A MODERN PULSE OF PLATE-VELOCITY EXHUMATION AND DIACHRONOUS CRUSTAL MELTING IN THE NANGA PARBAT MASSIF

The Nanga Parbat Massif (NPM) in the western Himalayan syntaxis exposes Earth’s most rapidly-exhuming continental crust, and has served as a natural laboratory for investigating feedbacks between surficial and deep crustal processes during orogenesis. Proposed driving mechanisms for extremely rapid (up to 13 mm/yr) exhumation of the NPM include: 1) rapid surface denudation, largely driven by the powerful Indus River, 2) crustal weakening due to biotite breakdown melting, and 3) plate-scale changes in tectonic forcing. We combine monazite petrochronology with thermal modelling to evaluate the relative roles of crustal melting, surface denudation, and tectonics in facilitating plate-velocity exhumation of the NPM. Our data show an acceleration of exhumation rates from 2-4 mm/yr to 9-13 mm/yr that began at ~1 Ma. The total time-averaged exhumation rate derived from our analyses is ~5 mm/yr, in agreement with previous long-term estimates of exhumation rates in the NPM, but our results show that the NPM may be undergoing a modern pulse of Pleistocene exhumation.

Further, our data suggest that biotite breakdown melting reactions occurred at different points on the exhumation path in different rocks, making it difficult to conclusively attribute late-stage, in-situ biotite-breakdown melting and attendant crustal weakening as the trigger for the ~1 Ma exhumation pulse. The antecedence of the Indus River suggests that fluvial erosion alone did not initiate this pulse. Though it is tempting to attribute increased glacial erosion associated with the Mid-Pleistocene Transition as a driver of this exhumation pulse, differences in glacial coverage/precipitation between the western and eastern syntaxes suggests they should have responded differently to climatic forcing, which is not observed. Rather, a nearly synchronous acceleration in exhumation rates of similar magnitude in both the eastern and western syntaxes is most consistent with an orogen-scale tectonic trigger. One-dimensional thermal modelling shows that the reconstructed pressure-temperature-time path of our samples is consistent with exhumation in the NPM being dictated by antecedent deep crustal flow. The results of our study ultimately suggest tectonic forces currently dictate the loci and tempo of rapid exhumation in the Himalayan syntaxes.