oxygen isotope calibration of the onset of ice rafting and history of glaciation in the north atlantic region we report here that dsdp site 552a cored with the hydraulic piston corer on the west flank of rockall bank recovered an undisturbed sequence of alternating white deep sea carbonate oozes and dark coloured layers that are rich in glacial debris oxygen isotope analysis of the sequence together with detailed nannofossil and palaeomagnetic stratigraphy shows that the first major horizon of ice rafting occurred at about 2 4 myr and was preceded by a minor pulse of ice rafting at about 2 5 myr the carbon isotope record shows that the site has been bathed by a water mass of similar characteristics to present day north atlantic deep water at least since 3 5 myr

the development of antarctic glaciation and the neogene paleoenvironment of the maurice ewing bank abstract a micropaleontologic magnetostratigraphic and sedimentologic analysis of 56 piston cores provides the basis of a geologic study of the late miocene to holocene depositional and erosional history of the intermediate depth maurice ewing bank meb located at the eastern extremity of the falkland malvinas plateau southwest atlantic ocean sedimentation on the meb is controlled presently by the position of the polar front and strong bottom currents beneath the axis of the antarctic circumpolar current acc fluctuations in the position of the polar front and in the intensity of the acc are inferred to have largely controlled deposition on the meb since the initiation of acc flow over the bank in the miocene the sedimentary record of the meb is used to infer the late miocene to holocene oceanographic and climatic conditions in the south atlantic sector of the southern ocean the meb suffered major erosion by intense acc flow during late miocene time which exposed cretaceous miocene sediment near the surface and shaped the present configuration of the bank this erosional event is inferred to have occurred during late miocene to early pliocene time 7 2 4 7 m y b p with the major phase of erosion occurring between middle magnetostratigraphic chron 7 and late chron 6 7 2 6 2 m y b p we suggest that the miocene sedimentary record of the meb and other paleoenvironmental evidence from the circum antarctic region precludes the existence of a west antarctic ice sheet or extensive ice shelves along the antarctic margin prior to the late miocene during the late miocene a west antarctic ice shelf formed in the former west antarctic sea and rapidly thickened until it grounded below sea level to form the west antarctic ice sheet formation of this ice sheet with floating and partially grounded extensions ice shelves in the ross and weddell embayments led to the first major production of true antarctic bottom water formed in a manner similar to today which permanently altered global abyssal circulation late miocene changes in oceanic ccd levels and a permanent shift in the oceanic 13 c 12 c composition may be attributed to this major change in abyssal circulation a decrease in acc velocity during an early pliocene amelioration of climate led to resumption of deposition on the bank from 4 7 to 3 9 m y b p intensification of the acc during the late gilbert and early gauss chrons again resulted in limited deposition and widespread erosion and or non deposition over most of the meb from 4 0 to 3 2 m y b p acc velocity apparently began to wane during the middle of the gauss chron and allowed widespread deposition on the bank during late gauss time 2 9 2 48 m y b p and over more limited areas during earliest matuyama time 2 48 to about 2 2 m y b p during middle to late matuyama time bottom current intensity reached its greatest post miocene level forming a regional disconformity between sediments of about 2 0 and 1 0 m y in age this increased circumpolar circulation and associated erosion is inferred to have peaked during the late matuyama chron 1 2 1 0 m y b p and is approximately the same age as the tentatively dated greatest patagonian glaciation in nearby southern argentina between 1 0 and 0 7 m y b p the bank was blanketed with a coarse erosion resistant layer of ice rafted detritus which armored the older sediment thereby protecting it from major subsequent erosion sedimentation on the meb during the bruhnes chron 720 000 yr b p present was intermittent and finally culminated with the deposition of a veneer of carbonate ooze during or since latest pleistocene time

recurrent uranium relocations in distal turbidites emplaced in pelagic conditions abstract the sediments of the madeira abyssal plain east of great meteor seamount are dominated by distal turbidite deposition while the turbidites exhibit a wide compositional range 25 80 caco3 individual examples can be correlated over a wide area and are relatively homogenous organic c oxidation by bottom water oxygen proceeds from the turbidite tops downwards after emplacement in pelagic conditions and the progress of this oxidation front is marked by a sharp colour contrast in the sediments wilson et al 1985 in turbidites with corg 0 5 redistribution of authigenic u occurs to form a concentration peak 4 9 ppm u just below the oxidation front or colour change several tens g u cm2 may be mobilised and in all examples studied 60 of the remobilised u is relocated into the peak following burial by subsequent turbidites such u concentration peaks are persistent as relict indicators of their extinct oxidation fronts for at least 2 105 years in the case of thin turbidites where labile corg is almost exhausted the u peaks may be located in underlying sedimentary units because of their relationship to the oxidation front a redox mechanism for u peak formation is suggested from these data rather than a complexation with organic matter

nd and sr isotopic systematics of central australian granulites chronology of crustal development and constraints on the evolution of lower continental crust nd and sr isotopic data are reported for a granulite terrain in the proterozoic arunta block of central australia sm nd data from a wide range of rock types define a crust formation age of 2 070 125 ma and provide further evidence for voluminous crustal growth in the proterozoic an nd value of 1 5 0 8 indicates a depleted mantle source for this crustal segment and there is no evidence for a large component of significantly older sialic crust field relationships geochemistry and rb sr data for mafic and felsic granulites indicate that intracrustal differentiation and polyphase deformation were followed by granulite facies metamorphism rb depletion at 1 800 ma rb sr data for strongly retrogressed granulites define an age of 1 700 ma which is interpreted as the time of retrograde biotite growth partial melting at the presently exposed crustal level and anatexis at deeper crustal levels were broadly coeval with the retrograde metamorphism sm nd and rb sr isotopic systematics of minerals indicate that the terrain cooled slowly did not experience significant uplift until 1 000 ma and remained at temperatures above 320 c until the late palaeozoic the mineral data are consistent with geological relationships and petrological evidence for a prolonged period of isobaric cooling followed by uplift late in the metamorphic evolution of the terrain the granulite protoliths appear to have formed in a rift which closed within 280 ma of initial separation deformation and granulite facies metamorphism at 1 800 ma are interpreted to be a consequence of collision between the continental blocks which defined the rift regional retrogression and granitoid magmatism at 1 700 ma are attributed to underthrusting of the granulites by lower grade rocks in the final stages of collision subsequent events in the cooling and uplift history appear to have been controlled by the presence of long lived major faults in the crust and a prolonged history of episodic compression in the continental lithosphere the results of this study suggest that granulite terrains in general cannot be equated with lower continental crust but instead represent assemblages of mainly supracrustal rocks which in some instances have been involved in major collision events

climate and bioproductivity control on carbonate turbidite sedimentation paleocene to earliest eocene gulf of biscay zumaia spain abstract the paleocene to lowermost eocene zumaia series situated along the basque coast northern spain consists of pelagic limestones and marlstones with interbedded mixed calcarenite turbidites we have compared the composition and frequency of turbidite beds in time as well as the clay mineral and bulk rock carbon isotope composition of the pelagic beds to discern the factors controlling turbidite deposition the pelagic beds are coccolithic mudstones with mainly planktic foraminifera the sources of detrital material to turbidites were the carbonate producing shelves bearing foramol associations around the evolving bay of biscay and to a minor extent the incipient pyrenean chain for siliciclastic components we have found close relationships between variations of turbidite facies and depositional rates with clay mineral assemblages and carbon isotopes in the pelagic beds the latter are in line with global trends it appears that in the early bay of biscay climate and nutrient availability controlled turbidite sedimentation the bulk rock late paleocene np 5 np 9 positive carbon isotope excursion coincides with a period of relatively cool and dry climate characterized by the highest frequency of turbidity flow we propose that during this period globally enhanced latitudinal temperature gradients and strong thermohaline ocean circulation increased bioproductivity in surface waters by upwelling and or by wind driven ekman pumping in contrast warm perennially wet periods such as the early paleocene np 1 np 4 and the paleocene eocene transition upper np 9 np 10 are characterized by low frequency of turbidity flow in the basin this may have been caused by sluggish thermohaline circulation and low carbonate productivity during this time period

sm nd and u pb dating of early proterozoic mafic felsic volcanism in northernmost sweden a sm nd mineral isochron of 1932 45 ma on a kiruna greenstone is defined by greenschist facies metamorphic minerals apparently older than the regional svecokarelian metamorphism 1880 ma however this age agrees within error with u pb zircon ages of 1909 1617 ma on quartz bearing prophyries which overlie but are in part intercalated with basic volcanics if these basic volcanics following previous studies are considered to be contemporary with the kiruna greenstones the present data suggest that eruption of both basic and acid volcanics and associated hydrothermal metamorphism took place over a short time interval close to 1 9 ga the greenstone sample is depleted in light rare earths and has high initial 145nd 144nd indicating a source in which such depletion has existed for a long time in contrast an albite diabase sample with a somewhat younger mineral age 1 80 0 08 ga is light rare earth enriched and has a correspondingly low initial nd isotopic composition

the bergslagen ore province sweden review and update of an accreted orocline 1 9 1 8 ga bp abstract we have revisited and re interpreted deformation structures in the classical ore province of west bergslagen part of the 1 9 1 8 ga old svecofennian orogen in south central sweden four successive phases and styles of deformation reflect 1 continental rifting associated with bimodal magmatism in continental back arc basins d1 2 closure of the rift s by upright tight to isoclinal folds on any scale d2 3 renewed rifting d3 with deposition of argillites conglomerates and submarine basalts in restricted rift basins and 4 intense buckling and folding of west bergslagen along variably but generally steeply plunging folds d4 strain localization during d4 initiated a network of conjugate retrogressive shear zones and closely related large scale vertical drag folds with the southwest northeast striking west bergslagen boundary zone system acting as the main tectonic boundary along which west bergslagen slipped relative to the central part of the province d4 transpression and variably oblique d4 shear may have caused the preservation of a low grade domain in west bergslagen as well as the exhumation of a granulite facies domain near the hinge of the orocline d4 structures include a regional scale oroclinal bend of 135 in southwest bergslagen connecting common southwest northeast strikes in west bergslagen to east west strikes to the south and east in central bergslagen the oroclinal bend tightens to 180 to the south it lines up with another orocline the two forming an s shaped pair of large scale buckle folds the orocline finds expression in the regional patterns of form lines and magnetic anomalies and in the spatial distribution of specific intrusive rock suites the northern structural boundary of the province is known as an east west striking zone of dip slip movement and local wrenching which we also interpret as a d4 effect the d4 structures imply the evolution of a separate ribbon like bergslagen micro continent at least 700 km long rifted from the former active continental margin during the d3 event and finally accreted slightly obliquely to pre existing fennoscandia to the present north by correlating d4 deformation structures with the areal distribution and previously determined radiometric ages of late orogenic anatectic granites we derive a provisional estimate 1 83 1 82 ga as the lower age limit to the initiation of terrane accretion d4 accretion is svecofennian orogeny proper in this part of the belt younger than the currently preferred estimate of 1 86 ga west bergslagen s synsedimentary hydrothermal ore deposits originating from 1 91 to 1 89 ga old volcanism and rifting have been remobilized to various extent during the late orogenic d4 phase and show pronounced concentration along d4 shear zones

