propagating rift pseudofaults zones of crustal underplating imaged by multichannel seismic reflection data two multichannel seismic reflection lines recorded over the northern juan de fuca plate off western canada provide images of the crustal structure of three pseudofaults the traces of propagating rifts anomalous reflectivity including both subhorizontal and dipping events at times below the normal crust mantle transition is prevalent in the region of the pseudofaults although the crustal thickness elsewhere on the lines varies by only about 10 crust associated with the pseudofaults is as much as 30 thicker than average this may be due to an increase in thickness of the interpreted oceanic crustal layer 3b or crustal underplating the results indicate that magma supply at propagating rifts may at times be large perhaps due to the effects of nearby hotspots or convective partial melting associated with the initiation of rifting

relic magma chamber structures preserved within the mesozoic north atlantic crust the north atlantic transect seismic reflection data collected southwest of bermuda have been reinterpreted following post stack migration and reveal two major intracrustal reflections the shallower of these two events located 1 s below the igneous basement is a subhorizontal undulating surface that in some places is continuous for as much as 10 km on the basis of its position within the section and its laterally discontinuous nature we believe that this upper crustal reflection corresponds to the intermittently sharp contact between the sheeted dikes and the underlying isotropic gabbro a second set of lower crustal reflections dipping 20 40 eastward is also prominent on the migrated profile and terminates downdip against the subhorizontal reflection moho several lines of evidence argue against these features being either artifacts or out of the plane events instead their presence may be ascribed either to crustal penetrating fault zones or to mafic ultramafic cumulate layers frozen into the oceanic crust at the time of formation at the paleo spreading center because of the laminated character of these events and their typical occurrence within 1 0 to 1 5 s of the reflection moho we prefer a compositional versus a structural interpretation for their origin the gradual thinning in the crust approaching the fracture zones is shown to be more complex than was originally inferred although the interpretation that the crust gradually thins toward fracture zones may still apply in a few localities significant departures are recognized elsewhere similarly the improved image on the migrated profile documents an increase in complexity across the localized region directly surrounding the blake spur fracture zone an interpretation advocating crustal thickening in this narrow zone is proposed as an alternative to the crustal thinning model of mutter and others

crustal architecture and tectonic evolution of the sergipe alagoas and jacuipe basins offshore northeastern brazil abstract the sergipe alagoas and the jacuipe basins are divergent margin basins located in the south atlantic ocean at the northeastern brazilian margin high quality 18 s two way travel time seismic reflection profiles which extend from the coastline towards the boundary with the oceanic crust were integrated with potential field data and calibrated with the results of several exploratory boreholes when combined these data provide evidence that the deep seismic reflectors in the deep water region are related to the crustal architecture of the basin rather than to sedimentary features arrays of high impedance reflectors previously interpreted as top of basement probably correspond to detachment planes near the base of the crust or to underplated igneous rocks above the moho the extensional processes that were active during rifting resulted in normal crustal faults that apparently detach near the seismic moho in the deep water region these crustal faults control and rotate syn rift sedimentary units along the margin the seaward portion of the rift seems to have been uplifted and highly eroded during a post break up tectonic event the deep seismic reflection profiles also indicate that the deep water province is characterized by wedges of reflectors that dip seawards these features probably correspond to seaward dipping reflectors sdr which are composed of volcanic rocks marking the transition to oceanic crust the sdr wedges are locally associated with volcanic intrusions several volcanic plugs in deep water region are aligned with oceanic fracture zones that apparently penetrate through the whole crust and reach the upper mantle on the other hand there are some diapiric structures located near the boundary between continental and oceanic crust that bear evidence for deep water salt tectonics

brazilian and african passive margins of the central segment of the south atlantic ocean kinematic constraints the thinning of passive continental margins is usually explained by models using pure stretching or simple shear these models imply hypothetical extensional structures and large horizontal movements between the two conjugate margins more than 250 km for the brazilian and angolan margins refraction reflection data together with the most recent and tightest pre opening fit using continental and oceanic geological and geophysical constraints show that the substratum of the sag basin is divided into an autochthonous part upper continental crust and an allochthonous part exhumed material the thinning process which evolves in an elevated position of the system until at least the break up seems to be depth dependent and to mainly concern the lower middle crust which we postulate is exhumed this exhumation does not explain the entire thinning of the system horizontal motions cannot alone explain the formation of the huge angolan brazilian basin similar observations are made on the whole central segment of the south atlantic ocean part of the lower continental crust is still missing and it seems most improbable that the continental crust maintains its integrity throughout the thinning process

complex structure along a mesozoic sea floor spreading ridge birps deep seismic reflection cape verde abyssal plain summary as part of an intensive study of a small area of oceanic lithosphere the british institutions reflection profiling syndicate birps acquired closely spaced deepseismic reflection profiles over the early cretaceous crust of the cape verde abyssal plain off west africa the survey consisted of profiles spaced at 4 km arranged into strike lines parallel to the old sea floor spreading axis isochron profiles and orthogonal dip lines oriented in the original direction of spreading flow profiles a large capacity well tuned airgun source and very quiet shooting conditions ensured a high signal to noise ratio for deep reflection devising a strategy for mitigating contamination from wrap around multiples arriving from previous shots enabled us to use the minimum possible shot point interval 50 m allowed for collecting long 18 s records data processing was oriented towards a medium with low root mean square velocity steeply dipping structure and pervasive low apparent velocity noise from diffraction at the top of the igneous crust the contrast between the isochron and flow profiles is striking isochron profiles are typically highly reflective throughout the igneous crust consisting of bright bidirectionally dipping reflection sets that extend in places from the top of the igneous basement down to the interpreted moho reflection these reflections do not offset intracrustal or top basement structure and thus are not interpreted as faults an igneous intrusive origin seems more likely flow profiles are more sparsely reflective but show individual steeply dipping reflections best developed in the upper igneous crust continuing down in places to the moho dipping reflections on the flow profiles are interpreted as major normal faults since they are clearly associated with offsets of the top of the basement as well as truncation of horizontal reflections within the igneous crust the dominant dip of these reflections is to the west towards the spreading ridge axis reflections from the vicinity of the moho while well developed in some places are not particularly prominent across the survey area moho reflections appear to show a different structural relation to crustal features on the isochron and flow profiles on isochron profiles dipping reflections occasionally flatten out into and may merge with the moho reflection on flow profiles as dipping crustal reflections approach the moho reflection they are usually abruptly cut off by it without extending deeper this survey shows how oceanic crustal structure can vary rapidly over relatively small areas provides convincing evidence that a structurally complex fabric dominates oceanic igneous crust and gives a conclusive observation of faults that penetrate the entire oceanic crust

tectonic history along the south gabon basin anomalous early post rift subsidence an integrated study of the south gabon margin south atlantic based on reflection seismic and well data has been performed to quantify tectonic activity a regional profile crossing the entire basin together with subsidence analysis highlights important aspects of the post rift history the most striking event in the margin evolution appears to be the anomalous extra subsidence during the early post rift period characterized by high sedimentation rates equivalent to one third of the syn rift subsidence although the presence of evaporite layers restricts knowledge of the underlying structures essentially composed of pre rift and syn rift sequences the outcome of this post rift tectonic study has strong implications for the rifting history the early post rift subsidence patterns can be related to a high thermal anomaly during the early rifting thermal state of the lithosphere these findings are highly relevant for petroleum system studies and have implications for hydrocarbon generation

listric faults imaged in oceanic crust deep seismic reflection profiling has provided exceptional images of listric faulting in oceanic basement of jurassic age off the coast of nova scotia the faults dip toward the mid atlantic ridge and sole near the base of the crust at a common depth of about 4 km presumably the brittle ductile transition at the time of deformation at least five rotated fault blocks with dips ranging from 0 at the surface to 45 at the base are observed along a 36 km flow line segment suggesting a stretching factor of 1 5 2 5 the faults can be traced upward with small displacement in the overlying sedimentary strata to approximately the middle cretaceous strata implying continued movement or consolidation for about 70 m y after crustal formation

geochemical processes in mill tailings deposits modelling of groundwater composition a general model is presented for geochemical processes occurring in the unsaturated zone of a carbonate depleted pyritic tailings deposit quantification of slow geochemical reactions using published empirical rate laws from small scale experiments on monomineralic samples and geochemical equilibrium reactions successfully reproduced the relative rates of field processes in the case study impoundment 1 in kristineberg reproduction of absolute rates was achieved by scaling down all laboratory derived mineral weathering rates by two orders of magnitude the sensitivity of the modelled groundwater composition and ph to rates of ph buffering processes and redox reactions indicated that inclusion and accurate quantification of all dominant geochemical processes on the field scale is necessary for reliable prediction of groundwater composition and ph

heat flow variations correlated with buried basement topography on the juan de fuca ridge flank systematic heat flow variations are commonly observed on sedimented flanks of mid ocean ridges these variations have been attributed to the convective circulation of pore fluids within the oceanic crust and have been used to estimate the scale and pattern of this circulation here we report similar systematic variations observed in a detailed heat flow and seismic reflection survey over sediment covered 3 myr old sea floor on the eastern flank of the juan de fuca ridge we find that heat flow peaks are located above buried basement ridges and adjacent to two minor basement outcrops heat flow lows are found where thick sediments fill basement valleys the simple inverse relationship between heat flow and sediment thickness defined by the data indicates that hydrothermal circulation sealed within the permeable portion of the oceanic crust is vigorous enough to maintain approximately isothermal 10 k conditions at the sediment crust interface as no systematic variations in estimated basement temperatures are observed it seems that at this location the heat flow variations result only from sediment thickness variations and do not reflect a particular convective pattern

