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

Abstract

The Tien Shan are the quintessential in- tracontinental range, situated more than 1000 km north of the suture between India and Asia. Their initiation and growth in the Cenozoic, however, remain poorly under- stood. In this study we present stratigraph- ic, detrital fission-track, and magnetostra- tigraphic results that provide a basis for reconstructing the Cenozoic tectonic evo- lution of the Kyrgyz Range and adjacent Chu basin in the northwestern Tien Shan. Detrital fission-track thermochronology in- dicates 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 fis- sion-track and (U-Th)/He ages from a sam- pling 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 Kyr- gyz Range. Between 10 and 3 Ma, bedrock- exhumation rates decreased to ,0.3 km/ m.y., while sedimentation rates decelerated initially to ;0.25 km/m.y. before acceler- ating to ;0.4 km/m.y. at 4-5 Ma. Detrital fission-track results indicate that by 4.5 Ma, the top of an exhumed apatite partial an- nealing zone (PAZ) was exposed in the Kyr- gyz Range, corresponding with ; 2k m 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. Fis- sion-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.