TWO PHASES OF INCREASED TECTONIC ACTIVITY OF THE QILIAN MOUNTAIN AND THE ALTYN TAGH FAULT: EVIDENCE FROM SEDIMENTARY BASIN ANALYSIS IN THE HEXI CORRIDOR BASIN, NE TIBET

The Qilian Mountain and the Altyn Tagh Fault in the northern rim of the Tibetan Plateau play a key role in adjusting and absorbing the huge deformation imposed by the Indo-Asian collision. Timing and characterization of deformation and uplift in this region is crucial to understanding the style and mechanism of growth of the Tibetan Plateau. The previously hypothesized Plio-Quaternary uplift of the Qilian Mountain has been challenged by recent bedrock thermochronology and basin analysis studies. Here we present results from comprehensive investigations (including paleomagnetism, seimic stratigraphy and sedimentary facies analysis, paleocurrent, and section balancing) of a thrust-fold system developed along the southern margin of the Hexi Corridor basin in the northern foot of the Qilian Shan and along the northeastern end of the Altyn Tagh Fault. The results indicate that the Hexi Corridor basin experienced two phases of flexure during 41-23 Ma and 9-8 Ma, respectively, indicating two phases of rapid tectonic activities of the Qilian Mountain and Altyn Tagh Fault. Accompanying these two episodes of enhanced flexure are rapid fault movements, facies changes, coarsening of sediments and sedimentation accumulation rate increase, strata tilting, and rotation of the basin. Punctuating these two phases of enhanced flexure are pulsed tectonic uplift events occurred at ca. 41, 33-31, 23, 9-8, 6.5, 3.6, 2.6, 1.8, 1.2, 0.8, and 0.14 Ma, as indicated by occurrence of strata unconformity, growth strata, and/or rotation of basin. We interpret the two phases of increased tectonic activities in the Qilian Mountain and the Altyn Tagh Fault as remote responses to the collision of India with Asia.