collapse of archean orogens and the generation of late to postkinematic granitoids intrusion of late stage granitoids into archean granite greenstone terranes may be spatially and temporally related to late orogenic upper crustal extensional collapse and formation of metamorphic core complexes the late to postkinematic granitoid magmas are derived through partial melting of upper mantle and lower crustal sources and are locally associated with late stage extensional structures thick roots of depleted lithospheric mantle of archean age tectosphere beneath archean cratons makes melting of the upper mantle difficult it is proposed that the archean tectosphere is made of imbricated slabs of buoyant archean oceanic crust and lithosphere together with trapped wedges of fertile mantle this would explain several seemingly contradictory phenomena decompression from upper crustal extension generates significant amounts of basaltic melts from the trapped wedges of fertile mantle these basaltic magmas intrude and partially melt the lower crust becoming more silicic in composition they migrate upward to solidify in the middle and upper crust forming the late to postkinematic granitoid suite the tectosphere in this model is less dense and colder than surrounding asthenosphere and is consistent with eclogitic xenoliths from subducted oceanic lithosphere found entrained in kimberlites the direction and magnitude of shear wave anisotropy in the upper mantle is also explained in this model through the orientation of olivine crystals in the underplated slabs of oceanic lithosphere

low pressure differentiation of tholeiitic lavas as recorded in segregation veins from reykjanes iceland lanzarote canary islands and masaya nicaragua segregation veins are common in lava sheets and result from internal differentiation during lava emplacement and degassing they consist of evolved liquid most likely replaced by gas filter pressing from a 50 crystallised host lava pairs of samples host lavas and associated segregation veins from the reykjanes peninsula iceland lanzarote canary islands and the masaya volcano nicaragua show extreme mineralogical and compositional variations mgo in host lava segregation veins and interstitial glass ranges from 8 10 wt 3 6 wt and to less than 0 01 wt respectively these samples allow the assessment of the internal lava flow differentiation mechanism since both the parental and derived liquid are known in addition to the last magma drops in the form of late interstitial glasses the mineralogical variation mass balance calculated from major and trace element composition and transitional metal partition between crystals and melts are all consistent with fractional crystallisation as the dominant differentiation mechanism the interstitial glasses are highly silicic sio2 70 80 wt and represent a final product of high degree 75 97 fractional crystallisation of olivine tholeiite at a pressure close to one atmosphere the tholeiitic liquid line of decent and the composition of the residual melts are governed by the k2o na2o of the initial basaltic magma the granitic minimum is reached if the initial liquid has a high k2o na2o whereas trondhjemitic composition is the final product of magma with low initial k2o na2o

effective velocities in fractured media a numerical study using the rotated staggered finite difference grid the modelling of elastic waves in fractured media with an explicit finite difference scheme causes instability problems on a staggered grid when the medium possesses high contrast discontinuities strong heterogeneities for the present study we apply the rotated staggered grid using this modified grid it is possible to simulate the propagation of elastic waves in a 2d or 3d medium containing cracks pores or free surfaces without hard coded boundary conditions therefore it allows an efficient and precise numerical study of effective velocities in fractured structures we model the propagation of plane waves through a set of different randomly cracked media in these numerical experiments we vary the wavelength of the plane waves the crack porosity and the crack density the synthetic results are compared with several static theories that predict the effective p and s wave velocities in fractured materials in the long wavelength limit for randomly distributed and randomly orientated rectilinear non intersecting thin dry cracks the numerical simulations of velocities of p sv and sh waves are in excellent agreement with the results of the modified or differential self consistent theory on the other hand for intersecting cracks the critical crack density porosity concept must be taken into account to describe the wave velocities in media with intersecting cracks we propose introducing the critical crack density concept into the modified self consistent theory numerical simulations show that this new formulation predicts effective elastic properties accurately for such a case

diapiric transfer of melt in kilauea iki lava lake hawaii a quick efficient process of igneous differentiation kilauea iki lava lake formed in 1959 is a large pond of picritic basalt average mgo content 15 34 by weight which has cooled and crystallized as a small self roofed magma chamber repeated drilling of the upper crust of the lake down to its molten core and more recent 1981 drilling through the thermal maximum and part way through the lower crust have made it possible to monitor the differentiation processes in the lake in detail differentiation processes recognized as active in the lake include rather inefficient settling of the larger 2 10 mm olivine phenocrysts formation of segregation veins and formation of diapir like vertical olivine rich bodies all processes which occur in one or more of the other kilauean lava lakes as well in addition most of the central part of kilauea iki has been affected by diapiric melt transfer in this process relatively low density liquid present at 1145 1160 c rose from within the loose crystal mush at the base of the lens of melt and intruded the equivalent thermal horizon at the top of the lens of melt passing through the hotter but denser melt in the core of the lake without mixing the source volume from which the low density liquid was extracted is depleted in tio 2 and other incompatible elements and enriched in feo and cao the upper part of the lake shows the opposite effects the crystalline assemblage present was olivine augite minor plagioclase the crystallinity of both the source and receiving layers was low enough that no obvious textural imprint was left by the transfer process the principal evidence for its occurrence is the chemical zonation of the lake seen in core from depths of 13 to 80 m diapiric melt transfer was active from 1960 to 1971 and has affected most of the central part of the lake from 13 m to at least 80 m the process ran simultaneously with the other three main differentia tion processes but started and stopped independently of the others calculations suggest that between 21 and 42 wt liquid has been extracted from the depleted zone at 56 78 m in the center of the lake making this a very efficient process of chemical differentiation

deformation pattern in a massive ponded lava flow at odp iodp site 1256 a core and log approach 1 a complete in situ section of upper oceanic crust from extrusive lavas through dikes into gabbros has been recently drilled for the first time in 15 ma old crust created at the superfast spreading east pacific rise epr the upper igneous basement cored at ocean drilling program odp integrated ocean drilling program iodp holes 1256c and 1256d consists of sheet flows and massive flows including a ponded lava flow unit lava pond interpreted as a thick lava flow delivered off axis and accumulated in a topographic depression fine scale structural analysis shows that the lava pond is much less fractured than the deeper lavas probably as the effect of structure spacing that is larger than in the underlying lava flows in hole 1256d structures are mainly subhorizontal in the lava pond and become steeper downhole toward the sheeted dike complex this reflects the distance of the lava pond from the stress field produced by the intrusion of dikes at depth inducing mostly subvertical eruptive fractures and fracture network and the distance of the site from active rifting at the epr contrasting structural patterns in the lava pond and in the deeper parts of the drilled section are reflected in contrasting physical properties patterns as obtained by shipboard and downhole logging data electrical resistivity compressional velocity and bulk density are generally higher whereas natural radioactivity and neutron porosity are lower in the lava pond than in the deeper hole the occurrence of a thick massive lava flow at the top part of the volcanic section affects the deformation pattern of the entire drilled crust and consequently its permeability regime which in turn controls the geometry and distribution of hydrothermal circulation and basalt alteration

submarine lava flow direction revealed by neutron diffraction analysis in mineral lattice orientation the ocean crust is formed by the rising of magma from mid ocean ridges and voluminous 1 30 km3 flows of lava away from ridge axes however our understanding of the emplacement kinematics of submarine lava is often limited to plan view geometries of near axis lava drilled cores provide in situ access to the intact internal structure of submarine lavas we used neutron diffraction to study off axis lava flows drilled into the uppermost crust of odp iodp site 1256 cocos plate we provide quantitative insights into submarine lava microstructures and strong evidence for a secondary lava injection into the interior of a solidifying flow of lava along the nw se direction parallel to the paleo ridge axis of the east pacific rise the dynamics of lava inflow are controlled by crystal abundance and the temperature of the lava crystal mixture rather than by local seafloor topography we provide a description of an in situ shear within submarine lavas revealed by composite shape and lattice preferred orientations accounting for a dominant laminar nonuniform type flow

finite difference modeling of wave propagation on microscale a snapshot of the work in progress digital rock methodology combines modern microscopic imagingwithadvancednumericalsimulationsofthephysicalproperties of rocks modeling of elastic wave propagation directly from rock microstructure is integral to this technology we survey recent development of the rotated staggered grid rsg finite difference fd method for pore scale simulation of elastic wavepropagationindigitalrocksamples includingthedynamic elastic properties of rocks saturated with a viscous fluid examination of the accuracy of this algorithm on models with known analytical solutions provide an additional accuracy condition for numerical modeling on the microscale we use both the elastic and viscoelastic versions of the rsg algorithm to study gas hydratesandtosimulatepropagationofbiot sslowwave weapply rsg method ology to examine the effect of gas hydrate distributions in the pore space of a rock we compare resulting p wave velocities with experimentally measured data as a basis for buildinganeffective mediummodelforrockscontaininggashydrates we then perform numerical simulations of biot s slow wave in a realistic 3d digital rock model fully saturated with a nonviscous fluid corresponding to the high frequency limit of poroelasticity and placed inside a bulk fluid the model clearly demonstrates biot s slow curve when the interface is open between the slab and bulk fluid we demonstrate slow wave propagation in a porous medium saturated with a viscous fluid by analyzing an idealized 2d porous medium represented alternating solid and viscous fluid layers comparison of simulation results withtheexactsolutionforthislayeredsystemshowsgoodagreementoverabroadfrequencyrange

perspectives on basaltic magma crystallization and differentiation lava lake blocks erupted at mauna loa volcano summit hawaii explosive eruptions at mauna loa summit ejected coarse grained blocks free of lava coatings from moku aweoweo caldera most are gabbronorites and gabbros that have 0 26 vol olivine and 1 29 vol oikocrystic orthopyroxene some blocks are ferrogabbros and diorites with micrographic matrices and diorite veins 2 cm cross cut some gabbronorites and gabbros one block is an open textured dunite the mgo of the gabbronorites and gabbros ranges 7 21 wt those with mgo 10 wt have some incompatible element abundances zr y ree positive eu anomalies lower than those in mauna loa lavas of comparable mgo gabbros mgo 10 wt generally overlap lava compositions olivines range fo83 58 clinopyroxenes have mg s 83 62 and orthopyroxene mg s are 84 63 all evolved beyond the mineral mg s of mauna loa lavas plagioclase is an75 50 ferrogabbro and diorite blocks have 3 5 wt mgo tio2 3 2 5 4 k2o 0 8 1 3 la 16 27 ppm and a diorite vein is the most evolved sio2 59 k2o 1 5 la 38 ppm they have clinopyroxene mg s 67 46 and plagioclase an57 40 the open textured dunite has olivine fo83 5 seven isotope ratios are 87sr 86sr 0 70394 0 70374 and 143nd 144nd 0 51293 0 51286 and identify the suite as belonging to the mauna loa system gabbronorites and gabbros originated in solidification zones of moku aweoweo lava lakes where they acquired orthocumulate textures and incompatible element depletions these features suggest deeper and slower cooling lakes than the lava lake paradigm kilauea iki which is basalt and picrite clinopyroxene geobarometry suggests crystallization at 1 kbar p highly evolved mineral mg s 75 are largely explained by cumulus phases exposed to evolving intercumulus liquids causing compositional shifts ferrogabbro and diorite represent segregation veins from differentiated intercumulus liquids filter pressed into rigid zones of cooling lakes clinopyroxene geobarometry suggests 300 bar p open textured dunite represents olivine melt mush precursor to vertical olivine rich bodies as in kilauea iki its fo83 5 identifies the most primitive lake magma as 8 3 wt mgo mass balancing and melts show that such a magma could have yielded both ferrogabbro and diorite by 50 fractional crystallization but under different fo2 fmq 250 bar led to diorite and fmq 250 bar yielded ferrogabbro these segregation veins documented as similar to those of kilauea testify to appreciable volumes of rhyolitic liquid forming in oceanic environments namely sio2 rich veins are intrinsic to all shields that reached caldera stage to accommodate various sized cooling differentiating lava lakes

