an experimental study of the relationship between joint spacing and layer thickness abstract two methods for measuring joint spacing are described and compared the area method is a constant for a given outcrop area and is not affected by joint distribution within that area in contrast the line method depends on the location of the linear traverse two kinds of joint sets are distinguished on bedding surfaces 1 a poorly developed set represents the early stages of development when typical joint lengths are less than typical spacing 2 a well developed set represents later stages when lengths are much greater than spacing the area method is applicable to both poorly and well developed sets whereas the line method produces inconsistent results for poorly developed joint sets surface textures on many joints indicate point fracture origins and propagation parallel to bedding whereas most numerical model studies of spacing assume linear origins and propagation perpendicular to bedding the laboratory experiments described in this paper do not suffer from these restrictions an important concept confirmed during these experiments is fracture saturation when the applied strain reaches a certain value fracture spacing stops evolving and remains nearly constant these fracture sets are well developed spacing at saturation is a function of layer thickness but is independent of strain whereas spacing before saturation varies strongly with applied strain thus plotting spacing vs thickness and comparing the slopes of lines fit to such data for poorly developed joint sets in different layers is unlikely to be a diagnostic test for differences in material properties on the other hand spacing may be a sensitive indicator of strain for layers with poorly developed joint sets assessing fracture saturation is a lirst order consideration when gathering spacing data

analogue modeling of overlapping spreading centers insights into their propagation and coalescence abstract the propagation and segmentation of mid ocean ridges is studied using centrifuged analogue models built with non linear materials the deformation of the brittle ductile model is controlled by diapiric uprise of buoyant analogous asthenospheric material induced by a centrifugal body force this linear upwelling laterally stretches the model mantle that in turn induces failure in the upper layer simulating the brittle crust arrays of fractures initiate within zones of high stress concentration above the diapir fractures propagate laterally in a direction perpendicular to the maximum tensile the minimum principal stress secondary tension cracks initiate in the vicinity of parent fracture tips through going fractures that crosscut the model surface develop by short fractures propagating toward each other and coalescing in different types of patterns the overlap overstep and inclination of fractures developed at the initial stage of extension control their subsequent growth and coalescence non overlapping sub parallel fractures propagate along nearly straight paths and coalesce to produce a single planar fracture if overlapping fractures are parallel they propagate towards each other along curved paths that enclose an intervening elliptical core of intact material fracture curvature in this case results from crack crack interaction and is similar to that of overlapping spreading centers oscs observed along mid ocean ridges overlapping non parallel fractures tend to coalesce by one of their tips propagating sub parallel to the spreading direction toward the other fracture such offsets can serve as models for the development of the orthogonal ridge transform fault patterns common along mid ocean ridges

an examination of the parameters that govern the acoustic behavior of sea bed sediments containing gas bubbles the acoustic properties of sea bed sediments containing occluded gas are dominated by the volume of gas contained in bubbles the size of bubbles and the elastic properties of the soil matrix this study evaluated current theory developed by anderson and hampton to determine the sound speed and resonance frequency of gassy soils and the models they used to determine the elastic properties of the soils it compared calculated sound speeds based on material properties simulated by the models with measured sound speeds on large bubble laboratory soils produced in a similar manner to natural sea bed gassy soils there was some evidence that the anderson and hampton equations accurately predicted sound speed at lower frequencies of bubbles resonance and below but results were sensitive to inappropriate values for the elastic and damping properties of the soil the bounds of sound speed based on the elastic properties of models that simulate compressible fluid or suspension behavior were grossly misl

fragmentation processes and xenolith transport in a proterozoic minette dyke grenville province quebec the mesoproterozoic rivard minette dyke provides a case example of dyke parallel fracturing induced by propagation of low viscosity melts at deep crustal levels the dyke constitutes a xenolith choked intrusion breccia and provides samples of an extensive section of the lithosphere underlying the southwestern grenville province of quebec at 1 08 ga the xenoliths occupy approximately 50 of the volume of the breccia they are extremely diverse and include pyroxenites and mafic granulites from the upper mantle and lower crust and grenvillian gabbroids gneisses and quartzites wall rock fragments are detached to various extents along dyke parallel fractures and apophyses the host lamprophyre magma has a calculated viscosity of ca 40 pa s rapid ascent of magma and turbulence are suggested by the presence of large dense fragments and the brittle to ductile deformation at the margins of some xenoliths a minimum ascent velocity of ca 50 cm s 1 is estimated from the settling velocity of a 40 cm long pyroxenite xenolith the progressive addition of xenoliths increased the effective viscosity of the liquid solid mixture to 103 pa s during emplacement this favoured bingham behaviour and laminar flow of the magma and xenolith suspension field evidence suggests that xenoliths were formed through 1 early fracturing of wall rock by inelastic deformation during dyke propagation and 2 continuous delamination of wall rock by intrusion of magma along the dyke parallel fractures this led to removal of chilled margins and to fluid infiltration partial melting and microbrecciation in the wall rock pre existing discontinuities played a minor role in the fracturing process xenoliths may thus be abundant in alkaline magmas not only because these magmas ascend rapidly and can transport xenoliths but possibly also because their low viscosities promote intense fracturing of wall rock

tidal height and frequency dependence of acoustic velocity and attenuation in shallow gassy marine sediments remote prediction of gassy marine sediment properties is important for geohazard assessment gas bubble resonance theory suggests that gassy sediments exhibit acoustic wave velocity frequency and attenuation frequency relationships that depend on gas bubble size gas content and sediment elastic properties an acoustic monitoring experiment to investigate gas bubble resonance effects was undertaken at an intertidal site at dibden bay southampton united kingdom a vertical hydrophone array was positioned to straddle the top of the gassy zone identified on acoustic reflection profiles at about 1 m below the seabed a miniboomer in the seabed above the array was used to generate broadband 600 hz to 3000 hz acoustic signals every 10 min during a 24 hour period with water depths varying between 0 m subaerial exposure at low tide and 2 35 m at high tide the calculated frequency spectra of compressional wave attenuation coefficient show an attenuation maximum over 200 db m that shifts in frequency from 1050 hz at low tide to 1250 hz at high tide thus for the first time providing direct evidence of in situ gas bubble resonance in marine sediments modeling suggests that effective gas bubble radii of 11 mm to 13 mm are responsible for the attenuation maximum supported by x ray computed tomography scan observations on a pressure core which also indicate that bubble shape depends on sediment type modeling of bubble size fluctuations due to pressure equilibration cannot reproduce the observed frequency shift of the attenuation maximum implying that gas diffusion and nonspherical bubbles are significant

a reconnaissance of the major holocene tephra deposits in the upper cook inlet region alaska abstract the upper cook inlet region of southcentral alaska would be significantly impacted by a major tephrafall owing to a widespread population and heavily travelled transportation corridors to evaluate the likelihood of such an occurrence the tephra deposits of the region have been inventoried approximately 90 deposits of holocene age are sufficiently thick to have been preserved for sampling the frequency of such major tephrafalls ranges from 1 every 200 years near sources on the west side of upper cook inlet to 1 every 1000 years on the more populated east side the volcanoes located on the west side of upper cook inlet are from north to south hayes spurr redoubt and iliamna hayes volcano produced the most extensive set of 6 to perhaps 8 tephra layers in the region about 3650 yr b p and produced one other less extensive tephra layer during holocene time spurr and redoubt volcanoes have produced respectively approximately 35 and 30 holocene layers which were dispersed eastward toward population centers no holocene tephra layers of iliamna have been recognized with certainty consequently several tephra layers which originated to the south of the region must have a source at augustine volcano or some more distant volcano tephra layers of hayes volcano are calc alkaline dacites most of the spurr deposits are tholeiitic basaltic andesites whereas those of redoubt volcano are calc alkaline andesites and dacites

emplacement and growth of alkaline dikes insights from the shonkinite dikes elchuru alkaline complex se india abstract the mesoproterozoic elchuru alkaline complex in the prakasam alkaline province pap eastern ghats belt se india is intruded by coeval shonkinite dikes magmas parental to both the host nepheline syenite and shonkinite dikes formed during ca 1350 ma continental rift related magmatism the shonkinite dikes show fine to medium grained equigranular to foliated textures with clinopyroxene biotite amphibole k feldspar perthite and nepheline as major mineral phases the dikes grew by inflation and coalescence of dike segments along intrusive steps anastomosis and subsequent coalescence and thermal mechanical erosion of the host rocks a few dikes resulted by filling the fractures created by shearing inflation of intrusive steps between magma segments by mode i opening is the major mechanism of shonkinite dike emplacement and growth however the shonkinite dikes also demonstrate the application of remote shear stress on the dike walls resulting in anastomosis entrapment and segmentation of tabular host rock xenoliths and in extreme cases brecciation of the host rocks under volatile rich and low viscous melt conditions fractal analysis of xenolith bearing dike further illustrates that thermal erosion was facilitated and accentuated by mechanical breakdown of host rock xenoliths due to interactions of tensile and shear forces cross cutting relationships between the shonkinite dikes suggest local deviation in the least compressive stress direction during dike emplacement the shonkinite dikes of elchuru demonstrate that spatially and temporally restricted high volatile low viscous alkaline magmas may have distinctly different styles of emplacement as controlled by interaction of near field magmatic tensile stress and far field tectonic shear stress in an evolving continental rift setting

