the relationship between the hawaiites and basalts of the itcha volcanic complex central british columbia volumetrically subordinate alkaline mafic lava flows form a late capping stage over the earlier felsic lavas that form the shield of the itcha volcanic complex ivc of the anahim volcanic belt avb in central british columbia b c the mafic capping stage of the ivc is dominated by hawaiites which are the earliest of the mafic lavas and are succeeded by alkali olivine basalts aob and then by basanites the alkali olivine basalts can be subdivided into high intermediate and low mgo aob groups all of which share similar hfse ratios e g nb zr with the hawaiites high al contents and sr zr ratios indicate that hawaiites and fe rich evolved aob were derived from primitive aob parental magmas by crystal fractionation of a wehrlitic assemblage at pressures on the order of 8 to 10 kbar high si and low fe contents indicate that the majority of the evolved aob lavas however do not represent an intermediate stage in the liquid line of descent to hawaiites but were most likely produced by gabbroic fractionation from primitive aob magmas at relatively low pressures the parental magmas of the majority of these lavas were distinct from those of the observed high mgo basalts having higher hfse contents and being more si under saturated the high al high sr zr signature of high pressure fractionation of a clinopyroxene dominated assemblage in the ivc is shared by hawaiites of other alkaline volcanic suites of the canadian cordillera such as the edziza volcanic complex in northern b c and appears to be a feature of hawaiites in many localities including hawaii and iceland viscosities calculated for both high and low pressure crystal fractionation models suggest that aphyric hawaiites are residual liquids escaped from a wehrlitic crystalline network at elevated pressures possibly at the base of the crust

the rainbow range british columbia a miocene peralkaline shield volcano abstract the rainbow range is a late miocene shield volcano 30 km diameter 370 km 3 whose stratiform flanks surround a complex central zone over a 2 m y interval extrusion of low viscosity silicic peralkaline lavas and minor basaltic lavas built up the gently sloping 5 8 flanks forming a shield volcano rather than a composite cone comenditic trachytes are the lowest flows exposed on the north flank of the volcano thin mugearite flows rest unconformably on the comenditic trachytes comendites unconformably overlie the mugearites and account for at least 75 of the volume of flows within the flank zone these lavas are distinguished from the comenditic trachytes by lower al 2 o 3 13 higher total iron as fe 2 o 3 7 and extremely depleted sr 1 10 ppm and ba 10 100 ppm strontium isotopic studies combined with petrologic data suggest that rainbow range lavas originated from alkali basalt magma trapped in an intracrustal magma chamber and tapped at several intervals after it underwent crystal fractionation a best fit mathematical model for the origin of the suite involves step wise derivation of the lavas in the order hawaiite mugeartite comenditic trachyte comendite with the major phases precipitating in the order olivine clinopyroxene plagioclase iron titanium oxides and alkali feldspar

early cretaceous uplift in the ellsworth mountains of west antarctica apatite fission track analysis of samples covering a 4 2 kilometer vertical section from the western flank of vinson massif antarctica s highest mountain indicates that the ellsworth mountains were uplifted by 4 kilometers or more during the early cretaceous following the initial separation of east and west gondwana and accompanying the opening of the weddell sea relief of at least 1 8 kilometers has persisted in the ellsworth mountains since the early cretaceous and a maximum of 3 kilometers of uplift has occurred since that time

autochthonous v accreted terrane development of continental margins a revised in situ tectonic history of the antarctic peninsula the allochthonous terrane accretion model previously proposed for the geological development of the antarctic peninsula continental margin arc is reviewed in light of recent data and the geology is reinterpreted as having evolved as an in situ continental arc this is based upon the following factors 1 the presence of early palaeozoic basement and stratigraphic correlation of sequences between the autochthonous and previously proposed allochthonous terranes 2 isotopic evidence for similar deep crustal structure across the different terranes 3 ocean island basalt magmas and deep marine sedimentary rocks formed during continental margin extension within the previously proposed accretionary wedge sequence i e not formed against an active oceanic arc 4 the distribution of magnetic susceptibility measurements and aeromagnetic data locating the palaeo subduction zone along the west of the peninsula 5 a lack of clear palaeomagnetic distinction between the terranes the following alternative tectonic history is proposed 1 amalgamation and persistence of gondwana 2 subsequent silicic large igneous province magmatism and extension 3 development and history of andean subduction until its cessation in the cenozoic a number of features in the antarctic peninsula correlate with those of other circum pacific margins supporting a global evaluation of allochthonous v autochthonous margin development to aid our understanding of crustal growth mechanisms

stratigraphy and geochemistry of the itcha volcanic complex central british columbia the itcha volcanic complex is the youngest and easternmost felsic shield volcano of the anahim volcanic belt of central british columbia the main body of the shield erupted over an area of 300 km2 forming itcha mountain and mount downton volcanism associated with the itcha shield extended 20 km south to the satah mountain area where lavas erupted along a north northwest south southeast fault system and covered an additional area of 250 km2 the itcha volcanic complex is characterized by a bimodal population of volcanic rocks which are dominated by felsic lavas there were two stages of volcanism i an early felsic shield building stage dominated by felsic lavas ranging in composition from phonolite to minor quartz normative trachytes which erupted as flows domes and pyroclastic deposits to form a low angled shield and ii a late mafic capping stage which comprises a thin veneer of hawaiite and more primitive mafic lavas ranging in composition from alkali olivine basalt to basanite the late

