geochemical evolution of peraluminous plutons in southern nova scotia canada a pegmatite poor suite three southern nova scotia plutons crystallized rapidly at 375 ma from magma containing both mantle and crustal components isotopic and chemical data suggest that the crustal contribution included both lower crustal material and cambro ordovician turbidites of the host meguma group despite local evidence of mixing and mingling of magmas the bulk of the plutons evolved by assimilation and fractional crystallisation evolved portions of the plutons have compositions appropriate for development of rare metal pegmatite fields but pegmatites are relatively rare and little differentiated like parental plutons pegmatites fall into biotite plagioclase and muscovite potassium feldspar assemblages the latter locally contain mn rich garnet biotite giving calculated p t conditions of pegmatite crystallisation of 620 c 0 44 gpa under water saturated conditions host rocks at the time of emplacement experienced p t conditions varying from 500 to 620 c at 0 44 gpa beryl bearing pegmatites occur only where host rocks were below sillimanite grade and pegmatites of any kind are scarce in sillimanite grade host rocks rarity of pegmatites can be ascribed to a combination of insufficient amounts of f li and b in the magma with rapid cooling which prevented extensive fractionation and undercooling our data suggest rather that peraluminous suites produced by rapid heating due to incursions of lithospheric mantle into supracrustal rocks and typified by biotite rich cordierite bearing plutons are not favorable locales for major rare metal enriched pegmatite fields such fields appear to require relatively prolonged anatexis resulting from crustal thickening or an areally extensive mafic underplate

collision along an irregular margin a regional plate tectonic interpretation of the canadian appalachians discussion stockmal et al recently presented a simple three dimensional plate tectonic model by which they attempted to explain the pre carboniferous evolution of the northern appalachians this model which in part follows the basic principles of the two stage collision model of colman sadd 1982 is mainiy based on the results of a marine seismic profile across northem newfoundland and on large scale geometrical considerations such models are useful because they provide geologists with a framework that can be tested against field data however in this case the existing geological data base in our opinion is incompatible with the model we will discuss the inconsistencies and present an altemative model the main thesis of the stockmal et al model is that the taconian and acadian orogenies are explained in terms of continuous southeastward subduction of iapetus oceanic lithosphere during the collision between the taconic magmatic arc and the irregular margin of north america the benioff zone clogged first in newfoundland and then in mainland canada the continuing convergence between the taconic arc and north america in newfoundland was then achieved by delamination of lithospheric mantle while b subduction continued between the taconic arc and north america in mainland canada evidence in support of this model includes the 300 400 km right hand offset of equivalent tectono stratigraphic zones between newfoundland and mainland canada and a difference in style and vergence of the acadian structures in newfoundland and new brunswick the idea of a diachronous collision is not a new one but the sense of diachronism remains unresolved see e g dallmey er 1977 clague et al 1981 lux 1986 considered the allochthonous ophiolites in the humber zone and argued in favour of a north to south diachronism his conclusion is consistent with the fact that the oldest fossil dated autochthonous flysch is younger in quebec than in newfoundland since there is no overstepping autochthonous cover in quebec stockmal et al argue that the timing of the final emplacement of the allochthons and the closure of iapetus is not well constrained there and they explain the acadian orogeny in mainiand canada as

the cold spring melange and a possible model for dunnage gander zone interaction in central newfoundland structural relationships at cold spring pond and the recognition of ophiolitic melange bear on the important questions of timing and style of structural superpositioning of dunnage zone rocks above gander zone rocks in central newfoundland the latest models emphasize ductile shear boundaries and orogen parallel movements previous models proposed west to east or head on obduction of dunnage ophiolitic rocks across the gander zone at the dunnage exploits subzone gander meelpaeg subzone boundary at cold spring pond discrete outcrop size ultramafic blocks and smaller quartzite blocks are randomly distributed and they are surrounded by or are embedded in homogeneous black graphitic shale or phyllite the ultramafic blocks are typical of nearby early ordovician dunnage ophiolite suites the quartzite blocks are typical of adjacent early ordovician or earlier gander clastic rocks and the matrix black shales are similar to those of middle or early ordovician age that occur throughout central newfoun

dynamic recrystallization and solution transfer in mylonitic rocks of the tetagouche group northern new brunswick canada a steeply dipping shear zone is present in augen schists and green phyllites of the ordovician tetagouche group a multiply deformed volcanic sedimentary complex in the canadian appalachians the shear zone developed early in the tectonic history and constituent rocks are marked by foliation transposition strong extension and mylonitic or phyllonitic microstructures dynamic recrystallization was an important deformation process but the l s tectonites also contain various generations of veins the deformation of these veins in combination with relict crack seal vein microstructures vein controlled alteration and the differentiation of the tectonic layering illustrates pervasive solution transfer during shear zone evolution various stages of brittle ductile deformation can be recognized in veins of different ages this reflects a microstructural competition between solute mass transfer and crystal plastic processes which is discussed in terms of wet versus dry mylonitization

transpression and extensional collapse steep belts and flat belts in the appalachian central mobile belt northern new brunswick canada models of the appalachian caledonian orogen infer oblique collision and an enigmatic reversal from sinistral silurian to dextral devonian transpression the miramichi highlands in canada are relevant to such models because 1 they are a subductional thrust complex in which the inferred reversal coincided with recumbent folding and 2 its kinematic indicators include rotated thrust structures and conjugate sets of coeval structures both are easily mistaken as evidence for sinistral and dextral transpression the thrust complex is part of the appalachian central mobile belt and is partitioned into steep belts and flat belts according to the attitude of the schistosity we argue that the steep belts are coeval with late silurian plutonism and low pressure metamorphism representing transpressional zones d 2 in which the extension lineation of d 1 thrusting was tilted twice first from gentle to steep then back to a gentle plunge the transpression was synchronous with exhumation as well as sedimentation flat belts represent a later flattening d 3 that probably resulted from extensional collapse and preceded early devonian dextral transpression d 4 along the central mobile belt our model for the flat belts differs principally from models proposed for equivalent flat belts in newfoundland

on the initiation of subduction estimates of shear resistance associated with lithospheric thrusting and convergence represent lower bounds on the force necessary to promote trench formation three environments proposed as preferential sites of incipient subduction are investigated passive continental margins transform faults fracture zones and extinct ridges none of these are predicted to convert into subduction zones simply by the accumulation of local gravitational stresses subduction cannot initiate through the foundering of dense oceanic lithosphere immediately adjacent to passive continental margins the attempted subduction of buoyant material at a mature trench can result in large compressional forces in both subducting and overriding plates this is the only tectonic force sufficient to trigger the nucleation of a new subduction zone the ubiquitous distribution of transform faults and fracture zones combined with the common proximity of these features to mature subduction complexes suggests that they may represent the most likely sites of trench formation if they are even marginally weaker than normal oceanic lithosphere

generation and obduction of ophiolites constraints from the bay of islands complex western newfoundland constraints provided by the bay of islands ophiolite complex boic western newfoundland suggest that its generation as well as its obduction is related to collision of the dunnage zone island arc system with the irregular continental margin of eastern north america the constraints include the supra subduction zone setting of the boic its extensional environment of generation the synchronous occurrence of ophiolite generation with initiation of drowning and deformation of the north american continental margin and the restriction of the boic as well as other appalachian ophiolites and associated taconian allochthons to reentrants in the north american margin during the early phases of collision of the dunnage arc with the north american margin subduction of old oceanic lithosphere trapped in reentrants will continue and allow the development of two potential sites of ophiolite generation the first develops through trench rollback leading to ophiolite generation in the overriding arc plate outboard of the reentrant whereas the second develops in a transtensional environment with spreading centers offset by strike slip faults which propagate through the overriding plate away from the indenter promontory the configuration of the boic spreading ridge normal to the continental margin and the direction of initial obduction parallel to the margin favor generation of the ophiolite in an overall strike slip regime with complete subduction of the oceanic remnant extension in the overriding plate ceases and the buoyant newly formed oceanic lithosphere is obducted onto the continental margin diachronous collision of the arc with the irregular continental margin accounts for the range of ages for ophiolite generation and obduction observed within the dunnage zone and for termination of arc volcanism along the north american margin

the margaree orthogneiss an ordovician peri gondwanan mafic felsic igneous complex in southwestern newfoundland the igneous protoliths of the margaree orthogneiss intruded the peri gondwanan rocks of the port aux basques complex in the southwestern corner of the hermitage flexure prior to regional deformation and metamorphism field relationships and u pb geochronology indicate that the margaree orthogneiss represents a 20 km long mafic felsic intrusive complex formed by amphibolite dioritic orthogneiss 474 14 4 ma hornblende bearing tonalitic orthogneiss with mafic enclaves 472 2 5 ma and 465 3 ma biotite bearing granitic orthogneiss and minor ultramafic rocks most amphibolite have the chemistry of volcanic arc tholeiite and are interpreted to be coeval with the intrusion of tonalitic and granitic magma with volcanic arc geochemical signatures locally mafic magmatism continued after the intrusion of the 465 ma granite the margaree orthogneiss and its country rock were overprinted by upper amphibolite facies metamorphism and deformation associated with the final closure of the iapetus ocean a late s

