evolution of the sanbagawa and shimanto high pressure belts in sw japan insights from k ar ar ar geochronology abstract the sanbagawa belt sensu lato in japan is one of the world s classical high pressure hp metamorphic belts and has long been considered to be a coherent hp belt of the cretaceous age however recent progress in detrital zircon geochronology together with the accumulation of abundant phengites k ar ar ar ages revealed that the belt is comprised of two petrotectonic belts sanbagawa sensu stricto hp belt and shimanto hp belt these two belts have distinctly different ages of peak metamorphism and different p t conditions of metamorphism the sanbagawa schists sensu stricto were metamorphosed in the conditions of the pumpellyite actinolite facies through the epidote blueschist to epidote amphibolite facies and up to the eclogite facies during a prograde stage in a subduction zone at 120 115 ma phengite k ar geochronology revealed that the subsequent exhumation after the peak metamorphism took placed in a manner that the higher grade rocks exhumed more rapidly in comparison with the lower grade rocks this is supported by a positive correlation between age and apparent metamorphic gradient that have formed during the post metamorphic peak hydration recrystallization moreover the schists have experienced intense ductile deformation and long term exhumation it took longer than 31 m y to reset the phengite k ar system in contrast the shimanto hp schists were metamorphosed in the epidote blueschist greenschist transitional facies to the epidote amphibolite facies and have experienced a short term deformation for less than 13 m y this short deformation formed a negative correlation between k ar age and apparent metamorphic gradient these two contrasting age temperature relationships suggest different exhumation processes between the two hp belts most likely due to a change in subducting oceanic plates in the cretaceous along the paleo japanese convergent margin

provenance of albian cenomanian rocks of the methow and tyaughton basins southern british columbia a mid cretaceous link between north america and the insular terrane basin analysis of albian cenomanian rocks of the tyaughton and methow basins suggests that the two basins were filled by three principle petrofacies during a regionally significant contractional ev

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

thm coupled finite element analysis of frozen soil formulation and application a fully coupled thermo hydro mechanical thm finite element fe formulation is presented that considers freezing and thawing in water saturated soils the formulation considers each thermal hydraulic and mechanical process and their various interactions through fundamental physical laws and models by employing a combination of ice pressure liquid pressure and total stress as state variables a new mechanical model has been developed that encompasses frozen and unfrozen behaviour within a unified effective stress based framework important frozen soil features such as temperature and porosity dependence of shear strength are captured inherently by the model potential applications to geotechnics include analysis of frost heave foundation stability or mass movements in cold regions the model s performance is demonstrated with reference to the in situ pipeline frost heave tests conducted by slusarchuk et al detailed consideration is given to fe mesh design the influence of hydraulic parameters an

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

strain and displacement across the pinaleno mountains shear zone arizona u s a abstract the 27 km long 0 5 km thick mylonite zone of the pinaleno mountains metamorphic core complex is interpreted as a normal displacement simple shear zone on the basis of major minor and microscopic scale structures shear strains calculated as a function of the angle between the foliation and the lower boundary of the shear zone range from 2 to 19 and have a mean value of 3 5 corresponding to a mean angular shear of 74 these values are the same as those calculated independently from rotations of sheared lithologic contacts integration of the calculated shear strains yields a minimum translation estimate of 2 9 km

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

a magma loading model for barrovian metamorphism in the southeast coast plutonic complex british columbia and washington barrovian metamorphism linked to crustal thickening in the southeast coast plutonic complex of british columbia and washington is proposed in this paper to be the product of magmatic loading relevant observations and arguments are the following 1 isotopic ages coupled with structural and fabric relations document that throughout the region barrovian metamorphism and plutonism are broadly coeval 2 baric patterns in country rock indicate steep sided crustal loads originating within the high grade parts of the orogen 3 patterns of subsidence and uplift are localized and diachronous precluding a single event regional thickening mechanism 4 country rock fabrics are dominated by steep orogen parallel foliations and shallow strike parallel lineations features not easily reconciled with tectonic thickening mechanisms 5 abroad zone of schists flanking the large 25 x 100 km scuzzy batholith in british columbia bears metamorphic isograds and foliations that track the border of the pluton across regional structural trends implicating pluton emplacement as the cause of metamorphism 6 the crustal loading event in the vicinity of the scuzzy pluton is bracketed by mineral and textural features as being coeval with emplacement of the pluton which occurred over a period of 7 m y 7 the regionally developed chiwaukum schist in washington has petrologic and structural features and a metamorphic age that suggest it was formed by and represents the floor of an eroded extension of the scuzzy pluton the proposed magma loading model invokes the diapir and ballooning concepts of previous workers in which rising plutons displace country rock downward this mechanism results in a type of convective crustal overturn and at least partially solves the space problem related to batholithic emplacement as a result of this process relatively broad tracts of country rock acquire barrovian mineralogy following plutonism magmatically thickened crust is eroded and isostatically up lifted to expose the plutonic roots and high grade country rock basement

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

chemical features of white micas from the piemonte calc schists western alps and implications for k ar ages of metamorphism abstract anomalously large chemical ranges in muscovite paragonite and muscovite celadonite systems are observed in white micas from the piemonte calcschists in the chisone valley area internal western alps the petrographical and chemical observations on white mica strongly suggest that most mica crystals with high na k ratios in the chlorite zone are of detrital origin and were derived from the pre alpine high temperature metamorphic sequence such the caledonian and or variscan submicroscopic muscovite ms paragonite pg composite aggregates occur in the chlorite zone and their epma analyses give an apparent chemical composition range from ms 0 6 pg 0 4 to ms 0 2 pg 0 8 in the rutile zone the paragonite content of the white micas is less than 20 suggesting that the white micas have been homogenized during the alpine metamorphism even if detrital white micas existed metamorphic mica is also very heterogeneous the total range in si content becomes wider with increasing of metamorphic grade 3 22 3 39 pfu for the chlorite zone 3 07 3 45 pfu for the chloritoid zone and 3 06 3 59 pfu for the rutile zone this clearly indicates that the micas have experienced significant retrogressive chemical reactions during cooling and exhumations of the host schists the detrital white mica in the chlorite zone has not reset well in its k ar system during the alpine subduction related metamorphism the wide range of the white mica k ar ages from 115 to 41 ma must be due to a mixture of various amounts of detrital white mica in the separates this feature is also observed in the chloritoid zone though the age variation is not so large as that in the chlorite zone in contrast the mica in the rutile zone which was higher than 450 c has been reset completely during alpine hp metamorphism

frost heave and thaw consolidation modelling part 2 one dimensional thermohydromechanical thm framework this paper presents a thermohydromechanical framework to model frost heave and thaw consolidation simultaneously in which effective and total stresses are taken as the stress variables for unfro

comparison of soil freezing curve and soil water curve data for windsor sandy loam unfrozen water content as a function of temperature was measured in the laboratory using pulsed nuclear magnetic resonance pnmr for a windsor sandy loam soil the pnmr data were related to previously measured soil moisture retention data through the modified clausius clapeyron equation with suitable adjustment for surface tension the transformed measured unfrozen water content data and the previously measured soil moisture retention data were expressed by a brooks and corey type of equation with the required set of regression parameters determined it was found that a single set of parameters were sufficient to correctly express the behavior of these data when suitable constraints were imposed on the unfrozen water content data additional insight into the traditional form of expressing unfrozen water content data is presented in terms of air or ice entry pressure

