Titanite petrochronology supports protracted transport along a high-level thrust within the Greater Himalayan Sequence, Central Nepal.

Abstract:

Zr-in-titanite thermometry and U-Pb geochronology offer potential to recover prograde and retrograde temperature-time (T-t) histories from moderate- to high-grade metamorphic rocks, which in turn can elucidate tectonic processes. In this study, titanite-based T-t paths were determined along a transect across the proposed trace of the ductile South Tibetan Detachment System (STDS) in central Nepal. Titanite-bearing calcsilicates were sampled from the upper Greater Himalayan Sequence (GHS) through the putative lower Tethyan Himalayan Sequence (THS) along the Marsyandi River in the Annapurna Himal. T’s decrease from 800 ºC to 650 ºC with increasing structural level, consistent with previous work that shows decreasing T structurally upwards towards the STDS. T-t trends separate into three groups, from structurally lowest to highest: I. increasing T from ~27 to ~15 Ma, II. constant T from ~50 to ~10 Ma, and III. Decreasing T from ~ 25 Ma to ~ 10 Ma. These data are consistent with synmetamorphic thrusting from ~25 Ma to ~15 Ma: Group I (structurally low) heated during burial, Group II (structurally intermediate) resided in the shear zone at quasi-static T, and Group III (structurally high) cooled during emplacement. After ~ 15 Ma, Groups I-III exhumed coherently, supported by decreasing T-t trends at all structural levels. These data are consistent with (a) titanite T-t paths from elsewhere in the Annapurna Himal, (b) the timing of heating of structurally lower GHS rocks from other Central Nepal locations, and (c) the occurrence of intra-GHS thrusts within the orogen. High-grade GHS transport more likely occurred through a series of in sequence thrusts, rather than ductile flow. Our data do not support a ductile STDS at this location, and instead indicate a GHS origin for rocks previously assigned to the Tethyan. Significant extensional throw appears absent at this structural level, whereas shortening within the GHS and Himalaya is greater than commonly assumed.