the geometry and emplacement of conical sandstone intrusions conical sandstone intrusions with a geometry comparable with that of many igneous sills have been identified using 3d seismic data from large areas of the north sea and faeroe shetland basins these intrusions are of reservoir scale ranging from 100 to 2000 m or so in diameter 50 300 m in height and 1 80 m in thickness aperture they are concentrated in specific stratigraphic intervals in the cenozoic fills of both basins two geometrical end members are recognised and defined here apical cones and flat based bowls the former consist of inward dipping conical inclined sheets meeting at a prominent apex and the latter of similarly dipping discordant margins climbing from the edges of a concordant sheet both end members are associated with domal folds that are interpreted as resulting from the hydraulic elevation of the overburden during intrusion and which are analogous to similar structures associated with bowl shaped igneous sills measurements of aperture versus distance exhibit systematic relationships with the structural relief of these folds offering a potentially predictive method for estimation of sandstone intrusion aperture and reservoir volume prior to drilling a growth model for these end member geometries is presented drawing on existing theory for igneous sill emplacement aperture versus distance plots x are used to illustrate two contrasting models for aperture inflation during propagation but these require much further data before any specific growth model can be adopted

magma transport through the crust via interconnected sill complexes three dimensional seismic data from the volcanic margin offshore norway have been used to map the complex three dimensional geometry of two major sill complexes these data show that the complexes consist of bowl shaped or inclined sheetlike discordant sills that intersect with one another over a vertical distance of 8 12 km and transect basement and sedimentary rocks of the middle to upper crust using a simple graphical technique we demonstrate the interconnectedness of the sill complex and hence the maximum time for sill emplacement we show that sills are interconnected by junctions occurring systematically at the lowest points of the overlying sills and hence that the sill complex could have acted as a single throughgoing magmatic plumbing system transporting mafic melts from mid lower crustal depths to near surface based on this imaging of the magmatic plumbing system we conclude that sills can form viable and efficient conduits for crustal magma transport in volcanic margins and may play a role in feeding large extrusive complexes

on steady homogeneous sand water flows in a vertical conduit in this paper an idealized model of the steady state phase of the flow in a vertical conduit leading to a sand volcano eruption is developed from first principles the model assumes that a sand water mixture flows upwards driven by an overpressure at the base of a vertical cylindrical conduit or a two dimensional fracture and opposed by gravity viscous resistance and turbulent drag the conditions for flow are analysed in detail and the mechanisms controlling the eruption rates are studied quantitatively the flow predictions are in accordance with our observations of analogous vigorous sand eruptions at deepwater oil fields for sufficiently high flow velocities u 10ut and small sand fractions s 0 2 the flow may be well mixed and homogeneous if these conditions are not met the flow may either become two phase or does not develop combining geological considerations with the steady homogeneous model it is possible to predict the behaviour of the vigorous quasi steady stage of a sand volcano eruption it is shown that based on the average density of the overlying sediments there are a range of overpressures for which sand volcanoes may develop

on the deep mantle origin of the deccan traps the deccan traps in west central india constitute one of earth s largest continental flood basalt provinces whose eruption played a role in the cretaceous paleogene extinction event the unknown mantle structure under the indian ocean at the start of the cenozoic presents a challenge for connecting the event to a deep mantle origin we used a back and forth iterative method for time reversed convection modeling which incorporates tomography based present day mantle heterogeneity to reconstruct mantle structure at the start of the cenozoic we show a very low density deep seated upwelling that ascends beneath the reunion hot spot at the time of the deccan eruptions we found a second active upwelling below the comores hot spot that likely contributed to the region of partial melt feeding the massive eruption

