Strike-slip Component Research Articles

A new analysis of the intraplate stress regime and stress ratio in numerically modeled mantle convection

I propose a new analysis method for determining the intraplate stress in geodynamic models using a series of numerical simulations of mantle convection in 3D spherical-shell geometry. In the present study, the intraplate stress was evaluated from numerically obtained velocity and stress fields of mantle, and quantitatively classified into nine types by analyzing the principal deviatoric stress axes and the “stress ratio,” which is a continuous parameter accounting for the stress regimes. The sensitivity of model parameters and physical conditions associated with the basic characteristics of mantle convection, such as internal heating ratio, viscosity stratification, and temperature-dependent viscosity of the mantle as well as viscoplastic rheology that causes plate-like surface motion, on the intraplate stress regimes were studied. The results demonstrated that the radial viscosity structure of the mantle interior strongly affected intraplate stress regimes, and the combination of increased viscosity in the lower mantle and the low-viscosity asthenosphere enhanced the pure strike-slip faulting regime in the stable part of plate interiors. The temporally averaged toroidal-poloidal ratio (T/P ratio) at the top surface of mantle convection with surface plate-like motion and the mantle's viscosity stratification generally ranged ~20–40%, which is comparable to the observed T/P ratio of present-day and past Earth. Under such Earth-like surface conditions, normal faulting regime with strike-slip component or strike-slip regime with normal faulting component, as well as pure strike-slip faulting regime, were broadly found in the stable parts of the plate interiors. From the definition of the stress regime in the present study, strike-slip faults on the real Earth are likely to occur where the strike-slip faulting component is dominant in the present models. The analysis method proposed herein is effective for evaluating the intraplate stress in research target regions, for which observation data is insufficient to determine the intraplate stress.

Neotectonic Fault Pattern of the Salair Area (Southern West Siberia): Relation with the Pre-Cenozoic Tectonic Framework

Abstract —Neotectonic activity in the area of the Salair Ridge (southern West Siberia) rejuvenated a system of large arc-shaped faults separating the Salair tectonic arc from the adjacent tectonic units. These regional faults, which make up the general tectonic framework of the Altai–Sayan Folded Area, originated in the late Paleozoic and were repeatedly reactivated in the Mesozoic. The deformation within the major Salair thrust sheet is mainly brittle and follows small fault planes that crosscut the margins of Paleozoic thrusts. The neotectonic faulting has controlled the erosion pattern of the territory and produced a reticulate drainage system. The Salair tectonic unit is a single 80 × 250 km block consisting of multiple neotectonic blocks, with relative vertical offset no more than 100 m in the block interior and 100–200 m in its southern, northern, and eastern borders. The northwestern and southeastern border faults have reverse slip geometry, while the motions on the en-echelon northeastern fault boundary include reverse and right-lateral strike-slip components. The thickness of the Salair thrust sheet estimated from magnetotelluric (MT) data increases in the western direction from 5 to 15 km in the northern block part and from 10 to >20 km in the south. The allochthon base is delineated by a low-resistivity zone interpreted as a horizontal detachment. This boundary formed in the Mesozoic and was rejuvenated at the neotectonic stage. The lithology and deformation of Jurassic sediments filling piedmont basins around the Salair Ridge indicate that the Cenozoic fault pattern generally inherits the Mesozoic framework but differs in about ten times smaller vertical offset.

Surface Rupture on a Secondary Fault Associated with the 8 August 2020 Mw 5.1 Sparta North Carolina Earthquake

Abstract On 8 August 2020, northwest North Carolina experienced an Mw 5.1 earthquake that caused damage to buildings and roads in the city of Sparta. A regional centroid moment tensor solution shows that the earthquake was the result of slip on a reverse fault with a minor strike-slip component. Interferometric Synthetic Aperture Radar (InSAR) data, from the Japan Aerospace Exploration Agency’s Advanced Land Observing Satellite #2 (ALOS2) satellite, reveal a deformation field that is more complex than expected from a single reverse fault earthquake. The data also reveal an apparent fault rupture at the Earth’s surface that caused damage to local roads. Modeling of the InSAR deformation field indicates the fault rupture is associated with a very shallow normal faulting event with an equivalent Mw of about 5.1 that overprinted the reverse fault deformation field and possibly occurred aseismically.

