evidence of early cretaceous collisional style orogenesis in northern fiordland new zealand and its effects on the evolution of the lower crust structural metamorphic and kinematic data from a well exposed section of lower crustal rocks in northern fiordland new zealand reveal a history of intense contractional deformation and high p metamorphism at the roots of a convergent orogen high p 14 kbars granulite facies garnet clinopyroxene bearing reaction zones occur adjacent to anorthositic veins within gabbroic and dioritic gneiss these veins and reaction zones were variably deformed by two phases of high p granulite facies deformation quantitative kinematic analyses conducted using systems of rotated veins and reaction zones indicate that the first phase produced steeply dipping shear zones within a sinistral pure shear dominated flow regime wk 0 69 this deformation occurred at conditions of p 14 0 1 3 kbars and t 676 34 c and resulted in subhorizontal arc parallel ne sw stretching and up to 60 subhorizontal shortening in high strain zones of the lower crust at depths 45 km the second phase of deformation occurred at p 14 1 1 2 kbars and t 674 36 c and produced vertically stacked gently dipping ductile thrust faults that accommodated arc normal nw directed displacement these features reflect major tectonic thickening of the crust oblique convergence and high p metamorphism during the collision of the roots of a convergent orogen represented by plutons of the median tectonic zone in eastern fiordland with the paleo pacific margin of gondwana represented by western fiordland distinctive kinematic styles suggest that this collision resulted in a partitioning of the arc parallel ne sw and arc normal nw se components of oblique convergence onto sinistral strike slip and ductile thrust faults respectively at lower crustal levels

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

thermomechanical evolution of the crust during convergence and deep crustal pluton emplacement in the western province of fiordland new zealand 1 fiordland new zealand contains exposures of high pressure p 12 14 kbar granulite facies rocks that form one of earth s largest exposed lower crustal roots of an early cretaceous magmatic arc these exposures allowed us to examine the mechanisms and processes that controlled crustal thickening and large vertical displacements at the deepest levels of a deforming arc system we present structural and metamorphic data that show how the root of this arc was tectonically thickened by imbricate granulite facies thrust zones during and after the emplacement of sheeted plutons into the middle and lower crust the imbricate thrust zones form part of a well exposed 12 km wide two sided fold thrust belt that preferentially developed in crust that was thermally softened by magmatism changes in metamorphic mineral assemblages and microstructural data from the contact aureole of a composite arc batholith called the western fiordland orthogneiss record thrust and pluton emplacement conditions of p 7 9 kbar paleodepths of 25 30 km these data show that thrust imbrication and tectonic loading at 30 km depth is a viable mechanism of large vertical displacements and crustal thickening at the deepest levels of magmatic arcs this mechanism produced a characteristic up pressure metamorphic history that is similar to that observed in many other large magmatic belts worldwide

three dimensional model for tides and surges with vertical eddy viscosity prescribed in two layers ii irish sea with bed friction layer summary using the theory of part i of this work a three dimensional numerical tidal model of the irish sea is formulated with eddy viscosity defined in two layers through the vertical in the upper layer the viscosity is uniform through the depth in the lower layer it reduces linearly with increasing depth to a small value at the sea bed where a no slip condition is imposed after experiments with varying frictional levels the model is verified against observations of tidal elevation throughout the irish sea and against observations of current at a position in liverpool bay to a first approximation the well known square law of bottom friction involving depth mean current is confirmed but only when the bottom roughness length is particularly related to the thickness of the lower layer

geomorphically driven late cenozoic rock uplift in the sierra nevada california geologists have long accepted that the sierra nevada california experienced significant late cenozoic tectonically induced uplift a flexural isostatic model presented here shows however that a large fraction of the primary evidence for uplift could be generated by the lithospheric response to coupled erosion of the sierra nevada and deposition in the adjacent central valley and therefore requires less tectonic forcing than previously believed the sum of range wide erosion and the resultant isostatic rock uplift would have lowered sierra mean elevation by 200 to 1000 meters since 10 million years ago and could also have increased summit elevations during the current period of relief production

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

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

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

role of plate kinematics and plate slip vector partitioning in continental magmatic arcs evidence from the cordillera blanca peru new structural and geochronological data from the cordillera blanca batholith in the peruvian andes coupled with nazca south american plate slip vector data indicate that oblique convergence and associated strike slip partitioning strongly influenced continental magmatic arc evolution both the strain field and mode of magmatism plutonism vs volcanism in the late miocene peruvian andes were controlled by the degree to which the arc parallel component of the plate slip vector was partitioned into the arc strong strike slip partitioning at ca 8 ma produced arc parallel sinistral shear strike slip intercordilleran basins and east west oriented tension fractures that facilitated emplacement of the cordillera blanca batholith ca 8 2 0 2 ma periods during which the strike slip component was not partitioned into the arc ca 10 and ca 7 ma were associated with roughly arc normal contraction and ignimbrite volcanism the data thus support the contention that contraction within continental magmatic arcs favors volcanism whereas transcurrent shear favors plutonism the tie between oblique convergence and batholith emplacement in late miocene peruvian andes provides a modern analogue for batholiths emplaced as the result of transcurrent shear in ancient arcs

