On isostasy at Atlantic‐type continental margins

Abstract

Cross‐spectral techniques have been used to analyze the relationship between free air gravity anomalies and topography of approximately 100 profiles over the continental margins of eastern North America, southwest Africa, and the Coral Sea/Lord Howe Rise. The resulting admittance functions for these margins have been interpreted in terms of the flexure model of isostasy. The relatively young Coral Sea/Lord Howe rise margin is associated with the lowest value of the effective elastic thickness (Te < 5 km), while the relatively old eastern North American margin is associated with the highest value (10 < Te < 20 km). Thus Te appears to increase with the age of the margin. These results, which are consistent with flexure studies from the major ocean basins, have been interpreted in terms of a simple model in which the flexural strength of the lithosphere increases with age. The model suggests that a low‐rigidity plate (or Airy‐type model) is most applicable early in rifting history, while a high‐rigidity plate is more applicable later in margin evolution. Thus sediments at a margin initially load a relatively thin and hot plate, while sediments later in margin evolution load a relatively thick and cold plate. The model explains a number of features of the tectonic‐stratigraphic evolution of margins. These include a change in flexural style from a narrow and deep basin to a broad and shallow basin, coastal plain onlap, and uplift in the continental interior. Furthermore, the flexure model of isostasy contributes in a major way to observed free air gravity anomalies at continental margins. In particular the model explains the existance of (Airy) isostatic gravity anomalies at margins. The amplitude and wavelength of these anomalies are a function of the sediment supply, the type of mechanisms controlling sediment deposition (up building and/or out building), and the thermal structure, history, and age of the margin.