Maximum Principal Stress Axis Research Articles

Tectonic Stress Analysis of Shivnath-Salena Area, Using Stress Response Structure

The geological mapping of the Shivnath-Salena area and structure analysis of the same area were conducted in the LesserHimalayan sequence, Far West Nepal, unveiling the characteristics of kinematics and the associated stress field in the region. Thearea comprises of the Salena Formation and Chachura Formation. The Salena Formation is sandwiched between the PachkoraThrust and Dudulakhan Thrust, and the Chachura Formation occurs between the North Dadeldhura Thrust and Pachkora Thrust.The Salena Formation is composed of light grey and white quartzite and black slates that are intensely deformed, with folds. The Malena Anticline and the Lamalek Syncline are the intraformational folds observed in the study area. Superimposed folding was also observed. The Chachura Formation comprises Paleocene to Late Eocene green sandstone, grey and purple shale, and approximately 1 m thick grey fossiliferous limestone. This study interprets stress-response structures to show the tectonic stress field in the Salena Formation. Twenty-seven fault slip data (slickenside) were collected in the field and used to determine the stress regime by applying the stress inversion technique. The direction of the maximum principal stress axes is interpreted as NE-SW. The average value of the stress index R' is about 0.30, indicating an extensional tectonic stress regime in the study area. As σ1 is vertical, R = R' in the study area suggests normal faulting in an extensional regime.

ПОЛЯ НАПРУЖЕНЬ ТА ГЕОЛОГІЧНА СТРУКТУРА ЗАХІДНОГО ЗАМИКАННЯ ГОРЛІВСЬКОЇ АНТИКЛІНАЛІ ДОНБАСУ ЧАСТИНА 2. ПОЛЯ НАПРУЖЕНЬ І ДЕФОРМАЦІЙ

Background. In tectonophysical studies, particular importance is placed on the mechanism of formation of large complex deformation structural elements, and the Horlivka anticline of the Donets Basin serves as such an example. Its importance lies not only in its complex structural composition and development mechanism but also in its potential impact on safe and efficient coal mining. This study aims to analyse the geological structure, stress and strain fields of the western closure of the Horlivka anticline that could be useful in forecasting geological factors for coal mining operations. Methods. The paleostress analysis was performed on the collected fault orientation and kinematic data using Gushchenko's kinematic method based on the analysis of tectonic displacement vectors along slickensides. Mesoregional stress field characteristics were reconstructed by statistical processing of local stereographic solutions. The relative age chronology and staging of tectonic stresses were studied using the stress monitoring method. The characteristics of the principal axes of the total strain field were processed using the GEOS software. Results. Mesoregional stress field is characterised by a subhorizontal NW–SE-oriented maximum principal stress axis and a subhorizontal NE–SW-oriented minimum principal stress axis. This stress regime is classified as a strike-slip regime and is the youngest one (Alpine orogeny) for the Donets Basin. The evolution of tectonic loading conditions of the studied structure is characterised by a deformation series of six stress regimes from the oldest (normal) to the youngest (strike-slip). Stretching axis of the strain ellipsoid is oriented NW and N–S, and shortening axis is oriented NE, nearly orthogonal to the anticline axis. Strike- and oblique-slip faulting regimes of the total strain field were determined for most of the study area, and the deformation has occurred under shear conditions, as indicated by the Lode–Nadai coefficient. Conclusions. A structural pattern of deformation elements, including a conjugate strike-slip fault system of the shear zone, a dome-shaped fold, and longitudinal thrusts in its limbs, may be interpreted as a single pattern of structural paragenesis developed by right-lateral displacements along the longitudinal strike-slip fault system within the Horlivka anticline paraxial part. Fragments of mutual symmetry between the stress and strain fields can be taken as evidence of their genetic relationship.

Present-Day Tectonic Stress Evolution in Southern Yunnan Based on Focal Mechanisms.

