Geologic framework and tectonic evolution in Western Victoria, Australia

Abstract

The Stawell Zone is interpreted to be part of the eastern extension of the Adelaide Fold Belt into Victoria. The Cambro-Ordovician turbidite sequence of the Stawell and Glenelg zones rests on a pile of Late Proterozoic metavolcanic rocks, rather than Cambrian metavolcanic units that are characteristic of the Lachlan Fold Belt. Early deformation events, D 1–D 3, in the Stawell Zone pre-date granite emplacement and were synchronous with regional fold-forming events that accompany thrust movements along discrete detachment surfaces. The thrust system follows a NE-SE-trending strike, with an E-directed translation of the tectonic units. The steep (≈ 60°) predominantly W-dipping thrusts represent high-strain zones localised in relatively weak Cambro-Ordovician quartz-rich turbidites, that are sandwiched along the boundaries of the Late Proterozoic metavolcanics. Overprinting the early thrust system are D 4–D 6 deformation events that include reverse, strike-slip and normal faults. The Grampians extensional basin, overlying the Late Proterozoic to Ordovician sequence, records a significant change in the tectonic regime operating during the Late Silurian-Early Devonian. It has been subjected to a Middle Devonian compressional deformation event, D 4, with the development of thrust and fold structures. This deformation is also superimposed on the thrusted and folded (D 1 to D 3) sequence to the east of the Grampians half-graben, producing further thrusts. A gradual change from a NE-SW to a N-S stress field produces oblique strike-slip faults. Normal faults of probably Early Cretaceous age transect the entire sequence. The Proterozoic-Cambrian sequence has been affected by a low- P/high- T, mid-greenschist facies regional metamorphic event. Peak-metamorphic conditions have been inferred from metavolcanic rocks and mafic units; derived from a strongly differentiated tholeiitic suite at Stawell. These have been calculated to be 1.7 ± 0.7 (2 σ) kbar and 450 ± 20 (2 σ)° C. The advective heat input necessary to create this low- P/high- T metamorphic event is attributed to penetrative deformation and strain partitioning between the crust and the mantle lithosphere which in turn causes crustal thickening, associated compression, and the observed D 1 to D 3 structures on the surface.