Svelocity structure and radial anisotropy in the Aegean region from surface wave dispersion

Abstract

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.

Receiver function images of the Hellenic subduction zone and comparison to microseismicity

“…7) along all profiles (dark red dashed lines). The depth of our LVZ is consistent with that derived from P-wave tomography and surface wave analysis in the northern and central Aegean (Drakatos et al, 1997;Papazachos and Nolet, 1997;Karagianni et al, 2002;Endrun et al, 2008;Bourova et al, 2005).…”

“…If the negative discontinuity at about 60 km depth represents the lithosphere-asthenosphere boundary (LAB) of the continental Aegean plate (in good agreement with Endrun et al, 2008), this will be a remarkable result as it indicates a very thin mantle lithosphere in the central and northern Aegean. Taking the error bounds of P receiver functions (±5 km) into account, we found a relatively thicker lithosphere in the central Aegean (60 km) rather than that seen by surface waves analysis beneath the southern Aegean between forearc and volcanic arc (40-50 km) (Endrun et al, 2008).…”

“…Meier et al (2007) identified two slab segments and proposed that they are associated with laterally varying properties of the African slab. Furthermore, in the Mediterranean southeast of Crete, Rhodes and south of western Anatolia, high velocities in the mantle lithosphere -typical for cold oceanic lithosphere -are found by tomographic studies (Bijward et al, 1998;Marone et al, 2004;Endrun et al, 2008;Legendre et al, 2012), whereas south of Peloponnese and western Crete velocities in the mantle lithosphere are lower Legendre et al, 2012).…”

“…In addition, surface wave dispersion analysis revealed a low-velocity layer with minimum velocities of 4.15 km s −1 at depths of 50-70 km between the Aegean Moho and the slab in the Cretan Sea and beneath the central Aegean (Karagianni et al, 2002;Endrun et al, 2008). An even more pronounced low-velocity layer was found by The signals between 10 and 20 s are crustal multiples.…”

“…Makris and Vees, 1977;Makris, 1978;Knapmeyer and Harjes, 2000;Bohnhoff et al, 2001;Li et al, 2003;Endrun et al, 2004Endrun et al, , 2008Meier et al, 2004;Tirel et al, 2004;Snopek and Casten, 2006). Recently, receiver function studies have obtained new constraints on the nature of the subducting African plate mainly in the western Hellenic arc (e.g.…”