Anomalous seafloor spreading of the southeast Indian ridge near the Amsterdam-St. Paul plateau

The Amsterdam-St. Paul Plateau is bisected by the intermediate-rate spreadi ng Southeast Indian Ridge, and numerous geophysical and tectonic anomalies arise from the interactions of the Amsterdam-St. Paul hotspot and the sprea ding center. The plate boundary geometry on the hotspot platform evolves ra pidly ton timescales <1 Myr), off-axis volcanism is abundant, the seafloor does not deepen away from the axis, and transform faults do not have fractu re zone extensions. Away from the hotspot platform the ridge-transform geom etry is typical of mid-ocean ridges globally. In contrast, the Amsterdam-St . Paul Plateau spreading segments are shorter, they often overlap each othe r significantly, and the intervening discontinuities are smaller, more ephe meral, and more migratory. Abyssal hills are smaller and less uniform on th e hotspot platform than on neighboring spreading segments. From gravity and isostasy analysis the average thickness of the platform crust is similar t o 10 km, approximately 50% thicker than that of typical oceanic crust. Most of the isostatic compensation of the hotspot plateau occurs at the Moho or within the lower crust, and the effective elastic thickness of the plateau lithosphere is similar to 1.6 km, less than half that of adjacent spreadin g segments. Away from the platform some transform faults contain intratrans form spreading centers; on the platform the two transform faults have valle ys which may be depocenters for abundant axial or off-axis volcanism and ma ss wasting. Although not well-constrained by magnetics coverage, the Amster dam-St. Paul hotspot appears to have been "captured" by the Southeast India n Ridge, enhancing crustal production at the ridge since about 3.5 Ma. Prio r to this time the hotspot formed a line of smaller, isolated volcanoes on older Australian plate. The underlying cause for the present-day crustal ac cretion anomalies is the effect of melt generation from separate sources of mantle upwelling (due to plate spreading and the hotspot) which has a cons equent effect of weakening the lithosphere.