crustal response to continental collisions between the tibet indian south china and north china blocks geochronological constraints from the songpan garze orogenic belt western china we report shrimp u th pb monazite conventional u pb titanite sm nd garnet and rb sr muscovite and biotite ages for metamorphic rocks from the danba domal metamorphic terrane in the eastern songpan garze orogenic belt eastern tibet plateau these ages are used to determine the timing of polyphase metamorphic events and the subsequent cooling history the oldest u th pb monazite and sm nd garnet ages constrain an early barrovian metamorphism m1 in the interval c 204 190 ma coincident with extensive indosinian granitic magmatism throughout the songpan garze orogenic belt a second higher grade sillimanite grade metamorphic event m2 recorded only in the northern part of the danba terrane was dated at c 168 158 ma by a combination of u th pb monazite and titanite and sm nd garnet ages it is suggested that m1 was a thermal event that affected the entire orogenic belt while m2 may represent a local thermal perturbation rb sr muscovite ages range from c 138 100 ma whereas rb sr biotite ages cluster at c 34 24 ma these ages document regional cooling at rates of c 2 3 c myr 1 following the m1 peak for most of the terrane however those parts of the terrane affected by the higher temperature m2 event e g the migmatite zone experienced initially more rapid c 8 c myr 1 cooling after peak m2 before joining the regional slow cooling path defined by the rest of the terrane at c 138 ma regional slow cooling between c 138 and c 30 ma is thought to be the result of post tectonic isostatic uplift after extensive crustal thickening caused by collision of the south and north china blocks the clustering of biotite rb sr ages marks the onset of rapid uplift across the entire terrane commencing at c 30 20 ma this cooling history is shared with many other regions of the tibet plateau suggesting that uplift of the tibet plateau including the songpan garze orogenic belt occurred predominantly in the last c 30 myr as a response to the continuing northwards collision of india with eurasia

kinematic analysis of the san juan thrust system washington shear related fabric in the middle to late cretaceous san juan thrust system washington is observed in imbricated blocks and in fault zones a few millimeters to tens of meters thick foliation and lineation are defined in tectonic blocks by alignment of 1 newly grown muscovite and chlorite and 2 flattened and stretched radiolaria mineral porphyroclasts and rock fragments thin argillaceous shear zones bear a pronounced lineation defined by lithologic streaks and slickenside corrugations the slickenside and stretching lineations show a well defined preferred orientation in a horizontal northwest southeast direction widespread s c fabrics asymmetric pressure shadows fault drag features and asymmetric folds in the x z plane yield a sense of tectonic transport at most localities of upper plate to the northwest aragonite prehnite lawsonite pumpellyite and rare na amphibolite and na pyroxene characterize the metamorphism during the thrusting the findings of this study indicate north west orogen parallel transport of the san juan thrust system we suggest that this structure is a large step over zone transferring dextral strike slip from the western cascades to the oceanward side of wrangellia

u pb zircon and monazite ages for the okanogan range batholith washington implications for the magmatic and tectonic evolution of the southern canadian and northern united states cordillera new u pb zircon and monazite dates for the okanogan range batholith a suite of trondhjemitic plutonic rocks in north central washington are interpreted to reflect minimum crystallization ages of 114 111 ma these new data suggest that the okanogan range batholith may represent an older southerly segment of the late albian spences bridge group which crops out in southern british columbia this would constitute a 300 km long north northwest trending belt of 130 100 ma volcanic and plutonic rocks along the western margin of the intermontane belt herein named the okanogan spences bridge arc the okanogan spences bridge arc and the gambier arc in the coast belt of british columbia were coeval widely separated early cretaceous magmatic belts in the southern canadian northern united states cordillera the cbronologic data require reevaluation of models proposing one versus two east dipping subduction zones to explain the origin of these two parallel magmatic belts neither model adequately reconciles all geologic observations

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

excess 40ar free phengite in ultrahigh pressure metamorphic rocks from the lago di cignana area western alps abstract ar ar analyses of phengites and paragonites from the ultrahigh pressure metamorphic rocks zoisite clinozoisite schist garnet phengite schist and piemontite schist in the lago di cignana area western alps were carried out with a laser probe step heating method using single crystals and a spot dating method on thin sections eight phengite and two paragonite crystals give the plateau ages of 37 42 ma with 96 100 of 39 ar released each rock type also contains mica crystals showing discordant age spectra with age fractions 20 35 ma significantly younger than the plateau ages phengite inclusions in garnet give ages of 43 2 1 1 ma and 44 4 1 5 ma which are significantly older than the spot age 36 4 1 4 ma from the matrix phengites and the plateau ages from the step heating analyses inclusion ages 43 and 44 ma are consistent with a zircon shrimp age 44 1 ma in this area these results suggest that the oceanic materials that underwent a simple subduction related uhpm form excess 40 ar free phengite and that the peak metamorphism is ca 44 ma or little older we suggest that matrix phengites experienced a retrogression reaction changing their chemistry contemporaneously with deformation related to the exhumation of rocks releasing significant radiogenic 40 ar from the crystals this has lead to the apparent ages of the matrix phengites that are significantly younger than the inclusion age

discrete ice lens theory for frost heave in soils the existing segregation potential sp method for frost heave prediction in soils is semiempirical in nature and does not explicitly predict the relationship between heave rate temperature gradient and other more fundamental soil properties the sp method assumes that the heave rate is directly related to the temperature gradient at the frost front but acknowledges that the sp parameter is dependent on pressure suction at the frost front cooling rate soil type and so forth this paper extends and modifies an approximate analytical technique of gilpin and accounts for the effects of distributed phase change within the freezing fringe in both the head and mass transfer components of the formulation the approach requires as input a relationship between frozen hydraulic conductivity and temperature and predicts the discrete location of each ice lens within the freezing soil the solution can be carried out quickly on a microcomputer to obtain the heave suction at the frost front ice lens temperatur

