comparison of dynamic flow models for the central aleutian and tonga kermadec subduction zones using three dimensional 3 d instantaneous dynamic models of the central aleutian subduction zone we show that a low viscosity low density deltarho 10 kg m3 region in the wedge is needed to match observations of topography the geoid and the stress state in the slab and overriding plate previous models of the tonga kermadec subduction zone also require a low viscosity low density deltarho 20 kg m3 region in the wedge a low viscosity region in the wedge decouples the overriding plate from the downward flow in the wedge caused by the sinking slab reducing the magnitude of negative dynamic topography on the overriding plate and changing the state of stress in the slab from down dip tension to down dip compression if the low viscosity region is large as in the tonga kermadec models then the decoupling may also cause the overriding plate to go into extension these are the first dynamic models to simultaneously match the state of stress in the slab and overriding plate and suggest that the combination of stress state within the slab and overriding plate may provide an additional constraint on the viscosity structure within subduction zones

episodic post rift subsidence of the united states atlantic continental margin sediment thickness paleobathymetry and chronostratigraphy from cost wells offshore from georgia and new jersey indicate periods of rapid subsidence superimposed on the slower thermal subsidence of the continental margin rapid subsidence occurred during the coniacian santonian the eocene and in the cost wells off new jersey since the end of early miocene once the maximum effects of water and sediment loading compaction and thermal cooling are removed the residual vertical movements caused by tectonic and sea level fluctuations can be analyzed because no global sea level change can account for all residual movements we propose that tectonism variously amplified by loading is responsible for the observed episodes of rapid subsidence synchroneity of subsidence with sea floor spreading changes in the north atlantic suggests a unified cause for these events recognition of episodic subsidence may have implications for fault timing petroleum potential and global sea level effects on passive margins

mesozoic aseismic ridges on the farallon plate and southward migration of shallow subduction during the laramide orogeny we propose that an aseismic ridge of late jurassic and early cretaceous age on the farallon plate was subducted beneath the unites states cordillera during the laramide orogeny the relative buoyancy of the aseismic ridge caused shallow subduction which resulted in a magmatic lull 70 40 ma in the existing near trench magmatic arc and laramide uplift and faulting of crystalline basement 1000 1500 km inland from the trench the timing and latitudinal limits of the subduction of the aseismic ridge are similar to the timing and latitudinal limits of both the laramide orogeny and the magmatic lull in the western cordillera because the north northeast trending aseismic ridge was moving northeast relative to the north american plate the point of collision of the aseismic ridge with north america and therefore the locus of shallow subduction migrated southward with time causing the well documented southward migration of the magmatic lull

mesozoic plate motion history below the northeast pacific ocean from seismic images of the subducted farallon slab the high resolution seismic imaging of subducted oceanic slabs1 2 has become a powerful tool for reconstructing palaeogeography3 the images can now be interpreted quantitatively by comparison with models of the general circulation of the earth s mantle4 here we use a three dimensional spherical computer model of mantle convection5 6 to show that seismic images of the subducted farallon plate provide strong evidence for a mesozoic period of low angle subduction under north america such a period of low angle subduction has been invoked independently to explain rocky mountain uplift far inland from the plate boundary during the laramide orogeny7 the computer simulations also allow us to locate the largely unknown kula farallon spreading plate boundary the location of which is important for inferring the trajectories of suspect terrain across the pacific basin8

khasib and tannuma oil sources east baghdad oil field iraq this work presents new insights of the generation quality and migration pathways of the hydrocarbons in the east baghdad oil field the khasib and tannuma formations in east baghdad are considered as oil reservoirs according to their high porosity 15 23 and permeability 20 45 md in carbonate rocks the hydrocarbons are trapped by structural anticline closure trending nw se gas chromatography analysis on these oil reservoirshave shown biomarkers of abundant ranges of n alkanes of less than c22 c17 c21 with c19 and c18 peaks this suggests mainly liquid oil constituents of paraffinic hydrocarbons from marine algal source of restricted palaeoenvironments in the reservoir the low non aromatic c15 peaks are indicative for slight degradation and water washing oil biomarkers of pr ph 0 85 c31 c30 1 0 location in triangle of c27 c29 sterane c28 c29 of 0 6 sterane oleanane of 0 01 and cpi 1 0 indicate an anoxic marine environment with carbonate deposits of upper jurassic to early cretaceous age four miospores seven dinoflagellates and one tasmanite species confirm affinity to the upper most jurassic to lower cretaceous chia gara and ratawi formations the recorded palynomorphs from the khasib and tannuma formations are of light brown color of tai 2 8 3 0 and comparable to the mature palynomorphs that belong to the chia gara and the lower part of ratawi formations the chia gara formation generated oil during upper cretaceous to early palaeogene and accumulated in structural traps of cretaceous age such as the khasib and tannuma reservoirs the chia gara formation generated and expelled high quantities of oil hydrocarbons according to their toc wt of 0 5 8 5 with s2 2 5 18 5 mg hc g rock high hydrogen index of the range 150 450 mg hc g rock good petroleum potential of 4 5 23 5 mg hc g rock mature tai 2 8 3 0 and tmax 428 443c kerogen type ii and palynofacies parameters of up to 100 aom amorphous organic matters this includes algae deposits in a dysoxic anoxic to suboxic anoxic environment alternative plays are discussed according to the migration pathways

localization of the gravity field and the signature of glacial rebound the negative free air gravity anomaly centred on hudson bay canada shows a remarkable correlation with the location of the laurentide ice sheet suggesting that this gravity anomaly is the result of incomplete post glacial rebound this region however is also underlain by higher than average mantle seismic velocities suggesting that the gravity low might result instead from dynamic topography associated with convective downwellings here we analyse the global gravity field as a simultaneous function of geographic location and spectral content we find that the hudson bay gravity low is unique with anomalously high amplitude in the spectral band where the power from the laurentide ice load is greatest and the relaxation times predicted for viable models of viscous relaxation are longest we estimate that about half of the hudson bay gravity anomaly is the result of incomplete post glacial rebound and derive a mantle viscosity model that explains both this gravity signature and the characteristic uplift rates for the central laurentide and fennoscandian regions this model has a jump in viscosity at 670 km depth comparable to that in dynamic models of the geoid highs over subducted slabs but lacks a low viscosity asthenosphere consistent with a higher viscosity in the upper mantle beneath shields than in oceanic regions

optical characteristics of amorphous kerogens and the hydrocarbon generating potential of source rocks abstract four recurring types of amorphous kerogen are optically identified in petroleum source rock samples and are related to hydrocarbon generating potential defined here as geochemically interpreted oil prone or gas prone at this time a genetic classification is not possible the different amorphous kerogen types are distinguished on the basis of microscopic textural differences type a chunky compact masses with mottled network or weak polygonal textures type b very small dense elongate oval or rounded individual grains type c clumpswith granular fragmented or globular textures type d thin platy or rectangular individual grains geochemistry defined oil prone samples generally contain types a and or d separately or combined geochemically defined gas prone samples rarely contain type a and vary in amounts of types b c and or d the samples in this study contain dominantly one of the four types of amorphous kerogen so the geochemical analyses of the whole rock or bulk concentrated kerogen samples are assumed to be analyses of the dominant amorphous type the samples were chemically analyzed by several techniques to determine the oil or gas generating potential the establishment of relations between specific optical and chemical properties of kerogen types can make visual kerogen analyses more meaningful in predicting the hydrocarbon generation potential of source rocks

assessments of oil characterization source affinities and hydrocarbon dynamic of east baghdad oil fields central iraq abstract bulk properties stable carbon isotopes whole crude gas chromatography and oil mode ratios of sterane and terpanes data of analyzed samples of oil from the cretaceous formation reservoirs of east baghdad oil field are from five producing wells eb21 eb68 eb92 eb5 and eb31 from reservoirs of zubair khasib and tanuma formations they are discussed with aids of characters hopane tricyclic terpane isotopic c13 saturate aromatics and pristine phytane diagrams the analized oil have indicated light and medium oil in the zubair reservoirs and heavy oil for the khasib and tannuma oil with about 15 asphalt and porpherine range of 16 7 35 1 ni and 68 117 ppm giant reservoirs the oil source environment and lithology are of marine algal type ii and deltaic type ii iii that are non biodegraded deposited in anoxic environments of carbonate and shale source maturation at the time of the oil generation aided by pristine phytane and tmax equivalent from mdr methyldibenzothiophene ratio diagrams are showing mature with tmax 430 435 c while source age assessment are taken from c13 and the calculated c28 c29 sterane ratio to be of middle and upper jurassic as well as lower cretaceous age that could be correlated mainly with the chia gara and zubair formations as well as the middle jurassic sargelu formation source rocks for oil that are accumulated in the reservoirs of zubair khasib and tannumah formations source rocks are mainly upper jurassic lower cretaceous chia gara formation with contribution from the middle jurassic sargelu and lower cretaceous zubair formations the richness of the source rocks mostly due to shale rich in toc and the type of kerogen is mixed type ii iii and type iii the formations have good petroleum potentiality pp and the om reaches the early of hydrocarbon generation oil and gas are the most probable product and the generated hydrocarbons can be expelled at this level of thermal maturity which is confirmed by petromod software basin modeling to suggest migration and accumulation during the upper cretaceous time hydrocarbon dynamic in east baghdad oil field are performed using seismic section logs and porosities model assessments of the hydrocarbon generation migration paths and accumulation sites as well as places of enriched oil are used to assess reservoir location and suggestion for drilling sites

