tectonic setting of synorogenic gold deposits of the pacific rim abstract more than 420 million oz of gold were concentrated in circum pacific synorogenic quartz lodes mainly during two periods of continental growth one along the gondwanan margin in the palaeozoic and the other in the northern pacific basin between 170 and 50 ma these ores have many features in common and can be grouped into a single type of lode gold deposit widespread throughout clastic sedimentary rock dominant terranes the auriferous veins contain only a few percent sulphide minerals have gold silver ratios typically greater than 1 1 show a distinct association with medium grade metamorphic rocks and may be associated with large scale fault zones ore fluids are consistently of low salinity and are co2 rich in the early and middle palaeozoic in the southern pacific basin a single immense turbidite sequence was added to the eastern margin of gondwanaland deformation of these rocks in southeastern australia was accompanied by deposition of at least 80 million oz of gold in the victorian sector of the lachlan fold belt mainly during the middle and late devonian lesser devonian gold accumulations characterized the more northerly parts of the gondwanan margin within the hodgkinson broken river and thomson fold belts additional lodes were emplaced in this flyschoid sequence in devonian or earlier palaeozoic times in what is now the buller terrane westland new zealand minor post devonian growth of gondwanaland included terrane collision and formation of gold bearing veins in the permian in australia s new england fold belt and in the jurassic early cretaceous in new zealand s otago schists collision and accretion of dozens of terranes for a 100 m y long period against the western margin of north america and eastern margin of eurasia led to widespread latest jurassic to eocene gold veining in the northern pacific basin in the former location late jurassic and early cretaceous veins and related placer deposits along the western margin of the sierra nevada batholith have yielded more than 100 million oz of gold additional significant ore forming events during the development of north america s cordilleran orogen included those in the klamath mountains region california in the late jurassic and early cretaceous the klondike district yukon by the early cretaceous the nome and fairbanks districts alaska and the bridge river district british columbia in the middle cretaceous and the juneau gold belt alaska in the eocene gold bearing veins deposited during the late jurassic and early cretaceous terrane collision that formed the present day russian far east have been the source for more than 130 million oz of placer gold the abundance of gold bearing quartz carbonate veins throughout the gondwanan north american and eurasian continental margins suggests the migration and concentration of large fluid volumes during continental growth such volumes could be released during orogenic heating of hydrous silicate mineral phases within accreted marine strata the common temporal association between gold veining and magmatism around the pacific rim reflects these thermal episodes melting of the lower thickened crust during arc formation slab rollback and extensional tectonism and subduction of a slab window beneath the seaward part of the forearc region can all provide the required heat for initiation of the ore forming processes

back arc with frontal arc component origin of triassic karmutsen basalt british columbia canada the largely basaltic 4 5 6 2 km thick middle to upper triassic karmutsen formation is a prominent part of the wrangellian sequence twelve analyses of major and minor elements of representative samples of pillowed and massive basalt flows and sills from queen charlotte and vancouver islands are ferrotholeiites that show a range of 10 2 3 8 mgo as normalized h2o and co2 free and related increases in tio2 1 0 2 5 zr 43 147 ppm and nb 5 16 ppm other elemental abundances are not related simply to mgo distinct groupings are evident in al2o3 na2o and cr but considerable scatter is present in feo feo 0 9fe2o3 and cao some of the variation is attributed to alteration during low rank metamorphism or by seawater including variation of ba rb sr and cu but high field strength elements sc ti y zr and nb as well as cr ni cu and rare earth elements ree s were relatively immobile ree s show chondrite normalized patterns ranging from light ree depleted to moderately light ree enriched on eleven discriminant plots these analyses fall largely into or across fields of within plate basalt wip normal or enriched mid ocean ridge tholeiite morb and island arc tholeiite iat karmutsen basalts are chemically identical to the stratigraphically equivalent nikolai greenstone of southern alaska and yukon territory these data and the fact that the karmutsen rests on sicker group island arc rocks of paleozoic age suggest to us that 1 1 the basal arc after minor carbonate shale deposition underwent near axial back arc rifting as e g the mariana arc rifted at different times 2 2 the karmutsen basalts were erupted along this rift or basin as arc rift tholeiitite and 3 3 after subsequent deposition of carbonates and other rocks and jurassic magmatism a large fragment of this basalt sediment covered island arc was accreted to north america as wrangellia the major and minor elemental abundances of karmutsen basalt is modeled by first mixing primitive arc magma with enriched basaltic liquid derived either from garnet peridotite or metasomatized mantle followed by fractionation of olivine pyroxenes plagioclase and spinel

impressions of algal mats from the middle proterozoic belt supergroup northwestern montana u s a irregular low relief bedding plane markings which probably represent impressions of thin wrinkled algal mats have recently been detected in mudstones from the 1100 m y old snowslip formation belt supergroup glacier national park montana algal mat impressions are rarely reported from ancient strata yet algal mats are common in certain modern environments originating primarily in moist settings and in relatively quiescent shallow water settings such structures may be of some value in interpreting ancient depositional environments

structural geometry and kinematic evolution of the eastern cascades foldbelt washington and british columbia the eastern cascades foldbelt is one of three structural domains lying within the complex collision zone between the insular and intermontane composite terranes in northern washington and southern british columbia the foldbelt resides between the high grade metamorphic backbone of the cascade orogen on the west and rocks of the composite intermontane terrane to the east it encompasses the stratigraphically coherent basalt floored jura cretaceous methow basin as well as more chaotically disposed permian jurassic oceanic rocks of the hozameen terrane methow basin rocks are thought to have been sutured above the oceanic rocks prior to the middle cretaceous contractional episode described in this report based on the analysis presented herein between ca 100 and 88 ma the rocks in the foldbelt underwent shortening in an east northeast west southwest direction by 50 km or more largely by displacement on the east vergent jack mountain chuwanten thrust system the early stages of contraction occurred by

intersecting intracontinental tertiary transform fault systems in the north american cordillera in central british columbia north trending dextral strike slip faults that cut late eocene granite also truncate northwest trending dextral strike slip faults the northwest trending strike slip faults bound the wolverine metamorphic complex wolverine complex which has been uplifted primarily by northwest southeast eocene crustal extension and somewhat by late eocene northerly extension the crustal extension is indicated by shallow dipping extensions faults dyke complexes and stretching lineations the wolverine complex and its bounding faults define a crustal pull apart in an en echelon dextral transform the northwest and north trending dextral strike slip faults in central british columbia are the continuations of faults that transect the interior of the north american cordillera and they represent at least two distinct plate boundaries intermittently active during the early to middle eocene and the late eocene to early oligocene each of these systems consists of en echelon strike slip fault

lead and strontium isotope geochemistry of the karmutsen formation vancouver island british columbia lead isotope whole rock ratios of the triassic flood basalts of the karmutsen formation are heterogeneous with 206pb 2mpb 18 72 21 51 207pb 204pb 15 56 15 77 and 208pb 204pb 38 16 40 15 whole rock lead initial ratios are coincident with galena analyses having 206pb 204pb 18 1 19 0 207pb 204pb 15 53 15 60 and 208pb 204pb 38 2 38 6 lead uranium and thorium concentrations are greater than for mid ocean ridge basalts less than for island arcs but similar to those in ocean island and continental flood basalts isotopic similarities exist between the columbia river chilcotin continental flood basalts and the karmutsen formation suggesting similar origins for these basalts the heterogeneity in whole rock ratios can be explained in terms of isotopic mixing between a lead rich average crustal lead type and an ocean island basalt type mantle source thus the karmutsen formation displays evidence for sediment contamination of the mantle

the southern coast mountains british columbia new interpretations from geological seismic reflection and gravity data the southern coast mountains of british columbia are characterized by voluminous plutonic and gneissic rocks of mainly middle jurassic to eocene age the coast plutonic complex as well as metamorphic rocks folds and thrust and reverse faults that mostly diverge eastward and westward from an axis within the present mountains and by more localized eocene and younger normal faults in the southeastern coast mountains mid cretaceous and younger plutons intrude bridge river cadwallader and methow terranes and overlap middle jurassic through early cretaceous marine clastic rocks of the tyaughton methow basin the combination of geological data with new or reanalyzed geophysical data originating from lithoprobe and related studies enables revised structural interpretations to be made to 20 km depth five seismic profiles show very cut up and chaotic reflectivity that probably represents slices and segments of different deformed and rearranged rock assemblages surface geology seismic interpretations ph

relay mountain group tyaughton methow basin southwest british columbia a major middle jurassic to early cretaceous terrane overlap assemblage the upper middle jurassic to lower cretaceous relay mountain group is the lower part of the northern tyaughton methow basin southwestern british columbia the relay mountain group consists of 2700 3400 m of clastic rocks that we subdivide into three formal formations the callovian and lower oxfordian tyoax pass formation is marine shale and sandstone turbidites the teepee mountain formation consists of upper oxfordian to valanginian shallow marine clastic rocks with common buchia and fluvial and marginal marine facies in the upper part of the unit in the northwest these rocks overlie the lower formation across an abrupt conformable to disconformable contact the hauterivian and barremian potato range formation consists of clastic rocks that are marine in the southeast mainly nonmarine to the northwest and derived from the west this unit displays an abrupt conformable to disconformable contact with the middle formation and locally rests above the lower formation across an angular unconformity t

the jackass mountain group of south central british columbia depositional setting and evolution of an early cretaceous deltaic complex121this article is one of a series of papers published in this special issue on the theme of new insights in cordilleran intermontane geoscience reducing exploration risk in the mountain pine beetle affected area british columbia 2earth science sector ess contribution 20100280 the lower cretaceous jackass mountain group of southwestern british columbia records a complex history of deltaic and proximal marine sedimentation in a tectonically active area of the canadian cor

early mesozoic southward migration of cordilleran transpressional terranes oblique plate convergence can cause forearc arc and back arc regions of active margin orogenic belts to behave as semi independent terranes relative to bordering continental and oceanic lithospheric plates these terranes called here transpressional terranes are bound by the subduction zone on one side by arc parallel strike slip faults in the forearc in the volcanic arc or in the back arc region on the other and are underlain by throughgoing basal decollements these terranes undergo shortening in a direction perpendicular to the subduction zone and migrate as coherent structural bodies parallel to the subduction zone along bounding strike slip faults many terranes of the north american cordillera may have behaved in this manner and perhaps did not travel across the pacific basin but rather were always peripheral to north america we propose that during the triassic to mid cretaceous portions of the cordillera borderland generally were displaced southward with respect to the north american craton due to left oblique convergence since the mid cretaceous they moved northward due to right oblique subduction the proposed displacements are consistent with structural paleomagnetic and paleontologic data and with the relative and absolute motions of north america and the pacific basin plates based on hotspot reference frame models

non recognition of continental tholeiites using the ti y zr diagram continental tholeiites cannot be identified using the ti y zr diagram two hundred and thirteen analyses from 16 localities representing 7 continental tholeiitic provinces all plot outside the within plate basalt field of this diagram

