An independent test of thermal subsidence and asthenosphere flow beneath the Argentine Basin

We have used primary precision depth recorder and single channel seismic data from three cruises of the R/V Conrad to test thermal subsidence and asthenosphere flow models for seafloor depth versus age in the Argentine Basin in the western South Atlantic. We found a region in the west central part of the basin where anomalously shallow depths, that can not be explained by any simple thermal or dynamic model, are associated with a local free-air gravity anomaly. Elsewhere, over ages ranging from 1 to 104 Ma, there is no evidence of the “flattening” of the depth/age trend that is characteristic of the plate cooling model for the thermal subsidence of the oceanic lithosphere. The halfspace thermal subsidence model accounts for nearly 98% of the variance of seafloor depth, but the slope, b=425±10 m Ma−1/2, implies improbably high mantle temperatures and/or low mantle densities. Moreover, there is some systematic misfit between the data and the halfspace model. A thermal subsidence model in which initial conditions vary with age accounts for the misfit, but also requires an implausible variation of mantle temperature and/or density. Alternatively, a model that includes the effect of induced flow in the asthenosphere eliminates the misfit and yields a reasonable rate of thermal subsidence b=330±20 m Ma−1/2. That the mantle temperature (Tm∼1150±70)°C implied by the subsidence rate is slightly lower than normal is consistent with the hypothesis that this region has not been affected by hot spots or mantle plumes. The viscosity of the asthenosphere derived from the model (3–4 × 1019 Pa s) is high, but consistent with viscosities estimated from plate dynamics models when the low mantle temperature is taken into account. Finally, the PMS flow model is consistent with measured heat flow in the region. These results lend weight to the hypothesis that the bathymetry of the Argentine Basin is influenced by induced flow in the asthenosphere, as well as by halfspace cooling of the upper mantle.