Strike-slip Stress Regime Research Articles

Late Cenozoic shift from extension to strike-slip stress regime in the west of the Biga Peninsula, NW Turkey

To the west of the Biga Peninsula, there are Paleozoic-Mesozoic metamorphic massifs and young fault systems that developed in different directions and periods. We investigated the types, formation times, and kinematics of these faults. Our field observations and kinematic studies show that there were three stress phases in the region between the Neogene and Quaternary. According to the analysis, in the first phase during the Miocene-Pliocene, the region was under NW-SE extension which affected western Anatolia. Right-lateral faults with left step-overs developed with the entrance of the North Anatolian Fault System into the Marmara region in the Middle-Late Pleistocene. This system started the second stage in the Late Pleistocene, and a short-lived NNE-SSW compression regime prevailed in the region. As a result, reverse faults and thrusts developed. In the third phase, During the Late Pleistocene-Holocene, the direction of extension turned to the NE-SW as the North Anatolian Fault System affected the region. The data we obtained from the region show that the West Anatolian Extensional System was effective during the Miocene-Pliocene. Then, particularly within the Late Pleistocene-Holocene period, the North Anatolian Fault System played the main role in the morphotectonic development of the Biga Peninsula and its surroundings.

Understanding present-day stress in the onshore Canning Basin of Western Australia

Western Australia’s Canning Basin is an underexplored prospective basin with proven petroleum systems and small-scale production. Recently, several formations within deeper depocentres have been investigated for unconventional hydrocarbon resources. Modern petroleum resource evaluation generally depends on an understanding of local and regional stresses that are a primary control over the formation and propagation of induced fractures. There are significant gaps in our understanding of these factors within the Canning Basin. This study characterises the regional stress regime of the onshore Canning Basin and presents modelled present-day stress, allowing for the identification of significant stress heterogeneity and natural fracture barriers. Interpretation of wireline data reveals a present-day state of stress with variation in magnitude and faulting-type. An approximately northeast–southwest regional present-day maximum horizontal stress orientation is interpreted, which is in broad agreement with the Australian Stress Map and previously published data. One-dimensional mechanical earth models, constructed for intervals from 15 Canning Basin wells, highlight the relationship between lithology and stress. Significant changes in stress within and between lithological units, owing to the existence of discrete mechanical units forming numerous inter- and intra-formational stress boundaries, likely to act as natural barriers to fracture propagation, particularly within those units currently targeted for their unconventional resource potential, are interpreted. A strike-slip faulting stress regime is interpreted broadly through the basin. However, when analysed in detail there are three distinct stress zones identified: (1) a transitional reverse- to strike-slip faulting stress regime in the top ∼1 km, (2) a strike-slip faulting stress regime from ∼1 km to ∼3.0 km and (3) a transitional strike-slip to normal faulting regime at depths greater than ∼3.0 km. This study is a component of the Australian Government’s Exploring for the Future initiative, which focusses on gathering new data and information about northern Australia’s resource potential. KEY POINTS New stress data, including 15 mechanical earth models, are presented for Western Australia’s onshore Canning Basin. Broadly, a strike-slip stress regime with a NE–SW-oriented maximum horizontal orientation is described. Significant stress heterogeneities and natural fracture barriers are identified.

Dependence of earthquake faulting type on fault strike across the Korean Peninsula: Evidence for weak faults and comparison with the Japanese Archipelago

