magmatic epidote and its petrologic significance epidote is a major magmatic mineral in tonalite and granodiorite in a belt coextensive with the mesozoic accreted terranes between northern california and southeastern alaska textural and chemical evidence indicates that epidote crystallized as a relatively late but magmatic mineral that formed through reaction with hornblende in the presence of a melt phase the observed relations concur with experimental data on crystallization of epidote from synthetic granodiorite at 8 kbar total pressure plutonic rocks bearing magmatic epidote must have formed under moderately high pressures corresponding to lower crustal depths under fairly oxidizing conditions

thermal expansion contraction and slope instability of a fumarole field inferred from geodetic measurements at vulcano between 1987 and 1993 fumarole temperatures at the fossa crater of vulcano italy were characterized by the highest values since the 1920 s increasing from about 300 c in 1987 to 690 c in may 1993 before decreasing to 400 c by 1996 1997 during 1990 vulcano s electronic distance measurement edm network was expanded to provide more detailed coverage of the northern sector of the fossa crater and in particular to monitor the movement of the northern flank the fossa cone measurements carried out between 1990 and 1994 showed shortening by about 6 to 7 cm along baselines measured to a small section of the northern rim over the following four years these baselines showed a slow extension by about 3 cm to gradually recover part of the previous deformation we believe that the shortening and lengthening of the edm baselines was respectively due to the increasing and decreasing temperature of the rock body lying close to the deforming area this caused thermal expansion followed by contraction the positive movement of the rim was not completely matched by a negative recovery suggesting that a non recoverable sliding movement was also responsible for some of the shortening of the baselines we verified our hypothesis by calculating the expected dilatation of a rock body as a function of the volume of rock heated and its thermal expansion coefficient and compared the expected deformation to that observed the geodetic investigation showed that the unstable portion affects a small length of the rim about 100 m long and involves a volume of about 0 8 106 m3 however this zone lies directly above a particularly unstable portion of the flank as well as the main village and port on the island

melatroctolite anorthositic gabbro complex cumberland rhode island petrology origin and regional setting an igneous complex exposed in cumberland rhode island is one of a series of mafic intrusions that outline a northeast trending lineament extending from the preston gabbro in connecticut to the boston massachusetts area the age of these intrusions is not well constrained but appears to be between 620 and 370 m y b p the mafic complex in northern rhode island consists of a small body of melatroctolite adjacent to a much larger body 1 km2 of anorthositic gabbro the contact between the two rock types is not exposed but inclusions of anorthositic gabbro and large cumulophyric crystal aggregates identical to the gabbro occur in the melatroctolite the latter rock is composed of 49 olivine fo63 32 titaniferous magnetite 15 plagioclase an59 ab40 or1 and minor ilmenite and aluminum spinel the plagioclase in the melatroctolite occurs as tabular cumulophyric crystal aggregates 0 2 cm 0 5 cm in a matrix of and often including finer grained 3 mm olivine and titaniferous magnetite the tabular plagioclase aggregates are oriented to give the rock a weak foliation the gabbroic rocks are composed of a framework of thin tabular 1 0 cm 0 2 cm cumulus plagioclase identical in composition to that in the melatroctolite interstitial volumes in the gabbro are filled by ca pyroxene zoned overgrowths on the plagioclase and smaller amounts of olivine fo57 52 magnetite ilmenite and apatite the prominent igneous lamination and parallel but weak rhythmic layering in the gabbro are not parallel to the lamination in the melatroctolite but the rocks have been subjected to one and possibly two episodes of deformation faulting and rotation accompanying these deformations may explain the fact that the two foliations are not parallel or the relatively weak lamination in the melatroctolite may have formed during postcumulus flow compositionally the gabbroic rocks range from a high alumina gabbro 19 5 a12o3 to anorthositic gabbro 25 a12o3 a porphyritic chilled margin locality on the northern edge of the gabbro contains plagioclase phenocrysts an61 as much as 8 cm across and oriented parallel to the contact in a finer grained 0 5 mm groundmass of plagioclase and altered ferromagnesiaia minerals the mineralogy whole rock chemistry cumulus textures and igneous laminations indicate that the melatroctolite and anorthositic gabbro are genetically related igneous rocks this evidence together with the presence of the gabbroic inclusions in the melatroctolite indicates that the two rocks formed simultaneously by gravitational settling of olivine titaniferous magnetite and cumulophyric plagioclase olivine aggregates whereas discrete plagioclase crystals did not sink the fine grained ground mass of the marginal gabbro sample is thought to represent the parent magma on the basis of the composition an61 and planar orientation of the plagioclase phenocrysts the calculated density of the proposed parent magma is 2 66 g cc at 1250 c and 1 atm which is between that of plagioclase 2 63 and the plagioclase olivine aggregates 2 68 as would be required the crystal settling occurred slowly enough to permit unzoned adcumulus overgrowths to completely fill the interstices in the melatroctolite according to magnetite ilmenite pairs in the melatroctolite and gabbro the final equilibration between these two minerals took place at 950 c 100 c and a fo2 of 10 12 5 1 0

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

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

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

transpressional shearing and strike slip partitioning in the late cretaceous sierra nevada magmatic arc california the rosy finch shear zone rfsz represents an example of synmagmatic strike slip tectonics in the east central sierra nevada and provides information on transpressional tectonics within magmatic arcs the rfsz 1 4 km wide and 80 km long is contained in late cretaceous granitoids 92 83 ma of the mono pass intrusive suite dextral strike slip deformation is indicated by a continuous band of orthogneiss mylonitic and cataclastic deformation characterized by subvertical foliations with subhorizontal lineations the style of dextral shearing in the rosy finch shear varies along strike from wide zones of ductile deformation in the youngest plutons to narrow zones of cataclastic deformation in the older plutons deformation of the youngest granitoids is synmagmatic as constrained by both field observations and isotopic studies dextral shearing occurs concurrently in contemporaneous intrusive suites to the north and south tuolumne and mount whitney suggesting strike slip movement along the axis of active plutonism field studies also suggest that contraction across the arc acted concurrently with strike slip movement consistent with a transpressional setting for the late cretaceous sierra nevada magmatic arc comparison of the field data with strain models of oblique plate convergence suggests that the rfsz is a preserved ductile signature of transcurrent motion of strike slip partitioning within a transpressional orogenic system recent plate reconstructions indicate a switch from sinistral to dextral oblique convergence at 95 ma which is consistent with the timing constraints on the dextral movement on the rfsz