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 metallogeny of late triassic rifting of the alexander terrane in southeastern alaska and northwestern british columbia a belt of unusual volcanogenic massive sulfide vms occurrences is located along the eastern margin of the alexander terrane throughout southeastern alaska and northwestern british columbia and exhibits a range of characteristics consistent with a variety of syngenetic to epigenetic deposit types deposits within this belt include greens creek and windy craggy the economically most significant vms deposit in alaska and the largest in north america respectively the occurrences are hosted by a discontinuously exposed 800 km long belt of rocks that consist of a 200 to 800 m thick sequence of conglomerate limestone marine clastic sedimentary rocks and tuff intercalated with and overlain by a distinctive unit of mafic pyroclastic rocks and pillowed flows faunal data bracket the age of the host rocks between anisian middle triassic and late norian late late triassic this metallogenic belt is herein referred to as the alexander triassic metallogenic belt the vms occurrences show systematic differences in degree of structural control chemistry and stratigraphic setting along the alexander triassic metallogenic belt that suggest important spatial or temporal changes in the tectonic environment of formation at the southern end of the belt felsic volcanic rocks overlain by shallow water limestones characterize the lower part of the sequence in the southern and middle portion of the belt a distinctive pebble conglomerate marks the base of the section and is indicative of high energy deposition in a near slope or basin margin setting at the northern end of the belt the conglomerates limestones and felsic volcanic rocks are absent and the belt is composed of deep water sedimentary and mafic volcanic rocks this northward change in depositional environment and lithofacies is accompanied by a northward transition from epithermal like structurally controlled discontinuous vein and pod shaped pb zn agba cu occurrences with relatively simple mineralogy to sulfosalt enriched vms occurrences exhibiting characteristics of vein diagenetic replacement and exhalative styles of mineralization and finally to cu zn co au occurrences with larger and more clearly stratiform orebody morphologies occurrences in the middle of the belt are transitional in nature between structurally controlled types of mineralization that formed in a shallow water near arc setting to those having a more stratiform appearance formed in a deeper water rift basin setting the geologic setting in the south is consistent with shallow subaqueous emplacement on the flanks of the alexander terrane northward the setting changes to an increasingly deeper back or intra arc rift basin igneous activity in the alexander triassic metallogenic belt is characterized by a bimodal suite of volcanic rocks and a previously unrecognized association with mafic ultramafic hypabyssal intrusions immobile trace and rare earth element ree geochemical data indicate that felsic rocks in the southern portion of the belt are typical calc alkaline rhyolites which give way in the middle of the belt to peralkaline rhyolites rhyolites are largely absent in the northern part of the belt throughout the belt the capping basaltic rocks have transitional geochemical signatures radiogenic isotope data for these rocks are also transitional basalts and gabbros end 4 9 and 87sr 86sr initial at 215 ma 0 7037 0 7074 together these data are interpreted to reflect variable assimilation of mature island arc crust by more primitive melts having the characteristics of either mid ocean ridge morb or intraplate within plate basalts wpb the ore and host rock geochemistry and the sulfosalt rich mineralogy of the deposits are strikingly similar to recent descriptions of active sea floor hydrothermal white smoker systems in back arcs of the southwest pacific ocean these data in concert with existing faunal ages record the formation of a belt of vms deposits and occurrences in a propagating intra to back arc rift tectonic setting during the late triassic a modern analogue having similar tectonic and metallogenic features is the southward projection of the lau basin from the active sea floor hydrothermal vents of the valu fa ridge to the taupo volcanic zone of the north island new zealand

topographic control on distribution of modern microbially induced sedimentary structures miss a case study from texas coast abstract modern back barrier tidal flats of galveston island follets island and matagorda peninsula of the texas coast are dominated by mud to fine sand sized siliciclastic sediments and prolific microbial mats these microbial mats modify sediment behavior and result in a variety of microbially induced sedimentary structures miss common structures include knobby surfaces reticulated surfaces gas domes mat cracks sieve like surfaces erosional pockets wrinkles and mat chips in general mat thicknesses increase from 1 mm in the upper supratidal to 3 cm maximum in the lower supratidal and then decrease to 2 mm in the lower intertidal areas this same wedge shaped pattern is displayed by detailed measurements of mat thicknesses from the rims into the deeper centers of depressions pools on the supratidal flats measurements of 175 mat cracks show that height of the curled edges of the mat crack polygons increases with increase in mat thickness similarly measurements of 150 gas domes reveal that the size of the gas domes also increases with increasing thickness of the associated mats because mat thickness varies with elevation on the tidal flat curl height of the mat cracks and size of the gas domes are also related to elevation six zones were identified based on the occurrence of miss within the supratidal zone i to upper subtidal zone vi areas at the highest elevation knobby surfaces characterize zone i whereas zone ii is defined by reticulated surfaces along with reticulated surfaces gas domes and mat cracks characterize zone iii and zone iv respectively association of sieve like surfaces with mat cracks typifies zone v whereas mat deformation structures and sieve like surfaces define zone vi boundaries between the miss zones in general are parallel and related to tidal zones the distribution patterns of the miss zones are strongly controlled by local topography of the sediment surface because the degree of inundation is the primary controlling factor for the mat growth and resultant miss therefore distribution of the microbially induced sedimentary structures in siliciclastics along with the dimension of the mat cracks and gas domes can be potentially helpful in interpretation of topography of paleodepositional surfaces

current aspects of basin analysis abstract current research in basin analysis focuses on interdisciplinary integration of geodynamics mathematical and physical modelling and sedimentary geology these efforts are organized into a research framework emphasizing basin formation nature of the basin fill maturation diagenesis of the basin fill and timing of events research in geodynamics permits development of a proposed classification of sedimentary basins in which basin types are discriminated according to nature of the plate margin basin position on or within a tectonic plate nature of the crust and geodynamic models or known process of formation a summary of current research developments on sedimentary basin fills focuses on the interdisciplinary solutions developed to understanding carbonate distribution through time origin of black shales sea level history and global sedimentary cycles maturation of basin fills requires an understanding of both thermodynamic aspects of mineral phase changes and changing character of the chemistry of flow of water through porous sediments during basin evolution summary case histories from the illinois basin and the appalachian basin illustrate these approaches

origin accretion and postaccretionary evolution of the bridge river terrane southwest british columbia the bridge river terrane of southwestern british columbia canada is one of a number of crustal fragments in the western part of the north american cordillera which originated elsewhere in continental margin or oceanic settings and have subsequently been incorporated into north america detailed geologic analysis of individual accreted terranes such as bridge river is essential in order to elucidate the processes of continental accretion and the postaccretionary evolution of accreted crust the bridge river terrane was probably derived from a mid mesozoic ocean margin setting and consists of three principal elements 1 the carpenter lake assemblage cla 2 bridge river schists brs and 3 the shulaps ultramafic body the cla is mid triassic to mid jurassic in age and consists of ribbon cherts both pillowed and tuffaceous greenstones limestone olistoliths and coarse volcaniclastic rocks which were metamorphosed in prehnite pumpellyite facies and are nonpenetratively deformed on a regional scale parts of the cla are intensely faulted the faulting in part records mesozoic accretion related shortening but also includes structures which offset dikes of probable tertiary age the brs separated from the cla by a late high angle fault was metamorphosed in prehnite pumpellyite through lower amphibolite facies is locally twice deformed and contains large scale recumbent folds a mesozoic phase of deformation and metamorphism was related to thrust emplacement of the shulaps ultramafic complex above the brs the thrust zone between the shulaps and the brs characterized by a thick serpentinite melange cuts lower plate metamorphic rocks which preserve an inverted metamorphic gradient tertiary deformation resulted in recumbent folding of the earlier thrust related fabric in the brs probably during a postaccretion strike slip deformational regime early phases of the deformation of all three elements of the bridge river terrane probably record accretion the current position and age of the bridge river terrane imply accretion later than the intermontane terrane and earlier than the insular terrane however its incorporation into north america may have been closely linked to one or both of these other crustal blocks early structures in the bridge river area have been strongly overprinted during late cretaceous and early tertiary strike slip faulting and northward transport

the hozameen fault system and related coquihalla serpentine belt of southwestern british columbia the hozameen fault of southwestern british columbia is associated with the coquihalla serpentine belt and separates two distinct crustal units northeast of the fault are greenstones of the early t

tatla lake metamorphic complex an eocene metamorphic core complex on the southwestern edge of the intermontane belt of british columbia the tatla lake metamorphic complex tlmc which lies on the southwest side of the intermontane belt imb in british columbia has characteristics typical of a metamorphic core complex anticlinorial amphibolite grade gneissic and migmatitic core underlying a 1 to 2 5 km thick zone of mylonite and ductilely sheared metamorphic rocks which is in fault contact beneath an upper plate of low metamorphic grade cover rocks of the 1mb ductile shearing in the tlmc involved tonalitic to granodioritic orthogneiss and structurally overlying amphibolite grade metasedimentary rocks and greenschist grade chlorite actinolite albite schist structures observed throughout the ductilely sheared rocks include a gently dipping mylonitic foliation ss containing a mineral lineation ls which trends toward 280 100 20 minor folds of variable trend fs almost exclusively confined to metasedimentary rocks are interpreted as synductile shear vergence of these folds defines a movement sense and direction of top toward 290 20 kinematic indicators from rocks not deformed by synductile shear folds indicate a tops to the west sense of shear while those within metasedimentary rocks deformed by fs folds yield conflicting results with a tops to the west sense predominating calculated a directions from fs folds which deform ls lineations indicate nearly horizontal ds movement subparallel to 290 110 the entire metamorphic core of the tlmc has been deformed by upright west to west northwest trending shallowly plunging map scale folds f3 the steeply dipping northwest trending yalakom fault cuts all units and forms the southwestern margin of the tlmc u pb zircon geochronology has documented the existence of cretaceous 107 79 ma in the core and eocene 55 47 ma in the mylonitic zone deformation and metamorphism in the tlmc k ar dates for biotite and hornblende of 53 4 45 6 ma record the uplift and cooling of the tlmc during early and middle eocene time 55 47 ma metamorphic rocks of the tlmc were carried to higher crustal levels along the footwall of the tlmc normal ductile shear zone final uplift and development of f3 folds post 47 ma are possibly related to dextral motion along the yalakom fault the tlmc has structural style and timing of deformation similar to other metamorphic core complexes in southeastern british columbia local and regional evidence is consistent with the formation of the tlmc in a regional extensional setting within a vigorous magmatic arc similar interlayered gneisses with near horizontal layering and foliation may underlie the entire southern imb

