Plastic Deformation of Plagioclase in Oceanic Gabbro Accreted at a Slow‐Spreading Ridge (Hole U1473A, Atlantis Bank, Southwest Indian Ridge)

Abstract

AbstractCrustal architecture at slow‐spreading oceanic ridges results from complex interactions between magmatism, hydrothermalism, and tectonics. IODP Hole U1473A was drilled during Expeditions 360 and 362T at the summit of the Atlantis Bank, a gabbroic massif exhumed at the Southwest Indian Ridge. In this study, we identify and quantify plastic deformation processes in gabbroic lithologies and active slip systems in plagioclase from 115 microstructural domains throughout Hole U1473A. We describe deformed zones using petrographic observations and electron backscattered diffraction analyses made all along the core. Ductile deformation is widespread, and in places strongly localized in mylonitic and ultramylonitic zones. Plagioclase represents ∼60% of rock's volume and is the dominant phase accommodating deformation in samples. It shows strong dynamic recrystallization accommodated by subgrain rotation in the dislocation creep regime, forming a fine‐grained matrix. Electron backscattered diffraction analyses reveal weak to moderate crystallographic preferred orientations of plagioclase as a result of plastic deformation and strain localization, producing a fabric characterized by (010) parallel to the foliation plane and [100] parallel to the lineation. The fabric strength is first increasing from slightly deformed lithologies to mylonites before decreasing significantly in ultramylonites. This could be explained by orientation scattering after recrystallization, and a change of active slip systems. Subsequent granular flow has likely occurred in some samples. A detailed investigation of intracrystalline misorientations measured at plagioclase subgrain boundaries reveals the activity of four dominant slip systems: [001](010), [100](001), ½[110](001), and ½[](001). These slip systems reflect decreasing temperatures during CPO development and subgrain wall formation.