effects of hydraulic roughness on surface textures of gravel bed rivers field studies of forest gravel bed rivers in northwestern washington and southeastern alaska demonstrate that bed surface grain size is responsive to hydraulic roughness caused by bank irregularities bars and wood debris we evaluate textural response by comparing reach average median grain size d50 to that predicted from the total bank full boundary shear stress t0 bf representing a hypothetical reference condition of low hydraulic roughness for a given t0 bf channels with progressively greater hydraulic roughness have systematically finer bed surfaces presumably due to reduced bed shear stress resulting in lower channel competence and diminished bed load transport capacity both of which promote textural fining in channels with significant hydraulic roughness observed values of d50 can be up to 90 smaller than those predicted from t 0 bf we find that wood debris plays an important role at our study sites not only providing hydraulic roughness but also influencing pool spacing frequency of textural patches and the amplitude and wavelength of bank and bar topography and their consequent roughness our observations also have biological implications we find that textural fining due to hydraulic roughness can create usable salmonid spawning gravels in channels that otherwise would be too coarse

evolution of the eastern waddington thrust belt and its relation to the mid cretaceous coast mountains arc western british columbia the eastern waddington thrust belt cuts the eastern margin of the coast mountains mid cretaceous magmatic arc thrust faults carried triassic rocks of the intermontane superterrane early cretaceous volcanic and clastic rocks and volcanic and plutonic rocks of the active arc to the northeast outward from the core of the arc minimum shortening across the thrust belt is estimated as 50 or 40 km synkinematic through postkinematic metamorphism produced an inverted metamorphic gradient with the structurally higher magmatic arc as the likely heat source radiometric thermochronologic and structural data indicate that the thrust belt was active at 84 ma and probably at 87 ma and suggest that the peak of postkinematic metamorphism occurred about 82 84 ma the thrust belt was intruded by postkinematic plutons in latest cretaceous and early tertiary time 68 ma and 58 ma the eastern waddington thrust belt is coeval with or slightly younger than a system of west directed thrusts in the western and southern parts of the coast mountains arc the prominence of these structures suggests that synmagmatic contraction played a major role in development of the arc the distribution and character of syn and postthrusting metamorphism indicate that this contraction rather than localized loading by magma produced the metamorphism

can reflected extra equatorial rossby waves drive enso abstract the possibility that the evolution of the enso phenomenon is determined by the reflection of extra equatorial rossby waves from the western boundary into the equatorial waveguide has been a subject of recent debate observations and some wind driven models suggest an apparent continuity of off equatorial signals and subsequent waveguide anomalies on the other hand coupled model results show that enso like behavior can be simulated with no involvement of the extra equatorial regions linear equatorial wave theory shows that significant reflection can only occur within about 8 of the equator with a sharp fall off in the reflectivity poleward of this latitude although the amplitude of the thermocline anomalies associated with observed enso forced extra equatorial rossby waves can be large it is the net zonal transport of these waves that is crucial to the reflectivity and this net transport decreases rapidly as rossby waves occur farther from the equator the zonal geostrophic flows associate

structural evolution of the alaska juneau lode gold deposit southeastern alaska the alaska juneau lode gold deposit is hosted by a series of polydeformed permian to late triassic volcanic pelitic volcaniclastic and mafic intrusive rocks rocks in the mine area have been sheared and metamorphosed to greenschist grade interpretation of rock fabrics indicates several generations of ductile and brittle deformation prior to mineralization reverse shear occurred along northwest striking and northeast dipping ductile shear zones mineralization consists of eocene auriferous quartz carbonate veins which cut the regional metamorphic fabrics mineralization was followed by reverse right lateral shear along northwest trending ductile brittle shear zones two northwest striking and steeply dipping vein sets host the bulk of the ore orientation of carbonate fibers within the quartz veins were used to determine the deformation regime that existed during mineralization plunge of the fibers indicate that down to the northeast extension occurred synchronous with mineralization structural da

dynamics and thermodynamics of a warming event in a coupled tropical atmosphere ocean model abstract a simple coupled ocean atmosphere model similar to that of zebiak and cane is used to examine the dynamic and thermodynamic processes associated with el nino southern oscillation enso the model is run for 300 years the interannual variability which results is regular with a period of either 3 or 4 years quantized by the annual cycle the amplitude 1 5 m s 1 wind and 2 c sst anomalies period and structure of the interannual variability compare well with observations the model warm event is initiated in the spring prior to the event peak and is well described as an instability of the coupled system during instability growth the sea surface temperature sst anomaly is primarily generated by vertical upwelling processes the sst anomaly can be approximately described by the expression t t kth t where t is the sst anomaly t time h the upper layer thickness pycnocline perturbation and an effective damping time which includes heat loss to the atmosphere kt parameterizes

paleomagnetism magnetic anisotropy and mid cretaceous paleolatitude of the duke island alaska ultramafic complex we report paleomagnetic results from layered igneous rocks that imply substantial post mid cretaceous poleward motion of the insular superterrane western canadian cordillera and southeast alaska relative to north america the samples studied are from the stratiform zoned ultramafic body at duke island which intruded rocks of the alexander terrane at the south end of the southeastern alaska archipelago at about 110 ma thermal and alternating field demagnetization experiments show that the characteristic remanence of the ultramafic rocks has high coercivity and a narrow unblocking temperature range just below the curie temperature of magnetite this remanence is likely carried by low ti titanomagnetite exsolved within clinopyroxene and perhaps other silicate hosts the duke island intrusion exhibits a well developed gravitational layering that was deformed during initial cooling but below 540 c into folds that plunge moderately to the west southwest the characteristic remanence clearly predates this early folding and is therefore primary the fisher parameter describing the concentration of the overall mean remanence direction improves from 3 to 32 when the site mean directions are corrected by restoring the layering to estimated paleohorizontal all samples exhibit a magnetic anisotropy that is strong but nonuniform in orientation across the intrusion and we show that it has no significant or systematic effect on the site mean directions of remanence at least some of the anisotropy derives from secondary magnetite formed during partial serpentinization the mean paleomagnetic inclination 56 10 corroborates paleomagnetic results from five coeval silicic plutons of the canadian coast plutonic complex to the south and southeast and implies 3000 km 1300 km of poleward transport relative to the north american craton between mid cretaceous and middle eocene time the insular superterrane and coast plutonic complex shared a common paleolatitude history with more poleward transport than coeval inboard terranes

