Age‐depth relationships and depth anomalies in the southeast Indian Ocean and south Atlantic Ocean

Abstract

The distributions of crustal depths as a function of their age have been analyzed for the Southeast Indian Ocean and South Atlantic regions. Average depth, sediment thickness, and age data, compiled and digitized for 1°×1° areas, are used to establish the relationships of adjusted crustal depths to crustal age and to map the planform of residual depth anomalies as referred to a new global, empirical standard. Both the Southeast Indian Ocean and the South Atlantic are characterized by systematic, intermediate wavelength (∼500–1500 km) depth anomalies that serve to define “tectonic corridors” trending parallel to spreading directions. The exact boundaries of these corridors have not yet been determined. Within each corridor, crustal depth increases linearly with age½ as predicted by simple cooling models. The crustal subsidence curves can vary significantly from corridor to corridor and with regard to the adopted global standard. Furthermore, the presence of asymmetric subsidence of oceanic crust within certain corridors is not in accord with the conventionally accepted theory describing crustal depth as a function of crustal age. Corridors exhibiting asymmetries coexist adjacent to corridors exhibiting no asymmetries, thus indicating that the mechanisms controlling these depth distributions can change fairly abruptly along strike of the midocean ridge. A qualitative model is presented that indicates that very small regional differences in the density of the asthenosphere (of the order of ±0.01 g/cm3) can give rise to ∓15% differences in the isostatically controlled equilibrium depth of the top of the lithosphere. Such density differences could easily exist if lateral variations of ∼100°C persisted in the upper asthenosphere. Since the lithosphere is generally perceived as a thermal boundary layer, this is equivalent to predicting small but systematic differences in the thickness of the lithosphere and hence the depth to the top of the lithosphere. An alternative dynamic model involving small‐scale convection (e.g., Buck and Parmentier, 1986) can also account for the observed depth‐age relationships. Both of the above explanations imply the existence of lateral temperature variations in the upper asthenosphere.