subseabed disposal of radioactive waste effects of consolidational fluid flow subseabed disposal of radioactive waste applies a multiple barrier concept with the sediment being the most important barrier for preventing a release of nuclides into the biosphere while many investigations have been carried out to analyze the risk potential in this type of disposal the effects of sediment consolidation and associated fluid flow have not fully been taken into consideration here possible effects of consolidational fluid flow in the penetrator disposal option and possible consequences to the transport of nuclides in the sediment are analyzed results of numerical experiments demonstrate that consolidation contributes to the transport of radioactive nuclides released from containers buried in the sediment and to the release of nuclides at the sediment water interface both depend on geological conditions and to a large extent on possible alterations of hydraulic conductivity i of the sediment in the vicinity of the entry path of a penetrator

sm nd isotope systematics of 1 9 1 8 ga granites from western bergslagen sweden inferences on a 2 1 2 0 ga crustal precursor the bergslagen ore province forms part of the svecofennian domain of the baltic shield the initial nd values of least altered and albitized granite samples of the 1 88 ga bergslagen older granite suite vary between 1 2 and 2 5 the granite samples of the 1 78 ga fellingsbro malingsbo suite vary between 0 3 and 7 7 the rb sr system of the older granite suite yields an errorchron of 1 70 0 01 ga with an initial 87sr86sr ratio of 0 7053 indicating tectonothermal disturbance perhaps due to the intrusion of younger granites of the transscandinavian igneous belt the initial nd isotopic composition of the 1 88 ga older granite suite can be interpreted in two extreme end member models a interaction between depleted mantle derived melts and archaean crustal material at 1 88 ga and b remelting of early proterozoic pre 1 88 ga crust previous studies in the baltic shield have advocated the former model and discarded the latter because of absence of 2 0 2 4 ga crust likewise in the case of western bergslagen no archaean crust is present it is hence argued that the nd isotopic data may also be interpreted in terms of generation of the older granite suite through remelting of a crustal precursor in this case the 2 1 2 0 ga depleted mantle model ages of the older granite suite could give an indication of the time that such a precursor was extracted from the depleted mantle

the growth of early proterozoic crust new evidence from svecofennian detrital zircons ion microprobe u pb analyses of zircons from the 1 9 gyr old svecofennian metasediments demonstrate the presence of both archaean and early proterozoic grains whereas all conventional multi grain analyses give apparent u pb ages of about 2 3 gyr most zircons are 1 9 2 0 gyr old suggesting that a major still unknown early proterozoic terrain older than 1 9 ga supplied detritus to the early orogenic svecofennian turbidites these results imply that the formation of early proterozoic felsic crust started c 100 myr earlier than has previously been inferred from isotopic data models in which nd model ages of 2 1 2 4 gyr have been interpreted as real crustal formation ages are not supported by our results

changes in redox conditions in deep sea sediments of the subarctic north pacific ocean possible evidence for the presence of north pacific deep water cores of upper quaternary and holocene sediment from the subarctic north pacific north of about 48 n contain one or more layers of oxidized brown sediment interbedded within predominantly reduced green sediment the brown layers are enriched in several trace elements especially mn mo ni and co relative to the green layers where multiple oxidized layers are present the intensity of the brown coloration and the magnitude of trace element enrichment often decrease with depth suggesting that the oxidized layers are unstable and are being chemically reduced at depth the oxidized layers represent a change in redox conditions between the north pacific red clay province and the subarctic biosiliceous green clay province the redox change may have been caused by an increase in supply of dissolved oxygen to bottom waters during glacial interglacial transitions as the result of the periodic formation of a seasonal bottom water mass in the northeastern pacific ocean

geochronology and structural relationships of mesothermal gold mineralization in the palaeoproterozoic jokisivu prospect southern finland the palaeoproterozoic svecofennian orogen in southern finland contains a number of orogenic gold occurrences the jokisivu gold deposit comprising auriferous quartz veins is hosted by syn tectonic quartz diorites to gabbros mineralization occurs in approximately wnw ese and wsw ene trending shear zones which probably branch from regional scale nw se trending shears ore zone fabrics post date regional scale folding and the metamorphic peak and can be correlated with late svecofennian regional shear tectonics d 6 1 83 1 78 ga indicating that mineralization formed during the late stages of orogenic evolution sims and tims u pb dating of three samples place tight constraints on the age of gold mineralization zircons from both unaltered and altered quartz diorites have ages of 1884 4 ma and 1881 3 ma respectively these are interpreted as the crystallization age of the rock and as providing the maximum age for mineralization zircon rims from an altered quartz diorite from the ore zone give ages of c 1802 15 ma which overlap with the 1801 18 ma titanite mean pb pb age from the ore zone the ages are similar to the age of the pegmatite dyke that cuts the ore zone and whose zircon age of 1807 3 ma is approximately the same as the 1791 2 ma monazite age tims giving the minimum age of the gold mineralization the mineralization and its structural framework can be correlated with coeval late svecofennian shear tectonics related to wnw ese oriented shortening in southern finland extensive c 1 8 ga granite magmatism shear zone development and associated gold mineralization are of regional importance also in the northern and western fennoscandian shield finnish lapand and sweden a cordilleran type setting can explain the widespread distribution of magmatism and gold mineralization associated with shortening as well as the required heat source triggering hydrothermal fluid flow along shear zones

post depositional migration of elements during diagenesis in brown clay and turbidite sequences in the north east atlantic abstract two cores from a ne atlantic pelagic clay area of low accumulation rate each contain a single turbidite with different age thickness and composition diagenesis following introduction of exotic turbidite material into the pelagic clay sequence has resulted in distinctive colour changes the diagenetic process is thought to be driven by bacterially mediated organic oxidation and compositional differences in the turbidites and clays allow examination of the effects on metal concentrations of this process in one core a long turbidite section emplaced 330 000 years ago is overlain by clay organic oxidation has apparently proceeded from the turbidite top downwards and has maintained a u concentration peak below the oxidation front marked by a colour change the u source is believed to be u initially associated with the organic matter in the oxidised section of the turbidite vanadium and cu behave similarly but upward migration of the redox sensitive metals mn and ni is also seen in the second core an 8 cm turbidite section was emplaced about 170 000 years ago in a clay column in this case organic consumption now appears complete but evidence for diagenetic effects is found in a 16 cm compositional alteration halo of the underlying pelagic clay fe ii has been enriched and the hydrogenous component of mn co ni and cu removed from the pelagic clay to form the halo

the behaviour of manganese in atlantic carbonate sediments abstract manganese porewater and solid phase data are considered from two different atlantic carbonate sediments both with good geochemical characterisation the first case is a continuously accumulated alternating pelagic marl carbonate ooze where there is a correlation in post oxic conditions between mn ii porewater levels and an acetate leachable mn fraction of the sediment this leachable mn appears to have an association with calcite and in this example the leached residue mn content approximates to that of shale in the second case a very recent compositionally homogeneous calcareous turbidite is underlain by mn oxyhydroxide at the former sediment surface which is now acting as a source of mn ii by reduction sorption of mn by the turbidite sediment and shale like leached residue mn levels in anoxic conditions are again observed as mn ii diffuses towards present oxic conditions although no unique equilibrium is achieved similar older buried turbidites have concentration decreases in solid phase mn content preserved from bottom to top as relict evidence that this process has occurred in these turbidites 54 64 of the total mn is leachable by the acetate method and therefore presumed to be sorbed by calcite the oldest example has persisted for 125 kyr it is suggested that the slow breakdown of metastable mn oxyhydroxide and sorption of mn ii to calcite surfaces are important controls on the behaviour of mn in pelagic carbonates in mildly reducing conditions where accumulation fluctuations occur these processes act together with accumulation to limit overall mn migration and to prevent the large mn enrichments seen near the sediment surface in more reducing environments

basalt columns large scale constitutional supercooling abstract the conventional interpretation of basalt column formation in terms of cracking of a homogeneous magma during cooling is questionable because cooling cracks in homogeneous materials do not tend to form quasi hexagonal patterns it is proposed that basalts develop inhomogeneous patterned compositions during cooling as a result of constitutional supercooling the latter is observed in metallic alloys and its mechanism is briefly described it is expected to produce a quasi hexagonal pattern of glassy interfaces these interfaces crack preferentially during uniaxial cooling

rift related igneous activity and metallogenesis in sw bergslagen sweden abstract the bergslagen ore belt in central sweden comprises a mid proterozoic complex of broadly coeval volcano sedimentary and plutonic rock units showing overlapping ages between 1 9 and 1 84 ga and formed in a rift tectonic setting the volcano sedimentary rock unit of this complex is referred to as the bergslagen supracrustal sequence and divided into the lower leptite the middle leptite and the upper leptite and slate groups in sw bergslagen the development of the plutono volcano sedimentary complex began about 1 9 ga ago with the early volcanic phase of extrusion of lower leptite group felsic volcanics in the subsequent initial rift phase branching basins were formed in which felsic volcanics and iron formations of the middle leptite group were deposited the succeeding rift phase was marked by the deposition of felsic volcanics volcaniclastic and other sediments of the upper leptite and slate group in rift phase basins the rift phase is characterized by bimodal mafic felsic volcanism and plutonism metabasic flows are found between the felsic metavolcanics and metasediments in the rift basins whereas metabasic sills and dykes are associated with granite intrusions in the horsts between the rift basins the metabasic rocks are referred to as the bergslagen metabasic rock suite and the granites as the bergslagen older granite suite the alkaline to subalkaline chemistry and anorogenic suite trace element enrichments of the older granites their close affinities to associated silicic volcanics the continental tholeiitic character of the metabasite rock suite and the outspoken bimodality of the rift phase magmatism all support the contention of a rift tectonic setting manganese and sulfide ores are found as syngenetic exhalative sedimentary deposits in the rift basins and epigenetic replacement deposits in older formations below the floor of and in the horsts between the rift basins these ore deposits and associated metasomatic alterations are related to widespread sub seafloor hydrothermal metamorphism in connection with hydrothermal convection systems driven by the heat of the various rift phase mafic and felsic intrusions and extrusions the rift phase terminated with cessation of volcanism and was followed by the post rift phase of tectonic compression and the intrusion of about 1 84 ga old hybride rocks of the hyttsjo gabbro tonalite suite

