new insights on the tectonics along the new hebrides subduction zone based on gps results 1 at the new hebrides nh subduction zone ridges born by the subducting australia plate enter the trench and collide with the overriding margin results from gps surveys conducted on both sides of the trench and new bathymetry maps of the nh archipelago bring new light on the complex tectonics of this area convergence vectors present large variations that are not explained by australia pacific a p poles and that define four segments vectors remain mostly perpendicular to the trench and parallel to the earthquake slip vectors slow convergence i e 30 40 mm yr is found at the central segment facing the d entrecasteaux ridge the southern segment moves faster than a p motion predicts 89 to 124 mm yr relatively to a western north fiji basin wnfb reference the northern and southern segments rotate in opposite directions consistently with the extension observed in the troughs east of both segments both rotations combine in central vanuatu into an eastward translation that bulldozes the central segment into the wnfb at 55 mm yr that model suggests that the motion of the central segment forced by the subduction collision of the d entrecasteaux ridge influences the motion of the adjoining segments the new caledonia archipelago is motionless with respect to the rest of the australia plate despite the incipient interaction between the loyalty ridge and the nh margin southeast of the interaction area convergence is partitioned into a 50 mm yr trench normal component accommodated at the trench and a 90 mm yr trench parallel component close to the a p convergence and presumably accommodated by a transform boundary at the rear of the nh arc

microstratigraphy trilobite biostratinomy and depositional environment of the lower cambrian ruin wash lagerstatte pioche formation nevada abstract the uppermost 43 cm of dyeran strata at the ruin wash lagerstatte chief range lincoln county nevada contain nonmineralized invertebrates and exceptionally preserved articulated olenelloid trilobites however the environmental factors responsible for the preservation of olenelloids in this unusual state at ruin wash have received little study and are therefore poorly understood microstratigraphic analysis of this interval reveals that the strata comprised almost exclusively of clay sized particles and rather monotonous and featureless in outcrop were deposited as a series of thin event beds interpreted as tempestites the energy of deposition progressively waned then waxed through the interval and benthic redox conditions shifted concurrently biostratinomic data for more than 500 trilobite specimens encountered in the measured section reveal that the trilobite fauna was autochthonous although winnowed surfaces rich in bioclasts occurring at the base and top of the section indicate intervals of prolonged pre burial exposure time and or in situ reworking trilobite biostratinomy was strongly influenced by subtle environmental shifts the degree of disarticulation and sclerite fracturing correlate positively with event bed energy and inferred bottom water oxygen content these results demonstrate that 1 preservation of articulated trilobites is favoured near the distal limit of scouring associated with depositional events where rapid sedimentation occurs in the absence of substrate reworking and 2 differential taphonomic overprint on trilobite preservation can occur within a monofacial succession driven by subtle environmental change insufficient to leave a distinct lithological signature this highlights the need for careful microstratigraphic sedimentological and ichnological analysis prior to comparison of palaeobiological or palaeoecological parameters inferred from fossil assemblages even when the assemblages occur within the same lithofacies

time specific black mudstones and global hyperwarming on the cambrian ordovician slope and shelf of the laurentia palaeocontinent abstract the early paleozoic featured nine intervals of strong expansion of an upper slope dysoxic anoxic d a water mass with eustatic rise or epeirogenic transgression strong expansion of this d a water mass led to deposition of time specific macroscale alternations of dark grey black mudstone within oxic green to red mudstone on the middle lower slope this d a facies even onlapped warm carbonate and cool water siliciclastic shelves as in the mesozoic d a muds were deposited in shallow water perhaps tens of metres deep with sea level rise these nine d a macroscale alternations correspond to intervals of global hyperwarming times of very intense greenhouse conditions that resulted from a feedback initiated by higher insolation and heat storage as shallow seas onlap tropical palaeocontinents warm epeiric seas heated the ocean and thermal expansion accelerated eustatic rise ever more extensive epeiric seas heightened oceanic and global temperature as heat storage capacity increased deep ocean circulation intensity fell below that of a greenhouse interval and lead to d a deposition low on the slope and on the platforms to provide the signature of global hyperwarming global hyperwarming differs from a hothouse interval as it does not require co 2 input from large igneous provinces to produce high temperatures and never shows deep sea anoxia late ordovician and late devonian black mudstones that cover much of laurentia record epeirogenic transgressions that led to global hyperwarming and suggest that cold water upwelling or plant terrestrialisation had nothing to do with epeiric sea anoxia global hyperwarming reduced oxygen solubility in these seas and erosion of orogens produced muddy water that limited light penetration and promoted shallow water anoxia the global hyperwarming hypothesis means that relative eustatic and epeirogenic sea levels complement the effect of global pco 2 on climate and sea level must also be regarded as a primary driver of phanerozoic climate

exceptional fossil preservation during co2 greenhouse crises abstract exceptional fossil preservation may require not only exceptional places but exceptional times as demonstrated here by two distinct types of analysis first irregular stratigraphic spacing of horizons yielding articulated triassic fishes and cambrian trilobites is highly correlated in sequences in different parts of the world as if there were short temporal intervals of exceptional preservation globally second compilations of ages of well dated fossil localities show spikes of abundance which coincide with stage boundaries mass extinctions oceanic anoxic events carbon isotope anomalies spikes of high atmospheric carbon dioxide and transient warm wet paleoclimates exceptional fossil preservation may have been promoted during unusual times comparable with the present co 2 greenhouse crises of expanding marine dead zones oceanic acidification coral bleaching wetland eutrophication sea level rise ice cap melting and biotic invasions

