Penetration of mid-crustal low velocity zone across the Kunlun Fault in the NE Tibetan Plateau revealed by ambient noise tomography

Abstract

The NE Tibetan Plateau, composed of the Mesozoic accretions of Lhasa, Qiangtang and Songpan-Ganze Terranes, are bounded by the east Kunlun–Qaidam Block in the north with the boundary delineated by the Kunlun Fault. The NE Tibetan Plateau is at a nascent stage of plateau growth resulting from the collision between the Indian and Eurasian plate starting ∼50 million years ago, and is one of the best areas to study the growth mechanism of the Tibetan Plateau. In this study, we process continuous ambient noise data collected from ∼280 stations during 2007 and 2010 and generate Rayleigh wave phase velocity maps at 10–60 s periods with a lateral resolution of ∼30–50 km for most of the study region. By adopting a Bayesian Monte Carlo method, we then construct a 3-D Vsv model of the crust using the Rayleigh wave dispersion maps. Our 3-D model reveals that strong LVZs exist in the middle crust across the NE Tibetan Plateau; and the lateral distribution of LVZs exhibit significant west–east variations along the Kunlun Fault. In the west of 98°E, LVZs are confined to regions of the Kunlun Fault and the eastern Kunlun Ranges, but absent beneath the Qaidam Basin; while in the east of 98°E, LVZs extend and penetrate northward into the east Kunlun and Qinling Orogens over ∼100 km across the Kunlun Fault. The strong contrast of the LVZs distribution along the Kunlun Fault may be related to the distinct neighboring tectonic units in the north: a strong crust of the Qaidam Basin in the west blocking the penetration of LVZs, but a weak crust in the Qinling Orogens facilitating the extrusion of LVZs. The distribution of LVZs in the NE Tibetan Plateau is consistent with the crustal channel flow model, which predicts a branch of north-eastward mid-crustal channel flow. Our 3D model clearly delineates the north extent of the mid-crustal LVZs, probably reflecting the status of channel flow in the NE Tibetan Plateau.