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Abstract The Wuhai Basin is located in the northwestern corner of the Ordos Block. Analyzing the geometry, along with the kinematic and dynamic characteristics of the boundary fault, the Zhuozishan West Piedmont Fault (ZWPF), will elucidate the regional tectonic environment and guide earthquake prevention and disaster reduction projects. Six representative sites were selected for topographic measurements, from the northern, middle and southern parts. Displacements of the ZWPF were calculated by measuring the top surface elevation of a widely distributed lacustrine layer in the footwall from outcrops at each site, and from boreholes in the hanging wall. The vertical slip rate of the ZWPF was then calculated based on the displacement and age of the lacustrine layer. Three to four normal fault-controlled terraces have developed on the footwall of the ZWPF, and the top surface of the lacustrine layer is at 1092–1132 m elevation. Data from boreholes showed that the top surface of the lacustrine layer is at an elevation of 1042–1063 m in the hanging wall. Vertical slip rates since 70 ka were estimated as 0.5 ± 0.2 to 1.0 ± 0.2 mm/a. The highest rate of vertical slip was observed at Fenghuang Ridge, in the central part of the fault system, and decreased to the south. In the northern Wuhai Basin, normal faulting still controls the piedmont landscape. However, NW-SE trending reverse faults and secondary folding have resulted from dextral strike-slip movement of the fault. The Wuhai Basin developed as a dextral-tensional negative flower structure. This study indicates that stress conditions of the northwestern margin of the Ordos Block include NE–SW compression and NW–SE extension, and an S-shaped rift zone has dominated the scale, structure, and evolution of the Yinchuan, Wuhai, and Hetao Basins, along with the active mode of faulting in these basins. Highlights • Vertical slip rates between 0.5 ± 0.2 and 1.0 ± 0.2 mm/a were calculated for the ZWPF. • The ZWPF has a larger strike-slip rate, resulting in development of reverse faults. • The Wuhai Basin has developed a dextral- tensional negative flower structure. • Stress conditions and S-shaped distribution dominate the development of basins. [ABSTRACT FROM AUTHOR]

On January 21st, 2016, a Ms 6.4 earthquake hit Menyuan County, Qinghai province, China. The nearest known fault is the Leng Long Ling (LLL) fault which is located approximately 7 km north of the epicenter. This fault has mainly shown sinistral strike-slip movement since the late Quaternary Period. However, the focal mechanism indicates that it is a thrust earthquake, which is different from the well-known strike-slip feature of the LLL fault. In this study, we determined the focal mechanism and primary nodal plane through multi-step inversions in the frequency and time domain by using the broadband regional seismic waveforms recorded by the China Digital Seismic Network (CDSN). Our results show that the rupture duration was short, within 0–2 s after the earthquake, and the rupture expanded upwards along the fault plane. Based on these fault parameters, we then solve for variable slip distribution on the fault plane using the InSAR data. We applied a three-segment fault model to simulate the arc-shaped structure of the northern LLL fault, and obtained a detailed slip distribution on the fault plane. The inversion results show that the maximum slip is 0.43 m, and the average slip angle is 78.8°, with a magnitude of Mw 6.0 and a focal depth of 9.38 km. With the geological structure and the inversion results taken into consideration, it can be suggested that this earthquake was caused by the arc-shaped secondary fault located at the north side of the LLL fault. The secondary fault, together with the LLL fault, forms a normal flower structure. The main LLL fault extends almost vertically into the base rock and the rocks between the two faults form a bulging fault block. Therefore, we infer that this earthquake is the manifestation of a neotectonics movement, in which the bulging fault block is lifted further up under the compresso-shear action caused by the Tibetan Plateau pushing towards the northwest direction. [ABSTRACT FROM AUTHOR]

