white mica crystallinity finite strain and cleavage development across a large variscan structure nw spain x ray diffraction analyses were performed on white micas formed under strain and low grade metamorphic conditions from sandstones and siltstones along a transect through the narcea antiform a large structure that separates the external from the internal zones of the variscan belt of nw spain the results indicate a close spatial relationship between the onset of metamorphism and the cleavage front at the cleavage front strain is characterized by oblate ellipsoids that have x y z ratios from 1 to 1 5 the x y z ratio increases westward up to 3 0 as does the grade of metamorphism and is accompanied by an increase in the 9crystallinity9 index of the white mica from very low to low metamorphic grade this increase corresponds qualitatively to an increase in penetrativeness of the axial planar cleavage in the western part of the narcea antiform the major deformation event was the development of kilometre scale reverse shear zones because of polyphase deformation these rocks are not suitable for quantitative strain analysis but qualitatively finite strain can be observed to be prolate and invariably larger in this region than in the eastern sector the metamorphic grade is higher in the western part of the narcea antiform with local maxima occurring next to the base of the shear zones the b 0 cell parameter a semi quantitative geobarometer measured in the white mica is chaotic in the eastern sector showing a clear detrital inheritance in the western sector it is rational due to complete re equilibration during thermal resetting the inverse correlation between finite strain and white mica 9crystallinity9 is close showing that the anchizone epizone limit can be related to the increase of finite strain above values of r s 1 5 the close relation between white mica 9crystallinity9 and strain is suitable for tracking strain variations in extensive fine grained siliciclastic rocks where no appropiate strain markers are found

magnetization acquisition in two mesozoic red sandstones abstract paleomagnetic samples have been collected from folds and other sedimentary structures produced by soft sediment deformation and from intra formational conglomerates at several locations of the chugwater formation of wyoming and the moenkopi formation of the colorado plateau both of triassic age most load casts and other small scale folds were magnetized after deformation slump folds were magnetized before or during folding two of the four conglomerates contain clasts that were magnetized before they were ripped up these observations indicate that most fine grained sandstones are magnetized after deposition but before burial by a meter or so of sediment some sandstones and finer grained samples can be magnetized very shortly after deposition if they are consolidated and dewatered by drying samples having more complicated magnetization histories can usually be identified by poor within sample grouping of remanence directions and by the behavior of individual samples during demagnetization

the guadalupian permian kamura event in european tethys article i nfo in order to document paleoenvironmental conditions of the equatorial western paleo tethys during the late middle permian prior to the end guadalupian mass extinction chemostratigraphic analysis using stable carbon isotopes was conducted for the guadalupian rocks at brusane in the velebit mtn central croatia by analyzing 72 carbonate samples of the capitanian upper guadalupian velebit formation we found an interval with unusually high 13 ccarb values 4 to 6 in the ca 150 m thick yabeina fusuline zone the present find clarifies that the primary productivity and burial rate into the sediments were considerably high during the capitanian in westernmost paleo tethys this chemostratigraphic signal is properly correlated with the kamura event detected in a mid panthalassan paleo atoll limestone in japan the present results identify the capitanian kamura event for the first time in european paleo tethys on the opposite side of the globe from the mid panthalassan paleo seamount and prove the global context of the event as well as its utility in chemostratigraphic correlation in order to enhance bioproductivity on a global scale the increase in nutrient supply is inevitable in addition to riverine run off from pangea active oceanic circulation in particular upwelling of deep sea water enriched in nutrients was likely vital both in panthalassa and paleo tethys the extinction of some guadalupian fauna in particular photosymbiotic community large tested fusulines large bivalves rugose corals was likely related to a temporary cooling coupled with eutrophication in low latitude shallow marine environments of paleo tethys and panthalassa

the lower cretaceous atzompa formation in south central mexico record of evolution from extensional backarc basin margin to carbonate platform abstract lower cretaceous depositional systems of southwestern oaxaquia in south central mexico were influenced by initiation of a continental arc on mainland mexico and subsequent accretion of the guerrero composite arc terrane to mainland mexico the atzompa formation defined herein which crops out in the sierra de tentzo constitutes a succession of conglomerate sandstone siltstone and limestone with early cretaceous fauna and detrital zircon maximum depositional ages that range 126 123 ma late barremian to early aptian the lower part of the atzompa records a transition from alluvial to deep lacustrine depositional environments suggesting the early stages of an extensional basin overlying deposits of anabranching axial fluvial systems that flowed to the ne se accumulated after a period of rapid subsidence in the tentzo basin also formerly undescribed fluvial facies grade up section to tidal deposits overlain in turn by a carbonate ramp succession that contains late barremian to early aptian fossils the ramp deposits of the uppermost atzompa formation are overlain on a sharp contact by basinal carbonates of early albian age the tentzo basin formed due to crustal extension of the overriding plate in a backarc setting was characterized by very high rates of sedimentation 3 6 mm yr during the early stages of basin formation rift initiation and rift climax and slower rates during the development of tidal systems and the carbonate ramp post rift stage regional and local subsidence took place in the backarc region of the zicapa magmatic arc which was established in the western margin of mexico by hauterivian time abrupt deepening following atzompa formation deposition is attributed to flexural subsidence related to collision of the guerrero composite volcanic terrane with the western margin of mexico following late aptian accretion of the guerrero terrane to oaxaquia the carbonate basin eventually shallowed to become a carbonate platform that faced the gulf of mexico

further evidence for inclination shallowing in red beds red beds and related shallow marine siliciclastic sediments within the extensional catalan neogene basins offer a unique opportunity to study the relationship between sediment anisotropy remanence acquisition and inclination shallowing on the basis of the dominant fabric features sedimentary facies are grouped into laminated massive and bioturbated the bulk susceptibility appears to be dominated by paramagnetic phyllosilicates and clay minerals the magnetic susceptibility fabric is dominantly foliated while the degree of anisotropy is very variable and is dependent on the facies type strong foliation dominates in the laminated sediments whereas weak anisotropy characterizes the bioturbated and massive mass flow facies moreover we observe a correlation between facies type and remanence inclination the undisrupted thinly laminated facies provides the most biased inclination values 20 as opposed to 60 of the expected geomagnetic field steeper inclinations are reported from bioturbated muds while the best record of the geomagnetic inclination is found in the weakly anisotropic mass flow deposits

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

thermal convection dolomitization induced by the emeishan large igneous province abstract the mechanism of dolomitization has long been debated due to complex sedimentary and diagenetic conditions large scale dolomite bodies were recently discovered in the permian cisuralian qixia formation 284 273 ma in the northwestern sichuan basin providing an opportunity to understand the dolomitization associated with the emeishan large igneous province elip traditionally the dolomites in the sichuan basin have been interpreted as hydrothermal dolomitization because of the hydrothermal impacts from the elip here we report new petrographic and geochemical data on the permian dolomites which we grouped into two categories matrix dolomites or md1 md3 and cement dolomites or cd that support thermal convection dolomitization the petrographic and geochemical features 13c 18o ree mn sr ratios 87sr 86sr ratios suggest that md1 were formed in penecontemporaneous seawater whereas md2 md3 and cd were formed in fluids resembling permian seawater during shallow burial stage of which both dolomitizing fluids are non hydrothermal high formation temperature inferred from fluid inclusion is consistent with the low 18o values of the md2 md3 and cd suggesting that the elip event may have provided the heat for the dolomitization in the qixia carbonate platform the formation of the widespread dolomite bodies was facilitated by the rapid convection exchange between warm pore seawater in the qixia strata of the platform margin and cold water from outside the slope therefore we demonstrate that the dolomites in the northwestern sichuan basin are mainly due to thermal convection dolomitization in the shallow burial stage this study provides important implications for the mechanism of dolomite formation linked to abnormal geothermal activity

early late permian coupled carbon and strontium isotope chemostratigraphy from south china extended emeishan volcanism abstract carbon isotope compositions of carbonates 13ccarb document a new 3 5 cie toward lower values concomitant with an emeishan large igneous province elip related drowning event mapojiao event southern guizhou during the early wuchiapingian organic carbon isotope data 13corg have a 2 shift toward higher values across the drowning event showing decoupling with the 13ccarb evolution rock eval and palynofacies analyses suggest an elevated flux of terrestrial organic matter om during the drowning episode therefore the decoupling between 13ccarb and 13corg is best explained by the mixing of different organic carbon pools in the 13corg curve strontium isotope data 87sr 86sr also show a transient shift from 0 70715 to 0 70694 associated with this early wuchiapingian carbon isotope excursion cie which is superimposed on the late permian prolonged global rising trend this short lived 87sr 86sr excursion is best interpreted as an enhanced hydrothermal flux related to a short pulse of elip related volcanism comparison of the mapojiao event with other c isotope records from south china and other parts of tethys reveals substantial discrepancies moreover older capitanian cies concomitant with elip related drownings are also not laterally reproducible in time the local distribution of cies across guadalupian lopingian g l interval indicates the influence of local factors such as bathymetry and increased burial rate of terrestrial om rather than perturbations in the global carbon cycle to due volcanism this prevents the c isotope record to be used as a global correlation tool during capitanian and wuchiapingian times comparison of palaeobiodiversity changes with perturbations in c isotope compositions across the g l interval shows that only the oldest cie was associated with a modest extinction event as documented in south china hence this study undermines any systematic coupling and causal relation between extinctions and iterative perturbations of the c isotope record during the g l interval

palaeoenvironmental change during the end guadalupian permian mass extinction in sichuan china abstract the end guadalupian mass extinction has been investigated in sichuan province sw china in the south of the province the platform carbonates of the maokou formation are overlain by the emeishan flood basalts a possible factor in the extinction event and in the north of the province the formation is succeeded by shallow marine carbonates of the wuchiaping formation late permian the extinction event is primarily recorded by the loss of fusulinaceans and possibly by species level turnover amongst calcareous algae the high diversity middle permian microfossil assemblages are last seen beneath a surface recording widespread emergence and karstification in northern sichuan emergence occurred early in the capitanian stage and the succeeding strata record restricted hypersaline deposition that preceded the deposition of a thick ash horizon the wangpo bed these mid capitanian arid conditions are replaced by humid conditions towards the end of the stage as indicated by the development of a widespread coal seam in southern sichuan only a thin development of this humid coaly facies is seen developed atop the maokou karstic surface and below the oldest flood basalt the guadalupian interval saw the development of large c isotope excursions in the carbonate record these include a positive then negative swing late in the capitanian and a newly discovered negative spike superimposed on an early capitanian heavy interval this spike may be a global phenomenon but the magnitude 8 may have been exaggerated by an increased contribution from light respired c derived from soils during relative sea level fall a plethora of environmental factors approximately coincide with the extinction event including regression acidic volcanism and flood basalt volcanism making it difficult to unravel the relative significance of each

paleomagnetism of the dewey lake formation late permian northwest texas end of the kiaman superchron in north america the remanent magnetization of red beds of the late permian dewey lake formation from northwest texas was investigated using thermal chemical and alternating field demagnetization after removal of a small present day viscous overprint the dewey lake shows univectorial decay of the remanence revealing a dual polarity characteristic direction of d 152 4 i 11 1 k 39 5 95 5 9 n 16 sites three polarity transitions were observed defining at least four polarity zones the observed polarity zones are roughly layer parallel and may correlate with the illawarra magnetozone at the end of the kiaman if this is so k ar age determinations of an interbedded ash layer kolker and fracasso 1985 suggest that the kiaman superchron ended before 251 4 ma the paleopole located at 51 4 n 126 2 e confirms the reliability of previously reported late permian poles from southwest united states which fall significantly off the late permian segment of the paleomagnetic euler pole pep track of gordon et al 1984 this implies that a more segmented set of small circles is required to model the north american apparent polar wander path diminishing the usefulness of the pep method