tectonic setting of the tetagouche group in northern new brunswick implications for plate tectonic models of the northern appalachians a regional compilation and reinterpretation of geochemical analyses of mafic igneous rocks from the ordovician tetagouche group in northern new brunswick indicates the presence of at least three mafic assemblages two groups of basalts and gabbros and one group of gabbros only the three groups can be separated on the basis of tio2 and trace element content group a basalts generally have mid ocean ridge basalt morb like compositions whereas group b basalts indicate a within plate setting wpb group c gabbros intrude into silicic volcanics that underlie the basalts but are rare or absent in the areas underlain by group a and b basalts the group c gabbros probably represent the earliest stages of mafic igneous activity and have morb like compositions or represent a transition between morb and arc basalts the association of group a and b basalts with abundant marine sediments such as thinly bedded turbidites and black and red shales and the fact that they erupted locally on top of continental crus

brunswick subduction complex in the canadian appalachians record of the late ordovician to late silurian collision between laurentia and the gander margin of avalon the brunswick subduction complex in the new brunswick part of the canadian appalachians records the late ordovician to late silurian collision between laurentia and the gander margin of avalon the brunswick complex is anomalously well preserved compared with equivalent rocks and structures elsewhere owing to its unique position in the deepest part of the quebec reentrant of the laurentian margin this part of the margin experienced less underthrusting and exhumation and overprinting by orogen parallel faulting than the adjacent promontories where collision started earlier the early southeast to east vergent thrust related structures represent a progressive d1 deformation that formed in response to northwestward subduction of the previously extended gander margin and subsequent tectonic unroofing of the subduction complex the original shallow northwestward dipping envelope to s1 was deformed in the late silurian by d2 upright folds and associated shear zones into a steep belt during terminal collision the d2 structures probably formed in response to sinistral transpression together d1 and d2 indicate that convergence was oblique and sinistral most rocks incorporated in the subduction complex formed in the tetagouche back arc basin that evolved from rifting of an arenig magmatic arc built on the gander margin into a wide marginal basin subduction was initiated in the late ordovician 455 ma in the back arc basin following collision of the middle ordovician popelogan arc with laurentia in the caradoc shortly after its opening in the late arenig 473 ma the closure history of the iapetus ocean involved more than one subduction zone and arc continent collision and rivals the southwestern pacific ocean in its complexities

a comparative analysis of pre silurian crustal building blocks of the northern and the southern appalachian orogen the new york promontory serves as the divide between the northern and southern segments of the appalachian orogen antiquated subdivisions distinct for each segment implied that they had lithotectonic histories that were independent of each other using new lithotectonic subdivisions we compare first order features of the pre silurian orogenic building blocks in order to test the validity of the implication of independent lithotectonic histories for the two segments three lithotectonic divisions termed here the laurentian iapetan and the peri gondwanan realms characterize the entire orogen the laurentian realm composed of native north american rocks is remarkably uniform for the length of the orogen it records the multistage neoproterozoic early paleozoic rift drift history of the appalachian passive margin formation of a taconic seaway and the ultimate demise of both in the middle ordovician the iapetan realm encompasses mainly oceanic and magmatic arc tracts that once lay within the iapetus ocean between laurentia and gondwana in the northern segment the realm is divisible on the basis of stratigraphy and faunal provinciality into peri laurentian and peri gondwanan tracts that were amalgamated in the late ordovician south of new york stratigraphic and faunal controls decrease markedly rock associations are not inconsistent with those of the northern appalachians although second order differences exist exposed exotic crustal blocks of the peri gondwanan realm include ganderia avalonia and meguma in the north and carolinia in the south carolinia most closely resembles ganderia both in early evolution and late ordovician silurian docking to laurentia our comparison indicates that to a first order the pre silurian appalachian orogen developed uniformly starting with complex rifting and a subsequent drift phase to form the appalachian margin followed by the consolidation of iapetan components and ending with accretion of the peri gonwanan ganderia and carolinia this deduction implies that any first order differences between northern and southern segments post date late ordovician consolidation of a large portion of the orogen

correction to petrology and porosity of an oceanic crustal site results from wave form modeling of seismic refraction data by paul spudich and john orcutt

influence of deformation and fluids on ar retention in white mica dating the dover fault newfoundland appalachians abstract white mica 40ar 39ar analyses may provide useful constraints on the timing of tectonic processes but complex geological and thermal histories can perturb ar systematics in a variety of ways ductile shear zones represent excellent case studies for exploring the link s between dynamic re neo crystallization of white mica and coeval enhanced fluid flow and their effect on 40ar 39ar dates white mica 40ar 39ar dates were collected from compositionally similar granites that record different episodes of deformation with proximity to the dover fault a terrane bounding strike slip shear zone in the appalachian orogen newfoundland canada 40ar 39ar data were collected in situ by laser ablation and by step heating single crystals results were compared to each other and against complementary u pb zircon and monazite and k ar fault gouge analysis although step heat 40ar 39ar is a widely applied method in orogenic settings this dataset shows that relatively flat step heat 40ar 39ar spectra are in contradiction with wide spreads in in situ 40ar 39ar dates from the same samples and that plateau dates in some cases yielded mixed dates of equivocal geological significance this result indicates that the step wise release of ar from white mica likely homogenizes and obscures spatially controlled ar isotope reservoirs in white mica from sheared rocks in contrast in situ laser ablation 40ar 39ar analysis preserves the spatial resolution of 40ar reservoirs that have been variably reset by deformation and fluid interaction this study therefore suggests that laser ablation is the best method for dating the timing of deformation recorded by white mica final interpretation of results should be guided by microstructural analysis estimation of deformation temperature chemical characterization of white mica and complementary chronometers overall the dataset shows that granitic protoliths were emplaced between 430 and 422 ma u pb zircon high strain deformation along the wing pond shear zone occurred between ca 422 405 ma u pb monazite and 40ar 39ar subsequent patchy ar loss in white mica occurred locally during low t shear

silurian extension in the upper connecticut valley united states and the origin of middle paleozoic basins in the quebec embayment pre silurian strata of the bronson hill arch bha in the upper connecticut valley nh vt are host to the latest ludlow comerford intrusive suite consisting east to west of a mafic dike swarm with sheeted dikes and an intrusive complex the rocks are mostly mafic but with compositions ranging from gabbro to leucocratic tonalite the suite is truncated on the west by the monroe fault a late acadian thrust that carries rocks of the bha westward over silurian devonian strata of the connecticut valley gaspetrough cvgt dikes intrude folded strata with a pre intrusion metamorphic fabric taconian but they experienced acadian deforma tion twenty fractions of zircon and baddeleyite from three sample sites of gabbro diorite spanning nearly 40 km yield a weighted 207 pb 206 pb age of 419 1m a greenschist facies dikes sampled over a strike distance of 35 km were tholeiitic basalts formed by partial melting of asthenospheric mantle with little or no influence from mantle or crustal lithosphere the dike chemistry is similar to mid ocean ridge within plate and back arc basin basalts parent magmas originated in the astheno sphere and were erupted through severely thinned lithosphere adjacent to the cvgt extensive middle paleozoic basins in the internides of the appalachian orogen are restricted to the quebec embayment of the laurentian rifted margin and include the cvgt and the central maine trough cmt separated from the bha by a silurian tectonic hinge the ne trending comerford intrusions parallel the cvgt cmt and the tectonic hinge and indicate nw se extension during post taconian convergence the irregular margins of composite laurentia and avalon permitted continued colli sion in newfoundland st lawrence promontory and coeval extension in the quebec embayment extension may be related to hinge retreat of the northwest directed brunswick subduction complex and rise of the asthenosphere following slab break off an alternative hypothesis is that the basins originated as pull apart basins between northwest trending left stepping sinistral strike slip faults along the southern flanks of the new york and st lawrence promontories

mechanisms of blueschist creation and preservation in an a type subduction zone mount olympos region greece deformation and metamorphism in the mount olympos region greece are the result of subduction of continental crust a type subduction during the alpine orogeny new structural and petrologic data from this region provide insight into the creation of low geothermal gradients in a type subduction zones and the preservation of blueschist facies assemblages early convergent deformation was accompanied by greenschist amphibolite facies metamorphism blueschist facies mineral assemblages overprinted higher grade assemblages during large scale episodic shortening that occurred during and after continental collision prehnite pumpellyite facies assemblages are postkinematic with respect to contractional fabrics but predate a prolonged period of extension that was not accompanied by metamorphism under plating and extension permitted unroofing and preservation of the blueschists with no over printing by greenschist facies assemblages

garnet rotation and the development of axial plane crenulation cleavage rotated helicitic garnets syntectonically overgrowing axial plane crenulation cleavage on the limbs of folds show the opposite sense of rotation to that undergone by the folded foliation during the development of the fold the garnet rotation is inconsistent with simple minded shear models for the development of the crenulation cleavage and it is useful to consider this problem in order to understand the development of crenulation cleavage better various models are considered in an attempt to explain both garnet rotation and cleavage morphology earlier models were based on the assumption that garnet rotation is driven by shear parallel to s2 they were mostly unsatisfactory but one involving migration of the crenulation hinges is viable when combined with other models described here the new models proposed here are based on the assumption that garnet rotation is driven by shear on the crenulated surface s1 the new foliation s2 is considered to be a passive structure at least in the early stages of its development these models satisfactorily explain garnet rotation and development of the cleavage including differentiation and they can be used to predict the morphological evolution of the cleavage microstructure as it should be preserved in garnets comparison of observed and predicted helicitic structures strongly supports the proposed models evidence is presented to show that where a garnet has an se that is a schistosity and an si that is a crenulation cleavage the schistosity can have developed by unfolding of the crenulations