Tectonic extrusion bypassing the eastern Himalayan syntaxis results in a significant increase in regional stress instability and the associated frequent occurrence of earthquakes in southern Yunnan, China. However, the stress field, and the relationship between the focal mechanism of earthquakes and stress evolution in southern Yunnan, remain enigmatic. In this paper, using a modified grid point test method, we calculated the focal mechanism of ML ≥ 2.5 earthquakes in southern Yunnan (22-25° N, 100-104° E) from January 2009 to June 2023. Utilizing the solutions of historical earthquake focal mechanisms, we obtained the present-day regional tectonic stress field in southern Yunnan via inversion. The results indicate complex and diverse seismic focal mechanisms, and the main types of earthquakes are strike-slip events, followed by normal fault and reverse fault events. The orientations of the maximum and minimum principal stress axes rotate in a clockwise direction from northeast to southwest. The internal stress orientation distribution of the rhombic Sichuan-Yunnan block in the study area is consistent, and the block boundary zone is the site where stress deflection occurs, and the regional tectonic stress field is influenced by the interaction among different blocks. The distribution of R-value in the Lamping-Simao block gradually increases from north to south, indicating that the compressive stress required for material transport becomes relatively small. Combined with the geological and tectonic background of the study area, our results suggest that the speed of block movement gradually decreases from north to south; the distribution of R-value in the South China block is significantly smaller than that of the interior of the Sichuan-Yunnan rhombus, and the proportion of compressive stresses is larger, indicating a stronger extrusion in this region, which may be related to the fact that the Sichuan-Yunnan rhombus is strongly resisted by the South China block in the east.

Spatial–Temporal Distribution and Focal Mechanisms for the Shallow Earthquakes in the Rongchang-Luxian Region in Southern Sichuan Basin, China

AbstractAn Ms 6.0 earthquake struck the Luxian County in southern Sichuan basin, China, on 15 September 2021, which was the largest local event thus far and caused heavy casualties and serious property losses. Using data of 107 temporary and six permanent stations from five months before to four and a half months after the time of the mainshock in the Rongchang-Luxian region, we utilize the machine learning-based workflow for phase picking, earthquake detection and relocation, and obtain precise hypocenter data for 22,806 events. We found that earthquakes increase significantly since July 2021, showing north-northeast-, northeast-, east-northeast- and east-southeast-directed seismic strips. Almost all events are located within the sedimentary layers at 1–7 km depth above the crystalline basement, and more than half of them occurred in Silurian and Ordovician formations. The Ms 6.0 Luxian earthquake sequence occurred in an area where no clear seismicity was observed before the mainshock. On map view, the aftershock zone shows a convex polygon with an overall strike of 112°. The focal depths of aftershocks southwest to the mainshock are distributed in a narrower range of 4–7 km and extend to a wider range of 2–7 km on the northeast side. It is speculated that the seismogenic fault is a blind thrust fault dipping to the southwest at 2–7 km depth. The focal mechanisms of 13 ML 3.0+ earthquakes show either pure reverse faulting or reverse faulting dominated mechanisms with a strike-slip component. The centroid depths fall in the range of 1.95–6.75 km. The maximum and middle principal stress axes are nearly horizontal; the azimuth of the maximum horizontal stress is about 120° and the stress shape ratio is 0.78. These results provide an important reference for reunderstanding the local seismic risk and for detailed studies on the seismological aspects about the unusual Luxian earthquake in the future.

Palaeostress analysis using fault-slip data from the Xiangdong tungsten deposit in the central south China continent: Evolution of intraplate stress in the Middle Mesozoic

In the central South China continent, the Xiangdong tungsten deposit associated with Late-Jurassic granitic intrusion offers an opportunity to examine the evolution of intraplate stress between the Late Jurassic and Early Cretaceous, which was a time of transition from regional shortening to regional extension. NE–ENE-striking, steeply-dipping, opening-mode tungsten-bearing quartz veins constitute the deposit. Favorable conditions for vein formation where densely spaced arrays have non-curving tips occur under remote stress with a large stress difference and hence in this case probably aligned with the horizontal maximum principal stress axis. Palaeostress analysis using fault-slip data measured on the side walls of underground mine levels yielded the optimal solution of two stresses characterized by an NNW–SSE-shortening strike-slip regime and an NNW–SSE extensional regime. Based on field observations, the first tectonic phase existed during meso- and epi-thermal mineralization, probably owing to the collision between the Siberia and Mongolia–Sino-Korean continents far to the north at the time, and the second tectonic phase appeared after hydrothermal mineralization, as a result of Cretaceous regional rifting. Therefore, multiple strike-slip deformations would have involved the study area during hypo- and epi-thermal mineralization in the Late Jurassic. This evidence does not support the mainstream extension model for Mesozoic tungsten mineralization in central South China.

