volcano ice sea interaction in the cerro santa marta area northwest james ross island antarctic peninsula abstract we present here the results of detailed mapping lithofacies analysis and stratigraphy of the neogene james ross island volcanic group antarctic peninsula in the cerro santa marta area northwest of james ross island in order to give constraints on the evolution of a glaciated volcanic island our field results included recognition and interpretation of seventeen volcanic and glacial lithofacies together with their vertical and lateral arrangements supported by four new unspiked k ar ages this allowed us to conclude that the construction of the volcanic pile in this area took place during two main eruptive stages eruptive stages 1 and 2 separated from the cretaceous bedrock and from each other by two major glacial unconformities u1 and u2 the u1 unconformity is related to antarctic peninsula ice sheet expansion during the late miocene before 6 2 ma and deposition of glacial lithofacies in a glaciomarine setting following this glacial advance eruptive stage 1 6 2 4 6 ma volcanism started with subaerial extrusion of lava flows from an unrecognized vent north of the study area with eruptions later fed from vent s centered at cerro santa marta volcano where cinder cone deposits and a volcanic conduit lava lake are preserved these lava flows fed an extensive 7 km long hyaloclastite delta system that was probably emplaced in a shallow marine environment a second unconformity u2 was related to expansion of a local ice cap centered on james ross island which truncated all the eruptive units of eruptive stage 1 concomitant with glacier advance renewed volcanic activity eruptive stage 2 started after 4 6 ma and volcanic products were fed again by cerro santa marta vents we infer that glaciovolcanic eruptions occurred under a moderately thin 300 m glacier in good agreement with previous estimates of paleo ice thickness for the james ross island area during the pliocene

an early lutetian carbon cycle perturbation insights from the gorrondatxe section western pyrenees bay of biscay turbidites and kaolinite a 3 decline in the bulk d 13 c record a 1 decline in benthic foraminiferal d 13 c followed by a gradual recovery a distinct deterioration in foraminiferal preservation high proportions of warm water planktic foraminifera and opportunistic benthic foraminifera and reduced trace fossil and benthic foraminiferal diversity thus recording a significant environmental perturbation the onset of the perturbation correlates with the c21r h6 event recently defined in the atlantic and pacific oceans which caused a 2c warming of the seafloor and increased carbonate dissolution the perturbation was likely caused by the input of 13 c depleted carbon into the ocean atmosphere system thus presenting many of the hallmarks of paleogene hyperthermal deposits however from the available data it is not possible to conclusively state that the event was associated with extreme global warming based on our analysis the perturbation lasted 226 kyr from 47 44 to 47 214 ma and although this duration suggests that the triggering mechanism may have been similar to that of the paleocene eocene thermal maximum petm the magnitude of the carbon input and the subsequent environmental perturbation during the early lutetian event were not as severe as in the petm

airborne gravity reveals interior of antarctic volcano abstract understanding antarctic volcanoes is important as they provide a window on magmatic and tectonic processes of the antarctic plate and contain datable records of ice sheet changes we present the results from the first detailed airborne radar and gravity surveys across james ross island northern antarctic peninsula which is dominated by mt haddington an ice covered miocene recent alkaline stratovolcano the surveys provide new insights into the subsurface structure of the volcano and hence its development which are unavailable from the surface geology alone we show that mt haddington is associated with a significant negative bouguer gravity anomaly 26 mgal which suggests that there has not been significant pooling and solidification of a dense shallow level mafic magma chamber during the growth of the volcano over at least the past 6 m y which is consistent with independent geochemical evidence simple flexural isostatic models cannot explain the localised negative bouguer anomaly 3d modelling techniques show that the negative anomaly is best explained by a shallow low density intra crustal body with its top close to or at the surface although comparable gravity anomalies are commonly associated with large 20 km ash filled calderas as seen at yellowstone or toba there is no geological evidence on james ross island for a similar structure we therefore propose that the james ross island volcanic edifice subsided into the thick underlying pile of relatively soft jurassic and cretaceous sediments which were displaced by low density hyaloclastite breccia the type of deformation envisaged is similar to that associated with concepciou or iwaki volcanoes in south america although mt haddington is much larger

origin of minor limestone shale cycles climatically induced or diagenetic the opinion is widely held that each limestone bed in minor limestone shale cyclic sequences however modified subsequently by diagenesis reflects an environmental event on the assumption that this is true attempts have recently been made to relate limestone shale cycles to climatic variations affected by milankovitch orbital cycles a test is proposed and illustrated mainly with reference to the blue lias formation of dorset england that appears to indicate that many limestones can be produced solely by rhythmic unmixing of caco 3 during diagenesis unless such limestones can be clearly distinguished from those that record genuine environmental signals orbital cycle analysis based on such sequences will give meaningless results

melt generation during rifting in the north sea in a forthcoming paper mckenzie and o nions1 invert rare earth element concentrations in mid ocean ridge basalts to calculate the variation of melt fraction with depth in the asthenospheric mantle beneath ocean ridges they find that melting persists to greater depths about 80 km than would be predicted by parameterizations of melting experiments2 about 50 km this result has important implications for predicting the volume and composition of melt produced from asthenosphere of normal potential temperature during continental rifting the north sea rift is a good location to test models for melt generation because the physical conditions of rifting are well constrained3 8 using the distribution of melt fraction with depth derived in ref 1 we calculate the volume and composition of melt produced from the asthenosphere in the most stretched part of the north sea the forties region when this melt is mixed in approximately equal proportions with an alkaline melt similar to that produced from metasomatized lithosphere by very small amounts of stretching on the rift flanks it accounts for the volume and the elemental and isotopic composition of the basalts in the most extended region

geochemical and sr nd isotopic constraints on the mantle source of neoproterozoic mafic dikes of the rifted eastern laurentian margin north central appalachians usa abstract abundant and widely distributed unmetamorphosed mafic dikes intrude mesoproterozoic rocks of the new jersey highlands the age of the dikes is imprecisely known but interpreted to fall between 615 and 576 ma which is consistent with the range of ages of mafic dikes from labrador and nova scotia south to pennsylvania that were emplaced along the rifted eastern laurentian margin new jersey highlands dikes are a few cm to 18 m wide and have lengths of as much as several km they have sharp largely discordant contacts against enclosing mesoproterozoic rocks and aphanitic chilled margins that grade into coarser grained interiors columnar joints are present locally and suggest emplacement at a shallow crustal level geochemical compositions of the dikes range from alkalic to less common tholeiitic basalt having generally high tio2 p2o5 zr nb y and la yb and low mgo cr and ni tio2 contents define high ti and low ti dikes that differ in high field strength elements hfse and light rare earth elements lree but overlap in abundances of most other elements dike magma evolved in an ascending mantle plume of oib like asthenosphere from enriched higher tio2 compositions to more depleted lower tio2 compositions subtle differences in the dike compositions are due to variations in the amount of partial melting within the plume and the depth of melt segregation sr nd isotope values of both dike compositions overlap and are characterized by end t of 1 5 to 3 8 and initial 87sr 86sr ranging from 0 7032 to 0 7077 higher sr isotope ratios are interpreted as resulting from local interaction of the dike magma with heterogeneous high 87sr 86sr lithospheric mantle having emi or emii like geochemical characteristics dikes form tabular structures that have long segments striking an average of n44 e and short segments striking about east west their regional geometries form right stepping rhomb shaped patterns due to emplacement into rift related dilational fractures likely formed through a combination of southeast directed extension and strike slip shear stresses geochemical compositions of the dikes are the same regardless of their structural trend or location implying they formed during a single magmatic event they along with other mafic dikes in the north central appalachians were emplaced in a within plate tectonic setting along the rifted margin of eastern laurentia prior to opening of the iapetus ocean

chemical weathering in antarctica an example of igneous rock particles in big lachman lake sediments james ross island jezerni sedimenty velkeho lachmanova jezera ostrov jamese rosse antarktida dominuji jilovite a prachovite castice r zneho petrografickeho slo eni ty jsou sopecneho p vodu a jsou odvozeni p eva n z mendelova souvrstvi intenzita chemickeho zv travani vulkanick ch castic v sedimentech byla vyjad ena chemick m indexem alterace cia kter popisuje p em nu ivc na jilove mineraly pozorovane mineralni asociace montmorillonitu illitu a kaolinitu pravd podobn pochazela z p em ny sopecneho skla a ivc na jilove mineraly mira zv travani vyjad ena pomoci cia z lakustrialnich sediment je podobna jako u spodnojursk ch na ti bohatach vulkanick ch hornin souvrstvi mount poster a st ednojursk ch na ti bohat ch vulkanick ch hornin ze souvrstvi mapple z antarktickeho poloostrova nami studovane sedimenty ukazuji dobrou shodu se slo enim st ednojursk ch ryolit ze souvrstvi mapple a naopak se castecn lisi od slo eni kenozoick ch bazalt vulkanicke skupiny ostrova jamese rosse zdrojov material z jezernich sediment pochazi z antarktickeho poloostrova a byl pravd podobn transportovan p es kanal prince gustava mal mi ledovci v pr b hu neogenu a kvarteru

covarying sedimentary and biotic fluctuations in lower middle eocene pyrenean deep sea deposits palaeoenvironmental implications abstract analyses of expanded 1975 to 3330 m thick deep marine ypresian lutetian successions from the pamplona and basque basins w pyrenees demonstrate a recurring pattern of coeval sedimentary and biotic changes intervals rich in resedimented deposits are typified by high latitude planktonic foraminiferal indices whereas intervals devoid of large scale resedimented deposits megabreccias are characterized by low latitude planktonic foraminiferal indices data from the megabreccia free intervals record a reasonably good correlation between the vertical trend of planktonic foraminiferal indices from the two pyrenean basins and the global curve of oxygen isotopes from deep sea records for instance the early eocene climatic optimum and the long lasting late ypresian lutetian cooling are respectively recorded in the studied successions by an increase and by a progressive decline in abundance in low latitude planktonic foraminiferal indices this correlation may support the assumption that changes of planktonic foraminiferal indices in the megabreccia free intervals were forced by global climatic changes the dominance of high latitude planktonic foraminiferal indices in the megabreccia bearing intervals is more difficult to interpret such dominance might record global cooling episodes that resulted in eustatic lowstand resedimentation events however the climatic cooling and sea level fall do not fully explain the characteristics of these intervals a reasonable scenario is that tectonically induced resedimentation processes resuspended large volumes of mud and organic matter which led to the eutrophication of seawater such eutrophication would preferentially affect low latitude oligotrophic planktonic foraminiferal species and lead to a relative or absolute increase in the cosmopolitan opportunistic species that typify high latitude groups