the early rhuddanian survival interval in the lower silurian of the oslo region a third pulse of the end ordovician extinction abstract ordovician silurian boundary layers with rhuddanian strata are exposed as a long continuously fossiliferous sequence in the solvik formation in the asker area central oslo region norway brachiopods belonging to benthic assemblage 5 are preserved in the lower parts of the formation this level is investigated for the presence of a survival interval after the last end ordovician extinction event the criteria for a survival interval include taxa that are dwarfed long ranging eurytopic often opportunistic as well as assemblages that show low density and diversity four species isorthis prima leangella scissa dicoelosia osloensis and eoplectodonta duplicata were collected and measured the three former were dwarfed compared to a younger aeronian fauna belonging to the same benthic assemblage detailed investigations showed a statistically significant two step reduction in size for all but d osloensis the first interval immediately above the last end ordovician extinction event displays dwarfed brachiopods but diversity is high and the number of long ranging and eurytopic species is low due to the presence of globally and locally relict ordovician species the second interval the upper parts of lower rhuddanian shows all the characteristics of a survival interval in which the sizes are statistically significantly smaller than both those in the interval below and those in the aeronian interval above no lithological change within lower rhuddanian strata could be linked to the pronounced reduction in size the second step in size reduction may be related to global occurrence of anoxia in the deep oceans with a pulse of anoxic water pushed onto the shelf at that time it is coeval with a short negative 13 c excursion found in some locales signifying a brief period of global warming the results suggest a protracted extinction event through parts of the early rhuddanian and a third and final extinction event followed by a clear survival interval

a reconnaissance teleseismic study of the upper mantle and transition zone beneath the archean slave craton in nw canada abstract the objective of this study is to investigate upper mantle structure below the archean slave craton site of the oldest known rocks on earth and numerous diamondiferous kimberlites and thence to gain an understanding of early craton formation and kimberlite genesis to this aim a temporary array consisting of 13 sites equipped with broadband seismometers recorded teleseisms between november 1996 and may 1998 this data set has been augmented with recordings from the yellowknife seismic array our three most robust observations and their interpretation are 1 p wave travel time tomography reveals the oldest part of the craton the central slave basement complex to be underlain by the fastest seismic velocities indicating that this block remains distinct well into mantle depths 2 receiver function analysis requires only the moho as major s wave velocity discontinuity and points to a fairly constant crustal thickness throughout the slave province and 3 sks splitting analysis shows little variation in delay times and fast polarization directions across the array leading us to conclude that the present day plate motion of north america is the primary cause for mantle fabric beneath the entire array furthermore the data let us rule out the possibility that the mackenzie plume had any seismologically detectable effect on the slave lithosphere more speculative results of our investigation namely a possible genetic link between a low seismic velocity anomaly at depth with the overlying lac de gras kimberlite field and a possible archean origin of two shallow low velocity anomalies will require further investigation

on isostasy at atlantic type continental margins cross spectral techniques have been used to analyze the relationship between free air gravity anomalies and topography of approximately 100 profiles over the continental margins of eastern north america southwest africa and the coral sea lord howe rise the resulting admittance functions for these margins have been interpreted in terms of the flexure model of isostasy the relatively young coral sea lord howe rise margin is associated with the lowest value of the effective elastic thickness te 5 km while the relatively old eastern north american margin is associated with the highest value 10 te 20 km thus te appears to increase with the age of the margin these results which are consistent with flexure studies from the major ocean basins have been interpreted in terms of a simple model in which the flexural strength of the lithosphere increases with age the model suggests that a low rigidity plate or airy type model is most applicable early in rifting history while a high rigidity plate is more applicable later in margin evolution thus sediments at a margin initially load a relatively thin and hot plate while sediments later in margin evolution load a relatively thick and cold plate the model explains a number of features of the tectonic stratigraphic evolution of margins these include a change in flexural style from a narrow and deep basin to a broad and shallow basin coastal plain onlap and uplift in the continental interior furthermore the flexure model of isostasy contributes in a major way to observed free air gravity anomalies at continental margins in particular the model explains the existance of airy isostatic gravity anomalies at margins the amplitude and wavelength of these anomalies are a function of the sediment supply the type of mechanisms controlling sediment deposition up building and or out building and the thermal structure history and age of the margin

pb isotopic composition of late archaean granites and the extent of recycling early archaean crust in the slave province northwest canada initial pb isotopic compositions have been determined for potassium feldspar from ca 2 58 to 2 62 ga plutonic rocks in the southern and central slave province of northwestern canada to evaluate the extent of recycling of ancient crust within the province large differences in initial pb compositions were measured which correlate with geographical areas of the province plutons in the east central part of the province have initial compositions only slightly more radiogenic than estimated mantle values 207pb 204pb 14 8 14 9 and were dominantly deruved from juvenile crustal sources in contrast plutons in the point lake and western contwoyto lake areas of the western slave province have radiogenic compositions 207pb 204pb 15 1 15 2 and indicate significant recycling of pre 3 5 ga crust the pb data support previous interpretations based on nd isotopes for a major isotopic boundary in the central part of the province granites from the southern part of the province near yellowknife have intermediate compositions which indicate 1 the age of the protolith to the granitoids in the yellowknife area is younger than at point lake but older than in the eastern slave or 2 the granitoids in the yellowknife area contain a mixture of an older point lake type component and younger crust the absence of pre 3 2 ga crust in the yellowknife area and lack of evidence for pre 2 8 ga inherited zircons in the yellowknife granitoids favour the former possibility evidence for recycling of ancient crustal sources such as the acasta gneiss is limited to a relatively small area of the west central part of the province suggesting that acasta aged or derived crust is not widespread in the province the marked regionality of isotopic composition may reflect a basement in the western part of the province which is itself a collage of crust of different age being younger ca 3 2 2 9 in the south relative to the point lake region 3 9 3 2 ga

wuchiapingian early lopingian permian global brachiopod palaeobiogeography a quantitative approach abstract a global presence absence database of 212 wuchiapingian early lopingian permian brachiopod genera from 30 stations is analysed by cluster analysis nonmetric multidimensional scaling and minimum spanning tree to document the global palaeobiogeographical patterns five core groups are revealed by the quantitative analysis and interpreted as representing five marine biotic provinces they are the cathaysian tropical western tethyan tropical himalayan warm temperate austrazean cold temperate and greenland svalbard provinces cold temperate the cathaysian province is composed of many isolated or semi isolated islands situated in the palaeotethys whereas the other four provinces occurred mainly on the continental shelves of pangea the western tethyan province along the western coast of the palaeotethys the himalayan province on the northern margin of gondwanaland the austrazean province along the southeastern margin of gondwanaland and the greenland svalbard province on the northern margin of pangea in addition nonmetric multidimensional scaling helped to identify key biogeographic determinants latitude related thermal gradient appears to have accounted for most of the variance in the data geographic distance and ocean circulation may have also played a major but subordinate role in the delineation and or enhancement of some of the provinces comparison with early and middle permian global marine provincialism indicates that marine biotic provinces had significantly reduced during the lopingian late permian in the lead up to the end permian mass extinction

brachiopod faunas after the end ordovician mass extinction from south china testing ecological change through a major taxonomic crisis classification of extinction events and their severity is generally based on taxonomic counts the ecological impacts of such events have been categorized and prioritized but rarely tested with empirical data the ecology of the end ordovician extinction and subsequent biotic recovery is tracked through abundant and diverse brachiopod faunas in south china the spatial and temporal ranges of some 6500 identified specimens from 10 collections derived from six localities were investigated by network and cluster analyses nonmetric multidimensional scaling and a species abundance model depth zonations and structure of brachiopod assemblages along an onshore offshore gradient in the late katian were similar to those in the latest ordovician earliest silurian post extinction fauna within this ecological framework deeper water faunas are partly replaced by new taxa siliciclastic substrates continued to be dominated by the more ordovician orthides and strophomenides shallow water carbonate environments hosted atrypides athyridides and pentamerides with the more typical ordovician brachiopod fauna continuing to dominate until the late rhuddanian the end ordovician extinctions tested the resilience of the brachiopod fauna without damage to its overall ecological structure that commenced later at the end of the rhuddanian

the effect of noise on estimates of the elastic thickness of the continental lithosphere by the coherence method 1 we model the lithosphere as a uniform elastic plate overlying an inviscid fluid and loaded with both surface and subsurface fractal loads to generate synthetic topography and gravity data to simulate data having low topographic signal to gravity noise ratio we use an algebraically larger exponent for the subsurface load in the spectral synthesis fractal algorithm the gravity power spectrum then decays less rapidly than that of topography the spectra resembling those for central australia we find that the coherence method using multitaper spectral estimation yields significant underestimates of plate thickness for both low and normal signal to noise ratio data unless the data window is larger than several times the true flexural wavelength we quantify this bias for the parameters used here and apply it as a correction to an effective elastic thickness estimate for central australia obtaining a value of 115 25 km index terms 8110 tectonophysics continental tectonics general 0905 8159 tectonophysics rheology crust and lithosphere 1236 geodesy and gravity rheology of the lithosphere and mantle 8160 citation swain c j and j f kirby the effect of noise on estimates of the elastic thickness of the continental lithosphere by the coherence method geophys res lett 30 11 1574 doi 10 1029 2003gl017070 2003

can the lilliput effect be detected in the brachiopod faunas of south china following the terminal ordovician mass extinction abstract in the immediate aftermath of global extinctions organisms were normally much smaller than those prior to these events this lilliput effect can be subdivided into two types 1 a specific type following the original definition of the effect which targets species level taxa associated with inhospitable environments and 2 a more general type related to the reactions of higher rank taxa above the species level the body sizes of brachiopods from south china through the ordovician and silurian transition late katian hirnantian and earliest rhuddanian are compared at generic superfamilial ordinal and class levels the results indicate that the body sizes of the taxa of lower rank e g genus level are highly variable within these different intervals the type of evidence for the lilliput effect through the end ordovician mass extinction is thus quite different from that of the end permian mass extinction probably reflecting differences in the intensity of these two major bioevents however the relationships between the contrasting trends in body size change of some taxa of higher rank e g at the ordinal level and the relative dominance of these taxa in the latest ordovician and earliest silurian suggest that the brachiopods of the two major ordovician groups the strophomenoids and orthoids adopted different survival strategies during and immediately after the crisis from those of the pentamerides and rhynchonellides that were common in silurian assemblages

accretion and oxidation of a superfast spread axial melt lens tims and sims zircon analyses of the iodp hole 1256d gabbros abstract iodp hole 1256d penetrates a complete section of upper oceanic crust spread from the east pacific rise at a full rate of up to 220 mm yr here we present u pb zircon data from hole 1256d gabbroic rocks using isotope dilution thermal ionization mass spectrometry id tims and secondary ion mass spectrometry sims three gabbro samples yielded th corrected weighted mean 206pb 238u sims dates of 15 03 0 14 ma 15 13 0 35 ma and 15 23 0 12 ma an id tims th corrected weighted mean 206pb 238u date of 15 191 0 040 ma from the first of the three samples provides verification and refinement of the timing of magmatism the u pb dates agree with the predicted age of the crust based on marine magnetic anomalies suggesting the gabbros most likely formed by intrusion and crystallization at or near the ridge axis zircons from two of the studied samples have anomalously high th u ratios likely reflecting oxidation of u prior to or during zircon crystallization the elevated o2 of late stage melts in the axial melt lens may be related to progressive crystallization assimilation of hydrothermally altered sheeted dikes and gabbros and or contamination of the melt lens by seawater derived saline brines

structural and petrophysical characterization of the upper basement crustal section at odp iodp site 1256 east pacific ocean 1 new ocean crust is constantly being formed from mid ocean ridge axis voluminous flows of lava are emplaced also away from the ridge axis adding off axis crustal layers to the crustal pile research on deep sea is of great importance to better understand the mechanisms and the nature of the crust forming and evolution in this work we decipher the first stages of the postmagmatic evolution of an intact volcanic section from the upper oceanic crust at ocean drilling program integrated ocean drilling program odp iodp site 1256 eastern pacific ocean using for the first time an innovative core log integration technique to match direct core related and indirect borehole related data by depth shifting and reorienting individual core pieces recovered by drilling we are able to identify the clusterization of structures and physical properties within distinct downdeep strong and weak lava zones reflecting the cooling and tectonic evolution of lavas rather than lithological variations we define the evolution of the structural zones that typically affect lava flows colonnades and entablature zones studying an off axis lava flow encountered in present day upper ocean crust for the same off axis flow we are also able to suggest the lava flow direction nw se and its relationships with the paleoridge axis despite the environmental difficulties in the study of the subseafloor under deep water and using only one dimensional data deriving from ocean drilling this work shows how an array of diverse data can be integrated into a coherent interpretation of lava flow history obtaining detailed information on the mechanism of submarine lava emplacement and flow

