Abstract

Late Pleistocene–Holocene sinistral slip-rates on several segments of the Kunlun Fault in northeastern Tibet have been determined. These determinations are based on the measured displacement of alluvial surfaces whose surface ages were determined by cosmogenic 26Al and 10Be dating of quartz pebbles, and by 14C dating of charcoal. In the west, at three sites along the Xidatan–Dongdatan segment of the fault, near 94uE, terrace riser offsets ranging from 24 to 110 m, with cosmogenic ages ranging from ∼1800 to ∼8200 yr, yield a mean left-lateral slip-rate of 11.7±1.5 mm yr-1. Field observations indicate minimum offsets of 9–12 m; this offset, when combined with the long-term slip-rate, indicates that great earthquakes (M∼8) rupture this segment of the fault with a recurrence interval of 800-1000 yr. At two sites along the Dongxi– Anyemaqin segment of the fault, near 99uE, terrace riser offsets ranging from 57 to 400 m with 14C ages ranging from 5400 to 37 000 yr BP yield a minimum slip-rate of ∼10 mm yr-1. At one site, the 1937 January 7, M=7.5 and the penultimate earthquakes produced 4 m of left-slip and 0.4 m of reverse-slip. The maximum recurrence interval of earthquakes with such characteristic slip is thus ∼400 yr. Farther east, near 100.5uE, along the Maqen segment of the fault, the 180 m offset of a lateral moraine, emplaced between the last glacial maximum (20 ka) and 11 100 yr BP, yields a mean slip-rate of 12.5±2.5 mm yr-1. The slip-rates are constant, within uncertainty, throughout the 600 km of the Kunlun Fault that we studied. The average slip-rate is 11.5±2.0 mm yr-1. Extrapolating this rate to the reminder of the fault, we conclude that most (80 per cent) of the 300 morphological offsets measured in the field or on SPOT satellite images post-date the Last Glacial Maximum. Most of the terraces we studied were deposited during the humidperiod of the Early Holocene Optimum (9–5 ka); the formation of younger terraces reflects Late Holocene climate change.