emergent long lived gondwanaland vs submergent short lived laurasia supercontinental and pan african heat imparts long term buoyancy by mafic underplating gondwanaland was buoyant as indicated by nonmarine facies and laurasia was depressed as indicated by marine facies as a supercontinent gondwanaland lasted much longer than laurasia and was hotter from internal heat moreover the pan african orogenic cycle confined to gondwanaland augmented the heat supply which possibly generated a permanently buoyant lower crust by underplating a crustal layer in the australian proterozoic shield with subhorizontal reflectors and velocity vp 7 5 km s is interpreted as mafic underplating beneath latest neoproterozoic flood basalt the pan african terrane in east africa also contains evidence of mafic underplating and most of gondwanaland but not laurasia was affected by terminal pan african 0 5 ga uplift and cooling

biomarkers from huronian oil bearing fluid inclusions an uncontaminated record of life before the great oxidation event we report detailed molecular geochemistry of oil bearing fluid inclusions from a ca 2 45 ga fluvial metaconglomerate of the matinenda formation at elliot lake canada the oil most likely derived from the conformably overlying mckim formation was trapped in quartz and feldspar during diagenesis and early metamorphism of the host rock probably before ca 2 2 ga the presence of abundant biomarkers for cyanobacteria and eukaryotes derived from and trapped in rocks deposited before the great oxidation event is consistent with an earlier evolution of oxygenic photosynthesis than previously thought and suggests that some aquatic settings had become sufficiently oxygenated for sterol biosynthesis by this time it also implies that eukaryotes survived several extreme climatic events including the paleoproterozoic snowball earth glaciations the extraction of biomarker molecules from paleoproterozoic oil bearing fluid inclusions thus establishes a new method using low detection limits and system blank levels to trace evolution of life through earth s early history that avoids the potential contamination problems affecting shale hosted hydrocarbons

the paleoproterozoic snowball earth a climate disaster triggered by the evolution of oxygenic photosynthesis although biomarker trace element and isotopic evidence have been used to claim that oxygenic photosynthesis evolved by 2 8 giga annum before present ga and perhaps as early as 3 7 ga a skeptical examination raises considerable doubt about the presence of oxygen producers at these times geological features suggestive of oxygen such as red beds lateritic paleosols and the return of sedimentary sulfate deposits after a 900 million year hiatus occur shortly before the 2 3 2 2 ga makganyene snowball earth global glaciation the massive deposition of mn which has a high redox potential practically requires the presence of environmental oxygen after the snowball new age constraints from the transvaal supergroup of south africa suggest that all three glaciations in the huronian supergroup of canada predate the snowball event a simple cyanobacterial growth model incorporating the range of c fe and p fluxes expected during a partial glaciation in an anoxic world with high fe oceans indicates that oxygenic photosynthesis could have destroyed a methane greenhouse and triggered a snowball event on timescales as short as 1 million years as the geological evidence requiring oxygen does not appear during the pongola glaciation at 2 9 ga or during the huronian glaciations we argue that oxygenic cyanobacteria evolved and radiated shortly before the makganyene snowball

tectonic geomorphology of the southeastern mississippi embayment in northern mississippi usa we analyzed drainage basin geometry in part of the southeastern mississippi embayment near the new madrid seismic zone in order to detect areas of lateral stream migration that could indicate recent tectonism toward this goal a field of drainage basin asymmetry vectors was generated from a digital terrain model and compared to geologic structure lithofacies seismicity remotely sensed lineaments and major drainage divides transverse topographical drainage basin asymmetry t index results define geomorphic domains in northern mississippi 33 to 35 n 88 5 to 90 25 w that may reflect deep crustal blocks bounded by active faults or flexures t index measurements a proxy for lateral stream migration of more than 4500 second order drainage basins produced a two dimensional field of 282 spatially averaged vectors from which we interpreted 12 domains some domain boundaries correspond to mapped faults of two paleozoic fault systems in our study area the northeast striking mississippi valley graben system and the northwest striking ouachita thrust belt and foreland faults several other prominent domain boundaries may be previously unmapped faults flexures parallel to these systems this interpretation is supported by satellite image analysis of the northeastern study area which reveals lineaments consistent with our domain boundaries that have strikes that reflect the regional subsurface structural grain

reappraisal of hydrocarbon biomarkers in archean rocks hopanes and steranes found in archean rocks have been presented as key evidence supporting the early rise of oxygenic photosynthesis and eukaryotes but the syngeneity of these hydrocarbon biomarkers is controversial to resolve this debate we performed a multilaboratory study of new cores from the pilbara craton australia that were drilled and sampled using unprecedented hydrocarbon clean protocols hopanes and steranes in rock extracts and hydropyrolysates from these new cores were typically at or below our femtogram detection limit but when they were detectable they had total hopane 37 9 pg per gram of rock and total sterane 32 9 pg per gram of rock concentrations comparable to those measured in blanks and negative control samples in contrast hopanes and steranes measured in the exteriors of conventionally drilled and curated rocks of stratigraphic equivalence reach concentrations of 389 5 pg per gram of rock and 1 039 pg per gram of rock respectively polycyclic aromatic hydrocarbons and diamondoids which exceed blank concentrations exhibit individual concentrations up to 80 ng per gram of rock in rock extracts and up to 1 000 ng per gram of rock in hydropyrolysates from the ultraclean cores these results demonstrate that previously studied archean samples host mixtures of biomarker contaminants and indigenous overmature hydrocarbons therefore existing lipid biomarker evidence cannot be invoked to support the emergence of oxygenic photosynthesis and eukaryotes by 2 7 billion years ago although suitable proterozoic rocks exist no currently known archean strata lie within the appropriate thermal maturity window for syngenetic hydrocarbon biomarker preservation so future exploration for archean biomarkers should screen for rocks with milder thermal histories

the tectonic evolution of the south atlantic from late jurassic to present an improved tectonic database for the south atlantic has been compiled by combining magnetic anomaly geosat altimetry and onshore geologic data we used this database to obtain a revised plate kinematic model starting with a new fit reconstruction for the continents around the south atlantic we present a high resolution isochron map from chron m4 to present fit reconstructions of south america and africa that require rigid continental plates result in substantial misfits either in the southern south atlantic or in the equatorial atlantic to achieve a fit without gaps we assume a combination of complex rift and strike slip movements 1 along the south american parana chacos basin deformation zone 2 within marginal basins in south america salado colorado basin and 3 along the benue trough niger rift system in africa these faults are presumed to have been active before or during the breakup of the continents our model describes a successive unzipping of rift zones starting in the southern south atlantic between 150 ma tithonian and approximately 130 ma hauterivian rifting propagated to 38 s causing tectonic movements within the colorado and salado basins subsequently between 130 ma and chron m4 126 5 ma the tip of the south atlantic rift moved to 28 s resulting in intracontinental deformation along the parana chacos basin deformation zone between chron m4 and chron mo 118 7 ma rifting propagated into the benue trough and niger rift inducing rift and strike slip motion after chron mo the equatorial atlantic began to open while rifting and strike slip motion still occurred in the benue trough and niger rift since chron 34 84 ma the opening of the south atlantic is characterized by simple divergence of two rigid continental plates

biomarkers brines and oil in the mesoproterozoic roper superbasin australia gas chromatography mass spectrometry of oil inclusions from the ca 1430 ma marine roper group in the roper superbasin australia provides a new source of information about the early biosphere and proterozoic petroleum systems oil most likely derived from an overlying shale was trapped at 60 c as abundant oil inclusions within transgranular microfractures in detrital quartz during mesoproterozoic basin inversion the oil is very mature and has a wide range of biomarkers derived mainly from cyanobacteria but lacks eukaryote biomarkers unlike associated solid bitumens the inclusion oil is nonbiodegraded evidently the inclusions remained closed systems sheltered from postentrapment alteration and contamination because fluid inclusions have preserved biomarkers for 1000 m y they constrain the diversity of primordial ecosystems whereas other forms of early precambrian organic matter are usually absent or metamorphosed

a reconstruction of archean biological diversity based on molecular fossils from the 2 78 to 2 45 billion year old mount bruce supergroup hamersley basin western australia abstract bitumens extracted from 2 7 to 2 5 billion year old ga shales of the fortescue and hamersley groups in the pilbara craton western australia contain traces of molecular fossils based on a combination of molecular characteristics typical of many precambrian bitumens their consistently and unusually high thermal maturities and their widespread distribution throughout the hamersley basin the bitumens can be characterized as probably of archean age accepting this interpretation the biomarkers open a new window on archean biodiversity the presence of hopanes in the archean rocks confirms the antiquity of the domain bacteria and high relative concentrations of 2 methylhopanes indicate that cyanobacteria were important primary producers oxygenic photosynthesis therefore evolved 2 7 ga ago and well before independent evidence suggests significant levels of oxygen accumulated in the atmosphere moreover the abundance of cyanobacterial biomarkers in shales interbedded with oxide facies banded iron formations bif indicates that although some archean bif might have been formed by abiotic photochemical processes or anoxygenic phototrophic bacteria those in the hamersley group formed as a direct consequence of biological oxygen production biomarkers of the 3 methylhopane series suggest that microaerophilic heterotrophic bacteria probably methanotrophs or methylotrophs were active in late archean environments the presence of steranes in a wide range of structures with relative abundances like those from late paleoproterozoic to phanerozoic sediments is convincing evidence for the existence of eukaryotes in the late archean 900 ma before visible fossil evidence indicates that the lineage arose sterol biosynthesis in extant eukaryotes requires molecular oxygen the presence of steranes together with biomarkers of oxygenic photosynthetic cyanobacteria suggests that the concentration of dissolved oxygen in some regions of the upper water column was equivalent to at least 1 of the present atmospheric level pal and may have been sufficient to support aerobic respiration

temporal and spatial correlations between changes in plate motions and the evolution of rifted basins in africa we present the results of a backstripping study incorporating over 200 wells in 14 different basins or groups of basins located on the african interior or continental margins these results presented as mean tectonic subsidence rates averaged over appropriate periods are compared with the opening histories and changes in plate motions of the atlantic and indian oceans and the tethys mediterranean area for seven time steps from the late jurassic until present the results indicate a temporal and spatial correlation between phases of rapid subsidence in african basins and changes in plate motions and therefore are promising in terms of applicability of large scale plate motions in the studies of paleo stresses in sedimentary basins especially at times of initial rifting and formation of subduction zones reactivation of the basins appears to be primarily controlled by the orientation of the basin and underlying basement structures with respect to the stress direction the basins are preferably formed within weaker zones mostly pan african shear zones and the distance from the active plate boundary to the weak zone plays a minor role in whether or not it will be reactivated the presence of three important hot spots which were active during the early stages of break up of gondwana may have been important in preweakening the lithosphere at their sites this may have played an additional role in the localization of the basin deformation

