Fig. 11.9 (a) Geologic map of the Mt. Edgar Dome and granitic complex in the Eastern Pilbara (after Van Kranendonk et al., 2007, with permission from Blackwell Publishing). Map shows the internal structure of the domes and the radial distribution of3.32-3.30 Ga metamorphic mineral elongation lineations (arrows) that converge in a vertical zone of sinking between the Mt. Edgar, Carunna Downs and Shaw (not shown) domes. These features are contemporaneous with the arcuate shear zones that formed along granite-greenstone contacts (Fig. 11.8). (b,c) Cross-sections showing the trends of foliation surfaces and shear zones within the Mt. Edgar Dome (after Collins, 1989, with permission from Elsevier).
et al., 2002). Successive groups of these strata were deposited in autochthonous basins that developed on synclines of older greenstones. Episodes of felsic volca-nism in these belts accompanied emplacement of the granitoid domes. The degree of metamorphism and the age of the strata gradually decrease away from the deformed margins of the domes and toward the cores of synclines where the greenstones are only weakly deformed. These weakly deformed areas preserve the delicate Archean stromatolites and other evidence of early life (Buick, 2001). The geometry of the synclines between the domes creates a high amplitude (~15 km), long wavelength (120 km) dome-and-keel structure that developed throughout the entire history of the Eastern Pilbara.
The contacts between the granitoid domes and the greenstones in the Eastern Pilbara vary from being intrusive to unconformities, ring faults and high grade shear zones. The shear zones and ring faults are concentric about the domes and generally display steep to subvertical orientations (Figs 11.8, 11.9b,c). Many of these shear zones, including the Mt. Edgar Shear Zone, formed during the period 3.32-3.30 Ga (Van Kranendonk et al., 2007). The central part of the craton contains a 5- to 15-km-wide zone of ductile deformation called the Lalla Rookh-Western Shaw structural corridor (Fig. 11.8). This zone formed during a period (~2.94 Ga) of contraction and is characterized by multiple generations of folds and ductile rock fabrics (Van Kranendonk & Collins, 1998).
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