Southern Extension Research Articles

Signatures of continental collisions and magmatic activity in central Brazil as indicated by a magnetotelluric profile across distinct tectonic provinces

Broadband and long-period magnetotelluric soundings were collected along a 560km E–W profile in a region of central Brazil subjected to Neoproterozoic collision tectonics and Archean to Cretaceous magmatic events. The profile crosses the northeastern portion of Phanerozoic sediments and volcanics of the Paraná basin, the southern extension of the Neoproterozoic metasedimentary rocks of the Brasília belt, locally pervaded by Cretaceous alkaline magmas of the Alto Paranaíba igneous province (APIP), and Neoproterozoic sedimentary cover and exposed Archean basement of the southern São Francisco craton. 2D conductivity structures derived by joint inversions of the TE and TM polarization modes and a separate inversion of the tipper components show signatures of the past tectonomagmatic events that affected the area. A gravity-defined suture zone beneath the Paraná basin is detected in the models as a subvertical conductor extending from crustal to upper mantle depths, which strengthen the interpretation of a Neoproterozoic collision of the São Francisco craton and a continental block beneath the sedimentary basin. Deep underthrusting of organic graphite-bearing metasedimentary rocks composing a fossil suture zone is proposed to explain such increase in electrical conductivity. A similar conductivity signature beneath the sedimentary covered region of the São Francisco craton can be tentatively interpreted as another similar cryptic suture zone. Other isolated high-conductivity anomalies at midcrustal depths below the Paraná and APIP provinces are interpreted as fossil residues representing precipitated graphite and sulfide derived from percolating volatiles during the emplacement of Cretaceous mafic-ultramafic volcanics. A very high conductivity wedge into the lower lithosphere is highlighted in the deep mantle beneath the APIP volcanic complex, coincident with a zone of low velocity defined by seismic tomography. Geochemical evidence indicates that the alkaline magmatism stemmed from a metasomatized upper mantle at only slightly raised temperatures. Thus, interconnected carbonatite melts of low melting point and graphite in the lithospheric mantle are the most likely candidates to explain this high conductivity. The seismic and conductivity anomalies are probably triggered by the same source mechanisms (incipient melting) because they are plausibly related to the same magmatic process. Conductivity anomalies at crustal and mantle depths in the southern segment of the São Francisco craton suggest that its lithosphere was significantly affected by the several tectonomagmatic episodes it has experienced throughout its geological history. Consequently, the enhanced conductivity in the lower crust can be genetically related to upwelling volatile-rich intrusions whereas upper mantle high conductivity can be related to refertilization by infiltrations of low-degree carbonatitic melts from deeper-sourced metasomatic processes.

Petrochemistry and tectonic setting of mafic volcanic rocks in the Chon Daen–Wang Pong area, Phetchabun, Thailand

AbstractThe mafic volcanic rocks and hypabyssal rocks in the Chon Dean‐Wang Pong area are possibly the southern extension of the western Loei Volcanic Sub‐belt, Northeast Thailand. They are least‐altered, and might have been formed in Permian–Triassic times. The rocks are commonly porphyritic, with different amounts of plagioclase, clinopyroxene, orthopyroxene, amphibole, Fe–Ti oxide, unknown mafic mineral, and apatite phenocrysts or microphenocrysts, and are uncommonly seriate textured. The groundmass mainly shows an intergranular texture, with occasionally hyalophitic, intersertal and ophitic–subophitic textures. The groundmass constituents have the same minerals as the phenocrysts or microphenocrysts and may contain altered glass. The groundmass plagioclase laths may show a preferred orientation. Chemically, the studied rock samples can be separated into three magmatic groups: Group I, Group II, and Group III. These magmatic groups are different in values for Ti/Zr ratios. The averaged Ti/Zr values for Group I, Group II, and Group III rocks are 83 ± 6, 46 ± 12, and 29 ± 5, respectively. In addition, the Group I rocks have higher P/Zr, but lower Zr/Nb relative to Group II and Group III rocks. The Group I and Group II rocks comprise tholeiitic andesite–basalt and microdiorite–microgabbro, while the Group III rocks are calc‐alkalic andesite and microdiorite. According to the magmatic affinities and the negative Nb anomalies on normal mid‐oceanic ridge basalt (N‐MORB) normalized multi‐element plot, arc‐related lavas are persuasive. The similarity between the studied lavas and the Quaternary lavas from the northern Kyukyu Arc, in terms of chondrite‐normalized rare earth element (REE) patterns and N‐MORB normalized multi‐element patterns, leads to a conclusion that the mafic volcanic rocks and hypabyssal rocks in the Chon Daen–Wang Pong area have been formed in a volcanic arc environment.