early cretaceous rifts of western and central africa an overview abstract the structure and evolution of early cretaceous rift basins in western and central africa are described two stages of rift development and fracturing have been identified 1 from neocomian to early aptian roughly e w and nw trending troughs upper benue n cameroon s chad sudan etc opened in response to a submeridian extensional regime in central africa while in western africa the n s trending transsaharian fault zone acted as a sinistral wrench 2 from middle aptian to late albian large northwest trending troughs e niger sudan sirte etc opened in response to a northeast extensional regime while the central african fault zone from benue to sudan exhibited strike slip movements generating pull apart basins these rift and fracture systems delimit three large blocks within the african plate a western block an arabian nubian block and an austral block the arabian nubian block tends to separate from the two other blocks migrating towards the north during the first stage of basin development and then towards the ne during the second stage the opening of the atlantic ocean was the dominant driving force for the western and austral blocks while the arabian nubian block probably moved in response to the opening of the indian ocean and to the evolution of the tethyan margin

geochemistry and sr nd pb isotopic composition of the central atlantic magmatic province camp in guyana and guinea abstract the central atlantic magmatic province camp is one of the largest igneous provinces on earth extending more than 5000 km north to south on both sides of the atlantic ocean its emplacement occurred about 200 ma ago at the triassic jurassic boundary and is linked to the initial breakup of pangaea two areas of the province are studied here french guyana surinam south america and guinea west africa in order to document the petrogenesis and geodynamical significance of high ti and low ti basaltic magmas from the camp in guyana doleritic and gabbroic dykes are located on the edge of the guiana shield and represent limited volumes of magma they display low sio2 47 50 high tio2 2 5 3 5 and high feo tholeiitic trends and show variably enriched trace element patterns la yb n 1 5 5 1 their isotopic signature and ratios of very incompatible elements endi 5 8 to 4 2 87sr 86sr i 0 703 0 705 207pb 204pb i 15 46 15 64 match a depleted prema prevalent mantle like source their genesis can be modeled by ca 15 partial melting of a lherzolite source and a subsequent limited fractional crystallization 5 10 or a slight upper crustal assimilation fractional crystallization afc r 0 1 proterozoic contaminant in guinea in contrast huge volumes of camp magmas were intruded along the rockelides suture and the west african craton forming the fouta djalon sills and the kakoulima laccolith the laccolith is more than 1000 m thick these features consist of gabbros dolerites diorites and mafic gabbro and ultramafic dunite wherlite cumulates guinean tholeiites show high sio2 51 58 low tio2 0 7 1 2 and feo trends with high lile hfse ratios and slight negative nb ta anomalies isotopic signatures endi 0 4 to 5 3 87sr 86sr i 0 705 0 710 207pb 204pb i 15 57 15 66 indicate a more enriched source than for guyana as well as a higher rate of magma upper crust interaction through an afc process r 0 3 birimian crust contaminant and probably an additional upper crustal contamination for the most differentiated sample this geochemical study supports the prevalence in guinea as for other low ti camp tholeiites of a lithospheric mantle source previously enriched during ancient subduction events and preferentially reactivated in late triassic times by edge driven convection between cratonic and mobile belt domains a larger contribution from a depleted asthenospheric source is required to generate high ti tholeiites in guyana which may reflect the development of camp rifting towards the initiation of the central atlantic oceanic crust

simulation and observations of stratospheric aerosols from the 2009 sarychev volcanic eruption 1 we used a general circulation model of earth s climate to conduct simulations of the 12 16 june 2009 eruption of sarychev volcano 48 1 n 153 2 e the model simulates the formation and transport of the stratospheric sulfate aerosol cloud from the eruption and the resulting climate response we compared optical depth results from these simulations with limb scatter measurements from the optical spectrograph and infrared imaging system osiris in situ measurements from balloon borne instruments lofted from laramie wyoming 41 3 n 105 7 w and five lidar stations located throughout the northern hemisphere the aerosol cloud covered most of the northern hemisphere extending slightly into the tropics with peak backscatter measured between 12 and 16 km in altitude aerosol concentrations returned to near background levels by spring 2010 after accounting for expected sources of discrepancy between each of the data sources the magnitudes and spatial distributions of aerosol optical depth due to the eruption largely agree in conducting the simulations we likely overestimated both particle size and the amount of so2 injected into the stratosphere resulting in modeled optical depth values that were a factor of 2 4 too high modeled optical depth due to the eruption shows a peak too late in high latitudes and too early in low latitudes suggesting a problem with stratospheric circulation in the model the model also shows a higher decay rate in optical depth than is observed showing an inaccuracy in stratospheric removal rates in some seasons the modeled removal rate of sulfate aerosols from the sarychev eruption is higher than the rate calculated for aerosols from the 1991 eruption of mt pinatubo

multiple sulfur isotopes from paleoproterozoic huronian interglacial sediments and the rise of atmospheric oxygen mass dependently fractionated mdf sulfur isotopes in sedimentary sulfides and sulfates can provide information on the past activity of microbial metabolisms and serve as a proxy for the concentration of seawater sulfate in conjunction with these proxies mass independently fractionated mif sulfur isotopes can be used to track the rise of atmospheric oxygen the anoxic oxic transition of the surface environment in the paleoproterozoic has been constrained by mif sulfur in 2 47 ga sulfides from western australian banded iron formations and the absence of mif in diagenetic sulfides from 2 32 ga black shales in the transvaal supergroup south africa we report new multicollector ion microprobe data for individual sulfides from water lain sedimentary units in the 2 45 2 22 ga huronian supergroup ontario canada the authigenecity of sulfides in these units is interpreted on the basis of host lithology mode of distribution sulfide chemistry and mdf sulfur isotopes sulfides interpreted to have an authigenicsedimentary origin from the mckim fm have a range of 0 7 in 33 s values and those in the pecors fm preserve small magnitude mif 33 s values up to 0 88 which we propose represents the last remnants of an anoxic atmosphere high 34 s values up to 31 2 and near zero 33 s values in authigenic sulfides from the stratigraphically higher espanola and gordon lake fms suggest local variations of seawater sulfate concentrations and or 34 ssulfate consistent with increased atmospheric o2 levels mif sulfur is absent from the marine record after the second huronian glaciation which is analogous to the record of the transvaal supergroup where mif is absent from the upper part of the interglacial sequence our results suggest that the possible simultaneous demise of mif sulfur isotopes at these two localities may serve as a useful global geochemical marker to correlate paleoproterozoic supergroups we propose a new biogeochemical model where enhanced weathering rates during paleoproterozoic post glacial thawing served as a critical stimulus for interglacial blooms of oxygenic photosynthesis the demise of methane and ultimately to the irreversible rise in atmospheric oxygen

differential opening of the central and south atlantic oceans and the opening of the west african rift system abstract plate tectonic studies of the development of the central and south atlantic oceans using seasat and geosat altimeter and magnetic anomaly isochron data now provide quantitative models of seafloor spreading through time such models enable an initial assessment of the differential opening between these two oceanic basins to be determined the equatorial atlantic is an integral part of this oceanic rifting process allowing stresses arising from the differential opening to be dissipated into both the caribbean and africa along its northern and southern boundaries respectively the tectonic model for the west african rift system based on geological and geophysical studies shows a series of strike slip fault zones diverging into africa from the gulf of guinea and dissipating their shear movement into the development of extensional basins orientated perpendicular to these faults zones the development of the west african rift system was contemporaneous with the early opening of the south atlantic continued to develop well after the final breakup of south america from africa and did not cease until the late cretaceous when there was a major phase of basin inversion and deformation santonian 80 ma deformation across the benue trough nigeria is broadly contemporaneous with dextral shear reactivation of the central african fracture system which in turn resulted in renewed extension in the sudan basins during the late cretaceous and early tertiary this paper illustrates the close linkage in both time and space between the history of the african rift basins and the opening of the atlantic both exhibit distinct phases of evolution with the rift basins developing in direct response to the differential opening between the central and south atlantic in order to dissipate stresses generated by this opening the mesozoic tectonic model proposed is therefore one of an intimate interaction between oceanic and continental tectonics

geochemistry and regional significance of the early cretaceous bimodal basalt felsic associations on grand banks eastern canada cretaceous mafic rocks are present in several wells on the grand banks of newfoundland near the southwest grand banks jurassic transform margin and along the trend of the collector magnetic anomaly in the brant and mallard wells felsic volcanic subvolcanic and pyroclastic rocks are several hundreds of meters thick seismic reflection profiles show that the volcanic rocks are locally bounded by rejuvenated faulted margins of early rift basins beneath the mid cretaceous postrift unconformity the early cretaceous age of the volcanic rocks is confirmed by k ar geochronology trace element and isotopic composition of both mafic and felsic rocks are quite different from those of plume related cretaceous rocks elsewhere on the eastern north american margin mafic rocks have a pb isotopic composition unusually enriched in 207 pb 204 pb which is similar to late triassic early jurassic mafic rocks along the eastern appalachians nd isotopic compositions are closer to typical ocean island basalt oib sources observed isotopic compositions suggest mafic magma was partially melted from continental margin lithospheric mantle enriched with small amounts of hydrous or silica rich phases of oib affinity whole rock geochemistry isotopic composition and mineral chemistry show that the felsic rocks resemble alkaline a type granite those from mallard have some crustal contamination those from brant which is located closer to the continental margin edge have almost pristine mantle isotopic characteristics indicating an oib source magma the spatial distribution of cretaceous volcanic products suggests that magma was generated along the reactivated transform margin and that major faults in continental crust acted as magma conduits partial melting of continental margin lithospheric mantle resulted from extensional decompression probably aided by a weak plume related thermal anomaly as on other transform margins magmatic products reflect a complex mantle history

