The Lachlan Orogen

Australia: The Land Where Time Began

The Lachlan Orogeny is said by the author¹ to be the keystone for the understanding of eastern Australia's tectonic and metallogenic evolution. He begins his paper by discussing the Lachlan Orogen within the context of eastern Australia. Later in the paper he focuses on its boundaries then on key aspects of its formation, key tectonic, structural and metallogenic aspects. He says his aim is to construct frameworks, plate tectonics and architectural, within which a discussion of mineral deposits can be placed.

The Tasmanides of eastern Australia, part of Gondwana, is comprised of Palaeozoic to Mesozoic age. The break-up, about 750 Ma to about 525 Ma, of the Rodinia Supercontinent is recorded in these Tasmanides. Following the break-up there was about 300 My, until the Late Triassic, of margin interaction, that was largely convergent, with the proto-Pacific plate.

According to the author¹ it is difficult to describe the chaotic geology that results from such interactions, especially as they occurred more than 480 Ma. It has been learned from the South Pacific that arcs form and are subducted in as little as few million years, and they suggest there are 2 main ways.

One way the author¹ suggest is to divide the Tasmanides into 2 areal packages, conventionally by subdividing them into a collage of orogenic belts, previous called fold belts, based on structure and the estimation of the areal extent of deformations of different ages. The Tasmanides are subdivided into 5 orogenic belts by this method. In the south of the Tasmanides these are the Delamerian Orogen, which is the Tyennan Orogen in western Tasmania, the Lachlan Orogen and the southern New England Orogen. In the north there are the Thomson Orogen, then northern New England Orogen and the North Queensland Orogen to the north. The New England Orogen is separated from the Lachlan Orogen and the Thomson Orogen by the Bowen-Gunnedah-Sydney Basin System, a rift-foreland basin system of Permian-Triassic age that is a complication. According to this subdivision system Delamerian Orogen rocks were deformed by the Delamerian Orogeny of the Middle to late Cambrian, and at the end of the Ordovician (Benambran Orogeny) the Lachlan Orogen was deformed as well as in the Middle Devonian, the Tabberabberan Orogeny, and again in the Early Carboniferous, when it was deformed by the Kanimblan Orogeny. Sedimentation and/or igneous activity cause complications when they separate 2 phases of an orogeny, and where stratigraphic rock units of post-orogenic age lie above rocks that have deformed in these defining orogenies, which make boundaries between orogens difficult to recognise and interpret.

The author¹ says that among ways problems arise from this system of classification is the question of how to classify the mafic and ultramafic rocks of Cambrian age characteristic of the Delamerian Orogen even the same rock types are also present to the east of the Delamerian Orogen. Along the Peel-Manning Fault System in the New England Orogen and in the Lachlan Orogen, such as in the hanging walls of the Heathcote, Mt Wellington and the Governor Fault Zones. According to the author¹ as it not believed that these rocks were deformed during the Delamerian Orogeny they can't be allocated to that orogeny.

Subdivision of the Tasmanides into packages based on time

The author¹ suggests a solution to this problem is to construct a series of tectonic cycles independent of geography, the concept of supercycles (Glen, 2005). Rock sequences and structures can then be constructed into plate tectonic elements in an individual time slice, a convergent margin possibly consisting of an arc, back arc basin, subduction complex, or an extensional margin or a strike-slip margin.

Sedimentation and igneous activity are included in a complete tectonic cycle, either 1 or a combination of, convergent, strike-slip or extensional or intraplate, after which the accretion of a terrane to Gondwana, that often appears to be a reflection of deformation, occurs such as an arc or subduction complex. The name of its terminal deformation is given to each cycle. According to the author¹ the Tasmanides temporally comprise 3 (super)cycles, each of which terminates in orogens and encompasses sedimentation and igneous activity.

