The new shear wave velocity and radial anisotropic models from surface wave tomography reveal the variations of lithospheric strengths along the strike of the Tien Shan orogenic belt

Abstract:

The Tien Shan orogenic belt, one of the greatest intracontinental orogenic belts on Earth, is surrounded by the Tarim Basin to the south and the Kazakh shield to the north. The Tien Shan mountain rejuvenated and experienced intense tectonic deformation due to the further field effect of the Indian/Asian plate collision in the Cenozoic and it is regarded as a natural laboratory to explore the formation and consequent lithospheric deformation of the orogens within plates. In this study, we constructed a new lithospheric three-dimensional (3-D) shear wave velocity and radial anisotropy models from previously obtained Rayleigh wave and Love wave phase velocity maps at 16 periods of 20-167 s (Liang et al., 2020) in order to provide critical constraints on the crust and upper mantle lithospheric structures of the central and western Tien Shan. A prominent low velocity anomaly accompanied with negative radial anisotropy (i.e. Vsv > Vsh) is imaged in the middle crust of central and western Tien Shan, probably reflecting the middle-crust in this region is weak and warm. Starting from the depth of 40 km, the low velocity anomaly can be traced downwards to 145 km in the central Tien Shan, whereas in the western Tien Shan this anomaly confines in its eastern part nearby the central Tien Shan down to the depth of 145 km. Noticing that relative high velocity anomalies are observed beneath the south and north of the central Tien Shan, we interpreted that this upper mantle low velocity probably result from hot mantle material induced by the subducted the Tarim basin and Kazakh shield. We cannot rule out the possibility of asthenospheric upwelling resulting from lithospheric delamination. Nevertheless, our shear wave velocity and radial anisotropy models suggest the variation of lithospheric strength along the strike of the Tien Shan orogenic belt. Combined with surface geologic and geochemical observations, this study will shed lights on the underlying dynamics of mountain building and its relationship with lithospheric strength.