on the shape and velocity of fluid filled fractures in the earth summary in this paper i present a model to estimate the shape and velocity of slowly ascending buoyancy driven fluid filled fractures the model considers elastic deformation linear fracture mechanics and fluid flow an attempt is made to incorporate the effect ofthe 2 d fluid pattern on the viscous pressure drop most other models assume that the viscous pressure drop can be approximated by flow through a channel with constant width although the form of the fracture is known to deviate from such a simple shape the 2 d flow in my model has an important consequence for the mechanism of buoyancy driven fracture propagation it predicts a large pressure gradient at the tail of the propagating fracture indicating that the tail of the fracture is most important in hindering the fracture propagation a singularity at the tail of the fracture can be avoided when a small amount of fluid trails in the channel left behind the propagating fracture the trailing and decoupling of fluids at the tail seems to be accompanied by small flow and shape instabilities which is indicated by the jerky movement of the tail observed in propagating air filled fractures in solidified gelatine and by numerical boundary element solutions of the coupled flow deformation fracturing problem by comparing the predictions for propagation velocities with laboratory observations of buoyancy driven fracture propagation in gelatine i derive a non dimensional effective thickness at the tail of the fracture for which trailing of fluids may occur the model is applied to earth relevant problems such as oil and water filled fractures in pressurized sediments or magma filled dykes in the lithosphere which are discussed in the paper

hydrologic connections of a stream aquifer vegetation zone in south central platte river valley nebraska field investigation and numerical modeling approaches were used to examine the hydrologic relations between the platte river and the adjacent alluvial aquifer and riparian zone in south central nebraska field methods include direct push techniques for coring and electric logging in the river channel permeameter tests for estimating the hydraulic conductivity of the streambed and monitoring of groundwater levels responding to changes in stream stages and to groundwater evapotranspiration the channel sediments consist mostly of coarse sand and gravels at the study site with large values of horizontal and vertical hydraulic conductivity groundwater in the riparian zone responded nearly simultaneously to the changes in stream stages and diurnal fluctuations of the water table are correlated with fluctuations of stream stage in summer all these indicate a well connected river aquifer vegetation hydrologic system numerical models based on the galerkin finite element method were developed to construct detailed flow nets for examining the changes in the patterns of groundwater flow dynamics resulting from the use of groundwater and stream water by riparian vegetation simulation results suggest that a number of hydrologic factors such as the thickness of the aquifer and vertical anisotropy of aquifer hydraulic conductivity also affect the flow patterns vertical flow is a major component more significant than the horizontal flow below the river and the vegetation zone in the growing season groundwater evapotranspiration can bring deeper groundwater to the water table by hydraulic lift this function of riparian vegetation could cause a complicated situation in an investigation of groundwater quality in riparian zones

dike deflection modelling for inferring magma pressure and withdrawal with application to etna 2001 case abstract we studied the stress effects of the topography load on dike propagation by considering the results from analogue experiments in addition we refined the results by applying numerical simulations using the finite element method fem in order to also consider the medium rheology and real topography we investigated the dike deflection observed during the final dike emplacement accompanying etna s 2001 eruption we cross related the information on the position of the dike from ground deformation modelling with the numerical simulation results with the aim of estimating the final excess pressure of the dike when it started to deflect which proved to be about 4 8 mpa assuming that the pressure decreases linearly with the volume of magma moving from the chamber into the dike we estimated 7 15 mpa as the initial overpressure accumulated at the intermediate magma chamber before its breakout although the previous modelling overestimated the stress the approach presented here leads to infer a compatible stress with the strength of the rocks

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

numerical modelling of dykes deflected into sills to form a magma chamber abstract most shallow magma chambers are thought to evolve from sills for this to happen several conditions must be met 1 there must be a discontinuity normally a contact that deflects a dyke or an inclined sheet into a sill 2 the initial sill must have a considerable thickness normally depending on dyke injection rates not less than some tens of metres 3 the resulting sill must receive magma through dykes frequently enough so as to stay liquid and expand into a chamber 4 the resulting magma chamber must remain at least partially molten and receive multiple magma injections over a given period of time to build up a volcano on the surface above in this paper we present numerical models based upon field data and geophysical data as to how sills are emplaced and may subsequently evolve into shallow magma chambers we suggest that most sills form when dykes meet contacts particularly weak ones which are unfavourable to dyke propagation a contact may halt arrest a dyke altogether or alternatively deflect the dyke into the contact the three main mechanisms for dyke deflection into a contact are 1 the cook gordon debonding or delamination 2 rotation of the principal stresses generating a stress barrier and 3 an elastic mismatch across a contact between adjacent layers elastic mismatch means that the layers have contrasting young s moduli and varying material toughness once a sill is initiated the developing magma chamber may take various forms many shallow magma chambers however tend to maintain a straight sill like or somewhat flat oblate ellipsoidal geometry during their lifetimes for a sill to evolve into a magma chamber there must be elastic plastic deformation of the overburden and to some extent of the underburden so long as the sill stays liquid subsequent dyke injections become arrested on meeting the sill some magma chambers develop from sill complexes for the sill complex to remain partially molten it must receive a constant replenishment of magma implying a high dyke injection rate alternatively an initial comparatively thick sill may absorb much of the magma of the dykes that meet it and evolve into a single shallow magma chamber

buoyancy driven fracture ascent experiments in layered gelatine laboratory experiments on air filled fracture propagation in solidified homogeneous and layered gelatine have been carried out providing an analogue model for magma filled dikes ascending in the crust the effects of layering on fracture velocity and shape have been analyzed in detail the free surface is found to accelerate approaching fractures layering accelerates or decelerates fractures approaching discontinuities of the elastic parameters depending on the value of the rigidity contrast the shape of fractures are strongly influenced as they pass from one layer to another the observed cross sectional shape when crossing a layer interface and the acceleration with decreasing rigidity can be explained with theoretical models our experiments also reproduce the arrest of fractures in proximity of joints and the formation of sills in the layer below the interface these findings could help in the interpretation of accelerated seismicity and deformation rates observed in volcanic areas

sill geometry and emplacement controlled by a major disconformity in the tarim basin china abstract igneous sills are widely distributed in sedimentary basins worldwide sill emplacement has significant impacts on structural and thermal evolutions of sedimentary basins and also provides important insights into volcanic and sub volcanic processes dyke deflection and sill formation are largely controlled by mechanical layering of the host rock and a number of mechanisms have been proposed however most models of sill formation are based much on numerical and analogue modeling for many natural sills the key mechanism governing the dyke deflection and sill formation remains to be discussed and in some cases controversial to better understand the control of mechanical layering on sill formation and test the existing models we study detailed geometries of igneous sills from the central part of the tarim basin china based on seismic reflection data and borehole data nineteen igneous sills which are expressed as packages of high amplitude reflections are observed and they were all emplaced in the upper ordovician strata at current burial depths of about 5 8 km the sills can be classified into three geometric types type 1 saucer shaped sills type 2 strata concordant sills and type 3 hybrid sills which have the characteristics of both type 1 and 2 the geometry of the saucer shaped sills is typical and comprises three distinct parts a flat inner sill inclined sheets and in many cases flat outer sills all the bases of the saucer shaped sills coincide with the disconformity between the middle and upper ordovician strata the m disconformity indicating that the disconformity controlled the sill geometry and emplacement depth in the basin the m disconformity is characterized by a lithological change from underlying limestones to overlying mudstones we suggest that among the three main mechanisms for the deflection of dykes into sills namely cook gordon debonding stress barriers and elastic mismatch the cook gordon debonding is the dominant mechanism in the study area this is a mechanism by which a weak contact opens up ahead of a vertically propagating dyke allowing the dyke to be deflected into a sill when the dyke meets the open contact this study indicates that the control of mechanical layering of the host rock on sill formation is largely depends on the host rock lithologies and confirms again that the saucer shape is the fundamental sill geometry under the control of layering

the inorganic geochemistry of cretaceous black shales dsdp leg 41 in comparison to modern upwelling sediments from the gulf of california

surface subsurface water interactions in an alluviated mountain stream channel data from an instream tracer experiment performed in north fork dry run shenandoah national park virginia illustrate that the transport of water and conservative solute in the stream is greatly influenced by interaction with water present in both the relatively shallow gravel bed and in the relatively deeper alluvial infill material the shape of concentration time curves from the field tracer experiment exhibit storage of tracer in the subsurface short term storage imposed by exchange with the gravel bed and long term storage by exchange with the deeper alluvial infill time series models fit the observed concentration data very well estimates of mixing volumes generated by the time series analysis indicate that a large volume of the gravel bed participates in rapid mixing with the surface stream at one point along the study reach a second order time series model is supported by the data suggesting two independent parallel flow paths observations of tracer in wells more than 10 m from the stream and the very extended recession of the tracer in the stream where it flows over a bedrock ledge support the contention that the deeper alluvium plays an important role in transient storage of solute in north fork dry run the work implies that physically based models to predict transport and storage in alluviated mountain catchments may have to account for interactions between the stream and large portions of its floodplain

