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In the 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 rifted margins. Obtaining geologic samples from COTs to directly constrain the diversity of rifting processes is a challenge because the igneous crust and mantle rocks are 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 geologic 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 Terrace). 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 magmatic intrusions. The mantle peridotites are significantly hydrated and weathered, resulting in the formation of 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 the Triassic–Paleozoic successions 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 mantle serpentinization and carbonation; to constrain the geometry and timing of the deformation that led to mantle exhumation; to study the fluid-rock interactions between seawater, sediment, and mantle peridotites; and to constrain geodynamic models of rifting and COT formation.
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Expedition 402 Scientific Prospectus: Tyrrhenian Continent–Ocean Transition
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.
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- Award ID(s):
- 1326927
- PAR ID:
- 10405105
- Date Published:
- Journal Name:
- Scientific prospectus
- Volume:
- 402
- ISSN:
- 2332-1385
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Book Chapter:
- https://doi.org/10.14379/iodp.sp.402.2023
