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In 2008, a Vibroseis seismic reflection survey was acquired by Geoscience BC across the eastern part of the volcanic- covered Nechako basin in central British Columbia, where Cretaceous sedimentary rocks have been exhumed along a NNW trend. Good signal penetration through the volcanic cover is indicated by lower crustal reflections at 8-12 s, which were recorded by the entire seismic survey. Comparison of the 2008 seismic survey with data from a previous survey indicates that the lack of reflectivity in the earlier surveys is generally representative of the subsurface geology. The seismic data show that ~1700 and ~2900 m thick sub-basins are present at the northern and southern ends of this trend, but the intervening Cretaceous rocks are discontinuous and relatively thin. The creation of a passive-roof duplex by Campanian or later low-angle thrusting is inferred within the thickest Cretaceous strata, but elsewhere faulting is likely related to Eocene extension or transtension. Seismic reflections are also recorded from folded volcanic stratigraphy, the base of the surface volcanic rocks, an underlying volcaniclastic stratigraphy, and intrusions projecting into a Quaternary volcanic cone. Seismic interpretation is complemented by coincident audiofrequency magnetotelluric surveys, from which faulting is inferred at offsets in a regional conductor. No regionally extensive stratigraphy can be identified within the seismic data, and the central Nechako basin appears to be a complex network of small, deformed sub-basins, rather than a single large basin. En 2008, Geoscience BC a acquis un releve sismique reflexion Vibroseis portant sur la partie est du bassin Nechako, recouvert de roches volcaniques; ce bassin est situe dans le centre de la Colombie-Britannique, ou des roches sedimentaires datant du Cretace ont ete exhumees le long d'une tendance NNO. Une bonne penetration du signal a travers le couvert volcanique est indiquee par les reflexions sur la croute inferieure a 8-12 s, lesquelles sont enregistrees dans tout le releve sismique. Une comparaison entre le releve sismique de 2008 et les donnees d'un releve anterieur indique que le manque de reflectivite dans les anciens releves est generalement representatif de la geologie souterraine. Les donnees sismiques montrent que des sous-bassins d'une epaisseur ~1700 et ~2900 m sont presents aux extremites nord et sud de cette tendance, mais que les roches cretacees entre les extremites sont discontinues et relativement minces. La creation, vers le Campanien ou plus tard, d' un << duplex >> a toit passif par chevauchement a angle faible est inferee dans les strates cretacees les plus epaisses, mais ailleurs, les failles sont plus probablement reliees a l'extension a l'Eocene ou a de la transtension. Des reflexions sismiques sont aussi enregistrees a partir de la stratigraphie de roches volcaniques pliees, de la base des roches volcaniques en surface, d'une stratigraphie volcano-clastique sous-jacente et a partir d'intrusions se projetant dans un cone volcanique quaternaire. L'interpretation sismique est completee par des releves magnetotelluriques de frequences audio coincidents, a partir desquels les failles sont inferees aux points de decalage dans un conducteur regional. Aucune stratigraphie regionale extensive ne peut etre identifiee dans les donnees sismiques et le bassin Nechako central semble etre un reseau complexe de petits bassins deformes plutot qu'un seul grand bassin. [Traduit par la Redaction], Introduction The Nechako basin, which is located in the interior plateau of British Columbia between the Coast Mountains and the Rocky Mountains (Fig. 1), has seen very little exploration for [...]

Abstract: Magnetotelluric (MT) soundings were made at more than 230 sites in the western Superior Province, in central Canada, as part of the Lithoprobe Western Superior Transect investigations. The Superior Province forms the nucleus of the North American continent. Its western part contains a regional pattern of east–west trending 100–200km wide granite–greenstone and metasedimentary belts or subprovinces. The objectives of the MT study are to define the electrical resistivity structure of the crust of the subprovinces and their boundaries, and to determine if there exists a signature in the upper mantle related to the formation of the craton. The exceptionally large MT database for the western Superior Province provides a three-dimensional sampling of the resistivity structure. In this paper, the primary MT responses from the data set, including geoelectric strike, apparent resistivity and phase, are used to define first-order geoelectrical structures in the region. Apparent resistivity responses indicate the ca 3.0Ga North Caribou terrane contains extremely resistive crust. Crust to the south, accreted at ca. 2.7Ga, is significantly more conductive, with resistivities similar to in the southeast Superior Province. The results suggest that the supracrustal component of the North Caribou terrane provides minimal enhancement of the crustal conductivity. At the southern margin of the former North Caribou superterrane, induction arrows define a conductivity anomaly that extends in an east–west direction for at least 600km and causes a reversal of arrows in a region 100–200km wide. This anomaly is interpreted to be caused by a conductive component of the 2.7Ga metasedimentary rocks of the English River subprovince. The observations demonstrate the ability of electromagnetic methods to map some Neoarchean continental margins. In the southwest Superior Province, geoelectric strike azimuths for periods corresponding to signal penetration into the upper mantle are aligned with the trend of subprovince boundaries and are interpreted to be related to fossil structure in the mantle lithosphere. This result provides support to the interpretation that the lithospheric mantle of the western Superior Province has remained intact since the 2.7Ga accretion of the subprovinces. The MT responses from the western Superior Province have thus defined a number of large-scale resistivity structures providing information on tectonic structure and history, and confirming the value of electromagnetic imaging of Archean lithosphere. [Copyright &y& Elsevier]

