New constraints on the width of the zone of active faulting on the East Pacific Rise 8° 30′ N – 10° 00′ N from Sea Beam Bathymetry and SeaMARC II Side-scan Sonar

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Sea Beam bathymetry and SeaMARC II side-scan sonar data are used to constrain the width of the zone of active faulting (plate boundary zone) to be ∼90 km (∼0.8 Ma) wide along the East Pacific Rise 8° 30′ N – 10° 00′ N. Fault scarps, identified on the basis of contoured, shaded relief and slope intensity maps of bathymetry, are measured. These scarp measurements, used in conjunction with data from a separate near-axis study, show that both inward- and outward-facing fault scarps increase in height away from the ridge axis, reaching average heights of ∼100 m at 0.8±0.2 Ma, 45±10 km from the ridge axis. Beyond this distance, there is no significant increase in scarp height. Earlier studies had suggested that the width of the zone of active faulting for outward-dipping faults might be significantly narrower than for inward-dipping faults. A lower crustal decoupling zone between brittle crust and strong upper mantle is predicted to exist out to ∼20–200 km from the ridge based on previously published lithospheric models. Such a decoupling zone may explain why outward-dipping faults continue to be active as far off-axis as inward-dipping faults. If the width of the zone of active faulting is controlled by the width of a lower crustal decoupling zone, our observations predict an ∼90 km wide decoupling zone in the lower oceanic crust at this location.