localized subcritical convective cells in temperature dependent viscosity fluids abstract numerical simulations of infinite prandtl number convection in the stagnant lid regime of temperature dependent viscosity convection demonstrate the existence of spatially localized stable convective cells below the critical rayleigh number subcritical convection these solutions are in stark contrast to the usual supercritical convective planforms where convective cells form in the entire layer the isolated cell has a shape of an axisymmetric dome with an upwelling at the center and thus appears as a very weak plume formation of these structures requires subcritical conditions and a localized initial temperature perturbation but does not require any spatial heterogeneity in the material properties or the heat flux when several localized plumes form they tend to attract to each other and form stable clusters this type of subcritical convection may play a role in the formation and longevity of localized features on planetary bodies including the crustal dichotomy and tharsis region on mars and the asymmetric pattern of volcanism on mercury

constraints on the formation of the martian crustal dichotomy from remnant crustal magnetism the martian crustal dichotomy characterizing the topographic difference between the northern and southern hemispheres is one of the most important features on mars however the formation mechanism for the dichotomy remains controversial with two competing proposals exogenic e g a giant impact and endogenic e g degree 1 mantle convection mechanisms another important observation is the martian crustal remnant magnetism which shows a much stronger field in the southern hemisphere than in the northern hemisphere and also magnetic lineations in this study we examine how exogenic and endogenic mechanisms for the crustal dichotomy are constrained by the crustal remnant magnetism assuming that the dichotomy is caused by a giant impact in the northern hemisphere we estimate that the average thickness of ejecta in the southern hemisphere is 20 25 km while such a giant impact may cause crustal demagnetization in the northern hemisphere we suggest that the impact could also demagnetize the southern hemisphere via ejecta thermal blanketing impact demagnetization and heat transfer from the hot layer of ejecta thus posing a challenge for the giant impact model we explore how the pattern of magnetic lineations relates to endogenic theories of dichotomy formation specifically crustal production via degree 1 mantle convection we observe that the pattern of lineations roughly corresponds to concentric circles about a single pole and determine the pole for the concentric circles at 76 5 e and 84 5 s which nearly overlaps with the centroid of the thickened crust in the southern hemisphere we suggest that the crustal magnetization pattern magnetic lineations and crustal dichotomy i e thickened crust in the highlands can be explained by a simple endogenic process one plume convection causes melting and crustal production above the plume in the southern hemisphere and strong crustal magnetization and magnetic lineations are formed in the southern hemisphere as crustal production fronts spread radially out from the plume center and as the newly created crust cools in the presence of a dynamo with polarity reversals

true polar wander driven by late stage volcanism and the distribution of paleopolar deposits on mars article i nfo the areal centroids of the youngest polar deposits on mars are offset from those of adjacent paleopolar deposits by 5 10 we test the hypothesis that the offset is the result of true polar wander tpw the motion of the solid surface with respect to the spin axis caused by a mass redistribution within or on the surface of mars in particular we consider the possibility that tpw is driven by late stage volcanism during the late hesperian to amazonian there is observational and qualitative support for this hypothesis in both north and south observed offsets lie close to a great circle 90 from tharsis as expected for polar wander after tharsis formed we calculate the magnitude and direction of tpw produced by mapped late stage lavas for a range of lithospheric thicknesses lava thicknesses eruption histories and prior polar wander events we find that if tharsis formed close to the equator the stabilizing effect of a fossil rotational bulge located close to the equator leads to predicted tpw of b2 too small to account for observed offsets if however tharsis formed far from the equator late stage tpw driven by low latitude late stage volcanism would be 6 33 similar to that inferred from the location of paleopolar deposits a volume of 4 4 1 3 10 19 kg of young erupted lava can account for the offset of the dorsa argentea formation from the present day south rotation pole this volume is consistent with prior mapping based estimates and would imply a mass release of co2 by volcanic degassing similar to that in the atmosphere at the present time the south polar layered deposits are offset from the present rotation pole in a direction that is opposite to the other paleopolar deposits this can be explained by either a sequential eruption of late stage lavas or an additional contribution from a plume beneath elysium we predict that significant volcanic activity occurred during the time interval represented by the basal unit planum boreum unconformity planum boreum postdates the promethei lingula lobe and that the north polar deposits span a substantial fraction of solar system history if the additional contribution to tpw from plumes is small then we would also predict that tharsis montes formation postdates the promethei lingula lobe of the south polar layered deposits we conclude with a list of observational tests of the tpw hypothesis

potential vorticity structure in the north atlantic western boundary current from underwater glider observations abstractpotential vorticity structure in two segments of the north atlantic s western boundary current is examined using concurrent high resolution measurements of hydrography and velocity from gliders spray gliders occupied 40 transects across the loop current in the gulf of mexico and 11 transects across the gulf stream downstream of cape hatteras cross stream distributions of the ertel potential vorticity and its components are calculated for each transect under the assumptions that all flow is in the direction of measured vertically averaged currents and that the flow is geostrophic mean cross stream distributions of hydrographic properties potential vorticity and alongstream velocity are calculated for both the loop current and the detached gulf stream in both depth and density coordinates differences between these mean transects highlight the downstream changes in western boundary current structure as the current increases its transport downstream upper layer potential vorticity is generall

organic geochemistry of oil and gas in the kuqa depression tarim basin nw china abstract the kuqa depression in the tarim basin nw china contains significant natural gas and condensate resources with only small amounts of black oil this study demonstrates that the primary reason for the accumulation of large natural gas reserves in the kuqa depression is the high maturity level of the jurassic coal bearing sequence that is currently at the peak stage of dry gas generation from the combined stable carbon isotopes and molecular biomarker data it is possible to identify two separate source rocks for the discovered hydrocarbon fluids the gases were primarily from the middle lower jurassic coals and associated clastic rocks and the oils were from the upper triassic lacustrine mudstones peak oil generation from the triassic source rocks occurred during the early miocene 23 12 ma b p these oils migrated laterally over relatively long distances 20 50 km reaching the outer periphery of the depression peak gas generation took place more recently perhaps during the past 5 ma the gases migrated mainly along faults over relatively short lateral distances resulting in accumulations adjacent to the over matured source kitchens different timings for the trap formation along the north and south margins and a late injection of gas into early oil accumulations provided favorable conditions for the formation of evaporative condensates and the preservation of gas pools in the more down dip reservoirs and oil pools in the more up dip locations

the structure and transport of the gulf stream at 73 w abstract between september 1980 and may 1983 16 sections of temperature and velocity were obtained with the pegasus instrument along a transect crossing the gulf stream at 73 w the mean temperature and velocity fields of the upper 2000 meters were calculated the transport above 2000 meters calculated from the sections had a mean of 87 8 17 3 106 m3 s 1 a comparison with mass flux measurements obtained by previous investigators at cape fear indicated that the addition to the transport of the stream in the region was uniform with depth above 800 meters the mean inflow measured at the pegasus line was found to be uniform above 800 meters and to decrese substantially below this depth the increase in transport at the line estimated from the mean cross stream velocity field was 15 4 5 8 106 m3 s 1 per 100 km downstream distance the temporal fluctuation of the pegasus transport measurements was consistent with an annual cycle with a maximum in april the mean eddy kinetic energy at the surface

the subtropical subpolar gyre exchange in the presence of annually migrating wind and a meandering jet water mass exchange abstract a simple barotropic double gyre jet ocean circulation model is developed driven by surface wind the model consists of a subtropical gyre in the south and a subpolar gyre in the north and a meandering jet between them using this ocean model the water mass exchange between two gyres is investigated by calculating the lagrangian trajectories of the water column the results show that the meandering jet will cause strong intergyre exchange and the exchange will be substantially enchanced when the wind is allowed to migrate north and south for example in the standard case adapted after the gulf stream it is found that about 10 subtropical water mass has been transferred into the subpolar gyre in 25 years when the wind is steady whereas it is increased to about 18 in the same period of time when the wind is migrating annually with a distance 800 km when the wind is steady the subtropical water mass enters the subpolar gyre mainly through the western boundary it flows eastward and then pen

