quaternary tectonic response to intensified glacial erosion in an orogenic wedge active orogens are thought to behave as internally deforming critical taper wedges that are in rough long term equilibrium with tectonic influx and erosional outflux spatial and temporal variations in climate are therefore hypothesized to have a significant influence on denudation topography and deformation of orogens thereby affecting wedge taper however the impact of the most severe transition in northern hemisphere climate during the cenozoic era the onset of glaciation has hitherto not been empirically documented here we analyse the spatial patterns of denudation and deformation and their temporal variations in the heavily glaciated st elias orogen in southern alaska low temperature thermochronometry thermokinematic modelling and offshore seismic reflection and borehole data suggest that the global scale intensification of glaciation in the middle pleistocene epoch enhanced glacier growth and caused ice streams to advance to the edge of the continental shelf this led to focused denudation across the subaerial reaches of the orogen and burial of the actively deforming wedge toe by the eroded sediment we propose that this climatically driven mass redistribution forced a structural reorganization of the orogen to maintain critical taper our empirical results thus support decades of numerical model predictions of orogenesis and provide compelling field evidence for the significant impact of climate change on tectonics intense glaciation during the middle pleistocene epoch led to focused denudation and mass redistribution within the st elias orogen in southern alaska and resulted in structural reorganization of the orogen the tectonic response of this orogen to climate change is consistent with predictions of numerical models

deposition and imbrication of a 2670 2629 ma supracrustal sequence in the indin lake area southwestern slave province canada new u pb age data from the southwestern slave province demonstrate that units of the indin lake supracrustal belt form an imbricated structural stack the oldest rocks of the belt are undated mafic volcanic flows of the hewitt lake group that are crosscut by a 2670 ma felsic sill itself coeval with mafic through felsic volcanic rocks of the 2668 2671 ma leta arm group the youngest rocks of the belt are 2647 2629 ma turbidites and felsic volcanic rocks of the unconformably overlying chalco lake group tonalite orthogneiss of the adjacent cotterill gneiss complex is 2680 ma suggesting that it does not represent in situ basement to the supracrustal belt intercalation of the older hewitt lake and leta arm groups with the younger chalco lake group is interpreted to result from d1 imbrication and folding between 2629 and 2609 ma the age of a crosscutting tonalite intrusion subsequent d2 folding and regional low pressure metamorphism occurred between 2609 ma and ca 2590 ma d3 normal faulting between the

archean greenstone belts are not oceanic crust archean greenstone belts with a preponderance of mafic volcanic rocks often preserved in tectonically complex sequences are obvious candidates in the search for remnants of archean ocean crust we review the tectonic setting and stratigraphy of a number of greenstone sequences previously interpreted as archean ophiolites and conclude on the basis of basal unconformities presence of xenocryst zircons geochemical and isotopic evidence for crustal contamination intrusive relationships with older basement and their internal stratigraphy that none of these examples is derived from archean oceanic crust thermal tectonic and isostatic constraints imply that archean oceanic basins did exist and were covered with several kilometers of water we consider it possible indeed probable that relict archean oceanic crust is preserved in granite greenstone terrains or other archean tectonic settings but that an unequivocal archean ophiolite has not yet been recognized

reorientation of structures near granitic plutons and orthogonal lineaments russell lake supracrustal domain southwestern slave province a complex array of folds and cleavages in metasedimentary and metavolcanic rocks in the russell lake domain could reflect regional compression recurrently reoriented across crustal lineaments several generations of steeply inclined folds trending northward across the domain curve into or interfere with folds parallel to margins of bordering granite batholiths aligned orthogonally northwest lnw and northeast lne sets of later subvertical cleavages strike northward northwest parallel and oblique to lnw and northeastward subparallel to lne the cleavages extend beyond the aligned margins indicating that the association with the granites is indirect the arrangements of folds and cleavages are consistent with intermittent redirection of subhorizontal compression of cover rocks perpendicular to orthogonal steep crustal fractures that were intruded by syndeformational granitic plutons additionally overturning of folds near plutons suggests local reorientation of compression near shallow dipping margi

model for the origin of the yakutat block an accreting terrane in the northern gulf of alaska a composite oceanic and continental terrane the yakutat block is currently colliding with and accreting to north america in the northern gulf of alaska marine geophysical data indicate that the block has moved with the pacific plate during pliocene and quaternary time but are indeterminant about its previous movement history a model that explains some observed geologic and structural features of the block is that it originated as a result of subduction of the kula farallon spreading center beneath north america in this model the spreading center that formed the basaltic basement of the yakutat block was offset from a spreading center to the south by a left lateral transform fault 400 500 km long subduction of the northern spreading center about 45 m y ago sliced off part of the north america continental margin attached it to the kula plate and initiated northward movement of the composite terrane as the yakutat block during eocene and oligocene time migration of the southern spreading center toward north america truncated the yakutat block on the south and left a fossil fracture zone that is now seen as the transition fault the southern spreading center subducted about 25 m y ago and the yakutat block has since traveled with the pacific plate to southern alaska

comment on an extensional model for the early development of greenstone belts with reference to a portion of the abitibi belt ontario canada by i l gibson r g roberts and a gibbs discussion de l article de gibson i l roberts r g gibbs a an extensional model for the early development of greenstone belts with reference to a portion of the abitibi belt ontario canada earth planet sci lett 79 159 167 1986

new insights into archean crustal development from geochronology in the rainy lake area superior province canada precise isotopic analyses of zircon titanite and rutile from units in the rainy lake area of northwest ontario are combined with structural investigations to develop new stratigraphic interpretations in which lithotectonic sequences were juxtaposed by thrusting prior to steepening by wrenching ages for the oldest volcanic and plutonic rocks in the area range from 2725 pm 2 ma to 2728 4 3 ma the age of metagray wackes of the coutchiching group is bracketed between 2704 pm 3 ma and 2692 pm 2 ma by detrital zircon and a crosscutting intrusion alluvial conglomerate of the seine group is bracketed between 2696 5 3 ma the age of a clast and 2686 2 1 ma the age of the undeformed ottertail lake pluton the coutchiching group is younger than structurally overlying metavolcanic rocks and is therefore allochthon ous these data suggest that sedimentation and deformation were controlled by collisional plate interactions and subsequent transpression at the subprovince boundary combined with dat

u pb zircon geochronology of the yellowknife volcanic belt nwt canada new constraints on the timing and duration of greenstone belt magmatism volcanic and sedimentary rocks in the slave province collectively termed the yellowknife supergroup have long been considered to represent laterally continuous and correlative stratigraphically coherent homoclinal sequences correlation based on the lithologic analysis of certain distinctive units has been most intensely applied to the yellowknife volcanic belt in the southern slave province u pb zircon data for seven samples collected from within the belt are reported here the distinctiveness of the townsite formation a term applied to three lenticles of rhyodacitic to dacitic tuffs breccias and pillowed flows in comparison to the overlying and underlying tholeiitic basalts kam group has led to its use as a key marker unit for stratigraphic correlations and palinspastic restorations of offsets on proterozoic faults in the area a sample from each of the three segments was collected for u pb zircon geochronology in addition numerous thinner tuffaceous beds from the kam group were also dated

geophysical insights into the transition fault debate propagating strike slip in response to stalling yakutat block subduction in the gulf of alaska on the basis of faulting mapped on seismic reflection and bathymetric data seismicity current plate motions and evidence that the yakutat block may be anomalously thick we propose a tectonic model for yakutat pacific interactions including the often debated transition fault to the east deformation associated with the queen charlotte fairweather fault system is extending offshore facilitating westward propagation of strike slip motion along the eastern segment of the transition fault to the west the oblique slip pamplona zone and transition faults merge at an embayment in the continental margin where a north south dextral strike slip fault within the pacific plate illuminated by the 1987 1992 earthquake swarm intersects the pacific yakutat tectonic boundary these fault patterns are consistent with modern plate motions and reflect a plate boundary reorganization that may be caused by resistance to subduction by the yakutat block a possible moderate sized oceanic plateau

geochemistry of the late archean banting group yellowknife greenstone belt slave province canada simultaneous melting of the upper mantle and juvenile mafic crust this study investigates the geochemistry and tectonic setting of the 2 66 ga banting group the younger sequence of volcanic rocks in the yellowknife greenstone belt and its relationship to older tholeiitic volcanic rocks of the 2 72 2 70 ga kam group the banting group includes a much higher proportion of felsic volcanic and volcaniclastic rocks than the kam group but mafic to intermediate volcanic rocks are common banting basalts are tholeiitic and are melts of archean depleted upper mantle as are basalts of the kam group in contrast banting dacites and rhyolites have much lower heavy rare earth element abundances and generally have higher initial 143nd 144nd than kam felsic rocks the chemistry of the felsic rocks provides a geochemical signature to distinguish rocks of kam versus banting age where complex structures have obscured the stratigraphy whereas kam felsic rocks evolved from mafic parents by assimilation fractional crystallization processes banting felsic rocks have compositions sim

crustal evolution of archean rocks in the kakagi lake area wabigoon subprovince ontario as interpreted from high precision u pb geochronology the evolution of the archean volcanic plutonic complex in the kakagi lake area occurred during a time interval of ca 32 ma the earliest age is from analysis of zircon and baddeleyite in a gabbro intruding the lowermost katimiagamak volcanic group an age of 2723 2 1 8 ma on a tonalite gneiss from the interior of the underlying sabaskong batholith is indistinguishable from a previously dated massive border phase of the batholith and shows no evidence for inheritance from an older sialic component an early tonalite phase from the adjacent aulneau batholith is dated at and the latest granodiorite phase is dated at this defines a time span of about 7 ma for intrusion of the bulk of the batholith and indicates that previously dated felsic volcanism from the uppermost sequence above the kakagi lake volcanic group is coeval with late plutonic activity in the aulneau batholith the end of regional deformation in the area is given by the ages of two late tectonic intrusions the heronry lake pluton and

