seismic petrophysics and isotropic anisotropic avo methods for unconventional gas exploration exploration and drilling for natural gas in north america has moved radically away from conventional reservoirs to focus on unconventional reservoirs such as tight gas sands and shales these reservoirs have low porosity and near zero permeability with gas stored in natural fractures and within the matrix porosity economic gas production requires hydraulic fracture stimulation to open connections to existing natural fractures or matrix porosity and successful stimulation depends on the formation s geomechanical brittleness being capable of supporting extensive induced fractures however despite adequate stimulation significant variations exist between wells in expected ultimate recovery eur due to the heterogeneity of these resource plays consequently predicting natural fractures or fracture prone sweet spots is essential to optimize development of such plays

lithospheric processes and products in the southern canadian cordillera a lithoprobe perspective analyses of lithoprobe and other data from southwestern canada provide new insights on how this portion of the cordillera formed during plate convergence along the western margin of north america crustal rocks are detached from their mantle lithosphere which must have been consumed during subduction detachment occurred at or near the base of the crust beneath the intermontane and or omineca belts probably along the tips of tectonic wedges while the rocks were still outboard of the relatively cool mechanically rigid north american craton during the late cretaceous and early tertiary rotation of detached rocks caught between the north american craton and the oceanic plates accounts for some apparently conflicting results between paleomagnetic data that indicate large northward translation of rocks in the western cordillera and regional geological features that appear to preclude comparable amounts of translation of rocks in the eastern cordillera during the same time interval transpression assoc

magmatic evolution of the eastern coast plutonic complex bella coola region west central british columbia the eastern coast plutonic complex in west central british columbia 51 54 n consists of a broad belt of jurassic to eocene granitic to dioritic intrusive rocks that collectively represent more than 140 m y of nearly continuous subduction related magmatism seven distinct intrusive suites have been identified in the bella coola region including from oldest to youngest the howe lake stick pass firvale desire fougner big snow and four mile suites pre late cretaceous rocks were produced by episodic subduction related magmatism characterized by partial melting of preexisting lower arc crust with minimal incorporation of evolved continental material the eastern coast plutonic complex experienced a magmatic lull between ca 110 and 95 ma coincident with sinistral transpression potentially linked to translational displacement of the wrangellia alexander composite terranes crustal thickening in late cretaceous time ca 100 90 ma possibly due to underplating of the wrangellia alexander terrane produced a deep crustal root 40 km that likely extended into the eclogite transition zone adakitic magmas ca 95 67 ma were produced by partial melting of an amphibolitic protolith in this thickened lower arc crust adakitic plutons are pre syn and postkinematic with respect to late cretaceous high angle shear zones and they are coeval and spatially associated with postkinematic two mica granites ca 70 75 ma having a hybrid a type geochemistry we suggest that delamination of this dense crustal root led to voluminous magmatism extension and crustal exhumation in paleocene eocene time the successive intrusive events in the eastern coast plutonic complex may reflect a predictable evolutionary progression common to all continental arc systems

fracture mapping using seismic amplitude variation with offset and azimuth analysis at the weyburn co2 storage site abstractcap rock integrity is an essential characteristic of any reservoir to be used for long term co2 storage seismic avoa amplitude variation with offset and azimuth techniques have been applied to map hti anisotropy near the cap rock of the weyburn field in southeast saskatchewan canada with the purpose of identifying potential fracture zones that may compromise seal integrity this analysis supported by modeling observes the top of the regional seal watrous formation to have low levels of hti anisotropy whereas the reservoir cap rock composite midale evaporite and ratcliffe beds contains isolated areas of high intensity anisotropy which may be fracture related properties of the fracture fill and hydraulic conductivity within the inferred fracture zones are not constrained using this technique the predominant orientations of the observed anisotropy are parallel and normal to the direction of maximum horizontal stress northeast southwest and agree closely with previous fracture studies

regional tilt of the mount stuart batholith washington determined using aluminum in hornblende barometry implications for northward translation of baja british columbia we have developed a new quantitative method to estimate paleohorizontal in granitic plutons using the aluminum in hornblende ah barometer the method is used to correct previously published paleomagnetic data from the 93 96 ma mount stuart batholith of the cascades mountains washington state for the effects of postemplacement tilting ah barometry was done on 46 samples from the batholith using the compositions of hornblende rims coexisting with the full mineral assemblage required for pressureestimation high contrastback scatteredelectronimagingwas usedtoensurethattheanalyzedhornblendeswerenotsignificantly affected by subsolidus alteration the success of the ah barometry is indicated by two observations first increases in the al content of the hornblendes are governed almost entirely by a pressure sensitive tschermak type substitution second amphibole plagioclase thermometry indicates that assemblage equilibration occurred at or very near magmatic conditions 650 c and that temperature has a negligible effect on our pressure estimates ah barometry results indicate that the depth of crystallization across the batholith decreases systematically from 0 3 gpa in the northwest to 0 15 gpa in the southeast consistentwithindependentbarometryforthecontactaureole ofthebatholithandregionalstructuralandstratigraphicrelations using a best fit planar tilt model and bootstrap analysis of uncertainties we estimate that the paleohorizontal plane has a strike of 43 630 4 and dip of 7 62 0 southeast 695 confidence our estimated paleohorizontal allows us to restore the paleomagnetic data of beck et al 1981 and to estimate the original paleolatitude of the mount stuart batholith beck et al found that the southern part of the batholith yielded a number of sites with a well defined high coercivity remanence the carrier of this remanence was not resolved but the following four lines of evidence stronglysuggestthatthepublisheddirectionswereacquiredshortly after emplacement of the batholith 1 the stable sites all came from the shallowest and most rapidly cooled portions of the batholith as indicated by our ah results and concordant k ar ages for hornblende biotite pairs 2 the high coercivity component was always normal in polarity which is consistent with emplacement of the mount stuart batholith at the end of the cretaceous long normal 3 sites from the batholith and the contact aureole gave similar directions 4 the directions show no indication of tilt related smearing after restoration the paleomagnetic data indicate 42 6 11 clockwise rotation and 3100 6 600 km of northward offset

geochemical and stratigraphic analysis of the chisana formation wrangellia terrane eastern alaska insights into early cretaceous magmatism and tectonics along the northern cordilleran margin

linked thrust and strike slip faulting during late cretaceous terrane accretion in the san juan thrust system northwest cascades orogen washington the san juan thrust system makes up the western elements of the late cretaceous northwest cascades orogen of washington a stack of metamorphic nappes with terranes ranging in age from early paleozoic to middle cretaceous the tectonic development of the nappes is an unresolved issue important to understanding plate convergent processes operating in the north american cordillera in the creta ceous as well as at other times two terrane bounding fault zones were studied in this project the lopez and the rosario fault zones the fault zone rocks were affected by two cataclastic plastic late cretaceous deformational events d1 and d2 that included accretion and lateral motion during blueschist facies metamorphic conditions d1 produced upright folds and a subvertical axial planar s1 foliation fabric in the lopez fault zone and southwest vergent folds and gently northeast dipping axial planar s1 foliation in the rosario fault zone in the lopez fault zone d2 produced northwest striking and subvertical strike slip shear zones s2 with dominant sinistral displacements whereas in the rosario fault zone d2 generated low angle s2 shear zones and various slip crenulation fabrics formed by northwest directed thrust motion the d1 fabrics are inferred to be related to initiation of subduction related accretion of the nappes by orogen normal southwest northeast shortening and accretionary tectonics the subsequent d2 fabrics developed as temporally overlapping subduction related low angle thrusts in the rosario fault zone and high angle sinistral strike slip faults in the lopez fault zone in an overall transpressive setting due to oblique convergence this model suggests that the high angle lopez fault zone d2 was located above a gently northeast dipping thrust detachment s2 expressed by the rosario fault zone that allowed the accreted nappes to move parallel to the orogen the kinematic model cannot validate regional scale dextral terrane translation of the san juan thrust system as inferred by paleomagnetic data elsewhere in the western united states but addresses important tectonic processes caused by oblique subduction and changing plate conditions

