applications of organic geochemistry to paleolimnological reconstructions a summary of examples from the laurentian great lakes the organic matter content of lake sediments contains information that helps to reconstruct past environmental conditions evaluate histories of climate change and assess impacts of humans on local ecosystems the elemental isotopic and molecular compositions of organic matter buried in sediment provide evidence of the biota that have lived in a lake and its catchment area and they serve as proxies of organic matter delivery and accumulation sedimentary records from the north american great lakes provide examples of applications of organic geochemistry to paleolimnological reconstructions the records of these lakes date from retreat of the laurentide ice sheet around 12 ka include the mid holocene hypsithermal and show consequences of recent human changes low corg ntotal ratios indicate that most of the sediment organic matter in the great lakes is from algal production yet changes in biomarker molecule compositions also show that varying amounts of land plant organic matter have been delivered to the lakes elevated algal productivity that accompanies nutrient enrichments of lake waters is recorded as excursions to less negative 13c values in the organic matter of sediments that were deposited in the 1960s and 1970s increased organic carbon mass accumulation rates mirror the isotopic excursions in most parts of the great lakes accumulations of petroleum residues and pyrogenic polycyclic aromatic hydrocarbons in sediments identify fluvial and eolian delivery of organic matter components to different parts of the great lakes emerging applications of multiple and novel organic geochemical proxies to paleolimnological reconstructions are promising yet some potentially important measurements remain underutilized

organic geochemical proxies of paleoceanographic paleolimnologic and paleoclimatic processes abstract organic matter constitutes a minor fraction of marine and freshwater sediments yet its important contribution to the sedimentary record can be used to reconstruct marine and continental paleoenvironments the organic matter content of sediments is the residue of past biota the amounts and types of organic matter present in sediments consequently reflect environmental conditions that impacted ecosystems at different past times general sources of the organic matter are inferred from bulk properties such as elemental compositions carbon and nitrogen stable isotope ratios rock eval pyrolysis data and organic petrography details of organic matter origins are refined by analyses of biomarker molecular compositions source changes are proxies for fluctuations in sea level oceanic surface currents and continental climates algal paleoproductivity rates are indicated by organic carbon mass accumulation rates and carbon and nitrogen stable isotopic compositions these parameters record past availability of nutrients and therefore are proxies of surface mixing in the oceans and amount of land runoff to lakes sea surface paleotemperatures are recorded by the number of carbon carbon double bonds in lipid biomarkers produced by marine algae larger proportions of the double bonds are proxies for the cooler surface waters that accompanied periods of global glaciation and intervals of enhanced upwelling the 13c and d values of plant organic matter record past concentrations of carbon dioxide in the atmosphere and changes in delivery of atmospheric moisture respectively diagenesis which causes the concentration and composition of organic matter in sedimentary settings to differ from those of the original biologically synthesized materials can bias organic geochemical paleoenvironmental records the magnitude of this potential source of misinformation must always be considered and evaluated comparison of multiple organic geochemical proxies of past conditions helps to compensate for the effects of diagenetic alterations and thereby to improve interpretations of paleoenvironmental change

novel tricyclic and tetracyclic terpanes in tertiary deltaic oils structural identification origin and application to petroleum correlation abstract tricyclic terpanes of the cheilanthane series in crude oils and sediments are routinely used for geochemical correlation the cheilanthanes are particularly useful to identify crude oils derived from marine clastic or carbonate source rock as well as input to the source rocks from marine algal or terrigenous organic matter however they sometimes have limitations for the correlation of oils expelled from different source rocks that received similar organic matter e g oils from source rocks dominated by terrigenous organic matter novel non cheilanthane tricyclic terpanes consisting of c21 and c25 compounds and a c27 tetracyclic terpane were identified in oils from the assam india beaufort mackenzie canada and the niger nigeria deltas nmr data reveal that all three compounds have an oleanane type structure with rings c d and e intact while rings a and b have been degraded leaving a side chain attached to ring c in the case of the tricyclics and only a degraded a ring in the tetracyclic on the basis of occurrence nmr structure and co variance with oleanane abundance these new tricyclic and tetracyclic compounds are believed to form from a ring functionalised oleanoid precursors in angiosperm flowering plants the distribution of these compounds in representative oils from the niger delta accumulations permits separation of the oils into at least three distinct source rock organofacies

refrigeration of the western cordilleran lithosphere during laramide shallow angle subduction the laramide orogeny has generally been attributed to a shift from normal angle to shallow angle subduction beneath the western margin of the north american plate in addition to important mechanical effects this shift may have had important thermal effects on the lithosphere in the western cordillera before the laramide geothermal gradients in the western cordillera were probably normal to high because of the presence below of a hot asthenospheric wedge with the shift to shallow subduction the wedge was probably expelled and replaced by a cold subducting slab that extended just below the cordilleran lithosphere this would shift the western cordillera into a cold forearc like thermal setting dominated by the refrigeration effects of the subducting slab thermochronologic data from the sierra nevada great basin mojave desert western arizona and perhaps the colorado plateau record latest cretaceous early tertiary cooling that may be evidence of such refrigeration if regional refrigeration occurred it would have several important implications 1 metamorphism in the western cordillera would have waned with the start of the laramide orogeny 2 the crust in the western cordillera would have strengthened and become much more resistant to deformation e g gravitational collapse and extension of thickened crust in the sevier hinterland would have been impeded and 3 latest cretaceous early tertiary isotopic cooling ages may not be valid indicators of a period of major regional uplift and unroofng

gravity anomalies and flexure of the brazilian shield beneath the bolivian andes abstract seismological and geological data suggest that the brazilian shield is now underthrusting beneath the sub andes in order to constrain the minimum amount of shortening that results from this underthrusting we analyzed bouguer gravity anomalies over the shield and the bolivian andes assuming that they are due primarily to variations in crustal thickness that in turn are due to the flexure of an effectively elastic plate brazilian shield loaded by the weight of the andean mountain range gravity anomalies show that airy type isostatic equilibrium does not exist the sub andes are overcompensated and the chaco plain to the east is undercompensated the flexure of the plate can help explain both the overcompensation and the undercompensation if the brazilian shield behaves as an essentially elastic plate that extends at least 150 km and probably 200 km beneath the sub andes and the cordillera oriental and if the flexural rigidity of the plate is between 0 1 and 2 10 24 n m the fit of observed and calculated anomalies however is far from perfect and additional sources of short wavelength anomalies appear to be present assuming that the sediments and sedimentary rocks in the sub andes and cordillera oriental were deposited on the brazilian shield and that later they were overthrust eastward onto one another causing the present topography and the deflection of the plate we computed a family of estimates for the minimum amount of crustal shortening these estimates depend strongly on the amount of sedimentary rock deposited before deformation which does not seem to be well constrained by published geologic mapping nevertheless the calculations suggest that at least 100 km of crustal shortening have occurred and probably more implying that the andes may have been built mostly by crustal shortening

zircon and whole rock nd pb isotopic evidence for a grenville age and a laurentian origin for the basement of the precordillera in argentina suggestions that the precordillera of argentina has an early paleozoic connection to the appalachian ouachita rifted margin of the southern u s have been based on cambro ordovician stratigraphy and faunal assemblages further support comes from comparing precambrian grenville appalachian basement with xenoliths brought up in precordil leran miocene volcanic rocks u pb zircon ages and whole rock pb and nd isotopic data from the xenoliths show that precordilleran basement along with some part of the westernmost sierras pampeanas pie de palo range to the east is essentially unique in south america and is best interpreted as being derived from laurentia a grenville age is substantiated by u pb zircon upper intersect ages of 1102 pm 6 ma for a mafic xenolith and near 1100 ma for acidic xenoliths both xenolith types contain pink doubly terminated prismatic igneous zircons with relatively high u and pb concentrations rounded colorless low u and pb zircons with overgrowths produced in a later pre