electric lithosphere of the slave craton the archean slave craton in northwestern canada is an ideal natural laboratory for investigating lithosphere formation and evolution and has become an international focus of broad geoscientific investigation following the discovery of economic diamondiferous kimberlite pipes three deep probing magnetotelluric surveys have recently been carried out on the craton using novel acquisition procedures the magnetotelluric responses reveal an unexpected and remarkable anomaly in electrical conductivity collocated with the kimberlite field that is modeled as a spatially confined upper mantle region of low resistivity 30 m at depths of 80 100 km and is interpreted to be due to dissolved hydrogen or carbon in graphite form this geophysically anomalous upper mantle region is also spatially coincident with a geochemically defined ultradepleted harzburgitic layer the tectonic processes that emplaced this structure are possibly related to the lithospheric subduction and trapping of overlying oceanic mantle at 2630 2620 ma

effect of supercritical co2 on carbonates savonnieres sample case study co2 geosequestration is an efficient way to reduce greenhouse gas emissions into the atmosphere carbonate rock formations are one of the possible targets for co2 sequestration due to their relative abundance and ability to serve as a natural trapping reservoir the injected supercritical co2 can change properties of the reservoir rocks such as porosity permeability tortuosity and specific surface area due to dissolution and precipitation processes this in turn affects the reservoir characteristics i e their elastic properties storage capacity stability etc the tremendous progresses made recently in both microcomputed x ray tomography and high performance computing make numerical simulation of physical processes on actual rock microstructures feasible however carbonate rocks with their extremely complex microstructure and the presence of microporosity that is below the resolution of microcomputed x ray tomography scanners require novel quite specific image processing and numerical simulation approaches in the current work we studied the effects of supercritical co2 injection on microstructure and elastic properties of a savonni eres limestone we used microtomographic images of two savonni eres samples i e one in its natural state and one after injection and residence of supercritical co2 a statistical analysis of the microtomographic images showed that the injection of supercritical co2 led to an increase in porosity and changes of the microstructure i e increase of the average volume of individual pores and decrease in the total number of pores the co2 injection residence also led to an increase in the mean radii of pore throats an increase in the length of pore network segments and made the orientation distribution of mesopores more isotropic numerical simulations showed that elastic moduli for the sample subjected to supercritical co2 injection residence are lower than those for the intact sample

sonic waveform attenuation in gas hydrate bearing sediments from the mallik 2l 38 research well mackenzie delta canada 1 the mallik 2l 38 research well was drilled to 1150 m under the mackenzie delta canada and penetrated a subpermafrost interval where methane hydrate occupies up to 80 of the pore space a suite of high quality downhole logs was acquired to measure in situ the physical properties of these hydrate bearing sediments similar to other hydrate deposits resistivity and compressional and shear sonic velocity data increase with higher hydrate saturation owing to electrical insulation of the pore space and stiffening of the sediment framework in addition sonic waveforms show strong amplitude losses of both compressional and shear waves in intervals where methane hydrate is observed we use monopole and dipole waveforms to estimate compressional and shear attenuation comparing with hydrate saturation values derived from the resistivity log we observe a linear increase in both attenuation measurements with increasing hydrate saturation which is not intuitive for stiffening sediments numerical modeling of the waveforms allows us to reproduce the recorded waveforms and illustrate these results we also use a model for wave propagation in frozen porous media to explain qualitatively the loss of sonic waveform amplitude in hydrate bearing sediments we suggest that this model can be improved and extended allowing hydrate saturation to be quantified from attenuation measurements in similar environments and providing new insight into how hydrate and its sediment host interact

wave propagation in heterogeneous porous media a velocity stress finite difference method a particle velocity stress finite difference method is developed for the simulation of wave propagation in 2 d heterogeneous poroelastic media instead of the prevailing second order differential equations we consider a first order hyperbolic system that is equivalent to biot s equations the vector of unknowns in this system consists of the solid and fluid particle velocity components the solid stress components and the fluid pressure a maccormack finite difference scheme that is fourth order accurate in space and second order accurate in time forms the basis of the numerical solutions for biot s hyperbolic system an original analytic solution for a p wave line source in a uniform poroelastic medium is derived for the purposes of source implementation and algorithm testing in simulations with a two layer model additional slow compressional incident transmitted and reflected phases are recorded when the damping coefficient is small this slow compressional wave is highly attenuated in porous media saturated by a viscous fluid from the simulation we also verified that the attenuation mechanism introduced in biot s theory is of secondary importance for fast compressional and rotational waves the existence of seismically observable differences caused by the presence of pores has been examined through synthetic experiments that indicate that amplitude variation with offset may be observed on receivers and could be diagnostic of the matrix and fluid parameters this method was applied in simulating seismic wave propagation over an expanded steam heated zone in cold lake alberta in an area of enhanced oil recovery eor processing the results indicate that a seismic surface survey can be used to monitor thermal fronts

effective elasticity of fractured rocks production related changes in subsurface stress are known to cause velocity variations that can be detected from time lapse seismic data and may provide valuable information for reservoir surveillance it has been long recognized that the stresses primarily influence the most compliant part of a rock by preferentially opening or closing the fractures walsh 1965 therefore examining wave propagation in fractured formations might give important clues about the stresses acting upon them as the length of seismic waves is usually much larger than that of the fractures deformation of cracked rocks can be treated as static

post extinction diversification patterns of brachiopods in the early middle llandovery silurian abstract network and frequency distribution analyses of global brachiopod occurrences in the earliest silurian rhuddanian aeronian revealed that brachiopod recovery from the end ordovician mass extinction during the early late rhuddanian was represented mainly by the reestablishment of late ordovician cosmopolitan holdover taxa brachiopods nearly doubled their generic diversity from rhuddanian to aeronian owing to the radiation of both endemic and cosmopolitan taxa associated with an overall post glacial amelioration of global environment and increased habitat heterogeneity as a result of marine transgressions and expansion of epeiric seas a drastic turnover from the ordovician type to silurian type brachiopod faunas took place in the aeronian several million years after the terminal ordovician mass extinction compared with the orders of orthides and strophomenides that radiated and predominated in the ordovician the atrypides and pentamerides displayed pulses of drastic diversification from the rhuddanian to aeronian mainly in tropically located paleoplates to become the most abundant and diverse brachiopod orders in the silurian atrypides recovered in the early rhuddanian whereas diversification of pentamerides was delayed until the aeronian and becoming widespread in south china kazakhstan terranes baltica avalonia and laurentia

resolution and limitations of faunal similarity indices of biogeographic data for testing predicted paleogeographic reconstructions and estimating intercontinental distances a test case of modern and cretaceous bivalves abstract faunas in biogeographic regions have been compared with descriptive indices such as the jaccard simpson and the probabilistic raup crick coefficients the question is which approach is the best for testing paleogeographic reconstructions based on modern geography and the location of predicted biogeographic boundaries such as deep ocean basins convergence of cold and warm water currents and land barriers i have plotted eleven faunal realms biogeographic data composed of presence or absence information on 996 species of modern bivalves within these realms were compiled and analyzed with the above listed indices land barriers and biogeographic regions separated by 6000 km of deep ocean or widely spaced latitudinally produced statistically significantly different faunas convergence of cold and warm currents did not produce significantly different faunas this may be due to seasonal movement of the convergence zone a natural logarithmic transformation of the jaccard s coefficient was found to show the best correlation between the bivalve fauna data and distance between biogeographic regions within an ocean basin this may be because the jaccard coefficient incorporates both faunal compositional and diversity differences in its measure the best fit of the faunal data and distance by the least squares method is given by the equation distance ln jc 0 75209 0 00036 when applied to a late cretaceous bivalve fossil dataset the distance obtained by the above equation gave a paleolongitudinal separation for north america and europe similar to that based on magnetic anomaly sea floor data

on bimodal differentiation by solidification front instability in basaltic magmas part 1 basic mechanics extensive coarse grained silicic lenses are common in the upper portions of large diabase sills lava lakes and gabbroic intrusions these lenses which often contain up to 10 wt more silica than their host rocks record a clear process of bimodal differentiation that may play a fundamental role in generating significant quantities of silicic magma in predominantly basaltic systems the spatial compositional textural and contact relations suggest that these silicic segregations form by infilling of tears formed during gravitational instability of the upper solidification front this process is investigated in some detail by exploring the thermal and mechanical conditions leading to internal failure of the front and segregation formation as the upper solidification front grows inward and thickens the local strength of the developing crystalline network must increase upward through the front sufficiently fast to offset the increasing weight of the front itself this necessary local variation in strength dictates how crystallinity must vary to prevent failure but the actual variation in crystallinity is determined by phase equilibria which in general varies in a fashion that is noncovariant with that required for mechanical stability it is this mismatch that apparently leads to failure and formation of silicic segregations the rate of filling of the tears by porous flow of interstitial melt from below regulates the rate of tear opening and the overall size of the tears is regulated by the local rate of advance of the solidification front segregations therefore thicken downward as the growth of the front slows and then die out altogether with approach of the lower solidification front this process irreversibly introduces siliceous noise into basaltic systems the silicic signal can be enhanced through subsequent reprocessing of the host rock to form volumetrically important masses of silicic magma this process may be fundamental to achieving the strong compositional diversity of the igneous rocks to yielding continental masses and to the differentiation of earth itself

flexural rigidity of the basin and range colorado plateau rocky mountain transition from coherence analysis of gravity and topography stochastic inversion for flexural loads and flexural rigidity of the continental elastic layer can be accomplished most effectively by using the coherence of gravity and topography however the spatial resolution of coherence analysis has been limited by use of two dimensional periodogram spectra from very large 105 km 2 windows that generally include multiple tectonic features using a two dimensional spectral estimator based on the maximum entropy method the spatial resolution of flexural properties can be enhanced by a factor of 4 or more enabling more detailed analysis at the scale of individual tectonic features this new approach is used to map the spatial variation of flexural rigidity along the basin and range transition to the colorado plateau and middle rocky mountains physiographic provinces large variations in flexural isostatic re sponse are found with rigidities ranging from as low as 8 7x102o n m elastic thickness t 4 6 km in the basin and range to as high as 4 1x1024 n m t 77 km in the middle rocky mountains these results compare favorably with independent determinations of flexural rigidity in the region areas of low flexural rigidity correlate strongly with areas of high surface heat flow as is expected from the contingence of flexural rigidity on a temperature dependent flow law also late cenozoic normal faults with large displacements are found primarily in areas of low flexural rigidity while deformation fronts of mesozoic tertiary overthrusts occur 0 to 100 km east of the low rigidity region the highest flexural rigidity is found within the archean wyoming craton where evidence suggests that deeply rooted cratonic lithosphere may play a role in deter mining the distribution of tectonism at the surface

elastic thickness of the lithosphere in the central canadian shield we have used the coherence between bouguer gravity and topography to determine the elastic thickness t e of the lithosphere in central canada in order to improve the resolution we have estimated the spectra with the maximum entropy method the variations in elastic thickness delineate three main regions 1 the interior platform where t e varies between 25 and 50 km 2 the shield includes several high rigidity cores in particular in the trans hudson orogen and in the hearne province where t e 80km 3 the hudson bay basin has an elastic thickness between 50 and 70 km with higher values in the eastern part the results suggest that t e is controlled by the thermal regime in the platform there is no difference in t e between the proterozoic and archean provinces of the shield in the platform the elastic thickness is lowest where sediments are thickest indicating that low rigidity has amplified the subsidence of the basin

