Wide-angle refl ections generated by fi ve controlled blasts and over 110 timed quarry blasts in the Southern Appalachians were used to test models for isostatic compensation of topog raphy. The profi les cross the Appa lachian gravity gradient and gravity low and sample the highest elevations within the orogen. Migration of P, SV, and SH refl ections suggests that crustal thickness varies from 35 to 39 km within the coastal plain and 37-39 km within the Carolina terrane. It increases northwestward from 40 to 45 km across the Inner Piedmont, and then thickens to 50-52 km along the southeastern fl ank of the Blue Ridge Mountains. Crustal thickness within the Blue Ridge Mountains ranges from 47 to 56 km. Receiver functions for broadband stations GOGA and MYNC show a similar pattern in crustal thickness. Assuming Airy compensation, the correlation between elevation and Moho depth suggests a range of 50-150 kg/m 3 for the density contrast between the crustal root and mantle. The greatest Moho depths are associated not with the tallest peaks, but rather with the broadest portions of the mountain chain. This observation is consistent with regional bending of the lithosphere. However, the planar basement surface suggests that the root either predates Alleghanian thrusting, and therefore is unrelated to the present topography, or formed in response to some other mechanism. Bounds on curvature of the basement surface suggest a lower bound of 30-40 km for the effective elastic thickness of the lithosphere. This is consistent with previous estimates for the southern Appalachians based on analysis of gravity data.

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Wide-angle recordings of small, multiple-hole blasts at dimension-stone quarries in the Elberton granite show coherent reflections at offsets between 7 and 15 km. In contrast with much larger blasts at crushed-stone quarries, this type of blast is fired without delays between holes. The data were recorded with an array of twenty portable seismographs using three-component, 4.5-Hz geophones. Migration of source gathers for two of the blasts shows a number of subhorizontal interfaces at depths between 2 and 4 km that may represent a layered complex near the base of the granite. Other reflectors include southeast-dipping interfaces at depths between 6 and 9.3 km and a more gently southeast-dipping complex at 9.5–11 km that may represent the master decollement. The results of the pilot study show that this type of quarry blast can be useful for imaging structure within the upper crust. Although preliminary, the wide-angle results support the interpretation that the Elberton granite is a tabular body less than 4 km thick.