from rifting to active spreading in the lau basin havre trough backarc system sw pacific locking unlocking induced by seamount chain subduction associated with pacific australia plate convergence the lau basin havre trough is an active back arc basin that has been opened since 5 5 ma by rifting and southward propagating oceanic spreading current back arc opening rates decrease from 159 mm yr 1 in the northern lau basin to 15 mm y 1 in the southern havre trough major tectonic changes occur at the transition between havre trough rifting and full oceanic spreading of the eastern lau spreading center elsc where the oblique to trench westward subducting louisville seamount chain lsc sweeps southwards along the tonga trench new swath bathymetry seismic reflection data and limited rock sampling in this area constrain a tectonic and kinematic back arc model that incorporates the effects of lsc subduction the elsc which extends southward to 24 55 s forms a deep rift valley propagating southward through older rifted arc basement present day seismicity and fresh and fractured pillow lavas at 23 42 s are consistent with rift valley neovolcanism conversely the northern havre trough has low seismicity and rifted volcanic basement ridges trending 25 45 oblique to the basin axis consistent with low levels of extensional tectonism and volcanism this latter structural fabric is interpreted as an early stage of rifting that is now locked due to compression on the arc exerted by lsc subduction while in the lau basin such effects have passed as the lsc swept along the tonga trench it is proposed that the lau havre back arc opening is controlled by tectonic constraints exerted at the limits of the system by the lsc subduction which determines the southward migration of the tonga arc pole of rotation and associated lau basin opening a discrete three stage back arc opening evolution is proposed comprising 1 an initial phase of back arc rifting along the whole length of the plate boundary beginning at 6 5 ma 2 a subsequent phase mostly present in the southern part of the back arc domain and still active in the havre trough of transpression and transtension starting at 4 ma in the north as the lsc starts to subduct and sweeps southward along the tonga trench and 3 a renewed opening phase in the northern segment of the back arc domain with rifting and spreading starting at 3 5 ma as subduction of the lsc along the northern tonga trench is progressively completed

characterization of various types of deformation and their corresponding deviatoric stress tensors using microfault analysis abstract considering an ideally prefractured material based on mohr s representation and bott s model we define the geometry and the peculiarities of the different theoretical fault domains which are likely to be reactivated under different tectonic regimes the theoretical striations associated with the different types of deformation are also calculated the compressive strike slip extensive strike slip radial extensive and constrictive types of deformation are specified in the second part of this paper using etchecopar s computer aided method a quantitative structural analysis of various types of superficial deformation has been carried out in different study areas southern france greece spain tunisia and nigeria the geometry of the microfault domains as well as the nature of the associated tensors are determined for each type of regional deformation the problem of the spatial homogeneity of the tensors deduced by the microtectonic analysis within the framework of a particular tectonic phase is also discussed

an analysis of loop current frontal eddies in a atlantic ocean model simulation abstractthe loop current frontal eddies lcfes refer to cyclonic cold eddies moving downstream along the outside edge of the loop current in the eastern gulf of mexico they have been observed by in situ measurements and satellite imagery mostly downstream of the campeche bank continental shelf their evolution simulated by a primitive equation and 37 level atlantic ocean general circulation numerical model is described in detail in this study some of the simulated lcfes arise with the passage through the yucatan channel of a caribbean anticyclonic eddy as weak cyclones with diameters less than 100 km near the yucatan channel they then grow to fully developed eddies with diameters on the order of 150 200 km while moving along the loop current edge modeled lcfes have a very coherent vertical structure with isotherm doming seen from 50 to 1000 m depth the caribbean anticyclone and lcfe are two predominant features in this numerical model simulation which account for 22 and 10 respectively

a proposed nomenclature for stages and series for the cambrian of laurentia an internally consistent succession of six named regional stages from youngest to oldest sunwaptan steptoean marjuman delamaran dyeran montezuman with their boundaries defined by regional extinction events within the trilobite record is proposed for the trilobite bearing rocks of laurentia sub trilobite stage s are recommended but unnamed the three youngest stages have been named previously but the lower boundary of the marjuman is shifted downward the three older stages within the trilobite record are new if deemed necessary four regional series are also suggested from youngest to oldest millardan lincolnian waucoban and begadean each of the three youngest series accommodates two stages

deep earthquakes in the southwest pacific a tectonic interpretation this paper examines the spatial distribution of deep earthquakes in the tonga fiji new hebrides region of the southwest pacific our interpretation emphasizes the complex cenozoic tectonics of the pacific indo australian plate boundary as a primary control on the distribution and deformation of subducted lithosphere most deep earthquakes in the interarc region are associated with the contorted pacific plate lithosphere subducted at the tonga trench however anomalous groups of deep earthquakes located west of the tonga zone are unrelated to the present plate configuration tectonic reconstructions of the region to 8 m y b p provide circumstantial evidence that 1 the anomalous events west of the tonga zone occur in two pieces of detached lithosphere subducted at the vitiaz and proto new hebrides trenches during the late miocene 2 the flattening of the inclined seismic zone in northernmost tonga is related to the rapid opening of the lau basin since 4 m y b p and 3 the sharp westward curvature of the tonga seismic zone in this area coincides with a preexisting bend in the late miocene vitiaz arc the sharpness of its present curvature appears to be a secondary effect of shear flow in the lower mantle and compression between detached vitiaz and attached tonga lithosphere thus much of the contortion of the subducted lithosphere beneath tonga fiji may be produced by local tectonic interactions rather than collision of the slab with an impenetrable boundary in the midmantle

