TEMPORAL EVOLUTION OF THE EASTERN NORTH AMERICAN STRESS FIELD: EVIDENCE FROM BRITTLE STRUCTURES IN THE CENTRAL VIRGINIA SEISMIC ZONE

The Central Virginia Seismic Zone (CVSZ) is a region of moderate intraplate seismicity in the Appalachian Piedmont. Recent earthquakes in the CVSZ, such as the August 2011 M=5.8 earthquake, have renewed interest in understanding the state of stress in the Piedmont and faulting in this passive margin setting. The 2011 earthquake was centered on a previously unknown NE-striking and SE-dipping blind reverse fault within the Chopawamsic Terrane. The central Virginia Piedmont includes a suite of suspect terranes such as the Goochland, Chopawamsic, and Carolina terranes cut by N- to NE-striking high-strain zones and Mesozoic rift basins recording initial stages of extension leading to the formation of the Atlantic Ocean. Older rocks are covered by, and faulted against, Cenozoic Coastal Plain sediments.

A systematic study of fractures in central Virginia (~20,000 km² area) reveals three regional fracture sets that record a changing stress field through time. The oldest fractures record dextral slip along NE/SW brittle/ductile faults in the eastern Piedmont, whereas NW/SE extension fractures are common in the western Piedmont and Blue Ridge. Both fracture patterns are consistent with σ₁ oriented WNW/ESE_{, ­}suggesting these fractures formed during the late Alleghanian orogeny (290-260 Ma). Younger normal faults and Mode I fractures oriented NW/SE record extension consistent with a σ_3­ oriented NW/SE. These fractures likely formed duing the late Triassic and include basin bounding faults. Later N/S extension fractures and diabase dikes record E/W extension and rotation of σ_{3 ­}to E/W, consistent with a Jurassic stress field rotation of almost 90°, likely caused by South America rifting away from North America. The most recent faults occur on NE/SW striking structures, many of which were reactivated as reverse faults due to a σ­_­­­­­₁ oriented NW/SE_­­­­­. Modern earthquake focal mechanisms suggest the stress field reactivating these faults persists to the present.

Fault rocks associated with these structures include cataclasite and pseudotachylyte. Cataclasite in Alleghanian shear zones and Mesozoic normal faults indicate slip occurred in the zeolite facies. Pseudotachylyte associated with basin bounding faults in the eastern Piedmont record major friction heating and M>6 earthquakes in the Mesozoic or Cenozoic.