Volcanic Markers of the Post-Subduction Evolution of Baja California and Sonora, Mexico: Slab Tearing Versus Lithospheric Rupture of the Gulf of California

The study of the geochemical compositions and K-Ar or Ar-Ar ages of ca. 350 Neogene and Quaternary lavas from Baja California, the Gulf of California and Sonora allows us to discuss the nature of their mantle or crustal sources, the conditions of their melting and the tectonic regime prevailing during their genesis and emplacement. Nine petrographic/geochemical groups are distinguished: “regular” calc-alkaline lavas; adakites; magnesian andesites and related basalts and basaltic andesites; niobium-enriched basalts; alkali basalts and trachybasalts; oceanic (MORB-type) basalts; tholeiitic/transitional basalts and basaltic andesites; peralkaline rhyolites (comendites); and icelandites. We show that the spatial and temporal distribution of these lava types provides constraints on their sources and the geodynamic setting controlling their partial melting. Three successive stages are distinguished. Between 23 and 13 Ma, calc-alkaline lavas linked to the subduction of the Pacific-Farallon plate formed the Comondú and central coast of the Sonora volcanic arc. In the extensional domain of western Sonora, lithospheric mantle-derived tholeiitic to transitional basalts and basaltic andesites were emplaced within the southern extension of the Basin and Range province. The end of the Farallon subduction was marked by the emplacement of much more complex Middle to Late Miocene volcanic associations, between 13 and 7 Ma. Calc-alkaline activity became sporadic and was replaced by unusual post-subduction magma types including adakites, niobium-enriched basalts, magnesian andesites, comendites and icelandites. The spatial and temporal distribution of these lavas is consistent with the development of a slab tear, evolving into a 200-km-wide slab window sub-parallel to the trench, and extending from the Pacific coast of Baja California to coastal Sonora. Tholeiitic, transitional and alkali basalts of subslab origin ascended through this window, and adakites derived from the partial melting of its upper lip, relatively close to the trench. Calc-alkaline lavas, magnesian andesites and niobium-enriched basalts formed from hydrous melting of the supraslab mantle triggered by the uprise of hot Pacific asthenosphere through the window. During the Plio-Quaternary, the “no-slab” regime following the sinking of the old part of the Farallon plate within the deep mantle allowed the emplacement of alkali and tholeiitic/transitional basalts of deep asthenospheric origin in Baja California and Sonora. The lithospheric rupture connected with the opening of the Gulf of California generated a high thermal regime associated to asthenospheric uprise and emplaced Quaternary depleted MORB-type tholeiites. This thermal regime also induced partial melting of the thinned lithospheric mantle of the Gulf area, generating calc-alkaline lavas as well as adakites derived from slivers of oceanic crust incorporated within this mantle.

The Tandilia System of Argentina as a southern extension of the Río de la Plata craton: an overview

The southernmost outcrops of the Río de la Plata cratonic region are exposed in the Tandilia System in eastern Argentina. The geological evolution comprises mainly an igneous-metamorphic Paleoproterozoic basement named Buenos Aires Complex, which is covered by Neoproterozoic to Early Paleozoic sedimentary units which display subhorizontal bedding. The basement of calc-alkaline signature consists mainly of granitic-tonalitic gneisses, migmatites, amphibolites, some ultramafic rocks, and granitoid plutons. Subordinate rock-types include schists, marbles, and dykes of acid and mafic composition. Tandilia was recognized as an important shear belt district with mylonite rocks derived mainly from granitoids. The tectonic scenario seems related to juvenile accretion event (2.25–2.12 Ga) along an active continental margin, followed by continental collision (2.1–2.08 Ga) after U–Pb zircon data. The collisional tectonic setting caused thrusting and transcurrent faulting favouring the anatexis of the crustal rocks. The tholeiitic dykes constrain the time of crustal extension associated with the last stages of the belt evolution. The basement was preserved from younger orogenies such as those of the Brasiliano cycle. After a long paleoweathering process, the Sierras Bayas Group (c. 185 m thick) represents a record of the first Neoproterozoic sedimentary unit (siliciclastic, dolostones, shales, limestones), superposed by Cerro Negro Formation (c. 150–400 m thick, siliciclastics) assigned to Upper Neoproterozoic age. The final sedimentary transgression during Early Paleozoic was the Balcarce Formation (c. 90–450 m thick) deposited over all the mentioned Precambrian units. Based on all the geological background, a tectonic evolution is offered.

