Multichannel and wide‐angle seismic data collected off Virginia during the 1990 EDGE Mid‐Atlantic seismic experiment provide the most detailed image to date of the continent‐ocean transition on the U.S. Atlantic margin. Multichannel data were acquired using a 10,800 in3 (177 L) airgun array and 6‐km‐long streamer, and coincident wide‐angle data were recorded by ten ocean bottom seismic instruments. A velocity model constructed by inversion of wide‐angle and vertical‐incidence travel times shows strong lateral changes in deep‐crustal structure across the margin. Lower‐crustal velocities are 6.8 km/s in rifted continental crust, increase to 7.5 km/s beneath the outer continental shelf, and decrease to 7.0 km/s in oceanic crust. Prominent seaward‐dipping reflections within basement lie within layers of average velocity 6.3–6.5 km/s, consistent with their interpretation as basalts extruded during rifting. The high‐velocity lower crust and seaward‐dipping reflections comprise a 100‐km‐wide, 25‐km‐thick ocean‐continent transition zone that consists almost entirely of mafic igneous material accreted to the margin during continental breakup. The boundary between rifted continental crust and this thick igneous crust is abrupt, occupying only about 20 km of the margin. Appalachian intracrustal reflectivity largely disappears across this boundary as velocity increases from 5.9 km/s to >7.0 km/s, implying that the reflectivity is disrupted by massive intrusion and that very little continental crust persists seaward of the reflective crust. The thick igneous crust is spatially correlated with the East Coast magnetic anomaly, implying that the basalts and underlying intrusives cause the anomaly. The details of the seismic structure and lack of independent evidence for an appropriately located hotspot in the central Atlantic imply that nonplume processes are responsible for the igneous material.

___ ________________________________________ Introduction. _ _____________________________________ Petrographic methods ___________________________ Definitions of some terms._______________________ Acknowledgments ______________________________ General geologic setting.____________________________ Cretaceous rocks.__________________________________ Pre-Robles rocks_ ______________________________ Robles Formation._____________________________ Bedded volcaniclastic rocks__________________ Rio Maton Limestone Member.______________ Lapa Lava Member._ ______________________ Las Tetas Lava Member.____ ______________ Some chemical peculiarities of the lavas____ __ Age ______________________________________ Source and conditions of deposition__ ________ Cariblanco Formation-__________________________ Volcaniclastic rocks_________________________ Conglomerate: mixtures of gravel, ash, and reworked pyroclastics_ ____________ Gravel. ___________________________ Conglomerate matrix and coarse volcaniclastic strata _________________ Mudstone, chert, and tuff___________.

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A regional seismic reflection line (I‐64) across the Virginia Piedmont has provided a stacked section suitable for an integrated interpretation of geophysical data in the region. A highly reflective upper crust, an allochthonous Blue Ridge Province, underlying thrust sheets including the Blue Ridge master decollement, and a basal decollement at a depth of about 9 km (3 s) are confirmed on the seismic data. Immediately east of the Blue Ridge Province, Appalachian structures plunge to as much as 12 km (4 s) depth. The Evington Group, Hardware terrane, and Chopawamsic metavolcanic rocks (Carolina terrane) crop out in the Piedmont Province, and numerous eastward dipping reflections originate from these rocks in the subsurface. These eastward dipping reflectors overlie a gently west dipping (10°–15°), highly reflective zone that varies in depth from 1.5 s (4.5 km) beneath the Goochland terrane to 4 s (12 km) beneath the rocks of the Evington Group. Some of the overlying eastward dipping reflections apparently root in this zone. The zone may include decollement surfaces along which the overlying rocks were transported. Relatively few reflections originate from within autochthonous Grenville basement at the western end of the profile. The Goochland granulite terrane is interpreted to be a westward thrust nappe structure that has overridden a portion of the Chopawamsic metavolcanic rocks. A broad zone of east dipping (20°–45°) reflections bounds the Goochland terrane on the east. These reflections may originate from deformation zones and continue to Moho depths. They appear to be correlative with similar events seen on other Appalachian lines. The pervasiveness of the zone of east dipping events on other seismic reflection lines and the continuity of the adjacent Piedmont gravity high suggest continuity of crustal features along the length of the Appalachians. A major conclusion of this study is that crustal thinning is responsible for the main components of the gravity field in Virginia, that is, the Appalachian gravity gradient and the Piedmont gravity high. The crust thins from about 52 km beneath the Appalachian mountains to about 35 km beneath Richmond, Virginia, and then rethickens by up to 10 km beneath the zone of east dipping reflections (mylonites?) east of Richmond. The I‐64 seismic data also contain a sequence of reflections at about 9–12 s, indicative of lower crustal layering; the base of this zone of reflections coincides almost exactly with the Mohorovicic discontinuity interpreted from earlier refraction work. The layering extends about 70 km west from Richmond, Virginia, and is interpreted as a lower crustal transition zone that is believed to persist across most of Virginia.

