Possible mechanism of mud volcanism at the prism-backstop contact in the western Mediterranean Ridge Accretionary Complex

The Eastern Mediterranean seafloor has numerous mud volcanoes, most of which form a well-defined belt within the Mediterranean Ridge (MedRidge) accretionary complex. However, mud volcano fields in the western MedRidge are less well known as those in the central and eastern MedRidge. This study investigates material cycling and fluid migration within the western MedRidge. We propose a possible ascent style of the ejecta forming the Médée-Hakuho Mud Volcano (MHMV) in the western MedRidge by applying the vitrinite reflectance technique to ejecta samples. First, we model the 2-D thermal structure in the western MedRidge, taking into account frictional heating on the plate interface, to help estimate the source depth of the MHMV ejecta. The result suggests an effective coefficient of friction of around 0.01, and a temperature of about 160 ± 15 °C along the plate interface at a distance of ~ 180 km from the deformation front, the location of the seaward toe of the Aegean backstop. Second, we evaluate the source depth of the MHMV ejecta using vitrinite reflectance in conjunction with the modeled thermal structure. The results suggest that the ejecta matrix showing vitrinite reflectance values of ~ 0.6% was subjected to a temperature of around 85 °C, corresponding to a depth of approximately 5 km below the seafloor (kmbsf), whereas older clasts of Aptian or earlier age, with vitrinite reflectance values of ~ 0.6-1.0%, are derived from much deeper depths. Most of the clasts are considered to have been lifted to the depth of 5 kmbsf as a result of underplating at the toe of a rigid backstop that had developed below MHMV after underthrusting related to plate subduction. At that depth, fluid pressures that are dramatically increased because of underplating promote the ascent of fluid-rich sediments and entrain clasts along an existing fault in the accretionary wedge.