Contemporary crustal movement of southeastern Tibet: Constraints from dense GPS measurements

Abstract

The ongoing collision between the Indian plate and the Eurasian plate brings up N-S crustal shortening and thickening of the Tibet Plateau, but its dynamic mechanisms remain controversial yet. As one of the most tectonically active regions of the world, South-Eastern Tibet (SET) has been greatly paid attention to by many geoscientists. Here we present the latest three-dimensional GPS velocity field to constrain the present-day tectonic process of SET, which may highlight the complex vertical crustal deformation. Improved data processing strategies are adopted to enhance the strain patterns throughout SET. The crustal uplifting and subsidence are dominated by regional deep tectonic dynamic processes. Results show that the Gongga Shan is uplifting with 1–1.5 mm/yr. Nevertheless, an anomalous crustal uplifting of ~8.7 mm/yr and negative horizontal dilation rates of 40–50 nstrain/yr throughout the Longmenshan structure reveal that this structure is caused by the intracontinental subduction of the Yangtze Craton. The Xianshuihe-Xiaojiang fault is a major active sinistral strike-slip fault which strikes essentially and consistently with the maximum shear strain rates. These observations suggest that the upper crustal deformation is closely related with the regulation and coupling of deep material.

P Wave Anisotropic Tomography of the SE Tibetan Plateau: Evidence for the Crustal and Upper‐Mantle Deformations

“…More detailed information on the 3‐D crustal anisotropy in the Tibetan Plateau is necessary to clarify this issue. However, we prefer the existence of the deep‐crustal flow, because the plateau uplift in the northwestern part of the study region is notable (Figure ; Liang et al, ; Pan & Shen, ). The focal mechanism solutions indicate that the upper crust is undergoing horizontal extension (Figure ; Xu et al, ; Zhao et al, ); hence, the plateau uplift cannot be caused by upper‐crustal deformations.…”

“…We used a 3-D ray tracing technique (Zhao et al, 1992) that combines the pseudo-bending algorithm (Um & Thurber, 1987) and Snell's law to compute theoretical traveltimes and ray paths, which also incorporates station elevations and an undulating Moho discontinuity. We applied the LSQR algorithm (Paige & Saunders, 1982) to solve the observational equations (equation (4)) with proper smoothing and damping regularizations (Lees & Crosson, 1989) based on the trade-off curve between the traveltime residuals and the model norms.…”

Spatiotemporal Distribution of Microearthquakes and Implications Around the Seismic Gap Between the Wenchuan and Lushan Earthquakes

“…Both focal solutions of the two recent main earthquakes (2008 Wenchuan and 2013 Lushan) and substantial industry reflection profiles illustrate listric thrust sheets, steeply dipping to the west in the uppermost crust, and merges into a detachment at 15–17 km depth in the southwestern segment of the LMSF (Hubbard & Shaw, ; Li et al, ). Successively, the GPS observed a markedly low convergence rate (<3 mm/year; Gan et al, ; Shen et al, ) but an anomalously high uplifting rate (8 mm/year in the central LMS range) and strong compressional strain rates (40–50 nstrain/year; Pan & Shen, ) among the LMS range. The southwest portion appears to be pure‐thrust faults and the dextral strike‐slip component increases gradually from SW to NE along the trend of the LMS fault, revealed by focal mechanisms (Yang et al, ) and coseismic rupture investigation (Xu et al, , ).…”

“…The convergence velocity of the SGT and Yangze block across the LMS range is distinctly lower (<3 mm/year) than the extension velocity of the interior Tibetan Plateau (Gan et al, ; Xu et al, ; Zhang et al, ). A three‐component Global Positioning System (GPS) velocity field reveals that the LMSF zone is uplifted beneath anomalous vertical velocity and compaction strain rates (Pan & Shen, ). The peculiarity of the crustal deformation and the occurrence of two recent destructive earthquakes (Wenchuan M s = 8.0, 2008, and Lushan M s = 7.0, 2013) throughout the LMSF zone illustrate the capability of cumulating tectonic stress to a critical level.…”

“…The Longmen Shan Fault (LMSF) system is the bounding unit separating the stable Sichuan Basin to the east and eastern Tibetan Plateau to the west. The fault system evolves into rapid uplifting with the modest shortening under a compressional regime (Pan & Shen, ; Tapponnier et al, ; Wang et al, ). Two devastating earthquakes, the 2008 Wenchuan earthquake and the 2013 Lushan earthquake, left an approximately 60‐km nonrupture zone referred to as an aftershock gap (Chen et al, ; Pei et al, ).…”