holocene coastal glaciation of alaska holocene uctuations of the three cirque glaciers on the seward peninsula and ve groups of tidewater and land terminating glaciers along the northernmost gulf of alaska provide a proxy record of late holocene climatic change furthermore the movements of the coastal glaciers were relevant to late holocene native american migration the earliest expansion was recorded about 6850 yr bp by hubbard glacier at the head of yakutat bay in the gulf of alaska however its down fjord advance to the bay mouth was delayed until 2700 bp similarly expansions of the icy bay bering and mccarty glaciers occurred near their present termini by 3600 3000 bp compatible with marked cooling and precipitation increases suggested by the alaskan pollen record decrease in glacier activity 2000 bp was succeeded by advances of gulf coastal glaciers between 1500 and 1300 bp correlative with early medieval expansions across the northern hemisphere a medieval optimum encompassing at least a few centuries prior to ad 1200 is recognized by general retreat of land terminating glaciers but not of all tidewater glaciers little ice age advances of land based glaciers many dated with the precision of tree ring cross dating were centered on the middle 13th or early 15th centuries the middle 17th and the last half of the 19th century a d strong synchrony of these events across coastal alaska is evident 2000 elsevier science ltd all rights reserved

late weichselian and holocene sedimentary palaeoenvironment and glacial activity in the high arctic van keulenfjorden spitsbergen high arctic fjords for example van keulenfjorden on spitsbergen provide valuable palaeoenvironmental archives as they typically contain landforms and sediment sequences that document past changes in glacial activity with high temporal resolution van keulenfjorden was covered with a grounded glacier during the last glacial and it was deglaciated between c 11 8 and 11 3 cal ka bp the retreat of the ice front accelerated from approximately 80 to 190 m a during the deglaciation the maximum late holocene glacier extent occurred after surge like advances of the glacier nathorstbreen between 2790 and 2610 cal yr bp i e during a period with the coldest climatic conditions on svalbard this maximum extent was reached approximately 2600 years earlier than inferred for most fjords on svalbard suggesting that surge like glacier advances on svalbard can occur under variable climatic conditions the time interval between the advances of nathorstbreen around 2 7 ka bp was approximately 100 150 years this is comparable to the last and only historically known quiescent phase of nathorstbreen of c 120 years between the late 19th century and the most recent surge from 2003 to 2012

glaciological and marine geological controls on terminus dynamics of hubbard glacier southeast alaska hubbard glacier located in southeast alaska is the world s largest nonpolar tidewater glacier it has been steadily advancing since it was first mapped in 1895 occasionally the advance creates an ice or sediment dam that blocks a tributary fjord russell fiord the sustained advance raises the probability of long term closure in the near future which will strongly impact the ecosystem of russell fiord and the nearby community of yakutat here we examine a 43 year record of flow speeds and terminus position to understand the large scale dynamics of hubbard glacier our long term record shows that the rate of terminus advance has increased slightly since 1895 with the exception of a slowed advance between approximately 1972 and 1984 the short lived closure events in 1986 and 2002 were not initiated by perturbations in ice velocity or environmental forcings but were likely due to fluctuations in sedimentation patterns at the terminus this study points to the significance of a coupled system where short term velocity fluctuations and morainal shoal development control tidewater glacier terminus position

geochemical constraints on the growth of the continental crust several lines of evidence indicate that the archean upper crust was considerably more mafic than the present day upper crust there has been no significant change in ree and th abundances in post archean clastic sedimentary rocks suggesting that there has been no change in the composition of the upper crust during the post archean this indicates that if there have been any additions to the post archean upper crust they must have had similar composition to the upper crust itself geochemical modelling of ree and th abundances in sedimentary rocks suggests that the minimum ratio of post archean to archean upper crustal composition required to eliminate the archean upper crustal trace element signature within analytical uncertainty is about 4 1 such a model also is supported by isotopic data using plausible assumptions regarding the volume of archean crust isostatic relations and extreme models of the earth s degassing history it is proposed that approximately 65 75 of the continental crust formed

stabilizing feedbacks in glacier bed erosion glaciers often erode transport and deposit sediment much more rapidly than nonglacial environments1 with implications for the evolution of glaciated mountain belts and their associated sedimentary basins but modelling such glacial processes is difficult partly because stabilizing feedbacks similar to those operating in rivers2 3 have not been identified for glacial landscapes here we combine new and existing data of glacier morphology and the processes governing glacier evolution from diverse settings to reveal such stabilizing feedbacks we find that the long profiles of beds of highly erosive glaciers tend towards steady state angles opposed to and slightly more than 50 per cent steeper than the overlying ice air surface slopes and that additional subglacial deepening must be enabled by non glacial processes climatic or glaciological perturbations of the ice air surface slope can have large transient effects on glaciofluvial sediment flux and apparent glacial erosion rate

sm nd age of kambalda and kanowna greenstones and heterogeneity in the archaean mantle a precise sm nd age of 2 790 30 myr and an initial ratio corresponding to nd 3 4 0 6 has been determined for the kambalda sequence from the eastern yilgarn block of western australia the high initial 143nd 144nd implies a long lived light rare earth element depleted high sm nd source for the kambalda volcanics and therefore provides definitive evidence for ancient heterogeneities in the archaean mantle

archean greenstone belts are not oceanic crust archean greenstone belts with a preponderance of mafic volcanic rocks often preserved in tectonically complex sequences are obvious candidates in the search for remnants of archean ocean crust we review the tectonic setting and stratigraphy of a number of greenstone sequences previously interpreted as archean ophiolites and conclude on the basis of basal unconformities presence of xenocryst zircons geochemical and isotopic evidence for crustal contamination intrusive relationships with older basement and their internal stratigraphy that none of these examples is derived from archean oceanic crust thermal tectonic and isostatic constraints imply that archean oceanic basins did exist and were covered with several kilometers of water we consider it possible indeed probable that relict archean oceanic crust is preserved in granite greenstone terrains or other archean tectonic settings but that an unequivocal archean ophiolite has not yet been recognized

seasonal fluctuations in the advance of a tidewater glacier and potential causes hubbard glacier alaska usa satellite imagery has been used to acquire seasonal terminus positions of tidewater hubbard glacier alaska usa from 1992 to 2006 during this 15 year time period the width averaged advance of the entire terminus has been 620 m at a mean rate of 35 m a 1 seasonal fluctuation of the terminus ranges from 150 to 200 m on average and varies spatially a section of the terminus near a narrow gap where the glacier has now twice closed off 40 km long russell fiord exhibited little to no mean advance during this time period but displayed seasonal fluctuations of 300 500 m seasonal variability in surface ice speeds and surface sea water temperatures was also observed both are potential forcing mechanisms for terminus fluctuations seasonal changes in sea water temperature of 10 128c as well as seasonal changes in subglacial freshwater discharge are inferred to influence calving and submarine melting at the terminus driving seasonal variations displacements of the medial moraine separating hubbard and valerie glaciers at the terminus suggest surge like pulses of the latter with a periodicity of several years the timing of these pulses suggests they may influence the hubbard terminus near gilbert point and have implications for future closures of russell fiord

zircon geochronology and hf isotopes of the dwalile supracrustal suite ancient gneiss complex swaziland insights into the diversity of palaeoarchaean source rocks depositional and metamorphic ages abstract we report the results of combined u pb and lu hf isotope analyses of detrital and metamorphic zircon grains from a variety of metavolcano sedimentary rocks of the dwalile supracrustal suite dss from the ca 3 66 to 3 20 ga ancient gneiss complex agc in swaziland the results indicate that the dss is made up by palaeo and mesoarchaean rocks which have been affected by a polymetamorphic overprint and became tectonically assembled during the neoarchaean the oldest dwalile rocks were formed during a phase of felsic to intermediate volcanism at ca 3 46 ga and are derived from the same magma source as the tsawela gneisses based on the combined u pb hf isotope data two groups of metasedimentary rocks can be distinguished an older dwalile group i that is similar in age to the hooggenoeg and kromberg formations 3 46 3 40 ga but with a different source area a younger dwalile group ii is coeval with the fig tree and possibly moodies groups the u pb hf isotope data and petrological observations demonstrate that the dss was affected by three stages of metamorphic overprint an event at 3 23 ga pervasive amphibolite facies metamorphism at 3 15 ga and low p granulite facies metamorphism at 2 99 ga metamorphic zircon in the amphibolite facies rocks resulted from new growth in aqueous fluids at 3 15 ga whereas in the granulite facies rocks new zircon was formed in the presence of melt the 3 23 ga event coincides with the last significant metamorphic overprint documented in the barberton greenstone belt bgb whereas as two younger events are restricted to the agc the data support the model suggesting amalgamation between the agc and bgb terranes at ca 3 23 ga in addition they require crustal thickening in the agc terrane at ca 3 15 ga causing barrovian type metamorphism in the dss and granulite facies metamorphism in the luboya kubuta terrane low p granulite facies metamorphism at ca 2 99 ga is most likely related to crustal extension during the onset of pongola basin rifting causing mantle upwelling mafic magmatism and heat transfer from the mantle into the thinned crust

geochemistry of precambrian carbonates 3 shelf seas and non marine environments of the archean a comprehensive whole rock study of mineralogical chemical and isotopic attributes of archean carbonates suggests that their lithologies and facies have been controlled by tectonic setting in the first two papers of this series we have shown that the dominant lithology of sedimentary carbonates in greenstone belt settings is limestone fe rich dolostone of ankeritic composition in older examples in this paper we suggest that the archean shelf sequences are mostly dolostone and the contemporaneous lacustrine playa lakes are characterized by limestone facies the present study is of the shelf environments of the archean represented by the pongola supergroup of south africa and the hamersley group wittenoom and carawine dolomite of australia the lacustrine playa examples have been sampled from the ventersdorp supergroup of south africa and the fortescue group of australia geological trace element and oxygen isotope considerations of the shelf carbonates suggest that their original mineralogy may have been aragonite and that the pongola dolostones probably represent a direct dolomitization product of this precursor in contrast the stabilization of the hamersley carbonates may have involved an additional step of transformation of a metastable precursor into limestone prior to dolomitization subsequently both sequences but the pongola supergroup much more so were subjected to pervasive thermal events which overprinted their chemical and isotopic signatures consideration of the best preserved samples yields 0 9 2 1 and 6 l pdb for the 13c and 18o respectively for the initial archean signature if compared to phanerozoic carbonates the archean 13c values are comparable but the oxygen isotope measurements again support the primary nature of 18o depletion in ancient marine sediments regardless of its ultimate cause the 87sr86sr of 2 5 ga old sea water was not more than 0 7033 if this value reflects the contemporaneous sea water it would indicate the commencement of a significant 87sr input from the growing and aging continents it is however entirely possible that the slight enrichment in radiogenic 87sr relative to the contemporaneous mantle represents only an unresolved artefact of post depositional alteration if so the 87sr86sr of the 2 5 ga old sea water may still have been within a mantle range the lacustrine limestones retain a strong facies control in their trace element attributes they are also 4 depleted in 13c compared to marine carbonates and strongly depleted in 18o 19 l pdb these light 18o values are somewhat enigmatic unless the lakes were of the high altitude latitude variety the 87sr86sr of 0 711 for the ventersdorp lakes suggests that their drainage basin incorporated not only volcanic rocks but also aged continental crust archean carbonates of all lithologies and facies are enriched in mn and fe compared to their phanerozoic counterparts

seismic stratigraphic evidence for glacial expanse during glacial maxima in the yakutat bay region gulf of alaska 1 modern shelf bathymetry bordering the gulf of alaska exhibits shelf crossing sea valleys that suggest focused pathways for ice flow during glacial conditions using an integrated seismic data set between the present yakutat and alsek sea valleys we investigate the glacial stratigraphic record in order to improve our understanding of the regional glacial system during maximum glacial conditions our investigations reveal four glacial unconformities of which the latter two are overlain by sediment packages hundreds of meters thick we suggest that these unconformities are indicative of ice advance phases during the little ice age lia the last glacial maxima lgm and two pre lgm advances with glacial retreat sequences preserved from the youngest two the advances were dominated by ice expanse from the malaspina glacial system and alsek river districts rather than the hubbard glacial system and only show distinctive morainal bank development near the shelf edge and near the mouth of or within modern bays fjords or river valleys this observation strongly supports rapid and continuous retreat from glacial maxima conditions during climatic warming all except the inferred lia sequence transgress the shelf and exhibit concentrated erosion in overdeepened troughs analogous to cross shelf troughs similar to those observed on other high latitude glaciated shelves the two alaskan troughs discussed here may be end member examples of high sediment flux systems due to the temperate glacial setting combined with an actively exhuming orogen

