the upper ordovician black shales of southern quebec canada and their significance for naturally occurring hydrocarbons in shallow groundwater abstract shale gas exploration in the st lawrence platform of southern quebec eastern canada focussed on the upper ordovician utica shale from 2006 to 2010 during which 28 wells were drilled 18 of which were fracked the st lawrence platform is thus considered as a pristine geological domain where potential environmental effects of fracking can be evaluated relative to the natural baseline conditions of the shallow aquifers in the saint edouard area southwest of quebec city it has been shown that groundwater carries variable and locally high levels of naturally occurring dissolved hydrocarbons in which thermogenic ethane and propane can be found fifteen shallow 30 147 m wells were drilled into bedrock and sampled cores and cuttings with the purpose of characterizing the shallow bedrock in a shale gas pre development context the shallow bedrock geology is made of three upper ordovician clastic formations the lotbiniere and les fonds formations are time and facies correlative with the utica shale present at a depth of 1 5 to 2 km in this area they are dominated by calcareous black shales with minor siltstone and micrite beds the nicolet formation is the youngest unit of the area and consists of gray to dark gray shales with locally abundant thick siltstone and fine grained sandstone beds the organic matter in the lotbiniere and les fonds formations is represented by solid bitumen with subordinate liptinite algae graptolites and chitinozoans representing normal marine type ii kerogen both formations are at the post peak hydrocarbon generation as indicated by the equivalent random vitrinite reflectance of 0 94 to 1 04 rock eval data support the type ii nature of the kerogen and the late oil window maturation level hydrocarbon extracts from the three formations have yielded high to low concentrations of c1 to c6 for all units an upward decrease in total volatiles c1 c2 c3 together with an increase in the gas dryness ratio c1 c2 c3 is recorded the transitions occurring at depths shallower than 50 m where the shales are more fractured the upward increase in the gas dryness ratio results from the more significant reduction of ethane and propane concentrations compared to that of methane consistent with the dryness ratio trend the 13 c vpdb values of methane change from thermogenic values 50 for deeper samples to more biogenic negative values 2 h vsmow trend of more negative values at shallower depths is noted the 13 c vpdb and 2 h vsmow values of rock hosted methane indicate that samples at shallow depth recorded a microbial influence it is proposed that diffusion and some microbial degradation of hydrocarbons are responsible for the decrease of rock volatiles and the in situ generation of biogenic methane in the shales at shallow depths to mix with the in situ thermogenic methane the utica shale is a very good source rock with high generation potential however thermogenic volatiles can also originate from shallower units with much shorter migration pathways the mixed thermogenic and biogenic methane in the groundwater results from fracture enhanced diffusion and biodegradation of volatiles at shallow depths

palaeoclimate perturbations before the sheinwoodian glaciation a trigger for extinctiond during the ireviken event telychian to sheinwoodian conodonts pterospathodus eopennatus ssp n 1 zone through the ozarkodina sagitta rhenana superzone were studied in detail for their oxygen isotope composition the upper kockelella ranuliformis and ozarkodina sagitta rhenana zones represent the peak interval of the sheinwoodian positive 18oapatite excursion reflecting the coolest conditions with marine sea water temperatures decreased by more than 6 c in the subtropics the 18oapatite data cover pre through post ireviken event strata the study includes also material from younger levels up to the late wenlock ctenognathodus murchisoni zone however this record is incomplete with one major hiatus reflecting the regression associated with the middle silurian mulde event the stratigraphic levels of the analyzed conodont samples from estonia can be correlated with high precision to the datum points of the ireviken event strata in the lower and upper visby formations on gotland sweden inferred short term climatic changes in the upper telychian part of the studied interval may have been caused by phases of major volcanic activity along the caledonian front across the ireviken event we observe variations in oxygen isotope ratios which allow a direct comparison to the stepwise extinctions and faunal reorganisations in different groups our data suggest that faunal extinctions are connected to time intervals of warming before the establishment of more stable and cooler conditions during the main sheinwoodian glacial the most severe extinctions and faunal turnovers at datum points 2 and 4 occurred during the early warmer interval of the event after the shift into the sheinwoodian icehouse in the lower kockelella ranuliformis zone reflected by rapidly decreasing sea water temperatures faunas restabilized and reefal communities started to flourish sedimentary evidence for the glaciation is discussed for different palaeolatitudes less

forces controlling the present day state of stress in the andes the present day state of stress in the andes is expected to be controlled primarily by two different types of forces 1 the resistive force exerted on the western overriding margin of the south american plate and 2 forces that arise from the thickened crust of the andes i e the effect of topography and its compensating crustal root we have studied these forces on the basis of a model for the dynamics of the entire south american plate in which the andes are embedded in this model a given set of forces is constrained by the criterion that the net torque on the plate should vanish a thin elastic shell representation is used to calculate the intra plate stress field associated with the various force distributions we define a reference model for the present study that incorporates a uniform magnitude for the resistance associated with convergence along the western plate margin fpcr plate contact resistance and does not include the effects of topography subsequently we investigate the effect of lateral variation in the magnitude of fpcr and add the topography related forces the main results are 1 a uniform magnitude of fpcr leads to a better match with the observations than a magnitude that is a function of the dip of the lithosphere subducting below the western plate margin 2 the amount of horizontal compression across the andes found in the case that ridge push is considered to be the only force driving the south american plate is small compared to the value required to sustain the andes

fault reactivation and structural inversion backarc and intraplate compressive deformations example of the eastern sunda shelf indonesia the tectonic evolution of the eastern sunda platform in indonesia is revealed by the study of seismic profiles and comprises three general tectonic periods during the cenozoic extension with graben infill from the paleogene to the early miocene a tectonically quiet period with only broad subsidence and a middle miocene recent period of folding inversion and thrusting these structures reveal changes in the intraplate stress field which was controlled by the changes in the boundary conditions of the plates collisions on the northern and eastern sides and interplate coupling southward along the java subduction zone inversions often occurred without reactivation of pre existing normal fault planes at the upper levels of these faults the factors controlling these deformations are discussed with special interest placed on the conditions for the potential reactivation of cohesionless normal faults inversion and reactivations were mostly controlled by the orientation of the pre existing fault planes in relation with the regional stress field and by the tectonic stress regime in a strike slip regime effective principal stresses 1 and 3 horizontal with 1 2 3 the optimum conditions for reactivation in isotropic rocks are pre existing faults that strike close to the potential two vertical conjugate failure planes the same mechanical properties as for pre existing fault prevail in such conditions strike slip movements and reverse strike slip movements and associated en echelon folds were generally observed in a compressive regime 1 and 2 horizontal with 1 2 3 typical steeply dipping normal faults are difficult to reactivate by slip movements we suggest that reverse slip movements occurred in the deeper levels of faults and were accommodated by flexure above the pre existing fault planes and the inversion of grabens the strongly inverted grabens are those whose orientation is perpendicular to the regional compression nevertheless with strong regional shortening large uplifts and thrust faults were observed with reactivation in the form of reverse or reverse strike slip movements along pre existing steeply dipping normal fault planes

les crises du devonien superieur par l etude des faunes d ostracodes dans leur cadre paleogeographique abstract ostracod evolution studied at a global scale can contribute to an evaluation of the palaeobilogical crises during the late devonian the mostly pelagic entomozoidae are not affected data on benthic neritic ostracods come from the western canadian platform europe the russian platform north africa and the middle east species extinction rates vary according to provinces and areas extinction occur at the givetian frasnian lower upper frasnian and frasnian famennian boundaries only five families seem to disappear during the late devonian the appearance of new assemblages since the earlisst famennian is worldwide on platforms the thuringian ecotype ostracod assemblages underwent a major change since the early famennian it is characterized by many species with spinous carapaces and is known in the paleotethys during famennian dinantian times it is related to a glacial event in the north of the gondwana which generated cold deep water currents invading the paleotethysian basins the thuringian ecotype allows to propose a paleobathymetric model its species composition shows a double origin by evolution of early devonian roots and with lineages proceeding from circalittoral species causes of these late devonian events can be understood in a particular paleogeographic reconstruction where the paleotethys has no western faunal communication since the givetian frasnian boundary warming water associated with an oxygen deficiency eliminated several species at the givetian frasnian and lower upper frasnian boundaries a first cooling at the frasnian famennian time was sufficient to decimate species strongly adapted to the frasnian conditions the generalized regression at this time probably increased this last phenomenon

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

frasnian famennian biotic crisis undervalued tectonic control the prime cause of the late devonian kellwasser crisis culminating in a mass extinction event near the frasnian famennian f f boundary remains conjectural nevertheless rapid sea level fluctuations of uncertain origin during tectono eustatic highstand paired with repeated oceanic anoxia and climatic changes are usually thought at present to be one of the main immediate triggers the cathles hallam model of stress induced changes in plate density accompanying rapid rift formation furnishes an alternative for understanding the enigmatic sudden eustatic variations in the non glacial time late devonian tectonic extension causing rifting and volcanicity appears to be strongly marked in several regions of eurasia particularly in kazakhstan and eastern laurussia all larger devonian continents were more or less tectonically affected a subtle record of this tectonic rearrangement is implied even for distant and apparently quiet carbonate platforms in local extensional block faulting and tilting hydrothermal mineralisation geochemical anomalies and localized blooms of siliceous biota interpreting the late frasnian regressive transgressive pattern in terms of the cathles hallam tectono eustatic model two major rifting events are hypothesized one at about the beginning of the rhenana zone and a second principal pulse in the late linguiformis zone that encompassed the f f transition developing the veimarn milanovsky scenario of the global extensional pulse it is assumed that the key endogenous factors were related to episodic super plume activity the tectonically triggered changes climaxed in thermal and nutrient pulses and induced the stepdown ecosystem destabilization observed in the f f bio crisis minor cometary strike s might have eventually participated in this prolonged multicausal environmental stress mainly due to additional thermal shocks but perhaps effective on a regional scale only

factors controlling sediment yield from small intensively cultivated catchments in a temperate humid climate abstract data on sediment yield from 26 small cultivated catchments 10 10 000 ha in central belgium were collected based on sedimentation rates in small flood retention ponds area specific sediment yield varies from 0 4 to 20 6 t ha 1 year 1 for catchments ranging from 7 to 4873 ha various catchment properties were analysed in order to understand this large variation in sediment yield catchment area alone explains 64 of the observed variance in area specific sediment yield although no other parameter could explain more it is concluded that catchment area is not a valid parameter to link with sediment yield overall catchment geomorphology e g presented by the hypsometric integral is much more important in controlling sediment yield the lack of data however made this not very clear a multiple regression model including three morphological catchment properties is a valuable tool to predict total sediment yield t year 1 from catchments in central belgium this model has an efficiency of 0 81 furthermore it was concluded that the within catchment spatial distribution of parameters for instance slope steepness is very important but difficult to obtain one single representative value the use of spatially distributed erosion and sediment delivery models is therefore necessary accurate sediment yield data like the one presented in this study however are needed to validate these models

the mechanical state of the lithosphere in the altiplano puna segment of the andes abstract information about topography the shape of the geoid seismicity neogene deformation and volcanism in the region of altiplano puna of western south america is used to analyse the state of stress across the convergent plate margin in terms of the effects of topography and simple models of its compensation an average elevation near 4 km is consistent with compensation by a yet unresolved combination of crustal root and hot uppermost mantle producing a geoid high of 22 27 meters average horizontal compressive stress in excess of a reference sea level lithostatic value of 390 bars in a 150 km thick lithosphere and an average shear stress of 170 bars along a 30 dipping interplate boundary the basis for these estimates is evidence for a neutral to extensional stress regime within the high plateau contrasted with a compressional regime on the eastern slopes and along the interplate boundary itself comparison with other plateaus in a convergent plate tectonic setting suggests an evolutionary sequence from compressional to extensional tectonics as elevation of the plateau increases

