Timing and tectonic significance of Late Archaean, sinistral strike-slip deformation in the Central Pilbara Structural Corridor, Pilbara Craton, Western Australia

Abstract

A north-south striking zone of sinistral strike-slip shear (20 × 100 km) and associated folding and thrusting within the central part of the Pilbara Craton is described. This zone, referred to herein as the Central Pilbara Structural Corridor (CPSC), was formed during regional D3 deformation and overprints two earlier sets of structures associated with granitoid doming. It is bound to the west by a continuous, north-south striking, curviplanar fault at greenschist-facies. In the east it is bound by a series of NE-SW striking fault segments located along sheared fold limbs, and by a kilometre-wide zone of amphibolite-facies mylonite within the western margin of the Shaw Batholith. Faults vary from narrow, high-level, brittle structures and retrograde breccia zones in the north, to broader, more ductile structures with S C mylonitic foliations in the south, suggesting a southerly increase in exhumation. A set of kilometre-scale, en-échelon folds with NE-SW striking, upright axial planes and NE-plunging axes characterize the interior of the CPSC and are wholly contained within the bounding faults, thus indicating coeval development with faulting during sinistral wrenching ( σ 1 = NW-SE). Strike-slip deformation in the CPSC is interpreted to have resulted from northwesterly indentation of Shaw Batholith (and contiguous rocks to the east). Westerly-directed tectonic escape of the Strelley Granite laccolith and associated supracrustal rocks from in between Shaw Batholith and North Pole Dome, was followed by ramping up the concave eastern margin of the Yule Batholith and eastward tilting during the transition from westerly to northerly displacement. Continued northward translation of the Strelley Granite at the leading edge of the system was terminated by constriction between the Carlindi Batholith and the North Pole Dome. Trailing greenstone successions were crumpled into a series of NE-trending, en-échelon folds and related faults. Kinematic evidence and structural geometry indicate that sediments of the Lalla Rookh Basin (De Grey Group) were deposited in advance of the north-moving Strelley Granite within a basin bounded by faults of opposite displacement, rather than in a pull-apart basin as previously suggested. The age of the D3 deformation is indicated by resetting of the ArAr and RbSr isotopic systems to ca. 2950 Ma in the Western Shaw region, and by UPb ages of syn-kinematic granitoid rocks.