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Kitasu Hill and MacGregor Cone formed along the Principe Laredo Fault on British Columbia&apos;s central coast as the Wisconsinan ice sheet withdrew from the Coast Mountains. These small-volume Milbanke Sound Volcanoes (MSV) provide remarkable evidence for the intimate relationship between volcanic and glacial facies. The lavas are within-plate, differentiated (low MgO < 7%) Ocean Island Basalts, hawaiites, and mugearites that formed from ~1% decompression melting of asthenosphere with residual garnet. Kitasu Hill, on glaciated bedrock, formed between 18 and 15 cal ka BP. Dipping, poorly stratified, admixed hyaloclastite, and glacial diamicton with large plutonic clasts and pillow breccia comprise its basal tuya platform (0-43 masl). Subaerial nested cinder cones, with smaller capping lava flows, sit atop the tuya. New marine samples show McGregor Cone formed subaerially but now sits submerged at 43-200 mbsl on an eroded moraine at the mouth of Finlayson Channel. Seismic data and cores reveal glaciomarine sediments draping the cone&apos;s lower slopes and show beach terraces. Cores contain glaciomarine diamictons, ice-rafted debris, delicate glassy air fall tephra, and shallow, sublittoral, and deeper benthic foraminifera. Dates of 14.1-11.2 cal ka BP show volcanism spanned ~2000years during floating ice shelf conditions. The MSV have similar proximal positions to the retreating ice sheet, display mixed volcano-glacial facies, and experienced similar unloading stresses during deglaciation. The MSV may represent deglacially triggered volcanism. The dates, geomorphic and geological evidence, constrain a local relative sea level curve for Milbanke Sound and show how ice gave way to fire. Key words: marine geology, physical volcanology, Wisconsinan deglaciation, relative sea level curve, tuya, British Columbia coast, Introduction Glaciovolcanism occurs where local glaciers or regional ice caps are present during volcanic activity, with numerous examples in British Columbia (BC) (Mathews 1947; Grove 1974; Hickson 2000; Edwards et [...]

A recent volcanic eruption occurred at Tseax volcano that formed a series of tephra cones in northwestern British Columbia, Canada. The explosive to effusive eruption also formed a 32 km long sequence of Fe-rich Mg-poor basanite-trachy-basalt lavas covering ~40 [km.sup.2]. Oral histories of the Nisga'a Nation report that the eruption may have caused as many as 2000 fatalities. The actual eruption date and question of whether there was one or multiple eruptive episodes in the 14th and 18th centuries are, as of yet, unresolved. New radiocarbon dating of wood charcoal from immediately beneath vent-proximal tephra deposits and complementary age information suggest an eruption in 1675-1778 CE (95.4% probability) was responsible for the formation of the tephra cone. New paleomagnetic and geochemical data from the tephra cone and lava flows suggest there is, in fact, no statistically significant difference in time between the explosive and effusive deposits and that they formed during a single eruptive episode. Key words: Tseax volcano, lava flow, tephra cone, paleomagnetism, radiocarbon dating, geochemistry. Une eruption volcanique recente a eu lieu au volcan Tseax, qui a produit une serie de cones de tephra dans le nord-ouest de la Colombie-Britannique (Canada). L'eruption explosive a effusive a egalement produit une sequence longue de 32 km de laves de basanite-trachybasalte riches en Fe et pauvres en Mg couvrant ~40 [km.sup.2]. Des recits oraux de la Nation Nisga'a mentionnent que l'eruption pourrait avoir cause jusqu'a 2000 morts. Les questions du moment exact de l'eruption et a savoir si elle comportait un seul ou plusieurs episodes eruptifs aux [14.sup.e] et [18.sup.e] siecles demeurent a ce jour sans reponses. De nouveaux resultats de datation au carbone radioactif de charbon de bois preleve immediatement sous des depots de tephra proximaux a la cheminee et de l'information temporelle complementaire indiqueraient qu'une eruption vers 1675-1778 EC (probabilite de 95,4 %) est a l'origine de la formation du cone de tephra. De nouvelles donnees paleomagnetiques et geochimiques sur le cone de tephra et les coulees de lave donnent a penser qu'il n'y a en fait aucune difference statistiquement significative dans le temps entre les depots explosifs et effusifs et qu'ils se sont formes durant un seul episode eruptif. [Traduit par la Redaction] Mots-cles: volcan Tseax, coulee de lave, cone de tephra, paleomagnetisme, datation au carbone radioactif, geochimie., Introduction Tseax volcano situated in northwestern British Columbia, Canada (55.11085[degrees]N, 128.89944[degrees]W), comprises several small tephra cones and a 32 km long basanite-trachybasalt lava (e.g., Hanson 1923; Sutherland Brown 1969). The [...]

