effective viscoelastic behaviour of rock mass with double scale discontinuities summary effective viscoelastic behaviour of a rock mass with double scale discontinuities is investigated the rock mass is modelled as a system of microdefected rock traversed by macrojoints the effect of microdefects on the dynamic behaviour is evaluated by a viscoelastic equivalent medium model implicitly whereas the macrojoints in the rock mass are modelled explicitly as physical discontinuities therefore a homogenization discontinuity combination approach is proposed to investigate the effective viscoelastic behaviour of rock mass under dynamic loading the effective wave propagation coefficients and the effective viscoelastic complex moduli storage modulus and loss modulus are analytically derived the effects of the double scale discontinuities on the energy dissipation and the phase velocity are also discussed it is shown that the effective behaviour of the rock mass with double scale discontinuities is two phase viscoelastic microdefect and macrojoint have different frequency sensitive ranges and the effective viscoelastic behaviour is highly related to the stiffness of the discontinuities

fluid overpressure estimates from the aspect ratios of mineral veins abstract several hundred calcite veins and mostly normal faults were studied in limestone and shale layers of a mesozoic sedimentary basin next to the village of kilve at the bristol channel sw england the veins strike mostly e w 239 measurements that is parallel with the associated normal faults the mean vein dip is 73 n 44 measurements field observations indicate that these faults transported the fluids up into the limestone layers the vein outcrop trace length 0 025 10 3 m and thickness 0 1 28 mm size distributions are log normal taking the thickness as the dependent variable and the outcrop length as the independent variable linear regression gives a coefficient of determination goodness of fit of r2 0 74 significant with 99 confidence but natural logarithmic transformation of the thickness length data increases the coefficient of determination to r2 0 98 indicating that nearly all the variation in thickness can be explained in terms of variation in trace length the geometric mean of the aspect length thickness ratio 451 gives the best representation of the data set with 95 confidence the true geometric mean of the aspect ratios of the veins lies in the interval 409 497 using elastic crack theory appropriate elastic properties of the host rock and the mean aspect ratio the fluid overpressure that is the total fluid pressure minus the normal stress on the fracture plane at the time of vein formation is estimated at around 18 mpa from these results and using the average host rock and water densities the depth to the sources of the fluids below the present exposures forming the veins is estimated at between around 300 m and 1200 m these results are in agreement to those obtained by independent isotopic studies and indicate that the fluids were of rather local origin probably injected from sill like sources water sills inside the sedimentary basin

deep seismic reflection images of the wharton basin oceanic crust and uppermost mantle offshore northern sumatra relation with active and past deformation 1 we present deep seismic reflection images along two profiles collected in 2006 in the wharton basin offshore northern sumatra the main profile is located subparallel to the sumatran trench at a distance of 32 66 km faulting of the entire sedimentary section strike slip deformation sometimes accompanied by a dip slip component is imaged over two fracture zones of the extinct wharton spreading center that prior studies have shown to be reactivated as left lateral faults the western fracture zone is associated with a wide region of strong basement topography a difference in crustal thickness of 1 5 km and an age offset of 9 ma the epicenters of the 11 april 2012 mw 8 6 great strike slip earthquake its mw 7 2 foreshock and mw 8 2 aftershock align along this major structure 100 km south of the profile intersection our high quality long offset seismic reflection data also reveal bright dipping reflections extending down to a maximum of 24 km into the oceanic mantle 37 km below sea level apparent dips are mostly 25 35 corresponding to 30 55 along either n s to nne ssw or e w to wnw ese directions which encompass the directions of plate fabric and nodal planes of the mw 8 6 event we suggest that these enigmatic reflections arise from presently inactive dip slip fault planes reaching for the deepest ones to the base of the brittle layer possible origins include extension related to plate bending or an episode of now inactive thrust type deformation reactivating paleonormal faults similar to that taking place in the central indian basin

mercury emissions and stable isotopic compositions at vulcano island italy abstract sampling and analyses methods for determining the stable isotopic compositions of hg in an active volcanic system were tested and optimized at the volcanic complex of vulcano aeolian islands italy condensed gaseous fumarole hg fum t plume gaseous elemental hg g 0 and plume particulate hg p ii were obtained at fumaroles f0 f5 f11 and fa the average total hg emissions based on hgt so2 in condensed fumarolic gases and plumes range from 2 5 to 10 1 kg y 1 in agreement with published values ferrara r mazzolai b lanzillotta e nucaro e pirrone n 2000 volcanoes as emission sources of atmospheric mercury in the mediterranean basin sci total environ 259 1 3 115 121 aiuppa a bagnato e witt m l i mather t a parello f pyle d m martin r s 2007 real time simultaneous detection of volcanic hg and so2 at la fossa crater vulcano aeolian islands sicily geophys res lett 34 l21307 plume hg p ii increases with distance from the fumarole vent at the expense of hg g 0 and indicates significant in plume oxidation and condensation of fumarole hg fum t relative to the nist srm 3133 hg standard the stable isotopic compositions of hg are 202hg fum t 0 74 0 18 2sd n 4 for condensed gaseous fumarole hg fum t 202hg g 0 1 74 0 36 2sd n 1 for plume gaseous elemental hg g 0 at the f0 fumarole and 202hg p ii 0 11 0 18 2sd n 4 for plume particulate hg p ii the enrichment of hg p ii in the heavy isotopes and hg g 0 in the light isotopes relative to the total condensed fumarolic hg fum t gas complements the speciation data and demonstrates a gas particle fractionation occurring after the gas expulsion in ambient t atmosphere a first order rayleigh equilibrium condensation isotope fractionation model yields a fractionation factor cond gas of 1 00135 0 00058

erosion record in lake la thuile sediments prealps france evidence of montane landscape dynamics throughout the holocene lake la thuile in the northern french prealps 874 m a s l provides an 18 m long sedimentary sequence spanning the entire lateglacial holocene period the high resolution multi proxy sedimento

deformation and fault activity in space and time in high resolution numerical models of doubly vergent thrust wedges a discrete element model is used to investigate the manner in which deformation and fault activity change in space and time during the development of a doubly vergent thrust wedge in the upper crust deformation is a result of shortening at a subduction slot in the base of the model a configuration which produces a dynamic backstop within the cohesionless frictional cover material a series of experiments with differing basal decollement friction are performed the distinct manners in which thrust wedges grow and the variability of fault development and activity in space and time are then examined both predicted large scale wedge geometries and individual fault fold structures are similar to those observed in sandbox models and show the complex manner in which shortening is accommodated and localized during the development of the thrust wedge when compared to a sandbox model with similar boundary conditions model results are strikingly similar in all cases deformation initiates above the subduction slot with the formation of an axial zone the wedge is then developed by displacement on a retro wedge thrust and propagation of deformation into the pro wedge region models with low coefficients of basal friction typically develop wide shallow wedges with distributed spaced deformation and rather symmetric box like structures whereas those with high coefficients of basal friction develop narrower steeper wedges consisting of a series of stacked pro wedge thrust sheets and a high displacement retro wedge thrust in general fault initiation and linkage is extremely complex in our models with several smaller faults operating until linkage occurs to form a major through going structure of particular interest is the observation that many of the faults do not develop at the basal decollement and propagate upwards through the cover but rather initiate at high levels in the cover and propagate link downwards with other deeper structures results also indicate the utility of the discrete element approach in modelling large displacement complex deformation of geological materials

deflection of dykes into sills at discontinuities and magma chamber formation abstract many rift zones contain composite volcanoes central volcanoes most of which are supplied with magma from shallow crustal magma chambers volcanotectonic studies of eroded rift zones such as in iceland as well as geophysical studies indicate that many shallow magma chambers evolve from sills here i provide field description of sills and extinct magma chambers in the quaternary and tertiary palaeorift zones of iceland focusing on the felsic pluton of slaufrudalur southeast iceland as a fine example of a magma chamber developed through the injection of sills in this paper i review analyse and develop further two related mechanisms for the deflection of dykes into sills and thus the potential initiation of a magma chamber namely a cook gordon debounding delamination and b stress barriers and propose a third mechanism c favourable material toughness ratios due to elastic mismatch difference in young s moduli or stiffnesses of layers in contact in the cook gordon mechanism a weak contact opens up as a result of dyke induced tensile stress this mechanism is likely to operate primarily at shallow depths as is supported by field observations and numerical models a stress barrier is a layer where the local stress is unfavourable to a particular type of rock fracture here a dyke field observations and numerical models show that on meeting a stress barrier a dyke either changes into a sill or becomes arrested the material toughness mechanism indicates that when the upper layer at a contact has the same or less stiffness than the lower layer hosting the dyke there is little tendency for the dyke to become deflected into the contact however when the upper layer is stiffer such as when a stiff basaltic lava flow is on the top of a soft pyroclastic layer the dyke tends to become deflected into the contact to form a sill field results suggest that all these mechanisms may cause dyke deflection into sills and may operate together particularly where the rock consists of alternating soft and stiff layers since the quaternary lava pile in iceland has many more soft hyaloclastite layers than the tertiary lava pile these mechanisms were probably very efficient in generating sills and by implication shallow magma chambers during the quaternary this may be one reason why in comparison with the tertiary lava pile the quaternary lava pile contains so many sills and extinct plutons and active shallow magma chambers

