stable organic carbon isotope stratigraphy across oceanic anoxic event 2 of demerara rise western tropical atlantic ocean drilling program odp leg 207 recovered expanded sections of organic carbon rich laminated shales on demerara rise western tropical atlantic high resolution organic carbon isotope and total organic carbon toc records are presented which span the cenomanian turonian boundary interval ctbi including the oceanic anoxic event oae 2 from four sites oriented along a nw striking depth transect these records represent the first high resolution carbon isotope records across oae 2 from the south american margin of the tropical atlantic due to the scarcity of age significant fossils the main purpose of this study was to develop a detailed carbon isotope stratigraphy in order to correlate the ctbi across the depth transect and to tie this to biostratigraphically well defined sections in the western interior basin pueblo usa boreal shelf seas eastbourne england and western tethys oued mellegue tunisia all four sections studied document a 6 increase of 13corg values at the base of the ctbi which is followed by an interval of elevated 13corg values and a subsequent decrease our results supply an important stratigraphic base for subsequent paleoceanographic studies on late cenomanian to early turonian sediments from demerara rise and elsewhere

mid cretaceous oceanic red beds in the umbria marche basin central italy constraints on paleoceanography and paleoclimate abstract detailed studies of the mid cretaceous sedimentary strata in the umbria marche basin in italy revealed that aside from several well known organic rich black shale horizons that record oaes several varicolored mainly reddish horizons indicate oxic conditions at the ocean bottom eight such horizons have been identified in aptian cenomanian sequences in the umbria marche basin the dysoxic oxic beds alternate regularly but not cyclically and seem to be the result of non random processes their occurrence is not a completely random process the duration of deposition of these oceanic red beds orbs varies from 0 13 my orb4 recorded in the ticinella primula zone to 4 54 my for orb1 which spans the globigerinelloides ferreolensis zone to the ticinella bejaouaensis zone in the piobbico core mid cretaceous orbs are not a local phenomenon because they occur in the tethyan deposits in the southern and austrian alps the carpathians the northern caucasus the central north atlantic in northeastern england and in the western himalayas they provide evidence for periodic changes in redox conditions at the ocean bottom such changes could have been caused by changes in bioproductivity basin geometry sedimentation rates paleocirculation and or production of bottom waters with higher content of dissolved oxygen in response to changes in paleoclimate we suggest that the periodic inflow of colder more oxygenated bottom waters was the probable cause of orbs development either as a result of changes in the ocean bottom topography or as a result of brief cool climate periods however reliable proxies for changes in deep ocean circulation are still lacking if orbs were the result of the paleoclimate and therefore indirectly caused by changes in co2 in atmosphere they document the increased sensitivity and instability of the mid cretaceous climate thus changes in ocean dynamics were on a scale of several hundred thousand years to several million years which has not been considered by most theories of co2 cycling mid cretaceous greenhouse paleoclimate and paleocean dynamics

a seismotectonic study of the burma and andaman arc regions using centroid moment tensor data abstract the concept of a mean slip angle is introduced this enables a classification of focal mechanisms in any region into predominantly strike slip thrust and normal categories based on this concept the harvard centroid moment tensor cmt data in the burma and andaman arc regions from 1977 to 1992 comprising 167 focal mechanism solutions are examined and categorized distinct trends on the surface and in depth sections emerge on examination of these categories for instance in the burmese arc there is a clear segregation along the slab between strike slip type mechanisms down to a depth of about 90 km and thrust events which occur exclusively below this depth in addition a study of p and t axis orientations indicates that the stress pattern in the subducted slab is different from that farther east whereas the p axes of the thrust and strike slip mechanisms in the slab show a predominantly nne trend commensurate with the direction of motion of the indian plate with respect to the eurasian plate those in the region to the east show interestingly an average e w oriented compressive regime the northern and southern parts of the andaman are region exhibit distinct tectonic patterns whereas in the southern part the disposition of focal mechanisms along the slab and the p and t axis orientations indicate active subduction the mechanisms in the northern part exhibit a peculiar segregation into clusters of thrust normal and strike slip types without conforming to the local trend of the arc a similar study in the western aleutian trench region indicates a much simpler subduction pattern

geochronology and calibration of global milankovitch cyclicity at the cenomanian turonian boundary new 40 ar 39 ar geochronology and global cyclostratigraphic calibration provide high resolution insights into the timing of geochemical fluctuations across oceanic anoxic event ii oae ii five new 40 ar 39 ar data from a single section in western canada suggest an age of 96 4 1 ma for the cenomanian turonian c t boundary this is 3 m y older than previously dated time series analysis e g wavelet transform reveals that oae ii was synchronously deposited worldwide over a period of 320 k y the preservation and stationarity of milankovitch cyclicity in sections across the atlantic indicates that no major fluctuations in sedimentation rate occur across the oae ii except for a minor short term decrease in the rate just before the c t boundary a 13 c org increase of 2 took place at the beginning of oae ii over a period of 110 k y and likely represents a period of worldwide excessive organic carbon burial during transgression leading to a 12 c depletion of seawater and sedimentation this interval was followed by a three step decrease to pre oae ii 13 c values

ductile brittle transition structures in the basal shear zone of a rhyolite lava flow eastern australia abstract structural and textural evidence indicates that the basal breccia of the tertiary minyon falls rhyolite of tweed volcano eastern australia formed mainly by fragmentation within the basal shear zone of the lava flow cooling at the flow base led to lava behaviour passing through the ductile brittle transition while subjected to progressive deformation the intact lavas of the basal zone record intense ductile shearing in multiple folds rotated phenocrysts strong alignment of crystallites and micro folding of these crystal alignments minor brittle structures including faults on fold limbs and microfaults occur in the intact lava immediately above the basal breccia these structures represent incipient brecciation and complete brecciation has occurred by intense faulting of previously intact lava brittle deformation migrated upward into overlying intact lava as cooling occurred this process is distinctly different from overriding and incorporation of surface breccias which is commonly invoked as the primary process of basal breccia formation

paleo co2 variation trends and the cretaceous greenhouse climate the cretaceous was one of the most remarkable periods in geological history with a greenhouse climate and several important geological events reconstructions of atmospheric co2 using proxies are crucial for understanding the cretaceous greenhouse in this paper we summarize the major approaches for reconstructing co2 based on paleobotanical or geochemical data and synthesize the co2 variations throughout the cretaceous the results show that atmospheric co2 levels remained relatively high throughout the cretaceous but were lower in the early cretaceous highest in the mid cretaceous and gradually declined during the late cretaceous however this overall trend was interrupted by several rapid changes associated with ocean anoxic events oaes and the end cretaceous catastrophic event new data on paleo co2 levels from paleobotanical and paleosol evidences support not only the overall trends indicated by geochemical models but provide more precise records of the short term fluctuations related to brief episodes of climate change temporal resolution within the long quiet magnetic period in the middle cretaceous is one of the obstacles preventing us from a more comprehensive understanding of the co2 climate linkage but new paleo co2 determinations and climatic data from stratigraphic sections of sediments intercalated with datable volcanic rocks will allow a better understanding of the relationships between fluctuations of atmospheric co2 climate change and geological events

enhanced productivity led to increased organic carbon burial in the euxinic north atlantic basin during the late cenomanian oceanic anoxic event 1 three cenomanian turonian c t 93 5 ma black shale sections along a northeast southwest transect in the southern part of the proto north atlantic ocean were correlated by stable carbon isotope stratigraphy using the characteristic excursion in 13c values of both bulk organic matter om and molecular fossils of algal chlorophyll and steroids all three sites show an increase in marine organic carbon oc accumulation rates during the c t oceanic anoxic event oae the occurrence of molecular fossils of anoxygenic photosynthetic green sulfur bacteria lack of bioturbation and high abundance of redox sensitive trace metals indicate sulfidic conditions periodically reaching up into the photic zone before as well as during the c t oae during the c t oae there was a significant rise of the chemocline as indicated by the increase in concentrations of molecular fossils of green sulfur bacteria and mo al ratios the presence of molecular fossils of the green strain of green sulfur bacteria indicates that euxinic conditions periodically even occurred at very shallow water depths of 15 m or less during the c t oae however bottom water conditions did not dramatically change as indicated by more or less constant v al and zn al ratios at site 367 this suggests that the increase in oc burial rates resulted from enhanced primary productivity rather than increased anoxia which is supported by stable carbon isotopic evidence and a large increase in ba al ratios during the c t oae the occurrence of the productivity event during a period of globally enhanced organic carbon burial rates i e the c t oae points to a common cause possibly related to the formation of a deep water connection between north and south atlantic basins

thermal erosion of felsic ground by the laminar flow of a basaltic lava with application to the cave basalt mount st helens washington 1 the thermal erosion of cold felsic ground by the steady laminar flow of a hot basaltic lava is examined theoretically and experimentally initially a chill layer is grown and then remelted at the base of the lava flow a steady thermal erosion velocity is then established which is limited by the buoyant instability of the melted ground or by the effective freezing temperature of the basaltic lava when the theoretical analysis is applied to the longest lava tube system of the cave basalt on mount st helens it is found that about 100 days of flow is sufficient to produce the observed ground erosion in little red river ape and lake caves

testing the cretaceous greenhouse hypothesis using glassy foraminiferal calcite from the core of the turonian tropics on demerara rise glassy turonian foraminifera preserved in clay rich sediments from the western tropical atlantic yield the warmest equivalent 18o sea surface temperatures ssts yet reported for the entire cretaceous cenozoic we estimate turonian ssts that were at least as warm as conservative mean 30 c to significantly warmer warm mean 33 c than those in the region today however if independent evidence for high middle cretaceous pco2 is reliable and resulted in greater isotopic fractionation between seawater and calcite because of lower sea surface ph our conservative and warm sst estimates would be even higher 32 and 36 c respectively our new tropical ssts help reconcile geologic data with the predictions of general circulation models that incorporate high cretaceous pco2 and lend support to the hypothesis of a cretaceous greenhouse our data also strengthen the case for a turonian age for the cretaceous thermal maximum and highlight a 20 40 m y mismatch between peak cretaceous cenozoic global warmth and peak inferred tectonic co2 production we infer that this mismatch is either an artifact of a hidden turonian pulse in global ocean crust cycling or real evidence of the influence of some other factor on atmospheric co2 and or ssts a hidden pulse in crust cycling would explain the timing of peak cretaceous cenozoic sea level also turonian but other factors are needed to explain high frequency 10 100 k y instability in middle cretaceous ssts reported elsewhere

water mass circulation on demerara rise during the late cretaceous based on nd isotopes abstract circulation in the north atlantic during the late cretaceous has implications for poleward heat transport and nutrient distribution during an extreme greenhouse interval with episodes of ocean anoxia nd isotopes of fossil fish teeth and debris represent one of the few water mass tracers that can be used to reconstruct deep ocean circulation we present nd isotopic data interpreted as bottom water values for 290 samples from three ocean drilling program sites on demerara rise sites 1258 1260 and 1261 along with 102 analyses from four other north atlantic sites cape verde goban spur bermuda rise and blake nose that provide additional geographic and bathymetric control our results confirm the presence of a water mass with low end values 14 to 17 that are believed to be influenced by continental material during local water mass formation at low latitudes this demerara bottom water dbw is the primary water mass in the region from the cenomanian to coniacian and from the late campanian through early maastrichtian following a hiatus of 10 my a positive 8 end unit excursion occurs during ocean anoxic event 2 oae2 that cannot be explained by changes in weathering inputs diagenesis or magmatic sources of nd instead it appears to represent an influx of bottom waters sourced from the tethys or north atlantic this replacement of dbw during oae2 argues for decreased production of dbw or enhanced production of tethys north atlantic waters during peak greenhouse conditions from the late campanian through early maastrichtian dbw becomes the only water mass recorded at demerara rise and it appears to expand to abyssal depths at cape verde in the campanian this water mass is ultimately replaced by waters that appear to be sourced from the north atlantic starting in the late maastrichtian observed variations in circulation in the tropical north atlantic during the late cretaceous can account for extensive deposition of black shales in the region prior to the hiatus as well as local warming during a global cooling event in the maastrichtian

generation of block and ash flows at the onset of silicic volcanism in the parana magmatic province brazil evidence from photoanalysis of caxias do sul breccias in this work we perform textural analysis on volcanic breccias related to the silicic magmatism in the region of sao marcos southern brazil parana magmatic province the breccias are matrix supported coarse grained poorly sorted with angular to rounded fragments in a fine grained matrix juvenile fragments are dense to pumiceous massive to banded dacites lithic fragments were not recognised the analyses were performed on field photographs grain size distribution gsd analysis showed maximum fragment sizes between 8 and 9 5 and smallest recognised sizes between 3 and 1 5 due to the spatial resolution limit of the photographs and the weathered condition of the matrix the fine fragments from the matrix were not analysed the gsd fractality could be quantified by a single power law with low moderate d values of 1 9 2 3 vesicle size distribution vsd and shape parameters of both fragments and vesicles were also measured the fragments are typically low vesicularity with median vesicle area fractions around 5 for all samples but pumiceous fragments with up to 43 area are found vsds exhibit single exponential concave upwards curves with vesicles becoming more deformed complex with increasing size suggesting a single continuous process of nucleation and growth under low degrees of shear regarding the fragments shape parameters all samples have high values of solidity which together with the low vesicularity of the fragments suggest brittle fragmentation of poorly vesicular material and the operation of secondary fragmentation processes during transport that round and smooth the clasts however field evidence for local conduit systems together with the gsd fractality suggest that these may be proximal deposits without substantial transport history based on a comparison of the deposits characteristics with several types of volcanic breccias we propose a block and ash flow source for these breccias that indicate the presence of nearby lava domes providing evidence for high viscosity low flux dacitic volcanism in the parana magmatic province

