crustal structure measurements near fram ii in the pole abyssal plain abstract an extensive refraction profiling program was carried out during the fram ii experiment march may 1980 in the eastern arctic ocean two structural areas were covered north of the ice camp 86 n 24 w into the basin of the pole abyssal plain and south onto the flanks of the morris jesup rise digital multichannel data on an 800 by 800 m 24 channel hydrophone array and a single 2 component ocean bottom seismometer obs were recorded for offsets from 2 5 to 100 km arrival times amplitudes and phase velocities of the seismic signals recieved on the hydrophone array were determined using high resolution array processing from these measurements and the obs data preliminary velocity structural models of the crust have been derived for the purposes of this paper 2 refraction lines have been analyzed a 40 km line on a flat region of the pole abyssal plain and an 86 km line on a slightly dipping region taken as the drifting ice camp shoaled on the morris jesup rise these preliminary analyses yield a sedimentary layer with a gradually increasing velocity 1 5 2 km thick this cover overlays a crust with a thin layer 2 and yields a total ocean bottom to mantle thickness of 4 7 km

an extensive region of off ridge normal faulting earthquakes in the southern indian ocean an unusually large number of off ridge earthquakes have occurred within a broad region astride the southeast indian ridge the source mechanisms of nine of these earthquakes in lithosphere from 4 to 27 m y old have been determined with a formal inversion technique based on matching observed and synthetic p and sh waves all events occur in the oceanic mantle from 8 to 22 km below the seafloor the seismic moment and duration of faulting obtained from the inversion constrain the stress drop to lie between a few tens of bars and a few hundred bars for most of the events all of the off ridge earthquakes are characterized by normal faulting mechanisms with the t axes nearly parallel to the nearby ridge axis in sharp contrast to typical mid ocean ridge or intraplate earthquakes a variety of possible tectonic processes which might produce normal faulting in young oceanic lithosphere are considered as possible causes including bending stresses thermal stresses of various origins differential subsidence at a fracture zone off ridge volcanism latitudinal plate motions on a nonspherical earth basal shear stress from small scale convection and plate wide perturbations of the stress field due to the continental collision between india and asia no single mechanism is adequate to explain all the observed off ridge normal faulting earthquakes an apparent asymmetry in the observed seismicity with respect to the ridge suggests that the earthquakes on the antarctic and indian plates are caused by different processes the antarctic plate events can be satisfactorily attributed to the release of thermal and bending stresses associated with a residual depth anomaly between the kerguelen plateau and the southeast indian ridge a region which has been suggested as the site of a horizontal pipe for flow in the asthenosphere from the kerguelen hot spot to the vicinity of amsterdam and st paul islands on the southeast indian ridge the off ridge earthquakes in the indian plate are speculatively attributed to part of the plate wide response of the lithosphere to the ongoing continental collision

geometry and mechanism of faulting of the 1980 el asnam algeria earthquake from inversion of teleseismic body waves and comparison with field observations the el asnam earthquake of october 10 1980 ms 7 3 provided a wealth of geological and seismological data and is an ideal event for comparing geologically and seismologically derived models the event produced extensive surface faulting in addition to the main tectonic deformation which is clearly a thrust widespread secondary normal faulting was observed at the surface in the southwest region the surface break of a thrust fault could be traced for 24 km in the northeast part of the fault zone normal faults with throws of several meters were observed although no clear thrust type surface breaks were observed in this region geodetic measurements and aftershocks indicate that thrusting was nonetheless the main tectonic mode of deformation in this paper the rupture process of the el asnam earthquake is investigated by inversion of teleseismic p and sh waves the effort is concentrated on the late part of the waveforms which carries information about the faulting in the least understood north eastern segment of the fault zone during the earthquake rupture initiated at the southern terminus of the southwestern fault segment and propagated northward as indicated by the epicentral location and the observable azimuthal directivity of the body wave shapes and amplitudes the seismogenic faulting in the southwestern segment has a thrust mechanism with the following average parameters seismic moment 3 9 1019 n m depth 6 km strike 220 dip 46 and rake 72 the duration of rupture on this fault segment was approximately 10 s which together with the observed fault length of 24 km indicates a rupture velocity of 2 4 km s unconstrained inversions suggest normal faulting in the northeast part of the source region that began immediately following the arrival of the rupture to the northeast region the normal faulting mechanism is in agreement with that observed for the large surface fault scarps in this region guided by the evidence from aftershocks and geodetic measurements a solution with a thrust type mechanism was also sought using a priori constraints based on the field observations the observed strike and slip angle we obtain the following thrust source mechanism for the northeast segment seismic moment 3 6 1019 n m depth 6 km strike 230 dip 20 and rake 91 this solution requires that the thrusting in the northeast be initiated 10 s after the arrival of the rupture from the southwest suggesting that some time is required to overcome the geometric barrier marked by the abrupt change in the dip and azimuth inversions are nonunique with respect to the seismic moment associated with normal faulting the ambiguity being caused by the limited bandwidth of the observed body waves the moment estimated from the surface waves at periods about 300 s however favors thrust faulting to be dominant in the northeast region

mechanisms of lithospheric extension at mid ocean ridges summary we examine the extensional deformation of oceanic plates at mid ocean ridges especially within an axial yield zone where pervasive faulting occurs thermal models of ridges are developed which include the effects of lithospheric thickening on the mantle flow the heat of magmatic crustal accretion at the ridge axis and the hydrothermal cooling due to seawater circulation in the crust when hydrothermal circulation occurs the brittle lithospheric plate at slow spreading ridges could be as thick as 8 9 km thicker than the crust while the plate at fast spreading ridges is only 1 2 km for a typical slow spreading ridge several kilometres of plate thickening are expected within a distance of 15 km from the ridge axis when subjected to the extensional force due to horizontal stretching shear failure by normal faulting will occur pervasively in an axial zone where the lithospheric plate is the thinnest adopting perfectly plastic rheology as a continuum description of deformation on the distributed faults we obtain approximate solutions for the stress distribution in the yield zone within this yield zone sea floor topography increases significantly away from the ridge axis so that the resuiting gravity sliding force balances the differential horizontal extensional force due to the thickening of the lithospheric plate basal stresses induced by the viscously deforming asthenosphere could significantly influence the stresses inside the lithospheric plate only if the mantle viscosity beneath the ridge is on the order of 10 pa s significantly higher than generally accepted values of 10 pa s model calculations reveal that although the seafloor topography at the mid atlantic ridge at 13 15 on is regionally compensated it is locally supported by stresses in the lithospheric plate the deviation of the lithospheric plate from its thermal isostatic equilibrium position can be explained by necking due to plastic stretching in the axial yield zone and elastic deflection of strong plates outside the yield zone the best fit models require the yield zones to have a half width of 10 15 km we find systematic variations in the gravity and topography of the east pacific rise which indicate strong influence of plate spreading rate on the ridge thermal and mechanical structure at the 16 17 on area where the half spreading rate is 4 3 cm yr a prominent axial topographic high and an axial mantle bouguer gravity low exist implying a crustal or sub crustal low density body the gravity low disappears at the 20 21 n area where the half spreading rate is 3 6 cm yr as the plate spreading rate decreases from 4 3 cm yr at the 16 17 on area to 2 7 cm yr at the 22 23 on area axial ridge topography changes from higher than the thermal isostatic equilibrium position to lower than the isostatic position the low axial topography at the 22 23 n area can be explained by the existence of a low amplitude median valley

