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The Tethyan Himalayan sequence (THS) is the structurally highest lithotectonic unit of Indian affinity within the Cenozoic Himalayan orogen. In the NW Himalaya of the Himachal Pradesh, India, the Neoproterozoic–Cretaceous THS is thought to have relatively modest deformation despite the unit commonly recording early collision-related shortening. This lack of significant deformation contrasts that of other Himachal lithotectonic units closer to the foreland. In addition, burial depths of the Himachal THS estimated from structural reconstructions (~10 km) and basal metamorphic pressures (7–8 kbar, ~28 km lithostatic burial) conflict. To address these issues, we performed geologic mapping, thermochronology, and restored new balanced cross sections along two transects across the Himachal THS to better constrain its deformation state and timing, stratigraphic thickness, and burial extent. Along the Spiti and Pin valleys, the THS is shortened by seven NE-dipping thrusts and one SW-dipping thrusts that mostly form fault-propagation folds. The Mata Nappe region (NE of Spiti Valley) has been reinterpreted as a thrust pop-up structure, consistent with structural observations. Along this transect, the estimated THS thickness measured from the basal Akpa granite and Haimanta Group to the uppermost-exposed Tandi Group is ~12.3 km. Restoration of one cross section along this transect yields a minimum shortening of ~30 km (~22% strain). Farther SE along Sutlej Valley, the THS is cut by three SW-dipping thrusts and several S-dipping normal faults. The estimated thickness of the exposed Akpa granite and Haimanta Group is ~8.5 km. Restoration of one cross section along this transect yields a minimum shortening of ~8 km (~21% strain). Thrusts mapped along both transects are interpreted to branch from a single decollement formed by the South Tibet detachment and Great Counter thrust. Our THS shortening estimates added to those for other Indian rocks in the Himachal Himalaya (Webb, 2013) yields a total minimum estimate of ~515–537 km. Preliminary zircon (U-Th)/He dates along Spiti and Pin valleys generally young towards the SW from ca. 42–5 Ma. These results confirm: (1) relatively minor shortening of the Himachal THS that was likely compensated by duplexing of other units; and (2) the discrepancy between THS burial estimates, which may be a product of non-lithostatic pressure. 

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.