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.

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SUMMARY We present a crustal cross‐section of the Western Mediterranean Ridge from the African margin to the Aegean Sea constrained by the morphology of the ridge, new seismic reflection and refraction results obtained during a two‐ship cruise performed in December 1988 (Pasiphae cruise), the distribution of seismicity and gravity modelling. Morphologically, the Western Mediterranean Ridge can be divided into three main units from the Sirte Abyssal Plain to the Hellenic Trench: a broad outer unit, a narrow central unit and an inner unit. the seismic front, or seaward extent of the interplate seismicity between the African lithosphere and the Cretan Arc, is located 50 km seaward of the Hellenic Trench. Seismicity is thus distributed widely over the innermost unit of the ridge, suggesting that brittle material is present at shallow depth beneath the inner portion of the Mediterranean Ridge. the refraction results of the Pasiphae cruise show that the crust in the Ionian Basin and beneath the outer and central units of the Western Mediterranean Ridge is oceanic and that the Moho discontinuity progressively deepens towards the Hellenic Arc, in agreement with the accretionary prism model. High velocity material occurs at shallow depth beneath the innermost unit of the ridge. Gravity modelling also indicates a significant increase in density beneath the inner unit. We consequently interpret the inner unit of the ridge as the backstop of the Western Mediterranean Ridge prism, possibly composed of Peloponnesus thrust sheets.