evidence for preindustrial variations in the marine surface water carbonate system from coralline sponges 1 coralline sponge skeletons are excellent tools for reconstructing the carbon isotope history of dissolved inorganic carbon dic in tropical surface waters carbon isotope records from coralline sponges clearly reflect the industrial 12c increase in atmospheric co2 with a precision that permits quantitative interpretations we find from a set of 13c records of four caribbean sponge specimens that the isotopic response of surface water dic to the changing isotopic composition of atmospheric co2 varied dynamically during the last century depending on the rate of atmospheric change three of our sponges provide 600 year long 13c records for the first time we can reconstruct surface water 13cdic for the full history of the industrial co2 release as well as the preceding preindustrial period back to the beginning of the little ice age this provides a well founded estimate of the anthropogenically uninfluenced preindustrial background level of surface water 13c 12c ratios our records show small but systematic variations that appear to be linked to the climate fluctuations of the little ice age

tectono thermal evolution magma emplacement and shear zone development in the caruaru area borborema province ne brazil abstract the neoproterozoic borborema province bp in northeastern brazil is characterized by a regional flat lying foliation and by abundant magmatic rocks commonly spatially associated with large transcurrent shear zones combined field micropetrographic thermobarometric and magmatic fabric studies carried out on plutons country rocks and shear zones in the eastern domain of the province the caruaru area reveal that 1 an early episode of regional deformation with a top to the ne displacement was followed by the development of conjugate strike slip shear zones 2 similar low pressure 650 c metamorphic conditions occurred during the two events 3 plutons intruded the flat lying foliation 4 magma emplacement slightly predated strain localization in transcurrent shear zones but 5 plutons underwent strike slip deformation before complete crystallization therefore regional deformation pluton emplacement and shear zone development were successive events occurring over a relatively short time span 40ar 39ar laser dating of amphibole 584 ma and biotite 545 553 ma single grains supports slow cooling 5 c ma of country rocks through the argon closure temperatures of these minerals complex sometimes discordant amphibole ages bracketed between 552 and 575 ma for shear zones may be attributed to episodic activity and or dike swarm intrusions into them which disturbed the ar system biotite ages for the shear zones 533 545 ma also imply a relatively low cooling rate these data show that the massive injection of magmas in the continental crust of the bp during the brasiliano pan african orogeny produced a long lived thermal anomaly of regional extent the 40ar 39ar ages in the caruaru area are older than in other segments of the bp with similar geological characteristics this indicates a tectono thermal history for the bp that is more complex than previously recognized with either diachronous deformation magmatism and associated low p high t metamorphism or contrasting cooling of the different domains possibly due to differential vertical movements

pluton emplacement in a strike slip fault zone the doctors flat pluton victoria australia the siluro devonian doctors flat pluton lachlan fold belt australia is a hornblende biotite granite i type that was emplaced into the early silurian omeo metamorphic complex it lies along the ensay fault a major dextral strike slip fault a solid state foliation is developed in much of the pluton with the most intense development at the outer parts grading into massive granite at the centre this foliation is approximately parallel to trace of the ensay fault and to the borders of the pluton it grades into a mylonitic fabric along the ensay fault in the eastern part of the pluton a second fabric is developed giving rise to s c fabrics indicating dextral shear some microfabrics in the pluton are typical of relatively high temperature deformation consistent with syntectonic emplacement but lower temperature microstructures overprint them and are dominant a possible mechanism of intrusion involves the pluton being emplaced into a dilational section of the ensay fault when a jog in the fault opened up into a pull apart structure as intrusion continued the northeastern branch of the pull apart took up all the movement with later intrusive material being less deformed along the ensay fault lower grade assemblages of chlorite epidote albite sphene and cataclastic textures developed at lower temperatures as fault displacement continued during and after cooling of the pluton

revisiting the late pleistocene mammal extinction record at tight entrance cave southwestern australia tight entrance cave tec in southwestern australia provides a pleistocene sequence documenting the extinction of 14 large mammal species this record has been interpreted as indicating that extinctions did not occur during or before the penultimate glacial maximum pgm and that humans played a primary role in the extinctions however it remains possible that the majority of extinct megafauna persisted no later than the pgm the tec extinctions correspond with vegetation change a cooling drying trend increased biomass burning and increasingly unstable small mammal communities the initiation of these trends predates human arrival on the continent and implies environmentally mediated extinctions

topographic relief driven by variations in surface rock density earth s surface topography is generated by tectonically induced variations in crustal thickness combined with erosion and to a lesser degree by vertical stresses caused by convection in the underlying mantle rock hardness and resistance to erosion are also commonly thought to influence topography because hard rocks such as granites and basalts usually form topographic highs in the landscape here we use analytical and numerical models to simulate the erosion induced isostatic rebound of rocks we find that the isostatic rebound that accompanies erosion causes denser rocks to occupy higher elevations in the landscape thereby creating topographic relief that is proportional to surface rock density differences rather than rock hardness we quantify this effect taking into account the flexural strength of the continental lithosphere we show that in a steady state erosional setting density dependent isostatic rebound can cause the densest rocks to be exhumed at double the rate of surrounding less dense rocks and has a stronger effect than typical rock hardness variations the results imply that denser rock formations should erode faster and therefore be characterized by younger thermochronological ages thermochronological data sets from the kinabalu granite in borneo and the shakhdara alichur gneiss domes in pamir confirm this counter intuitive result our findings imply that lateral variations in surface rock density have significant control on the shaping of the large scale features of earth s surface the high elevation in earth s topography of hard rocks such as granites and basalts was thought to be caused by their inherent resistance to erosion numerical modelling now demonstrates counterintuitively that erosion induced isostatic rebound of rocks which is density dependent causes granites and basalts to occupy high elevations because they are more dense than surrounding rocks

transcurrent tectonics and polycyclic evolution in the lower proterozoic of senegal mali abstract outcrops of the early proterozoic birimian series within the kedougou inlier straddling the mali senegal border have traditionally been described as being composed of a basal volcanic mako series overlain by a detrital carbonate dialle dalema series this geosynclinal context was later reinterpreted in terms of oceanization and subduction as the mako series contains successively basaltic pillow lavas of tholeiitic affinity and a calc alkaline complex however the sedimentary dialle dalema series lie in the core of a major anticlinorium and the rocks show overprinting of an earlier by a later tectonic phase the volcanic and sedimentary rocks of the mako series were affected only by the later tectonic phase in addition a fan delta deposit intercalated between the tholeiitic and calc alkaline rocks contains pebbles of schistose and folded sedimentary rock that clearly belong to the dialle dalema series these observations necessitate a complete revision of the local stratigraphy thus the clastic deposits of the dialle dalema are attributed to the lower birimian and are interpreted as having been deposited in intra cratonic basins the first tectonic phase took place before emplacement of the volcanic sereis which shows evidence of an extensional setting with major crustal fracturing the second deformation affected both the sedimentary and volcanic series structural analysis furthermore shows that the formation of the second cleavage s2 was contemporaneous with the emplacement of major granite bodies as well as with major transcurrent sinistral shear zones finally this evolution is shown to be comparable with that in other lower proterozoic terranes of the west african craton

slip preference on pre existing faults a guide tool for the separation of heterogeneous fault slip data in extensional stress regimes abstract synthetic fault slip data have been considered in the present paper in order to examine through a simple graphical manner the validity and use of the widely mentioned and applied criteria such as the slip preference slip tendency kinematic p and t axes transport orientation and strain compatibility the examination and description concern extensional stress regimes whose greatest principal stress axis 1 always remains in vertical position as in andersonian stress states in particular radial extension re radial pure extension re pe pure extension pe pure extension transtension pe trn and transtension trn are examined with the aid of the win tensor stress inversion software in all of these extensional stress regimes only extensional faults can be activated the lower dip angle of the reactivated faults is about 40 assuming that the coefficient of friction is no smaller than 0 6 the increase of the stress ratio and or the fault dip angle up to 70 results in the increase of the slip deviation from the normal activation based on the present examination of the slip preference and slip tendency in different extensional stress regimes a new simple and practical method is proposed herein in order to separate originally heterogeneous fault slip data into homogeneous fault groups by which different extensional stress regimes could be determined the application of the method on the already published fault slip data of lemnos island supports its validity since over 90 the resulted fault groups and stress regimes coincide to the already published ones

computational approaches to studying non linear dynamics of the crust and mantle we outline a mathematical formulation for mantle convection which can deal with the viscoelastic plastic rheology of the cool parts of the lithosphere this formulation is then analyzed to expose the numerical challenges inherent in the equations and a suitable solution strategy is outlined with this strategy in place we discuss a parallel implementation explaining how we maintain computational efficiency in a tiered and modular software environment we show two examples from geodynamics which demonstrate the capability of the numerical method and its implementation c 2007 elsevier b v all rights reserved

fault terminations and barriers to fault growth abstract field observations of strike slip faults in jointed granitic rocks of the central sierra nevada california combined with a mechanical analysis of fault interaction provide insight into how fault terminations vary with scale we document here a strike slip fault system 2 3 km long clustered about the west end of the fault system are several dozen faults that parallel the three main fault zones in the system we interpret this cluster of small faults as a barrier that inhibited growth of fault zones in the fault system a two dimensional mechanical analysis shows that a cluster of small faults flanking the tip of a large fault zone will tend to diffuse the stress concentration near the fault zone tip an analogous effect in engineering is known as crack tip shielding near tip stress concentrations promote fault growth and processes that decrease these stress concentrations inhibit fault growth as faults lengthen and grow they interact with features at greater distances and over a broader area so the potential for tip shielding effects will increase as fault length increases this effect can account for why the mechanisms and character of fault terminations would tend to vary as a function of scale

palaeoenvironmental records from fossil corals the effects of submarine diagenesis on temperature and climate estimates the geochemistry of coral skeletons may reflect seawater conditions at the time of deposition and the analysis of fossil skeletons offers a method to reconstruct past climate however the precipitation of cements in the primary coral skeleton during diagenesis may significantly affect bulk skeletal geochemistry we used secondary ion mass spectrometry sims to measure sr mg b u and ba concentrations in primary coral aragonite and aragonite and calcite cements in fossil porites corals from submerged reefs around the hawaiian islands cement and primary coral geochemistry were significantly different in all corals we estimate the effects of cement inclusion on climate estimates from drilled coral samples which combine cements and primary coral aragonite secondary 1 calcite or not vert similar2 aragonite cement contamination significantly affects sr ca sst estimates by 1 c and 0 4 to 0 9 c respectively cement inclusion also significantly affects mg ca b ca and u ca sst estimates in some corals x ray diffraction xrd will not detect secondary aragonite cements and significant calcite contamination may be below the limit of detection not vert similar1 of the technique thorough petrographic examination of fossils is therefore essential to confirm that they are pristine before bulk drilled samples are analysed to confirm that the geochemistry of the original coral structures is not affected by the precipitation of cements in adjacent pore spaces we analysed the primary coral aragonite in cemented and uncemented areas of the skeleton sr ca b ca and u ca of primary coral aragonite is not affected by the presence of cements in adjacent interskeletal pore spaces i e the coral structures maintain their original composition and selective sims analysis of these structures offers a route to the reconstruction of accurate ssts from altered coral skeletons however mg ca and ba ca of primary coral aragonite are significantly higher in parts of skeletons infilled with high mg calcite cement we hypothesise this reflects cement infilling of intraskeletal pore spaces in the primary coral structure

