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[1] We derive a slip rate for a thrust at the central Qilian Shan mountain front by combining structural investigations, satellite imagery, topographic profiling, luminescence dating, and 10 Be exposure dating. The seismically active Zhangye thrust transects late Pleistocene alluvial fan deposits and forms a prominent north facing scarp. The fault consists of two segments that differ in orientation, scarp height, and age. A series of loess-covered terraces records the uplift history of the western thrust segment. Loess accumulation on all terraces started at 8.5 ± 1.5 kyr and postdates terrace formation. Gravels from the highest terrace yielded a 10 Be exposure age of 90 ± 11 kyr, which dates the onset of faulting. With a displacement of 55-60 m derived from fault scarp profiles, this yields a vertical slip rate of 0.64 ± 0.08 mm yr À1 . Along the eastern thrust segment, three 10 Be ages from the uplifted alluvial fan constrain that faulting started at $31 ± 5 kyr. Together with a displacement of 25-30 m this leads to a vertical faulting rate of 0.88 ± 0.16 mm yr À1 . A dip estimate of 40°to 60°for the fault plane combined with lower and upper limits of $0.6 and $0.9 mm yr À1 for the vertical slip rate gives minimum and maximum horizontal shortening rates of 0.4 and 1.1 mm yr À1 across the Zhangye thrust. Our results are consistent with geologic and GPS constraints, which suggest that NNE directed shortening across the northeastern Tibetan Plateau is distributed on several active faults with a total shortening rate of 4 to 10 mm yr À1 .

In order to understand the dynamics of the India Asia collision zone, it is important to know the strain distribution in Central Asia, whose determination relies on the slip rates for active faults. Many previous slip-rate estimates of faults in Central Asia were based on the assumption that offset landforms are younger than the Last Glacial Maximum (approximately 20 kyr ago). In contrast, here we present surface exposure ages of 40 to 170 kyr, obtained using cosmogenic nuclide dating, for a series of terraces near a thrust at the northern margin of the Tibetan Plateau. Combined with the tectonic offset, the ages imply a long-term slip rate of only about 0.35 mm x yr(-1) for the active thrust, an order of magnitude lower than rates obtained from the assumption that the terraces formed after the Last Glacial Maximum. Our data demonstrate that the preservation potential of geomorphic features in Central Asia is higher than commonly assumed.

The confidence in surface exposure dating and related research, such as erosion rate studies or burial dating, strongly depends on the accuracy and precision of the currently used production rates of in situ-produced cosmogenic nuclides. Reducing the uncertainties of nuclide production rates by more accurate calibrations with independently dated natural rock surfaces is crucial for further improving the quantification of earth surface processes. Here we use surface samples from the 760±2 ka old Bishop Tuff in eastern California to quantify the Ne production rate, and (3) the assumption of steady-state erosion. Other assumptions, such as the applied scaling procedure, the muon contribution to nuclide production, or the attenuation lengths of neutrons and muons in rock, do not substantially affect the results. Based on 13 samples, the following average production rate ratios and conservative uncertainty estimates are obtained for sea level, high latitude, open sky, and rock surface: 0.249±0.009 or 0.232±0.009 for 10

Extreme flood events have the potential to cause catastrophic landscape change in short periods of time (10 0 to 10 3 h). However, their impacts are rarely considered in studies of long-term landscape evolution (>10 3 y), because the mechanisms of erosion during such floods are poorly constrained. Here we use topographic analysis and cosmogenic 3 He surface exposure dating of fluvially sculpted surfaces to determine the impact of extreme flood events within the Jökulsárgljúfur canyon (northeast Iceland) and to constrain the mechanisms of bedrock erosion during these events. Surface exposure ages allow identification of three periods of intense canyon cutting about 9 ka ago, 5 ka ago, and 2 ka ago during which multiple large knickpoints retreated large distances (>2 km). During these events, a threshold flow depth was exceeded, leading to the toppling and transportation of basalt lava columns. Despite continuing and comparatively largescale (500 m 3 /s) discharge of sediment-rich glacial meltwater, there is no evidence for a transition to an abrasion-dominated erosion regime since the last erosive event because the vertical knickpoints have not diffused over time. We provide a model for the evolution of the Jökulsárgljúfur canyon through the reconstruction of the river profile and canyon morphology at different stages over the last 9 ka and highlight the dominant role played by extreme flood events in the shaping of this landscape during the Holocene.bedrock erosion | extreme floods | knickpoints | Iceland | cosmogenic 3 He E xtreme floods in both terrestrial and extraterrestrial environments can cause abrupt landscape change that can have longterm consequences (1-5), especially when a geomorphic threshold is exceeded (6). The timescale over which this change is visible is controlled by the ability and efficiency of background processes to reshape the landscape. As a result, progress in understanding both short-term and long-term landscape evolution requires better knowledge of bedrock channel erosion processes and thresholds over the different scales at which geomorphological processes operate (7-10).The majority of research into extreme flood events has focused on the interpretation of deposited sediments (e.g., refs. 11 and 12) and the reconstruction of the hydraulic conditions prevailing during such events (e.g., refs. 13-15). Further work has defined the geomorphic impact of extreme flood events in proglacial areas close to the source of the flood water (e.g., refs. 16 and 17). Studies that examine the processes of bedrock erosion, especially large canyon formation, during extreme flood events can help establish a diagnostic link between formation processes and morphology in canyons in both terrestrial and extraterrestrial settings, but they remain scarce (e.g., refs. 18-20). Here, evidence for bedrock landscape change during extreme floods along the course of the Jökulsá á Fjöllum River (northeast Iceland) is used to test whether the contemporary landscape morphology reflects erosion during rare extreme eve...