geophysical insights into the transition fault debate propagating strike slip in response to stalling yakutat block subduction in the gulf of alaska on the basis of faulting mapped on seismic reflection and bathymetric data seismicity current plate motions and evidence that the yakutat block may be anomalously thick we propose a tectonic model for yakutat pacific interactions including the often debated transition fault to the east deformation associated with the queen charlotte fairweather fault system is extending offshore facilitating westward propagation of strike slip motion along the eastern segment of the transition fault to the west the oblique slip pamplona zone and transition faults merge at an embayment in the continental margin where a north south dextral strike slip fault within the pacific plate illuminated by the 1987 1992 earthquake swarm intersects the pacific yakutat tectonic boundary these fault patterns are consistent with modern plate motions and reflect a plate boundary reorganization that may be caused by resistance to subduction by the yakutat block a possible moderate sized oceanic plateau

sm nd isotopic and geochemical study of the archean tonalite amphibolite association from the eastern indian craton we report the results of a sm nd isotopic major element and rare earth element ree study of the older metamorphic group omg tonalite amphibolite association of the eastern indian craton the older metamorphic tonalite gneisses omtg have been previously dated to be 3 8 ga using sm nd isotope systematies and 3 2 3 4 ga by rb sr and pb pb dating the results of this study indicate that the protoliths of the omg amphibolites are 3 3 ga isochron age 3 30 0 06 ga nd 0 9 0 7 and therefore the omtg which intrude into the associated amphibolites cannot be any older than 3 3 ga the amphibolites display light ree enrichment ce yb n 2 2 6 7 la 30 100 x chondrite and nearly flat heavy ree patterns tb lu n 1 2 1 9 the basaltic parents of the amphibolites were probably generated by the partial melting of a spinel lherzolite mantle strong linear relationships between the amphibolites and tonalites in 147sm 144nd 143nd 144nd space isochron age 3 29 0 04 ga nd 0 8 0 8 imply that they are genetically related the tonalites display fractionated ree patterns la 100 300 x chondrite with moderate heavy ree depletions tb lu n 1 9 3 4 the isotopic major element and ree data are consistent with the derivation of the omtg from partial melting of omg amphibolites or equivalent rocks at amphibolegarnet stabilization depths an initial nd t value of 0 9 0 7 for the amphibolites indicates the presence of a slightly depleted mantle source at 3 3 ga with 147sm 144nd between 0 20 and 0 22 it is suggested that the growth of continental crust in the eastern indian craton occurred in response to magmatic underplating in a plume setting

bedform signature of a west antarctic palaeo ice stream reveals a multi temporal record of flow and substrate control abstract the presence of a complex bedform arrangement on the sea floor of the continental shelf in the western amundsen sea embayment west antarctica indicates a multi temporal record of flow related to the activity of one or more ice streams in the past mapping and division of the bedforms into distinct landform assemblages reveals their time transgressive history which implies that bedforms can neither be considered part of a single downflow continuum nor a direct proxy for palaeo ice velocity as suggested previously a main control on the bedform imprint is the geology of the shelf which is divided broadly between rough bedrock on the inner shelf and smooth dipping sedimentary strata on the middle to outer shelf inner shelf bedform variability is well preserved revealing information about local complex basal ice conditions meltwater flow and ice dynamics over time these details which are not apparent at the scale of regional morphological studies indicate that past ice streams flowed across the entire shelf at times and often had onset zones that lay within the interior of the antarctic ice sheet today in contrast highly elongated subglacial bedforms on sedimentary strata of the middle to outer shelf represent a timeslice snapshot of the last activity of ice stream flow and may be a truer representation of fast palaeo ice flow in these locations a revised model for ice streams on the shelf captures complicated multi temporal bedform patterns associated with an antarctic palaeo ice stream for the first time and confirms a strong substrate control on a major ice stream system that drained the west antarctic ice sheet during the late quaternary

compositional heterogeneity of archean mantle estimated from sr and nd isotopic systematics of basaltic rocks from north pole australia and the isua supracrustal belt greenland abstract compositional variability found in modern mid ocean ridge basalt morb and ocean island basalt oib reflects differentiation processes associated with material recycling in the mantle crust system to investigate the timing at which this recycling system was established and how it transformed into the present day system we present geochemical analyses of the archean basalts from north pole np in the east pilbara craton western australia and the isua supracrustal belt isb southern west greenland these rocks represent archean accretionary complexes with ages of 3 5 ga and 3 7 3 8 ga respectively we analyzed the trace element contents including rare earth elements rees and sr and nd isotopic compositions of the basalts which may represent morbs and oibs from np and isb their trace element compositions are broadly similar but show distinct geochemical characteristics particularly with respect to rees that probably reflect differences in both the source mantle and degree of melting such differences are also evident in their initial nd isotopic compositions which were estimated based on equilibrium partitioning of rees and well defined isochron ages in contrast the sr isotopic compositions of the np and isb basalts are highly variable and their isochron ages are inconsistent with previous studies furthermore the partitioning of rb and sr in the np basalts indicates disequilibrium suggesting that the rb sr system has been disturbed by post igneous alteration and metamorphism based on these observations we propose the following model to explain the temporal variations in the geochemical composition of the archean mantle i 3800 ma recycling of plate material and melting occurred quite readily and therefore morbs and oibs were produced from differentiated mantle sources ii 3460 ma to 3800 ma mantle crust mixing occurred as the result of an extreme event such as mantle overturning reducing the compositional variation of the mantle and iii after 3460 ma mantle heterogeneity gradually developed in the material recycling system re establishing the compositional differences between morbs and oibs this model requires an extreme event to drive the homogenization during stage ii which may provide new insights into the evolution of the crust mantle system

volcanic rocks of the witwatersrand triad south africa ii petrogenesis of mafic and felsic rocks of the dominion group abstract a bimodal suite of volcanic rocks builds the bulk of the dominion group which with an age of 2 72 ga is the oldest cover sequence overlying the granite greenstone archaean basement of the kaapvaal craton in the western transvaal south africa the basic lavas are relatively rich in sio 2 50 58 and aphyric and exhibit a large compositional range this variation is typically tholeiitic in that it is characterized by strong enrichment of ti fe and v in differentiated lavas the different amounts of relative enrichment of these elements allow the basic lavas to be subdivided into two groups one group is characterized by evolution to low ti v ratios in evolved lavas and the other by high ti v ratios for nearly all other elements the composition variation of the two groups is extremely similar and overlaps although the high ti v group contains the most evolved compositions and most of the primitive lavas mg numbers 60 belong to the low ti v group although recognition of the two groups is based on geochemistry they are interlayered in a consistent pattern within the lava pile theoretical modelling indicates that the variation in both lava groups can be explained by low pressure fractional crystallization of phases in a typical tholeiitic sequence primitive basalts of the low ti v group show a large range in major element compositions and a more restricted range of incompatible trace elements and ree abundances in particular fe 2 o 3 total ranges from 9 to 13 5 and correlates positively with mgo cr and ni and negatively with al 2 o 5 and sio 2 there are no correlations of mg number with incompatible trace elements such as zr and ce these lavas also have lree enriched ree abundance patterns and negative nd these features cannot be explained by crustal contamination but are consistent with derivation by uniform amounts of melting of a mantle source which was heterogeneous with regard to major elements particularly fe al and probably ca and na and uniformly enriched in incompatible trace elements at least 0 3 ga before melting the geochemical style of this enrichment is akin to that associated with modern subduction this enrichment is associated with the development of a thick archaean lithosphere the existence of which is corroborated by diamond age and diamond inclusion thermobarometric studies the geochemistry of the dominion basic lavas differs significantly from those of the 3 4 ga barberton komatiitic and tholeiitic suites this suggests that the changing pattern of crustal evolution from metastable greenstone tectonic styles to stable proterozoic styles is reflected in the changing character of the mantle sources tapped by basic magmas the porphyritic dominion rhyodacites or rhyolites exhibit limited compositional variation and are genetically unrelated to the basic lavas nd isotopes and la th and th co ratios suggest that they are partial melts of mafic crustal materials geochemical and petrological features of the dominion suite are entirely consistent with eruption in a continental environment characterized by rifting or lithospheric thinning this is consistent with some models for the tectonic development of the witwatersrand basin

fjords as temporary sediment traps history of glacial erosion and deposition in muir inlet glacier bay national park southeastern alaska glacimarine sedimentary deposits within the basins of muir inlet a 48 km long silled fjord are interpreted from complimentary sets of high resolution seismic reflection profiles using known glacial advance and retreat history two prominent glacial erosion surfaces are identified the lowest attributed to the last glacial maximum lgm advance and the upper coincident with the little ice age lia advance the lgm ice sheet which advanced onto the continental shelf was 1700 m thick in muir inlet and eroded bedrock whereas the thinner lia ice did not lgm deposits 300 m thick occur beneath the lia erosion surface in the deepest basins evidence for earlier neoglacial advances is present in subaerial deposits however neoglacial sediments preserved within the marine record are restricted to one depositional package on the entrance sill volumes of lia retreat sediments were calculated within basins an average annual sediment flux was calculated by modeling the duration of sediment contributed from muir glacier and from tributary glaciers and side entry sources the annual sediment flux ranged from 1 3 10 6 m 3 yr to 4 6 10 7 m 3 yr and increases logarithmically with increasing drainage basin area similar to fluvial systems this sediment flux does not only represent bedrock erosion additional sediment is contributed from persistent tributary glaciers and from lgm sediment stored within deeper basins basin wide reflections characterize the most common seismic facies and indicate that strata are horizontal and continuous across each basin confirming the importance of sediment gravity flows originating from sills and sloping fjord walls

a combined o sr nd and pb isotopic and trace element study of crustal contamination in central andean lavas i local geochemical variations systematic variations in o sr nd and pb isotopic ratios and in sr nd pb and rb trace element abundances in central andean andesitic lavas are analyzed in terms of combined crustal assimilation and fractional crystallization the results of integrated isotopic model calculations show that observed local isotopic and trace element variations within the arequipa and barroso volcanic groups of southern peru are best fit by a simple model involving shallow or intermediate depth crustal assimilation of charcani gneiss or an equivalent rock accompanied by dominantly plagioclase fractional crystallization a consequence of this result is that the rbsr pseudoisochrons reported for the arequipa and barroso volcanics are wholly artifacts of the combined processes of assimilation and plagioclase fractionation as are all of the pb isotopic variations measured in the same lavas already interpreted by tilton and barreiro 14 to be the result of crustal contamination isotopic and trace element compositions of the primary magmas for the arequipa and barroso volcanic series are obtained by correcting for assimilation and fractional crystallization the starting magmas are characterized over the entire range of permissible primary compositions by high87sr 86sr ratios low143nd 144nd ratios and high sr concentrations relative to normal island arc andesites these results appear to require prior contamination of the magmas but not involving plagioclase fractionation the uncertainty in pb and o isotope ratios computed for possible primary magmatic compositions is sufficiently large however that it is not possible to preclude a subcrustal origin for at least some regional variation in sr and nd isotopic ratios

variable history of quaternary ice sheet advance across the beaufort sea margin arctic ocean the seismic stratigraphy and architecture of the beaufort sea shelf and slope are investigated using a comprehensive grid of high resolution two dimensional seismic reflection data three cross shelf troughs representing locations of former ice streams draining a 1000 km long section of the laurentide ice sheet lis are examined the mackenzie amundsen gulf and m clure strait systems dynamics of these paleo ice streams influenced ice sheet configuration and may have forced abrupt climatic change through delivery of ice and freshwater to the arctic ocean a comprehensive understanding of their geometry and dynamics is crucial for constraining numerical models of the lis evidence for two quaternary ice advances to the shelf break is interpreted from the mackenzie trough by contrast seismic stratigraphy of the amundsen gulf trough 400 km east of the mackenzie trough records at least nine quaternary ice advances here the outer shelf consists of stacked till sheets extending to the shelf break and forming a trough mouth fan the contrasting glacial histories of these neighboring ice streams are explained by their positions within the lis the mackenzie trough ice stream was situated at the extreme northwest ice sheet margin whereas the amundsen gulf ice stream had a more central location and larger drainage basin supplying significant quantities of ice and sediment to the arctic ocean through much of the quaternary the m clure strait trough probably possesses a similar architecture to the amundsen gulf trough and an even larger trough mouth fan