remobilization of barium in continental margin sediments abstract the rate of ba release from california continental margin sediments has been measured using an in situ benthic flux chamber and the range of values 25 50 nmol cm 2 y 1 is larger than any previously published benthic flux estimate for this element the magnitude of the ba flux suggests that a significant fraction of the ba raining from the euphotic zone is recycled at the seafloor ba si regeneration ratios from these margin sediments increase with depth demonstrating that ba is decoupled from si during the earliest stages of diagenesis on the other hand ba regeneration rates and caco3 dissolution rates covary the coupling between these two constituents is supported by the observation that the ba caco3 dissolution flux ratio 1 7 0 4 10 3 is independent of bottomwater depth even in sediments underlying the oxygen minimum zone along the continental margin furthermore this flux ratio is consistent with both the water column ba alkalinity ratio for the world s ocean as well as the ba caco3 ratio in sediment trap solid phases from the equatorial pacific 1 1 2 2 10 3 however the constancy of the ba alkalinity ratio over geologic time remains in question because the mechanism that controls this relationship remains a mystery our flux measurements suggest that diagenesis does not significantly influence the ba ca ratio in the upper 0 5 mm of pacific sediments thereby supporting the idea of using the ba concentration in surface dwelling benthic forams as a proxy for deep water chemical conditions lea and boyle 1989 1990 on the other hand we predict that if a foraminifer lives 0 5 mm or more below this interface then diagenetic effects could influence the ba ca ratio that foram species would record the carrier phase of the particulate ba reactive during early diagenesis does not appear to be organic matter oxyhydroxides or calcium carbonate but rather a mineral phase related to marine barite or perhaps celestite

how hydrofractures become arrested fluids in the earth s crust are commonly transported by hydrofractures such as dykes and mineral veins many of which become arrested at various crustal depths hydrofractures are commonly arrested some showing blunt tips at contacts between soft low young s modulus and stiff high young s modulus layers for example many dyke tips are arrested at contacts between soft pyroclastic rocks and stiff basaltic lava flows and vein tips at contacts between soft marl and stiff limestone theoretical models indicate that overpressured buoyant hydrofractures in homogeneous isotropic host rocks should normally reach the surface in layered host rocks however abrupt changes in young s moduli horizontal discontinuities and layers with unusually high fracture perpendicular stresses encourage hydrofracture arrest it is proposed that for layer parallel loading stiff layers favour hydrofracture arrest during active compression but soft layers during extension it is concluded that for hydrofracture propagation to occur the stress field along its potential pathway must be essentially homogenous

spatial and depth variability of streambed vertical hydraulic conductivity under the regional flow regimes this study investigated the influence of the regional flow on the streambed vertical hydraulic conductivity k v within the hyporheic zone in three stream reaches of the weihe river in july 2016 the streambed k v with two connected depths was investigated at each test reach based on the sediment characteristics the three test reaches could be divided into three categories a sandy streambed without continuous silt and clay layer a sandy streambed with continuous silt and clay layer and a silt clay streambed the results demonstrate that the streambed k v mainly decreases with the depth at the sandy streambed without continuous silt and clay layer and increases with the depth at the other two test reaches at the sandy streambed with continuous silt and clay layer where streambed k v mainly decreases with the depth the regional upward flux can suspend fine particles and enhance the pore spacing resulting in the elevated k v in the upper sediment layers at another sandy streambed the continuous silt and clay layer is the main factor that influences the vertical distribution of fine particles and streambed k v an increase in streambed k v with the depth at the silt clay streambed is attributed to the regional downward movement of water within the sediments that may lead to more fine particles deposited in the pores in the upper sediment layers the streambed k v is very close to the bank in the sandy streambed without continuous silt and clay layer and the channel centre in the other two test reaches differences in grain size distribution of the sediments at each test reach exercise a strong controlling influence on the streambed k v this study promotes the understanding of dynamics influencing the interactions between groundwater and surface water and provides guidelines to scientific water resources management for rivers

the three dimensional geometry and growth of forced folds above saucer shaped igneous sills forced folds formed at the seabed immediately overlying shallow 1 km saucer shaped sills along the ne atlantic margin during the early paleogene examples of this sill fold relationship are exceptionally well imaged by high resolution 3d seismic datasets from the ne rockall basin the forced folds are domal in shape 2 4 km in diameter exhibit a structural relief of up to 350 m and comprise sediment volumes of ca 1 km3 a comparison of the thickness distribution across and volume of a saucer shaped sill with a high intrusion diameter to depth ratio and the structural relief and volume of its associated forced fold shows a remarkable equivalence this has the important implication that the structural relief on intrusion related forced folds can be used as an estimate of the thickness of the underlying sill the analysed forced folds are interpreted to have formed through three continual growth stages that are directly linked to the mechanical emplacement of the underlying saucer shaped sills their growth was associated with an increase in faulting of the overlying strata and influenced coeval or subsequent development of polygonal fault systems within the overburden these structures represent a new type of four way dip closed hydrocarbon trap

the effects of solidification on sill propagation dynamics and morphology sills are an integral part of the formation and development of larger plutons and magma reservoirs thus sills are essential for both the transport and the storage of magma in the earth s crust however although cooling and solidification are central to magmatism their effects on sills have been so far poorly studied here the effects of solidification on sill propagation dynamics and morphology are studied by means of analogue laboratory experiments hot fluid vegetable oil magma analogue that solidifies during its propagation is injected as a sill in a colder layered gelatine solid elastic host rock analogue the injection flux and temperature are maintained constant during an experiment and systematically varied between each experiment in order to vary and quantify the amount of solidification between each experiments the oil is injected directly at the interface between the two gelatine layers when solidification effects are small high injection temperatures and fluxes the propagation is continuous and the sill has a regular and smooth surface inversely when solidification effects are important low injection temperatures and fluxes sill propagation is discontinuous and occurs by steps of surface area creation interspersed with periods of momentary arrest the morphology of these sills displays folds ropy structures on their surface and lobes with imprints of the leading fronts that correspond to each step of area creation these experiments show that for a given constant injected volume as solidification effects increase the area of the sills decreases their thickness increases and the number of propagation steps increases these results have various geological and geophysical implications the morphology of sills such as lobate structures interpretation of 3d seismic studies in sedimentary basin and ropy flow structures field observations can be related to solidification during emplacement moreover a non continuous morphology as observed in the field does not necessarily involve multiple injections but could instead reflect a continuous yet complex morphology induced by solidification effects during emplacement also a discontinuous sill propagation induced by solidification effects should be associated with bursts of seismic activity finally our study shows that once a sill has initiated the dimensionless flux influences the sill thermal state and in turn its propagation and final extent and thickness in restricting the lateral extent of sills magma cooling and solidification are likely to impact directly the size of plutons constructed by amalgamated sills

biogenic flux of al to sediment in the central equatorial pacific ocean evidence for increased productivity during glacial periods we examined the flux of al to sediment accumulating beneath the zone of elevated productivity in the central equatorial pacific ocean along a surface sediment transect at 135 w as well as downcore for a 650 kyr record at 1 3 n 133 6 w across the surface transect a pronounced broadly equatorially symmetric increase in al accumulation is observed relative to ti with al ti ratios reaching values 3 4 times that of potential detrital sources the profile parallels biogenic accumulation and the modeled flux of particulate 234th suggesting rapid and preferential adsorptive removal of al from seawater by settling biogenic particles normative calculations confirm that most al is unsupported by the terrigenous fraction the observed distributions are consistent with previous observations of the relative and absolute behavior of al and ti in seawater and we can construct a reasonable mass balance between the amount of seawater sourced al retained in the sediment and the amount of seawater al available in the overlying column the close tie between al ti and biogenic accumulation as opposed to concentration emphasizes that biogenic sedimentary al ti responds to removal transport phenomena and not bulk sediment composition thus in these sediments dominated by the biogenic component the bulk al ti ratio reflects biogenic particle flux and by extension productivity of the overlying seawater the downcore profile of al ti at 1 3 n displays marked increases during glacial episodes similar to that observed across the surface transect from a background value near al ti of average upper crust the excursions in al ti are stratigraphically coincident with maxima in both bulk and caco3 accumulation and the excess al appears to not be preferentially affiliated with opaline or organic phases consistent with the similar behavioral removal of al and 234th the latter of which responds to the total particle flux the al flux reflects carbonate accumulation only because carbonate comprises the dominant flux in these particular deposits these results collectively indicate that 1 al in biogenic sediment and settling biogenic particles is strongly affected by a component adsorbed from seawater therefore the common tenet that al is dominantly associated with terrestrial particulate matter and the subsequent use of al distributions to calculate the abundance and flux of terrestrial material in settling particles and sediment needs to be reevaluated 2 the al ti ratio in biogenic sediment can be used to trace the productivity of the overlying water providing a powerful new paleochemical tool to investigate oceanic response to climatic variation 3 the close correlation between the al ti productivity signal and carbonate maxima downcore at 1 3 n suggests that the sedimentary carbonate maxima in the central equatorial pacific ocean record increased productivity during glacial episodes

