the traigh bhan na sgurra sill isle of mull flow localization in a major magma conduit field evidence points to localization of magma flowing in a tertiary doleritic sill on the isle of mull inner hebrides scotland regions of the sill in which flow was short lived have chilled margins a narrow or absent metamorphic aureole and alignment of plagioclase crystals resulting in pronounced flow parallel lineations on fracture surfaces type i regions prolonged flow resulted in blocky coarse grained dolerite with no chilled margins and an extensive metamorphic aureole type ii regions the distribution of type i and type ii regions shows no spatial pattern with stagnant solidifying sections immediately adjacent to sections with contemporaneous active and sustained flow the transition between type i and type ii regions occurs as the sill thickness exceeds 3 5 m our observation that sustained flow was only possible in regions of the sill thicker than 3 5 m is consistent with previously published theoretical models which predict a critical sill thickness in the range 2 2 5 m regions of the sill narrower than this experienced a single injection of magma simple models to determine flow duration from the width of the metamorphic aureole demonstrate that progressive focusing of flow into wider parts of the conduit created discrete channels active for up to 5 months

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

description of new dry granular materials of variable cohesion and friction coefficient implications for laboratory modeling of the brittle crust abstract cohesion and friction coefficient are fundamental parameters for scaling brittle deformation in laboratory models of geological processes however they are commonly not experimental variable whereas 1 rocks range from cohesion less to strongly cohesive and from low friction to high friction and 2 strata exhibit substantial cohesion and friction contrasts this brittle paradox implies that the effects of brittle properties on processes involving brittle deformation cannot be tested in laboratory models solving this paradox requires the use of dry granular materials of tunable and controllable brittle properties in this paper we describe dry mixtures of fine grained cohesive high friction silica powder sp and low cohesion low friction glass microspheres gm that fulfill this requirement we systematically estimated the cohesions and friction coefficients of mixtures of variable proportions using two independent methods 1 a classic hubbert type shear box to determine the extrapolated cohesion c and friction coefficient and 2 direct measurements of the tensile strength t 0 and the height h of open fractures to calculate the true cohesion c 0 the measured values of cohesion increase from 100 pa for pure gm to 600 pa for pure sp with a sub linear trend of the cohesion with the mixture gm content the two independent cohesion measurement methods from shear tests and tension extensional tests yield very similar results of extrapolated cohesion c and show that both are robust and can be used independently the measured values of friction coefficients increase from 0 5 for pure gm to 1 05 for pure sp the use of these granular material mixtures now allows testing 1 the effects of cohesion and friction coefficient in homogeneous laboratory models and 2 testing the effect of brittle layering on brittle deformation as demonstrated by preliminary experiments therefore the brittle properties become at last experimental variables

emplacement related fabric and multiple sheets in the maiden creek sill henry mountains utah usa the maiden creek sill is a small intrusion w 1k m 2 in map view that is part of the tertiary igneous complex of the henry mountains in southern utah field relationships indicate that 1 this sill has a complex map view geometry consisting of an elliptical main body from which radiate several finger like lobes 2 the intrusion is composed of two igneous sheets presumably representing distinct magma pulses and 3 the current exposure of the intrusion corresponds closely to the original intrusive geometry these relationships allow us to examine in detail the relationships between fabric development intrusive geometry and emplacement processes in this intrusion this sill was emplaced into the flat lying sediments of the colorado plateau during a time of tectonic quiescence fabrics within the intrusion can therefore be interpreted in terms of igneous emplacement processes fabric data are derived from field measurements image analysis x ray computed tomography and anisotropy of magnetic susceptibility in the center of each sheet magmatic fabrics are subhorizontal and poorly defined in the outermost 5 15 cm of each sheet solid state fabrics are developed sub parallel to the contact the two distinct intrusive sheets have similar composition thickness and areal extent we infer that emplacement of the first sheet established the initial map view geometry of the intrusion and the second sheet closely followed the contact between that first sheet and the host rock the example of the maiden creek sill demonstrates that sheeted emplacement can be an important process even in the early stages of construction of small intrusions in addition to other larger intrusions studied elsewhere q 2005 elsevier ltd all rights reserved

