geochronological constraints on paleoarchean thrust nappe and neoarchean accretionary tectonics in southern west greenland abstract major regional deformation and metamorphic events in the godthabsfjord region southern west greenland occurred at 3650 and 2820 2720 ma e g precambrian res 78 1996 1 new geochronological constraints u pb zircon sensitive high resolution ion microprobe shrimp and thermal ionisation mass spectrometry tims have been obtained from a stack of mylonitic crystalline thrust nappes in the footwall of the western part of the paleoarchean 3 8 3 7 ga isua greenstone belt isukasia a mylonitic tonalite sheet interpreted to have intruded synkinematically with respect to mylonitisation yields a magmatic crystallisation age of 3640 3 ma a cross cutting pegmatite and a post kinematic tonalite pluton yield magmatic crystallisation ages of 2948 8 and 2991 2 ma respectively accordingly we interpret the thrust nappe stack to have formed during the paleoarchean 3640 ma making it the oldest example known on earth the similarity of this structural regime to that of modern mountain belts suggests that paleoarchean and modern continental crust were comparable in terms of mechanical strength and constitution southern west greenland has been interpreted in terms of neoarchean accretion comparable with modern plate tectonics e g earth planet sci lett 142 1996 353 isukasia lies just east of a purported neoarchean accretionary boundary between the akia terrane to the northwest and the akulleq terrane to the southeast the akia terrane was previously considered to overthrust the akulleq terrane at 2820 2720 ma our geochronological and geological data indicate i that the two terranes as presently defined were stitched at 2991 2 ma and ii that thrusting across the boundary was directed toward the akia terrane therefore we suggest that the akia akulleq interface was not a fundamental tectonic structure during the neoarchean and we question its identification as an accretionary boundary

remnants of arc related mesoarchaean oceanic crust in the tartoq group of sw greenland the tartoq group located in sw greenland consists of supracrustal rocks of mainly tholeiitic basaltic composition including pillow lavas sills dykes and gabbros as well as ultramafic rocks metamorphic grade ranges from greenschist fades to granulite facies the tartoq group crops out as a series of blocks and slivers that are imbricated with originally intrusive mesoarchaean ttg orthogneisses the supracrustal rocks form part of a se vergent fold and thrust belt consistent with the imbrication of ttg gneisses and supracrustal rocks along a convergent margin la icp ms u pb zircon dating of an intrusive ttg sheet yields a minimum age of 2986 4 ma for the tartoq group this age is consistent with mc icp ms lu hf and sm nd isotopic whole rock data for mafic samples from different blocks of the tartoq group which yield errorchron ages of 3189 65 ma and 3068 220 ma respectively the mafic supracrustal rocks of the tartoq group have chondrite normalized ree patterns with la cn sm cn of 0 67 1 96 and rather flat primitive mantle normalized multi element patterns except for scattered lile contents and generally negative nb anomalies with nb nb of 0 26 1 31 th yb varies between 0 06 and 0 47 and nb yb between 0 45 and 4 4 indicative of an arc affinity when compared to rocks from modern settings the similar geochemistry of the different lithological units together with their coeval formation as evident from trace element geochemical trends supports a co magmatic origin for the rock assemblage and their formation as imbricated relics of oceanic crust accordingly we propose that the tartoq group represents remnants of mesoarchaean oceanic crust which formed in a suprasubduction zone geodynamic environment c 2011 international association for gondwana research published by elsevier b v all rights reserved less

late archaean terrane accretion in the godthab region southern west greenland tectono stratigraphic terranes fault bounded blocks of the earth s crust characterized by a geological history distinct from that of adjacent terranes1 are now widely recognized in phanerozoic and proterozoic orogenic belts and their present configurations are thought to result from plate tectonic processes similar to those in operation today2 4 there has been much debate about whether such processes operated in archaean times5 6 here we present new data from the archaean high grade gneiss complex of the godthab now nuuk region of southern west greenland which show that discrete tectonic units recognized in the south of the region7 can be more widely identified at least four terranes are recognized each of which evolved separately until their juxtaposition in the late archaean this suggests that at least some archaean high grade gneiss complexes may resemble more recent orogenic belts formed by plate tectonic processes

refolded nappes formed during late archaean terrane assembly godthabsfjord southern west greenland recent work has provided a new tectono metamorphic framework for the archaean evolution of the godthabsfjord region which can be divided into discrete tectonic units each characterized by different magmatic structural and metamorphic histories these tectonic units were assembled in the late archaean post 2750 ma the present structural patterns seen in the region date from the period 2750 2600 ma and do not reflect earlier structural events as was previously considered late archaean structural evolution began with a period of interthrusting of early archaean sialic and late archaean oceanic crustal segments coeval with or followed by isoclinal nappe formation under amphibolite facies conditions linear fabrics in early archaean rocks were rotated into parallelism with those newly formed parallel to f1 hinge lines in the late archaean rocks a second major thrusting event emplaced granulite facies rocks structurally on top of the amphibolite facies rocks subsequent to this event a series of f2 upright or steeply inclined basin and dome structures formed some of which have sheath morphology this second folding episode acting upon the isoclinal nappes produced the regional scale interference patterns observed the early linear fabrics were re oriented around these second generation structures data are produced from selected areas to illustrate the fabric patterns and structural continuity that exists throughout the region

stratocumulus cloud top height estimates and their climatic implications a depth dependent boundary layer lapse rate was empirically deduced from 156 radiosondes released duringsixmonth longresearchcruisestothe southeastpacificsamplingavarietyofstratocumulusconditions thelapse ratedependenceonboundarylayerheightisweak decreasingfromabestfitof7 6to7 2k km 21 as theboundarylayerdeepensfrom800mto2km ship basedcloud baseheightsupto800mcorrespondwellto liftingcondensationlevels indicatingwell mixedconditions withcloud bases 800moften 200 600m higher than the lifting condensation levels the lapse rates were combined with moderate resolution imaging spectrometer 11 mm derived cloud top temperatures and satellite microwave derived sea surface temperatures to estimate stratocumulus cloud top heights the october mean cloud top height structure of the southeast pacific was then spatially and diurnally characterized coastal shoaling is apparent but so is a significant along coast cloud top height gradient with a pronounced elevation of the cloud top heights above thearicabightat 208s diurnalcloud topheightvariations inferredfromirregular4 times dailysampling can locally reach 250 m in amplitude and they can help to visualize offshore propagation of free tropospheric vertical motions a shallow boundary layer associated with the chilean coastal jet expands to its north and west in the afternoon cloud top heights above the arica bight region are depressed in the afternoon which may mean that increased subsidence from sensible heating of the andes dominates an afternoon increase in convergence upward motion at the exit of the chilean coastal jet in the southeast atlantic during october thestratocumuluscloud topheightsaretypicallylowerthanthoseinthesoutheastpacific acoastaljetregion can also be identified through its low cloud top heights coastal shoaling of the south atlantic stratocumulus region is mostly uniform with latitude in keeping with the more linear namibian angolan coastline the southeast atlantic shallow cloudy boundary layer extends farther offshore than in the southeast pacific particularly at 158s

