CRUSTAL THICKNESS, POISSON'S RATIO, AND MOHO SHARPNESS BENEATH CENTRAL TIEN SHAN: CONSTRAINTS FROM RECEIVER FUNCTION STACKING

The Tien Shan is an approximately 2500-km long, 300-500-km wide chain of mountains located in central Asia. It is considered as one of the most tectonically active mountain ranges in the world and is a classic example of intraplate mountain building. Previous seismic and potential field studies were mostly concerned with lateral variations of crustal thickness, and systematic determinations of crustal Poisson’s ratio (which can be computed uniquely from Vp/Vs, the ratio of the P- and S-wave velocities) and Moho sharpness beneath Tien Shan have not be performed. Here we report preliminary results of such determinations using all the broadband seismic data archived at the IRIS Data Management Center, and use potential field data to constrain the interpretations of the seismic data. Results to date suggest that the average crustal thickness beneath Tien Shan from stacking of receiver functions is about 53 km which is consistent with previous geophysical determinations. The study area is characterized by a large Vp/Vs of 1.81±0.025 and a large overall stacking amplitude of the P-to-S converted phases beneath most stations, suggesting a mafic crust which is separated from the mantle by a sharp Moho. The thickest crust (about 68 km) is found beneath the northwestern corner of the Tarim basin. Distribution of earthquakes suggests that this area is among the seismically most active in Tien Shan. Stations in the vicinity of the Naryn Basin east of the Talas-Fergana strike-slip fault show low Vp/Vs ratios and anomalously thin crust of about 42 km, probably suggesting delamination of the lower crust. This interpretation is consistent with the positive regional Bouguer gravity anomalies which suggest a thinned crust beneath the area.