Stable rate of slip along the Karakax Valley Fault from observation of inter-glacial and post-glacial offset morphology and surface dating

The westernmost section of the Altyn Tagh Fault runs along the upper Karakax Valley in southern Xinjiang. The northern side of the valley is formed of imbricated alluvial terraces deposited by tributaries of the Karakax River. Quaternary movement along the fault has left clear scars into the terraces and displaced geomorphological features. Ages of terraces determined using cosmogenic dating and optically simulated luminescence methods span a broad time range from the Holocene ( 10 ka) to the Eemian interglacial period ( 125 ka). We use digital elevation maps obtained from TanDEM-X radar data and Pleiades tri-stereo optical images to quantify lateral displacement of quaternary features. At several sites along the valley the reconstruction of the conical shape of massive alluvial fans forming the Eemian surface shows consistent left-lateral offsets of 300 m. Successive episodes of incision have left cut terraces inset in wide canyons, 10-25 m below the upper surface. The incision phase has been followed by the deposition of a broad terrace of early Holocene age, which is in turn re-incised by modern stream channels. Two kilometers west of the village of Shanxili, a narrow valley is partially dammed by a shutter ridge displaced by the fault. A fill terrace deposited upstream from the ridge has an OSL age of 10±1 ka. The offset of 25-30 m of the riser incising the terrace indicates a minimum post-depositional amount of slip on the fault, yielding a Holocene rate of 2.5-3 mm/yr, consistent with the average rate of 2.4 mm/yr needed to displace the 125 kyr-old upper terrace by 300 m. Overall, these results indicate that the penultimate transition from glacial to interglacial (MIS-6 to 5) has resulted in a massive sediment discharge that has shaped the morphology of the Karakax valley and its side valleys, overshadowing the modest deposition that accompanied the last glacial to interglacial transition in the early Holocene. The glacial and following periods have only slightly modified the landscape, which explains why the trace of the active fault is so clearly preserved in the Karakax valley. A 2-3 mm/yr slip rate on the Karakax fault in the late Quaternary is a small fraction of the left-lateral slip rate of the Altyn Tagh fault to the east and implies a faster rate along the Longmu-Gozha fault splaying from the Altyn Tagh Fault to the south.