thermal structure beneath tohoku northeast japan combined with geophysical observations and petrological considerations we have studied the thermal models of the tohoku northeast japan subduction zone the temperature of the deeper part of the mantle wedge is constrained by the petrological requirements that demand a high temperature region 1400 c beneath the volcanic zone to satisfy this condition namely to explain the arc volcanism in the tohoku subduction zone a high temperature upper mantle beneath the japan sea possibly 1400 c at the depth shallower than 100 km is required heat flow distribution which constrains the temperature structure at shallower depth shows that low heat flow values ranging from 20 to 50 mw m2 are distributed between the trench and aseismic front and consistently high heat flow values 80 120 mw m2 prevail in the area landward of the volcanic front including the japan sea recent heat flow measurements indicate that the transition from low to high heat flow occurs between the aseismic front and volcanic front this heat flow distribution restricts the form of the flow in the mantle wedge and shear stress acting at the plate boundary a rigid non moving triangular region bounded by the subducting slab and the vertical plane beneath the aseismic front is necessary to explain the low heat flow zone between the trench and aseismic front shear stress of about 500 to 1000 bar is supposed to act at the interface between the subducting slab and rigid triangular mantle we also suggest that to explain the peak of heat flow near the volcanic front 120 mw m2 magma migration rate of about 0 05 cm3 cm 2 year 1 is necessary a comparison of the thermal structure of the tohoku subduction zone with that of the central south america which was preliminary calculated by the similar scheme shows that the temperature in the mantle wedge of the central south america is probably lower than that of the tohoku subduction zone this difference may result in that of the nature of volcanic rocks

preferential mode of gas invasion in sediments grain scale mechanistic model of coupled multiphase fluid flow and sediment mechanics 1 we present a discrete element model for simulating at the grain scale gas migration in brine saturated deformable media we rigorously account for the presence of two fluids in the pore space by incorporating forces on grains due to pore fluid pressures and surface tension between fluids this model which couples multiphase fluid flow with sediment mechanics permits investigation of the upward migration of gas through a brine filled sediment column we elucidate the ways in which gas migration may take place 1 by capillary invasion in a rigid like medium and 2 by initiation and propagation of a fracture we find that grain size is the main factor controlling the mode of gas transport in the sediment and we show that coarse grain sediments favor capillary invasion whereas fracturing dominates in fine grain media the results have important implications for understanding vent sites and pockmarks in the ocean floor deep subseabed storage of carbon dioxide and gas hydrate accumulations in ocean sediments and permafrost regions our results predict that in fine sediments hydrate will likely form in veins following a fracture network pattern and the hydrate concentration will likely be quite low in coarse sediments the buoyant methane gas is likely to invade the pore space more uniformly in a process akin to invasion percolation and the overall pore occupancy is likely to be much higher than for a fracture dominated regime these implications are consistent with laboratory experiments and field observations of methane hydrates in natural systems

north sea rift system abstract the mesozoic north sea rift forms an integral part of the arctic north atlantic rift system rifting activity in the north sea commenced during the earliest triassic during the mid jurassic the central north sea underwent a temporary doming stage crustal extension peaked during the late jurassic earliest cretaceous abated subsequently and terminated altogether during the paleocene the saucer shaped cenozoic post rift basin straddles the axis of the viking and central grabens this speaks for the applicability of a depth dependent pure shear model rather than a simple shear model geophysical data indicate an important discrepancy between upper and lower crustal attenuation across the north sea rift suggesting that during rifting processes the mono discontinuity was seriously destabilized this requires a basic modification of currently favoured crustal stretching models

paleomagnetic evidence for the miocene counter clockwise rotation of northeast japan rifting process of the japan arc paleomagnetic results have been obtained from more than 700 oriented samples in igneous rocks of cretaceous to miocene age from northeast japan the remanent magnetizations of welded tuffs with age between 32 ma and 21 ma old from 17 widely distributed sampling sites in northeast japan are fairly well grouped with a mean ofd 41 2 i 56 5 95 7 2 the remanent magnetization of welded tuffs of 14 ma and 11 ma from 4 sites shows a northerly direction d 11 7 i 60 5 95 15 3 these data imply that northeast japan was apparently subjected to a counter clockwise rotation through 47 14 about a vertical axis on the representative point of northeast japan 141 e 39 n during the past 21 ma comparing with the period of the clockwise rotation of southwest japan the differential rotation between northeast japan and southwest japan is concluded to have occurred concurrently during the period between 21 ma and 12 ma a rifting model of the japan arc is proposed northeast japan rotated counter clockwise through 50 about a northern pivot at 146 e 44 n while southwest japan rotated clockwise through 54 about a southern pivot at 129 e 34 n the differential rotation about different rotation pivots gives rise to the opening of a diamond shaped back arc basin the japan sea

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

provenance of pliocene sediments and paleoenvironmental changes in the southern north sea region using samarium neodymium sm nd provenance ages and clay mineralogy abstract an 800 m thick sedimentary succession from the late pliocene north sea has been investigated on the provenance of the sediments and on changes of the source regions related to transport processes and climate changes the lower zanclean to piacenzian part of the succession from 1250 to 1050 m depth and the middle early gelasian part from 1100 to 700 m depth belong to one distributory system with at least two contributing source areas provenance ages calculated on basis of sm nd content indicate one old archean and one young

structural characteristics of congelation and platelet ice and their role in the development of antarctic land fast sea ice ice cores were obtained in january 1990 from the land fast ice in mcmurdo sound for a study of variations in texture fabric sub structure composition and development two primary ice types were observed congelation and platelet with a minor amount of frazil ice congelation ice growth precedes platelet ice accretion congelation ice fabrics show frequent moderate to strong alignments a phenomenon believed to be due to water current control of selective ice crystal growth platelet ice originates at the base of the congelation ice initially as a porous latticework of tabular ice crystals which subsequently consolidate by congelation of the interstitial water interstitial congelation ice fabrics generally have little or no alignment indicating the reduced effect of currents within the platelet latticework prior to solidification platelet crystal textures range from small wavy edged forms to large blade like forms platelet crystal fabrics indicate that in addition to being randomly oriented the platelet latticeworks commonly include many crystals with their flat 0001 faces oriented both parallel and normal to the base of the overlying ice plate width data suggest that the interstitial congelation ice growth rates remain similar to those of the overlying congelation ice this effective increase in growth rates probably happens because the latticework of accumulating platelets ahead of the freezing interface ensures that the water within the platelet layer is at the freezing point and less heat has to be removed from platelet rich water than from platelet free water for a given thickness of congelation ice to grow the negative oceanic heat flux associated with platelet ice formation in mcmurdo sound explains why mcmurdo sound fast ice is thicker than ross sea pack ice and also why it reaches a greater thickness than arctic fast ice grown in a similar polar marine climate plate widths in the mcmurdo sound congelation ice suggest however that it grows no faster than arctic congelation ice

plate tectonics and the evolution of the british isles this paper is an essay on the rationalism and difficulties of the geological corollaries of plate tectonics with illustrations particularly from the geological history of the british isles there is special emphasis on the mechanics of continental lithosphere in extension when basins are formed and in convergence whereby continental lithosphere thinned in extension is shortened and restacked to form orogenic belts an upper lithosphere stress concentrating elastic lid suffers extremely complex inhomogeneous deformations above a simpler viscoelastic lower lithosphere whose deformation drives strain and displacement in the elastic lid the very nature of plate tectonics as relative motion among a multi plate mosaic of plates provides a rational semi quantitative framework for large scale geological complexity in space and time the caledonian and hercynian history of the british isles may be explained semi quantitatively in terms of plate boundary evolution but it is not possible to define unequivocal solutions the rationalism of plate tectonics is exhibited on a smaller more direct field scale by many of the complexities of ophiolite complexes such as ballantrae scotland that can be explained as the effects of the evolution of ridge ridge transforms and by diachronous deformation and sedimentation in subduction accretion prisms such as the southern uplands deformational and sedimentary patterns in nw europe from triassic times to the present day resulted from a complex interaction of extensional events related to diachronous atlantic opening and compressional events related to sequential alpine shortening phases those widespread events are typical of intracontinental plate boundary systems and create great difficulties for the geologist wishing to decipher relative plate motion from geological sequences because huge areas much larger than the british isles must be analysed and synthesized before much sense can be extracted further difficulties arise from the palinspastic and preservation problems of convergent boundaries orogenic shortening telescopes and partially subducts facies belts most orogenic belts were assembled as a collage of displaced and rotated terrain elements that were subsequently flattened elongated and straightened to an apparent but deceiving simplicity therefore while plate tectonics provides a rational geological framework against which for example british basin evolution can be tested and explained we cannot yet use the geological effects of plate boundary evolution to reconstruct relative plate motion quantitatively

evidence for constriction and pliocene acceleration of east west extension in the north lunggar rift region of west central tibet 1 the active north trending north lunggar rift in west central southern tibet exposes an extensional metamorphic core complex bounded by an east dipping low angle normal fault apatite and zircon u th he thermochronology and thermal modeling of the north lunggar rift document a minimum timing for rift inception at 10 ma and rapid footwall exhumation between 5 and 2 ma miocene footwall cooling and exhumation rates were initially slow to moderate at 400 c ma 1 and 4 10 mm a 1 footwall isotherms were significantly compressed during rapid exhumation resulting in an elevated transient geothermal gradient between 50 and 90 c km 1 the minimum magnitude of horizontal extension for the north lunggar rift is 8 1 12 8 km maximum is 15 20 km less in the south at 10 km mean pliocene extension rate is 1 2 2 4 mm a 1 in the 120 direction results for the north lunggar rift are similar in magnitude rate and orientation of slip to the kinematically linked lamu co dextral strike slip fault to the north this suggests a state of constrictional strain during pliocene time along this stretch of the bangong nujiang suture from which the lamu co fault emanates the onset of extension in this region may be explained by crustal thickening and gravitational orogenic collapse followed by accelerated rifting resulting from localized crustal stretching and increased magmatic activity potentially driven by the position and northward extent of underthrusting indian lithosphere

rapid intra arc rifting in miocene northeast japan although life times of major continental rifts are tens of millions of years intra arc rifting in miocene ne japan lasted for only about 3 m y before back arc spreading began the rate of initial subsidence was over 1 km m y much faster than that in major continental rifts thus this intra arc rift evolved 10 times as fast as major intracontinental rifts the large temperature dependence of lithosphere rheology and the hot thermal regime of island arc lithosphere account for the rapid rifting less than 1 m y before the rifting spreading transition the inner ne japan arc subsided from shallow marine depths to middle or lower bathyal depths 1 0 2 5 km the average total basement subsidence was 2 3 km during the rifting the rapid submergence occurred during one third of the duration of rifting but accounts for two thirds of the total subsidence during rifting namely the initial subsidence rate accelerated toward the time of breakup of the arc crust at the same time the region along the present pacific coast remained only shallow marine or terrestrial throughout presumably influenced by global eustacy

