Tyrrhenian Continent–Ocean Transition (Expedition 402)

Summary from the Scientific ProspectusThe discovery in the 1980s of exposed mantle in the continent–ocean transition west of Iberia started an ongoing debate on the mechanisms of lithospheric extension and mantle exhumation, on the definition of continental break-up and initial seafloor spreading, on the nature of the continent–ocean transition, and on the first emplacement of oceanic crust. Our imperfect understanding of continent–ocean transition processes and of the onset of seafloor spreading is largely due to the worldwide limitation on drilling basement rocks typically buried under several kilometers of sediments. There are four main reasons to drill the Tyrrhenian Basin: it is very young and consequently has a thin sedimentary cover; its bedrock lithology and stratigraphy is extremely well documented by more than 40 y of academic investigations; a 30 m section of partially serpentinized peridotite has been already recovered in the center of the basin; and extensive recent seismic refraction and reflection experiments suggest that most of the basement in the center of the Tyrrhenian is made of exhumed mantle. Expedition 402 will drill two perpendicular transects. An east–west transect will target the progression from magmatic crust to exposed mantle; a north–south transect will map the fault zone that exhumed mantle. Drilling will sample the mantle, the associated magmas, and the products of syntectonic, and possibly ongoing, fluid-rock interaction to evaluate the geochemical exchange between the lithosphere and the hydrosphere and potential related ecosystems.