seafloor weathering and the oxygen isotope ratio in seawater insight from whole rock δ18o and carbonate δ18o and δ47 from the troodos ophiolite abstract the controls on and history of the oxygen isotope ratio in seawater continue to be debated after many decades of research with the lack of consensus in large part reflecting uncertainty in the role of hydrothermal exchange between seawater and the oceanic crust we have investigated this using new carbonate 47 and 18 o data and whole rock o isotope data for samples from the lava section of the troodos ophiolite carbonate data confirm that fluid to rock ratios in the upper lavas during off axis hydrothermal circulation are generally sufficiently large that both the fluid 18 o and temperature are similar to those of bottom water however some samples require more complicated interpretations that could reflect changes in the rate of calcite formation whole rock data indicate that o isotope exchange in the lavas is directly linked to the major element exchange that leads to alkalinity production i e co2 consumption and both are dependent on bottom water temperature this means that the o isotopic composition of seawater is linked to the long term c cycle the data are used to parameterise a simple model of the evolution of the o isotopic composition of seawater driven by changes in solid earth co2 degassing alkalinity balance links the total extent of weathering of the continents and seafloor which are sinks for high 18 o material to co2 degassing rate and surface temperature the modelling suggests that if solid earth co2 degassing and the rate of formation of oceanic crust are linked the o isotopic composition of the ocean including any ice sheets is unlikely to have varied more than 1 over the phanerozoic

stable isotope compositions of serpentinite seamounts in the mariana forearc serpentinization processes fluid sources and sulfur metasomatism the mariana and izu bonin arcs in the western pacific are characterized by serpentinite seamounts in the forearc that provide unique windows into the mantle wedge we present stable isotope o h s and c data for serpentinites from conical seamount in the mariana forearc and s isotope data for torishima seamount in the izu bonin forearc in order to understand the compositions of fluids and temperatures of serpentinization in the mantle wedge and to investigate the transport of sulfur from the slab to the mantle wedge six serpentine mineral separates have a restricted range of 18o 6 5 8 5 antigorite separates have d values of 29 5 to 45 5 that reflect serpentinization within the mantle wedge whereas chrysotile has low d values 51 8 to 84 0 as the result of re equilibration with fluids at low temperatures fractionation of oxygen isotopes between serpentine and magnetite indicate serpentinization temperatures of 300 375 c two late cross fiber chrysotile veins have higher 18o values of 8 9 to 10 8 and formed at lower temperatures as low as 100 c aqueous fluids in equilibrium with serpentine at 300 375 c had 18o 6 5 9 and d 4 to 26 consistent with sediment dehydration reactions at temperatures 200 c in the subducting slab rather than a basaltic slab source three aragonite veins in metabasalt and siltstone clasts within the serpentinite flows have 18o 16 7 24 5 consistent with the serpentinizing fluids at temperatures 250 c 13c values of 0 1 2 5 suggest a source in subducting carbonate sediments the 34s values of sulfide in serpentinites on conical seamount 6 7 to 9 8 result from metasomatism through variable reduction of aqueous sulfate 34s 14 derived from slab sediments despite sulfur metasomatism serpentinites have low sulfur contents generally 164 ppm that reflect the highly depleted nature of the mantle wedge the serpentinites are mostly enriched in 34s median 34ssulfide 4 5 consistent with a 34s enriched mantle wedge as inferred from arc lavas

fluid driven nucleation and propagation of splay fractures from a permeable fault progressive development of opening mode splay or branch fractures along a permeable fault in an elastic medium subject to elevated fluid pressure from a constant influx fluid source is studied numerically using a plane strain hydraulic fracturing model that couples fracture deformation and fluid flow in situ stresses are imposed so that their resultant shear stress on the fault is lower than the frictional strength new splay fractures are initiated based on satisfying a dual criterion for both tensile strength and fracture toughness and meeting a minimum fracture spacing requirement numerical results demonstrate that spatial variations in permeability along faults can cause arrest of local slip and the created slip gradient can result in splay fracture initiation at a significant distance inward from the fault tips one splay fracture generally grows first and the kinematic coherence in displacements at the junction between it and the fault can reduce the downstream flow rate to prevent the nucleation of other splay fractures however the number of splay fractures can be increased when the branch growth extent is limited to a certain size when the fault is divided into many segments each with a linear distributed initial aperture and if the main fault is curved the generation of a number of splay fractures can act to increase the permeability of the rock mass when the splay fractures are constrained by two faults a rhomb shaped fault zone involving multiple high angle branches forms the development of the second generation splay fractures on a first generation one is promoted by fluid penetration multiple fluid driven splay fractures can be created under the condition that fluid pressure is below or slightly above the fault confining stress this implies that generation of complex splay fracture patterns requires less energy than generating a single opening mode fracture multiple fluid driven splay fracture nucleation and growth into a horsetail pattern occur when the fault parallel in situ normal stress becomes more tensile in addition the effects of fluid viscosity and tensile fracture toughness are investigated to determine their role in splay fracture initiation and growth

