sediment fluxes along the northeastern european margin inferring hydrological changes between 20 and 8 kyr abstract in an attempt to delineate past hydrographic conditions on the northeastern atlantic ocean margin along a latitudinal transect 37 63 n and link the eastern north atlantic ocean with the norwegian sea we have investigated planktonic and benthic stable oxygen isotopes as well as abundances and fluxes of planktonic and benthic foraminifera calcium carbonate organic carbon and detrital grains in six sediment cores studied by the enam project in our joint approach we focused on the major climatic events of the last 20 kyr that is the last glacial maximum lgm the heinrich event 1 h1 the younger dryas yd and the holocene climatic optimum hco simmax temperature estimates suggest glacial ssts off portugal similar to holocene values contradicting the proposed polar front position of climap project members 1976 besides increased planktonic and benthic foraminiferal fluxes in the norwegian sea and the faeroe islands cores point to nordic seas partially ice free with seasonally productive polynas multiple ird spikes observed at the faeroe islands and norwegian sea sites coeval with h1 suggest multiple pulses of the fenno scandian ice sheet at the same time the presence of ird on the portuguese margin cores confirms the southward influence of icebergs the increase in foraminiferal abundances and fluxes that parallel ird spikes at all sites suggests that times of increased productivity either succeed or occur simultaneously with the iceberg surges in the open north atlantic productivity conditions similar to those observed during h1 are also registered during the yd even though planktonic foraminiferal assemblages suggest less cold conditions

proterozoic continental volcanism in the belcher islands implications for the evolution of the circum ungava fold belt the belcher islands of eastern hudson bay expose a continuous sequence of early proterozoic volcanics and continental and shallow marine sedimentary rocks the volcanic rocks comprise two sequences of continental basalts the older eskimo formation and the younger flaherty formation the flows of both formations are composed of tholeiitic basalts mgo 9 wt which in each formation are divided into two groups based on the concentrations of high field strength elements both groups of eskimo flows have fractionated rare earth element ree patterns with negative nb anomalies relative to eskimo flows the flaherty flows have flatter ree profiles and higher nb contents the chemical variations observed in the eskimo flows are consistent with an assimilation fractional crystallization process involving a lower continental crust contaminant the major and most trace element trends of the flaherty flows are attributed to gabbroic crystal fractionation of two parental liquids which are interpreted to

a recipe for microcontinent formation accreted slivers of continental margins are common in the geologic record but the processes that lead to their formation are poorly understood we observe an association of plume related microcontinent isolation and subsequent long term asymmetries in oceanic crustal accretion based on four recent examples the seychelles in the indian ocean jan mayen in the norwegian greenland sea and the east tasman plateau and the gilbert seamount complex in the tasman sea these microcontinents formed by rerifting of a young continental margin 25 m y old in the vicinity of a mantle plume stem followed by asymmetric seafloor spreading two dimensional numerical stochastic basin modeling suggests that a yield strength minimum along the landward edge of a rifted margin thermally enhanced by heating from a mantle plume may cause a spreading ridge to jump onto this zone of weakness this action isolates a passive margin segment the association of large igneous provinces and microcontinents should be useful for identifying similar events in the geological record

younger dryas style cooling events at glacial terminations i vi at odp site 658 associated benthic δ13c anomalies constrain meltwater hypothesis high resolution planktonic and epibenthic stable isotope records from ocean drilling program site 658 off northwest africa provide a basis for a detailed study of glacial terminations i vi during the last 650 000 years the duration of the terminations was about one half to one quarter of an orbital precession cycle 5800 10 700 years when its amplitude was high at low amplitudes the terminations lasted longer than half an obliquity cycle 29 000 years marked climatic rebounds similar to the younger dryas each with a duration of 1000 2500 years subdivided all six terminations into distinct steps a b and c important parts of the deglacial steps were as brief as 700 1000 years the speed of climatic change suggests that special associations existed between orbital forcing and inherent instability of the ice sheets in harmony with published models the more rapid pulses of glacial meltwater incursions to the northern north atlantic led to one or more brief short term shut downs of north atlantic deep water formation this process is reflected by pronounced benthic 13c minima that precisely coincide in most cases with the end of the 18o deglaciation steps and immediately terminate with succeeding younger dryas style cooling events thus we conclude that the rebounds resulted from a short term antecedent estuarine circulation regime in the north atlantic except for the younger dryas itself which succeeded the allerod with its well ventilated atlantic deepwater circulation and hence continues to be an enigma

the wathaman batholith an early proterozoic continental arc in the trans hudson orogenic belt canada the wathaman batholith a major element of the trans hudson orogenic belt in northern saskatchewan and manitoba has an exposed strike length of 900 km this immense body separates dominantly archean rae hearne provinces to the northwest from early proterozoic arc related terranes of the orogen to the southeast major and trace element data for 42 samples from the saskatchewan part of the batholith are similar to those of phanerozoic arc batboliths for example sierra nevada and andean batholiths data plotted on trace element discrimination diagrams show that the batholith has continental arc affinities u pb zircon geochronology of five main phase granite samples suggests that most of the batholith formed between 1865 and 1850 ma and is coeval with smaller plutons of the la rouge arc the main deformation of the batholith is limited by the 1830 ma age of a late granite sheet that crosscuts folds and foliation of main phase granites the 15 to 25 m y crystallization span of the batholith is similar to the histories of many more recent continental magmatic arcs

early partitioning of quebec microcontinent formation in the paleoproterozoic the distribution of allochthonous versus parautochthonous carbonate platforms combined with the timing of initial continental rift magmatism versus the timing of subsequent rifting of the continental margin provides tectonostratigraphic evidence for the formation and isolation of the oldest documented microcontinent perhaps as a consequence of the impingement of a mantle plume onto a cratonic margin during the paleoproterozoic initial rifting of the archean nucleus of north america in eastern canada is constrained to have been diachronous between 2 17 and 2 03 ga renewed rifting of a segment of the continental margin 292 to 120 m y later was accompanied by the emplacement of ultramafic layered sills the accumulation of komatiitic and alkalic basalts the deposition of banded iron formations and the isolation of a microcontinent and its 1 93 ga continental shelf succession subsequently accreted to the telescoped continental margin during collisional orogenesis

on the late pleistocene ocean geochemistry and circulation a box model of the atmosphere and ocean was developed to investigate how geochemical distributions extant during the late pleistocene may have come about the model simulates the regional distribution of calcium carbonate dissolution as well as the chemical oceanography and atmospheric co2 13c and radiocarbon if the downward biological flux of particulate carbon increases by a factor of 2 to 3 in the antarctic and if this increase is combined with a relative increase of the atlantic sector antarctic bottom water aabw versus north atlantic deep water nadw source ratio from 1 3 to about 2 1 then the model predicts several changes that seem to be recorded in the sedimentary record as follows 1 a global redistribution of nutrients and 12c from the intermediate to deep water takes place with the atlantic intermediate water phosphate decreasing 0 6 mole kg 1 and the 13c increasing 0 3 to 0 5 2 the dissolved oxygen level of the deep sea is reduced from an average of about 180 to 70 mole kg 1 but the intermediate water oxygen declines only a small amount 3 the decrease in intermediate water nutrient concentration results in lower average organic carbon and calcium carbonate production in the warm surface ocean 4 the atmospheric co2 decreases by 90 to 110 ppm 5 initially a global increase in calcium carbonate dissolution occurs which is followed by a relaxation toward better preservation than exists for the present ocean in the model in this paper the reduction of nadw by itself does not produce these effects rather the nutrient decrease that does occur is found mostly in north pacific intermediate water and the model atmospheric co2 decrease is only 10 to 30 ppm it is observed that 92 of the atmospheric co2 change takes place according to a 200 year time constant in the model this corresponds to the response time of the upper ocean and atmosphere to a change in the stationary state atmospheric pco2 thus according to this model the time lag between the nutrient based cause and the atmospheric co2 response is not expected to be particularly large

late pleistocene ice age scenarios based on observational evidence ice age scenarios for the last glacial interglacial cycle based on observations of boyle and keigwin concerning the north atlantic thermohaline circulation and of barnola et al concerning atmospheric co sub 2 variations derived from the vostok ice cores are herein analyzed northern hemisphere continental ice sheets are simulated with an energy balance model ebm that is asynchronously coupled to vertically integrated ice sheets models based on the glen flow law the ebm includes both a realistic land sea distribution and temperature albedo feedback and is driven with orbital variations of effective solar insolation with the addition of atmospheric co sub 2 and ocean heat flux variations but not in their absence a complete collapse is obtained for the eurasian ice sheet but not for the north american ice sheet we therefore suggest that further feedback mechanisms perhaps involving more accurate modeling of the dynamics of the mostly marine based laurentide complex appears necessary to explain termination i 96 refs 12 figs 2 tabs

