REINTERPRETATION OF ADCOH AND COCORP SEISMIC REFLECTION DATA WITH CONSTRAINTS FROM DETAILED FORWARD MODELING OF POTENTIAL FIELD DATA – IMPLICATIONS FOR LAURENTIA-PERI-GONDWANA SUTURE

To better constrain the structure of the Laurentian – Peri-Gondwana suture zone, including footwall structures beneath the Blue Ridge/Inner Piedmont allochthon, maps and a two-dimensional regional cross-section model, constrained by seismic reflection (ADCOH and COCORP) and surface geology have been developed by forward and inverse modeling the aeromagnetic and gravity fields. The profile traverses the Southern Appalachians from the Valley and Ridge to the Coastal Plain of South Carolina, and is consistent with the seismic data, as well as the observed gravity and magnetic fields at long and short wavelengths. Spatially coincident gravity and magnetic anomalies limit the inherent non-uniqueness of the structural interpretation, especially for igneous bodies.

The Central Piedmont Suture (CPS) is the boundary between the Laurentian Inner Piedmont and the Peri-Gondwanan Carolina Terrane. The new forward model is consistent with previous seismic interpretations of CPS as a low-angle listric thrust fault ramping up from a mid-crustal detachment on the Alleghenian thrust at depths over 12 km. Unlike previous interpretations, the model explains the Appalachian paired gravity anomaly (APGA, the Appalachian low and the East Coast high) without a change in lower crustal density. The APGA is explained by the seismically well-constrained increase in crustal thickness northwest of the CPS and the observed higher densities of the Carolina Terrane relative to the Inner Piedmont and Blue Ridge.

Most surprisingly, the seismically imaged low-density metamorphic rocks in the Piedmont and Blue Ridge allochthon appear to over-thrust high-density footwall duplexes. The gravity anomalies are primarily correlated with the footwall folds and not rift basins in the underlying Grenville basement. The high densities suggest that the folded footwall reflectors may not represent Paleozoic shelf strata as previously interpreted, but may need to be reinterpreted as Grenville basement duplexes. This also implies that the eastern edge of Laurentian margin shelf sediments has been displaced westward beyond the Hayesville Fault. At the same time, the lack of need for a lateral density contrast in the lower crust opens the possibility that Grenville basement may extend as far east as the Coastal Plain.