deep basement seismic reflection profiling of the purcell anticlinorium using a land air gun source seismic reflection profiling in the purcell anticlinorium of southern british columbia canada using a land air gun source has revealed reflections from depths of at least 15 20 km interpretations of these data in conjunction with related geological and geophysical information confirm the allochthonous structure of the purcell anticlinorium and suggest that autochthonous north american basement at 15 20 km depth beneath the anticlinorium has a westward dip of 15 20 the thickness of the crust above the north american basement surface may be accommodated by structural repetition of purcell strata by thrust slices of north american basement rocks or by both the geometry observed on the reflection data implies that the purcell anticlinorium is cored by a crustal scale thrust anticline hanging wall anticline further the data demonstrate that land air guns are effective sources for crustal reflection work

ndsr isotopic study of proterozoic to triassic sediments from southeastern british columbia abstract sedimentary rocks from the southeastern canadian cordillera varying in age from mid proterozoic to triassic have been analysed for their nd isotopic composition e nd t values range from 1 5 to 17 2 and nd model ages t dm nd vary from 1 00 to 2 32 ga much in excess over their stratigraphic ages the difference between the two ages ranges from 0 43 to 1 93 ga decreasing as the sediments become stratigraphically older nd isotopic compositions of sediments suggest that there are two distinct continental sources of sediments in the cordilleran miogeocline a an older source e nd 0 from 20 3 to 23 8 with apparent crustal residence ages of 2 26 2 32 ga and b a younger source average e nd 0 14 0 with the crustal residence age of about 1 8 ga the older residence ages are interpreted to be due to mixtures of materials from the archean and proterozoic crusts sediments belonging to the laib formation lower cambrian and lower clastic part of the milford group namurian appear to have derived from an archean continental segment with some admixture of sediments from the early proterozoic source the younger crustal source which seems to be the principal supplier of sediments for most of the stratigraphic units aldridge formation middle proterozoic horsethief creek group upper proterozoic hamill and lardeau groups lower cambrian nelway formation middle cambrian active formation ordovician and part of milford group is not the archean canadian shield immediately to the east recent subsurface mapping of the alberta basement beneath the phanerozoic sedimentary cover using u pb zircon dating of igneous lithologies 1 has established the extensive archean and early proterozoic crustal provinces it is proposed here that the early proterozoic province of west central alberta is the most likely source of the easterly derived sedimentary rocks of the cordilleran miogeocline with t cr ages from 1 94 to 1 50 ga on the basis of sedimentologic tectonic and isotopic evidence we suggest that the younger sediments could be to some extent recycled older sediments for the triassic slocan and ymir groups eastern part of quesnel terrane sedimentologic lithologic tectonic and isotopic evidence suggest the source was a western one with about 1 8 ga crustal residence age comparison of the crustal residence ages with stratigraphic ages shows the best fit nd evolution curve for the cordilleran miogeocline approximately coincides with the continental growth curve deduced for the north atlantic province and similar to the growth curve for gondwana during the phanerozoic this curve also passes through the north american composite the levelling off of crustal residence ages for the mid proterozoic to triassic sediments at about 1 8 ga supports the idea of negligible crustal growth since at least 1 7 ga the average crustal residence age of the modern miogeoclinal and cratonic sediments

anomalous paleomagnetism of the crowsnest formation of the rocky mountains the volcanic crowsnest formation of albian age late early cretaceous from the rocky mountain fold and thrust belt of alberta has a stable remanent magnetization with a mean direction of 349 59 95 5 and paleopole at 78 n 108 e dm 7 dp 5 the inclination is lower than and the declination clockwise of the expected mid cretaceous paleogeomagnetic field for cratonic north america taken at face value the result indicates that the crowsnest formation and the thrust sheet in which it occurs have been transported from the south relative to cratonic north america by 17 6 about 1800 km and rotated 24 10 clockwise it is also possible that flattening of inclination is caused by magnetic anistropy but tests show this to be unlikely a third possibility is that the magnetization is secondary and of latest cretaceous age but there are good reasons for believing this is not so lastly it is possible that the unit could have been formed close to its present position relative to the craton

geochemical mineralogical and isotopic data relating to the origin and tectonic setting of the rossland volcanic rocks southern british columbia bulk rock and mineral chemical and isotopic analyses of rossland volcanic rocks are used to infer the nature of the magma extruded in the nelson rossland area of southern british columbia during th

