International Ocean Discovery Program
Expedition 402 Scientific Prospectus

Tyrrhenian Continent–Ocean Transition

Tyrrhenian Magmatism and Mantle Exhumation (TIME)¹

¹Zitellini, N., Malinverno, A., and Estes, E.R., 2023. Expedition 402 Scientific Prospectus: Tyrrhenian Continent–Ocean Transition. International Ocean Discovery Program. https://doi.org/10.14379/iodp.sp.402.2023

Abstract

A tenet of plate tectonics is that divergent plates cause the asthenospheric mantle to ascend, decompress, and melt, producing new magmatic crust. However, drilling west of Iberia in the 1980s discovered a continent–ocean transition (COT) made of exposed mantle, revising models of lithospheric thinning and melt generation and defining magma-poor margins. A long-standing argument about mantle in COTs concerns its nature as either subcontinental or being exhumed during ultraslow seafloor spreading. Additionally, two models attribute the apparent lack of melts either to slow extension resulting in low ascent rates with enhanced asthenospheric cooling and reduced melt production or to upwelling mantle originally too depleted to produce a significant melt fraction. The debate on COT models is limited by the scarce evidence obtained in ultra-deepwater drilling, restricted to a few basement highs. Thus, 30 y after its discovery, the nature and genesis of COTs is still controversial. The comparatively shallow water depth and thin sediment cover of the Tyrrhenian Sea provide an optimal location to test COT formation models by drilling. The Tyrrhenian is the only example where extensive modern geophysical data has accurately mapped basement domains of a conjugate pair of COTs. They can be characterized with unprecedented detail in a single drilling expedition to study the time and space evolution of COT processes. Expedition 402 will drill two perpendicular transects. An east–west transect will target the progression from magmatic crust to exhumed mantle; a north–south transect will map the fault zone that exhumed the mantle. Drilling will sample the complete sediment section including Messinian deposits, the sediment/basement interface, the mantle, the associated magmas, and the products of syntectonic, and possibly ongoing, fluid-rock interactions to evaluate the hydrosphere–lithosphere geochemical exchange and potential related ecosystems.

Plain language summary

The 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.