Role of mantle flow at the North Fiji Basin: Insights from anomalous topography

Abstract

The North Fiji Basin is a complex back‐arc system situated between the opposite‐verging Tonga and New Hebrides subduction zones. On the basis of two independent calculations, we find that the back‐arc region is anomalously high with topography residuals reaching up to 2000 m above what should be isostatically supported. Our study aims to test whether a hot buoyant upwelling is required beneath the North Fiji Basin to support the observed high elevation. The mantle thermal structure beneath the North Fiji Basin is inferred from the global tomographic model S20RTS, where seismic velocity anomalies are converted to density/temperature anomalies. This thermal structure is input into a three‐dimensional mantle convection code, CitcomS, and the flow‐induced surface topography is calculated and compared with the observed anomalous topography. In a series of experiments, we eliminate various portions of the mantle structure to consider what type of mantle flow can support the anomalous topography. The experiments suggest that the observed topography in the North Fiji Basin is consistent with the support of hot buoyant upper mantle upwelling, and specifically rising material in the depth range from 70 to 120 km is the most critical component. A relatively thin/weak lithosphere is required to produce the high‐amplitude and short‐wavelength topography residuals, and the presence of discrete breaks in the plate produces further localization of flow‐induced topography.