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PRECAMBRIAN TECTONICS AND THE SUPERCONTINENT CYCLE 361

tainties. Problems with diapiric models commonly high-grade metamorphic rocks that define large oro-
include uncertainties surrounding the timing of convec- genic belts. Both these groups contain distinctive suites
tive overturn and how an inverted density profi le could of igneous rocks.
be maintained or periodically established over a 750 The most common lithologic assemblage in the
million year history without thrust faulting (Van weakly deformed parts of Early–Middle Proterozoic
Kranendonk et al., 2004). How the stiff rheology of crust are quartzite-carbonate-shale sequences that reach
granitoids allows diapirism also is unclear. Problems thicknesses of some 10 km (Condie, 1982b). Quartz-
with horizontal tectonic models may include a lack of pebble conglomerates and massive, cross-bedded sand-
evidence of large-scale tectonic duplication of the stones also are common. Many of these sequences are
greenstones by thrusts in some areas and uncertainties intercalated with banded iron formations, cherts, and
surrounding how the formation of metamorphic core volcanic rocks. Other rock types that are either rare or
complexes could produce the distinctive ovoid patterns absent in Archean belts appeared at this time, including
of the granitoids. Horizontal tectonic models also com- extensive evaporites, phosphorous-rich sedimentary
monly encounter difficulty explaining the kinematics sequences, and red bed deposits (Section 3.4). These

and horseshoe-shaped geometry of shear zones that latter rocks generally are interpreted to have accumu-
border many granitoid domes (Marshak, 1999). lated in stable, shallow water environments after 2.0 Ga.
A comparison of the evolution of various Archean The appearance and the preservation of such thick
cratons has suggested that aspects of both horizontal sequences of sedimentary rock has been interpreted to

and vertical tectonic processes occurred in different reflect the stabilization of Precambrian continental
places and at different times. Hickman (2004) high- crust during Proterozoic times (Eriksson et al., 2001,
lighted numerous tectonic and metamorphic differ- 2005) (Section 11.4.2). In the Pilbara region of north-
ences between the Eastern and Western parts of the west Australia (Fig. 11.8) the deposition of 2.78–2.45 Ga
Pilbara craton prior to ∼2.95 Ga. He showed that, unlike coarse clastic sedimentary rocks and volcanic sequences
the more or less autochthonous dome-and-keel struc- in a shallow platform environment in the Hamersley

ture of the Eastern Pilbara, the Western Pilbara pre- Basin (Trendall et al., 1991) reflects this stabilization. By
serves a series of amalgamated terranes (Section 10.6.1) 1.8 Ga, the existence of large, stable landmasses and free
that are separated by a series of thrusts and strike-slip oxygen in the Earth’s atmosphere allowed all of the
shear zones (Fig. 11.8) and involved episodes of hori- well-known sedimentary environments that character-
zontal compression that resemble a Phanerozoic style ize Phanerozoic times to develop (Eriksson et al.,
of plate tectonics. These differences suggest that both 2005).
vertical and horizontal tectonics played an important The highly deformed regions of Proterozoic crust
role during the formation of the Pilbara craton. are divisible into two types (Kusky & Vearncombe,1997).
The first type consists of thick sedimentary sequences

that were deformed into linear fold-and-thrust belts
11.4 PROTEROZOIC similar to those in Phanerozoic orogens (Figs 10.5,
10.19). The second type consists of high-grade gneisses
TECTONICS of the granulite and upper amphibolite facies. Some of
the largest and best known of these latter belts form the
∼1.0 Ga Grenville provinces of North America, South
America, Africa, Antarctica, India, and Australia (Fig.
11.4.1 General geology of 11.19). Other belts (Fig. 11.12) evolved during the period
2.1–1.8 Ga (Zhao et al., 2002). These orogens contain
Proterozoic crust large ductile thrust zones that separate distinctive ter-
ranes. Some contain ophiolites (Section 2.5) that resem-
Proterozoic belts display two groups of rocks that are ble Phanerozoic examples except for the lack of highly
distinguished on the basis of their metamorphic grade deformed mantle-derived rocks at their bases in ophio-
and deformation history. The first group consists of lites older than ∼1 Ga (Moores, 2002). The presence of

thick sequences of weakly deformed, unmetamor- these features reflects the importance of subduction,
phosed sedimentary and volcanic rocks that were collision, and terrane accretion along Proterozoic
deposited in large basins on top of Archean cratons. continental margins (Carr et al., 2000; Karlstrom et al.,
The second group is composed of highly deformed, 2002).