paleomagnetism of jurassic radiolarian chert above the coast range ophiolite at stanley mountain california and implications for its paleogeographic origins upper jurassic red tuffaceous chert above the coast range ophiolite at stanley mountain california lat 35 n long 240 e contains three components of remanent magnetization the first component a removed by 100 200 c has a direction near the present day field for southern california and is probably a recently acquired thermoviscous magnetization a second component b removed between 100 and 600 c is identical to that observed by previous workers in samples of underlying pillow basalt and overlying terrigenous sedimentary rocks this component has constant normal polarity and direction throughout the entire section although these rocks were deposited during a mixed polarity interval of the geomagnetic field the b magnetization therefore is inferred to be a secondary magnetization acquired during accretion uplift or miocene volcanism prior to regional clockwise rotation the highest temperature component c removed between 480 and 680 c is of dual polarity and is tentatively interpreted as a primary magnetization although it fails a reversal test possibly due to contamination by b separation of the b and c components is best shown by samples with negative inclination c directions and a corrected mean direction using only these samples indicates an initial paleolatitude of 32 n 8 paleobiogeographic models relating radiolarian faunal distribution patterns to paleolatitude have apparently been incorrectly calibrated using the overprint b component few other paleomagnetic data have been incorporated in these models and faunal distribution patterns are poorly known and mostly unquantified the available data therefore do not support formation of the coast range ophiolite at stanley mountain near the paleoequator or accretion at 10 n paleolatitude as has been previously suggested based on paleomagnetic data but indicate deposition near expected paleolatitudes for north america 35 n 4 during late jurassic time

redox conditions across the g l boundary in south china evidence from pyrite morphology and sulfur isotopic compositions abstract the end guadalupian middle permian mass extinction recorded the disappearance of shelf faunas such as fusulinids small foraminifers brachiopods rugose corals which flourished in the warm shallow marine habitats after the late pennsylvanian early permian glaciation the causes for this mass extinction are still unclear marine anoxia often as a cause for other mass extinctions has been paid less attention in this extinction here we present a comprehensive study including pyrite morphology and sulfur isotopic compositions of pyrite across the guadalupian lopingian middle upper permian g l boundary at tianfengping in northern yangtze platform and at penglaitan in southern yangtze platform in south china our results show that the sulfur isotope ratios of pyrite mainly track the framboid size distributions in these two sections anoxia and intermittent euxinia occur at the g l boundary evidenced by the coincidence between reduction of framboid size and the extremely negative sulfur isotope values of pyrite involving disproportionation and corresponds to the main mass extinction of the end guadalupian suggesting a causal link between them long term dysoxic conditions during the middle capitanian are evidenced by the relative small framboid size and lower sulfur isotope ratios of pyrite and coincide with the first pulse of the end guadalupian mass extinction also suggesting a causal link between them the oxygen depletion conditions in the capitanian during the global sea level falling may be the main cause for the end guadalupian mass extinction

quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation abstract a new method of analysing isothermal remanent magnetisation irm acquisition curves based on cumulative log gaussian analysis robertson and france phys earth planet inter 82 1994 223 234 is proposed it is based on the curve fitting of the irm acquisition curve versus the logarithm of the applied field with i the acquisition curve on a linear scale ii the acquisition curve expressed as a gradient and iii the acquisition curve on a probability scale even when a sample is not saturated its magnetic properties can be defined although with less certainty the number of magnetic components required for an optimal fit to a measured irm acquisition curve is evaluated statistically the method discriminates on the basis of different mineral coercivity therefore additional rock magnetic tests are still required to separate minerals with similar coercivities

age and duration of the emeishan flood volcanism sw china geochemistry and shrimp zircon u pb dating of silicic ignimbrites post volcanic xuanwei formation and clay tuff at the chaotian section abstract the age and duration of the emeishan basalts sw china remain poorly constrained largely due to the severe thermo tectonic overprinting of the ar ar system and failure to obtain zircon separates from erupted basalts in an attempt to solve this problem geochemical analyses and shrimp zircon u pb dating have been carried out on rare felsic ignimbrite in the uppermost of the emeishan lava succession the xuanwei formation which immediately overlies the emeishan basalts and a clay tuff at the middle late permian boundary at the chaotian section clastic rocks of the lowermost xuanwei formation group 1 in eastern emeishan large igneous province lip have a geochemical affinity to the emeishan felsic volcanic rocks whereas the overlying sediments group 2 are compositionally more akin to mafic components of the emeishan traps this is the reverse of volcanic sequence of the central emeishan lip where the felsic extrusives sit above predominant mafic rocks it is likely that the clastic rocks are water transported sediments resulting from erosion of the volcanic rocks in the center of the emeishan lip this interpretation is further supported by the general agreement between the age of the lowermost xuanwei formation 257 4 ma 260 5 ma and that of the silicic ignimbrite 263 4 ma and the clay tuff at the middle late permian boundary at chaotian 260 5 ma these ages interpreted as the termination age of the emeishan volcanism are indistinguishable within error from the middle late permian boundary age 260 4 0 4 ma and the main stage 259 262 3 ma of the emeishan volcanism inferred from dating of mafic and alkaline intrusions in the emeishan lip all these suggest that the emplacement of the emeishan volcanism took place over a very short interval moreover the temporal link and geochemical analyses suggest that the chaotian clay at the middle late permian boundary was genetically related to the emeishan silicic volcanism this together with the fact at that both the emeishan basalts and the chaotian clay rest on the maokou formation leads us to infer that the emeishan basalt was emplaced at the middle late permian boundary in this sense the emeishan volcanism can be regarded as a boundary event and its age is presumed at 260 ma although more precise dating is required both stratigraphic correlation and chronologic data presented in this paper lend supports to the notion that the emeishan volcanism was one of likely causes of the end guadalupian mass extinction

the metamorphic aureole of the nisa alburquerque batholith sw iberia implications for deep structure and emplacement mode the nisa alburquerque granitic batholith southern variscan belt iberian peninsula has been studied by petrological structural and geophysical approaches obtaining contrasting models for its deep structure and emplacement sequence in order to test these models and gain knowledge on the thermal increase induced by the intrusion we have studied its contact aureole which was developed in similar country rock lithologies mica schists alternating with metasandstones and feldespatic schists all along the northern external contact of the batholith our results indicate no change in metamorphic grade and some variations in aureole width which narrows toward the western sectors of the batholith cordierite is the only contact metamorphic mineral developed together with a high temperature biotite probably related to the granite thermal input by considering these new data together with zircon saturation temperatures within the granite and previous petrological and geophysical studies we propose a model in which the feeder zones of the granitic magmas were an eastern main one and a western secondary one we have also made comparisons of the metamorphic grade in the country rocks and the xenoliths within the granite most of the xenoliths have the same metamorphic facies as the country rocks crd zone though some of them contain slightly different assemblages and crd which could be explained in different ways 1 differences in the primary schist compositions 2 increased time span of xenoliths in contact with the melt and 3 xenolith incorporation at slightly higher depths during final granite ascent

tectonic control of guadalupian lopingian cherts in northwestern sichuan basin south china abstact cherts have been considered to be formed mainly through processes involving biosilicification riverine inputs and hydrothermal activities in the permian massive cherts are widely distributed in south china bearing significant implications for the reconstruction of paleogeography and paleoceanography of the eastern paleo tethys ocean the origin of the permian cherts in south china however remains inconclusive due to a lack of systematic petrographic and geochemical study in this article we analyze the petrologic and geochemical characteristics of the cherts collected from five outcrops in the northwestern sichuan basin south china these cherts are found from the upper guadalupian maokou formation ca 268 259 ma and upper lopingian dalong formation ca 254 252 ma field and microscopic observations suggest that cherts from the maokou formation occur mainly in nodular forms and consist of microcrystalline quartz silicified bioclasts and siliceous fossil debris both petrographic observation and geochemical analysis suggest that the maokou cherts are mainly of hydrothermal origin near deep faults in the southwest and of biogenetic origin with less hydrothermal influence in the northeast of the studied area the dalong cherts occur in both nodular and bedded forms with more abundant siliceous fossil debris than the maokou cherts major trace and rare earth element data support that the dalong cherts are influenced by both biosilicification and hydrothermal activities the spatial heterogeneity of the cherts origin in south china is largely controlled by tectonic activities associated with the emeishan large igneous province elip event and the closure of the paleo tethys ocean the troughs formed by the extensional tectonic activity linked to the elip could provide accommodation space and activate deep faults allowing the rise of silicon rich hydrothermal fluids from the deep and nutrient rich water from volcanogenic upwellings subsequent cooling may have facilitated the precipitation of large amounts of silica and the formation of cherts in the study region this work demonstrates that the origin of the guadalupian lopingian cherts in south china is predominantly controlled by tectonic and biological activities which sheds light on the global permian chert event

approach to chemical equilibrium in diagenetic chlorites electron microprobe analyses were made on diagenetic chlorites in sandstones and mudstones from two deep wells according to the petrographic character of the chlorite occurrence as pseudomorphic phases rims on quartz or glauconite or as distinct phases in the clay matrix chlorite compositions do not depend upon crystallization site reacting phases making it apparent that new chlorites can form in an approach to chemical equilibrium at or near the surface 40 c 1 km depth comparison of this data with that for late diagenetic and early metamorphic chlorites indicates that the compositional range for different grains in the same thin section is similar for the samples throughout the 40 270 c temperature span compositional range decreases upon further metamorphism

new timing and geochemical constraints on the capitanian middle permian extinction and environmental changes in deep water settings evidence from the lower yangtze region of south china the capitanian guadalupian witnessed one of the major crises of the phanerozoic and like many other extinctions it coincided with the eruption of a large igneous province in this case the emeishan traps of sw china however the timing and causal relationships of this event are in dispute this study concentrates on the deep water chert mudstone strata of the gufeng formation and its transition to the yinping formation at chaohu zircons from tuffs in the uppermost gufeng formation yield a u pb age of 261 6 1 6 ma and comparison with sections around emeishan suggests that the tuffs appeared in the jinogondolella altudaensis conodont zone and persisted to the jinogondolella xuanhanensis zone this coincides with the emeishan eruptions and suggests that the tuffs probably derived from this province mineralogical and geochemical characteristics also show the tuffs are of acid volcanogenic origin and have a geochemical fingerprint of the emeishan large igneous province our dating shows that a crisis amongst radiolarian and a subsequent productivity decline occurred during the middle capitanian prior to the guadalupian lopingian boundary the emeishan eruptions began immediately before this indicating a likely causal relationship between these events major regression and marine anoxia euxinia are two other important extinction relevant environmental changes that occurred during this critical interval

detrital zircon u pb geochronology of southern guerrero and western mixteca arc successions southern mexico new insights for the tectonic evolution of southwestern north america during the late mesozoic late jurassic cretaceous arc related volcaniclastic rocks from the southern guerrero and western mixteca terranes of mexico were analyzed by u pb detrital zircon geochronology laser ablation multicollector inductively coupled plasma mass spectroscopy to place constraints on the depositional history and provenance of the rocks pre middle jurassic basement rocks and sandstone from the upper cretaceous mexcala formation were also analyzed to defi ne the origin and provenance of the prevolcanic substratum and the time of accretion of guerrero composite terrane sequences data from the taxco taxco viejo teloloapan and arcelia assemblages indicate that the youngest 129 141 ma zircon fraction in each sequence was derived from local volcanic sources whereas older populations ca 247 317 365 459 530 617 712 878 947 964 1112 1188 1350 1420 1842 1929 2126 2439 and 2709 3438 ma show sediment infl ux from varied sources most likely through grain recycling the major zircon clusters in these sequences match the populations recorded in the nearby acatlan complex in contrast the huetamo sample is dominated by lower cretaceous ca 126 ma zircons of local volcanic provenance and the zihuatanejo sample contains zircon clusters ca 259 ca 579 and ca 947 1162 ma comparable to major populations recorded in the underlying arteaga complex a sample from the middle triassic middle jurassic arteaga complex at tzitzio contains zircon clusters ca 202 247 ca 424 ca 600 ca 971 and ca 2877 ma consistent with an ultimate derivation from both north american and south american sources the sample from the las ollas suite contains comparable zircon populations ca 376 475 ca 575 ca 988 1141 and ca 2642 2724 ma and it is interpreted to be part of the prevolcanic basement in contrast the youngest zircon cluster ca 105 ma in the mexcala formation coincides with the major volcanic events in the taxco taxco viejo teloloapan and arcelia assemblages whereas the older clusters ca 600 ca 953 ca 1215 ca 1913 and ca 2656 2859 ma broadly match the major populations recorded in rocks from the acatlan complex these new data combined with available geochemical and isotopic data indicate that the taxco taxco viejo arc assemblage developed on continental crust the acatlan complex is the most plausible candidate the teloloapan and arcelia arc assemblages were developed on oceanic crust as offshore arcs facing the acatlan complex the zihuatanejo terrane assemblages were developed on the arteaga complex and evidence no infl uence from the acatlan complex this suggests that these assemblages were formed farther away or in a restricted basin the guerrero composite and mixteca arc successions are coeval with the alisitos arc of northern mexico and in part with the nevada and klamath ranges of the southwestern united states and with the arc series from the greater and lesser antilles and northern south america data indicate that during late mesozoic time southwestern north america was a site of intensive volcanism in a complex arc trench system similar to that of the east pacifi c our data are consistent with a diachronic accretion of the guerrero composite terrane sequences beginning during late cenomanian time with the amalgamation of the teloloapan and probably the arcelia assemblages and fi nishing at the end of cretaceous time with the accretion of the zihuatanejo terrane assemblages