impact of atmospheric co2 and galactic cosmic radiation on phanerozoic climate change and the marine δ18o record 1 a new model is developed and applied to simulate the phanerozoic evolution of seawater composition dissolved ca sr dissolved inorganic carbon alkalinity ph 18o marine carbonates sr ca 87sr 86sr 13c 18o atmospheric co2 and surface temperature the marine carbonate records sr ca 87sr 86sr 13c are used to reconstruct changes in volcanic tectonic activity and organic carbon burial over the phanerozoic seawater ph is calculated assuming saturation with respect to calcite and considering the changing concentration of dissolved ca documented by brine inclusion data the depth of calcite saturation is allowed to vary through time and the effects of changing temperature and pressure on the stability constants of the carbonate system are considered surface temperatures are calculated using the geocarb iii approach considering also the changing flux of galactic cosmic radiation gcr it is assumed that gcr cools the surface of the earth via enhanced cloud formation at low altitudes the 18o of marine carbonates is calculated considering the changing isotopic composition of seawater the prevailing surface temperatures and seawater ph repeated model runs showed that the trends observed in the marine 18o record can only be reproduced by the model if gcr is allowed to have a strong effect on surface temperature the climate evolution predicted by the model is consistent with the geological record warm periods cambrian devonian triassic cretaceous are characterized by low gcr levels cold periods during the late carboniferous to early permian and the late cenozoic are marked by high gcr fluxes and low pco2 values the major glaciations occurring during these periods are the result of carbon cycling processes causing a draw down of atmospheric co2 and a coevally prevailing dense cloud cover at low altitudes induced by strong gcr fluxes the two moderately cool periods during the ordovician silurian and jurassic early cretaceous are characterized by both high pco2 and gcr levels so that greenhouse warming compensated for the cooling effect of low altitude clouds the very high jurassic 18o values observed in the geological record are caused by low ph values in surface waters rather than cold surface conditions

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

mechanics of fluid driven fracture growth in naturally fractured reservoirs with simple network geometries 1 a numerical model has been developed for fluid driven opening mode fracture growth in a naturally fractured formation the rock formation contains discrete deformable fractures which are initially closed but conductive because of their preexisting apertures fluid flow that develops along fractures depends on fracture geometry defined by preexisting aperture distribution offsets along a fracture path and intersections of two or more fractures the model couples fluid flow elastic deformation and frictional sliding to obtain the solution which depends on the competition between fractures for permeability enhancement the fractures can be opened by fluid pressure that exceeds the normal stress acting on them and by interactions with intersecting closed fractures experiencing coulomb type frictional slip the newtonian fluid is assumed to flow through the conductive fractures according to a lubrication equation that relates the cube of an equivalent hydraulic aperture to fracture conductivity the rock material is assumed to be impermeable and elastic this paper provides the governing equations for the multiple fracture systems and the solution methods used flow distribution and fracture growth in conductive fracture sets are simulated for a range of geometric arrangements and hydraulic properties numerical results show that elastic interaction between fracture branches plays a controlling role in fluid migration although initial apertures can give rise to a preferential fluid flow direction during the early stage in the presence of offsets fracture segments subject to strong compression are difficult to open hydraulically and their resulting smaller permeability can increase overall upstream fracture pressure and opening the patterns of fluid flow become more complicated if fractures intersect each other a portion of injected fluid is lost into closed empty fractures that cut across the main hydraulic fracture and this delays the pressure increases required for fracture growth past the crosscutting fracture the nonlinear fluid loss rate depends on the geometric complexities of the fracture sets and on the fluid viscosity sometimes fracture growth can be accelerated by the fast fluid transport along an intersected relatively conductive natural fracture

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

stable isotopic composition of upper oceanic crust formed at a fast spreading ridge odp site 801 1 odp legs 129 and 185 sampled the upper 474 m of 170 ma ocean crust in the western pacific in order to investigate alteration processes in fast spread crust and to determine inputs to subduction fourteen composite bulk samples of altered upper oceanic crust from site 801 have 18o 8 7 25 7 d 69 4 to 90 4 and 13c 2 7 to 1 8 the intensity of alteration and the amount of sediment within the basement decrease with depth leading to corresponding decreases in 18o and d a super composite constructed to estimate the bulk composition of the upper crust has 18o 12 0 d 87 0 and 13c 0 7 compared to core descriptions and geophysical logs the super composite contains too much 18o rich sediment 18o 25 7 and a corrected 18o value of 10 8 is more reasonable for the upper crust at site 801 these values are higher than those for other bulk upper oceanic basement sections 18o 8 0 10 0 and result in part from 1 intense low temperature 100 c hydrothermal alteration of the upper 100 m of tholeiitic basement at site 801 that may not be representative of material subducting in the western pacific and 2 an aging effect whereby progressive addition of 18o rich secondary carbonate in veins and breccia cements contributed to the high bulk 18o of this old upper crustal section

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

oxygen isotope study of mid ocean ridge hydrothermal fluids implication for the oxygen 18 budget of the oceans new data are reported for the 18o composition of hot hydrothermal fluids from various sites with different tectonic settings the measured 18o in the range 0 53 2 3 confirm earlier data suggesting that mid ocean ridge hydrothermal vents are a major source of 18o to the ocean in light of this new data we attempt to re evaluate the marine 18o budget from direct quantification of the main sources and sinks of oceanic oxygen 18 within the uncertainty of the flux estimates 18o inputs and outputs are found to balance each other showing that oceanic 18o is presently at a steady state using these present fluxes we examine the past evolution of the 18o composition of the ocean and speculate that the initial 18o oceanic content may have evolved rapidly towards its present value this suggests as already pointed out in previous studies that ocean water may have had a rather constant 18o composition over most of the earth s history and that 18o measurements in ancient cherts and carbonates may primarily record the evolution of the ocean temperature