control of strike slip fault on dyke emplacement and morphology strike slip faults are commonly assumed to influence magma transport and emplacement in the earth9s crust however direct observations of magma conduits within strike slip faults are lacking here we provide some of the first detailed field observations of dykes emplaced within strike slip faults in the neuquen basin argentina we show how fault planes within strike slip fault zones affect the emplacement of dyke offshoots resulting in complex dyke morphology our study also emphasizes the importance of pre existing strike slip fault array on the development of dyke swarms showing that orientations of dyke swarms may not systematically relate to the principal tectonic stress axes

the geometry of echelon fractures in rock implications from laboratory and numerical experiments abstract the traces of echelon joints veins and dikes in rock range from curving to straight theoretical analyses using boundary element numerical methods have concluded that straight open fractures imply a significant remote differential stress whereas curving traces imply a more nearly isotropic stress we present a series of laboratory experiments which investigate the two dimensional propagation paths of echelon fractures in pmma plates as a function of the applied biaxial loading and the initial geometry of a simple fracture array the experimental results support the theoretical conclusions and verify the accuracy of the numerical method at the grain scale rock does not rigorously conform to the assumptions of isotropy homogeneity and linear elasticity demanded by the numerical method nevertheless results from crack path stability theory suggest that small scale deviations of a fracture from the ideal path generally do not affect its large scale behavior fracture paths which become kinked or curved by local heterogeneities are found to self correct to a path governed by competition between the fracture tip and remote stress fields provided the remote differential stress is non zero this enhances our confidence that the numerical method can be used to produce and explain realistic fracture geometries in rock an unexpected experimental result is the non perpendicular intersection of fractures grown in the laboratory free surface boundary conditions require that the walls of an open fracture be free of shear stress the principal stresses are therefore aligned with the crack wall and we might consequently expect an approaching opening mode fracture either to intersect the free surface at a right angle or to turn away and follow an asymptotic nonintersecting path experimental and numerical modeling however shows how the near tip stresses generated by an approaching fracture dominate the local stress field and allow it to propagate along an oblique path very close to intersection with the adjacent free surface

tectonics and magma dynamics coupling in a dyke swarm of iceland abstract to study the dynamics of dyke and fissure swarms in iceland we analyse the distribution of the dykes across the tertiary alftafjordur dyke swarm and that of open fissures and faults across the active krafla fissure swarm within these narrow 5 to 10 km wide and long length width ratio 10 swarms deformation is accommodated both by tectonics and magmatism the dyke swarm built during the westward regional tilting of the host rock lava pile the populations of dykes suggest that a characteristic thickness at about 3 m is involved during dyking dykes show a quasi periodic distribution across the swarm and form clusters spaced by about 2 5 km the analysis of the fissure distribution in the krafla volcanic system reveals a cluster spacing of about half of that of the dyke clusters we discuss the potential origins of these features as a result of magma dynamics host rock properties and tectonics clustering possibly reflects the intrinsic mechanical response to stretching of the upper levels of the brittle crust we propose a three dimensional model of dyke swarms in which magma likely migrates laterally within the upper crust from igneous centres where high magmatic supply rates locally perturb the regional stress field

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

thermal mechanical modeling of the role of thermal stresses and stoping in magma contamination mantle derived magmas emplaced in the upper crust depth 10 km are in thermal disequilibrium with the surrounding country rock as a consequence of the large temperature contrasts not only will the magma cool quite rapidly but the surrounding country rock will be heated and affected by large thermal stresses using coupled time dependent thermal mechanical models we find that a a single stage 3 km radius magma body has an eruptive life of on the order of 104 years b near the magma wall rock boundary large thermal stresses develop over short time spans 10 years and c material stoped into the chamber as a result of these thermal stresses will be assimilated either in the upper reaches of the chamber or at its base leaving a relatively uncontaminated central core these results indicate that contamination of a primary magma by stoped crustal blocks is an efficient mechanism for producing hybrid lava sequences

formation and interpretation of dilatant echelon cracks the relative displacements of the walls of many veins joints and dikes demonstrate that these structures are dilatant cracks we infer that dilatant cracks propagate in a principal stress plane normal to the maximum tensile or least compressive stress arrays of echelon crack segments appear to emerge from the peripheries of some dilatant cracks breakdown of a parent crack into an echelon array may be initiated by a spatial or temporal rotation of the remote principal stresses about an axis parallel to the crack propagation direction near the parent crack tip a rotation of the local principal stresses is induced in the same sense but not necessarily through the same angle incipient echelon cracks form at the parent crack tip normal to the local maximum tensile stress further longitudinal growth along surfaces that twist about axes parallel to the propagation direction realigns each echelon crack into a remote principal stress plane the walls of these twisted cracks may be idealized as helicoidal surfaces an array of helicoidal cracks sweeps out less surface area than one parent crack twisting through the same angle thus many echelon cracks grow from a single parent because the work done in creating the array as measured by its surface area decreases as the number of cracks increases in cross sections perpendicular to the propagation direction echelon cracks grow laterally each crack overlapping its neighbors until the mechanical interaction of adjacent cracks limits this growth dilation of each crack pinches the tips of adjacent cracks into an asymmetrical form and introduces local stresses that can cause lateral growth along a curving sigmoidal path sigmoidal echelon cracks may link at tip to plane intersections leaving a step in the through going crack wall the geometry of dilatant echelon cracks may be used to infer spatial or temporal changes in the orientation of principal stresses in the earth

the 3d geometry of regional scale dolerite saucer complexes and their feeders in the secunda complex karoo basin abstract dolerites in the karoo basin of south africa commonly represent kilometre scale interconnected saucer shaped structures that consist of inner sills bounded by inclined sheets connected to stratigraphically higher outer sills based on information from over 3000 boreholes and mining operations extending over an area of ca 500 km 2 and covering a 3 km vertical section from karoo strata into underlying basement rocks this paper presents the results of a 3d modelling exercise that describes the geometry and spatial relationships of a regional scale saucer complex locally referred to as the number 8 sill from the secunda coal mine complex in the northern parts of the karoo basin the composite number 8 sill complex consists of three main dolerite saucers dolerites a to c these dolerite saucers are hosted by the karoo supergroup and the connectivity and geometry of the saucers support a lateral sill feeding sill relationship between dolerite saucers a b and c the saucers are underlain and fed by a shallowly dipping sheet dolerite d in the basement rocks below the karoo sequence the 3d geometric strata model agrees well with experimental results of saucer formation from underlying feeders in sedimentary basins but demonstrates a more intricate relationship where a single feeder can give rise to several split level saucers in one regionally extensive saucer complex more localised dome or ridge shape protrusions are common in the flat lying sill parts of the regional scale saucers we suggest a mode of emplacement for these kilometre scale dome and ridge structures having formed as a result of lobate magma flow processes magma lobes propagating in different directions ahead of the main magma sheet undergo successive episodes of lobe arrest and inflation the inflation of lobes initiates failure of the overlying strata and the formation of curved faults magma exploiting these faults transgresses the stratigraphy and coalesces to form a ring like inclined sheet that subsequently feeds a central flat lying roof at a higher stratigraphic level on a regional scale the kilometre size saucer geometries reflect the lateral migration and transport of mafic magmas close to or at the level of the karoo supergroup fed by only isolated feeders in the basement on a more local scale the complex internal geometries within saucers mainly reflect the flow pattern of the magmas and wall rock accommodation structures

fracture propagation associated with dike emplacement at the skaergaard intrusion east greenland abstract extensive magmatic activity including several episodes of dike emplacement was associated with early tertiary continental rifting in eastern greenland exposures of the skaergaard intrusion host a large number of these dikes as well as a variety of hydrothermal veins one vein set has a characteristic outcrop expression and is filled with silica rich chlorites relative timing constraints as well as the distribution and geometric relations of these veins relative to the mafic dikes indicate that at least some of these fractures formed during the emplacement of the dikes that cut the intrusion calculations based on elastic theory permit modeling of dike geometries observed in the field and prediction of fracture geometries that could be associated with the inflation of these dikes extensional fractures may form in the area ahead of the dike termination for a variety of loading conditions the predicted width to length ratio for the dike as well as the size of the predicted fracture zone increases with increasing magma pressure within the dike and or increasing differential between the remote principal stresses for dike geometries similar to those observed in the field the predicted fracture zone is roughly elliptical in cross section and is centered in front of the dike termination the size of the zone perpendicular to the dike trend varies from less than one meter to several meters field observations indicate that dike parallel fractures are locally offset normal to the dike margin by distances of a few centimeters to a few meters our analysis implies that at least some of these fractures initiated and propagated ahead of the dikes as they were emplaced the cooling of these dikes initiated localized hydrothermal systems that formed the vein minerals currently filling the fractures

