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The Tethyan Himalaya (TH) fold-thrust belt comprises a deformed Neoproterozoic-Cretaceous section of sedimentary rocks that record the early stages of deformation of the Himalayan orogen. In the northwestern Himalaya, rocks at the base of the TH are metamorphosed and are useful for reconstructing the thermal evolution of the Himalaya during initial stages of crustal thickening. Here, we present results of multi-method thermobarometry (thermodynamic modelling, Si in white mica barometry, quartz in garnet barometry, raman spectroscopy of carbonaceous material (RSCM) thermometry) on metasedimentary samples from two transects across the TH, with apparently continuous stratigraphy separated along strike of the orogen by ~40 km. Samples from the Pin Valley region record peak pressure-temperature (P-T) conditions of 0.4-0.5 GPa, 600 °C, suggesting a paleo-geothermal gradient of 30-40 °C/km. These samples are from the base of a continuous ~10-12 km-thick TH section in which the stratigraphically highest units are undeformed, fossil-bearing sedimentary rocks. RSCM thermometry on samples from stratigraphically higher levels of the TH suggest a continuous ~40 °C/km geothermal gradient through the entire TH section in the Pin Valley region. In contrast, previous thermobarometric studies from the Sutlej Valley ~40 km to the east report peak P-T conditions of 0.7-0.8 GPa, 600-650 °C, suggesting a paleo-geothermal gradient of 20-25 °C/km. Our new data indicate significant along-strike variation in peak P-T conditions and paleo-geothermal gradients at the base of the TH. Possible explanations for this along-strike thermobarometric discrepancy include: 1) pre-Himalayan metamorphic assemblages preserved in the TH resulting in erroneous Himalayan peak P-T estimates, 2) along-strike structural differences that resulted in differential burial and exhumation during Himalayan orogenesis, or 3) non-lithostatic pressure during orogenesis. Thermobarometric work on samples from different stratigraphic levels of the basal TH in the Sutlej Valley is in progress to determine paleo-geothermal gradient continuity both across- and along-strike of the orogen. 

The thermal conditions during orogenesis exert first-order control on the style, magnitude, and extent of deformation. The Eocene Tethyan Himalaya (TH) thrust belt is the structurally highest part of the Himalayan orogen and deforms a ~10-km thick Neoproterozoic–Cretaceous stratigraphic section. The Pin Valley region preserves the northernmost exposed TH in the Himachal Himalaya, NW India, and is a classic site for stratigraphic, paleontological, paleoenvironmental, and structural reconstructions. The base of the TH in Pin Valley records minor garnet-grade metamorphism and relatively undeformed fossils throughout the middle to upper TH. However, thermobarometric data from the basal TH along the structurally continuous Sutlej Valley to the east (<20 km map distance) is consistently 7-8 kbar, indicative of deep intra-orogen burial to 26–30 km depths in the Eocene, which is inconsistent with structural and stratigraphic observations in Pin Valley. Ongoing geothermobarometry estimates and Ar thermochronology from Pin Valley are being conducted to constrain the timing and pressure of peak metamorphic conditions. Here, we integrate structural observations and geologic mapping, Raman spectroscopy of carbonaceous material (RSCM) thermometry, detrital zircon geochronology, and Ar thermochronology to place constraints on the geometry, kinematics, stratigraphy, and thermal structure along the Pin Valley transect. This, in turn resolves the viability of deep burial of the TH along the Sutlej Valley. Important observations show: (1) detrital zircon geochronology along the Pin Valley transect shows strong correlation with regional TH strata, which will be further compared with the TH section along the Sutlej Valley; and (2) temperature-depth relationships record a regionally elevated, but continuous, geothermal gradient (40 °C/km), which is inconsistent with gradients predicted by P-T estimates along the Sutlej Valley (≤25 °C/km). Preliminary results show no evidence for large magnitude burial of the upper crust, suggesting limited thickening of the Tethyan Himalaya thrust belt.