Pachycephalosauridae from the San Carlos and Aguja Formations (Upper Cretaceous) of West Texas, and observations of the frontoparietal dome

ABSTRACT Several fragmentary frontoparietal domes from the San Carlos and Aguja Formations (Campanian) of Brewster and Presidio Counties, Texas, are referable to Pachycephalosauridae (Dinosauria: Ornithischia) and represent a southern extension of the known range of these dinosaurs in North America. Although the specimens are insufficient for confident generic identification, the size, relative dome height and width, continuity of frontal and parietal portions of the dome, and obliteration of sutures between these bones suggest affinity with pachycephalosaurids as Stegoceras and Gravitholus. More than one taxon is probably represented in the collection. Sections taken from one of the domes reveal histological tissue zonation comparable to that found in other pachycephalosaurids and growth lines indicating that at least 5 years were required to attain adult size. The bone tissue is well suited for strengthening the dome to resist compressional loading, and is compatible with the hypothesis that these animals engaged in head-butting behavior.

Active tectonics of the Alaotra–Ankay Graben System, Madagascar: Possible extension of Somalian–African diffusive plate boundary?

The Lake Alaotra–Ankay rift valley of Central Madagascar forms a NE–SW oriented depression filled with Neogene to recent sediments and is part of a more regional post-Miocene graben system that strikes N–S across much of the central part of the island. The region is characterized by a number of small earthquakes, active hot springs, steep fault-scarp bounded valleys, and several levels of terraces. Young, deeply incised topography related to intense tropical weathering, is largely concentrated near active fault systems. The origin and evolution of this extensional structure and its morphological expressions, however, have not been clearly documented. Mountain front sinuosity values indicate that the eastern side of the rift valley is tectonically more active than the western side, and that rapid erosion under subtropical conditions is causing rapid weathering of fault scarps. The active river network within the Alaotra–Ankay rift shows clear signs of surface re-adjustment to recent tectonic activity. The drainage network is asymmetric, with most channels on the east sides of valleys and controlled by regional eastward tilting. First-order rivers flow parallel to the basin axis. Zones of intense lavaka (erosional gully) formation are correlated with active fault systems and seismicity. Extension in the Ankay–Alaotra rift is oriented roughly the same as in the morphologically similar East African rift system, located only 500 km to the west. The eastern edge of the African continent is moving somewhat independently from the rest of Africa and the portion of the African plate east of the East African rift is regarded as a separate plate, the Somali plate. However, few plate configurations have clearly defined the southern extension of the Africa-Somali plate boundary to where it must join the Southwest Indian Ocean ridge, a necessary requirement of any true microplate bounded by plate boundaries. Instead, the southeast part of the African plate, including the Somali microplate, seems to be dominated by a number of individual rift systems, overall defining a broad diffusive extensional plate boundary. We suggest that this diffusive plate boundary includes one segment that extends off the coast of Africa through the Comores, then cuts through northern Madagascar and extends down the active Ankay–Alaotra rift to the fault-block dominated southeastern coast of Madagascar, then extends southeastward to the Southwest Indian Ocean Ridge. Thus, the southeastern part of the African plate is considerably more fragmented by plume-related uplift and diffuse extensional boundaries than previously envisaged.

Spatial distribution of mercury and other trace elements in recent lake sediments from central Alberta, Canada: An assessment of the regional impact of coal-fired power plants

These have been growing concerns over the environmental impacts of the coal-fired power plants in the western Canadian province of Alberta, which collectively comprise one of the largest point sources of Hg and other trace elements nationally. The overall cumulative impact of the power plants since the beginning of their activities several decades ago has been a critical question for industry, government agencies, and the research community. This paper aims to delineate the cumulative geographic extent of impact by investigating the spatial distribution of mercury and other trace elements of environmental concern in nine freshwater lakes, which cover the large area surrounding the coal-fired power plants in central Alberta, Canada. 210-Lead dating was used in conjunction with physical evidence of deposited fly ash to determine the sediments' age and hence the depths corresponding to the onset of coal-fired power generation in 1956. Total mean concentrations and fluxes of elements of environmental concern with integrated values since 1956 were then determined. The concentration values do not reflect the catastrophic oil spill at Lake Wabamun in 2005. The post-1956 flux rates of As, Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, Sb, V, W, and Zn were generally highest in sediment cores obtained from two lakes adjacent to power plants. However, the variable prevailing wind directions played an important role in determining the aerial distribution of Hg and other trace elements to the southeast and to the west of the power plants. Post-1956 fluxes of most elements declined downwind (westward), consistent with strong easterly winds transporting metal pollution further to the west of the power plants. However, spatial interpolation of the data suggested a major southern extension to the area of maximum metal deposition, which has not been sampled by this or previous studies in the region. An atmospheric model estimate of total Hg flux in 2007 near the Genesee power plant was substantially lower than indicated by the most impacted lake sediments, which have integrated Hg fluxes since 1956. The most likely explanations for the model-sediment difference are either that a sharp spatial gradient of deposition exists in the region, and/or recent technology improvements by the power plants have substantially reduced Hg emissions.

