do transcurrent faults guide volcano growth the case of nw bicol volcanic arc luzon philippines this paper shows that although volcanoes can be aligned along transcurrent faults their growth can in reality be controlled by secondary structures that have a different orientation and kinematics assessing the relationships between local structures and volcano growth can be very useful for the evaluation of volcanic hazards and for economic purposes such as geothermal and hydrogeological exploration in the north western bicol volcanic arc luzon philippines the quaternary labo and caayunan volcanoes are aligned with the nw striking transcurrent philippine fault system they are exceptional examples of composite volcanoes mostly comprising lava domes where regional tectonic structures and hydraulic radial fracturing have alternated to control volcano growth field data and analogue modelling demonstrate that apical depressions affecting the domes reflect the geometry of magma feeding fractures these data together with alignments of domes and hot springs and other structural information suggest that magma rose at depth along the nw striking transcurrent faults but that in the uppermost crust magma rose along ne striking fractures parallel to the greatest principal stress

development of a subvolcanic structure by the interaction of liquid filled cracks abstract the interaction of liquid filled cracks may be an important process for magma accumulation and the development of subvolcanic structures the horizontal interaction was studied through the stress analyses around pressured cracks in various regional stress fields the vertical interaction was studied using gelatin experiments buoyaney driven cracks coalesce more easily than pressure cracks the theory of the three dimensional crack interaction was discussed there are two types of crack interaction the interaction between liquid filled cracks may cause coalescence of cracks the interaction between a liquid filled crack and a solid filled crack never causes coalescence the stress relaxation by ductile or brittle behavior or by the driving out of magma decreases the local stress field generated by the accumulation of magma filled cracks as the regional differential stress increases and the minimum compressive principal stress becomes tensile magma filled cracks become parallel to each other as the supply rate of magma filled cracks becomes relatively large or as the regional differential stress decreases and the minimum compressive stress increases the arrangement of magma filled cracks varies from an offset pattern to a radial one because the local compressive stress field cannot be decreased this process leads to the development of subparallel or radial dikes in two dimensions or a dike sheet complex in three dimensions a large magma chamber such as a sphere or a cylinder is not always necessary for the development of a basaltic volcano in case of the interaction between liquid filled cracks magma filled cracks coalesce easily into one large crack and diminish the local compressive stress field last the theory of the crack interaction is applied to developments of various volcanoes the fissure pattern of hekla volcano iceland is explained in terms of the variation of the stress relaxation the difference in fissure pattern between izu oshima and miyakejima volcanoes japan is interpreted by the variation of the differential stress the local stress balance including the stress relaxation is kept in the developments of etna volcano italy and kilauea volcano hawaii

evaluating sand and clay models do rheological differences matter abstract dry sand and wet clay are the most frequently used materials for physical modeling of brittle deformation we present a series of experiments that shows when the two materials can be used interchangeably document the differences in deformation patterns and discuss how best to evaluate and apply results of physical models extension and shortening produce similar large scale deformation patterns in dry sand and wet clay models indicating that the two materials can be used interchangeably for analysis of gross deformation geometries there are subtle deformation features that are significantly different 1 fault propagation and fault linkage 2 fault width spacing and displacement 3 extent of deformation zone and 4 amount of folding vs faulting these differences are primarily due to the lower cohesion of sand and its larger grain size if these features are of interest the best practice would be to repeat the experiments with more than one material to ensure that rheological differences are not biasing results dry sand and wet clay produce very different results in inversion models almost all faults are reactivated in wet clay and few if any are significantly reactivated in sand models fault reactivation is attributed to high fluid pressure along the fault zone in the wet clay a situation that may be analogous to many rocks sand inversion models may be best applied to areas where most faults experience little to no reactivation while clay models best fit areas where most pre existing normal faults are reactivated