turbulent fluxes of liquid water and buoyancy in partly cloudy layers abstract a simple convective mass flux model is used to derive expressions for the fluxes of liquid water and buoyancy in partly cloudy turbulent layers the results differ radically from those suggested in some previous studies physical interpretation is given and examples are presented implications for the dynamics of partly cloudy boundary layers are discussed and the aftermath of cloud top entrainment instability is analyzed

field and geochemical characteristics of the mesoarchean 3075ma ivisaartoq greenstone belt southern west greenland evidence for seafloor hydrothermal alteration in supra subduction oceanic crust the mesoarchean ca 3075ma ivisaartoq greenstone belt in southern west greenland includes variably deformed and metamorphosed pillow basalts ultramafic flows picrites serpentinized ultramafic rocks gabbros sulphide rich siliceous layers and minor siliciclastic sedimentary rocks primary magmatic features such as concentric cooling cracks and drainage cavities in pillows volcanic breccia ocelli interpreted as liquid immiscibility textures in pillows and gabbros magmatic layering in gabbros and clinopyroxene cumulates in ultramafic flows are well preserved in low strain domains the belt underwent at least two stages of calc silicate metasomatic alteration and polyphase deformation between 2963 and 3075ma the stage i metasomatic assemblage is composed predominantly of epidote now mostly diopside quartz plagioclase hornblende scapolite and occurs mainly in pillow cores pillow interstitials and along pillow basalt gabbro contacts the origin of this metasomatic assemblage is attributed to seafloor hydrothermal alteration on the basis of the common presence of epidote inclusions in diopside and the local occurrence of epidote rich aggregates the stage i metasomatic assemblage is interpreted as relict epidosite the stage ii metasomatic assemblage occurs as concordant discontinuous layered calc silicate bodies to discordant calc silicate veins commonly associated with shear zones the stage ii metasomatic assemblage consists mainly of diopside garnet amphibole plagioclase quartz vesuvianite scapolite epidote titanite calcite scheelite given that the second stage of metasomatism is closely associated with shear zones and replaced rocks with an early metamorphic fabric its origin is attributed to regional dynamothermal metamorphism the least altered pillow basalts picrites gabbros and diorites are characterized by lree enriched near flat hree and hfse especially nb depleted trace element patterns indicating a subduction zone geochemical signature ultramafic pillows and cumulates display large positive initial end values of 1 3 to 5 0 consistent with a strongly depleted mantle source given the geological similarities between the ivisaartoq greenstone belt and phanerozoic forearc ophiolites we suggest that the ivisaartoq greenstone belt represents mesoarchean supra subduction zone oceanic crust 2006 international association for gondwana research published by elsevier b v all rights reserved

buoyancy flux modeling for cloudy boundary layers the modeling of the buoyancy flux in a partly cloudy atmospheric boundary layer is complicated by its dependence on the correlation between clouds and vertical velocity elements which can vary significantly with the underlying layer dynamics e g stratocumulus versus shallow cumulus the buoyancy flux is sometimes modeled in terms of higher order statistics such as vertical velocity skewness or saturation variance to try to capture some of these dynamical effects in this work an approximate expression for the buoyancy flux is formulated solely in terms of the liquid potential temperature and total water profiles and their respective flux profiles the predictions compare favorably with the results of an extensive set of large eddy simulations les including simulations of stratocumulus shallow cumulus and transitional behavior in between this formulation is combined with previous results on the relation between cloud top entrainment rate and circulation structure to predict the behavior of quasi steady cumulus coupled boundary layers as a function of a basic set of physical input parameters these predictions also compare favorably with les results

geodynamic regimes of subduction under an active margin effects of rheological weakening by fluids and melts the dynamics of subduction under an active margin are analysed by using a 2d coupled petrological thermomechanical numerical model of an oceanic continental subduction system the model includes spontaneous slab bending dehydration of the subducted crust aqueous fluid transport partial melting of both crustal and mantle rocks as well as melt extraction processes resulting in magmatic arc crust growth based on our models the following five geodynamic regimes of subduction which may potentially form at active margins are identified i stable subduction with no backarc spreading centre and without plumes in the mantle wedge ii retreating subduction with a focused backarc spreading centre and without plumes iii retreating subduction with distributed intra arc extension and trans lithospheric sedimentary plumes iv advancing subduction with underplating laterally extending sub lithospheric plumes and v stable to advancing subduction with stationary laterally limited sub lithospheric plumes transitions between these different regimes are mainly caused by the concurrence of rheological weakening effects of i aqueous fluids percolating from the subducting slab into the mantle wedge and ii melts propagating from the partially molten areas formed in the mantle wedge towards the surface the aqueous fluids mainly affect the forearc region strong fluid related weakening promotes plate decoupling and reduces subduction drag and thus results in stacking of sediments in the accretion prism in contrast reduced weakening by fluids results in strong coupling of the plates and leads to advancing collision like subduction with enhanced subduction erosion thickening of the overriding plate and large sedimentary plumes in the mantle wedge are the consequences on the other hand melts extracted from the hot regions above the slab rheologically weaken the lithosphere below the arc which thus controls overriding plate extension and shortening strong rheological weakening by melts in combination with weak plate coupling trigger retreating subduction with a pronounced backarc spreading centre also weakening of the continental lithosphere by melts extracted from trans lithospheric sedimentary plumes generate a weak channel through which these structures may be emplaced into subarc crust if there is insufficient melt related weakening plumes cannot ascend but extend horizontally and thus underplate the lithosphere

