porous fluid flow enables oceanic subduction initiation on earth 1 although most of the presently active intra oceanic subduction zones are relatively young and initiated during the cenozoic subduction initiation process remains poorly understood previous models of subduction initiation assumed excessive weakening of tectonic plate boundaries that does not reconcile with laboratory rock strength measurements the weakening was assumed to be caused by fluids present along tectonic fractures however no self consistent solid fluid model of subduction initiation has been developed so far here we present new numerical hydro thermo mechanical model of spontaneous intra oceanic subduction initiation where solid rock deformation and fluid percolation are fully coupled based on 2 d numerical experiments we demonstrate that although subduction fails to initiate under fluid absent conditions it can naturally start when porous fluid is present inside oceanic crust and along the plate boundaries fluid percolation is localized along spontaneously forming faults where high fluid pressure compensates lithostatic pressure thus dramatically decreasing friction along the incipient subduction zone through the parametric study we conclude that the most important parameter for subduction initiation is the solid matrix permeability paradoxical at first lowering the permeability indeed favors subduction initiation by maintaining high fluid pressure and thus decreasing friction along active faults

propagating rift during the opening of a small oceanic basin the protector basin scotia arc antarctica abstract the opening of oceanic basins constitutes one of the key features of plate tectonics because it determines the rifting and displacement of the continental crustal blocks although the mechanisms of development of large oceans are well known the opening and evolution of small and middle size oceanic basins have not been studied in detail the protector basin located in the southern scotia sea is a good example of a small oceanic basin developed between two thinned continental blocks the pirie bank and the terror rise poorly studied up to now a new set of multibeam bathymetry multichannel seismic reflection and gravity and magnetic anomaly profiles obtained on the scan 2001 cruise led us to determine that the protector basin probably opened during the period comprised between c5dn 17 4 ma and c5acn c5abr chrons 13 8 ma forming a n s oriented spreading axis the end of spreading is slightly younger to the north the start of spreading is clearly diachronous with the most complete set of chrons up to c5dn in the southern profile c5cn in the middle section and only up to c5adn in the northern part of the basin the spreading axis propagated northwards during the basin development producing the wedge shape of the basin in addition at the ne part of the basin a reverse fault developed in the border of the pirie bank after basin opening accentuates the sharp northern end moreover the northwestern part of the pirie bank margin is an extremely stretched continental crust with n s elongated magnetic anomalies related to incipient oceanic southward propagating spreading axes the protector basin shows the oldest evidence of e w continental stretching and subsequent oceanic spreading during middle miocene related with the eastward development of the scotia arc that continues up to present the relative rotation of continental blocks during the development of small sized oceanic basins by continental block drifting favoured the opening of wedge shape basins like the protector basin and conjugate propagating rifts

an integrated stratinomic model for the genesis and concentration of small shelly fossil style phosphatic microsteinkerns in not so exceptional conditions abstract carbonate fluorapatite filled millimeter scale microsteinkerns are a key element of the small shelly fossil ssf fauna generally associated with the late proterozoic cambrian the purported disappearance of these fossils and this style of phosphatic preservation during the cambrian stage 2 botomian extinction has been linked to changes in ocean chemistry and or changes to the physical environment due to the evolution of metazoans as well to the changing role of metazoans in the marine environment through a qualitative meta analysis of existing literature we document a broader temporal distribution for ssf style preservation than previously recognized we examine these occurrences for common patterns of depositional conditions and we offer a conceptual model for both the origin of the phosphatic microsteinkerns and their eventual concentration in certain sediments based on known biogeochemical diagenetic processes of authigenic calcium fluorapatite precipitation phosphogenesis and subsequent concentration of phosphatic sedimentary particles this polycyclic phosphogenic condensation ppc model makes predictions as to how and where this form of preservation might occur in cambrian and non cambrian marine strata alike

dynamic causes of the relation between area and age of the ocean floor estimates for the area and age of the ocean floor are at odds with assumptions for mantle convection which imply that an older sea floor rather than a new one would be preferentially subducted over time previous efforts to explain these relationships have been based on geologic evidence and simple models coltice et al p 335 1 created numerical three dimensional convection models representing more realistic physical boundaries including a spherical earth the existence of continents and supercontinents over time and realistic rheologies a combination of continents and plate like behavior of the ocean floor sufficed to produce the observed relationship between plate area and plate age which explains why some old oceanic crust still remains 1 lookup doi 10 1126 science 1219120

origin of phosphatic stromatolites in the upper cretaceous condensed sequence of the polish jura chain abstract the turonian stromatolite bearing condensed sequence in the polish jura chain the european epicontinental basin provides good insight into the environment of formation of cretaceous phosphatic stromatolites owing to their purely phosphatic development and negligible post depositional alteration the sequence developed as a result of slow pelagic sedimentation and microbial mat phosphatization on a submarine swell surrounded by local basins with non condensed carbonate deposition diagenesis of organic matter and dissolution of biogenic apatite were the major sources of reactive phosphorus for the microbial mat phosphatization stromatolite growth occurred due to pulses of amorphous or poorly ordered calcium phosphate precipitation followed by crystallization of carbonate fluorapatite cfa the phosphogenic environment left an imprint on the isotopic composition of limestone carbon and lattice bound carbon and sulphur in cfa and on the light rare earth element lree distribution in cfa the 13c of the stromatolite bearing sequence shows a negative excursion 1 to 3 standing in marked contrast to positive carbon values of the surrounding basinal carbonate most of the 34s values of cfa 20 to 21 fit the value range of the coeval seawater sulphate and the lree distribution shows a well defined seawater pattern this geochemical signature is indicative of intense diagenesis of organic matter at the seafloor pelagic carbonate dissolution and prolonged exposure of the deposited phosphate towards the water column the enhanced deposition and diagenesis of organic phosphorus in the stromatolitic environment reflects elevated levels of the epicontinental basin nutrification related to sea level rises and the associated oceanographic and geochemical changes

influence of lateral slab edge distance on plate velocity trench velocity and subduction partitioning 1 subduction of oceanic lithosphere occurs through both trenchward subducting plate motion and trench retreat we investigate how subducting plate velocity trench velocity and the partitioning of these two velocity components vary for individual subduction zone segments as a function of proximity to the closest lateral slab edge dse we present a global compilation for 207 trench segments from 17 active subduction zones on earth and three dimensional numerical models of progressive free subduction of a single oceanic plate that subducts into a stratified mantle the results show that the subducting plate velocity is always high 5 1 cm yr models and 4 2 cm yr nature and trench velocity is always low 2 5 cm yr models and 1 7 cm yr nature in the center of wide subduction zones dse 2200 km only in regions close to lateral slab edges dse 1000 km be it for narrow or wide subduction zones can the trench velocity exceed 4 cm yr models and 6 cm yr nature and can the subducting plate velocity go below 4 cm yr models and 2 cm yr nature in general plate velocities trench velocities and subduction partitioning are much more variable near slab edges than in the center of wide subduction zones owing to other parameters that affect subduction kinematics we conclude that subduction kinematics can vary considerably along individual subduction zones and that the upper bound values for trench velocity and lower bound values for subducting plate velocity and subduction partitioning at individual subduction zone segments depend critically on dse

evolution of retreating subduction boundaries formed during continental collision retreating subduction boundaries formed where the rate of subduction exceeds the rate of overall plate convergence appear to be commonly developed features within regions of early or incomplete continent continent collision they are characterized by regional extension within the overriding plate and at their leading edge by thin skinned arcuate thrust belts that are concave towards the overriding plate as is illustrated by examples from the mediterranean region the formation of retreating subduction boundaries is intimately related to the process of continental collision during the early stages of collision retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary retreating plate boundaries can also form before the main collision and the associated thrust belts emplaced as precollisional accretionary assemblages because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab slab pull subduction usually ceases when and only when thick buoyant continental crust enters the subduction zone thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted in some cases retreating subduction boundaries may escape into the open ocean where they form nearly isolated local tectonic systems in these systems the rate of subduction is approximately compensated by the rate of upper plate extension and migration of the system across the oceanic region may be very rapid for example the horseshoe seamounts located about 800 km offshore in the eastern north atlantic may be the active expression of an east dipping westwardly migrating retreating subduction boundary that has evolved from the betic cordillera rif system active in miocene time and may now be progressing across the atlantic at approximately 50 mm yr an analogous situation may be represented by the scotia arc system a westward dipping retreating subduction system located between the south american and antarctic plates which may have escaped into the south atlantic ocean from a zone of crustal shortening in the andes and is now progressing across the atlantic at a rate of about 80 mm yr

microbial processes in the negev phosphorites southern israel microscopic and sem observations indicate that cyanobacterial activity was important during the formation of the campanian negev phosphorites one aspect of such activity is the development of phosphorites through the binding of phosphate particles these sediments often demonstrate a conspicuous smooth cryptalgal lamination the algal binder was fossilized as intertwined phosphatic sheaths that coat and seal off the phosphate grains different types of sheaths representing the remnants of an algal community were found thick apatite overgrowth commonly deforms the microbial fossils converting them into barely recognizable forms fabrication of phosphate particles is another aspect of the microbial activity phosphate algal coated grains are a common constituent of the negev phosphorites the algal coating consists of stacked phosphatic tubes and colonies of coccoid unicells with an apatite infilling fragmented phosphatic sheath bundles form another type of phosphate particle produced by algae obliteration of algal structures is common resulting in undifferentiated groundmasses of cryptocrystalline apatite the extremely close connection between microbial structures and phosphatization processes suggests cyanobacterial participation in phosphorus enrichment

upper triassic norian cephalopod limestones of the hallstatt type oman norian crinoidal brachiopod limestones and cephalopod limestones of the hallstatt type occur as blocks in a hettangian calcareous breccia of the haliw formation in the oman mountains crinoidal and brachiopod packstones up to 12 m thick prevail in the lower part of the sequence and were deposited on a substrate of norian forereef breccia the overlying cephalopod wackestones up to 4 9 m thick have a basal white bed followed by red limestones with abundant planar and scalloped corroded surfaces and local stromatolites upward red nodular wackestones and finally slumped grey wackestones follow the analysis of geopetal fabrics in orientated samples shows that bedding of these facies is in fact inclined bedding inclinations varied between 15 and 29 in addition the restored dip directions demonstrate rotation indicating deposition on a gliding block the preferred orientation of orthoconic cephalopods and imbrication of discoidal ammonoids coincide with the dip direction measured from geopetal fabrics such features generally interpreted as current induced are here interpreted as gravity induced the overall mud supported rock fabric thus indicates deposition under very low energy conditions the common mud supported texture of the rocks contrasts with evidence for current activity found in the scalloped surfaces and shell lags particularly in the crinoidal brachiopod facies and the lower stratigraphically condensed cephalopod limestones this indicates that deposition of lime mud alternated with periods of elevated current strength a comparison of the hallstatt type limestones and current influenced sediments on the northern slope of the little bahama bank suggests that condensed sequences of the hallstatt type are restricted to relatively shallow depths with strong fluctuation of contour following currents undersaturated with respect to aragonite along carbonate shelf margins facing the open ocean on steep slopes sediment bypassing may be an additional factor for stratigraphic condensation

