Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt, southeastern Australia

Abstract

The Lachlan Fold Belt of southeastern Australia developed along the Panthalassan margin of East Gondwana. Major silicic igneous activity and active tectonics with extensional, strike-slip and contractional deformation have been related to a continental backarc setting with a convergent margin to the east. In the Early Silurian (Benambran Orogeny), tectonic development was controlled by one or more subduction zones involved in collision and accretion of the Ordovician Macquarie Arc. Thermal instability in the Late Silurian to Middle Devonian interval was promoted by the presence of one or more shallow subducted slabs in the upper mantle and resulted in widespread silicic igneous activity. Extension dominated the Late Silurian in New South Wales and parts of eastern Victoria and led to formation of several sedimentary basins. Alternating episodes of contraction and extension, along with dispersed strike-slip faulting particularly in eastern Victoria, occurred in the Early Devonian culminating in the Middle Devonian contractional Tabberabberan Orogeny. Contractional deformation in modern systems, such as the central Andes, is driven by advance of the overriding plate, with highest strain developed at locations distant from plate edges. In the Ordovician to Early Devonian, it is inferred that East Gondwana was advancing towards Panthalassa. Extensional activity in the Lachlan backarc, although minor in comparison to backarc basins in the western Pacific Ocean, was driven by limited but continuous rollback of the subduction hinge. Alternation of contraction and extension reflects the delicate balance between plate motions with rollback being overtaken by advance of the upper plate intermittently in the Early to Middle Devonian resulting in contractional deformation in an otherwise dominantly extensional regime. A modern system that shows comparable behaviour is East Asia where rollback is considered responsible for widespread sedimentary basin development and basin inversion reflects advance of blocks driven by compression related to the Indian collision.

Scandium speciation in a world-class lateritic deposit

“…Similar lateritic profiles are developed over other ultramafic-mafic rocks in Australia (Anand and Paine, 2002), with most of the weathering profile directly inherited from the parent rock except for the transported level. The depth of the profile indicates that the weathering occurred over long time scales, possibly starting as early as 430 million years ago (Fergusson, 2010), in a stable tectonic context. Bulk assays show that Sc concentration increases upward, from ca.…”

Early Palaeozoic continental growth in the Tasmanides of northeast Gondwana and its implications for Rodinia assembly and rifting

“…Following intrusion of plutons related to the Benambran Orogeny, such as the Dido Tonalite and Gem Park Granite in the Greenvale Province and Anakie Province respectively, no evidence has been found for any Silurian igneous activity or sedimentation in the Thomson Orogen. This contrasts with the Lachlan Orogen where there was widespread Middle to Late Silurian sedimentation and igneous activity (Glen, 2005;Fergusson, 2010). The Mossman Orogen in north Queensland also has a substantial Silurian succession deposited in the Broken River Province following the Benambran Orogeny in a forearc setting (Henderson et al, 2011.…”

“…Major shortening and thickening of the crust occurred during the Benambran Orogeny at 450-425 Ma, with voluminous granitoid emplacement in its eastern part. Unlike the Delamerian Orogen, the Lachlan Orogen experienced a complex history post-dating the Benambran Orogeny with periods of extension and successor basin development separated by episodes of compression mainly during Tabberabberan (~390-370 Ma) and Kanimblan (~340 Ma) orogenesis (Glen, 2005;Fergusson, 2010).…”

Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt, southeastern Australia

“…We wish to take issue with only one aspect of the way he has tackled a difficult task and draw his attention to a paper (Rossiter & Gray 2008), which was published in this journal, and should have been considered in his discussion of the role of laterally displaced fragments. Fergusson (2010) argues against the hypothesis of large-scale strike-slip displacement along a Baragwanath Transform as proposed by VandenBerg et al (2000) and Willman et al (2002). He suggests that any Baragwanath Transform lying beneath the eastern faulted margin of the Melbourne Zone must curve in an improbable manner to continue up the eastern margin of the Hay-Booligal Zone ( Figure 1).…”