Rotation and deflection of 3D principal stress axes induced by a prefabricated single flaw in sandstone: A numerical investigation based on DEM

Fracture structure in the rock mass is an enduring research interest because of its negative impacts on the strength and safety of rocks. Therefore, a series of DEM simulations of two types of fractured sandstones are conducted based on the former published experimental test. It is proved that different fracture structures have various influences on the mechanical properties and failure patterns of sandstone samples, which lead to the redistribution of principal stress in surrounding rock. The main results and conclusions are as follows: 1) fractured sample without filler shows a relatively lower uniaxial compressive strength and a larger elastic modulus, and the flaw free surfaces make the secondary crack band illustrate a typical arc-shape distribution. 2) maximum principal stress concentrations in the open-type flawed sample occur at the left and right flaw tips, the corresponding axes show progressive deflection in the vertical direction, and the intermediate and minimum principal stress axes demonstrate various rotations as the positions move away from the flaw tip. 3) maximum principal stress axes in the cement-filled fractured sample are basically parallel to the external loading direction, the largest deflection occurs in the cement filler material, and the intermediate principal stress axes near the flaw tip are consistently vertical to the flaw strike direction. 4) initiation positions of tensile and shear cracks in the open-type flaw sample are different because of the non-negligible flaw thickness, while all secondary cracks start from the top and bottom tips of the cement-filled flaw.

Double-sided subduction with contrasting polarities beneath the Pamir-Hindu Kush: Evidence from focal mechanism solutions and stress field inversion

The Pamir-Hindu Kush region at the western end of the Himalayan-Tibet orogen is one of the most active regions on the globe with strong seismicity and deformation and provides a window to evaluate continental collision linked to two intra-continental subduction zones with different polarities. The seismicity and seismic tomography data show a steep northward subducting slab beneath the Hindu Kush and southward subducting slab under the Pamir. Here, we collect seismic catalogue with 3988 earthquake events to compute seismicity images and waveform data from 926 earthquake events to invert focal mechanism solutions and stress field with a view to characterize the subducting slabs under the Pamir-Hindu Kush region. Our results define two distinct seismic zones: a steep one beneath the Hindu Kush and a broad one beneath the Pamir. Deep and intermediate-depth earthquakes are mainly distributed in the Hindu Kush region which is controlled by thrust faulting, whereas the Pamir is dominated by strike-slip stress regime with shallow and intermediate-depth earthquakes. The area where the maximum principal stress axis is vertical in the southern Pamir corresponds to the location of a high-conductivity low-velocity region that contributes to the seismogenic processes in this region. We interpret the two distinct seismic zones to represent a double-sided subduction system where the Hindu Kush zone represents the northward subduction of the Indian plate, and the Pamir zone shows southward subduction of the Eurasian plate. A transition fault is inferred in the region between the Hindu Kush and the Pamir which regulates the opposing directions of motion of the Indian and Eurasian plates.

Using Seismic Data and Slip Tendency to Estimate Paleostress: A Case Study From Xicaogu Area of Bohai Bay Basin, China

Paleostress plays an important role in controlling the formation and distribution of reservoir fractures, which is a key factor controlling the production of oil and gas from unconventional reservoirs. At present, there are many methods to perform paleostress inversion, but these methods are mainly based on outcrop or directional core data, which are not easily available in the oil and gas industry, which mainly focuses on the study of underground geological structures. In this study, we use 3D seismic reflection data for the first time to perform slip-tendency-based stress inversion to determine the stress field of the Xicaogu area in the Bohai Bay Basin during the Neogene period. This slip-tendency-based stress inversion method is based on the analysis of slip tendency and fault throw and does not require the measurement of slip directions. The data for this method can easily be extracted from data sets routinely used in the oil and gas industry. The inversion results show that in the study area and the strata of interest, the maximum principal stress axis (σ1) is oriented vertically, the intermediate principal stress axis (σ2) is oriented N70o, the minimum principal stress (σ3) is oriented N160o, and σ2/σ1 = 0.69 and σ3/σ1 = 0.27. The results of this study serve as an important reference to clarify fault reactivation at the critical stage of hydrocarbon accumulation.

