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

three dimensional geometry of concentric intrusive sheet swarms in the geitafell and the dyrfjoll volcanoes eastern iceland sheet intrusions inclined sheets and dykes in the deeply eroded volcanoes of geitafell and dyrfjoll eastern iceland were studied at the surface to identify the location depth and size of their magmatic source s for this purpose the measured orientations of inclined sheets were projected in three dimensions to produce models of sheet swarm geometries for the geitafell volcano the majority of sheets converge toward a common focal area with a diameter of at least 4 to 7 km the location of which coincides with several gabbro bodies exposed at the surface assuming that these gabbros represent part of the magma chamber feeding the inclined sheets a source depth of 2 to 4 km below the paleoland surface is derived a second younger swarm of steeply dipping sheets crosscuts this gabbro and members of the first swarm the source of this second swarm is estimated to be located to the se of the source of swarm 1 below the present day level of exposure and deeper than the source of the first swarm for the dyrfjoll volcano we show that the sheets can be divided into seven different subsets three of which can be interpreted as swarms the most prominent swarm the njardvik sheet swarm converges toward a several kilometers wide area in the njardvik valley at a depth of 1 5 to 4 km below the paleoland surface two additional magmatic sources are postulated to be located to the northeast and southwest of the main source crosscutting relationships indicate contemporaneous as well as successive activity of different magma chambers but without a resolvable spatial trend the dyrfjoll volcano is thus part of a complex volcanic cluster that extends far beyond the study area and can serve as fossil analog for nested volcanoes such as askja whereas in geitafell the sheet swarms seem to have originated from a single focus at one time thus defining a single central volcanic complex such as krafla volcano

imaging and photogrammetry models of olduvai gorge tanzania by unmanned aerial vehicles a high resolution digital database for research and conservation of early stone age sites this paper presents the first aerial mapping of olduvai gorge tanzania using unmanned aerial vehicles and photogrammetric techniques to provide a detailed digital cartographic basis for this world renowned paleoanthropological site the survey covered an area of 32 km2 of olduvai gorge and through the use of aerial photos and ground control points from global navigation satellite systems an orthomosaic and digital surface model with a higher than 5 cm pixel ground resolution were produced the digital surface model was then denoised to calculate a digital elevation model and a high resolution imaging model of olduvai gorge was generated a preliminary morphometric characterization using geographic information systems shows the potential of this approach when analysing multiple topographic variables in large areas of paleoanthropological relevance including production of a new map template for olduvai gorge and new data for the investigation of sedimentary and tectonic processes these results constitute one of the first attempts to obtain high quality imagery from large geographic areas amenable to early stone age research and introduce new workflows for the creation of digital elevation models overall the digital dataset produced is intended to support archaeological and geological investigation in this area and provide new monitoring tools for the conservation of cultural heritage

new insights into the mechanics of sill emplacement provided by field observations of the njardvik sill northeast iceland sills are concordant sheet like bodies of magma their mechanics of emplacement is an important but still not fully understood topic the well exposed basaltic njardvik sill in the extinct tertiary dyrfjoll volcano in northeast iceland offers exceptionally clear insights into the mechanism of sill emplacement the sill is multiple and consists of at least 7 units sills all of which were emplaced along a sharp contact between a rhyolitic intrusion and adjacent basaltic lava flows each sill unit was supplied with magma from an inclined sheet the contacts between the sheets and the sill units are very clear and show that the sill units are much thicker than their feeder sheets since the njardvik sill consists of separate units it obviously did not evolve into a homogeneous magma body nevertheless the abrupt change in dip and thickness from inclined sheets to horizontal sills at this particular locality indicates that the earlier sills were influencing the stress field in their vicinity during the subsequent sheet injections the local stresses around the newly formed sill units forced each of the subsequently injected sheets to change into sills the njardvik sill can be followed laterally in a coastal section for 140 m until it ends abruptly at a fault that cuts the sill using these field observations as a basis a numerical model shows how an inclined sheet opens up the contact between the felsic intrusion and the basaltic lava pile along which the sill emplacement takes place the results suggest that sill emplacement is primarily the result of stress rotation at contacts between layers of contrasting mechanical properties there the orientation of the maximum principal compressive stress 1 is horizontal hence such contacts can represent interfaces along which sill emplacement is encouraged once a sill has been emplaced it extends the stress field with a horizontal orientation of 1 consequently inclined sheets and dykes injected near the sill will be deflected into sills the injection frequency of further sill units controls if the sill can grow into a larger magma body by mixing of the newly supplied with the initially injected magma in case of the njardvik sill the injection frequency was low so subsequently emplaced sill units can be distinguished

discovery of a magma chamber and faults beneath a mid atlantic ridge hydrothermal field crust at slow spreading ridges is formed by a combination of magmatic and tectonic processes with magmatic accretion possibly involving short lived crustal magma chambers the reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast spreading centres but it has been difficult to image such magma chambers beneath slow spreading centres owing to rough seafloor topography and associated seafloor scattering in the absence of any images of magma chambers or of subsurface near axis faults it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow spreading ridges here we report the presence of a crustal magma chamber beneath the slow spreading lucky strike segment of the mid atlantic ridge the reflection from the top of the magma chamber centred beneath the lucky strike volcano and hydrothermal field is approximately 3 km beneath the sea floor 3 4 km wide and extends up to 7 km along axis we suggest that this magma chamber provides the heat for the active hydrothermal vent field above it we also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward dipping faults cutting through the volcanic edifice suggesting continuous interactions between tectonic and magmatic processes

3d seismic imaging of the shallow plumbing system beneath the ben nevis monogenetic volcanic field faroe shetland basin seismic reflection data allow for the 3d imaging of monogenetic edifices and their corresponding plumbing systems this is a powerful tool in understanding how monogenetic volcanoes are fed and how pre existing crustal structures can act as the primary influence on their spatial and temporal distribution this study examines the structure and lithology of host rock as an influence on edifice alignment and provides insight into the structure of shallow sub volcanic monogenetic plumbing systems the anticlinal ben nevis structure located in the northerly extent of the faroe shetland basin ne atlantic margin was uplifted during the late cretaceous and early paleocene by the emplacement of a laccolith and a series of branching sills fed by a central conduit seismic data reveal that multiple intrusions migrated up the flanks of the ben nevis structure after its formation c 58 4 ma kettla equivalent and fed a series of scoria cones and submarine volcanic cones these monogenetic edifices are distributed around the crest of the ben nevis structure the edifices are fed from a complex network of sills and transgressive sheets involving lateral magma migration of tens of kilometres before extrusion at the surface this work highlights the importance of underlying basin structures in influencing the sites and development of subaerial monogenetic fields and the importance of lateral magma flow within volcanic systems supplementary materials the results of the statistical alignment analysis of the monogenetic edifices are available at https doi org 10 6084 m9 figshare c 3662809

geological constraints on the dynamic emplacement of cone sheets the ardnamurchan cone sheet swarm nw scotland cone sheets are a significant constituent of many central volcanoes where they contribute to volcano growth by intrusion and through flank eruptions although the exact emplacement mechanisms are still controversially discussed in particular it is not yet fully resolved whether cone sheets propagate as magma driven opening mode fractures or as shear fractures and to what extent pre existing host rock structures and different stress fields influence cone sheet emplacement to shed further light on the role of these parameters in cone sheet emplacement we use detailed field and remote sensing data of the classic ardnamurchan cone sheet swarm in nw scotland and we show that the cone sheets primarily propagated as opening mode fractures in the 1 2 plane of the volcanic stress field in addition more than one third of the ardnamurchan cone sheet segments are parallel to lineaments that form a conjugate set of nnw and wnw striking fractures and probably reflect the regional nw se orientation of 1 during emplacement in the palaeogene cone sheets exploit these lineaments within the ne and sw sectors of the ardnamurchan central complex which indicates that the local volcanic stress field dominated during sheet propagation and only allowed exploitation of host rock discontinuities that were approximately parallel to the sheet propagation path in addition outcrop scale deflections of cone sheets into sills and back into cone sheets also referred to as staircase geometry are explained by the interaction of stresses at the propagating sheet tip with variations in host rock strength as well as the influence of sheet induced strain as a consequence cone sheets associated with sill like segments propagate as mixed mode i ii fractures hence cone sheet emplacement requires a dynamic model that takes into account stress fields at various scales and the way propagating magma interacts with the host rock and its inherent variations in rock strength

