Shortening rates in the Nepalese Himalaya derived from quantitative geomorphic analysis

Abstract:

The rates and distribution of active shortening throughout the Himalayan foreland have been the subject of intense study for the past several decades. Despite substantial progress, the distribution of slip on specific faults remains poorly understood. Numerous studies suggest that fluvial systems developed above growing folds exhibit strong correlations with uplift rate. Here, we build on these studies to develop a quantitative, empirical calibration of the scaling between channel steepness index (k_sn – a measure of channel gradient normalized for drainage area) and uplift rate; we derive a geometric model of uplift rate using data of Lavé and Avouac (2000) and analyze channel profiles using a 30m DEM derived from SRTM data. Our analysis confirms a linear relationship between uplift rate and k_sn; we apply this calibration to regions of similar lithology along the Baisahi anticline in western Nepal. Here, channel profiles exhibit smooth profiles with similar concavity, consistent with spatially uniform uplift rates above a planar ramp in the Main Frontal Thrust fault (Mugnier et al., 1999). Channel steepness indices along the western anticline are invariant with position along the anticline, suggesting a uniform uplift rate of 9 mm/yr ± 2 mm/yr. Along the eastern portion of the anticline, steepness indices increase systematically from west to east, suggesting that uplift rates increase toward the east. However, structural data suggest that this reflects an increase in the dip of the fault at depth (Husson and Mugnier, 2003). Our analysis suggests that the Baisahi anticline absorbs 17.5 mm/yr ± 4 mm/yr of active shortening. Our results demonstrate how local calibration of channel profile adjustment to uplift rate can be used to place constraints on active deformation in regions where other geomorphic markers are sparse or absent.