spreading structure of the troodos ophiolite cyprus orientations of dikes within the sheeted dike complex of the troodos ophiolite reveal primary spreading structure produced at a complex ridge transform intersection three structural grabens are defined by listric and planar normal faults and rotated dikes that dip symmetrically toward graben axes faults flatten at depth into a detachment within the upper parts of the plutonic complex large exhalative massive sulfide deposits occur within the pillow sections of two of the grabens and appear to be associated with underlying altered and mineralized normal fault zones that channeled hydrothermal fluids we suggest that the grabens are fossil axial valleys produced by successive eastward jumps of an approximately north trending present coordinates slow spreading ridge crest a simple model for ridge migration indicates eastward jumps of 8 13 km changes in ridge orientation are suggested by changes in trends of dikes and graben axes comparison of the pattern of dikes near the arakapas fault zone with the structure of active ridge transform intersections suggests that the fault is a right offset sinistral transform in contrast to earlier models in which a ridge to the west of the exposed troodos complex was proposed

sm nd isotopic dating of proterozoic clay material an example from the francevillian sedimentary series gabon clay fractions of 0 4 m and 0 2 m and acid leachates of these clay fractions were separated from two different kerogen rich black shale samples from the francevillian sedimentary series in gabon they yielded identical sm nd isochron ages of2099 115 ma and2036 79 ma with initial nd values of 6 5 2 0 and 9 3 1 5 respectively these ages define multi episode illitization during early diagenesis as suggested by the differentsm nd ratios of the clay fractions the sm nd isotope data of the kerogen source rock up to 13 organic carbon plot between the two isochrons but close to the clay fractions suggesting that this organic matter reached nd isotopic equilibrium with respect to the clay minerals and the diagenetic fluids analyses of fracture filling bitumen showed that it had very elevated143nd 144nd and147sm 144nd ratios these values are higher than the most elevated values of the leachates but plot on the extension of the two isochrons this indicates that the bitumen reached nd isotopic equilibrium with the kerogen source rock from which it was expelled and the authigenic clay minerals secondary migration and chemical evolution of the bitumen appear to have caused the fractionation between sm and nd with an attendant increase in thesm nd ratio the results suggest that the sm nd isotope method has potential for both dating diagenetically related illitization using leachate residue pairs of small size clay fractions and reconstructing geochemical processes during diagenesis in oil producing sedimentary basins

transform fault earthquakes in the north atlantic source mechanisms and depth of faulting we have determined the centroid depths and source mechanisms of 12 large earthquakes on transform faults of the northern mid atlantic ridge from an inversion of long period body waveforms the earthquakes occurred on the gibbs oceanographer hayes kane 15 20 and vema transforms we have also estimated the depth extent of faulting during each earthquake from the centroid depth and the fault width for five of the transforms earthquake centroid depths lie in the range 7 10 km beneath the seafloor and the maximum depth of seismic faulting is 14 20 km on the basis of a comparison with a simple thermal model for transform faults this maximum depth of seismic behavior corresponds to a nominal temperature of 900 100 c in contrast the nominal temperature limiting the maximum depth of faulting during oceanic intraplate earthquakes with strike slip mechanisms is 700 100 c the difference in these limiting temperatures may be attributed to the different strain rates characterizing intraplate and transform fault environments three large earthquakes on the 15 20 transform have shallower centroid depths of 4 5 km and a maximum depth of seismic faulting of 10 km corresponding to a limiting temperature of 600 c the shallower extent of seismic behavior along the 15 20 transform may be related to a recent episode of extension across the transform associated with the northward migration of the triple junction among north american south american and african plates to its present position near the transform the source mechanisms for all events in this study display the strike slip motion expected for transform fault earthquakes slip vector azimuths agree to within 2 3 of the local strike of the zone of active faulting the only anomalies in mechanism were for two earthquakes near the western end of the vema transform which occurred on significantly nonvertical fault planes secondary faulting occurring either precursory to or near the end of the main episode of strike slip rupture was observed for five of the 12 earthquakes for three events the secondary faulting was characterized by reverse motion on fault planes striking oblique to the trend of the transform in all three cases the site of secondary reverse faulting is near a compressional jog in the current trace of the active transform fault zone we find no evidence to support the conclusions of engeln wiens and stein that oceanic transform faults in general are either hotter than expected from simple thermal models or weaker than normal oceanic lithosphere

a seismic refraction survey of the imperial valley region california the u s geological survey conducted an extensive seismic refraction survey in the imperial valley region of california in 1979 the imperial valley is located in the salton trough an active rift between the pacific and north american plates forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0 5 1 km more than 1300 recording locations were occupied and more than 3000 usable seismograms were obtained we analyzed five profiles using a standard ray tracing program constructed a contour map of reduced travel times from our most widely recorded shot point and modeled an existing gravity profile across the salton trough results are itemized 1 all models have in common a sedimentary layer vp 1 8 5 0 km s a transition zone vp 5 0 5 65 km s a basement vp 5 65 km s in the imperial valley 5 9 km s on the bordering mesas and subbasement vp 7 2 km s 2 the sedimentary layer ranges in thickness along the axis of the salton trough from 3 7 km salton sea to 4 8 km u s mexican border on the bordering mesas it is quite variable in thickness 3 the transition zone is about 1 km thick in most places in the imperial valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement on the bordering mesas however there is a discontinuity at the top of this zone 4 there are apparently two types of basement on the bordering mesas basement is crystalline igneous and metamorphic rocks in the imperial valley basement is mostly lower greenshist facies sedimentary rocks based primarily on the smooth transition in character from sediment to basement arrivals the low value of basement velocity and the fact that deep 4 km wells in the valley penetrate only the upper part of the known cenozoic stratigraphic column for the salton trough 5 the subbasement or intermediate crustal layer ranges in depth along the axis of the salton trough from 16 km salton sea to 10 km u s mexican border gravity modeling requires that this layer deepen and or pinch out beneath the bordering mesas and mountain ranges based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the imperial valley the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust 6 several structures are seen that affect basement transition zone and deeper parts of the sedimentary layer they include a scarp along the imperial fault as much as 1 km down to the northeast and a scarp passing roughly along the topographic boundary between the imperial valley and the bordering mesa to the west as much as 3 km down to the east we interpret the latter scarp to be the suture or rift boundary between the older crystalline basement on the mesa and the younger metasedimentary basement in the imperial valley 7 on a contour map of reduced travel time from our most widely recorded shot point subtle patches of early arrivals among otherwise late arrivals in the central imperial valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150 c apparently the salton trough is a location where new crust is being generated as the rift opens mafic intrusive rocks fill it from below as sedimentary rocks fill it from above rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth metasedimentary basement in the imperial valley and thus consolidates the new crust

