http://www.researchonline.mq.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:22334 25 page(s) 2012-10-25T23:52:25.826Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:22253 The distribution of lamprophyre-hosted Neoarchean diamond deposits in the southem Superior Province, Canada, corresponds to that of coeval giant(> 100 tAu) examples of "orogenic" gold deposits, which are typically associated with quartz-carbonate veining. This common association in the southern Superior Province, and at Yellowknife in the Slave craton, suggests that the occurrence of both diamonds and gold was promoted by the same geodynamic factors. A previously proposed subduction diamond model invokes flat subduction of buoyant oceanic plateau crust as the only means of entraining diamonds in shoshonitic lamprophyres, which are generally derived from relatively shallow mantle depths. Computer modeling of the thermal evolution and dehydration processes associated with this tectonic scenario clarifies observations made in presentday flat subduction settings and suggests many factors that should enhance the hydrothermal mineralization systems responsible for Archean and post-Archean orogenic gold deposits. The case for links among mantle plume-derived oceanic plateaus, crustal growth, and anomalously large gold deposits is strengthened by the newly recognized association of oxidized granites, lamprophyres, and diamonds at 3.1 Ga in the Kaapvaal craton and by evidence for similar recurring gold-diamond ± lamprophyre associations throughout the geologic record, including the Mother Lode deposits of California. 2012-10-23T00:31:03.537Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:9516 5 page(s) 2011-08-26T04:41:48.307Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:9517 Pressure‐release melting at mid‐ocean ridges generate compositional and rheological layering in the oceanic mantle that may control the evolution of the oceanic lithosphere. We use dynamic models coupled with melting and petrological models to explore 1) the influence of this layering on the development of small‐scale convection under the oceans, 2) its role in determining the thickness of oceanic lithosphere, and 3) its feasibility as responsible for the deviations of seafloor and surface heat flow from predictions by conductive models in mature oceanic lithosphere. Here we show that the existence of small‐scale convection is entirely compatible with experimental creep parameters and flow laws, and that the viscosity stratification due to melt extraction (i.e., H₂O removal) is the main factor controlling the plate's thermal evolution, its asymptotic thickness, and the flattening of seafloor and surface heat flow at ages ≳70 Ma. The effects of Al‐rich phase transitions and compositional layering are minor. 2011-08-26T04:41:47.728Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13420 A combined geophysical-petrological methodology to study the thermal, compositional, density, and seismological structure of lithospheric/sublithospheric domains is presented. A new finite-element code (LitMod) is used to produce 2-D forward models from the surface to the 410-km discontinuity. The code combines data from petrology, mineral physics, and geophysical observables within a self-consistent framework. The final result is a lithospheric/sublithospheric model that simultaneously fits all geophysical observables and consequently reduces the uncertainties associated with the modeling of these observables alone or in pairs, as is commonly done. The method is illustrated by applying it to both oceanic and continental domains. We show that anelastic attenuation and uncertainties in seismic data make it unfeasible to identify compositional variations in the lithospheric mantle from seismic studies only. In the case of oceanic lithosphere, plates with thermal thicknesses of 105 ± 5 km satisfy geophysical and petrological constraints. We find that Vp are more sensitive to phase transitions than Vs, particularly in the case of the spinel-garnet transition. A low-velocity zone with absolute velocities and gradients comparable to those observed below ocean basins is an invariable output of our oceanic models, even when no melt effects are included. In the case of the Archean subcontinental lithospheric mantle, we show that “typical” depleted compositions (and their spatial distribution) previously thought to be representative of these mantle sections are compatible neither with geophysical nor with petrological data. A cratonic keel model consisting of (1) strongly depleted material (i.e., dunitic/harzburgitic) in the first 100–160 km depth and (2) less depleted (approximately isopycnic) lower section extending down to 220–300 km depth is necessary to satisfy elevation, geoid, SHF, seismic velocities, and petrological constraints. This highly depleted (viscous) upper layer, and its chemical isolation, may play a key role in the longevity and stability of cratons. 