exxon global cycle chart an event for every occasion the basic premise of the recent exxon cycle chart that there exists a globally correlatable suite of third order eustatic cycles remains unproven many of the tests of this premise are based on circular reasoning the implied precision of the exxon global cycle chart is not supportable because it is greater than that of the best available chronostratigraphic techniques such as those used to construct the global standard time scale correlations of new stratigraphic sections with the exxon chart will almost always succeed because there are so many exxon sequence boundary events from which to choose this is demonstrated by the use of four synthetic sections constructed from tables of random numbers a minimum of 77 successful correlations of random events with the exxon chart was achieved the existing cycle chart represents an amalgam of regional and local tectonic events and probably also includes unrecognized miscorrelations it is of questionable value as an independent standard of geologic time

ice age at the middle late jurassic transition a detailed record of sea surface temperatures in the northern hemisphere based on migration of marine invertebrate fauna ammonites and isotopic thermometry 18o values of shark tooth enamel indicates a severe cooling at the middle late jurassic transition mljt about 160 ma ago the magnitude of refrigeration 1 3 c for lower middle latitudes and its coincidence in time with an abrupt global scale fall of sea level documented through sequence stratigraphy are both suggestive of continental ice formation at this time ice sheets may have developed over the high latitude mountainous regions of far east russia the drastic cooling just post dated the middle late callovian widespread deposition of organic rich marine sediments e g northwestern europe central atlantic and arabian peninsula this thermal deterioration can thus be ascribed to a downdraw in atmospheric co2 via enhanced organic carbon burial which acted as a negative feedback effect i e the inverse greenhouse effect the glacial episode of the mljt climaxed in the late callovian lasted about 2 6 myr and had a pronounced asymmetrical pattern composed of an abrupt 0 8 myr temperature fall opposed to a long term 1 8 myr stepwise recovery the glacial conditions at the mljt reveal that atmospheric co2 levels could have dropped temporarily to values lower than 500 ppmv during mesozoic times

the role of seamount volcanism in crustal construction at the mid atlantic ridge 24 30 n an analysis of approximately 6000 km2 of sea beam swaths indicates that the floor of the median valley of the mid atlantic ridge mar between 24 30 n is strewn with near circular volcanoes we have identified 481 seamounts in the height range 50 650 m because many of the features which appear to be seamounts do not meet our criteria fully they have not been counted and our numbers are likely a minimum estimate of the population the large abundance of seamounts on the median valley floor indicates that seamount volcanism plays an important role in the accretionary processes at this section of the mar the summit height distribution of the mar seamount population is consistent with the exponential frequency size distribution model for off axis eastern pacific seamounts jordan et al 1983 smith and jordan 1987 the model parameters for the mar population yield an average of about 195 seamounts per one thousand square kilometers and a characteristic height of about 60 m shape statistics compiled from the 481 seamounts give parameters that are exactly the same or cover the same range in values as those obtained from a study of pacific seamounts in the height range 140 3800 m smith 1988 implying a universal control on seamount construction based on the volcanic morphology we identify 18 spreading segments along the ridge all except two contain a prominent axial volcanic ridge many of the seamounts identified are associated with the axial ridges from the sea beam swaths we infer that the ridges are composed of piled up seamounts and hummocky flows and interpret them to be the primary sites of crustal construction this style of volcanism contrasts strongly with that observed at the east pacific rise where seamounts are virtually absent at the spreading axis the construction of seamounts and hummocky flows may be directly related to the fact that the mar is fed by magma chambers that are limited in size and frequency we suggest that small magma pockets with slow eruption rates produce seamounts either from pipes or initial fissure eruptions that collapse to construct a single edifice small magma bodies with somewhat higher eruption rates produce hummocky fissure fed flows using buoyancy arguments the heights of the mar seamounts are related to the depth of the magma bodies we hypothesize that small magma bodies rise buoyantly to the base of the hydrothermally cooled brittle lid where they are trapped and then erupt if this hypothesis is correct the thickness of the brittle lid controls seamount height and the exponential distribution of seamount summit heights implies that the thickness of the brittle hd follows an exponential distribution in time and space with characteristic thickness 1 4 1 9 km if seamounts and hummocky flows are fed each from their own discrete magma body trapped at the brittle ductile transition then the lower crust of the mar will be made up of the products of a multitude of small plutons