form and dimensions of dykes in eastern iceland abstract a study was made of about 700 dykes in eastern iceland the majority of these belong to three swarms about 73 dip within 10 of the vertical most strike between 10 and 40 ne the strike of the dykes within the southernmost swarm alftafjordur changes along its trace from almost n at the north end to nne ssw southward along the swarm the average thickness of the dykes is about 4 1 m and the thickness does not change notably along the alftafjordur swarm the thinner dykes tend to have smaller grains than the thicker dykes of five dykes followed laterally the longest is over 22 km the thickness of individual dykes changes irregularly along their length and the dyke is often offset where its thickness changes many dykes appear to be completely discontinuous but some parts are connected by veins where the dykes end in a vertical section most of them simply taper away only about 10 5 of the dykes occupy faults the mechanical and thermal effects of the dykes on the country rock are small many of the dykes appear to be non feeders i e dykes that never reached the surface to feed lava flows using the length width ratio the depth of origin of three dykes has been estimated the maximum depth of origin of these three dykes is 7 5 km 9 km and 10 km below the original surface

igneous sill and finger emplacement mechanism in shale dominated formations a field study at cuesta del chihuido neuquen basin argentina seismic reflection data and field observations have revealed the presence of voluminous igneous sill complexes emplaced into organic rich shale formations in sedimentary basins worldwide damage and structures associated with sills have major implications for fluid flow through basins constraining the distribution of these structures requires a good understanding of the sill emplacement mechanism however most mechanical models of sill emplacement assume elastic host behaviour whereas shale is expected to deform inelastically this contradiction calls for new field observations to better constrain sill emplacement mechanisms in this paper we report on detailed field observations of spectacularly exposed fingers and a sill emplaced in shale at cuesta del chihuido in the neuquen basin argentina exceptional outcrop conditions allow detailed descriptions of both 1 the entire cross section of the intrusions and 2 the deformation structures accommodating intrusion propagation in the host rock all intrusions exhibit irregular blunt or rectangular tips the structures accommodating the tip propagation are systematically compressional including reverse faults folding and imbricate thrust system our observations suggest that the studied intrusions have propagated by pushing the host rock ahead as a viscous indenter our observations suggest that the viscous indenter model is probably a dominant mechanism of sill emplacement in shale

the undrained shear strength of soils containing large gas bubbles measurements of undrained shear strength are reported from a series of triaxial tests on reconstituted soil samples containing large bubbles of methane gas the experimental results indicate that the undrained strength can be either increased or reduced by the presence of large gas bubbles depending upon the values of the consolidation pressure and the initial pore water pressure a conceptual model for soils containing large gas bubbles developed in the companion paper is analysed to provide theoretical predictions for the undrained shear strength the analysis leads to theoretical upper and lower bounds for the undrained strength which show good agreement with the experimental data the experimental and theoretical developments have led to a better understanding of the strength of soils containing large gas bubbles this could be of considerable benefit to the offshore construction industry if platforms pipelines and other structures are to be built in areas of gas bearing sediments l article pre

thermal and mechanical aspects of magma emplacement in giant dike swarms we consider the thermal history and dynamics of magma emplacement in giant feeder dikes associated with continental flood basalts for driving pressure gradients inferred for giant dike swarms thicknesses of 10 m would enable dikes to transport magma laterally over the distances observed in the field up to thousands of kilometers without suffering thermal lock up using time dependent numerical solutions for the thermal evolution of a dike channel under laminar and turbulent flow conditions in the presence of phase transitions we investigate the possibility that the observed dike thicknesses of the order of 100 m result from thermal erosion of the country rocks during dike emplacement this implies that the observed range of dike widths in giant dike swarms may reflect variations in the source volume and not the excess magma pressure it is found that the total volume of intruded magma required to produce an order of magnitude increase in dike width via wall rock melting broadly agrees with the estimated volumes of individual flows in continental flood basalts the presence of chilled margins and apparently low crustal contamination characteristics of some giant dikes may be consistent with turbulent magma flow and extensive melt back during dike emplacement in this case measurements of the anisotropy of magnetic susceptibility most likely indicate magma flow directions during the final stages of dike intrusion shear stresses generated at the dike wall when the dike starts to freeze strongly decrease with increasing dike width which implies that thicker dikes may have less tendency to produce consistent fabric alignment our results suggest that if the dike was propagating downslope off a plume related topographic swell the mechanism responsible for flow termination could possibly have been related to underpressurization and collapse implosion of the shallow magma plumbing system feeding the intrusion radial dikes that erupted at the periphery of the topographic uplift might have increased rather than decreased extensional stresses in the crust within the topographic uplift upon their solidification

tomographic inversion for three dimensional velocity structure at mount st helens using earthquake data tomographic inversion is applied to 17 659 p phase observations at 21 stations from 2023 earthquakes in the vicinity of mount st helens to study the three dimensional velocity structure block size for the inversion is 2 km horizontally and 2 km or more vertically locations of hypocenters are assumed known and are based on a reference one dimensional layered velocity structure a conjugate gradient technique lsqr is used to invert the large sparse system of equations augmented by regularization with a laplacian roughening matrix resolution is estimated by computing the impulse response of the inversion for various critical locations and uncertainties of the estimates are determined by a jackknife approach the results of the inversion show a remarkable correlation with known geological and geophysical features the spirit lake and spud mt plutons are characterized by high velocity regions extending to approximately 9 km depth the st helens seismic zone a band of diffuse seismicity extending nnw from the volcano is evident as a prominent low velocity lineation the change in character of the velocity anomalies south of st helens corresponds well with the near cessation of seismic activity there a low velocity anomaly beneath the crater from 6 to 16 km depths may represent modern magma accumulations

seismic imaging of broken bridges linking seismic to outcrop scale investigations of intrusive magma lobes three dimensional seismic datasets have provided unrivalled insights into magma flow within sub volcanic systems one of the key revelations is that sills appear to be constructed of a series of discrete magma lobes that form during the emplacement of magma into host rock we focus on a large sill within the faroe shetland basin north atlantic that is well imaged on seismic data and identify the presence of broken bridges within the sill developed between elongate magma lobes and reveal for the first time in three dimensions the development of broken bridges critically by relating the imaged structures to key outcrop scale examples we confirm that bridge and broken bridge structures are oriented perpendicular to the magma flow direction this work thus demonstrates a key link that can be made between seismic scale investigation of intrusions and sub seismic outcrop scale processes highlighting the seemingly scale invariant nature of the magmatic emplacement process

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

energy release in subvolcanic environments implications for breccia formation mechanical pdelta v energy is released from hydrous magmas in subvolcanic environments by the two generally sequential processes of second boiling and subsequent decompression the second boiling process h 2 o saturated melt crystals h 2 o vapor releases sufficient pdelta v energy in the model system previously established by burnham 1979 and burnham and ohmoto 1980 to cause fracture failure of even the strongest wall rocks tensile strength approximately 3 77 x 10 10 ergs kg super 1 to depths of 4 or 5 km following fracture failure moreover expansion of already exsolved h 2 o and exsolution of additional h 2 o upon decompression into the model fracture system releases additional pdelta v energy 9 7 x 10 10 ergs kg super 1 of magma sufficient to lift an equivalent mass of rock nearly 1 km in the absence of frictional resistance this latter energy although a conservative value is model dependent to the extent that it is affected by changing any one of the following six independent model parameters 1 bulk composition of the magma hornblende biotite granodiorite in the model 2 depth of emplacement of the magma body approximately 2 3 km p t 0 6 kb 3 mass fraction of h 2 o in the initial melt f o w 0 027 2 7 wt h 2 o 4 mass fraction of melt in the magma at the time of emplacement f o m 1 0 100 liquid 5 tensile strength of the wall rocks 3 77 x 10 10 ergs kg super 1 and 6 depth to the top of the fracture once formed approximately 2 3 km p f 0 3 kb although all of these parameters except the first are of major importance in the formation of hydrothermal breccias generally the mass fraction of h 2 o in the initial melt f o w is regarded as the single most important factor in the formation of breccia bodies in typical porphyry type systems f o w not only determines primarily whether or not the second boiling reaction releases enough pdelta v energy to produce fractures it also determines the magnitude of the decompressional energy released into the fractures if formed magmas in which f o w 0 02 to 0 04 at liquidus temperatures generally release sufficient pdelta v energy not only to produce extensive porphyry type fracture systems but also to produce the breccia bodies these systems commonly contain magmas in which f o w 0 02 on the other hand generally do not release sufficient energy for extensive breccia formation except in structurally weak wall rocks at depths shallower than approximately 2 km magmas in which f o w raquo 0 04 at near liquidus temperatures are incapable of emplacement in the subvolcanic environment

the generation and shape of feeder dykes from mantle sources the criterion for fracture propagation suggests that the primary magma generated in the mantle ascends in a stepwise fashion initially a pulse of magma forms a dyke above the mantle source and ascends for some distance whereafter it stops a subsequent pulse of magma joins the first one and the ascent of the accumulating body of magma continues for some distance each time a new pulse of magma reaches the body of magma situated in the top of the dyke in this manner a dyke is formed that may extend either partly or the whole way from the mantle source to the surface this model may explain the large pressures up to 2000 bars that prevail during basaltic eruptions the calculated shape of the ascending pulses of magma suggests that their heights and widths are similar