geochronology of the pre andean metamorphism and magmatism in the andean cordillera between latitudes 30 and 36 s abstract in the 30 36 s section of the andean cordillera the pre andean basement is composed by metamorphic igneous and sedimentary rocks of precambrian paleozoic and lower triassic ages they crop out in the morphostructural units named from east to west sierras pampeanas precordillera cordillera frontal and bloque de san rafael in argentina and cordillera de la costa in chile isotopic age determinations of the metamorphic rocks indicate a migration of the metamorphic events from east to west such as precambrian to lower paleozoic ages 900 m y and 650 400 m y in the sierras pampeanas lower paleozoic ages about 500 m y in the precordillera and cordillera frontal and upper paleozoic to lower triassic ages 340 210 m y in the cordillera de la costa these metamorphic belts contain a large variety of mafic and ultramafic rocks the distribution of the pre andean granitoids show no apparent chronological zonation however the plutonic activity persisted until the lower carboniferous in the sierras pampeanas to the lower triassic in the cordillera frontal and precordillera associated with an important vulcanism which also develops in the bloque de san rafael and until the jurassic in the cordillera de la costa

northeast trending folds in the western skeena fold belt northern canadian cordillera a record of early cretaceous sinistral plate convergence abstract the western portion of the skeena fold belt northern canadian cordillera contains northeast trending folds that are highly oblique to northwest trending folds in the eastern portion of the fold belt and to most mesozoic contractional structures in the northern cordillera the northeast trending folds locally interfere with the northwest trending folds and one region includes transected folds geometric relationships within and between the two fold sets are not easily reconciled by notions of the northeast trending folds resulting from vertical axis rotation of blocks influence of basement features or lateral variations in magnitude of shortening the northeast trending folds are inferred to result from sinistral plate convergence early in the history of the fold belt early cretaceous northeast trending folds in the skeena fold belt are the most conspicuous elements of a seldom studied group of similarly oriented contractional structures which collectively define a belt at least 1700 km long within and bordering the coast belt the extent of early cretaceous structures potentially related to sinistral convergence supports them having originated in response to the relative plate motion rather than local controls e g indentors this agrees with relative plate motion studies based on ocean floor reconstructions which suggest a mid cretaceous change from sinistral to dextral convergence

detrital zircon record of a mesozoic collisional forearc basin in south central alaska the tectonic transition from an oceanic to continental arc

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

mesozoic cenozoic deformation in the canadian cordillera the record of a continental bulldozer abstract a possible mesozoic cenozoic trajectory for the north american craton is outlined from latitude changes of the craton derived from a revised apparent polar wander path and from westward movement of the craton based on the assumption that africa has been the least mobile continent geographically since the latest paleozoic during each of five time intervals that span 220 million years the craton trajectory had a different vector each vector appears to be reflected in the canadian cordillera by the dominant style and orientation of structures formed during that interval for much of the past 220 million years the meridionally oriented western margin of the pangea laurasia north america plate has been the site of arc magmatism where weak arc back arc lithosphere sandwiched between strong ocean floor and craton lithospheres at times coupled across convergent or transform plate boundaries focused and recorded strain when the craton apparently moved due westward between 180 160 ma and 120 60 ma the dominant structures formed then record orogen normal compression and were accompanied by orogeny structures formed during the earlier episode are mainly in the eastern and interior cordillera and their formation shortly followed or coincided with accretion of most terranes to the craton margin in the later episode compressional structures span the entire cordillera which emerged as a tectonic and physiographic entity before and between these intervals when the craton apparently moved mostly northwestward mainly geological and paleomagnetic considerations indicate some terranes moved southward sinistrally relative to the craton after 60 ma southwestward movement of the craton was concurrent with large northward dextral strike slip faults that disrupted the newly established ancestral cordillera the coincidence between the age of structures that record dominant orogen normal compression at times when the craton apparently moved due westward and orogen parallel displacements when the craton had either northward or southward components of motion suggests the craton acting as a continental bulldozer was the primary driver of cordilleran deformation and orogenesis

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

seismic interpretation of crustal scale extension in the intermontane belt of the northern canadian cordillera the crustal thickness of the canadian cordillera which may have reached 50 65 km during the cretaceous is now only 32 38 km along its length in the south postorogenic extension during the eocene resulted in the formation of core complexes and crustal scale extensional shear zones whereas in the north evidence for postorogenic extension is either limited to relatively minor basin formation in a transtensional environment or has not been recognized using new images of lithoprobe seismic reflection line 2a in northern british columbia which has been previously interpreted in terms of terrane accretion i make an alternative interpretation of crustal scale postaccretion extension in the middle and upper crust as early as the late permian to early triassic the quesnellia and stikinia arc terranes were accreted to ancestral north america along a crustal ramp which is imaged in the lower crust an extensional shear zone which extends from close to the surface to 20 km depth is identified from the geometry of reflections in the overlying structural basin and a lateral change in seismic velocity the oceanic cache creek terrane which is 4 km thick in the vicinity of line 2a was preserved because it was imbricated with its overlap assemblage and dropped into the 10 km deep structural half graben which may correspond approximately to the combined extent of the whitehorse trough and the cache creek terrane in southern yukon and northern british columbia

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

comment on u pb and hf isotope analysis of detrital zircons from mesozoic strata of the gravina belt southeast alaska by yokelson et al 2015

p wave anisotropy from azimuthal avo and velocity estimates using 3d seismic data from saudi arabia a horizontal well is most productive in tight reservoirs when it intersects a large number of vertical fractures yet strata near the borehole remain mechanically stable azimuthal velocity analysis and p wave amplitude versus offset avo using 3d wide azimuth prestack surface seismic data provide a remote yet detailed way to map a fracture pattern away from well control we estimate fracture direction and relative fracture intensity from such data at two gas fields in saudi arabia our results show a small azimuthal variation in p wave velocity maximum 5 and a larger variation in azimuthal avo at the reservoir larger than 100 computed fracture attributes for field 1 show a consistent east west fracture direction however in field 2 fracture azimuth is variable but generally east west and north south with the strongly anisotropic north south orientation correlating with faults and areas of large structural curvature in the reservoir in both fields azimuthal avo analysis shows a more consisten