cretaceous alkaline intra plate magmatism in the ecuadorian oriente basin geochemical geochronological and tectonic evidence abstract small volumes of cretaceous alkaline basaltic magmas have been identified in the sedimentary infill of the ecuadorian oriente foreland basin they are characterized by a restricted range of compositional variation low lile hfse ratios and sr nd isotope values within the range of oceanic island basalts oib reflection seismic data show that a pre existing nne ssw triassic and jurassic rift controls the location and occurrence of these alkaline eruptive sites radiometric ages 40ar 39ar incremental heating method and the biostratigraphic record of their surrounding sediments indicate a nne ssw systematic age variation for the emplacement of this alkaline volcanism from albian 110 5 2 ma in the northern part of the oriente basin to campanian 82 2 2 0 ma in the west central part the geochemical geochronological and tectonic evidences suggest that asthenospheric mantle has upwelled and migrated to the ssw into the region underlying the pre existing triassic and jurassic rift thin spot we propose that subduction was abandoned subsequent to the accretion of allochthonous terranes onto the ecuadorian and colombian margin in the latest jurassic earliest cretaceous causing the relict slab material corresponding to the eastwards directed leading plate to roll back unmodified asthenospheric mantle migrated into the region previously occupied by the slab this resulted in partial melting and the release of magmatic material to the surface in the northern part of the oriente basin since at least aptian times then magmatism migrated along the ssw trending central wrench corridor of the oriente basin during the upper cretaceous probably as a consequence of the lateral propagation of the transpressive inversion of the triassic jurassic rift eventually the late cretaceous east dipping andean subduction system was renewed farther west and the development of the compressional retro foreland oriente basin system halted the cretaceous alkaline magmatic activity

effects of ridge crest trench interaction on antarctic phoenix spreading forces on a young subducting plate precise measurements of spreading rates on marine magnetic profiles collected to the west of the antarctic peninsula have enabled some consideration of the forces governing plate motion since antarctic phoenix motion has been controlled by the local rather than global force balance over the last 35 m y the total effective driving force per unit length of trench is calculated to have ranged between 2 6 and 3 6 1012 n m which is much less than is commonly thought necessary to support subduction conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench the low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed driving forces show a strong correlation with observed spreading rates which indicates that resistive forces were largely velocity dependent fluid migration up the subduction zone may elevate temperatures in and around the shear zone reducing resistive forces below the levels required by purely conductive models changes in convergence rate may affect the depths of both the brittle ductile deformation boundary and the basalt eclogite phase change causing a negative feedback which would appear as a velocity dependent resistive force the different driving forces acting on the ne and sw parts of the phoenix plate as a consequence of older oceanic lithosphere at the trench in the ne caused antarctic phoenix spreading to take place about a near pole to the sw since 21 m y ago and ultimately resulted in disruption of the phoenix plate about 9 m y ago spreading rates decreased abruptly about 6 m y ago probably because of e w compression across the long transform faults bounding the phoenix plate however spreading on the last three segments of the antarctic phoenix ridge continued at least until 4 m y ago either spreading stopped progressively from sw to ne or the final stage took place about a very near pole to the sw a magnetic quiet zone extends up to 95 km from the margin between the tula fracture zone and the north anvers fracture zone and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision the absence of a magnetic quiet zone associated with the most recent ridge crest trench collisions suggests a change in sedimentary regime during the late miocene anomalously fast apparent spreading rates between 23 and 21 m y ago are thought to indicate an error in this part of the magnetic reversal time scale

large volume silicic volcanism along the proto pacific margin of gondwana lithological and stratigraphical investigations from the antarctic peninsula jurassic magmatism in western gondwana produced the most voluminous episode of continental volcanism in the phanerozoic era during the early to middle jurassic some 2 5 3 million km3 of dominantly basalt and to a lesser extent rhyolite were erupted onto a supercontinent in the early stages of break up the major silicic portion of the gondwana magmatic province is exposed in patagonian south america the volcanic rocks of patagonia have been collectively termed the chon aike province and constitute one of the world s most voluminous silicic provinces the volcanic rocks are predominantly pyroclastic dominated by ignimbrite units of rhyolite composition volcanic rocks crop out sporadically across much of the once contiguous antarctic peninsula and are considered to form an extension of the chon aike province a continuation of the province to include the antarctic peninsula would extend its strike length along the active pacific margin by c 2000 km volcanic rocks exposed along the east coast of the antarctic peninsula defined here as the mapple formation are also dominated by rhyolitic ignimbrite flows with individual units up to 80 m in thickness and a total thickness of c 1 km the ignimbrites vary in degree of welding from high grade rheomorphic ignimbrites with parataxitic textures to unwelded lithic rich ignimbrites rhyolite lava flows air fall horizons debris flow deposits and epiclastic deposits are volumetrically minor occurring as interbedded units within the ignimbrite succession the lithology and stratigraphy of the jurassic volcanic rocks of the mapple formation are presented and comparisons are made to the chon aike province a consistent stratigraphy of permo triassic metasedimentary rocks unconformably overlain by terrestrial mudstone siltstone sequences which are in turn conformably overlain by largely silicic subaerial volcanic rocks is present at several localities along the antarctic peninsula and at localities in the chon aike province precise zircon u pb middle jurassic ages exist for two volcanic formations from the antarctic peninsula and a middle lower jurassic age has been suggested for the underlying sedimentary formations based on fossil flora analysis the antarctic peninsula chronostratigraphy coupled with lithological similarities indicate a close relationship to those sequences of the chon aike province

palaeoclimatic interpretation of clay minerals in marine deposits an outlook from the continental origin abstract as distribution of clay minerals in modern oceans seems to be controlled by contemporary climates marine clays in ancient deposits have been widely used to reconstruct palaeoclimates in this review we examine this question mainly from the continental origin of the clay minerals and show the difficulties of retrospective analysis of the influence of climate on common marine deposits we examine successively how soils develop and behave are reworked and palaeoweatherings are preserved focusing the review on kaolinitic soils indicative of tropical wet climates several stages intervene between the time of clay mineral development in soils and their final deposition in the basin 1 the persistence over geological times of huge amounts of kaolinitic palaeoweathering materials in the landscapes may seriously alter the palaeoclimatic signal of kaolinite in the sedimentary record although the kaolinite deposited in recent sediments of the world oceans is consistent with wet climatic zones this is mainly because the major kaolinitic and bauxitic palaeosurfaces coincide with the tropical areas where kaolinite is still developing at present 2 possible development of deep kaolinitic profiles at high latitudes and cold conditions under co 2 enriched palaeoatmospheres or even by unusual acidic weathering linked with dry climates is misleading palaeoenvironments for palaeoclimatic reconstruction 3 erosion and transport processes always cloud the signal to some extent because frequently erosion products from the bedrock and different soil horizons will be mixed and major drainage systems may include soils of different climatic zones 4 the arrival of the soil clay assemblages in the basin inevitably lags against their formation on the continent and may occur several million years after formation of the soils on the continent and climate may have considerably changed at that time it is pointed out that sedimentologists and palaeoclimatologists should consider the complexity of the relations between soil and sedimentary clay minerals soil formation rates are slow and therefore the resolution of the palaeoclimatic record in marine clay may not be closer than 1 or 2 ma at present the palaeoclimatic interpretations of marine clay assemblages are yielding at best nothing more than rather broad palaeoclimatic information this approach cannot achieve the same degree of resolution as other techniques such as isotope or microfossil studies clay mineral assemblages may provide integrated records of overall climatic impacts whereas other techniques are more likely to reveal local or temporary climates

the south american retroarc foreland system the development of the bauru basin in the back bulge province abstract the aim of this research is to understand the tectonic setting of the bauru basin this basin in central eastern south america has been classified as intracratonic but the basin fill geometry the involved subsidence mechanisms and the age of the deposits are poorly understood in this work the ranges of the fossil taxa are analyzed and ages are proposed for the lithostratigraphic units isopach maps were used to reconstruct the stratigraphic intervals of the basin fill the stratigraphy of the bauru basin is compared with that of the adjacent basins and the data are integrated with the available information on south american geodynamics the fossil record indicates that sediment accumulated from the cenomanian to early paleocene beginning after the mochica phase of the andean orogeny the basin fill geometry demonstrates migration of the depocenter through time which occurred simultaneously with migration of the andean basin and immediately after the orogenic events of the peruvian phase we propose that the bauru basin is a component of a retroarc foreland system developed during the early stages of the andean evolution and that it was developed in the back bulge province of this system the andean basin constitutes the foredeep depozone of this foreland system including the potosi oriente acre and maranon basins in addition the upper cretaceous of the parecis and solimoes basins were likely also developed in the back bulge province the thickness of the bauru accumulation indicates that other mechanisms might have overlapped the flexural subsidence in this back bulge province