We re-evaluate the stress state across the Korean Peninsula by analyzing the stress tensor inversion results, strike–rake relationships, and faulting-type indicator–magnitude relationships of published earthquake focal mechanism data. Previous studies have shown that the stress state is characterized by a strike-slip faulting stress regime with ENE–WSW maximum compression, NNW–SSE minimum compression, and vertical intermediate stress. The differential stress magnitudes are estimated to be on the order of 100 MPa under the assumption of the strong-fault earthquake with a high Coulomb friction coefficient of 0.85. Conversely, other studies have shown that the weak-fault model is applicable across the Japanese Archipelago, where the differential stress level is only a few tens of MPa. We, therefore, review previous studies, evaluate the focal mechanism data across the Korean Peninsula, and infer the absolute stress level. Our comparison of the geological and tectonic histories of the Korean Peninsula and Japanese Archipelago suggests that the NNW–SSE tensional stresses in the Korean Peninsula are caused by slab rollback of the Philippine Sea Plate at around 5 Ma, west of Kyushu Island, southwestern Japan. We also find that the earthquake faulting types across the Korean Peninsula are closely related to their fault strikes, confirming a spatially uniform distribution of the strike-slip stress regime with nearly equal tensional and compressional tectonic stresses. However, this situation leads to the apparent paradox of strike-dependent fault strength. We eliminate this paradox by proposing a model whereby reverse- and normal-faulting events are caused by local stress heterogeneities around the edges of strike-slip faults under a uniform low-strength strike-slip stress state for all faulting types. We also highlight the usefulness of strike–rake and faulting-type indicator–magnitude diagrams in considering stress states.

Study on In Situ Stress Distribution Law of the Deep Mine: Taking Linyi Mining Area as an Example

The variation of the in situ stress state is closely related to various factors. In situ stress state is also an important indicator to guide mining production. The study of in situ stress measurement and its distribution characteristics has always been a basic and very important work in mine production. In this study, the deep mines of Linyi Mining Area were considered as the research object. In this regard, the stress distribution law of each mine was studied. We found that the relationship between principal stresses was σH > σv > σh, which belongs to the strike‐slip stress regime. In this stress regime, the lateral Earth pressure coefficient was greater than one, and the magnitude of the three principal stresses all showed an increasing trend with the increase of depth. The maximum horizontal stress direction of the Gucheng Coal Mine, Guotun Coal Mine, and Pengzhuang Coal Mine was NW‐SE under the influence of regional geological structure, while the maximum horizontal stress direction of Wanglou Coal Mine was NE‐SW under the influence of local geological structure. Besides, the relationship between mine in situ stress and mine geological structure, the impact of original rock stress on stope stability, and the effect of original rock stress on floor water inrushing were also investigated. We believe that the research results are beneficial to mine disaster prevention and safety production.

Seismicity in a low-strain area: Example of the Irauçuba fault in NE Brazil

The present study illustrates an example of methods and results of fault examination in a slow strain and elastic attenuation area in intraplate Brazil. We investigate the Irauçuba seismogenic fault, which occurs in the Precambrian crystalline basement of the Borborema Province, the Equatorial margin of Brazil. The focal mechanisms in the region show normal, reverse, and mainly strike-slip faulting indicating a regional strike-slip stress regime in the upper crust up to 12 km depth with maximum horizontal compression (SHmax) roughly following the coastline trend. Under such context, we present earthquake data from a swarm sequence, the Irauçuba fault, at which the seismicity does not correlate with surface features. However, events with a magnitude as small as 1.5 mb are felt by the local population, and, in some cases, events with magnitude 2.5 mb or greater have damaged poorly built constructions. Therefore, suitable aftershock monitoring demands detailed macroseismic surveillance and engagement of the affected population and the Civil Defense. This engagement is crucial for awareness of the affected cities and the sensor network design and sustainability of the many stages of the aftershock monitoring such as deployment and maintenance. The earthquake data reveals that the Irauçuba NW-SE-striking fault occurs at depths between 8 and 9 km and dips ~70° to NE. This fault has not been previously reported and is solely defined by the spatial distribution of hypocentres that coincides with the focal mechanism solution, and no obvious surface expression is observed. The fault plane reveals an initial rupture at shallower depths that progressively migrated to deeper portions of the crust. Therefore, we suggest that the initial seismic period transferred stresses to other near-critical areas of the fault, which allowed pore pressure to migrate within permeable portions of the fault zone within the upper crust.