late gondwanide tectonic rotations within gondwanaland geologic and paleomagnetic evidence from the ellsworth whitmore mountains crustal block of west antarctica and from the falkland islands of south america indicates that tectonic displacement of major portions of the samfrau geosyncline occurred after the early mesozoic gondwanide folding but prior to seafloor spreading comparison with continental deformation in the collisional alpine himalayan belt offers a mechanism for the displacements while supporting the hypothesis that the gondwanian orogeny resulted from collision of a fore arc and magmatic arc terrane with the panthalassic margin of the gondwana craton during closure of a marginal basin the displacements had major consequences for the ensuing evolution of the south atlantic weddell sea ross sea region

new paleomagnetic data from the antarctic peninsula and their tectonic implications new paleomagnetic data presented here further constrain the relative motion of the antarctic peninsula relative to east antarctica during the mesozoic development of the southern ocean basins the antarctic peninsula ap is one of four crustal blocks that define west antarctica the others being the ellsworth whitmore mountains ewm thurston island eights coast ti and marie byrd land mbl a jurassic pole 155ma 124 e 64 s a95 7 1 n 10 vgps was obtained from the ap block which suggests that the ap block rotated clockwise between 175 and 155 ma due to significant early opening in the weddell sea basin a new early cretaceous paleomagnetic pole 182 e 74s a95 5 9 n 6 site mean vgps indicates that the ap block was in or near to its present day position with respect to east antarctica by 130 ma between 155 and 130 ma counterclockwise rotation of the ap ti blocks together with the southward motion of east gondwanaland probably resulted in subduction of weddell sea ocean floor beneath the southern ap block and initiated the palmer land deformational event a 130 ma pole from the ti block requires clockwise rotation of the ti and possibly the ewm blocks between 130 and 110 ma producing sinsistral strike slip motion between the ewm block and east antarctica and dextral transpressional motion between the ti ewm blocks and the ap block new ap block 110 ma and 85 ma poles from this study 199 e 74 s a95 6 9 n 13 vgps 152 e 86 s a95 7 5 n 6 vgps respectively are similar to equivalent age poles from east antarctica and suggest little or no relative motion between the antarctic peninsula and east antarctica northern and southern antarctic peninsula mid cretaceous poles are very much alike suggesting that the s shape of the antarctic peninsula is not due to oroclinal bending since 110 ma

tuyas a descriptive genetic classification we present a descriptive genetic classification scheme and accompanying nomenclature for glaciovolcanic edifices herein defined as tuyas positive relief volcanoes having a morphology resulting from ice confinement during eruption and comprising a set of lithofacies reflecting direct interaction between magma and ice melt water the combinations of lithofacies within tuyas record the interplay between volcanic eruption and the attending glaciohydraulic conditions although tuyas can range in composition from basaltic to rhyolitic many of the characteristics diagnostic of glaciovolcanic environments are largely independent of lava composition e g edifice morphology columnar jointing patterns glass distributions pyroclast shapes our classification consolidates the diverse nomenclature resulting from early isolated contributions of geoscientists working mainly in iceland and canada and the nomenclature that has developed subsequently over the past 30 years tuya subtypes are first recognized on the basis of variations in edifice scale morphologies e g flat topped tuya then on the proportions of the essential lithofacies e g lava dominated flat topped tuya and lastly on magma composition e g basaltic lava dominated flat topped tuya these descriptive modifiers potentially supply additional genetic information and we show how the combination of edifice morphologies and lithofacies can be directly linked to general glaciohydraulic conditions we identify nine distinct glaciovolcanic model edifices that potentially result from the interplay between volcanism and glaciohydrology detailed studies of tuya types are critical for recovering paleo environmental information through geological time including ice sheet locations extents thicknesses and glaciohydraulics such paleo environmental information represents a new innovative underutilized resource for constraining global paleoclimate models

the 2007 nazko british columbia earthquake sequence injection of magma deep in the crust beneath the anahim volcanic belt on 9 october 2007 an unusual sequence of earthquakes began in central british columbia about 20 km west of the nazko cone the most recent circa 7200 yr volcanic center in the anahim volcanic belt within 25 hr eight earthquakes of magnitude 2 3 2 9 occurred in a region where no earthquakes had previously been recorded during the next three weeks more than 800 microearthquakes were located and many more detected most at a depth of 25 31 km and within a radius of about 5 km after about two months almost all activity ceased the clear p and s wave arrivals indicated that these were high frequency volcanic tectonic earthquakes and the b value of 1 9 that we calculated is anomalous for crustal earthquakes but consistent with volcanic related events analysis of receiver functions at a station immediately above the seismicity indicated a moho near 30 km depth precise relocation of the seismicity using a double difference method suggested a horizontal migration at the rate of about graphic with almost all events within the lowermost crust neither harmonic tremor nor long period events were observed however some spasmodic bursts were recorded and determined to be colocated with the earthquake hypocenters these observations are all very similar to a deep earthquake sequence recorded beneath lake tahoe california in 2003 2004 based on these remarkable similarities we interpret the nazko sequence as an indication of an injection of magma into the lower crust beneath the anahim volcanic belt this magma injection fractures rock producing high frequency volcanic tectonic earthquakes and spasmodic bursts

a palaeomagnetic study of jurassic rocks from the antarctic peninsula and its implications palaeomagnetic measurements have been made on rocks of jurassic age from the antarctic peninsula to determine whether there has been relative movement between the region and east antarctica samples were collected from the oscar ii coast and the cole peninsula reliable results have been obtained from jurassic dykes dated in one area at 174 5 ma from an acid pluton of approximately the same age and from lavas of middle jurassic age combining these results a position for the palaeopole was obtained at 48 s 238 e if a reconstruction of gondwana consistent with sea floor spreading palaeomagnetic and geological data is adopted such as that of norton sclater 1979 then a well determined position for the antarctic peninsula within the reconstruction is obtained space considerations suggest that during the jurassic the relative positions of the antarctic peninsula and the ellsworth block were not very different from the present day though palaeomagnetic measurements watts bramall 1981 indicates that the orientation was different