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

demonstration of a multi technique approach to assess glacial microbial populations in the field the ability to perform microbial detection and characterization in field at extreme environments rather than on returned samples has the potential to improve the efficiency relevance and quantity of data from field campaigns to date few examples of this approach have been reported therefore we demonstrate that the approach is feasible in subglacial environments by deploying four techniques for microbial detection real time polymerase chain reaction microscopic fluorescence cell counts adenosine triphosphate bioluminescence assay and recombinant factor c assay to detect lipopolysaccharide each technique was applied to 12 subglacial ice samples 12 meltwater samples and two snow samples from engabreen northern norway using this multi technique approach the detected biomarker levels were as expected being highest in debris rich subglacial ice moderate in glacial meltwater and low in clean ice debris poor and snow principal component analysis was applied to the resulting dataset and could be performed in field to rapidly aid the allocation of resources for further sample analysis we anticipate that in field data collection will allow for multiple rounds of sampling analysis interpretation and refinement within a single field campaign resulting in the collection of larger and more appropriate datasets ultimately with more efficient science return

sill morphology and comparison of brittle and non brittle emplacement mechanisms magmatic sheet intrusions contribute significantly to the upper crustal magma transport network the emplacement mechanism of the magmatic sheets controls the final geometry of the intrusions and the characteristics of host rock deformation previous observations have highlighted the preponderance of brittle structures associated with shallow level sheet intrusions however recent studies have suggested that non brittle host rock behaviour also occurs particularly related to the formation of magma fingers during shallow level sill intrusion here we examine both brittle and non brittle intrusion mechanisms and expand upon them with field observations from a series of widespread and variable magmatic systems non brittle emplacement appears primarily associated with viscous flow of the host rock during intrusion and is therefore intimately linked to the contemporaneous host rock rheology as well as magma dynamics purely brittle and non brittle emplacement processes are found to be end members with many intrusions containing evidence of both behaviours deriving the host rock characteristics is therefore important for discerning potential diagnostic intrusion indicators and intrusion geometries both within the field and in modelling incorporation of variable host material behaviours in numerical and analogue modelling tuned using direct field observations may consequently further our understanding of the controls on shallow level intrusion

internal structure and emplacement of the papoose flat pluton an integrated structural petrographic and magnetic susceptibility study the papoose flat pluton of eastern california is commonly cited in the geologic literature as a classic example of a forcibly emplaced pluton although the relative importance attributed to magmatic versus tectonic processes in controlling the structural evolution of the pluton has been controversial reexamination of this late cretaceous 83 ma pluton using a combination of new structural petrographic microstructural and anisotropy of magnetic susceptibility ams data on the pluton wall rock system has shown that the pluton is an inclined and internally zoned tabular structure assembled by forcible intrusion of successive pulses of magma at a crustal depth of 12 15 km initial pluton formation involved magma ascent in a vertical west northwest striking feeder dike which was arrested at a stratigraphically controlled mechanical discontinuity in the overlying cambrian metasedimentary rocks leading to formation of a southwest dipping sill subsequent sill inflation accompanied by horizontal infilling from the feeder dike at the base of the sill resulted in deformation and vertical translation of previously emplaced magma pulses and local raising of the sill roof facilitated by thermal weakening as the wall rock temperatures progressively rose during emplacement of successive magma pulses cooling from the roof of the pluton downward resulted in cessation of vertical inflation on the west side of the pluton and promoted lateral expansion toward the northeast and floor depression below the eastern part of the pluton we have been unable to document any regional scale structures e g of equivalence to similar age synplutonic strike slip shear zones in the sierra nevada batholith to the west that may have controlled emplacement of the papoose flat pluton however this fact does not preclude the likelihood that the country rocks were subjected to a regional deviatoric stress field at this time indeed tectonic overpressuring was probably essential for producing sufficiently high magma pressures for laccolith formation at this midcrustal level simple thermal modeling using microstructural and thermobarometric data indicates that the total duration of emplacement of the pluton did not exceed 30 000 yr this rapid emplacement rate may explain why the pluton appears to be anorogenic even though it was emplaced during a period of regional scale deformation

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

the sonju lake layered intrusion northeast minnesota internal structure and emplacement history inferred from magnetic fabrics abstract the sonju lake intrusion sli in northeastern minnesota is a layered mafic complex of keweenawan age 1096 1 0 8 ma related to the midcontinent rift the cumulate paragenesis of the intrusion is recognized as broadly similar to the skaergaard intrusion a classic example of closed system differentiation of a tholeiitic mafic magma the sli represents nearly closed system differentiation through bottom up fractional crystallization geochemical studies have identified the presence of a stratabound 50 100 m thick zone anomalously enriched in au pge similar to the pge reefs of the skaergaard intrusion this pge enriched zone is hosted within oxide gabbro cumulates about two third of the way up from the base of the intrusion we present a petrofabric study using the anisotropy of magnetic susceptibility ams to investigate the emplacement and flow patterns within the sonju lake intrusion petrographic and electron microprobe studies combined with ams and hysteresis measurements indicate the primary source of the magnetic signal is pseudo single domain psd magnetite or titanomagnetite low field ams was measured at 32 sites within the sonju lake intrusion which provided information about primary igneous fabrics the magnetic fabrics in the layered series of the sonju lake intrusion are consistent with sub horizontal to inclined emplacement of the intrusion and show evidence that the cumulate layers were deposited in a dynamic environment well aligned magnetic lineations consistently plunging shallowly toward the southwest indicate the source of the magma is a vertical sill like feeder presumably located beneath the finland granite the finland granite acted as a density trap for the sonju lake magmas forcing lateral flow of magma to the northeast the strongly oblate magnetic shape fabrics indicate the shallowly dipping planar fabrics were enhanced by compaction of the crystal mush