local and regional controls on the lateral emplacement of the ben hiant dolerite intrusion ardnamurchan nw scotland abstract sub volcanic and related satellite intrusions record the delivery storage and accommodation of magma before eruption however when several volcanic centres are in close proximity the relationship between centre and satellites may be ambiguous here we examine the structure of the ben hiant dolerite satellite intrusion discuss its relationship with the ardnamurchan central complex located 2 km to the northwest and explore the possibility of a genetic connection with the next nearest centre the mull central complex 35 km to the southeast structural field observations and anisotropy of magnetic susceptibility ams fabric analyses reveal that the ben hiant dolerite was emplaced as a series of lobes in a sequence of stacked sheets the ams fabric data further indicate that the ben hiant dolerite intruded laterally and may have been sourced from either the ardnamurchan central complex or the mull central complex testing either model and discriminating between these two possible source reservoirs remains an outstanding challenge however the potential implications of the lateral movement of magma over many 10 s of kilometres between upper crustal magmatic centres suggests hidden complexities associated with volcanic plumbing systems that must be tested structurally as well as geochemically

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

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

relations between sill intrusions and bedding parallel extensional shear zones in the mid continent rift system of the lake superior region abstract the mid continent rift system is a continuous u shaped feature extending from kansas north through the lake superior region and then south through michigan the logan sills on the northwest side of the lake were intruded during the rifting stage 1109 ma deformation features in the country rock adjacent to the logan sills indicate two episodes of shear strain the first consists of minor thrusts and mesoscopic shear bands both suggesting top to the south southwest movement parallel to the bedding the second rotates the previously formed shear bands in the opposite sense top to the north northeast and is localized at the basal contact between the sills and the country rock we attempt to explain the relations observed with a tectonic model in which sill intrusions take advantage of bedding parallel shear zones that are the surface expression of a south dipping detachment plane the detachment plane steps up in the layered proterozoic sedimentary sequence and allows extension during rifting in this scenario the first episode of shear strain might be related to bedding parallel extension and the second to shear stress induced by the intrusion of magma along the detachment from the south the polarity of both events is supported by small scale kinematic indicators such as veins fiber growths and foliation

mechanisms and duration of non tectonically assisted magma emplacement in the upper crust the black mesa pluton henry mountains utah abstract a new study of black mesa pluton henry mountains colorado plateau utah usa indicates that it is a classic example of a small upper crustal pluton assembled over a few years by incremental amalgamation of discrete magma pulses the results of our petrostructural study of the pluton interior allow us to constrain the geometry kinematics and timing of the processes the symmetric internal fabric is interpreted as an evidence for a feeding by below and not laterally the observed rotation of the lineation from wnw ese on the very top to nne ssw below lead us to propose that the fabric at the base of the pluton is a record of magma infilling process and the fabric at the very top is a record of the strain due to the relative movement between magma and wallrocks a consequence is that except at the contact between pluton and wallrocks top and margins the stretching direction recorded by the lineation is not parallel to the flow direction of the magma i e displacement the black mesa pluton is a sheeted laccolith on its western edge and a bysmalith on its eastern edge this e w asymmetry in pluton geometry construction and the symmetrical internal fabric indicates that the apparently different west and east growth histories could have occurred simultaneously our field data indicate pluton growth through an asymmetric vertical stacking of sill like horizontal magma sheets one dimensional thermal models of the pluton provide maximum limits on the duration of its growth we have constrained the number the thickness and the frequency of magma pulses with our structural observations including 1 the emplacement of the pluton by under accretion of successive magma pulses 2 the absence of solid state deformation textures at internal contacts and 3 the apparent absence of significant recrystallization in the wallrocks our results suggest that the emplacement of the black mesa pluton was an extremely rapid event with a maximum duration on the order of 100 years which requires a minimum vertical displacement rate of the wallrocks immediately above the pluton greater than 2 m yr finally our data show that the rates of plutonic and volcanic processes could be similar a significant result for interpretation of magma transfer in arc systems

