great thrust earthquakes and aseismic slip along the plate boundary of the makran subduction zone the makran subduction zone of iran and pakistan exhibits strong variation in seismicity between its eastern and western segments and has one of the world s largest forearcs we determine the source parameters for 14 earthquakes at makran including the great mw 8 1 earthquake of 1945 the only instrumentally recorded great earthquake at makran we determine the loci of seismic and aseismic slip along the plate boundary and we assess the effects of the large forearc and accretionary wedge on the style of plate boundary slip we apply body waveform inversions and for small magnitude events use first motions of p waves to estimate earthquake source parameters for the 1945 event we also employ dislocation modeling of uplift data we find that the earthquake of 1945 in eastern makran is an interplate thrust event that ruptured approximately one fifth the length of the subduction zone nine smaller events in eastern makran that are also located at or close to the plate interface have thrust mechanisms similar to that of the 1945 shock seaward of these thrust earthquakes lies the shallowest 70 80 km of the plate boundary we find that this segment and the overlying accretionary wedge remain aseismic both during and between great earthquakes this aseismic zone as in other subduction zones lies within that part of the accretionary wedge that consists of largely uconsolidated sediments seismic velocities less than 4 0 km s the existence of thrust earthquakes indicates that either the sediments along the plate boundary in eastern makran become sufficiently well consolidated and de watered about 70 km from the deformation front or older lithified rocks are present within the forearc so that stick slip sliding behavior becomes possible this study shows that a large quantity of unconsolidated sediment does not necessarily indicate a low potential for great thrust earthquakes in contrast to the east the plate boundary in western makran has no clear record of historic great events nor has modem instrumentation detected any shallow thrust events for at least the past 25 years most earthquakes in western makran occur within the downgoing plate at intermediate depths the large change in seismicity between eastern and western makran along with two shallow events that exhibit right lateral strike slip motion in central makran suggest segmentation of the subduction zone two paleozoic continental blocks dominate the overriding plate the boundary between them is approximately coincident with the transition in seismicity although relative motion between these blocks may account for some of the differing seismic behavior the continuity of the deformation front and of other tectonic features along the subduction zone suggests that the rate of subduction does not change appreciably from east to west the absence of plate boundary events in western makran indicates either that entirely aseismic subduction occurs or that the plate boundary is currently locked and experiences great earthquakes with long repeat times evidence is presently inconclusive concerning which of these two hypotheses is most correct the presence of well defined late holocene marine terraces along portions of the coasts of eastern and western makran could be interpreted as evidence that both sections of the arc are capable of generating large plate boundary earthquakes if that hypothesis is correct then western makran could produce a great earthquake or it could rupture as a number of segments in somewhat smaller magnitude events alternatively it is possible that western makran is significantly different from eastern makran and experiences largely aseismic slip at all times a knowledge of the velocity structure and nature of the state of consolidation or lithification of rocks at depth in the interior portion of the forearc of western makran should help to ascertain whether that portion of the plate boundary moves aseismically or ruptures in large to great earthquakes a resolution of this question has important implications for seismic hazard not only for western makran but also for other margins such as the cascadia subduction zone of western north america where historical thrust events have not occurred

tectonically driven changes in the indian ocean circulation over the last 25 ma neodymium isotope evidence abstract changes in ancient oceanic current patterns have been strongly influenced by plate tectonics particularly by the opening or closing of gateways between different oceans such modifications of oceanic circulation have important consequences on climate we studied the nd seawater isotopic composition of indian and pacific ocean cores using nd isotopes which are good paleo oceanographic tracers we focused on the past 25 ma which are marked by the closure of the indonesian gateway as well as the mediterranean connection we show that a strong westerly oceanic surface current which we refer to the miocene indian ocean equatorial jet miojet linked the eastern and western indian ocean from 14 ma to 3 ma and infer that this major change in oceanic circulation probably induced important variations of global climate

nd and pb isotope time series from atlantic ferromanganese crusts implications for changes in provenance and paleocirculation over the last 8 myr high resolution nd and pb isotope time series for the last 8 myr are reported for four atlantic ferromanganese crusts dated by 10be 9be chronology these are compared to new high resolution and high precision pb isotope time series and recently published nd isotope time series for two previously studied crusts from the nw atlantic ocean these records allow a more detailed examination of atlantic deepwater variability over the time period of intensification of northern hemisphere glaciation nhg changes in the pb isotope time series started after 3 ma but were most dramatic over the last 1 8 myr coinciding with changes of pb isotopes in the arctic ocean this latter change post dates the intensification of nhg at 3 1 to 2 5 ma and reflects an increase in the input of material eroded from the archean shield of canada and greenland shifts in nd and pb isotope compositions in a crust from the blake plateau occur before 5 ma and most prominent at 8 ma suggest that water masses from either the pacific or southern ocean influenced the isotope composition of this crust the relatively high end values around 8 ma recorded by the blake plateau crust are explained by a contribution of eastward flowing pacific water through the panama gateway into the caribbean sea this high end signal decreased between 8 and 5 ma suggesting that the supply of pacific water into the caribbean became restricted this is earlier than the caribbean seawater salinity increase at 4 2 ma deduced from 18o data and may indicate that there was only a surface water connection between the caribbean and pacific between 5 and 4 2 ma the closure of the panama gateway to intermediate and deep water exchange 200 m depth apparently occurred much earlier than the intensification of northern hemisphere glaciation at 3 1 2 5 ma and cannot therefore have been a direct cause of this climatic change but may as recently argued only have been a necessary precondition

seismotectonics of southern iran the oman line seismotectonic characteristics of the transition area between the zagros continental collision zone and the makran ocean continent convergence zone the oman line are carefully examined a northeast trending zone of earthquakes terminates the zagros belt of seismicity just west of the oman line this seismic zone extends northeastward beyond the main zagros thrust and coincides with a surface escarpment clearly visible on landsat imagery events with accurately determined depths along this seismic zone occurred in the upper 20 km of the crust and possibly show a shallow 10 northeastward dip east of the oman line the zendan minab fault system is relatively aseismic the oman line region may be characterized by underthrusting of a wedge of arabian shelf edge beneath iranian crust or by an indentation of arabia into the iranian crustal block as a promontory available seismological and geological data cannot uniquely distinguish between these two possible tectonic settings the area located to the northwest of the oman line region now a zone of continental collision appears to have been characterized by an episode of neogene subduction a well located intermediate depth earthquake occurred in this area to the northeast of the main zagros thrust on november 9 1970 it was accurately located at 107 km depth beneath a line of quaternary volcanoes its focal mechanism may indicate downdip tension with nodal planes that strike closely parallel to the trend of the zagros arc comparison of this event and other neotectonic features in this part of iran with those in other active convergent zones suggests that a descending oceanic lithosphere beneath the zagros volcanic arc may still be attached to the colliding arabian plate

subsurface structure of the eastern edge of the zagros basin as inferred from gravity and satellite data a data set of 10 505 points of land gravity measurements from southeast iran obtained from the bureau gravimetrique international combined with landsat imagery was used to investigate crustal and cenozoic lithospheric structure interpretation of the bouguer anomalies reveals three primary structural features the zagros mountain belt is characterized by a progressive decrease in gravity values from 70 mgal near the persian gulf to 160 mgal over the suture zone between the arabian margin and central iran crustal blocks the second feature is marked by a backward l shaped pair of anomalies that extends from the eastern peripheries of the zagros basin and wraps around southern iranian shores these 15 to 20 km deep source anomalies with amplitudes of as much as 10 mga are interpreted as intrabasement intrusions demarcating an ancient rift axis the shallow 6 8 km east west trending anomalies are perhaps interbasement uplifts bordered by reverse faults the third structure observed on both gravity and landsat displays a north striking eastward facing topographic escarpment has a gravity gradient of 0 85 mgal km and is right laterally offset approximately 100 km by the zagros main recent fault on landsat this escarpment is manifest by a system of north trending faults that seem to diverge into a horsetail pattern toward the east and by a set of north trending dike swarms that cut jurassic volcano sedimentary rocks it separates two distinct structural and sedimentary styles the zagros foreland basin anticlines of shallow marine precambrian to pliocene rocks on the west and the makran deep water eocene flysch belt on the east a comparison of gravity features with surface structures on thematic mapper and landsat multispectral scanner imagery indicates that a northeast trending fault system is the result of post miocene pervasive transpressive stress coupled with clockwise rotation of underlying basement blocks following the collision of arabia and iran accommodation structures such as forced folds and rabbit ear anticlines may develop over and on the flanks of the basement blocks providing remigration and trapping mechanisms for new oil and gas plays

tectonics and magmatism in turkey and the surrounding area this volume contains 23 papers from a range of international contributors describing recent research into the tectonics and magmatism of turkey and its surroundings this region is sited at the collision zone between eurasia and afro arabia and as such provides an extraordinarily complete and well exposed record of the staged tectonic evolution of this sector of the alpine himalayan orogen the geological history of this area involves separation of continental fragments from the margin of gondwana their migration across the tethyan oceans the subsequent closure of these oceans and finally the development of the neotectonic regime which continues to evolve to the present day such a comprehensive record is relevant to the understanding of collisional zones worldwide the volume is divided into five sections tethyan evolution neotethyan ophiolites post tethyan basin evolution neotectonics and igneous activity the first two sections deal with tethyan oceans whose growth and subsequent closure dominated the geodynamic framework in the mesozoic and cenozoic the subsequent sections deal with more recent geological developments from the balkan peninsula in the west to the transcaucasus in the east that followed consumption of the tethyan oceans there is a broad mix of papers throughout the volume wide ranging review papers on ocean development and extensional tectonics are followed by detailed descriptions of petrology and geochemistry and geographically focused studies on basin evolution specific aspects of extensional and strike slip tectonics and discussions of the relationship of magmatic activity to the tectonic development of the area tectonics and magmatism in turkey and the surrounding area presents up to date results and ideas from a large number of international contributors on a wide range of current research activity in this region it is essential reading for all geoscientists with an interest in both academic and applied aspects of eastern mediterranean geology

origin of a thrust related fold geometric vs kinematic tests abstract geometric tests to determine the origin of fault related folds are common but as is typical in structural geology more than one fold origin may yield the final natural geometry thus the results of geometric tests are usually non unique in contrast kinematic tests of origin which employ both geometry and data about deformation commonly yield more constrained if not unique results unfortunately the necessary data collection requires much more work than for a geometric test in this study the thrust related barclay anticline is analyzed both geometrically and kinematically to determine which test is more effective geometric tests using angular relationships indicate three possible origins fault bend fault arrest and break thrust for the kinematic test predicted deformations for interlayer and flexural slip flexural flow simple shear and bending strains are compared to micro and mesostructural distributions solution strain and finite strain from the anticline strain measurements indicate that microscale deformation is uniformly distributed through the structure and is lithification dominated the microscale deformation does not match kinematic predictions and did not accommodate fold formation fold growth was achieved primarily through layer parallel slip restricted mostly to the forelimb and absent in the hinge which eliminates fault bend and fault arrest origins that require material transport from forelimb to backlimb the barclay anticline is therefore interpreted to be a break thrust structure interestingly a suite of contraction faults in the forelimb and hinge indicates material transport from forelimb to backlimb such transport has been discounted for break thrust folds the most important result of this study is that a kinematic test was a necessary step for distinguishing fold origin geometric testing alone was insufficient given that the barclay anticline has geometric characteristics typical of many thrust related folds kinematic testing appears necessary to determine their structural origin

the estuarine chemistry of rare earth elements comparison of the amazon fly sepik and the gulf of papua systems abstract a comparison of the amazon fly gulf of papua and sepik river systems has shown that there are two distinct processes operating on dissolved rare earth elements ree in estuaries large scale salt induced coagulation in the low salinity region and small to extensive release in the mid to high salinity region fractionation of the ree occurs during both removal and release the order of removal follows lree mree hree and the order of release follows hree mree lree where l m and h refer to light middle and heavy ree respectively hence estuarine reactions can modify the relative abundance of dissolved ree reaching the oceans operating in tandem the removal and release lead to the preferential removal of the lree and the preferential release of hree during estuarine mixing this combination in turn results in an effective river water flux river flux after modifications by estuarine chemistry which is evolved toward the ree composition of seawater which has a heavy ree enriched composition when normalized against the ree composition of rivers or the earth s continental crust the increase of dissolved ree in the mid to high salinity waters of the amazon and fly estuaries suggests a sediment and or suspended sediment source the lack of ree increases in the sepik river estuary strengthens this explanation as the mixing zone of this estuary is located over a deep shelf where there is little physical contact between bottom sediment and estuarine waters in marked contrast the amazon and fly estuaries are dominated by the active resuspension and deposition of bottom sediments the resupply of ree accompanied by fractionation may result from ree carbonate complexation during the reaction of seawater with suspended particles and or bottom sediment estuarine and shelf sediments may be important sources of dissolved nd to the oceans thereby helping to maintain the inter ocean differences in the nd isotopic composition of seawater by reducing the residence time of nd in the oceans