an example of lower proterozoic sediments the francevillian in gabon abstract new mineralogical and isotopic data have been obtained on diagenetic fine fractions of the francevillian sediments together with the rb sr age of a syenitic intrusive and already published isotopic ages determined by means of uranium ore and uranium fission products they allow us to present a more complete geological history of the francevillian the main steps are deposition of the fa formation before 2140 ma deposition of the fb fe formations between 2140 and 2050 ma siliceous diagenesis followed by nuclear reactions between 2050 and 1930 ma carbonate diagenesis at nearly 1870 ma end of cooling at 1700 ma and dolerite intrusion at ca 970 ma the francevillian sediments are to date the oldest true sediments known they are diagenetic lower proterozoic sediments

role of organic matter in the proterozoic oklo natural fission reactors gabon africa of the sixteen known oklo and the bangombe natural fission reactors hydrothermally altered elastic sedimentary rocks that contain abundant uraninite and authigenic clay minerals reactors 1 to 6 at oklo contain only traces of organic matter but the others are rich in organic substances reactors 7 to 9 are the subjects of this study these organic rich reactors may serve as time tested analogues for anthropogenic nuclear waste containment strategies organic matter helped to concentrate quantities of uranium sufficient to initiate the nuclear chain reactions liquid bitumen was generated from organic matter by hydrothermal reactions during nuclear criticality the bitumen soon became a solid consisting of polycyclic aromatic hydrocarbons and an intimate mixture of cryptocrystalline graphite which enclosed and immobilized uraninite and the fission generated isotopes entrapped in uraninite this mechanism prevented major loss of uranium and fission products from the natural nuclear reactors for 1 2 b y 24 refs 4 figs

natural nuclear fission reactors time constraints for occurrence and their relation to uranium and manganese deposits and to the evolution of the atmosphere abstract knowledge of the formation conditions of francevillian uranium and manganese ore deposits as well as natural fission reactors sheds light on the early evolution of the atmosphere between 1950 and 2150 ma ago the model explaining the formation of the oklo uranium deposits suggests that at the time of sediment deposition in the franceville basin 2150 million years ago the oxygen deficient atmosphere would have inhibited uranium dissolution dissolution of uranium was only possible during later diagenesis approximately 1950 ma reduction reactions in the presence of hydrocarbons allowed precipitation of dissolved uranium to u4 forming deposits with high enough uranium contents to trigger subsequent nuclear fission reactions such a model is in agreement with earlier suggestions that oxygen contents in atmosphere increased during a transition phase some 2450 2100 ma ago the manganese deposits were formed before the uranium deposits during the deposition of the black shales and very early diagenesis and thus at a time when oxygen content in atmosphere was very low carbon isotopes data of organic matter show decrease of 13c upward in the francevillian series 20 to 46 pdb reflecting the high ch4 and low o2 contents in the atmosphere during sediment deposition this favoured anoxic conditions during deposition of the basinal fb black shales and likewise the migration of mn over long distances the manganese precipitated first as mn oxides at the shallow edges of the franceville basin in photic zones where photosynthetic organisms flourished mn oxides were then reduced in the black shales forming mn carbonates when conditions became more reducing during transgression episodes and or the first stages of burial in the black shales reducing conditions prevailed until recent weathering allowing the good preservation of organic matter and the mn deposits the present day alteration is responsible for the dissolution of mn carbonates and precipitation of mn oxides at the water table to form the high grade mn ore 45 50 mn development of photosynthesizing organisms a volcanic source of the mn and favourable palaeogeography of the francevillian basins are all important parameters for the formation of the mn deposits for the occurrence of the natural nuclear reactors the age of 2 0 ga is the main parameter that controls the abundance of fissile 235u and the critical mass before 2 0 ga the 235u 238u ratio was sufficiently high for fission reactions to occur but conditions favourable for forming high grade uranium ores were not achieved then after 2 0 ga the increase of oxygen in the atmosphere commonly led to the formation of high grade uranium ores in which the 235u 238u ratio was too low to support criticality

the first stage in the formation of the uranium deposit of arlit niger role of a new non continental organic matter abstract the uranium deposits of arlit are located in fluvial sandstone rich in organic matter of continental origin type iii deposited in palaeochannels previous studies on arlit deposits documented that organic matter of continental origin type iii was the main contributor to the reduction of uranium from the oxidation state of uranium vi u vi to uranium iv u iv this study shows that migrated oils of non continental origin strongly contributed to the genesis of uranium mineralization the petrographic analyses showed that i solid bitumen resulting from the polymerization of migrated oils is present in abundance in the porosity of the sandstone and fractures at the level of the channels of the tarat formation tamou takriza southern taza and guezouman formation mine sector of akouta and ii uranium mineralization is mainly associated with this migrated organic matter currently expressed in the form of solid bitumen the association occurs at an infra micrometric scale in the solid bitumen the geochemical analysis rock eval py gc ms and gc ms and the analysis of different indicators and biomarkers n alkanes pr ph steranes indicate the presence of a second organic matter with a different signature to type iii the oils present in the porosity of the sandstone of the guezouman and tarat formations confirm that channels of these two formations are the traps of the oil migration

the composition and emplacement of basaltic magmas produced during the development of continental margin basins the gulf of california mexico the gulf of california is a region of active ensialic basin development and transform faulting legs 64 and 65 of the deep sea drilling project have demonstrated the diversity of basement stratigraphy drilling at sites 474 482 483 and 485 at the mouth of the gulf recovered massive basaltic flows and occasional pillow lavas and sills whereas drilling in the central part of the gulf in the guaymas basin recovered a sequence of sediments and massive doleritic sills very high sedimentation rates over 1000 m ma are characteristic of the guaymas basin and prevent extrusion of the basaltic magma onto the seafloor thus resulting in at least partial intercalation of oceanic layers 1 and 2 such a process of ocean crust formation could be locally important during the early stages of ocean and marginal basin development when sedimentation rates may be very high over 200 samples of basalts and dolerites from the gulf of california have been analysed by x ray fluorescence and neutron activation techniques basalts from the mouth of the gulf are tholeiitic and contain very low abundances of the more hygromagmatophile more hyg elements k rb ba th ta nb and light ree relative to the hyg elements y zr ti and heavy ree thus resembling depleted n type morb from elsewhere along the e pacific rise epr dolerites from the guaymas basin also have a tholeiitic chemistry and some epr basalt characteristics e g la n sm n

