SEISMIC STRUCTURE OF THE US MID-ATLANTIC CONTINENTAL-MARGIN

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 in(3) (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.