RATES OF DIFFERENTIAL ROCK UPLIFT ACROSS THE BAISAHI ANTICLINE, WESTERN NEPAL, DETERMINED FROM STREAM PROFILE ANALYSIS

Topography in erosional landscapes largely reflects the adjustment of stream profiles to differential rock uplift, relative to baselevel. Emerging datasets from orogens around the globe reveal that catchment-mean channel steepness indices (k_sn, a measure of channel gradient normalized for drainage area) increase monotonically with erosion rate (E) or rock uplift rate (U), provided that climate and lithology remain relatively uniform among catchments. Here we exploit this relationship to estimate differential rock uplift rates along an actively growing fold in the foreland of the Himalayan orogen. We focus on the Baisahi anticline in western Nepal; this structure is a fault-bend fold developed above the Main Frontal Thrust (MFT). The ramp is planar and dips to the north at ~30° (Mugnier et al., 1999) and thus uplift rates are expected to be relatively uniform across the anticline. Observations of channel profile form (n=18) are consistent with this expectation: channel profiles are relatively smooth and channel steepness indices (k_sn) exhibit little variation along strike of the range (k_sn ranges from 40 – 60 m^0.9). We utilize a local calibration of the k_sn – U relationship from central Nepal (Kirby and Whipple, 2012) that relies on the fact that channels appear to be adjusted to spatially-variable uplift rates in the region of the Bagmati River (Lavé and Avouac, 2000). This scaling relationship implies that uplift rates (U) along this range are ~9-11 mm/yr. Above a planar ramp, these rates imply that slip along the MFT beneath the Baisahi anticline likely accommodates 18-22 mm/yr of shortening. These values are consistent with slip rates along the MFT determined in central Nepal (Lavé and Avouac, 2000) and with long-term rates determined from an assumed age of fault initiation and total shortening (e.g., Mugnier et al., 2004). Although our results suffer from limited precision, they demonstrate that the morphology of erosional landscapes can, in certain settings, provide constraints on the rates and patterns of deformation.