Résumé
At fast-spreading oceanic ridges such as the East Pacific Rise, divergence between tectonic plates is accommodated almost exclusively by magmatic accretion. A robust understanding of magmatic accretion during seafloor spreading is therefore necessary to model the structure and composition of oceanic lithosphere and exchanges in heat and mass that occur are spreading ridges at a global scale over geological time. Whereas most models consider magmatic accretion in 2D, variations in ridge axial morphology and magmatic compositions highlight the occurrence of ridge-parallel variations in magmatic processes along fast-spreading ridges. This suggests that magmatic accretion may be a 3D process involving significant ridge-parallel magma transport. To constrain the process of magmatic accretion at fast-spreading ridges, we present our on-going investigation of magma transport in the Oman ophiolite. The first order structure and composition of the Oman ophiolite, defined by a sheeted dyke complex with an underlying axial melt lens of variable thickness on top of foliated gabbro, is analogous to the East Pacific Rise. The Oman ophiolite therefore provides a unique opportunity to investigate magmatic accretion processes in three dimensions at ridge segment- to grain-scales. We present new data that constrains the directions of magma transport in the crust of the Oman ophiolite. This includes analyses of foliated gabbros and sheeted dyke complexes from the Fizh, Salahi, and Sarami blocks, which define a complete ridge segment. Using anisotropy of magnetic susceptibility (AMS), we report the orientations of magnetic fabrics that serve as proxies for magmatic flow directions. Combining AMS data with paleomagnetic analyses of magnetic remanence directions allows us to restore magmatic flow directions to their paleo-ridge orientation, prior to obduction. Our results indicate that magmatic flow directions vary along the length of the ridge segment and cannot be explained by simple 2D models. Our data show that ridge-parallel lateral flow is a common phenomenon in both the sheeted dykes and foliated gabbros over the length of a ridge segment, which has important implications for competing models of crustal accretion.