Abstract

A southeastward extension onto the Coastal Plain of an earlier COCORP traverse, which confirmed large-scale, thin-skinned thrusting of crystalline rocks of the southern Appalachians, has provided some of the most spectacular reflections yet seen in crustal seismic data. Most of the reflectors can be interpreted as either fault surfaces or as metamorphosed strata of late Precambrian—early Paleozoic age. They are consistent with the hypothesis that a major detachment extends eastward beneath this part of the orogen, although other interpretations with a more complex pattern of detachments or sutures are also possible. Large-scale overthrusting provides a mechanism for incorporating sedimentary rocks into the lower crust and may help to explain many of the layered features on crustal seismic data. Reflections from deep beneath the Coastal Plain indicate that the structural configuration of the rocks is complex and that the remains of a collision zone are being observed. Several east-dipping horizons, which bear strong similarities to thrust faults in Valley and Ridge sedimentary rocks, are seen in the basement at shallow and mid-crustal levels beneath the Coastal Plain. The Augusta fault, for example, displays a reflection which extends at a low angle some 80 km or more southeast of its surface position. In conjunction with surface geologic information, these new data demonstrate that late Paleozoic compressive deformation was pervasive and resulted in lateral movements in the upper crust extending from the Valley and Ridge to the crystalline rocks beneath the Coastal Plain — a distance of 400 km or more. A large antiform, cresting at about 2.3 sec, or about 6 km below the surface, and other structures beneath the Coastal Plain of Georgia deserve further consideration for petroleum exploration, although metamorphism may have eliminated petroleum from these rocks. Refracted arrivals and fault geometries indicate two Triassic rift basins beneath Coastal Plain sedimentary rocks, one of which has apparently not been recognized previously.

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