changing fluvial environments across the permian triassic boundary in the karoo basin south africa and possible causes of tetrapod extinctions abstract the karoo supergroup is a 12 km thick succession of sedimentary rocks that accumulated in a large intracratonic retroarc foreland basin in southwestern gondwana the strata record 100 million years of almost continuous sediment accumulation from the permo carboniferous 300 ma through to the early jurassic 190 ma under a range of climatic regimes and within several tectonically controlled sub basins alluvial sediments dominate the succession from the late permian onwards fossils of synapsid reptiles and early dinosaurs are sufficiently common to be used in a ten fold biostratigraphic subdivision of these strata the permian triassic boundary in the karoo succession is marked by a major extinction of the herbivorous dicynodonts which co incides with a rapid change in fluvial facies this study uses field observations of the sedimentary facies palaeosols and in situ fossils of well exposed p tr boundary sequences in the southern karoo basin to provide evidence of environmental changes that may have caused the tetrapod extinctions in the main karoo basin below the p tr boundary sequence the sediments comprise drab greenish grey mudrocks with a few thick 3 12 m multistoried laterally accreted fine grained sandstone bodies the mudrocks host hydromorphic palaeosols with palustrine carbonate horizons and mostly disarticulated skeletal remains of dicynodon assemblage zone fauna above a transition zone of approximately 20 m thickness these strata change into reddish brown mudrocks with numerous thin 0 5 5 m ribbon sandstone bodies and more continuous sheet sandstones calcic palaeosols with rhizocretions nodular horizons and pervasive calcareous encrustation of articulated skeletons of lystrosaurus and associated fauna of the lystrosaurus assemblage zone within a 20 m thick interval of the transitional strata fossils of dicynodon and lystrosaurus occur together the facies transition that co incides with the latest permian dicynodont extinction is interpreted as a change in fluvial style from meandering to low sinuosity channels with general drying of the floodplain habitats and redistribution of vegetation belts the horsetails in particular became significantly reduced in areal distribution the changes in fluvial style in the karoo basin at the end of the permian were triggered by a pulse of thrusting in the southerly source area dated at 247 ma 2 ma which brought about rapid progradation of a large sandy braided fan system the katberg sandstone fm into the central parts of the basin a concomitant basinward migration of depositional environments allowed the lystrosaurus communities that were adapted to dry floodplains to colonise the entire basin to the detriment of most of the wet floodplain dicynodon fauna that lived there

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

palaeogeographical significance of clay mineral assemblages in the permian and triassic sediments of the se iberian ranges eastern spain the evolution of the palaeogeography of the se iberian basin during the permian and triassic represents a general evolution from continental to marine environments it has been recently studied from the sedimentological stratigraphical tectonic and palaeontological points of view in spite of these results many aspects of this palaeogeography are still a matter of discussion in this study clay mineralogy analysis complements previous studies representing a new aspect for understanding the evolution of the sedimentary environment and the palaeogeography of the iberian basin during the periods in question and thus of the palaeogeography and the location of the major high areas in the westernmost border of the tethys sea in spite of late diagenetic transformations the original clay mineral associations of the permian triassic sediments of the se iberian ranges can be reconstructed seventy seven samples of siliciclastic and carbonate sediments of these ages have been studied sem and xrd revealing six new aspects that help to precise the palaeogeographical interpretation of the area 1 two major mineral assemblages have been found illite kaolinite pyrophyllite in the continental facies and illite chlorite vermiculite mixed layer clays in the marine facies 2 the mg rich clay minerals are here considered to be of marine origin 3 active phases of basin boundary faults are marked in the sediments by the presence of pyrophyllite derived directly from the palaeozoic metamorphic basement 4 unconformities separating major depositional sequences also separate formations with different clay mineralogy 5 different groups of clay minerals can be separated clearly coinciding with the different palaeogeographical stages also distinguished in the westernmost border of the tethys sea 6 the clay mineral associations back up the data of a previous hypothesis of a humid climate for the end of the permian in the study area just prior to the first incursion of the tethys sea

composition and tectonic evolution of the chinese continental crust constrained by poisson s ratio article i nfo we have measured both p and s wave velocities vp and vs and poisson s ratios of 60 typical ultrahigh pressure uhp metamorphic rock samples from the chinese continental scientific drilling ccsd main and pre pilot holes and surface outcrops in the sulu dabie orogenic belt at hydrostatic confining pressures up to 850 mpa the experimental results together with those compiled in handbook of seismic properties of minerals rocks and ores ji s c wang q xia b 2002 handbook of seismic properties of minerals rocks and ores polytechnic international press montreal 630 pp reveal that except for monomineralic rocks such as quartzite serpentinite anorthosite limestone and marble the rest of the rock types have poisson s ratios falling along an upward convex curve determined from the correlations between elastic moduli and density poisson s ratios increase with density as the lithology changes from granite felsic gneiss and schist through diorite syenite intermediate gneiss and metasediment to gabbro diabase amphibolite and mafic gneiss and then decrease as the rocks become ultramafic in composition eclogite has a higher density but a lower poisson s ratio than peridotite the results were applied to constrain the crustal composition and tectonic evolution of the chinese continental crust based on crustal thickness h and poisson s ratio from 248 broadband seismic stations measured using teleseismic receiver function techniques the north china yangtze south china and northeast china blocks and songpan ganzi terrane are dominated by low b0 26 and moderate 0 26 b0 28 values n70 suggesting the dominance of felsic composition in the crust the lhasa terrane qiangtang terrane and indochina block are characterized by high proportions 33 42 of measurements with very high values 0 30 and h is found for the south china block northeast block lhasa block qiangtang terrane and indochina block indicating either tectonic thickening of the felsic upper and middle crust by folding and thrusting or the removal of mafic layers from the lower crust into the upper mantle by delamination

seismic properties of rock samples from the pikwitonei granulite belt god s lake domain crustal cross section manitoba laboratory measurements of compressional and shear wave velocity to confining pressures of 600 mpa for a suite of representative samples collected from the pikwitonei granulite belt and god s lake domain an archean crustal cross section in the northwestern superior province provide the basis of comparison of these terranes with the seismic characteristics of archean lower crust we found that felsic rocks in the pikwitonei granulite belt and god s lake domain which make up the bulk of these terranes have a similar average compressional wave velocity of 6 5 km s at 600 mpa indicating that felsic rocks show little velocity change across the amphibolite granulite facies transition compressional wave velocities for mafic rocks from each terrane are between 7 1 and 7 3 km s apparent poisson s ratio ranges from 0 24 to 0 26 and 0 26 to 0 28 for felsic and mafic rocks respectively these velocity data compare favorably with data for similar lithologies from the kapuskasing uplift using the relative abun

crustal fabric in the tibetan plateau based on waveform inversions for seismic anisotropy parameters 1 the tibetan plateau has the thickest continental crust on earth and fabrics within the crust that are anisotropic to seismic waves may provide clues to how it reached such extreme proportions and how it is currently deforming waveform modeling using a global minimization inversion technique applied to receiver functions computed from 11 stations spanning the north south length of the eastern plateau has yielded a suite of crustal models that include anisotropy these models suggest that the tibetan crust contains 4 14 anisotropy at different depths that is likely a result of both fossil fabrics and more recent deformation all models contain anisotropy in the surface layer and for most stations the alignment of the slow symmetry axis suggests a relationship with crustal fabrics associated with e w trending thrust faults or sutures middle to lower crustal anisotropy is present at most stations with a fast axis trending n s to nw se in the south nearly e w in the central plateau and n s to ne sw in the northern plateau this pattern appears consistent with recent ductile deformation due to both topographically induced flow and to boundary forces from subducting lithosphere at the northern and southern margins of the plateau the orientations of crustal anisotropy determined for most stations in this study are significantly different from shear wave splitting fast polarization directions implying distinct deformation in the crust and mantle

