c1c8 hydrocarbons in sediments from guaymas basin gulf of california comparison to peru margin japan trench and california borderlands surface seafloor sediments hydrothermal vent samples and deep sea drilling project sediments hole 481a from the guaymas basin were examined for c1c8 hydrocarbons the proportions of various classes of compounds were examined and compared to those from other geographic areas peru upwelling region and japan trench to gain insight into the relative importance of thermal generation migration and biodegradation concentrations of c2c7 hydrocarbons were about 10 10 000 times higher in geothermally warm estimated to have been exposed to maximum temperatures in the range of 30 150 c guaymas basin sediments in comparison to the low concentrations 0 1 10 ppb per compound typical of geothermally cold maximum thermal exposure less than 20 c seafloor and dsdp diatomaceous sediments however one sediment sample from dsdp site 477 estimated to have been exposed to temperatures of 300 c or higher in the past showed only a limited hydrocarbon composition consisting of c1c3 alkanes and aromatic hydrocarbons only alkene alkane ratios of 0 1 or greater were typical of both geothermally cold sediments and also of very hydrocarbon richalvin samples recovered from the seafloor because little or no alkene was generally detected in buried sediments exposed to geothermal temperatures greater than 30 c it is suggested that the alkenes are produced by biogenic processes normal alkenes predominated over cyclic and branched structures in geothermally cooler 20 c sediments with the proportion of cyclic and branched compounds increasing in hotter sediments concentrations ofgem dimethyl and aromatic compounds generally remained approximately constant or increased slightly with temperature in comparison with geothermally cold shallow sediments similarities in compositions of branched and cyclic compounds were observed in some pairs of bitumen rich guaymas seafloor samples recovered from different areas suggesting common mechanisms of light hydrocarbon generation and or migration localized increases in ratios of specific cycloalkane ratios were observed adjacent to sill intrusions

geochemistry of interstitial gases in quaternary sediments of the gulf of california abstract interstitial gases from the deep sea drilling project dsdp ipod leg 64 sites 474 477 479 and 481 in the gulf of california have been analyzed by gas chromatography and stable isotope mass spectrometry in order to evaluate their compositions in terms of biogenic and thermogenic sources the hydrocarbon gas c 1 c 5 concentrations were comparable to the shipboard data and no olefins could be detected the 13 c data for the ch 4 confirmed the effects of the thermal stress on the sedimentary organic matter in that the values were typically biogenic near the surface and become more depleted in 12 c vs depth at sites 474 478 and 481 the ch 4 at site 477 was the heaviest in 13 c and at site 479 it did not show a predominant high termperature component the co 2 at depth from most sites was of a primarily thermogenic origin from carbonate decomposition

research aircraft observations of a polar low at the east greenland ice edge abstract the structure of a subsynoptic scale cyclone polar low that formed along the cast greenland ice edge during the 1989 coordinated eastern arctic research experiment cearex is described using noaa wp 3d research aircraft and satellite observations satellite imagery showed a well defined 400 km wide comma cloud pattern during the time of the aircraft observations frontal zones with marked wind shifts and thermal gradients near the surface were associated with the polar low although the polar low s vorticity decreased rapidly with height between 950 and 800 mb a secondary vorticity maximum was found in the upper troposphere associated with a short wave trough doppler radar and aircraft observations showed the structure of the main precipitation band to be similar to that of other polar lows observed by research aircraft in general the structure of the polar low resembled except for horizontal scale the structure of midlatitude cyclones at a similar stage of cloud field evolution

s wave velocity structure of the canadian upper mantle abstract we investigate mantle structure beneath the canadian landmass using 500 surface wave waveforms recorded at stations of the canadian national seismic network and global seismic network the data are processed using the method of partitioned waveform inversion and afford a comprehensive sampling of s velocity structure beneath the north american craton and adjacent orogenic belts our study thereby extends results from previous regional s wave studies north of the coterminous us and provides a characterization of mantle structure beneath canada which is complementary to the body wave model of grand j geophys res 99 1994 11591 the resulting 3d velocity model is well resolved to a depth of 400 km over most of the region and displays two prominent large scale anomalies a high velocity structure beneath the canadian shield and platform associated with the cratonic keel and pronounced low velocities beneath the cordillera the contrast between these features is strong up to 10 and at more southerly latitudes is sharp with the transition occurring over distances of 600 km or less laterally coherent high velocities persist to depths of ca 250 km beneath the north american craton which we interpret to represent the base of the continental keel smaller scale variations are observed within the cratonic mantle but in general show little correlation with surface geological provinces moderately low velocities beneath the st lawrence valley region and labrador may be related to intracontinental volcanism and rifting respectively

high precision u pb ages for granulite metamorphism and deformation in the archean kapuskasing structural zone ontario implications for structure and development of the lower crust abstract precise u pb isotopic ages for two subhorizontal granulite gneisses in the kapuskasing structural zone indicate that formation and emplacement occurred more than 50 m y after cessation of volcanism in the steeply folded nearby greenstone belts by successive stages of ductile underplating across a zone of crust mantle delamination volcanism and batholith formation had ceased by 2700 and 2680 ma respectively while granulite formation occurred between 2660 and 2640 ma and ductile deformation continued until 2585 ma emplacement of the oldest granulite caused the development of shear planes and melting in the older overlying tonalite while emplacement of the younger granulite caused rupturing and rehydration of the overlying older granulite widespread tectonic underplating is implied since all superior province granulites have similar young ages anomalous ar ages that post date accretion by about 150 m y and scrambled pb pb and rb sr whole rock systems also suggest that late low temperature alteration accompanied this event temporal relationships and the presence of post volcanic 2670 ma conglomerate cobbles at granulite grade support a model of ductile underplating of a post accretion thermally softened back arc fore arc complex with components welded onto an overlying volcanic plutonic complex at successively deeper levels multistage deformation in active fault zones and hydrothermal emplacement of major gold deposits in greenstone belts are coeval with ductile underplating with ages between 2670 and 2585 ma anomalously abundant gold production from archean rocks may indicate that shallow underplating granulitization and devolatization was a widespread phenomenon at this time post accretion ductile underplating must be considered as a primary cause of seismic reflectors in archean terrains

