sea ice motion in response to geostrophic winds it is an old rule of thumb that sea ice moves with a speed of about 2 of the surface wind and about 45 to the right of the wind a similar relationship between the ice velocity and the geostrophic wind is examined here it is found that only about half of the long term several month average ice motion is directly related to the geostrophic wind the other half being due to the mean ocean circulation on shorter time scales and in all seasons more than 70 of the variance of the ice velocity in the central arctic ocean is explained by the geostrophic wind within about 400 km of the coasts the geostrophic wind is less successful in explaining the ice motion the spatial variations in ice velocity are also partly explained by the geostrophic wind about half of the variance in the large scale ice vorticity and shear are accounted for on the other hand none of the large scale ice divergence can be explained by the wind the long term average ocean current is estimated by subtracting the share of the ice motion caused by the wind from the total ice motion

space geodetic observations of nazca south america convergence across the central andes space geodetic data recorded rates and directions of motion across the convergent boundary zone between the oceanic nazca and continental south american plates in peru and bolivia roughly half of the overall convergence about 30 to 40 millimeters per year accumulated on the locked plate interface and can be released in future earthquakes about 10 to 15 millimeters per year of crustal shortening occurred inland at the sub andean foreland fold and thrust belt indicating that the andes are continuing to build little 5 to 10 millimeters per year along trench motion of coastal forearc slivers was observed despite the oblique convergence

timing and duration of partial melting and magmatism in the variscan montagne noire gneiss dome french massif central unravelling the detailed pressure temperature time deformation p t t d evolution of magmatic and metamorphic rocks provides essential insights into the timing and duration of partial melting and related plutonism during crustal flow and migmatitic dome formation the montagne noire axial zone mnaz is a migmatitic dome located within the variscan orogen in the southern french massif central the timing of the main thermal event that was responsible for intense partial melting is still highly debated in this study we present new laser ablation inductively coupled plasma mass spectrometry la icp ms age data on micaschists migmatites and granites that clarify the p t t d evolution of the mnaz structurally controlled samples were collected in order to constrain the timing of metamorphism migmatization and plutonism regarding the main structural pattern d1 d2 and d3 d1 and d2 correspond to nappe stacking and dextral transpression respectively d3 is related to vertical shortening and coaxial thinning with a preferential ne sw to e w directed stretching la icp ms analyses on the syntectonic angles soulie and martys granites yielded u th pb monazite ages of 305 1 5 306 1 9 and 314 2 ma respectively five migmatitic rocks sampled in the eastern and central espinouse area yielded in situ ages ranging between 312 2 and 301 2 ma along the dome envelope two garnet staurolite bearing micaschists near saint pons de thomieres village gave in situ u th pb ages of 312 1 2 1 and 309 0 3 1 ma a fine grained gneiss with a d3 fabrics in the eastern dome envelope yield a 208pb 232th mean age at 305 7 3 9 ma all ages obtained in this study for the micaschists migmatites and granites range between 315 and 301 ma we interpret this time span as the record of the high thermal event responsible for intense crustal partial melting within the lower and middle crust the onset of partial melting occurred at ca 315 ma that marked the beginning of transpressional deformation d2 based on structural and petrological studies our new u th pb results suggest that 1 partial melting may have started at ca 315 ma and lasted 15 10 myr and 2 d2 et d3 developed between 315 and 300 ma and were synchronous d1 deformation ended at 315 ma the onset and duration of d1 related to nappe stacking and crustal thickening is still uncertain

fore arc tectonothermal evolution of the el oro metamorphic province ecuador during the mesozoic the el oro metamorphic province of sw ecuador is a composite massif made of juxtaposed terranes of both continental and oceanic affinity that has been located in a fore arc position since late paleozoic times various geochemical geochronological and metamorphic studies have been undertaken on the el oro metamorphic province providing an understanding of the origin and age of the distinct units however the internal structures and geodynamic evolution of this area remain poorly understood our structural analysis and thermal modeling in the el oro metamorphic province show that this fore arc zone underwent four main geological events 1 during triassic times 230 225 ma the emplacement of the piedras gabbroic unit at crustal root level 9 kbar triggered partial melting of the metasedimentary sequence under an e w extensional regime at pressure temperature conditions ranging from 4 5 to 8 5 kbar and from 650 to 900 c for the migmatitic unit 2 at 226 ma the tectonic underplating of the arenillas panupali oceanic unit 9 kbar and 300 c thermally sealed the fore arc region 3 around the jurassic cretaceous boundary the shift from trench normal to trench parallel subduction triggered the exhumation and underplating of the high pressure oceanic raspas ophiolitic complex 18 kbar and 600 c beneath the el oro group 130 120 ma this was followed by the opening of a ne sw pull apart basin which tilted the massif along an e w subhorizontal axis 110 ma 4 in late cretaceous times an n s compressional event generated heterogeneous deformation due to the presence of the cretaceous celica volcanic arc which acted as a buttress and predominantly affected the central and eastern part of the massif

erosional control on the geometry and kinematics of thrust belt development in the central andes long term erosion rates in the central andes may have influenced not only mean elevation and relief but also the regional geometry and kinematic history of the orogenic belt a drastic along strike erosional gradient exists in the modern central andes from a high erosion region directly north of the 17 5 s bend in the andes to a low erosion region south of the bend this gradient has existed since 10 15 ma based on fission track analyses of middle miocene to holocene denudation and qualitative evaluations of the preservation potential of middle late tertiary volcanic edifices and synorogenic sediment global positioning system velocity data indicate conflicting patterns of active surface shortening north and south of the 17 5 s bend whereas present day shortening in the thrust belt north of the bend is distributed over much of the width of the belt south of the bend it is concentrated near the eastern frontal margin structural data suggest a similar kinematic situation during late miocene to holocene shortening an out of sequence chronology of thrusting in the narrow 200 km wide thrust belt to the north versus a forward breaking sequence of thrusting in the relatively wide 350 km wide thrust belt to the south the long term internal deformation and limited width of the thrust belt north of the bend are attributed to prolonged subcritical thrust wedge conditions induced by rapid erosion rates since 10 15 ma such conditions inhibit thrust front advance and favor distributed deformation within the thrust belt interior in the thrust belt south of the bend a progressive eastward migration of thrusting is interpreted to be the result of long term critical thrust wedge conditions promoted by extremely low rates of denudation since middle miocene time

rates of shortening propagation underthrusting and flexural wave migration in continental orogenic systems the rate of horizontal shortening in an orogenic wedge is the rate at which the length of undeformed crust decreases as it is incorporated into the orogen this rate equals the rate of convergence of the foreland lithosphere toward the central surface of the orogenic belt and the rate of subduction of foreland lithosphere beneath the central surface the rate of propagation of an orogenic wedge is the rate at which it elongates in the direction of horizontal shortening this rate is controlled by the rates of mass accretion to the orogenic wedge and erosion the orogenic belt drives a flexural isostatic wave through the foreland lithosphere at a velocity equal to the rate of propagation plus the rate of subduction or convergence or shortening in orogenic belts where the total amount of shortening cannot be reliably estimated from balanced regional cross sections it may be possible to determine total shortening from the distance of flexural wave migration in the foreland basin and the width of the orogenic wedge in addition orogenic wedges may accelerate solely in response to a reduction in taper

modelling granite migration by mesoscale pervasive flow abstract mesoscale pervasive magma migration leads to granite injection complexes common in hot crustal terranes pervasive migration is limited by magma freezing when intruding cold country rock here we explore numerically the feedback mechanism between magma intrusion and heating of the country rock which allows younger intrusive batches to reach increasingly shallower cooler levels this process relies on the higher solidus temperature of a rock compared to that of its melt once melt is segregated we define the free ride layer as the region above the melt source where magma may freely migrate because rock temperature is above melt solidus the top of the free ride layer which corresponds to the melt solidus t s isotherm is at the limiting depth z s after magma passes through the free ride layer the magma front is always at the limiting depth we modeled the thickening and heating of the crust above the source as melt at its liquidus t l intrudes it pervasively from below we found that a magma quickly warms crust below z s to about t l forming a step in temperature at z s b the front z s moves up through the crust as more magma is intruded c as magma is emplaced at the front a mingled layer of about half magma half crust forms below it so that the total rise of the front corresponds approximately to half of the thickness of magma added to the free ride layer d the rate of rise of the front depends on the temperature difference between crust and t l and slows down as the magma front rises e for most reasonable intrusion rates and volumes the crust above z s feels little influence of the intrusion because the diffusion time scale is much smaller than the rise rate of the front in summary pervasive migration is an efficient way of heating the lower to middle crust and can result in an injection complex several kilometers thick consisting of about half magma and half original crust

a parameterization of the ice ocean drag coefficient 1 a parameterization of the ice ocean drag coefficient cw was developed through partitioning the oceanic drag force into three components 1 form drag on the floe edge 2 form drag on the ridge keel and 3 skin friction on the ice bottom through these quantities cw was expressed as a function of observable sea ice geometric parameters sensitivity studies were carried out to investigate the influence of varying sea ice conditions on cw the results revealed that cw increases first and then decreases with increasing ice concentration a similar to the observations of the air ice drag coefficient and which is mainly attributed to the nonmonotonic variation of the form drag on the floe edge with ice concentration moreover the form drag on the floe edge is always the dominant component having a proportion of more than 60 in sea ice with a large aspect ratio draft length 1 100 indicating the necessity of including this term in sea ice dynamic models particularly for the marginal ice zone miz the form drag on the ridge keel becomes dominant only when the ridging intensity is extremely high depth spacing 1 20 additionally a large value of cw cannot be caused only by the inclusion of form drag terms but also by large skin friction over rough ice bottoms finally for typical situations in the miz with moderate ridging intensity the parameterization will underestimate cw by approximately 30 for a rough ice bottom and by over 80 for a smooth ice bottom if no form drags are considered

