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RESEARCH PRODUCT

The Mulgandinnah Shear Zone; an Archean crustal scale strike-slip zone, eastern Pilbara, Western Australia

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Abstract A large part of the deformation in the Archean Pilbara granitoid-greenstone terrain is localized in relatively narrow shear zones. The Mulgandinnah shear zone (MSZ) is a major one of these, with a width up to 8 km, that can be followed for over 70 km along strike in the Shaw Batholith in the eastern Pilbara. It forms part of the Mulgandinnah Lineament, that can be traced to the Lalla Rookh Basin and the Carlindi Batholith in the north, giving it a total length of over 150 km. The MSZ contains both mylonites and ultramylonites, both of which have foliations that are subvertical to steeply dipping, with the ultramylonitic foliation overprinting the mylonitic foliation to form more localized zones of high strain. Stretching lineations are mostly subhorizontal to shallow south plunging for both the mylonites and ultramylonites, but can vary between 50°N and 50°S. Both foliations formed under upper-greenschist to amphibolite grade conditions. The MSZ displays a consistent sinistral sense of shear, as indicated by σ and δ-type porphyroclasts, SC fabrics and rotated pegmatite veins. The mylonites and ultramylonites are interpreted to have developed as part of a progressive deformation event leading to strain localization in the ultramylonites. Timing constraints indicate that the MSZ has been active at ca. 2930 Ma, but it is likely to have been a long lived structure that was reactivated at that stage, when it acted as a conjugate shear during a NW-SE crustal shortening in the eastern Pilbara. The final juxtaposition of the various domains in the Pilbara granitoid-greenstone terrain occurred during this deformation phase