u pb zircon geochronology and tempo of the end permian mass extinction the mass extinction at the end of the permian was the most profound in the history of life fundamental to understanding its cause is determining the tempo and duration of the extinction uranium lead zircon data from late permian and early triassic rocks from south china place the permian triassic boundary at 251 4 0 3 million years ago biostratigraphic controls from strata intercalated with ash beds below the boundary indicate that the changhsingian pulse of the end permian extinction corresponding to the disappearance of about 85 percent of marine species lasted less than 1 million years at meishan a negative excursion in 13 c at the boundary had a duration of 165 000 years or less suggesting a catastrophic addition of light carbon

emeishan basalts sw china reappraisal of the formation s type area stratigraphy and a discussion of its significance as a large igneous province the late permian emeishan basalt formation of sw china is one of earth s lips large igneous provinces yet its basic geology remains poorly documented recent work on sections close to the type area in sichuan province enable us in part to rectify this descriptions of the formation and associated units at two areas one on the lower flanks of mt emei and another from a series of outcrops in ebian county 50 70 km to the sw are presented the basalt pile is 180 270 m thick and in both areas comprises 12 flows that were erupted in relatively quick succession it rests conformably upon shallow marine limestones lignites suggesting emplacement close to sea level the upper half of the youngest basalt was intensively weathered but not eroded prior to it being conformably succeeded by complex body of rocks c 30 m thick that includes thin basalts pyroclastic rocks tuffs and organic rich terrestrial sediments this unit which has previously been described as a sedimentary package presumably because intense weathering has obscured the primary lithological fabric in key outcrops is considered to mark the volcanic waning phase uppermost permian and triassic terrestrial sediments conformably overlie the terminal volcanic rocks the sub regional stratigraphy is compared as best it can be with that described from two sections 400 km to the se one section matches reasonably well the other does not indicating that regional correlations need to be developed carefully the information is discussed in the context of lip generator models several key features of the emeishan basalt terrain are at odds with those commonly encountered in lip s the most important conclusion is that the unit marks a prematurely terminated system in which full bloodied rifting leading to the development of an ocean basin never started

seismic velocities anisotropy and shear wave splitting of antigorite serpentinites and tectonic implications for subduction zones 1 antigorite the high temperature ht form of serpentinite is believed to play a critical role in various geological processes of subduction zones we have measured p and s wave velocities vp and vs anisotropy and shear wave splitting of 17 serpentinite samples containing 90 antigorite at pressures up to 650 mpa the new results combined with data for low temperature lt lizardite and or chrysolite reveal distinct effects of lt and ht serpentinization on the seismic properties of mantle rocks at 600 mpa vp 5 10 and 6 68 km s vs 2 32 and 3 67 km s and vp vs 2 15 and 1 81 for pure lt and ht serpentinites respectively above the crack closure pressure 150 mpa the velocity ratio of antigorite serpentinites displays little dependence on pressure or temperature serpentine contents within subduction zones and forearc mantle wedges where temperature is 300 c should be at least twice that of previous estimates based on lt serpentinization the presence of seismic anisotropy high pressure fluids or partial melt is also needed to interpret ht serpentinized mantle with vp 1 81 the intrinsic anisotropy of the serpentinites 3 8 16 9 with an average value of 10 5 for vp and 3 6 18 3 with an average value of 10 4 for vs is caused by dislocation creep induced lattice preferred orientation of antigorite three distinct patterns of seismic anisotropy correspond to three types of antigorite fabrics s l and ls tectonites formed by three categories of strain geometry i e coaxial flattening coaxial constriction and simple shear respectively our results are thought to provide a new explanation for various anisotropic patterns of subduction systems observed worldwide

shear wave velocities anisotropy and splitting in high grade mylonites shear wave velocities have been measured for 10 high grade granulite and upper amphibolite facies mylonites from the snowbird tectonic zone canada at various pressures up to 600 mpa in order to explore the kinematic significance of shear wave splitting in the lower crust at 600 mpa vs anisotropy ranges from 11 in amphibolitic mylonites to 2 in granulite facies pyroxene bearing and tonalitic mylonites hornblende bearing granitic mylonites display intermediate vs anisotropy 6 7 vs anisotropy is lower than vp anisotropy in hornblende rich mylonites whereas vs anisotropy is higher than vp anisotropy in feldspar rich mafic tonalitic granitic and diatexitic mylonites shear wave splitting is pronounced in rocks displaying strong s wave anisotropy in most of the high grade mylonites studied the polarization direction of the first arrival vs1 corresponds to the extension lineation x when propagation is parallel to y perpendicular to lineation and parallel to foliation or to the y direction when propagation is parallel to x for all other propagation directions the polarization direction of vs1 is oblique to the x y and z directions therefore it may be difficult to infer the structural implications of shear wave splitting in lower continental crustal shear zones if one does not know 1 the orientation of the propagation and fast vibration directions with respect to in situ lineation and foliation and 2 the lattice preferred orientation of in situ rock forming minerals

sequence stratigraphy of continental triassic strata of southernmost brazil a contribution to southwestern gondwana palaeogeography and palaeoclimate abstract the continental triassic succession of southernmost brazil comprises two second order depositional sequences the sanga do cabral early triassic and the santa maria middle to late triassic supersequences the first one includes ephemeral low sinuosity fluvial deposits developed on a low gradient plain based on fossil tetrapods especially procolophonids an upper induan age is estimated for this sequence facies association of the santa maria supersequence indicates low sinuosity fluvial rivers deltas and lakes this supersequence can be further subdivided into three third order sequences age provided by palaeovertebrate biostratigraphic data as follows santa maria 1 ladinian santa maria 2 carnian to early norian and santa maria 3 probably raethian or early jurassic sequences the gondwanides paroxysms i and ii in the sierra de la ventana cape fold belt are directly related to the development of both supersequences the source area of the sanga do cabral supersequence was located to the south it consisted of an uplifted peripheral bulge situated landward of the retro foreland system from where older sedimentary rocks were eroded the source area of the santa maria supersequence was also positioned southwards and related to the uplifted sul rio grandense and uruguayan shields the santa maria supersequence stratigraphic architecture is comparable to the triassic rift basins of western argentina diagenesis facies and palaeontology of the studied succession suggest a dominantly semiarid climate during the triassic

pattern of vertebrate extinctions across an event bed at the permian triassic boundary in the karoo basin of south africa the extinction of vertebrates around the time of the permian triassic boundary has long been regarded as a gradual event occurring over millions of years our new field investigations of fluvial strata in the central and southern karoo basin of south africa have revealed the presence of an event bed coinciding with a mass extinction of terrestrial fauna and flora the bed is in a sedimentary sequence that is marked by a reddening of flood plain mud rocks and a change from high to low sinuosity river channel systems here we show that the pattern of vertebrate taxa disappearing below this boundary and the subsequent appearance of new taxa above the boundary are consistent with a relatively sudden possibly catastrophic event perhaps of 50 000 yr duration or less

