extension subsidence and thermal evolution of the los angeles basin a two dimensional model abstract the los angeles basin is a narrow deep basin containing large oil fields its formation began in the middle miocene and its subsidence has been extraordinarily rapid we suggest that although the basin is in a predominantly strike slip tectonic environment the subsidence is largely a result of crustal extension simple one dimensional extension models ignoring the lateral flow of heat are inadequate to explain the rapidity of the subsidence we use a two dimensional numerical method to model the thermal evolution of the basin based on our model studies the central southwest northeast and palos verdes blocks are inferred to have been extended by 178 69 20 and 27 respectively the average crustal extension for the whole basin is 66 the lateral flow of heat out of the central block into adjacent cooler blocks accounts for its rapid subsidence during the early stage the deepest sediments are predicted to have been heated to about 210 c hydrocarbon maturity calculations indicate that the upper miocene sediments reached oil maturity a little over 4 m y b p no sediment is predicted to have achieved natural gas maturity

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

deep hydrocarbons in the northwestern junggar basin nw china geochemistry origin and implications for the oil vs gas generation potential of post mature saline lacustrine source rocks abstract deep 4500 m sedimentary units in petroliferous basins are a frontier field for petroleum exploration and exploitation worldwide this study focuses on the northwestern junggar basin of nw china and presents the results of a comprehensive geochemical analysis of the source rocks and hydrocarbons hosted by deep reservoirs in this region including data obtained from rock eval pyrolysis carbon isotope biomarker and light hydrocarbon analyses these data provide insights into the exploration potential of deep hydrocarbons in the study area and in other areas with similar geologic settings i e saline lacustrine petroleum systems results indicate that the deep hydrocarbons in the northwestern junggar basin are dominated by oil with both the oil and natural gas being derived mainly from the lower permian fengcheng formation this formation was deposited in a heterogeneous saline lacustrine environment which led to the generation of geochemically diverse oil compositions consequently fengcheng derived oils can be divided into subcategories based on source rock lithology i e hypersaline alkaline muddy dolomites saline dolomitic mudstones and brackish mudstones a small number of deep oils in the study area were derived from the carboniferous lower permian jiamuhe formation the deep hydrocarbons in the study area generally accumulated in near source environments indicating that prospective areas are present in the form of fengcheng derived oils in the depocenter of the mahu sag especially oils derived from hypersaline alkaline muddy dolomites and saline dolomitic mudstones these source rocks are likely to have generated oil even under the highly mature conditions associated with deep burial this might explain why discoveries within saline lacustrine basins are generally dominated by oil implying that these conditions might also exist within the deeper parts of saline lacustrine basins worldwide

lateral velocity variations in southern california i results for the upper crust from pg waves the plate boundary and major crustal blocks in southern california are imaged by a tomographic backprojection of the pg first arrivals recorded by the southern california array the method formulated specifically for local earthquake arrival times is a fast iterative alternative to direct least squares techniques with it we solve the combined problem of determining refractor velocity perturbations and source and station delays resolution and variance are found empirically by using synthetic examples a map showing lateral velocity variations at a depth of approximately 10 km is presented the results show a strong correlation with surface tectonic features clear velocity contrasts exist across the san andreas the san jacinto and the garlock faults the mojave region has the slowest velocities while the peninsula ranges have the highest the san jacinto block has velocities intermediate between mojave and peninsula range velocities and also has early station delays this may indicate that the san jacinto block has overridden mojave material on a shallow detachment surface no velocity variations are found associated with the transverse ranges which we interpret to mean that the surface batholithic rocks in this area do not extend to pg depths

lithologic control of the depth of earthquakes in southern california the depth distribution of southern california earthquakes indicates that areas underlain by schist basement rocks have a shallower 4 to 10 kilometers maximum depth of earthquakes than do areas with other types of basement rocks the predominant minerals in the schists become plastic at lower temperatures and thus at shallower depths than the minerals in the other basement rocks the lateral variations in lithology will control the depth extent and thus the magnitudes of potential future earthquakes these depths can be determined from the depth of the current background seismicity

trapping styles and associated hydrocarbon potential in the norwegian north sea exploration has been active in the norwegian north sea for almost 20 years resulting in recoverable reserves that are estimated at 1 5 billion cu m 9 5 billion bbl of oil and 2 5 trillion cu m 88 2 trillion cu ft of gas the 25 fields with reserves of more than 100 million bbl of oil equivalent each together account for 98 of the total recoverable reserves seven fields are giant with more than 1 billion bbl each the distribution of oil and gas fields is intimately linked with the mesozoic rift system of the viking and central grabens that in turn controlled the distribution and maturation of the upper jurassic source rock interval in the norwegian north sea hydrocarbon discoveries to date occur in four forms of traps structural unconformity salt supported compression related and stratigraphic traps end page 585

flow simulation with reactive transport applied to carbonate rock diagenesis abstract significant volumes of the known hydrocarbon reserves are found in carbonate rocks many of these dolomitized the spatial distribution of diagenesis on these rocks is one of the main challenges in oil reservoir modeling reactive transport models can be a powerful tool to understand the active diagenetic processes and their effects on the quality of these reservoirs in this study it was used for the first time the cmg gem simulator to model diagenetic evolution of a carbonate sequence subjected to compaction driven and geothermal flow in a simulated period of 200 thousand years it was simulated carbonate cementation dolomitization and dissolution with and without presence of faults among the analyzed variables the volume of circulating fluid was the most important factor for both mechanisms flow simulated velocities obtained had magnitudes smaller than 10 6 m day diagenesis was insignificant for these low speeds at simulated time interval only dolomitized facies presented relevant diagenesis in form of calcite dissolution and dolomite precipitation simulations with flow rates of 1 m day revealed a considerable increase in observed diagenesis especially in carbonate cementation and in porosity enhancement diagenesis was more pronounced in more permeable sediments highlighting the role of fluid flow in diagenetic reactions relative dissolution was greatly reduced during simulations performed with absence of dolomite and dolomitization reactions the presence of faults strongly influences spatial distribution of diagenesis especially relatively to dissolution more permeable facies were more dissolved near fault decreasing with increasing distance low permeability facies as mudstones are not dissolved even near fault spatial distribution of diagenesis would then be dependent mainly on the quality of original pore structure of fault presence and mineral composition of rock

relationship between petroleum composition and depositional environment of petroleum source rocks crude oils of nonmarine source can be distinguished from those of marine shale source and from oils originating in marine carbonate sequences by using a battery of geochemical parameters as demonstrated with a sample suite of nearly 40 oils a novel parameter based on the presence of c30 steranes in the oil was found to be a definitive indication of a contribution to the source from marine derived organic matter a second novel parameter based on monoaromatized steroid distributions was effective in helping to distinguish nonmarine from marine crudes and can be used to gauge relative amounts of higher plant input to oils within a given basin sterane distributions were similarly useful for detecting higher plant input but were less effective than monoaromatize steroid distributions for making marine versus nonmarine distinctions concentrations of high molecular weight paraffin can also be effective nonmarine indicators but are influenced by maturation and biodegradation processes certain algal derived nonmarine oils may show little high molecular weight paraffin response oils from carbonate sources with a few exceptions can be distinguished by having low pristane phytane ratios low carbon preference indexes and high sulfur contents gammacerane indexes and carbon isotope ratios of the whole crude are not effective in distinguishing these types of environmental differences on a global basis

deep seismic reflectors in the campos basin offshore brazil summary some deep crustal features underlying the campos basin are best recognized in a few reflection seismic sections that have been reprocessed recently to 10 s two way traveltime a prominent climbing to the basin reflector is interpreted as the moho and a relatively steep fracture zone is probably the first example so far of an extensional fault crossing the whole crust and offsetting the moho further constraints on the deep structure of the basin are provided by estimating the thinning of the crust from shallow seismic data and gravity modelling and by cross plotting backstripped subsidence curves against curves predicted by the lithospheric stretching model

structure and significance of c30 4 methyl steranes in lacustrine shales and oils abstract the structure of the abundant c 30 4 methyl steranes of ancient freshwater lacustrine sediments and derived oils is of interest as their precursors the 4 methyl and in particular 4 23 24 trimethyl sterols dinosterols are reported to be specific to dinoflagellates this study employs a novel liquid crystal gc phase combined with gc ms and gc ms ms to separate and characterise mixtures of c 30 4 methyl sterane standards not resolvable on non polar phases this technique is then applied to identify the c 30 4 methyl steranes in an ancient freshwater sediment a freshwater lacustrine and a marine derived oil a comparison is also made with the 4 methyl steranes in marine sediments containing varying amounts of dinoflagellate cysts the c 30 4 methyl steranes of the freshwater lacustrine samples have been shown to possess a 4 methyl 24 ethylcholestane structure while both 4 methyl 24 ethylcholestanes and 4 23 24 trimethyl 5 h cholestanes dinosteranes were found in the marine dinoflagellate rich sediments while still indicative of dinoflagellates 4 methyl 24 ethyl sterols are generally not the major dinoflagellate sterol and alternative explanations for the abundance of 4 methyl 24 ethylcholestanes in ancient lacustrine sediments should be considered possibly ancient freshwater dinoflagellates synthesised principally the 4 methyl 24 ethyl form alternatively a source from an unrecognized algal class is possible while a source from bacteria which are known to synthesise certain 4 methylsterols is discussed

geochemical and biological marker assessment of depositional environments using brazilian offshore oils abstract a combined geochemical and molecular characterization of a wide selection of oils from the major brazilian offshore basins has been undertaken the elemental sulphur nickel and vanadium and bulk oapi and 13c properties of each sample been considered together with its molecular composition determined using liquid and gas chromatography and quantitative biological marker investigations using gas chromatography mass spectrometry for alkanes the results reveal significant differences in the chemical features of the various oils which enable them to be divided into five groups the distinction of the groups appears to reflect differences in the depositional environment of the source rocks of the oils each group is correlated tentatively with source rocks laid down in a specific depositional regime namely lacustrine freshwater lacustrine saline water marine evaporitic marine carbonate or marine deltaic the diagnostic features that allow this classification are the relative abundance and carbon number distributions of n alkanes pristane phytane ratios sulphur nickel and vanadium contents carbon isotope data the absolute concentrations of hopanes and steranes and their abundance relative to 4 methylsteranes and also the occurrence and abundance of several specific biological markers including 18 h oleanane gammacerane carotane tricyclic terpanes higher acyclic isoprenoids 28 30 bisnorhopane and 25 28 30 trisnorhopane this investigation shows the value of a combined geochemical and molecular approach in the assessment of the palaeoenvironment of deposition of the source rocks which gave rise to the oils