numerical models on the influence of partial melt on elastic anelastic and electrical properties of rocks part ii electrical conductivity abstract partial melting influences both the elastic and electrical properties of mantle rocks significantly but in a different manner a set of theoretical models of the electrical conductivity of partially molten rocks is developed for different geometries of the melt distribution the models correspond to a similar set on elasticity and anelasticity presented in a preceding paper part i both model sets allow combined interpretations of seismic and geoelectric observations from anomalous hot mantle regions by a systematic variation of the melt geometry the following melt geometries are assumed in the conductivity models melt films tubes isolated ellipsoidal inclusions films and ellipsoidal inclusions with a variable degree of interconnection distributed geometries and superpositions of spheres with films and with tubes unexplored conduction mechanisms near the melting temperature film thicknesses of molecular sizes or effects of volatiles are not considered resistor network theory is utilized to account for a variable degree of interconnection and distributed geometries thereby a new possible interpretation of archie s law is found in terms of varying interconnection furthermore it is found that a reduced degree of interconnection may have an important influence on the conductivity at melt fractions characteristic for a weak to moderate decrease in seismic velocity the model results are compiled together with the elastic models of part i in a set of diagrams which may be applied systematically to observed electrical and seismic data to find constraints on the state of melting

the ramesses experiment iii controlled source electromagnetic sounding of the reykjanes ridge at 57 45 n summary a controlled source electromagnetic sounding survey centred on an axial volcanic ridge avr segment of the reykjanes ridge at 57 45aen was performed as part of the ramesses experiment low frequency 0 35 11 hz electromagnetic signals were transmitted through the crust to an array of horizontal electric field recorders at the seafloor to ranges of 15 km from the source which was a 100 m long horizontal electric dipole towed at heights of 50 80 m from the seafloor coincident seismic and magnetotelluric studies were conducted during the rest of the ramesses experiment data were interpreted using a combination of 1 d forward modelling and inversion and iterative forward modelling in two dimensions on the axis of the avr the resistivity at the seafloor is 1 v m there is a steep resistivity gradient in the upper few hundred metres of the crust with the resistivity reaching approximately 10 v ma t a depth of 500 m in order to explain the low resistivities the upper layer of the crust must be heavily fractured and saturated with sea water the resistivity increases with distance from the axis as the porosity decreases with increasing crustal age the most intriguing feature in the data is the large diverence in amplitude between fields transmitted along and across the avr axis a significant zone of low resistivity material is required at approximately 2 km depth beneath the ridge crest in order to explain this diverence it is coincident with the low velocity zone required by the seismic data and has a total electrical conductance in excellent agreement with the results of the magnetotelluric study the low resistivity zone can be explained by the presence of a body of partially molten basalt in the crust taken together these results provide the first clear evidence for a crustal magma chamber at a slow spreading mid ocean ridge the data constrain the melt fraction within the body to be at least 20 per cent with a melt volume suycient to feed crustal accretion at this segment of the ridge for of the order of 20 000 years since this body would freeze in the order of 1500 years this finding lends support to the hypothesis that at slow spreading rates crustal accretion is a cyclic process accompanying periodic influxes of melt from the mantle to a crustal melt reservoir

controlled source electromagnetic sounding of the oceanic lithosphere the attenuation of ionospheric signals in the frequency range 0 06 24 hz by sea water effectively precludes using the magnetotel luric method to study the electrical structure of the upper oceanic lithosphere we have carried out a dipole dipole electromagnetic sounding in the north pacific by injecting electromagnetic signals into the ocean and sea bed the crust at the site is 25 myr old and has a thin sediment cover the technique similar to that used in earlier work1 2 involves dragging a horizontal dipole antenna along the sea floor the electric fields that propagated through the resistive basement were detected by seafloor receivers at ranges of 10 65 km as the ambient electric field is very small varying from 10 18 v2 m 2 hz 1 at 0 1 hz to 10 24 v2 m2 hz 1 above 1 hz ref 3 the controlled source signals could be easily monitored our data are consistent with a simple one dimensional earth model consisting of a 3 7 km thick crustal layer of moderate conductivity 0 001 s m 1 underlain by a thicker region of very low conductivity 2 10 5 s m 1 the results suggest an upper mantle water content of at most 0 1 by volume

late paleozoic depositional sequences are synchronous and worldwide more than fifty transgressive regressive depositional sequences are present in carboniferous and permian shallow marine successions on stable cratonic shelves worldwide these were synchronous depositional events resulting from eustatic sea level changes that generally ranged from 100 to 200 m each transgressive regressive sequence is correlatable using current fossil knowledge they average about 2 m y and range from 1 2 to 4 m y in length the presence within these strata of numerous synchronous unconformities of considerable duration and worldwide extent suggests that the fossil record is very incomplete and that we are studying a punctuated fossil record and not a punctuated evolution based on a highly irregular mutation rate these late paleozoic transgressive regressive depositional sequences facilitate correlations because depositional histories of a rock succession can support interpretations of faunal assemblages and faunal similarities in evaluating age relationships