bimodal back arc alkaline magmatism after ridge subduction pliocene felsic rocks from central patagonia 47 s abstract volumetrically minor microsyenites alkali microgranite and related trachytic dykes intrude early pliocene oib like alkali basaltic and basanitic flows of the meseta del lago buenos aires in central patagonia 47 s 71 30 w and occur together with scarce trachytic lava flows whole rock k ar ages between 3 98 and 3 08 ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post plateau basaltic sequence that they intrude during a bimodal mafic felsic magmatic episode devoid of intermediate compositions chemically these rocks have a1 type granitoid affinities and are characterized by high silica and alkali contents 60 68 wt sio2 8 7 10 8 wt na2o k2o major and trace elements patterns evidencing evolution by low pressure fractional crystallization and sr and nd isotopic signatures similar to those of coeval basalts 87sr 86sr o 0 70488 0 70571 143nd 144nd o 0 512603 0 512645 nevertheless some of them have the most radiogenic sr values ever reported for a magmatic rock in the meseta and even in the whole neogene patagonian plateau lavas province 87sr 86sr o 0 70556 0 70571 143nd 144nd o 0 512603 0 512608 in addition very high contents of strongly incompatible elements in the most evolved rocks together with sr isotopic ratios higher than those of coeval basalts suggest the occurrence of open system magmatic processes continuous fractional crystallization from a primitive basaltic source similar to post plateau coeval basalts towards alkali granites combined with small rates of assimilation of host jurassic tuffs afc in a shallow magmatic reservoir best explains the geochemical and petrographic features of the felsic rocks therefore a1 type magmatic rocks can be generated by open system crystallization of deep asthenospheric melts in back arc tectonic settings in central patagonia these 3 4 ma old alkaline intrusions occur aligned along a n160 170 trending lineament the zeballos fault zone stacking the morphotectonic front of one segment of the patagonian cordillera intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area related to major regional tectonics occurring in possible relation with the collision of one segment of the chile spreading ridge with the trench

carbonate systems along nutrient and temperature gradients some sedimentological and geochemical constraints research over the past several decades has clearly demonstrated that changes in the ocean environment have had major impacts on carbonate systems changes in climate ocean circulation and seafloor spreading rates have influenced temperature and seawater chemistry including carbonate saturation state and nutrient availability and thereby have determined boundary conditions for the biota that form carbonate platforms in turn the biota determine accumulation rates and facies zonations thus controlling platform geometry and facies dynamics in the first section of this paper we examine how nutrient availability influences carbonate facies associations we first discuss the role of temperature and nutrient gradients in the modern ocean and their influence on biotic associations then we discuss how carbonate sedimentation can be characterized along nutrient gradients in the second section we review proxies currently used to reconstruct paleoproductivity in open ocean environments and discuss their applicability to neritic carbonate systems we highlight the variety of existing proxies and their limitations and suggest that multiple lines of evidence are needed for valid interpretations our short review discusses sedimentological biogenic and geochemical proxies that can be used to reconstruct past nutrient fluxes and to constrain paleoceanographic controls over the distribution of carbonate associations however it also reveals that more data and case studies are needed that integrate shallow and deep water carbonate sequences and elucidate the links between temperature vs nutrient supplies changes and facies in ancient carbonate sequences

antarctic peninsula continental magnesian andesites indicators of ridge trench interaction during gondwana break up subduction of young hot oceanic lithosphere and a major episode of lithospheric extension have been considered as key tectonomagmatic factors in the genesis of setouchi type continental magnesian andesites we present data on a new discovery of such andesites from the northern part of the antarctic peninsula and propose that subduction of newly created oceanic lithosphere occurred along the proto pacific margin of antarctica during the middle jurassic continental magnesian andesite magmatism was contemporaneous with the early stages of gondwana break up and we suggest that subduction of young hot oceanic lithosphere and ridge trench interaction may have contributed to the disintegration of gondwana by thermally weakening the lithosphere and developing a broad linear extensional zone along the proto pacific margin

the cenozoic subduction history of the pacific margin of the antarctic peninsula ridge crest trench interactions new magnetic anomaly identifications w of the shackleton fracture zone show 5 spreading sections separated by fracture zones in the 2 most southerly the ridge crest collided with a trench at the margin of the antarctic peninsula only 6 5 and 4 ma ago the latest of a series of collisions starting at the base of the peninsula so ma ago following each collision spreading and subduction both stopped opposite the south shetland trench and actively extending bransfield strait spreading also stopped 4 ma ago but before the spreading centre reached the trench the tendency of the subducting plate to continue sinking probably initiated the opening of the bransfield strait the parallelism between fracture zone orientation on the descending plate and convergence direction previously thought responsible for the tectonic segmentation of the peninsula was effective for only 10 30 ma before collision before that either the fracture zones did not extend to the trench or the subducting ocean floor formed at the farallon phoenix nazca aluk boundary with fracture zones oblique to the subduction direction the aleutian arc kula ridge model in which arc magmatism virtually ceases when ocean floor younger than 25 30 ma occurs beneath the arc fits the distribution of antarctic peninsula radiometric dates explaining a 50 60 ma gap between collisions and youngest ages in the s and possibly the migration of the youngest activity towards the trench thus gaps in the geologic record of arc magmatism need not imply cessation of subduction the progressive steepening of the peninsular margin towards the 4 ma collision site suggests tectonic erosion of the fore arc as the ridge crest approached

igneous history of the andean cordillera and patagonian plateau around latitude 46 degrees s from the middle jurassic onwards persistent igneous activity in the southern andes around 46 s was controlled by easterly dipping subduction along the pacific margin cogenetic plutonic rocks belonging to the patagonian batholith and calc alkaline volcanics ranging from basaltic andesites to rhyolitic tuffs and ignimbrites are the principal products erosion of the primary volcanics has led at various times to the development of thick volcaniclastic sequences for example in the cretaceous lower tertiary divisadero formation the coyhaique region marks the northerly extension of a narrow back arc basin in which the marine neocomian successions accumulated volcaniclastics from the island arc which presumably lay to the west are intercalated with the sediments although the marine basin was short lived a mildly extensional back arc regime may have existed through much of mesozoic recent times widespread basalt rhyolite volcanism on the eastern side of the cordillera seems to have been associated with this tectonic environment remnants of the patagonian basalt plateau at latitude 45 47 s extend from the argentine chile frontier to lago colhue huapi four principal age and compositional groups have been distinguished in the lavas i the oldest which are about 80 ma occur in sections at senguerr and morro negro they are almost exclusively tholeiitic but show some calc alkaline affinities and resemble in other respects basalts from marginal basins ii the second group 57 43 ma occur in the lower part of the chile chico section with a compositional spread from olivine tholeiites through alkali basalts to one occurrence of a basanite iii the upper part of the main plateau sequence where the flows are in the range 25 9 ma are dominantly of alkali basalt composition iv post plateau flows from small cinder cones on the surface of the plateau range in age from ca 4 ma to 0 2 ma or less they are mostly highly undersaturated basanites with occasional leucite basanites enriched in incompatible elements a few of the earlier tholeiites with calc alkali traits may have been closely associated with subduction or marginal basin processes the younger lavas are more alkalic intraplate types generated in the remote back arc extensional zone

short paper the significance of mid palaeozoic basement in graham land antarctic peninsula a mid palaeozoic basement to the antarctic peninsula is confirmed by a geochronological study of the gneissose country rock of the mesozoic magmatic arc of eastern graham land rb sr whole rock ages of 410 15 and 426 12 ma from samples of orthogneiss indicate a previously unrecognized episode of granitic magmatism in the antarctic peninsula during the silurian sm nd data on garnet whole rock pairs indicate that subsequent amphibolite facies metamorphism occurred in late carboniferous times probably as a result of plutonism

southern patagonian plateau basalts and deformation backarc testimony of ridge collisions abstract the distribution and volume of tertiary patagonian plateau basalts and the evolution of the patagonian fold and thrust belt between 46 and 49 s appear to be closely tied with ridge trench interactions along the continental margin to the west eocene plateau basalts south of 43 s are associated with a gap in the eocene arc and can be related to a proposed collision of the aluk farallon ridge against the trench at this time eruptions of late miocene to recent plateau basalts between 46 and 49 s can be related to time transgressive slab windows in the underlying mantle generated by the collision of segments of the chile ridge at 10 14 ma and 6 ma the principal deformation in the patagonian fold and thrust belt predates the eruption of the basalts and also appears to be related with late miocene ridge collision comparison of plateau basalts related to the collision of the three distinct ridge segments suggests that the volume of basalt erupted increases with the length of the collided ridge segment almost all eocene to recent plateau basalts between 46 and 49 s have oib like ocean island basalt trace element signatures showing that the mantle source region changed little with time plateau basalts with more arc like signatures appear to be contaminated by continental crust relative melting percentages inferred from the trace element chemistry of the plateau basalts correlate with volumes of erupted basalt implying spatial and temporal changes in temperatures in the mantle source that can be correlated with slab windows

melting of heterogeneous mantle in a slab window environment examples from the middle tertiary masset basalts queen charlotte islands british columbia voluminous basalts dominate a middle tertiary bimodal volcanic assemblage in the queen charlotte islands british columbia canada these moderately enriched mid ocean ridge basalt morb like magmas were associated with significant crustal extension related to the subduction of the pacific farallon ridge and the initiation of a slab window environment which persisted for 35 ma these asthenospheric melts were derived from a low degree of melting of a heterogeneous peridotite source mainly in the spinel stability field as inferred from fractional melting inversion calculations the basaltic magmas in turn gave rise to both tholeiitic and calc alkaline volcanic strata up to 3 5 km thick in local grabens the tholeiitic basalts resemble morb with rare earth element ree patterns ranging from slightly depleted to enriched in light ree lree the calc alkaline basaltic andesites are enriched in lree but are not strongly depleted in nb the rocks have overlapping nd sr and pb isotopic compositions sim

geochemical and mineralogical evidence for the occurrence of at least three distinct magma types in the famous region bulk rock major and trace element variations in selected basalts from the famous area in conjunction with a detailed study of the chemical compositions of phenocryst minerals and associated melt inclusions are used to place constraints on the genetic relationship among the various lava types the distribution of nio in olivine and cr spinel phenocrysts distinguishes the picritic basalts plagioclase phyric basalts and plagioclase pyroxene basalts from the olivine basalts for a given mg mg fe2 atomic ratio of the mineral the nio content of these phenocrysts in the former three basalt types is low relative to that in the phenocrysts in the olivine basalts the zr nb ratio of the lavas similarly distinguishes the olivine basalts from the plagioclase phyric and plagioclase pyroxene basalts and in addition distinguishes the picritic basalts from the other basalt types these differences indicate that the different magma groups could not have been processed through the same magma chamber and preclude any direct inter relationship via open or closed system fractional crystallization

