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January Bulletin 2024

Post-Miocene Erosion in Central Nepal Controlled by Midcrustal Ramp Position, Duplex Growth, and Dynamically Maintained Elastic StrainS. N. Johnston1 , J. M. Cannon1,2, and P. Copeland11Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, USA, 2Now at Department of Geological Sciences, University of Texas at San Antonio, San Antonio, TX, USA Abstract To better understand what drives erosion in central Nepal, we have mapped regions of preferen-tial erosion by comparing modern detrital muscovite 40Ar/39Ar data to bedrock data from the Narayani river catchment and two subcatchments in central Nepal. We compare our pattern of erosion to erosion patterns from previous studies. Each shows a zone of preferential erosion from 5 km south to 20 km north of the Main Central Thrust (MCT). Peak erosion occurs in a 5-km-wide, orogen-parallel band, north of the MCT, overlapping proposed ramps in the Main Himalayan Thrust (MHT). This location of peak erosion is consistent with the interpretation that erosion is tectonically driven. Although peak erosion is spatially coincident with the interpreted MHT ramps, the zone of enhanced erosion extends north of the ramps' downdip limits sug-gesting that an additional driver of rock uplift is at work. This can be explained by a duplex growing along the ramp back-tilting and uplifting older duplex faults. This region also coincides with a dynamically supported bulge in the High Himalaya where a reservoir of elastic strain appears to be maintained over numerous seis-mic cycles further enhancing erosion.

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