three dimensional p and s wave velocity structures beneath japan abstract we determined the three dimensional vp and vs structures beneath japan by applying seismic tomography to a large number of arrival times recorded at temporary stations in the japan sea and the pacific ocean as well as those at permanent stations on the japan islands as a result we obtained more precise seismic images than previous studies in the crust and the uppermost mantle southwestern honshu exhibited weaker heterogeneity than the other areas in japan corresponding to the distribution of active volcanoes stripe like heterogeneities exist in the subducting pacific slab relatively low velocity zones correspond to low seismicity areas in the pacific slab suggesting that the slab is possibly torn or thin around the areas the fact that nonvolcanic deep tremors associated with the subducting philippine sea slab beneath shikoku kii and tokai do not occur in zones of high vp high vs and low vp vs ratio may reflect the existence of fluids generated by the dehydration processes of the slab prominent and wide low vp and vs zones exist beneath central honshu at the depth range of 30 60 km where the volcanic front related to the subducting pacific plate is located and seismicity around the philippine sea plate is very low this condition may exist because magma genesis processes related to the subducting pacific plate activate the same processes around the philippine sea plate

heterogeneous strain regime at the west of the ogasawara plateau in the western pacific ocean from inversion of earthquake focal mechanisms abstract the ogasawara plateau has a major control on the geology of the region between the izu bonin and mariana subduction zones this region is important for understanding active collisional zones we attempt to illustrate the stress field in the wadati benioff zone of this region through seismic estimations 49 events m 3 5 are chosen for waveform inversion by the isola software to distinguish the stress field according to available focal mechanism solutions the heterogeneous stress field parameters are determined for several depth ranges based on the epicentral distribution and seismic stress field the subduction system at the west of the ogasawara plateau can be divided into three segments separated by 26 5 n and 24 n at depths of 0 50 km in the outer trench slope the slab is characterized by a varying stress filed the best fit stress model with horizontal 3 and vertical 1 is related to the plate bending and preexisting faults for the central segment the solutions present a compressional stress regime in the outer trench slope without preexisting faults induced by the resistance of the buoyant plateau the solutions for the depth range of 0 50 km in the forearc region 1 coincides with the plate convergence and 3 dips steeply to the west in the northern and central segments controlled by the friction effect and convergence of the plates however this depth range in the southern segment shows an extensional stress range associated with subduction of older slab segment relatively low rate of plate convergence and the morphological adjustment of the slab between different segments at depths of 50 200 km the inhomogeneous stress field exhibits a complicated extensional stress system with strike slip focal mechanisms at depths of 200 400 km the seismicity presents a pronounced seismic gap and the stress field cannot be determined the seismic gap may be attributed to the missing or tear of the subducting slab which is caused by the subduction of the buoyant ogasawara plateau the mechanism solutions are characterized by an inhomogeneous strain regime below 400 km driven by gravity and strong contortion of the slab and resistance to penetration of the 660 km discontinuity in contrast there are little earthquakes below 400 km in the southern region of the ogasawara plateau

bioturbation in burgess shale type lagerstatten case study of trace fossil body fossil association from the kaili biota cambrian series 3 guizhou china abstract cruziana gordia planolites rusophycus and trichophycus are common ichnological elements of the kaili biota new discoveries based on the examination of 323 specimens include eldoniids echinoderms trilobites monoplacophorans and non biomineralizing arthropods that are associated with trace fossils based on the observed effects of bioturbation on the preservation of five different animal groups it is clear that infaunal scavengers deposit feeders were periodically active on the kaili sea floor and were able to reach historic layers yielding exceptionally preserved fossils in general the average level of infaunal activity is absent to moderate ichnofabric index i i 1 to 3 in the kaili substrate by contrast the phycodes beds are completely disturbed by infaunal activity i i 5 observed burrow diameter ranges from diminutive 0 2 mm to normal up to 4 2 mm computed tomography allows us to visualize the precise geometry of the trace fossil body fossil association in three dimensions we concluded that although some kaili infaunal animals could bore through the biomineralized echinoderms they did not appear to have scavenged upon these echinoderms based on our three dimensional reconstruction furthermore kaili burrowers can reach the historic layers containing exceptionally preserved fossils without altering soft tissue preservation on the other hand we used conventional techniques to reveal that one burrow is filled with fecal pellets 100 200 m at its terminal end because most burrows are filled with yellow brown coarse sediment and because there is no compositional difference between the fecal pellets and the surrounding coarse sediment within the burrow the origin of the yellow brown coarse sediment is interpreted here as parautochthonous and or autochthonous and as a result of sediment reworking by deposit or suspension feeders the relative scarcity of soft part preservation in the kaili biota compared to the older chengjiang biota and the younger burgess shale biota may be the result of post burial bio disturbance due to the relatively high intensity of local bioturbation it is plausible that oxygenation in deeper sediments facilitated by bioturbation allowed carcasses in the kaili biota to undergo a more extensive period of normal decay prior to final burial than in other cambrian konservat lagerstatten

early palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms abstract stylophorans cornutes mitrates represent one of the most diverse classes of cambro ordovician echinoderms they were free living benthic non radiate forms closely related to asterozoans and crinoids taphonomic sedimentological and palaeosynecological data provide useful information on key aspects of stylophoran palaeoecology such a combined approach suggests that the rarity of stylophorans in proximal environments above storm wave base was probably original and does not exclusively result from the possession of a loosely articulated polyplated calcitic test conversely stylophorans were relatively abundant in deeper settings below storm wave base where they frequently occur in low diversity dense beds several tens to hundreds of individuals per square metre such assemblages were either largely dominated by stylophorans only middle cambrian darriwilian or by the association of both stylophorans and ophiuroids latest darriwilian silurian comparison with ecological requirements of similar modern and fossil dense populations of ophiuroids suggests that stylophoran beds occurred in well oxygenated oligotrophic waters with low predatory pressure and that sea temperature played a key role in the palaeobiogeographic distribution of stylophorans seven main stylophoran biofacies are identified and described the associated environments and tropical distribution of primitive heavily plated forms suggest that the initial radiation of the class probably took place in warm to temperate shallow settings earliest middle cambrian late cambrian however from this primitive stock stylophorans radiated several times towards deeper and or colder environments possibly because of the lower predation pressure in such settings cothurnocystid cornutes in the middle cambrian mitrates in the late cambrian hanusiids in the tremadocian as a consequence the fact that most cambro ordovician cornutes and mitrates were psychrospheric i e cold water adapted probably explains their relatively wide palaeobiogeographic distribution in high palaeolatitudes cambro ordovician stylophorans were abundant and diverse in shallow to deep environments but they were generally restricted to deep settings in low palaeolatitudes the great caradocian transgression favoured the extension of stylophorans from circum polar regions e g western margin of gondwana towards external settings at lower palaeolatitudes avalonia baltica and eastern margin of laurentia where one group of mitrates underwent a major radiation anomalocystitids and extended towards shallower and warmer palaeoenvironments in contrast all silurian stylophoran assemblages known so far are rare poorly diversified and restricted to shallow warm to temperate environments in tropical regions this distribution partly results from 1 the probable eradication of psychrospheric forms e g cornutes because of the prolonged anoxia of deep environments in the late hirnantian early silurian of west gondwana 2 the radiation of some mitrates anomalocystitids in shallow tropical settings in the late ordovician and 3 of strong taphonomic and or sampling biases numerous ghost lineages

crustal structure and evolution of the mariana intra oceanic island arc a new high resolution velocity model of the mariana arc backarc system obtained from active source seismic profiling demonstrates velocity variations within the arc middle and lower crusts of intermediate to felsic and mafic compositions the characteristics of the oceanic island arc crust are a middle crust with velocity of 6 km s laterally heterogeneous lower crust with velocities of 7 km s and unusually low mantle velocities petrologic modeling suggests that the volume of the lower crust composed of restites and olivine cumulates after the extraction of the middle crust should be significantly larger than is observed suggesting that a part of the lower crust especially the cumulates is seismically a part of the mantle