late cenozoic tectonic evolution of the los angeles basin and inner california borderland a model for core complex like crustal extension we propose that the los angeles basin inner california borderland lab ib and the southern borderland off baja california formed by large magnitude crustal extension beginning in latest oligocene earliest miocene time and that these regions represent major late cenozoic rifts the southern borderland is analogous to the gulf of california rift but the onset of rifting in this region may have preceded the opening of the gulf by as much as 15 to 20 m y the lab ib rift is more complex than the southern borderland because it formed in the wake of the rotating western transverse ranges wtr a large crustal block that has undergone more than 90 of neogene clockwise rotation total extension in both rifted regions is estimated to be in excess of 200 km or comparable to the magnitude of late cenozoic crustal extension now estimated for the basin and range province also like the basin and range the crustal floors of the basins in these highly extended domains are characterized by metamorphic core complexes and detachment faults voluminous miocene volcanic rocks and hanging wall fragments of pre rift strata and basement rocks stranded atop detachments during extension in support of this model we synthesize a wide variety of existing geologic data and present interpretations of new geophysical data seismic reflection profiles these profiles image features that we infer are detachment faults and overlying remnants of fragmented and rifted hanging wall rocks we also suggest that a similar but slightly earlier onset of crustal extension affected the santa maria basin and adjacent regions in central california and that the southern margin of this santa maria basin rift was overridden during neogene clockwise rotation of the wtr

shortening within underplated oceanic crust beneath the central california margin structures within a tectonically underplated layer of early miocene age oceanic crust indicate that the lower crust of offshore central california has been deformed under compression prior to the inception of the san andreas transform system along the continental margin shortening in the lower crust is inferred from the geometry of two structures the santa lucia monocline and the santa maria antiform delineated by an integrated interpretation of multichannel and wide angle seismic reflection data in conjunction with regional gravity data and teleseismic receiver functions the 700 km of multichannel seismic data and 300 km of wide angle seismic data from the 1986 pacific gas and electric edge seismic experiment form the primary data base used to establish the northwesterly trend and extent of the monocline and the wnw trend of the antiform in cross sectional view the santa lucia monocline is a 30 kink in the oceanic crust while the santa maria antiform is characterized by local thickening of the oceanic crust these structures are probably the result of convergence between the monterey and arguello microplates that took place as subduction along the margin waned the multichannel seismic reflection data image two miocene age shelf basins the santa lucia and offshore santa maria basins that formed within a transtensional stress regime the offshore santa maria basin directly overlies the santa maria antiform suggesting that either the upper and lower crustal stress regimes were completely decoupled in miocene time or that the two features formed in different places differential slip between the upper crust and lower crust may have been accommodated along a detachment zone located above oceanic crust convergence within the oceanic crust and apparent differential motion between upper and lower crust are indicative of the processes that can affect the transition from subduction to an oceanic continental transform zone

ophiolitic basement to a forearc basin and implications for continental growth the coast range great valley ophiolite california we present a compilation of 18 published models from the length of the great valley forearc basin california based on seismic reflection borehole seismic refraction gravity and aeromagnetic data to address long standing questions about the nature of the basement to the great valley its origin its tectonic history and it s mechanism of incorporation into the north american continental margin the geophysical models permit a 700 km long 70 km wide complete ophiolite sequence beneath the entire great valley in the northern great valley the ophiolite is overlain by ophiolitic breccia the ophiolite crust is 7 8 km thick and the mantle section is mostly unserpentinized beneath the southern great valley there is no ophiolitic breccia the crust may be up to 10 12 km thick and the mantle section if present at all is serpentinized to such a degree that it cannot be distinguished from sierran basement or mafic ophiolite members on the basis of velocity or density data geochemical petrological and paleomagnetic data support suprasubduction zone ophiolite formation at north american paleolatitudes and geological data and geophysical models are consistent with ophiolite formation by back arc spreading behind an east facing arc in the north this was apparently followed by obduction of back arc crust onto older continental basement during the late jurassic nevadan orogeny in the south the newly formed intraoceanic arc and back arc apparently collided with the continental margin during the nevadan orogeny but were not obducted onto it instead the arc and back arc docked with the continental margin leaving the arc itself to become the basement to the great valley basin cretaceous sierran magmatism then intruded plutons beneath the docked ophiolite and mafic arc irrespective of the detailed accretionary history our cross sections show a rapid pulse of continental growth by ophiolite accretion of more than 500 km km 1 in less than 10 myr

magmatism as an essential driving force for formation of active metamorphic core complexes in eastern papua new guinea the d entrecasteaux islands in eastern papua new guinea are composed of a number of active metamorphic core complexes which have been intruded by granodiorite plutons during their formation the plutons do not appear to have been intruded by diapiric processes as previously suggested late relatively undeformed plutons form flat lying bodies which crosscut structural boundaries and are strongly discordant to core complex shear zones granodiorite magmatism and the development of the metamorphic core complexes have occurred in a linear zone which coincides with a zone of thick crust and rugged topography it is proposed that plutonism facilitated deformation in ductile extensional shear zones which resulted in tectonic exhumation of deep crustal rocks and formation of the metamorphic core complexes the source of the plutons is thought to be related to a linear zone of mantle upwelling beneath the islands related to the propagation of the woodlark seafloor spreading center into continental crust it is suggested that a localized heat source of this type which can provide heat and magmatic material to the crust is essential for the development of metamorphic core complexes

three stage evolution of the los angeles basin southern california we propose that an episode of transtension dominated development of the los angeles basin area from 12 to 6 ma following middle miocene transrotation and prior to the modern transpressional regime transtension resulted from the releasing bend of the san gabriel chino hills cristianitos fault which acted as the primary transform boundary in southern california during this episode such an interpretation implies that significant transform motion did not occur on the southern san andreas fault prior to 6 ma and that the gulf of california has opened primarily since 6 ma we propose a three stage model for evolution of the los angeles basin and vicinity within the evolving transform fault system transrotation 18 12 ma transtension 12 6 ma and transpression 6 0 ma timing of these stages correlates with microplate capture events which occurred during conversion from a convergent margin to a transform margin

depth dependent stretching a different approach uplift of the crust during rifting is a major problem that has been left unresolved by uniform stretching models various depth dependent stretching models have been proposed to account for this but these require decoupling of the crust and mantle and result in space problems within the mantle a significant modification of these discontinuous nonuniform or depth dependent stretching models is proposed here that accounts for synrift uplift and doming but does not result in these problems the model involves continuous nonuniform stretching within a polygonal region bounded by symmetrical outward sloping boundaries within the mantle this geometry results in thinning of the lithosphere below crustal regions that are less stretched or unstretched crustal uplift resulting from asthenospheric upwelling below these regions this in turn results in stretching factors for the mantle that are less than those for the crust a relationship opposite to relationships derived by discontinuous nonuniform stretching models corollary predictions for mantle derived heat flow within and outside regions of stretching of these two different depth dependent models may allow them to be distinguished from each other and from regions where uniform stretching predominates

geodynamics of rifting and implications for hydrocarbon habitat abstract rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub lithospheric upper mantle combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system and by far filed stresses controlling the interaction of lithospheric plates moving relative to each other mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting therefore it is questioned whether a distinction between active and passive rifts is still justified lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust this could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data the duration of the rifting stage of palaeorifts and passive margins is highly variable depending on plate interaction crustal separation can be achieved after as little as 7 ma and as much as 280 ma of rifting activity the availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved the structural style of rifts is controlled by the physical state of the lithosphere the mode and amount of its extension and the lithological composition of pre and syn rift sediments subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere post rift basin subsidence is mainly governed by the decay of rift induced thermal anomalies densification of the lower crust and intra plate stresses provide overprinting effects in view of this it is unlikely that quantitative analyses of post rift basin subsidence give a direct measure of lithospheric extension tectonically active rifts palaeorifts and passive margin basins contain major hydrocarbon provinces their hydrocarbon charge can rely exclusively on pre rift syn rift or post rift sedimentary sequences or a combination thereof maturation of source rocks can be achieved during the syn and or post rift stage of basin evolution during rifting conductive and convective heat transfer accounts for elevated geothermal gradients these play an important role in the maturation of pre and syn rift source rocks thermal gradients decrease asymptotically during the post rift stage

upper crustal structure and neogene tectonic development of the california continental borderland multichannel seismic reflection data sonobuoy seismic refraction data and regional geology are used to define the upper crustal structure of the southern california continental borderland and to delineate the characteristics of the main lithotectonic belts of the region the catalina schist belt is separated on its west side from the gently deformed nicolas forearc belt by faults that have steep west dips and pronounced normal separations on its east side the schist belt is bounded by a large detachment fault that dips gently to the east beneath the west edge of the peninsular ranges belt at the coastline near oceanside the catalina schist was uplifted from middle crustal depths and exposed during a major event of extensional tectonism that started in early miocene time in conjunction with about 10 of clockwise rotation of the western transverse ranges belt part of the uplift of the catalina schist could have occurred on the detachment fault but it is thought to have mostly occurred on the steep faults that bound the west edge of the schist belt a large amount of uplift is required and it probably involved strong footwall flexural deformation in the wake of the translating and rotating western transverse ranges and nicolas forearc belts extension accompanied by probable large amounts of right slip continued in the borderland region during and after middle miocene time the later stage of extension was accompanied by rapid clockwise rotation of the western transverse ranges of at least 90 most of the borderland including the belt of schist that was uplifted in early miocene time was further deformed into numerous basins and ridges during this stage of oblique extension the primary driving force for the deformation is thought to have been derived from the rapid northwest motion of the pacific plate after it had become coupled to the farallon plate system which had previously been subducted beneath the borderland

