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Here, we use multi-method thermobarometric analyses (thermodynamic modelling, quartz in garnet barometry, Raman spectroscopy of carbonaceous material (RSCM) thermometry, and titanium in biotite thermometry) from samples throughout two transects in the Northwestern Tethyan Himalaya (TH) to constrain the pressure-temperature conditions of the basal TH. Peak metamorphic conditions from the basal TH indicate anomalously high pressures relative to the paleogeothermal gradients recorded along the two transects, suggesting non-lithostatic pressure conditions at the base of the Tethyan Himalaya. The 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. RSCM thermometry on samples along the Pin Valley and Sutlej Valley transects of the TH suggest a continuous ~1500 °C/GPa thermobarometric gradient through the entire TH section. These samples are from a continuous ~10-12 km-thick TH section in which the stratigraphically highest units are undeformed, fossil-bearing sedimentary rocks. Assuming lithostatic pressure, the basal TH is expected to record peak pressure-temperature (P-T) conditions of ~0.4-0.5 GPa and ~600 °C. However, quartz-in-garnet (QuiG) barometry and titanium-in-biotite thermometry of samples from the basal TH indicate peak P-T conditions of 0.94 ± 0.25 GPa and ~600°C, suggesting a paleo-geothermal gradient of 870-500 °C/GPa. These data constitute unexpectedly high peak pressure conditions along the basal TH. Possible explanations for these anomalously high basal TH pressures include pre-Himalayan metamorphic assemblages preserved in the TH resulting in erroneous Himalayan peak P-T estimates, or regional non-lithostatic pressure along the basal TH during Himalayan 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.
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This content will become publicly available on December 4, 2026
Optimizing methods for Raman spectroscopy of carbonaceous material thermometry: Best practices to develop high-fidelity thermal constraints for tectonic studies
There is a variety of published sample preparation and data acquisition techniques for Raman spectroscopy of carbonaceous material (RSCM) thermometry, which complicates systematic evaluation, assessment, and comparison of RSCM thermometry datasets. In particular, many modern studies are applying large-n RSCM analyses, acquiring high numbers of RSCM temperature estimates, often systematically distributed along transects, to quantify regional thermal structure or spatial temperature gradients associated with geologic structures. Given the significance of RSCM analyses to address numerous geologic questions in a variety of tectonic settings, it becomes imperative to develop a reproducible, standardized, and optimized workflow for RSCM analyses. Here, we introduce and test protocols for RSCM analyses using samples from the Papoose Flat pluton contact aureole, eastern California, western United States, to determine the reproducibility of automated peak-fitting and RSCM temperature estimation programs. Our RSCM results align with previously estimated temperatures derived from calcite-dolomite isotope exchange thermometry, phase equilibria temperature estimates, and quartz c-axis fabric opening-angle thermometry. Temperature estimates derived from two automated peak-fitting programs (i.e., Iterative Fitting of Raman Spectra software [IFORS] and AutoRaman_K2024) are comparable within analytical error. The RSCM temperature estimates were further validated with simple thermal modeling of pluton heat diffusion. Based on these data, we tested sample preparation and data acquisition parameters to optimize large-n workflows. Coupling large datasets at multiple structural levels with supplementary thermal models of varying complexity allows RSCM thermometry to serve as a robust method for tectonic studies.
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- Award ID(s):
- 2210074
- PAR ID:
- 10651570
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geosphere
- ISSN:
- 1553-040X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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