spatial and temporal variation in the isotopic composition of mercury in the south river va abstract historic point source mercury hg inputs from industrial processes on the south river waynesboro virginia ended many decades ago but sediment and surface water hg concentrations remain elevated relative to the regional background to understand hg sources mobility and fate in the south river we analyzed total hg concentrations and hg stable isotope compositions of streambed sediments bank soils suspended particles filtered surface waters and channel margin hyporheic zone pore waters hg isotopes allow for the identification of three distinct hg end member inputs to the south river consisting of a regional background source and two isotopically distinct contaminant sources hydrologic conditions are demonstrated to have an influence on within channel hg isotope fractionation and hg partitioning with no observed isotopic discrimination between suspended particulate hg and filtered surface water hg during elevated flow conditions channel margin hyporheic zone porewaters had significantly higher 202hg values than surface waters 202hg 0 52 0 44 and 202hg 0 89 0 20 respectively mean 1sd a subset of porewaters exhibited mass independent fractionation signatures 199hg 0 33 0 06 200hg 0 19 0 03 mean 1sd that are suggestive of a precipitation derived origin for the dissolved hg pool sediments from a floodplain profile were analyzed to explore the temporal variation in hg isotopic composition within the south river indicating brief excursions up to 202hg 0 61 from the average composition observed in modern samples 202hg 0 52 0 09 by improving understanding of the spatial temporal and hydrologic conditions that contribute to variations in hg isotopic composition this study provides insights into the processes that control hg isotopic end member sources hg loading to the channel during elevated flows and hg fate in the south river

effects of internal structure and local stresses on fracture propagation deflection and arrest in fault zones the way that faults transport crustal fluids is important in many fields of earth sciences such as petroleum geology geothermal research volcanology seismology and hydrogeology for understanding the permeability evolution and maintenance in a fault zone its internal structure and associated local stresses and mechanical properties must be known this follows because the permeability is primarily related to fracture propagation and their linking up into interconnected clusters in the fault zone here we show that a fault zone can be regarded as an elastic inclusion with mechanical properties that differ from those of the host rock as a consequence the fault zone modifies the associated regional stress field and develops its own local stress field which normally differs significantly both as regard magnitude and orientation of the principal stresses from the regional field the local stress field together with fault rock heterogeneities and interfaces discontinuities fractures contacts determine fracture propagation deflection along discontinuities interfaces and arrest in the fault zone and thereby its permeability development we provide new data on the internal structure of fault zones in particular the fracture frequency in the damage zone as a function of distance from the fault core new numerical models show that the local stress field inside a fault zone modelled as an inclusion differ significantly from those of the host rock both as regards the magnitude and the directions of the principal stresses also when the mechanical layering of the damage zone due to variation in its fracture frequency is considered the numerical models show abrupt changes in local stresses not only between the core and the damage zone but also within the damage zone itself abrupt changes in local stresses within the fault zone generate barriers to fracture propagation and contribute to fracture deflection and or arrest also analytical solutions of the effects of material toughness the critical energy release rate of layers and their interfaces show that propagating fractures commonly become deflected into and often arrested at the interfaces generally fractures propagating from a compliant soft layer towards a stiffer one tend to become deflected and arrested at the contact between the layers whereas fractures propagating from a stiff layer towards a softer one tend to penetrate the contact thus it is normally easier for fractures to propagate from the host rock into the damage zone than vice versa similarly it is easier for fractures to propagate from the outer stiffer parts of the damage zone to the inner softer parts and from the stiff host rock to the outer damage zone than in the opposite directions these conclusions contribute to increased understanding as to how fractures propagate and become arrested within fault zones and how the fault zone thickness is confined at any particular time during its evolution

tracing anthropogenic hg and pb input using stable hg and pb isotope ratios in sediments of the central portuguese margin abstract three short marine sediment cores from the cascais submarine canyon csc cores 252 32 and 252 35 and the estremadura spur core 252 16 on the central portuguese margin were analysed for hg pb al and mn concentrations and both pb and hg stable isotope compositions in order to reconstruct trends in hg and pb sources and accumulation sediment ages were determined based on 210pb measurements the studied cores reveal increasing hg and pb concentration independent of grain size variations since the middle of the 19th century towards the present concomitantly a decreasing trend of 206pb 207pb to less radiogenic values towards the surface was found indicating increasing anthropogenic inputs the lowest values of 202hg were generally observed in the older sediments characterized by low hg and pb concentrations suggesting a low input of anthropogenic metals odd mass number 199hg and 201hg hg isotopes in sediment samples from core 252 16 were characterized by positive mass independent fractionation mif while recent sediments from cores 252 32 and 252 35 did not reveal significant mif probably reflecting both the proximity to the source of anthropogenic hg contamination tagus estuary and the importance of the csc as a particle carrier the multi tracer approach based on both stable hg and pb isotopic signatures confirms anthropogenic hg and pb enrichment in recent marine sediments and also allows us to distinguish between areas dominated by detrital e g csc versus hemipelagic estremadura spur sedimentation

morphology and genesis of slow spreading ridges seabed scattering and seismic imaging within the oceanic crust a grid of 32 across axis and five axis parallel multichannel seismic mcs reflection profiles were acquired at an axial volcanic ridge avr segment at 57 45 n 32 35 w on the slow spreading reykjanes ridge mid atlantic ridge to determine the along axis variation and geometry of the axial magmatic system and to investigate the relationship between magma chamber structure the along axis continuity and segmentation of melt supply to the crust the development of faulting and the thickness of oceanic layer 2a seismic reflection profiles acquired at mid ocean ridges are prone to being swamped by high amplitude seabed scattered noise which can either mask or be mistaken for intracrustal reflection events in this paper we present the results of two approaches to this problem which simulate seabed scatter and which can either be used to remove or simply predict events within processed mcs profiles the 37 mcs profiles show clear intracrustal seismic events which are related to the structure of oceanic layer 2 to the axial magmatic system and to the faults which dismember each avr as it ages through its tectono magmatic life cycle and which form the median valley walls the layer 2a event can be mapped around the entirety of the survey area between 0 1 and 0 5 s two way traveltime below the seabed being thickest at avr centres and thinning both off axis and along axis towards avr tips both avr parallel and ridge parallel trends are observed with the pattern of on axis layer 2a thickness variation preserved beneath relict avrs which are rafted off axis largely intact each active avr is underlain by a mid crustal melt lens reflection extending almost along its entire length similar reflection events are observed beneath the offset basins between adjacent avrs these are interpreted as new avrs at the start of their life cycle developing centrally within the median valley the east west spacings of relict avrs and offset basins is 5 7 km corresponding to a life span of the order of 0 5 0 7 myr during which avrs appear to undergo multiple 20 60 kyr tectono magmatic cycles

microstructural observations on natural syntectonic fibrous veins implications for the growth process abstract syntectonic antitaxial and ataxial fibrous veins were investigated using sem microprobe cathodoluminescence cl and optical microscopy in antitaxial calcite veins fibres and surrounding selvage grew simultaneously with similar growth rates of crystallographically differently oriented grains new material precipitated at the vein margin in antitaxial and bi mineralic ataxial microstructures bridges of country rock material formed during vein growth in an initial en echelon vein system in our antitaxial and bi mineralic ataxial samples the spacing of solid inclusions does not reflect individual crack seal openings

a hundred year record of industrial and urban development in french alps combining hg accumulation rates and isotope composition in sediment archives from lake luitel this study reconstructs the history of multiple industrial and urban mercury hg emissions recorded in the sediment archive of lake luitel france from ad similar to 1860 to ad 2011 for this purpose we provide a well constrained short lived radionuclides continuous age depth relationship of the sediment sequence mean accumulation rate of 5 18 0 28 mm yr 1 with hg accumulation rates hg ar hg isotopic composition and extensive historical data hg ar were stable around 45 mu g m 2 y 1 from 1860 to wwi and rose to reach their maximum at the end of wwii 250 mu g m 2 y 1 followed by a gradual decreased to reach about 90 mu g m 2 y 1 in the current period normalization to a terrigenous hg proxy highlighted the dominance of atmospheric hg inputs to the lake the combination of hg ar with isotopic signatures through the use of binary mixing delta hg 199 vs 1 hg ar models and isotopic plots and comparison to literature data allowed us to identify the main industrial and urban historical inputs the major outcome of this study is that the hg mass independent fractionation mif signature did not enable the identification of particular anthropogenic sources but reflected an integrated pool of industrial and urban emissions which tend to shift to less negative mif values mean 0 15 0 04 during their period of maximum emissions temporal mif and hg ar variations depict the rising hg emissions from the industrial revolution 1860 1910 to the modern industrial and urban development period 1950 1980 mass dependent fractionation mdf signatures enabled the identification of major contributors in relation to their relative intensities lying between two endmember pools i the combustion activities smelters cement factories and urban heating with more negative delta hg 202 values and ii the chemical and electrometallurgical activities electrochemistry chlor alkali with higher delta hg 202 values unconformities of mif and mdf signatures were observed during wwi wwii and interwar period and were attributed to drastic and rapid changes in regional industrial activities finally recent laws regarding hg emissions 1998 2011 prove their efficiency as hg ar decreased with a return to more negative mif and mdf signatures such as during the industrial revolution period our study highlights that the combination of hg isotopic data with hg ar in sediment archives is a useful tool for reconstructing the history of anthropogenic hg emissions and has the potential to identifiy their relative contributions c 2016 elsevier b v all rights reserved

backscattered coherent noise and seismic reflection imaging of the oceanic crust an example from the rift valley of the mid atlantic ridge at 23 n seismic reflection surveys shot over oceanic crust are contaminated by source generated coherent noise that limits the identification and interpretation of subbasement reflections coherent noise arrivals are generated around the seismic line by scattering from the upper surface of the basaltic layer and by backscattering of turning waves from the underside of the same interface both types of arrival may be enhanced by conventional common midpoint cmp stacking some turning arrivals also exhibit negative moveout in a cmp gather due to the increase of crustal velocity with depth in unsedimented areas coherent arrivals scattered from the top of the seafloor can be aligned by dip moveout with or without a subsequent zero offset migration prestack and suppressed but turning arrivals will usually remain turning arrivals generated by linear features in the seafloor such as fault scarps appear on a stack section as nearly linear coherent arrivals the apparent dip of which is related to the features angle to the seismic line after migration these turning wave arrivals can extend close to the topography that produced them and continue to late times leading to the possibility that such arrivals might be incorrectly interpreted as intrusions that crosscut but do not offset the moho careful identification of crustal reflections on a seismic profile along the mark area rift valley indicates that the snake pit hydrothermal field marks the northern limit of identifiable crustal reflections at least some of which appear to be related to magma injection the recent extensive volcanism of the northern mark area may well be a result of faulting caused by the stong northward growth of the southern spreading center permitting melt to rise more easily to shallow crustal levels

