Chris McA Powell

South China in Rodinia: Part of the missing link between Australia–East Antarctica and Laurentia?

Stratigraphic correlations and tectonic analysis suggest that the Yangtze block of South China could have been a continental fragment caught between the Australian craton and Laurentia during the late mesoproterozoic assembly of the supercontinent Rodinia. The Cathaysia block of southeast China may have been part of a 1.9–1.4 Ga continental strip adjoining western Laurentia before it became attached to the Yangtze block around 1 Ga. This configuration provides a western source region for the clastic wedges in the Belt Supergroup of western North America which contain detrital grains of 1.8–1.6 Ga and 1.22–1.07 Ga. The breakup of Rodinia around 0.7 Ga separated South China (Yangtze plus Cathaysia blocks) from the other continents.

Models of Rodinia assembly and fragmentation

Abstract Amongst existing palaeogeographic models of the Rodinia supercontinent, or portions thereof, arguments have focused upon geological relations or palaeomagnetic results, but rarely both. A new model of Rodinia is proposed, integrating the most recent palaeomagnetic data with current stratigraphic, geochronological and tectonic constraints from around the world. This new model differs from its predecessors in five major aspects: cratonic Australia is positioned in the recently proposed AUSMEX fit against Laurentia; East Gondwanaland is divided among several blocks; the Congo-São Francisco and India-Rayner Cratons are positioned independently from Rodinia; Siberia is reconstructed against northern Laurentia, although in a different position than in all previous models; and Kalahari-Dronning Maud Land is connected with Western Australia. The proposed Rodinia palaeogeography is meant to serve as a working hypothesis for future refinements.

Paleozoic terranes of eastern Australia and the drift history of Gondwana

Abstract Critical assessment of Paleozoic paleomagnetic results from Australia shows that paleopoles from locations on the main craton and in the various terranes of the Tasman Fold Belt of eastern Australia follow the same path since 400 Ma for the Lachlan and Thomson superterranes, but not until 250 Ma or younger for the New England superterrane. Most of the paleopoles from the Tasman Fold Belt are derived from the Lolworth-Ravenswood terrane of the Thomson superterrane and the Molong-Monaro terrane of the Lachlan superterrane. Consideration of the paleomagnetic data and geological constraints suggests that these terranes were amalgamated with cratonic Australia by the late Early Devonian. The Lolworth-Ravenswood terrane is interpreted to have undergone a 90° clockwise rotation between 425 and 380 Ma. Although the Tamworth terrane of the western New England superterrane is thought to have amalgamated with the Lachlan superterrane by the Late Carboniferous, geological syntheses suggest that movements between these regions may have persisted until the Middle Triassic. This view is supported by the available paleomagnetic data. With these constraints, an apparent polar wander path for Gondwana during the Paleozoic has been constructed after review of the Gondwana paleomagnetic data. The drift history of Gondwana with respect to Laurentia and Baltica during the Paleozoic is shown in a series of paleogeographic maps.

Synorogenic hydrothermal origin for giant Hamersley iron oxide ore bodies

Geologic mapping, basin analysis, and calculated fluid compositions indicate that giant orebodies of microplaty hematite, and possibly martite-goethite, in the Hamersley province of Western Australia, were formed by heated fluids driven by early Paleoproterozoic orogenesis. Detrital grains of microplaty hematite in the McGrath trough, a foreland basin in front of the northward-advancing Ophthalmian fold belt constrain the age of the earliest microplaty hematite ore formation to 200 °C and locally up to 400 °C were involved. Regional circulation of hydrothermal fluids, including heated surface water, through reduced banded iron formations occurred during or soon after the Ophthalmian orogeny. We speculate that martite-goethite orebodies, previously considered Mesozoic–Cenozoic, could also be related to heated Paleoproterozoic meteoric fluids migrating northward away from the Ophthalmian fold belt.

Fault-valve behaviour in optimally oriented shear zones: an example at the Revenge gold mine, Kambalda, Western Australia

Abstract Quartz vein systems developed in and adjacent to shear zones host major gold deposits in the Kambalda region of the Norseman–Wiluna greenstone belt. At the Revenge Mine, two groups of mineralised reverse shear zones formed as conjugate, near-optimally oriented sets during ESE subhorizontal shortening adjacent to a major transpressional shear system. The shear zones developed at temperatures of about 400°C in a transitional brittle–ductile regime. Deformation was associated with high fluid fluxes and involved fault-valve behaviour at transiently near-lithostatic fluid pressures. During progressive evolution of the shear system, early brittle and ductile deformation was overprinted by predominantly brittle deformation. Brittle shear failure was associated with fault dilation and the formation of fault-fill veins, particularly at fault bends and jogs. A transition from predominantly brittle shear failure to combined shear along faults and extension failure adjacent to faults occurred late during shear zone evolution and is interpreted as a response to a progressive decrease in maximum shear stress and a decrease in effective stresses. The formation of subhorizontal stylolites, locally subvertical extension veins and minor normal faults in association with thrust faulting, indicates episodic or transient reorientation of the near-field maximum principal stress from a subhorizontal to a near-vertical attitude during some fault-valve cycles. Local stress re-orientation is interpreted as resulting from near-total shear stress release and overshoot during some rupture events. Previously described fault-valve systems have formed predominantly in severely misoriented faults. The shear systems at Revenge Mine indicate that fault-valve action, and associated fluctuations in shear stress and fluid pressure, can influence the mechanical behaviour of optimally-oriented faults.