palaeointensity and palaeomagnetic study of cretaceous and palaeocene rocks from western antarctica summary a combined palaeodirectional and palaeointensity study of a representative collection of plutonic rocks from the antarctic peninsula batholith from the western part of the antarctic peninsula near the ukrainian antarctic base academik vernadsky were carried out petrographically the collection includes gabbros diorites and quartz diorites tonalities granodiorites and granites the ages of igneous complex emplacement vary from 50 to 117 ma with most of the rocks belonging to the cretaceous normal superchron the characteristic remanent magnetizations were isolated by stepwise thermal demagnetization over the temperature interval 440 590 c and their intensities amount to 95 per cent of the nrm the geographic positions of palaeopoles do not contradict the key poles of the antarctic peninsula between 90 and 60 ma a significant part of the collection was subjected to coe modified thellier palaeointensity experiments with the ptrm checks which yielded seven reliable palaeointensity determinations for seven different locations the obtained vdms are relatively low for all sites being on average about half of the present day vdm the analysis of available palaeointensity data for the cretaceous miocene and middle jurassic indicates the existence of strong correlations between the mean vdm and vdm scatter versus the rate of reversals however due to the shortage of data the correlations are not significant at the 5 per cent significance level

intraglacial volcanism in the wells gray clearwater volcanic field east central british columbia canada small volume subaerial subaqueous and subglacial basaltic eruptions occurred in the wells gray clearwater area during quaternary time part of this time significant thicknesses of glacial ice were present dating of intraglacial volcanic features corroborates other evidence of an early pleistocene cordilleran wide ice sheet of the intraglacial volcanoes investigated three were studied in detail and of these two probably erupted during the fraser glaciation 11 20 ka when maximum ice level exceeded 2100 m elevation major element and sulphur concentrations were measured in glass from the volcanoes to provide insight into vent conditions at the time of eruption hyalo ridge 2102 m elevation whole rock k ar age of 0 02 0 01 ma is a small volcanic edifice capped by lava flows with coherent pillowed lavas and interbedded hyaloclastite exposed over nearly 400 m altitude on its east flank low sulphur content 0 03 wt in pillow rim glasses indicates that the lavas are degassed it is interprete

tertiary and quaternary volcanism in the massif central france summary continental alkaline volcanics of the french massif central range in age from 65 ma to 3450 years bp and can be divided by geography and age into 20 separate areas there is no correlation between the age of an area and the geographic locality two magma series are present a silica undersaturated basanite tephrite phonolite series and an alkali basalt trachyandesite trachyte rhyolite series the evolved members of which are silica saturated rare nephelinites are also found primitive magmas of both main series may be derived from 2 20 partial melting of a mantle enriched with light rare earth elements with variable amounts of garnet in the residue nephelinites were formed from a source which was isotopically and chemically distinct fractional crystallization of observed phases accounts for the diversity of intermediate and evolved products amphibole fractionation in basalts at depth causes the trend towards silica saturation while alkali feldspar fractionation dominates the final stages of crystallization removal of small quantities of sphene and zircon explains trace element depletions in trachytes phonolites and rhyolites up to 30 crustal contamination has occurred in the evolved magmas but contamination is generally absent from their basaltic parents sr nd and pb isotope data indicate that the most probable contaminant is undepleted meta sedimentary granulite facies lower crust variations in sr and nd isotope ratios also constrain petrogenetic hypotheses such as amphibole fractionation and magma mixing mineralogical disequilibrium compositional banding and emulsification textures indicate extensive magma mixing some flows are homogeneous hybrid lavas in which only the unusual composition and rare mineral disequilibria indicate the mode of formation

crystal fractionation of the basalt comendite series of the mount edziza volcanic complex british columbia major and trace elements abstract the mount edziza volcanic complex in north central british columbia includes a group of overlapping basaltic shields salic composite volcanoes domes and small calderas that range in age from 7 5 ma to less than 2000 years b p the volcanic assemblage is chemically bimodal comprising voluminous alkali olivine basalt and hawaiite a salic suite of mainly peralkaline trachyte and comendite plus a relatively small volume of intermediate rocks trachybasalt tristanite mugearite benmoreite the complex is the product of five cycles of magmatic activity each of which began with alkali olivine basalt and culminated with the eruption of salic magma the regular chemical variation shown by almost 100 major and trace element analyses suggests a genetic lineage between the basic and salic members of each cycle least squares mathematical modelling indicates that the salic rocks trachyte and comendite have formed by fractionation of observed phenocryst and cumulate nodule mineral phases from a common alkali olivine basalt parent magma hawaiite is thought to be a cumulate rock formed by partial fractionation and feldspar accumulation within rising columns of primary alkali olivine basalt fractionation leading from alkali olivine basalt through trachybasalt and trachyte to comendite is believed to have taken place where primary basalt became trapped in large crustal reservoirs the early removal of olivine clinopyroxene and plagioclase leading to a trachytic residuum and subsequent fractionation of mainly alkali feldspar leading to the peralkaline end members is consistent with major and trace element variation and with isotopic and ree data the chemical diversity of the complex is attributed to its location over a zone of crustal extension where mantle derived basalt trapped in large high level reservoirs underwent prolonged fractionation

