Deformation in the Ballarat Slate Belt, central Victoria, and implications for the crustal structure across southeast Australia

Publisher Website
Google Scholar
Abstract
Chevron folds, high‐angle reverse faults and spaced cleavage characterize crustal shortening above subsurface detachments in the quartz‐rich turbidite succession of the Ballarat Slate Belt of central Victoria. Part of an east‐vergent thrust‐belt with leading‐imbricate fan geometry, the succession is imbricated and allochthonous with respect to the underlying substrate. Palinspastic restoration of the belt gives an original width of 370 km compared with the present width of 110 km. Inferred detachments are within the Lancefieldian (Early Ordovician) and the lower part of the Cambrian succession. Section‐balancing and thrust‐sheet thickness require significant duplexing within the Cambrian metavolcanics (greenstone) that are considered to underline the quartz‐rich turbidite succession. Diachronous west‐to‐east deformation, a consequence of the thrust‐belt geometry, is supported by the 30–40 Ma age variation in older syn‐tectonic granite in the west to younger post‐tectonic granite in the eastern part of the slate belt. Assuming a uniform displacement rate the thrust‐belt can be emplaced over 30 Ma at a geologically reasonable displacement rate of ≈ 1 cm/a. Subvertical extension (20–120%) associated with approximately 50–60% subhorizontal shortening due to folding and cleavage formation are part of an overall plane strain, coaxial deformation sequence in the upper parts of thrust‐sheets, which constitute the major part of the exposed succession. Adjacent to major strike‐parallel faults intense phyllitic cleavage, decreased fold interlimb angles and west‐dipping axial surfaces are part of a non‐coaxial high strain deformation associated with the initiation and propagation of the detachment‐faulting and the emplacement of thrust‐sheets to shallow crustal levels. Overprinting sets of crenulation cleavages that occur within and adjacent to the major faults and fault zones suggest that the early thrusting event (D1), was followed by dextral strike‐slip motion (D2) and then sinistral strike‐slip motion (D3). Faults both within and bounding the structural zones of the Lachlan Fold Belt have undergone complex movement histories that reflect subsequent distribution of regional stresses during protracted deformation from the Late Silurian to the Middle Carboniferous. Such behaviour is a reflection of the composite nature of the crust of southeast Australia, where an allochthonous fragmented upper‐crustal continental margin sediment‐prism assemblage structurally overlies thinned Proterozic? continental basement blocks (granite basement terranes) of the lower crust.