Magnetic and gravity inversions are used to create 2D or 3D models of the magnetic susceptibility and density, respectively, using potential field data. Unconstrained inversions generate an output based on mathematical constraints imposed by the inversion algorithm. Constrained inversions integrate lithological, structural, and petrophysical information in the inversion process to produce more geologically meaningful results. This study analyses the validity of this assertion in the context of the NSERC-CMIC Mineral Exploration Footprints project. Unconstrained and constrained geophysical inversions were computed for three mining sites: a gold site (Canadian Malartic, Quebec), a copper site (Highland Valley, British Columbia), and a uranium site (Millennium--McArthur River, Saskatchewan). After initially computing unconstrained inversions, constrained inversions were developed using physical property measurements, which directly link geophysics to geology, and lithological boundaries extracted from an interpreted geological model. While each derived geological model is consistent with the geophysical data, each site exhibited some magnetic complexity that confounded the inversion. The gold site includes regions with a strong magnetic signature that masks the more weakly magnetic zone, thereby hiding the magnetic signature associated with the ore body. Initial unconstrained inversions for the copper site yielded solutions with invalid depth extent. A consistency between the constrained model and the geological model is reached with iterative changes to the depth extent of the model. At the uranium site, the observed magnetic signal is weak, but the inversion provided some insights that could be interpreted in terms of an already known complexly folded geological model. Des inversions magnetiques et gravimetriques sont utilisees pour creer des modeles 2D et 3D de la susceptibilite magnetique et de la densite, respectivement, en utilisant des donnees de champ potentiel. Les inversions non contraintes generent un extrant base sur des contraintes mathematiques imposees par le logarithme d'inversion. Les inversions contraintes integrent de l'information lithologique, structurale et petrophysique au processus d'inversion pour produire des resultats plus signifiants sur le plan geologique. Nous analysons la validite de cette affirmation dans le contexte du projet Empreintes (Footprints) de l'exploration miniere du CRSNG et du CCIM. Des inversions geophysiques non contraintes et contraintes sont calculees pour trois sites miniers, soit un site aurifere (Canadian Malartic, Quebec), un site cuprifere (Highland Valley, Colombie-Britannique) et un site uranifere (Millennium--McArthur River, Saskatchewan). Apres le calcul initial d'inversions non contraintes, des inversions contraintes sont obtenues en utilisant des mesures de proprietes physiques qui relient directement la geophysique a lageologie, et des limites lithologiques extraites d'un modele geologique interprete. Si chaque modele geologique obtenu concorde avec les donnees geophysiques, tous les sites presentent une certaine complexite magnetique qui complique l'inversion. Le site aurifere comprend des regions a forte signature magnetique qui masque la zone plus faiblement magnetique, cachant ainsi la signature magnetique associee au corps mineralise. Les inversions non contraintes initiales pour le site cuprifere donnent des solutions caracterisees par un domaine de profondeur invalide. La coherence du modele contraint et du modele geologique est obtenue par des changements iteratifs au domaine de profondeur du modele. Au site uranifere, le signal magnetique observe est faible, mais l'inversion fournit certains renseignements dont l'interpretation pourrait concorder avec un modele geologique existant caracterise par un plissement complexe. [Traduit par la Redaction], Introduction Geophysical inversion methods considered in this paper are mathematical techniques developed to create geophysical models in 2D and 3D of the subsurface from airborne, ground, or borehole geophysical survey [...]

