NSFC-Active crustal deformation around the Tibetan Plateau: The kinematics and dynamics

As an important factor that controls the relative movement of two walls of a fault, the mechanical properties of rock are closely related to the fault slip, stress state and the regional crustal velocity field. Subjected to the northward motion of the India plate, and blocked by the Ordos and Alashan blocks, the northeastern margin of the Tibetan plateau exhibits a complex pattern of tectonic deformation. In this paper, a 3-D finite element geodynamic model is established based on the synthetic investigation of the geodynamics environment, spatial distribution of active faults and the crustal rheological structure. Constrained with GPS data, this model is employed to investigate the fault shear modulus and its effect on the regional crustal velocity field. Furthermore, the stress state of the major active faults is determined with the best fitted model. The simulation results indicate that the regional crustal motion is closely related to the mechanical properties of eastern Altyn fault (AEJ) and the Generalized Haiyuan fault (G_HY), and the shear properties of these two faults are opposite to each other. The shear modulus of AEJ is as strong as the surrounding crustal media while that of the G_HY could be as low as 1/10000 of the surrounding intact rocks. Moreover, the Liupanshan fault (LPS) and the north-edge fault of the West Qinling (XQL) are not sensitivity to the crustal velocity field, but both exhibit relative strong shear property. In addition, the stress state analysis points out that AEJ, three segments of G_HY (Muli-Jiangcang Fault (MJF), Jinqianghe-Maomaoshan-Laohushan fault (JMLD) and LPS that are located in the western, central and eastern of G_HY respectively), and the middle to western section of the XQL are all at high shear stress rates currently. The region of high stress rate is consistent with the high fault locking area in previous studies. That the high stress rate in dynamics agrees well with high fault locking in kinemics manifests that these regions are at high seismic risk that deserve more attention in the further analysis.

As an important factor that controls the relative movement of two walls of a fault, the mechanical properties of rock are closely related to the fault slip, stress state and the regional crustal velocity field. Subjected to the northward motion of the India plate, and blocked by the Ordos and Alashan blocks, the northeastern margin of the Tibetan plateau exhibits a complex pattern of tectonic deformation. In this paper, a 3-D finite element geodynamic model is established based on the synthetic investigation of the geodynamics environment, spatial distribution of active faults and the crustal rheological structure. Constrained with GPS data, this model is employed to investigate the fault shear modulus and its effect on the regional crustal velocity field. Furthermore, the stress state of the major active faults is determined with the best fitted model. The simulation results indicate that the regional crustal motion is closely related to the mechanical properties of eastern Altyn fault (AEJ) and the Generalized Haiyuan fault (G_HY), and the shear properties of these two faults are opposite to each other. The shear modulus of AEJ is as strong as the surrounding crustal media while that of the G_HY could be as low as 1/10000 of the surrounding intact rocks. Moreover, the Liupanshan fault (LPS) and the north-edge fault of the West Qinling (XQL) are not sensitivity to the crustal velocity field, but both exhibit relative strong shear property. In addition, the stress state analysis points out that AEJ, three segments of G_HY (Muli-Jiangcang Fault (MJF), Jinqianghe-Maomaoshan-Laohushan fault (JMLD) and LPS that are located in the western, central and eastern of G_HY respectively), and the middle to western section of the XQL are all at high shear stress rates currently. The region of high stress rate is consistent with the high fault locking area in previous studies. That the high stress rate in dynamics agrees well with high fault locking in kinemics manifests that these regions are at high seismic risk that deserve more attention in the further analysis.

Constraining the seismotectonic faulting that occurred as a result of the 2016 Mw 6.0 Hutubi earthquake provides valuable information about the deformation in the northern Tien Shan region. However, due to the lack of surface rupturing and high quality near-field teleseismic data, the exact nature of the faulting remains controversial. In our study, we analyze the coseismic strain time series of the Mw 6.0 Hutubi earthquake using strain data collected from nearby borehole stations. The tensile and compressive coseismic strain steps account for most of the recorded borehole data in this region. Employing a numerical model that is based on elastic dislocation theory, we reproduce the observed tensile and compressive coseismic strain steps using source parameters that were generated through seismic wave inversion, seismic reflection data, and aftershock relocation. By conducting a comparative analysis between the predicted and observed coseismic strain steps, we study the seismogenic faulting of the Mw 6.0 Hutubi earthquake. The results indicate that when the source parameters are 292°/62°/80°(strike/dip/rake), the predicted tensile and compressive characteristics for 13/16 channels are consistent with the observational data. Based on these results, we infer that the seismogenic faulting, which is located near the Horgos-Manas-Tugulu fault, can be characterized as a high-angle blind back-thrust fault with a north-dipping fault plane. Providing constraints on the seismogenic faulting associated with the 2016 Mw 6.0 Hutubi earthquake also yields to understand the mechanism of the overall deformation pattern in the northern Tien Shan region.

