CONSTRAINTS FROM SEA BEAM BATHYMETRY ON THE DEVELOPMENT OF NORMAL FAULTS ON THE EAST PACIFIC RISE

We address the question of the horizontal extent of active upper crustal extension near a fast-spreading ridge by applying seafloor slope criteria for normal fault identification to twelve Sea Beam swaths that cross the axis of the East Pacific Rise between 9 degrees 10' and 9 degrees 50'N. Both the density of and the total throw on identified fault system increase with axial distance to at least 30 km from the rise, indicating that the half width of the zone of active normal faulting is 30 km or more. The densities of inward- and outward-facing normal fault scarps in this distance range appear to be similar. The distance from the rise to the nearest large-throw fault zone is approximately axisymmetric to the north of 9 degrees 23'N, but the mid-point between nearest large-throw fault zones is offset 2-3 km to the west of the bathymetric axis south of 9 degrees 23'N and thus tracks the center of the axial magma chamber reflector and presumably the axis of most recent magmatic emplacement.