crustal thickness variations in southern california from los angeles region seismic experiment passive phase teleseismic travel times the goal of the 1993 los angeles region seismic experiment larse93 passive phase was to collect waveform data from local and distant earthquakes to study lower crust and upper mantle structural features in southern california particularly under the san gabriel mountains and san andreas fault during larse93 approximately 88 stations were deployed in a 175 km long linear array across the los angeles basin san gabriel mountains and mojave desert northeast of los angeles during the four weeks of continuous recording teleseismic events recorded at each site provided a wide range of ray path backazimuths the teleseismic events included a number of intermediate magnitude earthquakes with epicenters in the aleutian island kamchatka kuril island mid atlantic ridge solomon island japan fiji island and chile regions this experiment was followed by larse94 which involved land refraction and deep crustal seismic reflection profiles from offshore and onshore explosion sources we have used the joint data sets in this study to distinguish upper crustal features from adjacent lower crustal and upper mantle structures p wave travel times were determined from 17 teleseisms and upper crustal residual signatures were removed by incorporating larse94 upper crustal velocity model results within each backazimuth range the resulting relative traveltime residuals increase from negative values 0 5 sec average recorded in the northern san gabriel valley southern san gabriel mountain foothills to positive values 0 2 sec average in the central and northern san gabriel mountains the residual patterns display variations for different backazimuths and incidence angles but show almost no lateral spatial shift of maximum or minimum residual along the array indicating that the dominant source of the residual pattern is shallow 50 km the patterns of residuals require a sharp lateral gradient in shallow velocities between the northern san gabriel valley located in the northernmost los angeles basin and the san gabriel mountains over a distance of less than 50 km most of the residual pattern can be explained by laterally varying crustal thickness and a high velocity anomaly in the upper mantle in our model of moho depth variations the northern san gabriel valley to the southwest of the array is underlain by thinned continental crust crustal thickness increases laterally by 12 km over a distance of less than 50 km into the san gabriel mountains this conclusion is supported by widespread surface evidence for rift style volcanism and crustal extension in southern california crustal rocks

time and temperature in petroleum formation application of lopatin s method to petroleum exploration n v lopatin in the soviet union has developed a method for taking both time and temperature into account as factors in thermal maturation of kerogen lopatin s time temperature index of maturity tti values correlate with the thermal regimes corresponding to generation and preservation of hydrocarbons because such information is potentially of great interest for oil exploration a calibration and evaluation have been made of lopatin s method within the limitations of the data presently available the following statements can be made 1 the rate of the chemical reactions involved in thermal maturation of organic material appears to double with every 10 c 18 f rise in temperature 2 threshold values of lopatin s time temperature index of maturity tti are 15 onset of oil generation 75 peak oil generation 160 end oil generation 500 40 oil preservation deadline 1 000 50 oil preservation deadline 1 500 wet gas preservation deadline 65 000 dry gas preservation deadline 3 tti values calculated from lopatin reconstructions consistently agree with other maturation parameters commonly used by petroleum geochemists potential applications of lopatin s method for oil exploration include timing of oil generation calculation of volume of hydrocarbons generated within a basin and determination of economic deadlines

acidic biomarkers from albacora oils campos basin brazil abstract acidic biomarkers of 8 different oil samples from albacora oil field campos basin brazil were analyzed in order to obtain additional information about biodegradation and thermal evolution for this purpose we selected oils lacustrine saline origin showing different biodegradation levels and at different distances from the fault that acted as migration pathway in the area previous analyses using hydrocarbon biomarkers suggested a mix of oil charges characterizing two distinct oil cycles interrupted by meteoric water circulation in these reservoirs analyses of the naphthenic acid biomarkers revealed a parallelism between the hydrocarbon and the acidic biomarkers notwithstanding there is evidence that the carboxylic acid biomarkers more resistent to biodegradation can provide information on past reservoir history when most important neutral biomarkers have been biodegraded the limitations to the use of these biomarkers are their de novo synthesis in the biodegradation process and the complexity of the sample manipulation prior to the analysis

a balanced cross section of the 1994 northridge earthquake southern california the northridge earthquake of 17 january 19941 was the latest in a series of very damaging thrust fault generated earthquakes to strike california following the san fernando2 1971 coalinga3 1983 and whittier narrows4 5 1987 events like the last two of these the northridge event occurred along a fault that did not reach the surface and which had not been detected by traditional seismic hazard methods6 7 balanced cross sections8 9 which flatten and remove the crustal deformation can be used to identify and quantify the seismic hazard posed by thrust faults here we present a balanced cross section through the northridge portion of the transverse ranges fold and thrust belt10 which shows that the earthquake occurred on what we call the pico thrust a cross section of this sort constructed before the earthquake would have revealed the fault although it would not have predicted the earthquake cross sectional modelling of the pico thrust yields an average slip rate of 1 4 1 7 mm yr 1 and a recurrence interval of northridge sized mw 6 7 earthquakes every 1 500 1 800 years we show that the pico thrust is the back thrust to the 170 km elysian park thrust3 4 which underlies some of the most densely urbanized portions of the los angeles basin

hydrocarbon source potential in brazilian margin basins abstract twenty thousand samples from the brazilian continental shelf basins were analyzed to characterize and evaluate the hydrocarbon source potential of the areas the geochemical evaluation of the rock and oil samples was performed by organic carbon determinations rock eval pyrolysis vitrinite reflectance thermal alteration index liquid and gas chromatographies gas chromatography mass spectrometry and carbon isotope analyses three source rock systems have been identified lower neocomian shales deposited in a continental environment upper neocomian shales grading from continental to lagoonal environment and aptian shales related to evaporitic and lacustrine sequences upper cretaceous and tertiary open marine slope sediments are not considered as source rocks locally these sediments present high organic carbon content but show an extremely poor hydrocarbon yield anoxic depositional conditions nevertheless can be traced locally along some levels of the santonian to cenomanian shales and marls these sediments are generally immature in the brazilian margin basins and no oil was generated from this section three oil families were distinguished through oil to oil and oil to source rock correlations the lower neocomian continental type the upper neocomian continental to lagoonal type and the aptian evaporitic to lacustrine related sequences the geochemical studies together with geologic and geophysical data provided the basis to display some models for the migration pathways and habitat of oils in the brazilian margin basins end of article last page 506

mapping and characterization of the api gravity of offshore hydrocarbon seepages using multispectral aster data abstract the objective of this work is the qualitative remote characterization api gravity degree of oil seepages on the ocean surface in order to achieve this goal multispectral data acquired by the advanced spaceborne thermal emission and reflection aster sensor are employed aster registered tracts of oil along oceanic sectors of the campos basin brazil and the bay of campeche gulf of mexico in several occasions numerous evidences indicate that these oil patches bear a straight link to oceanic seepages the delimitation and segmentation of these seepages in the aster imagery are accomplished making use of an unsupervised neural network fuzzy clustering algorithm spectra representative of the seepages are extracted from atmospherically corrected aster data pixels 9 bands spanning from visible to shortwave infrared wavelengths contained in the classified segments the aster spectra are checked against a predictive api partial least square regression model this model is established on the basis of oil spectra of known oapi varying from 13 to 47 yielded by laboratory measurements considering this model api gravity degrees of 19 6 1 37 and 15 9 2 9 are remotely estimated for the seepage in the campos basin and the bay of campeche respectively oils produced from campos and campeche fields typically show api varying from 17 24 and 12 16 5 correspondingly these results indicate the potential of the methodology proposed and of aster data and alike to remotely infer physical chemical properties of hydrocarbons since a close match was verified between predicted and true api gravity degrees for both study areas the data methods and experience gained in this research can be operationally tested in offshore oil exploration and likewise be adapted to environmental monitoring of oil spills in coastal regions

a tectonostratigraphic synthesis of the sub andean basins inferences on the position of south american intraplate accommodation zones and their control on south atlantic opening it has been shown in the accompanying paper that the sub andean foreland can be subdivided longitudinally into a number of tectonostratigraphic domains to test the hypothesis that changes in palaeo depositional setting rely on the presence of a series of transverse zones of structural accommodation data have been digitally compiled from across the south american continent spatial and temporal geological relationships have been analysed and evaluated as a means of identifying the position of tectonostratigraphic domain boundaries structural accommodation zones and patterns of subsidence and intraplate deformation the results suggest that individually these structural accommodation zones represent a composite of deep crustal fractures which on a regional scale interlink to form a transcontinental belt or zone that can accommodate intraplate deformation during episodes of plate reorganization their strong spatial relationship with mesozoic intraplate alkaline igneous activity suggests that they exerted an important control on lithospheric melt siting during gondwana breakup these localized zones of high heat flow have important implications for source rock maturity in the interior phanerozoic intracratonic basins of south america on the south atlantic margin the majority of these crustal lineaments correlate with failed arms of triple junction rifts and define the boundaries to tectonostratigraphic domains recognized along the south atlantic rift system

anomalous heat flow belt along the continental margin of brazil a comprehensive analysis of thermal gradient and heat flow data was carried out for sedimentary basins situated in the continental margin of brazil cmb the results point to the existence of a narrow belt within cmb where temperature gradients are higher than 30 c km and the heat flow is in excess of 70 mw m2 this anomalous geothermal belt is confined between zones of relatively low to normal heat flow in the adjacent continental and oceanic regions the width of the belt is somewhat variable but most of it falls within the range of 100 300 km the spatial extent is relatively large in the southern in the basins of pelotas santos and campos and northern in the basins of potiguar and ceara parts when compared with those in the central parts in the basins of south bahia sergipe and alagoas the characteristics of heat flow anomalies appear to be compatible with those produced by thermal sources at depths in the lower crust hence magma emplacement at the transition zone between lower crust and upper mantle is considered the likely mechanism producing such anomalies seismicity within the belt is relatively weak with focal depths less than 10 km for most of the events such observations imply that tectonic bonding between continental and oceanic segments at the transition zone of cmb is relatively weak hence it is proposed that passive margins like cmb be considered as constituting a type of plate boundary that is aseismic at sub crustal levels but allows for escape of significant amounts of earth s internal heat at shallow depths