aragonite cycles diagenesis caught in the act organic biomarker and nannofossil assemblages are used in combination with sedimentary petrology to identify the role of diagenesis for the formation of a rhythmic calcareous succession a record from the early pliocene of the maldives ocean drilling program odp leg 115 is presented containing precession and eccentricity cycles expressed as variable aragonite content additional sub milankovitch cycles are caused by rhythmic precipitation of calcite cement in the lower part of the interval comparison with palaeo productivity indicators nannofossils chlorin total organic carbon suggests that cementation occurs preferentially in intervals characterized by increasing or decreasing productivity the coupled variability in productivity and carbonate diagenesis is attributed to the effect of organic matter degradation in the sediment the observed combination of primary and diagenetic factors hampers the interpretation of the cyclicity on milankovitch and sub milankovitch scales diagenetically stable proxies for palaeo productivity such as nannofossil assemblage data were used to distinguish between palaeoclimate and diagenetic influences

combined sr nd pb and li isotope geochemistry of alkaline lavas from northern james ross island antarctic peninsula and implications for back arc magma formation abstract we present a comprehensive geochemical data set for a suite of back arc alkaline volcanic rocks from james ross island volcanic group jrivg antarctic peninsula the elemental and isotopic sr nd pb and li composition of these cenozoic basalts emplaced east of the antarctic peninsula is different from the compositions of the fore arc alkaline volcanic rocks in southern shetlands and nearby bransfield strait the variability in elemental and isotopic composition is not consistent with the jrivg derivation from a single mantle source but rather it suggests that the magma was mainly derived from a depleted mantle with subordinate oib like enriched mantle component em ii the isotopic data are consistent with mantle melting during extension and possible roll back of the subducted lithosphere of the antarctic plate magma contamination by triassic early tertiary clastic sediments deposited in the back arc basin was only localized and affected li isotopic composition in two of the samples while most of the basalts show very little variation in 7 li values as anticipated for mantle driven li isotopic composition these variations are difficult to resolve with radiogenic isotope systematics but li isotopes may prove sensitive in tracking complex geochemical processes acting through the oceanic crust pile including hydrothermal leaching and seawater equilibration

coccolith δ18o and alkenone records in middle pliocene orbitally controlled deposits high frequency temperature and salinity variations of sea surface water 1 the calcareous nannoplankton flora is strongly controlled by water masses and is sensitive to ambient nutrient levels in consequence fossil remains of calcareous nannoplankton are useful tools for paleoceanography although they are the major producers of carbonates in the pelagic realm their use as a seawater 18o recorder is not easy due to their small size and the difficulty in isolating them from the bulk carbonate using a granulometric separation technique minoletti et al 2001 it is now possible to concentrate coccoliths in monospecific fractions we applied this method on three precession cycles insolation cycles i 288 to i 282 from the mid pliocene punta piccola section south sicily which led to an almost pure pseudoemiliana lacunosa fraction it was then possible to measure the isotopic signatures 18o and 13c and alkenone unsaturation ratio on the same recorder p lacunosa this permits a precise reconstruction of environmental conditions temperature and salinity that prevailed in the upper photic zone during the cyclic deposition of these sediments our data provide a clear confirmation that sapropelic marl formation occurred during a wetter climate compared to the deposition of the marly limestones moreover these results point out that salinity variations are not as previously thought always the primary factor that controlled the deposition of these cyclic events this study is a step forward in the utilization of calcareous nannoplankton isotopic records in paleoenvironmental reconstructions

a preliminary assessment of the hydrocarbon potential of the larsen basin antarctica abstract the larsen basin on the northwest margin of the weddell sea formed as a mesozoic ensialic basin during gondwana breakup deposition was either in half grabens on the extending weddell sea margin or in a restricted back arc basin at the northern end of this basin 5 6 km of sedimentary rock crop out on james ross island exposing elements of a large potential hydrocarbon system aeromagnetic and outcrop data suggest that the basin structure inferred from james ross island can be recognised at least as far south as 70 s upper jurassic anoxic marine strata deposited prior to the main phase of arc development form a rich potential source t o c up to 3 5 with both marine and terrestrial kerogens arc derived volcaniclastic sediments of barremian oligocene age form a regressive megasequence basal strata represent slope apron and rudaceous submarine fan deposits proximal to the margin fan conglomerates form lenticular bodies hundreds of metres thick and tens of kilometres across enveloped in slope apron mudstones late cretaceous fault reactivation and uplift led to dramatic shallowing of the basin with deposition of shelf facies although there are many potentially attractive reservoir targets there may be problems of pore occlusion due to the abundant labile volcanic grains however there is evidence of more quartzose sandstone towards the top of the section and inferentially toward the basin centre in the larsen basin there is moderate potential for oil generated from upper jurassic source rocks and reservoired in cretaceous and tertiary sandstones and conglomerates in large stratigraphic or structural traps caused by partial basin inversion during deposition

geochemistry and origins of ueno and on take basaltic to andesitic rocks 3 ma produced by distinct contributions of subduction components central japan abstract ueno basaltic rocks 2 7 1 6 ma and on take volcanic rocks 0 75 ma crop out in a restricted area in central japan and were analyzed for major and trace elements and sr and nd isotopes the ueno rocks were erupted as numerous monogenetic volcanoes largely of basaltic compositions whereas the on take lavas form a composite stratovolcano dominated by andesites the ueno basalts are primarily geochemical mixtures of two components 1 sub alkalic common basalts generated from the lithospheric subcontinental mantle with 87sr 86sr 0 706 end 2 and k nb 1000 and 2 rare within plate basalts wpb of alkali basaltic composition with 87sr 86sr 0 705 end 5 k nb 180 subordinate ueno andesites were formed by assimilation fractional crystallization processes within the lower crust having 87sr 86sr 0 709 end 8 but without appreciable assimilation in the upper crust on the other hand the on take basalts have 87sr 86sr 0 705 0 0005 end 2 1 k nb 1200 and geochemical signatures characteristic of continental arc basalts the on take andesites appear to have been contaminated frequently by upper crustal materials it is proposed that the injection of a wpb type diapir yielded partial melting of the subcontinental mantle forming the common ueno basalt magmas the arc type basaltic magmatism which was probably the result of subduction of the pacific plate occurred at 1 ma after the ueno volcanism addition of slab derived components with 87sr 86sr 2300 modified the subcontinental mantle forming the source of on take basalts

the lutetian bartonian transition middle eocene at the oyambre section northern spain implications for standard chronostratigraphy abstract with the aim of finding a potential global stratotype section and point gssp for the base of the bartonian stage an integrated stratigraphic study was carried out in the acebosa fm exposed at the cape of oyambre cantabria spain while the oyambre section does comply with the majority of the requirements of the international commission on stratigraphy several shortcomings were also found which apparently make it unsuitable for the gssp for the base of the bartonian stage an unexpected finding was that the chronostratigraphic calibration of the oyambre planktonic foraminiferal events revealed major discrepancies with the standard timescale the lowest occurrence of globigerinatheka index was found in the upper part of chron c20r and not in the upper part of c20n as depicted in the standard chart being dated at 43 68 ma and not at 42 64 ma the lowest occurrences of turborotalia pomeroli and globigerinatheka euganea were found in the upper part of calcareous nannofossil zone cp14a 40 92 ma and not in the upper part of zone cp13c 42 21 ma finally the highest occurrences of guembelitrioides nuttalli and morozovelloides bandyi were also found in the upper part of zone cp14a 40 68 ma and not in the upper part of zone cp13c 42 07 ma the accuracy of these anomalous results was further substantiated with data from other areas strongly suggesting that the standard eocene chronostratigraphy was in need of amendment the oyambre findings if proven significant at a global scale would have two important chronostratigraphic outcomes firstly the standard eocene planktonic foraminiferal biostratigraphy would need revision as there would be practically no space to accommodate zones e9 and e11 secondly an array of sound biostratigraphic markers would now be defined around chron c19n which has been considered the best marker for the lutetian bartonian boundary thus facilitating correlation of a potential gssp

the use of paleoceanographic proxies in carbonate periplatform settings opportunities and pitfalls in a sediment core from the leeward side of the great bahama bank ocean drilling program odp leg 166 1006a 24h early pliocene several paleoceanographic proxies were compared with carbonate mineralogy the proxies measured on the planktonic foraminifera species globigerinoides sacculifer are mg ca and sr ca elemental ratios and 18o values these proxies are widely used in pelagic sediments to derive parameters of water masses such as paleo temperature and salinity to produce quantitative estimates it is necessary to verify that the primary shell mineralogy of the foraminifera was not altered diagenetically diagenetic alteration can be rapid in periplatform settings relative to pelagic deep sea sediments since the meta stable carbonate phases provide an additional driving force for recrystallisation we show that the sr ca ratio of foraminiferal calcite can be used to assess the degree of diagenetic alteration of their shell chemistry using this method we demonstrate that the primary signal in the oxygen isotope ratios is preserved in the studied core in the mg ca record the relative change of foraminiferal mg ca ratios seems to be preserved but absolute values are influenced by diagenesis the second set of proxies comprises 13c and c n ratios of organic carbon these are used in pelagic sediments in order to assess the origin of organic carbon terrestrial or marine phytoplankton it is demonstrated here that a simple two end mixing model cannot explain the observed co occurrence of relatively high 13corg and c n values as the 13corg values co vary with the aragonite content we attribute this signature to varying contribution of organic carbon from benthic algae which are also the major aragonite producers on the platform top