Pampia: A large cratonic block missing in the Rodinia supercontinent

The large cratonic block of Pampia, at present in southern South America, is commonly missing in recent comprehensive reconstructions of Rodinia. This block was the conjugate margin of the Amazonia craton that corresponds to the W-NW-trending segment of the Sunsás orogen. This segment developed between Sucre in Bolivia and Corumbá in Brazil as a result of a Mesoproterozoic collision. The western margin of Pampia partially coincides with the southern part of the Arequipa-Antofalla terrane that was also amalgamated during the Mesoproterozoic. The Cuyania terrane, a Laurentian rifted continental block, was accreted to the southern sector of this margin of Pampia in middle Ordovician times. The northern sector of the eastern margin of Pampia is the lower plate of the Brasiliano belt developed along the Paranapanema craton during the early Cambrian collision associated with the closure of the Clymene Ocean. The southern sector of the eastern margin partially coincides with the southern extension of the Transbrasiliano lineament, which juxtaposed the Rio de La Plata Craton with the Pampia cratonic block. Along this eastern margin the Pampean-Paraguay orogen was formed during the late Brasiliano cycle in latest Proterozoic-Early Cambrian time. The southern margin was generated by the collision of Patagonia during late Paleozoic times. This cratonic block of Pampia, so defined, has western and northern sectors formed by Mesoproterozoic orogens, while the eastern sector is partially formed by juvenile Neoproterozoic crust, although older rocks cannot be ruled out. The Pampia cratonic block was the locus of several magmatic belts associated with important metamorphism during the Paleozoic and preserved in the central part at lower crustal levels.

Petrography of cuttings from spa borings and their tectonic setting upon the Kanto Plain at Shimohara, Tsukuba city and at Hanai, Noda city

This report describes the subsurface geology, based on the petrography of rock cuttings taken from bore holes at spa sites, at Shimohara, Tsukuba City, Ibaraki Prefecture, and at Hanai, Noda City, Chiba Prefecture, both upon the Kanto Plain. The cutting samples from Tsukuba (four samples from 1241–1210 m depth) are hornfels, biotite-muscovite schist, and sillimanite-andalusite gneisses that resemble the Tsukuba metamorphic rocks. A single cutting sample from Noda (1295–1300 m depth) is mylonitized biotite granite that is similar in lithology to the Late Cretaceous Name Granite in the Shimonita area, Gunma Prefecture. These results provide new information on the eastern extension of the Ryoke Metamorphic Belt and the southern extension of the Tsukuba Metamorphic Belt. The occurrence of mylonite from Noda also provides important information in terms of constraining the eastern extension of the Median Tectonic Line.

Assessment of the origin of a Loggerhead Turtle, Caretta caretta, found in Kuwaiti waters, using mitochondrial DNA

306 base pairs from the control (D-loop) region of the mitochondrial DNA (mtDNA) of a Loggerhead Turtle, Caretta caretta Linnaeus, 1758, from Kuwait were sequenced in order to identify the origin of this turtle. Neighbour-joining tree analyses with sequences available in the GenBank showed that it had a close relationship with those of the Atlantic colonies. For adult Loggerhead Turtles of the Atlantic colonies which undertake long distance migrations, the southern extension of Africa might be less formidable as a continental barrier to their passage into the Indian Ocean and subsequently into the Arabian area because of their temperate distribution.