rb sr geochronology of graham land antarctica thirty five new rb sr whole rock and mineral isochrons are reported for igneous rocks from graham land magmatism was essentially continuous but peaks of activity occurred in early jurassic early cretaceous late cretaceous and eocene times the oldest dated granite is triassic 209 3 ma volcanic rocks were erupted throughout this history and some previously supposed upper jurassic volcanic rocks must be mid cretaceous or younger a change in magma type from uniformly acid to predominantly basic or intermediate combined with westward migration of the arc may be partly responsible for previously reported transverse geochemical variations there is a marked trend of decreasing initial 87 sr 86 sr ratio of the plutonic rocks with time from c 0 707 to 0 704 although volcanic rocks tend to have higher values this trend is ascribed to the waning influence of an as yet unidentified pre magmatic crustal basement due to increasing depth of magma generation close parallels to the observed age pattern imply continuity of the andean belt through southern south america into graham land back to early mesozoic times and are not compatible with reconstructions of gondwana in which the antarctic peninsula is located w of south america

interpretation of reconnaissance gravity and aeromagnetic surveys of the antarctic peninsula two dimensional models are presented for the crustal structure of the antarctic peninsula a mesozoic cenozoic magmatic arc on the southeast pacific margin the models are constrained by measured rock properties the west coast magnetic anomaly wcma is caused by a zone of mafic plutons which is 70 150 km wide and over 1500 km long in modelling of bouguer anomalies isostatic compensation is maintained by crustal thickness variations the model moho is deepest 35 km beneath the mid cretaceous axis of the arc and shallowest 20 km beneath a late cenozoic marginal basin positive isostatic residual gravity anomalies are associated with the wcma and a mafic terrane in the mesozoic accretionary complex three broad areas of consistent crustal structure are identified the junctions of these areas are sinistral fault zones which displace the curvilinear magnetic physiographic and gravity patterns by several tens of kilometers

subduction of diverging plates and the principles of slab window formation consumption of an ocean basin by subduction commonly brings a sea floor spreading ridge toward a deep sea trench if plate divergence and convergence continue after the ridge intersects the subduction zone a slab window forms between the subducted parts of the diverging oceanic plates producing anomalous thermal physical and chemical effects in the surrounding asthenospheric mantle in turn these conditions alter the tectonic and magmatic evolution of the overriding plate usually disturbing ordinary fore arc and arc regimes differential lithospheric stresses on opposite sides of the triple junction contribute to disturbances in the overriding plate anomalous magmatism from fore arc to back arc may be accompanied by fore arc metamorphism strike slip faulting uplift extension and in extreme cases rifting the shape and size of the window are controlled mainly by the pre subduction ridge transform trench configuration slab dip angles and vectors of plate convergence subducted ridge segments expand into windows whose margins approximately parallel the motion vectors between the triple junction and the subducting plates subducted transform faults continue to be active usually as oblique slip faults until the plates separate as transform faults subduct they become longer on the plate which occupies the acute angle between ridge and trench and shorter on the other plate trains of isolated windows produced by subduction of a segmented ridge transform system progressively expand during descent commonly merging together to form a composite slab window oblique subduction of a highly segmented ridge is likely to produce two or more fraternal slab windows one at each site of ridge trench intersection above a slab window arc volcanism diminishes and may be replaced by volcanism of mid ocean ridge or rift affinity the change in chemical character reflects various processes including elevated heat flow decreasing hydration of the upper mantle juxtaposition of supra and sub slab mantle reservoris asthenospheric upwelling and melting of the trailing plate edges if the slab window migrates the anomalous magmatic regime may be replaced by renewed arc volcanism identifying the effects of slab windows in ancient convergent margin assemblages requires an understanding of slab window principles and implications

a fixed sublithospheric source for the late neogene track of the yellowstone hotspot implications of the heise and picabo volcanic fields the heise and picabo volcanic fields of eastern idaho are part of the more extensive time transgressive yellowstone snake river plain hotspot track calderas associated with these two silicic volcanic fields are buried under 1 to 3 km of younger basalt so their locations and eruption record histories have been based on analysis of silicic units along the margins of the eastern snake river plain along with some limited geophysical data a 1 5 km borehole penetrating through basalt into underlying silicic rocks provides new data we used to reassess caldera locations and the timing of eruptions of these volcanic fields using these new caldera locations we calculate an extension adjusted rate of 2 35 cm yr for the north american plate over the last 6 66 m y and a velocity of 2 30 cm yr over the 10 27 m y recalculation of a previously determined plate velocity based migration of the deformation field surrounding the eastern snake river plain yields an extension adjusted rate of 2 38 0 21 cm yr these migration rates all fall within the previously published range of north american plate velocities of 2 2 0 8 cm yr 2 4 cm yr and 2 68 0 78 cm yr based on a global hot spot reference frame the consistency of these rates suggest that over the last 10 m y the yellowstone hot spot is fixed with respect to the motion of the north american plate and therefore consistent with a classical deep sourced hotspot model

late quaternary sea level changes and crustal movements in eastern england and eastern scotland an assessment of models of coastal evolution abstract the most comprehensive records of late quaternary coastal environmental change in eastern england and eastern scotland are obtained from the intercalated semi terrestrial peats and intertidal silts clays and sands that have accumulated in estuaries and coastal lowlands radiocarbon dated sea level index points from sites in north east england are evaluated with reference to first a conceptual model of coastal sedimentary environments and second a model of pollen assemblages in holocene coastal environments analysis of these new data indicates differential crustal uplift within north east england of ca 0 2 mm yr for the last 4000 to 5000 radiocarbon years this pattern of uplift is consistent with models of holocene crustal deformation in scotland and england the uplift is attributable to the combined effects of ice unloading and water loading