organic matter production and preservation and evolution of anoxia in the holocene black sea dating of sediments collected in gravity cores during leg 1 of the 1988 r v knorr expedition to the black sea suggests that the onset of water column anoxia at 7 5 ka was virtually synchronous across the basin over a depth range of 200 2250 m a finely laminated organic carbon oc rich sapropel unit ii was produced as a result of this anoxia the trigger for increased oc production and development of anoxia was the spillover of saline waters through the bosporus that probably began at 9 0 ka and peaked between 7 0 and 5 5 ka this spillover enhanced vertical mixing and nutrient cycling and caused a short term 2 3 kyr burst in surface water productivity during the early part of unit ii deposition continued incursion of saline waters enhanced vertical stability and inhibited mixing of nutrients into surface waters thus limiting primary production and decreasing the oc flux to sediments beginning 5 5 ka concentration accumulation rate and degree of preservation of organic matter all decreased in the upper part of unit ii as a result of decreasing productivity but anoxia persisted throughout most of the water column the end of unit ii sapropel deposition was synchronous across the black sea as the result of the first blooms of the coccolith emiliania huxleyi which presumably marked an increase in surface water salinity above 11 and the beginning of unit i deposition the high coccolith carbonate fluxes that occurred during deposition of unit i diluted the oc concentration in the sediments but oc accumulation rates are about the same as those in upper part of unit ii

radial dikes laccoliths and gelatin models the northern adel mountains of central montana provide several excellent examples of laccoliths tangentially attached to feeder dikes that are members of radial swarms the radial distribution of the dikes appears to be controlled by stresses imposed by the load of an overlying central volcano model experiments using colored gelatin to simulate magma injected into a tank of uncolored gelatin duplicate the geometry of the radial dikes and field relations between feeder dikes and laccoliths as observed in the field

downstream patterns of bed material grain size in a large lowland alluvial river subject to low sediment supply 1 a new data set of bimodal subaqueous channel bed sediments was analyzed for longitudinal patterns in grain size it yielded two interesting observations 1 separate fining trends in d50 exist for gravel and fines that overlap for 175 river kilometers and 2 this overlap in fining trends results in a protracted nonabrupt gravel to sand transition these suggest bed patchiness that is interpreted in the context of inherent grain size bimodality localized hydraulic sorting due to spatially heterogeneous and low sediment supply and local deviations in channel gradient the data also show that tributaries have a minor impact on main stem downstream fining due to basin shape and tectonic history in addition subaqueous channel sediments were observed to be finer than nearby sediments collected from exposed bars which may indicate a narrow active zone of transport where bars are disconnected from sediment movement this research has implications for bed material collection and theories of downstream fining

cenozoic sill intrusion in the central and southern east china sea shelf basin abstract cenozoic igneous rocks dominated by basalts are exposed sporadically onshore east china and they provide important information about the cenozoic evolution of the region importantly their geometries and distribution are still not well constrained in the 200 600 km wide offshore continental shelf region based on seismic profiles this study presents the geometric characteristics of 133 igneous sills that are widely distributed in the central and southern regions of the east china sea shelf basin ecssb sills in high amplitude continuous seismic reflections mostly occur in a time interval of 1 1 3 3 s in multiple seismic profiles corresponding to a depth range of 1 1 4 8 km they can be classified into five types saucer shaped sills strata concordant sills laccoliths inclined sheets and hybrid sills of these the saucer shaped sills predominate strata deformation associated with these igneous sills indicate that sill intrusion has become intense since the miocene and it potentially affected petroleum systems in the ecssb forced folds induced by the sills are prospective petroleum traps we infer that sill intrusion in the ecssb was due to magma upwelling caused by dehydration and or small scale convection in a large mantle wedge above the stagnant pacific slab the subduction of the philippine sea plate resulted in tectonic inversion in the ecssb and extension in the okinawa trough since the early miocene triggered more magmatism in the tectonically thinned ecssb thick sedimentary strata over a thin crust in the ecssb may have induced more intrusive than extrusive cenozoic magmatism

evidence for a stronger oxygen minimum zone off central california during late pleistocene to early holocene of 31 deep sea cores collected along the central california continental slope 18 have distinctly laminated sediment at depth but none have laminations in the top few centimetres the cores with laminated facies are restricted to water depths between 508 and 1508 m but not all cores taken from this depth interval have laminated facies 14 c dates yield an extrapolated age of 4700 b p for the top of the uppermost laminated unit comparisons of the diatom flora in the laminated couplets with diatom floras in a 13 month sediment trap record suggest that the laminations are varvelike couplets of seasonal sedimentation the laminated facies represent a period from the last global deglaciation to early holocene when the oxygen minimum zone along the northeastern pacific ocean was stronger than at present a stronger oxygen minimum zone during this time is inferred to be the result of intensified upwelling

inorganic geochemical indicators of glacial interglacial changes in productivity and anoxia on the california continental margin abstract evidence from sediments in cores collected from within the present oxygen minimum zone omz 600 1200 m on the central and northern california margins record several episodes during the last interstadial ois 3 ca 60 24 ka of deposition of laminated sediments containing elevated concentrations of several trace elements indicative of anoxic conditions e g mo ni zn and cu the presence of abundant well preserved organic matter as well as lack of bioturbation and the presence of elevated concentrations of mo and other trace elements all support the theory that the omz in the northeastern pacific ocean was more intense possibly anoxic at several times during the late pleistocene sediments of all ages in cores from the southern california margin contain elevated concentrations of mo suggesting that this area has always had higher rates of sulfate reduction than either the central or northern california areas most of the ba in sediments in all cores collected on the upper continental slope 200 2700 m off california and southern oregon is derived from detrital clastic material and this source did not change much in time however the amount of biogenic ba did vary with time and these variations closely follow the temporal variations in organic c corg mass accumulation rate using ba and corg mass accumulation rates as proxy variables for productivity all cores show that organic productivity under the california current upwelling system was highest during ois 3 and the holocene and lowest during the last glacial interval lgi ca 24 10 ka all paleoproductivity proxy variables indicate that the southern california area has always experienced higher productivity than other areas under the california current at least over the last 50 ky

the shapes of dikes evidence for the influence of cooling and inelastic deformation we document the shape of dikes from well exposed field locations in the isle of rum scotland 14 and helam mine south africa the basaltic rum dikes crop out on a smaller scale than the 15 helam kimberlite dikes and have a smaller length to thickness ratio 100 1 isle of rum 16 1000 1 helam mine we compare dike thickness field measurements with the geometry 17 predicted by elastic theory finding best fit models to estimate magma overpressure and regional 18 stress gradients at the time of dike emplacement most of the dike shapes fit poorly with elastic 19 theory being too thick at the dike ends and too narrow in the middle our calculated 20 overpressures and stress gradients are much larger than independent estimates based on rock 21 strength dike shape can be explained by a combination of host rock inelastic deformation and 22 magma chilling at the dike s tapering edges preventing its closure as magma pressure declines 23 during emplacement the permanent wedging of the dike edges due to chilling has implications 24 for crustal magma transport and strain response in the crust due to dike emplacement

why meter wide dikes at oceanic spreading centers abstract numerical models show that maximum dike width at oceanic spreading centers should scale with axial lithospheric thickness if the pre diking horizontal stress is close to the andersonian normal faulting stress and the stress is fully released in one dike intrusion dikes at slow spreading ridges could be over 5 m wide and maximum dike width should decrease with increasing plate spreading rate however data from ophiolites and tectonic windows into recently active spreading ridges show that mean dike width ranges from 0 5 m to 1 5 m and does not clearly correlate with plate spreading rate dike width is reduced if either the pre diking horizontal stress difference is lower than the faulting stress or the stress is not fully released by a dike partial stress release during a dike intrusion is the more plausible explanation and is also consistent with the fact that dikes intrude in episodes at iceland and afar partial stress release can result from limited magma supply when a crustal magma chamber acts as a closed source during dike intrusions limited magma supply sets the upper limit on the width of dikes and multiple dike intrusions in an episode may be required to fully release the axial lithospheric tectonic stress the observation of dikes that are wider than a few meters such as the recent event in afar indicates that large tectonic stress and large magma supply sometimes exist

reply to comment by k thomson on the three dimensional geometry and growth of forced folds above saucer shaped sills by d m hansen and j cartwright