bubble growth and rise in soft sediments the mechanics of uncemented soft sediments during bubble growth are not widely understood and no rheological model has found wide acceptance we offer definitive evidence on the mode of bubble formation in the form of x ray computed tomographic images and comparison with theory natural and injected bubbles in muddy cohesive sediments are shown to be highly eccentric oblate spheroids disks that grow either by fracturing the sediment or by reopening preexisting fractures in contrast bubbles in soft sandy sediment tend to be spherical suggesting that sand acts fluidly or plastically in response to growth stresses we also present bubble rise results from gelatin a mechanically similar but transparent medium that suggest that initial rise is also accomplished by fracture given that muddy sediments are elastic and yield by fracture it becomes much easier to explain physically related phenomena such as seafloor pockmark formation animal burrowing and gas buildup during methane hydrate melting

dynamics of retreating slabs 2 insights from three dimensional laboratory experiments 1 a laboratory analogue of a three layer linear viscous slab upper mantle lower mantle system is established in a silicone putty honey and crystallized honey tank experiment the same setup as in the numerical investigation part 1 is used we focus on the interaction of the slab with the induced passive mantle flow by widely varying the mantle volume flux boundary conditions in our numerical experiments the lateral volume flux was set to zero in interpreting the results relative to the real earth the base of the box is taken as the bottom of the mantle convection system while the lateral boundaries may be associated with the presence of other nearby slabs dynamic force equilibrium assessed on the basis of an analytical review of forces is described for four different phases 1 the subduction initiation instability 2 the accelerating dynamic free fall phase of the slab 3 the dynamic interaction with the 660 km discontinuity and 4 a final phase of steady state trench retreat phase 3 is an important feature not observed in the numerical experiments this highly dynamic phase of interrupted trench retreat can therefore be attributed to boundary conditions on mantle volume flux on the basis of integration constants of force equilibrium in phases 2 and 4 we identify two different classes of volume flux one in which the lateral boundary can be considered open and the other class where it is closed closed boundary condition cases are obtained if any of the lateral box boundaries are 600 km away from the slab assuming a one to one relation between trench retreat and back arc spreading enigmatic observations of episodic opening of back arc basins can be explained by our experimental observations

buoyancy driven fracture ascent experiments in layered gelatine laboratory experiments on air filled fracture propagation in solidified homogeneous and layered gelatine have been carried out providing an analogue model for magma filled dikes ascending in the crust the effects of layering on fracture velocity and shape have been analyzed in detail the free surface is found to accelerate approaching fractures layering accelerates or decelerates fractures approaching discontinuities of the elastic parameters depending on the value of the rigidity contrast the shape of fractures are strongly influenced as they pass from one layer to another the observed cross sectional shape when crossing a layer interface and the acceleration with decreasing rigidity can be explained with theoretical models our experiments also reproduce the arrest of fractures in proximity of joints and the formation of sills in the layer below the interface these findings could help in the interpretation of accelerated seismicity and deformation rates observed in volcanic areas

experiments on rift zone evolution in unstable volcanic edifices large ocean island volcanoes frequently develop productive rift zones located close to unstable flanks and sites of older major sector collapses flank deformation is often caused by slip along a decollement within or underneath the volcanic edifice we studied how such a stressed volcanic flank may bias the rift zone development the influence of basal lubrication and lateral flank creep on rift development and rift migration is still poorly constrained by field evidence here our analog experiments provide new insights we injected colored water into gelatin cones and found systematic orientations of hydro fractures dikes propagating through the cones at the base of the cone diverse friction conditions were simulated by variation of the basal creep conditions we modeled radial dike swarms collinear rift zones and three armed rift systems it is illustrated that a single outward creeping flank is sufficient to modify the entire rift architecture of a volcano the experiments highlight the general unsteadiness of dike swarms and that the distribution and alteration of weak substratum may become a major player in shaping a volcano s architecture

rheological properties of paraffin as an analogue material for viscous crustal deformation the rheological stratification induced by temperature variations within the crust requires analogue materials with temperature sensitive viscosity to properly simulate the change of mechanical properties with depth rheological calibration experiments on commercial paraffin wax with melting point of 53 1 c show that this material is a nonlinear fluid 1 6 n 2 1 for homologous temperatures t tm 0 70 and an approximately newtonian fluid 1 0 n 1 3 for t tm 0 70 the decrease in the stress exponent n is probably associated with the phase transition of paraffin effective viscosity values are strongly temperature dependent varying by nine orders of magnitude from 1010 to 10 pa s in the temperature range between 30 and 52 c the rheological properties of paraffin make this material useful for modelling crustal rheology and tectonic processes when linear dimensions and time scale down by a factor of 106 and 1011 respectively

