High-pressure Tethyan Himalaya rocks along the India-Asia suture zone in southern Tibet

Abstract

This study documents an early Cenozoic continental high-pressure (HP) metamorphic complex along the Yarlung (India-Asia) suture zone in southern Tibet. The complex is exposed in the Lopu Range, located similar to 600 km west of the city of Lhasa. HP rocks in the core of the complex have Indian passive-margin (Tethyan Himalaya Sequence) protoliths and are exposed in the footwall of a top-to-the-north, normal-sense shear zone. Phengite geobarometry, Zr-in-rutile geothermometry, and pseudosection modeling indicate that these rocks reached pressures >= 1.4 GPa at temperatures <= 600 degrees C. A meta-Tethyan graywacke yielded a garnet Lu-Hf date of 40.4 +/- 1.4 Ma, which is interpreted as the age of prograde metamorphism. Five Ar-Ar phengite ages between 39 and 34 Ma are interpreted to record the timing of exhumation to midcrustal depths (similar to 25 km) and concomitant retrogression. The structural geometry and pressure-temperature-time (P-T-t) history of Lopu Range rocks are similar to the Tso Morari and Kaghan Valley complexes, located >700 km to the northwest along the Indus suture zone. However, peak metamorphism and exhumation occurred similar to 6 m.y. later in the Lopu Range, and no ultrahigh-pressure assemblages have been identified. We propose a tectonic model that involves steep subduction of the Tethyan Himalaya continental margin at ca. 40 Ma, initial exhumation of HP metasedimentary rocks at ca. 39 Ma, and subsequent northward underthrusting of Greater Indian lithosphere shutting off Gangdese arc magmatism at ca. 38 Ma.