Abstract The continental slope off eastern United States is built across a zone of deeply buried (6-12 km) fault blocks that formed during the rifting of continents. Associated with these blocks are intrusives, salt diapirs, and reef complexes; all are underlain by a zone of thinned continental, transitional, and oceanic crust beneath the shelf, slope, and upper continental rise. Four basic sedimentary units appear to underlie much of the slope: (1) a probable terrigenous clastic-evaporite-volcanic sequence associated with fault blocks; (2) a carbonate platform reef sequence, which probably formed along the ancestral shelf-slope break; (3) a marine and non-marine sequence built over the carbonate rocks north of the Blake Plateau as the shelf in some areas prograded seaward during the Cretaceous; and (4) a thick rise prism of sediments of Tertiary and younger age that laps up on the base of the slope north of the Blake Spur. Much of the present slope is an erosional surface, cut into lower Tertiary and Upper Cretaceous strata during Cenozoic marine regressions as sediment moved downslope to build the rise prism; the shelf-slope break has retreated 5-30 km from the break that existed during the Late Cretaceous. Over the Blake Plateau, periodic scour by currents associated with the ancestral Gulf Stream caused a 300-km landward jump of the shelf break at about the end of Paleocene as the plateau continued to subside. Laterally, the slope varies in the distance that it retreated (maximum to the south) and in the degree to which reeflike masses are developed. Continuous well-developed carbonate banks or reefs controlled location of the shelf edge of the southern Blake Plateau; they were terminated both there and to the north near the end of the Early Cretaceous, although reef formation may have shifted to locations beneath the present continental shelf. Local progradation of the outer shelf and slope occurred during parts of the Jurassic and Cretaceous, depending on the sediment supply and the effects of transgression and regression of sea level. During the Tertiary, however, erosion and retreat of the shelf edge was dominant.

Abstract Presently, the continental margin of the southeastern United States (Fig. 1) forms a zone of transition between the actively building, steep-fronted carbonate platform of the Bahamas and the typical eastern North American terrigenous clastic-dominated, drowned, shelf-slope-rise configuration. This region of the continental margin is underlain by two major sedimentary basins—the Blake Plateau Basin and the Carolina Trough (Fig. 2)—which are different in shape, basement structure, and history. Indeed, the two southern basins show some of the greatest contrasts of any basins of eastern North America, especially in their early response to rifting and in the change from rifting to drifting. The region has experienced abrupt major changes in geological conditions, most notably the onset of Gulf Stream flow in the early Tertiary. Morphologically, the area is dominated by the broad, flat Blake Plateau at about 800-1,000 m water depth (Fig. 1). The plateau is bounded to the east by the extremely steep Blake Escarpment, descending to 5,000 m water depths. To the west, a short continental slope rises to a continental shelf. This Blake Plateau morphology characterizes the margin east of Florida and north of the Bahamas. North of Florida the margin merges into the typical shelf-slope-rise morphology. Just north of the Blake Escarpment and its northern projection, the Blake Spur, the Blake Ridge extends away from the continental slope at water depths exceeding 2,000 m (Fig. 1). This broad ridge is a Cenozoic, sedimentary drift deposit controlled by bottom currents. (For the reader who is beginning to wonder why half of the features of this region seem to be named "Blake", the Blake was a Coast Survey steamer from which investigations off the southeastern U.S. were carried out in 1877 to 1880. Ferromanganese nodules were discovered on the Blake Plateau at that time [Murray, 1885].)

Abstract The E5 transect extends southeastward from the Cumberland Plateau across the Appalachian orogen, the Atlantic Coastal Plain, Continental Shelf and Slope, and the Blake Plateau Basin; it is a transect through the Precambrian-early Paleozoic and Mesozoic-Tertiary continental margins of North America. The transect consists primarily of a 100-km-wide geologic strip map, a cross section, and supporting geophysical data. The cross section is based on surface geology, surface and subsurface data from Coastal Plain and offshore drill holes, shipboard and aeromagnetic data, and gravity and seismic reflection data, including the ADCOH and COCORP southern Appalachians lines. Elements of the map and cross section include: (1) the Appalachian foreland fold-thrust belt and western Blue Ridge Late Proterozoic-Paleozoic continental margin; (2) the eastern Blue Ridge-Chauga belt-Inner Piedmont oceanic-continental fragment terrane; (3) the volcanicplutonic Carolina terrane containing the middle to late Paleozoic high-grade Kiokee belt; and (4) a major geophysical ly defined terrane beneath the Coastal Plain. Three Paleozoic sutures may be present along the section line: the Hayesville thrust, the Inner Piedmont-Carolina terrane boundary (Taconic or Acadian suture?), and an eastern boundary of the Carolina terrane (Alleghanian? suture) in the subsurface beneath the Coastal Plain. The modern continental margin consists of the terrestrial clastics-filled Triassic-Jurassic basins and offshore marine Jurassic- Cretaceous clastic-carbonate bank succession overlain by younger Cretaceous and Tertiary sediments. Above the Late Cretaceous onshore unconformity lie Cenozoic sediments that represent seaward prograding of the shelf-slope, truncated by Miocene to recent wave abrasion and currents.

Abstract The detachment (“simple-shear”) model of continental extension predicts that opposing passive- margin pairs are asymmetrical. The asymmetry is reflected in the morphology, structure, uplift/ subsidence history and/or thermal evolution of the passive margins. This results in two broad classes of passive margins—upper-plate margins in the hanging wall of the detachment, and lower-plate margins in the footwall. The detachment model applied here incorporates pure-shear extension above and below the master detachment, and ramp-flat geometry for the detachment. The detachment model is applied to the United States Atlantic-northwest Africa and the southern Australia-Antarctica margin pairs in some detail, and to the southern and northern Atlantic Oceans, the Tasman Sea margins and the Provencal basin in lesser detail. Asymmetry of various characteristics has been recognized in all cases, and upper- versus lower-plate settings have been determined. In the case of the United States Atlantic-northwest Africa margins, a switch in asymmetry along the margin pair is interpreted across a major transfer fault. On the United States Atlantic margin, the Blake Plateau is considered to be an upper-plate margin, whereas the Carolina-Baltimore Canyon Trough segments exhibit lower-plate characteristics. An attempt is made to palinspastically reconstruct the southern Australia-Antarctica margins, including the continental extension of more than 200 km. Application of the detachment model to these passive margins has resulted in a number of outcomes of general interest. Most importantly, attempts to make a simple distinction between “pure-shear” and “simple-shear” models of extension should be abandoned in favor of determination of the three- dimensional finite deformation field across the whole extended region. The model supports the view that at least some of the magnetic quiet zones (MQZ) that occur oceanward of many passive margins consist largely of highly extended, metamorphosed and underplated (intruded) continental crust rather than oceanic crust. Subsequent subduction of this thin continental crust will potentially lead to arc magmas with distinct continental signatures. Predrift reconstruction of passive margins can be substantially refined by (1) careful determination of the lithospheric strain field (including detachment geometry) during continental extension, and (2) recognition of major transfer faults on opposing margins.