osmium isotope variations in the oceans recorded by fe mn crusts this study presents osmium os isotope data for recent growth surfaces of hydrogenetic ferromanganese fe mn crusts from the pacific atlantic and indian oceans in general these data indicate a relatively uniform os isotopic composition for modern seawater but suggest that north atlantic seawater is slightly more radiogenic than that of the pacific and indian oceans the systematic difference in the os isotopic composition between the major oceans probably reflects a greater input of old continental material with a high re os ratio in the north atlantic ocean consistent with the distribution of nd and pb isotopes this spatial variation in the os isotope composition in seawater is consistent with a residence time for os of between 2 and 60 kyr indian ocean samples show no evidence of a local source of radiogenic os which suggests that the present day riverine input from the himalaya tibet region is not a major source for os recently formed fe mn crusts from the tag hydrothermal field in the north atlantic yield an os isotopic composition close to that of modern seawater which indicates that in this area the input of unradiogenic os from the hydrothermal alteration of oceanic crust is small however some samples from the deep pacific 4 km possess a remarkably unradiogenic os isotope composition 187 os 186 os ratios as low as 4 3 the compositional control of os incorporation into the crusts and mixing relationships suggest that this unradiogenic composition is most likely due to the direct incorporation of micrometeoritic or abyssal peridotite particles rather than indicating the presence of an unradiogenic deep water mass moreover this unradiogenic signal appears to be temporary and local and has had little apparent effect on the overall evolution of seawater these results confirm that input of continental material through erosion is the dominant source of os in seawater but it is not clear whether global os variations are due to the input of mantle or meteoritic material or simply indicate that the continental source itself is not uniform 1999 elsevier science b v all rights reserved

radiogenic isotope fingerprint of wilkes land adelie coast bottom water in the circum antarctic ocean 1 wilkes land adelie coast bottom water encompasses the bottom waters formed around the coast of east antarctica in the area east of prydz bay and west of the ross sea 90 e 150 e here we show that these bottom waters have not only characteristic physical properties but also carry a distinct radiogenic isotope signal that traces their dispersal the nd hf and pb isotopic compositions of circum antarctic ferromanganese nodules record the isotopic composition of ambient seawater those with the lowest nd and hf isotopic ratios and the highest pb isotopic ratios are from the deep australian antarctic basin and they fingerprint wilkes land adelie coast bottom water the data illustrate the potential value of these isotope systems as tracers of present and past deep and bottom waters in the circum antarctic realm and elsewhere

weathering versus circulation controlled changes in radiogenic isotope tracer composition of the labrador sea and north atlantic deep water geological reconstructions and general circulation models suggest that the onset of both northern hemisphere glaciation 2 7 myr ago and convection of labrador sea water lsw were caused by the closure of the panama gateway 4 5 myr ago time series data that have been obtained from studies of ferromanganese crusts from the northwestern atlantic suggest that radiogenic isotopes of intermediate ocean residence time pb and nd can serve as suitable tracers to reconstruct these events however it has been unclear until now as to whether the changes that have been observed in isotope composition at this time are the result of increased thermohaline circulation or due to the effects of increased glacial weathering in this paper we adopt a box model approach to demonstrate that the shifts in radiogenic isotope compositions are unlikely to be due to changes in convection in lsw but can be explained in terms of increases of erosion levels due to the glaciation of greenland and canada furthermore we provide experimental evidence for the incongruent release of a labile fraction of strongly radiogenic pb and nonradiogenic nd from continental detritus eroding into the labrador sea this can be attributed to the glacial weathering of old continents and accounts for the paradox that one of the areas of the world most deficient in radiogenic pb should provide such a rich supply of radiogenic pb to the oceans an important general conclusion is that the compositions of radiogenic isotopes in seawater are not always a reflection of their continental sources perhaps more importantly the transition from chemical weathering to mechanical erosion is likely to result in significant variations in radiogenic tracers in seawater

a new approach to the nd residence time in the ocean the role of atmospheric inputs concentrations of rare earth elements ree and nd isotopic ratios were analyzed for seawater filtered suspension and sediment trap samples collected in the tropical atlantic ocean eumeli program eutrophic mesotrophic and oligotrophic sites 20 n 18 21 w this is the first ree nd dataset on solution and different sized particles collected at the same site we present direct evidence of the nd isotopic exchange between particulate lithogenic fraction and seawater without significant mass transfer this exchange is probably one of the main factors that simultaneously constrains the nd concentration and isotopic ratio budget we propose a new approach to estimate the residence time of nd in the ocean nd based on isotopic exchange 200 yr nd 1000 yr the exchange requires a partial dissolution of lithogenic nd we estimate that the fraction of soluble nd proportion in atmospheric dust is of the order of 20 based on the isotopic ratios we suggest that the partial dissolution of atmospheric fallout is probably one of the main ree sources of the ocean

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

the hf isotopic composition of ferromanganese nodules and crusts and hydrothermal manganese deposits implications for seawater hf we present hf and pb isotopic data and chemical compositions of the outermost layers of marine ferromanganese deposits of different types hydrogenous and hydrothermal with a worldwide distribution the hf isotopic compositions display a broad range and refine previously reported regional differences as follows atlantic ocean hf 4to 2 indian ocean hf 2to 4 pacific ocean hf 3to 10 the most radiogenic hf isotopic compositions in the pacific samples are for hydrothermal manganese deposits that also have low 207pb204pb demonstrating that this signature reflects a contribution from hydrothermal venting of hf leached from oceanic volcanic rocks rather than from riverine inputs volcanic ash or eolian dust hafnium concentrations in the deposits increase from 20 ppb to 20 ppm with decreasing hf the hf and pb isotopic compositions for ferromanganese crusts define an apparent mixing trend between literature values of average continental crust and morb the range in hf for ferromanganese crusts is narrower than it is for 206pb204pb compared to the differences in isotopic composition of the sources of hf and pb this is consistent with hf having a longer residence time than pb the concentration of hf in ferromanganese crusts has been found to co vary with growth rate and inversely correlates with hf isotopic compositions hf isotope ratios may be used to determine not only the source of hf but possibly the source of fe and mn measurements of hf and hf concentrations in nodule tops bottoms and associated sediments show that the hf of nodules is sensitive to sedimentary oxic and sub oxic diagenetic processes and thus most nodules may not reliably reflect the isotopic composition of hf in seawater

a data brief on magnesium isotope compositions of marine calcareous sediments and ferromanganese nodules 1 the magnesium isotope compositions of 26 ferromanganese nodules from various oceans and 22 calcareous sediment samples from ocean drilling program odp leg sites from the atlantic odp704 leg 114 and the pacific odp846 leg 138 have been analyzed by multicollector inductively coupled plasma mass spectrometer most nodules from the pacific and atlantic oceans have 26mg between 1 89 0 24 and 0 54 0 01 2 scatter around the average ocean water value of 0 86 0 12 indian ocean nodules display a large variation in 26mg ranging from 2 94 0 46 to 0 69 0 18 as for calcareous sediments 26mg variations range between 2 94 0 50 and 0 71 0 45 and between 4 00 0 31 and 1 63 0 09 for the atlantic and the pacific legs respectively this study allows the construction of a thread of global 26mg mapping in fe mn nodules and suggests that what controls the 26mg variations of marine calcareous sediments is complex and probably linked both to a mixture of different tests of micro organisms and to diagenesis

age and isotopic relationships among the angrites lewis cliff 86010 and angra dos reis results of a wide ranging isotopic investigation of the unique antarctican angrite lew 86010 lew are presented together with a reassessment of the type angrite angra dos reis ador the principal objectives of this study are to obtain precise radiometric ages initial sr isotopic compositions and to search for the erstwhile presence of the short lived nuclei 146sm and 26al via their daughter products the isotopic compositions of sm u ca and ti were also measured this allows a detailed appraisal to be made of the relations between and the geneology of these two angrites lew proves to be severely contaminated with modern terrestrial pb which is shown to result from terrestrial weathering nevertheless concordant pb pb model ages of pyroxene separates were obtained 2 4 55784 52 ga for lew and 4 55780 42 ga for ador uranium isotopic compositions are normal within error the inferred initial pb isotopic composition is within error of primordial pb as defined by canon diablo troilite because of the extreme upb ratios of both angrites this places an upper limit of 2 ma on the time between volatile element loss of the angrite parent body or its precursor planetesimals and the final crystallization of the angrites as differentiates initial 87sr86sr ratios were found to be indistinguishable in lew ador and the cumulate eucrite moore county the derived initial 87sr86sr for all three meteorites is 0 698970 15 2 a 147sm 143nd isochron for lew was obtained yielding an age of 4 553 34 ga with an initial 143nd144nd of 0 506682 49 2 146sm142nd isotope systematics were also measured anomalies on 142nd arising from extinct 146sm were clearly resolved resulting in an initial 146sm144sm of 0 0071 17 with nd142 2 57 0 62 at the time of isotopic closure both of these sm nd methods imply derivation of lew from a reservoir with chondritic smnd ratio they are also quite consistent with the previously reported systematics for ador overall the age and isotopic similarities between lew and ador are striking it suggests almost simultaneous production on the same asteroid even though recent experimental studies imply that the two are not comagmatic calcium and titanium do not exhibit enrichments in the n rich isotopes in contrast to cais making it unlikely that the angrite parent body is inherently rich in early condensates from the solar nebula no evidence was found for live 26al in lew when combined with the pb pb age and initial 87sr86sr data this firmly excludes 26al being an important heat source in the early solar system heat supplied for for example 1 the differentiation of the angrite and eucrite parent bodies and 2 metamorphism of the chondrite parent bodies must come from some other source the debate over live vs extinct 26al in the earliest solar system remains unresolved by this new data from lew however both the angrite and eucrite parent bodies have extremely low rbsr and high upb ratios compared to the solar nebula therefore the common initial 87sr86sr of 0 698970 can be firmly associated with the u pb angrite age above and the formation of both bodies this enables an absolute chronology of the early solar system to be established based upon 87sr86sr differences the oldest cais are a minimum of 11 4 ma older than the angrites i e 4 569 5 ga the age and isotopic constraints are discussed with respect to current collapse condensation and accretion timescales calculated for the solar nebula

nd and pb isotopes in atlantic and pacific water masses before and after closure of the panama gateway two nd and pb isotope time series of hydrogenous ferromanganese crusts one from the tehuantepec ridge in the deep eastern equatorial pacific and the other from blake plateau in the shallow northwestern atlantic which cover the past 7 8 m y show no variations coincident with the final closure of the panama gateway estimated as ca 3 5 ma the record of the atlantic crust located in the present day gulf stream shows a shift in isotope composition from ca 8 to 5 ma that is explained by a diminishing supply of pacific water it is argued that the major restriction of water mass exchange through the panama gateway occurred before 5 ma and thus cannot serve as a direct cause of the onset of northern hemisphere glaciation the absence of a significant signature in the isotope records from the pacific crust suggests that the volume of water exchanged with the atlantic through shallow archipelagic straits of the gateway during the 3 4 m y prior to closure was too small to influence the radiogenic isotope composition of pacific deep water

distribution and sources of pre anthropogenic lead isotopes in deep ocean water from fe mn crusts abstract the lead isotope composition of ocean water is not well constrained due to contamination by anthropogenic lead here the global distribution of lead isotopes in deep ocean water is presented as derived from dated ca 100 ka surface layers of hydrogenetic fe mn crusts the results indicate that the radiogenic lead in north atlantic deep water is probably supplied from the continents by river particulates and that lead in pacific deep water is similar to that characteristic of island and continental volcanic arcs despite a short residence time in deep water 80 100 a the isotopes of lead appear to be exceedingly well mixed in the pacific basin there is no evidence for the import of north atlantic deep water derived lead into the pacific ocean nor into the north indian ocean this implies that the short residence time of lead in deep water prohibits advection over such long distances consequently any climate induced changes in deep water flow are not expected to result in major changes in the seawater pb isotope record of the pacific ocean

sr nd and pb isotope constraints on the formation of the metalliferous sediments in the nereus deep red sea previous chemical analyses of sediments from nereus deep core 412 located in the northern part of the red sea at about 23 20 n have demonstrated the occurrence of v rich authigenic magnetites associated with fe zn and cu sulphides mostly located at the bottom of the core the evolutionary stage of nereus deep is intermediate between that of the immature shaban deep and the more evolved atlantis ii deep previous studies have proposed either morb basalts by analogy with east pacific rise sulphide deposits or sediments as possible sources of the metals in the red sea metalliferous sediments sr nd and pb isotope systematics observed at ten levels in nereus core 412 lead us to distinguish three types of sediments 1 with predominantly basaltic fragments associated with loose titanomagnetite crystals 0 70413 87sr86sr 0 70622 0 51271 143nd144nd 0 51300 18 618 206pb204pb 18 673 2 detrital or biogenic sediments with a continental isotopic imprint 0 70938 87sr86sr 0 71323 0 51242 143nd144nd 0 51262 18 848 206pb204pb 18 890 and 3 v rich metalliferous sediments 0 70872 87sr86sr 0 70895 0 51260 143nd144nd 0 51274 18 717 206pb204pb 18 851 on sr nd and pb pb isotope diagrams this last group of sediments defines a field located between the fields of the first two groups our data suggest that the sr nd and pb in the studied metalliferous sediments were derived from overlying sedimentary beds in which the two main constituent detrital components show very distinct origins red sea morb and continental arabo nubian shield these data clearly support a model of convective circulation within the nereus deep sedimentary pile this model involves leaching two main components from the upper sedimentary layers of the core and a downward circulation of a hot fluid promoting the growth of v rich magnetites and fe zn and cu sulphides when the fluid cooled by mixing with the pore water of sediments from the lower part of the core a driving force represented by magmatic activity located near the 412 core in the nereus deep is required to explain the presence of hot convective fluids these data also suggest that for sedimented ridges the isotopic imprint of a basaltic source in metalliferous sediments or a hydrothermal ore deposit may result by interaction of a hot fluid with detrital sediments that are enriched in basaltic clasts and loose magmatic crystals

