The Southwest-Indian Ridge (SWIR) is an ultraslow spreading midocean ridge (< 20 mm/a full rate). Petrological investigations revealed that centers of magmatic activity and magmatic quiet zones interchange. The poor amount of locally generated melt requires a lateral transport from greater depths and a concentration of magma at the active centers. This led to the hypothesis that the lithosphere-asthenosphere boundary (LAB) varies significantly along the ridge axis. Seis-mo¬logical observations at the ultraslow spreading Knipovich Ridge (Arctic) showed a distinct distribution of hypocenters. The depth of the deeper earthquakes draw a sinoidal contour between 8 and 25 km. Shallower hypocenter depths correlate with active centers, greater depths with an amagmatic section of the ridge. The maximum hypocenter depths follow most likely the LAB which in turn can be regarded as isotherm (~800°C). A significant depth change of LAB should therefore be reflected in the measured seafloor heat flow. Based on seismological studies from SWIR we can infer that there the LAB also shows a pronounced topography as seen at the Knipovich-Ridge. Heat flow measurements in conjunction with new seismological observations of AWI will provide important constraints of the concept of a strongly varying LAB.
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