Ductile extrusion of the Higher Himalayan Crystalline in Bhutan: Evidence from quartz microfabrics

Quartz textures measured from deformed quartz tectonites within the Lesser Himalaya and Higher Himalaya Crystalline of Bhutan show similar patterns. Orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense: the asymmetry of c-axis and a-axis patterns consistently indicates top-to-the-south shearing. The obliquity of the texture and the inferred finite strain (plane strain to moderately constrictional), suggest the strain regime had a combination of rotational and irrotational strain path. In most of the samples from the Bhutan Himalaya, the inferred deformation mechanisms suggest moderate- to high-temperature conditions of deformation that produced the observed crystallographic preferred orientation. Much higher temperature of deformation is indicated in the quartz veins from a leucogranite. The observed ductile deformation is pervasively developed in the rocks throughout the investigated area. The intensity of deformation increases only slightly in the vicinity of the Main Central Thrust. Simultaneous southward shearing within a large part of the Higher Himalaya Crystalline near and above the Main Central Thrust and normal faulting across the South Tibetan Detachment, is explained by the tectonically induced extrusion of a ductily deforming wedge. The process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes. Channel flow accounts for some observed phenomena in the Himalayan orogen such as inverted metamorphic sequences near the Main Central thrust, not related to an inversion of isotherms, and the syntectonic emplacement of leucogranites into the extruding wedge, locally leading to an inversion of isotherms due to heat advection.