dynamics of continental accretion three dimensional dynamic computer models show how accretionary tectonic margins evolve from the initial plate collision state through a period of plate margin instability and then re establish a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones where one plate dives under another become congested when they try to accommodate buoyant exotic crust louis moresi et al describe new numerical models of continental accretion that follow the entire process from the initial collision state through a period of plate margin instability to the re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones become congested when they try to consume buoyant exotic crust the accretionary mountain belts orogens that form at these convergent plate margins have been the principal sites of lateral continental growth through earth s history modern examples of accretionary margins are the north american cordilleras and southwest pacific subduction zones the geologic record contains abundant accretionary orogens such as the tasmanides1 along the eastern margin of the supercontinent gondwana and the altaides which formed on the southern margin of laurasia2 in modern and ancient examples of long lived accretionary orogens the overriding plate is subjected to episodes of crustal extension and back arc basin development often related to subduction rollback3 and transient episodes of orogenesis and crustal shortening4 5 6 7 coincident with accretion of exotic crust here we present three dimensional dynamic models that show how accretionary margins evolve from the initial collision through a period of plate margin instability to re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region the complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction we find geological and geophysical evidence for this process in the tasmanides of eastern australia and infer that this is a recurrent and global phenomenon

on the initiation of subduction estimates of shear resistance associated with lithospheric thrusting and convergence represent lower bounds on the force necessary to promote trench formation three environments proposed as preferential sites of incipient subduction are investigated passive continental margins transform faults fracture zones and extinct ridges none of these are predicted to convert into subduction zones simply by the accumulation of local gravitational stresses subduction cannot initiate through the foundering of dense oceanic lithosphere immediately adjacent to passive continental margins the attempted subduction of buoyant material at a mature trench can result in large compressional forces in both subducting and overriding plates this is the only tectonic force sufficient to trigger the nucleation of a new subduction zone the ubiquitous distribution of transform faults and fracture zones combined with the common proximity of these features to mature subduction complexes suggests that they may represent the most likely sites of trench formation if they are even marginally weaker than normal oceanic lithosphere

source analysis of the february 12th 2007 mw 6 0 horseshoe earthquake implications for the 1755 lisbon earthquake 1 while very large earthquakes are generally confined to subduction zones the sw iberian margin setting of the famous mw 8 5 8 7 1755 lisbon tsunami earthquake may be an exception to this rule evidence for active subduction is not conclusive here but instead plate convergence in old oceanic lithosphere with large brittle layer thickness can account for the occurrence of great earthquakes along moderate length faults we estimate the source parameters of the february 12th 2007 horseshoe earthquake regional moment tensor inversion yields an mw 6 0 reverse to strike slip faulting source in the upper mantle modelling teleseismic surface reflected body waves pp pwp sp indicates a source depth of 40 km beneath the seafloor analysing apparent source time functions allows identifying the preferred fault plane strike n245 e dip 55 rake 50 and estimating rupture area 53 km2 and average slip 0 27 m scaling the source characteristics to the size of the 1755 earthquake suggests a fault length of 230 315 km being compatible with the length of mapped faults in the area

cratonic and foreland early cambrian phosphorites of west africa palaeoceanographical and climatical contexts abstract in western gondwana two types of pan african brasiliano fold belts have been distinguished the first type is characterized by an old long lived tectonic cycle 1000 600 ma and is generally oceanic the closure of these oceans resulted in the aggregation of western gondwana in late neoproterozoic time the second type is characterized by a young short lived cycle about 600 520 ma and is mostly intracontinental fold belts of the second type have various geodynamic significances and represent folded foreland intermontane or impactogen type basins formed during or slightly after the deformational closure of the old long lived cycle phosphorites of west africa are associated with the young short lived tectonic cycle they occur together with the upper member of a distinctive formation known all over west africa this formation is made up of three members and consists from base to top of diamictites generally tillites with detrital intercalations carbonate rocks with barite and bedded cherts with local shale or siltstone intercalations in eastern senegal in the cratonic taoudeni basin the carbonate member is palaeontologically dated as early cambrian deposited during a glacio eustatic transgression it can be considered as a chronostratigraphic marker for the entire west africa phosphorites associated with or immediately following the eustatic transgression are also possibly of early cambrian age they are located either in the lower part of the chert member in the upper part of the carbonate member or sandwiched between these two units in fersiga and namel in the taoudeni basin phosphorites are associated with stromatolites characterizing the initial stage of the transgression they are overlain by evaporites which can be related to an isostatic rebound in the volta foreland basin phosphorites are associated with the chert member there they were deposited on a shelf ramp either on its proximal portion where they are strongly reworked kodjari and arli occurrences or in its distal part where they are interbedded with sediments deposited under anoxic conditions tapoa mekrou and aloub djouana occurrences the depth of marine sedimentation as indicated by the sedimentological characteristics of phosphorites and associated sediments was controlled by the combined effect of glacio eustatic sea level rise subsidence rate and isostatic rebound numerous textural features are similar in the phosphorites from the two basins phosphatic grains occurring as oncolites and stromatolitic fragments together with intraclasts show a noticeable biogenic contribution of microbial origin microbial build ups were formed in submergent emergent environments in the taoudeni basin and in the outer shelf of the volta basin apatites from these basins have similar ree contents which suggest a continental origin for the phosphorus early cambrian phosphorites of west africa were deposited soon after glacially derived sediments according to the available palaeocontinental maps both were laid down at palaeolatitudes of about 40 s west african early cambrian phosphorites thus formed at higher latitudes than their modern analogues which are confined within the inter tropical zone

the role of bacterial extracellular polymeric substances in geomicrobiology abstract the last two decades have seen geomicrobiology evolve into a broad field encompassing a wide range of environmentally significant processes such as proton and metal adsorption onto cell surfaces and the effect of microbes on mineralisation redox cycling and contaminant transport within this sphere research groups have conducted studies using bacteria fungi diatoms and sludges all of which can play a part in geochemical processes here we review research on the role played by microbial extracellular polymeric substances eps focussing on bacterial and cyanobacterial eps we conclude by outlining future research directions in order to investigate unresolved questions in the field the effect of eps on metal adsorption is complex whereas some studies report an increase in metal adsorption when cells contain eps some report no differences yet others report a decrease these discrepancies may reflect differences in molecular and functional group composition of the eps eps provides a template for adsorption of metal cations to which carbonate ions are attracted to induce local mineral supersaturation this may be behind observed changes in both crystal polymorphism favouring formation of less stable polymorphs and crystal morphology where the presence of eps promotes the formation of rounded smoothed crystals or spheroids the role of eps in mineral dissolution bioleaching and corrosion is equivocal eps alone appears to have little effect on mineral dissolution bioleaching or biocorrosion instead it appears that eps increases rates of mineral weathering and leaching by forming complexes with metallic ions released by the mineral surface which may themselves catalyse bioleaching in the case of sulphides eps in biofilms forms effective barriers to the transport of particulate phases and exerts important controls on the transport and deposition of natural and engineered nanoparticles much less is known about the role of eps in the cycling of redox sensitive trace metals intuitively and as observed in microbial mats with sulphate reducing bacteria eps is a labile electron donor for microbial metabolism that could affect metal biogeochemistry but this remains to be conclusively demonstrated in laboratory experiments hence in this and other areas further studies are required to develop a mechanistic basis for including the effects of eps in biogeochemical models

on the initiation of subduction zones analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere numerical model studies show that if after a short evolution of a passive margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin

morphotectonic characterization of major bathymetric lineaments in gulf of cadiz africa iberia plate boundary insights from analogue modelling experiments abstract new high resolution multi beam bathymetry data allowed the recognition of several bathymetric lineaments ca 100 km long trending wnw ese in seafloor sediments of the gulf of cadiz offshore sw iberia the interpretation of multi channel mcs profiles crosscutting these lineaments showed that they are controlled by underlying deep seated faults which have endured a polyphase reactivation history to get insights on the recent tectonic evolution of these structures we performed two sets of analogue modelling experiments assuming 1 right lateral strike slip basement faulting and coupled passive shearing affecting an overlying soft cover and 2 low angle transpressive deformation along a narrow shear band overlying the fault our results show a good correlation between the experimentally obtained structural patterns and the natural morphotectonic lineaments allowing the use of some of the observed natural features as strain gauges based on this we conclude that the study lineaments correspond to the bathymetric expression of ongoing dextral wrenching reactivation of wnw ese pre existing faults and we estimate the age of this tectonic reactivation as being ca 1 8 ma i e form late pliocene to present day these characteristics agree with the most recent kinematic models derived from geodetic observations indicating that present day convergence between nubia and iberia is subparallel to the newly identified lineaments and occurs at a 4 mm yr rate

morphotectonics and strain partitioning at the iberia africa plate boundary from multibeam and seismic reflection data abstract the gulf of cadiz off sw iberia and the nw moroccan margin straddles the cryptic plate boundary between africa and eurasia a region where the orogenic alpine compressive deformation in the continental collision zone passes laterally to the west to strike slip deformation a set of new multibeam bathymetry multi channel and single channel seismic data presented here image the main morphological features of tectonic origin of a significant part of the gulf of cadiz from the continental shelf to the abyssal plain these morphotectonic features are shown to result from the reactivation of deeply rooted faults that changed their kinematics from the early mesozoic rifting through the late cretaceous paleogene collision to the pliocene quaternary thrusting and wrenching the old faults control deep incised more than 100 km long canyons and valleys several effects of neotectonics on deep water seabed are shown these include i the complex morphology caused by wrenching on the 230 km long wnw ese faults that produced en echelon folds on the sediments ii the formation of up to 5 km wide crescent shaped scours at roughly 4 km water depth by reactivation of thrusts iii 10 km long creep folds on the continental slope and iv the formation of landslides on active fault escarpments the present day deformation is partitioned on ne sw thrusts and wnw ese to w e strike slip faults and is propagating northwards on n s trending thrusts along the west iberia margin from 35 5 n to 38 n which should be considered for seismic hazard

spatial geochemistry of upper jurassic marine carbonates iberian subplate abstract chemostratigraphy applied to ancient marine carbonates is commonly based on one dimensional stratigraphic sections or core data as demonstrated from modern oceans this approach underestimates the spatial complexity of physico chemical seawater properties here a several hundred kilometer long transect consisting of seven upper jurassic sections from settings ranging from proximal neritic middle shelf to the epioceanic distal fringe across the southern and eastern palaeomargins of the iberian sub plate reveals variability in sedimentologic stratigraphic and geochemical records the comparison of isotopic and elemental data from different carbonate materials matrix micrite carbonate cements in veinlets belemnite rostra and ammonite shells reveals differential diagenetic pathways microfacies cathodoluminescence and geochemical data retrieved from biostratigraphically well constrained sections reveal that epioceanic matrix micrite geochemical data provide valuable proxies for palaeo seawater properties our data are reviewed in the context of published late jurassic records the outcome shows a higher level of complexity including the potential admixture of marine continental and diagenetic geochemical signals in the epicontinental record the stratigraphic trend in carbon isotopes of epioceanic sections agrees upon that of upper jurassic reference sections from the northern tethyan margins while oxygen isotope ratios are relatively 18o enriched palaeo seawater temperatures across the transect investigated were estimated using 18o as tentative proxy for interpreting distance from shore differences in water masses relative depth variations and potential local forcing factors palaeoenvironmental conditions evaluated through the combined record of isotope 18o elemental mn and skeletal content contrast with relative fluctuations in sea level site specific changes in palaeoceanographic parameters such as water depth seawater temperature salinity and upwelling are considered in comparison to examples from ancient and modern oceans

