Page 375 - Global Tectonics

358 CHAPTER 11

et al., 2002). Successive groups of these strata were Vertical tectonic models describe the diapiric rise of
deposited in autochthonous basins that developed on hot granitoid domes as the result of a partial convective
synclines of older greenstones. Episodes of felsic volca- overturning of the middle and upper crust. Collins et al.
nism in these belts accompanied emplacement of the (1998) and Van Kranendonk et al. (2004) used strain pat-
granitoid domes. The degree of metamorphism and terns, a dome-side-up/greenstone-side-down sense of
the age of the strata gradually decrease away from the displacement in shear zones, and other features to link
deformed margins of the domes and toward the cores the formation of dome-and-keel structures to a sinking
of synclines where the greenstones are only weakly of the greenstones. The process begins with the
deformed. These weakly deformed areas preserve the emplacement of hot TTG suite (Section 11.3.2) granit-
delicate Archean stromatolites and other evidence of oids into an older greenstone succession (Fig. 11.10a).
early life (Buick, 2001). The geometry of the synclines Domes are initiated at felsic volcanic centers due to a
between the domes creates a high amplitude (∼15 km), laterally uneven emplacement of TTG magma. After a
long wavelength (120 km) dome-and-keel structure that hiatus of several tens of millions of years, the emplace-
developed throughout the entire history of the Eastern ment of thick piles of basalt on top of less dense gran-
Pilbara. itoids creates an inverted crustal density profi le (Fig.
The contacts between the granitoid domes and the 11.10b). The magmatism also buries the granitoids to
greenstones in the Eastern Pilbara vary from being mid-crustal depths where they partially melt due to the
intrusive to unconformities, ring faults and high grade build up of radiogenic heat and, possibly, the advection
shear zones. The shear zones and ring faults are con- of heat from mantle plume activity. Thermal softening
centric about the domes and generally display steep to and a reduction in mid-crustal viscosity facilitates the
subvertical orientations (Figs 11.8, 11.9b,c). Many of sinking of the greenstones, which then squeezes out the
these shear zones, including the Mt. Edgar Shear Zone, underlying partial melts into rising, high-amplitude
formed during the period 3.32–3.30 Ga (Van Kranen- granitoid domes (Fig. 11.10c). The convective overturn-
donk et al., 2007). The central part of the craton con- ing depresses geotherms in the greenstone tracts,
tains a 5- to 15-km-wide zone of ductile deformation resulting in local cooling and the preservation of
called the Lalla Rookh–Western Shaw structural corri- kyanite-bearing metamorphic rocks, which equilibrate
dor (Fig. 11.8). This zone formed during a period at moderate-low pressures (∼600 MPa) and tempera-
(∼2.94 Ga) of contraction and is characterized by mul- tures (500°C). This model explains the formation of the
tiple generations of folds and ductile rock fabrics (Van dome-and-keel structure without rigid plates or plate
Kranendonk & Collins, 1998). boundary forces and is similar to the sinking or sagduc-
tion models proposed for the formation of dome-and-
keel structures in the Dharwar craton of India (Chardon
11.3.5 Horizontal and et al., 1996).
Horizontal tectonic models for the Eastern Pilbara
vertical tectonics propose that the greenstones were affected by one or
more periods of horizontal contraction and extension
The origin of the unique dome-and-keel architecture of (Blewitt, 2002). In these interpretations, the contraction
the Archean cratons (Section 11.3.4) is important for results from episodes of Early Archean collision (Sec-
understanding the nature of Archean tectonics. In tions 10.4, 10.5) and terrane accretion (Section 10.6).
general, interpretations can be divided into contrasting Periods of horizontal extension result in the formation
views about the relative roles of vertical and horizontal of crustal detachments and the emplacement of the
displacements in producing this pattern. The Eastern granitoid domes. Kloppenburg et al. (2001) used obser-
Pilbara craton in western Australia illustrates how verti- vations of multiple cross cutting fabrics and unidirec-
cal and horizontal tectonic models have been applied to tional patterns of stretching lineations to suggest that
explain the dome-and-keel structural style. During this the Mt. Edgar Dome initially formed as an extensional
discussion, it is important to keep in mind that the metamorphic core complex (Sections 7.3, 7.6.3, 7.6.6).
crustal structure, as illustrated by the Pilbara example, An initial period of terrane collision and thrusting prior
is the product of multiple episodes of deformation, to 3.32 Ga thickens the Early Archean Warrawoona
metamorphism, and pluton emplacement rather than a Greenstone Belt and buries granitoid basement to mid-
single tectonic episode. crustal levels where it partially melts. Partial melting