haskell 1935 revisited the haskell 1935 value of 1021 pa s for mantle viscosity is a classic and enduring constraint on the rheology of the earth s interior we revisit this inference using spherically symmetric self gravitating maxwell viscoelastic earth models our inference is based on both forward and inverse analyses of decay times associated with uplift at two sites considered by haskell angerman river sweden and oslo norway rather than the raw relative sea level rsl data at these sites we demonstrate that predictions of the decay time associated with the angerman river data are insensitive to variations in both the late pleistocene ice load history and the lithospheric thickness of the earth model the predictions at oslo are sensitive to both these inputs and hence decay times at this site provide a remarkably robust constraint on mantle viscosity we derive a constraint on the average viscosity of the mantle of 0 65 1 10 1021 pa s where the average resolved by the data encompasses a region which extends from the base of the lithosphere to a depth of near 1400 km this indicates that many previous analyses which have invoked the haskell value of 1021 pa s as a constraint on the average upper mantle i e above 670 km depth viscosity alone have misinterpreted the resolving power of the inference furthermore our analysis indicates that a number of apparently contradictory inferences of viscosity based on fennoscandian data satisfy the new rigorous interpretation of the haskell constraint finally we demonstrate how the ambiguity in the upper mantle lower mantle viscosity contrast associated with the haskell average may be reduced by invoking decay time constraints estimated from rsl curves in hudson bay a preliminary inversion of decay times at angerman river and richmond gulf in hudson bay suggests a contrast of approximately an order of magnitude between the average viscosities of the upper mantle and the top 1000 km of the lower mantle however a conclusive analysis in this regard must await the determination of consistent decay time estimates for the hudson bay region

variations in the sterane carbon number distributions of marine source rock derived crude oils through geological time abstract the analysis of the sterane data of a large set of crude oils 414 derivedfrom marine carbonate 27 and siliciclastic source rocks 14 where influences of terrestrial or lacustrine derived organic matter can reasonably be excluded shows that there are increases in the relative content of c28 steranes and decreases in the relative content of c29 steranes through geological time there are no consistent variations in the relative content of c27 steranes through time with one major exception a proterozoic oil from oman palaeozolic and older crude oils are thus generally characterised by strong predominances of c29 steranes and low relative concentrations of c28 steranes significantly higher proportions of c28 steranes and lower proportions of c29 steranes occur in oils derived from jurassic and upper cretaceous source rocks these changes through time do not appear to reflect the chemical evolution of the sterols of one particular variety of marine organism the increase in c28 steranes may be related to the increased diversification of phytoplantonic assemblages in the jurassic and cretaceous possible sources of the c28 sterols necessary for the observed changes in crude oil steranes includes diatoms coccolithophores and dinoflagellates although the technique does not give an accurate means of determining the age of the source rock of a crude oil it is possible to distinguish younger crudes derived from the upper cretaceous and tertiary from palaeozoic and older crudes

long term sea level fluctuations driven by ocean basin dynamics earth9s long term sea level history is characterized by widespread continental flooding in the cretaceous period 145 to 65 million years ago followed by gradual regression of inland seas however published estimates of the late cretaceous sea level high differ by half an order of magnitude from 40 to 250 meters above the present level the low estimate is based on the stratigraphy of the new jersey margin by assimilating marine geophysical data into reconstructions of ancient ocean basins we model a late cretaceous sea level that is 170 85 to 270 meters higher than it is today we use a mantle convection model to suggest that new jersey subsided by 105 to 180 meters in the past 70 million years because of north america9s westward passage over the subducted farallon plate this mechanism reconciles new jersey margin based sea level estimates with ocean basin reconstructions

simultaneous inversion of mantle properties and initial conditions using an adjoint of mantle convection through the assimilation of present day mantle seismic structure adjoint methods can be used to constrain the structure of the mantle at earlier times i e mantle initial conditions however the application to geophysical problems is restricted through both the high computational expense from repeated iteration between forward and adjoint models and the need to know mantle properties such as viscosity and the absolute magnitude of temperature or density a priori we propose that an optimal first guess to the initial condition can be obtained through a simple backward integration sbi of the governing equations thus lessening the computational expense given a model with known mantle properties we show that a solution based on an sbi generated first guess has smaller residuals than arbitrary guesses mantle viscosity and the effective rayleigh number are crucial for mantle convection models neither of which is exactly known we place additional constraints on these basic mantle properties when the convection induced dynamic topography on earth s surface is considered within an adjoint inverse method besides assimilating present day seismic structure as a constraint we use dynamic topography and its rate of change in an inverse method that allows simultaneous inversion of the absolute upper and lower mantle viscosities scaling between seismic velocity and thermal anomalies and initial condition the theory is derived from the governing equations of mantle convection and validated by synthetic experiments for both one layer viscosity and two layer viscosity regionally bounded spherical shells for the one layer model at any instant of time the magnitude of dynamic topography is controlled by the temperature scaling while the rate of change of topography is controlled by the absolute value of viscosity for the two layer case the rate of change of topography constrains upper mantle viscosity while the magnitude of dynamic topography determines the temperature scaling lower mantle viscosity when upper mantle lower mantle density anomaly dominates the flow field this two stage scheme minimizes the tradeoff between temperature and lower mantle viscosity for both cases we show that the theory can constrain mantle properties with errors arising through the adjoint recovery of the initial condition for the two layer model this error is manifest as a tradeoff between the temperature scaling and lower mantle viscosity

the ups and downs of north america evaluating the role of mantle dynamic topography since the mesozoic the driving force for transient vertical motions of earth s surface remains an outstanding question a main difficulty lies in the uncertain role of the underlying mantle especially during the geological past here i review previous studies on both observational constraints and physical mechanisms of north american topographic evolution since the mesozoic i first summarize the north american vertical motion history using proxies from structural geology geochronology sedimentary stratigraphy and geomorphology based on which i then discuss the published physical models overall there is a progressive consensus on the contribution of mantle dynamic topography due to buoyancy structures associated with the past subduction at the continental scale a largely west to east migrating deformation pattern suggests an eastward translation of mantle dynamic effects consistent with models involving an eastward subduction and sinking of former farallon slabs since the cretaceous among the existing models the inverse model based on an adjoint algorithm and time dependent data constraints provides the most extensive explanations for the temporal changes of north american topography since the mesozoic at regional scales debates still exist on the predicted surface subsidence and uplift within both the western and eastern united states where discrepancies are likely due to differences in model setup e g mantle dynamic properties and boundary conditions and the amount of time dependent observational constraints toward the development of the next generation predictive geodynamic models new research directions may include 1 development of enhanced data assimilation capabilities 2 exploration of multiscale and multiphysics processes and 3 cross disciplinary code coupling

adjoint tomography of the southern california crust using an inversion strategy based on adjoint methods we developed a three dimensional seismological model of the southern california crust the resulting model involved 16 tomographic iterations which required 6800 wavefield simulations and a total of 0 8 million central processing unit hours the new crustal model reveals strong heterogeneity including local changes of 30 with respect to the initial three dimensional model provided by the southern california earthquake center the model illuminates shallow features such as sedimentary basins and compositional contrasts across faults it also reveals crustal features at depth that aid in the tectonic reconstruction of southern california such as subduction captured oceanic crustal fragments the new model enables more realistic and accurate assessments of seismic hazard

topographic asymmetry of the south atlantic from global models of mantle flow and lithospheric stretching the relief of the south atlantic is characterized by elevated passive continental margins along southern africa and eastern brazil and by the bathymetric asymmetry of the southern oceanic basin where the western flank is much deeper than the eastern flank we investigate the origin of these topographic features in the present and over time since the jurassic with a model of global mantle flow and lithospheric deformation the model progressively assimilates plate kinematics plate boundaries and lithospheric age derived from global tectonic reconstructions with deforming plates and predicts the evolution of mantle temperature continental crustal thickness long wavelength dynamic topography and isostatic topography mantle viscosity and the kinematics of the opening of the south atlantic are adjustable parameters in thirteen model cases model predictions are compared to observables both for the present day and in the past present day predictions are compared to topography mantle tomography and an estimate of residual topography predictions for the past are compared to tectonic subsidence from backstripped borehole data along the south american passive margin and to dynamic uplift as constrained by thermochronology in southern africa comparison between model predictions and observations suggests that the first order features of the topography of the south atlantic are due to long wavelength dynamic topography rather than to asthenospheric processes the uplift of southern africa is best reproduced with a lower mantle that is at least 40 times more viscous than the upper mantle