cocorp deep seismic reflection traverse of the interior of the north american cordillera washington and idaho implications for orogenic evolution deep seismic reflection data collected by the consortium for continental reflection profiling along a transect of the interior of the northwestern u s cordillera indicate the presence of crustpenetrating faults these faults developed during an orogenic cycle consisting of jurassic to paleocene accretion related crustal thickening followed by eocene crustal thinning the reflection data also suggest that the moho is a young dynamic feature which developed a relatively smooth flat geometry beneath a complexly deformed crust probably during an eocene episode of crustal extension the profiles reveal prominent west dipping reflection sequences and associated diffractions interpreted as a mesozoic thrust system of crustal dimensions that linked accretion related shortening in the western cordillera with the thrust belt in the eastern cordillera east dipping reflections that crosscut the west dipping reflections in the middle and lower crust and are interpreted as ductile deformation zones that accommodated eocene crustal extension and essentially flat moho reflections at 33 35 km beneath complexly deformed crust these features are consistent with a sequence of 1 late mesozoic crustal imbrication of transitional crust beneath the easternmost accreted terrane which overrode the north american passive continental margin along a jurassic thrust 2 propagation of crust penetrating thrusts to shallower levels in the east where they probably formed the sole of the rocky mountain thrust belt 3 eocene extension that disrupted these thrusts and led to uplift of core complexes along detachments which fed into broad deformation zones in the lower crust and 4 development of a flat moho geometry during cenozoic extension and magmatism these data provide new information about the structure of the entire crust in the cordilleran interior and offer a working model both for evolution of cordilleran core complexes and possibly for other collisional orogens

large scale stable isotope alteration around the hydrothermal carbonate replacement cinco de mayo zn ag deposit mexico carbonate hosted hydrothermal deposits typically show narrow visible mineralogical and textural alteration halos which inhibit exploration targeting in contrast hydrothermal modification of the country rock s stable isotope composition usually extends far beyond the limited visible alteration hence stable isotope studies should be an effective tool to aid exploration for carbonate hosted deposits here we present new insight into the development of a large stable isotope alteration halo based on 910 o and c isotope analyses of carbonate veins and hydrothermally altered limestone hosting the cinco de mayo pb zn ag au cu carbonate replacement deposit crd in chihuahua mexico our results demonstrate that stable isotope alteration is consistent with reactive magmatic fluid flow into unaltered limestone and represents a powerful tool for the characterization of these hydrothermal ore systems synmineralization veins are texturally and isotopically distinct from those formed during pre and postmineralization diagenesis and fluid flow and show distinct gradients along the direction of mineralizing fluid flow this appears to be a promising exploration vectoring tool downhole variations in wall rock isotope values reveal aquifers and aquicludes and outline the principal hydrothermal flow paths furthermore wall rock 18ovsmow systematically decreases toward mineralization from 23 to 17 over a distance of 10 km providing another vectoring tool the extent of the stable isotope alteration halo likely reflects the overall fluid volume and areal extent of a fossil hydrothermal system which may be expected to scale with the mineral endowment this suggests that constraining the size shape and degree of isotopic alteration has direct application to mineral exploration by outlining the system and indicating the potential size of a deposit

geometric test for late cretaceous paleogene intracontinental transform faulting in the canadian cordillera the tintina trench northern rocky mountain trench tt nrmt fault zone and the fraser river straight creek fr sc fault zone are separate en echelon concentric small circle fault segments of a composite intracontinental transform fault zone more than 2500 km long that cuts diagonally across the canadian cordillera from the outboard part of a tectonic collage of accreted foreign terranes in the south into the north american preaccretionary continental margin in the north most of the 450 km of right hand slip on the tt nrmt fault zone was transformed southward during the late cretaceous and paleocene into oblique convergence in the southern canadian rockies the remainder probably comprising less than 100 km was transformed southwestward during early and middle eocene time via a zone of distributed shear and east west crustal stretching into right hand slip on the en echelon fr sc fault zone these interpretations based on regional systematic mapping of geologic structures are in conflict with interpretations of paleomagnetic measurements that call for more than 1000 km of post mid cretaceous right hand displacement along the general locus of the tt nrmt fault zone involving foreign terranes and the parts of north america to which they had been accreted this paradox must be resolved