formation of accretionary prisms influenced by sediment subduction and supplied by sediments from adjacent continents mechanical models are used to investigate the formation of accretionary prisms and related basins fed by sediments supplied from adjacent continents and subjected to sediment subduction results show that prisms forming under the infl uence of a hinterland sediment fl ux exhibit markedly different characteristics compared to classic critical wedges with identical mechanical properties forming by frontal accretion of a preexisting layer the main differences are reduced surface slopes increased spacing between thrusts widespread wedgetop basins and the presence of buried structures these differences are explained by the ability of continuous sedimentation to reduce differential stresses in the wedge leading to stable supercritical geometries thus in regions of active sedimentation wedge geometries cannot be explained solely in terms of relative strengths of the wedge and its decollement as is the case for critical wedges results also show that variations in the relative rates of sediment supply e g linked to climate changes and sediment subduction may lead to pulsed growth and decline of wedges though time as has been evidenced for some natural wedges thus rather than viewing compressive plate margins as accretionary versus erosive the dominant mode may repeatedly switch back and forth through time in response to variations in relative rates of sediment supply and sediment subduction

lithospheric domains and controls on kimberlite emplacement slave province canada evidence from elastic thickness and upper mantle composition 1 we have mapped the deep structure of the slave craton by combining analysis the effective elastic thickness te with data on mantle samples from numerous kimberlites three dimensional mapping of the subcontinental lithospheric mantle sclm using mantle derived xenoliths and xenocrysts in kimberlites has shown that much of the craton is underlain by a strongly layered sclm a highly depleted upper layer low in basaltic components ca al fe is separated from a relatively fertile lower layer by a sharp boundary this boundary lies at 140 150 km depth in the lac de gras area and shallows to 100 km in the northern and southern parts of the craton weak lithosphere te 56 km in the younger eastern part of the craton is separated from the older western part by a zone of steep te gradient parallel to the major locus of kimberlite intrusion which may map the deep extension of the boundary between the two domains another strong te gradient across the kilohigok basin accompanies a marked compositional change in the upper layer of the lithospheric mantle the basin probably marks a major translithospheric fault correlations between te and mantle composition suggest that te is strongly influenced by the rheology of the upper mantle

bathymetric and isotopic evidence for a short lived late ordovician glaciation in a greenhouse period the end ordovician glaciation is distinct among phanerozoic glaciations in that co 2 levels were generally high yet major continental ice sheets accumulated on the gondwana supercontinent new oxygen isotopic data indicate substantial changes in sea water temperatures and ice volume coinciding with glacio eustatic changes in sea level reflecting the growth and decay of the gondwana ice cap major glaciation was apparently confined to the hirnantian and was 0 5 1 m y long rather than the 35 m y of earlier estimates carbon isotope values indicate significant changes in carbon cycling as the oceans changed from a state with warm saline bottom waters to a state with cold deep water circulation and then back again we believe that the changes in the carbon cycle effected a reduction in p co 2 levels in the oceans and atmosphere and thus promoted glaciation but were unable to sustain icehouse conditions in a greenhouse world

the siberian lithosphere traverse mantle terranes and the assembly of the siberian craton abstract the kimberlite fields scattered across the ne part of the siberian craton have been used to map the subcontinental lithospheric mantle sclm as it existed during devonian to late jurassic time along a 1000 km traverse ne sw across the archean magan and anabar provinces and into the proterozoic olenek province 4100 garnets and 260 chromites from 65 kimberlites have been analysed by electron probe major elements and proton microprobe trace elements these data and radiometric ages on the kimberlites have been used to estimate the position of the local paleo geotherm and the thickness of the lithosphere and to map the detailed distribution of specific rock types and mantle processes in space and time a low geotherm corresponding approximately to the 35 mw m2 conductive model of pollack and chapman tectonophysics 38 279 296 1977 characterised the devonian lithosphere beneath the magan and anabar crustal provinces the devonian geotherm beneath the northern part of the area was higher rising to near a 40 mw m2 conductive model areas intruded by mesozoic kimberlites are generally characterised by this higher but still cratonic geotherm lithosphere thickness at the time of kimberlite intrusion varied from ca 190 to ca 240 km beneath the archean magan and anabar provinces but was less 150 180 km beneath the proterozoic olenek province already in devonian time thinner devonian lithosphere 140 km in parts of this area may be related to riphean rifting near the northern end of the traverse differences in geotherm lithosphere thickness and composition between the devonian toluopka area and the nearby mesozoic kimberlite fields suggest thinning of the lithosphere by ca 50 60 km related to devonian rifting and triassic magmatism a major conclusion of this study is that the crustal terrane boundaries defined by geological mapping and geophysical data extended from outcrops in the anabar shield represent major lithospheric sutures which continue through the upper mantle and juxtapose lithospheric domains that differ significantly in composition and rock type distribution between 100 and 250 km depth the presence of significant proportions of harzburgitic and depleted lherzolitic garnets beneath the magan and anabar provinces is concordant with their archean surface geology the lack of harzburgitic garnets and the chemistry of the lherzolitic garnets beneath most of the other fields are consistent with the proterozoic surface rocks mantle sections for different terranes within the archean portion of the craton show pronounced differences in bulk composition rock type distribution metasomatic overprint and lithospheric thickness these observations suggest that individual crustal terranes of both archean and proterozoic age had developed their own lithospheric roots and that these differences were preserved during the proterozoic assembly of the craton data from kimberlite fields near the main archean proterozoic suture the billyakh shear zone suggest that reworking and mixing of archean and proterozoic mantle was limited to a zone less than 100 km wide

heat flow and deep lithospheric thermal structure at lac de gras slave province canada 1 temperature measurements in two drill holes near lac de gras in the slave province of the canadian shield yield heat flow values of 43 5 and 48 8 mw m 2 with an average of 46 6 mw m 2 heat production measured on samples from one of these holes is relatively high 1 9 w m 3 confirming measurements made on outcrop samples the surface heat flow and heat generation data imply that below a 10 km thick radioelement enriched layer heat production is comparable to that in other archean provinces of the canadian shield the lithospheric geotherm is very close to the geotherms derived from geothermobarometry on xenoliths variations in crustal heat generation account for 50 k differences in moho temperatures within the slave province

crystal mush compaction and the origin of pegmatitic segregation sheets in a thick flood basalt flow in the mesozoic hartford basin connecticut

the influence of mechanical anisotropy on the behaviour of the lower crust abstract the aim of this paper is to determine what can be deduced about the rheology of the lower crust undergoing extension the state of stress existing there and how the finite strain pattern is developed from a consideration of the theoretical analyses of layer normal compression analogue models and outcrops of rocks from the lower crust it is concluded that the extended lower crust contains units with a significant competence contrast has a high mechanical anisotropy and experiences bulk rheological changes during deformation based on a consideration of hydraulic fracturing in anisotropic materials it is argued that depending on the magnitude differential stress fluids will move through and out of the lower crust by fluid fracturing along the planar horizontal stretching fabric and gently inclined normal kink bands extensional shears or by the formation of vertical fractures

global paleobiogeography of brachiopods during the mississippian response to the global tectonic reconfiguration ocean circulation and climate changes abstract changes in mississippian global paleogeography derived from the reconfiguration of the continents a reversal in ocean currents and global cooling although the tectonic and climatic changes are well documented their effects on the distribution of brachiopod fauna are poorly documented here we present systematic quantitative analyses on global paleobiogeography based on a global brachiopod database from the mississippian i e tournaisian visean and serpukhovian the dataset consists of 2123 species of 344 brachiopod genera from 1156 localities our results reveal that global provincialism was not evident during the tournaisian and visean stages two realms i e the gondwanan and paleoequatorial realms are recognized during the tournaisian the paleoequatorial realm dominates during the visean stage whereas the gondwanan realm is not documented due to the absence of data points in contrast to the early and middle mississippian stages faunal provincialism is greatly enhanced in the serpukhovian stage with paleotethyan and north american realms easily distinguished this indicates that the rheic ocean was closed before the serpukhovian due to the collision between gondwana and laurussia that disrupted faunal interchange between the paleotethys and north america in addition the paleolatitude related thermal gradient was enhanced and the boreal realm was distinguished from the paleotethyan realm during the onset of the late palaeozoic ice age lpia in the serpukhovian the paleolatitude diversity gradient pattern further shows a distinct shift of diversity center from the southern tropic zone in the tournaisian and visean to the northern tropic zone in the serpukhovian

the late ordovician mass extinction abstract near the end of the late ordovician in the first of five mass extinctions in the phanerozoic about 85 of marine species died the cause was a brief glacial interval that produced two pulses of extinction the first pulse was at the beginning of the glaciation when sea level decline drained epicontinental seaways produced a harsh climate in low and mid latitudes and initiated active deep oceanic currents that aerated the deep oceans and brought nutrients and possibly toxic material up from oceanic depths following that initial pulse of extinction surviving faunas adapted to the new ecologic setting the glaciation ended suddenly and as sea level rose the climate moderated and oceanic circulation stagnated another pulse of extinction occurred the second extinction marked the end of a long interval of ecologic stasis an ecologic evolutionary unit recovery from the event took several million years but the resulting fauna had ecologic patterns similar to the fauna that had become e

late ordovician brachiopod communities of southeast china the upper ordovician mid ashgill xiazhen and changwu formations in the zhejiang jiangxi border region of southeastern china contain an abundant and diverse suite of brachiopods a cluster analysis of 59 key samples from 180 collections over 23 000 specimens allowed delineation of eight distinct brachiopod communities herein named the ectenoglossa minor community the eospirifer praecursor community the ovalospira dichotoma community the antizygospira liquanensis sowerbyella sinensis community the kassinella shiyangensis community the altaethyrella zhejiangensis community the tcherskidium jiangshanensis community and the foliomena folium community these communities occur predominantly in limey clayey to silty mudstone facies similar to ziegler s early silurian brachiopod communities in the welsh borderland on the basis of lithological faunal and paleogeographical evidence the eight communities are interpreted to have distributed along an onshore offshore gradient from ba1 to ba6 setting

neodymium isotopic evidence for the tectonic assembly of late archean crust in the slave province northwest canada the ca 2 7 2 5 ga slave province is a granitegreenstone terrane comprising deformed sedimentary and subordinate volcanic belts extensively intruded by granitoid rocks the nd isotopic data are reported for 58 samples of supracrustal and granitoid rocks exposed along a 400 km east west transect at 65 n across the structural grain of the province initial nd values reveal distinctly different crustal sources in the eastern compared to the western parts of the province as expected from tectonic assembly of the province through accretion of juvenile crust to older continental crust supracrustal sequences ca 2 71 2 65 ga from the central and eastern parts of the province have positive nd 1 values 0 3 to 3 6 consistent with juvenile sources and formation remote from significantly older crust syn to late deformation ca 2 63 2 60 ga mantle derived diorites and related tonalites type i from the central and eastern parts of the province have similar initial nd values 0 1 to 2 7 in contrast samples from the westernmost plutons which intrude exposed pre 3 1 ga crust have much lower nd 1 values 1 0 to4 6 suggesting contamination of these magmas by older crust the nd 1 values of post deformation granites s s type ii also vary systematically across the province values for granites west of longitude 110 30 w range from 0 2 to 5 3 those to the east range from 0 6 to 3 7 these data suggest mixed crustal sources dominated by mid to early archean material nd 2 6 to 17 at 2 6 ga for the western granitoid rocks and juvenile sources for the eastern granites the nd isotopic data are consistent with the geology of the province in that exposures of mid to early archean crustal rocks predating the principal 2 7 2 5 ga orogenic event are restricted to the western part of the province the asymmetric pattern defined by the nd isotopic data indicates the presence of distinct crustal rocks beneath the slave province similar isotopic variations observed across phanerozoic collisional orogens have been interpreted to reflect tectonic assembly of crust by accretion of juvenile crustal terranes to an older continental margin this process may also have been an important mechanism in the cratonization of the slave province

deformation of granitoids at low metamorphic grade i reactions and grain size reduction abstract deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones two from the alps corvatsch granodiorite and aiguilles rouges granite switzerland and one from eastern australia wyangala granite when examined by light and electron microscopy all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite quartz deformed plastically while both k feldspar and plagioclase fractured and recrystallized in conjunction with chemical change which had as its end point an assemblage albite quartz white mica caal silicates k feldspar and calcic plagioclase depending on fluid chemistry from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid since ca bearing plagioclase was not stable in these environments myrmekite did not replace k feldspar in any of the rocks examined recrystallization of feldspar which should perhaps be termed neocrystallization was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets in old feldspar grains particularly in plagioclases dislocations exist in wall like structures that are commonly associated with voids suggesting origins in microfractures these dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization we conclude that recrystallization mainly occurred via a classical nucleation mechanism with minor contributions from twin boundary migration we argue that the 450 550 c lower limit for recrystallization of feldspar referenced throughout the literature is not applicable to the rock systems under investigation and should be discarded as a universal limit microfracturing microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades see part ii of this paper neocrystallization fluid access and nucleation appear to have accompanied or immediately followed the first deformation increments and were primarily responsible for continued grain size reduction transgranular fracture was comparatively important only at the onset of deformation since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade in the presence of aqueous fluids in the earth s crust breakdown reactions and nucleation recrystallization in feldspars may be important influences upon localization of deformation in granitoids