p t t evolution of an early silurian medium grade shear zone on the west side of the famatinian magmatic arc argentina implications for the assembly of the western gondwana margin abstract the geodynamic evolution of the proto andean margin of gondwana during the paleozoic was characterized by repeated subduction processes associated with the docking of several terranes including the cuyania precordillera terrane and the chilenia terrane and the development of the calc alkaline famatinian continental magmatic arc in the sierra de la huerta 30 56 31 29 s and 67 17 67 32 w at the southwestern end of the western sierras pampeanas some mafic to ultramafic igneous bodies belonging to the famatinian arc were affected by regional metamorphism of the medium pressure granulite facies 7 7 5 kbar 850 c after this regional metamorphism local mylonitization under amphibolite facies along discrete nw se to nnw sse striking ductile shear zones occurred a 40 ar 39 ar plateau age of 432 4 ma was obtained on rather homogeneous hornblende porphyroclasts from a metagabbro mylonite the textures and mineral chemistry of the mylonitized metagabbro allow its p t t evolution from magmatic crystallization to mylonitization to be constrained geochronological data obtained from mylonite provide evidence that orogenesis was active at least until the early silurian this deformational event would have been related to uplift and decompression during the later stages of the orogenesis probably associated with the accretion of the precordillera terrane to the southwestern gondwana margin

palaeoenvironmental and palaeoecological change on the northern tethyan carbonate platform during the late barremian to earliest aptian a major shift from urgonian oligotrophic carbonate accumulation to orbitolinid rich mixed siliciclastic carbonate deposition is observed near the barremian aptian boundary in many sections both within and outside the shallow marine tethyan realm this important facies change in the swiss helvetic alps is documented here and interpreted in the context of general palaeoenvironmental change to achieve this a detailed micropalaeontological sedimentological mineralogical and geochemical study has been carried out on six sections across the upper part of the lower schrattenkalk member late barremian the rawil member formerly lower orbitolina beds earliest aptian and the lowermost part of the upper schrattenkalk member early aptian the sediments of the rawil member exhibit inner platform facies with rudists miliolids orbitolinids and dasycladals to outer platform facies characterized by small benthic foraminifera orbitolinids crinoids and bryozoans stratigraphic trends in microfacies environments and the composition of microfossil assemblages indicate that the rawil member includes a transgressive systems tract and the base of a highstand systems tract which are composed of an increasing number of parasequences in distal directions five to nine in the sections studied here the sea level rise discerned in the rawil member is coeval with increased detrital input and phosphorus burial with maximum values up to 80 times and 21 times the background values in the subjacent part of the lower schrattenkalk member respectively furthermore the rawil member records the appearance of kaolinite indicating a change towards tropical and more humid climate conditions this change may have led to an increase in continental weathering rates and an associated increase in detrital and nutrient fluxes towards the ocean the phase of climate change observed near the barremian aptian boundary may have been triggered by a phase of intensified volcanic activity linked with the onset of the ontong java large igneous province and the rawil member may be the expression of a precursor episode to oceanic anoxic event 1a in the shallow marine environment

hydrogen index as reflecting intensity of sulphidic diagenesis in non bioturbated shaly sediments abstract based on 13 published porewater h 2 s and sulphate profiles the amount of h 2 s escaping from non bioturbated shales varies between some few to 45 of the amount of bacterially generated h 2 s this finding permits calculation of the original organic carbon toc or content of immature nonbioturbated shales using toc and sulphur content data in two immature non bioturbated sequences from hungary toarcian and oligocene the first order correlation between hi and toc toc or was found to be stronger than that between hi and toc indicating that sulphate reduction was the leading process both in decrease in toc content and degradation of kerogen source potential

a case of post depositional aerobic degradation of terrestrial organic matter in turbidite deposits from the madeira abyssal plain abstract a set of oxidized and unoxidized sediment intervals from the f turbidite identified in two piston cores from the madeira abyssal plain map was analyzed for calcium carbonate total organic carbon toc total nitrogen tn stable carbon composition of toc 13c toc total hydrolyzable amino acids thaa total neutral sugars tsug lignin phenols lig two different lipid biomarkers of marine phytoplankton origin and vascular plantwax n alkanes and n acids comparison shows changes for all properties depicting the effect of post depositional aerobic oxidation the oxidation process re mineralized approximately 80 of the toc and approximately 60 of the tn in the original deposit and left the residual organic matter depleted in 13c by 1 7 2 9 thaa and tsug account for 10 or less of the toc in all samples leaving approximately 90 of the toc chemically unidentified thaa tsug lig marine phytoplankton biomarkers and plantwax n acids were lost from the oxidized deposit to an extent as great as that for toc two non protein amino acids alanine and aminobutyric acid and plant wax n alkanes displayed much less degradation 9 37 and concentrated 3 3 4 8 times relative to toc by the oxidation process these observations show that both marine and terrestrial components contribute to the re mineralized toc fraction a binary mixing approach was used to model quantitatively the change in 13c toc and plantwax n alkane concentration results suggest that terrestrial organic carbon contributes approximately 15 to the toc content of the original turbidite its abundance increases approximately 2 4 times as a consequence of the oxidation process and approximately 40 and 90 of the terrestrial and marine component of toc in the original turbidite respectively was destroyed by the oxidation process although selective preservation of terrestrial relative to marine organic carbon is a well documented phenomenon in sedimentary processes this study represents the first attempt to assess the sensitivity of terrestrial organic matter to oxidative degradation in a sedimentary environment

the mechanical state of the lithosphere in the altiplano puna segment of the andes abstract information about topography the shape of the geoid seismicity neogene deformation and volcanism in the region of altiplano puna of western south america is used to analyse the state of stress across the convergent plate margin in terms of the effects of topography and simple models of its compensation an average elevation near 4 km is consistent with compensation by a yet unresolved combination of crustal root and hot uppermost mantle producing a geoid high of 22 27 meters average horizontal compressive stress in excess of a reference sea level lithostatic value of 390 bars in a 150 km thick lithosphere and an average shear stress of 170 bars along a 30 dipping interplate boundary the basis for these estimates is evidence for a neutral to extensional stress regime within the high plateau contrasted with a compressional regime on the eastern slopes and along the interplate boundary itself comparison with other plateaus in a convergent plate tectonic setting suggests an evolutionary sequence from compressional to extensional tectonics as elevation of the plateau increases

gravity isostasy and andean crustal shortening between latitudes 30 and 35 s we started the work by analyzing an e w crustal section at 33 s latitude which extends from the western chilean trench crosses the andean belt and goes inland into argentina a two layer gravity seismic model was prepared here it provided a maximum moho depth of 62 km beneath the andean axis this model is consistent with a one layer standard crustal model with normal thickness t 33 km and densities of 2 9 g cm3 and 3 3 g cm3 for the crust and the upper mantle respectively then we analyzed the andean isostatic behaviour on four andean gravimetric sections at latitudes of 30 32 33 and 35 s based on one layer simple models these results show antiroots near the chilean trench and crustal thickenings beneath the andean belt which locate the moho discontinuity at depths of 70 km at 30 s 65 km at 32 and 33 s and 57 km at 35 s from the crustal gravimetric models the following provisional charts for the sector were prepared and compared 1 topography 2 bouguer gravity 3 isostatic corrections 4 moho contours these charts are consistent with each other and they point out that the compensation of the andean belt trends approximately to isostasy in the airy concept other results also confirm this 1 the remarkable decrease of the isostatic anomalies in each of the analyzed sections as regards the bouguer anomalies and 2 the relationships between the exceeding areas and the corresponding crustal root areas which are consistent with the airy numerical relations as to the perfect isostatic equilibrium thus the bouguer anomalies are mainly controlled by the m discontinuity finally shortenings in the andean sector were fixed based on the crustal thicknesses found from gravity inversion these are 150 km at 30 s decreasing southwards and reaching only 90 km at 35 s these shortenings explain the cenozoic andean uplift without requiring other mechanisms such as magmatic intrusions

sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization in two contrasting regions of the ocean the equatorial pacific and the southern ocean the 15n of core top sediments were strongly related to no3 in surface waters with distance from the equator in the equatorial pacific 15n increased from 7 to 16 as no3 decreased from 8 m to 0 1 m going from 60 to 30 s in the se indian ocean core top 15n increased from 5 to 11 as surface no3 decreased from 25 m to 0 1 m these results are strong evidence that sedimentary 15n in these regions is recording the increasing isotopic enrichment of near surface no3 with its depletion by phytoplankton in the case of the equatorial pacific 15n values for sinking particles collected at 150 m matched well the core top sediment values demonstrating little diagenetic alteration of the near surface generated isotopic signal these equatorial pacific data sets have variations with near surface no3 consistent with rayleigh fractionation kinetics for a fractionation factor u of 2 5 this value is substantially lower than previously found for temperate or polar regions perhaps as a result of differences in phytoplankton species assemblage or growth condition in the southern ocean south of the polar front comparison of 15n values for opal rich sediments south and sinking particles indicates an apparent 5 diagenetic enrichment relative to the surface generated signal that requires further investigation this exception aside our observations show that the surface water relationship of increasing 15n with increasing no3 depletion is generally transmitted to and preserved in the sediments an important requirement for further development and application of this important paleoceanographic tool

magnetic mineralogy of the lower continental crust rocks that were once present in the deep continental crust can be found in surface exposure as xenoliths as parts of high grade metamorphic and plutonic terrains and as obducted lower crustal cross sections a review of magnetic and mineralogic studies of samples representing all such occurrences shows that magnetite is the only magnetic phase present in significant amounts in these rocks thermodynamic considerations indicate that tipoor magnetite is the expected magnetic oxide under equilibrium conditions that exist in the lower crust regardless of the primary magmatic composition of the spinel phase furthermore it seems unlikely that serpentinites with metal alloys contribute significantly to the magnetization of the lower crust or mantle thus magnetite in the deep crust is the most likely candidate for the source of the long wavelength magnetic anomalies observed in regional aeromagnetic and satellite magnetometer surveys if so the entire crust should be magnetic in most continental areas including shields in areas of high heat flow the bottom of the magnetic crust may lie considerably above the moho

guidelines for assessing the petroleum potential of coaly source rocks using rock eval pyrolysis abstract coaly source rocks are sufficiently different from marine and lacustrine source rocks in their organic matter characteristics to warrant separate guidelines for their assessment using rock eval pyrolysis the rank threshold for oil generation is indicated by the increase in bi s1 toc at rank sr 9 10 tmax 420 430 c ro 0 55 0 6 and the threshold for oil expulsion is indicated by the peak in qi s1 s2 toc at rank sr 11 12 5 tmax 430 440 c ro 0 65 0 85 the pronounced rank related increase in hi s2 toc prior to oil expulsion renders the use of immature samples inappropriate for source rock characterisation a more realistic indication of the petroleum generative potential and oil expulsion efficiency of coaly source rocks can be gained from samples near the onset of expulsion alternatively effective hi values i e his near the onset of expulsion can be estimated by translating the measured his of immature samples along the maturation pathway defined by the new zealand or other defined coal band coaly source rocks comprise a continuum of coaly lithologies including coals shaly coals and coaly mudstones determination of the total genetic potential of coaly source rock sequences is best made using lithology based samples near the onset of expulsion

contourite drift construction influenced by capture of mediterranean outflow water deep sea current by the portimao submarine canyon gulf of cadiz south portugal abstract the margin of the gulf of cadiz is swept by the deep current formed by the mediterranean outflow water mow flowing from the mediterranean to the atlantic on the northern margin of the gulf algarve margin south portugal the mow intensity is low and fine grained contourite drifts are built up with an alongslope development from new sedimentological data this study emphasizes the presence of two types of contourite drifts separated only by a deep submarine canyon incising the slope with a north south orientation portimao canyon high resolution seismic and bathymetry interpretation shows that on the eastern side of the canyon the mow forms a thick and large detached drift albufeira drift prograding toward both north and west as shown in seismic profiles with a high sedimentation rate on this side of the canyon the mow intensity is high enough to erode the slope forming a moat channel alvarez cabral on the western side of portimao canyon the mow energy is lower preventing moat channel erosion only flat and thin drift develops portimao and lagos drifts with slow aggradation and a low sedimentation rate this difference in drift development is due to the presence of the canyon which generates an important change in hydrodynamic of the mow confirmed by temperature density measurements showing that mow flows down portimao canyon the canyon is responsible for the deviation of the direction of the mow as it partly catches the deep sea current flowing westward i e capture phenomenon it creates thus a decrease of the flow energy competency and capacity between the east and west sides of the canyon through this phenomenon of mow deep sea current capture the canyon constitutes a morphologic feature generating an important change in the contourite deposition pattern in addition to already known climatic and oceanographic influences our results show the role of canyons on contourite drift building this study provides new elements on autocyclic factors influencing the contourite sedimentation which are important to consider in future sedimentary paleo reconstruction interpretations

a high resolution local network study of the nazca plate wadati benioff zone under western argentina this paper presents a precise determination of the fine structure of a wadati benioff zone based on analysis of analog and digital seismic data recorded by a local network in the san juan province of western argentina the network is located above one of the subhorizontal segments of the subducted nazca plate without the complicating effects of a dipping slab on the locations of earthquakes the nearly flat geometry of the subducted plate provides an excellent opportunity to determine the thickness and structure of the wadati benioff zone we found that the depth of the wadati benioff zone beneath the network is 107 5 km for a set of digitally recorded events the distance between the shallowest and the deepest event gives a thickness of 20km with 90 of the data concentrated in a zone 12 km thick the spatial distribution of hypocenters shows no evidence of complex internal structure such as a double wadati benioff zone the local network data also suggest that in the region near 32 s where the dip of the wadati benioff zone changes from flat to steep the nazca plate flexes rather than tears at depths less than at least 125 km the intermediate depth seismicity is separated from an active zone of crustal seismicity by an aseismic region between depths of about 40 and 95 km the upper surface of the subducted plate inferred to be at about 90 km provides an upper limit to the thickness of the south american plate in this region an apparent westward dip of the wadati benioff zone as determined by the local network data can be accounted for by a 6 westward dip of the moho beneath the san juan region a result that is in agreement with regional gravity and topographic data

mechanical model for single thrust sheets part i taxonomy of crystalline thrust sheets and their relationships to the mechanical behavior of erogenic belts crystalline thrust sheets cts are those partially or wholly composed of metamorphic and or igneous rocks they have been known for more than a century we have recognized several types 1 thin skinned basement cover sheets 2 ophiolites 3 tectonic slides 4 basement uplifts and 5 composite thrust sheets the overthrust index ratio b may be obtained by dividing the total estimated displacement on cts by the distance from the 0 mgal contour on the prominent gravity gradient in an orogen to the last thrust in the foreland a fundamental property of cts is that the largest crystalline thrust sheet is always larger than the largest foreland thrust sheet in the same orogen this property is doubtlessly related to then relative strengths because they generally contain no pre existing subhorizontal weak zones cts are probably generated within the ductile brittle transition zone thicknesses of cts are thus determined by the geo thermal gradient and crustal lithology a mechanical model primarily for types 1 2 and 5 cts relates cross strike width x of thrust sheets to the compressive stress c friction b and dip of the basal thrust thickness of the thrust sheet at the hinter end t and average thickness t the model based upon an energy conservation argument predicts x c t b g t where g is gravity 980 gal and is average density of the allochthonous mass this equation can be used to relate data from ancient deeply eroded orogens to processes that were active during emplacement of cts for observed widths and thickness the model requires emplacement of composite type 5 thrust sheets by tectonic compression c 50 mpa and low friction thrusts b b 50 mpa