the cenozoic denali fault system and the cretaceous accretionary development of southern alaska the juxtaposition of disparate geologic terranes in southern alaska has been previously interpreted to be mainly the result of several hundred kilometers of right lateral offset along the denali fault system in cenozoic time recent geologic investigations in the healy quadrangle strongly suggest that cenozoic horizontal displacements of such magnitude along the denali fault system do not exist in the healy quadrangle isograds and metamorphic facies boundaries of an early late cretaceous metamorphic belt trend across the cenozoic mckinley strand of the denali system without significant horizontal offsets the present geologic makeup of most of southern alaska is primarily the result of the talkeetna superterrane consisting of the previously assembled peninsular terrane and wrangellia colliding with and subsequently being thrust upon the yukon tanana and nixon fork terranes of the ancient north american continent in about middle cretaceous time the leading edge of the talkeetna superterrane faces a wide complexly deformed zone that contains numerous northwestward thrust miniterranes tectonically intermixed with jurassic and cretaceous flysch the flysch is interpreted to have been deposited mostly in the narrowing and subsequently collapsed oceanic basin between the converging continental blocks the postcollisional denali fault system developed in cenozoic time across the already accreted continental margin in eastern alaska along an older cretaceous suture

orogenic pulses in the alberta rocky mountains radiometric dating of major faults and comparison with the regional tectono stratigraphic record radiometric ages from regionally distributed thrust fault gouge show that the eastward propagation of the southern rocky mountain fold and thrust belt in canada occurred in four orogenic pulses that correlate with tectonic events of the cordilleran interior and with depositional patterns in the adjacent foreland in the main ranges the pyramid 163 0 ma simpson pass 161 7 ma and johnson creek 145 7 ma thrusts were related to the initiation of thin skinned deformation from jurassic terrane accretion and were partly contemporaneous with development of the first clastic wedge in the foreland basin in the front ranges the emplacement of the greenock thrust 103 1 ma and broadview snake indian thrust 99 2 ma was contemporaneous with development of cenomanian deltaic deposits in the immediate foreland three thrusts in the front ranges rocky pass 74 8 ma sulfur mountain 75 6 ma and clearwater 74 2 ma thrusts define a campanian phase of tectonic loading that led to the last major transgression in the southern portion of the alberta foreland along the eastern margin of the front ranges the mcconnell thrust 54 0 ma together with the muskeg 52 4 ma brule 53 9 ma and nikanassin 52 1 ma thrusts in the foothills recorded the last phase of regional contraction the late jurassic mid cretaceous late cretaceous and early eocene deformation pulses are separated by relatively long periods 40 m y 20 m y and 10 m y of tectonic quiescence these spatially and temporally restricted fault motion pulses contrast with gradual forward fault propagation while regional eastward progression of deformation is preserved

eocene underplating along the kodiak shelf alaska implications and regional correlations structural geology and geophysical data from the kodiak shelf suggest that the mesozoic rocks exposed on the shelf are structurally underlain at about 12 km depth by several kms of eocene age strata kinematic data from the late cretaceous to paleocene ghost rocks formation indicate that this formation and probably all of the kodiak islands were uplifted vertically to nearly their present elevations landward tilting and imbrication are not indicated the age of uplift is indicated by a regional angular unconformity of early eocene to early oligocene age that separates deep sea rocks from shallow water to non marine rocks the uplift of the accretionary prism is believed to have been caused by underplating of an eocene sedimentary sequence because 1 a band of seismic reflections that occur 12 to 20 km beneath the shelf is interpreted as the top of the underplated material and 2 an obductively offscraped sequence of eocene deep sea rocks crops out on the seaward side of the kodiak shelf suggesting that a thick trench fill sequence may have been present prior to uplift of the prism the underplated material is interpreted to be part of either a previously unrecognized turbidite fan of early eocene age or a proximal equivalent of the zodiac fan of late eocene to early oligocene age other possible on land remnants of the underplated material may be present in prince william sound the montague belt the gulf of alaska lower sections of the yakutat block and in the coast ranges of oregon and washington the large volume of underplated material beneath the kodiak shelf suggests that underplating may be the dominant process in the growth of convergent margins