pattern and forcing of northern hemisphere glacier variations during the last millennium time series depicting mountain glacier fluctuations in the alps display generally similar patterns over the last two centuries as do chronologies of glacier variations for the same interval from elsewhere in the northern hemisphere episodes of glacier advance consistently are associated with intervals of high average volcanic aerosol production as inferred from acidity variations in a greenland ice core advances occur whenever acidity levels rise sharply from background values to reach concentrations 1 2 equiv h kg above background a phase lag of about 10 15 yr equivalent to reported response lags of alpine glacier termini separates the beginning of acidity increases from the beginning of subsequent ice advances a similar relationship but based on limited and less reliable historical data and on lichenometric ages is found for the preceding 2 centuries calibrated radiocarbon dates related to advances of non calving and non surging glaciers during the earlier part of the little ice age display a comparable consistent pattern an interval of reduced acidity values between about 1090 and 1230 a d correlates with a time of inferred glacier contraction during the medieval optimum the observed close relation between noothern hemisphere glacier fluctuations and variations in greenland ice core acidity suggests that sulfur rich aerosols generated by volcanic eruptions are a primary forcing mechanism of glacier fluctuations and therefore of climate on a decadal scale the amount of surface cooling attributable to individual large eruptions or to episodes of eruptions is simlar to the probable average temperature reduction during culminations of little ice age alacier advances ca 0 5 1 2 c as inferred from depression of equilibrium line altitudes

the archean proterozoic boundary in the kaapvaal province of southern africa abstract the kaapvaal province has long been considered unusual in that archean cover sequences developed over granite greenstone gneiss terranes 3060 25 ga whereas in other provinces the oldest cover sequences are generally 2500 ga however two unconformity bounded sequences 2 5 ga and of greater inter regional extent than the unconformably underlying archean sequences can now be recognized in the kaapvaal province the two basal unconformities of these sequences are candidates for the temporally defined 2 5 ga archean proterozoic boundary the older but geologically more discordant unconformity occurs at the base of the uppermost group of the ventersdorp supergroup the pniel group and in at least five other erosional remnants in central transvaal and adjacent botswana these other remnants include parts of the buffelsfontein and wolkberg groups which heretofore have been considered to be conformable protobasinal phases of the younger transvaal succession the pniel and the other remnants are truncated by the locally well known unconformity at the base of the transvaal succession on a world wide basis cratonization and the evolution of mineralogically mature cover sequences were and continue to be diachronous the kaapvaal province is unusual only in that it also contains archean cover sequences however with major conformities at about 2 5 ga and the presence of proterozoic unconformity bounded sequences the kaapvaal province seems little different than most other provinces that contain the archean proterozoic boundary

geochemical constraints on the evolution of the early continental crust the most important process affecting both major and trace element concentrations in the mantle and crust is melting producing silicate liquids which then migrate another process whose effects are becoming more apparent is the transport of elements by co 2 and h 2 o rich fluids due to the relatively small amounts of fluids involved they have but little effect on the major element abundances but may severely affect minor and trace element abundances in their source and the material through which they travel the archaean crust was a density filter which reduced the possibility of komatiite or high feo melts with relative densities greater than about 3 0 from reaching the surface those melts retained in the lower crust or at the crust mantle boundary would have enhanced the possibility of melting in the lower crust the high feo melts may have included the archaean equivalents of alkali basalt whose derivatives may form an important component in the archaean crust the occurrence of ultramafic to basic to alkaline magmas in some archaean greenstone belts is an assemblage most typical of modern ocean island suites in continental environments the rock types in the assemblage were modified by conditions of higher heat production during the archaean and thus greater extents of melting and melting at greater depths if modern ocean island suites are associated with mantle plumes which even now may be an important way to transport heat upward from the deeper mantle it is suggested that during the archaean mantle plumes were an important factor in the evolution of the continental crust it appears that the archaean continental crust was of comparable thickness to that of the present based on geobarometeric data if the freeboard concept applied then this would suggest that plate tectonics was also an active process during the archaean if so it is probably no more realistic to assume that all archaean greenstone belts had a similar tectonic setting than to assume that all modern occurrences of basic rocks have a common tectonic setting

hafnium isotope record of the ancient gneiss complex swaziland southern africa evidence for archaean crust mantle formation and crust reworking between 3 66 and 2 73 ga abstract combined u pb and lu hf isotope analyses of zircons from 16 tonalite trondjemite granodiorite ttg gneiss and granite samples from swaziland reveal that the oldest rocks of the ancient gneiss complex in southern africa formed by reworking of early archaean or perhaps late hadean crust at 3 66 ga and that new crust was extracted from a depleted mantle source during palaeoarchaean events between 3 54 and 3 32 ga this interpretation is supported by ehf t of 1 6 2 0 obtained from 3 66 ga ttg gneisses corresponding to hafnium model ages between 3 77 0 18 ga for a presumed hadean early archaean chondritic mantle and 4 08 0 18 ga for a presumed hadean depleted mantle reservoir with the first model age being the most likely in the light of recent data from worldwide sources furthermore it is reflected by superchondritic ehf t up to 2 2 2 0 for ttgs formed at 3 54 3 45 and 3 32 ga the new datasets additionally show that the palaeoarchaean crust formed between 3 54 and 3 32 ga was intensely reworked afterwards without significant addition of depleted mantle derived material during orogenic and intracratonic melting processes at 3 23 3 1 and 2 7 ga this is well reflected by an array of decreasing ehf t from 2 2 to 7 2 between 3 3 and 2 7 ga which can be forced by 176 lu 177 hf of 0 0113 which is similar to that of present day average continental crust and might result from lower crust zircon fractionation during archaean crust reworking supplementary material results of in situ u pb and lu hf isotope zircon analyses and concordia diagrams are available at www geolsoc org uk sup18465

lithofacies and seismic reflection interpretation of temperate glacimarine sedimentation in tarr inlet glacier bay alaska abstract high resolution seismic reflection profiles of sediment fill within tarr inlet of glacier bay alaska show seismic facies changes with increasing distance from the glacial termini five types of seismic facies are recognized from analysis of huntec and minisparker records and seven lithofacies are determined from detailed sedimentologic study of gravity vibro and box cores and bottom grab samples lithofacies and seismic facies associations and fjordfloor morphology allow us to divide the fjord into three sedimentary environments ice proximal iceberg zone and ice distal the ice proximal environment characterized by a morainal bank depositional system can be subdivided into bank back bank core and bank front subenvironments each of which is characterized by a different depositional subsystem a bank back subsystem shows chaotic seismic facies with a mounded surface which we infer consists mainly of unsorted diamicton and poorly sorted coarse grained sediments a bank core depositional subsystem is a mixture of diamicton rubble gravel sand and mud seismic reflection records of this subsystem are characterized by chaotic seismic facies with abundant hyperbolic diffractions and a hummocky surface a bank front depositional subsystem consists of mainly stratified and massive sand and is characterized by internal hummocky facies on seismic reflection records with significant surface relief and sediment gravity flow channels the depositional system formed in the iceberg zone environment consists of rhythmically laminated mud interbedded with thin beds of weakly stratified diamicton and stratified or massive sand and silt on seismic reflection profiles this depositional system is characterized by discontinuously stratified facies with multiple channels on the surface in the proximal zone and a single channel on the largely flat sediment surface in the distal zone the depositional system formed in the ice distal environment consists of interbedded homogeneous or laminated mud and massive or stratified sand and coarse silt this depositional system shows continuously stratified seismic facies with smooth and flat surfaces on minisparker records and continuously stratified seismic facies which are interlayered with thin weakly stratified facies on huntec records

basinal and shelf sedimentation in relation to the archaean proterozoic boundary abstract in most countries the archaean proterozoic boundary has been designated as an unconformity between the oldest shelf type quartz arenites and a basement consisting of greenstone belts intruded by granitic rocks or high grade gneisses the quartz arenites have been characterized as shallow water shelf deposits and sediments in the greenstone belts as resedimented basinal wackes of volcanogenic origin radiometric ages for rocks of the canadian shield led to initial designation of the archaean proterozoic boundary as thus defined at about 2 5 ga subsequent age determinations have clearly established that a number of quartz arenites originally assigned to the proterozoic are 2 7 2 94 ga old e g witwatersrand and pongola supergroups of south africa to accommodate these old quartz arenites some have suggested that the arch aean proterozoic boundary should be regarded as diachronous embracing a span of time about equivalent to that of the phanerozoic however quartz arenites also are abundant in 3 2 3 5 ga old greenstone belts and high grade metamorphic terranes of south africa western australia and the u s a likewise large regions of basinal volcanogenic sediments occur in post 2 5 ga terranes this greatly expands the diachroneity of geological units that were initially regarded as either uniquely archaean or proterozoic we therefore recommend that chronometric and chronostratigraphic sense be retained for the archaean proterozoic boundary divested of the requirement for all quartz arenites to be proterozoic according to this analysis of the precambrian record shelf sediments have increased in abundance relative to basinal sediments through time but at different rates in different areas this change is attributable to progressive cratonization of shield areas a process that was strongly diachronous on a worldwide scale

temperate glacimarine varves an example from disenchantment bay southern alaska abstract the sediment record of disenchantment bay southern alaska a large marine calving embayment contains distinctive annual deposits each year a glacimarine couplet forms with a clast rich stratified to massive diamicton deposited in winter by intense iceberg rafting and a summer meltwater deposit of thinly laminated mud and turbidite sand beds although iceberg rafting occurs throughout the year coarse debris is deposited in high concentrations and forms diamicton only during the winter because of minimal fine sediment from meltwater discharges and a longer residence time of icebergs in the bay due to winter fjord circulation and meteorological factors when meltwater discharge commences in summer laminated mud with dropstones is deposited spring and fall conditions are recorded a transitional phases between winter and summer by clast poor diamictons average sediment accumulation rates calculated from varve thicknesses range from 48 cm yr 3 4 km from hubbard glacier at the head of the bay to 14 cm yr 15 km away sediment accumulation rates estimated from 210pb dating of core sediments are in the same range deposits from the 1986 russell fiord outburst flood are identified from anomalously high 210pb activities in sediments indicating an alternative sediment source to disenchantment bay glacimarine varves are a dating tool for the barren proximal sediments commonly deposited near calving glacier termini they yield paleoclimatic information because they are a product of both glacial ice in the sea and laminated sediments deposited from significant melt water discharge found in a temperate to subpolar climate

analyses of deterioration of the cappadocian tuff turkey the cappadocian tuff contains unique erosional features the so called fairy chimneys some of which in the past were dwelled in and contain valuable wall paintings these historical heritages however are undergoing chemical and physical deterioration due to atmospheric effects for the conservation studies understanding of the deterioration phenomenon of the tuff is essential in this study engineering geological and physicochemical characteristics of the tuff were determined the durability of the tuff was assessed through wetting drying freezing thawing and salt crystallization the test results suggest that chemical weathering may be traced to a depth of 2 cm below lichen covered surfaces and 20 cm adjacent to discolored joint walls based on durability assessment methods the tuff may be classified as having poor to very poor durability

re os isotope and pge constraints on the timing and origin of gold mineralisation in the witwatersrand basin abstract analyses of the re os isotope system and pge abundances have been used to investigate the timing and origin of mineralisation in the vaal reefs section of the witwatersrand basin the world s largest gold deposit consistent interelement pge fractionation in both organic and sulphide phases intimately associated with gold mineralisation suggests that the re os isotope system was fractionated during gold mobilisation the re os isotope data on organic separates intimately associated with the gold in this part of the basin indicate open system behaviour until 2 26 0 19 ga 2 of the order of 450 myr younger than the minimum age of sedimentation the overlying ventersdorp volcanics preserve a re os isochron age of 2 43 0 21 ga 250 ma younger than their u pb zircon emplacement age these data indicate substantial post sedimentary noble metal mobility within portions of the witwatersrand basin re os model age calculations on os rich fractions suggest that noble metals were derived from a 3 0 3 1 ga terrain of granite greenstone affinities significantly younger than the barberton greenstone belt these data support a hybrid model involving post depositional mobilisation of ancient probably detrital noble metals for the origin of mineralisation in this part of the witwatersrand basin such a model is applicable to other areas of gold mineralisation in the basin that display similar textural characteristics and geochemistry