burial of terrestrial organic matter in marine sediments a re assessment 1 calculations based on recent observations indicate that approximately one third of the organic matter presently being buried in marine sediments may be of terrestrial origin with the majority of this terrestrial organic matter tom burial occurring in muddy deltaic sediments these calculations further suggest that the remineralization of terrestrial organic matter in the oceans is also much less efficient than that of marine organic matter these two underappreciated observations have important implications in terms of our understanding of the controls on the global carbon cycle from a paleoceanographic perspective the results presented here also suggest that changes in tom burial on glacial interglacial timescales have the potential to impact the global carbon cycle i e atmospheric co2 levels

uplift of the central andean plateau and bending of the bolivian orocline the topography of the central andes can be considered the primary tectonic signal of late cenozoic mountain building in an arid region where the effects of uplift and magmatism are little obscured by denudation the spatial coverage of the topographic signal is more complete than that for sparsely sampled geological and geophysical data a color coded image of digitized topography between 12 s and 37 s highlights the altiplano puna one of the world s most remarkable plateaus and reveals important physiographic clues about the formation of that major feature the topographic data combined with information on structure magmatism seismicity and paleomagnetism support a simple kinematical model for the late cenozoic evolution of the central andes the model does not require collisional effects or enormous volumes of intrusive additions to the crust but instead calls upon plausible amounts of crustal shortening and lithospheric thinning the model interrelates andean uplift a changing geometry of the subducted nazca plate and a changing outline in map view of the leading edge of the south american plate crustal shortening has accommodated convergence between the chilean peruvian forearc and the south american foreland the altiplano puna plateau can be constructed by a combination of crustal shortening and thickening and lithospheric thinning above a shallow dipping 20 30 subducted plate the seawardly concave bend of western south america the bolivian orocline was enhanced but not completely produced by an along strike variation in the amount of late cenozoic shortening maximum shortening in bolivia both produced the widest part of the plateau and increased the seaward concavity of the bolivian orocline the along strike variations of shortening are hypothesized to result from corresponding along strike variations in the width of a weakened zone in the overriding plate weakening occurs above the wedge of asthenosphere located between the subducted and overriding plates hence the width of the zone of weakening depends upon the dip of the subducted plate two types of shortening are recognized 1 a widespread basin and range laramide like shortening that characterizes modem activity in the sierras pampeanas and late miocene deformation of the altiplano puna and 2 on the eastern side of the cordilleras and plateau an east verging foreland fold thrust belt in which the underthrust foreland compresses and thickens the ductile lower crust and produces a plateau uplift of the upper crust the second type of shortening can be applied to plio quaternary deformations throughout the central andes but with a substantial narrowing of the region of plateau uplift in peru and south of 28 s a proposed monoclinal flexure of the upper crust on the western side of the plateau uplift explains the remarkably simple and regular morphology of the main western slope of the central andes the monocline is located above the tip of the asthenospheric wedge between the converging plates it is postulated to occur above the western limit of lower crustal thickening in the regions of horizontal subduction the monocline can be associated with a late miocene asthenospheric wedge tip

mesozoic unconformities in israel characteristics mode of origin and implications for the development of the tethys abstract nine breaks in the mesozoic stratigraphic column of israel are described in terms of their hiatus type of contact residual deposits and areal extent using these criteria and by reference to published global eustatic sea level curves three of the stratigraphic breaks are correlated with global eustatic sea level lowstands triassic jurassic boundary toarcian aalenian late turonian coniacian one probably represents a condensed sequence resulting from a rapid sea level rise middle callovian four are related primarily to rifting and extension tectonics of the incipient anisian carnian pliensbachian sinemurian or evolving early neocomian tethys and one reflects a folding phase of the syrian arc fold system related to the early closure stages of the tethys campanian maastrichtian by careful scrutiny of stratigraphic breaks on the margins of the arabian craton and by recognition of eustacy related events important stages in the tectonic evolution of the tethys can be reconstructed

controls on the oil and gas potential of humic coals abstract humic coals are highly heterogeneous both with respect to maceral content and chemical composition although the oldest known humic coals albeit gas prone are of devonian age the majority of humic coals capable of generating and expelling non volatile oil are of cretaceous and tertiary age from australia new zealand and southeast asia in contrast the middle jurassic humic coals of the sleipner and hugin formations in the south viking graben of the north sea are primarily gasprone but have the capability to expel aliphatic rich volatile oil these coals have toc contents up to 80 and hydrogen indices hi ranging from 200 to 400 they are vitrinite rich with some containing up to 45 inertinite liptinite concentrations are generally around 10 20 chiefly as spores pollen and abundant resinite the main control on the oil potential of humic coals is the concentration of long chain aliphatic hydrocarbons in the coal matrix although the aliphatics have a high h c ratio the inference on hydrogen content from the rock eval pyrolysis hi value can be misleading for hi values below 500 mg hc g oc there is no relation between hi and the potential of the coals to generate non volatile oil consequently the hi is a poor indicator of the non volatile oil potential of humic coals a more robust indicator is the relative amount of c8 aliphatic groups also the bulk measure of liptinite content may also prove misleading when assessing the oil potential of coals high resinite concentrations can lead to an overprediction of the non volatile oil potential as they contribute significantly to the hi value but will generate primarily volatile aromatic rich oils expulsion from the sleipner and hugin coals appears to have occurred in a gaseous phase with preferential release of short chain alkanes

origins and accumulation of organic matter in expanded albian to santonian black shale sequences on the demerara rise south american margin ocean drilling program leg 207 recovered thick sequences of albian to santonian organic carbon rich claystones at five drill sites on the demerara rise in the western equatorial atlantic ocean dark colored finely laminated cenomanian santonian black shale sequences contain between 2 and 15 organic carbon and encompass oceanic anoxic events 2 and 3 high rock eval hydrogen indices signify that the bulk of the organic matter in these sequences is marine in origin however d 13 corg values lie mostly between 30 and 27 and toc tn ratios range from 15 to 42 which both mimic the source signatures of modern c3 land plants the contradictions in organic matter source indicators provide important implications about the depositional conditions leading to the black shale accumulations the low d 13 corg values which are actually common in mid cretaceous marine organic matter are consequences of the greenhouse climate prevailing at that time and an associated accelerated hydrologic cycle the elevated c n ratios which are also typical of black shales indicate depressed organic matter degradation associated with low oxygen conditions in the water column that favored preservation of carbon rich forms of marine organic matter over nitrogen rich components underlying the laminated cenomanian santonian sequences are homogeneous dark colored lower to middle albian siltstones that contain between 0 2 and 9 organic carbon the organic matter in these rocks is mostly marine in origin but it occasionally

seismotectonics of thin and thick skinned deformation in the andean foreland from local network data evidence for a seismogenic lower crust local network data from san juan argentina provides new information about crustal seismicity in the andean foreland above a horizontal segment of the subducted nazca plate the authors find two areas of foreland seismicity one associated with the sierras pampeanas basement uplifts and the other beneath but not within the precordillera foreland fold thrust belt the precordillera seismicity provides direct evidence for basement deformation beneath the sediments of the thrust belt and supports the idea that its eastern part is significantly modified by underlying basement deformation in both areas events are concentrated between 15 and 35 km depth and have volumetric rather than planar fault like distributions the depth distribution is usually deep for intraplate earthquakes and suggests a brittle ductile transition near 30 35 km it also suggests the upper plate may have a geotherm similar to stable cratonic regions rather than a steeper one that may be expected due to a lithospheric thinning implied by the location of the subducted plate this can be understood as a transient effect of a neogene flattening of the dip of the subducted plate where a rapid thinning of the upper plate was followed with underplating by the cool subducted plate leaving the upper plate geothermmore unchanged while producing a thinned mechanically weaker lithosphere finally a spatial correlation of upper plate seismicity and structures with subducted plate seismicity raises the possibility that a major lithospheric structure cutting across the strike of the foreland near 31 degree s may be related to subduction of the juan fernandez ridge less