2011-08-26T04:32:32.718Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14200 The oldest bradoriid fauna from Australia, occurring in the lower Cambrian Ajax and Wirrapowie limestones of the Flinders Ranges, South Australia consists of eleven taxa, including one new genus and species, Quadricona madonnae gen. et sp. nov. and two new species, Liangshanella circumbolina sp. nov. and Zepaera jagoi sp. nov. In the Ajax Limestone, Liangshanella circumbolina sp. nov. occurs c. 20 m below the FAD of the zonal trilobite Abadiella huoi. This pre-trilobitic occurrence represents the oldest bivalved arthropod hitherto known from East Gondwana and suggests a lower Cambrian (Series 2, Stage 3) age for the assemblage. The recognition of distinct bradoriid assemblages associated with the Abadiella huoi (Atdabanian), Pararaia tatei, P. bunyerooensis and P. janeae (all Botoman) trilobite biozones in South Australia indicates great potential for future regional biostratigraphic correlation. Quantitative biogeographic analysis including new taxonomic data from the lower Cambrian of South Australia, highlights the strong endemism displayed by early Cambrian bradoriid communities and strengthens the close faunal affinities with South China and Antarctica. 2011-08-01T06:25:25.887Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:14231 The Malay Peninsula lies on two continental blocks, Sibumasu and East Malaya, which are intruded by granitoids in two provinces: the Main Range and Eastern. Previous models propose that Permian–Triassic granitoids are subduction-related and syn-to post-collisional. We present 752 U–Pb analyses that were carried out on zircons from river sands in the Malay Peninsula; of these, 243 grains were selected for Hf-isotope analyses. Our data suggest a more complex Sibumasu–East Malaya collision history. ¹⁷⁶Hf/¹⁷⁷Hfi ratios reveal that Permian–Triassic zircons were sourced from three magmatic suites: (a) Permian crustally-derived granitoids, (b) Early-Middle Triassic granitoids with mixed mantle–crust sources, and (c) Late Triassic crustally-derived granitoids. This suggests three Permian–Triassic episodes of magmatism in the Malay Peninsula, two of which occurred in the Eastern Province. Although the exact timing of the Sibumasu–East Malaya collision remains unresolved, current data suggest that it occurred before the Late Triassic, probably in Late Permian–Early Triassic. Our data also indicate that Sibumasu and East Malaya basements are chronologically heterogeneous, but predominantly of Proterozoic age. Some basement may be Neoarchaean but there is no evidence for basement older than 2.8 Ga. Finally, we show that Hf-isotope signatures of Triassic zircons can be used as provenance indicators. 2011-08-01T06:24:30.232Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13964 U/Pb dating, REE geochemistry and Lu/Hf isotopic studies utilizing LA-ICPMS on detrital zircon suites from Neoproterozoic siliciclastic rocks from the northeast periphery of the East European Craton (sandstones of Djejim Formation, Djejim-Parma Hills, Southern Timan Ridge and sandstones of the Engane-Pe Formation, Northern Engane-Pe uplift, Polar Urals) are used to assess the provenance of sediments and test tectonic models for the late Precambrian assembly of continents. The data support the conclusion that Neoproterozoic complexes of the Timan–Pechora region (TPR) are composed mainly of sedimentary rocks (SW Pre-Uralides–Timanides) eroded from Baltica and deposited along the passive margin of Baltica. However, late Precambrian–Early Cambrian volcanic-sedimentary and volcanic rocks, granitoids, and rare ophiolites of the TPR (NE Pre-Uralides–Timanides) comprise more juvenile material developed some distance from Baltica. Important differences exist between the U/Pb ages and Lu/Hf isotopic systematics of zircons from rocks of the NE Pre-Uralides–Timanides and the Neoproterozoic complexes of the Peri-Gondwanan terranes and do not support a Peri-Gondwanan origin for the NE Pre-Uralides–Timanides. In our interpretation, the SW Pre-Uralides–Timanides were deposited in the Neoproterozoic along the passive Timanian–Uralian margin of Baltica, but the NE Pre-Uralides–Timanides were formed along the active (Bolshezemel) margin of a paleocontinent called Arctida and were caught in the collision zone between the two paleocontinents, Arctida and Baltica. 