the tertiary geodynamical evolution of the aegean arc a paleomagnetic reconstruction abstract the paleomagnetic results obtained in the last 5 6 years from tertiary formations in the aegean domain indicate that the lower miocene arc was almost rectilinear with an e w trend and that its curvature has been acquired tectonically in two major phases during the middle miocene a first phase of deformation is characterized by rotations occurring at the two terminations of the arc clockwise in the west epirus anticlockwise in the east southeastern anatolia a second phase of rotation occurring in the last 5 ma about a pole situated in the southern adriatic sea has affected only the northwestern part the data also indicate that the antalya basin has not been involved in the geodynamical evolution of the arc so that the bey daglari represent its easternmost unit a more complex rotational pattern is observed in the internal zones the large rotations measured in evia 48 and skyros 26 probably occurred in the last 5 ma and result from rotation of fault bounded blocks in a shear zone connecting the north aegean trough to the external arc in the izmir region in the karaburun peninsula of western anatolia and in the island of lesbos significant counterclockwise clockwise or null rotations of coherent blocks have taken place during the neotectonic extensional regime furthermore the values of the inclination obtained from both sedimentary and volcanic oligo miocene formations are systematically shallower than expected on the basis of a geocentered dipole field this suggests that the entire studied area has undergone a northward drift of over 1000 km since the middle miocene

parametric and non parametric statistical approaches to the determination of paleostress from dilatant fractures application to an early miocene dike swarm in central japan abstract several methods have been proposed for determining paleostress states from orientations of dilatant fractures such as dikes and veins recently a stochastic inversion method was invented to objectively estimate the principal stress axes and the stress ratio whether a fracture is dilated or not is controlled by the balance of the fluid pressure and the normal stress acting on it the magnitude of normal stress depends on the fracture orientation which causes anisotropic orientation distribution of dilatant fractures the inversion method assumes that the orientation distribution of fractures can be approximated by a bingham distribution an exponential probability distribution on the unit sphere of which symmetric axes are interpreted as the principal stress axes however it is unknown if the exponential type of distribution function is suitable or not here we examine the distribution functions and propose two improved methods one method uses the shifted power law function as the shape of probability distribution which is more flexible than the bingham distribution and is applicable to various shapes of orientation distributions furthermore an index of the driving fluid pressure can be estimated with a confidence interval the other is a non parametric distribution free method which can avoid the a priori assumption on the shape of distribution function without significantly losing accuracy or precision the new methods were applied to an early miocene dike swarm formed during the back arc opening of the japan sea a normal faulting stress regime with the minimum principal stress axis trending roughly perpendicular to the arc was obtained from the dikes a moderately high stress ratio and a high fluid pressure were also estimated

timing of rotational motion of southwest japan inferred from paleomagnetism abstract paleomagnetic field directions for the period younger than 35 ma are obtained from igneous rocks distributed in the san in district inner zone of southwest japan the remanent magnetization of samples between 30 and 35 ma old are fairly well grouped with a mean direction of d 65 9 i 48 6 and 95 6 5 this result establishes that southwest japan rotated clockwise 56 12 during the past 30 m y a declination value of about 60 is observed in the rocks of 28 ma d 52 2 i 33 5 and 21 ma d 69 9 i 49 5 comparing this with results from dacitic rocks with an age of 15 ma in other areas of southwest japan suggests that rotational motion did not occur possibly until 15 ma these results require that the rotation of southwest japan occurred just after the shikoku basin had been created

extensional tectonics and stratigraphy of hibernia oil field grand banks newfoundland the tectonic evolution of the grand banks involved several episodes of rifting and sequential development of the continental margin the dominant period of rifting and basin formation in the central grand banks was late callovian to aptian extensional failure of the central grand banks crust probably occurred along a shear zone that dipped gently to the west listric normal basin forming faults merge at depth with this detachment system the listric murre fault soles at 26 km 16 mi creating the very deep jeanne d arc basin cross basin transfer faults accommodated different amounts and rates of extension and resulted in the conspicuous offset pattern and funnel shaped geometry of the jeanne d arc basin the hibernia oil field is associated with one of these structural salients the stratigraphy of the jeanne d arc basin and the hibernia oil field is a direct response to this tectonic and structural framework large scale unconformity bounded sequences correspond to major plate reorganizations and zipper type opening of the atlantic ocean smaller scale adjustments to episodic extension are expressed in multiple stacking of depositional sequences the broad scale evolution of the synrift succession involved the early deposition of limestones and oil prone shales about 10 12 m y later rifting climaxed resulting in floods of coarse terrestrial material from the rift shoulders the characteristics of these depositional systems reflect a pervasive structural control affecting the distribution of facies tracts dip slip movement on basin bounding faults and c oss basin transfer fault trends were equally important a profound structural modification of the hibernia structure occurred in late barremian time when a major detachment developed within the basin fill this structural modification and associated antithetic and synthetic faulting in the hanging wall largely controlled reservoir distribution and the maturation migration and trapping of hydrocarbons

a dynamical model of lithosphere extension and sedimentary basin formation convection in the upper mantle can give rise to normal stresses on the base of the continental lithosphere that may cause localized elevation of its surface and extensional deviatoric stresses this possibility is investigated assuming homogeneous biaxial strain in a continental lithosphere whose strength is governed by a combination of brittle failure and power law creep in the crust and by brittle power law or dorn law behavior in the mantle for a given geotherm and for rheologies based on current laboratory data the lithosphere responds to convection induced uplift in one of three ways depending primarily on the magnitude of the driving stresses these regimes are as follows 1 when the driving stress exceeds a threshold value the resulting strain rates are high enough greater than about 3 10 16 s 1 that the lithosphere strains almost isothermally its effective rheology is strain thinning and it undergoes accelerating extension the magnitude of the dynamic uplift required to produce the threshold value of the driving stress depends on the geotherm characterized here by the moho temperature and varies from about 1 km for a moho temperature of 700 c to around 3 km for a moho temperature of 550 c an uncertainty of at least 100 c is attached to these temperatures owing to the inaccuracies in extrapolating laboratory data to geological strain rates 2 at stress levels due to less than about half of the critical uplift extensional strain is negligible 3 at intermediate stress levels moderate uplift produces extension that is bounded by strength increases due to cooling of the ductile portion of the lithosphere during extension

a natural gas hydrate system on the exmouth plateau nw shelf of australia sourced by thermogenic hydrocarbon leakage abstract the identification of natural gas hydrates and shallow free gas in sedimentary basins is critical for understanding the organic carbon cycle dynamics in the shallow geosphere as well as for geohazard studies related to the development of commercial hydrocarbon fields in this study we report for the first time the evidence for the potential occurrence of a natural gas hydrate system along the continental margin of australia on the exmouth plateau nw shelf by the use of high quality 3d seismic data calibrated by downhole data from odp and industry boreholes we interpret a series of shallow high amplitude anomalies including patchy bottom simulating reflections as the seismic expression of localised accumulations of free gas beneath the base of hydrate stability and overlying high saturation hydrates or authigenic carbonates the hydrate bearing reservoir is constituted by neogene and quaternary fine grained carbonate nannofossil and foraminifera rich oozes the patchy distribution of the shallow free gas and hydrate accumulations which reflects the geometry of deeper fault blocks is hypothesised to result from the leakage of thermogenic gas from deeper reservoirs the interpretation of a thermogenic origin for the hydrate forming gases is supported by 1 the existence in the study area of several thermogenic gas discoveries within late triassic reservoirs 2 seismic evidence of cross stratal and stratal pathways for migrating gases in the overburden 3 the presence of free gas accumulations at depths intermediate between the deep reservoirs and shallow gas hydrate systems and 4 geochemical and lithological evidence that the conditions which favour the generation of shallow microbial gas are not present in the area the acquisition of downhole log and sample data through the observed seismic amplitude anomalies is necessary to test the interpretations and hypotheses presented in this manuscript

a rheological weak zone intensified by post rift thermal relaxation as a possible origin of simple shear deformation associated with reactivation of rifting abstract rheological heterogeneity brought about by the grain size refinement associated with the spinel plagioclase lherzolite reaction is discussed in the context of post rift thermal relaxation in continental rifts and passive margins above a critical temperature the effect of grain size refinement is not sufficient to obtain significant rheological heterogeneity a critical time scale for thermal relaxation defined as a period when the temperature at the boundary of a reduced grain size segment falls below the critical temperature is evaluated in this study it is found that a segmental rheological weak zone with a significant strength contrast can develop if the following conditions are satisfied 1 a substantial grain size reduction associated with spinel plagioclase lherzolite reaction occurs during an earlier extensional event and 2 a successive extensional event occurs after the critical time scale for thermal relaxation a few tens of million years the development of such rheologically weak zones may contribute to simple shear deformation in rift related sedimentary basins developed by multi rifting events

modeling of anisotropic electromagnetic reflection from sea ice the contribution of brine layers to observed reflective anisotropy of sea ice at 100 mhz is quantitatively assessed and a theoretical explanation for observed reflective anisotropy is proposed in terms of anisotropic electric flux penetration into the brine layers the sea ice is assumed to be a stratified dielectric consisting of pure ice containing ellipsoidal conducting inclusions brine layers uniformly aligned with their long axes perpendicular to the preferred crystallographic c axis direction the asymmetrical geometry of the brine layers is shown to produce an anisotropy in the complex dielectric constant of sea ice the contribution of these layers to the reflective anisotropy is examined with a numerical method of approximating the reflected power of a radar pulse incident on a slab of sea ice mixture dielectric permittivities are calculated for both the electric fields parallel and perpendicular to the c axis direction these permittivities are then used to calculate power reflection coefficients at each interface in the air sea ice sea water system significant bottom reflection r 0 09 occurs when the polarization is parallel to the c axis however when the polarization is perpendicular to the c axis the return is almost completely extinguished r 0 003 this extinction is due primarily to absorptive loss associated with the conducting inclusions and secondarily to an impedance match at the ice water interface that results in transmission rather than reflection of the wave

seal failure assessment of a major gas field via integration of seal properties and leakage phenomena we present a seismic and well based interpretation of a large leakage zone above the scarborough gas field exmouth plateau north west shelf of australia this leakage zone well imaged on three dimensional seismic data extends over a region of 100 km2 39 mi2 encompassing both the crest and flanks of the anticlinal trap and is termed here as distributed crestal leakage the present day gas water contact is 85 m 278 ft and the spillpoint is 110 m 328 ft below the crest implying that the trap is underfilled at present the leakage zone comprises more than 500 pockmarks at the present day seabed with no crosscutting or cannibalization suggesting that they formed in a short interval of time these are underlain by sediment remobilization features and amplitude anomalies consistent with a relatively high flux leakage of gas from the underlying cretaceous deep water sand rich reservoir by analyzing the geometrical relationship between the leakage zone the top seal properties and the gas water contact we conclude that the mode of leakage in this specific setting is not the result of gradual addition of gas charge but is instead consistent with a sudden increase of aquifer overpressure we suggest two alternative models for seal failure in this case study a conservative model consistent with a modest but rapid increase in aquifer overpressure leading to membrane seal failure and a model dominated by high aquifer overpressure leading to leakage through hydraulically dilated faults and fractures

