71-4 Assessing Climatic Impacts on Frontal Propagation of the Main Boundary and Main Frontal Thrusts, Central Nepal

Session: The Geodynamic Evolution of the Himalaya: From Mountain Building to Modern Seismicity and Climate Change (Posters)

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Abstract:

High topography and elevated precipitation in the Himalaya allow for complex feedbacks between climate and tectonics. Recent work suggests that the Narayani drainage basin of central Nepal, encompassing the high elevation Thakkhola Graben, low elevation Chitwan wedgetop basin, and intervening Kali Gandaki River, represents an apex of climatic-tectonic interactions, where climatically enhanced erosion modulated sediment flux and fold-thrust belt propagation since ca. 5 Ma (DeCelles and Carrapa, 2021). We present a new ~2.5 km thick section from the Chitwan Basin with climate and provenance proxies, which we compare to published records of hinterland basin development in the Thakkhola Graben, and new detrital zircon U-Pb data from Miocene to modern sediments across all sectors. The Chitwan Basin section spans the lower, middle, and upper Siwalik Group and detrital zircon U-Pb geochronology supports sourcing from the Tethyan, Greater, and Lesser Himalaya throughout. Though we are still working on age control, an excursion in the δ¹³C of pedogenic carbonates from strongly negative values (median= -9.8‰) towards less negative values (median= -4.9‰) at 725 meters near the lower to middle Siwalik boundary is interpreted to represent the late Miocene expansion of C₄ grasses, which occurs at ~7 Ma across Nepal. However, unlike most Siwalik Group sections, δ¹³C of pedogenic carbonates rapidly returns towards more negative values (median= -9.9‰) supporting a return towards C₃-dominant vegetation. Quartzite clasts appear in the upper Siwalik Group at ~1950 meters, attributed to slip on the Main Boundary thrust, dated in proximal sections to ca. 4 Ma, with an associated increase in ca. 1.8 Ga detrital zircons. Clay mineralogy and whole rock geochemistry of paleosol B-horizons, along with paleosol morphology, support an increase in chemical weathering in the upper Siwalik Group, broadly consistent with hypothesized rapid erosion of the Narayani drainage basin at this time (post-5 Ma). Future work using lag time thermochronology will allow evaluation of changes in erosion rates across this interval.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8925

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