correlation among lower to upper crustal components in an island arc the jurassic bonanza arc vancouver island canada the westcoast crystalline complex wcc island intrusions and bonanza group of vancouver island canada form three different crustal levels of the early to middle jurassic bonanza island arc differential uplift has exposed the plutonic roots and the volcanic carapace of the arc for a strike length of 500 km and for another 250 km on the queen charlotte islands at deeper crustal levels within the arc influx of mantle derived magmas was accompanied by metamorphism and melting of wrangellian basement rocks yielding the heterogeneous wcc upward mobilization and hybridization of magmas to shallower levels in the crust resulted in the batholiths of the island intrusions and the lavas and pyroclastic rocks of the bonanza group new u pb crystallization ages for plutonic rocks of the arc span an age range of 190 3 1 0 to 168 6 5 3 ma ages of the wcc and western island intrusions are indistinguishable and overlap with published fossil and isotopic ages for the bonanza group younger middle jurassic

measurements of the cretaceous paleolatitude of vancouver island consistent with the baja british columbia hypothesis a previously unsampled outcrop of gently dipping or flat lying upper cretaceous sedimentary strata in the vancouver island region which contains unaltered aragonitic mollusk fossils yielded a stable remanent magnetization that is biostratigraphically consistent with cretaceous magnetochrons 33r 33n and 32r these results characterized by shallow inclinations indicate an upper cretaceous paleolatitude of about 25 3 degrees north which is equivalent to that of modern day baja california these findings are consistent with the baja british columbia hypothesis which puts the insular superterrane well south of the oregon california border in the late cretaceous

magma emplacement in a convergent tectonic orogen southern revillagigedo island southeastern alaska the western metamorphic belt of the british columbia alaska coastal orogen consists of lithostratigraphic units assigned to the taku terrane and the gravina sequence on revillagigedo island and cleveland peninsula in the ketchikan quadrangle of southeastern alaska greenschist and amphibolite grade schists of the western metamorphic belt are intruded by mid cretaceous tonalite and leucotonalite plutons and sills a number of these igneous bodies are spatially associated with northwest trending shear zones developed along thrust faults that formed during the main phase of metamorphism of the schists observations of field relations and structural fabric in the igneous rocks and the country rock schist show the following i along the southern shore of revillagigedo island the moth bay pluton was emplaced along a shear zone magmatic and submagmatic flow foliation in the southern border zone of the pluton parallels foliations in the sheared country rock rocks in the interior of the pluton are general

upper crustal velocity structure of the southwestern canadian cordillera from explosion recordings on the wctn earthquake seismic net the digital seismic network used to monitor earthquakes in southwestern british columbia recorded explosions from lithoprobe s southern cordillera refraction experiment score in 1989 and 1990 and from the usgs pacific northwest refraction survey in 1991 both p and s waves were recorded the geographical distribution of the shots and receiver stations enabled a large three dimensional area to be sampled for ray paths considered to be in the top 10 km of the crust we used a tomographic inversion procedure to estimate the 2 d distribution of upper crustal p and s wave velocities within blocks of a refracting velocity grid located beneath a near surface velocity layer of uniform thickness the p wave velocity distribution mapped the subsurface location of several terranes in the southwest cordillera volcanic rocks of wrangellia terrane which is the principal component of the insular belt were characterized by upper crustal velocities 6 3 km s 1 whereas plutonic rocks of the coast belt had velocities 6 3 km s 1 east of southern vancouver island the velocity distribution indicated that wrangellia extends in the subsurface beneath the coast belt for about 130 km eastward of the surface location of the insular coast belt boundary a portion of the southern boundary of wrangellia was also delineated by the p velocity map which suggested that metamorphic rocks of the northwest cascades thrust system may extend beneath sedimentary cover as far west as south central georgia strait inverted s wave velocities were typically 3 6 3 7 km s 1 corresponding values of poisson s ratio were 0 27 0 28 over vancouver island and 0 20 0 24 in the coast belt the higher values for vancouver island likely reflect the high mafic content of wrangellia volcanics compared to the high quartz content of the coast belt plutonics

comparison of geomechanical deformation induced by megatonne scale co2 storage at sleipner weyburn and in salah geological storage of co2 that has been captured at large point source emitters represents a key potential method for reduction of anthropogenic greenhouse gas emissions however this technology will only be viable if it can be guaranteed that injected co2 will remain trapped in the subsurface for thousands of years or more a significant issue for storage security is the geomechanical response of the reservoir concerns have been raised that geomechanical deformation induced by co2 injection will create or reactivate fracture networks in the sealing caprocks providing a pathway for co2 leakage in this paper we examine three large scale sites where co2 is injected at rates of 1 megatonne y or more sleipner weyburn and in salah we compare and contrast the observed geomechanical behavior of each site with particular focus on the risks to storage security posed by geomechanical deformation at sleipner the large high permeability storage aquifer has experienced little pore pressure increase over 15 y of injection implying little possibility of geomechanical deformation at weyburn 45 y of oil production has depleted pore pressures before increases associated with co2 injection the long history of the field has led to complicated sometimes nonintuitive geomechanical deformation at in salah injection into the water leg of a gas reservoir has increased pore pressures leading to uplift and substantial microseismic activity the differences in the geomechanical responses of these sites emphasize the need for systematic geomechanical appraisal before injection in any potential storage site

geology and impact features of riachao structure northern brazil abstract riach ao located at s7 42 w46 38 in maranh ao state northeastern brazil is acomplex impact structure of about 4 1 km diameter formed in pennsylvanian to permiansedimentary rocks of the parnaiba basin sequence although its impact origin was alreadyproposed in the 1970s information on its geology and shock features is still scarce in theliterature we present here the main geomorphological and geological characteristics of theriach ao impact structure obtained by integrated geophysical and remote sensing analysis aswell as geological eld work and petrographic analysis the identi ed lithostratigraphicunits consist of different levels of the pedra de fogo formation and possibly the piauiformation our petrographic analysis con rms the presence of shock diagnostic planarmicrodeformation structures in quartz grains of sandstone from the central uplift asevidence for an impact origin of the riach ao structure the absence of crater lling impactbreccias and melt rocks shatter cones as well as the restricted occurrence of microscopicshock effects suggests that intense and relatively deep erosion has occurred since craterformation introductionthe riach ao structure centered at s7 42 sandw46 38 is located in a remote area of maranh ao statein northern brazil fig 1 it was recognized as ameteorite impact structure by mchone 1986 based onevidence of breccia occurrences and planar fractures inquartz but no diagnostic data such as bona de planardeformation features pdf were found at that time besides riach ao there are another ve con rmedimpact structures in brazil araguainha 40 kmdiameter varge ao 12 4 km vista alegre 9 5 km serra da cangalha 13 7 km and santa marta 12 km two other possible impact structures in thebrazilian territory are cerro do jarau 13 5 km andcol onia 3 6 km riccomini et al 1991 2011 cr osta2004 crosta et al 2010 crosta and vasconcelos 2013 uchoa et al 2013 the riach ao structure is a regionally anomalous somewhat circular area of highly deformed rocks of4 1 km diameter the whole structure is seeminglydeeply eroded in a region of at lying undeformedsedimentary rocks these sedimentary strata belong tothe carboniferous to permian sequences of the balsasgroup of the parnaiba basin fig 2 the rst reference to the circular structure atriach ao was made by ojeda and bembom 1966 these authors during an oil exploration projectconducted by petrobras a brazilian oil company during the 1960s and 1970s described the anomalousradial drainage pattern formed by the sereno river andits tributaries as for the possible origin for this