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 lack of relationship between overpressure and porosity in north sea and haltenbanken shales abstract disequilibrium compaction undercompaction is probably the most accepted mechanism for explaining overpressured formations this mechanism relies on the hypothesis that overpressured shales have higher porosity than normally pressured shales for a given depth converted to porosity units log response data from two jurassic shales at haltenbanken and nine cretaceous and tertiary shales in the norwegian sector of the north sea have been investigated to see whether such a relationship exists in these areas the depth of the studied shale formations varies between 0 5 5 km the degree of overpressuring within one formation typically varies by 20 mpa the log response data show that neither the porosity of massive north sea shales derived from sonic and density logs nor the porosity of haltenbanken intra reservoir shales derived from density and neutron logs differ significantly between the overpressured and normally pressured stratigraphic units distinct differences between the pressure regimes are however apparent in the resistivity and sonic log data from deeply buried shales at haltenbanken these differences do not reflect porosity variations and are interpreted to reflect textural changes resulting from the overpressuring these observations demonstrate that the haltenbanken and north sea shales do not conform to an undercompaction mechanism for overpressuring as the overpressured wells fail to show elevated porosity on the contrary each formation seems to have been individually compacted according to burial depth independent of pressure regimes

stratification and mixing in sea straits abstract quantifying the transport through relatively narrow straits connecting basins with differing fluid properties is an important issue in both surface and abyssal flows these flows range from large scale flows in ocean straits connecting the major ocean basins down to small scale regional flows such as in estuaries and lagoons connected to the coastal ocean by relatively narrow straits or channels the exchange rate through these straits is dependent on the topography mixing and stratification and for steady flows the interplay between these three effects can be quantified using the dimensionless parameter gr t a 2 where the grashoff number gr t and the aspect ratio a are evaluated within the strait when gr t a 2 is large the exchange is determined by the well known hydraulic solution as gr t a 2 becomes small the exchange rates progressively decrease below the hydraulic predictions toward values determined by diffusive control and characterized by strong vertical mixing for systems with simple topographic controls we show how the exchange rate in the strait may be linked to the external basin scale forcing characteristics we review recent work on extending this picture to consider the effects of unsteadiness in two forms time dependent forcing in the basin due to seasonal effects and tidal forcing with an associated barotropic forcing in the strait itself

discussion on mechanisms and controls on the formation of sand intrusions d m d james writes i congratulate the authors on a very clear and comprehensive account of a widespread and complex phenomenon of increasing economic importance however presumably for didactic ease they have made some simplifications that i feel are counterproductive for full physical understanding it is not entirely clear from the paper that sand intrusions require a concatenation of source seal properties that on the face of it are very rare these are that the source sand bed must have a negligible grain cohesion typical of burial at very low vertical effective stress not shallow depth necessarily and that the seal must have effectively negligible permeability typical of burial at very high vertical effective stress and often associated with high cohesion in most basins sands that can be fluidized are interbedded with leaky shales of high permeability for shales that do not sustain high overpressure during normal compaction however such high overpressure is vital because the intrusion is powered by a pressure decline at source that provides the mechanical energy to strain elastically the intrusion walls and to drive the ascent of the fluidized sand against frictional resistance this cannot happen if fracture takes place when fluid pressure reaches a value merely equal to that of the horizontal stress as illustrated by the authors in their figs 6 and 8 as this lies on the closure gradient of any fracture not its inception gradient how then can the requisite overpressure for fracture initiation and propagation be attained at the shallow depths normally associated with low vertical effective stress of the obvious ways around this problem the development of zero permeability diagenetic or clathrate seals seems unlikely to be a universal solution more likely and not mentioned in the paper is that the source pressure builds up at such a high

mechanisms and controls on the formation of sand intrusions sandstone intrusions are found in all sedimentary environments but have been reported most commonly from deep water settings they also appear to be more frequently developed in tectonically active settings where applied tectonic stresses facilitate development of high fluid pressures within the sediments a variety of mechanisms have been cited as triggers for clastic intrusions these include seismicity induced liquefaction application of tectonic stresses excess pore fluid pressures generated by deposition related processes and the influx of an overpressured fluid from deeper within the basin into a shallow sand body the formation of sandstone dykes and sills is investigated here by considering them as natural hydraulic fractures when the seal on an unconsolidated overpressured sand body fails the resulting steep hydraulic gradient may cause the sand to fluidize the fluidized slurry can then inject along pre existing or new fractures to form clastic intrusions the scale and the geometry of an intrusive complex is governed by the stress state depth and pre existing joints or faults within the sedimentary succession as well as the nature of the host sediments for the simplest tectonic setting where the maximum stress in a basin is vertical gravitational loading small irregular intrusions commonly result in the formation of sills at shallow depths within a few metres of the surface whereas at greater depth dykes and sills forming clastic intrusion networks are more typical a simple relationship is derived to calculate the maximum burial depth at which a dyke sill complex forms as a function of the source bed to sill height the bulk density of the surrounding sediments and the ratio of the vertical to horizontal effective stresses k 0 when applied to three examples of large scale dyke sill complexes developed within paleocene and eocene deep water reservoir sand bodies of the north sea maximum burial depths in a range of 375 to c 500 m 450 700 m and 550 850 m are estimated for intrusion of each of the three complexes