Fault kinematics and paleostress analysis using seismic data: A case study from the Archinsk field, West Siberian Basin, Russia

We present a methodology to determine the stress field based on the analysis of 3D seismic reflection data without additional information from wells. It involves the analysis of seismic horizons that were folded before the analyzed fault system. Fold hinges-faults intersections provide piercing points and fault slip vectors, which can be processed in the same way as slickenlines. Four seismic horizons and 51 faults in the Archinsk field, West Siberian Basin, were analyzed. The resulting fault-slip dataset is heterogeneous and has 126 slip vectors, corresponding to normal and strike-slip faulting. A normal stress regime with stress ratio (Φ) of 0.5 and a NE-trending minimum principal stress (σ3) is identified for the Late Triassic-Early Jurassic. A strike-slip stress regime with stress ratio (Φ) of 0.3, NW- and NE-trending maximum (σ1) and minimum principal stresses (σ3) corresponds to the present-day stress field.

An integrated petrophysical and geomechanical characterization of Sembar Shale in the Lower Indus Basin, Pakistan, using well logs and seismic data

Shale gas reservoirs are generally exploited through horizontal drilling and hydraulic fracturing. Petrophysical and geomechanical parameters help to determine horizontal well orientation, to select drilling mud densities for stable drilling operations, to assess the suitability of specific zones in a formation for hydraulic fracturing, and to assess required hydraulic fracturing pressures. The primary objective of this work was to characterize a shale interval in the Early Cretaceous-age Sembar Formation in the Lower Indus Basin of Pakistan, using only readily available data. A workflow was developed for the estimation and mapping of geomechanical properties using logs from multiple wells and relevant post-stack seismic reflection data. Mineralogy data from well cuttings, core testing results for elastic properties and hydraulic fracturing test data (mostly obtained from one key well) were utilized to constrain the values of the properties estimated from geophysical data. The following results obtained at the well-scale suggest that the Sembar Shale is favorable for development: high gas saturation, good porosity (up to 10%), moderate quantity of thermally mature organic matter (2%–4% TOC), a number of brittle intervals separated by thicker intervals that fall slightly below the brittle-ductile threshold, and a strike-slip stress regime. At the scale of the study area, robust statistical techniques were used to invert seismic stacks and develop a 3D mechanical earth model. This model shows a trend of increasing shale brittleness towards the northeastern portion of the study area, hence suggesting that this area might be most prospective for initial shale gas development. The results of sensitivity analyses are presented, which illustrate the potential errors in the estimated geomechanical properties. Future work to improve confidence in the shale gas potential of the Sembar Formation is given, including extensive coring and laboratory testing, in-situ stress and natural fracture characterization, and better delineation of shale thickness.

Seismotectonics and 1-D velocity model of the Greater Geneva Basin, France–Switzerland

SUMMARY The Greater Geneva Basin (GGB), located in southwestern Switzerland and neighboring France, is enclosed by the rotating northwestern edge of the Alpine front and the Jura mountains chain. Recently, this basin has received increasing attention as a target for geothermal exploration. Historical and instrumental seismicity suggest that faults affecting the basin may still be active. Moderate-magnitude earthquakes have been located along the Vuache fault, a major strike-slip structure crossing the basin. Before geothermal exploration starts, it is key to evaluate the seismic rate in the region and identify possible seismogenic areas. In this context, we deployed a temporary seismic network of 20 broad-band stations (from September 2016 to January 2018) to investigate the ongoing seismic activity, its relationship with local tectonic structures, and the large-scale kinematics of the area. Our network lowered the magnitude of completeness of the permanent Swiss and French networks from 2.0 to a theoretical value of 0.5. Using a new coherence-based detector (LASSIE - particularly effective to detect microseismicity in noisy environments), we recorded scarce seismicity in the basin with local magnitudes ranging from 0.7 to 2.1 ML. No earthquakes were found in the Canton of Geneva where geothermal activities will take place. We constructed a local ’minimum 1-D P-wave velocity model’ adapted to the GGB using earthquakes from surrounding regions. We relocated the events of our catalogue obtaining deeper hypocentres compared to the locations obtained using the available regional velocity models. We also retrieved eight new focal mechanisms using a combination of polarities and waveform inversion techniques (CSPS). The stress inversion shows a pure strike-slip stress regime, which is in agreement with structural and geological data. Combining the background seismicity with our catalogue, we identified seismogenic areas offsetting the basin.