substrate control of benthos in a middle cambrian near shore epeiric palaeoenvironmental setting late middle cambrian mail change limestone a thin b30 m lime mudstone unit of the epeiric middle and late cambrian georgina basin australia is unusually rich in epifaunal linguliform brachiopods especially acrotretids it is the only unit of this abundantly fossiliferous cambrian basinal succession where the representation of this group is more prolific than that of trilobites the diminutive size of acrotretids generally less than 3 mm in shell diameter in the case of common taxa from the mail change limestone raises questions of how an epizoic lifestyle was achieved on muddy substrates facies analysis indicates that lime mud substrates associated with deposition of the mail change limestone included microbial firm ground which developed in upper shoreface settings and ephemeral evaporate horizons of supratidal or high intertidal association during the shallow water phases of repeating parasequence cycles lime muds of shoreface association are brachiopod rich suggesting that firm ground substrates were favoured attachment sites for populations of anchoring epizoic acrotretids the conical shell form typical of this group evolved in part to minimize current drag reducing stress on pedicle attachment and in part to induce turbulence at the shell gape to facilitate feeding the high incidence of undamaged acrotretid valves representing a spectrum of growth stages obtained from the mail change limestone suggests that failure of pedicle attachment was a major contributor to mortality in this group firm ground was also characterised by ubiquitous fine scale planolites like open burrows which housed substantial populations of wormlike suspension feeders polychaetes which also took advantage of firm ground substrates near shore substrates were also host to suspension feeding lingulella and protospongia and a deposit feeder responsible for thalassinoides ichnofossils substrates associated with deposition of the mail change limestone were unfavorable to colonization by vagile seafloor feeding ptychopariid trilobites which are poorly represented in the unit in contrast agnostid trilobites of nektonic habit were less limited by trophic opportunities and are more commonly represented diverse agnostids ranged into very shallow marine shoreface environments represented by the mail change limestone the association of abundant epifaunal linguliforms and infaunal suspension feeders with firm ground suggests that substrate stability strongly influenced the nature and biomass of middle cambrian sessile benthos evolutionary strategies among sessile benthos to take full advantage of shallow marine level bottom environments were not realised until the ordovician biodiversity event

stress and the direction of slip on fault planes the shear stress direction on a fault plane depends only on four of the six components of the stress tensor assuming only that the slip direction marks the shear stress direction on any fault plane and that stress is homogeneous it is possible to estimate these four stress parameters from populations of fault planes with known slip directions as several workers have observed different formulations of the problem may yield varying best fitting stresses and estimates of uncertainty in the simplest case no assumptions are made regarding the orientations of fault planes relative to the stress tensor thus the technique allows for the possibility that the fault planes may be very weak here we present simple analytical and graphical descriptions of the field of admissible fault geometries relative to any four parameter stress model which can be used to illustrate the significance of various inverse strategies in particular this paper explores the effects of using two alternative measures of misfit between an observed fault datum and stress model 1 the pole rotation the angle between the observed and predicted slip direction on the observed fault plane and 2 the minimum rotation the smallest angle between the observed fault geometry and any fault geometry which is consistent with the model by allowing for variation of the fault plane as well as the slip vector the minimum rotation procedure generally achieves a more stable and presumably realistic estimate of the actual discrepancy between a fault observation and stress model than the pole rotation procedure in a test case using 17 earthquake focal mechanisms from the yuii region of eastern taiwan separate inversions based on the two misfit criteria yield different optimum stress models and uncertainty estimates additional constraints on the stress tensor such as the effect of friction can be superimposed on the ones used here

growth mechanisms and geochemistry of carbonate concretions from the cambrian wheeler formation utah usa carbonate concretions provide unique records of ancient biogeochemical processes in marine sediments typically they form in organic rich mudstones where a significant fraction of the bicarbonate required for carbonate precipitation is supplied from the decomposition of organic matter in the sediments as a result carbonates that comprise concretions are usually characterized by broad ranges in 13c and include values that are significantly depleted relative to seawater this article reports results from a physical petrographic and geochemical analysis of 238 concretions from the wheeler formation cambrian series 3 utah usa which are unusual in several respects most prominently they formed in organic poor mudstones total organic carbon 0 1 to 0 5 and are characterized by a narrow range of 13c that onlaps the range of contemporaneous seawater values subtle centre to edge trends in 13c demonstrate that concretion precipitation was initiated by local chemical gradients set up by microbial activity in the sediments but was sustained during growth by a large pool of inorganic bicarbonate probably derived from alkaline bottom waters the large inorganic pool appears to have been important in facilitating rapid precipitation of the concretion matrix which occurred via both displacive and replacive carbonate precipitation during early diagenesis stable isotope data from cogenetic pyrite 34s and silica 18o phases provide insight into the evolution of biogeochemical processes during concretion growth and suggest that concretions were formed almost entirely during sulphate reduction with only minor modification thereafter concretions of the wheeler formation appear to represent an end member system of concretion formation in which rapid growth was promoted by ions supplied from sea water as such they offer insight into the spectrum of processes that may influence the growth of carbonate concretions in marine sediments