reply to comment on u pb and hf isotope analysis of detrital zircons from mesozoic strata of the gravina belt southeast alaska by yokelson et al 2015

structural aspects of suspect terranes and accretionary tectonics in western north america abstract the regional structural relationships between the cordilleran suspect terranes and cratonic north america and within the suspect terranes themselves are reviewed for eastern alaska western canada and united states and northern mexico a distinctive characteristic of the relationships between the suspect terranes and the north american craton is that although much modified by post accretionary strike slip faulting thrust faulting and extensional detachment faults the generally more distal and or oceanic terranes are frequently found as rootless nappes or thin thrust sheets sitting upon more inboard terranes or upon the craton itself the amount of tectonic transport implied is large strike slip faulting has been a very significant aspect of the structural evolution of the north american cordillera with total distributed displacements of over 2000 km documented on geologic relationships alone and much more implied by paleomagnetic evidence at least since later mesozoic time to the present movements were such that the terranes moved northward post accretionary intraplate telescoping along thrust faults transpressional strike slip faulting and delamination at mid to upper crustal levels were important processes in cordilleran tectonic evolution

jurassic cretaceous basins along the canadian coast belt their bearing on pre mid cretaceous sinistral displacements the dominantly plutonic canadian coast belt is flanked on the northwest by a late jurassic early cretaceous intra or back arc basin in the south it contains strata of the same age deposited in an accretionary fore arc complex east of a coeval arc this distribution is ascribed to pre mid cretaceous sinistral movement of part of the jurassic cretaceous north american plate margin arc to a position west of the accretionary fore arc complex

bokan mountain peralkaline granitic complex alexander terrane southeastern alaska evidence for early jurassic rifting prior to accretion with north america the circular bokan mountain complex bmc on southern prince of wales island southernmost alaska is a jurassic peralkaline granitic intrusion about 3 km in diameter that crosscuts igneous and metasedimentary rocks of the alexander terrane the bmc hosts significant rare metal rare earth elements y u th zr and nb mineralization related to the last stage of bmc emplacement u pb zircon and 40ar 39ar amphibole and whole rock geochronology indicates the following sequence of intrusive activity i a paleozoic basement composed mainly of 469 4 ma granitic rocks ii intrusion of the bmc at 177 1 ma followed by rapid cooling through ca 550 c at 176 1 ma that was synchronous with mineralization associated with vertical wnw trending pegmatites felsic dikes and aegirine fluorite veins and late stage sinistral shear deformation and iii intrusion of crosscutting lamprophyre dikes at 150 ma and again at ca 105 ma the peralkaline nature of the bmc and the wnw trend of associated dikes

stress induced seismic azimuthal anisotropy sand shale content and depth trends offshore north west australia abstractseismic azimuthal anisotropy is apparent when p wave velocities vary with source receiver azimuth and downward propagating s waves split into two quasi s waves polarized in orthogonal directions not accounting for these effects can degrade seismic image quality and result in erroneous amplitude analysis and geologic interpretations there are currently no physical models available to describe how azimuthal anisotropy induced by differential horizontal stress varies with sand shale lithology and depth we develop a model that does so in unconsolidated sand shale sequences offshore north west australia our method naturally introduces two new concepts critical anisotropy and anisotropic depth limit critical anisotropy is the maximum amount of azimuthal anisotropy expected to be observed at the shallowest sediment burial depth where the confining pressure and sediment compaction are minimal the anisotropic depth limit is the maximum depth where the stress induced azimuthal anisotropy is ex

paleomagnetism and tectonic significance of albian and cenomanian turbidites ochoco basin mitchell inlier central oregon 1 understanding continental growth and convergent margin dynamics associated with terrane accretion and modification of the cordilleran margin of north america is prevented by conflicts in paleogeographic models for major terranes oceanic plates and the north american margin we present new paleomagnetic data suggesting that the blue mountains superterrane located at an inboard portion of the cordilleran margin has undergone substantial northward translation and clockwise rotation relative to north america since mid cretaceous time positive baked contact conglomerate and fold tests provide evidence that the magnetization of these rocks dates from their deposition these results yield a mean direction of d 10 8 i 58 5 k 127 95 4 1 and n 11 comparison of magnetic inclinations and magnetic fabrics shows no evidence for inclination error a paleolatitude of 39 2 4 8 4 3 n is obtained indicating a paleolatitude anomaly of 15 9 4 1 northward translation of 1760 460 km and cw rotation of 37 7 3 with respect to north america since circa 93 ma application of an ad hoc 5 inclination error yields translation of 1200 460 km through comparison with other units in the western cordillera we infer that the mitchell inlier was located adjacent to the sierra nevada arc during mid cretaceous time and that the blue mountains province may correlate with the intermontane superterrane in british columbia other interpretations such as large inclination errors or an independent displacement history for the mitchell inlier are possible but unlikely more work is needed to fully test these hypotheses and resolve major questions of cordilleran tectonics

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

electromagnetic images of crustal structures in southern and central canadian cordillera data from more than 400 magnetotelluric soundings made since the early 1980 s in the canadian cordillera over a 300 000 km2 area between 49 and 53 5 n are used to image qualitatively regional three dimensional crustal variation in electrical conductivity by means of phase maps phase frequency sections and maps of resistivity at depth two hundred of the soundings were acquired as part of lithoprobe southern canadian cordillera transect activities and their locations were coordinated with the seismic reflection and refraction experiments the lower crust has a generally pervasive low resistivity 1 100 m throughout the cordillera west of the foreland belt within this canadian cordilleran regional conductor the magnetotelluric data reveal both two dimensional structures with highest conductivities along the coast belt and omineca belt and three dimensional variation along geological strike this conductor mapped over a volume in excess of 106 km3 is most probably caused by fluids saline wat

using avo for fracture detection analytic basis and practical solutions tight low porosity reservoirs can produce significant amounts of hydrocarbons if fracture networks provide the necessary permeability characterization of naturally fractured reservoirs using surface seismic data is an important exploration problem that has attracted much attention in the literature tle for example had an entire issue august 1996 on this subject while the data and modeling in existing publications demonstrate that fracturing causes significant azimuthal variation in recorded seismic signatures the problem is how to infer fracture parameters from these data

timing of cache creek ocean closure insights from new jurassic radiolarian ages in british columbia and yukon and their significance for canadian cordillera tectonics the closure of the cache creek ocean during the jurassic led to the structuration of the canadian cordillera to reassess the timing of this major tectonic event this study proposes a biochronolog