sedimentary chromium isotopic compositions across the cretaceous oae2 at demerara rise site 1258 abstract in order to advance our understanding of the emerging chromium cr isotope system as a paleoredox proxy we measured the 53cr 52cr of black shales deposited before during and after cretaceous oceanic anoxic event 2 oae2 we observed a 1 coherent negative 53cr 52cr excursion and significant drawdown in cr enrichments during oae2 coincident with a large positive carbon isotope excursion our observed negative 53cr 52cr excursion during oae2 is most easily linked to an increase in the ratio of euxinic to reducing conditions additional work on other oae sections is needed to determine the spatial significance of this cr isotope trend

the parana etendeka continental flood basalt province a historical perspective of current knowledge and future research trends abstract the development of ideas concerning continental flood basalt provinces is not new and many studies were completed on specific provinces before the advent of plate tectonics the parana etendeka province is not an exception and actually is an example of a province that has been thoroughly studied for 100 years in this work i present a brief summary of various aspects of this province from a rather general point of view including many references of difficult access to a reader not versed on the portuguese language key features include the presence of alkaline volcanism along the edges of the main basin before and after a markedly tholeiitic event the uneven spatial distribution of eruptive products relative to the location of continental rupture the apparent lack of a pattern of temporal activity across the whole province and the close relationship between the structure of the underlying sedimentary basin and the distribution of volcanic rocks by bringing together information relevant to all of those key features an evolutionary model emphasizing the role played by the changing local structure is outlined this model is an example of how key observations many of which were overlooked for 50 years provide the required impetus for the completion of future research that has the potential to substantially change the form in which this province has been visualized for at least the past 30 years

does large igneous province volcanism always perturb the mercury cycle comparing the records of oceanic anoxic event 2 and the end cretaceous to other mesozoic events mercury hg is increasingly being used as a sedimentary tracer of large igneous province lip volcanism and supports hypotheses of a coincidence between the formation of several lips and episodes of mass extinction and major environmental perturbation however numerous important questions remain to be answered before hg can be claimed as an unequivocal fingerprint of lip volcanism as well as an understanding of why some sedimentary records document clear hg enrichment signals whilst others do not of particular importance is evaluating the impact of different volcanic styles on the global mercury cycle as well as the role played by depositional processes in recording global hg cycle perturbations here new mercury records of cretaceous oceanic anoxic event 2 oae 2 94 ma and the latest cretaceous 67 66 0 ma are presented oae 2 is associated with the emplacement of multiple predominantly submarine lips the latest cretaceous with subaerial volcanism of the deccan traps both of these connections are strongly supported by previously published trends towards unradiogenic osmium os isotope values in globally distributed sedimentary records hg data from both events show considerable variation between different locations attributed to the effectiveness of different sediment types in registering the hg signal with lithologically homogeneous records documenting more clear hg enrichments than sections with major changes in lithology such as limestones to claystones or organic rich shales crucially there is no geographically consistent signal of sedimentary hg enrichment in stratigraphic records of either oae 2 or the latest cretaceous that matches os isotope evidence for lip emplacement indicating that volcanism did not cause a global hg perturbation throughout the entire eruptive history of the lips formed at those times it is suggested that the discrepancy between os isotope and hg trends in records of oae 2 is caused by the limited dispersal range of hg emitted from submarine volcanoes compared to the global scale distribution of os a similar lack of correlation between these two proxies in uppermost cretaceous strata indicates that although subaerial volcanism can perturb the global hg cycle not all subaerial eruptions will do so these results highlight the variable impact of different volcanogenic processes on the efficiency of hg dispersal across the globe factors that could influence the impact of lip eruptions on the global mercury cycle include submarine versus subaerial volcanism volcanic intensity or explosivity and the potential contribution of thermogenic mercury from reactions between ascending magma and surrounding organic rich sediments

seismotectonics of subduction and back arc rifting under the andaman sea abstract active underthrusting of the indian plate below the burma plate produces seismic activity under the andaman and burmese arcs it is suggested that significant differences exist in the configuration of the benioff zone underlying the two arcs several parallel and linearly extensive fault zones are outlined by epicentral distribution in the andaman arc the andaman back arc spreading ridge is defined by a relatively narrow shallow seismic zone that orients nne to sse in north and south parts of the andaman sea respectively the spreading ridge is offset by active transform faults on the far north the spreading ridge appears to connect to a major transform fault in burma across the burmese coast focal mechanism study provides evidence for tensional stress near the inner wall of the andaman trench dominant compressive deviatoric stress within the overriding burma plate and tensional stress under the back arc spreading ridge

the northern gulf of mexico during oae2 and the relationship between water depth and black shale development despite their name oceanic anoxic events oaes are not periods of uniform anoxia and black shale deposition in ancient oceans shelf environments account for the majority of productivity and organic carbon burial in the modern ocean and this was likely true in the cretaceous as well however it is unlikely that the mechanisms for such an increase were uniform across all shelf environments some like the northwest margin of africa were characterized by strong upwelling but what might drive enhanced productivity on shelves not geographically suited for upwelling to address this we use micropaleontology carbon isotopes and sedimentology to present the first record of oceanic anoxic event 2 oae2 from the northern gulf of mexico shelf here oae2 occurred during the deposition of the well oxygenated inner neritic lower estuarine lower tuscaloosa sandstone the overlying organic rich oxygen poor marine tuscaloosa shale is entirely turonian in age we trace organic matter enrichment from the spinks core into the deepwater gulf of mexico where wireline log calculations and public geochemical data indicate organic enrichment and anoxia throughout the cenomanian turonian boundary interval redox change and organic matter preservation across the gulf of mexico shelf were driven by sea level rise prior to the early turonian highstand which caused the advection of nutrient rich oxygen poor waters onto the shelf this results in organic matter mass accumulation rates 1 2 orders of magnitude lower than upwelling sites like the nw african margin but it likely occurred over a much larger geographic area suggesting that sea level rise was an important component of the overall increase in carbon burial during oae2

deep sea paleotemperature record of extreme warmth during the cretaceous oxygen isotope analyses of well preserved foraminifera from blake nose 30 n paleolatitude north atlantic and globally distributed deep sea sites provide a long term paleotemperature record for the late albian maastrichtian interval that is difficult to reconcile with the existence of significant cretaceous ice sheets given reasonable assumptions about the isotopic composition of cretaceous seawater our results suggest that middle bathyal water temperatures at blake nose increased from 12 c in the late albian through middle cenomanian to a maximum of 20 c during the latest cenomanian and earliest turonian bottom waters were again 12 c during the middle campanian and cooled to a minimum of 9 c during the maastrichtian correlative middle bathyal foraminifera from other ocean basins yield paleotemperature estimates that are very similar to those from blake nose comparison of global bottom water temperatures and latitudinal thermal gradients suggests that global climate changed from a warm greenhouse state during the late albian through late cenomanian to a hot greenhouse phase during the latest cenomanian through early campanian then to cool greenhouse conditions during the mid campanian through maastrichtian

accumulation of organic carbon rich sediments in the late jurassic and cretaceous atlantic ocean a synthesis abstract organic carbon rich sediments black shales were deposited at some 40 deep sea drilling project dsdp sites in the atlantic ocean during late jurassic and cretaceous times in order to reconstruct the depositional environments of black shale formation we have studied in detail changes in the occurrence of black shales as well as the type and amount of organic matter in relation to changes in paleogeography paleooceanic circulation and paleoclimate we have calculated accumulation rates of marine and terrigenous organic carbon and combined these with organic carbon content sedimentation rate relationships to obtain information about paleoenvironments main factors causing the preservation and accumulation of organic carbon are 1 the supply of terrestrial organic matter 2 the supply of marine organic matter 3 the oxygenation of the deep water and 4 the bulk sedimentation rate changes in the importance of these different factors and changes in the interrelationships among them determine the complex system of black shale formation and their cyclic variations increased supply of terrestrial organic matter due to increased river discharge and humid climate in northeast america southwest europe and south africa were important in the hauterivian albian northwest and northern northeast atlantic and in the late jurassic aptian south atlantic in the latter anoxic deep water conditions due to restricted circulation in the nascent largely isolated basin as well as higher oceanic productivity may have reinforced the high preservation rates of marine organic matter during that time off northwest africa higher flux rates of marine organic matter were recorded in areas of higher productivity which may have been caused by fluvial nutrient supply during the hauterivian and by intensified upwelling during albian turonian times in general the depositional environment was oxygenated and the content of marine organic matter was a function of the sedimentation rate furthermore at some places redeposition by turbidity currents or slumps may have been important for the accumulation of organic matter in the deep sea basins the only episode of widespread anoxia in the western and eastern basins of the north atlantic may have occurred in the cenomanian turonian when high amounts of marine organic matter were also preserved in areas of low productivity and low sedimentation rates

lithofacies analysis of basic lava flows of the parana igneous province in the south hinge of torres syncline southern brazil abstract the parana igneous province records the volcanism of the earlier cretaceous that preceded the fragmentation of the gondwana supercontinent historically investigations of these rocks prioritized the acquisition of geochemical and isotopic data considering the volcanic pile as a monotonous succession of tabular flows this work provides a detailed analysis of the emplacement conditions of these basic volcanic rocks applying the facies analysis method integrated to petrographic and geochemical data the torres syncline is a nw se tectonic structure located in southern brazil where a thick sequence of the parana etendeka volcanic rocks is well preserved this study was performed in the south hinge of the syncline where the basaltic lava flows are divided into three lithofacies associations early compound pahoehoe early simple pahoehoe and late simple rubbly the first lavas that erupted were more primitive compound pahoehoe flow fields composed of olivine basalts with higher mgo contents and covered the sandstones of the botucatu formation the emplacement of compound pahoehoe flow fields is possibly related to intermittent low effusion rates whereas the emplacement of simple pahoehoe is related to sustained low effusion rates with continuous supply the thick simple rubbly lavas are associated with high effusion rates and were formed during the main phase of volcanism in the area the absence of paleosoils between the lavas and lithofacies associations suggests that the successive emplacement of the lava flows occurred in a relatively short time gap geochemically the lithofacies associations are low tio2 and belong to gramado magma type the lavas of the south hinge of the torres syncline have a similar evolution when compared to other continental basaltic provinces with earlier compound flows at the base and thicker simple flows in the upper portions

jiggling the tropical thermostat in the cretaceous hothouse modern open ocean sea surface temperatures rarely exceed 28 29 8c and the same has been thought to represent a rough maximum for past tropical climates however new isotopic estimates from the uppermost cenomanian in the tropical western north atlantic suggest that mixed layer temperatures reached 33 34 8 c 62 8c during the middle cretaceous hothouse uppermost cenomanian tropical sea surface temperatures may have been as much as 4 7 8c warmer than the highest modern mean annual temperatures such extreme conditions suggest that warm tropical oceans could have driven substantially intensified atmospheric heat transport near the cenomanian turonian boundary the tropical thermostat was set higher than today challenging the hypothesis of tropical climate stability

chemostratigraphy versus biostratigraphy data from around the cenomanian turonian boundary a detailed isotopic profile is presented for a stratigraphically expanded cenomanian turonian boundary section in chalk facies exposed at eastbourne sussex and compared with data from pueblo colorado in both sections macro and micropalaeontological markers first appearances and disappearances are well constrained and their relative positions and relationship to the structure of the carbon isotope curve are identical this consistent relationship between two independent phenomena one geochemical the other biostratigraphical is taken as evidence for the likely synchroneity of both the biostratigraphical markers and the carbon isotope excursion in these two areas this interpretation contrasts with suggestions made recently by other authors whose data have been taken to imply a lack of correlation between the carbon isotope excursion in europe and north america