influence of tectonism and climate on lithofacies distribution and sandstone and conglomerate composition in the archean beaulieu rapids formation northwest territories canada abstract the 0 2 1 km thick archean beaulieu rapids formation is a late orogenic tectonically controlled sedimentary sequence overlying the beaulieu river volcanic belt unconformably on the west and bordering the major n trending beniah lake fault on the east local porphyry stocks characteristic of late orogenic successions were emplaced along strike of the fault four lithofacies comprise the beaulieu rapids formation 1 the 20 320 m thick conglomerate i 2 the 10 305 m thick siltstone sandstone 3 the 20 120 m thick conglomerate ii and 4 the 40 320 m thick quartz rich sandstone lithofacies stacking displays two distinct large scale fining upward sequences represented by the 300 375 m thick conglomerate i and sandstone siltstone sequence a and the 300 400 m thick conglomerate ii and quartz rich sandstone sequence b fining upward sequences a bounding unconformity and the beniah lake fault are compelling indicators of tectonic influence on sedimentation conglomerate i was deposited by debris flows traction currents and sheetfloods on the proximal medial and distal portions of alluvial fans or fan deltas respectively the overlying siltstone sandstone contains two distinct lithological units a sandstone dominated unit formed from fluvial dune migration and overbank deposition in a sandy braidplain setting with scattered ponds whereas sedimentary structures in the siltstone dominated unit are consistent with lacustrine deposition a second phase of tectonic uplift produced sequence b conglomerate ii was deposited in a proximal braided stream setting characterized by abundant longitudinal gravel bars the quartz rich sandstone developed from dune migration on a sandy braidplain mineralogy and textures reflect high sediment influx limited transport distance local sources and climatic influence clast populations in conglomerate indicate a prevalent mafic volcanic source whereas sandstones contain prominent quartz av 76 with subordinate feldspar 13 and lithic fragments 11 the disparity between clast and sandstone composition is a function of chemical weathering typical of a humid climate source areas are restricted to the mafic dominated beaulieu river volcanic belt the 2 8 ga gneisso plutonic sleepy dragon complex and fault related porphyry stocks the humid weathering aggressive archean climate was responsible for destroying labile minerals and lithic fragments in proximal source rocks rich in plagioclase accounting for the high quartz component in the sandstones the beaulieu rapids formation is similar to modern strike slip basins that form along major fault zones in terms of basin geometry faulted and unconformable contacts lithological units and sedimentary facies stacking and organization fault controlled basins of this nature commonly develop during the last stage of terrane accretion on archean cratons

the transition from strike slip to oblique subduction in southeastern alaska from seismological studies abstract body waveform modeling of 11 moderate to large earthquakes within southeastern alaska has been incorporated with earthquake relocations and the results of previous seismicity studies to examine the transition from strike slip to oblique subduction in southeastern alaska in the sitka region of extreme southeastern alaska earthquakes indicate seismic slip is parallel to the direction of motion between north america and the pacific plate as the plate margin begins to bend to the west near cross sound and encounters the southeastern edge of the yakutat block partitioning of seismic slip is evident both onshore and offshore although the largest earthquakes magnitude 6 0 in this region have slip parallel to plate motion most moderate sized events have slip vectors rotated clockwise from plate motion offshore the 1973 cross sound sequence indicates that the southern end of the transition zone is seismogenic with the pacific plate being thrust beneath the yakutat block while onshore strike slip faulting has occurred along the fairweather border ranges and denali faults in the st elias region thrust faulting is occurring along and above the plate interface moderate sized events in the st elias show a mix of slip vector orientations in the pamplona zone slip vectors of offshore earthquakes and deformation directions determined onshore from gps studies show a counter clockwise rotation relative to plate motion suggesting that a change in strain field occurs just west of the st elias region

collisional model for rapid fore arc block rotations arc curvature and episodic back arc rifting in subduction settings 1 we review geophysical and geological data from 23 active and ancient plate boundaries to document a compelling spatial and temporal relationship between collision plate boundary curvature rapid tectonic rotations and the occurrence of back arc rifting our observations support a conceptual model where the change from subduction to collision causes rapid fore arc rotation when viewed in a reference frame relative to the bounding tectonic plates leading to marked plate boundary curvature our global compilation reveals that most active back arc rifts are associated with rapidly rotating fore arcs and nearby collisions thus we propose that collision and rapid fore arc rotations play a major role in the evolution and kinematics of back arc basins we conduct numerical modeling to better understand the physical processes behind these observed rapid tectonic rotations at convergent margins our results suggest that the presence of an indentor or choke point in the subduction system e g collision can generate rapid rotation of the fore arc about a nearby pole and lead to back arc rifting the rate of fore arc rotation and back arc rifting depends on the incoming indentor velocity and can be greatly enhanced by slab rollback and the presence of a low viscosity back arc where viscosity of the back arc is low fore arc rotation dominates where back arc viscosity is high the formation of strike slip faults and tectonic escape dominates our observational and model derived results illustrate that shallow crustal forces produced by the entry of buoyant features into subduction zones are a fundamental mechanism for the generation of fore arc block rotations plate boundary curvature back arc rift evolution and tectonic escape in subduction systems rollback of the subducting slab within the mantle will certainly enhance the processes we describe but it is not the only driving force

an archean metamorphic core complex in the southern slave province basement cover structural relations between the sleepy dragon complex and the yellowknife supergroup archean rocks in the fenton lake brown lake area southern slave province are subdivided into two lithotectonic domains a supracrustal domain which consists mainly of the archean yellowknife supergroup and a gneiss granite domain the latter is composed of gneissic and metaigneous rocks of the sleepy dragon complex determined to be basement to the yellowknife supergroup and granite plutons including the 2641 3 5 ma suse lake granite and the 2583 5 1 ma morose granite volcanic rocks of the cameron river belt and greywacke mudstone turbiditic metasedimentary rocks of the burwash formation constitute the supracrustal domain a late archean amphibolite to greenschist facies ductile to local brittle high strain zone separates the domains kinematic indicators demonstrate that the zone experienced two kinematically opposed episodes of displacement the older episode involved pre to synthermal peak thrusting of the supracrustal rocks over the gneiss granite domain thrusting is kinematically an

geochemical and nd pb isotopic systematics of late archean granitoids southwestern slave province canada constraints for granitoid origin and crustal isotopic structure new geochemical and nd pb isotopic data for 2 62 2 59 ga granitoids from the southwest slave province are used to determine the source s of granitoid magmas to evaluate the role of pre 2 8 ga basement during this magmatism and to refine the existing nd pb isotopic structure of the western slave province the pb isotopic data require crust older than 3 2 ga as a granitoid protolith whereas the nd isotopic data require input from juvenile crustal material this discrepancy is explained if the granitoid protoliths are mixtures of ancient basement and 2 7 ga juvenile crust in varying proportions specifically granitoids from the southwestern slave province require 10 30 basement whereas granitoids from other parts of the western slave province require 50 incorporation of basement as a protolith may be achieved indirectly by assimilation of basement during juvenile 2 7 ga magmatism or directly during 2 62 2 59 ga magmatism the granitoid isotopic data suggest that indirect basement input was

tectonic drivers of the wrangell block insights on fore arc sliver processes from 3 d geodynamic models of alaska intracontinental shear zones can play a key role in understanding how plate convergence is manifested in the upper plate in regions of oblique subduction however the relative role of the driving forces from the subducting plate and the resisting force from within intracontinental shear zones is not well understood results from high resolution geographically referenced instantaneous 3d geodynamic models of flat slab subduction at the oblique convergent margin of alaska are presented these models investigate how viscosity and length of the denali fault intracontinental shear zone as well as coupling along the plate boundary interface modulate motion of the wrangell block forearc sliver and slip across the denali fault models with a weak denali fault 1017 pa s and strong plate coupling 1021 pa s were found to produce the fastest motions of the wrangell block 10 mm yr the 3d models predict along strike variation in offs et along the denali fault changing from dextral strike slip motion in the eastern segment to oblique convergence towards the fault apex models further show that the flat slab drives oblique motion of the wrangell block and contributes to 20 models with a short fault and 28 models with a long fault of the observed quaternary slip rates along the denali fault the 3d models provide insight into the general processes of forearc sliver mechanics and also offer a 3d framework for interpreting hazards in regions of flat slab subduction

seismicity of the wrangell and aleutian wadati benioff zones and the north american plate along the trans alaska crustal transect chugach mountains and copper river basin southern alaska the configuration of the aleutian and wrangell wadati benioff zones in southern alaska between longitudes 142 w and 148 w is determined from regionally recorded earthquakes from 1971 to 1986 the earthquakes are located with a simple velocity model calibrated with seismic refraction measurements conducted for the trans alaska crustal transect tact west of the transect meridian approximately 145 20 w and beneath the chugach mountains shocks in the depth range 25 60 km compose the shallow part of the northwest to north dipping aleutian zone this zone can be traced updip and seaward beneath prince william sound east of the transect meridian and north of 61 n latitude the weakly active north northeast dipping wrangell zone extends to a depth of at least 100 km beneath the wrangell volcanoes and along strike for at least 150 km the two zones appear to be continuous at least in the depth range 20 45 km and may define adjacent limbs of a buckle in the subducted oceanic lithosphere north of 61 n focal mechanisms in the shallow aleutian zone are compatible with a uniform stress field characterized by in plane least compressive stress directed toward the steeply dipping deeper part of the aleutian zone north of cook inlet stress orientation in the wrangell zone is unresolved because of the low level of seismicity no focal mechanisms corresponding to slip on a gently northward dipping megathrust have been observed along the transect meridian a change of focal mechanism with depth near the top of the dipping seismic zone is inferred to reflect the interplate boundary beneath the chugach mountains this boundary coincides with a band of strong reflectors imaged by tact vertical seismic profiling the overriding plate appears to be aseismic beneath the northern chugach mountains but minor shallow seismicity occurs beneath the copper river basin first motion patterns for these shallow shocks suggest the most compressive stress within the overriding plate is oriented subhorizontally and approximately north south

new constraints on the motion of the fairweather fault alaska from gps observations 1 gps velocities from sites near to the fairweather fault in southern alaska are combined with line length data from geodetic surveys by the u s geological survey usgs to estimate the slip rate and locking depth of the fairweather fault using dislocation theory we perform a weighted least squares inversion of the geodetic data and obtain a best fitting slip rate of 45 6 2 0 mm yr and locking depth of 9 0 0 8 km the slip rate we estimate is one of the highest observed across any strike slip fault we also include the dalton creek segment of the denali fault in our model and estimate a slip rate of 3 8 1 4 mm yr for this section of the fault this is substantially lower than the rate observed for the denali fault segment to the northwest which ruptured in the november 3 2002 mw 7 9 denali fault earthquake

geochronological constraints on late archean magmatism deformation and gold quartz vein mineralization in the northwestern anialik river greenstone belt and igneous complex slave province n w t three phases of the anialik river igneous complex aric give u pb zircon ages in the range 2705 2683 ma and three units from the adjacent northwestern anialik river greenstone belt argb give ca 2678 ma ages titanite from unsheared aric rocks crystallized during localized metamorphism and deuteric alteration between 2693 and 2683 ma hydrothermal titanite in wall rocks to gold mineralized shear zones crystallized during early shear zone development 2670 1 ma and was subsequently locally altered to rutile with gold occurring within the rutile bearing assemblage unaltered second generation hydrothermal titanite overgrowing the rutile assemblage crystallized during later brittle ductile movement 2656 2 ma and provides a minimum age for gold mineralization relatively high 207pb 204pb ratios of pb in gold associated galena suggest that it was partly derived from significantly older crustal material possibly underlying the igneous complex and greenstone belt this interpretation is consisten

