Plate bending stresses at axial highs, and implications for faulting behavior

We develop flexural models of axial high topography which include the effects of plastic yielding, and use the results to examine the contribution of plate bending stresses and strains to fault patterns at the East Pacific Rise (EPR). The lithosphere responds to buoyancy near the ridge axis created by the presence of elevated temperatures and melt within the crust. Parameterizing these quantities, we find topography at wide axial highs can be fit assuming the crust cools to lithosphere temperatures to 3–4 km depth within ∼2 km from the axis, and the presence of 20–30% melt in the lower crust. A trade-off in buoyancy implies the greater the amount of crustal cooling, the less melt is required. Significant crustal cooling leads to lithospheric strengthening, but plastic bending effects allow the plate to deflect over the distance needed to fit the observed axial relief. The plate develops at the axis with non-zero curvature, and ‘unbends’ as it moves away from the axis, placing a section of the upper surface or seafloor into extension. Seafloor stresses are extensional beginning a few kilometers from the axis, and continuing up to distances of ∼35 km from the axis for models that fit wide axial high topography. A compilation of previous faulting studies at the EPR reveals that distance from the axis over which fault slip remains active appears to depend on the width of the axial high: for narrow highs, this region is confined to at most 15 km from the axis; but at wider axial highs it reaches distances of 20–45 km, comparable to regions of extension predicted by the flexural models. The model strain can be as large as 1.9%, amounting to half the observed strain assuming a fault slip angle of 45°, but most strain assuming a slip angle of 60°. These results strongly suggest that bending stresses contribute significantly to normal faulting on the flanks of fast-spreading ridges, and may have a strong influence on faulting patterns there.