Characteristics of Local Geomagnetic Field Variations and the Tectonic Stress Field Adjacent to the 21 May 2021, Ms 6.4 Yangbi Earthquake, Yunnan, China

The tectonic processes leading up to an earthquake and the occurrence of the earthquake itself will cause local changes in the geophysical field (geomagnetic field, stress field, etc.). In this paper, the variation characteristics of the tectonic stress field (TSF) and local geomagnetic field (LGF) before and after the Yangbi Ms 6.4 earthquake are studied. The regional stress tensor damping inversion method was used to invert the TSF using focal mechanism solutions (FMSs). The change characteristics of the TSF before and after the earthquake were analyzed. An annual variation model of the LGF was constructed, and the variation of the horizontal vector was analyzed. The azimuth and plunge of the maximum principal compressive stress axis of the TSF in the epicentral region before and after the earthquake were −4.4° and 2.7°, 172.7° and 6.6°, respectively. The variations in the declination, inclination and total intensity of the epicenter one year before and one month after the earthquake were −0.20′ (0.07′), 0.29′ (−0.12′), and −1.7 nT (−1.9 nT), respectively. The epicenter is located at the boundary of the “weak variation region” of the horizontal vector. This research is of great significance concerning the TSF background and incubation mechanism of earthquakes.

Characteristics of crustal stress field in Tienshan area

Based on 1143 focal mechanisms in Tienshan area from 1900 to 2019, according to the world stress map classification criterion, the main types of focal dislocations are thrust and strike slip; The results of parameter statistics show that the nodal plane of focal mechanism solution is basically consistent with the main tectonic strike in Tienshan area, and the orientation of P axis is perpendicular to the main tectonic strike in Tienshan area, which shows the horizontal nearly NS thrust. The direction of the maximum principal compressive stress is 175.1°, the minimum principal stress direction is 67.8°, the plunge angle of the maximum principal compressive stress axis is 4.3°, the plunge angle of the minimum principal compressive stress axis is 75.8°, the relative stress ratio R is 0.80, and the stress state is compressional, which indicates that the area is in the background of compressional orogeny.

POSSIBLE INFLUENCE OF A MAJOR INFERRED FAULT ON THE OCCURRENCE OF SINISTRAL TOP-TO-THE NW DEFORMATION IN THE ZAGROS HINTERLAND FOLD-AND-THRUST BELT, IRAN

In the present study, fault slip data, the geometry of en-echelon vein arrays (tension gash), and pressure-solution seams (stylolites), in the northeastern margin of Fars Province were analyzed. The results of this study indicate that in the time of the development of these structures, the maximum principal (σ1) stress axes were generally horizontal and directed towards NE-SW and ENE-WSW. This general direction is compatible with the expected directions of σ1 stress axes responsible for the occurrence of the sinistral top-to-the NW ductile and brittle-ductile shear zones of the area. This compatibility suggests a long-lasting stable stress condition over a long period and different pressure/temperature conditions. This longstanding constant stress state can be interpreted as the result of the occurrence of a major strike-slip NW-SE trending fault in the NE of the Zagros Hinterland Fold-and-Thrust Belt. The dextral activities of the Main Zagros Thrust and this inferred fault, which are subparallel, might result in the sinistral topto-the NW deformation in the area between this fault, which we named it Abarkuh Fault and the Main Deep Fault. This inferred fault has been covered by Quaternary alluvium of the Abarkuh plain, but the great age difference of rock units of the Esteghlal Anticline and its northeastern rock exposures, and the significant change in topography between the Abarkuh plain and its southwestern mountains can be two consequences of the existence of this probable fault.

