Post‐Miocene Erosion in Central Nepal Controlled by Midcrustal Ramp Position, Duplex Growth, and Dynamically Maintained Elastic Strain

Abstract

To better understand what drives erosion in central Nepal, we have mapped regions of preferential 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 suggesting 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 seismic cycles further enhancing erosion.

Megathrust Heterogeneity, Crustal Accretion, and a Topographic Embayment in the Western Nepal Himalaya: Insights From the Inversion of Thermochronological Data

“…PT2 exhibits along‐strike variations and is usually collocated with features that define the active‐uplift front of the Himalaya (e.g., Duncan et al., 2003; Harvey et al., 2015; Morell et al., 2015). The young cooling ages along this zone suggest rapid exhumation (e.g., Blythe et al., 2007; Copeland et al., 1991; Johnston et al., 2020; Wobus et al., 2003). The above‐mentioned bifurcation of the high‐slope zone in western Nepal Himalaya obscures the plateau‐taper pattern of the topography there (Figure 1).…”

Supplemental Material: Duplex kinematics reduces both frontal advance and seismic moment deficit in the Himalaya

“…Second, peak erosion and locally higher erosion patterns, indicated by the detrital muscovite dating, are spatially coincident with the interpreted midcrustal ramp and the ramp's north, which is suggested to reflect an uplift pattern caused by duplexing (e.g., Johnston et al, 2020). Third, repetitions of stratigraphic units, evidence of thrust sheets repeatedly breaking forward and stepping up from the common stratigraphic level, are found in geological cross-sections nearly the entire Himalayan arc (e.g., Mitra and Boyer, 2020;Long and Robinson, 2021)-from Pakistan (Jadoon and Frisch, 1997), the Kashmir Himalaya (Gavillot et al, 2018), the northwest Indian Himalaya (Yu et al, 2015), west, central andeast Nepal Himalaya (e.g., DeCelles et al, 2001;Robinson et al, 2006;Bhattacharyya and Mitra, 2009;Khanal et al, 2015;Ghoshal et al, 2020), to the Bhutan Himalaya (e.g., Long et al, 2011;McQuarrie et al, 2019).…”

“…Fourth, the continuously southward migration of the midcrustal ramps, related to the translation of the duplex, has also been detected via various indices of uplifting across a wide range of timescales (e.g., Grandin et al, 2012;Johnston et al, 2020).…”

Coupled Rapid Erosion and Foreland Sedimentation Control Orogenic Wedge Kinematics in the Himalayan Thrust Belt of Central Nepal

“…Cooling ages from low-T thermochronology can provide information on erosion and hence on tectonic and geomorphic processes (Braun et al, 2006). Thermochronological ages from central Nepal are generally Pliocene and younger, attesting to rapid, high-magnitude erosion (Blythe et al, 2007;Herman et al, 2010;Johnston et al, 2020;Nadin & Martin, 2012;Robert et al, 2011 (Blythe et al, 2007;DeCelles et al, 2020;Herman et al, 2010;Nadin & Martin, 2012;Robert et al, 2009Robert et al, , 2011Sakai et al, 2013;Streule et al, 2012;van der Beek et al, 2016). At regional scale, cooling ages in central Nepal are younger than cooling ages in western Nepal (Figures 6c and 6d).…”

“…The northern edge of the erosional bight coincides DECELLES AND CARRAPA 10.1029/2020JB021256 5 of 30 Wobus et al (2003), the approximate line north of which the geodetically derived interseismic coupling along the basal décollement drops to near zero (Ader et al, 2012), the Dang, Deukhury, and Chitwan wedge-top basins, and major rivers of the Karnali and Kosi River watersheds. with a nearly straight, ∼235 km long band of steep fluvial channel segments (e.g., Cannon et al, 2018;Johnston et al, 2020;Wobus et al, 2003) and abrupt hillslope steepening referred to as physiographic transition "PT 2 " by Hodges et al (2001) and Wobus et al (2003). The Narayani basin protrudes into topographic Tibet at its northwestern and northeastern corners by virtue of the high elevation Thakkhola and Gyirong grabens, respectively, which are Miocene-Holocene extensional basins bounded by steep normal faults that strike roughly north-south and dip inward toward the Narayani basin (Colchen, 1999;DeCelles et al, 2018;Hurtado et al, 2001;Shen et al, 2016;Xu et al, 2012).…”