Slipstream Slump, a well-preserved 3 km wide sedimentary failure from the frontal ridge of the Cascadia accretionary wedge 85 km off Vancouver Island, Canada, was sampled during Canadian Coast Guard Ship (CCGS) John P. Tully cruise 2008007PGC along a transect of five piston cores. Shipboard sediment analysis and physical property logging revealed 12 turbidites interbedded with thick hemipelagic sediments overlying the slumped glacial diamict. Despite the different sedimentary setting, atop the abyssal plain fan, this record is similar in number and age to the sequence of turbidites sampled farther to the south from channel systems along the Cascadia Subduction Zone, with no extra turbidites present in this local record. Given the regional physiographic and tectonic setting, megathrust earthquake shaking is the most likely trigger for both the initial slumping and subsequent turbidity currents, with sediments sourced exclusively from the exposed slump face of the frontal ridge. Planktonic foraminifera picked from the resedimented diamict of the underlying main slump have a disordered cluster of [sup.14]C ages between 12.8 and 14.5 ka BP. For the post-slump stratigraphy, an event-free depth scale is defined by removing the turbidite sediment intervals and using the hemipelagic sediments. Nine [sup.14]C dates from the most foraminifera-rich intervals define a nearly constant hemipelagic sedimentation rate of 0.021 cm/year. The combined age model is defined using only planktonic foraminiferal dates and Bayesian analysis with a Poisson-process sedimentation model. The age model of ongoing hemipelagic sedimentation is strengthened by physical property correlations from Slipstream events to the turbidites for the Barkley Canyon site 40 km south. Additional modelling addressed the possibilities of seabed erosion or loss and basal erosion beneath turbidites. Neither of these approaches achieves a modern seabed age when applying the commonly used regional marine [sup.14]C reservoir age of 800 years (marine reservoir correction ΔR = 400 years). Rather, the top of the core appears to be 400 years in the future. A younger marine reservoir age of 400 years (ΔR = 0 years) brings the top to the present and produces better correlations with the nearby Effingham Inlet paleo-earthquake chronology based only on terrestrial carbon requiring no reservoir correction. The high-resolution dating and facies analysis of Slipstream Slump in this isolated slope basin setting demonstrates that this is also a useful type of sedimentary target for sampling the paleoseismic record in addition to the more studied turbidites from submarine canyon and channel systems. The first 10 turbidites at Slipstream Slump were deposited between 10.8 and 6.6 ka BP, after which the system became sediment starved and only two more turbidites were deposited. The recurrence interval for the inferred frequent early Holocene megathrust earthquakes is 460 ± 140 years, compatible with other estimates of paleoseismic megathrust earthquake occurrence rates along the subduction zone. L'affaissement Slipstream, un glissement sedimentaire bien conserve, d'une largeur de 3 km, de la crete frontale du prisme accretionnaire Cascadia, a 85 km de l'ile de Vancouver, Canada, a ete echantillonne par le Navire de la Garde cotiere canadienne (NGCC) John P. Tully lors de sa campagne 2008007PGC le long d'un transect de cinq carottages par piston. La diagraphie des proprietes physiques et les analyses des sediments abord du navire ont revele 12 turbidites interstratifiees avec d'epais sediments hemipelagiques qui recouvraient le diamicton glaciaire affaisse. Malgre l'environnement sedimentaire different, au-dessus de la plaine abyssale alluviale, ce profil geologique est semblable en nombre et en age a la sequence de turbidites echantillonnee plus au sud a partir des systemes de chenaux le long de la zone de subduction Cascadia, sans turbidites supplementaires presentes dans ce profil local. Etant donne l'environnement physiographique et tectonique regional, les secousses de mega-seismes constituent le declencheur le plus probable pour l'affaissement initial et les courants de turbidite subsequents, les sediments provenant exclusivement de la face exposee de l'affaissement de la crete frontale. Les foraminiferes planctoniques cueillis des diamictons redeposes de l'affaissement principal sous-jacent forment une grappe desordonnee d'ages [sup.14]C variant entre 12,8 et 14,5 ka BP. En ce qui concerne la stratigraphie apres l'affaissement, une echelle de profondeur sans evenement a ete definie en retirant les intervalles de sediments de turbidites et en utilisant les sediments hemipelagiques. Neuf datations [sup.14]C des intervalles les plus riches en foraminiferes definissent un taux de sedimentation hemipelagique presque constant de 0,021 cm/an. Le modele des ages combines est defini en utilisant uniquement des datations de foraminiferes planctoniques et une analyse bayesienne avec un modele de sedimentation par processus de Poisson. Le modele de l'age de la sedimentation hemipelagique en cours est renforce par les correlations des proprietes physiques des evenements Slipstream avec les celles des turbidites du site canyon Barkley, a 40 km au sud. Des modelisations additionnelles ont traite des possibilites d'une erosion du plancher oceanique ou d'une perte et d'une erosion de la base sous les turbidites. Aucune de ces approches produit un age modele pour le plancher oceanique lorsque l'age [sup.14]C, de 800 ans (correction de reservoir marin ΔR = 400 ans), frequemment utilise, est applique au reservoir regional marin. Plutot, le sommet de la carotte semble etre 400 ans dans l'avenir. Un age plus jeune de reservoir marin, soit 400 ans (ΔR = 0 an) amene le sommet au moment present et produit de meilleures correlations avec la chronologie paleo-seismique du passage Effingham a proximite, en se basant uniquement sur le carbone terrestre qui ne necessite aucune correction de reservoir. La datation a haute resolution et l'analyse du facies de l'affaissement Slipstream dans cet environnement de bassin de pente isole demontre que c'est aussi un type de cible sedimentaire utile pour echantillonner le profil paleo-sismique, en plus des turbidites, plus etudiees, des canyons sous-marins et des systemes de chenaux. Les 10 premieres turbidites El l'affaissement Slipstream ont ete deposees entre 10,8 et 6,6 ka BP, apres quoi le systeme a manque de sediments et seulement deux autres turbidites ont ete deposees. L'intervalle de recurrence pour les mega-seismes, juges frequents a l'Holocene precoce, est de 460 [+ or -] 140 ans, ce qui concorde avec d'autres estimations du taux d'occurrence de paleo-mega-seismes le long de la zone de subduction. [Traduit par la Redaction], Introduction The Pacific coast of Canada off Vancouver Island is prone to periodic large megathrust earthquakes due to its location at the northern end of the Cascadia Subduction Zone. There [...]

