BUOYANCY OF SUBDUCTED LITHOSPHERE AND THE RECENT EVOLUTION OF THE HELLENIC SUBDUCTION SYSTEM

Young subduction systems in the Mediterranean exhibit widespread extension within the upper plate lithosphere and migration of the trench away from the rigid part of the over-riding plate. The primary driving mechanism for this subduction appears to be gravity acting on negatively buoyant subducted lithosphere. The Hellenic orogenic system provides an excellent example of this phenomenon. Between ~30 and ~10 Ma, subduction was of a shallow water, presumably continental lithosphere with an average subduction rate of 5-10 mm/yr. At approximately 8-10 Ma, oceanic lithosphere of the Ionian Sea entered the subduction system south of the island of Kephalonia, while continental lithosphere continued to be subducted to the north. Subsequently the rate of subduction of oceanic lithosphere has increased to 35-40 mm/yr while the continental lithosphere continues to be subducted at 5-10 mm/yr. Over the same time period the oceanic subduction boundary has migrated ~100 km to the southwest relative to the continental subduction boundary, forming a discrete dextral offset in the boundary. This zone of dextral offset is accommodated onshore (eastward) by a broad zone of right-slip with 100 km of dextral shear and clock-wise rotation of older geologic structures by 30°. Geodynamic modeling of the effects of slab density on subduction processes show that these recent change in subduction rate and trench retreat are consistent with the entry of dense lithosphere into the subduction system at 8-10 Ma. These results point to the very rapid response of such orogenic systems to changes in the density of the subducting lithosphere and suggest that many aspects of these orogenic systems can be understood as changes in buoyancy of the subducting lithosphere through time and along strike.