P-WAVE REFLECTIVITY OF THE SOUTHERN APPALACHIAN LITHOSPHERE: USING GLOBAL PHASES TO MAP VARIATIONS IN SEISMIC FABRIC WITHIN THE UPPERMOST MANTLE

One of the aims of the Southeastern Suture of the Appalachian Margin Experiment (SESAME) was to investigate the response of the subcrustal lithosphere to Alleghanian collision and subsequent extension. We use the global seismic phase PKIKP as a virtual source to construct normal-incidence reflection sections of the uppermost mantle that are analogous to those produced by active-source reflection profiling of the crust. Our goal is to produce detailed images of seismic fabric in order to map possible shear zones, compositional layering, contrasts in anisotropy, and partial melt.

We begin by deconvolving the source-time function for each earthquake. We then divide the traces in each profile into shorter-aperture gathers and slant stack to obtain objective measures of ray parameter and coherence of reflections. The data are migrated by downward continuation. The partial migrated images then are summed to construct a composite section. Gaps in subsurface coverage are filled by stacking results for multiple earthquakes.

Preliminary migrated sections along two N-S profiles across the Atlantic coastal plain show concentrations of reflections at depths of 50-75 km (just below Moho), 90-110 km, 140-160 km, 180-200 km, and 250-270 km. Beneath the eastern profile (length: roughly 400 km), apparent dips are gentle and dominantly to the north. Beneath the western profile (500 km), dip directions are more randomly distributed. We tentatively interpret the events between 90 and 110 km as the base of the lithosphere. This is consistent with results from surface wave tomography that show high velocities in the uppermost 70-120 km beneath the Coastal Plain.

A third profile (200 km) trending NW across the Carolina Terrane, Inner Piedmont, and Blue Ridge shows a similar clustering of reflections at depths greater than 90 km, but the zone of highly reflectivity at 50-75 km is absent. One possible interpretation is that the 50-75 km zone beneath the Coastal Plain represents layers within the uppermost mantle depleted by partial melting during Mesozoic extension and underplating of the crust. The absence of this zone beneath inboard terranes would be consistent with more limited extension. More detailed tracking of this zone could be used to help establish the NW extent of major alteration of the crust associated with the breakup of Pangaea.