seismic properties of rocks from an exposure of extended continental crust new laboratory measurements from the ivrea zone abstract compressional wave velocities for new samples from the ivrea verbano zone ivz and strona ceneri zone scz have been measured to 600 mpa in order to better understand the petrologic controls on velocity and to provide an expanded base for seismological models of the crust as represented by these terrains these data coupled with previous measurements show that preferred orientation of mica controls both the velocity hysteresis with pressure and seismic anisotropy of scz and ivz schists and gneisses ivz paragneisses form the basis of a linear velocity density relationship for metagraywackes of various metamorphic grade this relationship falls close to that expected for mafic rocks and overlaps velocity density data for ivz mafic gneisses one dimensional near vertical reflection models based on these data and recent structural reconstructions of the terrain show that lower crustal reflectivity can be related to high reflection coefficients associated with interlayered sequences of upper amphibolite granulite facies paragneiss mafic gneiss and ultramafic rocks transparent upper crustal levels in the model result from the overall similarity of velocities for schists gneisses and granites from the structurally and lithologically complex scz a mean lower crustal velocity of 7 1 km s is calculated for the part of the ivz composed of interlayered mafic gneiss and paragneiss and for another area dominated by a mafic layered igneous complex the velocity density analysis coupled with this observation emphasize the ambiguity inherent in interpretation of crustal composition from p wave velocity data in the absence of other constraints

correlations between compressional and shear wave velocities and corresponding poisson s ratios for some common rocks and sulfide ores abstract the correlations between v p and v s and corresponding poisson s ratios are important in modeling and interpreting seismic data in terms of chemistry and lithology data from high pressure laboratory measurements of compressional and shear wave velocities v p and v s are analyzed for 12 common categories of rocks amphibolite anorthosite basalt diorite eclogite felsic gneiss gabbro diabase granite intermediate gneiss limestone mafic gneiss and peridotite and 4 types of massive sulfide ores chalcopyrite pyrite sphalerite and pyrrhotite the analysis reveals that the linear correlation provides good descriptions for the v s v p and ln v s ln v p relationships poisson s ratio is linearly correlated with v s v p shear modulus g and young s modulus e for these rocks and sulfide ores a decrease in poisson s ratio is associated with increases in v s g and e however the variation of poisson s ratio with v p depends on the logarithmic ratio r s p i e ln v s ln v p poisson s ratio increases or decreases with v p when r s p 1 r s p varies systematically with lithology 0 300 for granite 0 573 for diorite 0 602 for felsic gneiss 0 631 for intermediate gneiss 0 721 for gabbro diabase 0 768 for mafic gneiss 0 866 for eclogite 0 890 for amphibolite and 1 391 for peridotite it is suggested that r s p can be used as a proxy for the composition of the deep continental crust and the upper mantle

timing of the permian triassic biotic crisis implications from new zircon u pb age data and their limitations abstract the most profound biotic crisis in the earth s history causing the near extinction of both terrestrial and marine life occurred at the end of the permian period about 253 myr ago and marks the paleozoic mesozoic era boundary the cause of this event is still a matter of vigorous debate with both brief and catastrophic as well as gradual mechanisms having been proposed similar to a recent landmark study this study uses the u pb method on zircons from the uppermost permian lowermost triassic ash fall deposits at meishan zhejiang province se china in order to examine time and rate constraints for these events the results of both this study and previous work show that for these ash layers the effects of pb loss are combined with varying amounts and sources of inheritance resulting in an age scatter which prohibits the extraction of a statistically robust age in many cases though the effects of pb loss on the zircons analyzed in this study were reduced by leaching the grains in hydrofluoric acid as opposed to commonly applied air abrasion prior to analysis the presence within a single ash layer of multiple generations of older xenocrysts in many cases only slightly older than the depositional age has made quantitative interpretation even more difficult when these combined phenomena bias individual zircon ages by less than a percent they are extremely difficult to deconvolute and if multi grain analyses are used can become impossible to recognize because of the resulting age averaging monte carlo simulations using actual measurements of individual zircon crystals show that age excursions due to pb loss and xenocrystic contamination for the meishan bentonites are easily homogenized to the point of undetectability when replicate analyses of multi grain zircon samples are compared thus this study uses only high precision analyses of single crystals whether from our work or that of previous studies three main conclusions have emerged first our data require a significant increase in the age of the permian triassic boundary by more than 2 myr compared to the previous study which shifts the age to a value older than 253 ma second neither our data nor those from previous work can confirm or negate the possibility of a very abrupt biotic crisis third even large suites of very high quality single zircon u pb analyses for these tuffs cannot in most cases yield objective reliable and robust dates with accuracies at the sub myr level though the temptation to perform arbitrary selection of subsets of the analyses for that purpose is almost irresistible the last conclusion is not an indictment of zircon u pb dating in general other rocks and other zircon populations can and do behave very differently and further technical advances will likely improve our ability to prepare grains or sub grains of adequately enhanced quality for analysis consequently the results of the present study strongly suggest that for problems requiring time scale accuracy inferences from zircon u pb dating must be based on sufficiently large suites of single crystal or crystal domain high precision analyses

sequence stratigraphic analysis and paleogeographic interpretation of the buntsandstein and muschelkalk facies permo triassic in the se iberian range e spain abstract the iberian ranges is an alpine structure trending nw se in central spain they first appeared as small intracratonic basins in the early permian and experienced different periods of extensional tectonics during the mesozoic and cenozoic controlled by ancient hercynian or older fracture systems and at least two periods of compressive inversion tectonics the permian and triassic of the se iberian ranges are of germanic type within the prevailing buntsandstein and muschelkalk facies eight formations are recognized pollen and spore assemblages and some ammonoid horizons allow for age assessments ranging from thuringian late permian to carnian late triassic two phases of sedimentation can be distinguished during the early extensional period the first phase rift phase led to the development of the lower buntsandstein facies bounded by two angular unconformities and subdivided into two depositional sequences ds 1 and ds 2 the lower one is dominated by alluvial fans deposited in half grabens and the upper one by sandy braided rivers in more symmetrical grabens the second phase thermal or flexural subsidence phase led to the deposition of the uppermost buntsanstein rot and muschelkalk facies mostly shallow marine carbonates subdivided into two depositional sequences ds 3 and ds 4 with well developed lowstand transgressive and highstand systems tracts lst tst and hst intervals the criteria for separating systems tracts and the interplay of tectonics eustatism and rate of sedimentation are discussed ds 3 and ds 4 broadly correspond to 2 1 and 2 2 third order cycles but their limits are about 3 4 m y younger this may be explained by local tectonic factors

the permo triassic extinction the end permian mass extinction brought the palaeozoic great experiment in marine life to a close during an interval of intense climatic tectonic and geochemical change improved knowledge of latest permian faunas coupled with recent advances in isotopic studies and biostratigraphy have greatly enhanced our understanding of the events of 250 million years ago and have begun to provide answers to many questions about the causes of extinction

the velocity structure of the britt domain southwestern grenville province from laboratory and refraction experiments the britt domain in the southwestern grenville province of the canadian shield is believed to be an exposure of high grade upper amphibolite facies mid crustal rocks of predominantly granitic and granodioritic composition a 270 km refraction line was conducted by lithoprobe across the britt domain and the grenville front tectonic zone to the north in order to determine the deep velocity structure under the region this data set demonstrates a uniform velocity structure to a depth of 15 km in the central britt domain with near surface p and s wave velocities of 6 15 and 3 55 km s respectively and linear vertical gradients at depth of 0 02 and 0 01 s 1 respectively the data also show that the grenville front tectonic zone is strongly anisotropic at shallow depths to determine the acoustic properties of rocks from the britt domain 80 velocity samples representing different lithologies in this area were measured at confining pressure up to 600 mpa these studies show that p wave velocities at 600 m