Fault Geometry and Mechanism of the Mw 5.7 Nakchu Earthquake in Tibet Inferred from InSAR Observations and Stress Measurements

Different types of focal mechanism solutions for the 19 March 2021 Mw 5.7 Nakchu earthquake, Tibet, limit our understanding of this earthquake’s seismogenic mechanism and geodynamic process. In this study, the coseismic deformation field was determined and the geometric parameters of the seismogenic fault were inverted via Interferometric Synthetic Aperture Radar (InSAR) processing of Sentinel-1 data. The inversion results show that the focal mechanism solutions of the Nakchu earthquake are 237°/69°/−70° (strike/dip/rake), indicating that the seismogenic fault is a NEE-trending, NW-dipping fault dominated by the normal faulting with minor sinistral strike-slip components. The regional tectonic stress field derived from the in-situ stress measurements shows that the orientation of maximum principal compressive stress around the epicenter of the Nakchu earthquake is NNE, subparallel to the fault strike, which controlled the dominant normal faulting. The occurrence of seven M ≥ 7.0 historical earthquakes since the M 7.0 Shenza earthquake in 1934 caused a stress increase of 1.16 × 105 Pa at the hypocenter, which significantly advanced the occurrence of the Nakchu earthquake. Based on a comprehensive analysis of stress fields and focal mechanisms of the Nakchu earthquake, we propose that the dominated normal faulting occurs to accommodate the NE-trending compression of the Indian Plate to the Eurasian Plate and the strong historical earthquakes hastened the process. These results provide a theoretical basis for understanding the geometry and mechanics of the seismogenic fault that produced the Nakchu earthquake.

TALDYK EARTHQUAKE on November 17, 2015 with KR=14.1, Mw=5.5 (Kyrgyzstan)

Information on the earthquake with KR=14.1, which occurred in Kyrgyzstan on November 17, 2015, is presented. Its epicenter is related to the South Fergana zone of the Osh region, in which felt earthquakes with intensity up to I=8–9 occurred repeatedly. This event was named Taldyk according to the settlement nearest to the epicenter. The earthquake was accompanied by numerous aftershocks: for the first day, 189 events were registered, for the second – 196, for the third – 84. Most part of the aftershocks is localized within the depth interval of 12–13 km, which is practically equal to the depth of the main shock (h=13 km). The focal mechanism of the main shock has a reverse type with strike-slip components. No serious investigation of the consequences of this earthquake carried out. Some macroseismic data are received from field reports of the station operators. For a more complete analysis of the possible impact of this earthquake and, first of all, for the needs of the Ministry of Emergency Situations of Kyrgyzstan Republic, a map of theoretical isoseismals was created.

SEISMICITY of the KOPETDAG REGION in 2015

In 2015, the seismicity of the Kopetdag region was monitored by the network of 32 seismic stations, including 28 digital and 4 analogue stations. The re-equipment of stationary analogue stations of Turkmenistan with digital GEOSIG equipment, which began in 2013, was continued in 2015 – 6 GEOSIG stations were added to 9 stations of this type, and the analogue equipment at the re-equipped stations was stopped. In 2015, the seismic activity A10 in the Kopetdag region was close to the background level for the period 1992–2014, while the number of weak events significantly exceeded the level of the previous year. The seismic activation along the boundaries of the crustal blocks in the north of the Iranian plate, which began in 2012, continued by the October 12, 2015 earthquake with KR=12.7, Mw=5.2. This strongest earthquake in the territory of Turkmenistan in 2015, named the Kenekesir earthquake by the name of the nearest settlement, accompanied by numerous aftershocks – more than 35000 events with KR=3–11 were located during 80 days after the mainshock within 30 km radius. The aftershock series lasted 186 days and ended in 2016. According to the complex of instrumental seismological and tectonic data, oblique-slip with equal normal and strike-slip components occurred in the source of the mainshock. The rupture plane had a southwestern strike and dipped to the northwest. The maximum ("Bath’s") aftershock occurred on November 16 with KR=11.1. Judging by its remoteness from the mainshock in space and time, and the difference in the type of movement in the source (upthrust), it was caused by stress relaxation in the environment.