the response of stratified frictional flow of shelf and slope waters to fluctuating large scale low frequency wind forcing abstract numerical and analytical models are developed to discuss large scale low frequency wind driven fluctuations over the continental shelf and slope in the presence of realistic density stratification and bottom friction the models employ the long wave assumption that frequencies are small relative to the inertial frequency and that alongshore scales are large relative to cross cross shelf scales an attempt is made to make the effect of bottom friction on the flow as realistic as possible by taking into account the influence of surface gravity waves on bottom stress the results were obtained for sinusoidal wind forcing having frequency and alongshore wavenumber l and are described below a general criterion for barotropic shelf water response for wind forcing frequencies in the weather band 2 few weeks to 2 few days can be derived for the purpose of rapid calculation this criterion reduces approximately tohere ns2 is a shelf averaged value of the buoyancy frequency squared is

three dimensional structure of vp vs and vp vs beneath northeastern japan implications for arc magmatism and fluids we estimated both p v p and s wave velocity v s structures beneath northeastern japan by applying a tomographic method to 169 712 p and 103 993 s wave arrival time data from 4338 local events the average value of v p v s ratio is 1 69 in the upper crust 1 75 in the lower crust and 1 77 in the uppermost mantle these differences in v p v s ratio may mainly reflect the lithological variations with depth low v p low v s and high v p v s zones are extensively distributed along the volcanic front in the uppermost mantle and are downward to the back arc side in the mantle wedge here the low v p low v s and high v p v s values are interpreted because of partial melting materials which imply the presence of a vast amount of melt in the uppermost mantle in the lower crust low v p low v s and high v p v s zones are not continuously visible along the volcanic front but are confined to individual volcanic areas which suggests that melting of lower crustal materials occurs just beneath each active volcano in contrast the upper crust beneath active volcanoes exhibits low v p low v s and low v p v s these results suggest that the partial melting zones under northeastern japan spread out from the uppermost mantle along the volcanic front up to the midcrust right beneath active volcanoes the low v p low v s and low v p v s features in the upper crust suggest the existence of h 2 o rather than melt right beneath active volcanoes

strong inhomogeneity beneath quaternary volcanoes revealed from the peak delay analysis of s wave seismograms of microearthquakes in northeastern japan summary high frequency s wave envelopes of microearthquakes well reflect the medium inhomogeneity of the earth defining the peak delay time as the time lag from the direct s wave onset to the maximum amplitude arrival of its envelope we use this quantity to evaluate the strength of multiple forward scattering and diffraction due to random inhomogeneities along the seismic ray path analysing peak delay times of many microearthquakes occurred along the subducting pacific plate for 2 4 4 8 8 16 and 16 32 hz frequency bands we find a clear path dependence of the peak delay time in relation to the distribution of quaternary volcanoes in northeastern japan peak delay times of less than 2 s are usually observed at most of the stations in the study area but large peak delay times of more than 5 s are observed in the backarc side stations for the case that s wave propagates beneath quaternary volcanoes the large peak delay times are inferred to be generated at a depth of 20 60 km beneath quaternary volcanoes by considering ray paths under a 1 d velocity structure these strongly inhomogeneous regions are located at low velocity and high vp vs regions revealed from tomography which suggests the inhomogeneity may be related to dykes and melts of ascending magma

three dimensional crustal structure of the mariana island arc from seismic tomography 1 a three dimensional 3 d seismic refraction survey was acquired over the mariana volcanic arc at 14 5 18 5n and 145 147e first arrival traveltimes from this survey and from a separate 2 d survey acquired approximately perpendicular to the arc have been simultaneously inverted for a 3 d p wave velocity model using seismic tomography subject to smoothness constraints the active arc which initiated only 3 4 ma ago has an average crustal thickness of 18 km approximately 40 km to the east the inactive remnant of the rifted eocene arc has an average crustal thickness of 21 km due primarily to a thicker lower crustal layer with velocities of 6 5 7 0 km s 1 crustal production clearly varies both temporally and spatially with some crustal layers including the igneous forearc crust varying in thickness by a factor of up to 2 along strike average p wave velocities within the upper crust of the modern arc are 240 360 m s 1 lower than in the eocene arc but are 280 m s 1 higher within the lower crust middle crust with velocities of 6 0 6 5 km s 1 is best developed beneath the eocene arc these results suggest an evolution of arc structure with increasing age we infer closure of fractures and porosity in the upper crust through hydrothermal circulation and a reduction in the mafic character of the middle to lower crust as a result of intracrustal differentiation although tonalitic rocks may predominate in the transition from upper to middle crust the bulk of the crust is essentially basaltic

structure and growth of the izu bonin mariana arc crust 1 seismic constraint on crust and mantle structure of the mariana arc back arc system 1 a high resolution seismic velocity model is presented for the crust and upper mantle of the mariana arc back arc system mabs based on active source seismic profiling the major characteristics are 1 slow mantle velocity of 8 km s 1 in the uppermost mantle especially and deep reflectors under the mariana arc ma and the west mariana ridge wmr 2 a deep reflector in the upper mantle beneath the relative thick crust of the mariana trough mt axis 3 distribution of lower velocity lower crusts 6 7 6 9 km s 1 beneath the volcanic front and adjacent to the mt and 4 high velocity lower crust 7 2 7 4 km s 1 beneath the boundary regions between the ma and mt and between the wmr and the parece vela basin pvb adding to structural characteristics of crust and upper mantle beneath the mabs of the characteristics described above characteristic 1 suggests that the origins of the slow mantle velocity and the deep reflectors be explained by transfer of the lower crustal residues to the upper mantle across the moho considering that the wmr is extinct arc currently on the other hand characteristic 2 suggests that the origin of deep reflectors beneath the mt axis might be lower velocity materials due to the diffractive signals with strong amplitudes characteristic 3 suggests that the lower velocity lower crust advanced crustal growth and characteristic 4 suggests that the high velocity lower crust beneath arc back arc transition zone is composed of mafic ultramafic materials created by extensive partial melting of mantle peridotites or last stage of the arc magmatism rather than serpentinized peridotite

thickness variation of the descending philippine sea slab and its relationship to volcanism beneath the kanto tokai district central japan 1 we use an iterative nonlinear travel time tomography technique to determine three dimensional p wave and s wave seismic velocity models for the region beneath the kanto tokai district of central japan salient points of the models are the following 1 the subducting philippine sea slab has thickness variation with a stepwise offset east of the izu peninsula the eastern beneath the kanto district and western beneath the area north of izu peninsula and the tokai district regions have respective thicknesses of 60 and 25 km the slab clearly subducts continuously to a depth of 100 km from the sagami trough beneath the kanto district although the tokai slab is indistinct at depths of 50 70 km 2 the oceanic crust at the top of the descending philippine sea slab transforms into eclogite at a depth of about 70 km beneath the kanto district 3 low velocity anomalies are associated with present and past volcanism they are located on the west of a 110 km depth contour of the wadati benioff zone rather than only on the backarc side of the volcanic front deflection of the volcanic front might result from the subducting philippine sea slab which shifts the dehydration reaction deeper as it subducts deeper

