S U M M A R YNew results from a two-ship refraction and oblique reflection deep seismic cruise are presented to discuss the nature of the crust in the Ionian, Sirte and Herodotus abyssal plains. These expanding spread profiles were processed and analysed in both the x-t and t -p domains. Arrival times of reflected and refracted branches are matched by ray tracing in both domains. In spite of a shallow evaporitic sequence (messinian evaporites) deposited on top of a thick sedimentary pile responsible for velocity inversions on many profiles, we obtain excellent velocity control down to Moho depth. P,P and P, (8.4-8.6kms-') arrivals are observed. The three basins have a relatively thin crust (8 to 11 km) overlain by a thick sedimentary cover, up to 10 km in the Herodotus abyssal plain. The Moho boundary and main crustal units identified in the basins can be followed beneath the Calabrian prism to the west, and beneath the Mediterranean Ridge to the east. The crustal structure is of oceanic type for both the lonian and Sirte basins, where typical oceanic layer 2 and 3 are recognized. The thin crust of the Herodotus basin may be interpreted either as oceanic or thinned continental crust (about 10 km thick). The top of the crust of the Herodotus basin is much deeper. Therefore, the Herodotus basin is probably significantly older than the Ionian basin, Triassic versus Early Cretaceous in age.
Seismic reflection data were collected and processed to 20 s two-way travel time along four lines which cross the rifted continent-ocean boundary off the Grand Banks region of eastern Canada specifically to examine the origin, age, and nature of this fundamental boundary. This represents the first regional study of its kind. The most important result is the presence of landward dipping reflectors near the foot of the continental slope. These occur where oceanic crust appears to terminate against the continent. We suggest that the dipping reflectors mark the continent-ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary. Sedimentary basins lie just landward of the continent-ocean boundary. Their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting, and this may be an important stage leading to continental breakup. Rift basins formed further landward on the Grand Banks do not exhibit this thinning. Other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent-ocean boundary.
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