a reappraisal of earthquake focal mechanisms and active shortening in the zagros mountains of iran summary the zagros mountains of sw iran are one of the most seismically active intra continental fold and thrust belts on earth and an important element in the active tectonics of the middle east surface faulting associated with earthquakes is extremely rare and so most information about the active faulting comes from earthquakes we use long period teleseismic p and sh body waves to determine the orientation and depth of faulting in 16 new earthquakes and then evaluate and synthesize all the available teleseismic data on earthquake source parameters in the zagros we use this information to investigate the style and distribution of active faulting in the zagros and how it contributes to the n s shortening of the arabia eurasia collision when the data are ranked in quality and carefully evaluated simple patterns are seen that are not apparent when routine catalogue data are taken at face value an important change in the fault configuration occurs along strike of the belt in the nw overall convergence is oblique to the trend of the belt and the surface anticlines and is achieved by a spatial separation partitioning of the orthogonal strike slip and shortening components on separate parallel fault systems by contrast in the se overall convergence is orthogonal to the regional strike and achieved purely by thrusting in the central zagros between these two structural regimes deformation involves parallel strike slip faults that rotate about vertical axes allowing extension along the strike of the belt the overall configuration is similar to that seen in other curved shortening belts such as the himalaya and the java sumatra trench all the zagros earthquakes we have been able to check have centroids shallower than 20 km and are confined to the upper crust many of the larger earthquakes are likely to occur in the basement beneath the sedimentary cover which is active even beneath areas of known shallow structural decollement such as the dezful embayment the dominant style of shortening is high angle reverse faulting with dips 30 though some lower angle thrusting occurs in places active thrust and reverse faulting is relatively confined to the lower topography on the sw edge of the belt today and only strike slip faulting affects the higher topography profound vertical changes in structural and stratigraphic level indicate that a similar style of deformation was once active across the width of the simple folded belt but has progressively migrated sw over the last 5 ma there is no evidence for a seismically active structural decollement such as a low angle thrust beneath the zagros nor is there any seismic evidence for active subduction either beneath the zagros or beneath central iran instead the arabian margin seems to have shortened by distributed thickening of the basement only in the syntaxis of the oman line at the se end of the zagros is there any evidence for a low angle thrust of regional extent here earthquakes continue 50 km north of the zagros thrust line the geological suture between the arabian margin and central iran reaching depths of 30 km and may represent thrusting of arabian basement beneath central iran to this extent

thrust surface geometry implications for thrust belt evolution and section balancing techniques abstract the application of modern concepts of thrust tectonics to orogenic belts has yielded many detailed restorable sections thrusts generally restore with a staircase geometry onto the undeformed stratigraphic template since the work of rich 1934 it has been generally assumed that thrusts have propagated with a staircase trajectory and that hangingwall structures result from the movement of the thrust sheet up footwall ramps however thrusts often cut through previously folded strata with a smooth trajectory smooth trajectory thrusts can be identified from footwall geometry and strain state once a smooth trajectory thrust has been identified it is insufficient to produce a balanced deformed section and its restored counterpart sequential balanced sections must be constructed reversing the deformation history the restoration of thrusts to a staircase trajectory has no mechanical significance as the thrust is merely acting as a marker if restored back beyond the time at which it propagated thrust ramps are not necessarily actual steps in the thrust surface they are often just zones where the thrust cuts through bedding in a smoothly curving trajectory ductile deformation in thrust belts can reduce bed length at depth thus removing the need for structures which are necessary to preserve constant bed lengths ductile deformation in the internal zones of orogenic belts renders the construction of balanced sections in these zones highly problematical

the origin of changes in structural style across the bandar abbas syntaxis se zagros iran abstract the bandar abbas syntaxis marks the transition between the zagros collisional belt and the makran subduction related prism and constitutes a major structural feature within the alpine himalayan mountain belt within the foreland part of the zagros the zagros fold thrust belt zftb the transition is marked by a sudden change in structural trends and style and a rapid decrease in seismicity and salt diapirism from west to east of the syntaxis the depth to decollement and lithology affect the geometry and size of the folds on each side of the syntaxis while different mechanical coupling along the basal decollement affects the degree of internal rock deformation thus the short and broad folds of the western side of the syntaxis are decoupled along the 8 km deep hormuz evaporites visible at surface in numerous salt diapirs strong seismicity in this area suggests that the basement is involved in the deformation by contrast the long and narrow folds of the eastern side of the syntaxis appear to be controlled by frictional slip along a 6 km deep detachment the absence of seismicity suggests that the basement is not involved in the deformation in this area we propose that the basal decollement for these structures corresponds to the eroded top of the nne ssw trending oman mesozoic nappes resulting from late cretaceous obduction of tethys on the arabian margin these structures are visible onshore in the northern tip of the oman peninsula and seen in offshore seismic profiles continuing northward beneath the strait of hormuz finally map view observations show that the main zagros thrust and the zendan fault separating the zftb from the internal zones of the orogen are shallow low angle thrust faults rather than steeply dipping faults

hydrothermal sediments as a potential record of seawater nd isotope compositions the rainbow vent site 36 14 n mid atlantic ridge 1 geochemical compositions and sr and nd isotopes were measured in two cores collected 2 and 5 km from the rainbow hydrothermal vent site on the mid atlantic ridge overall the cores record enrichments in fe and other metals from hydrothermal fallout but sequential dissolution of the sediments allows discrimination between a leach phase easily leachable and a residue phase refractory the oxy anion and transition metal distribution combined with rare earth element ree patterns suggest that 1 the leach fraction is a mixture of biogenic carbonate and hydrothermal fe mn oxy hydroxide with no significant contribution from detrital material and 2 99 5 of the ree content of the leach fraction is of seawater origin in addition the leach fraction has an average 87sr 86sr ratio indistinguishable from modern seawater at 0 70916 although we lack the nd value of present day deep water at the rainbow vent site we believe that the ree budget of the leach fraction is predominantly of seawater origin we suggest therefore that the leach fraction provides a record of local seawater nd values nd isotope data from these cores span the period of 4 14 ka 14c ages and yield nd values for north east atlantic deep water neadw that are higher 9 3 to 11 1 than those observed in the nearby madeira abyssal plain from the same depth 12 4 0 9 this observation suggests that either the iceland scotland overflow water isow and lower deep water contributions to the formation of neadw are higher along the mid atlantic ridge than in the surrounding basins or that the relative proportion of isow was higher during this period than is observed today this study indicates that hydrothermal sediments have the potential to provide a higher resolution record of deep water nd values and hence deepwater circulation patterns in the oceans than is possible from other types of sediments

u th pb systematics and ages of himalayan leucogranites south tibet abstract the age and origin of five leucogranites from the high and tethys himalaya and two country rock gneisses were investigated by u pb dating of zircon fractions and single grains and fractions of monazite additionally th u concentrations in whole rock powders and isotopic compositions of pb in leached k feldspars were determined monazites yield ages of 16 8 0 6 m y for the nialam migmatite granite 15 1 0 5 m y for the lhagoi kangri granite 14 3 0 6 m y for a granite from mt everest and 9 8 0 7 m y and 9 2 0 9 m y for two varieties of the maja granite these data together with monazite ages of 21 9 0 2 and 24 0 0 4 m y determined earlier on the makalu granite 1 substantiate a period of intracontinental granite emplacements from 24 to 9 m y ago i e from uppermost oligocene to late miocene times such a period of plutonic activity is consistent with the view that all these granites result from intracrustal melting following the collision of india with eurasia furthermore the individual ages together with structural relationships between granites and country rocks suggest that granite formation and tectono metamorphism occurred as alternating and strongly related processes with a periodicity of 7 to 9 m y inherited lead components present in all granite zircons point to large proportions of precambrian material in the magma source regions up to 2200 m y old th u systematics between monazite and country rocks indicate that u has been leached from most of the granites after crystallisation of monazite zircon dating of the kangmar granite gneiss which occurs in a window through the tethys himalayan sediments shows that this pluton transformed to a gneiss during the alpine orogeny crystallised in lowermost palaeozoic times 562 4 m y ago

cenozoic changes in atmospheric lead recorded in central pacific ferromanganese crusts the possible sources of pre anthropogenic pb contributed to the world s oceans have been the focus of considerable study the role of eolian dust versus riverine inputs has been of particular interest with better calibration of isotopic records from central pacific ferromanganese crusts using os isotope stratigraphy it now appears that deep water pb isotopic compositions were effectively homogeneous over a distance of 5000 km for the past 80 myr the composition shifted slightly from high 206pb 204pb ratios in the range of 18 87 0 02 before 65 ma to lower values of 18 62 0 02 by 45 ma and then gradually increased again very slightly to the present day ratio of 18 67 0 02 the regional homogeneity provides evidence of a dominant well mixed atmospheric source the most likely candidate for which is volcanic aerosols contributed either directly or as soluble condensates on eolian dust the slight shift in pb isotope composition of deep waters in the central pacific between 65 and 45 ma may be the result of a regional or perhaps global scale change in the sources of volcanic exhalations and volcanic activity caused by an increase in the importance of melting and assimilation of older continental crustal components over the cenozoic

the role of salt in fold and thrust belts davis d m and engelder t 1985 the role of salt in fold and thrust belts in n l carter and s uycda editors collision tectonics deformation of continental lithosphere tectonophysics 119 67 88 the style of deformation in thin skinned fold and thrust belts is critically dependent upon the resistance to sliding along the detachment between the mass of deforming sediments and the underlying rocks evaporites can provide an extremely weak horizon within which a basal detachment can form and along which only a relatively small shear traction can be supported fold and thrust belts that form atop a salt layer such as the appalachian plateau the franklin mountains in northwestern canada and the jura of the alps among others share several readily observable characteristics as predicted by a simple mechanical model for fold and thrust belts a detachment in salt permits a thrust belt to have an extremely narrow cross sectional taper in addition predicted orientations of the principal stress axes over a salt dtcollement are consistent with the commonly observed lack of a consistently dominant vergence direction of structures within the thrust belt other common attributes of salt basal thin skinned deformation include the presence of several widely but regularly spaced folds and abrupt changes in deformational style at the edge of the salt basin

variation of structural style and basin evolution in the central zagros izeh zone and dezful embayment iran abstract structural style and its relationship to sedimentary facies and the evolution of sedimentary depocenters since late cretaceous times have been studied on the basis of one regional balanced transect and several updated isopach maps in the izeh zone and the dezful embayment central zagros this study relies on fieldwork data existing geological maps seismic data and well information a new structural classification for part of the zagros sedimentary cover is presented to highlight the different mechanical behavior of the formations in the stratigraphic column it shows the existence of several local decollement levels activated during folding these decollement levels separate lithotectonic units which accommodate shortening in different ways the lower paleozoic is the basal decollement level throughout the studied area triassic evaporites albian shales eocene marls and miocene evaporites can act as intermediate decollement levels and present variable facies in the central zagros lateral facies and thickness variations the sedimentary overburden and the close relationship with inherited fault patterns influenced the wavelength amplitude and style of folding in the study area furthermore surface structures do not necessarily coincide with deeper objectives where these disharmonic levels are involved in folding the evolution of sedimentary depocenters from the late cretaceous obduction episode until early miocene collision is described based on updated isopach maps and tectonic subsidence curves this analysis shows continuous compression and movements along n s and nw se trending faults in the zagros basement which predate the neogene zagros folding and influence sedimentation history it also indicates a southwestward migration of the depocenters through time in addition to the basement involvement below some folds during the zagros orogeny

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

hydrothermal plumes at the rodriguez triple junction indian ridge water column anomalies of light transmission mn fe al and ch4 concentrations were searched in the central southeastern and southwestern indian ridge segments centered on the rodriguez triple junction rtj 25 32 s 70 02 e for the purpose of locating hydrothermally active areas in july to august 1993 we found an active zone in the central indian ridge segment 25 18 20 s approximately 12 miles north of the rtj where significant hydrothermal plumes were observed at 2 200 2 400 m depth intensive tow yo observations using a ctd rosette multi sampling system equipped with a transmissometer revealed that the plumes show temporal as well as spatial variations discrete water samples within the plumes were enriched in mn fe and ch4 with maximum concentrations of 9 8 nm 40 2 nm and 3 3 nm respectively judging from the spatial and chemical characteristics of the plumes especially from transmission anomalies andc h4mn ratios we speculate that the hydrothermal venting site might be not in the rift valley but on the eastern off axis zone several miles distant from the rift valley

tectonics of the zagros orogenic belt of iran new data and interpretations abstract the zagros orogenic belt of iran is the result of the opening and closure of the neo tethys oceanic realm and consists from northeast to southwest of three parallel tectonic subdivisions 1 1 the urumieh dokhtar magmatic assemblage 2 2 the sanandaj sirjan zone and 3 3 the zagros simply folded belt recent detailed and reconnaissance mapping and stratigraphie structural analyses in various parts of the belt indicate that the tectonics of the sanandaj sirjan zone is dominated by either large scale composite duplex structures or low and high angle ne dipping imbricate systems these thrust systems have transported numerous horses and sheets nappes of metamorphosed and non metamorphosed phanerozoic rocks for tens or possibly hundreds of kilometers stratigraphic evidence and synorogenic conglomerates indicate that thrusting was initiated in the late cretaceous and has continued episodically since that time shear sense indicators in various parts of the zone reveal a ne sw direction of transport thrust systems generated in each episode are superimposed upon the previous structures cutting across and displacing them and producing complicated stacks of thrust sheets southwestward transport and stacking of the thrust sheets has resulted in an increase of about 10 15 km in the thickness of the continental crust in the southwestern part of this zone the so called z main zagros thrust which has been traditionally considered as the boundary between the sanandaj sirjan zone and the zagros simply folded belt is by no means a single high angle reverse fault nor is it a narrow zone of crushed rocks but a series of low angle thrusts locally cut by normal faults these thrusts are indistinguishable from those mapped in the sanandaj sirjan zone and are geometrically part of the sanandaj sirjan zone structures there is no single thrust among them that can be singled out and referred to as the main one the boundary of the sanandaj sirjan zone with the urumieh dokhtar magmatic assemblage to the northeast is characterized by a series of structural depressions formed by compression rock sequences including ophiolite complexes exposed along this boundary are strongly dismembered sheared and mylonitized shearing planes and foliations are either subvertical or dipping at high angles to the northeast this zone is suggested to be the suture between the afro arabian and the iranian plates overall the intensity of deformation increases northeastward from the persian gulf mesopotamian basin where strata are undeformed and horizontal through the zagros simply folded belt which is characterized by en echelon doubly plunging flexural slip folds gentle to open in the southwest and close to locally sw overturned in the northeast the intensity of deformation continues to increase further northeastward through the sanandaj sirjan zone where thrust faulting is dominant this pattern suggests that the deformational front has migrated through time from the northeastern part of the sanandaj sirjan zone i e the suture zone to its present position in the central portion of the persian gulf mesopotamian foreland basin