calcareous nannofossils and mesozoic oceanic anoxic events abstract twenty five years ago mesozoic oceanic anoxic events oaes were documented and formalised as intervals of widespread to global deposition of organic matter the toarcian early aptian oae1a and latest cenomanian oae2 oaes are truly global in nature commonly carbonate poor and typically represented by organic carbon rich black shales in some areas these oaes are also characterised by abundant radiolarian sands and silts they are associated with negative and positive excursions in the 87sr 86sr record in addition to large global carbon isotope anomalies in carbonate and or organic matter caused by a major perturbation of the global carbon budget increased rates of volcanism during the formation of the ontong java and manihiki and caribbean plates and the karoo ferrar traps are believed to have caused the geological responses associated with oae1a oae2 and the toarcian oae respectively excess volcanogenic co2 in the atmosphere most probably turned the climate into a greenhouse mode accelerating continental weathering and increasing nutrient content in oceanic surface waters via river run off higher fertility in the global ocean was also probably triggered directly by submarine igneous events that introduced enormous quantities of biolimiting metals within hydrothermal plumes because mesozoic oaes are often represented by carbonate poor sediments quantitative studies of calcareous nannofossils have been applied to explore a the causes and effects of igneous tectonic events and climate changes relative to nannofloral increases and crises as well as b dissolution events and c diagenetic modifications characterization of calcareous nannofloras in oae intervals can improve our understanding of the marine ecosystem and biological processes such as photosynthesis biological pump and biomineralisation carbonate pump that affect the organic and inorganic carbon cycle as well as adsorption of atmospheric co2 in the oceans types and rates of nannoplankton production and evolution are interpreted to trace the impact of major palaeoceanographic and palaeoclimatic events in selected sections it has been documented that calcareous nannofloras rapidly reacted to new conditions of fertility and higher pco2 by drastically reducing calcification as in the modern oceans during oaes the increase of nutrients and atmospheric co2 induced higher abundances of nannoplankton producing small placoliths and inhibited the deep photic zone nannoconids and schizosphaerellids similarly to the nannoconid crisis preceding deposition of the early aptian oae1a black shales a schizosphaerellid crisis is detected prior to the toarcian oae both oaes are further characterised by a rapid nannofloral speciation beginning approximately 1 5 myr before the oae but without extinctions global changes during the latest cenomanian oae 2 exerted different influences on calcareous nannoplankton that experienced a turnover due to most extreme environmental conditions this event in fact was a time of extinctions followed by originations within calcareous nannofossils precise timing of the events before during and after oae1a oae2 and the toarcian oae indicates that they were intervals of enhanced oceanic productivity and that anoxia dysoxia post dated biotic changes

cenomanian turonian carbon isotope events recorded in terrestrial organic matter from northern japan carbon isotopic fluctuations recorded in terrestrial organic matter from the upper cretaceous cenomanian turonian succession of hokkaido japan show a remarkably similar pattern to isotopic fluctuation in carbonate carbon reported from southern england and the italian apennines chronological correlation for the cenomanian turonian section of japan is based on planktonic foraminiferal biostratigraphy with some additional data from macrofossils allowing a detailed comparison of isotope stratigraphies for japan and europe three distinct features of the isotope profile namely a positive spike near the cenomanian turonian boundary a shoulder in the lower middle turonian and a level segment in the middle upper turonian are observed in japan these features are biostratigraphically comparable and correlative with isotope profiles reported from europe the single factor influencing isotopic composition of both terrestrial organic carbon and marine carbonate carbon is isotopic change in the global co2 reservoir of the ocean atmosphere system if these three carbon isotope features are synchronous they may be inferred as having been caused by global phenomena and as providing global high resolution chemo stratigraphic markers carbon isotope stratigraphy could be an important bridge between terrestrial inland sequences and marine sequences

sedimentary mercury enrichments as a marker for submarine large igneous province volcanism evidence from the mid cenomanian event and oceanic anoxic event 2 late cretaceous oceanic anoxic event 2 oae 2 during the cenomanian turonian transition 94 ma was the largest perturbation of the global carbon cycle in the mid cretaceous and can be recognized by a positive carbon isotope excursion in sedimentary strata although oae 2 has been linked to large scale volcanism several large igneous provinces lips were active at this time e g caribbean high arctic madagascan ontong java and little clear evidence links oae 2 to a specific lip the mid cenomanian event mce 96 ma identified by a small 1 positive carbon isotope excursion is often referred to as a prelude to oae 2 however no underlying cause has yet been demonstrated and its relationship to oae 2 is poorly constrained here we report sedimentary mercury hg concentration data from four sites three from the southern margin of the western interior seaway and one from demerara rise in the equatorial proto north atlantic ocean we find that in both areas increases in mercury concentrations and hg toc ratios coincide with the mce and the oae 2 however the increases found in these sites are of a lower magnitude than those found in records of many other mesozoic events possibly characteristic of a marine rather than atmospheric dispersal of mercury for both events combined the new mercury data presented here are consistent with an initial magmatic pulse at the time of the mce with a second greater pulse at the onset of oae 2 possibly related to the emplacement of lips in the pacific ocean and or the high arctic

organic matter deposition and paleoenvironmental implications across the cenomanian turonian boundary of the subalpine basin se france local and global controls abstract the cenomanian turonian boundary is marked by global sea level highstands and one of the warmest periods of the mesozoic with high pco2 levels linked to a massive magmatic episode this is associated with the establishment of a worldwide marine anoxia the deposition of organic rich facies and disturbances of the global carbon cycle the so called oceanic anoxic event 2 oae2 the pont d issole section s subalpine basin se france sedimentary succession represents the reference record of oae2 in the tethyan domain although its organic fraction is extremely poorly studied therefore the main objectives of this study are the characterization of the pont d issole section s organic matter om for paleoenvironmental interpretation and the assessment of the om thermal maturity using organic petrology and organic geochemistry techniques the organic record characterizes a shallowing up marine system with varying redox and restriction conditions associated with periods of stratified oxygen regimen it displays the presence of two om enrichment episodes with different paleoenvironmental and geodynamic contexts the first om enrichment episode unit th1 is thought to be associated with the global mechanisms that trigger oae2 and is characterized by a palynofacies association co dominated by amorphous om and marine microplankton reflecting a context of high sea level this is followed by the plenus cold event unit th2 characterized by a decrease in amorphous om and increase in marine microplankton representing a reoxygenation associated with an increase in water level and an apparent decrease in temperature disappearance of classopollis sp the second om enrichment episode unit th3 and base of unit th4 is characterized by an association dominated by amorphous om the occurrence of freshwater microplankton and the development of a restricted environment with low sea level these depositional conditions are thought to be connected to the beginning phases of basin closure promoted by the regional tectonic framework compressive trend between africa and europe the molecular fraction showed the presence of unsaturated compounds monoaromatic steranes monoaromatic anthrasteroids and the biological hopanes indicating a low thermal evolution for the om this is corroborated by biomarkers ratios and organic petrographic data namely vitrinite reflectance 0 29 rr the presence of paleozoic reworked om in the samples was also identified presenting a more complex thermal history than the cretaceous om this om is possibly related to the subaerial erosion of hercynian massifs

modelling oceanic carbon and phosphorus fluxes implications for the cause of the late cenomanian oceanic anoxic event oae2 a two box ocean model with a coupled carbon and phosphorus cycle is used to test which processes can realistically generate a rapid positive 2 13c excursion in the global carbon reservoir internal oceanic processes enhanced preservation of organic matter under anoxic conditions enhanced upwelling cannot produce a substantial 13c excursion under the assumption that burial rates of c and p are coupled loss of nutrients to the sediment acts as a negative feedback mechanism that prevents a substantial perturbation of the carbon cycle a large and renewable source of nutrients is needed either from increased continental weathering or from enhanced regeneration of sedimentary organic phosphorus under anoxic conditions measurements of total phosphorus and organic carbon in sediments from the tarfaya basin morocco suggest that a large decrease in p burial occurred relative to organic carbon at the base of the cenomanian turonian oceanic anoxic event oae2 with the right magnitude to quantitatively explain a 2 shift in 13c it is concluded that the main underlying mechanism that allowed and sustained enhanced carbon burial during the mid cretaceous was a perturbation of the oceanic phosphorus cycle

a multiple proxy and model study of cretaceous upper ocean temperatures and atmospheric co2 concentrations foraminiferal d 18 o and mg ca suggests that the ratio of magnesium to calcium in the turonian coniacian ocean may have been lower than in the albian cenomanian ocean perhaps coincident with an ocean 87 sr 86 sr minimum the carbon isotopic compositions of distinct marine algal biomarkers were measured in the same sediment samples the d 13 c values of phytane combined with foraminiferal d 13 c and inferred temperatures were used to estimate atmospheric carbon dioxide concentrations through this interval estimates of atmospheric co2 concentrations range between 600 and 2400 ppmv within the uncertainty in the various proxies there is only a weak overall correspondence between higher lower tropical temperatures and more less atmospheric co2 the genesis climate model underpredicts tropical atlantic temperatures inferred from odp leg 207 foraminiferal d 18 o and mg ca when we specify approximate co2 concentrations estimated from the biomarker isotopes in the same samples possible errors in the temperature and co2 estimates and possible deficiencies in the model are discussed the potential for and effects of substantially higher atmospheric methane during cretaceous anoxic events perhaps derived from high fluxes from the oxygen minimum zone are considered in light of recent work that shows a quadratic relation between increased methane flux and atmospheric ch4 concentrations with 50 ppm ch4 genesis sea surface temperatures approximate the minimum upper ocean temperatures inferred from proxy data when co2 concentrations specified to the model are near those inferred using the phytane d 13 c proxy however atmospheric co2 concentrations of 3500 ppm or more are still required in the model in order to reproduce inferred maximum temperatures

chemical stratigraphy of the parana lavas south america classification of magma types and their spatial distribution a new classification scheme has been developed to assign the lava flows of the parana continental flood basalt province south america into geochemically distinct magma types with six basaltic major and trace element abundances and or ratios by mapping out the spatial distribution of these magma types within the lava sequences it has been possible to determine the internal stratigraphy of the lava pile on a regional scale previous studies on road profiles traversing the well exposed coastal serra geral escarpment of southern brazil are summarised together with results from some new sampled sections more widespread stratigraphical investigations of the parana lavas have been hampered by the lack of sufficient topographic relief and the cover of sedimentary rocks however access to drill core chippings from nine boreholes in the central parana region has provided a unique opportunity to investigate the stratigraphy of the otherwise inaccessible deeper levels of the lava pile and to map out stratigraphic variations in three dimensions the borehole samples have indicated cated a stacking of units of different magma types all overlapping towards the north which suggests that the main locus of magmatism moved northwards with time within the parana basin this migration could be related to the northward propagation of rifting during the initiation of the south atlantic ocean maps of the surface distribution of samples of each magma type show a pattern consistent with the stratigraphy inferred from the boreholes although suggesting that the shift in magmatism may have been towards the northwest on the basis of geochemical similarities between magma types and their inferred stratigraphical relationships it is proposed that the parana can be divided into two principal magmatic centres 1 an older one in the south comprising the gramado esmeralda and urubici magma types and 2 a younger one developed about 750 km to the north formed by the pitanga paranapanema and ribeira magma types

does ice drive early maastrichtian eustasy a large 30 40 m rapid 1 m y earliest maastrichtian sea level drop inferred from new jersey sequence stratigraphic records correlates with synchronous 18o increases in deep water benthic and low latitude surface dwelling planktonic foraminifera the coincidence of these events argues for the development of a moderate sized ice sheet during the early maastrichtian

a biomarker study of depositional paleoenvironments and source inputs for the massive formation of upper cretaceous lacustrine source rocks in the songliao basin china abstract the abundance and composition of total organic carbon aliphatic and aromatic hydrocarbons including biomarkers and the 13c composition of total organic matter and individual compounds in core samples of the nenjiang formation from the sk 1 borehole of the songliao basin provide information on the paleoenvironmental conditions of lacustrine sediments formed during the upper cretaceous the distribution of n alkanes and their 13c composition and 4 methylsteranes and dinosteranes in the first member of the nenjiang formation k2n1 indicate that the organic matter largely originated from algal organisms and is dominated by types i and ii kerogen the occurrence of gammacerane aryl isoprenoids isorenieratane and low pristane phytane and diasterane sterane ratios further suggest a salinity stratified water column associated with anoxic bottom waters as well as possible photic zone oxygen depletion conditions during major source rock deposition of the k2n1 the organic geochemical data also indicate significantly different depositional environments for the second member of the nenjiang formation k2n2 sediments which are interpreted to have been deposited under a uniform salinity non stratified oxic water column consequently although the organic matter in the k2n2 sediments was largely derived from algal and bacterial sources the kerogen types are dominantly iib and iii due to the oxidative degradation of organic matter combined with a gradual increase in land plant input furthermore the occurrence and shift of two saw toothed distribution patterns of the 13c composition of c29 to c33 n alkanes with w and m patterns being isotopically heavy odd and even numbered alkanes respectively is suggestive of organic input shift or an unknown mechanism of carbon uptake or isotopic fractionation in particular the heavy even carbon numbered high molecular weight n alkanes is rare and needs further investigation

