Evidence for structural repetition in the greenstones of the Kalgoorlie district,Western Australia |
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| Institution: | 1. Laboratory of Physiology, Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba 277-8564, Japan;2. Biological Laboratory, Faculty of Education, Wakayama University, Sakaedani, Wakayama 640-8510, Japan;3. Ibaraki Prefectural Oarai Aquarium, Oarai, Ibaraki 311-1301, Japan;1. Geological Survey of Canada, Continental Geoscience Division, Ottawa, Ontario, Canada;2. Département des Génies Civil, Géologique et des Mines, École Polytechnique de Montréal, Montreal, Québec, Canada |
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| Abstract: | Regional mapping and air-photographic interpretation of an area of about 20 000 km2 centred on Kalgoorlie provided compelling evidence that some previously published polycyclic stratigraphies cannot be substantiated. Large areas of rocks formerly proposed as younger cycles represent repetition, mainly by faulting, of a somewhat simpler stratigraphic sequence. The main lines of support are: (1) the similarity of older and younger mafic-ultramafic successions, (2) the tendency of ‘younger’ sequences to merge with ‘older’ ones when traced along strike, and (3) the abundant evidence of faulting along critical contacts. The subtle concordant nature of some of the faulting is consistent with an origin by thrusting at an early stage in the tectonic history, especially where repeat sequences are folded around major upright structures. Later reactivation of sheared contacts, and initiation of new ones during upright folding and faulting, and transcurrent shearing, is believed to have widely occurred. Gravitational gliding is proposed as a possible mechanism for thrust generation, this being consistent with evidence of earlier instability in the sedimentation style of turbidites, debris flows and olistostromes, however, conclusive evidence of a mechanism is lacking due to incomplete field evidence. The repetition is viewed as a rearrangement of recognisable elements of the local stratigraphy, rather than the result of a major collisional event at a plate margin. This tends to favour an intracratonic rather than an oceanic setting for the local greenstones, though the characteristic geological features of modern continental rifts have not been observed. |
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