cordilleria revisited with a three dimensional model for cretaceous tectonics in british columbia the cordilleria microcontinent theory for the tectonics of british columbia in the cretaceous and early tertiary and for the origin of the canadian rocky mountains and related features is reviewed new isotopic geological and paleomagnetic data support the hypothesis that the amalgamated superterranes of british columbia and alaska were not attached to the mainland of north america in the early cretaceous and were probably well south of their present relative position consideration of that evidence together with sedimentological evidence from the albian leads to a slightly earlier best estimate 115 ma for the time of initial collision in the yukon new geophysical data for southeastern british columbia and idaho lead to a comparison with the iapetus suture in northern england it is suggested that the cordilleria suture dips west from the present purcell thrust and related structures and should cut the moho a new model for regional deformation in se british columbia is proposed involving a flake

structural morphology and associated strain in the central cordillera british columbia and washington evidence of oblique tectonics the central cordillera developed during early jurassic to late cretaceous time as the continental edge was subducted under the exotic terranes of quesnellia and stikinia the earliest jurassic structures formed during prograde metamorphism and include cylindrical and double verging folds that trend subparallel to the orogen these structures are associated with mineral lineations and stretch values between 120 and 400 parallel to the fold axes later disharmonic structures formed during the emergence of the crust after the metamorphic peak and in response to decreasing p t conditions present tectonic models do not incorporate the kinematics capable of forming the early structures and thereby only address the emergent phase a scenario is offered which involves oblique subduction of the craton edge and which is kinematically compatible with the early structural morphology and associated strains

mise en evidence d un cisaillement ductile dextre d âge cretace moyen dans la region de tete jaune cache nord est du complexe metamorphique shuswap colombie britannique the boundary between the external zones rocky mountains and the internal zones of the eastern canadian cordillera is marked by a tertiary half graben the rocky mountains trench rmt in the south cordillera east of the shuswap metamorphic complex the fault limiting the trench is superimposed on an early major thrust the late jurassic purceli thrust on approaching this discontinuity the ductile deformation of the miette group a detrital precambrian suite is characterized by a subvertical foliation and a subhorizontal stretching lineation parallel to the fold axes the deformation intensity its noncoaxial characters and its geographic extension are interpreted as resulting from a dextral crustal shear parallel to the mapped trace of the purcell thrust and rmt the dextral slip is deduced from a microtectonic analysis of the observed rotational criteria and is consistent with the small angle occurring between the directions of the linear structure stretching lineations and fold axes and those

structure and tectonic development of the southern rocky mountain trench the rocky mountain trench is one of the youngest most prominent and most enigmatic structures of the canadian cordillera approximately 650 km of seismic reflection data providing regional three dimensional coverage over an area of 10 000 km include six crossings of the rocky mountain trench between 49 n and 50 15 n prominent reflections from mid proterozoic moyie sills outline thrust and fold structures of a late jurassic to early cretaceous fault system that was cut by the rocky mountain trench fault in the tertiary the near basement reflections outline a 10 km high west facing basement ramp the hinge line of which spatially coincides with the rocky mountain trench in this area this ramp is part of a mid proterozoic margin upon which the belt purcell supergroup was deposited and is preserved beneath the trench during mesozoic contraction the basal detachment of the foreland belt closely followed the craton cover contact across the basement ramp thrusting ceased and extension was initiated when a culmination of thick basinal strata was juxtaposed with the basement ramp in the eocene miocene the basement ramp and the culmination above it focused stress reactivating the basal detachment and causing extensional faulting in the southern rocky mountain trench the rocky mountain trench fault may be linked via the basal detachment to the flathead fault on the east and the eocene extensional faults that flank the omineca belt on the west

nd evidence for proterozoic crustal development in the belt purcell supergroup a knowledge of the chemical and isotopic characteristics of the continents as they existed in the past is essential to an understanding of their evolution1 7 vestiges of these characteristics remain in the sedimentary record for example the character and crustal residence age4 of ancient continental crust has been deduced from the sm nd isotopic patterns of clastic sediments that have survived to the present day4 6 8 we report here sm nd isotopic measurements from the belt purcell supergroup of western north america which form the first detailed isotopic investigation of a major proterozoic sedimentary succession the belt purcell has many features also seen in the detrital load of the world s major contemporary rivers including exceptionally homogeneous 147sm 144nd and 143nd 144nd ratios and a small range of crustal residence ages mantle additions to the continental crust seem to have been relatively minor since the middle proterozoic