high sedimentation rates and thrust fault modulation insights from ocean drilling offshore the st elias mountains southern alaska abstract the southern alaskan margin offshore the st elias mountains has experienced the highest recorded offshore sediment accumulation rates globally combined with high uplift rates active convergence and extensive temperate glaciation the margin provides a superb setting for evaluating competing influences of tectonic and surface processes on orogen development we correlate results from integrated ocean drilling program iodp expedition 341 sites u1420 and u1421 with regional seismic data to determine the spatial and temporal evolution of the pamplona zone fold thrust belt that forms the offshore st elias deformation front on the continental shelf our mapping shows that the pattern of active faulting changed from distributed across the shelf to localized away from the primary glacial depocenter over 300 780 kyrs following an order of magnitude increase in sediment accumulation rates simple coulomb stress calculations show that the suppression of faulting is partially controlled by the change in sediment accumulation rates which created a differential pore pressure regime between the underlying faulted strata and the overlying undeformed sediments

tectonic block motion and glacial isostatic adjustment in southeast alaska and adjacent canada constrained by gps measurements 1 we use data from campaign and continuous gps sites in southeast alaska and the neighboring region of canada to constrain a regional tectonic block model that estimates block angular velocities and derives a self consistent set of fault slip rates from the block motions present day tectonics in southeast alaska is strongly influenced by the collision of the yakutat block our model predicts a velocity of 50 3 0 8 mm a toward n22 9 0 6 w for that block our results suggest that the eastern edge of the yakutat block is deforming along this edge the fairweather fault accommodates a large portion of the pacific north america relative plate motion through 42 9 0 9 mm a of dextral slip further south along the queen charlotte fault our model predicts an average of 43 9 0 6 mm a of dextral slip and a southward increasing amount of transpression strain from the yakutat collision is transferred far to the east of the strike slip system this strain transfer causes the region north of glacier bay to undergo a clockwise rotation south of glacier bay and inboard of the queen charlotte fault a smaller but clearly defined clockwise rotation is observed the heterogeneous block motion north and south of glacier bay may indicate the area is undergoing internal deformation and could explain regional patterns of diffuse seismicity the northern cordillera of canada displays a small northeasterly motion our block model suggests that the entire southeastern alaska northwestern canada margin is mobile

exhumation along the fairweather fault southeastern alaska based on low temperature thermochronometry 1 the southern alaskan syntaxis marks the spectacular junction between the 1000 km long pacific north america transform margin and the chugach st elias belt where subduction and terrane accretion drive rapid convergent deformation and rock uplift new low temperature thermochronometry reveals that intense orogenic deformation is not restricted to one side of the syntaxis but extends nearly 300 km south along the dextral fairweather fault apatite and zircon u th he ages as young as 0 9 and 2 0 ma suggest maximum exhumation rates of nearly 2 mm a in close proximity 0 5 mm a along the entire plate margin we estimate that long term rock uplift accommodates 3 mm a of fault normal convergence in this area this suggests that the fairweather fault is slightly transpressive and highly partitioned analogous to the central san andreas fault this convergence only accounts for 1 5 of the obliquity between pacific plate motion and the continental margin however implying the deficit is taken up by 1 2 cm a thrust sinistral motion along the offshore transition fault additionally thermochronometry shows a marked increase in bedrock cooling coincident with onset of heavy glaciation similar to what has been observed in other parts of the pacific northwest the tectonically active fairweather corridor is distinguished however by the magnitude of the acceleration and the depth of exhumation since pliocene climate change

an extensional fault model for the early development of greenstone belts with reference to a portion of the abitibi belt ontario canada abstract detailed geological mapping of a small part of the archaean abitibi greenstone belt northwestern ontario suggests that the mafic and ultra mafic lavas were extruded in a submarine environment in which the volcanism was intimately associated with deformation of the growing volcanic pile the deformation led to the subsidence and progressive tilting of the lavas and to the development of discontinuities a review of selected areas of quaternary volcanism suggests that subsidence of the lava pile during volcanism is not unusual and that activity is frequently also associated with normal faulting in both volcanic and non volcanic extensional regimes deformation is dominated by the development of listric faults we argue that greenstone belts such as the abitibi were formed in extensional regimes and we show that a simple geometric model involving fissure volcanism and synvolcanic rotation of the lavas on listric faults accounts for the major primary features of greenstone belts and for some associated geophysical properties

geology and u pb geochronology of the neoarchean snare river terrane tracking evolving tectonic regimes and crustal growth mechanisms u pb zircon crystallization ages determined by isotope dilution thermal ionization mass spectrometry id tims and laser ablation microprobe inductively coupled plasma mass spectrometry lam icp ms for 13 intrusive units in the neoarchean snare river terrane srt provide tight constraints on the timing of crust formation and orogenic evolution seven metaluminous plutons were emplaced over 80 ma from ca 2674 to 2589 ma whereas six peraluminous bodies were emplaced in a 15 ma interval from ca 2598 to 2585 ma a detrital zircon study yielded an age spectrum with peaks correlative with known magmatic events in the slave province with the ca 2635 ma age of the youngest detrital zircon population providing a maximum estimate for the onset of sedimentation this age contrasts with evidence for pre 2635 ma sedimentation elsewhere in the srt indicating that sedimentation was protracted and diachronous evolution of the srt can be subdivided into four stages i 2674 2635 ma formation of a metal

the slave province a distinct archean craton the slave province is a relatively small late archean craton that exhibits distinctive rock associations and structures by comparison with the much larger superior province differences are evident in i the abundance of sedimentary versus volcanic rocks and of felsic versus mafic volcanic rocks ii the greater evidence for sialic basement iii the higher proportion of more evolved potassium rich granite iv the type setting and timing of gold and base metal mineralization and v a regional zonation of gold deposits that seems to be a unique feature of the slave province contrasts in structure are also significant the large scale linear belts and boundaries sutures that characterize the superior province have no obvious counterpart in the slave province despite some similarities with other archean cratons the distinct features of the slave province are important for they imply that no single archean craton should be used to develop a universal paradigm for the genesis and tectonic evol

a new look at the stratigraphy of the yellowknife supergroup at yellowknife n w t implications for the age of gold bearing shear zones and archean basin evolution based on recent detailed mapping a revised stratigraphic column is proposed for the rocks of the archean yellowknife supergroup in the yellowknife greenstone belt the mafic volcanic rocks of the kam formation previously thought to represent the oldest supracrustal rocks of the belt overlap remnants of an earlier volcanic sedimentary sequence here referred to as the octopus formation as its enormous thickness makes it too unwieldy to be described as a single formation the kam formation is raised to group status and subdivided into four formations it is proposed that the kam group should replace the beaulieu group in the yellowknife area the chan formation at the base of the kam group consists of multiple gabbroic intrusions that were emplaced into a carapace of pillowed flows the intrusions locally resemble sheeted mafic dyke complexes in phanerozoic ophiolites thought to represent evidence for sea floor spreading the crestaurum formation which overlies the chan formation is characterized b

the timing of yellowknife gold mineralization a temporal relationship with crustal anatexis in order to thoroughly evaluate the genesis of orogenic gold mineralization in the context of plate tectonics absolute age data are required to link mineralization with lithological evolution we show that pyrite associated with orogenic gold mineralization from the con deposit in the ca 2700 ma yellowknife greenstone belt has an re os isochron age of 2591 37 ma this is identical to the previously proposed timing of ca 2592 ma for s type pluton related lode gold within ca 2660 ma graywacke mudstone turbidites east of the greenstone belt the re os pyrite data indicate that orogenic gold mineralization was deposited ca 60 to 100 m y after the formation of the host rocks in the yellowknife district and when combined with the regional geologic framework indicate that the gold mineralization formed in a distinct geodynamic setting from that during host rock formation initial os values in the pyrite os i 0 78 0 17 indicate a significant crustal component of os to the ore system compatible with metals being derived from the older host rocks regionally preserved granulites migmatites and charnockites derived in part from the older supracrustal sequences formed at 2595 to 2585 ma corresponding to the timing of gold mineralization s type plutons with ca 2592 ma crystallization ages in the upper crust reflect this high temperature metamorphic event these plutons have turbidite hosted gold deposits associated with their metamorphic aureoles collectively the isotopic and regional geologic data indicate that gold mineralization was related to regional scale crustal anatexis which likely resulted from crustal accretion or collision outside what is now preserved of the craton

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

geochemical and nd isotopic constraints for the origin of late archean turbidites from the yellowknife area northwest territories canada abstract a detailed geochemical and isotopic study of late archean turbidites and volcanic rocks from the yellowknife area slave province was undertaken to constrain the nature of exposed crust at the time of 2 6 to 2 7 ga crustal consolidation the nd t values of the volcanic rocks range from 1 7 to 4 4 this variation can be produced by assimilation of pre 2 8 ga basement by a depleted mantle derived magma possibly followed by fractional crystallization the turbidites are typically metamorphosed to greenschist to amphibolite facies and where metamorphosed to greenschist facies different units of bouma sequence can be observed the different units of bouma sequence were sampled and analyzed separately to evaluate the possible differences in geochemical and isotopic signatures the geochemical data presented here is in accord with the previously proposed model that argues for a mixture of 20 mafic intermediate volcanic rocks 55 felsic volcanic rocks and 25 granitic rocks as a source of these turbidites however our revised calculation with the new data presented here argues for 1 to 2 input from an ultramafic source as well as somewhat higher input from mafic intermediate volcanic sources in the upper shale units compared to the lower sand units the nd t values of the turbidites generally are lower in the upper shale units compared to the lower sand units detailed inspection of trace element data suggest that this is not an artifact of rare earth element rich heavy minerals concentrating in the lower sand units of the turbidites but rather is a result of unmixing of detritus with different nd t values during sediment transportation and deposition the upper shale units of the turbidites are isotopically compatible with a derivation mainly from crustally contaminated volcanic rocks similar to those exposed in the yellowknife area the lower sand units contain a higher proportion of westerly derived plutonic rock detritus characterized by higher nd t suggesting that there are area s west of yellowknife not underlain by older 2 8 4 0 ga basement the trace element characteristics of these turbidites i e cr ni la th sc eu eu and gd n yb n are distinct from those of typical post archean turbidites this observation is consistent with the models that predict that the chemical composition of the upper continental crust was slightly different in the archean compared to post archean time