mass extinction of peat forming plants and the effect on fluvial styles across the permian triassic boundary northern bowen basin australia abstract the most spectacular extinction event in earth s history occurred across the permian triassic boundary in the northern bowen basin a major coal bearing sedimentary basin in eastern australia a long lived c 9 myr cold climate peat mire ecosystem collapsed at the permian triassic boundary when the vast majority c 95 of peat producing plants became extinct the environmental change marked by the permian triassic boundary is expressed as an abrupt and sharp change in sedimentary regime at the contact between the rangal coal measures and the sagittarius sandstone the stratigraphic record shows no diminution in the thickness lateral extent or spatial distribution of coal seams prior to the boundary event the abrupt ecological shift at the permian triassic boundary was coincident with and interrelated to a change in landscape attributes and fluvial style the boundary shift is considered to reflect a short period radical atmospheric change accompanied by an abrupt change in plant ecosystems however palynological data indicate that it was preceded by a more gradual gross taxonomic progression in the floral succession the boundary shift is unlikely to reflect change in the tectonic setting of the northern bowen basin because the detrital character of clastic sediment supply shows no provenance change within the boundary sequence the late permian fluvial style is characterised by large scale up to 1 km wide sandstone dominated low sinuosity trunk river channel deposits the trunk river channels were flanked by extensive levee composite crevasse splay systems channel tracts were relatively stationary in position over enduring periods and developed stacked sediment accumulations up to 30 m thick the constrained character of the late permian trunk river systems was most likely due to progressive compaction of thick tracts of peat substrate and the stabilising effect of vegetation adjacent to the channel complex the well developed crevasse splays coupled with the low sinuosity style of the fluvial channels might suggest a perennial fluvial system characterised by short discharge periods as common in high latitude settings the fluvial architecture of the sagittarius sandstone the basal formation of the lower triassic rewan group is characterised by sheet like elements suggestive of broad shallow channels in a deforested braid plain setting the channel deposits are considered to represent highly mobile sandy systems dominated by a flashy runoff regime the mass extinction of plants in the northern bowen basin at the permian triassic boundary thus had a significant impact on the early triassic landscape and fluvial architecture

large scale bed configurations macroforms platte river basin colorado and nebraska primary structures and formative processes large scale current bed forms defined here as bed configurations having dimensions commensurate with the size of the turbulent boundary layer are considered by many workers to be hydrodynamically equivalent to the regime bed forms these bed forms were investigated at formative flows bankfull flows on the south platte and platte rivers in colorado and nebraska to determine structures and formative processes results of this study suggest that large scale bed forms here termed macroforms are not hydrodynamically equivalent to the regime bed forms but constitute a unique hierarchical class of bed configurations produced by turbulent vortices that involve the entire boundary layer macroforms are oriented obliquely to the mean flow direction and are attached to one or both channel banks three members of a continuum of geometries are recognized in the channels of the platte river basin the internal stratification for each of the three macroform types recognized in the basin is similar and in its simplest form consists of the coarsening upward sequence apron laminae foreset laminae topset laminae although it may not be possible to differentiate the three macroform types solely on the basis of internal stratification the presence of a distinctive coarsening upward sequence in coarse grained fluvial systems offers a potentially powerful tool for identifying these environments in the geologic record

the late permian climate what can be inferred from climate modelling concerning pangea scenarios and hercynian range altitude abstract a major unsolved geodynamic problem is the permian pangea plate configuration at the end of paleozoic era around 255 ma while consensual geology indicates a plate arrangement close to that of the jurassic prior to the atlantic opening pangea a paleomagnetic data indicates a nearly 3000 km more eastward position of gondwana with respect to laurussia leading to major plate and relieves reorganisation during permo triassic times pangea b using an atmospheric general circulation model agcm we simulate the climatic response to both configurations we also test the fundamental role of paleo elevations through sensitivity experiments each simulated climate is then compared with the aim of constraining the best fit to a particular paleogeographic scenario main trends of simulated late permian climate in agreement with paleoclimatic indicators are 1 a warm temperate climate accompanied by a monsoon circulation over the eastern side of gondwana 2 a cold temperate climate marked by strong annual thermal amplitude at high latitudes in gondwana and in siberia 3 an arid belt in the subtropics over the western side of gondwana and laurussia and 4 strong climatic differences over both hemispheres respectively main variance is found over laurussia with a simulated tropical climate while an arid climate is suggested by paleodata simulating different paleo elevations of the appalachian and the variscan fold belts can solve this discrepancy the best model data fit is reached for a mean altitude of 4500 m in the appalachians and appears to be dependent of pangea configuration with a southern europe variscan range of some 3000 m in a pangea a configuration or only 2000 m in for pangea b taking into account the geodynamic context we argue that pangea b appears to be the more probable configuration

age of the emeishan flood magmatism and relations to permian triassic boundary events the permian triassic p t mass extinction the greatest biological mortality event in the earth s history was probably caused by dramatic and global forcing mechanisms such as the siberian flood volcanism here we present the first set of high precision 40 ar 39 ar dating results of volcanic and intrusive rocks from the emeishan traps south china which define a main stage of the flood magmatism at v251 253 ma and a subordinate precursory activity at v255 ma this time span is generally coeval with or slightly older than the age of the p t boundary estimated by the ash beds in the meishan stratotype section and the main eruption of the siberian traps our data reinforces the notion that the eruption of the emeishan traps rather than eruption of the siberian traps accounted for the formation of the p t boundary ash beds in south china the emeishan flood magmatism which occurred in the continental margin comprising thick marine limestone formations moreover may have triggered rapid release of large volumes of methane and carbon dioxide that could have been responsible for the global n 13 c excursion and associated environmental crisis leading to the mass extinction at the p t boundary fl 2002 elsevier science b v all rights reserved

how do gravel bed rivers braid sedimentary processes and bed forms leading to the onset of braiding were observed in small scale hydraulic models of gravel bed streams the laboratory streams had a variety of combinations of constant discharge and slope but identical bed material particle size distributions from initially straight channels braiding occurred by four different processes deposition and accumulation of a central bar chute cutoff of point bars conversion of single transverse unit bars to mid channel braid bars and dissection of multiple bars in these experiments the chute cutoff mechanism was the most common but the predominant braiding mechanism depends upon sediment mobility excess bed shear stress and the bed form regime at very low excess bed shear stress the central bar process dominates but at higher excess bed shear stress slip face unit bars are more common bed scour at confluences is more pronounced and propogation of alternate convergence scour and divergence deposition is more likely thus chu

a temporal link between the emeishan large igneous province sw china and the end guadalupian mass extinction abstract previous studies have suggested that there were two mass extinction events in the late permian one that occurred at the permo triassic p t boundary 251 ma and a second smaller mass extinction that occurred 5 8 myr earlier at the end of the guadalupian many workers have argued that there is a causal relationship between large scale volcanic activity and mass extinctions the major mass extinction event at the p t boundary coincides with the outpouring of huge quantities of lava that formed the siberian flood basalt province in russia courtillot et al earth planet sci lett 166 1999 177 195 and wignall earth sci rev 53 2001 1 33 suggested that the earlier late permian mass extinction coincided with the eruption of the lavas that formed the emeishan flood basalt efb province in sw china however the age of eruption of the efb lavas is poorly constrained using the sensitive high resolution ion microprobe to analyze zircons we have established the age of the xinjie intrusion believed to be a feeder to the main phase of efb volcanism to be 259 3 ma hence the formation of the efb is coincident with a proposed extinction event at 256 259 ma this result supports a temporal link between the emeishan large igneous province and the end guadalupian mass extinction

origin of the permian triassic iberian basin central eastern spain abstract the iberian basin was an intracratonic rift basin in central eastern spain developed since early permian times the basin boundary faults were normal listric faults controlling an asymmetric extension propagating northeast with time hercynian or older lineaments controlled the orientation of the iberian basin and extension was accommodated basically in the hanging wall block by the formation of secondary grabens and a central high the basin was related with the coeval ebro catalan and cuenca mancha basins and their connections are discussed subsidence curves show that the early permian early jurassic period of extension can be subdivided into three rifting episodes and a flexural one extension factor increases from 1 17 in the northwest to 1 29 near the mediterranean coast the increasing extension rate was accommodated by transfer faults trending nne ssw more important in the levante area the rift evolution is intermittent and seems to reflect distinct stress fields the collapse of the late hercynian orogen and related increased heat flux extension and rifting is the most probable origin of the iberian basin and related basins the origin of the catalan and the valencia prebetic basins is related to the southwards migration of the hesse burgundy rift along the eastern margin of the iberian microplate