anomalous opening of the equatorial atlantic due to an equatorial mantle thermal minimum abstract the geology of the equatorial atlantic is dominated by a broad east west megashear belt where a cluster of large fracture zones offsets anomalously deep segments of the mid atlantic ridge mar the origin and evolution of this megashear region may lie ultimately in an equatorial mantle thermal minimum the notion of a mantle thermal minimum in the equatorial atlantic is supported by an equatorial minimum of zero age topography a maximum in mantle shear waves seismic velocity and a minimum in the degree of melting indicated by the chemistry of mar basalts and peridotites this thermal minimum has probably been a stable feature since before the cretaceous separation of africa from south america it caused a pre opening equatorial continental lithosphere thicker and colder than normal the cretaceous benue trough in western africa and the amazon depression in south america are interpreted as morphostructural depressions created or rejuvenated by strike slip transpressional and transtensional tectonics ducing extension of the cold thick equatorial lithosphere the oceanic rift propagating northward from the south atlantic impinged against the equatorial thicker colder and therefore stronger than normal continental lithosphere that consequently acted as a locked zone this and a low magmatic budget due to the cold upper mantle caused a lower than normal rate of propagation of the oceanic rift into the equatorial belt with diffuse deformation during mostly amagmatic extension the thick cold lithosphere prevented major cretaceous igneous activity from the st helena plume eventually initial weak isolated nuclei oceanic lithosphere were emplaced separated by e w continent continent transforms opening occurred largely by strike slip motion along these initial transforms the consequences were that the equatorial atlantic opened prevalently along an e w direction in contrast to the n s opening of the north and south atlantic and that sheared continental margins are particularly well developed in the equatorial atlantic after further continental separation the cold equatorial mantle caused a low degree of melting with na rich morb and alkali basalt rather than normal morb and with undepleted mantle peridotities thin crust depressed ridge segments and a prevalence of amagmatic extension similar conditions still exist today long transforms offsetting short ridge segments kept sea floor spreading unstable and dominated by transform tectonics with transform migration transpression and transtension causing strong vertical motion emersion and subsidence of lithospheric blocks development of deep pull apart basins and preservation of relict slivers of old lithosphere occasionally even of continental lithosphere within younger crust the equatorial transforms are caused ultimately by a long lived thermal minimum in the upper mantle and not vice versa however they then create second order rebound thermal effects that help maintain the thermal minimum in the upper mantle it can be speculated that mantle thermal minima at the earth s equator might be related to true polar wander triggered by subduction of dense masses into the mantle

geochronology of the monteregian hills alkaline igneous province quebec three distinct groups of alkaline rocks can be recognized on the basis of silica content in the monteregian hills quebec strongly undersaturated slightly undersaturated to critically saturated and oversaturated the first two groups were derived from basanite and alkali picrite magmas respectively and yield distinct ages of intrusion clustered about 118 ma and 136 ma the age data are not readily explained by the translation of the north american plate across a fixed mantle hotspot but because the alkali picrite magmas were derived from greater mantle depths than the basanite magmas the data do suggest an upward migration of the zone of melting this change in the depth of melting may be a response to the upward transport of a metasomatic fluid perhaps with a coupled rise in mantle isotherms

a negative fold test on the lorrain formation of the huronian supergroup uncertainty on the paleolatitude of the paleoproterozoic gowganda glaciation and implications for the great oxygenation event previous paleomagnetic studies of the glaciogenic gowganda and lorrain formations have identified several low inclination magnetic components of high thermal stability which suggest low latitude glaciation during deposition of the huronian supergroup canada while extraordinary claims demand extraordinary proof prior authors have been unable to support their interpretations of these components conclusively with any of the classic field stability tests e g conglomerate fold and baked contact capable of demonstrating that the magnetization was acquired at or soon enough after the time of deposition to be used to constrain the paleolatitude of the gowganda or lorrain formations we report here the results of a fold test from the purple siltstone member of the lorrain formation near the town of desbarats ontario which indicate that none of the reported components dates to the time of deposition hence the paleolatitude of the gowganda glaciation is uncertain comparison of the lithostratigraphic paleomagnetic and radiometric constraints on the huronian sequence and the transvaal supergroup of southern africa implies that the one verified low latitude paleoproterozoic glacial event the makganyene glaciation transvaal supergroup south africa is younger than the three glacial units of canada with this correlation the physical rock record indicates that the great oxygenation event began in the time interval between the gowganda and makganyene glaciations these data are consistent with the sudden evolution of oxygenic photosynthesis destroying a methane greenhouse and thereby triggering the first snowball earth event in earth history

age migrations of anorogenic ring complexes in northern nigeria forty rb sr isotopic measurements on minerals and whole rock samples from 11 anorogenic complexes in northern nigeria complement the evidence for systematic age variations along a series of en echelon ene wsw and n s lineaments following two major fracture directions of extensional deformation in the precambrian basement the new ages presented in this paper confirm our earlier discoveries of a general southerly age trend but substantiate for the first time that major local migrations of magmatic activity were concentrated along at least two ene wsw zones from the overlap in analytical error the age patterns also suggest that contemporaneous anorogenic magmatism may have been locally derived from several simultaneous high level magma chambers connected to a common deeper source different models that have been proposed for the origin of linear mid plate magmatic activity are critically reviewed in our opinion magma generation was most likely initiated in the asthenosphere lithosphere decoupling zone or

the timing of late cretaceous alkalic igneous activity in the northern gulf of mexico basin southeastern usa upper cretaceous igneous rocks in the northern gulf of mexico basin are poorly understood due both to the paucity of material recovered from drillholes in the sedimentary basins and lack of geochronological data rocks from the alkalic igneous complexes at magnet cove and granite mountain arkansas the monroe uplift and the jackson dome louisiana and mississippi yielded 40ar 39ar ages ranging from 94 ma to 65 ma high precision plateau ages reveal that the magnet cove complex 94 ma predates granite mountain by 4 myr igneous activity in the monroe uplift may have been contemporaneous with that at magnet cove and continued sporadically for 30 myr a single specimen from the jackson dome indicates volcanism at 65 ma coincident with waning igneous activity at the monroe uplift based on geochemical and geochronological data the source for volcanoclastic material in the woodbine formation arkansas are rocks belonging to the magnet cove phase of magmatism cretaceous alkalic rocks of central ark

comparative microbial diversity and redox environments of black shale and stromatolite facies in the mesoproterozoic xiamaling formation abstract the composition of microbial communities and their relationship to ocean redox structure in the precambrian are topics of continuing interest in geobiology our knowledge of organismic diversity and environmental conditions during this time are mostly based on fragmentary paleontological and geochemical records and might be skewed accordingly in north china the xiamaling formation 1 37 ga is characterized by black shales of relatively low thermal maturity t max is 445 c and has been identified as a potential petroleum source rock to date however the biological sources of the organic matter and the environmental conditions prevalent during the deposition of these sediments remain unclear in this study we analyzed the hydrocarbon biomarker compositions of the xiamaling formation shales and a superjacent stromatolitic carbonate in order to shed light on the microbial diversity in the sedimentary environments they represent the hydrocarbons extracted from both sediments are dominated by low molecular weight n alkanes with a maximum at c 15 18 suggesting that bacteria and or algae were primary biotic precursors our inability to detect steranes in bitumen i and only traces of rearranged steranes in bitumen ii of black shales indicates that modern eukaryotic algae were either ecologically insignificant or not preserved due to a taphonomic bias the high relative concentration of hopanes and diahopanes ranging from c 27 to c 35 as well as monomethylalkanes suggests that cyanobacteria may have been the dominant primary producers and could have contributed to the biologically available nitrogen pool through n 2 fixation this observation is supported by the low nitrogen isotopic composition of the kerogens even though all facies zones appear to have been anoxic but not sulfidic on the basis of biomarker ratios and trace metals subtle but distinct molecular differences are observed between the stromatolite and the black shales which can be attributed to both lithologically controlled diagenetic rearrangements and differential biotic input the discrepancy between the presence of a large ucm and high abundances of alkyl lipids on one hand yet the absence of a stable carbon isotopic offset between lipids and kerogen on the other suggests that strong heterotrophic reworking might not be the sole source of the biodegraded fingerprint that is so typical for proterozoic bitumens and demands alternative explanations

structural segmentation and the influence of basement structure on the namibian passive margin passive margins are segmented into discrete elements by segment boundaries exhibiting a variety of structural styles the namibian margin is bounded by a hinge line that is offset by segment boundaries the most important is the cape cross segment boundary here pre existing basement lineaments and karoo rift elements were reactivated during late mesozoic rifting it marks the northern limit of the earliest of three rift sequences and relates to a temporal change in break up and formation of the south atlantic a similar change occurs across other important segment boundaries but smaller examples lack this major segment boundaries with a temporal change in rifting and break up exhibit a strong connection to the basement province

lithospheric sources of north florida usa tholeiites and implications for the origin of the suwannee terrane abstract sr nd and pb isotopic data for mesozoic tholeiites from six drillholes in northern florida suwannee terrane indicate derivation of the magmas from a source that was slightly incompatible element enriched with respect to bulk earth probably continental lithospheric mantle variably contaminated by more enriched crust initial isotopic compositions of the florida tholeiites show a considerable range 87 sr 86 sr 0 7059 to 0 7097 end 4 6 to 0 1 206 pb 204 pb 18 30 to 18 65 207 pb 204 pb 15 57 to 15 66 and 208 pb 204 pb 38 22 to 38 35 these compositions are distinctly different from those of mesozoic alkali basalts from the subsurface of southwestern florida heatherington a l mueller p a 1991 geochemical evidence for triassic rifting in southwestern florida tectonophysics 188 291 302 it is suggested that the north florida tholeiites were derived from within the continental lithosphere in contrast with mesozoic volcanic rocks from southwestern florida which have been previously suggested to be related to a mantle plume sm nd isotope systematics suggest derivation of the tholeiites from a 1 ga source a similar age to that proposed by pegram pegram w j 1990 development of continental lithospheric mantle as reflected in the chemistry of the mesozoic appalachian tholeiites usa earth and planetary science letters 97 316 331 for the source of tholeiitic basalts within the carolina terrane this supports proterozoic tectonic models which suggest that both the suwannee and carolina terranes had affinities with south american portions of gondwana containing 1 ga crust specifically the sunsas rondonian province nance murphy 1996 mueller p a kozuch m heatherington a l wooden j l offield t w koeppen r p klein t l nutman a p 1996 evidence for mesoproterozoic basement in the carolina terrane and speculations on its origin in nance r d thompson m d eds avalonian and related peri gondwanan terranes of the circum north atlantic gsa special paper 304 pp 207 218

mesozoic evolution of nw africa implications for the central atlantic ocean dynamics the central atlantic ocean opened during the early jurassic and represents the oldest portion of the atlantic ocean although the american margin has been well studied the onshore evolution of its african counterpart is poorly understood we investigated the evolution of a c 1300 km transect across the reguibat shield morocco mauritania algeria in the northern west african craton using low temperature thermochronology fourteen samples were dated using apatite fission track analysis nine of these samples were also dated using u th sm he analysis fission track ages range from 118 10 to 497 61 ma with mean track lengths between 11 2 0 4 and 12 5 0 2 m u th sm he single grain ages range from 32 3 to 396 32 ma through forward and inverse thermal modelling we demonstrate that the craton underwent kilometre scale exhumation between the early middle jurassic and the late cretaceous based on our new results published data on nw africa and data from the conjugate eastern north american passive margin we show that this post rift early middle jurassic to early cretaceous exhumation affected both margins to a similar areal extent and simultaneously transient mantle dynamic support is suggested to account for the major erosional phase recorded on both margins supplementary material the method for thermal modelling the thermal modelling for all samples and the relationship between apatite chemistry and u th he ages are available at https doi org 10 6084 m9 figshare c 3770918