TECTONIC STRESS FIELD AT INTERMEDIATE DEPTHS OF THE SOUTHERN FLANK OF THE KURIL-KAMCHATKA SEISMIC ZONE

The study has been done on the orientation of the principal axes of tectonic stress field at intermediate depths of earthquakes of the southern flank of the Kuril-Kamchatka subduction system separately for the upper and lower double seismic focal zones. Use has been made of the NIED and GlobalCMT catalogue data. The computation-based results are presented on schemes of the stressed state of the investigated areas and in tables. In the context of the southern Kuril Islands, evidence has been provided for predominance of the maximum compressive stresses along the slab plane in the upper layer and minimal compressive stresses (deviation extension) in the lower layer. However, the principal axes of maximum and minimum compression are displaced in direction relative to the slab dip: by 30–40° counterclockwise for the compression axis in the upper layer, which coincides with the direction of plate movement, and clockwise for the extension axis in the lower layer. This might be caused by the right-lateral strike-slip component of the Pacific Plate subduction. Unlike the general trend, the orientation of the principal axes of the stress field beneath the central Hokkaido-related segments in the upper layer is almost identical to that in the lower layer. There have also been found the segments exposed to shear stress, with the most extensive located opposite the northern Kunashir Island and beneath the southern Hokkaido Island. The results obtained for major large groups of clusters show good accordance with those published by other authors. The discrepancies relate primarily to small groups of isolated clusters showing local stress field heterogeneities.

Fracture Pattern Analysis of the Upper Cretaceous-Eocene Carbonates along with the Ghumawan Dome, Hazara Basin

Deformational history of the Hazara basin indicates a primitive collision of the two landmasses that undergoes an episodic deformation with NE-SW structural trend. Panjal Thrust (PT) and Main Boundary Thrust (MBT) demarcate the northern and southern extremities of the basin, respectively. The area bounded between these two thrusts is the core consideration of the present research. Different stratigraphic units juxtapose along the Hazara Kashmir Syntaxes (HKS), while the strike-slip component is indicated by imbrication due to thrusts. The study is amied to analyze the paleo-stresses along with developed fracture patterns. Field data were collected via Circle Inventory Method from various localities of the Ghumawan dome, Hazara basin. The zones of upper Cretaceous to Eocene carbonates were mainly targeted during the data collection. Win-Tensor was the key software that helps to analyze the paleo-stresses and fracture pattern of the study area. NW-trending fracture pattern was observed with a highly non-symmetric to dense fracture pattern. The local thrust system lead to severely de-shape the study area. N-S oriented σ1 indicated the compressional tectonic conditionthat prevailed during deformation of this area. Some segments also show extensional features i.e. normal faulting.

Paleoseismic history of the Manisa fault zone, Western Anatolia

The 45-km-long, E-W to NW-SE-striking Manisa Fault Zone, which constitutes the western section of the Gediz Graben, is characterized by a pure normal sense of motion with a minor strike-slip component. Even though there are numerous historical earthquakes have been listed in Western Anatolia Graben System, a few studies have been addressed on seismic sources of the earthquakes so far. According to existing literature, the western segment of the Manisa Fault Zone is well known characterized; however, the eastern segment of it has rarely been addressed. To decipher the Holocene seismotectonic behavior of the eastern segment of Manisa Fault Zone, trench-based paleoseismological analyses for the first time were performed along with it. To constrain the timing and frequency of past earthquakes, and elapse time from the last activation using the optically stimulated luminescence (OSL) and radiocarbon (14C) dating, we collected twenty-six colluvial and paleosol samples from the trenches. The obtained paleoseismic data show that (i) Manisa Fault Zone is responsible for six surface rupturing earthquakes since late Pleistocene-Holocene, which is occurred at 30.6 ± 8.8 ka (E1), 15.0 ± 5.0 ka (E2), 6.6 ± 1.3 ka (E3), 2.9 ± 1.3 ka (E4), 0.8 ± 0.4 ka (E5), and 0.1 ± 0.1 ka (E6), (ii) the recurrences interval of destructive earthquakes on Manisa Fault Zone was found between 0.95 kyr to 3.8 kyr for Holocene, and (iii) the elapsed time since the most recent surface ruptured earthquake on the Manisa Fault Zone is 159 year.