downsag and extension at calderas new perspectives on collapse geometries from ice melt mining and volcanic subsidence structures at calderas may form as a result of precursory tumescence subsidence due withdrawal of magmatic support resurgence and regional tectonism structural reactivation and overprinting are common to explore which types of structures may derive directly from subsidence without other factors evidence is reviewed from pits caused by the melting of buried ice blocks mining subsidence scaled subsidence models and from over 50 calderas this review suggests that complex patterns of peripheral deformation with multiple ring and arcuate fractures both inside and outside caldera rims topographic embayments arcuate graben and concentric zones of extension and compression may form as a direct result of subsidence and do not require a complex subsidence and inflation history downsag is a feature of many calderas and it does not indicate subsidence on an inward dipping ring fault as has been inferred previously where magmatic inflation is absent or slight initial arcuate faults formed during collapse are likely to be multiple and dip outwards to vertical associated downsag causes the peripheries of calderas undergo radial centripetal extension and this accounts for some of the complex peripheral fractures arcuate crevasses graben and some topographic moats the structural boundary of a caldera defined here as the outermost limits of subsidence and related deformation including downsag commonly lies outside ring faults and outside the embayed topographic wall it is likely to be funnel shaped i e inward dipping even though ring and arcuate fractures within it may dip outward inward dipping arcuate normal faults at shallow levels and steep inward dipping contacts between a caldera s fill and walls may both occur at a caldera that has initially subsided on outward dipping ring faults they arise due to peripheral surficial extension gravitational spreading and scarp collapse topographic enlargement at some calderas and the formation of embayments may reflect general progressive downsag and localized downsag respectively these processes may occur in addition to surficial degradation of oversteep ring fault scarps

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

fluidization and injection in the deep water sandstones of the eocene alba formation uk north sea the nauchlan member of the late eocene alba formation uk north sea consists of a deep water channel fill that was extensively modified by post depositional sand remobilization and injection sandstone textures facies associations and the geometry of the channel fill were affected a suite of sand rich facies was produced by large scale fluidization and injection within the channel fill and above it these facies termed here unstratified facies are characterized by the absence of stratification surfaces and by discordant relationships with bedding in the adjacent succession they reflect variable degrees of disruption of the primary sedimentary structures caused by escaping pore fluid the velocity of which is estimated at least in the order of 0 1 ms 1 adjacent mudstones were severely disrupted by hydraulic fracturing and fragments of fractured mudstone were incorporated into the fluidized sand average porosity was decreased in the sandstones affected by fluidization two main phases of sand injection are inferred to occur at different burial depths a shallow burial phase below 100 m produced thin dykes with ptygmatic folds the second phase occurred at the boundary between eocene and oligocene 300 m burial depth and resulted in large scale tabular wing like dykes that project from the edges of the channel fill the significant pore fluid overpressure which was required to hydraulically fracture the thick mudstone seal and to fluidize the large volume of sand was likely to be built up by static liquefaction of the source sand and was possibly enhanced by hydrocarbon gas influx

