A spatially distributed rate-of-erosion index (EI) based on models of bedrock river incision documents a strong spatial correspondence between areas of high erosion potential and young metamorphic massifs as well as structural highs throughout the Himalayas. The EI is derived from slopes and drainage areas calculated from a hydrologically corrected digital elevation model (GTOPO30) combined with precipitation data (IIASA) to generate synthetic annual stream discharges. These variables drive three generalized process models to produce EI maps that, while differing in detail, provide an internally consistent, spatially continuous index of large-scale erosion rates. The large spatial variation in potential erosion rates in the Himalayas suggested by the EI patterns contrasts with the uniform convergence of the Indian subcontinent. If these EI gradients persist through time, they support the emerging view of a positive feedback between localized, rapid erosion and upward advection of lower crust.
We performed a multibeam survey of Eel Canyon, offshore northern California. The survey revealed a signifi cant bend in the canyon that appears to be due to the oblique compressional tectonics of the region. A series of steps within a linear depression, ~280 m above the canyon fl oor, extends from the canyon rim at this bend to the subduction zone and a distinct fan-like topographic rise. We hypothesize that the linear depression is a distributary channel and the steps are cyclicstep bedforms created by turbidity currents. Our interpretation indicates that turbidity currents are able to run up and overspill the 280-m-high canyon wall, resulting in a partial avulsion of the canyon and the construction of a fan lobe that is offset from the canyon mouth. Simple hydraulic calculations show that turbidity currents generated in the canyon head from failure of 2-3 m of material would be capable of partially overfl owing the canyon at this bend, assuming steadyuniform fl ow, full conversion of the failed mass into a turbidity current, and a range of friction coeffi cients. These estimates are consistent with analyses of sediment cores collected in the head of Eel Canyon, which suggest that 2-3 m of material fails on decadal time scales. Our calculations show that the overfl owing parts of the currents would have large shear velocities (>10 cm/s) and supercritical Froude numbers, consistent with erosion of the distributary channel and formation of cyclic steps by turbidity currents.
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