BEDROCK GEOLOGIC FRAMEWORK OF THE 2011 M5.8 MINERAL, VA EARTHQUAKE

Following the August 23, 2011 M5.8 Mineral, Va. earthquake, a program of detailed geologic mapping was initiated in the epicentral region and a high-resolution airborne geophysics survey was flown. A LiDAR survey of the region after the earthquake did not reveal any clear surface ruptures for the M5.8 event. The aftershock pattern outlined a roughly planar, NE-SW-trending, southeast-dipping zone at 3-8 km depth that included the mainshock hypocenter and appeared to indicate the subsurface fault that moved during the quake, named the Quail fault by Horton et al. (2015). A surface projection of the Quail fault approximately coincides with a northeast-striking sheared contact between metagranodiorite of the Ordovician-Silurian Ellisville pluton and metavolcanic and metavolcaniclastic rocks of the Ordovician Chopawamsic Formation that we mapped as the Harris Creek fault, after exposures near the confluence of Harris Creek and the South Anna River. Foliations in the Ellisville and Chopawamsic rocks on either side of the contact dip southeast and formed under regional greenschist-facies metamorphic conditions, about 450 Ma according to ⁴⁰Ar/³⁹Ar data. The Harris Creek fault itself has a southeast-dipping to vertical mylonitic fabric defined by muscovite that overprints the older foliation and locally has right-lateral motion indicators. Mylonite along the Ellisville-Chopawamsic contact has been mapped as far NE as exposure permits, nearly to the North Anna River. Muscovite from this fabric yielded a ⁴⁰Ar/³⁹Arplateau age of 285+2 Ma. This age is interpreted to approximate the time of mylonitization along the Harris Creek fault during reactivation of the original Ordovician-Silurian intrusive contact. Trenching along the Harris Creek fault at its type area revealed small-scale brittle faults in saprolite, some of which appear to have extended from bedrock up into the C soil horizon, which would indicate fault activity as young as Quaternary. Gravity and magnetic modeling of the Ellisville-Chopawamsic contact supports a southeast-dipping contact to hypocentral depths between the pluton and metavolcanic rocks. The Mineral earthquake thus appears to have utilized an early Paleozoic lithologic contact that has been periodically reactivated as a fault since the late Paleozoic.