Gravity anomalies, flexure and crustal structure at the Mozambique rifted margin

Abstract

The free-air gravity “edge effect” anomaly associated with passive continental margins is one of the most distinctive features of the marine gravity field. Early studies attributed the edge effect to the juxtaposition of thick continental with thin oceanic crust. While thinning of the crust due to rifting at the time of continental break-up is a major contributor, it is now recognised that geological processes such as sedimentation and magmatic underplating also contribute. The different amounts of sediment and underplate at rifted margins help, in fact, to explain the diversity of the edge effect, although they obscure the mechanics, styles and geometry of rifting. In this paper, combined 2-dimensional flexural backstripping and gravity modelling techniques are used to quantify the contribution of rifting, sediment loading and magmatic underplating to the edge effect anomaly at the Mozambique margin. This margin formed as the result of a two-stage break-up, the first of which resulted in the separation of Africa and Madagascar in the Early Jurassic. By comparing the observed edge effect to calculations based on simple elastic plate models, we have been able to constrain the elastic thickness, T e, and, hence, flexural rigidity of the lithosphere that underlies the sediments which accumulated at the Mozambique margin following break-up. The model that best explains the shape (i.e. amplitude and wavelength) of the edge effect high is one in which the earliest Jurassic–Early Cretaceous sediments were deposited on relatively weak lithosphere ( T e∼17.5 km) while the later Miocene–Recent sediments accumulated on relatively rigid lithosphere ( T e∼40 km). These results suggest that sedimentation at the Mozambique margin has involved lithosphere which responds to loads in a similar manner as would oceanic lithosphere. Therefore, the thinned “transitional” crust that underlies much of the coastal plain of the Mozambique margin is probably of oceanic rather than continental origin.