KINEMATICS OF ACTIVE BASEMENT-INVOLVED DEFORMATION, SOUTHERN ISSYK-KUL BASIN, KYRGYZ TIEN SHAN

The Tien Shan of central Asia is an excellent example of active, contractional intracontinental deformation. As such, it provides a setting to test models of basement rock uplift, and offers insight into the fault kinematics and hazards of other regions of reverse faulting, such as the Puget Lowland and Los Angeles Basin of western North America. The southern margin of the Issyk-Kul basin is a particularly valuable location to study these processes, as faults developed in the granitic basement have initiated recently and thus retain more complete records of their kinematics.

Faults along the central southern margin of the Issyk-Kul basin generally verge to the south, away from the basin, and show an out-of-sequence pattern of propagation. The southernmost faults have over 2 km of structural relief and the northernmost structures are expressed as folds in the cover sediments. Many faults reverse vergence or die out into folds along strike. Up to 100 m offsets of late Pleistocene fluvial terraces demonstrate that the faults are active and are consistent with the high strain rates measured by GPS.

We have mapped ~50 km of the south-central Issyk-Kul thrust system and surveyed profiles of deformed Quaternary topographic surfaces using a Total Station and 1:25K topographic maps. Long north-dipping backlimbs defined by Tertiary sedimentary rocks in the hanging wall of a reverse fault with ~ 3 km of displacement show a consistent 10-12° dip. Hanging wall blocks of faults with greater displacements dip up to 50° to the north. A locally preserved angular unconformity within the Neogene section also records progressive northward rotation. Fluvial terraces along the Ak-Terek River consistently tilt northward where cut on north-dipping sedimentary rocks, along a 15 km transect from the southernmost fault to the shore of Lake Issyk-Kul and get progressively steeper with age. These observations suggest that the major basement faults are listric, causing hanging wall blocks to uplift by progressive rotation with slip along the fault. Based on the length of the tilted blocks, the curved portion of the fault must extend to at least 15 km depth. Although this zone is manifest as smaller structures that vary in style and development at the surface, the extent of rotation suggests linkage at depth into a fault capable of producing large ruptures.