plate tectonics and island arcs the plate tectonic concepts that developed rapidly in the late l960s made possible the understanding of island arcs before that time mobilistic concepts evolved slowly hindered particularly in the united states by an obstructionist geoscience establishment the volcanic belts of island arcs form about 100 km above subducting plates convergent plate boundaries evolve complexly with time and at any one time vary greatly along their lengths seismicity defines positions but not trajectories of descending slabs which sink more steeply than they dip and are overridden by advancing upper plates subduction occurs beneath only one side at a time of an internally rigid plate and the common regime in an overriding plate behind a surficial accretionary wedge is extensional except where a collision is underway back arc basin lithosphere is built behind or by migrating island arcs which lengthen and increase their curvatures a collision can involve two active arcs in which case intervening lithosphere sinks beneath both of them or an active margin and a passive one either type of collision generally is followed by the breaking through of new subduction beneath the composite mass of light crust from a new trench on the outside of the aggregate conversely a new subduction system commonly is a by product of collision a strip of back arc basin crust is in many cases left attached to the aggregate in front of the new trench and becomes the basement for a fore arc basin the leading edge of which is raised as melange is stuffed under it sedimentation in trenches is predominantly longitudinal and can be from distant sources accretionary wedges are dynamic being thickened at both toes and bottoms by tectonic accretion and thinned by gravitational forward flow melange is largely a product of tectonic imbrication and flowage driven by these conflicting processes not of submarine sliding high pressure metamorphic rocks form beneath overriding plates not within wedges in front of them arc magmas incorporate much material from the lithosphere through which they rise and vary correspondingly with the evolving composition of that lithosphere arc crust is inflated into geanticlines by intrusive rocks and thermal expansion submarine island arc volcanic rocks are widely spilitized with na enrichment and ca depletion by hydrothermal reaction with sea water the lower crust of mature island arcs consists of granulite facies rocks of mafic intermediate and felsic intermediate compositions the mohorovicic discontinuity may be primarily a constructional boundary representing the shallow limit off crystallization of voluminous rocks of ultramafic composition or plagioclase free mineralogy

foliation development and refraction in metamorphic rocks reactivation of earlier foliations and decrenulation due to shifting patterns of deformation partitioning reactivation of early foliations accounts for much of the progressive strain at more advanced stages of deformation its role has generally been insufficiently emphasized because evidence is best preserved where por phyroblasts which contain inclusion trails are present reactivation occurs when progressive shearing operating in a synthetic anastomosing fashion parallel to the axial planes of folds changes to a combination of coarse and fine scale zones of progressive shearing some of which operate antithetically relative to the bulk shear on a fold limb reactivation of earlier foliations occurs in these latter zones reactivation decrenulates pre existing or just formed crenulations generating shearing along the decrenulated or rotated pre existing foliation planes partitioning of deformation within these foliation planes such that phyl losilicates and or graphite take up progressive shearing strain and other minerals accom modate progressive shortening strain causes dissolution of these other minerals this results in concentration of the phyllosilicates in a simi lar but more penetrative manner to the for mation of a differentiated crenulation cleavage except that the foliation can form or intensify on a fold limb at a considerable angle to the axial plane of synchronous macroscopic folds reactivation can generate bedding parallel schistosity in multideformed and meta morphosed terrains without associated folds heterogeneous reactivation of bedding gen erates rootless intrafolial folds with sigmoidal axial planes from formerly through going struc tures reactivation causes rotation or refrac tion of axial plane foliations forming in the same deformation event causing reactivation in those beds or zones in which an earlier foli ation has been reactivated and results in destruction of the originally axial plane foli ation at high strains reactivation also provides a simple explanation for the apparently wrong sense but normally observed rotation of gar net porphyroblasts whereby the external foli ation has undergone rotation due to antithetic shear on the reactivated foliation alter natively the rotation of the external foliation can be due to its reactivation in a subsequent deformation event porphyroblasts with inclusion trails com monly preserve evidence of reactivation of earl ier foliations and therefore can be used to ident ify the presence of a deformation that has not been recognized by normal geometric methods because of penetrative reactivation reac tivation often reverses the asymmetry between pre existing foliations and bedding on one limb of a later fold leading to problems in the geo metric analysis of an area when the location of early fold hinges is essential the stretching lineation in a reactivated foliation can be rad ically reoriented potentially causing major errors in determining movement directions in mylonitic schistosities in folded thrusts geometric relationships which result from reactivation of foliations around porphyroblasts can be used to aid determination of the timing of the growth of porphyroblasts relative to deformation events other aspects of reac tivation however can lead to complications in timing of porphyroblast growth if the presence of this phenomenon is not recognized for example d grown porphyroblasts may be dis solved against reactivated s and hence appear to have grown syn d

kinematic evidence for extensional unroofing of the franciscan complex along the coast range fault northern diablo range california franciscan metagraywacke immediately below the del puerto ophiolite an outlier of the coast range ophiolite in the northern diablo range was sheared during top to the east displacement on the coast range fault this represents normal faulting and extensional offset it was accompanied by attenuation of the coast range ophiolite and great valley sequence in the hanging wall along layer parallel normal faults that sole into the coast range fault extension occurred as the franciscan complex moved relatively west out from under north american lithosphere and across the subducting ocean plate below this effected a lengthening and thinning in the wedge of material above the down going plate presumably in response to instability brought about by subduction shallowing krueger and jones 1989 and accretion of the franciscan central belt in the latest cretaceous to early paleocene as a result blueschist facies terranes of the uppermost part of the franciscan complex are now juxtaposed directly against hanging wall units that bear only low grade metamorphism

ganderia laurentia collision in the caledonides of great britain and ireland during terrane convergence an influx of clastic sediment from an upper plate onto a lower plate is an early indication of terrane juxtaposition in the caledonides of great britain and ireland units accreted to laurentia during the early palaeozoic era include peri gondwanan terrane assemblages that earlier separated from west gondwana however the southern uplands terrane contains detrital zircon populations apparently derived entirely from laurentia characterized by a large asymmetric mesoproterozoic peak and a scarcity of zircon at 600 ma and 2 1 ga in contrast cambrian and ordovician rocks from the lake district and the leinster massif of ireland show abundant grains with these ages together with a range of mesoproterozoic zircon these characteristics are shared with the monian terrane of anglesey and with ganderia in the appalachians indicating probable derivation from amazonia in west gondwana silurian sandstones from the lake district show an influx of laurentia derived zircon and lack the peri gondwanan signal this indicates that in the caledonides ganderia was not accreted to the laurentian margin until c 430 ma in contrast to the ordovician accretion of ganderian fragments recorded in the appalachians suggesting that the configuration of the closing iapetus ocean varied significantly along the strike of the orogen supplementary material details of sample coordinates analytical procedure and u pb detrital zircon analytical data are available at www geolsoc org uk sup18739

early paleozoic paleogeography and accretionary history of the newfoundland appalachians new paleomagnetic results were obtained from several units in the central mobile belt of the north central newfoundland appalachians combined with recently published data the results place important limits on interpretations of early paleozoic paleogeography and evolution of the appalachian caledonian orogen the ordovician appalachian margin of north america located at low paleolatitudes and facing roughly south was separated from the southern margin of iapetus avalon armorica by 3500 km primary directions in pillowed volcanic rocks of the moreton9s harbour group that are part of an early ordovician island arc show that these rocks were formed near the north american margin at a paleolatitude of 12 s early to middle ordovician volcanic rocks of the roberts arm chanceport and summerford groups on the other hand were extruded at paleolatitudes of 30 40 s unconformably overlying volcanics of the silurian botwood group were extruded subaerially at a paleolatitude of 24 s combined with tectonostratigraphic data these results suggest the following tectonic history in early ordovician time northerly subduction of the main iapetus basin was accompanied by the formation of a volcanic arc and a back arc basin near the north american margin middle ordovician emplacement of ophiolites on the north american margin reflects the closure of this back arc basin iapetus i and continued northerly subduction in silurian time closed the main basin iapetus ii