carbon isotope excursion near the wenlock ludlow silurian boundary in the anglo welsh area oxygen and carbon isotope analyses have been performed at close sampling intervals on rocks from several exposures that show the boundary between the wenlock and ludlow series in the classic silurian areas of wales the welsh borderland and the west midlands of england our data indicate a monotonic decline in 13 c from positive to negative values across the boundary between the much wenlock limestone formation wenlock series and the lower elton formation ludlow series and correlatives in all sites investigated a slightly earlier negative excursion is also present in the upper wenlock strata of two of these localities both negative 13 c excursions appear to be contemporaneous with episodes of increased ocean ventilation and eustatic sea level lowstand postulated by kemp at least one and possibly both of the late wenlock carbon isotope depletions can also be related to the decline in pelagic graptolite diversity the distribution of the carbon isotope depletion in both carbonate platform and off shelf facies implies that this trend is unlikely to have been caused by local diagenetic alteration isotope chemostratigraphy may also provide an extra correlation tool that can be integrated with traditional biostratigraphic methods for silurian strata

lower and middle ordovician chitinozoans from honghuayuan south china biodiversity patterns and response to environmental changes abstract a complete chitinozoan succession through the uppermost tremadocian to lower darriwilian lower and middle ordovician at honghuayuan northern guizhou province in south china was systematically studied and 47 species of 14 genera were identified two chitinozoan biodiversity acmes were recognized which occur in the lower part of the acrograptus filiformis uppermost hunghuayuan formation and middle part of the azygograptus suecicus middle member of the meitan formation graptolite biozones in the upper dapingian and lower darriwilian uppermost part of the meitan formation chitinozoan diversity was relatively high after a less conspicuous peak in the upper dapingian comparisons between diversity trends of chitinozoans acritarchs and brachiopods and the environmental changes revealed both similarities and differences in the floian the chitinozoan biodiversity curve at honghuayuan shows good correlation with sea level changes the group temporarily disappeared when the sea level was at the highest then reappeared and reached a biodiversity acme when sea level was low furthermore a large organic carbon isotope excursion is coincident with the absence of chitinozoans and a highstand period after the sea level rise in the lower and middle floian diversity curves show fewer connections with the sea level and organic c isotope undulations during the dapingian and early darriwilian new data suggest that the environmental changes played a significant role in the early stage of ordovician chitinozoan radiation roughly corresponding to the floian at the honghuayuan section in the dapingian and early darriwilian the intrinsic factors seem to have played a more important role in shaping biodiversity patterns of chitinozoans in south china

the multiple impacts hypothesis for mass extinction a comparison of the late devonian and the late eocene abstract application of the lag time multiple impacts hypothesis poag 1997b palaios 12 582 590 poag et al 2001 columbia univ press to the late devonian leads to the prediction that the frasnian famennian pulsed extinctions were triggered by a rapid drop in global temperature that followed an impact produced anomalous warm interval which interrupted the global cooling trend from the middle devonian greenhouse to the early carboniferous icehouse in actualistic comparison with the late eocene the lag time multiple impacts hypothesis would predict that a frasnian interval of multiple impacts should have occurred between 367 7 and 366 7 ma the fact that three impacts the alamo siljan and flynn creek do occur either within this predicted interval or close to it in time is corroborative evidence that the lag time multiple impacts hypothesis may indeed provide the causal mechanism for the frasnian famennian mass extinction based on the application of the lag time multiple impacts hypothesis to the frasnian famennian mass extinction it is here suggested that future searches for evidence of impact events in the late devonian be concentrated in strata that occur in the frasnian transitans to early hassi zonal interval and not in strata immediately below or above the frasnian famennian boundary

back arc rifting in the okinawa trough abstract geological and geophysical data reveal that the okinawa trough shows incipient continental rifting and crustal separation started from about 2 ma the early extensional movements in the trough are probably of miocene age in addition to the miocene phase two main periods of extension are recognized a pleistocene phase between 1 9 and 0 5 ma and the present day phase during the stage short central rifts central grabens were formed the opening however may have occurred only in the southern part of the trough basin having an average half spreading rate of 2 cm yr 1 since early pleistocene time producing its present width of several tens of kilometres these activities were well represented by igneous intrusions sedimentary facies and sedimentary structures in and around the okinawa trough the width of the zone affected by back arc extension defined as greater okinawa trough is larger than the present okinawa trough whose width is 200 250 km the present form of the greater okinawa trough started to form at the same time as that of the okinawa trough

soil erosion in small catchments of the quercy region southwester france during the holocene erosion rates in two small catchments of the quercy region southwestern france calculated from colluvial deposits trapped in karstic depressions show a progressive increase during the holocene rates lower than 0 2 t ha 1 yr 1 have been found for the period that covers the beginning of the holocene to the early iron age the soils buried in the depressions testify to the permanence of a forest cover until the late neolithic and to its replacement by a herbaceous vegetation during protohistorical times probably as a consequence of forest clearance and increased grazing by cattle after the early iron age erosion increased significantly and the mean rate reached 0 8 t ha 1 yr 1 before mediaeval times the associated coarse grained crudely stratified colluvium is thought to reflect ploughing of the surrounding fields erosion rate increased up to c 1 3t ha 1 yr 1 during mediaeval and modem times

back arc extension in the okinawa trough the okinawa trough lying to the east of china is a back arc basin formed by extension within continental lithosphere behind the ryukyu trench arc system middle to late miocene uplift associated with normal faulting of the initially adjacent ryukyu nonvolcanic arc and the taiwan sinzi folded belt corresponds to the first rifting phase the timing of rifting is supported by the presence of marine sediments of corresponding age drilled in the northern okinawa trough the rifting occurred after a major early miocene change in the motion of the philippine plate with respect to eurasia and ceased during the pliocene a second rifting phase started about 2 m y ago at the plio pleistocene boundary and has continued until the present time it has proceeded to a more advanced stage in the middle and southern okinawa trough than it has farther north detailed bathymetric sea beam seismic reflection and magnetics data collected during the pop 1 cruise of the r v jean charcot reveal the principal features of the extensional processes the back arc spreading phase started very recently in the southern and middle okinawa trough as exemplified by several en echelon and in some cases overlapping active central graben oriented n70 e n80 e some of these depressions are intruded by volcanic ridges of fresh back arc basalt with associated large magnetic anomalies transform faults between these en echelon active rifts are not obvious we suggest that the major part of the southern okinawa trough is underlain by a thinned continental crust and that except for the system of en echelon rifts of the southern okinawa trough the back arc basin oceanic domain is limited to a width of a few tens of kilometers or less in the axial portion of the trough the system of axial back arc volcanic ridges that occur in the rifts ends at the latitude of okinawa island whereas active volcanoes in the ryukyu arc occur only north of okinawa island we refer to this transition between active arc and back arc volcanism as the volcanic arc rift migration phenomenon vamp globally back arc volcanism propagated from the southern okinawa trough to the okinawa vamp area rifting continues to occur in the northern okinawa trough but is not yet accompanied by associated volcanism the okinawa vamp area is characterized by a series of parallel basaltic ridges oriented n75 e with associated linear magnetic anomalies characteristic of dyke intrusions we suggest that the formation of the back arc oceanic domain took place along the axial back arc extensional zone trending n75 e and that this zone presently ends at the southern extremity of the active volcanic chain the initial phase of formation of back arc basin oceanic crust is non steady state and is characterized by the lack of a developed fracture zone pattern the termination of the vamp area in the direction of the volcanic zone of the arc is consistent with the suggestion of molnar and atwater that the volcanic arc is a fundamental line of weakness which determines where initial back arc spreading occurs apparently back arc extension initially occurred within the continental lithosphere located westward of the ryukyu arc along its whole length but the subsequent back arc volcanism was initiated in the southernmost portion of the region and then moved northward this migration was accompanied by the shutting down of volcanic activity along the abandoned portions of the arc it is this transfer of volcanism that we call the vamp process thus arc and back arc basin volcanism seem to be associated in such a manner that spreading tends to migrate simultaneously with a cessation in volcanic activity along the arc this interplay of arc and back arc activity is probably linked to changes in the parameters of plate convergence since the plate motion in the phillippine sea is oblique to the trench at least in the southern part of the okinawa trough we suggest that the oblique resisting force applied to the edge of the overriding plate engenders the development of en echelon extensional features within and behind the arc the motion of the ryukyu platelet with respect to eurasia is consequently an extensional strike slip motion trending roughly parallel to the okinawa trough

carbon isotope geochemistry of the frasnian famennian transition abstract carbon isotope records for inorganic and total organic carbon toc as well as for individual organic biomarkers show two positive excursions in the late frasnian with comparable shape and amplitude in 13c the positive shifts in 13c can be correlated with the deposition of the kellwasser horizons the inorganic carbon isotope excursions are explained by an enhanced organic carbon burial that is expected to lower the concentration of oceanic dissolved co2 toc 13c values do not exhibit a larger shift than that recorded by inorganic carbon 13c values this is unexpected since a decrease in the dissolved co2 concentration should result in lower carbon isotope fractionation during photosynthesis ep 13c values of presumed algal biomarkers low molecular weight n alkanes pristane phytane although offset by 2 5 exhibit essentially identical records as toc and confirm this unexpected result it is proposed that high atmospheric and oceanic co2 concentrations during the devonian resulted in maximum photosynthetic fractionation any change in the co2 concentration would thus not affect ep as such 13c of primary organic carbon and 13ccarb will exhibit parallel excursions the data imply that carbon isotopes and the relationship between ep and co2 will not be effective to study changes in pco2 levels during the devonian since co2 concentrations were too high

earliest chitinozoans discovered in the cambrian duyun fauna of china chitinozoans are an enigmatic group of organic walled marine microfossils with an abundant lower ordovician to upper devonian fossil record they achieved maximum species diversity during the middle ordovician silurian and their history during the group s acme is well documented nevertheless information about their origin and early evolution is sparse here we report three phosphatized flask shaped vesicles recovered from the cambrian stage 5 similar to 510 m y old duyun fauna of southern china as the earliest known chitinozoans eisenackitina sp extending the record of the microfossil group back by at least 20 m y their exceptional occurrence within an orsten type lagerstatte might imply a benthic mode of life this discovery is significant in that it supplies critical data for evaluation of the basic morphology of cambrian chitinozoans and assessment of their early evolution

arc stresses determined by slabs implications for mechanisms of back arc spreading there is an anti correlation in stress field between the back arc area and the shallow portion of the slab generally slabs showing down dip compression have tensional back arcs and the reverse we explain this anti correlation by a force balance in the fore arc wedge between the slab pull collision force in the upper plate and the ridge push back arc spreading can occur as a passive response to the down dip compressional slab the mariana kyushu and aegean arcs are the exceptions to the anti correlation in these arcs the back arc is tensional although the slab is in down dip tension this may be because the fore arc is driven by the mantle drag toward the trench resulting in compression balanced with the slab pull the flow in the mantle would cause back arc spreading in this case

first record of the hirnantian upper ordovician δ13c excursion in the north american midcontinent and its regional implications the most prominent of the two major global delta c 13 excursions in the ordovician the hirnantian delta c 13 excursion hice which is previously recorded from the uppermost ordovician in a few sections in nevada quebec arctic canada baltoscandia scotland and china is documented for the first time from the north american midcontinent samples through the girardeau limestone and leemon formation in missouri and illinois show elevated delta c 13 values of 4 parts per thousand to 5 parts per thousand although not determined precisely the beginning of the hice is likely to be in the upper part of the orchard creek shale and it ends in the upper leemon formation being extraordinarily useful chronostratigraphically the presence of the hice makes it possible to provide a firm dating of the study interval whose age has long been controversial comparison between the study sections and coeval hice sequences in north america and europe show striking similarities especially in sea level history indicating that major local lowstands reflect eustatic sea level changes a comparison with hirnantian diamictite successions in north and south africa and argentina suggests that these lowstands correspond to two major gondwanan glacial episodes less