geochemistry and geochronology of the gneisses east of the southern rocky mountain trench near valemount british columbia petrographic geochemical and geochronological data are presented on the gneisses of the bulldog creek block the mount blackman block and the hugh allan creek block which lie to the east of the southern rocky mountain trench srmt south of valemount british columbia petrographical and geochemical data especially immobile trace element ratios nb y ti zr and cao versus y and afm plots are used to deduce the probable origins and protoliths of the gneisses the mount blackman block consists of a psammitic paragneiss probably derived from an immature arkosic sedimentary protolith intruded by sills of tholeiitic basalt now amphibolites the bulldog creek block consists of felsic orthogneisses of calc alkaline affinity which are structurally concordant with mafic orthogneisses of possible tholeiitic basalt parentage the hugh allan creek block consists of a felsic orthogneiss with a probable alkaline granite protolith rb sr and some u pb analyses show that each block has experienced a separate geological history the mount blackman block psammitic paragneisses are the only analysed gneisses east of srmt with a probable archean rb sr model crustal residence age u pb analyses on zircons from these gneisses give a 1950 ma minimum source rock age and rb sr whole rock analyses suggest a 1860 50 ma age for amphibolite facies metamorphism of both paragneisses and amphibolites the bulldog creek block gneisses have a metamorphic age of at least 640 ma but their rb sr systematics have been extensively disturbed possibly during mesozoic retrogressive metamorphism the hugh allan creek block gneisses have a rb sr model crustal residence age of 900 ma and a metamorphic age of 805 11 ma it is not possible to correlate any of these lithologies or events across the srmt with the malton block and it is concluded that the srmt is the site of a major fault or faults at this latitude nous prksentons les donnkes pktrographiques gkochimiques et gkochronologiques sur les gneiss des blocs bulldog creek mount blackman et hugh allan creek lesquels sont situks i i est du sillon des rocheuses du sud au sud de valemount en colombie britannique

correlation between the geologic record and computed plate motions for central california we have attempted to correlate geologic events in part of central california with computed motions of oceanic plates relative to cratonic north america three composite terranes are considered 1 the sierra nevada 2 the coast ranges northeast of the san andreas fault and 3 the salinian block in the sierra nevada jurassic plutonism ending about 147 m y b p and cretaceous plutonism 120 80 m y b p correlate with deduced farallon north america fa na convergence as does the nevadan orogeny 158 153 m y b p however the gap in plutonism 146 121 m y b p outlasted by far an apparent minimum in convergence 145 135 m y b p magmatism ceased in the sierra and uplift slowed during the laramide orogeny 75 45 m y b p when tectonic activity had moved east to the site of the rockies presumably because of a low angle of subduction but volcanisin resumed in the sierra about 33 m y b p indicating that a steeper angle was reestablished apparently east west compression slackened when the pacific north america pa na transform regime developed basalt was erupted in the southern sierra 12 3 m y b p and during this same time the range started its most rapid rise as a normal fault bounded block the coast ranges north east of the san andreas fault are underlain by the franciscan complex the older parts of the complex were probably assembled in a subduction zone far south of their present location but we believe that the same fa na convergence that was cited for sierran plutonism was responsible despite the disparity in latitude as the farallon plate was probably in contact with most of the western margin of north and south america a perceived strong pulse in convergence 100 85 m y b p evidently produced the coast range thrust which on geologic grounds alone most likely originated between 96 and 88 m y b p some of the northwesterly transport of franciscan rocks relative to north america is ascribed to oblique kula north america ku na convergence 85 43 m y b p during which period the coast range thrust was revived about 60 m y b p the farallon plate again influenced the california margin after the kuia plate had moved northward and fa na convergence caused the accretion of the coastal belt and king range terranes of the neo franciscan the advent of pacific north america pa na interaction was marked by local volcanism which advanced from the southeast toward the northwest in consonance with the passage of the mendocino triple junction dickinson and snyder 1978 in the salinian block plutons 107 82 m y old can be ascribed to the same fa na convergence as the contemporary cretaceous plutons of the sierra nevada although the salinian intrusives originated much farther south paleomagnetic results champion et al 1980 from the salinian related pigeon point formation 75 71 m y in age suggest about 2500 km of poleward essentially coastwise movement this postulated large movement during part or all of the last 75 m y poses the greatest dilemma in our study the kuia is the only known plate whose computed obliquely convergent motion is appropriate for the late cretaceous paleogene part of the migration and even this motion is inadequate for the estimated transport within the most likely time window furthermore the farallon plate seems ideally suited for the laramide orogeny 70 45 m y b p both plates could not have been in contact with the same part of the margin at the same time oblique convergence of the kula plate could possibly have caused large coastwise transport and also the laramide orogeny the coast ranges northeast of the san andreas fault and the salinian block both experienced strike slip faulting and en echelon folding correlative with pa na interaction indicating that they were close together by neogene time faulting and folding have been particularly marked since the inception of spreading in the gulf of california and attachment of westernmost california to the pacific plate about 4 m y b p the rise of individual coastal ranges commencing 3 1 m y b p may possibly have been caused by slight convergence which appears in computed pa na relative motions from 5 m y b p to the present

