P-wave Velocity Structure of the Mantle Transition Zone and 660km Discontinuity Beneath the Eastern Tianshan and Its Implications for Lithosphere Delamination
The Tianshan,one of the most active intracontinental orogenic belts globally,exhibits distinct geomorphological features and complex formation and reactivation processes.The velocity anomaly structure of the mantle transition zone,along with characteristics of mantle discontinuities,provides valuable insight into the deep dynamic processes of the Tianshan.However,due to limited station coverage in previous studies,the velocity structure of the Eastern Tianshan remains insufficiently explored.Using seismic waveform data from a temporary broadband array in central Mongolia,we obtained P-wave velocity profiles near the 660km discontinuity beneath the eastern Ili Basin and the western and central Eastern Tianshan by analyzing triplicated waveforms.Our findings reveal a high-velocity anomaly(HVA)in the mantle transition zone beneath the central Eastern Tianshan,with velocity increasing progressively with depth,reaching a 1％increase at the base of the mantle transition zone.The 660km discontinuity is a sharp interface,with a P-wave velocity contrast of 4.61％～5.78％.The discontinuity exhibits a depression of 5～15km,with an eastward increase in depth.These results suggest that lithospheric delamination or fracture material from the Tianshan is sinking into the mantle transition zone due to negative buoyancy,contributing to the observed HVA.Additionally,a low-temperature anomaly likely enhances the subsidence of the 660km discontinuity.Our findings provide new seismological evidence for lithospheric delamination or break-off beneath the Eastern Tianshan,offering valuable insights into the dynamic processes of this region.
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