TECTONIC EXHUMATION OF THE NANGA-PARBAT MASSIF, NORTHERN PAKISTAN

Structural analysis in the Nanga Parbat region of northern Pakistan indicates that extensional deformation has been, in part, responsible for the exhumation of gneissic rocks of the Indian plate basement complex that dominates the Nanga Parbat massif. This massif has been mapped as a syntaxis at the boundary between the Indian and Asian plates in the northwest Himalaya and has received recent attention for evidence of rapid exhumation within the last 10 Ma. Field work along the Main Mantle Thrust (MMT) and within the Indian plate rocks in the area between Babusar Pass and Toshe Gall, southwest of the peak of Nanga Parbat, provided us with evidence from shear fabrics and a shallowly WSW-plunging stretching lineation for a dominant phase of extensional deformation. During this deformation rock of the Kohistan sequence and the Indian plate cover sequence moved to the WSW relative to rocks of the Indian plate basement including the Nanga Parbat gneiss. A post-metamorphic ductile shear fabric pervades the MMT contact, the Indian plate cover sequence below the MMT, but decreases in intensity in the Indian plate basement and the Nanga Parbat gneiss. Using Ar-39/Ar-40 geochronology of hornblendes we have constrained the age of pre-deformational, amphibolite facies metamorphism to be less than or equal to 40-56 Ma. Published cooling ages from the Babusar Pass region further document that the extensional deformation was completed by similar to 20 Ma. We interpret this ductile, extensional shear as the mechanism responsible for significant tectonic exhumation of the gneissic and granitic rocks of the Nanga Parbat massif.