cathodoluminescence of late triassic terebratulid brachiopods implications for growth patterns abstract upper triassic terebratulid brachiopod rhaetina shows differential preservation of shell structure under cathodoluminescent cl and scanning electron microscope sem indicating intra and interspecific differences in brachiopod growth patterns in longitudinal sections the cl lines appear firstly in inner ontogenetically older parts of the shell and their outlines are parallel with the internal valve surface in posterior parts of the shell in anterior parts of the shell cl lines run diagonally towards the external valve surface and terminate in external growth lines the shell structure of external growth lines indicates that major growth lines associated with gradual reorientation of calcitic crystals due to reflection of the mantle edge and disturbance lines associated with abrupt mantle regression are present at the terminations of cl lines these data indicate that cl lines correspond to zones recording slowing cessation of the growth rate under sem shell layers that are luminescent can be locally recognized as amalgamated or altered relicts of the secondary fibres the origin of luminescence in these areas can be related to 1 the increased mn bearing diagenetic fluid flow favoured by higher porosity of altered secondary fibres or can be caused by 2 temporal changes in the redox state controlling the solubility of mn2 and fe2 during the mantle anaerobiosis similarly amalgamated altered shell structure could either point to 1 the primary modification of shell secretory regime during the slowing cessation in growth rate or could be 2 the product of diagenetic processes responsible for post depositional alteration of a shell structure due to the primary shell heterogeneity porosity in areas of growth discontinuities although presented data do not convincingly solve this uncertainty the important point is that cl lines of studied brachiopods reflect changes in the growth rate and can be used in the investigation of growth dynamics in brachiopods complementary to external growth lines and isotope or trace element analysis in order to verify the modification of shell structure and understand the origin of associated luminescence during the change in growth rate unbiased by diagenetic overprint it is necessary to examine modern brachiopods the distribution pattern of cl lines that correlates with growth rate fluctuations potentially reflects a short term environmental instability on the time scale of an individual life span the data show correlation between cl line pattern and different bathymetric settings regular repetition of cl lines is preserved in rhaetina gregaria rhaetina hybensis and rhaetina pyriformis that occur in relatively uniform depositional settings below a normal storm wave base in contrast irregular pattern of cl lines is typical of r gregaria from highly variable deposit associations derived from very shallow subtidal settings above a normal storm wave base in the case of r gregaria this difference reflects intraspecific variations in growth dynamics among bathymetric settings indicating environmental control on their growth rates rhaetina gregaria is mostly the only one brachiopod species present in benthic assemblages from very shallow marginal marine environments in the western tethys the implication is that r gregaria in contrast to most other brachiopods could live in physically stressed habitats characterized by higher environmental instability unpredictability

morphology internal architecture and emplacement mechanisms of lava flows from the central atlantic magmatic province camp of argana basin morocco abstract the morphology internal architecture and emplacement mechanisms of the central atlantic magmatic province camp lava flows of argana basin in morocco are presented the volcanic pile was produced by two volcanic pulses the first represented by the tasguint formation corresponds to a succession of 3 13 individual flows created by 1 8 eruptions the second alemzi formation is composed of 2 7 individual flows formed by 1 4 eruptions these formations geochemically distinct are separated by thin silty or sandy horizons or by palaeosols they include compound pahoehoe flows and simple flows the first type is almost exclusive of the lower formation while the second type dominates the upper formation the lava flows show clear evidence of endogenous growth or inflation the characteristics of the volcanic pile suggest slow emplacement during sustained eruptive episodes and are compatible with a continental basaltic succession facies model

boron carbon and oxygen isotopic composition of brachiopod shells intra shell variability controls and potential as a paleo ph recorder abstract the boron isotopic composition of biogenic carbonates has been used to reconstruct seawater ph and atmospheric pco 2 on pleistocene and cenozoic timescales because of their excellent preservation and extensive fossil record throughout the phanerozoic brachiopods are a promising candidate for extending the boron isotope record as far back as the cambrian here we present stable carbon oxygen and boron isotopic measurements of modern terebratulid brachiopod calcite in comparison with environmental ph estimates calculated from oceanographic data geochemical transects along the length and depth of single shells confirm previously published trends in carbon and oxygen isotopic composition in the outer surface primary and outermost secondary layers 11 b covaries with 13 c and 18 o with more negative values in the outer and more positive values in the middle of the shell however 11 b deviates from 13 c and 18 o in the inner part of the secondary layer where the 13 c and 18 o values are more positive and near equilibrium whereas 11 b returns to more negative values a comparison of different specimens of the species terebratalia transversa sowerby 1846 and laqueus californianus kuster 1844 microsampled from the middle part of the fibrous secondary layer demonstrates a clear correlation to ambient ph with a sensitivity similar to other empirical calibration curves for cultured planktic foraminifers corals and inorganic calcite the relationship in other species is less clear and significantly offset necessitating the use of a single species or a cross calibration method with other species in paleo ph reconstructions

meso cenozoic long term landscape evolution at the southern moroccan passive continental margin tarfaya basin recorded by low temperature thermochronology abstract this paper presents the first regional study of low temperature thermochronology to be undertaken in the tarfaya basin at the southern moroccan passive continental margin the basin is characterised by vast subsidence since mid triassic times whereby up to 12 km of meso to cenozoic sedimentary rocks accumulated the study focused on the post rift vertical movements along a typical passive margin and besides dealt with the timing and maximum temperature reached by potential source rocks of the basin to unravel the t t development thermochronological analyses were performed on 50 outcrop and well samples from meso cenozoic rocks thermochronological data reveal a continuous subsidence phase in the offshore basin from mid triassic to recent times in contrast apatite u th sm he and apatite fission track data as well as thermal modelling point to an inversion of the northeastern onshore basin starting in the palaeogene at 65 55 ma the rock uplift and exhumation period resulted in the erosion of a 1 0 1 4 km thick cretaceous palaeogene sedimentary pile contemporaneously with peak atlas surface uplift in the cenozoic the exhumation stage could be an explanation for the increasing periodic influx of detrital material into the offshore and southern onshore tarfaya basin since palaeocene detrital apatite fission track ages from 92 16 to 237 35 ma of the upper cretaceous neogene succession indicate no heating above 60 c confirming immature to early mature cenomanian to campanian and eocene source rocks in the onshore tarfaya basin

phosphate defects and apatite inclusions in coral skeletal aragonite revealed by solid state nmr spectroscopy abstract recent development of paleo nutrient proxies based on the phosphorus calcium p ca ratio in tropical and deep water corals also known as cold water corals require an understanding of the processes by which p is incorporated into the coral skeletal aragonite here we apply single and double resonance solid state nuclear magnetic resonance nmr spectroscopy to determine the speciation of p in coral aragonite the results show that the majority of p occurs as phosphate defects in the aragonite structure but in many samples a significant fraction of the p occurs also in crystalline hydroxylapatite inclusions quantification of the amount of hydroxylapatite indicates that its presence is not related simply to external environmental factors and that it can occur at varying abundances in different parts of the same corallite since there is currently no model available to describe the relationship between dissolved inorganic phosphate and its incorporation as apatite inclusions into carbonates careful screening of samples which contain only phosphate in the aragonite structure or selective microsampling could improve proxy development

textural elemental and isotopic variations among constituents in middle devonian limestones north america abstract biogenic components brachiopods crinoids and corals in middle devonian limestones were manually separated from their enclosing matrix and analyzed by textural elemental and isotopic methods in order to assess the preservational state of these components in general brachiopods in these rocks are better preserved texturally than other constituents as determined by sem and cathodoluminescence observations elemental variations among the major components are compatible with a differential response to diagenesis governed by the relative stability of the original mineralogy of the component however among the most stable low mg calcite brachiopods the relationship between textural preservation and trace element content is complex this apparent complexity may be due at least par ially to our method of textural characterization of whole brachipod samples the nonluminescent portions of a set of well preserved brachipods have textural characteristics and elemental contents which are virtually indistinguishable from modern representatives consequently their 18o values 3 7 0 2 are assumed to represent primary biogenic calcite these 18o values are in the heavy end of the range of values from texturally well preserved whole brachiopods 6 6 to 2 5 texturally altered whole brachiopods 6 7 to 3 7 and texturally altered crinoids and cora s 6 7 to 2 7 this comparison suggests that marine derived pore waters were a predominant component during the principal phase of diagenesis the relatively low l8o values of well preserved brachiopods compared with modern representatives implies that middle devonian seawater was either warmer by about 10 c or depleted in 18o by about 2 3 relative to modern seawater

ocean island densities and models of lithospheric flexure estimates of the effective elastic thickness te of the oceanic lithosphere based on gravity and bathymetric data from island loads are commonly significantly lower than those based on the wavelength of plate bending at subduction zones the anomalously low values for ocean islands have been attributed to the finite yield strength of the lithosphere to erosion of the mechanical boundary layer by mantle plumes to pre existing thermal stresses and to overprinting of old volcanic loads by younger ones a fifth possible contribution to the discrepancy is an incorrect assumption about the density of volcanic loads we suggest that load densities have been systematically overestimated in studies of lithospheric flexure potentially resulting in systematic underestimation of effective elastic thicknesses and overestimation of the effects of hotspot volcanism we illustrate the effect of underestimating load density with synthetic examples and an example from the marquesas islands this effect combined with the other effects listed above in many cases may obviate the need to invoke hotspot reheating to explain low apparent elastic thicknesses