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The northern Appalachian, dextral fault system of late Paleozoic age is also present in the central and southern Appalachians. An example of a dextral strike‐slip fault is the 4 km wide Brookneal shear zone in the southwest Virginia Piedmont. The shear zone is, in part, superimposed on the Melrose Granite where an S‐C mylonite was produced by dynamic recrystallization of all constituent minerals. The Arvonia metasedimentary and Charlotte belt metavolcanic rocks contain spaced dextral shear bands at a consistent +24° ± 3° to the shear zone boundary. A minimum displacement estimate of 17 km was obtained from rotated foliation measurements in the Melrose Granite. The age of movement on the Brookneal shear zone has been constrained by isotopic dating to between 324 and 300 Ma. Other faults in the southern Appalachians, including the Nutbush Creek and Modoc zones show similar ages and relative offsets. Possible plate tectonic models that could account for the late Paleozoic dextral fault system throughout the Appalachians include: (1) tectonic escape resulting from the collision of a plate with North America to the north of the Canadian Appalachians, (2) postcollision interplate readjustments involving counterclockwise rotation of Africa relative to North America, and (3) oblique convergence of eastern North America with an oceanic plate moving west.

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Recent investigations in south‐eastern Pennsylvania and northern Maryland have demonstrated a major anastomosing strike‐slip shear system. The Pleasant Grove‐Huntingdon Valley shear system emerges from beneath the coastal plain cover at Trenton, New Jersey, and extends to the area west of Baltimore, Maryland, where it is overlain by the Culpepper Mesozoic rift basin. The sense of offset across this system is dextral. In the Susquehanna River region and north of the shear zone, the rocks of the Octoraro Formation contain evidence for two metamorphisms and deformations prior to strike‐slip shearing, whereas south of the shear zone the Peters Creek Formation contains evidence for only one. The discordance in metamorphic and deformational history across the shear zone suggests the now juxtaposed rocks originated in different parts of the orogen. Although conclusive ages for the strike‐slip deformation do not exist at this time, the timing of deformation is loosely constrained where the shear system crosscuts known Taconian structures in the Piedmont. Comparison of deformation style with other regions in the Appalachian suggests the Pleasant Grove‐Huntingdon Valley shear system is related to Alleghanian transcurrent tectonics in the Piedmont. Palinspastic reconstruction of the Pleasant Grove‐Huntingdon Valley shear system reveals fundamental problems in current tectonic models for the central Appalachian Piedmont. A minimum of 150 km of dextral offset is proposed for the Pleasant Grove‐Huntingdon Valley shear system based on reconstruction of the Cambrian‐Ordovician shelf edge between northern Maryland and southeastern New York. Displacement of this magnitude can account for the previously proposed tectonic models that portray a failed Iapetan rift block and microcontinent that contains the Baltimore Grenvillian massifs. Even though a history of early orthogonal collision is preserved within discrete structural blocks, transcurrent shearing has greatly influenced the distribution of those blocks. Models not including the strike‐slip component of tectonic assembly need serious reconsideration, as evidence grows that the magnitude of orogen‐parallel displacement is equal to or larger than the orthogonal component.