the validity of two biostratigraphic approaches in sequence stratigraphic correlations brachiopod zones and marker beds in the jurassic abstract the recent emergence of sequence stratigraphic concepts has highlighted the way time is inferred and measured from depositional facies in sedimentary geology this paper aims at discussing the consequences of the methods involved in jurassic brachiopod biostratigraphy for which a zonal and a marker bed approach have been used brachiopod marker beds 1 account for a wider and more general significance within a sequence stratigraphic framework 2 represent an inherent character of the brachiopod stratigraphic record 3 allows the set up of a scale of biochronologic marker beds that characterize some mf s and 4 can accordingly be used as a constraining framework for higher resolution correlations finally the punctuated marker beds and the zonal approaches are tentatively compared the consistence of sequence stratigraphic correlations over the anglo paris basin testify to the relevance of the punctuated approach particularly within complex carbonate platform series where classical zonal biostratigraphy is ineffective the brachiopod case shows how biostratigraphic tools would be better understood and improved according to their capacity for fitting with the nature of the time record that is to characterize the cyclic intermittent sedimentary record of continuous time

archaean crustal evolution in southern west greenland a review based on observations in the buksefjorden region abstract thermal metamorphic and tectonic processes associated with the two major crust forming events expressed as voluminous generation of amitsoq and nuk tonalitic granitic gneisses ca 3750 ma and ca 2800 ma respectively in southern west greenland are reviewed briefly in the light of recent studies in the buksefjorden region

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

a new boundary layer mixing scheme part i scheme description and single column model tests a new boundary layer turbulent mixing scheme has been developed for use in the ukmo weather forecasting and climate prediction models this includes a representation of nonlocal mixing driven by both surface fluxes and cloud top processes in unstable layers either coupled to or decoupled from the surface and an explicit entrainment parameterization the scheme is formulated in moist conserved variables so that it can treat both dry and cloudy layers details of the scheme and examples of its performance in single column model tests are presented

continental subduction and exhumation of high pressure rocks insights from thermo mechanical laboratory modelling abstract thermo mechanical physical modelling of continental subduction is performed to investigate the exhumation of deeply subducted continental crust the model consists of two lithospheric plates made of new temperature sensitive analogue materials the lithosphere is underlain by liquid asthenosphere the continental lithosphere contains three layers the weak sedimentary layer the crust made of a stronger material and of a still stronger lithospheric mantle the whole model is subjected to a constant vertical thermal gradient causing the strength reduction with depth in each lithospheric layer subduction is driven by both push force and pull force during subduction the subducting lithosphere is heating and the strength of its layers reduces the weakening continental crust reaches maximal depth of about 120 km and cannot subduct deeper because its frontal part starts to flow up the subducted crust undergoes complex deformation including indicated upward ductile flow of the most deeply subducted portions and localised failure of the subducted upper crust at about 50 km depth this failure results in the formation of the first crustal slice which rises up between the plates under the buoyancy force this process is accompanied by the delamination of the crustal and mantle layers of the subducting lithosphere the delamination front propagates upwards into the interplate zone resulting in the formation of two other crustal slices that also rise up between the plates average equivalent exhumation rate of the crustal material during delamination is about 1 cm year the crust asthenosphere boundary near the interplate zone is uplifted the subducted mantle layer then breaks off removing the pull force and thereby stopping the delamination and increasing horizontal compression of the lithosphere the latter produces shortening of the formed orogen and the growth of relief the modelling reveals an interesting burial exhumation evolution of the sedimentary cover during initial stages of continental subduction the sediments of the continental margin are dragged to the overriding plate base and are partially accreted at the deep part of the interplate zone at 60 70 km depth these sediments remain there until the beginning of delamination during which the pressure between the subducted crust and the overriding plate increases this results in squeezing the underplated sediments out part of them is extruded upwards along the interplate zone to about 30 km depth at an equivalent rate of 5 10 cm year

on a gold mineralization in the precambrian tartoq group sw greenland in the precambrian tartoq group sw greenland gold occurs in three parageneses associated with arsenopyrite pyrite tennantite and chalcopyrite the gold bearing sulphide mineralizations appear as conformable quartz layers lenses and veins in greenstones of basaltic composition gold particles in the arsenopyrite pyrite layers have a true fineness of c 990 in the tennantite rich veins a fineness of c 720 and a fineness of c 500 in the chalcopyrite quartz veins the arsenopyrite pyrite layers are of submarine exhalative origin whereas the tennantite and the chalcopyrite quartz veins and lenses are of diagenetic and or metamorphic origin

sediment and shear hosted gold mineralization of the tartoq group supracrustals southwest greenland abstract gold mineralization occurs in the archaean tartoq group supracrustals which are dominated by greenschist amphibolite facies metamorphosed mafic rocks although deformed recognizable features of the stratigraphy pillowed lavas intercalated sills and interflow sediments suggest shallow to deep sea floor deposition large intensely carbonatized structural zones cut through the greenstones subparallel to regional layering and postdating major folding they are interpreted as major shear zones and are characterized by internal ductile deformation due to high fluid pressures with brittle deformation of peripheral zones erratic high grade gold mineralization is associated with quartz pyrite lenses and veins that occur subparallel to layering within the shear zones moderate but more consistent values are found in more extensive pyrite arsenopyrite quartz layers also subparallel to layering these lenses and layers occur adjacent to or within the major carbonatized shear zones microscopically gold is found as small 10 m grains in both pyrite and arsenopyrite low but consistently anomalous grades of gold are also found in strongly contorted banded muscovite rich quart zpyrite rocks these are thought tobe thin chemically deposited interflow sediments within the greenstone pile analogous to oxide facies banded iron formation which is found nearby metamorphism has obscured critical textures but the gold was either chemically deposited along with exhalative silica and sulphides or may have been introduced later during deformation and alteration of primary bif the tartoq group rocks are similar in most respects to those of other archaean greenstone belts being distinguished only by their excellent outcrop exposure current models for the origin of archaean gold deposits are adequate to explain the tartoq gold mineralization a multistage model of gold concentration is proposed gold is initially leached from mafic volcanics and possibly underlying gneisses and redeposited at anomalous levels in chemical sediments these may be further enriched by sulphidation reactions high grade gold mineralization is typically epigenetic in style where very large volumes of fluid are channeled through shear zones undergoing brittleductile deformation

