DUCTILE EXTRUSION OF THE HIGHER HIMALAYAN CRYSTALLINE IN BHUTAN - EVIDENCE FROM QUARTZ MICROFABRICS

Quartz textures measured from deformed quartz tectonites within the Le sser Himalaya and Higher Himalaya Crystalline of Bhutan show similar p atterns. Orientation and distribution of the quartz crystallographic a xes were used to confirm the regional shear sense: the asymmetry of c- axis and a-axis patterns consistently indicates top-to-the-south shear ing. The obliquity of the texture and the inferred finite strain (plan e strain to moderately constrictional), suggest the strain regime had a combination of rotational and irrotational strain path. In most of t he samples from the Bhutan Himalaya, the inferred deformation mechanis ms suggest moderate- to high-temperature conditions of deformation tha t produced the observed crystallographic preferred orientation. Much h igher temperature of deformation is indicated in the quartz veins from a leucogranite. The observed ductile deformation is pervasively devel oped in the rocks throughout the investigated area. The intensity of d eformation increases only slightly in the vicinity of the Main Central Thrust. Simultaneous southward shearing within a large part of the Hi gher Himalaya Crystalline near and above the Main Central Thrust and n ormal faulting across the South Tibetan Detachment, is explained by th e tectonically induced extrusion of a ductily deforming wedge. The pro cess of extrusive flow suggested here can be approximated quantitative ly by channel flow models that have been used to describe subduction z one processes. Channel flow accounts for some observed phenomena in th e Himalayan orogen such as inverted metamorphic sequences near the Mai n Central thrust, not related to an inversion of isotherms, and the sy ntectonic emplacement of leucogranites into the extruding wedge, local ly leading to an inversion of isotherms due to heat advection.