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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.
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Reconstructing Thermobarometric Gradients in the Northwestern Tethyan Himalaya: Testing the Lithostatic Pressure Paradigm
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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- Award ID(s):
- 2210075
- PAR ID:
- 10608790
- Publisher / Repository:
- Geological Society of America Abstracts with Programs
- Date Published:
- Volume:
- 56
- Issue:
- 1
- Format(s):
- Medium: X
- Location:
- Manchester, NH
- 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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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.more » « less
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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.more » « less
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1130/abs/2024NE-397336
