A MODERN PULSE OF ULTRAFAST MID-CRUSTAL EXTRUSION AND DIACHRONOUS CRUSTAL MELTING IN THE NANGA PARBAT MASSIF (Invited Presentation)

A feedback between surface denudation driven by fluvial processes, structural control, and crustal melting has previously been proposed to explain extremely rapid exhumation of the Nanga Parbat Massif (NPM) in the western Himalayan syntaxis. We present texturally- and chemically-characterized monazite Th-Pb dates and link them to the petrologic evolution of migmatites from the high-grade core of the NPM to constrain the role of crustal melting during rapid exhumation. These constraints, and structural and geometric considerations, are necessary to evaluate the feedbacks between surficial and deep crustal processes proposed by previous work.

Our data show diachronous late-stage melting histories between the studied samples, and an acceleration of exhumation rates at ~1 Ma from ~2 mm/yr to ~9-13 mm/yr. The total time-averaged exhumation rate of our samples is ~5 mm/yr, in agreement with previous long-term estimates of exhumation rates in the NPM, but our results show that this exhumation is a pulsed process, rather than steady-state. Similar pulses of mid-crustal extrusion are thought to have occurred in the central Himalaya during the Miocene. Biotite breakdown melting in one of our samples began prior to 4 Ma, followed by progressive melt crystallization between ~4.0 and 1.0 Ma, which was almost finished when extremely rapid exhumation began. Biotite breakdown melting in another sample beginning at 2.1 Ma may have been ongoing at this time. The data show that these melting reactions occurred at different points on the exhumation path in different rocks, which resulted in heterogeneous rock strength in the core of the massif at the onset of accelerated exhumation. It is thus difficult to conclusively attribute massif-wide crustal weakening associated with late-stage, in-situ melting as the trigger for the exhumation pulse at 1 Ma. Other mechanisms could have initiated accelerated exhumation, but our data cannot distinguish between their relative roles. Nonetheless, the antecedence of the Indus River suggests fluvial erosion alone did not initiate the modern pulse of mid-crustal extrusion in the NPM revealed by the petrochronologic data presented here.