time scales of hybridisation of magmatic enclaves in regular and chaotic flow fields petrologic and volcanologic implications this paper describes numerical models of advection diffusion between enclaves and host magmas applied with the aim of estimating time scales during which enclaves can be homogenised in particular advection was simulated using a numerical system consisting of regular and chaotic regions results indicate that the homogenisation time of enclaves in chaotic regions is several orders of magnitude faster than in regular regions for instance an enclave with a diameter of 100 cm may be homogenised in the chaotic region in 380 years assuming an advection velocity of 10 cm year whereas in the regular region it would require 6 5 105 years for complete homogenisation this implies that in the same magmatic system large differences in the degree of homogenisation may co exist generating magmatic masses with large spatial and temporal inhomogeneities

the diversity of flow structures in felsic dykes abstract numerous structures and textures which can be related to magma flow were observed in felsic dykes intruding late mesozoic granitoid plutons in the jiaodong peninsula in eastern shandong province eastern china these flow structures may be classified into two categories interior and peripheral the former group includes magmatic bands various types of folds e g injection sheath similar and disharmonic folds rotation of phenocrysts magmatic foliation or lineation and crenulation whereas the latter includes hot tool marks and quarrying structures magmatic banding resulted from shearing of mingled magma during magma flow in the dykes the magma seemed to flow rapidly probably triggering turbulence in some thick dykes interaction at the contact between the hot moving magma and the cold stationary wallrock sometimes produced the peripheral structures a few measurements of hot tool marks and of magmatic lineation reveal a roughly horizontal flow of magma within these dykes for the dominant ne sw striking dyke set in the laoshan granitoid pluton the felsic magma probably ascended on or to the sw of the pluton to feed the dyke swarm and then flowed laterally to drive the horizontal propagation of dykes

kinematics of lava flows based on fold analysis a kinematic analysis of the mingling folding structures occurring in a lava flow from porri volcano southern tyrrhenian sea has been performed in order to estimate the flow kinematic parameters folds have resulted from the deformation of enclaves and banding structures and consist of a single layer of a crystal rich andesitic magma hosted in a less competent basaltic magma results of the analysis indicate that the flow kinematic vorticity increases with the distance from the vent whereas the ratio between pure shear and simple shear strain decreases following a power law quantitative estimates of the kinematic parameters provide an opportunity to examine the deformation behaviour of the lava flow near the vent the lava is a non ideal shear flow away from the vent the lava is an ideal shear flow gravity driven mechanisms control the relative contribution of pure and simple shear to the bulk deformation

the dynamics of magma mixing in a rising magma batch the conditions under which two magmas can become mixed within a rising magma batch are investigated by scaling analyses and fluid dynamical experiments the results of scaling analyses show that the fluid behaviours in a squeezed conduit are determined mainly by the dimensionless number i mu 1 u g delta rho r 2 where 1 is the viscosity of the fluid u is the velocity g is the acceleration due to gravity is the density difference between the two fluids and r is the radius of the tube the parameter i represents a balance between the viscous effects in the uppermost magma which prevent it from being moved off the conduit walls and the buoyancy forces which tend to keep the interface horizontal the experiments are carried out using fluid pairs of various density and viscosity contrasts in a squeezed vinyl tube they show that overturning of the initial density stratification and mixing occur when i order 10 1 the two fluids remain stratified when i order 10 3 transitional states are observed when 10 3 i 10 1 these results are nearly independent of reynolds number re 1 rho 1 ur mu 1 and viscosity ratio mu 1 mu 2 in the range of 10 2 mu 1 mu 2 10 3 and re 1 300 applying these results to magmas shows that silicic to intermediate magmas overlying mafic magma will be prone to mixing in a rising magma batch this mechanism can explain some occurrences of small volume mixed lava flows

subsolidus physical and chemical mixing of granite and gabbro during mylonitization south victoria land antarctica abstract at dromedary massif southern victoria land antarctica a suite of coarse grained granite dykes cross cuts a gabbro pluton which has been partially metamorphosed at amphibolite facies during regional deformation strain has been inhomogeneously distributed through the gabbro pluton and has been concentrated in granite dykes in zones of relatively high strain the granite dykes have developed a mylonitic fabric a high strain gradient between granitic mylonite and metagabbroic host rock has induced isochemical mylonitization of the margin of the host this grain size reduction allowed chemical diffusion between granitic and metagabbroic mylonites resulting in a marginal zone of biotite rich mylonite with intermediate composition biotite rich mylonite decoupled from metagabbroic mylonite and flowed with granitic mylonite continued folding and transposition of granitic mylonite and biotite rich mylonite has produced compositionally banded mylonite zones through thorough and irreversible mixing of the two lithologies