thermally softened continental extensional zones arcs and rifts as precursors to thickened orogenic belts abstract intra continental deformation of soft zones during continental collision requires weak continental lithosphere which is able to be shortened across considerable width during later convergence this enables significant thickening with formation of an orogenic root we have examined models with a history of lithospheric thinning by pure shear during an earlier phase of intra continental extension with associated heating geologically this situation is appropriate to intra continental rifts and back arc basins if thinning of elevated thermal structure is decoupled from the thinning of lithology then a weak soft lower crust and sub arc rift mantle result this weak structure has a favoured rheology for subsequent convergent thickening while the lithosphere is still hot these regions are associated with formation of granulites and metamorphic assemblages typical of high temperature low pressure ht lp if convergence starts while the heat input is still active then the failed rifts and arcs are shut off by lateral wedging of the hard lower crust and upper mantle of shoulder regions into the softened arc rift domain such sites are ideal for the formation and for the exhumation of metamorphic core complexes subsequent thickening during convergence leads to ht eclogites when the previous arc rift was hot and to medium t eclogites for a thickened standard geotherm these p t paths are counterclockwise and their shapes are strongly dependent on the amount of previous thinning and type of initial geotherm if the compression starts long after cessation of the extensional event and associated thermal anomaly then the geotherm of the extended area relaxes and the whole region hardens in this case no homogeneous thickening occurs and deep continental roots cannot form

mid crustal flow during tertiary extension in the ruby mountains core complex nevada structural analysis and geochronologic data indicate a nearly orthogonal late eocene oligocene flow pattern in migmatitic infrastructure immediately beneath the kilometer thick extensional mylonitic shear zone of the ruby mountains metamorphic core complex nevada new u pb radiometric dating indicates that the development of a northward trending lineation in the infrastructure is partly coeval with the development of a pervasive west northwest trending lineation in the mylonitic shear zone u pb monazite data from the leucogranite orthogneiss of thorpe creek indicate a crystallization age of ca 36 39 ma zircon fractions from a biotite monzogranite dike yield an age of ca 29 ma the three dated samples from these units exhibit a penetrative approximately north south trending elongation lineation this lineation is commonly defined by oriented bundles of sillimanite and or elongated aggregates of quartz and feldspar indicating a synmetamorphic and syndeformational origin the elongation lineation can be interpreted as a slip line in the flow plane of the migmatitic nonmylonitic infrastructural core of the northern ruby mountains a portion of this midcrustal flow is coeval with the well documented west northwest sense of slip in the structurally overlying kilometer thick mid tertiary mylonitic shear zone lineations in the mylonitic zone are orthogonal to those in the deeper infrastructure suggesting fundamental plastic decoupling between structural levels in this core complex furthermore the infrastructure is characterized by overlapping oppositely verging fold nappes which are rooted to the east and west one of the nappes may be synkinematic with the intrusion of the late eocene orthogneiss of thorpe creek in addition the penetrative elongation lineation in the infrastructure is subparallel to hinge lines of parasitic folds developed synchronous with the fold nappes suggesting a kinematically related evolution the area is evaluated in terms of a whole crust extension model magmatic underplating in the lower crust stimulated the production of late eocene early oligocene granitic magmas which invaded metasedimentary and mesozoic granitic rocks of the middle crust the midcrustal rocks weakened by the magmatic heat influx acted as a low viscosity compensating material decoupled from an extending upper crust the fold nappes and lineation trends suggest large scale flow of the weakened crust into the study area the inflow pattern in the migmatitic infrastructure can be interpreted as a manifestation of midcrustal migration into an area beneath a domain of highly extended upper crustal rocks at present the inferred eocene early oligocene phase of upper crust extension remains unknown but available data on relative and geochronologic timing are not inconsistent with our model of return flow into an area already undergoing large scale upper crustal extension

role of ice dynamics in anomalous ice conditions in the beaufort sea during 2006 and 2007 1 a new record minimum in summer sea ice extent was set in 2007 and an unusual polynya formed in the beaufort sea ice cover during the summer of 2006 using a combination of visual observations from cruises ice drift and satellite passive microwave sea ice concentration we show that ice dynamics during preceding years included events that preconditioned the beaufort ice pack for the unusual patterns of opening observed in both summers intrusions of first year ice from the chukchi sea to the northern beaufort and increased pole ward ice transport from the western arctic during summer has led to reduced replenishment of multiyear ice older than five years in the western beaufort resulting in a younger thinner ice pack in most of the beaufort we find ice younger than five years melts out completely by the end of summer south of 76n the 2006 unusual polynya was bounded to the south by an ice tongue composed of sea ice older than 5 years and formed when first year and second year ice melted between 76n and the older ice to the south in this paper we demonstrate that a recent shift in ice circulation patterns in the western arctic preconditions the beaufort ice pack for increased seasonal ice zone extent

possible thermal buffering by crustal anatexis in collisional orogens thermobarometric evidence from the nepalese himalaya one dimensional thermal models suggest that large thrust faults may exert first order control on the thermal structure of collisional orogenic belts and an increasing body of petrologic data may be interpreted as consistent with these models we present new thermobarometric data from the central nepalese himalaya which imply that large sections of the himalayan orogen may have been at roughly uniform temperatures during active thrusting a phenomenon that is not predicted by existing models we attribute this phenomenon to thermal buffering as a consequence of widespread in situ anatexis if this process is common in the intermediate and lower crust of active orogens and if it is localized spatially one dimensional thermal models may produce an extremely oversimplified picture of heat transfer processes during collisional orogenesis

two circulation regimes of the wind driven arctic ocean the major goal of this paper is to demonstrate the existence in the arctic ocean of two regimes of wind forced circulation we simulated the vertically averaged currents sea level heights and ice drift in the arctic ocean from 1946 to 1993 using a two dimensional wind forced barotropic model that includes frictional coupling between the ocean and ice the model has a spatial resolution of 55 5 km and is driven by winds river runoff and an imposed but realistic sea level slope between the pacific and the atlantic oceans there is a good agreement between velocities from observed buoy motions and velocities of modeled ice drift even though the model lacks ocean baroclinicity and ice thermodynamics the results indicate that wind driven motion in the central arctic alternates between anticyclonic and cyclonic circulation with each regime persisting for 5 7 years based upon our analysis of the modeled sea level and ice motion anticyclonic wind driven motion in the central arctic appeared during 1946 1952 1958 1963 1972 1979 and 1984 1988 and cyclonic motion appeared during 1953 1957 1964 1971 1980 1983 and 1989 1993 shifts from one regime to another are forced by changes in the location and intensity of the icelandic low and the siberian high the two regimes may help explain the significant basin scale changes in the arctic s temperature and salinity structure observed recently the great salinity anomaly and the variability of ice conditions in the arctic ocean

the impact of variable sea ice roughness on changes in arctic ocean surface stress a model study the arctic sea ice cover is thinning and retreating causing changes in surface roughness that in turn modify the momentum flux from the atmosphere through the ice into the ocean new model simulations comprising variable sea ice drag coefficients for both the air and water interface demonstrate that the heterogeneity in sea ice surface roughness significantly impacts the spatial distribution and trends of ocean surface stress during the last decades simulations with constant sea ice drag coefficients as used in most climate models show an increase in annual mean ocean surface stress 0 003 n m2 per decade 4 6 due to the reduction of ice thickness leading to a weakening of the ice and accelerated ice drift in contrast with variable drag coefficients our simulations show annual mean ocean surface stress is declining at a rate of 0 002 n m2 per decade 3 1 over the period 1980 2013 because of a significant reduction in surface roughness associated with an increasingly thinner and younger sea ice cover the effectiveness of sea ice in transferring momentum does not only depend on its resistive strength against the wind forcing but is also set by its top and bottom surface roughness varying with ice types and ice conditions this reveals the need to account for sea ice surface roughness variations in climate simulations in order to correctly represent the implications of sea ice loss under global warming

on the remote sensing of oceanic and atmospheric convection in the greenland sea by synthetic aperture radar 1 in this paper we discuss characteristic properties of radar signatures of oceanic and atmospheric convection features in the greenland sea if the water surface is clean no surface films or ice coverage oceanic and atmospheric features can become visible in radar images via a modulation of the surface roughness and their radar signatures can be very similar for an unambiguous interpretation and for the retrieval of quantitative information on current and wind variations from radar imagery with such signatures theoretical models of current and wind phenomena and their radar imaging mechanisms must be utilized we demonstrate this approach with the analysis of some synthetic aperture radar sar images acquired by the satellites ers 2 and radarsat 1 in one case an ers 2 sar image and a radarsat 1 scansar image exhibit pronounced cell like signatures with length scales on the order of 10 20 km and modulation depths of about 5 6 db and 9 10 db respectively simulations with a numerical sar imaging model and various input current and wind fields reveal that the signatures in both images can be explained consistently by wind variations on the order of 2 5 m s but not by surface current variations on realistic orders of magnitude accordingly the observed features must be atmospheric convection cells this is confirmed by visible typical cloud patterns in a noaa avhrr image of the test scenario in another case the presence of an oceanic convective chimney is obvious from in situ data but no signatures of it are visible in an ers 2 sar image we show by numerical simulations with an oceanic convection model and our sar imaging model that this is consistent with theoretical predictions since the current gradients associated with the observed chimney are not sufficiently strong to give rise to significant signatures in an ers 2 sar image under the given conditions further model results indicate that it should be generally difficult to observe oceanic convection features in the greenland sea with ers 2 or radarsat 1 sar since their signatures resulting from pure wave current interaction will be too weak to become visible in the noisy sar images in most cases this situation will improve with the availability of future high resolution sars such as radarsat 2 sar in fine resolution mode 2004 and terrasar x 2005 which will offer significantly reduced speckle noise fluctuations at comparable spatial resolutions and thus a much better visibility of small image intensity variations on spatial scales on the order of a few hundred meters index terms 3314 meteorology and atmospheric dynamics convective processes 4275 oceanography general remote sensing and electromagnetic processes 0689 4279 oceanography general upwelling and convergences 4255 oceanography general numerical modeling

