Oceanic corrugated surfaces and the strength of the axial lithosphere at slow spreading ridges

We analyse the topography and gravity signature of 39 corrugated surfaces formed over the past 26 myrs in the footwall of axial detachment faults at the eastern Southwest Indian Ridge. These corrugated surfaces appear to have formed at a melt supply about half the global melt supply average for mid-ocean ridges, and we find that their presently elevated topography, relative to adjacent non-corrugated seafloor, was mostly acquired at the end of their formation, at the “termination stage”. This configuration, which also characterizes many off-axis corrugated surfaces in other oceans, suggests that the plate flexural rigidity was very low during the formation of the corrugated surface, and increased significantly at the termination stage. Following Buck (1988), we hypothesize that stresses related to bending of the plate cause internal deformation and damage in the footwall of the fault, which is associated with weakening. As a possible mechanism for enhanced footwall weakening while corrugated surfaces form, we propose the formation of weak shear zones coated with hydrous minerals such as talc, amphibole, chlorite and serpentine, in mantle-derived ultramafics next to gabbro intrusions. If this hypothesis is correct, the amount of footwall weakening and roll-over along axial detachment faults at slow spreading ridges may be controlled both by access to hydrothermal fluids in the footwall of the detachment, and by the abundance and distribution of gabbros intrusions in exhumed ultramafics.