The geometry of the Wadati-Benioff zone and lithospheric kinematics in the Hellenic arc

Accurate locations of 961 shallow and intermediate depth earthquakes, which occurred between 1956 and 1995 in the Hellenic are (34-39 degrees N, 19-28 degrees E), are used to define the plate boundaries in this area. Reliable fault plane solutions for 77 shallow and intermediate depth earthquakes are also used in order to define the interaction between the different plates in the are. An ocean-continent type of interaction occurs on a curved surface, which is defined by the shallow branch (20-100 km) of the Wadati-Benioff zone. The intersection of this zone with the earth's surface is a curve which follows the convex (outer) side of the sedimentary are (western Peloponnese-west of Cythera-south coast of Crete-east coast of Rhodes) and dips at low angle (similar to 30 degrees) to the Aegean sea. Coupling between the subducted oceanic crust and the overriding of the Aegean lithospheric plate takes place on this surface. The deep branch (100-180 km) of the Wadati-Benioff zone is dipping freely (without coupling) at a high angle (similar to 45 degrees) beneath the south Aegean trough and the volcanic are. The high shallow seismicity (h less than or equal to 20 km) which is observed in the southwestern convex (outer) side of the are (Ionian sea) is attributed to the fast southwestward motion of the Aegean plate. Location of strong deep earthquakes (h > 100 km) in the fore-are area of the southwestern part of the Hellenic are (west of Cythera) indicates that oceanic crust is 'destroyed' in this part of the Hellenic trench due to roll-back of the descending lithospheric slab. (C) 2000 Elsevier Science B.V. All rights reserved.