numerical models of martian mantle evolution induced by magmatism and solid state convection beneath stagnant lithosphere 1 numerical models of magmatism in a convecting mantle are presented to understand martian mantle evolution the viscosity is strongly temperature dependent and the lithosphere is stagnant magmatism is modeled as a permeable flow of basaltic magma generated by decompression melting when the initial mantle temperature is sufficiently high a magma ocean develops to generate a thick basaltic crust and create a compositionally layered mantle the upper layer of the mantle consists of compositionally buoyant residue of the basaltic crust while the lower layer consists of compositionally denser materials that are not as depleted in the basaltic component hot plumes grow from the lower layer making it thinner with time by erosion the hot plumes also induce magmatism which extracts heat producing elements hpes from the mantle the hpe extraction regulates plume magmatism activity and controls the duration of plume magmatism plume magmatism also keeps the mantle temperature below the solidus by efficiently extracting heat as soon as the mantle temperature exceeds the solidus the martian mantle is likely to have evolved as a relaxation from a compositionally layered state formed by the magma ocean and plume magmatism has probably played a crucial role in the relaxation process

the cross stream potential vorticity front and its role in meander induced exchange in the gulf stream abstract the potential vorticity pv front observed in the upper thermocline at the northern edge of the gulf stream has been thought of as an inhibitor to lateral motion of parcels on isopycnals the role of the pv front in relation to lateral motion and cross stream exchange was investigated by monitoring the evolving pv field in the vicinity of lagrangian parcels at the northern edge of the stream the observational study involved shipboard acoustic doppler current profiler adcp and conductivity temperature depth surveys to sample the velocity and density fields around water parcels as they flowed through meanders the observations reveal that the pv field in the vicinity of parcels moves laterally across the stream and evolves as parcels negotiate meander crests and troughs allowing parcels to move across the stream without changing their lagrangian pv this lateral motion of the pv front suggests that it is more a response to than an inhibitor of the lateral motion of parcels in the meanders the

continuous hydrocarbon accumulation over a large area as a distinguishing characteristic of unconventional petroleum the ordos basin north central china abstract global petroleum exploration is currently undergoing a strategic shift from conventional to unconventional hydrocarbon resources unconventional hydrocarbons in tight reservoirs show characteristics distinct from those of conventional hydrocarbon sources hosted in structural and stratigraphic traps the characteristic features include the following a hydrocarbon source and reservoir coexist porosity and permeability are ultra low nano pore throats are widely distributed hydrocarbon bearing reservoir bodies are continuously distributed there is no obvious trap boundary buoyancy and hydrodynamics have only a minor effect and darcy s law does not apply phase separation is poor there is no uniform oil gas water interface or pressure system and oil or gas saturation varies examples of unconventional hydrocarbon accumulations are the mesozoic tight sandstone oil province and the upper paleozoic tight sandstone gas province in the ordos basin north central china generally continuous hydrocarbon accumulation over a large area is a distinguishing characteristic of unconventional hydrocarbon sources because of the great potential of unconventional petroleum resources it is believed that research on such resources will be at the forefront of the future development of petroleum geology

gas geochemistry of the macuspana basin mexico thermogenic accumulations in sediments impregnated by bacterial gas a gas survey was performed in the macuspana basin south of mexico two distinct gas families were identified family 1 is represented by homogeneous and purely thermogenic gas associated with condensates expelled at the end of the oil window and family 2 is composed of a mixture of bacterial dry gas and thermogenic gas derived from source rocks at high maturity chemical and isotopic parameters as well as noble gas data provided information about the sources the proportions of mixing and directions of hydrocarbon migration of these two families considering the risk associated with further oil and gas exploration in the basin deeper zones present higher exploratory interest for family 1 gas fields and associated oil accumulations in contrast the bacterial gas fields should be considered a less promising target for future oil and gas exploration

are terrestrial plumes from motionless plates analogues to martian plumes feeding the giant shield volcanoes the near one plate planet evolution of mars has led to the edification of long lasting giant shied volcanoes unlike the earth mars would have been a transient convecting planet where plate tectonic would have possibly acted only during the first hundreds of million years of its history on earth where plate tectonic is active most of them are regenerated and recycled through convection however the nubian and antarctic plates could be considered as poorly mobile surfaces of various thicknesses that are acting as conductive lids on top of earth s deeper convective system in these environments volcanoes do not show any linear age progression at least for the last 30 ma but constitute the sites of persistent focused long term magmatic activity rather than a chain of volcanoes as observed in fast moving plate plume environments here the near stationary absolute plate motion probably exerts a primary control on volcanic processes and more specifically on the melting ones the residual depleted mantle that is left behind by the melting processes cannot be swept away from the melting locus over time the thickening of this near stationary depleted layer progressively forces the termination of melting to higher depths reducing the melt production rate such a process gradually leads both to decreasing efficient melt extraction and increasing mantle lithospheric melt interactions the accumulation of this refractory material also causes long term fluctuations of the volcanic activity in generating long periods of quiescence the presence of this residual mantle keel induces over time a lateral flow deflection which translates into a shift of future melting sites around it this process gives rise to the horseshoe like shape of some volcanic islands on slow moving plates e g cape verde crozet finally the pronounced topographic swells bulges observed in this environments may also be supported both by large scale mantle upwelling and their residual mantle roots most of these processes are likely similar to those observed on martian giant shield volcanoes the goal of this presentation will be to describe the essential characteristics of intra oceanic plumes on slow moving plates on the earth and to point out their similarities with those of the large shield volcanoes from the tharsis region

internal structure and early thermal evolution of mars from mars global surveyor topography and gravity topography and gravity measured by the mars global surveyor have enabled determination of the global crust and upper mantle structure of mars the planet displays two distinct crustal zones that do not correlate globally with the geologic dichotomy a region of crust that thins progressively from south to north and encompasses much of the southern highlands and tharsis province and a region of approximately uniform crustal thickness that includes the northern lowlands and arabia terra the strength of the lithosphere beneath the ancient southern highlands suggests that the northern hemisphere was a locus of high heat flow early in martian history the thickness of the elastic lithosphere increases with time of loading in the northern plains and tharsis the northern lowlands contain structures interpreted as large buried channels that are consistent with northward transport of water and sediment to the lowlands before the end of northern hemisphere resurfacing

geophysical evidence supports migration of tharsis volcanism on mars tharsis swell represents the largest area of volcanism on mars and sits adjacent to the dichotomy between the northern lowlands and southern highlands while a number of investigators have proposed that tharsis volcanism and topographic swell was caused by a stationary mantle plume both numerical and tank experiments find that a plume will rise beneath a region of thick lithosphere hence somewhere beneath the southern highlands on mars we present models of crustal thickness which show a region of thickened crust associated with previously mapped geologic units that have been suggested to represent the path of the tharsis plume we find a similarity in topography and structure between the proposed tharsis plume track on mars and the snake river plain yellowstone hot spot track in western north america

diapycnal mixing in western boundary currents in a previous study pelegri and csanady 1991 we found upward entrainment and two way exchange between the upper thermocline and surface layers of the gulf stream here we use isopycnic coordinates to analyze the mechanism of diapycnal shear induced mixing in western boundary currents a hydrographic section across the gulf stream off the mid atlantic bight is used for a case study it shows that the upper thermocline layers of the gulf stream are characterized by relatively small 16 m averaged gradient richardson numbers ri analysis of the jacobian j z p with z the depth and p the potential density total vorticity and potential vorticity fields shows that this region has low jacobian values associated with high values of potential vorticity as required theoretically for the case of dominant diapycnal mixing the density tendency wp dp dt is calculated from the vertical gradient of the reynolds density flux the latter parameterized as density eddy diffusivity divided by the jacobian we use munk and anderson s 1948 expression for the eddy diffusivity coefficient modified to conform to data of ueda et al 1981 maintaining a ri 3 2 dependence for large ri from this we obtain an explicit expression for wp in terms of the ri and j distributions the distributions of wp its diapycnal gradient wp p and the diapycnal velocity wd jwp are then calculated regions of high diapycnal convergence large negative wp p values coincide with the location of anomalously low jacobians in agreement with the idea that diapycnal mixing is the consequence of anomalies produced during frontogenesis in some phase of the meanders we suggest that this process taking place intermittently in space and time is responsible for two way exchange in western boundary currents

