Thermobarometry and geochronology of the Himachal Himalaya, NW India: Inverted metamorphism along the Sutlej and Pabbar Valleys

Understanding the spatial distributions of peak metamorphic P-T-t conditions helps constrain models of orogenesis. In this study, we quantified P-T conditions in Himachal Pradesh in the NW Indian Himalaya, along the Sutlej River valley and nearby Pabbar valley. From structurally lowest to highest, these rocks consist of the Lesser Himalayan Sequence (LHS), the Munsiari Sequence (MS), the Greater Himalayan Sequence (GHS), and the Tethyan Himalayan Sequence (THS). The Munsiari thrust (MT) emplaces MS over LHS, while the Main Central thrust (MCT) emplaces GHS locally over MS or LHS. In areas along strike, the South Tibetan detachment system (STDS) drops the THS down onto GHS, but the STDS is not noticeably exposed along the Sutlej or Pabbar valleys.

In this study, we calculated P-T conditions using garnet-biotite thermometry and garnet-plagioclase barometry from the upper ~1 km thickness of the MS, through the GHS (~10 km), and into the basal ~9 km of the THS. P-T conditions increase abruptly at the MCT, from ~600 °C and ~9 kbar in the upper MS to ~750 °C and ~12 kbar in the lower 2 km of the GHS (c. 100 °C/km; 1 kbar/km). P-T conditions then increase to ~800 °C and ~14 kbar in the middle of the GHS (c. 15 °C/km; ~0.5 kbar/km), and then decrease consistently to ~500 °C and ~5 kbar in the highest level of the THS analyzed (c. 25 °C/km; 0.75 kbar/km). Upper GHS and THS data are sparsely distributed, so we cannot rule out a metamorphic discontinuity across the STDS. However, our data are equally consistent with flattening strain distributed through the upper GHS and lower THS without a distinct STDS, unlike all other transects in the Himalaya farther east for >1500 km. Previously published U-Pb titanite ages from the THS indicate peak metamorphism until ~22 Ma, while new zircon ages from a GHS migmatite indicate rapid cooling since ~21 Ma, similar to many transects to the east. Additional geochronology is needed to delineate prograde and high-temperature cooling patterns across the GHS and THS.