mesozoic paleomagnetism and northward translation of the baja california peninsula paleomagnetic data from cretaceous igneous and sedimentary rocks of the peninsular ranges and continental borderland of the baja california peninsula yield an average pole position of 83 n 344 e a95 3 4 suggesting that the peninsula as a whole has moved northward 11 and rotated 32 clockwise with respect to the north american craton since cretaceous time only a small portion of this movement is due to the neogene opening of the gulf of california results from stratified rocks giving both positive and negative fold tests are included in the average cretaceous pole the rocks that fail fold tests have been deformed and remagnetized by nearby intrusives that are only slightly younger unconnected directions from these sites closely match corrected directions from sites where the fold tests are positive oceanic rocks were also sampled in the continental borderland from upper triassic and upper jurassic tectonostratigraphic terranes which are apparently allochthonous with respect to the western baja peninsula and each other these oceanic rocks show magnetizations indicating low paleolatitudes although the hemisphere of origin is presently unknown the cretaceous rocks of the baja california peninsula may have originally formed adjacent to the now truncated continental margin along the northern middle america trench northward transport of the peninsula probably occurred by transcurrent faulting associated with oblique subduction of the farallon plate beneath western north america between late cretaceous and late miocene time

do red beds indicate paleoclimatic conditions a permian case study abstract terrestrial red beds have long been interpreted as desert deposits by comparison with modern red deserts more recently red beds have been interpreted as evidence of seasonally dry conditions and a permo triassic pangean monsoon red beds of cala viola sardinia are identified as paleosols and used to reconstruct late permian paleoclimatic conditions reconstruction of paleoenvironmental conditions based on the paleosols of the cala viola indicates warm humid conditions with no evidence of dry conditions as in a desert or of extreme seasonality as in a monsoon instead it is suggested that the red color of the paleosols is a result of former good drainage and that red color in general does not indicate specific paleoclimatic conditions

constraining the cause of the end guadalupian extinction with coupled records of carbon and calcium isotopes abstract a negative c 13 excursion in carbonate sediments near the guadalupian lopingian middle late permian boundary has been interpreted to have resulted from a large carbon cycle disturbance during the end guadalupian extinction event ca 260 ma however the carbon isotope data alone are insufficient to uniquely determine the type and magnitude of perturbation to the global carbon cycle calcium isotopes can be used to further constrain the cause of a carbon isotope excursion because the carbon and calcium cycles are coupled via caco3 burial in this study we present coupled carbon and calcium isotope records from three guadalupian lopingian g l sections in china penglaitan and chaotian and turkey koserelik tepe the c 13 and ca 44 40 records differ among our studied sections and do not co vary in the same manner no section shows c 13 and ca 44 40 changes consistent with massive rapid volcanic co2 emissions or methane clathrate destabilization additionally many sections with large 3 changes in c 13 exhibit o 18 evidence for diagenetic alteration only one section exhibits a large excursion in the ca 44 40 of limestone but the absence of a similar excursion in the ca 44 40 of conodont apatite suggests that the limestone excursion reflects a mineralogical control rather than a perturbation to the global calcium cycle hence we interpret the large isotopic changes observed in some sections to have resulted from local burial conditions or diagenetic effects rather than from a large carbon and calcium cycle disturbance perturbations to the global carbon and calcium cycles across the g l transition were much less intense than the disturbances that occurred across the subsequent permian triassic boundary this finding is consistent with the much smaller magnitude of the end guadalupian extinction relative to the end permian

variations in relative abundances of goethite and hematite in bengal fan sediments climatic vs diagenetic signals abstract relative abundances of goethite and hematite in marine sediments have been increasingly used in reflectance spectrometry studies to infer the precipitation regime at the source area of detrital sediments this interpretation is contingent on the assumption that the ratio of these minerals is not modified by diagenetic processes our rock magnetic study of the 10 5 4 0 ma interval of the bengal fan sedimentary sequence indicates that this is not always the case we demonstrate that a statistical analysis of isothermal remanent magnetization acquisition curves provides an adequate estimate of the content of goethite and hematite in sediments we also identified distinctive biogenic and detrital components likely titano magnetite or maghemite in the studied samples the sediments of the bengal fan contain a significant change in relative abundance of goethite and hematite at 7 ma which is approximately coeval with a postulated climatic turnover in the sediment source region however systematic variations in the detrital and biogenic components as well as an increase in the total organic carbon content and the appearance of pyrite concretions at the same stratigraphic level suggest that this change in magnetic mineral content reflects a change in the degree of diagenetic alteration of the initial detrital assemblage rather than a climatic signal when assigning climatic interpretations to changes in the relative abundance of goethite and hematite in marine sediments in future studies the possibility of diagenetic modification should be evaluated with rock magnetic methods alteration of ancient sediments can be detected and the severity of alteration can be estimated based on the presence and grain size variation of the ferrimagnetic fraction magnetite maghemite which is more sensitive to early reductive dissolution than high coercivity minerals goethite and hematite

relationships between very low grade metamorphism and tectonic deformation examples from the southern central iberian zone iberian massif variscan belt abstract we have studied the syn kinematic very low grade metamorphism in a polyphase variscan deformed region using x ray diffraction techniques two phases of regional metamorphism are related to their respective episodes of penetrative deformation in the southern central iberian zone the data obtained suggest that the rocks did not reach metamorphic equilibrium but strain favoured the progress of mineral reactions in the more deformed parts the first deformation is devonian in age and consists in a heterogeneous ductile shearing coeval with large scale recumbent folding that were produced under high anchizone to epizone metamorphic conditions the heterogeneity of the shearing originated strain gradients that can be said to enhance the growth of new minerals and the illite polytype transformation in the highly strained overturned limb of the preserved pile of recumbent folds but illite crystallinity remained constant throughout the structure the second deformation is mid carboniferous in age and consists in an upright folding that took place under late diagenesis to low anchizone metamorphic conditions the distribution of mineral parageneses and illite crystallinity across one of the upright folds suggests that strain gradients favoured the metamorphic reaction progress from the hinge low strain towards the limbs high strain other characteristics of the region such as a metamorphic gap associated with an unconformity at the base of the lower carboniferous rocks or cryptic contacts aureoles surrounding volcanic intercalations and a large granitic batholith are also studied

remagnetization and northward translation of mesozoic red chert from cedros island and the san benito islands baja california mexico samples for paleomagnetic analysis were collected from red radiolarian ribbon chert in a franciscan like subduction complex western baja terrane exposed along the western margin of the baja california peninsula a 40 m thick section of chert was sampled on cedros island and a 30 m thick and two smaller sections of chert were sampled on the san benito islands to the west a site in pillow basalt underlying the chert also was sampled on cedros island the sections of chert on cedros island and the san benito islands contain radiolaria ranging in age from late triassic to early cretaceous all of these rocks have similar normal polarity direction of magnetization indicating that they were remagnetized long after deposition results from a chert site including an intrastratal fold also indicate that the magnetization is postdepositional a significantly improved grouping 95 confidence level of directions after correction for the structural attitudes of the cedros and san benito chert sections however implies that the remagnetization predates breaking up and incorporation of the chert into the subduction complex studies of modern subduction zones imply that the attitude of the ancient subducting slab including the chert sections was near horizontal dip angles

paleomagnetism of the moenkopi formation colorado plateau basis for long term model of acquisition of chemical remanent magnetism in red beds paleomagnetic studies of moenkopi samples from several sites on the colorado plateau including a complete 427 m section at sinbad valley western colorado reveal the following 1 the presence of both normal and reversed triassic components in many of the same samples at many localities 2 the close correlation of the magnetic polarity and intensity of magnetic remanence with the lithologic characteristics in the sinbad valley section 3 a streaked pattern of the remanence directions in any one member in the sinbad valley section similar to that expected from polar wander 4 a lack of progressive change from the base upward and 5 the common intermediate remanence directions of weak intensity suggesting the presence of two or more nearly balanced anti parallel components throughout the formation the results of our investigation combined with previously reported petrographic and paleomagnetic studies of the moenkopi formation have led us to conclude that the principal remanence carried by the formation is chemical remanent magnetism crm acquired diagenetically over a geologically long time interval from the available data we have formulated the following model to explain how the crm was acquired through natural processes acting intermittently at geologically reasonable rates when initially deposited the moenkopi sediments consisted primarily of first cycle detritus containing a few percent of iron bearing accessory minerals such as biotite hornblende magnetite and ilmenite at some time after deposition under favorable subsurface conditions intrastratal alteration began involving breakdown of iron bearing silicate and oxide minerals partial replacement of some iron rich silicate grains by hematite martitization of magnetite growth of hematite pods and blebs in ilmenite and biotite and precipitation of crystalline hematite in both primary and secondary voids the authigenesis spanned at least two polarity events and probably continued over several tens of millions of years stratigraphic units with grossly similar lithologies underwent similar diagenetic alterations at rates and to extents that were different from other stratigraphic units having different lithologies because of variations in the rate and duration of alteration and variations in the length of normal versus reversed polarity events any given lithologic unit tended to be dominated by one polarity the model given above seems to be capable of explaining not only those magnetic and petrographic features found in the moenkopi formation but also those of most other red beds

a recalibration of the chlorite biotite muscovite geobarometer massonne 1981 reported experiments on the composition of muscovite in the assemblage phlogopite muscovite k feldspar quart the experimental data have been used to recalculate the equilibrium conditions for the chlorite muscovite biotite quartz assemblage which has been proposed as a new geobarometer by powell and evans 1983 the recalculated equilibrium shows much steeperdp dt slopes than those calculated by powell and evans 1983 nevertheless with the necessary precautions it is suggested that the barometer may provide the critical user with valuable pressure estimates

detrital remanent magnetization in hematite heavy mineral placer layers within channel sandstones of the morrison formation western united states have been shown to contain a very stable accurate record of the jurassic geomagnetic field these laminae are composed almost entirely of hematite grains in particular they are largely the hematite pseudomorph of magnetite martite the laminae record the correct geomagnetic field direction during a period of rapid apparent polar wander which suggests an early acquisition of the remanence several other remanence characteristics imply that the remanance is probably a detrital remanent magnetization drm 1 there is large directional dispersion in one lamina which had been heavily burrowed by organisms measurement of individual burrows shows that the burrowing disrupted and randomized the magnetization held in the martite grains 2 directions of laminae that were deposited on sloping surfaces are significantly deflected from the directions of the flat lying laminae 3 more than 50 of the martite grains have high temperature oxidation features suggesting that they may have been martite at the time of deposition these characteristics in particular the burrowing disturbance strongly suggest that the martite was deposited as martite and that the remanence is a drm as opposed to deposition of magnetite grains which were altered to hematite after deposition examination of recent weathering debris on igneous outcrops shows a substantial amount of martite on these outcrops this observation provides further evidence in support of the idea that hematite and in particular martite can be a deposited mineral in sedimentary rocks and thus can carry a drm