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

deep seismic reflection study of a passive margin southeastern gulf of guinea a large grid of deep imaging marine seismic reflection data has been acquired in the gulf of guinea the data show that the architecture of old atlantic igneous oceanic crust and upper mantle is highly variable particularly if reflection moho is taken to be the base of the crust most abrupt changes in oceanic basement thickness and depth to moho can be correlated with fracture zone crossings but significant variations can occur between fracture zones and along flow lines especially near the ocean continent transition reflection moho is usually continuous from ocean to continent and does not display any systematic changes in character continuity or reflection time even beneath the innermost shelf areas there are several varieties of intra crustal reflectors including those that mark different levels within the oceanic gabbroic complex and events that diagonally link the top of oceanic seismic layer 2 and moho different types of sub moho dipping reflections also are observed some are associated with fracture zones some originate within continental crust and dip toward the ocean dissecting moho without offsetting it and still others originate at the oceanic moho and dip toward the continent the transition from oceanic to continental crust is generally quite sharp north of lat 1 s but the exact nature of the transition ranges from rift block geology to abrupt juxtapositions of oceanic and continental crustal rocks south of about lat 1 s the transition to continental crust is more gradual involving a progressive thickening of oceanic crust toward land this difference may relate to the occurrence of much more oblique initial rifting north of 1 s

detailed tectonics near the tip of the galapagos 95 5 w propagator how the lithosphere tears and a spreading axis develops a model for propagation of an active spreading axis into preexisting lithosphere has been developed based on intensive detailed studies using seamarc ii sea beam deep tow and alvin near the tip of the propagator growing westward at 52 km m y along the east west trending galapagos spreading axis near 95 5 w initial lithospheric rifting appears to be accommodated along reactivated roughly east west abyssal hill faults at the tectonic tip extension leads to downdropping of a keystone block which is pervasively cut by normal faults and fissures the initial volcanism initial volcanic tip occurs about 6 5 km behind the tectonic tip as basalt erupts through these fissures covering most of the keystone block the neovolcanic axial tip occurs another 4 5 km back at the western termination of a well defined axial pillow ridge along which volcanism is localized spreading along the propagator axis accelerates to the full rate 29 mm yr half rate at the full rate tip interpreted to be 10 km behind the neovolcanic axial tip or 21 km behind the tectonic tip a complex pattern of strain in the tip area results from the interaction of plate spreading transform shear stress crack propagation dynamic depression of the western end of the axial valley and the transform zone and variations in the strength thickness heterogeneity and anisotropy of the lithosphere southwesterly curvature of the neovolcanic axis near its tip is inferred to result primarily from the response of young thin weak relatively homogeneous lithosphere to counterclockwise rotation of the maximum tensional stress direction relative to the spreading direction due to the influence of shear stress from the transform zone similar stress interactions likely occur at the tectonic tip but are inferred to be accommodated in this older thick strong heterogeneous and anisotropic lithosphere by oblique slip along a set of left stepping reactivated preexisting east west abyssal hill structures although on a time scale of 106 years the overall process of propagation is mostly continuous rifting at the tectonic tip appears discontinuous at a scale of 1500 m or 30 000 years based on the east west spacing of grabens formed as rifting begins the scale of discontinuity of propagation of the neovolcanic axis is 3 5 km or 50 000 100 000 years based on the distribution of northeast trending features interpreted as beheaded western terminations of paleoneovolcanic axes the extension involved in rifting is 10 20 based on summing the horizontal normal separation of all normal faults and fissures integrated and averaged over the zone of rifting these values are approximately doubled if simple shear listric faulting is dominant rather than pure shear normal faulting

crustal structure of endeavour ridge segment juan de fuca ridge from a detailed seismic refraction survey a detailed seismic refraction survey was carried out over the endeavour segment of the juan de fuca ridge a medium rate spreading center which lies off western north america to investigate the creation and evolution of oceanic crust a bathymetric high and the presence of hydrothermal vents suggested that the study area was the most recent locus of spreading travel time and amplitude data from 15 in line air gun ocean bottom seismometer profiles were forward modeled using asymptotic ray theory to obtain two dimensional velocity models consisting of four primary layers which correlate well with classic models of oceanic crust significant lateral variations in thicknesses and velocities on the scale of a few to 10 km are superimposed on this basic velocity structure but they appear to be random rather than distributed symmetrically about the ridge we attribute them to fracturing which causes porosity changes hydrothermal circulation which fills voids and fractures with alteration products and variations in magmatic and or deformational processes at the spreading center layer 2a is found to have low 2 6 2 8 km s velocities to average 0 4 km in thickness with variations up to 0 2 km and to be bounded at its base by a sharp velocity increase to 4 8 km s along the axial ridge velocities 0 4 0 6 km s higher than average are interpreted for layers 2b and 2c but these values are confined to a 2 km wide zone centered below the ridge velocities along ridge parallel lines offset 10 km are normal indicating that maturation to off ridge structure has occurred within at most 0 3 ma layer 3 velocities decrease by 0 1 0 2 km s for arrivals traveling along and under the axial ridge perhaps caused by higher temperatures however we find no anomalously low velocities beneath the ridge indicating that no large crustal magma chamber exists on the basis of this study we conclude that magmatic accretion is a fully three dimensional process within ridge segments such as endeavour ridge

element discharge from pyritic mine tailings at limited oxygen availability in column experiments sulfide mineral oxidation in mine tailings deposits poses a long term threat to surrounding ground water and surface waters soil or water cover remediation aims at reducing the rate of sulfide min

atmospheric carbon sequestration in ultramafic mining residues and impacts on leachate water chemistry at the dumont nickel project quebec canada abstract passive carbon mineralization in ultramafic mining residues which allows the sequestration of co2 through carbonate precipitation is one of the options being considered to limit the accumulation of anthropogenic co2 in the atmosphere the dumont nickel project dnp will generate approximately 1 7 gt of utramafic mining residues over 33 years of production and the mine will release about 127 700 tonnes of co2 each year using two experimental cells filled with ultramafic waste rock ec 1 and milling residues ec 2 the impacts of carbon mineralization on leachate water quality were studied and the quantity of sequestered carbon was estimated hydrotalcite supergroup minerals aragonite artinite nesquehonite dypingite and hydromagnesite precipitated through atmospheric weathering while the inorganic carbon content of the weathered mining waste increased from 0 1 wt to 4 0 wt which indicates active co2 sequestration the leachate water sampled at the bottom of the experimental cells is characterized by an alkaline ph 9 5 a high alkalinity 90 to 750 mg l and a high concentration of magnesium 50 750 mg l which is typical from weathering of ultramafic rocks in a system open to co2 since 2012 the chemical composition of the leachate water has evolved seasonally these seasonal variations are best explained by 1 climatic variations over the year and 2 increased carbonate precipitation between may and july increased carbonate precipitation decreased the alkalinity and magnesium concentrations in the leachate water and produced pore waters which were undersaturated with respect to carbonate minerals such as artinite and hydromagnesite carbonate precipitation thus self limits carbon sequestration through a negative feed back loop the carbon sequestration potential of the dnp residues is also influenced by the hydrogeological properties of the residues in cell ec 2 a high liquid solid ratio which limits carbonate precipitation was maintained by the hydrogeological properties since 2011 an estimate of 13 kg of atmospheric co2 has been sequestered in the milling residues ec 2 which corresponds to a mean rate of 1 4 0 3 kgco2 tonne year using this mean rate the 15 mt of tailings produced each year during the planned 33 years of mining operation could potentially sequester 21 000 tonnes of co2 per year by passive carbon mineralization about 16 of the 127 700 tonnes of co2 annually emitted by the planned mining operation