2011-07-04T12:21:10.842Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13926 Rising as “the roof of the world” the Tibetan plateau is now underlain with the thickest continental crust on Earth. How and when was this crust formed, which would have exerted pivotal controls to the formation of the plateau, has long been an issue of hot debates. This paper reports zircon U–Pb ages and Hf isotope data for postcollisional (~ 30–9 Ma) adakites in the southern Lhasa terrane, southern Tibet. A comparative analysis of whole-rock rare earth element geochemistry and zircon Hf isotopes between the adakites and associated Gangdese igneous rocks suggests that the Tibetan crust underwent a major phase of tectonic thickening between ca. 45 and 30 Ma in the region. The lower part of the thickened crust consisted prevailingly of mafic lithologies, which we argue to have resulted from intense basaltic underplating and subsequent remelting that took place during the Late Cretaceous and Eocene time related to the Neotethyan subduction processes including breakoff of the subducted slab at ca. 50 Ma in the early stage of the India–Asia collision. These processes were responsible for not only the juvenile crust formation but also for the creation of a thermally softened lithosphere in the southern Lhasa terrane. The indentation of India, consequently, caused distributed lithospheric thickening with formation of an orogenic root beneath southern Tibet. Root foundering during the Oligocene gave rise to the adakitic magmatism, regional topographic uplift, and onset of northward underthrusting of the Indian plate that has since played a key role in forming the entire Tibetan plateau. 2011-07-01T09:41:21.417Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13933 Abundant primary sulfides occur as inclusions in silicates and as discrete grains in mantle-derived spinel lherzolite xenoliths from Miocene intraplate basalts on the Penghu Islands, Taiwan, which is located at the southeastern margin of Cathaysia Block. These sulfides are dominantly mixtures of Fe-rich and Ni-rich monosulfide solid solutions (MSS), with minor pentlandite, millerite and chalcopyrite, and are considered to represent sulfide melts crystallized at high temperatures (>900 °C). Some sulfides from the Tungchiyu (TCY) islet (37 out of 118 grains) have remarkably high Co contents resulting in subchondritic Ni/Co ratios (<21; 5–20, median = 12), distinct from the superchondritic values (Ni/Co = 48–157, median = 83) typical of mantle sulfides worldwide. The Co-rich nature of the TCY sulfides is considered to be a primary characteristic as no secondary processes can be identified to account for the feature. They are similar to Ni–Co-rich sulfides from Lac de Gras, Slave Craton (Aulbach et al. (2004) Chemical Geology 208, 61–88) interpreted as being derived from the lower mantle. Experimental studies suggest that the sulfide melt/silicate melt partition coefficient of Ni becomes lower than that of Co at pressures greater than 28 GPa, similar to recent estimates of the magma ocean conditions. Os model ages of the TCY Co-rich sulfides reveal four episodes of generation: 2.0, 1.7, 1.4 and 0.8 Ga; this is consistent with the age pattern of all Penghu sulfides, indicating significant lithosperic mantle formation, melt extraction or metasomatic events at these time periods. These events closely correspond to the global 1.9-Ga superplume event related to the assembly of the Nena/Columbia supercontinent, a minor 1.7-Ga superplume event in SW Laurentia prior to breakup of Nena/Columbia, the 1468 Ma Moyie event in the Belt Basin region in western Laurentia and the ~0.8 Ga breakup of Rodinia, with which the Cathaysia Block was associated at various stages during its Proterozoic evolution (Li et al. (2008) Precambrian Research 160, 179–210 and references therein). Olivine in a peridotite sample from the TCY locality has distinctly high ³He/⁴He (11 RA), whereas other peridotites from the KP and TCY localities have ³He/⁴He ~6.7 RA, lower than MORB. The high ³He/⁴He further suggests that materials from the deep mantle have interacted with the host peridotite of Co-rich sulfides. We thus propose that the Co-rich sulfide melts may have been trapped in the lower mantle during core–mantle differentiation and then transported to shallow depths by mantle plumes that entrained lower mantle materials at several different time periods. This study provides the first substantial evidence from the lithosperic mantle beneath the Cathaysia Block to support the activity of mantle plumes related to the breakup of the supercontinents Nena/Columbia and Rodinia in Proterozoic time. 2011-07-01T09:40:47.997Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13442 A model of the thermal, compositional, density, and seismological structure of the lithospheric and sublithospheric mantle along a 500-km transect across the Namibian volcanic passive margin is presented. This margin juxtaposes old oceanic lithosphere and a Precambrian continental domain. The model combines within an internally consistent framework data from petrology, mineral physics, and geophysical observables. The calculated mantle temperature and density distributions down to a depth of 400 km are consistent with available xenolith-derived data, and fit simultaneously the observed free-air