finite deformation in and around a fluid sphere moving through a viscous medium implications for diapiric ascent abstract different types of geodynamically active regions like mountain belts hot spots or areas of salt tectonics are characterized by diapirism one key for the reconstruction of the dynamic history of such structures is the progressive strain which can sometimes be determined from field observations successful interpretation of such observations requires a quantitative model of the finite strain in and around a rising diapir a constant viscosity fluid sphere of radius r rising through another isoviscous fluid is assumed to approximate the buoyant motion of a diapir known analytical solutions for the velocity fields are used to numerically evaluate the finite deformation in and around the sphere regions of very high strains are found in a tube with a radius 1 2 r behind the sphere and in a shell of thickness of 0 1r around the sphere the following three dimensional strain regimes can be identified in the half space above the sphere down to its equator finite strains are oblate behind the sphere they progressively change to plane strain in the diapiric source region they are prolate viscous drag at the sphere s surface leads to an internal circulation with one overturn after 10r of rise if the sphere has a relatively low viscosity finite strains within the fluid sphere show a continuous increase with superimposed cyclic straining and unstraining after several body radii of rise the strains become highly inhomogeneous inside the sphere except along the vertical axis and just inside the sphere s surface where strong prolate and oblate strains are observed respectively finite strain determinations in a falling ball experiment cruden 1988 are compared with the theoretical results at horizontal distances of a few body radii from the fall axis the effect of confining container walls is clearly seen in the experimental strain data the results are compared briefly with available strain data from the field which seem to be significantly lower than predicted proposed explanations include a short memory of the rock fabric and a lack of recognition of the strain concentration which would be expected for a temperature dependent rheology it might also be possible that few natural diapirs rise more than a few radii in the solid state

kinematic significance of mingling rolling structures in lava flows a case study from porri volcano salina southern tyrrhenian sea a basaltic andesite lava flow from porri volcano salina southern tyrrhenian sea is composed of two different magmas magma a 51 vol of crystals has a dacitic glass composition and magma b 18 vol of crystals a basaltic glass composition magma b is hosted in a and consists of sub spherical enclaves and boudin like banding and rolling structures rs four types of rs have been recognized type type complex types and transitional structures between sub spherical enclaves and rolling structures an analysis of the rs has been performed in order to reconstruct the flow kinematics and the mechanism of flow emplacement rolling structures have been selected in three sites located at different distances from the vent in all sites most rs show the same sense of shear kinematic analysis of rs allows the degree of flow non coaxiality to be determined the non coaxiality is expressed by the kinematic vorticity number wk a measure of the ratio sr between pure shear strain rate and simple shear strain rate the values of wk calculated from the measured shapes of microscopic rs increase with increasing distance from the vent from approximately 0 5 to 0 9 results of the structural analysis reveal that the rs formed during the early intermediate stage of flow emplacement they represent originally sub spherical enclaves deformed at low shear strain at higher strain rs deformed to give boudin like and stretched banding structures results of the kinematic analysis suggest that high viscosity lava flows are heterogeneous non ideal shear flows in which the degree of non coaxiality increases with the distance from the vent in the vent area deformation is intermediate between simple shear and pure shear farther from the vent deformation approaches ideal simple shear lateral extension processes occur only in the near vent zone where they develop in response to the lateral push of magma extruded from the vent lateral shortening processes develop in the distal zone and record the gravity driven movement of the lava the lava flow advanced by two main mechanisms lateral translation and rolling motion lateral translation equals rolling near the vent while rolling motion prevailed in the distal zones

enclaves provide new insights on the dynamics of magma mingling a case study from salina island southern tyrrhenian sea italy abstract three lava flows hereafter flows a b and c from salina island italy consist of basaltic andesitic enclaves dispersed in a dacitic matrix enclaves represent 8 12 vol of the erupted magma the number of enclaves and the surface covered by the enclaves at each outcrop do not vary significantly with the distance from the vent in the flows a and b these feature reflect the dynamics of magma mingling within the reservoir and not the kinematics of the lava flow in the flow c these parameters vary irregularly the statistical entropy s t of the enclaves which is a measure of their spatial distribution dispersion is estimated in outcrops located at different distance from the vent the kolmogorov sinai entropy rate k which describes the variations of s t with time is also determined in the lava flow a s t increases linearly with time t for 0 0 5 s t attains its maximum value and maintains constant with increasing t in the lava flow b s t linearly increases with t and k is 0 01 in the lava flow c there is not correlation between s t and t the comparison between the results from the analysis of the porri enclaves and those from numerical experiments on the variation of s t in chaotic advective mixing systems and from previous experimental models on magma mixing allow us to draw some conclusions on dynamics of the basaltic andesite dacite mingling in the magma chamber fully chaotic magma mingling systems show three evolution stages an initial stage which is unknown because of the disruption of the initial configuration of the interacting magmas a second stage characterized by a linear increase of the statistical entropy with time and a third stage in which the uniformity of the system is reached and the entropy does not vary with increasing time a system in which the uniformity is never attained is characterized by irregular variations of s t with time in the flows a and b the relations between s t and t are consistent with those of a fully chaotic dynamics possibly associated to convection the basaltic andesite was uniformly distributed in the dacitic host due to the occurrence of convective movements driven by the injection of the basaltic andesite within the dacitic chamber the mingling system recorded by the lava flow a evolved with a higher rate with respect to that of the flow b this suggests that chaotic advection stirring and folding is more efficient in the magmatic system a than in b on the contrary the mingling system c is characterized by a non uniform distribution of the basaltic andesite within dacite this reflects the occurrence of a dynamics in which stirring and folding processes do not operate efficiently and are unable to uniformly distribute the dispersed phase within the continuous one the decrease of k from a to b and the lack of a measurable k in c along with the observation that a and b were emitted before c indicate that the efficiency of advective movements within the porri magma chamber declined with decreasing time mingled magmas characterized by a homogeneous spatial distribution of enclaves or an initially inhomogeneous distribution evolving towards a homogeneous one are indicative of efficient advection processes that may favor magma mixing mingled magmas characterized by an inhomogeneous distribution of enclaves suggest low dynamical interaction between the two end members magma mixing is not allowed