planktonic benthonic foraminiferal ratios modern patterns and tertiary applicability abstract the abundance of planktonic specimens in foraminiferal assemblages was determined in numerous bottom samples from inner neritic to deep oceanic depths along the atlantic margin of the northeastern united states the results augment previous studies in other areas that have shown a general increase in percentage of planktonic specimens in total foraminiferal bottom assemblages as water depth increases the patterns found in this area of complex shelf bathymetry and hydrography illustrate the influence on the planktonic benthonic percentages of water depth distance from shore different water mass properties and downslope movement of tests in high energy areas the patterns found in the 661 samples from the atlantic margin were compared with results from 795 stations in the gulf of mexico pacific ocean and red sea the relative abundance of planktonic specimens and water depth correlates positively in all open oceanic areas even though taxonomic composition and diversity of the faunas from different areas is variable the variation of planktonic percentages in bottom samples within most depth intervals is large so that a precise depth determination cannot be made for any given value however an approximate upper depth limit for given percentages can be estimated for open ocean environments a decrease in planktonic percentages is seen in the lower salinity and higher turbidity coastal waters of the gulf of maine planktonic percentages intermediate between the lower values in the less saline coastal waters and the higher values in the normal open oceanic conditions occur in the transitional area between the gulf of maine and the open marine atlantic ocean to the east similarly lowered values in another area of restricted oceanic circulation occur in the high salinity clear but nutrient poor waters of the gulf of aqaba off the red sea a comparison of the similarity of modern planktonic percentage values to those found in earlier tertiary assemblages was made to confirm the usefulness of this measure in the fossil record in some stratigraphic sections in upper paleocene and lower eocene strata of the eastern gulf coastal plain water depths inferred from trends and values of planktonic percentages consistently match paleobathymetry constructed from physical stratigraphic characteristics and paleogeographic relationships

role of strike slip faulting in the tectonic evolution of the antarctic peninsula the antarctic peninsula mesozoic magmatic arc has had a long history of dextral strike slip deformation the deformation was initially associated with the development of a wide accretionary complex by the migration of fore arc slivers and the formation and inversion of a thick fore arc basin succession it also formed an important component within major shear zones in the arc and may have controlled the formation of sedimentary basins in the back arc region although some transcurrent motion within the fore arc region was related to a component of oblique subduction the main movement occurred during the breakup of gondwanaland and the formation of a major transtensional rift system a new reconstruction for this part of gondwanaland is presented taking this transcurrent motion into consideration

sub milankovitch cycles in periplatform carbonates from the early pliocene great bahama bank high resolution bulk sediment magnetic susceptibility and aragonite content and 18o records from two different planktonic foraminifera species were analyzed in an early pliocene core interval from the straits of florida ocean drilling program site 1006 the 18o record of the shallow dwelling foraminifera g sacculifer and the aragonite content are dominated by sub milankovitch variability in contrast magnetic susceptibility and the 18o record of the deeper dwelling foraminifera g menardii show precession cycles the relationship between the aragonite and the paleoproxy data suggests that the export of sediment from the adjacent great bahama bank was triggered directly by atmospheric processes rather than by sea level change we propose a climate mechanism that bears similarities with the semiannual cycle component of eastern equatorial pacific sea surface temperatures under present day conditions

δd and δ18o variations of the magmatic system beneath deception island volcano antarctica implications for magma ascent and eruption forecasting abstract geochemistry of volatiles in active volcanoes provides insights into the magmatic processes and evolution at depth such as magma evolution and degassing which can be implemented into volcanic hazards assessment deception island is one of the most active volcanoes in antarctica with more than twenty explosive eruptions documented over the past two centuries hydrogen and oxygen isotopic variations in the volatiles trapped in the deception island rocks glass and melt inclusions in phenocrysts provide essential information on the mechanisms controlling the eruptive history in this volcanic suite thus understanding the petrological and related isotopic variations in the island has the potential to foresee the possible occurrence and its main eruptive features of a future eruption the new isotopic results of this study combined with exhaustive petrologic knowledge reveal in deception island i fast ascent and quenching of most magmas preserving pre eruptive magmatic signal of water contents and isotopic ratios with local modification by rehydration due to glass exposition to seawater and by meteoric and fumarolic waters ii a plumbing system s variable with time and currently dominated by closed system degassing leading to explosive eruptions iii control on the interactions of ascending magmas with the surface waters producing hydrovolcanic activity throughout the two main fault systems in deception island and at different magma stagnation depths these results can be considered in further studies of volcanic monitoring to improve the capability to interpret geophysical data and signals recorded during volcanic unrest episodes and hence forecast volcanic eruptions and related hazards

geochemistry of holocene volcanic rocks associated with ridge subduction in baja california mexico although the geochemistry of magmatism associated with the subduction of oceanic lithosphere is well understood1 6 the geochemical signature of lavas produced shortly after subduction of an oceanic spreading centre has not been characterized the baja california peninsula in mexico provides an ideal scenario to study the latter process geophysical models indicate that in response to oblique collision of the ancestral east pacific rise epr with the western seaboard of north america at 29 myr subduction of the farallon plate was succeeded by the progressive southwards development of a transform fault system7 9 at 12 5 myr a considerable length of the epr was simultaneously subducted beneath baja thus terminating subduction processes along this segment10 by 3 5 myr the locus of transcurrent faulting had switched from the west of baja to within the developing gulf of california coupling baja to the pacific plate11 13 we present here data for holocene volcanic rocks from baja california norte which show that post ridge subduction volcanism has a distinctive geochemistry these results have important consequences for the interpretation of ancient orogenic belts

geochemistry and tectonic setting of alkaline volcanic rocks in the antarctic peninsula a review abstract the numerous miocene recent alkaline volcanic outcrops in the antarctic peninsula form a substantial volcanic province the least well known part of a major belt of alkaline volcanism that extends between south america and new zealand the outcrops consists mainly of aa and pahoehoe lavas and hyaloclastites which locally contain accidental nodules of spinel lherzolite and other mantle derived lithologies the province is predominantly basaltic with two major differentiation lineages 1 a sodic series of olivine and alkali basalt hawaiite mugearite trachy phonolite and trachyte and 2 a relatively potassic highly undersaturated series of basanite tephrite and phono tephrite all the lavas show varying effects of fractionation by crystallization of olivine and clinopyroxene joined by plagioclase in the hawaiites to trachytes fractional crystallization can probably explain most of the chemical variation observed within each outcrop but variable partial melting is necessary to account for the differences in incompatible element enrichment between the two series and between the individual outcrops the degree of partial melting may not have exceeded 3 as is the case for many other alkaline magmas the volcanism is an intraplate phenomenon but there is no correlation in timing between the cessation of subduction and the inception of alkaline volcanism the activity cannot be related to the passage of the coupled pacific antarctic plate over a stationary mantle hot spot although the precise causal relationship with tectonic setting is unknown regional extension was a prerequisite for giving the magmas rapid access to the surface

petrogenesis of the eocene and mio pliocene alkaline basaltic magmatism in meseta chile chico southern patagonia chile evidence for the participation of two slab windows the meseta chile chico mcc 46 48s is the westernmost exposure of eocene lower basaltic sequence lbs 55 40 ma k ar ages and mio pliocene upper basaltic sequence ubs 16 4 ma k ar ages flood basalt volcanism in patagonia the mcc is located south of the lago general carrera buenos aires lgcba southeast from the present day chile triple junction ctj east of the actual volcanic gap between southern south volcanic zone and austral volcanic zone ssvz and avz respectively and just above the inferred location of the south chile ridge segment subducted at 6 ma scr 1 erupted products consist of mainly ne normative olivine basalt with minor hy normative tholeiites basalt trachybasalt and basanite mcc lavas are alkaline 42 7 53 1 wt sio2 3 8 wt na2o k2o and relatively primitive ni 133 360 ppm cr 161 193 ppm co 35 72 ppm 4 16 5 mgo wt they have a marked oib like signature as shown by their isotopic compositions 87 sr 86 sro 0 70311 0 70414 and qnd 4 7 5 1 and their incompatible trace elements ratios ba la 10 20 la nb 0 46 1 09 ce pb 15 52 27 5 sr lab25 reflecting deep mantle origin ubs primitive lavas have characteristics similar to those of the eocene lbs basalts while ubs intermediate lavas show geochemical imprints la nbn1 sr lan25 low ce pb nb u compatible with contamination by arc slab derived and or crustal components we propose that the genesis and extrusion of

contrasting environmental effects of astronomically driven climate change on three eocene hemipelagic successions from the basque cantabrian basin several processes can contribute to the formation of hemipelagic limestone marl alternations as a consequence of astronomically driven climate change the aim of this work is to decipher which environmental factors governed the formation of three eocene hemipelagic successions of the basque cantabrian basin using a comprehensive set of physical and bulk carbonate geochemical data bed thickness mineralogy caco3 13c and 18o the results show that the significance of several environmental processes varied depending on the palaeogeographic setting and eccentricity modulated precessional seasonality in the sopelana starved deep sea basin limestones were formed as a consequence of high pelagic carbonate productivity during periods of warm seawater and sluggish circulation which corresponded to periods of low seasonality summers at aphelion conversely marls accumulated when pelagic carbonate productivity decreased during periods with cooler waters and more vigorous circulation which occurred when seasonality was higher summers at perihelion in the gorrondatxe submarine fan fringe marls accumulated when high seasonality produced significant continental rainfall and runoff causing the dilution of pelagic carbonate sedimentation with terrigenous supplies in the oyambre upper slope marls also accumulated when seasonality was high as pelagic carbonate productivity decreased due to both the expansion of low salinity waters on the ocean surface and the increase in continentally derived nutrients which caused detrimental seawater conditions for calcareous plankton both in gorrondatxe and oyambre limestones accumulated when boreal summer at aphelion caused low seasonality which allowed relatively stable conditions to prevail at minimum eccentricity when precession driven seasonality contrast diminished changes in pelagic carbonate productivity were significant in the three sections on the contrary at maximum eccentricity when seasonality peaked due to summers occurring at perihelion the effects of other environmental processes such as continental and oceanic currents became influential however the influence of these processes minimized when summertime coincided with aphelion at maximum eccentricity and seasonality was weakest this article is protected by copyright all rights reserved