upper jurassic lower cretaceous depositional environments and evolution of the bilecik sakarya zone and tauride carbonate platforms turkey abstract three late jurassic early cretaceous carbonate platform stratigraphic sections have been studied in central western turkey bilecik carbonate platform at bilecik and saricakaya and in the taurides in southern turkey tauride carbonate platform at beysehir by using sedimentological and cyclostratigraphical methods one section was analyzed by detailed magnetic susceptibility measurements peritidal carbonates on both platforms display cyclic patterns throughout the sections and do not contain any siliciclastic material this cyclicity is generally expressed as shallowing upward cycles and interpreted as records of short term sea level changes magnetic susceptibility ms analysis revealed that small scale changes in the ms values can be followed by cyclic alternation of facies and may indicate that varying detrital input during wet and dry climate alternation can be one of the controlling factors for short term sea level changes the large scale ms evolution records an increase just around the jurassic cretaceous boundary interval and a decrease above the boundary this curve pattern can be correlated with some european ms sections and seems to indicate a consistent ms pattern around this critical interval consequently this study implies that sea level variations on studied shallow carbonate platforms were on the order of few meters pelagic equivalents of these sea level records can be traced along the margins short term climatic control can be seen as one of the controlling factors for small scale sea level changes the large scale changes can be associated with oceanographic climatic third order sea level changes and the influence of regional tectonics

no global scale transgressive regressive cycles in the thanetian paleocene evidence from interregional correlation abstract a detailed examination of transgressions and regressions that occurred during the thanetian 58 7 55 8 ma may provide an important constraint on the global palaeoenvironment seven tectonically stable regions the eastern russian platform northwestern europe northwestern africa northeastern africa the arabian platform the northern gulf of mexico and southern australia represent exceptional records of transgressive regressive t r cyclicity their chronostratigraphic frameworks are sufficiently well constrained to permit accurate correlation surprisingly except for a generally regressive trend occurring in the late thanetian no common t r cycles can be delineated which indicates an absence of global scale t r cyclicity during the thanetian furthermore we find no clear correspondence between documented t r patterns and previously reported eustatic changes we suggest that a warm climate and an absence of major glaciations in the early middle thanetian coupled with only slow eustatic change expected from tectonic processes stabilized thanetian eustatic sea level regional subsidence or uplift possibly generated by mantle flow in the form of dynamic topography governed transgressions and regressions locally and resulted in an inconsistency between t r cycles in different parts of the globe the late thanetian regressive episode which preceded the paleocene eocene thermal maximum may be linked to an advance of glaciation

electrical resistivities of crustal fluids results of calculations on the electrical resistivities of crustal fluids as a function of temperature pressure and fluid composition indicate that crustal fluids can range in resistivity from approximately 1 m the dominant contribution to decreasing the resistivities of crustal fluids comes from increased concentrations of chloride and bicarbonate salts and increased temperature and density of the fluids calculations indicate that the sharpest decrease in fluid resistivity occurs at shallow depths 10 km and temperatures 300 c can not be readily explained as a product of variations in resistivities of crustal fluids these results indicate that a transition in mineralogy or pore shape geometry is more likely responsible for the observed decrease in electrical resistivity of the middle crust than a major change in fluid resistivity

the cenomanian turonian boundary event ctbe in northern lebanon as compared to regional data another set of evidences supporting a short lived tectonic pulse coincidental with the event abstract constrained by both foraminifera and isotope data the zeitoun and nahr ibrahim sections in northern lebanon record a steady flooding of the cenomanian carbonate platform during the uppermost cenomanian lower turonian interval the flooding began just before the inception of the ctbe 13c positive shift it is marked by the deposition of finely bedded to laminated fine grained limestone overlying platformal carbonates this scenario is entirely different to what has been published in northern israel where repeated emergences are instead recorded during the event a survey of published data from nearby middle east countries to north africa suggests that the ctbe is accompanied on a larger scale especially along the syrian arc by tectonic disturbances responsible for sequence stratigraphic heterogeneity and palaeogeographic changes detailed analysis of available data makes it possible to extract from the sedimentary record four sequence stratigraphic scenarios implying two phases of heterogeneity bounding the event and responsible for the outphasing of relative sea level changes in some sensitive areas these disturb a stepped early cenomanian to early turonian overall transgressive trend recorded in some large stable areas often used to support the concept of mesozoic eustasy the discussion focuses on the global processes that are possibly involved the hypothesized tectonic pulse does not explain the ctbe it is just another set of data to be taken into account in the search for causes