the mechanics and dynamics of sills in layered elastic rocks and their implications for the growth of laccoliths and other igneous complexes abstract the recent experimental work by kavanagh et al kavanagh j l menand t and sparks r s j 2006 an experimental investigation of sill formation and propagation in layered elastic media earth planet sci lett 245 799 813 shows that lithological discontinuities and rigidity contrasts can control the formation and dynamics of sills at interfaces separating upper rigid strata from lower weaker strata the present paper extends this work and focuses on its implications in terms of the length and time scales associated with the development of laccoliths and other igneous complexes sill formation controlled by rigidity contrasts is shown to provide a growth mechanism for laccoliths the formation of a sill provides favourable rigidity anisotropy for the emplacement of subsequent sills so that laccoliths grow by over accretion under accretion or even mid accretion of successive sills in accord with field data this model predicts that laccoliths grow mainly by vertical expansion representing the cumulative thickness of their internal sills while maintaining a comparatively constant lateral extend the model also predicts that the time scale over which laccoliths form is essentially determined by the cumulative time between successive sill intrusions also sill dynamics are controlled by viscous dissipation of the fluid along their length which have consequences for the size and shape of sills viscously controlled dynamics would enable sills to propagate further and thus to grow thicker than dykes of similar magmas these dynamics would also enable sills to propagate faster and thus to induce non elastic deformations in surrounding rocks that could deviate them from the interface they originally follow this would allow them to feed new sills along other interfaces and could assist in the formation of the step structures and saucer shapes that are commonly observed in sill complexes

crustal trace of a hot convective sheet the iceland plume has played an influential role in the evolution of the north atlantic ocean and margins over the past 60 m y it is believed that this plume formed at the conjunction of a tetrad of hot subvertical convective sheets the impingement of these hot sheets at the base of the lithospheric lid caused decompressional melting generating substantial quantities of high temperature magma that were injected into the cold overlying lid over the next 10 m y these sheets partly coalesced to form a crudely axisymmetric plume head here we analyze the lithospheric fingerprint of one of these hot convective sheets by forward and inverse modeling of densely sampled wide angle seismic data in conjunction with gravity observations we determined the three dimensional shape of magmatic underplating trapped within the lithosphere the injection of this melt into the lithosphere generated substantial permanent and minor transient uplift of earth s surface predicted and measured amounts of consequent denudation and sedimentation agree within error temporal variations in the patterns of deposition and oceanic circulation adjacent to the convective sheet show its evolution through time and space our results suggest that this linear sheet has probably been directly and indirectly responsible for cyclical events over 60 m y these events have 0 5 1 and 4 6 m y periodicities the existence of which may help to elucidate the dynamic behavior of convective sheets during and after impingement thus in particular circumstances surficial geological processes yield an indirect record of mantle convection and melt generation processes

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

temporal and spatial variation of local stress fields before and after the 1992 eruptions of crater peak vent mount spurr volcano alaska we searched for changes in local stress field orientation at mount spurr volcano alaska between august 1991 and december 2001 this study focuses on the stress field orientation beneath crater peak vent the site of three eruptions in 1992 and beneath the summit of mount spurr local stress tensors were calculated by inverting subsets of 140 fault plane solutions for earthquakes beneath crater peak and 96 fault plane solutions for earthquakes beneath mount spurr we also calculated an upper crustal regional stress tensor by inverting fault plane solutions for 66 intraplate earthquakes located near mount spurr during 1991 2001 prior to the 1992 eruptions and for 11 months beginning with a posteruption seismic swarm the axis of maximum compressive stress beneath crater peak was subhorizontal and oriented n67 76 e approximately perpendicular to the regional axis of maximum compressive stress n43 w the strong temporal correlation between this horizontal stress field rotation change in position of the 1 3 axes relative to regional stress and magmatic activity indicates that the rotation was related to magmatic activity and we suggest that the crater peak stress field rotation resulted from pressurization of a network of dikes during the entire study period the stress field beneath the summit of mount spurr also differed from the regional stress tensor and was characterized by a vertical axis of maximum compressive stress we suggest that slip beneath mount spurr s summit occurs primarily on a major normal fault in response to a combination of gravitational loading hydrothermal circulation and magmatic processes beneath crater peak online material regional and local fault plane solutions