granite magma generation ascent and emplacement within a transpressional orogen crustal thickening during transpressive orogenesis may produce anatectic granites which may then localize deformation leading to transcurrent movement granites may be transported from sites of generation through the mid crust in dyke like channelways within relatively narrow strike slip shear zones which link to major fault zones in the upper crust extensional jogs within fault systems provide developing sites for the assembly of plutons from magma arriving from below the model is based upon observations from the cadomian belt of nw france which exposes sections through middle and upper levels of the late precambrian crust within different elements of the orogen the mechanism provides a favourable alternative to diapirism and explains the common collocation of granites and shear zone fault systems within orogenic belts

pluton accommodation at high strain rates in the upper continental crust the example of the central extremadura batholith spain abstract emplacement in the tensional bridge of a stepped dextral shear zone system is proposed for the central extremadura batholith spain the country rocks show a pervasive anisotropy that conditioned the style of the structures developed as a consequence of the transference of displacement from the stepped shear zones to the releasing area the kinematic evolution of the resulting megakink fold provided the volume increase necessary for the granite emplacement thermal and kinematic models suggest that the growth of individual plutons took place in periods of no more than several hundred to a few thousand years fast strain rates 10 10 10 11 s 1 must concentrate in local structures e g initiation of kink folds even in zones deforming as a whole under typical strain rates 10 14 1 s 1 granite plutons might be used as strain rate gauges for syn plutonic structures

thickness composition and evolution of the indian precambrian crust inferred from broadband seismological measurements abstract we present seismological constraints to model the evolution of indian precambrian crust through receiver function analysis of teleseismic waveforms recorded on 22 broadband seismographs along a n s corridor from southern granulite terrain to delhi on the aravalli craton archean terrains eastern dharwar bastar bundelkhand and aravalli cratons are underlain by crust with felsic intermediate composition vp vs 1 65 1 75 in contrast to mafic composition vp vs 1 78 of crust beneath vindhyan basin godavari basin and the narmada son lineament nsl of proterozoic age the moho depth varies between 33 and 43 km with no definite relationship with age of the crust duplex moho with highly mafic underplated lower crust is characteristic feature of the nsl signifying its paleo rift character present day felsic character of the indian archean crust could be a consequence of post formation modification of the initial mafic lower crust through the process of lithospheric delamination this major tectonic event also created layered lithosphere beneath parts of india imprinted as positive velocity contrast at a depth of 80 km usually referred to as hales discontinuity the upper part of the lithosphere has shear velocity vs 4 52 km s and the lower one 4 77 km s we speculate that such a layered lithosphere could be due to a low density depleted spinel peridotite with archean affinity lying over higher density fertile garnet peridotite of proterozoic age

influence of trench width on subduction hinge retreat rates in 3 d models of slab rollback subduction of tectonic plates limited in lateral extent and with a free trailing tail i e free subduction is modeled in a three dimensional 3 d geometry the models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength width and thickness we investigate the time evolution of this progressive rollback subduction measure the accompanying return flow in the upper mantle and quantify the plate kinematics due to the 3 d geometry flow is allowed to accompany slab rollback around the lateral edges of the slab the toroidal component as opposed to 2 d geometry where material is forced to flow underneath the slab tip the poloidal component a simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate our results indicate most of the gravitational energy of the system i e the negative buoyancy of the subducting slab is converted into a toroidal flow 69 a much smaller amount goes into weakening the plate 18 and the remaining amount goes into driving flow parallel to displacement of the slab 13 for the trench widths w we investigate 1500 km a maximum trench retreat rate occurs for trenches 600 km wide which is attributed to the interaction between a plate of finite width and the induced flow which has a lengthscale in the horizontal direction these numerical results quantitatively agree with comparable 3 d laboratory experiments using analogue models with a purely viscous plate material schellart 2004a 2004b including correlations between increasing retreat rate with increasing plate thickness trench width for maximum retreat rate 500 km and estimated amount of slab buoyancy used to drive rollback induced flow 70 several implications for plate tectonics on earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle and endothermic phase transition prior to a flushing event into the lower mantle mantle avalanche

sr ca and mg ca vital effects correlated with skeletal architecture in a scleractinian deep sea coral and the role of rayleigh fractionation deep sea corals are a new tool in paleoceanography with the potential to provide century long records of deep ocean change at sub decadal resolution complicating the reconstruction of past deep sea temperatures mg ca and sr ca paleothermometers in corals are also influenced by non environmental factors termed vital effects to determine the magnitude pattern and mechanism of vital effects we measure detailed collocated sr ca and mg ca ratios using a combination of micromilling and isotope dilution icp ms across skeletal features in recent samples of desmophyllum dianthus a scleractinian coral that grows in the near constant environment of the deep sea sr ca variability across skeletal features is less than 5 2 relative standard deviation and variability of sr ca within the optically dense central band composed of small and irregular aragonite crystals is significantly less than the surrounding skeleton the mean sr ca of the central band 10 6 0 1 mmol mol 2 standard error and that of the surrounding skeleton 10 58 0 09 mmol mol are statistically similar and agree well with the inorganic aragonite sr ca temperature relationship at the temperature of coral growth in the central band mg ca is greater than 3 mmol mol more than twice that of the surrounding skeleton a general result observed in the relative mg ca ratios of d dianthus collected from separate oceanographic locations this large vital effect corresponds to a 10 c signal when calibrated via surface coral mg ca temperature relationships and has the potential to complicate paleoreconstructions outside the central band mg ca ratios increase with decreasing sr ca we explain the correlated behavior of mg ca and sr ca outside the central band by rayleigh fractionation from a closed pool an explanation that has been proposed elsewhere but which is tested in this study by a simple and general relationship we constrain the initial solution and effective partition coefficients for a rayleigh process consistent with our accurate metal ca measurements a process other than rayleigh fractionation influences mg in the central band and our data constrain a number of possible mechanisms for the precipitation of this aragonite understanding the process affecting tracer behavior during coral biomineralization can help us better interpret paleoproxies in biogenic carbonates and lead to an improved deep sea paleothermometer

shear zones between rock units with no relative movement abstract shear zones are normally viewed as relatively narrow deformation zones that accommodate relative displacement between two blocks that have moved past each other in opposite directions this study reports localized zones of shear between adjacent blocks that have not moved past each other such deformation zones which we call wakes form due to the movement of exotic blocks within a viscous medium denser blocks sinking within a salt structure the paths between separated boudins melt in partially molten surroundings melt movement during migmatisation or solid blocks sinking through a partially molten magma body stoping from the fluid dynamics perspective these shear zones can be regarded as low reynolds number deformation zones within the wake of a body moving through a viscous medium while compact moving bodies aspect ratio 1 1 1 generate axial symmetric cone like shear zones or wakes elongated bodies vertical plates or horizontal rod like bodies produce tabular shear zones or wakes unlike conventional shear zones across which shear indicators usually display consistent symmetries shear indicators on either side of the shear zone or wake reported here show reverse kinematics thus profiles exhibit shear zones with opposed senses of movement across their center lines or planes we have used field observations and results from analytical and numerical models to suggest that examples of wakes are the transit paths that develop where denser blocks sink within salt structures bodies of melt rise through migmatites between boudins separated by progressive extension and perhaps where slabs of subducted oceanic lithosphere delaminate from the continental crust and sink into the asthenosphere we also argue that such shear zones may be more common than they have been given credit for and may be responsible for some reverse kinematics reported in shear zones

cryptic meteoric diagenesis in freshwater bivalves implications for radiocarbon dating shells of freshwater bivalves are commonly used for radiocarbon dating late pleistocene archaeological and vertebrate fossil sites thus providing important constraints on late pleistocene human dispersal and megafauna extinction hypotheses the reliability of bivalve shells for dating rests partly on the ease with which subsequent diagenetic alteration can be recognized typically wherein original shell aragonite is replaced by calcite in meteoric environments here we document late pleistocene freshwater bivalve shells wherein meteoric diagenesis involved syntaxial overgrowth of aragonite cement on original aragonite shell biocrystals aragonite cement was identified in situ using raman microspectroscopy and formed rather than calcite as a result of unusually high mg ca ratios in local groundwaters thus altered shells contain diagenetic 14c rendering their dates unreliable but they may slip past common vetting techniques because 1 epitaxial cements are not readily apparent petrographically because they are in optical continuity with adjacent biocrystals 2 x ray diffraction indicates that no calcite is present 3 alteration is not apparent in cathodoluminescence studies and 4 stable isotopes of c and o are difficult to interpret in shells that originate in terrestrial meteoric environments hence although freshwater with a high mg ca ratio is not common groundwater chemistry should be considered before accepting bivalve based radiocarbon dates uncritically more broadly meteoric diagenesis in carbonate rocks is generally characterized by the dissolution of aragonite or its conversion to calcite our data show that such is not invariably the case even in fully terrestrial freshwater systems

would the australian megafauna have become extinct if humans had never colonised the continent comments on a review of the evidence for a human role in the extinction of australian megafauna and an alternative explanation by s wroe and j field a critical comment on a review of the evidence for a human role in the extinction of australian megafauna and an alternative explanation by s wroe and j field is presented the authors have highlighted a range of ideas under consideration and provided a selective interpretation which does not come to terms with biology and ignores or misinterprets current evidence they rely heavily on the ages reported by roberts et al 2001 to argue for a gradual attenuation of the megafauna they propose a staggered series of extinctions throughout the middle and late pleistocene with many taxa lost during the penultimate glacial maximum pgm 140 130 ka and relatively few species persisting they ignore measurement uncertainties associated with the ages which when properly considered means that 20 rather than eight of the species they list have last appearance ages consistent with 45 ka

strain localisation and population changes during fault system growth within the inner moray firth northern north sea abstract the evolution of fault populations is established for an area within the late jurassic inner moray firth sub basin of the north sea sedimentation rates outstripped fault displacement rates resulting in the blanketing of fault scarps and the preservation of fault displacement histories displacement backstripping is used to establish the growth history of the fault system fault system evolution is characterised by early generation of the main fault pattern and progressive localisation of strain onto larger faults this localisation is accompanied by the death of smaller faults and an associated change in the active fault population from power law to scale bound fault length populations evolve from a power law frequency distribution containing all faults to a power law distribution with a marked non power law tail containing the largest faults this change in population character is synchronous with the development of a fully connected fault system extending across the mapped area and the accommodation of displacements almost exclusively on the largest faults strain localisation onto fewer and better connected faults represents the most efficient means of accommodating fault related deformation and is considered to be a fundamental characteristic of the spatio temporal evolution of fault systems progressive strain localisation requires complementary changes in the characteristics of associated earthquake populations

trace metal mg ca and sr ca analyses of single coccoliths by secondary ion mass spectrometry here we present the first multi species comparison of modern and fossil coccolith trace metal data obtained from single liths we present both trace metal analyses sr ca mg and al and distribution maps of individual paleogene fossil coccoliths obtained by secondary ion mass spectrometry sims we use this data to determine the effects of variable coccolith preservation and diagenetic calcite overgrowths on the recorded concentrations of strontium and magnesium in coccolith calcite the analysis of coccoliths from deep ocean sediments spanning the eocene oligocene transition demonstrates that primary coccolith calcite is resistant to the neomorphism that is common in planktonic foraminifera from similar depositional environments instead where present diagenetic calcite forms distinct overgrowths over primary coccolith calcite rather than replacing this calcite diagenetic overgrowths on coccoliths are easily distinguished in sims analyses on the basis of relatively higher mg and lower sr concentrations than co occurring primary coccolith calcite this interpretation is confirmed by the comparable sims analyses of modern cultured coccoliths of coccolithus braarudii further with diagenetic calcite overgrowth being the principle source of bias in coccolith based geochemical records we infer that lithologies with lower carbonate content deposited below the palaeo lysocline are more likely to produce geochemical records dominated by primary coccolith calcite than carbonate rich sediments where overgrowth is ubiquitous the preservation of primary coccolith carbonate in low carbonate lithologies thus provides a reliable geochemical archive where planktonic foraminifera are absent or have undergone neomorphism