magmatism and rift margin evolution evidence from northwest australia deep crustal seismic observations from northwest australia demonstrate that the cuvier margin is a volcanic passive margin that formed as greater india rifted away from australia in the early cretaceous formation of the cuvier basin and initial sea floor spreading resulted in the emplacement of exceptionally thick oceanic crust while contemporaneous spreading off the adjacent exmouth plateau formed normal thickness oceanic crust crust of normal thickness was generated in the cuvier basin within 3 4 m y after spreading began there is no evidence for hot spot activity with the appropriate timing or in the appropriate spatial pattern to account for the difference in volcanism along the margin we believe that rapid rifting of the cuvier margin created strong lateral temperature gradients in the subjacent upper mantle at the time of breakup this induced small scale convection in the upper mantle that increased the rate of upwelling and thus the volume of melt generated by decompression partial melting in turn this resulted in a larger volume of magma at the initiation of sea floor spreading and thicker oceanic crust was initially emplaced rifting on the exmouth plateau happened over a much longer period of time and no such convective partial melting occurred the volume of melt associated with rifting was therefore entirely the result of passive upwelling and a normal thickness of oceanic crust was emplaced immediately adjacent to the exmouth plateau

neocomian to early aptian syn rift evolution of the normal to oblique rifted north gabon margin interior and n komi basins abstract the north gabon coastal rift basins consist of a set of 130 150 long segment asymmetrically tilted half grabens interior basin and 000 020 short segment en echelon half grabens n komi basin separated by 040 060 major transverse faults tectono sedimentary analysis of field and subsurface data reveals the control exerted by extensional tectonism over continental sedimentation during berriasian to early barremian times uniform uniaxial 040 060 extension was responsible for the stretching of the brittle upper crust over a 100 km wide domain during late barremian early aptian times the main locus of extension stepped westward resulting in severe end rift uplift and erosion of the failed interior and n komi rift basins early cretaceous coastal rifts in north gabon display a wide range of styles from oblique rifting n komi basin normal rifting interior basin to transform rifting the pre existing precambrian tectonic fabric exerts a strong control over the mode and over the 100 300 km scale segmentation of the rifting

coherent noise in marine seismic data three mechanisms that might produce noise patterns on stacked sections are examined direct and trapped waves that propagate outward from the seismic source cable motion caused by the tugging action of the boat and tail buoy and scattered energy from irregularities in the water bottom and sub bottom different noise patterns can be observed on shot profiles and common midpoint cmp gathers these patterns can be diagnostic of the dominant mechanism in a set of data field data suggest that the dominant noise is from waves scattered within the shallow sub bottom this type of noise is enhanced by cmp stacking moreover it can be as strong as surface wave noise on stacked land seismic data as well moveout filtering is best for suppressing the noise while preserving signal since the scattered noise does not exhibit a linear moveout pattern on cmp sorted gathers moveout filtering must be applied either to traces within shot records and common receiver gathers or to stacked traces our data example demonstrates that although it is more costly moveout filtering of the unstacked data is particularly effective because it conditions the data for the critical data dependent processing steps of predictive deconvolution and velocity analysis modified journal abstract

crustal structure of northern juan de fuca plate from multichannel reflection data the northern juan de fuca plate forms the basement of a young 8 ma ocean basin off western canada two subparallel multichannel seismic reflection lines totaling 230 km in length and trending obliquely to the spreading direction were recorded across the basin and provide high quality images of crustal structure the processing sequence applied to the data includes a prestack inside trace mute of common midpoint cmp gathers to reduce noise levels on the deep data cmp stack poststack dip filtering f k migration and band pass filtering coherency filtered stacks are helpful in tracing weaker reflectors the stacked sections reveal a horizontally layered sedimentary sequence overlying a rugged and prominent basement reflector which dips slightly landward topography of the basement was formed at or near the spreading ridge there is no evidence for subsequent deformation a strong fairly continuous reflection from the base of the crust oceanic moho at 2 0 2 2 s two way travel time below the basement surface generally mimics the basement topography and shows the characteristic multicyclic waveform seen in other surveys estimates of reflection coefficients for the moho at several locations indicate that in general the crust mantle transition is unlikely to occur as a discrete first order interface although much of the crust appears acoustically transparent a number of discontinuous intracrustal reflectors are observed over 40 km at the eastern end of the northern line some of these are interpreted to arise from the oceanic layer 2 3 and layer 3a 3b boundaries the noncontinuous nature of these reflectors is an indication of the complexity of the processes producing intracrustal reflectivity and the lateral variability of crustal formation however the interpreted crustal thickness varies by only about 10 away from the distal ends of spreading segments indicating that while the process of crustal formation may be locally variable it produces a broadly uniform result

odm an analytical solution based tool for reacting oxygen diffusion modelling in mine spoils a simple one dimensional analytical solution is presented to model oxygen diffusion through the pore space of mine spoils containing pyrite the model incorporates volumetric oxygen consumption terms due to pyrite oxidation oxidation of fe 2 to fe 3 and bacterial activity based on this analytical solution a graphical user interface gui tool is programmed and designed in matlab software this tool can be used to model transport of oxygen through the mine spoils either with or without a cap results of several simulation scenarios of sensitivity analysis showed a significant change in oxygen concentration with varying effective diffusion coefficient of oxygen transport model and simulation time efficiency and flexibility of the tool developed here is verified by modelling oxygen transport through the pore space of a coal waste pile case a and a copper mine tailings case b maximum depth of oxygen diffusion is obtained approximately equal to 2 and 1 5 m through the cases a and b respectively

seismic evidence for the migration of fluids within the accretionary complex of western canada multichannel deep seismic reflection data from the subduction zone of western canada delineate the wedge of accereted sediments and the principal terranes crescent pacific rim and wrangellia that form the convergent margin the top of the igneous oceanic crust is defined by subhorizontal reflections extending at least 100 km landward of the deformation front upon incorporation into the accretionary wedge the clearly defined stratigraphy of the incoming oceanic sedimentary section is destroyed over a distance of about 10 km initially an unreflective zone which correlates well with maximum fluid expulsion is formed farther landward a predominantly landward dipping reflectivity exists a number of reflections are thrust faults which appear to merge at depth with the subhorizontal reflections but most have another origin these reflections may be related to the movement of fluids generated by the compaction of sediments or possibly by the dehydration of the subducting plate they are strongest i

reactive transport modelling of the hydro geochemical behaviour of partially oxidized acid generating mine tailings with a monolayer cover abstract the efficiency of a monolayer cover to prevent acid mine drainage amd generation from two pre oxidized tailings impoundments was assessed using the min3p code a finite volume model for coupled groundwater flow oxygen diffusion and multi component reactive transport numerical simulations were validated using large column tests set up in the laboratory and monitored during 19 wetting and drying cycles over approximately two years results indicate that a monolayer cover made of either non acid generating tailings or a till was not able to prevent sulfide oxidation in the underlying reactive tailings both for the conditions applied in the laboratory and under conditions more representative of field observations the efficiency of the simulated monolayer cover was highly dependent on the position of the water table despite improved water quality the reactive tailings nonetheless continued to oxidize and generate amd even when the water table was close to the surface of the tailings 1 m depth the results from this study indicate that the efficiency criteria for cover systems on fresh unoxidized tailings may not be directly applicable for pre oxidized tailings the paper also presents a few modifications to the min3p code that were successfully applied to simulate the hydrogeological and geochemical response of pre oxidized already acidic and highly contaminated tailings

seismic reflection profiling in deep water avoiding spurious reflectivity at lower crustal and upper mantle traveltimes abstract the acquisition of seismic reflection data in deep water can be complicated by persistent multiple reflections from the seabed and sediments that continue to contaminate subsequent records beyond their initial source point these multiples may create both severe and subtle artefacts that could easily be misinterpreted as sub horizontal primary reflections from the lower crust or uppermost mantle contamination is particularly severe for shot repetition rates less than about 40 s i e for shot points spaced less than 100 m and in areas with a sedimentary cover that is thick and contains reflection coefficients that are large relative to the seabed under these circumstances shots fired at constant time increments maximise the likelihood of contamination in the stacked section in this study we use seismic reflection data from three sites in the north atlantic to show how and under what conditions such multiples are produced and how they may mislead the interpreter we also outline a field recording procedure to minimise their effect on the common mid point stacked profile

acid neutralization mechanisms and metal release in mine tailings a laboratory column experiment mining and milling of base metal ore deposits can result in the release of metals to the environment when sulfide minerals contained in mine tailings are exposed to oxygen and water they oxidize and dissolve two principal antagonistic geochemical processes affect the migration of dissolved metals in tailings impoundments sulfide oxidation and acid neutralization this study focuses on acid neutralization reactions occurring in the saturated zone of tailings impoundments to simulate conditions prevailing in many tailings impoundments 0 1 mol l sulfuric acid was passed continuously through columns containing fresh unoxidized tailings collected at kidd creek metallurgical site the results of this column experiment represent a detailed temporal observation of ph eh and metal concentrations the results are consistent with previous field observations which suggest that a series of mineral dissolution precipitation reactions control ph and metal mobility typically the series consists of carbonate minerals al and fe iii hydroxides and aluminosilicates in the case of kidd creek tailings the dissolution series consists of ankerite dolomite siderite gibbsite and aluminosilicates in the column experiment three distinct ph plateaus were observed 5 7 4 0 and 1 3 the releases of trace elements such as cd co cr cu li ni pb v and zn were observed to be related to the ph buffering zones high concentrations of zn ni and co were observed at the first ph plateau ph 5 7 whereas cd cr pb as v and al were released as the ph of the pore water decreased to 4 0 or less

