Seismic properties of naturally deformed quartzites of the Alaknanda valley, Garhwal Himalaya, India

Abstract

The present contribution summarizes the results of a study focusing on the influence of quartz microstructures on the seismic wave velocities in the quartzites of the Garhwal Himalaya. Quartzites being monomineralic were chosen for the present study so as to nullify the effect of other mineral constituents on the seismic velocity. Samples were collected from different tectonic settings of the Higher and Lesser Himalayas which are separated from one another by the major tectonic zone 'Main Central Thrust' (MCT). These are mainly Pandukeshwar quartzite, Tapovan quartzite and Berinag quartzite. The samples of Berinag quartzite were collected from near the klippen and the thrust, termed as Alaknanda Thrust. The vast differences in microstructures and associated seismic wave velocities have been noted in different quartzites. It has also been observed that quartzites of the MCT zone and Alaknanda Thrust have higher seismic velocities. This is because of their coarse-grained nature of the rocks as evidenced by the strong positive relation between seismic velocities and grain area. The coarsening is either due to the operation of grain boundary migration and grain area reduction process or high aspect ratio/shape preferred orientation. The quartzites located around Nandprayag Klippen have undergone static recrystallization and exhibit the lowest seismic wave velocities.