[1] High-pressure metamorphic rocks provide evidence that in subduction zones material can return from depths of more than 100 km to the surface. The pressure-temperature paths recorded by these rocks are variable, mostly revealing cooling during decompression, while the time constraints are generally narrow and indicate that the exhumation rates can be on the order of plate velocities. As such, subduction cannot be considered as a single pass process; instead, return flow of a considerable portion of crustal and upper mantle material must be accounted for. Our numerical simulations provide insight into the self-organizing large-scale flow patterns and temperature field of subduction zones, primarily controlled by rheology, phase transformations, fluid budget, and heat transfer, which are all interrelated. They show the development of a subduction channel with forced return flow of low-viscosity material and progressive widening by hydration of the mantle wedge. The large-scale structures and the array of pressure-temperature paths obtained by these simulations favorably compare to the record of natural rocks and the structure of high-pressure metamorphic areas.

S U M M A R YOn a SW-NE profile from the Libyan coast towards central Turkey phase velocity curves of the fundamental Rayleigh mode were measured using a two-station method. The inversion of phase velocity curves yields 1-D models of shear wave velocity down to approximately 200 km depths that may be interpreted as estimates of average models between neighbouring stations on the profile. Strong lateral variations in the shear wave velocity structure are imaged along the profile.The subducted oceanic African mantle lithosphere is indicated in 1-D models for the region around Crete by significantly enlarged shear wave velocities. It is also imaged by an average model of the structure between stations on Crete and Santorini. On a path crossing the Libyan Sea south of Crete the resulting model is slower than a model expected for 110 Myr old oceanic lithosphere. The passive African margin is thus assumed to extend northwards beneath the Libyan Sea. Anomalous low shear wave velocities are found for the uppermost mantle beneath central Turkey down to a depth of approximately 130 km.Using two stations on Crete the average depth of the oceanic Moho within the subducting slab is estimated to be at approximately 50 km beneath Crete. For this arc-parallel path, an enlarged standard deviation of the measured phase velocities of approximately 0.2 km s −1 between 10 and 30 mHz is observed that is probably caused by strong lateral heterogeneity related to the subducting slab. In addition, in this frequency range an anomalous propagation of the fundamental Rayleigh mode is detected that is indicated by measured phase velocities that are approximately one standard deviation faster than phase velocities expected from a great-circle approximation. An average shear wave velocity of approximately 3.5 km s −1 is observed above the oceanic Moho.In order to explain the recent lithospheric structure of the Hellenic subduction zone a tectonic model is assumed for the NE-SW striking profile considered. It is based on the calculated 1-D models, tectonic reconstructions and on a model derived from the metamorphic history of rocks exposed on Crete. The suggested model summarizes the tectonic development at a lithospheric scale starting in the Late Cretaceous. Accretion of crustal material of two microcontinents to Eurasia is assumed, while continuous subduction of the oceanic lithosphere of different ocean basins and possibly of the mantle lithosphere of the microcontinents resulted in a single slab. The length of the oceanic lithosphere that was subducted south of Crete is estimated to be not greater than approximately 550 km.

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Abstract. Fission track (FT) analysis is applied to assess the

Ó Springer-Verlag 2005 neath the forearc of the Andes, where the importance of subduction erosion is well documented, and where a strong upper crust forms a stable lid.

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