WIDE-ANGLE, THREE-COMPONENT SEISMIC SOUNDINGS OF THE CRUST BENEATH THE BLUE RIDGE AND ADJACENT PROVINCES, SOUTHERN APPALACHIANS

The goal of this study was to map variations in crustal thickness and velocity structure within the SW end of the Blue Ridge to test models for isostatic compensation of topography. We used a portable array of 20 digital recorders with 3-component, 4.5 Hz geophones and timed quarry blasts (51 blasts at 17 quarries) as seismic sources. Station spacings were 50-250 m; recording distances were 6-200 km.

Travel-time inversion suggests an average crustal velocity of 6.5-6.6 km/s. Vp/Vs ratios in the upper crust derived from S/P time ratios are about 1.45 for recording distances of 6-8 km; this value is consistent with thick sequences of quartzite that support the higher elevations. At greater offsets, ratios show an increase with offset, from 1.65-1.75 for raypaths restricted to the regional gravity low, and from 1.72-1.79 for raypaths that cross a local gravity high.

Migration of deconvolved wide-angle reflections suggests that crustal thickness increases from 38 km beneath the Carolina Terrane to 47-51 km along the southeastern flank of the Blue Ridge. Migrated sections within the Blue Ridge show a marked increase in the number of reflectors at depths of 20 km and 40 km. The latter depth marks the top of a zone with high apparent reflectivity that extends to depths of 50-55 km, suggesting a layered zone of variable thickness in the lower crust that is similar to models proposed for the Cumberland Plateau and the Adirondacks. A minimum crustal thickness of 46 km is observed beneath the floodplain of the French Broad River south of Asheville, suggesting that topography may be supported by Airy-type crustal roots. Prominent reflectors also occur at 5 km and 10 km, near the projected depths of the base of the Blue Ridge allochthon and top of North American basement. Receiver functions computed for broadband stations GOGA (Carolina Terrane/Inner Piedmont boundary) and MYNC (Blue Ridge) show a similar northwestward increase in crustal thickness. Upper mantle events that arrive shortly after the Ps Moho conversion for MYNC correlate with a peak in the number of migrated reflectors at a depth of 60 km.

These findings differ with earlier models that show a flat Moho dipping to a maximum depth of 43 km beneath the Blue Ridge. They are consistent, however, with gravity data and with prominent crustal roots imaged for other Paleozoic orogens such as the Urals.