Geomechanical Analysis for Deep Shale Gas Exploration Wells in the NDNR Blocks, Sichuan Basin, Southwest China

The abundant reserve of shale gas in Sichuan Basin has become a significant natural gas component in China. To achieve efficient development of shale gas, it is necessary to analyze the stress state, pore pressure, and reservoir mechanical properties such that an accurate geomechanical model can be established. In this paper, Six wells of Neijiang-Dazu and North Rongchang (NDNR) Block were thoroughly investigated to establish the geomechanical model for the study area. The well log analysis was performed to derive the in-situ stresses and pore pressure while the stress polygon was applied to constrain the value of the maximum horizontal principal stress. Image and caliper data, mini-frac test and laboratory rock mechanics test results were used to calibrate the geomechanical model. The model was further validated by comparing the model prediction against the actual wellbore failure observed in the field. It was found that it is associated with the strike-slip (SS) stress regime; the orientation of SHmax was inferred to be 106–130° N. The pore pressure appears to be approximately hydrostatic from the surface to 1000 m true vertical depth (TVD), but then becomes over-pressured from the Xujiahe formation. The geomechanical model can provide guidance for the subsequent drilling and completion in this area and be used to effectively avoid complex drilling events such as collapse, kick, and lost circulation (mud losses) along the entire well. Also, the in-situ stress and pore pressure database can be used to analyze wellbore stability issues as well as help design hydraulic fracturing operations.

Structural controls and metallogenic model of polyphase uranium mineralization in the Kiggavik area (Nunavut, Canada)

The Kiggavik area is located on the eastern boundary of the Paleo- to Mesoproterozoic Thelon Basin (Nunavut, Canada) and hosts uranium mineralization in Archean basement rocks. The major fault/fracture network in the Kiggavik area is mainly oriented ENE-WSW and NE-SW, consisting of polyphased fault zones initiated during the Thelon and Trans-Hudsonian orogenies (ca. 1900–1800 Ma). These faults were subsequently mineralized in four stages referred to as U0, U1, U2, and U3. The first event U0 is inferred to be of magmatic origin and is related to microbrecciation and weak clay alteration under a WSW-ENE σ1. U0 is a ca. 1830 event which predates intense quartz brecciation (QB) and veining at ca. 1750 Ma. QB is associated with emplacement of the Kivalliq Igneous Suite and caused pervasive silicification of former fault zones, which in turn controlled subsequent fracture development and behaved as barriers for later U mineralizing fluids (U1 to U3). U1, U2, and U3 postdate deposition of the Thelon Basin. U1 and U2 occurred under a regional strike-slip stress regime, with the direction of σ1 evolving from WNW-ESE (U1) to NE-SW /ENE-WSW (U2); both formed at ~ 1500–1330 Ma and are related to circulation of Thelon-derived uranium-bearing basinal brines. A post U2, but pre-Mackenzie dykes (ca. 1270 Ma), extensional/transtensional stress regime with σ3 oriented NE-SW caused normal-dextral offset of the orebodies by reactivating NNW-SSE and E-W trending faults. This fracturing event triggered circulation of hot (~ 300 °C), probably acidic, fluids that dissolved quartz, and caused illitization and bleaching of the host rocks. Finally, U3 records remobilization of the previous mineralization along redox fronts through percolation of low-temperature meteoric fluids during two main tectonic events at ca. 550 and 350 Ma. This study provides evidences for the presence of a primary, pre-Thelon Basin uranium stock within the Kiggavik prospects, and a strong structural control on mineralization in the Kiggavik area. Our study also shows a nearly similar evolution of uranium mineralization in this area compared to the world-class uranium district of the Athabasca Basin (Saskatchewan, Canada).

Postrift stress field inversion in the Potiguar Basin, Brazil – Implications for petroleum systems and evolution of the equatorial margin of South America