submesoscale streamers exchange water on the north wall of the gulf stream the gulf stream is a major conduit of warm surface water from the tropics to the subpolar north atlantic here we observe and simulate a submesoscale 20 km mechanism by which the gulf stream exchanges water with subpolar water to the north along isopycnals the front has a sharp compensated temperature salinity contrast with distinct mixed water between the two water masses 2 and 4 km wide this mixed water does not increase downstream despite substantial energy available for mixing a series of streamers detrain this water at the crest of meanders subpolar water replaces the mixed water and resharpens the front the water mass exchange accounts for a northward flux of salt of 0 5 2 5 psu m2 s 1 large scale diffusivity o 100 m2 s 1 this is similar to bulk scale flux estimates of 1 2 psu m2 s 1 and supplies fresher water to the gulf stream required for the production of 18 subtropical mode water

hydrocarbon accumulation processes and mechanisms in lower jurassic tight sandstone reservoirs in the kuqa subbasin tarim basin northwest china a case study of the dibei tight gas field the dibei gas field is a large tight gas field located in the kuqa subbasin tarim basin northwestern china the reservoir is within the lower jurassic ahe formation j1a and has porosity and permeability ranges of 2 8 and 0 01 1 md respectively two episodes of hydrocarbon charge are identified based on a detailed study of fluid inclusion petrography and microthermometry fluorescence spectroscopy characteristics and the thermal maturity of both gas and light oil low maturity oil as represented by hydrocarbon inclusions with yellow green fluorescence entered the reservoir circa 23 12 ma whereas high maturity hydrocarbons as indicated by hydrocarbon inclusions with blue white fluorescence have charged the reservoir since 5 ma the hydrocarbon charge process combined with porosity evolution determined the present gas water distribution characteristics in the dibei gas field porosity in the j1a sandstone reservoir was relatively high during the first episode of hydrocarbon charge which allowed oil to migrate upward and accumulate in structural highs under buoyancy from 5 ma to the present the dibei gas field experienced strong tectonic compression associated with intense thrust fault reactivation causing deformation and oil leakage from the reservoir continuous tight sand deposits along the slope areas located far away from the active faults became favorable accumulation sites for gas derived from the underlying triassic source rocks hydrocarbon accumulation along the slope area in the ahe formation is dominantly controlled by equilibrium between hydrocarbon generation pressure and capillary pressure

the average distribution of volume transport and potential vorticity with temperature at three sections across the gulf stream abstract average cross sections of downstream velocity and temperature obtained using pegasus current profiles at three locations along the gulf stream have been partitioned into 2 5 c temperature intervals to examine the distribution of transport increase versus temperature between the two southern sections 27 and 29 n and off cape hatters 73 w between 27 and 29 n the total transport of the florida current over the sections increased only by about 3 106 m3 s 1 3 sv but the current broadens by about 50 by cape hatteras the transport has increased nearly three fold to 93 7 sv of which two thirds of the increase is contained in the 19 5 17 0 c 18 layer and in water colder than the 7 c still temperature found at 27 n cross stream distributions of layer transport potential vorticity and thickness are estimated at each section the 10 10 7 m 1 s 1 contour tends to be a boundary independent of temperature between the region of relatively uniform layer potential vorticity on the a

exchange geometry revealed by float trajectories in the gulf stream in an effort to understand the extent to which lagrangian pathways in the gulf stream indicate fluid exchange between the stream and its surroundings trajectories of rafos floats are viewed in a frame of reference moving with the dominant zonal phase speed associated with the periodic flow in such a frame geometrical structures emerge that more clearly delineate the position of the parcel in relation to the jet core and its surroundings the basic premise of this work is that the pathways of fluid parcels in the vicinity of stagnation points as defined in the moving frame of reference are susceptible to changes in their pathways thereby facilitating fluid exchange between different regions of the flow field four representative rafos float trajectories are shown to exhibit the expected behavior in the vicinity of stagnation points to further examine the mechanism of exchange in the vicinity of these geometrical features concepts from dynamical systems theory are applied to a numerically simulated flow field the entrainment and detrainment of parcels from the jet core are explained in the context of stable and unstable manifolds and their associated lobes it is shown that the lagrangian pathways from the numerical flow and the observational trajectories exhibit a similarity based on the kinematics of a meandering flow field overall this study provides the first look at rafos float trajectories in a moving frame and provides insight as to how the temporal variability of a jet creates chaotic exchange

impacts of the tectonic stress field on natural gas migration and accumulation a case study of the kuqa depression in the tarim basin china stress fluid and temperature are three of the major factors that impact natural gas migration and accumulation in order to study the influences of tectonic stress field on natural gas migration and accumulation in low permeability rocks we take the kuqa depression as an example and analyze the evolution of the structure and tectonic stress field at first then we study the influences of tectonic stress field at different tectonic episodes on fractures and fluid potentials through the numerical simulation method on the section across the kl2 gas field we summarize two aspects of the impact of the tectonic stress field on natural gas migration and accumulation firstly under the effects of the tectonic stress field the rock dilation increases with the added stress and strain and when the shear stress of rock exceeds its shear strength the shear fractures are well developed on one hand the faults which communicate with the hydrocarbon source rocks become the main pathways for natural gas migration on the other hand these positions where fractures are well developed near faults can become good reservoirs for natural gas accumulation secondly because fluid potentials decrease in these places near the faults where fractures are well developed natural gas can migrate rapidly along the faults and accumulates the impact of tectonic stress fields on natural gas migration and accumulation allows for hydrocarbon migration and accumulation in the low permeability rocks in an active tectonic compressive setting

alignments of volcanic features in the southern hemisphere of mars produced by migrating mantle plumes abstract mars shows alignments of volcanic landforms in its southern hemisphere starting from the equatorial regions and converging towards the south pole and visible at global scale these composite alignments of volcanoes calderas shields vents heads of valley networks and massifs between the equatorial regions and the southern polar region define twelve different lines fitted by rhumb lines loxodromes that i propose to be the traces of mantle plumes the morphology of the volcanic centres changes along some of the alignments suggesting different processes of magma emplacement and eruptive style the diameters of the volcanic centres and of the volcanic provinces are largest at tharsis and elysium directly proportional to the number of alignments starting from them a minor presence of unaligned volcanic features is observed on the northern lowlands and on the highlands outside the 12 major alignments the heads of channels commonly interpreted as fluvial valleys are aligned with the other volcanic centres unaltered olivine is present along their bed floors raising severe doubts as to their aqueous origin several hypotheses have tried to explain the formation of tharsis with the migration of a single mantle plume under the martian lithosphere but the discovery of twelve alignments six starting from tharsis favours the hypothesis of several mantle plumes as predicted by the model of the southern polar giant impact spgi and provides a new view on the formation of the volcanic provinces of mars

potential vorticity dynamics of boundary currents in a quasi geostrophic ocean abstract boundary layer potential vorticity dynamics for a quasi geostrophic eddy resolving general circulation ocean model are studied using both lagrangian and eulerian analyses active western boundary layers are found not only in the upper wind driven layer but also in the lower layers despite the lack of a direct vorticity input to the deep ocean at the western wall dissipative and inertial boundary regimes are exclusively controlled by the time mean dynamics except for the deepest layer where eddy fluxes drive the mean flow across mean potential vorticity contours boundary layers formed at the southern wall in this model are dynamically distinct from the western boundary layers they are controlled solely by the eddy flux of potential vorticity found in this region of active baroclinic instability basin integrated vorticity balances reveal a strong contribution to the vorticity cycle by the lateral boundaries with such input overshadowed by the vorticity exchange across the midbasin gyre bounda

the early geodynamic evolution of mars type planets abstract it is not clear whether mars once possessed active tectonics yet the question is critical for understanding the thermal evolution of mars and the origin and longevity of its early dynamo to address these issues we have coupled mantle flow simulations together with parameterized core evolution models to simulate the early evolution of mars like planets and constrain the influence of early mobile lid tectonics on core evolution we have explored a wide parameter suite encapsulating a range of uncertainties in initial conditions rheological parameters and surface strength we present successful models that experience early mobile lid behaviour with a later transition into a stagnant lid mode which reproduce core dynamo histories similar to the magnetic history of early mars