abyssal water carbon 14 distribution and the age of the world oceans the carbon 14 distribution in the abyssal waters of the world oceans indicates replacement times for pacific indian and atlantic ocean deep waters more than 1500 meters deep of approximately 510 250 and 275 years respectively the deep waters of the entire world ocean are replaced on average every 500 years

a lead isotopic study of circum antarctic manganese nodules abstract lead isotope ratios in manganese nodules from the circum antarctic ocean display systematic geographical variations eastward in the direction of current movement from south of australia they progressively decrease in the pacific sector and into the western atlantic sector until 30 w from this longitude lead isotope ratios increase in the eastern atlantic sector and reach their highest values in the indian sector near 110 e south of western australia they abruptly decrease these two lead isotope domains correspond closely to the circum antarctic neodymium isotope domains previously recognized by albarede and goldstein 1992 however the intraocean isotopic variations are much clearer for pb than nd reflecting the shorter oceanic residence time of pb 102 y compared with nd 103 y and confirming that pb is a much more sensitive marine tracer throughout the circum antarctic ocean the lead isotopic changes follow the pattern of present day oceanic circulation the pb shows an overall continental signature which is strongest in the indian sector the most probable candidate as the source of the continental signal is north atlantic deep water nadw which enters the circum antarctic in the south atlantic as nadw mixes with circumpolar water lead isotope ratios increase eastward in the indian sector reflecting progressive loss of the pacific derived pb signal the abrupt decrease of lead isotope ratios between the indian and pacific sectors appears to be a consequence of addition of pacific seawater through the pacific lead isotope ratios gradually decrease eastward approaching values of east pacific rise metalliferous sediments ocean ridge basalts and south pacific mn nodules this trend results from an increasing influence of pacific ocean water containing a component derived from pacific volcanism the pacific signature persists into the argentine basin in the southwest atlantic ocean reflecting significant input of water from the drake passage northward into the atlantic the pacific pb component is seen in the atlantic and indian oceans near southern africa as well as in the atlantic and indian sectors of the circum antarctic lead and neodymium isotopes correlate in the atlantic indian sectors but not in the pacific sector reflecting differences in residence times and modes of water mass mixing the renewal of circum antarctic water occurs on a timescale which is short compared to the residence time of nd but long compared to pb as a result dissolved pb is subject to considerable changes over the timescale of circumpolar current movement while nd is less easily affected nadw is added locally in the south atlantic and moves eastward as a water mass as nadw mixes with circumpolar water cpw through the indian sector changes affecting nd pb in nadw and cpw occur in tandem and neodymium and lead isotopes are correlated through the pacific sector gradual addition of pacific seawater occurs over a large region and nd pb relationships are not coupled because simple isotopic mixing systematics are not conserved there neodymium and lead isotopes vary independently the consistency of lead isotope variations with present day ocean bottom water circulation is remarkable considering that the mn nodules as sampled reflect the integrated flux of pb over 105 106 y the oceanic mixing time is 103 y the residence time for pb is 102 y and the circulation time of the circumantarctic ocean is 30 y processes occurring over such different timescales can show such a relationship only if the system is stable over the longest time interval thus on average element sources and ocean circulation through the pleistocene have been similar to the present day despite significant perturbations during glacial periods

lead isotopic record of barremian aptian marine sediments implications for large igneous provinces and the aptian climatic crisis we present initial isotopic ratios of lead for early cretaceous barremian aptian sections from shatsky rise pacific and gorgo a cerbara italy our pb isotopic data track an interval representing oceanic anoxic event oae 1a which is characterized by quasi global deposition of organic carbon rich black shale pb isotopic compositions of sediments from shatsky rise decrease at the end of barremian time from radiogenic continental values to unradiogenic values and subsequently remained less radiogenic until the end of early aptian time we explain the isotopic shift by a significant increase in supply rate of unradiogenic pb most likely due to massive volcanism in contrast the pb isotopic compositions from the italian section which was situated at the western end of tethys are mostly identical to those of upper continental crust showing no significant change in supply rate of unradiogenic pb the discrepancy between two sites is attributed to quiescent deep submarine eruptions of pacific large igneous provinces lips such as the ontong java plateau ojp which severely limited dispersion of pb carrying particles out of the pacific ocean published os isotopic data from the italian section indicate two episodes of massive eruptions of ojp or contemporaneous manihiki and hikurangi plateaus starting from earliest aptian time slightly later than that indicated by the sedimentary pb isotopic record from shatsky rise differences in isotopic variations between pb and os likely reflect differences in their chemical behaviors in the oceans i e pb isotopic compositions would have varied in response to local or regional changes in sediment provenances whereas large scale changes in os inputs are required to explain variations in seawater os isotopic compositions our pb isotopic data together with the published os isotopic record provide new evidence for the eruptive history of ojp together with contemporaneous pacific plateaus and its environmental consequences starting from end barremian time and extending through early aptian time

implications of the deep circulation and ventilation of the indian ocean on the renewal mechanism of north atlantic deep water in the indian ocean sector of the great ocean conveyor scheme the north atlantic deep water nadw replacement is realized by the transformation of the sinking circumpolar deep water cdw from the southern ocean to a prevailingly upward flow in the northern indian ocean this water mass transformation scenario has been studied by using hydrographie data including potential temperature salinity dissolved oxygen and nutrients in a water mass mixing model and by calculation of dianeutral velocity the model comprises a mixing system of three major deep water masses nadw cdw and north indian deep water this third water mass is introduced as a virtual water mass to represent the aged cdw the assumption is feasible as the artificial water mass can be eliminated on the deepest n 28 12 neutral surface characterized by isopycnal mixing the experiment is fulfilled when a conservative variable initial phosphate po40 is introduced and all conservative parameters are used in the mixing scheme the n 28 12 surface provides a major isopycnal transition path of cdw to the northern indian ocean and thus cdw is transformed to an upward flow dominant dianeutral up welling is detected on all four neutral surfaces north of 30 s showing increasingly strong dianeutral velocity toward shallower surfaces with the water mass mixing pattern and spreading path dianeutral circulation and dynamical information of the acceleration potential 10 m2 s 2 mapped on neutral surfaces a schematic of deep water circulation and ventilation of the indian ocean emerges from this study the contribution of nadw is largely limited to the southwest indian ocean the northward cdw is found mainly in two paths one in the western indian ocean apparent between 2000 and 3000 m through the western crozet and madagascar basins and farther north into the somali basin and arabian basin and another in the central indian basin apparent below 3000 m where ridge blocking becomes effective in the west and nadw salinity reaches a maximum while the major path switches from west to the central basin in the deeper layer 3000 3500 m the flow in the western indian ocean reverses southward north of the southeast indian ridge the deep path bifurcates from which point the eastern branch feeds the deep western boundary flow in the west australian basin except for part of the transformed cdw which is contributed to the upward flow in the north the return flow of cdw is found on the eastern side of these basins

geochemical evidence of terrigenous influence in deep sea sediments up to 8 s in the central indian basin abstract thirty two representative deep sea sediment samples have been studied from three manganese nodule bearing environments in the central indian basin diatomaceous radiolarian ooze clay pelagic brown clay and calcareous ooze they have been analysed for major and minor elements such as si al ca mg na k fe mn p ti ba cu ni co v zn and cr r mode factor analysis reveals five important sources for these elements 1 detrital loaded with fe ti al mg mn p and k 2 combined hydrogenetic diagenetic mn ni cu co and fe 3 biogenic si 4 seasalts na and mg and 5 dissolution residue ba factor scores distributional trends of detrital elements al ti and fe concentrations and al al fe mn ratios provide new geochemical evidence for the effect of the terrigenous influx from the ganges brahmaputra rivers into the northern part of the basin up to 8 s which inhibits the growth of manganese nodules in this area in contrast incorporation of transitional metals into sediments and manganese nodules in the other two areas of the basin is governed by hydrogenetic and diagenetic processes

quaternary climate modulation of pb isotopes in the deep indian ocean linked to the himalayan chemical weathering we use reductive sediment leaching to extract lead pb from the authigenic fraction of marine sediments and reconstruct the pb isotope evolution of the deep central indian ocean over the past 250 thousand years at 3 kyr resolution temporal variations define a binary mixing line that is consistent with data from ferromanganese nodules and which records mixing between two well defined endmembers through time the unradiogenic endmember appears to represent a widely distributed pb source from mid ocean ridges or possibly volcanic aerosols while the radiogenic endmember coincides with the composition of ganges brahmaputra river sediments that are indicative of the himalayan weathering inputs glacial interglacial pb isotope variations are striking and can be explained by an enhancement of himalayan contributions by two to three times during interglacial periods indicating that climate modulates the supply of dissolved elements to the ocean while these changes could accurately record variations in the continental chemical weathering flux in response to warmer and wetter conditions during interglacials the relative proportions of pb derived from the ganges and brahmaputra appear to have been constant through time this observation may point towards particulate dissolved interactions in the estuary or pro delta as a buffer of short timescale variability in the composition and potentially flux of the fluvial inputs in addition the changes are recorded at 3800 m water depth and with the lack of deep water formation in the bay of bengal a mechanism to transfer such a signature into the deep ocean could either be reversible scavenging of dissolved pb inputs and or boundary exchange on the deep sea fan unless the mechanism transferring the pb isotope signature into the deep ocean was itself highly sensitive to global climate cycles and with the absence of a precessional signal in our pb isotope data we suggest that the indian climate and its influence on basin scale chemical weathering were strongly modulated by glacial versus interglacial boundary conditions

sedimentological and structural evolution of the arabian continental margin in the musandam mountains and dibba zone united arab emirates the northernmost extremity of the oman mountains the musandam peninsula is composed of an allochthonous sequence of permian to middle cretaceous shelf carbonates these are separated from ophiolitic rocks to the south by a northeast southwest trending belt known as the dibba zone this structurally complex belt is composed of allochthonous slope and basin facies sediments haybi volcanic rocks oman exotic limestones sub ophiolitic metamorphic rocks and ultra mafic slices lithofacies correlation confirms that fragmentation and rifting of a vast east facing carbonate platform occurred in middle to late triassic time resulting in the establishment of a shelf edge and small ocean basin this margin was typified from northwest to southeast by ooid skeletal lime sand shoals or small bioherms on the shelf edge a bypass foreslope of well laminated periplatform ooze and basin margin accumulations of carbonate turbidites or debris flows the shelf edge now coincides with the northern boundary of the dibba zone and appears to have remained stationary from middle triassic to late jurassic time periods of arrested carbonate sedimentation on the platform either because of exposure late triassic and middle cretaceous or because of drowning late jurassic are represented in the basin by starved sedimentation and deposition of radiolarian cherts a spectacular platform margin collapse occurred in the middle and late cretaceous as represented by massive conglomerates with clasts as young as albian above an unconformity that progressively removed all of the lower cretaceous the tethyan basinal and ophiolitic rocks of the dibba zone were emplaced from the east southeast during the turonian to lower maastrichtian after emplacement of these allochthonous units compressional deformation in the mid tertiary resulted in large scale open whaleback folds with wavelengths as much as 15 km generally with north south axes in places these folds which affect the complete shelf and allochthonous sequences are overturned toward the west and thrusting has caused previously lower tectonic units of the late cretaceous stacking order to be thrust over previously higher tectonic units thus reversing the late cretaceous tectonostratigraphy the maximum amount of translation on the later thrusts is in excess of 5 km on the hagab thrust where the complete shelf carbonate sequence of the musandam mountains has been thrust west northwest over the previously higher hawasina and haybi thrust sheets the tertiary folding and thrusting can be correlated in time and space with the zagros fold belt of southwestern iran

interpretation of magnetic and gravity surveys in the southern arabian gulf the strait of hormuz and the northwesternmost gulf of oman implications of pre permian basement tectonics abstract a 5 nt aeromagnetic chart and a 5 mgal bouguer gravity map of the area bounded by latitudes 25 30 27 10 n and longitudes 55 15 57 00 e in the southern arabian gulf the strait of hormuz and the entrance of the gulf of oman are collectively studied the total magnetic intensity map covering the coastal strip and the arabian side of the southern part of arabian gulf reveals three prominent anomalies of amplitudes ranging from 30 to 150 nt related to tectonically controlled igneous bodies emplaced in the deep seated pre permian basement of the arabian shield the bouguer gravity map indicates three well defined trends an effect duplicating a magnetic source in the basement a negative belt indicating an anomalous hormuz salt accumulation in the southern gulf salt basin and a continental margin gravity effect marking the transition from continental crust to the gulf of oman oceanic crust the magnetic and the gravity derived lineaments are shown to correlate with gemini interpreted strike slip basement faults in the iran s southeastern zagros fold zone

geographic control on pb isotope distribution and sources in indian ocean fe mn deposits abstract high precision pb isotope data have been obtained on indian ocean fe mn deposits with a large geographic coverage these data reveal a strong geographic control on the distribution of pb isotopes as well as sources of pb within this basin the provinciality of pb isotopes at the scale of the whole indian ocean as well as that of individual basins broadly matches the pattern of deepwater flow the existence of several sources of pb is best illustrated by the presence of three well defined pb isotopic arrays each of which is confined to clear cut geographic domains these arrays imply a dominance of binary mixing of pb sources within each of these domains the domains consist of the north indian n indian north of 20 s southwest indian sw indian 20 s to 50 s west of 45 e south indian s indian 20 s to 50 s east of 45 e and antarctic indian a indian south of 50 s and clearly exhibit a strong control by latitude the s indian domain and mozambique channel samples form a cluster suggesting that there are more than just two sources contributing in these regions we show that the n indian is dominated by sources of pb derived from the high himalayas and the trans himalayan complex and most likely inherited from interaction at the water interface with bengal and indus fan sediments the sw indian and a indian domains share a common unradiogenic component represented by circumpolar waters derived from the pacific ocean and flowing through the drake passage into the atlantic and indian oceans however the radiogenic sources of pb in these two domains are clearly distinct in the sw indian the radiogenic pb signal reflects the tongue of north atlantic deep water flowing around the tip of south africa while in the a indian the source of radiogenic pb is less certain but quite probably originates in the weddell sea region these data illustrate the complexity of the spatial pb isotopic distribution and pb sources within a single ocean basin at the present day which should be borne in mind when interpreting long term radiogenic isotope paleorecords from fe mn crusts