west antarctica in gondwanaland crustal blocks reconstruction and breakup processes abstract a combined bas usarp west antarctic tectonic project has provided new constraints on the crustal structure and geological evolution of west antarctica and its relationship to the rest of gondwanaland rb sr age dating has confirmed the presence of 1200 1000 m y old precambrian gneisses within west antarctica and aeromagnetic data have defined the extent of this precambrian crustal block paleomagnetic data have constrained the position of four of the five geologically distinctive west antarctic crustal blocks within gondwanaland they can be restored to their original position prior to breakup by 15 25 counter clockwise rotation with little or no relative displacement between the blocks a major period of within plate middle jurassic magmatism associated with an important crustal melting event that is clearly related to the breakup of the supercontinent has also been identified based on new isotopic and geochemical data

an early pleistocene proglacial succession in south central british columbia a succession of conglomerates sandstones siltstones and tills herein named the dog creek formation is sandwiched between flat lying basalts along dog creek lat 51 36 n long 122 02 122 12 w for about 15 km east of fraser river the sedimentary succession rests disconformably on underlying basaltic lavas herein referred to as the harpers creek formation which have yielded k ar dates of 1 3 2 9 ma and in one place on a glaciated surface carved in metavolcanic rocks of permian age the sedimentary succession is capped by basalt flows yielding k ar whole rock ages of 1 1 ma the occurrence of proglacial beds and a glaciated surface in south central british columbia 70 km away from any high mountains capable of supporting glaciers today testifies to a major glacial stage in early pleistocene time the record of sedimentation and volcanism sheds light on early incision of the nearby valley of fraser river

distribution nature and origin of neogene quaternary magmatism in the northern cordilleran volcanic province canada the northern cordilleran volcanic province encompasses a broad area of neogene to quaternary volcanism in northwestern british columbia the yukon territory and adjacent eastern alaska volcanic rocks of the northern cordilleran volcanic province range in age from 20 ma to ca 200 yr b p and are dominantly alkali olivine basalt and hawaiite a variety of more strongly alkaline rock types not commonly found in the north american cordillera are locally abundant in the northern cordilleran volcanic province these include nephelinite basanite and peralkaline phonolite trachyte and comendite the most mgo rich nephelinites basanites and alkaline basalts from throughout the northern cordilleran volcanic province show trace element abundances and isotopic compositions that are consistent with an asthenospheric source region similar to that for average oceanic island basalt and for post 5 ma alkaline basalts from the basin and range our petrologic observations help constrain the origin of northern cordilleran volcanic province magmatism as well as lithosphere changes between the four major basement terranes that underlie the province results from phase equilibria calculations and the spatial distributions of volcanic rock types and magmatic inclusions are more consistent with the existence of thicker lithosphere beneath stikinia which underlies the southern part of the northern cordilleran volcanic province than beneath the cache creek and yukon tanana terranes which underlie the northern part of the northern cordilleran volcanic province our results support a model for initiation of northern cordilleran volcanic province magmatism due to incipient rifting of the northern cordillera driven by changes in relative plate motion between the pacific and north american plates ca 15 10 ma

mesozoic evolution of west antarctica and the weddell sea basin new paleomagnetic constraints paleomagnetic data from the antarctic peninsula and our recent results from the ellsworth whitmore mountains block suggest that since the middle jurassic these two west antarctic blocks have undergone little relative movement and together have rotated relative to the east antarctic craton new data from lower cretaceous rocks from the thurston island region of west antarctica suggest that on the basis of paleomagnetic constraints the antarctic peninsula ellsworth whitmore mountains and thurston island blocks define a single entity which we call weddellia some motion between these blocks is possible within the limits of the paleomagnetic data between the middle jurassic and early cretaceous weddellia remained attached to west gondwanaland while east antarctica moved southward dextrally relative to weddellia from the early cretaceous to mid cretaceous weddellia rotated clockwise 30 and moved sinistrally approximately 2500 km relative to east antarctica to its present day position we suggest the early to mid cretaceous to be the time of the main if not initial opening of the weddell sea

new paleomagnetic and 40ar 39ar geochronological results for the south shetland islands west antarctica and their tectonic implications

pleistocene geology and geomorphology of the smoking hills upland and lower horton river arctic coast of mainland canada the smoking hills upland and lower horton river valley on the arctic coast of canada 300 km east of the mackenzie delta retain an extended record of subaerial erosion of nearly flat lying cretaceous shales this erosion led to the development of i a very gently sloping low relief upland surface early pleistocene ii a slightly steeper intermediate surface and iii younger steep valley walls terraces and broad valley bottoms no direct glacial contribution to any of these landforms can be recognized an early interglacial fluvial episode is recorded in plateau cap sediments suggestions of an early mid pleistocene or earlier glaciation overwhelming the smoking hills upland are found in i anomalies in drainage patterns ii disturbances in bedding believed to have been caused by ice thrusting and iii local occurrences of diamictons later probably early wisconsinan glaciation left meltwater channels in peripheral areas the late wisconsinan ice sheet did not reach the smoking hi

crustal growth of the antarctic peninsula by accretion magmatism and extension a subduction accretion model incorporating new geophysical data is presented to explain the geology of the antarctic peninsula from late palaeozoic to cenozoic time according to the model the peninsula consists of overlapping accretionary magmatic and extensional regimes that were diachronous across the peninsula and have built the crust to its present form the crust which contains a small proportion of sialic basement was mainly formed by accretionary and magmatic processes and modified to its present shape by extension the gondwanide orogeny for the antarctic peninsula is interpreted in terms of the accretionary processes