continental rifting at pre existing lithospheric weaknesses continental rifting is the poorly understood thermomechanical process by which continents break up and new ocean basins form two dominant styles of continental rifting are recognized1 crevice style rifts are characterized by initial graben formation local uplift of the graben shoulders and little or no volcanism arch volcanic rifts exhibit an initial doming of the crust extensive volcanism and late stage graben formation the prevailing expla nation for the two styles is that crevice rifts form in response to regional horizontal stresses principally derived from the inter action of the lithospheric plates along their edges whereas arch volcanic rifts are thought to be the result of mantle convection currents which upwell directly beneath the rift axis2 we propose an alternative working model of continental rifting in which both styles initiate in response to regional horizontal stresses the different surface expressions of the two styles are explained in terms of differences in the mode of failure at different kinds of pre existing weaknesses in the continental lithosphere rather than differences in the nature of the driving forces involved

seismic low frequency effects in monitoring fluid saturated reservoirs there is a complex relationship between seismic attributes including the frequency dependence of reflections and fluid saturation in a reservoir observations in both laboratory and field data indicate that reflections from a fluid saturated layer have an increased amplitude and delayed traveltime at low frequencies when compared with reflections from a gas saturated layer comparison of laboratory modeling results with a diffusive viscous theory model show that low 5 values of the quality factor q can explain the observations of frequency dependence at the field scale conventional processing of time lapse vsp data found minimal changes in seismic response of a gas storage reservoir when the reservoir fluid changed from gas to water lowfrequency analysis found significant seismic reflectionattribute variation in the range of 15 50 hz the field observations agree with effects seen in laboratory data and predicted by the diffusive viscous theory one explanation is that very low values of q are the result of internal diffusive losses caused by fluid flow this explanation needs further theoretical investigation the frequencydependent amplitude and phase reflection properties presented in this paper can be used for detecting and monitoring fluid saturated layers

high frequency sequence stratigraphy of upper cenozoic deposits in the central and southeastern north sea areas abstract during late cenozoic times southwestward prograding deposits filled in the central and southern part of the epicontinental north sea basin the ringkobing fyn high acted as a topographical high partly separating the central and southeastern north sea into two embayments which were connected towards the west by a deeper seaway located in the central graben area the stratal geometry deduced from seismic sections indicates different basin physiography subsidence rates and sediment influx between the two embayments eustatic sea level cycles of several frequencies interacted with low subsidence rates in the southern embayment to generate composite relative sea level cycles each composed of relative sea level cycles of higher frequencies forty five upper cenozoic sequences are identified in the southern embayment whereas only 21 sequences are recognized in the northern embayment major hiati and intervals of starved sedimentation may therefore be present in the northern embayment the sequences stack into nine composite sequences each reflecting a depositional cycle representing lowstand succeeded by transgressive highstand conditions in general a composite sequence is characterized by a period of increasing sediment accumulation rates

structure and development of porcupine seabight sedimentary basin offshore southwest ireland the porcupine basin is a deep sedimentary basin on the continental margin west of ireland all available geologic and geophysical data have been used to interpret the history of the basin it contains up to 8 km of cretaceous and younger sediments underlain by an unknown thickness of jurassic and triassic rock gravity and magnetic anomaly models have been used to investigate the nature of the crust beneath the porcupine basin these models suggest a thinned continental crust rather than the oceanic crust previously proposed by some authors although the nature of a narrow zone of crust 10 km thick in the central southern basin cannot be determined conclusively a high amplitude elongate gravity high associated with a negative magnetic anomaly in the axis of the n rthern porcupine seabight correlates with a volcanic ridge probably of early cretaceous age seen on seismic reflection profiles regional geologic and geophysical studies show that the porcupine basin was originally structured in the early mesozoic as part of a complex northeast to north northeast trending intracontinental rift system extending from the grand banks of newfoundland to the present northwest europe continental shelf the basin can be divided into a sequence of segments bounded by north northeast trending normal faults and northwest trending strike slip faults the degree of extension within the basin decreases northward across each strike slip fault accounting for the overall northward narrowing of the basin the early mesozoic rift fabric of the porcupine basin was reactivated during the early cretaceous when the basin became caught up in the northwest trending rift that later evolved into the n rth atlantic this second phase of extension led to subsidence and the accumulation of extreme thicknesses of cretaceous and younger sediments

pore throat sizes in sandstones tight sandstones and shales pore throat sizes in siliciclastic rocks form a continuum from the submillimeter to the nanometer scale that continuum is documented in this article using previously published data on the pore and pore throat sizes of conventional reservoir rocks tight gas sandstones and shales for measures of central tendency mean mode median pore throatsizes diameters are generally greater than2 mm in conventionalreservoir rocks range from about 2 to 0 03 mm in tight gas sandstones and rangefrom0 1to0 005 mminshales hydrocarbonmolecules asphaltenes ring structures paraffins and methane form another continuum ranging from 100 a 0 01 mm for asphaltenes to 3 8 a 0 00038 mm for methane the pore throat size continuum provides a useful perspective for considering 1 the emplacement of petroleum in consolidated siliciclastics and 2 fluid flow through fine grained source rocks now being exploited as reservoirs

subsidence history of australia s southern margin constraints on basin models the subsidence history of australia s southern margin is shown to consist of two distinct tectonic phases and has important implications for models of sedimentary basin formation the first phase is characterized by rapid tectonic subsidence about 50 m m y which lasted for about 25 30 m y during the early cretaceous the period of rapid subsidence represents the rifting phase of margin development this early stage is correlated with associated rifting phenomena faulting and extension of the brittle upper crust formation of grabens and half grabens and the rapid accumulation of volcanogenic detritus otway group the amount of extension 20 50 is estimated from a comparison of observed and modeled tectonic subsidence at several exploratory wells along the margin the model includes uniform extension and thinning of the continental lithosphere a prolonged extensional event followed by simple cooling horizontal and vertical conduction of heat and radioactive heat production by the crust the rift phase may have begun earlier but the most rapid and widespread subsidence occurred during the early cretaceous the end of rapid and widespread subsidence occurred during the early cretaceous the end of rapid subsidence coincides closely with the age of the oldest sea floor spreading magnetic anomaly about 90 ma adjacent to the continent ocean boundary cob the second stage of margin development is the post rift or drift phase a period of much slower rates of subsidence and relative tectonic quiescence subsidence during the post rift stage is modeled as cooling and contraction of extended continental lithosphere estimates of extension and thinning from subsidence modeling agree with seismic refraction measurements of the thickness of continental crust near the cob estimates of extension from the geometry of basement blocks near the cob do not agree with the analysis from subsidence modeling upper crustal extension seems to record only a small proportion of total lithospheric extension alternative or supplementary mechanisms of extension are explored with emphasis on implications from a model that includes localized shear along a lithospheric detachment system

genetic classification of petroleum systems our genetic classification of petroleum systems is founded on a simple working nomenclature that consists of combining qualifiers from each of the following three categories 1 charge factor supercharged normally charged or undercharged 2 migration drainage style vertically drained or laterally drained and 3 entrapment style high impedance or low impedance the charge factor is estimated on the basis of the richness and volumetrics of mature source rocks the source potential index spi which combines source rock richness and thickness into a single parameter is a convenient shortcut for comparing the petroleum potential of diverse source rocks containing dissimilar kerogen types and for rapidly estimating regional charging capacity in extensively explored basins a positive correlation exists between the magnitude of spi and basin wide petroleum reserves the migration drainage style is determined from the structural and stratigraphic framework of a basin vertical migration drainage which occurs mainly through faults and fracture systems breaching a seal is characteristic of petroleum systems contained within rift basins deltaic sequences salt dome provinces wrench basins and fold and thrust belts in contrast lateral migration drainage is dominant wherever stratigraphically continuous seal reservoir doublets extend over a very large area in a tectonically stable province e g commonly foreland or intracratonic platform basins recognition of the dominant migration style helps to predict the location of zones of petroleum occurrence in relation to the hydrocarbon kitchens the entrapment style which is also dependent on the structural framework and the presence and effectiveness of seals describes the degree of resistance i e impedance working against dispersion of the petroleum charge application of these working concepts should help to significantly reduce geologic risk particularly in new ventures type exploration

intraplate lithosphere deformation and the strength of the lithosphere summary the intraplate deformation of continental lithosphere in response to applied stress has been investigated using a mathematical model which incorporates the elastic ductile and brittle response of lithosphere material ductile deformation is assumed to be controlled in the crust by dislocation creep in quartz and in the mantle by dislocation creep and plasticity in olivine brittle failure is predicted using modified griffith theory a fundamental feature of the model is the redistribution of stress within the lithosphere following stress release by both ductile and brittle deformation this redistribution produces high levels of stress in the middle or lower crust immediately above the elastic ductile or brittle ductile transition lithosphere deformation is shown to be critically dependent on the temperature structure of which surface heat flow is a convenient indicator for higher geothermal gradients the release of stress in the lower lithosphere by ductile deformation is more rapid and complete and results in large stress levels in the upper lithosphere for sufficiently large applied stresses or steep geothermal gradients the stress levels in the upper and middle crust will cause complete fracture of the upper lithosphere whole lithosphere failure wlf then results by continued brittle and ductile deformation causing geologically significant strains the critical value of applied stress required to give wlf has been calculated for both tensional and compressional deformation as a function of surface heat flow the predicted lithosphere bulk strength is then compared with expected levels of intraplate stress arising from plate boundary forces and isostatically compensated loads which are thought to give net stress levels in the continental lithosphere in the range 0 25 to 0 25 kb using these expected maximum stress levels the model predicts significant extensional deformation in regions of moderate heat flow with q c 60mwm e g central europe as well as for areas of high heat flow like the basinand range province significant compressional deformation is predicted for areas of high heat flow with q c 75 mwm 2 but only for restricted conditions of stress combination these results are in good agreement with

three dimensional seismic attenuation structure beneath the japanese islands and its tectonic and thermal implications a three dimensional 3 d seismic attenuation structure beneath the japanese islands is determined by inversion of seismic intensity data the result clearly reveals a remarkable contrast in the attenuation structure and reflects the lateral variation of thermal structure beneath the japanese islands for the inversion the method developed by hashida and shimazaki is used the intensity which is assumed to be a measure of the maximum acceleration of the s wave at the observation point is inverted to estimate the attenuation structure and the acceleration at the seismic source the data consist of about 15 000 intensity readings for about 800 earthquakes reported by the japan meteorological agency between 1951 and 1983 by combining 3 d structures of eight districts in japan 3 d attenuation maps consisting of three layers are constructed to a depth of 90 km the obtained structure reveals the following features first a distinct difference exists in the crustal structure between ne and sw japan in ne japan low q high attenuation areas on the continental side of the arc are dominant whereas some high q low attenuation areas exist on the pacific side of the arc in sw japan except for the kyushu district high q areas are dominant and small low q areas exist mainly on the pacific coast these crustal features generally continue down to the uppermost mantle second the obtained crustal structure shows that attenuation is age dependent low qareas correspond to the distribution of quaternary volcanic materials and high q areas correspond to the geological provinces where pre cenozoic rocks are distributed since the thermal structure strongly depends on the crustal age this feature suggests that the attenuation structure reflects the thermal structure third in the upper mantle low q loci are found which correspond to the distribution of active and other quaternary volcanoes forming the arc volcanic chain it should be noted that even on the continental side of the volcanic front these loci do not exist beneath areas where no volcano exists this suggests that the upper mantle attenuation is controlled by the existence of upwelling diapirs which are considered to lie beneath a volcano or a group of volcanoes fourth high q zones in the upper mantle exist on the pacific side of the volcanic front along the kurile northeast honshu arc and along ryukyu arc the high q mantle lying above the sinking slab seems to reflect a cooling effect of the subducting cold oceanic plates the thickness of the lithosphere may reach 60 km or more in such outer arcs although the thickness has been considered to be about 30 km or so in sw japan where the high q uppermost mantle also lies a thick lithosphere of about 60 km may exist possible thermal cross sections are proposed for ne and sw japan based on the obtained 3 d attenuation structure and other geophysical considerations in the uppermost mantle of ne japan a higher temperature is expected than in sw japan the thermal structures reflect the difference of tectonic settings in the two regions such as the subducting plate overriding lithosphere and volcanic activity