paleoproterozoic high δ13c dolomites from the lastoursville and franceville basins se gabon stratigraphic and synsedimentary subsidence implications abstract new data for the isotopic composition of carbon in shallow marine sedimentary carbonates in southeastern gabon franceville and lastoursville sub basins indicate that the seawater value of 13 c in these sediments underwent a large positive excursion around 2 1 ga these carbonate rocks belong to the fc formation of the francevillian group deposited along the margin of the archean chaillu craton in open marine shelf and evaporite settings and experienced locally lower greenschist facies metamorphism four microfacies mf1 mf4 are recognized from subtidal cyanobacterial dolobindstones to inter and supratidal collapse breccias mf4 the dolomites exhibit elevated 13 c values most values are above 0 50 the highest value is 9 74 and the average value is near 4 vs vpdb consistent with deposition during the large worldwide positive excursion between 2 22 and 2 06 ga these positive 13 c values are independent of the microfacies type such high 13 c values are also recorded in reworked carbonates dolomites embedded in coarse grained sandstones and conglomerates interstratified in the black shales of the underlying fb formation suggesting that the dolomites were at least stratigraphically equivalent to the black shale deposits interaction between early structural development and sediment fill lead to the formation of several sub basins with distinct differences in terms of amount and timing of subsidence the 13 c enriched dolomites are probably linked to high burial rate of organic carbon starting with the deposition of the fb sediments in this extensional geotectonic context as a consequence the cherty and dolomitic fc formation cannot longer be considered as a marker interval the end of the positive excursion is located near the top of the francevillian series i e fe and could be used in future studies as a potential chemostratigraphic tool in the paleoproterozoic

the origin distribution and hydrocarbon generation potential of organic rich facies in the nonesuch formation central north american rift system a regional study abstract precambrian unmetamorphosed organic rich clastic rocks whilst uncommon in the geologic record are of interest with respect to their potential for hydrocarbon generation the 1 1 billion year old nonesuch formation is primarily a lacustrine deposit which is part of a thick sequence of volcanic and clastic sedimentary rocks that filled an aborted rift in central north america previous organic geochemical investigations of the nonesuch formation have been limited to isolated samples from the basal portion of the nonesuch formation at a copper mine white pine in northern michigan in the present study 183 outcrop and core samples of the nonesuch formation from northern wisconsin and michigan upper peninsula were collected for sedimentologic and organic geochemical analyses total organic carbon toc values for the samples ranged from 0 0 to 2 5 and showed a strong correlation between organic richness and depositional environment detailed petrographic analysis using incident white light and reflected blue light fluorescence revealed two major organic petrographies 1 and 2 which could be further distinguished using combined pyrolysis gas chromatography mass spectrometry py gc ms whereas the composition of organic petrography 1 kerogens is primarily aliphatic the composition of organic petrography 2 kerogens is dominated by aromatic and or phenolic constituents the geographic and stratigraphic distribution of rocks bearing the two organic petrographies indicate that variable source input or preservation superimposed over maturity differences is responsible for both organic petrographic and kerogen pyrolysate compositions

saturated hydrocarbon biomarkers in oils of late precambrian age from eastern siberia abstract large quantities of petroleum derived from source rocks of late precambrian early cambrian age have been discovered in the lena tunguska region of eastern siberia over the last twenty five years we have examined three oils with presumed late precambrian vendian source rocks from this region for c 15 saturated hydrocarbon biomarkers using gas chromatography gc and gas chromatography mass spectrometry gc ms our results show these oils to have many characteristics in common with other oils of similar age from eastern siberia and oman amongst the compounds detected in the three oils analyzed were 1 n alkanes which show a marine derived distribution 2 centrally branched monomethyl alkanes 3 acyclic isoprenoids pr ph 1 4 tricyclic and tetracyclic terpanes 5 hopanes 6 c 29 unrearranged steranes and 7 c 30 nuclear steranes all of these compounds with the exception of the 4 methylsteranes which on current biochemical knowledge are unexpected in oils of this age can be assigned a prokaryotic source this and other evidence such as the lack of diasteranes suggest microorganism were the major contributors to organic matter deposited as part of a carbonate source rock

precambrian oil inclusions in late veins and the role of hydrocarbons in copper mineralization at white pine michigan liquid oil was trapped as primary fluid inclusions in calcite crystals in late cu fe sulfide bearing veins that crosscut and offset the cupriferous shale deposit at white pine the age of the calcite 1047 35 ma is an entrapment age and thus a minimum age for the oil close temporal and spatial associations of oil and metallic sulfides in the late veins suggest that liquid and solid hydrocarbons may have been dominant controls of the main stage white pine copper mineralization

natural fission reactors of oklo fourteen natural fission reactors are known to date in three different ore deposits located in the franceville basin the most famous and the first discovered are those in the oklo uranium ore deposit petrographic geochemical and oxygen isotope studies have been performed in an attempt to answer these questions 1 what are the main characteristics of the uranium ores in which fission reactions took place 2 what are the processes that created such exceptionally high grade ores and 3 what are the effects of such fission reactions on the host rocks the ore which hosts the fission reactions at oklo is characterized by a gangue of clay minerals whereas the normal ore of oklo consists of sandstones these clay minerals are the result of hydrothermal alteration of sandstone hydrothermal circulation was induced by the heat produced by fission reactions the reactor ores are further characterized by very high uranium contents 20 60 and high uranium accumulation by means of silica depletion and by uranium enrichment of the normal ore without uranium depletion hydrothermal convective circulation induced by the fission reactions created very high grade ore next to the reactors hydrothermal processes are thus critical to the propagation of the fission reactions

the nansen ridge arctic ocean some geophysical observations of the rift valley at slow spreading rate abstract we have examined continous echo soundings across the nansen rift valley and analysed the first gravity and bathymetric data available across the northern flank of the nansen ridge for comparison with other parts of the mid ocean ridge system the observed width of the inner floor 1 7 5 5 km at a total opening rate of 1 5 cm yr supports the idea of progressive narrowing of the inner floor with decreasing spreading rate for slow spreading ridges however the observed distance between the outer walls 18 22 km or rift valley width 30 33 km suggests that these parameters at least to a first approximation tend towards a minimum value with decreasing spreading rate the relation between gravity and bathymetry along the drift track of ice station fram i tentatively suggests that the sea floor topography of this part of the nansen ridge is supported by a plate with an elastic thickness less than what is observed on the mid atlantic ridge

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

petrography and paragenesis of organic matter associated with the natural fission reactors at oklo republic of gabon a preliminary report sixteen known uranium rich pockets in the sediment hosted uranium ore deposits in the oklo area became nuclear fission reactors 1968 50 ma ago and operated as such for up to 1 ma nuclear criticality was caused by unique and fortuitous geological environments and events these included the localized high concentrations of uranium in small pockets in the ore bodies the fact that the relative abundance of the fissile 235u isotope was five times greater in uranium 2 ga ago than present the presence of water which acted as a moderator in the natural reactors and the absence of neutron poisons which are elements that can prevent nuclear chain reactions organic matter is present in all of the oklo natural reactors but it is abundant only in reactors 7 to 16 organic matter has been studied in natural reactors 7 to 9 and at locations at various distances from these reactors bitumen in the oklo reactors is the predominant organic phase it is now solid bitumen containing dispersed cryptocrystalline graphite in crystal domains smaller than optical microscopic size its immediate precursor was liquid bitumen generated from syngenetic kerogen in organic rich sedimentary rocks in the francevillian basin of gabon and in the natural reactors themselves by hydrous reaction mechanisms prior to during and after nuclear criticality a preliminary paragenesis of organic matter at oklo may be defined through the following sequence of events a protokerogen and kerogen evolve from abundant cyanobacteria in the tidal and deltaic sediments in the basal fa formation of the francevillian series during sedimentation and burial respectively b during subsequent subsidence of the basin the first generation of bitumen and petroleum occurred c during uplift the sedimentary strata were fractured and bitumen and petroleum migrated through fractures following this oxidizing aqueous solutions carrying uranyl ions migrated through the highly fractured rocks met and were reduced by organic matter and precipitated pitchblende or uraninite at the present sites of the oklo uranium ore deposits and natural reactors the natural reactors reached nuclear criticality they were heated and produced the second generation of bitumen through hydrous reactions from the by now solid first generation bitumen and remaining kerogen finally hydrothermal effects associated with the dolerite dike swarm intrusion 750 150 ma ago produced yet another localized generation of liquid bitumen from solid organic matter possibly other bitumen generations also occurred during and following this time interval the organic matter in the natural reactors soon became a solid and was able to prevent the loss of uraninite grains enclosed in it these uraninite grains trapped and then immobilized fission products until and to an extent even after a major tectonic igneous event the intrusion of the dolerite dike swarm