The North China Plate (NCP) underwent multi-phase intracontinental deformation during the Mesozoic, which is a much-debated research subject. Here we present a case study of this deformation inferred from regional-scale Helanshan superposed folds in the west of the NCP. New structural analyses and fault kinematic analyses in the Helanshan Structural Belt (HSB) indicate a two-stage syn-folding paleo-stress field, comprising early NNE–SSW compression, with mean orientations 190°/01° (plunge direction/plunge) for σ 1 , 283°/26° for σ 2 , and 100°/63° for σ 3 , and later WNW–ESE compression with mean orientations of σ 1 , σ 2 , and σ 3 at 124°/07°, 213°/04°, and 001°/75°, respectively. These results, combined with geochronological analysis, indicate two episodes of shortening in this belt during the Mesozoic. Late Triassic–Early Jurassic NNE–SSW shortening, which produced ∼8.1 km of horizontal shortening, led to the development of open WNW-trending fold structures. Subsequent Late Jurassic–Early Cretaceous WNW–ESE shortening, which produced ∼18.7 km of horizontal shortening, reshaped the early WNW-trending structures and generated close, overprinting NNE-trending folds that resulted in the development of the regional-scale superposed folds in this area. In terms of geodynamics, the early NNE–SSW shortening is likely to be related to Indosinian intercontinental convergence between the North China Plate and the Yangtze Plate, and the later WNW–ESE shortening is likely to have been related to subduction of the Paleo-Pacific Plate. The regional structural analysis indicates that the whole of the western North China Plate experienced two-phase contractional deformation during the Mesozoic, as manifest by the early NNE–SSW and the late WNW–ESE shortening that characterized Mesozoic intracontinental deformation in the western NCP. [ABSTRACT FROM AUTHOR]

The Shanxi Rift, an intracontinental belt along the eastern margin of the Ordos Block, North China Plate, has undergone multi-stage extensional deformation during the late Cenozoic. In this study, new fault kinematic analysis of boundary faults in the Shanxi Rift, constrained by geochronological data, defines a three-stage tectonic stress regime since the late Miocene, providing new structural evidence for deciphering the northeastward growth of the Tibetan Plateau. The earliest stage involved a transtensional stress regime of NW–SE extension and NE–SW compression in the late Miocene–early Pleistocene (ca. 10–1.8 Ma) and characterized by mean principal stress axes of σ 1 at 344.7°/87.2°, σ 2 at 217.6°/01.1°, and σ 3 at 128.1°/01.0°. The early transtensional stress regime was responsible for the opening of the Shanxi Rift and was very likely associated with intensive right-lateral pull-apart activity in the eastern Ordos Block due to the northeastward growth of the Tibetan Plateau. During the second stage, the stress regime changed to less intense NE–SW extension, with mean stress orientations of 323.3°/85.2° for σ 1 , 127.5°/02.9° for σ 2 , and 214.9°/02.6° for σ 3 in the early late Pleistocene (ca. 1.8–0.11 Ma). This stage is marked by a distinct shift from fluvio-lacustrine to fluvio-piedmont sedimentary facies at the end of the early Pleistocene, and the widespread eruption of basalt in the Datong Basin north of the Shanxi Rift. This short-lived extension appears to be the result of relaxation in the midst of crustal shortening during growth of the Tibetan Plateau. The third stage was transtensional, characterized by NNW–SSE extension and WNW–ESE compression ( σ 1 at 060°/88.9°, σ 2 at 074.9°/03.9°, and σ 3 at 164°/00°) since the late Pleistocene (ca. 0.11 Ma), causing dextral shear deformation in the Shanxi Rift and the disappearance of its paleo-lake. This stage resulted from regional ENE–WSW shortening due to northeastward growth of the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Convergent plate margins could have an immediate impact on intraplate deformation in the adjacent sedimentary basin. In the Tarim Basin, a strike-slip fault network in the Guchengxu High is adjacent to the Tethys (northern Tibetan Plateau). This paper uses high-quality 3-D seismic data to unravel the geometry and kinematics of the strike-slip fault network and the genetic relationship with the Tethys tectonics. Within the fault network, the left-lateral NNE- and ENE-oriented strike-slip faults are a result of sequential evolution rather than conjugate faults. The sub-vertical ENE-oriented strike-slip faults are widely spaced with stepping-segmented geometry in the west, whereas they are relatively clustered with larger vertical throws close to the Cherchen fault. They were interpreted as a series of the Cherchen fault-related structures due to oblique blocking of the Tazhong Uplift and block deflection. It responded to the Late Ordovician Altyn Tagh/Tarim collisional orogeny and the Cherchen fault. The NNE-oriented strike-slip faults are characterized by underlying sub-vertical faults and two layers of overlying normal faults. These NNE-oriented strike-slip faults have large transversal compressional components, commonly presenting reverse drags on both sides of the underlying sub-vertical faults. These NNE-oriented faults initiated on pre-existing discontinuities (extensional joints) and offset the ENE-oriented ones when NNW-oriented stress was transmitted to plate interiors during this Late Ordovician orogeny. Afterward, two layers of overlying normal faults formed due to dragging of the covers during the Middle Silurian–Early Carboniferous reactivation and the Late Carboniferous–Early Permian inversion, in response to subduction of the Kunlun Ocean and opening of the Paleo-Tethys Ocean, respectively. The strike-slip fault network in plate interiors evolved as immediate responses to the Tethys tectonics at plate boundaries. • The 3-D seismic data detailedly characterize the strike-slip fault network in the Guchengxu High. • The strike-slip fault network is a result of sequential evolution rather than a series of conjugate faults. • The strike-slip fault network in the Guchengxu High evolved in response to the Tethys tectonics at plate boundaries. [ABSTRACT FROM AUTHOR]