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

application of open source photogrammetric software micmac for monitoring surface deformation in laboratory models quantifying deformation is essential in modern laboratory models of geological systems this paper presents a new laboratory monitoring method through the implementation of the open source software micmac which efficiently implements photogrammetry in structure from motion algorithms critical evaluation is provided using results from two example laboratory geodesy scenarios magma emplacement and strike slip faulting micmac automatically processes images from synchronized cameras to compute time series of digital elevation models dems and orthorectified images of model surfaces micmac also implements digital image correlation to produce high resolution displacements maps the resolution of dems and displacement maps corresponds to the pixel size of the processed images using 24 mp cameras the precision of dems and displacements is 0 05 mm on a 40 40 cm surface processing displacement maps with matlab scripts allows automatic fracture mapping on the monitored surfaces micmac also offers the possibility to integrate 3 d models of excavated structures with the corresponding surface deformation data the high resolution and high precision of micmac results and the ability to generate virtual 3 d models of complex structures make it a very promising tool for quantitative monitoring in laboratory models of geological systems

seismic images of active magma systems beneath the east pacific rise between 17 05 and 17 35 s seismic reflection data from the east pacific rise between 17 059 and 17 359s image a magma lens that varies regularly in depth and width as ridge morphology changes confirming the notion that axial morphology can be used to infer ridge magmatic state however at 17 269s where the ridge is locally shallow and broad the magma lens is markedly shallower and wider than predicted from regional trends in this area submersible dives reveal recent volcanic eruptions these observations indicate that it is where the width and depth of the magma chamber differ from regional trends indicating an enhanced magmatic budget that is diagnostic of current magmatism

subsurface ice as a microbial habitat we determine the physicochemical habitat for microorganisms in subsurface terrestrial ice by quantitatively constraining the partitioning of bacteria and fluorescent beads 1 10 m between the solid ice crystals and the water filled veins and boundaries around individual ice crystals we demonstrate experimentally that the partitioning of spherical particles within subsurface ice depends strongly on size but is largely independent of source particle concentration although bacteria are shown consistently to partition to the veins larger particles which would include eukaryotic cells become trapped in the crystals with little potential for continued metabolism we also calculate the expected concentrations of soluble impurities in the veins for typical bulk concentrations found in natural ice these calculations and scanning electron microscope observations demonstrate a concentrated chemical environment 3 5 m total ions at 10 c in the veins where bacteria were found to reside with a mixture of impurities that could sustain metabolism our calculations show that typical bacterial cells in glacial ice would fit within the narrow veins which are a few micrometers across these calculations are confirmed by microscopic images of spherical 1 9 m diameter fluorescent beads and stained bacteria in subsurface veins typical bacterial concentrations in clean ice 102 103 cells ml would result in concentrations of 106 108 cells ml of vein fluid but occupy only a small fraction of the total available vein volume 0 2 hence bacterial populations are not limited by vein volume with the bulk of the vein being unoccupied and available to supply energy sources and nutrients

a numerical model of dyke propagation in layered elastic media summary we develop a mathematical model describing dyke propagation in proximity of an elastic discontinuity of the embedding medium the dyke is modelled as a fluid filled crack in plane strain configuration employing the boundary element method the pressure gradient along the crack is assumed proportional to the difference between the densities of the host rock and the fluid mass conservation is imposed during propagation and fluid compressibility is taken into account the path followed by the crack is found by maximizing the total energy release given by the sum of the elastic and gravitational contributions the mathematical simulations provide a sort of refraction phenomenon that is a sudden change in the direction of propagation when the crack crosses the boundary separating different rigidities if the dyke enters a softer medium its path deviates towards the vertical if the dyke enters a harder medium its path deviates away from the vertical and may even become arrested as a horizontal sill along the interface if the rigidity contrast is large gravitational energy plays a major role during propagation in particular in proximity of layer boundaries this role is enhanced by the shift of the centre of mass due to changes of dyke shape mathematical results were validated by laboratory experiments performed injecting tilted air filled cracks through gelatin layers with different rigidities

emplacement of a xenolith rich sill lajitas texas abstract a 2 15 m thick hawaiite sill in the cretaceous boquillas formation west of the big bend national park texas locally contains a zone 1 12 m thick containing 25 vol xenoliths the inclusions ranging in size from 2 cm to 1 m are dominated by cumulate gabbros and white quartzite the mafic inclusions broke from the sidewall cumulates in a reservoir that fed the hawaiite sill many xenoliths show contacts between quartzite and igneous rock marked by clinopyroxene crystals perpendicular to the contacts the quartzite xenoliths are permeated variably by quenched mafic liquid now devitrified glass with plagioclase clinopyroxene smectite and some quartz grains are fringed by inverted tridymite stratigraphic reconstruction indicates that the sill was emplaced at a depth shallower than 0 5 km but the source of the paleozoic quartzite and sidewall cumulate xenoliths was at least 1 km below the sill the high concentration of xenoliths their lack of imbrication orientation or sorting the preservation of unbroken thin slabs of limestone and the absence of xenoliths at the top and bottom of the sill all support emplacement by plug flow of xenolith charged magma in the center lubricated by less viscous xenolith free magma at the margins crystal size distribution of plagioclase indicates that mean crystal residence time for cumulates was about 75 years and crystallization time at the aphyric upper margin of the sill was about 2 years

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

lidar uav or compass clinometer accuracy coverage and the effects on structural models abstract light detection and ranging lidar and structure from motion sfm provide large amounts of digital data from which virtual outcrops can be created the accuracy of these surface reconstructions is critical for quantitative structural analysis assessment of lidar and sfm methodologies suggest that sfm results are comparable to high data density lidar on individual surfaces the effect of chosen acquisition technique on the full outcrop and the efficacy on its virtual form for quantitative structural analysis and prediction beyond single bedding surfaces however is less certain here we compare the accuracy of digital virtual outcrop analysis with traditional field data for structural measurements and along strike prediction of fold geometry from stackpole syncline in this case the sfm virtual outcrop derived from uav imagery yields better along strike predictions and a more reliable geological model in spite of lower accuracy surface reconstructions than lidar this outcome is attributed to greater coverage by uav and reliable reconstruction of a greater number of bedding planes than terrestrial lidar which suffers from the effects of occlusion irrespective of the chosen acquisition technique we find that workflows must incorporate careful survey planning data processing and quality checking of derived data if virtual outcrops are to be used for robust structural analysis and along strike prediction

neutral buoyancy and the structure of mid ocean ridge magma reservoirs the mechanical structure of the regions for shallow storage and lateral intrusion of mid ocean ridge basalt morb is explored in part by examining the density range appropriate for picritic to tholeiitic liquids and liquid crystal mixtures and its relationship to the nonlinear density depth structure of the ridge magma density spans the range 2 6 to at least 2 82 mg m 3 and incorporates the effects of fractional crystallization on the density evolution the incorporation of olivine phenocrysts and the contributions of water in melt phase density reductions the melt and melt crystal density band crosses the in situ rock density curve over the 1 to 3 km depth interval beneath the volcanic surface within that averaged interval picritic to tholeiitic melt and melt crystal mixtures are in approximate mechanical equilibrium with their surroundings and the interval is referred to as the morb horizon of neutral buoyancy hnb the detailed locations of the transition regions between the negative and neutral buoyancy and between the neutral and positive buoyancy regimes are dependent on melt phase composition as well as the presence of suspended mineral phases and their identity in the east pacific rise the hnb has a 1 1 correspondence with the inferred location of the sheeted dike complex and the seismically detected minimum compressional wave velocity region in ophiolite complexes the region corresponds to the sheeted dike complex and the uppermost isotropic gabbros above the hnb is the region of negative buoyancy seafloor to an averaged 1 km depth within which negative buoyancy forces may induce the descent of nonvesiculated melt this region corresponds to the pillow basalt sediment sequence the region of positive buoyancy extends from 3 km to the depth of melt generation and picritic melt ascends in dikes and veins of finite height 500 m to 11 km and relatively narrow width 20 mm to 100 cm the finite strengths of high temperature fluid weakened rocks mandate finite height melt parcels along the ascent pathway the magmatic source region is therefore never hydraulically connected with the surface nor with the near surface reservoir thus relatively locally integrated buoyancy has a natural role in the ascent of melt from the source region to the region of neutral buoyancy during the final stages of magma ascent hydrothermal fluid circulation enshrouds the hnb in a three dimensional envelope as suggested by the depth extent of alteration mineralogy in ophiolite complexes while hydrothermal cooling may play an important role in arresting the ascent of magma in very low spreading rate environments gravitational equilibrium represents the effectively dominant theme in intermediate and high spreading rate regions such as the east pacific rise elastic crack penetration of rheological transition zones near the hnb identify modes of crack tip morphologic changes as a function of young s modulus ratios e2 e1 above and beneath the transition and affect the style of intrusion while melt buoyancy determines the ultimate equilibrium resting place elastic crack stability considerations relate the vertical magma pressure gradients for tholeiitic pt and picritic pp magmas to the vertically varying gradient in the horizontal stress component h and suggest further subdivision into two horizons of neutral buoyancy one for tholeiitic magma hnbt and one for the picritic liquid olivine mixtures hnbp these intervals are very approximate but are estimated as 600 m to 1400 m and 1400 to 3000 m depth respectively beneath the east pacific rise axis as oceanic lithosphere is created at the rise axis and spreads laterally the horizons of negative buoyancy and neutral buoyancy ride with it and may continue to modulate the subsurface magma dynamics that influence off axis volcanism

