CONDITIONS OF DEFORMATION IN THE MATHER GORGE AND SYKESVILLE FORMATIONS, POTOMAC RIVER, SW MARYLAND AND N VIRGINIA

Current models of deformation in the central Appalachian Piedmont postulate that lithologies of the Mather Gorge Fm. were polydeformed prior to deposition of the Sykesville Fm. (Drake and Froelich, 1997). New structural analysis of Piedmont rock units exposed along the Potomac River SE of Great Falls, Md. confirms that the Mather Gorge and Sykesville Fms. exhibit a complex polygenetic history and provides evidence that both units experienced the same deformation events. Early recumbant tight isoclinal folds (F_a of Schoenborn, 2001) that are well developed in Mather Gorge metapelites are also observed in a sample of Sykesville (interlaminated muscvt schist - qtz-fsp granofels) collected from a construction site close to the Potomac River. Qtz c-axis pole figure patterns developed from 6 samples of Mather Gorge and Sykesville tectonites collected along the river exhibit an orthorhombic symmetry, a symmetrical broad small circle distribution about Z, and a lack of maxima parallel to Y, which are interpreted to be the result of predominantly coaxial non-rotational flattening strain related to later upright parallel folds (F_b). 2-D strain measurements from 51 elliptical markers in the Sykesville (biotite clasts within qtz fsp granofels) using the Rf/f' technique suggest a bulk finite strain of 7.0 which is consistent with 2-D strains measured from lithologies in the Mather Gorge Fm. Within the phyllonitic rocks of the Mather Gorge, cm-scale pods of qtz-fsp leucosome (former cumulate melt) between pulled-apart host rock, and metric-scale tabular bodies of dark banded agmatitic rock (sheared diatexite) suggest that initial shearing of the phyllonitic rocks occurred under high temperature near solidus conditions. F_b axial planes in phyllonitic schist are defined in thin section by smooth zonal mica cleavage domains that anastomose around lenticular qtz-muscvt-chlrt microlithons, which preserve older S_a cleavage at high angle to dominant S_b foliation. Discreet transition from cleavage domains to microlithons is commonly marked by dusty trains of opaque minerals and suggests pressure solution mechanisms operated during F_b folding. Greenschist mineralogy, meso- and microscopic textures, and broad small girdle patterns in quartz c-axis pole figures (30 deg. small circles), are consistent with late strain at lower temperatures.