the frasnian famennian boundary interval in the south polish moravian shelf basins integrated event stratigraphical approach abstract the late devonian mass extinction event near the frasnian famennian f f boundary has been analysed using conodont biostratigraphy and biofacies sedimentology magnetic susceptibility and geochemistry in reference sections of the south polish moravian shelf holy cross mountains cracow and brno areas high resolution biostratigraphic study revealed difficulties in the precise recognition of this natural stage boundary but confirmed the occurrence of a major third order sequence boundary in the f f transition in an active synsedimentary tectonic setting marked by erosional discontinuities hardgrounds and brecciation or omission surfaces conclusive evidence of an extraterrestrial impact has not been found among earth bound factors the main devastating role in the shelf habitats is ascribed to fluctuating anoxia and or nutrient dynamics in a disturbed greenhouse climatic setting the long term facies changes were determined by a conspicuous break in carbonate production accompanied by replacement of mature stromatoporoid coral reefs by pioneer shelly crinoid banks microbial mounds and localised oolitic bars the key f f passage interval was marked by intermittent but generally accelerated periplatform ooze debris input and severe storm events as well as by probably highly fluctuating oxygenation and biological overproduction best manifested in radiolarian silicisponge and cephalopod acmes eutrophication phenomena at least partly stimulated by various hydrothermal and volcanic processes were one of the major biogeochemical processes during this climax of the late devonian biotic crisis however significant oceanographic perturbations were also linked with the onset of the transgressive hypoxic upper kellwasser event still within the latest frasnian linguiformis zone i e well before the f f conodont collapse implied variations in the redox state of seawater through the kellwasser timespan probably in regionally highly variable temporal scales are in agreement with recent palaeoecological and biogeochemical inferences in particular suggesting recovering oxygenation just prior to the f f boundary in the other laurussian intrashelf basins all the data support a long time multicausal earth bound crisis instead a worldwide cosmic catastrophe

palaeoenvironmental changes at the frasnian famennian boundary in key european sections chemostratigraphic constraints abstract the study aims to put constraints on global environmental changes which took place during the frasnian famennian f f transition by using a geochemical multiproxy approach for this several key european f f boundary sections deposited in different palaeogeographic settings were selected including in addition to the stratotype and parastratotype sections of coumiac montagne noire france and steinbruch schmidt branau germany respectively also some other sections thoroughly characterized by other authors the drilling budesheimer bach prumer mulde germany kowala section holy cross mountains poland la serre france changes in the detrital flux along the sections were evaluated on the basis of both variations in mineral composition and in some diagnostic element ratios like ti al the biogenic flux was assessed as a function of the corg content the input of nutrients or the palaeoproductivity was determined by the p2o5 and ba in excess the degree of anoxia during deposition was constrained by determining the concentration of redox sensitive metals e g fe mn in the bulk sediment of specific element ratios e g v cr as well as the size of pyrite grains together with the degree of pyritization dop in the budesheimer bach core the hydrothermal and volcanic influence in the sedimentary record was detected using e g the al al fe mn and zr al2o3 ratios the kellwasser horizons are characterized by a general decrease in the detrital input together with the increase of al normalized concentration of some of the transition metals these changes are interpreted as reflecting a relative sea level rise and a higher hydrothermal influence in contrast to these the establishment of dysoxic anoxic conditions in the depositional environment as well as the increase of organic matter burial are not always restricted to the kellwasser horizons kwh moreover the hydrothermal influence can lead to misinterpretation with respect to redox conditions and productivity due to the additional input of some specific elements our results indicate the coincidence of sea level rise with hydrothermalism and volcanism during the deposition of the kwh in the area of south laurussia and north gondwana the resulting enrichment in metals and nutrients is coupled in several f f boundary sections with a higher microbial activity as already observed by some authors these effects coupled with a general lowering of the degree of oxygenation of the water bodies could have played a significant role in the demise of the biota

do all fractions of organic matter contribute equally in shale porosity a case study from upper ordovician utica shale southern quebec canada abstract in many unconventional hydrocarbon systems organic matter om substantially contributes to total porosity of the rock that led to a positive correlation between total organic carbon toc and porosity while organic porosity is an important reservoir property of unconventional hydrocarbon systems toc of these rocks consists of different macerals or petrographic fractions these toc fractions can be geochemically divided and quantified by the revised extended slow heating esh rock eval analysis this study discusses the contribution of different fractions of om on porosity variation in the mature to overmature utica shale in southern quebec using esh rock eval analysis helium pycnometery scanning electron microscopy sem and x ray diffraction xrd mineralogy selected samples from three wells at depths of 400 700 and 2000 m covering a range of thermal maturity from late peak oil window vroeqv 1 to dry gas zone vroeqv 2 1 were analyzed in this study the predominant om constituents are solid bitumen chitinozoans and minor graptolites with mean toc of 1 1 0 4 wt the mean porosity for oil and gas window samples decreases with depth from 3 9 0 9 to 2 6 0 5 respectively the sem images reveal that organic porosity is limited to pores developed in solid bitumen while regardless of thermal maturity chitinozoans have a non porous structure throughout the oil to dry gas window this observation suggests that the impact of thermal maturity on porosity is different for different om fractions i e macerals leading to different contribution of om fractions to total porosity the latter conclusion is further supported by the improved correlation observed between the solid bitumen fraction and porosity compared to toc porosity correlation for oil window samples our entire data set suggests that om i e bitumen fraction especially in the dry gas window is not the major controlling factor on the total porosity in the utica shale whereas intergranular porosity has the dominant control on total porosity this observation suggests that the hydrocarbon storage potential may largely reside in inorganic pores rather than organic pores in the analyzed intervals of the utica shale the decrease in total porosity observed from shallower samples i e oil window to deeper samples i e dry gas window also suggests that intergranular porosity is the dominant porosity component which decreases with burial

bransfield basin antarctic peninsula not a normal backarc basin the bransfield basin a marginal basin located northwest of the antarctic peninsula has been habitually considered as a backarc basin associated with the rollback process that took place along an inactive plate boundary the south shetland trench where the antarctic and the phoenix plates meet new geophysical and structural data discussed in this paper show that the basin opening is related to a sinistral simple shear couple between the scotia and the antarctic plates and not to the previously suggested rollback mechanism the widening of the bransfield basin and the lack of trench retreat are causing compression in the south shetland islands two different neotectonic stress directions with interchanged stress axes are found in the area of the south shetland block

the late alpine evolution of an extensional shear zone naxos greece the mid miocene structural evolution of the metamorphic complex on naxos involved deformation in a kilometre scale low angle ductile shear zone the structures described are consistent with exhumation of the metamorphic complex during crustal extension after and possibly during localized high temperature barrovian style metamorphism non coaxial fabrics developed throughout the structural history indicate that upper levels were transported to the north during ductile extension this shear sense is opposite that inferred previously for the development cordilleran style metamophic core complexes in the cvclades ductile deformation was accompanied during its later stages by e w directed compression

plate boundary forces at subduction zones and trench arc compression abstract viscoelastic finite element modelling has been used to study the state of stress in the overriding and subducting plates meeting at a subduction zone the subduction fault is included using the dual node technique it is demonstrated that substantial horizontal deviatoric compressive stress occurs in the trench arc region as a result of the downpull of the dense slab and the associated surface depression including the trench and other downflexing of the plates this may be masked at the trench by bending stress it is the lack of significant shearing stress along an unlocked subduction fault in the presence of this compressive stress that gives rise to the slab pull and trench suction plate boundary forces slab pull and trench suction were found to be of comparable magnitude within the range 1 0 to 4 0 10 12 n m in models studied with vertical subduction and there are indications that this may also apply when the slab dips at 45 as a result of viscous flow induced by rollback when the slab dips beneath the arc backarc region it is shown that horizontal deviatoric compression can occur in this region contemporaneous with plate interior tension produced by trench suction this suggests that backarc tension associated with marianas type trench arch systems may be related to the nearly vertical slab whereas backarc compression in the chilean type may result from the low dip and small downpull of the slab it is also shown that successive locking and unlocking of the subduction fault may give rise to large variations of stress in plate interiors

did cooling oceans trigger ordovician biodiversification evidence from conodont thermometry the ordovician period long considered a supergreenhouse state saw one of the greatest radiations of life in earth9s history previous temperature estimates of up to 70 c have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time we present a very different global climate record determined by ion microprobe oxygen isotope analyses of early ordovician silurian conodonts this record shows a steady cooling trend through the early ordovician reaching modern equatorial temperatures that were sustained throughout the middle and late ordovician this favorable climate regime implies not only that the oxygen isotopic composition of ordovician seawater was similar to that of today but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period

chitinozoan bio event in the frasnian famennian boundary beds at la serre montagne noire southern france abstract chitinozoans and other palynomorphs have been investigated in frasnian famennian boundary beds from trench c at la serre montagne noire southern france this section is located some 30 km from the coumiac quarry containing the global stratotype section and point gssp for the base of the famennian at la serre the frasnian famennian f f boundary falls within interbedded anoxic dark shales and limestones this lithology proves to be more suitable for palynological investigations than the reddish nodular limestones of the f f stratotype in the coumiac section which are virtually devoid of organic matter at la serre the uppermost meter of frasnian deposits corresponding to the upper kellwasser event level contains only a few chitinozoans but other palynomorphs especially tasmanaceans maranhites proliferate in some horizons conversely the most basal famennian bed yields an exceptional number of chitinozoans up to 19 000 specimens per gram of rock a few tracheids but virtually no tasmanaceans the absence or the extremely low numbers of miospores and acritarchs in these f f boundary beds seems related to the distal position of the deposition site we have compared the fluctuation of the chitinozoan abundance both with the abundance of the other palynomorphs and with that of the conodonts present in the same samples these data collated with sedimentological and palaeoecological information document the relationships between the anomalous abundance of the chitinozoans and the latest frasnian earliest famennian mass extinction it is concluded that the exceptional chitinozoan concentration in the most basal famennian bed at la serre is not related to a sorting process generated by turbidity currents or by a temporary increase in the hydrodynamic energy at the water sediment interface it corresponds to the conjunction of a high production of chitinozoans a very low rate of destruction of the vesicles and minimal dilution of the deposit by terrestrial mineral input the environmental factors leading to such an exceptionally high record of chitinozoans are discussed with a particular attention to consequences of the late frasnian mass extinction

a new ecological severity ranking of major phanerozoic biodiversity crises a new ecological severity ranking of the major phanerozoic biodiversity crises is proposed in which the capitanian crisis is ranked lesser than the frasnian late devonian but greater than the serpukhovian end mississippian and the famennian end devonian crisis is ranked as equal in ecological impact to the hirnantian end ordovician two new decouplings between taxonomic severity and ecological severity are revealed in these analyses the capitanian and famennian crises in which the ecological impact of the biodiversity loss was markedly different from the magnitude of the biodiversity loss these analyses also reveal that the great devonian interchange gdi invasive species event in the givetian biodiversity crisis may provide an important palaeoecological analog for the study of present day extinction and homogenization in ecosystems produced by modern invasive species

soil erosion and sediment deposition in the belgian oess belt during the holocene establishing a sediment budget for a small agricultural catchment a method to establish a holocene sediment budget for a 103 ha agricultural catchment representative for the belgian loess belt is presented soil erosion and sediment deposition were determined based on 185 coring locations and a large excavation in the valley bottom results were integrated in a gis and interpolation techniques applied to derive spatial patterns of erosion and sedimentation total soil erosion sediment deposition and sediment export were calculated and the results show that volumes are highly dependent on the interpolation technique used sediment delivery ratios between 20 and 42 are derived and are consistent with data reported in previous studies this clearly shows that the majority of the sediments produced during the holocene have been stored near their source area and have not been delivered to the downstream rivers the spatial distribution of soil erosion and sediment deposition within the catchment is strongly dependent on slope gradient and position within the catchment which suggests that since human impact began topography has been the main factor determining long term soil erosion and sedimentation