application of aromatic compounds as maturity indicators in source rocks and crude oils concentration ratios of aromatic compounds such as naphthalenes phenanthrenes and dibenzothiophenes are at present attracting increasing attention as maturity indicators in sedimentary rocks and crude oils generally these indicators rely either on an increase with maturity in the degree of alkylation of a given parent compound or a shift in the isomer distribution of alkyl aromatic homologues towards thermally more stable isomers a combination of both concepts in the methylphenanthrene index mpi resulted in an excellent maturity parameter as demonstrated for rock extracts by an improved correlation with mean vitrinite reflectance rm between 0 65 and 1 4 i e in the zone of oil formation in cases that rm measurements were ambiguous this empirical relationship has proved useful in the calculation of theoretical vitrinite reflectance rc values from the mpi on the other hand for potential petroleum source rocks reaching maturity at relatively low temperatures the influence of the organic matter type on the aromatic distributions cannot be neglected consequently organic facies effects should be taken into account in the interpretation of aromatic distributions of such source rocks and related crude oils since the impact of the organic facies is strongly reduced with increasing maturity aromatic maturity parameters are particularly useful in the maturity evaluation of post mature crude oils and condensates in contrast to biological markers substantially higher concentrations of the key alkyl aromatics persist at elevated maturation levels thus aromatic maturity parameters should be considered as versatile tools in organic maturation studies and can complement established maturity indicators based on steranes and terpanes

degree 1 mantle convection and the crustal dichotomy on mars the surface of mars consists of an old heavily cratered and elevated southern hemisphere and younger resurfaced and depressed northern hemisphere a feature often termed the crustal dichotomy the global crustal structure zuber et al 2000 revealed by topography and gravity data from the mars global surveyor spacecraft and the possible late formation of the boundary zone between the hemispheres mcgill and dimitriou 1990 have been proposed to indicate an endogenic origin for the dichotomy however degree 1 mantle convection that is required for any endogenic process to be viable cannot be produced with conventional mantle convection models schubert et al 1990 we have studied the role of radially stratified viscosity on mantle deformation by using rayleigh taylor instability analyses in a spherical shell geometry our analyses reveal that when mantle viscosity is stratified with a weak asthenosphere deformation at long wavelengths is more efficient than that at short wavelengths the weaker the asthenosphere the longer the wavelength at which the deformation is the most efficient a thicker asthenosphere also favors the deformation at long wavelengths both the rayleigh taylor instability analyses and numerical modeling of mantle convection show that degree 1 convection can be produced within the martian mantle provided that the mantle had a weak asthenosphere v500 km thick and v10 2 times weaker than the underlying mantle early in planetary history the degree 1 convection causes preferential heating of one hemisphere that may explain the primary features associated with the dichotomy in crustal structure fl 2001 elsevier science b v all rights reserved

early mars serpentinization derived ch4 reservoirs h2 induced warming and paleopressure evolution ch4 has been observed on mars both by remote sensing and in situ during the past 15 yr it could have been produced by early mars serpentinization processes that could also explain the observed martian remanent magnetic field assuming a cold early mars a cryosphere could trap such ch4 as clathrates in stable form at depth the maximum storage capacity of such a clathrate cryosphere has been recently estimated to be 2 x 10 19 to 2 x 10 20 moles of methane we estimate how large amounts of serpentinization derived ch4 stored in the cryosphere have been released into the atmosphere during the noachian and the early hesperian due to rapid clathrate dissociation and photochemical conversion of ch4 to h 2 these episodes of massive ch4 release may have resulted in transient h 2 rich atmospheres at typical levels of 10 20 in a background 1 2 bar co2 atmosphere the collision induced heating effect of h 2 present in such an atmosphere has been shown to raise the surface temperature above the water freezing point we show how local and rapid destabilization of the cryosphere can be induced by large events such as the hellas basin or tharsis bulge formation and lead to such releases our results show that the early mars cryosphere had a sufficient ch4 storage capacity to have maintained h 2 rich transient atmospheres during a total time period up to several million years or tens of million years having potentially contributed to the formation of valley networks during the noachian early hesperian

geochemical evidence for coal derived hydrocarbons and their charge history in the dabei gas field kuqa thrust belt tarim basin nw china abstract large to middle scale thrust structures are important reservoir plays for coal derived hydrocarbons in the foreland basins of nw china with both gas and some accompanying oil in the dabei gas field of the kuqa thrust however the oil and gas pools are vertically distributed in a quite unique way 1 liquid oil and some dissolved gas are present in the dawanqi anticline with the reservoir at 300 700 m depth forming the only oil field in the kuqa thrust 2 gas and minor accompanying oil are found in the deep reservoir of the dabei 1 and dabei 2 thrust traps around 5000 6000 m depth 3 an extremely dry gas pool is found in the dabei 3 thrust trap where the depth of the reservoir is over 7000 m geochemical data suggest that the hydrocarbons in the dawanqi anticline and the dabei thrust traps originated from a similar source i e the underlying jurassic coal measures with some contribution from jurassic lacustrine shales the jurassic source rocks did not start to generate oil until the miocene around the kangcun stage and extended into the pliocene the kuche stage with the main gas generation period in the pliocene the kuche stage and the quaternary because the traps formed relatively early the dabei 1 and dabei 2 thrusts could trap some of the early generated oils but most of the early charged oil was redistributed to the shallower dawanqi anticline during the kuche stage the dabei 3 thrust trap formed concurrently with major gas generation and thus could not trap liquid hydrocarbons the difference in the vertical distribution of the hydrocarbon accumulations in the dabei gas field resulted from a complex interplay of source variability structural evolution of the basin and thermal maturation

potential vorticity sources and sinks in a quasi geostrophic ocean beyond western boundary currents abstract potential vorticity dynamics for a quasi geostrophic eddy resolving general circulation model egcm are studied in order to determine the effects of mesoscale variability on the potential vorticity distribution of a wind driven ocean the study employs both eulerian and lagrangian analyses in the effort to describe the potential vorticity gain loss cycle along the path of a particle while the mean wind stress curl is the dominant potential vorticity source for the interior of the upper layer a redistribution of eddy potential vorticity creates sources of potential vorticity for the multiple gyres in the lower layers this redistribution is a result of the local generation of eddies via baroclinic instabilities these eddies are advected by the western boundary current into the midlatitude jet where they are responsible for a cross gyre potential vorticity exchange this exchange is concentrated at the entrance to the eastward jets where northward and southward boundary currents converge from

thermal and compositional evolution of the martian mantle effects of phase transitions and melting abstract we present new numerical models for the thermochemical evolution of the mantle of mars over the past 4 gy specifically we have developed a parameterized model of composition and thermoelastic properties of mantle material and combined it with a two dimensional anelastic compressible convection and melting algorithm in a spherical annulus geometry these models include a detailed treatment of the effects of solid solid phase transitions and of compositional changes that accompany generation and removal of mantle partial melt during magmatism among other questions we examine if a perovskite ferropericlase layer pv fp exists at the base of the martian mantle if long lived plumes can explain the volcanic provinces under which circumstances mars may still be volcanically active and how iron and radionuclide rich mars is results of the models are compared with geophysical and chemical observations from spacecraft and information from martian meteorites most models yield crustal thicknesses between 75 and 90 km ancient depths for the curie temperatures of possible magnetic carriers that include the entire crust and mechanical lithosphere thicknesses that increased from less than 100 km in the noachian to 200 250 km now generally models with a large core mg 0 75 and radionuclide contents derived from those suggested by wanke and dreibus chemistry and accretion history of mars philos trans r soc lond a 349 285 293 1994 tend to explain observations best but none of the models reproduces the full range of concentrations found in martian meteorites thus implicitly supporting the idea that ancient chemical heterogeneities survive in the mantle only a subset of the models develops a pattern of mantle convection that evolves towards two or three large long lived plumes and it takes at least 2 gy before this stage is reached whereas models with a larger number of plumes or with plumes that are less stationary are more common as tharsis and probably elysium are older than 2 5 3 gy either model assumptions more complex than those made in this study or a dynamical mode different from low degree convection with long lived large plumes is required to explain these major volcanic provinces the effect of the mid mantle phase transitions on mantle dynamics is rather weak so that whole mantle convection prevails however a basal pv fp layer would form a dynamically separate unit and decouple the mantle and core to some extent