paleomagnetism and geochronology of the ecstall pluton in the coast mountains of british columbia evidence for local deformation rather than large scale transport 1 samples for geochronologic geobarometric and paleomagnetic analyses were collected across the northern portion of the ecstall pluton southeast of prince rupert british columbia al in hornblende geobarometry indicates pressures from 740 10 to 840 30 mpa corresponding to crystallization depths of 25 to 30 km u pb analyses of zircons from western central and eastern localities within the pluton yield crystallization ages of 91 5 1 0 ma 90 8 1 0 ma and 90 5 1 0 ma respectively rock magnetic experiments reflected light microscopy and thermal demagnetization behavior suggest that natural remanent magnetism is carried by low ti titanohematite unblocking temperatures of the characteristic remanent magnetization chrm are dominantly in the 560 c to 630 c range with age of magnetization approximated by the 40ar 39ar hornblende ages of 84 2 0 10 ma on the western margin and 76 4 0 6 ma in the center of the pluton site mean chrm directions were isolated for paleomagnetic samples from 23 sites and are distributed along a small circle with subhorizontal axis at 340 azimuth chrm directions from the central portion of the pluton are concordant with the expected cretaceous magnetic field direction while chrm directions from the western margin are discordant by 70 folding of the ecstall pluton either during late cretaceous west directed thrust transport above the convex upward prince rupert shear zone or during younger deformation of the pluton and underlying shear zone can account for the paleomagnetic data and is consistent with the geochronologic geobarometric and structural geologic observations

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

paleogeographic and tectonic implications of the geology of the tiefort mountains northern mojave desert california geologic mapping petrologic and structural analyses and u pb dating in the tiefort mountains northern mojave desert california provide data on paleozoic paleogeography and mesozoic intra arc tectonics metasedimentary rocks consist primarily of quartzose schists and gneisses that are provisionally correlated with late precambrian cordilleran passive margin facies recognition of precambrian augen gneiss and metasedimentary rocks with craton margin protoliths suggests that the craton miogeocline boundary is in or near the tiefort mountains the presence of eugeoclinal rocks in ranges to the west implies truncation or telescoping of the miogeocline in this area the metasedimentary units are intruded by middle jurassic granite and diorite orthogneisses dated by u pb as ca 164 and ca 160 ma deformed rocks record amphibolite facies metamorphism and mylonitization during middle to late jurassic southeast vergent ductile shear pervasively annealed microstructures indicate that recrystallization outlasted deformation at south tiefort mountain the mylonitic fabric is deformed by northwest trending folds and crosscut by a swarm of mafic and felsic dikes dated as 148 14 ma at tiefort mountain synkinematic biotite granite gneiss is dated as ca 105 ma and mylonitic fabrics are folded about north trending axes the presence of the cretaceous orthogneiss suggests that two deformational events jurassic and cretaceous in age may have affected part of the study area dikes dated as ca 82 ma that crosscut all fabrics are interpreted to be part of a regional intrusive event the timing and vergence of shear zones at south tiefort mountain are similar to contractional shear zones in adjacent areas forming a belt of contractional deformation from the central mojave desert to the east sierran thrust system this deformation may be related to a period of late middle to late jurassic sinistral oblique subduction

beginning of foreland subsidence in the columbian sevier belts southern canada and northwest montana subsidence analysis and geometry of jurassic cretaceous foreland strata in northwestern montana and southern alberta and british columbia suggest that loading by the fold thrust belt in canada began as much as 40 m y earlier than in montana in canada early foreland basin deposits are late jurassic age thicken rapidly westward and are restricted to a narrow belt within 30 km of the thrust belt in western montana contemporaneous deposits are widespread and do not increase markedly in thickness toward the thrust belt the unconformity overlying these deposits also changes from canada where it is angular to a disconformity in western montana near great falls between these two areas foreland geometry is transitional over a distance of 250 km beyond the transition zone early foreland basin geometries are broadly consistent showing late jurassic foreland subsidence in southern canada and early cretaceous initial subsidence in the united states

kinematics and tectonic significance of transpressive structures within the coast plutonic complex british columbia abstract structural data from the coast plutonic complex near prince rupert british columbia are consistent with a deformational history dominated by dextral transpression from campanian to paleocene time penetrative east side up southwest directed ductile shearing produced moderately northeast plunging overturned kilometer scale isoclinal folds these folds are dextrally sheared and refolded into kilometer scale upright northwest plunging folds and steeply dipping transposed foliations with moderate to shallow northwest plunging lineations along their western side an east side up component to the transcurrent shearing is kinematically compatible with east side up shearing found within the great tonalite sill kinematic and geometric gradients and the spatial distribution of the finite stretching direction are interpreted to result from partitioning of transpression the location of these structures and overprinting relationships suggest the great tonalite sill intruded late kinematically into a crustal scale dextral transpressive shear zone these results indicate this shear zone could form part of the baja b c fault system that would have accommodated large northward displacements of the terranes making up western british columbia and southeast alaska this conclusion is based on 1 it is favorably located to accommodate the proposed displacements 2 deformation occurred during the time period of proposed large displacement 83 59 ma 3 the 15 km thickness of the shear zone indicates it records large displacements