hydrothermal effects on organic matter high vs low temperature components abstract hydrothermal activity and sea bed mounds have been explored in guaymas basin central gulf of california by the deep sea drilling project piston coring dredging and diving with the d s r v alvin the deep seated magmatic heat flow causes extensive thermal alteration of both organic and mineral matter especially in the case of the south rift of guaymas basin this results in hydrothermal circulation with concomitant migration of organic pyrolysate to the seabed the bitumen at depth of dsdp sites 477 and 481 contained thermogenic components such as primary olefins thermodynamically stabilized molecular markers 17 h hopanes and elemental sulfur with traces of polynuclear aromatic hydrocarbons pah the major components were n alkanes with no carbon number predominance and unresolvable complex branched and cyclic hydrocarbons hump which are typical of mature petroleum and were generated by the heat from the intrusives and magma dredge samples recovered from a mound in guaymas basin contained petroleum of a thermogenic origin as confirmed by the presence of gasoline range hydrocarbons a broad distribution of n alkanes c13 c33 no carbon number predominance hump pristane and phytane olefins and pah and large amounts of aromatic naphthenic and asphaltic material similar petroleums subsequently recovered from other active mounds in the southern rift by the d s r v alvin had analogous compositions ranging from volatile to heavy oils in pyrrhotite chimneys the extractable organic matter consisted of mainly higher molecular weight pah these petroleums are derived from the immature primarily marine organic matter at depth by thermal alteration rapid quenching by hydrothermal removal and migration followed by condensation at the sea bed results in the manifestations of oil as part of the sulfide formations exterior exposed oil samples are degraded and leached whereas interior samples are essentially unaltered the pah are high temperature resynthesis aromatization products that condense in the hottest parts of the vent systems

sediment hosted geothermal systems review and first global mapping abstract sediment hosted geothermal systems shgss are hybrid geological systems where volcanic and sedimentary domains interact leading to mixtures of inorganic and organic gases typically characterized by geothermal thermometamorphic or mantle derived co2 and biotic microbial or thermogenic ch4 shgss occur in sedimentary basins crossed by magmatic intrusions or involved in volcanic plumbing systems these systems can be of considerable interest for petroleum exploration and natural greenhouse gas emission studies but systematic studies for their characterization and worldwide distribution are missing here we provide a review of shgss identified so far and propose methodological criteria for their definition and identification based on integrated geological and gas geochemical parameters we find that shgss are typically characterized by a fluids dominated by mantle or decarbonation methamorphic co2 50 vol b considerable amounts of ch4 and heavier hydrocarbons at least 1 5 vol generally up to 30 40 vol produced by microbial or thermogenic degradation of organic matter hosted in sedimentary rocks c tectonically active sedimentary basins back arc rift zones and foredeep generally hosting petroleum fields and within 300 km from recent or ancient volcanic centers this analysis resulted in a global map including a first set of 33 shgss located in north america central and eastern europe far east eastern oceania and northern new zealand and a second set of potential shgs prone areas occurring also in south america north africa middle east and kamchatka the present shgs map can evolve on the basis of more detailed geological analysis and new gas geochemical data

convective instability of thickening mantle lithosphere mantle lithosphere being colder and therefore denser than the underlying mantle is prone to convective instability that can be induced by horizontal shortening numerical experiments on a cold layer with imposed horizontal shortening are carried out to examine the relative importance of mechanical thickening thermal diffusion and gravitational instability in deforming the layer this analysis is then used to develop a method for determining which of these styles dominates for a layer thickening at a given rate if viscosity is non newtonian the imposition of shortening decreases the lithospheric strength which causes perturbations to the lithosphere s temperature structure to grow exponentially with time once these perturbations become sufficiently large they then grow super exponentially with time eventually removing the lithospheric base because lithospheric viscosity is highly temperature dependent at most only the lower 30 per cent of the lithosphere participates in the downwelling associated with this initial super exponential growth event after this event however a downwelling develops that removes material advected into the region of downwelling by horizontal shortening the magnitude of this persistent downwelling depends on the rate and duration of shortening if the total amount of shortening does not exceed 50 per cent doubling of crustal thickness then this downwelling extends to a depth three to four times the thickness of undeformed lithosphere and forms a sheet significantly thinner than the width of the region undergoing shortening once shortening stops this downwelling is no longer replenished by the shortening process and should then detach due to its inherent gravitational instability the hottest 60 per cent of the mantle lithosphere could be removed in such an event which would be followed by an influx of hot buoyant asthenosphere that causes rapid surface uplift because more cold material is removed after the cessation of shortening than by the initial gravitational instability the former has a potentially greater influence on surface uplift the tibetan interior is thought to have been shortened by about 50 per cent in 30myr and afterwards at 8ma experienced a period of rapid uplift that may have resulted from the removal of a large downwelling finger of cold lithosphere generated by shortening

survey of hydrothermally generated petroleums from the guaymas basin spreading center abstract the guaymas basin spreading center and hydrothermal system are generating an assortment of petroleum like bitumens by pyrolysis of organic matter in the overlying uncosolidated sediments samples collected by d s v alvin from hydrothermal fields in the southern trough exhibit large variations in the quantities and character of the solvent soluble organic materials various pyrolytic regimes combined with fluctuating thermal gradients and migration velocities may impart a compositional fractionation possibly by differential gaseous solubilization and hydrodynamic alteration of the multi component fluids the data suggest that differential condensation solidification biodegradation and water washing cause selective removal of components at the seabed many hydrothermal oils are unlike normal reservoir petroleums because they contain significant concentrations of polar material due to their rapid genesis and transport and probable solubilization or advection of polar immature components the biomarker distributions confirm high temperature pyrolysis at depth with variable entrainment of less thermally mature bitumens during transport data for shallow core samples from the northern trough indicate that those locations experienced variable influx predominantly volatiles of a low temperature pyrolysate

a polar low pair over the norwegian sea abstract during the lofotes cyclone experiment lofzy 2005 two polar lows developed one behind the other inside a cold air outbreak from the north in the lee of spitsbergen on 7 march 2005 buoys ship and aircraft measurements as well as satellite imagery are applied to analyze the polar low bulk properties the horizontal and vertical structure and the mass moisture and heat budget the lifetime of the system until landfall at northern norway was 12 h the generation occurred under the left exit region of an upper level jet with 70 m s 1 both polar lows had a radius of 100 130 km and extended to a height of about 2 5 km the propagation speeds were within 14 17 m s 1 and correspond to the vertically averaged wind velocity of the lowest 2 5 km in the polar low centers the pressure was about 2 3 hpa lower and the air was 1 2 k warmer and drier than in the surroundings aircraft measurements in the second of the two polar lows show an embedded frontlike precipitation band north of the center here