botryococcane in a new class of australian non marine crude oils there are few documented examples of source specific hydrocarbons in non marine petroleum and most of these are related to land plant precursors1 3 one such biological marker of algal origin is botryococcane c34h70 a geologically rare acyclic alkane derived from the freshwater green alga botryococcus braunii4 5 first discovered in several sumatran crude oils6 7 and here identified in a suite of coastal bitumens from 10 different stranding sites between kingston south australia and portland victoria table 1 figs 1 2 biological marker and carbon isotope analyses of these bitumens provide evidence for a new class of australian non marine crude oils the bitumens are inspissated waxy residues of paraffinic and paraffinic naphthenic crude oils that emanate from submarine seeps in the western otway basin8 12 and are characterized by high concentrations of botryococcane unlike other high wax australian oils13 14 the coastal bitumens contain up to 2 6 sulphur and seem to be primarily of algal origin their geochemistry can be used to predict the occurrence of extensive organic rich source beds that were deposited in deep meromictic lakes along australia s southern continental margin

kinematics of present crust and mantle flow in southern california in the southern california region published geologic studies of prehistoric fault offsets yield at least 81 rates of slip covering various periods of time chiefly since the opening of the gulf of california these rates together with the known geometries of active faults are used to constrain a hand fit block model for present horizontal velocity of the crust the assumptions employed are that these rates apply to the present that the blocks are internally rigid down to the greatest depth of earthquakes and that the over all pacific north american plate motion is correctly given by global models the resulting model fits 67 data within their uncertainties 10 more with a small discrepancy 1 mm yr and fails to accommodate only 4 data 2 of which are inconsistent with other data nominal precision is good 3 mm yr but accuracy and uniqueness are both suspect because many block boundaries are questionable despite its many local defects this model suggests four reliable regional conclusions first the rate of shortening on the south side of the transverse ranges varies from 6 to 14 mm yr increasing eastward which implies an unrecognized seismic hazard unless the motion is taken up by folding or fault creep second the pattern of block rotation rates is quite different from the pattern of rotations shown by tertiary paleomagnetic declinations which implies some major pliocene reorganization of blocks third the great bend left step of the plate boundary fault system in the transverse ranges appears to be straightening gradually by a 3 m y clockwise rotation relative to both plates finally if it is assumed that plate shortening has been occurring at this bend since the early pliocene and that crust and upper mantle velocities are similar then this model predicts mantle lithosphere downwelling to at least 250 km depth beneath the transverse ranges this prediction is consistent with upper mantle seismic velocity structure

progressive growth of san clemente island california by blind thrust faulting implications for fault slip partitioning in the california continental borderland we find that the genesis of san clemente island and its surrounding submarine platform is consistent with progressive slip on two southeast striking southwest dipping blind thrust fault segments since their inception 2 to 5 ma 3 km of compression normal to the n150 e fault strike has been accommodated with 1700 m of domal uplift of the san clemente anticlinorium the existence of an extensive suite of pleistocene marine terraces provides evidence that slip and uplift are continuing today based on direct terrace fossil age determinations and correlations of terrace heights with global sea level curves we estimate that san clemente island is currently uplifting at between 0 2 and 0 5 mm yr 1 this translates into 0 6 1 5 mm yr 1 of thrusting on the causative blind thrusts beneath the island unlike the situation at nearby palos verdes where a simple twist in a regional strike slip fault accommodated both fault parallel and fault normal motions the shallow dips of the thrusts suggest that if regional strike slip motion on the san clemente fault exists it must be partitioned onto through going surfaces distinct from the thrusts current gps data are sparse and equivocal but they indicate that 1 4 mm yr 1 of compression and 4 7 mm yr 1 of strike slip are absorbed in the california continental borderland with the palos verdes fault taking some 3 mm yr 1 from the strike slip budget 1 4 mm yr 1 of motion could be present on a through going san clemente fault when translated into an annual moment release rate using kostrov s formula gps strains predict that between 2 5 and 4 9 times 1017 n m yr 1 of earthquake potential is available offshore from san diego to the santa barbara channel distribution of this moment budget among various earthquake magnitudes is arguable but we predict that m 6 quakes in the borderland could recur between 30 and 80 years and m 7 quakes might be found every 310 to 580 years

three dimensional vp and vp vs velocity models of the los angeles basin and central transverse ranges california we use p and s arrival times from 5225 earthquakes and 53 explosions recorded by the southern california seismographic network to invert for the three dimensional p velocity v p and the p and s velocity ratio v p v s in the central transverse ranges and the los angeles basin to model long wavelength features of the velocity structure we invert for the models by interpolating models determined successively from a sparse medium to a dense grid with 40 20 and 10 km spacing of horizontal grid nodes layers of grid nodes are placed at depths of 1 4 6 8 12 16 and 20 km the data variance decreased about 80 in the gradational inversion ample data from the 1994 northridge and other earthquake sequences the rich background seismicity and the dense station distribution along with controlled sources made the model well resolved except along the edges to the southwest in the offshore region and at depths greater than 20 km the v p model images the shape of the los angeles and east ventura basins down to depths of 8 and 12 km three low velocity areas at 1 km depth in the los angeles basin that coincide with recent sediment depositional areas are also imaged the north edge of the peninsular ranges the santa monica and the san gabriel mountains form discontinuous high velocity ridges extending to depths of 20 km the high v p v s ratios in the near surface are consistent with high pore fluid pressures in the basin sediments at depth beneath the east ventura basin the high v p v s and high v p suggest the presence of ophiolitic assemblages or mid miocene volcanics in contrast a body with normal v p and low v p v s that is identified in the upper crust beneath santa monica bay may be a fragment of quartz rich continental crust the v p model of both the ventura and the los angeles basins shows features that suggest deformation of the hanging wall or basin closure at seismogenic depths of 16 km the hypocenters of moderate sized and large earthquakes are located within or adjacent to high v p velocity bodies in most cases these high velocity bodies form the upper block consistent with shortening of the lower crust as described in thick skinned tectonic interpretations

crustal and uppermost mantle structure under southern california the travel times of pg and pn arrivals are used together as a new approach to estimate the station delays lateral velocity variations in the upper crust and upper mantle crustal thickness variations and pn anisotropy in the upper mantle under southern california the main advantage of this approach is that crustal effects are estimated directly from the data and therefore do not contaminate the results from pn arrivals significant negative station delays are found in the colorado desert san jacinto mountains san gabriel mountains central mojave desert and from the southern ventura basin to santa cruz island in addition to the positive delays associated with well known sedimentary basins lateral velocity variations in the upper crust exceed a few percent only in the coso ranges these variations show no clear association with surface faulting they also show little resemblance to the upper mantle velocity variations which have fast velocity under the mojave and near the offshore area and slow velocity in most other areas the southern california moho shows large variations large crustal thinning occurs near the salton sea northwest of the white wolf fault in the coso range near san diego and offshore while the san bernardino mountains western granite mountains and eastern mojave south of the ludlow fault are underlain by roots a significant amount 3 of pn anisotropy is present in the uppermost mantle with a fast direction n 75 4 w this could be explained very well by averaging the pn anisotropy estimates east and west of the san andreas

microplate capture rotation of the western transverse ranges and initiation of the san andreas transform as a low angle fault system tectonic rotation of the western transverse ranges block is explained by capture of the partially subducted monterey microplate by the pacific plate at about anomaly 6 time ca 20 ma as pacific monterey spreading slowed and eventually ceased the slip vector along the gently northeast dipping subduction interface beneath the california margin changed from slightly oblique subduction to transtensional dextral transform motion this change in slip vector and a shift of pacific plate motion eastward along the already subducted monterey plate interface imply that the san andreas transform began as a system of low angle faults that locally subjected the overriding continental margin to distributed basal shear and crustal extension this basal shear produced the rotated western transverse ranges this model helps explain the timing of initial rotation and basin formation the sudden appearance of widely distributed transform motion well inland of the margin in early miocene time why the western transverse ranges uniquely rotated as a large coherent crustal block and several fundamental structural characteristics of central and southern california the model also provides major constraints on the amount of pacific north america strike slip motion the position through time of offshore oceanic plates relative to onshore geology and a general explanation for what may happen as a spreading ridge approaches a trench and the subduction zone evolves into a transform system

a model for neogene crustal rotations transtension and transpression in southern california two episodes of crustal rotation have occurred in the regions of southern california and the mojave desert since the beginning of miocene time an early miocene episode of clockwise crustal rotation is associated with widespread north south to north northeast south southwest crustal extension throughout southeast california and western arizona this episode is apparently related to changing subduction parameters of the farallon plate a middle miocene and later episode of clockwise rotation in the transverse ranges and bordering areas is related to shear between the pacific and north american plates in both episodes clockwise crustal rotations first occur in a deforming zone which has a component of extension across its trend and shear along its trend the effects of extension are most pronounced in the earlier episode but clear in the later one also the pacific north american deforming zone began contracting in width during pliocene time but clockwise rotations continued and are probably in progress today

