Convergence and transpression along the paleo-Pacific margin of Australia during the Paleozoic: insights from the Pinnak Sandstone at Mallacoota, northeast Victoria
Creators: name: Wilson, C. J. L.
Publisher: name: Taylor & Francis
Year: 2024
Subjects: subject: Biophysics
subject: Physical Sciences not elsewhere classified
subject: Biotechnology
subject: Evolutionary Biology
subject: FOS: Biological sciences
subject: Environmental Sciences not elsewhere classified
subject: Chemical Sciences not elsewhere classified
subject: Sociology
subject: FOS: Sociology
subject: Biological Sciences not elsewhere classified
subject: Computational Biology

description: Underpinning almost every Paleozoic paleogeographic reconstruction and tectonic model of the Lachlan Orogen is the interpretation of the structural geology related to crustal collision and amalgamation. Bounding the eastern margin of the Lachlan Orogen is a north-trending corridor of Cambrian–Ordovician turbidites that in northeast Victoria are recognised as the Pinnak Sandstone or previously known as the Mallacoota Beds. This sequence contains an accretionary prism related early S* foliation, syn-sedimentary dykes, slump structures, early faults or décollements and early quartz veins. These indicate anisotropic compaction by fluid extraction in a shallow marine turbidite sequence that underwent localised deformation (D<sub>1</sub>), which can be attributed to the Benambran Orogeny (<i>ca</i> 450 Ma). There was a change in the rheological properties during the ensuing late Silurian D<sub>2</sub> Bindian Orogeny (<i>ca</i> 420 Ma). This event produced north–south-trending upright, east-verging folds (F<sub>2</sub>) with a marked partitioning of strain between the different sedimentary units. Folds plunge south at <i>∼</i>30°, are tight to isoclinal, with local collapsed hinges and limb thrusts. This D<sub>2</sub> deformation along the paleo-Pacific margin involved dextral transpression and a southerly propagation of the deformation front. Superimposed on these structures is an east–west paleostress pattern and two episodes of faulting (D<sub>3</sub> and D<sub>4</sub>). In the first, brittle strike-slip faults were accompanied by ductile folding related to east–west shortening during the Middle–Late Devonian Tabberabberan Orogeny (<i>ca</i> 385–370 Ma). These were accompanied by the emplacement of the Sandpatch Point Granite. The second episode (D<sub>4</sub>) of strike-slip faulting reactivated many of the earlier faults during further east–west shortening and was related to the Carboniferous Kanimblan Orogeny (<i>ca</i> 354–343 Ma). This is best seen close to the granite contact where the strong crystallised granite reinforced the contact metamorphosed greywacke sequence, inhibiting ductile deformation so that faults deformed in a brittle fashion. Eastern Australia was at a convergent plate boundary where a Middle–Late Ordovician turbiditic sequence was accreted to an accretionary prism. The Pinnak Sandstone contains a weak phyllosilicate alignment (S*), D<sub>1</sub> faults and folds from localised deformation in the accretionary prism. The Silurian Bindian Orogeny produced upright east-verging, south-plunging F<sub>2</sub> folds. Brittle faults overprint the Bindian tectonic features and are accompanied by minor folding, marking the transition into Middle Devonian and Carboniferous deformation and intrusive events. The kinematics and geometries associated with these deformation events reflect the changing tectonic regime along the paleo-Pacific margin of eastern Australia during east–west compression.