mechanisms of dike propagation in layered rocks and in massive porous sedimentary rocks the mechanisms of dike propagation are analyzed through detailed examination of small structures associated with the dike intrusion exposures in the ramon area israel offer an opportunity to evaluate theoretical models of tensile fracture propagation and to better understand the factors governing the dike geometry dike related structures are studied in two jurassic units the ardon formation a well stratified sequence of shales and dolomites and the inmar formation which is composed of porous massive sandstones with a few shale lenses it is shown that for the different host rocks the propagation mechanisms and structures differ significantly dikes in the ardon formation are dominated by 1 20 m wide segments confined to distinct layers the mechanisms of segment containment are determined by calculating the stress intensity factor for an extension crack approaching a mechanical interface it is apparent that dike segmentation and containment is controlled mainly by in situ stress and shear moduli differences between adjacent layers and partly by bedding plane slippage dikes in the inmar formation are dominated by smaller segments and by 1 10 cm wide and 10 100 cm long fingers with intermittent smooth patches and slickensides opposite dike walls display exact mirror images of these features dikes propagate in alternating stages of viscous flow and brittle deformation the viscous stages include fluidization of the sandstone by the dike related fluids and increase of the pore pressure in the rock a viscous instability develops between the highly viscous fluidized sandstone and the less viscous dike related fluids forming a zone of viscous fingering in front of the dike stages of brittle fracturing occur when the pore pressure is released and the flux of fluids is insufficient to fluidize the rock

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

oxygen isotope water relations in altered deep sea basalts low temperature mineralogical controls detailed analyses of oxygen isotope water relations in 3 13 and 109 ma old mid ocean ridge basalts reveal that to a first approximation the low temperature alteration of deep sea basalts in the upper few hundred metres of the igneous crust can be modelled as the partial conversion of basalt to smectite formed in isotopic equilibrium with seawater second order variations in 18o versus h2o plots are used to show that this approximation breaks down in detail because secondary mineral assemblages and mass transfer vary widely in response to subtle variations in temperature and or fluid flow strongly oxidized open systems can evolve relatively low i8o hydrous oxide rich assemblages if leaching is efficient dsdp deep sea drilling project site 396 or relatively anhydrous high 18o k feldspar rich assemblages if plagioclase breaks down site 417 compared with the clay mineral dominated assemblages common in somewhat more restricted flow environments zonal distribution and common superposition

anomalous nitrogen isotopes in ultrahigh pressure metamorphic rocks from the sulu orogenic belt effect of abiotic nitrogen reduction during fluid rock interaction modern nitrogen n fixation is primarily mediated by biological processes however in the early earth where biological activity was absent or limited abiotic n reduction in hydrothermal systems is thought to be a key process to transform atmospheric n2 and nox to ammonium an essential nutrient to support the emergence of life and also an n form that can be incorporated into rocks surprisingly evidence for abiotic n reduction in the rock record has not been clearly identified in this study we reported anomalously low n isotope compositions n15 values as low as 15 8 15 8 of mica samples in ultrahigh pressure metamorphic rocks from the donghai area in the sulu orogenic belt eastern china compared with mica samples with typical crustal n15 values 3 9 3 9 in similar metamorphic rocks from the western dabie orogen the 15n depleted mica samples from the sulu orogen are characterized by significant n enrichment 10 times higher and extreme 18o depletion o18 values as low as 9 9 these features can be best explained by assimilation of n from a source characterized by extremely low n15 values less than 16 16 the extremely low n15 value would be produced by abiotic n reduction during reaction of a meteoric hydrothermal fluid with crustal rocks before subduction this observation provides a clue to the occurrence of abiotic n reduction in continental supracrustal rocks and infer that abiotic n reduction process could be a fundamental process driving the geological n cycling in early earth

a complete oxygen isotope profile through the lower oceanic crust odp hole 735b abstract odp hole 735b located on the southwest indian ridge at 57 e is an in situ sampled long continuous section of lower oceanic crust oxygen isotope compositions of constituent minerals of leg 176 gabbros have been measured by uv laser oxygen isotope microprobe together with existing data from leg 118 a complete oxygen isotope profile through the lower oceanic crust has been obtained most clinopyroxenes and olivines have normal mantle values of 5 5 and 5 2 respectively while plagioclases show slight 18 o enrichment relative to its mantle value of 6 1 down hole variations of hole 735b gabbro indicate a downward decreasing 18 o profile with a kink at a depth of about 800 m below sea floor above this depth gabbros are depleted in 18 o relative to unaltered basalts while below 800 m they show nearly unmodified 18 o values abundant seawater penetration appears to be limited to the upper part of the lower crust at odp site 735 800 m into the gabbroic layer and 2 2 5 km into the oceanic crust from the top of pillow basalts mass balance calculations show that the lower crust formed under this ultra slow spreading ridge has an average 18 o value of 5 5 the whole crust at site 735 has an overall 18 o enrichment with 18 o values of 6 0 to 7 8 depending on the possible variation of the 18 o values of the upper pillow basalts and sheeted dykes the apparent difference in oxygen isotope compositions of ocean crusts formed with different spreading rates has important implications on the buffering of ocean water over geological time as well as on the oxygen recycling between crust and mantle through subduction the difference of seawater penetration between fast and slow spreading ridges could be related to their particular magmatic tectonic history during the formation and aging of the crust however more analyses on continuous sections through oceanic and ophiolitic crust in different tectonic settings are required to derive any predictive models