interpretation of crustal seismic velocities in the san gabriel mojave region southern california the increased concern over seismic hazards in the los angeles region has created interest in the crustal structure and mechanical properties associated with faults like the sierra madre and san andreas this study presents the seismic properties of major lithologies found in the vicinity of the san andreas fault zone in order to correlate with the los angeles region seismic experiment lars e results scientists from the larse group have recently acquired high quality seismic data along three transects thirty nine rock samples were collected along the larse line i which trends ne sw across the central mojave desert and the san gabriel mountains these rock samples represent the following lithologies precambrian gneisses san gabriel anorthosite mesozoic granitic intrusives pelona schist mendenhall granulite gneisses and san andreas fault zone cataclasites the seismic properties were determined by measuring compressional and shear wave velocities as a function of confining pressure average compressional and shear wave velocities at 150 mpa show a wide range of values the mendenhall gneiss and the san gabriel anorthosite complex have high velocities 6 5 km s and 6 4 km s respectively the pelona schist the mojave granitic intrusives and the san andreas cataclasitic rocks have average velocities of 5 8 km s 5 2 km s and 5 1 km s respectively average densities range from 2320 kg m3 for fault zone cataclasites to 3100 kg m3 for a mendenhall granulite gneiss gneisses and schists show significant seismic anisotropy 5 20 due to preferred mineral orientation while most other rocks are nearly isotropic poisson s ratios range from a high of 0 29 in the san gabriel anorthosite to a low of 0 22 in the san gabriel gneiss comparison of the measured seismic properties with the larse velocity model allows interpretation of crustal structure in terms of rock compositions the upper crust southwest of the san andreas fault contains granitic intrusives gneisses and schists while under the mojave desert the upper crust is dominated by gneisses high velocities under the san gabriel mountains represent either lower crustal granulites or anorthosite the velocity decrease beneath the san andreas fault zone originates from minor amounts of cataclasite or the presence of low velocity lithologies such as pelona schist 1998 elsevier science b v all rights reserved

depositional models of tertiary rift basins eastern china and their application to petroleum prediction abstract the tertiary bohai rift system of the eastern part of china consists of several rift basins and represents one of the important petroleum provinces of china four major aspects of sedimentology of the rift basins are summarized and discussed in this paper 1 facies types and their characteristics 2 the pattern of spatial distribution and temporal evolution of sedimentary facies facies associations and sequences of the basins the typical depositional models and facies associations have been established and are illustrated according to different evolutionary stages of the rift basins 3 two main controlling factors tectonics and paleoclimate and 4 relationships between the distribution of oil and gas and the sedimentary facies and depositional models of the rift basins and how to apply them to predict the occurrence of oil and gas

the helmut geophysical anomaly a regional left lateral transtensional shear zone system connecting santos and campos basins southeastern brazil abstract in the ultradeep water of santos and campos basins eastwards from the area of present day pre salt exploration in southeastern brazil there is a nearly 1000 km long curved magnetic anomaly here named helmut on a magnetic map the anomaly orientation switches from ne sw in santos basin to nearly n s in campos mimicking the curved neoproterozoic shear zone pattern of the basement exposed onshore from the comparison between the position of such anomaly and gravity derived modeling and residual maps it is suggested that the magnetic anomaly also corresponds to a major gravimetric boundary separating different crustal domains this boundary is slightly oblique to the rift related normal fault trend in santos basin based on interpreted 3d seismic data the nne trending normal faults are dragged to the ne direction close to the magnetic anomaly in southern santos basin where an en echelon array of oblique magnetic anomalies occurs which resulted mainly from the left lateral component of an overall transtensional displacement field in regional 2d deep reflection seismic profiles the helmut anomaly is correlated to major eastward dipping down to the basin stepped faults sometimes arranged in a fan like fashion in santos basin and as a landward dipping normal fault zone in campos basin north of the e w araruama transfer zone in some areas the fault zone displaces the base salt reflector and cross cuts the entire stretched continental crust the analysis of potential field and seismic data suggests that the helmut anomaly may be interpreted as a crustal scale shear zone system active after salt deposition during the late aptian regionally the helmut anomaly connects two approximately n s trending branches of the south atlantic rift system the northern branch that runs from espirito santo basin northwards up to the camamu south of the reconcavo tucano jatoba aborted trough and the southern branch running southwards from santos basin through pelotas basin in brazil as far as the basins offshore uruguay and argentina basins southwards extensional faulting of the salt reflector along early nne rift related normal faults and dextral transtension on the nw trending fault zones are all consistent with a sinistral transtensive motion on the ne trending helmut anomaly during the late aptian

nonlinear refraction and reflection travel time tomography we develop a rapid nonlinear travel time tomography method that simultaneously inverts refraction and reflection travel times on a regular velocity grid for travel time and ray path calculations we apply a wave front method employing graph theory the first arrival refraction travel times are calculated on the basis of cell velocities and the later refraction and reflection travel times are computed using both cell velocities and given interfaces we solve a regularized nonlinear inverse problem a laplacian operator is applied to regularize the model parameters cell slownesses and reflector geometry so that the inverse problem is valid for a continuum the travel times are also regularized such that we invert travel time curves rather than travel time points a conjugate gradient method is applied to minimize the nonlinear objective function after obtaining a solution we perform nonlinear monte carlo inversions for uncertainty analysis and compute the posterior model covariance in numerical experiments we demonstrate that combining the first arrival refraction travel times with later reflection travel times can better reconstruct the velocity field as well as the reflector geometry this combination is particularly important for modeling crustal structures where large velocity variations occur in the upper crust we apply this approach to model the crustal structure of the california borderland using ocean bottom seismometer and land data collected during the los angeles region seismic experiment along two marine survey lines details of our image include a high velocity zone under the catalina ridge but a smooth gradient zone between catalina ridge and san clemente ridge the moho depth is about 22 km with lateral variations

post breakup tectonics in southeast brazil from thermochronological data and combined inverse forward thermal history modeling the continental margin of southeast brazil is elevated onshore tertiary basins and late cretaceous paleogene intrusions are good evidence for post breakup tectono magmatic activity to constrain the impact of post rift reactivation on the geological history of the area we carried out a new thermochronological study apatite fission track ages range from 60 7 1 9 ma to 129 3 4 3 ma mean track lengths from 11 41 0 23 mm to 14 31 0 24 mm and a subset of the u th he ages range from 45 1 1 5 to 122 4 2 5 ma results of inverse thermal history modeling generally support the conclusions from an earlier study for a late cretaceous phase of cooling around the onshore taubate basin for a limited number of samples the first detectable period of cooling occurred during the early tertiary the inferred thermal histories for many samples also imply subsequent reheating followed by neogene cooling given the uncertainty of the inversion results we did deterministic forward modeling to assess the range of possibilities of this tertiary part of the thermal history the evidence for reheating seems to be robust around the taubate basin but elsewhere the data cannot discriminate between this and a less complex thermal history however forward modeling results and geological information support the conclusion that the whole area underwent cooling during the neogene the synchronicity of the cooling phases with andean tectonics and those in ne brazil leads us to assume a plate wide compressional stress that reactivated inherited structures the present day topographic relief of the margin reflects a contribution from post breakup reactivation and uplift

rifted margin architecture and crustal rheology reviewing iberia newfoundland central south atlantic and south china sea abstract crustal rheology controls the style of rifting and ultimately the architecture of rifted margins here we review the formation of three magma poor margin pairs iberia newfoundland the central segment of the south atlantic rift and the south china sea by integrating observational data into a numerical forward modelling framework we utilise a 2d version of the finite element code slim3d which includes nonlinear temperature and stress dependent elasto visco plastic rheology and is able to reproduce a wide range of rift related deformation processes such as flexure lower crustal flow and faulting extension in cold strong or thin crust is accommodated by brittle faults and ductile shear zones that facilitate narrow rifts with asymmetric fault geometries hot weak or thick continental crust is dominated by ductile deformation and often extends symmetrically into a wide rift system this simple recipe provides the standard framework to understand initial rift geometry however it is insufficient to account for the dynamics of intermediate and late rift stages that shape the final margin architecture asymmetric conjugate margins where one side is wide and the other narrow can be formed via both wide and narrow rift styles which we reproduce with weak and strong crustal rheologies respectively exemplified by the iberia newfoundland conjugates and the central south atlantic we define three characteristic rift phases an initial phase of simultaneous faulting an intermediate phase of rift migration that involves sequential fault activity and finally the breakup phase crustal rheology plays an overarching role in governing the dynamics of these asymmetric margins we illustrate that weak rheologies generally prolong the phase of simultaneous faulting while rift migration is enabled by initial fault asymmetry as well as relatively weak crust formation of the predominantly symmetric and wide margins of the south china sea was controlled by extraordinarily weak crust that extended the phase of simultaneous faulting until breakup the weak crustal rheology of this region relates to the south china sea s pre rift history where plate convergence lead to crustal thickening and magmatic additions in a back arc regime shortly before the onset of rifting

an alkaline lake in the late paleozoic ice age lpia a review and new insights into paleoenvironment and petroleum geology abstract the ca 300 ma fengcheng formation in the junggar basin of nw china is reported to develop a rare alkaline lake soda lake in this study based on an expanded literature survey and new insights from geological and geochemical data the development of this ancient alkaline lake resulted from a convergence of favorable paleotectonic paleogeographic and paleoclimatic conditions a detailed characterization of the alkaline lake includes evidence for a ph of 9 0 indicated by 15n data average 18 4 the presence of alkali minerals such as trona and shortite globular bacteria like fossils low contents of clay minerals organic and inorganic geochemistry indicating high salinity strongly reducing conditions and hydrothermal activity and a complete sedimentary evolution sequence of an archetypal alkaline lake the studied lake deposit is presented as the first such deposit worldwide to generate a large oil reserve thus allowing scientific insights to be gained into the controls on hydrocarbon generation in alkaline lakes the alkaline lacustrine source rocks have high contents of organic matter favorable organic matter type abundant oil and poor natural gas multiple phases of hydrocarbon generation and high hydrocarbon volumes the genesis and development of this alkaline lake were controlled jointly by evaporation and volcanic hydrothermal activity however some aspects of this type of lake remain unclear and future work should explore the integrated evolution of biological climatic petroleum geology systems with respect to alkaline lakes and their associated deposits and their coupling research with worldwide event then namely glacial episode iii of the late paleozoic gondwanan glaciation our data suggest that the fengcheng formation is a well preserved archive of late paleozoic ice age lpia geology and geochemistry around 300 ma

