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Autochthonous sediments equivalent to Valley and Ridge formations are interpreted to exist beneath crystalline rocks of the entire Blue Ridge and western portion of the Piedmont. Although hydrocarbon potential has not been determined, we have defined the southeastern extent of sediments beneath the allochthonous southern Appalachians. Truncation of underlying sediments by a subsurface fault occurs at 10 km (6 mi) depth approximately 55 km (34 mi) southeast of the Brevard zone. Farther southeast, a basal detachment might exist, but it would be within mylonitized Grenville basement. Blue Ridge basement rocks probably originated from this area. The interpretation is primarily based on detailed analysis of reprocessed COCORP seismic data, modeling potential fields, and the un isputable fact that Grenville rocks must be cut by a deep fault someplace beneath the southern Appalachians. All previous theories which attempted to correlate with the original interpretation of a regional detachment underlying the entire southern Appalachians should be reexamined. Although detachments and major thrust faults undoubtedly exist throughout the southern Appalachians, they do not form one continuous overthrust sheet and are not underlain by sediments deposited on the ancient continental margin of North America.
Geophysical studies of the southern Appalachians have determined them to be largely allochthonous. Seismic data show that a continuous master decollement underlies the Valley and Ridge, Blue Ridge, and Piedmont provinces. Other forms of geophysical data are consistent with the hypothesis of an extensive overthrust system. The question remains, however, how far east can we define a continuous master decollement? This question has been debated in many conferences in recent years where two extreme positions have developed. One side believes that the Brevard zone represents a cryptic suture continuing to depth beneath the inner Piedmont and terminating the master decollement at depth. The alternative position draws a detachment continuous from the western Valley and Ridge, beneath the ent re Appalachians to the southeast, deep beneath the coastal plain, and possibly out to the edge of the continent in the Atlantic Ocean. In this paper, a moderate position is presented which places a sloping master decollement root zone beneath the eastern Piedmont. The adjoining province of island arc assemblages is thus regarded as an accreted terrain which is generally regarded as autochthonous. Evidence for such a model is primarily from reflection profiles across Georgia. Additional support comes from gravity, aeromagnetics, magnetotellurics, and surface geology.
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