a multichannel seismic investigation of upper crustal structure at 9 n on the east pacific rise implications for crustal accretion reprocessed multichannel seismic profiles from the 9 n segment of the east pacific rise reveal prominent shallow subbasement events these events are identified as wide angle reflections from the base of seismic layer 2a based upon modeling of expanding spread profile data and velocity functions the layer 2a reflections typically increase from 0 15 s after the seafloor reflection at the rise axis to 0 3 0 45 s within 1 2 km of the axis corresponding to an increase in layer thickness of 200 600 m no further systematic increase in layer thickness is observed although lateral variability of the order of a few hundred meters in thickness is observed at greater offsets from the rise axis however the intermittent character of the imaged layer 2a reflection is attributed to focusing and defocusing of energy by the seafloor bathymetry rather than necessarily to intrinsic lateral variability at the base of the layer the base of layer 2a is interpreted as corresponding to the transition between the extrusive section pillow basalts and sheet flows and a sheeted dike complex the rapid thickening of the layer near the rise axis is attributed to successive lava flows burying the initially shallow top of the sheeted dike complex as the layer passes through the neovolcanic zone lateral variability of layer 2a can significantly affect the imaging of the underlying axial magma chambers as average velocities within layer 2a are approximately half that of layer 2b for an along axis profile apparent along axis variability in the depth of the axial magma chamber is traced to variability in the thickness of layer 2a caused by wandering of the profile relative to axis within the resolution of the data the time delay of the magma chamber reflection relative to the base of layer 2a is constant

6 kyr record of flood frequency and intensity in the western mediterranean alps interplay of solar and temperature forcing abstract the high resolution sedimentological and geochemical analysis of a sediment sequence from lake savine western mediterranean alps france led to the identification of 220 event layers for the last 6000 years 200 were triggered by flood events and 20 by underwater mass movements possibly related to earthquakes that occurred in 5 clusters of increase seismicity because human activity could influence the flood chronicle the presence of pastures was reconstructed through ancient dna which suggested that the flood chronicle was mainly driven by hydroclimate variability weather reanalysis of historical floods allow to identify that mesoscale precipitation events called east return events were the main triggers of floods recorded in lake savine the first part of this palaeoflood record 6 4 kyr bp was characterized by increases in flood frequency and intensity in phase with northern alpine palaeoflood records by contrast the second part of the record i e since 4 kyr bp was phased with southern alpine palaeoflood records these results suggest a palaeohydrological transition at approximately 4 kyr bp as has been previously described for the mediterranean region this may have resulted in a change of flood prone hydro meteorological processes i e in the balance between occurrence and intensity of local convective climatic phenomena and their influence on mediterranean mesoscale precipitation events in this part of the alps at a centennial timescale increases in flood frequency and intensity corresponded to periods of solar minima affecting climate through atmospheric changes in the euro atlantic sector

subsurface fractures and their relationship to stress history in east texas basin sandstone abstract subsurface fractures were sampled with more than 400 m of core from the lower cretaceous travis peak formation a tabular sandstone and shale unit approximately 600 m thick that was deposited in the gradually subsiding gulf coast basin depth to the top of the formation in east texas now ranges between 1800 and 2900 m fractures in sandstone exhibit features that are due to intermittent fracturing and sealing crack seal which suggest that the fractures are the result of episodes of fluid pressure in excess of hydrostatic correlation of the vein mineral precipitation sequence with the diagenetic history and the results of previous 18 o studies of quartz cement indicates that fractures propagated at depths of between 900 and 1500 m during the migration of quartz precipitating fluids fracture permeability provides a mechanism for the passage of large fluid volumes recorded by the precipitation of extensive quartz cement in the formation fractures stopped propagating before maximum burial depth was achieved during approximately 1500 m of subsequent burial fractures either remained partly open or were sealed by postkinematic ankerite and clay minerals

book reviewthe techniques of modern structural geology vol 1 strain analysis j g ramsay and m i huber academic press london 1983 xiii 307 pp us 55 00 28 00 hardcover

the biogeochemistry of mercury at the sediment water interface in the thau lagoon 2 evaluation of mercury methylation potential in both surface sediment and the water column abstract methylation rates of mercury have been determined in both surface sediments and the water column of a shallow coastal lagoon thau france using in situ incubation experiments the experiments were conducted in order to evidence and evaluate the significance of such pathways on the fate of methylmercury mehg as influenced by both benthic and pelagic dynamics isotopically labelled hg species have been used as chemical tracers allowing the direct determination of specific methylation and demethylation yields each experimental method cores experiments and water experiments has been carefully evaluated in terms of sensitivity and reproducibility of the transformation rates and has been demonstrated as a powerful method to investigate transformation processes although mercury methylation in surface sediments is a major process significant mehg formation in the water column has been measured for the first time in a coastal environment in spring conditions methylation yields are found to be higher in the water column 6 3 than in sediments 0 8 1 3 area integrated rates for the experimental site demonstrate however that mehg is mostly produced in surface sediments with a formation rate of 12 nmol m 2 d 1 compared to 1 8 nmol m 2 d 1 in the water column biological characteristics of the incubated samples indicate that sediment and plankton microorganisms are involved in hg methylation which is thus directly associated to the pelagic benthic turnover occurring in the lagoon

an analysis leading to the reduction of scattered noise on deep marine seismic records the rough and uneven basaltic basement in deep marine environments functions as a potential source of scattering the source generated noise not only interferes with the weak and deep primary reflections but often resembles primary reflections thereby confusing the seismic interpretation an analysis of the arrival time of scattered noise indicates that nmo and apparent dip of scattered noise exhibit azimuth dependence for isotropic scattering in line scattering at ranges near 0 degree azimuth and sideswipe from 90 degree azimuth should dominate the seismic section actual marine data displayed in various data gathers illustrate these characteristics of scattered noise and also correct a common misconception about the appearance of scattered noise the analysis suggests the use of cdp stacking and velocity filtering to attenuate the scattered noise the combined effect shows a total reduction of scattered noise by 21 db for the frequency bandwidth from 20 to 32 hz

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

bedding plane slip in initial stages of fault related folding abstract frictional slip along bedding planes contributes to fault related folding of layered rocks we use numerical experiments to investigate the deformation of frictional bedding planes near dipping faults under layerparallel contraction and extension within the numerical experiments contraction boundary conditions produce asymmetric anticlines and extension produces asymmetric synclines the fold shape may be used to infer dip of the underlying fault in situations where the fault may not be observable additionally sense of slip along bedding planes may indicate proximity to the fault tip under uniform remote tectonic strain fault slip induces deformation in both the hangingwall and the footwall at depths as shallow as 1 km there is no significant difference between fold amplitudes in the hangingwall and the footwall this result is contrary to many kinematic models currently in use kinematic models of fault related folds commonly include the development of flat ramp and flat ramp flat fault geometries which may be attributed to initial ramp thrusting and later flat development our mechanical models show that fault flats may be produced from fault ramps due to slip along frictional bedding planes near the thrust fault tips our numerical experiments also evaluate joint initiation joints perpendicular to bedding are promoted in extensional environments we compare the results of the mechanical model to kinematic models of fault propagation folds and conclude that mechanical models offer important insights to better understand the folding process

a 14 500 year record of the accumulation of atmospheric mercury in peat volcanic signals anthropogenic influences and a correlation to bromine accumulation a 14 500 calendar year record of mercury accumulation rates has been obtained from an ombrotrophic peat bog in the swiss jura mountains the range of natural pre industrial mercury accumulation rates varied from 0 3 to 8 0 w gm 32 yr 31 during the late glacial and holocene climatic and volcanic signals were evident in the mercury record mercury accumulation rates increased by a factor of ca 5 during the younger dryas cold period short term spikes in mercury accumulation rates which correspond in time to known volcanic eruptions occur during the late boreal and older atlantic periods when volcanic influences on mercury deposition appear to have been intensified due to increased atmospheric humidity a correlation of mercury to bromine accumulation is observed throughout the preanthropogenic period during the holocene mercury accumulation only exceeded the range of this correlation for a few short periods of elevated mercury deposition which correspond to known volcanic eruptions during the older atlantic during historical times mercury accumulation rates have exceeded the range of the correlation of mercury to bromine continuously since ca 1330 ad this excess in mercury accumulation is interpreted as an indication of pollution during the industrial period mercury accumulation rates reached 107 6 w gm 32 yr 31 of which 84 was mercury that exceeded the correlation range mercury accumulation rates peaked again during the 1970s with 78 8 wg m 32 yr 31 early 20th century pollution appears to have been dominated by non swiss emissions from coal burning whereas swiss mercury emissions appear to have been the dominant pollution source during the mid and late 20th century current mercury accumulation rates at the site are similar to those ca 10 years ago with modern deposition rates being ca 15 times their prehistorical average anthropogenic emissions of reactive brominated compounds could be contributing to increased atmospheric deposition rates of mercury a 2002 elsevier science b v all rights reserved

complex vein systems as a data source in tectonics an example from the ugab valley nw namibia abstract neoproterozoic metaturbidites in the lower ugab domain namibia contain a complex network of four sets of quartz calcite veins overprinted by km scale folds associated with four regional foliations the veins formed by fluid overpressure predating the main deformation deformation structures developed at the junction of two mobile belts during the assembly of gondwana the ns kaoko belt and the ew trending damara belt km scale ns trending folds were initiated during ew constriction in the kaoko belt while their further development and all subsequent events are related to constriction in the ew damara belt with coeval sinistral strike slip in the kaoko belt deformation of the veins and development of four orthogonal foliations are due to gradual changes in the bulk tectonic framework rather than separate orogenic events the veins are deformed in a complex manner allowing a full 3d reconstruction of regional sequence of events the local complex tectonics could be reconstructed because of the perfect local exposure and the multitude of veins it illustrates the potential complexity of tectonic events and structural evolution in apparently simple slate belts