a model for garnet and plagioclase growth in pelitic schists implications for thermobarometry and p t path determinations numerical models of the progressive evolution of pelitic schists in the ncmnkfmash system with the assemblage garnet biotite chlorite staurolite plagioclase muscovite quartz h2o are presented with the goal of predicting compositional changes in garnet and plagioclase along different p t paths the numerical models support several conclusions that should prove useful for interpreting the p t paths of natural parageneses i garnet may grow along p t vectors ranging from heating with decompression to cooling with compression p t paths deduced from garnet zoning that are inconsistent with these growth vectors are self contradictory ii there is a systematic relation between garnet and plagioclase composition and growth such that for most p t paths garnet growth requires plagioclase consumption furthermore mass balance in a closed system requires that as plagioclase is consumed the remaining plagioclase becomes increasingly albitic inclusions of plagioclase in the core of garnet should be more anorthitic than those near the rim and zoned matrix plagioclase should have rims that are more albitic than the cores complex plagioclase textures may arise from the local variability of growth and precipitation kinetics iii a decrease of fe fe mg in a garnet zoning profile is a reliable indicator of increasing temperature for the assemblage modelled however there is no single reliable p monitor and inferences about p can only be made by considering plagioclase and garnet together iv consumption of garnet during the production of staurolite removes material from the outer shell of a garnet and may make recovery of peak metamorphic compositions and p t conditions impossible low peak temperatures typically recorded by staurolite bearing assemblages may reflect this phenomenon v diffusional homogenization of garnet affects the computed p t path and results in a clockwise rotation of the computed p t vector relative to the true p t path

on the importance of time varying eddy viscosity in generating higher tidal harmonics by using a spectral modal expansion in the vertical a method is developed for determining tidal current profiles subject to either a slip or no slip bottom boundary condition an arbitrary vertical profile of eddy viscosity can be used in the calculation the magnitude of which can be related to the flow field the nature of the modal solution is such that the high shear bottom boundary layer can be accurately resolved using a few modes of order five compared with the order of 45 grid boxes on a logarithmic transformation which would be required in a finite difference approach calculations show the importance of a time varying eddy viscosity in generating higher harmonics when a no slip condition is applied at the seabed by relating the current one meter above the bed to the bed stress a value of friction coefficient c100 in reasonable agreement with observations was obtained from these no slip calculations however the value of friction coefficient was strongly dependent upon the height above the bed at which the current was calculated with friction coefficient increasing rapidly in the near bed region current profiles on the continental shelf determined using the modal method developed in the paper were in good agreement with observations with computed profiles in the celtic sea giving the observed midwater maximum

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

high temperature arc parallel normal faulting and transtension at the roots of an obliquely convergent orogen structural and kinematic data from northern coastal british columbia 54 5 n document intra arc deformation patterns at mid crustal levels during and after emplacement of the coast mountains batholith major arc parallel displacements occurred along high temperature 700 c ductile normal faults and steep sinistral transtensional shear zones within an obliquely convergent margin strain patterns and pluton emplacement were controlled by 1 structural anisotropies in host rocks that predate batholith emplacement and 2 kinematic compatibility requirements created by simultaneous motion on curved pluton bounding shear zones these controls superseded a partitioning of the arc parallel and arc normal components of oblique plate convergence onto strike slip and thrust faults respectively

records of the evolution of the himalayan orogen from in situ th pb ion microprobe dating of monazite eastern nepal and western garhwal abstract in situ th pb monazite ages from rocks collected along two transects the dudh kosi everest eastern nepal and the bhagirathi river garhwal himalaya india perpendicular to the main central thrust mct suggest a striking continuity of tectonic events across the himalaya the youngest age reported in this study 5 9 0 2 ma mswd 0 4 from matrix monazite grains collected beneath the mct in the garhwal region is consistent with several age data from rocks at similar structural levels in central nepal providing support for widespread late miocene mct activity the lateral parallelism of orogenic events is further manifested by the 20 7 0 1 ma age of a high himalayan leucogranite from an injection complex that outcrops along the dudh kosi everest transect resembling ages of these bodies reported elsewhere the youngest monazite grain analyzed along the dudh kosi everest transect is 10 3 0 8 ma the absence of 7 3 ma monazite ages in eastern nepal may reflect a different nappe structure which obscures the reactivated ramp equivalent exposed in the garhwal and central nepal garnets from the mct hanging wall greater himalayan crystallines and footwall lesser himalaya display different major element zoning and the patterns are useful for constraining the location of the thrust system that separates the two lithologies pressure temperature paths for two upper lesser himalayan garnets that contain monazite inclusions indicate the utility of an in situ methodology to constrain the metamorphic evolution of the shear zone along the dudh kosi everest transect upper lesser himalayan monazite grains from three rocks record a clear signal at 14 5 0 1 ma mswd 8 4 and the 23 ma age that characterizes the hanging wall is notably absent monazites collected within a large scale greater himalayan crystallines fold yield the 14 ma age consistent with the structure forming due to mct related compression paleo mesoproterozoic 1407 35 ma matrix monazite grains are found within an augen gneiss unit located beneath the mct whereas cambro ordovician 436 8 548 17 ma inclusions are preserved within garnets of the greater himalayan crystallines the presence of 45 2 2 1 ma grains from lower structural levels of the greater himalayan crystallines indicates the unit realized conditions conducive for monazite growth during the eocene