proterozoic flood basalts from the coppermine river area northwest territories isotope and trace element geochemistry the voluminous proterozoic continental tholeiites of the coppermine river province northwest territories which are coeval with the mackenzie magmatic event 1 27 ga old and were emplaced over a short period of time 5 ma have trace element and sr nd pb isotopic characteristics comparable to those of phanerozoic flood basalts the variations of the compositional parameters are inferred to be due to a mixing of at least two components mantle and crust in addition the mantle component probably includes two end members the first is a mantle plume whereas the other represents the base of the continental lithosphere the crustal component reflects contamination of the magma by precambrian basement during its ascent to the surface

imaging the continental upper mantle using electromagnetic methods abstract the internal structure of the continental lithosphere holds the key to its creation and development and this internal structure can be determined using appropriate seismic and electromagnetic methods these two are complementary in that the seismic parameters usually represent bulk properties of the rock whereas electrical conductivity is primarily a function of the connectivity of a minor constituent of the rock matrix such as the presence of a conducting mineral phase e g carbon in graphite form or of a fluid phase e g partial melt or volatiles in particular conductivity is especially sensitive to the top of the asthenosphere generally considered to be a region of interconnected partial melt knowledge of the geometry of the lithosphere asthenosphere boundary is important as this boundary partially controls the geodynamic processes that create modify and destroy the lithosphere accordingly collocated seismic and electromagnetic experiments result in superior knowledge than would be obtained from using each on its own this paper describes the state of knowledge of the continental upper mantle obtained primarily from the natural source magnetotelluric technique and outlines how hypotheses and models regarding the development of cratonic lithosphere can be tested using deep probing electromagnetic surveying the resolution properties of the method show the difficulties that can be encountered if there is conducting material in the crust examples of data and interpretations from various regions around the globe are discussed to demonstrate the correlation of electromagnetic and seismic observations of the lithosphere asthenosphere boundary also the observations from laboratory measurements on candidate mineralogies representative of the mantle such as olivine are presented

stratigraphy of the archean western superior province from p and s wave receiver functions further evidence for tectonic accretion the archean western superior province in canada represents the nucleus of the north american continent whose origin has been speculated to be the result of widespread crustal accretion some 2 7 ga ago in this paper crustal and upper mantle seismic discontinuities beneath the western superior province of the canadian shield are imaged with teleseismic p to s and s to p converted phases using the receiver function method three crustal discontinuities are observed the moho ranging in depth between 38 and 47 km and dipping to the south and two intra crustal discontinuities having depths of approximately 15 and 30 km the crustal discontinuities undulate laterally and often lose continuity possibly indicating an imbricated structure and or regions of velocity gradients in the shallow lithosphere a positive discontinuity is imaged at approximately 65 km depth and is consistent with earlier refraction and wide angle reflection results additionally two zones of negative receiver function amplitudes at 55 km depth are observed and are coincident with a region of anomalous tomographic low p and s wave velocities as well as a zone of high electrical conductivity the images for the crust and shallow upper mantle when integrated with previous geophysical studies are consistent with ideas of continental root formation due to imbrication of archean subducted material and accretion of island arcs observed in surface geology

deep water hydrocarbon seeps in guaymas basin gulf of california abstract acoustically discovered hydrocarbon seeps along a transform fault zone on the sonoran margin of guaymas basin and in the southern trough of guaymas basin were examined and sampled during dives of dsv alvin seepage of methane and heavier hydrocarbons occurs through shallow pockmarks along the eroding crest of a steep anticline 1600 m below sea level extensive ledges of aragonite crop out around the rims of the pockmarks isotopic analysis indicates that carbonate precipitation is a result of methane oxidation seepage zones within the pockmarks support dense communities of calyptogena half buried in the mud tubeworms lamellibracheae sp have colonized many of the aragonite outcrops though the margin site is only a few kilometers from a high temperature sulfide precipitating and petroleum discharging vent system at a nearby center its structural setting is more akin to pockmarks described from continental shelves and its chemosynthetic fauna is more like that around low temperature seeps on other continental slopes and margins e g oregon u s a and japan the seep in the southern trough is a condensate type petroleum c 1 c 40 and rises as a plume from a young vent mound system about 2000 m below sea level

deep rifts as sources for alkaline intraplate magmatism in eastern north america intracontinental magmatism in eastern north america does not exhibit the regular time progression associated with mantle plume activity instead the magmatism is sporadic but locally persistent suggesting that the magmas rise from a mantle source that drifts with the continental plate a spatial association with ancient rifts further suggests that this source is fertile9 melt generating material emplaced in the mantle lithosphere during the early stages of rifting

lithospheric assembly and modification of the se canadian shield abitibi grenville teleseismic experiment this paper presents the results of a joint lithoprobe incorporated research institutions for seismology iris program for array seismic studies of the continental lithosphere passcal teleseismic experiment that investigates portions of the grenville and superior provinces of the canadian shield along the quebec ontario border data from a 600 km long n s array of 28 broadband seismographs deployed between may and october 1996 have been supplemented with additional recordings from an earlier 1994 deployment and from stations of the canadian national seismograph network and the southern ontario seismic network relative delay times of p and s waves from 123 and 40 teleseismic events respectively have been inverted for velocity perturbations in the upper mantle and reveal a low velocity nw se striking corridor that crosses the southern portion of the line at latitude 46 n and lies between 50 and 300 km depth multievent s k s splitting results yield an average delay time of 0 57 0 22 s and a direction of fast polarization of n93 e 18 which is consistent with an earlier interpretation as being due to fossil strain fields related to the last major regional tectonic event subtle variations in splitting parameters over the low velocity corridor may suggest an associated disruption in mantle fabric profiling of radial receiver functions reveals large and abrupt variations in moho topography specifically a gradual thickening in crust from 40 to 45 km between latitudes 45 n and 46 n which is followed by an abrupt thinning to 35 km at 46 6 n some 65 km southeast of the grenville front this structure is interpreted as a subduction suture extending the full length of the front and punctuating a major pre grenvillian archean proterozoic episode of lithospheric assembly in the southeast canadian shield the low velocity mantle corridor by contrast is better explained as the extension of the monteregian white mountain new england seamount hotspot track below the craton and is here postulated to represent interaction of the great meteor plume with zones of weakness within the craton developed during earlier rifting episodes