tectonic history of the eastern edge of the alexander terrane southeast alaska rocks exposed west of the coast plutonic complex in southern southeast alaska form an imbricate thrust belt that overprints the tectonic boundary between two of the largest allochthonous crustal fragments in the north american cordillera the insular and intermontane composite terranes in the alexander terrane insular composite terrane lower paleozoic metavolcanic and metasedimentary rocks descon formation and dioritic plutons are unconformably overlain by lower devonian clastic strata karheen formation these rocks are overlain locally by upper triassic basalt rhyolite and marine clastic strata hyd group upper jurassic and lower cretaceous metavolcanic and metasedimentary strata of the gravina sequence unconformably overlie the alexander terrane the gravina sequence forms a structural package over 15 km thick and records intermittent arc volcanism along the eastern edge of the alexander terrane the gravina sequence is structurally overlain by upper paleozoic and lower mesozoic metamorphosed basaltic strata marble and argillite alava sequence and locally by lower paleozoic supracrustal rocks and orthogneiss kah shakes sequence together these constitute the taku terrane which we correlate with the intermontane composite terrane local unconformity of gravina sequence strata over the alava sequence demonstrates that the gravina sequence overlapped an earlier structural boundary between the intermontane and insular composite terranes the rocks were deformed in the mid cretaceous by west vergent thrusting that was was broadly coeval with arc magmatism deformation involved emplacement of west directed thrust nappes over the structurally intact and relatively unmetamorphosed alexander terrane basement mid cretaceous tonalite granodiorite and quartz diorite intrude rocks of the thrust belt and are locally affected by the deformation mid cretaceous deformation occurred during two episodes that were contemporaneous with the emplacement of large sill like plutons older structures record ductile southwest vergent folding and faulting regional metamorphism and development of axial planar foliation the second generation structures developed during the later stages of southwest directed thrust faulting which juxtaposed rocks of contrasting metamorphic pressures and temperatures structural stratigraphic and geochronologic data indicate that the two phases of regional thrusting in southeast alaska occurred between 113 ma and 89 ma rocks in the western part of the thrust belt were uplifted regionally by 70 ma deformation involved the collapse of a marginal basin s and a magmatic arc and overprinted the older tectonic boundary between the insular composite terrane and the late mesozoic western margin of north america at that time the intermontane composite terrane contractional deformation along the length of the thrust belt was broadly coeval with arc magmatism and thus records intra arc tectonism late paleocene to early eocene igneous activity and extensional deformation subsequently affected the thrust belt

the gravina sequence remnants of a mid mesozoic oceanic arc in southern southeast alaska fragments of upper jurassic to lower cretaceous volcanic and basinal strata constitute the gravina belt in southeast alaska in the ketchikan area the gravina belt is made up of two lithotectonic units the lower unit consists of coarse marine pyroclastic and volcaniclastic strata mafic flows breccia and fine grained tuff which are locally intruded by hypabyssal bodies of diorite and quartz diorite the volcanic rocks are characterized by tholeiitic arc basalts lack felsic volcanic strata and overlie upper triassic and older strata of the alexander terrane augite and or hornblende bearing porphyritic rocks are common and locally intrude the alexander terrane basement where they are thought to represent the intrusive equivalents of lavas within the section age constraints for the volcanic unit based on structural and stratigraphic relations with adjacent units are late middle to late jurassic the gravina belt upper unit consists of fine to coarse grained turbidites and related conglomeratic channel fill deposits the basinal rocks unconformably overlie permian and triassic rocks of the taku terrane and remnants of the lower volcanic part of the gravina sequence which overlie the alexander terrane the conglomerate units contain mostly volcanic and plutonic lithic clasts some of which yield pb u zircon ages of 154 158 ma the predominance of pyroclastic deposits interbedded with massive flows tuff breccia and argillaceous turbidites and the lithologic and chemical composition of the volcanic rocks indicate a submarine volcanic arc setting for the gravina sequence the basinal pyroclastic rocks are inferred to have been shed from submarine stratovolcanos during the late jurassic epiclastic rocks were deposited as submarine fans derived in part from erosion of a magmatic arc the presence of fine grained tuffaceous turbidites implies ongoing but distant volcanism the pyroclastic and volcaniclastic rocks represent remnants of a late jurassic oceanic arc constructed on a composite basement consisting of the alexander and taku terranes the strata accumulated in an intra arc basin on the eastern edge of the alexander terrane the volcanic and basinal rocks were deformed during a major mid cretaceous intra arc contractional event in conjunction with the emplacement of a distinctly younger arc related plutonic suite

on the role of equatorial ocean modes in the enso cycle abstract in order to test certain aspects of the enso mechanism proposed by suarez and schopf and by battisti and hirst we force a shallow water ocean with 28 years of observed fsu winds and decompose both the atmospheric forcing and ocean response into equatorial modes the proposed mechanism was verified in the following sense for each warm phase of the cycle a downwelling kelvin mode directly forced by the weakening of the trade winds in the central pacific was found the same wind anomaly forces an upwelling gravest rossby mode which propagates freely westward and reflects as a freely propagating kelvin mode at the western boundary this kelvin mode returns to the scene of the original warming and acts as a retarded forcing of opposite sign eventually switching the phase of the oscillation from warm to cold the cold phase then proceeds by the same mechanism but with modes of opposite signs to eventually switch to the warm phase the contribution of the higher rossby modes to this process is e

southern continuation of the coast shear zone and paleocene strain partitioning in british columbia southeast alaska this paper documents the newly recognized southern continuation of the early tertiary coast shear zone extending its known length by 350 km to more than 1200 km three sites along the shear zone in british columbia douglas channel bella coola and machmell river have similar histories during the period ca 65 55 ma the shear zone is 2 11 km thick and is defined by well developed mylonite zones that strike northwest and dip steeply northeast motion on the shear zone was predominantly reverse with the northeast side up synkinematic plutons are common in the shear zone lower plate rocks high grade gneiss derived from an ancient continental margin assemblage show little evidence of the extensive deformation and plutonism in the shear zone north of bella coola high grade gneiss forms the upper plate but to the south weakly metamorphosed rocks of stikinia compose the upper plate geochronologic data show that the shear zone was active between ca 60 and 55 ma at douglas channel ca 62 and 56 ma at bella coola and after 66 and before 56 ma near the machmell river these features match those of the coast shear zone in southeast alaska and adjacent british columbia together the shear zones formed a continental scale reverse ductile fault in paleocene time plate reconstructions show an oblique component to the dominantly dextral transcurrent paleocene margin partitioning of this motion across the continental margin produced a regional system of strike slip faults and contraction on the coast shear zone

mesozoic evolution of exotic terranes and marginal seas western north america abstract regional terrane analysis has been combined with a global evaluation of plate kinematics to produce a new tectonic model for the mesozoic evolution of western north america and its associated marginal seas the model employs a two tiered data reliability ranking system to resolve conflicts within data sets the lower tier of the ranking system involves the assignment of a numerical rank to paleomagnetic and or paleobiogeographic data based on each data set s reliability the upper tier of the ranking system places the paleomagnetic and paleobiogeographic data in perspective by assigning a relative order of importance to different types of data the most reliable data are considered to be departure rift and arrival collision times that are tightly constrained by independent data sets evidence of subduction and or strike slip motion also ranks high in the master ranking system paleomagnetic and vertebrate paleobiogeographic data come next in the hierarchy followed by invertebrate and floral paleobiogeographic data application of this approach to a case study the baja british columbia controversy has resulted in a coherent model for the entire region results derived from the application of this modelling approach to the problem of the mesozoic assembly of western north america suggest that its evolution may be modeled using a minimum of 57 large terranes oblique convergence may have been the dominant tectonic process along the western north american margin during the mesozoic causing large scale latitudinal transport of many exotic terranes the motions of the terranes produced a number of marginal seas yukonia etc that were short lived but important components of western margin tectonics a combination of collision events side swiping episodes and changes in plate kinematics led to the initiation of several regional tectonic upheavals including the nevadan and sevier orogenies

