modes of detachment at the inclusion matrix interface abstract with the help of a 2d theoretical model this paper analyses the modes of matrix detachment around a circular elliptical rigid inclusion under pure and simple shear bulk deformations three modes of matrix detachment are possible mode 1 the matrix is displaced normal to the inclusion boundary forming fissures at the interface mode 2 the matrix slips along the inclusion boundary without any separation mode 3 the detachment occurs by a combination of modes 1 and 2 in order to determine the onset of detachment at any point on the coherent inclusion matrix interface the tensile and shear stresses were derived at that point and compared with imposed critical values numerical simulations based on the theoretical model reveal that the three modes occur systematically along the inclusion matrix interface the geometrical dispositions of which depend on the aspect ratio r and orientation of the inclusion in the case of circular inclusions mode 1 and mode 2 domains are separated by a mode 3 domain and the disposition shows an internal symmetry on the other hand it is asymmetrical when the inclusions are elliptical and oriented oblique to the bulk extension direction in pure shear and to the shear direction in simple shear in simple shear mode 2 detachment with synthetic slip occurs dominantly when is less than 45 and greater than 135 the results obtained from numerical models are complemented with observations in physical experiments the paper also determines theoretically the critical stresses for detachment to occur as a function of r for different values revealing that for a given mechanical strength of the inclusion matrix interface a particular mode of detachment can take place only when the aspect ratio crosses a threshold value

development of structures under the influence of heterogeneous flow field around rigid inclusions insights from theoretical and numerical models rocks that are mechanically heterogeneous due to the presence of stiff or rigid inclusions floating in a ductile matrix commonly show a variety of micro to macro scale structures developing under the influence of heterogeneous flow field in the neighbourhood of the inclusions it is of fundamental importance to apprehend the nature of strain heterogeneity around inclusions to understand progressive development of structures associated with rigid inclusions such as strain shadow foliation drag porphyroclast mantle porphyroblast inclusion trails intragranular fractures etc the development of these diverse types of structures can be analyzed with the help of a suitable hydrodynamic theory in this paper we review different continuum models that have been proposed to characterize the heterogeneous flow field around rigid inclusions focusing on recent developments recent studies reveal that jeffery s proc r soc lond a 120 1922 161 theory dealing with the motion of ellipsoidal rigid bodies in an infinitely extended viscous medium is more general in nature and applicable for modeling the heterogeneous flow around both equant and inequant shapes of inclusions and ideal or non ideal shear deformation of the matrix the application of this theory therefore has advantages over other models based on lamb s lamb h 1932 hydrodynamics cambridge university press cambridge theory dealing with spherical inclusions the review finally illustrates numerical simulations based on hydrodynamic theories highlighting the controls of physical and kinematic factors on the progressive development of the structures mentioned above

geological and geochemical data from the proposed sirente crater field new age dating and evidence for heating of target the proposed sirente crater field consists of a slightly oblong main structure main crater 120 m in width and about 30 smaller structures satellite craters all in unconsolidated but stiff carbonate mud here we focus on the subsurface structure of the satellite craters and compare the sirente field with known meteorite crater fields we present a more complete outline of the crater field than previously reported information on the subsurface morphology of a satellite crater c8 8 m in width radiocarbon and thermoluminescence tl ages of material from this crater and evidence for heated material in both crater c8 and the rim of the main crater crater c8 has a funnel shape terminating downwards and evidence for soil injection from the surface to a depth of 9 m the infill contained dispersed charcoal and small irregular porous fragments of heated clay with a calibrated age of b p 1712 13c corrected radiocarbon age b p 1800 100 and a tl age of b p 1825 calculated error 274 together with previous radiocarbon age b p 1538 of the formation of the main crater i e target surface below rim a formation is suggested at the beginning of the first millennium a d although projectile vaporization is not expected in sirente sized craters in this type of target material we used geochemistry in an attempt to detect a meteoritic component the results gave no unequivocal evidence of meteoritic material nevertheless the outline of the crater field evidence of heated material within the craters and subsurface structure are comparable with known meteorite crater fields