structure and emplacement history of a multiple center cone sheet bearing ring complex the zarza intrusive complex baja california mexico the cretaceous zarza intrusive complex located in the peninsular ranges of baja california norte mexico is perhaps the bestpreserved multiple center cone sheet bearing ring complex documented in north america the 7 km 2 elliptical complex hosts three nested non concentric intrusive centers that are successively younger to the south the northern and central centers show the same evolutionary sequence of 1 intrusion of concentric gabbroic cone sheets 2 intrusion of massive core gabbros and 3 development of subvertical ductile ring faults ring fault kinematics indicate that both centers moved down relative to the surrounding country rocks suggesting collapse into an underlying magma chamber the southern center is composed of approximately equal proportions of gabbro and tonalite and lacks cone sheets aluminumin hornblende barometry on the tonalite indicates a maximum emplacement depth of 2 3 0 6 kbar the zarza intrusive complex is surrounded by a ductile deformation aureole and bedding is inward dipping and inward younging around the entire complex excellent preservation of the intrusive history allowed us to evaluate the origin of the aureole and the three most applicable models are 1 collapse of the complex into its underlying magma chamber 2 sinking of the complex and its chamber after solidification and 3 formation of the aureole prior to emplacement of the complex the preserved structural and intrusive relationships provide information on the dynamic evolution of subvolcanic magma chambers and suggest that the complex may have been overlain by a caldera

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

the laccolith stock controversy new results from the southern henry mountains utah domes of sedimentary strata at mount holmes mount ellsworth and mount hillers in the southern henry mountains record successive stages in the growth of shallow 3 to 4 km deep magma chambers whether the intrusions under these domes are laccoliths or stocks has been the subject of controversy according to g k gilbert the central intrusions are direct analogues of much smaller floored intrusions exposed on the flanks of the domes that grew from sills by lifting and bending of a largely concordant overburden according to c b hunt the central intrusions are cylindrical stocks sheathed with a zone of shattered sedimentary rocks and the small flanking sills and laccoliths grew laterally as tongue shaped masses from the discordant sides of these stocks new geologic mapping demonstrates that the sedimentary overburden now partially eroded from the domes was uplifted about 1 2 km at mount holmes 1 8 km at mount ellsworth and at least 2 5 km at mount hillers the radii of the domes are similar between 5 and 7 km the strata over the domes have a doubly hinged shape consisting of a concave upward lower hinge and a concave downward upper hinge a limb of approximately constant dip joins these two hinges and dips 20 at mount holmes 50 to 55 at mount ellsworth and 75 to 85 at mount hillers the distal portion of each dome is composed of a gently dipping peripheral limb 3 to 4 km long presumably underlain by sills and minor laccoliths although geologic cross sections and regional aeromagnetic data for the three domes are consistent with floored laccolithic central intrusions these data alone do not rule out the possibility of a stock at depth at mount hillers paleomagnetic vectors indicate that tongue shaped sills and thin laccoliths overlying the central intrusion were emplaced horizontally and were rotated during doming through about 80 of dip this sequence of events is not consistent with the emplacement of a stock and subsequent or contemporaneous lateral growth of sills and minor laccoliths growth in diameter of a stock from about 300 m at mount holmes to nearly 3 km at mount hillers as hunt suggested should have been accompanied by considerable radial shortening of the sedimentary strata and a style of folding which is not observed geologic and geophysical data and mechanical models support a laccolithic origin for the central magma chambers underlying the domes

a mohr circle construction for the opening of a pre existing fracture abstract the opening of a fracture is dependent on the fluid pressure and the local stress regime around the fracture for a fracture to open the fluid pressure must exceed the normal stress acting on the fracture the mohr circle provides a useful tool allowing the visualization of these relationships this construction determines the range of fracture orientations that are able to open as well as determining the opening direction of these fractures this is then used to produce ratios of the magma pressure and the principal stresses for some dykes from the lizard ophiolite u k

the alba field ocean bottom cable seismic survey impact on development alba field in the central north sea uk block 16 26 consists of eocene age high porosity unconsolidated turbidite channel sands sealed by low permeability shales at an average subsea depth of 1900 m the main channel is approximately 9 km long and 1 5 km wide and can be up to 100 m thick figure 1 the channel contains discrete bodies of intrareservoir shales that can cause significant drilling completion and production problems immediately overlying the main alba sand channel several wells have encountered thin discontinuous oil saturated brioc sands

