Pervasive compression and deep structure of the Hellenic subduction forearc, west of Crete, revealed by penetrative long offset multichannel seismic data

The Hellenic subduction system exhibits a fairly atypical structure, resulting from the intense radial extension undergone by the Aegean domain from Eocene to Miocene. It features a very wide fore-arc, that itself includes an external non-volcanic arc between southeastern Peloponnesus and southwestern Anatolia, with Crete occupying a frontal position midway along it. South of it, separated from the Aegean domain by a large escarpment, the Hellenic trenches marking the most advanced position of the forearc Alpine nappes and the wide abutting accretionary wedge are found. The Hellenic subduction is widely regarded as poorly coupled from historical seismicity. Yet, a large tsunamigenic destructive event is reported in 365 CE just west of Crete, for which various rupture processes are proposed. In order to gain better insight into the structures of this major event, some deep penetrating seismic acquisition was performed in 2015 during the SISMED project, using the leading-edge equipment of R/V Marcus Langseth along a 210 km profile crossing through the fore-arc; an 8 km-long streamer and a voluminous source were used. Pre-stack depth imaging was applied to the recorded data set, with a strong effort to build a reliable velocity model using common image focusing analysis, both in the time and depth domains. Good imaging conditions thus were reached down to a maximum depth of 25 km. From south to north, our results reveal that: (1) a 6-7 km-thick oceanic crust with hints of compressive deformation enters the subduction, carrying no sediment past the backstop; (2) it crosses the continental Moho as shallow as 13 km depth; (3) above, a set of reactivated antithetic reverse faults controlling the Matapan forearc basin is clearly imaged; (4) no evidence is found supporting a speculated inverse splay fault outcropping at the toe of the Hellenic scarp and tentatively related to the 365 CE event; (5) some steep faulting is observed at mid-escarpment, whose downward continuation coincides with a jump in the forearc Mohos depth, and that likely accommodates some of the required dextral strike slip partitioning motion; (6) possible clues are found for some compressive reactivation of the Maleas basin, north of the external arc.