plumes in a convecting mantle models and observations for individual hotspots the motion of hotspots and the deformation of their underlying plume conduits as calculated within models of global mantle flow are presented a new list of 44 possible hotspots with associated tracks has been compiled for all of them calculations have been performed under consideration of individual age and anomalous mass flux for three different models of plume buoyancy and mantle flow plume source depth has usually been assumed to be the top of d but an alternative source depth at the 670 km discontinuity has also been considered using models of relative plate motions and boundaries hotspot tracks on plates have been calculated and compared with age data ocean floor topography and distribution of volcanics on continents absolute plate motions have been redetermined under consideration of hotspot motion using a new least squares method for the hawaiian and yellowstone hotspots source locations and hotspot motion have been computed for a total of up to 23 different models the results show plume conduits being tilted with source regions at the d moving in the lowermost mantle flow generally toward large scale upwellings under southern africa and the south central pacific hotspot surface motion often represents the horizontal component of midmantle flow which is frequently opposite to plate motion toward ridges and away from subduction zones in particular almost all models tested predict southward motion of the hawaii and kerguelen hotspots and westward motion of the iceland hotspot for models including hotspot motion the agreement between calculated and observed hotspot tracks is frequently about as good as or better than for the fixed hotspot model but sometimes fixed hotspots give the best fit in some cases where the track ends at a subduction zone e g for the bowie hotspot results can give indications about the otherwise unknown age of the hotspot in other cases especially for the tahiti hotspot results suggest an origin shallower than d and in yet other cases particularly east africa the failure of the hotspot models used supports other evidence indicating the presence of comparatively broad upwellings rather than localized plumes

influence of climate change and uplift on colorado plateau paleotemperatures from carbonate clumped isotope thermometry the elevation history of earth s surface is key to understanding the geodynamic processes responsible for the rise of plateaus we investigate the timing of colorado plateau uplift by estimating depositional temperatures of tertiary lake sediments that blanket the plateau interior and adjacent lowlands using carbonate clumped isotope paleothermometry a measure of the temperature dependent enrichment of 13 c 18 o bonds in carbonates comparison of modern and ancient samples deposited near sea level provides an opportunity to quantify the influence of climate and therefore assess the contribution of changes in elevation to the variations of surface temperature on the plateau analysis of modern lake calcite from 350 to 3300 m elevation in the southwestern united states reveals a lake water carbonate temperature lct lapse rate of 4 2 0 6 c km analysis of miocene deposits from 88 to 1900 m elevation in the colorado river drainage suggests that the ancient lct lapse rate was 4 1 0 7 c km and temperatures were 7 7 2 0 c warmer at any one elevation than predicted by the modern trend the inferred cooling is plausible in light of pliocene temperature estimates off the coast of california and the consistency of lapse rates through time supports the interpretation that there has been little or no elevation change for any of the samples since 6 ma together with previous paleorelief estimates from apatite u th he data from the grand canyon our results suggest most or all of the plateau s lithospheric buoyancy was acquired 80 60 ma and do not support explanations that ascribe most plateau uplift to oligocene or younger disposal of either the farallon or north american mantle lithosphere

biomarker signatures of the ediacaran early cambrian origin petroleum from the central sichuan basin south china implications for source rock characteristics abstract hydrocarbons with an ediacaran early cambrian origin from the central sichuan basin south china were investigated for biomarker compositions they were obtained by solvent extraction from the upper ediacaran reservoir core samples which contain abundant solid bitumen the solid bitumen is widely believed to be formed by thermal cracking of early charged oils in the reservoirs which have presumed source rocks of an ediacaran early cambrian age thermal maturity parameters based on phenanthrenes and aromatic steroids suggest a very high thermal maturity for the extracted hydrocarbons which is consistent with the severe maturation undergone by their host reservoirs age diagnostic biomarkers including c30 24 isopropylcholestanes and mid chain monomethyl alkanes support source rocks of an ediacaran early cambrian age the extracted hydrocarbons have very similar biomarker signatures which include high relative abundance of acyclic isoprenoids and pristane phytane 1 and the presence of c26 norcholestanes methyl and ethylsteranes the presence of hopanoids including extended series and methylhopanes indicates contributions from prokaryotic bacteria while the presence of various steroids including regular steranes and methylsteranes indicates contributions from the eukaryotic algae parameters based on tricyclic terpane and hopane dibenzothiophene phenanthrene ratios and c27 diasteranes diasteranes regular steranes ratios suggest a major shale source but a mixed shale carbonate source cannot be completely excluded presence of c30 demethylsteranes low pristane phytane ratios high gammacerane index and presence of 28 30 dinorhopane and 2 methyhopanes indicate that the source rocks were deposited in a stratified anoxic marine depositional environment with enhanced salinity conditions

mantle circulation models with variational data assimilation inferring past mantle flow and structure from plate motion histories and seismic tomography summary mantle convection models require an initial condition some time in the past because this initial condition is unknown for earth we cannot infer the geological evolution of mantle flow from forward mantle convection calculations even for the most recent mesozoic and cenozoic geological history of our planet here we introduce a fluid dynamic inverse problem to constrain unknown mantle flow back in time from seismic tomographic observations of the mantle and reconstructions of past plate motions using variational data assimilation we derive the generalized inverse of mantle convection and explore the initial condition problem in high resolution 3 d spherical mantle circulation models for a time period of 100 myr roughly comparable to half a mantle overturn we present a synthetic modelling experiment to demonstrate that mid cretaceous mantle structure can be inferred accurately from fluid dynamic inverse modelling assuming present day mantle structure is well known even if an initial first guess assumption about the mid cretaceous mantle involved only a simple 1 d radial temperature profile we also demonstrate that convecting present day mantle structure back in time by reversing the time stepping of the energy equation is insufficient to model the mantle structure of the past the difficulty arises because such backward convection calculations ignore thermal diffusion effects and therefore cannot account for the generation of thermal buoyancy in boundary layers as we go back in time inverse mantle convection modelling should make it possible to infer a number of flow parameters from observational constraints of the mantle

flexural subsidence and basement tectonics of the cretaceous western interior basin united states the flexural subsidence history recorded in cenomanian to early campanian 97 to 80 ma strata in the cretaceous u s western interior basin was studied with two dimensional flexural backstripping techniques results indicate that the flexural subsidence resulting from thrust loading was superimposed on epeirogenic subsidence in the foreland basin the flexural component exhibits significant spatial and temporal variations along both the strike and dip relative to the sevier thrust belt the greatest cumulative subsidence occurred in southwestern wyoming and northern utah concurrent subsidence in northwestern montana and southern utah was insignificant temporal trends in subsidence also show a distinct regional pattern from the cenomanian to late turonian 97 to 90 ma subsidence rates were high in utah and much lower in wyoming and montana in contrast during the coniacian and santonian 90 to 85 ma subsidence accelerated rapidly in wyoming increased slightly in montana and decreased in utah we suggest that these spatially and temporally varying subsidence patterns reflect the interplay of several geodynamic factors including 1 temporal and spatial variation in emplacement of the thrust loads 2 segmentation of the basement into adjacent blocks with different rheological properties 3 reactivation of basement fault trends and 4 regional dynamic topographic effects

organic petrological and geochemical appraisal of the upper jurassic naokelekan formation kurdistan iraq abstract upper jurassic naokelekan formation being part of the intercepted strata of the well known miran oilfield in kurdistan region of northern iraq has been previously extensively evaluated as a potential effective source rock mainly by means of organic geochemistry in the present study a total of 60 samples were collected from four naokelekan fm outcrops those of banik galy derash barsarin and dwawa in order to conduct organic petrographical study and organic geochemical assessment including both biomarker and rock eval analyses the average recorded toc content for banik galy derash barsarin and dwawa section is considerable to very high corresponding to values of 27 5 9 0 3 5 and 5 2 wt respectively with the highest values representing the notably organic rich bottom part of the formation known as coal layer according to the petrographical analysis solid migrabitumens comprise the predominant component for all the analysed samples whereas the maceral groups of vitrinite liptinite and or inertinite showcase a minor to trace occurrence in terms of organic geochemistry each of the four sections of interest shows a different biomarker profile nevertheless a number of calculated parameters related to kerogen type and depositional conditions including the isoprenoids n alkanes ratios suggest a mixed terrestrial marine organic matter furthermore as indicated by the ro measurements along with rock eval parameters and the vitrinite reflectance equivalent based on aromatic compounds the maturity level is marginally mature in the cases of banik and galy derash whereas in the cases of dwawa and barsarin sections it is mature to post mature a synthesis of the organic petrography and geochemical appraisal applied in the present study suggests that the reported solid hydrocarbons in naokelekan fm particularly at its basal layers are non indigenous migrated and the precursor organic matter was deposited in a marine environment under oscillating conditions ranging from euxinic to well oxygenated accordingly the organic features of the notably bitumen rich coal bed fit the profile of a reservoir and or migration path

organic geochemistry and petroleum potential of early cretaceous garau formation in central part of lurestan zone northwest of zagros iran abstract deposition of organic rich black shales and dark gray limestones in the berriasian turonian interval has been documented in many parts of the world the early cretaceous garau formation is well exposed in lurestan zone in iran and is composed of organic rich shales and argillaceous limestones the present study focuses on organic matter characterization and source rock potential of the garau formations in central part of lurestan zone a total of 81 core samples from 12 exploratory wells were subjected to detailed geochemical analyses these samples have been investigated to determine the type and origin of the organic matter as well as their petroleum generation potential by using rock eval toc pyrolysis gc and gcms techniques the results showed that toc content ranges from 0 5 to 4 95 percent pi and tmax values are in the range of 0 2 and 0 6 and 437 and 502 c most organic matter is marine in origin with sub ordinary amounts of terrestrial input suggesting kerogen types ii iii and iii measured vitrinite reflectance rrandom values varying between 0 78 and 1 21 indicating that the garau sediments are thermally mature and represent peak to late stage of hydrocarbon generation window hydrocarbon potentiality of this formation is assessed fair to very good capable of generating chiefly gas and some oil biomarker characteristics are used to provide information about source and maturity of organic matter input and depositional environment the relevant data include normal alkane and acyclic isoprenoids distribution of the terpane and sterane aliphatic biomarkers the garau formation is characterized by low pr ph ratio