relationships between deformation and mixing processes in lava flows a case study from salina aeolian islands tyrrhenian sea the massive unit of a lava flow from porri volcano salina aeolian islands displays many unusual structures related to the physical interaction between two different magmas the magma a represents approximately 80 of the exposed lava surface it has a crystal content of 51 vol and a dacitic glass composition sio2 63 64 wt the magma b has a basaltic andesite glass composition sio2 54 55 wt and a crystal content of approximately 18 vol it occurs as pillow like enclaves banding boudin like and rolling structures which are hosted in magma a structural analysis suggests that banding and boudin like structures are the result of the deformation of enclaves at different shear strain the linear correlation between strain and stratigraphic height of the measured elements indicates a single mode of deformation we deduce that the component b deformed according to a simple shear model glass analyses of the a b boundary indicate that a and b liquids mix together at high shear strain whereas only mingling occurs at low shear strain this suggests that the amount of deformation i e forced convection plays an important role in the formation of hybrid magmas high shear strain may induce stretching shearing and rolling of fluids which promote both forced convection and dynamical diffusion processes these processes allow mixing of magmas with large differences in their physical properties

hybridization of mingling magmas with different densities abstract it is usually assumed that chemical exchange between mafic enclaves considered as blobs of mafic liquids and their host silicic melt proceeds through static diffusion however the chemical boundary layer should keep the interior of magmatic enclaves nearly pristine a feature which is not observed the surface layer of a mafic blob falling in a more silicic melt is likely to be continuously folded in with the inner part a numerical model shows that buoyancy driven forced convection is an efficient way of enhancing the rate at which melt inclusions become hydrous and hybridized by the host melt

rolling structures at large shear strain abstract at large shear strain shear criteria are often obliterated or become ambiguous from examination of both natural examples and experimental models we describe a new criterion called rolling structure widely represented in sheared rocks a typical rolling structure is composed of a rigid or competent object e g porphyroclast boudin fossil etc with two tails asymmetrically disposed around it in most cases tails are of the same material as that of the rotating object and result from strain softening and grain size reduction at the object periphery forming a mantle z and s asymmetries of rolling structures represent dextral and sinistral senses of shearing respectively tails must not be confused with pressure shadows which usually present an opposite asymmetry for a given shear sense besides the determination of the sense of shear the occurrence of these structures allows a minimum estimate of the strain intensity in strongly sheared rocks since rolling structure length is proportional to shear strain

methods of determining the rheological properties of magmas at sub liquidus temperatures non vesicular basaltic melts behave as newtonian fluids at temperatures above their liquidus and their viscosities can be calculated using a method developed by shaw 1972 and bottinga and weill 1972 because many igneous processes involve the flow of silicates at sub liquidus temperatures numerous attempts have been made to calculate the interactive effects of the crystal phase however current methods are appropriate only for relatively low crystal concentrations and they assume newtonian behaviour we argue that this assumption is invalid when bubble or crystal concentrations exceed 30 at higher concentrations factors other than concentration become increasingly important these include particle shape and size distribution a method which takes these factors into account is tested on a range of suspensions including magmas at sub liquidus temperatures and rheological properties calculated using this method agree closely with the measured values we also demonstrate that an equation which was introduced to explain large differences in measured apparent viscosities during the cooling and crystallisation of mount st helens dacite murase et al 1985 and which is currently used to calculate the rheological properties of crystallising lavas generates viscosities which may be in error by several orders of magnitude this difference is argued to be caused by a combination of factors including the ten orders of magnitude range in the strain rates utilised during the mount st helens measurements causing orders of magnitude difference in the resulting apparent viscosities rheological data on crystallising silicic melts are reinterpreted taking into account the non newtonian rheology of the magmas and changes in activation energy of flow during crystallisation

interactions between rhyolitic and basaltic melts unraveled by chaotic mixing experiments abstract magma mixing may operate at any stage in the evolution of a magmatic system the development of mixing is strongly controlled by fluid dynamics and its understanding requires a comprehensive physico chemical approach in order to identify and interpret its occurrence in nature here we experimentally study the physical and chemical interplays during the mixing of basaltic and rhyolitic natural melts from the snake river plains usa in particular we present the results of the first high temperature mixing experiments performed under controlled chaotic dynamic conditions providing a new methodological approach to constrain the complexities of the mixing process between natural silicate melts the mixing process is initially governed by the dynamics of stretching and folding of the melts producing alternating flow bands these bands increase the contact area between the end members which subsequently enhance chemical exchanges and thus contribute to the generation of regions with variable degrees of hybridization we quantified the mobility of major and trace elements across contact areas and analyzed the concentration variance decay induced by chemical diffusion the analysis shows that elements diffuse with different efficiencies as the chemical gradient evolves and therefore the achievement of hybrid compositions contrasts between elements the approach introduced in this study can in principle be applied to mixing trends observed in nature in order to estimate the time scales and degree of magma mixing evidenced across volcanic rocks deposits