granite intrusion in a metamorphic core complex the example of the mykonos laccolith cyclades greece abstract the aegean domain is a well suited place to study the formation of metamorphic core complex mcc and to investigate the role of syn tectonic granites on their development in the northern cyclades the mykonos delos rhenia mcc is characterized by the intrusion of a kilometer scale late miocene pluton of i type granitoids within a migmatitic gneiss dome new combined ams anisotropy of magnetic susceptibility and microstructural studies on the mykonos granitoids together with recently published thermochronological data allow us to use the granitoids as strain markers the mykonos granitoids form a laccolith like intrusion with a n70 e long axis the laccolith is strongly asymmetric with an outlying root zone to the sw and a major body mainly developed to the ne the laccolith construction is due to successive pulses of more or less differentiated magma that intruded the cycladic blueschist unit the attitude of stretching markers suggests an important about 60 vertical axis local rotation phenomenon in the cycladic upper crust during the exhumation of the mykonos mcc structural data suggest a four stage evolution of the mykonos mcc i a first stage characterized by flat shearing toward the n ne and by the formation of a domal structure in migmatitic paragneisses with multi scale generation of folds with axes either perpendicular or parallel to the regional stretching as a result of the interplay between regional n20 e directed extension and ew shortening ii a second stage marked by the emplacement of the mykonos laccolith at 13 5 0 3 ma at the top of the migmatitic paragneisses iii the third stage corresponding to the development of protomylonitic foliations and lineations in the whole laccolith in high to medium temperature conditions and iv the late stage marked by an acceleration of the exhumation of the mykonos mcc this exhumation was accommodated by important rotations of upper crustal blocks during the end of the exhumation processes around 10 ma deformation localized at the top of the laccolith in semi ductile conditions and then in brittle conditions in the major detachment plane our study shows that the cycladic plutonism event had no role on the initiation of the mcc however the geometry of the mykonos intrusion supports that the magmas are sucked into the direction of regional extension and that the intrusion of magmas has caused an acceleration of the last stages of the mcc development this acceleration was marked by a very fast exhumation of the laccolith after its emplacement

an alternative emplacement model for the classic ardnamurchan cone sheet swarm nw scotland involving lateral magma supply via regional dykes abstract sub volcanic intrusive networks of which cone sheets are recognised as a major constituent partially control volcano growth and eruption style the accepted cone sheet model is that these confocally dipping intrusions originate from an unexposed central magma chamber through dip parallel magma flow however the emplacement mechanism of cone sheets remains poorly understood the 58 ma cone sheet swarm of ardnamurchan nw scotland offers an excellent opportunity to further resolve the emplacement dynamics of cone sheets through studying magma flow and their importance in volcanic edifice construction structural measurements and anisotropy of magnetic susceptibility ams analyses have constrained a lateral magma flow pattern consistently oriented nw se in the majority of the ardnamurchan cone sheets field observations also highlight the importance of host rock structure and interference between competing local and regional stress fields in controlling intrusion geometry our observations suggest cone sheet formation may be linked to laterally propagating nw se trending regional dykes sourced from laterally adjacent magmatic systems likely the palaeogene mull central complex which are deflected around the central complex by stress field interference implicitly edifice construction and potential eruption precursors observed at a volcano may instigate or result from magmatic activity in laterally adjacent volcanic systems