anomaly, geoid height, surface heat flow, and elevation. The model also explains the anomalously thick oceanic crust and the depletion of the lithospheric mantle in the ocean–continent transition and in the Proterozoic continental domain. Seismic velocities predicted by the present model are in good agreement with values obtained from wide angle reflection/refraction and tomography experiments. The thermal lithospheric thickness is ~ 100 km in the oceanic domain, increasing gradually to ~ 125 km across the ocean–continent transition and then more sharply to ~ 175 km in the continental domain. The density distribution in the mantle differs significantly from current purely thermal approaches where density is assumed to be only temperature-dependent. Non-negligible compositional density differences are encountered between the oceanic, transitional, and continental domains. Results show that non-thermal effects such as composition and phase changes cannot be neglected in models of the upper mantle. 2011-06-01T02:20:11.105Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13434 The elastic properties and the coefficient of thermal expansion (CTE) of the lithospheric mantle are important parameters that affect the results of lithospheric modelling. However, there is still no consensus on which values are the most appropriate to model the lithosphere, and various average values are used for lithospheres of different age, thermal state, and composition. We present an integrated approach to calculate the elastic properties and the CTE of mantle rocks, based on the mineral physics of composites and considering the spatial heterogeneity of the lithospheric mantle. The method considers the dependence of parameters on pressure and temperature, following a procedure based on an extension of the shear-lag model and thermal expansivity systematics. 2011-05-31T05:10:08.288Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13429 The sub-Andean region of the North Patagonian Andes is located between the north Patagonian foreland and the highest elevations of the Andean Cordillera. Its Tertiary contractional structure, active since the upper Late Oligocene and through the upper Late Miocene, corresponds to the external sector of the North Patagonian Andes fold-and-thrust belt, which is characterized by east-vergent thrusts that affect stratified Cenozoic rocks. The units involved in the deformation correspond to El Maitén Oligocene volcanic belt, deformed between east-vergent thrusts and backthrusts, and Oligocene–Miocene sedimentary rocks of the Ñirihuau–Collón Curá Basin. The northern sector of the basin was structured as a thin-skinned fold belt that comprises the Ñirihuau fold belt and a frontal sedimentary wedge. To the south, sedimentary depocenters are confined between medium- to high-angle reverse faults with evidence of basement structural control. In both cases, the general tectonosedimentary framework shows the youngest units toward the east, in agreement with the advance of the orogenic wedge. Most of the Ñirihuau–Collón Curá Basin infill fits a foreland basin system model. Some sectors of the basin show evidence of structural control by normal faults in the initial sedimentation stages. This early extensional regime might be associated with late phases of Oligocene volcanism. Extensional structures probably were partially reactivated during the upper Late Oligocene, as suggested by seismic data. 2011-05-30T22:50:10.272Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13426 The lithospheric structure of the Western Carpathian–Pannonian Basin region was studied using 3-D modelling of the Bouguer gravity anomaly constrained by seismic models and other geophysical data. The thermal structure and density distribution in the shallow upper mantle were also estimated using a combination of petrological, geophysical, and mineral physics information (LitMod). This approach is necessary if the more complicated structure of the Pannonian Basin is to be better constrained. As a result, we have constructed the first 3-D gravity model of the region that combines various geophysical datasets and is consistent with petrological data. The model provides improved estimates of both the density distribution within the lithosphere and the depth to major density discontinuities. We present new maps of the thickness of major sedimentary basins and of the depth to the Moho and the lithosphere–asthenosphere boundary. In our best-fitting model, the Pannonian Basin is characterised by extremely thin crust and lithospheric mantle, both of which have low density. A low-density uppermost asthenospheric mantle layer is also included at depths of 60–100 km. The Western Carpathians have only a thin crustal root and moderate densities. In contrast, the European Platform and Eastern Alps are characterised by lithosphere that is considerably thicker and denser. This inference is also supported by stripped gravity anomalies from which sediment, Moho and asthenospheric gravity contributions have been removed. These residual anomalies are characteristically low in the Western Carpathian–Pannonian Basin region, which suggests that both the ALCAPA and Tisza–Dacia microplates are ‘exotic terranes’ that are markedly different to the European Platform. 