Quartz preferred orientations within a deformed pebble conglomerate from New Hampshire, U.S.A.

Abstract

The orientations of quartz c-axes within a deformed pebble conglomerate from Piermont, New Hampshire, have been measured and compared to finite strain estimates for both the pebbles and the surrounding matrix. Quartz grains within the relatively pure orthoquartzite pebbles show a strong c-axis preferred orientation while quartz grains within the matrix show a random distribution of c-axes. Comparisons of the preferred orientation of the pebbles with computer simulated fabrics of Lister and Hobbs (1980) yield estimates of the principal finite strains ( ϵ i ) = (1.00, 0.00, −0.50) which are consistent with previous strain estimates (Rumble and Oertel, 1979) derived from the pebble shapes (1.22, −0.13, −0.48). The random c-axis pattern within the matrix, however, is inconsistent with the strain estimates derived from fabric analysis of matrix phyllosilicates using the March model. While it is believed that intracrystalline slip of quartz was as prevalent in the matrix as in the pebbles, the presence of phyllosilicates in the matrix appears to have prevented the development of a quartz preferred orientation. It is proposed that slip and/or grain-boundary sliding of phyllosilicates in the matrix may have been responsible for quartz grain rotations which were random with respect to quartz c-axes.