detrital zircon geochronology and regional correlation of metasedimentary rocks in the coast mountains southeastern alaska u pb ages have been determined for 55 detrital zircon grains from a metasedimentary sequence along the west flank of the coast mountains in southeastern alaska these rocks belong to the port houghton assemblage which consists of upper paleozoic pelitic and psammitic schist metaconglomerate metabasalt and marble the port houghton assemblage rests unconformably on metamorphosed and deformed mid paleozoic arc type volcanics endicott arm assemblage which gradationally overlie upper proterozoic lower paleozoic continental margin strata tracy arm assemblage three main clusters of ages are present 330 365 ma 19 grains 1710 2000 ma 27 grains and 2450 2680 ma 6 grains additional grains are approximately 2334 2364 and 3324 ma comparison of these ages with detrital zircon ages in other cordilleran assemblages supports previous interpretations that metasedimentary rocks in the coast mountains i form a southwestern continuation of the yukon tanana terrane of eastern alaska and yukon

cache creek terrane entrapment oroclinal paradox within the canadian cordillera exotic and far traveled oceanic crustal rocks of the cache creek terrane cc are bordered by less exotic quesnel qn and stikine st arc terranes to the east north and west all of these terranes are enveloped by an arcuate belt of displaced continental margin rocks the kootenay ko nisling ns and parts of the yukon tanana ytt terranes that have indirect ties to ancestral north america na initial 87sr 86sr isopleths conform to this arcuate pattern such a pattern of concentric belts presents a geological conundrum how did the qn st and cc come to be virtually enveloped by terranes with ties to na past and current models that explain assembly of the canadian cordillera are deficient in their treatment of this problem we propose that early mesozoic qn and st were joined through their northern ends as two adjacent arc festoons that faced south toward the cache creek ocean panthalassa oceanic plateau remnants within the cc today were transported from the tethyan realm and collided with these arcs during subduction of the cache creek ocean counterclockwise oroclinal rotation of st and ns terranes in the late triassic to early jurassic caused enclosure of the cc rotation continued until these terranes collided with qn in the middle jurassic paleomagnetic declination data provide support for this model in the form of large average anticlockwise rotations for permian to early jurassic sites in st but moderate clockwise rotations for sites in qn specific modern analogues for the cordilleran orocline include the yap trench where the caroline rise is colliding end on with the mariana arc and the banda arc located on the southeastern tail of the asian plate which is being deformed into a tight loop by interactions with the australian and pacific plates

u pb geochronology of detrital zircons from a continental margin assemblage in the northern coast mountains southeastern alaska metamorphic rocks within and west of the northern coast mountains in southeastern alaska consist of an upper proterozoic to upper paleozoic continental margin assemblage that we interpret to belong to the yukon tanana terrane u pb geochronologic analyses of single detrital zircon grains from four samples of quartzite suggest that the zircons were shed from source regions containing rocks of 495 ma 750 ma 1 05 1 40 ga 1 75 2 00 ga 2 3 ga 2 5 2 7 ga and 3 0 ga multigrain fractions from two samples yield upper intercepts between 2 0 and 2 3 ga but the scarcity of single grains of similar age suggests that these fractions comprise a mixture of 2 3 ga grains zircons in these rocks generally overlap in age with i detrital zircons in metasedimentary rocks of the yukon tanana terrane in eastern alaska and yukon ii detrital zircons in strata of the cordilleran miogeocline and iii plutonic and gneissic rocks that intrude or are overlain by miogeoclinal strata in addition the p

contributions to the geology and geophysics of northwestern british columbia and southeastern alaska a symposium entitled l i l wansect northwestern in producing succinct yet data rich papers this special issue canadian cordillera was convened by robert anderson and contributions to the geology and geophysics of northwestern g h l at the l i l i i of canada british columbia and southeastern alaska is the result mineralogical association of canada gac mac annual general meeting held in vancouver in may 1990 an impetus for the symposium was the growing geological and geophysical database for the northwest cordillera the mature state of many of the geological and geophysical research projects being carried out by canadian and american university and federal and provincial geological survey groups was another important catalyst the symposium proved an excellent opportunity to bring researchers together and review the state of the art concerning the geology and geophysics of the insular coast and intermontane belts of northwestern british columbia and southeastern alaska as well a collection of papers reviewing the latest advances in the geology and geophysics of the northwest cordillera seemed long overdue not since publication of special volume 8 tectonic history and mineral deposits of the western cordillera by the canadian institute of mining and metallurgy in 1966 has there been a collection of topical papers by an international group of geoscientists encompassing recent geological mapping geophysical and geochemical results that contribute to a revised tectonic framework for the northwestern cordillera from the beginning symposium participants and john clague editor of the canadian journal of earth sciences cjes were enthusiastic about publication of the symposium proceedings in a special issue of cjes papers by h gabrielse and c a evenchick part of the gac mac special session 20 sedimentary and structural evolution of cordilleran interior basins were solicited as well the issue complements rather than supersedes the eagerly awaited cordilleran decade of north american geology dnag volumes e g gabrielse and yorath in press plaflcer and berg in press in providing a synopsis of new data and ideas ensuing from post dnag research to accommodate the anticipated number of contributions yet not monopolize the capacity of a multidisciplinary publication such as cjes authors were asked to limit submissions to 20 manuscript pages or less about two thirds the page limit normally allowed by cjes for articles through the judicious use of well crafted figures tables and data held separately in the depository of unpublished data canadian institute for scien

the tatshenshini shear zone new in southwestern yukon canada comparison with the coast shear zone in british columbia and southeastern alaska and implications regarding the shakwak suture the tatshenshini shear zone extends from klukshu lake in southwestern yukon southward to the yukon british columbia border deformation in the shear zone ranges from hundreds of meters to a few kilometers in width and is characterized by protomylonitic and mylonitic turbidites belonging to the dezadeash formation jurassic cretaceous that are overlain by protomylonitic granodiorite of the ruby range batholith late cretaceous tertiary the dezadeash formation ruby range batholith contact bedding and deformation and magmatic fabrics within the tatshenshini shear zone are all coplanar striking northwest southeast and dipping moderately to the northeast kinematic indicators from the dezadeash formation and ruby range batholith record a top to the southwest sense of shear collision of the insular superterrane with the intermontane superterrane by the mid cretaceous resulted in the eastward directed underplating of yukon tanana terrane equivalent rocks by the dezadeash formation and the formation of the tatshenshini shear zone the tatshenshini shear zone preceded and controlled emplacement of the ruby range batholith but magmatism ending 58 56 myr ago outlasted deformation the results of this work support the correlation of the ruby range batholith with the adjacent great tonalite sill in southeastern alaska it also suggests a linkage between the tatshenshini shear zone and the coast shear zone in southeastern alaska and along which the great tonalite sill was emplaced the tatshenshini shear zone and the coast shear zone represent the coda of long lived tectonism along the shakwak suture the accretionary boundary between the insular and intermontane superterranes

experimental deposition of carbonate mud from moving suspensions importance of flocculation and implications for modern and ancient carbonate mud deposition abstract flume experiments with fine grained carbonate particles 62 5 m show that they form floccules that travel in bedload form current ripples and deposit laminated sediments in order to compare our flume experiments to work on sand transport we ran flume experiments with medium sand and observed that the velocities at which sand grains started to move and form ripples were in the same range as those where floccules move and form ripples these ripples are in essence identical to those formed by clay mineral floccules or sand grains under similar conditions in light of previous experiments with clay minerals these results indicate that of the key controls on mud deposition flocculation and suspended sediment concentration are more important than particle mineralogy or water chemistry and are about as important as bottom shear stress for deposition suspensions of carbonate mud show the same pattern of flocculation ripple formation and bed accretion as observed previously in experiments with clay mineral suspensions the resulting carbonate mud deposits show internal low angle 2 5 degrees laminae and in plan view a pattern of ripple foresets that is identical to rib and furrow structure in sandstones just as previously assumed for terrigenous muds there has been a long standing notion that accumulation of abundant carbonate mud reflects quiescent conditions of offshore and deeper water environments these experiments demonstrate unequivocally that carbonate muds can also accumulate in energetic settings in the sedimentary record of carbonate rocks interbedded grainstones and lime mudstones may thus not necessarily reflect shifts in depositional energy or water depth but alternatively may imply a shift in supplied sediment type the observations we report suggest that published interpretations of ancient lime muds and derived paleoceanographic conditions may need to be reevaluated

