The Himalayan Main Central Thrust pile and its quartz-rich tectonites in central Nepal

Abstract

As a consequence of the India—Asia collision, a deeply rooted crustal slice has been moved southwards over the northern boundary of the Indian plate. At the present level of observation the Main Central Thrust pile, a rock-pile of several kilometers thickness, is affected by shear deformation which imprinted a flat foliation and a constant N—S trending lineation. These tectonites were deformed under a wide range of metamorphic conditions grading from lower greenschists up to the sillimanite field. The M.C.T. shear zone is studied from the view-point of the quartz-rich rocks: over 100 samples helped to define a microstructural zonation, roughly parallel to the thermal one; 35 samples, particularly rich in quartz, have been used for the preferred orientation study of their c-axes, and a few of them for their 〈a〉 axes. Strong lattice orientations, when undisturbed by (mainly) secondary recrystallization, attest to large plastic strains in quartz. The c-axes are mostly distributed along unequally populated crossed-girdles which may evolve into an inclined single girdle. The 〈a〉 axes concentrate perpendicular to the more populated girdle. Concerning quartz the following is suggested: 1. (1) the deformation has a dominant shear component responsible for the dominant stretching lineation. It could be simple shear in case of inclined single girdles 2. (2) one set of 〈a〉 axes line up parallel to the shear direction. Slip on various planes in the 〈a〉 direction may be an important deformation mechanism 3. (3) asymmetry in the lattice preferred orientations could be used to predict the sense of shear for the Himalayan thrusting in more than 80% of cases.