high resolution sr ca records in sclerosponges calibrated to temperature in situ ratios of strontium to calcium have been analyzed by laser ablation inductively coupled plasma mass spectrometry la icp ms in a skeletal section of the sclerosponge ceratoporella nichol soni the growth period representative of 3 yr was stained in the skeleton with a fluorochrome calcein temperatures were record ed at 2 h intervals within the shallow cryptic reef enclosure that the sclerosponge inhabited on the northern coast of jamaica al lowing the formulation of a direct empirical relationship between sr ca and temperature to verify this calibration sr ca ratios of two sclerosponges of the same species from depths of 67 m and 136 m in exuma sound bahamas were analyzed by la icp ms and compared to the temperatures from these depths over a decade prior to collection the result is an independently verified high resolution empirical calibration for the temperature sensitivity of sr ca ratios in the aragonite skeletons of sclerosponges from ja maica and the bahamas the calibration is a first for c nicholsoni and indicates that sclerosponges are more sensitive temperature recorders than zooxanthellate corals it represents an important step in establishing skeletal geochemistry of sclerosponges as a proxy of temperature in the upper 250 m of the ocean

natural variability of skeletal elemental phosphorus p ca lead pb ca and barium ba ca in the western pacific sclerosponges acanthoceatetes wellsi and astrosclera welleyana

coupled mantle dripping and lateral dragging controlling the lithosphere structure of the nw moroccan margin and the atlas mountains a numerical experiment abstract recent studies integrating gravity geoid surface heat flow elevation and seismic data indicate a prominent lithospheric mantle thickening beneath the nw moroccan margin lab 200 km depth followed by thinning beneath the atlas domain lab about 80 km depth such unusual configuration has been explained by the combination of mantle underthrusting due to oblique africa eurasia convergence together with viscous dripping fed by asymmetric lateral mantle dragging requiring a strong crust mantle decoupling in the present work we examine the physical conditions under which the proposed asymmetric mantle drip and drag mechanism can reproduce this lithospheric configuration we also analyse the influence of varying the kinematic boundary conditions as well as the mantle viscosity and the initial lithosphere geometry results indicate that the proposed drip drag mechanism is dynamically feasible and only requires a lateral variation of the lithospheric strength the further evolution of the gravitational instability can become either in convective removal of the lithospheric mantle mantle delamination or subduction initiation the model reproduces the main trends of the present day lithospheric geometry across the nw moroccan margin and the atlas mountains the characteristic time of the observed vertical movements the amplitude and rates of uplift in the atlas mountains and offers an explanation to the miocene to pliocene volcanism an abnormal constant tectonic subsidence rate in the margin is predicted

petrological and geochronological constraints on lower crust exhumation during paleoproterozoic eburnean orogeny nw ghana west african craton we present new petrological and geochronological data on high grade ortho and paragneisses from northwestern ghana forming part of the paleoproterozoic 2 25 2 00 ga west african craton the study area is located in the interference zone between n s and ne sw trending craton scale shear zones formed during the eburnean orogeny 2 15 2 00 ga high grade metamorphic domains are separated from low grade greenstone belts by high strain zones including early thrusts extensional detachments and late stage strike slip shear zones paragneisses sporadically preserve high pressure low temperature hp lt relics formed at the transition between the blueschist facies and the epidote amphibolite sub facies 10 0 14 0 kbar 520 600 c and represent a low 15 c km 1 apparent geothermal gradient migmatites record metamorphic conditions at the amphibolite granulite facies transition they reveal a clockwise pressure temperature time p t t path characterised by melting at pressures over 10 0 kbar followed by decompression and heating to peak temperatures of 750 c at 5 0 8 0 kbar which fit a 30 c km 1 apparent geotherm a regional amphibolite facies metamorphic overprint is recorded by rocks that followed a clockwise p t t path characterised by peak metamorphic conditions of 7 0 10 0 kbar at 550 680 c which match a 20 25 c km 1 apparent geotherm these p t conditions were reached after prograde burial and heating for some rock units and after decompression and heating for others the

eight glacial cycles from an antarctic ice core the antarctic vostok ice core provided compelling evidence of the nature of climate and of climate feedbacks over the past 420 000 years marine records suggest that the amplitude of climate variability was smaller before that time but such records are often poorly resolved moreover it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records here we report the recovery of a deep ice core from dome c antarctica that provides a climate record for the past 740 000 years for the four most recent glacial cycles the data agree well with the record from vostok the earlier period between 740 000 and 430 000 years ago was characterized by less pronounced warmth in interglacial periods in antarctica but a higher proportion of each cycle was spent in the warm mode the transition from glacial to interglacial conditions about 430 000 years ago termination v resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases but there are significant differences in the patterns of change the interglacial stage following termination v was exceptionally long 28 000 years compared to for example the 12 000 years recorded so far in the present interglacial period given the similarities between this earlier warm period and today our results may imply that without human intervention a climate similar to the present one would extend well into the future

sediment mixing at nonda rock investigations of stratigraphic integrity at an early archaeological site in northern australia and implications for the human colonisation of the continent archaeological excavations in sediments dating to between 60 000 and 40 000 years ago are rare in australia yet this is precisely the period in which most archaeologists consider that aboriginal people arrived on the continent in the few cases where such early sites have been investigated questions have invariably been raised as to the reliability of stratigraphic associations between cultural items and the surrounding sediments this paper describes a method for examining sediment mixing in a stratigraphic sequence using the optically stimulated luminescence osl signals from individual sand sized grains of quartz we apply this method to the archaeological site of nonda rock north queensland in combination with radiocarbon dating of charcoal fragments to construct chronologies for human occupation and for the preceding culturally sterile deposits our age estimates have implications for the timing of first human arrival in australia

interpreting environmental signals from the coralline sponge astrosclera willeyana coralline sponges sclerosponges have been proposed as a new source for paleo subsurface temperature reconstructions by utilizing methods developed for reef building corals however unlike corals coralline sponges do not have density variations making age determination difficult in this study we examined multiple elemental rations b mg sr ba u in the coralline sponge astrosclera willeyana we also measured skeletal density profiles along the outer living edge of the sponges and this data indicates significant thickening of skeletal material over intervals of 2 3 mm or 2 3 years this suggests that any skeletal recovered environmental record from astrosclera willeyana is an integration of signals over a 2 3 year period sponge sr ca seemed to hold the most promise as a recorder of water temperature and we compared sr ca from 2 sponges in the great barrier reef and one from truk in micronesia to their respective sea surface temperature record the correlations were not that strong approx r 0 5 but they were significant it appears that the signal smoothing due to thickening or perhaps even some biologic control on sr skeletal partitioning limits the use of sr ca as an indicator of water temperature in astrosclera willeyana

transient hot channels perpetrating and regurgitating ultrahigh pressure high temperature crust mantle associations in collision belts abstract two dimensional numerical modeling of early continental collision associated with subduction of the lithospheric mantle shows that the formation and exhumation of coesite and diamond bearing rocks metamorphosed at 700 to 900 c in the presence of dense supercritical silicate fluids and melts may be explained by a transient hot channel effect anomalously high temperature is caused by intense viscous and radiogenic heating in the channel composed of deeply subducted radiogenic upper crustal rocks especially sediments of passive margin origin and mantle rocks heating is associated with partial melting of crustal rocks caused by pervasive flow of aqueous fluids relieved by rapid dehydration deserpentinization of the overriding mantle lithosphere that has been hydrated during earlier subduction stages the channel penetrates the plate interface to the bottom of the lithosphere of the overriding plate 150 200 km and is characterized by metamorphic temperatures reaching 700 to 900 c low effective viscosity of rocks subsequent to increased temperature partial melting and fluid infiltration permits profound mixing of hydrated mantle and crustal rocks the hot channel exists only during early collision but rapidly produces large amounts of ultrahigh pressure high temperature rocks further collision closes the channel by squeezing rheologically weak partially molten buoyant rocks between the strong lithospheric mantles of the two colliding plates assemblages of complicated p t paths with repetitive loops characterize exhumation of ultrahigh pressure rocks in the convoluted flow pattern of the hot channel

re evaluating the late quaternary fossil mammal assemblage of seton rockshelter kangaroo island south australia including the evidence for late surviving megafauna it is widely accepted that most larger australian vertebrates were extinct by 40 ka the reliability of 20 ka radiocarbon 14c ages on charcoal stratigraphically associated with sthenurine short faced kangaroo tooth fragments from seton rockshelter kangaroo island have therefore proven contentious some researchers have argued these fossils were in situ while others have claimed they were reworked to address this we obtained new 14c ages on bones from the site these bone ages are not only consistent with earlier charcoal ages but are in near perfect stratigraphic order providing strong support for the site s stratigraphic integrity our analyses indicate units aged 21 17 ka were primarily accumulated by tasmanian devils sarcophilus harrisii and owls tyto species after which humans became the primary accumulation agent the tight chronology faunal trends and current lack of evidence for older layers from which specimens could have been reworked suggest the sthenurine remains may be in situ however because attempts to directly date sthenurine material failed we cannot confidently assert that they survived to this time therefore seton rockshelter may be best excluded from the pleistocene extinction debate until the site can be re excavated and more conclusive evidence collected including more complete or directly datable sthenurine remains copyright 2015 john wiley sons ltd

shear zone controlled magma emplacement or magma assisted nucleation of shear zones insights from northeast brazil abstract in the borborema province of northeast brazil neoproterozoic granitoids and large scale transcurrent shear zones are spatially associated suggesting a genetic link between magma bodies and shear zones in some cases magma emplacement was clearly favored by shear zone activity but for several plutons this model is not satisfactory in these plutons pre full crystallization strike slip deformation evidenced by parallelism of magmatic foliations and lineations with the solid state mylonitic fabric and by a transition from magmatic to solid state flow is restricted to the vicinity of the shear zones evidence of shear zone activity prior to magma emplacement is lacking and the magmatic foliation away from the shear zones is in most cases shallowly dipping and concordant with the slightly older gently dipping regional gneissic foliation field and anisotropy of magnetic susceptibility mapping together with petrographic and geochemical studies performed in one of the magmatic complexes of the borborema province have revealed a structure and a magmatic fabric incompatible with the shear zone controlled emplacement model away from the shear zones this complex has retained a stratification inherited from the mixing of crystal poor magmas of contrasting composition and a magmatic fabric characterized by low to moderate dip magmatic foliations bearing a nw trending lineation which contrast with vertical foliations bearing ne or e w trending stretching lineations in the shear zones and indicates that crystallization started prior to shear zone development based on evidence that magma emplacement predated strike slip shearing and on information about the transition from magmatic to solid state deformation observed in the studied plutons we suggest that incompletely solidified plutons within the crust represent rheological heterogeneities that may induce strain localization and favor shear zone nucleation we propose that in the studied cases deformation first concentrated within the plutons where shear zones nucleated and grew subsequently as the solid phase component increased the crystal mush began to behave as a solid and the shear zones propagated into the country rocks