geology u pb and pb pb geochronology of the lake harbour area southern baffin island implications for the paleoproterozoic tectonic evolution of northeastern laurentia geological and geochronological results of an investigation of the paleoproterozoic siliciclastic and carbonate supracrustal rocks of the lake harbour group lhg and surrounding tonalitic gneisses on southern baffin island are presented conventional u pb geochronology of monazite from rocks of the lhg suggest that penetrative deformation of these rocks occurred prior to or during peak metamorphic conditions at ca 1845 1840 ma conventional u pb zircon results indicate that much of the tonalitic gneiss ranges in age from to 1827 the tonalitic gneisses and lake harbour group units were tectonically imbricated by ca 1805 ma and are part of a southwest verging thrust belt interpreted from regional considerations to represent the northern continuation of the ungava orogen the present results indicate that current tectonic models for the evolution of northeastern laurentia that involve a dominantly archean southeastern rae province require revision it is proposed that much of the metaigneous mate

effects of earthquake mislocation on estimates of velocity structure abstract recently there have been several attempts at imaging slab structure using teleseismically observed travel times these times are a function of both event location and velocity structure teleseismically determined epicenters of subduction zone earthquakes which can occasionally be checked by comparison with accurately determined epicenters using local seismographs are systematically incorrect by as much as 40 km observed travel time residuals are calculated with respect to this incorrect location theoretical residuals on the other hand are usually calculated by initiating ray tracing at the teleseismically determined hypocenter in comparing the theoretical and observed times we make two errors first ray tracing should be initiated at the true location of the earthquake not at the location determined from teleseismic observations obviously this can rarely be done but fortunately this source of error produces only subtle differences in the theoretical residuals the second much more significant error results from the fact that the observed residuals are calculated with respect to a location that differs from the actual source whereas the theoretical residuals are calculated with respect to the same location used to initiate ray tracing this inconsistent handling of event locations produces profound differences in residuals which unless accounted for in the velocity determination could result in velocity estimates that are severely biased to demonstrate this we analyze travel time residuals from a small thrust earthquake in the central aleutians which produced impulsive teleseismic arrivals times from the local adak array provide tight constraints on its absolute location the isc location which is constrained primarily by teleseismic times is 40 km north of the local array location we compute residuals from the observed arrival times with respect to the local array and the teleseismic locations the residual distribution computed at the local array location is dominated by large average of 2 s negative residuals to the north as expected for a high velocity slab dipping to the north on the other hand the residual distribution computed at the teleseismic location has negative residuals averaging less than 0 5 s to the north and larger amplitude negative residuals to the south theoretical times are calculated by initiating ray tracing within a thermal model of the aleutian slab at the local array location when theoretical residuals are calculated with respect to the location used to initiate ray tracing these times agree well with observed times at the local array location the correlation coefficient is 0 84 however the correlation between these synthetic times and observed times at the teleseismic location drops to 0 04 this latter comparison is often used in the literature and can lead to biased estimates of velocity structure the former comparison is only possible when absolute event locations are known we suggest a technique to correct for this bias the procedure of calculating travel time residuals with respect to the hypocenter determined by least squares minimization of the residuals is equivalent to applying a specific high pass linear filter to the residuals mapped on the unit focal sphere in nearly all teleseismic problems this filter has been applied to the data to make a self consistent analysis we should also apply this filter to the synthetic times the correlation between filtered data and filtered synthetic for the aleutian event is 0 51

u pb geochronology of the eastern hall peninsula southern baffin island canada a northern link between the archean of west greenland and the paleoproterozoic torngat orogen of northern labrador abstract a corridor east of iqaluit that crosses several regionally significant lithologic boundaries has been mapped and sampled for geochronology zircon monazite and titanite were analysed by the isotope dilution u pb method to characterize igneous and metamorphic events and pb pb ages of detrital zircons were determined by laser ablation microprobe icp ms tonalitic gneisses at the eastern end of the corridor are archean with emplacement ages of 2920 8 6 2848 3 2844 6 5 and 2797 27 15 ma these samples record variable tectonothermal overprints at ca 2 77 ga and between 1844 and 1736 ma and may be related to similar rocks in west greenland a sequence of quartzites and semipelites associated with the archean gneisses contains exclusively archean detrital zircons and was metamorphosed at 1 88 ga in the central part of the corridor semipelitic and psammitic rocks contain almost exclusively paleoproterozoic detritus deposited after 1 93 0 05 ga and are interpreted as the northern continuation of the tasiuyak gneiss of the torngat orogen at the western end of the corridor monzogranites of the cumberland batholith intruded at 1869 9 3 1857 5 3 and 1850 3 2 ma the present data suggest that the north south nagssugtoqidian collision

paleomagnetism and u pb geochronology of diabase dyke swarms of minto block superior province quebec canada in the first collaborative study of paleomagnetism and precise u pb geochronology in the minto block of the superior province mafic dyke swarms with three widely divergent paleomagnetic signatures and isotopic ages have been identified the 2505 2 ma ptarmigan dykes trend north to northeast and have a virtual geomagnetic pole at 42 s 220 e similar to that of 2473 2446 ma matachewan dykes of the southern superior province the ca 2230 ma maguire dykes trend west to northwest and yield a paleopole at 9 s 267 e similar to those for 2216 8 4 ma senneterre dykes and 2217 2210 ma nipissing sills of the southern superior and southern provinces respectively the 2209 1 ma klotz dykes trend west northwest but do not carry a consistent magnetization direction finally 1998 2 ma minto dykes of west northwest to northwest trend identical in age to the 1998 ma 2 ma purtuniq ophiolite of the cape smith belt have a paleopole at 38 n 174 e the similarity of paleopoles for the ca 2 23 2 21 ga maguire

multi stage zircon and monazite growth revealed by depth profiling and in situ u pb geochronology resolving the paleoproterozoic tectonics of the trans hudson orogen on southeastern baffin island canada abstract understanding the growth and recrystallization of zircon and monazite during regional metamorphism is important for identifying the nature and timescales of tectonic processes in continental crust we present extensive in situ u pb monazite geochronology and u pb zircon depth profiling analyses from the mid crust exposed on southeastern baffin island canada in the internal zone of the paleoproterozoic trans hudson orogen in situ la icp ms u pb monazite dating on ten metasedimentary rocks totalling 300 analyses from different paleodepths reveals a dominant ca 1860 1820 ma monazite population that records regional mid amphibolite to granulite facies metamorphism with coeval fabric development subordinate monazite with ages between ca 1800 1750 ma are interpreted to have formed through dissolution reprecipitation the sims u pb depth profiling technique allowed dating of ca 1861 1833 ma zircon rims and less abundant ca 1800 1764 ma zircon rims that would be unresolvable using conventional spot dating trace element depth profiling via la icp ms shows that zircon rims typically have lower th u values higher hf contents and different ree compositions than zircon interiors zircon rim compositions are highly variable with no apparent link to u pb age and are interpreted as the combined result of metamorphic zircon growth and dissolution reprecipitation our data demonstrate that the timing of peak metamorphism is recorded by both monazite and zircon and that they are both sensitive to u pb age resetting at lower temperatures during late orogenic tectonism regional mid amphibolite to granulite facies metamorphism and east directed crustal shortening are bracketed between ca 1860 1820 ma these events are interpreted to result from the accretion of cratonic blocks in the internal zone of the trans hudson orogen the integrated geochronological approach on regionally distributed samples enabled us to document ca 1800 1750 ma monazite domains and zircon rims which we interpret to result from the terminal collision with the lower plate superior craton followed by late to post orogenic tectonothermal activity that continued for at least 70 myr after thermal peak this prolonged history is characteristic of the northeastern trans hudson orogen and implies that orogenic collapse may not be a hallmark of all hot thickened orogens

terrane accretion in the internal zone of the ungava orogen northern quebec part 1 tectonostratigraphic assemblages and their tectonic implications the tectonostratigraphic record of the ungava orogen contains evidence for the interaction of divergent transform and convergent plate boundaries over a 0 2 ga period in the early proterozoic three principal tectonic domains are recognized 1 autochthonous basement plutonic and supracrustal rocks of archean age 2 autochthonous and allochthonous sedimentary and volcanic units associated with a ca 1 99 1 92 ga rift to drift margin and 3 suspect crustal components of a ca 2 00 ga ophiolite and ca 1 90 1 83 ga island arc terrane domain 1 is the stratigraphic or structural basement to domains 2 and 3 field relationships indicate that domain 2 units were accumulated on or near domain 1 whereas no tectonostratigraphic linkages are known between domain 3 and the others accumulation of continent derived sedimentary rocks continental tholeiites komatiitic basalts and mafic lavas equivalent to modern normal mid oceanic ridge basalts n morb s in domain 2 is interpreted as recording continental rifting leading to the possible formation of oceanic crust pillowed basalts sheeted dykes and cumulate rocks suggest that the crustal portion of an ophiolite is preserved within domain 3 the ophiolite is interpreted as a suspect fragment of oceanic crust which may represent an earlier rifted portion of domain 2 possibly juxtaposed by transform plate boundary motions within domain 3 calc alkaline volcanic and plutonic units and sequences of volcanogenic and siliciclastic sedimentary rocks are thought to indicate the development of a magmatic arc and fore arc basin and thus the establishment of a convergent plate boundary the most internal lithotectonic packages in domain 3 are predominantly crystalline thrust sheets containing i an older intrusive suite interpreted as the plutonic foundation of the magmatic arc and ii younger plutons interpreted as the result of a reversal in subduction polarity the reversal may have followed attempted subduction of domains 1 and 2 beneath the upper plate arc assemblage of domain 3