an ocean modeling study of the cenomanian atlantic equatorial paleo upwelling organic rich sediments and the consequences for a connection between the proto north and south atlantic abstract a reduced gravity ocean modelis used to deter mine the pattern of upwelling in the cenomanian atlantic the deposition of organic rich sediments on the margins ofthe equatorial region is found to be consistent with the pat terns of upwelling in simulations which incorporate only ashallow 300m equatorial connection between the northand south atlantic fracture zone ridges were probably re sponsible for the e ective shallowness of the gateway if adeep equatorial channel is assumed the modeling results areconsistent only with the morerecent turonian sedimentaryrecord we suggest therefore that reduced gravity modelsprovide a mean of predicting areas of the continental mar gins which may be underlain by hydrocarbon rich sedimen tary rocks introduction the equatorial region of the atlantic is a major route forwater transfer between the two hemispheres studies of itssedimentary cover have provided abundant evidence for ma jor environmental changeswithin this critical oceanographicgateway from late jurassic time among the important sed imentary consequences ofcirculation and sea level variationswas the development of petroleum rich sequences along thecontinental margins and in the adjacent deep water basins on the present continental shelves these occur over a widestratigraphic range from the early cretaceous to oligocene reyre 1966 nairn and stehli 1973 akpati 1978 kesse 1984 dumestre and carvalho 1985 herbin et al 1986 koutsoukos et al 1991 while in the deep sea organic richshales ranging in age from albian to late cretaceous havebeen found at several deep sea drilling project dsdp sites figure 1 the deep sea deposits have been attributedto oceanwide anoxic events oaes pedersen and calvert 1990 arthur and sageman 1994 the stratigraphic rangeof the continental margin black shales is much broader thanthe deep sequences however we have to look for othermechanisms apartfrom general oceananoxia to explain this these sequences could be ascribed to enhanced primary pro ductivity arising from upwelling of nutrients into the eu photic zone barron 1985 parrish 1995 parrish 1995 be

geochemical and climatic effects of increased marine organic carbon burial at the cenomanian turonian boundary perhaps the most significant event in the cretaceous record of the carbon isotope composition of carbonate1 2 other than the 1 2 5 negative shift in the carbon isotope composition of calcareous plankton at the cretaceous tertiary boundary3 is the rapid global positive excursion of 2 13c enrichment which took place between 91 5 myr and 90 3 myr late cenomanian to earliest turonian c t boundary event 1 4 5 this excursion has been attributed to a change in the isotope composition of the marine total dissolved carbon tdc reservoir resulting from an increase in rate of burial of 13c depleted organic carbon which coincided with a major global rise in sea level5 during the so called c t oceanic anoxic event oae 6 here we present new data from nine localities which demonstrate that a positive excursion in the carbon isotope composition of organic carbon at or near the c t boundary7 8 is nearly synchronous with that for carbonate and is widespread throughout the tethys and atlantic basins fig 1 as well as in more high latitude epicontinental seas the postulated increase in the rate of burial of organic carbon may have had a significant effect on co2 and o2 concentrations in the oceans and atmosphere and consequent effects on global climate and sedimentary facies

the cenomanian turonian boundary at eastbourne sussex uk a proposed european reference section the cenomanian turonian boundary ctb section at eastbourne sussex england is the thickest at outcrop in the anglo paris basin is rhythmic throughout rich in macro micro and nanno fossils and shows less diagenetic alteration than other sections in nw europe geochemical and biostratigraphic data are all related to detailed rhythmic lithostratigraphy allowing correlation with the global boundary stratotype section to within 20 000 years and accurate estimates of rates of changes very detailed stable isotope curves for oxygen and carbon are presented and show that the build up phase of the ctb carbon excursion had two portions separated by a trough patterns of occurrence of coarse fraction benthic foraminifera 250 mm show two peaks of extinction corresponding to the two build up phases of the carbon excursion benthic and planktic foraminifera ostracods and calcareous nannofossils all show declines in diversity and abundance across the ctb foraminifera also decline significantly in size key biomarker events for the ctb include first occurrences of the ammonites fagesia catinus and mammites nodosoides the bivalve mytiloides the planktic foraminifera helvetoglobotruncana praehelveticaand marginotruncana renzi and the calcareous nannofossils quadrum intermedium q gartneri and eprolithus octopetalus and last occurrences of the ammonite sciponoceras the planktic foraminifera rotalipora greenhornensis and r cushmani the calcareous nannofossils corolithion kennedyi axopodorhabdus albianus lithraphidites acutus rhagodiscus asperand microstaurus chiastius cyclostratigraphy allows very precise correlation of isotopic and bioevents with other sections in europe and north america eastbourne is recommended as a nw european standard section 1999 elsevier science b v all rights reserved

seismotectonics of transverse lineaments in the eastern himalaya and its foredeep abstract the himalayan collision zone and the burmese subduction zone lie in rather close vicinity across northeast india their possible interaction produces complex tectonics and a high level of seismicity in the region several prominent transverse lineaments across the eastern himalaya and its foredeep appear to be seismically active quite a few of the active lineaments are regionally extensive even transgressing the bengal basin a focal mechanism study indicates that active lineaments are either normal or strike slip faults for the highly active shillong mikir massif a fragmented portion of the indian shield this correlation is less clear although it appears that some of the activity may be associated with the northeasterly lineaments crossing the massif into the eastern himalayan foredeep this presumably results due to drag experienced by the indian lithosphere near its margins under the himalayan and burmese arcs the assam valley which is the common foredeep for both the arcs is remarkably aseismic though it is surrounded by active regions mainly thrusting mechanisms characterize the earthquakes which originate from the himalayan and burmese arcs adjoining the valley a model of basement reactivation below the valley is proposed in order to explain the style of tectonic deformation and current seismicity in the himalayan and burmese orogens adjoining the valley

cretaceous oceanic red beds corbs different time scales and models of origin abstract the cretaceous oceanic red bed corb is a newly opened window on global oceanic and climate changes during the cretaceous greenhouse world as a result of the international geoscience programmes 463 494 and 555 2002 2010 corbs have been documented in many places by numerous publications the principle goal of this paper is to summarize scientific advances on corbs including chronostratigraphy sedimentology mineralogy elemental and isotopic geochemistry and their relationship to oceanic anoxic events oaes palaeoclimate and palaeoceanography we propose a new geochemical classification of the corbs using cao al 2 o 3 and sio 2 values which lithologically refer to marly clayey and cherty corbs respectively detailed mineralogical studies indicate that hematite goethite and mn 2 bearing calcite are the minerals imparting the red color of corbs hematite clusters of several to tens of nanometers in the calcite structure are the main cause of the red coloring of limestones and the mn 2 bearing calcite gives additional red color goethite was thought to form originally with hematite and was subsequently transformed to hematite during late diagenesis chronostratigraphic data allow the distinction of two groups of corbs by their durations short term corbs are generally less than 1 myr in duration and seem to be on the scale of milankovitch cycles during the deposition of cretaceous reddish intervals from odp cores 1049 and 1050 low primary productivity and relatively high surface temperature resulted in low organic carbon flux into the sediments which reduced oxygen demand and produced oxidizing early diagenetic conditions in such an oxic environment iron oxides formed imparting the reddish color the long term corbs depositional events lasted longer than 4 myr and may be a possible consequence of the oaes enhanced amounts of organic carbon and pyrite burial during and after the oaes would have resulted in a large and abrupt fall in atmospheric co 2 concentration which probably induced significant global climatic cooling during and after the oaes global cooling would have enhanced formation of cold deep water increasing its oxidizing capacity due to the greater content of dissolved oxygen and would promote formation of oceanic red beds sedimentological mineralogical and geochemical data indicate that corbs were deposited under highly oxic oligotrophic conditions probably at a low sedimentation rate the cretaceous red and white limestones from italy have similar compositions of terrestrial input sensitive elements al ti k mg rb zr higher contents of fe 2 o 3 and depleted redox sensitive elements v cr ni and u and micronutrient elements cu zn indicating similar provenance sources but red limestones were deposited under more oxic conditions at the sediment water interface than white limestones the cretaceous red shales such as those from the north atlantic and tibet have similar mineralogy and geochemistry as the late cenozoic red clays in the pacific ocean and the environment where both are formed was well oxidizing at a very low sedimentation rate we compiled seventeen published stratigraphic examples of phanerozoic oceanic red beds including the late cenozoic red clays in the pacific different hypotheses explain the origin of red pigmentation of limestones and shales including 1 detrital origin of iron derived from continental weathering 2 iron bacterial mediation at the time of sedimentation and 3 iron oxidation in oligotrophic highly oxic environment additional research on phanerozoic oceanic red beds is needed in order to better document their origin and palaeoceanographic and palaeoclimatic significance

structural context of the great sumatra andaman islands earthquake 1 a new three dimensional seismic model and relocated regional seismicity are used to illuminate the great sumatra andaman islands earthquake of december 26 2004 the earthquake initiated where the incoming indian plate lithosphere is warmest and the dip of the wadati benioff zone is least steep along the subduction zone extending from the andaman trench to the java trench anomalously high temperatures are observed in the supra slab mantle wedge in the andaman back arc the subducting slab is observed along the entire plate boundary to a depth of at least 200 km these factors contribute to the location of the initiation of rupture the strength of seismic coupling the differential rupture properties between the northern and southern segments of the earthquake and the cause of convergence in the andaman segment

lava sediment interaction in desert settings are all peperite like textures the result of magma water interaction abstract this study reports on lava sediment interaction in dry depositional settings focusing on the early cretaceous volcano sedimentary sequence in the huab basin nw namibia as a detailed example here an active aeolian sand sea erg system was progressively engulfed by lavas of the etendeka flood basalt province 133 ma bp this volcanic flooding resulted in 1 the unprecedented preservation of large parts of the active dune system 2 the development of a variety of sediment interlayers and 3 a record of excellent examples of the dynamic interaction between lava and aeolian sand the lava sediment interaction happens on a variety of scales from simple sediment interbeds within the lavas at a large scale down to complex breccia horizons and bulbous lava sediment contacts at small scales peperite like textures are found where hot lava has dynamically interacted with unconsolidated aeolian sands such interaction could involve lava cascading down the front of dune faces or active aa style flow fronts bulldozing into and oversteepening dune faces causing rapid influx of sand into the active flow front resulting breccia horizons have been found up to 5 m thick locally the aeolian sand is baked to a quartzite by the hot lava in contrast on other surfaces the lava leaves striations in the sand as it flows across it indicating local flow directions and less pronounced interaction in places wind ripples and topset beds are preserved on the stoss side of the dune indicating the passive nature of emplacement for some of the lava flows peperites are often thought to form due to the presence of water within the unconsolidated sediments that the juvenile magma comes into contact with however examples from arid environments demonstrate that peperite forming processes are diverse and may not always require water therefore it is suggested that the term peperite should be used to describe deposits consisting of sediments mixed with juvenile magmatic components where it is clear that there has been a dynamic interaction between the two this leaves the presence or lack of water as an issue for interpreting the processes by which the peperite formed

chaotic dynamics and fractals in magmatic interaction processes a different approach to the interpretation of mafic microgranular enclaves abstract fractals have been used for describing a wide range of natural processes however applications of fractal geometry to the study of magma mingling mixing processes is almost completely lacking in spite of the fact that such an approach could give new insights into the nature of magmatic interactions fractal analyses of two geochemically linked groups of mafic microgranular enclaves mme from the sithonia plutonic complex northern greece have been performed because of the difficulties involved in using contours of enclaves to estimate their fractal dimensions a different approach is used in this paper fractal dimension is estimated using distribution maps of chemical elements inside enclaves the results obtained indicate that a clear correlation between fractal dimensions of enclaves and geochemical evidences of interaction exists and hence the two groups of enclaves reflect different degrees of interaction from a dynamical point of view stretching and folding processes between two magmas with different rheologies are considered representative of magmatic interaction this dynamic system can easily generate chaotic patterns recognized by the presence of fractal structures the contemporaneous occurrence of well mixed regions together with poorly mixed regions in the same system is thought to be similar to the host enclave system found in many granitoid plutons in particular mme are believed to represent islands of partially mixed mafic fluid subjected to different degrees of interaction with the host granitoid whereas host rocks are believed to represent mixed regions where the mixing process acted more efficiently producing almost completely hybrids it is shown that fractal geometry and chaotic dynamics can be considered suitable techniques in studying complex petrological phenomena and they can represent helpful methods in the development of petrogenetic models

carbon and oxygen isotope stratigraphy of the english chalk and italian scaglia and its palaeoclimatic significance a detailed carbon and oxygen isotope stratigraphy has been generated from upper cretaceous coastal chalk sections in southern england east kent culver cliff isle of wight eastbourne and seaford head sussex norfolk coast and the british geological survey bgs trunch borehole norfolk data are also presented from a section through the scaglia facies exposed near gubbio italian apennines wherever possible the sampling interval has been one metre or less both the chalk and scaglia carbon isotopic curves show minor positive excursions in the mid cenomanian mid and high turonian basal coniacian and highest santonian lowest campanian there is a negative excursion high in the campanian in chalk sections that span that interval the well documented cenomanian turonian boundary spike is also well displayed as is a broad positive excursion centred on the upper coniacian a number of these positive excursions correlate with records of organic carbon rich deposition in the atlantic ocean and elsewhere the remarkable similarity in the carbon isotope curves from england and italy enables cross referencing of macrofossil and microfossil zones and pinpoints considerable discrepancy in the relative positions of the turonian coniacian and santonian stages the oxygen isotope values of the various chalk sections although showing different absolute values that are presumably diagenesis dependent show nonetheless a consistent trend the east kent section which is very poorly lithified indicates a warming up to the cenomanian turonian boundary interval then cooling thereafter regional organic carbon burial documented for this period is credited with causing drawdown of co 2 and initiating climatic deterioration inverse greenhouse effect data from other parts of the world are consistent with the hypothesis that the cenomanian turonian temperature optimum was a global phenomenon and that this interval represents a major turning point in the climatic history of the earth

