Exhumation of early Tertiary, coesite-bearing eclogites from the Pakistan Himalaya

Early Tertiary, coesite-bearing eclogites crop out in the internal zones of the Indian Plate in the Kaghan Valley of North Pakistan. Field and geochronological data suggest that UHP metamorphism, at c. 46 Ma, accompanied subduction of the leading edge of continental India beneath Kohistan. A new U–Pb rutile age of 44.1 ± 1.1 Ma can be interpreted as either a growth or a cooling age, given the uncertainty of the closure temperature of rutile for Pb, but clearly documents a very short-lived metamorphic peak, with cooling occurring between 44 and 40 Ma. Eclogitic assemblages are preserved in Permo-Triassic mafic rocks in both cover and basement sequences, with the best-preserved eclogites hosted in the anhydrous basement. The eclogite-facies rocks lie on the hanging wall of a major south-directed ductile thrust zone, which placed basement rocks that contain coesite eclogites southward onto lower-pressure cover metasediments. Shear criteria show that those sequences that contain eclogite-facies rocks are flanked above by a zone of pervasive north-directed extensional shearing. Mineral assemblages contained within both the thrust and extensional shear zones are consistent with them both having operated synchronously during the amphibolite- to greenschist-facies transition. Amphibole Ar–Ar cooling history data suggest that this was at c. 40–42 Ma. As in the Alps this implies exhumation from the metamorphic peak to greenschist-facies conditions within a few million years. This short time scale has important implications, as it is likely that both the thrust and extensional shear zones represent the late stage of a deformation continuum that commenced at UHP and brought the UHP rocks back to ‘mid’-crustal levels. This late Eocene extensional shearing, and associated exhumation of HP or UHP rocks, has not previously been recorded in the Pakistan Himalaya. The thrusts and shears were later folded by major east-trending folds, themselves cut by Miocene-aged, north-vergent extensional faults that currently separate the Indian Plate from the structurally overlying Kohistan island arc.