibers, suggest the existence of a progressive transformation from sepiolite to palygorskite. (2) Mg content in smectite suggests that a portion of the smectites within the fault plane could have an authigenic origin and may be the result of a transformation reaction from palygorskite, however, the similarity of the 2:1 layer compositions between the smectites in the two contexts do not allow to either confirm nor deny such possibility. (3) Chemical continuity of Mg-decrease and Al+Fe-increase in the octahedral cation content of the sepiolites, palygorskites, and smectites within the gouges indicate a sequence of mineral transformations that is favored by a depleted Mg content and an increase of Al content in the fluid. In this setting, deformation promotes grain size reduction and fluid-rock interaction with the wall rocks resulting in a local supply of Al to the fault gouge that drives phase transformations. Structural differences between smectites and fibrous clay minerals affect important chemical and physical properties of the gouge including their mechanical properties. We propose that the permeability of the gouges in the Galera Fault is strongly affected by their mineralogy. Furthermore, the extent of the mineral authigenesis and mineral transformations could be a controlling factor that progressively changes both the permeability and the strength of the fault.

Description: In the upper crust, where brittle deformation mechanisms dominate, the development of crack networks subject to anisotropic stress fields generates stress-induced elastic anisotropy. Here a rock specimen of Westerly granite was submitted to differential stress cycles (i.e., loading and unloading) of increasing amplitudes, up to failure and under upper crustal conditions. Combined records of strains, acoustic emissions, and P and S elastic wave anisotropies demonstrate that increasing differential stress promotes crack opening, sliding, and propagation subparallel to the main compressive stress orientation. However, the significant elastic anisotropies observed during loading (≥20%) almost vanish upon stress removal, demonstrating that in the absence of stress, crack-related elastic anisotropy remains limited (≤10%). As a consequence, (i) crack-related elastic anisotropies measured in the crust will likely be a strong function of the level of differential stress, and consequently (ii) continuous monitoring of elastic wave velocity anisotropy along faults could shed light on the mechanism of stress accumulation during interseismic loading. ©2018. American Geophysical Union. All Rights Reserved.

Description: Fractures are ubiquitous within the subsurface and play an important role in the fluid flow, elasticity, and strength of rocks. Because they are essential to geologic systems such as hydrocarbon and geothermal systems, they need to be properly imaged and monitored. We used the spectral-ratio technique to measure the P-wave Q factor ([Formula: see text]) and the S-wave Q factor ([Formula: see text]) of dry and water-saturated crystalline rock samples that were thermally fractured by heating to 250, 450, 650, and 850°C. Increasing the temperature during thermal treatment produces an increased fracture density that is isotropically distributed. The samples were subjected to hydrostatic pressures up to the pressure at which all fractures are closed, and they were axially loaded to 25% of the failure strength. Axial loading of the samples further closes fractures that are oriented perpendicular to the loading direction, but it opens those that are oriented parallel and oblique to the loading direction. Attenuation measurements were made in the frequency range of 0.8 to 1.7 MHz. At the fracture closure pressure, the sample with the highest fracture density showed a reduction in [Formula: see text] and [Formula: see text] when compared to the sample with the lowest fracture density of 84% and 24%, respectively, under dry conditions and by 33% and 5%, respectively, under saturated conditions. The [Formula: see text] and [Formula: see text] increase as the samples were axially loaded. The increase is more pronounced in [Formula: see text] because it is less influenced by the opening of parallel and obliquely oriented fractures. The opening of these fractures mostly affects the propagation of the S-wave and therefore reduces the increase in [Formula: see text]. The results suggest that [Formula: see text] is very sensitive to fracture closure at intermediate and high effective mean stresses. We determine that [Formula: see text] is a useful measure of the fracture density even at high pressures at which fractures might be expected to be fully closed.

Description: Petroleum-rich basins at a mature stage of exploration and production offer many opportunities for large-scale Carbon Capture and Storage (CCS) since oil and gas were demonstrably contained by low-permeability top-sealing rocks, such as shales. For CCS to work, there must be effectively no leakage from the injection site, so the nature of the top-seal is an important aspect for consideration when appraising prospective CCS opportunities. The Lower Cretaceous Rodby Shale and the Palaeocene Lista Shale have acted as seals to oil and gas accumulations (e.g., the Atlantic and Balmoral fields) and may now play a critical role in sealing the Acorn and East Mey subsurface carbon storage sites. The characteristics of these important shales have been little addressed in the hydrocarbon extraction phase, with an understandable focus on reservoir properties and their influence on resource recovery rates. Here, we assess the characteristics of the Rodby and Lista Shales using wireline logs, geomechanical tests, special core analysis (mercury intrusion) and mineralogical and petrographic techniques, with the aim of highlighting key properties that identify them as suitable top-seals. The two shales, defined using the relative gamma log values (or Vshale), have similar mean pore throat radius (approximately 18 nm), splitting tensile strength (approximately 2.5 MPa) and anisotropic values of splitting tensile strength, but they display significant differences in terms of wireline log character, porosity and mineralogy. The Lower Cretaceous Rodby Shale has a mean porosity of approximately 14 %, a mean permeability of 263 nD (2.58 × 10−19 m2), and is calcite rich and has clay minerals that are relatively rich in non-radioactive phases such as kaolinite. The Palaeocene Lista Shale has a mean porosity of approximately 16% a mean permeability of 225 nD (2.21 × 10−19 m2), and is calcite free, but contains abundant quartz silt and is dominated by smectite. The 2% difference in porosity does not seem to equate to a significant difference in permeability. Elastic properties derived from wireline log data show that Young’s modulus, material stiffness, is very low (5 GPa) for the most shale (clay mineral)-rich Rodby intervals, with Young’s modulus increasing as shale content decreases and as cementation (e.g., calcite) increases. Our work has shown that Young’s modulus, which can be used to inform the likeliness of tensile failure, may be predictable based on routine gamma, density and compressive sonic logs in the majority of wells where the less common shear logs were not collected. The predictability of Young’s modulus from routine well log data could form a valuable element of CCS-site top-seal appraisals. This study has shown that the Rodby and Lista Shales represent good top-seals to the Acorn and East Mey CCS sites and they can hold CO2 column heights of approximately 380 m. The calcite-rich Rodby Shale may be susceptible to localised carbonate dissolution and increasing porosity and permeability but decreasing tendency to develop fracture permeability in the presence of injected CO2, as brittle calcite dissolves. In contrast, the calcite-free, locally quartz-rich, Lista Shale will be geochemically inert to injected CO2 but retain its innate tendency to develop fracture permeability (where quartz rich) in the presence of injected CO2.