A FIRST COINCIDENT NORMAL-INCIDENCE AND WIDE-ANGLE APPROACH TO STUDYING THE EXTENDING AEGEAN CRUST

The Aegean Sea is a broad area of submerged continental crust undergoi ng active extension to varying degrees. A combined near-normal inciden ce and wide-angle seismic recording programme was conducted in the wes tern Aegean Sea in 1993, with the principal objective of testing the p opular hypothesis that lower crustal deformation (particularily extens ion) is expressed as a seismically ''layered lower crust'' (LLC), Acro ss the southern margin of the Cretan trough (i.e. North Cretan offshor e margin), a LLC was indicated by wide-angle arrivals that was not app arent on either the coincident near-normal-incidence profile or on old er low-frequency refraction records, North of the northern margin of t he Cretan Trough, beneath the Cyclades, a domain of strong reflectivit y is recorded from the middle to lower crust. Here, the near-normal in cidence sections also show this typical LLC reflectivity, On the wide- angle sections, a distinct interface is suggested in addition, at a la rger depth than that previously assumed for the Moho discontinuity. Th e structural images and interpretations derived from the new seismic d ata so far do not clearly support either a pure-shear crustal stretchi ng or an asymmetric simple-shear extension model for the Aegean Sea. O ur results appear to be consistent with a tectonic model, where middle crust mobilised by flow coincides spatially with upper crust that has been thinned by active extension of an orogenically thickened crust a nd expressed near the surface as an exhumed metamorphic core complex.