a physical model of sea wave period from altimeter data a physical model for sea wave period from altimeter data is presented physical roots of the model are in recent advances of the theory of weak turbulence of wind driven waves that predicts the link of instant wave energy to instant energy flux to from waves the model operates with wave height and its spatial derivative and does not refer to normalized radar cross section 0 measured by the altimeter thus the resulting formula for wave period does not contain any empirical parameters and does not require features of particular satellite altimeter or any calibration for specific region of measurements a single case study illustrates consistency of the new approach with previously proposed empirical models in terms of estimates of wave periods and their statistical distributions the paper brings attention to the possible corruption of dynamical parameters such as wave steepness or energy fluxes to from waves when using the empirical approaches applications of the new model to the studies of sea wave dynamics are discussed

oxygen isotope composition of a section of lower oceanic crust odp hole 735b 1 the oxygen and hydrogen isotope compositions of vein minerals and 18o values of 64 whole rocks from the deeper kilometer of the 1 5 km lower crustal section of odp hole 735b in the sw indian ocean were analyzed in order to investigate seawater penetration into and reaction with the lower oceanic crust vein amphiboles have d values of 35 to 65 indicating formation from seawater fluids and hydrothermal fluids containing a component of low d magmatic fluid crystal plastic shear zones reacted with 18o enriched fluids at high temperatures and have 18o values of 6 0 7 0 indistinguishable from surrounding rocks reaction with seawater dominated fluids over a range of temperatures 350 700 c led to local 18o depleted rocks but much of the section exhibits 18o enrichment 18o up to 7 6 as the result of static alteration of plagioclase over a range of temperatures 250 to 600 c and fluid compositions chloritic veins and associated alteration by 18o enriched fluids occurred at temperatures of 150 250 c and late carbonates and fe oxyhydroxides formed from cold 10 seawater associated with open fault related fractures bulk rock 18o values of the lower 1 km range from 3 7 to 10 2 but are mostly 5 6 7 6 and the section has a mean bulk 18o of 6 5 1 0 in contrast rocks from the shallower 500 m exhibit net 18o depletion 5 1 1 0 reflecting high temperature deformation and seawater hydrothermal alteration stakes 1991 the weighted average 18o for the entire gabbro section of hole 735b is 6 0 exhibiting little or no change compared to unaltered crust although tectonism at slow spreading rates may allow exposure of lower crust to greater amounts of low temperature alteration and 18o enrichment there is no evidence for a consistent difference in the bulk 18o of the plutonic sections of oceanic crust formed at fast versus slow spreading rates

initiation and growth of a hydraulic fracture from a circular wellbore abstract a two dimensional 2d model is presented for initiation and growth of one or more hydraulic fractures from a well that is aligned with either the maximum or intermediate principal in situ stress the coupling of fluid flow and rock deformation plays a key role in reorientation and pattern evolution of the fractures formed upon initiation the fracture may reorient itself as it extends from the wellbore until it becomes aligned with the preferred direction for fracture growth relative to the far field stresses we consider initiation and growth of multiple fractures to study their interaction and competition with each other in such cases some fractures are unable to extend at all or they arrest after limited growth but others can grow in length relative to earlier developed fractures for fractures that are driven by a uniformly distributed internal pressure which implies injection of an inviscid fluid fracture closure may occur at the portion of the fracture path adjacent to the wellbore the fracture closure does not typically occur when fluid viscous dissipation is introduced but a fluid lag zone develops and the near wellbore region of the fracture may have a narrowed width the reduced fracture width results in an increase in fluid viscous friction and an associated pressure drop near the wellbore which makes fracture stimulation more expensive and less successful and may reduce well productivity initiation of a fracture using a viscous fluid and a higher injection rate causes the fracture to curve more gradually as it seeks to align with the maximum principal stress direction a result from our model that is consistent with widely used tortuosity remedy methods a dimensionless parameter is developed that is shown to characterise near wellbore reorientation and curving of hydraulic fractures driven by viscous fluid for complex multiple fracture cases the results demonstrate that the misalignment angle and the number of initiated fractures are important in near wellbore fracture path selection

agents of low temperature ocean crust alteration 18o and87sr 86sr isotopic data from smectites calcites and whole rocks together with published isotopic age determinations alkali element concentration data and petrographic observations suggest a sequential model of ocean floor alteration the early stage lasts about 3 m y and is characterized by palagonite and smectite formation and solutions with a large basaltic component increasing with temperature which varies from 15 to 80 c at dsdp site 418a most carbonates are depositedafter this stage from solutions with a negligible basaltic sr component and temperatures of 15 to 40 c water of seawater sr and o isotopic composition is shown to percolate to at least 500 m into the basaltic basement no evidence was found for continuing exchange of strontium or oxygen after 3 m y

deflection and propagation of fluid driven fractures at frictional bedding interfaces a numerical investigation propagation of fluid driven or hydraulic fractures deflected at bedding interfaces in layered sedimentary rocks and subsequent fluid invasion is investigated numerically using a two dimensional boundary element model the fracture is driven by an incompressible newtonian fluid injected at a constant rate the frictional stress on the interface is assumed to obey coulomb s frictional law without cohesion the bedding interface can be given a non zero minimum fluid conductivity a numerical scheme that deals with both rock deformation and fluid flow is presented and its accuracy is verified in terms of comparisons with existing results to explore the mechanisms involved in fluid and fracture invasion into the interface parametric studies are carried out for different elastic modulus contrasts in situ stresses interfacial frictional coefficients distances from the injection point to the interface and fluid viscosities the results are provided as time dependent variations of displacements fluid pressures contact stresses and fluid fronts fracture deflection and fluid invasion into the interface are found to rely essentially on local stress and deformation states at the intersection point fluid invasion and fracture growth may be delayed or inhibited when the interface is subjected to large confining stress or when fluid viscosity is relatively low for cases where the fluid driven fracture originates in the softer layer in this case a greater layer parallel tensile stress is produced and can lead to fracture propagation through the bedding contact low to medium frictional strength is found to promote fluid penetration and t shaped fracture formation by interfacial opening if the hydraulic fracture originates in a stiffer layer fluid invasion into bedding contacts can occur smoothly without the occurrence of interface closure and the fracture is thus terminated by forming a t shaped fracture at the bedding interface for fracture deflection into and growth along the interface in the absence of interface closure the long time responses resemble the solution for a fluid driven fracture growing along a frictionless interface with vanishing toughness