2011-05-30T14:10:08.783Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13427 The relative importance of the contribution of the lower crust and of the lithospheric mantle to the total strength of the continental lithosphere is assessed systematically for realistic ranges of layer thickness, composition, and temperature. Results are presented as relative strength maps, giving the ratio of the lower crust to upper mantle contribution in terms of crustal thickness and surface heat flow. The lithosphere shows a “jelly sandwich” rheological layering for low surface heat flow, thin to average crustal thickness, and felsic or wet mafic lower crustal compositions. On the other hand, most of the total strength resides in the seismogenic crust in regions of high surface heat flow, crust of any thickness, and dry mafic lower crustal composition. 2011-05-30T08:34:19.082Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13083 Deep and fast wide-angle reflection arrivals observed at offsets over 180 km, and over a reduced time interval of 1–1.5 s, have been observed in a seismic experiment shot across SW Iberia as part of the IBERSEIS project. Using different modelling approaches, these reflections have been found to be consistent with a heterogeneous gradient zone located at 61–72 km depth that features an absolute P-wave velocity contrast from 8.2 to 8.3 km/s. Paradoxically, this interface has not been observed in coincident vertical incidence data, probably due to the change in the reflection coefficient with decreasing incidence angles, the lack of energy at high recording times for the near-vertical (vibroseis) data, and/or the different location of the CDPs in both experiments. Although the mantle is acknowledged to be highly heterogeneous and mantle lithologies are capable of giving impedance contrasts high enough as to be observed in seismic data, it is often seen as transparent from a seismic point of view. The short wavelength of mantle compositional heterogeneities is probably what hinders their identification with active source seismic data and only big and sharp discontinuities are imaged in vertical incidence experiments whereas regional transitional boundaries may be also observed at high incidence angles. Accordingly, we propose that deep reflectivity observed in SW Iberia must correspond to a regional–continental scale feature, not sharp enough as to be seen with vertical incidence energy. This feature, already identified in previous DSS experiments carried out in Iberia, has a depth, a P-wave velocity contrast and a transitional nature that match the characteristics proposed for the spinel-lherzolite to garnet-lherzolite phase transition, i.e. the Hales interface or gradient zone. This boundary is relatively narrow (at least 2–3 kb) in enriched mantle and appears deeper and along wider intervals when the mantle is depleted. In addition, it is a worldwide scale boundary already identified over large areas with different types of datasets. The variability in depth and sharpness of this interface, which is related to mantle chemistry, constrains the type of seismic techniques that should be used to identify it. 2011-05-25T21:48:52.764Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13207 Our paper presents the general overview of the current geophysical results, which helps to improve the geophysical image and the lithospheric structure of the Carpathian-Pannonian Basin region. Two different geophysical methods have been applied for the study of the structure and composition of the lithosphere as well as for determination of the lithospheric thermal structure. Firstly, integrated 2D modeling of gravity, geoid, topography and surface heat flow data was performed. Secondly, based on the results of the CELEBRATION 2000 seismic experiment, a large-scale 3D lithospheric gravity model was developed. The resulting map of the lithospheric thickness shows important variations in lithospheric thickness across the chain as well as along strike of the Carpathian arc. The sediment stripped gravity map is characterized by minima in the Eastern Alps and Western Carpathians. The maxima are observed in the Pannonian Back-arc Basin system, Bohemian Massif, Fore-Sudetic Monocline, Bruno-Silesian unit (BSU), Lublin Trough and partly in the Holy Cross Mts. and Malopolska unit. The Western Carpathian gravity minimum is a result of the interference of two main gravity effects. The first one comes from the lowdensity sediments of the Outer Western