NEW GEOLOGIC MAPPING AND GEOPHYSICAL SURVEYS IN THE EPICENTRAL REGION OF THE 2011 MINERAL, VIRGINIA EARTHQUAKE

New 1:24,000-scale geologic maps of two 7.5-minute quadrangles, combined with new airborne gravity, magnetic, K-U-Th radiometric, and LiDAR surveys, yield insights into the geologic setting for the mainshock and aftershock sequence of the 2011 M_w 5.8 Mineral, Virginia earthquake. NE-striking lithologic belts record the accretion of the Ordovician Chopawamsic volcanic arc to Laurentia, and intrusion of the Ordovician-Silurian Ellisville granodiorite as a stitching pluton across a 2-3 km wide, ultramafic-bearing suture zone that includes the pre- to early-metamorphic Chopawamsic fault zone (CFZ) and mélange zone III of the Mine Run Complex. Metasedimentary rocks of the Quantico Formation were deposited in a successor basin on top of Chopawamsic Formation metavolcanics and volcaniclastics; the timing of basin formation relative to accretion and Ellisville intrusion is still unknown. The epicenter of the 2011 mainshock is near the western edge of the Pendleton 7.5-minute quadrangle, and aftershocks delineate a NE-striking, SE-dipping plane that projects to the surface in the adjacent Ferncliff quadrangle, in or just southeast of where the Ellisville-Chopawamsic contact is cut by closely-spaced Alleghanian shear zones and faults. Magnetic data suggest that this contact may extend down to the southeast into the zone containing the mainshock hypocenter and aftershocks. Radiometric data clearly show one of the mapped faults, the Harris Creek fault, due to the relative abundance of K-rich recrystallized white mica in the fault zone. Trenching across this fault and the adjacent Roundabout fault reveals abundant evidence of brittle faulting and cataclasis that are probably Mesozoic, and fault offsets of well-developed soil horizons that are interpreted to indicate Quaternary neotectonism­­. LiDAR and field observations have not yet identified any fault scarps. Magnetic-high anomalies were used to map NNW-to-NNE-striking Jurassic diabase dikes and a NE-striking belt of Chopawamsic ferruginous quartzite. Magnetic anomalies also delineate a zone in the Chopawamsic of complex folding and local east-west foliation interpreted to be rotated by Paleozoic thrusting. The Quantico Formation produces distinctive radiometric anomalies that facilitate mapping of complex refolded structures in the belt.