Hard classification of polyphase fault-slip data: Improvement and application

The quickest approach to the separation of polyphase fault-slip data into single-phase subsets is perhaps the recognition in stress space of the hyperplanes, consisting of their corresponding datum vectors, whose normals are parallel to the solutions of the stress vector. However the uncertainty of the solution sign makes it impossible to distinguish the subsets with respect to the opposite real stress vectors. In order to overcome this drawback the existing hard-classification method was improved by incorporating the slip senses during the separation. A series of artificial two-phase fault-slip datasets with the measurement errors generated under the imposed stresses were taken to test the improved method. There is a high accuracy of the stress solution even when the measurement errors have a range of [-12°, 12°]. The new method is further applied to data of shear bands measured on the limbs of the NNE-trending syncline at the Shihtipin resort, east of Taiwan. Two stress estimates representing the strike-slip or compressional tectonic regimes were obtained on each limb. They indicate an anticlockwise rotation of the maximum principal stress axis from ESE–WNW or SE–NW to E–W during the folding, probably in response to the oblique collision between the Luzon volcanic arc and the South China continent.

Origin of synchronous extension and inversion in a rift basin: The Phitsanulok Basin, central Thailand

Abstract The Phitsanulok Basin is a Cenozoic rift basin in central Thailand. The basin opened in the Oligocene to Early Miocene with the development of N-S and NNW-SSE trending normal faults. In the Middle Miocene the northern part of the basin continued to experience extension, while the southern part was affected by compressive inversion. Extension resumed in the Late Miocene to Pliocene, followed by post-rift subsidence in the Quaternary. The Middle Miocene anomalous deformation is attributed to activities of the NE-SW Uttaradit Fault and NW-SE Mae Ping Fault, respectively at the northern and southern basin margins. Both faults had limited activities during the Oligocene to Early Miocene rifting, where E-W extension was probably accompanied by vertical maximum principal stress. In the Middle Miocene, the maximum principal stress rotated to a N-S direction and produced sinistral motion on the Uttaradit Fault and dextral motion on the Mae Ping Fault. The strike-slip motions contributed to a counter-clockwise rotation of the continental block west of the Phitsanulok Basin, creating the synchronous extension and contraction. The kinematic model introduces the possibility of local inversion in a rift basin being originated from short-term switch between the maximum and intermediate principal stress axes and reactivation of long-lived strike-slip faults, as opposed to major change in regional stress related to plate movements. The model implies that the segment of the Mae Ping Fault south of the Phitsanulok Basin was inactive during the Oligocene to Early Miocene and was reactivated as a dextral fault in the Middle Miocene.

Evolution of slip susceptibility of Quaternary faults in southeastern Korea from Quaternary to present day

We study the geomechanical stability of faults in southeastern Korea that have been active during the Quaternary (mostly Pleistocene), to investigate whether these faults can still be considered active today. We compile fault plane data and slip orientations for 40 Quaternary faults in southeastern Korea distributed along the two major fault systems, Yangsan Fault (group 1) and Ulsan Fault (group 2), and in the younger block to the east of the Ulsan Fault (group 3). Our aim is to determine the representative tectonic stress states in the Quaternary and in the present day, and to compare the slip susceptibility of these faults under the stress fields of these two periods. We estimate Quaternary stress from inversion of Quaternary fault data, and present-day stress from inversion of contemporary earthquake mechanisms. Both stress fields exhibit a maximum principal stress axis that is sub-horizontal and trends ENE-WSW, but the intermediate and minimum principal stress axes are switched, suggesting a change in stress regime from reverse-faulting during the Quaternary to strike-slip-faulting in the present day. We demonstrate that this change in stress regime is due to a marked reduction in the magnitude of horizontal tectonic stress from the Quaternary to the present day. We assess the slip susceptibility of the faults under the Quaternary and present-day stress fields using two indices: slip tendency and fault instability. This analysis shows an increase in slip susceptibility for most of group 1 faults and some of group 3 faults. Some of these faults exhibit a slip susceptibility comparable to the average obtained for contemporary earthquake focal planes, suggesting that these faults can be considered active in the present day.