magnetic fabric studies of the nipissing sill province and senneterre dykes canadian shield and implications for emplacement the 2 21 ga nipissing diabase sills intrude the huronian supergroup of the southern province coeval senneterre dykes in the adjacent portion of the superior province are potential feeders for the sills anisotropy of magnetic susceptibility ams data were obtained from both of these units to study flow patterns and test emplacement models the sills exhibit three distinct paleomagnetic directions that are likely primary and represent a time correlation tool within the magmatic event two of the three directions are also found in senneterre dykes sites in the northern cobalt embayment carry an n1 reverse paleomagnetic signature they possess maximum ams axes that trend north northwest south southeast interpreted as the direction of magma emplacement in one area the data rule out a cone sheet emplacement mechanism the n3 paleomagnetic signature is restricted to a sill and or dyke like bodies near lake timiskaming where the ams axes are dominantly north northwest south southeast similar to that of the nearby n1 reverse sites sites in a geographically restricted area farther south carry an n1 normal paleomagnetic direction the ams patterns in these sites exhibit very poor within and between site consistency compatible with the magma having crystallized statically in this volumetrically minor magmatic pulse in the sudbury elliot lake area the nipissing diabase is deformed and outcrops exhibit east west elongation parallel to the fold axes of the enclosing huronian sedimentary rocks ams fabrics are interpreted as reflecting this deformation rather than primary magmatic flow three of eight senneterre dyke sites show horizontal flow patterns whereas the remainder have more complicated ams patterns

deformation of basaltic shield volcanoes under cointrusive stress permutations we performed a microstructural study of piton des neiges la reunion island to understand how intrusions and stresses control each other in basaltic volcanoes our study reveals that three perpendicular intrusions trends coexisted during the 2 myr history of the volcano a n120 140 e rift zone a perpendicular dike trend and swarms of subhorizontal intrusions hereafter called sill zones independently the inversion of fault slip data shows that incompatible paleostress fields recurrently occurred along with the intrusions a dominant nne ssw extension a perpendicular extension and strike slip or compressional regimes the orientations of paleostresses are consistent with the orientations of the three perpendicular intrusion populations we propose that stress accumulation in the edifice under the effect of repeated magma injections resulted in permutations of the principal axes of the stress tensor causing a reorientation of subsequent intrusions stress permutations were cyclical each cycle started with dike injections in an extensional stress field reducing the deviatoric stress and switching the axes of principal stresses and finished with sill intrusions in a compressional stress field sill zones acted as detachment planes restoring the extensional stress field and initiating a new cycle of permutations our model of stress permutations is in agreement with the pattern of eruptions and deformation observed at piton de la fournaise in contrast with the hawaiian model of spreading on a decollement stress permutations in la reunion s volcanoes imply that the basal deformation of the edifices if any is not sufficient to compensate the reduction of deviatoric stress caused by intrusions

integrated stratigraphy from the vallcebre basin southeastern pyrenees spain new insights on the continental cretaceous tertiary transition in southwest europe abstract an integrated sedimentological magnetostratigraphic and paleontological study of the vallcebre section south eastern pyrenees spain is carried out in order to define and portray the transition from the cretaceous to the tertiary in a continental setting a robust magnetostratigraphy is correlated to the standard polarity scale in light of known biochronological constraints charophyte marine invertebrates eggshells and other dinosaur remains our results show that this section is among the thickest stratigraphic records for the continental maastrichtian in the old world sedimentology indicates a progressive regression from marine through lagoonal to entirely continental environments the section is dominated by mudstones deposited under low energy conditions exceptionally a basin wide regression maximum is recorded some time before the cretaceous tertiary boundary k t this regression maximum is marked by the input of coarse grained alluvial sediments that record a dramatic change in the landscape quiet mud plains changed to sandy floodplains deposited by high energy currents after a period of renewed quiescence following the regression maximum a cenozoic flooding took place such terminal cretaceous sequence of events has been recorded in shorter sections in several other basins from southwestern europe this energetic sediment input suggests that some time before the k t event a sudden paleoenvironmental reorganization took place in the continental basins of south western europe

shallow sill and dyke complex in western hungary as a possible feeding system of phreatomagmatic volcanoes in soft rock environment abstract neogene alkaline basaltic rocks in the western pannonian basin are eroded remnants of maars tuff rings tuff cones scoria cones and lava fields the erosion level of these volcanoes is deep enough to expose diatreme zones associated with the phreatomagmatic volcanoes the erosion level is deeper yet in the west exposing shallow dyke and sill swarms related to former intra plate volcanoes the basanitic sills are irregular in shape and their lateral extent is highly variable individual sills reach a thickness of a few tens of metres and they commonly form dome like structures with rosette like radial columnar joint patterns the largest sill system identified in this region is traceable over kilometres and forms a characteristic ridge running north east to south west elevation differences in the position of the basanitic sills within an otherwise undisturbed layer cake like siliciclastic succession indicate emplacement of the basanite magma at multiple levels over kilometre scale distances the margins of sills in the system are irregular at a dm to mm scale undulating contacts of the sills together with gentle thermal alteration in the host sediment over cm to dm distances indicate the soft but not necessarily wet state of the host deposits at the time sills were intruded parts of the sill complex show a complicated relationship with the host sediment in form of peperitic zones and irregularly shaped disrupted peperite textures this is interpreted to reflect inhomogenities in water content and rheology of the siliciclastic deposits during intrusion the current summit of the longest continuous ridge preserves a small diatreme that seems to cut through an otherwise disk like sill indicating of relationship between sill emplacement and phreatomagmatic explosive eruptions

