Geologic interpretation of geophysical maps of the pre-Cretaceous “basement” beneath the Coastal Plain of the Southeastern United States

Abstract

Salient features of the new aeromagnetic map of the Southeastern UnitedStates (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 125 on June 21, 2015 memoirs.gsapubs.org Downloaded from 126 M. W. Hìggins and I. Zietz 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 a...

Crustal structure of rifted and convergent margins : the U.S. East Coast and Aleutian margins

“…shown outlined in the magnetic anomaly map of Figure lc, is bounded to the south and east by the BMA and to the west and north by s hort-wavelength, northeast trending magnetic a nomalies associated with Piedmont metamorphic rocks. Various authors have commented on the possible significance of the Brunswick terrane [Popenoe and Zietz, 1977;Higgins andZeitz. 1983: Klitgord et a/ .. 1983;Williams and Hatcher, 1983].…”

Terranes and overlap sequences in the Central and Southern Appalachians, an expanded explanation for part of the Circum-Atlantic terrane map

“…It is tentatively portrayed by Horton and others (1989a, Fig. 1) as a modified version of the magnetic lineament which Higgins and Zietz (1983) term the Carolina-Mississippi fault.…”

“…REFERENCES- Chowns and Williams (1983); Daniels and others (1983); Higgins and Zietz (1983); others (1984,1989a); McBride and Nelson (1988); Nelson and others (1985aNelson and others ( , 1985bNelson and others ( , 1985c; Rankin (1977); Sinha and others (1980);Speer (1982); Tauvers and Muehlberger (1987); Thomas and others (1989); Hatcher (1982, 1983); Zietz and Gilbert (1980) …”

“…These could all be fault blocks of a single suspect terrane, but because of a lack of information about the stratigraphy and geologic history, Horton and others (1989a) tentatively consider the Roanoke Rapids as a separate terrane. REFERENCES-Farrar (1985a; ; Horton and others (1989a); Kite and Stoddard (1984); others (1983, 1987); Russell and others (1985) NA37d: CHARLESTON TERRANE The Charleston terrane of Horton and others (1989a) is equivalent to the "Charleston magnetic terrane" of Higgins and Zietz (1983) and the "Brunswick terrane" of Hatcher (1982, 1983) except for some boundary differences. The Charleston terrane is partly covered by overlapping Mesozoic strata of the South Georgia basin and is completely covered by Coastal Plain sediments.…”

“…Other circular short-wavelength anomalies have been attributed to plutons thought to be of Mesozoic age (Daniels and others, 1983). EVIDENCE FOR TERRANE IDENTITY AND TIME OF AMALGAMATION~The Charleston terrane is distinguished from the Carolina and Spring Hope terranes to the northwest as an area of broad-wavelength magnetic highs Hatcher, 1982,1983;Higgins and Zietz, 1983;Zietz and Gilbert, 1980). The magnetically high region cannot be explained by variations in depth to the moho and must be attributed to material at middle to upper levels of the crust (Horton and others, 1989a).…”