stretching lineations in transpressional shear zones an example from the sierra nevada batholith california abstract ductile shear zones and associated stretching lineations are generally considered to be the result of two dimensional simple shear deformations with stretching lineations interpreted to rotate into parallelism with the direction of tectonic transport with increasing deformation however stretching lineations perpendicular to the inferred tectonic transport direction are displayed by some shear zones field studies in the sierra nevada batholith have revealed a single shear zone the rosy finch gem lake shear zone that contains both steeply plunging stretching lineations in older metamorphosed sedimentary and granitic rocks and shallowly plunging stretching lineations in syntectonic granitoids dextral sense of shear indicators are found in all these units and deformation occurred simultaneously along the length of the shear zone theoretical strain modeling indicates that stretching lineations can be either horizontal or vertical within transpressional shear zones to investigate the kinematics of three dimensional deformation we examine the role of stretching lineations which form as a result of finite strain including the possibility of partitioning a component of the strike slip movement onto shear bands comparing the field data with the strain modeling we interpret the along strike variation in lineation behavior within the rosy finch gem lake shear zone to be a result of the differences in finite strain recorded by different units the older units showing steeply plunging lineation have recorded more finite strain across a narrower shear zone than the syntectonic granites which record shallowly plunging lineations we conclude that the orientation of the lineation within transpressional shear zones does not necessarily correlate with the transport direction but may reflect along strike variations in finite strain and or strain partitioning

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

late cretaceous truncation of the western idaho shear zone in the central north american cordillera the mesozoic boundary separating accreted oceanic rocks and continental north america in west central idaho forms a prominent right angle bend the origin and kinematic significance of which have had a great influence on models for the evolution of the north american cordilleran orogen based on structural and geochronologic relations we conclude that the bend in the steep crustal boundary is formed by the intersection of two shear zones with different ages structural fabrics of the north striking western idaho shear zone are overprinted by penetrative structures of the west northwest striking orofino shear zone u pb sensitive high resolution ion microprobe ages on metaplutonic rocks and late synkinematic leucocratic dikes constrain the age of deformation in the orofino shear zone to ca 90 70 ma distinctly younger than ca 120 90 ma magmatism and deformation within the western idaho shear zone the orofino shear zone is interpreted as a southwest vergent contractional relay structure that linked basement cored foreland thrusts of the middle and southern rocky mountains with the deep crustal duplex of the shuswap complex in the canadian rockies truncation ca 90 ma terminated late cretaceous margin parallel translation of terranes along the western idaho shear zone and required displacement to step outboard to poorly understood structures to the west

nd sr and pb isotopic characteristics of cretaceous intrusive rocks from deep levels of the sierra nevada batholith tehachapi mountains california the cretaceous gabbroic to granitic intrusive rocks of the tehachapi mountains were emplaced at depths of 25 30 km and thus afford a view of deep processes in the sierra nevada batholith they consist of the 115 ma tehachapi suite and the 100 ma bear valley suite new zircon u pb age data reveal the presence of the latter as far west as grapevine canyon the nd sr pb and o isotopic whole rock data and zircon pb inheritance patterns for the bulk of the suites suggest an origin by mixing between depleted mantle derived magmas and metasedimentary material with a substantial component of old continental material however this mixing is not evident in variations between isotopic ratios and chemical and lithologic parameters this implies that isotopic hybridization of magmas took place deeper than 30 km and that fractionation processes are likely responsible for the bulk of the chemical variation in this part of the sierra nevada batholith consideration of the isotopic data in the context of the sierra nevada batholith as a whole suggests that the well known east to west isotopic gradients in the batholith may reflect a change in the average isotopic character of the preintrusive framework rather than a change in amount of crustal component on the other hand the lack of areal gradients in sr and nd isotopic ratios in the main study area may indicate a lack of pronounced gradation at deep levels at least within the western batholith

contrast of metamorphic and structural histories across the work channel lineament coast plutonic complex british columbia a prominent topographic lineament along the western margin of the coast plutonic complex of southeast alaska and adjacent parts of british columbia separates terranes of contrasting metamorphic histories where it enters british columbia near prince rupert as the work channel lineament the western terrane preserves a regional medium high pressure metamorphic facies series increasing from chlorite grade in the west to kyanite muscovite migmatite near the lineament the metamorphic structural and age relations of the western terrane are consistent with a model of crustal thickening accompanied by westward transport of higher over lower grade metamorphic units during the jurassic this terrane was uplifted and cooled by cretaceous time 85 ma east of work channel lineament high temperature relatively low pressure assemblages are overprinted onto high pressure assemblages the low pressure metamorphic assemblages developed in the eocene during rapid uplift 2 mm yr of rocks which initially may have been metamorphosed at the same time and along the same pt gradient as those of the western terrane structures along the work channel lineament suggest vertical motion during uplift of the eastern terrane and after metamorphism of the western terrane because the metamorphic discontinuity across the lineament is 3 5 kbar at about 700 the lineament in the prince rupert area marks a major crustal break

the devils elbow ophiolite remnant and overlying galice formation new constraints on the middle to late jurassic evolution of the klamath mountains california the josephine ophiolite and its sedimentary cover galice formation of the western jurassic belt klamath mountains northern california are the expression of a late jurassic rift basin which developed between an active late jurassic arc to the west and a remnant middle jurassic are to the east the devils elbow ophiolite remnant deo and its sedimentary and volcanic cover represent the southern continuation of the josephine ophiolite and overlying galice formation the deo consists of a dike complex volcanic sequence and ophiolitic breccia overlying strata consist of terrigenously derived turbidites and andesitic to dacitic volcaniclastic rocks the following relations suggest formation of the deo in a marginal basin setting in accordance with the setting inferred for the josephine ophiolite basin 1 dikes and lavas are characterized by an abundance of clinopyroxene phenocrysts by crystallization of clinopyroxene before plagioclase and by trace element geochemistry which is transitional between iat and morb 2 overlying sedimentary rocks appear to have been derived from erosion of older klamath mountain terranes and were deposited into the deo basin shortly after and or during its formation 3 the texture and composition of the volcanic deposits interlayered with sedimentary strata indicate the presence of a nearby active volcanic arc source the period of rifting which culminated in the formation of the josephine ophiolite deo basin began by at least 164 ma the age of the deo but is unlikely to have begun much earlier as the region was experiencing a major period of compression just prior to this time rifting began at least 5 m y prior to cessation of magmatism in the rifted middle jurassic arc the overlap in rifting and arc magmatism can be explained if spreading in the josephine ophiolite deo basin was asymmetric relative to the axis of middle jurassic are magmatism and or if transform motion approximately parallel to the middle jurassic are was important in the spreading geometry of the basin the western boundary fault of the western jurassic belt and klamath mountain province is the south fork fault previously referred to as part of the coast range thrust in the southern western jurassic belt a serpentinite matrix meelange smm occurs along this fault blocks in the smm consist largely of greenstones with the trace element geochemistry of morb other block types include gabbroic to dioritic intrusive rocks diabase harzburgite amphibolite chert sandstone and shale generally similar serpentinite matrix melanges have been described from other locations along the south fork fault this spatial association suggests that the south fork fault provided a pathway for the bouyant rise of serpentinite or serpentinite matrix melange from a source at depth obvious sources for the smm include parts of the franciscan complex the josephine ophiolite deo or possibly the coast range ophiolite the geochemistry and relative abundances of different lithologies in the smm however indicate that none of these could represent the source in contrast the smm is nearly identical in block content and block geochemistry to serpentinite matrix melange of the rattlesnake creek terrane which structurally overlies the western jurassic belt to the east this suggests that a previously unrecognized part of the rattlesnake creek terrane may underlie the josephine ophiolite deo at depth as the rattlesnake creek terrane formed the western part of the basement for the rifted middle jurassic arc the part of the rattlesnake creek terrane which underlies the josephine ophiolite deo may represent a fragment of crust which was rifted oceanward with the opening of the josephine ophiolite deo basin and then subsequently thrust beneath the basin during the late jurassic nevadan orogeny