towards self consistent modeling of the martian dichotomy the influence of one ridge convection on crustal thickness distribution in order to find an explanation for the origin of the martian crustal dichotomy a number of recent papers have examined the effect of layered viscosity on the evolution of a degree 1 mantle convection e g roberts and zhong roberts j h zhong s 2006 j geophys res 111 e06013 and yoshida and kageyama yoshida m kageyama a 2006 j geophys res 111 doi 10 1029 2005jb003905 b03412 it was found that a mid mantle viscosity jump combined with highly temperature and depth dependent rheology are effective in developing a degree 1 convection within a short timescale such a layered viscosity profile could be justified by martian mineralogy however the effect of a degree 1 convective planform on the crustal thickness distribution has not yet been demonstrated it is not obvious whether a thinner crust due to sublithospheric erosion and crustal thinning or a thicker crust due to enhanced crustal production would form above the hemisphere of mantle upwelling also the general shape of the dichotomy which is not strictly hemispherical has not yet been fully investigated here we propose a model of the crustal patterns produced by numerical simulations of martian mantle convection using the finite volume multigrid code stagyy tackley p j 2008 phys earth planet int 107 7 18 doi 10 1016 j pepi 2008 08 005 a self consistent treatment of melting crustal formation and chemical differentiation has been added to models of three dimensional thermal convection this allows us to obtain global maps of the crustal thickness distribution as it evolves with time the obtained results demonstrate that it is indeed possible to form a crustal dichotomy as a consequence of near degree 1 mantle convection early in mars history we find that some of the observed patterns show intriguing first order similarities to the elliptical shape of the martian dichotomy in all models the region of thick crust is located over the region of mantle upwelling which itself is a ridge like structure spread over roughly one half of the planet a planform we describe as one ridge convection

early transient superplumes and the origin of the martian crustal dichotomy 1 the large temperature difference between the core and mantle of mars at the end of planetary accretion creates a hot internally convecting thermal boundary layer at the base of the mantle whose viscosity is several orders of magnitude lower than the viscosity of the mantle above it theoretical analysis and numerical simulations of the instability of this thermal boundary layer show that it is likely that only one large plume forms this superplume may play a role in the formation of crustal dichotomy and generation of the magnetic field in the early history of mars

two dimensional numerical studies on the effects of water on martian mantle evolution induced by magmatism and solid state mantle convection 1 numerical models of magmatism and mantle convection beneath a stagnant lithosphere are presented to understand how water affects martian mantle evolution magmatism is modeled as an upward permeable flow of the basaltic magma generated by decompression melting first a global partially molten layer gpml develops and generates a basaltic crust a layer of compositionally buoyant residue of melt extraction in the uppermost mantle and a denser layer with a higher content of the basaltic component in the deep mantle the gpml extracts most of the water initially contained in the upper layer but some water remains in the lower layer of the mantle subsequently hot plumes ascend from the lower layer to induce magmatism the water allows plume magmatism to continue for a long duration up to 5 gyr depending on the initial water content and the detail of the initial temperature distribution in the mantle provided that the mantle is initially not too hot just after planetary formation the plume magmatism is sufficiently active to cause significant crustal growth and dehydration of the crust and mantle in the early evolutionary stage when the internal heating is strong the amount of extracted water is equivalent to a water layer of up to several hundred meters in depth water can also enhance the extraction of heat producing elements hpes from the mantle reduce the heat flux from the mantle to the crust and make the lithosphere thicker both crustal growth and dehydration eventually subside as the hpes decay

basin centered gas systems basin centered gas systems bcgss are potentially one of the more economically important unconventional gas systems in the world in the united states they contribute as much as 15 of the total annual gas production these regionally pervasive gas accumula tions are different from conventionally trapped accumulations in several respects the basin centered gas accumulations bcgas associated with bcgss are typically characterized by regionally pervasive accumulations that are gas saturated abnormally pres sured commonly lack a downdip water contact and have low permeability reservoirs the accumulations range from single isolated reservoirs a few feet thick to multiple stacked reservoirs several thousand feet thick two types of bcgss are recognized a direct type characterized by having gas prone source rocks and an indirect type characterized by having liquid prone source rocks during the burial and thermal histories of these systems the source rock differences between the two types of bcgss result in strikingly different characteristics that impact exploration strategies the majority of known bcgas are the direct type exploration activity for bcgas is in the early stages and thus far has been focused in north america in other parts of the world concepts of basin centered gas systems are poorly known and exploration activity focused on basin centered gas accumulations is minimal

migration of tharsis volcanism on mars caused by differential rotation of the lithosphere as a single plate planet mars cannot support large scale motion of the lithosphere according to the standard theory of stagnant lid convection but such motion is required to explain the position of the tharsis rise modelling work suggests that differential rotation of the lithosphere can resolve this problem

a closer look at particle exchange in the gulf stream abstract the trajectories of 95 isopycnal floats deployed in the gulf stream in the last decade have shown that a substantial amount of particle exchange takes place between the gulf stream and the surrounding fluid at the level of the main thermocline this exchange is suggestive of significant cross stream eddy mixing but in order to accurately interpret the float exchange in terms of property exchange the location of float deployment was assessed relative to the strong potential vorticity front associated with the gulf stream the basic result of this analysis is that most of the observed float exchange is not representative of cross frontal exchange at the level where a strong potential vorticity front is present some fluid particles escape from the jet but most of them stay on the same side of the front in the deep main thermocline significant particle exchange is observed between the gulf stream and fluid on both sides of the jet but this exchange is indicative of particles circulating in a r

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

role of ocean continent contrast and continental keels on plate motion net rotation of lithosphere and the geoid three dimensional spherical shell models of mantle convection have been formulated to investigate the effects of ocean continent contrast i e different viscosity for continental and oceanic lithospheres and continental keels on plate motion net rotation of lithosphere i e degree 1 toroidal plate motion and the geoid the models include relatively realistic plate rheology i e strong plate interiors and weak plate margins and continental keel structure based on seismic models the mantle flow in the models is driven by slab buoyancy the models demonstrate that the ocean continent contrast and continental keels have minor effects on the long wavelength geoid for degrees 2 5 however continental keels overriding subducted slabs for example in north america can produce large negative gravity anomalies at a regional scale this may have important implications to interpreting the gravity anomalies in north america our models show that when plates bounded by weak plate margins have the same thickness neither weak plate margins nor the ocean continent contrast efficiently excite net rotation of lithosphere although they excite toroidal motion at higher harmonic degrees however plate thickness variations like continental keels excite the net rotation in spite of their ability to excite the net rotation continental keels as thick as 300 km cannot provide the necessary coupling to the deep mantle to produce the observed net rotation of lithosphere and slow continental motion if there is a weak asthenosphere underlying them if the thermomechanic structure of north american continental upper mantle derived from seismic and heat flow studies is representative our models suggest that the temperature and pressure dependent mantle rheology may not produce sufficiently high viscosity below continental keels that is needed to explain the observed plate motion our study hints the necessity to include more realistic treatment of plate boundaries in future studies to assess possible effects of plate plate coupling on plate motion

a geophysical perspective on the bulk composition of mars we invert the martian tidal response and mean mass and moment of inertia for chemical composition thermal state and interior structure the inversion combines phase equilibrium computations with a laboratory based viscoelastic dissipation model the rheological model which is based on measurements of anhydrous and melt free olivine is both temperature and grain size sensitive and imposes strong constraints on interior structure the bottom of the lithosphere defined as the location where the conductive geotherm meets the mantle adiabat occurs deep within the upper mantle 250 500 km depth resulting in apparent upper mantle low velocity zones assuming an fe fes core our results indicate 1 a mantle with a mg molar mg mg fe of 0 75 in agreement with earlier geochemical estimates based on analysis of martian meteorites 2 absence of bridgmanite and ferropericlase dominated basal layer 3 core compositions 13 5 16 wt s core radii 1640 1740 km and core mantle boundary temperatures 1560 1660 c that together with the eutectic like core compositions suggest the core is liquid and 4 bulk martian compositions that are overall chondritic with a fe si wt ratio of 1 63 1 68 we show that the inversion results can be used in tandem with geodynamic simulations to identify plausible geodynamic scenarios and parameters specifically we find that the inversion results are reproduced by stagnant lid convection models for a range of initial viscosities 1019 1020 pa s and radioactive element partitioning between crust and mantle around 0 001 the geodynamic models predict a mean surface heat flow between 15 25 mw m2

a simple kinematic mechanism for mixing fluid parcels across a meandering jet abstract recent observations of fluid parcel pathways in the gulf stream using isopycnal rafos floats revealed a striking pattern of cross stream and vertical motion associated with meanders bower and rossby 1989 in an attempt to explain the observed pattern a two dimensional kinematic model of a meandering jet has been developed which enables examination of the relationship between streamfunction patterns and fluid parcel trajectories the streamfunction fields are displayed in a reference frame moving with the wave pattern so motions of fluid parcels relative to the jet can be seen more easily the results suggest that the observed pattern of cross stream motion results primarily from the downstream phase propagation of meanders the model successfully reproduces several of the most distinctive features of the float observations 1 entrainment of fluid into the gulf stream occurs at the leading edges of meander extrema while detrainment takes place at the trailing edges 2 exchange between the gu

