Fluid-driven Cyclic Reorganisation in Shallow Basaltic Fault Zones

Faults represent a critical heterogeneity in basaltic sequences, yet their architectural and hydromechanical evolution is poorly constrained. We present a detailed multi-scale characterisation of passively exhumed fault zones from the layered basalts of the Faroe Islands, which reveals cyclic stages of fault evolution. Outcrop-scale structures and fault rock distribution within the fault zones were mapped in the field and in 3D virtual outcrop models, with detailed characterisation of fault rock microstructure obtained from optical and SE-microscopy. The fault zones record localisation from decametre-wide Riedel shear zones into metre-wide fault cores, containing multiple cataclastic shear bands and low strain lenses organised around a central principal slip zone (PSZ). Shear bands and the PSZ consist of (ultra-) cataclasites with a zeolite-smectite assemblage replacing the original plagioclase-pyroxene host rock composition. Low-strain lenses are hydrothermal breccias of weakly altered host rock, or reworked fault rocks. PSZ-proximal zones show significant late-stage dilatation in the form of hydrothermal breccias or tabular veins with up to decimetre apertures. We interpret these structures as evolving from alternating shear-compaction and dilation through hydrofracture. The fault core preserves PSZ reworking, evidencing repeated shear zone locking and migration. The alternating deformation styles of shear compaction and dilatation suggest episodic changes in deformation mechanisms driven by transient overpressure and release. The fault zone mechanical properties are thus governed by the combined effects of permanent chemical weakening and transient fluid-mediated mechanical weakening, alternating with cementation and healing.

Understanding the fractures connectivity control in NEB field, South Sumatra Basin: from seismic to geomechanics and Its Implication for further fractured basement exploration

The natural fractured basement reservoirs become an obsessive target in Jabung Block. Currently, there are two wells drilled in the block that targeting fractured basement reservoir. They are the NEB Base-1 well that located in western part of the NEB Field and the NEB Base-2 well which located 7 kilometers away to the East of NEB Base-1 well. The first well was technically success, however NEB Base-2 well shows no indication of hydrocarbon influx during the test. Interestingly, the fractures development in both wells shows almost the same condition of fractures orientation, dip-magnitude and fractures intensity. Furthermore, each fracture in both wells can be correlated into several zones, as they indicate similar fracture set orientation at each zone. These findings create a big question, why the similar fractures characters show a very different test result? This study is intended to have that question answered with the idea to focus on the following two workflows: the first is to re-evaluate the previous works starting from re-picking the seismic fault in detail. The second is to analyze the relationship between geomechanical forward modelling result with the structural evolution in Jabung Block through sandbox modelling. The geomechanical forward modelling in the NEB Field imply the critical stress stated that was predominantly located in the western part or within the NEB Base-1 area., This result is strongly correlate with the new basement fault map which shows an intensive faulting in the western area, and is characterized by couples of synthetic-antithetic Riedel shear as a result of the strike-slip faulting. In addition, the sandbox modelling shows a major oblique-slip fault movement was observed within the western area. Therefore, it can be concluded that the intensive strike slip fault plays an important role to enhance the connectivity between fault and fracture to the hydrocarbon storage as shown in the result of NEB Base-1 well. This idea could be used as a guidance to explore another fractured basement prospect within the block.

Strike-slip fault in a sandbox: insight of on- and off-fault deformation from analogue modelling