excavation induced microseismicity in the columnar jointed basalt of an underground hydropower station abstract large scale columnar jointed basalt cjb developments have had serious adverse effects on the stability of the project in progress to create the diversion tunnels at the baihetan hydropower station in china an in situ microseismic ms monitoring experiment was carried out to help understand the fracturing process in the cjb rock mass as a result of tunnel excavation ms monitoring performance accuracy of location estimation and system sensitivity is analyzed first then the spatiotemporal evolution of the ms activity associated with the baihetan cjb rock mass fractures along the tunnel axis and at the tunnel sidewalls during the entire excavation unloading process is obtained finally the forms and opportunities to deploy appropriate support are suggested according to the characteristics of the cjb microseismicity meanwhile the ms monitoring methods required for cjb or jointed hard rock engineering e g sensor types sensor spacing sensor array forms system protection and selection of analysis regions resulting from this exploration experiment are summarized

high resolution seismic stratigraphy of late quaternary deposits in manitounouk sound northern quebec effects of rapid postglacial emergence manitounouk sound contains a succession of postglacial deposits consisting of three acoustic units separated by disconformities from base to top these are 1 a stratified draped unit interpreted as of glaciolacustrine and glaciomarine origin 2 an onlapping basin fill unit of thick transparent beds interpreted as gravity flow deposits and 3 a divergent fill unit of stratified and transparent modern marine deposits the gravity flow deposits of unit 2 pass laterally into slide scars that truncate unit 1 pollen and dinoflagellate analysis of cores correlated with the acoustic sections indicate that unit 1 is of early holocene age and confirms a glaciomarine environment unit 3 contains modern assemblages of pollen and dinoflagellates the slide scar and gravity flow deposits of unit 2 correspond to the period of transition from glaciomarine to modern conditions and are interpreted to result from a period of relatively intense earthquake activity due to stress release along faults during or shortly

palaeoproterozoic accretionary processes in fennoscandia abstract accretionary processes contributed to major continental growth in fennoscandia during the palaeoproterozoic mainly from 2 1 to 1 8 ga the composite svecofennian orogen covers c 1 10 6 km 2 and comprises the lapland savo fennia svecobaltic and nordic orogens it is a collage of 2 1 2 0 ga microcontinents and 2 02 1 82 ga island arcs attached to the archaean karelian craton between 1 92 and 1 79 ga andean type vertical magmatic additions especially at c 1 89 and c 1 8 ga were also important in the continental growth the palaeoproterozoic crust is the end product of accretionary growth continental collision and orogenic collapse preserved accretional sections are found in areas where docking of rigid blocks has prevented further shortening the pirkanmaa belt represents a composite accretionary prism and other preserved palaeosubduction zones are identified in the gulf of bothnia and the baltic sea areas in the southern segment of the lapland savo orogen collision between the archaean continent lower plate and the palaeoproterozoic arc microcontinent assembly upper plate produced a special type of lateral crustal growth the archaean continental edge decoupled from its mantle during initial collision and overrode the arc and its mantle during continued collision

early proterozoic crust mantle interaction at a continental margin in northern sweden abstract on the basis of recent geochemical and sm nd isotopic data alternative models for the early proterozoic evolution of the archaean craton of the baltic shield and its marginal areas in northern sweden are discussed before the deposition of a craton cover and the subsequent svecofennian orogenic activity the archaean areas probably formed a continuous domain the 1 89 1 86 ga old early svecofennian granitoids found close to exposed archaean rocks were created largely by the remobilization of archaean crust to the south of the archaean domain however chronologically equivalent granitoids are made up of matter with short crustal residence times that area the skellefte district represents a new addition to the continental crust of the baltic shield 1 90 ga ago the particulars of the crustal accretion process are still controversial however recent data indicate different tectonic developments in the skellefte district and in the vicinity of the archaean proterozoic boundary farther north apparently the one time edge of the archaean craton was situated somewhere between these two areas but appears to have had a rather more northerly position than previously assumed

magnetic measurements used to identify paleoxidation fronts in deep sea sediments from the madeira abyssal plain we describe the results of rock magnetic measurements of a sediment core from the madeira abyssal plain which contains multiple turbidites previous studies of trace element e g u concentration and observations of sediment color changes have shown that turbidite emplacement has resulted in reductive diagenesis and the formation of paleoxidation fronts we show that variations in sediment lithology and postdepositional sedimentary processes can be characterized very sensitively using magnetic measurements low concentrations of magnetic minerals occur in turbidites with a high organic content reflecting the destruction of magnetic material via the process of reductive diagenesis in addition fossil redox fronts are readily identified by rock magnetic properties

early diagenesis in an organic rich turbidite and pelagic clay sequence from the cape verde abyssal plain ne atlantic magnetic and geochemical signals abstract processes of early diagenesis in sediments from the cape verde abyssal plain are investigated using the complimentary techniques of environmental magnetism and solid phase geochemistry the sequence captured by a 2 m long box core comprises an organic rich turbidite derived from the nw african continental margin emplaced ca 330 ka bp overlain by brown pelagic clay diagenesis in the pelagic clay mainly involves slow in situ oxidation of the iron oxide phases present which mossbauer effect data suggest are detrital haematite and magnetite possibly maghaemitised in the turbidite diagenesis has proceeded in two stages the first involving suboxic reductive processes following depletion of porewater o2 and the second involving oxidative processes associated with the slow descent of an oxidation front through the bed due to diffusion of o2 from the overlying seawater carbonate free magnetic parameters interparametric quotients and remanence acquisition demagnetisation and coercivity measurements of the core reveal that significant depletion of detrital ferrimagnetic iron fe2 fe3 oxide grains like titano magnetite has occurred within the turbidite during suboxic diagenesis and confirm previous suggestions that reductive dissolution of such components is a grain size selective process however the higher oxidation state fe3 canted antiferromagnetic iron oxide grains like haematite and goethite are much less susceptible to reductive diagenesis remobilisation of transition metal ions from below the oxidation front in the turbidite has led to authigenic precipitation of ferrimagnetic fe mn micronodules in a distinct layer immediately overlying the front and above this a further interval of mn enriched diagenetic laminae however within the suboxic organic rich zone of the turbidite well below the oxidation front there is clear evidence for the presence of a significant concentration of fine single domain ferrimagnetic iron oxide grains which are most probably associated with a population of live magnetotactic bacteria between this zone and the oxidation front there is a transitional interval in which the number of live magnetotactic bacteria declines rapidly and fossil magnetosomes in the sediment are being reductively dissolved progressively up sequence these results are broadly similar though differing in some respects to magnetic and geochemical studies of hemipelagic and high productivity pelagic sediments in which mn and fe redox boundaries have been recognised and further illustrate the value of environmental magnetic parameters for characterising early diagenetic processes in sediments of this kind

rates of sedimentation and stratigraphical completeness the most obvious result of sedimentation processes is the change in the thickness of accumulated sediment with time whenever the net change over a period of time is positive a record is left of a sedimentation system s existence a stratigraphical section for the stratigrapher the problem that such a section poses is to determine the detailed correspondence of thickness to time1 for the sedimentologist the problem is to infer the nature of the parent sedimentation system and to estimate its parameters a simple stochastic model is used here 1 to illustrate some basic stratigraphical results of clastic sedimentation processes 2 to offer an explanation for the commonly cited systematic decrease of mean sedimentation rate with increasing time span and 3 to assess the concept of stratigraphical completeness it will be assumed that all post depositional non erosional processes such as compaction can be identified and their effects on stratigraphical thickness allowed for

early proterozoic continental tholeiites from western bergslagen central sweden iii geodynamic inferences abstract the 1 88 ga old within plate continental tholeiites belong to the mainly metavolcanic bimodal bergslagen supracrustal series bss in the svecofennian domain of the baltic shield the bss evolved in an intracontinental rift despite only moderate lithospheric extension the tholeiites are the near surface expression of a 10 20 km thick magmatic crustal underplate that probably underlies most of the sw svecofennian domain and caused the svecofennian crust to be three layered thick but yet isostatically stable underplating strengthened the lithosphere sufficiently to control the time of cratonization geodynamic models for the origin of the bss consistent with our interpretations are gravitational collapse of an overthickened continental accreted terrane margin or back arc processes related to a marginal volcanic arc 250 km further south a tentative reconstruction of relative plate motions during svecofennian time indicates predominantly longitudinal motion of the plates in question and complies with the relatively slow latitudinal drift of the shield at the time recent ion microprobe u pb ages of proterozoic zircons in svecofennian metasediments imply about 0 2 ga of pre svecofennian crustal evolution while the nd model ages of the most primitive metatholeiites may be indicative of initial mantle differentiation as early as 2 4 ga bp accompanying protocrust formation

the svecofennian orogen a tectonic model abstract recent geological studies have provided new concepts concerning the timing of the final break up of the archaean craton age data on detrital zircons from sedimentary rocks in sweden and finland show marked similarities and indicate pervasive homogenization of accumulated material geochemical and isotopic studies in sweden and finland suggest the existence of two volcanic belts separated by an area dominated by sedimentary rocks however the ages of metamorphism and plutonism differ in particular the late post 1 85 ga granitoid magmatism was more voluminous in sweden than in finland the svecofennian crust in finland is anomalously thick up to 65 km and is characterized by a high velocity lower crust of variable thickness the evolution of the svecofennian orogen is modelled by opening of the pre svecofennian ocean at 1 95 ga and progressive accretion of two arc complexes to the archaean craton between 1 91 1 87 ga initial accretion at 1 91 ga led to thrusting of the slices of an island arc complex and proterozoic marine strata onto the thickening craton margin subduction reversal with simultaneous anti clockwise rotation of the converging plate or convergence of another plate resulted in subduction below the newly accreted arc complex the tectonic style at the craton margin changed from thrusting to lateral displacement in the east in finland in the west in sweden the subduction zone retreated along a transform fault a large sedimentary basin closed in the east 1 89 ga ago when another arc complex collided against the accreted one collision resulted in lithospheric thickening subsequent delamination of the thickened lithosphere and compensation by a hot mantle underplate and large scale granitoid magmatism as a result of mixing between mantle magma and remelted arc complex material the thick dense layer that formed in the lower crust as the result of underplating kept the thickened crust in isostatic balance thus the exhumation rate in this area was similar to the rate in areas of thinner crust subduction and sediment accumulation continued in the west until at least 1 87 ga when large areas in the east were already stabilized an intracratonic transpressional zone developed as the result of continued convergence subsequent extensional collapse along the transpressional zone caused melting of svecofennian sedimentary rocks migmatization and generation of the 1 84 1 82 ga anatectic granites of southern finland and central sweden the voluminous post 1 85 ga granitoid magmatism in sweden is tentatively associated with an east west extension that is not related to the extensional collapse

