242-15 Geochronology and thermobarometry in the Sutlej Valley of the Himachal Himalaya, NW India: Insights into the “missing” South Tibetan Detachment System

Abstract:

Understanding the spatial distributions of peak metamorphic P-T-t conditions helps constrain models of orogenesis. Here, we quantified P-T conditions and peak metamorphic ages in Himachal Pradesh, NW India, along the Sutlej River valley and the nearby Pabbar valley. From structurally lowest to highest, metamorphic rocks consist of the Lesser Himalayan Sequence (LHS), the Greater Himalayan Sequence (GHS), and the Tethyan Himalayan Sequence (THS). Within the LHS, the Jutogh thrust emplaces Jutogh Group rocks over LHS, while the Main Central thrust (MCT) emplaces GHS over LHS. The South Tibetan Detachment System (STDS), a north-dipping normal fault system, drops the THS down onto GHS, but is not noticeably exposed in the study area. We calculated peak PT conditions using garnet-biotite thermometry and garnet-plagioclase barometry; we also measured peak ages using monazite U-Th-Pb, and coupled them to petrogenetic and P-T history through REE concentrations.

Within Jutogh Group rocks, P-T conditions increase upward towards the MCT, from ~575 °C and ~10 kbar at ~500 meters below the MCT to ~750 °C and ~12 kbar in the ~200 meter-thick section of uppermost Jutogh Group and lowermost GHS. Along the Sutlej Valley, PT conditions increase from ~600 °C and 9 kbar at the MCT to ~825 °C and ~13 kbar in the middle of the GHS, then decrease steadily through the upper GHS to ~500 °C and ~5 kbar into the THS.  Lower GHS monazite gradually increases in HREE content from 40 to 25 Ma. Monazite from upper GHS migmatites shows low HREE from 29 to 23 Ma, then an abrupt increase in HREE to 21 Ma. THS monazite shows no trend of HREE, with ages spreading between 29 and 25 Ma.

P-T conditions are consistent with large lateral transport along the MCT (abrupt increases in P and T), but not across the GHS-THS contact; these data could be consistent with flattening strain distributed through the upper GHS and lower THS without a distinct STDS. Chronologic data suggest protracted high-T within the middle GHS from 40 to 25 Ma, and abrupt cooling and in situ melt crystallization in the upper GHS commencing ~23 Ma. This cooling could reflect either STDS movement or a flattening event. THS chronologic data are, as yet, too sparse for definitive interpretation.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-20902

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