neodymium isotopic evidence for the tectonic assembly of late archean crust in the slave province northwest canada the ca 2 7 2 5 ga slave province is a granitegreenstone terrane comprising deformed sedimentary and subordinate volcanic belts extensively intruded by granitoid rocks the nd isotopic data are reported for 58 samples of supracrustal and granitoid rocks exposed along a 400 km east west transect at 65 n across the structural grain of the province initial nd values reveal distinctly different crustal sources in the eastern compared to the western parts of the province as expected from tectonic assembly of the province through accretion of juvenile crust to older continental crust supracrustal sequences ca 2 71 2 65 ga from the central and eastern parts of the province have positive nd 1 values 0 3 to 3 6 consistent with juvenile sources and formation remote from significantly older crust syn to late deformation ca 2 63 2 60 ga mantle derived diorites and related tonalites type i from the central and eastern parts of the province have similar initial nd values 0 1 to 2 7 in contrast samples from the westernmost plutons which intrude exposed pre 3 1 ga crust have much lower nd 1 values 1 0 to4 6 suggesting contamination of these magmas by older crust the nd 1 values of post deformation granites s s type ii also vary systematically across the province values for granites west of longitude 110 30 w range from 0 2 to 5 3 those to the east range from 0 6 to 3 7 these data suggest mixed crustal sources dominated by mid to early archean material nd 2 6 to 17 at 2 6 ga for the western granitoid rocks and juvenile sources for the eastern granites the nd isotopic data are consistent with the geology of the province in that exposures of mid to early archean crustal rocks predating the principal 2 7 2 5 ga orogenic event are restricted to the western part of the province the asymmetric pattern defined by the nd isotopic data indicates the presence of distinct crustal rocks beneath the slave province similar isotopic variations observed across phanerozoic collisional orogens have been interpreted to reflect tectonic assembly of crust by accretion of juvenile crustal terranes to an older continental margin this process may also have been an important mechanism in the cratonization of the slave province

field relations u pb geochronology and sm nd isotope geochemistry of the point lake greenstone belt and adjacent gneisses central slave craton n w t canada data from the point lake area central slave craton suggest an intimate tectonic and paleogeographic association between volcano sedimentary supracrustal rocks and adjacent gneisses granite plutons and orthogneisses yield u pb zircon crystallization ages ranging from ca 3230 to 2818 ma numerous mafic dykes cut the gneisses and two have been dated by u pb zircon geochronometry at 2673 3 and 2690 3 ma ages similar to those of volcanic rocks in the point lake greenstone belt although high strain zones form the greenstone gneiss in most places a structural repetition of granite about 4 km east of keskarrah bay is cut by numerous mafic dykes and apparently overlain depositionally by pillow basalt mafic volcanic and plutonic rocks from point lake have initial 2 7 ga end values ranging from about 2 2 to 6 3 significantly lower than the depleted mantle at that time the nd data suggest either derivation from a more isotopically evolved reservoir or assimilation of crust similar to the granit

evolution of an archean basement complex and its autochthonous cover southern slave province canada abstract the sleepy dragon complex sdc is one of several antiformal exposures of a 4 0 2 85 ga basement block underlying the west central region of the archean slave province northwestern canada this basement block the central slave basement complex csbc is overlain by an autochthonous to locally parautochthonous dominantly quartz rich clastic sequence the 2 85 2 80 ga central slave cover group together these units comprised a fundamental building block during neoarchean growth and assembly of the slave craton the well exposed patterson lake morose lake area represents a type locality for study of the csbc central slave cover group and overlying rocks of the yellowknife supergroup field mapping and u pb geochronology employing both thermal ionisation mass spectrometry tims and sensitive high resolution ion microprobe shrimp document a prolonged crustal history oldest intact basement units consist of foliated to gneissic tonalite dated in two places at 2955 12 ma and 2944 9 ma indirect evidence for a ca 3150 ma basement component is obtained both from xenocrystic zircon in tonalite and detrital zircons from the overlying central slave cover group which here consists of the patterson lake formation a granite boulder from this formation is dated at 2934 3 ma and may be locally derived a sheared mafic volcanic unit along the basement cover contact contains 2942 3 ma metamorphic titanite which provides a minimum deposition age this volcanic unit is therefore also a part of the basement complex younger events in the sdc mainly reflect tonalite trondhjemite granodiorite plutonism during construction of the overlying cameron river and beaulieu river greenstone belts of the yellowknife supergroup a unit of k feldspar megacrystic granodiorite is dated at 2726 3 ma and tonalite and granodiorite bodies located toward the centre of the sdc have primary crystallization ages of 2683 3 2 ma 2677 2 ma and 2672 7 6 ma the core of the southern sdc is occupied by a late tectonic to post tectonic granite pluton previously dated at 2586 2 ma an intravolcanic unconformity along the southern margin of the sdc developed in part on 2 7 ga granodiorite is constrained by existing u pb data to have formed sometime between ca 2685 2660 ma an abundance of synvolcanic plutons of similar age and the presence of a 3 km thick mafic volcanic sequence overlying a less dense magmatically active felsic crust suggest that gravitational instability and magmatic diapirism may have played important roles in doming and uplift of the sdc prior to regional d 1 deformation the younger toward centre zonation of plutonic rocks in the sdc is documented in other domal granite gneiss complexes of archean age e g pilbara craton western australia where it is attributed to upward and outward displacement of older crust during repeated magma channeling into a growing mid crustal dome in the west central slave craton such a process might account for the regional presence of local unconformities of apparently similar age the domal basement plutonic complexes would have been further amplified after 2660 ma during the development of regional f 1 and f 2 fold belts

re os molybdenite ages from the archean yellowknife greenstone belt comparison to u pb ages and evidence for metal introduction at 2675 ma the late archean yellowknife greenstone belt in the southwestern slave province hosts world class orogenic gold deposits with total production of 15 million ounces moz of gold the timing of gold mineralization is estimated to be ca 2600 ma based on the relative timing of d 2 structures and associated formation of deposit hosting shear zones the majority of studies on the deposits indicate that gold was emplaced by metamorphically driven hydrothermal fluids depositing gold in chemical and structural traps alternatively based on ore mineral compositions proximity to plutons and increased gold grades in the vicinity of quartz feldspar porphyry dikes it has been suggested that the gold deposits may also be related to the intrusions in this study we present two new re os molybdenite ages from two intermediate to felsic plutonic rocks and compare them with u pb zircon crystallization ages the duckfish aplite contains intergrown molybdenite and yields a re os model age of 2611 7 ma the same as a u pb zircon crystallization age of 2611 2 1 5 ma for the aplite and within assigned error of a previously reported 2608 3 ma u pb titanite age for the duckfish pluton the subvolcanic ryan lake pluton has synplutonic quartz veining with intergrown molybdenite that yields an re os model age of 2679 8 ma repeat analysis of 2675 7 within assigned error of the 2671 6 5 ma u pb zircon crystallization age of the pluton in both cases the re os and u pb chronometers survived later low to moderate temperature low pressure contact and regional metamorphism the ca 2675 ma ryan lake pluton and associated quartz feldspar porphyry dikes have gold contents significantly higher than the least altered ore hosting mafic volcanic rocks but the pluton is significantly older than the ca 2600 ma relative age of the gold deposits the age data and the fact that an early pluton carries elevated gold suggest that similar plutons and associated quartz feldspar porphyry dikes may have fertilized the greenstone belt prior to formation of the orogenic gold deposits we suggest that when ca 2600 ma metamorphically derived fluids interacted with these older rocks the gold budget in the hydrothermal fluid could have been increased as evidenced by increased gold grades near porphyries an intrusion related early introduction of gold into the yellowknife greenstone belt may have implications for other archean gold deposits where there is a spatial relationship between gold and early plutonic rocks

neoarchean decoupling of upper and mid crustal tectonothermal domains in the southeast slave province evidence from the walmsley lake area the walmsley lake area of the southeastern slave province northwest territories exposes a crustal transect from greenschist through to upper amphibolite facies two tectonothermal crustal domains are proposed based on disparate tectonic histories and rock types within the upper tectonothermal domain d1 deformation reached lower amphibolite facies prior to 2614 ma and associated m1 metamorphic conditions outlasted d1 peak m2 conditions reached middle amphibolite facies prior to 2603 ma and outlasted d2 deformation deformation style and metamorphic sequencing during these two pre 2600 ma events are consistent with regional crustal shortening and thickening in the upper tectonothermal domain at mid crustal levels in the lower tectonothermal domain a third tectonothermal event d3 m3 produced uppermost amphibolite facies peak metamorphic conditions transposed preexisting fabrics to shallow dips and produced a subhorizontal foliation and recumbent folds at ca 2583 ma d3 structures are not present

intensity of the earth s magnetic field in late archean obtained from diabase dikes of the slave province canada abstract the geomagnetic field intensity in late archean times is evaluated from archean diabase dikes intruded into the yellowknife greenstone belt of the slave province n w t canada the dike set used is referred to as the 8a dikes and their age is bracketed between 2620 and 2642 ma by crosscutting relationships between the 8a dikes and adjacent granitic rocks dated by u pb method paleomagnetic directional data after thermal demagnetization thd and results of some field tests suggest that the characteristic remanent magnetizations chrm of the dikes have been acquired at ca 2 6 ga several rock magnetic studies indicate that the dike samples are suitable for paleointensity determinations thelliers method with consistency checks was used for the paleointensity determinations and two of the 8a dikes yielded the mean paleointensities of 30 9 1 1 and 43 9 1 2 t respectively the corresponding virtual dipole moments vdm of 6 3 0 2 1022 a m2 and 9 0 0 2 1022 a m2 are similar to the present day value suggesting that in the earth s core the dynamo process of comparable activity to that of the present day has already existed at ca 2 6 ga

multiple dikes in lower kam group yellowknife greenstone belt evidence for archean sea floor spreading a mafic intrusive extrusive complex at the base of the kam group in the archean yellowknife supergroup at yellowknife northwest territories canada grades from gabbros through a zone of multiple and sheeted dikes into massive and pillowed flows with thin beds of interflow sediments although rock types and primary structures resemble those in upper parts of phanerozoic ophiolites the kam group lacks an ultramafic base and is much thicker than typical ophiolites because similar mafic sections occur in certain atypical ophiolites and in plateau areas of modern ocean basins however the kam group is viewed as a remnant of archean oceanic crust rather than as a product of intracontinental rift volcanism the dikes are analogous to sheeted dikes of ophiolites and are suggestive of sea floor spreading in an archean marginal basin setting

evidence for past subduction earthquakes at a plate boundary with widespread upper plate faulting southern hikurangi margin new zealand abstract at the southern hikurangi margin new zealand we use salt marsh stratigraphy sedimentology micropaleontology and radiocarbon dating to document evidence of two earthquakes producing coseismic subsidence and in one case a tsunami over the past 1000 yrs the earthquake at 520 470 yrs before present b p produced 0 25 0 1 m of subsidence at big lagoon the earthquake at 880 800 yrs b p produced 0 45 0 1 m of subsidence at big lagoon and was accompanied by a tsunami that inundated 360 m inland with a probable height of 3 3 m distinguishing the effects of upper plate faulting from plate interface earthquakes is a significant challenge at this margin we use correlation with regional upper plate paleoearthquake chronologies and elastic dislocation modeling to determine that the most likely cause of the subsidence and tsunami events is subduction interface rupture although the older event may have been a synchronous subduction interface and upper plate fault rupture the southern hikurangi margin has had no significant m 6 5 documented subduction interface earthquakes in historic times and previous assumptions that this margin segment is prone to rupture in large to great earthquakes were based on seismic and geodetic evidence of strong contemporary plate coupling this is the first geologic evidence to confirm that the southern hikurangi margin ruptures in large earthquakes the relatively short time interval between the two subduction earthquakes 350 yrs is shorter than in current seismic hazard models online material historical accounts description of vertical deformation core names foraminifera census and abundance diatom census modern analog samples map of cores collected stratigraphic correlation diagram for all cores and detailed core logs