coordinated biotic and abiotic change during the great ordovician biodiversification event darriwilian assembly of early paleozoic building blocks abstract the ordovician period records an extraordinary biodiversity increase known as the great ordovician biodiversification event gobe which coincided with a series of environmental changes to the earth system notably a cooling global ocean increased oxygenation and increased nutrient supply from volcanism and continental weathering the co evolution of earth and its biota during this interval has been studied in various contexts on multiple paleocontinents emerging patterns depend on the lens of investigation here we summarize the current state of understanding by reviewing and synthesizing the fossil and sedimentary records recent paleontological studies mainly focused on rhynchonelliform articulated brachiopods bryozoa cephalopods trilobites graptolites echinoderms and reef organisms have documented details of diversification body size increase development of ecosystem complexity and intensification of inter continental dispersal from the late cambrian through late ordovician biomass increased markedly between the early and middle ordovician furthermore diversification rates increase statistically during the mid darriwilian age both globally and regionally coincident with these biotic changes geochemical proxies record significant changes to earth s physical system oceanic temperatures decreased and atmospheric oxygen levels increased to near modern levels in concert with the middle ordovician diversification of shelly fauna anoxic pulses ceased and evidence for deep ocean ventilation prevails in middle ordovician strata furthermore a major middle late ordovician change in oceanic strontium isotopic composition indicates increased weathering of juvenile volcanic rocks and delivery of nutrients to marine settings this multi proxy dataset records near simultaneous changes in fossil rich shallow marine environments during exactly the interval of greatest diversification by integrating biotic and geochemical datasets a clear picture of the co evolution of earth and its biota emerges indicating that the darriwilian was the critical interval facilitating increased capacity of ecosystems we therefore recommend restricting the term gobe to indicate this short interval of rapid diversification and ecosystem change and using ordovician radiation when referencing the sum of diversifications that occurred throughout the ordovician period

paleoecology of pelmatozoan attachment structures from a hardground surface in the middle silurian massie formation southeastern indiana abstract the contact between the basal limestone and overlying mudstone interval of the massie formation silurian wenlock sheinwoodian represents a major flooding surface that is well exposed at the new point stone quarry at napoleon southeastern indiana at this locality this surface is a locally microbiohermal hardground encrusted by numerous attachment structures of both crinoids and blastozoan echinoderms the distribution of attachment structure morphologies is nonrandom with respect to variations in substrate properties and microtopographic relief discoidal structures and thecal attachments predominantly representing diploporites are concentrated on fine grained winnowed crests of the undulatory surface reflecting the stability of this substratum complex root like and runner type structures attributable to long stemmed crinoids and rhombiferans dominate the coarser muddier poorly sorted hardground troughs reflecting the need for greater surface area in more unstable rubbly areas microbioherms are densely encrusted by thickened radix systems belonging to long stemmed pelmatozoans the cause of thickening is unclear but may represent a pathologic response to some biotic interaction or investment by encrusting taxa in crustose holdfasts in order to prevent dislodgement this study supports previous interpretations of pelmatozoan attachment paleoecology wherein solid permanent discoids preferentially encrusted stable homogenous substrata whereas more complex structures were needed to occupy more heterogeneous and presumably less stable substrata thus the hardground studied here highlights the strong influence of substrate variations in influencing paleoecological dynamics in topographically complex carbonate settings possibly leading to locally increased alpha diversity and development of biofacies not typically encountered in certain normal marine environments

hot fingers in the mantle wedge new insights into magma genesis in subduction zones abstract quaternary volcanoes in the northeast japan arc can be grouped into 10 volcanic clusters striking transverse to the arc these have an average width of 50 km and are separated by parallel gaps 30 75 km wide this clustering of volcanic centres topographic profiles low velocity regions in the mantle wedge and local negative bouguer gravity anomalies along the japan sea side of the volcanic arc are closely correlated all these observations may be related to locally developed hot regions within the mantle wedge that have the form of inclined 50 km wide fingers each of the 10 fingers recognised extends from deep mantle 150 km below the back arc region towards the shallower mantle 50 km beneath the volcanic front quaternary volcanoes are built immediately above the hot mantle fingers the volcanic basement along the fingers has been uplifted by repeated injection of magmas into the crust accompanied by quaternary volcanic activity at the surface although volcanic activity is rare along the japan sea coast tomographic results show that hot low velocity mantle fingers exist within the mantle wedge the negative bouguer anomalies at the rear of the volcanic arc could be caused by magmas supplied from the hot mantle fingers these have not yet been erupted but have accumulated at the moho discontinuity

seismological evidence for variable growth of crust along the izu intraoceanic arc 1 the processes that create continental crust in an intraoceanic arc setting are a matter of debate to address this issue we conducted an active source wide angle seismic study to examine along arc structural variations of the izu intraoceanic arc the data used were acquired over a 550 km long profile along the volcanic front from sagami bay to tori shima the obtained structural model showed the existence of felsic to intermediate composition middle crust with a p wave velocity vp of 6 0 6 5 km s 1 in its upper part and 6 5 6 8 km s 1 in its lower part the thickness of the middle crust varied markedly from 3 to 13 km the underlying lower crust also consisted of two layers vp of 6 8 7 2 km s 1 in the upper part and vp of 7 2 7 6 km s 1 in the lower part the upper of these layers was interpreted to consist of plutonic gabbro and the lower layer was interpreted to be mafic to ultramafic cumulates average crustal velocities calculated from our model showed remarkable lateral variation which correlated well with arc volcanism low average crustal seismic velocities 6 7 km s 1 due to thick middle crust were obtained beneath basaltic volcanoes e g o shima miyake jima hachijo jima aoga shima while higher average velocities 7 1 km s 1 were obtained beneath rhyolitic volcanoes e g nii jima kurose south hachijo caldera myoji knoll and south sumisu caldera we concluded from these observations that continental crust grows predominantly beneath the basaltic volcanoes of the izu arc and that rhyolitic volcanism may be indicative of a more juvenile stage of crustal evolution or remelting of preexisting continental crust or both

strike slip tectonics within the northernmost philippine sea plate in an arc continent collisional setting abstract the geological processes in the northernmost philippine sea plate which is bounded by the suruga and sagami troughs are a typical example of an active collision zone we attempt to illustrate the stress field through seismic estimations and geodetic analysis and propose the kinematic mode of the northernmost tip of the philippine sea plate seven events m 4 0 are chosen for waveform inversion by the isola software to distinguish the stress field in particular six of the chosen events which exhibit strike slip motion are distributed in the eastern area where few focal mechanisms have been reported by previous studies according to the available focal mechanisms strike slip faults with similar p and t axes are widely distributed in the study area the stress inversion suggests that the northern area is characterized by a nw se compression and a ne sw extension stress regime although some spatial differences exist as indicated by an analysis of the geodesy epicenters focal mechanisms gravity anomalies and velocity structure the deformation in the northernmost tip is mainly accommodated by several conjugate strike slip fault systems with steep dips that center on the izu volcanic line generally the maximum principal stress of the kinematics is derived from the collision between the philippine sea plate and central japan because of the different subduction angles rates and directions of the down going plate diverging slab pull forces along the suruga and sagami troughs may be causing the ne nne extension in most of the areas that are bounded by the two troughs the extension propagates southwards along the izu volcanic line and reaches the area adjacent to miyake jima

palaeogeographical and palaeoecological aspects of the cambro ordovician radiation of echinoderms in gondwanan africa and peri gondwanan europe abstract ecology and tempo of the lower palaeozoic radiation of echinoderms are discussed in this paper based on comparison of the diversity patterns observed in cambro ordovician faunas from laurentia and the northern gondwana margin the cambrian agronomic revolution triggered a global radiation of echinoderms with the progressive disappearance of biomat related lifestyles and the colonisation of new environments both in laurentia and on the northern gondwana margin soft substrate echinoderm assemblages related to cold and or deep environments were dominated by blastozoans and stylophorans these assemblages show a pattern of continuous diversification from the middle cambrian to the middle ordovician the major radiation of crinoids in the lower to middle ordovician of laurentia is only a local diversification correlated with the development of hardgrounds in warm shallow environments no comparable major ordovician radiation event occurred in the cooler environments of the northern gondwana margin

continental crust crustal underplating and low q upper mantle beneath an oceanic island arc a detailed structural model of the crust subducting slab and underlying upper mantle across the northern izu ogasawara bonin island arc system is derived from a marine seismic reflection and ocean bottom seismographic refraction survey and subsequent forward modeling combined with tomographic inversion the model indicates that the crust is thickest beneath the presently active rift zone and a granitic crust may have formed in the mid crust a highly attenuative mantle that is one with low quality q seems to be confined mainly beneath the presently active rift zone in contrast high p wave velocity persists in the lower crust between the forearc and eastern margin of the back arc basin suggesting a large scale magma input responsible for the arc formation