petrogenesis of the conejo volcanic suite southern california evidence for mid ocean ridge continental margin interactions the evolution of the southern california borderland during mid miocene time was dependent on the interactions between the north american continent and the converging east pacific rise evidence from geological and geochemical investigations of the conejo volcanic suite whose petrogenesis is linked to these interactions suggests an intimate relation between the subducting ridge and the volcanic rocks the whole rock chemistry of this suite k2o 0 4 87sr 86sr 0 70248 0 70372 average 0 70306 tio2 1 2 1 8 and restricted range of pyroxene and plagioclase compositions support a model in which active ridge volcanism fractionation and mixing of primitive liquids with later differentiates can account for the observed petrologic evolution the interaction of the ridge with the subduction zone may result in the cessation of subduction the preferential subduction of the trenchward ridge flank or localized volcanism in the vicinity of the ridge subduction zone intersection

re sedimented deposits in the rift section of the campos basin abstract an integrated petrologic sedimentologic stratigraphic seismic study of the lagoa feia group rift section of the campos basin has shown that rift sedimentation was dominantly intrabasinal carbonate and stevensitic deposits with siliciclastic deposits restricted to the proximity of graben border faults the bivalve bioclastic rudstones coquinas that constitute the rift reservoirs show commonly limited abrasion of the bioclasts and were deposited both on basement highs and lows throughout the rift section although in situ deposits of stevensite ooids and peloids occur dominantly at the base of the succession these particles are ubiquitous to the entire rift section mixed in variable proportion with siliciclastic and carbonate sediments the environmental conditions required for the formation of stevensite and the growth of bivalves are mutually exclusive as stevensite forms only at ph greater than 10 while bivalves cannot tolerate ph greater than 9 the common mixture of well rounded basaltic rock fragments with angular granitic gneissic rock fragments and feldspars in the sandstones and conglomerates indicates recycling of epiclastic deposits from the early rift section combined with first cycle contribution from the plutonic basement the studied cores show no evidence of subaerial exposure and there is a lack of bioturbation suggesting harsh environmental conditions the rift deposits are dominantly massive or faintly laminated with diffuse facies boundaries structures indicative of unidirectional or oscillatory flow are subordinate integration of seismic sedimentologic and petrographic evidence indicates that the campos basin rift section is formed mostly by re sedimented gravitational deposits the onset of the rift sedimentation occurred in synformal depressions where bivalve banks or stevensite ooids were formed in shallow lacustrine environments under variable alkalinity conditions with the development of half grabens and concentration of the tectonic activity along the border faults recurrent tectonic events promoted the mixing and gravitational re deposition of stevensitic clastic and bioclastic sediments in deeper fault bounded troughs large scale units hundreds of meters thick were generated by major tectonic events whereas compositional variations in the scale of meters were possibly a product of lake level climatic fluctuations thus due to syn rift and mainly to post rift erosion the neo aptian unconformity the preserved rift section of the lagoa feia group comprises mostly sediments deposited in the central troughs of the half graben structures marginal sediments were extensively eroded and re sedimented as gravity driven mixed deposits this new model constructed form the integration of seismic stratigraphic sedimentologic and petrologic data diverges substantially from the presently accepted model for the sedimentation of the rift section opening new possibilities for the exploration of campos basin as well as of similar settings as in the adjacent santos basin

quantitative seismic stratigraphic interpretation of the evaporite sequence in the santos basin abstract on the account of the absence of modern analogues the interpretation of evaporite bearing basins relies extensively on seismic and well data recent studies describe the intrasalt kinematic geometries and compositions relating the change of reflection patterns in seismic data to variations in evaporite sequences however these interpretations are only qualitative this study describes a step further in proceeding from qualitative to quantitative interpretation of an evaporite dominated interval it integrates geophysical and geological foundations by combining rock physics seismic inversion and statistical techniques to deliver seismic driven facies volume of the evaporite sequence in the santos basin offshore brazil synthetic seismogram demonstrate the ambiguities and uncertainties in relating the seismic amplitude to salt types rock physics analysis indicates that acoustic impedance is an effective differentiator of salt types in comparison to seismic amplitude therefore we perform seismic inversion to transform interface property into layer property the acoustic impedance volume mitigated significantly the risk of seismic interpretation in the salt sequence the bayesian classification integrates acoustic impedance and rock physics analysis to deliver the seismic driven facies volume which categorizes bittern salts halite and anhydrite this enables the refinement of the seismic stratigraphic interpretation of intrasalt sequences of the ariri formation based on the sedimentary cycles of evaporites additionally we calculate proportion maps to evaluate the spatial variability of salt deposition we observe an increase in bittern salts and anhydrite proportions from the bottom to the top of the sequence the comparison between well based and seismic based proportions testifies to the quantitative potential of seismic data the approach provides quantitative analysis of the evaporite sequence and can be applied to other salt bearing basins

sycamore knoll a wave planed pop up structure in a sinistral oblique thrust system southern california continental borderland at the boundary between the western transverse ranges province and inner continental borderland of southern california strain is partitioned across the sinistral oblique anacapa dume fault system sycamore knoll a pop up structure 20 km west of point dume in the hanging wall of the north dipping anacapa dume fault stands out as an anomalous bathymetric high along the southern california coast we synthesize new and existing geophysical datasets including multibeam bathymetry backscatter and single multichannel seismic to understand the morphology and tectonic implications of sycamore knoll we identify a complete eustatic low stand submerged marine terrace package encircling sycamore knoll platform that corresponds to the last glacial maximum lgm the marine terrace deposits and wave planed surface of sycamore knoll require a late quaternary uplift rate of 0 55 0 5 0 3 mm yr relative to sea level by combining our uplift rate with previous estimates of basin subsidence and a revised structural model of the anacapa dume fault we determine a maximum slip rate in latest quaternary time of 1 50 0 5 0 3 mm yr and hence a post lgm rate of convergence across the anacapa dume fault of 0 60 0 2 0 1 mm yr

major controlling factors on hydrocarbon generation and leakage in south atlantic conjugate margins a comparative study of colorado orange campos and lower congo basins abstract we present a supra regional comparative study of the major internal and external factors controlling source rock sr maturation and hydrocarbon hc generation and leakage in two pairs of conjugate margins across the south atlantic the brazil campos basin angola lower congo basin margins located in the central segment and the argentina colorado basin south africa orange basin in the southern segment our approach is based on the analysis and integration of borehole data 1d numerical modeling 2d seismic reflection data and published reports coupling of modeling results sedimentation rate calculation and seal bypass system analysis reveal that 1 oil window is reached by syn rift srs in the southern segment during the early to late cretaceous when thermal subsidence is still active while in the central segment they reach it in late cretaceous neogene during a salt remobilization phase and 2 early hc generation from post rift srs in the southern segment and from all srs in the central segment appears to be controlled mainly by episodes of increased sedimentation rates the latter seems to be associated with the andes uplift history for the western south atlantic basins campos and colorado and to a possibly climate driven response for the eastern south atlantic basins orange and lower congo additionally we observe that the effect of volcanism on sr maturation in the southern segment is very local the comparison of cretaceous mass transport deposit mtd episodes with hc peak of generation and paleo leakage indicators in the southern segment revealed the possible causal effect that hc generation and leakage have over mtd development interestingly paleogene leakage indicators which were identified in the argentina south africa conjugate margins occur contemporaneously to low sedimentation rate periods nonetheless present day leakage indicators which were also identified in both pairs of conjugate margins might be related to seal failure events linked to eustatic sea level drops

nonlinear refraction traveltime tomography a few important issues for performing nonlinear refraction traveltime tomography have been identified they include the accuracy of the traveltime and raypath calculations for refraction the physical information in the refraction traveltime curves and the characteristics of the refraction traveltime errors consequently we develop a shortest path ray tracing method with an optimized node distribution that can calculate refraction traveltimes and raypaths accurately in any velocity model we find that structure ambiguity caused by short and long rays in the seismic refraction method may influence the inversion solution significantly therefore we pose a nonlinear inverse problem that explicitly minimizes the misfits of the average slownesses ratios of traveltimes to the corresponding ray lengths and the apparent slownesses derivatives of traveltimes with respect to distance as a result we enhance the resolution as well as the convergence speed to regularize our inverse problem we use the tikhon

both differential and equatorial heating contributed to african monsoon variations during the mid holocene the sahara was significantly greener 11 5 kya and during multiple earlier interglacial periods but the mechanisms related to the greening of the sahara remain uncertain as most climate models severely underestimate past wet conditions over north africa the variations in the african monsoon related to the greening of the sahara are thought to be associated with the variations in the inter hemispheric differential heating of earth caused by orbital variations however how orbital variations affect regional climate is not well understood using recent theory that relates the position of the tropical rain belt to the atmospheric energy budget we study the effect of orbital forcing during the mid holocene on the african monsoon in simulations provided by the third phase of the paleo model intercomparison project pmip3 we find that energy fluxes in the african sector are related to orbital forcing in a complex manner contrary to generally accepted theory orbital modulation of seasonal differential heating alone is shown to be a weak driver of african monsoon variations instead net atmospheric heating near the equator which modulates the intensity and extent of seasonal migrations of the tropical rain belt is an important but overlooked driver of african monsoon variations a conceptual framework that relates african monsoon variations to both equatorial and inter hemispheric differential solar heating is presented