the new england batholith eastern australia geochemical variations in time and space the new england batholith australia is part of the upper paleozoic new england fold belt with most plutons intruded into the deformed trench complex metasedimetary rocks in the southeast part of the fold belt the batholith was emplaced in two major periods of plutonism the first during the upper carboniferous and the second during the upper permian and triassic with a major phase of metamorphism and deformation including westward overthrusting of the trench complex sedimentary rocks between the two periods on the basis of petrography geochemistry and isotopic characteristics the granitoids of the batholith are subdivided into five named intrusive suites and a group of leucoadamellites the differences between the six groups are considered to reflect differences in their source rock types the carboniferous granitoids are peraluminous s type and are divided into the bundarra plutonic suite a belt of very coarse grained adamellites with cordierite garnet and the hillgrove plutonic suite a belt of biotite rich garnet deformed adamellites and granodiorites both suites have 18o greater than 10 negative 34s 87sr 86sr initial ratios about 0 706 are ilmenite bearing and have low feo3 ratios the bundarra plutonic suite however is consistently sio2 rich greater than 70 contains cordierite and has higher 18o than the hillgrove plutonic suite the two s type suites are inferred to have formed by partial melting of the deepest parts of a wedge of trench complex sedimentary rocks oceanwards of an andean volcanic chain to the west analagous to the lower tertiary s type plutons of the sanak baranof plutonic belt of southern alaska of the remaining four groups of plutons emplaced during the upper permian and triassic two are metaluminous i type granitoids one the clarence river plutonic suite new name a group of k poor granodiorites and tonalites with low 87sr 86sr initial ratios average 0 7035 and the other the moonbi plutonic suite a group of k rich adamellites with low 87sr 86sr initial ratios average 0 7045 18o less than 10 and 34s positive the clarence river plutonic suite is inferred to have formed from the partial melting of a large ion lithophile element poor gabbro source region and the moonbi plutonic suite from a large ion lithophile element rich shoshonite source region a third group the uralla plutonic suite is less metaluminous has higher 87sr 86sr initial ratios average 0 706 18o only just less than 10 and negative 34s and is considered to have formed in a region containing a physical mixture of metaluminous and peraluminous source rocks gabbro and metasedimentary rocks plutons of the fourth group are leucoadamellites largely emplaced after the other suites with low 18o indicating an i type source it is argued that this group has resulted from low degrees of dry partial metling of an i type source region that had undergone an earlier melting event these four groups of plutons are largely confined to a narrow belt parallel to but 100 km west of the present rifted margin of australia their origin may be related to crustal thickening attendant on the overthrusting that occurred early in the permian and or a temperature increase in the lower crust due to the recovery of normal continental geotherms following the cessation of subduction

carbonaceous material in pelitic schists of the sanbagawa metamorphic belt in central shikoku japan abstract carbonaceous material in pelitic schists of the sanbagawa metamorphic belt in central shikoku japan was separated from the host rocks and its x ray diffraction and chemical composition were studied its crystal structure and chemistry change continously with increasing metamorphic grade and approach those of well ordered graphite near the biotite isograd as graphitization is a rate process the temperature of complete graphitization differs from one metamorphic terrain to another as a function of the duration of metamorphism in an individual metamorphic terraan however the degree of graphitization is a useful indicator of relative metamorphic temperature in lower grade rocks

coast plutonic complex a mid cretaceous contractional orogen early late cretaceous east vergent thrusts deform much of the eastern margin of the coast plutonic complex in western british columbia similarities in timing and style suggest that the faults represent a system of backthrusts to a west vergent thrust belt on the west side of the complex this geometry and the rarity of mid cretaceous strike slip faults indicate that the coast plutonic complex was a strongly contractional orogen in mid cretaceous time

nature of the contact zone between the north cascades crystalline core and the methow sequence in the ross lake area washington implications for cordilleran tectonics previous analyses of the accretionary history of nw washington sw british columbia have suggested the possibility of large scale dextral slip faulting events to explain the present day regional outcrop pattern the contact zone between the crystalline skagit complex and the volcano sedimentary methow sequence has been considered to be a major fault named the ross lake fault by misch 1966 and has been considered to be a possible large scale strike slip fault by later workers detailed mapping of this contact zone in the ross lake area shows that the brittle faults and mylonites thought by misch to express dextral shearing along the contact are not continuous throughgoing structures and that the contact between skagit orthogneisses and methow sequence strata is primarily intrusive furthermore the mylonites show both left and right lateral shear the skagit methow contact is therefore not a major terrane boundary but rather a tectonized intrusive contact that has not accommodated significant motion between the two regions based on these observations and regional synthesis of timing relations deformation in the contact zone is interpreted to express regional ene wsw shortening not major dextral slip in the early tertiary since no terrane bounding fault exists the cretaceous tertiary history of the ross lake area can be interpreted as follows 1 jurassic early cretaceous forearc rift basin sedimentation deposition of the lower methow sequence 2 110 95 ma imbrication of the lower methow sequence and its basement of oceanic crustal rocks by east vergent thrusting with concomitant development of a west derived foredeep in the upper methow sequence 3 95 85 ma arc magmatism with genesis of the skagit migmatite from methow and older basement protoliths and 4 paleocene early eocene folding faulting uplift and exposure of a relatively intact 25 km crustal section termed the skagit methow crustal section by kriens and wernicke 1987

cenozoic and mesozoic metamorphism in the longmenshan orogen implications for geodynamic models of eastern tibet new zircon u pb and mica 40ar 39ar dating combined with structural studies in the longmenshan orogen confirm that most of the upper crustal deformation in the eastern margin of tibet is mesozoic however at lower structural levels apatite u pb and monazite electron microprobe dating reveals a previously unknown domain of cenozoic ca 65 ma barrovian type metamorphism and deformation this discovery shows that the crust in the eastern margin of tibet was already a substantial thickness around the time of the india asia collision associated deformation has a n s oriented stretching lineation implying that deformation was not driven by topographic gradients in the tibetan plateau the observed moderate amounts of distributed postmetamorphic e w shortening can probably explain the present thickness of the continental crust in the area these results do not support models of crustal thickening caused by solid state lateral flow of midcrustal metamorphic rocks

middle ordovician high pt metamorphic rocks in eastern australia evidence from k ar ages phengite separated from a metagabbro three blueschist and one metabasite that occur as blocks in serpentinite melange in the new england fold belt eastern australia has yielded k ar ages of 465 480 ma significantly older than the age of most components of the belt the schists and metabasite contain either glaucophane and or crossite intergrown with the phengite providing the first conclusive evidence for early paleozoic subduction accretion associated with the east gondwana margin the tectonic affinity of the blocks is uncertain they and other small early paleozoic convergent margin elements in the new england fold belt may be allochthonous remnants of an arc system accreted in the devonian or carboniferous or they may be related to early paleozoic convergent margin components found farther inboard in the paleozoic fold belts of eastern australia

late cretaceous and early tertiary plutonism and deformation in the skagit gneiss complex north cascade range washington and british columbia the skagit gneiss complex forms a more or less continuous terrane within the northern more deeply eroded part of the north cascade range the complex comprises abundant plutons intruded at mid crustal depths into a variety of metamorphosed supracrustal rocks of both oceanic and volcanic arc origin a plethora of syntectonic pegmatite small plutons and granitic dikes gives the complex a migmatitic aspect u pb zircon ages from gneissic plutons within and near the skagit gneiss complex indicate magmatic crystallization between 75 and 60 ma deformation recrystallization and migmatization in part postdate intrusion of the 75 60 ma plutons this latest cretaceous and earliest tertiary plutonism and migmatization may reflect thermal relaxation following early late cretaceous orogeny documented else where in the north cascades the complex was ductilely extended northwest southeast shortly after intrusion of granite dikes at 45 ma but before emplacement of the earliest 34 ma plutons of the cascade arc outcrops of late cretaceous and earliest tertiary plutons migmatites of the skagit gneiss complex and rocks with young ductile deformation are roughly coextensive all apparently marking a region of greater middle eocene unroofing unroofing was apparently contemporaneous with east west extension in the okanogan region to the east and north south and northwest southeast strike slip faulting within the north cascades