Relationship between northwestern Tasmania and East Gondwanaland in the Late Cambrian/Early Ordovician: Paleomagnetic evidence

The 71 samples from 11 sites in the lower Upper Cambrian turbiditic red siltstone and mudstone of northwestern Tasmania have revealed a high-temperature component which passes a fold test, indicating an age older than Late Devonian. The mean direction of D = 58.3°, I=11.1°, α95 = 10.4°, k = 20.2, is interpreted as of primary or early diagenetic origin, and gives a paleomagnetic pole position at (19.4°S, 28.9°E) with DP = 5.3°, DM = 10.5°. This pole position falls on the Late Cambrian to Early Ordovician part of the East Gondwanaland apparent polar wander path, indicating that if northwestern Tasmania were separated from East Gondwanaland during the Neoproterozoic breakup of the supercontinent Rodinia, it had been accreted to East Gondwanaland by at least the Early Ordovician. Two over-print poles, one at (50.4°S, 177.4°E) with DP = 6.5°, DM = 6.9°, and the other at (62.0°S, 102.6°E) with DP = 9.9°, DM = 11.3°, suggest that there could have been more than one overprinting event in the region during the breakup between Australia and Antarctica in the late Mesozoic and early Tertiary.

Late Neogene loess deposition in southern Tarim Basin: tectonic and palaeoenvironmental implications

Abstract Uplift of the Tibetan Plateau during the late Cainozoic resulted in a thick apron of molassic sediments along the northern piedmonts of the Kunlun and Altyn Mountains in the southern Tarim Basin. Early Neogene sediments are characterised by sandstone, siltstone and red mudstone, representing floodplain to distal alluvial fan environments. The Early Pliocene Artux Formation consists of medium-grained sandstone and sandy mudstone with thin layers of fine pebbly gritstone. The Late Pliocene to Early Pleistocene Xiyu Formation is dominated by pebble to boulder conglomerate typical of alluvial fan debris flow deposits. Sedimentological investigation, together with grain size and chemical analyses of siltstone bands intercalated with sandstone and conglomerate in the Xiyu and Artux Formations, point to an aeolian origin, suggesting desertic conditions in the Tarim Basin by the Early Pliocene. The onset of aeolian sedimentation in the southern Tarim Basin coincided with uplift of the northern Tibetan Plateau inferred from the lithofacies change from fine-grained mudstone and sandstone to coarse clasts. Tibetan Plateau uplift resulted in the shift of sedimentary environments northwards into the southern Tarim Basin, and could well have triggered the onset of full aridity in the Taklimakan region as a whole.

An Early Carboniferous paleomagnetic pole for Gondwanaland: New results from the Mount Eclipse Sandstone in the Ngalia Basin, central Australia

Paleomagnetism of fourteen diamond-drill cores from the uppermost Devonian-Lower Carboniferous Mount Eclipse Sandstone in the Ngalia Basin, central Australia, has been studied. The samples were oriented using the borehole directions, bedding planes identified in the drill cores, and regional bedding attitudes. Three magnetic remanent components have been revealed: (1) a drilling-induced, low- to moderate-temperature remanence (C1) from all the drill cores, which is oriented parallel to the drill cores and is most likely a combination of isothermal remanent magnetization and piezoremanent magnetization; (2) a moderate- to high-temperature, syndeformational overprint (C2: D = 226.7°, I = 82.3°, α95 = 6.2°, k = 68.9) in 65 samples from nine drill cores, which was probably acquired during the mid- to Late Carboniferous Mount Eclipse movement; and (3) a moderate- to high-temperature, likely primary remanence (C3: D = 057.1°, I = −36.9°, α95 = 9.6°, K = 29.6) from 64 samples in nine drill cores, which exhibits a positive fold test. C2 and C3 are found only in reddish siltstones and sandstones, whereas C1 is best developed in grey reduced sandstones and siltstones. A palynological examination gives an early to mid-Visean (Early Carboniferous) age of deposition for samples carrying the interpreted primary component. C2 gives a mid- to Late Carboniferous pole at 32.1°S, 119.5°E with A95 = 11.9°, which is in agreement with results of a previous study on surface outcrops. C3, when combined with results from one drill core in a previous study, gives a Visean pole at 37.6°S, 52.6°E with A95 = 8.7°. This pole suggests a south polar position for central Africa during the Early Carboniferous and therefore indicates the existence of a large ocean between Gondwanaland and Laurussia at that time. The paleo-south pole shifted from central Africa in the Early Carboniferous (∼340±5 Ma) to East Antarctica in the mid- to Late Carboniferous (∼310±10 Ma) with a minimal continental drifting rate of 15–20 cm/yr.

Eolian and lacustrine evidence of late Quaternary palaeoenvironmental changes in southwestern Australia

Abstract Lake Lefroy and Lake Cowan occur in the relic Tertiary palaeodrainage that used to drain to the east in central western Australia. Lacustrine conditions prevailed during the late Neogene and the early part of the Quaternary. Strong aridity sets in during mid-Quaternary, resulting in extensive gypsum precipitation and eolian deposition in and around the playa lakes. Lakes Lefroy and Cowan are now hypersaline and deflating, with a salt crust developed on the lake floor and lunette dunes constructed along the eastern margins. Optical dating (green light-stimulated luminescence (GLSL) dating) of the lunette dunes along the eastern and southeastern margins of the playas and electron spin resonance (ESR) dating of relic gypsum dunes on the lake floor show that intensified eolian activity occurred during the last glacial period, with a major lunette construction phase occurring during the Last Glacial Maximum (LGM). 14 C dating of high-lake deposits, which are extensively developed along the margins of Lake Cowan, suggested a significant high-lake event during the early to mid-Holocene. The late Quaternary eolian and lacustrine evidence of palaeoenvironmental changes supports the model that during the Last Glacial Maximum, the subtropical high-pressure cell intensified and probably shifted southward, displacing the southern westerlies.