New constraints on the width of the zone of active faulting on the east pacific rise 8 degrees 30 ' N-10 degrees 00 ' N from sea beam bathymetry and SeaMARC II side-scan sonar

Sea Beam bathymetry and SeaMARC II side-scan sonar data are used to constra in the width of the zone of active faulting (plate boundary zone) to be sim ilar to 90 km (similar to0.8 Ma) wide along the East Pacific Rise 8 degrees 30' N - 10 degrees 00' N. Fault scarps, identified on the basis of contour ed, shaded relief and slope intensity maps of bathymetry, are measured. The se scarp measurements, used in conjunction with data from a separate near-a xis study, show that both inward- and outward-facing fault scarps increase in height away from the ridge axis, reaching average heights of similar to 100 m at 0.8 +/-0.2 Ma, 45 +/- 10 km from the ridge axis. Beyond this dista nce, 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 l ower crustal decoupling zone between brittle crust and strong upper mantle is predicted to exist out to similar to 20-200 km from the ridge based on p reviously 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 a n similar to 90 km wide decoupling zone in the lower oceanic crust at this location.