EXHUMATION OF EARLY TERTIARY, COESITE-BEARING ECLOGITES FROM THE KAGHAN VALLEY, PAKISTAN HIMALAYA

The preservation of coesite in ultra high pressure eclogites now at surface raises questions about the structural mechanisms by which they were exhumed. As UHP rocks metamorphosed early in an orogenic cycle probably come to surface through more than one exhumation phase, often few structures remain that relate to the early part of the exhumation path that brought them to "mid"-crustal depths. Here, part of that early path is revealed for coesite-bearing eclogites from the Indian Plate internal zones of N. Pakistan. Metamorphism, at 725 ± 25°C, 28 - 30 kbar, at c. 50 Ma, shortly post-dated subduction of the leading edge of continental India beneath Kohistan.

Structural restorations, field and petrographic data, show the eclogite-bearing rocks are flanked by thrusts below and extensional shears above and that thrusting, extension and the amphibolite to greenschist facies transition were synchronous. The eclogites lie immediately above a top-side-S thrust, S-C' fabrics related to which are overprinted by greenschist facies albite porphyroblasts. The dominant microstructures, though, are S-C' shear bands, penetratively developed throughout the eclogite facies metasedimentary sequences, which document a phase of pervasive top-side-N extension. Hornblende crystals locally parallel stretching lineations related to this extension. Albite porphyroblasts up to 10 cm in diameter overgrow extension related fabrics, although elsewhere they are flattened and stretched within those fabrics.

Top-side-S thrusting predated cooling through the amphibolite greenschist transition at 40 Ma, at which time the pervasive top-side-N extension was operating. This implies exhumation from metamorphic peak to greenschist facies within a few million years. It is likely that both the thrust and extensional sense shear zones represent the late-stage of a deformation continuum which commenced at UHP and brought the eclogites back to "mid" crustal levels. Subsequent Miocene extension brought the rocks near to surface.