Geological Survey of Victoria


Geological Survey of Victoria

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Robertson K.E.,University of Adelaide | Heinson G.S.,University of Adelaide | Taylor D.H.,Geological Survey of Victoria | Thiel S.,University of Adelaide | Thiel S.,Geological Survey of Western Australia
Australian Journal of Earth Sciences | Year: 2017

The magnetotelluric (MT) method was used to image the crust and upper mantle beneath the Delamerian and Lachlan orogens in western Victoria, Australia. During the Cambrian time period, this region changed from being the extended passive margin of Proterozoic Australia into an Andean-style convergent margin that progressively began to accrete younger oceanic terranes. Several broadband MT transects, which were collected in stages along coincident deep (full crust imaging) seismic reflection lines, have now been combined to create a continuous 500 km east–west transect over the Delamerian–Lachlan transition region in the Stawell Zone. We present the electrical resistivity structure of the lithosphere using both 3D and 2D inversion methods. Additionally, 1D inversions of long-period AusLAMP (Australian Lithospheric Architecture Magnetotelluric Project) MT data on a 55 km regionally spaced grid were used to provide starting constraints for the 3D inversion of the 2D profile. The Delamerian to Lachlan Orogen transition region coincides with the Mortlake Discontinuity, which marks an isotopic discontinuity in Cenozoic basalts, with higher strontium isotope enrichment ratios in the Lachlan Orogen relative to the Delamerian Orogen. Phase tensor ellipses of the MT data reveal a distinct change in electrical resistivity structure near the location of the Mortlake Discontinuity, and results of 3D and 2D inversions along the MT profile image a more conductive lower crust and upper mantle beneath the Lachlan Orogen than the Delamerian Orogen. Increased conductivity is commonly ascribed to mantle enrichment and thus supports the notion that the isotope enrichment of the Cenozoic basalts at least partially reflects an enriched mantle source rather than crustal contamination. Fault slivers of the lower crust from the more conductive Lachlan region expose Cambrian boninites and island arc andesites indicative of subduction, a process that can enrich the mantle isotopically, and also electrically, by introducing carbon (graphite) and water (hydrogen). © 2017 Geological Society of Australia

Robertson K.,University of Adelaide | Taylor D.,Geological Survey of Victoria | Thiel S.,University of Adelaide | Heinson G.,University of Adelaide
Gondwana Research | Year: 2015

The Delamerian Orogen in southeast Australia represents a Proterozoic continental rift margin, overprinted by convergent margin Andean-style subduction in the Cambrian. A detailed 150. km east-west magnetotelluric transect was collected across the orogen to investigate the electrical resistivity structure. The magnetotelluric transect follows an existing full crustal reflection seismic transect, of which interpretations support a westward-dipping Cambrian subduction model as derived from field mapping and geochemistry. A 2D inversion of the data from the 68 station broadband magnetotelluric transect imaged a heterogeneous crust with lateral changes as large as 10,000. Ω. m occurring over ~. 15. km. The crust within the western Glenelg Zone is resistive, in contrast to the eastern Glenelg Zone and the Grampians-Stavely Zone (above the paleo-subduction zone), which host three conductive pathways. The main low resistivity regions (~. 1-10. Ω. m) reside at mid-lower crustal depths (~. 10-30. km), extending up to the surface with a higher resistivity (~. 300. Ω. m), but still much less than surrounding resistivity (mantle. ~. 1000. Ω. m, crust. ~. 10,000. Ω. m). Fluids released from the upper mantle during the Cambrian west-dipping subduction are interpreted to have moved up crustal faults to create the observed low resistivity pathways by serpentinisation and magnetite creation in mafic-ultramafic rocks. The electrical conductivity of hand samples of serpentinised mafic-ultramafic rocks in the region was found to be much greater than most other rock types present. In addition to adding insight into the crustal structure, the magnetotelluric data also supports geological surface mapping, as the major Lawloit and Yarramyljup Faults that bound different geological domains also mark domains of different electrical structure. © 2014 .

Rawlinson N.,University of Aberdeen | Arroucau P.,University of Lisbon | Musgrave R.,Geological Survey of New South Wales | Cayley R.,Geological Survey of Victoria | And 2 more authors.
Geology | Year: 2014

We use ambient noise recordings from the largest transportable seismic array in the Southern Hemisphere to image azimuthal variations in Rayleigh wave phase anisotropy in the crust beneath southeast Australia. This region incorporates a transition from the Precambrian shield region of Australia in the west to younger Phanerozoic terranes in the east, which are thought to have been formed by subduction-accretion processes. Our results, which span the shallow to lower crust, show a strong and consistent pattern of anisotropy that is oriented north-south, approximately parallel to the former margin of East Gondwana. However, significant deviations from this trend persist through the period range 2.5 s to >10 s. One of the most notable deviations occurs along the edge of cratonic Australia, where the Curnamona Province forms a salient into the younger accretionary terrane, here, the fast axis of anisotropy follows the boundary almost exactly, and is virtually coincident with magnetic lineations extracted from aeromagnetic data. To the east of this boundary beneath the Lachlan orogen, a region masked by the Cenozoic Murray Basin, the fast axis of anisotropy becomes strongly curved and traces out a semicircular pattern with a radius of 200-250 km. Farther east, the fast axis of anisotropy returns to a dominantly north-south orientation. These new findings provide strong observational support to recent geodynamic modeling results that demonstrate how large-scale oroclinal structures can become embedded in accretionary mountain belts. © 2014 Geological Society of America.

