A "Great Deepening": Chronology of rift climax, Corinth rift, Greece

Continental rift deposits contain critical clues concerning the evolution of extensional tectonics, yet such evidence is often obscure due to poor geochronology, burial by younger deposits, or later tectonic overprinting. We revisit Corinth rift development, which began as distributed extension created synrift depocenters with rivers flowing into shallow (<50 m) lakes. Subsequent focused deformation initiated a “Great Deepening” event, evidenced by fan deltas prograding into 300–600-m-deep water. A chronology is provided for the event from 40Ar/39Ar dating of the Xylocastro ash by single-crystal CO2 laser fusion, yielding a precise age of 2.550 ± 0.007 Ma (1σ, full error propagation). Sedimentological data indicate that the ash-bearing sediments were deposited as turbidites and hemipelagites on sublacustrine fans fed from the Mavro fan delta at the faulted south-central rift margin. The ash age and turbidite provenance data enable stratigraphic constraints for an estimate of central rift climax occurring between 3.2 and 3.0 Ma. This is some 0.8–1.0 m.y. earlier than radioisotopic- and magnetostratigraphic-constrained estimates for the eastern Corinth rift. Central rift climax was probably triggered by initial counterclockwise rotation of the Peloponnesus block with respect to central Greece. The rotation pole of this block subsequently migrated to its present position as rift climax moved eastward in an “unzipping” action, with the southern active margin also migrating northward. These events are unlikely to be due to local or regional fault kinematics, but rather to the consequences of deep-seated interactions between the rapidly southward-moving Aegean continental forearc and the slowly northward-subducting African oceanic plate. A possible scenario involves forearc “pushback” with décollement on a low-angle subducting lower plate. This causes acceleration and counterclockwise rotation of Peloponnesus with respect to central Greece and strain localization across the boundary; the Corinth rift.