faulting in madeira abyssal plain sediments abstract interpretation of high resolution seismic data revealed the occurrence of faults in the sediments of two north atlantic abyssal plain environments the madeira abyssal plain map and the nares abyssal plain nap most of the data are from the map where faulting appears to be confined to particular locations from our data individual faults cannot be traced from one seismic section to another a positive correlation exists between the concentration of faults and the maximum sediment thickness although the mechanism that caused the faulting is still not understood the faults apparently are related to processes which have been active for a long period of time

early diagenesis in north atlantic abyssal plain sediments characterized by rock magnetic and geochemical indices processes of early diagenesis are investigated in a sequence of late quaternary turbiditic sediments from the madeira abyssal plain map west of the nw african continental margin in three sediment cores from this region solid phase geochemical indices of redox sensitive ion mobilization are compared with rock magnetic parameters as proxies for diagenesis characterization the mud dominated distal turbidites are emplaced mainly as a result of glacio eustatic changes in sea level and derive both from different sites on the nw african margin and from the flanks of the canary islands thus the turbidites are of variable composition especially in terms of carbonate detrital magnetic mineral and organic carbon content within organic rich 0 5 corg turbidite horizons remineralization of organic detritus proceeds in two stages the initial stage involves suboxic reductive diagenesis following depletion of pore water o2 while the later stage involves oxidative diagenesis associated with the slow descent of an oxidation front through the sediment due to diffusion of o2 from the overlying seawater rock magnetic parameters of these sediments when expressed on a carbonate free basis reveal that significant depletion of detrital ferrimagnetic iron fe2 fe3 oxide grains has occurred within the organic rich turbidites during suboxic diagenesis normalized quotients of magnetic parameters and remanence coercivity profiles of selected samples also show that reductive diagenesis is a ferrimagnetic grain size selective process but it has a minimal effect on the canted antiferromagnetic fe3 oxides in the sediment such components if present therefore become relatively enriched in magnetic assemblages as the ferrimagnetic grains are progressively dissolved in all three cores studied however there is clear evidence for the presence of ultrafine ferrimagnetic iron oxide grains at depth within the suboxic zones of organic rich turbidites beneath both active and fossil oxidation fronts these grains are probably associated with populations of live magnetotactic bacteria which inhabit such organic rich horizons and which form a link in the chain of bacterially mediated reactions normally associated with suboxic diagenesis these results comply with previous observations made in studies of suboxic diagenesis in hemipelagic sediments and demonstrate the value of rapid and non destructive rock magnetic measurements for illuminating redoxomorphic processes of diagenesis involving transformation of iron phases in deep sea sediments

rock magnetic characterization of early redoxomorphic diagenesis in turbiditic sediments from the madeira abyssal plain rock magnetic measurements of two sediment cores from the madeira abyssal plain map north atlantic are used to investigate post depositional changes in the concentration grain size and composition of magnetic minerals in the sediments that have occurred within organic rich turbidite horizons the changes are associated with an initial stage of suboxic reductive diagenesis following depletion of porewater o2 and a later stage of oxidative diagenesis associated with the slow descent of an oxidation front through the sediment as a result of diffusion of o2 from the overlying sea water the turbidites are of late quaternary age 18o stages 1 3 and derive both from different sites on the nw african continental margin and from the flanks of the canary islands thus the turbidites are variable compositionally especially in terms of carbonate detrital magnetic mineral and organic carbon content diagenetic changes in these sediments have been identified using solid phase geochemical data u mn corg and caco3 reported previously in more than one study rock magnetic parameters of the sediments when expressed on a carbonate free basis reveal that significant depletion of detrital ferrimagnetic iron fe2 fe3 oxide grains has occurred within organic rich turbidites during redoxomorphic diagenesis normalized quotients of magnetic parameters also show that reductive diagenesis is a ferrimagnetic grain size selective process but it has a minimal effect on the canted antiferromagnetic fe3 oxides in the sediment such components if present therefore become relatively enriched in magnetic assemblages as the ferrimagnetic grains are dissolved progressively and bulk magnetic concentration is thus depleted there is clear evidence in both cores for the existence of ultrafine ferrimagnetic grains at depth within the suboxic zone of the organic rich turbidites beneath both active and fossil oxidation fronts these grains are most probably associated with populations of live magnetotactic bacteria which commonly inhabit such organic rich horizons and play a part in the chain of bacterially mediated reactions normally associated with suboxic diagenesis these results show that simple and rapid rock magnetic techniques can be used to characterize early diagenetic processes involving iron phases in deep sea sediments at least as effectively as more laborious time consuming and sample destructive geochemical measurements

microseismic monitoring and stability analysis of the left bank slope in jinping first stage hydropower station in southwestern china risks associated with the left bank slope of jinping first hydropower station in southwestern china are growing as a result of excavation of the dam foundation ditch and various tunnels as well as grouting to soft strata to improve our understanding of the slope instability and resolve the complex subsurface conditions of the highly fractured rock mass a microseismic monitoring system was installed in june 2009 which is the first time such a technique has been introduced into stability analysis of a high steep rock slope in china the microseismic monitoring system consists of a data acquisition unit a data processing unit and a three dimensional array of uniaxial accelerometers in order to analyze the correlation between seismicity and potential slip surface of the slope a numerical model was implemented to analyze the failure processes and stability of the slope the monitoring results illustrate that microseismic events mainly occur as a strip along the spandrel groove of the dam which coincides with the potential slip surface obtained from numerical analysis especially at the bottom of the slope therefore the comprehensive method incorporating microseismic monitoring and numerical analysis has been proven to be very promising in instability prediction of rock slope subject to identification and delineation of potential slip surface and failure regions

calcium carbonate preservation patterns in the central north atlantic during the last 150 000 years abstract the dissolution record of the canary and eastern north american basins was examined in 15 cores spanning the last 150 000 years stratigraphic control was based on fluctuations in percent carbonate and 18 0 16 0 dissolution changes were determined based on variations in percent planktonic foraminiferal fragments and benthonic foraminifera results indicate three main dissolution events coincident with 18 0 stages 2 4 and 6 the lysocline apparently shallowed about 300 m in the canary basin at these times there is no simple linear relationship between dissolution intensity and ice volume dissolution was more intense during stage 4 than during stage 2 even though 18 0 records indicate the reverse is true for ice volume also carbonate preservation during the lower part of stage 3 is comparable to stages 1 and 5e a comparison of atlantic pacific carbonate dissolution patterns indicates a tendency for out of phase preservation patterns in 18 0 stages 1 and 2 and an in phase dissolution event during stage 4 the out of phase pattern may partially reflect variations in the production rate of north atlantic deep water the in phase dissolution peak correlates with a major cooling regression transport of organic carbon from transient shallow reservoirs into the deep sea could have contributed to formation of such a pattern however additional evidence from longer atlantic pacific records is needed before the shelf basin model can be more fully evaluated

determination of sediment volumes accumulation rates and turbidite emplacement frequencies on the madeira abyssal plain ne atlantic a correlation between seismic and borehole data the sedimentary infill history of the madeira abyssal plain map is established from correlation of odp leg 157 drillsites sites 950 952 with an almost regular grid of 7000 km of intermediate resolution seismic reflection profiles covering the central part of the abyssal plain the most conspicuous seismic reflectors bounding the seismostratigraphic units have been identified and mapped correlation between seismic and borehole data using synthetic seismograms allows the lithological attribution and dating of the reflectors and seismostratigraphic units lateral mapping and correlation of seismic units also allows both the volumes and rates of accumulation of sediments within each seismostratigraphic unit and equivalent time periods of deposition to be determined these calculations have been corrected for the effect of compaction calculated at around 40 at the base of the drillholes three main turbidite types have been identified at the drillsites and their emplacement frequency has been calculated for each site and time period our results show that cretaceous oceanic crust was draped with red pelagic clays and the fracture zone valleys were completely infilled and levelled in a geologically rather short time probably during the latest oligocene and early miocene by organic rich turbidites derived from the nw african continental margin at 16 ma the topography was levelled enough to allow large turbidity current flows to cover the entire plain during the middle and late miocene 16 5 9 ma organic rich turbidites were emplaced on the abyssal plain at a low rate of accumulation 12 m my in the uppermost miocene early pliocene 5 9 3 6 ma turbidite emplacement increased markedly in both frequency and accumulation rate e g 26 m my for organic rich turbidites during this time period emplacement of volcanic rich turbidites also increased in volume and frequency a trend that continued into the pliocene increased volcanic rich turbidite emplacement correlates well with increased volcanic activity on the canary islands and increased organic rich turbidite emplacement may correlate with periods of erosion on the nw african continental margin these erosional periods may be related to global cooling and falling sea level intensification of bottom water currents and enhanced upwelling on the margin

early proterozoic continental tholeiites from western bergslagen central sweden ii nd and sr isotopic variations and implications from smnd systematics for the svecofennian sub continental mantle abstract a subset of nine least altered least fractionated within plate continental metatholeiites from western bergslagen yields a sm nd isochron age of 1886 48 ma with an nd t of 1 1 this age is in agreement with previous u pb zircon dating of a coeval felsic intrusion variation of nd t in other subsets is shown to be caused by crustal contamination of the basic magma to explain the remarkable isotopic homogeneity of the least altered least fractionated metatholeiites and their combined lree enrichment and isotopic depletion three petrogenetic models are discussed the first model involves incipient melting of a depleted mantle in the garnet stability field the second model presumes complete homogenization of a depleted mantle derived melt contaminated by older continental crust and the third model proposes melting of an enriched mantle source in light of the depleted nature of the baltic asthenosphere absence of enriched archaean crust in the bergslagen area negative hfse zr hf ti anomalies in the metatholeiites and enriched isotopic characteristics of lamprophyres occurring 50 km to the west model three is preferred in a tentative scenario three time scales are inferred from sm nd isotope systematics to generate the metatholeiites 1 long term 1 ga lree depletion of the asthenosphere by extraction of continental crust 2 shorter term pre 1 88 ga bp lree enrichment of this depleted mantle by supra subduction zone modification of the mantle wedge during protocrust formation and 3 short term around 1 88 ga bp generation of basic magma by melting of this enriched mantle as a result of continental rifting of the early proterozoic protocrust