the upper permian boniches conglomerates formation evolution from alluvial fan to fluvial system environments and accompanying tectonic and climatic controls in the southeast iberian ranges central spain the boniches conglomerates fm upper permian represents the first episode of sedimentation in the southeast border margin of the iberian basin central spain the iberian basin commenced development as a rift basin running mainly nw se during the early permian and underwent its first general extensional period during the late permian the boniches fm was deposited as a small segment less than 100 km long and 9 10 km wide of the iberian basin that extended in a northeast direction during this period detailed mapping and palaeocurrent analysis have shown that the boniches fm is of a curvilinear wedge shaped morphology sediments close to the southwest main faulted border are of a maximum thickness of 230 m clear lateral variation in thickness controlled by the activity of secondary transfer faults perpendicular to the main nw se axis may be observed this network of faults permitted the formation of blocks limited laterally by the transfer faults these blocks experienced subsidence and rotation against the elevated southwest footwall block which resulted in the isolation of reduced sub basins with different sedimentary characteristics based on facies and facies association the boniches fm is thought to have evolved vertically from two main alluvial fan systems in the lowest part of the formation into fluvial braided systems flowing southeast as expected no flow orientations towards the footwall block are observed during the first episodes of the refill this may be explained by erosion or simply a lack of sedimentation due to the gentle slope the change from alluvial fan to fluvial environments is interpreted as a relatively sudden tectonic reorganization of the whole segment favouring new longitudinal fluvial drainage from a transfer zone located to the northwest three different reorganization episodes clearly marked in the boniches fm as main boundary surfaces mbs separate four members subdivided into 4 5 levels 34 m thick each level consists of fining upward successions less than 1 m thick tectonics and climatic factors clearly controlled sedimentation tectonic activity was probably of greatest importance and was conditioned by the movement of the main nw se and ne sw transfer faults the role of climate in ancient conglomerate sediments has not been well established palaeogeographical mineralogical sedimentological and clast analyses suggest that the boniches fm was deposited under humid conditions by running water the differentiation between proper alluvial fans and fluvial environments during the transition period of the boniches fro is not clear since in many recent alluvial fans braided fluvial systems are associated with fans

altered river morphology in south africa related to the permian triassic extinction the permian triassic transition in the karoo basin of south africa was characterized by a rapid and apparently basin wide change from meandering to braided river systems as evidenced by preserved sedimentary facies this radical changeover in river morphology is consistent with geomorphic consequences stemming from a rapid and major die off of rooted plant life in the basin evidence from correlative nonmarine strata elsewhere in the world containing fluvial permian triassic boundary sections suggests that a catastrophic terrestrial die off of vegetation was a global event producing a marked increase in sediment yield as well as contributing to the global 13 c excursion across the permian triassic boundary

mesozoic extensional tectonics stratigraphy and crustal evolution during the alpine cycle of the eastern iberian basin abstract sequence stratigraphy subsidence analysis and the integration of the basin fill data allow to the identification of four successive evolutionary stages in the basins of the eastern iberian margin during mesozoic extension 1 triassic rift late permian hettangian 2 early and middle jurassic postrift sinemurian oxfordian 3 late jurassic and early cretaceous rift kimmeridgian middle albian and 4 late cretaceous postrift late albian maastrichtian the present day crustal structure of the eastern iberian range and evidence of its evolution are deduced from the analysis of a new gravity map and other geophysical data a regional gravity low along the iberian range is interpreted in terms of crustal thickening beneath the orogen as a result of the collision between the ebro block and the iberian plate during the paleogene the boundary between these blocks coincides with a magnetic lineament that is interpreted as an inherited signature of the thinning that occurred during mesozoic extension a geodynamic scenario for the crustal evolution of the eastern part of the iberian peninsula based on the evaluation of mesozoic extensional tectonics and an analysis of the available geophysical data includes three main successive evolutionary stages 1 1 mesozoic crustal thinning 2 2 paleogene crustal thickening 3 3 neogene crustal thinning based on new geological and geophysical arguments this study offers an alternative and a more complete geodynamic history of the eastern iberian basins related to the evolution of the iberian plate and the central north atlantic

petrogenesis and alteration of tuffs associated with continental flood basalts from putorana northern siberia in the putorana region of the siberian continental flood basalt province tuffs form thin layers or small bodies between single basaltic lava flows based on our fieldwork these ash fall deposits have relatively small volumes 100 000 m 3 compared with the associated basaltic flows and the tuffs of the adjacent noril sk region in the north six tuff units with different degrees and modes of alteration were analysed by xrd sem and xrf in order to determine mineralogical and chemical changes which occurred during progressive alteration pyroxene compositions and immobile element ratios suggest that the tuffs were derived from a basaltic lava like the flood basalts post eruptive low temperature alteration processes have significantly changed the primary composition of the tuffs therefore the different tuffs have variable petrography mineralogy and geochemistry mgo 5 2 9 2 wt sio 2 33 6 69 1 wt and tio 2 0 6 1 6 wt compared to the tholeiitic basalts in which they are interbedded crystallization of secondary minerals such as carbonate clay minerals zeolites and analcime occurred from circulating fluids the results show that the fluid compositions varied significantly between the different tuffs this implies that the elements behaved differently depending on the circulating agent which led to the formation of different minerals in addition two samples show an input of quartz derived either by wind from terrigenous sediments or generated by magma sediment interaction and one sample was deposited in a subaqueous possibly lacustrine environment these results indicate that the conditions during alteration varied despite the close spatial relationship of the tuffs and that a generalization about the alteration processes cannot be made in the putorana region the analyses of the clay minerals show alteration of chlorite to vermiculite interestingly corrensite a regularly stacked 50 50 mixed layered clay mineral composed of chlorite and vermiculite is still preserved in two samples this indicates different alteration stages in the different tuff outcrops the state of alteration deduced from immobile element ratios plagioclase compositions and mineral assemblages shows contradictory results this suggests that the state of alteration is best deduced from clay minerals as one can follow this process step by step

age and timing of the permian mass extinctions u pb dating of closed system zircons the age and timing of the permian triassic mass extinction have been difficult to determine because zircon populations from the type sections are typically affected by pervasive lead loss and contamination by indistinguishable older xenocrysts zircons from nine ash beds within the shangsi and meishan sections china pretreated by annealing followed by partial attack with hydrofluoric acid result in suites of consistent and concordant uranium lead u pb ages eliminating the effects of lead loss the u pb age of the main pulse of the extinction is 252 6 0 2 million years synchronous with the siberian flood volcanism and it occurred within the quoted uncertainty

rapid and synchronous collapse of marine and terrestrial ecosystems during the end permian biotic crisis a newly studied permian triassic p tr boundary section in jameson land east greenland contains an abundant and well preserved marine fauna as well as terrestrial palynomorphs for the first time it is possible to compare the biotic crises of the marine and terrestrial realms using the same samples from the same section the sediments record a negative excursion in 13 c carb values of 8 9 and in 13 c org values of 10 11 the presence of the conodont hindeodus parvus combined with the 13 c carb record enables correlation with the proposed global stratotype section at meishan this shows that the greenland section is the most expanded p tr section known collapse of the marine and terrestrial ecosystems took between 10 and 60 k y it took a further few hundred thousand years for the final disappearance of permian floral elements collapse of the terrestrial and marine ecosystems began at the same stratigraphic level and preceded the sharp negative excursion in the 13 c record

