Depth, age and dynamic topography of oceanic lithosphere beneath heavily sedimented Atlantic margins

Our understanding of the thermal structure of oceanic lithosphere is based upon observations of water-loaded subsidence and plate age. These observations are predominantly from areas with sparse sedimentary cover, which are unperturbed by crustal thickening and by regional epeirogeny. Few such measurements are available from old oceanic lithosphere where sediment thickness is large. In order to address this omission, we have analyzed a legacy dataset of seismic reflection and wide-angle profiles from the margins of the Atlantic Ocean. Our aims are twofold. First, we determine the isostatically corrected age–depth variation of old oceanic lithosphere adjacent to continental margins and compare it with existing trends. Secondly, we isolate residual depth anomalies and compare them with dynamic topographic predictions made from long-wavelength gravity anomalies. Within the sedimentary pile, we have used a suite of coincident seismic reflection and wide-angle measurements to create a method for converting two-way travel time into depth and sediment density. We also correct for changes in oceanic crustal thickness. On the oldest portions of the Atlantic Ocean, the corrected age–depth variation is consistent with results from the Pacific Ocean, which suggests that the oceanic plate has a thickness of 90–95 km. Positive and negative deviations from the global age–depth trend are common and can be as large as ± 1 km. They mostly (but not always) correlate with long-wavelength gravity anomalies if the admittance is ~ 30 mGal km− 1. We propose that these deviations are primarily generated by convective circulation within the upper mantle.