Sharp Lateral Moho Variations Across the SE Tibetan Margin and Their Implications for Plateau Growth

Abstract

The tectonic uplift of the Tibetan Plateau is a focus in the geosciences. Middle‐lower crustal flow is a popular model to interpret the geodynamic mechanism on the margin of the Tibetan Plateau. The model predicts different surface and Moho topographies across the plateau boundary due to the different strengths of the surrounding blocks, that is, sharp boundaries on the eastern plateau boundary and gentle variations in the southeastern plateau boundary. Here, we employ receiver function and common conversion point stacking analysis with the seismic waveforms recorded by the dense ChinArray and other local seismic stations to accurately define the Moho topography in southeastern (SE) Tibet. We find that the Moho under the Tibetan Plateau is much deeper than that under the surrounding Yangtze Craton and Indochina block; abrupt Moho changes are found across the southeastern plateau margin, similar to that under the eastern plateau margin. We interpret these sharp Moho variations across the plateau margin to have developed when the Tibetan Plateau was extruded southeastward in the late Miocene. Subsequent gravity collapse resulted in crustal extension and gentle topographic variation, while the sharp Moho slope was preserved.

Mantle dynamics in the SE Tibetan Plateau revealed by teleseismic shear-wave splitting analysis

“…In contrast, recent high-resolution P wave anisotropic tomography found that the fast orientation is parallel to the plateau margin as well as the contour lines of the relief and Moho depths [Huang et al, 2018;Han et al, 2020;. These results would indicate that crustal flow has been blocked by the strong Yangtze and Indochina crustal blocks [Huang et al, 2018;Xu et al, 2020].…”

“…Many seismological studies have been conducted to reveal the deep architecture of the SE Tibetan Plateau. The crustal thickness decreases from more than 60 km in eastern Tibet to 30 km in the surrounding Yangtze and Indochina blocks to the southeast [Wang et al, 2017;Xu et al, 2020]. Seismic velocities in the mid-lower crust beneath the plateau are generally lower than those in the surrounding regions [Yao et al, 2010;Liu et al, 2014;Bao et al, 2015b].…”

“…However, focal mechanisms indicate that the stress field in the upper crust is controlled by the gravity potential [Xu et al, 2016;Xu et al, 2020].…”

Moho Complexity in Southern California Revealed by Local PmP and Teleseismic Ps Waves

“…However, when seismic stations situate above a relatively complex crust, the Moho‐related multiples could be mingled with other intracrust‐generated signals, resulting in ambiguity in the estimation of Moho depth and bulk velocity ratio (Figure S8 in Supporting Information S1). In this study, to account for crustal velocity heterogeneities, we first construct the Moho geometry beneath southern California in the 3D community velocity model CVM‐S4.26 (Lee et al., 2014; hereafter referred to as the CVM‐S model) by applying the common conversion point (CCP) stacking technique (Zhu, 2000; Xu et al., 2020) to P‐wave RFs. We call the Moho geometry constrained by the Ps data as the Ps Moho.…”

“…The locations of those discontinuities can be determined by migration techniques. In this study, we use the common conversion point (CCP) stacking method to image the Moho discontinuity (Zhu, 2000; Xu et al., 2020). The success of CCP stacking relies on the accuracy of the velocity model in which the time‐to‐depth mapping is performed.…”