GEOCHRONOLOGICAL AND MICRO-FABRIC CONSTRAINTS ON PULSED EXTRUSION OF THE METAMORPHIC CORE IN THE EASTERN HIMALAYA (BHUTAN)

In the Bhutan Himalaya, the South Tibetan detachment, the major extensional Himalayan structure, emplaces klippen of the Tethyan Sedimentary Series over high-grade Greater Himalayan rocks. A combined micro-fabric, geochemical (Th, Y zonation) and geochronological ((U, Th)/Pb, LA-ICPMS data) analysis of metamorphic monazite from across the South Tibetan detachment at the base of the Radi klippe, constrains top-to-the-north normal shear to between 25 and 20 Ma. This is the first evidence for late Oligocene-early Miocene movement on the South Tibetan detachment east of the Yadong-Gulu rift. With coeval thrusting on the Main Central Thrust, the tectonic evolution of the Bhutan Himalaya appears similar to tectonics in the central and western Himalaya over this time span. After 20 Ma, the South Tibetan detachment was folded and cut by out-of-sequence thrusting on the Kahktang thrust. Klippen of Tethyan Sedimentary Series were isolated in the footwall, and a pulse of extrusion of the hanging wall between 14 and 10 Ma reflects the migration of metamorphism and exhumation towards the orogenic hinterland, a process not observed elsewhere in the Himalaya and thus highlighting the need for a three-dimensional tectonic model of the orogen. Our data are consistent with a pulsed channel flow model (Hollister and Grujic, 2006) for the eastern Himalaya. However in the absence of evidence for a sustained channel flow process, other tectonic models featuring short (< 3 Ma), isolated pulses of extrusion e.g. critical taper, are equally compatible.

Hollister, L.S., and Grujic, D., 2006, Pulsed channel flow in Bhutan, in Law, R.D., Searle, M.P., and Godin, L., eds., Channel Flow, Ductile Extrusion and Exhumation in Continental Collision Zones, Volume 268, Geological Society, London, Special Publications, p. 415-423.