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

stable isotope evidence for microbial sulphate reduction at the bed of a polythermal high arctic glacier glacier beds may be host to a range of microbial communities which drive oxic waters towards anoxia along certain hydrological flowpaths chemical and isotopic signatures in meltwaters from finsterwalderbreen a polythermal glacier on sedimentary bedrock in svalbard show clear evidence for anoxia at the glacier bed increases in 34s and 18o of sulphate indicate that microbial sulphate reduction has resulted in significant decreases in sulphate concentration the 13c of the dissolved inorganic carbon dic is isotopically light 13c 8 which is consistent with the use of bedrock kerogen and or the necromass of sulphide oxidising bacteria as organic substrates for the sulphate reducing bacteria calculated rates of organic carbon mineralisation correspond to 10 of the total annual dic flux of the glacial meltwaters this microbial ecosystem is chemoautotrophically based ultimately being sustained by the kerogen and or bacterial necromass and sulphides in the bedrock this work suggests that glacier beds can be refugia for life when climatic and or atmospheric conditions are otherwise inclement and also supports the contention that microbial life is present in subglacial lake vostok

fracturing and flow investigations on the formation of shallow water sills on europa abstract double ridge tectonic features appear prominently and ubiquitously across the surface of jupiter s icy moon europa previous studies have interpreted flanking fractures observed along some of the ridges as indicators of stress resulting from the ridge loading and flexing of the ice shell above a shallow water body here we investigate a shallow water sill emplacement process at a time when the shell is cooling and thickening and explore the conditions that would make such a system feasible on timescales of ridge formation results show that fracture initiation and transport of ocean water to shallow depths can realistically occur although horizontal fracturing and sill lifetimes prove challenging finite element models demonstrate that mechanical layering or a fractured shell do not provide enough stress change to promote horizontal fracturing but tidal forcing does result in a small amount of turn assuming it is possible for a shallow sill to form a sill would convect internally and conduct heat out quickly resulting in a short lifetime in comparison to an estimated flexure timeframe of 100 kyr suggested required for double ridge formation consideration of heat transfer and residence in the overlying ice however extends the flexure timeframe and multiple sill intrusions or replenishment with warm ocean water could prolong the effective sill lifetime though challenges still remain for sill formation at europa these analyses constrain the potential mechanisms for emplacement and indicate sills can act as viable options for supplying the heat needed for surface flexure further analyses and future missions to europa will help to increase our understanding of these enigmatic processes

reinterpretation of palmate and semi palmate webbed fossil tracks insights from finite element modelling abstract a track from the late cretaceous previously described as being generated by a semi palmate bird was studied with the aid of high resolution laser scanning substrate conditions at the time of track formation were diagnosed fine grained soft waterlogged sediment and used to constrain a finite element track simulator the indentation of a non webbed virtual tridactyl foot in such conditions created a resultant track with features analogous to webbing between digits this webbing was a function of sediment deformation and subsequent failure in 3d specific to rheology variation of substrate conditions and interdigital angle was incrementally stepped apparent webbing impressions were clearly developed only within a limited range of sediment conditions and pedal geometry the implications of this work are that descriptions of webbed tracks should account for the possibility that webbing was indirectly formed through sediment failure and not necessarily the direct impression of a webbed foot additionally dating the earliest occurrence of webbed feet in the fossil record and potentially extending phylogenetic ranges should be treated with caution when based upon evidence from tracks

high resolution lidar and photogrammetric survey of the fumanya dinosaur tracksites catalonia implications for the conservation and interpretation of geological heritage sites increasing political and social awareness of the importance of protecting the geological heritage is compelling geoscientists to consider new methods for reconciling conservation and exploration of their research sites terrestrial light detection and range lidar imaging is an accurate method of collecting 3d spatial data that has so far been under utilized in the geological sciences this aim of this paper is to assess the value of integrated lidar and photogrammetric imaging as a tool for synchronizing scientific exploration with conservation of geological heritage sites fumanya catalonia is one of the most important cretaceous tracksites in europe but the nature of exposure of the track bearing surface has hindered quantitative documentation of the ichnites using integrated light detection and range lidar imaging and photogrammetry it has been possible to construct high resolution digital outcrop models dom of the tracksites photo textured doms are a powerful visualization tool and function as fully 3d interactive databases that preserve information about the site that would otherwise be lost to erosion lidar derived doms have the potential to contribute profoundly to future geoconservation projects particularly as a tool for documenting and monitoring heritage sites and promoting education and tourism lidar scanning also provides sufficient resolution to perform robust quantitative analysis of dinosaur tracks

structural control on the shape of intrusions in the koktokay ore district chinese altai north western china abstract the koktokay pegmatite type rare metal bearing ore district in the altai orogen is famous for both its large scale and its diversity of rare metals however the emplacement mechanisms of the ore bearing pegmatite intrusions in the koktokay ore district are still unclear based on field observations the emplacement of the ore bearing pegmatite intrusions falls into two types the first type is typical of the formation of dykes and sills whereby they intruded into fan shaped moderate dipping joints within plutonic rocks the second type involves the formation of a punched laccolith that was fed by a pegmatite sill magmatic stoping is the main mechanism of the laccolith emplacement the peripheral faults played an important role in helping the emplacement of the laccolith the trend of dykes and sills indicate two potential prospecting areas which are located in the western and northern regions of the koktokay ore district

comparative field study of shallow rhyolite intrusions in iceland emplacement mechanisms and impact on country rocks abstract shallow silicic intrusions are known to exist in many active volcanoes and can fuel both eruptions and hydrothermal fields however our knowledge of magma intrusions remains far from complete and processes occurring at intrusion margins are poorly understood in this field based study we characterise four shallows dissected rhyolitic intrusions at three sites in iceland njar vik dyrfjoll krafla and husafell central volcanoes we focus on the relationship between intrusion emplacement mechanisms and country rock response employing scanline mapping of fractures and in situ rock property measurements hardness and permeability along transects from the intrusion margins to damaged and undamaged country rocks we identify various scenarios of shallow intrusion emplacement style based upon their diverse geometry and lithofacies architecture additional information from rock properties and characteristics of fractures and vesicles indicates that initial country rock properties strongly influence the emplacement style we identify two discrete types of country rock response to magma injection the matrix permeability of weak porous and permeable lithologies conglomerate and hyaloclastite is reduced by 1 order of magnitude adjacent to intrusions due to pore occlusion stronger and denser low permeability lithologies basalt and welded ignimbrite undergo a decrease in hardness by a factor 2 related to an up to fivefold increase in fracture density with no significant change in matrix permeability our observations highlight the importance of robust characterisation of the mechanical properties of caldera filling or geothermal reservoir formations for appropriate forecasting of magma mobility geophysical data interpretation and geothermal resources characterisation

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

role of grain size in phyllonitisation insights from mineralogy microstructures strain analyses and numerical modeling abstract brittle y and p planes exist in an exposure of greywacke in the garhwal lesser himalaya india although y planes are well developed throughout the p planes are prominent only in some parts domain a and not elsewhere domain b to investigate why the p planes developed selectively the following studies were undertaken 1 clay separated xrd analyses clinochlore and illite are present in both the domains 2 strain analyses by rf method it deduces strain magnitudes of 1 8 for the ductile deformed quartz grains from both the domains a and b 3 grain size analyses of quartz clasts domain a is mostly composed of finer grains area up to 40 000 m2 whereas domain b consists of a population of coarser grains area 45 000 m2 a 2d finite element modeling of linear elastic material was performed using comsol software to investigate the control of grain size variation on the generation brittle shear planes the results of numerical modeling corroborate the known fact that an increase in grain size reduces the elastic strain energy density a broader grain size distribution increases the effects of diffusion creep and resists the onset of dislocation creep thus rocks with coarser grain population domain b tend to resist the generation of shear fractures unlike their fine grained counterpart domain a