A methodology for inversion of broadband seismic waveforms for elastic and anelastic structure and its application to the mantle transition zone beneath the Northwestern Pacific

We develop a quantitative and objective methodology for inversion of body-wave waveform data for elastic and anelastic structure and apply it to the upper mantle and mantle transition zone (MTZ) beneath the Northwestern Pacific, using PREM as the starting model. We use the singular value decomposition (SVD) as the basis for our inversion and use Akaike’s Information Criterion (AIC) to evaluate the statistical significance of the models. Although the resulting models are preliminary, we obtain an average positive S-wave velocity anomaly in the MTZ beneath the Philippine Sea Plate and the Japan Sea. We also find a negative S-wave velocity anomaly in the depth range 300–400 km beneath the subduction zone to the east of northern Japan and its southern extension beneath the Izu–Bonin–Marianas arc. Our inversions obtained a lower Q μ , as compared to PREM, for all of the subregions.

Organic geochemistry of Gadvan and Kazhdumi formations (Cretaceous) in South Pars field, Persian Gulf, Iran

Abstract In Southern Iran, Gadvan (Barremian-early Aptian) and Kazhdumi (Albian) formations are the most effective source rocks and have produced the majority of hydrocarbons reserved in the Zagros Basin especially in Dezful Embayment and Persian Gulf area. In this article, hydrocarbon potential of Gadvan and Kazhdumi formations is investigated in the South Pars field which is southern extension of the North field of Qatar Country. This field is located in Persian Gulf waters and is actually the northern extension of Qatar Arc Paleohigh where geological history of Gadvan and Kazhdumi formations is different from nearby area regarding depositional setting, burial history and source rock maturity. In this study, Gadvan and Kazhdumi formations as source rock candidates, which underlay Upper Dariyan and Mauddud members, respectively, were sampled in two drilled wells of the South Pars field for routine geochemical analysis to investigate hydrocarbon potential of these formations and source rock identification of trapped oil in the Upper Dariyan and Mauddud members. Several samples from top to the bottom of the formations were taken and analyzed by Rock-Eval pyrolysis. The average TOC content of Gadvan and Kazhdumi formations is 0.79 wt. % and 0.49 wt. %, respectively. Rock-Eval results (e.g. HI vs. Tmax) represent that TOC content of these formations contains type II-III kerogens which haven't suffered sufficient thermal maturity (Ro As it is believed, Gadvan and Kazhdumi formations to be source of trapped oil in the system, therefore, in the South Pars field reserved hydrocarbon in Upper Dariyan (Aptian) and Mauddud (late Albian) members which overlie Gadvan and Kazhdumi formations, respectively, are probably generated from Gadvan and Kazhdumi formations of the nearby through and flanks of the Qatar Arc where the burial depth and temperature increase then generated hydrocarbons in downdip area are migrated to the upper carbonate reservoirs in the crest part of the Qatar Arc. Long path migration of the hydrocarbon and source rock with lower organic matter are caused hydrocarbon accumulation in the South Pars Oil Layer (Al-Shaheen) which is approved by professional petrophysical and geological studies of the field.

The Kumaun and Garwhal Lesser Himalaya, India: Part 1. Structure and stratigraphy

Our understanding of the geologic evolution of the Himalaya remains incomplete, particularly in regard to structural and geochronologic details of the Proterozoic-Paleozoic Lesser Himalayan Sequence. We conducted an integrated field mapping, geochronological study, and geochemical analysis of the Lesser Himalayan Sequence strata in the Kumaun and Garwhal Himalaya of NW India (78° 00′–80°30′E). Structural observations reveal a systematic change in deformation styles from lower to higher structural levels in the Main Central Thrust footwall. In the south, and at lower structural levels, the Main Central Thrust footwall is characterized by parallel folding and sparse development of axial cleavage. Quartz microstructures indicate that deformation occurred at temperatures below 350 °C. In contrast, in the north and at higher structural levels close to the Main Central Thrust, deformation is characterized by replacement of original bedding, the development of penetrative cleavage, and schistosity locally. Folds are tight, in places isoclinal and overturned. The corresponding quartz microstructures indicate that deformation occurred above 350 °C. Dating of detrital zircons from Lesser Himalayan Sequence metasedimentary units, and igneous zircons from schistose gneisses throughout the Lesser Himalayan Sequence allow refinements to the stratigraphic framework. First, unlike has been observed in western Nepal immediately west of our study area, U-Pb zircon dating suggests the absence of Paleoproterozoic strata in the Ramgarh Thrust hanging wall. This suggests that the thrust cuts upsection laterally along strike. Second, U-Pb detrital zircon dating suggests that metasedimentary strata in the Ramgarh Thrust hanging wall are correlative with basal units of the Outer Lesser Himalayan Sequence, removing the necessity that the Ramgarh Thrust hanging wall was allochthonous with respect to its footwall Lesser Himalayan Sequence units. Schistose gneisses with U-Pb ages of ca. 1850 Ma are older than Lesser Himalayan Sequence units in the area, suggesting that they are thrust-emplaced Lesser Himalayan Sequence basement. We conclude that the Ramgarh Thrust is an out-of-sequence thrust postdating a folding event in its footwall. The earlier proposal that Neoproterozoic-Cambrian Lesser Himalayan Sequence strata are the southern extension of the Tethyan Himalayan Sequence requires that the Tons Thrust, which separates the distal and proximal facies of the Lesser Himalayan Sequence, be a major, south-directed structure with a slip magnitude of >50–100 km.

