Constraints on the structure and tectonic development of the Early Mesozoic South Georgia Rift, southeastern United States; Seismic reflection data processing and interpretation
Authors: McBride John H. 
Published: February 2008

Seismic reflection data collected by the Consortium for Continental Reflection Profiling (COCORP) over the buried Triassic‐Jurassic South Georgia rift, reprocessed in order to improve the upper 5 s of reflection time, provide a window into the regional deep structure of the rift and delineate variations in basin structure and depositional styles. Located at the southernmost extent of the North American east coast early Mesozoic rift system, the structure of the South Georgia rift basin has previously been known only from scattered drilling and potential field interpretation, leaving the internal and boundary, as well as the deep, structure unknown. The reflection data reveal that the South Georgia basin consists of a broad, complex terrane of isolated synrift grabens with intervening structural highs, in contrast with earlier concepts of a single narrow trough or an amorphous “sag” basin. Two main subbasin depocenters are observed in west and west central Georgia, the largest of which is more than 100 km in width, and the deepest at least 7 km at its inferred base. Analysis of the reflection data indicates that the distinct structural and seismic‐stratigraphic character of the reflection packages is analogous to other, better studied extensional provinces (e.g., North American Basin and Range). The reprocessing of the reflection data provides a view of contrasting styles of faulting and subsidence within the rift. The structural style of the subbasins varies drastically along strike, changing from (1) a narrow asymmetric graben bordered by a planar and more deeply penetrating normal fault to (2) a broad and roughly symmetric graben bordered by a shallower‐dipping, possibly listric fault, along which extension may have been accommodated at depth by a low‐angle detachment or by a more complex system of high‐angle faults. Only limited crustal extension is suggested by the narrower graben and planar fault (1), whereas much greater extension would be expected from the wider graben bordered by a listric/low‐angle fault (2). Such a dual style of basin formation is predicted by segmented plate models of variable extension along continental margins and intracratonic rifts, and may imply a significant change in crustal rheology along the developing rift. Integration of the interpretation of the regional structure of the South Georgia basin with that of the deeper crustal structure, as inferred from the 16‐s (reflection time) COCORP data, indicates that the synrift basins developed mainly over the “upper plate” of the Alleghanian (late Paleozoic) suture between North America and west Africa. The major subbasin border faults dip northward in an antithetic relation to the predominantly northward vergence of the suture zone. The border faults thus do not appear to have reactivated antecedant structure. Although the development of the South Georgia rift appears to have approximately coincided with early Mesozoic thinning of the crust in the southeastern United States, no horizontally “layered” or “laminated” lower crust is observed as has been associated with other, more fully rifted terranes (e.g., North American Basin and Range; continental shelf around British Isles). However, the Moho discontinuity appears to have formed as a dynamic feature superimposed on late Paleozoic compressional structure probably associated with early Mesozoic rifting. These findings thus support the interpretation that the South Georgia rift developed as a major early Mesozoic depocenter and possible incipient spreading center associated with the initial formation of the central Atlantic Ocean.