sulfide mineral oxidation and subsequent reactive transport of oxidation products in mine tailings impoundments a numerical model a versatile numerical model that couples oxygen diffusion and sulfide mineral oxidation pyrox has been developed to simulate the oxidation of pyrite in the vadose zone of mine tailings a shrinking core oxidation model and a finite element numerical scheme are used to simulate the transport of oxygen and oxidation of pyrite grains the rate of pyrite oxidation is assumed to be limited by the transport of oxygen to the reaction site the model determines the spatially variable bulk diffusion coefficient for oxygen on the basis of moisture content porosity and temperature all of which are variable input parameters the model pyrox has been coupled to an existing reactive transport model mintran which uses a finite element scheme for transport of contaminants and minteqa2 to solve for the equilibrium geochemistry the reactions described by mintran are subject to the local equilibrium assumption the resulting model mintox is capable of simulating tailings impoundments where the oxidation of pyrite or pyrrhotite is causing acidic drainage and where acid neutralization and attenuation of dissolved metals can be attributed to equilibrium reactions because mintox uses realistic boundary conditions and hydrogeological properties the potential benefits of various remediation schemes such as moisture retaining covers can be quantitatively evaluated

seismic structure of the southern east pacific rise seismic data from the ultrafast spreading 150 to 162 millimeters per year southern east pacific rise show that the rise axis is underlain by a thin less than 200 meters thick extrusive volcanic layer seismic layer 2a that thickens rapidly off axis also beneath the rise axis is a narrow less than 1 kilometer wide melt sill that is in some places less than 1000 meters below the sea floor the small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge crest thermal models however the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate these observations indicate that the paradigm of ridge crest magma chambers as small sill like midcrustal bodies is applicable to a wide range of intermediate and fast spreading ridges

incorrect when uncorrected reconstructing vegetation history using n alkane biomarkers in loess paleosol sequences a case study from the saxonian loess region germany abstract this study aims at contributing to the reconstruction of the paleoenvironment of the saxonian loess region by investigating sedimentary n alkanes as plant leaf wax derived biomarkers in two loess paleosol sequences lpss namely the gleina and the rottewitz lpss the weichselian paleosols are characterised by n alkane concentration maxima when compared to the loess layers when this coincides with high odd over even predominances oeps this points not only to increased n alkane input and or production but also to increased n alkane preservation in the soils namely in gleysols with water logged edaphic conditions by contrast the buried cambisols reveal low oeps indicating accelerated degradation under aerobic edaphic conditions the n alkane concentrations in the truncated eemian interglacial paleosol are very low this reflects the removal of the formerly n alkane enriched eemian topsoil low n alkane input into the b t s d horizon subsoil horizon of the eemian luvisol and or strong soil organic matter mineralization four investigated n alkane ratios show distinct variations during the weichselian tempting inferences of vegetation changes in terms of varying contributions of tree shrub derived dominated by n c 27 or n c 29 versus grass derived dominated by n c 31 or n c 33 n alkanes however using the oep as a proxy for degradation and considering that n alkane ratios are prone to degradation effects the above interpretation is very likely incorrect degradation can account for most variability of the n alkane ratios applying an endmember model based on a modern n alkane reference dataset allows estimating the contribution of tree shrub versus grass derived n alkanes semi quantitatively these modelling results suggest that both the gleina and the rottewitz lpss were formed under prevailing grassland minor contributions of trees shrubs are only likely for the gleinaer soil complex in rottewitz lps for the top of a brown gleysol in gleina lps and the holocene soil the weichselian vegetation changes as inferred from the uncorrected n alkane ratios are not confirmed overall the n alkane results support the idea that the gleinaer soil complex in gleina and rottewitz lps respectively are not identical but rather are chronostratigraphic equivalents more favourable regional climatic conditions presumably occurred in the southwesterly exposed rottewitz locality and allowed tree shrub vegetation to grow during favourable periods of the weichselian a major shortcoming of the n alkane biomarker method is that only vegetation changes of deciduous woodland to grassland and vice versa can be reconstructed because coniferous trees exhibit by two to three orders of magnitude lower n alkane concentrations

the behavior of inclined covers used as oxygen barriers covers with capillary barrier effects ccbe have been recently proposed as a viable option for gas migration barriers however the effect of geometry on ccbe performance has not been clearly demonstrated in this paper the results of a laboratory study performed with an original apparatus called the inclined box are presented the results obtained show that the hydraulic behavior of layered covers is influenced by the inclination of the slope generally the upper part of the slope contains less water than the lower part this means that the upper part is less efficient than the lower part for limiting gas migration the authors have also studied an existing site where a ccbe was built on a sloping surface after validation of the numerical model with in situ measured data the model was used to perform a parametric study to quantify the influence of the main ccbe parameters on its performance the results obtained confirm those measured in the lab and clearly show how geometry influences the performan

internal structure of a spreading segment of mesozoic oceanic crust we define the crustal architecture and internal structure of a 70 km long segment of mesozoic oceanic crust combined reflection and wide angle refraction profiles show the relations between reflectivity and seismic velocity within the crust the crust thins by 4 km from the segment center to the bounding fracture zones a 2 km thick lens at the base of the crust in the segment center exhibits enhanced reflectivity in the otherwise mainly opaque lower crust which coincides with abnormally high seismic velocities of 7 5 km s we attribute these characteristics to the presence of a melt injection region the reflection profiles in the 4 km spaced grid show that there are two distinct approximately orthogonal sets of dipping reflectors within the crust ridge normal reflectors dip at typically 25 in both directions the majority dip west toward the spreading center and their common association with basement steps suggests that they represent normal faults they terminate above or near the base of the crust ridge parallel reflectors are shallower dipping averaging 15 and planar many are restricted to the middle crust and we speculate that they represent magmatic rather than tectonic features a prominent dipping reflector beneath a fracture zone cuts at 20 into the upper mantle similar reflectors are found elsewhere beneath fracture zones in the north atlantic and may be caused by thermal stresses generated as the lithosphere cooled and moved away from the spreading axis

oceanic rift propagation a cause of crustal underplating and seamount volcanism we present the first seismic reflection data across a pseudofault zone the trace of a propagating rift away from a spreading axis strong reflections from the crust mantle transition are discontinuous across the pseudofault deeper reflections which originate near the base of the crust formed at the propagator tip dip beneath the older oceanic crust and become subhorizontal they are interpreted to represent the lower limit of an underplated subcrustal plutonic complex an anomalously smooth basement surface indicative of massive lava flows and a 1200 m high seamount are above the underplated zone the sill complex extends tens of kilometres ahead of and off axis from the former location of the propagating rift indicating that the magmatic supply to the propagator tip was unusually vigorous other seamounts in the northeast pacific lie close to pseudofaults and may have formed as a result of rift propagation

the continent ocean boundary at the rifted margin off eastern canada new results from deep seismic reflection studies seismic reflection data were collected and processed to 20 s two way travel time along four lines which cross the rifted continent ocean boundary off the grand banks region of eastern canada specifically to examine the origin age and nature of this fundamental boundary this represents the first regional study of its kind the most important result is the presence of landward dipping reflectors near the foot of the continental slope these occur where oceanic crust appears to terminate against the continent we suggest that the dipping reflectors mark the continent ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary sedimentary basins lie just landward of the continent ocean boundary their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting and this may be an important stage leading to continental breakup rift basins formed further landward on the grand banks do not exhibit this thinning other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent ocean boundary also the oceanic crust exhibits a complexity of reflections some of which may be due to compositional zonation during magmatic crystallization finally our results may have important consequences for continental predrift reconstruction as oceanic crust appears to extend farther landward in the newfoundland basin than some recent studies have indicated

transition from continental to oceanic crust on the wilkes adelie margin of antarctica the wilkes adelie margin of east antarctica a passive margin rifted in the early cretaceous has an unusually reflective moho which can be traced seismically across the continent ocean transition velocity models and depth sections were constructed from a combined set of u s and french multichannel seismic reflection lines to investigate the transition from continental to oceanic crust these data show that the boundary between oldest oceanic crust and transitional continental crust is marked by a minimum in subsediment crustal thickness and in places by a shoaling of moho the moho reflection is continuous across the edge of oceanic crust and gradually deepens landward under the continental edge

