LATE CENOZOIC SLIP ON THE TALAS-FERGHANA FAULT, THE TIEN-SHAN, CENTRAL-ASIA

Although Cenozoic crustal shortening and thickening by thrust faulting have built the present Tien Shan, active right-lateral shear on the n orthwest-trending Talas-Ferghana fault appears to be the most rapid lo calized deformation in the belt. Ephemeral stream valleys have been of fset right-laterally tens of metres. New and published radiocarbon dat es of organic material deposited in depressions blocked by offset ridg es place upper bounds on the average Holocene slip rate at 18 localiti es, Uncertainties allow 14 upper bounds to overlap the range of 8-16 m m/yr, and 95% confidence limits on such bounds at 11 sites are entirel y within this range. We infer that the rate of approximate to 10 mm/yr is not simply an upper bound, but applies to the late Holocene Epoch. Although the bounds on rates permit more rapid slip in the northwest than the southeast, they do not place a useful constraint on variation s in slip rate along the fault. Offsets of Paleozoic facies boundaries imply a total right-lateral shear of 180-250 km, but Early Cretaceous sedimentary rock appears to have been offset only 60 +/- 10 km, Publi shed paleomagnetic declinations of Cretaceous- Miocene rock demonstrat e 20 degrees-30 degrees of counterclockwise rotation of the Ferghana V alley, which lies just west of the Talas-Ferghana fault, with respect to stable parts of Eurasia and 20 degrees +/- 11 degrees with respect to the central Tien Shan east of the fault, These declinations are con sistent with a maximum northwestward translation of 70-210 km of the F erghana Valley at the Talas-Ferghana fault and, therefore, with a simi lar maximum horizontal shortening across the Chatkal Ranges, which lie between the Ferghana Valley and the Kazakh platform, Estimates of cru stal thickness beneath the Chatkal Ranges, however, permit only 60-100 km of Cenozoic shortening, If <100 km of slip on the Talas-Ferghana f ault accumulated at a constant rate of 10 mm/yr, it would imply an ini tiation of slip more recently than ca, 10 Ma, long after India collide d with the rest of Eurasia.