environmental and physiological influences on isotopic and elemental compositions of brachiopod shell calcite implications for the isotopic evolution of paleozoic oceans abstract brachiopods from the demissa bed middle devonian hamilton group biologically regulated the incorporation of mg sr and na into their shell calcite significant taxonomic differences in the elemental contents of three species athyris spiriferoides mediospirifer audacula and mucrospirifer mucronatus may be related to differences in calcification processes in contrast no significant difference or vital effects were observed between the isotopic values of mediospirifer audacula and athyris spiriferoides from erie 18o 0 03 p 0 949 13c 0 76 p 0 083 and genessee counties 18o 0 39 p 0 471 13c 0 06 p 0 854 this suggests that these brachiopods did not exert a biological control over their isotopic compositions and that their shell calcites reflect ambient physicochemical conditions isotopic compositions in unaltered shell calcites of brachiopods from genessee county which was close to the basin depocentre are heavy for carbon 13c x 5 01 pdb and oxygen 18o x 2 85 pdb compared to the species sampled at the basin s edge erie county 13c x 2 79 pdb 18o x 3 83 pdb there is a significant separation in isotopic values between the deeper and shallower water brachiopods of the basin 18o 0 98 p 0 0005 13c 2 22 p 0 0005 the 18o variation suggests a temperature salinity change with water depth whereas the change in 13c composition probably records an enrichment depletion of organic matter with water depth this observation has significant implications for paleozoic ocean isotopic evolution studies because many global changes in marine 18o and 13c are based on isotopic shifts of similar magnitude

heterogeneity cyclicity and diagenesis in a mississippian brachiopod shell of palaeoequatorial britain a shell of gigantoproductus okensis shows twenty growth lines with marked changes of fabric indicating periodical reduction of growth rates caused by environmental perturbations the number of growth lines suggests a lifespan of 20 years in agreement with the survival rates of extant brachiopods and with spiral deviation analysis geochemical analyses across the growth profile show a heterogeneous distribution of stable isotopes and trace elements it is possible to distinguish primary from altered carbonate and to interpret the isotopic data the oxygen isotope signal in the unaltered parts is periodical and annual with oscillation of 1 1 the higher values are at the growth lines winter and therefore most likely related to monsoon circulation during the visean the annual periodicity seems also present in the altered part of the shell suggesting that diagenesis could have reset the primary values but preserved their cyclicity

triassic early liassic extensional systems and their tertiary inversion essaouira basin morocco abstract nne ssw striking late triassic early liassic half grabens of the essaouira basin and the adjacent high atlas are linked by roughly ew striking transfer faults they are overlain by a wide considerably less faulted evaporitic sag basin and extensive basalt flows a pre pliensbachian unconformity truncates the triassic early liassic sequences on the e and ne margin of the basin and is overlain by a mid liassic to cretaceous platform sequence in the cap sim necnafa depression jurassic and early cretaceous salt tectonics caused widespread salt withdrawal structures tertiary nnw to sse compression caused the inversion of ne and nne striking structures and or the formation of salt anticlines the same compression formed ew striking reverse faults and anticlines suggesting the reactivation of late triassic early liassic transfer faults

the western termination of the jebilet high atlas system offshore essaouira basin morocco abstract industrial seismic profiles from the offshore essaouira basin reveal the previously unknown cap tafelney folded belt as the offshore termination of the jebilet high atlas system of morocco this evaporite based decollement folded belt involves a dramatically southward thickening mesozoic basinal sequence the cretaceous of this sequence was deposited in a flexural basin that responded to tectonic loading by the nascent atlas mountains to the south the ne sw striking cap tafelney folds are formed over a basal decollement which towards the east ramps down into the underlying paleozoic continental basement this over 100 km wide lateral ramp system is associated with a pronounced cretaceous tertiary shelf margin flexure smf which is superimposed on the jurassic lower cretaceous carbonate platform margin the deformation of the cap tafelney folded belt began in the mid cenomanian and lasted into the tertiary

geochemistry of modern brachiopods applications and implications for oceanography and paleoceanography abstract modern rhynchonellida terebratulida thecideida and a few inarticulated brachiopods were collected live or soon after death at 30 locations from depths of 2 3940 m from the equator to high latitudes covering the atlantic pacific indian caribbean and mediterranean oceans seas well preserved fibers of the secondary layer combined with sr 450 1928 mg kg mn 1 199 mg kg and fe 1 610 mg kg contents confirm with few exceptions the excellent preservation of the modern brachiopod material evaluation of the stable isotope compositions was conducted on 276 specimens and on 374 published samples for a total database of 650 samples from 80 locations it revealed that oxygen isotope incorporation into shell calcite secondary layer of modern articulated brachiopods in most cases occurs in equilibrium with the ambient water and as such reflects surrounding environmental conditions thecideida as a group shallow water and low latitude incorporate oxygen isotopes 3 04 to 0 35 into shell calcite in equilibrium with their ambient water a large majority of genera and species of the terebratulida shallow water and low mid latitude also incorporate oxygen 2 30 to 3 48 into shell calcite in isotopic equilibrium with their surrounding water in contrast equilibrium incorporation of oxygen isotopes into shell calcite by the rhynchonellida is uncertain but a vital effect probably applies to the craniida the question of equilibrium disequilibrium incorporation of carbon isotopes into shell calcite of modern brachiopods remains unresolved at this time isotope compositions range up to 4 5 5 6 and 1 5 for 13 c and up to 5 5 5 9 and 0 3 for 18 o for shallow water low mid and high latitude brachiopods respectively strontium isotopes of modern brachiopods as expected show no significant variation with location latitude water depth temperature salinity and general water conditions once local habitat and ecological influences of fossil brachiopods from similar latitudes are considered their isotopic elemental values should be valuable diagenetic indicators and important oceanographic chemostratigraphic proxies

stratigraphic record of the early mesozoic breakup of pangea in the laurasia gondwana rift system rift basins of the central atlantic margins cam of north america and morocco preserve largely continental sequences of sedimentary strata and less important minor basalt flows spanning much of the early mesozoic the best known is the newark basin of new jersey new york and pennsylvania where an astronomically calibrated magnetic polarity time scale is developed lacustrine cycles of milankovitch origin are commonly present in cam basins with the period changing from 10 ky paleoequator with coals to 20 ky 4 10n to perhaps 40 ky northward with evaporites cycles of100 ky 413 ky and2 my are also important four mostly unconformity bounded tectonostratigraphic sequences are present the anisian ts i is fluvial and eolian ts ii ts iv late triassic to early jurassic consist of tripartite lacustrine sequences caused by extension pulses the newark basin accumulation rate history allows comparison with quantitative rift basin models the north american plate s slow northward drift resulted in a relative shift of climate although the rapid humidification during the latest triassic and early jurassic is associated with a sea level rise the triassic jurassic mass extinction is of independent origin plausibly impact related

geochemistry of campanian maastrichtian brachiopods from the rordal 1 core denmark differential responses to environmental change and diagenesis abstract past marine environmental conditions can be reconstructed from geochemical signatures of both benthic and planktic organisms and particularly brachiopods have been extensively used for this purpose however it is seldom possible to investigate the links between environmental conditions and shell geochemistry in monospecific records over extended periods of time here we present geochemical data from the 100 m thick brachiopod rich upper campanian upper maastrichtian chalk 73 8 68 6 ma of the rordal 1 drill core northern denmark two species that lived attached to small substrate argyrotheca bronnii roemer and gisilina jasmundi steinich and one secondarily free living species magas chitoniformis schlotheim have been studied for element concentrations as well as c and o isotope signatures each species has a unique chemical signature with m chitoniformis showing lowest mg and mn concentrations a bronnii the highest mn concentrations and g jasmundi the highest sr concentrations all three species display decreasing mg ca ratios throughout the studied interval and distinct mn enrichments in the campanian maastrichtian boundary cmb interval in the campanian part of the succession the three species are slightly enriched in 13 c median 13 c values of 0 0 to 0 5 and 18 o median 18 o values of 0 3 to 0 6 with respect to coeval chalk in the course of the maastrichtian the two attached species acquire progressively lighter isotopic compositions with a bronnii reaching 13 c and 18 o values 1 0 lower than coeval chalk the secondarily free living species m chitoniformis on the other hand becomes isotopically heavier and reaches 13 c and 18 o enrichments of 1 0 at the top of the core the observed differences between the species illustrate species specific signatures of biomineralisation diagenesis and response to environmental change this geochemical complexity illustrates that a comprehensive geochemistry based picture of the late cretaceous chalk sea can only be painted using detailed multi species and multi proxy datasets

age of jurassic continental tholeiites of french guyana surinam and guinea implications for the initial opening of the central atlantic ocean abstract a detailed 40ar 39ar study of mineral separates from the jurassic atlantic continental tholeiites jact of guyana french guyana and surinam south america and guinea west africa related to the initial opening of the central atlantic has been carried out in french guyana plateau ages of 196 0 5 7 ma and 196 1 7 5 ma were obtained on single small amphibole grains from nnw sse trending dykes in guinea single biotite grains from intrusive formations from the kakoulima and fouta djalon areas yielded plateau ages of 200 4 0 2 ma and 194 8 0 5 ma concordant with high temperature apparent ages on other biotites the bulk plagioclase samples display disturbed age spectra due to alteration and excess argon however intermediate temperature weighted mean plagioclase ages are similar in both regions of guyana and guinea ranging from 200 2 2 4 ma to 188 7 1 9 ma partly in agreement with the amphibole and biotite data these data combined with previous 40ar 39ar and u pb results from the northern part of the central atlantic margins indicate intense magmatic activity distributed over a large area from iberia to liberia ca 4500 km long for a short period of time 204 195 ma perhaps less for the bulk of the magmatism during the initial break up of pangea continent these data do not support an initiation of the magmatism from a radial volcano tectonic system centred in the south of the region as suggested by may 1 and the initial break up seems to affect the whole central atlantic during a period of 9 ma