Comment on ‘Evidence for a large strike-slip component during the 1960 Chilean earthquake’ by H. Kanamori, L. Rivera and S. Lambotte

SUMMARY Based on numerous studies of the relevant geodetic data, a low-angle thrusting mechanism has been assigned to the 1960 Chile earthquake. Kanamori, Rivera and Lambotte recently suggested that a component of dextral slip comparable to the thrusting be included in the mechanism to satisfy long-period, teleseismic observations. The absence of geodetic evidence for that huge strike-slip component is the subject of this comment. The geodetic data are largely measurements of coseismic uplift associated with the earthquake but include eight measurements of the coseismic change in shear strain. Because strike-slip produces relatively little uplift except near the end points of the rupture, identification of that strike-slip component in the geodetic data depends upon the measured, shear-strain change. I consider elastic, half-space models of oblique slip on the plate interface possibly supplemented by simultaneous dextral slip on the nearby, intra-arc Liquiñe-Ofqui Fault Zone. Slip is assumed to be uniform along strike. The best fits to the geodetic data for these models furnish little evidence for strike-slip on those structures. To satisfy the long-period, teleseismic data, Kanamori et al. proposed six examples, each of which requires a large amount of dextral slip. Because the long-period, teleseismic data do not define the slip distributions, I have used the best fits of those examples to the geodetic data to define those distributions. The large thrusting near the deformation front required by those slip distributions implies large uplift there, contrary to the uplift inferred from the inversion of tsunami data. However, an acceptable fit to the geodetic data and the tsunami data for the six examples suggested by Kanamori et al. can be obtained if the seismic moments specified by them are reduced by a factor ∼1.8, a factor within the uncertainties in estimating seismic moments of the 1960 Chile earthquake. The presence of strike-slip in those reduced-moment examples despite the lack of geodetic evidence for strike-slip is due to a remarkable coincidence that requires careful balancing of contributions from the shallower (depths < 70 km) coseismic sources against those from the deeper coseismic sources to nullify the geodetic evidence for strike-slip. Such balancing is possible, but it is remarkable that the balancing is so nearly perfect that it nullifies the geodetic evidence for strike-slip and thereby confounds the interpretation of the geodetic data.

Classification of Zagros earthquakes based on focal mechanism

The Zagros suture zone is seismically active region in Iranian plateau. This region is of high importance in terms of seismicity, since it is a vast and populated region and in recent years the earthquakes with high intensities have frequently occurred and have caused extensive destruction and heavy human loss. The study of the focal mechanism is very important in understanding the seismotectonic characteristics. Focal mechanisms of Zagros were collected over a period of 20 years and they were classified by FMC software. Seven groups were considered for the type of faulting and Zagros was divided into three zones. For each zone, the frequency percentage of each group of faults was determined. The most of faulting are of the reverse and compression type with the strike-slip component. Finally, the role of nodal plane selection in determining the type of faulting was discussed and it was found that the selection of each nodal plane in determining the type of faulting has the same result.

Reply to ‘Comment on “Evidence for a large strike-slip component during the 1960 Chilean earthquake” by H. Kanamori, L. Rivera and S. Lambotte’ by J. C. Savage

Reply to ‘Comment on “Evidence for a large strike-slip component during the 1960 Chilean earthquake” by H. Kanamori, L. Rivera and S. Lambotte’ by J. C. Savage

Ombilin Basin as Inverted Oblique Rift in Barisan Mountains, Sumatra: Considerations on Subsidence Mechanisms and Fault Development