tidal computations for morecambe bay summary a numerical finite difference technique for computing tides in areas of shoaling water has been established and used to calculate the m2 tide in morecambe bay an essential feature of the method is the representation of drying banks which appear as the water level falls on the ebb and which are re submerged on the flood as the level rises again since the non linear advective acceleration should be important in shallow water schemes for incorporating these terms are examined and the influence of advection on the tide assessed in a series of tests the open sea tides applied to the outer boundary of the numerical bay model were taken first from existing tidal charts then from calculations with a larger scale encompassing model and finally from observations of current in each case the computed tidal elevations within the bay turned out to be in satisfactory agreement with those obtained from observations however it is significant that in the first case inaccuracy of the input tidal distribution deduced from charts gives rise to erroneous currents near the boundary

geometric test for late cretaceous paleogene intracontinental transform faulting in the canadian cordillera the tintina trench northern rocky mountain trench tt nrmt fault zone and the fraser river straight creek fr sc fault zone are separate en echelon concentric small circle fault segments of a composite intracontinental transform fault zone more than 2500 km long that cuts diagonally across the canadian cordillera from the outboard part of a tectonic collage of accreted foreign terranes in the south into the north american preaccretionary continental margin in the north most of the 450 km of right hand slip on the tt nrmt fault zone was transformed southward during the late cretaceous and paleocene into oblique convergence in the southern canadian rockies the remainder probably comprising less than 100 km was transformed southwestward during early and middle eocene time via a zone of distributed shear and east west crustal stretching into right hand slip on the en echelon fr sc fault zone these interpretations based on regional systematic mapping of geologic structures are in conflict with interpretations of paleomagnetic measurements that call for more than 1000 km of post mid cretaceous right hand displacement along the general locus of the tt nrmt fault zone involving foreign terranes and the parts of north america to which they had been accreted this paradox must be resolved

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

operator bias characterizing stream substrates using wolman pebble counts with a standard measurement template this study characterizes the variability associated with using a standard measurement template to perform wolman pebble counts of coarse stream substrates four 10 count and two 12 count replicate experiments were conducted by five operators at three sample sites within gravel bed streams using a gravel template results show significant differences between different operators sampling the same location as observed in previous studies conducted without a standard sampling template as well as significant variability within each operator s replicate samples variability was highest in clast estimates larger than d50 suggesting that these metrics cannot be consistently derived from wolman pebble count samples variability was also highest in the low order study site suggesting that the sedimentary characteristics of low order streams increase the chance for operator bias in light of our findings we suggest that traditional 100 particle wolman counts be utilized with more caution than is the current norm

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

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

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

post 10 ma uplift and exhumation of the northern coast mountains british columbia apatite u th he ages reveal three distinct periods in the exhumation history of the northern coast mountains 54 n a well developed helium partial retention zone indicates little or no exhumation between ca 30 and 10 ma beginning at 10 ma and extending to at least 4 ma the range underwent steady but slow exhumation of 0 22 mm yr after which the exhumation rate increased by at least 70 an 85 km long he age traverse across the orogen at sea level shows that vertical offsets on post 10 ma faults are minor furthermore the sea level he ages 14 2 8 ma inversely correlate with local mean elevation along the traverse these data suggest that the coast mountains first appeared as a significant topographic feature only within the past few million years this history is not consistent with suggestions that uplift of the range resulted from formation of the queen charlotte basin in early middle miocene time we speculate that intense erosion by alpine and continental glaciation after 2 5 ma triggered isostatic uplift and creation of the modern topography of the coast mountains

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

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

intracrustal subduction and gravity currents in the deep crust sm nd ar ar and thermobarometric constraints from the skagit gneiss complex washington isotopic and thermobarometric pressure temperature p t data on a gneissic quartz diorite combined with previous u pb ages and p t data on wall rock metapelites define a precise p t time series for a portion of the skagit gneiss complex a deep crustal crystalline complex within a zone of intracontinental collision concordant u pb and sm nd ages on zircon indicate crystallization at 68 ma which preceded metamorphism of nearby pelitic wall rocks garnet core assemblages and the pluton plagioclase hornblende coronas on garnet at 800 900 mpa 700 800 c following nearly isothermal decompression and sillimanite cordierite metamorphism of wall rocks garnet rims plus matrix at 300 500 mpa 650 700 c a sm nd mineral isochron including two garnet separates records an initial phase of cooling of the complex at 60 ma slow cooling 10 c m y extends from approximately 60 ma to 50 ma hornblende and biotite ar ar ages of 47 ma and 45 ma respectively define a second unroofing event cooling rates during this period were approximately 100 c m y late cretaceous deep burial followed by two distinct unroofing episodes is also observed in some cordilleran metamorphic core complexes and may suggest a common origin we propose that regional shortening in the cordilleran interior is accommodated by a process of intracrustal subduction i e the upper crust is forced downward into a crustal asthenosphere resulting in a cold buoyant root heating and weakening of the root cause the formation of gravity currents effective viscosity being in the range of 10 16 10 19 pa s spreading outward below nonfluid upper crust spreading of the gravity current which does not necessarily contribute to crustal thinning is driven by the small density contrast between the root and its mid crustal surroundings hence unroofing is neither a response to overthick crust nor controlled by the orientation of principal stress axes in the upper crust and upper mantle the root then cools slowly until final unroofing to near surface levels which may occur via either extension or shortening with consequent erosion

