Delineation of the main extrusion zone of the East Pacific Rise at lat 21°N

A detailed bathymetric and photographic investigation of the axial region of the East Pacific Rise (EPR) at lat 21°N indicates that the main extrusion zone is 2.5 km wide. Local relief is dominated by irregular, steep-sided volcanic extrusive forms. This central zone gives way laterally to sea floor that is characterized by lower local relief and is cut by open fissures and axial-dipping normal faults that trend parallel to the regional bathymetric outline of the rise crest. This lower local relief results from flooding by low-viscosity lavas after the crust has moved away from the central extrusion zone. Although the EPR at this locality is considered to have a spreading rate between rates for slow and fast spreading centers, the axial region reflects many similarities in the sequence of processes that have been proposed for the slow-spreading Mid-Atlantic Ridge in the FAMOUS area. © 1976 Geological Society of America.