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

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

early cenozoic saucer shaped sills of the faroe islands an example of intrusive styles in basaltic lava piles abstract the early cenozoic igneous activity of the north atlantic igneous province that generated widespread sill complexes in sedimentary basins at the nw european margins also generated various intrusive systems in the contemporaneous basaltic lava pile of the faroe islands the faroe island basalt group comprises seven formations with a total thickness of about 6 5 km of which four major formations are built up of tholeiitic lava flows each being several hundred metres thick and three thinner formations are mostly built up of volcaniclastic lithologies each being a few metres to a few tens of metres thick the largest sills are exposed as partly saucer shaped bodies in the three uppermost formations where inner gently dipping basal sill sections gradually give way to more steeply inclined discordant outer rims that commonly cut several hundred metres into overlying lava flows numerous subvertical and moderately inclined dykes ranging in thickness from c 0 5 to c 4 0 m intersect the areas affected by sill intrusion but only inclined dykes or sheets have been positively identified as sill feeders locally controlled rotations of least principal stress axes 3 during initial sill intrusion or propagation may have been an important contributing factor in determining the overall geometry of the investigated intrusions

experiments on the evolution of laccolith morphology in plan view abstract the ultimate shape of a laccolith is the result of competition between internal magmatic phenomena e g overpressure and the process of solidification and the state of the external host rock e g thickness of the overburden the presence of faults and regional tectonic stresses intrusion morphology can be utilized as an inferential tool to better understand emplacement dynamics and history one specific shape aspect rarely considered is the intrusion s ellipticity which is the ratio of its long to short axes in plan view here with a series of simple analog experiments that integrate the effects of flow along pre existing fractures and solidification during emplacement an assessment is performed of the existing models for formation of a laccolith s plan morphology i e growth dependent and feeder dependent the results of these experiments suggest that neither model fully captures the behavior of laccolith ellipticity evolution and an updated model of formation that takes into account the control of the feeder dike homogenization of overburden bending moments and the processes involving solidification is presented furthermore experimental results are compared with ellipticity measurements of natural laccoliths there are proportionately more high ellipticity laccoliths in nature than in experiments this is interpreted as an induced morphological effect by magma viscosity crustal heterogeneities and or incremental assembly

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

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

unravelling the magmatic system beneath a monogenetic volcanic complex jagged rocks complex hopi buttes az usa the jagged rocks complex is the eroded remnant of the plumbing systems of closely spaced monogenetic alkaline volcanic centres in the southern hopi buttes volcanic field az usa it contains different clinopyroxene populations with distinctive textures and geochemical patterns in the northwestern part of the complex which exposes the best developed system of conduits most of the clinopyroxenes consist of large to medium sized resorbed cores overgrown by euhedral rims type 1 small moderately resorbed greenish cores with the same overgrown rims type 2 and phlogopite as an accessory phase by contrast in the southern part of the complex the majority of clinopyroxenes are euhedral with oscillatory zonation type 3 and are accompanied by minor euhedral olivine the differences between these mineral assemblages indicate a composite history of crystallization and magmatic evolution for the two parts of the complex governed by different mechanisms and ascent patterns from a single source at 50 km depth 16 kbar the northwest system preserves a high pressure assemblage that cooled rapidly from near liquidus conditions suggesting direct ascent from the source to the surface at high to moderate transport rates average 1 25 m s by contrast the southern system represents magma that advanced upward at much lower overall ascent rates stalling at times to form small volume mid crustal storage zones e g sills or a network of sheeted intrusions this allowed the re equilibration of the magma at lower pressure 30 km 8 kbar and led to nucleation and growth of euhedral clinopyroxene and olivine phenocrysts

modification of the regional stress field by magma intrusion and formation of tabular granitic plutons granitic intrusions typically have large aspect ratios with an horizontal major axis about 4 8 times the vertical one combined structural and geophysical gravity data allows distinction between two main types of plutons flat floored plutons the most represented are rather thin 3 4 km and extend in every horizontal direction with a gently dipping floor toward several root zones these contrast with the thick 10 km wedge shaped plutons more elongated in one direction with a few root zones we interpret the shape of flat floored plutons as the result of a switch in the stress pattern caused by the emplacement of magma magma is preferentially emplaced into the plane 1 2 perpendicular to the least principal stress component 3 this plane is initially vertical except for compressional conditions this dilation causes a local re organization of the stress field by increasing the minor and intermediate principal stress components when they overcome the lithostatic load a drastic change in the orientation of the opening plane results switching from vertical to horizontal this constitutes a change from vertically oriented dike shaped intrusions to sub horizontal laccoliths crustal anisotropies vertical faults or horizontal zones of different rheology also contribute to modify the shape of the intrusions the continual exchange of the intermediate stress components also explains the rounded shape of intrusions in strongly extended regions our analysis suggests that the concept of magmas rising to a level of neutral buoyancy is not applicable to many settings rather we suggest that the feedback between displacements resulting from magma intrusion and the local stress pattern controls the geometry of magma emplacement

dyke arrest scenarios in extensional regimes insights from field observations and numerical models santorini greece abstract during a volcanic unrest period with magma chamber rupture fluid driven fractures dykes are injected either from deep reservoirs or shallow magma chambers subsequently the dykes follow propagation paths towards the surface some eventually reaching the surface to erupt while others become arrested here we study dyke paths resulting in eruption or arrest in an excellent 5 km wide exposure of the northern caldera wall of the santorini volcano in greece mapping of 90 dyke segments shows that they were emplaced in a host rock consisting of layers of breccia tuff scoria and lava with a wide variety of mechanical properties at the contacts some dykes are arrested or deflected and hence change their propagation paths here we combine the field data with numerical models to explore dyke paths resulting in 1 arrest and 2 eruption we investigate the effect of different host rock mechanical properties magmatic overpressures and tectonic loading on dyke paths we find that layers with unfavorable local stresses for dyke propagation namely stress barriers result from layer stiffness young s modulus contrast and thickness variations and are a common cause of dyke arrest the study also shows how the details of the dyke path and eventually dyke fed eruptions depend on the mechanical layering and local stresses in volcanoes the results are of great importance for understanding dyke propagation paths and the likelihood of eruption during unrest periods particularly in stratovolcanoes fed by shallow chambers such as santorini

two stage growth of laccoliths at elba island italy at elba island italy nine shallow level late miocene granite porphyry layers connected by feeder dikes built up three nested christmas tree laccoliths detailed mapping and reconstruction of tectonic history led to restoration of the original 5 km thick sequence and determination of the dimensional parameters of each intrusive layer the laccolith layers were emplaced at depths between 1 9 and 3 7 km exploiting physical discontinuities that served as crustal magma traps inside a stack of nappes the intrusive layers are 50 700 m thick with diameters between 1 6 and 10 km length to thickness relationships for individual laccolith layers show a power law correlation that does not fit the known dimensional distribution for laccoliths but instead fits a line with a slope typical of the theoretical vertical inflation stage of laccolith development this is interpreted as the first reported natural example of the occurrence of a vertical inflation stage during laccolith growth the dimensional data for elba intrusive layers also suggest that laccoliths and plutons commonly form by amalgamation of smaller sheet like bodies while multilayer laccoliths form when coalescence fails possibly owing to the large availability of crustal magma traps