the existence of a thin low viscosity layer beneath the lithosphere the horizontal temperature gradient at the base of the lithosphere at an oceanic fracture zone where plate of different ages is juxtaposed is expected to drive a local circulation the characteristics of which can be constrained by the amplitude wavelength and age dependence of the geoid two dimensional numerical models of convection in a fluid layer overlain by a solid conducting lid have been used to generate theoretical geoid profiles at right angles to the fracture zone only a thin low viscosity layer provides a reasonable fit to the data the best model so far obtained has a fluid layer 150 km thick with viscosity 1 5 1019 pa s under a 75 km lid such a layer which is incapable of transmitting strong horizontal shear stresses could provide the decoupling mechanism between plate and deep mantle flow required to balance the forces on the plates

viscoelastic behaviour of basaltic lavas the rheological properties of basaltic lavas from etna hawai i and vesuvius have been investigated at temperatures between 500 and 1150 c using a small strain oscillatory shear the viscoelastic response of the lavas to small forced sinusoidal torques 109 pa s could be measured at strain rates less than 10 2 10 1 s 1 at 800 c temporal variations in complex shear modulus and internal friction suggest that over durations of up to 120 h structural adjustments were occurring within some of the samples this time varying behaviour of lava samples may be attributed to the slow closing healing of microcracks and small pore spaces resulting in the apparent stiffening of lava samples under annealing thus those parts of lava flows that undergo slow cooling will have more elastic properties regions that cool faster possess smaller shear moduli and higher internal friction due to thermal microcracking and less cohesion between crystals and the bulk glassy matrix

tectonic stress controls on ascent and emplacement of magmas abstract the tectonic stresses can significantly affect the propagation of a magma filled crack it has been pointed out that the rheological boundaries control the emplacement of magmas through the effect of stress however it has not been clarified how the role of rheological boundaries depends on the regional tectonic and thermal states we have evaluated the role of rheological boundaries under various tectonic and thermal conditions and found that the level of magma emplacement may jump according to the changes in the tectonic force or the surface heat flow the stress profiles were estimated by a simple model of lithospheric deformation we employed a three layer model of the lithosphere the upper crust the lower crust and the upper mantle have different rheological properties a constant horizontal force is applied to the lithosphere and the horizontal strain is assumed to be independent of depth when realistic tectonic forces 10 11 n m are applied the rheological boundaries mainly control the emplacement of magma the emplacement is expected at the moho the upper lower crust boundary and the brittle ductile boundary for lower tectonic forces 11 n m the tectonic stress no longer plays an important role in the emplacement of magmas when the tectonic stress controls the emplacement the roles of rheological boundaries strongly depend on the surface heat flow when the surface heat flow is relatively high 80 mw m 2 the stress in the mantle is quite low and the moho cannot trap ascending magmas for relatively low heat flow 2 on the other hand the moho acts as a magma trap and the upper lower crust boundary acts as a magma trap only when the magma supply rate is sufficiently high our results suggest that the emplacement depth can change responding to the change in the tectonic force and or that in the surface heat flow this may provide us a key to understand the relation between the evolution of a volcanic region and its tectonic and or thermal history