p wave velocities of polymineralic rocks comparison of theory and experiment and test of elastic mixture rules abstract we compare the p wave velocities vp of 696 dry samples measured at pressures up to 0 6 1 0 gpa with values calculated from the volume fraction and room pressure elastic constants of each constituent mineral using 16 averaging methods the exceptional large number of samples covers almost all common lithologic types of igneous and metamorphic rocks the calculated vp data are in good agreement with laboratory values measured at about 300 mpa even though elastic constants of only 22 common minerals are used in the computation the mean vp of a polymineralic rock is exclusively controlled by the volume fractions of its constituent minerals while grain shape and crystallographic preferred orientations anisotropy and other perturbations have minimum effects the mean vp can be fairly well predicted as long as a relevant mixture rule is used and the volume fraction is accurately determined for each mineral however none of the mixture rules can simultaneously produce the best fit to measured p wave velocities for all the lithologic types one method may work well for one lithology but poor for other lithology even for a given lithology an averaging method may yield good agreement at moderate pressure but poor agreement at high pressure applications of an inappropriate mixture rule will potentially cause the misinterpretation of the crust and mantle vp data in terms of mineralogical compositions and structures

contrasting sources for upper and lower continental crust the greenstone connection abstract greenstones may be useful in tracking the role of oceanic plateaus in the growth of continents lithologic proportions th ta ratios and ni concentrations in greenstone basalts of all ages show that arc related greenstones greatly exceed oceanic plateau and morb related greenstones in abundance this distribution may be accounted for by the preferential obduction of arcs formed on top of oceanic plateaus during collision of plateaus with continents because thick oceanic plateaus resist subduction a significant volume may be accreted to continental margins during collisions and over time these plateaus may evolve into lower continental crust this idea has important implications for continental development in that the lower continental crust may come chiefly from accreted oceanic plateaus while upper continental crust forms by subduction related processes consistent with this model are high seismic wave velocity layers in the lower crust of proterozoic cratons similar to high velocity layer

seismic anisotropy due to preferred mineral orientation observed in shallow crustal rocks in southern alaska laboratory velocity measurements and structural field relations explain the observation of a significant seismic anisotropy within highly foliated rocks in southern alaska the orientation of the principal compressive stress indicates that this anisotropy is not related to extensive dilatancy but can be satisfactorily explained by the foliation of the metasedimentary rocks laboratory measurements indicate a significant anisotropy within these rocks even at pressures well above those thought to close microcracks these observations document that anisotropy related to preferred mineral orientation may be widespread in continental rocks thus pervasive foliation can play a more important role in causing anisotropy than dilatancy and inferences of principal stress orientations solely from anisotropy may be erroneous

eclogites of the snowbird tectonic zone petrological and u pb geochronological evidence for paleoproterozoic high pressure metamorphism in the western canadian shield eclogite occurs within the southern domain of the east athabasca mylonite triangle in northern saskatchewan situated at the boundary between the archean rae and hearne provinces of the western canadian shield the east athabasca mylonite triangle is a fundamental exposure of the 3 000 km long snowbird tectonic zone the eclogite occurs in association with a variety of lower crustal high pressure granulites that record a complex metamorphic history from 2 6 to 1 9 ga temperatures of the eclogite facies metamorphism are constrained by garnet clinopyroxene exchange thermometry at 920 1 000 c minimum pressure conditions are recorded by the jadeite quartz albite geobarometer at 1 8 2 0 gpa a near isothermal decompression path to granulite facies conditions is inferred from retrograde reaction textures involving the formation of granulite facies assemblages such as orthopyroxene plagioclase and pargasite plagioclase u pb idtims zircon geochronology of the eclogite yields a weighted mean 207pb 206pb date of 1 904 0 0 3 ma which we interpret as the time of peak eclogite facies metamorphism shrimp in situ analyses of metamorphic zircons included within omphacitic clinopyroxene support this interpretation with a weighted mean 207pb 206pb date of 1 905 19 ma inclusion suites of high pressure phases and the petrographic setting of zircon are a direct link between zircon growth and eclogite facies metamorphism zircon from one eclogite sample has older cores that are 2 54 ga which is a minimum age for the emplacement or earliest metamorphism of the gabbroic protolith u pb rutile data indicate slow cooling at 1 c ma below 500 c from 1 88 to 1 85 ga the formation and exhumation of the eclogites at ca 1 9 ga has important implications for the tectonic significance of the snowbird tectonic zone during the paleoproterozoic the eclogites described here are consistent with transport of continental crust to mantle depths during the paleoproterozoic followed by rapid buoyancy driven exhumation to normal lower crustal depths

temporal and spatial variations in tectonic subsidence in the iberian basin eastern spain inferences from automated forward modelling of high resolution stratigraphy permian mesozoic by subsidence analysis on eighteen surface sections and 6 wells which cover large part of the iberian basin e spain and which are marked by high resolution stratigraphy of the permian triassic jurassic and cretaceous we quantify the complex permian and mesozoic tectonic subsidence history of the basin backstripping analysis of the available high resolution and high surface density of the database allows to quantify spatial and temporal patterns of tectonically driven subsidence to a much higher degree than previous studies the sections and wells have also been forward modelled with a new automated modelling technique with unlimited number of stretching phases in order to quantify variations in timing and magnitude of rifting it is demonstrated that the tectonic subsidence history in the iberian basin is characterized by pulsating periods of stretching intermitted by periods of relative tectonic quiescence and thermal subsidence the number of stretching phases appears to be much larger than found by earlier studies showing a close match with stretching phases found in other parts of the iberian peninsula and allowing a clear correlation with discrete phases in the opening of the tethys and atlantic

growth and demise of permian biogenic chert along northwest pangea evidence for end permian collapse of thermohaline circulation abstract the permian chert event pce was a 30 ma long episode of unusual chert accumulation along the northwest margin of pangea and possibly worldwide the onset of the pce occurred at about the sakmarian artinskian boundary in the sverdrup basin canadian arctic where it coincides with a maximum flooding event the ending of high frequency high amplitude shelf cyclicity the onset of massive biogenic chert deposition in deep water distal areas and a long term shift from warm to cool water carbonate sedimentation in shallow water proximal areas a similar and coeval shift is observed from the barents sea to the northwestern usa a landward and southward expansion of silica factories occurred during the middle and late permian at which time warm water carbonate producers disappeared completely from the northwest margin of pangea biotically impoverished and increasingly narrow cold water carbonate factories characterised by non cemented bioclasts of sponges bryozoans echinoderms and brachiopods were then progressively replaced by silica factories by late permian time little carbonate sediments accumulated in the barents sea and in the sverdrup basin where the deep to shallow water sedimentary spectrum was occupied by siliceous sponge spicules by that time biogenic silica sedimentation was common throughout the world silica factories collapsed in the late permian abruptly bringing the pce to an end in northwest pangea the end permian collapse of the pce was associated with a major transgression and with a return to much warmer oceanic and continental climatic conditions chert deposition resumed in the distal oceanic areas during the early middle triassic anisian after a 8 10 ma interruption early triassic chert gap the conditions necessary for the onset expansion and zenith of the pce were provided by the thermohaline circulation of nutrient rich cold waters along the northwestern and western margin of pangea and possibly throughout the world oceans these conditions provided an efficient transportation mechanism that constantly replenished the supply of silica in the area created a nutrient and oxygen rich environment favouring siliceous biogenic productivity established cold sea floor conditions hindering silica dissolution while increasing calcium carbonate solubility and provided conditions adverse to organic and inorganic carbonate production the northwest margin of pangea was for nearly 30 ma bathed by cold waters presumably derived from the seasonal melting of northern sea ice the assumed engine for thermohaline circulation this process started near the sakmarian artinskian boundary intensified throughout middle and late permian time and ceased suddenly in latest permian time it led to oceanic conditions much colder than normally expected from the palaeolatitudes and the influence of cold northerly derived water was felt as far south southern nevada the demise of silica factories was caused by the rapid breakdown of these conditions and the establishment of a much warmer marine environment accompanied by sluggish circulation and perhaps a reduced input of dissolved silica to the ocean complete thawing of northern sea ice would have ended thermohaline circulation and led to warm and sluggish oceanic conditions inimical to the production accumulation and preservation of biogenic silica