The Delamerian Supercycle, Neoproterozoic to latest Cambrian, that was terminated by collision between the craton and a forearc or arc in the Middle to Late part of the Cambrian, resulting in post-collision granite and molasse. The Lachlan Supercycle, from the basal Ordovician to the Early Carboniferous, has been divided into 3 cycles each of which ends in an orogeny - the Benambran, the Tabberaberabberan and the Kanimblan, and the Hunter Bowen, from the Late Devonian to the Late Triassic, that was terminated by the accretion of an intraoceanic arc.

It has been shown by using the supercycles in this manner that the Tasmanides evolved by sedimentation, arc and back arc activity along the eastern margin of Gondwana over long periods from the Middle Cambrian to the Late Triassic, coupled with the formation of subduction complexes that were related to subduction that was generally west-dipping. The author¹ suggests these were punctuated by short-lived ?orthogonal, as well as highly oblique, accretion of turbidite terranes that were craton-derived, island arcs and subduction complexes that had developed along the eastern margin of Gondwana that also closed back arc basins, though the proto-Pacific never closed.

The rocks of the Tasmanides are generally younger from west to east away from cratonic Australia with the result that the rocks are generally older than the rocks of the New England Orogen¹. The author says this is in accordance with a model in which the accretion or rocks to the boundaries of plates was the mechanism by which the Tasmanides grew, though he says there are 2 caveats that are important (Glen, 2005). The first of these is that on the North Queensland Orogen there is no sign of accretion, rocks from the Neoproterozoic and Permian being stacked on top of each other that remain close to the cratonic margin. This contrasts strongly with the rocks of the southern Tasmanides where the Delamerian Orogen margin is about 400 km east of the Gawler Craton (cratonic Australia). The rocks of the Lachlan Orogen may be up to an extra about 700 km farther east. The New England Orogen is about 300 km further east than the Lachlan Orogen, these widths being deformed. The author¹ suggests that rollback of the proto-Pacific Plate may have occurred only in the south, as may be indicated by the important difference between the northern and southern Tasmanides, and must reflect segmentation or oblique spreading of the plate.

According to the second caveat rollback of the southern Pacific Plate is reflected by some of the outboard orogens that contain old rocks, such as the Delamerian Supercycle in the Lachlan Orogen and New England Orogen of the Lachlan Supercycle in the New England Orogen. These represent pieces of older cycles that were rifted off during rollback in the Ordovician, as well as in the Silurian-Devonian (Glen, 2005) and are now part of the basement in the new cycle.

Boundaries

The young rocks of the Sydney Basin and Gunnedah Basin obscure the eastern boundary between the Lachlan Orogen and the New England Orogeny. To the west lies the Delamerian Orogen, the boundary between these 2 orogens is controversial (Hallett et al., 2005; Hallett, ). It is suggested by cooling ages of about 500 Ma for white mica that were reported from parts of the Stawell Zone, western Victoria (Miller et al., 2005) that the boundary between the Lachlan Zone and the Delamerian Zone lies to the east of the Coongee Break - to the east of the Stawell gold mine, the author¹ taking it to lie along the Avoca Fault, west of the Bendigo Zone. the author¹ says there are considerable economic implications that result from extensions of the Stawell Zone and the Bendigo Zone into southwestern New South Wales. The northern boundary of the Lachlan Orogen with the Thompson Orogen, that is curvilinear, east-west-trending is a crustal scale thrust in the Tibooburra-Brewarrina area, far northwestern New South Wales. Seismic reflection profiling results across this contact are discussed by the author¹ elsewhere.

Delamerian Supercycle

The Lachlan Supercycle

The Kanimblan Cycle

The Hunter-Bowen Supercycle

Dick Glen

Geological Survey of New South Wales, Department of Primary Industries PO Box 344 Hunter Region Mail Centre NSW 2310

Sources & Further reading

Glen, Dick, The Lachlan Orogeny: New Boundaries, new data, new ideas, new deposits.
Glen, R.A., 2005, The Tasmanides of Eastern Australia. In: Vaughn, A. P. M., Leat, P. T. & Pankurst, R. J. Terrane Processes at the Margins of Gondwana. Special publication of the Geological Society, London, 246, 23-96.