ACTIVE INTRACONTINENTAL BASEMENT UPLIFT IN THE KYRGYZ TIEN SHAN

The Tien Shan Mountains of central Asia are one of the most active regions of intracontinental deformation on Earth, locally accommodating half of India-Asia convergence. Like the Laramide Rocky Mountains, the Tien Shan are far from the main plate boundary (over 1000 km north of the Himalayas). In contrast to the Laramide region, shortening in the Tien Shan is more spatially focused and consistent in orientation. These attributes of the Cenozoic Tien Shan result in part from the regional superposition on Paleozoic collision zones; however, major Paleozoic structures are rarely reactivated as range-bounding faults.

We focus on the mechanism of late Cenozoic deformation along the rangefront of the Terskey Range with the Issyk-Kul and Kochkor basins. In addition to good exposure of the deformed post-Paleozoic unconformity and late Cenozoic sedimentary rocks, the rangefront structures are traversed by numerous rivers with progressively deformed terraces. Combining the total deformation from the rock record with incremental deformation from Quaternary terraces offers strong constraints on both the geometry and kinematics of major structures.

The north margin of the Terskey Range exhibits a consistent north-down sense of displacement with structural relief > 7 km along the central Issyk-Kul basin. Despite overall north vergence of this system, surface faults nearly all verge south along most of the rangefront with the Issyk-Kul basin. The terraces record a progressive northward tilting along the Issyk-Kul portion of the rangefront that spans individual blocks bounded by south-vergent structures. This northward tilting panel is at least 7-12 km wide and 200 km long, suggesting a single underlying crustal-scale structure despite surface variations. The geometric and kinematic observations are consistent with a deforming wedge of material with geometric similarities to the trishear model. However, rather than the wedge experiencing fault-parallel shear, the back thrusts act like a bookshelf system, with antithetic motion accommodating northward rotation and uplift of the crustal-scale panel of basement rock. The kinematics of this active system may be similar to the evolution of other basement-involved structures with structural wedge geometries including parts of the Colorado Front Range.