The crustal structure of the Madagascar Ridge

Abstract

Summary. The Madagascar Ridge is an elongated aseismic plateau in the southwest Indian Ocean, which separates two ocean basins of mid‐to late Cretaceous age. From interpretations of a reversed refraction line on the ridge and a continuous gravity profile across it we conclude that it formed during the Cretaceous as thickened oceanic crust, at a mid‐ocean ridge which possibly overlay a mantle hot‐spot. The seismic data consist of record sections from five recording sonobuoys. Forty‐one shots were fired along a 210 km long NNE—SSW line across the central part of the ridge. Simple horizontal‐layer analysis of first arrivals reveals a lower crustal refractor with Vp= 7 kms−1 at a depth of 8–8.5 km and the Moho at an apparent depth of 18 km, but fails to characterize the crustal structure as either oceanic or continental. We obtain a definitively oceanic structure by using synthetic seismogram techniques together with Tau(p) analysis and second arrival correlations. The model which best fits the data consists of 2.2 km of water and 1 km of sediments overlying a velocity gradient from 4 to 7 kms−1. Beneath this, at a depth of 7.5 km, is a 9.5 km thick layer with Vp= 6.9–7.1 kms−1. A 7.5 kms−1 layer, not observed in the first arrival analysis, is found at the base of the crust. The Moho is a transition zone 1 km thick and occurs at ∼ 25 km depth. The gravity data consist of a continuous NE—SW profile across the ridge and on to the flanking basins. Two‐dimensional gravity modelling allows a structure consistent with the synthetic seismogram results, and supports the existence of a high‐density basal layer in the crust. We use in addition a time‐series analysis approach to estimate the depth of compensation assuming an Airy model, and lithospheric thickness assuming an elastic plate model. Convolution of bathymetry along the profile with a range of model filters gives minimum rms gravity residuals for a crustal compensation depth of 23 ± 5 km, and a plate thickness of less than 2–5 km. Copyright © 1981, Wiley Blackwell. All rights reserved