global analyses of brachiopod faunas through the ordovician and silurian transition reducing the role of the lazarus effect global analyses of 88 families and 284 genera of brachiopods from middle ashgill late ordovician to early middle rhuddanian early silurian indicate that 18 6 and 12 5 of families and 51 0 and 41 3 of genera were eliminated in the first and second phases of the end ordovician mass extinction respectively with the total loss of 28 4 of families and 69 0 of genera in the crisis new investigation demonstrates that brachiopods at both generic and familial levels suffered greater during the first phase than during the second phase four groups victims relicts survivors and new arrivals are distinguished by their stratigraphical ranges generic survivors occurring in the kosov province during the hirnantian can be split into three types with respect to their changing abundance increasing declining and lazarus taxa among the 88 genera that survived numerous declining genera occurred in the hirnantian 16 lazarus families and 18 lazarus genera are provisionally known and may be regarded

acoustic properties of sediments saturated with gas hydrate free gas and water we obtain the wave velocities and quality factors of gas hydrate bearing sediments as a function of pore pressure temperature frequency and partial saturation the model is based on a biot type three phase theory that considers the existence of two solids grains and gas hydrate and a fluid mixture attenuation is described with the constant q model and viscodynamic functions to model the high frequency behaviour we apply a uniform gas water mixing law that satisfies wood s and voigt s averages at low and high frequencies respectively the acoustic model is calibrated to agree with the patchy saturation theory at high frequencies white s model pressure effects are accounted by using an effective stress law for the dry rock moduli and permeabilities the dry rock moduli of the sediment are calibrated with data from the cascadia margin moreover we calculate the depth of the bottom simulating reflector bsr below the sea floor as a function of sea floor depth geothermal gradient below the sea floor and temperature at the sea floor

mineral inclusions and geochemical characteristics of microdiamonds from the do27 a154 a21 a418 do18 dd17 and ranch lake kimberlites at lac de gras slave craton canada a mineral inclusion carbon isotope nitrogen content nitrogen aggregation state and morphological study of 576 microdiamonds from the do27 a154 a21 a418 do18 dd17 and ranch lake kimberlites at lac de gras slave craton was conducted mineral inclusion data show the diamonds are largely eclogitic 64 followed by peridotitic 25 and ultradeep 11 the paragenetic abundances are similar to macrodiamonds from the do27 kimberlite davies r m griffin w l o reilly s y 1999 diamonds from the deep pipe do27 slave craton canada in gurney j j gurney j l pascoe m d richardson s h eds the j b dawson vol proc 7th internat kimberlite conf red roof designs cape town pp 148 155 but differ to diamonds from nearby kimberlites at ekati e g lithos 2004 tappert r stachel t harris j w brey g p 2004 mineral inclusions in diamonds from the panda kimberlite s p canada 8th international kimberlite conference extended abstracts and snap lake to the south dokl earth sci 380 7 2001 806 that are dominated by peridotitic stones eclogitic diamonds with variable inclusion compositions and temperatures of formation 1040 1300 c crystallised at variable lithospheric depths sometimes in changing chemical environments a large range to very 13c depleted c isotope compositions 13c 35 8 to 2 2 and an nmorb bulk composition calculated from trace elements in garnet and clinopyroxene inclusions are consistent with an origin from subducted oceanic crust and sediments carbon isotopes in the peridotitic diamonds have mantle compositions 13c mode 4 0 mineral inclusion compositions are largely harzburgitic variable temperatures of formation garnet tni 800 1300 c suggest the peridotitic diamonds originate from the shallow ultra depleted and deeper less depleted layers of the central slave lithosphere carbon isotopes 13c av 5 1 and mineral inclusions in the ultradeep diamonds suggest they formed in peridotitic mantle 670 km the diamonds may have been entrained in a plume and subcreted to the base of the central slave lithosphere poorly aggregated nitrogen iaa without platelets in a large number of eclogitic 67 and peridotitic 32 diamonds with similar nitrogen contents indicates the diamonds were stored in the mantle at low temperatures 1060 1100 c following crystallisation in the archean type iaa diamonds have largely cubo octahedral growth forms and type ii and type iaab diamonds with higher nitrogen aggregation states mostly have octahedral morphologies however no correlation between these groups and their mineral inclusion compositions c isotopes and n contents rules out the possibility of unique source origins and suggests eclogitic and peridotitic diamonds experienced variable mantle thermal states variation in mineral inclusion chemistries in single diamonds possible overgrowths of 13c depleted eclogitic diamond on diamonds with peridotitic and ultradeep inclusions and type i ultradeep diamond with low n aggregation is consistent with diamond growth over time in changing chemical environments

late devonian palaeobiogeography of marine organic walled phytoplankton abstract acritarchs and prasinophytes have generally been considered as organic walled phytoplankton and their distribution patterns play a significant role in palaeogeographical and palaeoclimatical reconstructions in this paper the palaeobiogeography of late devonian phytoplankton mainly famennian is quantitatively analyzed based on a global database consisting of 95 genera from 15 geographical units the data are analyzed using cluster analysis nonmetric multidimensional scaling and minimum spanning tree analysis using the jaccard ochiai kulczynski and yule s y similarity coefficients the results show that there was provincialism in the late devonian and three phytoplankton palaeobiogeographical realms could be identified the west gondwana east gondwana and boreal realms there is a high degree of similarity between phytoplankton assemblages in east gondwana especially those in australia and iran and euramerica portugal of the iberian armorican block was situated to the north of algeria and probably acted as a stepping stone between euramerica and west gondwana the closed oceanic surface circulation pattern in the proto tethys ocean between euramerica and east gondwana might have hindered the exchange of phytoplankton between east and west gondwana the phytoplankton province that had been typically confined to the higher latitudes was still present while its characteristic genera gradually dispersed into the lower latitudes in the late devonian there is a high similarity between phytoplankton assemblages from the western junggar of xinjiang nw china and euramerica latitude and current influenced palaeotemperature and oceanic circulation patterns are considered to have been the major determinants of the geographical distribution and evolution of marine phytoplankton in the late devonian

upper mantle structure beneath eastern siberia evidence from gravity modeling and mantle petrology 1 the spatial distribution of large scale lithospheric domains and the boundaries between them may control the emplacement of large ore bodies and as such regional mapping of the lithosphere is relevant to mineral exploration in this study we combine potential field geophysical data and mantle petrology to map major lithospheric structures on the eastern part of the siberian platform the platform consists of several archean and proterozoic terranes that have been mapped from regional magnetic data and basement exposures in the anabar shield we use garnet and chromite concentrates from a chain of paleozoic to mesozoic kimberlites across the platform to construct mantle sections which show significant lateral variation in rock type distribution within the lithospheric mantle these lateral variations correspond to the terranes mapped at the surface and indicate that the terrane boundaries are translithospheric archean terranes are underlain by depleted archean lithosphere more than 200 km thick while the proterozoic terranes are underlain by thinner and less depleted lithosphere geophysical data show more strongly negative bouguer anomalies and a more heterogeneous magnetic anomaly pattern over the archean terranes than on the proterozoic terranes the pattern of the gravity data reflects the lateral variation in mantle composition beneath the terranes as shown by mantle petrology studies we invert gravity and topography data to estimate the flexural strength or elastic thickness te of the lithosphere across the area although on a stable precambrian craton the te is relatively low 30 km across most of the area suggesting a relatively weak lithosphere comparable to that of tectonically much younger areas around the world a 150 km wide zone of very weak lithosphere te 10 km runs n s across the western part of the study area this weak zone coincides with a zone of thickened lower crust and abnormally high sub moho p wave velocities which suggest anisotropy in the upper mantle the kimberlite fields in the archean part of the platform are localized on the flanks of this zone of weak lithosphere we suggest that the low te zone may be a mantle shear zone which has been a preferred conduit for the emplacement of magmas into the lower crust and later has controlled the emplacement of kimberlites in the study area

high resolution stable isotope stratigraphy of upper ordovician sequences constraints on the timing of bioevents and environmental changes associated with mass extinction and glaciation the two phases of the late ordovician mass extinction are approximately coeval with the periods of rapid climate change associated with the onset and demise of the gondwanan glaciation in this paper we argue that the distinctive late ordovician carbon isotope profile provides a chronostratigraphic ruler against which a sequence of environmental and biotic events may be located the ruler also allows regional and global high resolution correlation of successions representing very different environments cores from the upper ordovician succession of estonia and latvia record a large 1 3 c c a r b o n a t e excursion up to 6 with a similar profile shape the consistent relationship between the chemostratigraphy and biostratigraphy in the baltic region suggests that the isotope profile has a regional chronostratigraphic value the presence of similar profiles in nevada united states suggests that the excursion is a global chronostratigraphic signal this interpretation enables a detailed correlation to be made between upper ordovician shallow marine and basinal sequences that have wholly different faunas successions in the baltic area and in canada that do not display the model profile are interpreted as incomplete reinter pretation of these important successions significantly modifies the global database used to assess the pattern of diversity change during the mass extinction key levels of environmental change have been located against the carbon isotope profile new oxygen isotope data from brachiopod and ostracode calcite set tight limits on the start of the glacial events cooling and sea level fall started at the same stratigraphic level as the start of the carbon isotope excursion the later rise in sea level and fall in oxygen isotope values record the end of the glaciation these restrict the duration of the main glaciation to only 1 5 graptolite zones we propose that models of the carbon cycle should be adapted to be consistent with the temporal relationships between carbon cycling sea level fall and temperature change documented here the chronostratigraphic ruler provided by the carbon isotope profiles is used as a scale to determine the sequence of biotic changes and to allow high resolution correlation of biotic events at different locations this approach identifies regional similarities and differences in the patterns of extinction the main phase of graptolite extinction in the monitor range nevada for example is synchronous with the chitinozoan extinction in the baltic region but chitinozoan taxa survive to higher levels in nevada the benthic faunas in the baltic region demonstrate that the main extinction event corresponded with the beginning of the isotope excursion at the start of the hirnantian the level at which the marine environment started to change rapidly but that there were further extinctions of species within the early hirnantian the cold adapted hirnantia fauna did not appear immediately after the extinction in this area the relationship between the second phase of extinction and the carbon isotope excursion is less clear but the available data suggest that the extinction coincides with a time of rapid environmental change but not at the inception of environmental change as happened in the first phase

effective elastic properties of randomly fractured soils 3d numerical experiments this paper is concerned with numerical tests of several rock physical relationships the focus is on effective velocities and scattering attenuation in 3d fractured media we apply the so called rotated staggered finite difference grid rsg technique for numerical experiments using this modified grid it is possible to simulate the propagation of elastic waves in a 3d medium containing cracks pores or free surfaces without applying explicit boundary conditions and without averaging the elastic moduli we simulate the propagation of plane waves through a set of randomly cracked 3d media in these numerical experiments we vary the number and the distribution of cracks the synthetic results are compared with several most popular theories predicting the effective elastic properties of fractured materials we find that for randomly distributed and randomly orientated non intersecting thin penny shaped dry cracks the numerical simulations of p and s wave velocities are in good agreement with the predictions of the self consistent approximation we observe similar results for fluid filled cracks the standard gassmann equation cannot be applied to our 3d fractured media although we have very low porosity in our models this is explained by the absence of a connected porosity there is only a slight difference in effective velocities between the cases of intersecting and non intersecting cracks this can be clearly demonstrated up to a crack density that is close to the connectivity percolation threshold for crack densities beyond this threshold we observe that the differential effective medium dem theory gives the best fit with numerical results for intersecting cracks additionally it is shown that the scattering attenuation coefficient of the mean field predicted by the classical hudson approach is in excellent agreement with our numerical results

