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 across the Carolina Trough on the U.S. Atlantic continental margin. Inversion of vertical-incidence and wide-angle travel time data has produced a velocity model of the entire crust across the continent-ocean transition. The margin consists of three structural elements: (1) rifted continental crust, comprising 1-4 km of post-rift sedimentary rocks overlying a 30 -34 km thick subsedimentary crust, (2) transitional crust, a 70- to 80-km-wide zone comprising up to 12 km of postrift sedimentary 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, represents an abrupt change in physical properties, with strong lateral increases in seismic velocity, density, and magnetic susceptibility. The transitional crust contains mid-crustal seaward-dipping reflections observed on the MCS section and has seismic velocities of 6.5-6.9 km/s in the 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 magnetic 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 that interpreted on continental margins of the North Atlantic (e.g., Hatton Bank and Voring Plateau), which formed in the vicinity of the Iceland hotspot. These observations, when combined with other transects across the margin, confirm previous suggestions that the U.S. Atlantic margin is strongly volcanic and further imply that the magmatism was not the result of a long-lived mantle plume.