the satah mountain and baldface mountain volcanic fields pleistocene hot spot volcanism in the anahim volcanic belt west central british columbia canada the satah mountain and baldface mountain volcanic fields smvf bmvf comprise more than three dozen small volcanic centers and erosional remnants thereof these fields are located in the chilcotin highland of west central british columbia canada and are spatially associated with the anahim volcanic belt avb a linear feature of alkaline to peralkaline plutonic and volcanic centers of miocene to holocene ages the avb has been postulated to be the track of a hot spot passing beneath the westward moving cordilleran lithosphere we test the avb hot spot model by applying whole rock 40ar 39ar geochronology n 24 and geochemistry whole rock chemical compositions of volcanic rock samples n 59 from these two fields suggest a strong geochemical affinity with the nearby itcha range shield volcano however smvf and bmvf centers are mostly small in volume 1 km3 and differ in composition from one another even where they are in close spatial proximity trace element and ree patterns of mafic avb lavas are similar to ocean island basalts oib suggesting a mantle source for these lavas the age ranges for the smvf n 11 2 21 to 1 43 ma and bmvf n 7 3 91 to 0 91 ma are largely coeval with the itcha range the distribution of volcanoes in these two volcanic fields is potentially consistent with the postulated avb hot spot track eruption rates in the smvf were high enough to build an elongated ridge that deviates from the e w trend of the avb by almost 90 this deviation might reflect the mechanisms and processes facilitating magma generation and ascent through the lithosphere in this tectonically complex region and may also indicate interaction of the potential hot spot with pre existing fracture systems in vicinity of the itcha range

nazko cone a quaternary volcano in the eastern anahim belt nazko cone located in central british columbia at the eastern end of the anahim volcanic belt is the product of at least three episodes of quaternary volcanic activity an eroded pleistocene subaerial flow at the base of the pile is overlain by a subglacial mound of hyaloclastite that is in turn partly covered by a younger composite pyroclastic cone and associated lava flows a whole rock k ar date of 0 34 0 03 ma on the oldest flow is consistent with a hotspot model for the anahim belt and implies absolute late neogene motion of 2 6 cm year for north america the hyaloclastite mound was erupted beneath the cordilleran ice sheet during the late pleistocene perhaps during the fraser glaciation 25 000 10 000 years bp radiocarbon dates from peat above and below nazko tephra in a bog near the cone suggest that the volcano last erupted about 7200 years bp nazko basalt has 10 15 normative nepheline and is classified as basanite this is significantly more undersaturated than basalts farther west in

late cenozoic volcanic rocks of the clearwater wells gray area british columbia the clearwater wells gray area of east central british columbia includes a succession of late cenozoic alkali olivine basalt flows that lie east of the extensive chilcotin lavas and define the eastern end of the anahim volcanic belt the rocks are petrographically similar to but less altered than the chilcotin basalts the volcanic activity spanned at least two episodes of glacial advance and produced both subaerial flows and a subaqueous facies comprising pillow lava pillow breccia and tuff breccia locally intercalated with fluvial gravels and sand four morphological assemblages have been recognized an early glacial assemblage characterized by tuyalike forms gives k ar dates of 0 27 3 5 ma these circular features are surrounded by a deeply dissected valley filling assemblage of subaerial and minor subaqueous flows and tuff breccia that rest locally on lag gravel and till subaerial flows in this assemblage give k ar dates of 0 15 0 56 ma whitehorse bluffs a volcanic centre composed o

the margins of the south orkney microcontinent marine geophysical studies of the south orkney microcontinent show two intersecting tectonic fabrics of different age the older fabric is oriented e w and represents the magmatic arc and fore arc created by subduction related processes at the pacific margin during the mesozoic the younger oriented nnw to sse comprises half graben related to e w rifting at both the eastern and western margins of the microcontinent both fabrics provide constraints on a 40 ma reconstruction which shows a highly systematic distribution of arc and fore arc provinces and points to the importance of e w strike slip in regional tectonic evolution the rifting apparently of oligocene age is associated with back arc extension which created the oceanic basins to either side of the microcontinent this strongly supports the existence of subduction with the same orientation as the present south sandwich arc as far back as 35 or 40 ma

the structure of west antarctica from geophysical studies the subglacial topographical and geological configurations of west antarctica and the boundary zone between east and west antarctica are poorly known yet they are crucial to our understanding of the tectonic evolution of the pacific proto atlantic margin of gondwanaland preliminary results are presented of simultaneous airborne radio echo and magnetic sounding of 106 km2 of west antarctica including detailed maps of ice sheet and bedrock surfaces magnetics radio echo and other available geophysical data suggest interpretations of sub glacial geology

paleomagnetism and tectonics of the south shetland islands and the northern antarctic peninsula new paleomagnetic data presented here from 61 sites in paleozoic mesozoic and cenozoic igneous and sedimentary rocks from the antarctic peninsula and the south shetland islands constrain the relative motion of the antarctic peninsula since the mid cretaceous and allow the quantification of tectonic rotation between the different blocks recognized within the area paleozoic and jurassic results failed the fold test and suggest an important remagnetization in the area the similarity between these results and those obtained from cretaceous intrusives indicates a mid cretaceous age for the remagnetization the paleopoles obtained for the different blocks and for different ages suggest that there is no relative rotation among them these combined results allow us to obtain a cretaceous 90 ma and paleocene 60 ma paleopole these paleopoles document a very low apparent polar wandering of the antarctic peninsula for the last 100 ma