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

fission track thermochronology of the ruhla crystalline complex new constraints on the post variscan thermal evolution of the nw saxo bohemian massif abstract to help clarify the relative and absolute timing of post variscan cooling and exhumation processes in the ruhla crystalline complex rcc situated in the nw part of the saxo bohemian massif eight zircon and eleven apatite fission track ages and nine apatite track length measurements have been obtained zircon fission track ages support an existing model of post variscan exhumation of the rcc as three discrete blocks the eastern and western blocks show cooling from peak metamorphic conditions to below 260 30 c by ca 300 ma and exposure by 296 ma the central block shows a later cooling to below 260 30 c by ca 270 ma and exposure at the surface by ca 256 ma apatite fission track data reveal late cretaceous accelerated cooling through the apatite partial annealing zone 120 to 60 c this can be correlated with tectonic inversion at this time the apatite data also indicate that the rocks of the rcc were at temperatures above 110 10 c before ca 85 ma this requires either an anomalously high palaeo geothermal gradient of ca 60 c km 1 during the late cretaceous or that the rcc region was originally covered by up to 1400 m of now eroded jurassic and cretaceous sedimentary rocks we favour a combination of both factors with accelerated erosion of missing overburden during a period of increased heat flow initiated at ca 85 ma

a phenomenological model for precursor volcanic eruptions intense explosions of relatively short duration frequently precede large explosive and effusive volcanic eruptions by as much as weeks to months in the case of very viscous magmas1 2 3 4 5 6 in some cases such pre eruption activity has served as a sufficient warning to those living in the vicinity to evacuate and avoid calamity1 precursor events seem to be related to the formation of a magma pathway to the surface but their precise interpretation is a long standing puzzle it has been inferred from theoretical studies that exsolution of volatiles might create an almost separate gas pocket at the tip of a propagating dyke7 8 9 here we explain the role that such a process may have using a laboratory study of the transient propagation of a liquid filled crack with a gas pocket at its tip that grows with time we show that once the gas pocket acquires sufficient buoyancy to overcome the fracture resistance of the host solid the dynamics of the gas pocket rather than those of the liquid determine the velocity of the crack tip furthermore we find that the gas can ultimately separate from the liquid we propose that fast moving gas rich pockets reaching the surface ahead of the main liquid filled fissure could be the origin of many precursor eruptions

viscoelastic behavior of partially molten granites effects of partial melting on complex shear modulus g and internal friction q 1 of granites have been investigated using a torsion deformation apparatus in the frequency range of 2 mhz 20 hz at 650 1250oc granite samples from kirchberg w erzgebirge germany og1 and fine grained quartz feldspar porphyry a

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

mineral dissolution and the evolution of k0 adequate knowledge of the in situ state of stress can be essential to the analysis of geotechnical systems however the measurement and prediction of k0 remain difficult in particular limited attention has been given to the evolution of k0 during the formation history of the soil and diagenetic processes such as mineral dissolution experimental and numerical results show that grain mass loss due to mineral dissolution produces a pronounced horizontal stress drop under zero lateral strain conditions the state of stress may reach the active shear failure ka condition and internal shear planes may develop while horizontal stress recovery often follows upon further dissolution marked differences in fabric are observed between the pre and postdissolution soil structures

a laboratory investigation of effects of trench migration on the descent of subducted slabs a laboratory investigation of viscous slabs subducted from a migrating trench reveals a range of possible behaviours the slab dip in a uniform mantle is found to be steady or oscillatory depending on the rates of descent and trench migration in addition density and viscosity interfaces are used to model the increased resistance to sinking of stiff slabs through the seismic discontinuity at a depth of about 660 km in earth s mantle if the slab is denser than the lower layer and its dip in the upper layer is steady it can continue to descend through the lower layer in a tabular form and in either a steady or oscillatory manner or it can be laid horizontally on the interface for a distance before descending in a chain of diapirs resulting from gravitational instability at the interface if the slab dip in the upper layer is unstable the slab sinks into the lower layer in a chain of large diapirs at a spacing determined by the frequency of oscillations set by instability of the slab within the upper layer the style of penetration depends on the trench migration speed and the ratio of sinking velocities in the two layers estimates for mantle slabs indicate that they may range across the major regime transitions the experimental system provides a gross simplification of mantle conditions but the results are a testimony to the possibility of a range of complex behaviour of subducted lithosphere they also indicate that the relationship between the structural features in the lower mantle revealed by seismic imaging and present day tectonic processes at the surface may not be obvious tomographic images of western pacific subduction zones and data for the migration rates of associated trenches suggest a dependence of slab behaviour on migration rate similar to that seen in the experiments