deformation and stress change associated with plate interaction at subduction zones a kinematic modelling summary the interseismic deformation associated with plate coupling at a subduction zone is commonly simulated by the steady slip model in which a reverse dip slip is imposed on the down dip extension of the locked plate interface or by the backslip model in which a normal slip is imposed on the locked plate interface it is found that these two models although totally dierent in principle produce similar patterns for the vertical deformation at a subduction zone this suggests that it is almost impossible to distinguish between these two models by analysing only the interseismic vertical deformation observed at a subduction zone the steady slip model cannot correctly predict the horizontal deformation associated with plate coupling at a subduction zone a fact that is proved by both the numerical modelling in this study and the gps global positioning system observations near the nankai trough southwest japan it is therefore inadequate to simulate the eect of the plate coupling at a subduction zone by the steady slip model it is also revealed that the unphysical assumption inherent in the backslip model of imposing a normal slip on the locked plate interface makes it impossible to predict correctly the horizontal motion of the subducted plate and the stress change within the overthrust zone associated with the plate coupling during interseismic stages if the analysis made in this work is proved to be correct some of the previous studies on interpreting the interseismic deformation observed at several subduction zones based on these two models might need substantial revision on the basis of the investigations on plate interaction at subduction zones made using the finite element method and the kinematic mechanical conditions of the plate coupling implied by the present plate tectonics a synthesized model is proposed to simulate the kinematic eect of the plate interaction during interseismic stages a numerical analysis shows that the proposed model designed to simulate the motion of a subducted slab can correctly produce the deformation and the main pattern of stress concentration associated with plate coupling at a subduction zone the validity of the synthesized model is examined and partially verified by analysing the horizontal deformation observed by gps near the nankai trough southwest japan

intra arc sedimentation in a low lying marginal arc eocene clarno formation central oregon abstract the clarno formation consists of andesitic volcaniclastic rocks interstratified with clayey paludal sediments and lava flows and cut locally by irregular hypabyssal stocks dikes and sills lateral lithofacies variations are pronounced and intrusive and extrusive volcanic rocks appear haphazardly emplaced throughout the formation a range of sedimentary environments is represented in the volcaniclastic sequences including near vent flow and breccia accumulations bouldery high gradient braided streams and relatively low gradient sandy tuff braidplains associated with paludal deposits we infer that the coarse grained volcaniclastic rocks of the clarno formation accumulated largely in volcanic flank and apron settings the formation s stratigraphy reflects an abundance of small volcanoes erupting into sedimentary lowlands and only a few scattered remnants of large central vent volcanoes are known the absence of systematic variation across a large outcrop belt argues against derivation of the succession from a line of volcanoes beyond the reaches of the present outcrop we infer that the arc was composed of small to medium size volcanoes arranged haphazardly across a broad area the sedimentary succession most probably accumulated in a series of intra arc depressions formed by diffuse block rotation driven by oblique subduction during the eocene

stresses and plate boundary forces associated with subduction plate margins primary tectonic stress in the lithosphere is predominantly caused by lateral density variations within the earth and associated topographical loading when such a stress system caused by major sublithospheric density anomalies is intersected by a weak zone which cuts across the lithosphere plate boundary forces develop and modify the plate interior stresses in this paper unite element analysis is used to model the stresses and plate boundary forces associated with subduction plate boundaries with dipping and vertical slabs extending to about 270 and 400 km depths having the subduction fault both locked and unlocked in such regions several types of horizontal deviatoric stress may occur including 1 local compression in the trench arc region caused by the dense sinking slab and the associated surface downflexure 2 plate interior tension which occurs when this compression is intersected by a weak subduction fault 3 local tension associated with thickening of the crust at the arc and elsewhere 4 local tension in the back arc region produced by the underlying low density upper mantle and 5 downbending stresses in the subducting slab thermal stresses and transmitted ridge push which are not included in the modelling here a gradient from compression in the forearc to tension in the back arc can be modelled in terms of these stresses when the fault is partially locked it is however the intersection of the local compression 1 by an unlocked or partially locked subduction fault that modifies the plate interior stresses and gives rise to the slab pull and trench suction plate boundary forces the state of stress in the interior of the overriding plate is also crucially influenced by back arc spreading where this occurs plate boundary forces have been evaluated for each of the models it is shown that slab pull and trench suction may be of comparable magnitude

establishing sediment budgets for two small lowland agricultural catchments in the uk sediment budgets have been established for two small 4 km2 lowland agricultural catchments by using 137cs measurements sediment source fingerprinting and more traditional monitoring techniques to quantify the individual components of the budget the gross and net erosion rates for the fields on the catchment slopes were estimated using 137cs measurements within selected fields which encompassed a representative range of slope angles slope lengths and land use these estimates were extrapolated over the entire catchment using a simple topographically driven soil erosion model terrain based gis tbgis superimposed on a dem to derive catchment average gross and net erosion rates suspended sediment yields were measured at the catchment outlets and sediment source fingerprinting techniques were used to establish the relative contributions from the catchment surface subsurface tile drains and eroding channel banks to the sediment yields in channel and wetland storage were quantified using both direct measurements and 137cs measurements the sediment budgets established for the catchments highlighted the importance of subsurface tile drains as a pathway for sediment transfer accounting for ca 60 and 30 of the sediment output from the two catchments erosion from channel banks contributed ca 10 and 6 of the sediment output from the two catchments although the suspended sediment yields from these catchments were considered high by uk standards ca 90 t km 2 year 1 the sediment delivery ratios ranged between 14 and 27 indicating that a major proportion of the mobilised sediment was stored within the catchments in field and field to channel storage were shown to be of similar magnitude but storage of sediment in the channel system and associated wetlands was relatively small representing 5 of the annual suspended sediment yield

rapid development and infilling of a buried gully under cropland central belgium abstract analysis of contemporary and past gully erosion and infilling processes allowed to reconstruct the long term evolution of a permanent gully system under cropland an active and a buried gully under cropland were investigated the recent sediment deposits within the active gully adjacent to the buried gully showed that the recent gully was filling in at a mean rate of 6 4 cm a 1 in the buried gully several erosion and deposition phases could be identified and the type of deposited sediments revealed a complex infilling history charcoal pottery and brick fragments of different sizes were found at all depths of the gully infilling their age indicates that the first gully incised after the midst of the 17th century most probably in the second half of the 18th century or the early 19th century gully morphology and analogy with the processes in the recent gully indicate that the buried gully filled in rapidly overall five cycles of cut and fill occurred in 350 years or less and four cycles even within little less than a few decades indicating that gully development and infilling under cropland can be very rapid processes

the nature of small scale flooding muddy floods and retention pond sedimentation in central belgium abstract this study investigates the spatial variation of small scale flooding and muddy floods in rural areas in a medium sized study area 5516 km2 and the linkage with controlling factors a questionnaire set up in central belgium indicates that 43 of the municipalities have to deal from time to time with muddy floods generated from direct runoff from arable land and 36 with flooding of permanent streams a strong relation exists between the nature of the problem and the site in relation to topography and landuse areas suffering from muddy floods have significantly steeper cultivated slopes compared to areas suffering only from small scale flooding the high spatial and temporal frequency of small scale flooding and muddy floods results in emotional and significant economic damage to private households as a control measure more than 100 retention ponds have been constructed with 50 more to be built in the near future the mean cost for the construction of a retention pond amounts to 380 000 euro these retention ponds store large quantities of sediment from runoff events and must thus be dredged regularly with costs of the order of 1 5 million euro yearly the dredged sediment volumes can be used to assess and predict sediment yield for these drainage basins values vary between 0 19 and 6 m3 ha 1 year 1 for basins ranging from 25 to 5000 ha

application of continuum models to deformation of the aleutian island arc continuum models were constructed to describe large scale deformation of the aleutian island arc over the past 5 m y these models consider the island arc as a continuum in the horizontal plane with the velocity boundary condition at the pacific edge stated as a fraction of pacific plate convergence transferred to the arc first a simple model of uniformly distributed strain is formulated to illustrate the mechanics of continuous deformation lineaments along the arc massif rotated about a vertical axis are matched by small element rotation calculated from the model however this model does not predict across arc variations in deformation and produces an unrealistic amount of crustal thickening after 5 m y a physically more meaningful model of deformation is the thin viscous sheet model based on averages of stress and rheology throughout the lithosphere the amount of motion transferred from the pacific plate to the arc is constrained by the rotated lineaments while the effective stress strain exponent n and the ability the lithosphere has to sustain crustal thickness contrasts the argand number are independent variables primarily bathymetry earthquake focal mechanisms and styles of faulting are used to evaluate the models the preferred model is one where the amount of motion transferred from the pacific plate is greater in an arc parallel direction than in an arc normal direction producing stresses consistent with strike slip faulting at the far western end of the arc and tensional stresses consistent with transverse normal faulting elsewhere in the arc massif this model agrees with observations of slip vectors by ekstr6m and engdahl 1989 who conclude that a portion of the arc parallel component of relative plate motion is taken up in the overriding plate this model implies that compressive stress transferred to the arc is small in comparison to along arc shear stress and that stresses conducive to strikeslip faulting are prevalent throughout the arc

island arc tectonics abstract tectonic stress states in arc backarc systems have been examined on simplified model structures corresponding to the chilean mariana and intermediate arc types two sources of tectonic stress are assumed the stress due to lateral density variations and the stress transmitted from the subducting oceanic plate to the forearc the average vertical stress zz of the former type has been calculated for each isostatically compensated model lithospheric plate using the difference from a reference structure from the sign of xx zz one can estimate whether the tectonic situation is extentional or compressional if we assume that xx is caused by mechanical coupling between the subducting oceanic slab and is laterally constant in one arc backarc system we can estimate at least the upper or lower boundary of the mechanical coupling for the chilean type arcs where the truly great interplate earthquakes occur and no backarc spreading takes place the transmitted average stress must be larger than 660 bar for the mariana type arcs where no large interplate earthquakes are recorded and spreading takes place in the backarc region xx should be smaller than 100 bar in any arc backarc system the most extentional feature is expected at the highest topography of the volcanic arc once spreading has started the viscosity under the backarc region drops dramatically and the spreading will continue in the backarc even if its topography is subdued in comparison with the now separated main and remnant arcs

high resolution early paleozoic ordovician silurian time scales for much of the geologic time scale resolving power appears to be limited by the duration of biostratigraphic zones and subzones yet these zones exploit the appearance and disappearance events of only a small fraction of the species they span computer algorithms can sequence an order of magnitude more events and make explicit the uncertainties that arise when calibrating the resulting time line an illustrative case history builds an ordovician and silurian time scale from the geologic record of the entire graptolite clade over 1900 species from more than 400 localities and sections worldwide the same approach can be applied to any stratigraphic interval with detailed biostratigraphic observations it provides the foundation for time scales and paleobiologic time lines using logic similar to graphic correlation optimizing algorithms search for a composite sequence of species first and last appearance events that minimizes the implied shortcomings of all the field observations the algorithms minimize the number of species coexistences that are implied but never observed and the net adjustments needed to bring local range charts into agreement with a single composite sequence of events after total section thicknesses have been rescaled to help normalize for variation in depositional rate mean stratigraphic thicknesses are used to scale the intervals between adjacent events in the composite sequence the resulting scaled composite sequence is converted to a relative geologic time scale by identifying stage and zone boundaries within the sequence of graptolite events this relative scale is in turn calibrated by dated volcanic ash beds that were incorporated in the search for the optimal sequence these dated events are also used to test for linearity of the scaled composite the final time scale has a potential resolving power of 0 02 0 1 m y more than ten times better than can be achieved by traditional zones graptolite zones vary widely in duration from as short as 0 1 m y to nearly 5 0 m y the mean duration of zones or zonal groupings calibrated here is 1 44 m y in the ordovician and 0 91 m y in the silurian the average uncertainty in locating zone boundaries in a single composite sequence is about one fourth of the mean zone duration the variance resulting from differences in time scales developed from differing numbers of field observations and in response to changes in the optimization criteria gives a very conservative measure of the overall robustness of the method these differences indicate an average uncertainty in the age of graptolite zone boundaries that is more nearly equal to the mean zone duration for zone duration the mean uncertainty amounts to about one half of the length of an average zone

