The Crust of the Western Mediterranean Ridge From Deep Seismic Data and Gravity Modelling

Abstract

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.