Microscopic Deformation Mechanisms and Flow Laws in Quartzites within the South Mountain Anticline

Abstract

Microscopic strain analysis involving the measurement of components of strain due to different mechanisms is applied to the Weverton formation in the South Mountain Anticline. The normal limb of the anticline at Catoctin Mountain consists of open low amplitude folds with long normal limbs and short overturned limbs with the concentration of strain around the hinges of the folds. The overturned limb at Harpers Ferry and South Mountain consists of tight overturned folds with undeformed normal limbs and strongly sheared overturned limbs, suggesting that the folding is associated with the development of ductile deformation zones on the overturned limbs. The strain ellipsoid is usually oblate, with a consistent deviation from a plane strain situation. The important deformation mechanisms are dislocation creep involving intracrystalline slip and pressure solution involving grain boundary diffusion. The dislocation creep strain () is determined using deformed rutile needles which act as internal strain markers within the quartz grains. The pressure solution strain () is determined using the area ratios of new crystals and fibers to the whole rock in principal sections. Pressure solution is generally the dominant deformation mechanism in the South Mountain Anti-cline, but dislocation creep becomes increasingly important on the strongly sheared overturned limbs at Harpers Ferry and South Mountain, suggesting the enhancement of dislocation creep at higher stresses. The operation of pressure solution is strongly controlled by the presence of rigid and insoluble constituents such as magnetite and micas, which increase the rates of solution and diffusion, and also create pressure shadow areas. The normal limb at Catoctin Mountain is characterized by a slightly higher proportion of than South Mountain, because of the increasing importance of dislocation creep at higher temperatures. The Weverton quartzites are inferred to have obeyed a combined flow law consisting of a linear component due to pressure solution and a power law component due to dislocation creep. The proportion of the two components depended on the stress, temperature and lithology, so that each point obeyed a unique flow law which may have varied with time.