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

drilling mud contamination of samples in x ray diffraction and atomic absorption analyses drilling mud contamination in well cuttings may complicate determinations of minerals and elements commonly analyzed na k ba cr pb sr co ag ni in addition to caving and overrepresentation of the more consolidated fragments drilling mud contamination may cause ambiguity in sedimentologic studies utilizing cuttings analyses

mesoscale and regional kinematics of the northwestern yalakom fault system major paleogene dextral faulting in british columbia canada the northwestern yalakom fault system yfs lies on the eastern margin of the coast belt in southwestern british columbia the yalakom fault is the most significant structure in the fault system with at least 115 km of dextral slip mostly of early tertiary age the nw yfs is up to 25 km wide and is bounded on the southwest by the tchaikazan fault zone which had 5 km or more of dextral slip kinematic and paleostress analysis of mesofaults and map scale structures shows that secondary faults within the nw yfs had both dextral and sinistral oblique slip three large synclines had increasing amounts of clockwise vertical axis rotation from southeast to northwest block rotation between two synthetic strike slip faults with large offset yalakom and moderate offset tchaikazan is similar to current kinematics between the san andreas and san jacinto faults in southern california the nw yfs was active from latest cretaceous to mainly eocene time and postdates early late cretaceous thrust faulting to the southwest early stage mesofaults are compatible with the thrust belt and suggest that open folds developed in the foreland of the thrust belt before strike slip faulting commenced the abrupt northwest termination of the yfs may be related to extension within the adjacent tatla lake metamorphic complex some dextral slip may have been transferred farther northwest through the coast belt but these structures have not been documented in west central british columbia most of the 120 km of dextral slip on the yfs was transferred to the north via a mosaic of normal and strike slip faults in central british columbia thus the yfs forms the southwestern margin of a broad zone of transtensional structures that extends to the tintina rocky mountain trench fault zone these structures accommodated northwestward displacement of the northwestern cordillera in paleogene time

comparative geochemistry and petrology of triassic basaltic rocks from the taku terrane on the chilkat peninsula and wrangellia upper triassic metabasalt from the chilkat peninsula in southeastern alaska is lithologically similar to the middle and or upper triassic nikolai greenstone from the wrangell mountains east central alaska both basaltic sequences show comparable petrologic and geochemical features the informally designated chilkat metabasalt which forms part of a triassic sequence between the denali fault and the chilkoot lineament is included in the taku terrane whereas the nikolai greenstone characterizes the allochthonous wrangellia terrane the chilkat metabasalt consists mostly of massive to inconspicuously layered flows with well developed pillow structures at the top of the section the sequence may reach a total thickness of 3000 m the metabasalt is predominantly amygdaloidal and contains secondary minerals typical of greenschist facies metamorphism the nikolai greenstone is a widespread mostly subaerial but locally pillowed amygdaloidal basaltic sequence that reaches a total thickness of more than 3500

microbially induced sedimentary structures a new category within the classification of primary sedimentary structures abstract cyanobacterial films and mats syndepositonally influence erosion deposition and deformation of sediments the biomass levels surface morphologies and microbial mats stabilize depositional surfaces and shelter the sediment against erosion or degassing growing microbial mats dredge grains from their substrate upwards whereas cyanobacterial filaments that are oriented perpendicular to the mat surface reach into the supernatant water and baffle trap and bind suspended particles these and similar biotic physical interactions are reflected in syndepositional formation of microbially induced sedimentary structures we distinguish structures on bedding planes leveled bedding surfaces wrinkle structures microbial mat chips erosional remnants and pockets multidirectional ripple marks and mat curls and internal bedding structures sponge pore fabrics gas domes fenestrae structures sinoidal laminae oriented grains benthic ooids biolaminites mat layer bound grain sizes we propose to place this group of microbially mediated structures as a fifth category bedding modified by microbial mats and biofilms in pettijohn and potter s 1964 existing classification of primary sedimentary structures

geochemical polarity of the early cretaceous gambier group southern coast belt british columbia the gambier group of the southern coast belt was developed on the eastern edge of the wrangellia terrane in early cretaceous time a u pb zircon age determination from rhyolite produced a date of 112 0 0 3 ma consistent with albian fauna in sedimentary units major element geochemistry of volcanic rocks from separate east northeast and southwest volcanic centres displays both tholeiitic and calc alkalic affinities across the axis of the coast belt however assemblages in the east northeast have higher feot mgo ratios highlighting a more pronounced tholeiitic trend and are characterized by high al basalts volcanic rocks in the southwest are enriched in k trace element variations feature a southwesterly increase in ba rb and zr ti towards more alkalic values and a decrease in sr when compared with across arc geochemical trends from numerous other arcs the data may be interpreted to suggest that the gambier arc was east facing above a west to southwest dipping subduction zone such an interpret

possible indicators of microbial mat deposits in shales and sandstones examples from the mid proterozoic belt supergroup montana u s a abstract it has been suspected for some time that microbial mats probably colonized sediment surfaces in many terrigenous clastic sedimentary environments during the proterozoic however domination of mat morphology by depositional processes post depositional compaction and poor potential for cellular preservation of mat building organisms make their positive identification a formidable challenge within terrigenous clastics of the mid proterozoic belt supergroup a variety of sedimentary structures and textural features have been observed that can be interpreted as the result of microbial colonization of sediment surfaces among these are a domal buildups resembling stromatolites in carbonates b cohesive behaviour of laminae during soft sediment deformation erosion and transport c wavy crinkly character of laminae d bed surfaces with pustular wrinkled appearance e rippled patches on otherwise smooth surfaces f laminae with mica enrichment and or randomly oriented micas g irregular curved wrinkled impressions on bedding planes h uparched laminae near mud cracks resembling growth ridges of polygonal stromatolites and i lamina specific distribution of certain early diagenetic minerals dolomite ferroan carbonates pyrite although in none of the described examples can it irrefutably be proven that they are microbial mat deposits the observed features are consistent with such an interpretation and should be considered indicators of possible microbial mat presence in other proterozoic sequences

the morrowan atokan pennsylvanian boundary problem recognition of the morrowan atokan series boundary of the pennsylvanian subsystem in north america is complicated by the presence of unconformities succeeded by poorly fossiliferous lithologies at that level in both type regions abandonment of the name atokan in favor of the alternatively used name derryan would not resolve the problem the appearance of the fusiform fusulinid profusulinella has been taken to mark the base of the atokan series since its proposal yet there is no evidence to support that conclusion a competing definition based on the appearance of the primitive fusulinids pseudostqffella and eoschubertella seems to define better a boundary that is consistent with what is known of the morrowan and atokan type regions and of other faunal groups a boundary stratotype is needed to fully resolve the problem

contrasting tectono stratigraphy and structure in the coast belt near chilko lake british columbia unrelated terranes or an arc back arc transect stratigraphy and structural styles vary greatly in two areas of the coast belt near chilko lake chilcotin ranges in the east and coast mountains in the west no definite continuity between the two belts has been established in the pre mid cretaceous geology but this area may be a long lived episodic magmatic arc and nearby arc related basin the stratigraphic contrasts may reflect inherent differences between an arc and related basinal sequence triassic volcanic arc sequences are part of the stikine western belt and cadwallader eastern belt terranes which may be part of the same arc the jurassic is represented by one dated pluton in the west compared with almost continuous deposition of volcanogenic clastic rocks in the east lower cretaceous sequences in the west and east may represent a volcanic arc and back arc basin the taylor creek group albian is the first definitive link between the two belts and represents an arc and intra arc or back arc basin the structural evolution of the two be

distribution of paleogene and cretaceous rocks around the nazko river belt of central british columbia from 3 d long offset first arrival seismic tomography across the nechako chilcotin plateau of british columbia the distribution of cretaceous sedimentary rocks which are considered prospective for hydrocarbon exploration is poorly known due to the surface cover of glacial deposits and tertiary volcanic rocks to constrain the subsurface distribution of these cretaceous rocks in 2008 geoscience bc acquired seven long up to 14 4 km offset vibroseis seismic reflection lines across a north northwest trending belt of exhumed sedimentary rocks inferred to be part of the taylor creek group p wave velocity models which are consistent with sonic logs from nearby wells have been estimated using three dimensional first arrival tomography to depths ranging from 1 to 4 km igneous basement can be identified on most lines using the 5 5 km s isovelocity contour which locates the top of the basement to an accuracy of 400 m where its depth is known in exploration wells there is no general distinction on the basis of seismic velocity between cretaceous sedimentary

lithoprobe crustal reflection structure of the southern canadian cordillera 2 coast mountains transect the lithoprobe seismic reflection transect across the southern coast mountains of the canadian cordillera images fundamental crustal structures presumably related to collision of the intermontane and insular composite terranes and deep levels in the upper plate of the offshore cascadia subduction belt the eastern part of the coast mountains are characterized by east dipping upper crustal reflectors that project to exposed faults and east dipping lower crustal reflectors they are truncated by subhorizontal to west dipping middle and upper crustal reflectors these geometric relationships are interpreted to have formed during an early phase of primarily west directed contraction that created the east dipping structures of the upper and lower crust and a later phase of east directed shortening caused by wedging of the intermontane belt into the lower and middle crust of the tectonic stack subsequently the coast belt may have been displaced eastward on contractional faults that ascend from the lower crust beneath the intermontane belt and surface in the omineca and foreland belts extensional faults bounding the east flank of the coast mountains and west flank of the central nicola horst in the intermontane belt flatten into the middle and lower crust of the intervening region and geometrically outline crustal boudinage within the western coast mountains east dipping reflections spanning the middle crust to upper mantle are traced updip to vancouver island and the underlying cascadia subduction zone the c reflector on vancouver island is believed to separate wrangellia from underlying accreted terranes and is correlated to the mainland where it forms the upper boundary of a reflective lower crustal wedge that flattens into the moho if the moho is not a young feature then some accreted material appears to have wedged into the continental framework above the crust mantle boundary possibly causing shortening in the overlying crust and creating midcrustal ramps observed on the reflection data the structurally lower e reflections interpreted as shear zones originate at the subduction contact offshore and project landward into sub moho reflections within the upper plate on the mainland the region between the e reflector and the descending oceanic plate is interpreted to be subducted lower continental crust and mantle

revised stratigraphic nomenclature and age determinations for mid cretaceous volcanic rocks in southwestern british columbia mid cretaceous volcanic and volcaniclastic rocks in southwestern british columbia east of the fraser fault system constitute two principal lithostratigraphic units the lower unit a composite succession of basaltic to rhyolitic lavas and various clastic rocks is exposed in a 215 km linear belt from near pavilion to south of princeton the upper unit mostly amygdaloidal andesite is restricted to the centre of the belt between spences bridge and kingsvale where it overlies the lower unit and contiguous basement rocks both units were deposited subaerially concurrent with folding and faulting and share a contact that varies from gradational near kingsvale to unconformable near spences bridge the names spences bridge group and kingsvale group were used by several authors for various parts of the volcanic stratigraphy we suggest revision of nomenclature whereby the lower and upper units are named pimainus formation and spius formation respectively together they constitute the spences bridge group the term kingsvale group is