On the Orientation of Fractures with Transpressional and Transtensional Wrenches in Pre-Existing Faults

The orientation of fractures with transpressional and transtensional wrenches in pre-existing faults has not been quantitatively determined. Based on Coulomb failure criterion and Byerlee’s frictional sliding criterion, this paper has indicated quantitative geometric relationships between the pre-existing fault and the local induced principal stress axes caused by the rejuvenation of the pre-existing fault. For a hidden pre-existing fault with some cohesion, the angles between the local induced principal stress axes and the pre-existing fault quantitatively vary with the applied stress and the cohesion coefficient, the ratio of the thickness of the cover layer to the thickness of the whole wrench body, whether transpressional or transtensional wrenches occur. For a surface pre-existing fault with zero cohesion, the angles between the pre-existing fault and the local induced principal stress axes are related to the rock inner frictional angle regardless of both the applied stress and the cohesion coefficient where transpressional wrenches occur, and the local induced maximum principal stress axis is identical with the applied maximum principal stress axis where transtensional wrenches occur. Therefore, the geometric relationships between the pre-existing faults and their related fractures are defined, because the local induced principal stress axes determine the directions of the related fractures. The results can be applied to pre-existing weak fabrics. They can help to understand and analyze wrench structures in outcrops or subsurface areas. They are of significance in petroleum exploration.

Electrical resistivity of three-phase cracked rock-soil medium and its anisotropic changes caused by crack changes

Electrical resistivity of sedimentary rock-soil is highly variable and governed mainly by a wide range of properties including resistivities of both solid matrix and fluid, cracks or pores, fluid saturation, and crack fabric (e.g. crack shape and alignment). An anisotropic, three-phase, effective medium resistivity model is proposed in this article to describe the resistivity of cracked rock-soil medium of sediments in macroscale. The resistivities of both solid matrix and fluid are assumed to be isotropic. Anisotropic features are therefore caused by cracks. The model permits both solid and fluid phases to have arbitrary range of resistivity. We derive the differential form between resistivity changes and crack changes. Expressions of the volumetric sensitivity and saturation sensitivity are obtained. An amplification matrix is defined to describe the anisotropic changes in resistivity. The three electrical principle axes have different volumetric sensitivity. Therefore, the resistivity shows anisotropic changes when the cracks change. For water-bearing rock and soil medium, the minimum electrical axis is most sensitive to crack changes. However, the anisotropic changes of anhydrous rocks are not obvious. Apparent resistivity has been continuously monitored at fixed stations in China for more than 50 years, using Schlumberger arrays. Apparent resistivity monitored in the direction perpendicular or nearly perpendicular to the maximum principal stress axis has recorded the maximum magnitude of changes before earthquakes. While the magnitude is minimum when apparent resistivity is monitored in the direction parallel or sub-parallel to the axis. There is an angle difference of between the resistivity changes and the apparent resistivity changes for Schlumberger array. The analytic results of anisotropic changes based on the model agree with the field monitoring results.

Calcite twinning strains from syn-faulting calcite gouge: small-offset strike-slip, normal and thrust faults