sources and hydrothermal alteration of organic matter in quaternary sediments a synthesis of studies from the central gulf of california abstract deep sea drilling in the central gulf of california a young and active spreading centre shows that the high heat flow typical of these regions causes extensive alteration of sediment organic matter especially near sills and above magma chambers where hydrothermal activity is concentrated even on the nearby passive margin where there are no sills heat flow is moderately high and hydrocarbon generation has begun in immature sequences migrating light hydrocarbons are detected especially where hydrothermal activity is concentrated thermogenic methane is more widespread though not in the passive margin bordering the spreading centre despite the incidence of hydrocarbon generation and migration the amounts of hydrocarbons involved are relatively small and apparently do not lead to commercially significant accumulations of petroleum the organic matter in these sediments is mostly marine because the gulf of california generally has low runoff from land and highly productive surface waters turbidites rich in terrigenous organic material are locally abundant in the mainly pelagic section in the guaymas basin the highest concentrations of organic matter are found in laminated diatomites deposited on the guaymas passive margin within the oxygen minimum zone

geological constraints on the origin of the mantle root beneath the canadian shield cratonic north america is composed of a cluster of archaean microcontinents centred on the canadian shield and juvenile proterozoic crust that lies mainly buried beneath the sedimentary cover of the western and southern interior platforms the shield is underlain by an anomalous low temperature mantle root that is absent beneath the platform as there appears to be no systematic difference in crustal thickness or density between the shield and the platform the long lived arching of the shield implies an intrinsic buoyancy imparted by the mantle root that more than offsets its colder temperature isotopic and seismic anisotropy data indicate an archaean age for the mantle root close to the time of formation of the overlying crust the preferential development of the mantle root beneath archaean crust is consistent with an origin by imbrication of partly subducted slabs of highly depleted oceanic lithosphere assuming that buoyant subduction was more common in the archaean formation of the mantle root was not dependent on collisional orogenesis as has been suggested but the archaean cratonic mantle was sufficiently buoyant and refractory to survive later tectonic thickening the mantle root persists beneath archaean crust that was transected by mafic dyke swarms and subjected to short lived episodes of post orogenic crustal melting but the root is reduced at mantle plume initiation sites the partitioning of archaean and proterozic crust between the shield and the platform respectively causes the shield to misrepresent precambrian crust as a whole studies of the shield falsely conclude that a high percentage of precambrian crust formed in the archaean and that the proterozoic was characterized by epicontinental volcanism and sedimentation and crustal reworking furthermore the isotopic ratios of detritus eroded from the craton may tend to overestimate the mean age of continental crust

teleseismic studies of the canadian landmass lithoprobe and its legacy national research council of canada nrc research press http www nrc cnrc gc ca notices e html copyright permission to reproduce materials on this web site were produced and or compiled by the national research council for the purpose of providing canadians with direct access to information about the programs and services offered by the government of canada the material on this site is covered by the provisions of the copyright act by canadian laws policies regulations and international agreements such provisions serve to identify the information source and in specific instances to prohibit reproduction of materials without written permission

tip jets and barrier winds a quikscat climatology of high wind speed events around greenland the high topography of greenland results in a number of orographically induced high wind speed flows along its coast that are of interest from both a severe weather and climate perspective here the surface wind field dataset from the nasa jpl seawinds scatterometer on board the quick scatterometer quikscat satellite is used to develop a wintertime climatology of these flows the high spatial resolution and the twice daily sampling of the seawinds instrument allows for a much more detailed view of the surface winds around greenland than has been previously possible three phenomena stand out as the most distinctive features of the surface wind field during the winter months the previously identified tip jets and reverse tip jets as well as the hitherto unrecognized barrier flows along its southeast coast in the vicinity of the denmark strait peak surface wind speeds associated with these phenomena can be as large as 50 m s 1 with winds over 25 m s 1 occurring approximately 10 15 of the time at each location a compositing technique is used to show that each type of flow is the result of an interaction between a synoptic scale parent cyclone and the high topography of greenland in keeping with previous work it is argued that tip jets are caused by a combination of conservation of the bernoulli function during orographic descent and acceleration due to flow splitting as stable air passes around cape farewell while barrier winds are a geostrophic response to stable air being forced against high topography it is proposed that reverse tip jets occur when barrier winds reach the end of the topographic barrier and move from a geostrophic to a gradient wind balance becoming supergeostrophic as a result of their anticyclonic curvature

late cenozoic deformation in the south caspian region effects of a rigid basement block within a collision zone abstract active deformation in the south caspian region demonstrates the enormous variation in kinematics and structural style generated where a rigid basement block lies within a collision zone rigid basement to the south caspian basin moves with a westward component relative both to stable eurasia and iran and is beginning to subduct at its northern and western margins this motion is oblique to the approximately north south arabia eurasia convergence and causes oblique shortening to the south and northeast of the south caspian basin thrusting in the alborz and kopet dagh is accompanied by range parallel strike slip faults which are respectively left and right lateral there are also arcuate fold and thrust belts in the region for two principal reasons firstly weaker regions deform and wrap around the rigid block this occurs at the curved transition zone between the alborz and talysh ranges where thrust traces are concave towards the foreland secondly a curved fold and thrust belt can link a deformation zone created by movement of the basement block to one created by the regional convergence west to east thrusts in the eastern talysh represent underthrusting of the south caspian basement but pass via an arcuate fan of fold trains into ssw directed thrusts in the eastern greater caucasus which accommodates part of the arabia eurasia convergence each part of the south caspian region contains one or more detachment levels which vary dependent on the pre pliocene geology buckle folds in the south caspian basin are detached from older rocks on thick mid tertiary mudrocks whereas thrust sheets in the eastern greater caucasus detach on mesozoic horizons in the future the south caspian basement may be largely eliminated by subduction leading to a situation similar to archaean greenstone belts of interthrust mafic and sedimentary slices surrounded by the roots of mountain ranges constructed from continental crust

structure of a shear line polar low during march 2013 a series of polar lows originated in a high vorticity 10 3 s 1 shear zone that was associated with a prolonged marine cold air outbreak over the norwegian sea a detailed analysis of one shear line polar low at the leading edge of the outbreak is presented using comprehensive observations from a well instrumented aircraft dropsondes scatterometer and cloudsat data and numerical modelling output from a convection resolving configuration of the met office unified model the maximum low level wind gradient across the shear line was 25 m s 1 over 50 km high windsto the north and west were within the cold air mass and were associated with large surface turbulent heat fluxes and convective clouds low wind speed to the south and east of the shear line were associated with low heat fluxes and a clear eye in the polarlow shear line meso scale instabilities merging into the polar low appeared important to its structure and development the model captured the shear line and the polar low structure very well in particular the strength of the horizontal shear and the mesoscale thermodynamic fields the spatialstructure of convective cloud bands around the polar low wassimulated reasonably well but the modelsignificantly underestimated the liquid water content and height of the cloud layers compared to the observations shear line polar lows are relatively common however this case is arguably the first to be examined with a wide range of in situ and remote observations allied with numerical model output

