Rj Scott

Re-evaluation of contact relationships between Ordovician volcanic belts and the quartz-rich turbidites of the Lachlan Orogen

Some published tectonic reconstructions of the eastern Lachlan Orogen in New South Wales have shown Ordovician volcanic and volcaniclastic rocks of the Macquarie Arc conformably overlying or interfingering with a coeval Ordovician quartz-rich turbidite sequence. Re-examination of key contacts between the volcanic and quartz-rich successions has found no evidence to support this interpretation, and suggests that the two packages are separate tectonostratigraphic terranes. The contacts between these two coeval successions are generally marked by major faults containing mylonites, cataclasites and, at some locations, fragments of mid-ocean ridge-type pillow basalt and chert. The quartz-rich turbidites are generally highly deformed and of higher metamorphic grade than the adjacent volcanics. At Oberon and Mudgee, contacts are faulted but there are no mylonites or significant differences in metamorphic grade. At Palmers Oaky and Black Springs, Silurian quartz-rich sandstones overlying the Ordovician volcanics have been mistakenly assigned to the Ordovician in previous studies. Throughout the Lachlan Orogen, there is no mixing of framework grains. Quartz-rich turbidite successions are dominated by quartz with lesser feldspar and rare tourmaline, zircon and monazite derived from recycled continental sources. In contrast, the volcaniclastic sandstones contain feldspar, clinopyroxene and lithic fragments derived from subduction-related clinopyroxene-phyric basalt and plagioclase-phyric andesite. Detrital-zircon populations also differ, with separate U/Pb age populations and almost no overlap. Comparison of the Ordovician sequences of the Lachlan Orogen with modern turbidites from continental- and arc-related sedimentary basins suggests that complete separation of sedimentary sources is only possible if the sandstones were deposited hundreds of kilometres apart, in separate tectonic environments. The two sequences were juxtaposed along major faults in the Late Ordovician or Early Silurian, probably when the Macquarie Arc collided with a thick Ordovician sedimentary wedge located on the Gondwanan continental margin.

Benambran Orogeny in the Eastern Lachlan Orogen, Australia

The Benambran Orogeny reflects the accretion of the intra-oceanic Macquarie Arc to the Gondwana Plate. The ultimate cause was the northwards strike-slip transport of the allochthonous Bega Terrane along the eastern margin of Gondwana, into a forearc position outboard of the Macquarie Arc. Ensuing oblique compression in the outboard part of the Gondwana Plate drove the Macquarie Arc into and under its backarc Wagga Basin, represented by the Girilambone – Wagga Terrane, and led to a combination of thrusting and major strike-slip faulting within, inboard and outboard of the arc over 10 million years. In this time frame, the Benambran Orogeny in the Eastern Subprovince of the Lachlan Orogen in New South Wales consists of two phases of exhumation – deformation, at ca 443 Ma (late Bolindian to early Llandovery) and ca 430 Ma (late Llandovery), separated by a relaxation/extensional event. Both phases involved deformation and exhumation of the Macquarie Arc and coeval quartz-rich turbidites and black shales of the Adaminaby Superterrane. Closure of the former backarc basin was facilitated by limited east-dipping subduction that generated the short-lived Fifield arc. The second phase of the Benambran Orogeny also involved deformation and exhumation of overlying Llandovery strata (e.g. the Yalmy Group) and syn- to post-tectonic emplacement of granitoids. In both phases, deformation of Adaminaby Superterrane rocks was more intense than deformation of arc rocks.

Junee – Narromine Volcanic Belt, Macquarie Arc, Lachlan Orogen, New South Wales: components and structure

The Junee – Narromine Volcanic Belt of Ordovician volcanic, volcaniclastic and intrusive rocks in central New South Wales is the most westerly structural belt of the now disrupted Macquarie Arc. Although more than 200 km long (far longer than other belts of Ordovician arc volcanics), most of this belt is concealed by younger Palaeozoic to Holocene cover. Most of our knowledge of this belt thus comes from scattered outcrops and drillhole information, augmented by the interpretation of aeromagnetic and gravity data and, to a lesser extent, information from deep seismic-reflection profiling. This combined geological and geophysical approach suggests that the Junee – Narromine Volcanic Belt consists of major separate igneous complexes, each consisting of lavas and volcaniclastic sediments intruded by various mafic to felsic intrusive porphyries. Variations in age and geochemical affinities are used to suggest that the Junee – Narromine Volcanic Belt approximates most closely the magmatic core of the Macquarie Arc. The present geometry of the Junee – Narromine Volcanic Belt reflects the interplay of two major ductile – brittle fault systems: the north-northwest-trending Gilmore Fault System mainly in the south and the north-trending Tullamore Fault System, mainly in the north and centre, together with local west-northwest cross faults. These systems were generated during deformation in the Early Silurian, Early Devonian and Carboniferous.

