Evidence of micro-continent entrainment during crustal accretion

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Inherited crustal deformation along the East Gondwana margin revealed by seismic anisotropy tomography

Acknowledgments We thank Mallory Young for providing phase velocity measurements in mainland Australia and Tasmania. Robert Musgrave is thanked for making available his tilt-filtered magnetic intensity map. In the short term, data may be made available by contacting the authors (S.P. or N.R.). A new database of passive seismic data recorded in Australia is planned as part of a national geophysics data facility for easy access download. Details on the status of this database may be obtained from the authors (S.P., N.R., or A.M.R.). There are no restrictions on access for noncommercial use. Commercial users should seek written permission from the authors (S.P. or N.R.). Ross Cayley publishes with the permission of the Director of the Geological Survey of Victoria.Peer reviewedPublisher PD

Fossil seaward-dipping reflector sequences preserved in southeastern Australia: a 600 Ma volcanic passive margin in eastern Gondwanaland

Evidence for a c. 600 Ma rifted passive margin in eastern Australia exists in the form of multiple belts of mafic volcanic rocks preserved along the western margin of the Tasman Fold Belt System, and giving rise to elongate magnetic anomalies. Outcrop, drillhole and geophysical evidence points to piles of lavas, volcaniclastic and intrusive rocks up to 6 km thick, extending for strike lengths of hundreds of kilometres in individual segments. The distinctive, unifying characteristics of these piles are apparent common formation ages (600–580 Ma), presence of early more landward transitional alkaline basalts, and more seaward abundant rift tholeiites, with high-temperature picrites and olivine-rich basalts at most localities. Despite later structural reorganization, these belts have close geochemical, geometric and lithological affinities with Mesozoic seaward-dipping reflector sequences along the North Atlantic, and northwestern Australian volcanic passive margins, and strongly imply the formation of a volcanic passive margin in eastern Gondwanaland at the close of the Neoproterozoic. Recognition of this event has implications for the position of an implied earlier rifted margin related to the break-up of Rodinia around 780 Ma. A rifting event at 600 Ma in eastern Gondwanaland helps explain both the lack of evidence for volcanism from Rodinia break-up, and a widespread 600 Ma population of inherited zircons within rocks of the Lachlan Orogen, which developed outboard of the passive margin in earliest Palaeozoic time.Nicholas G. Direen and Anthony J. Crawfor

Regional crustal setting of iron oxide Cu-Au mineral systems of the Olympic Dam region, South Australia: Insights from potential-field modeling

© 2007 Society of Economic GeologistsDetailed analysis and modeling of regional gravity and magnetic data from the Stuart Shelf around the Olympic Dam iron oxide copper-gold (IOCG) deposit, constrained by geologic observations obtained from deep drill cores, show that the eastern Gawler craton basement at 1.61 to 1.59 Ga consisted of an Archean core with two sequences of successively younger supracrustal rocks stepping out eastward from it. These were overprinted by the tectonothermal Hiltaba event. Forward models of potential-field data show no convincing evidence for the presence of widespread mafic rocks or extensional basin systems developed immediately prior to, or during, IOCG mineralization. The high intrusive level and sill-like geometry of Hiltaba Suite plutons emplaced immediately prior to IOCG mineralization is difficult to explain in terms of a genetic association with a dominantly extensional tectonic setting. Instead, the tectonic setting inferred from the basement architecture during mineralization is one of low-strain shortening, consistent with regional field observations. A case can be made for intrusion of the Hiltaba Suite in localized accommodation zones in an overall orogenic setting. This style of low-strain shortening (<10%) is consistent with maximum rates and volumes of fluid flow in the upper crust, and access of these fluids to a variety of basement rock types to buffer metal-scavenging fluids. This fluid-driving mechanism does not necessitate a role for regional mafic magmatism and volcanism in the formation of IOCG deposits. Modeling of regional gravity and magnetic data is generally applicable to exploration for IOCG systems, and other mineral systems, in covered terranes that have distinctive geophysical signatures resulting from fluid–wall-rock redox geochemical reactions

The Tasman Line: where is it, what is it, and is it Australia's Rodinian breakup boundary?

he Tasman Line, a much-discussed concept in the geology and tectonics of eastern Australia, has a long and chequered history of interpretation. This extends to current debates regarding the age and position of the Tasman Line in Gondwana-Rodinia reconstructions. We present constraints, from mapping, geochemistry and geophysics, on the interpretation of gravity and magnetic lineaments attributed to the Tasman Line in New South Wales, South Australia, Victoria and Tasmania. These pieces of evidence suggest a protracted and complex latest Neoproterozoic to Carboniferous geological history that produces a variety of geophysical responses, rather than a simple 'Line'. We also find no evidence of Rodinian breakup age activity responsible for any of the anomalies. In light of these findings, our preference is that the Tasman Line concept be abandoned as misleading, especially with regard to models of Rodinia-Gondwana breakup, which must have occurred elsewhere, possibly well to the east. Instead, the rocks preserved in the westernmost part of the Tasmanides are consistent with previously proposed 'Southwest Pacific'-style models for Neoproterozoic continental breakup, margin formation and reaccretion of continental fragments in the Early Palaeozoic.N.G. Direen and A.J. Crawfor

