Rapid crustal heating during exhumation of the Nanga Parbat massif

Victor Guevara, Amherst College
Joshua Garber, University of St. Andrews
Mark Caddick, Virginia Tech
Andrew Smye, Pennsylvania State University
Angelina Han, Amherst College
Michael Searle, University of Oxford
Asghar Ali, University of Peshawar
Sarah Clarridge, Amherst College

Abstract

The Himalayan syntaxes expose the youngest and fastest-exhuming high-temperature rocks in the Himalaya. The geodynamic mechanisms that cause rapid crustal exhumation (>10 mm/yr) in the syntaxes are debated; consensus around this debate has been hampered by a lack of clarity over their pressure-temperature-time (P-T-t) evolution. Here, we present new petrochronologic data to reconstruct the P-T-t evolution of rock samples from the core of the Nanga Parbat Massif (NPM) in the western Himalayan syntaxis. Our data suggest the NPM was heated to ~660 °C between ~10 and 5 Ma. Decompression to 0.3 GPa began at 3.0 Ma, which was accompanied by rapid heating (91±45 °C/Myr) to >720 °C by 2.2 Ma. Cooling from peak-T began at ~1.0 Ma. Exhumation rates during this time interval were 5.1±3.1 km/Myr. Thermal modelling suggests the interpreted P-T-t path requires external heat input a few Myr prior to the onset of decompression, which may be achieved by ultrahigh-temperature lower crustal flow or lithosphere removal beneath the NPM. Recent geophysical studies of the eastern syntaxis (Namche Barwa) suggest the presence of a localized slab tear/window, allowing asthenospheric upwelling and attendant topographic uplift and crustal heating. Though similar studies have not been done for the NPM, our petrochronologic data are consistent with the presence of such a feature beneath the NPM, which may explain similarities in the timing and tempo of metamorphism and exhumation of the Himalayan syntaxes.

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