tharsis as a consequence of mars dichotomy and layered mantle 1 the two most striking features of the martian surface topography are the tharsis rise and the crustal dichotomy closely associated with tharsis are several large volcanoes active in the geologically recent past indicating a longevity of volcanic activity at tharsis that is unique in the solar system using analogue laboratory experiments we examine the dynamical effects of the crustal dichotomy and a layered mantle on the thermal evolution of mars we show that in combination these two effects lead to the formation of a large scale upwelling under the southern highlands that appears early and endures for many billions of years the upwelling comprises several persistent narrow plumes which can explain the long lived tharsis volcanoes

tectonic consequences of martian dichotomy modification by lower crustal flow and erosion the martian hemispheric dichotomy shows compressional features a few hundred kilometers south of the dichotomy boundary and contemporaneous extensional features along the boundary two processes that may have contributed to the observed deformation are lateral lower crustal flow and erosion crustal flow causes a decrease in crustal thickness in the south and an increase in the north the result is compressional surface stresses in the highlands and extensional stresses around the dichotomy boundary the magnitude location and timing of the predicted stresses are consistent with the observed tectonic features and strongly suggest that moderate lateral flow occurred for the nominal heat flux and rheology sufficient lateral flow is generated for a mean crustal thickness of 70 km erosion generates extension in the south and compression in the north opposite to what is observed either erosion only resulted in local as opposed to regional distribution of highland material or the rigidity of the lithosphere was sufficiently high at the time of erosion that significant stresses did not develop

planetary science mars s rotating shell volcanism in the enormous tharsis region on mars migrated from south to north numerical modelling suggests that this migration as well as the current location of the region can be explained by net rotation of the lithosphere relative to the mantle

observations of the velocity and vorticity structure of gulf stream meanders abstract in september october 1988 and in april 1989 two 4 week cruises were organized to study the structure and dynamics of gulf stream meanders the first one focused on an anticyclonic crest and the second one on a cyclonic trough one objective of this program was to conduct a high resolution study of the upper ocean velocity and vorticity field using a ctd and a continuously profiling acoustic doppler current profiler adcp the cross stream sections of velocity exhibit the typical pattern of a single velocity maximum with a narrow zone of cyclonic shear and a broad region of anticyclonic shear the cyclonic velocity shear is larger and extends to greater depths in a crest than in a trough and can exceed 120 of the coriolis parameter f at depths as great as 300 m the vertically integrated transport to 250 m appears to be more symmetric in troughs than in crests the computed sea level difference across the stream is about 0 46 m greater in the trough than the crest after seasonal correction an

thermochemical structures within a spherical mantle superplumes or piles 1 heterogeneous compositional mantle models are frequently invoked to explain some observations obtained from geochemistry and seismology in particular two regions in the earth s lower mantle under the pacific and africa are often interpreted as being either piles or superplumes of dense material we perform numerical modeling of thermochemical convection in a three dimensional spherical geometry to investigate whether the geometry can focus a dense chemical component into a small number of isolated rounded piles or superplumes of material we study the effect of temperature and compositionally dependent viscosity and we find two different modes of dense layer deformation temperature dependent rheology leads to a low viscosity dense layer which is passively swept aside by downwellings into linear piles that are spread throughout the entire lower mantle the addition of compositionally dependent rheology in which the dense layer is more viscous results in active deformation of the dense material forming large isolated superplumes our results indicate that piles and superplumes are separate features which in general do not occur together and in order for isolated rounded superplumes to form an intrinsic compositional viscosity increase is required for the dense material

lagrangian studies of fluid exchange between the gulf stream and surrounding waters abstract between spring of 1988 and winter of 1990 75 rafos floats were released east of cape hatteras near the center of the gulf stream on the 26 8 t surface o 15 c the purpose of this sequential deployment was to investigate the spatial and temporal characteristics of the meandering stream and the lagrangian properties of fluid motion in the upper thermocline these new observations were also intended to provide a database for comparative studies of fluid motion in the upper thermocline with similar observations made earlier 1984 85 in the middle and lower thermocline sixty one of the 75 rafos float trajectories in the 13 16 c layers have been analyzed from both a lagrangian and eulerian point of view the results reconfirm the strong baroclinic structure of the gulf stream established earlier and reveal enhanced cross stream motion with increasing depth and over the new england seamounts float trajectories tend to be more convoluted east of the seamounts reflecting the large amplitude meand

convection underneath a crust inhomogeneously enriched in heat sources application to martian mantle dynamics abstract two and three dimensional spherical convection models are presented which investigate the convection structure beneath a crust inhomogeneously enriched in radioactive elements the resulting convection patterns show a focusing of upwellings under the enriched part of the crust and downwellings under the not enriched part that maintain large scale horizontal motion from the warmer to the cooler region beneath the lithosphere and vice versa above the core mantle boundary this flow pattern is observed in all examined models independent of the mantle heating mode it is however more pronounced for models with a considerable amount of internal heating than for models with almost total bottom heating thermal blanketing of the mantle beneath the enriched part of the crust plays only a minor role in the lateral heat flux distribution within the lithospheric mantle and is almost insignificant for the surface heat flux distribution which is dominated by crustal heat generation implications of the results for mars are discussed

characteristics and origin of natural gases in the kuqa depression of tarim basin nw china abstract significant gas condensate as well as some black oils have been discovered in the kuqa depression of tarim basin nw china dry gases with high 13 c values occur in the kelasu structural belt wet isotopically light gases in the yiqikelik belt whereas condensates are distributed mainly in the front uplift area kinetic modeling results show that the variation of methane and ethane isotopes with increasing vitrinite reflectance is independent of heating rates for a given source rock two maturity trends have been observed on a c 2 c 1 13 c 1 plot one for thermogenic gases associated with coal another for oil associated gases with minor contribution from biogenic gas it is most likely that the gases in the kelasu belt and front uplift area were derived from the jurassic coal measures and triassic lacustrine shales respectively with those in the yiqikelik belt being the mixtures of gases from the two sources both jurassic coals and triassic lacustrine shales appear to have contributed to the gases in the kela 2 kl2 and dina 2 dn2 the two largest gas fields in the tarim basin

degree 1 convection in the martian mantle and the origin of the hemispheric dichotomy 1 the surface of mars appears dramatically different between the northern and southern hemispheres any endogenic origin for this hemispheric dichotomy must involve a pattern of mantle convection that reflects the shape of the dichotomy primarily spherical harmonic degree 1 we investigated two mechanisms by which degree 1 convection may be initiated in the martian mantle 1 an endothermic phase change near the cmb and 2 viscosity layering in the mid mantle using two dimensional 2 d and 3 d spherical finite element convection models we explored the conditions under which each mechanism can produce degree 1 structures the phase transition is only effective at generating degree 1 structures when the mantle viscosity is constant or weakly temperature dependent activation energy 100 kj mol but the degree 1 pattern requires several billion years to develop increasing convective vigor in phase change models leads to reduced wavelengths for convective structures degree 1 convection can also develop in a layered viscosity mantle with temperature and depth dependent viscosity an overall sublithospheric radial viscosity variation of a factor of 100 including a factor of 8 25 jump in the midmantle can lead to formation of degree 1 structure in a timescale ranging from 100 my to several hundred my consistent with the timescale for the formation of the dichotomy neither convective vigor nor the internal heating rate greatly affects the formation of degree 1 structures we propose that degree 1 mantle convection induced by a layered viscosity structure may be responsible for the formation of the crustal dichotomy

analysis of lagrangian potential vorticity balance and lateral displacement of water parcels in gulf stream meanders abstract the balance of potential vorticity components following fluid parcel motion in gulf stream meanders was studied using rafos float data from the synop experiment by introducing curvature dependent variations to the velocity and density fields the authors relaxed the rigid field assumption used in earlier studies and examined closely 61 floats in the upper layers 13 16 c of the main thermocline float trajectories were segmented according to transition from crest to trough and trough to crest and grouped by their positions relative to the current center a total of 154 segments were collected to estimate the horizontal divergence and the mean lateral displacement of parcels under two distinct regimes growing and decaying meanders both spatial and temporal changes in curvature affect the regions of divergence in a meandering stream on the one hand horizontal divergence increases with increasing curvature magnitude while on the other hand the divergence pattern itself changes going from g