the middle caradoc facies and faunal turnover in the late ordovician baltoscandian palaeobasin abstract a late ordovician episode of remarkable biotic climatic sea level and facies changes named here as the middle caradoc facies and faunal turnover took place in the baltoscandian area this paper presents an integrated overview of these changes in the critical middle caradoc interval data are given on carbonate rock composition distribution and grain size composition of the siliciclastic material and the carbon isotopic composition of whole rock carbonates in cores of estonia and sweden the middle caradoc facies and faunal turnover can be described as a succession of related environmental changes the turnover began with a positive excursion in carbonate 13c and continued with sea level changes that led to a sedimentary hiatus on the shelf and a change from carbonate dominated to siliciclastic dominated sedimentation in the basin the turnover ended with an extinction event and associated microfaunal crisis the middle caradoc turnover in baltoscandia is comparable to a similar succession of changes in north america the turnover affected two palaeocontinents and reflects a widespread possibly global environmental change onset of glaciation on gondwana and or increased orogenic activity might have initiated the changes in ocean circulation and led to the initial carbon isotope excursion the following sea level rise and faunal changes affected several different continents

establishing fine grained sediment budgets for the pang and lambourn locar catchments uk summary an integrated approach to data collection combining the use of 137cs measurements sediment source fingerprinting bed sediment surveys and conventional river monitoring has been successfully employed to establish the fine grained sediment budgets of two lowland groundwater fed catchments in the uk gross surface erosion is higher on cultivated land pang 55 263 t yr 1 or 507 t km 2 yr 1 lambourn 79 997 t yr 1 or 437 t km 2 yr 1 than on pasture pang 1960 t yr 1 or 140 t km 2 yr 1 lambourn 1425 t yr 1 or 95 t km 2 yr 1 in both study areas and a substantial proportion of the mobilized sediment is sequestered within the fields pang 28 058 t yr 1 or 228 t km 2 yr 1 lambourn 55 575 t yr 1 or 281 t km 2 yr 1 and between the individual fields and the river channel network pang 28 672 t yr 1 or 233 t km 2 yr 1 lambourn 24 782 t yr 1 or 125 t km 2 yr 1 the sediment contribution from banks and subsurface sources is relatively low and typically ca 5 t yr 1 in the pang and ca 11 t yr 1 in the lambourn representing only about 1 of the suspended sediment output from each study catchment the mean level of fine grained sediment storage in the main channel system is equivalent to 38 pang and 21 lambourn of the respective mean annual suspended sediment yields of the two catchments the estimated sediment delivery ratio for both study catchments is ca 1

strontium isotope stratigraphy potential resolution and event correlation abstract a new generation 87sr 86sr isotope curve for the phanerozoic seawater based on a compilation of 3635 stratigraphically well defined and well preserved skeletal components results in a substantially narrower band with some discrepancies if compared to the curve of burke et al 1982 which is based mostly on whole rock samples the results obtained from low mg calcite shells foraminifers belemnites brachiopods and oysters are mostly superior to those from phosphatic skeletal components such as conodonts even if the latter are exceptionally well preserved cai 1 5 at this stage the phanerozoic data set based on fossils is patchy for the cambrian upper ordovician lower devonian and upper carboniferous triassic intervals the major practical constraint that precludes delineation of an unequivocal sr isotope curve for the phanerozoic seawater is the uncertainty in the assignment of absolute ages to the samples as a result the curve is in reality a band this is a reflection of the limitations of biostratigraphy and geochronology that are inherited by any derivative correlation and dating technique including isotope stratigraphy it is therefore unlikely that in the near future the sr isotope stratigraphy will surpass the resolution capability of biostratigraphy as a global correlation tool on the other hand if complemented by lithostratigraphy it is potentially a valuable tool for correlation of sequences on intrabasinal and regional scales taking into account the above limitations careful consideration must be given to assignment of higher order isotope peaks to specific events within a biozone and to the use of such events as a global correlation tool

a seismic stratigraphic analysis of mariana forearc basin evolution new seismic data collected in the 14 5 18 5 n mariana segment of the izu bonin mariana island arc system image six seismic stratigraphic sequences that can be mapped throughout the inner forearc these sediments were most likely deposited from 35 ma to the present the oldest stratigraphic units 1 2 and 3 are syn rift volcaniclastic deposits unit 4 deposits accumulated during a period of mild structural inversion which resulted in several isolated reverse faulted anticlines within the forearc sedimentary basin a late period of extensional deformation began near the end of unit 5 deposition and continued through unit 6 sedimentation to the present seismic lines show that the basement of the forearc is composed of large rotated fault blocks and half grabens with ne nw and nne trends fault offset calculations show that basement faults with dips between 45 and 50 account for only 4 total extension in the forearc south of 16 3 n normal growth faults initiated during basement extension offset the frontal arc high from a deep forearc basin from correlations with the known geologic history we hypothesize that extension during deposition of units 1 through 3 corresponds to rifting of the eo oligocene arc between 35 ma and 29 ma older deposits being too thin to be seismically resolvable localized compression during unit 4 accumulation occurred some time after eo oligocene rifting in the early miocene late stage normal faulting near the end of deposition of unit 5 and throughout unit 6 accumulation may be associated with the opening of the mariana trough backarc basin from 8 ma years to the present there is a higher density of these later faults in the inner forearc between 15 5 and 17 n and in the outer forearc between 14 n and 18 n recent extension is at least partially accommodated by reactivation of older basement faults with the same ne nw and nne trends stratigraphic relationships indicate that the inner forearc south of 16 3 n has differentially subsided and tilted trenchward possibly as a result of a recent change in subducting slab geometry or subducted relief under the forearc

where and why do large shallow intraslab earthquakes occur we try to find how often and in what regions large earthquakes m 7 0 occur within the shallow portion 20 60 km depth of a subducting slab searching for events in published individual studies and the harvard university centroid moment tensor catalogue we find twenty such events in e hokkaido kyushu sw japan s mariana manila sumatra vanuatu n chile c peru el salvador mexico n cascadia and alaska slab stresses revealed from the mechanism solutions of these large intraslab events and nearby smaller events are almost always down dip tensional except for e hokkaido manila and sumatra the upper plate shows horizontal stress gradient in the arc perpendicular direction we infer that shear tractions are operating at the base of the upper plate in this direction to produce the observed gradient and compression in the outer fore arc balancing the down dip tensional stress of the slab this tectonic situation in the subduction zone might be realized as part of the convection system with some conditions as shown by previous numerical simulations 2003 elsevier b v all rights reserved

heavy metal contamination in overbank sediments of the geul river east belgium its relation to former pb zn mining activities overbank sediments of the geul river east belgium are highly contaminated by the heavy metals pb zn and cd due to former pb zn mining activities in the drainage basin geochemical variations in vertical overbank sediment profiles sampled 1 km north of the mine tailings of plombieres allow metal fluxes back to the 17th century to be reconstructed the vertical profiles are subdivided into three major units corresponding to different industrial periods based on sedimentological criteria as well as on the distribution of contaminants alluvial sediments with the highest heavy metal concentrations correspond to the major period of mining activity of the 19th century the fact that zn mining at the la calamine open mine started before large scale mining of the pbs zns subsurface exploitations is reflected in the vertical profiles by an increase in zn content before a marked increase in pb and cu the regional extent of contamination in the alluvial deposits was evaluated on the basis of the geochemical analysis of sediments at depths of the 0 20 cm and 80 100 cm most of the upper samples are extremely contaminated significant local variations in heavy metal concentration in the lower samples are interpreted in terms of which overbank sediment horizon has been sampled at a depth of 80 100 cm this indicates that blind sampling of overbank sediments to characterize the degree of contamination in shallow boreholes can give very erratic results

early silurian glaciations in brazil abstract the opening of the brazilian paleozoic basins during the late ordovician was followed by a tectonic stabilization in the silurian at the time of the early silurian glaciations deposition of continental sandstones prevailed in the basins offshore environments were present in the amazonas basin and to a smaller extent in the parana basin the ice sheets advanced over fluviatile and littoral sediments except for the south margin of the amazonas basin where the glacial diamictites immediately overlie marine facies the ice produced much meltwater even when advancing and this was probably climatically controlled the ice movements were from the northwest at the northern margin of the amazonas basin and towards the northeast at the southern margin in the parnaiba basin and its outliers the jatoba basin and cariri valley the ice came from southeast and probably also from east further to the south in the parana basin it came from the north and northeast four glaciations have been dated to the early silurian the oldest is dated as latest ashgill and or earliest llandovery in the parana basin as evidenced from tillites in the upper rio ivai and lower vila maria formations including the former iapo formation it is contemporaneous with a glaciation in the cape basin south africa the next glaciation is dated as early middle llandovery gregarius graptolite zone and is present in the amazonas basin and the andean belt and probably also in the parnaiba basin and its outliers the two youngest tillites were widespread in the northern brazilian basins amazonas parnaiba and jatoba basins and cariri valley as well as in the andean belt and are dated as early late llandovery possibly starting already in the late middle llandovery and latest llandovery to earliest wenlock the latter tillite is the most widespread one assuming these glaciations are related to polar positions the apparent migration of the south pole ice cap must have been quite rapid from central north africa in late ashgill towards central brazil in the middle late llandovery the glaciation in the parana basin took place up to 45 from the late ashgillian south pole the cool climate continued during the silurian but no new glaciations are known before latest devonian

mountain building enhanced continental weathering and organic carbon burial as major causes for climatic cooling at the frasnian famennian boundary c 376 ma the late devonian was a period of drastic environmental changes as exemplified by a major biotic crisis at the frasnian famennian boundary ffb and the onset in famennian times of glaciations across southern gondwana worldwide evidence for the coeval development of the major acadian eovariscan belt led us to propose a model relating the late frasnian famennian environmental perturbations to extensive continental uplift through two atmospheric co2 depleting mechanisms 1 the intensification of silicate weathering on the continental areas as attested by a major rise in the 87 sr 86 sr composition of sea water at the ffb and 2 the massive burial of organic carbon kellwasser events in partially confined basins due to the collisional induced reduction of equatorial oceanic communications between the palaeotethysian and panthalassic oceans this process is also suggested to have been controlled by an important primary productivity connected to an increased nutrient availability triggered by the enhanced continental run off

epipelagic chitinozoan biotopes map a steep latitudinal temperature gradient for earliest late ordovician seas implications for a cooling late ordovician climate abstract the early mid ordovician has long been considered a super greenhouse world based largely on high relative global sea levels and light stable oxygen isotope data from bulk carbonates an alternative and largely untested hypothesis has suggested that at least in equatorial palaeolatitudes there was a steady cooling trend through the early ordovician reaching the range of modern equatorial sea surface temperatures by the mid ordovician this hypothesis though controversial is supported by palaeobiogeographical studies of the early late ordovician sandbian zooplankton graptolite biotopes these are comparable to the modern planktonic foraminifera provinces and suggest a latitudinal temperature gradient that is similar to the modern one chitinozoans are also an important group of palaeozoic marine microfossils but with a poorly known biological affinity here we present an analysis of chitinozoan species palaeobiogeography for the early late ordovician sandbian c 460 ma which confirms that these microfossils and likely their parent organisms were epipelagic unlike the graptolites chitinozoans had their highest diversity and abundance south of 35 s during this time which strongly suggests adaptation to temperate to cold waters the distribution of chitinozoan biotopes during the gracilis time slice allows us to identify the austral subtropical subpolar and polar belts key oceanographic boundaries are identified as the subtropical subpolar transition at 35 s and tentatively the polar front at 55 70 s again these are in a similar position to those in the modern southern ocean chitinozoan biotopes also map a steep latitudinal faunal gradient that is comparable to that of the graptolites and that of modern plankton this likely indicates a steep latitudinal temperature gradient for early late ordovician seas and questions the notion that this interval of earth history had a greenhouse climate

