Superswell and Darwin Rise: Thermal
Understanding the vertical crustal
motion in the Pacific Basin has been a challenge since
its identification by Darwin . Menard
 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  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
Figure 1: Map of the south Pacific
showing place-names mentioned in text. Click on image
McNutt & Fisher  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 
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  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. 
suggested that the Darwin Rise was uplifted during
the Cretaceous, and was similar to the present Superswell
until about 70 Ma. Larson  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
Bottom: heat flow predicted by thinning models, and
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.,
Calmant, S., J. Francheteau,
and A. Cazenave, Elastic layer thickening with age
of the oceanic lithosphere a tool for prediction
of age of volcanoes or oceanic crust, Geophys.
J. Int., 100, 59-67, 1990.
Castillo, P., The Dupal anomaly
as a trace of the upwelling lower mantle, Nature,
336, 667-670, 1988.
Darwin, C., The Voyage of
the Beagle (1975 reissue), J. M. Dent, London,
Hart, S. R., A large-scale isotope
anomaly in the Southern Hemisphere mantle, Nature,
309, 753-757, 1984.
Hart, S.R., Heterogeneous mantle
domains: signatures, genesis and mixingchronologies,
Earth Planet. Sci. Lett., 90,
Larson, R. L., Latest pulse of
Earth: Evidence for a mid-Cretaceous superplume,
Geology, 19, 547-550,
McNutt, M., and A. Bonneville,
A shallow, chemical origin for the Marquesas swell,
Geochem., Geophys., Geosys., 1,
McNutt, M. K., and K. M. Fisher,
The South Pacific Superswell, in Seamounts,
Islands, and Atolls, Geophysical Monograph
#43 edited by B. H. Keating, P. Fryer, R. Batiza
and G. W. Boehlert, pp. 25-34, American Geophysical
Union, Washington, D.C., 1987.
McNutt, M. K., and A. V. Judge,
The superswell and mantle dynamics beneath the South
Pacific, Science, 248,
McNutt, M., and H. W. Menard,
Lithospheric flexure and uplifted atolls, J.
Geophys. Res., 83, 1206-1212,
McNutt, M. K., E. L. Winterer,
W. W. Sager, J. H. Natland, and G. Ito, The Darwin
Rise: A Cretaceous Superswell?, Geophys. Res.
Lett., 17,1101-1104, 1990.
McNutt, M., D. Caress, J. Reynolds,
K. A. Jordahl, and R. A. Duncan, Failure of plume
theory to explain midplate volcanism in the southern
Austral Islands, Nature, 389,
Menard, H. W., Marine Geology
of the Pacific, McGraw-Hill, New York, 1964.
Menard, H. W., Darwin reprise,
J. Geophys. Res., 89,
Morgan, W. J., Deep mantle convection
plumes and plate motions, Am. Assoc. Petrol.
Geol. Bull., 56, 203-213,
Nishimura, C. E., and D. W. Forsyth,
Anomalous Love-wave phase velocitiesin the Pacific:
sequential pure-path and spherical harmonic inversion,Geophys.
J. R. astron. Soc., 81, 389-407,
Sleep, N. H., Hotspots and mantle
plumes, Ann. Rev. Earth Planet. Sci.,20,
Stein, C., and D. Abbott, Heat
flow constraints on the South Pacific Superswell,
J. Geophys. Res., 96,
Stein, C. A., and S. Stein, Constraints
on Pacific midplate swells fromglobal depth-age
and heat flow-age models, in The Mesozoic Pacific,Geophys.
Monogr. Ser. vol. 77, edited by M. Pringle, W. W.
Sager,W. Sliter and S. Stein, pp. 53-76, AGU, Washington,
D. C., 1993.
Vogt, P. R., On the applicability
of thermal conduction models to mid-plate volcanism:
comments on a paper by Gass et al., J. Geophys.
Res., 86, 950-960, 1981.