tectonic evolution of the north sea basin crustal stretching and subsidence summary the lithospheric stretching model for the formation of sedimentary basins was tested in the central north sea by a combined study of crustal thinning and basement subsidence patterns a profile of crustal structure was obtained by shooting a long range seismic experiment across the central graben the main axis of subsidence a seabed array of 12 seismometers in the graben was used to record shots fired in a line 530 km long across the basin the data collected during the experiment were interpreted by modelling synthetic seismograms from a laterally varying structure and the final model showed substantial crustal thinning beneath the graben subsidence data from 19 exploration wells were analysed to obtain subsidence patterns in the central north sea since jurassic times changes in water depth were quantified using foraminiferal assemblages where possible and observed basement subsidence paths were corrected for sediment loading compaction and changes in water depth through time the seismic model is shown to be compatible with the observed gravity field and the small size of observed gravity anomalies is used to argue that the basin is in local isostatic equilibrium both crustal thinning and basement subsidence studies indicate about 70 km of stretching across the central graben during the mid jurassic to early cretaceous extensional event this extension appears to have occurred over crust already slightly thinned beneath the graben and the seismic data suggest that total extension since the early permian may have been more than 100km the data presented here may all be explained using a simple model of uniform extension of the lithosphere

change in tectonic force inferred from basin subsidence implications for the dynamical aspects of back arc rifting in the western mediterranean abstract a method has been developed that allows temporal changes in tectonic force during rift basin formation to be inferred from observed tectonic subsidence curves and has been applied to the gulf of lions the provencal basin and the valencia trough in order to gain some understanding of the dynamical aspects of back arc basin rifting in the western mediterranean sea two distinct tectonic force regimes active at different times during the evolution of each of these back arc basins are identified the first which can be seen in both basins is characterized by tensional forces that gradually abate with time to vanish some 20 my after the onset of rifting the magnitude of tectonic force required to initiate the rifting process is significantly greater in the valencia trough than in the provencal basin subsequently the dynamic development of these back arc basins differs in the provencal basin there is a renewal of force with extensional deformation concentrated in the central part of the rift whereas in the valencia trough the second tectonic force regime is inferred to be one that causes compression that subsequently relaxes such temporal patterns of tectonic force are interpreted to be related to the causative driving processes allowing constraints to be placed on the transient interaction between the overriding and subducting plates in a back arc setting the models also allow inferences to be made about the rheological structure of the lithosphere a significant variation of initial crustal thickness is inferred for the provencal basin but not for the valencia trough in both basins a wet rheology is required in order to initiate rifting given currently accepted bounds on tectonic force magnitudes adoption of a dry rheology leads to insufficiently high strain rates for significant lithosphere extension in both cases

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

the initiation of rifting at constant tectonic force role of diffusion creep we have investigated the effect of diffusion creep on lithospheric extension using a one dimensional numerical model that assumes a constant force is available to drive extension the model is motivated by the fact that continental areas with average heat flow should be too strong to rift using standard estimates of lithospheric strength however such areas do rift and diffusion creep is a mechanism by which lithosphere may deform at a lower stress level than is required for the usually assumed dislocation creep we consider the evolution of strain rate and temperature at the center of an idealized pure shear rift the strain rate will either increase or decrease with time depending on whether lithospheric weakening or strengthening dominates as extension progresses given an initial thermal condition and an assumed lithospheric rheology the applied force must be such that the initial strain rate is greater than some critical value for weakening to dominate the force at which this condition is met we will term the critical force diffusion creep is more efficient at smaller grain sizes and the model results indicate that mantle grain sizes would have to be less than 1 mm for diffusion creep to significantly reduce the critical force we speculate that during the initial stages of continental rifting grain size reduction and diffusion creep deformation mechanisms may have an important effect on lithospheric strength

tectonomagmatic evolution of cenozoic extension in the north american cordillera the spatial and temporal distributions of cenozoic extension and magmatism in the cordillera suggest that the onset of major crustal extension at a particular latitude was confined to a relatively narrow belt 100 km pre extension and followed the onset of intermediate and silicic magmatism by no more than a few million years extension began in early eocene time in southern british columbia northern washington idaho and montana farther s extension began at about the eocene oligocene boundary in the great basin and slightly later in the mojave sonora desert region the intervening area at the latitude of las vegas remained quiescent until mid miocene time compositional and isotopic characteristics of most pre miocene magmas are consistent with their containing major components of melted continental crust in mid miocene time two major changes occurred widening of the area of extension and the widespread appearance of basaltic magmas the area affected by extension from southwestern montana to the lake mead region widened to several hundred kilometres by this time extension in southern british columbia northern washington and northern montana had ceased probably before the end of the eocene and extension s of lake mead except in the gulf of california had waned regions affected by the broader belt of extension during late miocene pliocene and quaternary time experienced basaltic magmatism which began along a central rift zone in the northern part of the region and which had within a few million years spread to include most of the region later basaltic activity has tended to concentrate in restricted zones especially near the margins of the extended area we recognize a correlation between the amount of earlier crustal thickening and cenozoic extension and between the length of time after shortening but before extension and the degree to which a given region was intruded by late cretaceous plutons the localization of extension in areas of previous crustal thickening and the dependence of the timing of extension on the thermal state of the overthickened crust is consistent with a simple thermal mechanical model developed in a companion paper sonder et al this raises the possibility that stresses inherent in the north american plate dominated over plate interaction forces as controls of the cenozoic tectonomagmatic evolution of the north american cordillera especially in its earlier stages

structure and dynamics of earth s inner core abstract the seismological exploration of the earth s inner core has revealed unexpected and puzzling structural complexities its elastic anisotropy is now well established and has been shown to vary spatially there is a well defined hemispherical dichotomy in anisotropy and attenuation and significant variations in the radial direction with weaker anisotropy in the uppermost regions and possibly different properties in an innermost inner core of radius 300 600 km perhaps even more puzzling is the observation of a stably stratified layer at the base of the outer core which has recently been proposed to result from melting of inner core material if confirmed the presence of this layer has major implications for our understanding of convection in the outer core inner core differential rotation with respect to the mantle remains controversial we will review here recent progress in understanding the dynamics of the inner core a number of different models have been proposed for the development of the inner core s structure and formation of the stable layer at the base of the outer core and particular care will be taken to clarify under which conditions the proposed mechanisms can be active

the northwest australian continental margin the northwest shelf of australia offers a typical example of a passive continental margin major intra cratonic basins of permian to middle jurassic age developed along the present coastline superimposed to either orthogonally trending palaeozoic basins or precambrian basement rocks in each of these depocentres distinct lithotectonic units can be recognized that are related to phases of rifting and subsequent continental break up the pre break up rift valley and intra cratonic basin stages are represented by a very thick permian to middle jurassic series of mainly fluvio deltaic sediments but with occasional marine incursions break up took place near the end of the middle jurassic and was accompanied by large scale block faultings with associated uplift and sub areal erosion gradually late jurassic to early cretaceous marine sediments transgressed over the eroded surface within the general transgressive episode late callovian late oxfordian to kimmeridgian late tithonian to early cretaceous marine incursions may be singled out open marine conditions related to the breakup of gondwanaland and opening of the indian ocean became widespread during the albian in the southern part of the australian northwest shelf and during the cenomanian in the northern part the deposition of a thick prograding wedge of mainly carbonate sedimentation since the mid eocene resulted in a northwesterly regional tilt of the shelf hydrocarbon occurrences are related to the tectonic evolution early triassic early middle jurassic late oxfordian kimmeridgian and early cretaceous marine incursions are directly related to the deposition of potential source rocks in restricted basins a regressive phase led to the deposition of triassic fluviatile sediments with excellent reservoir potential break up tectonism and subsequent marine transgression provided the relevant trapping mechanism and probably the migration paths for the major gas condensate discoveries of the rankin platform the prolonged high rate of subsidence and accompanying thick sedimentation have ensured that hydrocarbon generation occurred despite the low geothermal gradient

3d seismic geomorphology of a large plio pleistocene delta bright spots and contourites in the southern north sea abstract regional 2d seismic interpretation has been integrated with high resolution 3d interpretation in dutch offshore blocks a f in the southern north sea 17 horizons mmu s1 s4 s4 1 s4 2 s5 s5 1 s6 s13 and 14 seismic units were interpreted expanding upon studies by kuhlmann and wong 2008 the study presents detailed and extensive twt structure and thickness maps of key depositional units allowing paleogeographic reconstructions detailed analysis of incisional scour marks and hypothesised iceberg scours and constructional features contourites identified initially by bright spots on seismic data linked to bottom currents was undertaken paleo paleographic reconstructions of water depth emphasise the role of hypothesised tidally generated contour currents in the late stage deltaic infill of an epicontinental basin north sea

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

south malvinas falkland basin hydrocarbon migration and petroleum system abstract we report a detailed seismic interpretational study of direct hydrocarbon indicators dhis using 2d multichannel seismic reflection data the study includes analyses of four distinctive types of dhi 1 a bottom simulating reflection bsr 2 enhanced reflections ers 3 pipes and vertical anomaly clusters which indicate focused migration of hydrocarbon fluids and 4 flat spots which indicate a deeper source for the shallower hydrocarbon occurrences the interpretation of these dhis included analysis of spatial distribution amplitude variation with offset avo and instantaneous spectral decomposition these analyses are set in a tectono stratigraphic context in order to evaluate the relationship between intra basinal structures hydrocarbon occurrences and migration pathways the recent discovery of a gas condensate accumulation in the study area darwin well ca 190mmbbl confirms the presence of a mature source rock in the basin and supports a model whereby the observed dhis are connected via an active hydrocarbon plumbing system

hydrocarbon plumbing systems above the snohvit gas field structural control and implications for thermogenic methane leakage in the hammerfest basin sw barents sea abstract based on the analysis of the high resolution 3d seismic data from the sw barents sea we study the hydrocarbon plumbing system above the snohvit and albatross gas field to investigate the geo morphological manifestation and the dynamics of leakage from the reservoir fluid and gas escape to the seafloor is manifested in this area as mega pockmarks 1 2 km wide large pockmarks

iceberg ploughmarks illuminated by shallow gas in the central north sea 2014 the authors published by elsevier ltd this is an open access article under the cc by nc salicense http creativecommons org licenses by nc sa 3 0

