TROPICS

I. Overview of the Proposed Project
When compared to the other terrestrial planets, Earth’s continents, large areas
of thick silica-rich (“granitic”) crust, are unique. Understanding the mechanical
(accretion) and chemical (differentiation) processes of continent formation, and
whether there has been secular change are fundamental problems of the earth
science.
Any model for the formation and evolution of continents should explain their
distinctive lithospheric structure and chemical composition. Ultimately,
continental crust is derived from processing rocks formed by melting the upper
mantle, although unlike the mid-ocean ridge environment, multiple stages of
differentiation may be required. At present, some new continental crust may be
created at the edges of existing continents, e.g. in the Andes. This process is,
however, dependent on the prior existence of continental crust, and presents a
classical chicken-and-egg problem for the origin of the continents. An alternative
is to produce new continental crust through subduction-related magmatism.
However, the bulk crustal composition inferred for most intra-oceanic arcs is
basaltic and does not match either the geophysical structure or the composition
of continental crust. To solve this conundrum, a two-step procedure is often
envisaged, whereby thickened arc crust is melted and more silicic rocks are
concentrated in the upper crust while the lower mafic crust with cumulates is
removed via some mechanical process such as delamination.
Additional processes are necessary to account for the great thickness of the
lithosphere beneath continents. These lithospheric mantle “roots” are 200-250 km
thick under some cratons, the oldest (Archean) regions of the continents. While
this root is believed to record protracted melt extraction, the volume of extracted
melt is much larger then the volume of existing continental crust. Present-day
melting at ridges and arcs produces smaller volumes of less depleted, residual
mantle peridotite. Some authors have envisioned formation of cratonic mantle
roots via collision and imbrication when arcs and other oceanic tectonic elements
are accreted during plate convergence. This “underthrusting” scenario differs
from regular subduction in that a part of the lithosphere of the downgoing plate
do not sink into the mantle, but become attached to the overriding plate.
A number of lines of evidence suggest that oceanic plateaus, areas of
voluminous submarine volcanism, may play a significant role in the formation of
new continental crust. From a chemical standpoint, the trace-element budget of
the bulk continental crust differs from that of subduction zone products, and can
be matched by mixing them with a small but significant proportion rocks from
oceanic plateaus. From the physical standpoint, some plateaus are
“unsubductable” due to their combination of thick crust and buoyant residual
mantle. Some present-day stable continental regions (e.g. the Arabian shield) are
thought to represent agglomerations of oceanic plateaus. Edges of oceanic
plateaus are a very likely locus for new subduction zone formation, suggesting
that new arcs nucleate there. Finally, oceanic plateaus may have been more
common during the Archean, as slightly hotter mantle temperatures would have
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TPI 6838742