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

rabaul volcano papua new guinea seismic tomographic imaging of an active caldera abstract the recently active rabaul volcano on the island of new britain in eastern papua new guinea is associated with a subduction zone located near the triple junction formed by the pacific south bismarck and solomon lithospheric plates analysis of our 1997 seismic tomography survey of the rabaul caldera reveals the p wave velocity structure to a depth of about 12 km using both explosive and earthquake seismic sources the rabaul volcanic complex is formed by a series of caldera collapse structures and a group of basalt andesite volcanic centres with two currently active dacitic intra caldera cones tavurvur and vulcan the 1994 eruption of these intra caldera cones caused major infrastructure damage and required the evacuation of rabaul township ongoing minor activity of tavurvur continues to the present the 1997 three dimensional seismic tomography imaging identifies a 30 35 km 3 low velocity region magma reservoir at 3 6 km depth beneath the central rabaul caldera and gives an insight into the geological features of the caldera plumbing system on scales of a few kilometres the seismic survey highlights the heterogeneity in p wave velocity both laterally and vertically within the rabaul caldera indicating significant complexity within quite a small area the structural complexity is consistent with that observed in outcrop of palaeozoic eroded calderas in other parts of the world the low velocity region at rabaul 6 0 km s rock units around the caldera rims that are interpreted to indicate large volumes of mafic intrusive rock at shallow

lunar floor fractured craters modes of dike and sill emplacement and implications of gas production and intrusion cooling on surface morphology and structure abstract lunar floor fractured craters ffcs represent the surface manifestation of a class of shallow crustal intrusions in which magma filled cracks dikes rising to the surface from great depth encounter contrasts in host rock lithology breccia lens rigid solidified melt sheet and intrude laterally to form a sill laccolith or bysmalith thereby uplifting and deforming the crater floor recent developments in the knowledge of lunar crustal thickness and density structure have enabled important revisions to models of the generation ascent and eruption of magma and new knowledge about the presence and behavior of magmatic volatiles has provided additional perspectives on shallow intrusion processes in ffcs we use these new data to assess the processes that occur during dike and sill emplacement with particular emphasis on tracking the fate and migration of volatiles and their relation to candidate venting processes ffcs result when dikes are capable of intruding close to the surface but fail to erupt because of the substructure of their host impact craters and instead intrude laterally after encountering a boundary where an increase in ductility base of breccia lens or rigidity base of solidified melt sheet occurs magma in dikes approaching the lunar surface experiences increasingly lower overburden pressures this enhances co gas formation and brings the magma into the realm of the low pressure release of h2o and sulfur compounds both factors adding volatiles to those already collected in the rising low pressure part of the dike tip high magma rise velocity is driven by the positive buoyancy of the magma in the part of the dike remaining in the mantle the dike tip overshoots the interface and the consequent excess pressure at the interface drives the horizontal flow of magma to form the intrusion and raise the crater floor if sill intrusion were controlled by the physical properties at the base of the melt sheet dikes would be required to approach to within 300 m of the surface and thus eruptions rather than intrusions would be very likely to occur instead dynamical considerations strongly favor the sub crustal breccia lens as the location of the physical property contrast localizing lateral intrusion at a depth of several kilometers the end of lateral and vertical sill growth occurs when the internal magma pressure equals the external pressure the intrusion just supports the weight of the overlying crust dynamical considerations lead to the conclusion that dike magma volumes are up to 1100 km3 and are generally insufficient to form ffcs on the lunar farside the estimated magma volumes available for injection into sills on the lunar nearside up to 800 km3 are comparable to the observed floor uplift in many smaller ffcs and thus consistent with these ffcs forming from a single dike emplacement event in contrast the thickest intrusions in the largest craters imply volumes requiring multiple dike contributions these are likely to be events well separated in time rather than injection of new magma into a recently formed and still cooling intrusion we present a temporal sequence of 1 dike emplacement 2 sill formation and surface deformation 3 bubble rise foam layer formation and collapse 4 intrusion cooling and a synthesis of predicted deformation sequence and eruption styles initial lateral injection of the sill at a depth well below the upper dike tip initiates upbowing of the overburden leveraging deformation of the crater floor melt sheet above this is followed by lateral spreading of the sill toward the edges of the crater floor where crater wall and rim deposit overburden inhibit further lateral growth and the sill grows vertically into a laccolith or bysmalith uplifting the entire floor above the intrusion subsidiary dikes can be emplaced in the fractures at the uplift margins and will rise to the isostatic level of the initial dike tip if these contain sufficient volatiles to decrease magma density eruptions can also occur this initial phase of intrusion sill lateral spreading and floor uplift occurs within a few hours after initial dike emplacement during the subsequent cooling of the sill bubbles can rise hundreds of meters to the top of the intrusion to create a foam layer when drainage of gas bubble wall magma occurs in the foam layer a continuous gas layer forms above the foam gas formation and upward migration produces an increase in sill thickness while subsequent cooling and solidification cause a thickness decreases and subsidence the total topographic evolution history following an initial 2 km thick sill intrusion and floor uplift hours includes further floor uplift by gas formation and migration decades 30 m followed by cooling solidification and subsidence a century 350 m an initial 2 km thick sill is predicted to have a final thickness of 1 7 km this predicted sequence of events can be compared with the sequence of floor deformation and volcanism in ffcs in order to test and refine this model