evidence of uppermost proterozoic to lower cambrian miogeoclinal rocks and the mojave snow lake fault snow lake pendant central sierra nevada california this is the published version copyright 2010 american geophysical union all rights reserved

intracontinental subduction and hinged unroofing along the salmon river suture zone west central idaho the salmon river suture zone in west central idaho juxtaposes volcanic arc rocks of the wallowa terrane directly against cratonic north america detailed metamorphic studies along a 10 km traverse perpendicular to the suture indicate that the arc and two crystalline fragments thrust upon it each record different pressure temperature p t histories from lowest to highest structural level the wallowa terrane shows only subgreenschist metamorphism the rapid river plate rrp records unroofing and cooling from 8 kbar and 550 c to 6 kbar and 475 500 c and the pollock mountain plate pmp shows evidence for polymetamorphism and records burial and heating paths to final equilibration conditions of 9 11 kbar and 600 625 c ar ar hornblende ages combined with the p t data suggest that currently exposed levels of the rrp and pmp were juxtaposed against one another at 15 20 km depth at or prior to 118 ma indicating that 10 20 km of uplift and hence also the onset of collision related metamorphism occurred before 118 ma correlation of the metamorphic and age data with geometric constraints from the initial sr 0 706 line and the dimensions of the rrp and pmp permit construction of large scale retrodeformable sections of the west side of the suture from late jurassic through late cretaceous time the abrupt nature of the sr 0 706 line implies that the arc continent boundary extends vertically through most of the crust which requires sharp downwarping of the arc lithosphere in order to account for the pmp metamorphic data narrow zoned overgrowths on pmp garnets record this burial event and require initially rapid 3 km m y uplift rates in order to be preserved we suggest that the onset of rapid uplift resulted from the separation of the negatively buoyant lithospheric root from the downwarped arc allowing buoyant rise of fragments of thickened crust detachment of the root is suggested to change the environment of crustal shortening from one in which footwalls of thrusts or shear zones sink to one in which hanging walls rise this mechanism represents an alternative to cessation of shortening or onset of tectonic denudation as an explanation for the transition from burial to uplift of high pressure metamorphic terrains subsequent uplift appears to have been slow and to have occurred in a hinged fashion such that mineral and whole rock ages decrease systematically towards the suture zone the consumption of lithosphere during 40 km of shortening between two crustal blocks implies that the salmon river suture is the trace of an intracontinental subduction zone burial and collision apparently began before about 130 ma and thus any precollision strike slip faulting or tectonic escape of intervening terranes was likely accomplished in jurassic and earliest cretaceous time

direction and shear sense during suturing of the seven devils wallowa terrane against north america in western idaho a northeast dipping 1 5 km thick mylonite near dworshak dam marks the suture zone between precambrian north america and the seven devils wallowa terrane in western idaho the mylonite formed under amphibolite facies conditions from quartz diorite containing apparently synplutonic mafic and synkinematic pegmatite dikes of the kamiah plutonic complex mylonitic lineations and fold axes have a mean plunge of 48 toward 056 nearly down the dip of the mylonitic foliation shear sense given by offset of late stage crosscutting pegmatites is consistently top to the southwest reverse slip parallel to the mylonitic lineation folds that formed by progressive folding of the mylonitic foliation approach sheath fold geometry axial planes and fold limbs are nearly parallel to the mylonitic foliation mafic dikes that are apparently synplutonic in the undeformed quartz diorite immediately south of the mylonite zone and north of kamiah have variable dips and azimuths in the shear zone however these dikes lie nearly in the mylonitic foliation transposition of the dikes into near concordance with the foliation by simple shear requires high values of shear strain and suggests that cumulative top to the southwest reverse slip displacement across the mylonite zone is at least 27 km and likely more than 80 km this displacement involves underthrusting of the kamiah plutonic complex emplaced within the seven devils wallowa terrane beneath north america during late cretaceous docking with continental 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