capitanian late guadalupian permian global brachiopod palaeobiogeography and latitudinal diversity pattern a global database containing 3365 occurrences 821 species and 251 genera of the capitanian late guadalupian permian brachiopod faunas from 24 stations has been analyzed by cluster analysis using the jaccard and otsuka coefficients and the probabilistic index of similarity nonmetric multidimensional scaling and minimum spanning tree two supergroups three groups and six subgroups are revealed and interpreted as representing respectively two biotic realms the palaeoequatorial and gondwanan realms two regions and six provinces an additional realm the boreal realm based on the fauna from spitsbergen also appears recognizable although it also shows considerable similarities with southwestern north america and the northern margin of gondwana as revealed by the statistical analysis the palaeoequatorial realm can be further subdivided into the north america region and the asian tethyan region the six biotic provinces are the cathaysian province in the palaeotethys and mesotethys the greenland svalbard province in the arctic region the austrazean province in eastern australia and new zealand the grandian province in western north america and the two transitional zones the himalayan province in the southern temperate zone and the sino mongolian japanese province in the northern temperate zone polynomial regression analysis and rarefaction analysis indicate that the generic diversities of brachiopod faunas during the capitanian peaked in the palaeoequatorial cathaysian province and the two transitional zones himalayan province and sino mongolian japanese province but fell dramatically in the polar regions the generic diversity of the palaeoequatorial grandian province is apparently lower than in the two transitional zones of temperate palaeolatitudes suggesting that the generic diversity of capitanian brachiopod faunas does not exhibit a strict negative correlation with palaeolatitudes this in turn would suggest that biogeographical determinants such as geographical barriers inhabitable area and ocean currents other than latitude related temperature control may also have played an important role in the dispersal of some brachiopods and the characterization of some local provinces and high diversities the capitanian global brachiopod palaeobiogeography is generally comparable with those in the wuchiapingian and changhsingian but with some notable differences these include 1 that the grandian province of the capitanian in western north america vanished after the end guadalupian regression 2 that the western tethyan province of the lopingian could not be distinguished in the capitanian and 3 that the austrazean province was larger in area than either in the wuchiapingian or in the changhsingian

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

growth of granite greenstone terranes at convergent margins and stabilization of archean cratons abstract archean granite greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events most archean granite greenstone terranes appear to have acquired their first order structural and metamorphic characteristics at convergent plate margins where large accretionary wedges similar in aspect to the chugach makran and altaids grew through offscraping and accretion of oceanic plateaux oceanic crustal fragments juvenile island arcs rifted continental margins and pelagic and terrigenous sediments buoyant slabs of parts of archean oceanic lithosphere may have been underplated beneath these orogens forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere back stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas sanukitoid suite in the mantle wedge whereas other magmas tonalite trondhjemite granodiorite or ttg suite appear to have been generated by melting of hot young subducted slabs eventual collision of these juvenile orogens with other continental blocks formed anatectic granites then thickened the crust beyond its ability to support its own mass which initiated gravitational collapse and decompressional release of syn to late tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs and aided in the cratonization of the granite greenstone terranes deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid and upper crust forming ore deposits and plutons most preserved granite greenstone terranes have been tectonically stable since the archean and form the cratonic interiors of many continents

melting in feldspar bearing systems to high pressures and the structures of aluminosilicate liquids to test the possibility that aluminosilicate liquids exhibit pressure induced transformations particularly involving changes in the coordination of aluminum we determined melting relationships for the feldspar bearing systems naalsi 3 o 8 sio 2 kalsi 3 o 8 sio 2 and caal 2 si 2 o 8 sio 2 from 1 atm to 25 kbar albite and anorthite behave similarly in that they and presumably liquids of these compositions transform at high pressures to jadeite kyanite corundum and other structures with aluminum in six fold coordination releasing sio 2 component this results in a large increase in the activity of sio 2 component in the liquid a l qz which is manifested by a significant decrease in the melting point depression of albite and of anorthite by the addition of quartz at pressures above 15 kbar in contrast sanidine does not transform to denser phases at pressures below at least 100 kbar but it melts incongruently to leucite sio 2 rich liquid up to 15 kbar this produces a relatively large a l qz and a small freezing point depression by quartz below this pressure the opposite holds above 15 kbar these results support the concept that significant structural changes including coordination changes in aluminum occur in magmas in the upper mantle

emplacement origins of coarsely crystalline mafic rocks hosted in greenstone belts examples from the 2 7 ga yilgarn craton western australia coarsely crystalline mafic rocks commonly referred to as dolerite and diabase make up a substantial proportion of archean greenstone belts and are hosts to major orogenic gold deposits yet their internal architecture and contact relationships remain poorly documented and their emplacement origins are assumed rather than supported with evidence we present results from drill core logging of several late archean coarsely crystalline packages from the eastern goldfields superterrane of the yilgarn craton western australia including the golden mile dolerite these large mafic bodies comprise mostly medium to coarsely crystalline mafic rocks but also include a variety of other products of differentiation the bodies range from 75 to 460 m thick and extend for tens of kilometers are primarily hosted in mudstones and basalt and appear to be concordant to stratigraphy as such this study excludes mafic dykes and extremely thick and internally complex bodies such as the bushveld complex from base to top the internal stratigraphy generally consists of a lower unit of aphyric basalt dolerite gabbro and peridotite pyroxenite then a voluminous middle dolerite gabbro and an upper quartz dolerite gabbro and a topmost aphyric basalt granophyric veins and domains are common in the upper third of the mafic body contacts are gradational between all lithofacies with the exception of granophric veins which have sharp contacts within this overall stratigraphic framework there is variability in the relative thickness of lithofacies number of lithofacies as well as repetition of some lithofacies several examples also include basalt in the interior in well exposed examples the key evidence for distinguishing intrusions from extrusions is provided by the top contact however the top contact from these examples is ambiguous to overcome this gap we review the character of well studied thick ponded lavas from the literature most aspects of the internal character are not unique to intrusions although the maximum groundmass grain size excluding granophyric veins of the archean examples 3 mm is significantly coarser than that for any known ponded lavas 1 mm the ability to distinguish intrusions from extrusions has important implications for reconstructing archean greenstone stratigraphy and mineral exploration

accretion of the archean slave province continental rift models have long been applied to the archean slave province of northwestern canada a reassessment of these models shows them to be incompatible with observed geological relations and suggests that contractional tectonic models may be more appropriate than extensional ones regions composed of different rock suites e g orthogneisses vs mafic volcanics are separated by high strain zones recording large displacements it is proposed that the high strain zones separate four distinct terranes that have been juxtaposed during collisional orogenesis from west to east these include the anton terrane interpreted as an archean microcontinent the sleepy dragon terrane possibly an exhumed more eastern part of the anton terrane the contwoyto terrane a westward verging fold and thrust belt containing tectonic slivers of greenstone volcanics and the hackett river volcanic terrane interpreted as an archean island arc the contwoyto and hackett river terranes represent a paired accretionary prism and island arc system that formed above an east dipping subduction zone these collided with the anton microcontinent producing a basement nappe expressed as the sleepy dragon terrane during the main accretion event within the slave province the whole tectonic assemblage was intruded by late kinematic to postkinematic granitoids

the lithosphere below the rockall trough wide angle seismic evidence for extensive serpentinisation abstract the longer range data from wide angle seismic reflection experiments along an axial and transverse profile determined the seismic properties of the shallow mantle lithosphere beneath the rockall trough in the north atlantic two subcrustal p and s wave reflections are observed the first defines a layer 3 10 km thick below the moho where p wave velocities vary from 7 5 to 7 8 km s v p v s ratios increase within this layer from 1 80 in the north to 1 83 in the south of the basin along the axial profile the second reflection from approximately 34 km depth identifies a layer 15 20 km thick with a vp velocity of approximately 8 1 km s and a v p v s ratio of 1 73 these values are typical for normal mantle peridotites both p and s wave velocities and v p v s ratios constrain the possible composition of the first layer which is interpreted as a zone of partially serpentinised peridotites below the moho about 15 volume alteration of the parent mantle peridotite is required to produce the observed seismic properties this degree of alteration accounts for a systematic deficit in total tectonic subsidence when compared with that predicted from the variation in bulk crustal stretching along the axis of the basin syntectonic cooling occurred during differential lithospheric stretching as the upper to mid crust became more extended over a narrower region than the mantle lithosphere this served to rheologically couple the lower crust to the mantle near the final stage of deformation as the primary brittle ductile transition zone in the crust migrated downwards into the mantle lithosphere the resultant fracturing generated the permeability necessary to facilitate the seawater circulation which hydrated the cold mantle

facies patterns and geography of the yangtze region south china through the ordovician and silurian transition abstract lithofacies and biofacies changes through the ordovician and silurian transition on the yangtze platform are coincident with stepwise mass extinctions recovery events global sea level changes and regional palaeogeographic configurations facies patterns through three intervals the late mid ashgill hirnantian and early rhuddanian indicate that black shale occupied most of the yangtze platform region during the late mid ashgill and early rhuddanian while the hirnantian possesses more diverse facies types the black shale was replaced by carbonate facies during the hirnantian coincident with a global sea level low stand biofacies changes from the late mid ashgill graptolitic to the hirnantian shelly facies were apparently signalled by gradual and diachronous existence of a mixed facies the manosia facies in the yangtze region however the biofacies change was abrupt from the hirnantian to the early rhuddanian when graptolitic facies occupied most areas on the yangtze platform leaving only a very narrow limited belt of shelly facies along the southern margin of the platform in northern guizhou the early rhuddanian unified biofacies pattern is a result of a global sea level rise which may be synchronized with a recovery bioevent of graptolites after hirnantian mass extinction

isostatic response of the australian lithosphere estimation of effective elastic thickness and anisotropy using multitaper spectral analysis gravity and topography provide important insights regarding the degree and mechanisms of isostatic compensation the azimuthally isotropic coherence function be tween the bouguer gravity anomaly and topography evolves from high to low for increasing wavenumber a diagnostic that can be predicted for a variety of lithospheric loading models and used in inversions for flexural rigidity thereof in this study we investigate the isostatic response of continental australia we consider the effects of directionally anisotropic plate strength on the coherelce the anisotropic coherence function is calculated for regions of australia that have distinctive geological and geophysical properties the coherence estimation is performed by the thomson multiple slepian taper spectral analysis method extended to two dimensional fields our analysis reveals the existence of flexural anisotropy in central australia indicative of a weaker n s direction of lower te this observation is consistent with the suggestion that the parallel faults in that area act to make the lithosphere weaker in the direction perpendicular to them it can also be related to the n s direction of maximum stress and possibly the presence of e w running zones weakened due to differential sediment burial rates we also demonstrate that the multitaper method has distinct advantages for computing the isotropic coherence function the ability to make many independent estimates of the isostatic response that are minimally affected by spectral leakage results in a coherence that is more robust than with modified periodogram methods particularly at low wavenumbers our analysis elucidates the reasons for discrepancies in previous estimates of effective elastic thickness te of the australian lithosphere in isotropic inversions for te we obtain values that are as much as a factor of 2 less than those obtained in standard inversions of the periodogram coherence using bouguer gravity and topography but greater than those obtained by inversions that utilize free air rather than bouguer gravity and ignore the presence of subsurface loads however owing to the low spectral power of the australian topography the uncertainty on any estimate of te is substantial

the electrical structure of the slave craton the slave craton in northwestern canada a relatively small archean craton 600400 km is ideal as a natural laboratory for investigating the formation and evolution of mesoarchean and neoarchean sub continental lithospheric mantle sclm excellent outcrop and the discovery of economic diamondiferous kimberlite pipes in the centre of the craton during the early 1990s have led to an unparalleled amount of geoscientific information becoming available over the last 5 years deep probing electromagnetic surveys were conducted on the slave using the natural source magnetotelluric mt technique as part of a variety of programs to study the craton and determine its regional scale electrical structure two of the four types of surveys involved novel mt data acquisition one through frozen lakes along ice roads during winter and the second using ocean bottom mt instrumentation deployed from float planes the primary initial objective of the mt surveys was to determine the geometry of the topography of the lithosphere asthenosphere boundary lab across the slave craton however the mt responses revealed completely serendipitously a remarkable anomaly in electrical conductivity in the sclm of the central slave craton this central slave mantle conductor csmc anomaly is modelled as a localized region of low resistivity 10 15 v m beginning at depths of f80 120 km and striking ne sw where precisely located it is spatially coincident with the eocene aged kimberlite field in the central part of the craton the so called corridor of hope and also with a geochemically defined ultra depleted harzburgitic layer interpreted as oceanic or arc related lithosphere emplaced during early tectonism the csmc lies wholly within the ne sw