oxygen isotopes and mgco3 in brachiopod calcite and a new paleotemperature equation abstract modern brachiopods and their ambient seawater compiled from 34 localities covering shallow waters from the poles to the tropics were analyzed for their oxygen isotopic compositions and srco3 and mgco3 contents if the routine seawater 18o composition and the mgco3 effect on oxygen isotopes are considered in calculations calcification temperatures are concordant with measured ambient seawater temperatures the srco3 contents of modern brachiopods appear to be unrelated to ambient growth temperatures while their mgco3 contents exhibit some relationship with temperature and growth rate controlled in part by ambient productivity or clearance rates or local environment and by taxonomic affinity such as presence or absence of caeca linear least squares regression analysis yields a new oxygen isotope paleotemperature equation for brachiopod calcite which makes adjustment for shell mgco3 contents and covers habitats with water temperatures ranging from 2 to 32 c t c 16 192 3 468 c sw mg c n 319 r 2 0 98 the mg effect adjustment is based on the change in shell calcite 18o by 0 17 per mol mgco3 jimenez lopez et al 2004 this adjustment is critical to the determination of calcification temperatures for modern articulated brachiopods especially the ones from cold and warm water habitats and or the ones with exceptionally low or high mgco3 contents which otherwise may be offset lower or higher by as much as 7 c furthermore this adjustment may be critical for fossil brachiopods and other marine calcitic invertebrates with variable mgco3 contents and when using their 18o and 47 for determining ancient seawater 18o compositions and temperatures

results from a detailed aeromagnetic survey across the northeast newfoundland margin part ii early opening of the north atlantic between the british isles and newfoundland abstract poles of rotation for the north atlantic have been derived from the results of a new aeromagnetic survey northeast of newfoundland reconstruction of the north atlantic at anomaly 34 time shows a band of large amplitude magnetic anomalies which parallels anomaly 34 on both sides of the atlantic from flemish cap and goban spur to the azores gibraltar fracture zone a group of similar anomalies has also been identified in the bay of biscay north of goban spur and flemish cap these anomalies follow the ocean continent boundary poles of rotation derived for this anomaly show that it forms an isochron 100 110 m y during the long cretaceous normal polarity interval the cause of this anomaly is not definite but it may represent an increase in the magnetization of the crust during a limited time within the cretaceous magnetic quiet zone by a process such as replacement of thermoremanent magnetization by chemical remanent magnetization as proposed by raymond and labrecque the north atlantic has also been reconstructed at the time of the initial opening in the region between flemish cap and the charlie gibbs fracture zone using inferred ocean continent boundaries on the west and east sides it has been shown that the entire region could not have saparated at one time but that spreading between the british isles and newfoundland had to progress from south to north consequently when active sea floor spreading was taking place between goban spur and flemish cap about 110 m y the region to the north was still being stretched the calculated amount of stretching as derived from the reconstructions about 25 agrees well with the extension of the lithosphere obtained from modelling the subsidence history of this region and with the results of deep seismic studies active spreading in the north started about 100 m y ago

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

transects of the ancient and modern continental margins of eastern canada abstract rocks of the west flank of the northern appalachian orogen miogeocline record the history of the late precambrian early paleozoic passive continental margin of eastern north america the ancient margin was destroyed by ophiolite obduction and arc collision during the ordovician taconic orogeny the present sinuous form of the miogeocline is interpreted to reflect ancient promontories and re entrants of a previous orthogonal margin bounded by rifts and transforms four major terranes are recognized east of the miogeocline in newfoundland and nova scotia from west to east these are the dunnage gander avalon and meguma the dunnage and gander terranes were linked to the miogeocline during the middle ordovician taconian orogeny the avalon terrane arrived later possibly during the mid paleozoic acadian orogeny the meguma terrane of southern nova scotia had docked with the avalon terrane by carboniferous time the dunnage terrane contains arc volcanics which lie above an ophiolitic substrate the gander terrane comprises a thick sequence of clastic sedimentary rocks underlain by basement rocks with continental affinities it has been interpreted as a continental margin perhaps once on the eastern side of the paleozoic iapetus ocean the avalon terrane consists of belts of sedimentary and volcanic rocks which are probably underlain by grenvillian basement its tectonic affinities are unclear the meguma terrane comprises a thick sequence of sediments derived from the south east it is found only in southeastern atlantic canada the boundaries between terranes are compressional in the west and steep transcurrent faults in the east the surface extent of the geological terranes is grossly correlative with deep structural zones although no direct evidence exists for linking the two because most surface structures can be traced geophysically to only a few kilometres depth a striking feature of the deep crustal structure is a lower high velocity crustal layer beneath the dunnage and gander terranes the modern margin of atlantic canada developed by rifting and by transform motion between adjacent continents stretching and thinning of the lithosphere and the consequent production of basaltic magma that in places intrudes or underplates the thinned continental crust are the most likely processes responsible for the evolution of the modern margin these processes predict the observed deep sedimentary basins along the margin the thinning of continental crust and the high seismic velocities found within the ocean continent transition zones rifting adjacent to nova scotia began in late triassic early jurassic time between the present african and north american plates these plate motions are also responsible for the major transform margin south of the grand banks separation between iberia and the eastern grand banks occurred in mid cretaceous time before the late cretaceous opening of the labrador sea while the rifted segments of the margin exhibit deep sedimentary basins and thinned continental crust the grand banks transform segment is characterized by a sharp transition zone and a relatively thin sediment cover numerous volcanic seamounts are built on the ocean crust adjacent to this transform segment mimicry of paleozoic promontories and re entrants by modern rift and transform margin segments the location of mesozoic sedimentary basins on ancestral appalachian structures and the reactivation and propagation of major precambrian and paleozoic structural boundaries during the latest phase of ocean opening attest to ancestral controls of the modern margins the rift phase of both the ancient and modern passive margins is characterized by volcanism mafic dike intrusion and by the development of basins filled with clastic sediments the drift phase of both the ancient margin and the present nova scotia margin is marked by a change in sedimentary environment such that carbonates replaced the rift phase clastic sediments two of the markers used to delineate the ancient ocean continent transition zone carbonate banks and steep gravity anomaly gradients should be used with caution as the modern analogs of these markers may lie 100 km or more of this transition zone furthermore it is naive to view the ancient transition as simple and narrow for the modern margins exhibits complex transition zones between 30 and 300 km wide in general the evolution of the ancient and modern passive margins appear to be remarkably similar predictably closing the present atlantic will mimic the evolution of the appalachian orogen

2d petroleum system analysis of the tarfaya basin on offshore morocco north africa abstract this study presents a 2d basin model along a transect crossing the on and offshore tarfaya basin morocco in se nw direction the aim of the project is to investigate the thermal evolution of the basin fill and its influence on the petroleum system the modelling is based on a recently interpreted seismic section and includes new and published geochemical and organic petrological data from five wells core samples and cuttings provide information which was used for thermal calibration of the model vitrinite reflectance in the jurassic offshore basin fill reaches more than 1 the modelled subsidence history in combination with kinetic and geochemical data were used to model hydrocarbon generation in order to investigate the temperature hydrocarbon generation and its timing two 1d geohistory curves were extracted one for the onshore inner shelf and one for the offshore deep water area the modelling results suggest that deepest burial in the modern offshore area occurs at present day whilst deepest burial in the onshore area occurred during the oligocene depending on the location on the inner shelf based on the burial and thermal evolution significant hydrocarbon generation from lower jurassic source rocks took place from the early cretaceous until the neogene in the offshore area in the onshore area petroleum generation ceased in the late cretaceous as salt deposition may play an important role in the thermal history in the tarfaya basin a second model including salt was built indicating increased early cretaceous hydrocarbon generation in the offshore area

geochemistry of the brachiopod hemithiris psittacea from the canadian arctic implications for high latitude palaeoclimate studies abstract fossil shell material from high palaeo latitudes is increasingly studied for environmental forcings specific to these latitudes and as sensitive archives of climate change however more detailed studies of modern analogues are necessary for putting findings into global and high latitude perspectives here we present 13 c and 18 o values as well as mg sr na mn and fe contents for the rhynchonellide brachiopod hemithiris psittacea from friday harbor u s a and twenty five sites from canada ranging in latitude from 48 to 71 n median mn ca ratios are below 0 1 mmol mol and fe ca ratios are below 0 3 mmol mol with no strong correlation between ratios and no apparent geographical relationship mg ca and sr ca ratios vary within relatively narrow limits and yield median ratios of 3 7 mmol mol and 1 13 mmol mol na ca ratios vary widely amongst the sites with median values ranging from 9 to 20 mmol mol relatively uniform median 13 c values of 1 8 0 6 2 sd n 25 are observed for the canadian sites whereas at friday harbor the signature of 13 c depletion in local dic is imprinted in the shell calcite median 18 o values of 0 5 to 2 7 are more variable than carbon isotope signatures and point to precipitation of shell material in seawater diluted by variable amounts of freshwater and affected by seasonal sea ice our results imply that robust elemental patterns and carbon isotope signatures of high latitude brachiopod fossils can be expected for most palaeogeographic situations in contrast 18 o signatures of shell material from high latitude marginal seas particularly during glacial periods would yield unreasonably high seawater temperature estimates unless the oxygen isotopic composition of the ambient water is accurately accounted for in palaeotemperature calculations