comparing crustal and mantle fabric from the north american craton using magnetics and seismic anisotropy a central target in earth sciences is the study of deformation at various depth levels within the earth seismology has offered a remarkable tool for doing this via seismic anisotropy it is however not always clear how to interpret those observations a question of interest is to understand the relation between the deformation of the mantle and the crust and in studying the relation between the two mantle deformation is expressed in seismic anisotropy in this paper we seek an objective way of extracting information about crustal fabric as well to be able to compare with seismic anisotropy the magnetization of crustal rocks offers an attractive possibility for doing this we thus explore the use of magnetic data and we compare magnetic crustal fabric orientation with mantle fabric observations from seismic anisotropy we apply our technique to the north american craton for which we have an excellent magnetic dataset and we show that there is a clear relation between crustal and mantle fabric for the cratonic region this has important implications for crustal formation and for interpreting seismic anisotropy observations

increased subaerial volcanism and the rise of atmospheric oxygen 2 5 billion years ago the hypothesis that the establishment of a permanently oxygenated atmosphere at the archaean proterozoic transition approximately 2 5 billion years ago occurred when oxygen producing cyanobacteria evolved is contradicted by biomarker evidence for their presence in rocks 200 million years older to sustain vanishingly low oxygen levels despite near modern rates of oxygen production from approximately 2 7 2 5 billion years ago thus requires that oxygen sinks must have been much larger than they are now here we propose that the rise of atmospheric oxygen occurred because the predominant sink for oxygen in the archaean era enhanced submarine volcanism was abruptly and permanently diminished during the archaean proterozoic transition observations are consistent with the corollary that subaerial volcanism only became widespread after a major tectonic episode of continental stabilization at the beginning of the proterozoic submarine volcanoes are more reducing than subaerial volcanoes so a shift from predominantly submarine to a mix of subaerial and submarine volcanism more similar to that observed today would have reduced the overall sink for oxygen and led to the rise of atmospheric oxygen

archaean cratonic roots mantle shear zones and deep electrical anisotropy the extent to which the mantle participated in the growth and stabilization of ancient cratons is central to our understanding of the evolution of the continents1 the detection of seismic anisotropy beneath precambrian north america for example has been interpreted as showing that strain induced orientation of mantle minerals in subcontinental lithospheric mantle can preserve a record of ancient episodes of deformation2 here we present mag netotelluric measurements from the superior province of the canadian shield which reveal pronounced electrical anisotropy in the upper 100 km of the underlying mantle we argue that this anisotropy is best explained by conducting graphite films oriented within fractures or on grain boundaries and associated with metasomatism of the mantle roots of major archaean shear zones which transect the entire superior province the uppermost mantle beneath the canadian shield has therefore remained fixed to the crust and isolated from significant tectonic reworking since the late archaean

variations in the chemical and stable isotope composition of carbon and sulfur species during organic rich sediment alteration an experimental and theoretical study of hydrothermal activity at guaymas basin gulf of california organic rich diatomaceous ooze was reacted with seawater and a na ca k cl fluid of seawater chlorinity at 325 400 c 400 500 bars and fluid sediment mass ratios of 1 56 2 35 to constrain factors regulating the abundance and stable isotope composition of c and s species during hydrothermal alteration of sediment from guaymas basin gulf of california alteration of inorganic and organic sedimentary components resulted in extensive exchange reactions the release of abundant h2s co2 ch4 and corganic to solution and recrystallization of the sediment to an assemblage containing albitic plagioclase quartz pyrrhotite and calcite the 34scdt values of dissolved h2s varied from 10 9 to 4 3 during seawater sediment interaction at 325 and 400 c and from 16 5 to 9 0 during na ca k cl fluid sediment interaction at 325 and 375 c in the absence of seawater so4 h2s is derived from both the transformation of pyrite to pyrrhotite and s released during the degradation of organic matter in the presence of seawater so4 reduction of so4 contributes directly to h2s production sedimentary organic matter acts as the reducing agent during pyrite and so4 reduction requisite acidity for the reduction of so4 is provided by mg fixation during early stage sediment alteration and by albite and calcite formation in mg free solutions organically derived ch4 was characterized by 13cpdb values ranging between 20 8 and 23 1 whereas 13cpdb values for dissolved corganic ranged between 14 8 and 17 7 mass balance calculations indicate that 13c values for organically derived co2 were 14 8 residual solid sedimentary organic c showed small 0 7 depletions in 13c relative to the starting sediment the experimental results are consistent with the isotopic and chemical composition of natural hydrothermal fluids and minerals at guaymas basin and permit us to better constrain sources and sinks for c and s species in subseafloor hydrothermal systems at sediment covered spreading centers our data show that the sulfur isotope composition of hydrothermal sulfide minerals in guaymas basin can be explained by derivation of s from diagenetic sulfide and seawater sulfate basaltic s may also contribute to hydrothermal sulfide precipitates but is not required to explain their isotopic composition estimates of seawater sediment mass ratios based on sulfur isotopic composition of sulfide minerals and the abundance of dissolved nh3 in vent fluids range from 3 29 during hydrothermal circulation sources of c in guaymas basin hydrothermal fluids include thermal degradation of organic matter bacteriogenic methane production and dissolution of diagenetic carbonate

processes controlling water and hydrocarbon composition in seeps from the salton sea geothermal system california usa water mud gas and petroleum bearing seeps are part of the salton sea geothermal system ssgs in southern california seeps in the davis schrimpf seep fi eld 14 000 m 2 show considerable variations in water temperature ph density and solute content water rich springs have low densities 98 vol halogen geochemistry of the waters indicates that mixing of deep and shallow waters occurs and that near surface dissolution of halite may overprint the original fl uid compositions carbon isotopic analyses suggest that hydrocarbon seep gases have a thermogenic origin this hypothesis is supported by the presence of petroleum in a water dominated spring composed of 53 saturated compounds 35 aromatics and 12 polar compounds the abundance of polyaromatic hydrocarbons and immature biomarkers suggests a hydrothermal formation of the petroleum making the ssgs a relevant analogue to less accessible hydrothermal seep systems e g the guaymas basin in the gulf of california