the fezouata shale lower ordovician anti atlas morocco a historical review abstract exceptionally preserved fossils yield crucial information about the evolution of life on earth the fezouata biota from the lower ordovician of morocco is a konservat lagerstatte of major importance and it is today considered as an ordovician burgess shale this biota was discovered only some 15 years ago but geological studies of the area date back to the beginning of the 20th century pioneering geological investigations lead to the discovery of ordovician strata in the anti atlas 1929 and ultimately resulted in their formal subdivision into four main stratigraphic units 1942 in the agdz area the presence of fossiliferous tremadocian lower ordovician strata was suspected as early as 1939 but only definitively confirmed in 1955 in the 1960s 1990s jacques destombes provided the first detailed biostratigraphic framework for the lower ordovician of the anti atlas and collected thousands of fossils that were subsequently described in a series of monographs in the early 2000s exceptionally preserved fossils were discovered in the fezouata shale tremadocian late floian in the central anti atlas by mohamed ou said ben moula this biota now known as the fezouata biota is of utmost importance for it demonstrates the extent in the fossil record of non biomineralising animals typical of the cambrian explosion into the ordovician during the great ordovician biodiversification although most components are still in need of formal descriptions a fairly good picture of the composition and organisation of this biota and how it contributes to our understanding of the early evolution of metazoan communities can now be depicted moreover recent studies have substantially clarified the biostratigraphical and palaeoenvironmental context of the fezouata shale and are now being followed up by detailed investigations of the taphonomy geochemistry and micropalaeontology of this unique konservat lagerstatte these efforts will soon greatly benefit from the recovery of fresh unweathered samples from drill cores

whole mantle p wave travel time tomography abstract a method of tomographic inversion to obtain three dimensional velocity perturbations in the earth s whole mantle has been developed and applied to more than two million p wave arrival time data reported by international seismology center isc the model is parameterized with 32 768 blocks the divisions in latitude longitude and radius are 32 64 and 16 respectively horizontal cell size is 5 6 5 6 the layer thicknesses vary with depth 29 km just below the surface and 334 km just above the core mantle boundary starting from a spherically symmetric earth model we obtained a three dimensional model using the following iterative procedures first we relocated all the events second we backprojected the residual into the whole mantle third we refined the spherically symmetric earth model the solutions have been converged in five iterations we adopted the following techniques in the backprojection procedure the first order smoothness was introduced as a damping which makes the solution independent of the starting model and its apparent fluctuation minimal the basic equations for delay times and smoothness are solved using the conjugate gradient method an iterative method which guarantees the convergence of solution into the exact least squares solution the weight on the smoothness i e the damping factor was objectively determined by a simplified cross validation technique the final solution was obtained as an average of the ten solutions each of which was derived from one tenth of the total data set the reliability of the solution is examined in two ways 1 mapping the resolution given by the reconstruction of checkerboard patterns and 2 mapping the variance given from the gaussian noise input this is the first result which delineates the whole mantle structure with its reliability mapping the longest wavelength anomalies of the lower mantle are similar to those of previous studies by dziewonski 1984 and hager and clayton 1989 however the shorter wavelength patterns have many discrepancies the fast anomalies of the uppermost mantle beneath the shield and the slow anomalies beneath the active tectonic regions are clearly seen we also revealed the l 2 pattern at the transition zone reported by the previous surface wave and free oscillation studies the most striking feature is that of the fast anomalies lying along the extension of the large subducting plate up to depths of about 1000 km or more the resolving powers in these regions are generally good

wind driven currents on the west florida shelf abstract three weeks of current meter wind and sea level data off cedar key florida are analyzed currents and sea level are found to be coherent with alongshore wind stress in the synoptic band 0 05 0 25 cycle per day and to lag it by approximately half a day little coherence is found with cross shelf wind stress at the inshore mooring 22 m depth currents are nearly barotropic for these winter 1978 data a linear parameterization of bottom stress in the barotropic alongshore current leads to a bottom friction parameter r of 0 01 0 02 cm s 1 using coastal wind stress no significant steady alongshore slope is found during this short interval the dominant momentum balance in the alongshore direction is between wind and bottom stress the offshore frictional length scale csanady 1978 is estimated to be 75 100 km which implies a seaward extent to a depth of about 30 m at the offshore mooring 44 m depth there is vertical shear between the currents at 9 and 39 m the upper cross shelf compon

random inhomogeneities in the northern izu bonin arc estimated by tomographic inversion of peak delay times of s wave seismograms 1 random inhomogeneity in the lithosphere scatters seismic waves at high frequencies 1 hz the accumulated scattering effect of s waves can be measured from the peak delay time defined as the time lag from the s wave onset to the maximal amplitude arrival this study estimated the spatial distribution of the power spectral density function psdf of random velocity inhomogeneities in the northern izu bonin arc by inversion analysis of the peak delay times assuming von karman type spectrum peak delay times were measured at plural frequency bands 4 8 8 16 and 16 32 hz by using waveform data recorded by ocean bottom seismographs the inversion result at 0 30 km depth shows strong inhomogeneities beneath the quaternary volcanoes aoga shima sumisu jima and tori shima at 30 70 km depth strongly inhomogeneous regions were imaged beneath the quaternary volcanoes and weak inhomogeneities were imaged on the fore arc side the psdf beneath the quaternary volcanoes was estimated as p m 0 029 m 3 7 km3 at 0 5 m 50 km 1 where m is the wave number km 1 the psdf on the fore arc side is p m 0 0023 m 4 4 km3 the spectral roll off is significantly different between these two regions indicating that differences in random inhomogeneities become pronounced at smaller spatial scales petrological studies of the izu bonin mariana arc have suggested that magma and restite transport occurs across the moho we speculate that small magma inclusions or restite may account for the strong inhomogeneities beneath the quaternary volcanoes and that the stochastic characteristics of their spatial distribution have changed the spectral roll off there

settling strategy of stalked echinoderms from the kaili biota middle cambrian guizhou province south china abstract a total of 124 articulated echinoderms including sinoeocrinus lui n 30 s globus n 92 one indeterminate gogiid and one indeterminate edrioasteroid from the kaili formation middle cambrian eastern guizhou china was examined among them 73 of gogiid echinoderms are preserved attached to skeletal substrates suitable skeletal substrates for gogiid holdfasts in the kaili biota are organophosphatic brachiopods large trilobite fragments including cranidia free cheeks thoracic segments and pygidia hyoliths and scenella shells the high frequency of s globus attached to organophosphatic brachiopods is due to the high richness of organophosphatic brachiopods rather than a host specific association in the kaili biota however the possible discrimination against attachment to small trilobites such as pagetia requires further investigation based on 11 hand samples echinoderm population density epd varies from 0 3 to 4 5 echinoderms per 50 grind points 0 06 to 0 96 echinoderms per cm 2 depending on the percent coverage of skeletal debris crowding effect is evident when epd is approximately 0 32 echinoderms per cm 2 and the skeletal coverage is less than 20

subsurface jets in the northwestern gulf of mexico abstract subsurface jets defined as having velocity maxima 40 cm s 1 at depths between 100 and 350 m and being surrounded by much weaker near surface currents have been observed over the northwestern gulf of mexico continental slope the observations were from an array of 14 moorings equipped with upward looking 75 khz adcps deployed at 450 500 m a total of 10 jet events were observed in 18 adcp years of velocity profile data where these events were clearly not the result of downward propagating inertial internal waves the jets had durations from about 1 to 8 days and were usually associated with interactions between similarly sized cyclones and anticyclones over the slope or with the interaction of an eddy with upper slope topography the jets are associated with potential vorticity anomalies and their inferred length scales indicate that the dynamics depart from simple geostrophic balances observed anomalous density gradients present during the jets seem to involve the tilting of the vertical ax

