Neoproterozoic to Early Carboniferous
Regional magnetic image of southeastern Australia showing structural zone boundaries. Data sources: GSV, AGSO, PIRSA, MRT and GSNSW. Click on the image to view a larger version
The Palaeozoic basement is traversed by thrust faults more or less parallel to the north–south structural grain. The largest faults separate rocks with different ages and structural histories, and subdivide Victoria into three main structural rankings consisting of two fold belts (Delamerian and Lachlan), two terranes in the Lachlan Fold Belt (Whitelaw and Benambra), and ten structural zones (Glenelg, Grampians–Stavely, Stawell, Bendigo, Melbourne, Tabberabbera, Omeo, Deddick, Kuark, Mallacoota).
The Moyston Fault is the most important fault as it forms the terrane boundary between the Delamerian and Lachlan fold belts. These two fold belts show important differences. The Delamerian Fold Belt is mainly composed of Neoproterozoic–Cambrian rocks and was deformed in the Late Cambrian Delamerian Orogeny whereas the Lachlan Fold Belt contains mainly Cambrian–Devonian rocks with main deformations occurring in the Late Ordovician–Early Carboniferous interval. The first regional deformation to affect the Lachlan Fold Belt was the Benambran Orogeny, about 50 m.y. after the Delamerian Orogeny. Granites comprise 20% of the total exposed area of the Lachlan Fold Belt and fall within an age range of 440 to 350 Ma. Volcanics associated with the granites are also widespread and cover an additional 5%. Blocks of older crust consisting of Neoproterozoic–Cambrian rocks, such as the Selwyn Block in central Victoria, were deformed during the Late Cambrian Tyennan Orogeny prior to being incorporated into the Lachlan Fold Belt.
Map of eastern Australian fold belts and Victorian structural zones. Click on the image to view a larger version
The second major structural break in Victoria is the Baragwanath Transform, which occurs along the eastern side of the Selwyn Block. This transform fault divides the Lachlan Fold Belt into two terranes, the Whitelaw Terrane to the west and the Benambra Terrane to the east. The main difference between these is that orogen-parallel (north–south) transport was more prevalent in the Benambra Terrane, whereas convergent east–west transport orthogonal to the orogen was dominant in the Whitelaw Terrane.