the effect of seamount subduction on seismic coupling it has long been speculated that the subduction of seamounts may have profound mechanical consequences particularly on the generation of large subduction zone earthquakes we consider this problem as a special case of the seismic coupling model of scholz and campos the model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling in the case of coupled arcs this increased coupling will result in a large increase in the recurrence intervals of earthquakes such events will therefore appear rare in the instrumental record as is observed for decoupled arcs the subduction of a large seamount should cause them to become locally coupled three of the latter localities have been found with satellite altimetry data two on the tonga kermadec trench and one on the izu bonin trench the diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate in each case the small to moderate plate interface seismicity typical of decoupled seismic zones is almost completely absent in a 100 200 km arc segment indicating that the subduction zone is locally locked as predicted by the model the occurrence of large interplate earthquakes at both tonga sites confirms that they are seismically coupled

details of stress directions in the alaska subduction zone from fault plane solutions we used the cumulative misfit method to divide a data set with heterogeneous stress orientations into subsets with homogeneous stress directions we propose that slope changes of the cumulative misfit as a function of earthquake number pinpoint the boundaries of homogeneous stresses using a synthetic data set of 50 fault plane solutions composed of two halves corresponding to two incompatible stress tensors we tested the validity and efficacy of the cumulative misfit method our acceptance criteria for results of stress tensor inversions are 1 the directions must be well constrained that is the 95 confidence regions of the greatest and least principal stresses do not overlap and 2 there must be evidence for homogeneity in the sample that is the average misfit of the inversion is less than 6 we estimated stress directions in the alaska wadati benioff zone wbz using 470 fault plane solutions which we determined for earthquakes with ml 3 and 67 published fault plane solutions for earthquakes with ms 5 we succeeded in subdividing the data set of the small and large earthquakes into 25 and 3 subsets with the average misfit ranging from 3 2 to 5 5 the average misfits of the subsets we accept as satisfying the assumption of homogeneity are smaller than the average misfit of the overall data set f 10 by factors of 2 to 3 we estimated the stress fields at two scales along the alaska wbz the stress directions measured by the large earthquakes ms 5 were homogeneous over large volumes with extension downdip and the direction of greatest compression along strike this unusual orientation of the greatest principal stress is attributed to the bend of the slab under central alaska which generates compressive stresses along strike the stress orientations revealed by small earthquakes in contrast exhibited a great deal of heterogeneity as a function of space although their general trend confirms the overall stress directions obtained from the large events we propose that the ratio of the dimensions of the stress field sensed by earthquakes to the rupture dimensions is about 20 to 50 the estimated stress directions of the crustal earthquakes corresponded to the following mechanisms 1 strike slip faulting with the greatest principal stress oriented n s near fairbanks and 2 thrusting with the greatest principal stress oriented nw se near mount mckinley

burgess shale type biotas were not entirely burrowed away burgess shale type biotas occur globally in the cambrian record and offer unparalleled insight into the cambrian explosion the initial phanerozoic radiation of the metazoa deposits bearing except

diagnoses of an eddy resolving atlantic ocean model simulation in the vicinity of the gulf stream part i potential vorticity abstract output of an eddy resolving model of the north atlantic is diagnosed in the vicinity of the gulf stream gs using quasigeostrophic potential vorticity qgpv ertel s potential vorticity pv and particle trajectories time series of qgpv show strong input of qgpv by the gs in the top 1000 m of the model ocean vigorous wave motions are observed in the vicinity of the model gs mixing qgpv in the region the time mean horizontal qgpv structures show qualitative similarity to those of large scale climatological pv calculated from hydrographic data by keffer and that of lozier the top 1000 m of the model ocean is characterized by a tongue or an elongated island of high mean qgpv along the gs it is demonstrated that the tongue is a product of strong qgpv input by the gs vigorous mixing by eddies and dissipation of qgpv along the path of the gs at the intermediate depths 1000 2500 m a large region of nearly homogenized mean pv or weakly varying mean qgpv is found to the west of the mid atla

diagnoses of an eddy resolving atlantic ocean model simulation in the vicinity of the gulf stream part ii eddy potential enstrophy and eddy potential vorticity fluxes abstract output of an eddy resolving model of the north atlantic is diagnosed in the vicinity of the gulf stream gs using quasigeostrophic potential vorticity qgpv quasigeostrophic potential enstrophy qgpe and modified divergent eddy potential vorticity flux v hq d a tongue or an elongated island of large mean qgpv along the model gs in the top 1000 m is associated with predominantly downgradient v hq d suggesting that the horizontal eddy fluxes are balancing a sink of eddy qgpe in most of the tongue or island by converting the mean qgpe into eddy qgpe some large upgradient v hq d is observed to the north of the center of the tongue or island however suggesting that some of the eddy fluxes in the northern half of the tongue or island of high qgpv are balancing a source of eddy qgpe there by converting eddy qgpe into the mean qgpe at the intermediate levels of the model under the gs eddy qgpe is small and the role of eddies appears to be mixed they are forcing the mean to some