hydrogen isotope systematics of submarine basalts the d h ratios and water contents in fresh submarine basalts from the mid atlantic ridge the east pacific rise and hawaii indicate that the primary d h ratios of many submarine lavas have been altered by processes including 1 outgassing 2 addition of seawater at magmatic temperature and 3 low temperature hydration of glass decreases in d and h2o from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between d and h2o in basalts from the galapagos rise and the famous area are attributed to outgassing of ch4 and h2 a good correlation between d values and h2o is observed in a suite of submarine tholeiites dredged from the kilauea east rift zone where seawater added directly to the magma affected only the isotopic compositions of hydrogen and argon analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having d values as low as 100 d values vary with h2o contents of subaerial transitional basalts from molokai hawaii and subaerial alkali basalts from the society islands indicating that the primary d values were similar to those of submarine lavas extrapolations to possible unaltered d values and h2o contents indicate that the primary d values of most thoteiite and alkali basalts are near 80 5 the weight percentages of water are variable 0 15 0 35 for mor tholeiites about 0 25 for hawaiian tholeiites and up to 1 1 for alkali basalts the primary d values of 80 for most basalts are comparable to those measured for deep seated phlogopites these results indicate that hydrogen in marked contrast to other elements such as sr nd pb and o has a uniform isotopic composition in the mantle this uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the earth

the role of friction and secondary flaws on deflection and re initiation of hydraulic fractures at orthogonal pre existing fractures summary in this study we explore the nature of plane strain hydraulic fracture growth in the presence of pre existing fractures such as joints without or with secondary flaws the 2 d plane strain fracture studied can be taken as a cross section through the short dimensions of an elongated 3 d fracture or as an approximate representation of the leading edge of a 3 d fracture where the edge curvature is negligible the fluid driven fracture intersects a pre existing fracture to which it is initially perpendicular and is assumed not to immediately cross but is rather deflected into the pre existing fracture the intersection results in branching of the fracture and associated fluid flow into the pre existing fracture further growth results in opening and frictional sliding along the pre existing fracture fracture propagation in an impermeable homogeneous elastic medium and fluid invasion into a pre existing fracture are both driven by an incompressible newtonian fluid injected at a constant rate the frictional stress on the surfaces of pre existing fractures is assumed to obey the coulomb law the governing equations for quasi static fluid driven fracture growth are given and a scaling is introduced to help identify important parameters the displacement discontinuity method and the finite difference method are employed to deal with this coupling mechanism of rock fracture and fluid flow in order to account for fluid lag a method for separately tracking the crack tip and the fluid front is included in the numerical model numerical results are obtained for internal pressure frictional contact stresses opening and shear displacements and fluid lag size as well as for fracture re initiation from secondary flaws after fracture intersection the hydraulic fracture growth mode changes from tensile to shearing this contributes to increased injection pressure and to a reduction in fracture width in the presence of pre existing fractures the fluid driven cracks can be arrested or retarded in growth rate as a result of diversion of fluid flow into and frictional sliding along the pre existing fractures frictional behaviour significantly affects the ability of the fluid to enter or penetrate the pre existing fracture only for those situations where the fluid front is within a certain distance from the intersecting point importantly fluid penetration requires higher injection pressure for frictionally weak pre existing fractures fracture re initiation from secondary flaws can reduce the injection pressure but re initiation is suppressed by large sliding on pre existing fractures that are frictionally weak

alteration and mineralization of an oceanic forearc and the ophiolite ocean crust analogy mineralogical chemical and isotopic o c s and sr analyses were performed on minerals and bulk rocks from a forearc basement section to understand alteration processes and compare with mid ocean ridges mor and ophiolites ocean drilling program hole 786b in the izu bonin forearc penetrates 103 m of sediment and 725 m into volcanic flows breccias and basal dikes the rocks comprise boninites and andesites to rhyolites most of the section was affected by low temperature 100 c seawater alteration with temperatures increasing downward the rocks are partly 5 25 altered to smectite fe oxyhydroxide calcite and phillipsite and exhibit gains of k rb and p loss of ca variable changes in si na mg fe sr and y and elevated 18o and 87sr 86sr higher temperatures 150 c in the basal dikes below 750 m led to more intense alteration and formation of chlorite smectite corrensite albite k feldspar and quartz chlorite a 5 m thick hydrothermally altered and pyritized zone at 815 m in the basal dikes reacted with mixtures of seawater and hydrothermal fluids to mg chlorite albite and pyrite and gained mg and s and lost si and ca focused flow of hydrothermal fluids produced serialization halos na k sericite quartz pyrophyllite k feldspar and pyrite along quartz veins at temperatures of 200 250 c high 87sr 86sr ratios of chloritized 0 7055 and sericitized 0 7065 rocks indicate involvement of seawater via mixing with hydrothermal fluids low 34s of sulfide 2 to 5 5 and sulfate 12 5 are consistent with input of magmatic so2 into hydrothermal fluids and disproportionation to sulfide and sulfate alteration processes were generally similar to those at mors but the arc section is more intensively altered in part because of the presence of abundant glassy rocks and mafic phases the increase in alteration grade below 750 m and the mineralization in the basal dikes are analogous to changes that occur near the base of the volcanic section in mor and the troodos ophiolite