u pb zircon geochronology of lower jurassic and paleozoic stikinian strata and tertiary intrusions northwestern british columbia new u pb zircon ages are reported from western stikinia devonian and pennsylvanian ages of volcanic rocks at oweegee dome confirm the presence of pre permian strata and with paleozoic and triassic detrital zircons from lower jurassic sandstone they help to demonstrate pre lower jurassic deformation and uplift the absence of pre paleozoic inherited zircon from all samples is consistent with nd sr isotopic data which suggest that stikinia consists mainly of juvenile crust u pb ages for posttectonic intrusions suggest that structures in skeena fold belt in the kinskuch area formed prior to eocene time five ages for felsic volcanic rocks from stratigraphically well constrained upper parts of the hazelton arc are approximately 196 199 ma and suggest near contemporaneity for cessation of volcanism in the areas studied the sinemurian or late sinemurian early pliensbachian ages are older than previously reported u pb and biostratigraphic ages for presumed correlative rocks to the west and westward migra

equatorial wave sequence associated with warm pool displacements during the 1986 1989 el nino la nina in the pacific equatorial band the zonal displacement of the eastern edge of the warm pool subject to insignificant seasonal variations is dominated by strong interannual variations almost in phase with the southern oscillation index over the 1971 1973 el nino la nina period gill 1983 suggested that such displacement was caused solely by horizontal advection by zonal current anomaly basin wide geosat derived zonal surface current anomalies are available during the 1986 1989 period and they agree quite well with observed equatorial currents in the western and central pacific within the 4on 4 s equatorial band the cumulative contribution of first baroclinic kelvin and first symmetric m 1 rossby modes to surface zonal equatorial current anomalies account for most of the geosat derived current variability from these geosat derived currents it is demonstrated that the eastward westward displacement of the eastern edge of the warm pool was primarily due to horizontal advection by zonal current anomalies during the 1986 1987 el nino 1988 1989 la nina as a corollary the el nino la nina warm cold sea surface temperature sst anomaly in the central western pacific was the result of anomalous zonal advection the 1986 1989 displacement of the warm pool appeared as a low frequency movement resulting from high frequency forcing i e a succession of local wind forcing and its remote kelvin and m 1 rossby wave responses in the equatorial waveguide in the central western pacific a in 1 downwelling rossby wave issued from the eastern pacific shifted the displacement from eastward to westward and therefore seems to have been the cause for el nino to turn to la nina in boreal fall 1987 in the eastern pacific the simultaneous occurrence of free propagating downwelling upwelling kelvin waves with the seasonal sst warming cooling appeared necessary for the development of local el nino la nina sst anomaly over most of the equatorial pacific slow migration of sst anomalies clearly showed up distinct during el nino and la nina these zonal migrations appeared to result from the delayed association of two phenomena the zonal advection in the central western pacific and the arrival of downwelling upwelling kelvin waves into the eastern pacific synchronous with the seasonal sst warming cooling albeit different in terms of equatorial wave action as compared to the delayed action oscillator theory our results suggest that zonal advection and wave dynamics were both important for the generation and migration of el nino southern oscillation sst anomalies in 1986 1989 finally our observational results about warm pool displacements equatorial wave sequence the shift from el nino to la nina and the slow sst migrations are presented in a schematic form involving sequential ocean atmosphere coupling which appears continuously over the november 1986 to a

protolith relations of the gravina belt and yukon tanana terrane in central southeastern alaska metamorphic rocks west of the coast mountains batholith in central southeastern alaska are divided into the gravina belt taku terrane and newly defined ruth assemblage the ruth assemblage comprises metapelite quartzose metaclastic strata quartzite marble felsic metatuff mafic metavolcanic rocks and orthogneiss depositional and emplacement ages of 367 pm 10 ma and 345 pm 13 ma inferred from discordant u pb zircon analyses on felsic metatuff and granodioritic orthogneiss respectively require that at least portions of the ruth assemblage be late devonian and early mississippian in age the assemblage is similar in age and protolith to and thus correlated with the yukon tanana terrane the gravina belt is characterized by upper jurassic and lower cretaceous mafic volcanic rocks and tuffaceous turbiditic clastic strata that unconformably overlie the alexander terrane metamorphic rocks that structurally underlie the taku terrane and ruth assemblage are included in this assemblage trace

ancient continental margin assemblage in the northern coast mountains southeast alaska and northwest canada geologic relations indicate that quartz rich metasedimentary rocks in the northern coast mountains separate strata to the east that belong to the stikine terrane from strata to the west of the alexander wrangellia and taku terranes the quartz rich rocks structurally overlie western terranes along a mid cretaceous thrust fault and are overlain structurally originally stratigraphically by strata of the stikine terrace these rocks are interpreted to be a continental margin assemblage that belongs to the yukon crystalline terrane u pb and nd isotopic data indicate that the metasedimentary rocks were shed from a source terrane consisting at least in part of proterozoic rocks

a middle jurassic to early tertiary andean sierran arc model for the coast belt of british columbia regional tectonic features of the western canadian cordillera can be interpreted in terms of middle jurassic accretion of a single composite superterrane stikinia wrangellia alexander to ancestral north america closure of the intervening cache creek bridge river ocean moved the continental edge to a new position west of the accreted superterrane the coast belt is primarily a succession of superimposed middle jurassic to early tertiary magmatic arcs related to prolonged subduction of pacific ocean lithosphere beneath the new north american margin discrete magmatic pulses separated by lulls or periods of relative quiescence successively overprinted the new margin the insular and intermontane superterranes previously viewed as widely separate entities prior to mid cretaceous time were already amalgamated before an initial middle jurassic overprint produced an ancestral coast belt late jurassic to early cretaceous rift related marine basins mid to late cretaceous compressional structures and early tertiary extensional features coincident with the coast belt are subsidiary intraplate attributes that reflect external adjustments in plate motions within a primary subduction related andean magmatic arc

ocean model studies of upper ocean variability at 0 160 w during the 1982 1983 enso local and remotely forced response abstract a hindcast of the 1982 1983 enso event using a primitive equation ocean circulation model forced by monthly mean wind stresses based on the sadler pseudostress fields shows very good agreement with observations at 0 159 w between june 1982 and march 1983 the hindcast experiment is analyzed to explore the processes that caused the large accelerations decelerations and thermal changes observed during this time several hindcast experiments incorporating variations of the sadler wind field and several idealized experiments incorporating a western pacific westerly wind event are analyzed and compared with the 1982 1983 sadler hindcast to explore the importance of local and remote forcing the relative importance of zonal and meridional wind stress changes and the dynamical signatures of the processes at work meridional wind stress changes have little effect on either the zonal velocity or temperature fields local zonal wind stress variations can account for the qualitative changes in the uppe

