Great Lyell Fault, western Tasmania: a collage of Middle and Late Cambrian growth faults reactivated during Devonian orogenesis

Abstract

A new model accounts for the complex structural geometries observed at the contact between the Middle to Upper Cambrian Mt Read Volcanics and the Upper Cambrian Owen Conglomerate at the Mt Lyell Mine, Queenstown, Tasmania. Re-examination of the structure has revealed that the Great Lyell Fault comprises a series of Middle Cambrian, Late Cambrian and Middle Devonian aged faults. The Great Lyell Fault comprises a collage of Middle Cambrian faults active during emplacement of the Mt Read Volcanics and Late Cambrian growth structures that controlled deposition of the Owen Conglomerate. These faults were reverse reactivated during initial Middle Devonian orogenesis. The D1 shortening event reverse reactivated the Mt Read Volcanics fault, with shortening manifest by reverse faulting and folding. The Owen Conglomerate basin-margin fault at the Mt Lyell mine site is interpreted to have been dissected by a (D1) reverse-reactivated Mt Read Volcanics fault, which also led to local overturning of the Owen Conglomerate succession. Structural geometries along the Owen Conglomerate – Mt Read Volcanics contact are further complicated by the competency contrast between the Owen Conglomerate and the Mt Read Volcanics, with the development of lobe and cusp folding of the contact during pervasive D2 deformation. Folding associated with the D2 shortening event refolded the Owen Conglomerate – Mt Read Volcanics contact and extensively faulted the Owen Conglomerate, leading to the complex outcrop patterns identified in the Southern Corridor of the mine site. The model accounts for the complex local geology, particularly in light of recent advances in understanding of the regional tectonic evolution, such as the identification of a Late Cambrian extension event in western Tasmania.