Paleozoic and Grenvillian Structures in the southern Appalachians: Extended interpretation of seismic reflection data

Interpretive reprocessing of seismic reflection data has elucidated Paleozoic and Grenvillian structures in the southern Appalachians. The seismic data include a 7500‐km² grid of ADCOH, Seisdata, and COCORP reflection profiles that traverse the Blue Ridge and Inner Picdmont geologic provinces of North Carolina, South Carolina, and Georgia. Surface geology and potential field data were used to constrain the interpretation. The reprocessed seismic reflection data have delineated the internal and external geometry of the crystalline Blue Ridge‐Inner Piedmont allochthon, including the locations of the Blue Ridge master décollement, Haycsville fault, and Brevard fault zone. On the basis of the reprocessed data, all of the major faults within the allochthonous upper crust sole in the Blue Ridge master décollement. Reflections extending to the southeast from beneath the surface location of the Hayesville fault to the Blue Ridge thrust might be the seismic signature of a high strain zone. This implies that internal deformation of the Blue Ridge allochthon associated with the Alleghanian orogeny might have occurred farther to the west than has been previously documented from field studies. Relative amplitude seismic data enabled the discrimination between Blue Ridge‐Inner Piedmont crystalline rocks and underlying lower Paleozoic shelf strata, thereby delineating the Blue Ridge thrust. The interpreted geometry constrains the top of the shelf sequence beneath the Blue Ridge to depths of less than 3 km. This relatively shallow depth of the shelf strata together with the presence of duplex structures and bright spots that are imaged within the sequence might imply favorable conditions for hydrocarbon exploration beneath the Blue Ridge. Midcrustal reflections from within the upper‐to‐Iower crust are interpreted to originate from preserved Grenvillian structures that were reactivated at the basement surface during Late Proterozoic‐Early Cambrian extension. Reflection continuity is occasionally disrupted by interpreted post‐Grenvillian, pre‐Early Cambrian low‐density intrusions. Topography at the basement surface, possibly caused by the intrusions, is interpreted to have controlled the formation of some of the structures within the overlying allochthon, including Blue Ridge and Brevard fault zone ramps. Correlation of seismic time‐structure contour maps with available gravity data and two‐dimensional gravity modeling suggest that anomalies in the gravity field can be attributed to low‐density sources within the autochthonous crust. Discontinuous reflection packages from depths of 36–42 km are interpreted to originate from the Mohoroviĉiĉ discontinuity. The reflectors trend about N15°E with a true dip of approximately 15°NW.