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The geomorphic character of major river drainages in the Himalayan foothills of central Nepal suggests the existence of a discrete, west-northwest-trending break in rock uplift rates that does not correspond to previously mapped faults. The 40 Ar/ 39 Ar thermochronologic data from detrital muscovites with provenance from both sides of the discontinuity indicate that this geomorphic break also corresponds to a major discontinuity in cooling ages: samples to the south are Proterozoic to Paleozoic, whereas those to the north are Miocene and younger. Combined, these observations virtually require recent (Pliocene-Holocene) motion on a thrust-sense shear zone in the central Nepal Himalaya, ϳ20-30 km south of the Main Central thrust. Field observations are consistent with motion on a broad shear zone subparallel to the fabric of the Lesser Himalayan lithotectonic sequence. The results suggest that motion on thrusts in the toe of the Himalayan wedge may be synchronous with deeper exhumation on more hinterland structures in central Nepal. We speculate that this continued exhumation in the hinterland may be related to intense, sustained erosion driven by focused orographic precipitation at the foot of the High Himalaya.

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[1] An effective physics-based rule for the evolution of bedrock channel cross sections is required for quantitative modeling of the roles of climate, tectonics, and sediment supply in setting bedrock longitudinal profiles and landscape form. Here we propose a modeling strategy in which the spatial pattern of erosion rates in a channel cross section is calculated, allowing exploration of the origin of the channel cross-sectional profile, and of the dependence of channel width on flow discharge and channel slope. Our approach reproduces many of the scaling relationships observed in natural systems, including power-law width-discharge (W$Q 0.4 ) and width-slope (W$S À0.2 ) relationships. Models of channel cross-sections linked in series and subject to varying rock uplift (baselevel lowering) rates produce concave-up longitudinal profiles with power-law slope-uplift (S$B 1.31 ) and width-uplift (W$B À0.24 ) relationships. Our modeling strategy can easily be adapted to handle i) better representations of erosional processes, ii) better approximations of the flow structure, and iii) the role of non-uniform sediment mantling of the bed.

Recent convergence between India and Eurasia is commonly assumed to be accommodated mainly along a single fault--the Main Himalayan Thrust (MHT)--which reaches the surface in the Siwalik Hills of southern Nepal. Although this model is consistent with geodetic, geomorphic and microseismic data, an alternative model incorporating slip on more northerly surface faults has been proposed to be consistent with these data as well. Here we present in situ cosmogenic 10Be data indicating a fourfold increase in millennial timescale erosion rates occurring over a distance of less than 2 km in central Nepal, delineating for the first time an active thrust fault nearly 100 km north of the surface expression of the MHT. These data challenge the view that rock uplift gradients in central Nepal reflect only passive transport over a ramp in the MHT. Instead, when combined with previously reported 40Ar-39Ar data, our results indicate persistent exhumation above deep-seated, surface-breaking structures at the foot of the high Himalaya. These results suggest that strong dynamic interactions between climate, erosion and tectonics have maintained a locus of active deformation well to the north of the Himalayan deformation front.

[1] We document and characterize hanging valleys in a fluvially eroded landscape in eastern Taiwan. Our conceptual model for the initiation of hanging valleys builds on a recently proposed model of bedrock incision in which erosion actually becomes less efficient on very steep channel gradients. If a pulse of incision in the main stem outpaces the tributary response, the gradients at tributary mouths may therefore pass a threshold value beyond which erosional efficiency declines, giving rise to a mismatch between trunk and tributary erosion rates. This mismatch is expected at junctions with small tributaries, where a step function decrease in drainage area also leads to sharp contrasts in water and sediment flux between trunk and tributary channels. The occurrence of hanging valleys in actively uplifting fluvial landscapes such as the Central Range of Taiwan suggests that the most common parameterizations of bedrock erosion, which typically assume a monotonic positive correlation between channel gradient and incision rate, may be violated in very steep channels. In addition, hanging valleys could greatly increase the response time of landscapes to tectonic perturbations since catchments above these tributary mouths will be insulated from these perturbations until a new suite of processes (e.g., weathering and rock mass failure) wear through the hanging valley lip. The results of this study underscore the need for a more complete understanding of bedrock erosion processes and the incorporation of process transitions and threshold conditions into landscape evolution models.

[1] Erosion rates of permafrost coasts along the Beaufort Sea accelerated over the past 50 years synchronously with Arcticwide declines in sea ice extent, suggesting a causal relationship between the two. A fetch-limited wave model driven by sea ice position and local wind data from northern Alaska indicates that the exposure of permafrost bluffs to seawater increased by a factor of 2.5 during 1979-2009. The duration of the open water season expanded from ∼45 days to ∼95 days. Open water expanded more rapidly toward the fall (∼0.92 day yr −1 ), when sea surface temperatures are cooler, than into the mid-summer (∼0.71 days yr −1 ).Time-lapse imagery demonstrates the relatively efficient erosive action of a single storm in August. Sea surface temperatures have already decreased significantly by fall, reducing the potential impact of thermal erosion due to fall season storm waves.

[1] Although only recently recognized, hanging tributary valleys in unglaciated, tectonically active landscapes are surprisingly common. Stream power-based river incision models do not provide a viable mechanism for the formation of fluvial hanging valleys. Thus these disequilibrium landforms present an opportunity to advance our understanding of river incision processes. In this work, we demonstrate that thresholds apparent in sediment flux-dependent bedrock incision rules provide mechanisms for the formation of hanging valleys in response to transient pulses of river incision. We simplify recently published river incision models in order to derive analytical solutions for the conditions required for hanging valley formation and use these results to guide numerical landscape evolution simulations. Analytical and numerical results demonstrate that during the response to base level fall, sediment flux-dependent incision rules may create either temporary or permanent hanging valleys. These hanging valleys form as a consequence of (1) rapid main stem incision oversteepening tributary junctions beyond some threshold slope or (2) low tributary sediment flux response during the pulse of main stem incision, thus limiting the tributary's capacity to keep pace with main stem incision. The distribution of permanent and temporary hanging valleys results from four competing factors: the magnitude of base level fall, the upstream attenuation of the incision signal, the lag time of the sediment flux response, and the nonsystematic variation in tributary drainage areas within the stream network. The development of hanging valleys in landscapes governed by sediment flux-dependent incision rules limits the transmission of base level fall signals through the channel network, ultimately increasing basin response time.