geology of the hazelton volcanic belt in british columbia implications for the early to middle jurassic evolution of stikinia the hazelton group is an early to middle jurassic volcanic and sedimentary succession that was deposited on the stikine terrane stikinia of the canadian cordillera the group is distributed across the entire width of stikinia a distance of about 300 km prior to post hazelton contractional deformation the width across which volcanism had occurred was at least 450 km a review of chronologic and stratigraphic information corroborates an earlier suggestion that the hazelton group formed as a pair of coeval partly subaerial volcanic chains separated by a subsiding mainly sedimentary marine basin hazelton trough the trough developed in response to extension of stikinia during the latest triassic and or earliest jurassic and remained the locus of moderate extension during deposition of the hazelton group volcanic rocks of the group are diverse comprising subaerially and subaqueously deposited lavas and volcaniclastic rocks of mafic to felsic composition calc alkaline to tholeiitic characteristics and depletion of high field strength elements relative to alkali and alkaline earth elements indicate a subduction related origin the degree of alkalinity medium to high k is fairly consistent throughout the group the original width across which volcanism occurred is two to five times greater than widths of modern volcanic arcs the anomalous width cannot be accounted for by time transgression of a single volcanic arc because the two volcanic chains were coeval extremely low angle subduction of a single oceanic plate is also rejected as an explanation because alkalinity does not differ significantly between the two volcanic chains genesis of the hazelton group above a single subduction zone is therefore considered unlikely we propose a model in which a pair of oceanic plates were subducted beneath stikinia from opposite sides in a manner analagous to the tectonic regime of the philippine archipelago subduction angles of the two downgoing plates are postulated to be 30 to a depth of 150 km a configuration consistent with a postulated forearc width of about 175 km on each side stikinia is thereby considered to have been an 800 km wide microplate on which two island arcs developed the hazelton trough was a back arc region common to both volcanic arcs during deposition of the hazelton group stikinia was not part of the intermontane superterrane an amalgam composed of more inboard terranes rather it belonged to an oceanic microplate that accreted to the inboard terranes in late early jurassic to early middle jurassic time during which hazelton volcanism waned and ended

u pb geochronology of late cretaceous and early tertiary plutons in the northern coast mountains batholith u pb geochronologic studies demonstrate that steeply dipping sheetlike tonalitic plutons along the western margin of the northern coast mountains batholith were emplaced between 83 and 57 perhaps 55 ma less elongate tonalitic granodioritic bodies in central portions of the batholith yield ages of 59 58 ma coeval with younger phases of the tonalitic sheets large granite granodiorite bodies in central and eastern portions of the batholith were emplaced at 51 48 ma trends in ages suggest that the tonalitic bodies generally become younger southeastward and that at the latitude of juneau plutonism migrated northeastward across the batholith at 0 9 km ma variations in the age shape location and degree of fabric development among the various plutons indicate that late cretaceous paleocene tonalitic bodies were emplaced into a steeply dipping dip slip shear zone that was active along the western margin of the batholith postkinematic eocene plutons were emplaced at shallow crustal levels inhe

structural and geochronologic relations along the western flank of the coast mountains batholith stikine river to cape fanshaw central southeastern alaska abstract geologic and u pb geochronologic studies in central southeastern alaska provide constraints on the mid cretaceous to mid tertiary deformation west of the coast mountains batholith the ne dipping sumdum and fanshaw faults record the w directed emplacement of the ruth assemblage yukon tanana terrane and taku terrane over the subjacent gravina belt and alexander terrane ductile fabrics of the sumdum fanshaw fault system truncate early formed foliation and thrust faults in albian and older gravina belt strata u pb age data from syn and post tectonic plutons suggest that deformation was ongoing at 92 9 3 0 ma but had mostly ceased by 90 ma the sumdum fanshaw fault system marks the tectonic boundary between the alexander wrangellia terrane and inboard yukon tanana and stikine terranes and lies within a thrust belt that extends from southern alaska to northern washington mid cretaceous structures are truncated to the east by ne dipping ductile fabrics of the coast shear zone sheet like paleocene tonalites were emplaced into and deformed within the shear zone undeformed mid eocene dikes cross cut the tonalites kinematic relations in the coast shear zone suggest a complex displacement history that includes both east side up reverse and west side up normal shear the shear zone probably accommodated the collapse of overthickened crust developed during mid cretaceous shortening

geology of the western flank of the coast mountains between cape fanshaw and taku inlet southeastern alaska the western flank of the coast mountains batholith between cape fanshaw and taku inlet is underlain primarily by jura cretaceous strata of the gravina belt pre permian permian and triassic strata of the taku terrane and mid proterozoic to upper paleozoic rocks of continental margin affinity the continental margin rocks include mid proterozoic to lower paleozoic quartzite and marble of the tracy arm assemblage mid paleozoic metavolcanic and subordinate metasedimentary rocks of the endicott arm assemblage and upper paleozoic quartz rich metaturbidites and metaconglomerate of the port houghton assemblage we suggest that these three assemblages are correlative with components of the yukon tanana terrane which underlies a large region of yukon and eastern alaska rocks of the gravina belt taku terrane and yukon tanana terrane are juxtaposed along west vergent thrust faults of mid cretaceous age and are internally deformed and disrupted along latest cretaceous to early eocene dip slip and possibly strike slip shear zones these rocks and structures together with mid cretaceous to eocene plutons of the coast mountains batholith separate rocks of the alexander wrangellia terrane to the west from rocks of the stikine terrane to the east mid cretaceous thrust faults in the area belong to a system of thrusts that form the inboard margin of the alexander wrangellia terrane from central alaska to northwestern washington the continental margin rocks in the northern coast mountains may be part of the in situ north american margin that has been overthrust by the stikine and adjacent terranes alternatively these rocks may have 1 rifted from and then returned to the north american margin 2 moved 800 km along left lateral or right lateral faults from elsewhere along the north american margin or 3 originated near a continent other than north america

precambrian crust beneath the mesozoic northern canadian cordillera discovered by lithoprobe seismic reflection profiling 1 the cordillera in northern canada is underlain by westward tapering layers that can be followed from outcrops of proterozoic strata in the foreland belt to the lowermost crust of the orogenic interior a distance of as much as 500 km across strike they are interpreted as stratified proterozoic rocks including 1 8 0 7 ga supracrustal rocks and their basement the layering was discovered on two new deep seismic reflection profiles in the yukon line 3 650 km and northern british columbia line 2 1245 km in two segments that were acquired as part of the lithoprobe slave northern cordillera lithospheric evolution snorcle transect in the mackenzie mountains of the eastern yukon the layering in line 3 is visible between 5 0 and 12 0 s 15 to 36 km depth it is followed southwestward for nearly 650 km 500 km across strike and thins to less than 1 0 s 3 0 3 5 km thickness near the moho at the yukon alaska international boundary in the northern rocky mountains of british columbia the upper part of the layering on line 2 correlates with outcrops of proterozoic 1 76 1 0 ga strata in the muskwa anticlinorium at this location the layering is at least 15 km thick and is followed westward then southward into the middle and lower crust for 700 km 300 km across strike it disappears as a thin taper at the base of the crust 150 km east of the coast of the alaskan panhandle the only significant disruption in the layering occurs at the tintina fault zone a late to postorogenic strike slip fault with up to 800 km of displacement which appears as a vertical zone of little reflectivity that disrupts the continuity of the deep layering on both profiles 300 km apart the base of the layered reflection zone coincides with the moho which exhibits variable character and undulates in a series of broad arches with widths of 150 km in general the mantle appears to have few reflections however at the southwest end of line 3 near the alaska british columbia border a reflection dips eastward from 14 0 s to 21 0 s 45 to 73 km depth beneath exposed eocene magmatic rocks it is interpreted as a relict subduction surface of the kula plate our interpretation of proterozoic layered rocks beneath most of the northern cordillera suggests a much different crustal structure than previously considered 1 ancient north american crust comprising up to 25 km of metamorphosed proterozoic to paleozoic sediments plus 5 10 km of pre 1 8 ga crystalline basement projects westward beneath most of the northern canadian cordillera 2 the lateral 500 km by at least 1000 km and vertical up to 25 km extent of the proterozoic layers and their internal deformation are consistent with a long lived margin for northwestern north america with alternating episodes of extension and contraction 3 the detachments that carry deformed rocks of the mackenzie mountains and northern rocky mountains are largely confined to the upper crustal region above the layering 4 accreted terranes include thin klippen that were thrust over north american pericratonic strata e g yukon tanana and terranes such as nisling and stikinia that thicken westward as the underlying proterozoic layers taper and disappear 5 the ages of exposed rocks are not necessarily indicative of the ages of underlying crust a frequent observation in lithoprobe interpretations so that estimates of crustal growth based on surface geology may not be representative