influence of syntectonic sedimentation on thrust wedges in analogue models scaled analogue sandbox models have been used to simulate the growth of coulomb thrust wedges that include syntectonic sedimentation during thrusting increases in the syncontractional sedimentation rate dramatically change the shape of the coulomb thrust wedge decreasing the number of thrust slices the internal shortening and the critical taper of the wedge thrust trajectories change from ramp flat to sigmoidal with increased syncontractional sedimentation the analogue models show many similarities to thrust fault geometries in accretionary prisms and foreland fold and thrust belts in which thrust slices prograded into basins with active sedimentation

rheological and tectonic modeling of salt provinces abstract technological improvements and a more rigorous research approach have led the petroleum industry to renew and expand its interest in physical modeling of faulting associated with halokinesis salt tectonics and the formation of structural hydrocarbon traps can be reproduced in models using viscous polymers for the substratum and frictional plastic granular materials for the overburden the fault patterns simulated in these dynamically scaled models are remarkably similar to those in depth converted seismic sections scale models are therefore useful for generating new concepts and for reconstructing the structural evolution of hydrocarbon traps although even unsealed models may help generating ideas the full transfer of quantitative data requires that models be dynamically scaled to their natural prototypes previously published scaling theory for salt tectonics has erroneously assumed fluid behaviour of the overburden this paper provides a sound scaling approach for geological deformations involving brittle overburdens on ductile substrata rheological data from overburden rocks and salt are compiled and compared to model rheologies using dimensions of laboratory analogs and other intrinsic material properties scaling approaches are compared and rules are formulated for achieving dynamic similarity in models of salt tectonics including those with synkinematic sedimentation

dike propagation driven by volcano collapse a general model tested at stromboli italy 1 analogue experiments investigate how flank collapse affects dike propagation within volcanoes water magma analogue is injected within a gelatin cone volcano analogue with a lateral collapse the injections form dikes that away from the collapse become radial the dikes propagating nearby the collapse focus towards the collapse sides becoming subparallel to them because of the stress reorientation due to the unbuttressing only dikes formed along the collapse axis passing through the mean points of the collapse map view propagate radially within the collapse this general model is applied at stromboli the stromboli dikes cluster along and parallel to the collapse sides similar to the experiments nevertheless the current location of the conduit along the collapse axis leads to dike propagation within the collapse infill rather than at its sides this may have occurred during the 2002 03 eruption when diking in the collapse triggered a landslide and tsunami within the collapse

effects of volcano loading on dike propagation in an elastic half space we use laboratory experiments and numerical models to examine the effects of volcano loading on the propagation of buoyant dikes in a two dimensional elastic half space in laboratory experiments we simulate the propagation of buoyant dikes in an isotropic regional stress field by injecting air into tanks of solidified gelatin a weight resting on the surface of the gelatin represents a volcanic load a numerical model is used to simulate these experiments both experiments and numerical simulations show that as a dike ascends it begins to curve toward the load in response to the local stress field imposed by the load the lateral distance over which dikes curve to the load increases with the ratio of average pressure at the base of the load to the dike driving pressure for realistic volcano and dike dimensions this pressure ratio is going to be large suggesting that dikes can converge to a volcano over lateral distances several times the load width numerical calculations involving an anisotropic regional stress field however predict that the lateral extent of dike attraction shrinks as the regional horizontal compressive stress decreases relative to the vertical compressive stress dike focusing will be substantial if the regional differential stresses are less than the average pressure at the base of the load if this is the case then our models predict a positive feedback between the size of volcanoes and the area of dike attraction this feedback may promote the development of large discrete volcanoes and also predicts a positive correlation between the spacing and sizes of adjacent volcanoes to test this prediction we examine nearest neighbor pairs of the 21 largest volcanoes in the cascade range the 14 pairs examined show a large range in volcano spacing 6 115 km and a statistically significant correlation between spacing and average volcano height this result is consistent with our model results and suggests that the local compressive stress induced by these volcanoes may be an important factor in controlling magma transport in the lithosphere