Effects of the Permian–Triassic boundary on reservoir characteristics of the South Pars gas field, Persian Gulf

AbstractThe Permian–Triassic boundary (PTB) is a world‐wide event characterized by the most extensive mass extinction in the history of life. In the Persian Gulf, the rock record of this time interval host one of the most important hydrocarbon reserves in the world: the South Pars Gas Field and its southern extension, the North Dome (or North Field). These carbonate and evaporite successions were sampled in eight wells for petrographic, geochemical and porosity–permeability studies. An important characteristic of the Dalan and Kangan formations is the centimetre‐scale lithological heterogeneities caused by facies changes and diagenetic imprints that led to the compartmentalization of these reservoirs. These Permian–Triassic (P‐T) sediments were deposited in a shallow marine homoclinal ramp. The PTB in this hydrocarbon field is represented by a reworked coarse‐grained intraclastic/bioclastic grainstone facies deposited during a marine transgression. Prolonged subaerial exposure in the P‐T transition caused hypersaline and meteoric diagenesis, including extensive cementation, dolomitization and some dissolution, influencing reservoir characteristics of bordering units. Both δ18O and δ13C values in this succession mirror worldwide excursions typical of other P‐T sections, with some variations due to diagenetic alterations. A pronounced decline in 87Sr/86Sr values, reflective of global seawater geochemistry for most of the Permian is evident in our data. Reservoir quality declines through the late Permian, as a result of facies change and diagenesis. The Late Permian is succeeded by a Triassic transgressive facies and decline in reservoir quality. Copyright © 2009 John Wiley & Sons, Ltd.

Aeromagnetic exploration over the East Antarctic Ice Sheet: A new view of the Wilkes Subglacial Basin

The Wilkes Subglacial Basin represents an approximately 1400 km-long and up to 600 km wide subglacial depression, buried beneath the over 3 km-thick East Antarctic Ice Sheet. Contrasting models, including rift models and flexural models, have been previously put forward to explain the tectonic origin of this enigmatic basin, which is located in the largely unexplored hinterland of the Transantarctic Mountains. A major aerogeophysical survey was flown during the 2005–06 austral summer to explore the Wilkes Subglacial Basin. Our new airborne radar dataset reveals that the Wilkes Subglacial Basin contains several subglacial basins, which are considerably deeper than previously mapped. Major aeromagnetic lineaments are detected from total field, pseudo-gravity, tilt derivative and Euler Deconvolution maps. These aeromagnetic lineaments reveal that the Wilkes Subglacial Basin and its sub-basins are structurally controlled. Comparison between aeromagnetic signatures over the Wilkes Subglacial Basin region and the Cordillera in North America, suggests that the basin contains a former broad backarc basin and fold-and-thrust belts, forming the transition between a Precambrian craton and the Ross Orogen. The eastern margin of the Wilkes Subglacial Basin is imposed upon the Prince Albert Fault System and the Priestley Fault. These faults may have been reactivated in the Cenozoic, as major strike–slip faults. The western margin of the Wilkes Subglacial Basin is located along the southern extension of the Precambrian-age Mertz Shear Zone and marks the edge of the Terre Adélie Craton. High-frequency aeromagnetic anomalies in the Wilkes Subglacial Basin image large volumes of Jurassic tholeiites, which were intruded into and extruded over Beacon sediments in a possible rift setting. Depth-estimates of magnetic anomaly sources and forward modelling indicate that major Cretaceous and Cenozoic rift basins with thick sedimentary infill, comparable to the deep Ross Sea Rift basins, are however unlikely beneath this part of the Wilkes Subglacial Basin. More localised graben-like features, with up to 1.5 km of sedimentary infill are identified in the Central Basins. These grabens may be transtensional features related to regional Cenozoic intraplate strike–slip deformation, which has been extensively mapped over the adjacent Transantarctic Mountains, and/or older Cretaceous grabens. Over 3 km deep sedimentary basins are also imaged beneath the Western Basins and are inferred to contain older sedimentary infill of Ross-age, based on recent dating of glacial erratics between Mertz and Ninnis Glacier.