characteristics of the beaufort sea high abstract characteristics of the arctic ocean s beaufort sea high are examined using fields from the ncep ncar reanalysis at a 2 hpa contour interval the beaufort sea high appears as a closed anticyclone in the long term annual mean sea level pressure field and in spring in winter the beaufort sea region is influenced by a pressure ridge at sea level extending from the siberian high to the yukon high over northwestern canada as assessed from 6 hourly surface winds the mean frequency of anticyclonic surface winds over the beaufort sea region is fairly constant through the year while for all seasons a strong closed high can be interpreted as the surface expression of an amplified western north american ridge at 500 hpa there is some suggestion of a split flow where the ridge linked to the surface high is separated from the ridge to the south that lies within the main belt of westerlies the aleutian low in the north pacific tends to be deeper than normal when there is a strong beaufort sea high in

wind driven freshwater buildup and release in the beaufort gyre constrained by mesoscale eddies recently the beaufort gyre has accumulated over 20 km3 of freshwater in response to strong anticyclonic atmospheric winds that have prevailed over the gyre for almost two decades here we explore key physical processes affecting the accumulation and release of freshwater within an idealized eddy resolving model of the beaufort gyre we demonstrate that a realistic halocline can be achieved when its deepening tendency due to ekman pumping is counteracted by the cumulative action of mesoscale eddies based on this balance we derive analytical scalings for the depth of the halocline and its spin up time scale and emphasize their explicit dependence on eddy dynamics our study further suggests that the beaufort gyre is currently in a state of high sensitivity to atmospheric winds however an intensification of surface stress would inevitably lead to a saturation of the freshwater content a constraint inherently set by the intricacies of the mesoscale eddy dynamics

the interdependence of deformational and thermal processes in mountain belts crustal temperatures within collisional orogens are anomalously high compared with temperatures at comparable depths in stable continents which is evidence of thermal processes that are fundamental to orogenesis these temperatures can be explained by the redistribution of crust enriched in heat producing elements through the accretion of crust from the down going plate to the upper plate and surface erosion with the use of geologically reasonable rates the model results predict high temperatures over 600 c and inverted upper plate geotherms about 100 c over 20 kilometers at shallow depths 20 to 40 kilometers by 25 to 35 million years after collision this study emphasizes the interdependence of deformational surficial and thermal processes

archean granite greenstone tectonics at kolar south india interplay of diapirism and bulk inhomogeneous contraction during juvenile magmatic accretion 1 the structural study of the kolar greenstone belt and surrounding granite gneiss terrains combined with u pb dating reveals that the middle and lower crustal tectonoplutonic pattern of the eastern dharwar craton developed during a major magmatic accretion event between 2550 and 2530 ma the granite greenstone pattern resulted from the interference of gravity driven sagging of the greenstones i e diapirism e w bulk inhomogeneous shortening combined with horizontal n s stretching and syntectonic juvenile pluton emplacement bulk inhomogeneous contraction is accommodated by the synchronous development of a pervasive n s trending vertical foliation shallow stretching lineation and conjugate strike slip shear zone pattern within and outside the greenstone belt resulting in regional horizontal pure shear deformation the plutons around the greenstone belt record regional contraction by developing one set of strike slip c s fabrics of the shear zone pattern the development of the granite greenstone pattern was coeval and compatible with deformation during juvenile magmatic accretion melting and granulite metamorphism in the lower crust the kolar example points to a specific crustal rheology that allowed sagduction of the greenstones and regional distributed bulk inhomogeneous strain due to mechanical homogeneity and low viscosity provided by large scale melting during the accretion event this example further suggests specific boundary conditions to the craton that allowed e w inhomogeneous shortening to be accommodated by n s stretching and spreading of the crust without significant tectonic thickening such tectonoplutonic pattern is specific to the archean and may develop as a consequence of mantle plume activity in intracontinental settings

ekman circulation in the arctic ocean beyond the beaufort gyre data derived from satellite based observations with buoy based observations and assimilations are used to calculate ocean ekman layer transport and evaluate long term trends in the arctic ocean over the period 1979 2014 the 36 year mean of upwelling downwelling is 3 6 2 0 4 0 2 2 sv for the entire arctic basin with 0 4 sv net downwelling contributed mostly by the canadian region with regard to long term trends the annual mean upwelling downwelling over the entire arctic basin is increasing at a linear rate of 0 89 0 96 sv decade the canada alaska coasts and beaufort and laptev seas are regions of greatest ekman transport intensification the central arctic ocean and lincoln sea also have an increasing trend in transport the canadian and eurasian regions each account for about half the total vertical ekman variations in the arctic basin this article is protected by copyright all rights reserved

protracted coeval crust and mantle melting during variscan late orogenic evolution u pb dating in the eastern french massif central the late stages of the variscan orogeny are characterized by middle to lower crustal melting and intrusion of voluminous granitoids throughout the belt which makes it akin to hot orogens these processes resulted in the development of large granite migmatite complexes the largest of which being the 305 300 ma old velay dome in the eastern french massif central fmc this area also hosts a wide range of late variscan plutonic rocks that can be subdivided into four groups i cordierite bearing peraluminous granites cpg ii muscovite bearing peraluminous granites mpg iii k feldspar porphyritic calc alkaline granitoids kcg and iv mg k rich monzo diorites and lamprophyres vaugnerites new results of la sf icp ms u pb zircon and monazite dating on 33 samples from all groups indicate that both granites and mafic rocks emplaced together over a long period of 40 million years throughout the carboniferous as shown by intrusion ages between 337 4 1 0 and 298 9 1 8 ma for the granitoids and between 335 7 2 1 and 299 1 1 3 ma for the vaugnerites low zircon saturation temperatures and abundant inherited zircons with predominant late ediacaran to early cambrian ages indicate that the cpg and mpg formed through muscovite or biotite dehydration melting of ortho and paragneisses from the lower gneiss unit the kcg and vaugnerites contain very few inherited zircons if any suggesting higher magma temperatures and consistent with a metasomatized lithospheric mantle source for the vaugnerites the kcg can be explained by interactions between the cpg mpg and the vaugnerites or extensive differentiation of the latter the new dataset provides clear evidence that the eastern fmc was affected by a long lived magmatic episode characterized by coeval melting of both crustal and mantle sources this feature is suggested here to result from a lithospheric scale thermal anomaly triggered by the removal of the lithospheric mantle root the spatial distribution of the dated samples points to a progressive southward delamination of the lithospheric mantle perhaps in response to rollback following continental subduction or to retro delamination owing to the retreat of a south verging subduction zone

erosion and tectonics at the margins of continental plateaus the topography across the eastern margin of the central andean plateau north of 18 s beni region bears a strong resemblance to the topography of the southern margin of the tibetan plateau nepal himalaya with both regions featuring a steep frontal slope and high peaks at the plateau edge in contrast the topography of the eastern margin of the central andean plateau south of 18 s pilcomayo region tapers toward the foreland more gently and has no line of high peaks at the margin both the himalayan and the beni regions have been the sites for large amounts of denudation and in both regions geologic evidence suggests that erosion has been sufficiently vigorous for the physiographic plateau margin to have retreated toward the plateau interior during the neogene we hypothesize that the steep frontal slope and high peaks of the beni region and himalayan front largely reflect the high orographic precipitation and high erosion rates occurring in these regions and that the more gentle topography of the semiarid pilcomayo region reflects a tectonic landform only slightly modified by erosion we propose that orographic precipitation impinging on a plateau margin will generally tend to drop moisture low on the slope eroding back the plateau while enhancing or maintaining the steep long wavelength slope a numerical model coupling orographic precipitation erosion and tectonic uplift demonstrates the plausibility of this hypothesis the erosional efflux in both the beni and nepal himalaya have been considerable and simple mass balance calculations for the himalaya suggest that during the neogene the erosional mass efflux has generally outpaced the tectonic mass influx this contrasts with the apparent prior domination of tectonic influx and may reflect a decrease in the rate of tectonic addition during the same period and or increased late cenozoic erosion rates

the aerodynamic roughness of different types of sea ice the aerodynamic roughness for all major types of sea ice is specified on the basis of surface layer measurements from ship and ice floe platforms median neutral surface drag coefficients cdn 1000 for grease and nilas ice are 0 7 and 1 6 small medium large and fused pancake ice have cdn equal to 0 9 1 6 2 4 and 1 9 young ice cdn ranges from 2 3 to 3 1 very smooth first year or multiyear ice with no pressure ridges has cdn equal to 1 5 pack ice has median values of 2 0 and 2 2 for first year and multiyear ice marginal ice zone ice averages 3 1 and 3 4 first year multiyear if unaffected by wave action wave affected ice has cdn values of 4 2 and 4 6 first year multiyear extremely rough multiyear ice in intense shear or compaction regions has cdn equal to 8 0 open ocean areas within 100 km of an ice edge averaged 1 8 while ice free water downwind of ice with small or no waves is 1 4 these values can be adjusted if heat fluxes are present the accuracy of the cdn averages is estimated to be 20 or better