chemostratigraphy of upper cretaceous chalk in the danish subbasin the application of well cuttings to geochemical analyses was studied on the basis of the elemental distribution of mg sr mn and zn and the carbonate contents in upper cretaceous chalk sequences from six wells in the central part of the danish subbasin the cuttings were washed and dried at the well site with no other cleaning in the laboratory the analytical results from well cuttings were tested by comparison with data from corresponding cored intervals the results indicate that contamination by drilling mud is negligible in the material studied whereas caving occasionally may cause some veiling of the elemental distribution patterns the elemental data are therefore considered representative for carbonate sediments and the sequences studied a chemostratigraphic correlation of the sections is carried out on the basis of several chemostratigraphic markers each defined by one or more elements the chemostratigraphy hereby established for the central part of the danish subbasin is in accordance with the existing log stratigraphy and biostratigraphy in the area furthermore the geochemical profiles show remarkable similarity to corresponding profiles from the upper cretaceous in the north sea central graben the results emphasize the applicability of cuttings for geochemical analysis and of geochemistry as a useful tool for chemostratigraphy and basin analyses in the subsurface exploration for chalk as hydrocarbon reservoirs

lithostratigraphic and tectonic framework of jurassic and cretaceous intermontane sedimentary basins of south central british columbia1this article is one of a series of papers published in this special issue on the theme of new insights in cordilleran intermontane geoscience reducing exploration risk in the mountain pine beetle affected area british columbia the south central intermontane belt of british columbia has a complex architecture comprising late paleozoic to mesozoic volcanic and plutonic arc magmatic suites marine and nonmarine clastic basins high grade metamorphic complexes and accretionary rocks jurassic and cretaceous clastic basins within this framework contain stratigraphy with hydrocarbon potential the geology is complicated by cretaceous to eocene deformation dismemberment and dislocation the eocene to neogene history of the southern intermontane belt is dominated by non arc volcanism followed by pleistocene to recent glaciation the volcanic and glacial cover makes this a difficult region to explore for resources much recent work has involved re evaluating the challenges that the overlying volcanic cover has historically presented to geophysical imaging of the sedimentary rocks in this region in light of technological advances in geophysical data collection and analysis this paper summarizes the lithological and stratigraphic fra

paleomagnetism of the methow region north central washington structural application of paleomagnetic data in a complexly deformed variably remagnetized terrane jurassic through paleocene rocks of the methow pasayten belt were studied in order to use paleomagnetic directions to resolve the question of cretaceous northward transport in the end circumstanc

petrology and tectonic significance of gabbros tonalites shoshonites and anorthosites in a late paleozoic arc root complex in the wrangellia terrane southern alaska plutonic rocks intrusive into the late paleozoic tetelna formation of southern alaska are the underpinnings of the late paleozoic skolai arc of the wrangellia terrane there are four groups of intrusive rocks within the skolai arc 1 gabbro diorite plutons that contain gabbroic to anorthositic cumulates along with a differentiated series of gabbros and diorites of basaltic to andesitic composition 2 silicic intrusions including tonalite granodiorite and granite 3 monzonitic to syenitic plutonic rocks of the ahtell complex and related dikes and sills 4 fault bounded bytownite anorthosite of uncertain age and association these anorthosites may be related to post skolai nikolai greenstone magmatism the silicic rocks yield discordant u pb zircon ages of 290 320 ma early to late pennsylvanian relative age relations suggest that the oldest intrusive rocks are the gabbro diorite plutons the youngest are the monzonitic rocks and that the silicic rocks span this range the gabbro diorite pluto

northward translation of baja british columbia along the late cretaceous to paleocene margin of western north america paleomagnetic data suggest that a large part of british columbia and southern alaska was at the latitude of baja california mexico in early late cretaceous time this allochthonous block baja bc includes the insular talkeetna in alaska and intermontane composite terranes the miniterranes between them and possibly part of the omineca metamorphic belt assuming that baja bc was adjacent to north america by middle cretaceous an analysis using plate kinematics of the pacific basin produces a model of northward movement of baja bc which is compatible with the known geology of the reconstructed margin of western north america baja bc moved north 2400 km as part of the kuia plate along a transform margin with north america from 85 to 66 ma this time interval is marked by the cessation of igneous activity in the sierra nevada development of wrench faults and basins along coastal california and initiation of laramide style tectonics to the east within baja bc diminished igneous activity to the west was coeval with possible wrench tectonic basins along its eastern margin by the end of the cretaceous baja bc was positioned from oregon to northern british columbia between 66 and 56 ma baja bc became attached to north america in a dextral transpressive stage during which the kula north america plate boundary expanded to encompass the whole block with a series of northwest trending strike slip faults a fold and thrust belt in the east and large scale uplifts and the formation of an incipient volcanic arc on the west southern alaska was the site of convergence during the whole of the period of northward translation of baja bc a late cretaceous subduction zone in alaska evolved into a major compressional belt as the basin and miniterranes on the northern margin of baja bc collided with interior alaska the collision culminated in the paleocene with formation of the mckinley granites and coeval dextral slip on the denali fault system

stratigraphic record of triassic jurassic collisional tectonics in the blue mountains province northeastern oregon sedimentary and volcanic rocks in the blue mountains province bmp of northeastern oregon preserve a well studied record of triassic jurassic magmatism basin evolution and terrane accretion terranes of the bmp represent two magmatic arcs wallowa and olds ferry terranes an intervening oceanic subduction and accretionary complex baker terrane and a complex thick succession of sedimentary rocks commonly known as the izee terrane we divide volcanic and sedimentary rocks into two regionally correlative unconformity bounded megasequences 1 ms 1 late triassic to early jurassic deposits that change up section from 1a older volcanic and volcaniclastic deposits of the wallowa and olds ferry arcs to 1b marine turbidites shale and argillite with chert clast conglomerate and olistostromes derived from the emergent baker terrane and 2 ms 2 early to late jurassic marine deposits that overlap older rocks and structures and record 20 to 40 m y of deep crustal subsidence in a large marine basin many of the known stratigraphic relationships in the blue mountains cannot be explained using the existing model for a middle triassic to late jurassic west facing non collisional volcanic arc and forearc basin we propose a new tectonic model for the bmp based on prior studies and comparison to modern analogues which includes 1 middle triassic magmatism in the wallowa and olds ferry arcs during subduction and progressive closure of an ocean basin 2 late triassic collision between facing accretionary wedges of the wallowa and olds ferry arcs and growth of marine basins on both sides of the emergent baker terrane thrust belt 3 early to middle jurassic terrane continent collision which resulted in closure of a wide back arc basin crustal thickening and loading in nevada and growth of a large marine collisional basin in the bmp and 4 late jurassic thrusting regional shortening and final accretion of the basin and underlying terranes to western north america this analysis suggests that collisional tectonics may have played a significant role in plate interactions that drove triassic jurassic crustal thickening mountain building and basin development in the western north american cordillera

structural geology and accretionary history of the northwest cascades system washington and british columbia pre tertiary rock units on the western flank of the cascades in washington and southern british columbia constitute the northwest cascades system nwcs this litho tectonic package is characterized by predominantly oceanic lithologies subjected to high p t metamorphism and structurally mixed to form a regional melange although the focus of this report is on rocks of the nwcs as originally defined survey of the regional geology suggests broadening of the limits of this system to include the san juan islands the pacific rim complex and pre tertiary rocks on the western flank of the central and southern washington cascades the shuksan metamorphic suite a relatively coherent and broadly exposed unit of the nwcs contains a regionally consistent stretching lineation which provides evidence of east northeast directed underthrusting subduction during the early cretaceous blue schist metamorphism metamorphic f2 folds are approximately normal to the stretching lineation and appear to have developed late in the same deformational event the melange structure postdates the high pressure regional metamorphism it is characterized by a tectonic assemblage of diverse and unrelated rock units occurring as fragments as much as 10 or more kilometres in breadth and separated by a network of low to high angle north northwest striking faults pre fault restoration is impossible argillaceous or serpentinitic material commonly occurs in deformation zones between more competent lithologies mylonite zones as much as several metres thick are widely developed and display stretched clast lineations indicating a north northwest south southeast transport direction for both low and high angle faults the high angle faults are therefore strike slip in nature shear sense indicators are mixed in thrust fault zones but many indicate dextral movement for the strike slip faults k ar whole rock ages of mylonites range from 20 127 ma but two late cretaceous ages of 87 and 91 3 ma are considered as best representing the time of faulting the inferred shift in direction of tectonic transport from east northeast west southwest during early cretaceous metamorphism to north northwest south southeast during late cretaceous faulting and melange formation is correlated with a mid cretaceous change in plate interaction between the north american and farallon kula plates from high to low angle convergence the nwcs melange is interpreted to have formed during large dextral transform motion along the continental margin and to have reached final emplacement by thrusting against the southern end of wrangellia and the coast plutonic complex

strain partitioning in accretionary orogens and its effects on orogenic collapse insights from western north america changes in relative plate motions during the construction of accretionary orogens generally result in varying structural styles along the length of the orogen these disparate structural styles can be interpreted as having been formed by different tectonic regimes along the orogenic axis that formed at the same time if the orogen is considered at the large scale the differences in the way in which the crust responds during accretion can be explained by large scale strain partitioning within the same overall tectonic environment the westernmost canadian cordillera records the transition from late cretaceous dextral strike slip faulting to near orthogonal compression along the orogenic axis we postulate that the transition between strike slip dominated to compression dominated tectonics represents a late cretaceous partitioning of strain that resulted in a significant difference in crustal rheology along strike of the orogeny this had a dramatic effect on subsequent tertiary orogen scale extension we propose that plate readjustments in the tertiary led to orogen perpendicular collapse in portions of the orogen facilitated by decoupling between the middle and lower crusts along thermally weakened layers in contrast localized orogen parallel extension occurred in other portions of the orogen along kinematically linked large dextral strike slip faults where the upper crust remained coupled to the middle and lower crust new data indicate that partitioning of strain occurs across very large regions within any orogenic system and that the way in which strain is partitioned can lead to dramatic differences in future orogenic processes it becomes apparent from these data that orogens must be examined as a whole and that differing structural styles of similar ages are likely responses to the same overall tectonic regime

late paleozoic gabbroic rocks of the bridge river accretionary complex southwestern british columbia geology and geochemistry gabbroic bodies in the bralorne gold bridge area of southwestern british columbia are associated with the oceanic bridge river complex of the western canadian cordillera one of the suspect terranes accreted to north america in the jurassic the gabbros are locally cut by tonalites and are structurally interleaved with ultramafic rocks phyllites graphitic cherts and carbonate lenses that comprise the lower part of the bridge river complex their late carboniferous crystallization age overlaps the depositional age of affiliated supracrustal rocks mississippian jurassic some of which have been metamorphosed to blueschist facies compositionally the gabbros resemble mafic plutonic rocks of ophiolitic complexes and gabbroic rocks of the nearby shulaps range they display some affinity to oceanic island arc tholeiitic suites the bralorne and shulaps gabbros include cumulates and appear to have been derived from a single light ree depleted peridotitic source by melting and subsequent fractional crystallization accumulation of various combinations of plagioclase pyroxenes and olivine the tonalites are compositionally distinct from typical ophiolitic plagiogranites but might be related to the associated gabbros the gabbroic bodies occur within tectonic slivers derived from the oceanic crust that floored a deep ocean basin that existed during the late paleozoic and early mesozoic the bridge river complex comprises fragments of oceanic crust that were tectonically incorporated into an east verging accretionary prism during a middle late triassic to jurassic collisional event