is the southeast coast plutonic complex the consequence of accretion of the insular superterrane evidence from u pb zircon geochronometry in the northern washington cascades zircon u pb geochronometry of orthogneisses and plutons in the southwestern crystalline core of the north cascades coupled with fabric and textural studies of the orthogneisses plutons and their metamorphic host rocks indicates extensive synmetamorphic plutonism at 89 96 ma metamorphic mineral assemblages define a culmination composed of an axial kyanite sillimanite zone rimmed by lower grade zones high grade index minerals are typically syntectonic to posttectonic metamorphic fabrics are characterized by an orogen parallel northwest striking steep foliation that contains a subhorizontal stretching and mineral lineation interpreted to be the product of ductile strike slip deformation this fabric is crosscut by 96 92 ma plutons yet is imprinted on 92 89 ma orthogneisses suggesting spatially diachronous fabric development during orogeny documentation of the spatial and temporal coincidence of magmatism with the peak of orogeny together with the kinematic significance of the metamorphic fabric precludes generation of the metamorphic fabric and plutons in response to thrust loading the authors suggest that this part of the coast plutonic complex evolved as a transpressional magmatic arc

tectono metamorphic evolution of high p t and low p t metamorphic rocks in the tia complex southern new england fold belt eastern australia insights from k ar chronology the tia complex in the southern new england fold belt is a poly metamorphosed late paleozoic accretionary complex it consists mainly of high p low t type pumpellyite actinolite facies rare blueschist facies schists phyllite and serpentinite t 300 c and p 5 kbar and low p high t type amphibolite facies schist and gneiss t 600 c and p 5 kbar associated with granodioritic plutons tia granodiorite white mica and biotite k ar ages distinguish carboniferous subduction zone metamorphism and permian granitic intrusions respectively the systematic k ar age mapping along a n s traverse of the tia complex exhibits a gradual change the white mica ages become younger from the lowest grade zone 339 ma to the highest grade zone 259 ma in contrast si content of muscovite changes drastically only in the highest grade zone the regional changes of white mica k ar ages and chemical compositions of micas indicate argon depletion from precursor high p low t type phengitic white mica during the thermal overprinting and recrystallization by granitoids intrusions our new k ar ages and available geological data postulate a model of the eastward rollback of a subduction zone in early permian the eastward shift of a subduction zone system and subsequent magmatic activities of high mg andesite and adakite might explain formation of s type granitoids hillgrove suite and coeval low p high t type metamorphism in the tia complex

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

crustal structure of southwest japan revealed by the integrated seismic experiment southwest japan 2002 abstract a multi purpose seismic experiment named the 2002 integrated seismic experiment southwest japan was conducted in 2002 along a more than 240 km long line across southwest japan from the pacific coast to the japan sea coast its profile provides the first crustal scale cross section across the japanese island arc which highlights a number of significant points related to the structural development of the arc major outstanding points are that the japanese island arc is composed of two completely different crusts juxtaposed by the median tectonic line mtl and that the mtl started its activity associated with lower crustal thinning and formation of an upper crustal scale half graben in late cretaceous

discordant paleomagnetic poles from the canadian coast plutonic complex regional tilt rather than large scale displacement stratigraphic petrologic and isotopic data indicate that parts of the coast plutonic complex and the north cascade range have been tilted northeast side up by angles of approximately 30 degree about north northwest trending axes these tilts can account for discordant paleomagnetic directions observed in mid cretaceous plutons from these regions without large scale displacement relative to north america

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

deformation of granitic rocks across the brittle ductile transition abstract a microstructural analysis has been carried out on mylonites and mylonitic gneisses of the eastern peninsular ranges mylonite zone which were formed over a range of metamorphic conditions from lower greenschist to amphibolite facies composite planar fabrics in the form of c and s planes are found at all metamorphic grades fractured feldspars kinked biotites and ductile deformation of quartz characterize the lower greenschist facies mylonites at mid upper greenschist grade orthoclase grains show dynamic recrystallization textures whereas plagioclase exhibits low temperature plasticity with only minor recovery biotite ribbons form by progressive rotation and coalescence of kink band segments to produce chevron fold patterns at epidote amphibolite grade and above recovery processes and annealing recrystallization predominate in all minerals residual orthoclase porphyroclasts show strain related myrmekite formation along those sides of the grains that face the instantaneous shortening direction myrmekite formation due to replacement reactions cannot explain this geometry it is proposed that the myrmekites formed due to a combination of exsolution replacement and strain enhanced diffusion

reconciliation of two sided thrusting burial metamorphism and diachronous uplift in the cascades of washington and british columbia several recently published data sets have shed new light on many of the fundamental kinematic aspects of mid cretaceous orogeny in the cascades of northwestern washington and southwestern british columbia specifically a it is now clear that contractional deformation associated with this orogeny affected rocks east of the crystalline core of the range to a greater degree than had previously been suspected and b geobarometric data from the crystalline core of the range now permit a quantitative reconstruction of the crustal thickening event responsible for core metamorphism to be attempted i propose a new kinematic model for mid cretaceous orogeny that reconciles these data with previously collected data sets that focused on deformation in the northwest cascades san juan islands thrust system the model derived from this analysis suggests that the cascades underwent a minimum of between 400 and 500 km of east west shortening between about 115 and 85 ma owing to the collision of the greater insular terrane which probably included rocks of the chilliwack nooksack terrane with the western edge of the greater intermontane terrane which included the methow basin and bridge river hozameen terrane on the basis of their relative and absolute times of uplift i assign the deepest part of the metamorphic core the skagit gneiss to the greater insular terrane and the structurally higher carapace of the core the settler nason metapelites and the cogburn creek mad river metabasalt metachert to the westernmost intermontane terrane the contact between the skagit gneiss and the overlying assemblages must represent a fault or suture across which at least 120 km of displacement took place this fault is also the root zone from which many of the nappes in the northwestern cascades and san juan islands were derived i concur with previous workers who suggested that the collisional event was not precipitated by the closure of a several thousand kilometer wide ocean but was rather due to a short lived margin normal convergence event that both followed and was followed by periods during which the continental margin was highly influenced by oblique subduction and strike slip faulting

new insight into a subduction related orogen a reappraisal of the geotectonic framework and evolution of the japanese islands abstract the geotectonic framework and the evolutionary history of the japanese islands need revision in accordance with the various geophysical geological evidence gathered by new methodologies in the recent years including seismic tomography vibroseis ground breaking seismic experiments and detrital zircon chronology these investigations have addressed various themes such as 1 seismic profile of the crust and mantle beneath the japanese islands 2 high precision ages of the protoliths of high p t metamorphic rocks and 3 provenance of terrigenous clastics the results have led to a number of important findings including 1 detection of a large mass of slab around the mantle boundary layer suggesting the long term oceanic subduction beneath japan 2 confirmation of the subhorizontal piled nappe structure for the entire crust of japan 3 finding a new high p t metamorphosed accretionary complex unit that represents the youngest blueschist in japan 4 finding of the oldest early cambrian arc batholith and cover sediments and 5 the identification of plural arc batholiths which have already been erased from the surface based on a synthesis of these new data this article presents a re evaluation of the conventional geotectonic subdivision of the subduction related orogen in japan re definition of the elements and their mutual boundaries and re consideration of the geotectonic evolution of the japanese islands in particular the historical change in provenance suggests that proto japan has experienced large scale tectonic erosion in multiple stages and the corresponding large amounts of continental crust materials were subducted for understanding the orogenic growth of japan during the last ca 500 million years the significance of tectonic erosion coupled with continental contraction as well as the oceanward accretionary growth requires further attention