u pb geochronology of two pre tertiary plutons in the coast mountains batholith near ketchikan southeastern alaska u pb geochronologic studies indicate that the central part of the coast mountains batholith in the ketchikan area contains plutons of triassic and possibly mid paleozoic age in addition to the previously recognized early tertiary plutons zircons from a foliated granodiorite in burroughs bay yield an upper concordia intercept of 336 34 ma which is tentatively interpreted as the crystallization age and a lower intercept of 79 7 ma which may record extreme pb loss and growth of metamorphic zircon during amphibolite to granulite facies metamorphism zircons from a tonalite in boca de quadra yield an upper intercept crystallization age of 215 15 ma and a lower intercept pb loss age of 50 20 ma the presence of these triassic and possibly mid paleozoic plutons supports previous interpretations that pre tertiary rocks in this part of the batholith may belong to the yukon tanana terrane

tempo of burial and exhumation within the deep roots of a magmatic arc fiordland new zealand the u pb thermochronology of titanite apatite and rutile from a crustal profile through a mesozoic magmatic arc in fiordland new zealand is used to constrain the timing and duration of significant vertical movements during arc construction and evolution titanite data from deep crustal 12 13 kbar basement and cover rocks of central fiordland imply that contractional arc thickening 25 km occurred by 111 1 113 4 ma within a few million years of a major phase of mid to deep crustal magmatism this finding suggests that this cycle of magmatism arc thickening and high grade metamorphism occurred in 6 2 m y in contrast to rapid burial significant unroofing of the central fiordland gran ulites was more protracted requiring an additional 40 45 m y these new data are con sistent with continued residence of the granulites in thickened arc crust for 15 20 m y with subsequent major unroofing recorded by rutile cooling to 450 8c by ca 70 ma such temporal constraints are essential for comprehensive models of the growth modifi cation and unroofing of magmatic arc systems

metamorphism of the deepest exposed arc rocks in the cretaceous to paleogene cascades belt washington evidence for large scale vertical motion in a continental arc the swakane gneiss and the overlying napeequa complex in the north cascade range washington were metamorphosed and deformed during development of a cretaceous paleogene continental arc and are among the structurally deepest exposed rocks within the cordilleran arcs of north america peak metamorphic conditions in both the swakane gneiss and napeequa complex were c 640 750 c 9 12 kbar clockwise paths and widespread evidence for high p metamorphism in meta supracrustal rocks burial to 40 km document major vertical tectonic motion during arc construction and unroofing these and other moderately high pressure rocks in the north cascades coast mountains experienced a dramatically different tectonometamorphic history than metamorphic rocks within other cordilleran arcs the exhumed arc complexes of the sierra nevada and peninsular ranges are dominated by relatively low p metamorphic and plutonic rocks typically 6 kbar there is no evidence that the northern cordillera was thickened to a greater degree than these other belts suggesting that the greater magnitude of vertical motion in the cascades may have been related to exhumation mechanisms eocene extension in the northern cordillera vs erosional unroofing in the central and southern cordillera

paleogene history of the kula plate offshore evidence and onshore implications paleocene to middle eocene magnetic anomalies were mapped over oceanic crust that accreted at the kula pacific spreading center and is now obliquely entering the western aleutian trench between 179 e and 168 e the strike of anomalies and the pattern of abyssal hills and fracture zones changed abruptly during 56 55 ma when north south spreading veered to northwest southeast 310 130 kula pacific spreading ceased in 43 ma a 75 km long section of the fossil kula rift axis has avoided subduction although it now intersects the trench axis almost orthogonally near 171 5 e a narrow remnant of the former kula plate northwest of this fossil spreading center is bounded by a fossil kula pacific transform with a high transverse ridge alongside a sediment filled transform valley anomalies on this remnant show that eocene kula pacific spreading was highly asymmetric 2 1 the 56 55 ma change in kula plate rotation inferred from the change in spreading direction coincided with birth of the aleutian subduction zone and was probably a consequence of the resulting change in slab pull stresses on the oceanic lithosphere the change in direction of kula north american motion is a plausible explanation for the detachment of continental terranes from the pacific northwest and their migration around the gulf of alaska and for the early eocene demise of alaska range are volcanism the cessation of kula pacific spreading coincides with a major change in pacific plate rotation and the subsequent direction of convergence of the pacific plate with the aleutian arc was similar to the 55 43 ma direction of kula plate convergence

