| Summary: | The Suru Valley, NW India, is unique in the Himalayan mountain belt in preserving a widespread pressure-temperature-time-deformation (P-T-t-D) record of the early part of the collisional orogenic cycle, characterised by pervasive crustal thickening and Barrovian metamorphism. In addition, widespread magmatism and metamorphism (ca. 505–453 Ma) occurred on the Indian margin during the Bhimphedian orogeny and subsequent localised magmatism during the Permian (ca. 282-268 Ma). These records are deciphered by integrating petrographic and structural analysis, phase equilibrium modelling, and U-(Th)-Pb geochronology. During the India-Asia collision, the Indian margin experienced SW-vergent thrusting and folding (D1 and D2) with greenschist- to amphibolite-facies metamorphism. Mineral isograd patterns and the relationship between porphyroblast growth and fabric development suggest the thermal peak of metamorphism outlasted deformation within the Himalayan metamorphic core (HMC). Deformation and metamorphism propagated SW-wards with decreasing structural level and increasing metamorphic grade. Prograde conditions, syn-D2, are dominantly attained between ca. 38–30 Ma. Peak M1 conditions are recorded syn- to post-D2 between ca. 37–27 Ma. Average metamorphic field gradients are 36 °C/kbar during the early stages of metamorphism, with progressive M1 peak attained at 47 °C/kbar. In the deepest structural levels of the HMC, decompression initiates after ca. 28 Ma on a near-isothermal path towards sillimanite-grade conditions and peak M2 is recorded across grades between ca. 25–21 Ma, coincident with extension on the Zanskar Shear Zone (ZSZ, local equivalent of the South Tibetan Detachment) at around ca. 23 Ma. These temporal constraints and limited recorded displacement along the ZSZ suggest unroofing through erosion may first drive metamorphic decompression, prior to main phase of movement on the ZSZ. Consistent passive margin-stratigraphy, an inter-tectonic thermal peak, thick and melt-poor GHS, and minor D3 development and ZSZ displacement are not supported by a channel flow model for exhumation of the HMC.
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