THE ACS FORNAX CLUSTER SURVEY. V. MEASUREMENT AND RECALIBRATION OF SURFACE BRIGHTNESS FLUCTUATIONS AND A PRECISE VALUE OF THE FORNAX-VIRGO RELATIVE DISTANCE

We present g-z color and z-band surface brightness fluctuations (SBF) measurements for 43 early-type galaxies in the Fornax cluster imaged with HST/ACS. These are combined with measurements for Virgo cluster galaxies to derive a revised, nonlinear calibration of the z-band SBF absolute magnitude as a function of g-z. In all, we tabulate recalibrated SBF distances for 134 galaxies in Virgo, Fornax, the Virgo W' group, and NGC4697 in the Virgo Southern Extension. The calibration procedure yields a precise relative distance modulus for Fornax with respect to Virgo of 0.42+/-0.03 mag, or a distance ratio 1.214 +/- 0.017. The resulting Fornax distance modulus is 31.51 +/-0.03 +/-0.15 mag, 20.0 +/-0.3 +/-1.4 Mpc, where the second set of error bars reflects systematic uncertainty from our assumed Virgo distance of 16.5 Mpc. The rms distance scatter for the early-type Fornax galaxies is 0.49^{+0.11}_{-0.15} Mpc, consistent with its compact appearance on the sky. This translates to a depth scatter smaller than the intrinsic, or "cosmic", scatter sigma_cosmic in the SBF calibration, unlike in Virgo. As a result, we are able to place the first tight constraints on the value of sigma_cosmic, finding 0.06 +/- 0.01 mag, with a firm upper limit of sigma_cosmic<0.08 mag, at least for galaxies with g-z>1.02. We also present an alternative SBF calibration based on the `fluctuation count' parameter Nbar, a proxy for galaxy mass, which gives a consistent relative distance. Finally, we find no evidence for systematic trends of the galaxy distances with position or velocity (e.g., no current infall); the Fornax cluster appears both compact and well virialized.[abridged]

U–Pb detrital zircon ages and Sm–Nd isotopic features in low-grade metasedimentary rocks of the Famatina belt: implications for late Neoproterozoic–early Palaeozoic evolution of the proto-Andean margin of Gondwana

Abstract: The Famatina belt, Central Andes, is part of an ancient accretionary margin built along Western Gondwana in the early Palaeozoic. U–Pb ion microprobe analysis of detrital zircons and Sm–Nd whole-rock analysis of two early Palaeozoic low-grade metasedimentary units record the early evolution of this region. Detrital zircons in the Negro Peinado and Achavil formations have ages ranging from Palaeoproterozoic to Cambrian, consistent with derivation from Gondwanan sources. T DM ages suggest that the sedimentary rocks were derived from a composite source area, which separated from the mantle during the Palaeoproterozoic ( c . 1.8–1.6 Ga). Constraints from the youngest detrital grains indicate accumulation in a Mid- to Late Cambrian foreland basin adjacent to the inboard Pampean orogenic tract. The dominance of Cambrian ages in the Negro Peinado Formation suggests derivation principally from the eastern Pampean belt whereas the dominance of late Neoproterozoic ages in the Achavil Formation suggests that input from the Pampean belt was overwhelmed by older sources. The paucity of Palaeoproterozoic ages argues against direct input from older areas such as the Río de la Plata craton. The predominance of Meso- and Neoproterozoic ages over older sources suggests that a Brasiliano-age magmatic arc developed on a Mesoproterozoic basement, probably a southern extension of the Arequipa–Antofalla massif.