the paleomagnetic effects of reheating the ecstall pluton british columbia abstract paleomagnetic data of plutons from the western canadian cordillera are currently interpreted to indicate thousands of kilometers of latitudinal transport in situ tilting or post intrusion folding for the ecstall pluton of british columbia the inclination of magnetic remanence directions steepens to the east from 16 to 81 butler et al j geophys res 107 2002 between 24 and 12 km west of a thermal boundary with the coast mountains batholith cmb the cmb was at 700 800 c between 60 and 52 ma the stable remanent magnetization is in ilmenohematite grains with exsolved lamellae of ferrian ilmenite it has the characteristic properties of lamellar magnetism robinson et al nature 418 2002 517 520 which is acquired with exsolution in ilmenohematite below 390 c ghiorso phys chem minerals 25 1997 28 38 thus it is thermal chemical remanent magnetization tcrm our heat flow calculations show that the thermal effects of eocene cmb on the adjacent 91 ma ecstall pluton were enough to reset the remanent magnetization directions reheating to the temperatures necessary for tcrm is supported by k ar and ar ar cooling dates on hornblende and biotite which young from west to east towards the cmb in concert with the change in inclinations thus the progressive steepening of magnetization inclinations is attributed to reheating during the eocene of the 91 ma ecstall pluton by the cmb this reheating followed post solidification northward latitudinal displacement of the pluton the amount of translational displacement of the coastal terranes of british columbia prior to 60 ma hinges on the interpretation of discordant magnetizations from plutons that may have cooled slowly after emplacement in the mid to lower crust as was the case for the ecstall pluton many of these plutons contain ilmenohematite as an accessory magnetic phase our interpretation for the low temperature acquisition of tcrm in the ecstall pluton may have far reaching implications for understanding enigmatic discordant paleomagnetic directions reported from plutons of the western canadian cordillera

formation of new continental crust in western british columbia during transpression and transtension crustal growth in the coast mountains along the leading edge of the canadian cordillera was the result of processes associated with horizontal flow of material during transpression and subsequent transtension and the vertical accretion of mantle derived melts from 85 to 58 ma as exotic terranes were translated northward during transpression the crust was thickened to about 55 km and melt that originated from a mix of mantle derived basalt with partial melt of the thickened crust intruded into crustal scale transpressive shear zones when the orogen extended between 58 and 50 ma there was large scale decompression melting in the mantle and dehydration melting in the lower crust voluminous emplacement of sub horizontal sills facilitated by ductile flow of the gneissic country rocks partially filled space created as the crust was pulled apart and as 15 to 20 km of tectonic exhumation occurred across low angle normal ductile shear zones by 50 ma the final crustal thickness of the new continental crust was about 34 km comparison of seismic data with other crustal sections suggests that the crust forming processes identified in western british columbia have general applicability to models for the formation of continental crust

strain partitioning and batholith emplacement at the root of a transpressive magmatic arc abstract an integrative study of the high pressure ecstall batholith and its country rocks prince rupert area british columbia reveals the interplay of deformation and plutonism produced in the largest cordilleran magmatic arc i e the coast plutonic complex during mid cretaceous convergence between the farallon oceanic plate and north america the results emphasize the interference between three strain fields 1 an early 92 ma crustal wedge produced by orogen perpendicular sw vergent thrusting 2 an orogen parallel sinistral strike slip shear zone active until 87 ma and 3 the lateral expansion of the batholith 93 91 ma against the shear zone the batholith expanded in a direction oblique with respect to the shear zone trend 20 by extruding country rocks against its head in a direction normal to its expansion direction during sinistral strike slip partitioned transpression the batholith s emplacement combined far field batholith boundary normal translation extrusion related rotation and radial and concentric elongation in its structural aureole this case study may typify the process by which a modern magmatic arc grows longitudinally at depth during transpression the direction and sense of growth being determined by the direction of plate motion relative to the magmatic arc and the degree of strike slip partitioning of transpression

petrogenesis of the cretaceous cassiar batholith yukon british columbia canada implications for magmatism in the north american cordilleran interior the ca 100 ma cassiar batholith is the single largest plutonic body in the hinterland of the canadian cordillera and is part of widespread middle cretaceous to eocene magmatism that occurred in the omineca crystalline belt the cassiar batholith is dominated by muscovite biotite granite and biotite muscovite granodiorite along with subordinate biotite hornblende granodiorite quartz monzodiorite and quartz monzonite the batholith has a wide range of strontium neodymium and oxygen isotope compositions 87 sr 86 sr i 0 706 to 0 734 nd 2 7 to 17 1 t dm 2000 to 980 ma 18 o 7 6 to 11 6 but the batholith9s lead isotope compositions are limited 206 pb 204 pb 19 14 to 19 24 207 pb 204 pb 15 70 to 15 72 208 pb 204 pb 39 14 to 39 26 these data require that the cassiar magmas have been generated from at least two distinct sources an isotopically evolved felsic source and an isotopically more primitive source of mafic to intermediate composition we propose that the felsic source comprised mesoproterozoic to neoproterozoic metasedimentary rocks and paleoproterozoic basement rocks of the north american cratonic margin determining the mafic source is more problematic but most likely it was proterozoic basement rocks of mafic to intermediate composition such as have been reported from various parts of the omineca crystalline belt the cassiar batholith is lithologically geochemically and isotopically similar to other major batholiths in the cordilleran interior including the batholiths of southeastern british columbia and the idaho batholith we interpret these similarities to reflect melting of common crustal source materials namely proterozoic basement and cover rocks of western laurentia the cassiar batholith and other cordilleran interior granitoids are distinct from granitoids generated in known subduction zone environments in terms of their mineralogical geochemical and isotopic characteristics these differences reflect a predominantly or exclusively crustal source region for the cordilleran interior granitoids in contrast subduction zone magmatism contains a significant mantle input even in cases where subduction takes place beneath thick continental crust these results strongly suggest that cordilleran interior granitoids did not form directly from a subduction setting rather an intracontinental collisional setting seems more appropriate in which magma generation occurred in response to crustal thickening followed in some cases by extension