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

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

the laptev sea as a source for recent arctic ocean salinity changes this study was motivated by observations of significant salinification of the upper eurasian basin that began around 1989 observational data and modeling results provide evidence that increased arctic atmospheric cyclonicity in the 1990s resulted in a dramatic increase of the salinity in the eurasian basin two mechanisms account for the laptev sea salinization eastward diversion of russian rivers and increased brine formation due to enhanced ice production in numerous leads in the laptev sea ice cover both these mechanisms are linked to changes in wind patterns and were essential to the formation of salinity anomalies the resulting laptev sea salinity anomaly was advected to the central eurasian basin the strong salinization over the eurasian basin altered the formation of cold halocline waters weakened vertical stratification and released heat upward from below the cold halocline layer our analysis suggests that local processes in the laptev sea may have a basin wide impact on the thermohaline structure of the arctic ocean

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

an unexpected seasonal variability of salinity in the beaufort sea upper layer in 1996 1998 1 the salinity in the upper beaufort sea from the mixed layer to the thermocline layer was observed by drifting buoys from 1996 to 1998 the salinity in this depth range was lower in winter and higher in the summer the exact opposite from what one would expect from the seasonal cycle of the freshwater flux associated with the ice melting freezing and river runoff in this study we calculated the daily ekman transport and upwelling velocity in the beaufort sea using both satellite and buoy data in fall and winter months the offshore transport of low salinity water from the coastal area toward the interior where the buoys were located was observed to be strong this horizontal ekman transport led to the freshening of the surface ekman layer in the buoy location the convergence of the ekman transport resulted in a strong downwelling in the offshore regions and so the halocline and thermocline were pushed downward the downwelling then results in the freshening of the subsurface salinity as observed by buoys other processes such as lateral advection may have also played a role in the subsurface freshening the lack of in situ observations needed to estimate the salinity gradient makes it difficult to assess more accurately the contribution from lateral advection a scaling analysis using the salinity climatology suggests that the lateral salinity advection though considerably smaller than the vertical one may not be negligible

the geochemical behavior of f and cl during the weathering diagenesis metamorphism anatexis cycle insights from the clay fraction of fine sands from the amazon river mouth and metapelititic rocks from the serido belt brazil abstract the average contents f 788 ppm and cl 10 of the clay fraction from amazon river sediments are comparable only to cl poor morb and evolved granite and rhyolite the high content cl 180 ppm and low f cl 3 of average shale is due to interaction of the sediments with high salinity diagenetic fluids the serido schist belt is used to assess the behavior of f and cl during shale metamorphism and anatexis its ratio f cl 10 ppm is high compared to average shale and is related to the strong partitioning of cl into metamorphic fluids the obtained data suggest that the cl partitioned into shales during diageneses is released back into fluids produced during metamorphic conditions the experimental data indicate that anatexis of mica schist significantly fractionates f cl ratios producing schist residue with f cl 38 and peraluminous melts with f cl 6

near isothermal conditions in the middle and lower crust induced by melt migration simple one dimensional thermal models that include the effects of melt migration have been developed the models show that long lived plutonism results in a quasi steady state geotherm with a rapid temperature increase in the upper crust and nearly isothermal conditions in the middle and lower crust the thermal structure of the crust strongly influences deformation metamorphism and plutonism1 2 3 models for the geothermal gradient in stable crust predict a steady increase of temperature with depth this thermal structure however is incompatible with observations from high temperature metamorphic terranes exhumed in orogens1 4 5 6 global compilations7 of peak conditions in high temperature metamorphic terranes define relatively narrow ranges of peak temperatures over a wide range in pressure for both isothermal decompression and isobaric cooling paths here we develop simple one dimensional thermal models that include the effects of melt migration these models show that long lived plutonism results in a quasi steady state geotherm with a rapid temperature increase in the upper crust and nearly isothermal conditions in the middle and lower crust the models also predict that the upward advection of heat by melt generates granulite facies metamorphism and widespread andalusite sillimanite metamorphism in the upper crust once the quasi steady state thermal profile is reached the middle and lower crust are greatly weakened due to high temperatures and anatectic conditions thus setting the stage for gravitational collapse8 exhumation and isothermal decompression after the onset of plutonism near isothermal conditions in the middle and lower crust result from the thermal buffering effect of dehydration melting reactions that in part control the shape of the geotherm

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

intensification of geostrophic currents in the canada basin arctic ocean abstractcontinuous sampling of upper ocean hydrographic data in the canada basin from various sources spanning from 2003 through 2011 provides an unprecedented opportunity to observe changes occurring in a major feature of the arctic ocean in a 112 km radius circle situated near the center of the traditional beaufort gyre geopotential height referenced to 400 dbar increased by about 0 3 gpm from 2003 to 2011 and by the end of the period had increased by about 65 from the climatological value near the edges of the domain considered the anomalies in dynamic height are much smaller indicating steeper gradients a rough dynamic topography constructed from profiles collected between 2008 and 2011 shows the center of the gyre to have shifted south by about 2 in latitude along the 150 w meridian geostrophic currents are much stronger on the periphery of the gyre reaching amplitudes 5 6 times higher than climatological values at grid points just offshore from the beaufort and chukchi shelf slopes esti

barotropic wave propagation and instability and atmospheric teleconnection patterns abstract a global barotropic model linearized about the 300 mb climatological mean january flow is perturbed by applying a series of localized forcings distributed throughout the tropics and subtropics structures which resemble the observed pacific north american and east atlantic teleconnection patterns noted by wallace and gutzler 1981 tend to recur in the responses similar patterns are found to result from the dispersion of isolated initial perturbations placed at a variety of locations in the tropics and midlatitudes it is shown that these structures are related to the most rapidly growing mode associated with barotropic instability of the zonally varying climatological basic state in the absence of damping this mode has an e folding time of about a week and a period close to 50 days in localized regions the instantaneous growth rates can be competitive with those of baroclinic instability these episodes of rapid local barotropic growth are interspersed with intervals in which the local

salinity trends on the siberian shelves 1 we present an analysis of observed long term 100 year salinity trends on the freshwater dominated siberian continental shelves a multiple regression was performed in the white sea ws the kara sea ks the laptev sea ls and the east siberian sea ess since 1930 the ws has gained freshwater while the ess has lost it consistent with river discharge trends over this period over the past 20 years increases in both river discharge and direct precipitation can explain observed salinity decreases in the ws but not in the ks salinity trends in the ls and ess indicate that ocean circulation plays a dominate role in these areas where in recent years freshwater has been diverted eastward along the coast rather than northward toward the deep ocean

on the ice and heat balance in fram strait the sea ice margin northwest of spitsbergen is schematically represented by a stationary wedge maintained by opposing ice and warm atlantic water advection ice melt and cooling of the surface water generate a mixed layer with properties approximating those found elsewhere in the arctic basin the heat energy required to melt the advancing ice is roughly comparable to the rate of heat loss of the west spitsbergen current and suggests that direct heat exchange with the atmosphere is less important only a small fraction of the nearly fresh water from ice melt exits through eastern fram strait and most of it appears to return to the arctic basin whether or not it contributes to the formation and maintenance of the arctic mixed layer depends on the largely unknown circulation pattern north of fram strait an example of the large interannual variation of the mean wind stress field is shown to illustrate the likelihood of an equally large variance of the ice and surface water circulation

permian bimodal magmatism in the southern margin of the central asian orogenic belt beishan xinjiang nw china petrogenesis and implication for post subduction crustal growth abstract previous studies show that permian bimodal magmatism is widespread in the tianshan region the southern part of the central asian orogenic belt in northern xinjiang nw china we have found that such bimodal magmatism extended farther to the south in the beishan region our new zircon u pb age data show that the dolerite and granite dykes in the poyi area of the region were emplaced at 270 ma 20 myr after subduction ended in the region the dated dolerite dyke is characterized by pronounced negative nb ta anomalies high end t from 4 3 to 5 1 and low initial 87sr 86sr ratios from 0 7051 to 0 7060 for whole rocks plus high ehf t from 8 96 to 13 0 and elevated 18o from 5 8 to 6 5 for zircon crystals which indicate a subduction modified mantle source and moderate degrees of crustal contamination 25 myr and show no age progression in a single direction which are inconsistent with the results of mantle plume magmatism whole rocks from the permian granite dyke in the poyi area are characterized by strong negative nb ta anomalies negative end t from 2 4 to 1 6 and high initial 87sr 86sr from 0 7080 to 0 7088 magmatic zircon crystals from the granite dyke have ehf t from 0 86 to 5 64 and 18o from 6 5 to 7 2 the isotope compositions of the granite dyke are within the ranges of the pre permian crustal rocks in the region indicating a predominantly crustal source based on these data we propose that partial melting in the crust was induced by underplating of mantle derived mafic magmas some of which were emplaced in the upper parts of the crust to form the broadly coeval dolerite dykes and mafic ultramafic intrusions the results from this study support the view that post subduction bimodal magmatism is important for continental crustal growth and differentiation