isotopic and geochemical constraints on the evolution of the 1 93 1 79 ga svecofennian crust and mantle in finland the central part of the svecofennian domain in finland is characterized by thick lithosphere with crustal thickness up to 60 65 km major collisional events at 1 91 1 90 ga and 1 89 ga and a thrusting event at 1 86 1 84 ga the latter due either to continuation of shortening or to a separate intracrustal collision are considered the major causes of crustal thickening the origin of the svecofennian domain has been formerly attributed to mixing of depleted mantle melts with variable amounts of archaean crustal component via subduction but the new data and reinterpretation of older data imply that besides juvenile crust the svecofennian domain also contains older palaeoproterozoic crustal components no archaean component is found in the 1 93 1 91 ga gneissic tonalites and related felsic volcanics end t 1 4 which occur in the savo belt sb adjacent to the archaean craton the depleted mantle model ages vary from 1 94 ga to 2 33 ga with most values 2 0 ga the analysed syntectonic post thickening granitoids 1 89 ga in the sb exhibit positive end t values ranging from 2 8 to 3 0 pointing to a juvenile source the hypersthene granitoids are similar in age but show epsilon values around 1 which are only slightly lower than found in em derived mafic rocks the isotopic and geochemical data for the central finland granitoid complex and tampere schist belt indicate the occurrence of evolved thick crust 2 0 ga and associated lithospheric mantle already at 1 91 ga the southernmost part of the southern svecofennian in finland also shows evolved type crust 2 0 ga a tentative model for the development of crust and mantle is presented it includes the occurrence of continental nucleus surrounded by more juvenile island arc accretions two successive collisional events thickened both the crust and the subcontinental lithospheric mantle with the greatest relative amount of thickening occurring in the mobile zones convective removal of the lowermost parts of thickened lithosphere caused an upwelling of hot asthenosphere increase in the geotherm and melting of both the enriched subcontinental lithospheric mantle and the convective depleted asthenosphere mafic magmatism during the post thickening stage 1 89 ga thus varied from dominantly enriched to dominantly depleted mantle melts and was associated with variable crustal contamination the thickening of the lithosphere in the central part of the svecofennian domain was not followed by large extension suggesting that the thickness was already great 1 88 1 80 ga ago this together with the thick high velocity lower crust could have provided the isostatic balance needed to stabilize the thick crust

geochemical composition and inferred accretion rates of sediments and managanese nodules from a submarine hill in the madeira abyssal plain eastern north atlantic abstract the porewater and sediment composition of two boxcores and of a small gravity core taken on a manganese nodule covered hill and in the madeira abyssal plain proper respectively are compared the pore water study of the two boxcores indicates that oxic conditions prevail in both cores in addition it indicates that no detectable fluxes of mn or fe occur from the porewater to the ocean bottom water variations in the geochemical composition of the sediments can be explained by fluctuations in the amount of carbonate which acts as a diluting agent a clear carbonate minimum is observed at 20 22 cm depth in the two cores this minimum is likely to be associated with the last glacial period 10 20 kyr b p this association is supported by the sediment accumulation rate of 15 mm kyr as found by extrapolation from the rate for pelagic sediments in the madeira abyssal plain the bulk composition of the manganese nodules recovered from the submarine hill is chemically almost identical to the average composition of atlantic nodules the trace metal and rare earth elements composition indicate a hydrogenous origin for the manganese nodules of this study on the basis of the chemical composition and that of nodules relative to that of the adjacent sediments an average nodule accretian rate of 2 8 3 3 mm myr has been calculated although the analyses of the entire ferromanganese nodules that have been studied seem to indicate a homogenous composition internal structures of the nodules reveal great inhomogeneity both visually and chemically these fluctuations may be related to variations in the fluxes of mn and fe which in turn could be climate related

nature of a major tectonic discontinuity in the svecofennian province of northern sweden abstract the volcanic rocks of the skellefte ore district ca 1 89 1 87 ga like other volcanic complexes of similar age in the svecofennian orogenic province have been regarded as juvenile arc material developed over a contemporaneous subduction zone and subsequently accreted to the continent during a period of very rapid crustal growth a modelled geoelectric anomaly dipping ne beneath the district from the southern province has provided support for the subduction collision hypothesis we have used recently acquired geological and geophysical evidence to suggest an alternative to this model regional aeromagnetic and gravity data indicate that the skellefte district is the site of a major tectonic discontinuity the province to the north of this discontinuity provides evidence of an archaean basement to within 100 km of skelleftea the skellefte group on the southern margin of this province displays a narrow belt of strong upright folding trending w to wnw we correlate this folding with the second major deformation episode d 2 of similar trend in the metamorphic complex of the southern province here informally called the robertsfors group the family of d 2 structures on both sides of the province boundary defines the 80 km wide skellefte zone d 2 in the robertsfors group consists of discrete ductile shear zones and associated upright folds these zones are consistently steep display southside up kinematics and are superposed on earlier ne trending folding d 1 and migmatisation in the robertsfors group we infer from the style of d 2 that it could not be responsible for the accretion of an exotic volcanic arc to the continent upper crustal thickening was probably largely achieved during d 1 we conclude from the style and orientation of the d 1 structures in the robertsfors group their absence from the skellefte group and their relation to granitoid intrusions that d 1 pre dates deposition of the skellefte group we infer that the robertsfors group forms part of the previously unknown basement of the skellefte group the latter is therefore regarded as a supracrustal sequence deposited in a rift setting rather than a juvenile volcanic arc it is also concluded that the basement stratigraphic package which also occurs to the southwest of the skellefte ore district and has variable internal structures is responsible for the modelled geoelectric anomaly we therefore interpret the tectonic discontinuity as the boundary between pre and post 1 9 ga rock groups both of which have been intruded by post 1 9 ga granitoids and affected by the localised upright d 2 post 1 87 ga deformation in the skellefte zone the volcanism of the skellefte ore district and the ni bearing mafic intrusions of the southern province are attributed to an episode of extension between d 1 and d 2 which was probably related to a contemporaneous active margin lying west of the present exposure of svecofennian rocks d 1 in the robertsfors group is attributed to the closure of an extensive marginal basin possibly involving subduction prior to skellefte group deposition i e before 1 9 ga geochronological data from the robertsfors group on the ages of d 1 and d 2 and the regional significance of pre 1 9 ga metamorphic complexes are discussed in a companion paper

granites and early proterozoic crustal evolution in sweden evidence from smnd upb and o isotope systematics abstract u pb zircon and whole rock rb sr sm nd and o isotope results are reported for granitoids from three proterozoic provinces in sweden that represent different crustal environments the radiometric ages span the period 1 89 1 65 ga and record events ranging from original igneous crystallisation to late metamorphic resetting these events occurred at different times in the crustal segments the o isotope ratios identify different crustal source materials for the granitoids the sm nd results indicate that the major crustal source components common to most of the granitoids formed shortly before 1 9 ga close in time to the formation of the earliest granitoids contributions from recycled archaean basement are minimal the geographically widespread occurrence of such crustal sources indicates that the period around 2 ga was of great importance for the growth of new continental crust in sweden

shale compaction and statistical physics summary after a short historical outline of the development of the ideas on the compaction of clays and shales it is shown that the exponential porosity depth dependence of compacted shales expressed by athy s law can be derived using standard methods of statistical physics the main result of the paper states that the exponential compaction law expresses the maximum entropy equilibrium state of the pores in the rock that is compaction is an irreversible process where clay particles tend towards a statistically defined final equilibrium connections with the classical theory of consolidation are pointed out

origin of continental crust of 1 9 1 7 ga age nd isotopes in the svecofennian orogenic terrains of sweden initial nd isotopic ratios are reported for 57 samples representing all crustal components in the central and southern parts of the 1 9 1 7 ga old svecofennian orogenic terrains of sweden and the 1 8 1 7 ga old transscandinavian granite porphyry belt transscandinavian igneous belt u pb zircon ages are determined where the magmatic ages were not sufficiently constrained the nd isotopic results show that there are no archaean crustal blocks within the studied 1 9 1 7 ga old crustal segment this is consistent with results from finland the main input of recycled archaean crust to the svecofennian was in the form of sediments which are concentrated in the central svecofennian subprovince of north central sweden this is consistent with the vicinity of this subprovince to the archaean craton of the baltic shield these turbidite sediments contain up to 40 archaean material but s type granitoids associated with them have a smaller archaean component around 20 most of the quantitatively predominant i type granitoids and volcanics as well as all the basaltic rocks studied have nd from 1 to 3 and consist of 90 material newly derived from the mantle 1 9 1 7 ga ago we argue that the archaean material was introduced in the form of sediments but we cannot distinguish between pelagic sediments delivered on subducting oceanic crust and turbidite accumulations like the ones seen in north central sweden in either case an overall 15 archaean contribution for the swedish svecofennian was reasonably well homogenized with mantle derived magmatic products for this to occur an active near surface tectonic environment such as a complex of subduction zones must have been necessary

