INVESTIGATION OF P-WAVE REFLECTIVITY BENEATH THE SOUTHERN APPALACHIANS USING THE SESAME BROADBAND ARRAY

The Southeastern Suture of the Appalachian Margin Experiment (SESAME) is an investigation of the effects of Paleozoic collisional tectonics and subsequent Mesozoic rifting on the lithospheric structure of the southeastern United States. We use low-frequency, near-vertical reflections generated by the seismic phase PKIKP to image structure within the crust-mantle transition and uppermost mantle. Together with Consortium for Continental Reflection Profiling (COCORP) profiles, these data allow us to study the nature of the Moho and other discontinuities over a wide range of frequencies. Of particular interest are variations in the detailed structure of the crust-mantle transition from Grenville basement beneath the Valley & Ridge to accreted terranes beneath the Coastal Plain and across the boundary between Laurentian and Gondwanan lithosphere. Preliminary findings for a single earthquake (m_b=6.1) recorded along a profile trending northwest across the Carolina Terrane, Inner Piedmont, and Blue Ridge show discontinuous, dipping reflections in the crust and layered, continuous, relatively flat-lying reflections at a depth of roughly 70 km in the upper mantle. Possible explanations for the mantle reflections include contrasts in anisotropy generated by shearing associated with collision and/or extension. Ongoing work includes stacking of waveforms for multiple earthquakes to enhance signal levels and construction of images for two additional north-south trending profiles, one extending to the Blue Ridge and both crossing the Coastal Plain. In the latter region, deep structure is more difficult to image because of strong multiple reflections generated by very low-velocity, unconsolidated sediments and poorly consolidated sedimentary rocks. We experiment with a number of deconvolution strategies, including techniques similar to those used by the petroleum industry. A major Coastal Plain target is the Suwannee-Wiggins suture which is marked by a broad zone of southeast-dipping reflections on COCORP profiles. The resulting broadband images of P-wave reflectivity across the three profiles will be used in combination with models of S-wave reflectivity derived by other methods to place constraints on processes involved in the construction and subsequent rifting of the southern Appalachian orogen.