petroleum generation characteristics of heterogeneous source rock from chia gara formation in the kurdistan region northern iraq as inferred by bulk and quantitative pyrolysis techniques abstract this study is the rst attempt which provides information regarding the bulk and quantitative pyrolysis results of the chia gara formation from the kurdistan region northern iraq ten representative early mature to mature samples from the chia gara formation were investigated for toc contents rock eval pyrolysis pyrolysis gc and bulk kinetic parameters these analyses were used to characterize the petroleum generated during thermal maturation of the chia gara source rock and to clarify the quantity of the organic matter and its effect on the timing of petroleum generation pyrolysis hi data identi ed two organic facies with di erent petroleum generation characteristics type ii iii kerogen with hi values of 250 mg hc g toc and type iii kerogen with hi values bulk kinetic analysis of the chia gara source rock indicates a heterogeneous organic matter assemblage typical of restricted marine environments in general the activation energy distributions reveal relatively broad and high values ranging from 40 to 64 kcal mol with pre exponential factors varying from 2 2835 e 12 sec to 4 0920 e 13 sec the predicted petroleum formation temperature of onset tr 10 temperatures ranges from 110 to 135 c and peak generation temperatures geological tmax between 137 c and 152 c the peak generation temperatures reach a transformation ratio in the range of 42 50 tr thus the chia gara source rock could have generated and expelled significant quantities of petroleum hydrocarbons in the kurdistan of iraq

density of morb eclogite in the upper mantle in order to study the role of eclogite subducted oceanic crust in geodynamics we carried out high pressure experiments on natural mid oceanic ridge basalt morb using a multi anvil apparatus we determined phase relations of morb as a function of pressure and temperature in the temperature range of 1200 1700 c and pressures from 5 to 14 gpa the present results differ from a previous high pressure study of morb earth planet sci lett 77 1986 245 conducted at 1200 c as follows 1 coesite coe or stishovite st appears throughout the studied pressure range and 2 the clinopyroxene garnet cpx gar ratio is smaller particularly in the low pressure range 12 gpa our results show that the calculated bulk density of subducted oceanic crust increases discontinuously at around 9 gpa due to the coesite stishovite phase transition the density is considerably higher than estimates based on the previous experiments at pressures less than 12 gpa thus the role of eclogite in driving subduction should be more important than previously thought on the other hand the density difference between peridotite and eclogite in ascending mantle plumes becomes smaller as temperature increases a result of the increase in clinopyroxene garnet sio2 ratio in eclogite with increasing temperature as eclogite is still denser than peridotite even under subsolidus conditions recycled oceanic crust entrained in ascending plumes must be sustained by the upward flow of ambient peridotite in mantle plumes

comparison of petroleum generation kinetics by isothermal hydrous and nonisothermal open system pyrolysis this study compares kinetic parameters determined by open system pyrolysis and hydrous pyrolysis using aliquots of source rocks containing different kerogen types kinetic parameters derived from these two pyrolysis methods not only differ in the conditions employed and products generated but also in the derivation of the kinetic parameters i e isothermal linear regression and non isothermal nonlinear regression results of this comparative study show that there is no correlation between kinetic parameters derived from hydrous pyrolysis and open system pyrolysis hydrous pyrolysis kinetic parameters determine narrow oil windows that occur over a wide range of temperatures and depths depending in part on the organic sulfur content of the original kerogen conversely open system kinetic parameters determine broad oil windows that show no significant differences with kerogen types or their organic sulfur contents comparisons of the kinetic parameters in a hypothetical thermal burial history 2 5 c my show open system kinetic parameters significantly underestimate the extent and timing of oil generation for type iis kerogen and significantly overestimate the extent and timing of petroleum formation for type i kerogen compared to hydrous pyrolysis kinetic parameters these hypothetical differences determined by the kinetic parameters are supported by natural thermal burial histories for the naokelekan source rock type iis kerogen in the zagros basin of iraq and for the green river formation type i kerogen in the uinta basin of utah differences in extent and timing of oil generation determined by open system pyrolysis and hydrous pyrolysis can be attributed to the former not adequately simulating natural oil generation conditions products and mechanisms

geochemical characterization of source rocks and oils from northern iraq insights from biomarker and stable carbon isotope investigations abstract the origin of oil in the cretaceous tertiary reservoirs of northern iraq remains controversial consequently a detailed geochemical study including rock eval pyrolysis gas chromatography mass spectrometry and carbon isotope analysis was conducted on 65 source rock core and cuttings samples from the najmah chia gara and balambo formations northern iraq twenty four crude oils were also analyzed to provide new insights into source organic matter depositional environment and correlations between crude oils and their probable source rocks bulk geochemical analysis of candidate source rock samples indicates that the najmah formation has more petroleum potential than the chia gara and balambo formations these three candidate source rocks were deposited in slightly different environments and have different organic matter types as determined by screening analysis and biomarker data the najmah formation contains organic rich toc up to 4 2 wt oil prone type i ii kerogen deposited under anoxic conditions the chia gara and balambo formations contain mixed type ii iii and type iii kerogens oil and gas prone deposited under less reducing conditions with more terrigenous organic matter input unfortunately most of the rock samples contain less than 2 5 wt toc which likely excludes them as source rocks but more deeply buried equivalents may contain more organic matter biomarker and stable carbon isotope compositions suggest that cretaceous and miocene reservoired oils from the northern iraq basins although predominantly derived from marine carbonate organic matter constitute five genetically distinct oil families based on age specific biomarkers the oils were probably derived from three active petroleum systems in northern iraq jurassic jurassic early cretaceous and early to mid cretaceous biomarker maturity parameters indicate that most of the oils were generated in the early oil window similar to the extracts

mantle provinces under north america from multifrequency p wave tomography this is a survey of mantle provinces large scale seismic anomalies under north america from the surface down to 1500 1800 km depth the underlying p velocity model was obtained by multifrequency tomography a waveform based method that systematically measures and models the frequency dependence of teleseismic body waves a novel kind of three dimensional rendering technique is used to make the considerable structural complexities under north america accessible in the transition zone and below the north american mantle is dominated by seismically fast provinces which represent distinct subduction episodes of the farallon plate i attempt to date and interpret the various slab fragments by reconciling their present positions with paleotrench locations from plate tectonic reconstructions and with major geologic surface episodes differences in vertical sinking velocity have led to large vertical offsets across adjacent coeval slabs some of the mantle provinces have not been discussed much previously including 1 a seismically slow blanket overlying the oldest farallon subduction along the eastern continental margin 2 a transition zone slab coeval with the laramide orogeny ca 80 60 myr which i discuss in analogy to the stagnant slab subduction style commonly found in the western pacific today 3 the lower mantle root of present day cascadia subduction which may have started out as intraoceanic subduction 4 a lower mantle slab under arizona and new mexico the last material to subduct before strike slip motion developed along the san andreas boundary and 5 two narrow plate tears thousands of kilometers long one of which is the subducted conjugate of the mendocino fracture zone

eroding dynamic topography 1 geological observations of mantle flow driven dynamic topography are numerous especially in the stratigraphy of sedimentary basins on the contrary when it leads to subaerial exposure of rocks dynamic topography must be substantially eroded to leave a noticeable trace in the geological record here we demonstrate that despite its low amplitude and long wavelength and thus very low slopes dynamic topography is efficiently eroded by fluvial erosion providing that drainage is strongly perturbed by the mantle flow driven surface uplift using simple scaling arguments as well as a very efficient surface processes model we show that dynamic topography erodes in direct proportion to its wavelength we demonstrate that the recent deep erosion experienced in the colorado plateau and in central patagonia is likely to be related to the passage of a wave of dynamic topography generated by mantle upwelling

an inversion for radial viscosity structure using seismic tomography using a uniform velocity density scaling we have inverted the seismically inferred 3 dimensional structure of the whole manfie for the radial viscosity structure which best fits the geoid we are able to obtain 60 72 variance reductions for three different s wave tomographic models the resulting viscosity structures are remarkably similar showing a high viscosity lid a low viscosity zone in the transition region and a high viscosity lower manfie a resolution analysis indcates that the viscosity structure in the upper manfie is well resolved by the data however the resolution in the lower manfie is poorer our models axe in general agreement with previous studies except that our inversions prefer a low viscosity layer at 400 670 km as opposed to 100 400 kin

normal mode and free air gravity constraints on lateral variations in velocity and density of earth s mantle with the use of a large collection of free oscillation data and additional constraints imposed by the free air gravity anomaly lateral variations in shear velocity compressional velocity and density within the mantle dynamic topography on the free surface and topography on the 660 km discontinuity and the core mantle boundary were determined the velocity models are consistent with existing models based on travel time and waveform inversions in the lowermost mantle near the core mantle boundary denser than average material is found beneath regions of upwellings centered on the pacific ocean and africa that are characterized by slow shear velocities these anomalies suggest the existence of compositional heterogeneity near the core mantle boundary

