We present results from a coincident wide-angle and deep-penetrating, multichannel reflection seismic experiment conducted on the continental shelf of the Southeast Georgia Embayment. Over 5000 air gun shots were recorded by a 6-km-long, towed streamer and six ocean bottom seismic instruments along a ~250-km, north-south profile crossing the Brunswick magnetic anomaly (BMA). These data indicate a transition in seismic properties across the BMA, including higher seismic velocities south of the BMA, particularly in the upper crust (7--15 km depth), and a transition from reflective to transparent crust from north to south. We interpret this transition to indicate an increased mafic content in crust south of the BMA. Magnetic modeling based on our seismic results indicates that the BMA may be explained as an edge effect anomaly of the more mafic upper crust. We suggest that the increased mafic content toward the south is due to rift-related mafic intrusion. An alternative interpretation is discussed in which the seismic transition across the BMA is related to an Alleghanian suture. Although there is no direct indication in the data for the association of the BMA and an Alleghanian suture along our profile, such an association cannot be ruled out. The southern portion of the profile crosses the margin hinge zone, where we observe seaward dipping reflectors similar to those observed along the northern Blake Plateau Basin and Carolina Trough hinge zones, which have been interpreted as volcanic sequences. Our seismic data include no evidence for very high velocity (>7.0 km/s) mid-crust to lower-crust inboard of the hinge zone such as is observed in the Carolina Trough outboard of the hinge zone. This indicates that the volume of emplaced mafic material changes dramatically across the hinge zone and suggests that rift-related magmatic processes along the margin were highly focused. ¿ American Geophysical Union 1994