identification of contamination in a lake sediment core using hg and pb isotopic compositions lake ballinger washington usa abstract concentrations and isotopic compositions of hg and pb were measured in a sediment core collected from lake ballinger near seattle washington usa lake ballinger has been affected by input of metal contaminants emitted from the tacoma smelter which operated from 1887 to 1986 and was located about 53 km south of the lake concentrations and loadings of hg and pb in lake ballinger increased by as much as three orders of magnitude during the period of smelting as compared to the pre smelting period concentrations and loadings of hg and pb then decreased by about 55 and 75 respectively after smelting ended isotopic compositions of hg changed considerably during the period of smelting 202 hg 2 29 to 0 38 mean 1 23 n 9 compared to the pre smelting period 202 hg 2 91 to 2 50 mean 2 75 n 4 variations were also observed in 206 pb 207 pb and 208 pb 207 pb isotopic compositions during these periods data for 199 hg and 201 hg indicate mass independent fractionation mif of hg isotopes in lake ballinger sediment during the smelting and post smelting period and suggest mif in the ore smelted during the smelting process or chemical modification at some point in the past negative values for 199 hg and 201 hg for the pre smelting period are similar to those previously reported for soil peat and lichen likely suggesting some component of atmospheric hg variations in the concentrations and isotopic compositions of hg and pb were useful in tracing contaminant sources and the understanding of the depositional history of sedimentation in lake ballinger

dip moveout by fourier transform the conventional normal moveout nmo and common midpoint cmp stacking process enhances reflections having a particular moveout velocity while attenuating events such as multiple reflections having different moveout velocities unfortunately this process also acts as a dip filter applied to the cmp stack in other words nmo and stacking enhances reflections having a particular slope in the cmp stack while attenuating reflections having different slopes nmo and stacking like any dip filter degrades lateral resolution fortunately this dip filtering action can be suppressed by applying in addition to nmo a prestack process known variously as devilish prestack partial migration and dip moveout as the latter term implies this process is a dip dependent moveout correction that enables reflections from both horizontal and dipping reflectors to be stacked with the same nmo velocity stated another way nmo velocities estimated from dip moveout corrected seismograms are independent of the dips o

a three dimensional perspective on two dimensional dip moveout prestack migration in a constant velocity medium spreads an impulse on any trace over an ellipsoidal surface with foci at the source and receiver positions for that trace the same ellipsoid can be obtained by migrating a family of zero offset traces placed along the line segment from the source to the receiver the spheres generated by migrating the zero offset impulses are arranged to be tangent to the ellipsoid the resulting nonstandard moveout equation is equivalent to two consecutive moveouts the first requiring no knowledge of velocity and the second being standard normal moveout nmo the first of these is referred to as dip moveout dmo because this dmo nmo algorithm converts any trace to an equivalent set of zero offset traces it can be applied to any ensemble of traces no matter what the variations in azimuth and offset may be in particular this three dimensional perspective on dmo can be used with multifold inline data then it becomes clear that velocity independent dmo operates on rad

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

seismic structure of the endeavour segment juan de fuca ridge correlations with seismicity and hydrothermal activity 1 multichannel seismic reflection data collected in july 2002 at the endeavour segment juan de fuca ridge show a midcrustal reflector underlying all of the known high temperature hydrothermal vent fields in this area on the basis of the character and geometry of this reflection its similarity to events at other spreading centers and its polarity we identify this as a reflection from one or more crustal magma bodies rather than from a hydrothermal cracking front interface the endeavour magma chamber reflector is found under the central topographically shallow section of the segment at two way traveltime twtt values of 0 9 1 4 s 2 1 3 3 km below the seafloor it extends approximately 24 km along axis and is shallowest beneath the center of the segment and deepens toward the segment ends on cross axis lines the axial magma chamber amc reflector is only 0 4 1 2 km wide and appears to dip 8 36 to the east while a magma chamber underlies all known endeavour high temperature hydrothermal vent fields amc depth is not a dominant factor in determining vent fluid properties the stacked and migrated seismic lines also show a strong layer 2a event at twtt values of 0 30 0 09 s 380 120 m below the seafloor on the along axis line and 0 38 0 09 s 500 110 m on the cross axis lines a weak moho reflection is observed in a few locations at twtt values of 1 9 2 4 s below the seafloor by projecting hypocenters of well located microseismicity in this region onto the seismic sections we find that most axial earthquakes are concentrated just above the magma chamber and distributed diffusely within this zone indicating thermal related cracking the presence of a partially molten crustal magma chamber argues against prior hypotheses that hydrothermal heat extraction at this intermediate spreading ridge is primarily driven by propagation of a cracking front down into a frozen magma chamber and indicates that magmatic heat plays a significant role in the hydrothermal system morphological and hydrothermal differences between the intermediate spreading endeavour and fast spreading ridges are attributable to the greater depth of the endeavour amc and the corresponding possibility of axial faulting

cover deformation above steep basement normal faults insights from 2d discrete element modeling abstract a discrete element model is used to investigate progressive cover deformation above a steep 70 basement normal fault the cover materials are homogenous with frictional material behavior in the model shown here both normal and reverse faults in the cover accommodate displacement on the underlying basement fault the earliest faults are curved reverse faults which propagate upwards from the basement fault tip into the proto hanging wall these are replaced progressively towards the footwall by subvertical to steep normal faults and finally by a normal fault which dips at an angle predicted by mohr coulomb theory thus most early secondary structures are located in the hanging wall of the final through going fault this structural evolution produces an asymmetric triangular zone of deformation above the basement fault tip which superficially resembles that associated with trishear however its progressive development is quite different results also emphasize that the occurrence of reverse faults in extensional settings is not diagnostic of inversion

effects of bedding on the dynamic indirect tensile strength of coal laboratory experiments and numerical simulation abstract dynamic indirect tensile tests were carried out by using a split hopkinson pressure bar shpb for coal sampled from the datong mine in china the principal purpose was to explore the influence of bedding structure in the coal on its dynamic indirect tensile strength however to resolve some contradictions x ray micro ct high speed optical imaging and a discrete element based modelling approach were combined to analyze the test results the x ray micro ct was used to detect the actual bedding structure in the coal the high speed imaging captured failure patterns of the specimens with different bedding directions and the numerical modelling was utilized to investigate the influence of different bedding structures on dynamic strength the shpb and numerical results illustrate that dynamic indirect tensile strength reliably correlates with impact velocity in addition the dynamic indirect tensile strength is not only influenced by the bedding direction but also by the roughness and discontinuity of the bedding based on these findings a method is developed to further process the test data including a model to describe the dynamic indirect tensile strength of datong coal

post deposition diffusion of 137cs in lake sediment implications for radiocaesium dating peak activities of radiocaesium 137cs in lake sediments have frequently been used to infer the ages of sediments deposited in the 1960s 137cs derived from nuclear bomb testing or in 1986 chernobyl derived 137cs records of the vertical distribution of 137cs in sediments can thus be used to provide accurate dates for a critical period in which palaeoecological reconstructions often overlap contemporary monitoring data however knowledge regarding how the distribution of 137cs in sediments is affected by post depositional processes is limited to interpretations based on the 137cs distribution in sediments sampled at a single given date this study assesses the extent to which the 137cs record in annually laminated varved lake sediments is affected by post depositional diffusion using 11 archived sediment cores sampled between 1986 and 2007 the sediment record reveals how chernobyl 137cs incorporated into the 1986 varve diffused downwards in the core at a decreasing rate over time whereas the surface sediments continued to receive inputs of 137cs mobilized from the catchment soils or lake margin in spite of these processes all cores post dating the chernobyl accident had a clear and well resolved peak in the 1986 varve justifying the use of this feature as a fixed chronostratigraphic feature because of the very high levels of chernobyl fallout at this site downwards migration of chernobyl 137cs has however completely masked the nuclear weapons 137cs fallout peak that had been clearly preserved in the 1964 varve of a pre chernobyl core sampled just three weeks before the chernobyl accident in consequence the weapons fallout marker is likely to be of little use for determining 137cs dates in areas strongly affected by high levels of chernobyl fallout

extension fracture propagation in rocks with veins insight into the crack seal process using discrete element method modeling 1 we investigate how existing veins interact with extension fractures in rocks using 3 d discrete element method models with a geometry inspired by tension tests with notched samples in a sensitivity study we varied a the angle between the vein and the bulk extension direction and b the strength ratio between host rock and vein material results show a range of vein fracture interactions which fall into different robust structural styles veins which are weaker than the host rock tend to localize fracturing into the vein even at high misorientation angles veins which are stronger than the host rock cause deflection of the fracture tip along the vein host rock interface fractures are arrested at the interface from weak to stronger material when propagating from a stronger to a weaker material macroscopic bifurcation of the fracture is common complex interactions are favored by a low angle between the vein and the fracture and by high strength contrast the structural styles in the models show good agreement with microstructures and mesostructures of crack seal veins found in natural systems we propose that these structural styles form the basis for criteria to recognize strength contrasts and stress of crack seal systems in nature

suppression of sea floor scattered energy using a dip moveout approach application to the mid ocean ridge environment multichannel seismic mcs images are often contaminated with in and out of plane scattering from the sea floor this problem is especially acute in the mid ocean ridge environment where sea floor roughness is pronounced energy shed from the unsedimented basaltic sea floor can obscure primary reflections such as moho and scattering off of elongated sea floor features like abyssal hills and fault scarps can produce linear events in the seismic data that could be misinterpreted as subsurface reflections moreover stacking at normal subsurface velocities may enhance these water borne events whose stacking velocity depends on azimuth and generally increases with time making them indistinguishable from subsurface arrivals to suppress scattered energy in deep water settings we propose a processing scheme that invokes the application of dip moveout dmo to deliberately increase the differential moveout between sea floor scattered and subsurface events thereby facilitating the removal of unwanted energy in the stacked section after application of dmo all sea floor scatterers stack at the water velocity while subsurface reflections like moho still stack at their original velocity the application of dmo in this manner is contrary to the intended use that reduces the differential moveout between dipping events and allows a single stacking velocity to be used unlike previous approaches to suppress scattered energy dip filtering is applied in the common midpoint cmp domain after dmo moveover our dmo based approach suppresses out of plane scattering and therefore is not limited to removal of in plane scattering as is the case with shot and receiver dip filtering techniques the success of our dmo based suppression scheme is limited to deep water a few kilometers of water depth for conventional offsets where the traveltime moveout of energy scattered from the sea floor has a hyperbolic moveout with a stacking velocity that depends on the cosine of the scatterer steering angle in a manner analogous to how the moveout of a dipping reflector depends on the dip angle the application of dmo based suppression to synthetics and mcs data collected along the southern east pacific rise demonstrates the effectiveness of our approach cleaner images of primary reflectors such as moho are produced even though present shot coverage along the east pacific rise is unduly sparse resulting in a limited effective spatial bandwidth