eocene 51 ma end to northward translation of the coast plutonic complex paleomagnetism and k ar dating of the white pass dikes abstract the white pass mafic dikes are located in the coast belt inboard of the coast shear zone denali fault system east of skagway alaska astride the alaska british columbia border the dikes intrude eocene felsic plutons in the coast plutonic complex three k ar analyses date the dikes at 51 9 0 8 ma biotite 50 1 1 0 ma and 45 0 0 8 ma both whole rock structural analysis on these vertical dikes indicates that the study area has not been tilted significantly since dike emplacement conventional alternating field and thermal step demagnetization and saturation remanence paleomagnetic methods were used to analyse 407 specimens from 27 dike and five host rock sites three contact tests show that the specimens retain a primary thermoremanent magnetization trm carried in pseudosingle to multidomain magnetite although some magnetization in a few dikes is carried by pyrrhotite the dual polarity trm is mostly reversed and the 24 accepted dike sites give a pole position of 245 7 e 76 3 n dp 4 2 dm 4 4 indicating a paleolatitude discordance of 8 4 south and a clockwise rotation of 40 9 with respect to the north american craton these data along with other paleomagnetic data on eocene rocks indicate that the allochthonous eastern coast and intermontane belt terranes had halted their northward translation by about 51 ma but continued to be rotated clockwise by an average of 24 10 thereafter probably on underlying sole faults

correlation of settler schist with darrington phyllite and shuksan greenschist and its tectonic implications coast and cascade mountains british columbia and washington amphibolite facies settler schist in the southeastern coast mountains of british columbia has long been correlated with chiwaukum schist of the cascade metamorphic core north cascade mountains no

variability in the eastern equatorial pacific ocean during 1986 1988 we examine variability in the eastern equatorial pacific during 1986 1988 using conductivity temperature depth data velocity and temperature data from equatorial moorings between 110 w and 140 w and wind data from a basin scale zonal array of islands and moorings between 110 w and 165 e the period studied coincides with the el nino southern oscillation enso event of 1986 1987 and a subsequent cold event in 1988 weak warm sea surface temperature anomalies first appeared in the eastern equatorial pacific in mid 1986 and increased to 1 c in september november 1986 in association with a 30 cm s 1 weakening of the south equatorial current and a 20 to 40 m depression of the thermocline these warm anomalies lasted until early 1988 after which a large scale shoaling of the thermocline led to sea surface temperatures more than 3 c colder than climatology year to year fluctuations in the eastern pacific were related primarily to zonal wind variations in the central and western pacific westerly wind stress anomalies of 0 02 0 05 n m 2 were observed between 140 w and 165 e from the latter half of 1986 until the end of 1987 these were replaced by easterly wind anomalies of similar magnitude between 157 w and 165 e in 1988 energetic intraseasonal fluctuations with periods of 2 3 months were also prominent in zonal current temperature and dynamic height time series these fluctuations propagate eastward at approximately first baroclinic mode kelvin wave phase speeds and are forced west of the date line by episodes of westerly winds extrema in several oceanic variables occurred in association with these waves though their precise dynamical link to the enso cycle is unclear from our data sea surface temperature and thermocline depth anomalies at 0 110 w were less pronounced during the 1986 87 enso than during the 1982 1983 enso the equatorial undercurrent though weaker than normal in early 1987 did not disappear as it did in early 1983

resolving high precision u pb ages from tertiary plutons with complex zircon systematics success in establishing high precision crystallization ages with the u pb zircon method is highly dependent on the ability to isolate or remove the effects of inherited xenocrystic components or secondary lead loss or the combined effects of both current conventional single crystal and ion probe shrimp methods are successfully applied to paleozoic and older samples but typically suffer from imprecision when applied to younger mesozoic cenozoic samples due to generally reduced radiogenic lead concentrations here we apply a series of intense partial dissolution steps to multigrain zircon fractions from a series of paleocene tonalitic to granodioritic intrusive rocks from southeastern alaska the zircon systematics are complicated by both minor inheritance and postcrystallization lead loss physical and isotopic evidence demonstrate that the partial dissolution steps preferentially remove outer layers susceptible to lead loss as well as core regions containing inherited components the final residues are often hollow shells of low u primary igneous zircon that yield highly precise reproducible and concordant ages the resulting age determinations commonly statistically differ at the 95 confidence level from ages based on apparently concordant but less precise conventional isotope dilution analyses the uncertainties of which masked minor subtle isotopic complexities this observation strongly cautions against basing age assignments of samples yielding slightly discordant data on 1 single concordant analyses accompanied by an array of discordant data or 2 the mean of several 206pb 238u or 207pb 206pb ages the step wise dissolution technique allows age determinations on young relatively low u and pb zircons at a resolution not currently possible with techniques such as single grain conventional or spot ion probe analyses widespread application of the technique will likely prove instrumental in resolving detailed magmatic histories of igneous complexes where zircon systematics are typically complex

mid cretaceous transtension in the canadian cordillera evidence from the rocky ridge volcanics of the skeena group the age relations geochemistry and sedimentology of the rocky ridge formation of the skeena group are used to test competing tectonic reconstructions for the mid cretaceous canadian cordillera as well as the timing and location of the accretion of the insular superterrane pollen and macrofossil assemblages indicate that these intrabasinal basalts were erupted along the southern margin of the bowser basin in the early albian to early cenomanian single crystal fusion and step heating 40ar 39ar dating of hornblendes in one basalt flow from the uppermost part of the formation yielded middle cenomanian ages of 94 3 0 4 95 6 1 6 and 95 0 1 6 ma vesicular basalt flows interbedded with crystal rich tuff breccias contain evidence for hot emplacement as pyroclastic flows individual eruptive centers are identified by their proximal facies paleoflow indicators within the lava flows paleoflow indicators within interbedded volcaniclastic fluvial deposits geochemical differences and geographic isolation of volcanic deposits major and trace element geochemistry from 20 sampled lava flows indicates an alkali basalt composition for the volcanics the basalts of the northern rocky ridge volcanic center show enrichment of light rare earth and large ion lithophile elements with strong negative nb ta anomalies whereas the basalts of the southern tahtsa lake volcanic center show depletion to slight enrichment of light rare earth elements slight enrichment of large ion lithophile elements with minimal negative nb ta anomalies the geochemistry combined with paleogeographic and regional tectonic reconstruction suggests a continental arc setting with intraarc extension the presence of deeper marine facies to the west and the lack of a western sediment source in the skeena group indicate that the technically active insular superterrane was not west of the study area during mid cretaceous time thus we reconsider the omineca belt as the main axis of a mid cretaceous continental arc placing the intermontane superterrane in the intraarc to forearc position with the rocky ridge volcanics erupted along the forearc side of the omineca arc coeval regional strike slip faulting and reconstructed oblique plate convergence suggest a transtensional setting for rocky ridge intraarc extension an electronic supplement of tables a1 a2 may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type get and the name of the file to get it finally type exit to leave the system paper 95tc03496 mid cretaceous transtension in the canadian cordillera evidence from the rocky ridge volcanics of the skeena group kari n bassett and karen l kleinspehn diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order