location and orientation of eruptive fissures and feederdykes at mount etna influence of gravitational and regional tectonic stress regimes abstract lava extrusion from fissures on the flanks of the etna volcano is a common feature of recent activity as evidenced by the eruptions of 1971 1981 1983 and 1985 and was of similar importance during prehistoric times we propose here a model within which the location and orientations of the large majority of historic eruptive fissures on etna s flanks together with their feeder dykes are explained within the framework of a gravitational stress regime stress orientations within this regime are controlled largely by the gross morphology of the volcano but are significantly modified by local topographic anomalies we propose that the gravitational stress regime largely supercedes an earlier developed regional tectonic stress regime which still operates in the sub volcanic basement and may remain the dominant control of eruptive fissure and feeder dyke disposition at lower altitudes on the flanks the proposed model is supported by 1 detailed examination of fissure locations and orientations and the relationship between fissure azimuths and topography 2 precise horizontal distance measurement and vertical ground deformation studies of a recent eruption and 3 qualitative physical modelling experiments in which coloured water is injected into solidified gelatine solutions representing variously shaped volcanic constructs a cautious approach is advocated when extrapolating trends of spatially limited fissure segments for the purpose of locating potential sites of magma ascent and implications of the model for hazard analysis are outlined

analogue modelling of multiphase rift systems abstract analogue models of multiphase rift systems show that structures produced by the first phase of extension exertsignificant control on the geometries of the later phase extensional faults two experimental series series 1 and series 2 used different modelling media subjected to two phases of extension series 1 models used a simple homogeneous sandpack whereas series 2 models used a sand polymer multilayer in each series kinematic scenarios included orthogonal extension followed by oblique extension or vice versa extension directions for oblique rift kinematics varied from 75 to 30 to the axis of rifting in all models where oblique rifting superseded earlier orthogonal rift structures the second deformation phase produced segmented fault systems and reorientation of fault tips into high angles with respect to the extension vector higher angles of obliquity resulted in increased fault segmentation orthogonal extension superposed on earlier oblique rifting leads to coalescence of segmented faults although remnants of the inherited initial rift structure remained even for low angles of obliquity the sand polymer multilayer models produced more complex fault patterns than those in the simple homogeneous sandpack showing the possible complexities that can be generated during polyphase rifting persistent first phase rift faults caused boundary fault segmentation and salients and embayments along the boundary faults during the second deformation phase the natural occurrence of similar structural styles particularly in map view may indicate that previously unrecognised multiphase rifting has occurred in these natural examples

shear test results for cohesion and friction coefficients for different granular materials scaling implications for their usage in analogue modelling abstract laboratory tests have been carried out on dry granular materials such as quartz sand glass microspheres and sugar with different grain size rounding and sphericity the measurements have been made with a simple shear test machine for different values of normal stress 50 900 pa shear stress has been plotted against normal stress in order to determine the cohesion and coefficient of internal friction for the investigated materials resulting values of cohesion and coefficient of internal friction are mainly dependent on rounding and sphericity while grain size has a less significant influence further the behaviour of the materials for very small normal stresses 0 400 pa is more complex than previously assumed the fracture envelopes for all materials investigated are convex outward for this small range and converge towards a straight failure envelope with increasing normal stress finally in extensional faulting experiments there is no significant change in fault dip with increasing depth therefore the non linear behaviour for small normal stresses is best described as a dependence of the cohesion on the normal stress and not as a dependence of the coefficient of internal friction on the normal stress values for cohesion increase from 0 pa 15 pa at zero normal stress to 137 247 pa 15 pa for normal stresses greater than 250 400 pa the results show that well rounded spherical material is better suited to model brittle behaviour of rocks in crustal and lithospheric scale analogue models than less well rounded material since it has a smaller cohesion and a coefficient of internal friction which is closer to values of natural rocks

