DEEP-STRUCTURE OF THE UNITED-STATES ATLANTIC CONTINENTAL-MARGIN, OFFSHORE SOUTH-CAROLINA, FROM COINCIDENT OCEAN-BOTTOM AND MULTICHANNEL SEISMIC DATA

We present the results of a combined multichannel seismic reflection ( MCS) and wide-angle, ocean bottom seismic profile collected in 1988 ac ross the Carolina Trough on the U.S. Atlantic continental margin. Inve rsion of vertical-incidence and wide-angle travel time data has produc ed a velocity model of the entire crust across the continent-ocean tra nsition. The margin consists of three structural elements: (1) rifted continental crust, comprising 1-4 km of post-rift sedimentary rocks ov erlying a 30 -34 km thick subsedimentary crust, (2) transitional crust , a 70- to 80-km-wide zone comprising up to 12 km of postrift sediment ary rocks overlying a 10- to 24-km-thick subsedimentary crust, and (3) oceanic crust, comprising 8 km of sedimentary rocks overlying an 8-km -thick crystalline crust. The boundary between rifted continental and transitional crust, marked by the Brunswick magnetic anomaly, represen ts an abrupt change in physical properties, with strong lateral increa ses in seismic velocity, density, and magnetic susceptibility. The tra nsitional crust contains mid-crustal seaward-dipping reflections obser ved on the MCS section and has seismic velocities of 6.5-6.9 km/s in t he midcrust and 7.2-7.5 km/s in the lower crust. Modeling of potential field data shows that transitional crust also produces the prominent, margin-parallel gravity anomaly and the Brunswick and East Coast magn etic anomalies. These observations support the interpretation that the transitional crust was formed by magmatism during continental breakup . The prodigious thickness (up to 24 km) of igneous material rivals th at interpreted on continental margins of the North Atlantic (e.g., Hat ton Bank and Voring Plateau), which formed in the vicinity of the Icel and hotspot. These observations, when combined with other transects ac ross the margin, confirm previous suggestions that the U.S. Atlantic m argin is strongly volcanic and further imply that the magmatism was no t the result of a long-lived mantle plume.