MAPPING VARIATIONS IN CRUSTAL THICKNESS AND VP/VS IN THE SOUTHERN APPALACHIANS WITH WIDE-ANGLE SEISMIC DATA: RECONNAISSANCE PROFILES ACROSS THE CAROLINA TERRANE, INNER PIEDMONT, AND BLUE RIDGE

Wide-angle reflections are used to investigate variations in average crustal composition within the southern Appalachians and to test models for isostatic compensation. The profiles cross the Appalachian gravity gradient and the Appalachian gravity low and sample the highest elevations within the orogen. Timed quarry blasts were recorded with short-aperture arrays of three-component seismometers. P-wave velocities and Vp/Vs ratios estimated from travel-time ratios of P and S waves show trends that correlate with surface geology and that place constraints on deeper structures. At distances up to 30 km, Vp/Vs is sensitive to fractures in the upper crust. At greater distances, as raypaths penetrate to greater depths, the values correlate more closely with lithology. Values of 1.69-1.73 for the Kiokee Belt and 1.69-1.72 for the Inner Piedmont are consistent with exposures of granites and granite gneisses that dominate those terrains. For the Blue Ridge, the ratios show trends that correlate with variations in lithology within the thrust complex and underlying rocks. The highest values of Vp/Vs (1.78) correlate with the thickest sections of overthrust carbonates. Crustal averages for Vp and Vp/Vs also vary across the orogen. Average Vp is 6.5 km/s for the Carolina Terrane, decreases to 6.2 km/s for the Inner Piedmont, then increases to 6.5-6.6 km/s for the Blue Ridge. Crustal averages for Vp/Vs are 1.74-1.76 for the Carolina Terrane, 1.72-1.76 for the Inner Piedmont, and 1.74-1.80 for the Blue Ridge. In the Blue Ridge, the highest values correlate with a gravity high that bisects the Appalachian gravity low. Migration of deconvolved P and S reflections shows the crust thickening slightly from 37-38 in the Carolina Terrane to 39-41 km in the Inner Piedmont, then thickening more abruptly to 52 km along the SE margin of the Blue Ridge. Crustal thickness within the Blue Ridge varies from 47-59 km, with the smallest values beneath the Asheville Basin. When combined with a density contrast of 200 kg/m3 at the Moho, these thicknesses are consistent with values predicted for local compensation of the topography. These values also explain the long-wavelength component of Bouguer gravity anomalies and are consistent with the model proposed by Fischer (2002) to explain the persistence of crustal roots for orogens older than 100 m.y.