We have evaluated the stress–strain behavior of calcite precipitated and mechanically twinned in small-offset strike-slip, normal and thrust faults of a variety of ages and from a variety of tectonic settings (n = 3001 twin measurements, 63 strain analyses from 18 field sites). Five strike-slip faults with syn-faulting, horizontally striated calcite (rake = 0°) were studied and we report the orientations of the contemporaneous stress–strain field associated with each fault: intrusion of the Marathon Large Igneous Province mafic dikes (~ 2.1 Ga in Archean crust, Minnesota, USA); post-Keweenaw rift (1.1 Ga) faulting (Island Lake fault, central Ontario, Canada); subduction associated with metamorphic core complex formation (Cretaceous, China); subduction (Cretaceous to Miocene, Italy), and continental extension (recently active Furnace Creek fault, Death Valley, California, USA). Seven normal faults with synfaulting, dip-slip striated calcite were studied and are from the following tectonic settings: a normal fault slip surface in an Ordovician Piedmont fold, Appalachian’s; paleo-subduction associated with Cretaceous metamorphic core complex formation (China, 3 sites); the paleo-extensional Atlantic margin (~ 55 Ma, Ireland, 1 site with a U–Pb calcite age); continental extension (1 active site, Mojave desert); a transcurrent margin (Jamaica, 1 active fault site), and subduction [2 active faults along the Eur-African margin in Italy (with calcite U–Th disequilibria ages) and Crete, respectively]. Six thrust fault examples are all from convergent orogenic settings: the basal thrust of the Penokean (1850 Ma) fold-and-thrust belt; the Penokean orogen foreland in Mesabi Range banded iron formation folds; an offset breccia body in the Permian Gondwanide belt, Ellsworth Mountains, Antarctica; the frontal thrust of the Gondwanide Cape belt, South Africa; the Paleocene frontal Prospect thrust, Sevier belt, Wyoming, and an Alpine foreland back-thrust, Lulworth Cove, U.K. For each strike-slip fault system the twinning shortening strain is horizontal and at an angle of 0°–60° to the respective fault plane (dextral or sinistral) although in the majority of cases the shortening axis is parallel to fault strike (13 of 23 results). In each normal fault example, dip-slip kinematic striations dominate the faulted surface yet the orientation of the maximum principal compressive stress (σ1) and shortening strain axis (e1) are not 45° to the fault plane as predicted but are sub-horizontal and either strike-parallel (25 of 35 results) or strike-normal (10 of 35 results). Thrust faults preserve shortening strain axes parallel to the dip-slip kinematic direction, within the fault plane (plane strain) and not at 45° to the principal plane (5 of 5 results). None of the fault stress–strain field results reported here support the Andersonian or Mohr–Coulomb criteria for stress–strain relations predicted along faults.

Crustal movement and strain distribution in East Asia revealed by GPS observations

East Asia is bounded by the Indian plate to the southwest and the Pacific and Philippine plates to the east, and has undergone complex tectonic evolution since ~55 Ma. In this study, we collect and process three sources of GPS datasets, including GPS observations, GPS positioning time series, and published GPS velocities, to derive unified velocity and strain rate fields for East Asia. We observed southward movement and arc-parallel extension along the Ryukyu Arc and propose that the maximum principal stress axis (striking NEE) in North China could be mainly induced by westward subduction of the Pacific plate and the southward movement of the Ryukyu Arc. The large EW-trending sinistral shear zone that bounds North China has been created by eastward movement of South China to the south and westward subduction of the Pacific plate to the north. GPS velocity profiles and strain rates also demonstrate that crustal deformation in mainland China is controlled by northeastward collision of the Indian plate into Eurasia and westward subduction of the Pacific and Philippine Sea plates beneath Eurasia. In particular, the India-Eurasia continental collision has the most extensive impact, which can reach as far as the southern Lake Baikal. The viscous behavior of the subducting Pacific slab also drives interseismic deformation of North China. The crustal deformation caused by Philippine oceanic subduction is small and is limited to the region between the southeast coast of mainland China and Taiwan island. However, the principal compressional strain around eastern Taiwan is the largest in the region.

Cenozoic paleostress evolution in south central Vietnam: Implication for changing dynamics of faulting along the eastern Indochina continental margin