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

new evidence for cretaceous strike slip faulting in the united states cordillera and implications for terrane displacement deformation patterns and plutonism early to mid cretaceous dextral strike slip faulting involving up to 400 km displacement has been documented in the southern sierra nevada of california in western idaho the salmon river suture juxtaposes unrelated oceanic and continen tal rocks along a major near vertical shear zone that was active in the early to mid cretaceous these two structural zones have been treated as separate entities however new data indicate that they likely connect via a structural discontinuity in northwest nevada and southeast oregon herein called the western nevada shear zone wns current structures within the wns record dip slip contractional deformation however regional relations structural and stratigraphic indicate a mismatch in the early mesozoic geology of rocks across the wns that is best explained by dextral strike slip displacement of several 100 km these relations argue that an early strike slip structure along the wns was reactivated and obscured during later regional shortening deformation most likely related to the cretaceous sevier orogeny the wns is along strike and broadly coeval with the salmon river suture zone and strike slip boundaries in the sierra nevada we hypothesize that all these features are related elements of a major structural boundary in the western united states cordil lera this boundary is interpreted to have accommodated significant dextral strike slip displacement of outboard arc terranes of the cordillera in the early cretaceous followed by reactivation as a localized contractional zone in the mid cretaceous during the sevier orogeny cretaceous batholiths of the united states cordillera closely parallel the boundary which we suggest formed a major crustal flaw that localized and facilitated emplacement of arc magmas

strain partitioning in an obliquely convergent orogen plutonism and synorogenic collapse coast mountains batholith british columbia canada 1 we describe the crustal structure of the coast mountains batholith between 54 and 55 n within the canadian cordillera with emphasis on emplacement of the 7 km thick kasiks sill complex ksc kinematic patterns that developed during emplacement of the ksc are the result of interactions between magma transport magma accumulation and regional deformation the sills were emplaced during nw directed normal shearing and flattening of country rocks that host the ksc a 2 km thick shallowly ne dipping mylonite zone cuts the eastern side of the ksc kinematic indicators within the mylonite zone record top to the east normal displacements structural analysis shows that mylonite formation occurred during subvertical shortening and east northeast subhorizontal extension u pb zircon age dates show that ene directed normal shearing along the eastern side of the ksc and wnw directed normal shearing within the ksc occurred contemporaneously between 54 and 51 ma indicating strong strain partitioning between the mylonite and the ksc this pattern of strain partitioning is interpreted to have been driven by return flow of melt laden crust in response to tectonic denudation of the upper crust seismic profiling shows that many of these structures extend to mid and lower crustal depths comparison of our results with other regions within the canadian cordillera indicates that orogen scale right lateral strike slip faults deformed synchronously with wide spread magmatism and formation of extensional gneiss domes thus the crustal structure of the coast mountains batholith was the result of early tertiary batholith construction during dextral oblique convergence and synorogenic collapse

implications of magmatic epidote bearing plutons on crystal evolution in the accreted terranes of northwestern north america magmatic epidote in a plutonic rock signifies that the rock probably crystallized at pressures of at least 8 kbar 25 30 km depth in western north america undeformed late cretaceous plutons containing magmatic epidote occur within a mobile belt extending from northern california to southeastern alaska this belt has been interpreted as consisting of accreted terranes because the present crustal thickness within this belt is about 30 km the plutons imply a crustal thickness at the time of emplacement of 55 60 km comparable to that in the central andes the plutons further imply paleogeothermal gradients in the host country rock at the time of intrusion of not more than 20 c km and may be closer to 12 c km such cool thick crust suggests that subduction and piling of thrust sheets were probably the principal processes of crustal accretion in western north america the implications regarding the depth of crystallization of the plutons can be used to estimate the minimum rate of regional uplift it is on the order of 1 mm yr the rapid uplift of the mobile belt by and large did not affect the craton

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

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

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

intra arc strike slip fault exposed at batholithic levels in the southern sierra nevada california the kern canyon fault is a major north trending fault that is continuous for a distance of 140 km in the southern sierra nevada california previous geologic mapping and geochronological work along the northern third of the fault indicate that dextral offset occurred sometime after 80 ma and before 3 5 ma this offset was interpreted to be the result of cenozoic basin and range extension our new results from the central third of the fault kernville lake isabella region indicate an earlier right lateral movement history contemporaneous with emplacement of the largest plutons in the sierra nevada the older structure is termed the proto kern canyon fault zone the cenozoic fault trace is a narrow zone of brittle deformation whereas the cretaceous fault zone is a broad zone of ductile deformation u pb zircon geochronology on plutonic and metavolcanic rocks involved in the ductile deformation as well as a pluton that postdates ductile deformation demonstrate that the proto kern canyon fault zone was active at 85 ma and may have begun to move as early as 105 ma longitudinal strike slip faults are common in modern magmatic arcs where convergence is oblique the proto kern canyon fault zone may have originated in response to a moderate northward component in subduction of the farallon plate or perhaps a strong northward component for the kula plate

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

a petrogenetic grid for pelitic schists in the system sio 2 al 2 o 3 feo mgo k 2 o h 2 o a quantitative petrogenetic grid for pelitic schists in the system kfmash that includes the phases garnet chlorite biotite chloritoid cordierite staurolite talc kyanite andalusite sillimanite and pyrophyllite with quartz h2o and muscovite or k feldspar in excess is presented the grid is based on thermodynamic data of berman et al 1985 and berman 1988 for endmember kfash and kmash equilibria and natural fe mg partitioning for the kfmash system calculation of p t slopes and the change in fe fe mg along reactions in the kfmash system were made using the gibbs method in addition the effect on the grid of mno and cao is evaluated quantitatively the resulting grid is consistent with typical buchan and barrovian parageneses at medium to high grades at low grades the grid predicts an extensive stability field for the paragenesis chloritoid biotite which arises because of the unusual facing of the reaction chloritoid biotite quartz h2o garnet chlorite muscovite which proceeds to the right with increasing t in the kfmash system however the reaction proceeds to the left with increasing t in the mnkfash system so the assemblage chloritoid biotite is restricted to bulk compositions with high fe fe mg mn typical metapelites will therefore contain garnet chlorite at low grades rather than chloritoid biotite