Volcanology, geochemistry and structure of the Ordovician Cargo Volcanics in the Cargo – Walli region, central New South Wales

The Middle to lower Upper Ordovician Cargo Volcanics occur in a structural outlier immediately west of the contiguous Molong Volcanic Belt of the Ordovician Macquarie Arc. The sequence of basaltic to dacitic lavas, lava breccias and associated volcaniclastic rocks with medium-K calc-alkaline affinities has been interpreted as the subaqueous portion of a major intra-oceanic arc stratovolcano. The oldest part of the succession consists of massive to pillowed, poorly vesicular, aphyric to moderately plagioclase + clinopyxroxene-phyric basalt and andesite lavas and hypabyssal sills. The main part of the interpreted volcanic edifice is composed of massive and pillowed vesicular andesite and dacite, flanked by lenses of hyaloclastite, pillow breccias and interbedded with debris-flow and turbiditic deposits of crystal-rich and pebbly volcanic sandstones, siltstone and minor conglomerate. Uplift and erosion of the edifice prior to deposition of the overlying Eastonian Barrajin Group limestones is indicated by local aprons of thickly bedded subaqueous volcanic conglomerate and pebbly sandstone which appear to be separated from both the underlying andesite-dominant pile and overlying limestones by angular unconformities. The Cargo Volcanics, and the neighbouring Walli Volcanics further south, are clearly distinguished from other Macquarie Arc lavas by their high (>25) Zr/Nb values. However, the Walli Volcanics are readily differentiated from the Cargo Volcanics by their higher P2O5 contents and likely high-K calc-alkaline to shoshonitic affinities. The Cargo Volcanics are intruded by Cu – Au mineralised, plagioclase + hornblende + quartz-phyric dacites with medium-K calc-alkaline affinities. These dacites have lower TiO2 and higher MgO contents at any SiO2 level compared to the Cargo Volcanics, and are compositionally similar to Late Ordovician to Early Silurian (453 – 441 Ma) dacites at Copper Hill and in the Narromine Complex. However, at Cargo, clasts derived from the dacites are locally abundant in volcaniclastic conglomerate near the top of the Cargo Volcanics, indicating that the dacites were intruded and exhumed prior to deposition of the Barrajin Group limestones, which commenced at ca 454 Ma in this area. The dacites at Cargo are intruded by small, apparently unmineralised monzonitic intrusions with shoshonitic affinities, which also appear to pre-date deposition of the Barrajin Group. LA-ICPMS U – Pb dating of detrital zircons from the Ranch Member at the base of the Daylesford Limestone (basal Barrajin Group) revealed a dominant population with an average age of 453.0 ± 4.1 Ma (identical within error to the mid-Ea1 age for the host sediments based on fossil assemblages) and subordinate populations with average ages of 480 Ma and 505 Ma. The 453 Ma zircons were most probably derived from either the intrusive dacites or monzonites, suggesting that the Cargo region was rapidly exhumed following emplacement of the felsic intrusions. The angular unconformities near and at the top of the Cargo Volcanics and the broadly coincident change in magma chemistry suggest that this sector of the Macquarie Arc underwent major tectonic upheaval commencing in the latest Gisbornian – early Eastonian. The Cargo Volcanics are interpreted to have undergone major eastward translation at this time (from an original position now buried beneath the Cowra Trough). Their present juxtaposition with the contiguous Molong Volcanic Belt probably did not occur until the late Early Silurian during the final stages of the Benambran Orogeny.

Shortening in the upper plate of the Buckskin-Rawhide extensional detachment fault, southwestern U.S., and implications for stress conditions during extension

Commonwealth Post-graduate Research Award; U.S. Geological Survey National Cooperative Geologic Mapping Program under STATEMAP [G11AC20455]

The structure and metamorphism of the Red Point Metamorphic Complex—A newly discovered high-pressure metamorphic complex from the south coast of Tasmania

This study presents new data on the deformational and metamorphic history of previously unstudied Cambrian high-pressure metamorphic rocks exposed on the remote south coast of Tasmania. The Red Point Metamorphic Complex consists of two blocks of high-pressure, medium-grade metamorphic rocks including pelitic schist and minor garnet-bearing amphibolite, which are faulted against a sequence of low-grade phyllite and quartzite. The Red Point Metamorphic Complex records five phases of deformation, all of which except the first are expressed at a mesoscopic scale in both the medium- and low-grade rocks. Peak metamorphic conditions in the high-pressure epidote–amphibolite facies is recorded by medium-grade schist and amphibolite and was synchronous with the second major deformation event, which produced a pervasive schistosity and mesoscale isoclinal folds. The juxtaposition of the low- and medium-grade rocks is interpreted to have first occurred prior to the development of upright, opening folding associated with the third deformation. However, the present contacts between the two contrasting metamorphic sequences formed during widespread faulting and ductile-shear zone development associated with the fourth and fifth deformation events. The new data from the Red Point Metamorphic Complex provide insights into the structural and metamorphic history experienced by the medium-grade rocks of Tasmania during the Cambrian Tyennan Orogeny. This study demonstrates that Cambrian medium-grade metamorphic rocks are more widespread throughout Tasmania than previously realised, which represents an important step towards understanding the large-scale architecture of the Tyennan Orogen.

A practical approach to meter placement in distribution networks

This paper presents a novel approach to the optimal meter placement problem in distribution networks. Conventional methods place meters in order to achieve a predefined level of accuracy for the estimated parameters such as voltages and currents. However, there is no generic method to determine this desired accuracy value for the individual parameters. Hence, a uniform accuracy value is generally chosen. Considering that parameters which are not in proximity to their constraint do not need to be monitored with a high level of accuracy, using a uniform accuracy for every estimated parameter may not be optimal. The method proposed in this paper takes a new approach which accounts for the proximity of the estimated parameters to their constraints. This approach can potentially reduce the number of required metering devices compared to conventional meter placement methods, while providing a practical level of state estimation accuracy.