210pb scavenging in the north atlantic and north pacific oceans the radionuclide210pb shows significant geographic variations in the extent of its removal from the open ocean water column this texture of scavenging is defined by mapping 1 the integrated deficiency of210pb in the water column relative to its supply from the atmosphere and from in situ decay of dissolved226ra and 2 inventories of excess210pb in deep sea sediments the ratio of210pb deficiency to its supply termed the scavenging effectiveness is 20 in the north equatorial pacific and 50 in the north atlantic this variation is related to the combined effects of uptake of210pb onto sinking particles and lateral transport of210pb to areas of more intense removal sediment inventories of excess210pb normalized to the210pb deficiency in the overlying water column permit evaluation of the relative importance of these effects in the north equatorial pacific virtually all of the210pb removed from the water column is present in the underlying sediments but in the mid latitude north atlantic the sediments comprise only about 50 of the210pb removed the deficiencies of210pb in the mid latitude north atlantic sediments south of 50 n are qualitatively offset by surpluses in high latitude sediments north of 50 n higher primary productivity and new production in the surface waters of the high latitude north atlantic and north equatorial pacific relative to the oligotrophic central north atlantic may account for the greater fluxes of210pb to bottom sediments in those areas

recording changes in enadw composition over the last 340 ka using high precision lead isotopes in a fe mn crust we report high precision pb triple spike data and u th isotopic ages on a high resolution depth profile from a fe mn crust 121dk northeastern atlantic 24 n 21 w 2000 m our results bring important information particularly on the origin of the pb isotopic composition of the northeastern atlantic deep water and more generally on the use of pb isotopes as a paleo proxy to reconstruct weathering inputs to the north atlantic ocean growth rates determined using 230th ex and 230th 232th ex 2 9 0 2 and 3 0 3 mm ma respectively are found to be more reliable than 234uex sensitive to sea water contamination and are similar to the 10be results 3 0 3 mm ma very small variations in 206pb 204pb 18 942 18 964 208pb 204pb 39 057 39 083 and 207pb 204pb 15 696 15 701 are resolved defining linear trends in pb isotope diagrams high precision pb triple spike analysis of samples from crust bm1969 05 northwestern atlantic 39 n 61 w 1829 m are found to entirely reproduce the multi collector plasma source mass spectrometry data recently published this interlaboratory cross calibration establishes that the west north atlantic deep water wnadw does not represent a suitable pb contributor to the northeastern atlantic and rather suggests that the pb isotopic composition of wnadw is significantly altered before or during transport toward the northeastern atlantic basin the pb isotopic compositions of both suspended loads from the amazon mouth and offshore sediment cores are consistent on average with crust 121dk we therefore propose that the change in pb isotopic composition of wnadw may be caused by input of suspended particle loads from the amazon and possibly orinoco rivers or by interaction with sediments deposited by those rivers we speculate further that the short term compositional variations observed in this study may be caused by fluctuations in the relative contributions of amazon tributaries to the bulk composition supplied to the ocean because these tributaries sample terrains of drastically different ages phanerozoic andes and precambrian amazonian shield

closure of the central american isthmus and its effect on deep water formation in the north atlantic modern ocean thermohaline driven circulation influences global climate by transporting heat to high latitudes1 2 and by affecting the exchange of co2 between ocean and atmosphere3 north atlantic deep water nadw plays a key role in this circulation and quaternary climate cycles have been linked to changes in nadw flow4 general circulation model simulations indicate that before closure some 3 4 million years ago of the central american isthmus the narrow strip of land linking north and south america the direct flow of low salinity water from the pacific to the atlantic ocean would have led to a smaller nadw flow5 6 sedimentation patterns7 and nutrient proxies8 11 support these model results by indicating an increase in nadw flow around the time of isthmus closure but these records do not allow changes in different nadw sources to be distinguished and the overall effect of closure on global ocean circulation is poorly known here we present nd pb and sr isotope records preserved by a hydrogenous ferromanganese crust from the nadw flow path in the western north atlantic ocean these records indicate that the isotopic signal associated with nadw strengthened around 3 4 million years ago showing that deep water that formed in the labrador sea made a gradually increasing contribution to nadw flow these data taken together with those from the central pacific ocean12 indicate an increased nadw flow since isthmus closure and suggest that the closure established today s general pattern of ocean circulation

the geochemistry of rare earth elements in the amazon river estuary abstract the estuarine geochemistry of rare earth elements rees was studied using samples collected in the amazon river estuary from the amasseds amazon shelf sediment study cruise of august 1989 extensive removal of dissolved 0 22 m filtered trivalent rees from river water occurs in the low 0 6 salinity region removal by the salt induced coagulation of river colloids leads to fractionation among the ree iii series the order of removal is light rees middle rees heavy rees there also is the enhanced removal of ce relative to trivalent la and nd in the low salinity 0 6 zone and in the zone of high biological activity this is the first field observation of strong ce removal associated with coagulation of river colloids and biological productivity the argument is made that the decrease in the ce anomaly across a biological front is caused by biologically mediated oxidation of ce iii to ce iv coagulation of river colloids and biologically mediated oxidation of ce iii lead to fractionation of ree iii and redox modification of ce these processes result in the ree composition becoming fractionated relative to the amazon river water and crust and more evolved toward the ree composition of the oceans this study implies that reactions in estuaries play significant yet poorly understood roles in controlling the ree composition and ce anomaly of the oceans

impact of anthropogenic pb and ocean circulation on the recent distribution of pb isotopes in the indian ocean abstract pb and pb isotope ratios in the modern ocean have been altered significantly by anthropogenic pb inputs over the past century most studies of anthropogenic pb in the ocean have focused on the north atlantic and north pacific oceans and the impact of anthropogenic pb inputs to the indian ocean and processes controlling the distribution of pb in the indian ocean are poorly known this study presents the pb and pb isotopic composition 206 pb 207 pb 208 pb 207 pb of 11 deep stations from the indian ocean japanese geotraces cruise kh 09 5 from the bay of bengal and arabian sea to the southern ocean 62 s the pb isotope ratios of the indian ocean range 1 140 1 190 for 206 pb 207 pb and 2 417 2 468 for 208 pb 207 pb with lower ratios appearing in the shallow waters of the northern indian ocean and higher ratios appearing in the deep layers of the southern ocean this result agrees with a previous study on pb concentrations echegoyen et al 2014 showing that the indian ocean particularly its northern part is largely perturbed by anthropogenic pb inputs 206 pb 207 pb and 208 pb 207 pb of the indian sector southern ocean are still lower than natural pb showing this region was also affected by anthropogenic pb anomalously low or high 206 pb 207 pb and 208 pb 207 pb were observed in the thermocline and shallow waters of the southern indian ocean and the arabian sea which are ascribed to water mass distribution e g subantarctic mode water and evolving pb isotope ratios of this region as dominant anthropogenic pb sources change 206 pb 207 pb and 208 pb 207 pb in the bay of bengal are higher than those in the arabian sea which might be the result of the anthropogenic pb inputs from different provenance or seawater exchanging pb isotopes with natural particles derived from rivers and or sediments at the basin boundaries

pb nd and sr isotope mapping of grenville age crustal provinces in rodinia new pb nd and sr isotope data are presented for geochemically similar 1 1 1 2 ga granitoids and tonalitic granitic orthogneisses from antarctica southern africa and the falkland islands and adjoining plateau areas originally within the supercontinents of rodinia and gondwana these data support contentions for the presence of a mesoproterozoic 1 2 ga destructive plate margin running from namaqua natal southern africa through the displaced microplates of the falkland plateau and falkland islands the haag nunatak crustal block west antarctica and into western dronning maud land east antarctica the bulk of these granitoids represent juvenile mesoproterozoic additions to the crust except for in parts of east antarctica i e the sverdrupfjella where older paleoproterozoic or archean crust was involved in granitoid generation our isotope data permit plate reconstructions in which southern africa east antarctica and the falkland islands and plateau were adjacent within rodinia

tracing the history of submarine hydrothermal inputs and the significance of hydrothermal hafnium for the seawater budget a combined pb hf nd isotope approach secular variations in the pb isotopic composition of a mixed hydrogenous hydrothermal ferromanganese crust from the bauer basin in the eastern equatorial pacific provide clear evidence for changes in hydrothermal contributions during the past 7 myr the nearby galapagos rise spreading center provided a strong hydrothermal flux prior to 6 5 ma after 6 5 ma the pb became stepwise more radiogenic and more similar to equatorial pacific seawater reflecting the westward shift of spreading to the presently active east pacific rise epr a second previously unrecognized enhanced hydrothermal period occurred between 4 4 and 2 9 ma which reflects either off axis hydrothermal activity in the bauer basin or a late stage pulse of hydrothermal pb from the then active but waning galapagos rise spreading center hafnium isotope time series of the same mixed hydrogenous hydrothermal crust show invariant values over the past 7 myr hafnium isotope ratios as well as nd isotope ratios obtained for this crust are identical to that of hydrogenous equatorial pacific deep water crusts and clearly indicate that hydrothermal hf similar to nd does not travel far from submarine vents therefore we suggest that hydrothermal hf fluxes do not contribute significantly to the global marine hf budget

age cooling history and origin of post collisional leucogranites in the karakoram batholith a multi system isotope study u pb dating on zircon and monazite from different varieties of leucogranites in the karakoram batholith reveals two distinct pulses of plutonism at 25 5 0 3 0 8 and 21 4 0 3 0 6 ma in both granites these minerals contain inherited components documenting the presence of precambrian material in the magma source a crustal origin of the granites and long crustal residence times of the source rocks are confirmed by highly negative epsilon i nd strongly positive epsilon i sr and t dm nd in excess to 1 ga a biotite gneiss and a granitic dike from the karakoram metamorphic complex south of the batholith yield very similar epsilon i nd epsilon i sr t dm nd in a pb evolution diagram initial isotopic compositions for both the granites and the rocks from the metamorphic complex plot far above the reference field for morb documenting very close affinity to the eurasian tibetan continental crust the striking similarity of isotope signatures between leuco

plate tectonics radiogenic isotopic tracers and paleoceanography the case of the manganese crusts in the pacific abstract hydrogenous ferromanganese crust deposits on seafloor and volcanic seamounts are archives of past isotopic compositions of the ocean isotopic compositions of long life radiogenic isotopes that are dissolved in the ocean such as nd and pb are mixtures of different erosion products continents volcanics arcs ridges etc transported by oceanic currents or winds isotopic studies on mn crusts are thus potential paleogeographic and paleoceanographic tracers however global paleogeography can drastically evolve over the period of growth of crust as well as the latitude longitude and water depth of the mn crust thus each crust sampled a succession of geographic sites related to its spatial evolution through time for this reason valid information about the past ocean isotopic composition can only be extracted from mn crust under the condition that the successive paleogeographic and paleodepth positions can be reconstructed for each mn crust we followed this approach for six hydrogenous mn crusts from the pacific ocean that have recorded pb and nd isotopic signals for the past 80 myr the evolution of the isotopic data was considered within a proper geodynamical framework that takes into account the paleopositions of the pacific plate and surrounding continents of the hot spots and ridges as well as the paths of the six crusts the differences and similarities between the pb and nd revealed by our analysis emphasize their complementarity as paleoceanographic tracers the nd isotopes are strongly influenced by the exchange between indian and pacific waters specifically by the progressive closure of the indonesian gateway the closed central pacific characteristics are defined from the extrapolation of the isotopic trend this highlights the important role played by erosion of volcanic arcs in the nd isotopic value of seawater our approach also stresses the influence of local volcanic activity ridges or active hot spot for lead isotopes