dynamic sedimentation of paleoproterozoic continental margin iron formation labrador trough canada paleoenvironments and sequence stratigraphy abstract the paleoproterozoic sokoman formation ca 1 88 ga of the labrador trough eastern canada is a ca 100 m thick succession of interbedded iron formation and fine grained terrigenous clastic sedimentary rocks detailed examination of drill cores and outcrops indicates a dynamic paleoshelf where an oxygen stratified water column coastal upwelling of hydrothermally derived fe and si as well as tide and storm generated currents controlled lithofacies character vertical and lateral facies stacking patterns record deposition through two relative sea level cycles that produced seven distinct lithofacies comprising two unconformity bounded sequences sequence 1 reflects deposition of hematitic peritidal iron formation as deep as the upper shoreface sequence 2 is truncated by later erosion and encompasses the change to deeper water accumulation of magnetite and fe silicate rich iron formation the character and lateral distribution of redox sensitive facies indicate that iron formation accumulation was controlled as much by shelf hydraulics as oxygen levels the development of a suboxic surface ocean is interpreted to reflect photosynthetic oxygen production from a combination of peritidal stromatolites and cyanobacterial phytoplankton that flourished in nutrient rich upwelled waters offshore deposition of other continental margin iron formations also occurred on paleoproterozoic shelves that were favorably positioned for coastal upwelling variability between iron formations reflects intrinsic factors such as shelf profile fluvial contribution eolian input evaporation rates and coastal current systems which influenced upwelling dynamics and the delivery of fe si and nutrients aridity onshore was a primary depositional control since it governed the transport and type of diluting terrigenous clastics as well as evaporative precipitation along the coastline as in the phanerozoic unconformities and transgressive and maximum flooding surfaces frame iron formation sequences but with important differences the absence of trace and body fossils as well as lack of terrestrial vegetation can make the recognition of these surfaces difficult transgressive surfaces can also be easily mistaken for phanerozoic style maximum flooding surfaces since stratigraphic condensation was restricted to inboard environments during ravinement outboard the accumulation of fresh precipitates increased sedimentation to produce a maximum flooding surface not usually marked by a prominent depositional hiatus understanding these differences is essential for establishing an accurate sequence stratigraphic framework such context is the backdrop for properly interpreting the sedimentology oceanography microbial ecology and geochemistry of continental margin iron formations

oceanic mechanisms for amplification of the 23 000 year ice volume cycle situated adjacent to the largest northern hemispher ice sheets of the ice ages the mid latitude north atlantic ocean has an important role in the earth s climate history it provides a significant local source of moisture for the atmosphere and adjacent continents forms a corridor that guides moisture bearing storms northward from low latitudes and at times makes direct contact along its shorelines with continental ice masses evidence of major ice ocean air interactions involving the north atlantic during the last 250 000 years is summarized outflow of icebergs and meltwater initially driven by summer insolation over the ice sheets affects midlatitude ocean temperatures summer heat storage winter sea ice extent and global sea level these oceanic responses in turn influence the winter moisture flux back to the ice sheets as well as ablation of land ice by calving spectral data indicate that the oceanic moisture and sea level feedbacks in part controlled by glacial melt products amplify milankovitch insolation forcing of the volumetrically dominant mid latitude ice sheets at the 23 000 year precessional cycle

intraoceanic tectonics and the development of continental crust 1 92 1 84 ga evolution of the flin flon belt canada the flin flon belt is a complex amisk collage of distinct tectono stratigraphic assemblages that was brought together at a relatively early stage in the tectonic evolution of the paleoproterozoic trans hudson orogen four main tectono stratigraphic assemblage types are recognized isotopically juvenile oceanic arc 1 90 1 88 ga ocean floor ca 1 90 ga oceanic plateau ocean island and isotopically evolved arc 1 92 1 90 ga oceanic arc assemblages include tholeiitic calc alkaline and lesser shoshonitic volcanic and volcaniclastic rocks as well as turbidites and arc rift volcanic rocks the tectono stratigraphic assemblages were juxtaposed in an accretionary complex amisk collage at ca 1 88 1 87 ga probably as a result of arc arc collision collisions accretionary collage structures are largely obliterated by subsequent deformation and metamorphic events but can be inferred where cut by calc alkaline plutons related to a 1 866 1 838 ga arc coeval subaerial volcanism is recorded in ca 1 87 1 85 ga calc alkaline to shoshonitic volcaniclastic sequences unroofing of the accretionary collage development of a paleosol and deposition of continental alluvial fluvial sedimentary rocks occurred ca 1 85 1 84 ga coeval with the waning stages of post accretion arc magmatism stabilization of the flin flon belt by 1 85 1 84 ga as part of a philippines or japan like microcontinent is attributed to both tectonic and magmatic thickening of the lithosphere

trans hudson orogen of north america and himalaya karakoram tibetan orogen of asia structural and thermal characteristics of the lower and upper plates 1 the trans hudson orogen tho of north america and the himalaya karakoram tibetan orogen hkto of asia preserve a paleoproterozoic and cenozoic record respectively of continent continent collision that is notably similar in scale duration and character in tho the tectonothermal evolution of the lower plate involves 1 early thin skinned thrusting and barrovian metamorphism 2 out of sequence thrusting and high t metamorphism and 3 fluid localized reequilibration anatexis and leucogranite formation the crustal evolution of the indian lower plate in hkto involves 1 early subduction of continental crust to ultrahigh pressure uhp eclogite depths 2 regional barrovian metamorphism and 3 widespread high t metamorphism anatexis and leucogranite formation the shallow depths of the high t metamorphism in hkto are consistent with early to mid miocene ductile flow of an indian lower plate midcrustal channel from beneath the southern tibetan plateau to the greater himalaya melt weakening of the lower plate in tho is not observed at a similar scale probably due to the paucity of pelitic lithologies tectonothermal events in the upper plate of both orogens include precollisional accretion of crustal blocks emplacement of andean type plutonic suites and high t metamorphism syncollisional to postcollisional events include emplacement of garnet biotite muscovite leucogranites anatectic granites and sporadic metamorphism up to 90 myr following the onset of collision in tho comparing the type and duration of tectonothermal events for tho and hkto supports the notion of tectonic uniformitarianism for at least the later half of dated earth history and highlights the complementary nature of the rock record in an older exhumed orogen compared to one undergoing present day orogenesis

lithogeochemistry of volcano plutonic assemblages of the southern hanson lake block and southeastern glennie domain trans hudson orogen evidence for a single island arc complex this paper describes the geology geochemistry and age of two amphibolite facies volcano plutonic assemblages in the southern hanson lake block and southeastern glennie domain of the paleoproterozoic trans hudson orogen of east central saskatchewan the hanson lake assemblage comprises a mixed suite of subaqueous to subaerial dacitic to rhyolitic ca 1875 ma and intercalated minor mafic volcanic rocks overlain by greywackes similarly with modern oceanic island arcs the hanson lake assemblage shows evolution from primitive arc tholeiites to evolved calc alkaline arc rocks it is intruded by younger subvolcanic alkaline porphyries ca 1861 ma synvolcanic granitic plutons ca 1873 ma and the younger hanson lake pluton ca 1844 ma rocks of the northern lights assemblage are stratigraphically equivalent to the lower portion of the hanson lake assemblage and comprise tholeiitic arc pillowed mafic flows and felsic to intermediate volcaniclastic rocks and greywackes which can be traced as far west

review of crustal architecture and evolution in the ungava peninsula baffin island area connection to the lithoprobe ecsoot transect within the trans hudson orogen in northern quebec and southern baffin island parautochthonous archean basement and paleoproterozoic cover as well as allochthonous accreted paleoproterozoic units are exposed in a series of antiformal culminations and complementary synclinoria the parautochthonous rocks of the superior province margin and two assemblages of accreted paleoproterozoic units ungava and meta incognita crustal terranes define a basic tripartite crustal architecture which is characterized by polyphase deformation and metamorphic histories early structures and mineral assemblages are recognized in all three crustal components and shown to be temporally distinct the earlier structures and assemblages are overprinted by younger elements and mineral assemblages that are related to at least three regional contraction episodes and one thermal event that are common to all crustal components of the orogen close correlation of regional aeromagnetic data in northern quebec and southern baffin island with the three crustal components offers a first order approximation of the distribution of basement lithological units across hudson strait continuation of the aeromagnetic domains beneath ungava bay to northeastern quebec and northern labrador allows for the southward extrapolation of the tripartite trans hudson orogen crustal architecture into the lithoprobe eastern canadian shield onshore offshore transect area

