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Devonian

Early Devonian Bindian Orogeny (420-~410 Ma)

 Ensay Shear Zone mylonite. Thin section of mylonite derived from granite, showing dextral shear. Livingstone Creek.

Ensay Shear Zone mylonite. Thin section of mylonite derived from granite, showing dextral shear. Livingstone Creek.

At the end of the Silurian, rocks in the Cowombat Rift were faulted and tightly to isoclinally folded parallel to the rift margins, presumably controlled by the rigid rocks on the rift flanks. The volcanics were metamorphosed to lower greenschist facies and often were strongly cleaved. Newfabrics overprinted Benambran fabrics in the Omeo Metamorphic Complex. The most significant structures formed during the Bindian Orogeny include the Kiewa,Kancoona, Ensay and Indi faults.


The deformation occurred in a regime of mainly strike-slip movement. Several hundred kilometres of displacement between the Benambra and Whitelaw terranes was taken up by inferred dextral strike-slip displacement along the Baragwanath Transform. Elsewhere in the western and central parts of the Benambra Terrane, the effects of the Bindian Orogeny were concentrated along smaller strike-slip and thrust faults that accommodated the internal fragmentation of the Benambra Terrane as it moved southward.

No effects of the Bindian Orogeny are recorded in the Whitelaw Terrane of central and western Victoria.

Early Devonian rifting, volcanism and sedimentation (410-385 Ma)

 Little River gorge consists almost entirely of Gelantipy Ignimbrite which here is an intra-caldera ignimbrite within the Woongulmerang Caldera. Much thinner outflows continue south to Mt Murrinda.

Little River gorge consists almost entirely of Gelantipy Ignimbrite which here is an intra-caldera ignimbrite within the Woongulmerang Caldera. Much thinner outflows continue south to Mt Murrinda.

The Bindian Orogeny was followed by a second period of crustal extension that affected much of the eastern Lachlan Fold Belt. Bimodal (but dominantly felsic) volcanism occurred at several scales: in large rift-like basins where the volcanics are overlain by limestone, and in much smaller cauldrons.

The largest rift-like basins in eastern Victoria are the Buchan Rift and the Mitchell Basin, with smaller basins at Errinundra (the Boulder “graben”) and Bindi (the Bindi Syncline). The thick but poorly dated Mount Tambo Group may also have been deposited at this time. Cauldrons formed at this time include the Mount Burrowa, Dartella, Besford, Mount Elizabeth and Mount Gelantipy.

In the Buchan Rift there were two stages of rifting producing a mixture of volcanics and minor non-volcanic sediments (Snowy River Volcanics). Prolonged erosion was followed by a final phase of silicic to mafic olcanism and lacustrine sedimentation. Most of the sequence is subaerial, but in the southern half rifting and subsidence outstripped deposition, giving an important interval of submarine turbidites and dark shale. Pyroclastics, especially ignimbrites, make up much of the sequence but most are relatively thin. Sedolithic conglomerate and sandstone occurs at many levels, showing that the rift was a persistent topographic low and that rifting continued through the entire basin history.

In the Melbourne Zone, sedimentation continued without break (Yarra Supergroup). Along its western margin, shoreline facies are present. These may be coeval with rare conglomerates preserved in the eastern Bendigo Zone. To the east, the sediments were mostly deep marine except on the Waratah Bay Platform on which very shallow marine limestone accumulated. Not until late in the Early Devonian is there any evidence of sediment supply from the cratons of eastern Victoria. This suggests that either there was a major barrier to the sediment coming west or the cratons were not in their present position with respect to the Melbourne Zone.

In western Victoria, numerous granites, mostly I-types, were intruded. On the Delamerian craton, A-type volcanics (Rocklands Volcanics) were deposited over the already deformed and eroded Grampians Group.

Middle Devonian Tabberabberan Orogeny (385-380 Ma)

Syncline, with fault displaced sandstone bed in Liptrap Formation. Near Cape Liptrap.

Syncline, with fault displaced sandstone bed in Liptrap Formation. Near Cape Liptrap.

The Tabberabberan Orogeny was the first deformation that affected all parts of the Lachlan Fold Belt in Victoria. The orogeny marks the amalgamation of the Whitelaw and Benambra terranes and cessation of dextral strike-slip movement along the Baragwanath Transform. Its effects were most strongly felt in previously undeformed regions with thick sediment fills such as the Melbourne Zone, Mitchell Syncline, Boulder “graben” and Scrubby Creek Syncline. Deformation was also intense along the Governor Fault, the northern and eastern bounding structure to the Melbourne Zone. Within the Melbourne Zone, sedimentation ceased and the rocks were folded, generally into much more open patterns than the adjacent Bendigo and Tabberabbera zone rocks.



In the areas already substantially cratonised in the Benambran and Bindian orogenies, the Tabberabberan Orogeny mostly produced isolated brittle structures that had little effect on the overall structural framework.The Tabberabbera Zone is probably the only exception to this. More ductile structures, such as minor folds and cleavage, affected the previously deformed bedrock in the south of the zone and strong faulting occurred along its western margin during amalgamation with the Melbourne Zone. In the Kuark and Mallacoota zones the effects of deformation are poorly dated and it is possible that some ductile deformation occurred. Many older faults were reactivated, including the Kiewa, Kancoona, Indi and Ensay faults and the bounding faults to the Buchan Rift.


 Mount Useful Fault Zone. Cross-sections showing thin-skinned nature of structures in sedimentary rocks overlying Cambrian volcanics of the Selwyn Block.
Mount Useful Fault Zone. Cross-sections showing thin-skinned nature of structures in sedimentary rocks overlying Cambrian
volcanics of the Selwyn Block.


By the end of the Tabberabberan Orogeny, all of Victoria had been cratonised and the various crustal blocks were in more or less their present-day positions with respect to each other.

Late Devonian volcanism, plutonism and sedimentation (380-350 Ma)

 Cobaw Batholith. Total magnetic intensity and radiometric images showing the contrast between the component granites.

Cobaw Batholith. Total magnetic intensity and radiometric images showing the contrast between the component granites.

Volcanism and sedimentation began during the waning phases of the Tabberabberan Orogeny. Upper Devonian rocks are widespread in central Victoria and preserved in several large synclines farther east. The strong palaeogeographic differences that had existed in the various structural zones prior to the Tabberabberan Orogeny are no longer in evidence. They are thus relatively uniform across the fold belt and ignore structural zone boundaries.


However, two quite different provinces were formed. The Central Victorian Magmatic Province, which extends from the western edge of the Bendigo Zone across the Melbourne Zone into the westernmost Tabberabbera Zone, consists of granite plutons and silicic cauldron complexes.The Howitt Province, along the margin between the Melbourne and Tabberabbera Zones, contains a thick sequence of red-bed fluvial sediments and silicic volcanics and was originally connected with the East Gippsland Province where fluvial sediments predominate.