a review of nitrogen isotopic alteration in marine sediments key points use of sedimentary nitrogen isotopes is examined on average sediment 15n 14n increases approx 2 per mil during early burial isotopic alteration scales with water depth abstract nitrogen isotopes are an important tool for evaluating past biogeochemical cycling from the paleoceanographic record however bulk sedimentary nitrogen isotope ratios which can be determined routinely and at minimal cost may be altered during burial and early sedimentary diagenesis particularly outside of continental margin settings the causes and detailed mechanisms of isotopic alteration are still under investigation case studies of the mediterranean and south china seas underscore the complexities of investigating isotopic alteration in an effort to evaluate the evidence for alteration of the sedimentary n isotopic signal and try to quantify the net effect we have compiled and compared data demonstrating alteration from the published literature a 100 point comparison of sediment trap and surface sedimentary nitrogen isotope values demonstrates that at sites located off of the continental margins an increase in sediment 15n 14n occurs during early burial likely at the seafloor the extent of isotopic alteration appears to be a function of water depth depth related differences in oxygen exposure time at the seafloor are likely the dominant control on the extent of n isotopic alteration moreover the compiled data suggest that the degree of alteration is likely to be uniform through time at most sites so that bulk sedimentary isotope records likely provide a good means for evaluating relative changes in the global n cycle

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

origin and transformation of organic matter in pliocene pleistocene mediterranean sapropels organic geochemical evidence reviewed abstract drilling of deep water post messinian sedimentary sequences by odp legs 160 and 161 in the mediterranean sea has shown that occurrence of organic carbon rich sapropels and sapropel like sediments extends from the levantine basin westward into the alboran basin in the eastern mediterranean sapropel deposition started in the early pliocene whereas in the western basin the onset of sapropel formation occurred later in the early pleistocene precessional cycles are apparently the primary external forcing for sapropel formation nevertheless the pattern of sapropel occurrence suggests that the precessional influence is modulated by the glaciation cycles large differences were observed in the organic carbon contents of sapropels recovered in the eastern and western mediterranean correspondence between organic carbon contents rock eval hydrogen index values and elemental c n ratios indicate that both variations in the production and preservation of marine organic matter have led to the accumulation of high amounts of organic matter in sapropels molecular organic geochemical compositions of sapropels from the eastern mediterranean further confirm that the major fraction of organic matter in sapropels is derived from marine algal sources and has undergone variable oxidation enhanced marine productivity and improved preservation of organic matter is central to sapropel formation accumulation of increased amounts of land derived material at times of sapropel formation is also evidenced supporting the hypothesis of significant periodic freshwater discharges

structure of the subducting nazca plate beneath peru arrival times from intermediate depth 110 150 km earthquakes within the region of flat subduction beneath the subandean zone and foreland basins of east central peru provide constraints on the geometry and velocity structure of the subducting nazca plate hypocentral locations and origin times for these events were determined using observations from a 15 station digitally recording locator array deployed in the epicentral region of eastern peru observed p wave arrival times for coastal stations in peru some 3 6 from the epicenters are up to 4 s early relative to predicted arrival times based on the best fit velocity depth model used for hypocenter locations these large negative time residuals appear to be the result of propagation paths which have long segments in the colder higher velocity subducting plate p wave travel times were modeled for the effects of the slab using three dimensional 3 d ray tracing computed ray paths show that travel times to coastal stations for the eastern peru events can be satisfactorily modeled with average velocities relative to the surrounding mantle 6 lower within the uppermost slab assumed on the basis of other studies to be unconverted basaltic oceanic crust 6 km thick and 8 higher within the cold uppermost mantle of the slab ray tracing for this plate model shows that p wave shadow zones can occur if the source slab receiver geometry results in seismic rays passing through regions in which the dip angle of the slab changes significantly such geometries exist for seismic waves propagating to some coastal stations from intermediate depth earthquakes located east of the andes observed first arrival times for such cases do in fact have smaller negative residuals than those for geometries which allow for direct paths at similar distances modeling such arrivals as internally reflected waves propagating through the high velocity part of the plate produces a significant improvement in the travel time residuals for the slab velocities given above we obtain a model thickness of approximately 36 km for the cold slab interior and a slight northwest component of dip in the region of subhorizontal subduction

carbon isotope composition of organic compounds produced by abiotic synthesis under hydrothermal conditions although it is widely believed that production of organic compounds by fischer tropsch synthesis and related processes occurs in many geologic environments unambiguous identification of compounds with an abiotic origin in natural samples has been hampered by a lack of means to discriminate between abiotic compounds and organic matter from biological sources while isotopic compositions might provide a means to discriminate between biologic and non biologic sources of organic matter there are few data presently available to constrain the isotopic composition of compounds produced by abiotic processes in geologic systems here we report results of laboratory experiments conducted to evaluate the isotopic composition of organic compounds synthesized abiotically under hydrothermal conditions we find the organic products are depleted in 13c to a degree typically ascribed to biological processes indicating that carbon isotopic composition may not be a particularly effective diagnostic means to differentiate between biologic and non biologic sources furthermore our results suggest that the isotopic compositions of reduced carbon compounds found in many ancient rocks that have heretofore been attributed to biological sources could be consistent with an abiotic origin in a hydrothermal setting

significance of high c n ratios in organic carbon rich neogene sediments under the benguela current upwelling system corganic ntotal values change from 5 in middle miocene sediments deposited under the benguela current upwelling system to 15 in latest miocene and pliocene sediments although the change appears to record a larger proportion of land derived material in younger sediments bulk 13corg values indicate a predominance of marine origin for organic matter throughout the sequence marine paleoproductivity as represented by mass accumulation rates of caco3 and organic carbon increased over this time interval we postulate that the larger corganic ntotal values record a shift towards greater recycling rates of nitrogen rich relative to carbon rich organic matter components as the flux of total organic matter to the seafloor increased furthermore we propose that high corganic ntotal values may record enhanced paleoproductivity in other sedimentary sequences dominated by marine organic matter

applications of petroleum geochemistry to exploration and reservoir management abstract petroleum geochemistry improves exploration efficiency by accounting for many of the variables that control the volumes of crude oil and natural gas available for entrapment including source rock distribution richness and quality thermal maturity and the timing of generation migration accumulation relative to trap formation it is most powerful when used with other disciplines such as seismic sequence stratigraphy and reservoir characterization four key technology milestones form the basis for most modern applications of geochemistry to exploration these are the concepts and applications of 1 petroleum systems and exploration risk 2 biomarkers stable isotopes and multivariate statistics for genetic oil oil and oil source rock correlation 3 calibrated three dimensional thermal and fluid flow modeling and 4 controls on petroleum composition by secondary processes petroleum geochemistry offers rapid low cost evaluation tools to aid in understanding development and production problems some technology milestones in reservoir geochemistry include 1 assessment of vertical and lateral fluid continuity 2 determination of proportions of commingled production from multiple zones and leaky casing 3 prediction of oil quality in reservoir zones and 4 prediction of gas oil and oil water contact locations as described in the conclusions future research will continue a trend toward predictive geochemistry examples of predictive tools that draw major research support include piston core surveys to assess deepwater petroleum systems prior to drilling and three dimensional basin modeling to predict the regional timing of generation migration and accumulation of petroleum among other research objectives models are needed to better predict the distribution and quality of petroleum in reservoirs

preservation of elemental and isotopic source identification of sedimentary organic matter the amount and type of organic matter in the sediments of lakes and oceans contribute to their paleoenvironmental and paleoclimatological records only a small fraction of the initial aquatic organic matter survives destruction and alteration during sinking and sedimentation selective degradation modifies the character of the surviving small fraction of organic matter which becomes incorporated in bottom sediments organic matter alterations can continue to sub bottom depths of hundreds of meters corresponding to millions of years source and paleoenvironmental information nonetheless remains preserved in the molecular elemental and isotopic compositions of organic matter c n and 13c values of total organic matter in particular appear to retain paleoenvironmental information for multi myr time periods