age of the barramundi orogeny in northern australia by means of ion microprobe and conventional u pb zircon studies abstract u pb zircon geochronological studies are used to quantify the tectonostratigraphic history of a major crust forming event in early proterozoic terranes of northern australia magmatism and tectonic processes associated with this event the barramundi orogeny appear similar over very wide areas about one third of the australian continent the barramundi event marks the deformation metamorphism and cratonization of the earliest proterozoic 2 0 1 9 ga basinal successions and it was followed by and closely linked to extensive orogenic felsic magmatism the yaringa metamorphics are an inlier of paragneiss and schist that pre dates 1865 3 ma leichhardt felsic magmatism and younger basin formation in the mount isa inlier from an ion microprobe u pb zircon study of massive and migmatitic gneiss the time of yaringa metamorphism is established at 1890 8 ma recognized younger proterozoic deformations in the interval 1610 1550 ma are generally not evident in zircon u pb systems but have totally reset local rb sr whole rock values conventional u pb work on multi grain zircon samples of yaringa gneiss gives a geologically meaningless age 2090 40 ma that is an artifact of complex zircon growth part of which was initially inherited from a 2 55 to 2 2 ga provenance and the other major part of which was formed in situ during the 1890 ma high grade metamorphism predictably for simple zircon systems in low grade metavolcanic terranes conventional and ion microprobe zircon ages are in close agreement dating of volcaniclastic sequences above and below the barramundi tectonic break in the pine creek inlier brackets the orogeny in this terrane to a similar time frame 1885 1870 ma some zircons which crystallized at this time form selvedges around older cores whose age of 2100 2000 ma may be relics of an earlier deep crustal underplating the uniform chemistry of post barramundi felsic magmatism and the similar tectonic setting of the areally extensive barramundi event are mirrored by their temporal coherency cycles of geosynclinal deposition burial metamorphism and uplift took place during intervals of several million years or less implying comparable rates for early proterozoic orogenesis to those established for tertiary and quaternary settings

bimodal early proterozoic volcanism in the grythytte field and associated volcano plutonic complexes bergslagen central sweden the grythytte field sequence forms a nnw belt of acid basic metavolcanic and metasedimentary rocks the geology shows characteristics of a dynamic process of trough forming and infilling which to a large extent post dates the hjulsjo volcanoplutonic complex situated to the east in the latter complex acid metapyroclastics strike around a centre of granite and granophyre which cut and are cut by basic sills and dykes field and petrochemical evidence points to a distinct bimodal magmatism that ist manifested during one short phase of a rifting period

evidence for early proterozoic plate tectonics from seismic reflection profiles in the baltic shield plate tectonics provides the linking framework for all tectonic and magmatic activity seen today but it is not known when plate tectonics first developed on earth new deep seismic reflection and

deposition of ungraded muds from high density non turbulent turbidity currents turbidity current deposits of mud have recently been found up to 10m in thickness resulting from single depositional events1 2 turbidites 1 5 m thick from the madeira abyssal plain3 4 display long segments with little vertical change of grain size5 6 this suggests that the currents froze in some way making them unable to sort sediment here we suggest that a decline in flow speed of these currents led to a concentration gradient in silt close to the bed that was sufficiently steep to stratify the flow a concentration gradient of 18 kg m 3 over 1 5 m height gives a layer richardson number above the critical value of 0 25 for damping of turbulence we propose that with turbulence severely damped a dense suspension of fine material consolidates like a static suspension while continuing to flow downslope the current changes from a fully turbulent flow to a decelerating flow with suppressed turbulence and increasing density in the latter state interparticle forces prevent differential settling of the coarser grains

correlation frequency of emplacement and source directions of megaturbidites on the madeira abyssal plain abstract this paper describes the sediments of the whole madeira abyssal plain including the madcap area to the south previous studies have centred on the central sub basin which occupies most of the plain and which has become one of the most studied areas of seafloor the plain is made up of a series of large turbidites separated by thin pelagic layers which can be used to provide a high resolution stratigraphy back to about 750 000 years many of the turbidites were deposited close to oxygen isotope stage boundaries suggesting a strong relationship to sea level climate change although the exact initiation mechanism for the flows is not known data from the northern and southern extensions of the plain shows the effects of different entry points for the four main turbidite groups volcanic rich turbidites enter from the ne and e calcareous turbidites from the w one group of organic rich turbidites enters from the se and one group from the ne and e different turbidite sources operate for limited periods before switching to one of the other source areas it is very rare for two sources to provide turbidites at the same isotope stage boundary

implications of reduced turbidity current and landslide activity for the initial eocene thermal maximum evidence from two distal deep water sites previous studies propose that submarine landslides and turbidity currents may become more likely due to future rapid global warming determining whether global warming increases likelihood assists in assessment of landslide triggered tsunami hazards and risk to seafloor structures other studies propose that landslides helped to trigger past rapid climate change due to sudden release of gas hydrates two deep water turbidite records show prolonged hiatuses in turbidity current activity during the initial eocene thermal maximum ietm at 55 ma the ietm represents a possible proxy for future anthropogenically induced climate change it is likely that our records mainly represent large and fast moving disintegrative submarine landslides statistical analysis of long term 2 3 myr records shows that turbidity current frequency significantly decreased after the ietm our results indicate that rapid climate change does not necessarily cause increased turbidity current activity and do not provide evidence for landslides as a primary trigger for the ietm

some new lead isotope determinations from the proterozoic sulfide ores of central sweden lead isotope determinations were made on galenas from three strata bound sulfide ores in the early proterozoic svecokarelian bergslagen district of central sweden and four epigenetic deposits in the alvdalen and vansbro districts in the early to middle proterozoic post svecokarelian belt the leads from the strata bound bergslagen deposits show exceptional isotopic homogeneity over large areas their isotopic composition suggests the existence of a pre svecokarelian crust in the district and is consistent with exhalative sedimentary ore formation in an active continental margin environment the vansbro and alvdalen leads display constant compositions within each district but marked divergence between the districts their compositions preclude derivation exclusively from recycled svecokarelian lead and suggest a substantial lead contribution from a mantle like source the difference between model ages and geologic ages for many of the deposits with a small but significant excess of radiogenic lead suggests a significant deviation of the fennoscandian shield from conventional global lead evolution models

early proterozoic continental tholeiites from western bergslagen central sweden i petrology geochemical petrogenesis and geotectonic setting abstract the early proterozoic bergslagen supracrustal series bss of western bergslagen consists of a pile of felsic volcanites and intercalated carbonates with in the upper part an alternating succession of volcaniclastics and basic flows this pile is intruded by a bimodal sequence of granites and basic sills and dikes major and trace element data reveal the influence of several processes that have acted upon the basic magma crustal contamination or supra subduction zone ssz modification fractional crystallization and potassic and sodic alteration due to sub seafloor processes are recognized using geochemical and petrological criteria the basic rocks are divided in two least altered la and two alkali altered aa sets geochemically the least altered basalts resemble phanerozoic continental within plate tholeiites and thus contradict the interpretation by many previous authors of bergslagen as an active margin volcanic arc alkali alteration leads to pseudo calc alkaline or pseudo alkaline signatures however a ssz component observable in part of the la set indicates a relation in space and time to a convergent margin

origin sedimentary processes and depositional evolution of the agadir turbidite system central eastern atlantic the oligocene to recent agadir turbidite system occurs in the agadir basin abutting against the continental margin off south morocco central eastern atlantic this system is part of the submarine drainage system linking back to the anti and high atlas mountains through the agadir canyon and its sedimentary evolution is shown to comprise lateral shifts of canyon channel overbank and lobe deposits governed by local topography draped palaeorelief seamounts and a contourite ridge and channel avulsions in addition the turbidite system shows an overall retrogradational stacking migrating about 60 km upslope this probably reflects a decrease in gradient on the continental margin gradual basin filling and or a progressive decrease in energy and load of the sediment gravity flows that fed the system the latter change in sediment flux could result from the interplay between the progressive denudation of the source areas together with the climatic change which led to the present arid desertic conditions in the hinterland the seismic features observed with new ultra high resolution seismic tools allow the recognition of the present day sedimentary activity within the agadir turbidite system and this is related among other factors to the active seismicity of the area

rb sr and sm nd geochronology of lower proterozoic granite greenstone terrains in french guiana south america abstract nine samples of metavolcanic rock from the lower parts of greenstone belts in central french guiana the paramaca series and 14 granitic samples from the intrusive gneisses the degrad roche and arawa gneisses were selected for sm nd and rb sr analysis the sm nd results from the metavolcanic series including two tholeiites five peridotitic komatiites and two andesites yield an isochron age of 2 11 0 09 2 ga with an initial 143 nd 144 nd ratio ind of 0 51002 9 2 corresponding to nd t 2 1 1 8 this isochron is interpreted as representing the age of initial volcanism of the paramaca series acid intrusives were dated by the rb sr method a whole rock rb sr isochron including data points from both the degrad roche and arawa gneisses yields an age of 2 00 0 07 2 ga with initial 87 sr 86 sr ratio isr value of 0 7019 4 2 this result is considered to be the time of emplacement of the orthogneiss protoliths the positive end value 2 1 1 8 obtained from the metavolcanic rocks of french guiana suggests that their mantle sources have evolved in reservoirs slightly depleted in light rare earth elements lree this result confirms the possible existence of ancient lree depleted reservoirs within the lower proterozoic mantle moreover the high end t value for these rocks excludes any significant crustal contamination during magma genesis the french guianese orthogneisses yield a low isr value 0 7019 4 2 which together with geochemical considerations suggests that their granitic protoliths could have originated by partial melting of short lived crustal precursors of basaltic to granodioritic composition the present geochronological and isotopic study suggests that the guiana shield may represent a major continental accretion event during the lower proterozoic

the ree and end of 40 70 ma old fish debris from the west african platform ree concentrations and nd isotopic compositions have been determined in late cretaceous early cenozoic fish debris from west african and israeli platform sediments and show a strong regional control independent of time except for those from the northern morocco it is suggested that although diagenetic fluids do not contribute ree s directly to the phosphatic debris they may significantly change the ce anomaly and the end t value of the epicontinental seas which in turn control the ree in marine phosphates it is shown that over the investigated period the north moroccan shelf seawater was progressively flushed by deep north atlantic water

determination of turbidity current erosional characteristics from reworked coccolith assemblages canary basin north east atlantic turbidites contain mixtures of sediments of various ages these sediments can include both material that was eroded to form the initial turbidity current plus additions derived from erosion of the sea floor during transport it has been possible to interpret the age range of sediments incorporated into individual turbidites from the madeira abyssal plain by examining the proportions of microfossil coccolith species that they contain the pelagic record of coccoliths is well known for the quaternary period and shows a succession of dominant species or acmes each lasting a few tens of thousand years hence erosion of sediment representing more than a few tens of thousand years will produce coccolith mixtures not seen in the pelagic record but dependent upon the age range of the sediments that were eroded this age range can be estimated by comparison with synthetic ratios of coccolith species created by combining ratios of species from successively older layers in the pelagic record these can then be compared with the ratios found in individual turbidites until a match is found the results show age ranges of 54 500 kyr for the sediment mixture in seven turbidites from the madeira abyssal plain since the volumes of these turbidites are also known and accumulation rates in their source areas can be estimated it is possible to determine both the thickness and the area of the eroded sediment mass that generated the turbidity current minimum depths of erosion on the north west african continental margin vary from 8 to 50 m and minimum areas eroded from 1600 to 5800 km2 none of the turbidites examined contains a significant excess of surface sediment suggesting that once formed the turbidity currents that transported them were virtually non erosional and that they travelled several hundred kilometres in this state