seismic attenuation structure of the east pacific rise near 9 30 n we describe a spectral technique to measure the apparent attenuation of compressional waveforms recorded during an active seismic tomography experiment centered at 9 32 n on the east pacific rise over 3500 estimates of t are obtained from 0 4 to 0 7 s long windows aligned with crustal p phases including diffractions above and below a midcrustal magma chamber and moho reflections which cross the rise axis at a range of lower crustal depths we apply a smoothest model inversion algorithm to the t measurements to derive images of apparent crustal q 1 both 20 km off axis and within a 16 16 km area centered on the rise crest the models resolve regions of high attenuation in the uppermost crust and in a low q zone which extends from midcrustal to lower crustal depths beneath the rise axis off axis q values in the upper 1 km average 35 50 while at depths greater than 2 3 km q is at least 500 1000 the high levels of attenuation in the uppermost crust probably result from the combined effect of factional fluid flow and scattering mechanisms within 1 3 km of the rise axis q increases markedly in the uppermost 1 km to about 65 if the increase in attenuation off axis is entirely due to the 300 m increase in the thickness of layer 2a extrusives required by seismic velocity measurements then q in layer 2a must be about 10 20 no measurements of q are obtained in the immediate vicinity of the 1 6 km deep axial magma lens because no wave paths cross the rise axis through this region the diffractions beneath the magma chamber and the moho reflections require a low q region with minimum q values of 20 50 which extends from no more than 2 5 km depth to the base of the crust these values are similar to laboratory measurements of q obtained at solidus temperatures and constrain the low q region to contain no more than a few percent melt the axial magma chamber which comprises a melt lens and an underlying crystal mush zone must be confined to a narrow 1 km thick region through which no rise crossing paths pass inversions for along axis structure in the low q anomaly show a 20 25 increase in attenuation at 2 3 km depth north of 9 34 n but resolve no such trend at 4 6 km depth the along axis variations may reflect the recent history of volcanic eruptions and hydrothermal cooling and do not require systematic along axis variations in magma supply

a global scale plate reorganization event at 105 100 ma abstract a major plate reorganization is postulated to have occurred at approximately 100 ma however this reorganization has received limited attention despite being associated with the most prominent suite of fracture zone bends on the planet and many other geological events we investigate tectonic events from the period 110 to 90 ma and show that the reorganization occurred between 105 and 100 ma was global in scale and affected all major plates seafloor evidence for plate motion changes is abundant during this period with either fracture zone bends or terminations preserved in all ocean basins long lived eastern gondwanaland subduction ended along a 7000 km long section of the margin while elsewhere around the proto pacific rim subduction continued and there is evidence that compressional stresses increased in the overriding plates thrusting in western north america transpression and basin inversion in eastern asia and development of the present day andean style margin along western south america occurred contemporaneous with the development of an extensional regime in eastern gondwanaland basin instability in africa and western europe further demonstrates that lithospheric stress regime changes were widespread at this time considering the timing of the reorganization and the nature of associated plate boundary changes we suggest that eastern gondwanaland subduction cessation is the most likely driving mechanism for the reorganization subduction is the dominant driver of plate motion and therefore this event had the potential to strongly modify the balance of driving forces acting on the plates in the southwestern proto pacific and neighboring plates whereby producing widespread changes in plate motion and continental lithospheric stress patterns we propose that major changes in ridge trench interaction triggered the cessation of subduction the progressive subduction of two closely spaced perpendicular mid ocean ridges at the eastern gondwanaland subduction zone to the east of australia and new zealand respectively resulted in very young crust entering the trench and we suggest that by 105 100 ma there was insufficient negative buoyancy to drive subduction finally we propose that the plume push force of the bouvet plume that erupted near the african antarctic south american triple junction contributed to plate motion changes in the southern atlantic region

electrical conductivity of molten basalt and andesite to 25 kilobars pressure geophysical significance and implications for charge transport and melt structure electrical conductivities of molten hawaiian tholeiite and crater lake andesite were measured between 1200 c and 1400 c at atmospheric pressure and at pressures up to 17and 25 kbar respectively isobaric plots of log versus 1 t is electrical conductivity are linear with the exception of the zero pressure tholeiite melt data conductivities decrease with increasing pressure in both melts with the andesitic melt exhibiting a greater pressure dependence between 5 and 10 kbar abrupt decreases in the slopes of isothermal log versus p plots i e decreases in activation volume are observed for both rock melts this discontinuity probably reflects changes in melt structure as opposed to changes in conduction mechanism in each pressure range the data for each rock melt can be described reasonably well by an equation of the form 0 exp ea p v kt where 0 is a preexponential constant ea is the activation energy and v is the activation volume a qualitative model involving depolymerization of the melt with increased pressure leading to increased efficiency of packing can explain the observed discontinuity in activation volume as well as the observed pressure dependences of other melt physical properties such as viscosity and density conductivity versus melt fraction curves for partially molten peridotite are reevaluated using high pressure tholeiitic melt conductivities and crystalline conductivity values recently determined by other workers minimum melt fraction estimates of 5 10 are required to explain upper mantle regions of anomalously high electrical conductivity in terms of a partial melting hypothesis

geomorphology and quaternary stratigraphy the roles of morpho litho and allostratigraphy abstract the subdivision and ordering of quaternary sediments and landforms is very often based on geomorphology quaternary landforms such as moraines river terraces palaeoshorelines and indeed any other landform can be arranged and ordered in time based on their relative position in the landscape morphostratigraphy is the subdivision of sedimentary units primarily on the basis of surface form whilst surface form is often used as a criterion for separating sedimentary units morphostratigraphy also usually incorporates lithological criteria to aid stratigraphical subdivision and morpho and lithostratigraphy are often interlinked in quaternary science however another approach allostratigraphy has increasingly been applied to subdivide and order quaternary sediment landforms allostratigraphy uses discontinuities to subdivide sedimentary successions allostratigraphy enables lateral variations in lithology to be included within the same stratigraphical unit furthermore it enables lithologically similar sediments stacked on top of each other to be divided into separate units based on discontinuities this paper examines the applications of morpho litho and allostratigraphy to subdivide sediment landform assemblages and critically reviews their role in quaternary stratigraphy