continentally derived solutes in shallow archean seawater rare earth element and nd isotope evidence in iron formation from the 2 9 ga pongola supergroup south africa abstract the chemical composition of surface water in the photic zone of the precambrian ocean is almost exclusively known from studies of stromatolitic carbonates while banded iron formations ifs have provided information on the composition of deeper waters here we discuss the trace element and nd isotope geochemistry of very shallow water if from the pongola supergroup south africa to gain a better understanding of solute sources to mesoarchean shallow coastal seawater the pongola supergroup formed on the stable margin of the kaapvaal craton 2 9 ga ago and contains banded iron formations ifs that represent the oldest documented superior type iron formations the ifs are near shore pure chemical sediments and shale normalized rare earth and yttrium distributions reysn exhibit positive lasn gdsn and ysn anomalies which are typical features of marine waters throughout the archean and proterozoic the marine origin of these samples is further supported by super chondritic y ho ratios average y ho 42 relative to older isua ifs 3 7 ga from greenland and younger kuruman ifs 2 5 ga also from south africa the pongola ifs are depleted in heavy rare earth elements hree and appear to record variations in solute fluxes related to sea level rise and fall sm nd isotopes were used to identify potential sediment and solute sources within pongola shales and ifs the nd t for pongola shales ranges from 2 7 to 4 2 and nd t values for the coeval iron formation samples range 1 9 to 4 3 are generally indistinguishable from those of the shales although two if samples display nd t as low as 8 1 and 10 9 the similarity in nd isotope signatures between the shale and iron formation suggests that mantle derived rey were not a significant nd source within the pongola depositional environment though the presence of positive eu anomalies in the if samples indicates that high t hydrothermal input did contribute to their rey signature isotopic mass balance calculations indicate that most 72 of the nd in these seawater precipitates was derived from continental sources if previous models of fe nd distributions in archean ifs are applied then the pongola ifs suggest that continental fluxes of fe to archean seawater were significantly greater than are generally considered

what controls the location of ice streams ice streams influence ice sheet mass balance and stability but key aspects of their behaviour remain poorly understood this paper reviews and discusses one very important aspect what controls their location in an ice sheet seven potential controls on ice stream location are identified from the literature topographic focusing topographic steps macro scale bed roughness calving margins subglacial geology geothermal heat flux and subglacial meltwater routing for each control the theoretical basis for its link to rapid ice flow is introduced followed by discussion of the evidence of its influence on the location of both contemporary and palaeo ice streams based on this new synthesis topographic focusing subglacial geology meltwater routing and calving margins appear to be most commonly associated with fast ice flow it is clear that rather than a single control however there exist a number of potential controls of varying influence we propose a hierarchy of factors with those occurring at the top of the hierarchy exerting a stronger influence on ice stream location and where present beneath an ice sheet are very likely to be associated with fast flow those factors occurring lower down the hierarchy are less commonly associated with ice streaming but appear to be influential in the absence of more common controls in such a hierarchy topographic focusing in the presence of a calving margin is the primary control in the absence of this ice streams will preferentially occur in areas with favourable subglacial meltwater routing and subglacial geology in the absence of these bed roughness geothermal heat flux and topographic steps may promote ice streaming significantly the primary controls on a given ice stream location are likely to influence its spatial and temporal dynamics ice streams governed by the presence of meltwater routing and or calving processes might exhibit more variable behaviour because these controls can vary over relatively short time scales compared to controls that vary over longer time scales e g geothermal heat flux subglacial roughness geology and topography identifying the controls on ice stream location is therefore of paramount importance when understanding ice stream longevity and their past and future activity

seismic architecture and sedimentation in northwest spitsbergen fjords abstract acoustic records from several spitsbergern fjords yield a characteristic sedimentary architecture subaqueous fans 1 3 surface slope fed by tidewater glacier derived sediments are particularly well developed an upper fan has been built on an underlying fan in response to a shift in the position of the tidewater glacier sediment source the sequence of tidewater glacier fluctuations and sedimentation since the late weichselian interpreted from seismic records comprises the following 1 deposition of an acoustically transparent unit interpreted as either basal till deposited during late weichselian glaciation or a glacimarine unit formed during early rapid ice retreat 2 ice retreat up fjords by about 9000 10 000 yrs b p 3 deposition of a lower fan complex over several thousand years and slow infilling of bedrock controlled basins in more ice distal locations 4 ice readvance to 2 3 km beyond modern ice margins linked to little ice age climatic deterioration from the 13 14th century overriding of part of the lower fan complex inducing deformation 5 deposition of little ice age submarine terminal moraines and basal till by readvancing glaciers deposition of the upper fan during 500 yr little ice age ice advance 6 retreat of tidewater glacier termini to their present positions since little ice age termination at the end of the 19th century deposition of a thin veneer of sediment inside the little ice age submarine moraines as glacier retreat takes place

glacial outburst flood sediments within disenchantment bay alaska implications of recognizing marine jokulhlaup deposits in the stratigraphic record abstract glacial outburst floods have been observed or inferred in most glaciated regions of the world outburst flooding events along past ice sheet margins have been discussed as possible causes for changes in ocean circulation while subglacial lake drainage under ice sheets may influence ice sheet stability the recognition of these geologically instantaneous events in the marine sediment or rock record is critical in order to characterize the magnitude and frequency of past outburst events as well as to predict the likelihood of future occurrences in this paper we characterize outburst lithofacies and infer depositional processes from two outburst floods within disenchantment bay alaska and compare these deposits to other recognized outburst deposits within marine depositional settings here we show that the outburst deposits are lithologically similar and are composed of stratified mud sand and diamicton we interpret these facies to have been formed by sediment gravity flows and hypopycnal plume sedimentation these results show that the volume of sediment lithofacies and inferred depositional processes cannot be easily distinguished from normal sediment accumulation within the proximal temperate glacimarine depositional environment the paucity of fluvial subglacial discharge in polar glacimarine environments suggests the potential for recognition of such deposits in the antarctic

rapid erosion of soft sediments by tidewater glacier advance taku glacier alaska usa 2 0 0 1 m a 1 1940 2005 at l 3 km h 350 m and 1 5 0 2 m a 1 1952 2005 at l 1 5 km h 250 m detailed mapping over a 4 km 2 area of the terminus revealed a deeply incised channel in line with a major outlet stream glaciofluvial processes must play the dominant role in the subglacial erosion and removal of these unlithified sediments citation motyka r j m truffer e m kuriger and a k bucki 2006 rapid erosion of soft sediments by tidewater glacier advance taku glacier alaska usa geophys res lett 33 l24504 doi 10 1029 2006gl028467

pressure temperature and timing of mineralization of the sedimentary rock hosted orogenic gold deposit at klipwal southeastern kaapvaal craton south africa gold mineralization in the klipwal shear zone ksz at the klipwal gold mine is confined to laminated quartz carbonate lodes stringers and associated alteration in sandstone and siltstone of the delfkom formation in the upper mozaan group of the mesoarchaean pongola supergroup the moderately dipping brittle ductile ksz strikes n s with an oblique reverse sinistral sense of shear the deformational events that are recognized include an early compressional phase that produced anastomosing shears defined by shear fabrics with numerous shear parallel laminated quartz carbonate fault fill veins and in places extensional quartz vein stockworks and a late brittle reactivation phase that produced fault breccias displacing earlier extensional veins three closely spaced economic reefs lodes are developed the main r reef constitutes the ksz while the j and h reefs represent footwall splays alteration comprises chlorite muscovite epidote feldspar and carbonates along with pyrite arsenopyrite and chalcopyrite pyrrhotite an inner alteration zone is dominated by laminated quartz carbonate veins with alternating quartz carbonate rich and muscovite chlorite rich laminae whereas the proximal zone is characterized by alteration halos of k feldspar albite epidote chlorite and muscovite along with carbonates and associated quartz veins chlorite thermometry from the inner and proximal zones yielded temperatures of 267 to 312 c arsenopyrite compositions provide temperatures in the same range 255 to 318 c fluid inclusion microthermometry and raman spectrometry of quartz veins in the mineralized reefs reveal the presence of metamorphogenic aqueous gaseous fluid with an average salinity of 6 5 wt nacl equiv fluid compositions and estimated pressure temperature p t range 1 1 to 2 5 kbar at 255 to 318 c are typical of orogenic gold deposits devolatilization during the regional facies metamorphism of the pongola supergroup is considered the likely fluid forming event with fluid flow focused into a compressional jog of the ksz shear induced pressure fluctuations generated a phase separation of the initial aqueous gaseous fluid producing a gaseous and low salinity aqueous fluid this together with fluid rock interaction and a decrease in fo2 lead to sulfide and gold precipitation at klipwal re os data from six sulfide samples constrain the age of sulfide precipitation and by inference gold mineralization to 2563 84 ma with an initial 187os 188os 0 29 0 08 mswd 0 38 this age is distinctly younger than the post pongola granites 2863 2721 ma ruling out the association of granite emplacement with mineralization this would suggest that mineralization is linked to the regional d3 folding event which reactivated the ksz after emplacement of the post pongola granites and that final brittle post mineralization reactivation is related to karoo age faulting low initial os values suggest that ore fluid interacted with mafic rocks leaching non radiogenic os the likely source being the deeper seated nsuze group volcanics and or the greenstone belts that underlie the pongola supergroup

gneiss greenstone relationships in the ancient gneiss complex of southwestern swaziland southern africa and implications for early crustal evolution the ancient gneiss complex agc of swaziland is made up of a variety of archaean gneisses derived from granitoid intrusions of ttg composition emplaced between 3 6 and 3 2 ga ago and infolded amphibolite grade greenstone remnants locally referred to as dwalile supracrustal suite dss the greenstones were considered to postdate the oldest phases of the agc known as ngwane gneiss and this is now confirmed by conventional u pb zircon geochronology and shrimp single zircon ages on detrital grains from dwalile clastic metasediments suggesting that these rocks were derived from a granitoid source some 3 42 to 3 57 ga in age trondhjemitic ngwane gneiss structurally underlying the greenstones has a zircon age of 3521 23 ma and contains a 3683 10 ma old xenocryst this together with nd tdm model ages between 3 7 and 3 9 ga and a pronounced negative euanomaly for the metasediments strongly argues for the presence of very old crustal components in the agc nd isotopic systematics of dwalile metavolcanics suggest these rocks to be contaminated with older crustal material of ngwane type and resulting nd model ages 3 60 3 99 ga are likely to be too old the major and trace element chemistry of the dwalile metavolcanics is remarkably similar to that of onverwacht group rocks in the nearby barberton greenstone belt bgb and this together with a similar age for these two suites points to a genetic link and a similar mechanism of formation our data as well as field relationships do not favour an intra oceanic origin for the dss as has been inferred for the onverwacht group nor do they support a juvenile origin for the exposed ngane gneisses of the agc the ngwane precursors were apparently derived from older crustal protoliths whose subsequent modification particularly through remelting and deformation gave rise to the various compositional varieties now seen in the ngwane suite

the nsuze group pongola sequence south africa geochemical evidence for archaean volcanism in a continental setting abstract the archaean nsuze group in southeast south africa represents an important volcano sedimentary succession that is markedly different compositionally and lithologically from older and contemporaneous sequences elsewhere in southern africa the nsuze volcanic rocks cropping out in the vicinity of the pongola river in northern natal display a complete spectrum of chemical compositions from basalt to rhyolite with lavas of intermediate compositions predominant but ultramafic lavas absent flows of different compositions are complexly interdigitated the uppermost rocks of the nsuze group reflect a gradual decrease in volcanic activity accompanied by an increase in sedimentation the nsuze lavas are tholeiitic with total fe as fe 2 o 3 contents approaching 17 in the basaltic andesites all lithologies are characterized by moderate to strong light rare earth element ree enrichment heavy ree slopes range from moderate in the basalts and basaltic andesites to flat in the rhyolites with the exception of two basaltic andesites and a dacite which have flat heavy ree slopes basalts have small or no negative eu anomalies but increasingly larger negative anomalies are a feature of the intermediate and acid lavas variations in cr content appear to be related to stratigraphic position basalts and basaltic andesites in the lower part of the volcanic sequence have higher cr contents by a factor of six to eight times than lavas with similar mgo abundances in the upper part of the sequence provisional modelling of the available chemical data favours low pressure crystal fractionation from and evolved basaltic parent that could be derived by initial non modal melting of a garnet lherzolite source variable degrees of crustal contamination of the evolved magmas are considered probable

early archaean u pb fractionation and timing of late archaean high grade metamorphism in the saglek hebron segment of the north atlantic craton the archaean gneiss complex of the saglek hebron area in northern labrador is dominated by polyphase similar to 3 7 ga old tonalitic trondhjemitic granodioritic ttg uivak i gneiss that exhibit 3 8 3 9 ga inheritance these gneisses are interleaved with a heterogeneous group of supracrustal rocks known as the nulliak supracrustal assemblage early archaean ultramafic rocks occur either as tectonically emplaced slivers of lithospheric mantle along structural contacts within the uivak i gneiss or as layers of metakomatiite in nulliak supracrustal units the metakomatiites yield a pb pb isochron age of 3845 160 ma reflecting a significant change from an initial archaean mantle like mu 1 value of 7 9 to a range of mu 2 values of 2 0 4 2 members of the tectonically emplaced lithospheric mantle suite have less well correlated pb 206 pb 204 and pb 207 pb 204 compositions both ultramafic suites were collected in an area identified as transitional between granulite and amphibolite facies however model calculations show that the data scatter exhibited by the lithospheric mantle cannot be accommodated by post 3 85 ga u depletion evidence is provided that the lithospheric mantle suite reflects variable degrees of mixing between a depleted mantle and a high mu reservoir suggesting that the two ultramafic rock suites had different evolutionary histories timing of late archaean reworking during medium to high grade metamorphism was determined by mineral whole rock dating of a garnet pyroxenite from an area dominated by kiyuktok gneiss reworked similar to 3 75 ga uivak gneiss sm nd mineral whole rock data for this gneiss yielded an isochron equivalent to an age of 2570 17 ma significantly younger than its pb pb isochron age of 2735 43 ma the negative epsilon nd t 7 9 reflects a significant crustal residence time indicating that the sm nd system of the whole rock has been reset the internal pb mineral isochron is interpreted to reflect the maximum age of peak metamorphism the age discrepancy of ca 165 ma between the sm nd and the pb pb isochron ages suggests slow cooling at a minimum rate of 1 3 degrees c ma 1 this indicates that the saglek hebron segment of the north atlantic craton experienced gradual uplift during late archaean exhumation c 1999 elsevier science b v all rights reserved