further evidence for the development of photic zone euxinic conditions during mesozoic oceanic anoxic events periods in earth history characterized by extensive organic carbon deposition so called oceanic anoxic events oaes are the subject of considerable scrutiny insight into the extent of anoxic conditions in the water column has been provided by the reported occurrence of derivatives of isorenieratene a carotenoid found only in green sulphur bacteria and thus indicative of photic zone euxinia i e a water column containing free h 2 s in north atlantic deep sea sediments deposited during the cenomanian turonian boundary oae2 bonarelli event in this study we have used the distributions of chlorophyll and bacteriochlorophyll degradation products maleimides 1 h pyrrole 2 5 diones and high molecular weight porphyrins to examine further the cenomanian turonian boundary oae2 as well as other oaes of the early toarcian posidonienschiefer event and early aptian selli event in particular methyl isobutyl me i bu maleimide on structural grounds appears to be diagnostic of green sulphur bacteria this compound occurs in five of seven examined marls that record the early toarcian oae marche umbria italy further expanding the geographical range of toarcian sediments where evidence for photic zone euxinic conditions has been found me i bu maleimide occurs in three of six black shales spanning the livello selli marche umbria italy the type locality for the aptian oae1a providing the first evidence for photic zone euxinic conditions during this event with respect to the cenomanian turonian oae maleimide evidence for photic zone euxinic conditions was found in all the north atlantic sites investigated including those characterized by relatively organic lean sediments several tethyan sites and one site off western australia these data indicate that euxinic conditions were common in the water column during mesozoic oceanic anoxic events

structures and lithofacies of inferred silicic conduits in the parana etendeka lip southernmost brazil abstract extensive silicic units in the parana etendeka lip have been long interpreted as pyroclastic density currents rheomorphic ignimbrites derived from the messum complex in namibia in recent literature however they have been characterized as effusive lava flows and domes in this paper we describe structures and lithofacies related to postulated silicic lava feeder conduits at mato perso sao marcos and jaquirana cambara do sul areas in southern brazil inferred conduits are at least 15 25 m in width and the lithofacies include variably vesicular monomictic welded and non welded breccias in the margins to poorly vesicular banded spherulitic and microfractured vitrophyres in the central parts flat lying coherent vitrophyres and massive obsidian are considered to be the subaerial equivalents of the conduits large scale regional tectonic structures in southern brazil include the ne sw aligned porto alegre suture leao and acotea faults besides the antas lineament a curved tectonic feature accompanying the bed of antas river south of the antas lineament smaller scale nw se lineaments limit the exposure areas of the inferred conduits ne sw and subordinate nw se structures within this smaller scale lineaments are represented by the main postulated conduit outcrops and are parallel to the dominant sub vertical banding in the widespread banded vitrophyre lithofacies upper lava flows display flat lying foliation pipe like and spherical vesicles and have better developed microlites petrographic characteristics of the silicic vitrophyres indicate that crystal poor magmas underwent distinct cooling paths for each inferred conduit area the vitrophyre chemical composition is defined by the evolution of trachydacitic dacitic vitrophyres with 62 65 wt sio 2 to rhyodacite and rhyolite with 66 68 wt sio 2 the more evolved rocks are assigned to the latest intrusive grey vitrophyre outcropping in the center of the conduits degassing pathways formed during fragmentation and fracturing episodes within the conduits may have helped to inhibit the explosivity of the eruptions based on the documented lithofacies architecture we attribute the source of the silicic lava flows in the studied localities to tectonic controlled local conduits rather than pyroclastic density currents from distant vent areas

age and paleoenvironment of the cenomanian turonian global stratotype section and point at pueblo colorado biostratigraphy and stable isotopes indicate that the global stratotype section and point gssp at pueblo contains an essentially complete sedimentary record across the global ocean anoxic event oae 2 and the cenomanian turonian boundary the oae 2 d 13 c shift occurred over a period of about 90 ky and was accompanied by a major sea level transgression which at its peak was marked by an incursion of oxygen rich waters creating a benthic oxic zone that lasted about 100 ky a mid cenomanian d 13 c shift sea level transgression and faunal turnover occurred about 2 my before oae 2 d 18 o values of the planktic foraminifer hedbergella planispira and its relative abundance changes reveal cyclic variations in surface salinity due to alternating freshwater influx and marine incursions whereas dominance by the low oxygen tolerant heterohelix species indicates a well developed oxygen minimum zone omz for most of the middle to late cenomanian and early turonian profound faunal changes accompanied these oceanographic events including the extinction of 30 of the species assemblage and an equal gain in evolutionary diversification though the overall combined relative abundances of outgoing and incoming species were less than 2 and 4 respectively of the total assemblages the faunal turnover began with the sea level transgression and rapid increase in d 13 c values and accelerated with the influx of oxygen rich deep water increased water mass stratification and competition during the benthic oxic zone the incursion of oxygen rich deep waters at this time was also observed in morocco and may represent a global event of a still unknown source d 2003 elsevier b v all rights reserved

carbon isotope stratigraphy recorded by the cenomanian turonian oceanic anoxic event correlation and implications based on three key localities we present new detailed carbon isotope records for bulk carbonate total organic carbon toc and phytane from three key sections spanning the cenomanian turonian boundary interval eastbourne england gubbio italy tarfaya morocco with the purpose of establishing a common chemostratigraphic framework for oceanic anoxic event oae 2 isotope curves from all localities are characterized by a positive carbon isotope excursion of c 4 for toc and phytane and c 2 5 for carbonate although diagenetic overprinting appears to have obliterated the primary carbonate carbon isotope signal in at least part of the tarfaya section stratigraphically peak 13 c values for all components are followed by intervals of high near constant 13 c in the form of an isotopic plateau recognition of an unambiguous return to background 13 c values above the plateau is however contentious in all sections hence no firm chemostratigraphic marker for the end point of the positive isotopic excursion can be established the stratigraphically consistent first appearance of the calcareous nannofossil quadrum gartneri at or near the cenomanian turonian boundary as established by ammonite stratigraphy in conjunction with the end of the 13 c maximum characteristic of the isotopic plateau provides a potentially powerful tool for delimiting the stratigraphic extent and duration of oae 2 this oceanic anoxic event is demonstrated to be largely if not wholly confined to the latest part of the cenomanian stage

source area and rupture parameters of the 31 december 1881 mw 7 9 car nicobar earthquake estimated from tsunamis recorded in the bay of bengal 1 on the morning of 31 december 1881 a submarine earthquake beneath the andaman islands generated a tsunami with a maximum crest height of 0 8 m that was recorded by eight tide gauges surrounding the bay of bengal since the earthquake occurred 8 years before the construction of the world s first teleseismic recording seismometer little has been known about its rupture parameters or location waveform and amplitude modeling of the tsunami indicate that it was generated by a mw 7 9 0 1 rupture on the india andaman plate boundary resulting in 10 60 cm of uplift of the island of car nicobar the rupture consisted of two segments the northern 40 km long segment is separated from the southern 150 km long segment by a 100 km region corresponding to the westward projection of the west andaman spreading center the main rupture occurred between 8 5 n and 10 n with a total area of 150 km 60 km dipping 20 e with a mean slip of 2 7 m the recurrence time for 1881 type events is estimated to be 114 200 years on the basis of inferred gps convergence rates and inferred plate closure vectors although slip partitioning in the region may extend this estimate by as much as 30

toothpaste lava characteristics and origin of a lava structural type transitional between pahoehoe and aa toothpaste lava an important basalt structural type which illustrates the transition from pahoehoe to aa is particularly well displayed on the 1960 kapoho lava of kilauea volcano its transitional features stem from a viscosity higher than that of pahoehoe and a rate of flow slower than that of aa viscosity can be quantified by the limited settling of olivine phenocrysts and rate of flow by field observations related to the low angle slope on which the lava flowed much can be learned about the viscosity rheologic condition and flow velocity of lavas long after solidification by analyses of their structural characteristics and it is possible to make at least a semiquantitative assessment of the numerical values of these parameters

high resolution geochemical and biotic records of the tethyan bonarelli level oae2 latest cenomanian from the calabianca guidaloca composite section northwestern sicily italy abstract high resolution micropalaeontological and chemostratigraphic records for the upper cenomanian portion of the calabianca guidaloca composite section nw sicily provide new insight into the palaeoclimatic and palaeoceanographic evolution of the tethys ocean the bonarelli level equivalent was identified on the basis of lithology and well constrained by calcareous plankton biostratigraphy and radiolarian assemblages as well as by the 13 c curve showing a marked positive excursion up to 4 7 the bonarelli level equivalent deposition is characterized by highly eutrophic conditions as testified by radiolarian proliferation black shale samples from the calabianca guidaloca composite section contain very high toc up to 26 and moderate to high amounts of caco 3 on average 20 among the planktonic foraminifera hedbergellids and globigerinelloids tolerated the environmental stress induced by the oceanic anoxic event 2 oae2 the occurrence of some benthic foraminifera testifies to dysoxic rather than completely anoxic conditions at the sea floor during the deposition of some portions of the bonarelli level equivalent based on micropalaeontological results and geochemical proxies 13 c d rb ti v ni ba si cr the bonarelli level equivalent is interpreted as a high productivity event driven by increasingly warm and humid climatic conditions promoting an accelerated hydrological cycle we propose that periodically increased riverine influxes triggered by enhanced humidity resulted in a sluggish circulation mode and consequent anoxic euxinic conditions favouring the preservation of organic matter at the sea floor spectral analyses performed on selected geochemical signals reveal within the main high productivity event a strong orbital climatic forcing represented by a long term eccentricity steered humidity cyclicity that amplifies a short term precession controlled productivity fluctuation

relationships between deformation and mixing processes in lava flows a case study from salina aeolian islands tyrrhenian sea the massive unit of a lava flow from porri volcano salina aeolian islands displays many unusual structures related to the physical interaction between two different magmas the magma a represents approximately 80 of the exposed lava surface it has a crystal content of 51 vol and a dacitic glass composition sio2 63 64 wt the magma b has a basaltic andesite glass composition sio2 54 55 wt and a crystal content of approximately 18 vol it occurs as pillow like enclaves banding boudin like and rolling structures which are hosted in magma a structural analysis suggests that banding and boudin like structures are the result of the deformation of enclaves at different shear strain the linear correlation between strain and stratigraphic height of the measured elements indicates a single mode of deformation we deduce that the component b deformed according to a simple shear model glass analyses of the a b boundary indicate that a and b liquids mix together at high shear strain whereas only mingling occurs at low shear strain this suggests that the amount of deformation i e forced convection plays an important role in the formation of hybrid magmas high shear strain may induce stretching shearing and rolling of fluids which promote both forced convection and dynamical diffusion processes these processes allow mixing of magmas with large differences in their physical properties

seismicity and the nature of plate movement along the himalayan arc northeast india and arakan yoma a review abstract the himalaya together with arakan yoma form a well defined seismic belt to the north and east of the indian peninsula the seismicity along this belt is attributed mostly to collision between the indian and the eurasian plates however the exact nature of activity along the major thrusts and faults is not well understood the seismicity along the entire himalaya and northern burma has been studied in detail it has been found that besides the main boundary fault and the main central thrust several transverse features are also very active some of these behave like steeply dipping fracture zones along the arakan yoma most of the seismicity appears to be due to subduction of the indian lithosphere to the east analysis of focal mechanism solutions for the himalaya shows that although thrust movements are predominant normal and strike slip faulting is taking place along some of the transverse features in addition to thrusting strike slip faulting is also taking place along the arakan yoma orientation of p axes for all thrust solutions show a sharp change from predominantly east west along the burmese arc to n s and ne sw along the himalaya the direction further changes to nw se along the baluchistan arc it appears that the indian lithosphere is under compression from practically all sides the present day seismicity of northeast india and northern burma can be explained in terms of a plate tectonics model after nandy 1976 no simple model appears to be applicable for the entire himalaya

causes of oxic anoxic changes in cretaceous marine environments and their implications for earth systems an introduction abstract this special issue originated from a session at the 33th international geological congress oslo norway in august 2008 causes of oxic anoxic changes in cretaceous marine and non marine environments and their implications for earth systems igcp 463 and 555 convened by chengshan wang michael wagreich and bradley sageman the session aimed at bringing together contributions that shed light on the anoxic to oxic changes in the world oceans during the cretaceous including discussion on consequences for today s global change