<p>During large strike-slip earthquakes, the displacement at the ground surface, only partially measured, is often under-estimated in comparison with the amount of slip inferred at depth. The resulting concept of shallow slip deficit is challenged by the precise measurements of surface deformation of on- and off-fault deformation by space imaging techniques, showing that a significant amount of deformation might be accommodated through distributed damage in a zone several hundred meters to kilometers wide around the fault. In this study, analogue modeling is used to quantify the distribution of on/off-fault surface deformation along strike-slip faults over the long term and to understand how it relates to the deep structure of the fault.</p><p>To do so, we used a 1.5 m x 1.34 m PVC box, and studied the deformation of a homogeneous sand pack deposited above a straight basal fault, with sand thicknesses varying from 2 to 8 cm. During strike-slip fault experiments, the first structures to appear are the Riedel shears (R-shears) followed by the synthetic shears (S-shears). These structures eventually coalesce to form an anastomosed fault zone, made of a succession of segments separated by geometrical complexities of variable size. Optical imagery is used, at every stage of the strike-slip fault formation, to (1) describe the 3D surface displacement and (2) precisely quantify on/off-fault deformation. </p><p>At the initiation of the fault before the formation of the Riedels, a zone of diffuse deformation is highlighted by a positive divergence of the displacement. This diffuse zone is also characterized by a vertical deformation that forms a bulge.</p><p>When the displacement Ux parallel to the basal fault has a gradient dUx/dy >= 0.1, we consider that it is "on-fault" deformation, and it is "off-fault, when that gradient is between 0.02 and 0.1.</p><p>At the Riedel shear stage, we find 40% of off-fault deformation over a unique Riedel fault and about 60% if deformation is distributed over two Riedels.</p><p>Once the strike-slip fault is formed, the ratio drops between 0 to 5 % of off-fault deformation over a fault segment, but the ratio increases to 20% along geometrical complexities.</p><p>Moreover, we also show that off-fault deformation around the early Riedel structures partly control the long-lived segmentation and morphology of the strike-slip fault.</p><p>Experimental results are then compared to observations and measurements of near-field and far-field deformation obtained along the 2013 Mw 7.7 Balochistan earthquake by Vallage et al. (2015) and Gold et al. (2015). Azimuthal displacements measured in a relay zone  (Vallage et al. 2015) are consistent with those observed along our experimental relay zones. Although our experiments were only run with sand, we found a similar distribution of the deformation at the surface. These observations suggest that the distribution of the surface deformation of strike-slip fault earthquakes is inherent to the fault structure, possibly inherited from the Riedel shear stage, and not induced by earthquakes dynamics.</p>

Paleostress reconstruction of faults recorded in the Niedźwiedzia Cave (Sudetes): insights into Alpine intraplate tectonic of NE Bohemian Massif

Brittle structures identified within the largest karstic cave of the Sudetes (the Niedźwiedzia Cave) were studied to reconstruct the paleostress driving post-Variscan tectonic activity in the NE Bohemian Massif. Individual fault population datasets, including local strike and dip of fault planes, striations, and Riedel shear, enabled us to discuss the orientation of the principal stresses tensor. The (meso) fault-slip data analysis performed both with Dihedra and an inverse method revealed two possible main opposing compressional regimes: (1) NE–SW compression with the formation of strike-slip (transpressional) faults and (2) WNW–ESE horizontal compression related to fault-block tectonics. The (older) NE-SW compression was most probably associated with the Late Cretaceous–Paleogene pan-regional basin inversion throughout Central Europe, as a reaction to ongoing African-Iberian-European convergence. Second WNW–ESE compression was active as of the Middle Miocene, at the latest, and might represent the Neogene–Quaternary tectonic regime of the NE Bohemian Massif. Exposed fault plane surfaces in a dissolution-collapse marble cave system provided insights into the Meso-Cenozoic tectonic history of the Earth’s uppermost crust in Central Europe, and were also identified as important guiding structures controlling the origin of the Niedźwiedzia Cave and the evolution of subsequent karstic conduits during the Late Cenozoic.

Shear bands in dense fault gouge

Earthquakes happen with frictional sliding, by releasing all the stresses accumulated in the prestressed surrounding medium. The geological fault gouge, coming from the wear of previous slips, acts on friction stability and plays a key role in this sudden energy release. A large part of slip mechanisms are influenced, if not controlled, by the characteristics and environment of this tribological “third body”. A 2D granular fault (mm scale) is implemented with Discrete Element Modelling (DEM). A displacement-driven model with dry contact is studied to observe kinematics and properties of the slipping zone. Increasing the length of the granular media increases the slip needed to weaken the friction from friction peak to steadystate. Low-angle Riedel shear bands are mostly observed. Their number increases with the inter-particle friction coefficient, which also influences shear bands formation in their orientation angle (higher friction leads to higher angle with the main slip direction).

Control of pre-existing fabric in fracture formation, reactivation and vein emplacement under variable fluid pressure conditions: an example from Archean greenstone belt, India

