Pressure‐temperature‐time paths and the relationship between collision, deformation and metamorphism in the north‐west Himalaya

Abstract

AbstractTertiary collision in the NW Himalaya was between the Indian plate and the Kohistan island arc, the leading edge of the Indian plate being subducted beneath the arc along the Main Mantle Thrust (MMT). Porphyroblast‐matrix relationships show that metamorphism within Indian plate cover sediments exposed on the footwall of the MMT was synchronous with ductile shearing which marks the main deformation phase. The P‐T‐t paths, derived from inclusion assemblages and forward modelling of garnet zonation patterns, show that this metamorphism was along a path of increasing pressure. These data are consistent with Indian plate metamorphism occurring during active subduction beneath the Kohistan island arc. Geochronological data date the metamorphic peak as pre‐50 Ma. As initial collision cannot have pre‐dated 65 Ma, there was thus insufficient time for the long time‐scales of metamorphism generally predicted by conductive models of post‐thickening thermal relaxation. The cover sediments show a metamorphic inversion, argued previously as the result entirely of post‐metamorphic thrusting and imbrication. By contrast, basement gneisses exposed beneath the MMT have largely been unaffected by Tertiary metamorphism. The relationship between ductile shearing and metamorphism on the footwall of the MMT during active subduction, and the concentration of the highest metamorphic grades in the stratigraphically youngest rocks, suggests that early Tertiary metamorphism was controlled by dissipative shear heating distributed across the uppermost cover sediments of the subducting Indian plate. This generated a primary, inverted metamorphic sequence shortly after collision, an inversion reinforced by subsequent thrusting associated with accretion of the metamorphic pile onto the base of the over‐riding arc. This conclusion stresses the importance of dissipative shear heating in controlling metamorphism within thrust systems and provides a mechanism by which areas of apparently large regional extent can undergo regional metamorphism over short time‐scales, but in the absence of any magmatic heating.