3 5 ga old amphibolites from eastern hebei province china field occurrence petrography sm nd isochron age and ree geochemistry amphibolite blocks frequently occur in association with grey gneisses within a small area near the villages of caozhuang and huangbaiyu in eastern hebei they are interpreted as remnants of an earlier bimodal suite now engulfed in late archaean k rich granitic magma and are further cut by pegmatite dykes they are considered together as giant enclaves within late archaean granite plutons u pb zircon ages 2 5 2 6 ga the amphibolites have been dated by the sm nd isochron method the results yielded t 3470 107 ma ind 0 50827 11 and nd t 2 7 0 6 this age represents the time of protolithic basalt eruption and possibly suggests a period of formation of primitive mafic crust this is the oldest reliable age so far obtained for the archaean rocks of china the sm nd results of grey gneiss samples yielded model ages tdm of about 3 24 ga it is not clear whether this younger age in comparison with that of the amphibolites indicates the intrusive nature of these rocks or is a result of contamination by later granite about 2 5 ga ago the rb sr and common pb isotopic systems have been highly disturbed and no isochron ages could be established the common pb data show retarded growth of radiogenic pb and hence imply reduction of u pb ratios in the past this is consistent with the petrographic study which suggests that the amphibolites have undergone an earlier period of granulite facies metamorphism and were subsequently retrogressed to the present amphibolite facies mineral assemblages the positive nd t value of 2 7 for the amphibolites implies that the protolithic basalt magmas were generated from mantle sources which had undergone long term depletion in light rare earth elements lree the depletion event must have taken place prior to 4 ga ago if their source had a chemical nature similar to the modern oceanic mantle it could have occurred as early as 4 5 ga ago if their source was less impoverished in lree if the mantle depletion was related to extraction of continental or sialic material the sm nd data of the caozhuang amphibolites lend support to the existence of sialic crust if not of a continental size before 4 ga ago

age and isotope geochemistry of the archaean pongola and usushwana suites in swaziland southern africa a case for crustal contamination of mantle derived magma the igneous rocks of the pongola supergroup ps and usushwana intrusive suite uis represent a case of late archaean continental magmatism in the southeastern part of the kaapvaal craton of south africa and swaziland u pb dating on zircons from felsic volcanic rocks of the ps yields a concordia intercept age of 2940 22ma that is consistent with a sm nd whole rock age of 2934 114ma determined on the ps basalt rhyolite suite the initial end of 2 6 0 9 is the lowest value so far reported for archaean mantle derived rocks rb sr whole rock dating of the ps yields a younger isochron age of 2883 69ma which is not significantly different form the accepted u pb zircon age an internal cpx opx plag whole rock isochron for a pyroxenite from the younger uis yields an age of 2871 30 ma and initial 143nd 144nd that lies off the chur growth curve by end 2 9 0 2 however sm nd whole rock data for the uis yield an excessively high age of 3 1 ga that conflicts with firm geological evidence showing the uis to be intrusive into the ps the negative deviations of initialend from the chondritic nd evolution curve suggest significant contamination of the ps and uis melts by older continental crust a mixing process with continental crust after magma segregation is supported by a high initial 87sr 86sr ratio of0 703024 24 for a clinopyroxene sample from a uis pyroxenite compared with an expected value of 0 701 for the 2 9 ga mantle we therefore interpret the linear array of data points for the uis gabbros as a mixing line between 2 87 ga old magma and older continental crust parallel lree enriched ree patterns negative nb ti anomalies a distinctive and uniform ratio of ti zr 46 and a narrow span of initial nd indicate a common source for both the ps and uis suites which is different from primitive mantle

why was flood volcanism on submerged continental platforms so common in the precambrian abstract many archean and proterozoic greenstone belts contain volcanic rocks that erupted subaqueously but interacted with continental crust on their way to the surface this points to emplacement in a tectonic setting that was rare to absent throughout the phanerozoic the eruption of flood basalts onto submerged continental platforms to explain the situation requires that sea levels were high during periods of flood volcanism perhaps because the ocean basins were partially filled by extensive active and high standing oceanic ridges or by oceanic plateaus the common preservation of crust contaminated submarine basalts even in the oldest supracrustal sequences may indicate that continental crust was voluminous even at an early stage in earth history

sediment glaciohydraulic supercooling and fast glacier flow glaciers often advance over proglacial sediments which then may enhance basal motion forglaciers with abundant meltwater thermodynamic considera tions indicate that the sediment ice contact in the direction of ice flow tends toward an angle opposed to and somewhat steeper than the surface slope by slightly more than 50 a simple model basedonthis hypothesisyieldstheextentof over ridden sediments as a function of sediment thickness and strength a result that may be useful in guiding additional fieldwork forhypothesistesting sediment floored as wellasrock floored over deepenings are common features along glacier flow paths and are expected based on theories of glacier erosion entrainment transport and deposition

tectonic cycles in southern africa abstract the recurrence of major geological events within the 3600 ma stratigraphic record of southern africa suggests a periodicity of320 25ma for at least the last 3200 ma this periodicity is reflected in major episodes of continental volcanism granitoid plutonism orogeny reciprocity of terrestrial and marine dominated sequences platform carbonate sedimentation glaciation and first order changes in sea level these serve to define 11 completed megacycles each introduced by a major rifting event accompanied by listric faulting alluvial sedimentation in restricted fault bound troughs volcanism and a type granitoid plutonism prior to 3200 ma continental rift magmatism occurred subaqueously due to very high early archean sea levels resulting in pillow lavas and a lack of interbedded terrigenous siliciclastics the long term reciprocity of terrestrial and marine dominated sequences records eustatic first order changes in sea level with highstands characterized by generally humid warm continental climates low relief and tropical afforestation on the other hand lowstand climates were hot and arid with high relief and widespread desertification thick marine sequences accumulate in areally extensive epeirogenic basins without significant volcanism or faulting and commonly are divided into an early phase of open shelf sedimentation followed by deposition in a brackish marine foreland basin within openshelf environments an early phase of biochemical precipitation and or siliciclastic reworking along linear shorelines is separated from a later phase involving rapid deposition of immature siliciclastics along lobate shorelines evolution of the foreland basin is related to orogenic activity and the isolation of an epeiric sea which sometimes developed the characters of a barred basin marine dominated sequences display a deterministic stacking of lithofacies important glacial epochs commonly occur midway through first order regression sometimes with lesser events closely following peak transgression potentially they display 320 ma periodicity but with an unexplained mid proterozoic gap glaciation is related to the combined effect of climatic cooling associated with first order regression and depletion of atmospheric co2 by biological uptake associated with platform carbonate development terrestrial volcanism is believed to be the main contributor of atmospheric co2 climatic change is accompanied by cyclical patterns of cratonic weathering and pedogenesis

a smnd and pb isotope study of archaean greenstone belts in the southern kaapvaal craton south africa abstract a sm nd and pb study on a wide variety of lithologies in archaean greenstone belt fragments in the southern kaapvaal craton reveals a complex petrogenetic history the fragments are important because they represent a 350 km transect through the craton south of barberton to its southern margin the commondale greenstone belt yields a precise sm nd age of 3334 18ma on an exceptionally well preserved peridotite suite of komatiitic affinity the wide range of sm nd from 0 6 to 1 0 is attributed to the unusual occurrence of orthopyroxene in the spinifex bearing rocks a considerably younger age of about 3 2 ga is suggested for the nondweni greenstone belt close to the southern margin of the craton on the basis of separate sm nd isochrons on individual lithologies ranging from komatiite through komatiitic basalt and basalt to felsic volcanic rocks the most precise sm nd age for the greenstone belt of 3208 87ma is based on a combined mineral whole rock isochron for komatiitic basalts but the possibility cannot be excluded that this represents a later metamorphic age on the basis of the present study the greenstone belts appear to have been emplaced at progressively younger ages towards the southern margin of the craton of greater significance is that irrespective of age the individual lithologies of the nondweni sequence have isotopically distinct initial ratios combining isotopic data from these lithologies invariably yields apparent older ages the isotopic characteristics are consistent with a model of progressive contamination by older sialic crust thought to be the basement to the nondweni sequence

the cr th ratio in precambrian pelites from the kaapvaal craton as an index of craton evolution the cr th ratio in precambrian pelites from the kaapvaal craton in southern africa correlates with pelite th sc and co th ratios and appears to monitor changes in source composition it is a more sensitive indicator of provenance than other commonly used ratios including eu eu la yb and th u pelite cr th ratios are controlled most strongly by the effects of local provenance and local tectonics secular decreases in this ratio the most prominent of which occurred at 2 4 2 3 ga in pelites of the kaapvaal craton record rapid uplift exposing granite decreases in the ratio reflect increases in the abundance of komatiite basalt in sediment sources in response to arc collisions rift hotspot volcanism or intracratonic uplift of older greenstones relatively constant cr th ratios appear to require widespread uplift or and efficient sediment mixing during tectonic stability changes in cr th ratio of kaapvaal pelites reflect a complex tectonic history of the craton from about 3 3 ga to 1 8 ga and do not favor single stage cratonization in the late archean

can glacial erosion limit the extent of glaciation kaplan m r hein a s hubbard a 2009 can glacial erosion limit the extent of glaciation geomorphology 103 2 172 179

composition age and tectonic setting of amphibolites in the central bushmanland group western namaqua province southern africa abstract the western namaqua province wnp of southern africa is composed of several supracrustal assemblages and intrusive granitoid suites that were formed during the mid proterozoic 2 0 1 0 ga ago the bushmanland group bg has been the subject of intense study recently because of its considerable strategic metal potential hosting several giant stratiform base metal deposits e g gamsberg and aggeneys the bg contains amphibolites which represent metamorphosed tholeiitic basalts with mgo 9 4 and enriched in ni relative to modern basalts a feature reminiscent of archaean tholeiites post depositional mobility of elements is probably widespread and hampers recognition of parental magma composition and source region characteristics however the amphibolites have yielded a sm nd isochron age of 1649 90 ma nd t 0 48 0 89 which is interpreted as the time of extrusion of the basalt precursors and by inference a minimum sedimentation age for the underlying metalliferous bg sequence it follows that the bg sequence evolved during an intermediate period in the history of the namaqua province after major crustal stabilisation around 2 0 1 9 ga and before the last major tectonothermal event around 1 2 1 1 ga results of a parallel rb sr and u th pb study of the bg amphibolites indicate strong metamorphic resetting 1 2 1 1 ga ago thereby confirming earlier work on the wnp characteristics of the source region to the bg basalts have been inspected with morb normalised trace element abundances with the salient result being the bg basalts require derivation from a source that experienced variable geochemical modification similar to that proposed for the mantle wedge overlying modern subduction zones such a process is currently preferred over crustal contamination since the sm nd isochron for the bg amphibolites is remarkably linear but independent confirmation of the sm nd age is still required before this alternative model can be discounted to reconcile the requirement of subduction related metasomatism in the source to tholeiitic basalts erupted within what appears to be a cratonic sedimentary basin we suggest that back are continental extension could provide a satisfactory tectonic setting as such an environment is intrinsically linked to subduction

