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At fast-spreading centers, faults develop within the axial summit trough (AST; 0 to 250 m around the axis) primarily by diking-induced deformation originating from the axial magma lens (AML). The formation of the prominent abyssal-hill-bounding faults beyond the axial high (>2,000 m) is typically associated with the unbending of the lithosphere as it cools and spreads away from the AST. The presence of faults is rarely mapped between these two thermally distinct zones, where the lithosphere is still too hot for the faults to be linked with the process of thermal cooling and outside of the AST where the accretional diking process dominates the ridge axis. Here, we reveal a remarkable vertical alignment between the distinct morphological features of the magma body and the orientation of these faults, by comparison of 3-D seismic imagery and bathymetry data collected at the East Pacific Rise (EPR) 9°50’N. The spatial coincidence and asymmetric nucleation mode of the mapped faults represent the most direct evidence for magmatically induced faulting near the ridge axis, providing pathways for hydrothermalism and magma emplacement, helping to build the crust outside of the AST. The high-resolution seafloor and subsurface images also enable revised tectonic strain estimates, which shows that the near-axis tectonic component of seafloor spreading at the EPR is an order of magnitude smaller than previously thought with close to negligible contribution of lava buried faults to spreading. 

Abstract Oceanic detachment faults represent an end-member form of seafloor creation, associated with relatively weak magmatism at slow-spreading mid-ocean ridges. We use 3-D numerical models to investigate the underlying mechanisms for why detachment faults predominantly form on the transform side (inside corner) of a ridge-transform intersection as opposed to the fracture zone side (outside corner). One hypothesis for this behavior is that the slipping, and hence weaker, transform fault allows for the detachment fault to form on the inside corner, and a stronger fracture zone prevents the detachment fault from forming on the outside corner. However, the results of our numerical models, which simulate different frictional strengths in the transform and fracture zone, do not support the first hypothesis. Instead, the model results, combined with evidence from rock physics experiments, suggest that shear-stress on transform fault generates excess lithospheric tension that promotes detachment faulting on the inside corner.