deformation style way back when thoughts on the contrasts between archean paleoproterozoic and contemporary orogens abstract in many important ways archean and paleoproterozoic older orogens differ structurally from contemporary examples this essay examines the premise that contrasts between older orogens and contemporary orogens reflect long term changes in the temperature of the continental crust in the density of supracrustal sections and in exhumation rates for example if continental crust were warmer and exhumation rates faster earlier in earth history then higher grade rocks would occur closer to the surface of older orogens and the orogens would be lower and wider this situation might contribute to the formation of wide belts of high grade gneiss found in ancient crust if the high strength layer of the crust were thinner and supracrustal sequences denser earlier in earth history then regional extensional tectonism might lead to crustal scale boudinage and diapirism this situation might explain formation of the extensive dome and keel provinces found in ancient crust testing such speculations through the application of structural analysis coupled with petrologic studies dating and rheological modelling will constrain models of earth s long term physical evolution

the parameterization of the planetary boundary layer in the ucla general circulation model formulation and results abstract a planetary boundary layer pbl parameterization for general circulation models gcms is presented it uses a mixed layer approach in which the pbl is assumed to be capped by discontinuities in the mean profiles both clear and cloud topped boundary layers are parameterized particular emphasis is placed on the formulation of the coupling between the pbl and both the free atmosphere and cumulus convection for this purpose a modified coordinate is introduced in which the pbl top and the lower boundary are both coordinate surfaces the use of a bulk pbl formulation with this coordinate is extensively discussed results are presented from a july simulation produced by the ucla gcm pbl related variables are shown to illustrate the various regimes the parameterization is capable of simulating

geology of an archaean accretionary complex the structural record of burial and return flow in the tartoq group of south west greenland abstract this study presents structural and petrographic data from supracrustal slivers and associated ttg rocks of the mesoarchaen tartoq group in southwestern greenland that point to the burial and subsequent return flow of the mainly metamafic supracrustal and ttg rocks during subduction accretion along an obliquely convergent margin locally preserved high grade metamorphic up to lower granulite facies fabrics record the burial of the supracrustal rocks to depths 25 km during southeast directed subduction retrograde amphibolite and regionally widespread greenschist facies fabrics and structures indicate the subsequent exhumation of the rocks pro and retrograde fabrics are largely coaxial but retrograde fabrics record a reversal of shear and uniformly developed top to the east and southeast kinematics along extensional shear zones and overprinting brittle ductile fault zones extensional shearing and the concomitant retrogression are interpreted to be related to the northwest directed expulsion of the rocks in the subduction channel intrusive relationships between supracrustal slivers and ttg s leucogranites and pegmatites document the synkinematic emplacement of voluminous felsic magmas throughout this structural evolution while the structural evolution of rocks of the tartoq group shows the salient attributes of a modern convergent margin high t metamorphism pervasive ductile strains partial melting of the supracrustal rocks and syntectonic intrusion of voluminous granitoids are interpreted to reflect processes operating in a hotter archaean subduction zone slab melting and the formation of granitoids at relatively shallow depths in the subduction zone channel move the magmatic front to the fore arc region of the convergent margin the emplacement of felsic melts low strength and progressive disintegration of the downgoing slab choke subduction this has most likely resulted in only irregular and shortlived subduction disrupted by episodes of slab break off and subduction step back promoting tectonomagmatic accretion of ttg greenstones at relatively shallow levels but preventing subduction of rocks to larger depths this mode of shallow structural and magmatic accretion leads to lateral crustal growth documented by the at least 80 km wide accretionary complex of the tartoq group

boninite like volcanic rocks in the 3 7 3 8 ga isua greenstone belt west greenland geochemical evidence for intra oceanic subduction zone processes in the early earth abstract the 3 7 3 8 ga isua greenstone belt of southwest greenland is characterized by variably metamorphosed metasomatised and deformed lithotectonic successions of volcanic and sedimentary rocks the voluminous mafic volcanic rocks are composed primarily of pillow basalts intercalated with ultramafic units the sedimentary rocks consist mainly of banded iron formation cherts conglomerates and siliciclastic turbidites the least altered metavolcanic amphibolites the garbenschiefer unit from the central tectonic domain of the isua greenstone belt are characterized by high mg number 0 60 0 80 mgo 7 18 wt al2o3 14 20 wt ni 60 645 ppm and cr 60 1920 ppm contents but low tio2 0 20 0 40 wt zr 12 30 ppm y 6 14 ppm and rare earth element ree concentrations these compositional features collectively represent a coherent mafic to ultramafic suite chondrite normalized ree patterns are concave upward on the primitive mantle normalized extended trace element diagrams they are characterized by relative depletion of nb but with an enrichment of zr relative to neighboring rees alteration deformation and crustal contamination can be ruled out as the cause of the distinct and coherent composition the average initial end value of these metavolcanic rocks is 2 collectively these geochemical characteristics are comparable to those of phanerozoic boninites given the observation that in the tertiary boninites are exclusively associated with intra oceanic subduction environments e g izu bonin mariana subduction system this suggests that intra oceanic subduction zone like geodynamic processes were operating as early as 3 7 3 8 ga

on the emergence of plate tectonics at present young oceanic lithosphere is positively buoyant and it does not become negatively buoyant until it is older than about 20 m y if in the past the mantle was hotter the oceanic crust would have been thicker and the lithosphere9s age of neutral buoyancy greater on the other hand the hotter mantle would have had a lower viscosity and convected faster so the average age of oceanic plates at subduction would have been less than the present 100 m y at some time in the past average oceanic lithosphere would have only just become negatively buoyant as it reached a subduction zone it is estimated here that this condition occurred when the mantle was only about 50 c hotter than at present and as recently as 0 9 1 4 b y ago transformation of basaltic crust to eclogite might have enhanced the vigor of the plates but possibly only modestly and intermittently in earlier times plate tectonics could have operated but more slowly so that it could not by itself have accomplished the necessary rate of heat removal from the earth a different tectonic mode must also have operated perhaps subcrustal delamination or dripping plate tectonics would have gradually taken over from the earlier mode plumes are a omplementary mode of mantle convection driven by a lower thermal boundary layer and so they could have operated in conjunction with both regimes of the upper boundary layer

numerical simulations and a conceptual model of the stratocumulus to trade cumulus transition abstract a two dimensional eddy resolving model is used to study the transition from the stratocumulus topped boundary layer to the trade cumulus boundary layer the 10 day simulations use an idealized lagrangian trajectory representative of summertime climatological conditions in the subtropical northeastern pacific the sea surface temperature is increased steadily at 1 5 k day 1 reflecting the southwestward advection of the subtropical marine boundary layer by the trade winds while the free tropospheric temperature remains unchanged results from simulations with both a fixed diurnally averaged shortwave radiative forcing and a diurnally varying shortwave forcing are presented a two stage model for the boundary layer evolution consistent with these simulations is proposed in the first stage decoupling is induced by increased latent heat fluxes in the deepening boundary layer after decoupling cloud cover remains high but the cloudiness regime changes from a single stratocumulus layer to sporadic