Ombilin Basin is a NW-SE inverted oblique rift which is currently being part of Barisan Mountains in western Central Sumatra. Regarding its current position, Ombilin Basin can be one of the windows to see the evolution of Barisan Mountains since Paleogene. Two schools of thought, namely rift basin and pull-apart basin, have been established to explain the evolution of Ombilin Basin. This paper aims to present another perspective on the evolution of Ombilin Basin based on subsidence mechanisms and fault development. This study integrated remote sensing and subsurface interpretations. Remote sensing interpretation took the role to delineate surface fault lineaments using digital elevation model, while subsurface interpretation dealt with log and seismic interpretations, subsidence analysis, and palinspatic reconstruction. Fault lineaments derived both from remote sensing and seismic interpretations were combined to construct structural framework of the basin. Subsidence analysis generated geohistory and backstripped tectonic subsidence charts. Palinspatic reconstruction illustrated structural configurations through time. This study figured out that Ombilin Basin went through fault-controlled subsidence in Middle Eocene – Late Oligocene and thermal subsidence in Early Miocene – Late Pliocene. Each subsidence mechanism was terminated by an uplift. Subsidence mechanisms in Ombilin Basin represented the criteria of rift basin in terms of amount and rate of tectonic subsidence, duration of subsidence, and contribution of thermal subsidence. On the other hand, fault development captures extensional and strike-slip components during rifting and development of flower structures during inversion of the basin. Oblique rifting operates when dominant extensional component works together with strike-slip component. Therefore, subsidence mechanisms and fault development are in agreement to regard Ombilin Basin as inverted oblique rift.

Lineament mapping for a part of the Central Sulaiman Fold\u2013Thrust Belt (SFTB), Pakistan

This study aims to analyze the lineaments using the field data and a remote sensing approach, to describe their relationship with the folds, faults, and regional tectonic stress of the central Sulaiman Fold-Thrust Belt. Joint data, from nine anticlines, has been collected using the scanline method and classified into three sets (JS1, JS2, and JS3) based on their geometrical relationships. Lineaments extracted from the 30 m digital elevation model have been classified subsequently into three lineament sets (LS1, LS2, and LS3) based on the azimuths from the corresponding joint sets. A very high correlation coefficient (rs = 0.97), between the azimuths of the field joints and the remotely sensed lineaments, has been observed which validates that the lineaments are the regional representation of the local field joints. The geometrical relationship of the lineament sets with the fold hinges indicates that the older LS1 and LS2 are strongly related to the regional folding episode, while the younger LS3 is a result of local shears. The chronological interpretation of the deformational events responsible for the lineament sets is constrained by the presence of the Kingri Fault, which induces a strike-slip component within the study area. Furthermore, the controls on the joint and lineament sets, established using multivariate statistics to decipher the effects of lithological and structural contrasts on the lineament density, reveal that an increase in the lineament density can be attributed to the competence and thickness of the rock units as well as the variable local stresses across the different folds. Based on the orientations of these lineament sets, the cumulative direction of the compressive event in the NW-SE direction (310–320) coincides with the regional stress direction of the SFTB.

From neogene thin-skin to recent thick-skin deformation in Haiti fold-and-thrust belt (Western Hispaniola)

Haiti lies along the North Caribbean plate boundary, marked by two major left-lateral strike-slip faults. A southwest-verging Trans-Haitian fold-and-thrust belt (FTB), mostly affecting tertiary carbonate platform, developed in central Haiti between the two faults, south of the Massif du Nord and the San Juan–Los Pozos Fault Zone, as a result of north-south shortening. The large NW-SE antiformal Gonâve Island and Ridge mark the southwest termination of the FTB as it encountered the Southern Peninsula and Enriquillo-Plantain Garden Fault Zone. We show that this collision resulted in a tightening of the deformed wedge and thrust faults rooted deeper into the crust, leading to a transition from thin-skinned to thick-skinned tectonics in the Pliocene. These short- (3–6 km) and long- (15–30 km) wavelength folding deformations are temporally distinct in the north and more concomitant in the south, where compressional thin-skinned deformation is still active. The thick-skinned shortening was mostly expressed during the Quaternary, as shown by the formation of folds and uplift of terraces between 1.7 Ma and today. This thick-skinned detachment is characterised by a stronger imprint of strike-slip component, dragging the FTB to a more E-W direction in its northwest and southeast extremities. Understanding of the fault zone geometry of the recent 2010 Haiti earthquake (M = 7.2) could be reconsidered in the light of this tectonic transition.