three dimensional p and s wave velocity structure of redoubt volcano alaska the three dimensional p and s wave structure of redoubt volcano alaska and the underlying crust to depths of 7 8 km is determined from 6219 p wave and 4008 s wave first arrival times recorded by a 30 station seismograph network deployed on and around the volcano first arrival times are calculated using a finite difference technique which allows for flexible parameterization of the slowness model and easy inclusion of topography and source receiver geometry the three dimensional p wave velocity structure and hypocenters are determined simultaneously while the three dimensional s wave velocity model is determined using the relocated seismicity and an initial s wave velocity model derived from the p wave velocity model assuming an average vp vs ratio of 1 78 convergence is steady with approximately 73 and 52 reduction in p and s wave arrival time rms respectively after 10 iterations the most prominent feature observed in the three dimensional velocity models derived for both p and s waves is a relative low velocity near vertical pipelike structure approximately 1 km in diameter that extends from 1 to 6 km beneath sea level this feature aligns axially with the bulk of seismicity and is interpreted as a highly fractured and altered zone encompassing a magma conduit the velocity structure beneath the north flank of the volcano between depths of 1 and 6 km is characterized by large lateral velocity variations high velocities within this region are interpreted as remnant dikes and sills and low velocities as regions along which magma migrates no large low velocity body suggestive of a magma chamber is resolved in the upper 7 8 km of the crust

numerical modelling of magma transport in dykes the rheology and dynamics of an ascending pure melt in a dyke have been extensively studied in the past from field observations it is apparent that most dykes actually contain a crystalline load the presence of a crystalline load modifies the effective rheology of such a system and thus the flow behaviour indeed the higher density and viscosity of each crystal compared to the melt cause a decrease of the ascent velocity and modify the shape of the velocity profile from a typical poiseuille flow to a bingham type flow a common feature observed in the field is the arrangement of crystals parallel or at a very low angle to the edge of the dyke such a structural arrangement is often interpreted as the result of magma flow which caused the crystals to rotate and align within the flow direction but this process remains unclear another issue related to the introduction of a crystalline load concerns the possibility for crystals to be segregated from a viscous granitic melt phase during magma ascent the implications of such a process on magmatic differentiation have not previously been considered nor has such a process been previously investigated via numerical models in this study we examine the flow dynamics of a crystal bearing granitic melt ascending in a dyke via numerical models in our models both the crystal and melt phases are represented as highly viscous fluids in a stokes regime our results reveal that the presence of crystals in the melt modifies the magma velocity profile across the dyke furthermore we observe that whilst crystals continually rotate in the shear flow over one period of revolution their major axis has a high probability to be aligned parallel to the flow direction moreover some experiments showed that the melt phase can effectively be squeezed out from a crystal rich magma when subjected to a given pressure gradient range this demonstrates that crystal melt segregation in dykes during granitic magma ascent constitutes a viable mechanism for magmatic differentiation

modeling signal loss in surficial marine sediments containing occluded gas the presence of occluded gas in inland lakes harbor muds and surficial marine sediments is well documented surficial gassy sediments cause underlying beds to be acoustically impenetrable to seismic surveys therefore the modeling of signal loss arising from mudline reflection and transmission absorption is of particular interest the anderson and hampton j acoust soc am 67 1890 1903 1980 model for attenuation in gassy sediments was evaluated against the physical and acoustical properties of eight laboratory silty clay soils containing different amounts of occluded gas in bubbles of 0 2 to 1 8 mm diameter the model was shown to give good agreement with measured data over the lower frequencies of bubble resonance and above resonance it did not agree with measured data at frequencies below resonance for which the model did not simulate the bulk properties of the gassy soils the mackenzie j acoust soc am 32 221 231 1960 model for reflection loss was also examined for the gassy soils the maximum reflection losses of 6 db at a grazing angle of 40 does not wholly support speculation by levin geophysics 27 35 47 1962 of highly reflective pressure release boundaries arising from substantial reflection and absorption losses in gassy sediments it was found that mudlines formed from sediments with significant occluded gas may be successfully penetrated although the substantial absorption loss arising from signal transmission through the sediment prevents penetration of the surficial layers to much beyond a meter in depth

propagation and linkage of oceanic ridge segments we have investigated the propagation of spreading ridges and the development of structures that link ridge segments using an analogy between ridges and cracks in elastic plates the ridge propagation force and a path factor that controls propagation direction were calculated for echelon ridge segments propagating toward each other the ridge propagation force increases as ridge ends approach but then declines sharply as the ends pass so ridge segments may overlap somewhat the sign of the path factor changes as ridge ends approach and pass so the overlapping ridge ends may diverge and then converge following a hook shaped path the magnitudes of shear stresses in the plane of the plate and orientations of maximum shear planes between adjacent ridge segments were calculated to study transform faulting for different loading conditions simulating ridge push plate pull and ridge suction we identify a zone of intense mechanical interaction between adjacent ridge ends in which stresses are concentrated for all conditions the shear stress in the interaction zone increases as ends approach and remains large as the ends overlap thus crust in this zone may fracture and weaken in preparation for the formation of a through going transform fault the calculated shear planes rotate toward an orientation about 90 from the strike of ridges as the ends pass thus favoring the orthogonal arrangement of ridges and transforms the magnitudes of mean stresses in the plane of the plate and orientations of principal stress planes were also calculated the mean stress is tensile in the interaction zone so basins may form there except in the case of ridge push loading the planes across which the maximum tension acts are oblique to ridges thus favoring obliquely oriented normal faults bounding the transform valley

the measurement of soil stiffness in the triaxial apparatus this paper describes a simple technique for accurately measuring the mean local axial strains of triaxial samples over a central gauge length the technique makes use of an axial displacement gauge which is a development of one devised by burland symes 1982 which makes use of electrolytic levels the device can resolve to less than 1 m over a range of 15 mm is simple to mount on the specimen and is not damaged when the sample is taken to failure the results of undrained triaxial tests are presented for a wide spectrum of soil types ranging from sands through intact reconstituted and remoulded low plasticity till undisturbed london clay to intact unweathered chalk the test results show that conventional external measurements of displacement contain errors which are frequently so large that their use in the determination of soil stiffness at working levels of stress is invalid the errors mainly result from tilting of the sample bedding at the end platens and the effects of compliance in the app

upward and downward flow in a camptonite dike as recorded by deformed vesicles and the anisotropy of magnetic susceptibility ams abstract a 1 6 m wide cretaceous camptonite dike on higby mountain connecticut contains deformed vesicles that record an initial brief period of steeply upward intrusion followed by a prolonged vertical downward flow of magma prior to solidification the camptonite is unusual in that it is almost aphyric but it does contain abundant skeletal microphenocrysts 0 3 mm of magnetite which give the rock a strong magnetic susceptibility 127 10 3 si these crystals are slightly elongate and their long axes became preferentially aligned parallel to the dike walls but perpendicular to the magma flow direction which is indicated by the stretched vesicles consequently the rock has a strong anisotropy of magnetic susceptibility ams with the mean value of p being 1 03 the ams ellipsoid is everywhere prolate t 0 9 to 0 02 with the maximum susceptibility axis being almost parallel to the strike of the dike that is parallel to the walls of the dike but perpendicular to the magma flow direction in the outer part of the dike where the deformed imbricate vesicles indicate upward flow at an inclination of 70 75 the maximum susceptibility direction plunges 11 perpendicular to the flow direction in the interior of the dike where the vesicles indicate almost straight downward flow the maximum susceptibility direction is almost horizontal and again is perpendicular to the flow in many but not all samples the sense of flow i e up or down is indicated by imbrication of the intermediate magnetic susceptibility axis although the major magnetic susceptibility axis is commonly assumed to parallel the direction of flow of magma in dikes the present study indicates that at least in this camptonite dike the maximum susceptibility axis is perpendicular to the flow of magma

chilled margins in igneous rocks abstract when hot magma contacts cold solid rock a chilled margin forms because the heat flux from the magma cannot balance the very large initial conductive flux in the rock however the conductive flux decreases with time and can eventually be exceeded by the heat flux from the magma in these circumstances the chilled margin begins to remelt and can even disappear this process is modelled quantitatively for the case of a constant heat flux from the magma the results are applied to the turbulent steady flow of komatiite magma in dykes and as lavas the initial chilled margin with cold rock only survives for a few tens of seconds and so does not inhibit thermal erosion significantly the theory can also be applied to magmatic intrusions where the heat flux varies with time results are presented for the remelting of a chilled margin formed at the roof of a basaltic sill for sills with thicknesses from 10 to 1000 m the remelting of initial chilled margins from a few tens of centimetres to a few tens of metres is possible three kinds of contact are predicted at the margins of intrusions first contacts are developed where the chilled margin grows continually without any meltback second meltback can occur but does not completely redissolve the initial chilled margin this causes an internal contact to form within the intrusion third meltback is complete and the country rock can then be melted and assimilated the marginal rocks of such intrusions need not be fine grained

dyke sill relationships in karoo dolerites as indicators of propagation and emplacement processes of mafic magmas in the shallow crust abstract this paper describes the spatial and temporal relationships between karoo age ca 180 ma dolerite dykes and a regional scale saucer sill complex from the secunda coal mine complex in the northeastern parts of the karoo basin of south africa unlike parallel dyke swarms of regional extensional settings mafic dykes commonly show curved geometries and highly variable orientations short strike extents and complex cross cutting and intersecting relationships importantly the dyke networks originate from the upper contacts of the first order dolerite sill saucer structure and are not the feeders of the saucer complex cross cutting relationships indicate the largely contemporaneous formation of dykes and the inner sill and inclined sheets of the underlying saucer systematic dykes form a distinct boxwork type pattern of two high angle interconnected dyke sets the formation and orientation of this dyke set is interpreted to be related to the stretching of roof strata above elongated magma lobes that facilitated the propagation of the inner sill similar to the cracked lid model described for large saucer complexes in antarctica dyke patterns generally reflect the saucer emplacement process and the associated deformation of wall rocks rather than far field regional stresses