station coordinates baselines and earth rotation from lageos laser ranging 1976 1984 satellite laser range slr measurements to lageos from may 1976 to january 1984 have been used to compute earth rotation parameters and the geocentric positions of 57 laser tracking station sites occupied during this interval the station position determinations were part of a simultaneous solution which includes lageos orbit elements in 15 day arcs and earth rotation parameters in 3 day subarcs using residuals from a 7 7 year lageos long arc lla 8402 systematic differences between the terrestrial reference frames implied by the university of texas station coordinate solution lsc 8402 and by previous solutions computed at the university of texas ut and at goddard space flight center gsfc are reviewed the new solution resolves a 0 72 m z axis difference between the terrestrial reference system origins inferred from the gsfc solution sl5 and the previous ut solution lsc 8112 the implied origins of the lsc 8402 solution and the sl5 solution are consistent to less than 60 mm in all components and after removing orientation and scale differences the coordinates for 26 common tracking stations have an rms difference of less than 80 mm the rms difference in baseline length for 22 station pairs used in the recent crustal dynamics project very long baseline interferometry vlbi slr intercomparison experiment is 50 mm between the gsfc and ut laser solutions and 40 mm between the ut laser solution and the vlbi solution the earth rotation parameters computed in 3 day intervals have an internal precision of 2 marc sec 62 mm at the earth s surface for xp and yp and 0 15 ms 70 mm at the equator for ut1 the estimated accuracy of the series based on dual baseline vlbi intercomparisons is approximately 2 marc sec for xp and yp and 0 2 ms for ut1 the effects of plate motion on the determination of the secular drift in polar motion using the 7 7 year series is shown to be at the level of 1 marc sec yr finally baseline changes have been investigated by repeated station coordinate determinations using 60 day subsets of the 7 7 year interval of the lageos data set in each 60 day solution interval orbit elements were estimated at 15 day intervals and earth rotation parameters were determined at 5 day intervals for the 4 year period beginning in january 1980 baseline length changes inferred by the successive 60 day solutions were found to be in general agreement with the motions predicted by the am1 2 plate motion model of minster and jordan

lithosphere response to externally and internally derived stresses a viscoelastic stress guide with amplification summary lithosphere subjected to an externally derived horizontal stress undergoes creep in the lower lithosphere resulting in the decay of lower lithosphere stress and the associated amplification of stress within the upper lithosphere this stress response of lithosphere has been investigated for a lithosphere model with power law stress and temperature dependent viscoelastic properties the rate and extent of stress decay and associated stress amplification is greatly dependent on the lithosphere geotherm oceanic lithosphere subject to an applied stress of o 1 kb undergoes upper lithosphere stress amplification of 1 5 1 8 and 2 0 at 104 106 and 108 yr respectively at 106 yr the effective lithosphere thickness is reduced to approximately 40 km the stress decay and amplification proceeds more rapidly for an applied stress of 1 0 kb and in the case of a tensile applied stress results in some upper lithosphere fracture for continental and basin and range type lithosphere the comparable stress amplification at 106 yr for a stress of 0 1 kb is 2 0 and 6 5 with effective lithosphere thicknesses of 60 and 20 km respectively the large values of stress amplification for the basin and range lithosphere result in complete upper lithosphere fracture which gives rise to a cyclic process of upper lithosphere faulting and lower lithosphere creep in which extensive lithosphere deformation can occur the stress amplification process also occurs for stresses generated by lateral density contrasts for an isostatically compensated plateau uplift structure deviatoric stresses of the order of 1 kb can be generated in the upper lithosphere by this process and are sufficient to cause tensile fracture of the upper lithosphere the response of viscoelastic lithosphere to constant geometry bending stresses has also been examined and results in substantial but not complete reduction of the bending stresses

pliocene paleoenvironment evolution as interpreted from 3d seismic data in the southern north sea dutch offshore sector a high resolution 3d seismic survey from the dutch offshore sector has been interpreted and subsequently correlated with existing regional seismo stratigraphic concepts derived from conventional 2d seismic data sets the interpreted 13 seismic units have been related to a newly established chrono stratigraphic framework kuhlmann et al 2006a b chronostratigraphy of late neogene sediments in the southern north sea basin and paleoenvironmental interpretations palaeogeography palaeoclimatology palaeoecology 239 426 455 integrated chronostratigraphy of the pliocene pleistocene interval and its relation to the regional stratigraphical stages in the southern north sea region netherlands journal of geosciences geologie en mijnbouw 85 1 19 35 resulting in up dated age control for the seismic units the generation of amplitude maps time slices and isopach maps from the 3d seismic data enabled detailed spatial and temporal reconstruction regarding the paleoenvironmental and climatological development as depicted by kuhlmann et al 2006a chronostratigraphy of late neogene sediments in the southern north sea basin and paleoenvironmental interpretations palaeogeography palaeoclimatology palaeoecology 239 426 455 the lowermost seismic units s1 s4 comprise condensed middle miocene to piacencian sediments deposited under warm open marine conditions these sediments show a uniform seismic facies of low amplitude reflectors the boundary of seismic unit s4 s5 around 2 6 ma delineates a shift towards generally colder climate conditions that are connected to the onset of northern hemisphere glaciation seismic unit s5 includes alternations of warmer and colder periods during warmer periods bottom currents generated elongated structures 2 5 4 km long 300 500 m wide on the horizon display these layers show as well shallow gas accumulations with a more regional extent and are related to coarser grained sediments sealed by clayey sediments of the cold phases a homogenous seismic facies is characteristic for the colder periods within seismic units s6 and s7 internal fore set structures display the regressional trend related to the cooling of this interval seismic units s8 s11 display a pattern of unoriented lineaments which was probably caused by icebergs drifting into the north sea and souring the sea floor this finding is in accordance with the shallow marine arctic conditions of this interval within these units a bright spot of 3 by 7 km in size is interpreted to result from shallow gas accumulation related to a grounded iceberg this feature represents another more local area of gas accumulation that is closely related to iceberg activity within the study area the uppermost seismic units s12 and s13 belong to a fluvial paralic paleo environment that is characterised by aggradational seismic reflectors pleistocene glacial valleys were recognised in the upper 200 m below sea floor 2007 elsevier ltd all rights reserved

low angle detachment origin for the red sea rift system abstract the tectonic and magmatic history of the jizan coastal plain tihama asir southwest arabia suggests a two stage evolution a first stage of extension began during the oligocene and ended with uplift of the arabian graben shoulder which began about 14 ma ago it was followed by a period of approximately 10 ma characterized by magmatic and tectonic quiescence a second stage of extension began roughly contemporaneously with the onset of seafloor spreading in the southern red sea some 4 5 ma ago and is still active today the geometry of faulting in the jizan area supports a wernicke model of simple shear for the development of the southern red sea regional asymmetries of the red sea area such as the distribution of volcanism the marginal topography and asymmetries in the geophysical signatures are consistent with such a model available seismic profiles allow a rough estimate for values of the arabian red sea margin and were used to simulate subsidence history and heat flow of the red sea for classical two layer stretching models neither finite uniform nor finite non uniform stretching models can account for observed subsidence and heat flow data thus two model scenarios of whole lithosphere normal simple shear are presented for the geological history of the southwestern arabian margin of the red sea these models are limited because of the serravallian rearrangement in the kinematics of the red sea

fluid flow features in hydrocarbon plumbing systems what do they tell us about the basin evolution abstract fluid flow features such as sandstone intrusions mud volcano systems and pockmarks represent well known geological structures generated due to flow of fluids in the subsurface however the integration of fluid flow features in quantitative and qualitative basin analysis is still in its infancy this study presents a review and a qualitative integration of fluid flow features in basin analysis of the danish north sea area and of salt mini basins offshore angola based on results from six case studies focusing on the detection mapping and evaluation of features related to focused fluid flow in these areas in the study the diversity of seismically imaged fluid flow features in hydrocarbon plumbing systems is reviewed and their utility is demonstrated this is done by focusing on the aspects of fluid flow related features that can be integrated into basin analysis studies and how these may help us increase the understanding of basin evolution and to achieve the major goals of basin analysis such as understanding the depositional and tectonic context of the basin and the presence and extent of hydrocarbons particularly concerning the determination of fluid flow timing triggers sources and routes fluid flow features represent a major but underused tool in basin analysis

abrupt plate accelerations shape rifted continental margins rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved it is well known that strain rate dependent processes control rift evolution yet quantified extension histories of earth s major passive margins have become available only recently here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions we find that rifted margins feature an initial slow rift phase less than ten millimetres per year full rate and that an abrupt increase of plate divergence introduces a fast rift phase plate acceleration takes place before continental rupture and considerable margin area is created during each phase we reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions the extension models suggest that the two phase velocity behaviour is caused by a rift intrinsic strength velocity feedback which can be robustly inferred for diverse lithosphere configurations and rheologies our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force this mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes offering new insights into the balance of plate driving forces through time

the thermal and mechanical development of the wessex basin southern england summary the wessex basin of southern england is an e w trending late palaeozoic to tertiary extensional basin formed on early palaeozoic variscan crust extension occurred over a protracted period from late carboniferous to cretaceous times resulting in the evolution of four main depocentres or sub basins the channel basin winterborne kingston trough vale of pewsey and weald basins each sub basin was controlled fundamentally by the normal reactivation of basement thrust faults and wrench or transfer faults the basement faults represent primary structures associated with the fold and thrust terrain of the devonian carboniferous variscan orogeny subsequent but synchronous reactivation of both thrust and transfer faults tended to compartmentalize the basement leading to discrete depocentres and the production of general rhomboidal shaped sub basins characteristic of pull apart basins in strike slip terrains to investigate the thermo mechanical properties of the lithosphere during basin formation 50 boreholes within the wessex basin were back stripped to isolate the driving tectonic subsidence responsible for basin formation the resultant driving subsidence is characterized by a gentle sometimes negative exponential form with superimposed rapid discrete subsidence events as the geological development of the wessex basin was characterized by polyphase extension and the utilization of crustal detachments the lithospheric model used to determine crustal extension above the detachment and stretching below the detachment included the effects of depth dependent stretching and finite re rifting from the back stripping analysis the basin wide averaged upper crustal extension and lower crustal sub crustal lithospheric stretching were 11 and 5 respectively the distribution of is strongly influenced by the location of growth faults and hence basement thrust fault reactivation suggesting that is reflecting the brittle failure and collapse of hanging wall blocks repeated slip and block collapse resulted in the polyphase extension responsible for renewed basin subsidence in contrast is uniformly distributed across the basin suggestive of a ductile failure of the lower crust and responsible for the low magnitude thermal subsidence strain compatibility between the upper crust and lower crust sub crustal lithosphere is maintained partly within the basin 5 and partly external to the basin as the variscan basal thrust was the fundamental relay in the sense of wernicke responsible for the wessex basin formation we postulate that strain balancing induced a cumulative essentially thermal isostatic uplift of 60 m under the northern margin of the armorican massif

