Conjugate Strike-slip Fault System Research Articles

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.

Crustal structure of the Tibetan Plateau and adjacent areas revealed from ambient noise tomography

How the Tibetan Plateau deformed in the Indian-Eurasian continental collision has been long debated, more specifically, over the relationship between the deep processes and surface structural complexity. Here, we use ambient noise tomography to obtain a high-resolution crustal S-wave velocity model beneath the Tibetan Plateau and adjacent areas involving a comprehensive dataset from over 500 stations. Our images reveal that the crustal flow should be in a limited scale according to the intermittent low-velocity zones (LVZs) observed in the middle crust at 20–40-km depth of the Tibetan Plateau. The distributions and strengths of LVZs further imply that different deep processes promote the surface deformation in various regions of the Tibetan Plateau. The LVZs in the northern plateau, collocated potassic magmatism and low velocity anomalies in the upper mantle, should be originated from the lithospheric delamination. However, in the southern plateau, the S-wave velocity showed an apparent lateral segmentation feature correlated with the north–south trending rifts. The feature indicates that the LVZs were likely controlled by the lateral tearing of the subducted Indian mantle lithosphere, which promotes the rifting deformation. Moreover, the LVZ in the central Tibet should have contributed to the formation of the conjugate strike-slip fault system. In the Tarim Basin, our model showed a high-velocity anomaly in the lower crust that may be related to ancient mantle plume activity.

Seafloor Structures and Static Stress Changes Associated With Two Recent Earthquakes in Offshore Southern Batangas, Philippines

The 1994 Mw 7.1 Mindoro Earthquake and the 2017 Mw 5.9 Batangas Earthquake Sequence both occurred in offshore southern Batangas and devastated southern Luzon and Mindoro. These earthquakes exhibited NW-striking right-lateral slip in an area presumably defined by a WNW-striking left-lateral fault, therefore implying the existence of previously unmapped offshore faults. High resolution multibeam bathymetry grid and subbottom profiles revealed a conjugate strike-slip fault system under an approximately EW-directed extension. NW-striking right-lateral faults (F1 Faults: Central Mindoro Fault, Aglubang River Fault, and Batangas Bay Fault System) bound the western part of the study area. On the other hand, a series of almost parallel NE-trending left-lateral and normal faults (F2 Faults: Macolod Corridor, North Verde Fault System, Central Verde Fault System, South Verde Fault, and Northeast Mindoro Fault System) approach the F1 faults from the northeast. The distribution of the 1994 and 2017 earthquakes suggests that the possible rupture areas for these events are the Aglubang River Fault and the southwest Batangas Bay Fault System, respectively. These two traces appear to be connected and a restraining bend is suggested to have acted as a rupture barrier between the two events. Coulomb stress transfer modeling showed that the 1994 earthquake promoted the failure of the 2017 earthquake. Furthermore, results from the stress transfer models showed stress increase on the F1 faults (Batangas Bay Fault System and Central Mindoro Fault) and the northern F2 faults (North Verde Fault System and Central Verde Fault System). The newly recognized faults redefine the knowledge of the neotectonic structure of the area but are still consistent with the ongoing east-west extension in southern Luzon and the overall extension in northern Central Philippines. These faults pose seismic hazards, and more studies are needed to determine their seismogenic potential.

Present-day interseismic deformation characteristics of the Beng Co-Dongqiao conjugate fault system in central Tibet: implications from InSAR observations

SUMMARYNumerous V-shaped conjugate strike-slip fault systems distributed between the Lhasa block and the Qiangtang block serve as some of the main structures accommodating the eastward motion of the Tibetan Plateau. The Beng Co-Dongqiao conjugate fault system is a representative section, and determining its tectonic environment is a fundamental issue for understanding the dynamic mechanism of the V-shaped conjugate strike-slip fault systems throughout central Tibet. In this paper, we investigate the deformation rates of the Beng Co-Dongqiao conjugate faults using 3 yr of SAR data from both ascending and descending tracks of Sentinel-1 satellites. Only interferograms with a long temporal baseline were used to increase the proportion of the deformation signals. The external atmospheric delay product and the InSAR stacking strategy were employed to reduce various errors in the large-spatial-coverage Sentinel-1 data. The InSAR results revealed that the fault-parallel deformation velocities along the eastern and western segments of the Beng Co fault are 5 ± 1 mm/yr and 2.5 ± 1 mm/yr, respectively. The second invariant of the horizontal strain rates shows that the accumulated strain is centered on the eastern segment of the Beng Co Fault and Gulu rift. The velocity and strain rate fields show that the Anduo-Peng Co faults may be paired with the Beng Co fault to form a new conjugate system and the tectonic transformation between the Beng Co fault and Gulu rift. These results can better explain the tectonic deformation environment of the Beng Co-Dongqiao conjugate fault system and provide insights on the crustal dynamics throughout the entire plateau interior.