passive seismic monitoring of mine scale geothermal activity a trial at lihir open pit mine the lihir open pit mine in papua new guinea is located inside an old volcano where geothermal activity is strongly present outbursts of hot water and steam into the mining areas were a major safety concern passive seismic monitoring was carried out at the mine to investigate whether the geothermal activities could be detected and located using microseismic techniques in a mining environment in this trial sixteen triaxial geophones which can withstand temperature up to 200 c were used and installed in four deep boreholes inside the pit the microseismic events were discriminated using the sta lta triggering criterion during 6 weeks of monitoring more than 17 000 events were recorded approximately 12 of the events showed harmonic vibration characteristics similar to those observed in other geothermal and volcanic areas suggesting that the geothermal activity inside the pit was captured by the microseismic monitoring system more than 75 of the events present both p and s waves and they were interpreted to be associated with rock fracturing due to stress release near the bottom of the pit many geothermal type events were located in areas where shear events occurred implying that the detected geothermal events were not far from the mining area below the pit and they may also be associated with mining the borehole installation of the geophones significantly reduced the interference of mining noise and achieved good observation of the seismic events however equipment installation requires great attention as the geophones may be destroyed due to unexpected rising temperature within the boreholes

ore deposition in a proterozoic incipient rift zone environment a tentative model for the filipstad grythyttan hjulsjo region bergslagen sweden the 1 9 1 8 ga bergslagen supracrustal series comprises an early volcanic stage represented by the lower leptite group an initial rift stage by the middle leptite group a rift stage by the upper leptite halleflinta and slate group metabasites and the granite granophyre suite and a post rift stage by conglomerate beds remobilized granite granophyres and the hyttsjo gabbro tonalite suite the formation and subsequent alteration of iron manganese and sulfide skarn ores in the supracrustal series involve

a geochemical traverse across the north chilean andes evidence for crust generation from the mantle wedge major and trace element and sr and nd isotope analyses are presented on 186 0 ma magmatic rocks along an east west traverse across north chile at 22 s esr ranges from 25 to 100 and end from 6 to 9 but the low end and high esr values are in rocks generated in the last 15 ma it is argued that previous discussions of petrogenesis in north chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one but two processes one results in a progressive shift of esr from 25 to 20 and end from 6 to 6 in jurassic to recent rocks which is accompanied by increasing ta sm and sr and decreasing th ta the second is largely confined to the younger rocks and it is characterised by esr increasing up to 100 with increasing sio2 and decreasing sr and it results in relatively shallow trends on an end esr diagram the preferred interpretation is that trend 1 is due to the mobilisation of old late proterozoic mantle lithosphere as magmatism migrated eastwards and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust it follows that the mantle wedge is the principal site of crust generation and it is argued that 20 of the sr in the recent northern chile rocks is derived from the subducted ocean crust

age and tectonic framework of structurally controlled palaeoproterozoic gold mineralization in the hame belt of southern finland abstract a number of gold occurrences in the palaeoproterozoic svecofennian domain of southern finland show a clear structural control on mineralization and are classified as orogenic gold samples of pre syn and late to post mineralization rocks taken from two gold prospects in the somero tammela gold zone of the hame belt have been studied to gain absolute ages of mineralization by using sims u pb of zircon and titanite and ar ar single grain step heating of hornblende and biotite gold mineralization in the somero tammela gold zone is closely related to quartz veins and controlled by wsw ene to sw ne trending shear zones and faults formed during prolonged dextral oblique contraction in the late stages of svecofennian orogenic evolution zircons from a felsic volcanic host rock give an age of 1881 3 ma interpreted as the crystallization age of the rock and providing a maximum age for mineralization inherited zircons in a gold bearing quartz vein with the youngest zircon rim of 1832 15 ma give a maximum age of quartz vein emplacement a 1786 7 ma mean 207pb 206pb age of a pegmatitic quartz feldspar dyke providing a minimum age for mineralization is interpreted as reflecting lead loss whereas an 1815 4 ma zircon age is probably closer to the crystallization age of the dyke this age overlaps with 1805 4 5 4 ma 39ar 40ar age of hornblende from a quartz vein 1 78 and 1 76 ga 39ar 40ar biotite cooling ages mark the minimum age for mineralization in summary the age of mineralization can be estimated at ca 1 82 1 79 ga which is in good agreement with the age of dextral shearing in southern finland the controlling structures wsw ene to sw ne dextral shear zones and nw se oriented faults formed during approximately wnw ese oriented sub horizontal shortening during crossing ductile brittle boundary conditions the steep shear zones and faults and the intersection lines of the two sets impart a directional permeability allowing channelling of the mineralizing fluids in these structures this was supported by prolonged dextral oblique contraction enabling continuous or episodic slip along the faults and shear zones the late svecofennian tectonic episode could be described as an extension contraction cycle in a convergent plate margin setting above a retreating subduction zone oblique extension 1 86 1 83 ga caused by slab rollback was followed by a rapid switch to oblique contraction at ca 1 82 ga induced by arrival of positively buoyant crustal fragments e g oceanic ridge oceanic plateau or continental sliver at the subduction zone the contractional period characterized by dextral transpression provided a favourable setting for orogenic gold mineralization the termination of the svecofennian orogenic evolution in finland after 1 79 ga might be caused by lock up of the subduction zone and slab break off the striking nw se alignment of orogenic type gold occurrences in the svecofennian domain of southern finland suggests that the late svecofennian 1 82 1 79 ga gold mineralizing event may be regionally more widespread and of greater importance than hitherto expected

late miocene earliest pliocene glaciation in southern argentina implications for global ice sheet history abstract in south america at latitude 47 s till was deposited sometime between 7 m y and 4 6 m y ago at a time when the local climate was colder than today s during this same interval glaciers extended to sea level in southeast alaska and widespread cooling of the ocean surface in middle latitudes worldwide marine regression and change in the oxygen isotopic composition of ocean water occurred from the last three events major late miocene expansion of the antarctic ice sheet has been inferred on the assumption that the history of north atlantic ice rafting precludes the existence of northern hemisphere ice sheets until 3 m y ago this is disputed first because precipitation in antarctica would probably have decreased at temperatures below today s second because the antarctic ice sheet cannot expand appreciably until buildup of northern hemisphere ice sheets has lowered sea level third because virtually no late miocene sediments are present at the labrador sea dsdp sites that are critical to the reconstruction of north atlantic ice rafting history and fourth because the scale of late miocene glaciation in alaska is at least permissive for simultaneous buildup of ice at similar latitudes further east if global ice sheet volume at the end of the miocene was greater than it is today by an amount that would have lowered sea level by several tens of meters the excess ice did not accumulate in antarctica but in the northern hemisphere chiefly in north america away from the atlantic coast

petrology geochemistry and origin of a major australian 1880 1840 ma felsic volcano plutonic suite a model for intracontinental felsic magma generation in australia a major period of felsic volcanism and granite emplacement occurred between 1880 and 1840 ma towards the end of the early proterozoic these volcanics and granites covering at least 37 000 km2 are compositionally uniform over large areas the felsic magmas are predominantly i type being derived from infracrustal sources which have never been exposed to surficial weathering processes when compared with early archaean and phanerozoic i type granites the 1880 1840 ma suite has distinctively high levels of k2o rb zr la ce th u and low mgo and cao contents even at relatively low sio2 levels of 60 65 the 1880 1840 ma rocks are characterized by low initial 87sr 86sr ratios and negative nd values chemical and isotopic modelling on these volcanics and granites suggest that they formed from mantle derived sources that are around 2300 2000 ma old there is no evidence of a major archaean component in these granites the sources are interpreted to have formed as a result of polygonal small scale mantle convection which led to extensive underplating of archaean crust during this underplating event large volumes of incompatible element enriched mantle derived material accreted to the base of the lower crust and fractionated significantly the more fractionated part of this source was remelted between 1880 1840 ma to produce the voluminous felsic volcanics and comagmatic granites present in nearly every proterozoic domain of australia the actual underplating event between 2300 2000 ma is believed to have triggered the formation of early proterozoic sedimentary basins which occur in a polygonal pattern and appear to wrap around archaean nuclei the composition of these felsic melts and the tectonic setting operating during their emplacement precludes genesis by contemporaneous subduction of oceanic crust all are generated by processes of vertical accretion in an intracontinental environment

early organic diagenesis the significance of progressive subsurface oxidation fronts in pelagic sediments abstract porewater and solid phase geochemical data at two contrasting ne atlantic stations are reported station 10552 on the cape verde abyssal plain is a site of slow pelagic accumulation ca 0 4 cm kyr 1 molecular oxygen is present in the sediment column to at least 2 m and probably much deeper labile organic carbon is almost totally consumed in the upper few centimetres of the sediment by contrast at station 10554 on the madeira abyssal plain the pelagic sequence has been interrupted by the occasional deposition of organic rich turbidites porewater oxygen and nitrate profiles show that subsurface organic metabolism of the organic carbon associated with the uppermost turbidite layer is a significant fraction of the overall metabolism in the sediment column this metabolism occurs at a relatively thin reaction front which progresses deeper into the turbidite with time this phenomenon exerts a controlling influence on the present nutrient profile and redox succession in a less extreme form substrate distributions of this latter type are not uncommon in atlantic sediments a model has been developed which is controlled by both oxygen and nitrate data this model permits a vertical profile of metabolic activity to be derived and also gives estimates of the reaction rate constants and solid phase mixing rates at these two contrasting stations about 30 of the total activity at station 10554 is located within the turbidite at the deepening reaction front this is a non steady state condition in fact it is found that the integrated metabolic activity at the two stations is not dissimilar ca 1 2 10 13 moles cm 2 sec 1 the striking differences in redox profile are therefore primarily attributable to differences in the distribution of metabolic activity within the column