Kinematic partitioning in the Southern Andes (39\xb0 S\u201346\xb0 S) inferred from lineament analysis and reassessment of exhumation rates

The Southern Andes are often viewed as a classic example for kinematic partitioning of oblique plate convergence into components of continental margin-parallel strike-slip and transverse shortening. In this regard, the Liquiñe-Ofqui Fault Zone, one of Earth’s most prominent intra-arc deformation zones, is believed to be the most important crustal discontinuity in the Southern Andes taking up margin-parallel dextral strike-slip. Recent structural studies, however, are at odds with this simple concept of kinematic partitioning, due to the presence of margin-oblique and a number of other margin-parallel intra-arc deformation zones. However, knowledge on the extent of such zones in the Southern Andes is still limited. Here, we document traces of prominent structural discontinuities (lineaments) from the Southern Andes between 39° S and 46° S. In combination with compiled low-temperature thermochronology data and interpolation of respective exhumation rates, we revisit the issue of kinematic partitioning in the Southern Andes. Exhumation rates are maximal in the central parts of the orogen and discontinuity traces, trending predominantly N–S, WNW–ESE and NE–SW, are distributed across the entire width of the orogen. Notably, discontinuities coincide spatially with large gradients in Neogene exhumation rates and separate crustal domains characterized by uniform exhumation. Collectively, these relationships point to significant components of vertical displacement on these discontinuities, in addition to horizontal displacements known from published structural studies. Our results agree with previously documented Neogene shortening in the Southern Andes and indicate orogen-scale transpression with maximal vertical extrusion of rocks in the center of the transpression zone. The lineament and thermochronology data call into question the traditional view of kinematic partitioning in the Southern Andes, in which deformation is focused on the Liquiñe-Ofqui Fault Zone.

Cenozoic evolution of the Qaidam basin and implications for the growth of the northern Tibetan plateau: A review

As the largest depression in the Tibetan plateau, the Qaidam basin preserves over 10 km thick of Cenozoic sediments. Those sediments represent a unique archive of the evolution of the northern part of the Tibetan plateau and of intracontinental deformation in general. Yet, several critical issues associated with the evolution of the Qaidam basin remain controversial, preventing a full understanding of the evolution of the northern margin of Tibet during the Cenozoic. Here we comprehensively review these issues, synthesize records from structural geology, sedimentology, geochronology, and geophysics, and finally propose a holistic view of the Cenozoic evolution of the Qaidam basin. We infer that: 1) the traditional age model that assigned a Paleocene to Eocene basal age to the Cenozoic strata in the basin seems to fit reasonably well the deformation history obtained from combining growth-strata observations and thermochronology data across the basin; 2) basement-involved faults, with both dip-slip and strike-slip components, developed within the Qaidam basin. In particular, the Neogene initiation of strike-slip faulting along the south-dipping faults that roots into the Eastern Kunlun Shan played an important role in the evolution of the southern Qaidam basin; 3) The Eastern Kunlun Shan was already exhumed during the deposition of the Lulehe Formation, serving as a significant source of clastic material deposited in the Qaidam basin and separating the latter from the Hoh Xil basin to the south. We conclude that the Qaidam basin is a superimposed sedimentary basin that successively experienced flexural subsidence, outwards expansion, isolation and partitioning during the Cenozoic. Finally, the Cenozoic evolution of the basin results from a tight interplay between tectonics and climate.

Source characteristics and seismotectonic implications of the 26 September 2019Mw 5.7 Silivri High-Kumburgaz Basin earthquake and evaluation of its aftershocks at the North Anatolian Fault Zone (Central Marmara Sea, NW Turkey)