homo floresiensis and the late pleistocene environments of eastern indonesia defining the nature of the relationship abstract evidence from liang bua a limestone cave on the island of flores in east indonesia provides a unique opportunity to explore the long term relationship between hominins and their environment occupation deposits at the site span 95 ka and contain abundant stone artefacts well preserved faunal remains and evidence for an endemic species of hominin homo floresiensis work at the site included detailed geomorphological and environmental analysis which has enabled comparisons to be drawn between changes in the occupational intensity in the cave using stone tool and faunal counts and changes in the environmental conditions using the characteristics of the sedimentary layers in the cave and speleothem records these comparisons demonstrate that h floresiensis endured rapidly fluctuating environmental conditions over the last 100 ka which influenced the geomorphological processes in the cave and their occupational conditions the intensity of occupation in the cave changed significantly between 95 and 17 ka with peaks in occupation occurring at 100 95 74 61 and 18 17 ka these correlate with episodes of channel formation and erosion in the cave which in turn correspond with high rainfall thick soils and high bio productivity outside in contrast periods of low occupational intensity correlate with reduced channel activity and pooling associated with drier periods from 94 to 75 and 36 to 19 ka this apparent link between intensity of hominin use of the cave and the general conditions outside relates to the expansion and contraction of the rainforest and the ability of h floresiensis to adapt to habitat changes this interpretation implies that these diminutive hominins were able to survive abrupt and prolonged environmental changes by changing their favoured occupation sites these data provide the basis for a model of human environment interactions on the island of flores with the addition of extra data from other sites on flores this model will provide a greater understanding of h floresiensis as a unique human species

late quaternary milankovitch scale climatic change and variability and its impact on monsoonal australasia four pollen and charcoal records derived from marine cores around the northern perimeter of australia are examined to provide a regional picture of patterns causes and impacts of climate change over the last 100 300 ka the availability of radiocarbon dates and oxygen isotope records for the cores provides primary chronological control spectral analysis of components of these records demonstrates an overall importance of milankovitch frequencies with clear glacial interglacial cyclicity dominated by variation in precipitation in addition a number of pollen taxa as well as charcoal particles exhibit a 30 ka frequency that is considered from its relationship with biomass burning and with results of past modelling to reflect changes in the intensity of el nino southern oscillation enso variability pollen components of all records show a decline frequently stepwise in more fire sensitive vegetation and its replacement with more fire tolerant vegetation there is some evidence that this trend is linked to an onset or general increase in enso activity and perhaps also to variation in monsoon activity dating from about 300 ka bp that was caused by changes to oceanic circulation within the indonesian region the trend may have accelerated within the last 45 ka due to burning by indigenous people c 2003 elsevier b v all rights reserved

carbon isotope records from extant caribbean and south pacific sponges evolution of δ13c in surface water dic stable isotope records of demosponges from the caribbean and coral sea are described for the purpose of studying the influence of fossil fuel co2 on the carbon isotopic composition of dissolved inorganic carbon dic in surface water the slow growing sponges precipitate calcium carbonate in isotopic equilibrium with ambient sea water and are used to detect changes in 13cdic from pre industrial times early 19th century to the present we observed similar shapes and ranges in 13c curves measured on caribbean specimens collected from water depths of 25 84 and 91 m as well as a specimen collected in shallow waters off new caledonia the records reveal a highly significant correlation with atmospheric 13cco2 13cdic values for caribbean and coral sea surface waters were calculated using the 13c sponge records while 13c of atmospheric co2 decreased by about 1 4 from the early 19th century to 1990 13cdic of caribbean and coral sea surface waters decreased by 0 9 0 2 and 0 7 0 3 respectively no isotopic equilibrium between surface water dic and atmospheric co2 was observed either during the pre industrial steady state or during the last 100 years the lower amount of depletion in the surface water 13cdic with respect to the atmospheric anthropogenic signal is explained by the dilution of the surface waters by biologically altered subsurface water dic the lower 13c decrease in the coral sea points to a stronger influence of the subsurface water source compared to the caribbean

dip range for intracontinental reverse fault ruptures truth not stranger than friction abstract histograms of fault dips have been compiled for moderate to large m 5 5 reverse slip intracontinental earthquakes with the slip vector raking 90 30 in the fault plane the principal data set is restricted to earthquakes where the fault plane in the focal mechanism can be unambiguously distinguished from the auxilliary plane the reverse fault dips are bracketed within the range 12 1 the observed dip range is consistent with reactivation of faults possessing rock friction coefficients within byerlee9s 1978 range 0 85 s 0 6 undergoing frictional lockup at dips approaching 60 the broad 25 to 35 peak may arise from progressive domino steepening of imbricate reverse faults above the optimal dip for reactivation in regions undergoing bulk shortening paucity of very low angle thrusts implies that it is generally the steeper ramps within ramp flat assemblages that fail in moderate to large earthquakes the subsidiary peak at 45 to 55 likely results from compressional reactivation of former normal faults in areas undergoing tectonic inversion requiring some degree of fluid overpressuring the results are consistent with previous studies on the dip range for active normal faults that again demonstrate frictional lockup at reactivation angles approaching 60 together these analyses suggest that byerlee friction coefficients apply to faults with displacements of up to a few kilometers

palaeontology extinction promoted fire the extinction of megafauna in australia roughly coincided with shifts in vegetation and fire regimes sediment geochemistry shows that the vegetation shift followed the extinction indicating that the loss of browsers promoted fire and altered plant composition

man and megafauna in tasmania closing the gap abstract recent discussion on the late pleistocene extinction of the australian megafauna has revolved around interpretation of several key fossil sites in tasmania it has been suggested that humans did not arrive in tasmania until after the megafauna became extinct or did not hunt now extinct megafauna and therefore that humans cannot be implicated in the extinctions radiocarbon results from these sites indicate that the youngest extinct megafauna are close to charcoal ages from the oldest archaeological deposits although difficulties have arisen in establishing chronologies because most relevant sites have ages near the limit for radiocarbon analysis we report a series of new radiocarbon ages 13 c 15 n and c n ratios on collagen and dentine fractions from skeletal remains in the mount cripps karst area and the mowbray swamp both in northwestern tasmania and discuss the reliability of ages from these and other sites we also report the discovery of an articulated simosthenurus occidentalis skeleton at mt cripps that represents only the second directly dated extinct megafaunal taxon with a reliable age our results suggest that c n ratios measured on collagen or dentine are not an infallible guide to radiocarbon age reliability we confirm previous reports of a temporal overlap between the megafaunal and archaeological records in tasmania but the presence of archaeological evidence and megafauna with the same age at the same site has not yet been demonstrated at least two megafaunal taxa the now extinct protemnodon anak and a giant pleistocene form of the extant macropus giganteus were still present in tasmania after 43 ka when human crossing of the bassian landbridge from mainland australia first became sustainable

esr and u series analyses of faunal material from cuddie springs nsw australia implications for the timing of the extinction of the australian megafauna abstract the timing and cause of late pleistocene faunal extinctions in australia are subjects of a debate that has become polarised by two vigorously defended views one contends that the late pleistocene extinction was a short event caused by humans colonising the australian continent whereas the other promotes a gradual demise of the fauna over a period of at least 10 20 ka due to a combination of climatic changes and ecological pressures by humans cuddie springs is central to this debate as it is the only site known in continental australia where archaeological and megafauna remains co occur we have analysed more than 60 bones and teeth from the site by laser ablation icp ms to determine u and th concentrations and distributions and those with sufficiently high u concentrations were analysed for u series isotopes twenty nine teeth were analysed by esr these new results as well as previously published geochronological data contradict the hypothesis that the clastic sediments of stratigraphic unit 6 su6 are in primary context with the faunal archaeological and other materials found in su6 and that all have ages consistent with the optically stimulated luminescence osl estimates of 30 36 ka these young osl results were used to argue for a relatively recent age of the extinct fauna our results imply that su6 is either significantly older than the osl results or that a large fraction of the faunal material and the charcoal found in su6 was derived from older lateral deposits our u and th laser ablation icpms results as well as the ree profiles reported by trueman et al 2008 comparing rates of recystallisation and the potential for preservation of biomolecules from the distribution of trace elements in fossil bones c r palevol general paleontology taphonomy and fossilization 7 145 158 contradict the interpretation of previously reported rare earth element compositions of bones and the argument based thereon for the primary context of faunal material and clastic sediments in su6 layers

corals at their latitudinal limits laser ablation trace element systematics in porites from shirigai bay japan the rapid analytical technique of laser ablation inductively coupled plasma mass spectrometry la icp ms was used to measure the trace elements b mg sr ba and u in a high latitude coral colony porites lobata taken from shirigai bay japan 32 n a wide range of sea surface temperatures ssts 14 5 28 c and upwelling events influenced this coral cold winter ssts caused a decrease and or cessation of skeletal extension measurements of u ca and sr ca indicate an approximately linear response to ssts above 18 c and a non linear response below 18 c mg ca and b ca measurements both showed annual cycles broadly consistent with sst variations but also exhibited intra annual fluctuations not associated with temperature suggesting that the incorporation of mg and b into the coral skeleton was not simply regulated by temperature it is shown that ba ca ratios provide a proxy for wind induced seasonal upwelling this is inferred from the strong correlations between the strength of zonal winds 1 month prior to the sst minimum and the ba ca maximum secondary upwelling events occurred during the summers of 1982 1987 1991 and 1992 these summers were cooler than average and were associated with el nino southern oscillation events

slip localization and fault weakening as a consequence of fault gouge strengthening insights from laboratory experiments abstract a laboratory study of simulated quartz gouges was conducted to investigate how solution transfer processes influence the mechanical behaviour of fault wear products at high temperature hydrothermal conditions experiments were performed under nominally dry conditions as well as in the presence of an aqueous pore fluid at elevated temperatures 500 to 927 c and at effective confining pressure conditions 2 3 100 mpa to simulate on a laboratory timescale processes that may be important in fluid active fault zones at depth in the continental crust the mechanical data and microstructural analysis indicate that the kinetics of solution transfer processes can exert a fundamental control on the mechanical behaviour of fault wear products it is found that at nominally dry conditions gouges deform by cataclastic creep and distributed shear with strength and microstructures being relatively unaffected by temperature at moderately chemically reactive hydrothermal conditions 500 600 c coarse grain size or fast deformation rate the presence of a reactive pore fluid slightly reduces the shear strength with respect to dry conditions however at highly chemically reactive hydrothermal conditions 600 927 c small grain size and slow deformation rate rapid porosity reduction is accommodated by dissolution precipitation processes deformation under such conditions results in a fast increase of grain contact area and the development of cohesive bonds between adjacent particles which in turn inhibits cataclastic granular flow with increasing displacement and compaction of the quartz gouge there is a sudden transition from distributed cataclastic flow to slip localization at the interface between the gouge and one of the forcing blocks this deformation mode switch is associated with dramatic weakening up to 50 drop in shear resistance and changes in the apparent coefficient of friction from 0 7 to 0 4 stress drop occurs over many minutes in the laboratory it is speculated that solution assisted gouge compaction and consequent slip localization with associated slow yet dramatic stress drop could provide a mechanism for the occurrence of slow earthquakes

a biomineralization model for the incorporation of trace elements into foraminiferal calcium carbonate measurements of srca of benthic foraminifera show a linear decrease with water depth which is superimposed upon significant variability identified by analyses of individual foraminifera new data for cdca support previous work in defining a contrast between waters shallower and deeper than 2500 m measured element partition coefficients in foraminiferal calcium carbonate relative to sea water d have been described by means of a one box model in which elements are extracted by rayleigh distillation from a biomineralization reservoir that serves for calcification with a constant fractionation factor such that d 1 f 1 f where f is the proportion of ca remaining after precipitation a modification to the model recognises differences in element speciation the model is consistent with differences between dsr dba and dcd in benthic but not planktonic foraminifera depth variations in d for sr and ba are consistent with the model as are differences in depth variation of dcd in calcitic and aragonitic benthic foraminifera the shallower depth variations may reflect increasing calcification rates with increasing water depth to an optimum of about 2500 m observations of unusually lower dcd for some deep waters not accompanied by similar dsr or dba may be because of dissolution or a calcification response to a lower carbonate saturation state

