The young Gulf of Corinth rift in central Greece is an ideal place to study processes occurring during the initiation and early stages of continental extension. At the east end of the 100-km–long E-W rift, Holocene extension on N-dipping faults equates to geodetic extension rates. At the western end, however, estimated extension rates on N-dipping onshore faults account for only ∼20%–40% of the geodetically measured rates (∼10–15 mm/yr). We use high-quality, multichannel, seismic reflection and swath bathymetry data to investigate the tectonics of the western Gulf and quantify the contribution of offshore fault activity toward total-rift extension. Five major offshore faults generate the variable basement topography, and a ca. 0.4-Ma unconformity separates stratigraphy into two main packages. Basin-fill geometry indicates that during the early stages of subsidence of the offshore western Gulf, S-dipping faults were the dominant border-fault structure. This is still the case at the western end of the western Gulf; however, elsewhere, N-dipping faults on the south margin are now relatively more dominant and cause stratigraphy to tilt south. The temporally and spatially varying rift structure contrasts with the simple, consistently N-dipping, border-fault, half-graben geometry of the east Gulf of Corinth. Combined west Gulf offshore and onshore Holocene fault extension rates total 5–14.5 mm/yr, within the range calculated geodetically, and render enhanced slip on a low-angle detachment surface beneath the western Gulf unnecessary.

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