petrography geochemistry and tectonics of a rifted fragment of mainland asia evidence from the lasala formation mindoro island philippines petrological and geochemical investigations of the sedimentary lasala formation in northwest mindoro philippines offer new insights into the origin of this geologically contentious region mindoro island s position at the boundary between sundaland and the philippine mobile belt has led to variable suggestions as to how much of it is continent derived or not the eocene lasala formation overlies the jurassic halcon metamorphics a regionally metamorphosed suite generally thought to have formed as a result of arc continent collision processes the sedimentary formation consists mainly of sandstones and shales interbedded with mudstones basalt flows and subordinate limestones and conglomerates petrographic information on the lasala clastic rocks demonstrates a uniform framework composition that is predominantly quartzose major oxide trace element abundances and various elemental ratios similarly impart a strongly felsic signature these characteristics are taken to indicate a chiefly continental passive margin derivation and deposition of the lasala sediments during the eocene the weak indication of active margin influence is suggested to be an inherited signature supported by paleogeographic models of the southeastern asian margin area during the pre cenozoic

cordilleran slab windows the geometry and geologic implications of subducted spreading ridges are topics that have bedeviled earth scientists ever since the recognition of plate tectonics as a consequence of subduction of the kula farallon and east pacific rises slab windows formed and migrated beneath the north american cordillera the probable shape and extent of these windows which represent the asthenosphere filled gaps between two separating subducting oceanic plates are depicted from the late cretaceous to the present possible effects of the existence and migration of slab windows on the cordillera at various times include cessation of arc volcanism and replacement by rift or plate edge volcanism lithospheric uplift attenuation and extension and increased intensity of compressional tectonism eocene extensional tectonism and alkaline magmatism in southern british columbia and the northwestern united states were facilitated by slab window development

subduction of diverging plates and the principles of slab window formation consumption of an ocean basin by subduction commonly brings a sea floor spreading ridge toward a deep sea trench if plate divergence and convergence continue after the ridge intersects the subduction zone a slab window forms between the subducted parts of the diverging oceanic plates producing anomalous thermal physical and chemical effects in the surrounding asthenospheric mantle in turn these conditions alter the tectonic and magmatic evolution of the overriding plate usually disturbing ordinary fore arc and arc regimes differential lithospheric stresses on opposite sides of the triple junction contribute to disturbances in the overriding plate anomalous magmatism from fore arc to back arc may be accompanied by fore arc metamorphism strike slip faulting uplift extension and in extreme cases rifting the shape and size of the window are controlled mainly by the pre subduction ridge transform trench configuration slab dip angles and vectors of plate convergence subducted ridge segments expand into windows whose margins approximately parallel the motion vectors between the triple junction and the subducting plates subducted transform faults continue to be active usually as oblique slip faults until the plates separate as transform faults subduct they become longer on the plate which occupies the acute angle between ridge and trench and shorter on the other plate trains of isolated windows produced by subduction of a segmented ridge transform system progressively expand during descent commonly merging together to form a composite slab window oblique subduction of a highly segmented ridge is likely to produce two or more fraternal slab windows one at each site of ridge trench intersection above a slab window arc volcanism diminishes and may be replaced by volcanism of mid ocean ridge or rift affinity the change in chemical character reflects various processes including elevated heat flow decreasing hydration of the upper mantle juxtaposition of supra and sub slab mantle reservoris asthenospheric upwelling and melting of the trailing plate edges if the slab window migrates the anomalous magmatic regime may be replaced by renewed arc volcanism identifying the effects of slab windows in ancient convergent margin assemblages requires an understanding of slab window principles and implications

origin of limestone marlstone cycles astronomic forcing of organic rich sedimentary rocks from the cenomanian to early coniacian of the cretaceous western interior seaway usa abstract we present an integrated multidisciplinary study of limestone marlstone couplets from a continuously cored section including parts of the upper buda limestone the entire eagle ford group boquillas formation and lower austin chalk from the shell iona 1 research borehole texas usa which provides a 8 million year myr distal clastic sediment starved intrashelf basin record of the early cenomanian to the earliest coniacian stages results show that despite variable yet minimal diagenetic overprints several unambiguous primary environmental signals are preserved and support greater water mass ventilation and current activity promoting increased silica carbonate productivity during the deposition of limestone beds compared to deposition of marlstone beds which reflect greater organic matter productivity and preservation furthermore our astronomical analyses demonstrate that the limestone marlstone couplets in the iona 1 core reflect climatic forcing driven by solar insolation resulting from integrated milankovitch periodicities in particular we propose that obliquity and precession forcing on the latitudinal distribution of solar insolation may have been responsible for the observed lithological and environmental variations through the cenomanian turonian and coniacian in this mid latitude epicontinental sea setting our data also suggests that rhythmic lithological alternations deposited in greenhouse periods in general may simply reflect climate driven cycles related to earth sun dynamics without the need to invoke significant sea level variations

magnetostratigraphic and cyclostratigraphic calibration of a prospective palaeocene eocene stratotype at zumaia basque basin northern spain detailed magnetostratigraphic work at the palaeocene eocene boundary interval in the zumaia section has led to a precise delineation of the position of polarity chron c25n from 35 0 m or 35 7 m to 25 2 m below the base of the carbon isotopic excursion that marks the palaeocene eocene boundary this chronostratigraphic constraint suggests that the lithological carbonate marl couplets making up most of the studied section are the expression of orbital precession cycles and confirms the expanded and continuous nature of the section with mean sediment accumulation rates of 2 5 cm kyr 1 the new data render zumaia a unique land based marine section with available magnetostratigraphic data coupled to cyclostratigraphy the revised chronostratigraphic framework presented supports previous cyclostratigraphic calibration of the latest palaeocene geomagnetic timescale it should facilitate better assessment of the different chemobiostratigraphic events that span this critical interval as a consequence of past climate and oceanographic change consequently the zumaia section is proposed as a candidate global stratotype section and point gssp for the palaeocene eocene boundary

orbitally forced climate and sea level changes in the paleoceanic tethyan domain marl limestone alternations lower kimmeridgian se france abstract high resolution analysis of multiple climatic proxies was carried out on a 8 5 m lower kimmeridgian interval of a pelagic marl limestone succession at ch teauneuf d oze vocontian basin southeastern france the aim of the study was to characterize the orbitally controlled sedimentary cyclicity and to decipher paleoclimatic and paleoceanographic changes all analysed proxies magnetic susceptibility carbonate content manganese content and bulk carbon and oxygen stable isotopes 13c and 18o respond in synchrony to orbitally forced climate change precession index cycles modulated by short 100 kyr eccentricity cycles are strongly expressed in magnetic susceptibility carbonate and mn contents but less so in 18o obliquity cycles are expressed in manganese and particularly in 13c the orbital forcing was conferred to these rhythmic pelagic sediments via paleoclimatic and paleoceanographic changes as follows detrital input and marine carbonate production are recorded by the magnetic susceptibility and carbonate signals with strong precession cyclicity calcareous nannofossil analysis shows the omnipresence of coccoliths and debris of coccoliths in the marls and limestones suggesting that the carbonate production was in largest part in situ precession index may exert oscillations in the antagonist marine surface productivity and detrital flux processes via solar radiation change oxygenation of the seafloor leading to redox cycles is detected in the manganese signal and supported by the 13c record with a strong expression of the obliquity finally paleotemperature variations were deduced from the 18o record although this latter has been in part perturbed by diagenesis in sum the variations in the studied proxies suggest a depositional model of combined climate and sea level cycles forced in concert by earth s orbital parameters in particular the limestones represent warmer climates and higher sea levels and the marls colder climates and lower sea levels this depositional model concurs with a previous depositional model of deep water carbonates of the german kimmeridgian while is in opposition to a previous one of the same section in the vocontian basin

orbitally induced limestone marlstone rhythms in the albian cenomanian cismon section venetian region northern italy sedimentology calcareous and siliceous plankton distribution elemental and isotope geochemistry abstract a multidisciplinary study of the upper albian cenomanian portion of the cismon section venetian region northern italy was undertaken in order to characterize the cyclic alternations of carbonate rich and carbonate poor layers and to investigate their possible origin and cyclic patterns limestone semicouplets are characterized by abundant radiolarians and micarbs micron sized calcitic fragments common planktonic foraminifera strong bioturbation good oxygenation as expressed by the mn and v v ni parameters high si al ratio low k al in the absence of pyrite and organic matter the marlstone semicouplets are on the contrary frequently laminated rich in pyrite and organic matter with common fish remains in the absence of radiolarians the mn and v v ni parameters indicate that the marlstone deposition occurred in dysoxic to anoxic conditions which allowed the preservation of organic matter the low si al ratio and high k al ratio the latter related to abundant feldspars indicate that marlstones received a substantial contribution from a terrigenous source the abundance of biogenic silica suggests that during limestone deposition surface waters were rather fertile due to an efficient recycling of nutrients from deeper waters the enhanced fertility was coupled with an efficient current system at the welloxygenated sea floor which prevented the organic matter from accumulating during marlstone deposition surface waters were less fertile than in the limestone semicouplets but still within the mesotrophic spectrum as indicated by the presence of fertility index species among calcareous nannofossils whereas the bottom waters were oxygen depleted to accommodate fertility at the surface with stagnation at the bottom the scenario implies an enhanced stratification caused by an increase in freshwater runoff monitored by 18o depleted values in the meantime the water discharge supplied the nutrients to the surface waters necessary for the calcareous nannofossils to proliferate the cyclic organization of limestone marlstone couplets as revealed by power spectral analysis seems to represent climatic cycles the alternating conditions are 1 efficient mixing highly fertile surface and near surface waters and a well oxygenated seafloor to 2 enhanced water runoff or decreased evaporation or both and oxygen depleted bottom waters we suggest that this deterministic sedimentary cyclicity could be explained by periodic orbital climatic cycles in tune with the milankovitch cycles within this scenario the latest cenomanian bonarelli level is an exceptional event marked by a large positive 13c spike it is characterized by high to very high fertility conditions in surface waters caused by increased upwelling that engendered high nutrient levels in near surface waters the enhanced production of organic matter was in excess for the oxygen available at the bottom floor thus allowing the organic matter to accumulate in the sediments