concentration of gold during retrograde metamorphism of archean banded iron formations slave province canada gold bearing iron formations are widely distributed within extensive metasedimentary terranes of the archean slave province situated in the northwestern canadian precambrian shield mineralized iron formations occur within thick turbidite sequences overprinted by a protracted history of deformation metamorphism and plutonism economically significant gold prospects are specifically sited at structural culminations characterized by polyphase folding based on garnet biotite geothermometry on the stable prograde metamorphic assemblage of enveloping metapelites peak metamorphic conditions are approximated to be 570 c and 4 kbar 1 kbar 100 mpa diagnostic prograde mineralogy reveals that two facies of silicate iron formation are represented at the five gold occurrences investigated 1 amphibolitic iron formation aif characterized by quartz grunerite hornblende pyrrhotite garnet f graphite ilmenite and 2 pelitic iron formation pif consisting of quartz biotite garnet ilmenite f grunerite hornblende textures reveal that grunerite crystallization preceded hornblende and garnet within aif banded pyrrhotite is in textural equilibrium with prograde metamorphic minerals retrograde hornblende garnet zoisite apatite carbonate ferroactinolite and gold bearing sulphide minerals replace the prograde mineral assemblages on the margins of quartz veins that intensify at aif fold hinges it is hypothesized that the iron formation hosted gold deposits of the slave province are a result of multistage processes gold concentrated at high background levels within pyrrhotite bearing aif was remobilized during fluid migration into brittle aif fold hinges in subsequent metamorphic and deformational events metamorphic fluid ponded in fractured aif hinge domains caused retrogressive replacement quartz veining and gold bearing sulphide precipitation during waning temperature although the mineralized hinge zones commonly display evidence of late chloritization this alteration did not further affect gold distribution the gold precipitated with destabilization of thio complexes due to sulphidation prior to lowtemperature hydrothermal activity

u pb zircon ages for rocks from the island lake greenstone belt manitoba the archean hayes river group of the island lake greenstone belt superior province sachigo subprovince comprises mafic to felsic metavolcanics subvolcanics and associated metasedimentary rocks the hayes river group is intruded by granitoid rocks belonging to the early intrusive complex one such pluton the bella lake tonalite is intrusive into the metabasalt of the hayes river group and has a u pb zircon age of 2886 15 ma similar intrusives of this complex either internal or marginal to the greenstone belt yield zircon ages of 2801 8 ma pipe point tonalit and 2768 22 ma linklater island prophyry this suggests that the early intrusive complex was emplaced over an 120 ma long interval by at least three separate intrusive episodes subsequent to the emplacement of the early intrusive complex the isoclinally folded hayes river group and the early intrusive complex were uplifted eroded and followed by the unconformable deposition of the island lake group comprising fluvial to marine

effect of the northward migrating mendocino triple junction on the eel river forearc basin california structural evolution offshore northern california the gorda plate is subducting obliquely beneath north america the resulting complicated tectonic setting forms the southern end of the cascadia subduction zone the southern cascadia subduction zone and overlying eel river forearc basin lie just north of the unstable mendocino triple junction the neogene strata of the eel river basin record structural deformation caused by the underthrusting of the gorda plate as well as deformation generated by northward migration and encroachment of the mendocino triple junction three distinct deformation regimes are present in the eel river forearc basin 1 along the western margin of the forearc basin and within the foreslope of the accretionary prism thrust faults and anticlines record pliocene pleistocene shortening caused by subduction of the gorda plate 2 the southern part of the basin rotated counterclockwise in the late pleistocene resulting in modern transpressional deformation offshore humboldt bay the rotation and deformation are caused by north south convergence across the boundary between the pacific plate and the southernmost part of the forearc basin at the triple junction 3 the northeastern margin of the eel river basin is deformed by high angle faults with a component of strike slip motion that may represent the incipient northward propagation of the pacific north american transform system north of the triple junction

mapping the megathrust beneath the northern gulf of alaska using wide angle seismic data in the northern gulf of alaska and prince william sound we have used wide angle seismic reflection refraction profiles earthquake studies and laboratory measurements of physical properties to determine the geometry of the prince william and yakutat terranes the aleutian megathrust and the subducting pacific plate in this complex region the yakutat terrane is underthrust beneath the prince william terrane and both terranes are interpreted to be underlain by the pacific plate wide angle seismic reflection refraction profiles recorded along five seismic lines are used to unravel this terrane geometry modeled velocities in the upper crust of the prince william terrane to 18 km depth agree closely with laboratory velocity measurements of orca group phyllites and quartzofeldspathic graywackes the chief components of the prince william terrane to hydrostatic pressures as high as 600 mpa 6 kbar a landward dipping reflector at depths of 16 24 km is interpreted as the base of the prince william terrane this reflector corresponds to the top of the wadati benioff zone seismicity and is interpreted as the megathrust immediately beneath the megathrust is a 4 km thick 6 9 km s refractor which we infer to be the source of a prominent magnetic anomaly and which is interpreted by us and previous workers to be gabbro in eocene age oceanic crust of the underthrust yakutat terrane wide angle seismic data magnetic anomaly data and tectonic reconstructions indicate that the yakutat terrane has been underthrust beneath the prince william terrane for at least a few hundred kilometers wide angle seismic data are consistent with a 9 to 10 landward dip of the subducting pacific plate beneath the outer shelf and slope distinctly different from the inferred average 3 to 4 dip of the overlying 6 9 km s refractor and the wadati benioff seismic zone beneath the inner shelf our preferred interpretation of the geophysical data is that one composite plate composed of the pacific plate of a fairly uniform thickness and the yakutat plate of varying thickness is subducting beneath southern alaska

temporal distribution of granitoid plutonic rocks in the archean slave province northwest canadian shield granitoid rocks in the slave province consist of 4 0 2 8 ga granitoid gneisses that predate the 2 71 2 65 ga volcanic and turbiditic rocks of the yellowknife supergroup and 2 70 2 58 ga granitoid plutons that intrude the yellowknife supergroup u pb zircon ages and nd and pb isotopic data indicate that the older granitoids are restricted to the western part of the slave province granitoid gneisses in the eastern slave province previously suspected to predate the yellowknife supergroup are similar in age to the volcanic rocks in this paper the results of a detailed geochronological study of plutonic rocks of the contwoyto lake nose lake area of the northcentral slave province are reviewed and integrated with the available age data base for plutonic rocks elsewhere in the slave province the data indicate that the timing of the later 2 7 ga plutonism is bimodal consisting of two distinct magmatic periods separated by approximately 20 ma of apparent magmatic quiescence the first period was synvolc

the 1987 1992 gulf of alaska earthquakes abstract we present a study of the 1987 1992 gulf of alaska earthquake sequence using relocated seismicity data together with body wave analysis of selected larger events most of the sequence is located on a n s trending fault directly south of the yakataga seismic gap and consists of four main events of mw 7 2 7 8 7 7 and 6 8 and associated aftershocks the first earthquake is of left lateral strike slip type on an ene wsw trending fault the second event is a right lateral strike slip earthquake on the n s trending fault the fault plane of the third strike slip event was identified as an ene wsw trending fault located to the south of that for the first event in a previous study using body wave analysis we show that though the body wave study cannot unambiguously identify the fault plane the temporal development of the seismicity together with the pattern of aftershock distribution on the conjugate fault suggests that this event also occurred on the n s trending fault and is of right lateral type the seismicity on the conjugate fault is interpreted as being triggered by the increase of the shear stress in the direction of the normal to the fault plane due to the main shock the occurrence of the fourth main shock a right lateral strike slip event in 1992 which itself can be considered an aftershock of the 1987 1988 sequence with epicentres distributed along a n s trending fault favours this conclusion the first three events of the sequence have been described in earlier studies as having too short a rupture length for their seismic moment if the rupture lengths inferred from the aftershock zones in this study are used we find that this is not the case the events in the 1987 1992 sequence lie along magnetic anomaly 13 and related conjugate fracture zones indicating that the oceanic crust is rupturing along pre existing zones of weakness in response to plate boundary stresses at the southern end the seismic activity terminates near a seamount which rises 1 km above an otherwise relatively low relief ocean floor and close to a fracture zone perpendicular to the n s trend of the earthquake sequence suggesting that the n s striking rupture along the n s trending fault is terminated here by a barrier at the northern end the sequence terminates at the alaska trench near the yakataga seismic gap where there is a right lateral offset in the bathymetry along the transition fault this offset also being visible on the gravity map of the region thus this offset has the same right lateral sense as the n s trending strike slip fault planes of three of the main shocks and suggests that though there was no significant seismic activity in this region of the gulf of alaska since 1964 faulting here has been active in the past

growth of granite greenstone terranes at convergent margins and stabilization of archean cratons abstract archean granite greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events most archean granite greenstone terranes appear to have acquired their first order structural and metamorphic characteristics at convergent plate margins where large accretionary wedges similar in aspect to the chugach makran and altaids grew through offscraping and accretion of oceanic plateaux oceanic crustal fragments juvenile island arcs rifted continental margins and pelagic and terrigenous sediments buoyant slabs of parts of archean oceanic lithosphere may have been underplated beneath these orogens forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere back stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas sanukitoid suite in the mantle wedge whereas other magmas tonalite trondhjemite granodiorite or ttg suite appear to have been generated by melting of hot young subducted slabs eventual collision of these juvenile orogens with other continental blocks formed anatectic granites then thickened the crust beyond its ability to support its own mass which initiated gravitational collapse and decompressional release of syn to late tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs and aided in the cratonization of the granite greenstone terranes deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid and upper crust forming ore deposits and plutons most preserved granite greenstone terranes have been tectonically stable since the archean and form the cratonic interiors of many continents

u pb zircon ages for magmatism in the red lake greenstone belt northwestern ontario u pb dating was carried out on nine volcanic rocks and two felsic intrusions from the red lake greenstone belt in order to establish an absolute time framework for the magmatic evolution of the area and yield first indications on the time of deformation and gold mineralization the data indicate a protracted period of igneous activity spanning at least 270 ma felsic volcanic rocks near the top of the tholeiitic to komatiitic sequence in the eastern part of the belt yield ages of and a third unit dated at contains inherited zircons older than 2982 ma which casts some uncertainty on the validity of the inferred intercept age rocks in the western part of the belt previously believed to form a relatively young calc alkalic sequence but now known to be dominantly tholeiitic are shown to be relatively old with ages of and these two dates also bracket the age of stromatolites occurring in chemical sediments that are under and overlain by the dated units another volcanic horizon in the centre of the b

