No late Quaternary strike-slip motion along the northern Karakoram fault

Abstract

Models that treat long-term evolution of the Tibetan orogen in terms of interactions between rigid blocks require the right-slip Karakoram fault that bounds the western margin of the Tibetan plateau to be a long-lived, stable, high slip-rate feature. While the southern portion of the fault clearly remains active, recent work has proposed that the northern half of the Karakoram fault is currently inactive. New field observations and geochronologic results from the northern end of the Karakoram fault system confirm this interpretation and provide the first quantitative data on the minimum age for the termination of slip. In the southeast Pamir, gravel that yields a U-series age of 198±5 ka on secondary carbonate caps a non-deformed strath terrace that extends across the main strand of the Karakoram fault. The secondary Achiehkopai fault strand is overlain by undisturbed Hangdi glacial stage (24±6 ka) deposits and Dabudar glacial stage (penultimate glacial cycle, ∼150 ka, or older) deposits, which lack observable lateral displacement or deformation. Together, these observations show that the northern portion of the Karakoram fault system has not accommodated any detectable strike-slip deformation since at least 24±6 ka, and most likely since ∼200 ka or more. These results show that the Karakoram fault system no longer forms a continuous discrete kinematic boundary at the western margin of the Tibetan Plateau. This suggests that even long (>500 km) strike-slip faults within orogenic belts are inherently unstable features, consistent with models of continental collision zones involving relatively weak crust and distributed deformation.