petrology and tectonic significance of gates formation early cretaceous sediments in northeast british columbia moosebar gates sandstones are predominantly litharenites with some feldspathic litharenites both the light and heavy mineral suites indicate a mixed source characterized by clastic and carbonate sedimentary rocks acidic to intermediate plutonic and volcanic igneous rocks and metamorphic rocks the sediment sources all fall within a recycled orogenic provenance grouping histograms showing stratigraphic variation of mineral content do not indicate any significant progressive unroofing of more deeply buried source rocks the source area was very extensive regionally and extended well into the omineca crystalline belt and eastern margins of the intermontane belt zebraic chalcedony was derived from evaporitic rocks of the charlie lake formation situated east of the rocky mountain trench kyanite and almandine garnet were probably derived from the omineca crystalline belt west of the rocky mountain trench regional paleoslope dipped towards the north northwest restoration of strike slip on the rocky mou

structural geology of the lardeau group near trout lake british columbia implications for the structural evolution of the kootenay arc the lardeau group is a heterogeneous assemblage of lower paleozoic eugeoclinal strata present in the kootenay arc in southeastern british columbia it is in fault contact with lower paleozoic mioge

metamorphism of the spokane dome mylonitic zone priest river complex constraints on the tectonic evolution of northeastern washington and northern idaho metamorphic rocks in the core of the southern priest river complex record two distinct metamorphic events the earliest metamorphism occurred above 5 kb near the amphibolite granulite boundary and accompanied the formation of a steep crystalloblastic foliation the second less pronounced metamorphism was at lower pressure 3 5 kb in the middle amphibolite facies and accompanied mylonitization within a 4 km thick zone retrograde reactions during the second event include the formation of andalusite muscovite secondary green biotite and the breakdown of garnet biotite garnet geothermometry yielded widely variable and unrealistically high temperatures plagioclase garnet geobarometry is partially reset by the retrograde event and therefore provides minimum pressures the p t path is not known precisely but may include two episodes of prograde metamorphism perhaps related to continued accretion of terranes to the west the first metamorphism probably accompanied the jurassic accretion of quesnell

paleomagnetic evidence for displacement from the south of the coast plutonic complex british columbia the mid cretaceous spuzzum and porteau plutons of the coast plutonic complex of british columbia have two magnetizations a and b the a magnetization eight sites 83 specimens d 30 3 i 56 7 95 4 9 paleolatitude 37 5 n paleopole 65 0 n 14 9 w a95 6 2 is considered to have been acquired in the age range 105 90 ma this result differs from the field established for cratonic north america in this time range the difference could be caused either by previously undetected tilting about a horizontal axis of the plutons or by their rotation about a vertical axis and lateral displacement relative to the craton previously observed mid cretaceous magnetizations from other rock units from the western canadian cordillera and the cascades of washington united states are similarly discordant with respect to the craton this similarity over such a large area indicates that although local undetected tilting could be partly responsible it is unlikely to be the prime cause and we argue ther