Carpathians and Carpathian Foredeep. The second one is due to the thick low-density upper and middle crust, reaching up to 25 km. The sediment stripped anomaly in the Pannonian Back-arc Basin system is characterized by gravity high that is a result of the gravity effect of the anomalously shallow Moho. The most dominant feature of the complete stripped gravity map is the abrupt change of the positive anomalies along the Pieniny Klippen Belt zone. The complete residual anomaly of the Pannonian Back-arc Basin system and the Western Carpathian orogen is characterized by a long-wavelength gravity low. The lowest values are associated with the thick low-density upper and middle crust of the Inner Western Carpathians. The European Platform is characterized by significantly denser crust with respect to the less dense crust of the microplates ALCAPA and Tisza-Dacia. That is why we suggest that the European platform represents consolidated, while the Carpathian-Pannonian Basin region un-consolidated crust. 2011-05-25T21:43:45.738Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13262 A partly confined river is one along which the contemporary channel abuts the valley margin along 10–90% of its length. They occur along sections of longitudinal profiles that are transitional from fully bedrock confined to fully alluvial rivers. At this position in the landscape, there is sufficient accommodation space for discontinuous floodplain pockets to form. Segregation of the river's load and dissipation of flow energy result in sediments being stored outside the channel. Bedrock- and planform-controlled variants of these rivers, and associated floodplains, are differentiated. Terraces, fans, and piedmont zones act as secondary confining features on channel planform and floodplain formation in these valleys. The influence of these antecedent features on contemporary river character and behaviour in partly confined valleys of the upper Hunter catchment, New South Wales, Australia is appraised. The balance of formative and reworking processes, and associated capacity for geomorphic adjustment, is influenced by the site-specific configuration of antecedent controls at any floodplain pocket. Downstream patterns of these river types are analysed along all major tributaries of the upper Hunter catchment to highlight the extent of antecedent control on contemporary river forms and processes in this landscape setting. 2011-05-25T21:41:32.656Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:12914 The Gorringe Bank is a 5000 m high seamount near the Atlantic coast of Iberia characterized by a 9 m high geoid anomaly and a ∼120 mGal Bouguer anomaly relative to the surrounding abyssal plains. It has been linked to a NW directed thrust carrying exhumed upper mantle rocks and transitional crust on top of flexed-down Eurasian oceanic crust along the Tagus Abyssal Plain. However, estimations of crustal shortening have yielded dissimilar results, and the deep structure of the ridge remains highly unknown. We present a restored cross section and a new model of the lithospheric structure based on gravity, geoid, elevation, and the presence of serpentinized peridotites. At least 20 km of shortening took place along a flat-ramp-flat thrust fault, and the density structure of the lithosphere is consistent with mantle serpentinization varying from 70% at the surface to 20% at 14 km depth and 0% at 40 km. The topographic relief and gravity anomalies are explained by assuming a flexural isostatic model with an elastic thickness Te of ∼30 km. The evolution of the Gorringe Bank since the Late Jurassic is interpreted in relation to Eurasia-Africa-North America plate motion in four stages: (1) transtension between Newfoundland-Iberia and Africa, which generated small oceanic basins and mantle exhumation; (2) opening of the North Atlantic and seafloor spreading at the NW side of the exhumed Gorringe, which produced gabbro intrusions and serpentinization; (3) a quiescent tectonic period dominated by subsidence and sediment accumulation; and (4) a transpressional plate boundary between Eurasia and Africa with NW directed subcrustal thrusting and generation of the present Gorringe relief. 