middle and upper ordovician carbon isotope chemostratigraphy in baltoscandia a correlation standard and clues to environmental history abstract the baltoscandian epeiric sea was characterised by relatively slow but continuous carbonate deposition from the middle ordovician up to the silurian relatively good preservation of ordovician and silurian carbonate rocks in baltoscandia makes this region unique for chemostratigraphical and palaeoenvironmental studies previous palaeozoic carbon isotope studies have revealed isotopic events in different lithologies over the baltoscandian palaeobasin and some of them on different palaeocontinents we propose a composite carbon isotope curve for the baltoscandian region a new subdivision of the middle and upper ordovician succession into seventeen chemostratigraphic zones and an improved nomenclature of isotopic excursions based on analyses of 13 c carb data from 25 drillcore and outcrop sections of estonia latvia and sweden the composite carbon isotope curve is compared with biostratigraphical data on conodont and chitinozoan distribution the katian hirnantian interval with highly fluctuating isotopic values shows climatic cyclicity of about two million years in baltoscandia no abrupt changes occur in the dapingian sandbian part of the baltoscandian isotope curve but the mid darriwilian excursion nearly coincides with the onset of cooler temperatures similar to modern like equatorial sea surface temperatures and related global biodiversification increase suggesting a climatic background for this event

regional compensation of subducted lithosphere effects on geoid gravity and topography from a preliminary model abstract it is proposed that the anomalous mass of subducted lithospheric slabs is partially compensated by two mechanisms a by stresses transmitted up the slab to the surface lithosphere causing the trench and b by stresses transmitted through the mantle wedge overlying the slab which cause a broad shallow depression results calculated from a preliminary two dimensional model show that it can give a first order account of observed topography gravity and geoid anomalies in particular the model can reconcile the large mass anomaly predicted for the slab by thermal conduction theory with the smaller than expected gravity and geoid anomalies preliminary results indicate substantial compensation by mechanism a implying rather large stresses at the top of the slab these could be important for theories of earthquake mechanisms and plate driving mechanisms roughly 50 of the possible compensation by mechanism b is indicated the resulting depression the order of 500 m deep and over 500 km wide could explain anomalously deep marginal basins and some marine transgressions of continents

okinawa trough genesis structure and evolution of a backarc basin developed in a continent abstract the okinawa marginal basin was opened by crustal extension into the asian continent north of the taiwan collision zone it is located behind the ryukyu trench subduction zone and the ryukyu active volcanic arc if we except the andaman sea the okinawa trough is the only example of marginal backarc basin type opened into a continent at an early stage of evolution active rifting and spreading can be observed synthesis of siesmic reflection seismic refraction drilling dredging and geological field data has resulted in interpretative geological cross sections and a structural map of the ryukyu okinawa area the main conclusions of the reconstruction of this backarc basin volcanic arc evolution are 1 backarc rifting was initiated in the volcanic arc and propagated along it during the neogene it is still active at both ends of the basin remnants of volcanic arc are found on the continental side of the basin 2 there was synchronism between opening and subsidence of the okinawa trough and tilting and subsidence of the forearc terrace the late miocene erosional surface is now 4000 m below sea level in the forearc terrace above the trench slope retreat and subsidence of the ryukyu trench line relative to the asian continental plate could be one of the causes of tilting of the forearc and extension in the backarc area 3 a major phase of crustal spreading occurred in pliocene times 1 9 my ago in the south and central okinawa trough 4 en echelon rifting and spreading structures of the central axes of the okinawa trough are oblique to the general trend of the arc and trench the ryukyu arc sub plate cannot be considered as a rigid plate rotation of 45 to 50 of the southern ryukyu arc since the late miocene is inferred the timing and kinematic evolution of the taiwan collision and the south okinawa trough opening suggest a connection between these two events the indentation process due to the collision of the north luzon arc with the china margin could have provoked lateral extrusion clockwise rotation 45 to 50 according to palaeomagnetic data and buckling of the south ryukyu non volcanic arc tension in the weak crustal zone constituted by the south ryukyu volcanic arc and opening of the south okinawa trough similar lateral extrusions rotations buckling and tensional gaps have been observed in indentation experiments additional phenomena such as thermal convection retreating trench model or anchored slab model could maintain extension in the backarc basin such a hypothetical collision lateral backarc opening model could explain the initiation of opening of backarc basins such as the mariana trough bonin trough parece vela shikoku basin and sea of japan a new late cenozoic palaeogeographic evolution model of the philippine sea plate and surrounding areas is proposed

new insights into the chemical composition of chitinozoans the chemical structure of isolated chitinozoan specimens extracted from early silurian marine rocks has been investigated by micro ftir laser micro raman spectroscopy and laser micropyrolysis gas chromatography mass spectrometry lmpy gc ms the samples were subjected to relatively low thermal stress as attested by several indices lmpy gc ms and micro ftir indicate that chitinozoans consist mostly of a kerogen network dominated by aromatic groups with a low contribution of aliphatics and oxygen or nitrogen bearing compounds no chitin related product was found in the pyrolysate of the chitinozoans confirming previous findings on the occurrence of this biopolymer in the fossil record

alluvial and colluvial sediment storage in the geul river catchment the netherlands combining field and modelling data to construct a late holocene sediment budget abstract we used a combined approach of a two dimensional erosion and hillslope sediment delivery model watem sedem and detailed geomorphological reconstructions to quantify the different components in a sediment budget for the geul river catchment southern netherlands since the high middle ages hillslope erosion and colluvium deposition were calculated using the model while floodplain storage was estimated using field data our results show that more than 80 of the total sediment production in the catchment has been stored as colluvium mostly generated by hillslope erosion while almost 13 is stored in the floodplain since the high middle ages this situation resembles a capacity limited system model results for the period prior to the high middle ages with a nearly completely forested catchment show that far less sediment was generated and that most of the sediments were directly transported to the main river valleys or out of the catchment a supply limited system geomorphological analysis of a large alluvial fan shows the sensitivity of the study area to changes in the percentage of arable land our combined field data modeling study presents an elegant method to calculate a catchment sediment budget for a longer period and is able to identify and quantify the most important sediment storage elements furthermore it provides a valuable tool to calculate a sediment budget while only limited dated fluvial sediment sequences are available

early diversification of ordovician chitinozoans on baltica new data from the jagala waterfall section northern estonia abstract chitinozoans are a group of biostratigraphically valuable microfossils which appeared in the tremadocian and diversified during the rest of the great ordovician biodiversification event to become extinct in the devonian the early diversification of chitinozoans on the baltica palaeocontinent has remained poorly known due to preservation and collecting bias in this study we document the distribution of chitinozoans through the upper tremadocian to lower darriwilian strata in the jagala waterfall section from northern estonia in order to better understand the regional diversity patterns and biogeographic links fifty samples were studied from the 7 5 m succession of sandstones marls and carbonates revealing 47 chitinozoan species and 11 genera the lowermost productive samples attributed to the late tremadocian represent the earliest rich chitinozoan fauna from baltica combined with previous reports this shows a diversity peak in the tremadocian with balanced total diversity reaching 17 possibly followed by a decline in the floian and then a rather gradual increase through the dapingian and early darriwilian the new diversity curve shows more than three times higher values for the tremadocian and dapingian than indicated in previous studies and thus less abrupt radiation in the darriwilian the jagala section provided also new data on the regional biozonal species conochitina cucumis and cyathochitina regnelli which characterize the middle and upper volkhov and lower kunda regional stages respectively several other biostratigraphically significant taxa typical of other regions were recovered from the jagala section including conochitina decipiens and desmochitina bulla a new short ranging species laufeldochitina toilaensis sp nov is introduced having potential to become an index species for middle dapingian strata the early darriwilian assemblage from jagala includes rhabdochitina sp a with up to 2 7 mm long vesicle making it the largest chitinozoan ever reported

first documentation of the ordovician guttenberg δ13c excursion gice in asia chemostratigraphy of the pagoda and yanwashan formations in southeastern china the only published vc data from the ordovician of china are from the lower and upper ordovician and only the latter records include a significant excursion namely the hirnantian excursion hice our recent chemostratigraphic work on the upper ordovician sandbian katian pagoda and yanwashan formations at several localities oil the yangtze platform and chiangnan jiangnan slope belt has resulted in the recognition of a positive delta c 13 excursion that has values of similar to 1 5 parts per thousand above baseline values this excursion starts a few metres above a stratigraphic interval with b alobatus subzone conodonts as well as graptolites of the n gracilis zone the distinctive conodonts amorphognathus aff am ventilatus and hamarodus europaeus first occur at or very near the excursion interval because these conodonts appear in the stratigraphic interval of the guttenberg vc excursion gice in estonia we identify the chinese excursion as the gice this is the first record of the gice in the entire asian continent it confirms that gice is a global excursion and provides an illustration of how vc chemostratigraphy combined with new biostratigraphic data solves the problem of the previously controversial age of the pagoda formation and how this classical stratigraphic unit correlates with the baltoscandian and north american successions

the onset of the ordovician plankton revolution in the late cambrian abstract the great ordovician biodiversification event comprises the rapid diversification of marine organisms during the ordovician period it is now clear that this adaptive radiation started for some organisms already in the cambrian and continued for others beyond the end of the ordovician making the great ordovician biodiversification event part of a long term late proterozoic and early palaeozoic radiation that in part is expressed by the fossil record as the cambrian explosion a significant diversification of different groups of the plankton is observed in the late cambrian early ordovician interval leading to the subsequent ordovician plankton revolution the possible causes of this plankton revolution are currently debated they include changes in palaeoclimate palaeogeography or tectonic and volcanic activity as well as a modified nutrient supply in this context the steptoean positive carbon isotope excursion 13 c carb spice event in the late cambrian paibian stage furongian series has been related to a major increase in atmospheric o 2 from 10 18 to some 20 29 and to increased oceanic nutrient availability here we analyze the diversification of the planktonic groups during the late cambrian and early ordovician in particular in relation to the spice event our analyses include the changing diversities of the phytoplankton acritarchs diverse groups of zooplankton e g radiolarians graptolites chitinozoans and the switch to a planktonic mode of life of fossil groups e g arthropods molluscs that were part of the cambrian benthos and that later occupied pelagic niches in addition we focus also on data indicating evidence for a late cambrian to ordovician origin of planktotrophy in invertebrate larvae it can be concluded that none of the diversifications of the different planktonic organisms can be related directly to the spice event however a long term 10 20 million years oxygenation pulse related to the spice event might have fuelled the explosion of phytoplankton diversity observed in the latest cambrian early ordovician that led to completely modified trophic structures permitting an increase in diversity and abundance of plankton feeding groups during the ordovician

subduction of the lithosphere and back arc dynamics insights from physical modeling physical modeling of the subduction of an elastico plastic lithosphere underlained by a low viscosity asthenosphere and driven by both horizontal compressional force and gravitational sinking of the subducting plate has shown the subduction regime to be strongly affected by 1 a where a and 1 are the asthenosphere and lithosphere densities at 0 a steady state process develops which requires a horizontal lithosphere compression of h 0 3 0 4 s 108 pa where s is the lithosphere yield limit the central part of the leading wedge of the overriding plate is nonisostatically elevated in this case especially in the region of the frontal arc which corresponds to a free air gravity anomaly maximum of 200 400 mgal a decrease in 0 compression of the overriding plate is considerably less from the onset of subduction and then decreases during subduction at some point compression changes into tension tensional nonhydrostatic stresses grow during the process to a value near h 0 1 0 2 s 0 4 108 pa both subsidence of the overriding wedge below the isostatic equilibrium level and an increase in the benioif dip angle accompany this process the gravity anomaly maximum is displaced into the volcanic arc in this subduction regime where it is normally less than 150 mgal if some weaknesses are present in the overriding plate tensional stresses result in a failure of the lithosphere at these locations and the broken lithosphere segment the arc plate starts to move counter to the subduction plate the arc plate slides along the underthrusting plate s surface without loss of contact and causes back arc opening hydrostatic suction prevents the plates from being separated this effect is caused by the peculiarity of a mechanical system whereby a low viscosity asthenosphere and a plastic lithosphere which effective yield limit is considerably lower than the hydrostatic stresses in the plate if the overriding lithosphere is not weakened enough to fail under the existing tension then the hanging slab breaks down under its own weight when it reaches a specific length both interacting plates undergo isostatic recovery in this case and the whole cycle is repeated again the hydrostatic suction effect also provides back arc tension under other conditions such as when asthenospheric flow forces the subducted plate to move away from the island arc