subduction products according to experimental prediction knowledge of the thermal structure and processes in subduction zones is so uncertain that our more extensive knowledge of source materials and phase relationships remains insufficient for prediction of magmatic products the phase relationships do provide important constraints for testing geophysical and petrogenetic models especially when considered along with geochemical constraints physical questions such as the behavior of partially molten rocks are intimately related to the compositions of magmas from source to eruption site tentative predictions and conclusions include 1 andesite is not a primary magma from oceanic crust 2 it is unlikely that liquids from subducted oceanic crust yield andesites by fractionation except by melting of amphibolite in an especially warm subducted crust 3 it is more likely that hydrous siliceous magmas from the crust leak into the overlying mantle 4 andesite magmas can be generated in peridotite only under exceptional circumstances 5 h2o undersaturated basic magmas from peridotite modified by aqueous fluids or hydrous magma may yield andesite by fractionation 6 phlogopite peridotite could escape melting and yield alkalic magmas at deeper levels 7 andesite magmas could be produced in continental crust only by extreme heating through underplating of basalts or by mixing of basalt and rhyolite derived from crust most of these conclusions need revision if convection in the asthenosphere wedge raises the temperature of the subducted slab to 1250 c at 100 km as proposed by marsh 1979a this demonstrates the dependence of experimental predictions on geophysical models

mantle flow and dynamic topography associated with slab window opening insights from laboratory models abstract we present dynamically self consistent mantle scale laboratory models that have been conducted to improve our understanding of the influence of slab window opening on subduction dynamics mantle flow and associated dynamic topography over geological time scales the adopted setup consists of a two layer linearly viscous system simulating the subduction of a fixed plate of silicone lithosphere under negative buoyancy in a viscous layer of glucose syrup mantle our experimental setting is also characterized by a constant width rectangular window located at the center of a laterally confined slab modeling the case of the interaction of a trench parallel spreading ridge with a wide subduction zone we found that the opening of a slab window does not produce consistent changes of the geometry and the kinematics of the slab on the contrary slab induced mantle circulation quantified both in the vertical and horizontal sections using the feature tracking image analysis technique is strongly modified in particular rollback subduction and the opening of the slab window generate a complex mantle circulation pattern characterized by the presence of poloidal and toroidal components with the importance of each evolving according to kinematic stages mantle coming from the oceanic domain floods through the slab window indenting the supra slab mantle zone and producing its deformation without any mixing between mantle portions the opening of the slab window and the upwelling of sub slab mantle produce a regional scale non isostatic topographic uplift of the overriding plate that would correspond to values ranging between ca 1 and 5 km in nature assuming that our modeling results can be representative of the natural behavior of subduction zones we compared them to the tectonics and volcanism of the patagonian subduction zone we found that the anomalous backarc volcanism that has been developing since the middle miocene could result from the lateral flow of sub slab mantle and that part of the patagonian uplift could be dynamically supported

trench proximal volcanism following ridge crest trench collision along the antarctic peninsula on the basis of multichannel seismic reflection magnetics and bathymetry data a pleistocene holocene 0 1ma volcanic seamount has been identified at a location very close to the shelf edge off the west coast of the antarctic peninsula the seamount overlies the subducted trace of a transform fracture zone which divides segments of the margin along which collision took place 3 1 and 6 0 ma ago new trace element and isotopic data from dredged samples demonstrate that the seamount appears to be predominantly formed of relatively primitive undersaturated alkali basalt the basalts have geochemical signatures that are indistinguishable from ocean island basalts and some continental alkali basalts e g th ta 1 0 1 5 rb nb 0 5 87sr 86sr 0 70265 0 7028 143nd 144nd approximately 0 5129 and are totally lacking in any evidence for interaction with subduction enriched mantle geochemical similarities with other more trench distal postsubduction alkalic basalts along the antarctic peninsula are striking strongly implying that all the postsubduction basalts were derived from a chemically similar asthenospheric source region the basalts were most likely to have been generated as a result of the formation of a slab window beneath the antarctic peninsula following ridge crest trench collision subduction component free subslab asthenosphere upwelled into the incipient void left by the continued sinking of the leading plate following collision and decompressional melting resulted this type of trench proximal volcanism following ridge crest trench collision differs from that in other locations where calc alkaline volcanism persisted or ophiolite obduction occurred

tectonic setting of eocene boninite magmatism in the izu bonin mariana forearc abstract middle eocene boninites were simultaneously generated over a large region during the early history of the izu bonin mariana ibm arc however widespread boninite magmatism is not recognised in younger subduction zones of similar dimensions this suggests that an additional tectonic or thermal factor influenced the generation of the ibm boninite suite regional uplift ocean island basalt style magmatism and high heat flow also characterised the northern philippine sea plate psp at the start of the middle eocene these features are similar to those observed in large volume basaltic volcanic provinces such as the early tertiary north atlantic and suggest that the ibm boninite suite may have been produced because there was already a thermal anomaly in the mantle beneath the psp the reconstructed middle eocene location of the ibm arc and west philippine basin lies close to the present day manus basin where petrological and geochemical evidence indicate the presence of a mantle plume a calculated plume track linking these locations through time also passes close to the eauripik rise an aseismic ridge on the caroline plate during the oligocene and miocene therefore we propose that a thermal anomaly or mantle plume influenced the magmatic and tectonic development of the western pacific from the middle eocene until the present day

a geochemical traverse across the north chilean andes evidence for crust generation from the mantle wedge major and trace element and sr and nd isotope analyses are presented on 186 0 ma magmatic rocks along an east west traverse across north chile at 22 s esr ranges from 25 to 100 and end from 6 to 9 but the low end and high esr values are in rocks generated in the last 15 ma it is argued that previous discussions of petrogenesis in north chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one but two processes one results in a progressive shift of esr from 25 to 20 and end from 6 to 6 in jurassic to recent rocks which is accompanied by increasing ta sm and sr and decreasing th ta the second is largely confined to the younger rocks and it is characterised by esr increasing up to 100 with increasing sio2 and decreasing sr and it results in relatively shallow trends on an end esr diagram the preferred interpretation is that trend 1 is due to the mobilisation of old late proterozoic mantle lithosphere as magmatism migrated eastwards and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust it follows that the mantle wedge is the principal site of crust generation and it is argued that 20 of the sr in the recent northern chile rocks is derived from the subducted ocean crust

neogene patagonian plateau lavas continental magmas associated with ridge collision at the chile triple junction extensive neogene patagonian plateau lavas 46 5 to 49 5 s southeast of the modern chile triple junction can be related to opening of asthenospheric slab windows associated with collisions of chile rise segments with the chile trench at 12 ma and 6 ma support comes from 26 new total fusion whole rock 40ar 39ar ages and geochemical data from back arc plateau lavas in most localities plateau lava sequences consist of voluminous tholeiitic main plateau flows overlain by less voluminous 2 to 5 million year younger alkalic postplateau flows northeast of where the ridge collided at 12 ma most lavas are syncollisional or postcollisional in age with eruptions of both sequences migrating northeastward at 50 to 70 km ma plateau lavas have ages from 12 to 7 ma in the western back arc and from 5 to 2 ma farther to the northeast trace element and isotopic data indicate main plateau lavas formed as larger percentage melts of a garnet bearing oceanic island basalt oib like mantle than postplateau lavas the highest percentage melts erupted in the western and central plateaus in a migrating slab window model main plateau lavas can be explained as melts that formed as upwelling subslab asthenosphere which flowed around the trailing edge of the descending nazca plate and then interacted with subduction altered asthenospheric wedge and continental lithosphere alkaline postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window thermal problems of high pressure melt generation of anhydrous mantle can be explained by volatiles h2o and co2 introduced by the subduction process into slab window source region s an oib like rather than a mid ocean ridge basalt morb like source region and the lack of magmatism northeast of where ridge collision occurred at 13 to 14 ma can be explained by entrainment of weak plume s or regional variations in an ambient oib like asthenosphere

middle and late cenomanian oceanic anoxic events in shallow and deeper shelf environments of western morocco the response of shallow water sequences to oceanic anoxic event 2 and mid cenomanian events 1a and 1b was investigated along the west african margin of morocco north of agadir azazoul and correlated with the deep water sequence of the tarfaya basin mohammed beach based on biostratigraphy mineralogy phosphorus and stable isotopes in the deeper mohammed beach section results show double peaks in 13corg for mid cenomanian events 1a and 1b rotalipora reicheli biozone lower cc10a biozone the characteristic oceanic anoxic event 2 13c excursion rotalipora cushmani extinction top of cc10a biozone and laminated anoxic black shale in the shallow environment north of agadir a fluctuating sea level associated with dysoxic brackish and mesotrophic conditions prevailed during the middle to late cenomanian as indicated by oyster biostromes nannofossils planktonic and benthonic foraminiferal assemblages anoxic conditions characteristic of oceanic anoxic event 2 for example laminated black shales did not reach into shallow water environments until the maximum transgression of the early turonian climate conditions decoupled along the western margin of morocco between mid cenomanian event 1b and the cenomanian turonian boundary as also observed in eastern tethys north of agadir alternating humid and dry seasonal conditions prevailed whereas in the tarfaya basin the climate was dry and seasonal this climatic decoupling can be attributed to variations in the intertropical convergence zone and in the intensity of the north east trade winds in tropical areas

proximate environmental forcing in fine scale geochemical records of calcareous couplets upper cretaceous and palaeocene of the basque cantabrian basin eastern north atlantic abstract calcareous couplets are key elements in reconstructing the evolution of a sedimentary basin due to the influence of forcing mechanisms such as climate sea level and tectonism on their depositional patterns proposed forcing mechanisms however are often not mutually exclusive and even constraining the relative importance of different processes is problematic added to the question of discriminating forcing mechanisms a major challenge is to produce high sampling density so that observations lie within temporal resolutions equal to or finer than the timescales on which different forcing operates here we show fine scale 1 sample 2 cm caco3 18o and 13c records and sedimentological observations from three different sites isla de castro sopelana ma and sopelana da with calcareous couplets in the basque cantabrian basin eastern north atlantic to illustrate the potential of fine sampling strategies to help distinguish proximate environmental forcing partial redistribution of carbonate during burial diagenesis has been proposed for these sediments our caco3 18o and 13c data could thus be dismissed as diagenetic signals if only one sample was collected from each bed detailed observations of the fine scale geochemical records however challenge purely diagenetic explanations combined with sedimentology the caco3 18o and 13c values partially altered by diagenesis are interpreted to have resulted from alternating climates the proximate forcing through which alternating climates caused the geochemical patterns though was different in each section due to their specific palaeogeographic positions in the basin and the properties of the seawater masses the proximity of continental areas of high relief to the isla de castro section supported a high continental influence during its deposition the sopelana ma sediments are assigned to a transgressive system tract a condition that is interpreted to have promoted a high influence of oceanic processes in its depositional setting finally a relatively cool oxygen rich water mass with high oxidation potential influenced the geochemical and depositional patterns of sopelana da beyond the climatic and oceanographic dynamics inferred for a basin that linked the western tethys with boreal domains during major marine transgressions of the late cretaceous to palaeocene an implication of our work is that if similar fine scale geochemical records were applied to calcareous couplets spanning major events in earth s history e g ocean anoxic events alternative forcing scenarios leading to and out of these events could be discriminated