a conceptual model for soils containing large gas bubbles offshore site investigations have indicated the presence of undissolved gas bubbles in the soils forming the sea bed at various locations throughout the world the gas bubbles are typically much larger than the normal void spaces so they cannot be considered as occluded bubbles within the pore water which simply change the compressibility of the pore fluid the effect of these large bubbles on the engineering properties of the soil must be understood if offshore construction is to take place in areas of gas bearing sediments this paper describes a conceptual model for soils containing large gas bubbles the model consists of a matrix of saturated soil surrounding isolated gas filled cavities the cavities forming the bubble sites are fixed within the soil so that gas drainage occurs only by the movement of dissolved gas the behaviour of the model is complicated by the processes of localized consolidation and bubble flooding des recherches conduites en mer ont indique qu il y a partout dans le monde d

an acoustic study of soils that model seabed sediments containing gas bubbles the acoustic response of gassy seabed sediment is unique it is a dispersive and extraordinarily attenuative natural material at frequencies which cause gas bubble resonance it conceals the structure of the seabed from seismic profiling and it dampens acoustic signals that for example trigger acoustic mines in the past theoretical studies have formulated the probable cause of this response and crude experimental work has partially corroborated theory this study measures compressional wave velocity and attenuation in a laboratory soil simulating natural gassy soil and it investigates the structural properties that cause the unique acoustic response it was confirmed that below the frequencies which cause resonance the soil behaves as a compressible material containing gas and above as a relatively incompressible material containing no gas over the frequency range producing bubble resonance it is suggested that the soil should be modeled as a biphasic material of gas and a relatively incompressible saturated soil matrix particles and fluid velocities for gassy soil were found to be as low as 220 m s at frequencies below resonance and 1500 m s above resonance attenuations were found to be as high as 60 db cm for moderately gassy soil and as low as 1 db cm for soil with almost no gas

elastic moduli of soils containing large gas bubbles a conceptual model for soils containing large gas bubbles is analysed to provide theoretical expressions for the drained and undrained elastic moduli experimental values of the undrained moduli measured in triaxial tests at large strains show little agreement with the theoretical predictions this lack of agreement is probably caused by concen trated local plastic straining which is likely to occur in soils containing gas bubbles at all but the smallest strains the undrained behaviour at very small strains as represented by acoustic measure ments of the compression wave velocity shows much better agreement with the theoretical predictions improved understanding of the small strain behaviour of soils containing large gas bubbles could he of considerable benefit to the offshore industry for predicting the deformation of foundations constructed on gas hearing sediments on analyse un modele conceptuel pour des sols cootenant de grandes bulles de gaz afin d obtenir des expressions theoriques pour l

asymmetrical to symmetrical magnetic fabric of dikes paleo flow orientations and paleo stresses recorded on feeder bodies from the motru dike swarm romania the fabric in a dike is representative of the magmatic flow considered as newtonian the anisotropy of magnetic susceptibility of the rocks gives a good representation of the shape preferred orientation which in turn is a marker of the magmatic flow generally a symmetrical pattern of the fabric across the dike is in agreement with a flow of magma within a channel the flow direction is then reliable with this imbrication an asymmetrical fabric is dependent on the flow and displacement of the wall we present the case of both symmetrical and asymmetrical dike fabrics recording different emplacements from a pan african calc alkaline dike swarm of basaltic andesitic dacitic rhyolitic composition of the alpine danubian window from south carpathians romania two populations of dikes have been described thick 1 30 m n s trending dikes and thin 1 m e w dikes the first display asymmetrical fabric and record the regional sinistral movement of the walls in contrast the thin dikes are symmetrical and frequently display an arteritic morphology that limits the dike length with no cartographic extension we propose to relate the two types of dikes to the same regional stress field in a continuum of emplacement during a regional brittle event

coalescence of fractures under shear stresses in experiments a series of uniaxial compression tests were performed on gypsum specimens with preexisting fractures to study the failure mechanism of fractures and rock bridges in fractured rock masses the coalescence mechanism of two parallel and offset fractures was investigated by monitoring the process of fracture initiation and propagation with a video camera the tests showed that two inclined parallel fractures can coalesce by shear failure and or tensile failure under a uniaxial load the coalescence path and mechanism mainly depend on the relative position of the two fractures for instance when the two fractures are coplanar or slightly offset coalescence is generated by shear failure when they are overlapping in the loading direction coalescence is generated by mixed shear and tensile failure two types of preexisting fractures one without initial surface contact and hence frictionless and another with surface contact and friction were used to study the influence of fracture contact conditions on the coalescence path and load it was found that coalescence of fractures with surface contact and friction requires loads as much as 35 higher than that for coalescence of fractures without contact and friction a stress analysis was conducted in this study to explain the different coalescence mechanisms the analytical work indicated that different fracture geometries produce significantly different stress fields in the rock bridge area and hence result in different failure modes

the fractal and multifractal dimension of volcanic ash particles contour a test study on the utility and volcanological relevance abstract image processing analysis is used to check the ability of the fractal dimension for quantitatively describing the shape of volcanic ash particles digitized scanning electron microscopy images of fine pyroclasts from the eruptions of monte pilato rocche rosse lipari italy are investigated to test the efficiency of the fractal dimension to discriminate between particles of different eruptive processes multivariate analysis of multiple fractal components allows distinction between magmatic particles and phreatomagmatic particles which however is less significant than the discrimination obtained in previous studies by the use of simple adimensional shape parameters approximation of the actual particle boundary and the not rotation invariant nature of the fractal data frequently result in a significant scatter of data points in the mandelbrot richardson plot such behavior obscures in some cases the actual information of particle shape and renders the discriminating power of fractal analysis less effective than classical shape descriptors data less affected by scatter reveal that phreatomagmatic particles of the monte pilato rocche rosse eruptions are true mono fractals whereas magmatic particles are multifractals the textural small scale fractal of magmatic particles is similar to the fractal dimension value of phreatomagmatic particles and is attributed to the rheological behavior of melt upon brittle fragmentation the structural large scale fractal of magmatic particles refers to the walls of ruptured vesicles that lay on the particle outline the high difference between the values of the textural and structural fractals of magmatic particles of the monte pilato rocche rosse eruptions suggests two distinct and independent processes in the formation of such pyroclasts at the scales corresponding to the textural fractal the fragmentation process is independent of vesicles magmatic fragmentation is not simply related to growth expansion interference and explosion of vesicles but to a brittle cracking of the highly viscous melt likely related to rapid decompression

estimating the free gas content in baltic sea sediments using compressional wave velocity from marine seismic data a 2 d high resolution velocity field was obtained from marine seismic data to quantify free gas content in shallow muddy sediments at in situ pressure and temperature the velocities were acquired applying migration velocity analysis on prestack time migrated data compressional wave velocities are highly sensitive to free gas as very small amounts of gas can cause a significant decrease in the medium velocity the analyzed profile crosses a depression filled with organic rich holocene mud in the bornholm basin baltic sea the interval velocity field reveals two low velocity patches which extend from the reversed polarity reflections marking the top of the gassy sediment layer down to the base of the holocene mud average interval velocities within the gassy mud are lower than the seafloor migration velocity by up to 500 m s this decrease using a geoacoustic model is caused by an average 0 046 gas volume fraction the interval velocities in individual cells of the velocity field are reduced to 200 m s predicting up to 3 4 gas content the velocity field is limited in resolution due to velocity determination at and between reflections however together with the stratigraphic interpretation geological units containing free gas could be identified shallow gas occurs vertically throughout most of the holocene mud in the gassy area comparison with biogeochemical studies at other baltic sea sites suggests that the distribution of free gas is likely to be patchy in the sediment but the gas concentration may peak below the sulfate methane transition zone and gradually decrease below