u pb geochronology of detrital zircons in metasedimentary rocks from southern baffin island implications for the paleoproterozoic tectonic evolution of northeastern laurentia a geochronological investigation of metasedimentary rocks from southern baffin island using the geological survey of canada shrimp ii sensitive high resolution ion microprobe has characterized the ages of detrital zircon populations to determine their provenance bracket timing of deposition and distinguish potentially distinct sequences of rocks four lithologically and structurally distinct metasedimentary packages have been identified each appears to have been derived from a different source region in the structurally lowest package all analysed zircons are archean and 90 have ages between 2 83 and 2 63 ga these rocks are interpreted as the northernmost exposures of the paleoproterozoic povungnituk group of the cape smith belt northern quebec with detritus derived from the superior craton occupying the intermediate structural levels the most abundant supracrustal rocks on southern baffin island are siliciclastic and carbonate units of the lake harbour group and the tasiuyak paragneiss

a coupled climate ice sheet model applied to the quaternary ice ages a coupled climate ice sheet model is used in an attempt to simulate the northern hemispheric ice sheet volume variations implied by deep sea core records over the last several 105 years the climate model consists of a global one level diffusive energy balance equation with seasonal variation and land ocean contrast using a severely truncated series of associated legendre functions the ice sheet model uses a standard vertically averaged flow law and predicts ice thickness in a north south cross section bedrock deformation under the ice is included as linear flow confined to a thin asthenospheric channel the climate model is tuned to the modern climate and tested against results from atmospheric general circulation models for 18 000 and 10 000 yr b p it is suggested that the lack of sea ice variations causes the biggest departures from the gcm results the sensitivities to the orbital perturbations of the coupled model and of the ice sheet model alone with a prescribed snow budget distribution are examined some qualitative differences are found in the latitudinal forms of these sensitivities despite these differences however the two types of models produce similar ice age simulations over the last 700 thousand years in response to the orbital perturbations in particular fairly realistic simulations with strong 100 kyr cycles similar to those obtained by adding an ice calving mechanism to the ice sheet model in pollard 1982a are again produced in the coupled model

geochemistry of mafic magmas from the ungava orogen quebec canada and implications for mantle reservoir compositions at 2 0 ga the ungava orogen of northern quebec is one of the best preserved proterozoic mobile belts of the world recording 200 ma of plate divergence and convergence voluminous magmatism associated with rifting of the superior province basement 2 04 ga resulted in the development of a volcanic rift margin sequence and an ocean basin four distinct mafic magma suites were erupted 1 continental basalts eskimo formation western and central povungnituk group with moderate to high zr nb and negative nb anomalies which have interacted with the continental crust nd 2 0 ga from 7 4 2 mafic lavas from the flaherty formation eastern povungnituk group and some watts group lavas associated with passive margin rifting having slightly enriched isotopic signatures nd 2 0 ga 2 7 to 4 4 compared to the contemporaneous depleted mantle high nb y n and low zr nb ratios 4 4 and 8 9 respectively 3 a highly alkaline oib like suite nd 2 0 ga 2 3 to 3 2 nb y n 12 within the povungnituk group composed of nephelinites basanites and phonolites and 4 depleted mg rich basalts and komatiitic basalts nd 2 0 ga 4 5 to 5 5 with trace element characteristics of n morb but with higher fe and lower al than primitive morb chukotat group ottawa islands and some watts group samples the ocean basin into which these lavas were erupted was subsequently destroyed during subduction between 1 90 and 1 83 ga resulting in the development a magmatic arc narsajuaq terrane and parent group the ungava magmas provide a unique window into the mantle at 2 0 ga the chemical and isotopic similarity of these proterozoic magmas to modern day magmas provides strong evidence that the interplay between depleted mantle oib mantle and sub continental mantle during the proterozoic was comparable to that of the modern earth

dolomitization on an evaporitic paleoproterozoic ramp widespread synsedimentary dolomite in the denault formation labrador trough canada abstract the denault formation 2 1 1 9 ga crops out in the labrador trough northeastern quebec and western labrador rocks surrounding the town of schefferville quebec contain textural characteristics consistent with deposition on the middle and outer portions of a storm influenced shallow ramp mid ramp facies consist of intraclastic grainstones with hummocky cross stratification hcs swaley cross stratification scs current ripples and graded event beds further outboard grainstones grade into deeper water laminites that are composed of even mm scale couplets of flat lying organic and dolomudstone laminae scours within the laminites suggest periodic storm activity laminites gradually grade into outer ramp deep water shales an isolated eastern stromatolitic buildup is separated from these ramp facies by 50 km present day this succession can be interpreted as the remnant of a near continuous margin or may simply represent an isolated accumulation that developed on a pre existing topographic high the presence of gypsum pseudomorphs in all lithofacies indicates that the denault margin was restricted and evaporitic four paragenetic stages are recognized in the diagenetic evolution of the denault formation 1 carbonate deposition contemporaneous marine cementation authigenic gypsum growth and precipitation of authigenic chert 2 synsedimentary mimetic dolomite precipitation 3 pore rimming and pore occluding shallow burial dolomite cement and 4 fabric destructive sutured anhedral burial dolomite gypsum crystals occur in all lithofacies form the nuclei of interstitial dolomite rhombs average 10 m in length and often display swallowtail twinning paleoproterozoic ocean water had very low concentrations of dissolved sulfate and evaporation in restricted settings would have been required to form gypsum formation of microcrystalline gypsum across this restricted ramp facilitated dolomite precipitation by increasing pore water mg ca ratios and lowering its dissolved sulfate concentrations such an interpretation may explain why there is an abundance of synsedimentary dolostone in the precambrian and the relative paucity of phanerozoic analogs

filling in the juvenile magmatic gap evidence for uninterrupted paleoproterozoic plate tectonics abstract despite several decades of research on growth of the continental crust it remains unclear whether the production of juvenile continental crust has been continuous or episodic throughout the precambrian models for episodic crustal growth have gained traction recently through compilations of global u pb zircon age frequency distributions interpreted to delineate peaks and lulls in crustal growth through geologic time one such apparent trough in zircon age frequency distributions between 2 45 and 2 22 ga is thought to represent a pause in crustal addition resulting from a global shutdown of magmatic and tectonic processes the 2 45 2 22 ga magmatic shutdown model envisions a causal relationship between the cessation of plate tectonics and accumulation of atmospheric oxygen over the same period here we present new coupled u pb hf and o isotope data for detrital and magmatic zircon from the western churchill province and trans hudson orogen of canada covering an area of approximately 1 3 million km2 that demonstrate significant juvenile crustal production during the 2 45 2 22 ga time interval and thereby argue against the magmatic shutdown hypothesis our data is corroborated by literature data showing an extensive 2 22 2 45 ga record in both detrital and magmatic rocks on every continent and suggests that the operation of plate tectonics continued throughout the early paleoproterozoic while atmospheric oxygen rose over the same time interval we argue that uninterrupted plate tectonics between 2 45 and 2 22 ga would have contributed to efficient burial of organic matter and sedimentary pyrite and the consequent rise in atmospheric oxygen documented for this time interval

tomographic imaging of the p wave velocity structure beneath the kamchatka peninsula summary a total of 5270 shallow and intermediate depth earthquakes recorded by the 32 stations of the regional seismic network of the geophysical service of russia are used to assess the p wave velocity structure beneath the kamchatka peninsula in the western pacific the tomographic inversion is carried out in three steps first a 1 d tomographic problem is solved in order to obtain an initial velocity model based on the 1 d velocity model 3 d tomographic inversions with homogeneous and heterogeneous starting models are obtained the conrad 15 km depth and moho 35 km depth discontinuities determined from the 1 d tomographic inversion and the upper boundary of the subducting slab are taken into account in the heterogeneous starting model for the traveltimes and ray path determinations both velocity structure and hypocentral locations are determined simultaneously in the inversion the spacing of the grid nodes is a half degree in the horizontal direction and 20 50 km in the vertical direction a detailed p wave tomographic image is determined down to a depth of 200 km the resulting tomographic image has a prominent low velocity anomaly that shows a maximum decrease in p wave velocity of approximately 6 per cent at 30 km depth beneath a chain of active volcanoes at depth low velocity anomalies are also observed in the mantle wedge extending down to a depth of approximately 150 km these anomalies are apparently associated with the volcanic activity the sedimentary basin of the central kamchatsky graben to the west of the volcanic front and the accretionary prism at the trench correlate with shallow low velocity anomalies highvelocity anomalies observed at a depth of 10 km may be associated with the location of metamorphic basements in the ganalsky valaginskoe uplift and upper crust of shipunsky cape the results also suggest that the subducted pacific plate has p wave velocities approximately 2 7 per cent higher than those of the surrounding mantle and a thickness of approximately 70 km