paleomagnetism of the cretaceous morelos and mezcala formations southern mexico a paleomagnetic study of platform facies carbonate rocks of the mid cretaceous morelos formation and deep water carbonate rocks of the overlying upper cretaceous mezcala formation sampled at zopilote canyon in guerrero state southern mexico indicates that their characteristic magnetization was acquired contemporaneously with folding of these rocks during the late cretaceous laramide orogeny the remanence carrier is interpreted to be magnetite although other mineral phases of high coercivity carry recent secondary overprints the overall mean is of dec 323 1j and inc 36 5j k 162 7 a95 2 7j n 18 sites 64 unfolding comparison with the north america reference direction indicates that this area has experienced a small yet statistically significant counterclockwise direction of 19 2f4 0j similar rotations are documented in other localities from southern mexico rotations are linked to mid tertiary deformation associated with the left lateral strike slip fault system that accommodated motion of the chortis and xolapa blocks d 2002 elsevier science b v all rights reserved

proliferation of shallow water radiolarians coinciding with enhanced oceanic productivity in reducing conditions during the middle permian south china evidence from the gufeng formation of western hubei province the middle permian is a critical period when a possible cooling regime prevailed and massive deep water cherty sediments were deposited in the south china craton an integration of radiolarian paleoecologic and geochemical studies from the middle permian gufeng cherty succession of western hubei province south china tests the relationships among oceanic productivity redox conditions and radiolarian responses as well as origin of the cherts a total of 21 species and six undetermined species in 10 radiolarian genera are identified from the gufeng formation in the northern margin basin of the yangtze platform of the south china craton three radiolarian pseudoalbaillella globosa follicucullus monacanthus and follicucullus scholasticus zones were established and constrained the gufeng formation as roadian to middle capitanian in age the gufeng cherts embrace a biologic and hydrothermal origin in the northern marginal basin and southern margin basin the nanpanjiang basin of the south china craton respectively the high oceanic primary productivity in surface oceans resulted in the widespread reducing conditions in the bottom water column such high oceanic productivity event slightly predated the middle permian kamura cooling event of the panthalassian ocean both the elevated continent weathering due to pre eruptive crustal uplift of the emeishan flood volcanism and active oceanic upwelling may have stimulated high oceanic productivity and facilitated the deposition of the gufeng cherts all radiolarians bloomed in relatively oxic conditions only shallow water radiolarians mainly spherical forms proliferated in the reducing conditions which also coincided with an enhanced oceanic productivity the abundance of spherical radiolarians therefore is an ideal proxy indicating oceanic primary productivity

paleomagnetic evidence that the central block of salinia california is not a far traveled terrane new paleomagnetic results from late cretaceous 75 85 m y red beds on the central block of salinia indicate that salinia was located within 6 in latitude of its current cratonal north american position during the late cretaceous after correction for neogene san andreas fault transport the red beds formed as alluvial fan overbank deposits with hematite cement deposited directly on salinian granites in the la panza range paleomagnetic analysis shows two components of magnetization in the red beds a low blocking temperature present day overprint residing in goethite and a high blocking temperature 600 component residing in hematite the hematite magnetization is a chemical remanent magnetization which formed soon after deposition during pedogenesis the bedding corrected hematite remanence contains a magnetic polarity stratigraphy with antipodal normal and reversed directions twenty three class i sites 95 20 have an average hematite direction with inclination 54 4 and declination 18 2 95 6 1 after structural correction these paleomagnetic data suggest that salinia resided at about 35 n latitude during the late cretaceous within 6 of its current location adjacent to cratonal north america by contrast a summary of paleomagnetic data from the peninsular ranges terrane and the sur obispo terrane which are currently outboard of salinia shows northward transport of these terranes of 12 to 22 relative to their current locations in north america since the cretaceous the offsets increase systematically away from the craton with the most outboard sur obispo terrane which is composed of accretionary prism and distal forearc material showing the largest degree of northward translation

the carbon isotope excursion on gssp candidate section of lopingian guadalupian boundary an isotopic stratigraphically well documented outcrop in penglaitan guangxi province south china has been proposed as a candidate gssp for the lopingian guadalupian boundary correlatable outcrops from the west wing tieqiao section and east wing penglaitan section of the laibin syncline exhibit synchronous excursions in carbon isotopes the isotopic excursions 13c show the best placement of the boundary may lie at the base of bed 6k which coincides with the clarkina postbitteri conodont zone and with eustatic change 13c increases during the uppermost guadalupian jinogondolella granti conodont zone from the top of 3c to the base of 6i a 13c peak value of 5 is located at the transition between these two conodont zones and is suggested as a proxy for the transition from transgression to regression a gradual depletion of carbon isotopes occurs in the c postbitteri zone from bed 6e to 7b and this gradual 13c excursion also suggests the sequences around the guadalupian lopingian boundary at the candidate section are conformable during the middle later c postbitteri zone a 3 5 dramatic 13c depletion is recorded at the tieqiao section but only a 2 depletion at the deeper facies penglaitan section synchronous with conodont zones that mark eustatic changes

conodont biostratigraphy across the permian triassic boundary at chaotian in northern sichuan china abstract detailed conodont biostratigraphy of the permian triassic boundary ptb interval was analyzed at the fossiliferous chaotian section in northern sichuan china the ptb interval at chaotian is composed of the uppermost permian mudstone limestone of the dalong formation and the lowermost triassic marl limestone of the feixianguan formation both representing deep basinal facies the ptb interval is enriched in ammonoids and conodonts and is divided into 6 conodont zones including 1 clarkina changxingensis changxingensis c subcarinata assemblage zone 2 c postwangi c sp b assemblage zone 3 c taylorae c zhejiangensis c changxingensis yini assemblage zone 4 barren intra zone 5 hindeodus parvus zone and 6 isarcicella zone in ascending order the first four zones correspond to the changhsingian whereas the latter two to the lower induan griesbachian the first occurrence fo of h parvus in the lowermost feixianguan formation is regarded as the conodont biostratigraphic boundary from the uppermost dalong formation various taxa of the genus clarkina occurred abundantly and suddenly disappeared at the top of the dalong formation therefore the top of the dalong formation is interpreted as a horizon of the eventostratigraphic boundary on the basis of conodont zones the chaotian section is correlated with the shangsi and meishan gssp of the ptb sections in south china and the zal section in iran

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

diagenesis of clay minerals and k bentonites in late permian early triassic sediments of the sichuan basin chaotian section central china abstract detailed clay mineralogical analyses were carried out on late permian early triassic carbonate sediments exposed on the chaotian section sichuan basin central china the clay assemblages are dominantly composed of illite in platform carbonates and clay seams and illite smectite mixed layers i s in tuff layers k bentonites intercalated in the carbonate succession detrital and authigenic volcanogenic clay minerals have been partially replaced through illitisation processes during burial raising questions about diagenetic effects the precise determination of i s occurring in k bentonites shows that the sediments reached a temperature of about 180 c which is consistent with 1 previous estimates based on fluid inclusion homogenisation temperature analysis 2 the burial depth of the sedimentary series deduced from the post palaeozoic geological history of the sichuan basin and 3 the new data t max obtained on organic matter indicating the transition between oil and gas windows the wangpo bed located close to the guadalupian lopingian boundary is interpreted either as a volcanic acidic tuff or as a clastic horizon this controversial origin probably results from mixed volcanogenic and detrital influences the wangpo bed is therefore interpreted as a reworked bentonite as revealed by the occurrence of i s similar to those found in tuff layers together with preserved detrital kaolinite

tectonic evolution of the boundary between the central iberian and ossa morena zones variscan belt southwest spain three tectonometamorphic units can be differentiated in the boundary between the central iberian and ossa morena zones of the iberian massif variscan belt the three units have been named the northern unit central unit and southern unit the northern unit corresponds to the border of the central iberian zone it evolved at low temperature and intermediate or low pressure metamorphic conditions and was affected by top to the se ductile shearing the central unit placed under the northern unit preserves high pressure silurian eclogitic assemblages retrograded by high to medium temperature and finally low temperature ductile shearing with top to the nw sense of movement oblique left lateral the central unit is superposed on the southern unit the latter corresponds to the border of the ossa morena zone and underwent low pressure intermediate temperature metamorphism synchronous with right lateral top to the se ductile shearing the envisaged tectonic evolution is as follows after a stage of lower paleozoic rifting subduction of the central unit under the northern unit took place in silurian times as a result of the crustal thickening a gravitational instability developed giving way to left lateral extensional shearing that affected the entire central unit the combined action of oblique thrusting at the front of the wedge and oblique extensional shearing at the rear caused the exhumation of the high pressure rocks of the central unit this tectonic evolution reveals that the boundary between the central iberian and ossa morena zones is a suture of the variscan belt

permian palaeogeographic evolution of the jiangnan basin south china abstract the most striking geographic feature of the permian of south china is the jiangnan basin which intimately reflects the interplay between the yangtze and cathaysia blocks geographic and sedimentological evolution suggests that the jiangnan basin superposed on a precambrian collisonial megasuture and a folded early paleozoic deep basin was an extensional intracratonic basin it opened in the longlinian and became more passive and restricted in the latest maokouan the remarkably similar geographic patterns of the jiangnan basin in the early and late palaeozoic suggest that the yangtze and cathaysia blocks were coherent during the paleozoic

paleomagnetism of the moenkopi and chinle formations in central new mexico implications for the north american apparent polar wander path and triassic magnetostratigraphy in central new mexico red sedimentary rocks unconformably overlying permian carbonates of the san andres formation have been correlated with the early middle triassic moenkopi and late triassic chinle formations of the colorado plateau paleomagnetic samples from triassic sections exposed on basement cored uplifts along both the east and west side of the rio grande rift near albuquerque yield upon thermal and chemical demagnetization well defined high unblocking temperature dual polarity magnetizations carried by hematite the characteristic magnetization is interpreted as an early acquired chemical remanent magnetization based on a positive intraformational microconglomerate test and bedding parallel magnetization polarity zonation the moenkopi and lowermost chinle formations produced paleomagnetic poles respectively at 57 6 n 100 3 e n 36 sites k 74 1 a95 2 8 and 60 8 n 88 9 e n 17 sites k 130 3 a95 3 1 these data plus previously published and additional results from the underlying permian strata suggest that portions of central new mexico have experienced a small clockwise rotation i e less than 10 similar to that of the colorado plateau with respect to the north american craton the paleomagnetic directions of the chinle formation and related strata in eastern new mexico document about 12 great circle distance of rapid apparent polar wander during mid carnian to late norian times along a track which contains other cratonic poles of similar age we present a preliminary magnetic polarity time scale for the triassic that incorporates the present new mexico data and previously published data mostly form continental red bed sequences this magnetic polarity scale provides a basic framework which can be tested with future data from triassic sections where additional biostratigraphic control exists

the middle permian capitanian mass extinction on land and in the oceans abstract a middle permian mass extinction first discovered in 1994 has become known as the end guadalupian event in the literature however recent studies of foraminifera and brachiopod range truncations in conodont dated sections on the south china block have shown that the losses occur below this level in the middle of the capitanian stage extinctions were suffered by several other groups notably the corals whilst the mollusc record is more enigmatic a major bivalve crisis has been reported in some studies the giant alatoconchids being notable victims but not others gastropods were unaffected by the crisis whilst a roughly contemporaneous ammonoid mass extinction may have occurred in the early wuchiapingian a few million years after the main marine losses compilation of data from plant species in south china reveals a significant 24 loss suggesting that the capitanian crisis also occurred on land an intra capitanian extinction of 56 of plant species in north china block sequences may also have coincided with these losses correlation of these marine and terrestrial extinction events using the palaeomagnetic record provides two alternatives either turnover amongst plant species is contemporaneous with the marine extinction and the eruption of the emeishan flood basalt province in southwest china or plant losses post date the marine extinction and instead coincide with the waning phase of the igneous province current understanding of the major dinocephalian extinction suggests this event occurred during the preceding stage the wordian but future improvements in both sampling and dating this tetrapod crisis may reveal a synchronicity of plant animal and marine invertebrate extinctions the clear temporal link of the capitanian marine extinction with emeishan volcanism suggests that these flood basalt eruptions triggered the crisis a contemporaneous major negative c isotope excursion suggests that like many other mass extinction events methane release from hydrates may also be implicated however in the best dated chinese sections the main excursion is found to slightly post date the extinction which occurs at the end of an unusual and unexplained interval of exceptionally heavy 13c values other usual suspects for mass extinctions either lack geological and palaeontological evidence e g marine anoxia and global cooling or do not precisely correlate with the extinction e g major eustatic regression