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

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

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

the contorted new england orogen eastern australia new evidence from u pb geochronology of early permian granitoids 1 a series of sharp bends oroclines are recognized in the paleozoic to early mesozoic new england orogen of eastern australia the exact geometry and origin of these bends is obscured by voluminous magmatism and is still debated here we present zircon u pb ages that confirm the lateral continuation of early permian 296 288 ma granitoids and shed new light on the oroclinal structure orogenic curvature is defined by the alignment of early permian granitoids parallel to the structural grain of the orogen as well as the curved geometry of sub vertical deformation fabrics forearc basin terranes and serpentinite outcrops alternative geometrical interpretations may involve two bends texas and coffs harbour oroclines three bends manning orocline or even four bends nambucca orocline we argue that the model involving four bends is most consistent with available data although further kinematic constraints are required to confirm the existence of the manning and nambucca oroclines a subsequent phase of younger magmatism 260 ma cuts across the curved structural grain providing a minimum age constraint for orocline development assuming a structure of four oroclines we suggest a tentative tectonic model that involves an early stage of subduction curvature during slab rollback at 300 285 ma followed by bending associated with dextral transpression a final tightening of the curved structures was possibly obtained by e w shortening during the late permian to triassic 265 230 ma hunter bowen orogeny

permian evidence for trans tasman relationships between east australia new caledonia and new zealand abstract basaltic to basaltic andesitic tuff breccias agglomerates and subordinate lavas together with cogenetic dacitic tuffs and glassy flows comprise the tectonomorphically anomalous early permian submarine volcanic sequence of the gympie group southeast queensland the least accumulative gympie basalts and basaltic andesites show geochemical features which typify island arc tholeiites the absence of andesitic eruptives i e the essential bimodality of the volcanic sequence together with low levels of large ion lithophile elements lile and the absence of compelling evidence for amphibole or garnet fractionation are consistent with an immature submarine tholeiitic stage of arc development on relatively thin oceanic crust by contrast the mid carboniferous to permian calc alkaline camboon arc to the west is inferred to have developed at least in part on early carboniferous continental crust the volcanics and overlying sediments of the gympie group are compared with those of the volcanic arc and adjoining formations of the nelson eglinton takitimu areas of new zealand to reveal some striking geochemical and lithostratigraphic similarities dacitic volcanics in new caledonia may also have formed a relatively young part of the same volcanic arc we analyse the overlying permian sediments to show further similarities and facial differences between the three regions essentially we propose new zealand was locus for an actively spreading mid ocean ridge dun mountain ultramafics and patuki ophiolite complex which received a substantial thickness of marine sediment gympie lay towards the end of the mid ocean ridge and new caledonia close to the terminus of the volcanic arc and received more terrestrial sediment we analyse original and subsequent spatial interrelationships that developed before and during the late cretaceous opening of the tasman sea and suggest that the history of separation was complicated by strike slip and or transform movements

thermal hydro mechanical analysis of frost heave and thaw settlement abstractfrost heave and thaw settlement are the cause of substantial damage to infrastructure in regions of seasonal freezing as well as seasonal thawing permafrost while frost heaving of soils has been researched for decades less attention has been paid to quantitative estimates of settlement and loss of strength of soils due to thawing a constitutive model is developed capable of simulating freezing and thawing of soils and associated changes in the soil strength while the porosity growth parameters describing frost heaving have been calibrated and validated for a specific soil the description of the thawing phase has not been validated due to a lack of experimental data the yielding of the frozen soil is described using the critical state concept with the pore ice content being an important parameter affecting the yield function the model was calibrated using available laboratory test data and used in the simulation of a freezing thawing cycle in the soil with embedded footing

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

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

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

high pressure metamorphism in the western cordillera of north america the skagit gneiss a major component of the crystalline core of the north cascades was metamorphosed during a mid cretaceous to early tertiary high p event driven by the collision of the insular and intermontane superterranes maximum pressures recorded by metapelitic rocks are 8 10 kbar at 650 725 c high pressures are also indicated by coexisting staurolite and hornblende in amphibolites in the skagit gneiss and adjacent cascade river schist mineral reactions continued during nearly isothermal decompression from 8 10 kbar to c 3 5 kbar early high p minerals e g kyanite are present as armoured relics in garnet in gneisses that contain sillimanite and cordierite in the groundmass skeletal relics of kyanite are also present in the groundmass of lower grade staurolite bearing schists that contain texturally later cordierite this matrix kyanite may have been preserved as a result of rapid uplift following initial decompression at high temperature these results represent a revision of the metamorphic history of the skagit gneiss which was formerly thought to have experienced only relatively low p barrovian metamorphism qualitative estimates of metamorphic conditions based on stable matrix mineral assemblages result in an underestimation of maximum pressures because mineral reactions continued during decompression geobarometric results for the skagit gneiss are interpreted as evidence for major crustal thickening in the north cascades recognition that pressures of c 9 kbar were attained supports a contractional model for north cascades orogenesis and requires that tectonic syntheses account for the burial of the skagit gneiss protoliths to a depth of c 25 30 km

tilt and northward offset of cordilleran batholiths resolved using igneous barometry considerable controversy surrounds the suggestion based on palaeomagnetic evidence that large segments of the north american cordillera travelled long distances parallel to the coast during the latest cretaceous and early tertiary periods well after the amalgamation of exotic terranes discordant palaeomagnetic data from mid cretaceous plutonic rocks of the peninsular ranges batholith of coastal southern california and baja and the mount stuart batholith of the cascade range in washington play a pivotal role in the controversy the discordant data were originally interpreted to reflect northward transport of 1 000 km relative to cratonal north america after the batholiths cooled through their magnetic blocking temperatures1 5 more recently it has been argued that the discordances arise from local tilting of batholiths rather than northward offset6 7 here we present and implement new methods based on hornblende barometry8 for determining the palaeohorizontal in granitic batholiths and correcting palaeomagnetic data for tilting our results indicate that the peninsular ranges and mount stuart batholiths have undergone northward offsets of 1 000 450 and 2 900 700 km respectively and also significant tilting

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

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

geometry evolution and tectonic framework of the skeena fold belt north central british columbia the intermontane belt of the canadian cordillera has long been viewed as a passive relatively rigid block between two metamorphic plutonic belts the coast plutonic complex and the omineca belt however the skeena fold belt which spans most of the width of the northern intermontane belt exhibits shortening comparable with that in the rocky mountain fold and thrust belt the skeena fold belt has many features of thin skinned fold and thrust belts such as thrust faults which sole into a detachment a wide variety of structural styles which depend on rock type a foreland basin which was cannibalized by continued deformation a frontal triangle zone and a hinterland of metamorphic and plutonic rocks coast plutonic complex the skeena fold belt thus is comparable with the rocky mountain fold and thrust belt but rather than deforming a continental terrace wedge it developed in a terrane stikinia which had accreted to north america in the early mesozoic and in jurassic and cretaceous clastic successions bowser lake and sustut groups which overlie stikinia structural and stratigraphic relationships show that the earliest deformation occurred between oxfordian and albian time and that the last folds developed in latest cretaceous or early tertiary time as much as 160 km of northeastward shortening in the skeena fold belt was broadly contemporaneous with crustal thickening in the coast plutonic complex and omineca belt with dextral strike slip faulting east of the skeena fold belt and with shortening in the rocky mountain fold and thrust belt therefore between latest jurassic and early tertiary times horizontal shortening occured across most of the width of the northern canadian cordillera concurrent shortening across the cordillera suggests that a common detachment or detachments fed all of these zones as far east as the rocky mountain fold and thrust belt

