EUSTASY AS A TEST OF A CRETACEOUS SUPERPLUME HYPOTHESIS

Abstract

The mantle plume model has been used to explain a variety of geological and geodynamic events. For instance, a large plume, or 'superplume', under the Pacific basin has been proposed to account for a number of Cretaceous events, such as a global sealevel rise, an increase in global average temperature, and a pause in the reversals of the Earth's magnetic field. The primary evidence for this hypothesis is a model for the rate of production of ocean ridge material and ocean plateaus, with a high from about 120-80 Ma, mostly due to activity in the Pacific basin, which is claimed to have been driven by the upwelling of a large plume from the core-mantle boundary. The hypothesis requires that a large part of the primary evidence for this high productivity has disappeared by subduction. Here, we test this hypothesis by comparing the Cretaceous eustatic sealevel highstand which would result from the superplume model with generally accepted values. Our model includes estimates of eustatic sealevel change resulting from phenomena explicitly specified for the proposed plume, such as the volume of ocean crust produced and the extent of lithospheric swelling associated with the plume head, as well as other events known to affect eustatic sealevel, such as glaciation and continental collision. The estimated Cretaceous highstand resulting from all the modeled effects is 220-470 m, compared to the observed value of 120-200 m. This discrepancy indicates a probable overestimate of the rate of seafloor and plateau creation and of the size of plume that could have existed. The breakup of the Pangean supercontinent is a more viable explanation of the Cretaceous sealevel rise.