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces

3 d numerical modelling of the influence of pre existing faults and boundary conditions on the distribution of deformation example of north western ghana abstract high strain zones bound and separate the high grade tectono metamorphic terranes from low grade greenstone belts in the north western parts of ghana these belts are bounded by granitoid domains characterized by two main episodic pulses of magmatic intrusive events which occurred between 2213 ma and 2086 ma high strain zones are thought to play a significant role in creating fluid pathways particularly for partially molten material from lower crustal sources to the upper crust in this study a three dimensional thermo mechanical model has been used to explore the evolution of high strain zones and relief under compressional and simple shear boundary conditions different orientations of a system of branched strike slip faults were tested the effects of the frictional angle and density contrast on the evolution of relief were also compared in this study the resulting model indicates domains of tensile vs compressional strain as well as shear zones this shows that the internal fault zones as well as the host rock in between the faults behave as relatively weaker domains than the external regions under both applied compressive and simple shear boundary conditions these weakened domains constitute preferential zones of tensile and shear strain accommodations in the upper crust which may favour infilling by deeper partially molten rocks this processes is suggested by the authors as the most likely processes to have occurred in pre existing branched shear zones systems in north western ghana during the eburnean orogeny around 2 20 2 10 ga the orientations of faults in these models play an important role in controlling the evolution of relief and localized deformation in particular greatest the largest relief is obtained when faults dip parallel to each other and when they are inclined at depth as they thus facilitate strain rotation and material transfer from depth the host rock density does not play a primary role in producing relief compared to variations in friction angle at crustal scale of our model relief increases by 200 300 m when the host rock density is increased by 200 kg m3 whereas relief reduces by about 1200 m when decreasing the host rock friction from 20 to 10 this study suggests a model for interpreting the evolution and locus of exhumation of partially molten rocks in north western ghana

kinematics and flow patterns in deep mantle and upper mantle subduction models influence of the mantle depth and slab to mantle viscosity ratio 1 three dimensional fluid dynamic laboratory simulations are presented that investigate the subduction process in two mantle models an upper mantle model and a deep mantle model and for various subducting plate mantle viscosity ratios sp m 59 1375 the models investigate the mantle flow field geometrical evolution of the slab sinking kinematics and relative contributions of subducting plate motion and trench migration to the total rate of subduction all models show that the subducting plate is always moving trenchward resulting from slab pull furthermore all deep mantle models show trench retreat as do upper mantle models in the initial stage of subduction before slab tip transition zone interaction upper mantle models with a low sp m 66 217 continue to show trench retreat after interaction upper mantle models with a high sp m 378 709 show a period of trench advance after interaction followed by trench retreat upper mantle models with a very high sp m 1375 show continued trench advance after interaction the difference in trench migration behavior and associated slab geometries is attributed to both sp m and the mantle depth to plate thickness ratio tm tsp which both affect the slab bending radius to mantle thickness ratio rb tm four subduction regimes can be defined regime i with rb tm 0 3 trench retreat slab draping and a concave trench regime ii with 0 3 rb tm 0 5 episodic trench migration slab folding and a concave trench regime iii with rb tm 0 5 trench advance slab rollover geometries and minor trench curvature and regime iv with rb tm 0 8 trench retreat slab draping and a rectilinear trench in all models slab parallel downdip motion induces poloidal mantle flow structures in addition trench retreat and rollback motion of the slab induce quasi toroidal return flow around the lateral slab edges toward the mantle wedge rollback induced poloidal flow around the slab tip is not observed in any of the experiments finally comparison between the slab geometries observed in the upper mantle models and slab geometries observed in nature imply that the effective viscosity ratio between slab and ambient upper mantle in nature is less than 103 and of the order 1 7 102 with a best estimate of 1 3 102

numerical models of lithospheric deformation forming the southern alps of new zealand compression of the entire continental lithosphere is considered using two dimensional numerical models in order to study the influence of the lithospheric mantle on the geometry of continental collision in its initial stages the models are based on the central section of new zealand southern alps where continental collision has occurred along the alpine fault since about 7 ma they incorporate brittle elastic ductile rheology heat transfer surface processes and fault localisation the models are compared to the surface relief the gps convergence velocity the measured electrical conductivity and the geometry of the crustal root imaged from seismic velocity measurements the crustal deformation is characterized by localized uplift at the plate boundary alpine fault and by two secondary zones of faulting one is located about 60 to 80 km east of the alpine fault at the start of upper crust bending or tilting and the other is located about 100 to 130 km east of the alpine fault as a result from shear strain propagating to the surface through the ductile lower crust the observed asymmetric shape of the crustal root is best reproduced for a mantle lithosphere strength of the order of 200 mpa and an intermediate rate of strain softening a lower strength of the mantle lithosphere can produce symmetric thickening but provides an amplitude of the crustal root too small when compared to observations the observed 20 km offset between the maximum in surface relief and the crustal root was not satisfactorily reproduced this offset is most likely due to the three dimensionality of oblique collision in new zealand southern alps

magnesium isotope fractionation in biogenic and abiogenic carbonates implications for paleoenvironmental proxies abstract geochemical variations in marine biogenic carbonates that are preserved in the geological record serve as proxies of past environmental change however interpreting most proxies is complicated by biologically mediated vital effects highlighting the need to develop new tools for reconstructing paleoenvironmental change recently magnesium mg isotope variability in carbonates has been explored extensively to determine its utility as a paleoenvironmental proxy we review the results of these works which have yielded valuable information on the factors affecting mg isotope fractionation between carbonates and solution 26mgcarb sol in biogenic and abiogenic carbonate minerals strong evidence exists for a mineralogical control on 26mgcarb sol with the negative offset from 0 following the sequence aragonite 3 mol mol and saturation states 3 that are similar to seawater suggest that 26mgcarb sol has a temperature dependence of 0 01 c 1 and is insensitive to precipitation rate in contrast a significant precipitation rate dependence is observed in calcites precipitated from solutions with relatively low mg ca ratios

oceanic ph control on the boron isotopic composition of foraminifera evidence from culture experiments culture experiments were carried out with the species orbulina universa at four different ph values 7 70 0 05 8 15 0 05 8 60 0 05 and 9 00 0 10 in order to establish the ph dependence of boron isotope fractionation between seawater and foraminifera a clear relationship between the boron isotopic composition of the foraminifera and the ph of the seawater culture solutions was found showing heavier boron isotopic composition at higher ph this finding supports the viability of boron isotopes as a paleo ph tool it is important to note that orbulina universa cultured in natural seawater as well as those obtained from coretop samples have a significantly lighter boron isotopic ratio than globigerinoides sacculifer from coretop samples suggesting that at least for this species a vital effect is active

a new experimental material for modeling relief dynamics and interactions between tectonics and surface processes abstract we developed a new granular material mativ to study experimentally landscape evolution in active mountain belt piedmonts its composition and related physical properties have been determined using empirical criteria derived from the scaling of deformation erosion transport and sedimentation natural processes mativ is a water saturated composite material made up with 4 granular components silica powder glass microbeads plastic powder and graphite whose physical mechanical and erosion related properties were measured with different laboratory tests mechanical measurements were made on a modified hubbert type direct shear apparatus erosion related properties were determined using an experimental set up that allows quantifying the erosion sedimentation budget from tilted relaxation topographies for mativ we also investigated the evolution of mean erosion rates and stream power erosion law exponents in 1d as a function of slope our results indicate that mativ satisfies most of the defined criteria it deforms brittlely according to the linear mohr coulomb failure criterion and localizes deformation along discrete faults its erosion pattern is characterized by realistic hillslope and channelized processes slope diffusion mass wasting channel incision during transport eroded particles are sorted depending on their density and shape which results in stratified alluvial deposits displaying lateral facies variations to evaluate the degree of similitude between model and nature we used a new experimental device that combines accretionary wedge deformation mechanisms and surface runoff erosion processes results indicate that mativ succeeded in producing detailed morphological and sedimentological features drainage basin channel network terrace syntectonic alluvial fan geometric kinematic and dynamic similarity criteria have been investigated to compare precisely model to nature although scaling is incomplete it yields particularly informative orders of magnitude with all these characteristics mativ appears as a very promising material to investigate experimentally a wide range of scientific questions dealing with relief dynamics and interactions between tectonics erosion and sedimentation processes

insights into the damage zones in fault bend folds from geomechanical models and field data abstract understanding the rock mass deformation and stress states the fracture development and distribution are critical to a range of endeavors including oil and gas exploration and development and geothermal reservoir characterization and management geomechanical modeling can be used to simulate the forming processes of faults and folds and predict the onset of failure and the type and abundance of deformation features along with the orientations and magnitudes of stresses this approach enables the development of forward models that incorporate realistic mechanical stratigraphy e g the bed thickness bedding planes and competence contrasts include faults and bedding slip surfaces as frictional sliding interfaces reproduce the geometry of the fold structures and allow tracking strain and stress through the whole deformation process in this present study we combine field observations and finite element models to calibrate the development and distribution of fractures in the fault bend folds and discuss the mechanical controls e g the slip displacement ramp cutoff angle frictional coefficient of interlayers and faults that are able to influence the development and distribution of fractures during fault bend folding a linear relationship between the slip displacement and the fracture damage zone the ramp cutoff angle and the fracture damage zone and the frictional coefficient of interlayers and faults and the fracture damage zone was established respectively based on the geomechanical modeling results these mechanical controls mentioned above altogether contribute to influence and control the development and distribution of fractures in the fault bend folds

three dimensional distinct element modelling of relay growth and breaching along normal faults three dimensional numerical models of neutral i e slip parallel relay zones on normal faults that cut massive sandstone host rocks have been constructed using the distinct element method code particle flow code in 3 d pfc3d the models successfully reproduce the geometries displacement profiles and strains observed in natural relay zones in contrast to boundary element method simulations the modelled relay ramps dip towards the hanging wall consistent with observations of most natural relay zones the modelling shows that relay zones are stable structures that grow by progressive rotation of an approximately planar relay ramp without significant propagation of the relay bounding faults prior to breaching stable growth is terminated when a breaching fault propagates across the top or bottom of the relay ramp breaching fault propagation is not instantaneous and the ramp continues to rotate and therefore transfer displacement between the relay bounding faults until they become fully hard linked following hard linkage displacement is accommodated by slip on the through going fault surface the modelling results confirm previous conceptual models of relay growth and breaching based on geometric and kinematic analysis of natural relay zones

continuous 150 k y monsoon record from lake eyre australia insolation forcing implications and unexpected holocene failure our reconstructed history of lake eyre provides the first continuous continental proxy record of australian monsoon intensity over the past 150 k y this continental record s broad correspondence to the marine isotope record demonstrates that this very large catchment with its hydrology dependent on a planetary scale climate element responds to milankovitch scale climate forcing abrupt transitions from dry phases to wet phases ca 125 and 12 ka coincide with northern hemisphere winter insolation minima rather than southern hemisphere summer insolation maxima indicating that northern hemi sphere insolation exerts a dominant control over the intensity of the australian monsoon stratigraphic and dating uncertainties of other wet phases preclude conclusive correlation to specific insolation signals but within the uncertainties are consistent with northern hemisphere forcing regardless of the hemispheric forcing the low intensity of the early holocene australian monsoon by comparison with the last interglacial and particularly the last high level lacustrine event at 65 60 ka when all forcing elements were modest is an enigma that can be explained by a change in boundary conditions within australia