surface oceanography of broke west along the antarctic margin of the south west indian ocean 30 80 e abstract hydrographic ctd and adcp data were collected during the broke west research voyage january march 2006 in the south west indian ocean sector of the antarctic margin these data describe the large scale circulation water masses fronts and summertime stratification in the surface layer over the continental shelf slope and rise region between 30 and 80 e that forms ccamlr statistical area 58 4 2 the surface circulation matched the full depth circulation and consisted of the eastward flowing southern antarctic circumpolar current front to the north and the westward flowing antarctic slope current associated with the antarctic slope front along the continental slope to the south two sub polar gyres were detected south of the southern boundary of the antarctic circumpolar current the eastern weddell gyre in the cosmonaut sea 30 50 e and the greater prydz bay gyre in the cooperation sea 60 80 e in the eastern weddell gyre the seasonal mixed layer depths were shallower warmer and fresher relative to the regions to the east which were deeper cooler and more saline this spatial variability is found to be strongly correlated to the large scale pattern of sea ice melt retreat in the months preceding the voyage and the accumulated wind stress thereafter areas of upwelling warm deep waters into the surface layer are presented from positive anomalies of potential temperature and nutrient concentrations nitrate and silicate these anomalies were strongest in the eastern weddell gyre in the vicinity of the cosmonaut polynya embayment north of cape anne and near the southern boundary of the antarctic circumpolar current in the eastern sector of the survey the summertime stratification seasonal mixed layer seasonal pycnocline and tmin layer are discussed relative to the distributions of chl a and acoustically determined antarctic krill euphausia superba densities elevated chl a concentrations were found in the surface layer of the marginal ice zone and it is proposed that these are retained south of the fast narrow jet of enhanced antarctic slope current on the upper continental slope there is qualitative evidence of these maxima being subducted and transported north in the seasonal pycnocline in response to ekman convergence from the easterly winds in this region the seasonal mixed layer within the sub polar gyres had relatively low chl a concentrations with sub surface maxima in the seasonal pycnocline and the top of the tmin layer surface concentrations increased once again north of the southern boundary in the north east of the survey krill and chl a concentrations were both co located and decoupled at different locations across the survey there was no clear oceanographic boundary influencing the distribution of the krill surveyed though further work is necessary to properly synthesize this and other biological patterns with the oceanographic processes given the varying time and length scales and intrinsic sampling limitations

a hafnium isotopic record of magmatic arcs and continental growth in the iapetus ocean the contrasting evolution of ganderia and the peri laurentian margin abstract we test the sensitivity of combined u pb hf isotopic analyses of detrital and magmatic zircon to document complex processes in accretionary orogens using ganderia from the canadian appalachian orogen as an example ganderia hosted a long lived magmatic arc that began at least by 650 ma and continued until 450 ma during which time it drifted northward from the amazonian margin its isotopic record suggests arc activity began earlier possibly at 750 ma proto ganderia within an oceanic domain the u pb hf zircon isotopic array suggests that ganderia formed on a sliver of paleoproterozoic mesoproterozoic basement tdmc 2 2 1 1 ga a transition towards more evolved hf isotopic compositions between 650 and 600 ma is interpreted to coincide with its accretion to the amazonian margin of gondwana consistent with other geological evidence accretion was followed by increasing amounts of juvenile crustal inputs between 550 and 500 ma which is interpreted to reflect subduction roll back and the onset of drift from the gondwanan margin trench retreat culminated in the separation of the ganderia arc and formation of the exclusively oceanic 510 450 ma penobscot popelogan victoria arcs on ganderia accretion of ganderia to the active notre dame subzone along the laurentian margin occurred in the late ordovician and is marked by the abrupt termination of ganderian arc magmatism the notre dame arc evolved along the laurentian margin between 515 and 425 ma and was built on paleoproterozoic archean t dm c 2 9 2 5 ga crust supra subduction zone magmatism occurred at 490 460 ma during the closure of the taconic seaway and was dominated by the recycling of laurentian crust with progressive mixing of evolved and juvenile arc magmas over time the preservation of very evolved notre dame arc zircon in ganderian overstep sequences confirms the arrival of the leading edge of ganderia to laurentia by 450 ma the hafnium isotopic method is proven here to be a powerful tool when used in conjunction with geological constraints and is able to identify the changing nature of magmatism during the evolution of complex accretionary systems

a review of reflection seismic investigations in three major metallogenic regions the kevitsa ni cu pge district finland witwatersrand goldfields south africa and the bathurst mining camp canada abstract effective exploration for mineral deposits depends on a sound understanding of the processes and geological structures that contributed to their formation the reflection seismic method has proven to be a powerful tool that provides a high resolution image of the subsurface and information about structural and lithological relationships that control mineral deposits the method has also become an attractive geophysical tool for deep exploration and mine planning in this paper we review the use of reflection seismic methods to obtain a better understanding of the architecture and ore forming processes of three diverse mineral regions the kevitsa ni cu pge district in finland the goldfields of the witwatersrand basin south africa and the bathurst mining camp canada seismic data both 2d and 3d from the kevitsa deposit clearly image the 3d geometry of the ore bearing intrusion and provide information about its relationship to the host rock units and nearby intrusions within a larger tectonic framework 3d seismic data from the witwatersrand basin not only provide clear images of major structures including a distinct reflection that acts as a marker horizon for the gold bearing reef but also provide information that may be useful in resolving a long standing controversy regarding the origin of the gold in the basin for example it might be possible to show that dykes formed impermeable barriers thereby falsifying the epigenetic hydrothermal models 2d and 3d seismic data from the brunswick no 6 area in the bathurst mining camp suggest that the brunswick horizon which contains the bulk of the massive sulfide and associated iron deposits occurs within a reflective package that extends down to at least 6 7 km depth

structural evolution of sulphide tectonites and their host rocks stratmat mine new brunswick the sulphide orebodies at the stratmat mine in new brunswick are treated as tectonites because their primary characteristics have been so modified by deformation recrystallization and vein injection that most of their original features have been obscured pb zn cu sulphide orebodies at the stratmat mine consist of sulphide and sulphide silicate tectonites gneisses schists phyllites and slates produced by the mixing of two sulphide precursors and silicate host rocks by polyphase deformation much of which relates to progressive non coaxial deformation quartz vein injection during and after this period of deformation and the intermixing of nonsulphide lithotypes led to dilution of the initial ore composition both the deformation of the orebodies and fluid migration manifested by vein injection reflect processes that were operative in a major shear zone no indisputable primary characteristics of the orebodies are preserved although a number of tectonic and or tectonically modified features m

upper crustal investigation of the gulf of saint lawrence region eastern canada using ambient noise tomography we studied the 3 d shear wave velocity vs structure in the gulf of st lawrence gsl and adjacent onshore areas to 20 km depth by inverting rayleigh wave dispersion extracted from the vertical components of continuous ambient seismic noise waveforms the region is divided into three broad zones based on their vs characteristics in the northwest the grenville province i e the exposed edge of predominantly middle proterozoic laurentian crust is dominated by high vs except for well known anorthosite sites which are characterized by relatively lower vs in contrast the central segment of the gsl region corresponds to a belt with generally low vs at upper crustal levels in the southeastern part of the gsl prominent low vs in the uppermost crust are found to coincide with locations of subsidiary basins of the canadian maritime basin while higher vs characterize the accreted appalachian terranes where they are exposed on land the grenville province is wedged out at depth by the red indian line which is the suture between composite laurentia and peri godwanan ganderia in the canadian appalachians the geometry and vs characteristics of the south easternmost peri gondwanan terranes of avalonia and meguma suggest that they may be fully or partially structurally overlying a basement with distinct seismic characteristics which could be a vestige of the west african craton that was under thrust beneath composite laurentia during the terminal alleghenian continent continent collision in the middle of the gsl the 3 d geometry of the canadian maritimes sedimentary basins overlying the appalachian terranes shows that the depth to the top of basement is in excess of 8 km

the salinic orogeny in northern new brunswick geochronological constraints and implications for silurian stratigraphic nomenclature1this article is one of a series of papers published in this cjes special issue in honour of ward neale on the theme of appalachian and grenvillian geology the salinic orogeny is defined to encompass tectonic interactions that affect all elements of ganderia involved in the closure of the tetagouche exploits back arc basin between the late ordovician and early devonian hence the d1 and d2 deformations in the miramichi highlands and elmtree inlier of northern new brunswick are salinic events and onlap of lower silurian rocks onto exhumed parts of the brunswick subduction complex represents the earliest salinic a of three silurian unconformities in the region upper ordovician to lower silurian rocks of the matapedia successor basin contain widespread evidence of middle silurian tectonism e g disconformities angular unconformities and fold interference pat terns created by devonian overprinting of silurian folds lacking axial planar cleavage salinic b recent u pb radioiso topic dating of chemically abraded zircon from rhyolite just above the salinic b unconformity has yielded an age of 422 3 0 3 ma combined with late early silurian fossil ages just below the unconformity this indicates a ca 5 million year middle silurian hiatus finally upper silurian ludfordian rocks are locally disconformably overlain by polymictic conglomerates that form the base of the devonian section salinic c all silurian rocks in northeastern new brunswick have historically been included in the chaleurs group however unconformities and local stratigraphic variations especially compared with the type locality support the introduction of new higher rank names in new brunswick hence the quinn point group is introduced to incorporate lower silurian rocks the petit rocher group to include upper silurian sedimen tary rocks in the nigadoo river syncline and the dickie cove group for upper silurian volcanic rocks in the charlo jacquet river area upper silurian rocks west of campbellton that are contiguous with the chaleurs group in quebec will remain part of the chaleurs group

a new lower ordovician arenig faunule from the coy pond complex central newfoundland and a refined understanding of the closure of the iapetus ocean a newly discovered fossiliferous horizon within sediments belonging to the coy pond complex of the exploits subzone in central newfoundland yields the graptolite undulograptus austrodentatus s l and cyclopygid trilobite cyclopyge grandis brevirhachis this late arenig faunule constrains the upper age limit of the ophiolite complex and is the first record of an early ordovician cyclopygid trilobite in north america this is consistent with a paleogeographic affinity for south central newfoundland with the northern oceanic margin of avalonia in a peri gondwanan position during the early ordovician and contrasts with coeval shelly and graptolitic faunas from the notre dame subzone of central newfoundland which show marked laurentian affinities the exploits subzone is generally considered equivalent to the region of scotland lying south of the southern upland fault the newfoundland discovery which is supported by faunal data from elsewhere in newfoundland and in ireland suggests that the region around t