dem simulation of normal faults in cohesive materials abstract we performed discrete element method dem simulations of the formation of normal faults in a brittle cohesive material above a basement fault focusing on the formation of open fractures and cavities in areas of low normal stress i e near the surface of the model the simulations produce a wide range of fundamental structures similar to those observed in nature and in analogue models comparisons with these sandbox experiments show that the fundamental structures generated are similar between the analogue and the numerical models in a parameter study of the material properties and the model geometry we have investigated the effect of these parameters far beyond what is possible in analogue models over a range of parameters we show that the evolving structural style is robust i e all the models develop similar structures in a similar sequence results show a systematic dependence of some structural features such as location an style of faults on the simulation parameters in particular we observe that the maximum depth of the open fractures varies linearly with the cohesion suggesting that we can predict the depth of open cavities formed by tensile failure in natural fault systems if the material properties are known

pre historic changes in natural and anthropogenic heavy metals deposition inferred from two contrasting swiss alpine lakes abstract continuous high resolution sedimentary record of heavy metals chromium cr copper cu lead pb zinc zn manganese mn and mercury hg from lakes lucerne and meidsee switzerland provides pollutant deposition history from two contrasting alpine environments over the last millennia the distribution of conservative elements thorium th scandium sc and titanium ti shows that in absence of human disturbances the trace element input is primarily controlled by weathering processes i e runoff and erosion nonetheless the enrichment factor ef of pb and hg that are measured by independent methods and the pb isotopic composition of sediments from the remote lake meidsee which are proportionally more enriched in anthropogenic heavy metals likely detect early mining activities during the bronze age meanwhile the deposition of trace elements remains close to the range of natural variations until the strong impact of roman activities on atmospheric metal emissions both sites display simultaneous increases in anthropogenic trace metal deposition during the greek and roman empires ca 300 bc to ad 400 the late middle ages ca ad 1400 and the early modern europe after ca ad 1600 however the greatest increases in anthropogenic metal pollution are evidenced after the industrial revolution of ca ad 1850 at low and high altitudes during the twentieth century industrial releases multiplied by ca 10 times heavy metal fluxes to hydrological systems located on both sides of the alps during the last decades the recent growing contribution of low radiogenic pb further highlights the contribution of industrial sources with respect to wood and coal burning emissions

grain fracture in 3d numerical simulations of granular shear we present a new method to implement realistic grain fracture in 3d numerical simulations of granular shear we use a particle based model that includes breakable bonds between individual particles allowing the simulation of fracture of large aggregate grains during shear grain fracture simulations produce a comminuted granular material that is texturally comparable to natural and laboratory produced fault gouge our model is initially characterized by monodisperse large aggregate grains and gradually evolves toward a fractal distribution of grain sizes with accumulated strain comminution rate and survival of large grains is sensitive to applied normal stress the fractal dimension of the resultant grain size distributions 2 3 0 3 and 2 9 0 5 agree well with observations of natural gouges and theoretical results that predict a fractal dimension of 2 58 copyright 2005 by the american geophysical union

the avaatech xrf core scanner technical description and applications to ne atlantic sediments abstract x ray fluorescence xrf core scanning provides rapid high resolution down to 1 mm records of chemical composition on split sediment cores the measurements are non destructive and require very limited sample preparation the new avaatech xrf core scanner operational since 2002 covers the atomic mass range from al to u instrument parameters especially tube voltage can be adjusted to provide optimum settings for selected elements or sets thereof owing to the nature of the surface of split sediment cores particularly effects resulting from sample inhomogeneity and surface roughness results are semiquantitative yet provide reliable records of the relative variability in elemental composition downcore selected case studies from diverse sedimentary settings in the ne atlantic ocean illustrate a range of applications of xrf logging data these include preliminary stratigraphic interpretations glacial interglacial cycles provenance studies of the terrigenous sediment fraction lithological characterization early diagenetic processes and distinction between carbonate phases aragonite v calcite

simulating fracture and friction of aue granite under confined asymmetric compressive test using clumped particle model abstract fracture and friction behaviors of aue granite erzgebirge germany under confined asymmetric load are simulated using a clumped particle model in contrast to conventional circular bonded particle assemblies clumped particle models consist of groups of neighboring individual particles tied together to mimic unbreakable larger grains of irregular shape clumps an optimized clumped model is used for all calculations which is calibrated by the strength and deformation data from laboratory data of uniaxial and triaxial compression at a first order approximation the simulated fracture paths obtained from the clumped particle model match the observed fracture paths from laboratory experiments at various confinements under low confinement b values shows low b values in the fracturing regime and higher b values during stick slip and frictional sliding post failure regime which are in accordance with the laboratory findings

an isotopic record of mercury in san francisco bay sediment article i nfo we analyzed eight sediment cores from wetland and subtidal locations around san francisco sf bay terrace sediment from the yuba river ca and precipitation from the sf bay region we defined the hg isotopic com position of uncontaminated sf bay sediment two contributing endmember sediments contaminated by dis tinct hg sources and regional precipitation deep subtidal sediments with pre mining thg concentrations less than 60 ng g have 202

phase field study of grain boundary tracking behavior in crack seal microstructures in order to address the growth of crystals in veins a multiphase field model is used to capture the dynamics of crystals precipitating from a super saturated solution to gain a detailed understanding of the polycrystal growth phenomena in veins we investigate the influence of various boundary conditions on crystal growth in particular we analyze the formation of vein microstructures resulting from the free growth of crystals as well as crack sealing processes we define the crystal symmetry by considering the anisotropy in surface energy to simulate crystals with flat facets and sharp corners the resulting growth competition of crystals with different orientations is studied to deduce a consistent orientation selection rule in the free growth regime using crack sealing simulations we correlate the grain boundary tracking behavior depending on the relative rate of crack opening opening trajectory initial grain size and wall roughness further we illustrate how these parameters induce the microstructural transition between blocky crystals growing anisotropically to fibrous morphology isotropic and formation of grain boundaries the phase field simulations of crystals in the free growth regime in 2d and 3d indicate that the growth or consumption of a crystal is dependent on the orientation difference with neighboring crystals the crack sealing simulation results in 2d and 3d reveal that crystals grow isotropically and grain boundaries track the opening trajectory if the wall roughness is high opening increments are small and crystals touch the wall before the next crack increment starts further we find that within the complete crack seal regime anisotropy in surface energy results in the formation of curved oscillating grain boundaries instead of straight when the crack opening velocity is increased and wall roughness is not sufficiently high additionally the overall capability of phase field method to simulate large scale polycrystal growth in veins in 3d is demonstrated enumerating the main advantages of adopting the novel approach

seismic reflection imaging of an oceanic detachment fault atlantis megamullion mid atlantic ridge 30 10 n abstract we present multichannel seismic reflection data collected over the atlantis megamullion at the eastern ridge transform intersection of atlantis fracture zone on the northern mid atlantic ridge and over its conjugate crust these data image for the first time the internal structure of a young well developed megamullion dome formed by tectonic extension across a long lived oceanic detachment fault the exposed corrugated detachment fault surface exhibits a sharp coherent reflection that contrasts with less organized reflectivity of surrounding basaltic seafloor at the termination of the megamullion the fault is imaged 13 km along strike beneath a volcanic hanging wall block at a sub seafloor depth of 0 2 0 5 s two way travel time reaching north as far as 30 19 n the eastward dipping of the fault beneath the hanging wall block is estimated to be 6 14 the corrugated fault surface is underlain by a continuous strong and relatively smooth reflection d at 0 2 0 25 s sub bottom below the central axis of the dome this reflection deepens up to 0 6 s sub bottom beneath the western slope and it appears to intersect the seafloor on the eastern slope we suggest that atlantis massif formed by sequential slip on two different detachment faults that merged at depth with breakaways as little as 2 km apart the initial detachment is represented by reflection d and the second corresponds to the presently exposed fault surface in this interpretation much of the sliver between the faults is interpreted to be strongly serpentinized peridotite with reduced seismic velocity it lies in contact with less altered higher velocity mantle below the first detachment resulting in the strong smooth character of reflection d mantle rocks exposed in the megamullion indicate that the feature formed during a period of extreme tectonic extension and probably limited magmatism in conjugate crust corresponding to termination of the megamullion observed sub bottom reflections are interpreted as base of seismic layer 2a this layer is as thick as or thicker 570 900 m than layer 2a in normal atlantic crust and it suggests that relatively normal crustal accretion occurred by the time the megamullion stopped forming

diffraction enhancement in prestack seismic data seismic diffractions are often considered noise and are intentionally or implicitly suppressed during processing diffraction like events include true diffractions wave conversions or fracture waves which may contain valuable information about the subsurface and could be used for interpretation or imaging using synthetic and field data we examine workflows to separate diffractions from reflections that allow enhancement of diffraction like signals and suppression of reflections the workflows consist of combinations of standard processing modules most workflows apply normal moveout corrections to flatten reflection hyperbolas which eases their removal we observe that the most effective techniques are the decomposition of seismic gathers into eigensections and flows based on radon transformations