mineralogical and isotopic characterization of graphite deposits in the western part of the north qaidam orogen and east kunlun orogen northeast tibetan plateau china abstract medium to high grade metamorphic rocks comprising numerous graphite deposits are widely distributed in the periphery of the qaidam basin northern tibetan plateau china we present textural xrd and raman spectroscopy characteristics of graphite 13c results of graphite and cogenetic calcite and 18o results of calcite in three graphite occurrences located at the periphery of qaidam basin graphite displays hexagonal platy flaky irregular polygonal ovate morphologies with occasional spherulitic textures graphite morphologies combined with xrd and raman spectroscopy indicate that graphite has relatively high crystallinity raman microspectroscopy of graphite shows that peak metamorphic temperatures vary between 690 and 801 c graphite 13cv pdb values range between 26 3 and 21 6 indicating organic carbon as the major carbon source for graphite calcite 13cv pdb and calcite 18ov smow values range between 10 8 and 3 9 and between 6 6 and 29 5 respectively calcite 13cv pdb results show that organic carbon was mixed with inorganic carbon which resulted in an increase of 13cv pdb calcite 18ov smow values indicate that the graphite deposits experienced late stage water rock interaction the positive and negative anomalies of calcite 13cv pdb indicate that organic matter oxidation and recycling locally occurred during the final stage of the great oxidation event the wide distribution of regional metamorphic organogenic graphite deposits provides evidence for large scale paleoproterozoic biological activity in the periphery of the qaidam basin

resolving the richat enigma doming and hydrothermal karstification above an alkaline complex the richat structure sahara mauritania appears as a large dome at least 40 km in diameter within a late proterozoic to ordovician sequence erosion has created circular cuestas represented by three nested rings dipping outward from the structure the center of the structure consists of a limestone dolomite shelf that encloses a kilometer scale siliceous breccia and is intruded by basaltic ring dikes kimberlitic intrusions and alkaline volcanic rocks several hypotheses have been presented to explain the spectacular richat structure and breccia but their origin remains enigmatic the breccia body is lenticular in shape and irregularly thins at its extremities to only a few meters the breccia was created during karst dissolution and collapse internal sediments fill the centimeter to meter scale cavities alkaline enrichment and the presence of cretaceous automorphous neoformed k feldspar demonstrate the hydrothermal origin of these internal sediments and their contemporaneity with magmatism a model is proposed in which doming and the production of hydrothermal fluids were instrumental in creating a favorable setting for dissolution the circular richat structure and its breccia core thus represent the superficial expression of a cretaceous alkaline complex with an exceptionally well preserved hydrothermal karst infilling at its summit

anomalous negative excursion of carbon isotope in organic carbon after the last paleoproterozoic glaciation in north america early paleoproterozoic time 2 5 2 0 ga spanned a critical phase in earth s history characterized by repeated glaciations and an increase in atmospheric oxygen the great oxidation event goe following the last and most intense glaciation of this period marine carbonates record a large positive excursion of 13 c value termed the lomagundi event between about 2 2 and 2 1 ga coinciding with the global appearances of red beds and sulfates which suggest an accumulation of high levels of atmospheric oxygen here we report the discovery of large negative excursions of 13 c in organic matter down to 55 from quartzose sandstones of the marquette range and the huronian supergroups north america intermediate in age between the last paleoproterozoic glaciation and the possible onset of the lomagundi event the negative excursion is concomitant with the appearance of intensely weathered quartzose sandstones which may represent hot and humid conditions there are some interpretations that potentially explain the negative excursions 1 redeposition of older 13 c depleted kerogen 2 later post depositional infiltration of oil 3 active methane productions by methanogens in shallow marine environments or 4 dissociation of methane hydrate if the latter two were the case they would provide clues for understanding the environmental change connecting the intense glaciation and an increase in oxygen

characterisation of fluid inclusion oil in a mid proterozoic sandstone and dolerite roper superbasin australia detailed geochemical analysis has been carried out on fluid inclusion oils trapped for over 1 billion years in the roper superbasin northern australia in two different geological settings in healed microfractures in a quartz arenite and within partially altered plagioclase laths in a dolerite sill the oil inclusions and co occurring solid bitumens record multiple oil migration events postdating an intrusive episode ca 1300 ma most likely during neoproterozoic basin inversion solid bitumen represents the oldest migration event and occurs in secondary pore space within the bessie creek sandstone and in veins cross cutting the dolerite sill it is interpreted to represent thermally altered oil that may have at least partly been generated by contact metamorphism near the igneous intrusion inclusion oils in both the dolerite feldspar and in the transgranular fractures in the bessie creek sandstone are pristine nonbiodegraded fluids with a great similarity to other bitumens oils and sediment extracts from the roper superbasin biomarkers indicate that the oils were mainly sourced from prokaryotic cyanobacterial organic matter with a small contribution from eukaryotes based on migration distance organic richness geochemistry and similarity of source rock maturity the velkerri formation which directly overlies the dolerite sill and the bessie creek sandstone is the most likely source rock for the higher molecular weight hydrocarbons preserved in the fluid inclusion oils geochemical data indicate that the oil inclusions in clusters in albitised zones of labradorite crystals in the dolerite matrix represent a different migration event from oil inclusions in transcrystalline fractures in vein calcite in the dolerite sill and in transgranular fractures in quartz in the bessie creek sandstone the bessie creek sandstone fluid inclusion oil was generated in the peak to late oil window 0 9 1 0 vre the higher molecular weight c13 proportion of the dolerite feldspar fluid inclusion oil is of lower maturity 0 8 vre than the same molecular weigh range fluid in the bessie creek sandstone fluid inclusion oil both of these oils were most likely generated in the overlying velkerri formation however the volumetrically dominant low molecular weight hydrocarbons in the dolerite feldspar fluid inclusion oils have a higher maturity 1 3 vre and may have been generated from the deeper ca 1600 ma barney creek formation in the underlying mcarthur group this study shows that oil trapped in fluid inclusions has a greater potential to preserve pristine organic geochemical information than porosity bound oils and dispersed kerogen if oil inclusions occur in sufficient abundance the analysis of these tiny oil encapsulations is technically feasible and can give valuable information on the composition of the earth s early biosphere and precambrian petroleum systems

supracrustal faults of the st lawrence rift system quebec kinematics and geometry as revealed by field mapping and marine seismic reflection data abstract the st lawrence rift system from the laurentian craton core to the offshore st lawrence river system is a seismically active zone in which fault reactivation is believed to occur along late proterozoic to early paleozoic normal faults related to the opening of the iapetus ocean the rift related faults fringe the contact between the grenvillian basement to the nw and cambrian ordovician rocks of the st lawrence lowlands to the se and occur also within the grenvillian basement the st lawrence rift system trends ne sw and represents a se dipping half graben that links the nw se trending ottawa bonnechere and saguenay river grabens both interpreted as iapetan failed arms coastal sections of the st lawrence river that expose fault rocks related to the st lawrence rift system have been studied between quebec city and the saguenay river brittle faults marking the st lawrence rift system consist of ne and nw trending structures that show mutual crosscutting relationships fault rocks consist of fault breccias cataclasites and pseudotachylytes field relationships suggest that the various types of fault rocks are associated with the same tectonic event high resolution marine seismic reflection data acquired in the st lawrence river estuary between rimouski the saguenay river and forestville identify submarine topographic relief attributed to the st lawrence rift system northeast trending seismic reflection profiles show a basement geometry that agrees with onshore structural features northwest trending seismic profiles suggest that normal faults fringing the st lawrence river are associated with a major topographic depression in the estuary the laurentian channel trough with up to 700 m of basement relief a two way travel time to bedrock map based on seismic data from the st lawrence estuary and comparison with the onshore rift segment suggest that the laurentian channel trough varies from a half graben to a graben structure from sw to ne it is speculated that natural gas occurrences within both the onshore and offshore sequences of unconsolidated quaternary deposits are possibly related to degassing processes of basement rocks and that hydrocarbons were drained upward by the rift faults

petroleum generation and migration in the cambro ordovician laurentian margin succession of nw scotland fluid inclusion and organic biomarker data show that there was formerly a hydrocarbon system in the cambro ordovician laurentian margin rocks of nw scotland oil fluid inclusions occur in stylolitized eriboll formation sandstone in k feldspar cements with an 40 ar 39 ar age of 415 5 5 5 8 ma 2 analytical precision full external precision organic extracts from durness group black limestones yield biomarker ratios characteristic of high thermal maturity organic maturation to yield oil probably occurred during orogenic deformation along the moine thrust zone the recognition of a hydrocarbon system in scotland adds to a huge hydrocarbon province in laurentian rocks including north america and greenland supplementary material technical data for ar ar dating are available at https doi org 10 6084 m9 figshare c 3879616

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

oil and its biomarkers associated with the palaeoproterozoic oklo natural fission reactors gabon abstract the 2 1 ga fa formation sandstone of the franceville basin in gabon that hosts the oklo natural fission reactors has been discovered to contain abundant palaeoproterozoic oil bearing fluid inclusions inside which syngenetic biomarkers are preserved the oil occurs within h2o and co2 dominated inclusions trapped in syntaxial quartz overgrowths and intragranular and transgranular microfractures in detrital quartz homogeneous fluid inclusion assemblages with oil inside a brine and carbonic fluid indicate that there was intermixing of brines and co2 with liquid hydrocarbons prior to trapping textural relationships and microthermometry suggest that entrapment of oil occurred first during diagenesis of the sandstone at ca 115 to 200 c soon after deposition of the sequence at ca 2 1 ga and secondly during a hydrothermal event at 200 to 285 c linked to the operation of the reactors ca 1 98 ga a series of sequential outside rinse blanks of framework grains analysed by gas chromatography mass spectrometry gc ms prior to fluid inclusion crushing shows a first solvent rinse dominated by hydrocarbons derived from co occurring solid bitumens which have a different geochemical distribution compared to the fluid inclusion oil the crush leach extract containing oil from fluid inclusions is markedly richer than the final outside rinse blank displays significant geochemical differences in compound distribution and is interpreted to include only a very small contribution from solid bitumen the fluid inclusion oil has a high maturity is non biodegraded and lacks clear effects of radiolysis it is therefore likely that the oil that migrated at temperatures higher than those of diagenesis was generated by hydrothermal alteration caused by reactor heated fluids rather than by the criticality event itself biomarkers include hopanes 2 methylhopanes terpanes and steranes indicative of bacteria including cyanobacteria and of eukaryotes the relatively high concentration of regular steranes suggests that the source rock had a substantial eukaryotic contribution trace amounts of c30n propylcholestanes indicate a contribution from marine algae the oil was likely derived from conformably overlying marine black shales that were deposited during the great oxidation event during which the earth s atmosphere became oxygenated the biomarker content of the oil is remarkably similar to that of other oils formed in sedimentary basins between 2 7 and 1 4 ga it is thus suggested that hydrocarbons were derived from a uniform range of organisms in this time period despite apparent fluctuations of atmospheric o2