relationships between plate motions and late cretaceous to paleogene magmatism in southwestern alaska four major late cretaceous to paleogene magmatic belts have been recognized in southwestern alaska and tentative boundaries have been drawn for each the belts indicate minimum ages by which the tectonostratigraphic terranes they overprint must have been linked together though there has been significant postmagmatic tectonism the kuskokwim mountains belt 73 60 m y b p the alaska range belt 74 55 m y b p and the gulf of alaska belt 62 57 m y b p all may have been related to a period of rapid north northeastward motion of the kuia plate from about 74 to 56 m y b p the angle of convergence was low with respect to the present orientation of the belts suggesting the possibility of postmagmatic rotation of the belts a higher angle of convergence may have existed with respect to the alaska range belt a probable arc if postmagmatic rotation resulted from 300 400 km of right lateral displacement on the denali fault the gulf of alaska belt is in an anomalous forearc position with respect to the alaska range belt but probably formed by a convergence related mechanism the kuskokwim mountains belt is in an apparent back arc position but the mechanism of its origin is uncertain it may have been part of an unusually broad alaska range arc or less likely it may have been a separate arc magmatic quiescence occurred throughout south western alaska during a period of plate reorganization from about 56 to 43 m y b p a period which began with extremely rapid northward motion of the kuia plate magmatism resumed when the present period of rather slow northwestward pacific plate motion began at about 43 m y b p the aleutian arc probably formed at this time and has been episodically active in about the same location ever since

cordilleria a newly defined canadian microcontinent palaeomagnetic data and geological field evidence indicate that most of the western canadian cordillera lay 1 500 km south of its present position in the early cretaceous we suggest that this region comprised a microcontinent termed cordilleria which moved on the kula plate and collided with the craton 100 myr ago to form first the mackenzie mountains and then the rockies

offset of an upper pliensbachian geographic zonation in the north american cordillera by transcurrent movement the north to south climatic zonation of upper pliensbachian ammonites known in europe has been recognized in the north american cordillera this simple geographic arrangement has been disrupted by northwest trending dextral transcurrent faults with further documentation the amount of offset of these zones should provide a measure of the amount of movement on these faults since pliensbachian time and further document the allochthoneity of the western part of the cordillera

imbricated terranes of the cariboo gold belt with correlations and implications for tectonics in southeastern british columbia the cariboo gold belt of east central british columbia is divided into four fault bounded sequences of distinct stratigraphy they are from east to west the cariboo continental shelf sediments barkerville continental shelf sediments and intercalated volcanics slide mountain rift related submarine pillow basalt chert and diorite and quesnel island arc sediments and subaqueous volcanics terranes each is separated from others by thrust faults grit phyllite limestone and volcanics of the barkerville terrane may be correlative with the eagle bay formation near adams lake and the lardeau group near kootenay lake barkerville terrane may be part of a more regional rock package selkirk terrane which is defined to include kootenay terrane badshot formation and horsethief creek and hamill groups selkirk terrane is i separated everywhere by a low angle fault from the overlying age equivalent but stratigraphically and structurally different cariboo terrane and ii separated by a system of f

deep mantle matters the lower mantle lying between 670 and 2890 km depth comprises most of the rocky portion of earth convection processes within this region transfer heat from the iron core upward advecting the heat flow that drives the near surface tectonic engine as many of the largest volcanic events appear to be correlated with seismic features in the deep mantle 1 the deep lower mantle may also represent an occasional scourge of our surface environment in this issue two sets of challenging experiments yield new pictures for how different deep seismic anomalies might be generated on page 892 andrault et al 2 examine the melting temperature of oceanic crust basalt to core mantle boundary cmb pressures and temperature and use that to explain the genesis of areas with ultralow seismic velocities near the cmb on page 877 zhang et al 3 report the startling discovery of a new iron rich silicate phase that may be a major component of the lowermost 700 km of earth s mantle

walker creek fault zone central rocky mountains british columbia southern continuation of the northern rocky mountain trench fault zone walker creek fault zone wcfz well exposed in the western rocky mountains of central british columbia near 54 comprises a 2 km wide zone of variably deformed neoproterozoic and cambrian strata in fault bounded slivers and lozenges extensional shear bands subhorizontal extension lineations slickensides mesoscopic shear bands and other minor structures developed within and immediately adjacent to the fault zone consistently indicate right lateral displacement offset stratigraphic changes in correlative neoproterozoic strata indicate at least 60 km of right lateral displacement across the zone wcfz is the southern continuation of the northern rocky mountain trench nrmt fault zone it shows a through going moderate displacement strike slip fault system structurally links the nrmt and the north central part of the southern rocky mountain trench strike slip motion on the wcfz occurred in the late cretaceous to early eocene at the same time as northeast directed shortening in the fold and thrust