genetic characterization of gas shows in the east mediterranean offshore of southwestern israel drilling in the eastern continental margin of the levant basin southeastern mediterranean offshore southwestern israel encountered light oil shows numerous vertically stacked gas pockets and more recently commercial gas discoveries chemical and stable isotopic compositions obtained for 24 gas samples from middle jurassic to pliocene reservoir rocks suggest that they represent a few genetic systems which are distributed vertically in the section in accordance with the stratigraphy these groups include bacterial gas in the pliocene a mixture of bacterial and thermogenic gas in the lower cretaceous and thermogenic gas in the jurassic section the bacterial gases in the pliocene and the lower cretaceous appear not to be cogenetic and likewise the thermogenic gas in the lower cretaceous and the jurassic sections excluding one case where the thermogenic gas appears to represent phase separation from a probably cogenetic light oil the relationship of either the bacterial or other thermogenic gas samples to particular potential source rocks in the section or to light oil shows encountered in the middle jurassic section is at this stage inconclusive

early diagenetic remineralization of sedimentary organic c in the gulf of papua deltaic complex papua new guinea net loss of terrestrial c and diagenetic fractionation of c isotopes oceania supplies 40 of the global riverine flux of organic carbon approximately half of which is injected onto broad continental shelves and processed in shallow deltaic systems the gulf of papua on the south coast of the large island of new guinea is one such deltaic clinoform complex it receives 4 mt yr 1 particulate terrestrial organic carbon with initial particle corg loading 0 7 mg m 2 corg loading is reduced to 0 3 mg m 2 in the topset upper foreset zones of the delta despite additional inputs of mangrove and planktonic detritus and high net sediment accumulation rates of 1 4 cm yr 1 carbon isotopic analyses 13c 14c of co2 and corg demonstrate rapid 100 yr remineralization of both terrestrial 13c 28 6 and marine corg 13c 20 5 ranging in average age from modern bomb 14c 60 to 1000 yr 14c 140 efficient and rapid remineralization in the topset upper foreset zone is promoted by frequent physical reworking bioturbation exposure and reoxidation of deposits the seafloor in these regions particularly 20 m apparently functions as a periodically mixed suboxic batch reactor dominated by microbial biomass although terrestrial sources can be the primary metabolic substrates at inshore sites relatively young marine corg often preferentially dominates pore water co2 relative to bulk corg in the upper foreset thus a small quantity of young rapidly recycled marine organic material is often superimposed on a generally older less reactive terrestrial background whereas the pore water co2 reflects both rapidly cycled marine and terrestrial sources terrestrial material dominates the slower overall net loss of corg from particles in the topset upper foreset zone i e recycled marine corg leaves little residue preferential utilization of corg subpools and diagenetic fractionation of c isotopes supports the reactive continuum model as a conceptual basis for net decomposition kinetics early diagenetic fractionation of c isotopes relative to the bulk sedimentary corg composition can produce changes in 14c activity independent of radioactive decay in the gulf of papua topset upper foreset 14c of pore water co2 averaged 300 greater than corg sediment between 1 3 m depth in deposits diagenetic fractionation and decomposition aging of sedimentary corg compromises simple application of 14c for determination of sediment accumulation rates in diagenetically reactive deposits

sterane biomarkers as indicators of palaeozoic algal evolution and extinction events abstract algae are among the oldest organisms known to inhabit the earth s biosphere with a record dating back some 2 7 ga only few marine algae produce hard parts that can be preserved over geologic time and consequently their fossil record is incomplete molecular fossils or biomarkers add complementary information to the fossil palynomorph record steroids are important constituents of eukaryotic cell membranes and are preserved in sediments as steranes c28 and c29 steranes are indicators for the presence of green and c27 steranes for the presence of red algae respectively the relative abundance of steranes allows the investigation of the fossil record for palaeozoic algal diversification and evolution in this study 500 palaeozoic rock samples representing a broad facies variety from the upper ordovician to the p t boundary were investigated for sterane compositions in addition three profiles covering the hirnantian the f f and the hangenberg event were analyzed for possible shifts in the sterane distribution associated with extinction events a sharp increase of the c28 c29 sterane ratio from 0 70 at the devonian carboniferous boundary implies a fundamental change in the green algae assemblage from more primitive mainly c29 sterane producing algae to modern c28 sterane producing algae for all investigated extinction events a pronounced but short lived rise in the c28 sterane content occurs that is attributed to an episodic increase in prasinophytes the gradual radiation of algae may have been triggered by frequent mass extinctions in the upper devonian culminating with the massive decline of acritarchs at the d c boundary the coeval rise in the c28 c29 sterane ratio indicates the presence of an non encysting algal group and coincides with the global augmentation of numerous filamentous codiacea siphonales and the rise of euspondyle and metaspondyle dasycladales

the transformation of southern israel from a swell to a basin stratigraphic and geodynamic implications for intracontinental tectonics abstract the history of southern israel during the 100 myr period between the cretaceous and the eocene is an example of epeirogenic movements which were controlled by magmatism and heating initially this region was uplifted and became a local swell then it subsided more than its surroundings and became a local basin this study analyzes the subsidence history of the region during its cooling for two purposes first to understand the relations between the tectonic pattern of subsidence and the resulting stratal geometry and second to constrain the volume of magmatism that intruded the lithosphere and perturbed the isostatic equilibrium our results show that lateral changes in the intensity of the tectonically driven subsidence are not always expressed by simple corresponding changes in the thickness of sedimentary units the most prominent example of this was found in the cenomanian section which thickens northward in absolute contrast with the tectonic pattern of subsidence during its deposition the reason for this discrepancy lies in lateral differences in the compactibility of the substrate upon which these sediments were deposited the two most important factors controlling the compactibility of the substrate are the thickness of previously existing sediments and their history of uplift since sediments that are reburied after an uplifting phase do not compact significantly until they reach their previous maximal depth we emphasize the need to reveal the amount of exhumation from the amount of erosion hidden in unconformities our second aim was achieved only in part because we cannot distinguish the subsidence caused by midlithospheric intrusions from secondary factors one possible scenario that satisfies the observations is a combination of midlithospheric intrusions equivalent to a 15 km thick sill plus thermal thinning of the lithosphere of the same order

some outstanding issues in the geology of lebanon and their importance in the tectonic evolution of the levantine region abstract the concept of pervasive and disruptive late neogene quaternary strike slip tectonics in lebanon has made establishing a satisfactory pre neogene palaeogeographic and palaeotectonic history for the area difficult research has allowed clarification of three fundamental issues that permit the setting of lebanon against its adjacent regions to be more firmly assigned the first issue is the identification of the significance of the 100 km long nne ssw trending monocline of the western lebanon flexure this feature is interpreted as a modified and rejuvenated early mesozoic hinge line that is presumed to be the northern continuation of the carmel sinai hinge line to the south the second issue is the correlation of the mount lebanon bekaa and anti lebanon complex with the adjacent intracontinental palmyride fold belt of syria through recognition in each of common divisions in structural style between a higher northern zone dominated by broadly spaced box folds and a lower southern zone with more numerous closely spaced and more internally complex folds the third issue involves the identification of major two stage late cretaceous to palaeogene syrian arc tectonism within lebanon resolution of these issues suggests a tectonic model for the region whereby the levantine region formed in two main phases the first of these was the multiphase late palaeozoic early mesozoic rifting of the area which imposed a fundamental extensional fabric over much of the region the second tectonic phase was the subsequent inversion of these extensional structures in the two stage syrian arc deformation of senonian and late palaeogene age which gave rise to the main lebanon structures it is this syrian arc tectonism rather than the late cenozoic strike slip that was probably the main structuring agent which affected the lebanon area the transpression from the braided dead sea fault motion although locally complex is an essentially neotectonic overprint on an already existing fabric offset of syrian arc structures in lebanon suggests that the total sinistral motion of the yammouneh fault segment of the dead sea fault is around 47 km