The Warratta fault: geophysical architecture and landscape evolution significance

Nicole Anderson, Nicholas G. Direen & S.M. Hillhttp://crcleme.org.au/Pubs

Strong remanent magnetization in pyrrhotite: A structurally controlled example from the Paleoproterozoic Tanami orogenic gold province, northern Australia

Rocks associated with an orogenic gold system in the Paleoproterozoic Granites/Tanami Inlier of northern Australia exhibit strong natural remanent magnetism, identifiable in regional aeromagnetic data. Petrographical analysis indicates the two dominant magnetic minerals in the rocks of this region are multidomain magnetite and monoclinic pyrrhotite. Chemical analysis using an electron microprobe has determined that the magnetite and pyrrhotite are stoichiometrically pure, without significant elemental substitution. Alternating field demagnetization, Curie Temperature, and hysteresis analysis indicates that the remanently magnetized phase is monoclinic pyrrhotite with low magnetic susceptibility. In contrast, multidomain magnetites lack remanence and have high magnetic susceptibilities. Pyrrhotite is preserved in the rock mass along with other sulfide minerals as millimeter-scale veins parallel to regionally developed, penetrative mylonitic shear bands (c-planes) formed during sulfide remobilization in the waning, cooling stages of hydrothermal Au deposition. In contrast, magnetite grains exhibit porphyroblastic textures, consistent with formation during prograde to peak metamorphism, indicating formation prior to pyrrhotite, earlier in the orogenic cycle. Because of the low Curie Temperature (ca. 325 °C) of pyrrhotite, it is therefore likely that remanent magnetization effects observed in aeromagnetic images of the Tanami Inlier, are associated with structures intimately related to the late stages of Au mineralization during retrograde greenschist metamorphism after the peak thermal point of the orogenic cycle.Nicholas G. Direen, Kate M. Pfeiffer and Philip W. Schmid

Blind orogen: Integrated appraisal of multiple episodes of Mesoproterozoic deformation and reworking in the Fowler Domain, western Gawler Craton, Australia

The Fowler Domain in the western Gawler Craton in southern Australia is a poorly exposed region that can only presented be explored using a combination of information from drilling and potential field geophysics. Regional maps of the Total Magnetic Intensity (TMI) field of the Fowler Domain highlight an anastomosing system of terrain-scale shear zones that bound four crustal-scale tectonic blocks: from west to east, the Colona, Barton, Central and Nundroo blocks. Integrated thermobarometry and electron microprobe chemical dating of metamorphic monazites from drillholes in the Fowler Domain suggest that the Colona Block in the west underwent two mid-crustal amphibolite grade metamorphic events at ca. 1643 and 1600 Ma. The younger age corresponds to the timing of regional high-grade metamorphism in the Barton Block. Together, the age data suggest that the western Fowler Domain underwent a major tectonothermal event at ca. 1600 Ma. In contrast, regional lower crustal metamorphism in the Nundroo Block, which forms the eastern Fowler Domain, occurred at ca. 1545 Ma. In both the Barton and Nundroo Blocks, petrological relationships, mineral zoning, and pressure-temperature (P-T) modelling suggest the terrains cooled in the mid- to lower crust, rather than undergoing exhumation immediately following peak metamorphism. Age data from the geophysically defined shear zone systems that bound the blocks suggest that exhumation of these lower crustal domains occurred between ca. 1470 and 1450 Ma and was associated with transpressional reactivation of the terrain during the Coorabie Orogeny. A key finding of this study is that the tectonic evolution of the crustal blocks in the Fowler Domain was not in concert until at least 1500 Ma. Thus, the evolution of the individual blocks is unlikely to be related to the macroscopic character of the terrain defined by the regional-scale shear zone systems, which are one of the youngest tectonic imprints on the Fowler Domain. Coincident gravity and magnetic forward modelling of significant bounding faults suggests the shear zones form a steeply dipping transpressional array, consistent with the observed metamorphic field character of the different blocks. This study provides a demonstration of the integration of geophysical and petrological approaches to investigate the time-integrated tectonic evolution of poorly exposed basement terrains. © 2008 Elsevier B.V. All rights reserved.Jane L. Thomas, Nicholas G. Direen and Martin Han