Presents research which studied the electrical conductivity of the subcrustal lithosphere underlying the southern Archean Churchill Province (ACP) in western Canada. Conductivity compared with that of the nearby Paleoproterozoic crust; Conductivity based on olivine; Results of mantle xenolith and geological mapping; Effects of tectonically-induced metasomatism.

Abstract: The internal structure and growth history of six macro-diamonds from kimberlite pipes in Yakutia (Russia) were investigated with cathodoluminescence imaging and coupled carbon isotope and nitrogen abundance analyses along detailed core to rim traverses. The diamonds are characterised by octahedral zonation with layer-by-layer growth. High spatial resolution SIMS profiles establish that there is no exchange of the carbon isotope composition across growth boundaries at the μm scale and that isotopic variations observed between (sub)zones within the diamonds are primary. The macro-diamonds have δ13C values that vary within 2‰ of −5.3‰ and their nitrogen contents range between 0–1334at. ppm. There are markedly different nitrogen aggregation states between major growth zones within individual diamonds that demonstrate Yakutian diamonds grew in multiple growth events. Growth intervals were punctuated by stages of dissolution now associated with <10μm wide zones of nitrogen absent type II diamond. Across these resorption interfaces carbon isotope ratios and nitrogen contents record shifts between 0.5–2.3‰ and up to 407at. ppm, respectively. Co-variation in δ13C value–nitrogen content suggests that parts of individual diamonds precipitated in a Rayleigh process from either oxidised or reduced fluids/melts, with two single diamonds showing evidence of both fluid types. Modelling the co-variation establishes that nitrogen is a compatible element in diamond relative to its growth medium and that the nitrogen partition coefficient is different between oxidised (3–4.1) and reduced (3) sources. The reduced sources have δ13C values between −7.3‰ and −4.6‰, while the oxidised sources have higher δ13C values between −5.8‰ and −1.8‰ (if grown from carbonatitic media) or between −3.8‰ and +0.2‰ (if grown from CO2-rich media). It is therefore concluded that individual Yakutian diamonds originate from distinct fluids/melts of variable compositions. The diamond-forming fluids within the cratonic mantle beneath the Siberian Craton record significant variations in composition and volume and include both oxidised and reduced sources. These observations suggest that dating diamond inclusions using an isochron approach will best provide geologically meaningful ages if inclusions can be shown to be genetically (spatially) related. [Copyright &y& Elsevier]

• 3D resistivity structure of the Archean Superior Province from magnetotelluric data. • Sequential inversion produces internally consistent results on multiple scales of interest. • Inversion identifies low resistivity features at lower crustal-upper mantle depths. • The petrochemical source and timing of the low resistivity zones is inferred. • Archean magmatic-hydrothermal and deformation processes generate the low resistivity. The nature of lithospheric evolution and style of the driving tectonic processes during the growth and stabilisation of continental crust in the Archean remain enigmatic. The hotter, rheologically weak Archean crust would be unable to support thick orogens. Thus, gravitational collapse likely occurred when continental fragments became overthickened and/or when far-field stresses were relieved during the terminal stages of orogenesis. 3D magnetotelluric resistivity models of the Archean Superior Province, reveal the presence of low resistivity zones in the mid-lower crust that reflect a protracted history of magmatic-hydrothermal activity contemporaneous with construction and collapse. These include sub-vertical zones of low resistivity in the mid-upper crust, inferred to represent corridors of paleo-fluid flow along crustal-scale structural networks developed in response to terrane amalgamations. Subsequent orogenic collapse resulted in widespread lateral flow within the lower crust accommodated by sub-horizontal shear zones and included magmatic refertilisation. Thus, the preserved low-resistivity anomalies in the mid-lower crust represent an amalgamation of magmatic-hydrothermal and deformational processes that occurred during construction, peak orogenesis, and collapse in the Archean. [ABSTRACT FROM AUTHOR]