The Wilkes-Adelie margin of East Antarctica, a passive margin rifted in the Early Cretaceous, has an ususually reflective Moho which can be traced seismically across the continent-ocean transition. Velocity models and depth sections were constructed from a combined set of U.S. and French multichannel seismic reflection lines to investigate the transition from continental to oceanic crust. These data show that the boundary between oldest oceanic crust and transitional continental crust is marked by a minimum in subsediment crustal thickness and, in places, by a shoaling of Moho. The Moho reflection is continuous across the edge of oceanic crust, and gradually deepens landward under the continental edge. A marignal rift basin, some tens of kilometers in width, lies in the transition between continental and oceanic crust, contains an average of about 4 km of synrift sediment that is prograded in places, and has characteristics of a former rift valley, now subsided to about 10 km.

Three types of reflections in the seismic data are interpreted as volcanic deposits: (1) high-amplitude reflections that floor the marginal rift basin, (2) irregularly seaward dipping sequences that comprise an anomalously thick edge of oceanic crust, and (3) highly irregular and diffractive reflections from oceanic crustal basalts that cap a normal-thickness ocean crust. The present depth to the prerift surface of continental crust is compatible with passive margin subsidence since 95 Ma, corrected for its load of synrift and postrift sediment and mechanically stretched by factors of β=1.8 or higher. Comparison of seismic crustal thickness measurements with inferred crustal thinning from subsidence analysis shows agreement for areas where β4, measured thickness is greater than that inferred from subsidence analysis, a result that could be explained by underplating the crust beneath the marginal rift basin. ¿American Geophysical Union 1994