The Tasman Fold Belt System in Victoria

Abstract

To provide a background to a model of the evolution of the Tasman Fold Belt System in Victoria, brief descriptions are given of the various tectonic elements, arranged in eight structural zones and of the structural deformation affecting them. The evolution extended over four main depositional episodes, punctuated by orogenic activity. 1. (1) The Cambrian to Late Ordovician depositional episode Involved the diachronous formation of four large troughs (Stawell, Ballarat, Wagga, Melbourne Troughs). Early Cambrian mafic and ultramafic igneous activity was followed by thick diachronous flysch sedimentation. The Cambro-Ordovician Delamerian Orogeny stabilized part of the Stawell Trough and caused eastward shift in sedimentation into the newly formed Ballarat Trough. Late Middle Ordovician diastrophism led to cessation of sedimentation in the Ballarat Trough and to the formation of the Wagga and Melbourne Troughs. The episode was terminated by the Benambran Orogeny, which folded and stabilized the Ballarat and Wagga Troughs and caused onset of rapid subsidence of the Melbourne and newly formed Buchan and Grampians Troughs. 2. (2) Silurian deposition was confined to the Grampians, Melbourne and Buchan Troughs. The Melbourne Trough received a thick, relatively deep marine sequence, while in the other two troughs, acid volcanicity was followed by non-marine and shallow-marine sedimentation in the Grampians and flysch and shallow-marine sedimentation in the Buchan Trough. Widespread diastrophism and granite intrusion marks the Bowning Orogeny, which stabilized the Grampians Trough and changed the Buchan Trough from orogenic to transitional character. The Melbourne Trough remained relatively unaffected. 3. (3) Deposition during the Early Devonian episode was mainly confined to the Melbourne and Buchan Troughs. Eastward shift of the Melbourne Trough axis was followed by a gradual change from flysch to non-marine sedimentation. In eastern Victoria, formation of a large volcanic arch, superimposed on the Buchan Trough, was followed by widespread shallow-marine sedimentation. The Middle Devonian Tabberabberan Orogeny stabilized the Melbourne and Buchan Troughs and caused major translocation along the Indi—Long Plain Fault System in eastern Victoria. 4. (4) The Late Devonian to Early Carboniferous episode includes intrusion of granite in the Ballarat and Melbourne Troughs; extrusion of volcanics in large cauldron subsidences in the area of the former Melbourne Trough; volcanicity and “redbed” sedimentation in the newly formed Howitt Trough; and some “redbed” sedimentation in eastern Victoria. Mild deformation of these rocks in the Kanimblan Orogeny marked the close of the evolution of the Tasman Fold Belt System in Victoria.