δ18o and δ13c of permian brachiopods a record of seawater evolution and continental glaciation abstract two hundred sixty four 18 o and 13 c values of permian articulate brachiopod shells were analyzed and 81 of these values were characterized as well preserved and biostratigraphically well defined these were then utilized for construction of baseline oxygen and carbon isotope curves for the permian interval in addition 21 13 c whole rock values are reported for the wordian and capitanian the early permian asselian to artinskian times are characterized by 2 5 decrease in oxygen isotope values from 0 7 to 3 3 v pdb this is attributed to a 4 7 c increase in temperature in the southern urals concomitant with the retreat of the permo carboniferous ice sheets and return of the 18 o depleted melt water into the oceans the late permian samples from iran jolfa at kuh e ali bashi and china meishan yield 18 o values and presumably temperatures similar to those that followed the termination of the large scale glaciation in the lower permian in between the upper kungurian to capitanian samples from the delaware basin guadalupe mountains are enriched in 18 o at 1 5 to 3 we have no definitive explanation for these data but tentatively suggest that the anomaly can potentially be a result of evaporative enrichment of seawater in 18 o due to intracratonic arid setting of the basin the 18 o enriched nature of the zechstein samples 1 2 to 2 5 on the other hand is in all probability a reflection of the high evaporation rates in the zechstein sea the permian interval is characterized by a relatively constant 13 c at about 4 the exceptions are again the brachiopods from the delaware basin guadalupe mountains which show 1 6 increase in the guadalupian to values of up to 5 9 in the wordian a tentative explanation as in the case of oxygen is based on the proposition that the semi enclosed delaware basin was likely stratified with sequestration of the 13 c depleted carbon to the deeper water layers and a complementary 13 c enrichment in the upper oxygenated layer the coeval open ocean water dic may have been similar to that of the remainder of the permian interval at 4 as indicated by whole rock carbonate samples from oman sicily and iran in the latest permian the trend mimics the well known 13 c drop at the permian triassic boundary

carbon and conodont apatite oxygen isotope records of guadalupian lopingian boundary sections climatic or sea level signal abstract the guadalupian lopingian g l boundary 260 4 ma is one of the major extinction events in earth history that coincides with the emeishan large igneous province and palaeoclimatic changes carbon isotopes of whole rock samples were studied in order to document changes in the global carbon cycle in contrast to earlier studies we observe no major negative excursion in 13 c in the middle capitanian a positive 13 c excursion is observed in the latest capitanian with a 1 5 increase registered in the j xuanhanensis zone to c postbitteri hongshuiensis subzone followed by a decrease of 1 within the c postbitteri postbitteri subzone and a 2 decrease in the c dukouensis to c asymmetrica zone oxygen isotopes of conodonts from two g l boundary sections were measured in order to reconstruct conodont habitat and potential changes in water temperature oxygen isotope ratios of gondolellid conodonts are higher in comparison to oxygen isotope ratios measured on hindeodid conodonts suggesting that gondollelids lived in cooler and thus deeper waters compared with hindeodids the oxygen isotope record reconstructed from gondollelid conodonts suggests warming of water temperatures of about 4 c in the late capitanian j postserrata to j granti zone cooling of about 6 to 8 c across the g l boundary and in the earliest wuchiapingian and again significant warming in the wuchiapingian c dukouensis to c liangshanensis zone the temperature increase can be correlated with the main phase of emeishan volcanism suggesting that climatic warming may have resulted in an intensified hydrological cycle fertilisation of the oceans and enhanced primary productivity the latter documented in the positive late capitanian carbon isotope excursion however changes in sea level seem to parallel reconstructed water temperatures suggesting that changes in water depth in combination with superimposed climatic changes may be responsible for the observed temperature changes this study documents that oxygen isotope studies on permian conodonts should be performed on mono generic conodont samples and that oxygen isotopes not only provide valuable palaeoclimatic information but also may help to constrain the life habitat of conodonts

guadalupian cool versus warm water deposits in central iran a record of the capitanian kamura events arefifardcapitanian cooling event in iran

magnetostratigraphy of displaced upper cretaceous strata in southern california a magnetostratigraphic study of upper cretaceous marine strata from the santa ana mountains in southern california has identified a campanian reversed magnetozone this reversed interval corresponding to marine magnetic anomaly 33 34 chron 33r of campanian age can be correlated with a campanian reversed magnetozone that has been reported from strata of the great valley sequence in central california the late cretaceous paleolatitude of the santa ana mountains is estimated from this study to be 26 6 n this is significantly different from the region9s expected cretaceous paleolatitude of 43 8 n as part of the north american stable craton and indicates that this region part of the peninsular ranges terrane was 1900 km farther south in cretaceous time relative to the stable craton

development of a high productivity and anoxic euxinic condition during the late guadalupian in the lower yangtze region implications for the mid capitanian extinction event abstract widespread marine anoxia is considered one of the most important causes for the mid capitanian mass extinction however the mechanisms and subsequent sulfidic process for the mid capitanian mass extinction are still debated here we focus on a comprehensive study including marine productivity and redox conditions of the lower yangtze area across the mid capitanian mass extinction based on multiple geochemical indicators our results show that the redox conditions and marine productivity have gone through three stages the pre extinction early middle capitanian is characterized by anoxic conditions with intermittent euxinic episodes and moderate high productivity whereas the middle capitanian is dominated by stronger euxinic conditions and higher productivity subsequently an oxic shallow water environment and low productivity are recognized by significant decreases in all of these geochemical indicators across the mid capitanian the appearance and development of the anoxic euxinic conditions was probably caused by high productivity related to enhanced upwelling which led to the intensified heterotrophic respiration and subsequent oxygen depletion of the water column more sluggish oceanic circulation that resulted from the global warming associated with the eruption of the emeishan large igneous province and upward movement of the chemocline due to great regression also further exacerbated the euxinic conditions during the middle capitanian the development of these euxinic conditions may have played a major role in triggering the super anoxic ocean and is also coincident with the mid capitanian biotic crisis which suggests the possible causal link between them with comprehensive analysis of profiles across the globe sulfidic models showing that the sulfidic water first occurred in the deep water during the early middle capitanian and shoaled to shallow water in the middle late capitanian could be established despite the fact that the temporally and spatially heterogeneous sulfidic processes in the late guadalupian are still debated

chemical remanent magnetization due to deep burial diagenesis in oolitic hematite bearing ironstones of alabama abstract oolitic hematite bearing ironstones of the silurian red mountain formation of alabama are shown to carry a single component remanence stable enough to have survived major folding of probable permian age nevertheless the remanence direction ten sites yielding a paleopole at 38 0 n 132 4 e with dm 3 6 dp 1 9 its reverse polarity and a negative intraformational conglomerate test show that the remanence was very likely acquired during the pennsylvanian some 130 ma after deposition this remanence is likely a chemical remanent magnetization crm acquired during diagenesis induced by heating due to deep burial under a pennsylvanian clastic wedge two possible mechanisms for acquisition of crm during deep burial diagenesis are considered in hypothesis i the oolitic hematite transformed from original geothite when heated to about 80 c acquiring crm in hypothesis ii the oolitic hematite originated from ferrihydrite and was too fine grained to acquire stable crm until heat raised the solubility of hematite allowing grain growth hypothesis i explains the timing of remanence acquisition better but there is some evidence that oolitic goethites may be stable to considerably more than 80 c hypothesis ii has some difficulty explaining preliminary paleomagnetic results from oolitic hematite bearing ironstones of the silurian clinton group new york state we prefer hypothesis i but both hypotheses remain plausible both hypotheses warn that continental red beds may also acquire crm during diagenesis induced by deep burial heating long after deposition but before folding

the guadalupian lopingian boundary permian in a pelagic sequence from panthalassa recognized by integrated conodont and radiolarian biostratigraphy abstract guadalupian lopingian sedimentary rocks are widely distributed in accretionary complexes in japan but the guadalupian lopingian boundary g lb is not well documented from these pelagic sediments to identify the g lb and to better correlate an extinction event that occurred around the guadalupian lopingian boundary we examined the conodont biostratigraphy from a permian pelagic chert sequence in the gujo hachiman section gifu southwest japan age diagnostic conodonts including clarkina postbitteri postbitteri were found in this section the biostratigraphic occurrences of these age diagnostic conodonts can pinpoint the g l transitional zone in the gujo hachiman section by comparison with well studied sections from south china including the gssp section the transitional zone was recognized by the first occurrence horizons of both clarkina postbitteri hongshuiensis and c p postbitteri the g lb has been placed at or above the first occurrence horizon of the radiolarians albaillella yamakitai or albaillella cavitata in previous studies from china and japan we detected the first occurrence horizon of a yamakitai below the base of the g l transitional zone in the upper capitanian the conodont biostratigraphy is consistent with the radiolarian biostratigraphy in this section which can be correlated to relevant sections in china

the mid capitanian middle permian mass extinction and carbon isotope record of south china abstract conodont c isotope and fossil and facies data are presented for the capitanian middle permian mass extinction record seen in platform carbonates maokou and wuchiaping formations of south china where limestones interdigitate with the volcanic succession of the emeishan large igneous province the maokou formation provides an extinction record marked by the loss of keriothecal walled fusulinaceans and a turnover in calcareous algae in sections within the emeishan province this crisis occurs at the base of the oldest record of volcanism from the jinogondolella altudaensis conodont zone of mid capitanian age around the periphery of the emeishan province this extinction level lies within the upper part of the maokou formation at a level where platform carbonate deposition was frequently interrupted by thick volcanic ash depositional events the assemblages of the uppermost maokou formation are characterised by typical late permian taxa although these levels still lie within the middle permian guadalupian series disaster taxa such as earlandia and diplosphaerina are locally prolifically abundant in the aftermath of the mass extinction the crisis is particularly noteworthy amongst photosynthetic taxa such as calcareous algae and fusulinaceans that probably harboured photosymbionts therefore a kill mechanism involving cooling from explosive volcanism and potentially acid rain from sulphate aerosols appears appropriate a composite 13ccarb curve calibrated against a high resolution conodont biostratigraphy reveals a major intra capitanian negative excursion of 5 superimposed on typically heavy 4 5 middle permian values this curve can also be recognised in panthalassan seamount carbonates of japan although this requires reassignment of apparently upper permian limestones to the middle permian in both china and japan the capitanian mass extinction occurs during the early stage of this major intra capitanian negative excursion assuming typical platform carbonate accumulation rates the records of the maokou formation suggest 13ccarb values fell at 0 01 kyr suggesting a catastrophic origin such as gas hydrate destabilisation is unlikely although a volcanic source is possible