seismic imaging of a possible paleoarc in the izu bonin intraoceanic arc and its implications for arc evolution processes crustal evolution processes in intraoceanic arcs including crustal accretion and rifting have been long discussed to examine crustal evolution in the izu bonin intraoceanic island arc we conducted an active source wide angle seismic study along a north south profile 500 km long within a possible paleoarc in the rear arc i e the nishi shichito ridge about 150 km west of the present day volcanic front in this study the seismic velocity and reflectivity images are obtained using the wide angle seismic data for the seismic velocity imaging we applied refraction tomography in which 93 535 picks were used the overall root mean square rms misfit calculated from the initial model of the refraction tomography was 483 1 ms and those calculated from the final model were reduced to 66 7 ms the resultant seismic image shows marked variations of crustal thickness along the seismic profile thin crust 10 15 km thick in the northern part three discrete thick crustal segments 20 25 km thick in the central part and a moderately thick crust 15 km thick in the southern part these variations are mainly attributed to thickness variations of the middle crust having seismic velocity of 6 0 6 8 km s this variation of crustal thickness does not correlate with seafloor topography which is characterized by post miocene across arc seamount chains it does correlate well with crustal variations observed along the present day volcanic front of the izu bonin arc these findings suggest that the magmatic activity that created the across arc seamount chains had little effect on the rear arc crust and that the main part of the rear arc crust was created before the rear arc separated from the volcanic front by correlating the structural variations along the rear arc i e the variation of the average seismic velocity as well as the thickness of the middle crust and those along the present day volcanic front we found that the direction of rifting to separate the rear arc paleoarc from the present day volcanic front was north northeast

palaeoecology and evolution of marine hard substrate communities abstract marine organisms have occupied hard substrates since the archaean shells rocks wood and sedimentary hardgrounds offer relatively stable habitats compared to unconsolidated sediments but the plants and animals which inhabit them must develop means to gain and defend this premium attachment space hard substrate communities are formed by organisms with a variety of strategies for adhering to and or excavating the substrates they inhabit while mobile grazers organically attached and even soft bodied organisms may leave evidence of their former presence in ancient hard substrate communities a superior fossil record is left by sessile encrusters with mineralised skeletons and by borers which leave trace fossils furthermore encrusters and borers are preserved in situ retaining their spatial relationships to one another and to the substrate spatial competition ecological succession oriented growth and differential utilisation of exposed vs hidden substrate surfaces can all be observed or inferred hard substrate communities are thus excellent systems with which to study community evolution over hundreds of millions of years here we review the research on modern and ancient hard substrate communities and point to some changes that have affected them over geological time scales such changes include a general increase in bioerosion of hard substrates particularly carbonate surfaces through the phanerozoic this is at least in part analogous to the infaunalisation trends seen in soft substrate communities encrusting forms show an increase in skeletalisation from the palaeozoic into the mesozoic and cenozoic which may be a response to increasing levels of predation hard substrate communities considering borers and encrusters together show a rough increase in tiering through the phanerozoic which again parallels trends seen in soft substrate communities this extensive review of the literature on living and fossil hard substrate organisms shows that many opportunities remain for large scale studies of trends through time at the community and clade levels palaeontologists will especially benefit by closer integration of their work with that of neontologists particularly in aspects of ecology such as larval recruitment competition and succession

lateral variation in crustal structure along the lesser antilles arc from petrology of crustal xenoliths and seismic receiver functions abstract we reconstruct crustal structure along the lesser antilles island arc using an inversion approach combining constraints from petrology of magmatic crustal xenoliths and seismic receiver functions xenoliths show considerable island to island variation in xenolith petrology from plagioclase free ultramafic lithologies to gabbros and gabbronorites with variable proportions of amphibole indicative of changing magma differentiation depths xenoliths represent predominantly cumulate compositions with equilibration depths in the range 5 to 40 km we use xenolith mineral modes and compositions to calculate seismic velocities v p v s and density at the estimated equilibration depths we create a five layer model of crustal structure for testing against receiver functions rf from island seismic stations along the arc lowermost layer 5 comprises peridotite with physical characteristics of mantle xenoliths from grenada uppermost layer 1 consists of 5 km of volcaniclastics and sediments whose physical properties are determined via a grid inversion routine the three middle layers 2 to 4 comprise igneous arc crust with compositions corresponding to the xenoliths sampled at each island by inversion we obtain a petrological best fit for the rf on each island to establish the nature and thicknesses of layers 2 to 4 along the arc we see variations in the depth and strength of both moho and mid crustal discontinuity mcd on length scales of tens of km moho depths vary from 25 to 37 km mcd from 11 and 32 km the moho is the dominant discontinuity beneath some islands st kitts guadeloupe martinique grenada whereas the mcd dominates beneath others saba st eustatius along arc variability in mcd depth and strength is consistent with variation in estimated magmatic h2o contents and differentiations depths that in turn influence xenolith lithologies a striking feature is steep along arc gradients in v p similar to those observed at other oceanic arcs these gradients reflect abrupt changes in rates and processes of magma generation in the underlying crust and mantle we find no evidence for large interconnected bodies of partial melt beneath the lesser antilles instead the crustal velocity structure is consistent with magma differentiation in vertically extensive crystal mush dominated reservoirs along arc variation in crustal structure may reflect heterogeneous upwelling within the mantle wedge itself driven by variation in slab derived h2o fluxes

systematic variation in the depths of slabs beneath arc volcanoes summary the depths to the tops of the zones of intermediate depth seismicity beneath arc volcanoes are determined using the hypocentral locations of engdahl et al these depths are constant to within a few kilometres within individual arc segments but differ by tens of kilometres from one arc segment to another the range in depths is from 65 km to 130 km inconsistent with the common belief that the volcanoes directly overlie the places where the slabs reach a critical depth that is roughly constant for all arcs the depth to the top of the intermediate depth seismicity beneath volcanoes correlates neither with age of the descending ocean floor nor with the thermal parameter of the slab this depth does however exhibit an inverse correlation with the descent speed of the subducting plate which is the controlling factor both for the thermal structure of the wedge of mantle above the slab and for the temperature at the top of the slab we interpret this result as indicating that the location of arc volcanoes is controlled by a process that depends critically upon the temperature at the top of the slab or in the wedge of mantle immediately below the volcanic arc

taphonomy palaeoecological implications and colouration of cambrian gogiid echinoderms from guizhou province china based on rich material 381 specimens examined from two cambrian echinoderm faunas the early cambrian balang fauna and middle cambrian kaili fauna in guizhou province south china the taphonomy of gogiid echinoderms is described in detail and the preservation of stereomic microstructure and organic remains of cambrian gogiid echinoderms is reported here for the first time taphonomic considerations include entombment patterns decay sequences individual specific diagenetic histories unusual burial postures selective disarticulation patterns and postmortem elongation in particular five categories of gogiid entombment patterns are proposed to describe the multi directional orientations recorded at the burial time of articulated gogiids gogiid bearing slabs of guizhou material primarily 70 display the type 2 entombment pattern articulated gogiids preserved with fan shaped brachioles thus most guizhou gogiids were buried with brachioles preserved in feeding posture during obrution events balang gogiid faunas contain the oldest evidence of palaeoecological interactions among echinoderms and other indigenous taxa in addition to pre burial and post burial decay other potential causes for unusual disarticulation patterns exhibited by the gogiids from the lower cambrian balang formation include pre burial bio disturbance and post burial bioturbation based on ichnogenera including rusophycus and planolites chemical analyses reveal that carbon calcium manganese and iron are the major elements responsible for the variety of colours exhibited by guizhou gogiids three dimensional stereomic microstructure mean stereom pore size 8 4 8 7 m average trabecular thickness 4 5 4 6 m occurs on the external surfaces of thecal plates in two gogiid species stereom preservation in calcite suggests that the dissolution of calcareous echinoderm plates yielding characteristic mouldic preservation is sub recent after lithificaiton and exposure of gogiid bearing marine sedimentary successions on or near the land surface