The Cambrian Kalkarindji Large Igneous Province: Extent and characteristics based on new 40Ar/ 39Ar and geochemical data

The Early Cambrian Kalkarindji Continental Flood Basalt Province in northern Australia is an important Large Igneous Province (LIP) both in size and timing. Being the earliest Phanerozoic LIP it may have had a severe effect on the Early Cambrian biota. Here, we investigate the extent of this province by testing the hypothesis that the extensive Table Hill Volcanics in south-central Australia are a southern extension of the Kalkarindji province. The Table Hill Volcanics have long been considered coeval with the Antrim Plateau Volcanics and related volcanics in the Kalkarindji province; however precise age data is lacking. Here, we present new 40Ar/ 39Ar geochronological, and major and trace element data to investigate this possibility. One Table Hill sample yielded a plateau age of 504.6 ± 2.5 Ma (2 σ), within error of recently published age data from the Kalkarindji LIP. Samples from both the Table Hill Volcanics and the northern part of the province are low-Ti tholeiites with MgO-content ranging from 3 to 9 wt.% and all samples are highly enriched in incompatible elements compared to primitive mantle. This, coupled with a negative Nb anomaly, suggests crustal contamination at an early stage in magma evolution or a significant contribution from the sub-continental lithospheric mantle. Subsequent fractionation produced further elevation of the REE in the later stages of the eruptions. Subtle differences in some incompatible element ratios between the Table Hill Volcanics and the northern Kalkarindji indicate some variation likely related to the assimilated crustal component or minor mantle source heterogeneity. The new geochronology data and analogous geochemical signatures confirm that the Table Hill Volcanics are a southern counterpart of the Kalkarindji province. These new data permit the areal extent of the province to be estimated at > 2.1 × 10 6 km 2. This makes the Kalkarindji province one of the largest LIPs of the Phanerozoic. Volatile outgassing from a province of this size is likely to have had a significant impact on the climate and biosphere and may have played a dominant role in the Lower–Middle Cambrian mass extinction.

Ophiolite-Hosted Copper and Gold Deposits of Southeastern Turkey: Formation and Relationship with Seafloor Hydrothermal Processes

The paper documents evidence about the physical and chemical controls on the behaviour of Au and Cu sulphide deposits in fossil seafloor hydrothermal systems of SE Turkey. Observations from actively forming sulphides at mid-ocean ridges (MOR) and in back-arc environments are compared briefly with ancient analogues of gold and pyritic copper deposits such as Kisecik, Ergani and Siirt-Madenköy, formed at mid-oceanic ridges, or possibly at seamounts and back-arc settings. Many ophiolite-hosted Au- and Cu-deposits, of various sizes, are known to exist along the Bitlis-Zagros Suture Zone (BZS), the boundary between Anatolian and Arabian plates in SE Turkey. These deposits are associated with Cretaceous intra-oceanic supra-subduction zone ophiolitic slabs, such as Kızıldağ, the Bäer-Bassit massif (the southern extension of the Kızıldağ Massif in Syria), and the Yüksekova and Berit ophiolite bodies, which tectonically overlie the Mesozoic platform carbonates and Palaeozoic sediments of the Arabian plate. East of the BZS, ophiolitic segments of Tertiary age are strongly mineralized. Several Au and Co-Ni bearing pyritic Fe-Cu oxide and sulphide deposits associated with chloritized and spilitized basaltic pillow lavas or sheeted dykes crop out along the BZS. Volcanics associated with mineralisation closely resemble MOR sequences, sedimented ridges and back-arc environments in spreading centres of island arc systems. Ophiolitic rocks and the mineralogy of associated alteration are similar to characteristics of modern mid-oceanic ridge mineralization along the East Pacific Rise (EPR), Mid-Atlantic Ridge (TAG hyrothermal field) and Red Sea (Atlantis-II Deep).

Modes of the southern extension of the East Madagascar Current

Data sets from satellite observations and a nested high‐resolution model are used to study a source region of the Agulhas Current. Altimeter‐derived geostrophic surface currents are averaged over varying periods, providing evidence of the persistence of flow patterns in the extension of the southern branch of the East Madagascar Current (SEMC). South of Madagascar, the SEMC separates into one branch toward the Agulhas Current and into a second branch retroflecting and connecting to the Subtropical Indian Ocean Countercurrent (SICC). Good agreement is found between long‐term mean patterns of observational and model dynamic heights. Two basic modes are identified in the SEMC extension, with anticyclonic motion favoring retroflection in the northern Mozambique Basin when the extension is in a southwestward direction and cyclonic motion occurring in the case of the SEMC flowing westward along the southern Madagascar slope. A cross‐correlation sequence between model SEMC transports and the modal changes in the extension region displays a correlation at about 1‐month lag which agrees with eddy propagation time from the SEMC to the outflow region. Mean model SEMC transports are determined using floats released at 21°S, and the contribution of the SEMC to the SICC is obtained using floats injected at 55°E with the model running backward. Almost half of the SEMC volume transport contributes to the Agulhas system, and about 40% of SICC water originates from the SEMC.