nd evidence for extensive archean basement in the western churchill province canada to determine the extent of reworked archean crust in the western churchill province we have examined sm nd crustal residence ages of basement cores in southern alberta and southwestern saskatchewan along with crustal residence ages calculated for other sm nd data available from the churchill province the deep drill hole samples from the interior platform give sm nd crustal residence ages that average 2 8 ga granulites from northeastern alberta and composite gneisses from northern saskatchewan also provide archean crustal residence ages these data demonstrate that the presence of reworked archean crust in the western churchill province is more widespread than has been confirmed previously we suggest that sm nd crustal residence ages are valuable estimates of crust formation times especially in orogenic areas where other isotope systems have been disturbed

late cretaceous genesis of the kula plate abstract we propose that the pacific kula ridge began spreading approximately 85 m y b p during late cretaceous time extrapolation of the great magnetic bight backwards in time results in an implausible ridge configuration this implies that plate velocity vectors for the pacific kula and farallon plates were not constant during this interval evidence for splitting of the kula plate from the pacific plate along the chinook trough is the relationship between the north striking amlia and adak fracture zones the chinook and emperor troughs and the magnetic lineations south of the aleutian trench if this hypothesis is correct it will require that mesozoic reconstructions of the pacific basin and their relation to cenozoic reconstructions be re examined a previously unrecognized mesozoic plate may be required we propose calling this the izanagi plate

a critical review of tectonic processes at continental margin orogens abstract a cordilleran type orogen has previously been defined as contractional deformation of a continental margin due to subcontinental subduction the plate tectonic setting of cordilleran type orogenesis is fundamentally different from that of collisional orogenesis intracontinental orogens such as the himalayas and alps form by collision between a continental margin and a continental margin subduction zone orogens at continental margins may form due to subcontinental subduction cordilleran type orogeny or due to collision of a continental margin with an intra oceanic subduction zone which is typically followed by a reversal of subduction polarity in the latter case collisional orogenesis may proceed along with the reversed subcontinental subduction modern arcs are mostly extensional or neutral so subduction hangingwall contraction is not a necessary consequence of subduction crustal contraction in known collisional orogens lasts for tens of millions of years so collisional deformation at a continental margin may last for tens of millions of years after arc continent collision and subduction reversal collisional orogenesis is a predictable consequence of subduction and collisional deformation is kinematically linked to the kinematics of subduction noncollisional cordilleran type orogenesis is not predicted by plate theory a geologic history that would demonstrate noncollisional orogenesis is a magmatic arc built into a passive margin sequence with strong deformation but without an earlier arc or ophiolite accretion event this history has not been recognized in the geologic record the classic examples of cordilleran type orogens including the cretaceous cenozoic of the andes and the jurassic early tertiary of the western united states may be interpreted as arc continent collisions however the youngest increments of deformation in these two orogens occurred 100 million years after the youngest collisions and are thus difficult to ascribe to collisional processes notwithstanding the long time scales these deformations occurred at the end of space time continua of probable collisional orogenies alternatively these deformations may represent lithosphere scale fault bend folding duplex folding backthrusting and or out of sequence forethrusting during shallow subduction

the anvil plutonic suite faro yukon territory the anvil plutonic suite consists of three phases a peraluminous muscovite biotite granite mount mye phase and two metaluminous to peraluminous hornblende biotite granodiorite and minor granite intrusions orchay and marjorie phases the suite is massive or foliated equigranular or seriate and contains alkali feldspar megacrysts the marjorie phase is characteristically porphyritic geochemical trends are irregular for the suite and for individual phases high k2o low cao and low mgo compositions typify the silicic calc alkaline suite hornblende bearing phases contain less sio2 k2o and rb and more cafemic oxides tio2 sr ba and y than the mount mye phase and are compositionally similar to coeval south fork volcanics isochrons from some of the orchay phase whole rock samples t 99 2 5 ma 87sr 86sri 0 7161 0 0001 and from whole rocks and minerals of the mount mye phase t 100 2 ma 87sr 86sri 0 7405 0 0001 indicate they are coeval but not comagmatic accounting for the litho