post taconic blueschist suture in the northern appalachians of northern new brunswick canada a narrow belt of late ordovician early silurian blueschist at least 70 km long separates an allochthonous fragment of back arc oceanic crust of the middle ordovician fournier group from underlying rift related volcanic rocks of the middle ordovician tetagouche group in northern new brunswick canada the basalts on both sides of the blueschist belt are predominantly metamorphosed to greenschist facies conditions the blueschist belt is interpreted to be an out of sequence thrust zone that accommodated tectonic transport of higher pressure rocks on top of lower pressure rocks during post peak blueschist facies metamorphism the blueschists have higher fe sub 2 o sub 3 feo ratios and total iron contents in comparison to otherwise chemically equivalent basalts of the fournier and tetagouche groups that have been metamorphosed into greenschists the blueschist belt was probably the site of channelized flow of oxidizing fluids during active deformation ina subduction complex formed during the closure of a wide taconic back arac basin in late ordovician silurian time

geochemical variations in middle ordovician volcanic rocks of the northern miramichi highlands and their tectonic significance a detailed geochemical study of middle ordovician volcanic rocks undertaken in the northern miramichi highlands of new brunswick shows that 10 basaltic suites can be distinguished these suites are assigned to the tetagouche and fournier groups the contact between these two groups is a major thrust zone marked for over 70 km by a prominent blueschist zone all the tetagouche group volcanic rocks have chemistries consistent with extrusion in a continental rift but most fournier group basalts in the miramichi highlands have chemistries suggestive of an oceanic back arc setting the chemical signatures stratigraphic variations and structural data indicate that the northern miramichi highlands preserve a section across a telescoped middle ordovician back arc basin that initially opened as a result of asthenospheric injection near the rear part of a lower ordovician ensialic arc

pressure temperature time evolution of high pressure rocks of the acatlan complex southern mexico implications for the evolution of the iapetus and rheic oceans replyreply to comment by keppie et al

application of high field strength elements to discriminate tectonic settings in vms environments high field strength element ratios in felsic volcanic rocks reflect the tectonomagmatic evolution of a region th ta th hf ta hf th yb and ta yb ratios collectively help to define the tectonic environment of volcanism separating oceanic arcs active continental margins and within plate volcanic zones on the basis of the above empirical observations the tectonic environments of selected archean and post archean volcanogenic massive sulfide deposits were reexamined published data sets plotted on discriminant diagrams represent twelve volcanogenic massive sulfide vms deposits from two distinct tectonic environments the volcanic rocks of the kuroko deposits japan san nicolas mexico and bathurst new brunswick canada plot within the active continental margin field on all diagrams whereas volcanic rocks from archean deposits of the superior province canada kidd creek kamiskotia geco winston lake mattabi lyon lake noranda selbaie and south bay plot in the within plate volcanic zone field the discriminant diagrams demonstrate that although the chemical character of rhyolites has not changed significantly since the archean the range of geotectonic setting of vms deposits probably has it is suggested that archean deposits most likely formed in an ensimatic rift environment whereas post archean deposits formed in subduction related rift zones of arc environments subduction related rhyolites of vms deposits are variably enriched in th and the light rare earth elements with respect to ta nb and the heavy rare earth elements

crustal scale reflection seismic investigations in the bathurst mining camp new brunswick canada abstract the bathurst mining camp northern new brunswick canada contains the super giant brunswick no 12 massive sulphide deposit and the smaller now abandoned brunswick no 6 deposit discoveries of additional base metal deposits in the camp require a better understanding of geological structures at depth to this end reflection seismic data in the brunswick no 6 area were acquired along three 2d profiles in 1999 with a total length of about 30 km we have recovered processed and interpreted these seismic data in conjunction with petrophysical and geological data from the study area the seismic data and the borehole geophysical data allow a better understanding of both the shallow and deep structures to 9 km depth in the area the seismic data show steeply dipping structures of the brunswick no 6 area many of which reach the surface and allow for correlation with the surface and borehole geological information finite difference modeling of major geological formations constrained with borehole petrophysical measurements indicates good correlation between the observed seismic and the synthetic data a sequence of seismically reflective and transparent zones indicates a thrust stack in the brunswick no 6 area the contact between the reflective and transparent zones is a series of faults bringing the two units over each other a reflective package is observed in all three profiles and correlates well with the brunswick horizon the key mineralized zone in the study area the brunswick horizon extends down to depth greater than 3 km increasing the hope for discovery of deeper base metal deposits two other sets of reflections are also observed in all three profiles in the depth range of about 5 8 km we interpret them as two sets of thrusted sheets which could be an indication that the brunswick belt extends down to a maximum depth of 8 km

the ordovician volcanics of the elmtree belledune inlier and their relationship to volcanics of the northern miramichi highlands new brunswick an investigation of the geology and chemistry of the basic igneous rocks in the elmtree and belledune inliers in northern new brunswick shows that the bulk of the middle ordovician rocks of the ophiolitic fournier group are best interpreted as the products of volcanism and sedimentation in an extensive ensimatic back arc basin southeast of a volcanic arc the oceanic back arc basin igneous rocks form the basement to renewed arc related basaltic volcanism in late middle to late ordovician time the fournier group is separated from the structurally underlying shale dominated elmtree formation of the tetagouche group by an extensive tectonic melange which incorporates lenses of serpentinite mafic volcanic rocks and sedimentary rocks of both the tetagouche and fournier groups the mafic volcanic rocks in the elmtree formation correlate best with those intercalated with the lithologically similar sediments of the llandeilian caradocian boucher brook formation in the northern miramichi highlands the melang

stratigraphy and tectono sedimentary evolution of the late ordovician to middle devonian gaspe belt in northern new brunswick evidence from the restigouche area the gaspe belt in the restigouche area comprises three successions separated by a late silurian salinic disconformity and an early devonian angular unconformity the lower upper ordovician to lower silurian sequence consists of siliciclastic turbidites of the boland brook and whites brook formations grog brook group overlain by calcareous turbidites of the pabos and white head formations matapedia group and slope and shelf deposits of the upsalquitch and limestone point formations lower chaleurs group above the salinic disconformity the upper chaleurs group and the dalhousie group record a transgressive regressive cycle the former comprises pridolian carbonate rocks of the west point formation and overlying pridolian to lochkovian sedimentary rocks of the indian point formation the chaleurs group is conformably overlain by lochkovian to early emsian subaerial volcanic rocks of the dalhousie group val d amour formation which is unconformably overlain by alluvial lacustrine deposits of the late emsian campbellton formation acadian orogenesis began during the emsian and is characterized by open to closed folding heterogenous cleavage development and reverse and strike slip faults the salinic orogeny is manifested in extensional block faulting within plate volcanism and uplift and deep erosion of early silurian strata early devonian high level intrusion of the matapedia group white head clasts in indian point conglomerate and thermal maturation data all indicate an extended period of late silurian early devonian uplift in parts of the restigouche area thermal maturities of west point and indian point strata are within the oil and condensate windows and suggest potential for hydrocarbons in the study area

seismotectonics of new guinea a model for arc reversal following arc continent collision the structure and evolution of the northern new guinea collision zone is deduced from international seismological center isc seismicity 1964 1985 new and previously published focal mechanisms and a reexamination of pertinent geological data a tectonic model for the new guinea margin is derived which illustrates the sequential stages in the collision and suturing of the bewani toricelli adelbert finisterre huon new britain arc to central new guinea followed by subduction polarity reversal in the west east of 149 e the solomon plate is being subducted both to the north and south bringing the new britain and trobriand forearcs toward collision west of 149 e the forearcs have collided and together they override a fold in the doubly subducted solomon plate lithosphere which has an axis that is parallel to the strike of the ramu markham suture and that plunges westward at an angle of 5 beneath the coast ranges of northern new guinea active volcanism off the north coast of new guinea is related to subduction of the solomon plate beneath the bismarck plate active volcanism of the papuan peninsula and quaternary volcanism of the new guinea highlands are related to slow subduction of the solomon plate beneath the indo australian plate along the trobriand trough and the trough s former extension to the west respectively from 144 148 e seismicity and focal mechanisms reveal that convergence between the sutured bismarck and indo australian plates is accommodated by thrusting within the finisterre and adelbert ranges and compression of the new guinea orogenic belt together with basement involved foreland folding and thrusting to the south the finisterre block overthrusts the new guinea orogenic belt whereas the adelbert block is sutured to new guinea and overthrusts the oceanic lithosphere of the bismarck sea along the new guinea trench west of 144 e seismicity defines a southward dipping wadati benioif zone and focal mechanisms indicate oblique subduction only this oldest westernmost portion of the collision has progressed past suturing to a full reversal in subduction polarity