effects of subduction parameters on geothermal gradients in forearcs with an application to franciscan subduction in california geothermal gradients in forearcs are often suppressed below normal values because of the cooling effect of the relatively cold downgoing plate in this paper finite difference thermal modeling is used to evaluate the influence on forearc gradients of variations in six potentially important subduction zone parameters variations in forearc radiogenic heat production over the general range likely in forearc rocks 0 to 1 5 10 6 w m 3 cause only minor changes in temperatures at the base of the forearc about 2 to 35 depending on other parameters and thus have relatively little effect on pressure temperature pt conditions of metamorphism however their effect on gradients at shallow levels and thus on surface heat flows may be greater up to about 100 likely variations in forearc thermal conductivity 1 5 to 3 5 w m 1 c 1 similarly change temperatures at the base of the forearc only modestly 10 25 but change surface heat flows more substantially 1 45 1 75 times decreasing the subduction angle from 27 to 3 6 increases gradients by about a factor of 2 at both deep and shallow levels decreasing the subduction rate by a factor of 8 160 to 20 km m y increases gradients by about a factor of 2 when frictional heating at the plate contact is excluded and less when modest amounts of frictional heating are included forearc gradients increase virtually linearly with the presubduction geothermal gradient in the subducting slab and thus are higher above a younger slab when frictional heating is excluded forearc gradients are 2 0 2 8 times higher above a 10 m y old slab than above a 150 m y old slab again the relative effect is lessened when frictional heating is incorporated the amount of frictional heating at the plate contact potentially has the greatest influence on forearc gradients the frictional heat production h is given by h vs where is the shear stress along the plate contact and vs is the subduction rate the modeling shows that forearc gradients increase linearly with unless plate contact shear stresses are only a small fraction of those expected with frictional sliding of dry rocks outer parts of forearcs would be the sites of pervasive low p t metamorphism greenschist facies rather than high p t metamorphism blueschist facies very high fluid pressures along the plate contact are probably the way shear stresses are reduced pressure temperature conditions of blueschist facies metamorphism in the franciscan subduction complex of california are easily explained with typical subduction rates and slab ages with plate contact shear stresses of the order of 10 mpa equivalent to around 3 of lithostatic pressure but stresses within the range zero to a few tens of megapascals are probably permitted by the thermal constraints speculative application of the modeling results assuming a shear stress of 4 of lithostatic pressure to plate motion reconstructions for the franciscan forearc suggests that forearc gradients were about 8 c km around 85 ma when the subducting slab was perhaps 145 m y old and the subduction rate was perhaps 95 km m y gradients increased moderately through the latest cretaceous to middle tertiary as subduction became slower and the subducting slab became younger reaching about 16 c km at 28 ma when the slab age was about 11 m y and the subduction rate was about 48 km m y the slab age subduction rate and forearc gradient then remained fairly constant until 5 ma when subduction slowed to about 32 km m y and the slab age decreased to about 8 m y causing gradients to rise to about 20 c km

the san gabriel mountains bright reflective zone possible evidence of young mid crustal thrust faulting in southern california abstract during the los angeles region seismic experiment larse a reflection refraction survey was conducted along a line extending northeastward from seal beach california to the mojave desert crossing the los angeles basin and san gabriel mountains shots and receivers were spaced most densely through the san gabriel mountains for the purpose of obtaining a combined reflection and refraction image of the crust in that area a stack of common midpoint cmp data reveals a bright reflective zone 1 s thick that dominates the stack and extends throughout most of the mid crust of the san gabriel mountains the top of this zone ranges in depth from 6 s 18 km depth in the southern san gabriel mountains to 7 5 s 23 km depth in the northern san gabriel mountains the zone bends downward beneath the surface traces of the san gabriel and san andreas faults it is brightest between these two faults where it is given the name san gabriel mountains bright spot sgmbs and becomes more poorly defined south of the san gabriel fault and north of the san andreas fault the polarity of the seismic signal at the top of this zone is clearly negative and our analysis suggests it represents a negative velocity step the magnitude of the velocity step is approximately 1 7 km s in at least one location an event with positive polarity can be observed 0 2 s beneath the top of this zone indicating a thickness of the order of 500 m for the low velocity zone at this location several factors combine to make the preferred interpretation of this bright reflective zone a young fault zone possibly a master decollement 1 it represents a significant velocity reduction if the rocks in this zone contain fluids such a reduction could be caused by a differential change in fluid pressure between the caprock and the rocks in the sgmbs near lithostatic fluid pressure is required in the sgmbs such differential changes are believed to occur in the neighborhood of active fault zones where fault value action has been postulated less likely alternative explanations for this velocity reduction include the presence of magma and a change in composition to serpentinite or metagraywacke 2 it occurs at or near the brittle ductile transition at least in the southern san gabriel mountains a possible zone of concentrated shear 3 a thin reflection rising from its top in the southern san gabriel mountains projects to the hypocenter of the 1987 m 5 9 whittier narrows earthquake a blind thrust fault earthquake with one focal plane subparallel to the reflection alternatively one could argue that the bends or disruptions in the reflective zone seen at the san gabriel and san andreas faults are actually offsets and that the reflective zone is therefore an older feature possibly an older fault zone

overview tectonic implications of cenozoic volcanism in coastal california cenozoic volcanic rocks of coastal california were erupted west of the magmatic arc trend related to subduction along the continental margin two assemblages representing discrete pulses of mid tertiary oligocene miocene and mid miocene volcanism occupied relatively compact tracts as restored palinspastically prior to disruption and dispersal by tectonic displacements within the neogene san andreas transform system a third assemblage records migratory post mid miocene volcanism at centers located progressively farther north the distribution and petrologic character of all three assemblages reflect slab window magmatism triggered by decompression melting of upwelling mantle paleogeographic reconstructions based on analysis of magnetic anomaly patterns offshore and restoration of on land features prior to san andreas transform slip and associated transrotational deformation indicate the paleotectonic positions of the cenozoic volcanic fields before structural disruption the pulses of mid tertiary and mid miocene volcanism were related to transient episodes of mantle upwelling generated successively by rise trench encounters associated with subduction of the vancouver farallon and monterey arguello plates respectively along the continental margin off southern california migratory post mid miocene volcanism in central california accompanied the incremental expansion of a growing slab window as the mendocino triple junction migrated northward along the continental margin mid miocene columbia river basalt volcanism in the pacific northwest was coeval with the mid miocene pulse of coastal volcanism and may have reflected tectonism induced by the final demise of offshore microplates to allow initial full integration of the san andreas transform as a coherent plate boundary columbia river volcanism may have stemmed from mantle perturbation caused by torsional deformation of the continental block in response to shear imposed by the pacific plate

anatomy of a metamorphic core complex seismic refraction wide angle reflection profiling in southeastern california and western arizona the metamorphic core complex belt in southeastern california and western arizona is a nw se trending zone of unusually large tertiary extension and uplift midcrustal rocks exposed in this belt raise questions about the crustal thickness crustal structure and the tectonic evolution of the region three seismic refraction wide angle reflection profiles acquired and analyzed as a part of the u s geological survey s pacific to arizona crustal experiment were collected to address these issues the results presented here which focus on the whipple and buckskin rawhide mountains yield a consistent three dimensional image of this part of the metamorphic core complex belt the seismic refraction wide angle reflection data are of excellent quality and are characterized by six principal phases that can be observed on all three profiles these phases include refractions from the near surface and crystalline basement reflections from boundaries in the middle and lower crust and reflections and refractions from the upper mantle the final model consists of a thin veneer 2 km of upper plate and fractured lower plate rocks 1 5 5 5 kms 1 overlying a fairly homogeneous basement 6 0 km s 1 and a localized high velocity 6 4 km s 1 body situated beneath the western whipple mountains a prominent midcrustal reflection is identified beneath the whipple and buckskin rawhide mountains between 10 and 20 km depth this reflector has an arch like shape and is centered beneath or just west of the metamorphic core complex belt this event is underlain by a weaker approximately subhorizontal reflection at 24 km depth together these two discontinuities define a lens shaped midcrustal layer with a velocity of 6 35 6 5 km s 1 the apex of this midcrustal layer corresponds roughly to a region of major tectonic denudation and uplift 10 km defined by surface geologic mapping and petrologic barometry studies the layer thins to the northeast and is absent in the transition zone the 6 35 6 5 km s 1 velocities are compatible with a diorite composition or a mixture of mafic and silicic rocks this midcrustal layer is underlain by a higher velocity lower crustal layer that is modeled as only 3 6 km thick beneath the metamorphic core complex belt and regions to the southwest to the northeast however this layer thickens to 8 10 km as the midcrustal layer pinches out above it the velocity of the lower crust is constrained by traveltime modeling and is 6 6 0 15 kms 1 beneath the western transition zone and the metamorphic core complex belt higher velocities may be present farther to the southwest where the layer is thin the velocity of the lower crust is too low to accommodate significant amount of mafic underplating at the base of the crust instead we interpret the velocities to indicate that the lower crust is passively thinned beneath these regions without significant addition of mafic mantle derived intrusions the crust mantle boundary does not dome up beneath the core complexes but remains approximately subhorizontal at a depth of 26 28 km or in the case of the whipple mountains actually deepens a 3 km crustal root is modeled this lack of upward doming of the moho together with the vertical alignment of the metamorphic core complex belt over what are believed to be extension related structures in the middle and lower crust suggest that there is no lateral offset of upper crustal deformation from deeper zones of extension as one would expect if extension occurred along crust penetrating shear zones wernicke 1981 wernicke et al 1985 instead domed and inflated middle crust and thinned lower crust directly underlie the region of greatest thinning of the upper crust

large enigmatic crater structures offshore southern california summary digital mosaics of swath and conventional bathymetry data reveal large distinct near circular crater structures in the inner continental borderland offshore southern california two have maximum crater diameters that exceed 30 km and a third has a crater diameter of approximately 12 km all three features exhibit the morphology of large complex craters raised outer rim ring moat and central uplift yet their exact origin remains a mystery preliminary analyses of available seismic gravity and magnetic data over these structures reveal both similarities and distinct differences in geometry structure and geophysical signature to known impact sites all three crater structures however occur within the catalina terrane a highly extended volcanic and metamorphic province floored by catalina schist basement a likely alternative origin may thus involve explosive volcanism caldera collapse and resurgent magmatism and or possibly plutonism and schist remobilization associated with the catalina terrane no single model for crater formation whether impact caldera or pluton fully accounts for all of the present observations regarding the morphology internal structure and known geology of these near circular features timing of crater formation post dates the initial rifting and rotation of the western transverse ranges and appears to predate major right slip along the san clemente and san diego trough fault systems or approximately 18 to 16 ma regardless of their origin these complex craters represent some of the largest structures of their kind in western north america and provide a unique opportunity to better understand the development of unusual crater structures in a submarine environment