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

wide angle seismic imaging across accreted terranes southeastern alaska and western british columbia abstract this study addresses the question of crustal moho and uppermost mantle structure across an accreted terrane continental arc and fold and thrust belt in southeastern alaska and western british columbia the 186 km long portland canal line of the accrete wide angle seismic dataset across the coast mountains is analyzed using a combination of travel time analysis techniques including delay time tomography turning ray tomography reciprocal time analysis intercept time inversion and forward ray tracing additional constraints on the model are obtained from a coincident vertical incidence seismic section and from correlation with the mapping of geologic structures plunging into the seismic section the study reveals moderately high crustal velocities low velocity gradient in the middle crust and decreasing average crustal velocity and a north deepening moho termination of crustal reflectivity across a vertical zone the coast shear zone csz indicates that the csz is most likely a strike slip fault associated with a transpressive tectonic regime a mid cretaceous thrust system mapped on the surface to the west of the csz is imaged by several groups of mid to lower crustal reflectors extending close to the moho indicating it was a thick skinned thrust system ne dipping fabric imaged within the coast mountains batholith cmb is associated with a ductile deformation during early eocene crustal extension the crustal section under the cmb which has an average velocity of 6 55 km s and shallower than average crustal thickness of 31 km can be considered as corresponding to the lower two thirds of an average crustal section which has been inflated by intrusions of high velocity tonalite to gabbro sills

new geologic mapping and shrimp u pb zircon data in the peninsular ranges batholith baja california mexico evidence for a suture geologic and shrimp u pb zircon studies in the sierra san pedro martir area of northern baja california mexico suggest that the western and eastern parts of the peninsular ranges batholith originated as separate arcs they are now juxtaposed along a well exposed ductile thrust main martir thrust marked by an age discontinuity of at least 10 to 15 m y and sharp changes in rock type metamorphic grade and deformation history the east dipping thrust and adjacent highly deformed rocks were stitched by 108 97 ma plutons that were generated in a region of crustal thickening formed during juxtaposition of the two arcs lack of chemical isotopic and geologic evidence for continentally derived rocks in the western arc may preclude an origin by rifting of the continental margin or development of the arc on a prism of continentally derived sediments adjacent to the continental margin instead the western arc may have originated as an island arc above a subduction zone outboard of north america if so a second concurrent subduction zone along the north american margin must have driven convergence and suturing of the two arcs and the main martir thrust may mark a nonterminal suture within a wider convergence related deformation zone that formed ca 115 108 ma

burial rates during prograde metamorphism of an ultra high pressure terrane an example from lago di cignana western alps italy abstract estimation of burial rates and duration of prograde metamorphism of ultra high pressure uhp rocks t 590 630 c p 2 7 2 9 gpa of the zermatt saas ophiolite from lago di cignana italy may be made through combined lu hf and sm nd garnet geochronology in conjunction with petrologic estimates of the prograde p t path we report a lu hf garnet omphacite whole rock isochron age of 48 8 2 1 ma from the uhp locality at lago di cignana which stands in contrast to the sm nd age of 40 6 2 6 ma amato et al earth planet sci lett 171 1999 425 438 obtained from the same sample and mineral material the sm nd and lu hf ages as well as other ages determined on metamorphic garnet zircon and white mica amato et al earth planet sci lett 171 1999 425 438 mayer et al eur union geosci 10 1999 abstr 809 rubatto et al contrib mineral petrol 132 1998 269 287 dal piaz et al int j earth sci 90 2001 668 684 from lago di cignana and elsewhere in the zermatt saas ophiolite lie within a range of 50 38 ma which we interpret to encompass the duration of prograde metamorphism and possibly the duration of garnet growth the difference in measured sm nd and lu hf ages from cignana can be accounted for by expected core to rim variations in lu hf sm and nd the measured yttrium content in garnet which may be a proxy for lu is highest in garnet core and lowest in the mineral rim generally following a profile that is predicted by rayleigh fractionation preferential enrichment of lu in the core produces a lu hf age that is weighted toward the older garnet core sm nd ages as predicted by rayleigh fractionation of sm and nd during garnet growth however reflect later grown garnet as compared to lu hf ages the difference in sm nd and lu hf ages from a single sample should therefore be a minimum estimate for the duration of garnet growth and prograde metamorphism so long as sm nd and lu hf blocking temperatures were not exceeded for a long period of time based on the 12 myr duration of prograde garnet growth estimated in this study burial rates for rocks at lago di cignana were on the order of 0 23 0 47 cm yr these values correlate with continuous shortening rates of 0 4 1 4 cm yr between the european plate and the african adriatic promontory between 50 and 38 ma which is on the order of that calculated for plate velocities from plate reconstructions suggesting that the zermatt saas ophiolite may have remained well coupled to the down going slab to uhp conditions

sm nd garnet dating of polyphase metamorphism northern coast mountains south eastern alaska usa sm nd ages of garnet from the northern coast mountains of south eastern alaska usa constrain the timing of thermal events in polyphase metamorphic rocks of the western metamorphic belt and provide new data on the spatial extent of cretaceous regional metamorphism bulk garnet whole rock sm nd ages for a sillimanite zone amphibolite taku inlet and a biotite zone metapelite tracy arm are 77 17 ma and 59 12 ma respectively garnet core whole rock 80 9 ma core matrix 84 9 ma rim whole rock 59 4 ma and rim matrix 62 4 ma ages were obtained from a sample collected 200 m west of a palaeocene coast plutonic metamorphic complex sill like pluton that separates medium grade metamorphic rocks from high grade metamorphic rocks and voluminous tertiary plutons in the core of the orogen the garnet core ages of c 80 ma indicate that the regional metamorphic grade reached garnet zone prior to the intrusion of the plutons and high grade metamorphism of rocks to the east similar ages for the younger plutons the youngest garnets and the rim of a multistage garnet c 59 ma indicate a later episode of contact metamorphic garnet growth documentation of pre 71 ma garnet zone metamorphism along the western edge of the coast plutonic metamorphic complex confirms that albian to late cretaceous metamorphism associated with crustal thickening affected this part of the orogen the similarity of garnet sm nd ages to independent age estimates for metamorphic events confirms that this technique provides useful estimates for the timing of late cretaceous to tertiary thermal events the c 20 myr difference between garnet core and rim ages suggests that the sm nd isotope systematics of a single garnet grain can be used for distinguishing between multiple metamorphic events