electromagnetic fields induced at the seafloor by rayleigh stoneley waves we model oceanic acoustic and seismic disturbances as rayleigh stoneley waves in the layered medium consisting of ocean sediment and rock the waves induce electromagnetic fields because of motion of conducting materials through the geomagnetic field low frequency disturbances in deep water are efficient electromagnetic generators because the motions are coherent over large volumes of highly conducting seawater for wave frequencies below 0 1 hz in deep water the geomagnetic field is nearly frozen to the moving water consequently the electric field measured by a voltmeter attached to the seabed is approximately u f where u is the difference between the velocity of the water and of the voltmeter itself as it is moved by the motion of the seafloor and f is the geomagnetic field this result applied to love waves suggests that the electric field will result largely from the movement of the detector but detailed calculations have not been made although the fields are weak they should be detectable because the main interference is from ionospheric sources and the fields from these sources are greatly attenuated at the seabed by the overlying oceanic shield the effectiveness of the shield diminishes sharply at frequencies below 0 03 hz at high frequencies the principal limitation to detectability is in the inherent noise level of detectors whether electric or magnetic at present electric detectors are more effective than magnetic they appear to be competitive with accelerometers for seismic detection in the deep ocean in a restricted frequency band near 0 05 hz a layer of unconsolidated sediment underlying the ocean profoundly affects the dispersion relation of rayleigh stonely waves and thereby affects the electromagnetic induction process the shunting effect of a normal deep ocean sediment layer is not large on the electric field but the low shear modulus of the sediments influences sea bottom motions and therefore greatly changes the fields sensed by bottom mounted instruments especially at higher frequencies

the late jurassic tithonian a greenhouse phase in the middle jurassic early cretaceous cool mode evidence from the cyclic adriatic platform croatia well exposed mesozoic sections of the bahama like adriatic platform along the dalmatian coast southern croatia reveal the detailed stacking patterns of cyclic facies within the rapidly subsiding late jurassic tithonian shallow platform interior over 750 m thick ca 5 6 myr duration facies within parasequences include dasyclad oncoid mudstone wackestone floatstone and skeletal peloid wackestone packstone shallow lagoon intraclast peloid packstone and grainstone shoal radial ooid grainstone hypersaline shallow subtidal intertidal shoals and ponds lime mudstone restricted lagoon fenestral carbonates and microbial laminites tidal flat parasequences in the overall transgressive lower tithonian sections are 1 4ae5 m thick and dominated by subtidal facies some of which are capped by very shallow water grainstone packstone or restricted lime mudstone laminated tidal caps become common only towards the interior of the platform parasequences in the regressive upper tithonian are dominated by peritidal facies with distinctive basal oolite units and well developed laminate caps maximum water depths of facies within parasequences estimated from stratigraphic distance of the facies to the base of the tidal flat units capping parasequences were generally 4 m and facies show strongly overlapping depth ranges suggesting facies mosaics parasequences were formed by precessional 20 kyr orbital forcing and form parasequence sets of 100 and 400 kyr eccentricity bundles parasequences are arranged in third order sequences that lack significant bounding disconformities and are evident on accommodation fischer plots of cumulative departure from average cycle thickness plotted against cycle number or stratigraphic position modelling suggests that precessional sea level changes were small several metres as were eccentricity sea level changes or precessional sea level changes modulated by eccentricity supporting a global hot greenhouse climate for the late jurassic tithonian within the overall cool mode of the middle jurassic to early cretaceous

eustatic tectonic and climatic signatures in the lower cretaceous siliciclastic succession on the eastern russian platform abstract a methodical approach to identifying major abiotic events in the siliciclastic succession accumulated in the shallow epicontinental basin on the eastern russian platform during the early cretaceous is presented on the basis of a reliable chronostratigraphic framework a comparison between global and regional sea level curves was undertaken the intervals during which the global and regional sea level curve trends are similar correspond to a predominance of eustasy in the particular basin alternatively tectonic activity dominates during intervals when there is no similarity between the trends of the global and regional sea level curves three intervals of noncoincidences of trends of these two curves matched with major tectonic events that took place within the eastern russian platform in the early cretaceous the early hauterivian tectonic uplift subsequent late hauterivian subsidence and the late albian uplift the main consequences of the tectonic activity were two large regional unconformities and hiati the comparison of main global and regional sea level trends also reveals major climatic events the cold snaps that occurred during the early cretaceous greenhouse world hu et al 2012 coincided with simultaneous global and regional sea level lowstands peak shallowing of the basin and the almost complete absence of sediments the cold snap is identified in the late aptian sedimentary sequences on the eastern russian platform