archean oil evidence for extensive hydrocarbon generation and migration 2 5 3 5 ga archean sedimentary rocks from the pilbara craton australia contain evidence for petroleum generation and migration in the form of bitumen nodules produced by radiogenic immobilization of fluid hydrocarbons around detrital uraninite thorite and monazite grains the nodules are preserved in sandstones at several stratigraphic levels in the fortescue group 2 75 ga and lalla rookh formation 3 0 ga both nonmarine successions and in deltaic sediments of the mosquito creek formation 3 25 ga the most ancient evidence comes from the warrawoona group 3 46 ga where hydrocarbon droplets were apparently formed in situ from kerogenous sediments by flash maturation during early hydrothermal silicification bituminous relics of petroleum are also commonly preserved in shallow marine sandstones of the black reef formation 2 59 ga and the witwatersrand supergroup 2 85 ga from the kaapvaal craton south africa along with subeconomic methane accumulations in all cases the petroleum was apparently sourced from archean shales generated during the archean and migrated before the late archean or early early proterozoic metamorphism occluded fluid pathways given this widespread and abundant evidence for hydrocarbon generation and migration in archean depositional basins it seems that primordial bacterial biomass producing labile type i kerogen was often buried in sufficient quantities to successfully generate and expel petroleum depositional basins on ancient cratons clearly contained permeable rocks amenable to the migration and probably to the accumulation of petroleum thus the main factors precluding the discovery of economically exploitable hydrocarbon accumulations in archean basins are the subsequent destructive effects of deformation and metamorphism which causes trap breaching imperfect sealing or thermal obliteration however there are ancient stable cratons where such disruption may not have occurred and so petroleum explorers may wish to reassess the possibility of finding valuable hydrocarbon resources in archean rocks

sm nd age and isotopic systematics of the bimodal suite ancient gneiss complex swaziland studies of the development and stabilization of the archaean crust often focus on the relative temporal relationships between the metamorphosed basaltic to ultramafic volcanic units greenstone belts and the sialic gneiss terrains that make up the oldest sections of the terrestrial crust at the heart of this interest are the questions of the processes responsible for crust formation in the archaean and whether or not the various units of an archaean crustal section represent new additions to the crust from the mantle or are products of the reprocessing of even older crustal materials one area where this controversy has been particularly pronounced is the archaean crustal section of south west africa1 6 the oldest rocks in the kaapvaal craton consist of the onverwacht group of mafic to ultramafic metavolcanics of the barberton greenstone belt and a grey gneiss complex termed the ancient gneiss complex agc of swaziland we report here the results of a whole rock sm nd isotopic study of the agc and the implications these data may have for crustal evolution in the kaapvaal craton

hubbard glacier alaska 2002 closure and outburst of russell fjord and postflood conditions at gilbert point 1 hubbard glacier the largest temperate tidewater glacier in the world has been advancing since 1895 ad and has now twice dammed 60 km long russell fjord once in 1986 and more recently in 2002 this paper focuses on the 2002 event when a strong spring advance pushed shallow submarine proglacial sediments against gilbert point closing off russell fjord by late june as a consequence upstream ice flow decelerated from 5 m d 1 to 1 5 m d 1 with flow diverging to either side of gilbert point lake height reached 15 m asl before intense rains caused lake water to overtop the moraine dam on 14 august 2002 three cubic kilometers of water were released within 30 hours with peak discharge reaching 55 000 m3 s 1 24 hours after the flood began the discharge records for the 1986 and 2002 outbursts differ significantly and reflect differences in lake height 26 m versus 15 m and dam types ice versus moraine the 2002 outburst proceeded in two stages 1 relatively slow overtopping of the subaerial moraine with downward erosion rates of 1 2 m h 1 with little lateral expansion 2 followed by faster downward erosion of the submarine moraine up to 7 m h 1 with rapid lateral expansion of the channel by ice calving 7 m h 1 the annual average terminus position at gilbert point has remained constant since 2002 although there are seasonal variations of 100 200 m the deep channel strong tidal currents and seasonally warm ocean water appear to have prevented the advance of this segment of the terminus despite the glacier s continued advance elsewhere along its terminus sediments are slowly filling in the channel at a rate of about 4 m yr 1 and their steady accumulation may eventually trigger the next closure

seismic reflection characteristics of glacial and glacimarine sediment in the gulf of alaska and adjacent fjords abstract glaciation together with tectonism have been dominant factors affecting sedimentation in the gulf of alaska area from at least the late miocene throughout the quaternary the effects of tectonism are apparent in high mountains that border the gulf raised terraces of middleton island and the eastern gulf coastal zone and numerous active faults and related earthquakes glacial evidence includes magnificent glaciers and their onshore deposits spectacular fjords large sea valleys incised in the continental shelf submarine morainal ridges at mouths of bays and sea valleys and thick glacimarine sedimentary sequences diamicts that are exposed onshore and at the sea floor along the outer shelf seismic reflection profiling and sampling of the uppermost marine sedimentary sequences in the gulf of alaska and adjacent fjords and bays have allowed identification of three discrete glacially related stratigraphic units these units were delineated on the basis of seismic signature geometry physiographic location stratigraphic position and sedimentologic characteristics the oldest unit a quaternary diamict is portrayed on seismic profiles by irregular discontinuous reflections this unit probably includes till outwash and glacimarine sediment a geographically restricted unit one incorporating holocene end moraines at bay mouths and associated with some sea valleys consists of jumbled masses of discontinuous reflections and very irregular surface morphology the youngest unit a blanket of holocene sand to clayey silt prograding as a sediment wedge across the shelf contains nearly horizontal parallel reflections except where disrupted by mass movement although seismic reflection data alone cannot provide definitive proof of the presence of glacial sediment when combined with sea floor sampling seismic profiling is a powerful tool for determining the continuity of marine sedimentary units and relationships to past and modern glaciers

the crustal architecture of the kaapvaal crustal block south africa between 3 5 and 2 0 ga following terrane amalgamation of early oceanic lithosphere the southern and central parts of the kaapvaal craton were a coherent unit by 3 1 ga juxta position of the northern and western granitoid greenstone terranes including the murchison island arc was the result of terrane accretion that started at 3 1 ga the culmination of these events was the collision of the kaapvaal craton the pre cratonic zimbabwe block and the central zone to generate the limpopo granulite gneiss terrane coeval with these orogenic events the central kaapvaal craton underwent extension to accommodate the development of the dominion witwatersrand pongola and ventersdorp basins the craton scale thabazimbi murchison lineament development during the 3 1 ga accretion event and continued to influence the tectonic evolution of the kaapvaal block throughout the period under review as indicated by the syn sedimentary tectonics of the 2 64 ga wolkberg group overlying black reef formation and the transvaal sequence the transvaal and griqualand west basins developed in the late archaean 2 55 ga with basin dynamics influenced by far field stresses related to the limpopo orogeny during this period the thabazimbi murchison lineament lay close to the northern margin of the depository reactivation of the lineament between 2 4 and 2 2 ga resulted in inversion of the transvaal basin and formation of the northward verging mhlapitsi fold and thrust belt the half graben setting envisaged for the deposition of the pretoria group was influenced by the thabazimbi murchison lineament as was the emplacement and subsequent deformation of the bushveld complex

the mafic ultramafic complex near finero ivrea verbano zone ii geochronology and isotope geochemistry whole rock nd and sr isotopic compositions of the mafic ultramafic complex near finero demonstrate that the magma was derived from a depleted perhaps morb type mantle reservoir the sm nd data for the amphibole peridotite unit can be interpreted as an isochron with an apparent age of 533 20 ma which is consistent with a 207pb 206pb evaporation age of 549 12 ma of a single zircon grain from the internal gabbro unit however the interpretation of these apparent ages remains open to question we therefore retain the alternative hypotheses that the intrusion occurred either about 533 or 270 ma ago the latter being the most likely age of emplacement of the much larger magma body near balmuccia val sesia the implication of the older emplacement age if correct would be that the igneous complex may be related to the numerous amphibolite units which are intercalated with the metapelites of the overlying kinzigite formation and together with them may constitute an accretionary complex in this case the mafic ultramafic complex itself might also be part of such an accretionary complex as has been proposed for the balmuccia peridotite internal sm nd isochrons involving grt cpx plag and amph from the internal gabbro unit yield concordant ages of 231 23 226 7 223 10 214 17 and 203 13 ma these results confirm published evidence for a separate regional heating event about 215 15 ma ago initial end 533 values average 6 3 0 4 for six samples of the amphibole peridotite unit and 6 0 1 2 for ten samples of the external gabbro unit 87sr 86sr ratios require little or no age correction and range from 0 7026 to 0 7047 with two outliers at 0 7053 and 0 7071 strong correlations between 87sr 86sr and k2o and weaker correlations between initial end and k2o imply a comparatively minor 10 contamination of the external gabbro magma by crustal material and a later alteration by a crustal or seawater derived fluid these results contrast sharply with the isotopic composition negative end and high 87sr 86sr values of the associated mantle rocks the phlogopite peridotite unit which has been pervasively metasomatized by crustal fluids this type of metasomatism and its isotopic signature are never seen in the magmatic complex this evidence rules out any direct genetic relationship between the igneous complex and the mantle peridotite the crust mantle interaction is the opposite of that seen at balmuccia where the mantle peridotite is essentially pristine and the magmatic body has been extensively contaminated by assimilation of crustal rocks

imaging evidence for hubbard glacier advances and retreats since the last glacial maximum in yakutat and disenchantment bays alaska high resolution 2 d multichannel seismic data collected during the 2012 utig usgs national earthquake hazards reduction program survey of disenchantment and yakutat bays in southeast alaska provide insight into their glacial history these data show evidence of two unconformities appearing in the form of channels and are interpreted to be advance pathways for hubbard glacier the youngest observable channel thought to have culminated near the main phase of the little ice age lia is imaged in disenchantment bay and ends at a terminal moraine near blizhni point an older channel thought to be from an advance that culminated in the early phase of the lia extends from disenchantment bay into the northeastern edge of yakutat bay turning southward at knight island and terminating on the southeastern edge of yakutat bay our interpretation is that hubbard glacier has repeatedly advanced around the east side of yakutat bay in knight island channel possibly due to the presence of malaspina glacier cutting off access to central yakutat bay during times of mutual advance we observe two distinct erosional surfaces and retreat sequences of hubbard glacier in yakutat bay supporting the hypothesis that minor glacial advances in fjords do not erode all prior sediment accumulations interpretation of chaotic seismic facies between these two unconformities suggests that hubbard glacier exhibits rapid retreats and that disenchantment bay is subject to numerous episodes of outburst flooding and morainal bank collapse these findings also suggest that tidewater glaciers preferentially reoccupy the same channels in bay and marine settings during advances

influence of rapid glacial retreat on the rate of erosion by tidewater glaciers glacial erosion rates inferred from total sediment budgets in recently deglaciated fjords which are the highest reported erosion rates worldwide have received considerable attention in fields as diverse as tectonics glacial sedimentation and climate these record rates however are representative only of tidewater glaciers during their extensive retreat of the post little ice age period erosion rates averaged over glacial interglacial cycles and longer periods are likely to be substantially smaller we examine the influence of retreat rate on sediment yields from tidewater glaciers by reconstructing the history of sediment output from retreating glaciers necessary to produce sediment packages observed in contemporary fjords using a simple numerical model of proglacial sedimentation in front of a retreating glacier seismic profiles of proglacial sediments and the history of terminus retreat of muir glacier glacier bay alaska we calculate the sediment flux as a function of time from this glacier between 1900 and 1979 and conclude that sediment flux scales with retreat rate the corresponding basin wide erosion rate during this 79 yr period averages 37 mm yr and exceeds long term erosion rates by a factor of 5 1 for muir glacier and by inference for other calving glaciers the general drastic retreat and the marked regional drawdown of ice since the little ice age are both linked to unusually rapid calving and fast ice motion which is conducive to rapid erosion

origin and early growth rate of continents abstract data suggest that early continents formed over mantle sinks and grew rapidly between 2 7 and 3 0 ga ago nd isotopic results indicate that archean tonalites which are largely responsible for early continental growth are derived chiefly from ancient depleted mantle sources trace element and oxygen isotope data however indicate enriched sources for such tonalites these constraints can be explained if the mantle is fractionated into depleted dmr and enriched emr reservoirs during early degassing of the earth 4 4 ga blocks of emr are recycled back into the mantle and the emr dmr mixtures serve as sources for archean basalts these basalts are enriched in incompatible elements by deep sea alteration metasomatism in sinks or by other mantle processes and then undergo wet partial melting to produce tonalite magmas enrichment must occur not more than 150 ma prior to tonalite production basalt plateaus accumulate at sinks and as cooling continues geotherms at sinks pass into the eclogite stability field dense eclogite roots form on the basalt plateaus which pulls them into the mantle between 2 7 and 3 0 ga ago resulting in rapid tonalite production and thus in rapid continental growth