plate tectonics on the early earth limitations imposed by strength and buoyancy of subducted lithosphere the tectonic style and viability of modern plate tectonics in the early earth is still debated field observations and theoretical arguments both in favor and against the uniformitarian view of plate tectonics back until the archean continue to accumulate here we present the first numerical modeling results that address for a hotter earth the viability of subduction one of the main requirements for plate tectonics a hotter mantle has mainly two effects 1 viscosity is lower and 2 more melt is produced which in a plate tectonic setting will lead to a thicker oceanic crust and harzburgite layer although compositional buoyancy resulting from these thick crust and harzburgite might be a serious limitation for subduction initiation our modeling results show that eclogitization significantly relaxes this limitation for a developed ongoing subduction process furthermore the lower viscosity leads to more frequent slab breakoff and sometimes to crustal separation from the mantle lithosphere unlike earlier propositions not compositional buoyancy considerations but this lithospheric weakness could be the principle limitation to the viability of plate tectonics in a hotter earth these results suggest a new explanation for the absence of ultrahigh pressure metamorphism uhpm and blueschists in most of the precambrian early slabs were not too buoyant but too weak to provide a mechanism for uhpm and exhumation

arc generated blocks with crustal sections in the north atlantic craton of west greenland crustal growth in the archean with modern analogues abstract the ca 700 km long archean craton of west greenland consists of six meso neoarchean ca 3000 2720 ma shear zone bounded crustal blocks that display similar cross sections from south to north ivittuut kvanefjord bjornesund sermilik fiskefjord maniitsoq each block has a southerly upper and a northerly lower zone thus each faces upwards to the south upper zones have prograde amphibolite facies mineralogy and have never been in the granulite facies whereas lower zones reached granulite facies and were partly retrogressed to amphibolite facies upper and lower zones consist predominantly of tonalite trondhjemite granodiorite ttg orthogneisses geochemistry suggests generation by slab melting in subduction settings of island arcs and active continental margins the gneisses contain km thick metavolcanic amphibolite layers typically bordered by km thick layers containing anorthosite and leucogabbro most upper zones contain upper greenschist to amphibolite facies metavolcanic belts including volcaniclastic andesitic rocks the two most prominent metavolcanic belts in the fiskefjord block at qussuk andesitic volcaniclastic rocks garde a a 2007 a mid archaean island arc complex in the eastern akia terrane godthabsfjord southern west greenland journal of the geological society london 164 565 579 and ivisaartoq mafic ultramafic rocks and anorthosite leucogabbro from upper and lower parts of a supra subduction zone system polat a frei r appel p w u dilek y fryer b ordonez calderon j c yang z 2008 the origin and compositions of mesoarchean oceanic crust evidence from the 3075 ma ivisaartoq greenstone belt sw greenland lithos 100 293 321 have island arc geochemical signatures the 2 km thick fiskenaesset complex bjornesund block comprises chromite layered anorthosites leucogabbros and gabbros and local pillow bearing roof pendants from overlying metavolcanic amphibolite the style of deformation changes downwards within crustal blocks upper zones are characterised by linear metavolcanic belts deformed by mostly one major phase of isoclinal folding and lower zones by kilometre scale double triple fold interference patterns everywhere ttg protoliths have intruded anorthositic and volcanic rocks typically along ductile shear zones often so extensively that only anorthositic or amphibolitic lenses are preserved the meso neoarchean crust was thickened by a combination of thrusting isoclinal folding and continued ttg injection dissimilarities in the proportions of anorthositic and metavolcanic rocks in the six blocks suggest that they evolved in several different microcontinents but by similar processes these crustal blocks provide an exceptional example of how continents evolved in the meso neoarchean comparable archean examples in kapuskasing and pikwitonei canada and modern analogues in fiordland new zealand kohistan himalayas southern california batholith peruvian andes and hidaka japan demonstrate that processes of continental growth from island arc to continental arc magmatism and by implication to continental collision were broadly similar throughout most of earth history

adjacent terranes with ca 2715 and 2650 ma high pressure metamorphic assemblages in the nuuk region of the north atlantic craton southern west greenland complexities of neoarchaean collisional orogeny abstract in the gneiss complex of the nuuk region of the north atlantic craton in southern west greenland terrane juxtaposition was followed by neoarchaean folding under amphibolite facies conditions with widespread low pressure recrystallisation 5 kbar and 550 700 c the complex metamorphic overprinting requires that the p t history related to actual terrane assembly has to be extracted from very small relicts of older metamorphic assemblages combined with the u pb dating petrology and geochemistry of metamorphic zircons in the south of the region the faeringehavn terrane eoarchaean orthogneisses is tectonically overlain by a supracrustal package of amphibolites and paragneisses ca 2840 ma felsic volcano sedimentary protoliths this package is juxtaposed against a higher tectonic level represented by the tre brodre terrane 2825 ma orthogneisses and the tasiusarsuaq terrane 2920 2810 ma orthogneisses the terranes were assembled by 2710 2720 ma as shown by dating of granitic sheets intruded along the terrane boundary mylonites in the faeringehavn terrane and in the overlying 2840 ma supracrustal package relict early high pressure assemblages 12 8 kbar 700 750 c are clinopyroxene garnet plagioclase quartz hornblende in mafic rocks and garnet kyanite rutile bearing assemblages in paragneisses these are commonly replaced by lower pressure assemblages 7 5 kbar such as cordierite sillimanite garnet in paragneisses and hornblende plagioclase quartz garnet in mafic rocks in situ partial melting took place during both low and high pressure regimes metamorphic zircon in the high and low pressure assemblages yields dates of ca 2715 ma mostly with errors of the kapisilik terrane 3050 2960 ma orthogneisses and another supracrustal assemblage of amphibolites and ca 2800 ma quartzo feldspathic metasedimentary rocks are exposed north of the faeringehavn terrane and within fold cores along its western margin are bounded by folded neoarchaean mylonites the kapisilik terrane and this supracrustal assemblage include high pressure metamorphic remnants in amphibolites and metasediments metamorphic segregations with garnet clinopyroxene and kyanite respectively that formed at ca 2650 ma these remnants are overprinted by high temperature but lower pressure metamorphic events at ca 2630 2610 and 2580 ma thus remnants of early metamorphism reveal mutually exclusive ca 2715 and 2650 ma high pressure events in adjacent tectonically juxtaposed terranes repeated high pressure metamorphism in adjacent terranes is observed in younger complex collisional orogens

