Granitic intrusions enhance strain localization and rapid mantle exhumation along an oceanic detachment fault

Serpentinization and magmatism weaken the lithosphere and facilitate mantle exhumation during magma-poor rifting and ultraslow seafloor spreading. However, the complex interplay and timing among these competing mechanisms along detachment faults remain poorly constrained. The International Ocean Discovery Program (IODP) Expedition 402 drilled an incipient oceanic basin in the Tyrrhenian Sea offshore Italy. Recovered cores consist of one sequence of variably deformed granitic intrusions intercalated with slivers of peridotites and another of primarily serpentinized peridotites with heterogeneous deformation and local granitic intrusions. Geochronological and geochemical data reveal that the granites crystallized at 4 million years ago at a depth of ~7 to 9 kilometers and rapidly exhumed within ~0.5 million years, requiring exhumation rates of ~2 centimeters per year. Structural observations show that these granites accommodated significant postcrystallization strain and enhanced localization of detachment faulting. Stable isotopes record serpentinization temperatures of ~200°C, suggesting that serpentinization occurred after the emplacement and deformation of granites. We conclude that weak felsic rocks may enhance strain localization along detachment faults at intermediate depths and aid continental breakup and mantle exhumation.