Rifting and related normal stress regime in the equatorial continental margin of Brazil ceased during the Late Cretaceous, when the stress regimes in eastern South America and West Africa changed to induce strike-slip or reverse motion. In this study, we explore the postrift tectonic, geomorphic, magmatic, and sedimentary responses to stress changes using the Potiguar Basin, the easternmost basin in the equatorial margin of Brazil, as a case study. We use field and topographic data, 2D seismic reflection lines, vertical electric soundings, and geochronological and borehole data to constrain the stress evolution of the Potiguar Basin from the Late Cretaceous to the Quaternary, discussing the role of basin inversion on sedimentation and landforms. Our results indicate the presence of two strike-slip stress regimes after rifting. The first stress field (SF1) occurred from Late Cretaceous to the middle Miocene and consisted of a N-S-oriented maximum subhorizontal compression and an E-W-oriented extension. The second stress field (SF2) took place from the middle Miocene to the present day and included subhorizontal E-W to NW-SE compression combined with N-S and NE-SW subhorizontal extension. Emplacement of volcanic rocks occurred along transtensional faults, with a principal peak during SF1 at 20–30 Ma and a subordinate peak during SF2 at 5–10 Ma. In response to shortening during SF2, a 70-km-long and 50-km-wide dome formed, where marine Miocene strata were uplifted to ~250 m asl. This uplift induced the displacement of alluvial channels away from the dome. Anticlines formed by transpression along the main NE-SW-striking faults during both SF1 and ST2 acted as traps in the petroleum system. Similar shifts and stress field inversions documented in other areas of the Brazilian continental margin are consistent with the Neogene rise of the Andes and may have implications for reconstructing the tectonic history of the Equatorial Atlantic margin of South America.

Influence of tectonic stress regime on the magnitude distribution of induced seismicity events related to hydraulic fracturing

Hydraulic fracturing is today the primary means of initiating, increasing, and maintaining well productivity in unconventional shale gas reservoirs. However, hydraulic fracturing is attracting public concern over its rapid growth in use and environmental footprint, including potential hazards such as induced seismicity. To mitigate these concerns, there is a need to better understand the level of hazard present with respect to the magnitudes of the events possible, recognizing that these are not equal across different shale gas plays due to regional differences in the geological conditions or even within the same shale gas play due to local differences.An empirical study is presented here along with a series of numerical simulations to investigate the influence of the tectonic stress regime on the magnitude and magnitude distribution of induced seismicity events related to hydraulic fracturing practices. A database of determined earthquake focal mechanisms was first used to determine the tectonic stress regime for different North American shale gas basins. Next, induced events associated with hydraulic fracturing operations were identified to determine the magnitude distributions and b-values for these different shale gas basins. To support these empirical analyses, 3-D numerical modelling was performed to further investigate the mechanistic responses and event magnitudes under different simulated stress regimes. The empirical analysis results show that thrust faulting stress regimes have lower b-values than strike-slip stress regimes and therefore are more susceptible to larger induced seismicity events. The numerical simulations show that this is related to the stresses acting on a thrust fault (relative to the fault dip angle) being higher and more concentrated across a larger slip area. Slip in this case, and the stored strain energy being released, was observed to occur as a single large magnitude event. In contrast, numerical simulations for the other faulting types showed the stresses to be more distributed across the fault plane. This results in multiple slip events involving smaller slip areas and therefore with smaller event magnitudes, assuming all other factors are kept the same.

Seismicity of Valle Medio del Magdalena basin, Colombia

An analysis of seismicity of Valle Medio del Magdalena (VMM) Basin, Colombia was done over the period between 2014 and 2017 with the aim to obtain a baseline of the seismic activity in this region. Thirteen portable broad band seismic stations were deployed during the study period and were complemented by 15 permanent broad band seismic stations. Hypocenter distribution and focal mechanism analyses were performed.Hypocenter determination was performed for 7073 earthquakes with local magnitudes between 0.1 and 5.7. Those earthquakes were located at crustal as well as subduction depths. The former were related to fault systems that evidenced seismic activity, such as the Bucaramanga fault system and the Cimitarra, Curumaní and Arrugas faults. The latter delineated the shape of the subduction slab of the Caribbean plate beneath South American plate, which exhibited changes in the dip angle between 29° and 39° probably associated to deep ruptures.Inversion of focal mechanisms obtained by first motion P-wave analysis for crustal earthquakes showed that the VMM region was under a regional strike-slip stress regime.