mineral deposits in the archean slave structural province lithological and tectonic setting abstract the slave structural province an archean granite greenstone terrane covering 190 000 km 2 in the northwest territories of canada has an extensive history that goes back to earth s oldest preserved rocks it has been subjected to archean and proterozoic orogenies the former built the craton the latter destroyed or reworked large areas reducing the slave to its present size a mounting body of data suggests that primitive sialic crust had a major influence on the evolution of most slave rock types supracrustals of the slave province comprise abundant greywacke mudstone 80 and subordinate volcanics 15 that form two distinct types of volcanic belt yellowknife type dominated by tholeiitic basalts and hackett river type dominated by felsic intermediate calc alkaline pyroclastics slave volcanogenic massive sulphides have high silver lead zinc contents rare element pegmatites are associated with highly evolved granitoids in four widespread parts of the province and gold deposit types define zones trending northeasterly across the craton conglomerates quartz sandstones and quartz arenites formed in a shelf environment define a northerly trending zone locally coincident with a long lived fault and a major lead isotope boundary which together demarcate a western part of the slave containing old basement tectonic models for the slave comprise two types those that attempt to explain its formation in plate tectonic terms and those that suggest formation by rifting of primitive sialic crust with volcanics and granitoids derived largely by melting of that crust many features of the slave seem more easily explained by a rift related model rather than by plate tectonics

geochronological constraints on the tectonic evolution of a late archaean greenstone belt wabigoon subprovince northwest ontario canada abstract u pb isotopic age analyses have been carried out on zircon and titanite from rocks near the contact between the kakagi lake savant lake volcanic belt ksvb within the wabigoon subprovince and the english river subprovince in northwest ontario canada two tonalite gneisses intruded by mafic dykes at tannis lake and caribou lake have minimum zircon ages of 3051 ma and 3075 ma respectively this contrasts with a 2775 1 ma age on zircon from a felsic tuff located within a lower predominantly mafic volcanic sequence in the interior of ksvb these data combined with previous studies indicate that ksvb is presently flanked by older sialic crust in the sioux lookout and kashaweogama lake areas lower mafic volcanic sequences are unconformably overlain by sedimentary units that are in sheared contact with overlying calc alkaline volcanic units titanite and zircon analyses from clasts within a basal conglomerate indicate a maximum age for deposition of 2698 4 ma this late age is confirmed by a 2703 2 ma zircon age for an unconformably underlying porphyry intrusion one clast gives a zircon age of 2903 16 ma suggesting that the sediments were at least partly derived from older material in the english river subprovince data from previous studies show that the structurally overlying calc alkaline volcanic sequences are at least 30 ma older than the sediments the absence of zircon inheritance in all but the youngest ksvb sequences as well as geological and geochemical arguments suggest that the greenstone belt evolved largely in an ensimatic environment in isolation from older sialic material the intervening oceanic crust must therefore have been consumed by subduction which led to arc magmatism and eventual collision between the arc ksvb and continental material english river subprovince the sediments unconformably overlying the lowermost mafic volcanic sequences were deposited in a partly subaerial continental environment since chemical and textural evidence suggests the lower mafic volcanic sequences were deposited in an oceanic environment they must have been obducted onto the edge of a continental block this was perhaps followed by nappe and thrust emplacement on top of the sediments of calc alkaline sequences derived from earlier arc magmatism continued compression resulted in isoclinal folding of the supracrustal sequences into their present vertical positions

earthquakes in the pamplona zone yakutat block south central alaska the pamplona zone is a region of complex deformation and moderate seismicity located within the yakutat block a region that has been relatively aseismic since a series of large m 7 8 earthquakes in 1899 in 1970 a sequence of moderate to large sized earthquakes occurred within the pamplona zone largest event of mw 6 7 together with a mw 6 1 event in 1958 these events are the only m 5 5 events known to have occurred in the pamplona region since 1900 thus these events give important information on internal deformational processes within the yakutat block waveform modeling of three earthquakes in april 1970 showed rupture complexity along low angle thrust faults focal depths indicate that two of the events occurred above the wrangell aleutian megathrust while the largest event may have occurred on the megathrust events in 1958 and february 1970 indicate that deformation within the western pamplona zone is occurring along high angle 60 faults with reverse oblique motion we believe the pamplona spur the easternmost part of the pamplona zone may have behaved as an asperity during the 1899 sequence the location of the spur may be influenced by a north south trending fault zone in the subducting pacific plate that appears to be responsible for the 1987 1992 gulf of alaska sequence occurring 50 to 200 km south of the pamplona zone

peeling oceanic crust in subduction zones thin slabs of oceanic pillow basalt formed at ridge axes with associated pelagic sediments are common features in ancient accretionary complexes estimates of the average thicknesses of scraped off basaltic sections of oceanic crust in accretionary prisms vary from a few tens of metres to 300 m with a mean thickness of 80 100 m in the complexes studied the permeability contrast within the uppermost part of the oceanic crust is an important factor governing the thickness of delaminated oceanic crust this mechanism is an effective means of separating oceanic crust altered at low temperatures from the remainder of the downgoing slab and thus controlling geochemical budgets in subduction zones

origin and evolution of mid to late archean crust in the hanikahimajuk lake area slave province canada evidence from u pb geochronological geochemical and nd pb isotopic data abstract a geochronological major trace element geochemical and nd pb isotopic study of early to mid archean basement granitoids gneisses 2 68 ga greenstone belt and 2 58 2 62 ga late archean granitoids from the hanikahimajuk lake area of the northern slave province was undertaken to constrain the tectonomagmatic evolution of the north central area of slave province northwest territories canada the basement tonalites are precisely dated at 3378 1 ma by u pb zircon methods and are characterized by steeply fractionated ree patterns with no negative eu anomalies they were likely produced through partial melting of mafic crust at 0 8 gpa the endt of these rocks range from 0 3 to 2 1 implying involvement of even older crustal material in their genesis two different types of 2 68 ga mafic to intermediate volcanic rock are recognized in the hanikahimajuk lake area tholeiitic rocks with no ta nb depletions and calc alkaline rocks with slight negative ta nb depletions all of these rocks show clear evidence for contamination by the 3 4 ga basement tonalite thus precluding the possibility that some of these tholeiitic mafic rocks were produced in a mid oceanic ridge setting from the data presented here we conclude that the tholeiitic rocks were produced in a back arc setting likely similar to that of the japanese arc the calc alkaline rocks may either be a remnant of the volcanic arc or part of the same back arc system however the close proximity with the tholeiitic rocks in the field favours the latter possibility mafic to intermediate rocks with similar geochemical and isotopic characteristics are also found from the yellowknife area of the southwestern slave province and from the indin lake area of the westernmost slave province south of grenville lake area this observation may indicate that the mafic dominated volcanic belts of the western slave province may be a remnant of a single now collapsed and shortened back arc basin the existence of syn volcanic calc alkaline plutonic rocks with similar geochemical and isotopic features as the calc alkaline volcanic rocks supports this possibility the geochemistry of late archean granitoids suggests that they are produced mainly through partial melting of mafic igneous protoliths and clastic sedimentary rocks and possibly tonalitic rocks similarity in the end 2 6 values between these granitoids and the supracrustal rocks in the area is in accord with this model such protoliths are also consistent with the tectonic setting at which the 2 68 ga volcanic rocks were generated because mafic igneous rocks and clastic sedimentary rocks with lesser felsic igneous rocks are the major constituents of a back arc basin

archean oceanic flake tectonics assuming that the archean mantle was hotter and the oceanic crust thicker than at present we estimate that the archean oceanic lithosphere had a sandwich rheology with the lower crust behaving as a soft ductile layer accordingly the archean oceanic lithosphere had a rheological structure similar to present day continental lithosphere but unlike present day oceanic lithosphere the thickness of the ductile layer is a decreasing function of age while its strength is an increasing function of age the buoyancy problem in archean subduction could be overcome by delamination of oceanic upper crust but mass balance calculation indicates that most archean oceanic crust must have been recycled where the right dynamic conditions existed for delamination oceanic flake tectonics may have been an important process leading to the preservation of oceanic upper crust in greenstone belts that are characteristic of archean terrains

climatic and tectonic influences on fan deltas and wave to tide controlled shoreface deposits evidence from the archaean keskarrah formation slave province canada the 2 6 ga keskarrah formation located in the central slave province northwest territories canada is a late orogenic tectonically controlled sedimentary sequence that developed under unusual climatic and depositional conditions the formation is adjacent to the crustal scale north trending beniah lake fault and overlies the 3 15 ga augustus granite the 2 69 2 7 ga mafic volcanic peltier formation and the turbiditic contwoyto formation unconformably principal lithofacies in the keskarrah formation include conglomerate sandstone and siltstone sandstone the conglomerate lithofacies represents coalescing gravelly streamflow dominated fan deltas adjacent to topographic highs up section quartz rich arenites and quartz arenites of the sandstone lithofacies are interpreted to be shallow water shoreface deposits influenced by wave action and tides the overlying feldspathic litharenites of the siltstone sandstone lithofacies are consistent with a lower shoreface to proximal offshore environment dominated by wave and tide interaction tidal influence in both sandstone dominated lithofacies is inferred from the presence of mudstone laminae between bedforms and on foresets of cross beds as well as from abundant reactivation surfaces with local mudstone drapes intense chemical weathering during the archaean resulting from elevated atmospheric co2 levels higher temperatures and moist climatic conditions played an important role in the development of quartz rich arenites that appear to be first cycle deposits few lithic fragments and feldspar grains are preserved due to in situ host rock weathering chemical weathering during transport and wave and tide action hydraulic sorting and abrasion in the shoreface environment contributed to the continued breakdown and transport of labile minerals increased proportions of lithic fragments in sandstone beds of the conglomerate lithofacies are the result of shorter transport distances from source areas to the depositional environment abundant conglomerate with up to 4 m large granitic boulders derived from the adjacent augustus granite and mafic clasts from the peltier formation indicate high relief and fault related uplift and subsidence the intimate association of fan deltas and wave and tide influenced shallow marine deposits in association with quartz rich sandstones forming in a high relief area make the keskarrah formation remarkable in the rock record

integrated nd isotopic and u pb detrital zircon systematics of clastic sedimentary rocks from the slave province canada evidence for extensive crustal reworking in the early to mid archean abstract a combined u pb geochronology and nd isotopic analysis of clastic metasedimentary rocks from the archean slave province northwest territories canada is presented a series of clastic sedimentary rocks with deposition age of 3 13 ga to 2 58 ga was collected from the west central slave province these samples include conglomerates as well as finer sedimentary rocks such as greywacke arkose and quartzite although it is generally agreed that the nd model age t dm of clastic sedimentary rocks represents the average sedimentary provenance age t dm of samples studied here is generally older than the average u pb age of detrital zircons extracted from the same sample rather there seems to be a better 1 1 relationship between the t dm and u pb age of the oldest zircon from each sample a simple model relationship between u pb geochronology and t dm suggests that such correlation is a result of archean crustal evolution of the slave province being dominated by crustal reworking i e intracrustal processes rather than addition of juvenile materials from the mantle one exception is the period between 3 1 and 3 2 ga during which period a significant portion of detrital zircons have u pb ages exceeding the t dm indicating a large input of juvenile material and or decreasing level of crustal reworking an alternative explanation for the u pb vs t dm trend may be a mixing of older mafic juvenile crust containing no zircon and younger felsic juvenile crust however the general lack of extensive pre 2 8 ga mafic crust in the slave province makes this scenario unlikely the results of this study cannot constrain the change in the volume of continental crust at any given period of time since the addition of juvenile crust may always be counterbalanced by recycling of crustal material back into the mantle however a dominance of crustal reworking during the early to mid archean strongly implies that a large part of ancient crust was lost during the younger tectonomagmatic events since the majority of zircon may be lost during crustal reworking processes unless they are preserved as an inherited core absence of a large quantity of ancient detrital zircon cannot be used as evidence for the lack of large sialic crust existing on the early earth

