A creeping intracontinental thrust fault: past and present slip-rates on the Northern edge of the Tien Shan, Kazakhstan

We demonstrate that a continental interior reverse fault is deforming by aseismic creep, presently, and likely also in the long term. The Karkara Rangefront Fault, part of the larger Main Terskey Front, forms the northern boundary of the high Terskey Tien Shan in southeastern Kazakhstan and is a mature structure with evidence for high slip rates throughout the late Cenozoic. Combining field studies with a satellite stereo-image derived digital elevation model (2m resolution), we map a series of fluvial terraces along the rangefront which are uplifted by up to similar to 300 m above the present river level. Radiocarbon ages from one catchment constrain the ages of the lowermost two terraces to be similar to 4-5 ka and similar to 10-15 ka, consistent with prominent, regionally extensive terraces observed elsewhere in the Tien Shan. Based on conservative estimates for the fault dip under the displaced terraces, we estimate a slip rate along the fault plane of 3.5(-0.4)(+1.7) mmyr(-1) on the Karkara Rangefront Fault and a further > 0.8 mm yr(-1) on a fold structure in the Kegen basin that we infer is driven by a detachment from the main rangefront. We therefore estimate a minimum shortening rate across the rangefront of 1.1-3.3 mm yr(-1). Elastic modelling of the regional GPS velocity field suggests that the fault is presently creeping at similar to 3 mmyr(-1) (horizontal shortening), consistent with the upper limit of our Late Quaternary slip rate estimate. This is the fastest known slip rate in the northern Tien Shan and the only individual structure resolved in the regional velocity field. At present the fault is accumulating minimal strain, and there is evidence in the geomorphology that this creep is sustained in the long term, but whether or not it is also capable of generating earthquakes requires further study