Cenozoic remobilization of the Cambro-Ordovician proto-tectonic terrane boundary, South Tibetan Detachment System (STDS) in the NW Himalaya

Excellently exposed along the Dhauliganga valley, Uttarakhand, the STDS is a syn-contractional Low Angle Normal Fault (LANF) that separates the sillimanite grade Greater Himalayan Sequence (GHS) gneiss from the low to unmetamorphosed Tethyan Himalayan Sequence. The Early Paleozoic Gneissose granitoids intrude the high-grade mylonite-gneiss on the immediate footwall of the STDS. These granitoids are a part of an extensive Cambro-Ordovician (C-O) granite belt in the uppermost GHS between Sutlej River in Himachal Pradesh and Dhauliganga River in Uttarakhand.

The U-Pb zircon geochronology of gneissose granites produces crystallization ages ranging from 498.92 ± 2.7 Ma to 486.54 ± 1.1 Ma. The Th/U ratios (0.1-2.1) indicate that the Neoproterozoic (839.36 ± 2.5 Ma, 814.00 ± 3.3 Ma) and Mesoproterozoic (1900.00 ± 2.01 Ma) cores are magmatic in origin. Their Paleozoic rims show metamorphic character with Th/U ratios from 0.02 to 0.07. Paleozoic zircon cores (470 to 520 Ma) exhibiting a higher Th/U ratio (0.2-0.8) imply a prominent granite emplacement activity along the paleo-STDS during the Paleozoic.

Further, the STDS has witnessed a prolonged Cenozoic reactivation, controlling the decompression melting and leucogranite emplacement. The zircon growths are recorded in two distinct modes: the pulsative mode from Upper Eocene to Lower Oligocene, with peaks at 35, 31 and 25 Ma and the continuous mode, from 23 Ma to 13 Ma, in the Miocene.

New geochronological data postulates that the STDS, developed as a proto-tectonic extensional terrane boundary during the C-O Kurgiakh/Bhimphedian Orogeny, facilitated the prolific granite emplacement. This event also resulted in the footwall denudation and initiation of the Tethyan Basin. This study also records one of the oldest pulsative Cenozoic initiations during Upper Eocene and subsequent continuous activity till an abrupt cessation at 13 Ma.

These new inferences not only attempt to discord the previous opinions on the Cenozoic STDS activity but also suggest that the Cenozoic tectonics is a reactivation event of the Paleozoic tectonic activity along the STDS. Further geochemical and geochronological study is imperative to understand the precise tectonism and exhumation along the STDS.