sediment underthrusting within a continental magmatic arc coast mountains batholith british columbia nsf ear 0309885 ear 1338583 idaho state university romanian national science funding agency uefiscdi grant pn iii p4 id pce 2016 0127

simulation of enso with a global atmospheric gcm coupled to a high resolution tropical pacific ocean gcm abstract a global atmospheric general circulation model gcm coupled to an oceanic gcm that is dynamically active only in the tropical pacific simulates variability over a broad spectrum of frequencies even though the forcing the annual mean incoming solar radiation is steady of special interest is the simulation of a realistically irregular southern oscillation between warm el nino and cold la nina states its time scale is on the order of 5 years the spatial structure is strikingly different in the eastern and western halves of the ocean basin sea surface temperature changes have their largest amplitude in the central and eastern tropical pacific but the low frequency zonal wind fluctuations are displaced westward and are large over the western half of the basin these zonal wind anomalies are essentially confined to the band of latitudes 10 to 10 s so that they form a jet and have considerable latitudinal shear during el nino the associated curl contributes to a pair of pronounced minima in th

modeling tropical pacific sea surface temperature 1970 87 abstract an attempt is made to model the sea surface temperature sst of the tropical pacific ocean between january 1970 and august 1987 the sst is computed using the model and heat flux parameterization discribed in the climatological study of seager et al the model is forced with the observed winds as given by the florida state university analysis the results indicate that while short period variability in the model and observations is largely uncorrelated variability with periods greater than about one year is well represented in the model each el nino that occurred in this time period is captured by the model the main discrepancies in the evolution of the 1982 83 and 1986 87 model el nino events are the inability of the model equatorial sst anomaly to cool in early 1983 and the disappearance of the 1986 87 anomaly during 1987 the results suggest a number of conclusions in the east pacific equatorial kelvin waves excited by variations in the trade wind strength in the central and west pacific

two phase evolution of accretionary margins examples from the north american cordillera abstract the coast mountains orogen southeastern alaska and the salmon river suture zone western idaho record the accretion of oceanic crustal fragments to the north american cordilleran margin structural elements associated with these boundaries include a western accretion related thrust belt and steeply dipping post accretionary shear zone the shear zones the coast shear zone southeastern alaska and western idaho shear zone idaho are steeply dipping crustal scale structures characterized by steep isotopic and geophysical gradients the presence of syndeformational tabular calc alkaline tonalitic plutons concordant with shear zone fabrics locally preserved dextral strike slip indicators and pervasive down dip mineral and elongation lineations are consistent with an intra arc transpressional origin for both structures in addition to strike slip translation the structures accommodated exhumation of high pressure moderate temperature metamorphic rocks within the thrust belt to the west and moderate pressure high temperature batholithic rocks to the east similarities between the structures in alaska and idaho suggest that the observed two stage history of underthrusting of accreting crustal fragments followed by modification by intra arc transpressional shear zones is a common tectonic process our analysis implies that recognition of processes active during the accretion of crustal fragments to continental or cratonal margins is commonly inhibited by post accretion deformation and provides insight into lower crustal processes presently operative under modern strike slip accretionary boundaries in addition the intra arc transpressional shear zones e g the coast and western idaho shear zones are structures that could have accommodated paleomagnetically determined large magnitude transcurrent displacements

superimposed quesnel late paleozoic jurassic and yukon tanana devonian mississippian arc assemblages cassiar mountains northern british columbia field u pb and igneous petrochemical evidence allochthons in the cassiar mountains of northern british columbia contain assemblages belonging to two distinct canadian cordilleran terranes yukon tanana ytt and quesnellia these assemblages of pre late devonian devonian mississippian pennsylvanian permian and early jurassic age occur in intrusive and depositional as well as structural contact with each other the allochthons are gently dipping thrust panels interrupted by the mid cretaceous cassiar batholith a key element for correlation across the batholith is the mississippian and older pericratonic dorsey complex new devonian mississippian u pb ages for deformed plutons within it document an igneous suite like those in type yukon tanana exposures farther north other characteristics of the dorsey complex that ally it with ytt are orthoquartzites and grits and amphibolite bodies with transitional mid ocean ridge basalt morb to ocean island basalt oib petrochemical signatures unconformities deformed clasts in the late paleozoic sequences and a shared mid permian intrusive suite show that later arcs onlapped the mid paleozoic and older ytt assemblage the early jurassic intrusive suite cuts all major contacts and fabrics except the terrane bounding fault between the slide mountain and combined ytt quesnel terranes it represents a northern continuation of a plutonic belt that extends the length of the mesozoic quesnel magmatic arc these relationships carry important implications for cordilleran terrane history and the tectonic evolution of the north american margin at least some of the major terranes were not unrelated entities prior to their accretion to the continent but a system of superimposed and interconnected arcs that developed over a protracted time interval with complex and evolving paleogeographic configurations much like the modern western pacific province

tectonic framework of the upper paleozoic and lower mesozoic alava sequence a revised view of the polygenetic taku terrane in southern southeast alaska fragments of upper paleozoic and lower mesozoic metavolcanic and metasedimentary sequences of the taku terrane are exposed discontinuously along a narrow belt in southeast alaska and form a distinct lithostratigraphic package in the ketchikan area called the alava sequence crinoidal and argillaceous marble carbonaceous phyllite argillite mafic flows pillow breccia pyroclastic tuff and quartzite characterize the sequence these strata are unconformably overlain by upper jurassic to lower cretaceous fine to coarse grained epiclastic rocks of the gravina sequence the upper paleozoic part of the alava sequence may be correlative with the yukon tanana terrane whereas the middle and upper triassic portion of the alava sequence may represent a metamorphic vestige of the stikine terrane both parts are now exposed on the western flank of the coast plutonic complex in contrast with their correlatives to the east these relations suggest that the stikine and alexander terranes were juxtaposed prior to deposition of the gravina sequence the western boundary between rocks of north american affinity and allochthonous ensimatic crustal fragments of the alexander and wrangellian terranes lies west of the coast plutonic complex