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

viscosity of oceanic asthenosphere inferred from remote triggering of earthquakes a sequence of large interplate earthquakes from 1952 to 1965 along the aleutian arc and kurile kamchatka trench released accumulated stresses along nearly the entire northern portion of the pacific plate boundary the postseismic stress evolution across the northern pacific and arctic basins calculated from a viscoelastic coupling model with an asthenospheric viscosity of 5 x 10 17 pascal seconds is consistent with triggering of oceanic intraplate earthquakes temporal patterns in seismicity at remote plate boundaries and space based geodetic measurements of anomalous velocity over an area 7000 by 7000 kilometers square during the 30 year period after the sequence

centroid depths and mechanisms of mid ocean ridge earthquakes in the indian ocean gulf of aden and red sea we have determined the centroid depths and source mechanisms of 14 earthquakes along spreading centers in the indian ocean gulf of aden and red sea the earthquakes are all characterized by normal faulting on planes dipping at about 45 and generally striking parallel to the local trend of the spreading center for the ridge axis earthquakes in the indian ocean gulf of aden and central red sea seismic moments range from 3 to 9 1024 dyn cm source time functions are all of simple form and centroid depths of events with adequate azimuthal coverage are all very shallow between 1 and 4 km beneath the seafloor if the centroid depth typically marks the mean depth of fault slip then earthquake faulting extended to 2 8 km beneath the seafloor during these earthquakes the centroid depths for two earth quakes near the mouth of the gulf of suez support the view that seismic faulting accompanying continental rifting generally extends to greater depth than does seismic faulting along the axes of oceanic spreading centers when these findings are combined with our earlier results from the mid atlantic ridge the greatest centroid depths of earthquakes with well constrained source mechanisms tend to shoal with increasing spreading rate this result supports a general decrease with spreading rate in the maximum thickness of the zone of brittle behavior for slow spreading ridges

focal depths and mechanisms of large earthquakes on the arctic mid ocean ridge system as part of a global study of the source characteristics and tectonic implications of large earthquakes on mid ocean ridges we report on the focal depths and mechanisms of the six largest earthquakes that have occurred in the last 20 years on the arctic mid ocean ridge system for each earthquake we invert the long period p and sh waveforms to estimate the parameters of the best fitting point source including seismic moment centroid depth double couple source orientation and source time function three of the earthquakes occurred on the oceanic spreading center in the eurasian basin along ridge segments spreading at half rates of 4 6 mm yr these events have mechanisms very similar to those of ridge crest earthquakes on the mid atlantic ridge almost pure normal faulting on planes that dip at approximately 45 and strike parallel to the rift axis moments of 4 5 1024 dyn cm centroid depths of 1 2 km beneath the seafloor and water depths inferred from the predominant period of water column reverberations appropriate to epicentral locations within the median valley the remaining three earthquakes also characterized by normal faulting are associated with the continuation of the divergent plate boundary 2 3 mm yr half rate onto the continental shelf of the laptev sea where the crust becomes transitional in nature one of the largest known spreading center earthquakes august 25 1964 m0 1 1026 dyn cm occurred where the oceanic ridge intersects the outer edge of the continental slope waveform inversion for this event can resolve unilateral rupture from north to south landward along a fault at least 30 km in length the preferred centroid depth is 5 km beneath the seafloor in crustal material with an unusually low shear velocity but a centroid depth as great as 15 km cannot be ruled out two earthquakes beneath the continental shelf have significantly greater centroid depths 10 20 km than mid ocean ridge earthquakes indicating a thicker brittle regime and a cooler thermal structure than are typical of oceanic spreading centers the tectonic environment of these events is more representative of rifted continental lithosphere than of a mid ocean ridge

carbonaceous substances associated with the paleoproterozoic natural nuclear fission reactors of oklo gabon paragenesis thermal maturation and carbon isotopic and trace element compositions field relationships textural and reflectance characteristics and carbon isotopic compositions indicate that at least eight different categories of carbonaceous substances cs occur in the black shales uranium ores and fossil nuclear fission reactors of oklo the categories include kerogens of the mikouloungou coal in the fa formation the c1 pelites of the fa formation the black shale of the fb formation and bitumens present in veinlets in the basal black shale of the fb formation in the fa formation in normal ore and in hydraulic fractures in and near the fossil reactors and in late dolerite dikes bitumen migration occurred at oklo during at least three intervals 1 ca 2 0 ga ago when petroleum liquids derivatives of which are preserved as solid bitumen were generated by thermal maturation of black shale kerogen 2 ca 1 968 50 ma ago when 15 or more pockets of uranium ore achieved criticality and 3 977 to 981 27 ma ago when dolerite dikes intruded radionuclide migration occurred during intervals 2 and 3 bitumenization trends established for organic matter which has undergone thermal maturation fail to discriminate between heat affected and normally matured cs at oklo in and near oklo reactors where several types of solid bitumen commonly occur in close association carbon of the youngest low reflectance bitumen is isotopically heavier than that of older high reflectance bitumen due to thermal cracking this process together with enhanced radiolysis of bitumen in and near the reactors have combined to increase its 13c avg 25 8 compared to that of bitumen samples in normal ore avg 35 5 distant from the reactors graphitized bitumen inhibited migration of 235u and some fission products in uraninite of the natural reactors however alkaline earth elements such as ba sr and mo and alkaline elements rb and cs not compatible in the uo2 crystalline structure have migrated in hydrothermal solutions and in the once fluid oklo bitumens in situ analysis of solid bitumens by laser ablation icp ms detects differences in trace element composition developed as a result of their varying geological histories and evolutionary trends