tropical sst and sahel rainfall a non stationary relationship 1 sea surface temperature sst anomalies in the tropical atlantic have been associated with precipitation anomalies in west africa that form a dipole pattern with centers over the sahel and the gulf of guinea whilst this was clear before the 1970 s the dipole pattern almost disappeared after that date as the anti correlation between rainfall anomalies in the sahel and guinea dropped abruptly simultaneously the anti correlations between sahel rainfall and tropical pacific ssts strengthened it has been posited that these changes after the 1970 s developed as rainfall over west africa started to co vary with ssts in the global tropics in this co variability enhanced summer rainfall over west africa with a monopole pattern corresponds to warmer ssts in the tropical atlantic and maritime continent and colder ssts in the tropical pacific and western indian oceans the present paper describes the hitherto unexplored seasonal evolution of this co variability and the physical mechanisms at work sensitivity experiments with two atmospheric general circulation models demonstrate that after the 1970 s the impacts of sst anomalies in the indo pacific counteract those in the atlantic in terms of generating rainfall anomalies over the sahel and that this superposition of effects is primarily linear therefore at interannual timescales the change in the patterns of co variability between west african rainfall and tropical ssts can explain the non stationary relationship between the anomalies in these two fields

mass dependent cadmium isotopic variations in nature with emphasis on the marine environment abstract we report a survey of natural mass dependent cadmium isotope fractionation measured by thermal ionization mass spectrometry using a double spike technique ds tims over sixty samples of natural terrestrial cd from diverse environments including morb oib continental loess hydrogenic and hydrothermal ferromanganese deposits and sphalerites both oceanic and from major continental ore deposits were analysed our results are expressed in terms of e 112 110 cd which are deviations in 112 cd 110 cd from our in house jmc cd standard in parts per 10 4 the total e 112 110 cd variation is relatively small with a range of only 5 e units and is one to two orders of magnitude smaller than that previously found in meteorites the morb oib and loess e 112 110 cd values are similar and provide a good estimate for the bulk silicate earth bse value which is 0 95 0 12 relative to our cd standard e 112 110 cd 0 16 relative to munster jmc cd taken together these data suggest little cd isotope fractionation takes place during crust mantle segregation cd isotopic compositions of continental sphalerite zns deposits worldwide and high temperature oceanic hydrothermal sulphides show remarkably similar e 112 110 cd values consistent with our estimate for the bse in contrast mid temperature oceanic sulphides from a single extinct hydrothermal chimney display over 4 e units variation along with the most negative values these variations are most probably caused by precipitation redissolution of sulphide phases en route within the hydrothermal system the e 112 110 cd variability found in worldwide marine fe mn deposits reflects the seawater cd isotope signal upon precipitation from ambient seawater a decrease in e 112 110 cd is observed in passing from shallow water fe mn deposits to those from deeper waters 2000 m depth this shift is explained by biological fractionation related to the uptake of dissolved seawater cd by phytoplankton in the upper water column the relatively uniform e 112 110 cd values close to zero at great depths are consistent with regeneration and remineralization of cd at depth our data suggest that cd isotopes much like the cd ca ratio in foraminifera could potentially serve as a proxy for past changes in biological productivity the temporal cd isotope record in a fe mn crust archive at 2000 m depth from the ne atlantic suggests no gross long term changes in cd cycling took place over the past 8 ma

the geotectonic history of the makran and adjacent areas of southern iran abstract the makran mountain range in southern iran has been entirely covered by regional mapping on a scale of 1 250 000 but in contrast to the mapping of oman on the other side of the gulf the recent mapping is little known the cenozoic accretionary prism in the south fronts onto a mesozoic melange zone representing earlier subduction and to the north of this is an eastwards tapering microcontinental sliver which is followed by a zone of largely intact mesozoic ophiolite complexes to the north of this lies the jaz murian depression a desert expanse with no bedrock outcrop the sanandaj sirjan bajgan dur kan microcontinental sliver which extends from turkey through the zagros to the makran is one of a number of microcontinents which detached from gondwana no later than the early triassic nw iran yazd tabas lut birk helmand farah during the mesozoic these were enclosed by tracts of ocean now represented by ophiolites and the sectors closed at different times ranging from early cretaceous to end palaeogene an analogy is drawn with the present day mediterranean the sistan ocean in the east was infilled with an immense thickness of abyssal turbidites during the eocene before it closed the field evidence indicates that there was a second cenozoic accretionary prism here but sedimentation ceased after the eocene on its collision with the back of the main makran prism this eastern sistan trough now completely occluded is compared with the still open tract separating the makran from oman at the present time and its northern extension which has been similarly occluded by collision beneath the zagros some residual very shallow inland seas persisted inland from the makran through the neogene the occurrence of blueschists in the inner ophiolite belt of the makran is discussed abyssal turbidite sedimentation continued in the main southern zone of the makran into the early miocene to be succeeded by thick shallow water sequences including reefal limestones intense tectonic dislocation at the end of the miocene was accompanied by a shift of the subduction front to its present position out in the indian ocean and uplift leading to the deposition of spectacular pliocene pleistocene fanglomerates

changing mechanical response during continental collision active examples from the foreland thrust belts of pakistan abstract we have used data from teleseismic seismic reflection and field geologic studies along with both geomechanical and gravity modeling to contrast the tectonics of four active orogenic wedges in pakistan the kashmir himalaya the salt range potwar plateau foldbelt the sulaiman range and the makran accretionary wedge in makran oceanic crust is still being subducted and a thick pile of sediments is being accreted and underplated undercompaction and excess pore pressures can explain the narrow cross sectional taper and frontal aseismicity of this wedge beneath the sulaiman wedge continental crust is just starting to be underthrust indirect evidence suggests that fine grained carbonate rocks found in abundance deep in the stratigraphic section may be deforming ductilely at the base of the sulaiman wedge and provide a zone of ductile detachment the collision has proceeded to a much more mature stage in the salt range potwar plateau foldbelt and the kashmir himalaya isostatic response to underthrusting of continental crust has kept the sedimentary pile quite thin in both of these wedges so in that respect the two foldbelts are similar however thick eocambrian salt beneath the salt range and potwar plateau permits that foldbelt to be much wider in map view with a thinner cross sectional taper and a mixture of thrust vergence directions a major normal fault in basement causes the salt range to rise in front of the mildly deformed molasse basin of the southern potwar plateau much of the diversity among these mountain belts can be understood in terms of differences in the maturity of the collision process in each area the resulting thickness of the sedimentary pile encountered at the deformation front and the presence or absence of large contrasts in strength between the various layers of the stratigraphic section and basement relief

the sohar basin western gulf of oman description and mechanisms of formation from seismic and gravity data analysis of seismic lines coupled with gravity interpretation in the western gulf of oman shows that the rootless cretaceous ophiolite nappes are bounded to the north by the deep arcuate sohar basin two phases of sedimentary infill are clearly identified from late campanian to miocene and from miocene to quaternary the sohar basin is separated from the oceanic domain by a continental axis interpreted as the nw seaward prolongation of the saih hatat metamorphic unit we assume that creation of the sohar basin resulted from a late obduction uplift of the saih hatat axis followed by the re working of the basal thrust during a later neogene compressive event

lead isotopes in north pacific deep water implications for past changes in input sources and circulation patterns the sources of non anthropogenic pb in seawater have been the subject of debate here we present pb isotope time series that indicate that the non anthropogenic pb budget of the northernmost pacific ocean has been governed by ocean circulation and riverine inputs which in turn have ultimately been controlled by tectonic processes despite the fact that the investigated locations are situated within the asian dust plume and proximal to extensive arc volcanism eolian contributions have had little impact we have obtained the first high resolution and high precision pb isotope time series of north pacific deep water from two ferromanganese crusts from the gulf of alaska in the ne pacific ocean and from the detroit seamount in the nw pacific ocean both crusts were dated applying 10be 9be ratios and yield continuous time series for the past 13 5 and 9 6 myr respectively lead isotopes show a monotonic evolution in 206pb 204pb from low values in the miocene 18 57 to high values at present day 18 84 in both crusts even though they are separated by more than 3000 km along the aleutian arc the variation exceeds the amplitude found in equatorial pacific deep water records by about three fold there also is a striking similarity in 207pb 204pb and 208pb 204pb ratios of the two crusts indicating the existence of a local circulation cell in the sub polar north pacific where efficient lateral mixing has taken place but only limited exchange in terms of pb with deep water from the equatorial pacific has occurred both crusts display well defined trends with age in pb pb isotope mixing plots which require the involvement of at least four distinct pb sources for north pacific deep water the pb isotope time series reveal that eolian supplies volcanic ash and continent derived loess have only been of minor importance for the dissolved pb budget of marginal sites in the deep north pacific over the past 6 myr the two predominant sources have been young volcanic arcs one located in the northeastern part and one located in the northwestern part of the pacific margin from where material has been eroded and delivered to the ocean most likely via riverine pathways

evolution of nd and pb isotopes in central pacific seawater from ferromanganese crusts abstract hydrogenetic ferromanganese crusts incorporate elements from ambient seawater during their growth on seamounts by analysing nd pb and be isotope profiles within crusts it is possible to reconstruct seawater tracer histories depth profiles of 10be 9be ratios in three pacific ferromanganese crusts have been used to obtain growth rates which are between 1 4 and 3 8 mm ma nd and pb isotopes provide intact records of isotopic variations in pacific seawater over the last 20 ma or more there were only small changes in pb isotope composition in the last 20 ma this indicates a constant pb composition for the erosional sources and suggests further that erosional nd inputs may have been uniform too end values vary considerably with time and most probably reflect changes in ocean circulation the end values of the crusts not only vary as a function of age but also as a function of water depth from 25 to 0 ma crust va13 2 from 4 8 km water depth has a similar pattern of end variation to the two shallower crusts from 1 8 and 2 3 km but about 1 0 to 1 5 units more negative this suggests that end stratification in pacific seawater as demonstrated for the present day has been maintained for at least 20 ma each crust shows a decrease in end from 3 5 ma to the present which is interpreted in terms of an increase in the nadw component present in the pacific from 10 to 3 5 ma ago the crusts show an increase in end this suggests a decreasing role for a deep water source with end less than circum pacific sources in this regard the panamanian gateway restriction from 10 ma with final closure at 3 5 ma may have played an important role in reducing access of atlantic derived nd to the pacific

anthropogenic lead isotopes in antarctica we report the first measurements of pb isotopes in antarctic snow which show that even recent snow containing 2 3 pg g is highly polluted with anthropogenic pb this follows from a comparison of isotope abundances of pb in surface snow and terrestrial dust extracted from ancient antarctic ice dome c depth 308 m approximate age 7 500 a bp the latter being distinctly more radiogenic this result is independent of geochemical arguments based on measurements of al na and so 4 south america is suggested as a likely source of this anthropogenic pb the presence of significantly less radiogenic pb in the snow adjacent to two antarctic base stations indicates that there is contamination from station emissions although emission from australia is an alternative explanation for a site 33 km from dumont d urville

sources of osmium isotopes in manganese nodules abstract we have measured the 187 os 186 os and os concentrations of ferromanganese crusts forming on serpentinized peridotites from the atlantic ocean and also report major element data for these samples and six manganese nodules previously analyzed for os variations in the 187 os 186 os ratios of manganese nodules are the result of two processes operating on different spatial scales the large scale variations result from the impression of intense local os inputs from continental and oceanic sources on a generally homogeneous oceanic 187 os 186 os ratio fine scale variations may be understood as the result of the mixing of a constant fine grained debris flux of meteoritic os imposed on varying amounts of os precipitated from seawater

experimental modeling of role of gravity and lateral shortening in zagros mountain belt dynamically scaled analogs of the geologic structures of the zagros mountain belt are used to argue that different parts of the zagros mountains of iran record different combinations of the effects of a gravity driven overturn and a southwest northeast lateral shortening superimposed on the zagros overturn partially scaled material models have been used to simulate the zagros geodynamics which involve layer parallel compression of a 6 to 7 km thick phanerozoic carbonate cover containing a pattern of preshortening diapirs the folds in the zagros form rapidly 1 5 mm yr in a 20 to 30 km wide zone reactive some of the preshortening diapirs and generate new synshortening listric diapirs a third set of postshortening diapirs rises from the hormuz decollement behind the fold thrust front model buckle folds superimposed on diapirs or pillows tend to avoid and curve around preshortening diapirs which flatten in the synclines model profiles show that lateral shortening induces residual salt at depth to flow toward and rise through the anticlinal cores as synshortening or postshortening diapirs i suggest that any salt pillows in currently diapir free zones of the zagros fold thrust belt may surface as diapirs through the anticlines in the future in the absence of well data seismic and field observations i have used only three map pattern criteria to recognize the timing of diapirs these are locally obscured by extrusive salt sheets preshortening diapirs are generally small and elongate parallel to fold axes and are restricted to synclines synshortening diapirs are elongate perpendicular to the fold axes and are restricted to anticlines postshortening diapirs can be the same size as synshortening diapirs or larger and can be circular they are more common in the anticlines

out of sequence thrusts out of sequence thrusts oost are those thrusts which do not obey the foreland propagating or in sequence deformation style they include both isolated thrusts which develop hindward of the thrust front and sequences of break back thrusts which propagate from the foreland to the hinterland two end members of a series of oost types are recognized 1 older in sequence thrusts which are reactivated along their entire length and 2 completely new thrusts which propagate through already deformed thrust sheets between the two end members are thrusts composed partially of reactivated in sequence thrust sequences and partially of new entirely out of sequence segments oosts can be initiated for a variety of reasons including 1 keeping the orogenic wedge at critical taper 2 break back sequences from the suture zone in the overriding plate 3 ramping to overcome a sticking point which inhibits in sequence thrust propagation and 4 during simultaneous displacement along two stacked thrusts culminations which bow up segment of the upper thrust may be chopped through to permit continued displacement on the upper thrust many different types of thrust behavior including gravity sliding plucking and derivation of isolated horses from ramps may mimic some of the characteristics of oosts consequently it may be difficult to conclusively prove an oost origin for a complex thrust geometry