imaging a mantle plume beneath the french massif central abstract teleseismic tomography is now widely recognised as a very powerful tool for studying the velocity structure of the lithosphere asthenosphere system on both local and regional scales a prime objective is to identify velocity perturbations and to constrain their location and intensity here we use the arrival times of p waves from teleseismic events recorded by a dense network of mobile and permanent stations within the french massif central to derive a 3d p velocity model for an area of 425 km 300 km combined with knowledge of the basement geology of the area and the timing and geochemical characteristics of tertiary quaternary volcanic activity we consider that the velocity model provides unequivocal evidence for the ascent of a thermally and chemically anomalous mantle plume beneath the massif central during tertiary times our model is consistent with regional bouger gravity anomalies documented patterns of neogene recent uplift and deformation fabrics within lithosphere derived mantle xenoliths exhumed by the volcanics

the neogene alert bay volcanic belt of northern vancouver island canada descending plate edge volcanism in the arc trench gap abstract the alert bay volcanic belt trends northeasterly across northern vancouver island coincident with the trace of the subducted juan de fuca explorer plate edge volcanism began in the west at brooks peninsula about 8 ma ago but occurred in most centers 3 5 1 ma ago there is a suggestion of eastward migration of activity and shift from basalt to dacite or rhyolite with time most of the volcanism was coincident with a time of rapid changes in the geometry of subduction as inferred from offshore magnetic patterns and with a hiatus in mainland cascade volcanic arc activity geometry and chronometry suggest this is a descending plate edge volcanic belt where disruption of steady state plate consumption patterns triggered magma genesis chemically the rocks are quite variable with divergent fractionation trends one trend resembles that of mull hebrides with a plagiophyric basalt of transitional alkaline subalkaline mildly tholeiitic and aluminous character which differentiated to clinopyroxene andesite and eventually to tholeiitic rhyolite and mildly tholeiitic calc alkaline dacite both of k poor magma type the other trend is like the cascades with aluminous aphyric calc alkaline basalt hornblende and or hypersthene andesite and k poor dacite this divergent character is also evident in ba rb nb and zr fractionation trends major and trace element discriminant diagrams generally identify the basalts as within plate types the 87 sr 86 sr isotope ratio is relatively low averaging 0 70325 and shows no trend with rock type or differentiation series oxygen in the entire suite is relatively heavy 18 o averaging 7 1 even the basalts are 18 o enriched oxygen shows no trend with degree of hydration rock type or series these isotopic and chemical data are compatible with minor crustal contamination of mafic primary magmas followed by fractional crystallization under different oxidation and hydration conditions

new paleomagnetic data from thurston island implications for the tectonics of west antarctica and weddell sea opening palcomagnetic data from three west antarctic crustal blocks antarctic peninsula ap thurston island eights coast ti and the ellsworth whitmore mountains ewm indicate that there has been motion between the individual blocks and motion relative to east antarctica during the mesozoic a triassic palcomagnetic pole from the ti block 116oe 61os a95 19 4o n 3 vgps appears to indicate that the block has rotated 90 o relative to east antarctica between 230 ma and 110 ma our previously reported middle jurassic palcomagnetic pole from the ewm block indicates that a 90 o rotation relative to east antarctica occurred sometime between the cambrian and 175 ma we believe that the 90 o counterclockwise ewm rotation occurred between 220 ma and 175 ma related to the development of post gondwanide orogeny shear zones the motion of the ap ti and ewm blocks appears to be linked during the mid to late mesozoic to three major events in the evolution of the southem ocean basins opening in the mozambique somali weddell sea basins may have produced major counterclockwise rotation of the ti block with respect to east antarctica between the jurassic and early cretaceous based on new late jurassic 145oe 64 5os a95 7o n 5 vgps poles we believe that the ti rotation as well as deformation in the southem ap block was caused by collision and shearing of the ewm block against the other two as the ewm block moved southward with east antarctica an early cretaceous palcomagnetic pole 232oe 49os a95 7 9o n 5 vgps from the ti block requires that between the early and mid cretaceous there was clockwise rotation with respect to east antarctica of the ap ti ewm blocks an entity we call weddellia a change in the opening history of the weddell sea basin caused by initiation of spreading in the south atlantic ocean basin at 130 ma probably started weddellia s clockwise rotation two new 110 and 90 ma poles from the ti block 210oe 73 os a95 7 6o n 7 vgps and 161oe 81os a95 3 9o n 18 vgps respectively are similartoequivalent agepoles from the apblock and east antarctica and indicate that weddellia was at ornearits present day position with respect to east antarctica by 110 ma this corresponds to a time of major plate reorganizaion in the south atlantic and southeast indian oceans based on both the new ti palcomagnetic data and previously reported data from marie byrd land mbl dextral shearing would be expected to have occurred between mbl and weddellia since the mid cretaceous pine island bay the area between the ti and mbl blocks marks a fundamental and complex tectonic boundary in west antarctica that we propose has largely been a zone of transcurrent sheafing