major fracture zones of morocco the south atlas and the transalboran fault systems within the fracturai pattern of morocco 35 45 and 70 90 directions are predominant most of the faults originated already during the upper palaeozoic and were repeatedly reactivated later on two of the large fracture systems are considered here the south atlas fault system saf is composed of different faults with directions varying between 45 90 they were not functionally connected to the southern hinge of the high atlas until its miocene uplift today it seems to be inactive on the whole both the tectonic data and the facies distribution of mesozoic strata contradict clearly the interpretation that the saf is a huge wrench fault or even the mesozoic plate margin of africa the sw striking transalboran fault system taf is still active it crosses morocco from melilla to agadir showing again singular faults which are unified to extended lineaments only in a few areas the intervals between the faults are bridged by linear arrangements of earthquake hypocenters sinistral massflow within the deeper parts of the lithosphere seems to be compensated at the surface by movements along pre existing faults for the moroccan segment of the taf a sinsitral displacement is testified at least since the oligocene while the northeastern segments from the rif to spain did not even originate until the pliocene this would mean that sinistral shear was transmitted from africa into the accreted alboran and iberia blocks after the miocene collision

comparative study of the lower and mid cretaceous sequences on different maghrebian shelves and basins their place in the evolution of the north african atlantic and neotethysian margins abstract the lower and mid cretaceous sequences known from morocco coastal essaouira agadir safi basin and eastern rif foreland in western algeria da a mountains and finally in southern tunisia chott range and saharian shelf are presented from a biostratigraphic and sedimentologic viewpoint this work shows for the period considered common features in the geodynamic evolution of the different regions such as 1 generalization of the carbonate facies in the early cretaceous berrisian and at the end of the mid cretaceous cenomanian turonian times 2 delineation of important discontinuities in the valanginian and bedoulian times this evolution gives evidence of an undeniable influence of eustatism in this northern part of africa nevertheless detailed differences allow us to distinguish 3 an atlantic type evolution characterized by discontinuities with gaps safi or sudden variations of thickness essaouira agadir at the berriasian valanginian boundary related to the neocimmerian rifting and block faulting phase later on this evolution is underlined by a progressive acceleration of subsidence at the aptian albian boundary due to the removal of the mid oceanic doming 4 a neotethysian type evolution eastern rif foreland western algeria and southern tunisia where local tectonic movements have probably generated the rapid growth of the external marly bedoulian basins and the spreading of the wealdian type valanginian to barremian and utrillas albian terrigenous depositional systems the latest event is linked to the austrian tectonic phase responsible of variable gaps towards the aptian to albian transition

triassic liassic western moroccan synrift basins in relation to the central atlantic opening abstract the upper triassic lower liassic sequence of the atlantic margin of morocco is primarily composed of fine grained detrital sedimentary rocks containing basaltic intercalations its age is carnian to hettangian well data outcrop observations and seismic sequence analysis suggest that rifting started earlier carnian in the eastern and southern parts of the margin and later proceeded towards the west and the north resulting in the formation of the central atlantic rift on the moroccan margin the rift was composed of i eastern basins relatively narrow and bounded by e dipping faults which are reactivated hercynian structures ii a central horst and iii western basins bounded by w dipping faults the 020 trending basins are separated by w dipping faults the 020 trending basins are separated by 070 striking transfer faults estimations of the amount of lithospheric extension along the moroccan external rift system show a thinning ratio decreasing northward and probably associated to a necking of the lithosphere rift evolution on the part of the moroccan margin shows good correlation to the conjugate north american margin

accommodation and supply a dual control on stratigraphic sequences abstract it is widely accepted that both eustatic and tectonically controlled regional changes of sea level have contributed to the record of stratigraphic sequences i suggest that environmental change be added as a third autonomous control sedimentologic principles clearly indicate that sequences and their systems tracts are controlled by the interplay of two rates the rate of change in accommodation space available for sedimentation and the rate of sediment supply sea level has direct control on accommodation but its influence on sediment supply is remote and easily overshadowed by environmental factors for instance the record of the most recent sea level rise is a transgressive systems tract where supply is low it is a prograding highstand systems tract in deltas where the supply is high examples of sequence boundaries generated by changes in sediment supply include tectonically driven shifts in sediment input into basins changes in ocean currents pulsating supply from failure of submarine slopes and drowning of carbonate platforms by environmental stress furthermore the stratigraphic sequences in fluviatile continental basins are physically removed from sea level induced changes in accommodation and must have formed by changes in the rate and pattern of supply subaerial exposure of marine sediments at the sequence boundary is a most important criterion for recognizing sea level cycles as opposed to supply cycles other criteria include downstepping of shelf breaks and characteristic patterns in the spacing of time lines within sequences some third order cycles ca 0 5 3 ma duration meet these criteria others do not cycle stacking patterns and the shifting facies belts on cratons indicate that many second and third order cycles lack pronounced exposure unconformities and represent gradual changes superimposed on more rapid shorter oscillations seismic data yield poor images of these gradational changes because they lack resolution seismic models of outcrops show that near the limits of resolution the seismic tool tends to portray rapid changes in facies and bed thickness i e lateral variations in supply as unconformities seismic unconformities do not necessarily correspond to unconformities in outcrop the succession of stratigraphic sequences is best considered a composite record of sea level and environmental change separating the various controls requires carefully planned experiments work on the oxygen isotope curve another proxy for sea level has set an example in this respect

brachiopod shell spiral deviations ssd implications for trace element proxies abstract the importance of brachiopod shells as archives of past environmental changes via chemical proxies isotopes and trace elements will increase significantly if the age of specimens can be determined accurately here we evaluate the interpretation of spiral shell deviations throughout the ontogeny of the modern brachiopod species laqueus rubellus collected from sagami bay japan in combination with high resolution biogeochemistry the determination of spiral maxima and minima indicates that there is a different growth rate for ventral and dorsal valves as well as for juvenile and adult specimens and that the anterior shell regions provide a more accurate record of shell growth the combination of trace element proxies p ca and mg ca with shell spiral deviations indicates that the maxima represent prominent periods of shell growth that occurs during the beginning of the autumn by counting the maxima on dorsal valves of fully mature specimens these specimens can record shell growth up to 6 7 years which is confirmed by independent morphological and isotopic studies in the same brachiopod species the integration of statistical measurements of shell morphologies with chemical measurements represents an innovative methodology for specimen age determinations in brachiopods and provides a better understanding of trace element proxies overall the application of this approach to fossil brachiopods may open a new avenue for paleoseasonality and paleoceanography studies

paleostress evolution in the moroccan african margin from triassic to present abstract paleostresses were calculated from more than 1000 fault slip data on sites located in the rif moutains the middle and high atlas chains the occidental moroccan meseta and northeastern morocco we sorted the state of stress according to the main tectonic events in order to present new synthetic maps of paleostresses of morocco in the late triassic early cretaceous time nw se to ne sw extensions occurred they are consistent with the opening of the atlasic and atlantic rifts in the late cretaceous paleocene an e w to wnw ese compression took place conjugated strike slip faults developed in northeastern morocco and in the atlas chain this event was followed by a n s eocene compression where most of the reverse faults and the folds of the atlasic area developed together with strike slip faulting compression extended from oligocene to middle miocene with a ne sw direction in the rif chain the middle and high atlas and the coastal atlantic basins continuous deformation during the neogene in the atlasic area was responsible for shortening of the basement and detachment of the sedimentary cover thrust faults at the front of the atlas were either visible or blind in the lower tortonian nne ssw and ene wsw to e w compressions occurred respectively east and west of the nekor fault this tectonic regime led to the formation of post nappes basins separated by emerged lands in the tortonian early messinian the direction of compression varies from n s to nne ssw or ne sw east and west of the nekor fault respectively in the early messinian a large transgression occurred throughout the meseta domain and the middle atlas area in the late messinian a marine regression was synchronous with the mediterranean salinity crisis in the late miocene the atlas chain emerged and was progressively uplifted together with folding and thrusting of its foreland nappes involving paleozoic basement formed along the axis of the chain in the plio quaternary the compression varies from n s to n140 e and contributes to continuous uplift of the atlas chain

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

the mg ca temperature relationship in brachiopod shells calibrating a potential palaeoseasonality proxy brachiopods are long lived long ranging extant organisms of which some groups precipitate a relatively diagenetically stable low magnesium calcite shell previous work has suggested that the incorporation of mg into brachiopod calcite may be controlled by temperature brand et al 2013 here we build upon this work by using laser ablation sampling to de ne the intra shell variations in two modern brachiopod species terebratulina retusa linnaeus 1758 and liothyrella neozelanica thomson 1918 we studied three t retusa shells collected live from the firth of lorne scotland which witnessed annual temperature variations on the order of 7 c in addition to four l neozelanica shells which were dredged from a water depth transect 168 1488 m off the north coast of new zealand the comparison of intra shell mg ca profiles with shell ad18o confirms a temperature control on brachiopod mg ca and supports the use of brachiopod mg ca as a palaeoseasonality indicator our preliminary temperature calibrations are mg ca 1 76 0 27 e 0 16 0 03 t r2 0 75 for t retusa and mg ca 0 49 1 27 e 0 2 0 11 t r2 0 32 for l neozelanica errors are 95 confidence intervals

geochemistry and geochronology of early mesozoic tholeiites from central morocco from central morocco central high atlas middle atlas haute moulouya continental tholeiites were investigated geochemically and geochronologically these tholeiites are intercalated within continental redbeds of the early mesozoic triassic liassic the major trace and rare earth element contents classify these volcanic rocks as basaltic to andesitic basaltic quartz normative tholeiites some trace element ratios e g zr nb zr y y nb ti v suffer a heterogeneous source with a composition similar to morb p to n type the enriched lile contents the negative nb anomaly and the inital87sr 86sr ratios 0 7064 0 7069 reveal the presence of a crustal component up to 13 17 wt 87sr 86sr ratios of carbonate mineral separates from different lava flows show different cycles of alteration however the major and trace element chemistry together with sr isotope evidence indicate that the alteration phases are not submarine in origin 40ar 39ar age determinations on translucent plagioclase phenocrysts yield extrusion ages which range between 210 4 2 1 ma and 196 3 1 2 ma these ages correspond to a stratigraphic period between the norian rhaetian and the upper sinemurian

