Tectonic interaction between the Pamir and Tien Shan observed by GPS

Abstract

The complex tectonic interplay between the Central Asian Southwest Tien Shan and the north advancing Pamir as well as the role of the Pamir Frontal Thrust (PFT) separating these two orogens along the intervening Alai Valley is yet unclear. In this paper we present data of the newly installed Western Alai GPS profile (WAGP), capturing the deformation signal of both mountain ranges. The 20 km long WAGP records a maximum displacement rate of 9.3 ± 0.8 mm yr−1. The lion's share of displacement (6.0 ± 0.8 mm yr−1) is accommodated between the two stations located directly north and south of the PFT in 5 km distance. The WAGP data nicely complement the existing South Tien Shan and the Pamir GPS network data, which we present here in a combined reference frame and use it as input for horizontal block rotation/strain models. The model results show that both the Southwest Tien Shan and the Pamir behave as uniformly strained blocks and rotate counterclockwise (with respect to Eurasia) by 0.93 ± 0.11° Myr−1 and 0.62 ± 0.05° Myr−1, respectively. The Southwest Tien Shan undergoes NNE-SSW shortening of −22.1 ± 1.5 × 10−9 year−1 with an insignificant perpendicular extension. The Pamir is shortening with a rate of −10.2 ± 3.8 × 10−9 year−1 in a NNE-SSW direction, which is nearly 2.5 times less than its lateral extension rate. A band of increased deformation along the PFT is bounded to the north by the northern rim of the Alai Valley and extends up to 30–50 km south into the Pamir.