The tectonics of the Indochina continental margin before and after the Neogene-Quaternary basalt eruption is poorly understood. The analysis of 1500 striated faults observed in different rocks, mostly from volcano-plutonic rocks with ages ranging from Cretaceous to Quaternary, at more than 140 locations in south central Vietnam by direct inversion method allowed us to determine four paleostress regimes that affected eastern Indochina from Oligocene up to present day. The first tectonic paleostress regime with Oligocene age was responsible for the sinistral and dextral motions along the NW-SE and NE-SW striking faults respectively. This stress state is typical for a strike-slip tectonic regime with maximum principal stress (σ1) and minimum principal stress (σ3) axes oriented nearly in E-W and N-S direction and both plunging at average angle of 80 and 100, respectively. The paleostress ellipsoid shape ratio Φ (Φ = (σ2 − σ3)/(σ1 − σ3)) ranges from 0.23 to 0.77 with an average of 0.50 ± 0.15. The subsequent strike-slip tectonic stress state prevailing from the Late Oligocene to very Early Miocene, was characterized by σ1 and σ3 axes trending in NNW-SSE and ENE-WSW and plunging at 120 ± 90, and 120 ± 130, respectively. The paleostress ellipsoid shape ratio Φ ranges from 0.12 to 0.88 with a 0.52 ± 0.23 average. Under this paleostress regime, the N1100 to N1400 and N1600 to N200 striking faults moved with dextral and sinistral movements, respectively. The third paleostress state with assigned Early Miocene to Late Miocene age was responsible for dextral and sinistral motions along N1400 to N1600 and N200 to N400 striking faults, coeval with emplacement of N-S striking basaltic dykes. This pattern shows consistency and homogeneity of the σ1 and σ3 axes in N-S trend, plunging at 120 ± 90 and σ3 in E-W trend, plunging at 130 ± 110, respectively. The paleostress ellipsoid shape ratio Φ varies from 0.41 to 0.89 with an average of 0.53 ± 0.16. The fourth tectonic stress regime occurring since the Late Miocene to the present day is characterized by a consistency of sub-horizontal NNE striking σ1, plunging 110 ± 100 and ESE striking σ3, plunging at 130 ± 11, and an average of ellipsoid shape ratio Φ = 0.45 ± 0.24. Under the latest tectonic stress state, the N-S and N450 to N600 striking faults continue to move as dextral and sinistral faults, respectively. The first three paleostress regimes have evolved clockwise, and were induced by the force exerted from the eastern Himalayan syntaxis during the northward Indian plate motion, while the fourth tectonic stress regime is most likely linked to the force originated from the Sunda subduction.

Structural model and slip-dilation tendency analysis at the Copahue geothermal system: Inferences on the reservoir geometry

The Copahue volcano host one of the most attractive geothermal resources of Argentina. Several studies focused on both the volcanic activity and the related geothermal manifestations, including a feasibility report during the '90s. However, limited, unclear and usually contradictory structural interpretations are found in the literature, and the role of the main faults over the geothermal system remains thus unsolved. This paper provides the first structural characterization and role assessment of the faults over the fluid flow and the reservoir geometry. A 3D structural model has been constructed to achieve this goal, and a fault-plane striations survey has been carried out at Anfiteatro, Termas and Maquinitas hydrothermal zones. Using this dataset, a slip and dilation tendency analysis over the main structures have been performed. The 3D structural model indicates that the Copahue geothermal field is bounded by an N60°-trending fault set and constrained to the north by the Trolope fault and to the south by the Chancho-Co fault, both with WNW-ESE trend. The collected fault-plain striations data was inverted to obtain the paleostress tensor related to the formation of the Copahue village fault system (CVFS), the structure that controls the hydrothermal areas. The subvertical maximum principal stress axis obtained defines a well constrained tensional regime, with a subhorizontal NW-SE minimum principal stress, consistent in all the surveyed locations. Both the 3D model and the inverted paleostress tensor indicate that the geothermal field, and probably the entire northeastern slope of the volcano, is subjected to an extensional faulting regime. Our analysis shows that the N°60-trending fault set is critically stressed for dilation and probably behaves as an active hydrological fault system, whereas the N105° and N135° fault sets have intermediate and low dilation tendency, respectively. These tendencies might lead to a northeast-trend fluid path between the Chancho-co and the N135° faults and might define the extension of the steam cap. We propose a compartmentalized reservoir model made up by three fault segmented blocks. While a steam cap over a deeper reservoir controlled by the CVFS characterizes the central segment, a pressurized liquid-dominated reservoir could prevail to the west of Termas and the east of Maquinas hydrothermal zones. This model explains the differences between the fumaroles inside the CVFS (i.e., Termas to Maquinas fumaroles) and outside this structural system (i.e., Anfiteatro), is consistent with the steam cap identified by the exploration boreholes and is consistent with the 3D structural framework proposed. Also, the model suggests that the collapse of the eastern slope of the Chancho-co hill might constitute the initial state of the current configuration of the geothermal system. Additional studies are needed to verify the proposed reservoir model before defining deep exploration targets, including magnetotellurics, discrete fracture network analysis, and preliminary thermodynamical models.