morphochemistry of patterns produced by mixing of rhyolitic and basaltic melts abstract in this work we present the results of time series experiments performed by mixing basaltic and rhyolitic melts at high temperature using a device recently developed to trigger chaotic dynamics in a mixing system the morphology of mixing patterns is quantified at different times by measuring their fractal dimension and a linear relationship is derived between mixing time and morphological complexity the complexity of mixing patterns is also compared to the degree of homogenization of chemical elements during mixing and empirical relationships are established between the fractal dimension and the temporal variation of concentration variance of elements new concepts and tools to study the magma mixing process unfold from the experimental results presented in this work the first one is that the mixing patterns are fractals and they can be quantified by measuring their fractal dimension this represents a further step in the quantification of the magma mixing process the second outcome is that the relationship between the fractal dimension of the mixing patterns and mixing time is linear this has important volcanological implications as the analyses of the morphology of mixing patterns in volcanic rocks can be complemented by experiments to build a new chronometer to estimate the mixing to eruption time a further result from this work is the relationship between the fractal dimension of mixing patterns and concentration variance of chemical elements this represents the first morphochemical study in igneous petrology bringing with it the potential to infer the relative mobility of chemical elements during the time progression of mixing by analyzing the morphology of mixing patterns in the rocks

the mixing of magmas in plutonic and volcanic environments analogies and differences abstract magma mixing processes have been widely recognized in both the plutonic and volcanic environments but the quality and quantity of information that can be extracted from the two environments are substantially different understanding the advantages and limits associated with the study of plutonic and volcanic rocks is essential to establish precise methodological approaches to build the most complete conceptual model by merging information from these two complementary igneous environments in this work we review magmatic interaction processes in the plutonic and volcanic environments by considering several aspects of these geological phenomena in particular we first briefly report on the structural and geochemical evidence for magma mixing in both plutonic and volcanic rocks with the aim to provide a general picture of this natural phenomenon successively we discuss some recent results about magma mixing achieved using the concepts from chaos theory and discuss their potential impact on magma differentiation finally we attempt to build a general picture of this igneous process by merging present day information from both the plutonic and volcanic environments it emerges from the general picture that the time spent by the magmatic system in the molten or partially molten state is the crucial factor for the preservation of the fingerprints of magma mixing in the two environments we propose a conceptual model that may be useful to understand what kind of information we can obtain from volcanic and plutonic rocks and ultimately to maximize our knowledge about magma mixing

shear criteria in the grenville province ontario canada abstract the grenville province of canada is a major proterozoic orogenic belt mapping and structural analysis shows that the central ontario segment of this belt is composed of a stack of imbricated thrust nappes separated by mylonite zones the mylonite zones were generated at high metamorphic grades and contain a distinctive assemblage of mesoscopic and microscopic asymmetric structures these structures include rotated tectonic inclusions rotated boudins and pinch and swell structures sheath folds rotated single crystal porphyroclasts single crystal fish and shear band foliation the shear asymmetry indicated by these structures is highly consistent between members of the assemblage and over a large area of the grenville province and shows that the thrust nappes were transported to the northwest the consistency of the structures is thought to be due to cyclical dynamic recrystallization leading to steady state foliations in the mylonites

the monte guardia eruption lipari aeolian islands an example of a reversely zoned magma mixing sequence the monte guardia rhyolitic eruption 22 ka lipari aeolian islands italy produced a sequence of pyroclastic deposits followed by the emplacement of lava domes the total volume of dense magma erupted was nearly 0 5 km3 the juvenile clasts in the pyroclastic deposits display a variety of magma mixing evidence mafic magmatic enclaves streaky pumices mineral disequilibria and heterogeneous glass composition petrographic mineralogical and geochemical investigations and melt inclusion studies were carried out on the juvenile clasts in order to reconstruct the mixing process and to assess the pre eruptive chemico physical magmatic conditions the results suggest that the different mingling and mixing textures were generated during a single mixing event between a latitic and a rhyolitic end member a denser mixed magma was first erupted followed by a larger volume of an unmixed lighter rhyolitic one this compositional sequence is the reverse of what would be expected from the tapping of a zoned magma chamber the monte guardia rhyolitic magma stored below 200 mpa was volatile rich and fluid saturated or very close to this despite its relatively low explosivity in contrast to previous interpretations there exists the possibility that the rhyolite could rise and erupt without the trigger of a mafic input the entire data collected are compatible with two possible mechanisms that would generate a reversely zoned sequence 1 the occurrence of thermal instabilities in a density stratified salic to mafic magma chamber and 2 the intrusion of rising rhyolite into a shallower mafic sill dike

