THE BRUNSWICK MAGNETIC ANOMALY: GEOPHYSICAL SIGNATURE AND GEOLOGIC SOURCE

Rifted continental margins represent a dominant tectonic feature worldwide, and understanding continental break-ups and the development of sedimentary basins along divergent margins remain areas of active research. In particular, the nature and variability of the continent-ocean boundary recorded in early-forming 'transitional' crust, including late rift to early drift structures, and the role of magmatism during rifting and the inception of seafloor spreading, remain persistent problems. They involve complex interactions between plate setting, lithosphere mechanics, the thermal regime, and the presence of pre-existing structures.

We use integrated analyses and forward and inverse modeling of potential field data to characterize and provide a single geologic interpretation for the enigmatic and controversial Brunswick Magnetic Anomaly (BMA), a prominent negative magnetic lineation that is developed within the ocean-continent transition of the Southeast Georgia Embayment, and that lacks an equivalent along the West African margin. On the basis of magnetic profiles extracted from gridded data, residual magnetic maps, and simultaneous 2.5D forward and 3D Euler inverse modeling of the gravity and magnetic fields, we suggest that the source of the BMA both onshore and offshore is a series of semi-continuous to discrete late-stage rift-related mafic intrusions of Mesozoic age segmented by incipient fracture zones. This interpretation is supported by the observation that the BMA is independent of the East Coast Magnetic Anomaly, implying a pre-drift source, and that the amplitude and frequency of the anomaly changes nearshore across the Blake Spur fracture zone that divides the offshore BMA, where continental breakup occurred, from the onshore BMA. The fracture zone projects onshore to a crustal boundary separating exotic terranes accreted to the southern Laurentian margin during Appalachian orogenesis. Modeling demonstrates for the first time that a Mesozoic rift-related mafic body can explain the anomaly onshore. The lack of a BMA equivalent on the West African margin suggests a single lithosphere dislocation may have initiated the opening of the Atlantic, similar to crustal scale simple shear models for extension of continental lithosphere (e.g., Wernicke, 1981).