reassessment of archean crustal development in the barberton mountain land south africa based on u pb dating this paper presents u pb ages for 23 samples from the barberton mountain land an archean granite greenstone terrain in the kaapvaal craton of south africa a reexamination of the geology of this area in the light of previous u pb dating and the new ages indicates that much of its deformation can probably be accounted for by a single short lived compressional event within the time span 3230 3215 ma which was synchronous with deposition of the moodies group and the upper part of the fig tree group sediments this event may have been responsible for thrusting of much older sequences in the onverwacht group including slivers of 3 54 ga old gneiss circa 3 47 ga old komatiitic and tholeiitic sequences and circa 3 45 ga old felsic sequences key constraints on the timing of deformation are previously determined 3227 3225 ma ages for detrital zircons in thrusted units of the fig tree group a 3229 4 3 ma age for a deformed porphyry which is pre or synthrusting a previously determined 3227 3 ma age for an undeformed porphyry in an inferred thrust fault 3227 1 ma ages for both the kaap valley pluton and an ignimbrite in the stolzburg syncline both of which predate regional upright folding and a 3216 2 ma age for the dalmein pluton which appears to cut regional upright ne sw trending folds in the kromberg syncline part of the final phase of deformation associated with regional compression areally extensive granitoid sheets and syenogranitic plutons were subsequently emplaced into the barberton region at about 3105 ma the linear configuration of the syenogranitic plutons which are also aligned with the gabbroic usushwana complex as well as the geometry of the granitoid sheets suggests that the circa 3105 ma magmatism was caused by incipient rifting around a nw se trending crustal fracture this episode was probably coeval with a late regional transtensional phase of deformation recorded in the greenstone belt some of the above interpretations are in conflict with previous models for early development of the barberton greenstone belt a 3352 6 ma age from a metagabbro which is comagmatic with a proposed ultramafic sheeted dike complex is over 100 m y younger than the probable age of volcanic rocks which host the dikes this conflicts with the suggestion that much of the onverwacht group comprises an ophiolitic section obducted in an intraoceanic setting at circa 3 45 ga that partially melted to form trondhjemitic magmas which intruded along active thrust faults present data suggest that despite its antiquity and the diversity of ages and lithologies found within it the barberton greenstone belt can generally be understood in terms of actualistic tectonic processes at convergent margins

chilled margins and commingling of magmas in the bressanone brixen hercynian granodiorites eastern alps northern italy abstract the brixen hercynian pluton is the result of a multiple intrusion of crustal anatectic magma granodiorite granites and two mica cordierite bearing leucogranites and subcrustal magma gabbro diorites and fayalite bearing leucogranites some adjacent granodioritic stocks show chilled margins containing globular inclusions in the size of several centimetres of a coarse grained leucogranite the granodiorite margins also show narrow zoning with more acidic external borders this phenomenon is correlated to differentiated processes such as filter pressing and the soret effect which were active during the rise of these stocks the strong petrographic and chemical analogies between the globular inclusions of some chilled margins and the fayalite leucogranites suggest their common subcrustal origin the granodiorite containing these inclusions is of crustal origin the occurrence of globular inclusions inside it thus indicates only mechanical mingling between crustal anatectic and subcrustal magmas as shown by the absence of hybrids and chemical interactions

chronological correlations between the pilbara and kaapvaal cratons abstract the early geological development of the pilbara and kaapvaal cratons has many features in common attempts have been made to correlate geologically similar features of the two cratons and it has been postulated that they originated as contiguous components of a single continent vaalbara during this time the early geological histories of the pilbara and kaapvaal cratons are here compared in detail and the evidence that they were initially contiguous is assessed these comparisons indicate significant differences in the chronologies of magmatic events within the granite greenstone crusts of the pilbara and kaapvaal cratons in addition igneous correlatives emplaced during ca 2985 and 2782 ma magmatic events on the kaapvaal craton have not been identified on the pilbara craton and a well defined 2760 ma magmatic event manifest as widespread emplacement of granitic rocks into the pilbara granite greenstone basement and eruption of flood basalts of the lower part of the fortescue group is absent from the kaapvaal craton furthermore similarities in first and second order transgression regression cycles within the sedimentary supracrustal sequences may be attributable to global sea level fluctuations and thus may be irrevelant to the question of former contiguity however similarities in some aspects of the geological development of the pilbara and kaapvaal cratons imply that there were periods extending for between 60 and 200 ma of the archaean era during which the style of crust formation intensity of volcanism and subaerial erosion and magnitude of sea level fluctuations may have varied on a global scale such similarities include the overall duration of formation of the granite greenstone crusts from ca 3650 to 3100 ma the onset of craton wide erosion in the interval ca 3125 to 3000 ma the major episodes of flood basaltic volcanism between 2760 and 2680 ma the predominance of chemical carbonate and banded iron formation sedimentation between ca 2630 and 2440 ma and the transition to widespread clastic sedimentation within the interval 2440 to 2200 ma

sm nd and pb isotopic study of mafic rocks associated with early proterozoic continental rifting the peropohja schist belt in northern finland the perapohja schist belt in northern finland rests unconformably on archaean granitoids and marks the early stages of proterozoic crustal evolution in the fennoscandian baltic shield 2440 ma old layered mafic intrusions predate the supracrustal and ca 2200 ma old sills of the gabbro wehrlite association intrude the lowest quartzites and volcanics runkaus of the sequence the sm nd mineral isochron of the penikat layered intrusion gives an age of 2410 64 ma the initial nd values of the penikat intrusion nd 2440 1 6 and the runkausvaara sill nd 2200 0 suggest that these mafic magmas were contaminated by older crustal material the sm nd and pb isotopic results on the 2 44 2 2 ga old runkaus volcanics indicate mobility of pb fractionation of sm nd during late greenschist facies metamorphism and crustal contamination the pb pb data provide an age of 1972 80 ma with a high initial 207pb 204pb ratio 1 8 49 while scattered sm nd data result in an imprecise age of 2330 180 ma with an initial nd value of about zero secondary titanite gives an u pb age of ca 2250 ma the jouttiaapa basalts in contrast ascended from the mantle without interaction with older crust these lree depleted tholeiites mark a break in continental sedimentation and yield a sm nd age of 2090 70 ma their initial nd 4 2 0 5 implies that the subcontinental early proterozoic mantle had been depleted in lree for a long period of time the first lava flows are strongly depleted in lree suggesting that their source was significantly more depleted than the source of mid ocean ridge basalts today

morainal bank progradation and sediment accumulation in disenchantment bay alaska response to advancing hubbard glacier 1 morainal banks are primary features at the margins of advancing and stable to quasi stable temperate tidewater glaciers yet their roles in glacier dynamics and terminus stability are poorly defined by submarine observations analysis of new and archival multibeam data and landsat images of the advancing hubbard glacier southeast alaska reveal that between 1978 and 2010 the ice face and morainal bank advanced together at an average rate of 34 m yr varying spatially and temporally between 14 and 80 m yr morphological features including gullies and a boulder lag suggest cyclical deposition and gravitational erosion on the proximal slope of the morainal bank 15 18 and possible ice pushing in an area without recent sedimentation in contrast the morainal bank of the nearby quasi stable surging turner glacier advanced steadily since 1978 by proximal sedimentation on the steep fjord wall below its hanging valley sedimentation in the deep 220 m basin of disenchantment bay increased from 0 88 m yr spanning 1978 to 1999 to 1 22 m yr thereafter this change appears to be a combined response to glacier advance and sediment dispersal farther down fjord and to an increase in sediment yield from other glacial and non glacial sources analysis of hubbard glacier illustrates the direct correlation between movement of the terminus and morainal bank in advancing the grounding line of a marine terminating glacier and that morainal banks provide a fundamental stabilizing role for advance into a deep water fjord compensating for changes in water depth at the grounding line

the late archean to mesoproterozoic major unconformity bounded units of the kaapvaal province of southern africa reply abstract the late archean to mesoproterozoic 3 1 1 4 ga sedimentary and volcanic successions in the kaapvaal province of southern africa previously have been subdivided into numerous lithostratigraphic units that generally were believed to have been deposited in local or regional basins however these lithostratigraphic units are remnants of thirteen unconformity bounded sequences or units 10 3 m thick with composite strike lengths of 250 1100 km and with durations of 10 7 to 10 8 yr all of these units rest somewhere on the crystalline basement the stratotypes but not the full geographic extents of these sequences from the base upward are the dominion group west rand group of the witwatersrand supergroup central rand group of the witwatersrand supergroup klipriviersberg group of the ventersdorp supergroup platberg group of the ventersdorp supergroup pniel group of the ventersdorp supergroup black reef quartzite formation and chuniespoort group of the transvaal supergroup pretoria group of the transvaal supergroup and the interval represented by the dullstroom formation rooiberg group and the loskop formation three or more major unconformity bounded units occur in younger proterozoic rocks in the kaapvaal province but are not definitive of it two occur in the soutpansberg group and one is most of the waterberg group some of these unconformity bounded units are remarkably similar to phanerozoic cratonic and miogeoclinal sequences they are the natural building blocks for reconstructing archean and proterozoic depositional basins plate tectonic events continents and a provincial time scale

coupled stratigraphic and structural evolution of a glaciated orogenic wedge offshore st elias orogen alaska 1 the st elias orogen is the result of 10 myr of oblique convergence and flat slab subduction in the gulf of alaska between north america and the yakutat microplate extensive glaciation and a complex tectonic environment make this region a unique case study in which to examine the details of terrane accretion and the possible coupled influence of climate and tectonic drivers on the structural and topographic evolution of an orogenic wedge reflection seismic profiles across the offshore pamplona zone fold thrust belt the frontal st elias orogenic wedge provide constraints for quantifying pleistocene deformation recorded in the glaciomarine yakataga formation the total amount of pleistocene shortening observed varies from 3 to 5 mm yr compared to the current gps derived yakutat north america convergence rate across the st elias orogen of 45 mm yr growth strata and kinematic fold analysis allow comparison of relative timing of fault activity which reveals temporal and spatial shifting of active deformation during the glacial period faulting localized adjacent to the coastline and at the current submarine deformation front the abandoned currently inactive region is colocated with the major glacial depocenter in the region the bering trough these observations imply that glacial processes such as sediment loading and focused erosion during advance retreat cycles has a direct effect on the evolution of individual faults within the pamplona zone and the overall deformation pattern in the offshore st elias margin this information provides key constraints for understanding how climatic shifts may have affected the evolution of margin architecture during pleistocene glacial interglacial periods

variations in sediment yield over the advance and retreat of a calving glacier laguna san rafael north patagonian icefield abstract bathymetric and sub bottom acoustic data were collected in laguna san rafael chile to determine sediment yields during the little ice age advance and subsequent retreat of san rafael glacier the sediment volumes and subaqueous landforms imaged are used to interpret the proglacial dynamics and estimate erosion rates from a temperate tidewater glacier over a complete advance retreat cycle sediment yields from san rafael glacier averaged 2 7 107 m3 a since the end of the little ice age circa ad 1898 corresponding to average basin wide erosion rates of 23 9 mm a the highest erosion rates 68 23 mm a occurred at the start of the retreat phase and have since been steadily decreasing erosion rates were much lower during glacial advance averaging at most 7 mm a than during retreat such large glacial sediment yields over two centuries of advance and retreat suggest that the contribution of sediments stored subglacially cannot account for much of the sediment being delivered to the terminus today the detailed sub bottom information of a proglacial lagoon yields important clues as to the timing of erosion deposition and transfer of glacigenic sediments from orogens to the continental shelves and the influence of glacier dynamics on this process

isotope and geochemical constraints on proterozoic crustal evolution in south eastern africa proterozoic exposures south of the imbricate thrust terrane in the eastern sector of the namaqua natal belt of southern africa are dominated by granitoid plutons intrusive into older supracrustal gneisses many of these granites have geochemical characteristics similar to a type and rapakivi suites as previously described in australia and the northern hemisphere ages obtained from the natal units range from 1 1 to 0 88 ga nd tdm values cluster about 1 4 ga the oldest granitoids have volcanic arc chemical signatures but subsequent to 1 1 ga a type granites are common this suggests a change in tectonic environment or melting process at about this time initial 87sr 86sr ratios and nd values for the older suites approach values normally associated with a depleted mantle reservoir whilst values are similar to those found in orogen environments these data clearly indicate that the proterozoic crust of south eastern africa was not derived by extensive reactivation of archaean or early proterozoic protoliths low 87sr 86sr ratios in metasedimentary carbonates and 1 8 ga nd tdm ages for metapelites from the belt support this model and preclude the formation of the belt in an intracratonic setting all the data thus support models involving substantial crustal accretion and differentiation primarily subsequent to 1 5 ga