the cretaceous peri atlantic alkaline pulse paap deep mantle plume origin or shallow lithospheric break up abstract although the early jurassic initial phases of atlantic rifting were marked by the tholeiitic basalt flows and dikes of the central atlantic magmatic province camp the cretaceous era was characterised by widespread alkaline igneous activity on widely separated peri atlantic continental margins nearly half the peri atlantic alkaline rocks dated between 250 to 50 ma fall within the 125 80 ma range this period of intense alkaline activity during the mid cretaceous formed the peri atlantic alkaline pulse paap the mechanisms for the opening of the atlantic ocean as well as for the camp and later paap magmatic events have been attributed either to deep mantle plumes or to a combination of tensional forces lithospheric rifting and structural controls we describe the paap event for the first time and test the plume and structural models the paap displays evidence of strong structural control and appears to be related to reactivation of pre existing lithospheric structures throughout the peri atlantic zone early stages of continental separation and abrupt changes in plate motion were the most likely mechanisms for the reactivation of favourably oriented pre existing structures mid cretaceous pulses in alkaline magmatic activity were closely associated with major atlantic tectonic events the opening of the south atlantic the change in intensity and direction of both seafloor spreading and plate motion and the consequent fluctuation in intra plate stresses were responsible for two spikes in peri atlantic alkaline magmatism at 125 ma and 85 80 ma the general lack of evidence for a deep hotspot associated with the peri atlantic zone seems incompatible with a mantle plume origin on the other hand the spatial distribution of the paap along passive continental margins suggests an ideal setting for the development of edge driven convection we propose that the periodic reactivation of deep seated pre existing zones of weakness during the major stages of atlantic tectonic evolution combined with coeval asthenospheric upwelling due to edge driven convection and continental insulation flow enhanced the ascent of alkaline magmas this mechanism of shallow small scale upwelling during periodic structural reactivation provides a more likely explanation for the paap than the commonly proposed deep mantle plume scenario

the indigenous origin of witwatersrand carbon in the witwatersrand approximately 40 of the gold is intimately associated with so called carbon in carbon seam reefs which occur in over a dozen paleoplacers many of them concentrated at two stratigraphic levels in the 7000 m thick succession of archean siliciclastic sedimentary rocks this is reduced carbon present as kerogen admixed in various proportions with derivative now solid bitumen s oil generation and migration were active geological processes during early earth history numerous possible source rocks for oil generation including the carbon seams themselves occur within the witwatersrand basin in the witwatersrand ore oil bearing fluid inclusions are also present derived like the bitumen by thermal maturation of the kerogen the presence of kerogen and bitumen in the witwatersrand sedimentary rocks together with a wealth of observations on the spatial distribution of the carbon seams confirm that the carbon originated in situ from living organisms in microbial mat cover as opposed to flowing in from elsewhere as liquid hydrocarbons as some researchers have suggested paleochannels which truncated auriferous carbon seams early in the depositional history are of widespread occurrence and micro synsedimentary faults offset carbon seams the carbon seams are thus indigenous biogenic markers that grew contemporaneously with placer development the various features highlighting the nature and spatial distribution of witwatersrand carbon seams provide a classic case where field evidence trumps laboratory data in the reconstruction of geological processes

late archean molecular fossils from the transvaal supergroup record the antiquity of microbial diversity and aerobiosis abstract cores recovered during the agouron griqualand drilling project contain over 2500 m of well preserved late archean transvaal supergroup sediments dating from ca 2 67 to 2 46 ga bitumen extracts of these strata were obtained using clean drilling sampling and analysis protocols that avoided overprinting syngenetic molecular fossil signatures with contaminant hydrocarbons comparisons of biomarker contents in stratigraphically correlated intervals from diverse lithofacies in two boreholes separated by 24 km as well as across a 2 gyr unconformity provide compelling support for their syngenetic nature the suite of molecular fossils identified in the late archean bitumens includes hopanes attributable to bacteria potentially including cyanobacteria and methanotrophs and steranes of eukaryotic origin this molecular fossil record supports an origin in the archean eon of the three domains of cellular life as well as of oxygenic photosynthesis and the anabolic use of o2

redox conditions in the atmosphere and shallow marine environments during the first huronian deglaciation insights from os isotopes and redox sensitive elements article history the paleoproterozoic 2 5 2 0 ga is one of the most important periods in earth s history and was characterized by a rise in atmospheric oxygen levels and repeated at least three severe glaciations the huronian glaciations in this study we investigate redox conditions in the atmosphere and in shallow marine environments immediately after the first huronian glaciation based on the isotopic composition of os and the abundance of redox sensitive elements os re and mo in sedimentary rocks from the huronian supergroup canada we found no significant authigenic enrichment of os in the sedimentary rocks deposited during the first huronian deglaciation the initial isotopic composition of os in the sediments was close to that of chondrite at the time of deposition 187 os

an early cambrian deltaic fluvial model for an iapetan rift arm drainage system southeastern quebec based on the model that during the rifting stage beginning at ca 590 ma the ottawa graben an iapetan failed arm localized a large river that flowed into the nascent iapetus a search was made for the related fluvial and deltaic deposits the search led to the identification of fluvial deposits as predicted by the model deltaic deposits were also identified despite complications brought about by deformation metamorphism and thrusting although they probably belong to a late phase of delta buildup in the early cambrian older deltaic deposits of the river probably lie buried beneath a volcanic shield that built up at the proximal end of the graben ca 554 ma the successful application of the model supports the paradigm of rift arm failed arm rivers and their deltas as proposed by k burke and j f dewey nearly two decades ago investigations based on this paradigm in appropriate geological situations may provide insights into problems related to continental rifting and breakup and provide inform

a paleoweathering profile from griqualand west south africa evidence for a dramatic rise in atmospheric oxygen between 2 2 and 1 9 bybp a core drilled near wolhaarkop in griqualand west south africa intersected highly oxidized kuruman iron formation below red beds of the gamagara formation the lateral equivalents of the kuruman iron formation in this drill hole consist largely of siderite ankerite magnetite greenalite and quartz the oxidation of the kuruman iron formation in wol 2 occurred almost certainly during weathering prior to the deposition of the gamagara formation the date of this weathering episode is bracketed between about 2 2 and 1 9 bybp by the age of the ongeluk lavas in the transvaal sequence below the unconformity and by the age of the hartley lavas in the olifantshoek group above the unconformity the ratio of iron to sio2 in the several facies of the weathered kuruman iron formation in wol 2 is nearly the same as that in their unweathered equivalents since sio2 loss during weathering was almost certainly minor the similarity of the fe sio2 ratio in the weathered and unweathered bif indicates that nearly all the feo in the kuruman iron formation was oxidized and retained as feo3 during weathering such a high degree of iron retention is best explained by an o2 content of the atmosphere or 0 03 atm at the time of weathering such an o2 pressure is very much greater than that suggested by the composition of paleosols developed on basalt or 2 2 bybp but is consistent with the highly oxidized nature of the 1 85 by flin flon paleosol the new data suggest that po2 rose dramatically from about 1 percent pal present atmospheric level to or 15 percent pal between 2 2 and 1 9 bybp

composition and syngeneity of molecular fossils from the 2 78 to 2 45 billion year old mount bruce supergroup pilbara craton western australia shales of very low metamorphic grade from the 2 78 to 2 45 billion year old ga mount bruce supergroup pilbara craton western australia were analyzed for solvent extractable hydrocarbons samples were collected from ten drill cores and two mines in a sampling area centered in the hamersley basin near wittenoom and ranging 200 km to the southeast 100 km to the southwest and 70 km to the northwest almost all analyzed kerogenous sedimentary rocks yielded solvent extractable organic matter concentrations of total saturated hydrocarbons were commonly in the range of 1 to 20 ppm g g rock but reached maximum values of 1000 ppm the abundance of aromatic hydrocarbons was 1 to 30 ppm analysis of the extracts by gas chromatography mass spectrometry gc ms and gc ms metastable reaction monitoring mrm revealed the presence of n alkanes mid and end branched monomethylalkanes cyclohexylalkanes acyclic isopre noids diamondoids tri to pentacyclic terpanes steranes aromatic steroids and polyaromatic hydrocarbons neither plant biomarkers nor hydrocarbon distributions indicative of phanerozoic contamination were de tected the host kerogens of the hydrocarbons were depleted in 13 c by 2 to 21 relative to n alkanes a pattern typical of although more extreme than other precambrian samples acyclic isoprenoids showed carbon isotopic depletion relative to n alkanes and concentrations of 2 methylhopanes were relatively high features rarely observed in the phanerozoic but characteristic of many other precambrian bitumens molecular parameters including sterane and hopane ratios at their apparent thermal maxima condensate like alkane profiles high mono and triaromatic steroid maturity parameters high methyladamantane and methyldiaman tane indices and high methylphenanthrene maturity ratios indicate thermal maturities in the wet gas gener ation zone additionally extracts from shales associated with iron ore deposits at tom price and newman have unusual polyaromatic hydrocarbon patterns indicative of pyrolytic dealkylation the saturated hydrocarbons and biomarkers in bitumens from the fortescue and hamersley groups are characterized as probably syngenetic with their archean host rock based on their typical precambrian molecular and isotopic composition extreme maturities that appear consistent with the thermal history of the host sediments the absence of biomarkers diagnostic of phanerozoic age the absence of younger petroleum source rocks in the basin and the wide geographic distribution of the samples aromatic hydrocarbons detected in shales associated with iron ore deposits at mt tom price and mt whaleback are characterized as clearly archean based on their hypermature composition and covalent bonding to kerogen copyright 2003 elsevier ltd

petrology of the alkaline volcanism of gorringe bank southwest portugal abstract alkaline volcanic rocks of the gorringe bank overlie typical oceanic crust made up of spilitized dolerites differentiated gabbroic and ultramafic rocks two main series are distinguishable in alkaline volcanics one from nephelinite to phonolite the other from lamprophyre to trachyte each sequence is differentiated by fractional crystallization with separation of clinopyroxene in the first one and kaersutite and mica in the second an intermediate suite is rich in lamprophyric and trachytic ocelli and in cognate ijolitic inclusions hypotheses on the origins of the more mafic lavas and on the influence of fluids like h 2 o and co 2 are discussed these series have a similar petrology to that of the sines and monchique complexes which intrude the continental crust at the same time on the same structural trend the alkaline rocks of gorringe are among the most alkaline series of the atlantic islands close to those of fernando de noronha and trindade where similar rocks occur