accretion and recent deformation of sediments along the northern cascadia subduction zone new multichannel seismic reflection profiles acoustic imagery and swath bathymetric data from the vancouver island continental margin reveal the surface morphology cross sectional geometry and deformation style of this part of the cascadia accretionary prism the incoming 3 to 5 km thick sediment section fails most commonly along landward dipping thrusts that penetrate to or very close to the top of the underlying oceanic crust the faults are listric with approximately circular cross sections and hanging wall sediments are back tilted in a planar manner high near lithostatic pore pressures in the deeper part of the section are inferred from the flattening of the faults near the decollement and from the long distances over which the decollement has propagated seaward the faults dip more steeply at 40 45 near the surface where pleistocene turbidites are displaced similar folds have developed where the turbidites are particularly thick 4 km and near hydrostatic pore pressures are inferred to occur in the turbidite part of the section an increase in pore pressure with depth is expected as a consequence of sedimentation and tectonic loading particularly because the deeper hemipelagic sediments are probably far less permeable than are the overlying turbidites the top of the downgoing juan de fuca plate which defines the base of the accreted sedimentary wedge beneath the margin is well imaged in the new seismic reflection profiles and is further defined by land reflection data seismic refraction and earthquake hypocenters the landward dipping continental backstop to the prism is also well located if these constraints are used for the geometry of the accretionary prism a general balance is found between the present prism volume and the estimated total sediment supply over the approximately 42 m y accretion history this result along with the observation that deformation front faults penetrate most or all of the sediment section suggests that little or no sediment has been subducted since initiation of the present phase of subduction in the eocene application of the critically tapered wedge model of davis and others to the observed prism geometry suggests that high average pore pressures occur in the prism particularly the seawardmost part beneath the lower slope beneath the continental shelf the dip of the subducting slab increases to greater than 11 a dip that according to the critically tapered wedge model is too steep to allow upward growth of an accretionary prism this provides a simple explanation for ongoing subsidence of the tofino sedimentary basin over the older part of the accretionary complex to the southeast the juan de fuca plate dips much more gently beneath the olympic peninsula of northern washington allowing the accretionary prism to grow upward well above sea level to form the olympic mountains

structure and variability of oceanic crust on the flanks of the east pacific rise between 11 and 13 n we use over 500 seismograms collected by five ocean bottom seismometers to examine the structure and variability of 0 5 m y old crust in the rivera ocean seismic experiment rose area of the east pacific rise between 11 and 13 n there are no significant differences among the first arrival travel times for 10 independent refraction line segments along the isochron amplitude patterns recorded on the individual lines however suggest that structural boundaries within the upper crust vary by hundreds of meters in depth over lateral distances of tens of kilometers within this 200 km segment of lithosphere on the basis of the modeling of amplitude patterns at ranges of 5 10 km and of comparisons with the upper crust sampled at deep sea drilling project hole 504b and in ophiolite complexes layer 2 can be divided into three units 2a corresponds to extrusive volcanics with a steep velocity gradient 2b to extrusives with a near zero velocity gradient and 2c to a complex region of transition from extrusives to sheeted dikes the total thickness of layer 2 varies by about 0 7 km over the length of the line another amplitude peak which varies in range between 14 and 21 km may be related to the downward transition from sheeted dikes to isotropic gabbros within layer 3 the variability of these two amplitude patterns provides direct evidence that the along strike structure of oceanic crust in the rose area is definably heterogeneous and that the process of crustal accretion is likewise variable on the scale resolved by amplitude and travel time information

seismic structure of oceanic crust in the western north atlantic we examine the seismic structure of cretaceous aged ocean crust north of the blake spur fracture zone in the western north atlantic by using a combination of multichannel seismic and wide angle reflection refraction data although the oceanic crust in this area is characterized by a relatively uniform thickness and seismic velocity structure it displays large variations in crustal reflectivity on both ridge normal and ridge parallel profiles the upper crust is highly reflective and contains both subhorizontal or shallow dipping events and more steeply dipping reflectors subhorizontal reflectors are typically present between 1 5 and 2 km depth and are in some cases laterally continuous for distances of 15 to 20 km generally these events do not correlate with the depth of the seismic layer 2 layer 3 boundary determined from refraction data and their origin is still poorly understood the steeply dipping events are generally confined to the upper 2 5 km of the crust we interpret these reflectors as the subsurface expression of ridge parallel extensional faults commonly mapped at mid ocean ridges the lower crust in this area exhibits alternating regions of high and low reflectivity the highly reflective zones are made up of packages of linear or concave upward dipping reflectors that flatten out in a diffuse zone of high reflectivity near the base of the crust some of these dipping reflectors appear to cut through the whole crustal section although most fade out in the acoustically transparent midcrust on ridge normal profiles the majority of these events dip to the east toward the paleo spreading center whereas on ridge parallel profiles the events typically dip south toward the blake spur fracture zone the base of the crust is generally not associated with a strong discrete moho reflection but is a comparatively indistinct boundary usually associated with a 1 to 2 km thick band of diffuse high reflectivity we interpret the lower crustal and whole crustal dipping events as the subsurface expression of major fault systems that have ruptured the entire crustal section down to depths of 8 10 km with the available data we cannot unambiguously determine the geometrical relationship between the dipping lower crustal reflectors on the ridge normal and ridge parallel profiles we may be imaging a single major detachment surface that dips both toward the ridge axis and away from accommodation zones linking major boundary faults alternatively these events may represent two different classes of fault systems that form during different stages of the emplacement and aging of oceanic lithosphere

carbonate mineral water interactions in sulfide rich mine tailings the chemical composition and mineralogy of coatings on carbonate minerals from mine tailings have been studied using aqueous geochemical methods time of flight laser ionization mass spectrometry tof lims and transmission electron microscopy tem the goal is to study major and trace element partitioning between the aqueous and solid phase and to infer mechanisms that control the concentrations of elements in the pore water of sulfide rich mine tailings pore water samples and carbonate mineral grains were collected from four geochemically distinct zones within the tailings oxidation of sulfide minerals near the surface results in a large range in pore water ph 3 85 to 6 98 and aqueous concentrations of metals and sulfate with increasing depth in the tailings mineral water interactions lead to increasing ph and decreasing concentrations of metals and sulfate calculated mineral saturation indices trends in the abundance of ca fe mg and mn in tof lims profiles through the secondary coatings and electron diffraction patterns obtained from the coatings suggest that precipitation dissolution of jarosite group minerals gypsum goethite akaganeite amorphous fe oxyhydroxides and siderite control the aqueous ca fe na k and so4 concentrations the occurrence of secondary coatings on primary minerals is widespread and reactions with the secondary minerals rather than the primary mineral substrate probably represent the principal controls on trace element distributions in the pore water the data indicate that adsorption surface complexation and co precipitation reactions are important controls on the concentrations of trace elements in the pore water the occurrence of siderite coatings on the surface of ankerite grains suggests that fe bearing dolomite structure carbonate minerals dissolve incongruently this corroborates inferences made by previous workers that solubility differences between calcite and siderite lead to calcite dissolution and siderite precipitation during acid neutralization the accumulation of secondary coatings will diminish the rate of mineral dissolution reactions involving the primary minerals consequently the use of kinetic rate constants obtained from the literature for simulating the dissolution of primary mineral phases such as the carbonates and alumino silicates will likely overestimate the actual rates

reactive transport controls on sandy acid sulfate soils and impacts on shallow groundwater quality disturbance or drainage of potential acid sulfate soils pass can result in the release of acidity and degradation of infrastructure water resources and the environment soil processes affecting shallow groundwater quality have been investigated using a numerical code that integrates bio geochemical processes with water solute and gas transport the patterns of severe and persistent acidification ph 4 in the sandy carbonate depleted podzols of a coastal plain could be reproduced without calibration based on oxidation of microcrystalline pyrite after groundwater level decrease and or residual groundwater acidity due to slow vertical solute transport rates the rate of acidification was limited by gas phase diffusion of oxygen and hence was sensitive to soil water retention properties and in some cases also to oxygen consumption by organic matter mineralization despite diffusion limitation the rate of oxidation in sandy soils was rapid once pyrite bearing horizons were exposed even to a depth of 7 5 m groundwater level movement was thus identified as an important control on acidification as well as the initial pyrite content increase in the rate of fe ii oxidation lead to slightly lower ph and greater accumulation of fe iii phases but had little effect on the overall amount of pyrite oxidized aluminosilicate kaolinite dissolution had a small ph buffering effect but lead to the release of al and associated acidity simulated dewatering scenarios highlighted the potential of the model for risk assessment of bio geochemical impacts on soil and groundwater over a range of temporal and spatial scales

