High‐frequency seismic wave propagation beneath the Indian Shield, Himalayan Arc, Tibetan Plateau and surrounding regions: high uppermost mantle velocities and efficient Sn propagation beneath Tibet

Abstract

This paper reports the regional propagation characteristics of high‐frequency Pn, Pg, Sn and Lg waves (0.5 to 2 Hz) and Pn, Pg and Sn velocities for several regions in Central Asia, Tibet and India. An important result is that Sn waves propagate efficiently in the uppermost mantle beneath most of the Tibetan Plateau. Also, Sn transmission in Tibet is not affected by crossing many physiographic and tectonic boundaries, such as the Indus‐Tsangpo suture. These observations are similar to observations of Sn propagation in stable continental and shield regions. Efficient Sn propagation is also observed across the stable Tarim Basin, along the Tien Shan and Himalayan Mountains and across the Indian Shield. Sn is strongly attenuated across the northern Tibetan Plateau (Chang Thang terrane). Such strong attenuation suggests the existence of a low‐Q zone in the uppermost mantle beneath that region. This interpretation is supported by recent geological observations of widespread Quaternary to Recent basaltic volcanism in the Chang Thang terrane. Lg propagates efficiently across the Tarim Basin, the Indian Shield and along the strike of the Himalayan Mountains. Lg is also observed, although less pronounced, for paths crossing large portions of the complicated structures of Tien Shan, Karakoram and Pamir. Lg is not observed for paths crossing the Tibetan Plateau. The southern boundary of the region of inefficient Lg propagation in Tibet lies in the vicinity of the Indus‐Tsangpo suture zone. The lack of Lg can be attributed to attenuation within the Tibetan crust; however, other possibilities including scattering by structural discontinuities at the margins of Tibet are equally plausible. Pg and Sg are the most prominent features on most short‐period WWSSN seismograms at epicentral distances less than about 4|Mo. Apparent Pg velocity determined for the area of the Himalayan Mountains is 6.0 |Mp 0.2km s‐1. Apparent velocities of Pn and Sn waves that have at least 50 per cent of their paths in the uppermost mantle beneath the Tibetan Plateau are 8.42 |Mp 0.10 and 4.73 |Mp 0.06 km s‐1, respectively. These velocities are quite similar to those determined for the Himalayan Mountains (Pn= 8.45 |Mp 0.08 km s‐1, Sn= 4.74 |Mp 0.05 km s‐1) and the Indian Shield (Pn= 8.40 |Mp 0.08 km s‐1, Sn=4.69 |Mp 0.04 km s‐1). Pn and Sn velocities for the Tien Shan region are 8.23 |Mp 0.07 and 4.64 |Mp 0.06 km s‐1, respectively. Efficient Sn propagation and high uppermost mantle P and S velocities beneath the Tibetan Plateau are consistent with a model in which the Indian continental lithosphere is being underthrust beneath Tibet. Although few other tectonic models cannot be ruled out solely on the basis of our new seismic observations, our data, combined with available geological and geophysical data, are most easily explained by the underthrusting model. Copyright © 1983, Wiley Blackwell. All rights reserved