tectonic implications of syn and post emplacement deformation of the mount stuart batholith for mid cretaceous orogenesis in the north cascades the 93 96 ma mount stuart batholith intruded across the boundary between the northwest cascades thrust system and the crystalline core of the north cascades although previously considered posttectonic the northeast margin of the mount stuart batholith and its wall rocks have been involved in syn to post emplacement southwest directed thrusting and folding and west northwest stretching contraction ended shortly after emplacement as indicated by high temperature recrystallization in thrust related mylonites of the pluton and by geochronological data whereas west northwest stretching continued for an unknown period of time this is the best documented mid cretaceous contractional belt in the main part of the crystalline core the shortening direction and timing are identical to that of southwest vergent thrusts in the offset continuation of the core in british columbia the contractional belt provides a link between thrusting in the northwest cascades thrust system and deformation in the crystalline

upper paleozoic to lower mesozoic strata and their conodonts western coast plutonic complex british columbia six lithologic units including two newly named formations were mapped on randall dunira and nearby islands the islands are characterized by greenschist facies metamorphism and westerly directed thrusting the oldest unit is a late mississippian massive limestone on ducie island the dunira formation composed of thin bedded limestone and siltstone is early and middle pennsylvanian in age it is unconformably overlain by limestone and dolomite of the upper triassic randall formation the randall formation grades upwards into a green phyllitic unit of late triassic age rhyolitic and more mafic volcanic rocks may represent a bimodal volcanic suite of early jurassic age based on a u pb date of 188 ma on zircons these five units correlate with rocks in the alexander terrane in southeastern alaska the sixth and presumed youngest unit consists of flysch like sedimentary rocks of probable middle jurassic to early cretaceous age that may correlate with rocks of the gravina nutzotin belt the three ol

hydrothermal alteration in the baca geothermal system redondo dome valles caldera new mexico thermal fluids circulating in the active hydrothermal system of the resurgent redondo dome of the valles caldera have interacted with their diverse host rocks to produce well zoned alteration assemblages which not only help locate permeable fluid channels but also provide insight into the system s thermal history the alteration shows that fluid flow has been confined principally to steeply dipping normal faults and subsidiary fractures as well as thin stratigraphic aquifers permeability along many of these channels has been reduced or locally eliminated by hydrothermal self sealing alteration from the surface through the base of the miocene paliza canyon formation is of three distinctive types argillic propylitic and phyllic argillic alteration forms a blanket above the deep water table in formerly permeable nonwelded tuffs beneath the argillic zone pervasive propylitic alteration is weakly developed in felsic host rocks but locally intense in deep intermediate composition volcanics strong phyllic alteration is commonly but not invariably associated with major active thermal fluid channels phyllic zones yielding no fluid were clearly once permeable but now are hydrothermally sealed high temperature alteration phases at baca are presently found at much lower temperatures we suggest either that isotherms have collapsed due to gradual cooling of the system that they have retreated without overall heat loss due to uplift of the redondo dome that the system has shifted laterally or that it has contracted due to a drop in the water table the deepest well b 12 3423 m in the dome may have penetrated through the base of the active hydrothermal system below a depth of 2440 m in this well hydrothermal veining largely disappears and the rocks resemble those developed by isochemical thermal metamorphism the transition is reflected by temperature logs which show a conductive thermal gradient below 2440 m this depth may mark the dome s neutral plane which separates an upper permeable zone of extensional fracturing from a lower less permeable compressional regime the baca hydrothermal system is similar to those which have formed ore deposits in other calderas particularly creede colorado type epithermal silver base metal veins and stockworks recent scientific drilling has also intersected a deep zone of strong phyllic alteration and molybdenum mineralization in the valles caldera s ring fracture system a setting which localized a large stockwork molybdenite orebody in the nearby questa caldera

paleomagnetism of the upper cretaceous strata of mount tatlow evidence for 3000 km of northward displacement of the eastern coast belt british columbia this study contributes the first paleomagnetic data from dated cretaceous units of the coast belt 51 3 n 123 8 w in which paleohorizontal bedding surfaces can be identified with certainty we sampled 38 sites through a 1 5 km stratigraphic interval of cenomanian silverquick sedimentary strata and cenomanian to campanian powell creek volcanics in the tatlow syncline in the tyaughton methow basin magnetizations at 21 sites have high unblocking temperatures tub and pass the tilt test precision k 5 before untilting maximum k 48 after 100 untilting magnetic minerals of volcanics show original textures mean declinations in volcanic and sedimentary rocks are statistically identical and polarity is consistently normal the mean inclination 55 4 3 4 probability p 0 05 of this pretilting magnetization is 20 1 4 0 shallower than that expected if the rocks were deposited in their present position relative to cratonic north america field and laboratory observations rule out other causes for these anomalous shallow inclinations these rocks therefore have been displaced northward by 3000 500 km relative to cratonic north america since mid cretaceous time this displacement is almost 3 times greater than that estimated from mid cretaceous volcanics in the now adjacent intermontane belt evidently the two belts were not amalgamated until latest cretaceous or early tertiary time these results are inconsistent with all previous geologically based tectonic reconstructions of the cordillera magnetizations at eight sites in silverquick and powell creek beds have low tub and directional characteristics similar to those in three younger diorite intrusions reversed and normal polarities occur the rocks are commonly highly altered and maximum precision occurs after 40 untilting the magnetizations therefore are interpreted as overprints of latest cretaceous age acquired during folding

interplay of plutonism and regional deformation in an obliquely convergent arc southern coast belt british columbia the coast plutonic complex is an extensive zone of continental growth that formed along the mesozoic convergent margin of northwestern north america the orogeny creating this belt involved terrane accretion and assembly massive upward transfer and emplacement of sial in the form of batholiths constituting a magmatic arc and transformation of broad tracts of sedimentary and volcanic rocks into crystalline continental crust all operating in more or less the same space and time the mechanisms and interplay of these orogenic processes are well displayed in the harrison lake area of the southern coast belt british columbia great structural relief across the area exhibits a present day architecture defined by thin thrust stacked terrane sheets and early concordant pluton sheets folded on a macroscopic scale all truncated by oblique dextral reverse faults and overlain by later floored plutons construction of this complex began with terrane assembly on orogen normal thrusts during a lull in plutonism in the early cretaceous low grade metamorphism during this event indicates only modest crustal thickening subsequent plutons intruded into the assembled terranes appear to be composites of sheets early pluton sheets are now steeply dipping due to folding but were likely intruded as horizontal bodies large ovoid post folding plutons are mostly subhorizontal floored bodies at least in part sheeted these plutons are underlain by barrovian mineralogic aureoles that indicate downward vertical displacement of 10 km or more during plutonism suggesting pluton emplacement by vertical inflation magmatic fabrics in these bodies and the discordant relation of plutons to regional structures preclude emplacement in active fault zones penetrative strain aureoles flanking plutons are mostly limited to zones a few hundred meters wide and regional tectonic structures are widely preserved tectonic deformation of the arc is characterized by contraction and strike slip not orogen normal extension plutons played a greater role than terrane accretion in causing crustal thickening and high grade metamorphism

plutonism oblique subduction and continental growth an example from the mesozoic of california major episodes of mesozoic plutonism in california correlate with periods of oblique subduction and trench parallel transport of western california along intrabatholithic faults major episodes of plutonism occurred in the late jurassic during left oblique convergence and in the mid cretaceous during right oblique convergence in contrast a conspicuous lull in plutonism but continuation of volcanism in the earliest cretaceous coincides with a time when the north america farallon convergence vector although large in magnitude was oriented perpendicular to the trench this correlation suggests that plutonism is facilitated by strike slip faulting within the batholithic belt one explanation which helps to solve the plutonic room problem is that plutons are passively emplaced at releasing bends in the strike slip faults and volume is conserved by thrusting at the trench if this correlation is generally applicable then it implies that mid crustal plutonism is limited beneath areas in which the convergence vector is subperpendicular to the trench continental growth in such areas may occur dominantly by volcanism

stratigraphy and tectonic setting of the upper part of the cadwallader terrane southwestern british columbia the upper triassic to middle jurassic cadwallader terrane lies on the northeastern edge of the coast plutonic complex in southwestern british columbia previous work on the cadwallader group the basal unit of the terrane suggested it was an upper triassic carnian to middle norian volcanic arc and related clastic rocks volcanism ceased in early norian time a detailed study of the upper part of the cadwallader terrane tyaughton group and overlying last creek formation shows that it is a sedimentary sequence deposited on the fringe of the inactive cadwallader magmatic arc the upper triassic middle to upper norian tyaughton group consists of nonmarine to shallow marine clastic rocks and limestones that show sudden changes in depositional setting the lower to middle jurassic last creek formation a transgressive sequence of clastic rocks disconformably overlies the tyaughton group the clastic rocks in the two units were derived from a mixed volcanic and plutonic source region that also included a

interpretation of structures in the southeastern nechako basin british columbia from seismic reflection well log and potential field data1this article is one of a series of papers published in this special issue on the theme of new insights in cordilleran intermontane geoscience reducing exploration risk in the mountain pine beetle affected area british columbia 2geological survey of canada contribution 20100002 the structure and stratigraphy of the southeast nechako basin which are poorly understood primarily because of substantial volcanic cover are investigated in an analysis of seismic reflection well and potential field data formation and development of the se nechako basin resulted in sub basins containing cretaceous and eocene rocks interpretation reveals that dextral transtension in the early to middle eocene created nnw trending en echelon strike slip faults linked by pull apart basins which locally contain a thickness of eocene volcaniclastic rocks of 3 km this structural pattern is consistent with regional observations that suggest the transfer of slip from the yalakom fault to the north via a series of en echelon strike slip faults in the middle to late eocene faults associated with a change in the direction of stress echoed by the north trending right lateral fraser fault reactivated and cut earlier structures a simple model agrees with local observations that northeast directed comp