the chronological position of the lohne soil in the nussloch loess section re evaluation for a european loess marker horizon abstract the loess section of nussloch in southwestern germany is a key profile for the reconstruction of the terrestrial palaeo environment of central europe at the time of the last glacial and interglacial recently the significance of the site for palaeo environmental and geoarchaeological research increased additionally with the discovery of unique palaeolithic cultural remains from anatomically and culturally modern humans that invaded southwestern germany during the weichselian wurmian pleniglacial period at nussloch a chrono stratigraphical key position is taken by a middle pleniglacial cambisol remain which under the designation lohne soil serves as an important pedostratigraphic marker horizon for the greater area repeatedly greenland interstadial gis 8 was suggested as a likely period of soil formation for the lohne soil this interpretation is yet not justified on the basis of published chronometric data critical assessment of the data points to a later period of soil formation likely during gis7 to gis5 this conclusion is supported by a new set of radiocarbon ages for the nussloch site which are presented here for the first time consequences of a revised chronology for correlations of pleniglacial cryosols below and above the lohne soil with greenland interstadials are discussed the implications are important for european loess research as the nussloch section serves as a reference base for loess sections throughout europe

reconstruction of the late quaternary paleoenvironments of the nussloch loess paleosol response to comments by g wiesenberg and m gocke

deep crustal seismology of continental margins this review discusses results from recent seismic experiments at continental margins conducted by u s investigators as well as those by non u s investigators necessary to place the results of u s workers into context continental margins may be characterized as either rifted transform or convergent rifted or passive margins form where continents are torn apart during continental drift transform margins accommodate transform plate motions along strike slip faults such as the san andreas fault at convergent margins oceanic crust is being thrust beneath or subducted under the continental crust all three types of margins are found in north america

structure of the gabon margin from integrated seismic reflection and gravity data in the south gabon basin deep multi channel seismic reflection and gravity modeling analysis have shed light on key features of the structure of the margin the thinned continental crust beneath the gabon margin appears to be composed of two distinct layers separated by a clear strong and more or less continuous reflector running in the 7 10 s twt window the lower crust is characterized by a higher density intermediate between the lower values of the upper crust and the denser values of the mantle the lower crust is irregularly shaped and presents lateral thickness variations along the direction of thinning and along the coast in the offshore thinned continental domain the lower and upper crust form a 20 25 km thick body crustal thicknesses point to a relatively sharp and narrow transition along a few tens of kilometers between the unthinned and the thinned continental crust the high density layer identified offshore gabon presents similar characteristics in density geometry and spatial distribution as the underplated magmatic bodies observed along volcanic margins e g along the south atlantic namibia margin or the north atlantic voring margin although this lower crustal body could possibly represent ultra mafic serpentinized rocks or high grade metamorphic crustal rocks we suggest that it could be composed of mafic rocks magmas resulting from partial melting during rifting may underplate the crust and or be intruded in the lower crust through a system of dykes and sills in this view the present day crustal thicknesses along rifted margins characterized by magmatic underplating and or intrusion are not representative of the thinning that the crust experienced during rifting results of this study point to relatively shallow sedimentary basins along the south gabon margin the deepest offshore depocenters located under the westernmost side of the continental platform appear to be associated with the deepest syn rift basins these basins seem to extend along 20 to 40 km in the ne sw direction with a present day average thickness of 7 3 km offshore gabon whereas the crustal thinning appears significant the syn rift deposit are not thick we suggest that the area was anomalously uplifted during the rifting phase due to an elevated thermal lithospheric gradient the conclusions derived from our seismic and gravity analysis are consistent with the implications such a thermal anomaly would have on the tectonic evolution of a rifted margin with 1 an underplated high density lower crustal layer 2 shallow depth syn rift basins associated with a relatively thin crust and subsequently 3 elevated recorded subsidence rates in the initial post rift stages research highlights the gabon margin appears characterized by an underplated high density lower crustal layer shallow depth syn rift basins are associated with a relatively thin crust seismic and gravity analysis points to an anomalous uplift of the gabon margin during the rifting phase crustal structure syn rift infill and post rift subsidence are consistent an uplift of the gabon margin during the rifting phase the uplift of the gabon margin is possibly due to an elevated thermal lithospheric gradient

deep high amplitude reflections from a major shear zone above the subducting juan de fuca plate seismic reflection data show that a regionally extensive band of landward dipping reflections exists above the subducting juan de fuca plate at the western canadian continental margin the reflections truncate at depth a major terrane boundary mapped near the surface and must have a structural origin many reflections are very strong having reflection coefficients locally as high as 10 20 these amplitudes are consistent with theoretical calculations of reflections from sharp poirosity contrasts if the pores are very thin however seismic data from the accreted sedimentary melange farther seaward from which much of the deep reflective zone is derived suggest that significant porosity contrasts do not occur alone because higher reflection amplitudes are found in regions of inferred high strain both geometric and amplitude constraints imply that the deep reflective layering represents intensely sheared rocks possibly originating close to the subduction decollement

the northern cascadia subduction zone at vancouver island seismic structure and tectonic history the structure and tertiary tectonic history of the northern cascadia subduction zone have been delineated by a series of new multichannel seismic lines acquired across the continental shelf to the

wide and narrow margins of the brazilian south atlantic two contrasting end members of passive continental margins here referred to as wide and narrow margins are present in the south atlantic although some margin segments are intermediate between these two styles narrow margins are characterized by a large bounding fault 4 km throw near the shelf edge a sharp transition zone 10 20 km wide from normal thickness to substantially thinned continental crust or oceanic crust where the total width of extended continental crust is usually less than 100 km wide continental margins are characterized by a broad continental shelf and a wide zone 50 600 km of thinned crust with an even distribution of faults wide margins usually have a more gentle continental shelf and slope narrow narrow wide wide and wide narrow conjugate margin pairs exist in the south atlantic and are suggested to be controlled mainly by the opening kinematics and the thermal structure of the lithosphere during rifting basement structure and strain rate appear to have exerted little control on the style of margin extension although basement architecture appears to control the trend of the initial rifts the positions of fracture zones which separate narrow and wide segments and the rate of lateral rift propagation

reactive transport modelling of mine tailings columns with capillarity induced high water saturation for preventing sulfide oxidation abstract a series of laboratory column tests on reactive mine tailings was numerically simulated to study the effect of high water saturation on preventing sulfide mineral oxidation and acid mine drainage amd the approach also known as an elevated water table ewt is a promising alternative to full water covers for the management and closure of sulfidic tailings impoundments and for the long term control of acid mine drainage the instrumented columns contained reactive tailings from the louvicourt mine quebec and were overlain by a protective sand cover over a 13 19 month period the columns were exposed to atmospheric o 2 and flushed approximately every month with demineralized water a free draining control column with no sand cover was also used during each cycle water table elevations were controlled by fixing the pressure at the column base and drainage water was collected and analyzed for ph and eh major ions and dissolved metals fe zn cu pb and mg the columns were simulated using the multi component reactive transport model min3p which solves the coupled nonlinear equations for transient water flow o 2 diffusion advective dispersive transport and kinetic geochemical reactions physical properties and mineralogical compositions for the material layers were obtained from independent laboratory data the simulated and observed data showed that as the water table elevation increased the effluent ph became more neutral and so 4 and dissolved metal concentrations decreased by factors on the order of 10 2 10 3 it is concluded that water table depths less than or equal to one half of the air entry value aev can keep mine tailings sufficiently saturated over the long term thus reducing sulfide oxidation and amd production

a predictive model for the water retention curve application to tailings from hard rock mines it is now commonly recognized that many geotechnical problems are related to the unsaturated behavior of soils such is the case for phenomena occurring at shallow depths in the so called vadose zone unsaturated flow is also very important for the management of milling wastes especially for seepage in and around tailings ponds suction properties constitute a key element for the functional representation of unsaturated flow conditions these are usually shown by the water retention curve which gives the relationship between volumetric water content theta and matric suction psi laboratory measurements have been performed on three different tailings from hard rock mines in the abitibi region to study the effect of various factors including grain size and porosity on the theta psi relationship the main results are presented in this paper the data are first analyzed using a well known descriptive model the experimental results are also used to estimate the air entry value according to vario

effectiveness of various cover scenarios on the rate of sulfide oxidation of mine tailings long term environmentally sound disposal of the millions of tons of mining residue is a serious challenge to the international mining industry this paper evaluates through a numerical investigation the potential performance of desulfurized tailings as a cover material for the reduction of acidic drainage from sulfidic tailings this evaluation is facilitated through a comparison of various cover types as decommissioning options the cover types considered consist of a desulfurized tailings material cover exposed to ambient climate conditions a water cover flooded tailings and a combination cover type as part of the evaluation of cover performances the effect of climatic variability on the potential rate of sulfide oxidation in tailings with an open ground surface was also assessed the numerical analysis was conducted using the model pyrox which was modified to allow for variably saturated conditions time varying moisture contents and to account for the temperature dependence of henry s law and gas diffusion in the case study presented here the benign cover material consists of a low sulfide waste stream cassiterite float tails cft a by product of the production of tin concentrate cassiterite sno2 modelling results after a simulation period of 100 years indicate that a water cover alone or an exposed cft cover alone are both less effective options than the combined cover type a water cover alone leads to a reduction of approximately 99 1 in the oxidation rate relative to uncovered tailings while the combined cover type results in the lowest potential extent of sulfide oxidation after mine closure an approximately 99 8 reduction importantly a cft cover exposed to ambient environmental conditions can still substantially reduce the sulfide oxidation rate by approximately 75 82 over a 100 year time period relative to uncovered tailings variability in precipitation and hence percent saturation of the surface layer had less of an effect on the potential sulfide oxidation rate than did the cover type the performance of the exposed cft cover varied by less than 10 within the range of climatic conditions expected at the renison bell mine site in southwest tasmania australia although the modelling results indicate that the combined water and cft cover is the best option this approach achieves only a minor improvement over the water cover alone