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

seismic reflection investigations of the mount cayley bright spot a midcrustal reflector beneath the coast mountains british columbia an unusually high amplitude midcrustal reflector was discovered by a lithoprobe seismic reflection survey crossing a predominantly plutonic region in southwestern british columbia canada the bright spot was observed adjacent to mount cayley a large quaternary volcanic center associated with the cascadia subduction zone magmatic arc characterization of the bright spot may advance our understanding of either the midcrustal structure of magmatic supply systems or the presence and storage of saline fluids in the midcrust the location and three dimensional geometry of the reflector were determined using travel time inversion the resulting suite of models consistently describe a northwest dipping reflector that lies 12 5 to 13 km beneath the mount cayley volcanic complex the areal extent of the reflective surface is 3 km by 1 km and the structure is less than 1 6 km thick the transition between the body and the country rock must be rapid 200 m and exhibit a large impedance contrast in order to generate reflections of such high frequencies 30 hz and relative amplitudes 8 0 db our preferred interpretation for the reflector is a fossil sill complex associated with the volcanic development of mount cayley however the data do not rule out the possibility that the reflective structure represents melt lenses or saline fluid produced by the dewatering of the subducted slab the mount cayley bright spot lies just below the brittle upper crust as do the midcrustal reflectors detected beneath japanese subduction arc volcanoes this correlation suggests that thermal and rheological properties just below the brittle ductile crustal transition may be favorable for reflector formation

inversion of three dimensional wide angle seismic data from the southwestern canadian cordillera seismic refraction wide angle reflection data were recorded on a triangular array in southwestern british columbia centered on the boundary between the coast belt to the southwest and the intermontane belt to the northeast the experiment part of the lithoprobe southern cordillera transect enabled determination of the three dimensional 3 d velocity structure of the crust and upper mantle an algorithm for the inversion of wide angle seismic data to determine 3 d velocity structure and depth to reflecting interfaces is developed the algorithm is based on existing procedures for the inversion and forward modeling of first arrival travel times and forward modeling of reflection travel times including 1 forward modeling using a 3 d finite difference algorithm and 2 a simple velocity model parameterization for the inversion which eliminates the need to solve a large system of equations the existing procedure is extended to allow 1 the inversion of reflection times to solve for depth to a reflecting interface and or velocity structure 2 the inversion of first arrival travel times to solve for depth to a refracting interface and 3 layer stripping application of the algorithm to southern cordillera data uses pg to constrain upper crustal velocity structure pmp to constrain lower crustal velocity structure and depth to moho and pn to constrain upper mantle velocities and depth to moho the 3 d velocity model for the southwestern canadian cordillera is characterized by 1 significant lateral velocity variations at all depths that do not in general correlate with surface geological features or gravity data 2 a relatively high velocity middle and lower crust in the southwestern part of the study area which correlates with a strong relative gravity high and outlines the eastern extent of lower wrangellia an accreted terrane forming the insular belt to the west 3 a narrow zone of slower velocity in the lower crust and change in crustal thickness associated with the fraser fault system lending additional support to the view that it is a crustal penetrating fault 4 an average upper mantle velocity of 7 85 km s and 5 a depth to moho of 33 36 km in the intermontane belt and 36 38 km throughout most of the coast belt decreasing in the west to 33 km near the insular coast contact horizontal velocity structure slices and an interpreted cross section based on these and other results show the complexity of crustal structure in the region

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

recent advances in determination of fracture strike and crack density from p wave seismic data confidence in the ability of avaz analysis amplitude variation with angle and azimuth to capture information about fractures in a reservoir has steadily increased over the last four years because this technique detected seismic anisotropy in many different reservoirs and the results consistently tied other information about fractures

imaging the aquistore reservoir after 36 kilotonnes of co2 injection using distributed acoustic sensing abstractaquistore is a geologic co2 storage project that is using a deep saline formation as a storage reservoir from april 2015 to february 2016 approximately 36 kilotonnes of co2 were injected into the reservoir at a depth of 3130 3350 m we have developed an analysis of distributed acoustic sensing das 3d vertical seismic profiling data acquired in february 2016 marking the first seismic survey since injection began the vsp data were processed in parallel with baseline preinjection data from a november 2013 survey with the objective of detecting and characterizing the subsurface co2 plume and evaluating the repeatability of das in a reservoir monitoring project a single processing sequence was devised that 1 accurately imaged the reservoir for the baseline and monitor data and 2 attained adequate repeatability to observe time lapse differences related to the presence of co2 repeatability was somewhat compromised by the less advanced noise cancellation methodology of the das system used for

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

azimuthal avo methodology and data examples azimuthal avo avoa has become a popular method of fracture detection in both carbonate and sandstone reservoirs previous methods often relied on stacking the multiazimuth data into offset and azimuth bins often using only two azimuth sectors perpendicular to each other this can mix the data over a significant azimuth range and does not allow for meaningful error estimates once the data have been stacked into azimuth bins information on azimuthal distribution is lost each azimuth sector is treated as if it has equal weight even though one sector may have significantly lower fold and thus a lower signal to noise ratio it has also been suggested that the results of azimuth sectoring analyses are often influenced by acquisition geometry

evolving heavy mineral assemblages reveal changing exhumation and trench tectonics in the mesozoic chugach accretionary complex south central alaska the gulf of alaska is one of the largest accretionary complexes on earth in this study we examined the earliest phase of accretion in the mesozoic mchugh complex and valdez groups exposed in se alaska the oldest preserved fragment the mesomelange assemblage is jurassic ca 160 140 ma and consists of an 3 km thick structural package of strongly deformed shaley materials with slices of oceanic cherts and basalts heavy minerals indicate dominant erosion from a magmatic arc source uplifted after the collision of the wrangellia and the talkeetna oceanic arc a tectonic erosion event affected the forearc just prior to ca 120 ma and was likely caused by seamount collision ridge subduction or both this was followed at 105 ma by mass wasting of sandstone and conglomerates preserved as the graywacke conglomerate assemblage ca 105 83 ma heavy minerals indicate continued flux from arc sources but with significant changes suggesting a larger more diverse catchment area erosion of deeper crustal sources provided high mg diopside and garnets to the trench faster sediment flux was caused by rock uplift triggered by final accretion of the wrangellia peninsula terrane to north america the start of large scale accretion in alaska roughly coincided with the initiation of shimanto complex accretion in japan and can be understood as primarily linked to sediment supply driven by plate margin tectonics rather than climatically induced erosion onshore

early to middle cretaceous paleogeography of north central british columbia stratigraphy and basin analysis of the skeena group the lower middle cretaceous skeena group records the early cretaceous evolution of the southern margin of the jura cretaceous bowser basin in north central british columbia we formalize skeena group nomenclature and present interpretations of three distinct paleogeographic and tectonic phases during the first phase neocomian aptian skeena deposition was limited to a restricted tidal basin represented by laventie formation black shale deposits surrounded by coal swamp deltas of the lower bulkley canyon formation the lower skeena group correlated to the mcevoy formation bowser lake group in the northern basin represents final filling of the bowser foredeep produced by jurassic accretion of the intermontane superterrane to north america in the second phase early albian early cenomanian marine deposition transgressed eastward and southward accompanied by intrabasinal rocky ridge volcanism shedding volcanic detritus into the kitsuns creek member of the bulkley canyon formation the rocky ridge

source rock potential of lower to middle jurassic black clastic sequences of the intermontane belt1this 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 lower to middle jurassic clastic sequences are widespread within the interior of the canadian cordillera these successions cap waning jurassic volcanism in many cordilleran arc terranes and are succeeded by clastic sequences of the intermontane basins fine grained carbonaceous lithologies which locally contain elevated levels of organic carbon characterize these clastic successions these include sections of the spatsizi formation abou member and smithers formation in northern and western bowser basin respectively ashcroft formation and equivalent strata in central quesnellia and last creek formation and junction creek rocks below the tyaughton methow basin these rocks locally contain metre thick sections with total organic carbon toc levels 5 wt and others have thicknesses approaching 100 m with toc between 3 and 5 wt thermal maturation levels are high in many sections suggesting original organic contents were greater and that these rocks may have been excellent source beds associate