age of the richmond gulf group and implications for rifting in the trans hudson orogen canada abstract the aphebian richmond gulf group of the east coast of hudson bay is a fluvial sandstone sequence containing basalt flows it was shed eastward from a rifting related thermal dome situated in the present central part of hudson bay sandstone from the base of the group contains a uraniferous diagenetic apatite cement u pb and pb pb isochron analysis from this material show the age of the apatite haematite cement to be 2025 25 ma old these data provide an older age bracket for the deposition of the richmond gulf nastapoka and belcher groups they also date the formation of the richmond gulf graben an aulacogen related to the initial rifting of the trans hudson orogen

long lived continent ocean interaction in the early proterozoic ungava orogen northern quebec canada the early proterozoic ungava orogen contains evidence for the development of a longlived 80 m y and presumably wide ocean that was subsequently destroyed during plate convergence leading to arc continent collision a magmatic arc was built on 2 00 ga oceanic crust above a north dipping subduction zone between about 1 90 and 1 86 ga development of a south verging continental thrust belt containing units associated with prior rifting of the archean superior province basement probably records impingement of the upper plate against the underthrust continental margin at ca 1 87 ga subduction of the superior province continental crust is inferred to have led to a reversal in subduction polarity a younger south dipping subduction zone is suggested by a suite of 1 85 1 83 ga quartz diorite to granite plutons that intrude the upper plate oceanic crust and the older arc the arc continent collision resulted in a minimum of 65 km of tectonic overlap of the composite arc9s plutonic core on the autochthonous footwall basement

petrological and geochemical characteristics of a paleoproterozoic magmatic arc narsajuaq terrane ungava orogen canada and comparisons to superior province granitoids abstract the narsajuaq terrane is a major component of the paleoproterozoic ungava trans hudson orogen of northern quebec and is a well exposed mid crustal block interpreted to be the plutonic roots of a magmatic arc the narsajuaq terrane is comprised of a series of hornblende pyroxene bearing plutons and supracrustal rocks which were accreted to the archean superior province basement during the paleoproterozoic on the basis of geological field relationships and geochronological data the plutonic rocks of the narsajuaq terrane are divided into four main groups 1 the cape smith suite 1898 1839 ma consists of various deformed and undeformed plutons that are found intrusive into the cape smith belt lithologies 2 the gneissic suite which ranges in age from 1863 1844 ma and consists of a well layered sequence of diorite and tonalite intruded by and interlayered with variably deformed and metamorphosed granitic veins 3 the younger suite consisting of discrete kilometre size bodies of metamorphosed plutons diorite monzodiorite tonalite granite intrusive into the gneissic suite ranging in age from 1836 1821 ma and 4 granitic sheets of the anatectic suite which intruded the narsajuaq terrane ca 1800 ma a series of metamorphosed sedimentary rocks predominantly semi pelites of the sugluk group are also found in the narsajuaq terrane and contain zircons of both archean and paleoproterozoic age plutons of the gneissic and younger suites are predominantly calc alkaline and metaluminous to mildly peraluminous and contain variable but generally moderate to high concentrations of the large ion lithophile elements and light rare earth elements rees pronounced negative nb and ti anomalies and fractionated rees comparable to modern arc granites a range in isotopic compositions is noted for the suites with the gneissic and younger suites having initial end and 87sr 86sr i values ranging from juvenile 4 0 and 0 7020 respectively to enriched 10 7 and 0 7048 the anatectic suite granites range to extremely enriched isotopic compositions with initial end up to 18 5 and 87sr 86sr i 0 7064 the cape smith suite has a much more restricted isotopic composition which is predominantly juvenile initial end 3 2to 1 5 and 87sr 86sr i 0 7020 to 0 7024 and suggests a minor role for crustal contamination primarily via sediment subduction the isotopic signature of the narsajuaq terrane plutons indicate that at least three sources were involved in their petrogenesis depleted mantle average 2 8 ga superior province crust and older 3 1 ga superior province crust when compared to plutonic suites from the ne and se superior province minto block abitibi pontiac and opatica belts the narsajuaq terrane suites define a similar chronology and evolution indicating a commonality and dominance of arc accretionary processes for crustal formation in the precambrian shield of canada higher trace element abundances in the narsajuaq terrane plutons compared to the archean samples may be related to different subduction zone fluxes which may reflect different bulk compositions of subducted oceanic sediments during the proterozoic

the role of vertical chemical fractionation in controlling late quaternary atmospheric carbon dioxide the 30 decrease in atmospheric carbon dioxide during glacial maxima must be driven by some change in the chemical circulation of the ocean here a new model for late quaternary co2 variability is presented which resolves some problems occurring in previous models including the timing of carbon dioxide response and changes in the oxygen content of the deep ocean the primary driving factor in this model is a rearrangement of chemical distributions in the ocean whereby labile nutrients and metabolic co2 are concentrated in deep waters rather than in intermediate waters as observed in the modern ocean this new bottom heavy chemical structure does not affect atmospheric carbon dioxide directly instead co2 induced acidity lowers the deep ocean carbonate ion concentration and temporarily increases carbonate dissolution rates oceanic alkalinity then rises until the deep ocean carbonate ion is restored to its steady state value the resulting increase in oceanic alkalinity draws co2 out of the atmosphere into the ocean alkalinity and atmospheric co2 lag several thousand years behind the change in oceanic chemical structure this delayed response is determined by the limited rate of continental weathering and deep ocean carbonate dissolution relative to the oceanic alkalinity inventory it is proposed that characteristic deglacial and preglacial states of the ocean are leading factors driving glacial interglacial climate changes this concept reinterprets the carbon isotope contrast between surface water and deep waters 13c p b as due to a shift of light metabolic carbon from intermediate waters into deep waters the process is illustrated here by a simple equilibrium five box ocean model the observed intermediate depth nutrient depletion is not sufficient in itself to determine which of several possible mechanisms are operating and some mechanisms are not as effective in changing atmospheric co2 as others the largest response is seen from deepening the regeneration cycle of organic carbon the least response is seen from converting north atlantic deep water into intermediate water

three dimensional velocity models of focal zones and refinement of hypocenter parameters we develop an iterative procedure to calculate three dimensional p wave velocity fields by earthquake observations in areas of dense seismicity we describe the principal features of the algorithm considering the stability of the experimental data and of the derived velocity fields as well as problems of averaging results results are presented for the kuril kamchatka seismic zone

a paleoclimate model of northern hemisphere ice sheets abstract a model for predicting the growth and decay of ice sheets based on the astronomical theory of climate change is presented the purpose of the study in part is to isolate the role of the ice sheet physics and earth response under varying ice load by simplifying to the extreme the role of the hydrosphere atmosphere ice sheet physics and the response of the lithosphere asthenosphere under the ice load are modeled explicitly insolation anomalies taken at a fixed latitude directly force latitudinal displacement of the snow line accumulation rate a and ablation rate a evaluated at mean sea level are specificed as external constants a a decrease linearly with ice sheet elevation rough tuning of the model to the general shape of the ice volume record of the last two major glacials determines the external constants model predictions of the ages of several events in the last major glaciation compare well with the radiological ages the six glacial terminatios i vi over the last 600 000 yr are identified and the predicted ages compare reasonably well with the 18 o record for two deep sea cores a direct comparison of model power spectra of ice volume as a function of period with spectra of the 18 o record shows apparent underprediction of power near 100 000 yr when a quantitative but heuristic method for taking into account the red noise spectrum evident in the geological records is used a much more favorable comparison is possible the model prediction lends support to the hypothesis that the nonlinearity of the ice sheet physics is responsible for the 100 000 yr periodicity in the geological record of the late pleistocene

u pb ages for magmatism and deformation in the new quebec orogen the new quebec orogen is a paleoproterozoic fold and thrust belt active from 2 17 to 1 77 ga bordering the eastern margin of the archean superior province the orogen developed on a long lived passive continental margin and comprises two volcanic sedimentary sequences the ages presented in this study help to bracket the magmatic and deformational history of the orogen a glomeroporphyritic gabbro from the hellancourt formation was dated at 1874 3 ma and a rhyodacite from the upper murdoch formation yielded an identical age of 1870 4 ma these data allow the correlation of the two units and indicate that they are 10 14 ma younger than the estimated age for the sokoman formation these ages together with already published work indicate that magmatism related to the younger magmatic event occurred at 1884 1870 ma albitized pelites from the murdoch formation contain euhedral detrital zircons 2707 2690 ma old indicating that the source rocks were archean an undeformed monzonitic intrusion occurr