brachiopods recording seawater temperature a matter of class or maturation calcite fibres of the innermost secondary layer of low magnesium calcite brachiopod shells are in oxygen isotope equilibrium with ambient seawater previous work on terebratalia transversa indicates that the first formed calcite fibres of the secondary layer are not in isotopic equilibrium while the later fibres of mature valves are in oxygen isotope equilibrium with seawater here secondary ion mass spectrometry sims analyses reveal that this trend of oxygen isotope equilibrium being attained towards the innermost shell rather than being a feature throughout the secondary layer occurs in brachiopods from two orders and two sub orders this suggests a general pattern among brachiopods with low magnesium calcite shells and leads to the recommendation that only the innermost fibres of mature valves be included in proxy calculations of seawater temperature although the trend is common in the three species studied here the extent of the isotopic range is much less marked in the species that lacks punctae

senonian basin inversion and rejuvenation of rifting in africa and arabia synthesis and implications to plate scale tectonics abstract the late paleozoic to tertiary stratigraphic record of much of the african plate reflects the effects of continental rifting and passive margin development several short lived but widespread and tectonically important compressional or wrench dominated events occurred however during the permian to recent evolution of africa we focus here on the best documented of these events which occurred during the late santonian at that time older sedimentary basins mostly ene wsw trending were folded and inverted including some basins along the tethyan margin from morocco to the syrian arc and the intraplate benue southern chad basins and the lugh mandera basin in oman ophiolites were obducted following the santonian tectonism an extensive phase of rifting occurred in central and north africa and northern arabia spanning from campanian to maastrichtian or paleocene times rejuvenation or acceleration of subsidence occurred in several basins located along the tethyan margin along the atlantic and indian ocean margins as well as within the intraplate domain rifting was sometimes accompanied by magmatic activity especially in offshore northern libya and along the indian ocean margin compressional deformations rejuvenated or developed by latest maastrichtian times along the tethyan margin and locally within the intraplate domain one of the most remarkable attributes of the santonian tectonic event which we consider representative of the major trans african stress field changes is the rapidity with which the change was reflected stratigraphically across north and central africa and its short duration the cause of the santonian compressional event is directly linked to the change in poles of rotation for the opening of the atlantic at 83 85 ma the end of the cretaceous normal magnetic quiet zone this was also synchronous with the obduction of ophiolites along the northeast margin of arabia oman the onset of separation between india and madagascar with formation of the mascarene oceanic basin and the development of the european alpine chain we interpret these phenomena as causally related aspects of a global tectonic event the santonian compressional event as well as the campanian maastrichtian rifting event and the end cretaceous compressional event illustrate the critical connection between intraplate tectonic histories and processes occurring at sometimes very distant plate boundaries

intraspecific variations in carbon isotope and oxygen isotope compositions of a brachiopod basiliola lucida collected off okinawa jima southwestern japan abstract this study presents intraspecific variations in carbon isotope 13 c and oxygen isotope 18 o compositions of nine specimens of a subtropical brachiopod basiliola lucida collected west of okinawa jima ryukyu islands southwestern japan the 13 c values of samples collected along the maximum growth axis ontogenetic samples from two modern and seven older pre 1945 cal ad shells show no seasonal changes the modern shells which were collected from comparable depths have similar 13 c values that fall within the range of calcite precipitated in isotopic equilibrium with ambient seawater equilibrium calcite 13 c ec values and their mean 13 c values are 1 1 1 6 less than those from the older shells this decrease in 13 c values is similar in magnitude to the decreases in atmospheric co 2 and the oceanic dissolved inorganic carbon at the sea surface in recent years 13 c suess effect suggesting that the effect can even be detected at water depths of 200 300 m in the subtropical northwestern pacific ocean the 18 o values fluctuate within a narrow range 0 26 0 41 with no seasonal changes and they exhibit small 0 14 0 51 offsets from those of equilibrium calcite 18 o ec values a statistically significant negative linear relationship is established between seawater temperature and mean 18 o values of the nine shells but the slope 0 31 c is steeper than those of equilibrium calcite 0 23 c and other calcareous organisms 0 15 to 0 26 c the cross plots of the 13 c and 18 o values suggest that the degree of the vital effect varies among individuals in this species the 13 c and 18 o values of b lucida are potentially useful for reconstructing the 13 c and 18 o evolution of ancient oceans because both values show small intraspecific variations the former is identical to the 13 c ec values and the latter shows small within shell variations and small nearly constant offsets from the 18 o ec values

mesozoic sequence of fuerteventura canary islands witness of early jurassic sea floor spreading in the central atlantic the fuerteventura jurassic sedimentary succession consists of oceanic and clastic deposits the latter derived from the southwestern moroccan continental margin normal mid oceanic ridge basalt n morb flows and breccias are found at the base of the sequence and witness sea floor spreading events in the central atlantic these basalts were extruded in a postrift environment post late pliensbachian we propose a toarcian age for the atlantic oceanic floor in this region on the basis of the presence higher up in the sequence of the bositra buchi filament microfacies aalenian bajocian and of clastic deposits reflecting tectono eustatic events e g late toarcian to mid callovian erosion of the rift shoulder the s 1 sea floor oceanic magnetic anomaly west of fuerteventura is therefore at least toarcian in age the remaining sequence records atlantic tethyan basinal facies e g callovian oxfordian red clays aptian albian black shales alternating with clastic deposits e g kimmeridgian berriasian periplatform calciturbidites and a lower cretaceous deep sea fan system the fuerteventura n morb outcrops represent the only early jurassic oceanic basement described so far in the central atlantic they are covered by a 1600 m nearly continuous sedimentary sequence which extends to upper cretaceous facies

late paleozoic middle latitude gondwana environment stable isotope records from western australia abstract one hundred and twelve permian fossil brachiopod samples from two western australian basins carnarvon and canning basins have been analyzed for oxygen and carbon isotopes to study the middle paleolatitude environment in the permian all samples were thin sectioned and examined using petrography and cathodoluminescence microscopy for evaluating shell preservation a total of 249 isotopic analyses from the best preserved portion non luminescent nl of shells were found useful for inferring paleoenvironments for the carnarvon basin average 13 c values of nl shells were between 4 2 and 4 5 for the callytharra formation n 123 late sakmarian early artinskian jimba jimba calcarenite n 7 early artinskian coyrie madeline formations n 71 both late artinskian early kungurian and wandagee formation n 18 late kungurian average 13 c values of nl shells were 5 0 0 4 n 9 for the quinnanie shale cundlego formations both kungurian for the canning basin mean 13 c values of nl shells were 5 1 1 1 n 11 5 3 0 2 n 4 and 4 8 0 5 n 6 for the noonkanbah formation artinskian kungurian lightjack formation roadian capitanian and hardman formation wuchiapingian respectively within the uncertainty of the stratigraphical correlation the carbon isotope values spanning late sakmarian to kungurian in middle latitude western australia are comparable to those of coeval low latitude urals average 18 o values of nl shells were about 0 0 1 to 0 1 for the callytharra formation jimba jimba calcarenite coyrie madeline formations quinnanie shale cundlego formations noonkanbah formation and hardman formation and were 0 5 0 7 and 0 3 0 7 for the wandagee formation and lightjack formation respectively these oxygen isotope values are overall higher than the coeval values reported for low latitude regions and for eastern australia the positive oxygen isotope values are here interpreted to indicate a cool temperature and or a possibly moderately high salinity condition for the two studied basins in western australia during the permian

inversion tectonics and the evolution of the high atlas mountains morocco based on a geological geophysical transect an edited version of this paper was published by the american geophysical union agu copyright 1998 agu see also http www agu org pubs crossref 1999 1998tc900015 shtml http atlas geo cornell edu morocco publications beauchamp1999 htm

chemical variation in pennsylvanian brachiopod shells diagenetic taxonomic microstructural and seasonal effects abstract to improve our ability to use minor and trace element mte variation in biotic carbonates as diagenetic and paleoenvironmental indicators we performed electron probe microanalysis on more than 100 late pennsylvanian brachiopod shells from texas kansas missouri and new mexico texturally preserved specimens of the genera crurithyris composita and neospirifer from all three regions were analyzed as were eridmatus specimens from texas twenty measurements were made in two transects across each shell shell microstructure and cathodoluminescence were described for each spot analyzed three modern shells were analyzed for comparison diagenesis as indicated by cathodoluminescence and or absence of microstructure tends to enrich shells in fe and mn x ca 0 7 mmol mol and deplete shells in na and s mg content shows no consistent trend with diagenesis in fabric retentive nonluminescent shell areas mg na and s contents vary twofold to sevenfold depending on taxonomy microstructure and season overall taxonomy is the dominant factor controlling mte composition na and s concentrations are consistently highest in crurithyris and eridmatus intermediate in neospirifer and lowest in composita in taxa with mixed microstructure composita neospirifer secondary fibrous layer calcite contains 1 5 to 2 times more na than does interlayer prismatic calcite thus whole shell na contents of these taxa depend on the proportion of fibrous and prismatic shell seasonal cycles are revealed in mte transects across growth lines mg na and s contents commonly vary by more than a factor of two between maxima presumably summer and minima winter within the same shell retention of taxonomic microstructural and seasonal effects in shell chemistry argues for preservation of original chemistry in fabric retentive nonluminescent paleozoic brachiopod shells

