Tectonostratigraphic architecture and uplift history of the Eastern ...

WestraliaThe Westralia pit is the largest in the Mt Morgan’s district, and it produced nearly 1 million ouncesof gold (Vielricher et al. (1998). Gold is hosted by thin steeply NE–dipping BIF and porphyrydykes, in a mainly tholeiitic basaltic sequence. Talc–chlorite schists after komatiite occur in thewest and lamprophyre dykes are also common in the pit (Beardsmore, 1999).Westralia is close to the Celia Shear Zone and it provides a likely proxy for its kinematics. Thestructural event history appears complex (Fig. 23), but is simply reflecting a pulsing orogen. Elevendiscrete stages have been identified on the basis of overprinting relationships. The early history,Stage 1 to Stage 8 inclusive, involves west–directed thrusting and dextral faults switching withextension under a predominantly E–W contractional event (see photograph). This behaviour may bea function of how thrusts propagate (surging and/or post motion relaxation?). Gold timing is poorlyconstrained within this complex contraction–extension interplay. Stage 9 sees a switch to NW–SEcontraction, with minor sinistral strike–slip faults, and was followed by typical late collapse withdevelopment of normal faults and flat–lying crenulations.Figure 23: Summary structural stratigraphy of WestraliaEventnumberCharacteristicRegions structuresof σ1of deformationSimplifiedview of σ1‘D5’ reverseshearing‘D3’ Fold axial surface11101115Crenulations& FoldsNormalfaultsBIF‘D1’ Fold closure913Sinistralfaults‘D2’ porphyryintrusion875322Reverse,sinistralshearsDextralshearsAu?View north of complex folds and thrusts atWestralia651540NormalshearsReversefaultsAu?417Normalshears32168120Folds &FoliationPorphyryFolds &FoliationKey Point:Good example of the complex interplay betweencontraction and extension (rapid switching) in athrust régime0Bedding735