TECTONIC EXHUMATION OF THE NANGA-PARBAT MASSIF, NORTHERN PAKISTAN

Structural analysis in the Nanga Parbat region of northern Pakistan in dicates that extensional deformation has been, in part, responsible fo r the exhumation of gneissic rocks of the Indian plate basement comple x that dominates the Nanga Parbat massif. This massif has been mapped as a syntaxis at the boundary between the Indian and Asian plates in t he northwest Himalaya and has received recent attention for evidence o f rapid exhumation within the last 10 Ma. Field work along the Main Ma ntle Thrust (MMT) and within the Indian plate rocks in the area betwee n Babusar Pass and Toshe Gall, southwest of the peak of Nanga Parbat, provided us with evidence from shear fabrics and a shallowly WSW-plung ing stretching lineation for a dominant phase of extensional deformati on. During this deformation rock of the Kohistan sequence and the Indi an plate cover sequence moved to the WSW relative to rocks of the Indi an plate basement including the Nanga Parbat gneiss. A post-metamorphi c ductile shear fabric pervades the MMT contact, the Indian plate cove r sequence below the MMT, but decreases in intensity in the Indian pla te basement and the Nanga Parbat gneiss. Using Ar-39/Ar-40 geochronolo gy of hornblendes we have constrained the age of pre-deformational, am phibolite facies metamorphism to be less than or equal to 40-56 Ma. Pu blished cooling ages from the Babusar Pass region further document tha t the extensional deformation was completed by similar to 20 Ma. We in terpret this ductile, extensional shear as the mechanism responsible f or significant tectonic exhumation of the gneissic and granitic rocks of the Nanga Parbat massif.