Along-Strike Variability in Erosion of the Nepalese Himalayan Thrust Belt

Abstract:

We present 22 apatite fission track (AFT) and 82 single grain zircon He (ZHe) ages for thirty samples from Greater Himalayan (GH) and Lesser Himalayan (LH) rocks and AFT ages for two detrital samples from the Kaligandaki River and Modi Khola in central and western Nepal. At regional scale ages decrease from central to western Nepal. Ages in central Nepal are < 5 Ma with many < 2 Ma, whereas ages west of the Kaligandaki are 5-14 Ma. The sand samples are dominated by 8-6 Ma detrital AFT age populations. Thermal modeling of AFT and ZHe ages in western Nepal shows an episode of rapid cooling between 12 and 8 Ma during emplacement of the Ramgarh thrust. Rocks from lower structural positions have younger (< 8 Ma) ages, consistent with growth of the Lesser Himalayan duplex. Old cooling ages in western Nepal correlate with lower erosion rates, drier climate, and an upward-convex to gently sloping topographic profile. Precipitation in western Nepal is concentrated at the front of the high-grade, GH klippen, which orographically shelter the thrust belt to the north. Younger ages in central Nepal instead correlate with higher erosion rates, northward penetration of intense monsoonal precipitation into the interior of the range, and development of an upward-concave topographic profile indicative of relatively intense erosion. This region lacks GH rocks south of the trace of the Main Central thrust, and the area between the Main Boundary and Main Frontal thrusts is lower than 1 km allowing precipitation to penetrate farther north. Higher erosion in central compared to western Nepal is also supported by the geology: resistant GH rocks in southern central Nepal are limited to the Kathmandu klippe, whereas in western Nepal they cover a larger area in the Dadeldhura and Jajarkot klippen. Cooling ages in western Nepal are interpreted to date the timing of thrust-sheet emplacement as supported by a southward decrease of ages. In central Nepal the correlation between intense precipitation and young cooling ages suggests transient climate-enhanced headward erosion and is supported by a northward decrease of ages. We suggest that in central Nepal the cooling signature related to emplacement of the Main Central and Ramgarh thrusts as well as growth of the Lesser Himalayan duplex has been removed by recent erosion.