THE TECTONIC EVOLUTION OF THE HELLENIC SUBDUCTION WEDGE IN CRETE, GREECE

We present a tectonic synthesis of the Hellenic Subduction wedge, Greece, based upon our recent studies into the structural, metamorphic, and thermal evolution of the high-pressure low-temperature rocks exposed in Crete. We focus on the Phyllite-Quartzite unit, a package of Carboniferous to Triassic siliciclastic sediments and minor volcanics that was subducted to a depth of about 35 km at 25 Ma, underplated, metamorphosed, deformed, and exhumed to the surface by 9 Ma. The lithology of the Phyllite-Quartzite is well-suited for both structural and thermochronologic analyses, so the unit is ideal for investigating the deformation and exhumation processes that occur within the subduction wedge.

Ductile deformation in the phyllites that comprise the bulk of the unit occurred primarily by the solution mass-transfer (pressure solution) mechanism. Regional strains estimated using a tensor average approach show a co-axial flattening ductile flow, with sub-vertical shortening of nearly 40% and minor radial extension in the horizontal. All bedrock units in Crete are cut by numerous faults developed after the rocks passed into the shallow brittle part of the wedge. Inversion of fault-slip data from western and central Crete reveals a brittle strain-rate tensor with a co-axial flattening and vertical shortening direction. The geometric similarity of the ductile and brittle strain results suggests the strain field maintains a steady orientation and symmetry throughout the entire wedge. We argue the pattern of within-wedge deformation reflects the mode of accretion; in the case of Crete, the large underplating flux destabilizes the wedge and causes vertical thinning and horizontal extension.

The exhumation of the high pressure rocks of the Phyllite-Quartzite unit from maximum depths of about 35 km will occur by a combination of erosion and tectonic thinning. Published cooling data from the shallowest tectonostratigraphic units in Crete suggest erosion accounted for no more than 6-10 km of exhumation. Modeling of our strain results indicate that ductile thinning contributed to no more than 5-6 km of exhumation. Thus, we conclude that the bulk (~14-19 km) of exhumation was accommodated through brittle deformation. Metamorphic temperature data in central Crete demonstrate a modest thermal offset across the Cretan Detachment Fault, suggesting this structure did not play a dominant role in exhuming the Phyllite-Quartzite unit. We conclude that the bulk of exhumation occurred by pervasive brittle stretching in the upper levels of the subduction wedge.

Because exhumation of the high-pressure units in the Hellenic wedge is controlled by tectonic processes, the rate of exhumation must reflect the pace of within-wedge thinning. In the absence of significant erosion, the deformation rate in a wedge with a steady cross-sectional area scales with its accretionary flux. In the Hellenic wedge, this flux is primarily controlled by the subduction velocity of the African plate. We argue that the Hellenic wedge has been in a flux steady-state since at least 25 Ma, meaning that the volume of material added to the wedge through frontal offscraping and underplating is balanced by the material that leaves, primarily by advection out the rear of the wedge. We discuss a revised cooling history for the Phyllite-Quartzite unit that shows a history of punctuated cooling related to the history of subduction. A period of slow cooling and exhumation beginning at ~19 Ma coincides with a period of slow subduction, while rapid cooling at about 12 Ma reflects with the onset of accelerated rollback. Together, these results indicate the structural, thermal, and exhumational history of wedges is controlled by the material fluxes.