a reply to the discussion of our paper on the structure of the precambrian and lower paleozoic basement of the central andes between 22 and 32 s lat as presented by a p willner and h miller

structural development of the western metamorphic belt adjacent to the coast plutonic complex southeastern alaska evidence from holkham bay the coast plutonic complex in southeastern alaska and british columbia is bordered on the west by low to medium grade metamorphic rocks of the western metamorphic belt this belt includes intensely deformed rocks underlying a prominent topographic low the coast range megalineament structural data from the northern part of the belt near holkham bay southeastern alaska constrain the shear sense during polyphase deformation tectonite fabrics within the western metamorphic belt record at least three deformational events variation in foliation development fold geometry and metamorphism indicate that deformation was heterogeneously partitioned four tectonic zones can be defined the peninsular coast range megalineament coherent and straight zones the earliest recognized deformation resulted in folding that transposed bedding into a tectonite layering s1 throughout the western metamorphic belt these structures are inferred to have developed during late cretaceous crustal thickening that resulted in moderate to high pressure metamorphism later deformation probably early tertiary resulted in abundant mesoscopic and macroscopic f2 folds of s1 and development of shear bands in the coast range megalineament zone the vergence and variation in plunge of f2 folds are compatible with subvertical east side up displacement across the coast range megalineament zone uplift of the coast plutonic complex shear bands indicate right lateral strike slip motion occurred during the waning stages of metamorphism probably following development of f2 folds brittle faults parallel to the coast range megalineament contain subhorizontal slickensides suggesting additional strike slip movement

linking microseismic event observations with geomechanical models to minimise the risks of storing co2 in geological formations abstract for carbon capture and storage ccs in geological formations to be scientifically viable we must be able to model and monitor the effects of geomechanical deformation on the integrity of the caprock excess deformation may open fractures providing pathways for co2 leakage from the reservoir an acceptable geomechanical model must provide a good match with field observations microseismic activity is a direct manifestation of mechanical deformation so it can be used to constrain geomechanical models the aim of this paper is to develop the concept of using observations of microseismic activity to help ground truth geomechanical models microseismic monitoring has been ongoing at the weyburn co2 storage and monitoring project since 2003 we begin this paper by presenting these microseismic observations less than 100 events have been recorded documenting a low rate of seismicity most of the events are located close to nearby producing wells rather than the injection well a pattern that is difficult to interpret within the conventional framework for injection induced seismicity many events are located in the overburden without geomechanical simulation it is difficult to assess what these observations mean for the integrity of the storage formation to address these uncertainties we generate numerical geomechanical models to simulate the changes in stress induced by co2 injection and use these models to predict the generation of microseismic events and seismic anisotropy the initial geomechanical model that we generate using material properties based on laboratory core measurements does not provide a good match with either event locations or s wave splitting measurements made on the microseismic events we find that an alternative model whose reservoir is an order of magnitude softer than lab core sample measurements provides a much better match with observation as it leads shear stresses to increase above the production wells promoting microseismicity in these areas and generates changes in effective horizontal stresses that match well with s wave splitting observations this agreement between geophysical observations and a softer than lab measurements reservoir model highlights the difficulties encountered in upscaling lab scale results there is a strong need to link geomechanical models with observable manifestations of deformation in the field such as induced seismicity for calibration only then can we accurately assess the risks of leakage generated by mechanical deformation

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

application of lu hf garnet dating to unravel the relationships between deformation metamorphism and plutonism an example from the prince rupert area british columbia abstract the tectonic history of the prince rupert area is marked by profound crustal thickening from thrusting that produced inverted metamorphic field gradients and transpression in crustal scale shear zones syn tectonic garnet in the prince rupert area has lu hf ages of 102 6 3 7 ma and 108 3 4 1 ma 2 porphyroblast matrix relationships in these samples and samples from the same outcrops indicate syn tectonic garnet growth these relationships imply that the garnet ages directly date the development of the metamorphic foliations a third sample of migmatitic garnet amphibolite from the contact aureole of the 94 90 ma ecstall pluton had complex isotope systematics interpreted to indicate a garnet growth episode 105 ma similar to the ages obtained from the other samples and a growth or equilibration event at 94 ma during pluton emplacement the data show that the older lu hf garnet ages date prograde metamorphism during foliation development and modification during pluton emplacement the ecstall pluton was emplaced 10 to 15 m y after regional metamorphism and thrust stacking in the prince rupert area in order to place our samples in a regional tectonic context we compare our results to patterns of regional deformation metamorphism and plutonism throughout the north american cordillera in the time period between 110 and 85 ma contractional and transpressional deformation occurred throughout much of the north american cordillera at this time from southeastern alaska to the baja peninsula in mexico left lateral transpression dominated the canadian cordillera whereas right lateral transpression affected areas south of the idaho salmon river suture zone including the sierra nevada batholith this reversal in kinematics in the northern and southern cordillera within coeval magmatic belts appears to be a first order feature of the geology of the north american cordillera during the cretaceous

dispersed remnants of a northeast pacific fringing arc upper paleozoic terranes of permian mccloud faunal affinity western u s two fragmentary subparallel belts of terranes within the western north american cordillera contain upper paleozoic rocks and are characterized by contrasting lithotectonic assemblages and contrasting permian faunal affinity these two belts are a volcanic arc related successions of permian mccloud faunal affinity mccloud belt and b subduction related accretionary complexes of permian tethy an faunal affinity cache creek belt this paper supports the hypothesis that the fragmentary terranes of the mccloud belt once constituted parts of a northeast pacific fringing arc system and in constrast to some earlier interpretations concludes that the volcanic arc evolved above an eastward dipping subduction zone the absolute distance between this arc and western north america during the late paleozoic cannot be constrained however there is little evidence to suggest closure of a major 10 km ocean basin or protracted periods of westward dipping subduction parts of devonian to permian volcanic island arc sequences of the western u s cordillera are represented in the northern sierra eastern klamath bilk creek grindstone and chilliwack terranes these scattered volcanic arc remnants share several fundamental characteristics 1 the sequences were constructed across continental affinity basement assemblages 2 they underwent similar tectonic evolution during late paleozoic time such as coeval pulses in volcanism and related depositional histories 3 they contain early permian mccloud type fauna of distinctive biogeographic affinity 4 mccloud belt terranes are spatially and possibly genetically related to westward lying accretionary complexes of the cache creek belt which contain fragments of upper triassic blueschist and permian limestone blocks bearing tethyan permian fusulinids and corals based on the presence of distinctive early permian mccloud fauna the island arc remnants discussed in this paper are referred to as the mccloud belt the similarities between mccloud belt terranes suggest that they were once parts of a long lived late paleozoic volcanic arc complex the stratigraphy of these terranes records the tectonic evolution of the volcanic arc volcanism within the paleozoic arc began as early as the early devonian and was succeeded or accompanied by carbonate and epiclastic sedimentation during the middle and late devonian an influx of detritus derived from sedimentary and metasedimentary rocks during the late devonian and mississippian records the uplift and erosion of arc basement sequences an event that was at least locally accompanied by ongoing volcanism a pulse in volcanism during the pennsylvanian and coeval uplift led to the establishment of shallow marine and deltaic volcanogenic and carbonate sedimentation and locally subaerial exposure which persisted into the early early permian elsewhere during the early permian shallow water carbonate deposition was established with only minor influx of volcanogenic debris during the late permian and locally as early as late early permian a major pulse in volcanism occurred and is now represented by voluminous sequences of volcanogenic debris and lavas most of these sequences are overlain by upper triassic carbonate platformal sequences arc volcanism was recorded at least locally until the middle jurassic paleomagnetic data are used to constrain a schematic pre cretaceous reconstruction of these volcanic arc sequences and their associated subduction assemblages the inferred paleogeography of island arc sequences and the adjacent accretionary terranes suggests that volcanism occurred above an east dipping subduction zone with a long lived paleogeographic relation to north america the current structural position of these terranes is largely the result of mesozoic and locally younger deformation

