Intraplate earthquakes, like the recent moment magnitude, M w 5.8 Mineral, Virginia (USA), earthquake of 23 August 2011, are sobering reminders of how little we know about the lithosphere and active tectonics of plate interiors, including passive continental margins. Unlike plate boundaries, plate interiors typically lack clear geologic evidence of the tectonic stresses that lead to earthquakes, the deformation of rocks, and the building of topography. Inspired by the Mineral earthquake, preliminary geophysical and geomorphic work in the Central Virginia seismic zone, centered in Louisa County, resolves apparent surface deformation expressed in geomorphic markers and river channel patterns. This deformation is consistent with the location and sense of slip of the reverse fault that ruptured in the Mineral earthquake. Surface deformation is recorded by the modern and ancient longitudinal profile of the South Anna River that flows above the epicenter of the earthquake and orthogonal to the structural grain of the Appalachian Piedmont. Sinuosity of the South Anna River channel is greatest in the uplifted hanging wall of the fault that ruptured in the Mineral earthquake. More dramatically, preliminary correlations of terrace deposits of the paleo–South Anna River suggest that they are several meters higher above the modern channel in the hanging wall with respect to the footwall of the Mineral fault rupture plane. The implication is that persistent faulting in the vicinity of the Mineral earthquake, over many millennia, has resulted in several meters of cumulative rock and surface uplift of the eastern hanging-wall block with respect to its western footwall. This is a rare example of crustal and surface deformation directly linked to observed seismicity in eastern North America, where earthquakes have long been observed to cluster spatially. If confirmed by ongoing mapping, these results serve as a surface deformation field constraint for geodynamic models of intraplate earthquake generation and associated hazards in a passive margin setting.

Abstract This field guide covers a two-day west-to-east transect across the epicentral region of the 2011 M5.8 Mineral, Virginia, earthquake, the largest ever recorded in the Central Virginia seismic zone. The field trip highlights results of recent bedrock and surficial geologic mapping in two adjoining 7.5-min quadrangles, the Ferncliff and the Pendleton, which together encompass the epicenter and most of the 2011–2012 aftershocks. Tectonic history of the region includes early Paleozoic accretion of an island arc (Ordovician Chopawamsic Formation) to Laurentia, intrusion of a granodiorite pluton (Ordovician Ellisville pluton), and formation of a post-Chopawamsic successor basin (Ordovician Quantico Formation), all accompanied by early Paleozoic regional deformation and metamorphism. Local transpressional faulting and retrograde metamorphism occurred in the late Paleozoic, followed by diabase dike intrusion and possible local normal faulting in the early Mesozoic. The overall goal of the bedrock mapping is to determine what existing geologic structures might have been reactivated during the 2011 seismic event, and surficial deposits along the South Anna River are being mapped in order to determine possible neotectonic uplift. In addition to bedrock and surficial studies, we have excavated trenches in an area that contains two late Paleozoic faults and represents the updip projection of the causative fault for the 2011 quake. The trenches reveal faulting that has offset surficial deposits dated as Quaternary in age, as well as numerous other brittle structures that suggest a geologically recent history of neotectonic activity.

Abstract In 2014, the geomorphology community marked the 125th birthday of one of its most influential papers, ‘The Rivers and Valleys of Pennsylvania’ by William Morris Davis. Inspired by Davis’s work, the Appalachian landscape rapidly became fertile ground for the development and testing of several grand landscape evolution paradigms, culminating with John Hack’s dynamic equilibrium in 1960. As part of the 2015 GSA Annual Meeting, the Geomorphology, Active Tectonics, and Landscape Evolution field trip offers an excellent venue for exploring Appalachian geomorphology through the lens of the Appalachian landscape, leveraging exciting research by a new generation of process-oriented geomorphologists and geologic field mapping. Important geomorphologic scholarship has recently used the Appalachian landscape as the testing ground for ideas on long- and short-term erosion, dynamic topography, glacial-isostatic adjustments, active tectonics in an intraplate setting, river incision, periglacial processes, and soil-saprolite formation. This field trip explores a geologic and geomorphic transect of the mid-Atlantic margin, starting in the Blue Ridge of Virginia and proceeding to the east across the Piedmont to the Coastal Plain. The emphasis here will not only be on the geomorphology, but also the underlying geology that establishes the template and foundation upon which surface processes have etched out the familiar Appalachian landscape. The first day focuses on new and published work that highlights Cenozoic sedimentary deposits, soils, paleosols, and geomorphic markers (terraces and knickpoints) that are being used to reconstruct a late Cenozoic history of erosion, deposition, climate change, and active tectonics. The second day is similarly devoted to new and published work documenting the fluvial geomorphic response to active tectonics in the Central Virginia seismic zone (CVSZ), site of the 2011 M 5.8 Mineral earthquake and the integrated record of Appalachian erosion preserved on the Coastal Plain. The trip concludes on Day 3, joining the Kirk Bryan Field Trip at Great Falls, Virginia/Maryland, to explore and discuss the dramatic processes of base-level fall, fluvial incision, and knickpoint retreat.