geology alteration and lithogeochemistry of the hood volcanogenic massive sulfide vms deposits nunavut canada the hood volcanogenic massive sulfide vms deposits are hosted by the 2 68 ga amooga booga volcanic belt abvb in the northwestern archaen slave craton and consist of three deposits hood 10 41 and 41a and three occurrences 46 461 and 462 the mineralized zones consist of massive to semi massive pyrrhotite pyrite chalcopyrite sphalerite and galena hosted predominantly by felsic volcanic flows within the predominantly mafic abvb the mineralized lenses occur at different stratigraphic levels and have textural alteration and stratigraphic features consistent with formation via subseafloor replacement the felsic volcanic rocks in the hood deposits can be subdivided into groups based on immobile trace element geochemistry the main felsic types a and b are petrographically indistinguishable type a has higher high field strength element hsfe and rare earth element ree contents than type b suggesting a higher temperature of formation type a rocks also have higher nb ta values indicative of a greater mantle input in their genesis compared to type b rocks mineralization is more closely associated with type a than type b rocks the two mafic volcanic rock types previously identified in the abvb type i and type ii both occur within the hood deposits the type ii mafic group is interpreted to be the result of variable crustal contamination of type i magma the volcanic rocks of the abvb are interpreted to have formed in a continental margin arc back arc setting the genesis of these magmatic suites involved magmatic underplating and emplacement through pre existing sialic basement that resulted in crustal melting mantle crust mixing and contamination leading to the aforementioned geochemical features in both mafic and felsic suites this type of extensional tectonic environment was likely associated with high heat flow and is similar to global vms environments proximal to extending continental margins e g sturgeon lake bathurst and finlayson lake the similarities of the abvb to other slave craton greenstone belts further highlights the overall potential for greenstone hosted vms mineralization in the slave craton chlorite sericite quartz is the dominant hydrothermal alteration assemblage in the hood deposits and is typical of vms style mineralization mass change calculations illustrate that elemental changes are typical of vms environments with gains in fe2o3 mgo and base metals associated with chlorite alteration near mineralized zones k2o gains associated with sericite alteration and losses of na2o in both alteration types

mafic pegmatites intruding oceanic plateau gabbros and ultramafic cumulates from bolivar colombia evidence for a wet mantle plume

the bell lake group and anton complex a basement cover sequence beneath the archean yellowknife greenstone belt revealed and implicated in greenstone belt formation u pb zircon and monazite geochronology indicates the presence of a 2 93 ga basement anton complex and 2 8 ga cover sequence bell lake group beneath the 2 70 to 2 72 ga kam group of the yellowknife greenstone belt in the southern slave province the bell lake group comprises remnants of a quartzite rhyolite banded iron formation succession the ages of detrital zircons from the quartzite unit constrain its deposition to be younger than 2 92 and range up to 3 7 ga u pb ages for gneisses beneath the bell lake group are in excess of 2 93 ga and they are locally overlain by the quartzite the contact between the tholeiitic to calc alkaline kam group and the bell lake group is poorly exposed and equivocal approximately 6 km higher in the section the ranney chert a felsic volcaniclastic layer separates 2 70 2 72 ga tholeiitic to calc alkaline upper kam group rocks from a lower tholeiitic section containing sheeted dike complexes zircons from the ranney chert yield pb pb ages ranging from 2 72

the nature of mesoarchaean seawater and continental weathering in 2 85 ga banded iron formation slave craton nw canada abstract banded iron formations bif have been extensively used as proxies to infer the chemical composition of ancient bulk seawater however their proximity to ancient crust suggests that they might also be used to reveal the composition of emergent continental landmass at the time of their deposition here we use the combination of geochemistry and sm nd isotopes on a layer by layer basis to interpret the relative contributions of hydrothermal hydrogenous and terrestrial input to one of the oldest documented superior type bif in the world the 2 85 ga central slave cover group bif is deposited within a rift basin related to a continental margin and is found associated with basement gneisses as well as shoreline and shallow shelf type facies such as fuchsitic quartzite and pebble to cobble conglomerate that confirm a near shore depositional setting for the bif the bif ranges from a pure chemical oxide magnetite silicate grunerite actinolite sediment with low al 2 o 3 2 o 3 10 wt and the occurrence of ferro hornblende biotite and garnet the silica bands have low trace metal content e g ni low ree average of 6 ppm and a shale normalized rare earth and yttrium rey pattern that is hree to lree enriched with pr yb sn values reaching sn of 0 5 during active rifting of the basement excess of eu 2 impacted the basin yielding seawater with eu anomalies eu eu sn as high as 3 85 average 2 75 larger than similarly aged bif high resolution geochemistry shows that there is more silica 19 4 wt sio 2 in the iron bands than iron 8 7 wt fe 2 o 3 in the silica bands implying that dissolved fe 2 came to the bif site in pulses and that silica likely represents background deposition consistently radiogenic e nd t values for the iron bands average 1 7 show that the dissolved rey in the source water during ferric iron precipitation was provided by submarine hydrothermal fluids with relatively uniform 143 nd 144 nd the silica bands by contrast reveal high variation in seawater 143 nd 144 nd as evident from the bimodal e nd t distribution with one segment exhibiting negative e nd t values averaging 1 1 and another with positive e nd t values averaging 2 5 this suggests input of dissolved rey into the upper seawater from weathering of isotopically different crustal components in the source region collectively we speculate that the low rey in the upper seawater and the overall low ni content implies a highly weathered crustal surface that was unable to contribute a significant dissolved load to the shelf environment

coupled stratigraphic and structural evolution of a glaciated orogenic wedge offshore st elias orogen alaska 1 the st elias orogen is the result of 10 myr of oblique convergence and flat slab subduction in the gulf of alaska between north america and the yakutat microplate extensive glaciation and a complex tectonic environment make this region a unique case study in which to examine the details of terrane accretion and the possible coupled influence of climate and tectonic drivers on the structural and topographic evolution of an orogenic wedge reflection seismic profiles across the offshore pamplona zone fold thrust belt the frontal st elias orogenic wedge provide constraints for quantifying pleistocene deformation recorded in the glaciomarine yakataga formation the total amount of pleistocene shortening observed varies from 3 to 5 mm yr compared to the current gps derived yakutat north america convergence rate across the st elias orogen of 45 mm yr growth strata and kinematic fold analysis allow comparison of relative timing of fault activity which reveals temporal and spatial shifting of active deformation during the glacial period faulting localized adjacent to the coastline and at the current submarine deformation front the abandoned currently inactive region is colocated with the major glacial depocenter in the region the bering trough these observations imply that glacial processes such as sediment loading and focused erosion during advance retreat cycles has a direct effect on the evolution of individual faults within the pamplona zone and the overall deformation pattern in the offshore st elias margin this information provides key constraints for understanding how climatic shifts may have affected the evolution of margin architecture during pleistocene glacial interglacial periods

evidence for archean ocean opening and closing in the southern slave province archean mafic volcanic and plutonic rocks of the cameron and beaulieu river greenstone belts outcrop in the southern slave structural province of the canadian shield lower portions of the greenstone belts have ophiolitic characteristics but young felsic units may represent superimposed arc magmatism the greenstone belts are separated from older circa 2 7 3 2 ga quartzo feldspathic gneisses of the sleepy dragon complex by a regional decollement zone patterson lake structural complex consisting of deformed mafic volcanic rocks thin slivers of gneiss ultramafic rocks and banded iron formation mixed in a highly deformed phyllonitic and phacoidally cleaved metapelitic matrix high synmetamorphic strains are indicated by abundant intrafolial folds transposed layering rare sheath folds and by the parallelism of elongated primary features and metamorphic mineral lineations the patterson lake structural complex is similar to melanges of phanerozoic mountain belts adjacent quartzo feldspathic gneisses are strongly mylonitized the melange and deeper mylonites formed when the greenstone belts were emplaced as thrust sheets wildflysch shed from tips of emergent thrusts was in part incorporated into the basal melange as the allochthon overrode its own detritus continued compression involved the basement in the deformation uplifting the sleepy dragon complex in a structural culmination mantled by the imbricated greenstone stack late granodioritic to granitic intrusions formed regional thermal aureoles which overprint earlier metamorphic fabrics

archaean sedimentary sequences of the slave craton abstract the slave craton is composed of five archaean terranes that accreted during the neoarchean the central slave superterrane is the only tectonic block containing evidence of four major sedimentary successions including 1 a quartz arenite dominated succession deposited on an extensive tide influenced coastline along a rifted continental margin 2 a turbidite dominated succession that developed as submarine fans in a fore arc or intra arc basin 3 a second turbidite succession representing deep water deposition in a back arc basin and 4 late conglomerate dominated successions representing fluvial alluvial fan and shallow water depositional settings in strike slip basins deposition of the sedimentary rocks took place between ca 3 1 ga and 2 6 ga indicating a complex tectonic history an unconformity between the gneissic basement and the overlying quartz arenite succession marks a marine flooding surface consistent with a first order sea level change a major hiatus lies between the quartz arenite successions and arc backarc related volcanic deposits with interstratified and conformably overlying turbidite strata late conglomerate successions are exposed along north trending lineaments and overlie all sedimentary and volcanic rock units unconformably this depositional hiatus represents a fundamental change from compression and uplift to transcurrent motion during the final stages of craton stabilization between ca 2630 and 2580 ma the slave province experienced considerable pluton emplacement which disaggregated much of the once continuous stratigraphy a wheeler diagram illustrates the major tectonic events that occurred during formation of the central slave superterrane however remaining uncertainties concerning the evolution of the entire slave craton reflect the need for additional detailed field investigations

seismicity of the bering strait region evidence for a bering block newly compiled russian and u s seismological data support an independent bering block in motion relative to the north american plate this motion is likely to be driven by the westward extrusion of southwestern alaska resulting from compression in southern alaska due to subduction of the pacific plate and terrane accretion seismicity extends from central alaska through the bering strait and into chukotka in eastern chukotka several southwest trends are evident some of which continue through the koryak highlands to kamchatka the seismicity outlines the bering block which includes most of the bering sea chukchi peninsula seward peninsula and parts of western alaska focal mechanisms young basaltic volcanism and normal faults in western alaska and chukotka indicate that the bering strait is under northeast southwest extension this in conjunction with thrust faulting in the koryak highlands indicates that the bering block is rotating clockwise relative to the north american plate