dinosterane and other steroidal hydrocarbons of dinoflagellate origin in sediments and petroleum abstract the steroidal alkanes of a selection of sediments and oils have been examined by gc ms with multiple metastable reaction monitoring specific 4 methyl sterane isomers have been identified by comparison with isomers synthesized from sterols isolated from dinoflagellates an immature marine oil shale and two mature marine oils of triassic to early cretaceous age contained high concentrations of c30 steranes comprising desmethyl 24 ethyl 4 methylcholestane and 4 23 24 trimethylcholestane dinosterane isomers an immature non marine oil shale and two non marine oils of cretaceous to eocene age contained stereoisomers of 24 ethyl 4 methylcholestane as the dominant c30 steranes reaction monitoring analyses in gc ms are particularly suited to unravelling complex distributions of homologous and stereoisomeric steroids encountered in oils and their source rocks

earthquake hazards of active blind thrust faults under the central los angeles basin california we document several blind thrust faults under the los angeles basin that if active and seismogenic are capable of generating large earthquakes m 6 3 to 7 3 pliocene to quaternary growth folds imaged in seismic reflection profiles record the existence size and slip rates of these blind faults the growth structures have shapes characteristic of fault bend folds above blind thrusts as demonstrated by balanced kinematic models geologic cross sections and axial surface maps we interpret the compton los alamitos trend as a growth fold above the compton ramp which extends along strike from west los angeles to at least the santa ana river the compton thrust is part of a larger fault system including a decollement and ramps beneath the elysian park and palos verdes trends the cienegas and coyote hills growth folds overlie additional blind thrusts in the elysian park trend that are not closely linked to the compton ramp analysis of folded pliocene to quaternary strata yields slip rates of 1 4 0 4 mm yr on the compton thrust and 1 7 0 4 mm yr on a ramp beneath the elysian park trend assuming that slip is released in large earthquakes we estimate magnitudes of 6 3 to 6 8 for earthquakes on individual ramp segments based on geometric segment sizes derived from axial surface maps multiple segment ruptures could yield larger earthquakes m 6 9 to 7 3 relations among magnitude coseismic displacement and slip rate yield an average recurrence interval of 380 years for single segment earthquakes and a range of 400 to 1300 years for multiple segment events if these newly documented blind thrust faults are active they will contribute substantially to the seismic hazards in los angeles because of their locations directly beneath the metropolitan area

active faulting and growth folding in the eastern santa barbara channel california we develop new methods to identify blind thrust fault systems determine fault slip rates and estimate potential earthquake magnitudes and recurrence intervals in active fold and thrust belts these methods are applied to compressive folds along the offshore oak ridge and blue bottle trends which overlie active blind thrust faults in the eastern santa barbara channel these folds and their causative faults are interpreted using fault bend fold theory and are represented in balanced models and cross sections that integrate surface and subsurface data the structures are mapped using a new technique of axial surface mapping in seismic reflection grids which defines three dimensional structural geometries and shows changes in slip and subsurface fault geometry along strike analysis of syntectonic growth sediments yields pliocene and quaternary fault slip rates of 1 3 mm yr on a deep thrust 16 km and 1 3 mm yr on shallower faults 2 5 km the combined 2 6 mm yr slip rate represents only part of the 6 mm yr of shortening measured by geodesy across the channel and estimated from relative pacific north american plate motions across the transverse ranges additional shortening is probably accommodated on other active thrusts in the western transverse ranges and in the northern channel along the santa barbara coast deformed seafloor sediments and a swarm of axial surface seismicity along the fold trends indicate that the underlying thrusts are active and may pose significant earthquake hazards to coastal southern california unsegmented fault surfaces are used through empirical relationships between fault surface area and rupture magnitude to estimate the sizes of potential earthquakes this analysis suggests that a ramp in the channel islands fault beneath the offshore oak ridge trend is capable of rupturing in a m s 7 2 earthquake earthquakes of this magnitude may release 2 m of slip which when combined with the estimated slip rate 1 3 mm yr yields an earthquake recurrence interval of 1500 yr for this channel islands fault ramp

global positioning system constraints on fault slip rates in southern california and northern baja mexico we use global positioning system gps estimates of horizontal site velocity to constrain slip rates on faults comprising the pacific north america plate boundary in southern california and northern mexico we enlist a simple elastic block model to parameterize the distribution and sum of deformation within and across the plate boundary we estimate a pacific north america relative plate motion rate of 49 3 mm yr one standard deviation consistent with nuvel 1a estimates we are able to resolve robust slip rate estimates for the southernmost san andreas san jacinto and elsinore faults 26 2 9 2 and 6 2 mm yr respectively and for the imperial and cerro prieto faults 35 2 and 42 1 mm yr respectively accounting for about 86 of the total plate motion the remaining 14 appears to be accommodated to the west of these fault systems probably via slip along the san clemente fault and or the san miguel vallecitos rose canyon and newport inglewood fault systems these results are highly consistent with paleoseismic estimates for slip rates implying that off fault strain accumulation within the deforming zone of the plate boundary is largely elastic we estimate that the seismically quiescent southernmost san andreas fault has incurred about 8 2 m of slip deficit over the last few hundred years presumably to be recovered during a future large earthquake

lacustrine carbonate reservoirs from early cretaceous rift lakes of western gondwana pre salt coquinas of brazil and west africa abstract barremian aptian lower cretaceous pre salt lacustrine carbonates host large petroleum reservoirs in south atlantic rift basins yet remain poorly studied in part due to the scarcity of lacustrine carbonate reservoirs throughout the geological record this review provides an integrated discussion of the current state of knowledge on the coquina play having been deposited in extensive rift lakes during the early stages of the opening of the south atlantic thick successions of coquina are recorded from the santos to sergipe alagoas basins and the kwanza to gabon basins on the brazilian and west african continental margins respectively coquinas exhibit similarities in depositional processes across the south atlantic however have distinct diagenetic histories and stratigraphic stacking patterns having been deposited in lakes with different geologies climates and water chemistries west african coquinas exhibit depositional characteristics that are indicative of deposition on distally steepened ramps and rimmed platforms and accordingly facies are somewhat different to those in brazilian basins which were deposited on low angle ramps as petroleum exploration continues into ultra deep waters on the brazilian and west african margins accurate facies models will be important for the predictability of carbonate reservoir intervals to this end onshore outcrop in the sergipe alagoas basin has revealed that the molluscan faunas have marine origins suggesting that facies patterns may be more complex than previously considered it is hoped that this review will provide the impetus for further research on the coquinas especially given the presence of poorly studied easily accessible onshore outcrops

geophysical evidence for the evolution of the california inner continental borderland as a metamorphic core complex we use new seismic and gravity data collected during the 1994 los angeles region seismic experiment larse to discuss the origin of the california inner continental borderland icb as an extended terrain possibly in a metamorphic core complex mode the data provide detailed crustal structure of the borderland and its transition to mainland southern california using tomographic inversion as well as traditional forward ray tracing to model the wide angle seismic data we find little or no sediments low 6 6 km s p wave velocity extending down to the crust mantle boundary and a thin crust 19 to 23 km thick coincident multichannel seismic reflection data show a reflective lower crust under catalina ridge contrary to other parts of coastal california we do not find evidence for an underplated fossil oceanic layer at the base of the crust coincident gravity data suggest an abrupt increase in crustal thickness under the shelf edge which represents the transition to the western transverse ranges on the shelf the palos verdes fault merges downward into a landward dipping surface which separates basement from low velocity sediments but interpretation of this surface as a detachment fault is inconclusive the seismic velocity structure is interpreted to represent catalina schist rocks extending from top to bottom of the crust this interpretation is compatible with a model for the origin of the icb as an autochthonous formerly hot highly extended region that was filled with the exhumed metamorphic rocks the basin and ridge topography and the protracted volcanism probably represent continued extension as a wide rift until 13 m y ago subduction of the young and hot monterey and arguello microplates under the continental borderland followed by rotation and translation of the western transverse ranges may have provided the necessary thermomechanical conditions for this extension and crustal inflow

a kinematic model for the late cenozoic development of southern california crust and upper mantle a model is presented for the young and ongoing kinematic deformation of southern california crust and upper mantle that accounts for both the known late cenozoic geologic record and upper mantle structure in this model the high velocity upper mantle transverse ranges anomaly is created through the convergence and sinking of the entire thickness of subcrustal lithosphere using typical scaling relations we estimate this anomaly to be 500 c colder and 1 more dense than average southern california mantle of the same depth we infer that subcrustal lithosphere from both sides of the convergence zone participates nearly equally in the construction of the anomaly the low velocity upper mantle anomaly beneath the salton trough region is attributed to high temperatures and 1 4 partial melt related to adiabatic decompression during mantle upwelling our model has crustal motions in the central transverse ranges region differing from mantle motions while the subcrustal lithosphere south of the transverse ranges moves toward and sinks beneath the transverse ranges the corresponding crust largely avoids convergence by adopting a more westerly direction of motion the differing velocity fields necessitates of a decoupling zone immediately south of the transverse ranges near the moho we interpret the upper mantle anomalies as results of small scale convection beneath southern california to which we attribute much of the recent tectonic activity in the region

salt geology and tectonics of selected brazilian basins in their global context this work addresses the geological and geophysical interpretation of salt structures in selected brazilian sedimentary basins from intracratonic palaeozoic evaporites in the amazon and solimoes basins to divergent margin evaporite basins formed during the mesozoic break up of gondwana there is an intriguing correlation between evaporite basins and hydrocarbon accumu lations in all the selected basins discussed the solimoes and amazonas basins developed eva porite depositing environments as the middle carboniferous sea was closing during a plate convergence phase the salt basin along the eastern brazilian and western african margins devel oped along the mesozoic rifts of the south atlantic regional seismic interpretation and potential field gravity and magnetic data over the eastern brazilian and west african margins suggest a very thick autochthonous salt layer deposited over rifted continental crust and particularly above the thick sag basin sediments over the hyperextended crust that marks the transition from continen tal to oceanic crust most of the hydrocarbon discoveries in the eastern brazilian and western african margins are in post salt turbidite and carbonate reservoirs but recent discoveries in the deepwater salt basins along the southeastern brazilian margin indicate that pre salt plays will rep resent an important contribution to hydrocarbon production in the near future