LAUSR

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 6 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. *Corresponding author: [email protected] Plain Language Summary The 700-km long Talas-Fergana fault is one of several large faults linked to the collision between the Indian and Eurasian tectonic plates, and cuts obliquely across the Tien Shan mountain range north of the Himalayas. Satellite-based GPS data show the Tien Shan range is being rapidly compressed tectonically at the rate of about 20 mm per year, yet indicate a slip rate on the Talas-Fergana of less than 2 mm per year. However, our study for the first time employs intensive field geologic techniques to determine that the slip rate on this fault has averaged 11-16 mm per year over recent millennia. With increasing reliance on satellitebased measurements investigating such a profound mismatch is important for reliable earthquake hazard assessments on the Talas-Fergana fault, which appears locked, and is at an unknown point in its earthquake cycle. Reconciling the two datasets suggests significant variability in slip rate on the Talas-Fergana through time, a conclusion that may have additional implications for models of lithosphere behavior.