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TitleSubduction metamorphism in the Himalayan ultrahigh-pressure TsoMorari massif: An integrated geodynamic and petrological modelling approach
AuthorPalin, R M; Reuber, G; White, R W; Kaus, B J P; Weller, O M
SourceEarth and Planetary Science Letters vol. 467, 2017 p. 108-119,
Alt SeriesEarth Sciences Sector, Contribution Series 20150475
Mediapaper; on-line; digital
File formatpdf
AreaTso Morari; India
Lat/Long WENS 77.0000 79.0000 34.0000 33.0000
Subjectsstructural geology; tectonics; regional geology; subduction zones; tectonic evolution; metamorphism; tectonostratigraphic zones; modelling; metamorphism, prograde; crustal studies; continental crust; eclogites; overpressure; metastability
Illustrationsphase diagrams; geological sketch maps; geophysical profiles
ProgramBaffin Bedrock Mapping, GEM2: Geo-mapping for Energy and Minerals
AbstractThe Tso Morari massif is one of only two regions where ultrahigh-pressure (UHP) metamorphism of subducted crust has been documented in the Himalayan Range. The tectonic evolution of the massif is enigmatic, as reported pressure estimates for peak metamorphism vary from 2.4 GPa to 4.8 GPa. This uncertainty is problematic for constructing large-scale numerical models of the early stages of India-Asia collision. To address this, we provide new constraints on the tectonothermal evolution of the massif via a combined geodynamic and petrological forward-modelling approach. A prograde-to-peak pressure-temperature-time (P-T-t) path has been derived from thermomechanical simulations tailored for Eocene subduction in the northwestern Himalaya. Phase equilibrium modelling performed along this P-Tpath has described the petrological evolution of felsic and mafic components of the massif crust, and shows that differences in their fluid contents would have controlled the degree of metamorphic phase transformation in each during subduction. Our model predicts that peak P-Tconditions of 2.6-2.8 GPa and 600-620degC, representative of 90-100 km depth (assuming lithostatic pressure), could have been reached just 3 Myr after the onset of subduction of continental crust. This P-Tpath and subduction duration correlate well with constraints reported for similar UHP eclogite in the Kaghan Valley, Pakistan Himalaya, suggesting that the northwest Himalaya contains dismembered remnants of what may have been a 400-km-long UHP terrane comparable in size to the Western Gneiss Region, Norway, and the Dabie-Sulu belt, China. A maximum overpressure of 0.5 GPa was calculated in our simulations for a homogeneouscrust, although small-scale mechanical heterogeneities may produce overpressures that are larger in magnitude. Nonetheless, the extremely high pressures for peak metamorphism reported by some workers (up to 4.8 GPa) are unreliable owing to conventional thermobarometry having been performed on minerals that were likely not in equilibrium. Furthermore, diagnostic high-Pmineral assemblages predicted to form in Tso Morari orthogneiss at peak metamorphism are absent from natural samples, which may reflect the widespread metastable preservation of lower-pressure assemblages in the felsic component of the crust during subduction. If common in such subducted continental terranes, this metastability calls into question the reliability of geodynamic simulations of orogenesis that are predicated on equilibrium metamorphism operating continuously throughout tectonic cycles.
Summary(Plain Language Summary, not published)
The Tso-Morari massif is located in India in the Himalaya mountain range. The massif exposes rocks that were buried to over 80 km and then exhumed to the surface during the formation of the Himalaya. Such rocks are relatively rare at the Earth¿s surface, and studying them yields important insights into processes related to the formation of mountain belts. Currently estimates of the exact depth that the rocks reached vary from ~80 to 150 km. In this study, a new approach to modeling the rock¿s history is applied, which suggests that the lower bound to this range is the most appropriate. The results have important implications for models of the formation of the Himalaya, and more generally for understanding how rocks are transformed (metamorphosed) during mountain building events.