Segment interaction and linkage evolution in a conjugate strike-slip fault system from the Tarim Basin, NW China

Abstract Through new 3-D seismic data, we present relationship between maximum throw and fault length and fault segment length in fault zones, and relationship among length, width, throw and separation in overlap and tip zones of a conjugate strike-slip fault system in the intracratonic Tarim Basin, NW China. The results show that: (1) the major fault zones are in range of 23–76 km in length, and compose of 3–8 segments with low maximum throw less than 140 m; (2) a good power–law-scaling relationship of throw-segment length but not of throw-fault length; (3) more mature in NNW trending faults than in NNE trending faults; (4) the hard-linked overlap zones concentrate most deformation and exceptional high throws, with better linear correlation between throw and overlap zone length rather than soft-linked segments; (5) length/width, length/throw and separation/throw ratios from the transpressional overlap zones varies in a range of 4.6–11.2, 59–130 and 6.1–12.3, which are quite larger than those from extensional settings. Our study suggest that: (1) the fault growth depends on the stages of segment linkage (maturity); (2) the linear trends of scaling relationship between fault elements are mainly controlled by mechanical properties, while the scattered distribution of data is related to the stage of fault linkage and overlapping; (3) the deformation and displacement are localized in the overlap zones after hard-linkage following fault segments interaction; (4) fault elements could be useful to understanding fault growth and evolution stages in the deep subsurface. These results provide new insight into a better understanding of fault linkage and interaction from the subsurface seismic data in strike-slip fault zones.

Co-seismic surface ruptures in Qiangtang Terrane: Insight into Late Cenozoic deformation of central Tibet

Recently large earthquakes mostly occurred at the plateau margins along major lithospheric faults. However, a co-seismic rupture has been discovered in central Tibet along the Burgar Co Fault (BGCF), a 150 km-long northwest-trending right-lateral strike-slip fault within the hinterland of the Qiangtang terrane, central Tibet. The activity of the BGCF is marked by a 50 km-long fresh surface rupture from 83°38′57″E, 33°58′54″N to 84°09′50″E, 33°47′42″N, restrained by a bend with a 17° counter-clockwise angle. Geomorphic offsets measured in the field and from high-resolution satellite images indicate the lateral displacements cluster at 1.4 ± 0.1 m, 2.7 ± 0.4 m, 5.4 ± 0.6 m, 9.2 ± 0.8 m and 12.5 ± 1 m. The smallest value (average 1.4 ± 0.1 m, maximum 1.8 m) is defined as the most recent co-seismic offset of a magnitude Mw = 7 earthquake. Other clusters are interpreted as the cumulative events which demonstrate that earthquakes frequently took place along the BGCF. The seismic behavior of the BGCF together with widespread sinuous north-trending grabens and NE, NW strike slip faults around western and central Qiangtang indicates that the most recent active structures have deformed scattered in the internal part of this terrane. Additionally, the conjugate strike slip fault systems along the Bangong-Nujiang Suture define an intangible southern boundary, altogether implying that the Qiangtang terrane has undergone significant deformation thus can no longer be treated as an integral block. Finally, compared with the continental slip in eastern Qiangtang, an alternative geodynamic model for Qiangtang is inferred: a weak channel flow decoupling the upper and middle-lower crust which facilitates the southeastwards extrusion as an entire block in the east, and continuous deformation as scattered small crustal wedges without expansive upper and middle-lower crust decoupling underground in the western and central areas. The Qiangtang terrane serves as the dominant place for extensive material to migrate eastward, accommodating simultaneous E-W extension and N-S compression.