a treatment for the stratocumulus to cumulus transition in gcms numerical models of climate have great difficulties with the simulation of marine low clouds in the subtropical pacific and atlantic oceans it has been especially difficult to reproduce the observed geographical distributions of the different cloud regimes in those regions the present study discusses mechanisms proposed in previous works for changing one regime into another one criterion is based on the theory of stratocumulus destruction through cloud top entrainment instability due to buoyancy reversal situations in which the mixture of two air parcels becomes denser than either of the original parcels due to evaporation of cloud water another criterion is based on the existence of decoupling in the boundary layer when decoupled the stratocumulus regime changes to another in which these clouds can still exist together with cumulus in a les study the authors have suggested that a combination of those two criteria can be used to diagnose whether at a location the cloud regime corresponds to a well mixed stratocumulus regime a shallow cumulus regime or to a transitional regime where the boundary layer is decoupled the concept is tested in the framework of an atmospheric general circulation model gcm it is found that several outstanding features of disagreement between simulation and observation can be interpreted as misrepresentations of the cloud regimes by the gcm a novel criterion for switching among regimes is proposed to alleviate the effects of these misrepresentations

u pb zircon study of tectonically bounded blocks of 2940 2840 ma crust with different metamorphic histories paamiut region south west greenland implications for the tectonic assembly of the north atlantic craton abstract new fieldwork map interpretation petrography and single zircon u pb geochronology has allowed the identification of different crustal blocks in the paamiut region in the southern portion of the west greenland archaean craton changes of metamorphic grade from only amphibolite facies to granulite facies some subsequently retrogressed corresponds with zones of archaean high strain ductile deformation mylonites u pb zircon dates are presented for the ttg tonalite trondhjemite granodiorite protoliths from each block in the paamiut region and the southern portion of the previously identified tasiusarsuaq terrane lying to the north the southern part of the tasiusarsuaq terrane contains 2880 2860 ma ttg rocks and underwent amphibolite facies metamorphism structurally underneath the tasiusarsuaq terrane to the south is the sioraq block containing 2870 2830 ma ttg rocks partly retrogressed from granulite facies structurally underneath and to the south is the paamiut block dominated by 2850 2770 ma granodioritic rocks that have only undergone amphibolite facies metamorphism also structurally overlying the paamiut block but cropping out separately from the sioraq block is the neria block this appears to be dominated by 2940 2920 ma gneisses that have been totally retrogressed from granulite facies and strongly deformed in the southernmost part of the region the neria block overlies the greenschist to lowermost amphibolite facies sermiligaarsuk block that contains the 2945 ma tartoq group rocks from all the blocks record ancient loss of pb from zircons and some new zircon growth at 2820 ma interpreted to indicate a high grade metamorphic event at that time including granulite facies metamorphism in the sioraq and neria blocks the blocks of different metamorphic grade are interpreted to have moved to their current positions after the 2820 ma metamorphism explaining the change in metamorphic history across some mylonites and ductile shear zones which deform and retrogress granulite facies textures the juxtaposed blocks and their contacts were subsequently folded under amphibolite facies conditions the contacts are cut by undeformed palaeoproterozoic dolerite dykes which post date amphibolite facies metamorphism these results together with previously published data from the godthabsfjord region north of paamiut shows that the north atlantic craton in west greenland from ivittuut in the south to maniitsoq in the north 550 km consists of a mosaic of ductile fault bounded packages that attained their present relative positions in the late archaean

transition from subduction to exhumation related fabrics in glaucophane bearing eclogites oman evidence from relative fabric chronology and 40ar 39ar ages abstract controversy over the age of peak metamorphism and therefore the tectonic evolution of the arabian margin relates to the polydeformed and polymetamorphosed nature of glaucophane bearing eclogites from the saih hatat window beneath the allochthonous samail ophiolite in ne oman these eclogites contain relicts of earlier fabrics structures and metamorphic assemblages and provide a record of change from subduction to exhumation the eclogites are part of a mafic layer that was disrupted into boudins up to 0 5 km in length within a lower plate shear zone as sifah shear zone the megaboudins not only preserve the relicts of the highest grade of metamorphism but also an early ene trending lineation and sheathlike isoclines enveloped by the flat lying schistosity the boudin bearing layer is isoclinally folded with calc schist mafic schist and quartz mica schist where the regional folds have axes parallel to the ne trending stretching lineation a type folds textural evidence suggests multiple growth events for garnet and clinopyroxene requiring polymetamorphism of the mafic layers that formed the eclogite megaboudins the surrounding calc schist and quartz mica schist are both intensely deformed with transposition foliation containing an ne trending lineation in phengite and asymmetric shear indicators such as c type shear bands and asymmetric pressure shadows around garnets that give top to the ne sense of shear although consistent ene trending lineations in all the boudins suggest that they have largely acted as passive nonrotating rigid bodies the presence of ne vergent asymmetric mesofolds extensive dynamic recrystallisation multiple generations of phengites and a range of 40ar 39ar apparent ages within the megaboudins suggest however that they have not acted entirely passively during the later deformation phengites isolated from the high p low t fabrics show groupings in 40ar 39ar apparent ages interpreted as distinct metamorphic cooling intervals at 140 135 120 98 and 92 80 ma microstructural relations suggest that age groupings younger than 100 ma reflect phengite growth during exhumation with the top to the ne shearing the older ages 120 110 ma from fabrics that give top to the s shear sense may reflect growth during the subduction phase the combination of groupings of apparent argon ages older than the crystallisation age of the samail ophiolite the suggestion of different geothermal gradients and superposed metamorphism suggest that the eclogites and garnet blueschists formed as a result of underthrusting along a break that was not directly related to the metamorphic sole of the ophiolite the glaucophane eclogites are interpreted as having formed at different times under varying pressure temperature conditions during underthrusting with variations in the rate of underthrusting allowing thermal equilibration and or rapid cooling at different crustal levels

archean oceanic flake tectonics assuming that the archean mantle was hotter and the oceanic crust thicker than at present we estimate that the archean oceanic lithosphere had a sandwich rheology with the lower crust behaving as a soft ductile layer accordingly the archean oceanic lithosphere had a rheological structure similar to present day continental lithosphere but unlike present day oceanic lithosphere the thickness of the ductile layer is a decreasing function of age while its strength is an increasing function of age the buoyancy problem in archean subduction could be overcome by delamination of oceanic upper crust but mass balance calculation indicates that most archean oceanic crust must have been recycled where the right dynamic conditions existed for delamination oceanic flake tectonics may have been an important process leading to the preservation of oceanic upper crust in greenstone belts that are characteristic of archean terrains

