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Tectonics and crustal evolution
GSA TODAY | SEPTEMBER 2016 Chris J. Hawkesworth, Department of Earth Sciences, University peaks and troughs of ages. Much of it has focused discussion on
of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, the extent to which the generation and evolution of Earth’s crust is
UK; and Department of Earth Sciences, University of St. Andrews, driven by deep-seated processes, such as mantle plumes, or is
North Street, St. Andrews KY16 9AL, UK, c.j.hawkesworth@bristol primarily in response to plate tectonic processes that dominate at
.ac.uk; Peter A. Cawood, Department of Earth Sciences, University relatively shallow levels.
of St. Andrews, North Street, St. Andrews KY16 9AL, UK; and Bruno
Dhuime, Department of Earth Sciences, University of Bristol, Wills The cyclical nature of the geological record has been recog-
Memorial Building, Queens Road, Bristol BS8 1RJ, UK nized since James Hutton noted in the eighteenth century that
even the oldest rocks are made up of “materials furnished from
ABSTRACT the ruins of former continents” (Hutton, 1785). The history of
the continental crust, at least since the end of the Archean, is
The continental crust is the archive of Earth’s history. Its rock marked by geological cycles that on different scales include those
units record events that are heterogeneous in time with distinctive shaped by individual mountain building events, and by the
peaks and troughs of ages for igneous crystallization, metamor- cyclic development and dispersal of supercontinents in response
phism, continental margins, and mineralization. This temporal to plate tectonics (Nance et al., 2014, and references therein).
distribution is argued largely to reflect the different preservation Successive cycles may have different features, reflecting in part
potential of rocks generated in different tectonic settings, rather the cooling of the earth and the changing nature of the litho-
than fundamental pulses of activity, and the peaks of ages are sphere. In this contribution, we explore the extent to which
linked to the timing of supercontinent assembly. Isotopic and changes in tectonic processes have shaped the geological record
elemental data from zircons and whole rock crustal compositions and the surface environments through Earth’s history. Where
suggest that the overall growth of continental crust (crustal addi- possible these are linked to changing thermal conditions as the
tion from the mantle minus recycling of material to the mantle) earth cooled.
has been continuous throughout Earth’s history. A decrease in the
rate of crustal growth ca. 3.0 Ga is related to increased recycling The cooling earth influenced the depths and hence the
associated with the onset of plate tectonics. geochemical signatures at which melt generation takes place
(McKenzie, 1984; Nisbet et al., 1993) and the rheology of the crust
We recognize five stages of Earth’s evolution: (1) initial accre- and lithosphere (Gerya, 2014; Sizova et al., 2010). That in turn
tion and differentiation of the core/mantle system within the first influenced tectonic processes, including the initial onset of
few tens of millions of years; (2) generation of crust in a pre-plate subduction and the subsequent onset of “cold” subduction that
tectonic regime in the period prior to 3.0 Ga; (3) early plate was prevalent throughout the Phanerozoic (Brown, 2006, 2014;
tectonics involving hot subduction with shallow slab breakoff over Stern, 2005), which shaped the surface environments on Earth.
the period from 3.0 to 1.7 Ga; (4) Earth’s middle age from 1.7 to Subduction and plate tectonics resulted in the development of
0.75 Ga, characterized by environmental, evolutionary, and litho- supercontinents and enhanced cooling that led to thickening of
spheric stability; (5) modern cold subduction, which has existed for the lithosphere and increased crustal reworking. This, in turn,
the past 0.75 b.y. Cycles of supercontinent formation and breakup resulted in higher erosion fluxes, and changes in the Sr isotope
have operated during the last three stages. This evolving tectonic ratios of seawater and the chemistry of the oceans (Cawood et al.,
character has likely been controlled by secular changes in mantle 2013; Flament et al., 2013; Shields, 2007; Spencer et al., 2014). The
temperature and how that impacts on lithospheric behavior. development of the continental crust is illustrated schematically
Crustal volumes, reflecting the interplay of crust generation and in Figure 2. Magma oceans may have persisted for 5–10 m.y. after
recycling, increased until Earth’s middle age, and they may have initial accretion of the earth, and a crust, which is likely to have
decreased in the past ~1 b.y. been mafic in composition, will have developed at a late stage in
the differentiation and solidification of the magma ocean (e.g.,
BACKGROUND Elkins-Tanton, 2008). The mafic crust is thought to have been
thickened by continuing mafic and ultramafic magmatism until
The geological record is incomplete—some rock types are more remelting and the generation of felsic magmas could occur,
likely to be preserved than others, and breaks in the rock archive resulting in the bimodal silica distribution that is a feature of
are marked by breaks in the depositional record in the upper crust Archean crust (Fig. 3; Kamber, 2015; Kamber et al., 2005). The
and deformational and metamorphic events in the deeper crust. residual garnet signature (low heavy rare earth elements [HREE])
The result is an inhomogeneous distribution of ages of rock units, in most tonalite-trondhjemite-granodiorite (TTG) associations
as strikingly seen in the peaks and troughs of U-Pb crystallization indicates that remelting took place at pressures >10–12 kb (Rapp
ages that appear to be a feature of the geological record (Fig. 1). and Watson, 1995). The late Archean was characterized by TTG
This distribution of ages is unexpected in a planet whose history is magmatism, the remelting of intermediate to felsic crust and the
thought to have been dominated by the continuous action of plate generation of more potassic granites, and the stabilization of
tectonics, and there is considerable debate over the causes of the continental crust and mantle lithosphere (Carlson et al., 2005).
GSA Today, v. 26, no. 9, doi: 10.1130/GSATG272A.1.
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