proto polymorphs of jimthompsonite and chesterite in contact metamorphosed serpentinites from japan proto polymorphs of jimthompsonite and chesterite occur in metamorphosed serpentinites from two japanese ultramafic complexes the lattice constants of the proto triple chain silicate measured by x ray diffraction are a 0 93605 208 b 2 72560 588 and c 0 53160 89 nm whereas those of the mixed double and triple chain silicate are a 0 94202 78 b 4 54402 392 c 0 53440 45 nm and 90 026 18 the lattice constants and systematic extinctions revealed by selected area diffraction patterns are consistent with proto triple chain silicate pbcn and mixed double and triple chain silicate a 2 m am but not with the ortho and clino polymorphs high resolution transmission electron microscopy 016 images of the triple chain silicate and 0 1 15 images of the mixed chain silicate indicate they have a x configuration proto forms of wide chain pyriboles might be geologically widespread

tectonics of slow spreading mid ocean ridges and consequences of a variable depth to the brittle ductile transition geophysical evidence especially microearthquakes that extend into the upper mantle beneath the inner floor of the mid atlantic and gorda ridges indicates that the axial magma chamber episodically freezes beneath slow spreading ridges freezing of the axial magma chamber will result in a variable thermal structure that has important tectonic consequences most importantly large near axis faults will intersect as the brittle ductile transition deepens causing some of the faults to become locked the topographic effect of fault locking will be a variation in the width of the inner floor from narrow 10 km when a magma chamber is absent the deep microearthquakes also imply that faults at slow spreading ridges extend into the upper mantle the nature of these faults at depth is unknown but the following hypotheses are presented 1 the faults may be planar and pass downward into aseismic ductile shear zones and 2 the faults may sole into a flat detachment fault defining a sharp boundary between brittle faulting and homogeneous ductile flow such low angle normal faults are common in continental extensional domains large scale tilting 50 70 is probably common at slow spreading ridges and implies either listric faulting or rotation of planar faults and fault blocks similar to toppling dominoes calculations of temperatures at the brittle ductile transition are made assuming olivine and diabase rheologies for the mantle and crust respectively the temperature is approximately 800 c when the transition is at 8 km depth and approximately 600 when the transition is at 2 km depth the strength of the lithosphere varies dramatically with the rise and fall of the brittle ductile transition as magma chambers form and subsequently freeze strong lateral temperature gradients will be present when a magma chamber is formed resulting in localization of faulting within thin weak crust above the chamber when the magma chamber freezes lateral temperature gradients will be small and the strength of the lithosphere will be similar over a wide area this results in active faulting occurring over a width of as much as 60 km some ophiolites possibly provide insights to the tectonic processes at slow spreading ridges the entire crustal section of the josephine ophiolite was tilted by approximately 50 prior to deposition of overlying pelagic sediments this tilting probably took place by faulting suggesting that oceanic faults extended into the upper mantle possible oceanic faults have been identified in the josephine ophiolite but their oceanic origin cannot be established beyond doubt

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

structural and metamorphic evolution of the gabon orogenic belt collision tectonics in the lower proterozoic abstract the archaean granite gneiss terrain of central gabon comprises medium to high grade gneisses metamorphosed at 3 15 ga forming the monts cristal and underlying the mitzic region intruded by and enclosed as enclaves in granitoids 3 0 2 6 ga old that form the extensive chaillu and north gabon blocks these rocks are unconformably overlain in the east of gabon by early proterozoic francevillian sedimentary rocks between 2 3 and 2 0 ga old forming the franceville and booue basins the francevillian is overthrust on its west side by the ogooue metamorphics themselves considered to be of early proterozoic age these rocks crop out in a broad synclinorium consisting essentially of paragneisses with orthogneisses appearing locally at the base the overall tectonic setting is discussed in terms of the evolution of the various domains from west to east 1 1 on the west flank of the synclinorium is a migmatitic granulitic dome the abamie dome in which structures diverge away from the core metamorphism is of high temperature intermediate to low pressure type 2 2 on the east flank of the ogooue synclinorium the medium grade ogooue metamorphics sillimanite biotite kyanite staurolite staurolite muscovite biotite have been thrust eastwards on to the low grade kyanite sericite francevillian 3 3 the francevillian is parautochthonous being widely detached from its archaean basement owing to the effects of the same eastward directed tangential tectonics 4 4 locally slices of archaean basement have been tectonically incorporated the structural cross section illustrates a history of diapiric uprise passing eastward into tangential tectonics that is typical of classical collision belts the suture marking the collision is interpreted to be located in the abamie migmatite dome where the nappes are rooted the gabon orogenic belt is correlated with the collision belts of about the same age on the eastern margin of the congo craton and with the trans amazonian orogenic belt of the guyana shield

characterization of insoluble organic matter from the lower proterozoic huronian supergroup elliot lake ontario abstract elliot lake organic matter falls into two main groups 1 stratiform kerogens with reflectance from 2 63 to 7 31 ro max exhibit a high level of aromaticity and low atomic h c ratios 0 41 0 60 2 less aromatic disseminated globules most resembling solid bitumen are of lower maturity with ro max from 0 72 to 1 32 and atomic h c ratios 0 71 0 81 stratiform kerogens originated from cyanobacteria have 13c values from 15 to 25 one exception with 13c 31 8 may have originated from methanogenic source biota thus two genetically distinct populations of kerogen are tentatively recognized in elliot lake metasediments globular bitumens remobilized by hydrous pyrolysis from kerogen exhibit 13c values ranging from 25 tp 33 0 the overall isotopic bimodality among elliot lake organic material is explained by analogy to petroleum generation from particulate kerogen that matured under conditions of catagenesis to metagenesis and was subjected to radiation over a long time barium u th au pt pd and ree contents of elliot lake stratiform kerogen greatly exceed of average black shales except for as pt pd and au all metals ti v cr mn fe co ni zn se rb sr ag sb cs ba hf ta w hg th and u are more concentrated in stratiform kerogens than in the derivative bitumen this signals a significant role for a low temperature transfer of noble metals to migrating bitumen

review and update of the applications of organic petrology part 2 geological and multidisciplinary applications abstract the present paper is focused on organic petrology applied to unconventional and multidisciplinary investigations and is the second part of a two part review that describes the geological applications and uses of this branch of earth sciences therefore this paper reviews the use of organic petrology in investigations of i ore genesis when organic matter occurs associated with mineralization ii the behavior of organic matter in coal fires self heating and self combustion iii environmental and anthropogenic impacts associated with the management and industrial utilization of coal iv archeology and the nature and geographical provenance of objects of organic nature such as jet amber other artifacts and coal from archeological sites and v forensic science connected with criminal behavior or disasters this second part of the review outlines the most recent research and applications of organic petrology in those fields

nanoscale relationships between uranium and carbonaceous material in alteration halos around unconformity related uranium deposits of the kiggavik camp paleoproterozoic thelon basin nunavut canada concentrations of 7 u and 1 cu were identified in massive brecciated and amorphous carbonaceous materials cm characterized by strongly negative values of carbon stable isotopes 13c 39 1 relative to pdb the anomalies are restricted to clay alteration halos developed in neoarchean woodburn lake group metagreywacke that is the predominant host of unconformity related uranium u deposits in the kiggavik exploration camp petrographic and microstructural analyses by sem x ray diffraction hr tem and raman spectroscopy identified carbon veils best described as graphene like carbon upon which nano scale uraninite crystals are distributed cm are common in u systems such as the classic cretaceous roll front deposits and the world class paleoproterozoic unconformity related deposits however the unusual spatial and textural association of u minerals and cm described herein raises questions on mechanisms that may have been responsible for the precipitation of the cm followed by crystallization of u oxides on its surfaces based on the characteristics presented herein the cms at kiggavik are interpreted as hydrothermal in origin furthermore the nanoscale organization and properties of these graphene like layers that host u oxide crystallites clearly localized u oxide nucleation and growth