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

muscovite k ar ages of the sanbagawa schists japan and argon depletion during cooling and deformation seventy muscovites from schists in the sanbagawa terrain in central shikoku were dated by the k ar method the muscovite ages are consistently older with increasing metamorphic grade within the same zone the ages are significantly younger in schists which have been more severely deformed these k ar age variations could be due to systematic argon depletion during deformation i e to the dynamic recrystallization of muscovites during ductile deformation that formed a large scale recumbent fold during the uplift and cooling argon loss was greater in schists that were more extensively deformed and in the lower grade zone that experienced a longer period of low temperature deformation than the higher grade zone the relationships between age and grain size in a pelitic schist suggest that coarse grained muscovites lost more argon than the finegrained ones there was no significant resetting of ages in the vicinity of major strike slip faults such as the median tecotonic line or near thrust faults the combination of geochronological and geological data constrains the cooling rate of the sanbagawa schists to 9 12 c ma in the oligoclase biotite zone in central shikoku japan

structure and chronology of the oval peak batholith and adjacent rocks implications for the ross lake fault zone north cascades washington the 200 km long ross lake fault zone rlfz forms the northeastern border of the crystalline core of the north cascades this late cretaceous and paleogene structure is an 15 km wide system of faults and fault zones several of which deform the paleocene 65 ma u pb zircon oval peak batholith the southern segment of the gabriel peak tectonic belt of the rlfz is defined by strongly deformed rocks of the foliated southwestern margin of the oval peak batholith structurally underlying pre late cretaceous twisp valley schist and structurally highest part of the subjacent skagit gneiss which here consists of late cretaceous 87 ma and paleocene 60 ma orthogneiss kinematic indicators in moderately steeply northeast dipping mylonitic orthogneiss record reverse slip the strong solid state flattening fabric in the foliated margin of the oval peak batholith formed during or shortly after emplacement and overprinted a magmatic fabric observed in the core of the batholith foliation and associated stretching lineation within the gabriel peak tectonic belt are oblique to the regional structural grain but concordant with the contacts of the batholith these patterns in part record emplacement of the batholith as an expanding diapir into the tectonic belt reverse slip in the tectonic belt occurred before during and shortly after emplacement but ended by 55 58 ma the core of the oval peak batholith intruded a large tongue of twisp valley schist on the north porphyroblast microstructures and ar ar and k ar geochronology indicate that amphibolite facies dynamothermal metamorphism of the schist occurred during intrusion the foggy dew and twisp river fault zones separate the batholith and its wall rocks on the northeast from weakly metamorphosed strata of the methow basin the foggy dew fault zone contains mylonitic gneiss derived from the oval peak batholith skagit gneiss and tonalitic rock that displays u pb zircon systematics suggestive of an eocene age sillimanite schist and amphibolite are also present mylonites record dextral strike slip with a component of normal slip down to the northeast movement in the fault zone continued subsequent to deformation in the gabriel peak tectonic belt and ended by 48 ma mylonites in the twisp river fault zone also reveal dextral strike slip but apparently with less normal slip the rlfz experienced a diachronous history from at least 65 to 45 ma this history was dominated by dextral strike slip but there were significant components of normal slip and reverse slip on some structures during the latter part of this interval 57 45 ma the rlfz probably recorded a transtensional tectonic regime transitional between extension to the east in omineca metamorphic core complexes and dextral strike slip to the west related to oblique convergence along the continental margin early cenozoic plutonism metamorphism and ductile deformation have strongly overprinted any of the major cretaceous deformation postulated in previous tectonic models for the rlfz

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

metamorphism and plutonism in the central part of the okanogan range washington this report is concerned with the bedrock geology of a 1 000 km 2 area in the central part of the okanogan range in north central washington most of the rocks underlying this area were formed under conditions of high grade amphibolite facies metamorphism and or plutonism the metamorphism accompanied by metasomatic granitization and anatexis occurred between late triassic and late early cretaceous time plutonic rocks in the area are grouped into three temporal suites 1 an early premetamorphic suite represented by inclusions of mafic to granitic basement rock in gneisses of a trondhjemitic batholith which also is believed to have served as basement to supracrustal rocks deposited in a late paleozoic early mesozoic geosyncline 2 a synmetamorphic suite of gneisses including both concordant and discordant intrusions and 3 a post metamorphic suite of massive rocks which generally are discordant the igneous rocks range in composition from peridotitic to granitic trondhjemites and granodiorites are most abundant many of these units are somewhat leucocratic for their respective compositions younger intrusive rocks are more felsic than older ones after the plutonic rocks had been deroofed probably in early tertiary time the eastern part of the area was covered by quartz keratophyre flows these flows widely scattered diatremes and other hypabyssal intrusives are the youngest bedrock elements structures generated during metamorphism and plutonism generally trend northwest and indicate lateral compression some younger middle and late tertiary structures followed the established northwest grain but most formed athwart it these structures represented a basin and range type block faulting episode in the region a period of presumed lateral extension the strike of these fault block structures seldom varies more than a few degrees from north

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

monitoring of heat transmission from buildings into geological environment and evaluation of soil deformation consequences in foundation engineering in engineering geology a number of factors affecting foundation conditions are taken into consider ation during engineering geological investigations this article deals with the factor of heat sourced from a structure brick kiln as a restrictive factor in foundation engineering in clay soils and introduces a documentation of soil deformation observations as an impact of the heat trans mission into the geological environment it was carried out in southern moravia in the czech republic where the dominant foundation soils are neogeneous clays where differential settlements of a tunnel kiln structure occurred as a result of ignoring the boundary conditions of tem perature changes in the soil environment the brick kilns caused heterogeneous spatial changes in the subsoil tem peratures this consequently resulted in differential set tlements due to temperature changes originating from the kilns the differential settlements reached as much as 150 mm the major objective of the article is to highlight the importance of the heat transmission from buildings into the geological environment as a factor which should be considered in engineering geology and its application in planning a new procedure for reducing or elimination of ground movements sourced from underlying clayey soils depending on the heat changes was also suggested in this context