strontium in coral aragonite 2 sr coordination and the long term stability of coral environmental records we have used x ray diffraction xrd and sr k edge extended x ray absorption fine structure exafs to determine the structural state of sr in a suite of coral aragonite samples our samples encompassed a selection of coral species porites lobata porites lutea pocillopora eydouxi montastrea annularis pavona gigantea and pavona clavus including some commonly used for palaeoenvironmental reconstruction aragonite was the only carbonate observed by xrd we refined the isolated exafs against structural models for sr in aragonite and two phase strontianite aragonite mixes our data are indistinguishable from sr ideally substituted in aragonite and strontianite was present below detection levels estimated at 5 of sr present comparisons of recent and ancient coral aragonite show no sign of exsolution either by spinodal decomposition or by the direct nucleation of strontianite domains published diffusion rates of sr in ionic solids support the view that exsolution would occur prohibitively slowly coral aragonites are metastable materials with slow diffusion kinetics that have the potential to encode environments over timescales of millions of years

the effect of convergence angle on the kinematic evolution of strain partitioning in transpressional brittle wedges insight from analog modeling and high resoluiton digital image analysis 1 using analog modeling aided by digital image analysis dpiv we constrained the long term kinematic evolution of strain partitioning in transpressional brittle wedges as a function of convergence angle we ran a series of dry quartz sand experiments representing highly oblique continent continent collision convergence angles of 4 to 30 the digital image analysis provided high resolution constraints on the long term kinematic evolution of these wedges which could be subdivided in distinct kinematic stages comprising 1 an initial distributed strain stage and 2 an oblique wedge stage before 3 the stage of strain partitioning is reached thus we document the evolution of different deformation stages from a single plate tectonic boundary condition in addition the relationship between convergence angle kinematic stages and wedge geometry including fault dips and fault hierarchy was established the modeling results show that smaller convergence angles lead to steeper faults besides for a constant convergence angle the proshears that evolved during the strain partitioning stage were less steep than those formed during the oblique wedge stage the fault slip vector on individual fault segments was derived from the dpiv data set for each time increment quantifying the magnitude and orientation of slip on fault segments during the different kinematic stages in addition in the 7 5 and 15 models rotation of the slip vector by up to 40 was observed on a single proshear during the strain partitioning stage these observations allow to some degree a validation of existing analytical models of strain partitioning in particular the assumption of steady state

paleoenvironmental investigations chemical analysis and characterization of underwater strata of marigondon cave abstract the submarine marigondon cave mc is not only one of the most spectacular diving caves but also an unstudied submerged rock shelter with potential for intact stratigraphy the marigondon cave yielded two rock samples one coral limestone fossil with a travertine mantle and a sclerosponge specimen that provided evidence to understand the external and environmental conditions to which the cave might have been exposed the study s aimed to determine the links between the cave s underwater conditions and the samples studied analytical methodologies such as ir spectroscopy x ray diffraction optical microscopy sem edx and u series isotope analyses were carried out to determine the chemical composition of two rock samples and their crystal structures and formation ages results concluded that the two samples were composed mainly of the two calcium carbonate polymorphs calcite and aragonite however the sclerosponges contained a low percentage of mg calcite illustrating a period of submergence at other periods marigondon cave would have been above water and favorable for pleistocene fauna as well as possibly human settlement as demonstrated by the travertine deposit on the coral limestone this suggests that mc provides prospects for discovery of fossil assemblages that may be preserved in terrestrial deposits in the caves during periods of emergence these periodic submersions conform to eustatic sea level changes during the past 130 000 years during the late pleistocene and demonstrate that in that period a marine coral reef platform was the substrate within which the marigondon cave formed as a flank margin cave during nearly 80 000 years of emergence when sea level was lower than the present elevation of the cave

experimental measurement of boron isotope fractionation in seawater abstract the boron isotopic composition of marine carbonates is considered to be a tracer of seawater ph use of this proxy benefits from an intimate understanding of chemical kinetics and thermodynamic isotope exchange reactions between the two dominant boron bearing species in seawater boric acid b oh 3 and borate ion b oh 4 however because of our inability to quantitatively separate these species in solution the degree of boron isotope exchange has only been known through theoretical estimates in this study we present results of a spectrophotometric procedure wherein the boron isotope equilibrium constant 11 10 k b is determined empirically from the difference in the dissociation constants of 11 b oh 3 and 10 b oh 3 in pure water 0 6 mol kg 1 h 2 o kcl and artificial seawater within experimental uncertainty our results show no dependence of 11 10 k b on temperature but 11 10 k b at 25 c in pure water was statistically different than results obtained in solutions at high ionic strength 11 10 k b of the seawater s 35 b t 0 01 mol kg 1 h 2 o at 25 c is 1 0272 0 0006 this result is significantly larger than the theoretical value used in numerous paleo ph studies 11 10 k b 1 0194

sr ca ratios and oxygen isotopes from sclerosponges temperature history of the caribbean mixed layer and thermocline during the little ice age we investigate aragonitic skeletons of the caribbean sclerosponge ceratoporella nicholsoni from jamaica 20 m below sea level mbsl and pedro bank 125 mbsl we use d18o and sr ca ratios as temperature proxies to reconstruct the caribbean mixed layer and thermocline temperature history since 1400 a d with a decadal time resolution our age models are based on u th dating and locating of the radiocarbon bomb spike the modern temperature difference between the two sites is used to tentatively calibrate the c nicholsoni sr ca thermometer the resulting calibration points to a temperature sensitivity of sr ca in c nicholsoni aragonite of about 0 1 mmol mol k our sr ca records reveal a pronounced warming from the early 19th to the late 20th century both at 20 and 125 mbsl two temperature minima in the shallow water record during the late 17th and early 19th century correspond to the maunder and dalton sunspot minima respectively another major cooling occurred in the late 16th century and is not correlatable with a sunspot minimum the temperature contrast between the two sites decreased from the 14th century to a minimum in the late 17th century and subsequently increased to modern values in the early 19th century this is interpreted as a long term deepening and subsequent shoaling of the caribbean thermocline the major trends of the sr ca records are reproduced in both specimens but hardly reflected in the d18o records

updating martin s global extinction model australia has been cited as a weak link in anthropogenic models of megafauna extinction but recent work suggests instead that the evidence for rapid extinction shortly after human arrival is robust the global model is revisited based on the contention that late pleistocene megafauna extinctions took place rapidly on islands and some islands such as australia and the americas are much larger than others modern dating methods are increasingly able to refine chronologies and careful scrutiny suggests that hundreds of dates should be deleted from archives an updated summary of results from new zealand north america and australia is presented with a brief discussion on why temperate refugia offering protection from climate change ultimately did not work strongly supporting the global extinction hypothesis pioneered by paul martin

structural evolution of a three dimensional finite width crustal wedge abstract we present the results of three dimensional numerical experiments designed to study the response of a layer of crustal material subjected to convergence through an imposed basal velocity discontinuity and to surface processes erosion sedimentation we focus on the three dimensional response of the system arising from the finite width of the imposed velocity discontinuity in particular we describe the complex structures that develop around the wedge and their interactions with the loading unloading produced by the surface processes we show that the pro and retro shear zones that develop in a doubly vergent two dimensional wedge curve around the end of the velocity line discontinuity to merge into the strike slip structure that naturally develop i e as a consequence of the imposed boundary conditions along the edge of the wedge along the retro shear zone the stress orientation rotates along a vertical axis which implies that the retro shear zone is a pure thrust along all of its curved length whereas along the pro shear zone stresses rotate along a horizontal axis which in turn implies that the pro shear zone progressively evolves towards an oblique thrust in its curved section furthermore the outward motion i e perpendicular to the direction of imposed shortening along the curved section of the retro shear zone is accommodated by oblique extension along a secondary kinked structure antithetic to the retro shear zone we also show the complex evolution of the wedge when ductile flow and ductile strain softening is activated by increasing the imposed basal temperature in this situation the wedge is broader as structures develop at finite distances on either side of the line discontinuity and its dynamics resembles more that of a vise like orogen at the surface a flat plateau forms that accumulates sediment from the surrounding actively deforming mountain ranges until a channel breaks through one of the sides and flushes the inward draining basin of its sedimentary content the complex numerical models provide a detailed insight into the geometry and complex three dimensional structures within and at the margins of an orogen and how they can be affected by erosional processes at the surface

formation of conical fractures in sedimentary basins experiments involving pore fluids and implications for sandstone intrusion mechanisms large sand intrusions often exhibit conical morphologies analogous to magmatic intrusions such as saucer shaped or cup shaped sills whereas some physical processes may be similar we show with scaled experiments that the formation of conical sand intrusions may be favoured by the pore pressure gradients prevailing in the host rock before sand injection our experiments involve injecting air into a permeable and cohesive analogue material to produce hydraulic fractures while controlling the pore pressure field we control the state of overpressure in the overburden by applying homogeneous basal pore pressure and then adding a second local pore pressure field by injecting air via a central injector to initiate hydraulic fractures near the injection point in experiments involving small vertical effective stresses small overburden or high pore fluid overpressure the fracturing pressure lambda fract is supralithostatic and two dipping fractures are initiated at the injection point forming a conical structure from theoretical considerations we predict that high values of lambda fract are due to strong cohesion or high pore fluid overpressure distributed in the overburden such conditions are favoured by the pore pressure stress coupling induced by both pore pressure fields the dips of cones can be accounted for elastic stress rotation occurring around the source contrary to magmatic chamber models the aqueous fluid overpressure developed in a parent sandbody and prevailing before the formation of injectites may diffuse into the surrounding overburden thus favouring stress rotation and the formation of inclined sheets far from the parent source for experiments involving higher vertical effective stresses thick overburden or low pore fluid overpressure the fracturing pressure is lower than the lithostatic stress and a single fracture is opened in mode i which then grows vertically at a critical depth the fracture separates into two dilatant branches forming a flat cone we make use of a p i v particle imaging velocimetiy technique to analyse plastic deformation showing that these inclined fractures are opened in mixed modes close to the surface they change into steep shear bands where fluids can infiltrate the final morphology of the fracture network is very similar to the common tripartite architecture of various injection complexes indicating that different mechanisms may be involved in the formation of dykes feeder dykes under the sill zones may open as tensile fractures while overlying dykes may be guided by the deformation induced by the growth of sills these deformation conditions may also favour the formation of fluid escape structures and pockmarks c 2011 elsevier b v all rights reserved

a model comparison study of large scale mantle lithosphere dynamics driven by subduction modelling subduction involves solving the dynamic interaction between a rigid solid yet deformable plate and the fluid easily deformable mantle previous approaches neglected the solid like behavior of the lithosphere by only considering a purely fluid description however over the past 5 years a more self consistent description of a mechanically differentiated subducting plate has emerged the key feature in this mechanical description is incorporation of a strong core which provides small resistance to plate bending at subduction zones while simultaneously providing adequate stretching resistance such that slab pull drives forward plate motion additionally the accompanying numerical approaches for simulating large scale lithospheric deformation processes coupled to the underlying viscous mantle flow have been become available here we put forward three fundamentally different numerical strategies each of which is capabable of treating the advection of mechanically distinct materials that describe the subducting plate we demonstrate their robustness by calculating the numerically challenging problem of subduction of a 6000 kin wide slab at high resolution in three dimensions the successfuly achievement of which only a few codes in the world can presently even attempt in spite of the differences of the approaches all three codes pass the simple qualitative test of developing an s bend trench curvature previously observed in similar models while reproducing this emergent feature validates that the lithosphere mantle interaction has been correctly modelled this is not a numerical benchmark in the traditional sense where the objective is for all codes to achieve exact agreement on a unique numerical solution however we do provide some quantitative comparisons such as trench and plate kinematics in addition to discussing the strength and weaknesses of the individual approaches consequently we believe these developed algorithms can now be applied to study the parameters involved in the dynamics of subduction and offer a toolbox to be used by the entire geoscience community c 2008 elsevier b v all rights reserved