geochemistry of uraniferous bitumen in the southwest athabasca basin saskatchewan canada the sediment hosted unconformity type southwest athabasca uranium mineralization in northern saskatchewan canada contains uraniferous bitumen as observed elsewhere in the athabasca basin uraninite is present either as micron sized grains dispersed in bitumen uraniferous bitumen or as larger grains situated in the pore space between bitumen globules the spatial and textural relationships between uraninite and bitumen suggest that they were co precipitated la icp ms u pb dating of uraniferous bitumen yielded an age of 1575 53 ma similar to that of other unconformity type uranium mineralization in the athabasca basin indicating that the bitumen was formed early in the history of the basin nitrogen carbon and sulfur isotope analyses of uraniferous bitumen indicate that its source material was probably kerogen produced by cyanobacteria in a clastic environment

solid bitumens an assessment of their characteristics genesis and role in geological processes familiar since antiquity and subject in contemporary times to various characterization schemes the exact nature of solid bitumen is not yet fully known bitumens have random polymer like molecular structures are mobile as highly viscous fluids or were once fluids but have since turned into solids solid bitumens consist mainly of large moieties of polyclyclic aromatic hydrocarbons occasionally with finely admixed fine grained cryptocrystalline graphite solid bitumens are distinguished from kerogen which is the syngenetic and generally finely dispersed particulate organic matter in sedimentary rock that virtually does not migrate following its deposition occurrences of solid bitumens are relevant to petroleum exploration as well as the search for and evaluation of a variety of metallic mineral deposits genesis of bitumen is in many cases linked to the thermal and hydrothermal history of organic matter in sedimentary rock apparently bitumen or more specifically organic acids generated along with bitumen during diagenesis may alter porosity of reservoir rocks or otherwise prepare the ground for ore deposition bitumen is also relatively sensitive to alteration processes some of which such as oxidative weathering water leaching biodegradation contact metamorphism and ionizing radiation may likewise affect its nature elemental composition of bitumen commonly reflects the nature of mineral deposits is is possible that in petroleum exploration trace metal abundances of bitumen may eventually allow prediction of crude oil types and volumes anticipated from a given source rock beside transition elements notably ni and v highly anomalous concentrations of u pt and au occur in some solid bitumens during the generation of petroleum from kerogen the trend in 13c is toward lighter values the opposite seems to occur when liquid petroleum is subjected to thermal cracking and or related processes yielding solid bitumen enriched in 13c and isotopically light methane in fact except for deasphalting and possibly some irradiation processes the result of thermal cracking oxidation water leaching inspissation drying and bacterial degradation of crude oil is that lower molecular weight hydrocarbons are removed leaving bitumen residues enriched in aromatic hydrocarbons heteroatomic compounds nso and 13c such phenomena are relevant to bitumen paragenesis in petroleum reservoir rocks to certain phanerozoic occurrences of multiple generations of bitumens and to bitumens in mineral deposits

testing for fullerenes in geologic materials oklo carbonaceous substances karelian shungites sudbury black tuff fullerenes have been reported from diverse geologic environments since their discovery in shungite from karelian russia our investigation is prompted by the presence of onionskin like structures in some carbonaceous substances associated with the fossil nuclear fission reactors of oklo gabon the same series of extractions and the same instrumental techniques laser desorption ionization and high resolution mass spectroscopy electron impact mass spectroscopy were employed to test for fullerenes in samples from three different localities two sites containing putative fullerenes sudbury basin and russian karelia and one new location oklo gabon we confirm the presence of fullerenes c 6 0 and c 7 0 in the black tuff of the onaping formation impact breccia in the sudbury basin but we find no evidence of fullerenes in shungite samples from various locations in russian karelia analysis of carbonaceous substances associated with the natural nuclear fission reactors of oklo yields no definitive signals for fullerenes if fullerenes were produced during sustained nuclear fission at oklo then they are present below the detection limit 100 fmol or they have destabilized since formation contrary to some expectations geologic occurrences of fullerenes are not commonplace

basaltic sill sediment complexes in young spreading centers genesis and significance in contrast to mature mid oceanic ridges where magmatic activity is little affected by the slow accumulation of sediments in young spreading centers such as that of the guaymas basin in the gulf of california the basaltic magma of great magmatic pulses forms dikes and sills within the uppermost few hundred metres of soft sediments in general younger dikes and sills are injected next to or on top of the contact zone of the older ones in this manner a distinctive sill sediment complex is built up the sediments of which are rather compacted and partly metamorphosed despite the low burial depth the thickness of this transitional zone between the sheeted dike complex seismic layer 2 and younger sediment seismic layer 1 is controlled chiefly by the rates of sedimentation and spreading if the half spreading rate is approximately one order of magnitude greater than the sedimentation rate the sill sediment complex can reach a thickness of only a few hundred metres and the depth of the spreading trough remains approximately constant sedimentation rates approaching or surpassing the spreading rate cause filling up of the basins which probably hampers the injection of magma into sediments sill sediment complexes similar to those in the gulf of california are also expected to occur at the passive margins of older oceanic basins as well as orogenic belts

characterization of organic matter associated with uranium deposits in the francevillian formation of gabon lower proterozoic abstract elemental analysis organic petrography and high resolution transmission electron microscopy were used to study organic matter in lower proterozoic rocks of the francevillian series in africa results show a convincing relationship between solid bitumens derived from thermal alteration of crude oil and deposition of uraninite ores evidence is presented that suggests the presence of migration paths for crude oil in associated sandstones moreover the solid bitumens appear to have been further altered by radiation damage as a consequence of oxidation and uranium mineralization