2011-05-02T23:06:02.503Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:12609 Our ignorance about the tectonic affinity of the western Qinling-Songpan-Ganzi tectonic region, which is strategically located between the northeastern corner of the Tibetan Plateau, the northwestern corner of the Yangtze block, and the southwestern corner of the north China block, limits our understanding of the tectonic evolution of east Asia. Basaltic volcanic rocks in the Duofutun area within the west Qinling terrane in Qinghai Province (China), the northernmost part of the Songpan-Ganzi region, contain coeval magmatic zircons that constrain the eruption age of the host basalts to ∼14 Ma. More significantly, the basalts have entrained zircon xenocrysts from the deep crust that record the presence of unexposed Neoarchean (2.7–2.5 Ga) basement. U-Pb and Hf isotope data from the xenocrysts reveal that this basement has undergone a complex evolution that includes the addition of new mantle-derived material at ∼2.7–2.4 and 1.1–0.8 Ga and crustal reworking events at ∼1.8 and 1.4 Ga. Phanerozoic thermal events at 320–300, 230, and 160 Ma have also modified (reworked) the basement. Using these data, we interpret at least the western part of the west Qinling orogenic terrane as a microcontinental block that originally separated from the north China block, closed with the northern Yangtze block during the Meso-Neoproterozoic, and then redocked with the southern part of the north China block in the Phanerozoic (i.e., early Paleozoic). The west Qinling terrane was then affected by the northward subduction and collision of the Yangtze block in the Paleozoic and early Mesozoic and underwent lithospheric extension in Jurassic time. 2011-04-13T10:40:29.516Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:12231 31 page(s) 2011-03-21T05:01:30.031Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:11993 Determining the timing and source of gas transfer during intermittent intracaldera volcanism can aid in our understanding of degassing in these large systems. Using (²¹⁰Pb/²²⁶Ra) ratios, (parentheses denote activity ratios) as a time-sensitive tracer, injections of ²²²Rn and the subsequent time scales of gas accumulation and loss can be determined. Variations in (²¹⁰Pb/²²⁶Ra) have been measured for 15 volcanic products erupted at Rabaul Caldera over the period 1994 to 2001. In addition, one basaltic enclave from the 1937 eruption was also analyzed. Water and carbon dioxide contents determined from olivine hosted melt inclusions erupted in 1997 are < 1% and suggest extensive shallow-level degassing. Both ²¹⁰Pb excesses and deficits are found in andesites and dacites, whereas the basaltic enclave displays an (²¹⁰Pb/²²⁶Ra)0 ratio of 7. Between 1994 and 1997 three samples with (²¹⁰Pb/²²⁶Ra) deficits were erupted which indicate open system gas loss since 1992 and 1994. No correlation exists between (²¹⁰Pb/²²⁶Ra) and lava chemistry, eruptive style or date. ²¹⁰Pb excesses are more common than deficits in Rabaul samples but cannot be explained by plagioclase feldspar accumulation, Pb sublimate accumulation or differentiation. Instead, a model of intra-magma ²²²Rn transfer can produce ²¹⁰Pb excesses of the appropriate magnitude if gas transfer occurs over 1–5 years from an underlying body of magma that is 2–10 times larger than the volume of erupted material and that is consistent with geophysical estimates. Although intermittent gas transfer events can be inferred by the development of ²¹⁰Pb excess, there is no evidence at Rabaul for a direct link between eruptive style, gas flux and (²¹⁰Pb/²²⁶Ra). 2011-03-01T04:31:07.149Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:10673 The growth of thickened crustal units both depletes the mantle of heat producing elements, and thermally insulates it. These effects either cool or heat the mantle, respectively, and play an important role in subsequent crustal formation. We employ mantle convection models including thickened crustal units of variable heat production to show that for increasing crustal extent, mantle temperatures and melt production can either increase of decrease, depending on the degree of enrichment of the crust with respect to the mantle, the total heat production, and the Rayleigh number. The formation of the continents on Earth efficiently cooled the upper mantle, resulting in lower subsequent rates of melt production and continental formation. In contrast, the growth of the Martian highlands would have raised the temperatures of the Martian mantle, increasing rates of melt production, and leading to runaway crustal growth. This would have continued as long as the lithosphere of Mars was mobile. 2010-11-23T05:01:05.325Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:10612 Shock-crossing data obtained from spacecraft are used to test the shock location models derived by Chapman and Cairns [2003] . Three sets of data are considered: (1) ISEE 1 for 24–25 September 1987, (2) Wind, Geotail, IMP 8, and Interball for the intervals 26–27 April and 10–13 May 1999, and (3) IMP 8, Geotail, Magion-4, and Cluster during the period 1973–2003 from the bow shock database (available at http://nssdc.gsfc.nasa.gov/ftphelper/bowshock.html). Derived from MHD simulations, the two shock models are for angles θ IMF = 45° and 90° between the upstream magnetic field B IMF and solar wind velocity v sw . These models have azimuthal asymmetries, and they depend explicitly on the upstream Mach number M A and ram pressure P ram. We also test Cairns et al.'s [1995] rotationally symmetric shock model. The new models perform better on average than the rotationally symmetric model, providing some evidence and support for the shock's shape being strongly dependent upon M A and θ IMF. We also compare our analyses here with model/spacecraft comparisons performed by Merka et al. [2003a] and discuss the importance of filtering on the model predictions. 