lower crustal deformation during terrane dispersion along strike slip faults abstract after accretion terranes caught between obliquely converging plates are often displaced several hundreds of kilometers how this motion is accommodated at depth is not well known spurred by our trans alaskan crustal transect tact seismic studies of the yukon tanana terrane alaska i propose that the lower crust responds ductilely to accommodate differential movement between upper crust and upper mantle during margin parallel translation the locations of these ductile regions are dependent on temperature composition and strain rate for geotherms based on moderate surface heat flow 60 90 mwm 2 and reasonable geological strain rates 100 in 3 ma typical lower crustal rocks can deform ductilely the resultant subhorizontal layered fabric should be detectable with seismic techniques two terranes that have experienced hundreds of kilometers of margin parallel translation the yukon tanana terrane sampled by our seismic refraction wide angle reflection profiles and the salinian block california sampled by both seismic refraction and reflection have reflective lower crust both of these terranes have moderate to high surface heat flows have relatively thin crust and are interpreted seismically to be underlain by a quartzo feldspathic lower crust conditions favorable for ductile deformation in the lower crust interpretations of lower crustal layering for these two terranes is based on an increase in lower crustal reflectivity this layering is attributed to tectonic banding consistent with ductile deformation

eocene rotation of the coast plutonic complex and intermontane belt paleomagnetism of eocene plutons along the klondike highway paleomagnetic results are reported for the 59 ma skagway 54 ma fraser 53 ma summit lake and 48 ma clifton felsic plutons of the eastern coast plutonic complex cpc that outcrop along the south klondike highway in alaska and british columbia thermal and alternating field step demagnetizing methods yielded stable characteristic remanent magnetization chrm directions for all 29 sites of normal reversed and mixed polarity the chrm resides in single or pseudosingle domain magnetite and or pyrrhotite that is shown to be primary by contact tests with the 47 ma vertical white pass mafic dikes paleopoles from six 56 to 50 ma mean 52 2 ma intermontane belt yukon tanana terrane imb ytt units that cannot be explained by tectonic tilt are compared with nine clustered 59 to 46 ma mean 52 4 ma eastern cpc paleopoles both paleopole populations show nonsignificant poleward northward translation relative to north america imb ytt 3 7 5 3 n cpc 4 3 6 4 s overall 1 2 4 9 s ind

distribution and characteristics of metamorphic belts in the south eastern alaska part of the north american cordillera the cordilleran orogen in south eastern alaska includes 14 distinct metamorphic belts that make up three major metamorphic complexes from east to west the coast plutonic metamorphic complex in the coast mountains the glacier bay chichagof plutonic metamorphic complex in the central part of the alexander archipelago and the chugach plutonic metamorphic complex in the northern outer islands each of these complexes is related to a major subduction event the metamorphic history of the coast plutonic metamorphic complex is lengthy and is related to the late cretaceous collision of the alexander and wrangellia terranes and the gravina overlap assemblage to the west against the stikine terrane to the east the metamorphic history of the glacier bay chichagof plutonic metamorphic complex is relatively simple and is related to the roots of a late jurassic to late early cretaceous island arc the metamorphic history of the chugach plutonic metamorphic complex is complicated and developed during and after the late cretaceous collision of the chugach terrane with the wrangellia and alexander terranes the coast plutonic metamorphic complex records both dynamothermal and regional contact metamorphic events related to widespread plutonism within several juxtaposed terranes widespread moderate p t dynamothermal metamorphism affected most of this complex during the early late cretaceous and local high p t metamorphism affected some parts during the middle late cretaceous these events were contemporaneous with low to moderate p high t metamorphism elsewhere in the complex finally widespread high p t conditions affected most of the western part of the complex in a culminating late late cretaceous event the eastern part of the complex contains an older pre late triassic metamorphic belt that has been locally overprinted by a widespread middle tertiary thermal event the glacier bay chichagof plutonic metamorphic complex records dominantly regional contact metamorphic events that affected rocks of the alexander and wrangellia terranes widespread low p high t assemblages occur adjacent to regionally extensive foliated granitic dioritic and gabbroic rocks two closely related plutonic events are recognized one of late jurassic age and another of late early and early late cretaceous age the associated metamorphic events are indistinguishable a small late devonian or early mississippian dynamothermal belt occurs just north east of the complex two older low grade regional metamorphic belts on strike with the complex to the south are related to a cambrian to ordovician orogeny and to a widespread middle silurian to early devonian orogeny the chugach plutonic metamorphic complex records a widespread late late cretaceous low to medium high p moderate t metamorphic event and a local transitional or superposed early tertiary low p high t regional metamorphic event associated with mesozonal granitic intrusions that affected regionally deformed and metamorphosed rocks of the chugach terrane the chugach complex also includes a post late triassic to pre late jurassic belt with uncertain relations to the younger belts

jurassic and cretaceous plutonic and structural styles of the eagle plutonic complex southwestern british columbia and their regional significance the eagle plutonic complex is an elongate north northwest trending body of deformed middle to late jurassic and middle cretaceous rocks which underlies the southwestern margin of the intermontane terrane new mapping of the complex and its country rocks in concert with geochronometry has defined episodes of contractional ductile deformation in the middle to late jurassic and middle cretaceous as well as brittle deformation in tertiary time synkinematic middle to late jurassic eagle tonalite at the eastern margin of the eagle complex intrudes mylonitic nicola group rocks and structurally overlies them along a southwest dipping belt of high strain eagle shear zone with a structural thickness of 1 km and a strike length of 100 km in the central and western eagle complex eagle tonalite grades into tonalite orthogneiss eagle gneiss and both are crosscut by mid cretaceous muscovite bearing plutons of the fallslake plutonic suite fallslake suite rocks are themselves ductilely deformed along the

nd isotopic characterization of metamorphic rocks in the coast mountains alaskan and canadian cordillera ancient crust bounded by juvenile terranes nd isotopic data are reported for 52 samples from the crustal region between the alexander wrangellia terrane and the stikine terrane of the alaskan and canadian cordillera this region is composed of the gravina belt a jurassic cretaceous assemblage of volcanic and clastic sedimentary rocks the taku terrane a terrane of probable early permian to late triassic age and four assemblages of metamorphic rocks that occur to the west of and within the coast mountains batholith the gravina belt has end t values that range from 1 1 to 8 3 similar to values of the underlying alexander terrane and consistent with the interpretation that it is a juvenile belt that formed in a back arc or intra arc basin within the alexander terrane mid cretaceous plutons that were emplaced into the gravina belt have end t values of 4 4 to 5 7 and were probably produced by mantle derived melts that incorporated some alexander terrane crust the taku terrane has end 0 values that range from 5 5 to 3 3 with corresponding depleted mantle model tdm ages of 440 to 1430 ma a mid cretaceous pluton intruding the taku terrane has an end t value of 5 1 a value indistinguishable from those determined for cretaceous plutons intruding the gravina belt metamorphic rocks east of and structurally overlying the taku terrane are divided into the tracy arm assemblage end 0 26 to 0 tdm 800 2450 ma the endicott arm assemblage end 0 10 to 1 3 tdm 950 1500 ma the port houghton assemblage end 0 9 4 to 1 1 tdm 550 1500 ma and the ruth assemblage end 0 9 4 to 2 0 tdm 650 1300 ma these isotopic signatures indicate that a substantial component of each metamorphic assemblage was derived from precambrian continental crust the metamorphic rocks from these assemblages are lithologically very similar to rocks of the yukon tanana ytt terrane of eastern alaska and yukon territory and have such similar u pb detrital zircon ages and nd isotopic compositions to ytt rocks that they are considered part of that terrane possible tectonic scenarios that can explain the present geometry of the ytt with respect to the alexander wrangellia and stikine terranes include 1 the ytt is the upturned stratigraphic basement of the stikine terrane 2 part of the ytt was structurally emplaced beside the stikine terrane in a transpressive tectonic regime 3 the stikine terrane and other inboard terranes are huge sheets that were thrust over the margin of the ytt before the final accretion of the alexander wrangellia terrane