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 d eformation. Different generations of tension gashes have been rotated by va riable angles with respect to the mylonitic foliation, forming associated f ringe folds. These record the late stage brittle-ductile flow and reveal th at a strong pure shear component of deformation occurred throughout the MCT Z. To characterize earlier deformation increments, fabrics from highly defo rmed quartz ribbons were analyzed. Well-developed shape- and lattice-prefer red 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 t he samples from the base of the MCTZ show almost perfectly symmetrical Type I crossed girdles. Deformation within the MCTZ probably started close to s imple shear flow at higher temperatures, which progressively became a more general shear during cooling, and ended in a pure shear dominated flow duri ng the final stages of brittle-ductile deformation (i.e. a decelerating str ain path). Using the Higher Himalaya Crystalline as an example, a kinematic model for the extrusion of crustal wedges above major thrust zones is sugg ested. (C) 1999 Elsevier Science Ltd. All rights reserved.