seismic anisotropy beneath stable continental interiors a robust knowledge of seismic anisotropy beneath the continents is essential to our understanding of plate tectonic theory as anisotropy provides a unique constraint on the character of past and present deformation in the lithosphere and sublithospheric mantle this review paper summarizes the range of techniques currently available to image seismic anisotropy with passive source seismic data and addresses current issues surrounding the observation and interpretation of continental seismic anisotropy to this end we present case studies for four regions where seismic anisotropy has been extensively investigated in recent years eastern north america the canadian shield australia and southern africa based on this full suite of results we infer that stable continental regions generally contain seismic anisotropy that is clearly located within both the lithosphere and the sublithospheric mantle usually to depths of at least 200 km and perhaps more an implication of these results is that tectonic plates are at most only partially coupled to the underlying mantle the results from these case studies also demonstrate that while remarkable progress in seismic anisotropy imaging has been achieved in recent years it is clear that much more work will be required to adequately understand the origin of continental seismic anisotropy we suggest that a more robust characterization of anisotropic parameters can only be achieved by integrating complementary seismic datasets and by incorporating constraints from key related datasets from mineral physics magnetotellurics gravity and geodesy

convective instability of a boundary layer with temperature and strain rate dependent viscosity in terms of available buoyancy summary cold mantle lithosphere is gravitationally unstable with respect to the hotter buoyant asthenosphere beneath it leading to the possibility that the lower part of the mantle lithosphere could sink into the mantle in convective downwelling such instabilities are driven by the negative thermal buoyancy of the cold lithosphere and retarded largely by viscous stress in the lithosphere because of the temperature dependence of viscosity the coldest and therefore densest parts of the lithosphere are unavailable for driving the instability because of their strength by comparing theory and the results of a finite element representation of a cooling lithosphere we show that for a newtonian fluid the rate of exponential growth of an instability should be approximately proportional to the integral over the depth of the lithosphere of the ratio of thermal buoyancy to viscosity both of which are functions of temperature and thus depth we term this quantity available buoyancy because it quantifies the buoyancy of material sufficiently weak to flow and therefore available for driving convective downwelling for non newtonian viscosity with power law exponent n and temperature dependent pre exponential factor b the instabilities grow superexponentially as described by houseman molnar 1997 and the appropriate timescale is given by the integral of the nth power of the ratio of the thermal buoyancy to b the scaling by the available buoyancy thus offers a method of determining the timescale for the growth of perturbations to an arbitrary temperature profile and a given dependence of viscosity on both temperature and strain rate this timescale can be compared to the one relevant for the smoothing of temperature perturbations by the diffusion of heat allowing us to define a parameter similar to a rayleigh number that describes a given temperature profile s tendency toward convective instability like the rayleigh number this parameter depends on the cube of the thickness of a potentially unstable layer therefore mechanical thickening of a layer should substantially increase its degree of convective instability and could cause stable lithosphere to become convectively unstable on short cales we estimate that convective erosion will in 10 myr reduce a layer thickened by a factor of two to a thickness only 20 to 50 per cent greater than its pre thickened value thickening followed by convective instability may lead to a net thinningof a layer if thickening also enhances the amplitude of perturbations to the layer s lateral temperature structure for the mantle lithosphere the resulting influx of hot asthenosphere could result in rapid surface uplift and volcanism

a major change in the thermal state of the earth at the archean proterozoic boundary consequences for the nature and preservation of continental lithosphere

lithospheric structure beneath the archaean slave province and proterozoic wopmay orogen northwestern canada from a lithoprobe refraction wide angle reflection survey the slave geological province a relatively small craton in nw canada that includes the oldest rocks in the world is one of five cratons that form the archaean continental core of north america the palaeoproterozoic wopmay orogen lying between the craton and the phanerozoic northern canadian cordillera represents crustal growth through addition of a series of north south oriented magmatic arcs and accreted terranes as part of lithoprobe s multidisciplinary studies line 11 of the snore 97 seismic refraction wide angle reflection survey investigated the structure of the lithosphere in this region interpretation of crustal and mantle arrivals from 13 record sections followed an iterative procedure of traveltime inversion and forward modelling results show that crustal thickness at 32 35 km remains relatively constant from the archaean regions in the east to proterozoic domains in the west contrary to average values of crustal thicknesses from global compilations that suggest thicker proterozoic crust at the westernmost end of the line where crustal extension and basin formation are inferred the crust thins to 30 km upper crustal velocities to about 12 km depth lie in the range of 5 8 6 2 km s 1 the middle crust of thickness 8 10 km has velocities between 6 2 and 6 6 km s 1 average velocities for the lower crust differentiated as a 10 km thick layer range between 6 6 and 6 9 km s 1 with a higher velocity region 7 1 km s 1 in the transition zone between the fort simpson magmatic arc and hottah accreted terrane in the wopmay orogen where a palaeoproterozoic subduction zone has been inferred from coincident multichannel reflection data in the same area anomalously low upper mantle velocities 7 5 km s 1 have been found indicating that the effects of collision subduction and accompanying physical changes in the rocks comprising the mantle lithospheric wedge above the underthrusting structure may have been retained for 1800 ma data from most record sections with offset distances greater than 250 km show continuous wide angle reflections from upper mantle depths of about 75 km and almost 100 km although poorly constrained velocities in this depth range vary from 8 4 to 8 8 km s 1 interpretation of these phases and mantle reflections from equivalent depths in coincident near vertical incidence multichannel data relates them to subduction and underthrusting of oceanic crust and subsequent phase changes that occurred during the assembly of the craton and the evolution of the palaeoproterozoic orogen

precambrian plate tectonics seismic evidence from northern hudson bay canada the canadian shield is one of the largest exposures of precambrian rocks on earth it is a mosaic of several archean terranes that were brought together during a series of paleoproterozoic orogens culminating in the so called trans hudson orogen which is thought to have been similar to the himalayan orogen in scale and nature the tectonic evolution and lithospheric subdivisions of this region are poorly understood but new seismic networks in northern hudson bay provide fresh opportunity to place constraints on the precambrian processes that formed and shaped it we show via a study of seismic anisotropy that the lithosphere of the northern hudson bay region retains a strong signature of archean paleoproterozoic tectonics we map the boundary between the upper churchill and lower superior plates that collided ca 1 8 ga and identify back azimuth dependent splitting parameters t on baffin island that indicate complex anisotropy e g dipping fabric beneath the region our results support the view that significant lithospheric deformation occurred during the paleoproterozoic and that modern day plate tectonic processes were thus in operation by at least ca 1 8 ga