precambrian crustal evolution seismic constraints from the canadian shield whether or not plate tectonic processes operated on a younger hotter earth remains ambiguous seismic data from new networks in the hudson bay region of the canadian shield where the precambrian geological record spans more than 2 billion years offer fresh scope to address this problem using receiver function analyses we show that the crust of the rae domain which exhibits ages of paleo to neoarchean 3 9 2 7 ga is likely felsic to intermediate in composition average vp vs 1 73 and seismically transparent with a sharp moho there is little evidence for modern style plate tectonics and based on the simplicity and spatial extent of the felsic crust models favouring vertical tectonic processes such as crustal delamination or plume activity appear better suited to the results data from the hearne domain which exhibits widespread 2 7 ga granite and greenstone geology show a more complex crust with higher vp vs ratios consistent with a greater mafic component the trans hudson orogen tho proposed to be a himalayan scale mountain belt during the paleoproterozoic is thought to have formed during the 1 8 ga collision of the superior and churchill plates results from the quebec baffin island segment of the tho appear to map out the first order shape of the underthrusting superior plate with elevated vp vs ratios likely representing the rifted margin of the superior craton consistently thicker crust is observed beneath central and southern baffin island 43 km coincident with widespread high grade metamorphic surface geology these features can be explained by crustal thickening due to stacking of accreted terranes during continent continent collision analogous to the present day tibetan plateau followed by erosion when reviewed in light of age and compositional constraints from the geological record our seismic observations point towards secular crustal evolution from non plate tectonic during the paleo to mesoarchean evolving towards fully developed modern style plate tectonics during the paleoproterozoic

a simple ice sheet model yields realistic 100 kyr glacial cycles records of global ice volume for the past 700 kyr based on oxygen isotopic data from deep sea cores and reflecting mainly the changing northern hemispheric ice sheets show a dominant cycle of roughly 100 kyr period the records also show smaller amplitude oscillations with spectral peaks at roughly 40 and 20 kyr periods which are well correlated with the milankovich insolation variations due to perturbations in the earth s orbital parameters however no model has accurately simulated the 100 kyr glacial cycle recently birchfield et al 1 and oerlemans2 have obtained encouraging agreement with some features of the glacial cycle by using a simple ice sheet model with a realistic time lag in the response of the bedrock to the ice load this study extends their basic model first by including topography to represent high ground in the north improved results can then be obtained but only with unrealistic parameter values and for some aspects of the record further improvements areobtained by crudely parameterizing possible calving at the equatorward ice sheet tip during deglaciation by proglacial lakes and or marine incursions from the atlantic as emphasized by andrews3 the resulting ice volume curves agree fairly well with the observed records and their power spectra over the past 700 kyr

a study of the climatic regimes of the pleistocene using a stochastic resonance model a global energy balance model employing the stochastic resonance mechanism previously used to explain the climatic variability of the late pleistocene has now been extended to account for the climatic variations over the full pleistocene the possibility that extremely long term changes of the order of millions of years in the boundary conditions of the climate system have altered the response of the pleistocene climate to the external orbital forcing has been investigated it is shown that by slowly changing the only free parameter of the model the system can undergo a pitchfork bifurcation the bifurcation point separates a linear regime identified with the early pleistocene climate from a strongly nonlinear regime the late pleistocene where the stochastic resonance mechanism produces rapid and symmetric transitions between the two stable steady states of the system the main differences in the dynamic features of the two regimes are the change in amplitude of the oscillations the relative importance of the stochastic forcing the change in shape of the probability distribution and the corresponding change in the power centered around the 100000 year cycle in qualitative agreement with the observed geological record with the introduction of the external orbital forcing now spectrally complete and included without requiring any additional hypothesis the model reproduces the previous results namely the good correlation with the isotopic record the appearance of the dominant spectral peaks as well as the redness of the power spectrum in particular it is shown that the orbital forcing in eccentricity acts as a pacemaker of the major glacial cycles of the late pleistocene through the mechanism of stochastic resonance a stochastic sensitivity analysis is then applied to validate the significance of the results and to investigate the predictability of the climate system over the time scales of the orbital cycles

upper and lower plate juxtaposition deformation and metamorphism during crustal convergence trans hudson orogen quebec baffin segment canada abstract the lower plate of the quebec baffin segment of paleoproterozoic trans hudson orogen comprises parautochthonous archean crystalline basement of the superior province 3 22 2 74 ga and imbricated paleoproterozoic rift margin strata 2 04 1 92 ga the collisional upper plate contains 1 allochthonous ophiolitic units 2 00 ga 2 a continental arc terrane narsajuaq arc 1 86 1 82 ga 3 shelf sequence strata ca 1 86 ga and basement orthogneisses ca 1 95 ga 4 foreland basin deposits 1 86 ga and 5 a batholith which intrudes the shelf and foreland basin rocks cumberland batholith 1 86 1 85 ga lower and upper plate assemblages were juxtaposed during a 1 82 1 79 ga accretion collision event and deformed into regional scale folds during two post collisional folding episodes ca 1 76 1 74 ga as a result of the structural relief generated by the post collisional folding and subsequent exhumation all orogenic elements can be directly examined along a 400 km across strike transect in the external zone northern quebec lower plate rift margin units were internally imbricated cape smith belt and translated southward across the superior province basement paleoproterozoic amphibolite facies mineral growth is syn to post thrusting tectonic imbrication has resulted in a structural inversion of the accreted i e upper plate ophiolite stratigraphy north of the cape smith belt mid crustal plutonic units of the narsajuaq arc form an upper plate crystalline wedge whose sole thrust is localized near the thrust belt s basal decollement within narsajuaq arc syn magmatic granulite facies assemblages are retrogressed to amphibolite facies assemblages in proximity to major accretion collision faults in the internal zone southern baffin island a tripartite orogen scale tectonostratigraphy is exposed on the flanks of the post collisional fold structures at the lowest structural level lower plate superior province orthogneisses are imbricated with paleoproterozoic rift margin units and mineral growth is syn thrusting at intermediate structural level upper plate orthogneisses are correlated with the narsajuaq arc in northern quebec at the highest structural level a north facing shelf sequence is structurally overlain by an assemblage of clastic rocks interpreted as a foreland basin sequence dominantly monzogranite plutons of the cumberland batholith are restricted to the upper structural level and indicate pre emplacement imbrication of tectonostratigraphic units within both upper and lower plates granulite facies assemblages are retrogressed to upper amphibolite assemblages in proximity to 1 82 1 79 ga accretion collision structures exposures of superior province basement across the orogen document a progressive increase in the extent of reworking during accretion of upper plate units from a narrow shear zone in the external zone to km scale imbrication and more pervasive shear deformation in the internal zone a minimum of 160 km of basement underthrusting is constrained by the structural overlap of upper plate allochthons on lower plate units basement involvement during post collision shortening is manifest as thick skinned folding indicating coupling across the collisional basal decollement

u pb geochronology of reactivated archean basement and of hudsonian metamorphism in the northern labrador trough the labrador trough is a proterozoic orogenic belt bordering the eastern margin of the archean superior province the volcano sedimentary sequences of the trough were deformed and metamorphosed during the hudsonian orogeny ca 1800 ma the eastern basement present as domal inliers was remobilized during this orogeny whereas the western basement remained unaffected in this study we present u pb ages of the western and eastern basement of the proterozoic labrador trough orogen between leaf river and koksoak river and of the northernmost extension of the depas batholithic complex farther east from the western basement the leaf bay granodiorite has an age of 2721 3 ma zircon from the gneiss domes of the eastern basement the ages are as follows lac moyer 2883 6 ma zircon 1793 5 ma monazite and 1746 ma minimum age rutile lac boulder an upper intercept age of 2868 8 ma and a lower intercept age of 1783 11 ma zircon titanite regression and an age of 1740 5 ma rutile lac olmst

purtuniq ophiolite cape smith belt northern quebec canada a reconstructed section of early proterozoic oceanic crust the crustal part of a dismembered ophiolite is preserved in the structurally highest thrust sheets of the early proterozoic cape smith belt in northern quebec canada the rocks of the ophiolite represent two magmatic suites each with a mantle source that is nd isotopically distinct the older suite comprises pillowed and massive volcanic rocks sheeted dikes dominantly gabbroic layered cumulate rocks and rare plagiogranite sills and dikes in the volcanic sequence this 5 km thick tholeiitic suite is ca 1998 ma and is compositionally and petrographically similar to rocks formed at modern oceanic spreading ridges the younger suite 4 km comprises sheeted mafic dikes and layered ultramafic to mafic cumulate rocks that are similar to tholeiitic rocks found in modern plume generated oceanic islands such as hawaii the composite section is in excess of 9 km although the total thickness of the purtuniq ophiolite may not be typical of that for oceanic crust formed at early proterozoic spreading ridges elsewhere it does suggest that ancient ridge formed crust was at least as thick as that in modern oceans

