KINEMATICS OF FAULTS AND FOLDS IN THE JUMGAL BASIN, KYRGYZ TIEN SHAN

The Tien Shan mountain range is one of the most active intracontinental orogenic belts in the world and exhibits well-preserved, actively-growing folds that provide a natural laboratory for the study of thrust-related folding. This mountain belt is ~1200 km north of the Himalayan front, and is currently accommodating nearly half of the total crustal shortening rate between India and Eurasia. Our study focuses on the intermontane Jumgal basin of Kyrgyzstan, which is bounded to the north by crystalline Paleozoic basement rock in the hanging wall of a south-vergent reverse fault. A prominent anticline is present along the axis of the basin, exposing Neogene sedimentary rocks and warping well-preserved Quaternary geomorphic surfaces. Fluvial channels change in form across the fold, and windgaps show evidence of progressive defeat of some transverse drainages.

We are using deformed river terraces to evaluate: (1) geometry and kinematics of active structures, and (2) how slip rates are partitioned between folds growing in the basin and range-bounding reverse faults. Fieldwork summer 2016 will map the geometry of folded Neogene sedimentary rock, which records the full history of late Cenozoic folding. Surveying late Quaternary fluvial terraces and prehistoric irrigation channels that cross active folds will yield incremental records of folding deformation. We will integrate the surface geology with the progressive terrace deformation record to constrain geometry and kinematics of fault-related folding. Preliminary remote analysis of the intra-basin anticline near the town of Aral suggests the fold could be growing by fault-bend folding or trishear fault-propagation folding, and we will test the kinematic and geometric predictions of these models using field observations.

We plan to estimate slip rates using radiocarbon ages of deformed terrace surfaces coupled with structural models of the faults and folds. The relative slip rates of the basin-bounding fault and the fault underlying the intra-basin anticline have implications for fault mechanics of intracontinental basement-involved deformation. At a broader scale, this study will contribute to resolving how strain rates are partitioned spatially and temporally within the Tien Shan and the associated seismic hazard.