on determining paleohorizontal and latitudinal shifts paleomagnetism of spences bridge group british columbia the spences bridge group of late albian age palynology and u pb zircon date of 104 5 ma is situated in the intermontane belt of the southern canadian cordillera zeolite grade andesite lavas and interbedded volcaniclastic rocks have been sampled through a thickness of about 2900 m sampling has been confined to localities whose bedding attitudes are well defined and to rocks that have not undergone internal deformation of the 27 sites studied 25 gave single component magnetizations with well grouped directions high coercivities and high unblocking temperatures applying corrections for geological tilt to all 25 sites yielded minimum dispersion after 65 untilting analysis indicates that this is a consequence of the interaction of two modes of behavior one exhibited by y magnetizations at 8 sites in which minimum dispersion occurs after 35 untilting and one x magnetizations at 17 sites in which minimum dispersion occurs after 100 correction actually the dispersion is essentially constant in the range 88 to 100 correction the y magnetizations are uninterpretable in terms of the paleofield because no estimate of paleohorizontal can be made because of their positive response to the tilt test the x magnetizations were acquired before tilting and it is argued that geologically determined bedding planes are good estimators of paleohorizontal at the time of remanence aquisition there may be errors because of paleoslope but arguments are presented for believing that these are small on this basis the best estimate of the mean direction of the paleofield in spences bridge time is estimated to be d i declination inclination 38 7 63 9 17 sites k 44 compared with k 11 in situ 95 5 4 with a paleopole at 64 4 n 321 0 e k 24 compared with k 5 in situ a95 7 5 when compared with the cratonic reference field for mid cretaceous time this pretilting magnetization indicates a clockwise rotation of the study area of 66 3 11 8 p 0 05 and an apparent displacement from the south of 15 5 7 1 1725 790 km when these and other data previously obtained from cretaceous rocks of the pacific northwest are compiled they show an increase in apparent displacement from east to west across the cordillera this result is discussed in terms of three hypotheses 1 no translation but systematic tilts of about 30 to the southwest 2 translation of a substantial part of the pacific northwest referred to as baja british columbia and is defined more exactly in the text by variable amounts up to more than 2000 km from the south together with clockwise rotations and 3 lesser translation of baja british columbia by about 1500 km combined with variable tilting predominantly to the south it is concluded that paleomagnetic data as they are presently known are consistent with hypotheses 2 and 3 but unless serious sources of error are present are not consistent with hypothesis 1 the most likely candidate for such errors is the uncertainty in estimating initial dip of beds paleoslope in volcanic rock units

geology of the cadwallader group and the intermontane insular superterrane boundary southwestern british columbia several lower mesozoic fault bounded units separate the intermontane and insular superterranes in southwestern british columbia detailed study of one of these mesozoic units the cadwallader group helps clarify the boundary between the superterranes and establish the tectonic evolution of southwestern british columbia the cadwallader group is the oldest unit in an upper triassic through middle jurassic volcanic and sedimentary tectono stratigraphic terrane two formations the pioneer and the hurley compose the cadwallader group the previously recognized noel formation is no longer considered valid the pioneer formation contains pillow basalt flows and basalt breccia siltstone sandstone conglomerate and minor amounts of limestone megabreccia and basalt belonging to the hurley formation conformably overlie the pioneer the hurley spans latest carnian or earliest norian to middle norian time two episodes of deformation affected the cadwallader and a thrust fault separates the group from sligh

mid cretaceous strike slip and contractional fault zones in the western intermontane terrane washington and their relation to the north cascades southeastern coast belt orogen structural and geochronologic data from the 111 114 ma okanogan range batholith in north central washington are used to characterize the timing and style of deformation during the early stages of the mid to late cretaceous north cascades southeastern coast belt nc secb orogen the pasayten fault zone bounds jurassic cretaceous sedimentary and volcanic rocks of the methow basin to the west against predominantly mesozoic igneous rocks of the intermontane terrane to the east the pasayten fault zone accommodated intrusion of the western units of the okanogan range batholith during high angle slip then approximately 20 km of left lateral strike slip at 109 95 ma south of the pasayten fault zone the red shirt and methow river thrust zones accommodated 1 west vergent contraction at amphibolite grade that began by 113 ma and ended by 112 ma based on new u pb zircon dates for deformed and undeformed granitoids and 2 northwest vergent contraction in the methow river thrust zone followed by west vergent contraction in the red shirt thrust zone both at greenschist grade prior to 104 ma these contractional displacements thrust the methow basin 10 20 km southeastward beneath the western okanogan range batholith deformation in the nc secb thus began by earliest albian time earlier than previously thought and was characterized by coeval sinistral transcurrent and west vergent contractional faulting sinistral strike slip in the pasayten fault zone may have reflected the pre 100 ma sense of oblique plate convergence or relatively minor southward tectonic escape of the adjacent methow basin during contractional deformation the slip history of the pasayten fault zone is undocumented between approximately 80 ma and 60 ma when on the basis of paleomagnetic data some workers have proposed that the methow basin moved 1700 km northward to its present position relative to the intermontane terrane however significant late cretaceous to early eocene dextral slip on the pasayten fault zone is unlikely because the western intrusive units of the okanogan range batholith continue without disruption across its southeastward trend

paleogeographic evolution of the united states pacific northwest during paleogene time paleogeographic reconstructions for oregon and washington during paleogene time illustrate a major transition from a dominantly compressional prior to middle eocene time to an extensional tectonic regime this transition resulted in the development of three phases of paleogene basin evolution in the united states pacific northwest during the initial phase basins formed along the continental margin during collision of oceanic islands sediments in these basins were derived from nearby orogenic highlands the second phase of basin development began in middle eocene time and consisted of rapid subsidence of individual basins that formed within a broad forearc region nonmarine basins that formed during this phase were caused by extension possibly associated with transcurrent faulting rapid sedimentation in both marine and nonmarine basins during this time consisted dominantly of sandstone derived from cretaceous plutonic sources far to the east the final stage of basin development was the modification

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

comment on paleomagnetism of the upper cretaceous strata of mount tatlow evidence for 3000 km of northward displacement of the eastern coast belt british columbia by p j wynne et al and on paleomagnetism of the spences bridge group and northward

impact des ecosystemes microbiens sur la sedimentation impact of microbial ecosystems on sedimentation abstract the microbial communities which often give rise to the fine lamination in sediments are very diverse both in species composition e g phototrophic and chemotrophic bacteria fungi algae and metabolism e g oxygenic and anoxygenic photosynthesis organic and inorganic chemosynthesis anaerobic and aerobic respiration organic and inorganic fermentation these microbial ecosystems have separated for more than 3 5 ga 3500 m y they intervene in the biostructuration of sediments by active mineral deposition particle trapping and sediment stabilisation they build bioconstructions stromatolites oncolites oolites and microbial bioherms further they are active in a number of biogeochemical biomineralisation processes including phosphatogenesis carbonate genesis and the formation of iron ore and sulfur deposits and petroleum and kerogen they are also active agents in the biocorrosion and bioerosion of sedimentary rocks under marine and terrestrial conditions since they have an enormous capacity for proliferation microbial laminated communities invade new habitats practically instantaneously where suitable energy sources and gradients for electron flow are available in addition such microbial systems are excellent and sensitive paleoecological indicators in order to study their impact on sedimentation and fossilisation a close collaboration between geologists and microbiologists is needed only in this way can the nature of fossil microbial ecosystems and the metabolisms that were at work be recognized and reconstructed

paleontology of proterozoic shales and mudstones examples from the belt supergroup chuar group and pahrump group western usa abstract proterozoic black shales and other fine grained siliciclastic rocks contain a variety of fossils these fossils are illustrated by examples from three proterozoic units in the western usa the belt supergroup middle proterozoic the chuar group late proterozoic and the upper pahrump group late proterozoic a variety of microfossils and megafossils occur in the belt supergroup western montana acritarchs and filamentous microfossils are moderately well preserved in sub greenschist facies black shale in the little belt mountains the biologic affinity of the acritarchs is uncertain they could represent the vegetative or encystment stage of eukaryotic algae or the outer envelope of colonial coccoid cyanobacteria most of the filaments in these shales are of a size and morphology that is consistent with prokaryotic affinities however rare 20 and 40 m wide filaments could be either prokaryotic or eukaryotic higher grade sub greenschist facies dark gray to black mudstones in glacier national park contain poorly preserved acritarchs and spheroidal and filamentous pseudomicrofossils they are of interest because they provide information useful for distinguishing authentic microfossils from pseudofossils in mudstones and shales that have undergone substantial burial metamorphism several authentic megafossils occur in lower belt shales in the little belt and big belt mountains montana one of these is the carbonaceous compression beltina danai which could represent either fragments of a microbial mat or fragments of a sheet like megascopic alga the megafossil grypania spiralis has the form of a coiled ribbon and tentatively is interpreted as a megascopic alga green to purple mudstones and muddy sandstones in the lower belt supergroup in glacier national park contain markings that resemble a string of beads they are interpreted as megafossils that may have eukaryotic algal affinities black shales in the late proterozoic kwagunt formation chuar group grand canyon arizona contain a variety of acritarchs and filamentous microfossils of special interest is the vase shaped microfossil melanocyrillium which is interpreted as the sporangium of a eukaryotic and probably megascopic alga melanocyrillium also occurs in black mudstone in the late proterozoic pahrump group southeastern california

p t t evidence for mechanisms of vertical tectonic motion in a contractional orogen north western us and canadian cordillera the 1800 km long coast mountains north cascades orogen of the canadian cordillera and north western us developed as a continental magmatic arc metamorphic rocks in the orogen contain widespread evidence for burial of supracrustal rocks to depths of c 40 km followed by nearly isothermal decompression to depths of 10 km near many shallowly emplaced mid cretaceous plutons low pressure contact metamorphic effects were overprinted by high pressure regional metamorphic minerals and textures as evidenced by kyanite staurolite pseudomorphs after andalusite in metapelitic rocks therefore near pluton rocks record the loading history of the orogen metapelitic rocks not associated with plutons only preserve evidence for high pressure conditions and or high temperature decompression as indicated for example by sillimanite and cordierite after kyanite and garnet respectively petrological evidence for burial and decompression is therefore recorded in different rocks various regions of the orogen differ in timing of metamorphism the overall shape of p t paths and the relative timing and regional extent of the high pressure event but most of these data and observations are consistent with thrusting and or pure shear thickening as primary loading mechanisms throughout the orogen as opposed to magma dominated loading this interpretation is further supported by comparison with thermal models which demonstrate that the p t paths are consistent with simultaneous thrusting and folding at a high initial geothermal gradient 35 40 c km 1 in much of the orogen a high geothermal gradient supports tectonic models invoking intra arc contraction and suggests that magmatism played an important role in regional temperature time paths this tectonic thermal history may be typical of other contractional orogens and illustrates the importance of large vertical displacement of crust in magmatic arcs