measurements of friction coefficients and cohesion for faulting and fault reactivation in laboratory models using sand and sand mixtures abstract simple laboratory tests have been made on dry quartz sand and on mixtures of sand and clay and sand and cement tests were made on both low and high density specimens of each mixture as determined by physical handling techniques the tests included measurements of density estimates of the coefficient of internal friction based on fault orientation and estimates of the friction coefficients and cohesion values for fault initiation and reactivation in a shear frame apparatus which was also used to test artificial fault zones made from pure clay glass microspheres and wheat flour the results suggest that density is determined more by handling technique than by composition and that friction coefficients and perhaps cohesion both increase with increasing density the shear testing revealed a consistent drop in cohesion for fault reactivation in all densely packed mixtures tested although small this drop may be significant for some types of laboratory models

folds with axes parallel to the extension direction an experimental study abstract experimental studies of single and multilayer folding have generally considered shortening of layers oriented perpendicular to the maximum extension direction x i e layers parallel to yz or in a more limited number of cases oblique layers still containing the intermediate y axis few experimental studies have considered the case where the extension direction x lies within the layer itself although in nature folds with axes parallel to x are quite commonly seen these folds have often been ascribed to passive rotation of fold axes during continued shear but it has been shown both theoretically and experimentally that active buckle folds can also develop with axes parallel to x single and multilayer analogue model experiments were performed on planar layers oriented initially perpendicular to the intermediate y axis and with the extension direction x lying within the layer itself all experiments were conducted in plane strain either in pure shear or simple shear paraffin waxes of different melting ranges were used as analogues for rocks with a power law rheology stress exponent around 2 3 with a viscosity ratio of ca 30 1 no measurable fold amplification was discernible for shortening of 36 or shear strain of 3 6 neither domed initial perturbations with circular sections parallel to the layer nor cylindrical perturbations elongate parallel to the initial stretching direction were significantly amplified only at much higher viscosity ratios ca 600 1 did active buckle folding develop this folding at high viscosity ratio was associated with flow of the matrix in the x direction around the layer developing a strong linear fabric parallel to x in the matrix immediately adjacent to the layer the development of this flow discontinuity between matrix and layer may be characteristic of active buckling of layers parallel to xz with fold axes parallel to x

average lithospheric stresses induced by thickness changes a linear approximation abstract density moments and horizontal deviatoric stresses in the lithosphere related to lateral thickness variations are calculated analytically under the assumption of linear geotherms the moments are generated by displacements of major structural boundaries and changes in geotherms during thickening results given as total moments and related stresses for various lithospheric configurations and thickening factors confirm that density induced stresses play a significant role in geodynamics there is an upper limit to crustal shortening determined by the average lithospheric strength estimated to be in the 50 100 mpa range total or partial detachment of the lower part of the thickened lithosphere generates tensile stresses which can be a factor in the tectonic inversion from compression to extension observed in several orogenic belts

gelatins as rock analogs a systematic study of their rheological and physical properties abstract in a laboratory model the behavior of the selected analog material must be properly scaled to nature here we systematically investigate both rheological and physical properties of a wide range of gelatins as functions of temperature composition concentration ageing and applied strain rate results show that gelatins behavior changes gradually from purely elastic to visco elasto brittle to purely viscous nonlinear rheology going from the gel to the sol state the rheological variability of gelatins appears promising for the potential use of these analog materials to simulate crustal and lithospheric rheological behavior in particular we found that pig skin 2 5 wt at 10 c has the required rheological properties for a suitable experimental set up to model crustal deformation

power law viscous materials for analogue experiments new data on the rheology of highly filled silicone polymers abstract the selection of appropriate analogue materials is a central consideration in the design of realistic physical models we investigate the rheology of highly filled silicone polymers in order to find materials with a power law strain rate softening rheology suitable for modelling rock deformation by dislocation creep and report the rheological properties of the materials as functions of the filler content the mixtures exhibit strain rate softening behaviour but with increasing amounts of filler become strain dependent for the strain independent viscous materials flow laws are presented while for strain dependent materials the relative importance of strain and strain rate softening hardening is reported if the stress or strain rate is above a threshold value some highly filled silicone polymers may be considered linear visco elastic strain independent and power law strain rate softening the power law exponent can be raised from 1 to 3 by using mixtures of high viscosity silicone and plasticine however the need for high shear strain rates to obtain the power law rheology imposes some restrictions on the usage of such materials for geodynamic modelling two simple shear experiments are presented that use newtonian and power law strain rate softening materials the results demonstrate how materials with power law rheology result in better strain localization in analogue experiments