elongated thoracic spines as potential predatory deterrents in olenelline trilobites from the lower cambrian of nevada abstract repaired injuries offer the opportunity to study predation in the fossil record and test hypotheses concerning predation intensity anti predatory characters and predator selectivity many studies have used the late palaeozoic and mesozoic records but quantitative explorations of early palaeozoic predation especially during the cambrian are lacking trilobites a group of biomineralised arthropods with a high early palaeozoic disparity and diversity are an ideal study system for exploring repaired injury frequencies the ruin wash lagerstatte nevada usa stage 4 cambrian series 2 provides a bulk sample of closely related but morphologically distinct taxa the five most abundant taxa olenellus chiefensis o fowleri o gilberti o terminatus and nephrolenellus geniculatus differ in the spinosity of the cephalon axial lobe of the thorax and thoracic pleural lobes a total of 26 repaired injuries on 330 articulated trilobite cephalothoraces reveal that o terminatus has the highest incidence of repaired injuries repair frequency 0 18 and n geniculatus has the lowest repair frequency 0 043 the role of enlarged spines on the third thoracic segment which are most elongate in n geniculatus are suggested as predation deterrents predator selection for prey of a larger size may have played a minor role though we did not find statistical support for this idea three cephala with repaired injuries were noted in a sample of 685 specimens a much lower repair frequency compared to thoraces this indicates that cephalic attacks were more often fatal when compared to thoracic attacks our study provides the first quantitative support for species specific predation on cambrian trilobites

seismotectonics of the fiji plateau and lau basin abstract focal mechanism solutions of shallow earthquakes in the fiji plateau and lau basin provide significant contributions to the understanding of the recent tectonic movements beneath these marginal seas left lateral shear zones are identified on the linear shallow seismic zone extending from north of the fiji islands to the northern end of the lau basin in the central area of the fiji plateau and in the northern part of the lau basin nearly nw se trending opening tectonics is suggested nearly e w striking compressional tectonics is inferred near the western edge of the hazel holme fracture zone

a three plate kinematic model for lau basin opening 1 we present revised compilations of bathymetry magnetization acoustic imagery and seismicity data for the lau basin and surrounds we interpret these data to more precisely locate the plate boundaries in the region and to derive a three plate kinematic model for the opening of the lau basin during the brunhes chron our tectonic model includes the niuafo ou microplate which is separated from the australian plate along the peggy ridge lau extensional transform zone central lau spreading center and from the tongan plate along the newly discovered fonualei rift and spreading center our model shows that australia tonga plate motion between 15 5 s and 19 s is partitioned across the microplate and it resolves the former apparent conflict between geodetic versus seafloor spreading velocities regionally this implies that the angular rate of opening of the lau basin has been faster than that of the havre trough to the south and that the rapid current rates of pacific subduction at the tonga trench 240 mm yr at 16 s have been sustained for at least 0 78 m y

mineral composition of host sediments influences the fossilization of soft tissues cambrian lagerstatten host rocks are frequently composed of kaolinite and chlorite in varying amounts accordingly our goal was to study the preservation potential of crustaceans in these two clays we conducted long term experiments 12 18 months the longest duration of actualistic taphonomy experiments from published literature on the decay of artemia salina in these clay sediments the degree of preservation transformed mineralogical composition of the sediments and the elemental composition of the nauplial remains were examined we demonstrate that the kaolinite and chlorite sediment enhanced the preservation in the kaolinite the effect was considerably higher than in the chlorite compared with the sediment free control ph inside the sediments dropped to 6 5 7 1 and was even lower 4 around the buried carcasses facilitating the dissolution of clays this phenomenon was confirmed by mineralogical analyses of the experimental sediments which showed mineralogical signatures of such dissolutio

taphonomy and palaeoecology of the emuellid trilobite balcoracania dailyi early cambrian south australia monospecific assemblages of the trilobite balcoracania dailyi occur in lower cambrian strata within the adelaide geosyncline in south australia biostratinomic data from single bedding plane assemblages within the warragee and coads hill members of the billy creek formation and white point conglomerate reveal a range of taphonomic signatures from census to within habitat timeaveraged assemblages these assemblages are interpreted as having inhabited protected shallow marginal marine environments size frequency distributions coupled with taphonomic data show that the warragee member census assemblage represents a living population in a physically stressful environment within a tidally influenced lagoon while the original population structure of the coads hill member and white point conglomerate assemblages has been lost due to varying degrees of taphonomic overprinting integration of taphonomic stratigraphic and sedimentological data supports the interpretation of b dailyi as representing an opportunistic species a preserved body cluster from the warragee member assemblage is considered to characterise a congregation formed for the purpose of synchronous reproduction and ecdysis representing one of the oldest examples of gregarious behaviour in the arthropod fossil record furthermore by analogy with modern horseshoe crabs the high number of juveniles i e protaspides and early meraspides within the same assemblage are believed to be constituents of a nursery within the intertidal zone with adults migrating into the shallows to copulate and spawn preserved moult ensembles from the coads hill member and white point conglomerate have enabled the description of exuviation techniques for b dailyi 2007 elsevier b v all rights reserved