climatic control of turbidite deposition on the madeira abyssal plain studies of sedimentation in the southwestern part of the madeira abyssal plain have revealed sequences of turbidites correlatable over thousands of square kilometres turbidites are sediments transported by turbidity currents and represent rapidly accumulated sedimentary units1 in this case in the distal part of the flow they are very fine grained and homogeneous occasionally with silty bases individual turbidites are separated by thin pelagic units which alternate from oozes to pelagic clays a stratigraphy for these pelagic units has been produced from combining coccolith distributions and lithological changes which enables comparison to cores containing only pelagic sediments from outside the turbidite province these comparisons show that turbidity currents cause no significant erosion in the area the major turbidites occur between lithological alternations which reflect global climate changes associated with glaciations and related eustatic sea level fluctuations turbidites have been linked with eustacy on previous occasions but only with low stands of sea level2 3 our work shows that turbidites are more likely to occur during regressions and transgressions rather than in the quiet periods between

extraction of pore water from marine sediments a review of possible artifacts with pertinent examples from the north atlantic abstract great care must be taken in extracting pore water if results are to be reliable errors can arise by inadequate storage conditions prior to pore water extraction by a difference between extraction temperature and the in situ temperature by oxygen contamination and possibly by high squeezing pressures three different extraction methods have been applied to samples from 30 m long piston cores collected in two abyssal plains in the north atlantic ocean the results demonstrate that all of the above mentioned errors have occurred these errors are discussed ad examples are shown

mechanical anisotropy of columnar jointed basalts an example from the baihetan hydropower station china abstract columnar jointed basalts are characterized by a self organized joint network of discontinuities and pose a difficult challenge for geotechnical engineering design due to their inherent anisotropic nature we explore here the anisotropic characteristics of the columnar basalt formation in the chinese baihetan hydropower station including aspects of the geometrical rock structure deformation and strength anisotropy site investigations reveal that a typical cross section of the columnar basalt blocks is mainly quadrangular and pentagonal with an average edge length of 0 152 m three types of joints with different macro and micro characteristics form the columnar jointed rock mass and are identified by scanning electron microscopy in situ ultrasonic tests further confirm that the columnar basalt prisms exhibit transverse isotropy with the plane of isotropy perpendicular to the column axis as can also be observed in the field strength anisotropy is confirmed by point load and uniaxial compression tests the mechanical and anisotropic characteristics of baihetan s jointed basalt mass can be explained by its multi scale joints pattern by means of goodman s stiffness equation we find that the anisotropic coefficients obtained for its deformability and strength are similar for all testing methods employed including in situ p wave velocity point load tests and uniaxial compression tests this result confirms the strong anisotropic effect in these rock masses and facilitates an issue that must be considered when assigning input parameters for various classification and numerical analyses schemes

textural and dispersal patterns of thick mud turbidites from the madeira abyssal plain abstract the madeira abyssal plain is the site of a thick sequence of thick pleistocene turbidites that are almost entirely of material less than 63 m in grain size the major turbidites are 1 to 5 m in maximum thickness concentrating on three turbidites cored at 24 locations detailed patterns of thickness and vertical and lateral grain size changes reveal flow paths on the plain in general the bases of the units are of graded silt showing thick and thin planar lamination and occasionally ripple cross lamination a new idealised sequence of changes in size and structures with distance along the flow path is developed closer to source the beds comprise a graded structured silty base overlain by structureless mud which may also be graded down current the silty base becomes thinner with less apparent structure and the upper homogeneous part becomes thicker and more uniform in an overall thickening trend at the distal end uniform beds up to 5 m in thickness are found in one turbidite the modal size structure shows some silt modes which are hydrodynamically sorted and change size vertically and spatially while one particular mode is constant in size due to a particular species of coccolith the turbidites originated from slumps on the northwest african continental margin and from the vicinity of madeira and the canaries some of which exceeded 100 km3 in volume

the depositional evolution of the svecofennian supracrustal sequence in finland and sweden abstract on the basis of six new zircon ages from metavolcanic rocks together with other isotopic geophysical and geological data the chronological and tectono stratigraphic evolution of the early proterozoic svecofennian supracrustal rocks in finland and sweden are discussed these data suggest that a nw se displacement from the bothnian bay to lake ladoga and a sinking of the southwestern block was initiated 1950 2000 ma ago accumulation of pelitic sediments and greywackes on the subsiding archaean basement continued until about 1860 ma in south central and northern sweden shallow submarine and subaerial volcanics were extruded 1880 1890 ma ago the thick sedimentary piles and volcanic rocks were subsequently intruded by plutonic rocks forming large batholiths 1860 1890 ma ago

viscoelastic thermal stress in cooling basalt flows polygonal fractures commonly form in basaltic lava flows during cooling of the molten lava a transient thermomechanical model is constructed to apply the temperature history in a cooling basalt flow as a boundary condition in a mechanical model of viscoelastic stress in cooling basalt nearly all past modeling of stress in cooling basalt has treated the material as purely elastic below a well defined glass transition temperature an important distinction of this model is that the viscosity of the material is allowed to be temperature dependent and the effects of viscoelastic relaxation are included on the basis of this research faster cooling and higher strain rates lead to markedly higher effective glass transition temperatures consistent with previous observations by researchers the total thermal strain in the system is constant yet the percentage of strain that is not relaxed by viscous flow varies with position relative to the cooling surface this elastic strain partition is the portion of the total strain that creates stress in the material and it is directly dependent on cooling rate thus a genetic link between cooling rate and magnitude of stress is identified

nd isotopic evidence from wopmay orogen for 2 0 2 4 ga crust in western north america abstract nd isotopic data from the 1 9 ga wopmay orogen n w t canada strongly suggest the involvement of crust in the range of 2 0 2 4 ga remnants of three compositionally diverse magmatic arcs 1940 1902 ma 1890 1880 ma and 1875 1840 ma are preserved in wopmay orogen each of the three arcs originated west of the limit of exposed archean crust nineteen samples of plutonic and volcanic rocks from the three arcs yield a narrow range of initial e nd from 1 8 to 0 3 there is no apparent correlation between e nd and geographic distribution nor with whole rock chemistry sio 2 54 75 two basalt samples from west of the archean have initial e nd s between 2 and 3 two 1 87 ga granites which intrude through archean crust have much lower initial e nd s of 5 9 and 8 6 the data from the three magmatic arcs are best interpreted in terms of a source region which had a very limited range of e nd at 1 9 ga which we interpret as 2 0 2 4 ga crust presumably hottah terrane basement wopmay fault zone is therefore the surface manifestation of a suture between 2 0 2 4 ga hottah terrane and the archean slave craton there is a growing body of data which suggests that much of western north america may be underlain by 2 0 2 4 ga crust geochronologic and isotopic data from other precambrian cratons also suggest that 2 0 2 4 ga crust may be much more widespread than previously thought models of crustal growth which interpret 1 9 1 7 ga crust as mixtures of archean crust and proterozoic depleted mantle may be overestimating the amount of juvenile material added to the crust in this time period

early diagenesis of marine organic matter alteration of the carbon isotopic composition in two distal carbonate turbidites labelled a and e from the madeira abyssal plain oxidation of organic matter om has been accompanied by a reduction in hydrogen index hi and change in the isotopic composition of carbon since the emplacement of turbidite 0 5 ka ago 35 of the om in the upper part has been oxidised by oxygen and nitrate diffusing from sea water into the sediment there is an accompanying decrease of 0 3 in 13c om and a decrease of 25 in hi turbidite e was emplaced about 73 ka and was exposed to bottom water for 12 3 ka before being buried beneath later turbidites during exposure oxygen and nitrate diffused into its upper part and oxidised 50 of the toc without an accompanying change in isotopic composition further oxidation of the more refactory organic components decreased toc by a further 25 and decreased 13c by 2 5 oxidation also reduced hi by 50 from originally low values unless corrected for the isotopic changes accompanying extended oxic and suboxic degradation of organic matter in sediments will give rise to erroneous estimates of marine terrestrial om ratios

a study on a water inrush incident at laohutai coalmine abstract an underground water inrush incident at laohutai coalmine in china was investigated using an integrated approach based on data from microseismic monitoring hydrochemistry isotope analysis meteorology and mining induced subsidence it was found that the incident was closely related to a snow storm and the rapid melting of the snow and was attributed to the rapid flow of surface water through subsidence induced fractures into the ground and the capacity of storing a significant amount of water temporarily in the mined out areas near the active mining stopes we found that in both a long range of 60 days and a short range of 10 h before the water inrush incident there were a few distinct abnormalities in the seismic data such as irregularity of p wave to s wave velocity ratio v p v s in the low value range irregularity of s wave to p wave amplitude ratio a sh a ph in the high value range irregularity of the wave dominant frequency in the low value range change of waveforms and reduction of microseismic activities before the fracturing of the walls that bore the inrushed water it is hence proposed to use the microseismic monitoring technique to monitor and analyze information carried in the seismic waves generated from either mining induced rock fracturing or active blasting so as to detect unknown water bodies and reduce water inrush risk in underground mines

carbonate dissolution and abyssal hydrography in the atlantic ocean abstract surface sediment samples from two regions in the eastern atlantic angola basin and cape verde basin and two regions in the western atlantic brazil basin and vema channel were examined to evaluate the relationship among carbonate dissolution abyssal hydrography and carbonate chemistry of the water column at each of these locations the position of the lysocline was determined carbonate saturation 1 was calculated and major deep water mass boundaries were identified as previously observed the correlation among these variables is strikingly different between the eastern and western atlantic in the vema channel and the brazil basin there is a first order correlation among the position of the lysocline the benthic thermocline aabw nadw boundary and the transition from saturation to undersaturation with respect to carbonate this relationship breaks down in the eastern atlantic where there is no noticeable benthic thermocline and the lysocline occurs significantly deeper than the saturation undersaturation transition

lead isotopic study of early proterozoic wopmay orogen nw canada role of continental crust in arc magmatism lead isotopes are sensitive indicators of crustal interaction in arc magmatism because of the large contrast in lead concentration between mantle derived magmas and continental crust and the three independent decay paths in the u th pb system preserved within wopmay orogen 1 94 to 1 84 ga are the remnants of three magmatic arcs the isotopic composition of lead in alkali feldspars separated from a diverse suite of samples from all three arcs fall in a relatively tight cluster with 206 pb 204 pb 15 59 15 88 207 pb 204 pb 15 27 15 36 and 208 pb 204 pb 35 15 35 54 except for samples from the only plutonic suite within wopmay orogen observed to intrude archean crust plutons intruding archean crust have 206 pb 204 pb 15 29 15 49 206 pb 204 pb 15 23 15 31 and 208 pb 204 pb 35 21 35 30 lead isotopic compositions do not correlate with rock composition both diorites and peraluminous granites have the same lead isotopic composition this isotopic composition