complex subduction and small scale convection revealed by body wave tomography of the western united states upper mantle abstract new high resolution p and s wave tomography of the united states upper mantle from the pacific coast to the great plains reveals strong multi scale heterogeneity closely correlated with tectonic and magmatic activity we invert teleseismic travel time residuals from the earthscope transportable array and more than 1700 additional temporary and permanent stations for 3 d velocity perturbations to a depth of 1000 km the inversion uses recent advances in western u s crust models to better isolate the mantle component of travel time residuals and frequency dependent 3 d sensitivity kernels to map travel time residuals measured in multiple frequency bands into velocity structure in addition to separate v p and v s models we jointly invert the two datasets for v p v s perturbations by imposing a smoothness constraint on the ln v s ln v p field the joint inversion helps us identify regions where partial melt is probable the amplitude of v p v s and v p v s variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small scale convection partially molten mantle is inferred beneath yellowstone and the eastern snake river plain srp the salton trough and the clear lake volcanic field the inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern srp and yellowstone despite continuous subduction since the cretaceous the distribution of sub lithospheric high velocity anomalies is dissected similar to other recent studies based on our new tomography models western u s geologic history and plate tectonic reconstructions we infer patchy and incomplete removal of the flat subducting laramide slab and slab tearing associated with eocene accretion in the northwestern u s

late cretaceous subsidence in wyoming quantifying the dynamic component the late cretaceous western interior basin of north america is generally considered to have been a retroarc foreland basin flexural backstripping of the stratigraphic record from 97 2 ma to 73 4 ma along a section perpendicular to the wyoming idaho salient of the sevier belt clearly demonstrates that there were components of subsidence in addition to those driven by the thrust and associated sediment loads the simulation demonstrates that the flexural foredeep is only 180 120 km wide and shows a forebulge located near the subsequent laramide rock springs uplift foredeep strata between the rock springs uplift on the east and the thrust front to the west were mostly involved in the eastward overlapping thrust belt or deeply truncated by inferred rebound after 78 5 ma the difference between the observed decompacted cumulative subsidence and the amount of subsidence that can be explained by simple flexural loading the residual subsidence increases from 800 m in eastern wyoming to 1 8 km near the thrust belt the westward thickening upper cretaceous sediment wedge filled the accommodation space generated by downward tilting to the west of the north american cratonic margin by underplating and mantle flow associated with the shallowly subducted farallon plate

discovery of oil bitumen co existing with solid bitumen in the lower cambrian longwangmiao giant gas reservoir sichuan basin southwestern china implications for hydrocarbon accumulation process abstract natural gas in the recently discovered lower cambrian longwangmiao giant gas reservoir of the sichuan basin in southwestern china has been considered to be from oil cracking and sourced from the underlying lower cambrian qiongzhusi shales similar to the underlying neoproterozoic dengying giant gas reservoir however geological considerations and differences in geochemistry between the longwangmiao and dengying gases bitumens imply different scenarios organic petrology geochemistry and trace element analyses of widely occurring bitumen were conducted to evaluate this hypothesis results show the coexistence of solid bitumen and oil bitumen in this reservoir for the first time which implies a new hydrocarbon accumulation process and associated exploration strategies the two types of bitumen were termed type a solid bitumen and type b oil bitumen which were sourced from the lower cambrian qiongzhusi formation shale and the lower silurian longmaxi shale respectively the type a solid bitumen shows little fluorescence and its color under plane polarized light is almost black this indicates that the type a solid bitumen is likely of high maturity it typically occurs in dissolution pores and accounts for approximately 90 of all bitumen in the reservoir in contrast the type b oil bitumen is brown black in color under plane polarized light and yellow green under fluorescence light indicating that it is mature it occurs in the residual spaces of the solid bitumen and represents approximately 10 of the total bitumen content this suggests that type a bitumen charged the reservoir earlier than did the type b bitumen during the indosinian epoch of the triassic oil generated from the mature qiongzhusi formation migrated into the longwangmiao and dengying reservoirs following this the dengying reservoir reached the temperature threshold for oil cracking since the late cretaceous and it is suggested that oil could have started to crack to gas which is the critical moment for the gas formation however the longwangmiao oils had not been cracked at this stage because the reservoir temperature had not reached the threshold later during the himalayan epoch the gas in the dengying reservoir as well as some kerogen cracking gas from the qiongzhusi shales migrated into the longwangmiao reservoir along faults gas gravity drainage and phase separation took place in the longwangmiao reservoir forming the type a solid bitumen and giant gas accumulation later oil generated from the longmaxi shales migrated into the longwangmiao reservoir forming the type b oil bitumen the case here is an example of a multi stage oil and gas accumulation future exploration targets should include middle upper cambrian reservoir rocks

fracture patterns in the zagros fold and thrust belt kurdistan region of iraq abstract fracture data have been collected in the kurdistan region of iraq which is a poorly accessible and unexplored area of the zagros pre to early folding ne sw striking extensional fractures and nw se striking contractive elements represent the older set affecting the exposed multilayer of the area these latter structures are early syn folding and followed by folding related mesostructural assemblages which include elements striking parallel to the axial trend of major folds longitudinal fractures bedding perpendicular joints and veins and extensional faults belonging to this second fracture set are located in the outer arc of exposed anticlines whilst longitudinal reverse faults locate in the inner arcs consistently these elements are associated with syn folding tangential longitudinal strain the younger two sets are related to e w extension and nne ssw to n s shortening frequently displaying reactivation of the older sets the last shortening event which is described along the entire zagros belt probably relates with the onset of n s compression induced by the northward movement of the arabian plate relative to the eurasian plate in comparison between the inferred palaeostrain directions and the kinematics of recent gps measurements we conclude that the n s compression and the partitioning into nw se trending folds and nw to n trending strike slip faults likely remained unchanged throughout the neogene tectonic history of the investigated area

origin of crude oils from oilfields in the zagros fold belt southern iraq relation to organic matter input and paleoenvironmental conditions crude oil samples from cretaceous and tertiary reservoir sections in the zagros fold belt oil fields southern iraq were investigated using non biomarker and biomarker parameters the results of this study have been used to assess source of organic matter and the genetic link between oils and their potential source rocks in the basin the oils are characterized by high sulphur and trace metal ni v contents and relatively low api gravity values 17 4 22 7 api this indicates that these oils are heavy and generated from a marine source rock containing type ii s kerogen this is supported by their biomarker distributions of normal alkanes regular isoprenoids terpanes and steranes and the bulk carbon isotope compositions of their saturated and aromatic hydrocarbons the oils are characterized by low pr ph ratios 1 high values of the c35 homohopane index and c31 22r c30 hopane ratios relatively high c27 sterane concentrations and the predominance of c29 norhopane these biomarkers suggest that the oils were generated predominantly from a marine carbonate source rock deposited under reducing conditions and containing plankton algal and microorganisms source input the presence of gammacerane also suggests water column stratification during source rock deposition the biomarker characteristics of the oils are consistent with those of the middle jurassic sargelu carbonate as the effective source rock in the basin biomarker maturity data indicate that the oils were generated from early maturity source rocks

middle and upper jurassic hydrocarbon potential of the zagross fold belt north iraq abstract structured organic matters of the palynomorphs of mainly dinoflagellate cysts are used in this study for dating the limestone black shale and marl of the middle jurassic bajocian bathonian sargelu formation upper jurassic upper callovian lower oxfordian naokelekan formation upper jurassic kimeridgian and oxfordian gotnia and barsarine formations and upper jurassic lower cretaceous tithonian beriassian chia gara source rock formations while spore species of cyathidites australis and glechenidites senonicus are used for maturation assessments of this succession materials used for this palynological study are 320 core and cutting samples of twelve oil wells and three outcrops in north iraq terpane and sterane biomarker distributions as well as stable isotope values were determined for oils potential source rock extracts of jurassic lower cretaceous strata to determine valid oil to source rock correlations in north iraq two subfamily carbonate oil types one of middle jurassic age sargelu carbonate rock and the other of mixed upper jurassic cretaceous age chia gara with sargelu sources as well as a different oil family related to triassic marls were identified based on multivariate statistical analysis hca pca middle jurassic subfamily a oils from demir dagh oil field correlate well with rich marginally mature sargelu source rocks in well mk 2 near the city of baiji in contrast subfamily b oils have a greater proportion of c 28 c 29 steranes indicating they were generated from upper jurassic lower cretaceous carbonates such as those at gillabat oil field north of mansuriyah lake oils from gillabat field thus indicate a lower degree of correlation with the sargelu source rocks than do oils from demir dagh field palynofacies assessments are performed for this studied succession by ternary kerogen plots of the phytoclast amorphous organic matters and palynomorphs from the diagram of these plots and maturation analysis it could be assessed that the formations of chia gara and sargelu are both deposited in distal suboxic to anoxic basin and can be correlated with kerogens classified microscopically as type a and type b and chemically as type ii the organic matter comprised principally of brazinophyte algae dinoflagellate cysts spores pollen foraminifera test linings and phytoclasts in all these formations and hence affected with upwelling current these deposit contain up to 18 wt total organic matters that are capable to generate hydrocarbons within mature stage of thermal alteration index tai range in stalplin s scale staplin 1969 of 2 7 3 0 for the chia gara formation and 2 9 3 1 for the sargelu formation case study examples of these oil prone strata are one 7 m 23 ft thick section of the sargelu formation averages 44 2 mg hc g s2 and 439 c t max rock eval pyrolysis analyses and 16 wt toc especially in well mk 2 whereas one 8 m 26 ft thick section of the chia gara and 1 m 3 ft section of naokelekan formations average 44 5 mg hc g s2 and 440 c t max and 14 wt toc especially in well aj 8 one dimension petroleum system models of key wells using ies petromod software can confirm their oil generation capability these hydrocarbon type accumulation sites are illustrated in structural cross sections and maps in north iraq

