Stratigraphic continuity and early deformation of the central part of the Coppin Gap Greenstone Belt, Pilbara, Western Australia

Abstract

This article focuses on the discrepancy between the often advocated stratigraphic continuity of greenstone belts in the Pilbara Craton (Australia) on the one hand, and the observations of syndepositional deformation and other tectonic disturbance on the other hand. The Coppin Gap Greenstone Belt of the East Pilbara consists predominantly of mafic, intermediate and felsic rocks, with minor amounts of silicified sedimentary rocks of the early Archaean Warrawoona Group. Several well-defined zones subparallel to the bedding show intense brittle to ductile deformation, testifying to differential movements of the tectono-stratigraphic slices they delimit. These deformation zones were at least partly related to west-block-down normal faults and accommodated upper block to the west transport. However, new U–Pb zircon dates, geochemical analyses and detailed field observations show that the overall stratigraphic succession in the Coppin Gap Greenstone Belt has not been fundamentally disturbed. The normal faults occur in several irregularly superposed arrays of km-spaced faults, part of which were active during deposition. Normal faults also occur throughout the entire belt. Combined, the early deformation structures indicate east–west extension over a minimum period of 20 Ma, from ∼3469 Ma onwards. Likely, the extension took place over an even longer period of time, during much of the time of deposition of the Warrawoona Group, from ∼3490 Ma onwards. Syndepositional extension of the Warrawoona Group was geometrically unrelated to the doming of granitoid batholiths. Surficial (i.e. uppermost crustal) gravitational collapse during or shortly after deposition may have caused the apparent contradictory co-existence of deformation zones and stratigraphic continuity. Although the deformation has not caused major disruptions of the stratigraphic succession, the precision of the U–Pb SHRIMP ages is insufficient to rule out excisions or duplications of stratigraphy on the order of a few million years.