the elemental composition and decay series radionuclide content of plankton from the east pacific mixed plankton samples collected from both the east pacific and nearshore areas off southern california were analyzed for the elements na mg cl k ca sc v cr mn fe co zn se br sr ag sb i ba la ce sm eu th and u and the decay series nuclides 238u 234u 232th 230th 228th and 226ra the concentration populations for most elements 22 out of 26 were found to be normally distributed cross correlation coefficients calculated for all possible pairs of elements showed the following associations to be highly significant na 1bcl fe 1bco ra 1bcr ca 1bsb 1bzn sr 1bree 1bth 1bk 1bv while br and se were inversely correlated with ash content principal components analysis identified members of sea salt detrital silicate and authigenic element groups enrichment factors vs pelagic clay sea salt corrected also identified those elements that are biophilic concentration factors were calculated for each element relative to seawater indicating the following relative reactivity in the photic layer th ce fe sc la sm eu mn cr zn ba ag co se sb ra v i u sr ca br k mg na cl an inverse relationship was found to exist between the log of this concentration factor and the log of the oceanic mean residence time for each element calculated from river input data suggesting biologic transport as an important mechanism for cycling these elements between their introduction into the ocean and their ultimate incorporation into the sediments in addition a biologic mechanism appears likely as an explanation for the ce depletion in seawater relative to the other ree based on its higher concentration factor in marine plankton

dike propagation through layered rocks 1 dike penetration through a succession of upper crustal layers with different densities is studied with a new numerical code for an individual layer to significantly affect dike ascent its thickness must be of order 1 when scaled to the characteristic length scale for the inflated nose region that develops below the dike tip this characteristic length is l q 1 6 g 1 1 2 g 2 3 where and are the viscosity and buoyancy of magma g and are elastic moduli for the encasing rocks q is the magma flow rate and g gravity for basaltic dikes l is 1 km which is of the same order of magnitude as the typical thickness of sedimentary strata and volcanic deposits in such conditions dike ascent proceeds irregularly with large changes of velocity and width at an interface scaling laws for the ascent rate and dike width are derived penetration through low density layers is determined by a local buoyancy balance in the inflated nose region of the dike independently of the total buoyancy of the magma column between source and tip in such conditions a dike develops an internal overpressure that may be large enough to generate a horizontally propagating sill for this to occur the thickness of the low density layers must exceed a threshold value which depends only on the rock strength and on the average negative buoyancy of magma for basaltic melt we estimate that this threshold thickness cannot be less than about 700 m and is 2 km on average

saucer shaped sill with lobate morphology revealed by 3d seismic data implications for resolving a shallow level sill emplacement mechanism we use 3d seismic data to describe the 3d geometry of a large igneous intrusion the solsikke compound sill and address a number of issues related to sill emplacement the solsikke compound sill formed by amalgamation of a number of sills and exhibits a complex internal morphology dominated by saucer shaped depressions and linear discontinuities one of the saucer shaped sub elements of the compound sill the solsikke lobate sill has a previously unrecognized morphology it has a basal feeder and consists of a bifurcating network of interlinked lobe shaped sill segments we propose two models for the development of this intrusive style based on analogues from igneous systems and hydrofracturing experiments the lobate pattern indicates that the solsikke lobate sill was fed at its deepest point and adopted its geometry through outwards and upwards propagation the feeder location is coincident with a fault intersection suggesting that magma transport from the underlying source exploited the zone surrounding the intersection

fracture mode analysis and related surface deformation during dyke intrusion results from 2d experimental modelling surface deformation analysis in volcanic edifices in response to shallow magma intrusion is crucial for assessing volcanic hazards in this paper we discuss the effect of dyke propagation mode on surface deformation through 2d laboratory models our experimental setup consists of a hele shaw cell in which a model magma is injected into a cohesive model crust using an optical image correlation technique particle imaging velocimetry we measured the surface deformation the displacements and the strain field induced by magma emplacement within the country rock we identify two types of intrusion morphologies types a and b which exhibit two evolutional stages during the first stage both types resulted in a vertical dyke at depth its propagation was controlled by both shear deformation and tensile opening the model surface lifted up to form a smooth symmetrical dome resulting in tensile cracks during the second stage types a and b experiments differ when the dyke reaches a critical depth in type a the intrusion gradually rotates forming an inclined sheet dipping between 451 and 651 this rotation results in asymmetrical surface uplift and shear failure upon the tip of the dyke in type b the dyke tip interacts with tensile cracks formed during the first stage this fracture controls the subsequent propagation of the dyke toward the surface in both types of experiments intrusions result in surface uplift which can be accommodated by reverse faults our study suggests that dykes propagate as viscous indenters rather than linear elastic fracturing

conditions for the arrest of a vertical propagating dyke magma ascent towards the earth s surface occurs through dyke propagation in the vast majority of cases we investigate two purely mechanical effects unrelated to cooling or solidification that lead to the arrest of propagation so that no eruption occurs the first is that the input of magma from the source is not maintained continuously such that a fixed volume of magma is released laboratory experiments show that in this case the dyke stops at a finite distance from the source this behaviour is specific to the fracturing process in 3 d we derive a relationship for the minimum magma volume required for an eruption as a function of magma buoyancy and source depth when large magma volumes are available eruption may also be prevented by a thick low density layer in the upper crust numerical studies of dyke propagation show that the dyke continues to rise even though it is negatively buoyant magma accumulates in a swollen nose region at the interface between the low density layer and the dense basement magma overpressure is largest at this interface and increases with increasing penetration into the upper layer it may become large enough to induce horizontal fractures in the dyke walls and lateral intrusion of a sill which prevents eruption this requires that the thickness of the low density layer exceeds a threshold value that depends on the density contrast between magma and host rock if the magma volume is smaller than a threshold value neither sill intrusion nor eruption are possible and magma gets stored in a horizontal blade shaped dyke straddling the interface scaling laws for variations of ascent rate and for the minimum magma volume allow diagnosis of a failed eruption

dykes cups saucers and sills analogue experiments on magma intrusion into brittle rocks magma is transported in the crust by blade like intrusions such as dykes sills saucers and also collects in thicker laccoliths lopoliths and plutons recently the importance and great number o

a new model for the growth of basaltic shields based on deformation of fernandina volcano galapagos islands article history space geodetic measurements of surface deformation produced by the most recent eruptions at fernandina the most frequently erupting volcano in the galapagos archipelago reveal that all have initiated with the intrusion of subhorizontal sills from a shallow magma reservoir this includes eruptions from fissures that are oriented both radially and circumferentially with respect to the summit caldera a synthetic aperture radar sar image acquired 1 2 h before the start of a radial fissure eruption in 2009 captures one of these sills in the midst of its propagation toward the surface galapagos eruptive fissures of all orientations have previously been presumed to be fed by vertical dikes and this assumption has guided models of the origin of the eruptive fissure geometry and overall development of the volcanoes our findings allow us to reinterpret the internal structure and evolution of galapagos volcanoes and of similar basaltic shields furthermore we note that stress changes generated by the emplacement of subhorizontal sills feeding one type of eruption may control the geometry of subsequent eruptive fissures specifically circumferential fissures tend to open within areas uplifted by sill intrusions that initiated previous radial fissure eruptions this mechanism provides a possible explanation for the pattern of eruptive fissures that characterizes all the western galapagos volcanoes as well as the alternation between radial and circumferential fissure eruptions at fernandina the same model suggests that the next eruption of fernandina will be from a circumferential fissure in the area uplifted by the 2009 sill intrusion just southwest of the caldera rim 2013 elsevier b v all rights reserved

the roles of elastic properties magmatic pressure and tectonic stress in saucer shaped sill growth near surface igneous sills commonly exhibit saucer like shapes formed due to interaction with the earth s surface once some critical sill length is reached relative to its depth sill geometry has been strongly linked to the host material conditions particularly in terms of the elastic properties and shear cohesion of the host rock operating as primary controls on sill geometry here we conduct dynamic numerical simulations for sill growth in the near surface in which we vary the host rock properties magma pressure profile internal to the sill p and the externally applied tectonic stress r to consider their contributions to sill geometry we find that elastic properties alone have little impact on sill geometry saucer shapes reflect the additive stress components of the magma overpressure within the sill and the tectonic stress controlled by the locus of the maximum energy release rate gmax initially gmax is in plane with the sill but deflects to 25 at a critical base length rc relative to depth due to interaction between the sill and the free surface increasing r decreases this angle and increases rc of the sill p controls sill growth rate but has little effect on overall geometry host rock cohesion and elastic properties control the absolute magnitudes of r required to effect a change in sill geometry plain language summary horizontal magma pathways sills are a crucial part of volcanic plumbing systems acting as potential feeder conduits to volcanic eruptions and as magma storage systems saucer shaped sills which exhibit a flat inner region and inclined outer region are a common type of magma pathway in the shallow crust models for saucer shaped sills both as physical analogue models or numerical simulations typically under predict the length of the inner flat region and over predict the outer inclined region models are typically too short and too steep here we use numerical simulation to investigate parameters that may control sill shape we find that the dominant controls on sill shape are the competing effects of 1 bending of the rocks above the sill which promotes a transition to inclined growth typically at 25 and 2 plate tectonic shortening which serves to decrease the angle of incline towards 0 when the horizontal force is high increasing the applied horizontal tectonic force can produce sills that are up to five times longer in the inner region before growing as inclined sills at 5 this matches very closely with observations of natural sills indicating that tectonic forces are an important consideration in the growth of sills

