Mechanical anisotropy control on strain localization in upper mantle shear zones

Mantle rocks at oceanic spreading centers reveal dramatic rheological changes from partially molten to solid‐state ductile to brittle deformation with progressive cooling. Using the crustal‐scale Wadi al Wasit mantle shear zone (SZ, Semail ophiolite, Oman), we monitor such changes based on quantitative field and microstructural investigations combined with petrological and geochemical analyses. The spatial distribution of magmatic dikes and high strain zones gives important information on the location of magmatic and tectonic activity. In the SZ, dikes derived from primitive melts (websterites) are distributed over the entire SZ but are more abundant in the center; dikes from more evolved, plagioclase saturated melts (gabbronorites) are restricted to the SZ center. Accordingly, harzburgite deformation fabrics show a transition from protomylonite (1100°C), mylonite (900–800°C) to ultramylonite (<700°C) and a serpentine foliation (<500°C) from the SZ rim to the center. The spatial correlation between solid‐state deformation fabrics and magmatic features indicates progressive strain localization in the SZ on the cooling path. Three stages can be discriminated: (i) Cycles of melt injection (dunite channels and websterite dikes) and solid‐state deformation (protomylonites‐mylonites; 1100–900°C), (ii) dominant solid‐state deformation in harzburgite mylonites (900–800°C) with some last melt injections (gabbronorites) and ultramylonites (<700°C), and (iii) infiltration of seawater inducing a serpentine foliation (<500°C) followed by cataclasis during obduction. The change of these processes in space and time indicates that early dike‐related ridge‐parallel deformation controls the onset of the entire strain localization history promoting nucleation sites for different strain weakening processes as a consequence of changing physicochemical conditions.