Rock physical properties provide the link between geophysical surveys and their geological interpretation. The British Columbia rock physical properties database, compiled by the Geological Survey of Canada, now comprises 3876 values of density, 930 values of electric resistivity, 12 356 values of magnetic susceptibility, and 2576 values of magnetic remanence and Koenigsberger ratio. The measurements are linked to sample locations, lithologies, and geological formation or unit. Maps, histograms, and biplots are used to demonstrate useful links between lithology and physical properties, and serve as a background for future rock physical properties studies. As expected, density and resistivity are controlled mostly by porosity and mineralogy. Magnetic susceptibility has a bimodal distribution (maxima at 4 x [10.sup.-4] and 2 x [10.sup.-2] SI) controlled by magnetite concentration. Magnetic remanence is shown to be more important than usually considered in magnetic survey analysis, with Koenigsberger ratios greater than unity in 42% of the samples. A case study of the Chilcotin Group basalts is highlighted, as they form a significant barrier to mineral exploration in central British Columbia. These rocks are magnetically distinct from other basalts in British Columbia, distinguished by magnetic susceptibilities having a range of values concentrated around 3 x [10.sup.-3] SI and falling in the valley between the two susceptibility modes. These basalts are also characterized by very high Koenigsberger ratios (96% above unity), probably caused by a preponderance of fine-grained single-domain magnetite. The database provides a wealth of petrophysical properties that can help constrain analysis of several types of geophysical surveys and, in particular, modelling of anomalies in the quest to determine the three-dimensional distribution of rock units. Les proprietes physiques des roches constituent le lien entre les leves geophysiques et leur interpretation geologique. La base de donnees de proprietes physiques des roches de la Colombie-Britannique, compilee par la Commission geologique du Canada, comprend maintenant 3876 valeurs de densite, 930 valeurs de resistivite electrique, 12 356 valeurs de susceptibilite magnetique et 2576 valeurs de remanence magnetique et de rapport de Koenigsberger. Les mesures sont reliees aux sites d'echantillonnage, aux lithologies et aux formations ou unites geologiques. Des cartes, des histogrammes et des diagrammes de double projection sont utilises pour demontrer des liens utiles entre la lithologie et les proprietes physiques; ils servent aussi en tant que cadre de reference pour de futures etudes des proprietes physiques des roches. Tel qu'attendu, la densite et la resistivite sont controlees principalement par la porosite et la mineralogie. La susceptibilite magnetique a une distribution bimodale (maximums a 4 x [10.sup.-4] and 2 x [10.sup.-2] SM) controlee par la concentration en magnetite. La remanence magnetique est demontree etre plus importante que normalement consideree dans les analyses de leves magnetiques avec des rapports de Koenigsberger superieurs a l'unite dans 42 % des echantillons. Une etude de cas des basaltes du Groupe de Chilcotin est mise de l'avant, puisque ces basaltes restreignent fortement l'exploration minerale au centre de la Colombie-Britannique. Ces roches ont un magnetisme distinct des autres basaltes de la Colombie-Britannique; elles ont en effet des susceptibilites magnetiques dont la plage des valeurs est concentree autour de 3 x [10.sup.-3] SM, tombant donc dans le creux entre les deux modes de susceptibilite. Ces basaltes sont aussi caracterises par des rapports de Koenigsberger tres eleves (96 % superieurs a l'unite), probablement en raison de la preponderance de magnetite a grain fin a domaine unique. La base de donnees fournit un tresor de proprietes petrophysiques qui peuvent aider a encadrer l'analyse de plusieurs types de leves geophysiques, particulierement la modelisation des anomalies, dans le but de determiner la distribution idimensionnelle des unites rocheuses. [Traduit par la Redaction], Introduction Geophysical surveys provide a powerful tool for probing three-dimensional distributions of geological units and structures, which otherwise are only revealed through surface or drill-core mapping. They are particularly important [...]

