Cambrian‐Silurian oceanic rocks, upper Howqua River, eastern Victoria: Tectonic implications

Abstract

A Cambrian‐Silurian succession, with basal mafic volcanic rocks and chert along with much more widespread upper units of quartz‐rich turbidites and black siliceous shale, is exposed in the upper Howqua River area of eastern Victoria and forms much of the basement of the Tabberabbera Zone. This succession is typical of the stratigraphy of upper oceanic successions that are widely preserved in circum‐Pacific orogenic belts and, along with similar sections on the south coast of New South Wales and in central Victoria, provides evidence that much of the Lachlan Fold Belt developed on oceanic basement. The Early Ordovician turbidites indicate that the ocean floor lay adjacent to a continent (eastern Gondwana). Silurian turbidites were derived from a western source reflecting Early Silurian reactivation and eastward growth of the former Delamerian mountain chain in western Victoria. The Cambrian‐Silurian succession contains one major phase of folds, F1, which are northwest trending and open to close with steep axial planes; fold vergence indicates tectonic transport towards the southwest, which is characteristic of the Tabberabbera Zone. A major fault zone occurs along the southwestern contact of the Cambrian volcanic rocks and highly deformed Ordovician‐?Silurian rocks to the southwest are part of the Mt Useful Slate Belt. The deformation occurred in a Silurian compressional backarc setting distant from a convergent margin to the east (New England Fold Belt) and was a consequence of crustal weakening over time as the proportion of weak strata in the total crustal column increased and the region was subjected to thermal softening.