relative rheological evolution of chemically contrasted coeval magmas example of the tichka plutonic complex morocco the tichka plutonic complex presents many unusual structures related to physical interaction between mafic and felsic coeval magmas in this paper a rheological approach is used to attempt to understand the various observed structures after a general presentation of rheological laws and a discussion about the present state of our understanding of the rheology of suspensions we analyse the main steps of rheological evolution of magmas during crystallization taking into account the effect of cooling increase of silica content and increase of crystallinity on viscosity during thermal evolution of magma a general rheological diagram is presented two main rheological thresholds at about 35 crystallinity th1 and about 65 70 th2 allow us to distinguish three major steps in the rheological behaviour of magmas during cooling comparative study of evolution curves for viscosity of coeval mafic m and felsic f magmas shows that an inversion temperature ti exists at which viscosities of both magmas are the same this point has major petrologic significance because the viscosity ratio m f inverses during cooling when the temperature of magma falls below the ti point a model for the evolution of the tichka magmas during cooling has been derived from the knowledge of the chemical and mineralogical composition of the different rocks representing the original m and f magmas as well as some hybrid and differentiated samples viscosities and crystal fractions of magmas as functions of temperature have been calculated from chemical and mineralogical data curves representing the evolution of apparent viscosity of the three initial magmas of the complex are presented from these vicosity temperature curves and from the calculted temperatures corresponding to rheological thresholds th1 and th2 for the different coeval magmas five main evolution stages are defined the different structures observed at the contracts between m and f magmas are developed throughout the entire cooling history of the pluton each structure being related to one of the main evolution stages

a microstructural indicator of shear sense in volcanic rocks and its relationship to porphyroblast rotation in metamorphic rocks an asymmetrical arrangement of microfolds in flow layering adjacent to phenocrysts indicates the sense of shear and is a potential indicator of the local flow direction in flow layered volcanic rocks this microstructure is analogous to a similar arrangement of microfolds around rotated porphyroclasts in some metamorphic rocks it is also relevant to the incorporation of inclusion trails in porphyroblasts that grow while undergoing rotation relative to the matrix foliation suggesting that asymmetrical microfolds may form outside porphyroblasts during their growth and relative rotation asymmetrical microfolds adjacent to garnet porphyroblasts have been observed in some metamorphic rocks

experimental simulation of magma mixing at high pressure abstract magma mixing features are observed in many plutonic and volcanic environments they result from the juxtaposition of two chemically contrasted magmas usually not only during the replenishment of a magmatic reservoir but also syn eruptively within the conduit despite its ubiquity only a few experimental studies have explored mixing between magmas existing data have been mostly acquired at atmospheric pressure and high shear rates 10 1 s 1 which differ from those accompanying magma mixing in reservoirs to fill this gap we performed high pressure mixing experiments at strain rates ranging from 4 10 4 to 1 10 3 s 1 layers of a synthetic crystal free haplotonalite and a natural partially molten basalt were juxtaposed in a paterson apparatus at 300 mpa and deformed between 900 and 1200 c the experiments shed light on the first stages of magma mixing and illustrate the role and behaviour of crystals either pre existing or newly grown experiments evidence a rheological threshold for mafic material disruption which sets in abruptly as its melt fraction exceeds 50 which in the experiments occurs in the narrow temperature interval 1160 1170 c below this threshold plagioclase crystals in the mafic magma form a rigid touching network and all the deformations are accommodated by the less viscous felsic layer above it the crystal network collapses allowing typical mingling mixing features to appear altogether such as enclaves melt filaments and single xenocrysts isolated into the felsic end member coexisting with newly grown phases plagioclase and pyroxene whose compositions spread out over considerable ranges the pre existing fabric of the mafic magma is only slightly affected by deformation altogether providing few clues on either the regime or geometry of applied deformation during the magmatic stage

microgranitic enclave shapes and magmatic strain histories constraints from drop deformation theory microgranitic mafic enclaves are common components of plutonic bodies and may represent the result of mingling of mafic and felsic magmas these enclaves are often ellipsoidal or elongate in shape in order to constrain the conditions under which such enclaves were generated and deformed by their host pluton we apply drop deformation theory to model their distortion our model takes into account the change in viscosity of both the host and enclave with temperature composition and the amount of suspended solids in enclave host systems and it assumes that simple shear flow predominates in the plutonic system we focus on the deformational history of enclaves over the time period in which they remain liquid and are either more or less viscous than their granitic host we find that the most stable enclaves with respect to being broken up by their entraining flow are generally the most mafic enclaves those with silica contents near those of the host granite are highly unstable this is in reasonable accord with field observations of the abundance of enclaves of different chemistry most deformation of microgranitic enclaves is likely to occur at relatively high temperatures probably between 950 c and 1050 c and therefore the enclaves record magmatic strain of the host over only a limited temperature time range in the host s cooling history furthermore the observation of apparent deformation of these enclaves in a liquid regime coupled with our calculated dependences of enclave deformation on flow velocity implies that magmatic flow velocities are likely to be below 10 m yr in enclave bearing plutonic systems

along strike variability in the seismogenic zone below nicoya peninsula costa rica 1 at the subduction zone in northwestern costa rica the seismogenic zone lies directly beneath the nicoya peninsula allowing for near source seismic studies of earthquake activity we located 650 earthquakes along the seismogenic plate interface using a dense seismic network in the vicinity of the nicoya peninsula using these data we constrained the updip limit of the seismogenic zone there and found a transition in depth 10 km in the south to 20 km in the north that occurs where the subducting oceanic crust changes from warmer cocos nazca spreading center cns origin to colder east pacific rise epr origin we argue that the temperature of the incoming oceanic crust controls the seismogenic updip limit beneath nicoya costa rica subducting colder oceanic crust deepens the seismogenic updip limit

