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SUMMARY The lithospheric structure of the Aegean region is investigated by analysis of Rayleigh‐wave fundamental mode dispersion measurements. Isotropic 1‐D models for almost 100 two‐station ray paths across the region display distinct variations in the Moho depth and crustal S‐wave velocities. The descending slab of the subducting African plate can be resolved down to 120 km depth beneath the volcanic arc. Three different regions are distinguished in terms of Moho depth: (1) The forearc, with large crustal thicknesses between 38 and 48 km and an average of 43 km, (2) the northern Aegean, with an average Moho depth of 28 km and (3) the southern Aegean (central volcanic arc, i.e. Cyclades, and Sea of Crete) with an even thinner crust of around 25 km. Lateral variations in structure between 25 and 55 km depth indicate a marked difference between the western and eastern forearc, collocated with pronounced changes in trench and slab geometry as well as published deformation rates. S velocities between 25 and 55 km depth are low everywhere beneath the forearc but increase from the Peleponnesus to Crete. An abrupt change occurs between western and central Crete in terms of the visibility of the Aegean Moho and the seismic structure of the lithospheric mantle wedge: An Aegean mantle wedge with S velocities above 4.4 km s−1 is only observed to the east of central Crete, whereas to the west velocities of less than 4.0 km s−1 occur down to the plate contact. These low velocities above the slab may indicate the presence of a melange of metamorphic rocks at the depths. A low‐velocity asthenospheric layer is observed beneath the Sea of Crete and the Cyclades below 40 km depth, between the thinned lithosphere above and the slab below. The observed radial anisotropy in the northern part of the Aegean is likely to be due to preferred orientation of anisotropic minerals within the lower crust, possibly caused by lateral ductile flow associated with recent lithospheric extension.

The island of Crete represents a horst structure located in the central forearc of the retreating Hellenic subduction zone. The structure and dynamics of the plate boundary in the area of Crete are investigated by receiver function, surface wave and microseismicity using temporary seismic networks. Here the results are summarized and implications for geodynamic models are discussed. The oceanic Moho of the subducted African plate is situated at a depth of about 50–60 km beneath Crete. The continental crust of the overriding Aegean lithosphere is about 35 km thick in eastern and central Crete, and typical crustal velocities are observed down to the upper surface of the downgoing slab beneath western Crete. A negative phase at about 4 s in receiver functions occurring in stripes parallel to the trend of the island points to low-velocity slices within the Aegean lithosphere. Interplate seismicity is spread out about 100 km updip from the southern coastline of Crete. To the south of western Crete, this seismically active zone corresponds to the inferred rupture plane of the magnitude 8 earthquake of ad 365. In contrast, interplate motion appears to be largely aseismic beneath the island. The coastline of Crete mimics the shape of a microseismically quiet realm in the Aegean lithosphere at 20–40 km depth, suggesting a relation between active processes at this depth range and uplift. The peculiar properties of the lithosphere and the plate interface beneath Crete are tentatively attributed to extrusion of material from a subduction channel, driving differential uplift of the island by several kilometres since about 4 Ma.

SUMMARY We present a case study of lithospheric structure in the forearc of a retreating subduction zone for the Hellenic Arc. Lateral structural variations along the arc beneath the island of Crete are jointly investigated by receiver functions and Rayleigh phase velocities. Data from temporary short‐period networks amend previous results from broad‐band stations by broadening the frequency range available for phase‐velocity determination and increasing the spatial coverage of receiver function profiles. Both receiver functions and dispersion analysis reveal distinct structural differences between western and central Crete. Western Crete is characterized by nearly constant S‐velocities of 3.72–3.75 km s−1 from 10 km depth down to a depth of 50 km and no distinct continental Moho signal. Meanwhile, central Crete shows lower S‐velocities equal to 3.3 km s−1 in the crust between 10 and 20 km depth which are followed by the Aegean Moho in about 30 km depth and a mantle wedge with an S‐velocity of 4.35 km s−1. Both methods lead to an average depth of 55 km for the subducted oceanic African Moho beneath Crete. This means that the slab is separated from the Aegean crust by a mantle wedge beneath central Crete, while beneath western Crete the corresponding depth region is characterized by crustal velocities. This thickened crust in the forearc might be formed by crustal material of the Aegean Plate dragged down with the subducting African lithosphere. Furthermore, rocks extruded from a melange circulating in a subduction channel might accumulate between a depth of 20 and 50 km and contain low‐velocity material, e.g. in the form of serpentinized Aegean mantle. In addition, the lateral extent of a prominent negative phase observed around 4 s differential time in receiver functions from western Crete is mapped. This phase might point to low‐velocity material around 30 km depth which could be extruded from a subduction channel. An important property of the forearc found in this study is its strong lateral heterogeneity.