mechanical role of backstops in the growth of forearcs we use numerical and laboratory modeling to analyze the mechanical role of backstops in the overriding plate at subduction zones a backstop is defined as a region within a forearc that has significantly greater shear strength than the sediment lying farther trenchward it can be thought of as the bulldozer behind an accretionary wedge we calculate the stress and displacement fields within forearcs for various backstop models using the finite element method and we simulate deformation over a backstop using a small scale laboratory model in this way we model the effects of the mechanical properties and geometry of a backstop on forearc structures we find that the growth of an outer arc high the development of an inner deformation belt with landward vergence and the seemingly paradoxical presence of an undeformed forearc basin within an otherwise highly deformed forearc can be explained by assuming the existence of a geometrically simple backstop with geologically reasonable contrasts in mechanical properties compared to the sediments just trenchward of it the structures produced in our numerical models are quite insensitive to the rheology boundary conditions and exact mesh geometry employed the general types of structures observed in the laboratory models depend only weakly upon the strength and the geometry of the backstop these results suggest that a detailed picture of an underlying backstop cannot be determined from surface information alone backstops in which the contact with the accretionary wedge dips arcward rather than trenchward however should produce only slightly different forearc structures with less development of both the outer arc high and the landward vergent inner deformation belt although natural forearcs are far more complex than our simple models they exhibit many of the same features indicating that relatively simple backstop mechanics may be a very important factor in the overall growth of forearcs

middle ordovician conodonts from the buchans group central newfoundland and their significance for regional stratigraphy of the central volcanic belt reply conodonts have been recovered from carbonate clasts within breccia units in the buchans group of central newfoundland the assemblage is characterized by specimens of cordylodus horridus barnes and poplawski and histiodella holodentata ethington and clark these and other species recovered suggest a whiterockian latest arenig early llanvirn age for the clasts this is somewhat older than the whole rock rb sr age of 447 18 ma on volcanics of the buchans group the carbonate clasts are considered to be derived from the area of deposition of the buchans group rather than from some exotic source this interpretation is based on knowledge of local tectonics inferred paleogeogaphy and geographic distribution of the conodont assemblage all other clasts in the breccia units are considered of local origin if the conodonts are of local buchans group origin then they indicate a whiterockian age for the unit in view of this current regional models for the central volcanic belt may require substantial revi

tectonic history of subduction zones inferred from retrograde blueschist p t paths many phanerozoic convergent plate junctions are marked by discontinuous blueschist belts reflecting relatively high pressure p prograde trajectories common blueschist paragneisses such as those of the western alps exhibit widespread overprinting by greenschist and or epidote amphibolite facies assemblages for this type of high p belt retrograde metamorphism involved fairly rapid nearly isothermal decompression some terranes underwent continued heating during early stages of pressure release uplift probably occurred as a consequence of the entrance of an island arc oceanic plateau or segment of continental crust into the subduction zone collision resulting in marked deceleration or cessation of underflow and buoyant approximately adiabatic rise of the stranded recrystallized subduction complex other high p belts such as the franciscan of western california preserve metamorphic aragonite and lack a low p overprint retrogression approximately retraced the prograde p t temperature path or for early formed high grade blocks occurred at even higher p t ratios parts of this type of metamorphic belt evidently migrated slowly back up the subduction zone in response to isostatic forces during continued plate descent and refrigeration upward motion took place as tectonically imbricated slices as laminar return flow in melange zones and perhaps partly a lateral spreading extension of the underplated accretionary prism retrograde p t trajectories of high pmore belts therefore provide important constraints on the tectonic evolution of convergent plate junctions less

structure origin and concentration of the brunswick 12 and 6 orebodies the origin structure and history of the brunswick iron formation and massive sulfides of the brunswick 6 and 12 mines are reviewed new evidence concerning structure stratigraphy and alteration of the orebodies is presented five generations of folds are described of which the earliest three have an important influence on the large scale structure of the mine areas the earliest folds f 1 are large and overturned to the north and are related to coeval thrusting these folds have not been recognized previously the main orebody folds are upright isoclinal locally downward facing structures of the f 2 generation the mineralization is interpreted as syngenetic but evidence is presented to show that previously interpreted feeder pipes do not exist in a presently recognizable form and that previously interpreted syngenetic alteration is in fact a later diagenetic or postdiagenetic feature remobilization of the sulfide mineralization continued until after the f 3 deformation and concentration of the sulfides may be a relatively late feature but the possibility of the main concentration being syngenetic cannot be ruled out the evidence is obscured by later deformation and alteration the present form of the orebodies is largely a function of deformation and the sulfide layers are thickened locally by folding the orebodies are located in tight structural basins formed by interference of f 1 and f 2

postcollision extension in arc continent collision zones eastern indonesia postcollisional extension superimposed on arc continent collision complexes is a common feature in the orogenic belts of eastern indonesia in the southern banda arc the arc continent collision event is essentially a pliocene and younger feature but already an element of extension is being superimposed on the compressional structures of the collision complex it is thus likely that the extension commences within a very short period 5 m y after the initiation of collision related compressional deformation a similar history of fold belt development immediately followed by extension can be inferred for the lengguru fold belt and the adjacent wandamen wondiwoi terranes of irian jaya and for the gulf of bone in sulawesi it is suggested that the extension results from decoupling of the subducting oceanic lithosphere from the unsubductable continental lithosphere and that the superimposition of extension is a virtually unavoidable consequence of the arc continent collision

petrogenesis of siluro devonian rhyolites of the tobique group in the northwestern appalachians northern new brunswick canada tectonic implications for the accretion history of peri gondwanan terranes along the laurentian margin

old pacific crust near hawaii a seismic view an expanding spread profile esp over 80 ma pacific ocean crust near hawaii was modeled with a one dimensional solution for travel times relative amplitudes and sometimes frequency content the resulting profiles of compressional and shear wave velocities and poisson s ratio are similar to those determined by other ocean crustal studies except that the shear wave q in the sediments is very low 10 and some unusual high frequency high phase velocity arrivals require a laminated structure within the mid crustal section at about 3 km below the base of the sediment the laminations are probably alternating layers of gabbro and ultramafic cumulates the cumulates may be fractionation products from an upper level magma chamber that was present at the time of crustal formation reflections which may correspond to these laminations have been observed in reflection records both near this esp and in the north atlantic comparison of different ophiolite models for ocean crust reveal that the most commonly used velocity lithology model is probably in error for the pillow and dike basalts resulting in an interpretation of overly thick layers in previous studies of refraction lines the preferred interpretation is that the average total thickness of the pillow and dike sequences is between 1 0 and 1 5 km

a two dimensional seismic investigation of crustal structure under the hawaiian islands near oahu and kauai two expanding spread profiles esps were analyzed in detail to investigate the deep structure of the volcanoes and the underlying oceanic crust near oahu one esp was located on the southeastern submarine flank of kauai esp 10 and the other was located directly over the ridge axis between oahu and molokai esp 5 two dimensional models of the entire crust were obtained by iterative comparison of both travel times and relative amplitudes of arrivals on the ray theoretical synthetic seismograms with those in the data a 2 4 km vertical offset in the preexisting crust inferred to be a large fault zone was found near the midpoint of esp 10 this fault may form the southern wall of a graben or half graben in which kauai sits the layering and seismic velocities of the preexisting oceanic crust near kauai have remained unmodified by the load of the volcanic pile esp 5 is located over a shallow 4 km thick basin filled with volcaniclastics and possibly volcanic flows bordered by the islands of oahu and molokai penguin bank and two buried rift zones under this basin the main volcanic edifice is 4 to 5 km thick the top of the preexisting oceanic crust is 8 to 9 km below sea level and is 3 to 4 km lower than the crust away from the islands the underlying oceanic crust is 6 to 7 km thick excluding the moho transition zone which is of normal thickness and at a depth from 15 to 16 km the existence of a large subcrustal plutonic body is refuted

3d imaging challenges in steeply dipping mining structures new lights on acquisition geometry and processing from the brunswick no 6 seismic data canada we have analyzed and processed a 38 km 2 nonorthogonal 3d surface reflection seismic data in the brunswick no 6 area to better understand the effect of acquisition geometry on the resultant image

age and origin of the boil mountain ophiolite and chain lakes massif maine implications for the penobscottian orogeny the boil mountain ophiolite complex of west central maine is widely interpreted to mark the lower ordovician penobscottian suture between the dunnage chain lakes and gander terranes the ophiolite consists of two distinct volcanic groups including a lower island arc tholeiite sequence and an upper mid ocean ridge basalt sequence a new middle ordovician 477 1 ma u pb age on a tonalite sill that intrudes the lower volcanic gabbroic sequence is younger than other ca 500 ma age constraints for the ophiolite and represents a maximum age for the ophiolite prior to final emplacement over gneissic rocks of the chain lakes massif a comparison of ages and paleogeography of the boil mountain ophiolite with ophiolitic sequences in quebec and newfoundland indicates that the taconian and penobscottian orogenies and ophiolite obduction occurred simultaneously although on different margins of the iapetus ocean the taconian ophiolite sequences were obducted onto the appalachian margin of laurentia during its col