SUMMARYThe Central Marmara Sea region hosts the northwestern branch of the North Anatolian Fault Zone (NAFZ) with its known seismic gap between the 1912 Ganos (Mw 7.2) and 1999 Izmit (Mw 7.4) major devastating earthquakes and thus poses a significant seismic hazard potential for the megacity Istanbul. The 26 September 2019 Mw 5.7 Silivri High-Kumburgaz Basin (central Marmara Sea) earthquake ruptured a thrust fault with a minor strike-slip component at the north of the eastern end of this gap relatively in the shallow depth (h= 8 km) range. Thus, in this study, we examine source properties of the main shock activity and coseismic behaviour of the failure, and the pattern of post-seismic deformation based on the aftershock distribution to have an insight into the role of the subsidiary and main fault structures on the crustal kinematics along this complicated branch of the NAFZ. The relocated epicentres are aligned in the E–W direction and tend to propagate towards the segments to the east of the main shock. The detected aftershock activity appears to focus on the east side of the main shock and almost no seismic activity was observed to the west of the epicentre. Independent investigations from coda-wave fitting, point-source, and finite-fault slip modelling agree on the moment magnitude of Mw5.7 for the 26 September 2019 main shock. The kinematic rupture model of this event implied that the main rupture nucleated around the hypocentre, and then propagated bilaterally along the E–W direction but with significant progress towards the east. The distribution of the slip vectors indicates that the rupture evolved on a dextral thrust fault plane. The spatio-temporal behaviour of the overall aftershocks sequence, their focal mechanism solutions and our kinematic slip model clearly shows that the existing secondary structures developed in simple shear dextral deformation are likely responsible for the main shock activity. We conclude that such type of deformation model results in a motion in response to the thrust faulting with strike-slip component with an N89°W (271°) orientation and 33°NE dipping at left stepover transpressional region on the NAFZ.

Geometric and Dynamic Patterns of the Golmud Segment in the Southern Marginal Fault of the Qaidam Basin

The southern marginal fault of the Qaidam Basin (SMQBF) is a block-bounding border fault that has played a key role in the structural evolution of the Kunlun Fault. However, its geometric and dynamic deformation patterns since the Late Pleistocene have not been clearly observed. Field investigations, combined with high-resolution imagery and shallow seismic profiles, show that the SMQBF is a thrust fault with a sinistral strike-slip component composed of several secondary faults. Its Late Quaternary deformation pattern is characterized by piggyback thrust propagation, and the frontal fault may not be exposed to the surface. Due to the flexural slip of the hanging strata of the secondary fault, sub-parallel faults with widths of thousands of meters have formed on high terraces; these are important when assessing the seismic hazard of this area. Based on high-resolution topographic data obtained using an unmanned erial vehicle and optically stimulated luminescence chronology, the slip rates of several secondary faults were obtained. The vertical and strike-slip rates of the SMQBF were determined to be 0.96 ± 0.33 mm/a and 2.66 ± 0.50 mm/a, respectively, which may be the minimum rates for the fault. Considering that the SMQBF is composed of several secondary faults, these rates possibly correspond to minimum deformation only. The evident sinistral strike-slip of the SMQBF indicates that although the sinistral slip of the Kunlun Fault system is concentrated in main fault of this system, the branch faults have a significant influence on the lateral extrusion of the Qinghai-Tibet Plateau.

Structural framework of the transpressive Mahanadi Shear Zone and reevaluation of the regional geology in the northern sector of the Eastern Ghats Mobile Belt, Eastern India

Mahanadi Shear Zone (MSZ) is located in the northern segment of the Eastern Ghats Mobile Belt (EGMB), represented by extensive mylonitization. WNW-trending mylonitic foliation has a moderate-to-steep dip towards NNE. Kinematic indicators such as asymmetric folds, shear bands, quarter structures, suggest dextral ductile shear sense of the dominant strike–slip component and reverse sense of the less prominent dip slip component. Predominance of foliation over lineation indicates flattening strain. Reverse movement and flattening strain confirm shortening. The MSZ is a transpressional zone with simple shear along and pure shear (shortening) perpendicular to the mylonitic fabric. Wide variation in plunge of stretching lineation, presence of back-rotated clasts and 0 < Wk < 1 on two sections oblique to the mylonitic foliation, suggest triclinic symmetry of deformation. This study demonstrates that the width of the MSZ—located in the northern part of the Phulbani Domain—extends beyond 38 km. It covers the entire ‘Tikarpara Domain’ in the north of Phulbani Domain and put the very existence of Tikarpara Domain in question. Lithounits of Phulbani Domain continue to the north across the southern boundary of the MSZ, through Tikarpara Domain and Angul Domain up to the Northern Boundary Shear Zone (NBSZ). It is proposed that the entire area comprising the Phulbani Domain, the MSZ, and the region further north up to the NBSZ, form part of a single domain—Phulbani Domain. MSZ is an outcome of intra-terrane transpression. Thus, this study also questions the credibility of the domainal classification of the EGMB.