Seismic constraints and geodynamic implications of differential lithosphere-asthenosphere flow revealed in East Asia.

During the last 50 Ma, the East Asian continent has been a zone of massive continental collision and lithospheric deformation. While the consequences of this for Asian surface and lithospheric deformation have been intensively studied over the past 4 decades, the relationships between lithospheric deformation and underlying asthenospheric flow have been more difficult to constrain. Here we present a high resolution 3-D azimuthal anisotropy model for the northeastern Tibetan Plateau and its eastward continuation based on surface-wave tomography and shear-wave splitting measurements. This model shows that eastward lateral flow of asthenosphere beneath the northeastern Tibetan Plateau is being blocked by thick Ordos and Sichuan cratonic keels. The damming effect of these keels induces flow to first rotate around the Ordos keel and then transition into strong east-west flow beneath the thinner lithosphere that forms the lithospheric suture between the two cratonic keels. We further find that asthenosphere flow directions can differ from those of overlying lithosphere, with the asthenosphere neither being passively dragged by overlying lithosphere, nor being able to drag the overlying plate to mimic its subsurface flow. Finally, the region of eastward-channeled asthenospheric flow from Tibet underlies a belt of stronger intracontinental deformation in eastern China.

Correlated crustal and mantle melting documents proto-Tibetan Plateau growth.

The mechanism that causes the rapid uplift and active magmatism of the Hoh-Xil Basin in the northern Tibetan Plateau and hence the outward growth of the proto-plateau is highly debated, more specifically, over the relationship between deep dynamics and surface uplift. Until recently the Hoh-Xil Basin remained uncovered by seismic networks due to inaccessibility. Here, based on linear seismic arrays across the Hoh-Xil Basin, we present a three-dimensional S-wave velocity (VS) model of the crust and uppermost mantle structure beneath the Tibetan Plateau from ambient noise tomography. This model exhibits a widespread partially molten crust in the northern Tibetan Plateau but only isolated pockets in the south manifested as low-VS anomalies in the middle crust. The spatial correlation of the widespread low-VS anomalies with strong uppermost mantle low-VS anomalies and young exposed magmatic rocks in the Hoh-Xil Basin suggests that the plateau grew through lithospheric mantle removal and its driven magmatism.

Structure-controlled asperities of the 1920 Haiyuan M8.5 and 1927 Gulang M8 earthquakes, NE Tibet, China, revealed by high-resolution seismic tomography.

Detailed crustal structure of large earthquake source regions is of great significance for understanding the earthquake generation mechanism. Numerous large earthquakes have occurred in the NE Tibetan Plateau, including the 1920 Haiyuan M8.5 and 1927 Gulang M8 earthquakes. In this paper, we obtained a high-resolution three-dimensional crustal velocity model around the source regions of these two large earthquakes using an improved double-difference seismic tomography method. High-velocity anomalies encompassing the seismogenic faults are observed to extend to depths of 15 km, suggesting the asperity (high-velocity area) plays an important role in the preparation process of large earthquakes. Asperities are strong in mechanical strength and could accumulate tectonic stress more easily in long frictional locking periods, large earthquakes are therefore prone to generate in these areas. If the close relationship between the aperity and high-velocity bodies is valid for most of the large earthquakes, it can be used to predict potential large earthquakes and estimate the seismogenic capability of faults in light of structure studies.