Love Wave Tomography of the Central Tien Shan from Ambient Seismic Noise

The Tien Shan is the world’s largest and most active intracontinental orogeny. Despite numerous geological and geophysical studies conducted in the Tien Shan, the mountain-building mechanisms remain debatable. There are three competing hypotheses: (1) underthrusting of the Kazakh and Tarim lithosphere on both sides of the Tien Shan, (2) coherent shortening in the Tien Shan lithosphere, and (3) up-welling of hot mantle beneath the central Tien Shan. To better understand the formation of the Tien Shan crust and distinguish the competing models, I applied the ambient noise tomography method to Love waves in the central Tien Shan at 40 seismic stations and obtained phase velocities at a period range of 8 s-35 s. The noise data were first processed at individual stations with instrument response removal, component rotation and spectral whitening. Then cross-correlations of ambient noise were computed on a daily basis and then stacked for each station pair. Love wave phase velocities were computed from the stacked waveforms.

Four Love wave phase velocity anomaly zones are imaged in the central Tien Shan. A high velocity anomaly is consistently present in the Kazakh Shield. Another high velocity zone is observed at the boundary between the Tarim basin and the Tien Shan from 18 to 35 s. This anomaly becomes stronger and extends more towards the Tien Shan at longer periods. Such high velocity anomalies most likely reflect strong crust beneath the Kazakh shield and the Tarim basin. The northward extension of the second fast anomaly suggests that the Tarim lithosphere probably underthrusts beneath the Tien Shan. Two significant low velocity zones are imaged in the southwest and central-east of the study area, which correlate with high deformation mountain ranges. These low phase velocities might be due to high temperature or high volatile content in the Tien Shan crust, suggesting a weak crust beneath the Tien Shan. The slow anomaly at relatively long periods could also reflect a thick crust root beneath the mountain ranges. These results suggest that the Tien Shan was formed by coherent shortening of its weak lithosphere which is sandwiched between the two relatively strong lithospheres of the Kazakh shield and the Tarim basin.