Fracture attributes in reservoir-scale carbonate fault damage zones and implications for damage zone width and growth in the deep subsurface

The width of reservoir-scale fault damage zone governs fluid flow of the tight reservoirs in subsurface petroliferous basins. Together with cores, thin sections, geophysical well logs, and production data from more than 60 wells, we describe fracture attributes and fault damage zones associated with a conjugate strike-slip fault system in tight Ordovician carbonate rock at depths of between 6500 m and 7500 m in the Tarim intracratonic basin in NW China. Fracture frequency, aperture, and oil production in wells reveals a power-law relationship with the distance from fault cores, with a variation in two orders of magnitude. The cumulative data of fracture frequency, aperture and oil production show multiple gradients in slope with the distance. Except for sampling bias, the varied gradients are possibly related to the overlaying multiple fault zones or multiple sections along a large fault zone, which are useful to constrain damage zone width. Together with fracture frequency, cumulative fracture frequency, aperture and oil production, four major damage zone boundaries are identified at ∼200, ∼420 m, ∼780 m and ∼1100 m. Therefore, a wide fault damage zone with a width of more than 2 km in the carbonate rocks could be inferred, which mostly depends on the interaction and superposition of secondary faults. In addition, there is a relatively high fracture frequency decay exponent (>1) with distance, which is possibly related to fault size, fault splays, background fracture frequency and the lithology. In turn, the growth of wide damage zone also resulted in a large scattered distribution of the fracture attributes with the distance from fault cores. Attributes of fractures of various scales from the deep subsurface provide powerful tools for assessing damage zone width and growth.

Investigation into the basement morphology and tectonic lineament using aeromagnetic anomalies of Parts of Sokoto Basin, North Western, Nigeria

This study, which spans latitude 5°N–5.30IN and Longitude 13°E–13.5°E with an approximate area of about 3400 km2 within the Northwestern Sokoto basin of Nigeria, aims to delineate possible source of stress regime, determining the structural features and their corresponding trend directions, delineating lithologic boundaries as evidenced by magnetic disturbances caused by different rock types and estimating the hydrocarbon viability within the study area by interpreting the behavior of the geophysical data obtained from the Nigeria Geological Survey Agency, NGSA in half degree sheet. The aeromagnetic data was subjected to both qualitative and quantitative analytical techniques using Oasis Montaj software, version 8.1. The result of the qualitative analysis generated some magnetic gradient maps showing dominant and minor magnetic structures trending E-W, NE-SW and NW-SE. These trends are possible fault indicators within the region. Similarly, the qualitative analysis revealed lithologic boundaries trending in the E-W and NW-SE direction. The dominant E-W trend possibly highlights the stress regime and also represents the Pre-Pan African Orogeny that was characterized by small brittle deformation which resulted in the development of conjugate strike-slip fault systems while the truncated NE-SW, NW-SE trends indicate Pan-African Orogeny. On the other hand, quantitative analysis performed on the aeromagnetic data by means of Source Parameter Imaging (SPI) quantitative method reveals depth to deep seated magnetic sources to range from 54.1 m to 3384.4 m but with an average of about 739 m. The dominant E-W tectonic magnetic trending structures and a thickness average of 739 m limit the possibility of oil and gas exploration. Thus the possibility of hydrocarbon exploration even when seismic sections are run within the study area will be very minimal.

Strike-slip tectonics within the northernmost Philippine Sea plate in an arc-continent collisional setting

The geological processes in the northernmost Philippine Sea plate, which is bounded by the Suruga and Sagami troughs, are a typical example of an active collision zone. We attempt to illustrate the stress field through seismic estimations and geodetic analysis and propose the kinematic mode of the northernmost tip of the Philippine Sea plate. Seven events (M≥4.0) are chosen for waveform inversion by the ISOLA software to distinguish the stress field. In particular, six of the chosen events, which exhibit strike-slip motion, are distributed in the eastern area, where few focal mechanisms have been reported by previous studies. According to the available focal mechanisms, strike-slip faults with similar P and T axes are widely distributed in the study area. The stress inversion suggests that the northern area is characterized by a NW-SE compression and a NE-SW extension stress regime, although some spatial differences exist. As indicated by an analysis of the geodesy, epicenters, focal mechanisms, gravity anomalies and velocity structure, the deformation in the northernmost tip is mainly accommodated by several conjugate strike-slip fault systems with steep dips that center on the Izu volcanic line. Generally, the maximum principal stress of the kinematics is derived from the collision between the Philippine Sea plate and Central Japan. Because of the different subduction angles, rates and directions of the down-going plate, diverging slab-pull forces along the Suruga and Sagami troughs may be causing the NE-NNE extension in most of the areas that are bounded by the two troughs. The extension propagates southwards along the Izu volcanic line and reaches the area adjacent to Miyake-jima.