mechanism for joint saturation in mechanically layered rocks an example from southern israel abstract over 700 joint spacings were measured along a 190 m scanline within a single limestone bed of the turonian gerofit formation southern israel by investigating joints belonging to a single systematic set confined to a bed of uniform thickness one can factor out the major influences of bed thickness lithology and mechanical properties on joint set development joint spacing distributions and fracture spacing ratios fsr vary dramatically along the length of the scanline and correlate directly with throughgoing fracture zones belonging to the same tectonic stress regime regions cross cut by fracture zones display narrower median joint spacings fsr of 1 3 and unimodal joint spacing distributions where fracture zones are absent joint spacings have wider medians fsr of 0 8 and multimodal distributions because fracture zones most likely represent areas of high strain and laboratory experiments demonstrate a systematic decrease in fracture spacing with increasing strain regions of the bed cross cut by fracture zones are likely at a more advanced stage of joint set development and hence closer to saturation than other regions joint saturation in the gerofit formation may be achieved by a shift in scale from single layer to multi layer jointing as the development of throughgoing joints prevents the formation of additional bedding confined joints

the intersection of a pressurized crack with a joint a model is described for considering the partial slip that occurs when a pressurized crack intersects a joint to which it is perpendicular the crack is assumed not to penetrate into the undamaged material rather opening of the crack occurs at the juncture through interfacial slip within the interfacial slip region the resistance to slip is accounted for by prescribing the shear stress there results are obtained as the hydraulic loading in the crack is increased and the subsequent crack growth is investigated it is found that for a prescribed critical value of stress intensity factor the crack length remains fixed until a certain value of pressure is attained above this value the crack length can increase with increased pressure

mechanics of two interacting magma driven fractures a numerical study to understand magma focusing from broad melting zones in the crust propagation in brittle rocks of two interacting dikes ascending from a single deep source has been modeled using a time dependent plane strain hydraulic fracturing model the source is assumed to generate a constant influx rate to feed the dike growth which is also aided by buoyancy effects in contrast to uncoupled model results the simultaneous parallel growth of two dikes to a certain distance is found to occur provided that the two dikes are initially of different heights which might be produced either by previous magma intrusion or during nucleation the shorter dike will chase the longer one and they can either progressively merge or continue subparallel growth at a reduced spacing depending on the deviator stress between the vertical and horizontal stresses and the initial lateral separation with parameters given in dimensionless forms numerical results reveal the mechanics for simultaneous ascents of two subparallel dikes in that dike interaction can produce a low stress field which is just above the tip of the shorter dike favorable to growth of the shorter dike the energy analysis indicates that the energy required for two subparallel dikes is less than that for a single dike during the late time buoyancy viscosity propagation stage where considerable ascent occurs this growth behavior might provide the mechanism for simultaneous instead of sequential growth of various dikes in a single set

finite element analysis of the stress distribution around a pressurized crack in a layered elastic medium implications for the spacing of fluid driven joints in bedded sedimentary rock bedding perpendicular joints confined to individual beds in interbedded sedimentary rocks commonly exhibit spacings which are proportional to the thickness of the jointed bed and which vary according to lithology or structural position the mechanical explanation for this relationship is well understood when the joints are driven by far field crack normal tensile stresses but poorly understood for cracks driven by elevated fluid pressures where the crack driving stress is the difference between the crack normal compression and the fluid pressure in the crack through a series of finite element numerical models we investigate how various parameters influence the driving stress distribution around pressurized cracks in layered media and thereby identify factors influencing the spacing of fluid driven joints for the situation we modeled we observe that 1 crack driving stress is reduced in the vicinity of pressurized joints and that the extent of the stress reduction depends on the contrast in elastic properties between the layers and 2 crack driving stress distribution depends on the ambient pore pressure during jointing these results indicate the spacing of fluid driven joints should depend on lithology and pore pressure

no slab derived co2 in mariana trough back arc basalts implications for carbon subduction and for temporary storage of co2 beneath slow spreading ridges the southern mariana trough is particularly well suited to study mass balance in subduction zones because the flux of material recycled from the subducted slab has been shown to diminish to negligible levels in the southernmost part of the area we present new he and ar concentration and isotopic data for 16 back arc basaltic glasses and combine these with previously published co2 and h2o concentration and 13c data to explore the recycling of carbon and light noble gases in the mariana back arc degassing has affected all samples and is particularly extensive in more water rich samples i e those containing the largest recycled component the degassing history features three stages 1 deep degassing which commenced when the melt reached saturation of co2 and noble gases in the mantle 2 preeruptive degassing during storage in the crust mantle transition zone which involved addition of extraneous co2 to the vapor phase and 3 eruption co2 released during stage 1 was at least partially incorporated into wall rock and subsequently remobilized during stage 2 degassing of later magma batches reconstructed parental values for 3he 4he 13c co2 3he and co2 40ar are indistinguishable from those of mid ocean ridge basalt this implies that there is negligible recycling of subducted carbon helium or argon into the source of mariana trough basalt

