Mantle Convective Significance of Argentine Passive Margin Dynamic Topography

Abstract

<p>Convective circulation of the asthenospheric mantle deforms the Earth's surface and produces long wavelength (~1000 km) topography that varies as a function of space and time. To obtain accurate measurements of this dynamic topography, it is important to correct for other sources of elevation arising from lithospheric isostasy and flexure. Such measurements are relatively straightforward in the oceanic realm since its lithosphere has a simpler structure than more heterogeneous continental lithosphere. We use a comprehensive database of seismic reflection profiles from the South American passive margin to obtain oceanic residual bathymetric measurements, which are a proxy for dynamic topography. Corrections are applied in order to remove effects of sedimentary loading, anomalous crustal thickness variations, subsidence of oceanic lithosphere as a fuction of age, and non-hydrostaic geoid height variations. We present a  map of oceanic residual bathymetry for a substantial part of the South Atlantic Ocean which defines an extensive negative anomaly corresponding to the Argentine Abyssal Plain. This anomaly is associated with negative free-air gravity and fast shear wave velocities anomalies. The residual bathymetric depression has a roughly circular planform  (~2000 km along its major axis) and reaches more than -1500 m at its lowest point. It also expresses itself  within fringing sedimentary basins offshore Argentina. Seismic reflection images from the continental shelf adjacent to the abyssal plain reveal prograding clinoformal strata of Neogene age that constrain the temporal evolution of this residual bathymetric anomaly. Our results suggest that the Argentine Abyssal Plain depression developed within the last 10 million years. This inference has significant dynamic topographic implications.</p><p> </p>