Kinematics of the Greater Himalayan sequence, Dhaulagiri Himal: implications for the structural framework of central Nepal

In the Dhaulagiri region of central Nepal quartz-rich specimens sampled from the Greater Himalayan sequence yield well-defined quartz c-axis fabrics with a dominant top-to-the-SW sense of shear. These fabrics reflect pervasive crystal-plastic deformation extending more than 8 km structurally below previously mapped locations of the Main Central thrust fault. Quartz c-axis fabric opening angles suggest deformation temperatures of c. 500 +/- 50 degrees C within the lower portion of the Greater Himalayan sequence and up to c. 670 +/- 50 degrees C within the migmatitic upper portion. These temperatures generally plot within error of garnet-biotite temperature estimates for metamorphic assemblages and arc interpreted to reflect Tertiary deformation during extrusion of the mid-crust. The quartz c-axis data, and a new, detailed tectonostratigraphic map, constrain the position of the Ramgarh thrust in the Dhaulagiri region to be coincident with the Main Central thrust at the base of file pervasively deformed exhumed mid-crustal core. Mean kinematic vorticity numbers (W-m) measured in specimens sampled from the lower portion of the Greater Himalayan sequence range between 0.49 and 0.80 (c. 66-41% pure shear) with air average value of 0.67 (c. 53%, pure shear). These data indicate that extrusion of the mid-crust was facilitated by a significant component of pure shear strain.