lateral velocity variations in the andean foreland in argentina determined with the jhd method abstract lateral velocity variations in the andean foreland near san juan province argentina were detected by analyzing the station corrections computed as part of the joint location of local earthquake data the events recorded by a portable network panda have intermediate and shallow depths about 100 km and less than 40 km respectively the jhd technique was applied to intermediate depth events and to two subsets of shallow events that concentrate on two distinct geologic units precordillera and pampean ranges the computed station corrections have a strong dependence on the data processed analysis of the different patterns of corrections shows that the corrections are consistent with the presence of large scale lateral velocity variations bounded by major fault systems based on these corrections a qualitative velocity model is proposed that has relatively high velocities between the eastern precordillera and a westerly dipping plane on the eastern edge of pie de palo one of the mountain blocks of the pampean ranges velocities are lower immediately east of this plane and between the central and eastern precordillera this model has important tectonic implications because it suggests that there is a major boundary between the central and eastern precordillera and that the pampean basement whose extent is unknown underlies the eastern precordillera the existence of large velocity variations across the precordillera is consistent with a westward shift of almost 10 km of the relocated intermediate depth events and with the removal of an apparent westward dip opposite to the direction of subduction observed in the wadati benioff zone as determined from single event location the analysis of the intermediate depth events also exemplifies the deleterious effect on the jhd results introduced by the cancellation of small singular values in the computation of the station corrections

geophysical study of the valle fertil lineament between 28 45 s and 31 30 s boundary between the cuyania and pampia terranes abstract a gravimetric and magnetometric study was carried out in the north eastern portion of the cuyania terrane and adjacent pampia terrane gravimetric models permitted to interpret the occurrence of dense materials at the suture zone between the latter terranes magnetometric models led to propose the existence of different susceptibilities on either side of the suture the curie temperature point depth representing the lower boundary of the magnetised crust was found to be located at 25 km consistent with the lower limit of the brittle crust delineated by seismic data this unusually thick portion of the crust is thought to release stress producing significant seismicity moho depths determined from seismic studies near western sierras pampeanas are significantly greater than those obtained from gravimetric crustal models considering mass and gravity changes originated by the flat slab nazca plate along cuyania and western pampia terranes it is possible to reconcile moho thickness obtained either by seismic or by gravity data thus topography and crustal thickness are controlled not only by erosion and shortening but by upper mantle heterogeneities produced by a the oceanic subducted nazca plate with normal slope also including asthenospheric materials between both continental and oceanic lithospheres b flat slab subducted nazca plate as shown in this work without significant asthenospheric materials between both lithospheres these changes influence the relationship between topographic altitudes and crustal thickness in different ways differing from the simple airy system relationship and modifying the crustal scale shortening calculation these changes are significantly enlarged in the study area future changes in nazca plate slope will produce changes in the isostatic balance

nitrogen cycling during the cretaceous cenomanian turonian oceanic anoxic event ii low 15n values ranging from 1 2 to 3 9 and atomic carbon nitrogen c n ratios of 25 50 are characteristic of black shales deposited during the cenomanian turonian boundary interval and oceanic anoxic event ii oae ii an observed antithetic relationship between c n ratios and 15n could suggest a predominance of terrestrially derived organic matter or a diagenetic control on 15n variability shifting bulk 15n values lower however hydrogen indices hi generally 450 and a positive correlation of hi with c n mitigates against a significant terrestrial organic matter fraction high c n values are likely the result preferential degradation of labile n rich compounds during early diagenesis and loss of n as ammonium from sediments through time a hypothetical model that considers the degradation of a 15n enriched labile protein fraction yields only small 1 2 negative shifts in 15n however 15n depletion during diagenesis is contrary to normal isotope kinetics which should result in 15n enrichment of bulk organic matter therefore we conclude that the bulk 15n values in this study reflect primary changes in the nitrogen cycle the 15n data support the hypothesis of expanded nitrogen fixation driven by upwelling of nutrient nitrogen poor phosphorus replete waters during oae ii and from the mid cenomanian to santonian at demerara rise the low 15n values which are significantly lower than bulk 15n values in modern regions where nitrogen fixation is known to be important probably result from a more significant fraction of dissolved inorganic nitrogen being produced by nitrogen fixation during the peak of oae ii a marked shift to lower 15n values is observed this shift possibly reflects greater utilization of 15n depleted ammonium during a chemocline upward excursion cue dominance of low 15n values from other periods of more widespread marine anoxia is likely the result of similar processes

earthquake deformation cycle in the andean back arc western argentina repeated leveling surveys conducted in 1938 1967 1975 1976 1978 1979 1980 and 1981 provide information on preseismic coseismic and postseismic movements associated with the ms 7 4 1977 caucete argentina earthquake sequence the spatial pattern and amount of uplift 1 m that accompanied the earthquake suggest 4 m of slip on a north south trending west dipping fault buried 17 km below the surface with a dip of 35 a downdip width of 24 km and a length of 80 km these fault parameters are generally consistent with the seismic focal mechanism the moment of the double main shock and the distribution of aftershocks vertical movements during the first preseismic period 1938 1967 while an order of magnitude smaller have a spatial pattern similar to the observed coseismic movements the second period of preseismic deformation 1967 1976 is characterized by movements opposite those which accompanied the earthquake 2 1 cm of relative subsidence during the first postseismic interval 1978 1980 deformation continued in the same sense as the coseismic movements an additional 10 1 cm of relative uplift subsequent postseismic movements 1980 1981 were opposite in sense to the coseismic deformation 12 1 cm of relative subsidence we suggest that the preseismic movements for the 1938 1967 time period result from reverse slip 50 cm on a downdip section of the 1977 earthquake fault this slip may have been induced by a ms 7 5 earthquake that occurred near san juan in 1944 or may represent coseismic slip for a m 6 5 earthquake that occurred in 1941 deformation during the second preseismic period 1967 1976 is interpreted as being due to steady strain accumulation on the fault 9 mm yr 1 the initial postseismic movements 1978 1980 are roughly consistent with afterslip on the 1977 earthquake fault 40 cm in the same sense as the coseismic slip the second phase of postseismic deformation may represent the period of rapid postseismic recovery i e strain accumulation observed previously in interplate areas there is some evidence that strain accumulation following the earthquake began on the updip section of the fault this process of strain accumulation may be responsible for the fact that the 1977 main shock initiated at the updip end of the coseismic fault assuming that the time predictable model of earthquake occurrence is applicable to this area we estimate that the next earthquake on the 1977 earthquake fault will occur in 200 600 years

crustal thickness variation in the andean foreland argentina from converted waves abstract local network three component digital data from the san juan area argentina provide the first seismological images of the deep crustal structure in the andean foreland above a horizontal segment of the subducted nazca plate we have identified s to p seismic phases converted on the moho by analysis of seismograms formed by taking the product of the radial and vertical components r z from intermediate depth earthquakes in the benioff zone directly beneath the network under the sierras pampeanas the moho is estimated to be at a depth of about 52 km beneath the eastern precordillera the moho deepens to 55 57 km and further west beneath the central precordillera to 60 km we therefore estimate a 5 to 10 westward dip to the moho under the andean foreland in both the sierras pampeanas and precordillera provinces the thickness of the ductile lower crust is comparable to that for a stable continental crust while the brittle upper crust implied from the spatial distribution of the seismicity shows an important thickness increase this rheological behavior is interpreted to be a consequence of the flattening of the dip of the subducted plate the east west deepening of the seismicity the moho and the depth to basement under the precordillera all suggest that the andean foreland lithosphere is underthrusting beneath the andes the associated shortening is estimated to be about 15 across the sierras pampeanas it is estimated to be at least 38 across the precordillera for the part of the crust beneath the decollement while it is up to 70 for the part above the decollement

