Deformation, fluid circulation, and strain localization during mantle exhumation in the Tyrrhenian back-arc basin

The exhumation of lithospheric mantle at magma-poor rifted margins requires efficient strain localization and sustained weakening of ultramafic rocks. In the Tyrrhenian back-arc basin, recent IODP drilling by Expedition 402 has provided unprecedented access to mantle exhumed ≤4 Ma, revealing a complex interplay between deformation, magmatic intrusions, and hydrothermal fluid circulation.We assess how hydrothermal fluids and syn-rift intrusions influence the mechanical evolution of the exhumed mantle in the Tyrrhenian Sea. Structural and microstructural observations from drilled mantle sections document a transition from high-temperature ductile deformation (mylonitization) to brittle faulting (e.g., brecciation). This evolution is accompanied by serpentinization and localized carbonation veins. These veins follow pre-existing lithological contacts such as felsic and mafic intrusions, which probably act as rheological barriers and as preferential pathways for fluid flow.We integrate structural analysis, microstructural characterization and mineralogical constraints, and 3D tomography (synchrotron µCT) to evaluate how porosity distribution, connectivity of veins, reaction front, linked to fluid infiltration and fluid-driven mineral transformations, modify mantle rheology. Fluid-assisted weakening and reaction-induced volume changes may promote the development of localized shear zones and, ultimately, detachment faults. Preliminary observations indicate that magmatic intrusions (felsic and mafic) localize strain; subsequent serpentinization further reduces rock strength and facilitates the late stages of exhumation.Our results suggest that mantle exhumation in the Tyrrhenian basin reflects complex coupled deformation-magmatism- fluid processes rather than tectonic extension alone. This provides new constraints on strain localization mechanisms at magma-poor rifted margins and on the mechanical evolution of continent–ocean transitions.