Seismic investigation of the transition from continental to oceanic subduction along the western Hellenic Subduction Zone

Abstract

The western Hellenic subduction zone (WHSZ) exhibits well‐documented along‐strike variations in lithosphere density (i.e., oceanic versus continental), subduction rates, and overriding plate extension. Differences in slab density are believed to drive deformation rates along the WHSZ; however, this hypothesis has been difficult to test given the limited seismic constraints on the structure of the WHSZ, particularly beneath northern Greece. Here, we present high‐resolution seismic images across northern and southern Greece to constrain the slab composition and mantle wedge geometry along the WHSZ. Data from two temporary arrays deployed across Greece in a northern line (NL) and southern line (SL) are processed using a 2D teleseismic migration algorithm based on the Generalized Radon Transform. Images of P‐ and S‐wave velocity perturbations reveal N60E dipping low‐velocity layers beneath both NL and SL. The ∼8 km thick layer beneath SL is interpreted as subducted oceanic crust while the ∼20 km thick layer beneath NL is interpreted as subducted continental crust. The thickness of subducted continental crust inferred within the upper mantle suggests that ∼10 km of continental crust has accreted to the overriding plate. The relative position of the two subducted crusts implies ∼70–85 km of additional slab retreat in the south relative to the north. Overall, our seismic images are consistent with the hypothesis that faster sinking of the denser, oceanic portion of the slab relative to the continental portion can explain the different rates of slab retreat and deformation in the overriding plate along the WHSZ.

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

“…Suckale et al, 2009;Pearce et al, 2012). Our results are also consistent with those derived from regional body-wave travel-time tomography for the western Hellenic subduction zone (e.g.…”

“…Recently, receiver function studies have obtained new constraints on the nature of the subducting African plate mainly in the western Hellenic arc (e.g. Sodoudi et al, 2006;Suckale et al, 2009;Gesret et al, 2011;Pearce et al, 2012). The base of the subducting African lithosphere has been demonstrated down to a depth of about 225 km beneath the volcanic arc using converted Sto-P waves (e.g.…”

“…Studies of converted or scattered waves in the Aegean were mainly obtained from small networks mostly covering the western part of the Hellenic arc (e.g. Suckale et al, 2009;Gesret et al, 2011;Pearce et al, 2012).…”

The dynamics of laterally variable subductions: laboratory models applied to the Hellenides

“…The formation of the fault itself results from the entry in the subduction zone of a very negatively buoyant oceanic plate to the south, while in the north the subduction of a continental unit of increasingly positive buoyancy proceeds, as the buoyancy difference of the two plate units suffices to trigger a laterally variable regime along the trench . P-and S-wave velocity perturbations show that the subducted crusts for two sections north and south of the Kefalonia Fault are dextrally offset, implying an additional ∼70-85 km retreat of the southern segment of the slab (Pearce et al, 2012). The Aegean Sea then migrated along with the trench to the southwest (with respect to stable Eurasia), as an apparently almost rigid unit (McClusky et al, 2000;.…”

“…Cloos, 1993) and that of the continental lithosphere between −35 and −43 kg m −3 (corresponding to a continental lithosphere with a crust thickness of ∼ 16 km and a density of 2.75, in the range of crustal thickness found for the subducting lithosphere beneath the Northern Hellenides (20 km; Pearce et al, 2012)) (Table 1). When present, the overriding lithosphere is positively buoyant with ρ = −35 kg m −3 .…”