dissolution susceptibility of paleocene eocene planktic foraminifera implications for palaeoceanographic reconstructions abstract we investigated shell characteristics and differential dissolution susceptibility of planktic foraminiferal species derived from upper paleocene and lower eocene deep sea sequences ocean drilling program odp site 865 allison guyot and sites 1209b 1210b and 1212a shatsky rise in the north pacific ocean the purposes of this study are 1 assessing the effects of differential dissolution on upper paleocene lower eocene planktic foraminiferal assemblages at species level and within different biozones to quantify dissolution susceptibility of genera and species 2 investigating the differences in shell characteristics 3 revealing the relationship between shell parameters and dissolution robustness of taxa and 4 identifying the key shell parameter s influencing the dissolution susceptibility of foraminiferal taxa two independent experiments were carried out one focusing on gradual qualitative deterioration of taxa by dissolution and the other documenting the weight loss of taxa shell parameters such as wall thickness porosity and pore size were determined through scanning electron microscopy sem and image analysis jmicrovision we found that the large muricate acarinina and morozovella are most resistant followed by the cancellate subbotina and the small muricate igorina confirming results of previous work at species level the thick walled acarinina soldadoensis acarinina subsphaerica and the large morozovella subbotinae are the most resistant species most of the large morozovella species such as morozovella aequa morozovella formosa gracilis morozovella velascoensis and morozovella pasionensis together with acarinina nitida show intermediate dissolution resistance whereas the small muricate igorina species the cancellate subbotina velascoensis and the thin walled morozovella acuta and morozovella occlusa are the most vulnerable species we propose a formula for calculating the dissolution resistance of taxa based on their wall thickness and size two key parameters in dissolution resistance of a species application of this formula reveals good agreement between the calculated and measured dissolution resistance indicating its robustness furthermore the agreement between our experimental results in situ experimental results on live foraminifera and natural quantitative qualitative records suggests that our experiments accurately mimic natural dissolution processes consequently these experimental results strongly bear on interpretations of foraminiferal dissolution in natural environments especially in studies on early paleogene climatic events that are often associated with dissolution phenomena more generally a proper assessment of taphonomic alteration by dissolution should be part of every paleoenvironmental reconstruction based on quantitative foraminiferal records

analysis of james ross island volcanic complex and sedimentary basin based on high resolution aeromagnetic data abstract high resolution aeromagnetic surveys provide a geophysical tool to help image the subsurface structure of volcanoes and their tectonic framework here we interpret high resolution aeromagnetic data and models for james ross archipelago and surrounding regions to provide a new perspective on neogene magmatism emplaced along the eastern margin of the antarctic peninsula based on the analysis and modelling of magnetic anomalies we were able to image the subglacial extent of miocene to recent alkaline rocks of the james ross island volcanic group and map tectonic structures that appear to have exerted important controls on neogene magmatism high amplitude linear anomalies detected over mount haddington stratovolcano were modelled as caused by subvertical feeder bodies extending to a depth of at least 3 km these feeder bodies may have been emplaced along a n s oriented volcano tectonic rift zone we also identified several effusive subglacial centres and imaged two concentric magnetic arcs which we related to neogene volcanic and subvolcanic lineaments likely controlled by mid cretaceous strike slip fault belts and associated deformation zones the regional magnetic quiet zone that encompasses james ross island is caused by the thick sedimentary infill of the larsen basin and a low amplitude magnetic high within the basin is inferred to reflect a basement push up structure associated with strike slip faulting along the eastern edge of the antarctic peninsula in the offshore regions magnetic anomalies southwest of tabarin peninsula and west of vega island may reflect recent volcanic structures that have yet to emerge from the sea floor

geology geochronology and tectonic setting of late cenozoic volcanism along the southwestern gulf of mexico the eastern alkaline province revisited a nnw trending belt of alkaline mafic volcanic fields parallels the gulf of mexico from the u s border southward to veracruz state in eastern mexico previous studies grouped this volcanism into the so called beastern alkaline provinceq eap and suggested that it resulted from gulf parallel extensional faulting migrating from north to south from oligocene to present on the basis of new geologic studies forty nine unspiked k ar and two 40 ar 39 ar ages we propose a new geodynamic model for the volcanism along the southwestern gulf of mexico we studied in detail four of the six recognized fields of mafic alkaline volcanism in veracruz state 1 the lavas flows of tlanchinol area 7 3 5 7 ma 2 the alamo monogenetic field and sierra de tantima 7 6 6 6 ma 3 the poza rica and metlatoyuca lava flows 1 6 1 3 ma and 4 the chiconquiaco palma sola area 6 9 3 2 ma other two mafic volcanic fields may represent the continuation of alkaline volcanism to the southeast the middle miocene lavas at anegada high offshore port of veracruz and the middle to late miocene volcanism at the los tuxtlas the existence of major neogene extensional faults parallel to the gulf of mexico i e n s to nnw sse proposed in previous works was not confirmed by our geological studies elongation of volcanic necks vent alignment and faults mapped by subsurface data trend dominantly ne to ene and nw to nnw these directions are parallel to transform and normal faults that formed during the late jurassic opening of the gulf of mexico ascent of mafic magmas was likely facilitated and controlled by the existence of these pre existing basement structures

new insights into geochemical behaviour in ancient marine carbonates upper jurassic ammonitico rosso novel proxies for interpreting sea level dynamics and palaeoceanography elemental concentrations in phanerozoic sea water are known to fluctuate both in time and space with regard to carbonates precipitated from marine fluids elemental concentrations in the carbonate crystal lattice are affected by a complex array of equilibrium and non equilibrium as well as post depositional alteration processes to assess the potential of carbonate elemental chemostratigraphy seven upper jurassic sections were investigated along a proximal to distal transect across the south east iberian palaeomargin the aim was to explore stratigraphic and spatial variations in calcium strontium magnesium iron and manganese elemental abundances the epicontinental geochemical record is influenced by the combination of continental runoff and a significant diagenetic overprint in contrast the epioceanic geochemical record agrees with reconstructed open marine sea water values reflecting a moderate degree of syn depositional to early marine pore water diagenesis establishing a fair degree of preservation of matrix micrite a thorough statistical approach was applied and elemental associations tested for their environmental significance principal component and hierarchical cluster analyses revealed a persistent relation between carbonate magnesium iron and strontium abundances processes related to early diagenetic nodulation in ammonitico rosso facies most probably account for the incorporation of these elements in the calcium carbonate lattice the clear decoupling of carbonate manganese abundance with respect to the remaining elements is documented and related to high sea floor spreading rates and hydrothermal activity during the late jurassic the investigation of potential time fluctuation of geochemical patterns was approached through variogram computation the observed temporal behaviour is most likely to be forced by relative sea level dynamics reflecting late jurassic palaeoceanographic conditions and potential planetary interactions the data obtained in this study highlight the utility of elemental data from carbonate matrix micrites as geochemical proxies for studying the influence of remote trigger factors

orbitally forced sedimentary rhythms in the stratigraphic record is there room for tidal forcing the imprint of orbital cycles which result from the varying eccentricity of the earth s orbit and changes in the orientation of its axis have been recognised throughout the phanerozoic rock record variations in insolation and their effect on climate are generally considered to be the sole transfer mechanism between the orbital variables and cyclic sedimentary successions common oceanographic principles however show that the ocean tide also responds to variations in the orbital parameters the ocean tide has not yet been considered to be a valid additional transfer mechanism for the orbital variations in geological studies of milankovitch cycles in sedimentary successions the insolation paradigm offers satisfying explanations and the role of long term variations of the ocean tide has not yet been appreciated variations in the ocean tide related to changing eccentricity at present 0 0165 theoretical maximum 0 0728 affect a variety of oceanographic and sedimentary processes in addition to the widely accepted paradigm of orbitally forced insolation changes the tidal transfer of orbital signals may explain certain less well understood aspects of orbitally induced cycles in the stratigraphic record related to ocean mixing organic productivity and tidal processes in shallow seas and deep water variations of the ocean tide in relation to the 18 6 year lunar nodal cycle which has no insolation counterpart by which they may be obscured indeed show that these relatively small variations can produce significant effects in sedimentary environments that are sensitive to variations in the strength of the ocean tide in analogy with the 18 6 year lunar nodal cycle orbital variations of the tide on milankovitch time scales are likely to have affected sedimentary systems in the past

the scotia sea gateway no outlet for pacific mantle abstract the scotia sea in the south atlantic holds a prominent position in geodynamics because it has been proposed as a potential outlet of asthenosphere from under the shrinking pacific into the mantle beneath the opening atlantic shear wave splitting and geochemical studies have previously tested this hypothesis here we take a different approach by calculating present day dynamic topography of the region in search for a systematic trend in dynamic topography decreasing from west to east in response to a flow related pressure gradient in the sublithospheric mantle to this end we reconstruct the kinematic history of the scotia sea which is characterized by complex back arc spreading processes active on a range of time scales our plate reconstructions allow us to derive an oceanic age grid and to calculate the associated residual dynamically maintained topography of the scotia sea by comparing present day isostatically corrected topography with that predicted from our reconstruction the results provide no indication for a systematic trend in dynamic topography and we conclude that the material needed to supply the growing subatlantic mantle must be derived from elsewhere