a record of appalachian denudation in postrift mesozoic and cenozoic sedimentary deposits of the u s middle atlantic continental margin abstract the complex interplay between source terrain uplift basin subsidence paleoclimatic shifts and sea level change left an extensive sedimentary record in the contiguous offshore basins of the u s middle atlantic margin salisbury embayment baltimore canyon trough and hatteras basin isopach maps of 23 postrift lower jurassic to quaternary a allostratigraphic units coupled with a revised stratigraphic framework reveal that tectonism by regulating sediment supply accumulation rate dominated the interplay of forcing mechanisms tectonic pulses are evidenced by abruptly accelerated sediment accumulation marked latitudinal shifts in the location of depocenters and regional changes in lithofacies relatively rapid tectonic subsidence during the early and middle jurassic history of the basins may have enhanced sediment accumulation rates beginning in the late jurassic however subsidence rates decreased significantly though occasional short pulses of subsidence may have effected relative sea level rises sea level change heavily influenced the distribution and redistribution of sediments one they reached the basins and paleoclimate regulated the relative abundance of carbonates and evaporites in the basins we conclude that source terrains of the central appalachian highlands were tectonically uplifted intensely weathered and rapidly eroded three times since the late triassic 1 early to middle jurassic aalenian to callovian 2 mid early cretaceous barremian and 3 late cenozoic middle miocene intervals of tectonic quiescence following these three tectonic pulses provided conditions suitable for the formation of regional erosion surfaces geomorphic features commonly reported to characterize the central appalachian highlands this series of three irregularly spaced tectonic quiescent cycles does not however match the traditional four cycle concept of post triassic appalachian peneplanation

segmentation of the farallon slab abstract recent tomography images reveal a complex 3d mantle structure beneath western united states with feature morphology varying rapidly with depth by assimilating plate motion history paleo age of sea floor and paleo geography of plate boundaries in a 3 d numerical model we simulate the farallon juan de fuca subduction during the past 40 ma we find that the highly segmented upper mantle structure of western u s is a direct result of the farallon subduction we show that the tilted horseshoe shaped fast seismic anomaly beneath nevada and utah at 300 600 km depth range is in fact a segment of curled slab subducted since 15 ma and the shallower linear slab beneath the cascades is younger than 5 ma the distinct morphology between these two parts of the subduction system indicates the strong influence of the fast trench rollback since 20 ma the northward migrating jf pa na triple junction and the toroidal flow around slab edges the observed mantle structures are used to constrain the rheology of the upper mantle through matching the shape depth and location of modeled subducted slab segments the inferred viscosity for the asthenosphere is 5 10 19 pa s and those for the transition zone and lower mantle are 1 5 10 21 pa s and 2 10 22 pa s respectively the slab is found to be about 2 orders of magnitude stronger than the ambient mantle above 410 km depth but of similar order of magnitude viscosity in the transition zone

laramide orogenic influence on late mesozoic cenozoic subsidence history western deep gulf of mexico basin the deep part of the gulf of mexico basin which is underlain by oceanic crust has generally been regarded as a tectonically stable part of the basin since its formation by sea floor spreading and thermal subsidence during the late jurassic early cretaceous subsequent subsidence is believed to have been caused mainly by thermal cooling and sediment loading a backstripping study however has revealed a tectonic event in the western deep basin with tectonic loading effect today one possible explanation is laramide fold thrust loading along the western flank of the basin during 66 40 ma

rejuvenation of rbsr mica ages during shearing on the northwestern margin of the nanga parbat haramosh massif abstract the shear zone bordering the western margin of the nanga parbat haramosh massif nphm separates terranes with distinct but well documented cooling histories this study was undertaken in order to examine isotopic behaviour and if possible to date deformation or cooling within the contact zone itself rb sr muscovite and biotite ages and petrographic descriptions are presented for a range of metamorphic lithologies and granite sheets which have been affected to varying degrees by greenschist facies extensional crenulation this deformation has been kinematically linked to the differential uplift of the nphm ages for metamorphic lithologies and granite sheets are similar and are considered together determined muscovite ages mostly lie in the range 9 22 ma whilst the majority of biotite ages lie in the range 2 11 ma mica ages from the shear zone are significantly more variable than ages determined for unsheared lithologies from outside the shear zone even closely spaced samples yield variable mica ages and in some cases there is a significant difference between muscovite and biotite ages determined for the same sample muscovite ages yield a bimodal distribution with groups of ages in the range 17 21 ma and 8 12 ma the older group is thought to reflect the original purely thermostatic cooling of lithologies through the muscovite closure temperature prior to formation of the nphm structure in contrast the younger group of muscovite ages reflects variable degrees of isotopic resetting during shearing at temperatures below the muscovite closure temperature associated with the relative uplift of the nphm the majority of biotite ages also reflect partial to complete resetting during shearing however biotite samples from structurally competent amphibolite horizons preserve older 16 ma ages indicating that there was locally a strong lithological or fluid control on the degree of resetting the flanks of the nphm cooled through the muscovite closure temperature significantly earlier than the deeper structural levels presently exposed within the core of the nphm greenschist grade deformation was occurring within the shear zone at the same time that deeper levels of indian crust were passing through the muscovite closure temperature suggesting that the shearing reflects the relatively passive extensional movement of rocks sliding off the flank of the actively growing nanga parbat structure the results highlight the isotopic complexities of long lived shear zones present in orogenic belts but show that useful radiometric age information may be obtained if the regional tectonic history is well understood and if the samples for dating come from well constrained structural settings

biomarker signatures from neoproterozoic early cambrian oil western india abstract the neoproterozoic early cambrian time is the cradle of multicellular eukaryotic life and thereafter metazoan life started populating the planet biomarkers which record the chronicles of biotic events on earth have been investigated using gas chromatography mass spectrometry from a suite of nine oil samples extracted from oil stained sediments and a crude oil of neoproterozoic early cambrian age from bikaner nagaur basin western india the biomarker distribution is characterized by high concentration of both tricyclic terpanes and pentacyclic hopanes the predominance of c 29 regular sterane over c 27 and c 28 steranes indicates green algal contribution and may imply the prominence and diversification of the same algal group the low pristane phytane ratio and occurrence of substantial quantity of gammacerane monomethyl 2 methyl 2 2 4 8 trimethyltridecyl chroman and 1 1 7 8 tetramethyl 1 2 3 4 tetrahydrophenanthrene are indicative of stratified water column as well as anoxic and enhanced salinity condition of the environment of deposition this paper reports for the first time the presence of methyltrimethyltridecyl chromans mttcs from neoproterozoic early cambrian time putative c 19 norsteranes probably indicative of sponge input are recorded in the studied samples and also reported from other infracambrian oils and sediments normal alkanes n c 17 and n c 18 and isoprenoids pristane and phytane are highly depleted in 13 c the overall biomarker distribution and carbon isotope data of oils from bikaner nagaur basin show similarities with those of other infracambrian oils like huqf oils from oman and baykit high oils from eastern siberia

the distribution and relationship between palynomorphs in aerosols and deep sea sediments off the coast of northwest africa abstract palynomorphs in eolian dust and bottom sediments from the northeastern atlantic off the coast of northwest africa were studied primary examination was for terrestrial plant detritus pollen spores opal phytoliths and freshwater diatoms but observations about marine entities were also made three distinct geographic palynofloras were detected a northern palynoflora consisting of pollen derived primarily from the mediterranean basin a central or saharan palynoflora characterized by pollen derived from desert plants and a southern tropical equatorial palynoflora composed of pollen derived from tropical plants but including up to 50 allochthonous pollen from the drier interior of west africa palynomorph distributions are related closely to both source vegetation and to atmospheric and oceanic transport mechanisms and in many cases distribution patterns in bottom sediments and aerosols are complementary the quantity of pollen and spores per gram of bottom sediment ranges from greater than 2000 off the saharan coast in mauritania to less than 50 in deep ocean basins pollen and spores in mediterranean aerosols may exceed 40 per cubic meter of air during the summer and range between 4 to 6 for tropical aerosols during the winter tropical aerosols and bottom sediments contained the greatest abundance of fungal spores the distribution of opal phytoliths and freshwater diatoms in both aerosols and bottom sediments indicates that dust storms are a major transporting agent for these entities from the interior of west africa to the gulf of guinea these storms are also an important agent for the transport of pollen to the tropical atmosphere distances of transport may exceed 5000 km for mediterranean pollen and even greater distances for palynomorphs in eolian dust from interior west africa the abundance of dinoflagellates and microforaminifera in bottom sediments is directly related to the area of upwelling off the west african coast

mass balanced paleogeographic reconstructions paleogeography paleotopography and paleobathymetry of a closed erosion depositional system can be reconstructed by restoring sedimentary masses to elevated surfaces in a drainage basin based on the inverse of present erosion equations and adjusting for isostasy sea level changes sediment compaction and thermal subsidence the erosion deposition history of the northwestern gulf of mexico margin and its western central north american source area during the cenozoic is used to explore the sensitivity of mass balance reconstructions and changes in assumptions concerning erosion rate parameters and sea level

