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

Abstract

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.

Thermal Structure of the Ionian Slab

“…There appears to be continuity between the slab and the Ionian lithosphere which lies beneath the fronts of the Calabrian and Hellenic wedges and in the central part of the Ionian basin. Two abrupt changes in the sea-floor depth, the Apulia and Malta escarpments, mark the borders of the Ionian plate with the Adriatic to the north, and the African plates to the south, respectively (FINETTI, 1985;TRUFFERT et al, 1993).…”

Tectonic setting and processes of mud volcanism on the Mediterranean Ridge accretionary complex: evidence from Leg 160

“…Gravity modeling and ESP studies of the Mediterranean Ridge in the west (i.e., southwest of the Peloponnese; de Voogd et al, 1992;Truffert et al, 1993) suggest that a backstop of Cretan-type crust extends about 100 km oceanward of the Ionian branch of the Hellenic Trench in this area. However, it is unlikely that a continental crustal backstop extends beneath the Olimpi field in the central area of the Mediterranean Ridge as clasts of continental-type basement are not seen in the recovered mud breccias.…”

Deformation and stress regimes in the Hellenic subduction zone from focal Mechanisms

“…In a second step, this layer was subdivided into three segments along the Hellenic arc from West to East. The dip of each segment was adjusted following structural models of the forearc region that were derived from wide angle seismics (Bohnhoff et al, 2001;Brönner, 2003), surface wave and receiver function analysis (Li et al, 2003;Meier et al, 2004a;Endrun et al, 2004) and moving source profiles (Truffert et al, 1993). Furthermore, we implement data from a Moho map of the Eastern Mediterranean (Marone et al, 2003).…”

“…Below the Libyan Sea an accretionary wedge with a sedimentary cover of up to 15 km is located between the active and passive continental margins. Recent microseismicity and surface wave studies (Meier et al, 2004b) as well as active seismic lines (Truffert et al, 1993;Bohnhoff et al, 2001;Brönner, 2003) allowed to refine the structural model along the Hellenic subduction zone (see inset in Figure 1) exemplifying the complex geometry along a strongly curved plate boundary. The overall seismic activity of the Hellenic subduction zone is small compared to other subduction zones as can be observed from global earthquake catalogues.…”