GEOLOGY OF THE PENDLETON QUADRANGLE, VIRGINIA, WITH NOTES ON THE AUGUST 2011 MINERAL EARTHQUAKE

Quadrangle mapping in Virginia’s piedmont is part of a continuing effort to support economic development and mitigate risks from natural hazards. The Pendleton 7.5-minute quadrangle is of particular interest because it is within the developing I-64 corridor and contains the epicenter of the August 2011 M_w 5.8 Mineral, Virginia earthquake. Mapping has been aided by high-resolution aeromagnetic, aeroradiometric, and LiDAR data collected after the earthquake. The quadrangle lies within the Chopawamsic terrane, an Ordovician-Silurian volcanic-plutonic arc that was docked to Laurentia during one or more Paleozoic transpressional events. Bedrock is composed primarily of felsic to mafic volcanic rocks of the Chopawamsic Formation that have been metamorphosed to amphibolite facies. In the northwest corner of the quadrangle, the Chopawamsic is intruded by granodiorite and gabbro of the ca. 444 Ma Ellisville Pluton. A two km-wide zone of ductile deformation and brittle faulting overprints the eastern contact of the pluton. Recent mapping extends the Long Branch Fault southwestward across the north half of the quadrangle, within the Chopawamsic Formation. The fault is marked by a narrow zone of ductile deformation, localized siliceous breccia, and a change in structural style from a single generation of foliation to polydeformation fabrics from northwest to southeast. The Quantico formation, consisting of a basal quartzite overlain by graphitic garnet-biotite-muscovite schist, unconformably overlies the Chopawamsic Formation in a complex, SW-NE striking syncline. East of the Quantico syncline, mafic and intermediate metavolcanic rocks become progressively coarser-grained and are intruded by pegmatite that may be associated with a poorly exposed granitic pluton of unknown age that is present along the eastern quadrangle boundary. The epicenter of the Mineral earthquake, located in the southwestern quarter of the quadrangle, lies within a partially fault-bounded domain of east-west-trending intraformational folds that are oblique to regional strike by 55 degrees. Structural interpretation to the depth of the hypocenter indicates that the earthquake occurred within the Chopawamsic terrane, possibly along the fault zone that forms the contact between the Ellisville Pluton and the Chopawamsic Formation.