Tectonic activity along the inner margin of the South Tibetan Detachment constrained by syntectonic leucogranite emplacement in Western Bhutan

Chiara Montomoli (*), Rodolfo Carosi (**), Daniela Rubatto (***), Dario Visonà (****) & Salvatore Iaccarino (*)
(*) Dipartimento di Scienze della Terra, via S. Maria, 53, 56126, Pisa, Italy.
(**) Dipartimento di Scienze della Terra, via Valperga Caluso, 35, 10125, Torino, Italy. Corresponding author e-mail: rodolfo.carosi@unito.it.
(***) Research School of Earth Sciences, Australian National University, Canberra, Australia.
(****) Dipartimento di Geoscienze, via Gradenigo 6, 35131, Padova, Italy.

In Western Bhutan Himalayas leucogranite dykes emplaced in sub-vertical hybrid fractures that cut across the high-grade rocks of the upper Greater Himalayan Sequence just below to the South Tibetan Detachment. The granitic dykes dip to the north often showing a mylonitic deformation with a top-down to-the-N sense of shear. The high-angle fractures are interpreted to be related to the evolution of the South Tibetan Detachment toward a brittle regime of deformation. U-Pb monazite ages constrain the leucogranite emplacement at 13.9±06 Ma implying that brittle-ductile deformation of the South Tibetan Detachment was active at that time. NNE-SSW to nearly E-W trending large scale antiforms and synforms mapped in NW Bhutan affected the Greater Himalayan Sequence and South Tibetan Detachment only after 14 Ma.

South Tibetan Detachment System, leucogranite,
Bhutan Himalaya, monazite geochronology.