THE AXIAL TOPOGRAPHIC HIGH AT INTERMEDIATE AND FAST SPREADING RIDGES

An axial topographic high is commonly observed at both fast spreading ridges and some segments of intermediate spreading ridges. At fast rates the axial high is primarily created by the buoyancy of hot rock and magma beneath the rise. As newly formed crust is transported off axis, little vestige of an axial high is observed on the ridge flanks. In contrast, at intermediate rates, a significant component of the positive topography may be a volcanic construction, preserved on the ridge flanks as abyssal hills, which are split axial volcanoes. We suggest this difference in the nature of the axial high reflects a lithosphere strong enough to support construction of a volcanic crestal ridge at intermediate spreading rates, but only rarely at fast rates. Relict overlap ridges, found within the discordant zones left by overlapping spreading centers, is one class of ridge-flank topography which appears to have a significant volcanic constructional component even at fast spreading ridges. Unlike topography away from these discontinuities, the relief and shape of overlapping spreading centers is preserved as relict ridge tips are rafted onto the ridge flanks. Reduced magma supply at these discontinuities may give rise to an axial lithosphere strong enough to support volcanic construction of overlap ridges. Low axial lithospheric strength may also account for the lack of normal faults within the innermost 1-2 km of fast, and some intermediate, spreading ridges. With a thin/weak brittle layer at the ridge crest, tensile failure will predominate and few normal faults will form. Depths to the axial magma chamber reflector observed in multi-channel seismic data limit the thickness of the brittle layer on axis to less than 1-2 km for much of the EPR. This depth is comparable to depths over which tensile failure within the oceanic crust will predominate, estimated from the Griffith criteria for fracture initiation (similar to 0.5-1.5 km). As the brittle layer thickens/strengthens away from the ridge, shear failure begins and the large-scale normal faults associated with abyssal hill relief develop.