Late Cenozoic tectonic evolution of the northwestern Tien Shan: New age estimates for the initiation of mountain building

The Tien Shan are the quintessential intracontinental range, situated more than 1000 km north of the suture between India and Asia. Their initiation and growth in the Cenozoic, however, remain poorly understood. In this study we present stratigraphic, detrital fission-track, and magnetostratigraphic results that provide a basis for reconstructing the Cenozoic tectonic evolution of the Kyrgyz Range and adjacent Chu basin in the northwestern Tien Shan. Detrital fission-track thermochronology indicates that the northwestern Tien Shan was tectonically quiescent for much of the Cenozoic. Prior to uplift and exhumation in the late Miocene, the Kyrgyz Range was buried by sediments shed from highlands to the south and/or east. Paired bedrock fission-track and [U-Th]/He ages from a sampling transect of 2.4 km relief demonstrate that rapid exhumation commenced at ca. 11 Ma. Initial thrusting in the hinterland was followed by evaporite accumulation (;0.4 km/m.y.), which coincided with erosion of the pre–11 Ma strata that mantled the Kyrgyz Range. Between 10 and 3 Ma, bedrockexhumation rates decreased to ,0.3 km/ m.y., while sedimentation rates decelerated initially to ;0.25 km/m.y. before accelerating to ;0.4 km/m.y. at 4–5 Ma. Detrital fission-track results indicate that by 4.5 Ma, *Present address: ExxonMobil Exploration Company, GP3, Benmar Street, Houston, Texas 77060, USA; e-mail: [email protected]. †Present address: Department of Geological Sciences, Building 526, University of California, Santa Barbara, California 93106-9630, USA. the top of an exhumed apatite partial annealing zone (PAZ) was exposed in the Kyrgyz Range, corresponding with ;2 km of total exhumation. Continued exhumation of the Kyrgyz Range resulted in exposure of fully reset rocks from below the exhumed PAZ (;4–5 km depth) by ca. 1.5 Ma. Fission-track ages from two modern stream samples collected ;75 km apart along the range front indicate that exhumation in the Kyrgyz Range commenced in the west and then propagated to the east.