geochemistry and provenance of sediments from the pongola supergroup south africa evidence for a 3 0 ga old continental craton abstract the 3 0 ga pongola supergroup of south africa is one of the oldest well preserved supracrustal successions the sequence is divided into the nsuze chiefly volcanics quartzites and pelites and the overlying mozaan groups chiefly quartzites and pelites relative to north american shale composite nasc most pongola pelites have similar la yb and eu eu ratios and large ion lithophile elements lile high field strength elements hfse ree v and sc contents ni 30 300 ppm and cr 100 800 ppm contents are greater than those of nasc while cr ni ratios are generally higher than shales and pelites of all ages these trends may reflect residual cr enrichment during intense chemical weathering of komatiitic source rocks local populations of nsuze pelites based on major and trace element data may result from source heterogeneities and or contemporaneous volcanic input into the sedimentary basin pongola quartzite compositions and detrital modes are similar to those of sandstones from phanerozoic passive margin and basement uplift tectonic settings ree lile and hfse contents in pelites suggest that pongola source rocks were remarkably similar in composition to average phanerozoic upper continental crust trace element geographic and isotopic constraints favor 3 2 to 3 0 ga hood granites as a major source for pongola detritus moderate contributions from mafic sources are also required whereas relatively minor contributions from tonalite and komatiite are allowed these results indicate the existence of a chemically evolved felsic craton in southern africa by 3 0 ga this craton appears to have been more evolved than sources for either older moodies 3 3 ga or most of the younger witwatersrand sediments 2 8 2 7 ga pelites from the kaapvaal craton record at least two mafic to felsic source area compositional cycles in pelites deposited from the mid to late archean additional cycles may be masked by a depositional or erosional hiatus between the moodies and pongola sequences these sedimentary patterns suggest that crustal development in southern africa was episodic in nature during the early precambrian

crustal influence in the generation of continental flood basalts the suggestion that primordial undifferentiated material may exist in the earth s mantle has recently been revived on the strength of nd isotope data for two types of young continental rocks flood basalts1 and kimberlites2 the limited published data show a clustering of nd isotopic compositions close to those for meteorites with chondritic relative rare earth ree abundance in contrast we now present data for samples from the columbia flood basalt province of the northwestern united states which show large isotopic variability suggestive of mixing processes acting after the separation of the primary magmas from their mantle source

modelling landscape evolution under ice sheets we use an ice sheet model glimmer with an erosion component to examine the evolution of landscapes under ice sheets over long time scales in hypothetical situations glimmer is a fully coupled thermomechanical 3 dimensional ice sheet model with a sliding component the model thus has the ability to predict ice thickness the distribution of areas at pressure melting point and basal velocities the erosion rate is assumed to be a simple linear function of the basal velocity and therefore erosion only occurs where the bed is not frozen and basal sliding is possible in addition sliding velocities are assumed to be related to basal water production thus giving a mechanism for smooth temporal and spatial transitions between eroding and non eroding portions of the bed the eroding glacial landscape system is modelled for long term persistent glaciation assuming a variety of initial landscapes results indicate that without the inclusion of basal water production in the calculation of sliding velocities any thermal feedbacks are exaggerated because of steep horizontal basal temperature gradients additionally bed morphology exerts a greater influence under low basal slip conditions because ice cannot respond readily to thermal instabilities we suggest that erosion and valley overdeepening may aid stabilisation of the thermal regime of an ice sheet and that it is possible to generate glacial signals in previously fluvial systems after only 100 kyr of glaciation at conservative erosion rates

precise u pb baddeleyite age dating of the usushwana complex southern africa implications for the mesoarchaean magmatic and sedimentological evolution of the pongola supergroup kaapvaal craton the usushwana complex of the south eastern kaapvaal craton south africa and swaziland intrudes the ca 3 6 3 1 ga basement of the craton as well as the mesoarchaean volcanic and sedimentary cover succession of the pongola supergroup new high precision u pb dating of gabbros belonging to the piet retief suite of the usushwana complex yield ages of 2989 1 ma 2990 2 ma and 2978 2 ma the piet retief suite represents part of an intricate magmatic feeder to a major volcanic event which gave rise to the oldest known continental flood basalts on earth the nsuze volcanic rocks broadly coeval se trending dolerite dykes of the barberton badplaas dyke swarm in the larger region of the south eastern kaapvaal craton formed along the same structural trend as the usushwana complex one such dyke is dated herein to 2980 1 ma using the high precision u pb geochronological data the nsuze volcanic rocks can now be resolved into at least two magmatic episodes which can be correlated with parts of the pongola supergroup the first episode at ca 2 99 2 98 ga is broadly coeval with the pypklipberg nhlebela volcanic rocks whereas the second at ca 2 97 2 96 ga was near synchronous to the agatha volcanic rocks a dolerite sill intruding into the mozaan group of the pongola supergroup thought to be part of the usushwana complex was dated to 2869 5 ma and is instead coeval with the hlagothi complex further to the south and provides a new minimum age for deposition of the mozaan group c 2015 elsevier b v all rights reserved less

sm nd isotopic mapping of lithospheric growth and stabilization in the eastern kaapvaal craton whole rock sm nd isotope systematics of 79 archean granitoids from the eastern kaapvaal craton southern africa are used to delineate lithospheric boundaries and to constrain the timescale of crustal growth assembly and geochemical differentiation c 3 66 2 70 ga offsets in end values for 3 2 3 3 ga granitoids across the barberton greenstone belt bgb are consistent with existing models for c 3 23 ga accretion of newly formed lithosphere north of the bgb onto pre existing c 3 66 ga lithosphere south of the bgb along a doubly verging subduction margin the nd isotopic signature of c 3 3 3 2 ga magmatic rocks show that significant crustal growth occurred during subduction accretion after c 3 2 ga however the nd signature of intrusive rocks c 3 1 and 2 7 ga is dominated by intracrustal recycling rather than by new additions from the mantle signalling cratonic stability

vaalbara earth s oldest assembled continent a combined structural geochronological and palaeomagnetic test the only remaining areas of pristine 3 6 2 7 ga crust on earth are parts of the kaapvaal and pilbara cratons general similarities of their rock records especially of the overlying late archean sequences suggest that they were once part of a larger vaalbara supercontinent here we show that the present geochronological structural and palaeomagnetic data support such a vaalbara model at least as far back as 3 1 ga and possibly further back to 3 6 ga vaalbara fragmented prior to 2 1 ga and possibly as early as 2 7 ga suggesting supercontinent stability of at least 400 myr consistent with neoproterozoic and phanerozoic analogues

seismic stratigraphy sedimentary architecture and palaeo glaciology of the mackenzie trough evidence for two quaternary ice advances and limited fan development on the western canadian beaufort sea margin abstract a comprehensive grid of high resolution 2 d seismic reflection data from the western canadian beaufort sea margin is used to reconstruct past ice stream dynamics within the mackenzie trough eight seismic facies and five sequences divided into two megasequences are identified from the mackenzie trough stratigraphy evidence for two quaternary ice advances to the shelf break is provided by two sequences of acoustically chaotic to semi transparent facies interpreted as subglacial till buried landforms interpreted as lateral moraines and a grounding zone wedge record the positions of still stands or re advances in the ice margin the continental slope beyond the trough is characterised by canyons separated by inter canyon ridges and thin glacigenic debris flows correlation with the onshore record suggests that the older of the two ice advances which excavated the mackenzie trough probably occurred during the illinoian or early wisconsinan glaciations the younger ice stream advance is interpreted to have occurred during the last late wisconsinan glaciation the onset of cross shelf glaciation on the western canadian beaufort sea margin is inferred to have been initiated significantly later than on the eastern beaufort sea and eastern canadian arctic margins which have a longer history of ice advance and were less peripheral to the ice sheet centre the architecture of the slope beyond the mackenzie trough reflects this comparatively short history of ice advance and lacks the progradational architecture and major glacial sedimentary depocentre or trough mouth fan that is characteristic of slopes seaward of cross shelf troughs on formerly glaciated margins

the age origin and tectonics of the grao para group and associated rocks serra dos carajas brazil archean continental volcanism and rifting the grao para group which is host to the large serra dos carajas fe deposits is located in the eastern part of the central brazil shield ssw of belem brazil it is composed of a lower sequence of metavolcanic rocks a middle sequence of iron formation carajas formation and an upper sequence of mixed metavolcanic and metasedimentary rocks the grao para group is overlain by an unnamed sedimentary dominantly sandstone unit and unconformably overlies the older xingu complex the lower metavolcanic sequence of the grao para in the region of the serra norte fe deposits is bimodal with dominant metabasalts metabasaltic andesites and metatrachyandesites and subordinate 10 15 metarhyolitic tuffs and flows most of the metavolcanic rocks are at low to medium metamorphic grade with more intense metamorphism and deformation along the northern contact with the xingu complex the thickness of the metavolcanic section is not known but may be 4 6 km the mafic metavolcanic rocks are enriched in rb ba and k with respect to oceanic basalts and basalts of greenstone belts and are relatively depleted in th sr ta ti and p the mafic rocks also show an enrichment in lrees with flat hree patterns the hrees are correlated with mg number u pb zircon analyses of the metarhyolites from the grao para group yield an age of 2758 39 ma rb sr wholerock analyses of the mafic rocks yield an isochron with an age of 2687 54 ma with an initial ratio of 0 7057 0 0010 the results indicate a late archean age of volcanism in the grao para group much older than previously thought rb sr results from the metarhyolites have been partially reset yielding an age 2497 62 ma sm nd analyses from the grao para metavolcanic rocks were too scattered to yield an isochron but nd values relative to chur at 2758 ma indicate values similar to depleted mantle at this time although with some scatter the trace element data and the sr and nd isotopic results from the mafic rocks are consistent with contamination of mantle derived melts by significant but variable amounts of diverse older continental crust the stratigraphy geochemical and isotopic compositions show that the grao para group originated on extending older continental crust during the archean an age terrane boundary must exist between this archean continental terrane and the apparently ensimatic early proterozoic greenstone belts of the northern amazonian craton similarities to other sequences e g the fortescue group australia suggest that similar tectonic settings were widespread at this time many major iron formations were deposited within these environments indicating significant iron formation development on sialic crust in the late archean u ph zircon analyses from the serra dos carajas granite which intrudes the unnamed sedimentary unit overlying the grao para group yield an age of 1820 49 ma in agreement with previous age determinations this age is similar to the ages for the uatuma supergroup volcanic and shallow plutonic rocks which occur throughout the amazonian craton

holocene history of hubbard glacier in yakutat bay and russell fiord southern alaska stratigraphic and geomorphic data defined by radiocarbon ages tree ring dates and historical observations provide evidence of three major holocene expansions of hubbard glacier early in each advance the hubbard glacier margin blocked russell fiord to create russell lake raising base level and causing stream beds and fan deltas throughout the russell drainage basin to aggrade each hubbard glacier expansion continued with an ice lobe advancing through disenchantment and yakutat bays in the west and an eastern lobe advancing into russell fiord the earlier two holocene expansions were respectively under way at 7690 and 5600 calibrated yr b p and each advance culminated more than 1 k y later the late holocene advance was under way by 3100 yr ago and reached 13 km farther south in russell fiord than the preceding two expansions late holocene deglaciation of yakutat and disenchantment bays was complete before a d 1791 nunatak glacier flowing from neves east of russell fiord became the primary ice source to the russell fiord lobe at or before this date ice retreat from the southern end of russell fiord began in the late eighteenth century and the penultimate russell lake drained ca a d 1860 the relatively slow advances and more rapid retreats of hubbard glacier are consistent with the model of the iceberg calving glacier cycle hubbard glacier is currently advancing and will likely reestablish russell lake in the near future affecting local fisheries however glacier lobes are unlikely to reach the area of the town of yakutat built on late holocene glacial deposits in the next 1 k y

eastern indian 3800 million year old crust and early mantle differentiation samarium neodymium data for nine granitic and tonalite gneisses occurring as remnants within the singhbhum granite batholith in eastern india define an isochron of age 3775 89 x 10 6 years with an initial 143 nd 144 nd ratio of 0 50798 0 00007 this age contrasts with the rubidium strontium age of 3200 x 10 6 years for the same suite of rocks on the basis of the new samarium neodynium data field data and petrologic data a scheme of evolution is proposed for the archean crust in eastern india the isotopic data provide evidence that parts of the earth s mantle were already differentiated with respect to the chondritic samarium neodymium ratio 3800 x l0 6 years ago