new seismological constraints on growth of continental crust in the izu bonin intra oceanic arc the process by which continental crust has formed is not well understood though such crust mostly forms at convergent plate margins today it is thus imperative to study modern intra oceanic arcs such as those common in the western pacific ocean new seismic studies along the representative izu bonin intra oceanic arc provide unique along strike images of arc crust and uppermost mantle to complement earlier cross arc lithospheric profiles these reveal two scales 1000 10 km scale of variations one at the scale of the izu versus bonin thick versus thin arc crust the other at the intervolcano 50 km scale these images show that 1 the bulk composition of the izu bonin arc crust is more mafic than typical continental crust 2 the middle crust with seismic velocities similar to continental crust is predominantly beneath basaltic arc volcanoes 3 the bulk composition beneath basaltic volcanoes changes little at thick and thin arc segments and 4 a process to return lower crustal components to the mantle such as delamination is required for an arc crust to evolve into continental crust continued thickening of the izu bonin crust accompanied by delamination of lowermost crust can yield velocity structure of typical continental crust

heterogeneity within the subducting pacific slab beneath the izu bonin mariana arc evidence from tomography using 3d ray tracing inversion techniques distinct zones of seismic heterogeneity along the izu bonin mariana arc have been investigated using complimenting regional bulk sound shear wave speed and p wave tomographic images the distribution of seismic anomalies and the inferred geometry of the subducting pacific plate have been modelled in unprecedented detail using joint tomography and new p wave model using a 3d inversion algorithm the use of 3d ray tracing techniques and smaller cell parameterizations have greatly enhanced the resolution of gradients therefore the models show much more detail about the structure and physical properties of the subduction zone the well defined features from the multiple wave speed images are used to elucidate the distinct morphology change between slab beneath the izu bonin horizontal slab and mariana vertical arcs and the distribution of physical properties in the mantle and the subducting oceanic lithosphere changes in physical properties within the slab tear at the southern end of the izu bonin arc identified as a bgap or thinningq in the tomographic images could be the result of the distortion of the pacific plate as its shape transforms between near horizontal to near vertical a decrease in the rigidity and strength of the lithosphere the subduction of the marcus necker ridge change in subduction velocity or a combination of all these factors strong slow anomalies exist both in the mantle wedge above and below the slab beneath the izu bonin arc but are not present beneath the mariana arc at the junction between the izu bonin and mariana arcs it appears that the there are two separate pieces of the pacific plate the torn slab north of 268 n and a buckled near vertically dipping slab south of the tear the transition between the two distinct slab morphologies coincides with the location of the ogasawara plateau and the trench d 2005 elsevier b v all rights reserved

tonga ridge and lau basin crustal structure from seismic refraction data 1 the crustal structure across the tonga lau arc back arc system from the lau ridge to the pacific plate 178 170 w 18 19 s is modeled using data from an 840 km long air gun refraction line over 19 ocean bottom seismometers and one land station the data reveal that the pacific plate crust is 5 5 km thick with a velocity structure similar to that found at the present day east pacific rise epr beneath tonga ridge an intermediate velocity layer 6 7 km s is up to 7 5 km thick and has a velocity depth distribution similar to andesitic rocks found in continental crust the crust is abnormally thin 4 km at the boundary between the tonga ridge and the lau basin at the east end of lau basin the crust is 5 5 6 5 km thick and resembles crust formed at the epr except for a thicker sheeted dike section 2 3 km and thinner lower crust 2 km the lau basin crust thickens to 7 8 km near the central lau spreading center clsc mostly through thickening of the lower crust the crust thickens again to 8 5 9 5 km at 50 km west of the clsc mostly through thickening of the midcrust in the thick westernmost section the crustal structure is uniform even though one part of this section formed through extension of arc type crust while the rest was created at an oceanic spreading center the relative homogeneity of these rocks suggests that their petrology may be dominated by postemplacement magmatic infilling from a mantle source west of the spreading center

rise of echinoderms in the paleozoic evolutionary fauna significance of paleoenvironmental controls echinoderms were a major enduring component of paleozoic marine faunas beginning with the rapid diversification of crinoids and other classes in the early ordovician this diversification was triggered by increased availability of habitable areas accompanying sea level rise and changed sedimentation styles newly collected early ordovician echinoderms facilitate determination in detail of life modes and important adaptations during this transition interval substrate consistency and stability were crucial spatial limiting factors that resulted in heterogeneous distribution patterns for echinoderm classes crinoids were overwhelmingly large food groove suspension feeders with relatively open filtration fans attached by small discs to hard ground surfaces in shallow water they were preadapted for this life mode rhombiferans were mobile small food groove suspension feeders they lived on soft substrates at various depths several important crinoid clades underwent a second phase of diversification during the middle ordovician by developing holdfasts adapted to soft substrates and densely pinnulate arms with small food grooves leading to dominance of this class among paleozoic echinoderms our findings are in basic agreement with the overall onshore diversification pattern of most paleozoic benthic invertebrates but we argue for an extrinsic environmental control substrate availability for the observed echinoderm distribution

three dimensional p and s wave velocity structures beneath the japan islands obtained by high density seismic stations by seismic tomography abstract we construct fine scale 3d p and s wave velocity structures of the crust and upper mantle beneath the whole japan islands with a unified resolution where the pacific pac and philippine sea phs plates subduct beneath the eurasian eur plate we can detect the low velocity low v oceanic crust of the pac and phs plates at their uppermost part beneath almost all the japan islands the depth limit of the imaged oceanic crust varies with the regions high vp vs zones are widely distributed in the lower crust especially beneath the volcanic front and the high strain rate zones are located at the edge of the extremely high vp vs zone however vp vs at the top of the mantle wedge is not so high beneath northern japan we can image the high v subducting pac plate using the tomographic method without any assumption of velocity discontinuities we also imaged the heterogeneous structure in the pac plate such as the low v zone considered as the old seamount or the highly seismic zone within the double seismic zone where the seismic fault ruptured by the earthquake connects the upper and lower layer of the double seismic zone beneath central japan thrust type small repeating earthquakes occur at the boundary between the eur and phs plates and are located at the upper part of the low v layer that is considered to be the oceanic crust of the phs plate in addition to the low v oceanic crust the subducting high v pac plate is clearly imaged to depths of approximately 250 km and the subducting high v phs zone to depths of approximately 180 km is considered to be the phs plate beneath southwestern japan the iso depth lines of the moho discontinuity in the phs plate derived by the receiver function method divide the upper low v layer and lower high v layer of our model at depths of 30 50 km beneath kyushu the steeply subducting phs plate is clearly imaged to depths of approximately 250 km with high velocities the high vp vs zone is considered as the lower crust of the eur plate or the oceanic crust of the phs plate at depths of 25 35 km and the partially serpentinized mantle wedge of the eur plate at depths of 30 45 km beneath southwestern japan the deep low frequency nonvolcanic tremors occur at all parts of the high vp vs zone within the zone the seaward side and the landward side where the phs plate encounters the mantle wedge of the eur plate we prove that we can objectively obtain the fine scale 3d structure with simple constraints such as only 1d initial velocity model with no velocity discontinuity

silicic magmas in the izu bonin oceanic arc and implications for crustal evolution abundant rhyolitehaseruptedfromthe izu bonin marianavolca nic arc ibm arc from its earliest stage eocene to the present geochemically three types of quaternary rhyolites exist in the izu bonin arcfront and theyare closely related tovolcanotypeand crus tal structure the dominantly basaltic islands of the volcanic frontproduce small volumes of rhyolites that we call r1 the submarinecalderas of the volcanic front erupt mostly rhyolite that we call r2 seamounts knolls and pillow ridges in thebackarcextensionalzoneare mostly basaltic but also contain rhyolites that we call r3 thethickest total crust and the thickest intermediate compositionmiddle crust occurs below the dominantly basaltic volcanoes whereas the intermediate composition middle crust tends to be thin nerbeneaththesubmarinecalderas r1rhyolitesmaybederivedfromquaternary andesitic sources whereas r2 and r3 rhyolites may bederivedfromoligoceneones thehighercao al

