Late Cenozoic tectonics of the Kepingtage thrust zone: Interactions of the Tien Shan and Tarim Basin, northwest China

Abstract

The Kepingtage (Kalpin) thrust zone, northwest China, is an actively deforming part of the India-Asia collision system. It lies south of the Tien Shan, has an area of ∼16,000 km², and consists of arcuate, emergent imbricates. Overall vergence is toward the interior of the Tarim Basin to the south. Thrust sheets typically expose Upper Cambrian to Permian platformal strata. Thrusting is largely thin-skinned; thin Upper Cambrian evaporites are likely to be the main decollement horizon. The Kepingtage thrust zone is the only margin of the Tarim Basin to deform in this style in the Cenozoic. It is replaced to the east and west by thrust zones which have propagated shorter distances into the interior of the Tarim Basin. Thick (up to 10 km) Mesozoic and Cenozoic clastic successions are present and deformed in these regions. Such successions are not present in the Kepingtage thrust zone or the adjacent Bachu Uplift within the Tarim Basin, because of episodic activity on steep, northwest-southeast trending thrusts which define the margins of the Bachu Uplift. These Mesozoic-Cenozoic strata may have suppressed the ability of regions along strike from the Kepingtage thrust zone to deform by thin-skinned thrusting utilizing the Upper Cambrian decollement. The along strike variation in active thrusting at the southern margin of the Tien Shan is an example of syntectonic sedimentation controlling thrust belt deformation style. A balanced section across the thrust zone indicates ∼28% shortening, equivalent to ∼35 km. This is equivalent to an average slip rate of ∼1.8 mm yr⁻¹ and a strain rate of ∼4.4 × 10⁻¹⁶ s⁻¹, assuming deformation began at circa 20 Ma. Active deformation is focused along the frontal thrust, the Kepingtage Fault. The northern boundary of the Kepingtage thrust zone is formed by the South Tien Shan Fault. This major, north dipping thrust is seismically active, with published earthquake focal depths at midcrustal levels (14 and 18 km). It juxtaposes upper Carboniferous sedimentary rocks of different facies and different Paleozoic deformation histories. Speculatively, it represents a reactivation of a late Paleozoic thrust, which originated at the boundary between the platformal interior of the Tarim Block and a deeper-water foreland basin to its north.