modern and historic atmospheric mercury fluxes in both hemispheres global and regional mercury cycling implications 1 using two different natural archiving media from remote locations we have reconstructed the atmospheric deposition of mercury hg over the last 800 1000 years in both hemispheres this effort was designed 1 to quantify the historical variation and distributional patterns of atmospheric hg fluxes in the midlatitudes of north america at nova scotia n s and at a comparable midlatitude region in the southern hemisphere at new zealand n s 2 to identify and quantify the influence of anthropogenic and natural hg contributions to atmospheric hg fluxes 3 to further investigate the suitability and comparability of our two selected media lake sediments and ombrotrophic peat for hg depositional reconstructions and 4 to assess the relative importance of wet and dry deposition to the study areas significant findings from the study include the following 1 the lake sediments examined appear to faithfully record the contemporary flux of hg from the atmosphere e g 1997 n s lakes approximately 8 3 g m 2 yr 1 n s rain 8 g m 2 yr 1 the upper 10 cm approximately 10 yr of ombrotrophic peat cores from nova scotia were dated using a biological chronometer polytrichum and were also consistent with the flux data provided by current direct sampling of precipitation these observations place limits on the contribution of dry deposition 40 50 of wet flux unfortunately the peat samples could not be dated below 10 cm this was due to the apparent diagenetic mobility of the geochronological tracer 210pb 2 there is no evidence of a significant enhancement in the atmospheric hg flux as a result of preindustrial 1900 c e common era activities such as the extensive au and ag mining in the americas 3 a factor of 3 and 5x increase in the deposition of hg to the lake sediment archives was observed since the advent of the industrial revolution in new zealand and nova scotia respectively suggesting a worldwide increase in the atmospheric deposition of hg furthermore this increase is synchronous with increases in the release of co2 from combustion of fossil fuels on a global scale the magnitude of increase since industrialization appears larger in nova scotia than in new zealand this may be due to enhanced deposition of hg as a result of either regional emission of hg or enhanced regional oxidation of hg

the structure of cretaceous oceanic crust of the nw pacific constraints on processes at fast spreading centers a 725 km long transect along a flow line in the nw pacific provides new images of the internal structure of cretaceous oceanic crust formed at fast spreading rates reflections from the moho transition zone document changes in crustal travel time along the profile variations with a wavelength of 100 150 km may representing fluctuations in the magma supply with a 2 3 m y periodicity to the south the crust may be several kilometers thicker than normal in places possibly resulting from increased magma supply due to the distal effects of the shatsky hotspot at about the depth expected for the boundary between seismic layers 2 and 3 sub horizontal reflections are imaged over much of the profile the clearest are positive polarity and of an amplitude consistent with a reflection coefficient of 0 1 implying a sharp velocity and density discontinuity northward ridgeward dipping reflections in the lower crust are imaged over much of the profile particularly toward the north away from the region possibly affected by the hotspot these reflections stop at the moho reflection but die out perhaps 1 s beneath top basement they probably represent lithological layering as predicted by models of ductile flow accompanying passive upwelling during plate separation although they could represent secondary shears generated by basal drag during active mantle upwelling beneath the spreading center the discrete character and in places regular spacing every 8 km of these features hint at a cyclicity in the layering and thus at some episodicity in magmatic processes over a timescale of 0 1 0 2 m y

dynamic responses and failure modes of stratified sedimentary rocks abstract understanding the dynamic responses and failure modes of stratified sedimentary rocks is critical to predict ground motion in sedimentary rock formations during underground excavation we experimentally investigated the dynamic properties of sandstone specimens siltstone specimens and composite specimens with sealed sandstone siltstone interfaces using a split hopkinson pressure bar system we also numerically examined the effects of rock type strain rate particle size and interface cementation degree on the dynamic properties of sedimentary rock specimens using the discrete element method our results show that the dynamic strengths of sandstone and siltstone increase with higher strain rate in a similar manner but the dynamic strength of sandstone is larger than that of siltstone at the same strain rate although the size of deposited particles may not affect the dynamic strength of lithified rocks the fracture surface of sandstone with large particle size is relatively rough the degree of cementation at the interface between siltstone and sandstone has a negligible effect on the dynamic strength of sedimentary rocks but remarkably affects the failure modes micro cracks first initiate in the low strength siltstone and coalesce with those subsequently developed in the sandstone and at the sealed interface to form meso fractures

inferring paleostresses from natural fracture patterns a new method we introduce a method to infer the remote differential stress magnitude from the curvature of overlapping echelon fracture traces the curving paths of overlapping echelon cracks imply the predominance of local crack induced stresses over remote stresses during propagation nearly straight crack paths imply the controlling influence of a remote compressive crack parallel differential stress this method is used to interpret complex joint patterns mapped in sedimentary rock it is also applied to the problem of fracture pattern generation using computer models

no evidence from multichannel reflection data for a crustal magma chamber in the mark area on the mid atlantic ridge the role of crustal magma chambers in the construction of oceanic crust along the slowly spreading mid atlantic ridge mar is still highly disputed1 some investigators have argued for small short lived crustal magma bodies2 whereas others have postulated the presence of permanent reservoirs centred between ridge offsets which expand and contract in response to a fluctuating magma supply3 thermal modelling4 5 detailed seismic refraction experiments at several different locations along the mar6 9 and micro earthquake10 11 and teleseismic source mechanism studies12 preclude the existence of large steady state magma chambers under the entire mar rift valley however smaller localized crustal magma bodies could be present at least intermittently here we use multichannel seismic reflection profiles to look for evidence of a crustal magma chamber along the mar south of the kane fracture zone in one of the most magmatically robust and hydro thermally active areas presently known along the mar although some subbasement reflectors are imaged we do not see a shallow large amplitude intracrustal reflector that might be associated with a crustal magma body if a crustal magma chamber was associated with the most recent volcanic episode along this part of the mar our results suggest that it was extremely short lived

effects of sedimentary interfaces on fracture pattern linkage and cluster formation in peritidal carbonate rocks abstract the permeability of a reservoir is particularly dependent upon the proportion of its fractures that penetrate or are arrested at interfaces such as contacts and discontinuities here we report on fracture penetration and fracture arrest in lower cretaceous peritidal deposits exposed in the pizzicoli quarry gargano promontory southern italy we measured more than 2000 fractures in the field and using lidar data of which 564 fractures from the field and 518 from lidar studies are the focus of this paper fracture arrest deflection and penetration depend much on the effects of peritidal cycle interfaces such as paleosol horizons laminated carbonate mudstones and stylonodular horizons the laminated mudstones have the greatest effect 63 99 of the fractures are deflected or arrested at such interfaces whereas 63 90 are deflected arrested at paleosols and 20 35 at stylonodular horizons in the mudstones many fractures are arrested at thin internal laminae such that few penetrate the entire laminated layer and fewer still the boundaries between the layers paleosol interfaces deflect arrest more than 60 of all fractures however when small offset fractures above and below paleosols are regarded as penetrating they are evenly spaced non clustered so that fracture related fluid transport may occur across the entire paleosol stylonodular horizons deflect arrest and split some fractures but generally have little effect compared with the other types of interfaces we present three main mechanisms for fracture deflection and or arrest 1 the fracture induced tensile stress ahead of its tip referred to as the cook gordon debonding mechanism 2 rotation of the principal stresses at and across the interface resulting in the formation of stress barriers and 3 large elastic mismatch particularly as regards young s moduli between layers across an interface all these mechanisms are likely to have operated during fracture propagation and arrest in the carbonate rocks of the pizzicoli quarry

the evolution of crack seal vein and fracture networks in an evolving stress field insights from discrete element models of fracture sealing veins are ubiquitous in upper and middle crustal rocks due to strength and stiffness contrast to the host rock veins can influence crack propagation here we present discrete element models to investigate crack vein interactions by simulating cycles of fracturing of a rock mass sealing the cracks to form veins and refracturing the rock mass after rotating the stress field we observe different styles of interaction between new fractures and existing veins depending on the strength ratio between vein and host rock and on the changes in the stress field between the different deformation stages if the orientation of stress field does not change between deformation stages ataxial crack seal veins are produced if the veins are weak and a bundle of subparallel microveins if the veins are strong if the stress field is rotated between deformation stages the interactions include reactivation fracture deflection and crosscutting reactivation of weak veins occurs even if the vein orientation is highly unfavorable relative to the stress field relays of fractures between reactivated veins form at a higher angle to the veins than expected this demonstrates that the orientation of secondary veins does not reflect the regional stress field in a simple manner and that veins can strongly influence fracture connectivity with implications for paleostress analysis and basin modeling simulation results compare well with field examples of multiphase vein networks in carbonates from jebel akhdar oman

clay minerals and geochemistry record from northwest mediterranean coastal lagoon sequence implications for paleostorm reconstruction abstract the paleostorm history of coastal lagoon environments pierre blanche lagoon central part of the gulf of lions has been established by integrating clay mineralogy and geochemical analysis clay mineralogy combined with bulk major and trace elements concentrations allow for the definition of four different end members the mosson drainage basin the sandy barrier the biogenic and the anthropic components the two main sedimentary sources of the lagoonal system are the mosson drainage basin which has a high concentration of smectite and al 2 o 3 and the sandy barrier characterized by high contents of illite chlorite sio 2 and zr smectite illite chlorite sio 2 al 2 o 3 and zr al 2 o 3 ratios can be used to reconstruct the past storm history of the pierre blanche lagoon our results indicate that the sensitivities of the clay mineralogy and geochemistry proxies are different these sensitivities could be related 1 first to the comparison between different grain size for geochemistry and for clay mineralogy 2 second to the complexity of the mix four end members for the geochemistry and two end members for the clay minerals and 3 third by the storm s ability to transport sediment of different sizes