role of microbial and phytoplanktonic communities in the control of seawater viscosity off east antarctica 30 80 e abstract despite the long standing belief that seawater viscosity is driven by temperature and salinity biologically increased seawater viscosity has repeatedly been reported in relation to phytoplankton exudates in shallow productive coastal waters here seawater viscosity was investigated in relation to microbial and phytoplanktonic communities off the coast of east antarctica along latitudinal transects located between 30 e and 80 e in sub surface waters and at the deep chlorophyll maximum dcm the physical component of seawater viscosity observed along each transects ranged from 1 80 to 1 95 cp while the actual seawater viscosity ranged from 1 85 to 3 69 cp this resulted in biologically increased seawater viscosity reaching up to 84 9 in sub surface waters and 77 6 at the dcm significant positive correlations were found between elevated seawater viscosity and i bacterial abundance in sub surface waters and ii chlorophyll a concentration and the abundance of flow cytometrically defined auto and heterotrophic protists at the dcm among the 12 groups and 108 species of protists identified under light microscopy dinoflagellates and more specifically alexandrium tamarense and prorocentrum sp were the main contributors to the patterns observed for elevated seawater viscosity our observations which generalised the link previously identified between seawater viscosity and phytoplankton composition and standing stock to the southern ocean are the first demonstration of increases in seawater viscosity linked to marine bacterial communities and suggest that the microbially increased viscosity might quantitatively be at least as important as the one related to phytoplankton secretion

numerical modeling of water waves recorded by a subbottom seismometer in the southwest pacific the amplitudes of water waves generated by shallow explosive sources and recorded by a subbottom seismometer provide constraints on velocity and attenuation parameters of oceanic sediments and upper oceanic crust analysis of data collected during the 1983 ngendei seismic experiment in the southwest pacific ocean illustrates this method the data were recorded by a triaxial geophone emplaced 54 m into the basaltic basement synthetic seismograms are computed using wave number integration for several crustal models to determine the sensitivity of water wave amplitudes to specific parameters of the sediments and oceanic crust the synthetic seismograms fit the recorded data to better than a factor of 2 for frequencies as high as 15 hz and ranges out to 30 km the amplitude of the direct water wave beyond critical incidence direct wave root indicates that the sediment p wave velocity is near 1 8 km s modeling of the attenuation of shear wave reverberations in the sediments indicates that sediment q is between 30 and 50 modulations in water wave amplitude as a function of receiver offset are related to the velocity of the upper oceanic crust the distances at which amplitude maxima occur indicate that the shear wave velocity of the upper crust at the ngendei site is 2 4 0 1 km s this is an important result since reliable estimates of the shear wave velocity of upper oceanic crust are rare the distances at which amplitude minima occur are used to constrain the compressional wave velocity of the uppermost crust however the modulations do not resolve the compressional wave velocity of the crust as well as the shear wave velocity the relative amplitudes of successive water waves indicate that the reflection coefficient at the sediment basement interface is 0 66 this further constrains the compressional wave velocity of the upper crust using estimates of the sediment properties and crustal shear wave velocity the compressional wave velocity of the upper crust at the ngendei site is estimated from this reflection coefficient to be 4 3 4 4 km s a method for estimating the velocity gradient in the upper kilometer of the crust from the interference of water waves with upgoing crustal refractions is presented application of the method to these data indicate that the shear wave velocity gradient is about 1 0 s 1 which is indistinguishable from the previous determination by shearer and orcutt 1986 using a quite different methodology the velocity of the upper crust at the ngendei site is consistent with velocities reported for other regions of similar crustal age 100 m y and is much greater than velocities typically found for young oceanic crust 5 m y this lends support to current theories regarding the evolution of the uppermost oceanic crust that attribute higher velocities in older crust to the closing of cracks and pores

avalonian proximity of the ordovician miramichi terrane northern new brunswick northern appalachians paleomagnetic evidence for rifting and back arc basin formation at the southern margin of iapetus a paleomagnetic investigation of the middle ordovician tetagouche group in northern new brunswick was undertaken to determine the paleogeo aphi position of the miramichi terrane stepwise thermal demagnetization of pillow basahs reveals a high temperature characteristic gnet tion with a mean direction of b w z 69 k 22 aas 13 tih corrected n 7 sites 73 samples a positive fold test and the presence of antipodal normal and reversed polarity directions indicate that this ancient direction is ordovician in age with a paleopole position of 52 n 352 e the corresponding pateolatitude of 535 places these vokanic rocks near the southern margin of the iapetus ocean at paleolatitudes similar to those revealed by avalon for the middle to late ordovician the mafic and felsic volcanic rocks and marine sedimentary rocks of the tetagouche and fournier groups have been interpreted to be remnants of a rifted continental margin and a middle ordovician back arc basin our results show that the process of rifting and back arc basin formation occurred at the avalonian margin of iapetus which implies that ordovician subduction was not restricted to the laurentian margin but also marks the southern margin of japetus

40ar 39ar age pattern associated with differential uplift along the eastern highlands shear zone cape breton island canadian appalachians abstract the eastern highlands shear zone in cape breton island is a crustal scale thrust it is characterized by an amphibolite facies deformation zone 5 km wide formed deep in the crust that is overprinted by a greenschist facies mylonite zone 1 km wide that formed at a more shallow level hornblende 40ar 39ar plateau ages on the hanging wall decrease towards the centre of the shear zone in the older zone over 7 8 km from the centre the ages are between 565 and 545 ma in the younger zone within 4 5 km of the centre they are between 425 and 415 ma and in the transitional zone in between they decrease abruptly from 545 to 425 ma pressures of crystallization of plutons in the hanging wall based on the al in hornblende barometer and corresponding to depth of emplacement increase towards the centre of the shear zone and indicate a differential uplift of up to 28 km associated with movement along the shear zone the age pattern is interpreted to have resulted from the differential uplift the pressure data show that rocks exposed in the younger zone were buried deep in the crust and did not cool through the hornblende ar blocking temperature 500 c until differential uplift occurred the 40ar 39ar ages in the zone 425 415 ma thus date shear zone movement or the last stage of it in contrast rocks in the older zone were more shallowly buried before differential uplift and cooled through the blocking temperature soon after the emplacement of 565 555 ma plutons in the area long before shear zone movement the transitional zone corresponds to the ar partial retention zone before differential uplift the 40ar 39ar age pattern thus reflects a neoproterozoic to silurian cooling profile that was exposed as a result of differential uplift related to movement along the shear zone a similar k ar age pattern has been reported for the alpine fault in new zealand it is suggested that such isotopic age patterns can be used to help constrain the ages kinematics displacements and depth of penetration of shear zones

discussion on no exploits back arc basin in the iapetus suture zone of ireland journal of the geological society london 172 740 747 mcconnell et al 2015 claimed that the absence of middle ordovician back arc volcanic rocks in the bellewstown terrane in the irish caledonides implies that the tetagouche exploits back arc basin never existed in ireland the emphasis on the absence of evidence surprised us because absence of evidence is not evidence of absence however if they are correct their interpretation has major implications for the tectonic processes responsible for the final closure of iapetus

contrasting tectonic history of the arc continent suture in the southern and middle urals implications for the evolution of the orogen the main uralian fault has been considered the original arc continent suture for 2000 km along the uralide orogen the symmetry of the tectonic units across it suggested a consistent east dipping polarity for the palaeosubduction zone which together with its topographic and aeromagnetic signature supported the idea of a single suture however several characteristics vary at different latitudes in the middle urals it is a strike slip fault zone with moderately deformed and metamorphosed volcanic arc fragments in its hanging wall and low grade metamorphic rocks of the east european craton in its footwall here it has a prominent nnw trending magnetic signature which cross cuts north trending anomalies in its hanging wall and a pronounced reflection seismic signature that can be traced to the top of the middle crust at c 5 s twt in the southern urals it is a serpentinite melange zone of ambiguous kinematics with a weakly deformed and metamorphosed volcanic arc in its hanging wall and moderately metamorphosed to high pressure rocks of the east european craton in its footwall in this part of the orogen it has a weak reflection seismic character and a magnetic signature that parallels that of its hanging wall on the basis of an integrated analysis of these different data sets we suggest that the main uralian fault as it is currently defined is not a single entity but rather the original arc continent suture in the south and the western strand of a strike slip fault system that reworked the original suture in the middle urals