earthquakes along the ryukyu kyushu arc strain segmentation lateral compression and the thermomechanical state of the plate interface we systematically determined the focal depths and mechanisms of 49 large to moderate sized earthquakes mb 5 4 that occurred along the ryukyu kyushu arc since 1963 by inverting the waveforms and amplitudes of p and sh wave trains at teleseismic distances the results are sufficiently precise to delineate seismogenic structures near the plate interface in detail revealing features not predicted by plate kinematics in contrast to previous studies of this arc and those along other subduction zones shallow seismicity along the plate interface is systematic showing two distinct layers of activities the second layer of seismicity is delineated by a few earthquakes that occurred at depths between 50 and 65 km some 10 20 km directly beneath the seismogenic portion of the interplate thrust zone these earthquakes indicate lateral compressional strain within the subducted slab as their p axes are subparallel to the local strike of the arc not perpendicular to the arc as one would expect from the direction of plate convergence the occurrence of these events cannot be accounted for by membrane stress due to the geometry of the subducted slab to our knowledge similar earthquakes occurred only beneath the northern indoburman ranges and along the banda arc where subducted slabs as part of the indian plate are being dragged northward with their northern edges bumping into east west trending collision zones nearby by drawing an analogy between the tectonic settings of these three regions we interpret events beneath the plate interface along the ryukyu kyushu arc as a consequence of ongoing collision between the philippine sea plate and eurasia near taiwan the interplate thrust zone is largely aseismic down to a depth of approximately 30 km a large number of earthquakes showing low angle thrust faulting commence at this depth and are accompanied by two events that show antithetic thrust faulting at a slightly shallower depth of 20 25 km these observations suggest that the strength of the plate interface increases significantly below the depth of proximately 25 5 km based upon available heat flow measurements between the trench axis and the volcanic arc we estimated the temperature field and magnitude of shear traction along the interplate thrust zone the deepest interplate earthquakes 40 50 km correspond to temperatures of approximately 730 980 c comparable to the limiting temperature of intraplate mantle earthquakes 800 c and that of intermediate and deep focus earthquakes at wadati benioff zones potential temperature 900 k therefore the cessation of seismicity in general is probably controlled by similar temperature conditions regardless of the tectonic settings of source regions the magnitude of shear traction along the seismogenic portion 40 50 km depth of the interplate thrust zone seems to be of the order of 100 mpa which in turn implies a low average coefficient of friction of only 0 10 0 05 considerably less than those observed for laboratory specimens the inception of intermediate focus earthquakes within the subducted slab is at a depth of about 100 km whereas shallow earthquakes show no apparent variations along the entire arc intermediate depth earthquakes indicate downdip extension along the northern end of the arc near kyushu but abruptly change to downdip compression along the rest of the arc all available evidence indicates that this is a spatial pattern unrelated to earthquake cycles at the moment tectonic interpretation of such a distinct strain segmentation along the arc is enigmatic without the presence of clear discontinuities in the age of the subducted slab the sudden switch in the strain field of the slab is difficult to explain by the continuously varying rate of subduction along the arc in any case this pattern of strain segmentation requires the presence of a major accommodation structure such as a tear fault in the subducted slab near the tokara channel appendix is available with entire article on microfiche order from american geophysical union 2000 florida ave n w washington dc 20009 document b91 004 2 50 payment must accompany order

geochemical and petrographic characterization of the upper ordovician utica shale southern quebec canada abstract the utica shale is a potential unconventional tight gas reservoir from southern quebec the utica shale is a calcareous shale with intervals of fine grained siltstone with total organic content of 1 2 wt median n 408 the major organic matter constituents are migrabitumen and zooclasts chitinozoan and graptolite macerals the primary kerogen is characterized as marine due to the presence of chitinozoan and marine derived prasinophyte algae such as tasmanites a significant portion of the bulk organic matter is dominated by migrabitumen the present hydrocarbon potential s2 mg hc g rock of the utica shale is largely controlled by distribution of solid migrabitumen throughout the unit hydrogen index hi declines with increasing maturity which is controlled by greater burial from shallow to deep sections oxygen index oi remains low in most samples which is attributed to lack of terrestrial input however organic lean intervals show significantly inflated oi due to interference of mineral carbon possibly siderite random reflectance measurements on matrix solid bitumen and chitinozoans are robust indicators of thermal maturity the maturity ranges from the late oil window in the shallow utica to the wet and dry gas windows in the intermediate and deep utica respectively the results suggest that matrix bitumen formed during migration and dissemination of hydrocarbon into the clay fraction of the rock this is associated with significant bacterial sulfate reduction possibly in the early stages of generation and migration of oil it appears that the porous siltstone facies in the deeply buried section of the utica shale acts as a reservoir for the migrated bitumen from organic rich strata within the section and or possibly overlying source rock

palynofacies around the frasnian famennian boundary in the holy cross mountains southern poland abstract palynofacies have been analysed in two representative frasnian famennian f f localities kowala quarry and agow plucki trench in the holy cross mountains poland with the aim of reconstructing sedimentary conditions and palynofloral history over the extinction boundary interval leiospheres are the most common palynomorphs taxonomically impoverished acritarch assemblages and a low proportion of land derived material indicate offshore sedimentation in a mid shelf environment with some evidence of an earliest famennian regression increased productivity of phytoplankton near the f f boundary was possibly related to eutrophication of the epeiric seas thus causing anoxic conditions close to and on the seafloor no significant changes in palynofacies have been observed through the f f boundary deposits in these laurussian shelf basins in agreement with data from other parts of the devonian world

variation of downdip limit of the seismogenic zone near the japanese islands implications for the serpentinization mechanism of the forearc mantle wedge abstract it has been proposed that the downdip limit of interplate thrust type earthquakes in subduction zones dlt is determined either by the brittle ductile transition around 350 450 c of the crustal material at the plate interface or the moho of the forearc of the upper plate the latter is due to possible serpentinization of the forearc mantle wedge above the subducting plate which makes the thrust zone slide in a stable manner i examine this criterion in subduction zones near japan where the pacific and philippine sea plates are subducting i show that in most of n honshu where the old pacific plate is subducting and in kanto and s kyushu where the izu bonin and kyushu palau ridges of the philippine sea plate are subducting the dlt is located at a depth of 60 70 km which is deeper than the moho of the upper plate this shows that the criterion does not hold and the mantle wedge is not likely to be serpentinized in these regions in contrast in the region off iwate in n honshu and in bonin where the old pacific plate is subducting the dlt is located at a depth of 20 30 km around the moho of the upper plate suggesting that the mantle wedge is serpentinized in these regions in sw japan where the young shikoku basin is subducting the dlt is around 30 km in depth which coincides with the moho but also with the 350 c temperature at the plate interface although seismic tomography indicates that the mantle wedge is serpentinized the dlt here does not help to examine the criterion assuming that hydration of the mantle wedge is conducted through hydro fracturing by water released from the dehydrating subducted crust i propose that the stress state of the wedge would determine the extent of serpentinization of the mantle wedge in the reverse fault type stress regime of n honshu kanto and s kyushu hydro fracturing is not favored and fractures in the thrust zone associated with repeated interplate earthquakes provide conduits for water to flow in the updip direction resulting in no serpentinization of the mantle wedge no serpentinization of the mantle wedge in turn causes interplate earthquakes in contrast in the normal fault type stress regime of the region off iwate and bonin and the strike slip fault type stress regime of sw japan hydro fracturing is favored resulting in serpentinization of the mantle wedge the frictional property at the thrust zone is governed by stable sliding resulting in no interplate earthquakes deeper than the moho thus the stress regime of the mantle wedge would determine bifurcation whether it is serpentinized or not and then the depth of dlt

ground truthing late ordovician climate models using the paleobiogeography of graptolites 1 the 18o records obtained from conodonts suggest that during the mid ordovician equatorial temperatures stabilized at close to the present day a hypothesis broadly supported by published climate models however the degree to which equatorial temperatures represent global climate state and varied between different climatic modes greenhouse icehouse interglacial glacial and to what extent ordovician 18o and climate models truly reflect the global climate remain to be tested here we present paleobiogeographical data for the planktonic graptolites from the gracilis time slice i e early sandbian stage that just postdates the postulated onset of climate stabilization twinspan analysis and constrained seriation provide robust ecological groupings in paleobiogeographical presence absence data the highest level groups reflect tropical subtropical assemblages and a high southerly latitude assemblage constrained seriation defines latitude distinct biotopes that are considered equivalent to modern zooplanktonic provinces the distribution pattern of graptolite biotopes in the gracilis time slice show 1 that models explaining local graptolite ecological assemblages using lateral differentiation e g upwelling are to be favored above those using depth stratification and 2 a steep faunal gradient from equator to pole which is typical of a cool nongreenhouse world and comparable to the modern situation we therefore broadly support the climate stabilization hypothesis paradoxically the climate of the early late ordovician appears similar to that of the present day despite the higher atmospheric pco2 levels graptolite species were indicative of temperature controlled ocean water masses in much the same way as the modern zooplankton

chitinozoan biostratigraphy of the silurian wenlock ludlow boundary succession of the long mountain powys wales systematic collecting through the upper wenlock upper homerian and lower ludlow gorstian and lowermost ludfordian silurian rock succession of the long mountain powys wales identifies some 48 chitinozoan species that distinguish four biozones two subzones and an interregnum consideration of the chitinozoan biozones together with those of the graptolites enables a local three fold subdivision of the late homerian lundgreni graptolite biozone and the distinction of lower and upper intervals for the gorstian incipiens graptolite biozone the base of the ludlow series in the long mountain more or less equates to the base of the cingulochitina acme chitinozoan biozone although no key chitinozoan first or last appearance datums are associated with the series boundary itself the new graptolite chitinozoan biozonation allows enhanced correlation between upper wenlock and lower ludlow sedimentary deposits of the lower palaeozoic welsh depositional basin and those of the palaeo shelf in the stratotype wenlock and ludlow areas of shropshire chitinozoans seem affected by the phenomena that caused the late wenlock mulde extinction in graptolites but with the final disappearance of 9 species and re appearance of 11 species following an interval of overall low diversity they seem to have suffered less severely than their macro zooplanktonic contemporaries

possible effects of collisions on plate motions abstract a large scale collision at a plate boundary is expected to play an important role not only in the deformation at the boundary but also in the motion of the plate carrying the buoyant material to be accreted possible changes in rates and directions of such motions may be calculated provided that certain assumptions are made about the nature of the driving forces in this model we shall assume basically that 1 1 an oceanic plate is driven by slab pull and ridge push being resisted by basal asthenospheric drag and slab resistance and 2 2 because of detachment slab pull is lost upon collision if however the calculated motion following collision has a convergent component at the boundary a new subduction zone with an increasing slab pull force forms seaward of the accreting buoyant material calculations were first made on an idealized planar octagonal plate results indicated that so long as the scale of collision is limited changes in motion take place but the motion returns to the initial state however for large scale collisions the plate motion suffers a large directional change which also changes the nature of some boundaries and hence the motion approaches a new steady state motion irreversibly calculations were then conducted on an idealized philippine sea plate on a spherical earth for a sizable collision along the taiwan philippine mindanao boundary this boundary and perhaps the bonin mariana boundary may become a transform fault provided that slab pull ridge push is the driving force these results provide some support to uyeda mccabe model 1983 of episodic spreading of the philippine sea basins as well as the predominance of strike slip motion along collision boundaries finally in agreement with prior studies subduction of a ridge system assumed to have been located along the southwestern rim of the paleo pacific plate was shown to be consistent with the change in direction of motion of that plate at 43 m y b p calculations conducted in this study show that predicted changes are model dependent so that comparisons with observed or inferred motions may be useful as a means of estimating the relative importance of the various potential driving forces