Mesozoic and Cenozoic deformations in the Raggyorcaka area, Tibet: implications for the tectonic evolution of the North Qiangtang terrane

The Mesozoic and Cenozoic deformations of the North Qiangtang terrane reflect a portion of the collisional tectonics of the Tethys Ocean, involving Late Triassic, Early Cretaceous and Cenozoic intraplate structures of shallow to superficial tectonic levels. The structural mapping of the Permian–Triassic structural layer indicates that the Mesozoic structural style is dominated by Early Cretaceous horizontal contractional deformations characterized by NW–SE-trending thrust faults and flexural-slip folds. These folds and thrusts were superimposed on Late Triassic buckle folds. Structural analysis of deformations of the Neogene and Quaternary sedimentary layers indicates that there was an obvious transition in the Cenozoic tectonic evolution of the North Qiangtang terrane, which can be divided into two deformation stages. In episode I (50–40 to 18 Ma), the crust experienced large-scale north–south horizontal shortening and vertical thickening shown by buckle folds and thrust faults. In episode II (18 Ma–present day), east–west passive extension formed conjugate strike-slip fault systems and pull-apart basins. During episode II, the Qiangtang basin was extruded eastwards, and a number of superficial north–south-trending buckle folds were formed. Supplementary material: Supplementary material is available at http://www.geolsoc.org.uk/SUP18787 .

Mechanics of V-shaped conjugate strike-slip faults and the corresponding continuum mode of continental deformation

V-shaped conjugate strike-slip faults occur widely on Earth, Venus, and Mars in the solar system. They commonly lie at 60°–75° in map view from the maximum compressive stress (σ 1 ) direction. This fault pattern cannot be explained directly by the Coulomb fracture criterion, which predicts the formation of X-shaped shear fractures at 30° from the σ 1 direction. Possible explanations of this odd fault geometry include rotation of early formed Coulomb fractures or reactivation of preexisting weakness. Here, we show that none of these mechanisms is feasible for the formation of a late Cenozoic conjugate strike-slip fault system in central Tibet. Instead, its initiation can be best explained by distributed deformation during the formation of two parallel and adjoining shear zones that have opposing senses of shear. Our suggestion is based on the current global positioning system (GPS) velocity field in Tibet, which can be divided into two east-trending shear zones: a northern left-slip zone consisting of active ENE-striking left-slip faults, and a southern right-slip zone consisting of active WNW-striking right-slip faults. The correlation between the GPS strain field and the fault pattern suggests that the central Tibet conjugate faults may have initiated as two sets of Riedel shears in the two parallel but separate shear zones. Because the two east-trending shear zones also experience north-south contraction, we refer to this mechanism of conjugate-fault formation as paired general-shear (PGS) deformation. Assuming a Newtonian fluid, the observed Tibetan GPS velocity field requires the paired shear zones to have formed either by gravitational spreading of the Tibetan lithosphere or a horizontal shear at the base of the upper crust or mantle lithosphere. We demonstrate the feasibility of the two inferred mechanisms for the formation of V-shaped conjugate faults using simple sandbox experiments. Our paired general-shear (PGS) model implies that the combined effect of the state of strain and the state of stress favors only one set (i.e., Riedel shear) of Coulomb conjugate shear fractures under general shear flow. It also requires continuum deformation rather than discrete extrusion tectonics as the most dominant mode of deformation during the late Cenozoic development of the central Tibetan Plateau.

A conjugate strike-slip fault system within the extensional tectonics of Western Turkey

SUMMARY Three main shocks M-1, M-2 and M-3 (17 October 2005 at 05:45 UTC, M w 5.4; 17 October at 09:46 UTC, M w 5.8 and 20 October at 21:40 UTC, M w 5.9) and their associated after- shocks within the Gulf of Si˘ gacik, 50 km southwest of Izmir, Turkey were studied in detail. A temporary seismic network deployed during the activity allowed the hypocentre of M-3 and subsequent aftershocks to be determined with high accuracy. A relative relocation technique was used to improve the epicentres of M-1 and M-2. All three main shocks have strike-slip mechanisms which agree with the linear trends of the aftershock locations. Two distinct zones were illuminated by the aftershock locations. The zones contain clear echelon patterns with slightly different orientations from the trend of the aftershock distribution. M-2 and M-3 rup- tured along of the eastern rupture zone which aligns N45 ◦ E. However the strike direction of M-1 is not clearly identified. The alignment of the two rupture zones intersect at their southern terminus at an angle of 90 ◦ . The fault zones form conjugate pair system and static triggering is considered as a probable mechanism for the sequential west to east occurrence of M-1, M-2 and M-3. This earthquake sequence provides seismological evidence for conjugate strike-slip faulting co-existing within a region dominated by north-south extension and well-developed east-west trending normal faults.