the dynamics of rifting deformation of the lithosphere by active and passive driving forces summary the thermal and mechanical behaviour of the lithosphere during rifting is investigated using a two layer model for its rheology and different driving forces for the rift process the uppermost crustal layer is assumed to deform by brittle failure while the lower lithosphere including the lower crust is assumed to behave as a newtonian viscous fluid results of numerical modelling of the deformation flow and heat transport in the viscous layer are presented in which viscosity is a strong function of depth and temperature deformation of the upper brittle layer is complex and difficult to quantify thus deformation of this upper layer was approximated by a simple kinematic model in which the layer is thinned by an amount proportional to the amount of extension it experiences two fundamentally different driving forces were investigated they correspond to active rifting in which the litho sphere is thinned by an upwelling limb of a convection cell or mantle plume and to passive rifting or stretching of the lithosphere the results show that for zones of stretching which are wider than the thickness of the viscous layer thinning at all levels in the lithosphere can be described by a single parameter as in earlier stretching models this is not always true when the rift zone is narrow or near the edges of the rift zone initial subsidence due to isostatic and thermal changes during stretching slows as stretching progresses giving an exponential character to the rift stage subsidence active rifting is associated with thermal uplift in this case lithospheric deformation is confined to a narrow zone near the base of the lithosphere and thinning of the crust does not occur until the base of the lithosphere approaches that level the rate of lithospheric thinning and width of the rift zone depend on the convection velocities in the underlying asthenosphere and on the viscosity depth gradient near the lithosphere asthenosphere boundary also contributing to the elevation changes during rifting are dynamic stresses which result from the flow these dynamic affects may significantly modify the uplift or subsidence of the rift zone another important process which results from viscous flow is the generation of secondary convection in the upwelling asthenosphere ths will further thin the lithosphere and can cause depth dependent stretching in which the lower lithosphere is thinned more than the upper lithosphere both of these properties of the flow dynamic stresses and secondary convection critically depend on absolute values of viscosity and so are not well constrained at present the model results presented here exhibit the degree of complexity which results from viscous flow in the lower lithosphere and which is matched by the wide variety of observations on rifting

burial and thermal history of the west bashkirian sedimentary basins abstract the galo system is applied to the numerical reconstruction of burial and thermal histories of the west bashkirian lithosphere from the riphean to the present an analysis of the variation in tectonic subsidence of the basin during its development is utilized to estimate approximately the mantle heat flow variations our variant of basin evolution suggests that after cooling in the early riphean the rather weak thermal reactivations have not led to considerable heating of the lithosphere in the study region surface heat flow decreased from relatively high values in the early riphean 60 70 mw m 2 in the eastern area and 40 50 mw m 2 in the western part to present day values of 32 40 mw m 2 in spite of the relatively low temperature regime of the basin as a whole a syn rifting deposition of more than 10 km of limestone shale and sandstone in the riphean resulted in rather high temperatures 180 190 c at the base of present day sedimentary blanket in the eastern area in agreement with the observed data computed present day heat flow through the sediment surface increases slightly from 32 to 34 mw m 2 near the west boundary of the region to 42 mw m 2 near the boundary of the ural foldbelt whereas the heat flow through the basement surface decreases slightly from 28 32 to 24 26 mw m 2 in the same direction the mantle heat flow is only 11 3 12 7 mw m 2 which is considerable lower than mean heat flow of the russian platform 16 18 mw m 2 and comparable with the low heat flow of precambrian shields

the mesozoic tertiary evolution of the aquitaine basin the aquitaine basin situated in southwest france with an area of about 60 000 km 2 has the form of a triangle which opens towards the atlantic bay of biscay and is limited to the north by the hercynian basement of brittany and the massif central and to the south by the pyrenean tertiary orogenic belt beneath the tertiary sequence 2 km thick and which outcrops over much of the basin a mesozoic series up to 10 km thick rests generally on a tectonized hercynian basement but locally it covers narrow nw se trending post orogenic trenches of stephano permian age the mesozoic history can be subdivided into four major structural sedimentary episodes 1 during a triassic taphrogenic phase a continental evaporitic complex developed with associated basic magmatism 2 throughout the jurassic a vast lagoonal platform developed initially lower lias as a thick evaporitic sequence followed by a uniform shale carbonate unit indicating a relative structural stability 3 the end of the jurassic and the lower cretaceous saw a fragmentation of this platform due to an interplay between the iberian and european tectonic plates resulting in an ensemble of strongly subsident sub basins 4 during the upper cretaceous and until the end of the neogene the evolution of the aquitaine basin was influenced by the pyrenean orogenic phase with the development towards the south of a trench infilled by flysch which from the upper eocene is succeeded by a thick post orogenic molasse complex the main hydrocarbon objectives in the basin are situated in the jurassic platform e g the lacq giant gas field and the cretaceous sub basins e g the cazaux and parentis oil fields to date production has been about 4 times 10 7 m 3 of oil and about 15 times 10 10 m 3 of gas since the first gas discovery st marcet in 1939

the development and seismic expression of synsedimentary features within the chalk of southern england seismic reflection data acquired onshore to the north of bournemouth southern england image clearly a series of prominent concave up troughs or depressions and concave down mounded structures within the white chalk subgroup the chalk lithostratigraphy in the area of study is established from borehole geophysical log correlations these correlations then provide calibration of the seismic reflection data indicating that the structures are concentrated at levels between the lewes nodular chalk formation and tarrant chalk member and towards the base of the portsdown chalk formation onlap downlap and truncation of reflectors are observed with the most dramatic features forming a stacked series directly overlying faulting of jurassic strata similar stratal geometries have been described from comparable levels in the chalk of the north sea and in outcrop in britain and france to many they represent channels formed by submarine erosion and redeposition of the chalk during relative sea level falls linked to tectonics we also interpret a number of apparently incisive structures and slumped infill sequences described here as being of primary syndepositional origin the slumping and the close association of these features with underlying faults and proximity to nearby major crustal faults may be indicative of submarine erosion of the chalk caused by sea level changes linked to syndepositional fault movements however evidence of toplap downlap pairs is also suggestive of more aggradational mounding or laterally migrating surfaces or facies belts the presence of any such structures implies rapid lateral variations in the chalk that would have implications not only for aquifer management and future storage schemes but also during the processing of seismic reflection data and general studies and correlations of the chalk across southern britain

sequential vertical gas charge into multilayered sequences controlled by central conduits four sets of stacked amplitude anomalies are described from a three dimensional seismic survey acquired in block a of the dutch north sea the amplitude anomalies aas have a subcircular planform and within each set they are stacked vertically they have a high degree of spatial overlap in the vertical succession these sets of aas are interpreted as vertical anomaly clusters vacs composed of five to seven major aas each seismic interpretation of the vacs and quantitative analysis of the reservoir intervals reveal that the vacs are gas bearing silt rich reservoirs hosted in the upper section of the upper north sea group a mixed clastic succession of pliocene to pleistocene age novel analysis based on the geometry of the individual anomalies reveals that these are likely to be the result of a gas migration process characterized by sequential upward gas charge into reservoir units and that the flow across the seals separating these reservoirs is controlled by central regions of focused fluid flow these regions function as seal bypass systems and are most likely formed by hydraulic fracturing

spatio temporal evolution of strain accumulation derived from multi scale observations of late jurassic rifting in the northern north sea a critical test of models for lithospheric extension abstract we integrate observations of lithospheric extension over a wide range of spatial and temporal scales within the northern north sea basin and critically review the extent to which existing theories of lithospheric deformation can account for these observations data obtained through a prolonged period of hydrocarbon exploration and production has yielded a dense and diverse data set over the entire viking graben and its flanking platform areas these data show how syn rift accommodation within the basin varied in space and time with sub kilometer scale spatial resolution and a temporal resolution of 2 3 myr regional interpretations of 2d seismic reflection refraction and gravity data for this area have also been published and provide an image of total basin wide stretching for the entire crust these image data are combined with published strain rate inversion results obtained from tectonic subsidence patterns to constrain the spatio temporal evolution of strain accumulation throughout the lithosphere during the 40 myr 170 130 ma period of late jurassic extension across this basin for the first 25 30 myr strain localisation dominated basin development with strain rates at the eventual rift axis increasing while strain rates over the flanking areas declined as strain rates across the whole basin were consistently very low 16 s l thermally induced strength loss cannot explain this phenomenon the strain localisation is manifest in the near surface by a systematic migration of fault activity the pattern and timing of this migration are inconsistent with flexural bending stresses exerting an underlying control especially when estimates of flexural rigidity for this area are considered the best explanation for what is observed in this time period is a coupling between near surface strain localisation driven by brittle or plastic failure and the evolving thermal structure of the lithosphere we demonstrate this process using a continuum mechanics model for normal fault growth that incorporates the strain rate dependence of frictional strength observed in laboratory studies during the final 10 myr of basin formation strain accumulation was focused within the axis and strain rates declined rapidly replacement of weak crust by stronger mantle material plus crustal buoyancy forces can adequately explain this decline

stratigraphy of the rotliegend group in the danish part of the northern permian basin north sea a stratigraphic study of the rotliegend offshore denmark has resulted in a coherent lithostratigraphic framework and a better understanding of the mid permian evolution of the eastern northern permian basin the rotliegend group is subdivided into the karl formation composed of syn rift volcanics and volcaniclastic sediments and the younger post rift auk formation dominated by aeolian and fluvial sandstones radiometric age data based on k ar dating of plagioclase from the karl formation volcanics combined with previously published age data suggests that volcanism took place during two separate events 276 281 ma and 261 269 ma both events post date the lower rotliegend volcanism 288 300 ma in the southern permian basin suggesting that the northern permian basin was formed during a later tectonic event the main volcanic event in the northern permian basin was broadly coeval with volcanic activity in the altmark area of ne germany reflecting simultaneous rifting and onset of upper rotliegend 2 sedimentation in the northern and southern permian basins

seismic characteristics of fluid escape pipes in sedimentary basins implications for pipe genesis abstract fluid escape pipes were first documented from 3d seismic data over a decade ago and have subsequently been identified in many petroliferous basins worldwide they are characterized on seismic data by vertical to sub vertical zones of reduced reflection continuity that have a columnar geometry in three dimensions the upper terminations of these pipes commonly coincide with pockmarks or palaeo pockmarks signifying a close connection of pipe formation with a high flux fluid expulsion process dimensions range from tens to hundreds of metres in diameter and hundreds to over a thousand metres in height and the slenderness ratio defined as height diameter ranges from 0 8 to over 20 pipes are frequently associated with sub vertical clustering of amplitude anomalies on seismic data related either to the presence of free gas or to cementation linked to the passage of hydrocarbons three mechanisms have been suggested to explain pipe genesis 1 hydraulic fracturing 2 erosional fluidisation and 3 capillary invasion we suggest a further two possible mechanisms in the form of localised collapse by volume loss and synsedimentary flow localisation we review all five mechanisms and conclude that it is unlikely that a single mechanism applies but that combinations of these processes may all occur in particular contexts fluid escape pipes may be far more widespread that currently appreciated and they may play a critical role in secondary hydrocarbon migration and in providing leakage pathways for trapped hydrocarbons through overlying seals

generation migration entrapment and leakage of microbial gas in the dutch part of the southern north sea delta understanding the shallow gas system is critical for assessing its potential as an energy source for evaluating the possible hazard of shallow gas for drilling and wind farm locations and for evaluating the effect of gas emissions at the seabed on marine ecosystems and climate this paper presents the key elements and processes of the microbial shallow gas system in the plio pleistocene dutch southern north sea delta based on recent findings from different projects geochemical and carbon isotopic composition of shallow gas occurrences in the delta are indicative of a microbial origin of the gas shallow gas mainly occurs in stratigraphic traps and stacked anticlinal structures above salt structures as indicated by identified direct hydrocarbon indicators on seismic such as bright spots organic matter in the delta deposits is of predominantly land plant origin with toc values varying between 1 and 5 and mostly between 1 and 2 simulations of temperature and burial history in combination with dedicated 1d simulations of microbial gas generation reveal that gas generation in the delta started in early pleistocene calabrian times and is still ongoing simulated volumes of gas generation are more than enough to fill published estimated volumes of shallow gas prospects in the delta the geometry of the e w prograding delta sequences and the close interbedding of interglacial silty sandy and glacial clayey silty sediments focus gas migration updip through the foresets towards the topsets of the delta sequences and ultimately into the anticlinal stacked traps grain size based calculations of the permeability and capillary seal capacity of clayey silty seals of the stratigraphic and anticlinal traps provided first estimates of permeability values ranging from 2 8e 20 m2 to 1 1e 18 m2 and capillary seal capacity values between 10m and 24m comparison of gas column heights derived from grain size based calculations cross plots of neutron and density logs and pressure measurements with trap heights derived from seismic bright spots suggests that many traps in stacked bright spots are not filled to structural spill point this suggests that filling of the stacked reservoirs is not related to fill spill migration but rather to leakage through the top seal in absence of fault and fracture zones crossing the seal this leakage is related to the capillary seal capacity and permeability of the top seal the leakage extends locally to the seabed the microbial gas system in the delta today is a highly dynamic system driven by ongoing burial of the delta sediments and microbial gas generation 2018