A normal-faulting stress regime in the Bavarian Foreland Molasse Basin? New evidence from detailed analysis of leak-off and formation integrity tests in the greater Munich area, SE-Germany

Leak-off and formation integrity test data from the central part of the Bavarian Foreland Molasse Basin have been investigated in detail to infer information about the stress regime of the Cenozoic basin fill. The detailed analysis of leak-off test data from the Bavarian Foreland Molasse Basin and lithology-dependent analysis of leak-off test and formation integrity data is the first of its kind in a published study. Only test data from shale-rich sequences have been considered. All data yield minimum principal stresses that are smaller than an estimated vertical stress range. In combination with critical stress and frictional equilibrium theory, the data indicate that the stress regime in the greater Munich area and possibly the far-field stress regime of the Bavarian Foreland Molasse Basin are most likely of an extensional nature (normal-faulting stress regime). Under the assumption of frictional equilibrium, a friction coefficient between 0.2 and 0.4 best explains failure in shale-dominated sections of the central part of the Bavarian Foreland Molasse Basin and can be used to estimate the minimum horizontal stress SHmin. However, even in the spatially restricted domain of the greater Munich area the stress regime might vary towards a strike-slip stress regime; most likely in the vicinity of fault zones and/or due to variations in mechanical rock strength. The results of this study have great impact and relevance to improved planning of drilling campaigns, future numerical modelling and the general understanding of the evolution of the Bavarian Foreland Molasse Basin. Additional leak-off tests and extended leak-off tests are recommended to fully unravel the spatial variation and geologic control factors of the stress regime of the entire Bavarian Foreland Molasse Basin.

Preliminary seismic hazard assessment for the proposed Bohai Strait subsea tunnel based on scenario earthquake studies

In this paper we present a preliminary seismic hazard assessment for the proposed Bohai Strait subsea tunnel (BSST) based on scenario earthquakes in a strike-slip stress regime. We studied two scenarios: 1) a right-lateral, strike-slip faulting earthquake event with surface wave magnitude (Ms) 7.4 occurred on the north to northeast (NNE) oriented Tan-Lu fault zone (TLFZ) with a location about 100 km away from BSST (the far field); and 2) a left-lateral strike-slip event with magnitude Ms4.1 occurred in the direct vicinity to BSST (the near field) on the northwest to west (NWW) Zhangjiakou-Penglai fault zone (Zhang-Peng Fault Zone, or ZPFZ). Selections of these two scenario events are based on the characterization of the actual seismo-tectonic settings and representation of earthquake history in the Bohai region. We studied the impacts of these two scenario events on BSST in terms of longitudinal compression, bending, and tunnel cross-section's ovaling deformation. As a preliminary study, we excluded the analysis of formation-structure interaction and focused on the ground motion parameters expressed as the maximum displacement, the peak ground velocity (PGV) and peak ground acceleration (PGA) of the free seismic wave field surrounding the tunnel walls. This simplification is acceptable for the fact that BSST is planned to be constructed in the granite bedrock formation in which the inertial contrast between the formation and the structure is minimal. Seismic hazard assessment for tunnels is completely different from the assessment for above-ground buildings. It is also substantially different from assessment for tunnels built in soil formations, for which the formation (soil)-structure interaction evaluation is critical. We use semi-analytical approach for assessing the seismic impact of the far field, large event to BSST; whereas use the two-dimensional (2D) finite difference time domain (FDTD) numerical simulation technique for assessing the response of BSST to the seismic load of the near field, smaller event. The preliminary results suggest that 1) the impact of the low frequency, far field event has very subtle impact to BSST in terms of longitudinal compression, bending, and cross-section ovaling; and 2) the impact of the high-frequency, near field event has little impact to BSST in terms of longitudinal compression and bending; while has a measurable impact to cross-section ovaling. However, this kind of deformation is within the limit of tolerance under earthquake-resistant design. Over all, the seismic risk posed by regional large earthquakes and local small events to BSST is within allowable risk threshold and manageable with the enforcement of earthquake resistant code.