multichannel seismic images of the oceanic crust s internal structure evidence for a magma chamber beneath the mesozoic mid atlantic ridge near the base of the mesozoic oceanic crust of the western north atlantic we recognize a discontinuous diffractive layer horizon r moho is a distinct uniform and more continuous horizon beneath variations in seismic thickness of the crust occur primarily through changes in the r to moho layer the maximum thickness occurring midway between fracture zones we propose that horizon r may mark the top of a layered cumulate sequence that formed on the floor of a relatively persistent crustal magma chamber the crust having minimum thickness is spatially restricted to a small strip near fracture zones and comprises only the super r unit we further propose that this implies the sustained accretion of normal oceanic crust at the mesozoic mid atlantic ridge required the presence of a magma chamber

northeastern brazilian margin regional tectonic evolution based on integrated analysis of seismic reflection and potential field data and modelling abstract integration of regional seismic reflection and potential field data along the northeastern brazilian margin complemented by crustal scale gravity modelling is used to reveal and illustrate onshore offshore crustal structure correlation the character of the continent ocean boundary and the relationship of crustal structure to regional variation of potential field anomalies the study reveals distinct along margin structural and magmatic changes that are spatially related to a number of conjugate brazil west africa transfer systems governing the margin segmentation and evolution several conceptual tectonic models are invoked to explain the structural evolution of the different margin segments in a conjugate margin context furthermore the constructed transects the observed and modelled moho relief and the potential field anomalies indicate that the reconcavo tucano and jatoba rift system may reflect a polyphase deformation rifting mode associated with a complex time dependent thermal structure of the lithosphere the constructed transects and available seismic reflection profiles indicate that the northern part of the study area lacks major breakup related magmatic activity suggesting a rifted non volcanic margin affinity in contrast the southern part of the study area is characterized by abrupt crustal thinning and evidence for breakup magmatic activity suggesting that this region evolved partially with a rifted volcanic margin affinity and character

uniform accretion of oceanic crust south of the garrett transform at 14 15 s on the east pacific rise using migrated common depth point reflection profiles we find the structural differences along the ultrafast spreading 150 mm yr east pacific rise south of the garrett fracture zone are second order suggesting a remarkably uniform process of crustal accretion the rise axis south of the garrett transform is underlain by a narrow 1 0 km melt lens which shows great along strike continuity the depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading east pacific rise at 9 30 n this observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent melt sill width however shows little variation along the east pacific rise suggesting no dependence of magma chamber size on spreading rate the melt reservoir decreases in width toward across the 14 27 s ridge axis discontinuity by a modest 250 300 m and appears to be continuous across this feature given the small aspect ratio 1 0 km by 50 m by tens of kilometers of the axial melt lens the previously recorded jump in mgo content across the 14 27 s offset is likely the result of a mixing boundary which is sustained through an along strike impedance in convection wide angle reflections originating at the base of seismic layer 2a assumed to coincide with the extrusive layer reveal a twofold to threefold increase 200 250 to 500 600 m in thickness within 1 2 km of the rise axis the pattern of extrusive thickening imaged south of the garrett transform is similar to that observed along the slower spreading 110 120 mm yr east pacific rise at 9 n outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile this implies a degree of temporal stability of the along strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone the inferred uniformity of off axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14 27 s discontinuity are caused by long term reductions in magma supply second order differences in the style of extrusive thickening may be related to structural differences within the low velocity zone underlying the rise axis and or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis images of mono on cross axis profiles may be traced to within 1 0 km of the melt sill edge this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber

comment on a new scheme for the opening of the south atlantic ocean and the dissection of an aptian salt basin by trond h torsvik sonia rousse cinthia labails and mark a smethurst p torsvik et al recently presented a revised model for the opening of the south atlantic ocean according to these authors this new plate tectonic model is internally consistent and consistent with globally balanced plate motion solutions and takes into account realistic intraplate deformation however this model shows a number of kinematic and geological problems that we underline here together with some comparisons with the new kinematic model recently proposed by moulin et al

the african plate a history of oceanic crust accretion and subduction since the jurassic we present a model for the jurassic to present evolution of plate boundaries and oceanic crust of the african plate based on updated interpretation of magnetic gravity and other geological and geophysical data sets location of continent ocean boundaries and age and geometry of old oceanic crust jurassic and cretaceous are updated in the light of new data and models of passive margin formation a new set of oceanic palaeo age grid models constitutes the basis for estimating the dynamics of oceanic crust through time and can be used as input for quantifying the plate boundary forces that contributed to the african plate palaeo stresses and may have influenced the evolution of intracontinental sedimentary basins as a case study we compute a simple model of palaeo stress for the late cretaceous time in order to assess how ridge push slab pull and horizontal mantle drag might have influenced the continental african plate we note that the changes in length of various plate boundaries especially trenches do not correlate well with absolute plate motion but variations in the mean oceanic crust age seem to be reflected in acceleration or deceleration of the mean absolute plate velocity

asymmetric deep crustal structure across the juan de fuca ridge a multichannel line was shot across the endeavour segment of the juan de fuca ridge careful processing of 25 km of the line centered about the rise axis shows that young crustal structure is asymmetric an intermittent reflector from the top 1 km of crust follows topography on both the pacific and the juan de fuca plates it could be either the pillow basalt diabase dike contact or a metamorphic front within the pillow basalts a reflection exists 2 3 km below the rift valley floor diffractions from its edges indicate that the rift valley9s velocity structure is markedly different from the structure of the flanks the deep crustal structure is highly asymmetric moho exists within 4 km of the rift valley on the pacific plate in contrast to the juan de fuca plate which shows a reflector at intermediate depths dipping away from the rise to reach moho depth 12 km from the rift valley this reflector9s origin is enigmatic we speculate that the thermal history and structure of the two plates differ because of local differences in mantle structure caused by either ridge migration or a melting anomaly

nineteenth canadian geotechnical colloquium the soil water characteristic curve a historical perspective the constitutive relationship between water content or degree of saturation and suction is called the soil water characteristic curve the soil water characteristic curve provides a conceptual framework in which the behavior of unsaturated soils can be understood a historical review illustrates how the work of early researchers in soil science and geotechnical engineering laid the foundation for our current understanding of this relationship key elements of these early studies were a conceptual understanding of the soil water characteristic curve as a relationship between the mass or volume of water stored within the soil and the energy in the water phase it was on the basis of this conceptual model that current methods of measuring the soil water characteristic curve were developed interpretative models for the distribution and geometry of the water phase in an unsaturated soil based on the capillary model have provided a useful conceptual model for understanding the effects of soil texture gradatio

propagation as a mechanism of reorientation of the juan de fuca ridge we present a revised model of the tectonic evolution of the juan de fuca ridge by propagating rifting the new model has three different relative rotation poles covering the time intervals 17 0 8 5 ma 8 5 5 0 ma and 5 0 ma to the present the rotation pole shifts at 8 5 and 5 0 ma imply clockwise shifts in the direction of relative motion of 10 to 15 at each of these shifts the pattern of propagation reorganizes and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges the model containing a total of seven propagation sequences shows excellent agreement with the isochrons inferred from the magnetic anomaly data except in areas complicated by the separate explorer and gorda plates the agreement between model and data near the explorer plate breaks down abruptly at an age of about 5 ma indicating that the probable cause of the rotation pole shift at that time was the separation of the explorer plate from the juan de fuca plate

south atlantic volcanic margins the early cretaceous south atlantic continental break up and initial sea floor spreading were accompanied by large scale transient volcanism emplacing the parana etendeka continental flood basalts and voluminous extrusive constructions on the conjugate margins south of the torres arch abutment plateau on the north namibia margin we interpret four main tectono magmatic crustal units 1 oceanic crust 2 thickened oceanic crust covered by huge seaward dipping wedges 3 a c 150 km wide break up related rift zone partly covered by the dipping wedges and 4 thicker continental crust partly deformed by palaeozoic extension east of the early cretaceous rift similar settings also characterize other south atlantic margin segments we infer an up to 300 km wide and 2400 km long rift zone representing lithospheric extension leading to breakup and formation of the south atlantic volcanic margins comparison with other volcanic margins demonstrates in spite of local and regional differences gross similarities in tectono magmatic style crustal units and dimensions