the role of abiotic factors in the cambrian substrate revolution a review from the benthic community replacements of west gondwana abstract the cambrian substrate revolution refers to a substantial and rapid change to the nature of marine sedimentary substrates in the early cambrian and is widely interpreted as a biologically driven event a direct response to evolutionary innovations in metazoan burrowing and the development of new shelly faunas however abiotic factors such as tectonic and climatic evolution also had the potential to restructure cambrian substrates and are here shown to be more plausible drivers of change in the benthic faunas of western gondwana the western mediterranean region underwent a southward drift during cambrian times which drove a switch from subtropical carbonates to temperate siliciclastic substrates with short term episodes of temperate carbonate productivity as a result microbial and shelly carbonates disappeared diachronously in a stepwise manner across the lower middle cambrian boundary interval archaeocyathan microbial reefs were replaced by chancelloriid eocrinoid spiculate sponge meadows in which the stepwise immigration of new echinoderm taxa was primarily controlled by extensional tectonic events first recorded in rifting settings and later in passive margin platforms availability of new kinds of substrate was thus the primary factor that controlled where and when evolutionary innovations in benthic strategies arose examples of this include the early cambrian colonization of phosphatic hardgrounds and thrombolite crusts by chancelloriids archaeocyathan and spiculate sponges and the exploitation by benthos to the increasingly widespread availability of shelly grounds and carbonate firmgrounds by early diagenetic cementation a microbial mat epifaunal antagonistic relationship is demonstrated for echinoderm pelmatozoans based on the non overlapping palaeogeographic distributions of microbial reefs and mats versus mud sticker pelmatozoans cambrian benthic communities thus evolved in parallel with substrates in response to abiotic factors rather than being the primary drivers of substrate change

structure and composition of the aleutian island arc and implications for continental crustal growth we present results of a seismic reflection and refraction investigation of the aleutian island arc designed to test the hypothesis that volcanic arcs constitute the building blocks of continental crust the aleutian arc has the requisite thickness 30 km to build continental crust but it differs strongly from continental crust in its composition and reflectivity structure seismic velocities and the compositions of erupted lavas suggest that the aleutian crust has a mafic bulk composition in contrast to the andesitic bulk composition of continents the silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the aleutian intraoceanic arc therefore if island arcs form a significant source of continental crust the bulk properties of arc crust must be substantially modified during or after accretion to a continental margin the pervasive deformation intracrustal melting and delamination of mafic to ultramafic residuum necessary to transform arc crust into mature continental crust probably occur during arc continent collision or through subsequent establishment of a continental arc the volume of crust created along the arc exceeds that estimated by previous workers by about a factor of two

seismic attenuation tomography of the mariana subduction system implications for thermal structure volatile distribution and slow spreading dynamics 1 the anelastic structure of a subduction zone can place first order constraints on variations in temperature and volatile content we investigate seismic attenuation across the western pacific mariana subduction system using data from the 2003 2004 mariana subduction factory imaging experiment this 11 month experiment consisted of 20 broadband stations deployed on the arc islands and 58 semibroadband ocean bottom seismographs deployed across the fore arc island arc and back arc spreading center we compute amplitude spectra for p and s arrivals from local earthquakes and invert for the path averaged attenuation for each waveform along with the seismic moment and corner frequency for each earthquake additionally we investigate earthquake source parameter assumptions and frequency dependent exponents ranging from 0 to 0 6 tomographic inversion of nearly 3000 t estimates at 0 27 for 2 d qp 1 and qp qs structure shows a 75 km wide columnar shaped high attenuation anomaly with qp 43 60 beneath the spreading center that extends from the uppermost mantle to 100 km depth a weaker high attenuation region qp 56 70 occurs at depths of 50 100 km beneath the volcanic arc and the high attenuation regions are connected at depths of 75 125 km the subducting pacific plate is characterized by low attenuation at depths greater than 100 km but high attenuation is found in the plate between 50 and 100 km depth the fore arc shows high attenuation near the volcanic arc and beneath the serpentinite seamounts in the outer fore arc qs structure is less well resolved than qp because of a smaller data set but qp qs ratios are significantly less than 2 throughout the study region as temperatures estimated from qs 1 are unusually high we interpret the arc and wedge core anomalies as regions of high temperature with enhanced q 1 due to hydration and or melt the slab and fore arc anomalies as indicative of slab derived fluids and or large scale serpentinization and the columnar shaped high qp 1 anomaly directly beneath the back arc spreading center as indicative of a narrow region of dynamic upwelling and melt production beneath the slow spreading ridge axis

palaeoecological aspects of the diversification of echinoderms in the lower ordovician of central anti atlas morocco abstract echinoderms are one of the major components of benthic faunas in the lower ordovician sequence near zagora central anti atlas morocco the fezouata shale tremadocian late floian has yielded numerous exquisitely preserved echinoderm assemblages ranging through several stratigraphic levels and palaeoenvironmental conditions these associations offer a unique opportunity to document both evolutionary and palaeoecological aspects of echinoderm diversification in high latitude siliciclastic dominated western gondwana sediments where rapid in situ burials facilitated excellent faunal census conditions lower shoreface deposits of the fezouata shale provide the most complete record of successive echinoderm faunas in late tremadocian times these relatively shallow shelf deposits show the progressive replacement of low diversity opportunistic cambrian like dwarfed communities dominated by cornute stylophorans in unhospitable dysoxic environmental conditions by higher diversity benthic assemblages more typical of the great ordovician biodiversification event and dominated by blastozoans on well oxygenated sea floors the turnover of the ordovician radiation was apparently slightly delayed in more proximal settings eocrinoid meadows persisted in shallower environmental conditions up to the middle floian in the late floian they were replaced by diploporite dominated communities typical of later ordovician high latitude peri gondwanan faunas from a palaeobiogeographic point of view low diversity assemblages display relatively strong affinities with cosmopolitan late cambrian echinoderm faunas whereas high diversity communities are dominated by peri gondwanan taxa

the cambrian substrate revolution and the early evolution of attachment in suspension feeding echinoderms abstract the cambrian characterized by the global appearance of diverse biomineralized metazoans in the fossil record for the first time represents a pivotal point in the history of life this period also documents a major change in the nature of the sea floor neoproterozoic type substrates stabilized by microbial mats were replaced by unconsolidated soft substrates with a well developed mixed layer the effect of this transition on the ecology and evolution of benthic metazoans is termed the cambrian substrate revolution csr and this is thought to have impacted greatly on early suspension feeding echinoderms in particular according to this paradigm most echinoderms rested directly on non bioturbated soft substrates as sediment attachers and stickers during the cambrian epoch 2 as the substrates became increasingly disturbed by burrowing forming a progressively thickening mixed layer echinoderms developed new strategies for attaching to firm and hard substrates to test this model we evaluated the mode of attachment of 83 cambrian suspension feeding echinoderm species attachment mode was inferred based on direct evidence in fossil specimens and interpretations of functional morphology these data were analyzed quantitatively to explore trends in attachment throughout the cambrian in contrast to previous studies the majority of cambrian clades eocrinoids helicoplacoids helicocystoids isorophid edrioasteroids and solutes are now interpreted as hard shelly substrate attachers only early edrioasteroids attached directly to firm substrates stabilized by microbially induced sedimentary structures contrary to the predictions of the csr paradigm our study demonstrates that cambrian echinoderms were morphologically pre adapted to the substrate changes that occurred during the cambrian allowing them to fully exploit the appearance of hardgrounds in the furongian we find no support for the claim that the csr explains the peak in echinoderm diversity during cambrian epochs 2 3 or that it caused the extinction of helicoplacoids

a method for estimating wind driven frictional time dependent stratified shelf and slope water flow abstract friction the alongshore pressure gradient and time dependent effects are all of lowest order importance in the dynamics of wind driven fluctuating currents and sea levels on continental shelves previous work has shown that when all these effects are included the ocean response can be described by an infinite sum of coastal trapped waves whose amplitudes satisfy a fully coupled infinite set of forced first order wave equations we present a practical method for solving this coupled set of equations for general low frequency large scale wind stress forcing as input convergence properties of the solution are examined analytically for the same accuracy more modes are required to describe alongshore currents than sea level and fewer modes are required to describe barotropic than depth dependent motion as an example numerical calculations were carried out for a model of the west florida shelf the sea level field was effectively described by one mode but the alongshore velocity field was not