three dimensional numerical models of the influence of a buoyant oceanic plateau on subduction zones abstract we have investigated potential effects on a subduction zone during oceanic plateau trench collision using three dimensional numerical models an oceanic plateau of varied density was embedded in the slab which was pinned at one end the plateaus strongly influenced the shape of the trench for a plateau with a higher density the trench retreat rate was reduced in the region surrounding the plateau and the plateau subducted along with the slab for lower density plateaus the trench in the region of the plateau advanced and the plateau compressed and resisted subduction spreading laterally along the trench with a weaker slab rheology the arcuate shape of the trench towards the free end of the trench was enhanced beneath the most buoyant plateaus a tear formed in the subducted portion of the slab soon after the slab tip reached the top of the lower mantle we compare the model results with a region in the northwest pacific where the ogasawara plateau meets the trench of the izu bonin mariana subduction zone

the effect of multiple pre existing weaknesses on formation and evolution of faults in extended sandbox models abstract high resolution three dimensional seismic data show that fault distribution and evolution in rift basins can be very complicated and inconsistent with the classic model of fault formation and propagation fault development in the presence of multiple pre existing weaknesses is still not clearly studied trying to understand the role of multiple pre existing weaknesses in sand models two series of experiments were prepared in the first series of models the mechanical properties of pre existing weaknesses were the same but their orientations were different whereas in the second series of models both mechanical properties and orientations were different the basic characteristics of fault formation and evolution in a rift basin revealed by the models are as follows 1 weakness reactivated faults faults that formed directly along and above the pre existing weaknesses and weakness related faults faults which are not located above the pre existing weaknesses but have similar orientation form much earlier than weakness unrelated faults the order of formation of different weakness reactivated faults is determined by the relative reactivation tendency factors fa of the weaknesses 2 weakness reactivated faults and weakness related faults develop mainly along or parallel to pre existing weaknesses in contrast weakness unrelated faults develop nearly perpendicular to the extension direction 3 a complicated fault system can form during a single extension phase in the presence of multiple pre existing weaknesses and 4 the mode of fault formation and propagation and the increase in length and number of faults have close ties to the nature of pre existing weaknesses fault displacement and length do not have a power law relationship for the majority of faults and there is no unique relationship for different kinds of faults in addition to providing new insights to the formation of complex fault systems the experiment results presented here could be reasonably explained using the reactivation tendency theory they can also be applied to the complicated fault system in rift basin

strontium distribution in the shell of the aragonite bivalve arctica islandica 1 the behavior of sr in the aragonite shells of the bivalve arctica islandica has been explored to determine its potential for high resolution environmental reconstruction the structural state of sr in the shell was investigated using x ray absorption fine structure xafs and sr ca variations across the prismatic layer in the umbo and outer shell were determined by secondary ionization mass spectrometry sims and laser ablation inductively coupled plasma mass spectrometry la icp ms respectively xafs data suggest that sr randomly substitutes for ca within the shell aragonite sr ca in both the umbo and the outer shell exhibits significant seasonal variation but does not correlate with seawater temperature sr ca varies significantly between parallel transects across the same annual bands sr concentrations are highest in transects along the maximum growth axis of the outer shell but lowest in these transects in the umbo scanning electron microscopy indicates changes in crystal morphology both within and between transects we do not however observe a clear relationship between shell growth rate and sr ca fluctuations in sr ca are most likely to be controlled by a complex interaction of biological and kinetic processes

the skeletal geochemistry of the sclerosponge astrosclera willeyana implications for biomineralisation processes and palaeoenvironmental reconstruction abstract to investigate the controls on the geochemistry of aragonitic sclerosponge skeletons we used secondary ion mass spectrometry sims to analyse an astrosclera willeyana specimen the high spatial resolution of sims allows the independent analysis of the two key crystal structures in the skeleton i e the fused spherulites formed intracellularly and fused together at the surface of the skeleton and the epitaxial backfill deposited extracellularly at the base of the sponge tissue we analysed sr ca mg ca and ba ca across a short 5 mm transect of fused spherulites which represented several years growth we observe cyclical variations with a length of 0 1 to 0 6 mm in both sr ca and mg ca in some but not all sections of the transect the observed ranges of sr ca and mg ca over the presumed seasonal cycles are 9 5 to 11 5 mmol mol 1 and 0 6 to 1 0 mmol mol 1 respectively the annual seawater temperature range at the study site is 4 3 c so the inferred temperature sensitivity of skeletal sr ca and mg ca is 0 5 mmol mol 1 c or 5 c 1 and 0 1 mmol mol 1 c or 13 c 1 respectively this is higher than observed in most previous sclerosponge studies or anticipated from studies of synthetic aragonite this indicates that the chemistry of the a willeyana skeleton is affected by one or more additional influences besides temperature which are currently unresolved the ph of the precipitating fluid estimated from skeletal 11b is 8 1 8 2 for both fused spherulites and epitaxial backfill epitaxial backfill contains significantly higher sr ca mg ca and b ca and significantly lower ba ca than the fused spherulites but sr ca and mg ca are positively correlated by the same relationship in both skeletal features this suggests that the geochemistry of each feature is predominantly controlled by a common process this is unlikely to be rayleigh fractionation which is indicated by negative correlations between sr and mg in aragonite

differentiation of late archean crust in the eastern dharwar craton krishnagiri salem area south india the late archean crust south of krishnagiri tamil nadu consists of tonalitic trondhjemitic granodioritic ttg gneisses with mafic and sedimentary enclaves formed between 2 7 and 2 5 ga and metamorphosed at amphibolite facies in the north to granulite facies in the south close to 2 5 ga migmatization occurred at all grades and numerous small granite bodies were emplaced near the amphibolite to granulite facies horizon this nearly syn accretion meta morphism affected the entire crust and left a chemically differentiated section later exposed by uplift and erosion detailed chemical and petrographic study of 60 samples across a 90 km traverse provides evidence for an essentially unbroken crustal cross section paleopressures range from 4 kbar corresponding to 12 14 km paleodepth in the north to 8 kbar corresponding to 24 28 km paleodepth to the south corresponding paleotemperatures vary between 650 and 800 c across the section mineralogic grade monitors particularly increasing tio 2 con

strain partitioning during partial melting and crystallizing felsic magmas abstract the rheology of partly molten felsic systems changes dramatically during the onset of melting and through crystallization solid to liquid and liquid to solid transitions are not rheologically identical contradicting the concept of a unique rheological critical melt percentage we use four thresholds separating the successive stages according to the melt fraction for the melting transition 1 the liquid percolation threshold connects the melt pockets at about 8 of melt thus allowing local magma displacement at the metric scale and 2 for greater melt percentages 20 25 a melt escape threshold corresponds to the onset of transport over large distances of the melt and part of the residual solid phase leading ultimately to granitic bodies for the solidification transition 3 the first formation of a rigid framework of crystals which can accumulate stress although the melt can still flow corresponds to the rigid percolation threshold 55 solids and 4 the system becomes totally locked at about 72 75 solid which corresponds to the particle locking threshold the present paper focuses on strain partitioning induced by the coexistence of two contrasting rheological phases between these thresholds on a variety of scales non coaxial deformation is preferentially accommodated by the lower viscosity melt which increases the vorticity in the melt phase and reinforces asymmetries in the flow pattern strain partitioning also increases the scale length of the weakest phase during melting and magma ascent whereas strain partitioning decreases the scale length during crystallizing strain partitioning also affects the migration of melt by raising the particle locking threshold and lowering the melt escape threshold for three dimensional deformations such as transpression the direction of melt segregation e g horizontal may differ from the direction of melt migration e g vertical

seawater ph control on the boron isotopic composition of calcite evidence from inorganic calcite precipitation experiments experiments involving boron co precipitation with calcite have been carried out inorganically under controlled ph conditions 7 9 0 05 8 3 0 05 and 8 6 0 05 to determine the dependence of the boron isotopic composition 11b of calcite on the ph of seawater another purpose of these experiments was to estimate the magnitude of the biogenic influence on the 11b value of foraminifera by comparing their boron isotopic composition with that of the inorganic calcite over a common ph range the results show a clear relationship between 11b of inorganic calcite and the ph of artificial seawater the variation of boron isotopic fractionation between seawater and calcite with ph estimated from these experiments is similar to that estimated for cultured o universa and the theoretically predicted trend the results also support the hypothesis that b oh 4 is the dominant species incorporated into the calcite structure however the boron isotopic fractionation between seawater and inorganic calcite is lower than that estimated for o universa indicating the presence of a biogenic effect on the boron isotopic composition at least of this species of foraminifera most importantly the results imply that in spite of a small biogenic influence on the boron isotopic composition of foraminifera the variation in 11b of foraminiferal shells with ph at least for o universa is comparable to that for inorganic calcite supporting the potential of this isotopic signature in foraminifera as a reliable paleo ph proxy

petrogenesis of juvenile type birimian paleoproterozoic granitoids in central cote d ivoire west africa geochemistry and geochronology abstract the birimian paleoproterozoic granitoids of central cote d ivoire are associated either with greenstone belts or with basin sedimentary rocks field data indicate two stages of granitoid emplacement which according to singlezircon and monazite 207 pb 206 pb age determinations occurred within a short period of time the first generation of granitoids 2123 2108 ma intruded and metamorphosed the greenstone belts whereas the second generation 2108 2097 ma intruded and metamorphosed the sedimentary pile and subordinate andesitic volcanic rocks of the bandama basin unconformably overlying the greenstone belts geochemical and isotopic data show that the various granitoids belong to four magmatic groups 1 sodic calc alkaline nacg 2 potassic calc alkaline kcg 3 peraluminous alg and 4 alkaline alkg all the granitoids show juvenile features with mantle like low initial sr isotopic compositions 0 700 0 702 and positive epsilon nd values 1 5 2 7 these data indicate that a the nacg may have formed by partial melting of a garnet amphibolite crust b the kcg and alkg derived from partial melting of crustal mafic to intermediate metaigneous rocks and c the alg epsilon nd 1 5 represent infracrustal melting of young sialic igneous materials during a regional crustal shortening in a transcurrent tectonic context sm nd model ages 2 25 2 4 ga preclude the existence of an archean crust reworked in the birimian magmas an underplating model is discussed

analogue modelling of intraplate strike slip tectonics a review and new experimental results abstract intraplate strike slip zones commonly display intricate 3 d geometries with rapid changes in structural style along strike and with depth strike slip deformation typically results in complex vertical and horizontal sections that can be difficult to interpret coherently physical modelling of strike slip fault systems is a powerful and graphic tool to help in providing a unified picture of the evolution of strike slip zones with considerable spatial and temporal detail a large number of experimental studies have investigated different aspects of strike slip tectonics using materials such as dry sand wet clay or silicone the choice of analogue material and experimental design exerts a strong control on the structures that form in the model here we present a review of different experimental setups used to investigate intraplate strike slip tectonics from the classical riedel experiment to more sophisticated setups using brittle and viscous analogue materials we review our current understanding from models of distributed shear transtension transpression pull apart basins formed in releasing stepovers and popups formed in restraining stepovers in addition we present the results of two new experimental series that investigated 1 the effect of crustal weak zones on strike slip fault zone segmentation and 2 strike slip and transpressional reactivation of extensional basins