a new parameterization for shallow cumulus convection and its application to marine subtropical cloud topped boundary layers part i description and 1d results abstract a new parameterization of shallow cumulus convection is presented the parameterization consists of a mass flux scheme based on a buoyancy sorting entrainment detrainment plume model the mass flux scheme is coupled to a 1 5 order turbulence closure model with an entrainment closure for convective boundary layers model performance is verified using single column model simulations at relatively high vertical resolution of pure trade cumulus convection and a cumulus to stratocumulus transition mixing rates cloud cover and vertical flux profiles as deduced from previously published large eddy simulation studies are well reproduced by the parameterization the model is used to demonstrate that height variations of lateral mixing rates can be successfully captured by a simple implementation of a buoyancy sorting mechanism in the updraft cloud model a companion paper describes the implementation of the scheme in a mesoscale model

structural evolution metamorphism and restoration of the arabian continental margin saih hatat region oman mountains abstract 170 m y of relatively stable passive margin sedimentation along the northern continental margin of arabia was abruptly terminated during the cenomanian turonian 95 ma when the oman continental margin collapsed and subsided rapidly aruma basin to accommodate obduction of the semail ophiolite complex and underlying thrust sheets haybi and hawasina complexes in the oman mountains the ophiolite was emplaced at least 200 km over the passive margin probably over 450 km in total from ne to sw during the later stages of obduction the leading edge of the continental margin was subducted to depths where carpholite bearing rocks 6 8 kbar blueschist 12 15 kbar and eclogite ca 20 kbar facies metamorphism formed in a ductile deforming ne dipping subduction zone along the northeastern margin of the saih hatat culmination in ne oman the mesozoic shelf carbonates are deformed by recumbent folding on all scales the present day structure is dominated by four sw vergent thrust slices in carpholite grade rocks at higher structural levels overlying lower units of crossite bearing blueschist hulw unit garnet glaucophane chloritoid blueschists and garnet clinopyroxene phengite glaucophane eclogites 15 20 kbar the largest shear zone separates 7 8 kbar hulw unit blueschists above from 12 15 kbar retrogressed eclogites and ca 20 kbar eclogites of the as sifah unit below further inboard sw a major shear zone separates the lower plate hulw blueschist facies rocks from the upper plate sedimentary rocks upper plate lower plate discontinuity which also show carpholite grade hp metamorphism towards the ne restoration of the entire saih hatat continental margin shows that the upper plate lower plate discontinuity cuts down section to the sw as far as the proterozoic hatat schists ne directed extensional crenulation schistosity and nne oriented stretching lineations in the eclogite and blueschist facies rocks are consistent with sw directed exhumation of footwall hp rocks ne facing folds and spectacular sheath folds with greatly attenuated limbs in the upper plate sediments are interpreted as antithetic backfolds with shortening in the upper plate balanced by the subduction of the lower plate consistent with a ne directed subduction of the continental margin rocks beneath the sw obducting ophiolite haybi and hawasina thrust sheets recent suggestions of a nascent sw directed subduction beneath the oman margin are not consistent with the sedimentary evolution of the shelf and slope carbonates or the geological structure of saih hatat two main phases of tectonic uplift of saih hatat and jebel akhdar are firstly the latest cretaceous ramp culmination associated with foreland propagating thrusts in the sw and backfolds in the ne and secondly post late eocene oligocene uplift by ca 2000 m

buoyancy reversal decoupling and the transition from stratocumulus to shallow cumulus topped marine boundary layers the transition in a marine boundary layer mbl from stratocumulus topped to shallow cumulus topped is investigated by using a large eddy simulation les model the experiments performed aim to examine the influence on the transition of 1 the probability of buoyancy reversal at the mbl top i e situations in which the mixture of two air parcels becomes denser than either of the original parcels due to phase change or other nonlinear processes involved in the mixing and 2 the degree of decoupling in the mbl i e the strength of a shallow stably stratified layer near cloud base our results suggest that a stratocumulus topped mbl is most likely to transit to a cumulus topped one when 1 there exists high probability of buoyancy reversal at the mbl top and 2 the mbl is decoupled due to large surface evaporation we argue that a parameterization that includes representation of those two effects combined has the potential to provide a simple way of predicting the mbl transition in climate models

timing of late archaean terrane assembly crustal thickening and granite emplacement in the nuuk region southern west greenland abstract a breakthrough in understanding the structural and metamorphic evolution of archaean gneiss complexes occurred with the recognition that the nuuk region of southern west greenland comprised separate terranes assembled in the late archaean from northwest to southeast these are the akia 3220 2970 ma akulleq 3870 3600 and 2820 ma and tasiusarsuaq 2920 2860 ma terranes the minimum time of assembly is recorded by the first event common to all component terranes using shrimp u pb zircon geochronology the oldest events common to all terranes including emplacement of crustally derived granites contemporaneous metamorphism and anatexis have been dated at 2710 2725 ma in the akulleq terrane areas where in situ diatexite formed abundant granitoid sheets were intruded and common growth of metamorphic zircon mostly low th u in most lithologies occurred in the akia and tasiusarsuaq terranes there was only intrusion of a lesser number of 2720 ma granitoid sheets because metamorphic zircons of that age have not been found the 2720 ma event is interpreted as marking or shortly following terrane assembly when the diverse components of the akulleq terrane were tectonically juxtaposed with the other two this new documentation of metamorphic and associated igneous events within an accreting cratonic region is an illustration that the stabilisation of extensive areas of archaean gneisses can be due to accretionary tectonics long after the individual components were first formed

