TOWARD A RECONCILIATION OF SEISMIC REFLECTION AND GEOLOGICAL OBSERVATIONS ACROSS AN INTERNAL BASEMENT MASSIF, SOUTHERN APPALACHIANS, USA: PINE MOUNTAIN WINDOW

The Pine Mountain window of the southern Appalachian orogen provides a window through the allochthonous Inner Piedmont that exposes Grenvillian basement and its Paleozoic(?) cover. Controversy, however, has surrounded the question of whether the window has been transported above the master Appalachian décollement or is an autochthonous block that has been overridden by the décollement. Reprocessed deep seismic reflection data from west-central Georgia (pre- and post-stack noise reduction, examining amplitude variations, and pre-stack depth migration) indicate that a significant band of reflections occurs almost continuously beneath the window collinear with the originally recognized décollement reflections north of the window. However, the marked variation of the décollement image from strong and coherent north of the window to more diffuse directly beneath the window is likely a consequence of the varying geology between the Inner Piedmont and the window but may also involve imaging problems related to changes in topography and fold of cover (i.e., signal-to-noise ratio). Two alternative tectonic models for the Pine Mountain window can account for the observed variation in the décollement reflectivity: (1) the window is undercut by a relatively smooth continuation of the décollement beneath an allochthonous basement duplex or horse thrust upward from the south along the Late Proterozoic rifted continental margin or (2) the window represents localized exhumation of basement and cover sequence along a zone of distributed intra-basement shearing directly beneath the window. In either model, just south of the southern margin of the window (Dean Creek fault) is the Iapetan suture between North American-affinity Piedmont rocks and the exotic Carolina (Avalon) terrane to the south. This suture dips southward and roots in the lower crust beneath the Coastal Plain along what subsequently became the late Paleozoic Alleghanian suture zone between the Appalachians and Gondwana.