an experimental investigation of sill formation and propagation in layered elastic media a series of experiments are described where dyed water a magma analogue was intruded into solid gelatine a crustal analogue to investigate the formation of sills we considered a layered gelatine system with contrasting adjacent layers by varying the density and rigidity of the gelatine we found that experimental sills form when the upper layer is more rigid than the lower layer with intrusion occurring in a plane directly below the interface experimental dykes were observed to propagate to the surface when the young s modulus ratio of upper to lower gelatine layers was less than one experimental dyke arrest occurred when the upper layer was more rigid and the interface was strong two varieties of experimental sill formed when the upper layer was more rigid than the lower layer and the interface was sufficiently weak the form of the intrusion depends on the balance of driving pressures and the young s modulus ratio of contrasting adjacent layers when the rigidity ratio is high and there is a large driving pressure the experimental feeder dyke completely converts to propagate as a sill however when the rigidity ratio and driving pressure are both close to one a dyke sill hybrid forms under these conditions the experimental sill formation is accompanied by contemporaneous dyke intrusion into the overlying more rigid layer during sill propagation deformation structures such as faults and en echelon fractures are formed into the lower layer experimental sill propagation dynamics are controlled by viscous dissipation along the length of the sill causing acceleration with increasing length our study suggests that rigidity contrasts may play a major role in the location of sills and development of igneous complexes in ancient cratonic areas the moho is a suitable site for the preferential formation of sills with higher rigidity continental crust overlying weaker mantle mantle plumes impacting ancient continents provide a situation in which large sills can form to fractionate prior to eruption of flood basalts the boundary between the upper and lower crust conrad discontinuity may provide a preferential focus for the emplacement of sheets of silicic magma at continental arcs where the lower crust is weakened by prolonged heating and possible hydration

use of vegetable oil and silica powder for scale modelling of magmatic intrusion in a deforming brittle crust in the upper crust intrusive bodies adopt different configurations depending on the tectonic setting this paper describes a new technique for analogue modelling of such intrusions specifically of low viscosity magma into a deforming brittle crust proper dynamic scaling is an important consideration if the crust has a coulomb failure envelope the model material should have a cohesion c of 40 pa to 7500 pa if the intruding magma is highly mobile the model fluid should have a viscosity of 4 10 9 pa s to 75 pa s for the model crust we have used crystalline silica powder si crystal siliceous microspheres si sphere of grain size 30 m and a mixture si mix of both materials the mechanical properties of these powders have been obtained by shear and tension shear tests si crystal powder is cohesive c 300 pa angle of internal friction 45 and represents competent rock si sphere powder is much less cohesive c 10 pa 25 and represents incompetent rock si mix has intermediate properties the model magma is a vegetable oil which solidifies at room temperature its viscosity when molten is 2 10 2 pa s at 50 c using these materials we have done some preliminary experiments to investigate the intrusion of low viscosity magma into sedimentary basins in non deformed settings intrusions were saucer shaped cone sheets in horizontal extension they were steep dykes finally in horizontal shortening intrusions were basal sills that branched into thrust faults

an experimental study on the formation of composite intrusions from zoned magma chambers abstract we present a model which accounts for the common but paradoxical arrangement of composite intrusions i e silicic core and mafic margins on the basis of analogue experiments using gelatin and aqueous solutions the present model involves simultaneous flow out of the upper and lower magmas from a longitudinal crack along the chamber wall experimental results suggest that the mafic magma from the lower layer leaks from the side wall of the chamber and travels faster than the silicic magma because of its lower viscosity so that the mafic magma reaches the tip of the crack first once the mafic magma reaches the crack tip then the rate of dyke propagation becomes determined by the viscosity of the less viscous mafic magma and so it can advance rapidly the viscous silicic magma can flow efficiently into the center of the dyke being lubricated by the mafic magma margins this model accounts for the common arrangement of composite intrusions and gives an efficient mechanism of flow of viscous silicic magmas