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

emplacement of dikes sills and crustal magma chambers at divergent plate boundaries abstract a model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1 3 km of the oceanic crust the number and lengths of dikes in any particular swarm should increase but that the thickness should decrease with depth in the crust these predictions are supported by field observations it is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that at a certain level in the crust subsequent dikes change into sills such sills may develop into crustal magma chambers either when many nearby sills combine into a large one or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid for the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m whereas at the slowest spreading ridges the initial thickness must exceed 170 m because thin sills are much more likely to form than thick sills the probability of sills developing into crustal chambers increases with dike intrusion frequency hence with spreading rate furthermore the model predicts first that with increasing spreading rate the length of the chamber along the ridge axis should increase and second that the most likely site for magma chambers is at depths of 2 4 km both these predictions are supported by available data

igneous sills record far field and near field stress interactions during volcano construction isle of mull scotland abstract sill emplacement is typically associated with horizontally mechanically layered host rocks in a near hydrostatic far field stress state where contrasting mechanical properties across the layers promote transitions from dykes or inclined sheets to sills we used detailed field observations from the loch scridain sill complex isle of mull uk and mechanical models to show that layering is not always the dominant control on sill emplacement the studied sills have consistently shallow dips 1 25 and cut vertically bedded and foliated metamorphic basement rocks and horizontally bedded cover sedimentary rocks and lavas horizontal and shallowly dipping fractures in the host rock were intruded with vertical opening in all cases whilst steeply dipping discontinuities within the sequence i e vertical fractures and foliation in the basement and vertical polygonal joints in the lavas were not intruded during sill emplacement mechanical models of slip tendency dilation tendency and fracture susceptibility for local and overall sill geometry data support a radial horizontal compression during sill emplacement our models show that dykes and sills across mull were emplaced during nw se horizontal shortening related to a far field tectonic stress state the dykes generally accommodated phases of ne sw horizontal tectonic extension whereas the sills record the superposition of the far field stress with a near field stress state imposed by emplacement of the mull central volcano we show that through detailed geometric characterisation coupled with mechanical modelling sills may be used as an indication of fluctuations in the paleostress state

variation in the form and distribution of dykes in the mull swarm scotland the dykes of the mull swarm part of the british tertiary igneous province have been examined in a series of well exposed coastal sections traverses through the dyke swarm show systematic changes in the thickness and spacing of dykes both of which are typically log normally distributed both the geometric mean thickness and spacing of dykes increase with distance from the mull centre concordant with a decrease in crustal extension variations in the thickness and spacing result from changes in character of the magma flow it is inferred that the magma flux was fairly evenly distributed near the mull centre but farther away from the source it becomes focused into larger dykes the form orientation and opening direction are also observed to vary with distance away from the volcanic centre on mull the dykes intrude a range of pre existing fractures the range of fractures used decreases with distance from the centre this together with other features of the form of the dykes have been interpreted in terms of the varying magma pressure and regional stress which is expressed as the r ratio analysis of the variable strikes of fracture filled dykes is consistent with decreasing magma pressure with distance from the mull centre assuming that regional stresses were relatively invariant across the same distance

inferring paleostresses from natural fracture patterns a new method we introduce a method to infer the remote differential stress magnitude from the curvature of overlapping echelon fracture traces the curving paths of overlapping echelon cracks imply the predominance of local crack induced stresses over remote stresses during propagation nearly straight crack paths imply the controlling influence of a remote compressive crack parallel differential stress this method is used to interpret complex joint patterns mapped in sedimentary rock it is also applied to the problem of fracture pattern generation using computer models

structural signatures of igneous sheet intrusion propagation cm acknowledges a junior research fellowship funded by imperial college london jdm acknowledges national science foundation grant ear 1654518 ken mccaffrey paul bons and sandy cruden are thanked for their constructive reviews

development of symmetrical and asymmetrical fabrics in sheet like igneous bodies the role of magma flow and wall rock displacements in theoretical and natural cases abstract two orientations of fabrics are recognised in vertical dykes a symmetric orientation associated with magma injected into non deforming wall rocks and an asymmetric orientation usually associated with external non coaxial shearing or magma injected into an active fault the notion of two fabric arrangements stems from two planes of symmetry the dyke symmetry plane dsp and the fabric symmetry plane fsp this paper uses conceptual models to simulate all possible orientations of magma flow indicators that might be produced within a vertical dyke emplaced under coeval internal and external stresses the effect of magmatic buoyancy related stresses are portrayed in terms of magma flow velocities mfv and the external non coaxial tectonic related stresses in terms of wall displacement velocities divided by two wmv 2 the tectonic setting is that of a sinistral transcurrent fault using these assumptions five cases are proposed mfv wdv 2 mfv wdv 2 mfv wdv 2 mfv

formation of dykes feeder dykes and the intrusion of dykes from magma chambers the paper discussed the formation of dykes and applies the results to iceland it is postulated that dykes follow the pathway of least work and of least tensile strength and thereby intrude the subvertical joints in lava flows it is suggested that dykes form in magma pulses where each dyke is split in two by the next magma pulse and so on in iceland the estimated time between successive magma pulses is of the order of several hundred days statistical considerations indicate that in iceland the probability of seeing dykes end upwards is only 1 2 which agrees with observations it is concluded that many and probably the majority of dykes are non feeders this together with the low probability of finding the connection between feeder and lava flow explains the scarcity of observed feeder dykes it is concluded that overpressure in shallow magma chambers needed to drive magma through crustal fractures in iceland is usually smaller than the tensile strength of the host rock several mpa which thereby is the critical factor in dyke intrusion

finite element modelling of shear zone development in viscoelastic materials and its implications for localisation of partial melting the development of shear zones initiating on random weaker initial perturbations is modelled numerically for low deborah number viscoelastic materials considering the influence of effective viscosity contrast power law rheology strain softening and different imposed bulk deformation geometries ranging from pure to simple shear conjugate shear zones initiate at 90 to one another and rotate with increasing bulk deformation the basic pattern not being markedly influenced by the vorticity of imposed deformation the rate of propagation of individual conjugate shear zones is little affected by increased effective viscosity contrast between matrix and inclusion but is promoted by power law rheology however the most marked effect is observed for strain softening behaviour where rapid propagation produces straighter and narrower shear zones the localisation of strain is reflected in a correspondingly heterogeneous stress distribution in particular mean stress or pressure is higher in the extending near planar weaker zones of localised shear melting of gneissic or pelitic compositions is pressure dependent with free water present increased pressure promotes melting whereas the opposite is true for water absent melting for water present conditions a positive feedback could develop between localised shearing increased pressure and partial melting this is potentially more effective in concentrating melt in shear zones than shear heating where melt related softening has a negative feedback effect

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

temporal change of the magma plumbing system at galeras volcano colombia revealed by repeated seismic tomography in 2009 2018 abstract we performed repeated three dimensional seismic tomography in 2009 2018 for the area influenced by the intersection of the silvia pijao buesaco and cauca almaguer fault systems beneath galeras volcano colombia we analyzed two periods 2009 2012 and 2013 2018 which were characterized by explosive eruptive activity and relative quiescence at galeras respectively we used p and s wave arrival times of 2848 2009 2012 and 2289 2013 2018 earthquakes that occurred beneath the volcano p and s wave velocity vp and vs respectively anomalies and their changes between the two periods were interpreted to be associated with magma movement in the volcanic plumbing system which was controlled by the intersection of the faults two possible pathways of magma ascent and accumulation were identified in the fault systems one located sw of the active crater consisted of areas with high vp vs values 1 9 at depths of 2 to 4 km below sea level and the other located ne of the active crater with high and low vp vs values the low vp vs

seismic structure changes beneath redoubt volcano during the 2009 eruption inferred from local earthquake tomography we present seismic velocity models of the area beneath redoubt volcano alaska corresponding to two time periods before and after the strong eruption that occurred in march 2009 the calculations were based on tomographic inversion of p and s arrival time data recorded by 19 stations using the local earthquake tomography code lotos we performed thorough analysis of the results based on synthetic tests in each of the time periods that allow real time variations in structure to be distinguished from artifacts caused by changes in the observation system configurations in the resulting images corresponding to the period before the eruption the summit area is characterized by higher values of both p and s velocities and moderate values of the vp vs ratio this may correspond to the rigid igneous rocks composing the body of the volcano that lack significant liquid content in the second episode corresponding to the time of the eruption the p velocity remained high whereas the s velocity became very low the anticorrelation of p and s anomalies in the summit area produced a vp vs ratio as high as 2 2 which is seen down to 2 3 km depth this indicates presence of mobile phase at shallow depth beneath the volcano which can be either in the form of partial melt or fluid saturated rocks

komatiites i eruption and flow because of their high eruption temperatures and ultrabasic composition komatiite lavas had low viscosities which typically ranged from 0 1 to 10 pa s a major consequence of this low viscosity is that most lavas erupted as turbulent flows an analysis of their ascent through the lithosphere suggests ascent velocities in the range of 1 to over 10ms 1 and reynolds numbers much greater than the critical value of 2000 the lavas would have remained turbulent for most or all of their subsequent flow and emplacement typical horizontal flow rates are estimated to range from 0 5 to 100 m2 s per unit width of flow such turbulent lava flows would have lost their heat by convection to the surroundings at rates which are orders of magnitude greater than the rates for laminar flows which cool by conduction a quantitative analysis of the cooling of komatiites indicates cooling rates from over 1000 c hr to a few c hr 1 while the flows remained turbulent these rates are in an appropriate range to cause phenomena such as high nucleation rates strong supersaturation of the lava delayed nucleation of olivine and skeletal or dendritic crystal morphologies komatiites often flowed over ground composed of rocks with lower melting temperatures it is proposed that the turbulent lavas melted the ground to form deep thermal erosion channels melting rates at the lava source are calculated at several metres per day and deep troughs with depths of several metres to hundreds of metres and lengths of several kilometres probably formed laboratory experiments performed to simulate thermal erosion show qualitative agreement with the theory with channel depth decreasing downstream the experiments also revealed that the channel margins become undercut during thermal erosion to form overhanging sides of the channel some sinuous rilles observed in the mare regions of the moon are thought to have formed by thermal erosion hulme 1973 they provide analogues of the channels postulated to form in komatiite eruptions where conditions were in fact more favourable for thermal erosion an assessment of the role of olivine crystals precipitated in the cooling turbulent flows indicates that they will remain in suspension until the lava has come to rest contamination of komatiite lava by underlying rock can be as much as 10 per cent some illustrative calculations show how the major element and trace element compositions of residual melts can be significantly modified by combined assimilation and fractional crystallization in a moving flow assimilation of tholeiitic basalt into a komatiite can cause incompatible trace element ratios such as ti zr and y zr and the rare earth patterns of derivative lavas to vary substantially some of the variations in such geochemical parameters which are often ascribed to mantle heterogeneity also could have resulted from assimilation of the ground assimilation could have modified the isotope geochemistry of lava suites and led to apparent ages which differ from the true eruption age the thermal erosion model also provides an explanation of the formation of some nickel sulphide ores found at the bottom of thick komatiite flows it is proposed that ores can form by assimilation of sulphur rich sediment which combines with ni from the komatiite to form an immiscible liquid