remineralization of particulate authigenic trace metals in the middle atlantic bight implications for proxies of export production abstract samples collected by time series sediment traps deployed in the middle atlantic bight were studied to better understand the formation and preservation of particulate authigenic forms of trace metals cu ni ba that hold potential to serve as proxies in the sedimentary record of past changes in the flux of biogenic detritus sinking from the surface ocean into the deep sea export production particulate biogenic and authigenic phases are extremely labile as evidenced by the observation that as much as 70 of the particulate fluxes of organic carbon and of certain metals cu ni and mn and up to 25 of the particulate fluxes of authigenic ba and of opal collected by sediment traps are released rapidly into solution during the time period between particle collection and trap retrieval further remineralization on the seabed reduces concentrations of authigenic cu and ni in surface sediments below the limit of detection approximately 80 of authigenic ba is remineralized during early diagenesis on the seabed much more than is expected for conditions of high sediment mass accumulation rate that exist in the study area extensive remineralization during early diagenesis combined with large corrections required to remove the aluminosilicate contribution to the concentrations of cu ni and ba in sediments preclude the successful use of down core profiles of these trace metals to reconstruct past changes in export production of the middle atlantic bight similar problems are likely to plague paleoproductivity reconstructions in other ocean margin regions or wherever high fluxes of aluminosilicate phases occur

geometry and growth of sill complexes insights using 3d seismic from the north rockall trough doleritic sill complexes which are an important component of volcanic continental margins can be imaged using 3d seismic reflection data this allows unprecedented access to the complete 3d geometry of the bodies and an opportunity to test classic sill emplacement models the doleritic sills associated with basaltic volcanism in the north rockall trough occur in two forms radially symmetrical sill complexes consist of a saucer like inner sill at the base with an arcuate inclined sheet connecting it to a gently inclined commonly ragged outer rim bilaterally symmetrical sill complexes are sourced by magma diverted from a magma conduit feeding an overlying volcano with an elongate concave upwards trough like geometry bilaterally symmetrical sills climb away from the magma source from which they originate both sill complex types can appear as isolated bodies but commonly occur in close proximity and consequently merge producing hybrid sill complexes radial sill complexes consist of a series of radiating primary flow units with dimensions up to 3 km each primary flow unit rises from the inner saucer and is fed by primary magma tube primary flow units contain secondary flow units with dimensions up to 2 km each being fed by a secondary magma tube branching from the primary magma tube secondary flow units in turn are composed of 100 m scale tertiary flow units a similar branching hierarchy of flow units can also be seen in bilaterally symmetrical sill complexes with their internal architecture resembling an enlarged version of a primary flow unit from a radial sill complex this branching flow pattern as well as the interaction between flow units of varying orders provides new insights into the origin of the structures commonly seen within sill complexes and the hybrid sill bodies produced by their merger the data demonstrate that each radially symmetrical sill complex is independently fed from a source located beneath the centre of the inner saucer grows by climbing from the centre outwards and that peripheral dyking from the upper surface is a common feature these features suggest a laccolith emplacement style involving peripheral fracturing and dyking during inner saucer growth and thickening the branching hierarchy of flow units within bilaterally symmetrical sill complexes is broadly similar to that of primary flow units within a radially symmetrical sill complex suggesting that the general features of the laccolith emplacement model also apply

magma and sediment i emplacement mechanism of late carboniferous tholeiite sills in northern britain president s anniversary address 1981 like other great tholeiite sheets of the world the midland valley sill and whin sill were emplaced in thick accumulations of sediments which at the time of intrusion were not much older nor had been orogenically deformed however the carboniferous sediments of northern britain had been deposited under the influence of continuous differential subsidence so that they were already disposed in syn sedimentary basins and swells when intrusion began by drawing palinspastic contours of the sills relative to a late westphalian marine band approximating to the surface at the time of emplacement it is established that the dolerites mainly follow bedding planes dipping at angles of as much as 5 down to the bottoms of the basins isopachs of the sills indicate a further relationship for the intrusions are thickest at the bottoms of the basins these facts are not explained by any intrusion mechanism previously postulated though there seems to be broad agreement as to the processes which have given rise to thinning and en echelon lensing at leading edges as well as to step and stair transgression of sediment bedding it is proposed that emplacement must have been controlled in part by gravitational flow down dip from feeder dykes which extended to between 0 5 and 1 0 km below the contemporary surface thereafter hydrostatic equilibrium was attained first by accumulation at the bottom of the basins and then by further advance up dip under pressure of head the sheets are so similar in aspect to flood basalts that it must be assumed that the sediments offered only low frictional resistance to splitting and flow

pleistocene to holocene contrasts in organic matter production and preservation on the california continental margin organic matter in sediments from cores collected from the upper continental slope 200 2700 m off california and southern oregon shows marked differences in concentration and marine character between the last glacial interval ca 24 10 ka and either holocene time or last interstadial oxygen isotope stage 3 ca 60 24 ka in general sediments deposited during holocene time and stage 3 contain higher amounts of marine organic matter than those deposited during the last glacial interval and this contrast is greatest in cores collected off southern california the most profound differences in stage 3 sediments are between predominantly bioturbated sediments and occasional interbeds of laminated sediments the sediments are from cores collected within the present oxygen minimum zone on the upper continental slope from as far north as the oregon california border to as far south as point conception these upper pleistocene laminated sediments contain more abundant hydrogen rich type ii marine algal organic matter than even surface sediments that have large amounts of nonrefractory organic matter the stable carbon isotopic composition of the organic matter does not change with time between bioturbated and laminated sediments indicating that the greater abundance of type ii organic matter in the laminated sediments is not due to a change in source but rather represents a greater degree of production and preservation of marine organic matter the presence of abundant well preserved organic matter supports the theory that the oxygen minimum zone in the northeastern pacific ocean was more intense and possibly anoxic during late pleistocene time as a result of increased coastal upwelling that enhanced algal productivity

constraints on shallow mantle viscosity from morphology and deformation of fast spreading ridges 1 we show that the morphology and the deformation of a fastspreadingridgeconstrainshallowuppermantleviscosity a fast spreading center is simulated in a numerical model that couples tectonic deformation due to plate spreading and periodic dike emplacements the amount of magma intruded into dikes or extruded is enough to make a 7 km thick crust and the axial lithosphere is fixed at 2 km thickness for low viscosities 10 19 pa s the model spreadingcenter is marked by an axial high with relief governed by the axial density structure for high viscosities 10 20 pa s the axial morphology is a valley and extensional brittle deformation is distributed away from the near axis region the observed morphology and deformation of fast spreading ridges are not consistent with the high mantle viscosity estimated for very dry olivine as some suggest may result from partial melting related dehydration citation choi e and w r buck 2010 constraints on shallow mantle viscosity from morphology and deformation of fast spreading ridges geophys res lett 37 l16302 doi 10 1029 2010gl043681

numerical modeling of shallow magma intrusions with finite element method abstract a numerical approach for simulation of magma intrusion process considering the couplings of the stress distribution the viscous fluid flow of magma and the fracturing of host rock has been developed to investigate the mechanisms of fracture initiation and propagation in host rock during magma intrusion without pre placing a set of fractures the study focused on the dike intrusions filled with injected viscous magma in shallow sediments a series of numerical modellings were carried out to simulate the process of magma intrusion in host rocks with particular attention on the magma propagation processes and the formation of intrusion shapes the model materials were mohr coulomb materials with tension failure and shear failure the scenarios of both stochastically heterogeneous host rocks and layered host rocks were analyzed the injected magma formed intrusions shapes of a dyke b sill c cup shaped intrusion d saucer shaped intrusion the numerical results were in agreement with the experimental and field observed results which confirmed the adequacy and the power of the numerical approach

hydrogeologic controls on disconnection between surface water and groundwater 1 understanding how changes in the groundwater table affect surface water resources is of fundamental importance in quantitative hydrology if the groundwater table below a stream is sufficiently deep changes in the groundwater table position effectively do not alter the infiltration rate this is referred to as a disconnected system previous authors noted that a low conductivity layer below the surface water body is a necessary but not sufficient criterion for disconnection to occur we develop a precise criterion that allows an assessment of whether surface water groundwater systems can disconnect or not we further demonstrate that a disconnected system can be conceptualized by a saturated groundwater mound and the development of a capillary zone above this mound this conceptualization is used to determine the critical water table position at the point where full disconnection is reached a comparison of this calculated critical water table position with a measurement of the water table depth in a borehole allows the assessment of the disconnection status a sensitivity analysis of this critical water table showed that for a given aquifer thickness and river width the depth to groundwater where the system disconnects is approximately proportional to the stream depth and the hydraulic conductivity of the streambed sediments and inversely proportional to the thickness of these sediments and the hydraulic conductivity of the aquifer the conceptualization also allows the disconnection problem to be analyzed using both variably saturated and fully saturated groundwater models and provides guidance for numerical and analytical approaches

