Age-Depth Analysis of Heavily Sedimented Old Ocean Floor

Analysis of the relationship between ocean age and tectonic subsidence has long been used to study mantle processes beneath oceanic lithosphere. However, there is a lack of observations on old ocean floor adjoining continental margins because of the large and often uncertain sediment load. In order to address this problem, we have analysed a comprehensive database of seismic reflection and wide-angle profiles on ocean crust abutting the Atlantic continental margins. Where possible, we have made measurements of sea-floor depth, sediment thickness and crustal thickness. We have also used coincident reflection and wide-angle profiles to develop a method for estimating sediment thickness where data are presented only in two-way travel time. On each line, we have estimated the equivalent water-loaded depth to basement and compared these data to an average global age-depth curve. When there is good control on both sediment and crustal thickness, we are able to quantify residual depth and assess the validity of the thermal plate model near the continent-ocean boundary. We find that dynamic perturbations to the plate model adequately explain the observed subsidence and are highly correlated with the long-wavelength gravity anomalies, given an admittance of 25-30 mGal km-1. These observations can further be used to constrain theoretical models of dynamic topography calculated using seismic tomography. Where data are of poorer quality, we instead turn the approach around and use available observations to estimate crustal thickness or illuminate errors in the published interpretation.