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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]