Opposite facing dipping structure in the uppermost mantle beneath the central Tien Shan from Pn traveltime tomography

The Tien Shan orogenic belt is located in the western region of central Asia, which has been previously considered to have formed in the Paleozoic and reactivated in the Cenozoic. However, the dynamic processes and deep detailed structure of the Tien Shan orogen remain elusive. Seismic studies are expected to provide profound clues on the deep structure and dynamic mechanism of the mountain-building process. Regional Pn traveltime tomography provides opportunities for constructing a more detailed P-wave velocity model of the uppermost mantle linking the shallow crust and the upper mantle. In this work, a 3-D P-wave high-resolution velocity model in the uppermost mantle of the western and central Tien Shan is constructed by inverting the 20,940 Pn arrival time of the 1108 local and regional earthquakes retrieved from the ISC-EHB catalog. The 3-D fast-marching tomography (FMTOMO) package is used to invert the 3-D uppermost mantle velocity model with a resolution of 1 degrees x 1 degrees. We clearly observe descending high-velocity anomalies, extending from the lower crust down to 85 km, suggesting double subduction of the Kazakh lithosphere in the north and Tarim lithosphere in the south beneath central Tien Shan. Furthermore, a prominent low-velocity feature in the upper mantle of central Tien Shan might result from asthenospheric upwelling. The high-velocity anomaly in the upper mantle of the western Tien Shan may suggest the southward subduction of the Kazakh plate to the western Tien Shan and Fergana basins. A significant low-velocity anomaly in the uppermost mantle beneath the southwestern Tarim Basin might result from plume intrusion with evidence of mantle xenolith or plume-lithosphere interaction and its consequent upwelling.