metapelite phase equilibria modeling in mnnckfmash the effect of variable al2o3 and mgo mgo feo on mineral stability a series of mno na2o cao k2o feo mgo al2o3 sio2 h2o mnnckfmash metapelite pseudosections highlights the dependence of predicted mineral assemblages on bulk rock al2o3 and mg mgo mgo feo t xal pseudosections portray the dependence of staurolite biotite and aluminum silicate on al2o3 content allowing the distinction between high al and low al pelite as commonly portrayed with kfmash modeling the mnnckfmash system also shows the effect of al2o3 on plagioclase and zoisite stability which cannot be done in the kfmash system comparison of mnnckfmash to kfmash pseudosections highlights the consequence of ignoring the important rock constituents mno na2o and cao when constructing pseudosections kfmash cannot model important phases such as plagioclase and zoisite and there are significant differences in predicted garnet biotite and chloritoid stability in the two different systems in particular kfmash does not model garnet stability appropriately at low pressures and temperatures because it cannot account for the stabilizing effect of mn the comparisons also show that the method of calculating a kfmash bulk rock composition equivalent to a real rock composition is problematic and has significant implications for the predicted pseudosection assemblage stability comparison of the mnnckfmash pseudosections to natural assemblages observed in the waterville fm maine indicates that the mnnckfmash system comes very close to modeling naturally developed mineral assemblages successfully the only major discrepancy between predicted and observed assemblages is the inability to predict the paragenesis staurolite andalusite using an average or natural waterville fm composition garnet thermobarometric results from the waterville fm are in poor agreement with pseudosection topology for an average waterville fm composition this suggests that if quantitative p t path information is to be derived through a combination of pseudosections and thermobarometry samples will have to be investigated on an individual basis in more detail than was done in this study

thermobarometric constraints on the structural evolution of the coast mountains batholith central southeastern alaska thermobarometric data from amphibolite facies metamorphic rocks west of the coast mountains batholith provide important constraints on the structural evolution of the mid cretaceous sumdum fanshaw

on extracting tidal current profiles from vertically integrated two dimensional hydrodynamic models a numerically accurate and computationally efficient method is developed for extracting tidal current profiles from vertically integrated two dimensional models the method can incorporate an arbitrary profile of eddy viscosity and determines tidal current profiles satisfying a no slip condition at the sea bed comparisons with observations and current profiles computed with full three dimensional models confirm the accuracy of the method bottom boundary layer thicknesses determined from these current profiles clearly show regions of the european continental shelf where the bottom boundary layer extends to the sea surface the boundary between these regions and other areas where the bottom layer is less than the water depth corresponds quite closely to the known position of tidal fronts

driving mechanisms for 40 km of exhumation during contraction and extension in a continental arc cascades core washington 1 in the nw north american cordillera the cascades core region of the coast plutonic complex underwent late cretaceous 96 ma to locally 73 ma sw ne contraction and crustal thickening followed by dextral transpression 73 to 55 ma then transtension 3 mm yr by local thrusting in regions undergoing crustal thickening in the central part of the core chelan block 40 km of exhumation occurred between 91 and 45 ma about half of which occurred during early contraction driven by thrusting and half during top to north arc oblique shear during reactivation of a midcrustal cretaceous thrust the dinkelman decollement the footwall of this thrust consists of the swakane biotite gneiss a cretaceous metaclastic assemblage with recorded pressures of 10 12 kbar no arc related magmatism and structures dominated by pervasive top to north shearing the hanging wall consists of the napeequa complex an oceanic assemblage with recorded pressures of 6 12 kbar voluminous arc related magmatism and complex structures indicating early top to wsw shearing younger top to north shearing and widespread folding in the napeequa top to north shearing started by 73 ma during melt present conditions at pressures 6 kbar top to north shearing in both hanging wall and footwall continued during exhumation 1 6 mm yr and cooling to greenschist facies conditions during which slip became increasingly localized eventually resulting in formation of pseudotachylite on discrete slip surfaces we suggest that arc oblique extension was driven by along arc heterogeneity in displacements erosion initially during transpression and underplating of continental sediments and later during transtension

magmatism and deformation southern revillagigedo island southeastern alaska the mid cretaceous cale alkaline lree enriched plutons on southern revillagigedo island southeastern alaska form two distinct groups 1 biotite epidote hornblende tonalite of the moth bay pluton and 2 biotite bearing hornblende absent leucotonalite higher total ree and lower na 2 o al 2 o 3 and sr distinguish the moth bay tonalite from the leucotonalite the plutons are surrounded by narrow contact metamorphic aureoles containing kyanite and staurolite pressure estimates for emplacement based on metamorphic conditions of contact rocks and from igneous amphibole compositions are 8 9 kbar detailed analyses of pluton host rock relationships indicate that pluton emplacement was synchronous with orogen deformation pre emplacement thrusting placed the metavolcanic and metasedimentary rocks of the taku terrane structurally above the younger metasedimentary rocks of the gravina belt the contact between these units was subsequently cut by the moth bay pluton the southern margin of the moth bay pluton has been transformed to blastomylonite contact metamorphic textures and undeformed dikes which cut the blastomylonite indicate that shearing did not outlast crystallization of the pluton a still later phase of deformation transposed the pluton related structures and generated a northeast dipping ductile thrust fault southern revillagigedo shear zone along the southern shore of revillagigedo island the shear zone and earlier structures all trend approximately northwest fission track age determinations and track length measurements in apatite were used to determine the cooling histories of the plutonic units fission track ages from the western part of the study area are 51 3 4 0 ma apatite 57 8 4 4 ma zircon and 69 8 4 2 ma sphene eastward toward the coast batholith fission track ages are 10 m y younger these younger ages may result from a tectonic thermal event associated with batholith emplacement this same event may have generated north trending structures that overprint older northwest trending structures on the east side of the study area