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

digital outcrop models applications of terrestrial scanning lidar technology in stratigraphic modeling abstract laser ranging is extremely accurate and efficient terrestrial scanning lidar light detection and ranging applied to outcrop stratigraphic mapping enables researchers to capture laser range data at a rate of thousands of individual x y z and laser intensity points per second these data in conjunction with complementary remotely and directly sampled data are used to conduct high precision facies characterization and to construct 3d geological computer models outcrop data are presented here to explain our workflow and to discuss the construction of rock based 3d digital outcrop models doms reproducibility and quantification are the drivers of this methodology high resolution terrestrial lidar acquisition processing interpretation and visualization are discussed and applied to mapping of geological surfaces in three dimensions laser generated models offer scientists an unprecedented visualization medium in a quantitative 3d arena applications of this technology include constructing and visualizing complex 3d earth models from outcrops for improved reservoir modeling flow simulation in hydrocarbon and aquifer systems and property modeling to constrain forward seismic modeling

solidification effects on sill formation an experimental approach sills represent a major mechanism for constructing continental earth s crust because these intrusions can amalgamate and form magma reservoirs and plutons as a result numerous field laboratory and numerical studies have investigated the conditions that lead to sill emplacement however all previous studies have neglected the potential effect magma solidification could have on sill formation the effects of solidification on the formation of sills are studied and quantified with scaled analogue laboratory experiments the experiments presented here involved the injection of hot vegetable oil a magma analogue which solidified during its propagation as a dyke in a colder and layered solid of gelatine a host rock analogue the gelatine solid had two layers of different stiffness to create a priori favourable conditions to form sills several behaviours were observed depending on the injection temperature and the injection rate no intrusions extreme solidification effects dykes stopping at the interface high solidification effects sills moderate solidification effects and dykes passing through the interface low solidification effects all these results can be explained quantitatively as a function of a dimensionless temperature which describes the experimental thermal conditions and a dimensionless flux which describes their dynamical conditions the experiments reveal that sills can only form within a restricted domain of the parameter space these experiments demonstrate that contrary to isothermal experiments where cooling could not affect sill formation the presence of an interface that would be a priori mechanically favourable is not a sufficient condition for sill formation solidification effects restrict sill formation the results are consistent with field observations and provide a means to explain why some dykes form sills when others do not under seemingly similar geological conditions

fracture evolution in oil producing sills of the rio grande valley northern neuquen basin argentina oil producing sills are commonly considered atypical reservoirs although they can hold significant exploration potential the need for a better understanding of fracture properties and petroleum system characteristics for this and similar igneous rock plays is the main motivation of our study we explore the evolution of this play type by an analysis of the los cavaos oil field located in the malargue fold belt of the neuquen basin argentina integrating multiscale fracture data from outcrops and subsurface the field was created by a combination of intrusions and mild miocene pliocene inversion production stems from thick cavity zones in naturally fractured andesitic sills emplaced in upper jurassic shale source rocks orientation patterns fracture spacing and length of fracture sets in the sill are consistent over several orders of magnitude large multiply connected and weakly cemented fractures are responsible for excellent interconnectedness in the reservoir fracture density is correlated with fault proximity indicating a cogenetic evolution during active deformation abundant fractures in core with strike slip to oblique striations support transpressional overprint during and after fracture formation although it is challenging to separate cooling from tectonic fractures we propose two phases of fracturing marked by a coexistence of subvertical and oblique fractures together with transpressional striae petrographic evidence suggests initial local oil expulsion and migration through microfractures with opening displacements of 0 01 to 1 mm followed by subsequent charging of the evolving intrasill cavity system as well as the bulk fracture system during cooling and mild deformation we suggest that the observed patterns may be extrapolated to sills in similar geotectonic settings

influence of bedrock mineral composition on microbial diversity in a subglacial environment microorganisms in subglacial environments drive the chemical weathering of bedrock however the influence of bedrock mineralogy on the composition and activity of microbial assemblages in such environments is poorly understood here using a combination of in situ mineral incubation and dna fingerprinting techniques we demonstrate that pyrite is the dominant mineralogical control on subglacial bacterial community structure and composition in addition we show that the abundance of fe in the incubated minerals influences the development of mineral associated biomass the ubiquitous nature of pyrite in many common bedrock types and high so 4 2 concentrations in most glacial meltwaters suggest that pyrite may be a dominant lithogenic control on microbial communities in many subglacial systems mineral based energy may therefore serve a fundamental role in sustaining subglacial microbial populations and enabling their persistence over glacial interglacial time scales

monogenetic volcanoes fed by interconnected dikes and sills in the hopi buttes volcanic field navajo nation usa although monogenetic volcanic fields pose hazards to major cities worldwide their shallow magma feeders 500 m depth are rarely exposed and therefore poorly understood here we investigate exposures of dikes and sills in the hopi buttes volcanic field arizona to shed light on the nature of its magma feeder system shallow exposures reveal a transition zone between intrusion and eruption within 350 m of the syn eruptive surface using a combination of field and satellite based observations we have identified three types of shallow magma systems 1 dike dominated 2 sill dominated and 3 interconnected dike sill networks analysis of vent alignments using the pyroclastic massifs and other eruptive centers e g maar diatremes shows a nw se trend parallel to that of dikes in the region we therefore infer that dikes fed many of the eruptions dikes are also observed in places transforming to transgressive ramping sills estimates of the observable volume of dikes maximum volume of 1 90 106 m3 and sills minimum volume of 8 47 105 m3 in this study reveal that sills at hopi buttes make up at least 30 of the shallow intruded volume 2 75 106 m3 total within 350 m of the paeosurface we have also identified saucer shaped sills which are not traditionally associated with monogenetic volcanic fields our study demonstrates that shallow feeders in monogenetic fields can form geometrically complex networks particularly those intruding poorly consolidated sedimentary rocks we conclude that the hopi buttes eruptions were primarily fed by nw se striking dikes however saucer shaped sills also played an important role in modulating eruptions by transporting magma toward and away from eruptive conduits sill development could have been accompanied by surface uplifts on the order of decimeters we infer that the characteristic feeder systems described here for the hopi buttes may underlie monogenetic fields elsewhere particularly where magma intersects shallow and often weak sedimentary rocks results from this study support growing evidence of the important role of shallow sills in active monogenetic fields

widespread bacterial populations at glacier beds and their relationship to rock weathering and carbon cycling bacterial populations found in subglacial meltwaters and basal ice are comparable to those in the active layer of permafrost and orders of magnitude larger than those found in ice cores from large ice sheets populations increase with sediment concentration and 5 24 of the bacteria are dividing or have just divided suggesting that the populations are active these findings 1 support inferences from recent studies of basal ice and meltwater chemistry that microbially mediated redox reactions may be important at glacier beds 2 challenge the view that chemical weathering in glacial environments arises from purely inorganic reactions and 3 raise the possibilities that redox reactions are a major source of protons consumed in subglacial weathering and that these reactions may be the dominant proton source beneath ice sheets where meltwaters are isolated from an atmospheric source of co 2 microbial mediation may increase the rate of sulfide oxidation under subglacial conditions a suggestion supported by the results of simple weathering experiments if subglacial bacterial populations can oxidize and ferment organic carbon it is important to reconsider the fate of soil organic carbon accumulated under interglacial conditions in areas subsequently overridden by pleistocene ice sheets