a synoptic and statistical description of the gulf stream and subtropical gyre using sofar floats abstract trajectories of 17 sofar floats ballasted to nominal depth of 700 and 2000 m and tracked for 15 months have been analyzed to produce synoptic and statistical descriptions of the gulf stream and subtropical gyre sofar floats launched in and near the gulf stream along with positions of the surface gulf stream and satellite tracked buoys demonstrate strong baroclinic shear nevertheless the horizontal patterns of the gulf stream meanders and gulf stream rings are the same at the surface and at 700 and 2000 m depth above the thermocline particle speeds greater than the propagation speed of the meanders cause parcels of water to move with the flow pattern at greater depths the velocities are at most equal to the propagation speed and the deep trajectories are free to pass through the gulf stream or ring strong temporal variability is found near the tail of the grand banks instances of flow both southward into a narrow gulf stream recirculation and northward into the north atlantic current can

timing of the tianshan mountains uplift constrained by magnetostratigraphic analysis of molasse deposits abstract the tianshan mountains lie in the actively deforming part of the india asia collision zone the thrust faulting and folding of the neogene and quaternary sediments forming linear roughly east west anticlines at the northern and southern borders of the orogenic belts indicate crustal shortening and thickening in late cenozoic time detailed magnetostratigraphic studies of the molasse deposits of the folded neogene and quaternary strata in the northern edge of the tianshan mountains indicate that the onset of uplift occurred at 7 ma deposition of the thick conglomerates between 7 and 2 58 ma in the studied region was mainly in response to the tectonic uplift while the accumulation of the early pleistocene conglomerates xiyu formation was mostly controlled by both tectonics and climatic cooling our results indicate that the present high relief of the tianshan mountains is the result of two dominant phases of uplift occurring at about 7 2 58 ma and the early pleistocene

genetic characterization of natural gases natural gases can be characterized genetically using four properties c2 concentration carbon and hydrogen isotope variations in methane and carbon isotope variation in ethane three diagrams for genetic characterization of gases have been designed in which the carbon isotopic composition of methane is correlated with the other parameters in these diagrams compositional fields have been defined for primary gases biogenic thermogenic associated and thermogenic nonassociated and for gases which result from mixing of these gases these fields are strictly empirical and comprise compositional variations found in about 500 natural gases the isotopic and compositional variations in natural gases can be described in terms of 1 processes during formation of the gases such as bacterial fermentation or maturation of organic matter and 2 processes during secondary migration mixing of primary gases is an important and common process migration of gases predominantly affects the c2 concentration whereas the isotopic properties of gaseous hydrocarbons primarily remain unchanged allowing an assessment of the origin of migrated gases and properties of their source rocks the formation of gas from humic organic matter and coals is not yet clear from published data the diagrams use data from various basins and areas interstitial gases from the gulf of california are entirely of bacterial origin traces of thermogenic gases are formed only in the vicinity of dolerite sills gases in the south german molasse basin and in the vienna basin are of bacterial mixed and thermogenic origins data from the north italian po basin provide examples for genetic characterization of migrated gases

review of characteristics of low permeability gas reservoirs in western united states tight gas reservoirs generally are defined as gas bearing rocks with an in situ permeability to gas of less than 0 1 md in 1980 the national petroleum council estimated that undiscovered recoverable gas resources in low permeability tight rht reservoirs in the united states excluding eastern gas shales are about 574 tcf 16 1012 m3 the possible recovery of this volume of gas was based on 9 mcf 1979 dollars and a level of recovery technology presently unknown advanced technology most of this resource is in the western united states the two main types of tight rocks are shallow high porosity reservoirs and deep low porosity reservoirs the high porosity rocks are tight because of their fine grain size the low porosity rocks are tight because diagenetic alteration has resulted in them having small microvugs porosity poorly connected by sheet or cracklike very thin capillaries the low porosity rocks are very stress sensitive and may be overpressured and all tight rocks are susceptible to formation damage to varying degrees during drilling and treatment the western tight gas reservoirs can be placed in two major geologic and engineering categories blanket and lenticular blanket reservoirs generally respond to artificial stimulation in a predictable manner whereas the response of lenticular reservoirs is mostly unpredictable many obstacles complicate the identification and recovery of gas from these unconventional reservoirs accurate borehole geophysical log analysis is generally difficult and economic stimulation commonly artificial fracturing of the tightest rocks is difficult or impossible with present technology geologists and engineers must better understand the characteristics of these reservoirs work together to improve present recovery technology and develop new innovative drilling and stimulation techniques

geological evidence for a migrating tharsis plume on early mars abstract the tharsis bulge is the largest magmatic volcanic center on mars and in the solar system having a volume of 3 10 8 km 3 with the majority of its mass emplaced more than 3 7 billion years ago the igneous history of tharsis has been implicated in the generation of an atmosphere and hydrosphere capable of clement conditions on early mars it has been proposed that an early plume migration from the southern highlands of mars relative to a one plate lithosphere led to the development of the tharsis bulge in its current location along the martian crustal dichotomy we used geologic mapping crustal magnetic data crater age dating and crater morphometry to detail a previously unidentified path of putative plume migration our results indicate that extensive volcanic resurfacing occurred from a location near the present south pole to the equator around 3 8 billion years ago obliterating older cratered terrains the resurfacing path is manifest as smooth volcanic plains embaying ancient massifs and infilling large impact craters as well as a lack of a magnetic signature in this portion of the crust our results have significant ramifications for mantle dynamics and the early geologic history of mars

long wavelength stagnant lid convection with hemispheric variation in lithospheric thickness link between martian crustal dichotomy and tharsis 1 a dynamic link between the early evolution of tharsis and the crustal dichotomy on mars was recently proposed by zhong 2009 we address in detail the fundamental aspects of the proposed model using 3 d spherical shell modeling of convection we investigate the conditions under which a spherical harmonic degree 1 flow is produced in the mantle of mars in layered viscosity models with different depths of viscosity layering and different viscosity ratios between the bottom and top layers we find that a thinner weak layer requires a larger viscosity decrease in order to produce degree 1 planform in qualitative accordance with existing analytical studies a lithospheric keel of hemispheric extent which may represent melt residue after dichotomy formation process is then introduced in the models as a consequence thermal upwellings are first formed and oriented below the center of the melt residue cap for keels of maximum thickness 100 km a rotation of the entire single plate lithosphere relative to the underlying mantle follows until the plume is stabilized near the edge of the keel which can be identified as the dichotomy boundary this model may explain the early migration of the tharsis volcanic center from southern latitudes to the vicinity of the dichotomy boundary for keels 200 km thick the modeled time scales agree with observationally inferred migration rate if the tharsis migration is caused by the rotation of the lithosphere the requirement on the amount and location of upper mantle melting would argue for an endogenic origin of the dichotomy

the contributions of atmosphere and ocean to north atlantic subtropical mode water volume anomalies abstract subtropical mode water stmw in the western north atlantic ocean or eighteen degree water edw as it is commonly known is formed near the gulf stream in the wintertime is dissipated by mixing and is removed by subduction the ability of edw to store and discharge large quantities of heat over periods of several years contributes to the memory of the climate system to complement the 2 year field and modeling program of the clivar mode water dynamics experiment climode with a perspective of interannual to decadal variability 1985 2007 we used a simple box model to hindcast observed edw volume anomalies in two regions one in which edw is formed and an adjacent region of subducted edw estimates of the relative contributions of heat flux anomalies vertical mixing from ekman advection mixing and circulation are examined using proxy variables derived from winds sea surface temperature hydrographic data and altimetric sea level the importance of each process is evaluated by its contribution to observed edw volume anomalies in two regions the study produced some robust conclusions 1 anomalies of formation by surface heat fluxes are clearly reflected in edw volume anomalies with some contributions by ekman advection 2 of the newly formed edw about 65 is lost by mixing and about 35 is transferred to the subducted region 3 mixing losses are well parameterized by the meandering of the nearby gs and 4 transfer and losses from the subducted region can be parameterized by the geostrophic surface flow