upper mantle stratigraphy beneath the southern slave craton a large teleseismic data set comprising 724 broadband three component p wave seismograms has been compiled for the southern slave craton with the objective of characterizing underlying mantle stratigraphy coherent p to s wave conversions are identified by simultaneously deconvolving seismograms as functions of epicentral distance and along theoretical moveout curves corresponding to possible mantle phases clear pds conversions are observed from the 410 and 660 km discontinuities at times that are only slightly faster than those predicted from the iasp91 model and over 1 0 s slower than corresponding times observed at other stations on the canadian shield and the south african kaapvaal craton the pds times show very little azimuthal variation implying an absence of major lateral velocity variations in the lithospheric mantle underlying the slave craton and adjacent wopmay orogen and taltson magmatic zone considered in light of other geophysical and geological evidence these results suggest that th

lithospheric growth at margins of cratons abstract deep seismic reflection profiles collected across proterozoic archean margins are now sufficiently numerous to formulate a consistent hypothesis of how continental nuclei grow laterally to form cratonic shields this picture is made possible both because the length of these regional profiles spans all the tectonic elements of an orogen on a particular cratonic margin and because of their great depth range key transects studied include the lithoprobe snorcle 1 transect and the babel survey crossing the slave and baltic craton margins respectively in most cases the older archean block appears to form a wedge of uppermost mantle rock embedded into the more juvenile proterozoic block by as much as 100 200 km at uppermost mantle depths and archean lithosphere is therefore more laterally extensive at depth than at the surface particularly bright reflections along the moho are cited as evidence of shear strain within a weak low viscosity lower crustal channel that lies along the irregular top of the indenting wedge the bottom of the wedge is an underthrust subduction zone and associated late reversal in subduction polarity beneath the craton margin emerges as a common characteristic of these margins although related arc magmatism may be minor

deep mantle contributions to the surface dynamics of the north american continent abstract the regional and continental scale dynamics of north america and vicinity are explored using a high resolution model of mantle flow the model is constrained by simultaneously inverting global seismic and mantle convection data sets and it includes an explicit treatment of the positive chemical buoyancy of the continental tectosphere moreover it adopts a depth dependent mantle viscosity structure which reconciles both glacial isostatic adjustment gia and convection data the flow model successfully reproduces plate velocities and observations of surface gravity and topography including the continent scale quasi linear depression after corrections for gia and crustal heterogeneity extending from northern alaska to venezuela the predictions also match lithospheric flow and stress fields inferred from local and regional measurements of seismic anisotropy and surface deformation we demonstrate that these signals are largely driven by viscous flow coupled to density anomalies in the deep portions of the upper mantle and within the lower mantle where the latter may be associated with the descent of the ancient kula farallon plate system and an active mantle upwelling below the pacific margin of the north american plate more importantly the flow calculations elucidate how these large scale heterogeneities give rise to regional scale flow and stress patterns below the southwestern u s and below the central u s

upper mantle velocity structure of the lower great lakes region abstract the lithospheric root beneath north america contains a prominent indentation beneath the lower great lakes region that is approximately aligned with the track of the new england seamounts by combining data from the recently installed polaris network in southern ontario canada with data acquired in 1996 during the abitibi grenville teleseismic experiment we have performed a tomographic inversion using 4543 p wave traveltimes from 213 events 5 0 m b 6 6 and 1860 s wave traveltimes from 98 events 5 0 m b 6 6 to obtain high resolution images of the upper mantle beneath the lower great lakes two salient features of the 3 d models are 1 a patchy nnw trending low velocity region and 2 a linear ne striking high velocity anomaly s wave images show that the low velocity anomaly changes from an arcuate feature at 400 km depth to a nw striking linear feature at 100 km depth beneath the neoproterozoic ottawa bonnechere graben the linear high velocity anomaly extends to at least 300 km depth and strikes parallel to surface geological belts and the laurentian continental margin we interpret the high velocity anomaly as a possible relict slab associated with ca 1 35 1 3 ga subduction beneath the composite arc belt whereas the low velocity anomaly is interpreted as a zone of alteration and metasomatism associated with the ascent of magmas that produced the late cretaceous monteregian plutons our data support an interpretation of these plutons as melts generated by the passage of north america across a mantle plume rather than a far field response to opening of the north atlantic

thermogenic hydrocarbons in surface sediments of the bransfield strait antarctic peninsula antarctica is the only continent from which petroleum deposits have not been encountered despite the occurrence of potential hydrocarbon basins onshore and around its margins we have discovered thermogenic hydrocarbons in unconsolidated recent sediments from the king george basin bransfield strait west antarctica these sediments possessed a marked petroliferous smell throughout the length of the 8 6 m gravity core the basin sediments are glacial marine deposits dominated by turbidites and contain abundant autochthonous organic matter as a result of high seasonal primary productivity in this most fertile part of the circumpolar ocean1 the maturation of this material into hydrocarbons may have been accelerated by the high geothermal gradient in the basin associated with back arc spreading created by the subduction of the drake plate into the south shetland trench2 sediments in this rifted basin are frequently intruded by volcanic sills and dykes3 giving rise to acoustic features shown in fig 1 the presence of these thermogenic hydrocarbons provides the first demonstration of active source rocks in antarctica

generation accumulation and resource potential of biogenic gas biogenic gas is generated at low temperatures by decomposition of organic matter by anaerobic microorganisms more than 20 of the world s discovered gas reserves are of biogenic origin a higher percentage of gases of predominantly biogenic origin will be discovered in the future biogenic gas is an important target for exploration because it occurs in geologically predictable circumstances and in areally widespread large quantities at shallow depths in rapidly accumulating marine sediments a succession of microbial ecosystems leads to the generation of biogenic gas after oxygen is consumed by aerobic respiration sulfate reduction becomes the dominant form of respiration methane generation and accumulation become dominant only after sulfate in sediment pore water is depleted the most important mechanism of methane generation in marine sediments is the reduction of co2 by hydrogen electrons produced by the anaerobic oxidation of organic matter co2 is the product of either metabolic decarboxylation or chemical decarboxylation at slightly higher temperatures the factors that control the level of methane production after sediment burial are anoxic environment sulfate deficient environment low temperatu e availability of organic matter and sufficient space the timing of these factors is such that most biogenic gas is generated prior to burial depths of 1 000 m in marine sediments most of the biogenic gas formed can be retained in solution in the interstitial pore waters because of higher methane solubility at the higher hydrostatic pressures due to the weight of the overlying water column under certain conditions of high pressures and or low temperatures biogenic methane combines with water to form gas hydrates biogenic gas usually can be distinguished from thermogenic gas by chemical and isotopic analyses the hydrocarbon fraction of biogenic gas consists predominantly of methane the presence of as much as 2 of heavier hydrocarbons can be attributed to admixture of minor thermogenic gas due to low temperature degradation of organic matter the amounts of hydrocarbon components other than methane generally are proportional to temperature age and organic matter content of the sediments biogenic methane is enriched in the light isotope 12c dgr13c1 lighter than 55 ppt owing to kinetic isotope fractionation by methanogens the variations in isotopic composition of biogenic methane are controlled primarily by dgr13c of the original co2 substrate which reflects the net isotopic effect of both addition and removal of co2 the methane isotopic composition also can be affected by mixing of isotopically heavier thermogenic gas the possible complicating factors require that geologic chemical and isotopic evidence be considered in attempts to interpret the origin of gas accumulations accumulations of biogenic gas have been discovered in canada germany italy japan trinidad the united states and ussr in cretaceous and younger rocks at less than 3 350 m of burial and in marine and nonmarine rocks other gas accumulations of biogenic origin have undoubtedly been discovered however data that permit their recognition are not available