neogene clockwise tectonic rotation of the eastern transverse ranges california suggested by paleomagnetic vectors results from a paleomagnetic study of neogene volcanic rocks situated within and around the pinto basin region of the eastern transverse ranges suggest that the ranges have undergone a clockwise tectonic rotation of 41 4 7 7 since late miocene time both normal and reversed polarities are observed and inclination data indicate no significant flattening the rotation was accompanied by progressive sinistral slip on the several east west trending faults which bound and transect the eastern transverse ranges a computation using the observed amount of cumulative left slip on the east west trending faults of the eastern transverse ranges 50 km and the spacing between the pinto mountain and salton creek faults which bound the region 70 km indicates that 35 of clockwise rotation is required which roughly agrees with the paleomagnetic data removal of the eastern transverse ranges rotation in a reconstruction shows that the distributed dextral shear across the mojave desert region to the north must have been on the order of 100 km since late miocene time from the results of this study we infer that crustal rotation of the eastern transverse ranges began later and is of less magnitude than that of the western transverse ranges

relationship between median destructive field and remanent coercive forces for dispersed natural magnetite titanomagnetite and hematite summary remanent acquisition curves remanent hysteresis curves and alternating field demagnetization curves were determined for a number of artificial rock specimens containing well defined grain size fractions between 5 and 250 m of natural magnetite titanomagnetite and hematite from these curves the remanent acquisition coercive force h cr the remanent coercive force hcr and the median destructive field of irm h i were determined theoretically these parameters should be the same for an assembly of non interacting homogeneously distributed randomly oriented single domain grains for a given hematite specimen h cr hcr and h i have about the same value in spite of the strong grain size dependence of these parameters for each specimen of magnetite and titanomagnetite the value of h cr is larger than hcr which again is larger than h i however the ratios h cr hcr and h i hcr appear to have a different constant value an interesting relationship which appears to hold for dispersed magnetite titanomagnetite or hematite grains between 5 and 250 m independently of grain size quantity and packing density of the magnetic material is

are late permian carbon isotope excursions of local or of global significance we present a new biostratigraphically calibrated organic and inorganic c isotope record spanning the basal late permian to earliest triassic from southern guizhou nanpanjiang basin south china after fluctuations of a likely diagenetic overprint are removed three negative carbon isotope excursions cies persist these include a short lived cie during the early wuchiapingian a protracted cie ending shortly after the wuchiapingian changhsingian boundary and a third cie straddling the permian triassic boundary comparison of our new c isotope record with others from the same basin suggests that influences of local bathymetry and of the amount of buried terrestrial organic matter are of importance comparison with other coeval time series outside of south china also highlights that only the negative cie at the permian triassic boundary is a global signal these differences can be explained by the different volumes of erupted basalts between the late permian emeishan and the younger siberian large igneous provinces and their distinct eruptive modalities emeishan volcanism was largely submarine implying that sea water was an efficient buffer against atmospheric propagation of volatiles the equatorial position of emeishan was also an additional obstacle for volatiles to reach the stratosphere and benefit from an efficient global distribution consequently the local significance of these cies calls into question global correlations based on c isotope chemostratigraphy during the late permian the timing of the late permian chinese cies is also not reflected in changes in species diversity or ecology unlike the sudden and global permian triassic boundary crisis and subsequent early triassic upheavals

nitrogen isotope chemostratigraphy across the permian triassic boundary at chaotian sichuan south china abstract nitrogen isotopic compositions of upper permian to lowermost triassic rocks were analyzed at chaotian in northern sichuan south china in order to clarify changes in the oceanic nitrogen cycle around the permian triassic boundary p tb including the entire changhsingian late late permian prior to the extinction the analyzed ca 40 m thick interval across the p tb at chaotian consists of three stratigraphic units the upper wujiaping formation the dalong formation and the lowermost feixianguan formation in ascending order the upper wujiaping formation ca 10 m thick is mainly composed of dark gray limestone with diverse shallow marine fossils such as calcareous algae and brachiopods deposited on the shallow shelf in contrast the overlying dalong formation ca 25 m thick is mainly composed of thinly bedded black mudstone and siliceous mudstone containing abundant radiolarians deposited on the relatively deep slope basin absence of bioturbation substantially high total organic carbon contents up to 15 and abundant occurrence of pyrite framboids in the main part of the dalong formation indicate deposition under anoxic condition the lowermost feixianguan formation ca 5 m thick is composed of thinly bedded gray marl and micritic limestone with minor fossils such as ammonoids and conodonts deposited on the relatively shallow slope 15ntn values are in positive values around 1 to 2 in the upper wujiaping formation implying denitrification and or anammox in the ocean 15ntn values gradually decrease to 1 in the lower dalong formation and are consistently low around 0 in the middle dalong to lowermost feixianguan formation no clear 15ntn shift is recognized across the extinction horizon the consistently low 15ntn values suggest the enhanced nitrogen fixation in the ocean during the changhsingian at chaotian composite profiles based on previous and the present studies demonstrate the substantial 15n variation on a global scale in the late permian to earliest triassic a systematic 15n difference by low and high latitudes is particularly clarified although the enhanced nitrogen fixation throughout the changhsingian at chaotian was likely a regional event in northwestern south china the composite 15n profiles imply that the sea area in which fixed nitrogen is depleted has gradually developed worldwide in the changhsingian possibly acting as a prolonged stress to shallow marine biota

middle permian organic carbon isotope stratigraphy and the origin of the kamura event abstract large carbon cycle perturbations associated with the middle permian capitanian mass extinction have been widely reported but their causes and timing are still in dispute low resolution carbon isotope records prior to this event also limit the construction of a middle permian chemostratigraphic framework and global or local stratigraphic correlation and hence limit our understanding of carbon cycle and environmental changes to investigate these issues we analyzed the 13corg values from the middle permian chert mudstone sequence gufeng formation in the lower yangtze deep water basin south china and compared them with published records to build a chemostratigraphic scheme and discuss the underlying environmental events the records show increased 13corg values from late kungurian to early guadalupian followed by a decrease to the late wordian early capitanian the early mid capitanian was characterized by elevated 13corg values suggesting the presence of the kamura event an interval of heavy positive values seen in the 13ccarb record we propose that these heavy capitanian 13c values may be a response to a marked decline in chemical weathering rates on pangea and associated reduction in carbonate burial which we show using a biogeochemical model the subsequent negative 13c excursion seen in some carbonate records especially in shallower water sections and in a muted expression in organic carbon coincide with the capitanian mass extinction may be caused by the input of isotopically light carbon sourced from the terrestrial decomposition of organic matter

middle upper permian carbon isotope stratigraphy at chaotian south china pre extinction multiple upwelling of oxygen depleted water onto continental shelf abstract in order to examine the causal relationships between the carbon cycle in a shallow euphotic zone and the environmental changes in a relatively deep disphotic zone at the end guadalupian middle permian isotopic compositions of carbonate carbon 13ccarb of the guadalupian lopingian upper permian rocks were analyzed in the chaotian section in northern sichuan south china by analyzing exceptionally fresh drill core samples a continuous chemostratigraphic record was newly obtained the ca 65 m thick analyzed carbonate rocks at chaotian comprise three stratigraphic units i e the limestone unit of the guadalupian maokou formation the mudstone unit of the maokou formation and the lower part of the wuchiapingian lower lopingian wujiaping formation in ascending order the limestone unit of the maokou formation is characterized by almost constant 13ccarb values of ca 4 followed by an abrupt drop for 7 to 3 in the topmost part of the unit in the mudstone unit of the maokou formation the 13ccarb values are rather constant around 2 although distinct three isotopic negative excursions for 3 from ca 2 to 1 occurred in the upper part of the unit in the lower part of the wujiaping formation the 13ccarb values monotonously increase for 5 from ca 0 to 5 the present data newly demonstrated four isotopic negative excursions in the topmost part of the maokou formation in the capitanian late guadalupian at chaotian it is noteworthy that these negative excursions are in accordance with the emergence of an oxygen depleted condition on the relatively deep disphotic slope basin on the basis of litho and bio facies characteristics they suggest multiple upwelling of oxygen depleted waters with dissolved inorganic carbon of relatively low carbon isotope values along the continental margin from the deeper disphotic slope basin to the shallower euphotic shelf slightly before the end guadalupian extinction although the negative excursions at chaotian are apparently correlated with the previously proposed large negative excursion in the middle capitanian in south china the age difference according to the biostratigraphic constraints clearly exclude this interpretation the isotopic negative excursions at chaotian are unique and no similar isotopic signal in the same period has been reported elsewhere the multiple upwelling of oxygen depleted waters onto the euphotic shelf may have represented local phenomena that occurred solely around northwestern south china

a chlorite solid solution geothermometer the los azufres mexico geothermal system chlorite constitutes a major hydrothermal alteration product of metamorphism of andesites in the active geothermal system of los azufres mexico electron microprobe analyses performed on a set of crystals from each sample show wide variations in composition correlation coefficients among chemical constituents were calculated it is shown that the tetrahedral charge is positively correlated with the octahedral vacancy and negatively with the iron content and there is almost no correlation with the octahedral aluminium and magnesium content a procedure is proposed to select end members and substitution vectors and to give a general formula for these chlorites

a chronology of paleozoic sea level changes sea levels have been determined for most of the paleozoic era 542 to 251 million years ago but an integrated history of sea levels has remained unrealized we reconstructed a history of sea level fluctuations for the entire paleozoic by using stratigraphic sections from pericratonic and cratonic basins evaluation of the timing and amplitude of individual sea level events reveals that the magnitude of change is the most problematic to estimate accurately the long term sea level shows a gradual rise through the cambrian reaching a zenith in the late ordovician then a short lived but prominent withdrawal in response to hirnantian glaciation subsequent but decreasingly substantial eustatic highs occurred in the mid silurian near the middle late devonian boundary and in the latest carboniferous eustatic lows are recorded in the early devonian near the mississippian pennsylvanian boundary and in the late permian one hundred and seventy two eustatic events are documented for the paleozoic varying in magnitude from a few tens of meters to 125 meters

regional significance of kilometric scale north east vergent recumbent folds associated with east to south east directed shear on the southern border of the central iberian zone hornachos oliva region variscan belt iberian peninsula on the southern border of the central iberian zone there are two sectors with different styles of deformation to the south west in the hornachos sector large scale recumbent folds associated with ductile shearing can be seen this shearing is characterized by a direction of movement parallel to the fold axes and can be correlated for 150 km along strike the k values of the strain ellipsoid range from 0 8 to 2 0 stretching in the x direction parallel to the recumbent fold axes is more than 100 to the north east in the oliva sector first phase folds are upright and the strain intensity is lower than in the hornachos sector metamorphic geometric and kinematic considerations lead us to conclude that the shearing in the hornachos sector is better explained as conjugate to a main shear zone along which the southern border of the central iberian zone is moved onto the ossa morena zone this main thrust is at present obliterated by a left lateral extensional shear zone that affects a high pressure exotic unit located between the central iberian and the ossa morena zones this high pressure unit constitutes a suture of the variscan belt in the iberian peninsula

multiple sulfur isotope records at the end guadalupian permian at chaotian china implications for a role of bioturbation in the phanerozoic sulfur cycle abstract a recent study on quadruple sulfur isotopes 32s 33s 34s and 36s of sedimentary pyrite suggested that the end guadalupian extinction was caused by shoaling of the sulfidic deep water this scenario is based on the assumption that sulfur isotopic compositions of pyrite from hosting sediments were controlled by benthos activities thus by the redox conditions of the sedimentary environments nonetheless the relationship between the sulfur isotope records and redox conditions reconstructed from litho and bio facies are poorly known in order to examine the effect of bioturbation in sediments quadruple sulfur isotopic compositions of sedimentary pyrite from the end guadalupian succession in chaotian south china were analyzed black mudstones of deep water facies immediately below the extinction horizon have consistently high 33s values of ca 0 079 clearly suggesting a sulfate reduction in the anoxic water column our new data are consistent with the emergence of a sulfidic deep water mass prior to the end guadalupian extinction the upwelling of the toxic deep water may have contributed to the extinction in contrast shallow marine bioclastic limestones with burrows deposited under oxic conditions have negative 33s values this anomalous isotopic signal indicates the mixing of two distinct types of pyrite one generated during the sulfate reduction in an open system and the other in a closed system we interpret that bioturbation supplied sulfate in the sediments and promoted sulfate reduction and in situ sulfide precipitation within the sediments the negative 33s values of oxic sediments in chaotian are inconsistent with the previous model and demonstrate that the sedimentary sulfur cycle associated with bioturbation was more complicated than previously thought our study also implies that more generally the role of bioturbation in increasing seawater sulfate concentration in the phanerozoic may have been overestimated in the previous studies because bioturbation may have enhanced sulfide burial or sulfur output from the oceans