Annually laminated sediments from the anoxic inner basin of Effingham Inlet, Pacific coast of Vancouver Island, British Columbia, Canada, yield a high-resolution 42 m paleoenvironmental record, from the present to about 14 ka [sup.14]C BP (17 ka cal BP). A new age model, based on 68 radiocarbon dates from twigs and small plant material, from the 40 m core MD02-2494 and 2 m freeze cores from the surface, is anchored by the Mazama Ash and varve counting. A Poisson-process sedimentation model is used, applying a new method to determine the Poisson k value, giving a realistic age model compatible with the multi-proxy core data. Twenty-one 'seismites', which are lithofacies in the Effingham cores that may be representative of seismically triggered mass-wasting events, are identified and dated precisely, then compared with the chronology of the deep-sea turbidite record farther south in the Cascadia Subduction Zone (CSZ), to determine if regional sediment disturbances can be identified. With 16 proposed correlations, Effingham seismite ages are 169 ± 206 years older than turbidite ages estimated largely by radiocarbon analysis of foraminifera in hemipelagic deposits. Des sediments lamines annuels provenant du bassin interieur anoxique d'Effingham Inlet, sur la cote Pacifique de l'lle de Vancouver, Colombie Britannique, Canada, ont fourni un enregistrement paleoenvironnemental a haute resolution de 42 m, depuis le present jusqu'a il y a environ 14 ka [sup.14]C avant le present (17 000 annees calendrier avant le present). Un nouveau modele d'age, base sur 68 datations radiocarbone effectuees sur des brindilles et de la matiere vegetale de petites plantes provenant de la carotte MD02-2494 (d'une longueur de 40 m) et de carottes gelees de 2 m, prelevees a la surface, est fixe par la cendre Mazama et le comptage des varves. Un modele de sedimentation a ete utilise, base sur un processus de Poisson; il applique une nouvelle methode pour determiner la valeur k de Poisson et donne ainsi un modele d'age realiste compatible avec les donnees provenant de plusieurs carottes. Vingt et une seismites, qui constituent des lithofacies dans les carottes Effingham et qui pourraient representer des evenements de mouvements de masse declenches par des seismes, sont identifiees et datees avec precision. Les seismites sont ensuite comparees a la chronologie des donnees sedimentaires enregistrees dans des turbidites en eau profonde situes plus au sud dans la zone de subduction Cascadia, afin de determiner si les perturbations aux sediments regionaux peuvent etre identifiees. Avec 16 correlations proposees, les seismites Effingham sont plus vieilles de 169 [+ or -] 206 ans que les turbidites, dont les ages ont ete en general estimes par des analyses radiocarbone des foraminiferes dans les depots hemipelagiques. [Traduit par la Redaction], Introduction Central to the interpretation of event stratigraphy is the development of an accurate age model which defines the age of sediment deposition as a function of the depths of [...]

