Deformation mechanisms of granulite-facies mafic shear zones from hole U1473A, Atlantis Bank, Southwest Indian Ridge (IODP Expedition 360)

Abstract

Lower-crustal shear zones from hole U1473A (IODP Expedition 360) were studied via quantitative microstructural analysis and thermodynamic modelling to constrain deformation conditions during detachment faulting. Porphyroclasts of clinopyroxene and orthopyroxene, plagioclase and olivine are included in a fine-grained, polyphase matrix that contains plagioclase-rich layers. Microfractures occur in orthopyroxene, and core-mantle structures are common in all porphyroclasts. Crystallographic fabrics in clinopyroxene clasts indicate activation of (010)[001] slip system, whereas the rimming neoblasts show activity of both (010)[001] and (001)[100]. Fabrics of plagioclase-rich layers suggest the activation of the (010)[100] slip system. Phase mixing and weak crystallographic fabrics in the polyphase matrix point to oriented-growth during diffusion-assisted grain boundary sliding. Thermodynamic modelling indicates that the gabbroic shear zones formed at ~900–920 °C and 2.2–2.7 kbar, under melt-present conditions, and re-equilibrated down to 835 °C during exhumation, as indicated by hornblende–plagioclase thermometry. Our results suggest that deformation in the lower parts of Atlantis Bank was accommodated by a combination of brittle fragmentation and viscous flow during in-situ melt-consumption back-reaction. Such mechanisms effectively resulted in strain localisation in fine-grained, polyphase shear zones that contributed to the weakening of the ocean crust during detachment faulting and subsequent exhumation of the Atlantis Bank core complex.