unusually dumbbell shaped guposhan huashan twin granite plutons in nanling range of south china discussion on their incremental emplacement and growth mechanism abstract the guposhan huashan twin plutons crop out as an unusually dumbbell shaped batholith within the middle mesozoic central nanling range granitic belt in south china the batholith was controlled by the east west striking yishan quannan deep crustal fault zone and spatially associated with two regional anticlinoria both plutons show concentric zonation patterns and are composed of two emplacement sequences separated by an approximately 9 m y magmatic lull the 160 163 ma niumiao lisong wanggao sequence and the 148 151 ma xinlu huamei sequence based on field mapping and observations geochronology and geochemistry in a proposed incremental emplacement and growth model the early magmas exploited physical discontinuities such as the contacts between rocks with contrasting competence and densities as well as saddles of regional scale anticlinoria assisted by dominant vertical heat flow and uplifting of the roofs the niumiao lisong wanggao sequence magmas were subsequently emplaced in an over accretion style as distinguishable increments during an approximately 3 m y span gravimetric data observations of field relationships as well as analysis of finite strain and country rock rheology confirm that the plutons became thickened considerably due to largely vertical assembling and growth and most of their space was created by vertical displacement of their country rocks interestingly the coeval twin plutons show contrasting development and growth modes during their assembling and thickening process the guposhan pluton was built on an outward building mode with a reverse zoning pattern while the huashan pluton on an inward building mode with a normal zoning pattern we propose that the contrasting modes largely resulted from their divergent thermal evolution path for example the incremental inward building mode helps retain thermal energy and maintain a high temperature in older units which facilitates injection of subsequent younger increment units of the same sequence within the older host units the independent newer sequence of xinlu huamei magmas however were controlled by shallow brittle faults developed in the older sequence units and emplaced as stocks and dykes by a dyke propagation mechanism

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

magma rheology variations in sheet intrusions of the ardnamurchan central complex scotland inferred from gabbro inclusion characteristics entrainment of xenoliths and their consequent assimilation are key processes in modifying the crystallization kinetics and magma dynamics of conduit systems here an integrated textural and mineral chemical study of the evolution of a suite of gabbroic inclusions within a set of sheet intrusions from the ardnamurchan central complex nw scotland is presented the key findings are as follows 1 the host magma sheets and inclusions are not cognate 2 there are microstructural and mineral chemical similarities between the gabbroic inclusions and the textures and mineralogy of the major hypersthene gabbro on ardnamurchan 3 orientations of magnetic fabrics within the host sheet groundmass and within the gabbroic inclusions are virtually identical field observations suggest that the inclusions were derived from the hypersthene gabbro and were entrained in a few laterally restricted magma segments that subsequently coalesced with inclusion free segments into continuous sheets using stokes law and adaptations thereof we calculate that the magma within the inclusion free segments behaved as a newtonian fluid with a potential settling velocity of50 028 m s in contrast the presence of gabbro inclusions probably modified the magma dynamics to bingham like behaviour we infer that this variation in the magma rheology of separate segments continued after coalescence and internally partitioned the magma sheet preventing lateral mixing and inclusion transport