orogenesis without collision stabilizing the terra australis accretionary orogen eastern australia the neoproterozoic to end paleozoic terra australis orogen extended along the gondwana margin of the paleo pacific ocean and it now provides a detailed record of orogenic activity and continental stabilization within an ongoing convergent accretionary plate margin new geochronological data from end paleozoic plutonic and volcanic rocks associated with the gondwanide orogeny in the new england region of eastern australia integrated with information on the nature and timing of associated sedimentation deformation and metamorphism allow resolution of a high fidelity record of orogenesis at the end of the carboniferous around 305 ma convergent margin magmatism which had been active along the western margin of the new england region terminated and was followed by a short pulse of regional compressional deformation and metamorphism marking the commencement of the tablelands phase of gondwanide orogenesis deformation was almost immediately followed by the onset of clastic sedimentation and local calc alkaline volcanism dated at 293 ma in the extensional barnard basin emplacement of the two new england s type granitic suites the bundarra and the hillgrove suites along with localized high temperature low pressure metamorphism was essentially contemporaneous ranging in age from 296 to 288 ma and overlapped in time with i type magmatism and the switch from regional compression to extension and barnard basin rifting the hunter bowen phase of the gondwanide orogeny commenced with contractional deformation resulting in termination of sedimentation in the barnard basin and regional deformation and metamorphism across new england and into the sydney and gunnedah basins to the west at around 265 260 ma contractional loading of the sydney and gunnedah basins resulted in their conversion from extensional to foreland basins which received ongoing pulses of sediment from the new england orogenic welt until 230 ma the hunter bowen phase was associated with widespread i type plutonism and volcanic activity in new england that ceased around 230 ma marking the termination of gondwanide orogenesis orogenesis occurred in an evolving convergent plate margin setting s and i type magmatic activity ranging in age from ca 300 to 230 ma represents a stepping out of arc magmatism from the western margin of new england prior to 305 ma into the preexisting arc trench gap there is no evidence that deformation was related to the collision of the convergent margin with a major lithospheric mass and the widespread development of extensional basins in the eastern third of australia in the early permian indicates control by phenomena acting on a continental scale probably changing plate kinematics associated with the amalgamation of pangea

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

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

cretaceous isochron ages from k ar and 40ar 39ar dating of eclogitic rocks in the tso morari complex western himalaya india abstract the tso morari complex which is thought to be originally the margin of the indian continent is composed of pelitic gneisses and schists including mafic rock lenses eclogites and basic schists eclogites studied here have the mineral assemblage grt omp ca amp zo phn pg qtz rt they also have coesite pseudomorph in garnet and quartz rods in omphacite suggesting a record of ultrahigh pressure metamorphism they occur only in the cores of meter scale mafic rock lenses intercalated with the pelitic schists small mafic lenses and the rim parts of large lenses have been strongly deformed to form the foliation parallel to that of the pelitic schists and show the mineral assemblages of upper greenschist to amphibolite facies metamorphism the garnet omphacite thermometry and the univariant reaction relations for jadeite formation give 13 21 kbar at 600 c and 16 18 kbar at 750 c for the eclogite formation using the jadeite content of clinopyroxene x jd 0 48 phengites in pelitic schists show variable si al and na k ratios among grains as well as within single grains and give k ar ages of 50 87 ma the pelitic schist with paragonite and phengite yielded k ar ages of 83 5 ma k 4 9 wt for paragonite phengite mixture and 85 3 ma k 7 8 wt for phengite and an isochron age of 91 13 ma from the two dataset the eclogite gives a plateau age of 132 ma in ar ar step heating analyses using single phengite grain and an inverse isochron age of 130 39 ma with an initial 40 ar 36 ar ratio of 434 90 in ar ar spot analyses of phengites and paragonites the cretaceous isochron ages are interpreted to represent the timing of early stage of exhumation of the eclogitic rocks assuming revised high closure temperature 500 c for phengite k ar system the phengites in pelitic schists have experienced retrograde reaction which modified their chemistry during intense deformation associated with the exhumation of these rocks with the release of significant radiogenic 40 ar from the crystals the argon release took place in the schists that experienced the retrogression to upper greenschist facies metamorphisms from the eclogite facies conditions

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

intra arc crustal loading and its tectonic implications north cascades crystalline core washington and british columbia widespread high pressure cretaceous metamorphism in the coast plutonic complex of southeast alaska and british columbia and its southeast extension the cascades crystalline core is commonly attributed to thrust loading during superterrane collision however terrane boundaries within the cascades core are intruded by relatively shallow mid cretaceous plutons and crustal loading of 2 5 kbar postdates these plutons these observations are not consistent with proposed collisional models and we suggest that loading occurred by structural and or plutonic processes operating within a magmatic arc such loading may be an important process in arc tectonics although it has not been widely reported

analysis of frost heave mechanisms in a high speed railway embankment the harbin dalian high speed railway in northeastern china has a significant portion of track foundation built on seasonally frozen ground wide spread frost heave was observed during the first win

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

subsoil investigation of ice lensing at the calgary canada frost heave test facility several frost heave mitigation modes were studied at the calgary canada chilled pipeline frost heave test facility these included deeper burial to increase the pressure on the frost front below the pipe and replacement of the silty soil around the pipe with gravel for a noninsulated pipe frost heave at the deep burial section and at the gravel section was less than the heave at a control section other pipe sections tested the effects of insulation of the pipe on the long term frost heave as well as the effects of replacing the silt around an insulated pipe with gravel summer thawing of the frost bulb around the insulated pipe results in seasonal thaw settlement of the pipe thus reducing the long term pipe heave at least for the warmer ground temperature environment at the calgary facility thermal simulations of the frost bulb growth and predictions of frost heave using the segregation potential model agree well with the observations recent excavation of two frost bulbs in silty soil led to fiel

damage and mitigation of railway embankment drainage trench in warm permafrost a case study abstract warm permafrost sites such as those extensively existing on the qinghai tibet plateau present a challenging geological environment for infrastructure engineering a reliable drainage system is crucial for long term stability of railway embankment in such sites this paper presents a case study of the drainage trench along the qinghai tibet railway embankment the damage patterns to the precast u shaped concrete lining originally installed in 2008 is described a coupled thermal hydro mechanical finite element model and its results including temperature and stress distributions during ground freezing are presented parametric study was also conducted to assess the sensitivity of these results to soil water content and the stiffness of the u shaped concrete lining based on the modeling results a new design including a coarse grained transition layer and impermeable geotextile was proposed for field experiment and its field performance six years after its construction is presented the damage to the original trench was characterized by disjointed lining segments and extensive longitudinal cracks mostly located below the mid height of the linings factors contributing to the damage include challenging construction conditions uneven thaw settlement and frost heave stress modeling results show that significant horizontal frost heave stress can develop on the outside surface of the u shaped lining and hence results in large tensile stress at the outside edge of the u shaped lining the tensile stress may exceed the lower bound of the concrete flexural strength and induce longitudinal cracks at depths below the mid height of the lining which agrees well with field observation field performance of the new trench design during a period of six years shows that the improved design performed well and can be adopted for future engineering practice in similar challenging geology conditions