diachronous post orogenic magmatism within a developing orocline in iberia european variscides 1 u pb zircon crystallization ages of 52 late variscan granitoid intrusions from nw iberia 19 from new data 33 from previous studies constrain the lithospheric evolution of this realm of the variscan belt of western europe and allow assessment of the relationship between oroclinal development and magmatism in late carboniferous early permian times the u pb ages in conjunction with a range of geological observations are consistent with the following sequence of events i oroclinal bending starts at 310 305 ma producing lithospheric thinning and asthenospheric upwelling in the outer arc of the orocline accompanied by production of mantle and lower crustal melts ii between 305 and 300 ma melting continues under the outer arc of the orocline central iberian zone of the iberian variscan belt and mid crustal melting is initiated coevally the lithospheric root beneath the inner arc of the orocline thickened due to progressive arc closure iii between 300 and 292 ma foundering of the lithospheric root followed by melting in the lithospheric mantle and the lower crust beneath the inner arc due to upwelling of asthenospheric mantle iv cooling of the lithosphere between 292 and 286 ma resulting in a drastic attenuation of lower crustal high temperature melting by 285 ma the thermal engine generated by orocline driven lithospheric thinning delamination had cooled down beyond its capability to produce significant amounts of mantle or crustal melts the model proposed explains the genesis of voluminous amounts of granitoid magmas in post orogenic conditions and suggests that oroclines and similar post orogenic granitoids common constituents of numerous orogenic belts may be similarly related elsewhere

response of continental magmatic arcs to regional tectonic changes recorded by synorogenic plutons in the middle crust an example from the coast mountains of british columbia abstract the coast mountains of british columbia record an increase in magmatic activity acceleration in exhumation rate and a change from transpression to extension between 60 and 52 ma structural analysis of fabrics in three mid crustal plutons and country rocks leads to conclusions about pluton emplacement mechanisms and strain partitioning during changing tectonic conditions the quottoon and kitlope plutons 60 ma have steep foliations and lineations consistent with partitioned transpressional deformation the chief matthew s pluton 57 55 ma intruded during the formation of a sub horizontal transposition foliation and has radially distributed lineations consistent with sub vertical flattening during extension the change in orientation of the foliation represents an almost orthogonal rotation of the shortening direction from sub horizontal to sub vertical the chief mathew s pluton is interpreted to intrude initially into gently dipping fractures perpendicular to the steeply dipping foliation these melt filled fractures acted as conduits for melt triggering horizontal flow and eventually formed kilometer scale sills the steeply dipping fabrics of the coast shear zone provided pathways for melt to ascended from lower to middle crustal depths partitioning deformation into three dimensional domains of flattening simple shear and constrictional strain created the space for plutons this pattern of deformation reflects the interaction of regional deformation with magma emplacement

thermal modelling in low pressure high temperature metamorphic belts abstract low pressure high temperature metamorphic belts occur throughout the world they display a characteristic metamorphic style which has remained unchanged with time they are found in disparate tectonic settings including magmatic arcs regions of extension regions of thickened crust in continent continent collision zones and accretionary wedges they are commonly characterized by an abundance of granitoid plutons pressures of metamorphism are primarily less than or equal to that of the aluminosilicate triple point approximately 4 kbar while temperatures are in the range 500 750 c with rare exceptions inferred ptt paths are nearly isobaric and recrystallization was rapid and prograde metamorphic thermal gradients are commonly in the range 60 150 c km these conditions necessitate the presence of a large component of advective heat transfer and or anomalously high basal heat flow during metamorphism the large magmatic fluxes associated with the formation of magmatic arcs provide an adequate heat source for low pressure metamorphism lpm lithospheric extension is likely to produce lpm only if the degree of thinning is large 3 and there is additional heat from magmas the generation of lpm by crustal thickening requires either a large component of advective heating by magmas anomalously high basal heat flow or pre thickening lithospheric extension lpm in this environment must necessarily be accompanied by plutonic activity advection of heat by aqueous fluids may provide a significant contribution to the thermal budget but acting along is unlikely to produce lpm regional lpm induced by advective sources rarely occurs along a geotherm which increases monotonically with depth or synchronously across the region at a given depth it occurs over time as a series of overlapping temporally separate and spatially localized thermal events

from crustal thickening to orogen parallel escape the 120 myr long ht lp evolution recorded by titanite in the paleozoic famatinian backarc nw argentina

silurian granulite facies metamorphism and coeval magmatism and crustal growth in the tongbai orogen central china abstract it is intriguing whether the formation of granulite in an orogenic belt is related to magmatism and crustal growth the tongbai orogen is one of the particular areas in the qinling tongbai dabie sulu orogen that contains both granulites and associated arc magmatic rocks and thus is an optimal place to address this issue however the formation age and genesis of the granulites and their relations to the magmatic rocks have not yet been well constrained in situ la mc icpms u pb dating and trace element and lu hf isotope analyses were undertaken on zircon grains from gneiss granulite gabbro and granodiorite samples most zircons in three granulite samples and a gneiss sample have core rim structures the cores have oval or rounded shapes oscillatory zones high th u ratios and variable u pb ages suggesting a detrital genesis the youngest group of the detrital zircons yields a weighted mean 206 pb 238 u age of 450 5 ma which is interpreted as the maximum depositional age of their protoliths a major age cluster is ca 450 490 ma corresponding to prolonged arc magmatism in the northern tongbai and qinling orogens another group of ages range from 660 to 950 ma implying that the qinling group of the tongbai orogen belongs to the south china block scb metamorphic zircons in the granulites have weak or no zoning low th u ratios and ree contents negative eu anomalies and relatively flat hree patterns which indicate that they formed contemporaneously with garnet and plagioclase and thus under granulite facies metamorphic conditions their weighted mean age of 424 4 ma is taken as the best estimated age of the granulite facies metamorphism zircons in a leucosome sample have significant negative eu anomalies and relatively flat hree patterns with a formation age of 428 4 ma indicating partial melting coeval with the granulite facies metamorphism metamorphic zircons in the gneiss show apparently negative eu anomalies and variable th u ratios and hree contents they yield a weighted mean 206 pb 238 u age of 438 4 ma which was suggested to record the timing of the prograde metamorphism zircon grains in a gabbro and a granodiorite yield u pb ages of 432 4 and 424 4 ma respectively which are interpreted as their formation ages and identical to the ages of the granulite facies metamorphism they have positive e hf t values of up to 5 1 showing clear evidence for crustal growth during their formation the appearance of the silurian granulite facies metamorphism and magmatism in the tongbai orogen provides a good paradigm of coeval granulite facies metamorphism magmatism and crustal growth in an arc continent collision orogenic belt

mechanical consequences of granite emplacement during high t low p metamorphism and the origin of anticlockwise pt paths high temperature t low pressure p metamorphic belts preserve evidence of metamorphism under extremely perturbed and hence transient thermal regimes pt paths are commonly anticlockwise that is the maximumt was attained prior to or at the same time as the maximump with cooling at near constant or increasingp and in many terrains peak metamorphic temperatures prevailed during convergent deformation using a simple coupled thermal mechanical model that assumes a strongly temperature dependent rheology for the continental lithosphere we show that the coincidence of metamorphism with convergent deformation in high t low p terrains may reflect the thermal weakening attendant with rapid advective movement of heat within the lithosphere via granite magma ascent the model shows that the magnitude of the thermal weakening effect is sensitive to the critical temperature for granite segregation tm crit the depth of emplacement of the magmas and most importantly the granite forming mechanism during the prograde cycle material points attain maximum temperatures during a transient 3 ma high strain rate pulse withe up to 10 14 s 1 at levels near the site of granite emplacement the attainment of maximum pressures follows peak temperatures with rapid cooling paths that are close to isobaric or involve slight compression

the force balance of sea ice in a numerical model of the arctic ocean the balance of forces in the sea ice model of hibler 1979 is examined the model predicts that internal stress gradients are an important force in much of the arctic ocean except in summer when they are significant only off the northern coasts of greenland and the canadian archipelago a partition of the internal stress gradient between the pressure gradient and the viscous terms reveals that both are significant although they operate on very different timescales the acceleration term is generally negligible while the sum of coriolis plus sea surface tilt is small thus the seasonal average force balance in fall winter and spring is mostly between three terms of roughly equal magnitudes air drag water drag and internal stress gradients this is also true for the monthly average force balance however we find that there is a transition around the weekly timescale and that on a daily basis the force balance at a particular location and time is often between only two terms either between air drag and water drag or between air drag and internal stress gradients the model is in agreement with the observations of thorndike and colony 1982 in that the correlation between geostrophic wind forcing and the model s ice velocity field is high this result is discussed in the context of the force balance we show that the presence of significant internal stress gradients does not preclude high wind ice correlation a breakdown of the internal stress gradient into component parts reveals that the shear viscous force is far from negligible which casts strong doubt on the theoretical validity of the cavitating fluid approximation in which this component is neglected finally the role of ice pressure is examined by varying the parameter p we find a strong sensitivity in terms of the force balance as well as ice thickness and velocity