high resolution stable isotope records from the upper cretaceous rocks of italy and spain glacial episodes in a greenhouse planet high resolution 13c and 18o records from upper albian to lower santonian pelagic carbonates of the contessa quarry section in italy exhibit large positive oxygen isotopic excursions of 1 in the lower cenomanian and upper turonian coniacian strata within the uncertainties of biostratigraphic correlation these positive excursions appear to correspond to times of large sea level regressions in global sequence stratigraphic sea level curves several lines of evidence suggest that the major 18o excursions in contessa reflect episodes of global cooling and not differential diagenesis numerical models of oxygen isotope exchange during diagenesis show that a high contrast in the degree of alteration would be required to produce these signals as artifacts of diagenesis and lithological data provide no evidence for such large contrasts in the degree of alteration furthermore although precise correlation with a section in the south of spain is hampered by stratigraphic complexities the general sequence of major positive 18o excursions is reproduced it is unlikely that differential diagenesis would produce similar artifacts in multiple sites one explanation for the link between episodes of global cooling and sea level falls is that global cooling events led to polar ice sheet accumulation lowering sea level although ice free conditions have been inferred from evidence for a much warmer climate in late cretaceous time our results suggest that the relationship between continental high latitude ice sheets and overall climatic warmth warrants further examination

a euxinic southern north atlantic ocean during the cenomanian turonian oceanic anoxic event abstract during mid cretaceous times large amounts of organic carbon corg became sequestered in black shales possibly due to oceanic anoxic events characterized by the development of an extended oxygen minimum zone omz here we provide the first direct evidence for an open ocean omz in the cenomanian turonian c t southern north atlantic and in fact show that the base of the photic zone was euxinic as revealed by molecular fossils from photosynthetic green sulfur bacteria in c t black shales this together with evidence for bottom water anoxia and accumulation of redox sensitive trace metals and hydrogen rich organic matter indicates a continuously euxinic water column concurrent with the high corg accumulation rates which are 15 150 times greater in the southern than in the northern north atlantic and the low biological productivity this suggests that preservation controlled the accumulation of corg in c t black shales

organically bound iodine as a bottom water redox proxy preliminary validation and application abstract carbonate associated iodine i ca has been used as a proxy of local upper ocean redox conditions and has successfully demonstrated highly dynamic spatial and temporal patterns across different time scales of earth history to further explore the utility of iodine as a paleo environmental proxy we present here a new method of extracting organically bound iodine iorg from shale using volumes of samples on the order of tens of milligrams thus offering the potential for high resolution work across thin shale beds the ratio of iorg to total organic carbon i toc in modern surface and subsurface sediments decreases with decreasing bottom water oxygen which may be used to reconstruct paleo redox changes as a proof of concept we evaluate the i toc proxy in holocene sediments from the baltic sea landsort deep iodp 347 and discuss those data within a framework of additional independent redox proxies e g iron speciation and mo the results imply that i toc may be sensitive to hypoxic suboxic conditions complementary to proxies sensitive to more reducing anoxic euxinic conditions then we test the usage of i toc in sediments deposited during late cretaceous cenomanian turonian oceanic anoxic event oae 2 from 94 million years ago ma we generated i toc and iorg records from six oae 2 sections tarfaya morocco furlo central italy demerara rise western equatorial atlantic cape verde basin eastern equatorial atlantic south ferriby uk and kerguelen plateau southern indian ocean which provide a broad spatial coverage generally i toc decreases over the interval recorded by the positive carbon isotope excursion the global signature of oae 2 suggesting an expansion of more reducing bottom water conditions and consistent with independent constraints from iron speciation and redox sensitive trace metals e g mo relatively higher i toc values thus more oxic conditions are recorded at two high latitude sites for oae 2 supporting previous model simulations cgenie that indicated higher bottom water oxygen concentrations in these regions our results also indicate that organic rich and oxygenated seafloors are likely a major sink of iodine and correspondingly influence its global seawater inventory

intraplate response to the great 2004 sumatra andaman earthquake a study from the andaman segment abstract the 1300 km rupture of the 2004 interplate earthquake terminated at around 15 n in the northernmost segment of the andaman nicobar subduction zone this part of the plate boundary is noted for its generally lower level seismicity compared with the southern segments based on the global centroid moment tensor cmt and national earthquake information center neic data most of the earthquakes of m w 4 5 prior to 2004 were associated with the andaman spreading ridge asr and a few events were located within the forearc basin the 2004 event was followed by an upward migration of hypocenters along the subducting plate and the andaman segment experienced a surge of aftershock activity the continuing extensional faulting events including the most recent earthquake 10 august 2009 m w 7 5 in the northern end of the 2004 rupture suggest the reduction of compressional strain associated with the interplate event the style of faulting of the intraplate events before and after a great plate boundary earthquake reflects the relative influences of the plate driving forces here we discuss the pattern of earthquakes in the andaman segment before and after the 2004 event to appraise the spatial and temporal relation between large interplate thrust events and intraplate deformation this study suggests that faulting mechanisms in the outer ridge and outer rise regions could be indicative of the maturity of interplate seismic cycles

the evidence and implications of polar ice during the mesozoic abstract the mesozoic perhaps representing the longest period of warmth during phanerozoic earth history contains in general sparse and frequently equivocal evidence for polar ice although this time is undoubtedly punctuated by oscillations in climate whether sufficient to lead to polar cooling and the formation of polar ice an inviting mechanism to account for faunal and floral distribution patterns and large scale sea level change has been widely debated mesozoic evidence for glacial conditions includes abraded rock surfaces generally unsorted stone rich beds and the presence of dropstones rafted by ice within a finer grained host sediment faunal and floral evidence has also been utilised to determine the presence or absence of cold or sub freezing polar conditions together with more indirect evidence for glacial conditions derived from general circulation model gcm simulations of climate the analysis of clay mineral distributions glendonite abundance and palaeontological and sedimentological evidence for globally synchronous sea level change the extent of possible glacial environments during the mesozoic has been established by plotting published reports of glacial sediments on palaeogeographic reconstructions the general clustering of evidence at high palaeolatitudes suggests that the extent of polar ice during the mesozoic is likely to have been approximately one third the size of the present day based on such evidence a number of episodes of cold or sub freezing polar climates during the bajocian bathonian tithonian volgian valanginian and aptian are recognised evidence exists possibly also for a cold episode during the early jurassic pliensbachian although poorly constrained owing to limited biostratigraphical control the longevity of these events may be represented by a cold snap within a stage although undoubtedly the possible smearing of ages may have had the effect of lengthening and hence promoting the importance of these proposed events the climate regime of the earth during these times may be hypothesised to be characterised by a relatively steep pole to equator temperature gradient where low latitude regions are as warm or warmer than today the evidence to support such reasoning includes sharp increases in the areal distribution of both glendonites and deposits with affinities to glacial tillites and dropstones for these times coincidental falls of sea level arid events and an increased bipolarity of faunas at these times imparts further confidence that these events were of a magnitude to effect the earth as a whole

quartz latite rheoignimbrite flows of the etendeka formation north western namibia the etendeka formation of north western namibia consists of a sequence of interbedded quartz latites and tholeiitic basalts and forms part of the karoo igneous province in southern africa the age of the etendeka formation is approximately 130 135 ma the quartz latites make up a significant proportion of the stratigraphic succession 25 of the total stratigraphic thickness and form as much as 60 of the outcrop area in the southern etendeka apart from some systematic differences between pitchstones and devitrified quartz latite largely explained by alteration processes individual quartz latite units exhibit remarkably uniform compositions with no significant vertical or lateral variation geochemistry can be used as a primary criterion for the correlation of major quartz latite units over much of the southern etendeka area enabling the reconstruction of the etendeka formation stratigraphy in this region individual quartz latite units occur as voluminous 400 2600 km3 widespread up to 8800 km2 sheet like deposits typically between 40 and 300 m thick each unit consists of basal main and upper zones the main zone generally constitutes over 70 of the thickness of the unit and typically consists of texturally featureless devitrified quartz latite in contrast the basal and upper zones of the flow are characterised by flow banding pitchstone lenses and breccia with rare occurrences of pyroclastic textures the quartz latites are sparsely porphyritic 10 phenocrysts with glassy or devitrified groundmass textures the phenocrysts consist of plagioclase pyroxene titanomagnetite and rare ilmenite pyroxene geothermometry indicates high 1000 1100 c temperatures of crystallisation which coupled with the absence or primary hydrous phases indicates that the quartz latites were relatively hot h2o undersaturated magmas the quartz latites display features common to both rhyolite lavas and ignimbrites and are clearly the products of an unusual eruption style the local preservation of pyroclastic textures and the broad areal extent of these units lead to the conclusion that the quartz latites are high temperature rheomorphic ignimbrites i e rheoignimbrites a combination of high eruption temperature and relatively low viscosity helps to explain the often completely welded and homogeneous textures observed in most quartz latite outcrops in the etendeka area

anisotropy of magnetic susceptibility of silicic rocks from quarries in the vicinity of sao marcos rio grande do sul south brazil implications for emplacement mechanisms abstract the parana etendeka large igneous province includes acid volcanic rocks that can be found throughout its extension several aspects concerning those rocks remain controversial including their mechanism of emplacement and location of their eruptive sources opening of several quarries of dimension stone near the city of sao marcos rio grande do sul brazil offers a unique opportunity to study in detail the acid products here we present the results of a study of the anisotropy of magnetic susceptibility ams completed in some rocks that had been interpreted as the roots of volcanic conduits our results and reexamination of the textural features of the rocks lead to a reinterpretation that suggests that these rocks were emplaced subaerially and involved assimilation and remelting of clastic components of previous products due to the inferred conditions of emplacement it is unlikely that the eruptive vents are located far from the area of study therefore ruling out the long travelled nature of these products

an experiment in systematic study of global seismicity centroid moment tensor solutions for 201 moderate and large earthquakes of 1981 data from the global digital seismograph network were used to obtain centroid moment tensor solutions using the method of dziewonski et al 1981 results were obtained for 201 earthquakes ranging in seismic moment from 7 1023 to 3 1027 dyne cm the wide dynamic range of the sro asro stations allows us to investigate using the same algorithm series of events among which the smallest and the largest may differ in moment by a factor as large as 1000 among the events studied is a particularly interesting series in january 1981 off the island of honshu the main shock was preceded by three foreshocks and followed by four aftershocks all falling within the range of our analysis there is an indication that the source mechanism changes with the size of the event statistical analysis of the results for all earthquakes reveals that the half duration of shallow and intermediate events is approximated by 1 6 10 8 m01 3 the shifts in origin time for deep focus earthquakes are indicative of the general complexity of these events which depends only weakly on the seismic moment the extreme departures from a double couple mechanism seem to depend both on the moment and on the focal depth while for shallow earthquakes these departures decrease from a maximum at about 1025 dyne cm with increasing moment the reverse appears to be true with respect to deep focus earthquakes shallow earthquakes north of new guinea and along the solomon islands show systematic and substantial departures from the double couple mechanism

the middle cenomanian event in the equatorial atlantic the calcareous nannofossil and benthic foraminiferal response abstract in addition to oceanic anoxic event 2 oae2 other perturbations of the carbon cycle occurred during the cenomanian and turonian of which the middle cenomanian event mce is the most prominent one in palaeoecological publications however this event is strongly underrepresented in contrast to the well studied oae2 in order to fill this gap we have studied early cenomanian to late turonian calcareous nannofossil and benthic foraminiferal assemblages of ocean drilling program site 1260 at demerara rise western equatorial north atlantic in order to decipher biotic changes throughout this interval and especially across the mce our data show distinctive changes in the relative abundance of certain calcareous nannofossil taxa and a drastic decrease in benthic foraminiferal diversities and abundances associated with the mce in the lower part of the studied section and prior to the mce a mixed water column with high nutrient availability in the upper photic zone is suggested based on very high relative abundances of the mesotrophic eutrophic nannofossil species biscutum constans around the mce interval certain nannofossil taxa e g rhagodiscus asper show a distinctive decrease in relative abundance while others become more dominant taxa which increase in relative abundance after the mce e g eprolithus floralis are interpreted as either having favoured less eutrophic surface waters or having inhabited deeper parts of the photic zone in a well stratified water column this interpretation is supported by published oxygen isotope and tex86 data which suggest increased water column stratification starting with the mce and lasting to the end of the cenomanian stage as a result of the implementation of a saline intermediate to deep water mass during this interval our study shows that the mce was a significant biotic event in the cenomanian turonian equatorial atlantic characterised by a lasting change of surface and bottom water ecosystems