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

methylmercury in tailings ponds of amazonian gold mines french guiana field observations and an experimental flocculation method for in situ remediation sites of monomethylmercury mmhg production in amazonian regions have been identified in hydraulic reservoirs lake sediments and wetlands but tailings ponds have not yet received sufficient attention for this purpose this work evidenced high mmhg production within the water column and the interstitial water of two tailings ponds of french guiana au mines located i in a small scale exploitation combat where hg was used for au amalgamation and ii in an industrial on going au mine yaoni processing without hg the mmhg d maximum 2 5 ng l 1 occurred in the oxic water column above the sediment water interface swi of the most recent tailings pond combat where the substrate was fresh the redox transition was sharp and the pool of total hg was large in the yaoni pond the mmhg d maximum concentration 1 4 ng l 1 was located at the swi where suboxic conditions prevailed using the mmhg d concentration as a proxy for hg methylation rates the present results show that hg methylation may occur in various redox conditions in tailings ponds and are favored in areas where the organic matter regeneration is more active a 3 month long laboratory experiment was performed in oxic and anoxic boxes filled with high turbidity waters from the combat au mine to simulate tailings ponds slaked lime was added in an experimental set 2 mg l 1 and appeared to be very efficient for the reduction of suspended particulate matter spm to environmentally acceptable concentrations however at the end of the experiment large mmhg d concentrations were monitored under treated anoxic conditions with the mmhg d maximum located at the swi above the fe reducing zones no mmhg d was detected in oxic experiments the use of slaked lime for spm decantation appears to be an efficient and non onerous process for au miners to avoid hg methylation in tailings ponds when it is combined with rapid drainage of the mine waters a subsequent human intervention is however necessary for the recovery of soil structure through the cover of dried ponds with organic rich materials and reforestation to avoid the stagnation of rain waters and the occurrence of anoxia

enhanced mercury deposition by amazonian orographic precipitation evidence from high elevation holocene records of the lake titicaca region bolivia abstract very few studies have focused on historical mercury hg deposition in the southern hemisphere especially for the pre mining period therefore the respective contributions of hg emissions from anthropogenic and natural sources are not fully constrained and the long term influence of climate variability is still in debate in this study we reconstructed hg depositions over the last 13 500 years based on two sedimentary records located at the fringe between amazonia and the altiplano multiproxy analyses carried on the same sedimentary archives i e hg accumulation rates ar major elements total organic carbon 13c and pollen analyses show evidence that hg deposition has risen according to amazonian orographic moisture and precipitation during the holocene except for the last 200 years for which modern industry and goldmining overwhelmed the natural signal comparisons with existing hg records indicated that metallurgy and volcanic emissions were not perceptible due to the eastern down wind position of the sites altogether it argues that the historical variability of hg deposition in the central eastern andes has been primarily driven by changes in amazonian moisture at least six main phases of higher hg ar 2 6 1 4 g m 2 y 1 compared to baseline values 0 9 0 3 g m 2 y 1 were identified over the holocene during the early and late holocene phases of higher hg ar coincided with wet episodes recorded in lake titicaca and sajama records during the middle holocene arid phase reconstructed wetter episodes by hg together with vegetation dynamics were attributed to the set up of moisture niches in the eastern flanks of the andes comparisons to other paleoclimatic records supports the role of north atlantic sea surface temperature in forcing precipitation during the late holocene period while other mechanisms allowing moisture niches by orography are discussed for the mid holocene dry phase in the eastern andes region

discrete element modeling of boudinage insights on rock rheology matrix flow and evolution of geometry 1 we use discrete element model simulations to model the full boudinage process from initial fracturing of intact material to post fracture flow of material into gaps between fragments and to investigate the role which the material properties of the weak and strong layers play in this process the models are deformed in 2d plane strain under a range of confining stresses in coaxial bulk flow results show that the material properties i e mohr coulomb or quasi viscous for the matrix and elastic brittle for the competent layer lead to the development of natural looking boudin morphologies and deformation patterns in the matrix the details of the matrix rheology only have a minor influence on the morphology of the boudins by varying the material properties of the competent layer between fully brittle and semi ductile we obtain a wide range of deformation patterns ranging from pinch and swell structures to a variety of boudin types including drawn shear band and straight sided torn boudins in a number of models we observe rotation of the boudin blocks despite the applied deformation being purely coaxial these rotations are generally related to asymmetrical rhombic boudin shapes some features observed in natural boudins such as concave block faces or the formation of veins between fragments are not modeled because pore fluids are not yet included in our model

upper crustal structure and axial topography at intermediate spreading ridges seismic constraints from the southern juan de fuca ridge 1 we use multichannel seismic reflection data to image the upper crustal structure of 0 620 ka crust along the southern juan de fuca ridge the study area comprises two segments spreading at intermediate rate with an axial high morphology with narrow cleft and wide vance axial summit grabens asg along most of the axis of both segments we image the top of an axial magma chamber amc the amc along cleft deepens from south to north from 2 0 km beneath the ridge cleft observatory and hydrothermal vents near the southern end of the segment to 2 3 km at the northern end near the site of the 1980s eruptive event along the vance segment the amc also deepens from south to north from 2 4 to 2 7 km seismic layer 2a interpreted as the basaltic extrusive layer is 250 300 m thick at the ridge axis along the cleft segment and 300 350 m thick along the axis of the vance segment however off axis layer 2a is similar in both segments 500 600 m indicating 90 and 60 off axis thickening at the cleft and vance segments respectively half of the thickening occurs sharply at the walls of the asg with the remaining thickening occurring within 3 4 km of the asg along the full length of both segments layer 2a is thinner within the asg compared to the ridge flanks previous studies argued that the asg is a cyclic feature formed by alternating periods of magmatism and tectonic extension our observations agree with the evolving nature of the asg however we suggest that its evolution is related to large changes in axial morphology produced by small fluctuations in magma supply thus the asg rather than being formed by excess volcanism is a rifted flexural axial high the changes in axial morphology affect the distribution of lava flows along the ridge flanks as indicated by the pattern of layer 2a thickness the fluctuations in magma supply may occur at all spreading rates but its effects on crustal structure and axial morphology are most pronounced along intermediate spreading rate ridges

geometry of extensional faults developed at slow spreading centres from pre stack depth migration of seismic reflection data in the central atlantic canary basin we present depth images from portions of profiles that are close to flow lines of cretaceous oceanic crust in the eastern central atlantic compared with post stack time migrations the images illustrate the improvement resulting from the application of pre stack depth migration the images document the scale and geometry of normal faulting in oceanic crust formed over 25 myr at a half spreading rate of less than 10 mm yr 1 and the variation in extensional style with position within the spreading segment away from major fault zones fzs most faults are subplanar dip more than 35 are associated with moderate basement relief 0 2 1 km relief and may penetrate to deep crustal levels these faults could be related to the lifting of the lithosphere out of the median valley to the flanking mountains also observed away from fzs are gently dipping to subhorizontal reflections in the upper crust which resemble detachment faults in contrast the inside corner crust is more rugged with basement highs rising up to 2 km above the intervening basins this larger scale topography is associated with a different style of faulting the depth images reveal gently dipping 35 faults that are rooted in the deep crust and that project to the ridgeward flank of the dome shaped large basement highs 1 2 km vertical relief the faults seem to continue as the ridge facing flank of these highs and some may extend over the crest of the high to breakaways beyond in this case the domal highs that form the exhumed footwall to the faults can be described as oceanic core complexes these controlling faults are up to 20 km long and have a heave of 10 km sufficient to have accommodated up to 50 per cent extension and to have exhumed deep crustal and perhaps even mantle rocks we suggest that similar faults can explain the structure and lithologies found at megamullion structures oceanic core complexes at inside corners near the present day spreading ridge

kinematics of crystal growth in syntectonic fibrous veins abstract detailed observations of a set of fibrous antitaxial calcite veins in a slate reveal that some of the calcite fibres do not connect material markers on both sides of the vein and can therefore not have tracked the full opening trajectory during vein growth this calls for a better understanding of the mechanisms of fibre formation and reliable criteria to test the tracking hypothesis based on surface roughness characteristics of the vein wall we develop a simple model for shape growth of crystals in a crack seal environment which can account for both tracking and non tracking behaviours and propose a set of tracking criteria for antitaxial veins finally we discuss ways by which the model can be tested against natural examples

experimental studies on the effects of fracture on the p and s wave velocity propagation in sedimentary rock calcarenite del salento abstract experimental studies were carried out in laboratory in order to investigate the effects of fracture on compressional p wave and shear s wave velocity propagation and therefore the relations between seismic properties and rock mass parameters the discontinuity index i d fracture density parameter c linear fracture parameter and the rock quality designation rqd were used to describe the rock mass parameters these parameters are analyzed and then related to the seismic properties four vertical aligned fractures were created on an intact calcarenite block 0 6 m long 0 4 m thick and 0 4 m width by sawing the measures were carried out in four different blocks of cacarenite having the same physical properties and in four different phases in first block the fractures were filled with air in the second block the fractures were filled with terra rossa in third block the fractures were filled with wet terra rossa and in the fourth block the fractures were filled with clay the test results were statistically analysed using the method of least squares regression and polynomial relationships with high correlation coefficient were found between the fractured rock parameters and p wave s wave velocities and v p v s ratio the investigations suggest that the p wave and s wave velocities decrease with increasing the fracture parameters while the v p v s ratio increases with decreasing the fracture parameters furthermore the results of the experimental studies were applied on the seismic refraction tomography data acquired in a great measurements campaign undertaken in the adriatic salentina coast south italy in order to monitor the coastal erosion the geophysical results using the polynomial relationships between the fractured rock parameters and p wave velocity are in good agreement with the geomorphological and geological results

a simple analysis of oil induced fracturing in sedimentary rocks abstract geometric form of kerogen patches control oil induced fracturing in impermeable source rocks thin elongate kerogen flakes cause lateral fracturing kerogen has to occur in spherical or cylindrical forms to induce vertical fractures if length to width ratio of kerogen flakes is sufficiently large then lateral fractures are definitely initiated during oil generation in impermeable source rock oil and rock compressibilities and lateral to vertical stress ratio are decisive factors in vertical fracturing in impermeable rocks lateral to vertical stress ratio is generally higher than the maximum for oil induced vertical fracturing in tectonically relaxed sedimentary basins oil generation cannot initiate vertical fractures except when the lateral to vertical stress ratio is reduced below critical level either by tectonic tension or erosion of overburden when the source rock is permeable then the rate of oil seepage from kerogen into the surrounding rock is crucial in fracture initiation oil induced fractures cannot form unless permeability and hence rate of oil seepage from kerogen into the surrounding rock is non existent or negligible most source rocks have some permeability therefore oil induced fracture initiation should be a rare phenomenon