Abstract: The NE-trending Bayanwula Shan–Lang Shan is an important tectonic belt lying between the North China Plate (NCP) to the east and the Alxa block to the west. An understanding of its nature and the timing of deformation are essential to understand the relationship between the NCP and the Alxa block. Two phases of ductile deformation have been observed in this belt. Large-scale top-to-the-west ductile thrusting characterized the early deformation in the Bayanwula Shan–Lang Shan. Nearly east–west trending quartz stretching lineations and lineations formed by amphibole and biotite are well developed. Different types of sheath and oblique folds with east–west trending fold hinges are also developed in the region. The shear strain of this ductile thrust is up to 17. The ductile deformation may have resulted from the top-to-the-west thrusting of the northern part of the NCP over the Alxa block, and may have occurred ca. 351 Ma (biotite 40Ar/39Ar age). Later ductile deformation was expressed as NE-trending sinistral shear along the entire Bayanwula Shan–Lang Shan and likely occurred ca. 250 Ma (biotite and muscovite 40Ar/39Ar ages); this shear may have resulted from the collision between the Yangtze and North China plates to the south during the Triassic. Combined with recently obtained detrital zircon U–Pb ages for the area, the ductile deformation events in the eastern Alxa block indicate that the block may not have been part of the NCP, at least before the end of the Devonian. Both blocks were located in the Paleo-Asian Ocean during the Paleozoic and collided or amalgamated with each other at the end of the Devonian. [Copyright &y& Elsevier]