seismic reflection profiles across deep continental crust exposed in the kapuskasing uplift structure seismic imaging of precambrian cratons holds the promise of deep structural mapping and interpretation of fundamental crustal construction processes deep structures may be identified by tracing reflections to the surface1 particularly in exposed crustal cross sections2 elevated along deeply penetrating faults but many such cross sections have structural orientations too steep for successful reflection imaging like the ivrea zone3 or occur in plate boundary settings not representative of continental interiors by contrast the archaean greenstone terrain exposed in the kapus kasing uplift of canada4 5 has gently dipping structures and an intracratonic setting within the superior province thus providing an opportunity to examine the third dimension of continental crust down to 25 30 km palaeodepth6 here we present new seismic reflection data which although generally supportive of structural models based on surface geology4 gravity modeling5 and seismic refraction studies7 8 also indicate that the kapuskasing structure is a relatively thin thrust sheet containing seismically reflective sequences of high grade rock which may be analogues of seismi cally reflective in situ lower crust

source and migration of light hydrocarbons indicated by carbon isotopic ratios carbon isotopic ratios can distinguish among different sources of methane and can be used to correlate maturity of sediments with the methane this technique has been applied to several wells and the isotopic values have been used to suggest the sources of the methane and whether the methane migrated into the area comparison of the isotopic ratio changes in ethane propane and butanes indicated that isotopic fractionation of methane by migration is not a major factor the significant changes in methane isotopic ratio and the small changes in the isotopic ratios of these heavier hydrocarbons suggest that although diffusion is not the sole process in migration the time for migration is moderate

implications of melting for stabilisation of the lithosphere and heat loss in the archaean abstract the increased depth and volume of melting induced in a higher temperature archaean mantle controls the stability of the lithosphere heat loss rates and the thickness of the oceanic crust the relationship between density distributions in oceanic lithosphere and the depth of melting at spreading centres is investigated by calculating the mineral proportions and densities of residual mantle depleted by extraction of melt fractions the density changes related to compositional gradients are comparable to those produced by thermal effects for lithosphere formed from a mantle which is 200 c or more hotter than modern upper mantle if archaean continental crust formed initially above oceanic lithosphere the compositional density gradients may be sufficient to preserve a thick archaean continental lithosphere within which the archaean age diamonds are preserved the amount of heat advected by melts at mid ocean ridges today is small but heat advected by melting becomes proportionally more important as higher mantle temperatures lead to a greater volume of melt and as the rate of production of oceanic plates increases archaean tectonics could have been dominated by spreading rates 2 3 times greater than now and with mantle temperatures between ca 1600 c and 1800 c at the depth of the solidus mid ocean ridge melting would produce a relatively thick but light refractory lithosphere on which continents could form protected from copious volcanism and high mantle temperatures

formation of the rocky mountains western united states a continuum computer model one hypothesis for the information of the rocky mountain structures in late cretaceous through eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the north american lithosphere the horizontal components of the motion of this plate are known from paleomagnetism and the edge of the region of flat slab can estimated from reconstructed patterns of volcanism new techniques of finite element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the north american plate a model that has a realistic temperature dependent rheology and a simple plane layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present successful prediction of this model include i the location amount and direction of horizontal shortening that has been inferred from laramide structures ii massive transport of lower crust from southwest to northeast iii the location and timing of the subsequent extension in metamorphic core complexes and the rio grande rift and iv the total area eventually involved in basin and range style extension in a broad sense this model has predicted the belt of laramide structures the transport of crust from the coastal region to the continental interior the subsequent extension in metamorphic core complexes and the rio grande rift and the geographic region of late tertiary basin and range extension its principal defects are that i many events are predicted about 5 million to 10 million years too late and ii the wave of crustal thickening does not travel far enough to the east reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects the correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the laramide orogeny is probably correct the rocky mountain thrust and reverse faults formed in an environment of east west to northeast southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the north american crust nonuniform crustal thickening by simple shear transport also caused relative uplifts therefore this model is consistent with both of the range forming mechanisms that have been inferred 1 a new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional basin and range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away

helium and carbon gas geochemistry of pore fluids from the sediment rich hydrothermal system in escanaba trough abstract ocean drilling program odp leg 169 which was conducted in 1996 provided an opportunity to study the gas geochemistry in the deeper part of the sediment rich hydrothermal system in escanaba trough gas void samples obtained from the core liner were analyzed and their results were compared with analytical data of vent fluid samples collected by a submersible dive program in 1988 the gas geochemistry of the pore fluids consisted mostly of a hydrothermal component and was basically the same as that of the vent fluids the he isotope ratios r ra 5 6 6 6 indicated a significant mantle he contribution and the c isotopic compositions of the hydrocarbons 13c ch4 43 13c c2h6 20 were characterized as a thermogenic origin caused by hydrothermal activity on the other hand the pore fluids in sedimentary layers away from the hydrothermal fields showed profiles which reflected lateral migration of the hydrothermal hydrocarbons and abundant biogenic ch4 helium and c isotope systematics were shown to represent a hydrothermal component and useful as indicators for their distribution beneath the seafloor similarities in he and hydrocarbon signatures to that of the escanaba trough hydrothermal system were found in some terrestrial natural gases which suggested that seafloor hydrothermal activity in sediment rich environments would be one of the possible petroleum hydrocarbon generation scenarios in unconventional geological settings