the red bed controversy revisited shape analysis of colorado plateau units suggests long magnetization times several triassic and earliest jurassic sedimentary units from the colorado plateau region have distributions of virtual geomagnetic poles vgps that are highly elongate along the path of apparent polar wander apw this suggests that the remanent magnetizations measured in these units were acquired over an extended period of time possibly approaching 35 m y and are not precisely coeval with the stratigraphic age of the rock comparison with other paleomagnetic studies shows that the observed elongation is not a general attribute of the age of the rock nor is it related to paleolatitude the rocks that yield elongate vgp distributions are dominantly red to brown mudstones and it is possible that their remanence is dominated by a slowly acquired chemical remanent magnetization as suggested by larson et al j geophys res 87 1982 1081 and other authors however several superficially similar units from the colorado plateau have nearly circular vgp distributions the process by which remanence is acquired in clastic sedimentary rocks merits further study d 2002 elsevier science b v all rights reserved

end permian extinction and volcanism induced environmental stress the permian triassic boundary interval of lower slope facies at chaotian south china abstract in order to reveal environmental changes across the permo triassic boundary ptb the detailed lithostratigraphy of the ptb interval is analyzed at chaotian in northern sichuan china the studied section is composed of the changhsingian upper permian dalong formation and the induan lower triassic feixianguan formation of a lower slope facies deposited on the northwestern margin of the yangtze carbonate platform the 12 m thick interval across the ptb consists mainly of bedded carbonates and mudstone and is lithologically divided into 7 units i e units a to g in ascending order the main extinction horizon of permian taxa is recognized at the unit d e boundary where various fossil metazoans and protists such as ammonoids brachiopods bivalves conodonts and radiolarians rapidly disappeared or became scarce the complete disappearance of radiolarians at the unit d e boundary emphasizes that the ptb extinction affected not only various late permian benthic and free swimming metazoans but also planktonic protozoans the lowest induan index conodont hindeodus parvus first occurs at the base of unit f marking the biostratigraphically defined ptb horizon unit e composed of unique bedded marl between the main extinction horizon and the first occurrence of triassic taxon represents a period of strong environmental stresses that suppressed productivity both of silica and carbonate secreting organisms by changing their size radiolarians reacted most sensitively to the environmental change that already started in the late changhsingian appreciably before the final extinction event the frequent intercalation of rhyo dacitic tuff beds particularly in unit d and the lower part of unit e across the main extinction horizon suggests that intermittent felsic volcanism and relevant environmental change may have been responsible for the mass extinction of the permian taxa and for the prolonged post extinction lag time before the initial recovery the frequent ash falls during the late changhsingian indicate that the volcanism induced environmental change had already started earlier than the main extinction all the biological production carbonate silica and organic matter collapsed at the unit d e boundary when the environmental stresses may have passed a critical threshold for maintaining ecological stability the ptb interval between the extinction and the first appearance of triassic taxon at chaotian is ca 1 4 m thick apparently almost eight times thicker than that at the global stratotype section and point of ptb in meishan 19 cm the chaotian section as well as the neighboring shangsi section in northern sichuan may provide a better chance for high resolution chemostratigraphic analyses that may allow detection and correlation of subtle environmental changes across the ptb

a positive c isotope excursion induced by sea level fall in the middle capitanian of south china abstract a new carbon isotope excursion was recovered from the capitanian marine carbonates at the rencunping rcp section of south china significantly a pronounced excursion with elevated 13ccarb values over 5 was coeval with the conodont jinogondolella prexuanhanensis zone and resembles the kamura event recorded in tethys the excursions in 13ccarb constrained by conodont biostratigraphy however present inconsistent carbon cycle behaviors especially between the separated basins in south china and evidently reflect regional litho facies controls in addition a transitional environment in association with a fall in sea level was recovered from deposits of anoxic cherty carbonates subsequent to shallow water carbonates around the positive excursion in 13ccarb accordingly instead of this being a signal of global scale climatic cooling we suggest that the positive excursion in 13ccarb can be attributed to eutrophication effects regionally along a continental shelf in this scenario the increasing dissolved o2 level in the mixing zone that induced by the initial sea level fall will efficiently impede denitrification and increase the bio available n in n p ratio to satisfy the demands of primary producers in surface waters subsequently deposits of shallow water carbonates comprising calcareous algae and massively bedded lime muds accumulated widely around south china these deposits appear to represent an unusual environment and ecosystem fertilized perhaps by the weathering products from the earliest stages of volcanism prior to the main emeishan flood basalt eruptions at the guadalupian lopingian boundary glb

an early triassic smithian stromatolite associated with giant ooid banks from lichuan hubei province south china environment and controls on its formation this study is partly supported by the 111 program of china b80210 two research grants from the state key laboratory of biogeology and environmental geology bgeg gbl11206 and state key laboratory of geological processes and mineral resources gpmr201302 china university of geosciences and two nsfc grants 41272023 41572091

the structure of a major suture zone in the sw iberian massif the ossa morena central iberian contact abstract we have investigated the stratigraphy structure and metamorphism of the boundary between the ossa morena zone omz and the central iberian zone ciz two significant continental portions of the variscan iberian massif the omz ciz contact is marked by a strongly deformed and metamorphosed nw se trending narrow band namely the central unit in which partially retrogressed eclogites are included during the middle late devonian the ciz overthrust the omz and in the footwall km scale recumbent folds and thrusts developed with decoupling and underplating of the lower crust at the same time in the hanging wall there took place intense though localized back folding and back shearing in the early carboniferous a transtensional tectonic regime sank the overthrust block resulting in the exhumation of eclogites these eclogites probably came from the underthrust omz lower crust and they are at present included in the suture zone central unit of this continental collision the extension is responsible for the origin of a basin and bimodal magmatism on the southern border of the ciz a late episode of folding and fracturing significantly contributed to the final complex picture of this suture boundary

the composition of phyllosilicates in precambrian low grademetamorphic clastic rocks from the southern hesperian massif spain used as an indicator to metamorphic conditions abstract in order to obtain an indication of metamorphic conditions we have studied the mineralogy of the clastic rocks in particular the phyllosilicates from three precambrian series in the central iberian and ossa morena zones of the southern hesperian massif the esquisto grauvaquico complex the so called serie negra and the azuaga formation for this purpose we have used the crystal chemical parameters obtained via x ray diffraction d 001 b 0 ratios of the basal intensity of the white micas and chlorites and the crystallinity index of the mica together with electron microprobe data from selected areas obtained via backscattered electron images the mineral assemblages commonly quartz muscovite albite chlorite biotite and the values obtained for the illite crystallinity index between 0 18 and 0 20 2 in rocks with non diagnostic mineral assemblages point to at least greenschist facies metamorphic conditons the phengite content of the mica indicates an overall pressure regime near the limits between low and intermediate pressure but some significant differences exist between the series studied the azuaga formation shows the lowest pressure about 4 kbar while the serie negra indicates a pressure of at least 1 kbar more in the esquisto grauvaquico complex when the samples are considered overall the results are similar to those of the serie negra but there is inhomogeneity in the rocks from the two sections studied the various types of chlorite both textural and optical are homogeneous in composition in each rock specimen and their chemical composition depends primarily on the composition of the host rock

constraining paleo latitude of a biogeographic boundary in mid panthalassa fusuline province shift on the late guadalupian permian migrating seamount abstract the guadalupian paleo atoll limestone iwato formation in sw japan was primarily formed in low latitude mid panthalassa and was later tectonically accreted to south china japan margin during the jurassic the present biostratigraphic study clarified that the iwato formation consists of 5 biostratigraphical intervals i e four fusuline assemblage zones assemblage zones 1 to 4 and a barren interval on the top assemblage zones 1 to 4 correspond to the neoschwagerina craticulifera zone n margaritae zone yabeina globosa zone and lepidolina multiseptata zone of the conventional tethyan fusuline stratigraphy respectively the present study newly clarified the following significant aspects of paleobiogeography of the permian fusulines as to the extinction of large tested taxa in the latest guadalupian 1 the long unknown stratigraphic relationship was documented for the first time between the yabeina dominant interval and the overlying lepidolina dominant one within a single limestone unit 2 the occurrence of lepidolina cf kumaensis kanmera the unique last runner of large tested fusuine was detected for the first time in mid oceanic paleo atoll limestones 3 with respect to the northbound migration history of the paleo seamount capped by the iwato formation the development of the two coeval fusuline biogeographic territories in the low latitude panthalassa i e the yabeina territory on the south and the lepidolina territory on the north was confirmed 4 the paleo latitude of the biogeographic boundary between the yabeina and lepidolina territories is constrained around 12 in the southern hemisphere on the basis of the latest geomagnetic data from the same limestone this new approach utilizing biostratigraphy on ancient migrating seamounts coupled with geomagnetic paleo latitude data is applicable to other cases in different time space co ordinates and of other fossil groups for constraining position of ancient biogeographic boundaries within lost oceanic domains of deep past

restoration of reef ecosystems following the guadalupian lopingian boundary mass extinction evidence from the laibin area south china abstract the guadalupian lopingian boundary glb also middle late permian boundary mass extinction severely destroyed metazoan reef ecosystems although some studies argued that both biotic and environmental turnover across the glb are not so obvious when compared with prolifically developed reefs in the capitanian the wuchiapingian reef examples appear depauperate and almost 89 of the carbonate production in these bioconstructions was lost here we report a typical sponge reef from the wuchiapingian stage in the tieqiao section central guangxi province south china the tieqiao reef might represent the only example of a wuchiapingian metazoan reef in the eastern palaeo tethys region it provides insight into ecosystem restoration following the glb extinction major constructors of the tieqiao reef are sponges peronidella parauvanella sollasia tabulozoa and amblysiphonella algae anthracoporella archaeolithporella permocalculus gymnocodium and tubiphytes this reef is well constrained as middle late wuchiapingian in age by the clarkina orientalis conodont zone carbonate carbon isotope excursions experienced negative spikes near the glb and multiple perturbations in the early middle wuchiapingian and remained relatively stable during re establishment of metazoan reefs in middle late wuchiapingian conodont oxygen isotopes showed that the sea surface temperature sst was extremely high 30 c during late capitanian time punctuated by a short cooling event and global regression associated with the glb extinction and followed by high sst and rapid rise in sea level in the early wuchiapingian the reemergence of the tieqiao reef coincided with the initial cooling in surface oceans and sea level fall during the middle late wuchiapingian accordingly reef ecosystems experienced a long term depletion worldwide in early middle wuchiapingian time and then recovered 2 5 myr after the glb extinction based on estimate in integration of both conodont biostratigraphy and radiometric ages

episodic euxinia in the changhsingian late permian of south china evidence from framboidal pyrite and geochemical data abstract a multiproxy study of a new upper permian lower triassic section xiaojiaba in sichuan province china documents large changes in marine productivity redox conditions and detrital input prior to the latest permian mass extinction marine productivity as proxied by total organic carbon content toc biogenic sio 2 and excess barium displays a long term decline through most of the changhsingian stage late late permian culminating in very low values around the permian triassic boundary concurrently redox proxies including pyrite framboid 34 s py mo auth and u auth and c org p document a shift from suboxic to dysoxic oxic conditions that was interrupted by several episodes of benthic euxinia and detrital siliciclastic proxies al hf nb and rees suggest an increased flux of weathered material from land areas the long term changes in productivity redox conditions and terrigenous detrital fluxes were probably caused by a regional sea level fall across the south china craton on the other hand the brief euxinic episodes occurring during the late permian had oceanographic causes probably related to the transient upward expansion of the chemocline at the top of the oceanic oxygen minimum zone these euxinic episodes may have been harbingers of the more widespread anoxia that developed concurrently with the latest permian mass extinction and that may have played a major role in triggering the largest biotic crisis of the phanerozoic

