STRUCTURAL GEOMETRY, KINEMATICS, AND STRAIN IN PIEDMONT ROCKS, SOUTHWESTERN MARYLAND AND NORTHERN VIRGINIA

Recent models of the Piedmont propose early NW-convergent tectonics with late dextral transpression (e.g., Valentino, 1999). In the central Piedmont, detailed mapping of metamorphic rocks along the Potomac river has led to a reinterpretation of the deformation sequence there. Both the Mather Gorge and Sykesville Fms. have been deformed by two fold sets first recognized by Drake et al. (1997): (1)early tight to isoclinal folds (F_a) with penetrative foliation (S_a); and (2)later upright, mostly parallel folds (F_b) trending NNE-SSW with spaced foliation and gentle dip. F_b folds passively transported F_a fold elements with subvertical displacement oblique to their original recumbent orientation. The rectilinear locus of deformed L_a lineations suggests that shear-slip and subsequent homogenous strain (flattening) were likely deformation mechanisms during F_b folding. Minimum total ellipticity (R_f of 2-D strain) as calculated from deformed quartz layers (S₀), varies from 13-20 in metapelites and 6-10 in lithic arenites. The mean stretch history of amphibolite and lithic arenite as calculated from late veinlets using the technique of Passchier (Tectonophysics, 1990) suggests that flattening produced a minimum R_f=6-8, with 30-40% area loss.

Mappable rock units of the Mather Gorge and Sykesville Fms. are displaced together along meter- to cm-scale mylonite zones that are interpreted to be high-angle reverse faults or thrusts. An E-directed sense of tectonic movement is defined by asymmetric meso- and microtextures including C-type shear band cleavage, grain-shape alignment, winged s-porphyroclasts, and mica fish. The mylonites occur in mica-rich layers and are interpreted to be the result of resurgent movement along closely spaced F_b axial planes in rock units favorably disposed to deformation. Riedel shears in syndeformational quartz enclaves show evidence of sinistral slip. Microtextures, including dark seams of insoluble material along S_b cleavage planes suggest that layers accommodated strain during F_b folding via solution transfer and volume loss. E-directed movement with sinistral slip is inconsistent with existing NW-convergent transpressional models.