Correlation between tectonic stress regimes and methane seepage on the western Svalbard margin

Abstract. Methane seepage occurs across the western Svalbard margin at water depths ranging from < 300 m, landward from the shelf break, to > 1000 m in regions just a few kilometres from the mid-ocean ridges in the Fram Strait. The mechanisms controlling seepage remain elusive. The Vestnesa sedimentary ridge, located on oceanic crust at a depth of 1000–1700 m, hosts a perennial gas hydrate and associated free gas system. The restriction of the occurrence of acoustic flares to the eastern segment of the sedimentary ridge, despite the presence of pockmarks along the entire ridge, indicates a spatial variation in seepage activity. This variation coincides with a change in the faulting pattern as well as in the characteristics of the fluid flow features. Due to the position of the Vestnesa Ridge with respect to the Molloy and Knipovich mid-ocean ridges, it has been suggested that seepage along the ridge has a tectonic control. We modelled the tectonic stress regime due to oblique spreading along the Molloy and Knipovich ridges to investigate whether spatial variations in the tectonic regime along the Vestnesa Ridge are plausible. The model predicts a zone of tensile stress that extends northward from the Knipovich Ridge and encompasses the zone of acoustic flares on the eastern Vestnesa Ridge. In this zone the orientation of the maximum principal stress is parallel to pre-existing faults. The model predicts a strike-slip stress regime in regions with pockmarks where acoustic flares have not been documented. If a certain degree of coupling is assumed between deep crustal and near-surface deformation, it is possible that ridge-push forces have influenced seepage activity in the region by interacting with the pore-pressure regime at the base of the gas hydrate stability zone. More abundant seepage on the eastern Vestnesa Ridge at present may be facilitated by the dilation of faults and fractures favourably oriented with respect to the stress field. A modified state of stress in the past, due to more significant glacial stress for instance, may explain vigorous seepage activity along the entire Vestnesa Ridge. The contribution of other mechanisms to the state of stress (i.e. sedimentary loading and lithospheric flexure) remain to be investigated. Our study provides a first-order assessment of how tectonic stresses may be influencing the kinematics of near-surface faults and associated seepage activity offshore of the western Svalbard margin.

Regional stress field in Yunnan revealed by the focal mechanisms of moderate and small earthquakes

We determined focal mechanism solutions of 627 earthquakes of magnitude M ≥ 3.0 in Yunnan from January 2008 to May 2018 by using broadband waveforms recorded by 287 permanent and temporary regional stations. The results clearly revealed predominantly strike-slip faulting characteristics for earthquakes in Yunnan, with focal depths concentrated in the top 10 km of the crust. The earthquake mechanisms obtained were combined with the global centroid moment tensor solutions of 80 additional earthquakes from 1976 to 2016 to invert for the regional variations of stress field orientation by using a damped regional-scale stress inversion scheme. Results of the stress field inversion confirmed that the Yunnan region is under a strike–slip stress regime, with both maximum and minimum stress axes being nearly horizontal. The maximum compressional axes are primarily oriented in a northwest-southeast direction, and they experience a clockwise rotation from north to south, whereas the maximum extensional axes are oriented largely northeast-southwest. The maximum compressional axes are in line with the global positioning system–inferred horizontal velocity field and the southeastward escape of the Sichuan–Yunnan Rhombic Block, whereas the maximum extensional axes are consistent with anisotropy derived from SKS splitting. Against the strike–slip background, normal faulting stress regimes can be seen in the Tengchong volcanic area as well as in other areas with complex crisscrossing fault zones.

Basement fabric controls rift nucleation and postrift basin inversion in the continental margin of NE Brazil

In passive continental margins, the brittle reactivation of shear zones and their role in the deformation and deposition of sedimentary basins are still a matter of debate. In this research, we investigated the role of the brittle reactivation of Precambrian shear zones in the nucleation of rift and postrift faults in the onshore portion of the Sergipe-Alagoas and Pernambuco basins in northeastern Brazil. We combine and interpret a dataset of aeromagnetic and topographic data, associated with reflection seismic and borehole data, to analyze the evolution of a portion of the Atlantic continental margin of Brazil. Our results indicate that in the crystalline basement, the magnetic lineaments are correlated with ductile structures as shear zones, and the continuity of these lineaments in the Sergipe-Alagoas and Pernambuco basins is interpreted as the shear zones below the sedimentary cover of these basins. We document the following phases of the brittle reactivation of basement shear zones: (1) the opening of the South Atlantic Ocean in the Early Cretaceous under an extensional stress regime and (2) tectonic inversion induced by the Mid-Atlantic Ridge push and the Andean Cordillera rise in the Neogene-Quaternary under a predominantly strike-slip stress regime. During the rift phase, the brittle reactivation of the shear zones controlled the locations and architectures of the rifts. These structures acted as zones of weakness and were reactivated as normal faults. The brittle reactivation of shear zones was still active during the postrift phase and was responsible for the development of compressional structures. The reverse faulting and related folding pattern indicate tectonic inversion in the Late Cretaceous-Cenozoic. The structures formed during the postrift phase under a strike-slip regime are consistent with the present-day stress field, indicating that tectonic inversion is an active phase of the Brazilian margin.