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

regionally extensive mid cretaceous west vergent thrust system in the northwestern cordillera implications for continent margin tectonism the intermontane insular superterrane boundary zone represents a fundamental crustal boundary separating the two largest allochthonous crustal fragments in the north american cordillera structural stratigraphic and geochronologic relations along this boundary indicate that substantial west vergent compression and concomitant crustal thickening occurred there in mid cretaceous time this orogenic zone extends for more than 1200 km along strike length between southern southeast alaska and northern washington in southern southeast alaska and northwest british columbia rocks of the insular superterrane were imbricated along a series of west to southwest vergent thrust faults in northern washington and southwestern british columbia a wide zone encompassing the margins of the two superterranes as well as numerous intervening smaller fragments was shortened principally along west vergent thrusts known geologic relations do not discriminate among existing tectonic models that explain the origin of the mid cretaceous thrust system

possible pre cenozoic left lateral slip on the yalakom fault southwestern british columbia the yalakom fault strikes northwest for nearly 250 km in southern british columbia immediately north of the town of lillooet between the yalakom and fraser faults structures within jurassic and cretaceous clastic rocks suggest that the yalakom fault had a history of left lateral followed by high angle contractional motion north and northeast trending folds offset by west northwest trending left lateral and normal faults plus numerous small conjugate left and right lateral faults with associated extension fractures fit a strain ellipse for left lateral slip on the yalakom fault younger right lateral bedding plane faults a large vertically plunging open fold and a thrust fault adjacent to the yalakom fault zone suggest later high angle contractional motion left lateral motion preceded whereas contractional motion probably coincided with 50 ma fraser fault deformation to the northwest along the yalakom fault and on the pasayten fault in washington left lateral slip possibly occurred between about 100 and 95 ma left lateral motion may prove an important element in the pre tertiary history of southern british columbia

geochronometry of the eagle plutonic complex and the coquihalla area southwestern british columbia new u pb k ar and rb sr dates from the eagle plutonic complex and adjacent map units place timing constraints on intrusive and deformational events along the southwestern margin of the intermontane belt u pb zircon minimum dates for eagle tonalite and gneiss 148 6 156 4 and 157 4 ma document previously unrecognized middle to late jurassic magmatism and syn intrusive deformation along the eastern margin of the eagle plutonic complex and the southwestern margin of the intermontane terrane widespread mid cretaceous albian cenomanian resetting of k ar and rb sr isotopic systematics in jurassic and older rocks is coeval and cogenetic with emplacement of plutons of the fallslake plutonic suite 110 5 2 ma u pb which crosscut jurassic plutons and structures but were themselves ductilely deformed along the pasayten fault during sinistral east side up reverse displacement k ar and rb sr cooling dates for the fallslake suite of ca 100 ma including dates from mylonites along the pasayten fau

a permafrost test on intact gneiss rock changes in permafrost conditions in high mountain rocks have increased the risk of dangerous instabilities ice segregation within the rock mass has been interpreted as one of the mechanisms involved in high mountain bedrock degradation a long term laboratory test on a cube of intact gneiss has been designed to reproduce field temperature gradients and water supply conditions test results demonstrate that ice crystallization in a permafrost fringe t 0 degrees c to 3 degrees c leads to the formation of continuous ice filled cracks which explain the loss of rock continuity and the observed rock failures a coupled thermo hydro mechanical model which incorporates the thermodynamics of ice water mixtures has been used to reproduce test results the model which follows existing formulations for unsaturated porous media was capable of capturing the main observations derived from the experiment calculated tensile stresses are close to the gneiss tensile strength the analysis performed is a step forward in understanding field observations and in the application of computational tools to real cases c 2015 elsevier ltd all rights reserved

structural metamorphic evolution of the tia complex new england fold belt thermal overprint of an accretion subduction complex in a compressional back arc setting rocks in the tia complex underwent a simple clockwise p t t deformation path moving from mid crustal levels at blueschist facies conditions d1 d2 6 kbar 200 c to upper crustal levels at high t amphibolite facies conditions d4 d5 2 5 kbar 600 c in a relatively short time 15 ma the rocks remained under these conditions for approximately 40 ma d6 before they cooled and were brought to the surface d7 d8 during d6 permian rift basins were opening as thermal gradients in the tia complex reached a maximum d1 2 deformation was associated with accretion subduction processes and was followed by progressively less intense mainly e w oriented compression and uplift during d3 5 bringing the rocks to within 2 kbars of the surface d6 is characterized by limited extensional deformation and final uplift took place during d7 8 along shear zones that have orientations similar to d3 5 thrusts the shift from subduction dominated processes to thermally dominated processes occurred during d3 4 uplift of 4 kbar around 310 300 ma this coincided with a change in tectonic setting of the tia complex from an accretionary prism to a back arc position as the subduction zone stepped eastward an increased heat flow associated with back arc processes gradually caused thermal weakening of the lithosphere which due to external subduction related compressive forces resulted in uplift d3 with continued heating of the crust thermal doming resulted and uplift stopped d4 5 whilst thermal weakening led to limited crustal collapse as permian basins opened d6 the extremely high heat flow that was maintained for 40 ma suggests a heat source external to the crust possibly associated with thermal relaxation dehydration and detachment of the remnant stab left beneath the new england orogen after the subduction zone shifted east a stable crustal thermal structure was re established by 260 ma as the crust strengthened and could further support subduction induced compressional structures d7 8

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

spatial and temporal relations between early tertiary shortening and extension in nw washington based on geology of the pipestone canyon formation and surrounding rocks detailed mapping of the deformed paleocene pipestone canyon formation and surrounding cretaceous rocks in nw washington suggests that uplift erosion and unroofing of adjacent rocks was influenced by shortening at high angle to the continental margin during mid cretaceous circa 88 110 ma and early tertiary 47 65 ma time although significant shortening is mid cretaceous in age additional shortening expressed as reverse faults and major folds is early tertiary in the map area and regions to the west and northwest the skagit methow western okanogan area the shortening is essentially unmodified by extensional deformation whereas in regions east and northeast of the map area the omineca and eastern okanogan regions widespread eocene extension has obscured the shortening and caused additional unroofing as well the conglomeratic pipestone canyon formation lies unconformably on intruded early cretaceous rocks of the lower methow sequence approximately 3 5 km of mid cretaceous methow sequence strata are missing at the unconformity yet are present 15 km to the west and northwest these observations indicate late cretaceous erosion and unroofing in the southeastern methow area prior to deposition of the pipestone canyon formation furthermore conglomerate clasts in the pipestone canyon formation record sources resulting from relative uplift of the crystalline okanogan complex east of the basin and slightly later from relative uplift of the volcano sedimentary methow sequence and structurally underlying migmatitic skagit complex west to northwest of the basin following deposition which ended sometime between 55 and 65 ma the pipestone canyon formation and underlying methow sequence were tightly folded and faulted and juxtaposed with the okanogan complex by reverse faulting the map pattern of these folds and faults suggests ene wsw shortening and concomitant uplift crosscutting relations north of the map area constrain this deformation to be pre 47 ma similarity of pipestone canyon fold geometry to folds elsewhere in the methow and skagit areas supports the interpretation that a significant fold belt of circa 47 65 ma age developed in these areas and overprinted the mid cretaceous folding and thrusting regional data suggest the circa 47 65 ma folding formed a synclinorium in the methow area and an anticlinorium in the skagit area roughly concurrent with cooling ages for deep 8 9 kbar skagit rocks for the skagit methow western okanogan region as a whole the 47 65 ma shortening and unroofing were accompanied by uncertain amounts of fold belt parallel strike slip faulting timing estimates for shortening in the skagit methow western okanogan area are generally older but overlap with those established for extension farther east and northeast in the okanogan complex and omineca belt supporting interpretations that the belt of extension began to develop during or very soon after regional shortening existing thermal rheological models of extension immediately after shortening point to the need for heat sources other than mantle conduction and crustal radioactivity to facilitate the extension apparently the skagit methow fold belt failed to extend because it lacked the necessary heat