Geologic interpretation of geophysical maps of the pre-Cretaceous “basement” beneath the Coastal Plain of the Southeastern United States
Michael W. Higgins, Isidore Zietz, 1983. "Geologic interpretation of geophysical maps of the pre-Cretaceous “basement” beneath the Coastal Plain of the Southeastern United States", Contributions to the Tectonics and Geophysics of Mountain Chains, Robert D. Hatcher, Jr., Harold Williams, Isidore Zietz
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Salient features of the new aeromagnetic map of the Southeastern United States (Zietz and Gilbert, 1980) are: (1) the Charleston magnetic terrane that is generally high magnetically and has numerous distinct, very high magnetic anomalies within it; (2) the northern Florida magnetic terrane that is virtually identical magnetically to the Charleston magnetic terrane; (3) a linear series of magnetic low anomalies within a continuous magnetic low anomaly, collectively called the Altamaha magnetic anomaly, that is more than 1,150 km long and that arcs across the Continental Shelf, from about 33°30’N., 76°30’W., to the Georgia coastline at the mouth of the Altamaha River and trends inland across southern Georgia and Alabama; (4) the southern end of the East Coast anomaly and its shoreward branch, the Brunswick anomaly, which is part of the northern Florida terrane; (5) the characteristic magnetic terranes of the Piedmont and Valley and Ridge provinces; (6) the southern end of the New York-Alabama lineament and the magnetically high terrane west of it; and (7) the lineament formed by the nearly straight northwestern margin of the Charleston magnetic terrane in South Carolina and Georgia, the northwestern margin of the Altamaha anomaly, and the southern margin of the magnetically high terrane northwest of the New York-Alabama lineament in Alabama.
A newly compiled gravity map of part of the Southeastern United States shows that the Charleston and northern Florida magnetic terranes have mixed gravity expression, with gravity high anomalies coinciding with the distinct, very high anomalies on the aeromagnetic map. In general, the trace of the Altamaha magnetic anomaly either coincides with or lies just south of gravity low anomalies that are linear and form a linear series along or closely parallel to the magnetic anomaly. Near its northeastern end, the Altamaha magnetic anomaly lies along a relatively steep, northwestward-sloping gravity gradient. The East Coast anomaly coincides with linear gravity high anomalies.
The Charleston and northern Florida magnetic terranes are interpreted as being virtually the same geologic terrane, a predominantly mafic terrane, intruded by mafic plutons of batholithic proportions, that, until late Paleozoic time, was part of the African or African/South American plate. The northwestern margin of the Charleston terrane is interpreted as the Alleghanian suture between Africa and North America, and also as a major strike-slip fault, the Carolina-Mississippi fault, that extends to the southwest along the northwestern border of the Altamaha anomaly and the southern border of the magnetically high terrane northwest of the New York-Alabama lineament. The Altahama anomaly is interpreted as being caused by a fault-bounded, sediment-filled trough, locally as deep as 6 km, that divides the Charleston-northern Florida terrane. The Carolina-Mississippi fault truncates the Charleston terrane, the Piedmont and Valley and Ridge provinces, the New York-Alabama lineament, and the magnetically high terrane northwest of the New York-Alabama lineament, and the absence on the southeastern side of the fault of magnetic terranes found on the northwestern side suggests right-lateral displacement.