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The Superswell and Darwin Rise: Thermal no longer?

Carol A. Stein1 & Seth Stein2

1Dept. Earth & Environmental Sciences, University of Illinois at Chicago, Chicago IL 60607-7059 USA

2Department of Geological Sciences, Northwestern University, Evanston, IL 60208, USA

Understanding the vertical crustal motion in the Pacific Basin has been a challenge since its identification by Darwin [1845]. Menard [1964] proposed that a large shallow region (Figure 1) in the Western Pacific, which he termed the Darwin Rise, underwent major volcanism and uplift during the Cretaceous. The origin of the Darwin Rise has generally been related to mantle plumes [Morgan, 1972]. Menard [1984] proposed that the Cretaceous Darwin Rise was similar to the present area from the East Pacific Rise to the Society Islands. He considered this area, termed the Polynesian Plume Province (PPP) [Vogt, 1981], to contain both a broad regional uplift and a number of hot spot swells, including the Cook-Austral, Marquesas, Pitcairn, and Society seamount chains.


Figure 1: Map of the south Pacific showing place-names mentioned in text. Click on image to enlarge.

McNutt & Fisher [1987] proposed the term Superswell for the PPP, and further developed the concept of this area as a present analog to the Darwin Rise. The Superswell is shallower than expected for its age, and the effective elastic thicknesses of the lithosphere calculated from the loading of seamounts are less than expected for the age of loading [McNutt & Menard, 1978; Calmant et al., 1990]. McNutt & Fisher [1987] suggested that the shallow bathymetry resulted from the lithosphere in the area having the temperature structure of an anomalously thin 75-km thick thermal plate. McNutt & Judge [1990] further suggested that the weak flexural strengths, low surface wave velocities [Nishimura & Forsyth, 1985], and geochemical anomalies [Hart, 1984, 1988; Castillo, 1988] were consequences of the combined effects of a thin thermal plate and a deeper low-density plume. In this model, the lithosphere is thinned by enhanced heat flux from the mantle and low viscosity beneath the plate, such that the weak plate is easily penetrated by hot spot volcanism.

Extending this analysis, McNutt et al. [1990] suggested that the Darwin Rise was uplifted during the Cretaceous, and was similar to the present Superswell until about 70 Ma. Larson [1991] termed the Cretaceous event a Superplume, which produced both the Darwin Rise and very large amounts of lithosphere at midocean ridges. In this model, the present Superswell reflects the Superplume's waning phase.

Constraints on such models can be derived by examining how depth and heat flow in the Darwin Rise and Superswell compare to those of comparable age lithosphere elsewhere. As shown in Figure 2 (top), ocean depths in the Superswell are shallower than elsewhere. Hence they can be modeled as resulting from the lithosphere being thinned, as illustrated for plates with a 60 or 75 km thermal thickness. However, the thinning models predict heat flow much higher than observed (bottom). In fact, heat flow in the Superswell is no higher than for comparable age lithosphere elsewhere [Stein & Abbott, 1991; Stein & Stein, 1993].

Figure 2: Top: Bathymetric depths of Superswell and the Pacific elsewhere of the same age.
Bottom: heat flow predicted by thinning models, and observed.

Figure 3

Similarly, as shown in Figure 3, both the depths and heat flow in the Darwin Rise are similar to other sites in the Pacific [Stein & Stein, 1993]. Hence the Superswell and Darwin Rise are no longer considered to be due to shallow heating by plumes. Alternative explanations include the dynamic effect of mantle plumes [Sleep, 1992], or the presence of a buoyant volcanic layer just beneath the Moho [McNutt & Bonneville, 2000]. The low effective elastic thicknesses may be due to mechanical weakening by the volcanism, intraplate stresses [Stein & Stein, 1993], or an interaction of the flexural effects of volcanoes of different ages [McNutt et al., 1997].


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