Collisional Processes in the Crust of the Northern Tien Shan Inferred From Velocity and Attenuation Tomography Studies

Based on seismic data from existing seismic networks in Kyrgyzstan, we have constructed new crustal models of seismic velocity and attenuation for P and S wave s beneath the Kyrgyz Tien Shan. With data from more than 6,000 events recorded by the international KNET network, the most detailed structures were detected in the central-northern part of the study region, where the Kazakh Shield collides with the northern Tien Shan. The independently computed 3-D distributions of P and S wave attenuation show features that are consistent with the main structural elements. The high-attenuation areas correspond to folded areas of the northern Tien Shan, whereas the partitions of the stable Kazakh Shield and the Issyk Kul block match with the low-attenuation areas. The velocity model reveals some structures that help to determine the details of the collision processes in the northern Tien Shan. In the upper crust, we observe the alternation of several higher- and lower-velocity anomalies that likely represent the interaction of brittle and ductile crustal layers of the collided Kazakh and Tien Shan plates. In deeper sections, both P and S wave velocity models show a prominent low-velocity anomaly just beneath the northern boundary of Tien Shan. We propose that this anomaly represents an anomalous crustal thickening at the point of underthrusting of the Kazakh Plate beneath Tien Shan.