eolian inputs of lead to the north pacific we evaluate the importance of natural eolian pb to the dissolved oceanic pb budget by measuring the isotopic composition of pb in 35 holocene and late quaternary sediment samples from the north pacific and in 10 samples of chinese loess when the pacific is divided into sediments provinces based on published end and sedimentological data pb from the central north pacific tends to be the most radiogenic and homogeneous due to the dominance of eolian chinese loess lead from the marginal north pacific and the sparsely sampled regions south of 5 n are less radiogenic and more variable owing to hemipelagic inputs from various volcanic arcs and older continental crust located around the pacific rim 208pb 204pb ratios provide the most distinctive provenance information due to the relatively high ratios in chinese loess the chinese loess samples come from 3 localities and span up to 2 myr of time acetic acid leachate bulk loess and loess silicate fractions were analyzed separately leachate pb is considerably less radiogenic than silicate pb the isotopic composition of the silicate component closely matches the sediment data from the central north pacific confirming the dominance of eolian loess in this region we divided up a suite of published hydrogenous pb isotope data from the pacific ocean according to their locations within the three independently defined sediment provinces these data define three distinct fields differentiated primarily by their 206pb 204pb ratios which increase going from the central to southern to marginal provinces this relationship with sediment province strongly suggests that natural eolian and probably hemipelagic inputs significantly impact the seawater pb budget direct support for the dominance of eolian chinese loess in the central north pacific dissolved pb budget comes from the close match between loess leachate pb and the central province hydrogenous pb data eolian inputs are likely to dominate local seawater pb budgets only where eolian pb fluxes are high relative to hemipelagic pb fluxes and where the flux of pb carried by deep water advection is low

geochemical investigations of the origin of the manaslu leucogranite himalaya nepal the himalayan orogeny is characterized by an intraplate subduction zone the thrusting of the tibetan slab over the foreland is probably responsible for the origin of the manaslu type leucogranites this geochemical study tries to examine the crustal origin of the granite to verify whether the paragneisses of the tibetan slab are the most probable parent material and to determine the subsequent petrogenetic processes the rb sr systematics suggest that 1 1 the scatter of data points in the 87sr86srvs 87rb86sr diagram may be due to the heterogeneity of the initial 87sr86sr ratios 2 2 the initial isotopic ratios are very high 0 730 0 760 and are in agreement with those of the gneisses of the tibetan slab at the time of the melting pb isotopic ratios are homogeneous they are also consistent with a crustal prehistory the ree and th contents are unexpectedly low in comparison with the much higher contents of the parent material monazite left in the residue of melting or more probably extracted in an early stage of the magmatic history may explain the observed ree and th abundances alternatively field observations show that the emplacement of the granite has been accompanied by a very strong hydrothermal activity in consequence the low ree and th contents of the granite could also be due to the loss by solution during prolonged fluid circulation throughout the granitic melt and the sr and pb isotope distribution to the effect of late to post magmatic fluids the isotopic character of which was determined by the underlying tibetan slab

world map of nd isotopes in sea floor ferromanganese deposits the nd isotopic compositions of hydrogenous ferromanganese encrustations and nodules largely reflect those of the overlying seawater integrated over the few millions of years over which they grow available nd isotope data on ocean floor hydrogenous deposits suggest control by nd from particulates blown from the continents and released by volcanic activity associated with subduction zones and midocean ridges the regional nd isotope variations display a marked similarity to the broad patterns of present day deep ocean circulation suggesting that the particulates that are the sources of oceanic nd travel along with water masses despite rapid 10 100 ka perturbations during glacial periods the long term patterns of water mass movements and oceanic fluxes of particulates have remained essentially stable over the last few million years

climate forcing of the pb isotope record of terrigenous input into the equatorial atlantic abstract pb isotopic compositions of bulk sediment cores from the tropical atlantic are used here to infer variations in the provenance of terrigenous input to the tropical atlantic during pleistocene climate cycles the 200 ka high precision 2 100 ppm pb isotope records of the ceara rise western atlantic and sierra leone rise eastern atlantic cores show a clear glacial interglacial cyclicity reflected by alternating unradiogenic pb and radiogenic pb in both cores the glacial interglacial pb isotopic contrast is also observed in pb pb space and can be explained in terms of binary mixing variations along the mixing lines reflecting changes in the relative proportions of the glacial unradiogenic and interglacial radiogenic pb source s the pleistocene pb isotopic variability of the terrigenous input to the ceara rise can be linked to changes in the weathering styles in the amazon basin and as a result in the amazon river discharge these changes are reflected by a greater contribution from the highlands andes pb source during glacial times and strengthening of the lowlands shield pb source during interglacials on the other hand pb isotopic variations in the sierra leone rise core indicate increased pb inputs from the saharan dust plume during glacial times in agreement with the wind patterns over africa furthermore this result bears some implications on the cause of the enhanced glacial terrigenous fluxes to the tropical atlantic which we infer to be due to increase in winter wind transport rather than glacial hyperaridity the cyclicity of the ceara rise and sierra leone rise pb isotopic records together with changes in the proportions of mixing sources throughout the last 200 ka monitor changes in the hydrological cycle over south america and the wind systems over africa respectively both of which are linked to the seasonal latitudinal migration of the intertropical convergence zone the correlation found between pb isotope cycles and other paleoclimate proxies suggests that pb isotopes may be responding to variations in earth s orbital parameters

progressive unconformities within an evolving foreland fold thrust belt zagros mountains a major angular unconformity between the bakhtyari conglomerates and the underlying agha jari formation has long been interpreted as indicating that orogeny in the zagros simply folded zone took place in plio pleistocene times this study uses field evidence of unconformities between older units in conjunction with geological maps and cross sections to argue that the front of the zagros simply folded zone has propagated in time and space these unconformities indicate that deformation started as early as end eocene in the northeast of the simply folded zone and propagated progressively to the southwest where unconformable contacts are only seen between younger units as shortening continued the southwest migration of the deformation front drove the foreland basin in front of it to its present position along the persian gulf and mesopotamia the climax of orogeny took place at end pliocene time when the most extensive unconformity in the zagros simply folded zone developed between the upper bakhtyari formation and older units active seismicity and documented present uplift imply that the simply folded zone is still propagating southwestward

a nd isotopic study of southern sourced waters and indonesian throughflow at intermediate depths in the cenozoic indian ocean we present nd isotopic data for fossil fish teeth recovered from the past 40 m y at odp site 757 currently located at 1650 m water depth on the ninetyeast ridge in the indian ocean although site 757 sits in a region strongly influenced by weathering inputs from the himalayas and volcanic inputs from the indonesian arc the pattern of nd isotopic variations does not appear to respond to these potential sources of nd instead secular variations correlate to changes in the composition of intermediate to deep water masses bathing the site and circulation patterns in the indian ocean from 40 to 10 ma nd values and the pattern of change at site 757 closely match those of odp site 1090 a deep water site in the atlantic sector of the southern ocean comparison to data from several fe mn crusts in the indian ocean suggests that intermediate to deep water flow paths were similar to the modern distribution of circumpolar deep water at approximately 10 ma nd isotopic values increase in a step function by 2 nd units suggesting that plate motions had carried site 757 into a region influenced by indonesian throughflow estimates of the vertical and horizontal position of this site at 10 ma imply that indonesian throughflow extended as far south as 20 s and to a depth of 1500 m from 10 to 0 ma nd isotopic variations at site 757 appear to record variations in indonesian throughflow from 10 to 5 5 ma values at site 757 overlap with those from crusts located in the southwest pacific indicating extensive flow through the indonesian seaway from 5 5 to 3 4 ma nd values become less radiogenic at site 757 and more radiogenic in the southwest pacific suggesting increasing closure of the seaway and concomitant rerouting of equatorial pacific waters beginning at 3 4 ma nd values become more radiogenic again at site 757 which may be attributed to enhanced opening of the seaway or to a change in the source of throughflow waters from a southern to a northern pacific region

how much deep water is formed in the southern ocean three tracers are used to place constraints on the production rate of ventilated deep water in the southern ocean the distribution of the water mass tracer po4 phosphate star in the deep sea suggests that the amount of ventilated deep water produced in the southern ocean is equal to or greater than the outflow of north atlantic deep water from the atlantic radiocarbon distributions yield an export flux of water from the north atlantic which has averaged about 15 sv over the last several hundred years cfc inventories are used as a direct indicator of the current production rate of ventilated deep water in the southern ocean although coverage is as yet sparse it appears that the cfc inventory is not inconsistent with the deep water production rate required by the distributions of po4 and radiocarbon it has been widely accepted that the major part of the deep water production in the southern ocean takes place in the weddell sea however our estimate of the southern ocean ventilated deep water flux is in conflict with previous estimates of the flux of ventilated deep water from the weddell sea which lie in the range 1 5 sv possible reasons for this difference are discussed

the accommodation of arabia eurasia plate convergence in iran continental convergence between arabia and eurasia is taken up by distributed deformation in iran at wavelengths large compared with the thickness of the lithosphere this deformation is best described by a continuous velocity field the only quantitative source of information on the spatial distribution of strain rates within iran is the record of earthquakes we find that we can reproduce the style of deformation observed in the seismicity by simply minimizing the rate of work in a continuous viscous medium that has to accommodate the arabia eurasia plate motion between the defined shapes of iran s rigid borders when in addition we specify central iran azerbaijan and the southern caspian basin to be relatively rigid blocks within the deforming zone then the fit to the style of the observed strain rate distribution is even better we conclude that much of the pattern of deformation in iran is predetermined by the shape of its rigid borders and by the disposition of relatively rigid blocks within it this is likely also to have been a common occurrence in older orogenic belts we confirm earlier suggestions that earthquakes between 1909 and 1992 can account for only a small part 10 20 of the total deformation required by the convergence between the arabia and eurasia plates we then show that the whole plate motion can be accommodated by a velocity field with the same orientations and relative magnitudes of principal strain rates seen in the earthquakes but with larger absolute magnitudes there is therefore no requirement that the large proportion of aseismic deformation in iran is substantially different in style orientation or distribution from that released seismically in the earthquakes

on the origins of deep and bottom waters of the indian ocean the characteristics of the deep and bottom waters of the indian ocean when illustrated on potential density anomaly surfaces indicate that the waters enter from both the atlantic and pacific oceans the paths of spreading are constrained by the complex topography and characteristics are seen to be altered by exchange with the overlying and underlying water and with the sediments especially in the northern indian ocean the weddell sea contributes to the densest waters found in the western basins and the ross sea and adelie coast to the densest waters found in the eastern basins both dense water varieties are altered by and incorporated in the less dense water above initially water carried by the circumpolar current then water from the north atlantic and finally by deep water whose characteristics are derived in the northern indian ocean contact with the sediments increases the silica content of the bottom water in the southern ocean in the northern indian ocean the sediments alter the silica of the water at the bottom and together with enhanced salinity from diffusion of saline overflows from the marginal seas above imprint unique markers to the deep water that flows back to the south at middepths the series of ridges between madagascar and australia confine the flow to a series of gyres that carry characteristics from the circumpolar current equatorward and the northern indian ocean characteristics southward within the circumpolar current low oxygen deep water from the pacific is carried across the atlantic and into the indian ocean south of africa part flows around the cyclonic weddell sea gyre and part extends across the southern ocean water from another pacific source can be seen near 2000 m extending westward from the tasman sea south of aus tralia and across the indian ocean and perhaps to the aghulas current region southeast of africa in this study we show some maps of deep and bottom water characteristics of the indian ocean which may be used in a qualitative sense to infer pathways of spreading of water below a depth of about 2000 m the indian ocean has numerous ridges and basins figure 1 that severely restrict possible pathways of deep and bottom wa ter flow in the following we will begin by looking at char acteristics at the bottom of the ocean to reveal the path ways of spreading of the densest bottom waters from basin to basin then a series of six potential density anomaly surfaces at progressively shallower depths from about 4000 m up to about 2000 m will be shown to see what character istics influence the deep waters and how pathways of flow are constrained by the bathymetry of the indian ocean the choice of the specific density levels table 1 was based primarily on various property extrema shown by the 18os atlantis ii cruise 93 colored sections that accompany warren s 1981 detailed discussion of the major features of the central indian ocean between madagascar and aus tralia the sections approximately midway between antarctica and the northern indian ocean boundaries are well positioned to show features originating from both the north and the south as well as some brought into the indi an ocean by the antarctic circumpolar current the full ocean area isopycnal maps provide essential evidence on the origins and fates of features seen in the sections and