organic carbon cycling and modern phosphorite formation on the east australian continental margin an overview during 1987 the australian bureau of mineral resources conducted a multidisciplinary investigation of the modern phosphorites on the continental margin of southeastern australia between 28 and 32 s the objectives of the work were to examine the processes controlling the cycling of organic carbon and bioactive elements nitrogen phosphorus sulphur and iron in the sediments and to investigate the roles which these processes played in the formation of the modern phosphorites bacterial productivities sulphate reduction rates sedimentary oxygen and pore water concentrations of nitrate ammonia phosphate iron sulphate and fluoride were measured at sea the highest rates of microbial productivity were found in the surficial 0 20 mm sediments of the modern phosphorite zone in 350 460 m water depth these rates were about double those in shallower shelf 300 m sediments and 3 4 fold those rates in mid slope 600 1000 m sediments aerobic and anaerobic oxidation rates of organic matter calculated from sediment oxygen profiles and sulphate reduction rates were highest in the surface sediments in the modern phosphorite zone the recycling of sedimentary iron via reductive dissolution of iron oxyhydroxides and reprecipitation at the oxic anoxic boundary results in a near surface sedimentary trap for iron in the phosphorite zone sediments phosphate released from organic matter in the interfacial sediments and fluoride from seawater are scavenged by iron oxyhydroxides in the top few centimetres of sediment phosphorus in this way is decoupled from organic carbon in the near surface sediments and linked to the redox cycling of iron phosphate and fluoride scavenged onto iron oxyhydroxides and concentrated in the surficial sediments are subsequently released to pore waters in the anoxic sediments when iron oxyhydroxides are buried and dissolve the recycling process releases phosphate and fluoride for incorporation into apatite fluoride is depleted from pore waters at depths 18 cm phosphorite nodules form within anoxic sediments at depths 18 cm and continue to accumulate iron and phosphorus while resident in the mixed layer combinations of rapid sediment mixing rates a slow sedimentation rate and a mixed layer to about 18 cm result in an average particle residence time in the phosphorite zone sediments which is about ten fold that of the mid slope sediments long residence times and rapid mixing promote the oxidation of organic carbon and release of phosphate while the continuous recycling of iron and phosphate concentrates the phosphorus for apatite precipitation and accumulation into phosphorite nodules phosphorite nodules are not found in mid slope sediments probably because of combinations of relatively rapid sedimentation rates ineffective iron phosphorus and fluoride recycling and trapping mechanisms plus dilution and dissemination of any incipient apatite

geochemistry of the rare earth elements in ferromanganese nodules from domes site a northern equatorial pacific abstract the distribution of rare earth elements ree in ferromanganese nodules from domes site a has been determined by instrumental neutron activation methods the concentrations of the ree vary markedly low concentrations characterize samples from a depression the valley in which quaternary sediments are thin or absent high concentrations are found in samples from the surrounding abyssal hills the highlands where the quaternary sediment section is relatively thick moreover the valley nodules are strongly depleted in the light trivalent ree lree and ce compared with nodules from the highlands some of the former showing negative ce anomalies the ree abundances in the nodules are strongly influenced by the ree abundances in coexisting bottom water some controls on the ree chemistry of bottom waters include a the more effective removal of the lree relative to the hree from seawater because of the greater degree of complexation of the latter elements with seawater ligands b the very efficient oxidative scavenging of ce on particle surfaces in seawater and c the strong depletion of both ce and the lree in or a larger benthic flux of the hree into the antarctic bottom water aabw which flows through the valley the distinctive ree chemistry of valley nodules is a function of their growth from geochemically evolved aabw in contrast the ree chemistry of highland nodules indicates growth from a local less evolved seawater source

compressional deformation at the ocean continent transition in the ne atlantic local zones of compressional deformation spatially coincident with the ocean continent transition occur in the ne atlantic off western iberia on the northern margin of the bay of biscay and in the southern rockall trough the deformation zones are typically broad oceanward facing monoclines several tens of kilometres in width the amount of shortening is small although the structures may also accommodate some strike slip motion deformation peaked during the mid late eocene in biscay off galicia bank and in rockall trough and in the mid miocene off western iberia these deformation pulses were contemporary with the pyrenean and betic orogenies in western europe and may have resulted from structural reorganizations within the evolving orogens driven by changes in plate motion possible mechanisms which might concentrate deformation at the ocean continent transition include slip within serpentinized peridotite underlying the transitional region and reactivation of a pre existing detachment fault within the thinned continental crust our observations may imply that rifted continental margins when reactivated can develop into new sites of plate subduction

generation of plumes under a localized high viscosity lid in 3 d spherical shell convection we conduct numerical simulations of incompressible infinite prandtl number convection in a spherical shell with a single localized high viscosity lid hvl on the top surface to understand the possible effects of the continental lithosphere on plume generation the temperature under the hvl increases rapidly after the emplacement of the hvl on constant viscosity convection with internal and bottom heatings later upwellings at the bottom merge into a large scale flow and a large plume emerges under the hvl this time scale depends on the existence of phase changes in our model whose rayleigh number is 10 6 the large plume is formed on a time scale of 1 gyr a simple scaling suggests that this time scale may be reduced by a several times if the rayleigh number is around 10 despite the complicated 3 d thermal structure it is dominated by an i 1 pattern controlled by the position of the hvl

bacterially mediated authigenesis of clays in phosphate stromatolites authigenic clays in close textural relation to carbonate fluorapatite within finely laminated phosphate stromatolites of upper jurassic age have been studied using scanning electron microscopy sem transmission electron microscopy tem and analytical electron microscopy aem stromatolite laminae consist of hexagonal prisms of francolite sizes ranging between 0 1 and 1 m that are surrounded by poorly crystalline smectite and amorphous fe si al oxyhydroxides microanalyses show that smectite is fe rich with highly variable composition particularly regarding fe and si contents smectite has significant beidellitic montmorillonitic and non tronitic substitutions although the lack of fringe contrast in some areas adjacent to the smectite packets with 1 0 1 3 nm spacing is due to differences in orientation of layers textural and analytical data clearly indicate the presence of fe si al amorphous phases intimately intergrown with smectite the occurrence of poorly crystalline smectite and associated amorphous phases within microbially precipitated stromatolite laminae both as envelopes around and as pore fillings between extremely small calcium phosphate crystals demonstrates authigenic smectite growth from a precursor fe si al amorphous material this material is formed in close association with a phosphate rich precursor the textural and structural relations the preservation of chemical precursors of glauconite such as nontronitic montmorillonite and the presence of fe si al amorphous mineral phases imply crystallization of the observed crystalline phases from synsedimentary bacterially precipitated amorphous precursors during early diagenesis in postoxic environments carbonate fluorapatite was the first phase to crystallize from the primary gel smectite and associated amorphous fe si al oxyhydroxides were the residual material of the crystallization process the slow rate of transformation at low temperatures from fe si al rich gels to smectite explains the textural relations between the poorly crystalline phases and the phosphate crystals as well as the preservation of amorphous substances in relation to clays authigenic smectite represents the first step in glauconitization

slab pull mantle convection and pangaean assembly and dispersal two global scale mantle convection cells presently exist on earth centred on upwelling zones in the south pacific ocean and northeast africa one cell panthalassan contains only oceanic plates the other pangaean contains all the continental plates they have remained fixed relative to one another for 400 ma a transverse rheic tethyian subduction system splits the pangaean cell poloidal plate motion in the oceanic cell reflects circumferential pull of panthalassan slabs but toroidal flow in the pangaean cell reflected by vortex type motion of continents toward the altaids of central east asia throughout the phanerozoic has resulted from the competing slab pull forces of both cells the combined slab pull effects from both cells also controlled pangaean assembly and dispersal assembly occurred during palaeozoic clockwise toroidal motion in the pangaean cell when gondwana was pulled into pangaea by the ne trending rheic subduction zone forming the appalachian variscide altaid chain pangaean dispersal occurred when the rheic trench re aligned in the jurassic to form the nw trending tethyside subduction system which pulled east gondwanan fragments in the opposite direction to form the cimmerian himalayan alpine chain this re alignment also generated a new set of indian mid ocean ridge systems which dissected east gondwana and facilitated breakup 100 200 myr long phanerozoic wilson cycles reflect rifting and northerly migration of gondwanan fragments across the pangaean cell into the rheic tethyian trench pangaean dispersal was amplified by retreat of the panthalassan slab away from europe and africa which generated mantle counterflow currents capable of pulling the americas westward to create the atlantic ocean thermal blanketing beneath pangaea and related hotspot activity were part of a complex feedback mechanism that established the breakup pattern but slab retreat is considered to have been the main driving force the size and longevity of the two cells organised and maintained by long lived slab pull forces favours deep mantle convection as the dominant circulation process during the phanerozoic

bacterial origin of east australian continental margin phosphorites many models of the origin of marine phosphorites require a sediment rich in organic matter which by decomposition releases phosphorus for the precipitation of carbonate fluorapatite1 7 this concept is useful in explaining contemporary phosphorite in areas of very high productivity off the coasts of peru chile and south west africa8 10 where there are high organic matter fluxes to the sea floor4 7 11 it does not explain the origin of marine phosphorite deposits which formed in regions of limited oceanic upwelling and productivity12 14 the east australian continental margin an area of phosphogenesis throughout the late pleistocene and holocene13 14 is a modern analogue of an east coast phosphogenic province15 17 with low productivity over the upper slope region where the most recent phosphorites have been found18 evidence is reported here for the carbonate fluorapatite in the east australian phosphorites being located within bacterial cellular structures and a model proposed for the origin of these deposits through the slow bacterial assimilation of phosphorus from seawater in an area of restricted sedimentation

a regionalized upper mantle rum seismic model seismic velocity heterogeneity in the earth s mantle is strongly concentrated near its top the shallow heterogeneity of the mantle correlates strongly with surface tectonics we use these observations as constraints of a tomographic experiment aimed at building a regionalized upper mantle rum reference model we use a select set of teleseismic travel times to minimize the mapping of mislocation into structure the data selection emphasizes the robustness of individual picks the form of the rum model is a set of velocity profiles as functions of depth through the upper mantle for each of the different tectonic provinces of earth together the profiles constitute a three dimensional model which incorporates considerable structural detail but is described by only 90 parameters and has only about 22 degrees of freedom this is achieved by irregularly sampling a detailed regionalization of the globe by detailed mapping of subducted lithosphere in the mantle as defined by seismicity and by combining these structures in an irregular grid in which bookkeeping is efficiently handled the resulting rum model includes subducting slabs as sharp fast features in the upper mantle old continents are fast young oceans are slow models have been derived for both compressional and shear velocity the rum model is designed to represent as much of upper mantle heterogeneity as seen by body wave travel times as possible with a simple model it can be useful as a reference model for individual tectonic regions travel times are efficiently generated for the rum model mislocations of explosions of known location are significantly reduced when corrections for the rum model are applied to travel time residuals for a spherically symmetrical earth model

thin crust at the western iberia ocean continent transition and ophiolites western iberia is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments the age of breakup decreases from south to north seismic refraction and reflection profiles and magnetic and gravity data from each segment show a consistent pattern of geophysical observations across the ocean continent transition oct zone which is a few tens of kilometers wide we emphasize here the discovery of thin 2 4 km oceanic crust underlain by 7 6 km s 1 material within the oct the available evidence favors the suggestion that the 7 6 km s 1 layer is serpentinized peridotite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental breakup this thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin affinities