upper palaeozoic basins of the british isles caledonide inheritance versus hercynian plate margin processes hypotheses to explain the origin and location of upper palaeozoic sedimentary basins are presented group a basins were those old red sandstone basins whose location and subsidence were controlled by caledonide features including active and remnant fore arc basins midland valley and major transcurrent movements orcadian basin newer granite plutonic magmas in the southern uplands subduction complex post dated subduction and resulted from post collision cratonization processes as yet little understood group b basins were those whose initial positions in space were controlled by palaeo plate factors in particular the location of granitoid plutons but whose subsidence was controlled by active hercynian neoplate processes some group a basins were regenerated by this process midland valley a new hypothesis is presented to account for the origin of a carboniferous rift to sag trend similar to that proven in modern continental margin basins such as the north sea the hypothesis is an adaptation of mckenzie9s lithospheric stretching model some of whose major features are well shown in the carboniferous history of the north and central british isles group b processes were controlled by ligurian bretonic plate consumption in the s brittany massif central vosges areas group c basins in the rheno hercynian zone also owe their location and origin to back arc rifting and continental crustal oceanization processes caused by liguro bretonic subduction

microfacies zonation and spatial evolution of a carbonate ramp marginal moroccan phosphate sea during the paleogene the paleoenvironment of the southwestern portion of the paleogene moroccan phosphate sea has been reconstructed in detail by microfacies studies for a complete transgressive regressive cycle in front of the anti atlas forming the southern border of the atlantic gulf a bioclastic and phosphatic carbonate ramp extended to the north the ramp was a homoclinal very gentle dipping surface four stages of ramp evolution can be distinguished they are 1 the marginal marine initial stage 2 the marine transgressive stage 3 the marine regressive stage and 4 the final silting up stage the ramp is characterized by a distinct facies zonation a sabkha plain documented by mudstones and dolocretes was situated between the arid hinterland and the ramp nearshore open lagoons existed populated by chlorophycean algae and miliolid foraminifera these environments were protected from the open sea by a chain of oyster banks a belt of echinoid enrichments probably marked the wave base sea areas with deeper and only temporarily highly agitated water are characterized by a benthic faunal assemblage of varied molluscs with bryozoans becoming more abundant to the east the carbonates are intensively bioturbated and interfinger with phosphorite sands which dominated in the deepest part of the ramp all phosphorites studied are clastics poor in matrix and are not associated with phosphate source rocks a process of extensive reworking transport and redeposition which is related to transgressions and regressions has to be assumed

constraining mesozoic early post rift depositional systems evolution along the eastern central atlantic margin abstract extensive mesozoic outcrops exposed in the aaiun tarfaya basin in the atlantic margin of morocco have been overlooked for decades original subdivisions and dating of rocks performed more than five decades ago have survived until today virtually without any update in the north eastern margin of the basin key mesozoic sections have been revisited its coastal position next to the variscan anti atlas orogen allows insight into the interplay between local tectonics and eustasy during deposition of sedimentary sequences on passive margins high resolution stratigraphy and sedimentology have allowed for an improved understanding of the depositional systems and evolution during the early opening of the atlantic and provided a valuable control point recording the syn and early post rift evolution of the basin three formations are described i a basal continental red bed unit unconformably overlying the palaezoic variscan basement of the ifni inlier ii a unit dominated by intertidal clastics and microbial carbonates and iii a shallow marine unit comprising mixed siliciclastic carbonates despite the sedimentological affinity of these outcrops to triassic or lower jurassic deposits elsewhere in morocco these units had previously been assigned an early cretaceous age based on limited non conclusive fossil fauna and basin scale lithostratigraphic correlation new collections of macrofauna have yielded a bathonian age for the upper part of the succession and a possible triassic early jurassic for the lower part the whole succession displays an overall transgressive trend not consistent with the eustatic bathonian sea level fall local post rift tectonic pulses exhuming the western anti atlas coupled with eustasy are responsible for increased sediment supply at this time enhanced tectonic subsidence overcoming sea level fall is interpreted to be the main driver for creation of accommodation and preservation of this sequence resumed sedimentation on structural heights offshore suggests exhumation of the western anti atlas ceased by the early callovian reducing clastic input to the region and allowing the development of a carbonate platform variable regional exhumation and local tectonics along the moroccan margin during the early post rift stage influenced the highly variable nature of clastic vs carbonate deposition thickness distribution and depositional environments at this time

the southern moroccan passive continental margin an example of differentiated long term landscape evolution in gondwana abstract the ene trending anti atlas mountain belt reveals variation in recent topography with elevations of 3 km indicating a differentiated long term landscape evolution to unravel the subsidence and exhumation history low temperature thermochronology has been applied on 21 samples from the precambrian basement of the irherm kerdous ifni and bas dr a inlier thermochronological data suggest a common geological evolution of the western anti atlas zircon fission track ages from 287 23 to 331 24 ma point to a main exhumation in the late carboniferous early permian assigned to the variscan folding and post folding erosion zircon u th he 205 287 ma and apatite fission track 120 219 ma data are related to the central atlantic rift and drift stage as the anti atlas formed the exhumation shoulder of the rift system the rock uplift and exhumation phase lasted up to the early cretaceous whereby 9 km of precambrian palaeozoic overburden has been eroded thermal modelling reveals a minor sedimentary burial 1 5 2 5 km throughout the cretaceous in accordance with the widespread transgression across the northern africa in the cenomanian turonian apatite u th sm he ages between 49 3 and 164 10 ma confirm recent minor exhumation in the western anti atlas with an increasing impact of the atlasian orogeny and potentially a mantle plume anomaly from southwest to northeast reflecting the differentiated topography in the area

analysis of growth and form in aerothyris kerguelenensis rhynchonelliform brachiopod shell spiral deviations microstructure trace element contents and stable isotope ratios abstract brachiopods are usually attached benthic marine invertebrates with a shell exoskeleton that archives modifications of the environment via chemical proxies growth lines regularly occurring i e excluding random ones at the shell s surface reflect phenotypically controlled interruptions or changes in secretion williams et al 1997 modifications in the secretion rate and reorientation of shell elements within the thickness of the shell induce marked changes in shell morphology providing elements for estimating age throughout ontogeny for specimens of aerothyris kerguelenensis davidson 1880 formerly known as waldheimia kerguelensis davidson 1978 from the southern indian ocean maxima and minima of deviations from underlying shell spiral growth ssd plus maps sem eds and electron microprobe wds of mg s ca mn fe that are components of shells along with scanning electron microscopy sem all show evidence of rhythms at different time scales including annual ones growth rates differ in young and adult forms as well as between dorsal and ventral valves additionally the amplitudes of mg peaks are higher early in ontogeny and in the primary shell layer progressively decreasing with sporadic peaks anterior regions of shells are more informative regarding external growth i e major growth marks and mainly record later events in the life of the individual posterior regions are informative for internal morphology cumulative growth and reveal mg bands and related concentrations that are consistent with the main extrema maxima and minima observed when using ssd and wds analyses mg ca ratios indicate fluctuations in temperature and food supply during the growth period seawater temperatures calculated from the oxygen isotope compositions of the secondary layer of calcite in shells of a kerguelenensis are close to those measured in situ in the environment in which they live however the difference in carbon isotope composition between the two studied shells reveal that these specimens record isotopic compositions most likely resulting from a combination of kinetic vital isotope effects that may sometimes mimic equilibrium values

oxygen isotope and mg ca composition of late visean mississippian brachiopod shells from sw iberia palaeoclimatic and palaeogeographic implications in northern gondwana abstract latest visean brachiopod shells gigantoproductus sp from the pedroches basin of sw spain were sampled in order to obtain chemical and isotopic evidence that may help first to assess if primary geochemical and isotopical signals are preserved in these shells and second to reconstruct temporal variations in palaeoceanographic and palaeoclimatic conditions in northern gondwana during this time span microtexture cathodoluminescence cl and ultrastructure under sem of the brachiopods were examined to avoid sampling of shell portions altered after deposition shell portions classified as non luminescent nl and non luminescent to slightly luminescent nl sl present geochemical signatures within the ranges considered normal for recent brachiopods that have not undergone significant diagenetic modification these include low to undetectable fe and mn contents high sr values and 18o values similar to those of carbonates in equilibrium with seawater on the other hand valve areas that are luminescent l or moderately luminescent with luminescent portions ml l present higher fe and mn contents lower sr concentrations and lower 18o values than those corresponding to nl and nl sl shells this suggests a greater diagenetic alteration of these portions and that they acted as microzones of reaction with the diagenetic fluids temporal trends through the studied interval show a spread in 13c values much lower than that observed for contemporaneous samples up to 1 6 thus obscuring the 13c global signal at the time scale resolution of this study in contrast the 18o record of brachiopod calcite 18oc shows an overall increase of 1 relative to v pdb from the oldest to the youngest samples this is accompanied by a 30 decrease in mg ca ratios of the brachiopod calcites confirming that at least part of this shift in 18oc was related to cooling combination of mg ca and 18oc proxies established in this paper although not full proof for brachiopods yet allowed for the calculation of a temperature drop of 3 3 c and a change in seawater 18ow that becomes 0 4 more positive during this interval the data are consistent with regional seawater cooling that accompanied an increase in continental ice volume over a relatively short time period of

