A traverse of the ionian islands front with coincident normal incidence and wide-angle seismics

Abstract

Marine vertical reflection profiling with a powerful airgun source, augmented with a few wide-angle seismometer stations on land, has been applied along a 180-km line across the presently active deformation front of the subduction of the Ionian Sea plate beneath the western Hellenides. East of the Ionian islands, there is limited evidence for reflectivity down to 23 km interpreted as the base of a rather thin continental-type crust. Under the western slope of the islands, a major normal-incidence reflector dips eastward, first gently to the west of the islands, then more steeply under them. This reflector may be extrapolated southwestwards beneath a bulge that is thought to represent the modern pre-Apulian front located over the subduction zone. The continuity, signature, and geometry of this reflector suggest that it may act as the lower limit of the western Hellenides where they override the Ionian Sea quasi-oceanic crust, rather than just an intracrustal interface of a pre-Apulian crust. The location of the previous deformation front, the Ionian thrust proposed in previous models, can be constrained by the new seismic data. The new data raise the possibility of a larger area of evaporite mobility than previously considered, insofar as active block motion apparently related to halokinesis is recognized west of the Zakynthos anticline. Diapirism, décollement, and westward-directed over-thrusting in the pre-Apulian crust may have been brought on by Late Pliocene-Quaternary reactivation of regional extension associated with the separation of the Peloponnesus from northern Greece as it was captured by the Anatolian-Aegean rotation and associated also with the fast clockwise rotation of the Ionian islands as they were sheared off the Apulian domain to the southwest by the initiation of the Kefallinia transform.