a doubly vergent fan structure in the peninsular ranges batholith transpression or local complex flow around a continental margin buttress 1 the jura cretaceous peninsular ranges batholith prb of southern and baja california contains a remarkable example of a crustal transition between juxtaposed oceanic and continental floored arcs jura cretaceous deformation in the prb is focused along this lithospheric discontinuity and defines a zone of contractional deformation that stretches at least 800 km along the batholith the western edge of this zone is associated with a dramatic eastward increase in maximum recorded metamorphic pressures from 2 to 6 kbar and temperatures from 650 c elsewhere in the north american cordillera similar lithospheric boundaries are thought to accommodate various amounts of orogen parallel displacements of outboard tectonic fragments to test this proposal in the prb we examined an unusually well preserved part of the transition zone in the southern sierra san pedro martir sspm here a spectacular 20 km wide doubly vergent fan structure which developed over a period 40 m y during both jurassic and cretaceous magmatism occurs along the transition zone this structure consists of moderately inward dipping outward vergent fold and thrust belts with widespread mylonitic fabrics that steepen toward the center where both mylonitic and magmatic fabrics occur in syntectonic vertically sheeted plutons mineral lineations change from steeply pitching on the sides of the fan to moderately pitching in the center a number of constraints on possible displacements rotation of units and duration of fabric formation limit the amount of orogen parallel displacement to a few kilometers across the fan structure in the sspm instead we propose that the fan structure is a local feature that results from a combination of tectonic wedging caused by a cusp of rigid pre mesozoic continental margin crust at this location and vise tectonics that arose from strong lateral rheological gradients resulting from extensive heating by the sheeted tonalite complex focused deformation along the lithospheric discontinuity and subsequent strain softening we suggest that these mechanical controls on deformation during orthogonal convergence may be common in other orogens where similar fan structures have been attributed to transpression emphasizing the need for caution in interpreting such features as zones of significant orogen parallel displacement

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

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

minimal climatic control on erosion rates in the sierra nevada california climate is widely thought to regulate erosion rates but the relationships among precipitation temperature and erosion rate have remained speculative because long term erosion rates have been difficult to measure we used cosmogenic nuclides to measure long term erosion rates at climatically diverse sites in the sierra nevada california spanning 20 180 cm yr in annual precipitation and 4 15 8c in mean annual temperature average erosion rates vary by only 2 5 fold across these sites and are not correlated with climate indicating that climate only weakly regulates nonglacial erosion rates in mountainous granitic terrain

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

melt enhanced deformation a major tectonic process convergent tectonics between continental crustal blocks result in deep burial and anatexis of supracrustal rocks anatectic and or mantle derived melts combine to form a melt weakened zone in the thickened lower crust the accumulation of melt eventually leads to crustal failure along melt lubricated shear zones rapid 1 mm yr vertical component movements of large crustal blocks result we refer to these crustal displacements as tectonic surges the melt lubricated shears are characterized by the close association of sheared country rocks with foliated or massive igneous sills and plutons rocks that formed at different crustal levels are juxtaposed across these shear zones one result of surges with large lateral component of movement is metamorphic inversion with high pressure and high temperature metamorphic rocks structurally over lower pressure and temperature assemblages large and rapid vertical surges may displace crust containing abundant melt and may result in high t p including granulite facies metamorphism and preservation of metamorphic textures caused by rapid decompression

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

long term glacial erosion of active mountain belts example of the chugach st elias range alaska an emerging paradigm that equates glaciers to buzz saws of exceptional erosional efficiency has been strengthened by short term 10 2 yr sediment yields from southern alaska new low temperature cooling ages from this area the glaciated chugach st elias mountains constrain long term 10 6 yr exhumation rates vertically averaged exhumation rates reach 3 mm yr but are an order of magnitude lower than rates based on short term sediment yields whereas these exhumation rates are not exceptional for oro genic belts denudation patterns are strongly correlated with the distribution of glaciers and erosion appears to keep pace with con vergence and uplift these findings imply a coupling between gla cially dominated erosion and tectonics

field geochemical and isotopic evidence for magma mixing and assimilation and fractional crystallization processes in the quottoon igneous complex northwestern british columbia and southeastern alaska the quartz dioritic quottoon igneous complex qic is a major paleogene 65 56 ma magmatic body in northwestern british columbia and southeastern alaska that was emplaced along the coast shear zon