the 3 d distribution of random velocity inhomogeneities in southwestern japan and the western part of the nankai subduction zone 1 seismic waves at high frequencies 1 hz show collapsed and broadened wave trains caused by multiple scattering in the lithosphere this study analyzed the envelopes of direct s waves in southwestern japan and on the western side of the nankai trough and estimated the spatial distribution of random inhomogeneities by assuming a von karman type power spectral density function psdf strongly inhomogeneous media have been mostly imaged at shallow depth 0 20 km depth in the onshore area of southwestern japan and their psdf is represented as p m 0 05m 3 7 km3 with m being the spatial wave number whereas most of the other area shows weak inhomogeneities of which psdf is p m 0 005m 4 5 km3 at hyuga nada in nankai trough there is an anomaly of inhomogeneity of which psdf is estimated as p m 0 01m 4 5 km3 this psdf has the similar spectral gradient with the weakly inhomogeneous media but has larger power spectral density than other offshore areas this anomalous region is broadly located in the subducted kyushu palau ridge which was identified by using velocity structures and bathymetry and it shows no clear correlation with the fault zones of large earthquakes in past decades these spatial correlations suggest that possible origins of inhomogeneities at hyuga nada are ancient volcanic activity in the oceanic plate or deformed structures due to the subduction of the kyushu palau ridge

deep structure of the northeastern japan arc and its relationship to seismic and volcanic activity evidence for possible deep seated magmatic activity beneath northeastern japan can be obtained from seismic observations tomographic inversions of p wave velocity data show low velocity zones distributed in the crust and upper mantle beneath active volcanoes in or around these zones anomalously low frequency micro earthquakes perhaps caused by magmatic activity are found at depths of 25 40 km distinct s wave reflectors corresponding to the upper surfaces of magma bodies are found at depths of 10 18 km large crustal earthquakes also occur around the low velocity zones these observations reveal the distribution of magma reservoirs at depths and elucidate its relation with shallow seismicity beneath volcanic arcs

along arc variation in seismic velocity structure related to variable growth of arc crust in northern izu bonin intraoceanic arc the izu bonin arc is an intraoceanic island arc where the pacific plate subducts beneath the philippine sea plate along arc variation in thickness of the arc crust has been observed by previous active seismic surveys beneath the izu bonin arc we have imaged three dimensional 3 d seismic velocity structures in the northern izu bonin arc using arrival time data of the earthquakes during 80 days of observations by ocean bottom seismographs and three permanent island seismic stations our 3 d velocity model indicates heterogeneous structure in the mantle wedge along the arc low velocity anomalies related to upwelling flow in the mantle wedge are not uniform beneath the volcanic front low velocity anomalies extending down to the subducting slab beneath the volcanic front coincide with thicker parts of the arc crust north of aoga shima and south of sumisu jima this coincidence suggests that heterogeneous mantle upwelling flow fundamentally controls the growth of the arc crust along topographic highs in the forearc low velocity anomalies in the crust and uppermost mantle coincide with positive magnetic anomalies suggesting the presence of a remnant arc providing further evidence of this view of arc crust formation

wind forced sea level variability along the west florida shelf winter 1978 abstract coastal tide gage and meteorological records from pensacola to key west for the period january april 1978 have been examined for low frequency fluctuations the dominant 6 day period signals in sea level alongshore wind stress and atmospheric pressure were coherent over the entire shelf and propagated southward consistent with the movement of cold fronts through the area sea level response tagged the local wind stress by 18 h in the north to 9 h in the south in response to a 1 dyn cm 2 alongshore stress sea level amplitudes were largest 60 cm where the shelf is widest 200 km and undergoes an abrupt bend and were 30 cm elsewhere large transient alongshore sea level slopes on the order of 10 6 were thus set up a linear steady state shelf circulation model hsueh 1980 is used to explore the sea level distribution that is in frictional equilibrium with a wind stress of given orientation for a bottom resistance coefficient of 0 014 cm s 1 a value suggested by an analysis of fe

edrioasteroids on corals taphonomic feedback and sedimentary processes control the ecology of a late ordovician katian cincinnatian richmondian community in central kentucky usa abstract communities of organisms that encrusted hardgrounds and skeletal material developed and became widespread during the great ordovician biodiversification event and this community structure was a well established part of the marine ecosystem by the late ordovician here we describe and investigate an example of skeletal encrustation from the bardstown member of the drakes formation upper ordovician katian richmondian near mount washington kentucky this occurrence exhibits three dense coral beds near its base which primarily contain the problematic colonial organism tetradium the colonial rugose coral cyathophylloides and the solitary rugose corals grewingkia and streptelasma as well as several species of stromatoporoids bryozoans brachiopods cephalopods and trilobites in one locality the corals in the uppermost bed are fragmented reoriented and encrusted demonstrating reworking and taphonomic feedback of the skeletons tetradium skeletons created hard substrate clasts that were exploited by a community of sclerobionts including stromatoporoids bryozoans and edrioasteroid echinoderms the skeletons of cyathophylloides abundant in the underlying coral beds are in life orientation and host a lower diversity of encrusters corals at other exposures of the bardstown member are not significantly reworked and do not host encrusters suggesting localization of submarine erosion here we describe the stratigraphy paleoecology and comparative taphonomy of the bardstown coral beds at two localities to assess 1 the burial and taphonomic feedback of the coral skeletons 2 the community structure and evolution attachment strategies and substrate preferences of the encrusting community and 3 the spatial patterns of submarine erosion and disturbance at the bardstown coral bed horizons

genesis modification and preservation of complex upper ordovician hardgrounds implications for sequence stratigraphy and the great ordovician biodiversification event abstract hardgrounds from the upper ordovician strata of eastern laurentia exhibit a wide range of morphologies though the development of their complex surface topographies remains poorly understood these early lithified seafloors which can be laterally very extensive may provide key information about basin dynamics and global ocean fluctuations based on spectacular exposures and newly excavated material we examined a suite of hardgrounds from the upper ordovician katian of southern ontario and the cincinnati arch region and documented the processes of genesis development and modification of simple complex and block hardgrounds in this study we used extensive field work petrology and cathodoluminescence to determine the evolution and shaping of hardgrounds as a result of intense submarine erosion and prolonged exposure of these surfaces and reveal a progression from simple to complex hardgrounds and ultimately to isolated cobble to boulder sized block hardgrounds we survey the distribution of hardgrounds in a sequence stratigraphic framework to determine allocyclic controls on hardground development highlight the important role that mixed siliciclastic carbonate environments play in promoting the development of heterogeneous hardground topographies and analyze the effects of high surface reliefs on local depositional and erosional regimes in addition we interpret the impact of increasingly abundant complex hardgrounds throughout the ordovician on hard substrate community diversification and their contribution to the great ordovician biodiversification event we also discuss time specific aspects of the late ordovician that made it a peak interval of hardground proliferation

high temperature anomalies oceanward of subducting slabs at the 410 km discontinuity abstract our p wave whole mantle tomography revealed a low velocity region oceanward of the northern honshu slab of the pacific plate at depths around the 410 km seismic discontinuity resolution tests and scrutiny of the traveltime residuals for the ray paths passing through the low velocity region indicate that this anomaly is a resolvable feature and not an artifact due to the strong slab anomalies the existence of the slow anomalies is also supported by the analysis of the p wave records from the j array a large aperture seismic array in japan for a bonin earthquake the p arrivals to northern honshu at epicentral distances of 13 20 are strongly triplicated because of the 410 km discontinuity the later arrivals along the retrograde branch where ray paths pass through the low velocity region are anomalously slow comparison of the observed and synthetic waveforms indicates not only slow anomalies but also depression of the 410 km discontinuity this depression represents the direct evidence for the low velocity zone of primarily thermal origin an excess temperature of 200 k and the associated fractional melt of less than 1 can explain both the results of the tomographic and waveform analyses