taphofacies models for epeiric sea environments middle paleozoic examples abstract skeletonized organic remains display various modes of occurrence which may be used to better understand the sedimentary dynamics of strata within which they occur aspects of skeleton biostratinomy and early diagenesis are distributed nonrandomly within the stratigraphic record and therefore are useful in discriminating taphonomic facies for purposes of taphonomic comparisons skeletonized marine taxa are reduced to five generally defined types massive arborescent univalved bivalved and multielement different skeletal types display different attributes which reflect specific responses to physical chemical and or biological processes the taphonomic properties of disarticulation reorientation and sorting fragmentation corrosion and abrasion skeletal dissolution and early diagenesis display patterns which are predictable with respect to a variety of inter related physico chemical parameters these can be quantified through indices which evaluate relative frequencies of occurrence sedimentary events alter the temporal and spatial homogeneity of processes which generate background taphofacies and superimpose event related properties thereby generating composite taphofacies such events produce taphonomic signature which in themselves correspond to specific environmental conditions i e water depth and event intensity indeed background and event related processes interact to generate complex but nonetheless diagnostic facies attributes useful in reconstructing paleoenvironments the comparison of standard indices among beds should allow the recognition of stratigraphic patterns and facilitate the diagnosis of systematic gradients among taphonomic properties deductively derived taphofacies models corroborated by empirical data from paleozoic strata illustrate the distribution of taphonomic properties with respect to environmental energy turbulence background sedimentation rate and sediment oxygenation these parameters also indirectly influence other less obvious factors including bioturbation and sediment chemistry trends in the distribution of taphonomic properties taphonomic gradients permit us to map otherwise unrecognized trends in these environmental parameters across a hypothetical onshore offshore transect within a generalized epeiric sea setting taphofacies models provide the quantifiable basis by which evolution of taphofacies may be gauged and then related to biotic and abiotic phenomena moreover taphofacies models evaluate the preservability of skeletal remains in different environmental settings and therefore provide information important in paleoecological studies

testing patterns of association between brachiopod bioclast deposits and stratigraphic discontinuities a case study in middle upper devonian carbonate dominated settings of north america abstractnondepositional hiatuses tend to favor the dense accumulation of skeletal material because they are associated with minimal sedimentary dilution of bioclast input however bioclast durability or production must be sufficient to counterbalance the rigors of delayed burial and especially the repeated events of seafloor exposure and reworking that bioclasts can experience during sedimentary bypass or starvation associated with the formation of hiatuses detailed field investigations of skeletal concentrations in middle upper devonian brachiopod rich carbonate stratigraphic records from the continental margin of nevada and the cratonic interior of iowa reveal a surprising lack of abundant skeletal material mantling major stratigraphic discontinuities rather the distribution of densely packed brachiopod deposits tends to be determined by physical processes that operate within the timescale of deposition of a single lithofacies producing skeletal concentrations that are mostly associated with beddin

cambrian evolutionary radiation context correlation and chronostratigraphy overcoming deficiencies of the first appearance datum fad concept abstract use of the first appearance datum fad of a fossil to define a global chronostratigraphic unit s base can lead to intractable correlation and stability problems fads are diachronous they reflect species evolutionary history dispersal biofacies preservation collection and taxonomy the cambrian evolutionary radiation is characterised by diachronous fads biofacies controls and provincialism of taxa and ecological communities that confound a stable lower cambrian chronostratigraphy cambrian series and stage definitions require greater attention to assemblage zone successions and non biostratigraphic particularly carbon isotope correlation techniques such as those that define the ediacaran system base a redefined basal cambrian trichophycus pedum assemblage zone lies above the highest ediacaran type biotas vendobionts putative metazoans and calcareous problematica such as cloudina and the basal asteridium tornatum comasphaeridium velvetum zone acritarchs this definition and the likely close correspondence of evolutionary origin and local fad of t pedum preserves the fortune head newfoundland gssp of the cambrian base and allows the presence of sub cambrian branched ichnofossils the sub tommotian equivalent base of stage 2 a suggested laolinian stage should be defined by the i l4 zhuce 13 c positive peak bracketed by the lower ranges of watsonella crosbyi and aldanella attleborensis molluscs and the skiagia ornata fimbrioglomerella membranacea zone acritarchs the w crosbyi and a attleborensis fads cannot define a stage 2 base as they are diachronous even in the newfoundland type w crosbyi zone the series 2 base cannot be based on a species fad owing to the provincialism of skeletalised metazoans in the terreneuvian series 2 boundary interval and global heterochrony of the oldest trilobites a series 2 and stage 3 a suggested lenaldanian series and zhurinskyan stage new gssp base is proposed at the siberian lower atdabanian 13 c iv peak which correlates into south china avalonia and morocco and assigns the oldest trilobites to the terminal terreneuvian series