intra oceanic subduction shaped the assembly of cordilleran north america the western quarter of north america consists of accreted terranes crustal blocks added over the past 200 million years but the reason for this is unclear the widely accepted explanation posits that the oceanic farallon plate acted as a conveyor belt sweeping terranes into the continental margin while subducting under it here we show that this hypothesis which fails to explain many terrane complexities is also inconsistent with new tomographic images of lower mantle slabs and with their locations relative to plate reconstructions we offer a reinterpretation of north american palaeogeography and test it quantitatively collision events are clearly recorded by slab geometry and can be time calibrated and reconciled with plate reconstructions and surface geology the seas west of cretaceous north america must have resembled today s western pacific strung with island arcs all proto pacific plates initially subducted into almost stationary intra oceanic trenches and accumulated below as massive vertical slab walls above the slabs long lived volcanic archipelagos and subduction complexes grew crustal accretion occurred when north america overrode the archipelagos causing major episodes of cordilleran mountain building a new explanation for the origin of the accreted terranes that form the mountainous cordillera of western north america is proposed and tested stationary intra oceanic subduction deposited massive slab walls in the mantle and grew volcanic archipelagos at the surface which were overridden by and accreted to north america during cretaceous times the generally accepted account of how the geology of western north america developed involves the oceanic farallon plate acting as a conveyor belt sweeping crustal terranes onto the continental margin as it moves beneath the north american plate this model fails to explain many aspects of this accreted terrane and here karin sigloch and mitchell mihalynuk present an alternative model based on new tomographic images of lower mantle slabs and modelling of deep mantle structure in the region they propose that during the cretaceous period there were a series of island arcs west of north america beneath which proto pacific plates subducted into almost stationary trenches long lived volcanic archipelagos and subduction complexes grew above the slabs and then accreted onto north america as it overrode the archipelagos causing episodes of cordilleran mountain building

charging of elk hills reservoirs as determined by oil geochemistry crude oils from miocene and pliocene reservoirs from the elk hills field in california s san joaquin basin were analyzed for stable carbon isotopes and biomarkers cluster analysis of geochemical variables defines five principal oil families all derived from different organic rich facies of the miocene monterey formation carbon isotope analysis indicates no contribution from the basin s other major source rock the eocene kreyenhagen formation oil families show a strong correspondence to stratigraphic intervals oils from pre monterey reservoirs were probably generated from the lowermost organic rich facies of the monterey and are the most thermally mature upper miocene stevens zone turbidite reservoirs contain oils of various thermal maturity stages but mature light ends are abundant and are likely generated from the floors of the adjacent subbasins located north and south of elk hills the relatively minor presence of low thermal maturity biomarkers that are typically characteristic of monterey oils may indicate that stevens traps did not form until after the source intervals were at a higher level of thermal maturity all oils in stevens porcelanite reservoirs contain a higher concentration of low maturity biomarkers which may indicate derivation from more localized areas on the flanks of the elk hills anticlines the shallow pliocene oils have suffered biodegradation to different degrees and the lowest api gravities occur on the flanks of the anticline the carbon isotopic composition of these oils suggests yet another monterey source facies that charged the pliocene reservoirs and is not simply the result of vertical leakage from the older miocene reservoirs john e zumberge has been vice president and cofounder of geomark research in houston since 1991 he was manager of geochemical and geological research for cities service occidental general manager for ruska laboratories and director of geochemical services for core laboratories he has global experience in petroleum geochemistry focusing on crude oil biomarkers he obtained a b s degree in chemistry from the university of michigan and a ph d in organic geochemistry from the university of arizona judy a russell is a senior geological advisor for occidental and works on worldwide exploration and production with an emphasis on the integration of geochemistry she has extensive experience in many of occidental s projects in latin america far east the united states and the middle east she has a b s degree from phillips university and an m s degree from the new mexico institute of mining and technology stephen anthony reid is a senior geological advisor for occidental and works on international exploration with an emphasis on deep water projects he has extensive experience in california s san joaquin basin including the elk hills field where he conducted studies on stevens turbidite and monterey porcelanite reservoirs he has b s and m s degrees from california state university northridge

the dynamics of plate tectonics and mantle flow from local to global scales plate tectonics is regulated by driving and resisting forces concentrated at plate boundaries but observationally constrained high resolution models of global mantle flow remain a computational challenge we capitalized on advances in adaptive mesh refinement algorithms on parallel computers to simulate global mantle flow by incorporating plate motions with individual plate margins resolved down to a scale of 1 kilometer back arc extension and slab rollback are emergent consequences of slab descent in the upper mantle cold thermal anomalies within the lower mantle couple into oceanic plates through narrow high viscosity slabs altering the velocity of oceanic plates viscous dissipation within the bending lithosphere at trenches amounts to 5 to 20 of the total dissipation through the entire lithosphere and mantle

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

absolute plate motions since 130 ma constrained by subduction zone kinematics abstract the absolute motions of the lithospheric plates relative to the earth s deep interior are commonly constrained using observations from paleomagnetism and age progressive seamount trails in contrast an absolute plate motion apm model linking surface plate motions to subducted slab remnants mapped from seismic tomography has recently been proposed absolute plate motion models or reference frames derived using different methodologies different subsets of hotspots or differing assumptions of hotspot motion have contrasting implications for parameters that describe the long term state of the plate mantle system such as the balance between advance and retreat of subduction zones plate velocities and net lithospheric rotation previous studies of contemporary plate motions have used subduction zone kinematics as a constraint on the most likely apm model here we use a relative plate motion model to compute these values for the last 130 myr for a range of alternative reference frames and quantitatively compare the results we find that hotspot and tomographic slab remnant reference frames yield similar results for the last 70 myr for the 130 70 ma period where hotspot reference frames are less well constrained these models yield a much more dispersed distribution of slab advance and retreat velocities by contrast plate motions calculated using the slab remnant reference frame or using a reference frame designed to minimise net rotation yield more consistent subduction zone kinematics for times older than 70 ma introducing the global optimisation of trench migration characteristics as a key criterion in the construction of apm models forms the foundation of a new method of constraining apms and in particular paleolongitude in deep geological time

constraining cretaceous subduction polarity in eastern pacific from seismic tomography and geodynamic modeling interpretation of recent mantle seismic images below the america ignited a debate on the cretaceous subduction polarity in the eastern pacific ocean the traditional view is that the massive vertical slab wall under eastern north america resulted from an eastward farallon subduction an alternative interpretation attributes this prominent seismic structure to a westward subduction of the north american plate against a stationary intraoceanic trench here i design quantitative subduction models to test these two scenarios using their implied plate kinematics as velocity boundary conditions modeling results suggest that the westward subduction scenario could not produce enough slab volume as seismic images reveal as is due to the overall slow subduction rate 2 5 cm yr the results favor the continuous eastward farallon subduction scenario which with an average convergence rate of 10 cm yr prior to the eocene can properly generate both the volume and the geometry of the imaged lower mantle slab pile the eastward subduction model is also consistent with most cretaceous geological records along the west coast of north america

solid bitumen bituminite and thermal maturity of the upper jurassic lower cretaceous chia gara formation kirkuk oil field zagros fold belt kurdistan iraq abstract the upper jurassic lower cretaceous chia gara formation a proven marine source rock characterised by predominance of amorphous organic matter aom is known from few outcrops in ne iraq and a number of exploration wells in the zagros fold belt this study presents comprehensive organic petrographic and mineralogical investigations of the dispersed organic matter and rock forming minerals of the tithonian berriasian chia gara formation intersected by the kirkuk well k 109 drilled in the giant kirkuk oil field ne iraq the organic petrographic analyses including incident light microscopy and confocal laser scanning microscopy clsm were applied to carbonate source rocks with low to moderate organic carbon contents our data demonstrate the predominance of bituminite and solid bitumen among the examined dispersed organic matter followed by oil exudates and oil globule inclusions as well as the presence of minor contributions of alginite i e tasmanites and leiospheridia liptodetrinite and vitrinite based on optical characteristics and clsm spectral fluorescence measurements examinations of bituminite point towards the presence of bituminite type iii the bituminite type iii encloses dark orange fluorescent amorphous components suggesting a genetic relationship either with liptodetrinite and or with solid bitumen the qualitative assessment of solid bitumen reveals several dissimilar forms identified in the analysed samples the photometric and spectral fluorescence measurements point to distinct reflectance values ranging between 0 19 and 0 43 sbrr and lambda max values extending from 515 nm to 590 nm respectively the corresponding results of the solid bitumen exhibit 1 a lack of a relationship with depth and 2 suggest its poor property as a reliable maturity indicator for the examined samples the presence of oil exudates oil globule inclusions and their association with solid bitumen suggests a genetic relation with the examined residues of liquid hydrocarbon phase the performed thermal maturities assessments of dispersed organic matter point towards early mature to peak mature oil window with vitrinite reflectance values ranging between 0 52 vrr at 2801 m to 0 83 vrr at 3089 m depth mineralogical analyses revealed the predominance of calcite over dolomite in the examined limestone to marl lithofacies e g clear rimmed dolomite with localised intensive dolomitisation in the phyllosilicate rich matrix predominance of calcite and siderite is observed followed by zinc blende and pyrite the composition of organic matter in the examined chia gara formation with predominance of bituminite and solid bitumen makes the major marine source rock of iraq s cretaceous petroleum systems distinctive in terms of hydrocarbon generation and migration at an oil window stage as it provides suitable explanations for its generally low hydrogen index hi with only minor terrestrial input in addition the results presented herein can be helpful for future evaluation of thermal maturities and characterisation of optical appearances of dispersed organic matter in carbonates by incident light microscopy and especially by clsm during hydrocarbon prospecting