tectonic reconstructions for paleobathymetry in drake passage abstract a minimum complexity tectonic reconstruction based on published and new basin opening models depicts how the scotia sea grew by cenozoic plate divergence dismembering a jurassic sheared margin of gondwana part of the jurassic early cretaceous ocean that accreted to this margin forms the core of the central scotia plate the arc plate above a trench at the eastern end of the scotia sea which migrated east away from the antarctic and south american plates a sequence of extensional basins opened on the western edge of the central scotia plate at 50 30 ma decoupled from the south american plate to the northwest by slow motion on a long transform fault succeeding the basins seafloor spreading started around 30 ma on the west scotia ridge which propagated northwards in the 23 17 ma period and ceased to operate at 6 ma the circuits of plate motions inside and outside the scotia arc are joined via rotations that describe antarctic central scotia plate motion in powell basin until 20 ma and along the south scotia ridge thereafter the modelled relative motion at the northern edge of the scotia sea is thus constrained only by the plate circuit but nonetheless resembles that known coarsely from the geological record of tierra del fuego a paleobathymetric interpretation of nine time slices in the model shows drake passage developing as an intermediate depth oceanographic gateway at 50 30 ma with deep flow possible afterwards initially this deep flow would have been made tortuous by numerous intermediate and shallow barriers a frontal pattern resembling that in the modern scotia sea would have awaited the clearance of significant barriers by continuing seafloor spreading in the scotia sea at 18 5 ma at shag rocks passage and after 10 ma southeast of south georgia

stress pattern in portugal mainland and the adjacent atlantic region west iberia the portuguese mainland territory is located close to the azores gibraltar plate boundary in a tectonic setting responsible for significant neotectonic and seismic activities however few data concerning the present regional lithospheric stress field were available as testified by recently published maps of stress indicators for the europe and mediterranean regions one of the authors already presented a synthesis on this subject cabral 1993 where geological and geophysical stress indicators were considered in this paper we introduce new information mainly a considerable amount of borehole breakout data the updated data set comprises 32 reliable stress indicators showing a mean azimuth of 145 standard deviation 21 for the maximum horizontal stress direction shmax on the average the geological data are rotated clockwise and the focal mechanism data deviated anticlockwise to that azimuth while the borehole elongation results are consistent with the mean shmax trend these differences in stress trend suggest a regional progressive rotation of the shmax direction from nnw sse to wnw ese since the upper pliocene to estimate stress trajectories new and published stress indicators in the adjacent atlantic area and northern africa were also investigated showing a very uniform nw se shmax trend in west iberia a high level of horizontal compressive stress acting oblique to the western portuguese continental margin is inferred and interpreted in view of a proposed regional geodynamical model of activation of this passive margin with the nucleation of a subduction zone in the atlantic sw of iberia at the gorringe submarine bank which is propagating northward along the base of the continental slope at the transition between thinned and normal continental crust

depositional controls on glaucony texture and composition upper jurassic west siberian basin this study examines textural inhomogeneity and variable chemical composition of upper jurassic glaucony in relation to small scale synsedimentary and postsedimentary authigenic processes controlled by the palaeonvironmental and palaeogeographical context four glaucony types with complex textural and compositional features have been recognized in cores of the georgiev formation of the west siberian basin samples exclusively made of light green type 1 glaucony k2o 6 5 the less mature type richer in glauconite smectite mixed layer formed under dysoxic conditions in the deepest distal marine environments of the northern sectors of the west siberian basin dark green type 2 glaucony is the most mature richest in glauconitic mica k2o up to 8 5 is sometimes associated with type 1 glaucony and is typical of high bottom areas with a low sedimentation rate within the central sectors of the basin type 3 glaucony is formed by brown grains poorer in k and fe but richer in al and si than type 2 glaucony and is only present in strongly condensed successions of the central eastern sectors of the west siberian basin type 4 glaucony is much richer in fe than any other type shows fresh yellowish green cores slightly less mature than type 2 glaucony and brown rims and cracks with composition similar to that of type 3 grains it was formed in western sectors of the west siberian basin close to urals weathering under a subtropical to temperate climate and erosion of badly drained peneplaned lowland areas around the basin provided al rich terrigenous clays as substratum for glauconitization which explains al and si enrichment in siberian glaucony maturation from glauconite smectite to glauconitic mica is monitored by a change from light to dark green colour related to decrease in al si mg ca and na and to increase in k and fe brown rims of type 4 glaucony and brown type 3 grains formed after leaching of fe and k from mature glauconite with formation of clays and fe oxyhydroxides as reaction products as a result of free oxygen exposure related to a hydrodynamic regime and temporary sea level fall glauconitization stopped and diagenetic pyrite formed due to basin deepening and burial under black shales during the latest jurassic earliest cretaceous transgression this study demonstrates that due to the complex nature of glaucony the authigenesis of glauconitic minerals in the rock record cannot be correctly understood if the palaeoenvironmental context and the palaeogeographical context of glaucony bearing sediments are not considered

subduction initiation by thermal chemical plumes numerical studies abstract prior suggestions for the initiation of subduction have included sediment loading compression and plate reconfiguration as potential triggers here we investigate the possibility of subduction initiation by the interaction of the lithosphere with a buoyant mantle plume numerical testing of this hypothesis has been conducted in 2d with the i2vis thermo mechanical code accounting for phase transitions and a viscoplastic model of a thin oceanic lithosphere hit by a partially molten thermal chemical or purely thermal plume we demonstrate that a mantle plume can break the lithosphere and initiate self sustaining subduction provided the plume causes a critical local weakening of the lithospheric material above it the intensity of the required weakening depends on the plume volume plume buoyancy and the thickness of the lithosphere and is the highest for the least buoyant purely thermal plumes another necessary condition is the presence of high pressure fluids at the slab upper interface reducing the effective friction coefficient there to very low values based on our results we suggest that sheet like instabilities of the archean mantle convection could have initiated subduction on earth where ocean was already present in less stable tectonic settings provided that mantle plumes sheets at that time were rich in water and melt which could drastically reduce the effective friction coefficient in the lithosphere above the plume our numerical models are also in good agreement with suggested concepts for corona formation on venus

microbially influenced formation of phosphate nodules and megafossil moulds negev southern israel abstract phosphate nodules and megafossil moulds occur at the top of the late cretaceous mishash formation in southeastern israel their palaeogeographic distribution as well as the microstructure of the moulds and the nodules suggests that the former represent rapid lithification of the infilling sediment by apatite precipitating bacteria before the dissolution of the calcareous skeletons the nodules on the other hand are partly lithified portions of microbial mats formed in a lagoonal environment disrupted rolled and embedded in granular phosphorite carbonate replacement by apatite does not seem to be involved in the formation of this pebbly phosphate

primary bedded phosphorites in the campanian mishash formation negev southern israel abstract thin lentiform beds up to 1 cm thick of primary non pelletal phosphate have been found sporadically in the highly condensed anticlinal sections of the late campanian mishash formation of the central negev southern israel these primary phosphate beds are typically associated with iron rich montmorillonites of probable authigenic origin and occur at the top of upward fining phosphate dominated sequences the clay rich and primary phosphate levels are commonly truncated by amalgamated beds of high energy coarse grained phosphorites which form the basal part of these sequences with ripped up clasts of the primary phosphate commonly re incorporated in these rocks sem eds study of both the primary phosphate levels and the reworked phosphate clasts shows that they are built by a crudely stratified network of closely packed and gently compressed capsule like phosphate cells 2 5 m in external diameter and with walls of 0 5 m thick these structures are interpreted as being the synsedimentarily phosphatized remnants of former mat forming coccoid prokaryotes now represented as extracellular envelopes which intermittently colonized the mishash sea bottoms at the end of the high energy episodes differential degradation of the periphytic mat community possibly coupled with fe p redox cycling across the surficial mat iron was trapped as iron rich montmorillonites in the anoxic zone below the living mat probably resulted in an excess of dissolved phosphate immobilized as solid phases on decay resistant microbial remains which escaped total degradation development of the mat forming microbes on the mishash bottoms and their mineralization to form primary phosphorite beds occurred during periods of halted deposition which resulted in a high residence time of the mat on the seafloor and hence in intensified interface linked biogeochemical reactions ending in phosphate precipitation the restriction of primary phosphate occurrences to the condensed mishash sections of marginal palaeohigh positions strengthens previous assumptions that these zones were probably the main locus of mishash phosphate production it also points to a phosphate grain supply from marginal to basinal sites leading to the accumulation of pelletal phosphorites in the latter areas

how weak is the subduction zone interface several lines of evidence suggest that subduction zones are weak and that the unique availability of water on earth is a critical factor in the weakening process we have evaluated the strength of subduction zone interfaces using two approaches i from empirical relationships between shear stress at the interface and subduction velocity deduced from laboratory experiments and ii from a parametric study of natural subduction zones that provides new insights on subduction zone interface strength our results suggest that subduction is only mechanically feasible when shear stresses along the plate interface are relatively low less than 35 mpa to account for this requirement we propose that there is a feedback mechanism between subduction velocity water released from the subducting plate and weakening of the fore arc mantle that may explain how relatively low shear stresses are maintained at subduction interfaces globally

the subduction initiation rule a key for linking ophiolites intra oceanic forearcs and subduction initiation we establish the subduction initiation rule sir which predicts that most ophiolites form during subduction initiation si and that the diagnostic magmatic chemostratigraphic progression for sir ophiolites is from less to more hfse depleted and lile enriched composi tions this chemostratigraphic evolution reflects formation of what ultimately becomes forearc lithosphere as a result of mantle melting that is progressively influenced by subduction zone enrichment during si the magmatic chemostratigraphic progression for the izu bonin mariana ibm forearc and most tethyan ophiolites is specifically from morb like to arc like volcanic arc basalts or vab boninites or bon because si progressed until establishment of a mature subduction zone morb like lavas result from decompression melting of upwelling asthenosphere and are the first magmatic expression of si the contribution of fluids from dehydrating oceanic crust and sediments on the sinking slab is negligible in early si but continued melting results in a depleted harzburgitic residue that is progressively metasomatized by fluids from the sinking slab subsequent partial melting of this residue yields typical ssz like lavas in the latter stages of si if si is arrested early e g as a result of collision morb only ophiolites might be expected consequently morb and ssz only ophiolites may represent end members of the si ophiolite spectrum the chemostratigraphic similarity of the mariana forearc with that of ophiolites that follow the sir intimates that a model linking such ophiolites oceanic forearcs and si is globally applicable

testing the influence of far field topographic forcing on subduction initiation at a passive margin abstract despite favourable gravitational instability and ridge push elastic and frictional forces prevent subduction initiation from arising spontaneously at passive margins here we argue that forces arising from large continental topographic gradients are required to initiate subduction at passive margins in order to test this hypothesis we use 2d numerical models to assess the influence of the andean plateau on stress magnitudes and deformation patterns at the brazilian passive margin the numerical results indicate that plateau push in this region is a necessary additional force to initiate subduction as the se brazilian margin currently shows no signs of self sustained subduction we examined geological and geophysical data to determine if the margin is in the preliminary stages of subduction initiation the compiled data indicate that the margin is presently undergoing tectonic inversion which we infer as part of the continental oceanic overthrusting stage of subduction initiation we refer to this early subduction stage as the brazilian stage which is characterized by 10 km deep reverse fault seismicity at the margin recent topographic uplift on the continental side thick continental crust at the margin and bulging on the oceanic side due to loading by the overthrusting continent the combined results of the numerical simulations and passive margin analysis indicate that the se brazilian margin is a prototype candidate for subduction initiation