aphebian mafic ultramafic magmatism in the labrador trough new quebec its age and the nature of its mantle source the magmatic events occurring within the two main cycles in the south central part of the labrador trough new quebec have been dated in cycle 1 a granophyre dike related to the cramolet lake gabbro sill which intrudes the seward subgroup has a u pb zircon age of 2169 2 ma in cycle 2 the tholeiitic basalts of the willbob hellancourt formation and the related mafic ultramafic sills are dated at ca 1900 ma by the pb pb method these data confirm the existence of at least two main magmatic cycles separated by about 270 ma the magma source was depleted upper mantle and the magma did not experience any significant crustal contamination as indicated by the 1 ratio 7 9 and 4 for the tholeiitic basalts the 1 value for the intrusive rocks 8 03 and the average value for the gabbroic rocks of cycle 2 2 8 and for the granophyre of cycle 1 1 05 could be the result of slight crustal contamination or of mantle heterogeneity whatever the cause of these values the data indicate the prolong

vostok ice core provides 160 000 year record of atmospheric co2 direct evidence of past atmospheric co2 changes has been extended to the past 160 000 years from the vostok ice core these changes are most notably an inherent phenomenon of change between glacial and interglacial periods besides this major 100 000 year cycle the co2 record seems to exhibit a cyclic change with a period of some 21 000 years

u pb ages from the nimish formation and montagnais glomeroporphyritic gabbro of the central new quebec orogen canada three u pb zircon dates were obtained from two igneous suites associated with cycle 2 sedimentation in the central new quebec orogen labrador trough in the dyke lake area of the western part of the orogen schefferville zone the nimish formation includes a polymictic conglomerate containing quartz syenite cobbles that crystallized at 1877 8 1 3 ma these cobbles are petrographically and geochemically linked to the nimish volcanics which are intercalated with the sokoman iron formation consequently the syenite date gives an approximate age for both nimish magmatism and ironstone deposition in the schefferville zone a date of ca 2 65 ga obtained from a nimish trachyte in the same area indicates that zircons in the sample are xenocrysts farther east at howse lake howse zone a montagnais plagioclase glomeroporphyritic gabbro sill crystallized at 1884 0 1 6 ma the howse lake sill which intrudes turbidites of the menihek formation is considered comagmatic with the basalts that cap the form

upb geochronology of the western cape smith belt canada new insights on the age of initial rifting and arc magmatism abstract the cape smith belt of northern quebec is the foreland to hinterland transition of the ungava orogen a component of the paleoproterozoic trans hudson orogen of north america u pb ages for rift and volcanic arc related units from the cape smith belt are presented and used to evaluate models of its tectonic evolution a gabbroic sill intrusive into the rift related sediments near the base of the povungnituk group is 2038 2 4 ma old indicating that rifting of the superior province was initiated prior to that time 80 m y earlier than previously reported a granodiorite intrusive into the volcanic rocks of the povungnituk group is dated at 1991 2 ma indicating that magmatism associated with the rifting event was active at that time 30 m y earlier than formerly thought the age of parent group arc related magmatism ranges from 1874 3 4 ma the age of a diorite intrusion to 1860 2 ma the age of another diorite intrusion and of a rhyolite a granodiorite intrusive in the parent group yields an age of 1845 6 ma magmatism of the parent group is thought to be related to northward dipping subduction of the superior province margin below the watts group a gabbroic intrusion in the 2000 ma old watts group is dated at 1858 2 ma these ages are consistent with a tectonic model in which the rocks of the cape smith belt formed in a single continental rift to oceanic progression followed by subduction which led to an arc continent collision a comparison of u pb ages for similar events in different sectors of the trans hudson orogen in canada reveals that early events are diachronous and range from 2169 ma to 2038 ma the main collisional events are coeval in the ungava and new quebec orogens and in some of the western sectors at 1830 1860 ma younger ages of 1800 1820 ma are obtained for the late collisional stages throughout the trans hudson orogen

griffin gabbro sills 2 11 ga hurwitz basin nunavut canada long distance lateral transport of magmas in western churchill province crust circa 2 11 ga sills and rare dykes herein named the griffin gabbros extend discontinuously across an area 400 by 125 km in the hearne domain of the western churchill province in northern canada sills form poorly connected tongue like tabular bodies primarily within paleoproterozoic intracratonic basin deposits of the lower hurwitz group dykes are restricted to the southern limit of the griffin suite and have not been found in the interior of hurwitz basin or in archean basement incompatible element patterns and low end2111 values 0 1 1 0 indicate an enriched asthenosphere source similar to many modern hotspot related ocean island basalts the gabbros display enrichments in nb and la suggesting a mantle plume origin heavy rare earth element depletions and tio2 values averaging 3 40 wt suggest partial melting of a garnet peridotite source at depths of 70 100 km mgo values averaging 5 20 wt indicate fractionation after melting concentrations of both the incompatible and compatible elements scatter widely with respect to mg number indicating that compositional variation does not represent fractional crystallization from a common parent consistent with derivation from different magma chambers related to the same mantle plume the gabbros fail to show signs of an archean enrichment event recorded by other paleoproterozoic mafic rocks in the hearne domain this together with the absence feeder dykes in basement indicate derivation external to hurwitz basin we interpret that a mantle plume located beyond the present southern margin of the hearne domain was active during 2 1 ga rifting that led to opening of the manikewan ocean the griffin gabbros were likely injected because of the hydraulic head created by topographic doming above this plume analogous to the ferrar sills jurassic in antarctica the gabbros may have been fed by cryptic dykes in which magmas traveled laterally for at least 250 km across the southern hearne domain and then controlled by sedimentary layering spread as sills for hundreds of km within hurwitz basin

north atlantic thermohaline circulation during the past 20 000 years linked to high latitude surface temperature during a surface cooling event 10 000 to 12 000 years ago higher cd ca and lower 13c 12c ratios are observed in benthic foraminifera shells from rapidly accumulating western north atlantic sediments data from sediment cores show that marked nutrient depletion of intermediate waters occurs in association with reduced glacial north atlantic deep water flux it is proposed that cold high latitude sea surface temperatures enhance intermediate water formation at the expense of deep water formation

new seismic images of the crust in the central trans hudson orogen of saskatchewan abstract a reprocessing program to enhance the correlation between the surface geology and the seismic data has been completed for seismic line 9 eastern 100 km and line 10 in the central region of the trans hudson orogen of saskatchewan canada the new seismic images through lateral continuity of reflectivity provide sufficient detail to resolve the discrepancy between the low dipping layer parallel and dextral reverse nature of the sturgeon weir shear zone line 9 observed in the field and its steeply dipping apparent normal displacement character interpreted on the basis of the initial processing furthermore the new interpretation provides a strong confirmation of the role of pelican thrust as a major detachment zone the main sole thrust along which juvenile allochthons have been carried across the archaean microcontinental block the images are also refined enough to suggest a a boundary within the pelican thrust between its internal and external suites b a possible boundary separating a lower older archaean basement from its upper younger counterpart and c sub moho events m2 which reveal possible involvement of the upper mantle in the collisional tectonic process in addition to the well defined moho m1 which probably represents the youngest of the post collisional detachments

early proterozoic 1 88 1 87 ga tholeiitic magmatism in the new quebec orogen the new quebec orogen contains two volcano sedimentary sequences bounded by unconformities each sequence records a change from continental sedimentation and alkaline volcanism to marine sedimentation and tholeiitic volcanism the first sequence records 2 17 ga rifting and the development by 2 14 ga of a passive margin along the eastern part of the superior craton the second sequence developed between 1 88 and 1 87 ga in pull apart basins that reflect precollisional dextral transtension along the continental margin second sequence magmatism comprises i carbonatitic and lamprophyric intrusions and mildly alkaline mafic to felsic volcanic rocks ii widespread intrusion of tholeiitic gabbro sills and submarine extrusion of plagioclase glomeroporphyritic basalts and younger aphyric basalts and picrites and iii late stage mafic to felsic volcanism and intrusion of carbonatites crustal thinning allowed primitive tholeiitic magmas to equilibrate at progressively lower pressures before more buoyant

enriched mantle components in proterozoic continental flood basalts of the cape smith foldbelt northern quebec magmatism associated with ca 2 04 ga rifting of the superior province resulted in the formation of a succession of tholeiitic and alkaline basalts as well as minor rhyolites collectively known as the povungnituk group cape smith foldbelt the tholeiites are not typical continental flood basalts cfb as they lack significant negative nb and ta anomalies indicating that crustal contamination has not played a major role in their evolution although crystal fractionation along a gabbroic cotectic can account for much of the compositional variation in the povungnituk basalts it cannot explain the range of observed traceelement ratios which is similar to that from tholeiitic to highly alkaline lavas observed in modern ocean island basalt suites variations of la sm and nb zr indicates the presence of a highly enriched component and that mixing between this enriched component and a non enriched end member can account for the range of trace element ratios in the povungnituk lavas although the most enriched tholeiites are spatially associated with an alkaline volcanic center enriched tholeiites are widely interspersed in the povungnituk group and are also found in other parts of the eastern trans hudson orogen the paleoproterozoic mantle source appears to have contained a dispersed but variable enriched component that was responsible for the variation of degree of enrichment of the povungnituk tholeiitic basalts d 2003 elsevier b v all rights reserved