temperatures of granulite facies metamorphism constraints from experimental phase equilibria and thermobarometry corrected for retrograde exchange this study assesses temperatures of formation of common granulites by combining experimental constraints on the p t stability of granulite facies mineral associations with a garnet orthopyroxene grt opx thermobarometry scheme based on al solubility in opx corrected for late fe mg exchange we applied this scheme to 414 granulites of mafic intermediate and aluminous bulk compositions our findings suggest that granulites are much hotter than traditionally assumed and that the p t conditions of the amphibolite granulite transition portrayed in current petrology textbooks are significant underestimates by over 100 c for aluminous and intermediate granulites mean corrected temperatures based on our method are 890 17 and 841 11 c respectively uncertainties reported as 95 confidence limits on the mean consistent with minimum temperatures for orthopyroxene production by fluid absent partial melting in these bulk compositions in contrast mean temperatures based on grt opx fe mg exchange equilibria using the same thermodynamic data are 732 22 and 723 11 c respectively well below the minimum temperatures for opx stability for mafic granulites the mean corrected temperature using our method is 816 12 c similar to the mean temperature of 793 13 c from fe mg exchange reasons for the differences between the mafic granulites and aluminous intermediate granulites are unclear but may be due to the lower al concentrations in opx in the mafic rocks and possible deficiencies in the thermodynamic modelling of these low concentrations we discuss a number of well known granulite terrains in the context of our findings including the adirondacks the acadian granulites of new england the incipient charnockites of southern india and sri lanka and the kerala khondalite belt our findings carry implications for thermotectonic models of granulite formation a computer program to perform our thermobarometry calculations rclc is available from the journal of petrology website at http www petrology oupjournals org or from the authors at http www geo ucalgary ca pattison drm pattison rclc htm

convection in a partially molten metasedimentary crust insights from the el oro complex ecuador the el oro complex southwestern ecuador is a tilted section of the metasedimentary ecuadorian forearc which was partially molten during triassic time due to gabbroic magma emplacement pressure and maximum temperature estimates show that the metamorphic gradient during anatexis was 45 c km in the upper crust and 10 c km in the 7 8 km garnet bearing migmatitic lower crust controlled by biotite breakdown melting reactions our petrological and geochemical studies indicate that melts produced during biotite breakdown 5 15 vol were trapped and pervasively distributed in the garnet bearing migmatite based on these results we carried out one dimensional thermal modeling to characterize the heat transfer processes that led to the establishment of such a low thermal gradient during partial melting our results show that neither diffusive nor upward melt transfer models account for the low metamorphic gradient in the garnet bearing migmatite we demonstrate that in the el oro complex convection of the garnet bearing migmatitic layer is the most likely heat transfer process that explains all the petrological geochemical and metamorphic data

anatexis and metamorphism in tectonically thickened continental crust exemplified by the sevier hinterland western north america the generation of granitoid magmas by partial melting of crustal rocks during continental thickening events is well documented in many geological provinces throughout the world including the late mesozoic sevier belt of western north america we present a thermal and petrologic model of anatexis and metamorphism in regions of crustal thickening where the only mantle contribution is the normal conductive supply of heat through the base of the lithosphere i e advection of mass and energy are excluded we distinguish between formation of migmatites and generation of mobile granitoid magmas and examine the temporal and spatial relationships between these two distinct anatectic processes between anatexis and regional deformation and between anatexis and metamorphism a fundamental conclusion is that if protoliths rich in hydrous minerals are present regional anatexis is the end product of classical barrovian metamorphism in thickened continental crust even in the absence of a free water rich fluid phase barrovian metamorphic facies series are predicted with thickening ratios maximum crustal thickness attained initial crustal thickness as low as 1 3 but mobile granitoid magmas are not formed if this ratio is less than approximately 1 5 above these lower bounds barrovian metamorphism and anatectic granitoid magmatism occur independently of the magnitude of thickening and of the way in which thickening is accomplished both processes are sensitive to a diminished heat supply lowering either moho heat flow or crustal radioactive heat production results in blueschist eclogite metamorphism and inhibits the formation of mobile granitoid magmas we model anatexis under fluid absent conditions and show that with such a constraint migmatization is always a syn kinematic process relative to the crustal thickening event whereas generation of mobile granitoid magmas is in most cases post kinematic relative to crustal thickening but can be syn kinematic if thickening takes more than approximately 50 myr the typical time intervals for melting are consistent with geological observations mobile granitoid magmas are predicted by most of our models within approximately 10 myr of the end of the crustal thickening event this incubation period results primarily from the temperature increase required for the dehydration melting reactions capable of producing large melt fractions to occur the energetic requirements of anatexis are relatively minor compared to conductive crustal thermal budgets as shown by the fact that once the necessary p t conditions are attained melting reactions are completed within time intervals on the order of 1 myr i e 1 2 orders of magnitude smaller than the characteristic time scales of the tectonic processes involved in crustal thickening

collision vs subduction related magmatism two contrasting ways of granite formation and implications for crustal growth earth s continental crust is dominantly made of buoyant felsic igneous material granitoids that were ultimately extracted from the mantle as a result of earth s differentiation since felsic melts are not in chemical equilibrium with the mantle they can originate either from melting of older crustal lithologies or from differentiation of a primitive mantle melt only the latter case will contribute to crustal growth to understand the mechanisms of continental crust growth and differentiation through time it is therefore necessary to unravel the respective contribution of these two different mechanisms in the genesis of granitoid suites in modern earth granitoids are chiefly generated in convergent plate boundaries subduction and collision this paper examines the granitic suites in a late collision environment the variscan french massif central fmc and compares them with the suites found in an oceanic arc we therefore describe and compare two end members sites of granite generation in the fmc several main types of granites are described muscovite and cordierite bearing peraluminous granites resp mpg and cpg contain large amounts of inherited zircons and their chemistry demonstrates that their sources were older crustal material resp metasediments and metaigneous on the other hand potassic calc alkaline granites kcg associated to potassic diorites vaugnerites do not contain inherited zircons and ultimately derive from the vaugnerites the vaugnerites in turns form by partial melting of a mantle contaminated by the regional crust therefore although they are isotopically similar to the crust the kcg are net contributors to crustal growth thus we conclude that although late orogenic settings are dominated by crustal melting and recycling they may be sites of net crustal growth even though this is not visible from isotopes only in contrast arc granitoids are purely or almost purely mantle derived however the preservation potential of arcs is much smaller than the preservation of late orogenic domains such that at the scale of a whole orogenic belt late orogenic magmatism is probably as important as arc magmatism finally we speculate that the situation may have been similar in the archaean or even more skewed towards late orogenic sites or similar environments dominated by melting of a altered mafic protocrust owing to the hotter mantle and less stable subductions during that period

cross shelf transport of warm and saline water in response to sea ice drift on the laptev sea shelf 1 oceanographic moorings and conductivity temperature depth ctd surveys from september 2009 to september 2010 are used to describe recent changes in the laptev sea hydrography and to highlight wind and ice driven surface ekman transport as the mechanism to translate these changes from the outer to the inner shelf bottom waters in february 2010 moored oceanographic instruments recorded a sudden increase in temperature 0 8 c and salinity 3 near the bottom of the inner laptev sea shelf such warm and saline waters had not been previously observed on the inner shelf in winter they likely originated from the basin and were first observed during a summer 2009 ctd survey in the northwestern shelf break region subsequently spreading east and shoreward across the laptev sea shelf the changes were introduced to the mooring site by the first of a series of bottom intensified flow events with velocities reaching 20 cm s 1 topographically guided along a relic submarine river valley each of the flow events coincided with negative pressure anomalies at the mooring site and offshore directed upwelling favorable winds and ice drift we suggest that the observations to first order resemble a simplified two dimensional two layered ocean where offshore surface ekman transport is compensated for by a barotropic shoreward response flow near the bottom in this paper we use one of the first comprehensive long term laptev sea datasets to highlight ice ocean atmosphere interactions in early and late winter and discuss the role of freshwater stratification and ice mobility on under ice circulation on the laptev sea shelf

hot orogens and supercontinent amalgamation a gondwanan example from southern india abstract the southern granulite terrane in southern india preserves evidence for regional scale high to ultrahigh temperature metamorphism related to the amalgamation of the supercontinent gondwana here we present accessory mineral zircon and monazite geochronological and geochemical datasets linked to the petrological evolution of the rocks as determined by phase equilibria modelling the results constrain the duration of high to ultrahigh temperature 900 c metamorphism in the madurai block to be c 40 ma with peak conditions achieved c 60 ma after the formation of an orogenic plateau related to the collision of the microcontinent azania with east africa at c 610 ma a 1d numerical model demonstrates that the attainment of temperatures 900 c requires that the crust be moderately enriched in heat producing elements and that the duration of the orogenic event is sufficiently long to allow conductive heating through radioactive decay both of these conditions are met by the available data for the madurai block our results constrain the length of time it takes for the crust to evolve from collision to peak p t i e the prograde heating phase then back to the solidus during retrogression this evolution illustrates that not all metamorphic ages date sutures

persistent artifacts in the nsidc ice motion data set and their implications for analysis in this study we evaluate weekly ease grid sea ice velocities released by the national snow and ice data centre nsidc we identify persistent eulerian and lagrangian features that arise solely as an artifact of the method used in the incorporation of buoy data this in turn significantly impacts calculation of sea ice motion gradients including divergence convergence and shear our numerical experiments and comparison with observations further demonstrate the impact of these artificial features on climatological assessments including age of ice studies in particular we find that age of ice studies using this dataset significantly underestimate multi year ice extent by an average of 0 8 million km2 in the month of march

