Topographic and lithologic control on catchment-wide denudation rates derived from cosmogenic 10Be in two mountain ranges at the margin of NE Tibet

Abstract

Fault-bounded mountain ranges with along-strike variations in mean slope angle and local relief offer the opportunity to investigate how denudation depends on these topographic parameters. We determined catchment-wide denudation rates along two actively growing, fault-bounded mountain ranges (the Yumu and western Longshou Shan) from 10Be concentrations in quartz from stream sediments. Both ranges expose (1) low-grade metamorphic Paleozoic rocks in their center, and (2) unmetamorphosed Cretaceous red beds near the tips of the range-bounding thrust faults. Field observations document debris flows, shallow landsliding, and surface wash as dominant mechanisms of mass transport on the hillslopes. Denudation rates obtained for the Paleozoic metasediments range from ∼100 to ∼300mmka−1 and correlate linearly with mean slope angle and local relief. In contrast, Cretaceous sediments exhibit mainly higher and more variable denudation rates between ∼30 and ∼500mmka−1, although slope and relief values are lower for these catchments. The higher denudation rates in the Cretaceous red beds are interpreted as resulting from their lower mechanical strength as compared to the resistant Paleozoic rocks. This important lithologic control on denudation demonstrates that uniform catchment lithologies and rock strength are essential for deciphering the effect of topographic indices on the rate of denudation.