fusuline biotic turnover across the guadalupian lopingian middle upper permian boundary in mid oceanic carbonate buildups biostratigraphy of accreted limestone in japan abstract two sections in upper middle to lower upper permian shallow water limestones at kamura and akasaka in southwest japan were analyzed for detailed lithostratigraphy and biostratigraphy both sections represent ancient seamount capping carbonate buildups developed on a basaltic basement in a mid oceanic environment the occurrence of abundant tethyan fusulines allows the recognition of well defined biostratigraphic zonation in both sections and their mutual correlation the upper guadalupian middle permian lepidolina yabeina zone is overlain conformably by the lower lopingian upper permian codonofusiella reichelina zone with a 13 m thick transitional interval barren of index taxa the guadalupian lopingian g l boundary is marked by the first appearance datum fad of the lopingian codonofusiella reichelina assemblage in both sections this study recognizes for the first time the g l boundary horizon in a mid oceanic shallow water environment in addition the shallow water carbonates in the study sections record the extinction of the middle permian large sized fusuline family verbeekinidae at the g l boundary in mid panthalassa as well as in shallow water tethyan shelf areas demonstrating positively that the g l boundary mass extinction occurred on a global scale the abrupt elimination of large shelled fusulines followed by the domination of small shelled fusulines may indicate that environmental stress occurred at the end of guadalupian the dying out of symbiotic algae may have caused the selective extinction of the large shelled fusulines

progress problems and prospects an overview of the guadalupian series of south china and north america abstract the guadalupian epoch is marked by the formation of the pangean supercontinent global sea level change rifting and drifting of the cimmerian continents formation of large igneous provinces and dramatic biotic changes a high resolution biostratigraphic chemostratigraphic and high precision geochronologic framework of this critical transition is fundamental to understanding these events extensive studies of the latest cisuralian and guadalupian series in both south china and north america reveal the same conodont lineages but the conodont interval zones based on jinogondolella within the guadalupian series are slightly diachronous high precision u pb geochronological studies ca id tims method calibrate the base of the guadalupian series base roadian at 273 01 0 14 ma a previously reported age from an ash bed overlying the emeishan flood basalts 259 51 0 21 ma is adopted for the guadalupian lopingian boundary glb based on recently published geochronology and bayesian age modeling from the guadalupian series the base of the capitanian is constrained at 264 28 0 16 ma and the base of the wordian is interpolated to be 266 9 0 4 ma the illawarra reversal is of early middle wordian age both north america and south china possess a distinct negative 13ccarb excursion of 3 5 at the latest kungurian and early roadian lk er cie which coincides with the early stages of a significant 3rd order sea level rise the large end guadalupian 13ccarb negative excursion may have been affected by post depositional diagenesis or a warming event associated with the emeishan volcanism the 87sr 86sr ratios in both regions declined from the latest kungurian to the late capitanian but have different ratios and reveal several fluctuations in the middle guadalupian measured 18oapatite values suggest that the delaware basin was 3 4 c cooler than the eastern yangtze block analysis of a new high resolution database of marine taxa indicates only a minor pre lopingian diversity drop from 261 04 ma to 259 98 ma which coincides with the peak emeishan volcanism the widely perceived end guadalupian mass extinction in north america is evidently masked by and possibly an artefact of a stratigraphic truncation effect due to rapid lithofacies changes from limestone to laminated evaporites with the closure of the west texas basins

a new geobarometer for the assemblage biotite muscovite chlorite quartz the equilibrium thermodynamics of the reaction 4kmga1si40 o oh mg5a1 si30 o oh muscovlte chlorite muscovite biotite ka13si 0 oh 3kmg3aisi 0 0 oh 7si0 4h o quartz fluid can be formulated as a geobarometer thermo dynamic data for the mg a1 celadonite end member of muscovite is derived from the reversed mineral equilibria experiments of velde i 965 using the calculation method of powell 1978 giving afh 1298 5823 5 kj s 1298 0 3060kjk v 1298 13 8kjkbar

permo triassic boundary superanoxia and stratified superocean records from lost deep sea pelagic cherts of japan and british columbia canada recorded a long term and worldwide deep sea anoxic oxygen depleted event across the permo triassic or paleozoic and mesozoic boundary 251 2 million years ago the symmetry in lithostratigraphy and redox condition of the boundary sections suggest that the superocean panthalassa became totally stratified for nearly 20 million years across the boundary the timing of onset climax and termination of the oceanic stratification correspond to global biotic events including the end guadalupian decline the end permian extinction and mid triassic recovery

permian triassic extinction pattern revealed by foraminifers and geochemical records in the central persian gulf southern iran abstract permian triassic boundary ptb foraminifers are studied from the upper part of the dalan formation and the base of the kangan formation in four wells a b e f on and near the qatar arc persian gulf iran isotope studies including 13c and 18o values from wells b and e and 87sr 86sr ratios in wells b e and f have also been carried out to possibly provide a high resolution extinction pattern of foraminifers in the ptb interval the latter has been thoroughly investigated in iran turkey south china vietnam slovenia carnic alps and bukk mountains kashmir greenland and serbia the distribution of the last appearance of foraminiferal genera from the ptb interval in all of the wells is very similar featuring a stepwise extinction from 3 to 2 m before the boundary and compares to that observed in some of the sections from elsewhere i e south china vietnam slovenia and serbia the 13c value in wells b and e shows a negative shift at the base of a thrombolite unit immediately after the considered ptb and another negative shift at about 11 m below it in well e the thrombolite unit is assumed as earliest triassic but questionable with no supportive conodonts the 18o isotope values in wells b and e demonstrate the same negative upward trend due to dysoxic or anoxic conditions in the end permian oceans the uniform reduction without any sharp incline in the 18o profile suggests concurrent global warming the highest 87sr 86sr ratio fluctuations in wells b e and f coincide with the appearance of thrombolites changes in the profiles of carbon oxygen and strontium isotopes close to the ptb in wells b e and f seem to correspond to lithofacies variations from limestone to dolostone and a reduction in foraminiferal diversity these changes substantiate a clear perturbation in the marine environmental and global geochemistry elements through the ptb interval the 13c 18o and 87sr 86sr values obtained from the studied wells are comparable to those in other ptb stratigraphic sections in the paleo tethyan region

wasp waisted hysteresis loops mineral magnetic characteristics and discrimination of components in mixed magnetic systems rock magnetic studies of complex systems that contain mixtures of magnetic minerals or mixed grain size distributions have demonstrated the need for a better method of distinguishing between different magnetic components in geological materials hysteresis loops that are constricted in the middle section but are wider above and below the middle section are commonly observed in mixed magnetic assemblages such wasp waisted hysteresis loops have been widely documented particularly with respect to rare earth permanent magnets basaltic lava flows remagnetized paleozoic carbonate rocks and an increasingly wide range of other rocks our modelling combined with a review of previous work indicates that there are several conditions that give rise to as well as magnetic properties that are characteristic of wasp waisted hysteresis loops first at least two magnetic components with strongly contrasting coercivities must coexist this condition can arise from either mixtures of grain sizes of a single magnetic mineral or a combination of magnetic minerals with contrasting cocrcivities or a combination of these two situations second materials that give rise to wasp waisted hysteresis loops will have relatively high ratios of the coercivity of remanence to coercive force b cr b c because b0 is controlled by the soft low coercivity component whereas bcris controlled by the hard high coercivity component third values of b cr b c 10 usually only occur for strongly wasp waisted loops when the low coercivity component comprises an overwhelmingly large fraction of the total volume of magnetic grains fourth a given mixture of superparamagnetic and single domain sd grains is more likely to give rise to wasp waisted hysteresis loops than an equivalent mixture of sd and multidomain grains fifth our results provide empirical confirmation that the total magnetization of a material is the sum of the weighted contributions of each component in the absence of significant magnetic interaction between particles thus to contribute significantly to wasp waisted behavior a mineral magnetic component must give rise to a significant portion of the total magnetization of the rock as a result minerals with weak magnetic moments such as hematite need to occur in large concentrations to cause wasp waistedness in materials that also contain ferrimagnetic minerals we outline a method for determining the magnetic components that can give rise to wasp waisted hysteresis loops this method is based on high and low temperature magnetic measurements that are used to identify the dominant remanence bearing mineral s and on mineral magnetic techniques that are used to discriminate between different magnetic domain states the method is illustrated with several examples from archaeological geological and synthetic materials

interpreting carbon isotope excursions carbonates and organic matter abstract variations in the carbon isotopic compositions of marine carbonate and organic carbon provide a record of changes in the fraction of organic carbon buried through time and may provide clues to changes in rates of weathering and sources of organic carbon paired carbonate and organic carbon isotope determinations provide a possibility of interpreting not only changes in the global carbon cycle through time but changes in atmospheric pco2 as well interpretations of these types of data are typically rather qualitative a quantitative basis is required to develop a better understanding of changes in the carbon cycle for this purpose we employ a simple model of the global carbon cycle which is subjected to a number of different perturbations each lasting 500 ky i e much longer than the residence times of carbon and phosphorus in the ocean atmosphere system in addition to standard considerations of carbon mass and isotopic fluxes to the ocean atmosphere system from weathering and volcanism and fluxes of organic carbon and carbonate carbon to sediments the model incorporates sensitivity of the photosynthetic carbon isotope effect to changes in pco2 the inclusion of this parameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial a series of simple to more complex simulations illustrates the significant effects of varying differences between the carbon isotopic composition of sedimented carbonate and organic carbon b with constant b a 50 increase in organic carbon burial produces a parallel increase in carbonate and organic carbon isotopic compositions however the same simulation with b responsive to pco2 changes produces an initial parallel 13 c increase but this is followed by an even greater 13 c enrichment in organic carbon because pco2 falls in response to increased organic carbon burial the counterintuitive overall result of the enhanced organic carbon burial event is that the carbonate carbon isotopic composition actually decreases because of the more substantial increase in 13 c org in addition we illustrate the effects on carbon isotopic compositions of the oceanic inorganic carbon reservoir and buried organic matter of a 50 increase in volcanic co2 outgassing a 50 increase in weathering rate with coupled phosphate and riverine carbon flux responses a 50 decrease in shale associated organic carbon weathering a 50 decrease in silicate weathering rate and the possible effects of the rise in abundance of c4 plants in the late miocene to recent we compare the model simulated carbon isotopic responses for some of these experiments to paired carbonate and organic carbon records to illustrate how these records might be interpreted in light of the model response

rotation of the southernmost sierra nevada california structural paleogeographic and magmatic trends of the sierra nevada batholith swing from a characteristic nnw trend in the northern and central parts of the batholith to a nearly e w trend in the tehachapi and san emigdio mountains paleomagnetic results from eight sites in the upper cretaceous 80 86 m y bear valley springs pluton in the tehachapi mountains yield a mean direction d 022 i 56 k 97 95 5 6 this mean direction is significantly different from the expected late cretaceous field direction for this locality and suggests that the pluton has undergone a clockwise rotation of 45 14 in accord with the change in structural trend in that area lower to middle miocene volcanic and sedimentary rocks of the kinnick formation overlie the plutonic rocks and yield a mean direction d 354 i 64 a paleomagnetic pole derived from this mean direction is not significantly different from the expected miocene pole for north america this constrains the time of rotation of the pluton to be between about 80 and 20 m y probably predating establishment of the present san andreas transform in this area we suggest that the observed rotation is part of a large oroclinal bend affecting to different degrees the entire southern sierra nevada the driving mechanism for such a bending may be right lateral shear along a proto san andreas transform for which there is now a significant body of evidence