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

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

regional electrical resistivity structure of the southern canadian cordillera and its physical interpretation the regional geoelectric crustal structure of the southern and central canadian cordillera of western canada is interpreted from the inversion of magnetotelluric data along five profiles crossing the physiographic morphogeological belts with emphasis on the intermontane and omineca belts decomposition of the tensor impedance response estimates demonstrates that large scale regional structures can be reasonably approximated along each profile as two dimensional with dominant geoelectric strikes of either 30 or 15 depending on profile location these profile specific strike directions are consistent with a local clockwise rotation of crustal structures in the southern intermontane and omineca belts suggested by others based on paleomagnetic data and palinspastic reconstructions comparing the resistivity models derived from two dimensional inversions of the distortion corrected data along each profile allows us to construct an orogen scale three dimensional resistivity model for southern british columbia generally the model shows a resistive upper crust overlying a conductive lower crust the resistivity of the lower crust beneath the intermontane belt is independent of latitude and is similar for all profiles in stark contrast a 2 orders of magnitude variation in lower crustal resistivity is observed along strike in the omineca belt with higher conductivities to the south in the region of eocene extension and lower conductivities to the north in the unextended part of the belt such spatial association has noteworthy implications for the cause of lower crustal resistivity in active or recently active young regions our preferred interpretation of the observed lower crustal resistivities with other geophysical data is in terms of fluids with brines dominating for the most part but partial melt possible at the base of the crust in specific localities we attribute the along strike variation in the omineca belt mostly to variation in fluid content and interconnectivity with the lowermost crust of the southern omineca belt being partially molten this physical state difference is a consequence of degree of extension and implies that mantle derived fluids are important for lower crustal resistivity

fracture characterization at valhall application of p wave amplitude variation with offset and azimuth avoa analysis to a 3d ocean bottom data set the delineation and characterization of fracturing is important in the successful exploitation of many hydrocarbon reservoirs such fracturing often occurs in preferentially aligned sets if the fractures are of subseismic scale this may result in seismic anisotropy thus measurements of anisotropy from seismic data may be used to delineate fracture patterns and investigate their properties here fracture induced anisotropy is investigated in the valhall field which lies in the norwegian sector of the north sea this field is a chalk reservoir with good porosity but variable permeability where fractures may significantly impact production e g during waterflooding to investigate the nature of fracturing in this reservoir p wave amplitude variation with offset and azimuth avoa is analyzed in a 3d ocean bottom cable obc data set in general 3d ocean bottom seismic obs acquisition leads to patchy coverage in offset and azimuth and this must be addressed when considering such data to overcome this challenge and others associated with 3d obs acquisition a new method for processing and analysis is presented for example a surface fitting approach which involves analyzing azimuthal variations in avo gradients is used to estimate the orientation and magnitude of the fracture induced anisotropy this approach is also more widely applicable to offset azimuth analysis of other attributes e g traveltimes and any data set where there has been true 3d data acquisition land or marine using this new methodology we derive high resolution maps of p wave anisotropy from the avoa analysis for the top chalk reflection at valhall these anisotropy maps show coherent but laterally varying trends synthetic avoa modeling using effective medium models indicates that if this anisotropy is from aligned fracturing the fractures are likely liquid filled with small aspect ratios and the fracture density must be high furthermore we show that the fracture normal direction is parallel to the direction of most positive avo gradient in other situations the reverse can be true i e the fracture normal direction can be parallel to the direction of the most negative avo gradient effective medium modeling or comparisons with anisotropy estimates from other approaches e g azimuthal variations in velocity must therefore be used to resolve this ambiguity the inferred fracture orientations and anisotropy magnitudes show a degree of correlation with the positions and alignments of larger scale faults which are estimated from 3d coherency analysis overall this work demonstrates that significant insight may be gained into the alignment and character of fracturing and the stress field variations throughout a field using this high resolution avoa method

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

late cretaceous to early tertiary transtension and strain partitioning in the chugach accretionary complex se alaska abstract shear zones in the late cretaceous sitka graywacke of the chugach accretionary complex in southeast alaska record constrictional finite strains with maximum principal stretches plunging shallowly subparallel to strike of the shear zones macrostructural analysis indicates the finite strain formed during one deformation event microstructural analysis of the shear zones shows that this deformation is ductile promoted mostly through deformation of low strength lithic clasts and pressure solution kinematic indicators from some of the shear zones indicate dominantly dextral motion although multiple scenarios can explain constrictional finite strains in a shear zone these dextral strike slip shear zones must have experienced a component of extension across them in order to generate constrictional finite strains therefore the shear zones are dextral transtensional shear zones an uncommon tectonic regime in an accretionary complex the transtensional shear zones reflect strike slip motion related to partitioning of late cretaceous to early tertiary right oblique convergence between north america and the farallon plate the extensional component that was superposed on the strike slip shear zones to generate transtension resulted from contemporaneous collapse of the forearc following thickening related to underplating

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

lithospheric structure in the southern canadian cordillera from a network of seismic refraction lines lithospheric velocity structure and its relationship to regional tectonics and development of the southern canadian cordillera are derived from a synthesis of interpretations from nine in line seismic refraction wide angle reflection profiles and broadside data recorded during the lithoprobe southern cordillera refraction experiment score and other refraction experiments across southern british columbia and one profile in northwestern washington consistency of the score two dimensional models at their intersection positions is achieved through application of a simultaneous inversion of all relevant traveltime data the cross sectional and map presentations demonstrate the strong degree of three dimensional heterogeneity within the crust and upper mantle a first order characteristic is the continuous increase in crustal velocities westward from the foreland belt to the insular belt the variations do not correlate with the morphogeological belts they do correspond with large scale geological and or

4d seismic monitoring of co2 flood in a thin fractured carbonate reservoir the miscible co2 flood in weyburn field is a massive process for enhanced tertiary oil recovery and greenhouse gas sequestration the largest of this kind ever in western canada this field a mature paleozoic carbonate pool of 1 4 billion barrel reserves has had nearly 48 years of conventional production history mostly through waterflooding since october 2000 and in an effort to arrest production decline and improve recovery the field operator has pipelined 50 95 mmcf of co2 each day as by products of a coal gasification operation in north dakota and pumped it into the 30 m fractured reservoir at a mean depth of 1450 m over a 25 year project life an estimated 14 million tons of the greenhouse gas which would otherwise be vented into the atmosphere will be injected and stored in the field s producing formations