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

a back arc basin formed within continental lithosphere the central volcanic region of new zealand abstract the central volcanic region is an area of quaternary volcanism and tensional tectonics within the north island of new zealand that appears to be a direct continuation of a young oceanic back arc basin the havre trough into the continental structure of new zealand volcanism is predominantly rhyolitic yet the region possesses many of the geophysical characteristics often associated with oceanic back arc basins upper mantle seismic velocities are low pn and sn velocities of 7 4 and 3 95 km s respectively the crust is anomalously thin for its continental setting and the heat flow almost all of which is expressed in the discharge of hot water through geothermal systems is about 700 mw m2 i e some twelve times greater than normal continental heat flow in order to account for such a high heat flow a large scale mass transfer of heat is required it is proposed therefore that the central volcanic region represents a site of active back arc spreading within a continental lithosphere

constraints on extension of continental lithosphere the extension of the continental lithosphere is considered in terms of the deformation of a thin viscous sheet with a temperature dependent rheology which is governed primarily by the strength of the upper mantle the lithosphere is subjected either to a constant strain rate or to a strain rate decreasing with time corresponding to the stretching of a region of fixed volume with a constant velocity boundary condition the progress of the extension is influenced by the decrease in strength of the lithosphere owing to its attenuation and by an increase in strength resulting from cooling the relative importance of these two effects depends on two parameters a dimensionless strain rate and the total strain for a given strain rate the extending lithosphere goes through two phases as the strain increases in the first the effect of thinning the lithosphere is predominant and the average strength of the lithosphere remains close to and perhaps slightly below its original strength in the second the effect of cooling of the lithosphere leads to very rapid increase in strength as a function of strain resulting presumably in an effective limit on the degree of stretching that the lithosphere experiences for the case of strain rate decreasing with time when the initial strain rates are of order 10 15s 1 this maximum extension is between 100 and 150 for a strain rate of 10 l4 s 1 the maximum lies around 400 to 500 at higher strain rates than this very large extension may occur before cooling significantly affects the strength of the lithosphere this model accords well with the constraints available from the continental margins of the north atlantic

odp leg 107 in the tyrrhenian sea insights into passive margin and back arc basin evolution leg 107 of the ocean drilling program drilled a west northwest east southeast transect of seven sites across the tyrrhenian sea the youngest of the sub basins of the mediterranean sea sites 654 653 652 and 656 document the rifting and subsidence of the sardinia passive continental margin on the upper margin site 654 we cored a classic transgressive sequence subaerial conglomerates overlain by oyster bearing sands overlain by marine marl comparison between the recovered lithologies and seismic reflection profiles suggests that the synrift sediments on the upper margin are tortonian late miocene to messinian latest miocene in age whereas synrift sediments on the lower margin are messinian to pliocene in age during the messinian desiccation of the mediterranean sites 654 and 653 now on the upper sardinian margin apparently occupied a basinal setting where they received nannoplankton bearing clays interbedded with laminated gypsum sites 656 and 652 now on the lower sardinia margin were apparently higher standing during the desiccation event their messinian facies are subaerial and lacustrine respectively we infer from these lines of evidence that tilting and subsidence occurred more than a million years earlier on the upper margin than on the lower margin such diachroneity can be interpreted in terms of migration of the zone of maximum extension above a rolling back subduction zone or in terms of extension of continental crust by shear along a deep detachment fault sites 655 651 and 650 were drilled into two small basalt floored basins of the central and eastern tyrrhenian emplacement of basaltic crust in the central tyrrhenian vavilov basin apparently began more than a million years before emplacement of basaltic crust in the eastern tyrrhenian marsili basin this observation is compatible with previous suggestions that the tyrrhenian has grown southeastward in response to rollback of the down going slab that currently dips northwestward under the toe of italy at the easternmost site high vesicularity of the basalt and benthic foram assemblages in the oldest sediments imply that the basalt erupted in water shallower than 2 500 m it has apparently subsequently subsided to its present depth of 4 100 m below sea level nearly three times as fast as normal subsidence of crust formed at a mid ocean ridge

redistribution of the lithosphere deformation by the emplacement of underplated mafic bodies implications for microcontinent formation abstract sedimentary basin migration and microcontinent formation could be the result of deformation redistribution in which the emplacement of an underplated mafic body upmb may play an important role in this study a 2d finite element model is used to examine the redistribution of the deformation that can be triggered by a upmb it is shown that three modes of deformation redistribution can exist 1 a shift completed mode where the deformation is completely redistributed into the newly weakened region 2 a transition mode where the deformation is redistributed but extension is accommodated by thinning in both regions 3 a shift failed mode where the deformation is not redistributed whether or not the deformation is redistributed depends on the configuration of the upmb and on the initial rheological heterogeneity in the initially deformed region however it becomes difficult for any upmb to initiate the redistribution of the deformation once the stretching factor in the first deformed region exceeds a critical value the microcontinent formations observed in the indian ocean in the norwegian greenland sea in the red sea afar region and in the western mediterranean are particularly discussed in relation to the transition mode providing some important geodynamic implications

physical properties of summer sea ice in the fram strait the physical properties of sea ice in the fram strait region of the greenland sea were examined during june and july 1984 in conjunction with the marginal ice zone experiment field program most of the ice sampled within fram strait during this period was multiyear thicknesses and other properties indicated that none of the multiyear ice was older than 4 to 5 years snow cover on the multiyear ice averaged 29 cm while that on first year ice averaged only 8 cm deep this difference may be related to enhanced sublimation of the snow on the thinner first year ice the salinity profiles of first year ice clearly show the effects of ongoing brine drainage in that profiles from cores drilled later in the experiment are substantially less saline than earlier cores thin section examinations of crystal structure indicate that about 75 of the ice consisted of congelation ice with typically columnar type crystal structure the remaining 25 consisted of granular ice with only a few occurrences of snow ice the granular ice consisted primarily of frazil found in small amounts at the top of floes but mainly observed in multiyear ridges the horizontally oriented crystal c axes showed various degrees of alignment ranging from no alignment to strong alignments in which the alignment direction changed with depth implying a change in floe orientation with respect to the ocean current at the ice water interface during ice growth evidence of crystal retexturing was observed in the upper meter of nearly every multiyear core this retexturing consisting of grain boundary smoothing and nearly complete obliteration of the ice platelet brine layer substructure is attributed to summer warming

formation and structure of refrozen cracks in land fast first year sea ice 1 this study characterizes the healing process and structure of undeformed linear parallel sided flooded cracks in land fast sea ice field investigations and refreezing experiments were performed in mcmurdo sound ross sea antarctica between 1998 and 2002 data from a two dimensional thermistor array are used to show that the ice water interface of freezing cracks is arch shaped due to bidirectional heat flow to the surrounding ice cover and to the atmosphere ice growing laterally into the crack is found to desalinate over a prolonged period of time until the isotherms are approximately horizontal superposition of heat flow to the atmosphere and to the host sea ice sheet allows the refreezing progress to be modeled analytically close to the ice air interface the salinity is higher at the sides of wide refrozen cracks than it is at the center however deeper down and in narrower cracks in general the salinity is higher at the center than at the sides a finite volume computational fluid dynamics cfd model reproduces the generally arch shaped alignment of brine pockets this pattern is attributed to convection in the mushy layer crystals are found to grow upstream into the crack due to a salinity gradient in the buoyant convective flow

acceleration of continental rifting due to a thermomechanical instability abstract we have made a numerical experiment on continental rifting based on the rheology of rocks constituting continental lithosphere the model assumes homogeneous pure shear stretching of a three layered lithosphere we also assume constant extensional force in the model to explore acceleration or deceleration of rifting the main parameters which affect the evolution of continental rifting for an initial geotherm are the applied constant extensional force and the thermal diffusivity whereas continental extension elevates the isotherms in continental lithosphere leading to its softening the cooling of the extended continental lithosphere by thermal diffusion leads to its strengthing therefore the applied force and the thermal diffusivity play counteracting roles which cause a stretching instability to separate the final states of rifts i e breakup or failed the transition from the breakup to a failed rift in continental lithosphere is caused by a subtle change in the constant extensional force or in the thermal diffusivity similar to a phase transformation we have shown that a non dimensional number that is modified form the peclet number determines the final state

chronostratigraphy of late neogene sediments in the southern north sea basin and paleoenvironmental interpretations a chronostratigraphic framework has been established for upper neogene sediments from the northern netherlands offshore sector southern north sea data of dinoflagellate cysts pollen and foraminifers from eight boreholes are integrated and the resulting biostratigraphic events are connected to an additionally retrieved paleomagnetic record the sedimentary succession comprises at the base late to middle miocene sediments and spans the piacenzian to gelasian stages from 3 6 to 1 8 ma including the paleomagnetic gauss matuyama boundary and the olduvai subchron sedimentation rates vary between 30 cm kyr in the lower part of the succession and reach up to 84 cm kyr in the upper part these rates are high enough to record short paleomagnetic events and for the first time the presence of the so called x event has been recorded and dated at around 2 44 ma in a sedimentary setting the name north sea event is proposed here the paleoclimatic and paleoenvironmental information pictures the onset of northern hemisphere glaciation and the subsequent intensification of glacial conditions with its impact on the north sea sea region the paleoenvironmental changes are characterised by a stepwise development and five successive intervals are distinguished during the first interval a warm climate and an open marine setting prevailed the second interval is characterised by dominantly cold climatic conditions with occasional warmer periods during the third interval the environment became restricted marine and a stable freshwater layering occurred which facilitated the development of sea ice in the shallow sea within the fourth interval extreme cold arctic conditions were predominant and shallowing continued the fifth interval shows fluvial to deltaic paralic conditions from then on facies differences from the nearby delta system overruled the basin wide signals at the studied location 2006 elsevier b v all rights reserved