geophysical correlations in the ungava bay area we used gravity and magnetic data to extend geological features across ungava bay and hudson strait from northern quebec and labrador to baffin island tectonic domains identified on land were extended offshore by visual inspection of the potential fields the boundaries identified on colour shaded relief maps were drawn on the original marine magnetic profiles to verify the robustness of the conclusions our interpretation in ungava bay remains speculative but gives some insight into the architecture of the northeastern canadian shield onshore where trends are well marked the magnetic fabric of the superior province reappears east of the new quebec orogen in the western core zone geochronology and geochemical data also suggest the affinity of the western core zone with the superior province on the gravity map the new quebec orogen appears to vanish offshore a major trend in magnetic and gravity gradients can be followed offshore possibly as far north as hudson strait it has been identified as the extension of the george river and moonbase shear zones the de pas batholith is associated with a linear trend west of this shear zone to the east several features appear distinctly in the magnetic map a narrow northwest trending anomaly can be followed from quebec all the way to baffin island on land this feature marks the western limit of the lake harbour group two wide north trending bands that can be followed from the torngat orogen all the way to baffin island are interpreted as the extensions of the lac lomier complex and tasiuyak domain resume nous avons utilise des donnees gravimetriques et magnetiques pour prolonger les caracteristiques geologi ques a travers la baie d ungava et le detroit d hudson du nord du quebec et du labrador jusqu a l ile de baffin les domaines tectoniques identifies sur terre ont ete prolonges au large par une inspection visuelle des champs de poten tiels les limites indiquees sur des cartes topographiques en couleurs ont ete tracees sur les profils magnetiques marins originaux afin de verifier la solidite des conclusions dans la baie d ungava notre interpretation demeure speculative mais elle donne un apercu de l architecture du nord est du bouclier canadien sur terre ou les tendances sont bien marquees la texture magnetique de la province du superieur reapparait a l est de l orogene du nouveau quebec dans la zone noyau occidentale la geochronologie et les donnees geochimiques suggerent aussi une affinite entre la zone noyau occidentale et la province du superieur sur la carte gravimetrique l orogene du nouveau quebec semble dispa raitre vers le large une tendance majeure dans les gradients magnetiques et gravimetriques peut etre suivie au large possiblement aussi loin que le detroit d hudson cette tendance a ete identifiee comme la prolongation des zones de ci saillement de la riviere george et de moonbase le batholite de depas est associe a une tendance lineaire a l ouest de cette zone de cisaillement vers l est plusieurs caracteristiques sont distinctes sur la carte magnetique une etroite ano malie a tendance nord ouest peut etre suivie de quebec jusqu a la terre de baffin sur terre cette caracteristique marque la limite ouest du groupe de lake harbour deux larges bandes a tendance nord qui peuvent etre suivies de l orogene torngat jusqu a l ile de baffin sont interpretees comme etant des extensions du complexe du lac lomier et du domaine de tasiuyak

terrane boundaries within trans hudson orogen quebec baffin segment canada changing structural and metamorphic character from foreland to hinterland within the trans hudson orogen in northern quebec and southern baffin island parautochthonous archean basement and paleoproterozoic cover as well as allochthonous accreted paleoproterozoic units are exposed in a series of antiformal culminations and complementary synclinoria the parautochthonous rocks and two assemblages of accreted paleoproterozoic units crustal terranes are separated by major tectonic boundaries and associated deformation zones field petrological and isotopic data document an increase in the width of the deformation zones and a concomitant increase in p t conditions of metamorphism from orogenic foreland to hinterland tectonostratigraphic and geochronological criteria allow footwall and hangingwall units to be consistently distinguished in spite of the changing structural and metamorphic aspect of the terrane boundaries across the width of the orogen

age and tectonic implications of paleoproterozoic mafic dyke swarms for the origin of 2 2 ga enriched lithosphere beneath the ungava peninsula canada abstract mafic dyke swarms represent short lived magmatic events that carry important temporal and chemical constraints for the evolution of the lithospheric mantle five baddeleyite u pb isotopic analyses of paleoproterozoic mafic dyke swarms intruding the ungava peninsula yielded ages of 2508 6 2220 1 2212 3 2199 5 and 2149 3 ma these ages fall within the 2 5 2 0 ga period previously defined by five other swarms which together comprise a magmatic record of more than 500 ma the similar age composition and surface area of two 2 51 ga swarms along with the lack of coeval supracrustal rocks suggest they correspond to failed rifts an important magmatic hiatus of 280 ma follows but ends with the intrusion of numerous dykes between 2 23 and 2 20 ga in the thinnest lithosphere of the northern ungava peninsula these dykes exhibit chemical signatures that vary from enriched older 2229 2212 ma dykes nd t 4 to relatively depleted 2209 2199 ma voluminous younger dykes nd t 1 4 la yb ca 2 21 2 20 ga later dykes emplaced ca 2 17 ga within a rejuvenated paleoproterozoic lithosphere also have depleted chemical compositions and map the extent of rifting coeval with the emplacement of early basalts in the labrador trough dykes emplaced in the 2 15 2 00 ga period further to the south are proximal to coeval continental basalts with which they share enriched isotopic compositions nd t 1 5 to 7 and a positive correlation between la yb and zr nb la yb 4 10 zr nb 7 20 that suggest assimilation of an archean crustal component a progressive decrease in the nd t values of these magmas with decreasing age further implies increasing contamination until the archean lithosphere failed ca 2 0 ga and permitted the eruption of isotopically depleted basalts the ca 2 2 ga depleted northern dykes and the 2 1 1 9 ga povungnituk basalts of the cape smith foldbelt share a negative correlation between zr nb and la yb that is expected for the incorporation of an enriched lithospheric component this component is represented by the alkaline rocks comprised in the circum ungava supracrustal belts carbonatites lamprophyric lavas alkaline basalts and by the lac aigneau lamprophyric dykes 1941 3 ma intruding the ungava peninsula these alkaline magmas collectively have 2 2 2 1 ga depleted mantle nd model ages t dm similar to a 2 15 ga sm nd isochron defined by the basaltic rocks of the supracrustal belts although these nd t dm ages and isochron are 100 300 ma older than magmatic ages they are identical with the ca 2 2 ga mafic dykes of the ungava peninsula this time correlation along with the similar chemical signature of the northern dykes and younger povungnituk basalts suggest that the composition of younger lavas and alkaline magmas was influenced by an enriched component associated with the pervasive metasomatism of the lithosphere coeval with the emplacement of the many ca 2 2 ga dykes

deciphering the paleoproterozoic cooling history of the northeastern trans hudson orogen baffin island canada using 40 ar 39 ar step heating and uv laser thermochronology abstract the previously unstudied cooling and exhumation history of mid crustal rocks exposed on southeastern baffin island canada provides new insights into the post orogenic evolution of the paleoproterozoic trans hudson orogen tho new 40 ar 39 ar step heat analyses of biotite muscovite and phlogopite and core to rim intra grain 40 ar 39 ar analyses of muscovite have a range of apparent ages compatible with slow regional cooling following peak metamorphism twenty nine amphibolite to granulite facies rocks were dated using the 40 ar 39 ar step heating laser co 2 method 40 ar 39 ar spot analyses were performed across muscovite grains from three samples using an ultraviolet uv laser to investigate intra grain 40 ar 39 ar age variations step heating apparent ages range from ca 1788 1622 ma for biotite 1720 1630 ma for phlogopite and 1729 1657 ma for muscovite uv spot 40 ar 39 ar analyses in the three muscovite grains range from ca 1661 1640 ma 1675 1645 ma and 1680 1652 ma with core to rim apparent age gradients of 20 30 myr previous studies resolved peak metamorphism in this region to between ca 1860 and 1820 ma and identified late to post tho zircon and monazite populations at ca 1800 1750 ma numerical diffusion models for ar in muscovite were conducted to test different proterozoic cooling and exhumation scenarios comparisons with our 40 ar 39 ar ages attest to cooling rates of 1 2 c myr following peak metamorphism and 1 5 2 5 c myr after ca 1740 ma anomalously old apparent 40 ar 39 ar ages in cases equivalent to u pb zircon rim and monazite ages likely result from incorporation of excess ar the results suggest that mid crustal rocks on southeastern baffin island remained hotter than 420 450 c for 150 200 myr after peak metamorphism with subsequent slow cooling and denudation rates that are typical of proterozoic orogens the apparent absence of orogenic collapse implies that despite high temperatures and estimated maximum crustal thicknesses comparable to those of large hot orogens the tho remained gravitationally stable during its terminal phase

evidence from southern ocean sediments for the effect of north atlantic deep water flux on climate evidence from southern ocean sediments for the effect of north atlantic deep water flux on climate