i ca evidence for upper ocean deoxygenation during the petm anthropogenic global warming affects marine ecosystems in complex ways and declining ocean oxygenation is a growing concern forecasting the geographical and bathymetric extent rate and intensity of future deoxygenation and its effects on oceanic biota however remains highly challenging because of the complex feedbacks in the earth ocean biota system information on past global warming events such as the paleocene eocene thermal maximum petm 55 5 ma a potential analog for present and future global warming may help in such forecasting documenting past ocean deoxygenation however is hampered by the lack of sensitive proxies for past oceanic oxygen levels throughout the water column as yet no evidence has been presented for pervasive deoxygenation in the upper water column through expansion of oxygen minimum zones omzs we apply a novel proxy for paleoredox conditions the iodine to calcium ratio i ca in bulk coarse fraction sediment and planktonic foraminiferal tests from pelagic sites in different oceans and compared our reconstruction with modeled oxygen levels the reconstructed iodate gradients indicate that deoxygenation occurred in the upper water column in the atlantic indian oceans and possibly the pacific ocean as well during the petm due to vertical and potentially lateral expansion of omzs

the mantos blancos copper deposit an upper jurassic breccia style hydrothermal system in the coastal range of northern chile the upper jurassic mantos blancos copper deposit 500 mt at 1 0 cu located in the coastal range of northern chile displays two superimposed hydrothermal events an older phyllic alteration probably related to felsic magmatic hydrothermal brecciation at 155 ma and younger 141 142 ma potassic propylitic and sodic alterations coeval with dioritic and granodioritic stocks and sills and dioritic dikes main ore formation is genetically related to the second hydrothermal event and consists of hydrothermal breccias disseminations and stockwork style mineralization associated with sodic alteration hypogene sulfide assemblages show distinctive vertical and lateral zoning centered on magmatic and hydrothermal breccia bodies which constitute the feeders to mineralization a barren pyrite root zone is overlain by pyrite chalcopyrite and followed upwards and laterally by chalcopyrite digenite or chalcopyrite bornite the assemblage digenite supergene chalcocite characterizes the central portions of high grade mineralization in the breccia bodies fluid inclusions show evidence of boiling during the potassic and sodic alteration events which occurred at temperatures around 450 460 c and 350 410 c and salinities between 3 53 and 13 45 wt nacl eq respectively the hydrothermal events occurred during episodic decompression due to fluid overpressuring hydrofracturing and sharp changes from lithostatic to hydrostatic conditions sulfur isotope results of hypogene sulfide minerals fall in a narrow range around 0 per mil suggesting a dominance of magmatic sulfur carbon and oxygen isotopic data of calcites from propylitic alteration suggest a mantle derived carbon and oxygen isotope fractionation due to low temperature alteration

dynamics of dikes versus cone sheets in volcanic systems igneous sheet intrusions of various shapes such as dikes and cone sheets coexist as parts of complex volcanic plumbing systems likely fed by common sources how they form is fundamental regarding volcanic hazards yet no dynamic model simulates and predicts satisfactorily their diversity here we present scaled laboratory experiments that reproduced dikes and cone sheets under controlled conditions our models show that their formation is governed by a dimensionless ratio 1 which describes the geometry of the magma source and a dynamic dimensionless ratio 2 which compares the viscous stresses in the flowing magma to the host rock strength plotting our experiments against these two numbers results in a phase diagram evidencing a dike and a cone sheet field separated by a sharp transition that fits a power law this result shows that dikes and cone sheets correspond to distinct physical regimes of magma emplacement in the crust for a given host rock strength cone sheets preferentially form when the source is shallow relative to its lateral extent orwhen the magma influx velocity or viscosity is high conversely dikes form when the source is deep compared to its size or when magma influx rate or viscosity is low both dikes and cone sheets may form fromthe same source the shift fromone regime to the other being then controlled by magma dynamics i e different values of 2 the extrapolated empirical dike to cone sheet transition is in good agreement with the occurrence of dikes and cone sheets in various natural volcanic settings

formation of crystal magma chambers in iceland formation of crustal magma chambers in iceland may be facilitated by the occurrence of stress barriers that lead to formation of thick sills such sills absorb the magma of all dikes that enter them and may evolve into magma chambers ideal sites for stress barriers and hence for magma chambers are rock formations where individual layers have different elastic properties the rocks formed during the pleistocene have notably different elastic properties and when buried in the volcanic zones they form more promising sites for magma chambers than the tertiary rocks this may explain why the number of magma chambers indicated by the number of corresponding central volcanoes during the late pleistocene i e during the past 0 7 m y appears to be proportionally greater than the number of chambers i e central volcanoes active during tertiary time