The Tintina Trench in west-central Yukon has preserved an extensive record of late Cenozoic preglacial, glacial, and interglacial deposits. These deposits comprise multiple sequences of tills, outwash, loesses, and paleosols. The sediments that were laid down directly by ice (tills) are of both local (montane) and regional (Cordilleran) provenance. The Tintina Trench area was impacted repeatedly by montane ice from the southern Ogilvie Mountains to the northwest (2500 m above sea level (asl)), and also repeatedly along its southern extent by Cordilleran ice from the Selwyn Mountains to the east (2759 m asl), the latter forming the continental divide in this region. We report here the magnetostratigraphy of three sections: Rock Creek (64°13'N, 139°07'W), West Fifteenmile River (64°29'N, 139°55'W), and East Fifteenmile River (64°23'N, 139°48'W). The majority of the units identified at these sections record late Pliocene to mid-Pleistocene glaciations, although relatively thin surficial sequences of late middle Pleistocene to late Pleistocene loesses and tills are present as well. Of the 11 units described in the Tintina Trench, seven have normal polarity, three have reversed polarity, and one has an undefined polarity. These units span about 3.0 million years. It appears that most of the polarity chrons and subchrons of the late Cenozoic are present and that the sequence of six reversals record at least 10 glaciations (three in the Brunhes Chron and seven in the Matuyama Chron), and 11 interglaciations (four in the Brunhes Chron and seven in the Matuyama Chron). The interglacials are recorded as either paleosols or unconformities between glacial or loess units having opposite polarity. While not all Matuyama Chron glacial and interglacial cycles recorded in marine isotopic records are seen on land, the terrestrial records found in the Tintina Trench have thus far proven to be the most complete in terms of the polarity record. While no absolute ages were obtained from the sediments in the trench, the extensive polarity sequence constrains the timing of glaciations to a considerably greater degree than was previously possible for this region. The magnetostratigraphy of the trench sites are compared with the glacial, glaciofluvial, and loessic deposits at the nearby Klondike River valley and Fort Selkirk sites, central Yukon, where tephras and basalts provide absolute ages, and stratigraphic units contain an extensive late Cenozoic climate proxy for northwestern North America (eastern Beringia). In this study, we present new paleomagnetic polarity data and establish a magneto-lithostratigraphy describing preglacial, glacial, and interglacial deposits in the Tintina Trench. These deposits are referred to as the West Tintina Trench Allogroup and provide a broad framework for establishing a paleoclimate record for the northern Canadian Cordillera. Le sillon de Tintina dans le centre ouest du Yukon a preserve un enregistrement complet de depots preglaciaires, glaciaires et interglaciaires datant du Cenozolque. Ces depots comprennent de multiples sequences de tills, de depots fluvio-glaciaires, de loess et de paleosols. Les sediments qui ont ete deposes directement par la glace (tills) ont une provenance locale (subalpine) et regionale (de la Cordillere). Le sillon de Tintina a subi l'impact de la glace de la Cordillere de nombreuses fois; cette glace provenait des monts Ogilvie meridionaux situees au nord-ouest (2500 m au-dessus du niveau de la mer (ASL)). Il a aussi subi l'impact repete le long de son etendue sud par de la glace de la Cordillere provenant des monts Selwyn a l'est (2759 m ASL); ces monts forment la ligne continentale de partage des eaux dans cette region. Nous rapportons ici la magneetostratigraphie de trois sections : Rock Creek (RC) (64[grados]13'N, 139[grados]07'O), West Fifteenmile River (WFR) (64[grados]29'N, 139[grados]55'O) et East Fifteenmile River (EFR) (64[grados]23'N, 139[grados]48'O). La plupart des unites identifiees a ces sections enregistrent des glaciations datant du Pliocene tardif au Pleistocene moyen, bien qu'il subsiste encore en surface des sequences relativement minces de loss et de tills datant du Pleistocene moyen tardif au Pleistocene tardif. Des 11 unitesdecrites dans le sillon de Tintina, sept ont une polarite normale, trois ont une polarite inversee et la polarite d'une unite; est indefinie. Ces unites couvrent environ 3,0 millions d'annees. Il semble que la plupart des chrons et des sous- chrons polaires du Cenozolque tardif sont presents et que la sequence de six inversions documente au moins 10 glaciations (trois dans le chron Brunhes et sept dans le chron Matuyama) et 11 interglaciations (quatre dans le chron Brunhes et sept dans le chron Matuyama). Les interglaciaires sont enregistres en tant que paleosols ou discordances entre les unites glaciaires ou de loss a polarite; opposee. Bien que les cycles glaciaires et interglaciaires du chron Matuyama enregistres dans les donnees isotopiques marines n'aient pas ete; tous vus sur le continent, les enregistrements terrestres trouves dans le sillon de Tintina se sont averesetre les plus complets en termes d'enregistrement de polarite. Bien qu'aucun age absolu n'ait ete obtenu des sediments du sillon, la sequence extensive de polarites encadre beaucoup plus le moment des glaciations qu'il n'eetait possible auparavant pour cette region. La magnetostratigraphie des sites du sillon est compareea celle des depots glaciaires, fluvio-glaciaires et loessiques des sites avoisinants de la vallee de la riviere Klondike et de Fort Selkirk, dans le centre du Yukon, oil des tephras et des basaltes fournissent des ages absolus et les unites stratigraphiques contiennent beaucoup de donneees indirectes du climat au Cenozolque tardif pour le nord-ouest de l'Ameerique du Nord (Beringie orientale). Dans cette etude, nous presentons de nouvelles donneees de polarites paleeomagneetiques et nous eetablissons une lithostratigraphie magnetique decrivant les depots glaciaires et interglaciaires dans le sillon. Ces depots sont nommees l'Allogroupe du sillon de Tintina occidental et ils fournissent un cadre geneeral pour eetablir des donneees paleoclimatiques pour le nord de la Cordillere canadienne. [Traduit par la Redaction], Introduction The terrestrial glacial record of northwestern North America is one of the most extensive of any preserved worldwide. This record was left by Cordilleran, montane, continental, and plateau glaciers. [...]