continental subduction and a mechanism for exhumation of high pressure metamorphic rocks new modelling and field data from oman abstract the physical model presented reveals two principal regimes of continental subduction a highly compressional hc regime and a low compressional lc regime characterised by high and low pressure between the overriding and subducting plates respectively the pressure is inversely proportional to the pull force which depends on the difference between average density of the subducting lithosphere and density of the mantle the subducting continental crust reaches a maximum depth which is proportional to the strength of the crust inversely proportional to the interplate pressure and is ca 200 km on average the crust then fails forming a major crustal thrust the location of failure depends on the regime of subduction on the interplate pressure in the hc regime failure occurs near the front of the subduction collision zone whereas the crust fails at greater depth under the base of the overriding plate in the lc regime the failure is followed by buoyancy driven uplift of the subducted crustal slice while the lithospheric mantle keeps subducting the uplift causes a normal sense displacement formation of a normal fault along the upper surface of the crustal slice for the lc regime failure of the crust near the base of the overriding plate a rapid spontaneous crustal uplift intrusion into the interplate zone brings the deeply subducted crust to shallow depths under the pressure of this crust the frontal part of the overriding plate undergoes local extension and then fails forming a tectonic window the rising material the high pressure rocks is exhumed within this window in the hc regime uplift of the subducted crust after its failure in front of the overriding plate is possible only with erosion of the relief which provides an unloading effect allowing the subducted crustal slice to rise up the exhumation depth is generally smaller in this regime but the volume of the exhumed material is larger the hc subduction regime has been shown earlier to match the himalayan situation the lc regime fits the situation in the oman mountains considered in this paper

a mid archaean island arc complex in the eastern akia terrane godthabsfjord southern west greenland a relict oceanic island arc complex in the eastern akia terrane godthabsfjord southern west greenland constitutes a magmatic and geotectonic link between c 3 05 3 0 ga tonalitic orthogneiss and enclaves of older supracrustal amphibolite the relict arc forms isoclinally folded panels of volcaniclastic meta andesite with major and trace element island arc signatures intercalated with volcano sedimentary schist tholeiitic amphibolite and opx rich cumulate rocks a zircon u pb age of 3071 1 ma obtained from a volcano sedimentary schist is marginally older than the main orthogneisses and is the first depositional age reported from a supracrustal rock within akia terrane granite sheets were emplaced at c 3005 2980 ma synkinematically with the isoclinal folding and were followed by peak metamorphism at 2990 2970 ma with substantial recrystallization of volcanic zircon and mobility of large ion lithophile elements the identification of the arc complex provides new insight into mid archaean continental crustal accretion in west greenland and substantiates previous ideas that the orthogneisses are products of slab melting in convergent plate tectonic settings the presence of the arc complex also implies that archaean high grade orthogneiss amphibolite associations may not represent plate tectonic environments distinct from granite greenstone associations but expose deeper sections of the same convergent systems

an overview of the geochemistry of eoarchean to mesoarchean ultramafic to mafic volcanic rocks sw greenland implications for mantle depletion and petrogenetic processes at subduction zones in the early earth abstract this study reviews the geochemical characteristics of eoarchean to mesoarchean ultramafic to mafic volcanic rocks now amphibolites in sw greenland and compares them with those of cenozoic oceanic island arc basalts to evaluate archean subduction zone petrogenetic processes emphasis is placed on the th ree hfse zr ti and nb systematics of the ca 3800 and ca 3700 ma arc suites in the isua greenstone belt the ca 3075 ma ivisaartoq ujarassuit greenstone belt and amphibolites associated with the ca 2970 ma fiskenaesset layered anorthosite complex on n morb normalized diagrams the isua ivisaartoq ujarassuit and fiskenaesset volcanic rocks are all characterized by depletion of nb relative to th and lree consistent with a supra subduction forearc arc backarc geodynamic setting similarly on the th yb nb yb projection these suites plot within the field of cenozoic oceanic island arc basalts on log transformed immobile trace element ratio nb th la th sm th and yb th diagrams they display a trend projecting from morb mid ocean ridge basalt to iab island arc basalt on the iab crb continental rift basalt oib ocean island basalt morb diagram as for cenozoic oceanic island arc basalts accordingly these trace element compositions are interpreted as reflecting the enrichment of archean depleted upper mantle morb source by subduction derived melts and fluids following the initiation of intra oceanic subduction and arc migration concentrations of mgo and ni in sw greenland archean basalts overlap with but extend to 2 to 4 times higher than those in cenozoic oceanic island arc counterparts in contrast the majority of archean basalts have ree and hfse concentrations 2 to 4 times lower than cenozoic oceanic island arc basalts consistent with more depleted sub arc mantle wedge peridotites in the archean than in cenozoic counterparts such depletion reflects the extraction of large volumes of mafic to ultramafic melts from hotter archean mantle we infer that less refractory compositions of the present day depleted upper mantle the source of morb and conservative hfse in arc basalts have resulted from re mixing with the less depleted to enriched deeper mantle material transferred to shallower depths by mantle convection and plumes in post archean times archean basalts in sw greenland share the negative nb and ti anomalies of cenozoic oceanic island arc basalts on average archean basalts however have lower th nb la nb sm tio2 and gd tio2 ratios than cenozoic oceanic island arc counterparts given that rutile controls the nb and ti budgets in arc magmas the lower th nb la nb sm tio2 and gd tio2 ratios in sw greenland basalts are attributed to the mobility of nb and ti in slab derived melts involving rutile fusion in archean subduction zones these elements are less mobile in fluids originating from cenozoic subducting oceanic crust where rutile appears to be generally stable accordingly it is suggested that higher geothermal gradients in the archean may have provided optimized conditions for slab melting and metasomatism of the sub arc mantle wedge by slab derived melts

factors influencing cloud area at the capping inversion for shallow cumulus clouds large eddy simulations of marine cumulus clouds are performed aspects of the simulations are varied in order to identify those parameters that play a role in determining when the cumulus clouds spread out under the capping inversion into stratocumulus these include the strength of the surface fluxes which also affects the strength of the cumulus mass transport the strength of the capping inversion the moisture content of the free atmosphere the strength of cloud top radiative cooling and whether there is significant precipitation or sedimentation of cloud drops tests are also performed to evaluate sensitivity to the model resolution as in previous studies having sufficient resolution at the capping inversion is found to be important for robust simulation of any stratiform cloud that forms there in addition the cloud cover is found to be sensitive to many of the physical processes investigated in the end though a clear and robust dependence emerges on the cloud top entrainment instability parameter for large a typical small shallow cumulus cloud cover is seen for below the threshold for buoyancy reversal the cumulus spreads into a solid sheet of stratocumulus beneath the inversion consistent with previous studies there is no abrupt transition and the cloud cover decreases smoothly as increases the conclusion is reached that varying other parameters in the simulations affects the cloud cover through their impact on the size of the jumps across the inversion and thence the implications for turbulent mixing parametrizations in general circulation models are discussed crown copyright 2009 reproduced with the permission of hmso published by john wiley sons ltd