the central andean altiplano puna magma body receiver function analysis of 14 teleseismic events recorded by 6 temporary passcal broadband stations within the altiplano puna volcanic complex apvc shows a consistent 2 s negative polarity p to s conversion for all stations for all available azimuths forward modeling of the largest amplitudes suggests that this conversion is produced by the top of a very low velocity zone at a depth of 19 km with a vs 0 5 km s and a thickness of 750 810 m we interpret the characteristics of the low velocity zone low vs areal extent and flatness to be consistent with a sill like magma body on the basis of additional data from the german ancorp experiment the altiplano puna magma body appears to underlie much of the apvc and it may therefore be the largest known active continental crustal magma body

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

formation of conical fractures in sedimentary basins experiments involving pore fluids and implications for sandstone intrusion mechanisms large sand intrusions often exhibit conical morphologies analogous to magmatic intrusions such as saucer shaped or cup shaped sills whereas some physical processes may be similar we show with scaled experiments that the formation of conical sand intrusions may be favoured by the pore pressure gradients prevailing in the host rock before sand injection our experiments involve injecting air into a permeable and cohesive analogue material to produce hydraulic fractures while controlling the pore pressure field we control the state of overpressure in the overburden by applying homogeneous basal pore pressure and then adding a second local pore pressure field by injecting air via a central injector to initiate hydraulic fractures near the injection point in experiments involving small vertical effective stresses small overburden or high pore fluid overpressure the fracturing pressure lambda fract is supralithostatic and two dipping fractures are initiated at the injection point forming a conical structure from theoretical considerations we predict that high values of lambda fract are due to strong cohesion or high pore fluid overpressure distributed in the overburden such conditions are favoured by the pore pressure stress coupling induced by both pore pressure fields the dips of cones can be accounted for elastic stress rotation occurring around the source contrary to magmatic chamber models the aqueous fluid overpressure developed in a parent sandbody and prevailing before the formation of injectites may diffuse into the surrounding overburden thus favouring stress rotation and the formation of inclined sheets far from the parent source for experiments involving higher vertical effective stresses thick overburden or low pore fluid overpressure the fracturing pressure is lower than the lithostatic stress and a single fracture is opened in mode i which then grows vertically at a critical depth the fracture separates into two dilatant branches forming a flat cone we make use of a p i v particle imaging velocimetiy technique to analyse plastic deformation showing that these inclined fractures are opened in mixed modes close to the surface they change into steep shear bands where fluids can infiltrate the final morphology of the fracture network is very similar to the common tripartite architecture of various injection complexes indicating that different mechanisms may be involved in the formation of dykes feeder dykes under the sill zones may open as tensile fractures while overlying dykes may be guided by the deformation induced by the growth of sills these deformation conditions may also favour the formation of fluid escape structures and pockmarks c 2011 elsevier b v all rights reserved

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

rethinking the emplacement and evolution of zoned plutons geochronologic evidence for incremental assembly of the tuolumne intrusive suite california new u pb geochronologic data indicate that the tuolumne intrusive suite california was assembled over a period of at least 10 m y between 95 and 85 ma and that the half dome granodiorite intruded over a period approaching 4 m y simple thermal considerations preclude the possibility that a magma chamber the size of the half dome pluton could have existed as a liquid at shallow crustal depths for that long rather field evidence for sheeting along the margins of the suite the range of ages and the regular decrease of ages toward the center of the suite and within individual plutons suggest incremental assembly geochronologic evidence for incremental assembly is consistent with the failure of geophysical methods to detect large magma chambers with more than 20 melt even in active volcanic areas because it is unlikely that the individual plutons composing the tuolumne ever coexisted as liquid rich magmas the chemical evolution of the suite cannot be the result of simple fractionation and or mixing between exposed units but instead must reflect processes occurring during magma generation