time evolution of chemical exchanges during mixing of rhyolitic and basaltic melts we present the first set of chaotic mixing experiments performed using natural basaltic and rhyolitic melts the mixing process is triggered by a recently developed apparatus that generates chaotic streamlines in the melts mimicking the development of magma mixing in nature the study of the interplay of physical dynamics and chemical exchanges between melts is carried out performing time series mixing experiments under controlled chaotic dynamic conditions the variation of major and trace elements is studied in detail by electron microprobe and laser ablation icp ms the mobility of each element during mixing is estimated by calculating the decrease in the concentration variance in time both major and trace element variances decay exponentially with the value of exponent of the exponential function quantifying the element mobility our results confirm and quantify how different chemical elements homogenize in the melt at differing rates the differential mobility of elements in the mixing system is considered to be responsible for the highly variable degree of correlation linear nonlinear or scattered of chemical elements in many published inter elemental plots elements with similar mobility tend to be linearly correlated whereas as the difference in mobility increases the plots become progressively more nonlinear and or scattered the results from this study indicate that the decay of concentration variance is in fact a robust tool for obtaining new insights into chemical exchanges during mixing of silicate melts concentration variance is in a single measure an expression of the influence of all possible factors e g viscosity composition and fluid dynamic regime controlling the mobility of chemical elements and thus can be an additional petrologic tool to address the great complexity characterizing magma mixing processes

natural rotated inclusions in non ideal shear abstract kinematic analysis of rotated winged inclusions is often couched in terms of ideal simple shear with reference to a specific model based upon porphyroclasts of circular section however many natural winged inclusions are not porphyroclasts commonly their geometries as well as the orientation distribution of the inclusions themselves reflect the non ideal nature of the shearing deformation during which they formed stair stepped sigma shaped geometries may reflect the immaturity of a structure developed in non ideal shear or a mature structure developed in ideal simple shear stair stepped delta shaped geometries indicate that deformation approximated to ideal simple shear in plane delta shaped geometries and skewed inclusion orientation distributions are indicators of non ideal shear

the use of asymmetric pressure shadows in mylonites to determine the sense of shear abstract asymmetric pressure shadows aps on both sides of a rigid porphyroclast are commonly observed in mylonites along the median tectonic line mtl in japan it is one of the most noticeable asymmetric microstructures showing that the porphyroclasts have rotated during non coaxial laminar flow in a ductile shear zone the shadow domains are filled with recrystallized quartz and k feldspar excepting aps various asymmetric microstructures in the mylonites indicate a sinistral sense of displacement throughout the ductile shear zone along the mtl based on the shape analysis of aps in xz section parallel to the mylonitic lineation and normal to the mylonitic foliation the following results were obtained 1 the relative position of the aps with respect to a porphyroclast is not a reliable criterion for deducing the sense of shear and 2 the drag angle of the shadow boundaries with respect to the mylonitic foliation in each quartered domain is diagnostic of the sense of shear when the shearing is sinistral in upper right and lower left hand side of a porphyroclast is larger than in upper left and lower right hand side and vice versa for dextral shearing these results demonstrate that the drag patterns of aps around porphyroclasts in mylonites are highly reliable indicators for the determination of the sense of shear

diffusive fractionation of trace elements by chaotic mixing of magmas abstract recent research on magma mixing systems has shown that the mixing process is governed by chaotic dynamics and that this process is responsible for the generation of fractal structures that propagate within the magmatic mass from the meter to the micrometer length scale laser ablation icp ms trace element analyses have been performed on rock samples with evidence of chaotic mixing phenomena results indicate that trace elements with similar values of diffusion coefficient display good correlations in inter elemental plots whereas as the difference between diffusion coefficients increases the correlation is progressively lost in addition a large variability of ree patterns is observed with the remarkable feature of the presence of positive and negative eu anomalies occurring at short length scale of the order of few mm given the chaotic nature of magma mixing structures the mixing process has been simulated by coupling a chaotic advection and a chemical diffusion numerical scheme by considering several trace elements with variable diffusivities simulations indicate that such a model explains with good approximation the variable correlations among trace elements observed in natural samples in addition the same patterns of ree observed in natural samples including the occurrence of positive and negative eu anomalies at short length scale are observed indicating that a chaotic advection diffusion dynamic system is a suitable model to explain natural data results presented in this contribution indicate that at the micrometric length scale small volumes of magmas are strongly influenced by the coupled action of chemical diffusion and chaotic flow fields and hence they do not represent magmas de facto present in the magmatic system because their compositions may have experienced a diffusive fractionation process these results may have important petrogenetic implications for instance if such melts were trapped as melt inclusions they would provide misleading information about melt compositions it is suggested that the approach of studying the degree of correlation among trace elements may be a possible method to test if melt inclusion compositions commonly used as petrogenetic indicators display evidence of such a diffusive fractionation process

geological strain analysis a manual for the rf φ technique by richard j lisle pergamon press oxford 1985 viii 99 pp 15 hard cover 8 95 soft cover

