Abstract

Bathymetric, seismic-reflection, seismic-refraction, and magnetic-anomaly data suggest that the Madagascar Ridge consists of two different domains. North of 31°S, both the sea floor and the basement topography appear to be locally and regionally complex. Small sediment-filled pockets are present between numerous basaltic basement highs. Large-scale normal faulting shapes the western flank; on the east, the Late Cretaceous fracture zones of the Madagascar Basin penetrate deeply into the ridge. South of 32°S, an extensive area of thick undeformed sediments is found over the central part of the ridge. Two prominent seismic horizons — the upper one being attributed to an unconformity between lower Eocene–upper Oligocene and lower Miocene sediments — can be mapped throughout this area. The steep western slope reflects the Mozambique Basin fracture-zone trends. Both the topography and sediment cover at the southeastern limit are closely related to the north-south–trending fracture zones of the Southwest Indian Ridge. The character of magnetic anomalies is in accord with this regional subdivision. An east-west–trending rise situated near 32°S and marked by the 4,000-m isobath marks the former boundary of two spreading systems that are related to the Central Indian Ridge and the Southwest Indian Ridge. The shallow structural data and our interpretation of those data favor an oceanic nature for the entire Madagascar Ridge.

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