Kinematics of the Himalayan Metamorphic Slab: Implications for the Structural Framework of Central Nepal

In the Kali Gandaki and Budhi Gandaki valleys of central Nepal, the Himalayan metamorphic slab comprises amphibolite-to-greenschist facies rocks of the Greater Himalayan series and Lesser Himalayan metamorphic series thrust over unmetamorphosed rocks of the Lesser Himalayan sedimentary series along the Main Central thrust (MCT) fault. Quartz-rich specimens sampled from the lower half of the Himalayan metamorphic slab yield well-defined cross-girdle quartz c-axis fabrics characterized by a dominant top-to-the-southwest sense of shear. These crystallographic preferred orientations reflect pervasive crystal-plastic deformation throughout the Lesser Himalayan metamorphic series and extend more than 8 km structurally below the Greater Himalayan series / Lesser Himalayan metamorphic series contact. The quartz c-axis data, in conjunction with new, detailed structural mapping, constrain the position of the Ramgarh thrust to be coincident with the MCT at the base of the pervasively deformed Lesser Himalayan metamorphic series in central Nepal. Quartz c-axis fabric opening angles from specimens sampled within the Lesser Himalayan metamorphic series suggest deformation temperatures of c. 500 ± 50 ° C. Temperatures increase up to c. 670 ± 50 ° C within the migmatitic Greater Himalayan series. These temperatures generally plot within error of geothermometric estimates from metamorphic assemblages interpreted to be coincident with the extrusion of the Himalayan metamorphic slab. Thus, the quartz c-axis fabrics preserve evidence of the deformation incurred during extrusion of the mid-crustal core of the orogen. Neutral kinematic vorticity numbers (Wn) estimated from within the Lesser Himalayan metamorphic series range between 0.29 and 0.80 (c. 81-41% pure shear) with an average value of 0.65 (c. 55% pure shear). Like the quartz c-axis fabrics, these data are interpreted to reflect strain conditions during the extrusion of the mid-crust between ~ 22 and ~ 18 Ma. The vorticity data indicate that this extrusion was facilitated by a significant component of pure shear strain.