seismic anisotropy changes across upper mantle phase transitions the mantle transition zone is believed to play an important role in the thermochemical evolution of our planet and in its deep water cycle constraining mantle flow at these depths can help elucidate its nature and better understand mantle dynamics and the history of plate tectonics seismic anisotropy i e the directional dependence of seismic wave velocity provides us with the most direct constraints on mantle deformation its detection below 250 km depth is challenging and it is often assumed that the deep upper mantle is seismically isotropic due to a change in mantle deformation mechanism here we present a global model of azimuthal anisotropy for the top 1000 km of the mantle we used a dataset composed of fundamental and higher mode rayleigh wave phase velocity maps which provides resolution of azimuthal anisotropy to much greater depths than in previous studies our model unravels the presence of significant anisotropy in the transition zone challenging common views of mantle deformation mechanisms and reveals a striking correlation between changes in anisotropy amplitudes and in the fast direction of wave propagation where upper mantle phase transitions occur the newly found relation between anisotropy changes and phase transformations gives new insight on the nature of the mtz and suggests that the anisotropy originates from the lattice preferred orientation of anisotropic material while the interpretation of our results in terms of mantle deformation is not straightforward due to many possible scenarios possible explanations for our findings include recrystallization during phase transformations or changes in the slip system across the mtz boundaries which in turn could be explained by changes in the water content of mantle material consistent with the idea that the transition zone acts as a water filter 2013

kinematic significance of morphological structures generated by mixing of magmas a case study from salina island southern italy morphological features of mixing mingling structures of heterogeneous juveniles from the 13 ka old upper pollara eruption salina island southern italy are studied these heterogeneous rocks result from the mixing between an andesitic and a rhyolitic magma concentration patterns generated by magma mixing are analyzed on digital images and results have been compared with those obtained from numerical simulations of mixing processes coupling chaotic advection and chemical diffusion results show that the upper pollara rocks were produced by mixing 24 49 of andesite and 76 51 of rhyolite multifractal analysis of the mixing structures is performed and the mixing intensity i e the degree of interaction between the two end member magmas has been deduced from the analysis of the singularity spectrum results show that narrower is the multifractal spectrum the higher is the degree of homogenization of magmas reynolds number re during mixing has been estimated from the geometrical analysis of mixing structures by using experimentally defined relationships between shape parameters and re results show that re is between ca 500 and 7000 there is a positive correlation between the estimated initial percentage of mafic magma in the different analyzed samples and re in agreement with the observation that the higher is the percentage of the mafic lower viscosity magma the higher is the turbulence of the mixing system in addition our analysis reveals an unexpected inverse relationship between the calculated re and the degree of magma homogeneity suggesting that energy dissipation may have had a major role in the controlling mixing process because dissipation is inversely proportional to the mixing efficiency results suggest that mixing processes between the andesitic and rhyolitic magmas mainly developed in the conduit it is also suggested that mixing occurred in a shear layer type or pipe type flow

anisotropy of magnetic susceptibility of silicic rocks from quarries in the vicinity of sao marcos rio grande do sul south brazil implications for emplacement mechanisms abstract the parana etendeka large igneous province includes acid volcanic rocks that can be found throughout its extension several aspects concerning those rocks remain controversial including their mechanism of emplacement and location of their eruptive sources opening of several quarries of dimension stone near the city of sao marcos rio grande do sul brazil offers a unique opportunity to study in detail the acid products here we present the results of a study of the anisotropy of magnetic susceptibility ams completed in some rocks that had been interpreted as the roots of volcanic conduits our results and reexamination of the textural features of the rocks lead to a reinterpretation that suggests that these rocks were emplaced subaerially and involved assimilation and remelting of clastic components of previous products due to the inferred conditions of emplacement it is unlikely that the eruptive vents are located far from the area of study therefore ruling out the long travelled nature of these products

volcanological evolution of the rivi capo volcanic complex at salina aeolian islands magma storage processes and ascent dynamics lava flows and pyroclastic deposits from strombolian fallout related to the activity of the rivi and capo volcanoes which are representative of early subaerial volcanoes on salina aeolian islands have been investigated through a geological petrological approach our geological field survey shows that rivi and capo volcanoes are part of a single n50 e aligned volcanic complex here named rivi capo volcanic complex rcvc stratigraphically specific rock sampling has allowed reconstruction of the magma feeding processes through time whole rock major element compositions together with core to rim profiles of plagioclase and clinopyroxene crystals show a general evolution toward more basic compositions through the three formations constituting the capo volcano and within the rivi center melts simulations and mass balance modeling suggest that the rcvc rocks are the result of fractional crystallization of plagioclase clinopyroxene and olivine ca 45 of solid removed from a primary magma in addition to fractional crystallization continuous recharge and mixing with more basic magma coming from deeper parts of the magmatic plumbing system contributed to the final volcanic rock compositions our textural and microanalytical data on plagioclase and clinopyroxene crystals allow the definition of a multilevel magmatic storage system with reservoirs at 20 and 3 km below sea level when processes of magma differentiation ascent and storage are considered together with the stratigraphic position of each sample a history of continuous modification of the rcvc plumbing system can be constructed volcanism may have been characterized by fissure type eruptions during the early stages lower capo lower rivi and middle capo formations gradually changing later to central type volcanism upper capo and upper rivi formations

microshear zones in a miocene submarine dacite dome of southwest japan microscopic shear zones have been found in the groundmass of glassy rocks of a miocene submarine dacite dome in southwest japan similar textures have been reported previously but only in dykes these textures give a valuable insight into the deformation of the dome during its emplacement by recording the orientations of the principal strains detailed textural analysis indicates that the microshear zones formed as a result of flattening and stretching of the magma simultaneously with quenching measured stretching directions are near parallel suggesting the magma flowed in one dominant direction rather than in a radial pattern the strain is believed to also be influenced by high magma pressure inside the dome being imposed on the high viscosity outer part of the dome