segmentation volcanism and deformation of oblique spreading centres a quantitative study of the reykjanes ridge abstract new multi beam swath bathymetry charts and high resolution side scan sonar imagery provide detailed evidence for volcanic and magmatic segmentation along the reykjanes ridge by combining these data sets we have quantified some relationships between different orders of segmentation various characteristics of volcanism and changes in tectonism along the ridge axis three orders of segmentation along the reykjanes ridge have been recognised 1 a long wavelength depth variation or swell that is related to the shoaling of the ridge towards iceland 2 intermediate wavelength rises of the order of 40 120 km and 3 a short wavelength volcanic segmentation of the order of 5 30 km the long wavelength depth variation which is probably controlled by the temperature of the underlying asthenosphere has a maximum positive deviation from a predicted depth curve that forms a break in slope at 59 60 n this feature migrated southward along the ridge at between 5 and 10 cm yr at least four times in the past 10 ma the intermediate wavelength rises which are also temporally and spatially unstable are probably related to local rayleigh taylor style asthenospheric instabilities the short wavelength segments have an evolutionary life cycle starting with eruption via fissure volcanoes and progressing by increasing magma flux to the formation of conical and then shield like volcanoes these segments are tectonically dismembered by extensional faulting following a rapid decrease in magma supply we propose a model for the construction of the volcanic layer of the reykjanes ridge which involves a cyclicity of magmatic and tectonic activity

en echelon axial volcanic ridges at the reykjanes ridge a life cycle of volcanism and tectonics abstract new deep towed sidescan records and multibeam bathymetric data along the reykjanes ridge allow us to assess the varying thermal effect of the icelandic hotspot the increasing predominance of faulting speculated to be derived from mantle upwelling and outflow over simple extensional neotectonics is traced from northeast to southwest numerous en echelon volcanic ridges built along the plate boundary are seen in various stages of growth and decay and a cycle of alternating construction and dismemberment is proposed unfaulted short narrow lensoid ridges occasionally in small groups are interpreted as immature and developing axial volcanic ridges broad convex flanked ridges appear to be mature sites of volcanic construction where the magma supply has been maintained narrow bow form ridges occasionally with rectilinear flanks represent axial volcanic ridges which have ceased construction and are in initial stages of disaggregation isolated polygonal blocks generally seen off axis represent the most advanced stage of dismantling of the surface crustal accretionary features while this cyclicity is speculated to be related to periodic magmatic and tectonic focusing and defocusing along the axis due to the restricted magma supply at this slow spreading ridge it is recognised that this needs modelling although the oblique spreading of the reykjanes ridge allows one to clearly distinguish between structures attributable to plate spreading from those related to small scale accretionary processes we propose that the major processes identified in this study are directly applicable to all slow spreading sections of mid ocean ridges

two dimensional topographic responses in magnetotellurics modeled using finite elements we simulate the magnetotelluric response to two dimensional earth topography using finite elements linear interpolation of the secondary field parallel to strike over triangular elements allows accurate modeling of inclined resistivity boundaries including topographic surfaces to avoid discontinuities in field derivatives or resistivity care must be taken that the nodal values of the field parallal to strike used to obtain the auxiliary secondary fields are kept within uniform earth media the nodal locations may be shifted but the derivatives still are evaluated at the field points of interest correct values may be returned at gentle breaks in slope as well as along straight surfaces the finite element program is verified by comparison with the analytic transverse magnetic response of a hemicylindrical depression and with rayleigh scattering and transmission surface results for transverse electric and transverse magnetic polarization agreement with the other methods generally is excellent with t

a guide to phanerozoic cold polar climates from high latitude ice rafting in the cretaceous the high latitude extent of warm climate indicators at certain times in earth history has been considered as evidence that the globe was ice free for long intervals despite theoretical considerations and results from numerical modelling experiments1 2 indicating that this was unlikely one of the warmest periods the cretaceous displays faunal and floral evidence for cool temperate to sub tropical conditions very near to the poles however our studies of lower cretaceous mudstones of central australia that contain outsized exotic blocks have led to the conclusion that the blocks were emplaced by ice rafting implying that high latitude ice was present at sea level strata of a similar origin of mid jurassic to mid cretaceous age occur on other continents that were positioned between 65 and 78 palaeolatitude indeed there is a record of high latitude ice rafting throughout the phanerozoic suggesting that ice was present on earth for much of its history and that ice free conditions could have been at most only episodic over the past 600 myr