quantitative fluorescence techniques for detecting residual oils and reconstructing hydrocarbon charge history abstract two new fluorescence techniques quantitative grain fluorescence qgf and quantitative grain fluorescence on extract qgf e can detect palaeo oil zones and current residual oil zones in petroleum wells respectively qgf and qgf e measure the fluorescence emission spectra of trace fluorescence from reservoir grains and their extracts qgf is measured on dry disaggregated reservoir grains after a cleaning procedure involving solvent hydrogen peroxide and acid whereas qgf e is measured on the solvent extract from the qgf cleaned grains palaeo oil zones have distinct qgf spectra with spectral peaks between 375 and 475 nm and elevated fluorescence intensities compared with spectra for rocks that have always been water saturated current and residual oil zones have distinct qgf e spectra with spectral peaks at around 370 nm and elevated qgf e intensities compared with that for current water zones both qgf and qgf e responses increase upwards from the palaeo oil water and residual oil water contacts respectively an oil producing well from timor sea nw australia was investigated in detail using the qgf and qgf e techniques to illustrate how they can be used to reconstruct hydrocarbon migration history

what can olympus mons tell us about the martian lithosphere abstract under gravitational loading a volcanic edifice deforms and the underlying lithosphere downflexes this has been observed on earth but is equally true on other planets we use finite element models to simulate this gravity driven deformation at olympus mons on mars eleven model parameters including the geometry and material properties of the edifice lithosphere and underlying asthenosphere are varied to establish which parameters have the greatest effect on deformation values for parameters that affect deformation at olympus mons mars are constrained by minimising misfit between modelled and observed measurements of edifice height edifice radius and flexural moat width our inferred value for the young s modulus of the martian lithosphere 17 8 gpa is significantly lower than values used previously suggesting that the martian lithosphere is more porous than generally assumed the best fitting values for other parameters edifice density 2111 2389 kg m 3 and lithosphere thickness 83 3 km are within ranges proposed hitherto the best fitting values of model parameters are interdependent a decrease in lithosphere young s modulus must be accompanied by a decrease in edifice density and or an increase in lithosphere thickness our results identify the parameters that should be considered within all models of gravity driven volcano deformation highlight the importance of the often overlooked young s modulus and provide further constraints on the properties of the martian lithosphere namely its porosity which have implications for the transport and storage of fluid throughout mars history

martian sub crustal stress from gravity and topographic models the latest martian gravity and topographic models derived from the mars orbiter laser altimeter and the mars global surveyor spacecraft tracking data are used to compute the sub crustal stress field on mars for this purpose we apply the method for a simultaneous determination of the horizontal sub crustal stress component and the crustal thickness based on solving the navier stokes problem and incorporating the vening meinesz moritz inverse problem of isostasy results reveal that most of the martian sub crustal stress is concentrated in the tharsis region with the most prominent signatures attributed to a formation of tharsis major volcanoes followed by crustal loading the stress distribution across the valles marineris rift valleys indicates extensional tectonism this finding agrees with more recent theories of a tectonic origin of valles marineris caused for instance by a crustal loading of the tharsis bulge that resulted in a regional trusting and folding aside from these features the martian stress field is relatively smooth with only a slightly enhanced pattern of major impact basins the signatures of active global tectonics and polar ice load are absent whereas the signature of the hemispheric dichotomy is also missing the long wavelength spectrum of the stress field comprises the signature of additional dichotomy attributed to the isostatically uncompensated crustal load of tharsis volcanic accumulations these results suggest a different origin of the earth s and martian sub crustal stress whereas the former is mainly related to active global tectonics the latter is generated by a crustal loading and regional tectonism associated with a volcanic evolution on mars the additional sub crustal stress around major impact basins is likely explained by a crustal extrusion after impact followed by a moho uplift

mars paleomagnetic field as the result of a single hemisphere dynamo mars crustal magnetic field was most likely generated by dynamo action in the planet s early history unexplained characteristics of the field include its strength concentration in the southern hemisphere and lack of correlation with any surface features except for the hemispheric crustal dichotomy we used numerical dynamo modeling to demonstrate that the mechanisms proposed to explain crustal dichotomy formation can result in a single hemisphere dynamo this dynamo produces strong magnetic fields in only the southern hemisphere this magnetic field morphology can explain why mars crustal magnetic field intensities are substantially stronger in the southern hemisphere without relying on any postdynamo mechanisms

potential vorticity balances and horizontal divergence along particle trajectories in gulf stream meanders east of cape hatteras abstract trajectories of 37 isopycnal rafos floats launched in the gulf stream off cape hatteras have been analyzed to examine the dynamics of meanders from a lagrangian viewpoint using the float data in conjunction with information on the structure of horizontal velocity shear from the pegasus study halkin and rossby variations in planetary curvature and shear vorticity have been estimated along the float trajectories changes in fluid layer thickness were then inferred assuming potential vorticity is conserved following the floats this analysis shows that curvature vorticity changes are typically 10 20 of f planetary vorticity as fluid parcels travel between meander troughs and crests lateral shear changes on the order of 20 30 of f are common as parcels move laterally relative to the jet axis between meander extrema although changes in these two terms are usually of opposite sign and tend to compensate significant layer thickness changes do occur with some parcels exhibiting 30 change

gas washing of oil along a regional transect offshore louisiana abstract gas chromatogram data for 219 oils in a 190 km n s transect offshore louisiana reveal a spatially coherent pattern of compositional change which is caused by gas washing near the louisiana shoreline as much as 91 of the original n alkanes have been removed from the oils the maximum intensity of depletion decreases southward in a nearly regular fashion to nil at the jolliet field 190 km offshore the oils show a parallel change in the maximum carbon number of the removed n alkanes implying that the pressure at which gas washing took place also decreased in the offshore direction the systematic change in maximum extent of depletion crosscuts tectonostratigraphic boundaries as well as oil source provinces models of gas washing suggest that the maximum depth of washing reflects the distribution of deeply buried continuous sands suggesting deep sands may have provided sites for efficient gas oil interaction

origin of the martian dichotomy and tharsis from a giant impact causing massive magmatism the origin of the ancient martian crustal dichotomy and the massive magmatic province of tharsis remains an open problem here we explore numerically a hypothesis for the origin of these two features involving both exogenic and endogenic processes we propose a giant impact event during the late stage of planetary formation as the source of the southern highland crust in a second stage the extraction of excess heat by vigorous mantle convection on the impacted hemisphere leads to massive magmatism forming a distinct tharsis like volcanic region by coupling short term and long term numerical simulations we are able to investigate both the early formation as well as the 4 5 gyr evolution of the martian crust we demonstrate numerically that this exogenic endogenic hypothesis is in agreement with observational data from mars

new perspectives on ancient mars mars was most active during its first billion years the core mantle and crust formed within 50 million years of solar system formation a magnetic dynamo in a convecting fluid core magnetized the crust and the global field shielded a more massive early atmosphere against solar wind stripping the tharsis province became a focus for volcanism deformation and outgassing of water and carbon dioxide in quantities possibly sufficient to induce episodes of climate warming surficial and near surface water contributed to regionally extensive erosion sediment transport and chemical alteration deep hydrothermal circulation accelerated crustal cooling preserved variations in crustal thickness and modified patterns of crustal magnetization

cenozoic uplift exhumation and deformation in the north kuqa depression china as constrained by u th he thermochronometry placing spatial and temporal constraints on deformation uplift and exhumation using thermochronology methods can shed light on cenozoic orogenic evolution in the kuqa foreland new apatite u th he and previously published apatite fission track thermochronology are combined with vitrinite reflectance data and modeling to compile a low temperature multi stage thermal history of tectonic uplift thrust and exhumation a rapid uplift and exhumation event originated in south tianshan during the eocene 46 ma with systematic younging of mean apatite u th he ages from north to south from the margin towards the interior uplift and exhumation south edge of the biyoulebaoguzi anticline in the northern monocline belt at 36 ma with more than 3400 m of stratum denuded subsequently deformation and uplift south edge of the biyoulebaoguzi anticline in the northern monocline belt at 36 ma with more than 3400 m of stratum denuded subsequently deformation and uplift extended to the northern edge of kelasu yiqikelike at 10 ma and then at 6 ma the kanyaken anticline formed and uplifted resulting in 3000 3700 m of overburden erosion the data support sequential southward propagating deformation and uplift inferred exhumation in the north kuqa depression during the cenozoic probably as a result of the collision of india with asia far to the south of the tianshan