determining magma flow in sills dykes and laccoliths and their implications for sill emplacement mechanisms three dimensional seismic data from the faeroe shetland basin provides detailed information on the relationships between sills dykes laccoliths and contemporaneous volcanic activity the data shows that sills are predominantly concave upwards being complete or partial versions of radially or bilaterally symmetrical forms that possess flat inner saucers connected to a flat outer rim by a steeply inclined sheet such morphologies are only partially modified by pre existing faults sills can be sourced from dykes or the steep climbing portions of deeper sills both sills and dykes can provide magma to overlying volcanic fissures and sills can be shown to feed shallow laccoliths magma flow patterns as revealed by opacity rendering suggest that sills propagate upwards and outwards away from the magma feeder as an individual sill can consist of several leaves emplaced at different stratigraphic levels and as a sill or dyke can provide magma to volcanic fissures other sills and laccoliths the data suggests that neutral buoyancy concepts may not provide a complete explanation for the mechanism and level of sill emplacement instead the data suggests that the presence of lithological contrasts particularly ductile horizons such as overpressured shales may permit sill formation at any level below the neutrally buoyant level

the 2007 8 volcanic eruption on jebel at tair island red sea observed by satellite radar and optical images we use high resolution optical images and interferometric synthetic aperture radar insar data to study the september 2007 january 2008 jebel at tair eruption comparison of pre and post eruption optical images reveals several fresh ground fissures a new scoria cone near the summit and that 5 9 0 1 km2 of new lava covered about half of the island decorrelation in the insar images indicates that lava flowed both to the western and to the northeastern part of the island after the start of the eruption while later lavas were mainly deposited near the summit and onto the north flank of the volcano from the insar data we also estimate that the average thickness of the lava flows is 3 8 m resulting in a bulk volume of around 2 2 107 m3 we observe no volcano wide pre or post eruption uplift which suggests that the magma source may be deep the co eruption interferograms on the other hand reveal local and rather complex deformation we use these observations to constrain a tensile dislocation model that represents the dike intrusion that fed the eruption the model results show that the orientation of the dike is perpendicular to the red sea rift implying that the local stresses within the volcanic edifice are decoupled from the regional stress field

effects of hyporheic processes on streambed vertical hydraulic conductivity in three rivers of nebraska 1 streambed hydraulic conductivity is of great importance in the analysis of stream aquifer interactions and stream ecosystems we investigated streambed vertical hydraulic conductivity kv with two connected depths in three rivers of nebraska our results demonstrated that streambed kv in the upper sediment layer was much higher than that in the sediment of the lower layer we speculate that hyporheic processes can result in larger streambed kv in the upper layer specifically water exchange through upwelling and downwelling zones can lead to bigger pore spaces and a more unconsolidated structure of sediments in the upper layer the upward movement of gas produced by redox processes can loosen the sediments and further enlarge pore spaces in the upper layers also permeability can increase as a result of expanded pore spaces caused by invertebrate activities in the upper part of streambed the higher kv will likely enhance exchange processes between stream and sediments

statistical and geostatistical features of streambed hydraulic conductivities in the platte river nebraska this paper presents streambed hydraulic conductivities of the platte river from south central to eastern nebraska the hydraulic conductivities were determined from river channels using permeameter tests the vertical hydraulic conductivities k v from seven test sites along this river in south central nebraska belong to one statistical population its mean value is 40 2 m d however the vertical hydraulic conductivities along four transects of the ashland test site in eastern nebraska have lower mean values are statistically different from the k v values in south central nebraska and belong to two different populations with mean values of 20 7 and 9 1 m d respectively finer sediments carried from the loup river and elkhorn river watersheds to the eastern reach of the platte river lowers the vertical hydraulic conductivity of the streambed correlation coefficients between water depth and k v values along a test transect indicates a positive correlation a larger k v usually occurs in the part of channel with deeper water experimental variograms derived from the vertical hydraulic conductivities for several transects across the channels of the platte river show periodicity of spatial correlation which likely result from periodic variation of water depth across the channels the sandy to gravelly streambed contains very local silt and clay layers spatially continuous low permeability streambed was not observed in the river channels the horizontal hydraulic conductivities were larger than the vertical hydraulic conductivities for the same test locations

rise and emplacement of magma during horizontal shortening of the brittle crust insights from experimental modeling 1 magmatic activity tends to concentrate at plate margins at divergent margins extensional tectonics provide steep conduits for magma to reach the surface at rapidly convergent margins such as the andes one might imagine that horizontal compression prevents the rise of magma nevertheless volcanoes are also common in order to study the mechanisms by which magma rises in a compressional context we resorted to laboratory experiments in which a brittle crust was shortened while magma was intruding our model materials were 1 cohesive fine grained silica powder representing brittle crust and 2 molten low viscosity vegetable oil representing magma in general horizontal shortening and injection were coeval but independent processes thrust faults accommodated the shortening while overpressured oil formed hydraulic fractures in those experiments where there was no shortening injection resulted in a saucer shaped intrusive body in the other experiments where there was shortening oil formed a basal sill before rising along thrust faults once in place the sill lubricated the base of the model so that arcuate thrusts formed at the leading edge of a plateau uplift of the plateau promoted further intrusion of oil at depth in general the pattern of deformation and intrusion depended on the kinematic ratio r between rates of shortening and injection the lengths of the basal sill and plateau increased with decreasing r on the basis of these results we have reexamined two natural examples of magmatic complexes which were emplaced in compressional tectonic settings tromen volcano in argentina and the boulder batholith of montana

intrusion triggering of the 2010 eyjafjallajokull explosive eruption the ash cloud that brought chaos to european air traffic in april this year was the result of emissions from an icelandic volcano that had been intermittently active for about 18 years a combination of detailed space based geodetic measurements and seismic monitoring of the eyjafjallajkull volcano in the run up to the ash producing explosive summit eruption reveals an unusual deformation pattern that may be attributed to its off rift setting with relatively cool subsurface structure and limited magma at shallow depth as to the predictability of such a dramatic reawakening the immediate precursors to the initial eruption of the volcano in 2010 were subtle and difficult to detect but the clear signs of volcanic unrest during the weeks months and years preceding it may provide better clues to the catastrophic explosion

increased capture of magma in the crust promoted by ice cap retreat in iceland retreating ice caps overlying volcanoes are thought to relieve surface pressure causing enhanced magma generation and volcanic activity analysis of crustal stress during a dyke intrusion event associated with retreat of iceland s largest ice cap indicates that ice retreat could instead promote storage of magma in the crust rather than eruption at the surface

experimental modelling of shallow magma emplacement application to saucer shaped intrusions abstract saucer shaped dolerite and sandstone intrusions are common in sedimentary basins world wide we have conducted a series of scaled experiments simulating the process of magma emplacement in sedimentary basins with particular attention on the formation of saucer shaped sills the model materials were 1 cohesive fine grained silica flour representing brittle crust and 2 molten low viscosity oil representing magma the experiments were performed in both homogeneous and layered models in all the experiments oil injection resulted in doming of the surface in the homogeneous models the injected oil formed cone sheets and sub vertical dykes cone sheets formed for shallow injection 1 3 cm and vertical dykes formed for deeper injection 4 5 cm in layered models the injected oil always formed saucer shaped intrusions our experimental results show that 1 sill intrusion results in the formation of a dome with melt erupting at the rim 2 layering controls the formation of sills and saucer shaped sills 3 saucer shaped sills are fed from the bottom and the fluid flows upward and outward and 4 the diameter of saucer shaped sills increase with increasing emplacement depth the systematic relation between domes and sills and the depth dependence of sill diameters show that saucer shaped intrusions result from the interaction between a growing flat lying shallow sill and doming of the free surface we conclude that saucer shaped intrusions represent fundamental geometries formed by shallow magma intrusion in stratified basins

