Microfractures and their role in deformation of a quartz arenite from the central Appalachian foreland

Abstract

Microfractures are pervasive in the Massanutten Sandstone, a quartz arenite exposed in a doubly-plunging synclinorium in the central Appalachian foreland. They are present as fluid inclusion planes (FIPS) and microveins parallel the FIPS. FIPS occur in three orthogonal sets: horizontal; vertical and normal to the fold axis; and vertical and parallel to the fold axis. The geometry, distribution and age of FIPS relative to other microstructures shows that microfractures developed during folding. Microfracturing followed layer-parallel shortening by dislocation flow and was coeval with the peak of pressure solution that marked the final stages of folding. Microfracture densities are extremely high (> 160 mm −1) in many samples. Rapid closure of microfractures suppressed brecciation and prevented microfractures from propagating unstably to form joints. Microfracture opening accounts for significant finite strain with extensional strains of up to 11.1% and associated volume increases of over 16.0%. The general lack of shortening microstructures indicates that a net volume increase is likely and that the Massanutten Sandstone may have served as a sink for dissolved phases from adjacent units undergoing volume-loss deformation.