suspected microbial mat related crack like sedimentary structures in the palaeoproterozoic magaliesberg formation sandstones south africa abstract the occurrence of patchy preservation of ripples and of palimpsest ripples on many sandstone bed surfaces of the c 2 1 ga magaliesberg formation pretoria group transvaal supergroup south africa suggests that microbial mats grew within the upper parts of sandy deposits of the braid deltaic tidally controlled epeiric marine coastline palaeoenvironment inferred for this stratigraphic unit recently limited occurrences of petee ridges cracked sand layers elephant skin textures wrinkle structures and manchuriophycus like cracks have been found substantiating the envisaged role for microbial mats binding sandy sediment and providing cohesion during active sedimentation by fluvial and tidal processes ripple marks associated with most of these biogenic sedimentary structures enable evaluation of genetic processes for the ripples currents waves wind as well as estimation of wave heights and water depths cf tanner w f 1967 ripple mark indices and their uses sedimentology 9 89 104 tanner w f 1971 numerical estimates of ancient waves water depths and fetch sedimentology 16 71 88 the petee ridges are interpreted as reflecting spring low tidal desiccation which cracked the mats followed by disruption of sand beneath the mat due to loading from incoming tides or subsequent deposits slightly sinuous sand cracks on ripple crests are related to a similar combination of low tidal desiccation and formation of wind ripples with subsequent high tides providing the deeper water conditions and larger wave heights calculated from the tanner w f 1971 numerical estimates of ancient waves water depths and fetch sedimentology 16 71 88 formula synaeresis cracking is inferred for both elephant skin cracks and manchuriophycus structures these inferred suspect microbial sedimentary structures when studied in relation to data derived from associated ripples thus enable better estimation of depositional conditions within the magaliesberg palaeoenvironment and indicate minimum epeiric tidal ranges of c 112 cm in general these structures are found associated with evidence for shallower water and higher energy conditions which probably protected the microbial mats from degradation by heterotrophic bacteria through fast burial a possible association of such mat related structures with epeiric marine or passive margin coastline conditions may be pertinent for much of the precambrian clastic record

paleoenvironmental and chronological constraints on the mount tatlow succession british columbia first recognition of radiolarian and foraminiferal faunas in the intermontane cretaceous back arc basins of western canada1this article is one of a series of papers published in this special issue on the theme of new insights in cordilleran intermontane geoscience reducing exploration risk in the mountain pine beetle affected area british columbia 2geological survey of canada contribution 20100279 the cretaceous succession at mount tatlow british columbia is a cornerstone of cordilleran stratigraphy preserving a mostly continuous record of upper lower cretaceous to lower upper cretaceous sedimentary strata the succession is capped by volcanic strata of the powell creek formation lithofacies assemblages within the mount tatlow succession reflect sedimentation in a deep water submarine fan system at the base of the section to overlying submarine fan and to pro deltaic deposition and finally to delta plain sedimentation at the top of the succession radiolarian and foraminifer fossils from the lower part of the mount tatlow section are the first recovered from the intermontane basins of british columbia and indicate a middle albian to cenomanian age most likely cenomanian the presence of these fossils indicates that open marine conditions existed locally in the basin at this time but the strongly altered and pyritized nature of the fauna suggests that a reducing environment fostered early diagenetic pyritization processes in the subsurface sediments detrital zircon populations collected from the succession are in agreement with the paleontological ages

mid cretaceous thrusting in the southern coast belt british columbia and washington after strike slip fault reconstruction a major thrust system of mid cretaceous age is present along much of the coast belt of northwestern north america thrusting was concurrent and spatially coincided with emplacement of a great volume of arc intrusives and minor local strike slip faulting in the southern coast belt 52 to 47 n thrusting was followed by major dextral slip faulting which resulted in significant translational shuffling of the thrust system in this paper we restore the displacements on major dextral slip faults of the southern coast belt and then analyze the mid cretaceous thrust system two reconstructions were made that use dextral faulting on the yalakom fault 115 km castle pass and ross lake faults 10 km and fraser fault 100 km the reconstructions differ in the amount of dextral offset on the straight creek fault 160 and 100 km and how much the ne part of the cascades crystalline core expanded 30 km and 0 km during eocene extension reconstruction a produces the best match of lithotectonic units and thrust systems our synthesis shows that the southern coast belt thrust system was 250 180 km wide after thrusting the thrust system was mainly southwest vergent but had a belt of northeast vergent back thrusts on the northeast side associated with the tyaughton methow basin which may indicate large scale tectonic wedging thrust faults are commonly low to moderate angle but high angle faults also occur especially as late stage out of sequence structures involving plutons the amount of thrust displacement across the system is unknown but must be at least 100 km and may be many hundreds of kilometers most thrusting occurred from 100 to 80 ma and did not migrate systematically until after 90 ma when thrusting and magmatism shifted to the northeast for a few million years widespread thrusting occurred both near plutons and where there are no or small plutons which strongly suggests that thrust faulting was caused by regional to plate scale forces such as rapid plate convergence and or arc continent collision

magma emplacement during arc perpendicular shortening an example from the cascades crystalline core washington it is increasingly inferred by others that active arcs at all crustal levels are undergoing arc perpendicular extension and that magma utilizes this extension or extension along arc parallel strike slip faults during emplacement however a review of stress fields in modern arcs and deformation which occurred during magmatism in ancient arcs and our own emplacement studies of individual plutons indicate that magma emplacement typically occurred during arc perpendicular shortening without the aid of strike slip faulting this conclusion raises the question of how host rock is displaced during magma emplacement in shortening and thickening orogens evaluation of host rock material transfer processes that operated during emplacement of the 93 ma 10 to 14 km deep mount stuart batholith washington provides one example and includes the following 1 ductile downward flow in a narrow 0 2 body radii structural aureole 2 stoping and of lesser importance 3 regional folding thrusting and subhorizontal arc parallel extension we see no evidence that arc perpendicular extension or local extension along strike slip faults made space for this pluton nor for most other plutons in contractional arcs instead we argue that both localized and rapid around plutons and regional and slower downward flow during growth of crustal roots is a common process in such arcs

metamorphic and structural evidence for significant vertical displacement along the ross lake fault zone a major orogen parallel shear zone in the cordillera of western north america results of an investigation of the petrology and structure of the skymo complex and adjacent terranes constrain the amount timing and sense of motion on a segment of the 600 km long late cretaceous early tertiary ross lake fault zone rlfz a major orogen parallel shear zone in the cordillera of western north america in the study area in the north cascades washington state the rlfz accommodated significant pre middle eocene vertical displacement and it juxtaposes the skymo complex with upper amphibolite facies 650 690 c and 6 7 kbar skagit gneiss of the north cascades crystalline core to the sw and andalusite bearing phyllite of the little jack terrane intermontane superterrane to the ne the two main lithologic units of the skymo complex a primitive mafic intrusion and a fault bounded block of granulite facies metasedimentary rocks are unique in the north cascades granulite facies conditions were attained during high temperature 800 c low pressure 4 kbar contact metamorphism associated with intrusion of the mafic magma p t estimates and reaction textures in garnet orthopyroxene gneiss suggest that contact metamorphism followed earlier higher pressure regional metamorphism there is no evidence that the skagit gneiss experienced high t low p contact metamorphism in the little jack terrane however texturally late cordierite spinel and partial replacement of andalusite by sillimanite near the terrane s fault contact with skymo gabbro suggest that the little jack terrane experienced high t 600 c low p 4 kbar contact metamorphism following earlier low grade regional metamorphism similarities in the protoliths of metasedimentary rocks in the skymo and little jack indicate that they may be part of the same terrane differences in pressure estimates for the little jack versus skymo for regional metamorphism that preceded contact metamorphism indicate vertical displacement of 10 km west side up on the strand of the rlfz that now separates the two structural blocks high angle faults in the study area are dextral reverse mylonitic shear zones that experienced later brittle normal slip vertical motion on these shear zones before intrusion of skymo gabbro can account for metamorphic discontinuities indicated by p t results the terranes have also been internally deformed by nonintersecting but coeval dextral and sinistral shear zones that formed after the terranes were brought together in the rlfz and intruded by eocene dikes these results show that the rlfz has accommodated significant vertical displacement but perhaps no more than tens of kilometers of early tertiary lateral movement structural evidence for earlier large magnitude strike slip displacement is not preserved

orogen parallel extension in the north cascades crystalline core washington metamorphic stretching lineations throughout the crystalline core of the north cascades washington show a strong preferred orientation in a horizontal nw se direction foliations strike nw and dip steeply preliminary observations of shear sense features indicate noncoaxial right lateral motion we interpret these observations to indicate that the crystalline core was a broad right lateral shear zone the ages of the fabrics as recorded by metamorphic minerals and syntectonic plutons range from pre 85 ma in the south to post 45 ma in the north thus the shear regime was active during an extended interval of metamorphism and plutonism in the orogen a similar kinematic pattern is suspected for at least part of the coast plutonic complex british columbia which lies on strike with the crystalline core to the northwest

syntectonic remagnetization in the southern methow block resolving large displacements in the southern canadian cordillera 1 the upper cretaceous ventura member of the goat wall unit in the southern methow block of southern british columbia and northern washington state holds a syntectonic magnetization eight new sites from manning park in british columbia give a mean direction of d 27 5 i 60 1 k 304 7 95 3 2 after optimal partial tilt correction of five groups of bedded sites from farther south in the basin reported by bazard et al 1990 four have a syntectonic remanence with a direction similar to what we observe the exception is one group which has optimal concentration of remanence directions on 100 untilting and an abherent direction which must be rejected combining the accepted sites the optimal differential syntilting direction is d 11 8 i 61 5 k 39 3 95 3 4 n 47 giving a mean pole of 79 8 n 359 2 e k 19 5 and a95 4 8 the age of the remagnetization is constrained to be between 88 and 80 ma compared to cratonic north america this result indicates that the southern methow block was displaced from the south by 1800 500 km meaning it lay south of the sierra nevada subduction zone but well north of other paleomagnetically constrained cretaceous rock units from the insular superterrane including correlative strata of the mount tatlow area in the northern methow block among several possibilities to reconcile this discrepancy the most plausible has the whole methow block translated coherently but with the southern methow block strata remagnetized during transit

evidence for mid cretaceous wrench faulting in the methow basin washington tectonostratigraphic setting of the virginian ridge formation the cretaceous methow basin of north central washington is the southernmost of a series of mesozoic successor basins in the canadian cordillera the albian campanian sedimentary sequence there documents a restricted asymmetric basin filled by a west derived laterally amalgamated fan delta system the virginian ridge formation and an east derived stable fluvial deltaic system the winthrop formation followed by a locally derived volcanic sequence the midnight peak formation this study focuses on the depositional and tectonic setting of the albian cenomanian virginian ridge formation the mid cretaceous methow basin is interpreted as a wrench fault basin generated in response to transpressive dextral offset and uplift of a highland to the west stratigraphy and sedimentology of the albian cenomanian methow sequence indicates that 1 the basin was asymmetric normal to its axis in that the western margin was a mobile zone generating considerable relief while the eastern margin was passive stable and of relatively low relief 2 the basin was asymmetric along its axis documenting transpression and dextral offset along the western margin 3 the relatively short and steep fan delta system deposited along the western margin was laterally amalgamated and possibly time transgressive to the north this basin and similar coeval successor basins to the north may record extensive transtensile transpressive mobility along the cordillera in middle to late cretaceous time