late permian to middle triassic correlations and palaeogeographical reconstructions in south western european basins new sedimentological data from minorca balearic islands spain description of sedimentary sections and interpretations of sequence stratigraphic evolution of permian triassic red beds of minorca allow a new proposal of palaeoenvironment reconstructions the permian sediments mainly correspond to floodplain and pond deposits with sheetflood sand bed rivers environment de 1 within a warm and semi arid climate and the preserved succession forms one complete stratigraphic cycle the retrogradational phase of this cycle is represented by alluvial colluvial deposits grading upwards to environment de 1 and to extensive floodplain and pond deposits the progradational phase is characterized by increasing fluvial influence with palaeocurrent mainly oriented to the sw palaeosol horizons and preservation of a late permian palynoflora the uppermost part of this succession is truncated by a major erosional surface the sediments above this unconformity are characterized by braided river deposits environment de 2 with palaeocurrent oriented to the ssw within the more basal conglomeratic facies and to the wnw in overlying units the absence of extensive floodplain deposits a paucity of evidence for biogenic activity and palaeosols together with the presence of ventifacts indicate an arid climatic setting for the succession this environment evolved from braided rivers palaeocurrents mainly oriented to the se environment de 3 with in situ palaeosols and reworked breccias to fluvial coastal and shallow marine deposits environment de 4 dated as upper anisian to ladinian this succession characterizes a retrogradational trend although the transition between de 2 and de 3 was not observed a switch in palaeocurrent directions may indicate a change in source area a comparison at a regional scale with other peri tethyan basins allows the succession to be placed within a broader palaeoenvironmental and stratigraphic context during late permian to middle triassic during the late permian sedimentation occurred in alluvial playa lake or ponded environments where fluvial systems generally flowed southward except in south eastern france oriented to the ne within these sw european basins such as minorca the late permian succession shows a major retrogradational evolution from fluvial or alluvial fan deposits to extensive lake playa or floodplain deposits and a progradational trend fluvial or alluvial fan deposits the permian triassic transition corresponds to an unconformity overlain by braided river deposits with arid climate indicators aeolian deposits ventifacts and aeolian dune sedimentation at the scale of western europe this arid episode is dated as smithian and the induan age sedimentation deposits seem to be preserved only in the central part of the germanic basin as with all other peri tethyan basins environment de 2 of minorca above the major erosional surface is attributed to the smithian in the upper part of the studied succession braided river deposits indicative of less arid climatic conditions are preserved this succession contains the earliest mesozoic palaeosols dated as anisian by palynomorphs and expresses a vertical evolution from fluvial to open marine depositional environments attributed to the muschelkalk transgression

a new calibration of seismic velocities anisotropy fabrics and elastic moduli of amphibole rich rocks 1 a large portion of the middle to lower crust beneath the continents and oceanic island arcs consists of amphibolites dominated by hornblende and plagioclase we have measured p and s wave velocities vp and vs and anisotropy of 17 amphibole rich rock samples containing 34 80 vol amphibole at hydrostatic pressures p up to 650 mpa combined petrophysical and geochemical analyses provide a new calibration for mean density average major element contents mean vp p and vs p coefficients intrinsic vp and vs anisotropy poisson s ratios the logarithmic ratio rs p and elastic moduli of amphibole rich rocks the vp values decrease with increasing sio2 and na2o k2o contents but increase with increasing mgo and cao contents the maximum 0 38 0 40 km s and minimum s wave birefringence values occur generally in the propagation direction parallel to y and normal to foliation respectively amphibole plays a critical role in the formation of seismic anisotropy whereas the presence of plagioclase quartz pyroxene and garnet diminishes the anisotropy induced by amphibole crystallographic preferred orientations cpos the cpo variations cause different anisotropy patterns illustrated in the flinn diagram of vp x vp y vp y vp z plots the results make it possible to distinguish in terms of seismic properties the amphibolites from other categories of lithology such as granite granodiorite diorite gabbro diabase felsic gneiss mafic gneiss eclogite and peridotite within the earth s crust hence amphibole aligned by dislocation creep anisotropic growth or rigid body rotation is the most important contributor to the seismic anisotropy of the deep crust beneath the continents and oceanic island arcs which contains rather little phyllosilicates such as mica or chlorite

a small alluvial crevasse splay abstract a crevasse splay deposited on the floodplain of the clarence river in northeastern new south wales australia by a small ephemeral tributary provides a guide to the geometry and sedimentary structures of crevasse splays in general and to the processes affecting their final form the splay consists of a main lobe of sand with smaller sand fingers that end in slipfaces around its edges water seeping out of the channel reworks part of the splay into a gently thinning sheet small scale facies models derived from different parts of this splay explain the origins of solitary planar cross sets and gently inclined bedding in some ancient crevasse splay sandstones

emeishan basalts sw china and the end guadalupian crisis magnetobiostratigraphic constraints abstract a magnetostratigraphic investigation of the permian emeishan lip large igneous province was carried out on a composite section in ebian county close to the type area in sichuan sw china the main succession of twelve flows 175 m thick carries a normal polarity whilst the one reliable site from the overlying 30 m thick volcanic waning sequence is marked by a reverse polarity the data enable a correlation to be proposed with an emeishan basalt sequence in western guizhou c 400 km to the se from our knowledge of the geomagnetic field9s reversal behaviour during the permian it suggests that the main lava pile along the eastern half of the emeishan basalt outcrop belt was erupted within a half to one million years using magnetobiostratigraphic data from the adjacent south china platform the normal polarity magnetozone is correlated with the normal polarity chron associated with the upper part of the maokou limestones constrained by conodont data the main emei basalts appear to be at least two biozones older 1 1 5 ma than the mid late permian boundary it is possible however that the emei basalt waning zone sequence which represents an explosive volcanic phase might be coeval with the end guadalupian biotic crisis thus arguments implicating emei basalt volcanism as the causal mechanism behind this major global event have to accommodate the new relative age constraints

high pressure high temperature seismic velocity structure of the midcrustal and lower crustal rocks of the ivrea verbano zone and serie dei laghi nw italy the ivrea verbano zone and the adjacent serie dei laghi in the inner arc of the western alps nw italy display an upended cross section through much of the thickness of the continental crust as it existed in that region in triassic jurassic time we collected a suite of oriented rock samples that represent most of the volume of the rocks of the region p and s wave velocity measurements were made in three orthogonal directions related to the mesoscopic fabric of the rocks at room temperature and up to 550 mpa confining pressure combined high temperature and high pressure measurements were made up to 700 c on a subset of the samples the lithologic units were divided into 23 different principal rock types and we present velocity data averaged in terms of these groups vertical and horizontal velocity and anisotropy sections are computed from the measured velocity data based on a restored geologic section these show that vp and vs both increase and anisotropy decreases systematically with depth of burial reflecting variations in rock type and metamorphic grade with depth vp and vs anisotropy arises mainly because of crystallographic preferred orientation or mineralogical banding with the slowest direction tending to be normal to the foliation or banding the vp velocity sections were used to compute synthetic seismic reflection profiles for the region when it lay at the bottom of the continental crust these correspond well with contemporary deep reflection profiles for regions of comparable tectonic evolution