the ordovician chitinozoan biodiversification and its leading factors abstract regional and global dataset now available for almost all of the ordovician fossil groups have led to a search for the causes controlling the major biodiversification events that occurred during the ordovician period a review of the physico chemical state of the ordovician world is presented with an emphasis on the regional or global changes contemporaneous with the origination and extinction events that could have influenced the chitinozoan diversity this study focuses on the chitinozoans because these enigmatic organic walled microfossils are an important component of the ordovician palaeoplankton and provide one of the best documented dataset the intrinsic factors that initiated the ordovician biodiversification of this group are not discussed because the chitinozoans are regarded as reproduction stages of cryptic chitinozoan animals whose biological characteristics are speculative at best this study has two main goals i the evaluation of the impact of regional and global physico chemical events on chitinozoan diversity ii the comparison of the biodiversification patterns of the chitinozoans with other selected benthic and pelagic ordovician fossil groups the chitinozoan diversification was progressive and showed similar patterns in laurentia baltica and northern gondwana from the tremadocian to the late darriwilian when the group reached its acme in baltica and northern gondwana from the middle upper ordovician boundary onward the biodiversification pattern documented in laurentia diverged drastically from the two other regions in the late ordovician the contribution of the laurentian chitinozoans to the global curve was high suggesting major faunal inputs from the two other regions which significantly lowered the endemic character prevalent in the early middle ordovician in the late ordovician large anti clockwise oceanic currents developed as the result of a thermohaline circulation this marine circulation was likely to have been driven by a global cooling concomitant with a major palaeogeographic reorganisation of the southern hemisphere these oceanic climatic changes intervened in the breakdown of the existing chitinozoan endemism globally the chitinozoan biodiversity was not much higher in the mid late ordovician than in the late darriwilian diversity peak the most obvious feature is the progressive decrease in diversity during the late ordovician long before the hirnantian glaciation the influences of cosmic and volcanic parameters are excluded as they appear to have had too low impact on the late ordovician decrease of chitinozoan diversity correlation is noticed between some diversification events and sea level changes at least on a regional scale more globally however a climatic control is favoured a durable greenhouse environment gave an efficient support to the diversification conversely the onset of an icehouse environment in the early late ordovician onward culminating with the hirnantian glaciation is interpreted as a limiting factor for the chitinozoan diversification the reasons behind the onset of this icehouse episode are not fully understood but they appear to be linked to changes in the palaeogeography e g progressive closure of the iapetus northward drift of avalonia and its docking with baltica and in the carbon cycle e g high organic carbon burial with a lowering of the p co 2

magmatic processes under arcs and formation of the volcanic front temperature structure and stress under arcs are simulated in a two dimensional cross section taking into account the flow induced by the subducting slab in the mantle wedge results of the calculations show three important features with respect to magmatic processes under arcs 1 temperature structure in the crust and the mantle wedge under arcs is insensitive to the angle and velocity of slab subduction the temperature structure of the slab and that of the back arc region this indicates that physical conditions such as temperature and pressure are similar under various arcs it is thus inferred that primary magmas generated under various arcs should have similar chemical compositions if chemical composition and the flux rate of fluid from the slab are similar and the chemical compositions of mantle wedge materials are the same 2 calculated deviatoric stress magnitude is relatively large more than a few tens of megapascals in the partially molten mantle cracks may open under high differential stress and magma can easily segregate and accumulate through interconnected cracks while the buoyancy driven compaction of partially molten mantle proceeds 3 the deviatoric stress values in the region over the partially molten mantle are relatively large and the direction of the principal stress changes horizontally the direction of the maximum compressional stress is nearly vertical under the volcanic zone and is nearly horizontal under the fore arc region it is considered that magma segregated in partially molten mantle migrates upward through the brittle mantle and crust by the magma fracturing mechanism the propagation direction of magma filled fissures is controlled by the stress field in the crust and mantle and is parallel to the maximum principal stress the calculated stress is highly compressive horizontally on the trench side while it becomes tensile on the back arc side the location of this stress transition coincides with that of the volcanic front the location of this transition indicates that the volcanic front marks a change in the ease of upward migration of the magma filled cracks under relatively high differential stress field

geochemistry of the volcano hydrothermal system of el chichon volcano chiapas mexico the 1982 eruption of el chichon volcano ejected more than 1 km3 of anhydrite bearing trachyandesite pyroclastic material to form a new 1 km wide and 300 m deep crater and uncovered the upper 500 m of an active volcano hydrothermal system instead of the weak boiling point temperature fumaroles of the former lava dome a vigorously boiling crater spring now discharges 20 kg s of cl rich 15 000 mg kg and sulphur poor 200 mg kg of so4 almost neutral ph up to 6 7 water with an isotopic composition close to that of subduction type magmatic water d 15 18o 6 5 this spring as well as numerous cl free boiling springs discharging a mixture of meteoric water with fumarolic condensates feed the crater lake which compared with values in 1983 is now much more diluted 3000 mg kg of cl vs 24 030 mg kg less acidic ph 2 6 vs 0 56 and contains much lower amounts of s 200 mg kg of so4 vs 3550 mg kg with 34s 0 5 4 2 17 in 1983 agua caliente thermal waters on the southeast slope of the volcano have an outflow rate of approximately 100 kg s of 71 c na ca cl water and are five times more concentrated than before the eruption b r molina unpublished data relative n2 ar and he gas concentrations suggest extensional tectonics for the el chichon volcanic centre the 3he 4he and 4he 20ne ratios in gases from the crater fumaroles 7 3ra 2560 and agua caliente hot springs 5 3ra 44 indicate a strong magmatic contribution however relative concentrations of reactive species are typical of equilibrium in a two phase boiling aquifer sulphur and c isotopic data indicate highly reducing conditions within the system probably associated with the presence of buried vegetation resulting from the 1982 eruption all cl rich waters at el chichon have a common source this water has the appearence of a partially matured magmatic fluid condensed magmatic vapour neutralized by interaction with fresh volcaniclastic deposits and depleted in s due to anhydrite precipitation shallow ground waters emerging around the volcano from the thick cover of fresh pumice deposits red waters are ca so4 rich and have a negative oxygen isotopic shift probably due to ongoing formation of clay at low temperatures

relationship between phosphogenesis and sequence architecture sequence stratigraphy and biostratigraphy in the middle ordovician of the armorican massif nw france abstract in the armorican massif nw france the middle ordovician formations contain abundant phosphatic p beds associated with terrigenous sediments laid down on a storm dominated shelf facies analysis and sequence stratigraphy carried out on sections located in the crozon peninsula w armorican massif allow us to distinguish four p facies 1 beds with p nodules and crusts contained within sediments laid down on the distal and median parts of the inner shelf 2 amalgamated beds with p nodules and crusts and 3 beds with lingulid brachiopods heavy minerals and p clasts associated with retrogradation phases 4 p clast bearing conglomerates associated with progradation phases the genesis of these facies and their location in the depositional sequences are controlled by the fluctuation of terrigenous inputs during sea level changes sediment volume partitioning we propose a genetic model integrating the superposition of very high frequency vhf and high frequency hf eustatic signals in this model the amalgamated beds and beds with lingulid brachiopods heavy minerals and p clasts result from the superposition of several sea level rise episodes of vhf cycles occurring within the retrogradation phase of a hf cycle the lateral extension of p facies was investigated in the eastern part of the armorican massif benioc domfront les tanneries and st aubin du cormier sections several early darriwilian transgressive events associated with phosphogenesis episodes are dated through high resolution chitinozoan studies some of these transgressive events are at least of regional scale e g ornensis and protocalix transgressive events the topmost calix transgressive event is clearly identified over the northern gondwanan and avalonian domains and may reflect a global eustatic signature

very large plant and root traces from the early to middle devonian implications for early terrestrial ecosystems and atmospheric p co2 plant and root traces from the fort prevel member of the battery point formation late early devonian emsian gaspe bay quebec canada are larger and more complex than previously postulated for land plants of this time the traces are preserved as clay and silt lined casts in or near growth position and provide evidence that early vascular land plants achieved substantial stature 2 3 m and were capable of deep rooting to nearly 1 m the root traces and alluvial deposits in which they occur suggest increased landscape stabilization and root system and paleosol morphologies that were influenced by a water stressed episodically energetic environment early devonian plants of such large stature may have been partly responsible for initiation of a steep decline in atmospheric p co 2 through organic carbon burial and accelerated terrestrial weathering

emergence biodiversification and extinction of the chitinozoan group chitinozoans are considered as reproductive bodies of marine invertebrates called chitinozoophorans these chitinozoophorans were most likely to have been small pelagic or necto pelagic soft bodied probably wormlike animals and judging from the size of chitinozoans they probably measured from a few millimetres to a few centimetres in length the chitinozoophorans most likely survived by grazing on phytoplankton there is no evidence of a large colonization of the pelagic niche in the cambrian but from the early ordovician onward this niche was exploited chiefly by graptolites and chitinozoophorans both groups inhabited nearshore and offshore habitats but in contrast to the graptolites the chitinozoans displayed their highest diversity at high latitude in less distal that is upper and lower offshore environments the chitinozoan group evolved rapidly during the ordovician and reached its maximum ordovician diversity in the late darriwilian from the first occurrence of chitinozoans in early tremadocian times to the biodiversity crisis in latest ordovician times nearly 80 of the morphological innovations took place until their extinction in the latest devonian chitinozoans survived through several biodiversity crises in the early late ordovician late hirnantian late wenlock earliest emsian and in the latest frasnian kellwasser event during the melting of the hirnantian ice sheet most ordovician genera and species became extinct but some genera extended beyond the boundary e g spinachitina belonechitina cyathochitina ancyrochitina the hirnantian glaciation was not directly responsible for the dramatic extinction of organic walled microfossils but it certainly accelerated the extinction of lineages that had already been weakened since the early to mid katian the late wenlock and earliest emsian graptolite crises affected the chitinozoophorans to a lesser degree and the latest frasnian kellwasser event did not greatly affect chitinozoophorans the disappearance of the chitinozoan group at the end of the famennian resulted from a combination of factors for example the chitinozoophorans probably no longer had the genetic potential for successful adaptations to successive drastic environmental changes only one species is known from the latest famennian their usual niche was invaded by more efficient groups and their usual food supply disappeared or was no longer sufficient the latter factor is supported by the contemporaneous decline in phytoplankton

north american plate dynamics deformation within the north american plate in response to various tectonic processes is modeled using an elastic finite element analysis a coarse grid contains 328 elements and 190 nodes while a fine grid contains 718 elements and 396 nodes for a spatial resolution of 2 3 abundant information about the present day state of intraplate stress constrains the modeling the dominant pattern that must be fit by all acceptable models is an ene trend for the maximum compressive stress for most of the plate east of the rocky mountains the tectonic processes considered in the modeling include ridge forces associated with the normal thermal evolution of oceanic lithosphere shear and normal stresses transmitted across transforms normal stresses transmitted across convergent boundaries stresses due to horizontal density contrasts within the continent and shear tractions applied along the base of the plate model stresses are calculated with respect to a lithostatic reference stress state distributed ridge forces of magnitude 2 1012 n m predict deviatoric stresses of the order of 20 40 mpa that are capable of accounting for the dominant observed ene stress trend driving drag models also fit the trend but are not preferred because of a predicted tenfold increase in compressive stress magnitudes from east to west across the plate assuming that ridge forces account for the dominant ene stress trend bounds may be placed on other possible forces for example shear stresses transmitted across transform boundaries along the san andreas and caribbean are small of the order of 5 10 mpa also compressive stresses of the order of 5 10 mpa transmitted across the major transforms improve the fit to the data compressive stresses across convergent margins along the aleutians and the middle america trench are important

human and climate impact on catchment development during the holocene geul river the netherlands interest in the holocene development of small to medium sized river catchments in western and central europe in relation to changes in land use and climate has increased over the past years in this study we reconstruct the holocene landscape development and fluvial dynamics of the geul river the netherlands and the main forcing mechanisms of environmental change field studies were carried out and we used osl and 14c dating methods to reconstruct the holocene valley development our study shows that 2 periods of deforestation during the roman period and the high middle ages led to severe soil erosion and increased floodplain sedimentation in the catchment of the geul river possibly combined with periods of increased wetness during the high middle ages alluvial fans have been active since the roman deforestation phase our results show that the geul catchment is highly sensitive to changes in land use