structural patterns and tectonic evolution of supracrustal domains in the archean slave province canada a new plate tectonic model accounts for lithological relations and regional structural patterns in late archean supracrustal domains of the slave province multiple dykes and pillowed mafic flows most common in the western part of the province suggest sea floor spreading the mafic volcanics lying in narrow homoclinal belts stratigraphically below more extensive turbidites are viewed as megaxenolithic remnants of oceanic crust preserved on the periphery of granitoid plutons and blocks of sialic crust closure of an oceanic basin was marked by emplacement of the granitoid plutons and coeval felsic volcanics the latter predominating over mafic volcanics in northeastern domains the felsic calc alkaline magmas may have risen from a shallow dipping subduction zone westerly verging folds westerly convex fold arcs and inclinations of later foliations particularly in lower level rocks of higher metamorphic grade are in accord with underthrusting to the east the zone of underthrusting shifted progressi

exhumation rates in the st elias mountains alaska abstract reprocessing and interpretation of seismic profiles from the gulf of alaska in the vicinity of the st elias mountains indicate that over 3000 km 3 of sediment were deposited on the ocean shelf since the end of the last glacial epoch approximately 10 000 years ago this estimate of the sediment volume does not include material moved out of the region by long shore currents transportation from the shelf to deeper waters or material trapped in fiords and basins on and near land thus an estimated sedimentation rate offshore of 7 9 mm year and the average erosion rate of 5 1 mm year from the st elias mountains derived from the estimated sedimentation rate represent minimum estimates we infer that these estimated rates are not typical of the approximately 5 m y history of mountain building in the area but rather they represent the result of efficient erosion by glaciers during this interglacial period

recognizing distinct portions of seamounts using volcanic facies analysis examples from the archean slave province nwt canada abstract archean volcanic rocks in the mafic dominated ca 2 66 2 69 ga point lake and beaulieu river belts slave province northwest territories are significant in demonstrating the facies that characterize specific portions of pillow volcanoes or seamounts irrespective of tectonic setting three distinct localities mapped in detail display facies consistent with 1 proximal deep water 2 medial to distal deep water and 3 medial shallow water seamount settings the proximal facies in the point lake belt include a 55 m thick non vesicular pillowed sequence cut by numerous mafic dykes and sills dykes contain multiple chilled margins indicating successive magma pulses which contributed to edifice construction abundant feeder conduits in addition to the absence of fragmental facies and vesicles are typical of the central deep water portion of seamounts where growth is initiated the medial to distal deep water facies in the point lake belt are represented by a 30 80 m thick assemblage of disorganized pillow breccia and pillowed and massive flows with 5 27 vesicularity massive non vesicular hyaloclastite intermingled with sedimentary material fluidal peperite in addition to thin shale units interstratified with pillow breccia and hyaloclastite indicate that sedimentation and volcanism were contemporaneous an increase in fragmental units and vesicularity relative to the proximal deep water facies is suggestive of the medial to distal part of a seamount in shallower water bedded tuffs laterally along strike with massive flows are the results of turbidity current deposition immediately following localized subaqueous eruptions a medial shallow water seamount setting is represented in the beaulieu river belt by a 5 85 m thick sequence of vesicular lobate pillowed and massive flows stratified pillow breccia and hyaloclastite and mafic dykes vesicularity ranges from 21 49 in pillowed flows 5 40 in massive flows and 20 35 in pillow breccia and hyaloclastite stratified pillow breccia developed along steep flow fronts in shallow water whereas bedded hyaloclastite formed during reworking and redeposition of autoclastic hyaloclastite on seamount flanks in shallow water the volcanic facies associations in the study areas are analogous to those of modern seamounts associated with the mid atlantic ridge and east pacific rise as well as mesozoic cenozoic seamounts in the canary islands fiji southwest japan the sea of japan and cyprus volcanological studies in the point lake and beaulieu river volcanic belts and subsequent comparisons with phanerozoic analogues demonstrate the manner in which distinct portions of ancient seamounts can be recognized in similar archean terranes

geology and geochemistry of an archean mafic dike complex in the chan formation basis for a revised plate tectonic model of the yellowknife greenstone belt an archean mafic dike complex in the chan formation at the base of the mafic volcanic section of the yellowknife greenstone belt consists of multiple metagabbro dikes and sills separated by screens of pillowed mafic volcanic rocks which are cut by a younger postvolcanic metadiabase dike swarm field relationships and geochemical characteristics are compatible with a comagmatic origin for the metagabbro and metavolcanic rocks that were fed through and deposited on an older rift related supracrustal sequence of the dwyer group the synvolcanic metagabbro dikes have extended the strike length of the volcanic section by at least 100 the mafic rocks of the chan formation are geochemically similar to mid ocean ridge basalt possibly with a minor subduction zone component preliminary end values for metagabbroic rocks are consistent with the derivation of magmas predominantly from a normal depleted mantle source the chan formation is interpreted to have formed in a marginal basin like setting adjacent to

late archean tectono magmatic evolution of the central slave province northwest territories an early tectono stratigraphic and late plutonic assemblage are distinguished in the central part of the archean slave province contwoyto lake area on the basis of their relationship to deformation and metamorphic episodes and by geochemical characteristics the older assemblage which predates the onset of intense deformation and the thermal peak of metamorphism comprises 2 68 2 65 ga calc alkaline volcanic and plutonic rocks as well as extensive turbidites the younger assemblage comprises ca 2 61 2 58 ga plutonic units that were emplaced during and after deformation d d3 and the thermal peak of metamorphism compositions of the late plutonic units vary systematically with time of intrusion from an early calc alkaline diorite tonalite suite to late peraluminous granites the early assemblage is interpreted to represent remnants of an island arc or arcs overlain by turbidites and accreted during d1 and d2 the later assemblage is interpreted as a product of late to post accretion suprasu

u pb zircon geochronology in the southwestern abitibi greenstone belt superior province the abitibi belt is the largest continuous greenstone belt in the superior province of the canadian shield it comprises several composite komatiitic tholeiitic calc alkalic and sedimentary sequences that are folded transected by major faults and intruded by various generations of plutonic rocks precise u pb geochronology has been carried out in the belt for the past decade to solve chronostratigraphic and metallogenetic problems this paper presents new zircon ages and reassesses previously published ones now refined by the addition of abraded and concordant zircon analyses volcanic and subvolcanic units of the timmins area yield the following ages 2727 1 5 ma for a tuff of the upper deloro formation 2703 1 5 ma for a tuff of the upper tisdale formation and 2698 4 ma for the krist fragmental assigned to the top of the tisdale formation the age of a dunite intrusive into the upper deloro formation is revised at 2707 3 ma whereas the aquarius diorite east of timmins yields a poorly define

laser 40 ar 39 ar thermochronology of archean rocks in yellowknife domain southwestern slave province insights into the cooling history of an archean granite greenstone terrane laser 40 ar 39 ar ages for hornblende muscovite and biotite from archean plutonic rocks of yellowknife domain southwestern slave province complement u pb ages from minerals with higher closure temperatures zircon monazite titanite together the data indicate that the plutonic suites of different ages defeat prosperous morose coring three major thermal culminations in the domain have distinct archean cooling histories until below approximately 250 c biotite closure the cooling trajectories for successive suites appear to document a decrease in cooling rate over the course of orogeny possibly reflecting the build up of heat within the orogen coupled with a transition to lower rates of exhumation and or erosion greater structural relief over the sleepy dragon basement culmination may be responsible for slower cooling of the morose granite situated in its core at or below the closure temperature of biotite the cooling curves converge with one another documenting a transition from localized to regional cooling the fact that cooling was locality specific until at least the biotite closure temperature suggests that just as intrusion and related peak metamorphism were diachronous across the region so was cooling to approximately 250 c uniform regional isotherms thus were not established until 200 ma after the youngest archean plutonism the variable disturbance of biotite ar spectra but generally not muscovite or hornblende indicates that the paleoproterozoic thermal disturbance of archean rocks in yellowknife domain did not exceed 200 300 c

timing of plutonism deformation and metamorphism in the yellowknife domain slave province canada u pb zircon titanite and monazite ages for plutonic rocks are used to constrain the timing of post 2660 ma deformational and metamorphic events in the yellowknife domain southern slave province diorite and tonalite plutons of the defeat suite yield ages of 2628 3 and 2624 3 ma older than previous less precise estimates for this suite of ca 2618 2620 ma the defeat suite plutons crosscut and postdate upright f1 folds in turbidites of the ca 2680 2660 ma burwash formation d1 deformation is therefore bracketed between 2660 and 2630 ma the d1 event significantly predates deposition of polymict conglomerates of the jackson lake formation a second folding event f2 represented by northwest to north trending steeply plunging folds of bedding and d1 structures affected the area after intrusion of the hidden lake pluton at 2606 2 ma and synchronously with intrusion of the 2596 2 ma sparrow lake pluton a member of the two mica granite prosperous suite the time of initiation and the duratio

evolution of 3 1 and 3 0 ga volcanic belts and a new thermotectonic model for the hopedale block north atlantic craton canada a new model for evolution of the archean hopedale block based on mapping and supporting u pb geochronological and geochemical studies is highlighted by i ca 3 25 ga emplacement of igneous precursors of maggo gneiss ii g iii emplacement of the hopedale mafic dykes iv 3 1 ga deposition of hunt river volcanic rocks v ca 3 0 ga deposition of florence lake volcanic rocks vi 2 88 2 96 ga greenschist to amphibolite facies fiordian metamorphism and formation of penetrative northeast striking fiordian structures and vii emplacement of a suite of 2 89 2 83 ga tonalite to granite intrusions which partially overlap and locally postdate fiordian metamorphism and deformation the hunt river and florence lake volcanic sequences are different in age but similar in most other respects the former consists mainly of amphibolite facies mafic metavolcanic rocks and lesser amounts of komatiite flows and metasedimentary and 3105

the central slave basement complex part i its structural topology and autochthonous cover new field and geochronological data are used to define the distribution of mesoarchean basement rocks in the south central slave province this distribution reflects a single contiguous basement terrane that we propose to call the central slave basement complex it shows a structural topology that is internally consistent and compatible with known regional folding and faulting events a sample of a proposed basement gneiss below the courageous lake greenstone belt central slave province has been dated by u pb methods and yields an age of 3325 8 ma consistent with the new basement distribution this sample also contains 2723 3 ma metamorphic zircon and ca 2680 ma titanite the central slave basement complex is overlain by a thin discontinuous but distinctive cover sequence that includes minor volcanic rocks clastic sedimentary rocks and banded iron formation all previously known and some new occurrences of this distinctive cover sequence occur in the immediate stratigraphic hanging wall of the