Most of the upper crustal fluid flows are strongly influenced by the pre-existing fractures/foliations in the rocks under a certain state of tectonic stress and fluid pressure condition. In the present study, we analyzed a wide range of crosscutting fractures that are filled with quartz veins of variable orientations and thicknesses, from the gold-bearing massive metabasalts (supracrustals) of the Chitradurga Schist Belt adjacent to the Chitradurga Shear Zone (CSZ), Western Dharwar Craton, southern India. The study involves the following steps: (1) analyzing the internal magnetic fabric, using anisotropy of magnetic susceptibility (AMS) studies, and determining strength of the host metabasalts, (2) quantifying the fluid pressure condition through lower hemisphere equal area projection of pole to veins by determining the driving pressure ratio (R′), stress ratio (ϕ), and susceptibility to fracturing, and (3) deciphering the paleostress condition using fault-slip analysis. We interpret the NNW–SSE to NW–SE (mean 337/69∘ NE) oriented magnetic fabric in the rocks of the region as having developed during regional D1/D2 deformation on account of NE–SW shortening. However, D3 deformation manifested by NW–SE to E–W shortening led to the sinistral movement along CSZ. As a consequence of this sinistral shearing, fractures with prominent orientations formed riedel shear components, with CSZ as the shear boundary. Subsequently, all the pre-existing fabrics along with the riedel shear components were reactivated and vein emplacement took place through episodic fluid pressure fluctuation from high to low Pf at shallow depth (∼ 2.4 km). However, NNW–SSE orientations were prone to reactivate under both high- and low-Pf conditions, thereby attaining maximum vein thickness along these orientations. The deduced paleostress from fault-slip analysis along with the kinematics of the fractures and veins are in good agreement with previously estimated regional tectonics. Thus, integrating multiple domains of studies helps in the logical interpretation of fluid flow conditions and vein emplacement mechanisms in the study area that has not been ventured before.

New structural model of strike-slip tectonics of the Gulf of Aqaba, southern Dead Sea Transform

<p>The Dead Sea Transform is an active left lateral, strike-slip plate boundary. The Gulf of Aqaba corresponds to its southern segment, where the largest amount of opening is observed. The gulf itself is deformed by a set of en echelon faults which are bounded by normal faults. These en echelon faults show structural styles of Riedel shears which are typically observed in strike-slip tectonics. However, their orientation is the opposite to the one observed in well described models or natural cases. In this study, we compare a compiled dataset to analogue models which simulate the displacement in various strike-slip systems. This comparison to a sandbox model highlights the importance of the tectonic load in a strike-slip fault system. The model is composed of two base plates with only one straight velocity discontinuity. X-Ray Computed Tomography is used as a technique to carry out a 4D analysis of internal fault structures of the model. The 10°-transtensional model generates a set of Riedel shear faults, which merge during the later stages of deformation. The 30°-transtensional tectonic load shows two major steep bounding faults with a dip-slip component and a set of en echelon faults - opposite Riedel shears in between them. A higher amount of transtension rotates the classic Riedel shear faults to the opposite position. This fault pattern is very similar to the one observed in the Gulf of Aqaba, where the internal fault system is composed of opposite Riedel shears bounded by normal faults. These observations can increase the understanding of the structural styles seen in the Gulf of Aqaba. Moreover, our study describes a new strike-slip fault system.</p>

Control of pre-existing fabric in fracture formation, reactivation and vein emplacement under variable fluid pressure conditions: An example from Archean Greenstone belt, India

Most of the upper crustal fluid flows are strongly influenced by the pre-existing fractures/foliations in the rocks under a certain state of tectonic stress and fluid pressure condition. In the present study, we analyze a wide range of crosscutting fractures that are filled with quartz veins of variable orientations and thicknesses, from the gold bearing massive metabasalts (supracrustal) of the Chitradurga Schist Belt adjacent to the Chitradurga Shear Zone (CSZ), western Dharwar craton, south India. The study involves the following steps: 1) analyzing the internal magnetic fabric using anisotropy of magnetic susceptibility (AMS) studies, and strength of the host metabasalts, 2) quantifying the fluid pressure condition through lower hemisphere equal area projection of pole to veins by determining the driving pressure ratio (R'), stress ratio (ϕ), and susceptibility to fracturing, and 3) deciphering the paleostress condition using fault slip analysis. We interpret that the NNW-SSE to NW-SE (mean 337°/69° NE) oriented magnetic fabric in the rocks of the region developed during regional D1/D2 deformation on account of NE-SW shortening. However, D3 deformation manifested by NW-SE to E-W shortening led to the sinistral movement along CSZ. As a consequence of this sinistral shearing, fractures with prominent orientations formed riedel shear components, with CSZ as the shear boundary. Subsequently, all the pre-existing fabrics along with the riedel shear components were reactivated and vein emplacement took place through episodic fluid pressure fluctuation from high to low Pf at shallow depth (~ 2.4 km). However, NNW-SSE orientations were susceptible for reactivation under both high and low Pf conditions leading to a much greater thickness along the same. The deduced paleostress from fault-slip analysis, along with the kinematics of the fractures and veins are in good agreement with the previously revealed regional tectonics. Thus, integrating multiple domains of studies, help in the logical interpretation of fluid flow condition and vein emplacement mechanism in the study area that has not been ventured before.