volcanology and petrology of mathews tuya northern british columbia canada glaciovolcanic constraints on interpretations of the 0 730 ma cordilleran paleoclimate petrological volcanological and geochronological data collected at mathews tuya together provide constraints on paleoclimate conditions during formation of the edifice the basaltic tuya was produced via pleistocene glaciovolcanism in northern british columbia canada and is located within the tuya volcanic field 59 195 n 130 434 w which is part of the northern cordilleran volcanic province ncvp the edifice comprises a variety of lithofacies including columnar jointed lava pillow lava massive dikes and volcaniclastic rocks collectively these deposits record the transition from an explosive subaqueous to an effusive subaerial eruption environment dominated by pleistocene ice as is typical for tuyas the volcaniclastic facies record multiple fragmentation processes including explosive quench and mechanical fragmentation all samples from mathews tuya are olivine plagioclase porphyritic alkali olivine basalts they are mineralogically and geochemically similar to nearby glaciovolcanic centers from the southeastern part of the tuya volcanic field e g ash mountain south tuya tuya butte as well as the dominant ncvp rock type crystallization scenarios calculated with melts account for variations between whole rock and glass compositions via low pressure fractionation the presence of olivine microphenocrysts and the absence of pyroxene phenocrysts constrain initial crystallization pressures to less than 0 6 gpa the eruption of mathews tuya occurred between 0 718 0 054 ma and 0 742 0 081 ma based on 40ar 39ar geochronology weighted mean age of 0 730 ma the age determinations provide the first firm documentation for large 700 m thick pre fraser wisconsin glaciers in north central british columbia 0 730 ma and correlate in age with glaciovolcanic deposits in russia e g komatsu et al geomorph 88 352 366 2007 and with marine isotopic evidence for large global ice volumes 0 730 ma

morphology of the explorer juan de fuca slab edge in northern cascadia imaging plate capture at a ridge trench transform triple junction the explorer plate is a young oceanic microplate that accommodates relative motion between the pacific juan de fuca and north america plates near northern vancouver island canada the northern limit of explorer plate juan de fuca subduction and the fate of the slab in northern cascadia are poorly understood we use passive teleseismic recordings from an array of polaris broadband seismic stations to image crustal and upper mantle structure beneath northern vancouver island into the interior of british columbia a clear signature of subducted material extends northeast from the brooks peninsula at crustal levels beneath georgia strait and the mainland deep into the mantle to 300 km depth complexity in slab morphology results from juan de fuca ridge subduction and toroidal flow around the slab edge in agreement with geophysical and geological data we propose a tectonic model for the explorer plate in which its separation from the juan de fuca plate is caused by the thermomechanical erosion of the slab edge and slab thinning at shallow levels both of which slow convergence with north america and lead eventually to plate capture

constraints on past plate and mantle motion from new ages for the hawaiian emperor seamount chain 1 estimates of the relative motion between the hawaiian and louisville hot spots have consequences for understanding the role and character of deep pacific mantle return flow the relative motion between these primary hot spots can be inferred by comparing the age records for their seamount trails we report 40ar 39ar ages for 18 lavas from 10 seamounts along the hawaiian emperor seamount chain hesc showing that volcanism started in the sharp portion of the hawaiian emperor bend heb at 47 5 ma and continued for 5 myr the slope of the along track distance from the currently active hawaiian hot spot plotted versus age is constant 57 2 km myr between 57 and 25 ma in the central 1900 km of the seamount chain including the heb this model predicts an age for the oldest emperor seamounts that matches published ages implying that a linear age distance relationship might extend back to at least 82 ma in contrast hawaiian age progression was much faster since at least 15 ma and possibly as early as 27 ma linear age distance relations for the hawaii emperor and louisville seamount chains predict 300 km overall hot spot relative motion between 80 and 47 5 ma in broad agreement with numerical models of plumes in a convecting mantle and paleomagnetic data we show that a change in hot spot relative motion may also have occurred between 55 ma and 50 ma we interpret this change in hot spot motion as evidence that the heb reflects a combination of hot spot and plate motion changes driven by the same plate mantle reorganization

palaeomagnetic evidence for a displaced terrain in western antartica an outstanding problem in antarctic tectonics is the relationship between the east antarctic craton and the late palaeozoic cenozoic orogenic belts that comprise western antarctica1 antarctic peninsula ellsworth land eights coast and marie byrd land see fig 1a related problems are the place of western antarctica in reconstructions of gondwanaland and the evolution of the pacific margin of gondwanaland we present here palaeomagnetic data derived from folded cambrian argillites that crop out in the ellsworth mountains which were collected during the 1979 80 austral field season although the early palaeozoic palaeomagnetic field of gondwanaland is characterized by rapid apparent polar wander at the present level of coverage the data are consistent with rotation of the ellsworth mountain block with respect to the east antarctic craton as postulated by schopf2 this result supports a microplate nature of western antarctica

chronology of the peralkaline late cenozoic mount edziza volcanic complex northern british columbia canada the late cenozoic mount edziza volcanic complex covers an area of about 1 000 km 2 in north central british columbia 300 km east of the transcurrent boundary between the north american and pacific plates it is made up of a group of overlapping basaltic shields and intermediate to salic peralkaline composite domes flows and central volcanoes that are associated with extensional structures in the underlying basement new k ar and fission track dates 45 and rb sr and sr isotope analyses 12 from the mount edziza volcanic complex are reported the age dates are for the most part consistent with the stratigraphy and indicate that frequent eruptive activity occurred during the past 8 m y five major magmatic cycles each began with the eruption of basalt and culminated with the eruption of oversaturated peralkaline magma low 87 sr 86 sr initial ratios 0 7028 0 0001 indicate a mantle source for the basalts low sr contents and high 87 sr 86 sr ratios in the salic end members suggest that the oversaturated rocks were derived from the basaltic magma by crystal fractionation in crustal reservoirs rb sr isochrons suggest that residence times for the fractionating magma were about 0 7 to 1 m y early removal of large amounts of plagioclase followed by fractionation of potash feldspar can account for most of the observed petrological and isotopic relationships a few individual compositions and one suite of mainly intermediate samples contain anomalously large amounts of both 87 sr and radiogenic argon this indicates that contamination with crustal material and possibly mixing of parental basalt with partly fractionated magma from previous events may have produced the relatively small volume of intermediate rocks