Structural and metamorphic evolution of the Moornambool Metamorphic Complex, western Lachlan Fold Belt, southeastern Australia

G. Phillips, John Miller, C.J.L. Wilson

    Research output: Contribution to journalArticle

    27 Citations (Scopus)

    Abstract

    The wedge-shaped Moornambool Metamorphic Complex is bounded by the Coongee Fault to the east and the Moyston Fault to the west. This complex was juxtaposed between stable Delamerian crust to the west and the eastward migrating deformation that occurred in the western Lachlan Fold Belt during the Ordovician and Silurian. The complex comprises Cambrian turbidites and mafic volcanics and is subdivided into a lower greenschist eastern zone and a higher grade amphibolite facies western zone, with sub-greenschist rocks occurring on either side of the complex. The boundary between the two zones is defined by steeply dipping L-S tectonites of the Mt Ararat ductile high-strain zone. Deformation reflects marked structural thickening that produced garnet-bearing amphibolites followed by exhumation via ductile shearing and brittle faulting. Pressure-temperature estimates on garnet-bearing amphibolites in the western zone suggest metamorphic pressures of similar to0.7-0.8 GPa and temperatures of similar to540-590degreesC. Metamorphic grade variations suggest that between 15 and 20 km of vertical offset occurs across the east-dipping Moyston Fault. Bounding fault structures show evidence for early ductile deformation followed by later brittle deformation/reactivation. Ductile deformation within the complex is initially marked by early bedding-parallel cleavages. Later deformation produced tight to isoclinal D-2 folds and steeply dipping ductile high-strain zones. The S-2 foliation is the dominant fabric in the complex and is shallowly west-dipping to flat-lying in the western zone and steeply west-dipping in the eastern zone. Peak metamorphism is pre- to syn-D-2. Later ductile deformation reoriented the S-2 foliation, produced S-3 crenulation cleavages across both zones and localised S-4 fabrics. The transition to brittle deformation is defined by the development of east- and west-dipping reverse faults that produce a neutral vergence and not the predominant east-vergent transport observed throughout the rest of the western Lachlan Fold Belt. Later north-dipping thrusts overprint these fault structures. The majority of fault transport along ductile and brittle structures occurred prior to the intrusion of the Early Devonian Ararat Granodiorite. Late west- and east-dipping faults represent the final stages of major brittle deformation: these are post plutonism.
    Original languageEnglish
    Pages (from-to)891-913
    JournalAustralian Journal of Earth Sciences
    Volume49
    DOIs
    Publication statusPublished - 2002

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