TWO-PHASE EXHUMATION OF ULTRA HIGH-PRESSURE AND MEDIUM-PRESSURE INDIAN PLATE ROCKS FROM THE PAKISTAN HIMALAYA

Abstract

The Indian Plate rocks of NW Pakistan contain evidence for both Eocene and Miocene phases of post peak metamorphic exhumation. The Eocene phase shortly followed peak synchronous ultra high-pressure (UHP) and Barrovian metamorphism and was driven by the rapid return towards the surface of deeply buried, positively buoyant coesite-bearing UHP rocks, flanked by thrusts below and extensional shears above. Uplift of the UHP rocks contributed to crustal thickening and resulted in internal imbrication of the Barrovian metamorphic rocks onto which they were thrust. The Eocene and Miocene events were separated by a phase of large-amplitude and -wavelength folding. Upright folds related to this event have shallow WNW or ESE plunges. Quartz c-axis data suggest that the maximum stretching direction paralleled the fold axes. During the Miocene the Main Mantle Thrust was reactivated as a major top-side-north extensional fault zone. Cascading folds on its hanging wall and cascading folds and a variety of ductile to brittle top-side-north meso- and microstructures on its footwall document significant top-side-north movement. The driving force for Miocene extension is unlikely to be channel flow as suggested for the central Himalaya. Instead, rapid shortening of the overriding plate following Late Oligocene slab break-off could have destabilized the wedge and driven extension in its upper parts.