sound speed and attenuation measurements in gassy sediments in the gulf of mexico the speed and attenuation of sound in gassy marine sediments in the mississippi delta area have been measured by means of small 1 to 500 mg explosive charges buried 30 m below the mud line the respective values obtained were 800 m sec and 1 4 db khz m effective gas content was estimated to be approximately 0 06 percent

magmatic system geometry at mount st helens modeled from the stress field associated with posteruptive earthquakes earthquakes following the may 18 1980 and june 12 1980 explosive eruptions of mount st helens were concentrated at depths below 6 km the hypocenters of these posteruption earthquakes define two seismic lobes separated by an aseismic volume of 2 km width that is located beneath the position of the crater the timing and location of the posteruption seismicity suggests that the earthquakes occurred in the country rock surrounding a magma body located within the aseismic volume the withdrawal of magma caused a pressure decrease within the reservoir and the earthquakes occurred as a brittle response to the resulting stress change focal mechanism solutions for posteruption earthquakes distributed within the seismic lobes are calculated using polarity and amplitude data results show that a perturbation from the regional stress field occurs between depths of 7 and 11 km this anomalous stress field is successfully modeled by a decrease in pressure within a cylindrical magma body located between the seismic lobes and subject to conditions representing the regional shear stress regime in the best fitting model the magma body is centered at 46 204 n 122 187 w 600 m to the north of the present dome and is under a regional shear stress of 20 bars the radius of the chamber is calculated to be between 0 65 and 0 75 km when using pressure drops within the cylinder of 220 and 150 bars respectively the magma body extends from 7 to 11 km and has a volume of between 5 and 7 km3 posteruption earthquakes located below 11 km fall on a northeast striking fault that is preferentially aligned with the regional tectonic stress regime this fault may be a conduit to transport magma to the shallow reservoir from greater depths within the crust

tomographic inversion of local earthquake data from the hengill grensdalur central volcano complex iceland parameters is generally low however a quantitative analysis of resolution provides an unambiguous assessment of well resolved volumes within the better resolved regions of the model the averaging volumes are 1 2 km and 2 4 km in vertical and horizontal extent respectively results of tomographic inversion image three distinct bodies of anomalously high velocity two of these extend from near the surface to a depth of about 3 km these high velocity volumes are located directly beneath the surface expressions of the extinct grensdalur volcano and the extinct husmuli basalt shield the third high velocity structure occurs in the depth range of 3 4 km but does not extend to the surface these three high velocity bodies are interpreted to be solidified magmatic intrusions relatively low velocities underlay limited portions of the trace of the present accretionary axis and a low velocity body is imaged in the roots of the active hengill volcano the volume of lower velocities located beneath the surface expression of the hengill volcano is interpreted to be a region of partial melt

paleostress history of the basin and range province in western utah and eastern nevada from healed microfracture orientations in granites healed microfractures in quartz grains in 14 to 169 ma granites from the basin and range province in western utah and eastern nevada record a history of paleostress the microfracture data are in good agreement with the orientations of paleostresses determined for this same region by using orientations of dikes emplaced into only slightly older igneous host rocks samples generally have two or three steeply dipping microfracture sets one set is related to the tectonic stress field at the time of magma emplacement it strikes northeast southwest in 31 169 ma samples and east northeast west southwest in 25 30 ma samples rotates to east west by about 22 ma and strikes northwest southeast in the youngest samples about 14 ma plutons younger than about 14 ma are not exposed at the surface in western utah and eastern nevada other crack sets in the samples could have formed during cooling of the pluton during a post cooling regional thermal or tectonic event or during some local stress event a strong north south trend in these other crack sets may suggest a formation during basin and range extension

strike slip faulting normal faulting and lateral dike injections along a single fault field example of the gljufura fault near a tertiary oblique rift transform zone borgarfjorður west iceland 1 the crustal block of borgarfjor ur in western iceland was caught between the propagating reykjanes langjokull rift zone rlrz and the receding snaefellsnes rift zone srz during a tertiary rift jump at 7 ma this tertiary rift jump configuration presents an analogy with the presently unstable plate boundary in south and southwest iceland field examination of the gljufura fault in the tertiary lavas of borgarfjor ur reveals a complicated history of movements and magmatic activity during its evolution since tertiary time this n s fault acted as follows 1 the fault acted as a possible dextral strike slip fault as judged from riedel fractures in the oldest associated fault breccia 2 then the fault acted as a normal fault resulting in vertical displacement new brecciation of part of the oldest breccia hydrothermal activity and mineralization dominantly in n s mode i veins 3 the fault acted again as a dextral strike slip fault when the fault was injected laterally by dikes magma was injected mainly into the n s strike slip fault and secondarily into adjacent nne normal faults evidence of this stage are the en echelon geometry of the northerly dikes cooling cracks oblique to dike edges flow lineation elongated vesicles and soft striations on dike edges as well as bending and displacement of preexisting dikes 4 after the emplacement and partial erosion of the northerly dikes the fault acted dominantly as a strike slip fault with possible reactivation as an open fissure the gljufura fault strikes obliquely to the nne trending rifting structures active during crustal formation in this area its activity was initially related to a transform zone or an oblique rift connected to the srz then a shift in the plate boundary occurred around 7 6 ma when the rlrz became active during this shift the gljufura fault probably played a role similar to the n s strike slip faults of the presently active plate boundary within the south iceland seismic zone and the oblique rift of the reykjanes peninsula these faults may similar to the gljufura fault change mode and be injected by dikes if they are sufficiently close to a magma source

seismic image of the mount spurr magmatic system the three dimensional p wave velocity structure of mount spurr is determined to depths of 10 km by tomographic inversion of 3 754 first arriving p wave times from local earthquakes recorded by a permanent network of 11 seismographs results show a prominent low velocity zone extending from the surface to 3 4 km below sea level beneath the southeastern flank of crater peak spatially coincident with a geothermal system p wave velocities in this low velocity zone are approximately 20 slower than those in the shallow crystalline basement rocks beneath crater peak an approximately 3 km wide zone of relative low velocities correlates with a near vertical band of seismicity suggestive of a magmatic conduit no large low velocity zone indicative of a magma chamber occurs within the upper 10 km of the crust these observations are consistent with petrologic and geochemical studies suggesting that crater peak magmas originate in the lower crust or upper mantle and have a short residence time in the shallow crust earthquakes relocated using the three dimensional velocity structure correlate well with surface geology and other geophysical observations thus they provide additional constraints on the kinematics of the mount spurr magmatic system

numerical models of volcanotectonic earthquake triggering on non ideally oriented faults 1 calculation of inflation induced coulomb stress changes on faults of varying orientation indicates that local faults of almost any orientation are susceptible to slip in response to dike inflation additionally as dikes typically inflate in the direction of regional minimum compressive stress the sense of inflation induced strike slip faulting predicted by coulomb stress change models is opposite to that expected from regional tectonic forces observations of volcanotectonic vt earthquake fault plane solutions with pressure p axes oriented 90 to regional maximum compression confirm this prediction in detail imperfect agreement between coulomb stress change models and the ranges of fault plane orientations observed in vt fault plane solution studies and observed locations of vt seismicity indicates that the orientation and location of pre existing faults around the conduit ultimately determine the seismic response of host rock to an inflating dike

dike intrusion under shear stress effects on magnetic and vesicle fabrics in dikes from rift zones of tenerife canary islands a theoretical model predicting how anisotropy of magnetic susceptibility ams and vesicle fabrics are modified by shear stress resolved on the dike walls prior to the final cooling of magma is developed for vertical dikes the resulting fabrics are asymmetric with respect to initial fabrics assumed to be symmetric application of this model together with collected data on magma flow direction dike propagation direction and mechanism and shear sense allow us to interpret dike fabrics in terms of shear resolved on the dike walls during intrusion en echelon arrangement offsetting and dike curvature the interpretation of ams and vesicle fabrics of the margins of four dikes shows a reasonable agreement with the proposed theoretical models suggesting that asymmetric fabrics can be used to infer magma flow and may provide valuable information on the shear resolved on the dike walls during intrusion

