T108 Advances and Applications of Thermochronology in Tectonic, Magmatic, Basin, and Geomorphic Studies II

Monday, 10 October 2022
1:30 PM - 5:30 PM
601

THE EXHUMATION HISTORY OF THE GREATER HIMALAYAN SEQUENCE, CENTRAL NEPAL

Paper number: T108-6
Program: Technical Sessions
Time: 2:35 PM – 2:50 PM

Authors

Abstract

One of the fundamental questions faced by the geosciences today is the relative influences of climate and tectonics on how landscapes have evolved through geologic time. Understanding the spatial and temporal variation in the exhumation history across a tectonically active mountain range can help shed some light on this problem.

The Himalaya is one of the planet’s most useful natural laboratories to work on this issue. The South Tibetan detachment system (STDS), a collection of Oligocene-Miocene aged normal sense structures, stretches over 2000 km across the entire range and outcrops at roughly the location of a dramatic topographic and orographic transition. The Annapurna region in central Nepal is home to excellent exposures of these structures, as well as footwall rocks rich in accessory minerals allowing detailed thermochronometric and geochronometric studies.

Here we present new results from thermochronology transects in the footwall of basal structures of the STDS in both the Kali Gandaki and Marsyandi drainages (the Annapurna and Chame detachments). New constraints based on (U-Th)/Pb dating of accessory minerals in variably deformed leucogranites within the Annapurna and Chame detachment zones are consistent with the interpretation that the two are corelative and were active simultaneously during the mid-Miocene.

One dimensional inverse thermal-kinematic modeling using Pecube-D of 40Ar/39Ar muscovite and biotite, U/Pb titanite, and (U-Th)/He titanite, zircon and apatite dates reveal a decrease in exhumation rate at ~12 Ma followed by a dramatic increase at ~3 Ma in both drainages. Given the temporal agreement with the age of deformation on the Annapurna detachment, we suggest that this initial decrease in exhumation rate was due to cessation of slip on the Chame and Annapurna detachments. The timing of the dramatic ~ 3 Ma increase in exhumation rate coincides with the initiation of the modern Indian summer monsoon and the increase of the East Asian winter monsoon.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-380519

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