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

dinosaurs nesting on tidal flats abstract the discovery of a dinosaur clutch in upper cretaceous tidal flat deposits in the tremp formation in biscarri isona lleida spain provides insight into the mode of life of some dinosaurs the clutch occurs in grey marls that were originally waterlogged muddy sediments the seven elongated eggs are parallel to each other and with their long axis almost vertically oriented a mound nest is inferred in contrast to hole nest described so far for similar eggshell types the permeability of the eggshell oospecies megaloolithus siruguei was highly enhanced by a braided network of multicanaliculate pore canals the nest style eggshell porosity and tidal flat environment point to a periaquatic habitat for these dinosaurs a modern analogue occurs in crocodiles alligators and some birds living in periaquatic environments with nesting on plant mounds above waterlogged soils or even on floating vegetation

inelastic deformation during sill and laccolith emplacement insights from an analytic elastoplastic model numerous geological observations evidence that inelastic deformation occurs during sills and laccoliths emplacement however most models of sill and laccolith emplacement neglect inelastic processes by assuming purely elastic deformation of the host rock this assumption has never been tested so that the role of inelastic deformation on the growth dynamics of magma intrusions remains poorly understood in this paper we introduce the first analytical model of shallow sill and laccolith emplacement that accounts for elasto plastic deformation of the host rock it considers the intrusion s overburden as a thin elastic bending plate attached to an elastic perfectly plastic foundation we find that for geologically realistic values of the model parameters the horizontal extent of the plastic zone lp is much smaller than the radius of the intrusion a by modeling the quasi static growth of a sill we find that the ratio lp a decreases during propagation as 1 a 4 p with p the magma overpressure the model also shows that the extent of the plastic zone decreases with the intrusion s depth while it increases if the host rock is weaker comparison between our elasto plastic model and existing purely elastic models shows that plasticity can have a significant effect on intrusion propagation dynamics with e g up to a doubling of the overpressure necessary for the sill to grow our results suggest that plasticity effects might be small for large sills but conversely that they might be substantial for early sill propagation 2

phanerozoic magmatism in the parnaiba basin characterization of igneous bodies well logs and 2d seismic sections geometry distribution and sill emplacement patterns

toward more realistic formulations for the analysis of laccoliths abstract the published laccolith analyses are based on the linear plate bending theory and the a priori assumption that the width of the laccolith is fixed this is not the case in an actual situation the dimension of the laccolith in the horizontal plane has to result from an additional matching condition at the separation lines the published analyses are generalized by dropping the a priori assumption that the width of the laccolith is prescribed by assuming that the magmatic pressure is not constant and by taking into consideration the vertical compressibility of the overburden plate and base in the contact region in order to determine the magnitude of the magmatic pressure a condition is postulated that equates the measured volume of the intruded magma in a laccolith with the corresponding analytical expression for the volume the obtained closed form solution appears to satisfy many of the intuitive expectations it was evaluated numerically and the results are presented as graphs it may be concluded that even very small laccoliths may exist provided the magmatic pressure is sufficiently larger than the overburden weight we also show the dependence of the laccolith size on its stratigraphic position the thicker the overburden h the larger the size of the laccolith for an overburden plate of given thickness the larger the volume v of the intruded magma the larger the laccolith width 2a and its height the paper concludes by discussing a published analysis for laccolith with flexible underburden and overburden it is shown that this analysis is based on a formulation that is of questionable validity

emplacement mechanism of the great whin and midland valley dolerite sills the great whin and midland valley dolerite sill complexes appear to have been emplaced laterally from the walls of feeder dykes the overall thickness of each sill increases with depth as would be expected if the magma finally reached a state of hydrostatic equilibrium variations in the thickness of the sills with estimated intrusion depth imply that the head of magma was about 100 m below the contemporary ground surface in the areas of the present day outcrops at the end of the intrusive episodes before hydrostatic equilibrium was established the magma pressure would probably have been somewhat greater so it is likely that intrusion of the sills was accompanied by the extrusion of flood basalts step and stair transgressions of the bedding are commonly found within the sills mostly stepping downwards in the direction of bedding dip the reason for this directionality is that the weight of sediments floating on an intruding sill has a downdip component that applies a tensile stress to intersecting fractures below the sill when magma is moving downdip and to intersecting fractures above the sill when magma is moving updip

use of low cost multistripe laser triangulation mlt scanning technology for three dimensional quantitative paleoichnological and neoichnological studies abstract the purpose of this paper is to test the application of a new low cost 2995 00 us multistripe laser triangulation mlt scanner and three dimensional 3d software for semiquantitative and quantitative analyses of ichnofossils and modern traces the goal of this research is to improve on existing analytical techniques and apply new methods to 3d digital models of ichnofossils and modern traces objectives are to 1 provide researchers with new ways to develop and test hypotheses quantitatively in the fields of paleoichnology neoichnology sedimentology and soil science and 2 discuss uses advantages and limitations of mlt technology related to ichnology we scanned and created digital models of a variety of mostly continental ichnofossils and modern terrestrial traces produced by invertebrates and vertebrates visual methods applied include making uniformly colored specimen surfaces stereo pairs anaglyph stereo images animations and cross sections quantitative methods applied include measuring distances tortuosity indices and angles and producing contour maps of tracks two of the most useful properties measurable from digital models are surface area sa and volume v these are used rarely in ichnology because they are difficult to measure with traditional methods we use sa to calculate area exploited and introduce a method of quantifying surface roughness adapted from research on soil surfaces we measure v of burrows tracks and coprolites as well as introduce a new measure termed volume exploited we hypothesize that different tracemakers make burrows with characteristic v to sa ratios this is partially supported by statistical tests of previously published data we also use v to sa ratios to determine relative compactness a metric adapted from building physics digital models ensure perpetuity of specimens because they preserve 3d data that can be used to make physical copies placed in museums and disseminated easily to researchers and educational institutions of all levels data from digital models can be used to interpret ichnocoenoses bioturbation rates and pedogenic properties and processes in soils and paleosols note that mlt scanning digitizes only surfaces of objects so it is best suited for exogenic traces and casts of endogenic traces

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

two models for the formation of magma reservoirs by small increments abstract it is now clear that magma reservoirs develop over long time intervals out of a large number of individual intrusions evidence from some large volcanic deposits including those of the bishop tuff california and fish canyon tuff colorado indicates that such reservoirs may experience pervasive and rapid heating before catastrophic eruptions injection of hot primitive melt in the reservoir is often invoked to explain these observations but this requires the rapid emplacement of a large magma volume at least 25 km 3 in less than 100 years for the bishop tuff and efficient heat transfer through a thick cumulate pile within a magma body that grows incrementally temperatures rise progressively and may eventually lead to a permanent volume of melt temperature and crystallization follow two completely different paths depending on the thickness of individual intrusive sheets in one limit corresponding to thick sheets crystallization proceeds at equilibrium initial magma batches crystallize completely and a permanent body of melt develops progressively once ambient temperatures in the complex rise above the solidus in the other limit corresponding to thin sheets crystallization is kinetically controlled such that initial intrusions do not crystallize completely and preserve a glassy residue once temperatures within the growing magma pile reach a certain threshold value nucleation and growth of crystals get reactivated in the residual glass crystallization then proceeds catastrophically in a positive feedback loop involving latent heat release and temperature rise thus an initial phase with no evolved melt present ends with the sudden formation of a large volume of evolved melt in a crystal mush after this initial phase new intrusions get emplaced in heated surroundings and follow a thermal path similar to that of the equilibrium case with melts that become increasingly more primitive with time evidence for kinetic controls on crystallization in the natural environment is provided by the significant amounts of glass up to 40 that are found in thin isolated basaltic sills and dykes owing to sluggish crystallization kinetics more glass should be generated in andesitic and dacitic magmas emplaced in similar conditions devitrification of such quantities of glass accounts for the rapid heating of a thick sill complex the two models of magma reservoir formation involve different requirements the intrusion thickness must be smaller than a critical value in the kinetic model and the average magma input rate must exceed a threshold value in the equilibrium model for a given injection rate a permanent magma reservoir forms in a shorter time interval and over a smaller cumulative magma thickness in the kinetic model than in the equilibrium one the kinetic model is such that the amount of evolved melt generated increases with decreasing injection rate in contrast to the equilibrium model for which the opposite relationship holds