rare earth elements in foraminifera tests abstract the concentrations of nine rare earth elements ree la ce nd sm eu gd dy er and yb have been determined in mixed species assemblages of foraminifera tests taken from atlantic ocean sediment core tops reductive cleaning techniques have revealed that ree are present in three phases in foraminifera tests collected from sea floor sediments ree included in the foraminiferal calcite matrix termed lattice ree ree associated with an authigenic femn rich phase adsorbed onto the surface of the test following the death of the organism termed coating ree and ree associated with alumino silicate detritus termed detrital ree which commonly infills the test chambers after burial in the sediment the concentrations of ree in the nondetrital lattice plus coating and lattice phases have been measured in this study approximately 90 of the ree measured in the non detrital phase reside in the coating phase the remainder being present in the lattice phase these data have been used to investigate the relationship between the distribution of dissolved ree in the ocean and in the coating and lattice phases in addition to the ree the concentrations of mn fe cu al and po4 have been measured as an aid to characterisation of the various phases

quaternary phosphorites from the continental margin off chennai southeast india analogs of ancient phosphate stromatolites abstract pleistocene phosphorites occur on the continental margin off chennai abundantly in the depth range of 186 293 m they are associated with outer shelf glauconites and carbonate skeletals including large shells of molluscs and rhodoliths these are primary phosphate deposits and are similar to phosphate stratiform stromatolites this is the first report of phosphate stratiform stromatolites during the quaternary and modern times and they provide a quaternary analog for ancient phosphorites thin sections of phosphorites exhibit laminated phosphatized microbial mats consisting of mechanically deposited clastic particles or of phosphatic molds clay particles are scattered throughout the matrix sem studies reveal that the laminated phosphate matrix is made of thin sheets of tightly packed apatite globules densely packed intertwined microfilaments or filament molds phosphate with obscured laminae is composed of phosphate tubules cell like structures resembling coccoid cyanobacteria or their botryoidal aggregates are common carbonate fluorapatite and low magnesium and high magnesium calcites are the major mineral phases the structural co2 content of apatite ranges from 4 to 5 5 quartz feldspar and goethite are accessory minerals the microbial mats were formed on the outer shelf most probably during conditions of low sea level in the quaternary and microbial processes played a major role in direct phosphatization of these mats the primary phosphorus source seems to have been the continental supply and degradation of organic matter associated with benthic microbial communities the phosphate stratiform stromatolites thus formed were subsequently reworked into a shelf margin depression and resulted in the occurrence of condensed phosphorites

rare earth elements in the phosphatic enriched sediment of the peru shelf abstract apatite enriched materials from the peru shelf have been analyzed for their major oxide and rare earth element ree concentrations the samples consist of 1 the fine fraction of sediment mostly clay material 2 phosphatic pellets and fish debris which are dispersed throughout the fine grained sediment 3 tabular shaped phosphatic crusts which occur within the uppermost few centimeters of sediment and 4 phosphatic nodules which occur on the seafloor the bulk ree concentrations of the concretions suggest that these elements are partitioned between the enclosed detrital material and the apatite fraction analysis of the fine grained sediment with which the samples are associated suggested that this detrital fraction in the concretions should have shale ree values the analysis of the fish debris suggested that the apatite fraction might have seawater values the seawater contribution of ree s is negligible in the nodules and crust in which the apatite occurs as a fine grained interstitial cement that is the concentration of ree s and the ree patterns are predominantly a function of the amount of enclosed fine grained sediment by contrast the ree pattern of the pelletal apatite suggests a seawater source and the absolute ree concentrations are relatively high the ree p 2 o 5 ratios of the apatite fraction of these samples thus vary from approximately zero in the case of the crust and nodules to as much as approximately 1 2 10 3 in the case of the pellets the range of this ratio suggests that rather subtle variations in the depositional environment might cause a significant variation in the ree content of this authigenic fraction of the sediment pelletal glauconite was also recovered from one sediment core its ree concentrations closely resemble those of the fish debris

how was the iapetus ocean infected with subduction because subduction in the iapetus ocean began only 35 m y after the end of rifting spontaneous foundering of mature passive margins is an unlikely subduction initiation mechanism subduction is more likely to have entered the iapetus from the boundary with the external paleo pacific similar to the incursion of the scotia caribbean and gibraltar arcs into the modern atlantic the subduction zone probably became sinuous entraining fragments of the gondwanan margin along its complex sinistral southern boundary where oblique collision caused monian penobscottian deformation following taconian grampian collision of part of the subduction system with laurentia remaining parts of the iapetus were progressively infected with subduction leading to silurian closure

anomalies in rare earth distributions in seawater gd and tb we have measured profiles of the rare earth elements ree in atlantic and pacific ocean waters the data normalized versus shales exhibit a pronounced anomaly in gd relative to its neighbors eu and tb in the ree series such that the gd concentrations are high by 30 50 closer inspection reveals that the anomaly is made up of both elevated gd and depressed tb concentrations likely associated with solution chemistry shifts in the transition from an exactly half filled 4f electron shell anomalies in gd and tb solution complexation are also indicated by the turner whitfield dickson speciation model the overall trend of heavy ree iii enrichment in seawater and the gdtb anomaly described here tend to support scavenging as an important removal mechanism for the ree from seawater

early diagenetic phosphate cements in the albian condensed glauconitic limestone of the tatra mountains western carpathians early diagenetic phosphate cements are described from the albian condensed glauconitic limestone of the tatra mountains western carpathians with regard to their macro and micromorphology distribution classification and genesis the cements occur within stratigraphically condensed semi pelagic foramini feral glauconitic layers and are associated with mature hardgrounds within the tatra albian limestone phosphate cement fabrics consist of crypto to microcrystalline carbonate fluorapatite and they occur as i rim envelopes ii infillings of intraparticle porosity iii rim cement iv multiple rim cement v palisade fabric and vi cluster cement micromorphological variability of the cement fabrics results from varying texture of the cemented sediment the nature of original porosity as well as from presence of associated microbial fabrics the microbial fabrics are interpreted as fossilized coccoid cyanobacteria phosphate cementation developed under peculiar early diagenetic conditions within semi closed microenvironments rich in organic matter in the marine phreatic environment the cementation contributed to the formation of phosphatic fossils and hardgrounds the accretion of the cements was due to concentration of biologically uptaken phosphorus near the sediment water interface enrichment of pore fluids with respect to phosphate and its precipitation within restricted microenvironments phosphate cementation post dated seafloor formation of pelletal glauconite but predated partial decomposition of organic matter as well as dissolution or neomorphism of aragonite and high mg calcite phosphate cementation occurred on a carbonate platform following the submersion of urgonian reefal build ups episodes of phosphate cementation were repeated during the sedimentation of the tatra albian limestone as a response to rapid relative sea level rises and increased influence of nutrient rich tethyan waters

global tectonics and eustasy for the past 2 billion years abstract continental freeboard and eustasy as gauged by the relative position of the world shelf break with respect to sea level have varied by 250 m from today s ice free shelf break depth of 200 m during the past 600 ma assuming constant or uniformly accreting continental crust and ocean water volume in an ice free world sea level fluctuations can be attributed to variation in the world ocean basin volume caused by changes in either its area or its depth relative to the world shelf break an increase in volume and lowering of sea level occur as 1 the world ocean floor ages cools and subsides 2 accreting continents collide thicken and decrease in area and 3 poorly conductive continental platforms become thermally elevated due to a size induced stasis over the mantle conversely a decrease in the age of the world ocean floor attenuation of continental crust during rifting and an increase in continent number and mobility will reduce the world ocean basin volume and raise sea level theoretical sea level calculated from these principles correlates well with calibrated first order cycles of eustatic sea level change for the phanerozoic the record closely fits a simple model of retardation and acceleration of terrestrial heat loss during alternating periods of supercontinent accretion and fragmentation calibrated to sea level highstands successive first order marine transgressions and orogenic pangea regressions characterize a self sustaining 440 ma plate tectonic cycle for the late precambrian and phanerozoic the cycle can be recognized as far back as 2 ga from the tectonic evidence of continental collision and rifting recorded in global orogenic peaks and mafic dike swarms and may be related to major episodes of glaciation and evolutional biogenesis

thrust wrench interference tectonics in the gulf of cadiz africa iberia plate boundary in the north east atlantic insights from analog models abstract in the gulf of cadiz key segment of the africa iberia plate boundary north east atlantic ocean three main different modes of tectonic interference between a recently identified wrench system swim and the gulf of cadiz accretionary wedge gcaw were tested through analog sand box modeling a an active accretionary wedge on top of a pre existent inactive basement fault b an active strike slip fault cutting a previously formed inactive accretionary wedge and c simultaneous activity of both the accretionary wedge and the strike slip fault the results we obtained and the comparison with the natural deformation pattern favor a tectonic evolution comprising two main steps i the formation of the gulf of cadiz accretionary wedge on top of inactive tethyan related basement faults middle miocene to 1 8 ma ii subsequent reactivation of these basement faults with dextral strike slip motion 1 8 ma to present simultaneously with continued tectonic accretion in the gcaw these results exclude the possibility of ongoing active swim wrench system cross cutting an inactive gcaw structure our results also support a new interpretation of the swim wrench system as fundamentally resulting from strike slip reactivation of an old tethyan related plate boundary

focal mechanisms for sub crustal earthquakes in the gulf of cadiz from a dense obs deployment 1 an eleven month deployment of 25 ocean bottom seismometers provides an unprecedented opportunity to study low magnitude local earthquakes in the complex transpressive plate boundary setting of the gulf of cadiz known for the 1755 lisbon earthquake and tsunami 36 relocated earthquakes ml 2 2 to 4 8 concentrate at 40 60 km depth near the base of the seismogenic layer in 140 ma old oceanic mantle lithosphere and roughly align along two perpendicular nne ssw and wnw ese striking structures first motion focal mechanisms indicate compressive stress for the cluster close to the northern horseshoe fault termination which trends perpendicular to plate convergence focal mechanisms for the second cluster near the southern termination of the horseshoe fault indicate a strike slip regime providing evidence for present day activity of a dextral shear zone proposed to represent the eurasia africa plate contact we hypothesize that regional tectonics is characterized by slip partitioning

microstructure of apatite replaceing carbonate in synthesized and natural samples abstract the structure of the apatite replacing carbonate shells and oolites in experimental apatite synthesis by bacterial activity has been shown to be microspherical size a fraction of one micron to several microns and type of the microspheres spherical or ovoid ovoid or cauliflowerlike isolated siamese twinlike or in chains fit both the size and type of monocellular organisms very well the pseudomorphic replacement of carbonates by apatite is well known in natural phosphorites however the detailed structure of the replacing apatite has not yet caught the eye of petrographers bacterial structures in phosphorites have been reported recently but these structures were found isolated rather than accurately located within phosphatized shells our results show that while the initial aragonitic structure of phosphatized shells is prismatic the pseudomorphic apatite is microspherical and phosphatization appears to spread beyond the shell walls towards the center of the chambers all stages between the almost empty and the totally infilled apatite phosphatized shells could be observed in one sample from morocco the similarity of the apatite microstructures in experimental and natural samples lends further support to the genesis model we proposed earlier that combination of phosphorus from rna released by bacteria and calcium from carbonate is one of the possible genesis models of apatite in nature