age of mesozoic igneous rocks in northwesternnamibia and their relationship to continental breakup mesozoic igneous rocks of northwestern namibia comprise continental flood basalts and quartz latites of the parana etendeka province and a suite of central sub volcanic intrusions the damaraland complexes conventional k ar ages published for these rocks range from 377 88 ma with the majority of samples yielding ages of 150 110 ma this large spread of ages is interpreted as being due to disturbance of the k ar system by low temperature alteration which is ubiquitous among the volcanic rocks rb sr and 40 ar 39 ar dating methods which are less susceptible to such disturbance yield a narrower range of ages 132 124 ma with one outlier at 149 ma new rb sr internal isochron ages of 129 8 3 8 ma 126 8 1 3 ma and 126 1 7 3 ma have been determined for an etendeka quartz latite and nepheline syenite intrusions in the messum and okorusu complexes respectively in addition 40 ar 39 ar release spectrum ages of 137 0 0 7 ma 135 0 0 7 ma and 126 9 0 6 ma have been determined for samples from the paresis comendite cape cross nepheline syenite and okorusu nepheline syenite complexes respectively a 13 ma period between 137 and 124 ma is considered to be the best estimate of the timing of mesozoic igneous activity in northwestern namibia the continental flood basalt volcanism was probably limited to the early part of this period and is inferred to have been relatively short lived 1 2 ma magmatic activity among the damaraland complexes was contemporaneous with the onset of flood basalt volcanism but continued for approximately 15 ma and terminated after the onset of sea floor spreading in this area 130 125 ma both the flood basalt volcanism and the magmatic activity of the damaraland complexes are thought to have occurred in response to rifting across the upwelling tristan mantle plume the linear distribution of intrusions in damaraland is interpreted as being due to magmatism focused along a structural discontinuity between the pan african damara terrain to the south and proterozoic cratonic basement to the north

the shallow structures of the guinea and ivory coast ghana transform margins their bearing on the equatorial atlantic mesozoic evolution abstract recent geological and geophysical data from a marine survey conducted off guinea and the ivory coast ghana region equatorial atlantic are discussed we interpret the southern guinean margin firstly as an area linked to the central jurassic atlantic and secondly as an area affected by the progressive transform opening of the equatorial atlantic in the early cretaceous off the ivory coast and ghana we have demonstrated distinct structural domains interpreted as consequences of the progressive transform contact between the west african and brazilian cratons also during the early cretaceous on both margins we observe a clear tectonic unconformity of probable late albian early cenomanian age we propose that this unconformity results from the final disruption between the two continental west african and brazilian crusts the geological structures generated along transform margins can be considered as good geological tools to constrain early phases of plate motions particularly around the equatorial atlantic

tectonics and surface effects of the supercontinent columbia abstract assembly of the supercontinent columbia at about 1 85 1 90 ga coincided with several events that affected the entire earth the oldest worldwide network of orogenic belts formed at the same time although some granite granodiorite gg suites had formed earlier the gg suites became common in the 1 8 1 9 ga orogenic belts these suites succeeded the older tonalite trondhjemite granodiorite ttg suites which were not produced after 1 8 ga changes on the earth s surface at 1 8 1 9 ga include rapid increase in the concentration of oxygen in the atmosphere and oceans and probably the evolution of eukaryotes all of these surface changes occurred as columbia accreted and the assembly of columbia may have contributed to the drastic changes in the earth s surface environment as well as to the evolution of primitive life forms

a plume head melting under a rifting margin abstract a large igneous province lip in the form of a long narrow band of thickened oceanic crust runs along the atlantic margin of north america abutting the rifted continental shelf we propose that this like many other lips has a mantle plume origin there is evidence that when the central atlantic ocean opened the rift was underlain by the flattened head of a mantle plume and that the rift site had drifted away from the plume tail by the time of the rifting so that the tail took little part in the formation of the lip we carried out numerical simulations in which we rifted the lithosphere over various model plume heads and calculated the volumes of melt produced we found that the thickness and width of the resulting thickened oceanic crust is very sensitive to the thermal structure directly under the rift and the structure of the lithosphere to fit observations of the lip a thin flat plume head is required such a plume head results when a mantle plume with temperature dependent viscosity passes through a significant step reduction in the background mantle viscosity at 660 km depth however an extensive layer of low viscosity under the rift results in a region of thickened crust much wider than the layer is deep by decoupling the mantle flow from the lithosphere to avoid decoupling we propose that there must be significant topography on the lithosphere and the rift site is a region of thinned lithosphere very thick crust next to the margin can be explained by lithospheric necking and the resulting fast initial upflow under the rift

40ar 39ar ages for plutons of the monteregian hills quebec evidence for a single episode of cretaceous magmatism 40ar 39ar data for biotite and amphibole from six plutonic complexes of the monteregian petrographic province in quebec canada are presented generally uncomplicated and concordant incremental heating spectra are observed although there is evidence for minor excess 40ar at several localities biotites and amphiboles are concordant consistent with rapid cooling of the high level complexes ages for different phases of individual complexes are analytically indistinguishable and suggest that the various principal lithologies are cogenetic the 40ar 39ar ages of six monteregian complexes st hilaire rougemont johnson shefford brome megantic all fall within the very restricted range of 124 1 m y it is suggested that all of the monteregian plutons with the exception of oka were formed within a short interval 1 2 m y during the cretaceous consistent with paleomagmetic results the new ages along with geochemical characteristics support derivation of the alkaline magmas by a fixed mantle hot spot which also produced broadly similar complexes in new england and the chain of new england seamounts

the late cretaceous alkaline igneous province in the iberian peninsula and its tectonic significance abstract the iberian province consists of the following the three subvolcanic syenitic major intrusive complexes of monchique sines and sintra in w and sw portugal together with their basanitic lamprophyric minor intrusive suites basanitic volcanic complexes around lisbon at least some of a widespread suite of basanitic to theralitic minor intrusives in west central portugal about 80 small basanitic lamprophyric to nepheline syenitic intrusions scattered through the pyrenees ne spain the french corbieres and off the coast of nw spain and the ormonde seamount of the gorringe bank off the sw coast of portugal most of these occurrences have been dated isotopically or from field evidence as late cretaceous geological and petrological details of the various occurrences are compiled and reviewed primary basanitic magmas were probably parental to the entire province and generated syenitic magmas by differentiation processes oversaturated rocks were produced by alkali loss and perhaps also by crustal involvement the iberian province is related to the opening of the n atlantic specifically that of the bay of biscay

microbial remains in some earliest earth rocks comparison with a potential modern analogue abstract carbonaceous matter cm from ca 3 5 ga hydrothermal black cherts of the pilbara craton of western australia and the barberton greenstone belt of south africa yielded transmission electron microscopy tem images that are suggestive of microbial remains and possible remnants of microbial cell walls these are compared to a potential modern analogue the hyperthermophilic methanocaldococcus jannaschii derived from an active seafloor hydrothermal environment and cultured under similar conditions a striking resemblance to the early archaean forms was evident in wall structure and thermal degradation mode cell disintegration of the cultures occurred at 100 c marking the limits of life complete disintegration deformation and shrinkage occurred at 132 c a multidisciplinary approach to the characterisation of the cm was undertaken using organic petrology tem coupled with electron dispersive spectral analysis eds high resolution tem hrtem to determine molecular ordering and elemental and carbon isotope geochemistry reflectance measurements of the cm to determine thermal stress yielded a range of values corresponding to several populations and pointing to different sources and processes the 13c values of dresser formation cm 36 5 to 32 1 are negatively correlated with toc 0 13 0 75 and positively correlated with c n ratio 134 569 which is interpreted to reflect the relative abundance of high ro oxidised recycled cm and preferential loss of 12c and n during thermal maturation tem observations inferred carbon isotopic heterogeneity and isotope fractionations of 27 to 32 are consistent with the activity of chemosynthetic microbes in a seafloor hydrothermal system where rapid silicification at relatively low temperature preserved the cm

late cretaceous reactivation of major crustal shear zones in northern namibia constraints from apatite fission track analysis abstract namibia s passive continental margin records a long history of tectonic activity since the proterozoic the orogenic belt produced during the collision of the congo and kalahari cratons in the early proterozoic led to a zone of crustal weakness which became the preferred location for tectonism during the phanerozoic the pan african damara mobile belt forms this intraplate boundary in namibia and its tectonostratigraphic zones are defined by ductile shear zones where the most prominent is described as the omaruru lineament waterberg thrust oml wt the prominance of the continental margin escarpment is diminished in the area of the central and northern zone of the damara belt where the shear zones are located this area has been targeted with a set of 66 outcrop samples over a 550 km long 60 km broad coast parallel transect from the top of the escarpment in the south across the damara sector to the kamanjab inlier in the north apatite fission track age and length data from all samples reveal a regionally consistent cooling event thermal histories derived by forward modelling bracket this phase of accelerated cooling in the late cretaceous maximum palaeotemperatures immediately prior to the onset of cooling range from ca 120 to ca 60 c with the maximum occurring directly south of the omaruru lineament because different palaeotemperatures indicate different burial depth at a given time the amount of denudation can be estimated and used to constrain vertical displacements of the continental crust we interpret this cooling pattern as the geomorphic response to reactivation of basement structures caused by a change in spreading geometry in the south atlantic and south west indian oceans

the structure of the guinean continental margin implications for the connection between the central and the south atlantic oceans the results of a recent geological geophysical survey conducted off guinea are combined with previous data to establish a preliminary stratigraphy and provide a structural sketch of this portion of the west african continental margin three sectors are distinguished a northwestern portion of the margin comprises a wide and deeply submerged plateau the guinean marginal plateau underlain by a thick sedimentary sequence and facing westward toward the gambia abyssal plain scismic stratigraphy and structures show clear analogies to the jurassic margins of the central atlantic including the presence of a cretaceous paleoslope covered by cenozoic deposits a southern area of the margin comprises a series of aligned w e trending acoustic basement features extending along the slope and bounding the guinean plateau to the south these features basement ridges and volcanic piles are related to a fracture zone system also documented by magnetic anomalies and gravity data the bordering deep sierra leone abyssal plain also dissected by e w trending oceanic fracture zones contains a sedimentary cover apparently not older than middle cretaceous between both sectors and between two nw se trending scarps lies an intermediate area the seismic profiles show that here the margin is dissected by faults creating a series of asymmetric horst and graben features progressively narrowing towards the s e and covered by untectonized but partly eroded upper cretaceous to cenozoic sediments

