Spatial transition from compression to extension in the Western Mediterranean Ridge accretionary complex

Abstract

The Mediterranean Ridge has been described since 1982 as an accretionary prism related to the Hellenic subduction zone. However, little attention has been paid to the actual mechanical boundary between the sedimentary wedge and the crustal backstop of the outer Hellenic arc. Using seismic velocity structures from recent ESP's (Pasiphae cruise), we propose a new interpretation of existing geological and geophysical data around the western Hellenic trenches. The velocity structure of the inner unit of the Mediterranean Ridge suggests that the Late Neogene to Quaternary sequences were deposited above a high-velocity rigid body. We propose to correlate the boundary of this rigid body (backstop of the accretionary wedge) with a series of oblique reflectors dipping to the southwest, near DSDP sites 126 and 377. The backstop extends further south than previously thought and decreases by one third the total width of the compressional wedge; it has thus major implications on the volume of accreted material. The short-wavelength folding of Plio-Quaternary sediments observed on the outer wall of the South Matapan trench is probably a consequence of gravity-gliding layers above the high velocity body, gently dipping (2°) to the northeast using the Messinian evaporite as a décollement. On the other hand, the Hellenic trenches appear to be narrow half-grabens affected by the widespread extensional deformation active on the outer Hellenic arc. They are situated on the landward boundary of a wider and poorly sedimented fore-arc basin. We conclude that extensional deformation extends at least over the width of the Hellenic trenches, on the continental side of the backstop, whereas compressional deformation stops near the edge of the backstop itself, 100 km to the southwest.