the anisotropy of magnetic susceptibility of lava flows an experimental approach measurements of the anisotropy of magnetic susceptibility ams of natural lavas have shown that ams varies with depth within a lava flow we have investigated the reasons for such variation by studying the effects of temperature and strain rate on the ams of recent lava in the laboratory samples of lava from kilauea were melted and subjected to a range of strain rate and cooling histories the results show that the degree of anisotropy is a function of both the thermal and shearing history of a sample high degrees of anisotropy were found only in samples that were deformed at temperatures close to those encountered during eruption and then rapidly quenched lavas subjected to similar shear stresses at high temperatures had low degrees of anisotropy if allowed to cool down slowly without further deformation additionally lava subjected to complex shearing yield a lower degree of anisotropy even when high strain rates were imposed on it these results lead to the conclusion that only the last phase of deformation is detectable using ams and that high strain rates will not result in high degrees of anisotropy if either deformation ends while lava is still fluid or if the orientation of the maximum shear stress varies with time the relation between the orientation of the principal susceptibilities and that of shear is less sensitive to variation on shear with time consequently flow directions can be inferred confidently with this type of measurements

the dynamics of magma mixing in a rising magma batch the conditions under which two magmas can become mixed within a rising magma batch are investigated by scaling analyses and fluid dynamical experiments the results of scaling analyses show that the fluid behaviours in a squeezed conduit are determined mainly by the dimensionless number i mu 1 u g delta rho r 2 where 1 is the viscosity of the fluid u is the velocity g is the acceleration due to gravity is the density difference between the two fluids and r is the radius of the tube the parameter i represents a balance between the viscous effects in the uppermost magma which prevent it from being moved off the conduit walls and the buoyancy forces which tend to keep the interface horizontal the experiments are carried out using fluid pairs of various density and viscosity contrasts in a squeezed vinyl tube they show that overturning of the initial density stratification and mixing occur when i order 10 1 the two fluids remain stratified when i order 10 3 transitional states are observed when 10 3 i 10 1 these results are nearly independent of reynolds number re 1 rho 1 ur mu 1 and viscosity ratio mu 1 mu 2 in the range of 10 2 mu 1 mu 2 10 3 and re 1 300 applying these results to magmas shows that silicic to intermediate magmas overlying mafic magma will be prone to mixing in a rising magma batch this mechanism can explain some occurrences of small volume mixed lava flows

correlation of early cretaceous carbon isotope stratigraphy and platform drowning events a possible link the early cretaceous carbonate carbon isotope record is marked by three positive high amplitude 1 5 excursions each covering time spans of more than 106 years they are of late valanginian hauterivian and early and late aptian age in a case study along a transect across the western tethys ocean we identified a coincidence between 13c excursions black shale formation and widespread carbonate platform drowning events we conclude that the 13c excursions reflect a change in partitioning of carbon between the organic and carbonate carbon sinks which was triggered by climate induced ecological changes in cretaceous pelagic and neritic environments episodes of intensified greenhouse climate conditions led to an increase in weathering erosion and runoff rates and to elevated nutrient transfer rates from continents into oceans the resulting increase in oceanic nutrient levels favoured marine phytoplankton production and black shale deposition while conditions for carbonate producing biotas became unfavourable partial choking of carbonate production along river influenced coasts resulted in widespread carbonate platform drowning during times of sea level rise in the valanginian and aptian widespread contemporaneous black shale deposits and drowned carbonate platforms therefore reflect the contrasting response of the marine organic and carbonate carbon pumps to nutrient enhanced phytoplankton productivity the change in marine carbon partitioning is mirrored in a shift of the 13c record towards more positive values the transition to the 13c peak values lasted up to several hundred thousand years the peaks of the excursions also covering a time span of up to several hundred thousand years reflect a new stabilisation of the carbon partitioning between carbonate and organic carbon sinks a renewed intensification of carbonate sedimentation under mesotrophic conditions was facilitated by stabilisation of the sea level rise at a high level decreasing 13c values record increasing carbonate carbon burial rates at constant or decreasing organic carbon accumulation rates these changes contributed to the stabilisation of the marine carbon budget and the global carbon cycle up to millions of years after its initial perturbation

late cretaceous carbon isotope stratigraphy in europe correlation and relations with sea level and sediment stability abstract upper cretaceous upper cenomanian to lower coniacian stable carbon isotope stratigraphy of german hemipelagic marls borehole dresden blasewitz pelagic carbonates quarries salzgitter salder and sohlde and of two turonian to santonian pelagic sections in the northern alpine helvetic zone supplemented by published carbon isotope data kent southern england gubbio italy are used for correlations of northern temperate boreal and tethyan sections general carbon isotopic trends are different in pelagic and hemipelagic carbonates probably in response to the input of terrestrial organic carbon to the inner shelf carbon reservoir including sediments the global component in the carbon isotope stratigraphy is best recorded in pelagic carbonates sufficient biostratigraphic control is present to correlate all sections across facies boundaries and between the two biogeographic provinces hiatuses produce breaks in the gradual carbon isotopic trends and their duration can be estimated relative to complete sections a broad 13c minimum straddles the turonian coniacian boundary at the proposed boundary stratotype salzgitter salder with its center about 0 5 1 m below the biostratigraphic reference level first occurrence of c rotundatus fiege sensu troger non fiege increases and maxima of pelagic 13c values occur during phases of sediment accumulation decreasing pelagic 13c values and minima characterise phases of sediment erosion the amplitudes of these stratigraphic fluctuations may indicate the intensity of sediment reworking changes in the sediment accumulation erosion ratio and accompanied carbon isotopic variation may be related to short term sea level fluctuations and their effect on fine grained sediment stability

transcurrent movements in the burma andaman sea region the drag produced by the northward movement of the indian plate has caused the indoburman ranges an accretionary prism to decouple from the burma plate along the kabaw fault and the burma plate itself has decoupled from the eastern highlands of burma along the sagaing fault the indoburman ranges have probably been dragged north slightly ahead of the burma plate although at times the two plates may have locked together to move as one unit it is suggested that the double arc formed by the indoburman ranges and the andaman nicobar sumatra islands was formerly one large single arc northward translation of this larger single arcuate structure has caused the tail end of the arc i e the andaman nicobar sumatra section to rotate into the burma plate causing a bending of the arc to form the present double arc and compression within the burma plate to form the pegu yoma folding northward movement of the burma plate has also resulted in offset by over 400 km of the irrawaddy river from its former connection through the channel of what is now the chindwin river

the former presence of organic matter caused its later absence burn down of organic matter in oceanic red beds enhanced by bioturbation eocene variegated shale carpathians eocene oceanic red beds that formed in a well oxygenated setting at low sedimentation rates below the calcite compensation depth are effectively barren of organic carbon in the present state recurrent distal low erosive turbidites preserve the bioturbated zone underneath that documents seasonal and long term fluctuating accumulation of considerable amounts of organic matter on the sea floor as evidenced by scolicia the producers of this trace fossil exploited nutritious organic matter conserved in turbidite buried sea floor deposits over the long term slow average sedimentation of hemi pelagic oxic red mud led to long oxygen exposure times and low burial of organic matter consequently trace fossils representing persistent sediment feeding modes are of small size although the food limited setting appears appropriate for producers of graphoglyptids such stationary burrows have not been encountered because seasonal deposition of organic matter fostered at least temporary surface layer feeding organisms for instance producers of nereites irregularis that intensively reworked the sediment and hence hindered graphoglyptid production these findings confirm palaeoceanographic modelling results that suggest upwelling in the study area during the eocene this article is protected by copyright all rights reserved

effusive silicic volcanism in the parana magmatic province south brazil evidence for locally fed lava flows and domes from detailed field work abstract lava flows and dome complexes of silicic composition were identified in the lower cretaceous parana magmatic province pmp at rio grande do sul state southern brazil detailed mapping and image analysis reveals significant volumes of effusive deposits aligned according to main lineaments likely representing the fissural systems that fed the three palmas type silicic units different structures indicative of effusive emplacement lava domes lobated flows sheet flows and autobreccias are very common in the study area and are probably also more abundant than previously thought in whole pmp silicic magmatism in fact effusive deposits seem predominant in the three distinct silicic units identified in the area since no remnants of pyroclastic components have been identified the vitreous dacites that make up the upper flows of the basaltic andesite to dacite barros cassal sequence are clearly effusive as indicated by their occurrence as thin sheet flows the much thicker early caxias do sul dacites occur mostly as lava flow lobes and pancake like of low to moderate viscosity and lava domes the younger high sio2 santa maria rhyolite unit shows unequivocal examples of effusive deposits at its lower portion as lobated flows formed by vesicle rich obsidian in spite of higher viscosities relative to the previous units 106 pa s it is probable that the very low h2o contents 1 wt of these rhyolite melts associated with high discharge rates resulted in an effusive nature in most to this unit

sampling strategies and the anisotropy of magnetic susceptibility of dykes abstract most studies of the anisotropy of magnetic susceptibility ams of dykes have assumed that susceptibility axes should define a unique orientation relative to the dyke walls and magma flow direction theoretical considerations however predict systematic variations of ams as a function of the amount of shear experienced by the flowing magma although this feature of ams might seem undesirable at first sight based in a theoretical model it has been suggested that actually we can take advantage of such variation in the orientation of the ams axes to infer magma flow directions more confidently than until now if proper attention is given to the scheme followed during sample collection in this work we report the results of a pilot study made to test those theoretical predictions we resampled two dykes from the ko olau dyke complex in o ahu and for which magma flow direction was previously inferred albeit with some uncertainty the resampling was made following the guidelines suggested by the theoretical work as closely as possible i e collecting more than one profile parallel to the dyke walls and across the dyke the results of our measurements provide the first empirical evidence supporting the occurrence of a cyclic fabric acquisition during the emplacement of at least some dykes the procedure described in this paper therefore constitutes a self contained approach that can be followed to test in a case by case basis the occurrence of such fabric behavior this in turn should provide enough elements to increase the reliability of the interpretation in the obtained results

stress in the indo australian plate cloetingh s and wortel r 1986 stress in the ind austr ian plate in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 49 67 we modelled the state of stress in the indo australian plate in order to investigate quantitatively variations observed in tectonic style the numerical procedure incorporates the dependence of slab pull and ridge push on the age of the oceanic lithosphere estimates are presented for the average net resistive forces at the himalayan collision zone the suction force acting on the overriding indo australian plate segment at the tonga kermadec trench and the drag at the base of the lithosphere our modelhng shows a concen ation of compressive stresses of the order of 3 5 lcbar in the ninetyeast ridge area the effects of the compressive resistance associated with himalayan collision and subduction of young lithosphere off the northern part of the sunda arc are focused in this region the stress field as calculated gives a consistent explanation for the observed concentration of seismic activity stein and okal 1978 and significant deformation in the oceanic crust weissel et al 1980 mcadoo and sandwell 1985 in the area the calculated stress field in the area adjacent to the southeast and central india ridges is characterized by tension parallel to the spreading axis this explains the concentration of near ridge normal faulting seismicity with t axes subparaile1 to the spreading ridge in the indian ocean as recently observed by bergman et al 1984 and wiens and stein 1984 the regional stress field along the strike of the sunda arc varies from compression seaward of and parallel to the sumatra trench segment to tension perpendicular to the java flores segment this explains the selective occurrence of well developed grabens seaward off the java flores segment of the trench observed by hilde 1983 our modelling shows that the observed rotation of the stress field denham et al 1979 in the australian continent is mainly the consequence of its geographic position relative to the surrounding trench segments and the variations of the forces acting on the down going slab in each of these the state of compression in west and central australia is induced by the action of resistive forces at the himalayan and banda arc collision zones the joint occurrence of high levels of compression in the plate s interior and normal faulting seismicity in the near ridge areas is a transient feature unique to the present dynamic situation of the indo australian plate

tectonic segmentation of the burmese indonesian arc abstract several lines of evidences which include change in trend and offset in arcs bathymetry and sedimentation faulting in the region change in composition and trend of the line of volcanoes spatial distribution of earthquakes and change in dip of the benioff zone suggest a tectonic segmentation of the burmese indonesian arc two transverse boundary zones viz the north andaman boundary zone and the sunda boundary zone separating segments of the arc are identified larger than usual thickness of the oceanic crust beneath the ninetyeast ridge reduces the average density of the lithosphere carrying the ridge the buoyancy effect caused by the reduced average density inhibits the subduction of an approximately triangular shaped lithosphere between the burmese indonesian arc and the ninetyeast ridge which in turn causes tectonic segmentation along the arc the model predicts and some available focal mechanism solutions confirm left lateral northeast faulting within the sunda boundary zone

peperite formed by lava flows over sediments an example from the central parana continental flood basalts brazil pahoehoe flows interbedded with sediments have been identified in the superior portion of parana continental flood basalts pcfb west portion of parana state southern brazil in the study area peperites are generated by the interaction between lava flows and wet lacustrine sediments silt and clay evidence that the sediments were unconsolidated or poorly consolidated and wet when the lava flowed over them includes vesiculated sediment sediment in vesicles and fractures in lava flow and in juvenile clasts in the peperite and soft sediment deformation hydrodynamic mingling of lava and wet sediments coarse mingling is predominant and volcanic rocks and textures related to explosive phase of molten fuel coolant interaction mfci are not observed in study area locally centimeter sized areas display direct contact between ash sized juvenile clasts and sediments formed by the collapse of a vapor film the textures of fluidal peperites in the central pcfb indicate that the relevant factors that led to a coarse mingling between lava sediment are 1 lava properties low viscosity 2 fine grained unconsolidated or poorly consolidated wet sediment and 3 a single episode of interaction between lava flows and sediment