oil bearing co2 ch4 h2o fluid inclusions oil survival since the palaeoproterozoic after high temperature entrapment the 2 45 ga fluvial quartz arenites and conglomerates of the matinenda formation at elliot lake canada contain evidence for two episodes of oil migration with entrapment of oil bearing fluid inclusions the first episode was during diagenesis the second during a subsequent sub greenschist facies metamorphic event at ca 2 2 ga at temperatures exceeding 250 c remnants of the migrating hydrocarbon fluids are preserved as radiogenic pyrobitumen nodules occupying former inter granular pore spaces and as complex fluorescing oil bearing fluid inclusions the latter include two dominant types 1 aqueous oil inclusions with a minor oil phase located within original detrital grains and clasts and within rare quartz overgrowths and 2 aqueous carbonic inclusions with a minor oil phase light hydrocarbons and solids located within microfractures in detrital quartz grains and clasts the majority of type 1 inclusions were trapped with a high salinity fluid up to 25 wt nacl equivalent at relatively low temperatures between 80 and 200 c and pressures of less than 2 kbar early in the burial history of the matinenda formation type 2 inclusions were trapped during metamorphism with a much lower salinity aqueous carbonic fluid 1 2 to 17 wt nacl equivalent at temperatures between 280 and 350 c and pressures between 1 and 1 5 kbar under these conditions oil may have been miscible in the mixed fluid 13c of 25 5 and 24 9 for the pyrobitumen nodules and fluid inclusion hydrocarbon gas indicate a biogenic origin for the petroleum 13c of 16 2 for the bulk inclusion gas suggests a value of 12 for the carbonic phase hence a mix of inorganic co2 and organic co2 from either thermal alteration of organic matter or in situ chemical oxidation of hydrocarbons this study has implications for the depth and temperature at which oil may be present suggesting that deep old reservoirs may not be completely barren of petroleum the fact that hydrocarbons in the matinenda formation fluoresce indicates that aromatic compounds are able to survive temperatures of up to 350 c in a suitable geological environment

petrogenesis of jurassic a type granites associated with cu mo and w sn deposits in the central nanling region south china relation to mantle upwelling and intra continental extension abstract the central nanling region in south china was the site of uplift extensional deformation granite magmatism and hydrothermal activity from 180 to 150 ma information on the age petrological characteristics major and trace element contents whole rock sr and nd and zircon hf isotopic compositions of granitic plutons were synthesized to constrain the sources evolutionary processes and uplift cooling rates of intrusions that host cu mo and w sn deposits in this region the older 180 160 ma cu mo related granitoids are less differentiated a1 type alkaline rocks with faster cooling and uplift rates they have younger nd and hf two stage model ages and higher end t and ehf t values that indicate mantle derived magmas were moderately contaminated by upper crust the younger 160 150 ma w sn related granites are more differentiated a2 type aluminous rocks with slower cooling and uplift rates they have older nd and hf two stage model ages and lower end t and ehf t values that require more upper crustal contamination the time space distribution and characteristics of these granitoids and ore deposits are indicative of centralized mantle upwelling and gradational crustal extension the older rapidly uplifted less contaminated granitoids with cu mo mineralization occur near the center of an intra continental extension bulge produced by mantle upwelling whereas the younger slowly uplifted more contaminated granitoids with w sn mineralization occur on the periphery the results suggest that metals in the cu mo deposits may have come from the mantle whereas those in the world class nanling w sn polymetallic deposits were derived from early mesoproterozoic 1 4 1 6 ga crustal basement of the cathaysia block in south china

the permo jurassic alkaline province of tadhak mali geology geochronology and tectonic significance the tadhak alkaline complexes located along the eastern edge of the west african craton in northeastern mali intrude the 2 ga old basement or the overlying pan african nappes and are covered by cretaceous sediments this magmatic activity was accompanied by doming and rifting northnortheast trending normal faults define two crustal segments exhibiting variously sized 1 km to 17 km intrusive complexes these comprise eroded undersaturated ring complexes with occasional carbonatites in the west and high level phonolitic plugs with shallow intrusive nepheline syenites in the east new rb sr isotopic data indicate 100 ma of within plate magmatic activity in the area without any obvious age progression in addition to the adrar tadhak complex 262 7 ma the following intrusions have been dated tirkine 215 11 ma anezrouf 184 14 ma and tidjerazraze in imanal 161 5 ma a clear break thus exists with the cambrian alkaline granite province of the iforas located on the opposite side of the pan african suture 87sr 86sr initial ratios are similar throughout between 0 7044 and 0 7049 which indicates a bulk earth isotopic composition the conclusion of exclusively mantle origin dupal composition previously demonstrated for the adrar tadhak complex can now be extended to the other complexes this implies repeated tapping of a single mantle source throughout the life of the province the described magmatic and tectonic permo jurassic activity has been triggered by intraplate stresses focussed along the pre existing pan african suture zone together with similar features in europe and africa the permo jurassic tadhak alkaline province heralds the progressive break up of pangaea and the opening of the atlantic ocean

a note on the geochronology of the iberian alkaline province abstract a critical assessment of previously published ages and new k ar results for two complexes in the iberian alkaline province monchique and sintra lead to the conclusion that two separate magmatic episodes occurred around 72 and 82 ma since emplacement of these alkaline rocks is undoubtedly related to rifting and subsequent changes in the atlantic ocean floor spreading geometry accurate timing of magmatic events within the iberian alkaline province is of fundamental significance

the opening of the atlantic the mesozoic new england igneous province and mechanisms of continental breakup abstract plume and tensionally induced rifting models have been proposed as causes of continental rifting and breakup mantle plumes should produce early lithospheric doming events associated with alkaline magmatism however the triassic alkaline rocks on the east north american e n a margin are structurally localized the uplifts are mostly jurassic in age and voluminous plateau magmatism post dates the initiation of taphrogenesis by 10 to 20 ma in addition there are no chains of anorogenic igneous rocks leading to any of the postulated triassic plumes these facts argue against the plume generated rifting model for the e n a margin but are consistent with the predicted effects of the tensionally induced rifting model the genesis of linear chains of alkaline rocks has been attributed to passage of a lithospheric plate over a mantle plume this explanation has been proposed for the cretaceous monteregian and white mountain intrusive series and the kelvin seamount chain however these rocks are distributed along structurally controlled e w trending lineaments the monteregians are localized by the presence of a paleorift ottawa bonnechere graben in the underlying grenvillian basement and the kelvin seamounts are emplaced along the early jurassic kelvin fracture zone the structures localizing the southern white mountain lineament are less well defined the spatial temporal and geochemical relationships along and between these lineaments are consistent with an origin as a southeastward propagating tensional event the intra plate stresses are probably the result of shifts in the pole of rotation of the north american plate due to irregular northward extensions of seafloor spreading during the cretaceous

a plate tectonic setting for mesozoic rifts of west and central africa abstract africa by virtue of its central position within gondwana has recorded much of the complex history of plate interactions which have progressively fragmented this supercontinent since the early mesozoic continental reconstructions reveal both a temporal and spatial relationship between the development of the continental margins of africa and the formation of rifted sedimentary basins deep within the african continent the multi stage opening of the atlantic ocean and associated rifting in west and central africa provide one of the most impressive examples of ocean continent tectonic interactions during the early cretaceous the onset of rifting along the future margins of the south atlantic is contemporaneous with intra continental rifting generating both strike slip and extensional basins within west and central africa the syn rift phase of intra continental basin development continued until the santonian by which time africa and south america had been physically separated for approximately 10 ma except for minor rifting during the senonian and tertiary the short lived phase of deformation at about 80 ma marks the transition into the post rift or sag phase of basin development this deformational event can be correlated with a period of plate motion changes recognised from fracture zone geometries seen in both the central and south atlantic oceans using present day stress analogies cretaceous rifting in africa can provide a means of indicating the regional palaeostress directions within africa at this time

chronology and geodynamic setting of cretaceous cenozoic rifting in west and central africa abstract the development of the early cretaceous to palaeogene west and central african rift system which extends from nigeria benue trough to kenya anza trough can be related to the build up of intraplate tensional stresses during the break up of gondwana which caused reactivation of pre existing zones of lithospheric weakness repeated changes in the intraplate stress regime of africa are reflected by phases of crustal extension alternating with episodes of compression many of these events can be correlated with changes in rates of seafloor spreading in the central and south atlantic oceans as reflected in flowline patterns the west and central african rifts can be considered as typical passive rifts which evolved in response to the build up of intraplate stresses however the st helena hot spot appears to have been located beneath the equatorial plate boundary at approximately 120 ma and may have played an important role in weakening the lithosphere during extension

raman spectroscopy and biomarker analysis reveal multiple carbon inputs to a precambrian glacial sediment extractable biomarkers can help elucidate the environment and biota of ancient glaciations although the method must be applied with care as glacial sediments have a potential for incorporation of older detrital carbon in phanerozoic glacial sediments the distinct elemental molecular and isotopic compositions of the terrestrial and marine biomass allow discrimination between primary marine and redeposited terrestrial organic matter however as the proterozoic biosphere was largely microbial and marine biomarker and isotopic analyses are insufficient for distinguishing primary organic matter from secondary reworked organic matter here we report the combined application of raman spectroscopy and biomarker analysis to precambrian glacial sediments which together allows discrimination between mixed pools of organic carbon and provides a promising new approach for rapidly screening precambrian sediments for immature organic matter amenable to biomarker analysis

using petroleum inclusions to trace petroleum systems a review abstract petroleum bearing fluid inclusions are small encapsulations of oil and gas that offer an invaluable opportunity to better constrain the evolution of petroleum systems insights into palaeo fluid compositions complement observations on present day fluid compositions which represent only the end point of complex cumulative processes throughout basin history in this contribution we review a wide range of approaches used to extract geochemical information from petroleum inclusions and how these can be used to better constrain petroleum systems these techniques can be grouped into optical spectrographic and thermometric non destructive methods or destructive chemical analyses of bulk samples or individual inclusions typically optical methods documenting the distribution and visual properties of petroleum inclusions are used to provide petrographic context for subsequent specialised geochemical analyses of petroleum inclusions additional non destructive techniques such as raman spectroscopy can then be applied to provide some further insights into the composition of the trapped fluids although the complex nature of petroleum generally requires direct access to the fluid for a more complete understanding of geochemical aspects a variety of destructive techniques have been developed initially to analyse bulk samples released by mechanical crushing and more recently through ablation type techniques that allow the composition of individual inclusions to be characterised screening geochemical techniques that utilise mechanical crushing of bulk samples to analyse petroleum inclusions using mass spectrometry without prior chromatographic separation have become routine analyses other geochemical techniques more geared towards detailed molecular information such as biomarkers utilise chromatographic separation prior to mass spectrometry evaluation of the isotopic composition of petroleum inclusions is also possible for both bulk samples and compound specific analyses the use of lasers to open individual inclusions allows the released contents to be analysed by thermal extraction gas chromatography mass spectrometry gc ms or mass spectrometric mapping of minerals using time of flight secondary ion mass spectrometry tof sims a surface sensitive analytical method that uses ion beams to ablate into minerals the continued evolution of techniques to analyse the incredibly small volume of hydrocarbons trapped within fluid inclusions has progressed to a point where there is little that can be done to evaluate a live oil or gas sample that cannot be achieved for a fluid inclusion sample the full power for tracing petroleum systems is however only realised where there is an effective integration of fluid inclusion data with a more conventional approach to petroleum systems analysis

