TROPICS

island arc crust via some mechanical process driven by the density contrast between the
dense residues of andesite differentiation and less dense upper mantle peridotite
(Herzberg et al., 1983; Kay and Kay, 1988; Arndt and Goldstein, 1989; Jull and Kelemen
2001). Ducea and Saleeby (1996), Lee et al. (2000), and Kelemen et al. (2003a) discuss
observational evidence for this having taken place in the past. Behn and Kelemen (2006)
estimate the likely seismic properties of arc lower crust that is denser than underlying
mantle for comparison with observed seismic profiles of modern arcs. They conclude
that most arcs do not include much dense crust, and suggest that such material has
already foundered into the underlying mantle, or else is “hidden” beneath the crustmantle
transition since its high seismic velocities
approach those of mantle peridotite. Davidson and
Arculus (2005) and Takahashi et al. (2006) also
favor the notion that dense by-products of andesitic
crust production in arcs are simply not recognized
as such since their geophysical properties are close
to those of the mantle.
Figure 5. A relationship between the compressional wave
speed, Vp, and density of typical lower crustal arc rocks
(Behn and Kelemen, 2006). A grey zone shows a range of
Vp in the central Aleutian Arc. In Costa Rica all
estimates of crustal Vp are lower than 7.5 km/s.
2.2 Oceanic plateaus and their role in the formation of continents
Oceanic plateaus are areas of massive submarine volcanism that stand above the
surrounding seafloor (Coffin and Endholm, 1994), and that have crust which may be 3 or
more times thicker than the average 7 km of the oceanic crust (Kerr, 2003). Seismic
properties of oceanic plateau crust are similar to those of oceanic crust, but often with a
prominent lowermost layer of faster wave speed likely representing more dense
magnesium- and iron-rich rocks (Walther, 2003). Plateaus are believed to result from
initiation of mantle plumes within oceanic lithosphere, and, along with areas of
subaerial basaltic flood volcanism, they are termed Large Igneous Provinces (LIPs).
A number of lines of evidence link oceanic plateaus to formation of continents. First,
plateaus represent abrupt mechanical discontinuities in oceanic plates. They are heated,
and hence elevated, upon formation, and then slowly subside, remaining much
shallower than the oceanic lithosphere on which they are built (Ito and Clift, 1998). Thus
plateau edges represent compositional buoyancy contrasts that are likely to serve as the
locus of new subduction zones (Stein and Goldstein, 1996; Niu et al., 2003).
Furthermore, as plateau crust becomes thicker due to higher degrees of melting and/or
fast mantle upwelling, the respective volume of depleted (and thus buoyant) mantle
rock beneath oceanic plateaus grows as well. Abbott et al. (1998) suggest that at a critical
thickness of about 25 km, oceanic plateaus become "unsubductable", and thus must
become a part of a future continent. However, only one present-day plateau (the
Ontong-Java) is known to have crust in excess of 25 km thick (Gladczenko et al., 1997;
Miura et al., 2004; Walther, 2003).
Budgets of some trace elements in the bulk continental crust are different from those
found in intra-oceanic volcanic arcs. Rudnick (1995), as updated by Plank & Langmuir,
(1998) Barth et al. (2000), and Rudnick & Gao (2003) suggested that 5 to 20% of the bulk
continental crust could have originated through "intraplate volcanism". The intraplate
input could have occurred via incorporation of oceanic plateaus. And indeed, a number
of examples of former oceanic plateaus within stable continental crust are described in
the literature, including the classic example of the entire Arabian plate, the smallest and
youngest craton on the planet (e.g., Stein and Goldstein, 1996). On a smaller scale, the
C-10
TPI 6838742