the source of sulfate in brachiopod calcite insights from μ xrf imaging and xanes spectroscopy abstract geochemical signatures preserved in sedimentary carbonate strata are often used as archives for paleoenvironmental reconstructions however diagenetic overprinting and or muting of primary geochemical signatures complicates the interpretation of these data to avoid this issue geochemical techniques strive to target unaltered or minimally altered carbonate components the multi layer low mg calcite shell composition of articulate brachiopods are often the target in such studies because they are commonly more robust to recrystallization than the shells of other biomineralizers here we have combined s k edge xrf imaging xanes spectroscopy and petrography to determine the source of sulfate in the calcite lattice carbonate associated sulfate cas of ancient and extant brachiopods in order to establish their suitability for use in reconstructing seawater sulfate 34s throughout the phanerozoic both the extant brachiopod terebratalia transversa and fossil brachiopods display intra specimen variability in sulfate abundance parallel to the primary fabric likely corresponding to variations in growth rate xanes spectroscopy identifies a majority of the sulfate as inorganic additionally xanes spectroscopy detected low abundances of both reduced and oxidized organic sulfur species thiol thioether sulfoxide and sulfate esters in all t transversa samples and lesser abundances in a few of the fossil brachiopods in t transversa inorganic sulfate and sulfate ester abundance increase towards the hinge of the valves bulk 34scas of the samples containing the fossil brachiopods are consistently more positive than time equivalent brachiopod only values likely reflecting a mixture of cas signals from homogenization of carbonate components differentially affected by depositional environment and diagenesis in contrast bulk 34scas of modern t transversa is approximately 1 more positive than coeval seawater although organic sulfate esters are found within the brachiopod shells they are only ever present as trace components our findings indicate that the vast majority of sulfate in brachiopod shells is inorganic sourced from coeval seawater this result supports the use of brachiopods as a potential archive for a faithful cas record of seawater sulfate throughout the phanerozoic the characterization of in situ organic sulfur compounds in both extant and fossil brachiopods indicates the potential importance of various organic sulfur compounds in mineralogical determination and therefore fossil preservation and crystal orientation during brachiopod biomineralization throughout geologic time

quantitative meso cenozoic development of the eastern central atlantic continental shelf western high atlas morocco abstract the well exposed late paleozoic to cenozoic succession in the western high atlas morocco documents the early rift to mature drift development of the eastern central atlantic continental shelf basin vertical sections depositional geometries and unconformities have been used to reconstruct the basin architecture prior to atlasian inversion two dimensional reverse basin modeling has been performed to quantitatively analyze the development of the continental shelf between the latest paleozoic to early cenozoic basin evolution stages include i early rift late permian to anisian ii rift climax ladinian to carnian iii sag norian to early pliensbachian iv early drift late pliensbachian to tithonian v mature drift berriasian to cenomanian vi mature drift with initial atlasian deformation turonian to late eocene vii atlasian deformation late eocene to early miocene viii atlasian uplift and basin inversion early miocene to recent the late permian to late cretaceous basin development comprises eight subsidence trends of 10 35 myr duration trends were initiated by changes in thermo tectonic subsidence which in turn triggered positive and negative feedback processes between sediment flux flexural and compaction induced subsidence plate tectonic reconfigurations in the atlantic domain controlled the thermo tectonic subsidence history and the basin development of the agadir segment of the northwest african passive continental margin 1 major shifts in the sea floor spreading axis 2 significant decreases in sea floor spreading rates 3 the stepwise migration of crustal separation and sea floor spreading in and beyond the central atlantic 4 african eurasian relative plate motions and convergence rates during the early pliensbachian toarcian to cenomanian the first three plate tectonic reconfigurations triggered changes in ridge push forces and modulated the extensional stress field of central atlantic plate drifting since the turonian african eurasian relative plate motions and convergence rates represented the dominant control on the thermo tectonic subsidence history in the agadir basin major variations in sediment flux and total subsidence characterize the development of the northwest african passive continental margin the explanation of typical stratigraphic sequences as caused predominantly by sea level fluctuations and rough assumptions on sediment input production and subsidence is not necessarily applicable to passive continental margins the methodology applied in this study including the newly developed tool of compositional accommodation analysis allows to develop more rigorous genetic models for the development of continental shelf basins

kinematic evolution of the tethys belt from the atlantic ocean to the pamirs since the triassic abstract we present an updated series of kinematic reconstructions of the major plates around the tethys from the atlantic ocean to the pamirs between the early jurassic and the present this set is used elsewhere as a basis for paleogeographic maps of the entire region the problems related to the positions of the continents in the lower triassic are also discussed no direct analyses of magnetic anomalies and fracture zones in the atlantic have been made rather all available poles and rotations have been tested in order to eliminate or minimize possible kinematic errors the reconstructions are shown for nine key geological periods which correspond to well recognized magnetic anomalies except for the jurassic cretaceous boundary and the cretaceous tertiary boundary which correspond to interpolated positions paleolatitudes have been drawn using the study of westphal et al 1986 an attempt has been made to take into account the displacements caused by formation of the continental margins and basins by stretching the resulting relative vector of motions along the northern boundary of the tethys shows a significant change 80 m y ago left lateral motion with compression dominates before whereas right lateral motion with compression dominates after to the east rates of motion vary by a factor of three with time and four maxima can be clearly related to tectonic events in the late jurassic late cretaceous eocene and post middle miocene to the west north of apulia on the contrary the motion rate has not changed significantly since the early cretaceous and is close to 1 cm yr as an average these rather complex adjustments in rates and directions of relative motion are produced in great part through a complex migration of the africa eurasia pole of rotation and seem to be mostly governed by the tectonics of the tethys plate boundary

ontogenetic trace element distribution in brachiopod shells an indicator of original seawater chemistry articulated fossil brachiopod shells have been used extensively to extract primary chemical information of phanerozoic seawater despite the selection of well preserved shells using trace element microstructure and cathodoluminescence criteria there are still concerns as to whether the selected brachiopod shells do indeed contain original seawater signals analyzed in situ by laser ablation inductively coupled plasma mass spectrometry laicpms sr na mg mn b and ba distribute symmetrically in shell transects of the modern brachiopods magellania flavescens and terebratulina septentrionalis symmetry of the trace element distribution pattern is considered an intrinsic and original ontogenetic property of the brachiopod shell chemistry the trace element distribution is symmetrical in a well preserved shell of the devonian brachiopod independatrypa lemma indicating that the selected shell by the conventional criteria has preserved its original seawater signal for 400 ma in another specimen of i lemma that is considered diagenetically altered trace element concentrations are asymmetrically distributed in the shell the agreement between the distribution criteria and the conventional methods indicates the latter can be used to select brachiopod shells with original seawater chemistry the average element concentration in the whole shell of unaltered brachiopods should be a reflection of the seawater chemistry while its change in different part of a shell reflects ontogenetic effect and its high frequent fluctuations in a transect are results of changes in environmental parameters with seasonal or annual characteristics

stable isotope profiles ca o c through modern brachiopod shells of t septentrionalis and g vitreus implications for calcium isotope paleo ocean chemistry brachiopod shells are widely used as an archive to reconstruct elemental and isotopic composition of seawater studies focused on oxygen and carbon isotopes over the last decades are increasingly extending to the emerging calcium isotope system to date only little attention has been paid to test the reliability of fossil brachiopods on their modern counterparts in this context the present study investigates two modern brachiopods terebratulina septentrionalis eastern canada 5 30 m depth 7 1 c seasonal temperature variation two layer shell and gryphus vitreus northern mediterranean 200 m depth constant all year round temperature three layer shell both species were sampled along the ontogenetic growth direction and calcium oxygen and carbon isotopes as well as elemental concentration were measured calcium isotopes were analyzed on tims the elemental composition was analyzed by la icp ms and icp aes the results indicate an intra specimen 44 40ca variation ranging from 0 16 to 0 33 pointing to a fairly homogenous distribution of calcium isotopes in brachiopod shells however in the light of the suggested 0 7 increase in calcium isotopes over the phanerozoic such intra specimen variations constrain ocean reconstruction 44 40ca values of t septentrionalis do not seem to be affected by growth rate calcium isotopic values of g vitreus are heavy in the central part of the shell and trend towards lighter values in peripheral areas approaching the maximum isotopic composition of t septentrionalis the maximum inter species 44 40ca difference of 0 62 between t septentrionalis and g vitreus indicates that care should be taken when using different taxa species with different strontium content or brachiopods with specialized shell structure such as g vitreus for ocean water reconstruction in terms of ca isotopic composition t septentrionalis may record ca isotopic fractionation related to seasonal seawater temperature variations in its shell but this is difficult to resolve at the current analytical precision average 18o derived temperatures of the two investigated species are close to on site measured temperatures

isotopic evaluation of ocean circulation in the late cretaceous north american seaway the role of the north american seaway in creating and maintaining late cretaceous global greenhouse conditions has been unclear an isotopic analysis of marine turtle and fish fossils from western kansas and the mississippi embayment reveals that the inflow of tethyan surface waters to the seaway was limited during the mid and late cretaceous period north america was split by the north south oriented western interior seaway its role in creating and maintaining late cretaceous global greenhouse conditions remains unclear different palaeoceanographic reconstructions portray diverse circulation patterns1 2 3 the southward extent of relatively cool low salinity low 18o surface waters critically distinguishes among these models but past studies of invertebrates could not independently assess water temperature and isotopic compositions here we present oxygen isotopes in biophosphate from coeval marine turtle and fish fossils from western kansas representing the east central seaway and from the mississippi embayment representing the marginal tethys ocean our analyses yield precise seawater isotopic values and geographic temperature differences during the main transition from the coniacian to the early campanian age 87 82 myr and indicate that the seaway oxygen isotope value and salinity were 2 and 3 lower respectively than in the marginal tethys ocean we infer that the influence of northern freshwater probably reached as far south as kansas our revised values imply relatively large temperature differences between the mississippi embayment and central seaway explain the documented regional latitudinal palaeobiogeographic zonation4 5 and support models with relatively little inflow of surface waters from the tethys ocean to the western interior seaway2 3