potential links between ocean plateau volcanism and global ocean anoxia at the cenomanian turonian boundary the cenomanian turonian boundary is marked globally by the extinction of marine invertebrates and an increase in the accumulation and preservation of organic carbon rich sediments black shales the timing of this boundary also coincides with large scale volcanism from at least three large igneous provinces the caribbean and ontong java oceanic plateaus and the madagascar flood basalts in this paper we assess the possibility that hydrothermalism associated with large scale submarine magmatism was responsible for the reduction of dissolved o 2 in the oceans we investigate two potential mechanisms the oxidation of reduced material in hydrothermal effluents and the stimulation of primary productivity in the water column due to the injection of hydrothermal fe into surface waters with the first mechanism we find that a 10 000 km 3 submarine basalt eruption three orders of magnitude larger than recent midocean ridge eruptions could release enough reduced hydrothermal material to consume at least 6 percent of the dissolved o 2 in seawater in a well ventilated ocean such as the present ocean with the second mechanism we calculate that even a small percentage of the hydrothermal fe released from a single large flow could have led to a significant increase in primary productivity in areas where fe was the biolimiting nutrient such as open ocean areas in the pacific the potential impact of both of these o 2 reducing mechanisms at the cenomanian turonian boundary would have been significantly greater if cretaceous oceans were less well ventilated than the present oceans

enhancement of magma mixing efficiency by chaotic dynamics an experimental study magma mixing is common in the earth understanding the dynamics of the mixing process is necessary for dealing with the likely consequences of mixing events in the petrogenesis of igneous rocks and the physics of volcanic eruptive triggers here a new apparatus has been developed in order to perform chaotic mixing experiments in systems of melts with high viscosity contrast the apparatus consists of an outer and an inner cylinder which can be independently rotated at finite strains to generate chaotic streamlines the two cylinder axes are offset experiments have been performed for ca 2 h at 1 400 c under laminar fluid dynamic conditions re 10 7 two end member silicate melt compositions were synthesized 1 a peralkaline haplogranite and 2 a haplobasalt the viscosity ratio between these two melts was of the order of 103 optical analysis of post experimental samples reveals a complex pattern of mingled filaments forming a scale invariant i e fractal distribution down to the m scale as commonly observed in natural samples this is due to the development in space and time of stretching and folding of the two melts chemical analysis shows strong non linear correlations in inter elemental plots the original end member compositions have nearly entirely disappeared from the filaments the generation of thin layers of widely compositionally contrasting interfaces strongly enhances chemical diffusion producing a remarkable modulation of compositional fields over a short length scale notably diffusive fractionation generates highly heterogeneous pockets of melt in which depletion or enrichment of chemical elements occur depending on their potential to spread via chemical diffusion within the magma mixing system results presented in this work offer new insights into the complexity of processes expected to be operating during magma mixing and may have important petrological implications in particular 1 it is shown that in contrast with current thinking rheologically contrasting magmas can mix i e with large proportions of felsic magmas and high viscosity ratios thus extending significantly the spectrum of geological conditions under which magma mixing processes can occur efficiently 2 the mixing process cannot be modeled using the classical linear two end member mixing model and 3 the chemical compositions on short length scales represent snapshots within the process of mixing and therefore may not reflect the final composition of the magmatic system this study implies that microanalysis on short length scales may provide misleading information on the parental composition of magmas

an ams study of different silicic units from the southern parana etendeka magmatic province in brazil implications for the identification of flow directions and local sources abstract in the southern portion of the parana etendeka magmatic province in brazil extensive silicic dacite rhyolite deposits occur at the top of a sequence of low ti pahoehoe to rubbly basalts the internal architecture of the silicic deposits and their eruptive style as well as the location of their sources are still unsatisfactorily known in an attempt to provide independent evidence for flow directions in deposits previously characterized as effusive and test the hypothesis of local sources we carried out anisotropy of magnetic susceptibility ams studies on the two main silicic units caxias do sul dacites and santa maria rhyolites with the best exposures in an area previously mapped in detail magnetic anisotropies were determined on oriented cylindrical specimens from a total of 28 sites rock magnetism properties indicate that pseudo single domain magnetite carries the fabrics and the remanence magnetic fabrics were determined by applying anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanent magnetization aarm both ams and aarm tensors are coaxial indicating that the ams fabric is not affected by the effect of magnetite single domain grains magnetic data from several dacitic coulees caxias do sul unit indicate flows from se to nw the location and spatial distribution of these lavas support the hypothesis of local sources aligned along a ne sw trend these data are in agreement with the alignments of structures dome shaped hills observed in field work and dem images on the other hand magnetic data obtained in santa maria rhyolites indicate that flow directions in two different areas are distinct towards nw ne and w suggesting that they derived from different emission centers so regarding the silicic volcanism in the studied region our data do not support the model which classifies the entire silicic volcanism of the province as extensive rheomorphic pyroclastic deposits released from a central conduit in contrast we propose the occurrence of local volcanic events implying in the existence of different sources possibly characterized by local emission centers

magmatism and continental break up in the south atlantic high precision40ar 39ar geochronology abstract a detailed ar ar study of the parana etendeka continental flood basalts cfb has been undertaken using laser spot analysis data provide information not only on the age of the samples but also on the variability of the non radiogenic ar component and state of alteration the results indicate the parana etendeka cfb were erupted over 10 million years between 137 and 127 ma providing a minimum overall eruption rate of 0 1 km3 yr 1 this is an order of magnitude less than that previously proposed for this the deccan and siberian cfb provinces but it is similar to estimates for hawaii iceland and the columbia river cfb in detail the new ar ar analyses indicate that magmatism within the parana etendeka province migrated from nw to se prior to and during the opening of the south atlantic providing an explanation for the asymmetry of the cfb lavas about the south atlantic moreover chemically defined magma types were erupted at different times in different places and so within the paranathey may not be used as reliable chronostratigraphic units rather such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle and indicate that partial melting took place over a wide area under the paranabasin the nw se migration of the onset of magmatism might be interpreted as a plume trace but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the rio grande rise it is argued that the onset of magmatism reflected extensional tectonics normal to the ponta grossa dyke swarm given that magmatism preceded the main phase of rifting and that extension across the ponta grossa was moderate current plume models have difficulty in predicting sufficient melting within the asthenosphere

structure and origin by injection of lava under surface crust of tumuli lava rises lava rise pits and lava inflation clefts in hawaii tumuli are positive topographic features that are common on hawaiian pahoehoe lava flow fields particularly on shallow slopes and 75 measured examples are presented here to document the size range tumuli form by up tilting of crustal plates without any crustal shortening and are thus distinguished from pressure ridges which are up buckled by laterally directed pressure the axial or star like systems of deep clefts that characterize tumuli are defined here as lava inflation clefts their tips advanced into red hot lava and they widened as uplift proceeded and while the lava crust was thickening flat surfaced uplifts formed like tumuli by injection of lava under a surface crust were previously called pressure plateaus but lava rise is proposed instead the pits that abound among lava rises previously attributed to collapse or subsidence are generally formed because the lava around them rose and the name lava rise pit is proposed unique examples of tumuli and lava rises from which lava drained out under a surface crust 1 5 to 2 5 m thick are described from kilauea caldera these examples show that in tumuli and lava rises the crust floats on considerable bodies of fluid lava and is able to do so because of its higher vesicle content the fluid lava loses many of its gas bubbles during residence beneath the crust the bulk densities of samples from tumuli show a general downward increase the form of the density profile is consistent with the relationship that for any given crustal thickness the density of fluid lava closely matched the average density of that crust suggesting that the lava was stably density stratified it is inferred that stable stratification was regulated by out flows of the more vesicular lava fractions loss of bubbles through the lava inflation clefts and entry of injected lava at its level of neutral buoyancy below the uppermost meter the downward decrease in vesicularity closely conforms with that expected by compression of a uniform mass of gas per unit mass of lava

the cenomanian turonian oceanic anoxic event i stratigraphy and distribution of organic carbon rich beds and the marine δ13c excursion marine strata deposited during late cenomanian and early turonian time display lithological faunal and geochemical characteristics which indicate that significant parts of the world ocean were periodically oxygen deficient at or very close to the cenomanian turonian boundary between 90 5 and 91 5ma ago oxygen deficiencies were particularly marked over a period of 1ma this short lived episode of oceanic oxygen deficiency has been termed the cenomanian turonian oceanic anoxic event oae the widespread distribution of anoxic sediments deposited synchronously during such a short lived event indicates that such sediments are not simply the product of coincidental local climatic or basinal water mass characteristics but are the result of a global expansion and intensification of the cenomanian turonian oxygen minimum zone related to feedback between sea level rise and regional palaeoceanography the palaeoceanography of the cenomanian turonian oae is discussed in detail in a companion paper by arthur et al 1987 from authors

mingling in mafic magma chambers replenished by light felsic inputs fluid dynamical experiments abstract we investigate by means of laboratory analogues the fluid dynamics of magma hybridisation caused by the intrusion of felsic magma into a mafic magma chamber the input felsic magma is modelled by a cold low density fluid and the resident mafic magma by a layer of fluid wax close to its solidification point the cold rising input drives convection and solidification in the resident fluid and the solidification causes significant perturbations to an otherwise simple input flow greatly enhancing mixing and mingling between the two model magmas the dimensionless parameters controlling the evolution of the system are determined and different morphologies of the solid mass fit into clearly defined fields in the parameter space the most important implication of the experiments is that the freezing of mafic chambers due to felsic intrusion may give rise to intense hybridization of large volumes of magma this potentially important mechanism for hybridization has to date received little attention in the literature

seismo tectonics of the andaman nicobar islands abstract the epicentres of 345 earthquakes that occurred between january 1967 and december 1982 in a region bounded by 4 n to 16 n latitudes and 90 e to 98 e longitudes were plotted in order to study the seismicity of this region most of the earthquakes in this region are shallow focus the greatest focal depth being about 250 km in the northern part of this region north of 10 5 n tne earthquake foci are aligned along two branches which strike northeastwards no intermediate depth earthquakes are associated with the eastern branch south of 10 5 n the foci are located along a zone striking north northwest focal mechanism solutions are reported for 24 earthquakes that occurred in this region during this period the first motions of p and pkp phases were used to determine the mechanism solutions five of these events occurred in northern sumatra and the rest occurred in the andaman nicobar islands the mechanism solutions of the shallow focus earthquakes in northern sumatra are characterized by a predominant component of thrust and strike slip faulting the inferred orientation of slip vectors deduced from mechanisms of shallow focus events is nearly northeast mechanism solutions of 13 shallow focus events that occurred in the andaman nicobar islands are characterized by thrust normal and strike slip faulting the orientation of slip vectors deduced from thrusting mechanisms is consistent with the underthrusting of the indian ocean under the andaman nicobar islands in a northeasterly direction normal faulting mechanism solutions of these islands suggest that they are tectonically similar to island arcs the results reported in this study show for the first time using mechanism solutions the existence of underthrusting along the andaman nicobar islands for which there is ample geological evidence

comprehensive probabilistic seismic hazard analysis of the andaman nicobar regions an updated catalog of earthquakes has been prepared for the andaman nicobar and adjoining regions the catalog was homogenized to a unified magnitude scale and declustering of the catalog was performed to remove aftershocks and foreshocks eleven regional source zones were identified in the study area to account for local variability in seismicity characteristics the seismicity parameters were estimated for each of these source zones and the seismic hazard evaluation of the andaman nicobar region has been performed using different source models and attenuation relations probabilistic seismic hazard analysis has been performed with currently available data and their best possible scientific interpretation using an appropriate instrument such as the logic tree to explicitly account for epistemic uncertainty by considering alternative models source models maximum magnitude and attenuation relationships the hazard maps for different periods have been produced for horizontal ground motion on the bedrock level

seismicity and tectonic stress field of a part of the burma andaman nicobar arc abstract the nature of instrumentally recorded seismicity for a part of the burma andaman nicobar arc bounded by latitudes 2 and 24 n and longitudes 91 and 99 e was investigated based on the spatial distribution of the earthquakes and their focal mechanism solutions four tectonic units were identified from the south going northward these are the sumatra trench region characterized by strike slip and thrust faulting the andaman spreading ridge characterized by normal and a few strike slip faults the andaman nicobar ranges where faulting is mostly thrust type and the indo burman ranges region dominated by thrust faulting the stress orientations in these segments and their changes with depth were studied in relation to the tectonics of the region the direction of maximum compression in the burma andaman nicobar region is ne sw to n s compatible with the postulated motion of the indian plate changes in stress orientations are attributable to local tectonics