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

simulation of storm surges using a three dimensional numerical model an application to the 1977 andhra cyclone a three dimensional numerical model is developed for the simulation of storm surges generated by tropical cyclones off the east coast of india experiments are performed using wind stress forcing data representative of the 1977 andhra cyclone and the results are compared with earlier simulations using a depth averaged model the three dimensional model incorporates a turbulence energy closure scheme and is fairly sophisticated in comparison with the depth averaged approach even with the apparently more refined treatment of the frictional processes there is no substantial difference between simulations performed with the two models

gneiss domes and orogeny many gneiss domes record positive feedback between decompression and partial melting of orogenic middle crust exhumed orogens are riddled with gneiss domes cored by migmatites that underwent dehydration melting during decompression the decreasing buoyancy associated with increasing melt fraction drives further decompression at near isothermal conditions as the partially molten crust rises diapirically this combination of processes may explain the generation and retention of large volumes of crustally derived melt recorded in many deep seated migmatite terranes and inferred for active orogens in exhumed orogens the signature of the rapid ascent of partially molten crust is a gneiss dome cored by migmatite granite the large volume of material involved in the vertical transfer of partially molten crust indicates that the formation of gneiss domes is an efficient mechanism for heat advection during orogenesis

the great tonalite sill emplacement into a contractional shear zone and implications for late cretaceous to early eocene tectonics in southeastern alaska and british columbia the late cretaceous to early tertiary great tonalite sill of southeast alaska and british columbia is a very long 1 000 km and thin 25 km orogen parallel composite batholith which lies between the insular superter rane including the alexander and wrangellia terranes and the intermontane superterrane including the stikine and cache creek terranes the batholith is delineated by many steep sheet like plutons which are dominated by northwest southeast striking concordant fabrics with steep lineations that formed during deformation in a country rock shear zone prior to the complete crystallization of the magmas deformation in this shear zone is dominated by northeast southwest directed contraction orthogonal to the orogenic strike associated with a component of northeast over southwest high angle shear the steep multiple dike like nature of the body and its emplacement during orogenic contraction imply that ascent and emplacement have been achieved by dike wedging mechanisms along the deep reaching shear zone the remarkable narrowness of the great tonalite sill is probably the result of melting at the base of a very localized zone of thickened crust produced by the associated narrow contractional shear zone extending along the orogen length such a shear zone of late cretaceous to early tertiary age lying along 800 km of the possible boundary between the insular and intermontane superterranes implies that it may represent the actual boundary between them if this hypothesis is correct it implies that the large scale tectonic regime during emplacement of the great tonalite sill was predominantly orthogonal and not obliquely dextral as has been indicated from paleomagnetic data

deformational history of an outlier of metasedimentary rocks coast plutonic complex british columbia canada the khutzeymateen assemblage records a portion of the polyphase deformation experienced by rocks within the core of the coast plutonic complex this series of deformational events probably took place during late cretaceous to early eocene regional orogenic activity the khutzeymateen assemblage is dominated by metamorphosed graywackes and volcaniclastic material the earliest recognizable deformation involves thrust faulting that juxtaposed rocks of the khutzeymateen assemblage and central gneiss complex the next deformational event produced isoclinal folds f1 a penetrative foliation s1 and a strong mineral lineation l1 both f1 and l1 have a 340 15 orientation peak metamorphism p 450 50 mpa t 650 50 c was synchronous with this isoclinal folding event f1 folding was followed by a brittle chevron folding event f2 with a 335 20 orientation there is a strong lithologic control on the development of f2 minor folds which are developed predominantly within regularly layered qua

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

influence of lithological heterogeneity mechanical anisotropy and magmatism on the rheology of an arc north cascades washington abstract many aspects of crustal dynamics are dependent on changes in rheology and strength with depth in the lithosphere several of the controlling factors for rheology are difficult to study experimentally particularly lithological heterogeneity mechanical anisotropy and magmatism and we focus on these in a study of the deformation patterns in a thick crustal section 5 to 40 km paleodepth through the cretaceous cascades core of the nw cordillera this magmatic arc consists of metamorphosed oceanic and arc terranes intruded by magmatic bodies ranging from heterogeneous brittle deformation marked by serpentinite melange characterizes the shallowest part of the crustal section and the remainder of the section is characterized by heterogeneous fold dominated ductile deformation early tight to isoclinal recumbent folds and associated axial planar fabrics are refolded by one or more cycles of nearly coaxial open to isoclinal upright to overturned folds layering played a mechanically active role during folding at all levels as indicated by cleavage refraction boudinaged layers and kinematic indicators that record fold related shear ductile deformation intensifies in the narrow structural aureoles of plutons and sw directed reverse shear was partitioned into some of the aureoles the poor strain memory of these magmatic bodies makes it difficult to determine if deformation was focused in the pluton magma chambers before they reached the solidus as commonly predicted all of the plutons have magmatic foliations that at least in part reflect regional strains and these foliations are strong in the deeper plutons the thinner sheets acted as competent bodies during folding and boudinage after they reached the solidus but generally did not cause marked strain gradients in their hosts a relative strength profile constructed for the cascades crustal section shows an overall decrease in strength with depth for the ductile part of the arc that fits idealized strength profiles however in more detail relative strengths are markedly variable units were able to accumulate large ductile strains but even small variations in the physical properties of interlayered rock types exerted a strong influence on deformation patterns throughout the mid to deep crustal part of the profile this profile thus emphasizes the complex vertical rheological stratification of arcs at the crustal to thin section scale and should be applicable to many other magmatic arcs