detrital zircon geochronology of the taku terrane southeast alaska u pb geochronologic studies have been conducted on 60 detrital zircon grains from permian and triassic metasandstones of the taku terrane in central southeast alaska the resulting ages are mainly in the range 349 387 ma with five additional grains that yield probable ages ranging from 906 to 2643 ma these ages are similar to the ages of detrital zircons in carboniferous and older rocks of the yukon tanana terrane which lies directly east of the taku terrane in contrast these ages are different from the ages of detrital zircon grains in the alexander terrane to the west the data are accordingly consistent with models in which the taku terrane is a western component of the stikine and yukon tanana terranes and that this crustal fragment is separated by a fundamental tectonic boundary from rocks of the alexander and wrangellia terranes to the west

estuarine biofilm patterns modern analogues for precambrian self organization this field and laboratory study examines whether regularly patterned biofilms on present day intertidal flats are equivalent to microbially induced bedforms found in geological records dating back to the onset of life on earth algal mats of filamentous vaucheria species functionally similar to microbial biofilms cover the topographic highs of regularly spaced ridge runnel bedforms as regular patterning is typically associated with self organization processes indicators of self organization are tested and found to support this hypothesis the measurements suggest that biofilm induced sediment trapping and biostabilization enhance bedform relief strength and multi year persistence this demonstrates the importance of primitive organisms for sedimentary landscape development algal covered ridges consist of wavy crinkly laminated sedimentary deposits that resemble the layered structure of fossil stromatolites and microbially induced sedimentary structures in addition to layering both the morphological pattern and the suggested formation mechanism of the recent bedforms are strikingly similar to microbialite strata found in rock records from the precambrian onwards this implies that self organization was an important morphological process in times when biofilms were the predominant sessile ecosystem these findings furthermore emphasize that self organization dynamics such as critical transitions invoking ecosystem emergence or collapse might have been captured in fossil microbialites influencing their laminae this notion may be important for paleoenvironmental reconstructions based on such strata

reply to comment on paleomagnetism of the upper cretaceous strata of mount tatlow evidence for 3000 km of northward displacement of the eastern coast belt british columbia by p j wynne et al and on paleomagnetism of the spences bridge group and northward displacement of the intermontane belt british columbia a second look by e irving et al

variation in bed thickness in a turbidite succession dezadeash formation jurassic cretaceous yukon canada evidence of thinning upward and thickening upward cycles abstract analysis of the vertical variation in thickness of turbidites in an 883 in thick stratigraphic section of the dezadeash formation jurassic cretaceous yukon canada indicates that the turbidites are comprised of alternating sequences of thick beds and thin beds visual inspection reveals that 40 thick bedded cycles or megasequences are present fourier analysis indicates 38 cycles five different methods i e freehand signs of differences test correlation analysis least squares and moving averages used to determine the trend in bed thickness of the megasequences demonstrate that asymmetric cycles are present and most of these methods show that thickening upward trends are more common than thinning upward trends the best objective method for determining the asymmetry of megasequences appears to be correlation analysis viz pearson s spearman s and kendall s correlation coefficients and possibly the least squares method this particular section of the dezadeash formation is characterized by asymmetric cycles that are both common and statistically convincing and thickening upward sequences are more plentiful than thinning upward sequences

cretaceous sedimentation and tectonics tyaughton methow basin southwestern british columbia the mesozoic tyaughton methow basin straddles the fraser yalakom pasayten straight creek fypsc strike slip fault zone between six tectono stratigraphic terranes in southwestern british columbia data from hauterivian cenomanian basin fill provide constraints for reconstruction of fault displacement and paleogeography the early cretaceous eastern margin of the basin was a region of uplifted jurassic plutons and active intermediate volcanism detritus shed southwestward from that margin was deposited as the marine jackass mountain group albian inner to mid fan facies of the jackass mountain group can be correlated across the yalakom fault suggesting 150 25 km of post albian dextral offset deposits of the jackass mountain group overlap the major strike slip zone fypsc if that zone represents the eastern boundary of the tectono stratigraphic terrane wrangellia then accretion of wrangellia to terranes to the east occurred before late early cretaceous time the western margin of the basin first b

lithofacies and depositional environments of the telkwa coal measures central british columbia canada abstract albian coal measures of the skeena group in the telkwa coalfield comprise more than 500 m of conglomerate sandstone siltstone mudstone and coal that were deposited during two regressive transgressive cycles the stratigraphic sequence is divisible into four lithostratigraphic units the basal unit unit 1 is about 100 m thick and comprises conglomerate sandstone mudstone coal and seatearth deposited in a fluvial environment on a deeply eroded volcanic basement gravel and sand were deposited in braided channels and bars and mud accumulated in floodplains peat accumulated in poorly drained backswamps in the northern part of the study area coal seams thin and split a result of periodic flooding of peat swamps with sediment laden water from nearby streams deposition of unit i ended with marine transgression unit ii consists of up to 140 m of silty mudstone bioturbated and cross bedded chert and muscovite rich sandstone and rare thin carbonaceous mudstones deposited in a shallow marine environment the sands were deposited as distributary channels and mouth bars muds were deposited in bays and thin discontinuous peat accumulated in local salt marshes unit iii averages 90 m in thickness and comprises bioturbated and rippled chert and muscovite rich sandstone siltstone carbonaceous mudstone and thick coal seams deposited in a variety of low energy paralic environments sand and mud were deposited and biogenically reworked in tidal flats and siltstone accumulated in a restricted nearshore marine environment to the east peat accumulated in freshwater coastal domed mires which periodically prograded over tidal flats all but the lowermost coal seams pinch out eastward into brackish and nearshore marine sediments the ash yield of many coals increases towards their seaward margin locally the sulphur yield of the coal is high reflecting occasional inundation by brackish water the coals were deposited during regressive maximum and are characterized by an essentially vertical stacking pattern comprising an aggradational parasequence set the major coals are considered to have been deposited at the turnaround between progradation and retrogradation unit iv is over 150 m thick and is composed of chloritic sandstone overlain by silty mudstone deposited in a marine environment the basal sandstone is a transgressive lag deposit silty mudstone is the predominant lithofacies and was deposited in a nearshore shallow marine environment

kinetic control of metamorphic imprint during synplutonic loading of batholiths an example from mount stuart washington the superimposition of barrovian type metamorphism on low pressure contact aureoles in magmatic arcs is subject to the kinetic constraints of temperature and influx of h 2 o and may be cryptically developed or not at all the spatial distribution of detectable overprint will reflect these kinetic factors rather than the geometry of the imposed load many of the features believed indicative of magma loading including apparent steep baric gradients may be produced in this way on the basis of adding areas of cryptic metamorphism to the area of obvious barrovian overprint on the northeast side of the mount stuart batholith we conclude that loading 2 3 m y after emplacement was probably much more extensive sheetlike than believed previously

the scotia quaal metamorphic belt a distinct assemblage with pre early late cretaceous deformational and metamorphic history coast plutonic complex british columbia the scotia quaal metamorphic belt extends from hawkesbury island to work channel between the early late cretaceous ecstall and the paleogene quottoon plutons the belt consists of a proterozoic paleozoic metasedimentary and metavolcanic sequence the middle devonian big falls orthogneiss early jurassic orthogneiss and jurassic or cretaceous mafic and ultramafic intrusive rocks the assemblage may be correlative with nisling terrane lithologies and may have shared a common history at least from early jurassic time on with rocks of stikine terrane strong planar and linear fabrics abundant folds and scarcity of kinematic indicators characterize the belt s deformational style development of a strong foliation followed by three episodes of folding occurred between emplacement of the middle devonian big falls orthogneiss and early late cretaceous ecstall intrusion paleogene fabrics occur in the quottoon pluton and in the easternmost 1 5 km of the scotia quaal belt if a major paleogene shear zone is po

cretaceous crustal section through the proposed insular intermontane suture north cascades washington the north cascades of washington consist of rocks that were deformed and recrystallized during a middle cretaceous orogeny that was driven by the collision of the insular and intermontane superterranes we reconstruct a composite vertical crustal section through the orogen on the basis of geobarometric data from the skagit gneiss and on stratigraphic and structural data from supracrustal rocks exposed east of the skagit gneiss we propose that at the end of the orogeny the upper 30 km of crust consisted of skagit gneiss overlain by two structural couplets each composed of methow basin strata underlain by hozameen terrane rocks we suggest that the insular intermontane suture lies between the skagit gneiss and the overlying hozameen terrane rocks

fracture controlled magma conduits in an obliquely convergent continental magmatic arc magnetic fabric patterns of two mid cretaceous nested plutons 102 1 and 96 3 ma in the central sierra nevada batholith provide evidence that felsic magma emplacement and ascent occurred via north trending steeply dipping planar fracture conduits oriented obliquely to the arc u pb geochronology data indicate that emplacement of the plutons was separated by 2 to 10 m y and that they were emplaced in part via the same conduit magnetic fabrics in the younger pluton are related to its final emplacement which was strongly influenced by a system of host rock joints formation of the north trending conduits at 100 ma can be related to a small arc parallel dextral shear component that produced tension fractures and that was associated with high angle oblique convergence by 90 ma convergence had become sufficiently oblique for the development of nearly arc parallel structures which were then favored as magma conduits

tectono stratigraphic terranes and the metamorphic history of the northeastern part of the crystalline core of the north cascades evidence from the twisp valley schist the twisp valley schist tvs is important for both the correlation of terranes in the cascades crystalline core and for the determination of the metamorphic pressure temperature time history of the northeastern part of the core the tvs is a chaotically mixed unit of mainly siliceous schist metachert with significant amounts of metabasite calc silicate rock and marble and minor ultramafite and metapsammite geochemical analyses of metabasites indicate that the tvs contains both ocean island basalts oib and mid ocean ridge basalts morb the tvs is lithologically similar to the napeequa unit of the chelan mountains terrane in the cascades core and is broadly correlative with units outside of the core including the mississippian jurassic bridge river hozameen terrane of the eastern coast belt and coeval oib bearing terranes in the northwest cascades thrust system much of the northeastern cascades core thus consists of oceanic rocks that probably originally lay between the insular and the inter

revised metamorphic history for the chiwaukum schist north cascades washington a revised metamorphic history for the chiwaukum schist north cascades washington consisting of dynamic contact metamorphism of buchan style associated with the mount stuart batholith followed closely by regional metamorphism of barrovian style is similar to that for rocks adjacent to the sloan creek intrusion 30 km to the north and on the opposite west side of the straight creek fraser river fault system to the settler schist in the vicinity of the spuzzum and scuzzy intrusions 200 270 km north in british columbia the late cretaceous regional increase in lithostatic pressure implied by this history whether caused by tectonic or magmatic processes may be the same as that responsible for the crystallization of magmatic epidote in certain other intrusives in the north cascades such as the tenpeak more radiometric crystallization ages as opposed to cooling ages are needed to resolve this point