10Be and OSL dating of Pleistocene fluvial terraces along the Hongshuiba River: Constraints on tectonic and climatic drivers for fluvial downcutting across the NE Tibetan Plateau margin, China

Accurate geochronologic constraints of fluvial surfaces along tectonically active mountain fronts can help clarify river responses to tectonic uplift and rapid climate change. In this study, we re-constrained abandonment ages of four fluvial terraces along the Hongshuiba River on the northern margin of the Qilian Shan, NE Tibet using 10Be depth profiles to date fluvial deposits, and OSL dating of capping loess. Comparison of chronological ages of the same surface derived from both boulders and depth profiles provides an opportunity to evaluate the suitability of different sampling methods for determining landform ages in arid climate. Correlating terrace ages with regional climate proxies suggests that terraces formed during an interglacial stage or during glacial to interglacial transitions, or during a period of strong summer monsoon or monsoonal weakening. The initial period of fluvial incision may primarily result from climate changes (e.g, monsoon evolution), and other factors that influence the ratio of sediment flux to water discharge. Similarities between fluvial incision rates and incremental tectonic uplift rates suggest a tectonic control on continuous river downcutting before ∼26 ka. In summary, climate forcing drives the initial abandonment of fluvial terraces in this region, while the amplitude of fluvial incision was mainly controlled by progressive tectonic uplift. However, comparison of regional fluvial incision rates (e.g. ∼6mm/a of the Hongshuiba River), exhumation rates from the Qilian Shan (∼0.5mm/a), denudation rates from the river catchments (0.3-0.8mm/a), and vertical slip rates of the northern Qilian Thrust (∼1mm/a) all suggest climate change driven forcing of incision since the Late Pleistocene to Holocene.