two stage exhumation of midcrustal arc rocks coast mountains british columbia 1 new geologic data from the central gneiss complex along douglas channel help delineate the burial and exhumation of the late cretaceous to eocene coast mountains magmatic arc arc plutonism was on going between 90 and 60 ma replaced by formation of dikes to 52 ma supracrustal rocks were buried to midcrustal levels by 90 ma and garnet and kyanite grew from 90 through 70 ma subsequent exhumation was nearly isothermal and took place in two stages the first occurred from 70 to 59 ma as the arc contracted obliquely and sillimanite replaced kyanite exhumation was slow 0 5 mm yr probably accomplished by erosion aided by coaxial crustal thinning exhumation rates about doubled in the second stage of exhumation after 59 ma and before 52 ma penetrative deformation ended prior to intrusion of the quottoon pluton at 59 ma and 6 5 kbar production of cordierite rims on garnet at 4 5 kbar and 700 c signaled the end of near isothermal decompression prior to 52 ma rapid cooling 100 c 106 years of the central gneiss complex followed cordierite growth temperatures dropped to 250 by 48 ma the increase in exhumation rate and the subsequent rapid cooling are attributed to excision of 6 km of crust on a detachment system on the northeastern side of the central gneiss complex comparison to other parts of the coast mountains arc the sierra nevada and fiordland new zealand shows that the amount and tempo of exhumation vary greatly within and between arcs suggesting that the processes accommodating exhumation vary significantly

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

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

batholith emplacement at mid crustal levels and its exhumation within an obliquely convergent margin abstract emplacement of the central part of the coast mountains batholith of northern coastal british columbia occurred within a regime characterized by oblique convergence between the farallon kula and north american plates we use new structural kinematic and u pb isotopic data to show that the locations geometry and mechanisms of pluton emplacement within this batholith were controlled by displacements within a network of normal faults and transtensional shear zones these data also show that the most active period of pluton emplacement from 67 to 51 ma coincided with a change in style of deformation within the batholith prior to 67 ma plutons were emplaced within an arc dominated by regional scale contractional shear zones in contrast emplacement of 67 51 ma plutons occurred in an arc increasingly dominated by normal faults with arc parallel to oblique displacement and by sinistral transtensional shear zones we have identified and mapped the structure of three plutonic complexes composed of 67 to 51 ma plutons the khyex sill complex arden lake plutonic complex and quottoon plutonic complex shear zone controlled emplacement of plutons within the batholith accounts for the widely different orientations and structural features that characterize plutons within these three complexes during and after this latest cretaceous paleogene period of intense plutonic activity and accompanying deformation the deep roots of the batholith were rapidly unroofed by ductile normal faulting prior to 50 ma

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

40ar 39ar constraints on the emplacement uplift and cooling of the coast plutonic complex sill southeastern alaska a 700 km long and 5 to 15 km wide aggregation of sill like tertiary plutons separates low to medium grade greenschist to amphibolite facies metamorphic rocks to the west from medium to high grade upper amphibolite to granulite facies gneisses and granitic plutons to the east along the western boundary of the coast plutonic complex in southeastern alaska and british columbia 40ar 39ar analyses of hornblende biotite and plagioclase samples from a sea level traverse and a 1 to 2 km elevation traverse across one of the sills near holkham bay southeastern alaska yield dates that vary systematically with position samples collected on the eastern margin of the sill are 3 to 6 m y younger than samples of the same mineral collected at the same elevation from the western margin of the sill indicating that the western margin cooled earlier samples collected at sea level yield 40ar 39ar dates on hornblende biotite and plagioclase that are consistently younger than those collected at 1 and 2 km elevation dates from the center and eastern margin of the sill define isotherm migration rates km m y of 0 8 1 3 at 60 56 ma hornblende 0 8 0 6 at 56 53 ma biotite and 0 4 at 54 50 ma plagioclase 40ar 39ar dates from a single sea level sample collected 15 km farther south indicate no significant variation in dates parallel to the southeasterly strike of the sill for at least 30 km the younger dates calculated for the east side relative to the west side and metamorphic temperatures suggest that the country rock was hotter to the east than to the west when the pluton cooled through mineral blocking temperatures the rapid isotherm migration rates calculated from the vertically distributed samples and the suggestion of hotter country rock to the east during cooling is consistent with an interpretation that the pluton intruded during uplift of the rocks to the east because the relative precision of the 40ar 39ar technique is sufficient to detect subtle difference in the cooling ages its application to spatially distributed samples within a homogeneous plutonic body as in this study may yield new insights into the timing of metamorphism magmatism and tectonism of young mountain belts