The Xunhua, Guide and Tongren Basins are linked with the Laji Mountain and the northern West Qinling thrust belts in the Xunhua-Guide district. Basin depositional stratigraphy consists of the Oligocene Xining Group, the uppermost Oligocene-Pliocene Guide Group and the Lower Pleistocene. They are divided into three basin phases by unconformities. Basin phase 1 is composed of the Xining Group, and Basin phase 2 of the Zharang, Xiadongshan, Herjia and Ganjia Conglomerate Formations in the Guide Group, and Basin phase 3 of the Gonghe Formation and the Lower Pleistocene. Three basin phases all develop lacustrine deposits at their lower parts, and alluvial-braided channel plain depositional systems at upper parts, which constitute a coarsening-upward and progradational sequence. Basin deposition, paleocurrent and provenance analyses represent that large lacustrine basin across the Laji Mountain was developed and sourced from the West Qinling thrust belt during the stage of the Xining Group (Basin phase 1), and point-dispersed alluvial fan-braided channel plain deposition systems were developed beside the thrust and uplifted Laji Mountain and sourced from it, as thrusting migrated northwards during the stage of the Guide Group (Basin phase 2). Evolution of basin-mountain system in the study area significantly indicates the growth process of the distal Tibetan Plateau. The result shows that the Tibetan Plateau expanded to the northern West-Qinling at Oligocene (29–21.4 Ma) by means of northward folded-and-thrust thickening and uplifting and frontal foreland basin filling, and across the study area to North Qilian and Liupan Mountain at the Miocene-Pliocene (20.8–2.6 Ma) by means of two-sided basement-involved-thrust thickening and uplifting and broken foreland basin filling, and the distant end of Tibetan Plateau behaved as regional erosion and intermontane basin aggradational filling during the Pliocene and early Pleistocene (2.6–1.7 Ma).

The channel flow model was gradually being accepted with the more important multidisciplinary evidences from geology and geophysics, but how the lower crustal flow influenced the surface deformation quantitatively was unknown. Here, we develop a three-dimensional viscoelastic model to explore the mechanical relations between the lower crustal flow and the surface deformation in western Sichuan. Based on numerous tests, our results show that the modeled results fit well with the observed GPS data when the lower crust flows faster than the upper crust about 11 mm/a in the rhombic block, which can be useful to understand the possible mechanism of the surface deformation in western Sichuan. Moreover, taking the Xianshuihe fault as an example, we preliminarily analyze the relation between the active fault and stress field, according to the boundary constraints that deduced from the best model. The results show that the maximum shear stress on the Xianshuihe fault zone is mainly located in the fault terminal, intersections and the bend of the fault geometry, the stress level on the northwestern segment that has the high slip rate is relatively high. Additionally, with the reduction of the Young’s modulus in the fault zone, it’s conducive to generate the greater strain distribution, hence forming the high stress level.

The cross-fault baselines at the Zhuwo and Xuxu stations show anomalous characteristics following the 2013 Lushan Ms 7. 0 earthquake, which are confirmed by the preliminary analysis. These provide a unique opportunity for us to explore the possible relation between the anomalous change and the Lushan earthquake. In this paper, we firstly analyze the characters of different baseline lengths along the northwestern Xianshuihe fault before and afterthe Lushan earthquake. Then, a three-dimensional nonlinear viscoelastic finite element model of the Xianshuihe fault and adjacent regions is constructed. Based on the defined multiple models with different lower crust rheology, we further explore the temporal characters of different cross fault baselines before and after the Lushan earthquake in terms of the present GPS data and coseismic static slip data, which are applid successively at the model boundary and relative inner nodes, respectively. The results show that the baseline length expresses the coordinated change, which decreases when the Lushan earthquake occurs, and shows the subsequent rapid reverse recovery process in a short time, also a continual increase tendency according to the original observation data at Zhuwo and Xuxu stations. The Xianshuihe fault model successfully reproduces the sinistral strike-slip feature with the block kinematics from GPS-derived velocity constrained. The modeling results suggest the rapid decrease tendency of the baseline length when earthquake occurs, indicating the dextral characteristic of the fault, which coincides with the result from observation data analysis. In a short time following the Lushan earthquake, the modeling result suggests that the baseline length exhibits an increasing tendency due to the viscoelastic relaxation when the lower crust viscosity is assumed to be 10(18)Pa . s and 10(19)Pa . s, respectively. Comparing the modeling results with the actual observation data, we can conclude that the stress adjustment deduced from. the viscoelastic relaxation and tectonic loading effects might be responsible for the observed baseline length changes.

The characteristics of crustal deformation and its dynamical mechanisms in the Sichuan-Yunnan region are of interest to many researchers because they can help explain the deformation pattern of the eastern Tibetan Plateau. In this paper, we employ a precise three-dimensional viscoelastic finite element model to simulate the crustal deformation in the Sichuan-Yunnan region, southeastern Tibetan Plateau. We investigate the influence of lower crustal flow and rheological variations by comparing the modeled results with GPS observations. The results demonstrate that lower crustal flow plays an important role in crustal deformation in the Sichuan-Yunnan region. The best fitting is achieved when the flow velocity of the lower crust is approximately 10–11 mm/a faster than that of the upper crust. Additionally, crustal rheological properties affect regional crustal deformation. When the viscosity of the middle and lower crust in the South China block reaches 10 22 and 10 23 Pa·s, respectively, the modeled results match observations well, especially for the magnitude of crustal motion within the South China block. Finally, our dynamic model shows that the maximum principal stress field of the Sichuan-Yunnan region exhibits clear zoning, gradually shifting from an approximately east-west orientation in the northern Bayan Har block to southeast in the South China block, southwest in the western Yunnan block, and a radially divergent distribution in the Middle Yunnan and Southern Yunnan blocks.