a review of the broad pattern of jurassic sea level changes and their possible causes in the light of current knowledge abstract an assessment is made of the current state of research into jurassic sea level changes based on new insights into depositional patterns in relation to these changes and new stratigraphic information from several widely separated regions across the world the bearing of this information on alternative models respectively involving sea level rise interrupted by stillstands or falls is then evaluated the gross pattern appears to be one of more or less gradual rise of sea level through the period interrupted by episodes of comparative stillstand rather than eustatic fall and several episodes of significant regression are shown to be the result of regional tectonics major episodes of eustatic rise took place in the early hettangian early sinemurian early pliensbachian early toarcian early and late bajocian middle callovian and late oxfordian to kimmeridgian a significant episode of rapid and very extensive regression possibly global took place at the end of the triassic but other regressive episodes in the late aalenian bathonian late oxfordian kimmeridgian and tithonian are clearly only regional in extent there is no evidence for glacioeustasy and most if not all of the regional or global changes recognised can readily be related to plate tectonics the events at the triassic jurassic boundary seem to be the result of mantle plume activity centred on the central atlantic region

isotope major minor and trace element geochemistry of late magmatic fluids in the peralkaline ilimaussaq intrusion south greenland abstract the persodic 1 16 ga old ilimaussaq intrusion consists of alkali granites syenites and agpaitic nepheline syenites these rocks are cut by late magmatic veins and lenses consisting of albite aegirine ussingite na2alsi3o8 oh fluorite and occasionally quartz oxygen isotope compositions of quartz indicates both orthomagmatic 18oqtz 8 5 and country rock influenced 18oqtz 10 origin while albite and aegirine do not contain fluid inclusions suitable for investigations ussingite contains secondary hydrocarbon fluid inclusions fluorite contains brine inclusions of primary and secondary origin with up to 26 3 wt naclequiv and quartz contains predominantly secondary brine inclusions with up to 29 7 wt naclequiv or ch4 h2o nacl mixtures these fluids are interpreted to reflect fluids in equilibrium with late stage melts at ilimaussaq at 450 c the carbon and hydrogen isotope composition of ch4 in ussingite 13c 6 to 3 d 121 is indicative of a magmatic abiogenic origin however the isotopic composition of ch4 in fluid inclusions in quartz veins resembles the signature of thermogenic ch4 13c 43 to 23 d 176 to 121 but the higher hydrocarbons are mostly 13c depleted in relation to ch4 up to 7 3 which is typical of abiogenically derived hydrocarbons ion chromatography of fluid inclusion leachates from the late stage veins reveals cl br ratios of 100 as such values seem to be typical of peralkaline magmatic rocks at least in the gardar province of south greenland it is suggested that this ratio is typical of gardar magmatic fluids and may be characteristic of the cl br ratio of the lithospheric mantle from which these alkaline melts are derived

review short term sea level changes in a greenhouse world a view from the cretaceous abstract this review provides a synopsis of ongoing research and our understanding of the fundamentals of sea level change today and in the geologic record especially as illustrated by conditions and processes during the cretaceous greenhouse climate episode we give an overview of the state of the art of our understanding on eustatic global versus relative regional sea level as well as long term versus short term fluctuations and their drivers in the context of the focus of unesco iugs igcp project 609 on cretaceous eustatic short term sea level and climate changes we evaluate the possible evidence for glacio eustasy versus alternative or additional mechanisms for continental water storage and release for the cretaceous greenhouse and hothouse phases during which the presence of larger continental ice shields is considered unlikely increasing evidence in the literature suggests a correlation between long period orbital cycles and depositional cycles that reflect sea level fluctuations implying a globally synchronized forcing of eustatic sea level fourth order depositional sequences seem to be related to a 405 ka periodicity which most likely represents long period orbital eccentricity control on sea level and depositional cycles third order cyclicity expressed as time synchronous sea level falls of 20 to 110 m on 0 5 to 3 0 ma timescales in the cretaceous are increasingly recognized as connected to climate cycles triggered by long term astronomical cycles that have periodicity ranging from 1 0 to 2 4 ma future perspectives of research on greenhouse sea level changes comprise a high precision time scale for sequence stratigraphy and eustatic sea level changes and high resolution marine to non marine stratigraphic correlation

seismic mapping of a magma chamber beneath the valu fa ridge lau basin we analyze a dense grid of seismic normal incidence and wide angle data collected over an active back arc spreading center in the lau basin of the southwest pacific our survey area covers the whole of a 35 km long morphological segment and a small overlapping spreading center a bright reflector which is coincident with a velocity inversion at a depth of 3 2 0 2 km below the seafloor is observed on every one of the 40 across axis profiles we interpret this reflector as being the roof of a crustal magma chamber the widest magma chamber reflector occurs beneath the overlapping spreading center where it extends up to 4 km being imaged beneath both ridges and in places beneath the overlap basin elsewhere the width of the reflector varies between 0 6 and 2 3 0 4 km the narrowest reflectors are observed beneath deviations devais of the ridge axis in addition to being observed on each of the kilometer spaced across axis profiles the reflector is also seen as a continuous event for at least 10 km on a profile along the ridge axis the reflection coefficient of the event is between 0 34 and 0 65 this requires the melt at the top of the chamber to have a p wave velocity less than 2 7 km s if the interface is planar or less than 3 8 km s if the interface is layered