structures textures and cooling histories of columbia river basalt flows flood basalt flows of the columbia river basalt group commonly exhibit well developed colonnade and entablature structures formed during cooling of individual flows the colonnade refers to the well defined columnar structure of many flows which appears in the lowermost 10 to 30 and in places the uppermost 10 to 20 entablature refers to those parts of a flow that have smaller column diameters and more irregular pattern fractures than those of colonnade entablature commonly occupies the central 60 to 70 of a flow in some cases however the entablature is interrupted by colonnade in such cases the entablature and colonnade may be repeated once or twice within a single flow in other cases flows may lack an entablature entirely petrographic examination reveals a pronounced correlation between structural and textural features thin sections of colonnade basalt have relatively coarse cruciform or octahedral fe ti oxide crystals and 20 glassy mesostasis the central entablature basalt in contrast has much more mesostasis as much as 60 and has textural features such as feathery fe ti oxide crystals indicative of more rapid cooling these features are the reverse of what might be expected for normal conductive cooling because cooling rates at the flow margins should be greater than those in the flow interior we reconciled the observed basalt textures with thermal models by taking into account rainfall and deranged drainage during crystallization of the flow downward migration of water through cracks would cause convective cooling of underlying unsolidified lava this effect and its consequences have been modeled numerically in order to clarify the physical constraints on the mechanism by which the central part of the lava can be quenched we have found that a water infiltration front can explain the presence of a thick quenched flow interior provided that the infiltration front moves downward just behind the lava solidification front despite heavy rainfall on hawaii for example 250 cm yr at kilauea iki exposed prehistoric lava flows on that island exhibit only colonnades we deduce from this that heavy rainfall alone is inadequate to quench flow interiors extensive flooding by deranged drainage or perhaps extremely heavy rainfall 250 cm yr is apparently required to produce entablatures the model also can be used to explain the occurrence of repeated entablature and colonnade by considering the effects of intermittent inundation the imposition of boundary conditions that mimic the removal and reoccurrence of flooding yields cooling histories that are consistent with the development of multiple colonnades and entablatures

environmental controls on marine ecosystem recovery following mass extinctions with an example from the early triassic abstract the recovery of marine ecosystems following a mass extinction event involves an extended interval of increasing biotic diversity and ecosystem complexity the pace of recovery may be controlled by intrinsic ecosystem or extrinsic environmental factors here we present an analysis of changes in marine conditions following the end permian mass extinction with the objective of evaluating the role of environmental factors in the protracted 5 myr long recovery of marine ecosystems during the early triassic specifically our study examines changes in weathering productivity and redox proxies in three sections in south china chaohu daxiakou and zuodeng and one in northern india mud our results reveal 1 recurrent environmental perturbations during the early triassic 2 a general pattern of high terrestrial weathering rates and more intensely reducing marine redox conditions during the early griesbachian late griesbachian mid smithian and more weakly the mid spathian 3 increases in marine productivity during the aforementioned intervals except for the early griesbachian and 4 stronger and more temporally discrete intervals of environmental change in deepwater sections chaohu and daxiakou relative to shallow and intermediate sections zuodeng and mud our analysis reveals a close relationship between episodes of marine environmental deterioration and a slowing or reversal of ecosystem recovery based on metrics of biodiversity within community alpha diversity infaunal burrowing and ecosystem tiering we infer that the pattern and pace of marine ecosystem recovery was strongly modulated by recurrent environmental perturbations during the early triassic these perturbations were associated with elevated weathering and productivity fluxes implying that nutrient and energy flows were key influences on recovery more regular secular variation in deepwater relative to shallow water environmental conditions implies that perturbations originated at depth i e within the oceanic thermocline and influenced the ocean surface layer irregularly finally we compared patterns of environmental disturbance and ecosystem recovery following the other four big five phanerozoic mass extinctions to evaluate whether commonalities exist in general the pace of ecosystem recovery depends on the degree of stability of the post crisis marine environment

felsic segregation during crystallization of a subaqueous lava field odp iodp site 1256 east pacific rise inferences from structure and petrography abstract massive basalt flows have been mainly observed and studied in subaerial environments but data collected during sonar surveys and drilling cruises in ocean highlighted that thick lava fields up to 70 m in thickness are widespread in the oceanic crust produced along fast spreading ridges most of the thickest lava flows include felsic differentiates consisting of na plagioclase quartz drilling at odp iodp site 1256 encountered a 30 m and 70 m tick massive lava flow near the top of the basement in holes 1256c and d respectively rare 1 to 3 late magmatic veins lmvs and late magmatic domains lmds of felsic material occur within this very thick lava flow lmvs and lmds occur in a range of different orientations attitudes and textures and cut the basalt magmatic assemblage inducing a deformation ranging from brittle to ductile fine scale structural microstructural and petrographic analyses from the giant lava flow suggest that segregation and migration of felsic melt through host basalt were strictly related to the cooling and crystallization of the lava flow which represents a single stage magmatic event

effective elasticity of fractured rocks a snapshot of the work in progress exploration and development of naturally fractured reservoirs rely on understanding and interpretation of certain signatures associated with seismic waves propagating through cracked rocks this understanding comes primarily from the effective media theories that predict an overall elastic behavior of a solid containing many inhomogeneities cracks in particular whose sizes are too small to be seen individually by the waves to model seismic responses of fractured formations a geophysicist typically has a choice between the effective media schemes of hudson and schoenberg while the two predictions usually deviate slightly for liquid filled cracks the differences are significant when the fractures are dry explaining the origin of these differences and selecting a more accurate scheme is the first goal of this tutorial our second more challenging task is to prove that simply adding the compliance contributions of cracks as if they were isolated and noninteracting remains sufficiently accurate even for fractures that grossly violate the basic theoretical assumption of penny shaped cracks real fractures have notoriously irregular shapes might be partially closed and often form interconnected networks yet these details of fracture microgeometry turn out to be unimportant for the effective elasticity given a typical noise level in seismic data no closed form theory exists for irregular fracture shapes however take into account finite element simulations on so called digital rocks demonstrate which features of the crack geometry have to be taken into account because they influence propagation of long compared to the size of fractures seismic waves and which features can be ignored

channelized lava flows at the east pacific rise crest 9 10 n the importance of off axis lava transport in developing the architecture of young oceanic crust 1 submarine lava flows are the building blocks of young oceanic crust lava erupted at the ridge axis is transported across the ridge crest in a manner dictated by the rheology of the lava the characteristics of the eruption and the topography it encounters the resulting lava flows can vary dramatically in form and consequently in their impact on the physical characteristics of the seafloor and the architecture of the upper 50 500 m of the oceanic crust we have mapped and measured numerous submarine channelized lava flows at the east pacific rise epr crest 9 10 n that reflect the high effusion rate and high flow velocity end member of lava eruption and transport at mid ocean ridges channel systems composed of identifiable segments 50 1000 m in length extend up to 3 km from the axial summit trough ast and have widths of 10 50 m and depths of 2 3 m samples collected within the channels are n morb with mg indicating eruption from the ast we produce detailed maps of lava surface morphology across the channel surface from mosaics of digital images that show lineated or flat sheets at the channel center bounded by brecciated lava at the channel margins modeled velocity profiles across the channel surface allow us to determine flux through the channels from 0 4 to 4 7 103 m3 s and modeled shear rates help explain the surface morphology variation we suggest that channelized lava flows are a primary mechanism by which lava accumulates in the off axis region 1 3 km and produces the layer 2a thickening that is observed at fast and superfast spreading ridges in addition the rapid high volume flux eruptions necessary to produce channelized flows may act as an indicator of the local magma budget along the epr we find that high concentrations of channelized lava flows correlate with local across axis ridge morphology indicative of an elevated magma budget additionally in locations where channelized flows are located dominantly to the east or west of the ast the ridge crest is asymmetric and layer 2a appears to thicken over a greater distance from the ast toward the side of the ridge crest where the channels are located

a 220 km2 recently erupted lava field on the east pacific rise near lat 8 s seamarc ii records show a 220 km2 area of unusually high reflectivity on the east pacific rise epr near lat 8 s which we interpret to be a recently erupted lava field this is the only segment of the epr exhibiting high reflectivity over such a large area in our continuous seamarc ii coverage from lat 8 n to 18 n and 3 s to 23 s the lavas appear to issue from the axial neovolcanic zone between lat 8 15 and 8 17 s the lava field extends up to 18km off axis and inundates scarps up to 100 m high on the basis of measurements of scarp heights just outside areas where they are buried we estimate the average thickness of the field to be 70 20 m thus the volume of the lava field is 15 4 km3 in comparison the average annual volcanic budget for the earth is approximately 4 5 km3 and the largest historic basaltic eruption from the icelandic laki fissure in 1783 was approximately 12 3 km3 when did the eruption occur comparative studies of acoustic reflectivity and photographed bottom characteristics elsewhere along the epr suggest that the lava field has little or no sediment cover and is very young this is also one of the only areas on the epr where earthquakes occur that do not appear to be caused by transform faulting if the earthquakes are related to the lava field then major eruptions may have occurred in 1964 1965 and 1969 a large helium plume near lat 13 s may be caused in part by eruptions of the lat 8 s lava field this is consistent with estimates of currents at 2 3 km depth which are such that a plume generated at lat 8 s would eventually appear off axis at lat 13 s our observations suggest that this spreading segment which is bounded by small ridge axis discontinuities at lat 8 05 s and 8 47 s may have been the most volcanically active segment along the epr during the past 25 yr

depth shifting and orientation of core data using a core log integration approach a case study from odp iodp hole 1256d abstract studies of the evolution and architecture of the oceanic crust s basement are significantly hindered by the lack of sample orientation with respect to geographic coordinates generally the length of cores recovered in oceans is not equal to the drilled length recovery percentage 100 thus data such as structural measurements are either lost or not suitable to reliably interpret many aspects of the geological environment core log integration enables the correction of each core piece to the appropriate in situ depth position and reorientation of core structures with respect to magnetic north by correlating core measurements to downhole high resolution resistivity images and acoustic images this technique was tested on results from the shallowest section of odp iodp hole 1256d that drilled through a complete in situ section of upper ocean crust formed at ultra high spreading rate

multivariate data analysis in palaeoecology and palaeobiogeography a review abstract palaeoecologists and palaeobiogeographers are often confronted with large multivariate data sets from which fossil communities assemblages or associations or faunal provinces are to be recognized the complex multidimensional nature of these data sets the recognition of the enormous variation of communities and environments both in space and time and the difficulties of dealing with these problems by the human mind justify the use of multivariate statistical methods in palaeoecology and palaeobiogeography concepts and basic procedures of several commonly used multivariated statistical methods of palaeoecology and palaeobiogeography are reviewed in the wider context of quantitative community ecology and biogeography basic assumptions about data structure and theoretical and practical limitations are discussed the emphasis is on evaluation of binary similarity coefficients and two multivariate approaches cluster analysis and ordination thirty nine binary similarity coefficients are evaluated against nine criteria jaccard s coefficient of community is found most suitable as a similarity measure between samples under the conditions tested algorithmic procedures of cluster analysis especially the agglomerative hierarchical cluster analysis techniques and indirect ordinations particularly polar ordination principal component analysis principal coordinate analysis correspondence analysis detrended correspondence analysis and nonmetric multidimensional scaling are outlined and their applicability to palaeoecological and palaeobiogeographical data is discussed where the data are appropriate an integration of cluster analysis and ordination is suggested to be applied to the same data