a new burgess shale type assemblage from the thin stephen formation of the southern canadian rockies a new burgess shale type assemblage from the stephen formation of the southern canadian rocky mountains is described herein it occurs near stanley glacier in kootenay national park 40 km southeast of the type area near field british columbia while at least a dozen burgess shale localities are known from the thick stephen formation the stanley glacier locality represents the first discovery of burgess shale type fossils from the thin stephen formation the cathedral escarpment an important regional paleotopographic feature has been considered important to the paleoecologic set ting and the preservation of the burgess shale biota however the stanley glacier assemblage was preserved in a distal ramp setting in a region where no evidence of an escarpment is present the low diversity assemblage contains eight new soft bodied taxa including the anomalocaridid stanleycaris hirpex n gen n sp new genus new species pelagic or nektobenthic predators represent the most diverse group whereas in relative abundance the assemblage is dominated by typical cambrian shelly benthic taxa the low diversity of both the benthic taxa and the ichnofauna which includes diminutive trace fossils associated with carapaces of soft bodied arthropods suggests a paleoenvironment with restrictive conditions the stanley glacier assemblage expands the temporal and geographic range of the burgess shale biota in the southern canadian rockies and suggests that burgess shale type assemblages may be common in the thin ste phen formation which is regionally widespread

seismicity and state of stress within the overriding plate of the tonga kermadec subduction zone 1 to reassess the main tectonic units and to quantify the slip partitioning within the overriding plate of the tonga kermadec subduction zone a seismotectonic study was performed using global seismicity and focal mechanisms catalogs 1 new tectonic features were identified within the lau basin and the volcanic arc by remarkable shallow hypocenters alignments 2 the centroid moment tensor solutions catalog was processed in order to map the stress tensor variation in the upper plate we found the tectonic features characterized by a diffuse seismicity are subjected to a composite stress regime and they are interpreted as diffuse immature plate boundaries controlled by the high thermal anomaly lying beneath the lau basin 3 we quantified the margin parallel rates of motion using the azimuth of the maximum compressive stress component computed within the interplate zone the results highlight a major tectono kinematic segmentation related to the subduction of the louisville seamount chain

bathymetry and magnetic anomalies in the havre trough and southern lau basin from rifting to spreading in back arc basins abstract the havre trough and lau basin are active back arc basins related to pacific australian plate convergence these back arc basins provide insight into the evolution of rifting through various stages including a final stage of oceanic spreading we carried out a swath bathymetry and magnetic anomaly survey focused on the less studied havre trough and southern lau basin the western portion of the whole havre trough is deeper and more thickly sedimented whereas shallow bathymetry and volcanic edifices are dominant in the east we suggest that crustal rifting progresses asymmetrically with respect to the trough axis the active tectono magmatic zone migrates trenchward during ongoing widening rifting along normal faults trending in the longitudinal trough strike occurs in the southern havre trough volcanism is restricted to a line along the normal faults circular or elongated knolls in the northern havre trough indicate that en echelon type segmented rifting of short strike length and successive volcanic intrusion takes place volcanism in this area is dominated by diapir like magma intrusion into an extended and thinned arc crust the southernmost tip of the spreading center of lau basin is defined by the deep graben striking at n20 e located at 24 00 s 177 10 w

trilobites in early cambrian tidal flats and the landward expansion of the cambrian explosion reply the timing of the early invasion of the continents the routes to the land and the environmental breadth of the cambrian explosion are important topics because they are at the core of our understanding of early evolutionary breakthroughs illuminating some aspects of these problems are trilobite trace fossils in tidal flat deposits from the lower cambrian rome formation in the southern appalachian mountains of tennessee usa morphologic details and size range of the trace fossils suggest production by olenellid trilobites which occur as body fossils in the same unit the occurrence of this ichnofauna together with physical structures indicative of periodic subaerial exposure desiccation cracks and deposition within the intertidal zone flat topped ripples shows that trilobites forayed into the upper intertidal zone during the cambrian our finding supports the migration of subtidal organisms into marginal marine intertidal settings at the dawn of the phanerozoic suggesting that trilobites contributed to the establishment of the intertidal ecosystem during the cambrian the sequence of events involved in the colonization of early paleozoic tidal flats is consistent with the idea that most terrestrial taxa originated from marine rather than freshwater ancestors and that direct routes to the land from marginal marine ecosystems were involved in the colonization of continental environments early in the phanerozoic

diffuse back arc deformation in the southwestern pacific earthquake distribution and focal mechanisms from the lau and north fiji back arc basins indicate a diffuse and shear dominated system of deformation at odds with traditional plate tectonic models of lithospheric accretion in back arc basins the entire back arc region between the tonga and new hebrides arcs is more realistically modelled as a giant internally deformed pull apart basin formed along a left step in the transform boundary between the pacific and indo australian plates

herpetogaster from the early cambrian of nevada series 2 stage 4 and its implications for the evolution of deuterostomes the pioche formation of se nevada preserves a diverse soft bodied fauna from the early and middle cambrian series 2 3 stage 4 5 while the fauna is dominated by arthropods animals belonging to other taxa can be found here we document the first occurrence of herpetogaster collinsi outside the burgess shale further the specimens are from the nephrolenellus multinodus biozone and thus represent the oldest occurrence of the species as well as possibly the earliest soft bodied deuterostomes in laurentia

