The South Pacific superswell

Seafloor depths in a broad area of French Polynesia are 250 to 750 m shallower than lithosphere of the same age in the North Pacific and the North Atlantic. The area of shallow seafloor also correlates with a region of high density of volcanoes, low seismic velocity in the upper mantle, and a reduction in the thickness of the elastic plate supporting the volcanoes. The Marquesas fracture zone marks an abrupt transition between normal lithosphere to the north which follows the thermal subsidence curve for a 125-km-thick plate and shallow lithosphere to the south which behaves as though it is only 75-km thick. This age dependence in the French Polynesian depth anomalies, the low elastic plate thickness, and the change in depth at the Marquesas fracture zone, a lithospheric discontinuity, require elevated temperatures in the lithosphere. The pattern and amplitude of the depth anomaly is not consistent with the notion that it results from lithospheric thinning beneath a number of overlapping hot spot swells. Rather, we propose that hot spot traces cluster in this region because the lithosphere is already thinner and more vulnerable to magma penetration. The reduction in the thickness of the thermal plate is presumably due to enhanced small-scale convection resulting from the thermal and/or chemical effect of a broad mantle up welling beneath the South Pacific as imaged by seismic tomography. The morphologic and petrologic characteristics of this superswell resemble those that existed in the mid-Cretaceous over H. W. Menard's Darwin Rise, a region of the Pacific which includes the Mid-Pacific Mountains, the Marshall Islands, Magellan Seamounts, and Wake Guyots. We propose that the South Pacific superswell is the modern-day equivalent of the Darwin Rise, and that it may be merely an extreme example of global variability in lithospheric thermal structure as a function of temperature, chemistry, and/or state-of-stress in the upper mantle.