Abstract: The boundaries of a plate are usually correlated with the locations of major already-known faults, which are well defined by local geology, regional seismicity. It facilitates for the case of the Tibetan Plateau with boundaries along the large scale faults, such as the Altyn Tagh fault. Nevertheless, for smaller scale regions, it might become much more difficult to determine the block boundaries especially when lacking of neither active faults nor historical earthquake records. The geodetic including GPS observations may become useful tools to determine the potential faults. For the purpose of obtaining the kinematic pattern across the Longmenshan fault, we performed K-means clustering analysis for 145 GPS sites in the Longmenshan region (longitude 99°–110°E, and latitude 28°–35°N) and obtained a modified ten-block system almost best-fits the GPS velocity fields. Rigid rotations of these blocks are well determined, and the velocity filed can be explained well by rigid block motion and fault slip across block boundaries. The mean residual velocity is 1.26mm/year, less than the 2–3mm/year uncertainty of the GPS data with 90% confidence. All the block boundaries, except for these of AH/LMS and GZ/KD, are along major faults. The AH/LMS boundary is regarded to be along Longriba fault and translates ~5.5mm/year right-lateral rates and reduces the southeastward velocity of the plateau as shortening rate of <2.0mm/year and right-lateral slip rate of ~1.6mm/year across the Longmenshan fault. The shortening rate across Minjiang fault is of ~2.0mm/year. Even though there are neither already-known active faults nor active historical seismicity along GZ/KD boundary, the right-lateral slip rate is of ~5.6mm/year. We inferred it as an undiscovered active fault due to the geomorphic feature and the focal mechanism of earthquakes occurred at ~30°N. However, it needs to be validated in the future. The left-lateral slip rates along the Xianshuihe, Anninghe and Kunlun faults are ~9.0mm/year, ~6.9mm/year and ~2.8mm/year, respectively. The results indicate that the motion of the eastern Tibetan plateau has been accommodated significantly through the bounding faults, such as the Xianshuihe fault, Longriba fault, Kunlun fault and GZ/KD boundary. Further studies combining observations from surface surveys and depth detections are required and may provide insights into the crustal (i.e. deep into ~20km or deeper) deformation pattern of the Longmenshan region and the cause of the Wenchuan earthquake. [Copyright &y& Elsevier]

Abstract: The NE–SW-trending Hepu–Hetai shear zone extends for about 480km along the boundary between Guangdong and Guangxi provinces in the southern part of South China. Field mapping and kinematic analyses indicate that the zone is characterized by dextral ductile strike-slip deformation, with an estimated displacement along the fault zone in excess of 500km. Studies of synkinematic phengite and gas–liquid inclusions in quartz within mylonite suggest that the ductile shear deformation occurred under medium temperature/pressure conditions of the greenschist facies, and 40Ar/39Ar analyses of muscovite yield ages of 213–195Ma. Based on these data and Late Paleozoic–Mesozoic paleogeographic data for southern China, we propose that the Hepu–Hetai shear zone originated via the penetration of the Yunkai Promontory of South China into Indochina during the Triassic. [Copyright &y& Elsevier]

• Two stages of folds are identified on the northern margin of the North China Craton. • The D 1 folds were crossly superimposed by the D 2 folds and both formed in Paleozoic. • A dynamic transition occurred in middle Paleozoic in the northernNorth China Craton. Accretionary kinematic history of the Central Asia Orogenic Belt (CAOB) remains unclear in previous geological studies. Based on detailed structural analysis of newly identified folds on the northern margin of the North China Craton (NCC), we propose a tectonic model suggesting that Paleozoic accretion of the orogenic belt includes two phases of deformation. The first phase of deformation is oblique collision of the Bainaimiao Arc and the NCC, which was followed by orthogonal subduction of the Paleo-Asian Ocean in the second phase of deformation. The change of plate kinematics is supported by two generations of folding, namely D 1 (D B) and D 2 (D A). Both D 1 (D B) and D 2 (D A) folds developed in the Bayan Obo Group (BOG), a Meso- and Neoproterozoic rift sequence deposited on the passive continental margin of the NCC. D 1 (D B) folds were superimposed by the east-west trending D 2 (D A) folds. The D 1 (D B) folds developed as a result of oblique collision of the Bainaimiao arc and the NCC. The D 2 (D A) folds developed in response to the southward orthogonal subduction of the Paleo-Asian Ocean Plate. They formed between latest Silurian and Early Permian, coeval with the two accretion events at the northern edge of the NCC. Our study suggests that multiphase deformation of sedimentary layers on the subducting plates provide important constraints for kinematic evolution of plate tectonics. [ABSTRACT FROM AUTHOR]