gold in magmatic hydrothermal solutions and the rapid formation of a giant ore deposit the ladolam hydrothermal system on lihir island papua new guinea hosts one of the youngest and largest gold deposits in the world several deep more than 1 kilometer geothermal wells were drilled beneath the ore bodies to extract water at 275 c and to facilitate open pit mining using a titanium down hole sampler we determined that the deep geothermal brine of magmatic origin contains 15 parts per billion gold at the current gold flux of 24 kilograms per year this deposit could have formed within 55 000 years the combination of sustained metal flux and efficient metal precipitation led to the formation of a giant hydrothermal gold deposit in a short period

shell structure patterns and trends of oxygen and carbon stable isotopes in modern brachiopod shells this study investigates 13c and 18o variations in the shells of modern brachiopods representing all extant groups of calcite precipitating brachiopods collected live from 8 locations sem examinations determined the ultrastructural characteristics of each species prior to isotope analyses 13c and 18o analyses of shell carbonate were carried out with samples representing disparate morphological features and ultrastructural shell layers of both ventral and dorsal valves generally 18o values from the fibrous secondary or prismatic tertiary shell layers of the articulated terebratulida and rhynchonellida species were in oxygen isotopic equilibrium with ambient seawater isotopic temperatures extrapolated from these values are close to measured annual mean seawater temperatures 18o values were relatively unaffected by shell specialisation the only exception was antarctic species liothyrella uva which did not have a complete tertiary shell layer typical of this genus and had 18o values of the innermost layer strongly correlated with 13c and mostly not in oxygen isotopic equilibrium with ambient seawater with the exception of the rhynchonellid notosaria nigricans the outer primary layer material was depleted in 13c and 18o and highly variable inclusion of this material even as part of a whole shell sample could lead to misinterpretation of seawater temperature therefore only fossil secondary layer material should be used the anomalous articulated thecideidine brachiopod thecidellina barretti is composed of mainly primary shell material and was not in oxygen isotope equilibrium 18o values from the laminar secondary layer material of the inarticulated craniida are highly variable therefore these species are not recommended for use as palaeoenvironmental proxies this study suggests caution when employing fossil brachiopod shells with similar ultrastructures to modern craniid and thecideidine brachiopods the carbon isotope composition is highly variable in all of the brachiopods studied analysis of samples from specialised regions of the secondary shell layer show a pattern of depletion in 13c relative to non specialised secondary material the carbon isotope variability is independent of 18o and is repeated in most of the articulated species regardless of geographical location this is possibly a vital effect produced by metabolic prioritisation

a revision of the eurasian african plate boundary in the western mediterranean the current picture of the plate configuration in the western mediterranean suggests that africa and europe are separated by a transcurrent fault striking east west through the strait of gibraltar and the alboran sea such a fault has been generally assumed because the azores transform fault approaches the horseshoe seamounts north of madeira and therefore could possibly extend through the strait of gibraltar however detailed geological studies in the arc of gibraltar and the alboran sea reveal no major east west fault transecting the crust there this finding is used here to revise the boundary between the african and eurasian plates it is suggested that the northern boundary of the african plate in the western mediterranean is formed by the hayes atlas fault instead of the azores transform

carbon and oxygen isotopic compositions of modern brachiopod shells from a warm temperate shelf environment sagami bay central japan this study presents carbon 13c and oxygen 18o isotopic compositions of shells of an articulated brachiopod laqueus rubellus living in a warm temperate shelf environment in japan high resolution three dimensional samplings were conducted 1 to clarify variations in the isotopic compositions within a single shell 2 to evaluate offset of the compositions from those of the calcite precipitated in isotopic equilibrium with ambient seawater equilibrium calcite and 3 to specify shell portions that reflect the isotopic compositions of ambient seawater the 13c and 18o values of samples from the secondary shell layer which constitutes the main body of the brachiopod shell are in and or out of those of the equilibrium calcite the isotopic compositions of samples from the outermost part of the secondary shell layer outer surface of the secondary shell layer correlated well with growth rates positive correlations are recognized between the 13c and 18o values of these samples which can be ascribed to a kinetic fractionation effect although limited data on 13c of total dissolved inorganic carbon dic and 18o of seawater at the brachiopod sample site enable only approximate estimates of the 13c and 18o of the equilibrium calcite they are comparable with those of the brachiopod shell the 18o values of samples from the outermost part of the secondary layer along a growth axis of laqueus rubellus are a reliable environmental proxy because they mostly fall in the 18o range for the equilibrium calcite although samples from the inner series of l rubellus are depleted partially enriched in 13c 18o by 0 8 0 3 relative to the equilibrium calcite both isotopic variations are quite small therefore the isotopic compositions in this series would also be useful if the offsets are corrected this study clearly illustrates that although the 13c and 18o of modern brachiopod shells are influenced by the kinetic fractionation effect appropriate selection of species and shell portions reflecting the isotopic compositions of ambient seawater enables the reconstruction of secular variations in 13c or 18o in the oceans throughout geologic time

syn and postrift evolution of the el jadida agadir basin morocco constraints for the rifting models of the central atlantic the el jadida agadir basin shows an evolution related to that of the central atlantic rifting history the synrift period late early triassic to earliest jurassic is dominated by deposition of red clastic sediments and evaporites which are overlain by or intercalated with basalt flows two intense extensional phases are recognized the first probably in middle triassic times is oriented north northwest south southeast and resulted in the formation of east northeast west southwest oriented grabens and half grabens the second a widespread northwest southeast trending extension is ladinian to earliest liassic and induced the formation of westward dipping half grabens extensional tectonics initiated in the southern part of the basin and migrated northwards contrary to classical ideas proposing rifting from north to south the postrift evolution is characterized by jurassic to eocene subsidence tectonism appears to have been restricted to a narrow area close to the present day coast subsidence h

magnetic evidence for slow seafloor spreading during the formation of the newfoundland and iberian margins there is considerable debate concerning the nature and origin of the thin crust within the ocean continent transition oct zones of many passive non volcanic continental margins located between thinned continental and true oceanic crust this crust is usually found to be underlain by upper mantle material of 7 2 7 4 km s velocity at shallow depths 1 2 km it has been proposed that such crustal material could have originated either by exhumation of upper mantle material during rifting of continents or by slow seafloor spreading one of the examples of occurrence of such a crust are the conjugate margins of newfoundland and iberia here we present an interpretation of magnetic data from these regions to show that their oct zones are underlain by crustal material formed by slow seafloor spreading 6 7 mm yr soon after iberia separated from the grand banks of newfoundland in the late jurassic similarities in the magnetic anomalies and velocity distributions from these regions with those from the sohm abyssal plain a region lying immediately south of the newfoundland basin and formed by seafloor spreading at a similar rate of spreading give further support to such an interpretation the idea that these regions were formed by unroofing of upper mantle during rifting of iberia from newfoundland may be likely but the presence of weak magnetic anomalies in these regions which bear all the characteristics of seafloor spreading anomalies makes it difficult to ignore the possibility that these regions could be underlain by oceanic crust formed during slow seafloor spreading the similarities in velocity structure and the presence of small amplitude magnetic anomalies both across this pair of conjugate margins of the north atlantic and that of the labrador sea suggest that this oct velocity structure may be the norm rather than the exception across those passive non volcanic margins where the initial seafloor spreading was slow furthermore the existence of similar velocity distributions along a few active spreading centers raises the possibility of formation of similar crust across slow spreading ridges fl 2000 elsevier science b v all rights reserved

tertiary compressional structures on the faroe rockall plateau in relation to northeast atlantic ridge push and alpine foreland stresses abstract a number of compressional anticlinal structures are identified in the western and northern part of the faroe rockall plateau these structures occur on that part of the faroe rockall plateau which was above sea level during the latest phase of paleocene plateau basalt extrusion three post basalt compressional phases have affected the plateau most of the compressional structures in the northern part of the plateau are related to ne sw to ene wsw oriented stress which we date to late paleocene early eocene the oligocene phase is interpreted as resulting from n s directed compressional stress which also mainly affected the compressional structures on the northern part of the plateau compressional stress from the northwest seemed to affect the whole of the faroe rockall plateau and we suggest it to be of miocene age it is proposed that during the late paleocene early eocene phase of compression local structure and anomalously oriented gravitational ridge push from the now extinct aegir axis contributed to a local ne sw compressional stress system the two later deformation phases were apparently connected to the regional northwest european stress system with small local modifications

tropical marine climate during the late paleozoic ice age using trace element analyses of brachiopods abstract the late paleozoic ice age can be considered an important analogue to the modern ice age but comparisons between the two time intervals have been hampered by the difficulty of resolving climatic changes that occur over short e g seasonal time scales in the paleozoic record as a first step toward overcoming these limitations this study employed mg ca trace element ratios sampled across the growth bands of six specimens of the brachiopod composita to assess differences in tropical marine mean temperature and seasonality in visean and moscovian time these time intervals bracket the onset of the late paleozoic ice age which occurred in mid serpukhovian time and thus provide a snapshot of climate changes from a greenhouse to an icehouse world shells were screened for diagenetic alteration by cathode luminescence and by comparing the values of mn sr na and b to those reported for modern brachiopods in addition a 87sr 86sr value measured for one specimen agreed with a published strontium isotope curve for the late paleozoic era both the mean and range of mg ca values were larger in visean than moscovian time suggesting colder and less seasonal temperatures during the late paleozoic ice age the decline in seasonality was due to a greater proportional reduction of maximum values which presumably represent summer temperatures than minimum values these relative differences between the pre ice age and ice age tropical climate provide a first step toward understanding the broad patterns of seasonal scale changes in temperature during a critical time in earth s history