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Theory suggests the possibility for significant deviations between total pressure (or dynamic pressure) and lithostatic pressure during crustal metamorphism. If such deviations exist, the implications for orogenic reconstruction would be profound. Whether such non-lithostatic pressure conditions during crustal metamorphism are recorded and preserved in the rock record remains unresolved, as direct field evidence for this phenomenon is limited. Here, we investigate the Paleogene Tethyan Himalaya fold-thrust belt in Himachal Pradesh, northwestern India, which is the structurally highest part of the Himalayan orogen and deforms a ~10–15 km thick Neoproterozoic–Cretaceous passive margin stratigraphic section. Field-based kinematic studies demonstrate relatively moderate shortening strain across the Tethyan Himalaya. However, basal Tethyan strata consistently yield elevated pressure-temperature-time (P-T-t) estimates of 7–8 kbar and ~650°C, indicative of deep burial during Himalayan orogeny (ca. 20–45 Ma, 25–30 km depths). These P-T-t conditions can be reconciled by: (1) deep Cenozoic burial along cryptic structures and/or significant flattening of the Tethyan strata; (2) basal Tethyan strata recording metamorphism and deformation related to pre-Himalayan tectonism; or (3) non-lithostatic pressure conditions (i.e., tectonic overpressure). To test these models, we systematically mapped the Tethyan fold-thrust belt along the Pin Valley transect in northwestern India, a classic site for stratigraphic, paleontological, paleoenvironmental, and structural reconstructions. The Pin Valley region provides an opportunity to study a structurally continuous metamorphic field gradient from the near-surface to structural depths between 10–15 km, which should reflect P conditions ≤4 kbar if lithostatic. We integrate a multi-method approach combining detailed geologic mapping with quantitative analytical techniques (e.g., thermometry, finite strain analyses, thermo/geochronology, and thermobarometry) to quantify the magnitude, kinematics, thermal architecture, and timing of regional deformation, metamorphism, and subsequent exhumation. Results show: (1) throw on shortening structures is moderate to low (≤4 km); (2) temperature-depth relationships record a continuous, but regionally elevated, upper-crustal geothermal gradient of ≥40 °C/km, which is inconsistent with deep burial models (≤25 °C/km); (3) minimal flattening of basal Tethyan strata; (4) upper Tethyan strata yield pre-Himalayan low-temperature thermochronology dates, further refuting deep Cenozoic burial; and (5) basal Tethyan P-T-t estimates confirm elevated mid-crustal conditions of ~7 kbar, 630°C at 10–15 km depths during the Cenozoic. Preliminary volume expansion calculations are minimal; therefore, mechanisms involving non-hydrostatic thermodynamics, deviatoric stresses, rock strength contrasts, and tectonic mode switching are being explored. 

Theory suggests the possibility for significant deviations between the total pressure (or dynamic pressure) and lithostatic pressure throughout Earth’s crust. Whether such non-lithostatic pressure conditions are recorded and preserved in the rock record remains unresolved, as direct field confirmation is limited, yet the implications for orogenic reconstruction are profound. Here we investigate the Paleogene Tethyan Himalaya fold-thrust belt in Himachal Pradesh, NW India, which is the structurally highest part of the Himalayan orogen and deforms a ~10–15 km thick Neoproterozoic–Cretaceous passive margin stratigraphic section. Field-based kinematic studies demonstrate relatively moderate shortening strain estimates across the Tethyan Himalaya, yet basal Tethyan strata consistently yield elevated pressure-temperature-time (P-T-t) estimates of 7–8 kbar and ~650°C, indicative of deep burial during Himalayan orogeny (25–30 km depths). These P-T-t conditions can be reconciled by: (1) deep Cenozoic burial along cryptic structures and/or significant flattening of the Tethyan strata; (2) basal Tethyan strata recording pre-Himalayan deformation related to Pan-African orogeny; or (3) non-lithostatic pressure conditions (i.e., tectonic overpressure). To test these models, we systematically mapped the Tethyan fold-thrust belt along the Bhaba Pass-Pin Valley transect in NW India, a classic site for stratigraphic, paleontological, paleoenvironmental, and structural reconstructions. We integrate a multi-method approach combining detailed geologic mapping with quantitative analytical techniques (e.g., finite strain analyses, thermometry, thermobarometry, thermochronology, and geochronology) to quantify the magnitude, kinematics, thermal architecture, and timing of regional deformation, metamorphism, and subsequent exhumation of the Tethyan fold-thrust belt. Our preliminary observations refute deep Cenozoic burial of the Tethyan Himalaya, suggesting either the preservation of non-lithostatic pressures in the rock record or relicts of pre-Himalayan metamorphism. Either scenario demonstrates that caution is required in using Himalayan P-T-t estimates to reconstruct the Cenozoic Himalayan orogeny. 

Tectonic models for the development of the Himalaya, Earth's largest active collisional mountain belt, have been developed and tested through pressure, temperature, and time (P-T-t) information collected from exposed metamorphic rocks. Inferred deep burial and subsequent exhumation of these rocks are usually justified by observable structures (e.g., Main Central thrust) and mapping relationships. However, regions where pressure estimates are at odds with field-based reconstructions are reconciled with hypothesized cryptic structures that have since been completely eroded. Such field versus thermobarometric discrepancies significantly impact interpretations on the geometry, magnitude, and distribution of deformation. Here, we conducted detailed field mapping of the Paleogene Tethyan fold-thrust belt in the Himachal Himalaya, NW India, which is the structurally highest part of the Himalayan orogen and deforms a ~10–15 km thick Neoproterozoic–Cretaceous passive margin section. In this region, P-T estimates yield 6–8 kbar and ~650°C, which suggests burial to depths of ~25–30 km. To assess the viability of this deep burial, we constructed a 1:200,000 scale geologic map of the Bhaba Pass-Pin Valley region. Geologic mapping was focused on the stratigraphy, structural configuration, and metamorphic isograds of the basal Tethyan strata. Detailed field mapping aided the construction of balanced cross sections, which guided subsequent multi-method analytical approaches that fit into a coherent structural framework. Our field observations and map relationships show no major structures, abrupt changes in metamorphic grade or composition that would suggest deep burial of the stratigraphically continuous basal Tethyan group. Balanced cross sections throughout the study area suggest moderate amounts of shortening strain (~30–36%). This contribution highlights the importance of detailed field mapping to interpret P-T estimates. Ongoing analytical methods are being conducted to constrain the thermal architecture and metamorphic history of the Tethyan fold-thrust belt. 

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.