LAURENTIAN - PERI GONDWANAN SUTURE ZONE - REINTERPRETATION OF ADCOH AND COCORP SEISMIC REFLECTION DATA WITH CONSTRAINTS FROM NEW POTENTIAL FIELD DATA

To better constrain the structure of the Laurentian – Peri-Gondwana suture zone and footwall structures beneath the Blue Ridge/Inner Piedmont allochthon, maps and a 2-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 Central Piedmont Suture (CPS) is the boundary between the Laurentian Inner Piedmont and the Peri-Gondwanan Carolina Super Terrane. The new forward model is consistent with previous seismic interpretations of CPS as a low-angle thrust fault ramping up from a mid-crustal detachment on the Alleghanian thrust at depths of 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 Super Terrane relative to the Inner Piedmont.

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 eastward as the coastal plain, and the Laurentia-Gondwana collisional suture may continue as a low angle thrust fault under the coastal plain. Apparent Alleghanian uplifts of 5 to 10 km in the Blue Ridge, Inner Piedmont, and Carolina Super Terrane can be accommodated on low angle thrust faults ramping up from the Appalachian decollement and do not require significant extensional exhumation as previously suggested.