Earth stress and reservoir quality evaluation in high and steep structure: The Lower Cretaceous in the Kuqa Depression, Tarim Basin, China

Knowledge of the state of the in-situ stress field provides important insights into the petroleum engineering and geology. The in-situ stress orientation and magnitudes in the high and steep structure of Lower Cretaceous Bashijiqike Formation in Kuqa Depression were analyzed based on well log calculations, and the impacts of in-situ stress field on reservoir quality (macroscopic reservoir properties and microscopic pore structure as well as fracture aperture) were investigated. The maximum horizontal stress (SHmax) orientation was determined as nearly north-south according to the image log derived drilling induced fractures and borehole breakouts. The vertical stress (Sv), SHmax, and minimum horizontal stress (Shmin) magnitudes were calculated via well logs. The result reveals a dominant strike-slip stress regime (SHmax > Sv > Shmin) in the Bashijiqike Formation in the Kuqa Depression. The Sv at the depth ranging from 5000 to 7100 m is 150–180 MPa, and horizontal stresses (SHmax and Shmin) vary significantly from 130 to 210 MPa. The horizontal stress difference (Δσ) is in the range from 40 to 70 MPa, and fluid overpressures are commonly encountered. The effective stress (σe) has a positive relationship with the Δσ when the Δσ is less than 50 MPa. Intergranular pores can be preserved in layers with low Δσ values. The microscopic pore structure will be complex, and macroscopic reservoir quality will be poor with the increasing Δσ values. In addition, fractures are mainly corresponding with the layers where the Δσ value is low, and the fracture aperture and porosity decreased significantly with the effective stress σe. By determining the magnitude and orientation of in-situ stresses around the wellbore, insights can be provided into the wellbore instability, optimum drilling trajectories and reservoir quality evaluation.

Structural style and kinematic evolution of Al Ji'lani area, Ad Dawadimi terrane, Saudi Arabia

This article discusses the structural control of the Precambrian Al Ji'lani basic intrusion, which is one of numerous oval igneous intrusions scattered within different terranes of the Arabian-Nubian Shield. The area in the Ad Dawadimi vicinity exposes 75 km long northwest-trending sinistral en echelon fault systems. During East African Orogen, the master fault segments propagate as sinistral system creating a rhomboidal shape releasing stepover area, defined in this work as Al Ji'lani pull-apart structure. The fault kinematic analysis shows significant variation in stress tensors between data measured outside and inside Al Ji'lani pull-apart structure. The stress tensor, computed from fault population inside the pull-apart, typify a NW-SE to NNW-SSE pure tectonic extension. Outside the pull-apart structure, the stress tensor illustrates a pure strike slip stress regime with a minimum stress axis that varies from NW-SE to NNW-SSE, and maximum stress axis that varies from ENE-WSW to NE-SW. The pull-apart northwestern corner develops, at the end stage, a circular to elliptic basic intrusion known as the Al Ji'lani layered gabbro intrusion. The intrusion is located in the releasing stepover's corner and characterized by inward dipping layers, suggesting an evolution genetically related to the pull-apart growth. Field data and faults kinematic analysis reveal that the Al Ji'lani structure is an ancient Ediacaran–Cryogenian gabbro intrusion developed in pull-apart corner, widely recognized as preferential crustal weakness zone in recent systems of fault-related-volcanism. The results support a predominant relationship between brittle tectonic and volcanism rather than shearing as it was previously accepted.