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

determining paleo flow directions from groundmass fabrics in the lyttelton radial dykes new zealand abstract the poles to 010 of groundmass feldspar in radial dykes cutting the dissected miocene volcano lyttelton banks peninsula form generally oblique girdles about the paleo flow direction the obliquity is a response to velocity gradients of magma within the dykes and the sense of obliquity relative to each dyke wall provides a unique determination of the sense of flow the fabrics give support to the model of blanchard et al 1979 for the movement of rigid particles in flowing magmas this method of flow determination overcomes the problem inherent in most others which provide a flow line but no sense of movement along that line methods of sampling and examples of the feldspar fabrics are described together with the relationship of these fabrics to other dyke features such as secondary shears and stretched vesicles

dynamics of obsidian flows inferred from microstructures insights from microlite preferred orientations the flow of obsidian lava leads to crystal alignments that reflect both the accumulated strain and the type of flow across the surface microlite preferred orientations are used to investigate the emplacement dynamics strain history and structural evolution of obsidian dome eastern california measurements of three dimensional microlite trend and plunge in samples from the near vent region distal flow and flow front show 1 the flow directions along the dome margins 2 the deformation style e g pure versus simple shear at the dome margins and 3 the variation in strain as a function of position within the flow microlites form well developed lineations in the plane of flow banding in all samples stereographic projections indicate that lineations trend normal to the western flow front and plunge shallowly away from the margin the radial flow pattern indicated by measurements made along the western margin suggests that extrusion was from a roughly elliptical vent these results highlight a strong correlation between microlite trend and the bulk flow direction inferred from the geometry of the flow along most of the eastern periphery lineations trend parallel to the margin and likely reflect the local flow direction as influenced by compression against the thickening flow crust marginal talus piles and topography orientation distributions imply that radial spreading accompanied by flattening was the dominant mechanism for flow emplacement comparisons of measured orientation distributions with theoretical predictions suggest that microlite fabrics in flow front and nearvent samples developed in a pure shear flow microstructures in a sample from near the distal flow base records a component of simple shear variance in microlite trend provides a measure of the amount of strain acquired during flow standard deviation in trend decreases from the near vent region to the flow margins reflecting progressive alignment of microlites during transport pure shear strain inferred from orientation distributions increases from approximately 0 3 near the vent to about 1 1 at the flow front the difference between these strains 0 8 is an estimate of the strain associated with flow emplacement such strain is similar in size to that estimated from mesoscopic structures v1 and for horizontal spreading of a fluid whose volume is equal to that of obsidian dome v1 6 this suggests that flow on the surface was sufficient to produce observed microlite preferred orientations in the flow front these techniques can be applied to interpret older dissected lavas where erosion has erased much of the original flow front or where larger scale structures indicative of flow directions are poorly preserved fl 2002 elsevier science b v all rights reserved

bounds on strain in large tertiary shear zones of se asia from boudinage restoration we have used surface balanced restoration of stretched boudinaged layers to estimate minimum amounts of finite strain in the mylonitic gneisses of the oligo miocene red river ailao shan shear zone yunnan china and of the wang chao shear zone thailand the layer parallel extension values thus obtained range between 250 and 870 we discuss how to use such extension values to place bounds on amounts of finite shear strain in these large crustal shear zones assuming simple shear these values imply minimum total and late shear strains of respectively 33 6 and 7 3 at several sites along the red river ailao shan shear zone for the wang chao shear zone a minimum shear strain of 7 4 is deduced assuming homogeneous shear would imply that minimum strike slip displacements along these two left lateral shear zones which have been interpreted to result from the india asia collision have been of the order of 330 60 km red river ailao shan and 35 20 km wang chao

bone shaped boudins in progressive shearing abstract some peculiar bone shaped boudins are observed in two areas of progressive shearing they are characterized by large quartz filled gashes that separate thinner boudins these gashes often have a consistent obliquity suggesting a rotation synthetic to the shear sense these may be formed by rigid material that crystallizes in tension gashes between the boudins during further ongoing deformation these behaved as rigid inclusions in a more ductile matrix and show systematic rotations the sense of rotation of the gashes constitutes a shear criterion

a model for mixing basaltic and dacitic magmas as deduced from experimental data it has been demonstrated experimentally that basaltic and dacitic magmas can be easily mixed to form both banded dacite and homogeneous andesite in less than a few hours the presence of phenocrysts larger than 0 5 mm increased considerably the efficiency of mixing flow patterns in the experimental system were visualized using pt spheres which indicated that convection occurs in basalt melt but not in dacite melt the reynolds numbers of the basaltic and dacitic melts in the experimental system were calculated to be about 10 3 and 10 6 respectively mixing proceeds initially by mechanical mixing of the two magmas in a large scale but later by coupling interfacial convection and mutual diffusion thus depending on the depth where vesiculation and following disruption of the magma occurs banded pumice homogeneous pumice and homogeneous andesite lava are erupted the observed textures of mixed rocks of plinian type eruption and the limiting occurrence of banded pumice are satisfactorily accounted for on this model