circulation of summer pacific halocline water in the arctic ocean 1 we present an analysis of arctic ocean hydrographic and sea ice observations from the 1990s with a focus on the circulation of water that originates in the north pacific ocean previous studies have shown the presence of two varieties of relatively warm summer halocline water in the vicinity of the chukchi sea i e the relatively fresh alaskan coastal water acw and the relatively saltier summer bering sea water sbsw here we extend these studies by tracing the circulation of these waters downstream into the arctic ocean we find that acw is generally most evident in the southern beaufort gyre while sbsw is strongest in the northern portion of the beaufort gyre and along the transpolar drift stream we find that this separation is most extreme during the early mid 1990s when the arctic oscillation was at historically high index values this leads us to speculate that the outflow to the north atlantic ocean through the canadian archipelago and fram strait may be similarly separated as arctic oscillation index values fell during the later 1990s acw and sbsw began to overlap in their regions of influence these changes are evident in the area north of ellesmere island where the influence of sbsw is highly correlated with a 3 year lag with the arctic oscillation index we also note the presence of winter bering sea water wbsw which underlies the summer varieties all together this brings the number of distinct pacific water types in our arctic ocean inventory to three acw sbsw and wbsw

observations of seasonal upwelling and downwelling in the beaufort sea mediated by sea ice abstractwe present observational estimates of ekman pumping in the beaufort gyre region averaged over the canada basin the results show a 2003 14 average of 2 3 m yr 1 downward with strong seasonal and interannual variability superimposed monthly and yearly means range from 30 m yr 1 downward to 10 m yr 1 upward a clear seasonal cycle is evident with intense downwelling in autumn and upwelling during the winter months despite the wind forcing being downwelling favorable year round wintertime upwelling is associated with friction between the large scale beaufort gyre ocean circulation and the surface ice pack and contrasts with previous estimates of yearlong downwelling as a consequence the yearly cumulative ekman pumping over the gyre is significantly reduced the spatial distribution of ekman pumping is also modified with the beaufort gyre region showing alternating moderate upwelling and downwelling while a more intense yearlong downwelling averaging 18 m yr 1 is identified in the northern

determination of the fluid absent solidus and supersolidus phase relationships of morb derived amphibolites in the range 4 14 kbar experiments on a morb mid ocean ridge basalt derived amphibolite have been conducted in order to determine fluid absent solidus and supersolidus phase relationships the solidus has been determined in the range 4 14 kbar by using the incremental heating technique tested against classical procedures peritectic clinopyroxene was produced below 10 kbar and epidote was the stable peritectic phase at pressures greater than 10 kbar for temperatures near the solidus garnet was the peritectic phase for temperatures above 800 c and pressures of 10 to 14 kbar the shape of the solidus was similar to previous estimations but the temperature interval between the high and low pressure regions was strongly reduced with these new estimations the possibilities for melting of the oceanic crust at p below 10 kbar are increased and consequently the number of tectonic scenarios needed for magma generation from amphibolite sources is enlarged in the absence of free water the solidus in the pressure region above 10 kbar is at higher t than previous estimations

mechanisms of pacific summer water variability in the arctic s central canada basin pacific water flows northward through bering strait and penetrates the arctic ocean halocline throughout the canadian basin sector of the arctic in summer pacific summer water psw is modified by surface buoyancy fluxes and mixing as it crosses the shallow chukchi sea before entering the deep ocean measurements from ice tethered profilers moorings and hydrographic surveys between 2003 and 2013 reveal spatial and temporal variability in the psw component of the halocline in the central canada basin with increasing trends in integrated heat and freshwater content a consequence of psw layer thickening as well as layer freshening and warming it is shown here how properties in the chukchi sea in summer control the temperature salinity properties of psw in the interior by subduction at isopycnals that outcrop in the chukchi sea results of an ocean model forced by idealized winds provide support to the mechanism of surface ocean ekman transport convergence maintaining psw ventilation of the halocline

thermal buffering effects at the solidus implications for the equilibration of partially melted metamorphic rocks abstract a simple thermal model is used to investigate to what extent buffering effects of latent heat of fusion may prolong the thermal evolution of a one dimensional statically cooling partially or completely melted heat source for a melting model appropriate for crustal rocks the time of cooling of rocks through the solidus may be about tripled by this process depending on the melting model cooling may halt completely near the solidus for time spans comparable to the thermal time constant of the heat source in order to test the influence of this buffering effect on the equilibration of partially melted metamorphic rocks these thermal model results are coupled with a simple diffusion model that relates the degree of equilibration of a simple assemblage to temperature and cooling rate it is shown that for a broad range of initial heat source temperatures freezing of selected mineral equilibria may occur in a comparably narrow temperature range at and around the solidus the calculations may have some relevance to low p high t metamorphic terranes in as much as they may provide an explanation for the widely observed equilibration and partial re equilibration of rocks near the crystallisation temperatures of partial melts

generation and evolution of palaeoarchaean continental crust in the central part of the singhbhum craton eastern india abstract palaeoarchaean granitoids are exposed over wide area in the singhbhum craton whose origin and role in crustal evolution are not well constrained this study presents whole rock and mineral chemical data coupled with zircon u pb dating and hf isotope results on such granitoids from the central part of the craton to understand their petrogenesis tectonic setting and role in continental crustal evolution the first phase of granitoid magmatism in this area is represented by a 3 47 ga na rich gneissic tonalite belonging to the archaean ttg tonalite trondhjemite granodiorite suite this rock is characterized by high hree heavy rare earth elements negative eu anomaly low sr y ratio and positive zircon hf isotope signature ehft 2 1 to 4 8 it is interpreted to be formed by shallow melting of a juvenile mafic source at 3 35 ga a silicic ferroan porphyritic biotite granite formed it shows variable k na low y and high sr y moderately fractionated hree and positive zircon ehft 1 8 to 4 0 and is explained as a product of high temperature melting of a heterogeneous juvenile source consisting of tonalites and mafic rocks at lower crustal depth the final phase of granitoid magmatism is marked by a 3 30 ga non porphyritic ferroan silica rich biotite granite geochemical characteristics like moderately high k moderately fractionated hree low ca and sr y and zircon ehft 0 8 to 3 7 suggest that the granite was formed by high pressure melting of a tonalite dominated source with short crustal residence all the three granitoid phases display low mg ni and cr contents and magnesium number mg precluding direct involvement of mantle in their origin rather crustal reworking caused by episodic plume related mafic ultramafic magma underplating and intraplating in an oceanic plateau setting is suggested as the possible mechanism for formation of the granitoids successive crustal reworking events involved progressively greater amount of previously formed felsic crust generating more evolved k rich granitoids it appears to be a globally important process which led to effective crustal differentiation and maturing of the cratons during palaeoarchaean

gps measurements of present day convergence across the nepal himalaya the high elevations of the himalaya and tibet result from the continuing collision between india and asia which started more than 60 million years ago1 4 from geological and seismic studies of the slip rate of faults in asia5 it is believed that approximately one third of the present day convergence rate between india and asia 58 4mmyr 1 is responsible for the shortening uplift and moderate seismicity of the himalaya great earthquakes also occur infrequently in this region releasing in minutes the elastic strain accumulated near the boundary zone over several centuries and accounting for most of the advance of the himalaya over the plains of india the recurrence time for these great earthquakes is determined by the rate of slip of india beneath tibet which has hitherto been estimated indirectly from global plate motions6 from the slip rates of faults in asia7 8 from seismic productivity9 and from the advance of sediments on the northern ganges plain10 here we report geodetic measurements using the global positioning system gps of the rate of contraction across the himalaya which we find to be 17 52 2 mm yr 1 from the form of the deformation field we estimate the rate of slip of india beneath tibet to be 20 5 2 mmyr 1 strain sufficient to drive one or more great himalayan earthquakes with slip similar to that accompanying the magnitude 8 1 bihar nepal 1934 earthquake may currently be available in western nepal

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

syn collisional channel flow and exhumation of paleoproterozoic high pressure rocks in the trans north china orogen the critical role of partial melting and orogenic bending within the paleoproterozoic trans north china orogen the high pressure belt hpb is made of high pressure 15 kbar mafic granulites hosted in migmatitic gneisses in this contribution we document a set of structural analyses acquired over the whole hpb we also proposed a morphological subdivision of the partially molten rocks that compose the hpb according to changes in melt fraction a compilation of the p t and radiochronological data carried out over the last 15 years is presented the results highlight the concurrent effect of oroclinal bending and partial melting in controlling the exhumation of the deeply buried continental crust during ongoing compression of the thickening orogenic root onset of partial melting at peak metamorphism is responsible for a first strength drop that enhanced an eastward lateral flow radiometric ages show that the deep crust was partially molten over a 50 ma lasting period during which it evolved in a diatexite core mantled by metatexites this was responsible for a second strength drop with strain concentrated along the diatexite metatexite boundaries as exemplified by the newly documented datong chengde shear zone a 400 km long normal shear zone with a sinistral strike slip component that accommodated the final uprise of the high pressure rocks

controls on low pressure anatexis low pressure anatexis whereby rocks melt in place after passing through the andalusite stability field develops under more restricted conditions than does low pressure metamorphism our thermal modelling and review of published work indicate that the following mechanisms operating alone may induce anatexis in typical pelitic rocks without inducing wholesale melting in the lower crust i magmatic advection by pervasive flow ii crustal scale detachment faulting and iii the presence of a high heat producing layer of these only magmatic advection by pervasive flow and crustal scale detachment faulting have been shown quantitatively to provide sufficient heat to cause widespread melting combinations of the above mechanisms with pluton scale magmatic advection shear heating removal of the lithospheric mantle or with each other provide additional means of developing suitable high temperatures at shallow crustal levels to generate low pressure anatexis