ore related breccias in volcanoplutonic arcs an overview of breccias related to a variety of base metal precious metal and lithophile element deposits in volcanoplutonic arcs permits definition of six possible mechanisms for subsurface brecciation 1 release of magmatic hydrothermal fluids from high level hydrous magma chambers during second boiling and subsequent decompression generates a spectrum of breccia types in which fragments may suffer collapse and or ascent single or multiple intrusion related breccia pipes and pre and intermineral breccias in porphyry copper deposits provide widespread examples 2 magmatic heating and expansion of meteoric pore fluids may lead to brecciation commonly of late or postmineral age and including pebble dikes in porphyry type and related deposits magmatic heating of rocks saturated with seawater may cause submarine hydrothermal eruptions late in the emplacement histories of kuroko type massive sulfide deposits many of the resultant breccias underwent limited sedimentary transport overpressuring of heated fluids beneath semipermeable partly self sealed cap rocks may lead to brecciation and subaerial hydrothermal eruptions in shallow epithermal precious metal settings magmatic heating or tectonic disturbance may have triggered brecciation 3 interaction of cool ground waters with subsurface magma can generate phreatomagmatic explosions postmineral phreatomagmatic diatremes associated with porphyry systems and premineral diatremes with epithermal precious or base metal deposits were generated in this manner these attained the palcosurface to produce pyroclastic base surge and fall deposits that accumulated as tuff rings around maar craters 4 magmatic hydrothermal brecciation may lead to disruption of rocks through to the paleosurface decompression and fragmentation and eruption of the top part of an underlying magma chamber pre and postmineral magmatic diatremes of this sort are inferred to accompany a few porphyry type and other base and precious metal systems they were manifested at the palcosurface by accumulations of pyroclastic fall and flow deposits 5 breccias may result from mechanical disruption of wall rocks during subsurface movement of magma any intrusion related deposit may include such intrusion breccias 6 tectonic breccias resulting from fault displacement may accompany any type of ore deposit a continuum exists between many of these breccia types and it is difficult to identify unique criteria for their unambiguous distinction

structure and development of an anastomosing network of ductile shear zones a detailed structural analysis of an anastomosing shear zone network in metagabbros from the archean rainy lake zone canada revealed the existence of prevalent dextral and minor sinistral conjugate shear zones with the obtuse angle 130 facing the main shortening direction a typology of shear zone intersections confluences and other features shows that all shears formed during a single deformation event with dextral and sinistral shears being active together or in an alternating fashion in spite of the difficulty of establishing a complete kinematic sequence early and late shears can be distinguished the final angular pattern between dextral and sinistral shears is not an original feature dextral and sinistral shears formed at nearly right angles and the angles progressively opened towards the extension direction as a result of increasing strain the obtuse angles were achieved by the combined effects of continued shearing on newly forming shears and internal deformation of the lozenge shaped domains of lesser deformed rock bounded by the shears through time there was an increasing prevalence of dextral shears over sinistral ones the studied pattern and sequential analysis indicate that the bulk deformation was noncoaxial with a deformation regime evolving from a pure shear dominated dextral transpression to a higher vorticity dextral transpression

using magma flow indicators to infer flow dynamics in sills abstract fabrics from anisotropy of magnetic susceptibility ams analyses and shape preferred orientation spo of plagioclase are compared with field structures such as bridge structures intrusive steps and magma lobes formed during magma intrusion in jurassic sills this is to constrain magma flow directions in the sills of the karoo igneous province along the kwazulu natal north coast and to show how accurately certain structures predict a magma flow sense thus improving the understanding of the karoo sub volcanic dynamics the ams fabrics are derived from magnetite grains and are well constrained however the spo results are commonly steeply inclined poorly constrained and differ to the ams fabrics both techniques resulted in asymmetrical fabrics successful relationships were established between the ams fabric and the long axes of the magma flow indicators implying adequate magma flow prediction however where numerous sill segments merge either in the form of magma lobes or bridge structures the coalescence process creates a new fabric between the segments preserving late stage magma migration between the merged segments overprinting the initial magma flow direction

dikes emplaced into fractured basement timna igneous complex israel dikes are usually envisioned as arrays of parallel segments dilated perpendicular to the direction of the least compressive stress we describe here four dikes of highly irregular shape intruded in the fractured basement in the timna igneous complex southern israel the dikes include a doleritic dike 2 3 km long and 1 6 m to 32 m thick and three andesitic dikes up to 1 5 km long and 8 m thick the dikes each display significant variations of dip up to 60 strike up to 160 and thickness the thickness variations correlate better with the segment attitude than with the position along the dikes we show that the irregular shapes of the timna dikes are the result of emplacement into fractured host rock under different paleostress states and driving pressures three dilation styles that differ by the geometry of the initial cracks are analyzed an array of randomly oriented cracks style a a single linear crack style b and an array of interconnected nonparallel cracks style c the analysis of style a provides the stress state during dike emplacement including the orientations of the three principal stresses 1 2 3 the stress ratio 2 3 1 3 and the normalized driving pressure r pm 3 1 3 the stress ratio indicates the shape of the stress ellipsoid and it ranges from 0 for 2 3 prolate ellipsoid to 1 for 1 2 oblate ellipsoid the normalized driving pressure r indicates the relative magnitude of the internal magma pressure pm with respect to the tectonic stresses and it ranges from r 0 for pm 3 to r 1 for pm 1 we found that for three dikes in timna 0 25 indicating small differences between the two horizontal principal stresses and for one dike 0 9 indicating a large difference between the two horizontal principal stresses the normalized driving pressure r is about 0 08 in two horizontally propagating dikes and about 0 25 in two vertically propagating dikes style b predicts an elliptical thickness profile along the dike due to dilation of a linear crack this prediction agrees with the profile of one of the dikes the predicted thicknesses due to dilation of the interconnected array of cracks style c are in good agreement with the thickness variations of the doleritic dike and in fair agreement with two of the andesitic dikes deviations from the ideal geometry suggest separate stages of propagation and dilation in some of the dike segments

geometry formation and development of tectonic fractures on the reykjanes peninsula southwest iceland abstract a study was made of 903 fractures most of which are of postglacial age on the reykjanes peninsula some 120 fractures belonging to the vogar fissure swarm he completely within the same postglacial lava flow and were studied in great detail for the vogar fractures 75 are pure tension fractures and 25 are normal faults the average length of the vogar fractures is 611 m the average width is 0 6 m the average spacing measured perpendicular to the average strike n54e of fractures is 456 m the estimated average depth is of the order of several hundreds of meters and the average throw of the faults is 2 3 m all major fractures on the peninsula appear to have formed by coalescence of smaller fractures as a result of absolute tensile stress the fractures group into four fissure swarms each being a part of a volcanic system which it is proposed is fed by an elongated partially molten magma reservoir located at a depth of about 8 km beneath the peninsula it is suggested that a slight magmatic pressure increase in these reservoirs and resulting uplift and tensile stress at the surface is the main cause of postglacial fracture formation

microcracks in rocks a review abstract in the past decade the number of studies about microcracks in rocks has rapidly increased this review of recent work concentrates on microcracks in rock as separate entities emphasizing microcrack morphogenesis kinematics dynamics population statistics and observational techniques cracks are produced when the local stress exceeds the local strength the local stress may be augmented by twin lamellae interactions kink bands and deformation lamellae stress concentrations at grain boundary contacts and around intracrystalline cavities local strength may be reduced along cleavage planes along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids dislocations appear not to be a significant factor for crack nucleation below about 500 c in silicates spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities the amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity the application of hydrostatic pressure between 100 and 200 mpa effectively closes most cracks but the closure may not be uniform if crack wall asperities exist hydrostatic pressure appears to stabilize cracks and make crack growth more difficult the number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures the application of a deviatoric stress field on the boundaries of a rock mass results on a microscopic scale in a very complex stress system which greatly affects nucleation and propagation paths the relative amount of intragranular and intergranular cracking appears to depend upon mineralogy rock type and stress state the vast majority of stress induced microcracks in rocks appear to be extensional statistically they are predominantly oriented within 30 of the macroscopic maximum stress direction crack densities increase as macroscopic deviatoric stress increases above a threshold level crack size distributions may be either lognormal or exponential fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks not the growth of a single crack some crack configurations are more favorable for coalescence than others as deviatoric stress increases and rock failure is approached the microcrack population changes spatially from random to locally intense zones of cracking away from the fault the crack density dies off rapidly to the background level a few grains away under lesser deviatoric stresses slow subcritical microcrack growth can occur as a result of stress aided corrosion at the crack tip the rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip important details still remain to be worked out

seismicity and stress in the vicinity of mount spurr volcano south central alaska focal mechanism solutions and hypocenters for earthquakes near mount spurr volcano in south central alaska reveal spatial perturbations in the regional stress field and in the maximum depth of seismicity at the volcano practically all the shocks during the dormant decade 1981 1991 have depths shallower than 5 km and are characterized by nearly pure normal slip in contrast earthquakes located outboard of the volcano mostly concentrate in the depth range 3 18 km and exhibit predominantly strike slip oblique reverse slip or reverse slip the regional stress field is characterized by subhorizontal maximum principal stress directed n30 w concordant with the direction of convergence between the north american and pacific plates whereas the maximum principal stress is probably more nearly vertical beneath the volcano we suggest that the shoaling of seismicity beneath the volcano reflects localized elevation of the depth to the midcrustal transition from brittle failure to plastic flow we attribute this localized elevation to relict magmatic heat associated with past volcanic eruptions the available data are not sufficient to determine the cause of the suggested rotation of the maximum principal stress axis beneath the volcano possible mechanisms include an increase in the vertical stress from the weight of the mount spurr massif and a decrease in the maximum horizontal stress associated with doming of the shallow crust from magmatic processes