STRUCTURE OF THE LITHOSPHERE ACROSS THE APPALACHIAN OROGEN: REFLECTION PROFILING USING EARTHQUAKES

The passage of USArray across North America has revealed lithospheric structures with unprecedented detail. The results have provided insights into the nature of the continental lithosphere-asthenosphere boundary (LAB), the nature of flow in the asthenosphere, the evolution of lower lithosphere, and the possible role of upper mantle processes in supporting topography. Much of the analysis of USArray data to date has focused on the use of P to S conversions to image major discontinuities. We focus instead on P-wave reflectivity. We use the global phase PKIKP, observed for earthquakes at distances of 118°–140°, as a virtual source for generating near-vertical incidence reflections from the crust and upper mantle.

Recent results based on data from the Southeastern Suture of the Appalachian Margin Experiment (SESAME) arrays reveal coherent events at depths of 120-130 km interpreted as reflections from the LAB and uppermost asthenosphere beneath the southeastern Atlantic coastal plain, events at 50-70 km possibly related to Mesozoic extension, and primary and multiple reflections from the base of coastal plain sediments. Events interpreted as reflections from the Moho increase in travel time from roughly 10-11 s beneath the coastal plain northwestward to over 17 s beneath the Blue Ridge Mountains of Georgia and North Carolina, consistent with previous models showing a localized root beneath the highest elevations. We are currently using data from USArray to extend this coverage along the strike of the Appalachians and into the continental interior, to investigate the detailed structure and continuity of the LAB and crust-mantle transition.