timing constraints on building an intermediate plutonic arc crustal section u pb zircon geochronology of the sierra valle fertil la huerta famatinian arc argentina 1 the sierra valle fertil range in northwestern argentina exposes a tilted crustal section through the ordovician famatinian arc from 25 km to shallow crustal paleodepths fourteen new u pb zircon crystallization ages of magmatic rocks from sierra valle fertil area show that this section of the arc was built over a short time interval during the ordovician between 485 and 465 ma zircon rim ages demonstrate that high grade metamorphism and migmatization were synchronous with magmatic emplacement inherited ages in some of the plutonic rocks as well as detrital zircons in the metasedimentary framework suggest that the famatinian arc was emplaced into a thick miogeoclinal cover to the thinned margin of the proto south american continent in the ordovician which represents a part of gondwana docking of the precordilleran terrane outboard of proto south america led to the cessation of arc magmatism in the valle fertil area and preservation of the arc in its early stages after 60 myr of subduction during the pampean and famatinian magmatic stages our age data from the sierra valle fertil indicate that wet mafic magmas were emplaced into the section at a rate typical for modern island arcs 30 km3 km 1 myr 1 the production of intermediate to felsic magmas in the upper plate was a result of partial melting of the metasedimentary framework and hybridization with mantle derived melts and resulted in the generation of tonalites granodiorites and granites in the upper crust at a rate of 100 km3 km 1 myr 1 similar to major magmatic flare ups in the mature arcs of north american cordillera the sierra valle fertil arc section is a type example of an immature cordilleran arc before major crustal thickening and crustal overturn takes place in the upper plate the famatinian arc section may also be an equivalent to the crustal architecture of more mature modern island arcs such as parts of the aleutians caribbean and japan

genesis of zixi gold deposit in xuefengshan jiangnan orogen south china age geology and isotopic constraints abstract the medium sized zixi gold deposit is hosted in a meta gabbro diabase dike and located geologically in the xuefengshan belt of the jiangnan orogen in south china gold bearing quartz carbonate sulfide veins and the ore hosting meta gabbro diabase dike are structurally controlled by the ne trending first stage faults which are secondary faults to the regional xupu jingzhou fault metallic minerals at zixi include arsenopyrite and pyrite along with minor chalcopyrite sphalerite galena bournonite and native gold ore related alteration includes silicic carbonate sericite and chlorite based on vein crosscutting relationships and mineral paragenesis the alteration mineralization is divided into three stages 1 arsenopyrite pyrite native gold quartz mineralization 2 polymetallic sulfides native gold chlorite carbonate quartz mineralization and 3 post ore carbonate quartz alteration to constrain any temporal relationship between the gold mineralization and the ore hosting meta gabbro diabase dike we obtained a re os isochron age for the gold bearing arsenopyrite 425 28 ma n 5 mswd 0 47 which is 350 myr younger the meta gabbro diabase zircon u pb age 783 08 0 97 ma n 30 mswd 0 39 this precludes the dike from any direct metallogenetic link with the gold deposit the ore forming fluids have calculated 18oh2o and d values of 3 5 to 9 1 and 68 1 to 55 5 respectively indicating a metamorphic source probably through dehydration breakdown of chlorite sulfur isotope compositions of the gold bearing pyrite 5 8 to 3 6 are distinct from those of the lengjiaxi group metamorphic rocks but marginal overlap with those from the banxi group metamorphic rocks lead isotope compositions of the gold bearing pyrite 206pb 204pb 17 08 to 17 63 207pb 204pb 15 48 15 54 208pb 204pb 37 64 37 94 suggest that the lead was sourced from metamorphic rocks the h o s pb isotopic data indicate that ore forming materials were derived from a metamorphic source however the precise nature of that source is unclear the zixi gold mineralization displays many typical features of orogenic gold deposits such as being structurally controlled and have similar alteration and mineralization styles ore mineral assemblages and h o s stable isotopic features the zixi gold deposit was likely formed during the early paleozoic intracontinental orogeny of the jiangnan orogen

igneous emplacement in a shear zone termination the biotite granite at strontian scotland deformed xenolith strain data together with other structural information indicate that the biotite granite body 90 sq km of the strontian complex scotland was emplaced in the extensional termination of a dextral transcurrent shear zone this shear zone is a splay of the major great glen fault which lies along the southern boundary of the granite siting of the shear zone splay was probably controlled by a a slight releasing bend in the great glen fault in this area and b a large pre existing asymetric synform in proterozoic metasedimentary country rocks which intersects the great glen fault trace at a high angle a model is proposed in which moine thrust caledonian compression at ca 435 ma activated the great glen fault dextrally dextral movements around the releasing bend detached a flat segment from the inside fault wall and the biotite granite was emplaced side ways at depths of about 15 km as a sheet into this extensional listric fault bounded cavity

atmospheric boundary layer structure and drag coefficients over sea ice this paper estimates the air sea drag coefficient for first year ice from recent aircraft measurements and reconciles the range of observed drag coefficients 103cd 1 2 3 7 referenced to 10 m for all sea ice types based on ice roughness and seasonal meteorology for the purpose of sea ice modeling it is necessary to define an effective drag coefficient which relates regional stress to regional wind because sea ice is heterogeneous on scales less than 20 km regional stress is influenced by the distribution of surface roughness the buoyancy flux from quasi periodic leads and external atmospheric conditions principally the inversion height 103cd is 1 3 1 5 for smooth ice but is much greater for nonflat surfaces for wind speeds greater than 5 m s and air temperatures below freezing the effective 103cd is 2 5 3 0 for nearly continuous pack ice such as first year ice in seasonal ice zones and central arctic basin the range of values of 103cd is 3 0 3 7 for unstable boundary layers typical of off ice winds in the marginal ice zone miz or even greater if the ice has been broken by a recent storm cd values at the lower end of these ranges are associated with low inversion heights these coefficients are confirmed by 103cd of 2 9 2 5 and 3 1 for first year sea ice calculated from gust probe data collected by the noaa p 3 aircraft interior to the inner miz in the bering sea during the marginal ice zone experiment mizex west in february 1983 and 103cd of 2 6 for the arctic calculated from arctic ice dynamics joint experiment aidjex aircraft data for february 1976 the effective drag coefficient with the presence of even a small concentration of sea ice is greater than the oceanic value as shown by a 103cd of 2 2 calculated from noaa p 3 gust probe data over a 40 km track of 0 4 ice concentration in the outer miz of the greenland sea in june 1984 during mizex 84 the relation of surface wind and stress to the geostrophic wind for shallow inversion heights typical of high latitudes is reviewed with a turbulent closure atmospheric boundary layer model the winter arctic is typified by low inversions 100 m low geostrophic drag coefficients and large geostrophic surface wind turning angles with low functional dependence on surface roughness seasonal ice zones are typified by a more modest influence of the inversion on boundary layer characteristics

formation of the maturin foreland basin eastern venezuela thrust sheet loading or subduction dynamic topography 1 the maturin basin in eastern venezuela is considered a good example of a peripheral foreland basin earthquake and tomographic data indicate that eastern venezuela is affected by the oblique subduction of the south american plate underneath the caribbean plate new forward flexural isostatic modeling of eastern venezuela has been carried out in order to determine whether the maturin basin was generated purely by thrust sheet loading from the serrania and monagas foreland thrust belts a sequence of forward models from middle miocene to present was generated for 3 profiles across the serrania del interior thrust belt the monagas foreland thrust belt and the maturin foreland basin the predictions of these models are constrained using seismic reflection and well data the flexural isostatic modeling shows that thrust sheet loading associated with the serrania del interior and monagas foreland thrust belts is insufficient to generate the observed subsidence within the maturin basin dynamic fluid flow modeling of subduction related dynamic topography of eastern venezuela has been used to investigate the influence of south american plate subduction on the generation of the accommodation space observed in the maturin basin fluid flow modeling of subduction related dynamic topography suggests that the subduction of the south american lithospheric mantle caused downward deflection of the south american crust affecting the maturin basin and the serrania thrust belt this modeling suggests that the maturin basin subsidence has two components 55 related to thrust sheet loading and 45 driven by continental subduction

beaufort gyre freshwater reservoir state and variability from observations 1 we investigate basin scale mechanisms regulating anomalies in freshwater content fwc in the beaufort gyre bg of the arctic ocean using historical observations and data collected in 2003 2007 specifically the mean annual cycle and interannual and decadal fwc variability are explored the major cause of the large fwc in the bg is the process of ekman pumping ep due to the arctic high anticyclonic circulation centered in the bg the mean seasonal cycle of liquid fwc is a result of interplay between the mechanical ep and thermal ice transformations factors and has two peaks one peak occurs around june july when the sea ice thickness reaches its minimum maximum ice melt the second maximum is observed in november january when wind curl is strongest maximum ep and the salt input from the growing ice has not yet reached its maximum interannual changes in fwc during 2003 2007 are characterized by a strong positive trend in the region varying by location with a maximum of approximately 170 cm a 1 in the center of ep influenced region decadal fwc variability in the period 1950 2000 is dominated by a significant change in the 1990s forced by an atmospheric circulation regime change the center of maximum fwc shifted to the southeast and appeared to contract in area relative to the pre 1990s climatology in spite of the areal reduction the spatially integrated fwc increased by over 1000 km 3 relative to climatology