variability in the vertical hyporheic water exchange affected by hydraulic conductivity and river morphology at a natural confluent meander bend river confluences and their associated tributaries are key morphodynamic nodes that play important roles in controlling hydraulic geometry and hyporheic water exchange in fluvial networks however the existing knowledge regarding hyporheic water exchange associated with river confluence morphology is relatively scarce on the 14th and 15th of january 2016 the general hydraulic and morphological characteristics of the confluent meander bend cmb between the juehe river and the haohe river in the southern region of xi an city shaanxi province china were investigated the patterns and magnitudes of vertical hyporheic water exchange vhwe were estimated based on a one dimension heat steady state model while the sediment vertical hydraulic conductivity kv was calculated via in situ permeameter tests the results demonstrated that six hydrodynamic zones and their extensions were observed at the cmb during the test period these zones were likely controlled by the obtuse junction angle and low momentum flux ratio influencing the sediment grain size distribution of the cmb the vhwe patterns at the test site during the test period mostly showed upwelling flow dominated by regional groundwater discharging into the river the occurrence of longitudinal downwelling and upwelling patterns along the meander bend at the cmb was likely subjected to the comprehensive influences of the local sinuosity of the meander bend and regional groundwater discharge and finally formed regional and local flow paths additionally in dominated upwelling areas the change in vhwe magnitudes was nearly consistent with that in kv values and higher values of both variables generally occurred in erosional zones near the thalweg paths of the cmb which were mostly made up of sand and gravel this was potentially caused by the erosional and depositional processes subjected to confluence morphology furthermore lower kv values observed in downwelling areas at the cmb were attributed to sediment clogging caused by local downwelling flow the confluence morphology and sediment kv are thus likely the driving factors which cause local variations in the vhwe of fluvial systems

large scale conical sandstone intrusions and polygonal fault systems in tranche 6 faroe shetland basin abstract kilometre scale sandstone intrusions are important components of many deep water clastic systems they result from the remobilisation and injection of sand during early stages of burial and are commonly found in fine grained mudrocks containing a pervasive polygonal fault network this paper uses three dimensional seismic data from the faroe shetland basin on the uk atlantic margin calibrated to nearby wells to examine the relationship between an extensive system of large downward tapering conical sandstone intrusions to the pervasive polygonal fault network present within the surrounding encasing host mudstones the intrusions were injected upwards during the late miocene through eocene oligocene claystones with a pre existing polygonal fault network to form conical bodies up to 300 m high within the 700 m thick claystone sequence this study shows that polygonal faults and the discordant limbs of conical sandstone intrusions form two distinct dip populations with little overlap that have average dips of 58 and 26 respectively polygonal faults display decreasing dip with increasing depth however conical sandstone intrusion dip is uniform throughout the eocene oligocene succession these observations lead us to conclude that the geometry of conical sandstone intrusions is not greatly affected by the presence of a pre existing polygonal fault network nor is it affected by increased overburden pressure with depth therefore the 10 50 dip range displayed by the limbs of conical sandstone intrusions observed within the study area is a fundamental property of conical sandstone intrusions the presence of a widespread region of conical sandstone intrusions in the faroe shetland basin has important implications for vertical fluid migration in the region the eocene oligocene claystones in the region have very low permeability but the presence of large scale conical sandstone intrusions and any possible feeder systems is likely to provide high permeability fluid conduits through this sequence in effect the otherwise good regional caprock provided by the eocene oligocene claystone sequence is breached by a network of sandstone intrusions possibly resulting in reduced hydrocarbon trapping potential beneath the seal

on origin of near axis volcanism and faulting at fast spreading mid ocean ridges at fast and superfast spreading mid ocean ridges such as the east pacific rise a plate boundary is defined by a narrow tens to hundreds of meters wide neovolcanic zone within which the bulk of the upper oceanic crust is created however detailed near bottom observations indicate that the volcanic construction may occasionally persist several kilometers off of the ridge axis it has been proposed that off axis volcanism manifests tapping of a wide melting region that supplies magma to the ridge axis or spatial migration of magmatic sources in the crust and upper mantle we demonstrate that off axis eruptions may be a natural consequence of variations in magma supply rate even if the ridge axis is stationary in space and the magma delivery is perfectly focussed at the ridge axis theoretical modeling and field observations indicate that off axis volcanism may result from magma emplacement in sills that propagate toward the surface after their characteristic horizontal size exceeds their emplacement depth volcanic construction and faulting due to sill intrusions may contribute to the formation of abyssal hills arguably the most abundant relief form on earth fl 2001 elsevier science b v all rights reserved

constraints on magma chamber geometry at sierra negra volcano galapagos islands based on insar observations abstract we investigate the problem of estimating magma chamber geometry using insar observations of sierra negra volcano galapagos ascending and descending interferograms are combined to determine vertical and one horizontal component of displacement the ratio of maximum horizontal to vertical displacement suggests a sill like source spherical or stock like bodies are inconsistent with the data we estimate the geometry of the sill assuming a horizontal uniformly pressurized crack with unknown periphery and depth the sill is discretized into small elements that are either open and are subjected to the pressure boundary condition or remain closed we find the best fitting sill to be located beneath sierra negra s inner caldera at a depth of about 2 km using boundary element calculations we show that any magma chamber with a flat top coincident with the sill model fits the data equally well the data are insensitive to the sides and bottom of the magma chamber

craters of elevation revisited forced folds bulging and uplift of volcanoes the eighteenth nineteenth century craters of elevation theory required magma to uplift strata doming the surface and creating a central down fallen crater or graben exponents of craters of elevation attempted to apply it to explain the origin of all volcanoes and rapidly the contemporary competing craters of eruption theory replaced it as the paradigm for volcano construction several historic examples have shown that intrusions can cause uplift termed bulges and can create features like those proposed for craters of elevation e g at usu 1944 bezymianny 1955 and mt st helens 1980 work on sedimentary basins that have had igneous activity has shown that intrusions create forced folds that uplift and deform strata in a similar way to that originally proposed for craters of elevation in view of the above we investigate large scale intrusion related topographic changes at two sites where the craters of elevation theory was developed the monogenetic volcanoes of the chaine des puys france and the teide stratovolcano tenerife we combine observations of such features with examples of forced folding to integrate the two fields of research our observations in the chaine des puys show that 1 the petit puy de dome has a bulge of up to 150 m uplift the uplift has a central depressed area a graben a dense network of normal faults basal thrusts and an aborted landslide 2 the grosmanaux volcano is a forced fold created by uplift of a previously flat lying area and has dense faulting and a graben on the resultant topographic bulge it was the site also of a major vulcanian eruption from the associated kilian crater 3 the gouttes volcano was uplifted by an intrusion like the petit puy de dome but then collapsed to generate a landslide and lateral blast 4 excavation in the lemptegy volcano exposes intra eruption intrusions with associated uplift providing examples in cross section of the internal deformation likely to be found inside other chaine des puys uplifted bulges on teide a bulge near the summit shows similar structures and surface tilting as seen on the petit puy de dome and this bulging may have formed during the eruption of the lavas negras the most recent activity on the summit area fault scarps on teide also expose small cryptodomes like those seen at lemptegy these examples coupled with field studies on eroded intrusions data on forced folds in basins and analogue models show how large scale topographic remodelling and structural change can be created by intrusions these can rapidly and significantly change the volcanic edifice a crater of elevation bulge or forced fold that is stabilised by the cooling of the intrusion will remain an important structural element in a volcano this process starts even at the small scale of monogenetic volcanoes and could occur through the lifetime of any growing stratovolcano such activity may be commonplace but may be masked by concomitant eruption or removed by subsequent collapse monitoring and hazard strategies should be ready to deal with such large scale events that will seriously modify the eruptive activity and stability of a volcano within days or weeks

an experimental investigation of sill formation and propagation in layered elastic media a series of experiments are described where dyed water a magma analogue was intruded into solid gelatine a crustal analogue to investigate the formation of sills we considered a layered gelatine system with contrasting adjacent layers by varying the density and rigidity of the gelatine we found that experimental sills form when the upper layer is more rigid than the lower layer with intrusion occurring in a plane directly below the interface experimental dykes were observed to propagate to the surface when the young s modulus ratio of upper to lower gelatine layers was less than one experimental dyke arrest occurred when the upper layer was more rigid and the interface was strong two varieties of experimental sill formed when the upper layer was more rigid than the lower layer and the interface was sufficiently weak the form of the intrusion depends on the balance of driving pressures and the young s modulus ratio of contrasting adjacent layers when the rigidity ratio is high and there is a large driving pressure the experimental feeder dyke completely converts to propagate as a sill however when the rigidity ratio and driving pressure are both close to one a dyke sill hybrid forms under these conditions the experimental sill formation is accompanied by contemporaneous dyke intrusion into the overlying more rigid layer during sill propagation deformation structures such as faults and en echelon fractures are formed into the lower layer experimental sill propagation dynamics are controlled by viscous dissipation along the length of the sill causing acceleration with increasing length our study suggests that rigidity contrasts may play a major role in the location of sills and development of igneous complexes in ancient cratonic areas the moho is a suitable site for the preferential formation of sills with higher rigidity continental crust overlying weaker mantle mantle plumes impacting ancient continents provide a situation in which large sills can form to fractionate prior to eruption of flood basalts the boundary between the upper and lower crust conrad discontinuity may provide a preferential focus for the emplacement of sheets of silicic magma at continental arcs where the lower crust is weakened by prolonged heating and possible hydration

