International Ocean Discovery Program
Expedition 402 Preliminary Report
Tyrrhenian Continent–Ocean Transition: Tyrrhenian Magmatism and Mantle Exhumation (TIME)1
Alberto Malinverno, Nevio Zitellini, Emily R. Estes, and the Expedition 402 Scientists
1 Malinverno, A., Zitellini, N., and Estes, E.R., 2025. Expedition 402 Preliminary Report: Tyrrhenian Continent–Ocean Transition. International Ocean Discovery Program. https://doi.org/10.14379/iodp.pr.402.2025
Abstract
In a classical view of tectonic rifting, divergent lithospheric plates cause the asthenospheric mantle to ascend, decompress, and melt, eventually producing new magmatic crust. This view has been updated by drilling results that found exhumed mantle at the continent–ocean transition (COT), leading to the definition of magma-poor continental margins. Obtaining samples and data from drilling in magma-poor COTs is a challenge because the exposed mantle is typically buried under a thick sediment cover. The Tyrrhenian Sea provides an optimal location to test COT formation models by drilling because it has a comparatively thin sediment cover, allows for studying a conjugate pair of COT margins in a single drilling expedition, and has been mapped in unprecedented detail with recent geophysical measurements.
The key objective of International Ocean Discovery Program Expedition 402 was to determine the nature of the geological basement in the central Vavilov Basin, where exhumed mantle peridotites were expected, and in the conjugate margins to the west (Cornaglia Terrace) and east (Campania Basin). In the Vavilov Basin, Sites U1614 and U1616 recovered an exceptional variety of mantle rocks, including lherzolites, harzburgites, plagioclase-bearing lherzolites and harzburgites, dunites, and minor amounts of pyroxenites and mantle intrusions. The mantle peridotites are significantly hydrated and weathered, resulting in the formation of low-temperature serpentine and carbonate veins. In contrast, Site U1612 recovered at the sediment/basement interface an unconsolidated breccia with clasts of basalt, peridotite, and granite, followed by variably deformed mylonitic gneisses that transition downhole to granitoid quartz-diorite rocks. On the western Tyrrhenian margin (Cornaglia Terrace), Site U1613 sampled a sediment sequence dating back to the Messinian (late Miocene), resting on much older sedimentary rocks akin to those outcropping in Sardinia, supporting the hypothesis that the margin consists of extended continental crust. On the conjugate margin to the east (Campania Terrace), Site U1617 did not reach the basement but recovered a complete sequence of Messinian evaporites, including halite.
The samples and data collected during Expedition 402 provide an extensive new data set to determine the heterogeneity of the mantle, the nature and history of melt production and impregnation, and the extent and evolution of serpentinization and carbonate formation; to constrain the geometry and timing of the deformation that led to mantle exhumation; to study the fluid-rock interactions between seawater, sediment, and the serpentinizing mantle; and to constrain geodynamic models of rifting and COT formation.
Plain language summary
The Earth’s mantle rocks, known as peridotites, are generally inaccessible to direct sampling because they are buried tens of kilometers below the Earth’s crust. Consequently, even though the Earth’s mantle makes up 84% of the volume of our planet, our direct knowledge of it is still inadequate. In some continental margins, faults active during the separation of the continents uplifted the deep mantle rocks to the seafloor. However, access to these peridotites is still a challenge because, after continental separation, the margins are buried by sediments that can be several kilometers thick. The geologically young Tyrrhenian Sea is an ideal target for attempting to sample these mantle rocks because not enough time has passed to deposit thick sediments.
Recent seismological surveys predict that peridotites make up most of the basement in a 3600 m deep central Tyrrhenian Basin under a 200–300 m thick sedimentary cover. International Ocean Discovery Program (IODP) Expedition 402 succeeded in drilling through about 150 m of basement rocks at two sites in the central basin, recovering a wide variety of peridotites and some granitoid rocks belonging to the continental crust. Moreover, a drill site on the western margin of the Tyrrhenian Basin reached continental rocks similar to those that outcrop on the island of Sardinia. Another site on the eastern margin sampled deposits of gypsum and salt that formed when the Mediterranean Sea dried up about 5.5 million years ago.
The samples and data collected during IODP Expedition 402 will be used to understand the history of magma generation in the mantle peridotites, the chemical interactions between seawater and the mantle rocks, and the deformation processes that unroofed the deep mantle and led to the formation of the continent–ocean transition.