Competing processes of clastic deposition and compartmentalized inversion in an actively evolving transpressional basin, western Mongolia

The Dariv Basin is an actively evolving intracontinental transpressional basin located on the eastern flank of the Mongolian Altai. The basin occupies a complex tectonic position between a restraining bend, a thrusted basement block, and two major conjugate strike-slip fault systems. Structures and sedimentary strata exposed within the Dariv Basin suggest a Mesozoic and Cenozoic two-stage evolution. Jurassic–Cretaceous strata fine upward and record alluvial fan, fluvial and lacustrine depositional environments. The distribution of Mesozoic sedimentary rocks and the presence of a suspected Jurassic normal fault array suggest that the Dariv Basin initially formed as an extensional basin. Following Palaeogene tectonic quiescence, Oligocene–Recent basin fill is dominated by alluvial fan sediments derived from basin-flanking ranges. The modern basin is deforming by thrusting, normal fault inversion and folding along discrete belts expressed as intrabasinal ridges and domes. These belts define a rhomboid of active deformation that compartmentalizes the basin. Sediments derived from these discrete deforming belts and from basin flanking ranges continue to accumulate in the basin centre. Thus, modern fans contain reworked older basin fill and competing processes of sedimentation, deformation, erosion and resedimentation can be observed. The Dariv Basin is an excellent example of a transpressional piggyback basin in the early stages of basin inversion and destruction.

The post-Triassic sedimentary cover of Tunisia: Seismic sequences and structure

A series of geological cross sections based on data from the new geological map of Tunisia coupled with stratigraphic and structural studies indicate that a folded, post-Triassic sedimentary cover was separated from the basement by a decollement zone located in Triassic evaporitic deposits. Four depositional basins were formed during the Triassic and Liassic in an extensional regime, on a presumably thinned continental crust. Geological fieldwork, studies of metamorphic grade and gravimetric and magnetic surveys indicate that their depocentres shifted with time from basin to basin, finally resulting in today's Tellian and Tunisian troughs to the north, Gafsa Trough to the west of Gabes and the Tuniso-Libyan Trough to the east. Subsidence in the latter trough commenced at the end of the Cretaceous and is still continuing today. There are four well-defined seismic stratigraphic sequences comprising Mesozoic and Cenozoic sedimentary rocks, each with distinctive lithoseismic characteristics. We display the distribution and structure of these seismic sequences in a series of cross sections. In addition, we note that the regional tectonic style of Tunisia has been greatly influenced by a conjugate strike-slip fault system. The role of wrench faulting has been well-documented in the Kasserine and other areas by detailed structural studies.

An example of very consistent brittle deformation over a wide intracontinental area: The late Pan-African fracture system of the Tuareg and Nigerian shield

Brittle deformation during the latter part of the Pan-African orogeny in the Hoggar and Nigeria was characterized by a conjugate strike slip fault system, consisting of NE—SW dextral and NW—SE sinistral trending sets of faults. This system is considered to be the result of a horizontal maximum stress axis σ 1 orientated E—W. In the crystalline basement, the faults have offset the N—S trending Pan-African mylonitic shear zones, and have cut the late orogenic granites (Taourirt) and ring complexes of Adrar des Iforas. In the western Hoggar, folding of the post-orogenic molassic “série pourprée” considered as Cambrian in age (Caby, 1970), is probably associated with the same stress regime. A similar fault system also affects the Phanerozoic sedimentary cover of the North-Saharian platform, the Ténéré desert, and the Djado basin. Later reactivation of this fault system by phanerozoic epeirogenic events, and recent uplift is demonstrated. The geometry of this late Pan-African brittle deformation is very consistent over a large area and may reflect the stress field operative during indentation of a “plastic” Pan-African shield, by a more rigid West-African craton.