Rapid exhumation of mantle rocks along detachment faults facilitated by felsic granitoid intrusions at a continent-ocean transition drilled in the Tyrrhenian Sea

Mantle exhumation mechanisms at continent-ocean transitions (COTs) are similar to those at slow and ultraslow spreading ridges, where plate divergence is also accommodated by a combination of magmatic processes and detachment faulting. However, the timescales of exhumation at COTs are poorly constrained because of the thick sediment cover blanketing basement rocks along mature passive margins. IODP Exp. 402 drilled the Tyrrhenian Sea COT and successfully recovered in situ sections of mantle exhumed during Late Cenozoic extension in this back-arc basin. Onedrill site sampled a sequence of variably deformed granitic gneisses intercalated with ~cm-thick slivers of peridotites and basalts, and another drill site sampled a heterogeneous section of heavily serpentinized peridotites with granitoids between the ultramafics. Structural observations and core recovery trends indicate localized deformation along the granitoids, with fabrics varying from undeformed to mylonitic. The presence of both peridotites and felsic granitoids provides a unique opportunity to acquire precise ages for the exhumation and deformation stages that have not yet been resolved in detail.

Zircon and apatite U-Pb geochronology of granitoids yields similar Pliocene ages (<4 Ma), coeval with the biostratigraphic ages of the basal overlying sediments, requiring crystallization at depth followed by rapid exhumation. Thin section microstructures and Electron Backscatter Diffraction data suggest that these granitoids accommodated significant strain during exhumation along a detachment fault. Quartz and feldspar in the mylonites are deformed by dislocation creep, with quartz exhibiting grain boundary rotation and migration, and feldspar displaying bulging, suggesting deformation at temperatures of ~450°C. In contrast, quartz in the protomylonite shows polygonal-shaped grains, indicating static recrystallization at high temperatures with low strain. Ti in quartz analyses yields temperatures of ~400°C for both mylonites and protomylonites, suggesting that the differences in the microstructures are strain dependent and that shear was localized within a ~5-m-thick zone. These chronological and microstructural constraints require >1 cm/year exhumation rates after granitoid emplacement. Lastly, stable isotope constraints from the surrounding peridotites give serpentinization temperatures of ~200°C, with higher temperatures adjacent to granitic intrusions. These results, together with microstructural observations, suggest that serpentinization occurred at shallower depths, after most of the unroofing. Overall, we show that felsic lithologies facilitate most of the exhumation prior to serpentinization and demonstrate that heterogeneous lithologies and pre-existing structures have a major influence on the slip behavior of faults at COTs.