Velocity variations in the crust and upper mantle beneath the Tien Shan inferred from Rayleigh wave dispersion: Implications for tectonic and dynamic processes

Abstract

We used data from nuclear explosions and earthquakes recorded by the Kyrgyzstan telemetered broadband seismic network to examine crustal structure in the Tien Shan region of central Asia. Seventy‐nine measurements of fundamental and first higher mode Rayleigh waves were analyzed using the single‐station multiple filtering technique to determine group velocity dispersions. We found that there was a distinct difference in Rayleigh wave group velocity values between different regions, but the data could be combined in three groups: (1) paths from the west and south across the Turan platform and western Tarim Basin, (2) paths across the eastern Tarim Basin, and (3) nuclear explosions from the Lop Nor test site. Group velocity values were inverted to derive one‐dimensional shear wave velocity models for the Tien Shan region. Results reveal crustal thickness varying from 50 to 60 km with thickness increasing toward the eastern Tien Shan region in agreement with published crustal thickness maps for the region. Array analysis indicates that paths along the eastern Tien Shan and Tarim‐Tien Shan are affected by lateral refractions and multipathing, but we argue that these have a small effect on the measured dispersion curves. Our results indicate that upper mantle velocities in the eastern Tarim and Tien Shan are significantly lower than those beneath the western Tien Shan and Turan platform. Our results suggest that the boundary between these two regimes lies somewhere west of the depocenter of the Tarim Basin and east of Kashgar, China.