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

structure and evolution of the heath steele base metal sulfide orebodies bathurst camp new brunswick canada a structural analysis of the base metal sulfide deposits at the heath steele mines northern new brunswick is presented the structural similarity of the deposits is defined and their relationships to one another discussed the deposits are spatially associated with ordovician sedimentary rocks which are situated in an igneous rock complex comprising felsic and basic volcanics volcaniclastics and intrusives the complex has been subjected to low grade metamorphism and five generations of deformation d 1 d 5 the first two generations of deformation d 1 and d 2 are represented by transposition foliations s 1 and s 2 the present attitude of the s 2 foliation is a result of recumbent folding d 3 and two generations of open folds with vertical axial planar cleavages striking northwest s 4 foliation and northeast s 5 foliation the s 1 foliation is axial planar to conical f 1 folds regionally such folds have been associated with thrusting d 1 along the earliest recognized layering s 0 foliation vein swarms illustrate the local importance of solute mass transfer during this event at the heath steele mines culminating in fracture controlled alteration along a brittle ductile deformation d 1 shear zone near the sulfide deposits and development of metasomatic breccia in the sulfide deposits overall the deposits occur along the s 0 foliation the contact between augen schists and the metasedimentary rocks but massive sulfides are concentrated in f 1 fold closures forming pods and slabs parallel to the s 1 foliation this concentration reflects either mobilization of preexisting sulfides during d 1 or deposit formation by mineralization during d 1 if the contact in s 0 is nontectonic a stratigraphic control on the location of the deposits is indicated and their association with iron formation argues for a syngenetic origin conclusive evidence for these models has been obliterated by the intense deformation and solution transfer

paleomagnetism of the moreton s harbour group northeastern newfoundland appalachians evidence for an early ordovician island arc near the laurentian margin of iapetus paleomagnetic results have been obtained from mafic volcanic units in the upper part of the moreton s harbour group which is part of an accreted island arc terrane that is preserved in the notre dame bay subzone of the central mobile belt of the newfoundland appalachians detailed thermal and alternating field demagnetization reveals a stable characteristic component of magnetization carried by magnetite at a large number of sites in pillow basalts and coeval basaltic dikes the intrusives contain both polarities and a primary age for the characteristic magnetization is indicated by a positive contact test for one of the dikes and by a positive structural test involving a correction for block rotations strike correction the overall mean direction for the characteristic component after tectonic tilt and strike correction flows and intrusives d 171 i 22 k 22 6 95 6 5 pole 29 n 135 e flows d 166 i 22 k 33 2 95 6 5 corresponds to an early ordovician paleolatitude for the arc of 11 s which is indistinguishable from the expected paleolatitude of the north american margin this implies that the arc formed at or near the margin of the craton in contrast the avalon block which formed the other margin of iapetus was widely separated from the arc and the craton at this time the early ordovician paleolatitude of the arc terrane supports a tectonic model in which coeval ophiolitic sequences inboard of the arc were formed in a narrow ocean basin between the arc and the craton subsequent convergence between the arc and laurentia in middle ordovician time resulted in closure of this narrow ocean basin and obduction of the back arc basin oceanic crust onto the ancient margin of the craton thus giving rise to the taconic orogenic pulse in the newfoundland appalachians

geology and paleobiology of islands in the ordovician iapetus ocean review and implications a history of islands in the iapetus ocean and their destruction is recorded by widespread occurrences of lower to upper ordovician volcanic and epiclastic rocks and some limestone through the axial parts of the appalachian caledonide orogen in north america and northwestern europe assemblages of arenigian and llanvirnian benthic shelly fossils chiefly brachiopods from these rocks constitute the celtic biogeographic province that is distinguished by its endemic genera and by some that are older than in other biogeographic provinces some that are younger than elsewhere and others that have not hitherto been found together evidence from these fossils and from the geology of the rocks pertaining to them suggests that north american terranes that included islands of arenigian llanvirnian age lay at middle to high latitudes both in mid ocean and within the armorican continental margin whereas coeval rocks in west central ireland and norway that have toquima table head shelly fossils were originally part of the laurentian continental margin eradication of the celtic assemblage and the reduction of ordovician faunal provinciality in the appalachian caledonide region coincide with llanvirnian obduction subduction of hundreds of kilometres of iapetus ocean crust late llanvirnian and younger ordovician rocks in the remnant basin including some of peri insular origin have brachiopods of scoto appalachian affinities no discrete islands of ashgillian age have been recognized although fossiliferous volcaniclastic and polymictic conglomerates indicate intermontane basins in some places shelly fossils in these rocks and those in the siltstones and limestones from the gaspe peninsula are like those of ashgillian age elsewhere

zoned sodic amphibole petrologic indicator of changing pressure and temperature during tectonism in the bathurst area new brunswick canada amphibole zoned from an actinolite core through a barroisite layer to a crossite or ferroglaucophane rim coexists with epidote chlorite albite quartz phengite sphene magnetite and calcite in metabasites of the tetagouche group near bathurst new brunswick stratigraphic structural and regional metamorphic evidence suggests that these rocks were situated in the hangingwall of a se dipping subduction zone during the taconian orogeny using microprobe analyses of the coexisting minerals a dehydration reaction and a subsequent hydration reaction have been balanced in the system sio2 al2o3 tio2 fe2o3 fe mg mn o cao na2o k2o h2o so as to describe the development of the zoned amphiboles relatively large coefficients for magnetite in these reactions accord with the absence of sodic amphibole in associated magnetite free metabasites indicating that ferric iron has a significant effect on the transition from greenschist to epidote blueschist fades relatively small residuals of feo mgo and mno show that these components are only weakly partitioned between the reactant and product assemblages the procedure of holland and richardson 1979 has been used to calculate apparentp t paths that trace the growth histories of the zoned amphiboles from a low p environment to a high p environment although the apparent pressure change is implausibly large and the final temperature implausibly low it is clear that the bathurst metabasites followed a very differentp t path than those of the austrian alps despite their identical mineral assemblage

structural and tectonic evolution of the acatlan complex southern mexico its role in the collisional history of laurentia and gondwana 1 correlation of deformational phases and thermal events in the acatlan complex permits definition of nine major tectonic events seven events are related to the evolution of the iapetus and rheic oceans by the early ordovician the xayacatlan suite of laurentian affinity was metamorphosed to eclogite facies and exhumed before colliding with the el rodeo suite from the late ordovician to silurian the upper ordovician ixcamilpa suite with peri gondwanan affinity was metamorphosed to blueschist facies exhumed and subsequently was overthrust by the xayacatlan el rodeo block part of the esperanza suite is an early silurian intra iapetian continental arc that collided with and subducted beneath continental crust generating a silurian eclogitic event by the late devonian collision of the three high p t suites closed the iapetus ocean the resulting composite terrain was thrust over the gondwana bordering cosoltepec suite to close the rheic ocean during the earliest mississippian the collision of the acatlan complex and the oaxaca terrane uplifted and deformed the composite terrane by the early permian deposition of the chazumba suite occurred in a pull apart basin a latest early permian event produced thrust shear zones with n s directed shortening during the middle jurassic plume rise caused regional doming magmatism and amphibolite facies metamorphism during the late cretaceous paleogene folds and thrust faults overprint the acatlan complex most events in the acatlan complex correlate with events in the appalachian chain suggesting that the acatlan complex was located near the appalachians until the jurassic when it was displaced by the gulf of mexico opening

the circulation and water masses of the antarctic shelf and continental slope between 30 and 80 e abstract the circulation and water masses from the antarctic continental shelf to 62 s between 30 and 80 e are described using hydrographic data collected on seven hydrographic sections during the baseline research on oceanography krill and the environment west broke west experiment the eastern limb of the weddell gyre dominates circulation between 30 and 40 e and is significantly cooler and fresher than the region to the east the antarctic circumpolar current acc extends from the north into the survey region east of 40 e reaching as far south as 65 5 s at 60 e this results in increasing observed maximum temperature and salinities progressively towards the east peaking at 80 e due to the intrusion of the southern acc front saccf to 63 s this southward extension is steered by the southern end of kerguelen plateau causing a horizontal shear of over 0 15 m s 1 between the eastward acc and westward flowing antarctic slope current asc the asc is observed at all six meridional sections immediately north of the shelf break it is strongly barotropic and transports a total of 15 8 7 4 sv westwards while the bottom referenced baroclinic component only contributes 1 3 0 3 sv at each section this current intensifies to a narrow westward jet with absolute velocities up to 0 3 m s 1 over the steepest shelf slope gradients at 70 e a v shape is observed in the asf this and the nearby presence of denser shelf water and ice shelf water is characteristic of antarctic bottom water aabw formation but no new aabw is found on this section instead significantly warmer saltier and less oxygenated aabw to the east and newly formed aabw high on the continental slope immediately to the west suggest a formation region just west of 70 e this newly formed aabw progressively becomes warmer and saltier west of 60 e and is observed extending offshore and moving westward below eastward flowing water masses acc frontal positions are found to be 1 2 farther north in the survey region than suggested by historical climatology

relict subduction zones in canada 1 relict subduction zones that are interpreted on 10 lithoprobe deep seismic reflection profiles in canada provide key observational evidence that lithosphere plate interactions have operated with essentially the same resultant geometry for at least 2 7 gyr subduction zone reflections are generally found beneath known collisional belts and their orientations are often consistent with the locations of exposed arcs sutures and subduction complexes they project from the lower crust into the upper mantle and provide key geometric relationships between the crust moho and upper mantle the interpretation of subduction zone reflections as being syntectonic establishes a minimum age for the overlying lower crust and mantle wedge and precludes inferred formation of the moho and upper mantle by postorogenic delamination in these areas the orientation of subduction zone reflections may be synthetic or antithetic to exposed sutures indicating juxtaposition by subcretion or by delamination respectively the mantle wedge above the subduction zone reflections may exhibit sparse reflectivity subduction zone reflections commonly dip toward the craton indicating a preferred dip of subduction during the last stages of convergence within an orogen or else a higher likelihood of preservation beneath older domains interpreted ages of the canadian examples range from recent to neoarchean 2 7 ga providing observational evidence that plate tectonics or similar processes have been active since at least the neoarchean the locations and geometries of these relict subduction zones provide a snapshot of accretion boundaries in the mantle and establish a framework for the neoarchean recent accretionary architecture of the north american lithosphere