EXTENSION AND EXHUMATION OF THE HP/LT ROCKS IN THE HELLENIC FOREARC RIDGE

Localized intensive ductile and ductile-brittle extension of along-the-arc orientation is prominent in Phyllite-Quartzite unit (PQU) rocks in Kythera and the adjacent southeastern Peloponnese but declines in prominence to the north and to the south along the Cretan-Peloponnese ridge. Using zircon and apatite fission track dating we find that this structural characteristic is correlated with the youngest zircon fission track cooling ages of 14 to 9 Ma, and with the highest pressure-temperature condition recorded in the metamorphic rocks below the structural detachment. The cooling ages of the high pressure Phyllite-Quartzite unit (PQU) rocks along the Hellenic forearc ridge show that exhumation migrated from both Crete and from the Peloponnese to the area with the youngest ages, in Kythera and the southeastern Peloponnese. Starting in the Early Miocene, and continuing to the present, trench-rollback and slab retreat expanded the Hellenic arc, and this bending of the arc from an initial more rectilinear geometry increased areas of oblique convergence and we suggest this localized. the subsequent arc-parallel extension and local enhanced exhumation of HP-rocks. The Zircon FT exhumation ages from the arc-parallel stretching episode restrict the ductile part of this episode in exposed rocks to between about 14 and 9 Ma. Apatite FT ages suggest brittle along-arc extension continued to about 7 Ma. This episode is proposed to be the result of a temporary higher rate of rollback and slab retreat. Younger normal faults, including those which define the margins of the present Hellenic Arc, show return to arc-normal extension.