kinematic evolution of the tethys belt from the atlantic ocean to the pamirs since the triassic abstract we present an updated series of kinematic reconstructions of the major plates around the tethys from the atlantic ocean to the pamirs between the early jurassic and the present this set is used elsewhere as a basis for paleogeographic maps of the entire region the problems related to the positions of the continents in the lower triassic are also discussed no direct analyses of magnetic anomalies and fracture zones in the atlantic have been made rather all available poles and rotations have been tested in order to eliminate or minimize possible kinematic errors the reconstructions are shown for nine key geological periods which correspond to well recognized magnetic anomalies except for the jurassic cretaceous boundary and the cretaceous tertiary boundary which correspond to interpolated positions paleolatitudes have been drawn using the study of westphal et al 1986 an attempt has been made to take into account the displacements caused by formation of the continental margins and basins by stretching the resulting relative vector of motions along the northern boundary of the tethys shows a significant change 80 m y ago left lateral motion with compression dominates before whereas right lateral motion with compression dominates after to the east rates of motion vary by a factor of three with time and four maxima can be clearly related to tectonic events in the late jurassic late cretaceous eocene and post middle miocene to the west north of apulia on the contrary the motion rate has not changed significantly since the early cretaceous and is close to 1 cm yr as an average these rather complex adjustments in rates and directions of relative motion are produced in great part through a complex migration of the africa eurasia pole of rotation and seem to be mostly governed by the tectonics of the tethys plate boundary

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

improved methods for isolating and validating indigenous biomarkers in precambrian rocks hydrocarbon biomarkers in highly mature precambrian rocks have the potential to provide important information about the diversity ecology and evolution of early life but studying them presents special analytical challenges extractable hydrocarbons are present in archean and most paleoproterozoic sedimentary rocks in such trace concentration that even slight contamination from petroleum derived materials in situ or during drilling storage sampling handling and laboratory analysis would compromise the results and thereby any consequent inferences here we report protocols that we have developed for the analysis of cores from several recently completed deep time scientific drilling initiatives by paying special attention to cutting cleaning crushing and extraction it is possible to significantly reduce laboratory blanks to acceptable levels when these methods are utilized meaningful variations in the patterns of biomarkers over stratigraphic and lithologic boundaries provide compelling evidence for syngeneity

in search of early life carbonate veins in archean metamorphic rocks as potential hosts of biomarkers abstract the detection of early life signatures using hydrocarbon biomarkers in precambrian rocks struggles with contamination issues unspecific biomarkers and the lack of suitable sedimentary rocks due to extensive thermal overprints importantly host rocks must not have been exposed to temperatures above 250 c as at these temperatures biomarkers are destroyed here we show that archean sedimentary rocks from the jeerinah formation 2 63 billion yrs and carawine dolomite 2 55 billion yrs of the pilbara craton western australia drilled by the agouron institute in 2012 which previously were suggested to be suitable for biomarker studies were metamorphosed to the greenschist facies this is higher than previously reported both the mineral assemblages carbonate quartz fe chlorite muscovite microcline rutile and pyrite with absence of illite and chlorite geothermometry suggest that the rocks were exposed to temperatures higher than 300 c and probably 400 c consistent with greenschist facies metamorphism this facies leads to the destruction of any biomarkers and explains why the extraction of hydrocarbon biomarkers from pristine drill cores has not been successful however we show that the rocks are cut by younger formation specific carbonate veins containing primary oil bearing fluid inclusions and solid bitumens type 1 veins in the carawine dolomite consist of dolomite quartz and solid bitumen whereas type 2 veins in the jeerinah formation consist of calcite within the veins fluid inclusion homogenisation temperatures and calcite twinning geothermometry indicate maximum temperatures of 200 c for type 1 veins and 180 c for type 2 veins type 1 veins have typical isotopic values for reprecipitated archean sea water carbonates with 13 c vpdb ranging from 3 to 0 and 18 o vpdb ranging from 13 to 7 while type 2 veins have isotopic values that are similar to hydrothermal carbonates with 13 c vpdb ranging from 18 to 4 and 18 o vpdb ranging from 18 to 12 evidently the migration and entrapment of hydrocarbons occurred after peak metamorphism under temperatures congruous with late catagenesis and from fluids of different compositions the relatively high temperatures of vein formation and the known geotectonic history of the rocks analysed suggest a probable minimum age of 1 8 billion yrs paleoproterozoic our results demonstrate that post peak metamorphic veins provide an exciting opportunity in the search for evidence of early life the integration of petrological and organic geochemical techniques is crucial for any future studies that use biomarkers to reconstruct the early biosphere

thermal evolution of the central atlantic passive margins continental break up above a mesozoic super plume a near synchronous early jurassic 200 ma magmatic event along the north american and west african passive continental margins can be explained by the upwelling of a large scale mantle plume super plume beneath the west african craton lateral deflection of the outflow from the plume to the ne by the ambient sub lithospheric mantle flow could account for tholeiitic basaltic magmatism extending over a distance of nearly 5000 km from brazil to southern spain continental break up is considered to occur above the stagnation streamline along which cooler material eroded from the base of the continental lithosphere by the plume is returned to the convecting mantle the dynamic model is a scaled version of that recently proposed for the hawaiian plume and satisfactorily explains the apparent lack of involvement of thermally anomalous mantle in the generation of central atlantic oceanic crust

exceptional preservation of fossil plant spores in high pressure metamorphic rocks it is commonly believed that high grade metamorphism erases all traces of fossils and more generally of biogenicity in rocks to address this issue we report in this study x ray spectromicroscopy raman microspectroscopy and electron microscopy observations at the nm scale of fossil lycophytes megaspores found in triassic metasedimentary rocks from the french alps these rocks have undergone high pressure metamorphic conditions not vert similar 14 kbar not vert similar 360 c during the alpine orogeny corresponding to a burial of not vert similar 35 km during subduction despite such metamorphism the fossil megaspores are remarkably well preserved texturally an unusual mineralogical zonation consisting of magnetite ankerite and calcite is superposed on the structures of the spore walls in parallel chemical heterogeneities of the organic matter om composing the spore walls were detected we discuss different interpretations for our observations in particular a scenario in which these features are remnants of the original structural heterogeneities in the spores and provide insights on the fossilization processes under high grade metamorphic conditions whatever the processes leading to such heterogeneities our observations oppose the usual assumption that high grade metamorphism erases all indications of biogenicity in rocks and open new avenues for investigation at the nm scale of the evolution of organic matter during diagenesis and metamorphism

hydrothermal alteration of organic matter in uranium ores elliot lake canada implications for selected organic rich deposits abstract organic matter in the uraniferous matinenda formation elliot lake is preserved in the forms of syngenetic kerogen and solid bitumen as it is in many of the oklo uranium deposits and in the witwatersrand gold uranium ores the elliot lake kerogen is a vitrinite like material considered to be remnants of precambrian cyanobacterial mats the kerogen at elliot lake has reflectances in oil ranging from 2 63 7 31 romax high aromaticity relatively low 0 41 0 60 atomic h c ratios and it contains cryptocrystalline graphite bitumen present primarily as dispersed globules up to 0 5 mm dia has reflectances from 0 72 1 32 romax atomic h c ratios of 0 71 0 81 and is somewhat less aromatic than the kerogen overall similarity in molecular compositions indicates that liquid bitumen was derived from kerogen by processes similar to hydrous pyrolysis the carbon isotopic composition of kerogen 15 62 to 24 72 and the now solid bitumen 25 91 to 33 00 are compatible with these processes despite having been subjected to several thermal episodes ca 2 45 ga old kerogen of microbiological origin here survived as testimony of the antiquity of life on earth u pb isotopic data from discrete kerogen grains at elliot lake form a scattered array intersecting concordia at 2139 100 ma correspond to the nipissing event u pb systems were totally reset by this event uranium and lead show subsequently partial mobility the average of which is indicated by the lower concordia intersect of 550 260 ma the migrated bitumen contains virtually no uranium and thorium but has a large excess of 206pb which indicates that the once liquid bitumen must have acted as a sink for mobile intermediate decay products of 238u emplacement of the nipissing diabase may have been responsible for producing the bitumen and indirectly for its enrichment in 206pb as a result of outgassing of 222rn except for noble metals and arsenic the kerogen contains higher abundances of trace elements than bitumen ree distributions vary sympathetically and hree depletion occurs in both organic matter types significant relative enrichment of noble metals in bitumen resulted from low temperature processes a phenomenon manifest in various platinum group element organic matter associations at other localities

analysis of single oil bearing fluid inclusions in mid proterozoic sandstones roper group australia hydrocarbons and organic biomarkers extracted from black shales and other carbonaceous sedimentary rocks are valuable sources of information on the biodiversity and environment of early earth howe

nature and composition of the lower continental crust in central spain and the granulite granite linkage inferences from granulitic xenoliths xenolith bearing alkaline ultrabasic dykes were intruded into the of the hercynian basement of the spanish central system in early mesozoic times the suite of lower crustal xenoliths in the dykes divided into three groups felsic peraluminous granulites metapelitic granulites and charnockitic granulites the felsic granulites form 95 of the total volume of the xenoliths whereas the charnockitic and metapelitic granulites are much less abundant 0 01 5 respectively thermobarometric calculations based on mineral paragenesis indicate equilibration conditions around 850 950 c 7 11 kbar thus the xenoliths represent lower continental crustal material superimposed on this high t high p assemblage is a high t low p paragenesis represented mainly by kelyphitic coronas reflecting re equilibration during transport in the clearly restitic mineral assemblages with up to 50 garnet and 37 sillimanite major and trace element modelling supports the idea that the late hercynian peraluminous granites of central spain represent liquids in equilibrium with restitic material of similar composition to the studied lower crustal xenoliths 87sr 86sr and end of the felsic xenoliths calculated at an average hercynian age of 300 ma are in the range 0 706 0 712 and 1 4 to 8 2 respectively these values match the isotopic composition of the outcropping late hercynian granites the sr isotopic composition of the xenoliths is lower than that of the outcropping mid crustal lithologies orthogneisses pelites a major contribution from the lower crust to the source of hercynian granites greatly reduces the necessity of invoking a large mantle contribution in models of granite petrogenesis the felsic nature of the lower continental crust in central spain contrasts with the more mafic lower crustal composition estimated in other european hercynian areas suggesting a non underplated crust in this region of the hercynian orogenic belt