Quantitative kinematic flow analysis from the Main Central Thrust Zone (NW-Himalaya, India): implications for a decelerating strain path and the extrusion of orogenic wedges
Quantitative kinematic indicators from the Main Central Thrust Zone (MCTZ) in the NW-Himalaya have been used to characterize the type of flow during deformation. Different generations of tension gashes have been rotated by variable angles with respect to the mylonitic foliation, forming associated fringe folds. These record the late stage brittle-ductile flow and reveal that a strong pure shear component of deformation occurred throughout the MCTZ. To characterize earlier deformation increments, fabrics from highly deformed quartz ribbons were analyzed. Well-developed shape- and lattice-preferred orientation patterns show a systematic change of the glide systems and suggest inverted palaeotemperatures within the MCTZ. Investigations of the c-axes patterns reveal a strong asymmetry at the top of the MCTZ, whereas the samples from the base of the MCTZ show almost perfectly symmetrical Type I crossed girdles. Deformation within the MCTZ probably started close to simple shear flow at higher temperatures, which progressively became a more general shear during cooling, and ended in a pure shear dominated flow during the final stages of brittle-ductile deformation (i.e. a decelerating strain path). Using the Higher Himalaya Crystalline as an example, a kinematic model for the extrusion of crustal wedges above major thrust zones is suggested.