cenozoic tectonics of western north america controlled by evolving width of farallon slab the basin and range province of western north america exemplified by the alternating mountain ridge and valley landscapes across nearly the entire u s state of nevada started to form 50 million years ago through a series of extensions of the continental crust prior to that massive mountain building collisions at the boundary between north america and the pacific ocean formed a subduction zone and compressed the continent by combining observations of global subduction zone velocities with numerical modeling schellart et al p 316 1 demonstrate that the thinning of the subducting portion of the oceanic plate controlled how and when the transition from compression to extension occurred even today where the much smaller remnant oceanic plate continues to slowly subduct below north america in the pacific northwest the width of the slab and not its age controls the velocity of subduction indeed this relation may explain the dynamics of other modern subduction zones from south america to japan 1 lookup doi 10 1126 science 1190366

the late quaternary decline and extinction of palms on oceanic pacific islands late quaternary palaeoecological records of palm decline extirpation and extinction are explored from the oceanic islands of the pacific ocean despite the severe reduction of faunal diversity coincidental with human colonisation of these previously uninhabited oceanic islands relatively few plant extinctions have been recorded at low taxonomic levels recent faunal extinctions on oceanic islands are concentrated in larger bodied representatives of certain genera and families fossil and historic records of plant extinction show a similar trend with high representation of the palm family arecaceae late holocene decline of palm pollen types is demonstrated from most islands where there are palaeoecological records including the cook islands fiji french polynesia the hawaiian islands the juan fernandez islands and rapanui a strong correspondence between human impact and palm decline is measured from palynological proxies including increased concentrations of charcoal particles and pollen from cultivated plants and invasive weeds late holocene extinctions or extirpations are recorded across all five of the arecaceae subfamilies of the oceanic pacific islands these are most common for the genus pritchardia but also many sedis fossil palm types were recorded representing groups lacking diagnostic morphological characters

new u th ages for pleistocene megafauna deposits of southeastern queensland australia arguments over the extinction of pleistocene megafauna have become particularly polarised in recent years causes for the extinctions are widely debated with climate change human hunting and or habitat modification or a combination of those factors being the dominant hypotheses however a lack of a spatially constrained chronology for many megafauna renders most hypotheses difficult to test here we present several new u th dates for a series of previously undated megafauna bearing localities from southeastern queensland australia the sites were previously used to argue for or against various megafauna extinction hypotheses and are the type localities for two now extinct pleistocene marsupials including the giant koala phascolarctos stirtoni the new dating allows the deposits to be placed in a spatially and temporally constrained context relevant to the understanding of australian megafaunal extinctions the results indicate that the joint texas caves megafaunal assemblage is middle pleistocene or older 292 ky the cement mills gore megafaunal assemblage is late pleistocene or older 53 ky and the russenden cave bone chamber texas caves megafaunal assemblage is late pleistocene 55 ky importantly the new results broadly show that the sites date prior to the hypothesised megafaunal extinction window i e 30 50 ky and therefore cannot be used to argue exclusively for or against human climate change extinction models without first exploring their palaeoecological significance on wider temporal and spatial scales

blueschists eclogites and subduction zone tectonics insights from a review of late miocene blueschists and eclogites and related young high pressure metamorphic rocks abstract in this paper late miocene blueschist and eclogite belts including the world s youngest blueschist belt in timor tanimbar island chain eastern indonesia and the world s youngest coesite bearing eclogite papua new guinea together with selected cenozoic high pressure and ultrahigh pressure metamorphic rocks are reviewed from a synthesis of the geology metamorphism and chronology of these rocks the formation and exhumation process are evaluated and the significance on tectonics at convergent plate boundaries is discussed

what can the data on late survival of australian megafauna tell us about the cause of their extinction abstract roberts et al 2001a new ages for the last australian megafauna continent wide extinction about 46 000 years ago science 292 1888 1892 concluded that the extinction of australia s late pleistocene megafauna was an abrupt event that took place about 46 ka ago throughout the continent by placing the extinctions soon after the arrival of people in australia but before climate changes associated with the last glacial maximum this study implicated human impact as cause however the study was controversial because it excluded evidence mainly from archaeological sites containing disarticulated megafauna remains for survival of some megafauna well past 46 ka ago here i ask how our interpretation of the extinctions would be changed if this evidence were accepted contrary to climate change models of extinction the young megafauna sites are not concentrated in mesic refuges around the coast these sites do suggest that relatively small bodied megafauna species were the last to disappear as predicted by a version of the overkill hypothesis more work is needed to test the evidence for late survival of megafauna species in australia but at present this evidence supports overkill not climate change as the cause of the extinctions

early human occupation at devil s lair southwestern australia 50 000 years ago abstract new dating confirms that people occupied the australian continent before the earliest time inferred from conventional radiocarbon analysis many of the new ages were obtained by accelerator mass spectrometry 14c dating after an acid base acid pretreatment with bulk combustion aba bc or after a newly developed acid base wet oxidation pretreatment with stepped combustion abox sc the samples charcoal came from the earliest occupation levels of the devil s lair site in southwestern western australia initial occupation of this site was previously dated 35 000 14c yr b p whereas the aba bc ages are indistinguishable from background beyond 42 000 14c yr b p the abox sc ages are in stratigraphic order to 55 000 14c yr b p the abox sc chronology suggests that people were in the area by 48 000 cal yr b p optically stimulated luminescence osl electron spin resonance esr ages u series dating of flowstones and 14c dating of emu eggshell carbonate are in agreement with the abox sc 14c chronology these results based on four independent techniques reinforce arguments for early colonization of the australian continent

the role of crustal strength variations in shaping orogenic plateaus with application to tibet 1 the tibetan plateau is the type example of a large orogenic plateau formed as a result of continent continent collision the morphology of the plateau and its margins suggests that preexisting variations in crustal strength have influenced the growth of the plateau we have developed a three dimensional numerical model of deformation in a viscous crust in order to investigate the effects of lateral heterogeneities on plateau growth the model includes a two layer crust and allows for lateral variation of viscosity in both the upper and lower layers model results indicate that crustal strength variations have a dramatic effect on the morphology and dynamics of a developing plateau a region of strong crust is characterized by a very steep plateau margin that propagates extremely slowly does not accommodate significant shortening strain and is subparallel to local upper and lower crustal velocities a weak crustal region develops a gently sloping margin uplift propagates rapidly across the weak zone and crustal material within the plateau is diverted toward the low strength region with a relatively simple distribution of strength variations corresponding to strong tarim and sichuan basin crust and a weak southeastern corner our model produces a plateau with many similarities to the tibetan plateau including the overall morphology rotation around the eastern syntaxis and e w extension analysis of model results suggests that e w extension of the central plateau may be related to the rapid eastward flow of crustal material into a weak zone in the se corner without significant change in plateau elevation

a new metamorphic constraint for the eburnean orogeny from paleoproterozoic formations of the man shield aribinda and tampelga countries burkina faso abstract progressive metamorphic zones developed in pelites around the aribinda granite burkina faso indicate contact metamorphism at 5 6 kbar biotite chloritoid staurolite kyanite zones are distributed concentrically around the aribinda pluton on its northern margin but are displaced by later thrust faults in the south of the aribinda area and in the nearly tampelga area these metamorphic and structural characteristics provide new constraints for the geodynamical evolution of the eburnean orogeny in the north part of the baoule mossi domain they show the leading part of the granitoid emplacement in the thermic structuration of the greenstone belts and the post metamorphic character of the tangential tectonic in burkina faso the eburnean orogeny underwent 1 initial greenschist facies metamorphism 2 emplacement of granitoids at mid deep crustal levels leading to unusual medium pressure metamorphism 3 later thrusting

intra annual variation in the stable oxygen and carbon and trace element composition of sclerosponges 1 this paper presents data to support the presence of 1 intra annual signals in the chemical composition 18o and sr ca of the skeletons of sclerosponges from the bahamas and 2 variable rates of skeletal accretion these conclusions are based on data obtained by using a microsampling method for the stable oxygen and carbon isotopes in which material was extracted at a resolution of one sample every 34 m and a laser microprobe which obtained trace element data every 20 m sr mg and pb an age model was established using a combination of changes in the concentration of pb the change in the 13c of the skeleton of the sclerosponges and u th isotopic measurements these methods yield a mean growth rate of 220 m yr but suggest that the growth rate in this particular sclerosponge was not constant the calculated growth rate is within error identical to that determined by u th methods the variable growth rate was confirmed through spectral analysis of the 18o and sr ca data that showed peaks corresponding to the annual cycle in these parameters as well as peaks corresponding to growth rates of approximately 128 212 270 and 400 m yr the presence of these additional frequencies suggests a growth rate between approximately 100 and 300 m yr these conclusions were supported by modeling of oxygen isotopic data measured on a scleractinian coral as well as model isotope data generated on synthetic time series these findings have important implications for the use of sclerosponges as proxies of paleoclimate because they emphasize the need for a precise yearly chronology in order that proxy data can be compared with climatic variables

three dimensional numerical experiments of strain partitioning at oblique plate boundaries implications for contrasting tectonic styles in the southern coast ranges california and central south island new zealand a three dimensional numerical model is used to investigate the nature and distribution of crustal scale deformation and strain partitioning across obliquely convergent transpressive plate boundaries a uniform elastic plastic crustal layer obeying murrell s extension to three dimensions of the griffith failure criterion was deformed under basal kinematic boundary conditions that correspond to the oblique convergence between two mantle lithospheres one of which detaches and subducts beneath the other the results show deformation concentrated in relatively narrow shear zones the character planar versus curved and dip of these shear zones or crustal scale faults depend on the relative amount of convergence versus transcurrent motion imposed across the plate boundary in models dominated by transcurrent boundary conditions two separate sets of coexisting structures develop 1 a pair of planar relatively shallow dipping thrust zones arranged in a v structure that accommodate convergent motion and 2 a set of curved near vertical strike slip zones arranged in a flower structure that accommodates the transcurrent motion in models dominated by convergence both contractional and transcurrent deformations are accommodated by oblique slip across a single pair of structures that have the same geometry as the thrust shear zones there is a gradational behavior between these end member partitioned and nonpartitioned modes for intermediate boundary conditions in all cases the dip of the shear zones predicted by the numerical model is in good agreement with the dip of ideal faults predicted by mohr s incipient faulting hypothesis the two basic modes are interpreted to underlie the contrasting tectonic styles of the southern coast ranges california and central south island new zealand factors other than obliquity of convergence which play a role in strain partitioning are discussed

mammalian responses to pleistocene climate change in southeastern australia resolving faunal responses to pleistocene climate change is vital for differentiating human impacts from other drivers of ecological change while 90 of australia s large mammals were extinct by ca 45 ka their responses to glacial interglacial cycling have remained unknown due to a lack of rigorous biostratigraphic studies and the rarity of terrestrial climatic records that can be related directly to faunal records we present an analysis of faunal data from the naracoorte caves in southeastern australia which are unique not only because of the species richness and time depth of the assemblages that they contain but also because this faunal record is directly comparable with a 500 k y speleothem based record of local effective moisture our data reveal that despite significant population fluctuations driven by glacial interglacial cycling the species composition of the mammal fauna was essentially stable for 500 k y before the late pleistocene extinctions larger species declined during a drier interval between 270 and 220 ka likely reflecting range contractions away from naracoorte but they then recovered locally persisting well into the late pleistocene because the speleothem record and prior faunal response imply that local conditions should have been favorable for megafauna until at least 30 ka climate change is unlikely to have been the principal cause of the extinctions