CRUSTAL STRUCTURE ACROSS THE BRUNSWICK MAGNETIC ANOMALY, OFFSHORE GEORGIA, FROM COINCIDENT OCEAN-BOTTOM AND MULTICHANNEL SEISMIC DATA

We present results from a coincident wide-angle and deep-penetrating, multichannel reflection seismic experiment conducted on the continenta l shelf of the Southeast Georgia Embayment. Over 5000 air gun shots we re recorded by a 6-km-long, towed streamer and six ocean bottom seismi c instruments along a similar to 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 veloci ties 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 con tent in crust south of the BMA. Magnetic modeling based on our seismic results indicates that the BMA may be explained as an edge effect ano maly of the more mafic upper crust. We suggest that the increased mafi c content toward the south is due to rift-related mafic intrusion. An alternative interpretation is discussed in which the seismic transitio n across the BMA is related to an Alleghanian suture. Athough there is no direct indication in the data for the association of the BMA and a n Alleghanian suture along our profile, such an association cannot be ruled out. The southern portion of the profile crosses the margin hing e zone, where we observe seaward dipping reflectors similar to those o bserved along the northern Blake Plateau Basin and Carolina Trough hin ge zones, which have been interpreted as volcanic sequences. Our seism ic data include no evidence for very high velocity (> 7.0 km/s) mid-cr ust to lower-crust inboard of the hinge zone such as is observed in th e Carolina Trough outboard of the hinge zone. This indicates that the volume of emplaced mafic material changes dramatically across the hing e zone and suggests that rift-related magmatic processes along the mar gin were highly focused.