SHORTENING RATES ALONG THE NEPALESE HIMALAYA INFERRED FROM GEOMORPHIC ANALYSIS

Using stream channel analyses, we estimate uplift and shortening rates along the Main Frontal Thrust (MFT) of western Nepal. The MFT in the Himalaya is a system of thrust faults and associated folds that represents the surface trace of the primary décollement between India and Eurasia. In central Nepal, coincidence between geodetic velocities and Holocene slip rates along the MFT suggests that interseismic strain accumulation of ~20 mm/yr is eventually released during co-seismic ruptures. Geologic constraints on slip rates elsewhere along the MFT are sparse, however, and limited to only 3 studies where deformed fluvial terraces are preserved across active anticlines. In western Nepal, the frontal thrusts have not ruptured since at least 1505; slip rates along these structures are poorly known. To estimate shortening rates in western Nepal we analyzed more than 400 channels along 500 km of the MFT, from Ramnagar, in western India, to just west of the Bakeya River of eastern Nepal, with newly released and publicly available 30m SRTM data. Channels are automatically selected by minimum drainage area, and channels within the auto-selection are randomly analyzed by hand for quality control. We exploit a previously developed empirical calibration of the scaling between channel steepness index (k_sn – a measure of channel gradient normalized by contributing drainage area) and rock uplift rate using data of Lavé and Avouac (2000) for channels within an MFT-derived fold just south of Kathmandu (after Kirby and Whipple, 2012). We use our empirical calibration to estimate uplift rates from k_sn, and shortening rates are inferred from structural constraints on fault geometry at depth. Our previous stream channel analyses at the Baisahi Anticline found that rates of incision and subsequently calculated shortening are in agreement with geologic shortening rates at 17.5 mm/yr ± 4 mm/yr. Applying this calibration farther west, our preliminary results suggest that Holocene shortening rates along the MFT remain high as far west as Ramnagar. Our results support the contention that slip rates along the MFT in western Nepal are comparable to those in the Gorkha rupture region of eastern Nepal. Thus, our results support suggestions that western Nepal is primed for a large or great earthquake.