synchrony and causal relations between permian triassic boundary crises and siberian flood volcanism the permian triassic boundary records the most severe mass extinctions in earth9s history siberian flood volcanism the most profuse known such subaerial event produced 2 million to 3 million cubic kilometers of volcanic ejecta in approximately 1 million years or less analysis of 40 ar 39 ar data from two tuffs in southern china yielded a date of 250 0 0 2 million years ago for the permian triassic boundary which is comparable to the inception of main stage siberian flood volcanism at 250 0 0 3 million years ago volcanogenic sulfate aerosols and the dynamic effects of the siberian plume likely contributed to environmental extrema that led to the mass extinctions

significant seismic anisotropy beneath the southern lhasa terrane tibetan plateau 1 shear wave splitting measurements using teleseismic pks skks and sks phases recorded by station lsa on the southern part of the lhasa terrane of the tibetan plateau reveal significant azimuthal anisotropy with a splitting time of up to 1 5 s a conclusion that is contradictory to previous studies which suggested isotropy or weak anisotropy in addition systematic variations of the splitting parameters fast polarization direction and splitting time with the arriving azimuth of the seismic ray path are observed suggesting a model of anisotropy that is more complicated than a single layer with horizontal axis of symmetry the measurements are consistent with a model with two layers of anisotropy the top layer has a ne sw fast direction which is consistent with gps revealed direction of surface movement and can be associated with lattice preferred orientation of middle lower crustal minerals such as amphibole the lower layer has a nearly e w fast direction and can be the consequence of either the n s directed compressional stress originated from the india eurasia collision or flow in the asthenosphere related to the absolute motion of eurasia this study underlines the importance of a long duration of deployment of seismic stations in resolving complex anisotropy

deep root of a continent continent collision belt evidence from the chinese continental scientific drilling ccsd deep borehole in the sulu ultrahigh pressure hp uhp metamorphic terrane china abstract the chinese continental scientific drilling ccsd deep borehole which reached a depth of 5158 m in the sulu ultrahigh pressure uhp metamorphic terrane provides a new window into the deep root of a continent continent collision belt and the tectonic processes by which supracrustal material is recycled into the mantle by subduction and then uplifted to the surface major research themes of the ccsd project were to 1 determine the three dimensional composition structure and geophysical character of the deep root of this orogenic belt 2 investigate the nature and timing of the uhp metamorphism 3 investigate the processes of crust mantle interaction involved in the formation and exhumation of the uhp rocks 4 study the process of fluid circulation and mineralization during subduction and exhumation 5 study the rheological properties of the various rocks during subduction and exhumation 6 develop and refine dynamic models for deep subduction and exhumation of crustal rocks and 7 establish a long term natural laboratory for the study of present day crustal dynamics e g stress strain fluid activity the ccsd has developed precise oriented profiles of the main borehole in terms of lithology geochemistry oxygen isotopes zircon shrimp u pb ages 40ar 39ar ages deformation rheology mineralization physical properties of the rocks petrophysical logs seismic reflections and underground fluids the present paper summarizes the integrated research results of this project especially the new findings concerning the deep root of a continent continent collision

vertebrate extinction across permian triassic boundary in karoo basin south africa distinct assemblages of paleosols above and below the permian triassic boundary in the karoo basin of south africa are evidence for reorganization of ecosystems following this greatest of all mass extinctions the permian triassic boundary is recognized from the last appearance of dicynodon and from a series of negative excursions in the isotopic composition of carbon within therapsid tusks pedogenic carbonate nodules and organic matter the boundary is also marked by laminated beds with very weakly developed paleosols a change from purple 10r to brownish red 2 5yr paleosols and a thin 10 cm claystone breccia of reworked soil clasts paleoclimatic changes include a shift from arid and highly seasonal paleoclimate inferred from diffuse and shallow calcareous nodules in permian paleosols to semiarid and less seasonal paleoclimate inferred from deep and well focused calcic horizons in triassic paleosols an earliest triassic shift to warmer and wetter paleoclimate is also indicated by increased chemical weathering abundance of lycopsids and diversity of labyrinthodonts permian paleosols have root traces comparable to those of open shrubland and riparian woodland whereas triassic paleosols have root traces and profile forms like soils of open woodland this is a significant paleoenvironmental change but not as dramatic a change as would be expected from the devastating extinctions of 88 of fossil vertebrate genera latest permian therapsid reptiles were diverse and ecologically specialized in contrast the principal earliest triassic therapsid lystrosaurus was a burrower with no specific habitat preference its short internal nares barrel chest and high neural spines would have given it greater aerobic scope than preexisting therapsids and may have been an advantage under conditions of hypercapnia and hypoxia these adaptations and associated ecosystem changes are compatible with widespread vertebrate mortality by acidosis and pulmonary edema in a post apocalyptic greenhouse created by the voluminous release of methane from shallow marine and permafrost clathrates

amphibole and lower crustal seismic properties abstract measurements of seismic anisotropy frequently offer insights of the deformation kinematics in situ within the earth in the continental crust seismic anisotropy is commonly ascribed to regionally aligned lattice preferred orientation lpo of mica crystals however determinations of composition suggest that the deep continental crust contains rather little mica in this contribution the role of amphibole a far more plausible constituent especially for deep continental crust of basic composition is assessed a field analogue approach is adopted where the lpo are measured correlated with the strain state of the rocks and used to calculate the seismic properties the seismic properties may be up scaled through the specimen using the modal proportions of the constituent mineral phases to examine the role of composition in controlling the intensity of seismic anisotropy seismic properties are computed using an array of modal compositions so called rock recipes the case history comes from the central block of the lewisian of nw scotland where a regional basic dyke swarm the scourie dykes is locally deformed within amphibolite facies shear zones conditions that are representative of the deep crust shear strain profiles across a 100 m wide shear zone using deflected banding in the host gneisses yield values of 0

elastic wave velocities chemistry and modal mineralogy of crustal rocks sampled by the outokumpu scientific drill hole evidence from lab measurements and modeling the outokumpu scientific deep drill hole intersects a 2500 m deep precambrian crustal section comprising a 1300 m thick biotite gneiss series mica schists at top followed by a 200 m thick meta ophiolite sequence underlain again by biotite gneisses mica schists 500 m thick with intercalations of amphibolite and meta pegmatoids pegmatitic granite from 2000 m downward the dominating rock types are meta pegmatoids pegmatitic granite average isotropic intrinsic p and s wave velocities and densities of rocks were calculated on the basis of the volume fraction of the constituent minerals and their single crystal properties for 29 core samples covering the depth range 198 2491 m the modal composition of the rocks is obtained from bulk rock xrf and mineral chemistry microprobe using least squares fitting laboratory seismic measurements on 13 selected samples representing the main lithologies revealed strong anisotropy of p and s wave velocities and shear wave splitting seismic anisotropy is strongly related to foliation and is in particular an important property of the biotite gneisses which dominate the upper and lower gneiss series at in situ conditions velocity anisotropy is largely caused by oriented microcracks which are not completely closed at the pressures corresponding to the relatively shallow depth drilled by the borehole in addition to crystallographic preferred orientation cpo of the phyllosilicates the contribution of cpo to bulk anisotropy is confirmed by 3d velocity calculations based on neutron diffraction texture measurements for vertical incidence of the wave train the in situ velocities derived from the lab measurements are significantly lower than the measured and calculated intrinsic velocities the experimental results give evidence that the strong reflective nature of the ophiolite derived rock assemblages is largely affected by oriented microcracks and preferred crystallographic orientation of major minerals in addition to the lithologic control c 2009 elsevier b v all rights reserved