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

models of garnet differential geochronology abstract rayleigh distillation models are developed to describe theoretical growth zoning of lu hf rb sr sm and nd in typical garnet crystals from metapelites and metabasites effects of diffusion limited transport within the matrix and intracrystalline diffusion are also considered qualitatively theoretical zoning profiles show strong depletions of lu in garnet rims compared to cores but virtually invariant hf rb sr sm and nd profiles generally consistent with natural profiles for lu and hf and previously published models theoretical isochron diagrams for lu hf exhibit distinctive arcuate distributions and high mswds consistent both with himalayan data and with expectations that garnet growth durations exceed chronologic resolution by as much as an order of magnitude predicted isochron diagrams for sm nd and rb sr exhibit vertical arrays for garnet and high mswds that are generally lower than for lu hf in metapelites inherent chronologic resolution for bulk separates is best for lu hf in metapelites and rb sr but analytical considerations favor rb sr or sm nd for chronologic zoning studies diffusion limited transport in the rock matrix strongly influences zoning profiles but does not change the main trends on isochron diagrams intracrystalline diffusion will initially rotate lu hf isochrons to steeper slopes giving older apparent ages the natural himalayan data indicate growth of garnet in one rock from the greater himalayan sequence at 34 ma consistent with previously measured monazite ages from the same rock data from another himalayan rock suggest polymetamorphism that includes a paleozoic component

rare earth diffusion kinetics in garnet experimental studies and applications abstract we determined the diffusion coefficient of sm in almandine garnet as function of temperature at 1 bar and fo 2 corresponding to that of wustite iron buffer and to a limited extent that of a few other selected rare earth elements in almandine and pyrope garnets both garnets were demonstrated to have metastably survived the diffusion annealing at conditions beyond their stability fields the experimental diffusion profiles were analyzed by secondary ion mass spectrometry and in addition by rutherford back scattering spectroscopy for two samples transmission electron microscopic study of an almandine crystal that was diffusion annealed did not reveal any near surface fast diffusion path using reasonable approximations and theoretical analysis of vacancy diffusion the experimental data were used to develop an expression of rare earth element ree diffusion coefficient in garnet as a function of temperature pressure fo 2 ionic radius and matrix composition calculation of the closure temperature for the sm nd decay system in almandine garnet in a metamorphic terrain shows very good agreement with that constrained independently modeling of the ree evolution in melt and residual garnet suggests that for dry melting condition the ree pattern in the melt should commonly conform closely to that expected for equilibrium melting however for much lower solidus temperatures that would prevail in the presence of a h 2 o co 2 fluid the concentration of light ree in the melt could be significantly lower than that under equilibrium melting condition a reported core and rim differences in the ree content of a garnet crystal in a mantle xenolith in kimberlite have been reproduced by assuming that the ree zoning was a consequence of entrapment in a magma derived from an external source for 32 000 yr before the eruption

exhumation and tilting of the western metamorphic belt of the coast orogen in southern southeastern alaska the uplift history of the western metamorphic belt in southern southeastern alaska can be traced using emplacement depths and cooling ages of mid cretaceous plutons which intruded the metamorphic belt during the period 101 ma to 89 ma during the period 101 ma to 94 ma the rocks of the metamorphic belt now at the surface were buried to a depth of approximately 30 km pressure data indicate that by 89 ma the western portion of the metamorphic belt was at a shallower crustal level 23 km within the study area while the eastern side remained at a depth of approximately 30 km cooling curves constructed from minerals in mid cretaceous plutonic rocks indicate that the western side of the metamorphic belt cooled earlier than the eastern side an observation consistent with the inferred uplift history southwestward transport of metamorphic belt rocks within the hanging walls of thrust and high angle faults may have provided one mechanism for uplift during mid to late cretaceous time the eastern portion of the western metamorphic belt cooled rapidly at 55 ma probably in response to a period of rapid uplift the coast shear zone an early tertiary high angle fault with west side up displacement bounds the east side of the metamorphic belt we suggest tertiary uplift occurred by tilting of the metamorphic belt due to movement along this fault

lu hf apatite geochronology of mafic cumulates an example from a fe ti mineralization at smalands taberg southern sweden abstract we have employed lu hf apatite chronology to determine the age of a magnetite rich melatroctolite in southern sweden known as the smalands taberg fe ti ore deposit the melatroctolite is poor in incompatible elements and minerals forming at more advanced stages of differentiation which precludes the use of minerals baddeleyite zircon titanite or apatite commonly used for dating the emplacement of igneous rocks enclaves of leucogabbro in the melatroctolite have ree patterns parallel with the host melatroctolite and identical initial hf and nd isotope compositions these characteristics are conclusive evidence for a common parental magma the leucogabbro is a slightly more evolved facies that underwent limited fractionation of olivine and titanomagnetite the two major mineral phases in the melatroctolite apatite and plagioclase separated from the leucogabbro plus a whole rock sample define a lu hf isochron with a slope corresponding to an age of 1204 3 1 8 ma this result is close to the lower age boundary previously reported for this magmatic event comprising dolerites syenites and granites the lu hf apatite chronometer merits attention as a new tool in generating accurate and precise ages for igneous silica undersaturated rocks that may be difficult to date with traditional techniques

the isotopic composition of yb and the precise and accurate determination of lu concentrations and lu hf ratios by isotope dilution using mc icpms 1 we report yb isotopic data for the purpose of accurately correcting measurements of lu and lu hf ratios during multicollector inductively coupled plasma mass spectrometer mc icpms analysis the yb isotopic ratios agree with recent results of chu et al 2002 and amelin and davis 2004 when corrected for mass fractionation to the same reference values protocols for lu measurement involving simultaneous subtraction of yb interference and application of a fractionation correction based on yb are described it is shown that lu concentrations and lu hf ratios determined by mc icpms using this methodology are more precise and potentially more accurate than results based on analysis of lu by thermal ionization mass spectrometry tims a number of mc icpms lu hf isotopic analyses of standard rocks and other geological samples previously analyzed by tims are presented these document reproducibility of lu hf within 0 2 where most discrepancies can be assigned to the unknown lu mass fractionation during tims analysis

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

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

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