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

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

the structure of the precambrian and lower paleozoic basement of the central andes between 22 and 32 s lat three major diastrophic cycles defined by their structural style and their spatial distribution are recognized in the andean basement of this region the oldest structures are related to the panamerican orogeny 500 to 700 m a which produced in the central craton multiply deformed complexes of schists gneisses and granites that are covered discordantly by unmetamorphosed cambrian and ordovician beds west of the central craton ordovician sedimentary beds are folded with a simple structural style and intruded by granites both the sedimentry beds and the granites are covered discordantly by undeformed devonian sequences the folding of the ordovician is attributed to the ocloyic phase of the caledonian movements west of the ocloyic belt is another foldbelt consisting of strongly folded devonian beds attributed to the chanic phase hercynian the chanic belt is intruded by carboniferous and permian granites and covered discordantly by carboniferous and permian sequences

an amplitude based multiazimuthal approach to mapping fractures using p wave 3d seismic data we have tested an amplitude based multiazimuthal approach for mapping fractures which requires only a simple azimuth offset sorting process by displaying the amplitudes of all traces collected within a superbin the method predicts fractures by mapping p wave amplitude variations in which a lineation within the map indicates the presence and the orientation of fractures within the superbin test results using physical model and field data sets suggest that the amplitude based multiazimuthal approach could help to determine the presence of multiple fracture sets in a single layer which may be expressed through subtle variations in p wave multiazimuthal seismic reflections our experiments with a physical model containing manmade vertical fractures suggest that transmission effects could be one of the dominant factors which control azimuthal amplitude versus offset avo behavior the technique described in this paper can operate on any 3d p wave seismic data with wide azimuth and offset distributions

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

rates of floodplain accretion in a tropical island river system impacted by cyclones and large floods abstract fluvial processes especially rates of floodplain accretion are less well understood in the wet tropics than in other environments in this study the caesium 137 137cs method was used to examine the recent historical sedimentation rate on the floodplain of the wainimala river in the basin of the rewa river the largest fluvial system in fiji and the tropical south pacific islands 137cs activity in the floodplain stratigraphy showed a well defined profile with a clear peak at 115 cm depth our measured accretion rate of 3 2 cm year 1 over the last ca 45 years exceeds rates recorded in humid regions elsewhere this is explained by the high frequency of tropical cyclones near fiji 40 since 1970 which can produce extreme rainfalls and large magnitude floods since the beginning of hydrological records large overbank floods have occurred every 2 years on average at the study site the biggest floods attained peak flows over 7000 m3 s 1 or six times the bankfull discharge concentrations of suspended sediments are very high max 200 500 g l 1 delivered mainly by channel bank erosion in the future climatic change in the tropical south pacific region may be associated with greater tropical cyclone intensities which will probably increase the size of floods in the rewa basin and rates of floodplain sedimentation

subglacial sediment distribution from constrained seismic inversion using multi software examples from midtdalsbreen norway fast ice flow is associated with the deformation of subglacial sediment seismic shear velocities vs increase with the rigidity of material and hence can be used to distinguish soft sediment from hard bedrock substrates depth profiles of vs can be obtained from inversions of rayleigh wave dispersion curves from passive or active sources but these can be highly ambiguous and lack depth sensitivity our novel bayesian transdimensional algorithm multi circumvents these issues by adding independent depth constraints to the inversion also allowing comprehensive uncertainty analysis we apply multi to the inversion of a rayleigh wave dataset acquired using active source multichannel analysis of surface waves techniques to characterise sediment distribution beneath the frontal margin of midtdalsbreen an outlet of norway s hardangerjokulen ice cap ice thickness 0 20 m is constrained using co located gpr data outputs from multi suggest that partly frozen sediment vs 500 1000 m s 1 overlying bedrock vs 2000 2500 m s 1 is present in patches with a thickness of 4 m although this approaches the resolvable limit of our rayleigh wave frequencies 14 100 hz uncertainties immediately beneath the glacier bed are 280 m s 1 implying that multi cannot only distinguish bedrock and sediment substrates but does so with an accuracy sufficient for resolving variations in sediment properties

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

crustal structure of the cascadia subduction zone southwestern british columbia from potential field and seismic studies the northern cascadia subduction zone is a region of convergence between the oceanic explorer and northern juan de fuca plates and the continental north american plate potential field and new seismic reflection data coupled with previous seismic results and geology enable derivation of a series of density magnetic susceptibility cross sections and a block density model from the ocean basin to the volcanic arc from 2 5 and 3 dimensional interpretations the lateral extent and thickness of the accreted wedge vary significantly along the zone the narrow metasedimentary pacific rim terrane lies immediately west of wrangellia and extends the length of vancouver island consistent with its emplacement by strike slip faulting following the accretion of wrangellia the ophiolitic crescent terrane is a narrow slice lying seaward of the pacific rim terrane but not extending northward beyond the juan de fuca plate in this region the crescent terrane was emplaced in a strike slip or obliquely convergent style

magnetism and age of the porteau pluton southern coast belt british columbia evidence for tilt and translation the porteau pluton is a variably foliated quartz diorite to granodiorite intrusion in the southern coast belt of the canadian cordillera 49 6 n 123 2 w 40ar 39ar ages are 95 5 ma from biotite and 101 5 0 7 ma from hornblende which together with an earlier u pb zircon age of 100 2 ma indicate that the body was emplaced uplifted and cooled rapidly in mid cretaceous time the rocks contain high coercive force hard remanent magnetizations with unblocking temperatures between 500 and 600 c close to those of ar in hornblende indicating that remanence was acquired at or close to the hornblende plateau age the hard remanence directions have an elongate distribution in agreement with the predictions of m e beck regarding magnetization acquired during tilting uplift and cooling of plutons no part of the distribution agrees with the direction expected from observations from rocks of mid cretaceous age from cratonic north america the elongate distribution defines the axis of tilt 347 east

large scale transpressive shear zone patterns and displacements within magmatic arcs the coast plutonic complex british columbia the coast plutonic complex is the largest magmatic arc of the north american cordillera extending from northwestern washington state to eastern alaska it forms the transition between two tectonic domains that are suspected to have undergone several phases of large several thousands of kilometers orogen parallel displacement during the mesozoic and early cenozoic a compilation of fabric data published isotopic ages and new structural observations shows that the western coast plutonic complex was affected by subvertical orogen parallel crustal scale shear zones these shear zones mainly reflect sinistral transpression and were sequentially active from 110 to 87 ma during the intrusion of voluminous batholiths sinistral shearing was roughly coeval with the development of the thrust belts flanking the coast plutonic complex between 101 and 85 ma suggesting plate scale transpression was a first order process in the construction of the coast mountains orogen these shear zones separate panels with distinct plutonic and cooling histories suggesting the sinistral displacements between crustal blocks were large greater than tens to hundreds of kilometers this transpressive shear system likely reflects the jurassic to early late cretaceous migration of outboard cordilleran terranes to the south suggested by paleomagnetic evidence and plate reconstruction models this example from the coast orogen shows how transpression is partitioned between a thermally weakened magmatic arc and outwardly vergent fold and thrust belts our analysis further shows that the 2000 km long late cretaceous to early tertiary coast shear zone has a minimum extent toward the south to at least 51 30 n

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