seamount volcanism at the reykjanes ridge relationship to the iceland hot spot the axial zone of the reykjanes ridge is covered with small 0 5 3 km in diameter volcanoes that pile together to form larger axial volcanic ridges this style of volcanism is similar to that at the mid atlantic ridge mar and may be common to slow spreading ridges despite proximity of the reykjanes ridge to the iceland hot spot in this study we quantitatively investigate the population of seamounts in three study areas at the reykjanes ridge areas a and b are centered at 62 n and 60 n respectively area c is centered at 58 n and is located south of the transition in ridge morphology from an axial high to an axial graben using multibeam bathymetry data 541 seamounts summit height h 50 m were identified in the three areas and their size and shape statistics were compiled additionally 105 seamounts in areas b and c were recognized in deep towed side scan images and their surface morphologies hummocky or smooth were recorded on the basis of estimated population parameters we find that seamounts at the reykjanes ridge are more abundant 310 20 per 103 km2 on average than at the mar between 24 and 30 n 200 10 per 103 km2 significant along axis variations exist at the reykjanes ridge however which are not simply related to distance from the hot spot area b has nearly twice the abundance of seamounts as either area a or area c variation in the characteristic height of the seamount population is also observed between the reykjanes ridge 68 2 m and the mar 58 2 m but no significant variation is found between our three study areas a dramatic change in seamount surface morphology occurs between areas b and c there are no side scan data from area a area c has 78 hummocky seamounts similar to the proportion observed at the mar while area b has 83 smooth seamounts on the basis of these results we present a conceptual model for building the shallow crust at the slow spreading reykjanes ridge that takes into account the possible influence of the iceland hot spot on the crustal melt delivery system and its influence on variables that control seamount abundances sizes shapes and surface morphologies in this model we suggest that the increased seamount production and proliferation of smooth seamounts in area b may be associated with a pulse of hot spot material in the form of asthenosphere of higher temperature that has recently affected area b

how global are the jurassic cretaceous unconformities the reality of the global scale sedimentation breaks remains controversial a compilation of data on the jurassic cretaceous unconformities in a number of regions with different tectonic settings and character of sedimentation where new or updated stratigraphic frameworks are established permits their correlation unconformities from three large reference regions including north america the gulf of mexico and western europe were also considered the unconformities which encompass the jurassic cretaceous the lower upper cretaceous and the cretaceous palaeogene transitions are of global extent other remarkable unconformities traced within many regions at the base of the jurassic and at the santonian campanian transition are not known from reference regions a correlation of the jurassic cretaceous global scale sedimentation breaks and eustatic curves is quite uncertain therefore definition of global sequences will not be possible until eustatic changes are clarified activity of mantle plumes is among the likely causes of the documented unconformities

jurassic transgressions and regressions in the caucasus northern neotethys ocean and their influences on the marine biodiversity abstract in the jurassic the caucasus presently located in the southwest of russia georgia armenia and azerbaijan was located on the northern active margin of the neotethys ocean facies interpretation in all 62 areas distinguished by differences in facies allows to semi quantitatively evaluate jurassic regional transgressions and regressions for this region major transgressive regressive cycles took place in the hettangian aalenian bajocian bathonian and callovian tithonian each transgression was more extensive than the previous the same cycles are established in the greater caucasus basin deep marine environments were common in the pliensbachian late aalenian and late bathonian whereas they were very restricted in the late jurassic the jurassic transgressions and regressions in the caucasus coincided with the proposed global eustatic changes however some differences were caused by the regional tectonic activity although transgressions and regressions cause some changes in marine biodiversity it seems that only ammonites might have been directly influenced by them diversity of bivalves brachiopods and belemnites was driven by other factors however global changes in marine biodiversity were more closely related to the eustatic fluctuations than it was documented on a regional scale

sea level and climatic controls on aptian depositional environments of the eastern russian platform abstract on the basis of a high resolution chronostratigraphic framework of the eastern russian platform a comparison between late barremian aptian global and regional sea level trends was performed the detailed evaluation of the long term 3rd order aptian sea level cycle results in the recognition of sea level and climate as controlling factors on depositional environments in the basin the rising part of the aptian sea level cycle lasted from the deshayesites tenuicostatusis zone to the deshayesites deshayesi zone and transgression is responsible for the local development of anoxia on the eastern russian platform the lower aptian bituminous shales and sheeted calcite concretions associated with the eastern russian platform are interpreted as being a regional manifestation of oceanic anoxic event oae 1a the late aptian cold snap that occurred during the early cretaceous greenhouse world coincided with a simultaneous global and regional sea level lowstand peak shallowing of the basin and the almost complete absence of sediments due to subaerial exposure in the studied region the global distribution of the lowstand gives clear evidence for sea level fluctuations and intrinsic climate control on sequences in the study area