stable carbon isotope compositions of crude oils application to source depositional environments and petroleum alteration the carbon isotopic composition of the c15 saturate and aromatic hydrocarbon fractions and the gas chromatographs of the c15 saturate fraction of 339 oils were geochemically and statistically evaluated results of this study show that the average isotopic compositions of the c15 aromatic hydrocarbon fractions of oils sourced from terrigenous or marine organic matter are identical the average isotopic composition of the c15 saturate fraction of oils derived from terrigenous organic sources is slightly more negative 0 9 pmil than the average for marine oils but this difference is insufficient to be used as a reliable source indicator reports in the literature stating that marine oils are isotopically more positive than te rigenous oils and that the difference can be utilized to distinguish between the two are therefore not supported by this study the isotopic differences between oils sourced from terrigenous and marine organic matter do not manifest themselves in the range of absolute values of one oil fraction but rather in the isotopic relationship between the saturate and aromatic hydrocarbon fractions the isotopic relationship is the following oils of terrigenous organic source dgr13caro 1 12 dgr13csat 5 45 oils of marine organic source dgr13caro 1 10 dgr13csat 3 75 the difference between the two equations was evaluated statistically and a statistical parameter cv the canonical variable cv 2 53 dgr13csat 2 22 dgr13caro 11 65 was used to distinguish between marine and terrigenous oils cv values larger than 0 47 indicate predominantly a terrigenous organic source for the oil whereas cv values smaller than 0 47 indicate mostly a marine organic source thermal maturity differences in oils and isotopic variations in the source beds can cause as much as 2 pmil variations in the isotopic composition of one family of oils finally stable carbon isotopes can in some situations help correlate biodegraded oils to their nondegraded counterparts

reconstructing earth history in three dimensions an inverse model elucidates the connection between plate tectonics at earth s surface and the dynamics of the underlying mantle

p and s wave tomography of the mantle beneath the united states mantle seismic structure beneath the united states spanning from the active western plate margin to the passive eastern margin was imaged with teleseismic p and s wave traveltime tomography including usarray data up to may 2014 to mitigate artifacts from crustal structure 5 40 s rayleigh wave phase velocities were used to create a 3 d starting model major features of the final p and s models include two distinct low velocity anomalies at depths of 60 300 km beneath the central and northern appalachians and passive margin the central appalachian low velocity anomaly coincides with eocene basaltic magmatism and the northern anomaly is located along the cretaceous track of the great meteor hot spot at depths of 300 700 km beneath the central and eastern u s large high velocity anomalies are inferred to be remnants of the farallon slab that subducted prior to 40 ma during the laramide orogeny

an uplift history of the colorado plateau and its surroundings from inverse modeling of longitudinal river profiles 1 it is generally agreed that a region encompassing the colorado plateau has been uplifted by sub crustal processes admittance calculations tomographic studies and receiver function analyses suggest that dynamic support is generated by some combination of convective upwelling and lithospheric thickness changes notwithstanding advances in our understanding of present day setting uplift rate histories are poorly constrained and debated an improved history will aid discrimination between proposed models here we show that a regional uplift rate history can be obtained by inverting longitudinal river profiles we assume that the shape of a river profile is controlled by uplift rate and moderated by erosion in our model uplift rate is allowed to vary smoothly as a function of space and time upstream drainage area is invariant with time simultaneous inversion of river profiles from the colorado rio grande columbia and mississippi catchments shows that three phases of regional uplift occurred the first phase occurred between 80 and 50 myrs when 1 km of uplift was generated at a rate of 0 03 mm yr a second phase occurred between 35 and 15 myrs when 1 5 km of uplift was generated at a faster rate of 0 06 mm yr a final phase of uplift commenced 5 myrs ago these distinct phases of late cretaceous and oligocene uplift are corroborated by stratigraphic considerations by thermochronometric data and by stratigraphic evidence of periodic clastic efflux delivered into the gulf of mexico an episodic uplift history is consistent with staged removal of thick lithospheric mantle beneath a large region which is currently centered on yellowstone

geochemical characterisation of tithonian berriasian chia gara organic rich rocks in northern iraq with an emphasis on organic matter enrichment and the relationship to the bioproductivity and anoxia conditions abstract tithonian berriasian chia gara organic rich rocks located in kurdistan northern iraq were analysed based on inorganic and organic geochemistry to define the origin type of organic matter and the influencing factors of organic matter om enrichment including the om input and preservation and their relationship to the paleoenvironment conditions the 13 c org values of the chia gara rocks range from 29 99 to 26 93 with average values of approximately 28 8 indicating that the organic matter has a predominantly marine origin enhanced biological productivity within the photic zone of the water column during deposition of the chia gara fm is indicated by consistently high organic carbon content in most samples average 3 wt as well as by bulk geochemical and biomarker characteristics major contributions by aquatic algae and microorganisms with a minor terrigenous organic matter contribution are indicated by the n alkane distribution patterns and the composition of the hopanoids steroids and tricyclic terpenoids strongly reducing bottom water is indicated by low pristane phytane ratios homohopane distribution patterns and the redox sensitive trace elements geochemistry salinity stratification with alkaline bottom waters is indicated by high sr ba ratios and the presence of gammacerane in the analysed chia gara samples these data indicate that om enrichment in the tithonian berriasian chia gara rocks results from the combination of enhanced biological productivity and salinity stratification with anoxic bottom water conditions which favoured om preservation

apatite 4he 3he and u th he evidence for an ancient grand canyon the grand canyon is one of the most dramatic features on earth yet when and why it was carved have been controversial topics for more than 150 years here we present apatite 4he 3he thermochronometry data from the grand canyon basement that tightly constrain the near surface cooling history associated with canyon incision 4he 3he spectra for eastern grand canyon apatites of differing he date radiation damage and u th zonation yield a self consistent cooling history that substantially validates the he diffusion kinetic model applied here similar data for the western grand canyon provide evidence that it was excavated to within a few hundred meters of modern depths by 70 million years ago ma in contrast to the conventional model in which the entire canyon was carved since 5 to 6 ma

the role of oceanic plateau subduction in the laramide orogeny the cause of the laramide phase of mountain building remains uncertain conceptual models implicate the subduction of either ocean ridges or conjugates of the buoyant hess or shatsky oceanic plateaux independent verification of these models has remained elusive because the putative ridges or plateaux are no longer at the earth s surface inverse convection models have identified two prominent seismic anomalies on the recovered farallon plate here we combine inverse convection models with reconstructions of plate motions to show that these seismic anomalies coincide palaeogeographically with the restored positions of the shatsky and hess conjugate plateaux as they subducted beneath north america specifically the distribution of laramide crustal shortening events tracked the passage of the shatsky conjugate beneath north america whereas the effects of the hess conjugate subduction were restricted to the northern mexico foreland belt we propose that continued subduction caused the oceanic crust to undergo the basalt eclogite phase transformation during which the shatsky conjugate lost its extra buoyancy and was effectively removed increases in slab density and coupling between the overriding and subducting plates initially dragged the surface downward followed by regional scale surface rebound we conclude that laramide uplift resulted from the removal rather than emplacement of the shatsky conjugate

three dimensional numerical models of the influence of a buoyant oceanic plateau on subduction zones abstract we have investigated potential effects on a subduction zone during oceanic plateau trench collision using three dimensional numerical models an oceanic plateau of varied density was embedded in the slab which was pinned at one end the plateaus strongly influenced the shape of the trench for a plateau with a higher density the trench retreat rate was reduced in the region surrounding the plateau and the plateau subducted along with the slab for lower density plateaus the trench in the region of the plateau advanced and the plateau compressed and resisted subduction spreading laterally along the trench with a weaker slab rheology the arcuate shape of the trench towards the free end of the trench was enhanced beneath the most buoyant plateaus a tear formed in the subducted portion of the slab soon after the slab tip reached the top of the lower mantle we compare the model results with a region in the northwest pacific where the ogasawara plateau meets the trench of the izu bonin mariana subduction zone

subsidence in intracontinental basins due to dynamic topography the origin of anomalous tectonic subsidence ats of large intracontinental basins long after their most recent phase of extension and last thermal perturbation is the subject of a long standing debate we show that deep earth processes may contribute to the subsidence of these tectonically stable basins by analysing the tertiary mantle convection driven topography history of a global set of more than 220 intracontinental basins integrated into a plate kinematic framework most basins are affected by increasing negative dynamic topography over the last 70 myr due to the motion of many continents away from large mantle upwellings and towards downwellings previously located along the perimeter of the supercontinent pangaea during continental dispersal increasing negative dynamic topography causes dynamic subsidence of the basins creating additional accommodation space we utilise a parameter from a global crustal analysis of intraplate basins termed anomalous tectonic subsidence to quantify sediment accumulation not related to crustal stretching we propose that dynamic subsidence due to plate motions relative to the underlying mantle as well as variations in the large scale convection patterns can significantly contribute to the creation and destruction of accommodation space in intraplate basins