constraining the depth and timing of large scale conical sandstone intrusions kilometer scale sandstone intrusions a recently recognized end member of the spectrum of soft sediment deformation structures are important components of many deep water sandstone reservoirs they result from the remobilization of considerable volumes of sand during early stages of burial this paper uses three dimensional seismic data calibrated to nearby wells to describe an extensive system of large downward tapering conical sandstone intrusions from the faeroe shetland basin on the uk atlantic margin the intrusions are derived from a middle eocene submarine fan and are injected vertically upward through overlying highly faulted claystones to form conical bodies that interconnect across the 700 m thick claystone interval to feed a set of shallow intrusions located within a few hundred meters of the paleoseabed the forcible intrusion of these shallow sandstone bodies produced complementary folding of the contemporaneous seabed the subsequent onlap of these seabed domal folds took place immediately after the intrusion event and by correlation to biostratigraphic data from nearby wells we date these intrusions as late miocene this is the first time that both the age and emplacement depth range of a system of large scale sandstone intrusions have been constrained

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

regional fractures i a mechanism for the formation of regional fractures at depth in flat lying reservoirs 1 regional fractures may form slowly at depth in extension and parallel to regional horizontal tectonic compression such fracture sets can be common in reservoirs despite the absence of flexure of the strata and are commonly important contributors to reservoir permeability such fractures make good targets for deviated wells high formation pore pressure is crucial to creating low effective confining stresses and brittle rock properties in the reservoirs conditions conducive to fracturing despite deep burial but pore pressure does not exceed the least principal stress and does not cause tensile fracturing the mechanism of natural hydraulic fracturing is incompatible with the characteristics of most regional fracture sets rather many regional fractures form duri g far field compression initiate at locally induced tensile stresses caused by flaws in the rock and propagate in the plane of the maximum and intermediate compressive stresses the differential stress required for initiation and propagation of regional fractures is well below that necessary for shear failure in the presence of high pore pressure fractures can be held open at depth by a tectonic basinwide dilatancy of the strata and the open void space commonly becomes mineralized

eocene sandstone intrusions in the tampen spur area norwegian north sea quad 34 imaged by 3d seismic data the eocene of the northern north sea is characterised by fine grained mudstones and isolated sandbodies three dimensional 3d seismic data reveal that the eocene mudstones are intensely deformed by polygonal faulting and contain numerous discordant amplitude anomalies the anomalies identified in the tampen spur area are similar to discordant anomalies previously described from the eocene in the south viking graben and the outer moray firth detailed mapping shows that the anomalies are often conical in three dimensions with dimensions ranging from 100 to 200 m height and a few hundred metres to a few kilometres lateral extent well calibrations show that the anomalies arise from sandstones tens of metres thick encased in mudstone the discordant amplitude anomalies are interpreted as large scale conical sandstone intrusions fed from deeper paleocene sandbodies individual conical intrusions may contain millions of cubic metres of sandstone and may provide fluid migration paths potential reservoirs and pose a drilling risk when exploring for deeper targets

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

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

episodic basin wide hydrofracturing of overpressured early cenozoic mudrock sequences in the north sea basin a deformational fabric associated with the early cenozoic mudrock dominated sequences of the north sea is described seismic data show that the mudrock sequences are pervasively faulted over a large area of the basin axis in the central and northern north sea mapping the faults using regional two and three dimensional data shows that the faults are minor extensional structures arranged in stratigraphically bound tiers in the deep water sequences and bounded by regional condensed sequences seals the geometry of the faulting is complex faults are organized into cellular networks comprising polygonal prismatic and pyramidal forms a gravitational mechanism of downslope collapse faulting is rejected on the grounds that the distribution of the deformed sequences extends over large areas of formerly horizontal basin plain a mechanism involving the repeated development of basin wide overpressured compartments is proposed in which the cellular structure is related to deformation patterns associated with a density inversion between the overpressured units and the overlying seal hydrofracturing is invoked to account for the almost random strike distribution of the faulting the region affected by the pervasive fluid expulsion faulting encompasses almost half of the north sea basin basin wide collapse of the overpressured compartments could have resulted in a geologically rapid flux of enormous volumes of pore fluids this mechanism has implications for concepts of steady state compaction heat flow in basins hydrocarbon transport and the genesis of lead zinc ore deposits