Pn velocity and anisotropy at the lithospheric mantle wedge beneath the middle-eastern Gangdese Metallogenic Belt, southern Tibet, China

Abstract

The lithospheric mantle wedge beneath the middle-eastern Gangdese Metallogenic Belt (GMB), located at the forefront of the India-Eurasia collision-subduction zone retains many traces of structure evolution, which can be revealed by the velocity and anisotropy structure of the uppermost mantle of this region. In this study, high-resolution structures of the Pn velocity and anisotropy beneath the middle-eastern GMB are presented using interstation Pn differential travel-time data recorded by a dense broadband seismic array. The average Pn-wave velocity of 8.13 km/s beneath the GMB is close to that of the peridotite. The Pn velocity and anisotropy show prominent differences across the Jiali dextral strike-slip fault (JLF, ∼31° N) on the east of Shenza-Xietongmen rift, as indicates that the JLF (31°N) is an uppermost mantle trace of the Shiquan River-Nam Tso Mélange Zone. Low Pn velocity (<8.0 km/s) and a nearly EW-oriented Pn fast-wave direction to the north of the JLF, suggesting the EW extension of the Eurasian plate. NE-oriented Pn fast-wave direction with strange intensity (>0.03) to the south of the JLF indicates the direct influence of the Indian plate northward subduction. The rifts located at the boundary of high and low-velocity anomalies imply that the rift system formed under the influence of lithospheric mantle activity. Based on the NE compression caused by the Indian plate’s northeastward subduction, some of the forced upwellings of thermal material generated within the mantle wedge tip caused an EW extension of the crust. Subsequently, the rifts system was locally formed.