on the chemical variability of phosphatic particles from jordanian phosphorite deposits abstract the phosphate particles pellets intraclasts and skeletal fragments from thirteen friable samples representing the economic phosphorite horizons in jordan were hand picked and then cleaned a total of 39 particle fractions were chemically analysed for their major and certain trace elements the chemistry supported by petrography proved that the rounded particles are not faecal pellets they are either rounded intraclasts or the rounded internal molds of bone cavities the bone fragments have higher contents of co2 na20 and s03 and lower p205 and f compared with the pelletal intraclast particles these two groups of particles are readily separated chemically since the environment of deposition is the same these differences are postulated to be due to the initial chemical composition of the bone material as carbonate hydroxyapatite dahlite and early diagenetic pathway conversion to carbonate fluorapatite francolite versus the direct chemical precipitation as francolite of the pelletal intraclast particles from the sediment pore water

phosphatization of calcium carbonate in phosphorites microstructure and importance fabrics of phosphatized calcium carbonate particles in various phosphorites have been studied using scanning electron microscopy coupled with x ray dispersive microanalysis replacement of calcium carbonate by apatite has been observed in bivalve shell fragments and in foraminiferal tests replacement proceeds at constant volume with excellent preservation of the original microtextures in some deposits replacement of carbonate by apatite is the main phosphogenic process however in general the process seems to be far less important than might be believed purely on the basis of thin section observations in many phosphorites internal or external apatite moulds of bioclasts are common including very small particles such as coccoliths in phosphatized chalks apatite precipitation was typically followed by carbonate dissolution later apatite precipitation within the dissolution voids may produce partial or total phosphate pseudomorphs of the original carbonate grain in these examples direct replacement of carbonate by phosphate cannot be demonstrated

why is terrestrial subduction one sided subduction of the lithosphere at convergent plate boundaries takes place asymmetrically the subducted slab sinks downward while the overriding plate moves horizontally one sided subduction in contrast global mantle convection models generally predict downwelling of both plates at convergent margins two sided subduction we carried out two dimensional 2 d numerical experiments with a mineralogical thermomechanical viscoelastic plastic model to elucidate the cause of one sided subduction our experiments show that the stability intensity and mode of subduction depend mainly on slab strength and the amount of weak hydrated rocks present above the slab two sided subduction occurs at low slab strength sin 0 15 the weak interface is maintained by the release of fluids from the subducted oceanic crust as a consequence of metamorphism the resulting weak interplate zone localizes deformation at the interface and decouples the strong plates facilitating asymmetric plate movement our work suggests that high plate strength and the presence of water are major factors controlling the style of plate tectonics driven by self sustaining one sided subduction processes

integration of 4d seismic data and the dynamic reservoir model reveal new targets in gannet c two sets of 4d seismic data gave major new insights into the structure and dynamic behavior of the gannet c oil and gas reservoir in the u k central north sea the 4d data revealed major extensions of reservoir units previously presumed to be absent or thin over much of the reservoir furthermore in a subsea field with significant uncertainty of production allocation 4d also proved an invaluable history matching parameter for the dynamic model together the dynamic model and the 4d data gave rise to the identification of one recompletion opportunity and two infill well opportunities to produce oil volumes in existing and newly identified reservoir sands these now have been realized roughly trebling oil production from the field

rare earth distributions with a positive ce anomaly in the western north atlantic ocean from the vertical distributions of the rare earth elements ree in the sargasso sea we now report the first profiles of pr tb ho tm and lu in seawater together with profiles for la ce sm eu and yb the first observations of positive ce anomalies in seawater are ascribed to reducing inshore sediments as a source for ce ref 1 all vertical profiles are consistent with adsorption of trivalent rare earths by settling particles possibly siliceous or calcareous followed by their release at or near the sea floor on dissolution of the carriers the very different ce profile demonstrates the additional effects of oxidation reduction reactions

anatomy and origin of a cretaceous phosphorite greensand giant egypt late cretaceous epicontinental phosphorites porcelanites cherts dark coloured shales glauconitic sandstones and bioclastic and fine grained carbonate rocks in egypt are examined in terms of their overall depositional and diagenetic framework and stable isotopic and organic geochemical characteristics two main depositional realms are interpreted and correlated through sequence stratigraphic analysis 1 a shallow hemipelagic environment accompanying initial stages of marine transgression and conducive to the formation of organic carbon rich shales biosiliceous sediments and thick phosphorites and 2 a relatively high energy depositional regime accompanying sea level fall during which deltas advanced glauconites were reworked seaward and prograding oyster banks became periodically exposed to episodes of fresh water diagenesis thereby promoting solution collapse phenomena in associated cherts lenticular to massive phosphorites are viewed as the result of current winnowing and concentration of authigenic grains initially precipitated in associated reducing shales and biosiliceous sediments in eastern egypt the phosphorites form winnowed lag layers some of which may have been redeposited down slope in structural lows in the west these sands were concentrated into giant phosphorite sand waves built by reworking of penecontemporaneously deposited phosphatic muds during marine transgression carbon isotopic results substantiate interpretations from modern deposits for limitation of phosphate mineral precipitation with depth in sediments as a result of lattice poisoning however direct desorption of phosphorus to pore waters from detrital iron oxyhydroxide phases also may have been important in the cretaceous setting the iron reduced in this process being available for incorporation in glauconites the main locus for authigenic glauconite precipitation appears to be where iron fluxes from regions of lateritic weathering were highest and near the boundary between oxygenated and reduced waters this study suggests a model for the common coexistence of glauconites and phosphorites in the geological record although upwelling is often advocated as the origin of nearly all giant phosphorite deposits we suggest that some of these may have been strongly influenced by fluvially derived phosphorus borne on particulates and desorbed from these compounds upon flocculation and or reduction in bottom waters or pore waters

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

extrapolating oceanic age distributions lessons from the pacific region abstractextrapolation of the age distribution of oceanic lithosphere has played a significant role in assessments of variations in global mean spreading rate global mean ocean basin depth and implications for global mean sea level subduction has already removed 50 of oceanic lithosphere younger than 55 7 ma making some level of extrapolation a necessary part of global plate reconstructions an area equal in size to the pacific basin oceanic lithosphere must be extrapolated for ages older than 29 1 ma three modes of extrapolation are identified mode 1 extrapolation uses the preserved history as recorded on one plate to infer the history of the previously adjacent plate this mode of extrapolation is exemplified by the inferred history of the farallon vancouver nazca and cocos plates relative to the pacific plate on which this record is preserved mode 2 involves extrapolation beyond the preserved age extent of a given ridge system no observable data exist that directly constrain the motions bey

tectonic and magmatic evolution of the andes of northern argentina and chile abstract two orogenic cycles both with different evolution are developed in the western margin of the south american continent in northern argentina and chile the paleozoic hercynic cycle and the meso cenozoic andean cycle the hercynic cycle a wide marine basin extending westward of the cordillera oriental which developed in cambrian ordovician times marks the beginning of this cycle in contrast to peru and bolivia where this basin developed between two precambrian blocks the western margin of this basin in northern argentina and chile is still unknown the ordovician sedimentation and accompanied volcanism ends with the ocloyic deformation phase and its synkinematic granitic plutonism two basins were developed in the silurian devonian separated by the puna arc uplifted during this ocloyic phase the shallow water marine terrigeneous sediments which were deposited in them were deformed by a new tectono magmatic associated phase chanic phase carboniferous to lower permian marine carbonates were deposited west of the puna arc and red beds east of it later on during the permian to triassic a magmatic belt was developed along the cordillera occidental the rhyolites ignimbrites and the granitic to granodioritic related intrusives are well represented in chile although the overall geologic history of this period is known many problems concerning its origin and relations to plate tectonics are still unsolved the andean cycle during this cycle a series of magmatic arc systems related to the subduction of the pacific crust was built up along the western margin of south america huge volumes of calc alkaline lavas and related plutons were emplaced since the jurassic along belts parallel to the present coastline showing a general eastward migration trend up to the lower cretaceous an ensialic back arc basin was formed east of this magmatic arc thousands of meters of marine and continental sediments were deposited in it this basin disappeared during the middle cretaceous probably as a result of the final opening of the atlantic and the active westward movement of the south american plate since middle cretaceous times the magmatic arc has been the fundamental paleogeographic element the arc progressively migrated stepwise eastward each step marked by a tectonic phase subduction related crustal erosion could explain the lack of fore arc petrotectonic assemblages the eastward magmatic polarity which characterizes this section of the andes could also be explained by such a mechanism

ultra fine structures and genesis of the campanian negev high grade phosphorites southern israel ultra fine structures of the negev high grade phosphorites provide valuable clues to unravelling the genesis of these rocks the question of their differential areal distribution and the biosedimentary mechanisms involved in their considerable enrichment peculiar to these phosphorites is an intergranular phosphate matrix for the most part constituted by a variety of phosphatic microbial tubules displaying a range of spatial micro organizations the phosphate particles fixed by this filamentous meshwork also consists of internally organized and non organized packed microbial remains of different types analogies between the fabrics of the matrix and the corpuscles lead to conception of a two stage depositional scheme for these phosphorites based on rhythmical repetition of two sedimentary mini events in a slightly oscillating very shallow marine system 1 a low energy event of microbial colonization of the mishash bottoms followed by early phosphatization of the organic structures mainly in marginal situations and 2 a higher energy event which broke up the phosphatic mats into debris redepositing them as clastic layers in nearby basinward sites while becoming bound by a new meshwork of filamentous microphytes the differential phosphatization of the intergranular microbial binder again occurring mostly in marginal localities produced highly enriched phosphorites minor truncations and redepositions leading to amalgamation of the successive layers account for the massive fabric now displayed by most of these rocks examining the structural and textural features the validity of the sedimentary mechanisms of recent phosphorite formation for the campanian negev rocks is discussed

the future of earth s oceans consequences of subduction initiation in the atlantic and implications for supercontinent formation subduction initiation is a cornerstone in the edifice of plate tectonics it marks the turning point of the earth s wilson cycles and ultimately the supercycles as well in this paper we explore the consequences of subduction zone invasion in the atlantic ocean following recent discoveries at the sw iberia margin we discuss a buoyancy argument based on the premise that old oceanic lithosphere is unstable for supporting large basins implying that it must be removed in subduction zones as a consequence we propose a new conceptual model in which both the pacific and the atlantic oceans close simultaneously leading to the termination of the present earth s supercycle and to the formation of a new supercontinent which we name aurica our new conceptual model also provides insights into supercontinent formation and destruction supercycles proposed for past geological times e g pangaea rodinia columbia kenorland