the mohorovicic discontinuity in ocean basins some observations from seismic data abstract since the late 1970s studies of the oceanic crust using airgun sources and towed hydrophone arrays have been conducted in a wide range of ocean basin settings including mid ocean ridges and old oceanic crust the very earliest studies were performed at the fast spreading east pacific rise epr and revealed a very distinct almost continuous vertical incidence reflection event at a depth corresponding to the crust mantle transition as inferred from seismic refraction studies this suggested that the transition was quite sharp in comparison to the source wavelength that moho was observed very close to or even exactly beneath the ridge crest implied that it was formed at essentially zero age since then many experiments using progressively improving airgun arrays and streamer systems have expanded these observations here we review the literature presenting studies of normal oceanic crust produced at mid ocean ridges with the objective of assessing the age of formation of moho and the nature of variability of moho signature in multi channel seismic data moho is observed as a consistent feature for all spreading rates but appears quite variable being very distinct in some areas complex in form in others and absent in many regions as much as 40 although fast spread crust is associated with the strongest simplest and most laterally continuous moho images we see significant variability at almost all spreading rates and ages where moho is absent from vertical incidence data this cannot be fully explained by the effect of scattering or attenuation in the crustal section above moho can be observed at zero age in only a small number of areas at or near overlapping spreading centers on the epr after three decades of marine seismic studies many fundamental questions remain concerning the nature of the moho that will require targeted experiments to solve

the geochemical behavior and isotopic composition of hg in a mid pleistocene western mediterranean sapropel abstract the concentrations of mercury hg and other trace metals ni cu zn mo ba re u and the hg isotopic composition were examined across a dramatic redox and productivity transition in a mid pleistocene mediterranean sea sapropel sequence characteristic trace metal enrichment in organic rich layers was observed with organic rich sapropel layers ranging in hg concentration from 314 to 488 ng g avg 385 with an average enrichment in hg by a factor of 5 9 compared to organic poor background sediments which range from 39 to 94 ng g hg avg 66 comparison of seawater concentrations and sapropel accumulations of trace metals suggests that organic matter quantitatively delivers hg to the seafloor near complete scavenging of hg from the water column renders the sapropel hg isotopic composition representative of mid pleistocene mediterranean seawater sapropels have an average 202hg value of 0 91 0 15 n 5 1 sd and 199hg value of 0 11 0 03 n 5 1 sd background sediments have an average 202hg of 0 76 0 16 n 5 1 sd and 199hg of 0 05 0 01 n 5 1 sd which is indistinguishable from the sapropel values we suggest that the sapropel isotopic composition is most representative of the mid pleistocene tyrrhenian sea

velocity structure near iodp hole u1309d atlantis massif from waveform inversion of streamer data and borehole measurements seismic full waveform inversion fwi is a promising method for determining the detailed velocity structure of the igneous oceanic crust especially for locations such as the mid atlantic ridge with significant lateral heterogeneity and seafloor topography we examine the accuracy of fwi by inverting after downward continuation to datum just above the seafloor a multichannel seismic mcs profile from atlantis massif oceanic core complex at 30 n that passes close to integrated ocean drilling program iodp hole u1309d and comparing the results against borehole measurements and existing on bottom refraction data the comparisons include the results of iodp expedition 340t which extended the sonic logging and vertical seismic profiling to the bottom of the borehole at 1400 m below seafloor compared to travel time tomography the refinement in velocity and velocity gradient produced by fwi significantly improves the overall match to the borehole measurements and allows the multilevel pattern of deformation and alteration of the detachment footwall seen in hole u1309d to be extrapolated across the central dome prestack depth migration of the profile using the fwi velocities reveals the top and edges of the high velocity gabbroic core of the massif it also indicates that the comparatively uniform gabbroic rocks drilled at hole u1309d extend to 2 5 km below seafloor but overlie an extended 2 km thick mantle transition zone

geochemical monitoring system ii prototype gms ii installation at the acqua difesa well within the etna region first data during the 1999 volcanic crisis abstract the geochemical monitoring system ii gms ii prototype was designed assembled tested and installed at the acqua difesa test site near belpasso ct within the etna region in the frame of an ec funded program named automatic geochemical monitoring of volcanoes the easily changeable and versatile configuration of the remote station provides actual data every 10 min recording groundwater temperature electrical conductivity ph redox potential dissolved co 2 he atmospheric pressure and air temperature the radon sensor was assembled despite no jet added to the remote station an automatic sampler collects every 72 h 3 days two bottles of water acidified and not for further laboratory analyses the choice of the acqua difesa test site arises from preliminary volcanological studies deserving special interest i e for the high co 2 degassing for the high he isotopic values exhibiting a direct mantle signature of the volcanic gases and for the peculiar sensitivity to volcanic activity as tested by discrete monitoring in the past and confirmed by the present study during the discussed monitoring period october 1998 june 1999 an enhanced volcanic activity of etna was observed it started on july 1998 with enhanced plume degassing and since september 1998 with periodical lava fountains as well as with episodic low energy earthquakes and seismic swarms m max 3 1 till june 1999 the activity culminated with a summit crater fracture involving a sub terminal eruption on 04 02 1999 the summit crater fracture was not accompanied by enhanced seismicity despite moderate seismic energy release occurring in the succeeding months in that monitoring period the acqua difesa gms ii station recorded a few geochemical anomalies mostly in temperature ph co 2 electrical conductivity and eh as well as in the concentration of some major elements trend mostly in mg and hco 3 the geochemical continuous and discrete data set is discussed in comparison with the available information on the ongoing volcanic and seismic activity tentatively enhancing the comprehension of the aquifers mass heat transport properties of the volcano during crises the experimental observations may be defined at present the forerunners of the etna eruptions affecting the single out acqua difesa well it is nowadays a self consistent site with its own response to the pre eruptive processes defined sensitive to the etna aquifer modifications induced by the volcanic activity in a recurrent way thus we defined the typical answer of this specific test site to the pre eruptive processes and to the associated seismicity further data may modify this statement the station prototype configuration and the results may give hints for future volcanic eruption and seismic activity forecasting

crack seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures abstract an analysis of crack seal fibre growth mechanisms has shown two important implications of the crack seal process for the interpretation of microstructures in low grade metamorphic rocks 1 1 the ability of fibre growth to track the incremental separation history of crack walls depends on details of the nucleation and growth mechanisms 2 2 fibrous crystals may develop cristallographic preferred orientations in certain crack seal situations the microstructures of layer silicates and associated phases developed in several examples of syntectonic intragranular microfracture sites and veins indicates that layer silicate 001 and grain shape preferred orientation can develop during crack seal deformation by oriented growth mechanisms during successive crack seal increments preferred orientation may develop in response to an interaction between anisotropic growth kinetics and the displacement history resulting in preferential rejoining by syntaxial overgrowth of pulled apart grains having fast growth directions parallel to the incremental displacement direction across a microcrack preferred orientation may also be developed and enhanced in crack seal growth sites by overgrowth of previously oriented layer silicates in the microcrack walls in this case the crystallographic preferred orientation need not be simply related to the displacement history during crack seal fibre growth since crack seal processes may operate on all scales down to the minimum size of a microfracture in a deforming rock such mechanisms of layer silicate preferred orientation development are expected to be very significant in developing and enhancing foliation during deformation involving microfracture and solution transfer processes

isotope geochemistry of mercury in source rocks mineral deposits and spring deposits of the california coast ranges usa abstract we present here the first study of the isotopic composition of mercury in rocks ore deposits and active spring deposits from the california coast ranges a part of earth s crust with unusually extensive evidence of mercury mobility and enrichment the franciscan complex and great valley sequence which form the bedrock in the california coast ranges are intruded and overlain by tertiary volcanic rocks including the clear lake volcanic sequence these rocks contain two types of mercury deposits hot spring deposits that form at shallow depths the franciscan complex and great valley sequence contain clastic sedimentary rocks with higher concentrations of mercury than volcanic rocks of the clear lake volcanic field mean mercury isotopic compositions 202hg for all three rock units are similar although the range of values in franciscan complex rocks is greater than in either great valley or clear lake rocks hot spring and silica carbonate mercury deposits have similar average mercury isotopic compositions that are indistinguishable from averages for the three rock units although 202hg values for the mercury deposits have a greater variance than the country rocks precipitates from spring and geothermal waters in the area have similarly large variance and a mean 202hg value that is significantly lower than the ore deposits and rocks these observations indicate that there is little or no isotopic fractionation

structure of young oceanic crust at 13 n on the east pacific rise from expanding spread profiles we present the results of the analysis of expanding spread profiles esps collected on and near the axis of the east pacific rise at 13 n these profiles were collected at 0 1 1 2 1 3 6 and 9 5 km from the rise axis and all but the most distant profile show a distinct low velocity zone lvz located within layer 3 of the oceanic crust at the ridge crest the top of the magma chamber is at the base of layer 2 while 3 6 km off axis the roof of the lvz is 1 1 km below the top of layer 3 the profile farthest from the ridge could possibly have a residual lvz confined to the lower 1 5 km of the crust the total width of the lvz as determined from the esp data is at least 6 km and possibly much greater this wide lvz apparently contradicts multichannel seismic data which show cross axis reflections from the magma chamber with a width of 5 km we suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt the esp data are sensitive to this larger region while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast we identify the presence of a layer at the top of the oceanic crust with initial p wave velocities between 2 35 and 2 6 km s while the s wave velocity is estimated as being 0 8 km s the layer thickness lies between 100 and 200 m these velocities are consistent with previous estimates for the pacific and recent results for the atlantic the thickness of this layer is consistent with that of layer 2a determined from geophysical measurements at deep sea drilling project hole 504b