tectonic analysis and paleo stress determination of the upper lava section at odp iodp site 1256 east pacific ocean abstract research on the deep sea is of great importance for a better understanding of the mechanism of magma emplacement and the tectonic evolution of oceanic crust however details of the internal structure in the upper levels of the oceanic crust are much less complete than that of the more fully studied sub aerial areas for the first time this study proposes a dynamic analysis using the inversion method on core data derived from the drilled basement of the present day intact oceanic crust at odp iodp site 1256 in the cocos plate the research is based on an innovative core reorientation process and combines different stress hypothesis approaches for the analysis of heterogeneous failure slip data via exploitation of two distinct techniques from the analysis of the failure slip data both techniques produce 5 distinct subsystem datasets all calculated subsystems are mechanically and geometrically admissible interpretation of the results allows the researchers to note a complex local and regional tectonic evolution deriving from the interplay of 1 the ridge push and rotation of both the east pacific rise and the cocos nazca spreading center 2 the effect of the slab pull of the middle america trench 3 the influence of lava emplacement mechanisms and 4 intra plate deformation

determination of the volcanostratigraphy of oceanic crust formed at superfast spreading ridge electrofacies analyses of odp iodp hole 1256d 1 the objective of this study is to construct a representative volcanostratigraphy of ocean drilling program hole 1256d the first complete penetration of intact upper oceanic crust formed at a superfast spreading rate an accurate knowledge of the volcanostratigraphy is vital to understand processes of crustal construction and submarine magmatism and to estimate chemical exchange with seawater but this is rarely achieved due to very low recovery rates in most basement holes we used two approaches to determine the rock types that form the wall rocks in the basement sections of hole 1256d 1 user guided interpretations of electrofacies acquired by imaging tools combined with other wireline tools and 2 the use of an artificial neutral network to objectively classify the responses of all available logging information great availability of formation microscanner fms images provided superior coverage of the borehole wall compared to previous attempts at core log integration this has resulted in more confident and detailed lithologic classifications such as with the distinction between pillows and different styles of breciation ten lithology types are suggested for a volcanostratigraphy model massive flows ponded lava fractured massive flows fragmented flows thin flows or thick pillows pillows breccias dikes in dike complex isolated dikes and gabbros three major lithology types in the extrusive section are massive flows both massive and fragmented massive flow 22 fragmented flows 32 and breccias 19 pillow lavas make up only 1 9 of the volcanic section and are confined to a 100 m interval below the extrusive section subvertical contacts interpreted to be dike margins are typically observed every 1 to 2 m with brecciated zones along the contacts the dikes dip steeply to the northeast indicating slight rotation away from the ridge axis we used an artificial neural network ann approach to determine a quantitative lithostratigraphy the ann is most strongly influenced by porosity and alteration degrees and the resulting stratigraphy most closely resembles the above classifications when clustered by fms texture as opposed to lithologic interpretation the ann thus provides a porosity based stratigraphy of the basement rather than the traditional lithology based stratigraphy

spatial distribution of kimberlite in the slave craton canada a geometrical approach abstract exploration within the slave craton has revealed clusters of kimberlite intrusions commonly with internally consistent geochemical and temporal characteristics translation diagrams fry analysis allow an unbiased geometrical examination of the distance and direction between each kimberlite occurrence and all others in the database recurrent patterns are visually accentuated due to the square function in data density circular histograms quantify the azimuthal density of kimberlite at various distances for this study the database comprises the geographic position of 212 kimberlite occurrences of which 70 are from the lac de gras field ldg analyses are presented separately for the ldg data and for all non ldg data in order to test for regional variations and to avoid overwhelming the craton scale studies by the high density of ldg data empirical grouping of kimberlite locations results in delineation of five elliptical clusters that encompass all but four kimberlite occurrences clusters within the western part of the craton are elongate to the north northeast and align within a narrow zone western corridor elsewhere the clusters are elongate to the northwest or west northwest and appear to be arranged en echelon within a poorly defined north northwest trending zone central corridor geometrical spatial analyses of kimberlite locations highlight the craton scale pattern of emplacement within the two main corridors at regional and local scales individual intrusions are preferentially located towards the west northwest ca 280 and north northeast ca 015 of other intrusions and these orientations are interpreted to reflect upper mantle trends in magma generation at local scales 10 25 km kimberlite of the central and southern craton tends to be located to the northeast ca 045 and possibly weakly to the east northeast ca 070 of other intrusions and these orientations correspond to major crustal fractures systems it is proposed that kimberlite emplacement is controlled primarily by the interaction of elongate 280 and 015 source regions with near surface deviations influenced by crustal fracture systems the 015 trend evident at craton regional and local scales is parallel to a swarm of alkaline diabase dykes that are concentrated in a ca 30 km wide corridor passing through lac de gras a profound spatial association between significantly diamondiferous kimberlite and the margins of the dyke corridor suggests the corridor is the surface expression of a mantle depth structure it remains unclear whether the proposed mantle structure coincides with a diamond rich zone near the base of the lithosphere or delineates pathways favorable for diamond preservation during emplacement the linear array of kimberlite within the western craton forms a parallel corridor that may be an analogous mantle structure but which to date has failed to yield economic diamond concentrations

detection and estimation of gas hydrates using rock physics and seismic inversion examples from the northern deepwater gulf of mexico natural gas hydrates composed primarily of water and methane are solid crystalline ice like substances found in permafrost areas and deepwater basins around the world as the search for oil and gas extends into ever deeper waters particularly within the northern gulf of mexico gas hydrates are becoming more of a focus in terms of both safety and as a potential energy resource

global palaeobiogeographical patterns in brachiopods from survival to recovery after the end ordovician mass extinction abstract extinction events usually disrupt palaeobiogeographical patterns but there have been few detailed studies of the early silurian survival recovery period after the late ordovician mass extinction this paper summarizes and revises published information on the recovery of early and late rhuddanian brachiopod faunas the global dataset consists of 137 occurrences 72 genera and 13 localities of early rhuddanian survival interval and 271 occurrences 91 genera and 26 localities of the late rhuddanian early recovery interval the data are analyzed using cluster analysis nonmetric multidimensional scaling and minimum spanning tree using yule s y coefficient and the raup crick coefficient probabilistic index of similarity the results display palaeolatitudinal distribution patterns for brachiopods during their survival and recovery frequency analysis of the data indicates that cosmopolitan taxa before the mass extinction decreased in their post extinction distribution the survivors were mostly confined to one palaeoplate or even one locality in the survival interval but expanded progressively in the subsequent recovery period

lithosphere development in the slave craton a linked crustal and mantle perspective the late tectonic evolution of the slave craton involves extensive magmatism deformation and high temperature low pressure ht lp metamorphism we argue that the nature of these tectonic events is difficult to reconcile with early pre 2 7 ga development and preservation of a thick tectosphere and suggest that crust mantle coupling and stabilization occurred only late in the orogenic development of the craton the extent and repetitiveness of the tectonic reworking documented within the mesoarchean basement complex of the western slave together with the development of large volume extensional mafic magmatism at 2 7 ga within the basement complex argue against preservation of a widespread thick cool mesoarchean tectosphere beneath the western slave craton prior to neoarchean tectonism broad scale geological and geophysical features of the slave craton including orientation of an early f1 fold belt distribution of ca 2 63 2 62 ga plutonic rocks and the distribution of geochemical petrological and geophysical domains within the mantle lithosphere collectively highlight the importance of an ne sw structural grain to the craton these trends are oblique to the earlier ca 2 7 ga north south trending boundary between mesoarchean and neoarchean crustal domains and are interpreted to represent a younger structural feature imposed during northwest or southeast vergent tectonism at ca 2 64 2 61 ga extensive plutonism in part mantle derived crustal melting and associated ht lp metamorphism argue for widespread mantle heat input to the crust a feature most consistent with thin 100 km lithosphere at that time we propose that the mantle lithosphere developed by tectonic imbrication of one or more slabs subducted beneath the craton at the time of development of the d1 structural grain producing the early 2 63 2 62 ga arc like plutonic rocks subsequent collision external to the present craton boundaries possibly accompanied by partial delamination of some of the underthrust lithosphere produced widespread deformation d2 and granite plutonism throughout the province at 2 6 2 58 ga an implication of this model is that diamond formation in the slave should be neoarchean in age d 2003 elsevier b v all rights reserved

survival strategies of brachiopod faunas from the end permian mass extinction abstract global reviews reveal that 102 brachiopod species including 40 indeterminate species belonging to 43 genera within 27 families survived the end permian mass extinction in south china the himalayan regions southern tibet nepal salt range and kashmir southern alps italy arctic canada western usa and western australia of these the surviving brachiopods are most abundant and widespread in south china where they occur in 32 of the 42 permian triassic p t boundary sections around the world brachiopod faunas are the most abundant and diverse post extinction survivors and their appearance above the end permian mass extinction horizon somewhat masks recognition of the end permian extinction these surviving brachiopods occur in three stratal intervals corresponding to the mixed fauna beds mfb hereafter 1 3 of the p t boundary beds a sharp drop in diversity of the brachiopod faunas coincides with the widely accepted end permian mass extinction horizon the post extinction brachiopods were also affected by a subsequent crisis corresponding to the boundary between mfb 2 and mfb 3 so that most survivors were extinct approximately 0 7 ma after the end permian mass extinction the surviving brachiopods were mainly productida followed by spiriferida in particular small chonetids chonetid like productellids and smooth thin shelled ambocollid spiriferids are most abundant lingula is extraordinarily abundant and widespread approximately 0 7 ma after the greatest extinction when many regions became devoid of articulate brachiopods widespread broadly adapted and small sized taxa preferentially survived frequent intrageneric speciation of widespread widely adapted generalist genera enabled survival brachiopods to occupy rapidly and efficiently vacant ecospace in the aftermath of the end permian extinction the ecological habitats of the post extinction brachiopods are subdivided into nearshore epeiric sea restricted carbonate platform open carbonate platform ramp and outer shelf environments of these the survival brachiopods are most diverse in the open platform environments the nearshore zone provided ideal habitats for the nonarticulated brachiopods to occupy whereas the survivors in the outer shelf are predominantly chonetids and chonetid like productellids the most successful survivors the productid brachiopods are widely distributed in six types of ecological habitats and thus have no apparent preference for specific environments the end permian extinction resulted in the movement of biogeographical provinces towards the palaeo equator the brachiopods survived mostly in the palaeo equatorial provinces and along the southern margins of the palaeo tethys ocean

seismic velocity and q anisotropy in fractured poroelastic media a fluid saturated poroelastic isotropic medium with aligned fractures behaves as a transversely isotropic and viscoelastic tiv medium when the predominant wavelength is much larger than the average distance between fractures a planar fracture embedded in a fluid saturated poroelastic background medium can be modeled as a extremely thin and compliant porous layer p waves traveling in this type of medium induce fluid pressure gradients at fractures and mesoscopic scale heterogeneities generating fluid flow and slow diffusion biot waves causing attenuation and dispersion of the fast modes mesoscopic loss a poroelastic medium with embedded aligned fractures exhibits significant attenuation and dispersion effects due to this mechanism which can properly be represented at the macroscale with an equivalent tiv medium in this work we apply a set of compressibility and shear harmonic finite element fe experiments on fractured highly heterogeneous poroelastic samples to determine the five complex and frequency dependent stiffnesses characterizing the equivalent medium the experiments consider brine or patchy brine co 2 saturated samples and a brine saturated sample with a heterogeneous fractal skeleton with fractures we show that fractures induce strong seismic velocity and q anisotropy both for qp and qsv waves enhanced either by patchy saturation or frame heterogeneity

the central slave basement complex part i its structural topology and autochthonous cover new field and geochronological data are used to define the distribution of mesoarchean basement rocks in the south central slave province this distribution reflects a single contiguous basement terrane that we propose to call the central slave basement complex it shows a structural topology that is internally consistent and compatible with known regional folding and faulting events a sample of a proposed basement gneiss below the courageous lake greenstone belt central slave province has been dated by u pb methods and yields an age of 3325 8 ma consistent with the new basement distribution this sample also contains 2723 3 ma metamorphic zircon and ca 2680 ma titanite the central slave basement complex is overlain by a thin discontinuous but distinctive cover sequence that includes minor volcanic rocks clastic sedimentary rocks and banded iron formation all previously known and some new occurrences of this distinctive cover sequence occur in the immediate stratigraphic hanging wall of the