the subduction of young lithosphere studies using a variety of techniques of the subduction beneath western south america and the southwest honshu kyushu region of japan indicate that volcano free segments occur where the subducted and continental lithospheres remain in contact without intervening asthenosphere the subduction is initiated at normal dip angles but the plate deforms at some depth 100 km under central peru to travel horizontally immediately beneath continental lithosphere the most plausible reason for this geometry is that the subducted plate is buoyant a model is developed constrained by age of the plate bathymetry and heat flow estimates of the density of oceanic plates as a function of age show that younger ocean floor may be less dense than the asthenosphere into which it subducts if the high density tranformation of crustal basalt to eclogite is retarded by low temperatures the plate can remain buoyant to considerable depth for long periods of time heat flow data from western south america are consistent with this model

basement seismicity beneath the andean precordillera thin skinned thrust belt and implications for crustal and lithospheric behavior data from a digitally recording seismic network in san juan argentina provide the first images of crustal scale basement faults beneath the precordillera this seismicity is near the boundary between the precordillera a thin skinned thrust belt and the sierras pampeanas a region of thick skinned basement deformation two seismically active tectonic provinces of the andean foreland the seismicity data support models for this region in which crustal thickening rather than magmatic addition or thermal uplift plays the dominant mountain building role the precordillera seismicity occurs in three segments distributed north to south the southern segment is an area of diffuse activity extending across the precordillera and eastward into the sierras pampeanas that shows no patterns in map or cross section the northern and central segments have well defined dipping planes that define crustal scale faults extending from 5 to 35 km depth it is clear from the relative fault geometries that the overlying precordillera is not simply related to the basement activity the seismicity here may result from reactivation of an ancient suture between the precordillera and pampeanas terranes or be occurring in basement of unknown affinity west of the suture the seismicity provides the first constraints on basement fault geometries and we present models integrating this information with the surface geology these basement faults may have been responsible for the 1944 ms 7 4 earthquake that destroyed the city of san juan the imaging of these faults suggests that seismic risk estimates for san juan made on the basis of surface geologic studies may be too low

curie point depths beneath precordillera cuyana and sierras pampeanas obtained from spectral analysis of magnetic anomalies abstract a gravity and magnetic survey has been carried out with the purpose of investigating geophysical features of the crusts beneath three geological provinces in western argentina cuyo precordillera the sierras pampeanas of san juan and la rioja and famatina system the results of which are displayed in three maps bouguer anomaly total field magnetic anomaly and total field reduced to the pole the top and bottom boundaries of the magnetized crust were calculated from power density spectra of the total field anomalies from our terrestrial database in 90 2d windows the depths obtained for the bottom of magnetized crust are assumed to correspond to curie point depths the values thus obtained for the precordillera range between 29 and 40 km whereas for the sierras pampeanas in the sierra de pie de palo and other mountain chains along the bermejo desaguadero lineament or valle fertil lineament such depth ranges between 20 and 35 km these results are consistent with curie point depths determined on different continental regions in the world a map of regional heat flow has been prepared in the present work based on the depth of the curie point isotherm which shows that heat flow patterns in precordillera are different from those found in sierras pampeanas a significant heat flow extending in northeast southwest direction seems to be directly related to juan fernandez ridge trace the results of the present investigations also point out a possible relationship between the base of the magnetized crust and the boundary separating the brittle from the ductile crustal regime

significance to hydrocarbon exploration of terrestrial organic matter introduced into deep marine systems insights from the lower cretaceous in the levant basin abstract oil shows from wells in the levant basin and egypt are suggestive of potential economic discoveries within unpenetrated mesozoic reservoirs in the eastern mediterranean analysis of some of these oils indicate a non marine source of mixed deltaic terrestrial or lacustrine origin for the organic matter data from barremian aptian sediments from the mnuph 1 core in northern israel opens a window on potential importance of terrestrial sourced kerogen deposited in source rocks in a marine depositional environment core analyses of the barremian nabi sa id fm demonstrate the occurrence of a gas prone immature source rock of 77 m gross thickness averaging 4 6 toc peaking at 37 toc rock eval pyrolysis biomarker n alkane sterane and tricyclic terpane indices atomic c n ratios and 13corg ranges indicate mixed terrestrial marine organic source rocks for the barremian while higher up the aptian hidra fm contains source rocks with organic matter derived dominantly from carbohydrate rich land plants the interval at mnuph 1 includes abundant evidence for off shelf mass transport of terrestrial and proximal marine sediments into the deep marine basin these features are used to explain the anomaly of a terrestrial source for organic matter in this setting mass transport and consequent mixing of distal marine proximal marine and land derived sediment is in fact the norm at the foot of continental slopes globally the dominantly type iii source rocks described here would be unlikely to expel oil but might produce thermogenic gas thermogenic gas identified in similar marine settings might wrongly be interpreted as having been generated from marine organic matter misleadingly indicative of a prospective oil source when prospecting for deep rooted hydrocarbon accumulations in offshore marine settings an alternative system dominated by basinward sediment transport of gas prone terrestrial source rocks into strategically positioned toe of slope marine settings is supported here and could well be producing gas in the levant basin

organic geochemistry of the early toarcian oceanic anoxic event in hawsker bottoms yorkshire england a comprehensive organic geochemical investigation of the hawsker bottoms outcrop section in yorkshire england has provided new insights about environmental conditions leading into and during the toarcian oceanic anoxic event t oae 183 ma rock eval and molecular analyses demonstrate that the section is uniformly within the early oil window hydrogen index hi organic petrography polycyclic aromatic hydrocarbon pah distributions and tricyclic terpane ratios mark a shift to a lower relative abundance of terrigenous organic matter supplied to the sampling locality during the onset of the t oae and across a lithological transition unlike other ancient intervals of anoxia and extinction biomarker indices of planktonic community structure do not display major changes or anomalous values depositional environment and redox indicators support a shift towards more reducing conditions in the sediment porewaters and the development of a seasonally stratified water column during the t oae in addition to carotenoid biomarkers for green sulfur bacteria gsb we report the first occurrence of okenane a marker of purple sulfur bacteria psb in marine samples younger than 1 64 ga based on modern observations a planktonic source of okenane s precursor okenone would require extremely shallow photic zone euxinia pze and a highly restricted depositional environment however due to coastal vertical mixing the lack of planktonic okenone production in modern marine sulfidic environments and building evidence of okenone production in mat dwelling chromatiaceae we propose a sedimentary source of okenone as an alternative lastly we report the first parallel compound specific c13 record in marine and terrestrial derived biomarkers across the t oae the c13 records of short chain n alkanes acyclic isoprenoids and long chain n alkanes all encode negative carbon isotope excursions cies and together they support an injection of isotopically light carbon that impacted both the atmospheric and marine carbon reservoirs to date molecular c13 records of the t oae display a negative cie that is smaller in magnitude compared to the bulk organic c13 excursion although multiple mechanisms could explain this observation our molecular petrographic and rock eval data suggest that variable mixing of terrigenous and marine organic matter is an important factor affecting the bulk organic c13 records of the t oae

paleomagnetic confirmation of the laurentian origin of the argentine precordillera abstract several recent tectonic models have portrayed the argentine precordillera has been recently portrayed as an early paleozoic laurentian derived exotic terrane now found in southwestern south america these models have primarily been based on strong biogeographic and stratigraphic evidence however no paleomagnetic data have previously been available to independently test them a paleomagnetic study was therefore carried out on the early cambrian cerro totora formation exposed in the northern reaches of the argentine precordillera after stepwise thermal demagnetization a pre folding remanence was identified in ten sites of this formation yielding a paleomagnetic pole ct at 37 0 n 314 1 e a95 5 8 this pole is not consistent with the latest proterozoic early paleozoic apparent polar wander path for gondwana but it agrees with the early cambrian section of the laurentian path if the argentine precordillera is positioned as the conjugate margin of the ouachita embayment in southeast laurentia this result confirms that the precordillera is an allochthonous terrane derived from laurentia in cambrian times that was later accreted to gondwana probably in middle ordovician times