the diffuse plate boundary of nubia and iberia in the western mediterranean crustal deformation evidence for viscous coupling and fragmented lithosphere a spatially dense gnss based crustal velocity field for the iberian peninsula and northern africa allows us to provide new insights into two main tectonic processes currently occurring in this area in particular we provide for the first time clear evidence for a large scale clockwise rotation of the iberian peninsula with respect to stable eurasia euler pole component n42 612 w1 833 clockwise rotation rate of 0 07 deg myr we favor the interpretation that this pattern reflects the quasi continuous straining of the ductile lithosphere in some sectors of south and western iberia in response to viscous coupling of the nw nubia and iberian plate boundary in the gulf of cadiz we furnish evidence for a fragmentation of the western mediterranean basin into independent crustal tectonic blocks which are delimited by inherited lithospheric shear structures among these blocks an oceanic like western algerian one is currently transferring a significant fraction of the nubia eurasia convergence rate into the eastern betics se iberia and likely causing the eastward motion of the baleares promontory these processes can be mainly explained by spatially variable lithospheric plate forces imposed along the nubia eurasia convergence boundary

sedimentology geochemistry and origin of phosphatic chalks the upper cretaceous deposits of nw europe santonian lower campanian and lower maastrichtian phosphatic chalks in northern france southern england and belgium are europe s largest sedimentary phosphatc deposits the stratigraphy sediment ology petrography mineralogy and geochemistry of the lithofacies are reviewed and new data presented depositional and diagenetic models for phosphatic chalk deposits are developed using published experimental work and from observations of modern high and low productivity phosphogenic systems it is concluded that phosphatic chalks were deposited in well oxygenated current swept environments phosphatization required a delicate balance to be maintained between moderate organic carbon and carbonate sedimentation rates reduced bulk sediment accumulation rate and an enhanced rate of bioturbation precipitation of carbonate fluorapatite francolite accompanied the bacterially mediated decomposition of organic matter occurring within centimetres of the sediment seawater interface and taking place preferentially within microbial bodies and coatings in addition to the organically derived component pore water phosphate levels were enhanced by phosphate absorbed on ferric oxyhydroxides which was liberated during iron reduction mineralization was probably a dominantly post oxic process but occurred in a thick sediment mixed layer in which marine organic matter was undergoing intense mixed aerobic and anaerobic microbial degradation phosphogenesis occurred predominantly on the ne margins of the anglo paris basin where shallower sea floors and suitable palaeoceanographic conditions prevailed phosphogenic episodes were limited by sea level fluctuations which controlled the effectiveness of the erosional currents that formed and maintained the phosphatic basins and may have stimulated local productivity

cephalopod limestone deposition on a shallow pelagic ridge the tafilalt platform upper devonian eastern anti atlas morocco as a result of early variscan tectonic movements and of differential subsidence a platform and basin topography was created along the northern margin of the sahara craton during the late devonian in the moroccan anti atlas mountains the tafilalt platform is an approximately n s running ridge which developed since the late middle devonian it separated a slowly subsiding shallow basin in the east tafilalt basin from a rapidly subsiding furrow in the west mader basin platform deposits are characterized by highly reduced thicknesses shallow subtidal to supratidal deposits in the late frasnian and by unconformities at the lower upper frasnian and the frasnian famennian boundaries after a local transgression over emergent areas in the north water depth probably never reached more than several tens to about 100 m in the lower famennian cephalopod limestones of this age deposited on the platform represent a very diverse facies pattern comprising quartz rich brachiopod coquinas crinoidal limestones thick bedded cephalopod limestones and nodular limestones sedimentation rates ranged from 1 to 5 mm 1000 yr in the late famennian more uniform marl and nodular limestone facies suggest slightly deeper environments platform margins are characterized by higher rates of subsidence debris flow deposits and slump structures in the relatively shallow tafilalt basin marls with intercalated nodular limestones were deposited in the mader basin sandy and calcareous turbidites suggest deeper water conditions in the late devonian during the strunian tournaisian the whole area was overwhelmed by a thick deltaic sequence the general facies distribution is in agreement with depositional models of other upper devonian and lower carboniferous cephalopod limestones in the european variscan orogenic belts in all these cases condensed cephalopod limestones occupy a distinct palaeogeographic position in predictable facies sequences that reflect pre orogenic phases in the variscan geodynamic cycle moreover close parallels exist with condensed sequences in the triassic and jurassic that occur in a very similar position within the alpine orogenic cycle

stability of active mantle upwelling revealed by net characteristics of plate tectonics global scale mantle flow patterns can be deduced from the net behaviour convergence and divergence of surface plate motions persistent quadrupole divergence in central africa and the central pacific suggest sustained stationary upwelling beneath these locations in the mantle earth s long history of intense geological activity mountain building and plate tectonic motion included is correlated with viscous flow of the underlying mantle clinton conrad and co authors show here that despite their apparent complexity the net characteristics of plate tectonics reflect simple and steady large scale patterns of viscous flow within the underlying mantle they tracked the geographic locations of net convergence and divergence from tectonic reconstructions covering the past 250 million years and found that the positions of divergence have not moved significantly and are found above two large upwellings beneath africa and the pacific ocean the authors conclude that these lowermost mantle regions may have remained relatively stationary over geological time and thereby organized global mantle flow viscous convection within the mantle is linked to tectonic plate motions1 2 3 and deforms earth s surface across wide areas4 5 6 such close links between surface geology and deep mantle dynamics presumably operated throughout earth s history but are difficult to investigate for past times because the history of mantle flow is poorly known7 here we show that the time dependence of global scale mantle flow can be deduced from the net behaviour of surface plate motions in particular we tracked the geographic locations of net convergence and divergence for harmonic degrees 1 and 2 by computing the dipole and quadrupole moments of plate motions from tectonic reconstructions8 9 extended back to the early mesozoic era for present day plate motions we find dipole convergence in eastern asia and quadrupole divergence in both central africa and the central pacific these orientations are nearly identical to the dipole and quadrupole orientations of underlying mantle flow which indicates that these net characteristics of plate motions reveal deeper flow patterns the positions of quadrupole divergence have not moved significantly during the past 250 million years which suggests long term stability of mantle upwelling beneath africa and the pacific ocean these upwelling locations are positioned above two compositionally and seismologically distinct10 regions of the lowermost mantle which may organize global mantle flow11 as they remain stationary over geologic time12

micro environmental controls on biomineralization superficial processes of apatite and calcite precipitation in quaternary soils roussillon france calcium hydroxyapatite and calcite precipitates around bacteria were observed in 2 week old alluvial topsoil roussillon area se france this observation prompted a laboratory study of ca2 and po43 incorporation into hydroxyapatite and ca2 into calcite mediated by bacteria using similar topsoil material but free from apatite and calcite subsamples were prepared using three different grain sizes and experiments were undertaken using sucrose and different contents of ca2 and po43 mineralization experiments proceeded over 5 days calcium and po43 sorption onto clay influenced the ca p ratio in the solutions hydroxyapatite and calcite precipitation only occurred in unsterilized samples the presence of clay minerals promoted biomineralization

4d seismic for oil rim monitoring in the central north sea gannet a field a 50 ft oil rim is overlain by a gas cap of variable thickness oil is produced from horizontal wells which initially produced dry oil but as the field became more mature a significant water cut was seen in several wells a dedicated 4d seismic monitor survey was acquired in order to assess the remaining distribution of oil reserves by forward modelling the synthetic seismic response to parameters such as contact movement and residual saturations using 2d and 3d wedge models and comparing the results with real seismic data we are able to decipher the contact movements across the field it is shown that in one part of the field the increased water cut is caused primarily by the vertical displacement of the entire oil rim into the initial gas cap this oil rim displacement produces a very different 4d seismic response from the case of a static gas oil contact and rising oil water contact normal production as a result of these observations we are able to optimize field production by both re perforation of existing wells and by drilling sidetracks into the displaced rim a brown field development opportunity that might otherwise be missed

adaptive scaling for an enhanced dynamic interpretation of 4d seismic data in this study importance is drawn to the role of engineering principles when interpreting dynamic reservoir changes from 4d seismic data in particular it is found that in clastic reservoirs the principal parameters controlling mapped 4d signatures are not the pressure and saturation changes per se but these changes scaled by the corresponding thickness or pore volume of the reservoir volume that these effects occupy for this reason pressure and saturation changes cannot strictly be recovered by themselves this being true for all data interpretation this understanding is validated both with numerical modelling and analytic calculation interestingly the study also indicates that the impact of gas saturation on the seismic can be written using a linear term but that inversion for gas saturation can yield at best only the total thickness pore volume of the distribution the above provides a basis for a linear equation that can readily and accurately be used to estimate pressure and saturation changes quantitative updates of the static and dynamic components of the simulation model can be achieved by comparing thickness or pore volume scaled changes from the simulator with the corresponding quantities on the inverted observations

scotia sea regional tectonic evolution implications for mantle flow and palaeocirculation abstract the scotia sea and surrounding scotia arc have evolved over the past 40 ma by extension behind an east migrating subduction zone at the boundary between the south american sam and antarctic ant plates the considerable data set now available regional geology and geophysics earthquake seismology satellite altimetry global plate analyses suggest why east migrating subduction began what has been the driving force that has sustained it and what other processes have controlled the mode of back arc extension in the scotia sea a suite of six reconstructions has been developed based on this data set the reconstruction to 40 ma creates a compact cuspate continental connection between south america and the antarctic peninsula at the subducting pacific margin with fragments now dispersed around the scotia arc occupying positions within it compatible with their known geology the driving force has been subduction of south american ocean floor which began as a result of southward migration of the pole of south american antarctic plate rotation and a key modulator of back arc extension has been collision of ridge crest sections of the south american antarctic plate boundary with the east advancing trench cenozoic regional tectonic evolution has two other likely consequences which greatly increase its importance firstly this region saw the tectonic disruption of the final barrier to complete circum antarctic deep water flow that may have had a profound effect on palaeoclimate secondly it is possible that the rapid roll back of the hinge of subduction is related to shallow eastward flow in the sub lithospheric mantle both of these consequences are explored the reconstructions show that rapid roll back of the subduction hinge averaging 50 mm a over the last 40 ma with respect to the south american plate has been a feature of all of scotia sea evolution and provide a history of motion of several oceanic microplates most of which are now welded together within the scotia sea this will guide the location of seismometers and or dredge hauls to test the hypothesis of shallow mantle flow and help interpret the results the reconstructions also allow an assessment of the creation of deep water pathways that would have permitted the development of the present day antarctic circumpolar current acc an early miocene onset within the period 22 17 ma seems likely for the acc depending on the structure and palaeo elevation of davis bank and aurora bank sections of the north scotia ridge however the study shows there was a delay of one or more million years between initial provision of a deep water pathway and the major mid miocene change in global climate involving the general level of antarctic glaciation that may have been related if these changes were related then the delay suggests that other factors possibly rough elevated ocean floor but also non tectonic factors such as atmospheric co2 were important in determining palaeoclimate

rare earth element zonation in pacific ferromanganese nodules abstract the lower surfaces of ferromanganese nodules from the north equatorial pacific ocean which are enriched in mn cu and ni and the upper surfaces which are enriched in fe p and co have been analyzed for la ce nd sm eu gd dy er and yb the ree contents are lower and the ce anomaly is smaller in the lower surfaces than in the upper surfaces the magnitude of the ce anomaly increases with decreasing mn fe ratio indicative of a seawater origin the zonal distribution of the other ree supports the conclusion derived previously from inter nodule and nodule sediment relationships that diagenetic fixation of rare earths in sediments affects their enrichment by nodular iron oxyhydroxides