source mechanisms of earthquakes near mid ocean ridges from body waveform inversion implications for the early evolution of oceanic lithosphere to investigate the early tectonic evolution of oceanic lithosphere we present a synthesis of the characteristics of near ridge earthquakes i e earthquakes which occur off the axis of major mid ocean spreading centers but in lithosphere less than 35 m y old near ridge seismicity for the past two decades has been rather evenly distributed along the major ridge systems with the exception of the central indian ocean and the eastern part of the cocos plate which have experienced a high rate of earthquake activity during that time for 32 near ridge earthquakes we determined the double couple orientation seismic moment centroid depth and source time function using a formal inversion technique based on matching synthetic and observed p and sh waveforms all types of faulting styles are observed seismogenic deformation in young oceanic lithosphere is concentrated in the first 15 m y and occurs almost entirely in the uppermost mantle from just below the moho to depths at which the temperature estimated from standard thermal models exceeds 800 c thrust faulting is not observed at depths greater than 10 km below the seafloor the deepest near ridge earthquakes are generally characterized by normal faulting focal mechanisms with the t axis oblique or perpendicular to the local spreading direction most of these earthquakes occur in the central indian ocean the transition from mechanisms characteristic of ridge axes to those characteristic of the near ridge environment occurs within the first few million years for lithosphere less than 35 m y old horizontal compressive stress associated with the cooling and subsidence of the lithosphere ridge push does not appear to be the dominant source of stress released in earthquakes this result may be used to place a lower bound of several hundred bars on the typical stress difference in young oceanic lithosphere this bound together with the observed depth of faulting implies that the strain rate associated with seismic deformation is locally at least 10 15 s 1 we propose that thermoelastic stresses associated with plate cooling play an important role in near ridge earthquakes this source of stress can account for the mechanisms and depths of many near ridge earthquakes and for the concentration of activity in very young lithosphere the comparative aseismicity of the oceanic crust may reflect the concentration of thermoelastic stress below the moho due to different material properties for the crust and upper mantle or the early release of thermoelastic stresses through hydrothermal circulation and cracking throughout the crust at or near the ridge axis the higher level of near ridge seismicity in the indian ocean may be the result of modification or enhancement of the more typical near ridge stress state by such local processes as the continental collision between india and asia or active secondary convection beneath the young lithosphere of the indian plate

tectonic rotations in extensional regimes and their paleomagnetic consequences for oceanic basalts paleomagnetic samples of many deep sea drilling project dsdp basalts have shallower inclinations than would be inferred from the paleolatitude at which the basalts were extruded at mid ocean ridges rather than arising from peculiarities of the geomagnetic field the anomalous magnetic directions may result from tectonic rotations along the listric normal faults which often characterize extensional tectonic regimes several lines of evidence support this conclusion observations of the basin and range province an extensional tectonic regime occurring in continental crust observations of the troodos complex cyprus an oceanic extensional tectonic regime now exposed on land and analysis of possible fault patterns associated with the magma chambers postulated to occur at some mid oceanic ridges the tectonic rotations resulting from movement along the listric normal faults occur about a horizontal axis generally perpendicular to the extension direction and typically amount to 30 to 50 although rotations as large as 70 to 90 are not uncommon the paleomagnetic consequences of these rotations can be calculated for dsdp basalts if the original strike and latitude of a ridge are known at four dsdp sites for which adequate data are available namely sites 319 332 410 and 417 rotations ranging from 50 to 70 are required to explain the paleomagnetic results if the original strike and location of the ridge are not known as is the case for most of the older parts of the pacific ocean the paleomagnetic data and the inferred rotation determine a relationship between possible ridge strikes and locations this information combined with a similar but independent relationship derived from the marine magnetic anomalies puts severe constraints on the original strike and location of the ridge

the occurence of unusual c27 and c29 sterane predominances in two types of oman crude oil abstract two types of oman crude oils reveal unusual sterane distributions type a which is the more common 74 examples is characterised by a predominance of c 29 iso and normal steranes and generally none or only very low relative concentrations of rearranged steranes the triterpanes are characterised by the predominance of the c 29 17 h 21 h norhopane over the c 30 17 h 21 h hopane and non predominant c 20 c 30 tricyclic terpanes the c 29 steranes of this type of crude were not derived from the c 29 sterols of land plant origin frequently proposed as the source of c 29 steranes since there is good geological evidence that these crudes were generated from a pre cambrian source rock a geological period when land plants did not exist the type b crude oil 11 known examples is characterised by a strong predominance of c 27 iso normal and rearranged steranes relatively lower concentrations of 17 h 21 h hopanes and relatively high concentrations of c 20 c 30 tricyclic terpanes the remarkably different biomarker characteristics of these crude oils imply that the organisms active in the depositional environment of the respective source rocks were significantly different

source characterization of two reykjanes ridge earthquakes surface waves and moment tensors p waveforms and nonorthogonal nodal planes well constrained fault plane solutions from p wave first motions for mid ocean ridge normal faulting earthquakes usually require nonorthogonal nodal planes local structural effects and or departures from a double couple source mechanism have been invoked to explain this phenomenon in order to obtain an independent determination of the source mechanisms for the april 24 1970 and april 3 1972 events on the southern reykjanes ridge we invert the rayleigh wave radiation pattern to obtain the source moment tensor the moment tensor formulation should be particularly well suited to this problem because it is not restricted a priori to a double couple source mechanism a potential drawback of the technique however is the requirement that phase velocities along the earthquake station paths be known very accurately in order to obtain the source phase from the observed phase and an objective of this study was to determine whether a regionalized phase velocity model compiled from published dispersion curves is adequate the results of the moment tensor inversion for both events indicate shallow normal faulting with the tension axis approximately horizontal and perpendicular to the local strike of the ridge apparent departures from a pure double couple source seem to result from errors in the data and the poor resolution of the mxz and myz components of the moment tensor for shallow sources after performing the inversion under a series of increasingly more stringent constraints we conclude that the data for both events are compatible with pure double couple sources with moments of 4 8 and 7 5 1024 dyn cm respectively we then show that interference between p pp and sp due to shallowness of the source can account for the observed nonorthogonality and match the observed p waveforms for the april 3 1972 event with theoretical seismograms calculated for a shear fault whose orientation is consistent with the surface wave solution the best fit to the data is obtained for a long narrow fault 13 km by 3 km with rupture initiating near the seafloor the moment indicated by the p waves is 7 5 1024 dyn cm these source parameters give an average displacement of about 60 cm and a stress drop of 30 60 bars taking into account various uncertainties although we might expect attentuation to be high in the mid ocean ridge environment the average attenuation required to fit the teleseismic data is not higher than normal t 1 s the p waves from the april 24 1970 earthquake were too small to be suitable for quantitative modeling by synthetic seismograms but are qualitatively consistent with a shallow fault model similar to that for the larger event we conclude that the faulting process described by these two earthquake mechanisms is directly related to the formation of rift valley topography

the neodymium isotope system as a tool for petroleum exploration abstract mineralogical petrological geochemical and neodymium isotopic investigations strongly suggest isotopic and chemical exchange between authigenic silicate phases and petroleum alteration of organic matter sulfur reduction generation of aqueous acidic solutions with the resultant dissolution of k feldspar leads to in situ crystallization of illite and chlorite as well as neodymium isotopic equilibration between the silicate phases and petroleum these findings are of significance not only to understand the mechanisms leading to neodymium isotopic equilibration between authigenic clay minerals in detrital sediments but also to source rock petroleum in reservoir correlations in particularly when biomarker molecules i e isoprenoid hydrocarbons have been obliterated by prolonged severe diagenesis

novel type of crust produced during continental rifting a normal oceanic crust cannot be generated after continental crust splitting of rifts filled by a thick pile of young sediments the magma released by the rising mantle is trapped mainly in these sediments as basalts sills and plugs the sediments are themselves metamorphosed by the heat flux from the underlying mantle a metasedimentary crust is formed in direct contact with an abnormal mantle vp 7 5 km s l produced by partial serpentinization