the behavior of particle reactive tracers in a high turbidity environment 234th and 210pb on the amazon continental shelf excess 234th and 210pb seabed inventories were measured in cores collected from the amazon continental shelf to examine particle scavenging and seabed dynamics typical excess 210pb inventories range from 100 to 300 dpm cm 2 and the total excess 210pb inventory for the amazon shelf was determined to be 2 7 1017 dpm the 210pb measurements indicate that particle reactive species are scavenged not only from the amazon river but also from the lateral advection of offshore water in order to sustain the 210pb inventories the volume of water supplied by the lateral advection from offshore must be approximately five to ten times the water discharge of the amazon river this lateral advection supplies about 67 of the total excess 210pb to the amazon continental shelf with relatively small contributions from riverine input 31 atmospheric fallout 2 3 and in situ production 0 1 the 234th inventories were measured on four cruises which occurred during periods of differing river discharge wind stress and flow rates of the north brazil current the 234th excess seabed inventories show large spatial and seasonal variability with a range from 0 to 22 dpm cm 2 ratios of predicted 234th inventories over observed 234th inventories indicate that a large portion of the inventories on the inner and mid shelf occur in the fluid mud layer or the suspended sediments the 234th excess seabed inventories are controlled by resuspension cycling the inventories between the water column and the seabed 210pb and 234th excess seabed inventories are compared directly using corrections for the different time scales and offshore concentrations this approach indicates that for most of the shelf the inventories of the shorter term tracer 234th are less than predicted by the inventories of the longer term tracer 210pb there are two explanations for this trend the first is that a larger portion of the 234th inventory occurs in the fluid muds or the water column relative to 210pb the second is that the supply of offshore water scavenging efficiency and or deposition have been lower over the two year study period relative to the last one hundred years

structure and evolution of the northern oman margin gravity and seismic constraints over the zagros makran oman collision zone abstract the obduction process in oman during late cretaceous time and continental to oceanic subduction along the zagros makran region during the tertiary are consequences of the arabian eurasian collision resulting in construction of complex structures composed of the oman ophiolite belt the zagros continental mountain belt and the makran subduction zone with its associated accretionary wedge in this paper we jointly interpret bouguer anomaly and available petroleum seismic profiles in terms of crustal structures we show that the gravity anomaly in northern oman is characterized by a high amplitude negative positive couple the negative anomaly is coincident with late cretaceous fiqa and tertiary pabdeh foreland basins and with the zagros oman mountain belts whereas the positive anomaly is correlated to the ophiolite massifs the bouguer anomaly map indicates the presence of a post late cretaceous sedimentary basin the sohar basin centred north of the batinah plain we interpret the negative positive couple in terms of loading of the elastic arabian lithosphere we estimate the different cretaceous to recent loads including topography ophiolite nappes sedimentary fill and the accretionary prism of the makran trench a new method using mindlin s elastic plate theory is proposed to model the 2d deflection of the heterogeneous elastic arabian plate taking into account boundary conditions at the ends of the subducted plate we show that remnant ophiolites are isolated from tethyan oceanic lithosphere in the gulf of oman by a continental basement ridge a nw prolongation of the saih hatat window loading the northward limited ophiolite blocks explains the deflection of the fiqa foredeep basin west of the musandam peninsula the tertiary pabdeh foredeep is probably related to the emplacement of a 8 km thick tectonic prism located on the musandam peninsula and in the strait of hormuz final 2d density models along profiles through the oman mountain belt and the gulf of oman are discussed in the framework of late cretaceous obduction of the tethys and synchronous subduction and exhumation of the oman margin

submarine hydrothermal venting related to volcanism in the lesser antilles evidence from ferromanganese precipitates 1 radiogenic isotope compositions sr nd pb hf and os of sediment hosted seafloor ferromanganese crusts and sediments incrusted with ferromanganese oxyhydroxides from the lesser antilles island arc were measured to distinguish between hydrogenous seawater derived and hydrothermal metal sources the ages of the precipitates range between recent last few thousand years and a few 100 kyr as deduced from 10be and co concentrations evidence from the presence of bladed todorokite and nontronite together with the major element and ree composition suggests that a significant proportion of these sediment hosted precipitates formed at relatively low temperatures from a mixture of seawater and hydrothermal fluids associated with island arc volcanism the radiogenic isotope compositions of all metals mentioned above except pb show large differences in hydrothermal versus hydrogenous contributions over space and time in contrast to precipitates of high temperature fluids which mainly scavenge their ree contents from seawater the crusts of this study show 143nd 144nd of up to 0 512817 nd 3 5 this is close to the signature of the nearby island arc rocks and far above the expected local seawater ratio of 0 51209 nd 10 7 these crusts also show high 176hf 177hf up to 0 283102 low 87sr 86sr up to 0 7069 and low 187os 188os up to 0 16 compared with local seawater as expected from hydrothermal island arc derived metal contributions in contrast the pb isotope signatures of the crusts cannot be explained by mixing between seawater and hydrothermal sources it is suggested that pb was either removed from the ascending fluids within the sediment column before they reached seawater or the temperatures were too low to leach significant amounts of pb from the rocks or sediments external sources such as saharan dust particulate inputs from the orinoco river or even incongruent release of pb isotopes from the island arc rock derived particles must have contributed to the observed pb isotope variability our results suggest that submarine hydrothermalism originating from intraoceanic island arc volcanism creates distinct geochemical environments for the dispersion of hydrothermal fluids and may be an important mechanism to supply metals of hydrothermal origin to seawater

constraints on archaean crustal evolution of the zimbabwe craton a u pb zircon sm nd and pb pb whole rock isotope study the u pb ages of zircons from seven felsic volcanic and plutonic rocks from northern zimbabwe combined with field data and pb pb and sm nd whole rock isotope data constrain the timespan of development of the harare shamva granite greenstone terrain and establish the relative involvement of juvenile mantle derived and reworked crustal material basement cover field relationships and isotope and geochemical data demonstrate that the greenstones were deposited onto 3 2 2 8 ga basement gneisses in ensialic continental basins geodynamic models for the generation of the areally extensive bimodal magmatic products and growth of the pre existing crustal nucleus consistent with our interpretations are rift related 1 intracontinental rifting related to mantle plume activity or 2 rifting in a back arc environment related to a marginal volcanic arc the data in conjunction with field evidence do not indicate the presence and accretion of an older ca 2 70 ga and a younger ca 2 65 ga greenstone sequence in the harare part of the greenstone belt as was recently postulated on the basis of shrimp zircon ages zircon ages for basal felsic volcanics 2715 15 ma and a subvolcanic porphyry 2672 12 ma constrain the initiation and termination of deposition of the greenstone sequence the timespan of deposition of the upper bulawayan part of the greenstone sequence corresponds well with radiometric ages for upper bulawayan greenstones in the central and southern part of the craton and supports the concept of craton wide lithostratigraphic correlations for the late archaean in zimbabwe zircon ages for a syn tectonic gneiss 2667 4 ma and a late syn tectonic intrusive granodiorite 2664 15 ma pinpoint the age of deformation of the greenstone sequence and compare well with a pb pb age of shear zone related gold mineralization 2659 13 ma associated with the latter intrusive phase the intimate timing relation of greenstone deformation and granitoid emplacement but also the metamorphic evidence for a thermal effect of the batholiths on the surrounding greenstone belts and the structural and strain patterns in the greenstone sequence around and adjacent to the batholiths imply that the intrusion of the granitoids had a significant influence on the tectono thermal evolution of the greenstone belt prolonged magmatic activity is indicated by the zircon ages of small post tectonic plutons intrusive into the greenstone belt with a mineralized granodiorite dated at 2649 6 ma and an unmineralized tonalite at 2618 6 ma the 2601 14 ma crystallization age of an external chilimanzi type granite agrees well with existing radiometric ages for similar granites within the southern part of the craton demonstrating a craton wide event and heralding cratonization the similarity between u pb zircon ages and tdm model ages 2 65 2 62 ga and the positive ndt values 3 to 2 for the late syn tectonic and post tectonic intrusive plutons within the greenstone belt indicate magmatism was derived directly from the mantle or by anatexis of lower crustal sources with very short crustal residence times and minor contamination with older crust the rather high model 1 values 8 2 8 6 are unlikely to indicate the involvement of significant amounts of older crust and may be inherited from a high u pb mantle source as was suggested by previous workers for the archaean mantle beneath southern africa the older tdm ages for the felsic volcanics 3 0 2 8 ga and the porphyries 2 8 2 7 ga suggest that these felsic magmas were derived by partial melting of a source that was extracted from the mantle ca 200 ma prior to volcanism or may indicate interaction between depleted mantle derived melts and older crustal material

lead isotopes behavior in the fumarolic environment of the piton de la fournaise volcano reunion island abstract the recent activity of the piton de la fournaise volcano offers a rare opportunity to address the issue of pb isotope behavior in volcanic fumaroles as the composition of the degassing source is accurately and precisely known gas sublimates formed between 2007 and 2011 at temperature ranging from 400 to ca 100 c include na k sulfate aphthitalite ca cu sulfate e g gypsum na sulfate thenardite ca mg al fe fluoride e g ralstonite and native sulfur the high temperature deposits show trace element patterns typical of volcanic gas with pb concentration up to 836 ppm while the low temperature deposits are depleted in most volatile elements pb only for low temperature fluoride samples do pb isotope compositions plot significantly outside the field of lavas the isotopic shift is ascribed to leaching ubiquitous unradiogenic phases e g sulfides by acidic gas condensates the similarity in pb isotope signature between lavas and sublimate samples more representative of the gas phase sulfates indicates that the net fractionation of pb isotopes resulting from volatilization and condensation processes is smaller than the precision of pb isotope measurements better than 60 ppm a m u the absence of net fractionation could result from negligible isotope fractionation during pb volatilization followed by extensive condensation of gaseous pb with possibly significant isotopic fractionation at this stage although this scenario has to be refined by more direct measurement of the gas phase and its general applicability tested it suggests that a small fraction resolving very small isotopic differences between magmas and their gaseous products remains an analytical challenge high precision pb isotope measurements rest not only on instrumental performance but also on high yield chemistry as pb isotopes drastically fractionate 800 ppm a m u upon elution on anionic resin for 50 pb recovery the estimated isotopic bias is plus or minus 60 80 ppm a m u depending on which of the early isotopically light or late isotopically heavy pb fraction is lost

origin and fate of sulfide liquids in hotspot volcanism la reunion pb isotope constraints from residual fe cu oxides immiscible sulfide liquids in basaltic magmas play an important role in trace metal transport and the sulfur budget of volcanic eruptions however sulfides are transient phases whose origin and fate are poorly constrained we address these issues by analyzing sulfide destabilization products preserved in lavas from la reunion island iron oxide globules and coatings typically 20 80 m in size were found to occur in vesicles of differentiated lavas from piton des neiges and recent pumice samples from piton de la fournaise field and mineralogical evidence indicates that the iron oxides are syn eruptive phases not resulting from hydrothermal processes samples were first studied by scanning electron microscopy the globules were separated whereas the smaller spherules and coatings were concentrated by magnetic sorting and acid leaching and samples were processed through wet chemistry the fe oxide phases comprise 49 74 wt fe 26 40 wt o and up to 6 wt cu 811 ppm ni 140 ppm bi and 8 5 ppm pb compared to the host lava cu ni and bi are enriched by a factor of 101 103 systematic pb isotope disequilibrium between 500 ppm and 2 9 for 206pb 204pb exists between fe oxides and host rocks with fe oxides generally displaying less radiogenic ratios unradiogenic pb is a typical signature of sulfide which tends to concentrate pb but not its parent elements u and th thus both the chemical and isotopic compositions of the vesicle hosted fe oxides suggest that they are more or less direct products of the destabilization of immiscible sulfide liquids although pb dominantly partitions into the gas phase during sulfide breakdown the original pb isotope signature of sulfide is preserved in the residual oxide the composition estimated for the parent sulfides 206pb 204pb 18 20 18 77 207pb 204pb 15 575 and 208pb 204pb 38 2 38 8 precludes a genetic link with the la reunion plume and suggests a lithospheric or crustal origin it is estimated that magma ascent velocities at piton de la fournaise are high enough to counterbalance the settling velocities of millimeter size sulfides despite their high density sulfide liquids are thus transferred upward during eruptions and their destabilization contributes to so2 emanations assimilation of foreign sulfides from the lithosphere can explain why so2 emissions sometimes e g during the april 2007 eruption exceed those predicted from the s content of melt inclusions

the pb isotope geochemistry of granitoids from the himalaya tibet collision zone implications for crustal evolution abstract pb isotopic data on 80 k feldspars from six individual granitoid belts of the himalaya tibet collision zone are reported the isotopic trends observed in the trans himalayan complex which outcrops along the indus zangpo suture zone are best explained by a mixture of mantle derived pb component s with crustal derived pb these confirm that the batholith was built upon sialic continental crust as a result of the northward subduction of mesozoic oceanic crust beneath southern tibet the pb isotopic compositions of k feldspars from the three granitoid belts located within the lhasa block to the north of the indus zangpo suture zone are all radiogenic and indistinguishable from those of k feldspars extracted from basement rocks no pristine mantle derived component was identified and the granitoids are interpreted as being wholly derived from the partial melting of the continental crust furthermore the isotopic compositions of the granitoids are remarkably similar over the entire lhasa block implying that they were equilibrated by fluid remobilization during granulite facies metamorphism in the lower crust to the south of the indus zangpo suture zone and within the indian shield the lead isotopic composition of k feldspars from two granitoid belts although significantly more radiogenic than those of the lhasa block are identical to that of the basement gneisses indicating also that the granitic melts are the products of the anatexis of the continental crust in both the lhasa block and the indian shield the generation of granitic melts occurred as a consequence of large scale intracontinental thrusting events that created crustal repetition and induced the development of thermal anomalies at depths due to the burial of meta sedimentary rocks rich in water and radioactive heat producing elements the lhasa block and the indian shield define two distinct pb isotopic provinces which have undergone complex episodes of u th pb fractionation and are largely composed of precambrian material furthermore the correspondence between distinct pb isotopic provinces and discrete continental segments might indicate that the blocks originated from different sectors of the gondwanian continent