Abstract
Slow-spreading ridges are characterized by heterogeneous architecture and seafloor geology. The association of serpentinized mantle rocks and lower crustal gabbros with basalts and marine sediments is common at the seafloor in the vicinity of slow- and ultraslow-spreading ridges. These rocks are exhumed through low-angle normal faults, referred to as detachment faults, which develop in response to the weak magmatic activity at the ridge axis. A significant part of the ridge spreading is accommodated by detachment faulting that progressively evolves into a domed edifice, known as an oceanic core complex, hosting the exhumed lithologies. These core complexes are key localities for investigating tectono-magmatic processes operating at slow-spreading ridges axes. To examine the characteristics of plastic deformation occurring at slow-spreading ridges, I studied a gabbroic sequence drilled on the Atlantis Bank (57°E, Southwest Indian Ridge), and gabbros sampled in a fossil oceanic core complex, the Bracco-Gabbro Complex (Internal Ligurides, Northern Apennines, Italy).I performed microstructural and petro-geochemical studies of gabbros from the Atlantis Bank and the Bracco-Gabbro Complex and complemented them by thermodynamic modeling (Atlantis Bank gabbros). My thesis focusses on IODP Hole U1473A drilled to ~800 m below seafloor in 2016 on the Atlantis bank. The widespread plastic deformation observed in gabbros throughout the core was essentially achieved by dislocation creep, inducing significant dynamic recrystallization of plagioclase, olivine, and clinopyroxene. This deformation is related to emplacement and cooling of the gabbros in the lithosphere and to their subsequent exhumation by detachment faulting. The four main successive episodes of deformation are : (I) hypersolidus crystal-plastic deformation (> 1050 °C, 200-300 MPa) resulting from spreading-related forces at the ridge; (II) solid-state plastic deformation associated to detachment faulting, locally accompanied by melt channeling (1000-1050 °C, 200-300 MPa). Where present, the melt induces melt-rock reactions and enhances strain localization associated with melt-assisted grain boundary sliding; (III) solid-state mylonitization and intracrystalline hardening during strain localization (800-860 °C, 130-150 MPa); (IV) solid-state to semi-brittle strain localization associated to hydrothermal fluid circulation (700 °C, 100 MPa). A change to fluid-assisted grain boundary sliding occurs in highly localized shear zones.This continuum of deformation was essentially achieved by dislocation creep from hypersolidus to amphibolitic conditions, and is associated with the dominant activity of the [100](010) slip system in plagioclase grains. Decreasing temperature during exhumation and synchronous ductile deformation results in the activation of different slip systems from granulitic to amphibolitic conditions.The Bracco-Gabbro Complex is an Alpine Tethys fossil oceanic core complex exposed in the Internal Ligurides. In contrast to the Atlantis Bank, high-grade plastic deformation is poorly developed in the exhumed gabbro pluton. Deformation episodes are documented to occur under granulitic (> 850 °C) to greenschist and sub-greenschist conditions (< 550 °C) and are assumed in previous studies to represent a continuous evolution path during detachment faulting. Dislocation creep is dominant at HT conditions, leading to the dynamic recrystallization of constitutive minerals, assisted by the dominant activation of the [100](010) slip system in plagioclase, and the development of a granulitic foliation. I propose that the Bracco-Gabbro Complex corresponds to a “late-capture OCC”, with higher shear stresses associated to the development of high-grade shear zones, in opposition to the “early-capture OCC” such as the Atlantis Bank. The gabbro pluton was then almost fully crystallized before the detachment faulting initiation.