Paleomagnetic secular variation and a hiatus defined by two tephra layers confirm that tens of floods from Glacial Lake Missoula, Montana, entered Washington's Yakima and Walla Walla Valleys during the last glaciation. In these valleys, the field evidence for hiatuses between floods is commonly subtle. However, paleomagnetic remanence directions from waterlaid silt beds in three sections of rhythmically bedded flood deposits at Zillah, Touchet, and Burlingame Canyon display consistent secular variation that correlates serially both within and between sections. The secular variation may further correlate with paleomagnetic data from Fish Lake, Oregon, and Mono Lake, California, for the interval 12,000-17,000 [sup.14]C yr B.P. Deposits of two successive floods are separated by two tephras derived from Mount St. Helens, Washington. The tephras differ in age by decades, indicating that a period at least this long separated two successive floods. The beds produced by these two floods are similar to all of the 40 beds in the slack-water sediment sequence, suggesting that the sequence is a product of tens of floods spanning a period of perhaps a few thousand years. Keywords: Glacial Lake Missoula, jokulhlaups, tephra, paleomagnetic secular variation.

The Late Cretaceous Carmacks Group, a thick subaerial volcanic succession that once covered much of southwest Yukon, was deposited on an uplifted terrane and is divisible into a lower fragmental unit and an upper flood basalt unit. Coeval hydrothermal activity resulted in widespread alteration and gold mineralization. The lavas are shoshonites, enriched in large ion lithophile and light rare earth elements, but depleted in high field strength elements. Ankaramitic absarokite flows in the upper Carmacks Group range up to 15 wt% MgO, requiring a high liquidus temperature (1400 [degrees] C at 1 bar, dry). High [K.sub.2]O contents (> 3%) of these magnesian lavas indicate that the potassic character of the volcanic suite was established in the mantle. Although previously interpreted as subduction related, the Carmacks Group was erupted during a Cordilleran-wide magmatic lull and lacks coeval calc-alkalic batholiths. The lavas are petrologically similar to plume-related Eocene to Pliocene potassic lavas of the western United States. New paleomagnetic collections, combined with previous work, place the Carmacks Group 17.2 [degrees] [+ or -] 6.5 [degrees] (1900 [+ or -] 700 km) south of its present position relative to the craton during deposition, near the paleolocation of the Yellowstone hotspot. The spatial coincidence, similarity of tectonic setting, and lithologic similarity of the Carmacks Group and Yellowstone volcanic successions suggest that the Carmacks Group is the 70 Ma effusion of the Yellowstone hot-spot. Subsequent northward displacement of the Carmacks Group is attributed to coupling with the Kula plate. Correlation of the Carmacks Group and the Yellowstone hotspot fixes the paleolatitude and the paleolongitude of the terranes of the northern Intermontane belt at 70 Ma.