type of faulting and orientation of stress and strain as a function of space and time in kilauea s south flank hawaii earthquake focal mechanisms of events occurring between 1972 and 1992 in the south flank of kilauea volcano hawaii are used to infer the state of stress and strain as a function of time and space we have determined 870 fault plane solutions from p wave first motion polarities for events with magnitudes m l 2 5 and depth ranging between 6 and 12 km faulting is characterized by a mixture of decollement reverse and normal faults most large earthquakes with magnitude m 7 rupture the decollement plane since it is the only surface large enough to generate magnitude 7 or larger earthquakes the percentage of reverse faulting events is high compared to the decollement and normal faulting mechanisms for the period 1972 1983 the percentage of decollement type focal mechanisms becomes dominant after 1983 this pattern of faulting activity suggests that pressure was building up within kilauea s rift zone prior to the 1983 puu oo eruption overall a single stress orientation with the maximum compressive stress oriented se perpendicular to the rift and dipping at 45 is compatible with the coeval existence of decollement reverse and normal faults however in a crustal volume east of longitude 155 10 w we find a change of the orientation of 1 from nearly horizontal to plunging 45 se occurring in 1979 this stress rotation suggests magma movements within the aseismic part of kilauea s east rift zone the strain and stress orientations are coaxial in the south flank except within the volume where the stress rotation is observed we observe a change in the relationship between stress and strain directions caused either by the shifting of seismic activity from reverse faults to decollements while stress stays constant or by a rotation of stress while strain remains constant assuming that the model of a noncohesive coulomb wedge is appropriate for kilauea s south flank we find that high pore pressures are prevalent along the decollement and within the wedge for a coefficient of friction equal to 0 85

newly identified segments of the pacific australia plate boundary along the north fiji transform zone abstract the north fiji transform zone a 1500 km long and 200 km wide transform segment of the pacific australia plate boundary is one of the major transform fault systems of the earth new data collected during the alaufi cruise march 2000 on board the r v l atalante make it possible to define more accurately the geometry and kinematics of this transform plate boundary three spreading centers or extensional zones the north cikobia spreading center the futuna spreading center and the southeast futuna volcanic zone and a strike slip fault zone the futuna transform fault have been discovered over a distance of 500 km along the eastern north fiji transform zone from the north of the fiji platform to the east of the futuna archipelago the futuna transform fault oriented 100 has been mapped over a distance of 250 km it must be considered to be an important tectonic element of the transform plate boundary pure strike slip as well as transpression and transtension motions are responsible for the complex morphology of this feature the uplifted futuna alofi ridge represents a major compressional relay along the futuna transform fault the futuna spreading center trending 20 30 is composed of a series of en echelon left stepping spreading segments it represents a 200 km long extensional relay between the futuna transform fault and the western part of the north fiji transform zone the fiji transform fault which bounds the fiji platform to the north the opening rate at the futuna spreading center is estimated at 4 cm yr although the north cikobia spreading center and the southeast futuna volcanic zone have been only partly mapped bathymetric and reflectivity data clearly reveal that active extension also takes place along these two features a spreading rate of 2 cm yr is inferred at the north cikobia spreading center therefore the north fiji transform zone appears to be composed of two main overlapping transform segments relayed by parallel extensional zones the three active extensional zones have an ene wsw to nne ssw orientation while compressive features along the futuna transform fault are nw se to nnw sse oriented in accordance with the present day left lateral transform motion along this part of the pacific australia plate boundary

seismological evidence for tearing of the pacific plate at the northern termination of the tonga subduction zone we present seismological evidence for tearing of the pacific plate at the northern termination of the tonga subduction zone seismicity cross sections indicate 1 abrupt termination of intermediate depth seismicity along an east west trending zone coincident with the northern termination of the subduction zone and 2 shoaling of the subducted slab at its northern termination earthquake distribution and source mechanism determinations indicate progressive downwarping and tearing of the pacific plate as it enters the northernmost segment of the tonga subduction zone source mechanisms for 57 events located along a narrow belt of high seismicity in northernmost tonga indicate dip slip faulting along near vertical planes oriented 285 coinciding with the observed direction of plate convergence precisely determined depths for 21 events 18 57 km suggest that the plate is downwarped 15 km prior to initiation of tearing depths of the 7 april 1995 m s 8 event and aftershocks suggest tearing of the plate may extend through the entire thickness of the oceanic lithosphere

spreading mode of backarc basins in the western pacific abstract a number of phases are suggested in backarc spreading extension of the arc lithosphere is followed by spreading and formation of oceanic crust in the backarc basin in some basins a third phase of rifting with a reorientation of the spreading direction occurs during the second and third phases spreading is governed by rotation about a nearby pole and spreading axes are oblique to the transform faults this contrasts with the geometry of mid oceanic ridges where the ridges are on great circles perpendicular to the transform faults three spreading models are defined according to the various angular relations between transform faults and spreading axes backarc basins facing the pacific plate tend to open with axes spaced among adjacent ridge segments on projections parallel to the spreading axes type i spreading mode in some cases new spreading occurs in the countiguous area forming overlapped spreading axes type ii spreading mode initial spreading to form a backarc basin tends to spread with both spaced and overlapped axes type iii spreading mode inner backarc basins intercalated with other basins between the pacific plate also tend to spread with type iii spreading mode spreading modes and possible mechanisms for these formations in the backarc basins in the western pacific are discussed