how are saucer shaped sills emplaced constraints from the golden valley sill south africa 1 magmatic sill intrusions develop saucer like geometries in layered sedimentary basins intruded by large volumes of magma saucer shaped sill emplacement models are based on the analysis of the intrusion geometry and their spatial relationships with potential feeders not on magma flow pattern the karoo basin of south africa hosts hundreds of saucer shaped sills among these the golden valley sill is well exposed and displays connections with adjacent and nested saucers a combination of detailed field observations and anisotropy of magnetic susceptibility measurements ams were used to identify strain markers that can be interpreted in terms of magma flow directions a total of 113 localities six specimens per site mostly including pairs of opposite sill margins were sampled for anisotropy of magnetic susceptibility analyses the magnetic properties were determined by measuring hysteresis loops and k t curves the majority of the localities display well defined magnetic foliations that consistently dip outward from the center of the golden valley sill this orientation of the magnetic foliation most likely represents inflation deflation cycles of the intruding sill that interacts with nonstatic enclosing walls in addition magma channels and ropy flow structures were identified and display an imbrication of the magnetic foliation indicating an outward magma flow direction the observed magma flow geometries derived from macroscopic flow indicators and the ams data correlate well and constrain an emplacement model for the golden valley sill complex our emplacement model integrates our observations with dynamic emplacement processes characterized by inflation and deflation cycles

use of vegetable oil and silica powder for scale modelling of magmatic intrusion in a deforming brittle crust in the upper crust intrusive bodies adopt different configurations depending on the tectonic setting this paper describes a new technique for analogue modelling of such intrusions specifically of low viscosity magma into a deforming brittle crust proper dynamic scaling is an important consideration if the crust has a coulomb failure envelope the model material should have a cohesion c of 40 pa to 7500 pa if the intruding magma is highly mobile the model fluid should have a viscosity of 4 10 9 pa s to 75 pa s for the model crust we have used crystalline silica powder si crystal siliceous microspheres si sphere of grain size 30 m and a mixture si mix of both materials the mechanical properties of these powders have been obtained by shear and tension shear tests si crystal powder is cohesive c 300 pa angle of internal friction 45 and represents competent rock si sphere powder is much less cohesive c 10 pa 25 and represents incompetent rock si mix has intermediate properties the model magma is a vegetable oil which solidifies at room temperature its viscosity when molten is 2 10 2 pa s at 50 c using these materials we have done some preliminary experiments to investigate the intrusion of low viscosity magma into sedimentary basins in non deformed settings intrusions were saucer shaped cone sheets in horizontal extension they were steep dykes finally in horizontal shortening intrusions were basal sills that branched into thrust faults

geostatistical characterization of groundwater flow parameters in a simulated aquifer an analog method is used to simulate a discrete transmissivity field free of the artifacts inherent in conventional multivariate gaussian statistical generation methods the simulated field provides a realistic model of a formation exhibiting the spatial continuity of extreme transmissivities which gives rise to barriers and preferential channels for flow the heterogeneous transmissivity field and the corresponding steady state head and discharge fields are characterized in a geostatistical framework and observed spatial statistics are compared with theoretical results from the stochastic hydrogeology literature the simulated transmissivity field exhibits a bimodal distribution and strong directional anisotropy with nested scales of heterogeneity despite these significant departures from standard models the observed head covariance and head log transmissivity cross covariance agree well with theory the spatial covariance of specific discharge is highly anisotropic reflecting preferential channeling in the mean direction of flow and the conservation of flux along streamlines the effective transmissivity of the field is predicted more accurately by the spatial geometric average than by theoretical models despite the flow channeling and the anisotropy of heterogeneity tracer spreading modeled by particle tracking is non fickian at displacements of up to 13 4 times the log transmissivity integral range because transport behavior is dominated by convection along a small number of preferential channels the observed scale dependence of apparent longitudinal dispersivity is predicted generally well by theory results of this study suggest that ensemble theoretical models based on perturbation approaches can provide reasonable estimates of general flow and transport properties of single field realizations under moderate conditions of heterogeneity

comparison of vertical hydraulic conductivity in a streambed point bar system of a gaining stream summary vertical hydraulic conductivities k v of both streambed and point bars can influence water and solute exchange between streams and surrounding groundwater systems the sediments in point bars are relatively young compared to the older sediments in the adjacent aquifers but slightly older compared to submerged streambeds thus the permeability in point bar sediments can be different not only from regional aquifer but also from modern streambed however there is a lack of detailed studies that document spatial variability of vertical hydraulic conductivity in point bars of meandering streams in this study the authors proposed an in situ permeameter test method to measure vertical hydraulic conductivity of the two point bars in clear creek nebraska usa we compared the k v values in streambed and adjacent point bars through 45 test locations in the two point bars and 51 test locations in the streambed the k v values in the point bars were lower than those in the streambed kruskal wallis test confirmed that the k v values from the point bars and from the channel came from two statistically different populations within a point bar the k v values were higher along the point bar edges than those from inner point bars grain size analysis indicated that slightly more silt and clay particles existed in sediments from inner point bars compared to that from streambed and from locations near the point bar edges while point bars are the deposits of the adjacent channel the comparison of two groups of k v values suggests that post depositional processes had an effect on the evolution of k v from channel to point bars in fluvial deposits we believed that the transport of fine particles and the gas ebullition in this gaining stream had significant effects on the distribution of k v values in a streambed point bar system with the ageing of deposition in a floodplain the permeability of point bar sediments can likely decrease due to reduced effects of the upward flow and gas ebullition

saucer shaped intrusions occurrences emplacement and implications abstract magmatic intrusions and fluidized injected sands represent the two main types of sheet intrusions in the earth s crust in this paper we show that both intrusion types often display a saucer like geometry as revealed by 3d seismic imaging and field observations saucer shaped intrusions are fairly common in sedimentary basins as for example offshore in the norwegian and north sea basins and onshore in the karoo basin of south africa the formation of the saucer geometry is controlled by the low viscosity of the injected fluid and by the interaction between a growing shallow hydraulic fracture and overburden deformations statistics gathered from observations and modelling show a linear relationship between the depth of emplacement and the size of the saucer shaped intrusions we anticipate future cross disciplinary studies aiming to discover other occurrences of saucer shaped intrusions and to identify the physical processes controlling the development of this fundamental geometry

paleoproductivity and carbon burial across the california current the multitracers transect 42 n the multitracers experiment studied a transect of water column sediment trap and sediment data taken across the california current to develop quantitative methods for hindcasting paleoproductivity the experiment used three sediment trap moorings located 120 km 270 km and 630 km from shore at the oregon california border in north america we report here about the sedimentation and burial of particulate organic carbon corg and caco3 in order to observe how the integrated caco3 and corg burial across the transect has changed since the last glacial maximum we have correlated core from the three sites using time scales constrained by both radiocarbon and oxygen isotopes by comparing surface sediments to a two and a half year sediment trap record we have also defined the modern preservation rates for many of the labile sedimentary materials our analysis of the corg data indicates that significant amounts 20 40 of the total corg being buried today in surface sediments is terrestrial at the last glacial maximum the terrestrial corg fraction within 300 km of the coast was about twice as large such large fluxes of terrestrial corg obscure the marine corg record which can be interpreted as productivity when we corrected for the terrestrial organic matter we found that the mass accumulation rate of marine corg roughly doubled from the glacial maximum to the present because preservation rates of organic carbon are high in the high sedimentation rate cores corrections for degradation are straightforward and we can be confident that organic carbon rain rate new productivity also doubled as confirmation the highest burial fluxes of other biogenic components opal and ba also occur in the holocene productivity off oregon has thus increased dramatically since the last glacial maximum caco3 fluxes also changed radically through the deglaciation however they are linked not to caco3 production but rather to changes in deepwater carbonate chemistry between 18 ka and now

buoyancy driven magma fracture a mechanism for ascent through the lithosphere and the emplacement of diamonds magma driven fractures must be an important mechanism for the transport of magma through the crust and lithosphere dikes are pervasive throughout the crust and clearly play an important role in active volcanoes a magma fracture is the only mechanism that allows sufficiently high magma velocities to prevent complete solidification during transport through cold country rock the analysis of magma driven fractures is complex because it involves flow in the fracture often turbulent the elastic deformation of the country rock surrounding the fracture and the propagation of the fracture for vertically propagating magma fractures buoyancy forces may be the dominant driving mechanism in order to simplify the analysis of this problem we neglect the elastic distortion of the country rock and the fracturing process we consider a buoyant fluid initially in an elliptical two dimensional cavity the shape of the upwardly deforming cavity is shown to satisfy the nonlinear kinematic wave equation for laminar flow the cavity initially deforms but does not move upward at a critical time the solution near the top of the crack becomes multivalued indicating the presence of an upwardly propagating shock wave the tail of the upwardly propagating crack closes but does not move upwards solutions have also been obtained for turbulent flow velocities of upward propagation for appropriate parameter values indicate this mechanism can explain the observed characteristics of kimberlite eruptions inclusion of elastic and fracture properties will determine the structure of the shock wave that is the crack tip but would not effect the applicability of our solution