We evaluated imaging alteration associated with porphyry copper style mineralisation using aeromagnetic data in the Highland Valley Copper (HVC) district as part of the Natural Sciences and Engineering Research Council of Canada (NSERC) and Canada Mining Innovation Council (CMIC) Footprints project. The first step of the investigation involved construction of a 3D geological model based on surface and regional information, and petrophysical data of the host rocks. From the petrophysics, it was observed that hydrothermal alteration affected the magnetic susceptibility of some batholithic host rocks at HVC, and low susceptibility values were associated with the altered rocks. From this observation, an alteration index was devised, differencing, based on the magnetic susceptibility, between unaltered and altered batholith host rocks. Three-dimensional inversions were conducted using geological constraints obtained from surface and borehole geology as well as physical property data. Then the alteration index methodology was tested using 3D inversions and residual models within five overlapping blocks covering portions of the batholith. The indices inferred from inverted susceptibility were compared to surface mapped alteration. For some felsic facies of the batholith, the alteration index derived from inverted magnetic susceptibility corresponds to the mapped alteration and delineates the known porphyry Cu system district. However, it is not true for others where alteration was less magnetite-destructive and more focused in structures. [ABSTRACT FROM AUTHOR]

As mineral exploration seeks deeper targets, there will be a greater reliance on geophysical data and a better understanding of the geological meaning of the responses will be required, and this must be achieved with less geological control from drilling. Also, exploring based on the mineral system concept requires particular understanding of geophysical responses associated with altered rocks. Where petrophysical datasets of adequate sample size and measurement quality are available, physical properties show complex variations, reflecting the combined effects of various geological processes. Large datasets, analysed as populations, are required to understand the variations. We recommend the display of petrophysical data as frequency histograms because the nature of the data distribution is easily seen with this form of display. A petrophysical dataset commonly contains a combination of overlapping sub‐populations, influenced by different geological factors. To understand the geological controls on physical properties in hard rock environments, it is necessary to analyse the petrophysical data not only in terms of the properties of different rock types. It is also necessary to consider the effects of processes such as alteration, weathering, metamorphism and strain, and variables such as porosity and stratigraphy. To address this complexity requires that much more supporting geological information be acquired than in current practice. The widespread availability of field portable instruments means quantitative geochemical and mineralogical data can now be readily acquired, making it unnecessary to rely primarily on categorical rock classification schemes. The petrophysical data can be combined with geochemical, petrological and mineralogical data to derive explanations for observed physical property variations based not only on rigorous rock classification methods, but also in combination with quantitative estimates of alteration and weathering. To understand how geological processes will affect different physical properties, it is useful to define three end‐member forms of behaviour. Bulk behaviour depends on the physical properties of the dominant mineral components. Density and, to a lesser extent, seismic velocity show such behaviour. Grain and texture behaviour occur when minor components of the rock are the dominate controls on its physical properties. Grain size and shape control grain properties, and for texture properties the relative positions of these grains are also important. Magnetic and electrical properties behave in this fashion. Thinking in terms of how geological processes change the key characteristics of the major and minor mineralogical components allows the resulting changes in physical properties to be understood and anticipated. [ABSTRACT FROM AUTHOR]

Magnetic Fabric: Methods and Applications Edited by F. Martin-Hernandez, C. Luneburg, C. Aubourg and M. Jackson The Geological Society of London Special Publication No. 238, 2004 ISBN-13: 978-1-86239-170-3 Price 135.00 [...]

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The Upper Cretaceous Ventura Member of the Goat Wall unit in the southern Methow block of southern British Columbia and northern Washington State holds a syntectonic magnetization. Eight new sites from Manning Park in British Columbia give a mean direction of D = 27.5[degrees], I = 60.1[degrees], k = 304.7, [[alpha].sub.95] = 3.2[degrees] after optimal partial tilt correction. Of five groups of bedded sites from farther south in the basin reported by Bazard et al. [1990], four have a syntectonic remanence with a direction similar to what we observe. The exception is one group which has optimal concentration of remanence directions on >100% untilting and an abherent direction which must be rejected. Combining the accepted sites, the optimal differential syntilting direction is D = 11.8[degrees], I = 61.5[degrees], k = 39.3, [[alpha].sub.95] = 3.4[degrees] (N = 47), giving a mean pole of 79.8[degrees]N, 359.2[degrees]E, K = 19.5, and [A.sub.95] = 4.8[degrees]. The age of the remagnetization is constrained to be between 88 and 80 Ma. Compared to cratonic North America, this result indicates that the southern Methow block was displaced from the south by 1800 [+ or -] 500 km, meaning it lay south of the Siena Nevada subduction zone but well north of other paleomagnetically constrained Cretaceous rock units from the Insular superterrane, including correlative strata of the Mount Tatlow area in the northern Methow block. Among several possibilities to reconcile this discrepancy, the most plausible has the whole Methow block translated coherently but with the southern Methow block strata remagnetized during transit. INDEX TERMS: 1525 Geomagnetism and Paleomagnetism: Paleomagnetism applied to tectonics (regional, global); 8102 Tectonophysics: Continental contractional orogenic belts; 9350 Information Related to Geographic Region: North America; 9609 Information Related to Geologic Time: Mesozoic; 1533 Geomagnetism and Paleomagnetism: Remagnetization; KEYWORDS: paleomagnetism, orogenic belts, North America, Mesozoic, remagnetization