modelling the effect of chemical heterogeneity on acidification and solute leaching in overburden mine spoils abstract the generation of acid mine drainage from overburden spoil piles at open pit lignite mines is impacting the quality of groundwater and surface water bodies in large parts of the lusatian mining area in germany values of ph as low as 1 have been observed in the groundwater after decommissioning mine pits are generally converted to lakes which may also be acidic owing to the acidic groundwater discharge the acidic effluent is generated by sulphide oxidation in the unsaturated zone of the spoil pile which generally extends to large depths as a result of dewatering the long term evolution of the acidification is still largely unknown our research focuses on the effects of physical and chemical heterogeneity caused by mixing of soil materials that may have already been oxidized to different degrees during the deposition of the spoil pile processes considered include variably saturated groundwater flow oxygen diffusion in the soil gas kinetic pyrite oxidation and acidic effluent generation advective dispersive transport of the aqueous components equilibrium geochemical reactions between the chemical components and the soil minerals and possible buffering and acid neutralization several existing numerical codes were coupled to represent the complete set of processes simulations were carried out in one and two dimensions using representative characteristics of mine spoil piles with the two dimensional representation being based on spatially heterogeneous random fields of hydraulic conductivity and sulphide mineral fractions results show the long term evolution of the oxidation front the mass flux of oxidation products and the effects of system heterogeneity under conditions of constant flow the system is found to return to neutral conditions over a time period on the order of several decades further work including sensitivity analyses with respect to the controlling parameters and model calibration using site specific field data will be necessary to obtain a full understanding of the complex interacting mechanisms the simulation approach shows promise as a potentially useful tool for predicting the effects of sulphide oxidation on the water quality of pit lakes

an experimental study of iron sulfides weathering under simulated calcareous soil conditions in calcareous sites hard rock mining activities release pyrite fes2 pyrrhotite fe1 xs and other sulfides to soils the sulfides then undergo weathering processes generating acid rock drainage and secondary compounds despite the potentially important environmental impacts very few studies have considered the mechanisms of pyrite and pyrrhotite weathering and the transformation of secondary compounds under neutral alkaline carbonated conditions in this study we used an experimental approach combining electrochemical microscopic and spectroscopic techniques to examine the interfacial processes involved in pyrite and pyrrhotite weathering under simulated calcareous soil conditions the results showed an initial oxidation step with the formation of variable amounts of surface sulfur compounds e g polysulfides sn2 and elementary sulfur s0 and acid generation leading to significant modification of the oxidative behavior of the minerals the surface changes that occurred as a result of mineral weathering provoked transient enhancement of pyrite reactivity and progressive passivation in the pyrrhotite system iron sulfides weathering was found to involve the formation of an intermediate siderite feco3 like compound preceding the predominant formation of k jarosite k fe3 so4 2 oh 6 and or ferric oxyhydroxide feooh compounds depending on the surface acid condition reached in the systems mechanisms of pyrite and pyrrhotite weathering in calcareous soils are suggested on the basis of surface characterization and chemical analysis of the leachates generated and the environmental implications are discussed

thrusting in oceanic crust during continental drift offshore niger delta equatorial africa 1 two and three dimensional 3 d seismic reflection data acquired over oceanic crust in the deepwater west niger delta reveal convincing evidence for compressional tectonics during oceanic crustal spreading using the 3 d seismic data set we describe numerous inclined seismic reflections that dissect the entire oceanic crust from the top of the crust to the level of the moho that are interpreted as thrusts thrust propagation results in the development of associated hanging wall anticlines and footwall synclines these structures are orthogonal to and clearly postdate normal faults that formed during the accretion of oceanic crust during continental drift and strike at right angles to them the charcot ridge is located 140 km south of these thrusts and is a significantly larger structure it is a triangular shaped uplifted region of oceanic crust measuring 80 by 150 km and is located along the ne sw oriented charcot fracture zone two interpretations are possible for the role of the fracture zone in the development of the charcot ridge 1 a thin skinned model whereby the oceanic crust west of the fracture zone has been thrust southeastward with detachment occurring close to the level of the moho the ridge forms as a result of translation and folding above a crustal scale ramp flat thrust geometry 2 a thick skinned model where there is no detachment close to the moho with the thrust fault being much steeper penetrating the crust and probably the mantle lithosphere in this interpretation the structure formed owing to the compressional reactivation of the fracture zone approximate dating of onlapping reflections on either side of the ridge constrains the timing of its formation as between 25 and 120 ma ago the charcot ridge represents one of the largest thrust structures to be identified in a passive margin setting many other compressional folds with the same orientation formed to the northeast in the benue trough probably during the santonian as a result of a change in the spreading direction during south atlantic rifting we speculate that the same causal mechanism applies for the formation of the charcot ridge

lateral and depth variation of loess organic matter overprint related to rhizoliths revealed by lipid molecular proxies and x ray tomography abstract terrestrial sediments like loess are well known for their paleoenvironmental significance although organic carbon contents are commonly very low loess and organic matter lom thereof is regarded as important terrestrial archive for vegetation and climate during deposition however the lom signal is prone to contamination by om of other age and origin than the synsedimentary vegetation e g by postsedimentary deep rooting plants we hypothesized that the influence of rhizosphere effects related to deep rooting plants varies with depth in quality quantity and distance to the former root the 13 m thick late pleistocene loess paleosol sequence at nussloch sw germany contains rhizoliths calcified roots of holocene age with the carbonatic encrustation leading to preservation of former root deposits rhizoliths allow for assessment of rhizosphere processes that occurred during the root s lifetime several horizontal transects comprising rhizoliths surrounding loess rhizoloess up to a distance of 11 cm from rhizoliths and root free reference loess from different depth intervals were analyzed for their carbon c alkane and fatty acid fa composition alkane proxies like carbon preference index cpi average chain length acl as well as n c27 n c31 and n c29 n c31 ratios indicated grass vegetation as origin of lom while rhizoliths derived from woody vegetation several lipid molecular proxies e g short chain long chain alkanes long chain alkane composition and long chain very long chain fa indicated the incorporation of considerable amounts of root and rhizomicrobial om the rhizosphere effect i e postsedimentary overprint of initial lom in the vicinity of roots was not restricted to few mm around the former root but notable to distances of at least 5 cm and possibly more this was confirmed by x ray tomography analyses enabling identification of small calcified root remains and non calcified root channels in the former rhizosphere which could not be identified under field conditions in depth intervals with high rhizolith frequency this entailed lateral overprint of large parts of loess which in depths of 1 95 and 3 2 m below present surface can cover the whole loess due to extension of the former rhizosphere of 5 cm regarding the high rhizolith frequency maximizing at 200 m 2 at 2 6 m depth at nussloch these findings raise the importance of deep rooting plants as potential source for significant postsedimentary overprint of loess paleosol sequences

geochemical transport modelling of drainage from experimental mine tailings cells covered by capillary barriers the use of covers with capillary barrier effects ccbes for reducing acid mine drainage amd from sulphidic mine tailings is simulated using the min3p finite volume model for coupled groundwater flow o2 diffusion and multi component reactive transport the model is applied to simulate five pilot scale in situ test cells containing reactive tailings from the manitou mine site val d or que canada four of the cells were constructed with ccbes over the tailings while the fifth tailings cell was left uncovered observed and simulated discharge from the base of each cell showed that the capillary barrier covers significantly reduced sulphide oxidation and amd compared to acidic discharge from the uncovered cell discharge from the four ccbe covered cells had neutral ph levels and 1 7 orders of magnitude lower concentrations of so4 fe zn cu and al the simulations showed that the moisture retaining layer of the ccbes reduced amd by inhibiting o2 diffusion into the underlying reactive wastes provided the moisture retention layer of the ccbe remains close to saturation its thickness had a relatively minor effect under such near saturated conditions o2 availability is limited by its diffusion rate through the bulk porous medium and not by the diffusion rate through the oxidized grain shells the model is providing important new insights for comparing design alternatives for reducing or controlling amd

absolute reflection coefficients from deep seismic reflections abstract many deep seismic reflection profiles show bright sub horizontal layered reflections in the lower continental crust accurate estimates are obtained for normal incidence reflection coefficients for the brightest of these reflections by using a marine seismic acquisition system that has been calibrated using deep water sea bottom multiples minimum bounds are placed upon these reflection coefficients by assuming minimum losses due to scattering anelastic attenuation and geometric spreading reflection coefficients of around 0 1 are obtained from the layered lower crust and around 0 15 from the moho it is argued that constructive interference between successive reflections from a layered sequence of reflectors is often unimportant consequently these coefficients can be used to place useful limits on the absolute acoustic impedance contrasts present at depth