hydrocarbon system analysis in a rift basin with mixed marine and nonmarine source rocks the south viking graben north sea the south viking graben of the north sea is a prolific oil and gas province that has recoverable reserves of approximately 176 x 109 sm3 standard cubic meters 6 2 tcf gas 58 x 106 t 412 million boe natural gas liquids and 12 5 x 106 sm3 78 million bbl of black oil contained in the main fields sleipner vest sleipner ost and volve these fields are located primarily within block 15 9 in the norwegian sector and extend into neighboring blocks 15 6 and 16 7 the principal source of black nonvolatile oil is the late jurassic draupne formation which has a predominance of marine algal organic matter the lower section of the draupne formation together with the heather formation are organically leaner and contain a mixture of marine algal and terrigenous organic matter resulting in a potential to generate both oil and gas the middle jurassic hugin and sleipner formations contain humic coals and coaly shales with potential to generate gas and some light liquids these coals contain on average 80 90 vitrinitic woody material with occasional enrichment of resinite high resinite concentrations can lead to an overprediction of oil potential as they contribute significantly to the hydrogen index hi but generate primarily low molecular weight aromatic compounds all source rock facies types have reached maturities sufficient to generate oil and gas basin modeling suggests that onset of oil and gas generation started during the latest cretaceous early paleocene these source rocks have continued to yield oil and gas to the present day in many parts of the catchment area for the sleipner fields detailed geochemical analyses identified five main oil and condensate families family a comprises condensates and oil located in the northernmost part of sleipner vest and dagny generated from a marine clastic source with a predominance of type ii algal organic matter family b is condensates in the middle to southern part of sleipner vest generated from a source or a contribution from several source facies with mixed terrigenous higher plant organic matter and marine algal material family c consists of condensates reservoired in the jurassic of sleipner ost except well 15 9 a15 generated from a mixed algal terrigenous source but with a higher contribution of hydrocarbons from a marine algal source as compared with the sleipner vest family b family d comprises condensates in the jurassic triassic of loke and gungne in well 15 9 a15 from the crest of the sleipner ost structure as well as condensates within the paleocene sands of sleipner ost these condensates have a mixed terrigenous higher plant and marine algal signature and are derived from pre upper draupne formation sources family e is the black oil present in the jurassic volve field derived from a marine calcareous shale with type ii to ii s organic matter the source rock for this oil is unique in the greater sleipner area and is likely located in the isolated subgraben between the northern parts of sleipner vest and sleipner ost the hydrocarbon gases are broadly similar and are interpreted to have been generated from coals and coaly shales of the sleipner and hugin formations as well as those parts of the pre upper draupne section that have a predominance of terrigenous higher plant organic matter

noble gases solubility models of hydrocarbon charge mechanism in the sleipner vest gas field abstract noble gases are chemically inert and variably soluble in crustal fluids they are primarily introduced into hydrocarbon reservoirs through exchange with formation waters and can be used to assess migration pathways and mechanisms as well as reservoir storage conditions of particular interest is the role groundwater plays in hydrocarbon transport which is reflected in hydrocarbon water volume ratios here we present compositional stable isotope and noble gas isotope and abundance data from the sleipner vest field in the norwegian north sea sleipner vest gases are generated from primary cracking of kerogen and the thermal cracking of oil gas was emplaced into the sleipner vest from the south and subsequently migrated to the east filling and spilling into the sleipner ost fields gases principally consist of hydrocarbons 83 93 co2 5 4 15 3 and n2 0 6 0 9 as well as trace concentrations of noble gases helium isotopes 3he 4he are predominantly radiogenic and range from 0 065 to 0 116 ra reported relative to air ra 1 4 10 6 clarke et al 1976 sano et al 1988 showing predominantly 98 crustal contributions consistent with ne 20ne 22ne from 9 70 to 9 91 21ne 22ne from 0 0290 to 0 0344 and ar isotopes 40ar 36ar from 315 to 489 air derived noble gas isotopes 20ne 36ar 84kr 132xe are introduced into the hydrocarbon system by direct exchange with air saturated water asw the distribution of air derived noble gas species are controlled by phase partitioning processes in that they preferentially partition into the gas i e methane phase due to their low solubilities in fluids therefore the extent of exchange between hydrocarbon phases and formation waters that have previously equilibrated with the atmosphere can be determined by investigating air derived noble gas species we utilize both elemental ratios to address process i e open vs closed system and concentrations to quantify the extent of hydrocarbon water exchange i e volumetric gas water ratios these data are discussed within the framework of several conceptual models i total gas stripping model which assumes all noble gases have been stripped from the water phase thus defining the minimum volume of water to have interacted with the hydrocarbon phase ii equilibrium model which assumes equilibration between groundwater and hydrocarbon phase at reservoir p t and salinity and iii open and closed system gas stripping models using concentrations and elemental ratios by applying these models to ne ar data from sleipner we estimate volumetric gas water ratios v g v w between 0 02 and 0 07 which are lower than standard geologic gas water estimates of 0 24 estimated by combining gas in place estimates with groundwater porosity estimates sleipner vest data can be best approximated by an open system model which predicts more than an order of magnitude more groundwater interaction during migration than geologic estimates indicating a dynamic aquifer system and or a hydrous migration pathway in an open system the extent of gas loss can be estimated to be between 8 and 10 reservoir volumes which have passed through the system and been lost i e filled and spilled

effect of acceleration on sea ice growth field studies1 and laboratory experiments2 3 have shown that sea currents align the crystallographic c axes in sea ice illustrating the profound effect of fluid motion on the freezing of brine the role of buoyancy forces in the melt is further investigated here by solidifying nacl solution in a centrifuge when the ice is grown under negative accelerations choosing the solidification direction as positive the salt concentration in the ice decreases monotonically with depth whilst with positive acceleration it has the characteristic c shaped profile found in natural sea ice in addition the concentration decreases as the acceleration increases at any given depth this behaviour has been found to be due to variations in the initial trapping of brine rather than to subsequent buoyancy driven drainage accelerations up to 250 ms 2 25 5g reduce the scale of features within the solid while maintaining the essential characteristics of a sea ice cover

evolution of the pannonian basin system 2 subsidence and thermal history analysis of sedimentation and thermal data indicates that for most parts of the pannonian basin observed heat flow thermal gradients rates of thermal subsidence and vitrinite reflectance are about twice those predicted by assuming a uniform extension model for basin formation in late miocene time a modified extension model whereby large amounts of heat are added to the uppermost mantle during extension roughly equivalent to replacing the entire sub crustal lithosphere by asthenosphere produced results in good agreement with observation and could also account for periods of uplift observed during extension crustal extension beneath most parts of the basin is estimated to have been about 80 to 170 1 8 to 2 7 although in the deepest parts of the basin deeper than 4 km crustal extension could have been much greater analysis of extension within the danube and zala basins indicates that the late miocene extension in these basins involved the entire lithosphere but subsidence and heat flow data are not well enough constrained to determine if extra heating is required during extension on the basis of tectonic and structural data the vienna basin has been interpreted as the result of thin skinned extensional tectonics above a shallow detachment surface within the upper crust royden et al this issue analysis of thermal and subsidence data from the northern vienna basin suggests that little or no heating occurred during basin extension these data are consistent with a thin skinned extensional origin for the vienna basin and are in better agreement with this interpretation than with the assumption of a uniformly extended lithosphere beneath the basin this study emphasizes the need for good thermal subsidence and structural data in making reconstructions of extensional and thermal histories of sedimentary basins when such data are available it is found that a simple uniform extension model does not explain very well the evolution of some sedimentary basins such as the vienna and pannonian basins in the first instance the basin is thought to be the result of upper crustal extension only due to its special tectonic setting in the second instance the extra heating during extension is due to some unknown processes in the lower lithosphere that may be a general feature of lithospheric extension thus lithospheric extension and associated heating may be distributed in a variety of ways not all extensional sedimentary basins form in the same way application of a uniform subsidence model to basins for which good thermal subsidence and structural data are not available or not properly analysed can lead to reconstructions of thermal histories in error by a factor of 2 or more

the aegean sea we summarize briefly the main features of a kinematic model of evolution of the aegean area since 13 ma the formation of the aegean sea by extensional tectonics is related to the subduction of the mediterranean floor below the hellenic arc we then make a quantitative estimate of vertical movements in the aegean area on the basis of geological data and demonstrate that the outer arc was built since the beginning of this episode of subduction by vertical uplift the easiest way to explain the uplift is by underplating finally we discuss briefly the dynamics of the subduction marginal sea formation processes in this continental collision framework

a laboratory model for solidification of earth s core abstract to better understand the influence of rotating convection in the outer core on the solidification of the inner core we have constructed a laboratory model for solidification of earth s core the model consists of a 15 cm radius hemispherical acrylic tank concentric with a 5 cm radius hemispherical aluminum heat exchanger that serves as the incipient inner core onto which we freeze ice from salt water long exposure photographs of neutrally buoyant particles in illuminated planes suggest reduction of flow parallel to the rotation axis thermistors in the tank near the heat exchanger show that in experiments with rotation the temperature near the pole is lower than near the equator unlike for control experiments without rotation or with a polymer that increases the fluid viscosity the photographs and thermistors suggest that our observation that ice grows faster near the pole than near the equator for experiments with rotation is a result of colder water not readily convecting away from the pole because of the reversal of the thermal gradient we expect faster equatorial solidification in the earth s core such anisotropy in solidification has been suggested as a cause of inner core elastic and attenuation anisotropy though the plausibility of this suggestion will depend on the core nusselt number and the slope of the liquidus and the effects of post solidification deformation previous experiments on hexagonal close packed alloys such as sea ice and zinc tin have shown that fluid flow in the melt can result in a solidification texture transverse to the solidification direction with the texture depending on the nature of the flow a comparison of the visualized flow and the texture of columnar ice crystals in thin sections from these experiments confirms flow induced transverse textures this suggests that the convective pattern at the base of the outer core is recorded in the texture of the inner core and that outer core convection might contribute to the complexity in the seismically inferred pattern of anisotropy in the earth s inner core

back arc rifting initiated with a hot and wet continental lithosphere abstract how back arc rifting is initiated is studied by inferring the tectonic force and strain rate regimes from published subsidence data for nine back arc basins in the european tethyan belt and in the western pacific the results show that rifting only begins after some particular magnitude of tectonic force is reached this implies that subduction has already progressed to a point such that the total negative buoyancy of the slab becomes so large that the relevant tectonic force exceeds its bending strength once back arc rifting has been initiated the force required to maintain it gradually decays this indicates that the processes governing back arc extension are fast enough that lithosphere weakening due to an increase in geothermal gradient exceeds strengthening due to the thermal relaxation and crustal thinning this is achieved given currently accepted estimates for the maximum force 7 tn m if the thickness of the thermal lithosphere is significantly less than 125 km and if the lithosphere has a wet rheology meaning that a weak continental lithosphere is required for the initiation of back arc rifting the presence of significant strength in the uppermost mantle is not precluded but it is not required