TOWARD A RECONCILIATION OF SEISMIC REFLECTION AND FIELD GEOLOGIC MAPPING FROM THE PINE MOUNTAIN BELT: PRELIMINARY RESULTS

The Pine Mountain belt (PMB) of western Georgia and eastern Alabama is the southernmost internal Appalachian massif, and it provides a window through the allochthonous Blue Ridge-Piedmont that exposes Grenville basement and its Paleozoic(?) cover. The results of several years of geologic mapping of the PMB and its boundary faults imply that the Appalachian detachment must continue at depth from the Inner Piedmont beneath the PMB southward until it presumably roots beneath the Coastal Plain. Field relationships indicate that the faults framing the window are dominated by Alleghanian early thrusting and later dextral strike-slip. These results seem to contradict seismic reflection interpretations across the PMB in western Georgia that suggest that the detachment, typically expressed as a band of reflections up to ~0.4 seconds duration below the Piedmont and slate belts, does not continue southward beneath the PMB. Instead, the detachment supposedly is displaced upward to near the surface by normal faulting along the Towaliga dextral strike-slip fault. In order to re-evaluate this interpretation in light of field relationships, we have reprocessed the available seismic reflection data (acquired and originally processed by COCORP). This effort has so far included techniques aimed at reducing noise, improving lateral reflection coherency, and reflection migration as well as application of alternative methods of data visualization (e.g., examining amplitude variations and reflection strength). The preliminary results indicate that a thin band of reflectors may in fact be imaged continuously beneath the PMB collinear with the originally recognized detachment reflectors north of the Towaliga fault. Although the image of the reflectors beneath the PMB is much weaker than that just north of the fault, the abrupt variation in the reflectivity of the detachment is apparently no different than that seen elsewhere beneath the Inner Piedmont or Blue Ridge in western Georgia and eastern Alabama. Our preliminary results therefore suggest that the PMB may be undercut by a relatively smooth continuation of the Appalachian detachment beneath an allochthonous block (horse?) thrust upward from the south along the old Grenville continental margin.