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 rat es 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 positi ve topography may be a volcanic construction, preserved on the ridge f lanks as abyssal hills, which are split axial volcanoes. We suggest th is difference in the nature of the axial high reflects a lithosphere s trong enough to support construction of a volcanic crestal ridge at in termediate spreading rates, but only rarely at fast rates. Relict over lap ridges, found within the discordant zones left by overlapping spre ading centers, is one class of ridge-flank topography which appears to have a significant volcanic constructional component even at fast spr eading ridges. Unlike topography away from these discontinuities, the relief and shape of overlapping spreading centers is preserved as reli ct ridge tips are rafted onto the ridge flanks. Reduced magma supply a t these discontinuities may give rise to an axial lithosphere strong e nough to support volcanic construction of overlap ridges. Low axial li thospheric strength may also account for the lack of normal faults wit hin the innermost 1-2 km of fast, and some intermediate, spreading rid ges. With a thin/weak brittle layer at the ridge crest, tensile failur e will predominate and few normal faults will form. Depths to the axia l 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 mu ch of the EPR. This depth is comparable to depths over which tensile f ailure within the oceanic crust will predominate, estimated from the G riffith criteria for fracture initiation (similar to 0.5-1.5 km). As t he brittle layer thickens/strengthens away from the ridge, shear failu re begins and the large-scale normal faults associated with abyssal hi ll relief develop.