fluid driven multiple fracture growth from a permeable bedding plane intersected by an ascending hydraulic fracture 1 in bedded sedimentary rocks the energy for spontaneous growth of multiple vertical fractures from a bedding plane may be provided by an overpressurized sublayer fracture that connects a fluid source to the bedding plane in this paper using our coupled deformation and flow model we study the processes and mechanisms involved in the formation and interaction of closely space fractures from preexisting flaws or starter fractures located along the bedding plane fracture growth from multiple flaws can be convergent parallel or divergent depending on the factors like contrasts in moduli and far field stresses flaw sizes and locations and initial bed conductivity fluid viscosity and injection rate as well as time the results presented here have been obtained for conditions where fluid viscous dissipation is dominant in contrast to other results available in literature based on uniform pressure assumption equivalent to use of an inviscid fluid it is demonstrated that the earlier a hydraulic fracture starts to extend the more likely it is to become the primary fracture in a system of closely spaced fractures the fracture closest to the fluid source typically grows faster as a result of a higher pressure level because viscous dissipation results in a decrease in pressure with distance from the fluid source but its development does not completely inhibit the growth of other hydraulic fractures simultaneous growth of closely spaced fractures is supported by the local stress and energetic analyses and the fracture distance can be very small their length to spacing ratio is accordingly much larger than that predicted previously under certain circumstances a longer and more permeable fracture may grow to a greater extent than a shorter fracture closer to the fluid source which may grow toward and merge with the longer fracture to create fracture clusters adjacent to a bedding plane

escape of fluid driven fractures from frictional bedding interfaces a numerical study fluid driven or hydraulic fractures either natural or man made which are propagating vertically in horizontally layered rocks may interact with and deflect into bedding interfaces that they intersect pre existing secondary flaws located along the frictional bedding contact defined as the bedding plane can serve as nucleation sites for new fractures that are extended first by interface slip and then by fluid if the flaw propagates a stepped fracture forms in this paper a two dimensional numerical fracture model is used to analyse the coupled rock deformation and fluid flow in such fractures numerical results show that fracture escape from the interface is likely under the conditions of fracture growth from stiff to soft rocks small layer to layer far field stress contrasts and moderately low fluid viscosity and small parent fracture lengths and offset distances the change of the fracture propagation direction at the bedding plane gives rise to a variety of fracture and fluid flow patterns fracture permeability and internal pressure vary with time and location vertical fracture growth can impede fluid flow along the offset by inducing additional compressive stress on it if the offset channel is of relatively large permeability most fluid will be directed into the new fracture to facilitate fracture escape alternatively the interaction and geometry of the fracture branches can lead to low opening compliance on the offset and localised pinching of the fracture surfaces which restrict fluid flow

generation of layering in the lower stratosphere by a breaking rossby wave 1 an airborne ozone lidar has been used to observe layering in the ozone field between 350 and 400 k northward of the subtropical jet stream an ozone poor layer between 370 and 400 k was observed over western europe on 21 april 1999 on this day a rossby wave was observed to be breaking over the eastern north atlantic and western europe with both poleward and equatorward components we propose that the low ozone at 400 k was caused by a poleward breaking rossby wave stretching from spain to the south coast of britain isentropic back trajectory calculations suggest a tropical upper tropospheric origin for the air within this wave over north america we suggest that rossby waves generate layering in the lower stratosphere because of vertical gradients in the background wind fields which govern the growth in amplitude and breaking characteristics of the waves

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

analytical predictions for a natural spacing within dyke swarms dykes often grow next to other dykes evidenced by the widespread occurrence of dyke swarms that comprise many closely spaced dykes in giant dyke swarms dykes are observed to maintain a finite spacing from their neighbors that is tens to hundreds of times smaller than their length to date mechanical models have not been able to clarify whether there exists an optimum or natural spacing between the dykes and yet the existence of a natural spacing is at the heart of why dykes grow in swarms in the first place here we present and examine a mechanical model for the horizontal propagation of multiple closely spaced blade like dykes in order to find energetically optimal dyke spacings associated with both constant pressure and constant influx magma sources we show that the constant pressure source leads to an optimal spacing that is equal to the height of the blade like dykes we also show that the constant influx source leads to two candidates for an optimal spacing one which is expected to be around 0 3 times the dyke height and the other which is expected to be around 2 5 times the dyke height comparison with measurements from dyke swarms in iceland and canada lends initial support to our predictions and we conclude that dyke swarms are indeed expected to have a natural spacing between first generation dykes and that this spacing scales with and is on the order of the height of the blade like dykes that comprise the swarm

numerical investigation of hydraulic fracture network propagation in naturally fractured shale formations abstract hydraulic fracture network hfn propagation in naturally fractured shale formations is investigated numerically using a 3d complex fracturing model based on the discrete element method to account for the plastic deformation behavior of shales the drucker prager plasticity model is incorporated into the fracturing model parametric studies are then conducted for different young s moduli horizontal differential stresses natural fracture nf properties injection rates and number and spacing of perforation clusters numerical results show that horizontal differential stress primarily determines the generation of a complex hfn the plastic deformation of shale can reduce the stimulated reservoir volume this is more obvious with young s modulus of less than 20 gpa in addition a higher injection rate could largely increase the fracture complexity index fci moreover increasing perforation cluster numbers per fracturing stage is beneficial for increasing the fci but it also increases the potential merging of neighboring fractures which may lead to non uniform development of hfn in far wellbore regions to achieve uniform development of hfn within a fracturing stage the distribution of nfs should be fully considered the results presented here may provide improved understanding of hfn generation and are favorable for optimizing fracturing treatment designs for shale formations

