Heat flow constraints on the South Pacific Superswell

Abstract

The South Pacific superswell has been defined as a large region of anomalously shallow bathymetry, low Love wave velocities, and low effective elastic thicknesses relative to those predicted for its age. These phenomena have been interpreted as reflecting a combination of lithospheric reheating and thinning, and dynamic uplift due to mantle flow. We use heat flow data to better constrain the thermal structure of this region and examine the predictions of various possible models. The average heat flow for the superswell region does not differ significantly from that for lithosphere of similar ages elsewhere on the Pacific plate. Given their uncertainties, the heat flow data imply that thermal lithospheric thickness exceeds 60 km, but cannot discriminate between greater thicknesses. The lack of observed high heat flow appears not to be explained by biases due to water circulation in the thin sediment cover, since the superswell heat flow is not higher than for sites elsewhere with similar sedimentary environments. The Darwin Rise has been proposed as a fossil superswell in the Cretaceous, on the basis of the many similar characteristics to the South Pacific superswell. We find that the Darwin Rise heat flow values do not exceed those for similar ages elsewhere in the Pacific and Atlantic. This observation suggests that any thermal effects associated with the formation of the Darwin Rise are no longer present, and it is consistent with the idea of a fossil superswell. The surface heat flow data thus provide no evidence that the temperatures in the uppermost portion of the lithosphere; are significantly higher in the entire superswell region than in other areas of comparable age. This observation is intriguing given the suggestion that the thin effective elastic thicknesses inferred from seamount loading may reflect reheating of the lithosphere. Models in which the plate thicknesses and/or the basal temperatures are increased to yield temperatures high enough to explain the low effective elastic thicknesses predict surface heat flow much higher than observed. Reheating the lithosphere, as is proposed for hot spots, yields temperatures adequate to explain the effective elastic thicknesses only if reheating occurs at very shallow depths, and again implies a surface heat flow much greater than observed. Hence, unless shallow reheating is somehow localized beneath the seamounts, the thinner elastic thicknesses may reflect mechanical, rather than thermal, weakening of the lithosphere.