Geologic slip-rate determinations on the Talas-Fergana fault: mismatch with geodetic slip rate

Detailed new paleoseismic field investigations at two sites on the Talas-Fergana fault, a poorly known strike-slip structure that transects the Tien Shan mountain range, document late Holocene slip rates of 11 – 16 mm a^-1. This prominent structure is distinctive in striking obliquely NW-SE across the Tien Shan, which is otherwise dominated by contractional structures striking generally E-W. Moreover, a satellite-based GPS network spanning the Tien Shan orogen records active N-S contraction rates of ~20 mm a^-1, but limits slip on the Talas-Fergana fault to <2 mm a^-1. This profound mismatch between long-term geologic and short-term geodetic slip rates, which may suggest temporal variability in slip, highlights the importance of field-based investigations as a complement to remotely-sensed data, particularly in evaluating models of lithosphere behavior and earthquake probabilities on presently locked faults such as the Talas-Fergana.