helium 3 and deuterium in back arc basalts lau basin and the mariana trough samples of fresh basalt glass from the mariana trough and the lau basin were analyzed for their isotopic composition of water and helium in order to identify the sources of the volatiles in back arc basin basalts in the mariana trough basalts the concentration 0 64 2 1 wt and d h ratio d 46 to 32 of the water provide important evidence for a water rich component from the subducting lithosphere extrapolation to infinite water content gives an end member d h ratio of d 25 3he 4he ratios are in general similar to morb values 8 ra and indicate that mariana trough basalts represent a mixture of a morb type mantle and an alkali and water rich component from the descending slab in contrast the lau basin produces both hydrous 1 3 wt h2o and relatively anhydrous 0 12 0 35 wt basalts the d h ratio d 43 in the hydrous lau basin basalt resembles those of mariana trough lavas the low water contents 0 12 0 35 wt and morb like d h ratios d 63 to 70 in three of five lau basin samples show that the water rich component observed in all mariana trough lavas is absent in some lau basin lavas and not essential for the production of back arc basalts the high3he 4he ratio of rochambeau bank in the northern lau basin 11 ra confirms an earlier analysis by lupton and craig and provides evidence for an enriched mantle source region beneath the lau basin which is perhaps related to the high 3he samoan hotspot

architecture of the volcanic layer from the costa rica rift constraints from core log integration 1 an understanding of the architecture of the ocean crust s volcanic layer is critical since this records the variations in volcanic activity during formation and subsequently controls the movement of hydrothermal fluids through this layer unfortunately core recovery in the majority of ocean drilling program odp deep sea drilling program basement holes is generally low and biased toward more massive lithologies consequently the identification and estimation of lithological diversity within the volcanic layer is only possible through the integration of core and wire line logging results wire line logging provides nearly continuous records of both physical and chemical properties of the borehole wall which can be used to identify the various volcanic sedimentary lithologies which then can be calibrated in regions of enhanced core recovery using this approach the architecture of the volcanics from odp hole 896a has been reconstructed this new model illustrates a hiatus in volcanism marked by a thick breccia horizon which probably marks the transition from axis to off axis volcanism

reinitiation or termination of fluid driven fractures at frictional bedding interfaces 1 a two dimensional numerical model for coupled elastic deformation and fluid flow has been developed to examine reinitiation or termination of a vertical fluid driven fracture in bedded rocks the rocks on both sides of the bedding interface are assumed to be impermeable and a newtonian fluid whose viscous dissipation cannot be neglected is injected at a constant rate crack nucleation on the frictional interface is controlled by a tensile stress criterion a fracture approaching the interface or terminating on it can generate a large bed parallel tensile stress in the uncracked layer in excess of the local tensile strength to initiate a new fracture the propagation of the nucleated fracture is assisted by interface sliding and pressurized fluid the bedding interface can provide a conductive channel to connect the parent and the new fractures fluid partitioning among fracture branches depends on local stress states and triggers their competition to become the main fracture the three types of fracture patterns generated numerically by the model are crosscutting through terminating at and offsetting at the interface a large modulus or toughness contrast across the interface can lead to containment of the hydraulic fracture on the interface for offsetting fractures the presence of the bedding plane can reduce fracture permeability and any associated high stress barrier acts to slow or arrest further fluid driven fracture growth propagating the new fracture results in high excess pressure consistent with measured abnormal pressure in commercial fracture treatments the predicted offset distances are on the order of centimeters and their continued growth is perpendicular to the bedding contacts

geology of the saucer shaped sill near mahad western deccan traps india and its significance to the flood basalt model an 22 m thick saucer shaped sill occurs near mahad and is exposed as a curvilinear miniature ridge within the deccan traps the sill has variable dips 42 55 it has a 7 1 km long axis and 5 3 km short axis aspect ratio of 1 4 and is larger than the mv sill of the golden valley sill complex south africa and the panton sill australia the sill has distinct glassy upper and lower chilled margins with a coarse grained highly jointed core the samples from the margin are invariably fractured and iron stained because of deuteric alteration the rock from the sill is plagioclase phyric basalt at least three thick sill like apophyses emanate from the base of the main sill the apophyses change direction because of bending and thinning from a horizontal concordant sheet at the top to a discordant inclined form that bends again to pass into a lower horizontal concordant sheet we interpret such features as nascent saucer shaped sills that did not inflate to form nested sills geochemically the sill consists of poorly differentiated tholeiitic basalt that has a restricted geochemical range critical trace element ratios and primitive mantle normalised trace and ree patterns indicate that the sills have geochemical affinities to the poladpur chemical type and that the pahoehoe flow they intrude belongs to the bushe formation calculated magmatic overpressures during sill emplacement range from 8 4 to 11 3 mpa for young s modulus e 5 gpa and 16 7 to 22 5 mpa for e 10 gpa and depth to magma chamber ranges from 8 5 to 11 5 km e 5 gpa and 17 1 to 22 9 km e 10 gpa consistent with petrological and gravity modelling the volume of the mahad sill is approximately 276 km3 and is constant irrespective of the variations in the values of host rock young s modulus in 1980 cox j petrol 21 629 650 1980 proposed a conceptual model of the crust mantle section beneath the karoo cfb which is considered as the fundamental model for flood basalt volcanism our paper confirms the presence of a sill plus the inferred substructure beneath mahad that are compatible with predictions of that model in lips saucer shaped sills are formed in areas experiencing extensional tectonics where processes such as the cook gordon delamination and dundurs elastic extensional mismatch between layered sedimentary rocks or lava flows are responsible for the deflection of dykes into sills a similar process is envisaged for the formation of the mahad sill