TRANSITION FROM CONTINENTAL TO OCEANIC-CRUST ON THE WILKES-ADELIE MARGIN OF ANTARCTICA

The Wilkes-Adelie margin of East Antarctica, a passive margin rifted i n the Early Cretaceous, has an unusually reflective Moho which can be traced seismically across the continent-ocean transition. Velocity mod els and depth sections were constructed from a combined set of U.S, an d French multichannel seismic reflection lines to investigate the tran sition from continental to oceanic crust, These data show that the bou ndary 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 contin ental edge. A marginal rift basin, some tens of kilometers in width, l ies in the transition between continental and oceanic crust, contains an average of about 4 km of synrift sediment that is prograded in plac es, and has characteristics of a former rift valley, now subsided to a bout 10 km. Three types of reflections in the seismic data are interpr eted as volcanic deposits: (1) high-amplitude reflections that floor t he marginal rift basin, (2) irregularly seaward dipping sequences that comprise an anomalously thick edge of oceanic crust, and (3) highly i rregular 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 subside nce since 95 Ma, corrected for its load of synrift and postrift sedime nt and mechanically stretched by factors of beta = 1.8 or higher. Comp arison of seismic crustal thickness measurements with inferred crustal thinning from subsidence analysis shows agreement for areas where bet a < 4. In areas where beta > 4, measured thickness is greater than tha t inferred from subsidence analysis, a result that could be explained by underplating the crust beneath the marginal rift basin.