2010-11-17T04:30:32.467Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:10338 Earth's bow shock changes its three-dimensional (3-D) location in response to changes in the solar wind ram pressure Pram, Alfvén Mach number MA, magnetic field orientation, fast mode Mach number Mms, and sonic Mach number MS. Using shock locations from global 3-D ideal MHD simulations [ Cairns and Lyon, 1995 ], empirical models are derived for the 3-D shape and location of Earth's bow shock in the near-Earth regime as a function of solar wind conditions. Multiple simulations with different MA and Pram but two orientations of the interplanetary magnetic field BIMF are analyzed: θIMF = 45° and 90° with respect to the solar wind direction vsw. Models for the (paraboloid) flaring parameter bs as a function of MA, azimuthal angle ϕ, and θIMF = 45° or 90°, show bs decreasing with MA, corresponding to the shock becoming blunter and less swept back (with a larger cross section), as expected. Together with models for the shock's standoff distance (which increases with decreasing MA) the models for bs(MA, ϕ) predict the shock's 3-D location. Variations of bs with ϕ represent eccentricities in the shock's cross section (i.e., a departure from circularity), with the shock extending further perpendicular to vms (the fast mode speed) than parallel, as MA → 1. An additional effect is observed in which the shock shape is “skewed” for θIMF = 45° (but not for θIMF = 90°) in the plane containing BIMF and vsw. These latter two effects are consistent with the fast mode velocity varying with propagation direction relative to BIMF. 2010-10-29T02:42:08.481Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:10337 The location and geometry of Earth's bow shock vary considerably with the solar wind conditions. More specifically, Earth's bow shock is formed by the steepening of fast mode waves, whose speed v ms depends upon the angle θ bn between the local shock normal n and the magnetic field vector B IMF, as well as the Alfvén and sound speeds (v A and c S ). Since v ms is a minimum for θ bn = 0° and low Alfvén Mach number M A , and maximum for θ bn = 90° and high M A , this implies that as θ IMF (the angle between B IMF and v sw ) varies, the magnitude of v ms should vary also across the shock, leading to changes in shape. This paper presents 3-D MHD simulation data which illustrate the changes in shock location and geometry in response to changes in θ IMF and M A , for 1.4 ≤ M A ≤ 9.7 and 0° ≤ θ IMF ≤ 90°. Specifically, for oblique IMF the shock's geometry is shown to become skewed in planes containing B IMF (e.g., the x − z plane). This is also emphasized in the terminator plane data, where the shock is best represented by ellipses, with centers translated along the z axis. For the θ IMF = 90° simulations the shock is symmetric about the x axis in both the x − y and x − z planes. Simulations for field-aligned flow (θ IMF = 0°) show a dimpling of the nose of the shock as M A → 1. The simulations also illustrate the general movement of the shock in response to changes in M A ; high M A shocks are found closer to Earth than low M A shocks. Farris et al.'s [1991] magnetopause model is used in the simulations, and we discuss the limitations of this, as well as the expected results using a self-consistent model. 2010-10-28T23:30:05.456Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:6702 Our understanding of the core-mantle boundary (CMB) region has improved significantly over the past several years due, in part, to the discovery of the post-perovskite phase. Sesimic data suggest that the CMB region is highly heterogeneous, possibly reflecting chemical and physical interaction between outer core material and the lowermost mantle. In this contribution we present the results of a new mechanism of mass transfer across the CMB and comment on possible repercussions that include the initiation of deep, siderophile-enriched mantle plumes. We view the nature of core-mantle interaction, and the geodynamic and geochemical ramifications, as multiscale processes, both spatially and temporally. Three lengthscales are defined. On the microscale (1-50 km), we describe the effect of loading and subsequent shearing of the CMB region and show how this may drive local flow of outer core fluid upwards into D". We propose that larger scale processes operating on a mesoscale (50-300 km) and macroscale regimes (> 300 km) are linked to the microscale, and suggest ways in which these processes may impact on global mantle dynamics. 2010-01-27T22:14:41.256Z ]]>