rare earth geochemistry of phosphorites abstract the abundance of rare earth elements ree s in francolite increases with increasing age as a result of post depositional enrichment abundances are higher in pelletal samples than in non pelletal ones because the higher surface mass ratio of pelletal francolite promotes enrichment during diagenesis onshore francolite has higher ree abundances than that found offshore distribution patterns in francolite may be similar to those reported for open ocean water or shale depending upon the dominant source of the ree s different patterns result from weathering and deep burial diagenetic modification of the original patterns and abundances negative ce anomalies occur in francolites containing structural sulphur that is isotopically indistinguishable from seawater of the same age zero or positive anomalies occur in francolites which contain sulphur that is isotopically lighter than seawater

evidence for active subduction beneath gibraltar we report on a marine seismic survey that images an active accretionary wedge west of gibraltar ramp thrusts offset the seafloor and sole out to an east dipping decollement indicating ongoing westward vergent tectonic shortening new traveltime tomographic re sults image a slab of oceanic lithosphere descending from the atlantic domain of the gulf of cadiz passing through intermediate depth 60 120 km seismicity beneath the west ernmost alboran sea and merging with a region of deep focus earthquakes 600 660 km below granada spain together these new data provide compelling evidence for an active east dipping subduction zone

cyclic phosphate rich successions in the upper cretaceous of colombia abstract upper cretaceous neritic to hemipelagic successions from the eastern colombian cordillera display frequent and rhythmic intercalations of phosphate rich sediment their accumulation is attributed to a back arc setting between the andean arctrench system and the guayana cratonic shield in three examined sections near tausa tunja and iza all north of bogota respectively the phosphate rich sediments occur in 1 15 m thick coarsening upward series ideally consisting from the base to the top of porcelanite organic rich claystone siltstone sandstone and a condensed and thoroughly burrowed top bed phosphatic particles appear either in thin gravity flow deposits or in pristine in situ occurrences near the base of these successions intercalated in fine grained biosiliceous or clay rich sediment or in the condensed top bed the major portion of this coarsening upward series porcelanite to sandstone is considered a shallowing upward succession and the thin condensed phosphatic top bed a deepening upward succession these rhythmic successions are interpreted as parasequences resulting from fourth order relative sea level changes based upon biostratigraphic age estimates the time span of formation of these parasequences range between approximately 100 000 and 200 00 yr the allochthonous phosphate intercalations near the base of the parasequences are derived from condensed phosphatic top beds which may have been exposed at the sediment water interface in proximal directions this suggests that the parasequence boundaries i e marine flooding surfaces are diachronous and become younger in onshore directions using the vertical stacking patterns of these parasequences we distinguish between transgressive and highstand systems tracts tst and hst tst s are characterized by the dominance of phosphatic sediment laminated and organic rich claystone and laminated porcelanite this suite of sediments documents high nutrient fluxes and the presence of an oxygen minimum zone both probably induced by coastal upwelling hst s include laminated to well bioturbated siliciclastic successions which may reflect a weakening or basinward shift of upwelling cells and higher levles of bottom water oxygenation the dominance of siliciclastics in hst s is indicative of high detrital fluxes which outpaced sediment accomodation rates on the shelf upper campanian ammonoids have been found in three levels of the lower plaeners member of the guadalupe formation in the section near tausa nostoceras nostoceras liratum sp n exiteloceras jenneyi whitfield 1887 and libycoceras sp e jenneyi is an important zonal marker in the u s western interior that is also known from the basal mount laurel sand of delaware usa its occurrence at tausa is the first record outside the usa and provides an important datum for intercontinental correlation the type of libycoceras sp encountered in tausa is also known from the upper campanian of peru and angola together with the presence of andalusiella polymorphia malloy 1972 a dinoflagellate cyst an age range is given for the formation of the lower plaeners member at tausa late campanian to early maastrichtian

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

distribution of rare earth and some trace elements in tertiary phosphorites from the senegal basin and their weathering products abstract the distribution of rare earth and some trace elements is studied in the eocene phosphate deposits of farim saliquinhe lam lam and n diendouri ouali diala from the mauritania senegal guinea basin the depth of weathering varies between the three deposits the strong weathering of lam lam is marked by the leaching of apatites ca phosphate and the appearance of epigenetic crandallites al ca phosphate in the void spaces that have resulted from leaching of apatites and then of crandallites ferruginous and kaolinitic fillings are epigenised in millisites al ca phosphate all the studied phosphates have high ree ba and sr abundances and present shale normalized patterns with a negative ce anomaly of variable intensity clay minerals of sediments smectites and of weathered rocks kaolinites are characterized by low ree and high v zn cu cr sc and zr abundances they have no ce anomaly the comparison of the ree patterns and of the trace element abundances lead to three important conclusions at first the genesis of al ca phosphates is marked by specific leaching of heavy ree relative to the light ones then millisites have a ree abundance and a ce anomaly less pronounced than those of apatites and crandallites they show also v zn cu cr sc and zr abundances like clay minerals these facts can be related to their epigenetic formation from clay minerals last but not least the negative ce anomaly of epigenetic crandallites follows the same trend as in apatites therefore this anomaly cannot constitute a proof of the primary character for apatites if such variations should be related to the paleogeographic setting of studied deposits they must be considered with prudence

quaternary phosphorites off the southeast coast of india abstract detailed petrological mineralogical geochemical and radiogenic u sr nd and stable isotope c o s studies have been carried out on the quaternary phosphorites of the continental margin off chennai southeast coast of india these phosphorites are formed as a result of trapping and binding of sediments by microbial mats and are similar to phosphate stratiform stromatolites detrital and biogenic constituents enclosed in the phosphorites controlled the major and minor element composition except for sr and u the concentrations of most of the trace metals are lower than those in average shale and phosphorite middle rare earth element mree enriched patterns are the characteristic feature the u th dating method indicates that the ages of the phosphorites are beyond 300 000 years 87 sr 86 sr ratios of the phosphorites are higher than that of present day seawater and end values are more negative than those of seawater of the atlantic ocean carbon isotope ratios are within the range expected for the oxic suboxic zone but sulfur isotope ratios indicate suboxic conditions during phosphorite formation these results imply that the benthic microbial mats thrived on the shallow shelf during the quaternary low sea level conditions periodic or episodic sedimentation onto the mats led to their death the bacteria associated with decaying microbial mats utilised phosphorus supplied by continental sources and rapidly precipitated phosphate the availability of a high percentage of phosphorus in seawater seems to be an important controlling factor for the formation of phosphate stromatolites the composition of these phosphorites differs from the modern phosphorites in upwelling regions but are similar to cambrian apatite stromatolites these phosphorites provide evidence that the replicates of ancient phosphate stromatolites do exist in the quaternary

some examples and mechanical aspects of continental lithospheric folding abstract recent theoretical and observational advances in understanding the dynamics of lithosphere processes have yielded strong evidence in favour of the existence of large scale lithosphere folds intraplate compressional stresses generated by plate tectonic forces can reach magnitudes that approach the plastic buckling strength of mechanical models of lithosphere with brittle ductile rheological stratification the models predict lower plastic buckling stresses for continental lithosphere than for oceanic lithosphere intraplate folding of relatively strong oceanic lithosphere documented extensively in the northeastern indian ocean occurs where stresses are concentrated by restricted geometric and dynamic conditions resulting in unusually high stress levels several hundreds mpa on continents where geophysical data suggest that lithosphere folding may have occurred during the development of sedimentary basins in northern canada and central australia buckling stresses can have magnitudes that are of a similar order to those thought typically to occur in the lithosphere several tens mpa folding of continental lithosphere although more difficult to document is likely to be an important mechanism of large scale continental deformation in particular thinned continental lithosphere with a history of superposition of thick sedimentary successions might be the preferential locus of such folding lithosphere folding could also be a controlling factor in the near surface deformation of rifted basins as reflected by tectonic basin inversion

phosphate stromatolites from condensed cephalopod limestones upper jurassic southern spain upper jurassic phosphate stromatolites of the almola sierra southern spain encrust macrofossils and hardgrounds and form oncoids included within pelagic condensed fossiliferous limestones their accretion was determined by bacterially mediated precipitation of phosphate by the trapping and binding of fine siliciclastics and pelagic biomicrite and by the encrustation of benthonic foraminifera phosphorous trace elements and rare earth elements were concentrated from degraded organic matter and seawater by stromatolite building communities which mediated the formation of phosphate rich and fe al si rich organic gels under oxic conditions favouring the precipitation of amorphous mineral precursors acp and fe al si oxyhydroxides the observed ce enrichment for some stromatolites is explained by oxidative scavenging of ce4 from seawater by fe mn oxyhydroxides the bacterially mediated gels were able to migrate and fill the voids of the stromatolite structure and later changed to carbonate fluorapatite haematite and poorly crystalline fe rich clays under postoxic conditions phosphatization of trapped carbonate particles also occurred the phosphate stromatolites formed on a sediment starved pelagic swell during periods of no carbonate sedimentation and hardground development stromatolite lamination provides evidence for rhythmic alternation between bacterially mediated phosphogenesis sedimentation and erosion suggesting episodic changes in the sedimentary environment although some of the parameters that controlled phosphate precipitation associated with the stromatolites local high organic productivity sediment starvation moderate depth of deposition and physicochemical conditions were similar to those found in modern and ancient phosphogenic settings the palaeogeographical framework and the intensity of sedimentary processes were different to those of the world s major phosphorite deposits

temporal and bathymetric resolution of nautiloid death assemblages in stratigraphically condensed oozes new caledonia cephalopod shells can be affected by postmortem transport and biostratigraphic condensation but direct estimates of the temporal and spatial resolutions of cephalopod assemblages are missing amino acid racemisation calibrated by 14c demonstrates a centennial scale time averaging 500 years of nautilus macromphalus in sediment starved epi and mesobathyal pelagic environments the few shells that are thousands of years old are highly degraded the median occurrence of dead shells is at 445 m depth close to the 300 400 m depth where living n macromphalus are most abundant therefore dead shells of this species accumulate at a centennial temporal resolution and with excellent bathymetric fidelity dead nautilus shells exist for only a few hundred years on the seafloor in contrast to the biostratigraphically condensed mixture of extant foraminifers and foraminifers that went extinct during the pleistocene cephalopod shells that do not show any signs of early diagenetic cementation are unlikely to be biostratigraphically condensed

global imaging of polar cap patches with dual airglow imagers during a 2 h interval from 2240 to 2440 ut on 12 november 2012 regions of increased 630 0 nm airglow emissions were simultaneously detected by dual all sky imagers in the polar cap one at longyearbyen norway 78 1 n 15 5 e and the other at resolute bay canada 74 7 n 265 1 e the resolute bay incoherent scatter radar observed clear enhancements of the f region electron density up to 1012 m 3 within these airglow structures which indicates that these are optical manifestations of polar cap patches propagating across the polar cap during this interval of simultaneous airglow imaging the nightside dawnside dayside duskside half of the patches was captured by the imager at longyearbyen resolute bay this unique situation enabled us to estimate the dawn dusk extent of the patches to be around 1500 km which was at least 60 70 of the width of the antisunward plasma stream seen in the super dual auroral radar network convection maps in contrast to the large extent in the dawn dusk direction the noon midnight thickness of each patch was less than 500 km these observations demonstrate that there exists a class of patches showing cigar shaped structures such patches could be produced in a wide range of local time on the dayside nearly simultaneously and spread across many hours of local time soon after their generation