PubMed | Geological Survey of New South Wales, Geological Survey of Victoria, University of Tasmania, Australian National University and 3 more.
Type: | Journal: Scientific reports | Year: 2015

Simple models involving the gradual outboard accretion of material along curvilinear subduction zones are often inconsistent with field-based evidence. A recent study using 3-D geodynamic modelling has shown that the entrainment of an exotic continental fragment within a simple subduction system can result in a complex phase of growth. Although kinematic models based on structural mapping and high-resolution gravity and magnetic maps indicate that the pre-Carboniferous Tasmanides in southeastern Australia may have been subjected to this process, to date there has been little corroboration from crustal scale geophysical imaging. Here, we apply Bayesian transdimensional tomography to ambient noise data recorded by the WOMBAT transportable seismic array to constrain a detailed (20km resolution in some areas) 3-D shear velocity model of the crust beneath southeast Australia. We find that many of the velocity variations that emerge from our inversion support the recently developed geodynamic and kinematic models. In particular, the full thickness of the exotic continental block, responsible for orocline formation and the tectonic escape of the back arc region, is imaged here for the first time. Our seismic results provide the first direct evidence that exotic continental fragments may profoundly affect the development of an accretionary orogen.

Huston D.L.,Geoscience Australia | Mernagh T.P.,Australian National University | Hagemann S.G.,University of Western Australia | Doublier M.P.,Geoscience Australia | And 7 more authors.
Ore Geology Reviews | Year: 2015

Tectono-metallogenic systems are geological systems that link geodynamic and tectonic processes with ore-forming processes. Fundamental geodynamic processes, including buoyancy-related processes, crustal/lithospheric thinning and crustal/lithospheric thickening, have occurred throughout Earth's history, but tectonic systems, which are driven by these processes, have evolved as Earth's interior has cooled. Tectonic systems are thought to have evolved from magma oceans in the Hadean through an unstable "stagnant-lid" regime in the earlier Archean into a proto-plate tectonic regime from the late Archean onwards. Modern-style plate tectonics is thought to have become dominant by the start of the Paleozoic. Mineral systems with general similarities to modern or geologically recent systems have been present episodically (or semi-continuously) through much of Earth's history, but most of Earth's present endowment of mineral wealth was formed during and after the Neoarchean, when proto- or modern-style plate tectonic systems became increasingly dominant and following major changes in the chemistry of the atmosphere and hydrosphere. Changes in the characteristics of some mineral systems, such as the volcanic-hosted massive sulphide (VHMS) system, reflect changes in tectonic style during the evolution towards the modern plate tectonic regime, but may also involve secular changes in the hydrosphere and atmosphere.Whereas tectono-metallogenic systems have evolved in general over Earth's history, specific tectono-metallogenic systems evolve over much shorter time frames. Most mineral deposits form in three general tectono-metallogenic systems: divergent systems, convergent systems, and intraplate systems. Although fundamental geodynamic processes have driven the evolution of these systems, their relative importance may change as the systems evolved. For example, buoyancy-driven (mantle convection/plumes) and crustal thinning are the dominant processes driving the early rift stage of divergent tectono-metallogenic systems, whereas buoyancy-driven processes (slab sinking) and crustal thickening are the most important processes during the subduction stage of convergent systems. Crustal thinning can also be an important process in the hinterland of subduction zones, producing back-arc basins that can host a number of mineral systems. As fundamental geodynamic processes act as drivers at some stage in virtually all tectonic systems, on their own these cannot be used to identify tectonic systems. Moreover, as mineral systems are ultimately the products of these same geodynamic drivers, individual mineral deposit types cannot be used to determine tectonic systems, although mineral deposit associations can, in some cases, be indicative of the tectono-metallogenic system.Ore deposits are the products of geological (mineral) systems that operate over a long time frame (hundreds of millions of years) and at scales up to the craton-scale. In essence, mineral systems increase the concentrations of commodities through geochemical and geophysical processes from bulk Earth levels to levels amenable to economic mining. Mineral system components include the geological (tectonic and architectural) setting, the driver(s) of mineralising processes, metal and fluid sources, fluid pathways, depositional trap, and post-depositional modifications. All of these components link back to geodynamic processes and the tectonic system. For example, crustal architecture, which controls the spatial distribution of, and fluid flow, within mineral systems, is largely determined by geodynamic processes and tectonic systems, and the timing of mineralisation, which generally is relatively short (commonly <. 1. Myr), correlates with local and/or far-field tectonic events.The geochemical characteristics of many mineral systems are a consequence of their geodynamic and tectonic settings. Settings that are characterised by low heat flow and lack active magmatism produce low temperature fluids that are oxidised, with ore formation caused largely by redox gradients or the provision of external H2S. The characteristics of these fluids are largely governed by the rocks with which they interact, rocks that have extensively interacted with the hydrosphere and atmosphere, both environments that have been strongly oxidised since the great oxidation event in the Paleoproterozoic. In settings characterised by high heat flow and active magmatism, ore fluids tend to be higher temperature and reduced, with deposition caused by cooling, pH neutralisation, depressurisation and fluid mixing. Again, the characteristics of these fluids are governed by rocks with which they interact, in this case more reduced magmatic rocks derived from the mantle or lower crust. © 2015.

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