a rheologic model for wet crust applied to strike slip faults the strength and stability of crustal faults are not adequately addressed by the widely used two mechanism rheologic models of the crust based on byerlee s law and power law creep the models neglect the fluid assisted mechanisms of deformation that are important to strength of the crust and do not describe the variation in rate dependence of friction that governs the stability of fault slip several distinct mechanisms of frictional slip are defined on the basis of variations in frictional behavior and microfabric of simulated fault gouge in laboratory experiments state variable constitutive relations are used in a multiple mechanism formulation to describe the rate and temperature dependence of three friction mechanisms in wet quartz gouge at elevated temperatures this multimechanism description of friction is substituted for byerlee s law in the two mechanism models to generate a multimechanism rheologic model for the crust the multimechanism model is used to predict the rate and temperature dependence of crustal strength and the slip rate and depth temperature conditions over which each mechanism dominates application of the model to strike slip faults in the crust illustrates that the thickness of the actively shearing zone within faults is a critical parameter governing fault strength and stability the model predicts that frictional strength at midcrustal depths is significantly reduced relative to byerlee s law only for thick fault zones the reduction in strength is due to the operation of a strain rate sensitive friction mechanism involving combined cataclasis and solution transfer the rheologic model predicts that only very thin faults display the rate dependent characteristics necessary for initiation of seismic slip to any significant depth in the crust

natural deformation fabrics of plagioclase implications for slip systems and seismic anisotropy abstract the optical and tem microstructures and fabrics of twelve naturally deformed plagioclase containing rocks of compositions from an0 an80 have been studied with a view to determining the deformation mechanisms the optical indicatrix fabrics have been recorded in all samples with respect to the foliation and lineation in general and form a girdle in the foliation plane with a high concentration of normal to the foliation in some cases the orientation of 010 and 001 cleavages or albite and pericline twins has allowed the construction of pole figures in all cases 010 has been shown to be the dominant slip plane with either 001 or 100 the dominant slip direction tem shows that the dominant slip system is 010 001 with secondary 010 100 the seismic p wave anisotropy has been calculated for the first time from the lpo data of plagioclase the anisotropy coefficient is high at around 15 the minimum p velocity is normal to the lineation in the foliation plane and the maximum is sub parallel to the foliation normal the combined information on deformation kinematics and seismic anisotropy is valuable for interpretations of deep crustal seismic sections

the lomonosov crater impact event a possible mega tsunami source on mars recent research suggests that major meteorite impact events into a late hesperian early amazonian ocean likely produced a mega tsunami that would have resurfaced coastal areas in northwestern arabia terra the orientations of the associated lobate deposits a conspicuous type of landforms called thumbprint terrain suggests that if an impact event triggered the mega tsunami the most likely location of the source crater is within the northern plains regions situated north of arabia terra this study focuses on the identification of impact craters that impacted into the ocean and are likely to have produced the tsunami we selected 10 complex impact craters based on their diameters location and geomorphic characteristics of those the late hesperian 120 km diameter lomonosov crater exhibits a unique topographic plan view asymmetry compared to other similar sized and similar aged craters in the northern plains such as micoud korolev and milankovic we attribute its broad and shallow rim in part to an impact into a shallow ocean as well as its subsequent erosion from the collapsing transient water cavity the likely marine formation of the lomonosov crater and the apparent agreement in its age with that of the thumbprint terrain unit 3 ga strongly suggests that it was the source crater of the tsunami these results have implications for the stability of a late northern ocean on mars

high temperature shear zone formation in carrara marble the effect of loading conditions abstract rock deformation at depths in the earth s crust is often localized in high temperature shear zones occurring at different scales in a variety of lithologies the presence of material heterogeneities is known to trigger shear zone development but the mechanisms controlling initiation and evolution of localization are not fully understood to investigate the effect of loading conditions on shear zone nucleation along heterogeneities we performed torsion experiments under constant twist rate ctr and constant torque ct conditions in a paterson type deformation apparatus the sample assemblage consisted of cylindrical carrara marble specimens containing a thin plate of solnhofen limestone perpendicular to the cylinder s longitudinal axis under experimental conditions 900 c 400 mpa confining pressure samples were plastically deformed and limestone is about 9 times weaker than marble acting as a weak inclusion in a strong matrix ctr experiments were performed at maximum bulk shear strain rates of 2 10 4 s 1 yielding peak shear stresses of 20 mpa ct tests were conducted at shear stresses of 20 mpa resulting in bulk shear strain rates of 1 4 10 4 s 1 experiments were terminated at maximum bulk shear strains of 0 3 and 1 0 strain was localized within the carrara marble in front of the inclusion in an area of strongly deformed grains and intense grain size reduction locally evidences for coexisting brittle deformation are also observed regardless of the imposed loading conditions the local shear strain at the inclusion tip is up to 30 times higher than the strain in the adjacent host rock rapidly dropping to 5 times higher at larger distance from the inclusion at both bulk strains the evolution of microstructural and textural parameters is independent of loading conditions our results suggest that loading conditions do not significantly affect material heterogeneity induced strain localization during its nucleation and transient stages

cratering and modification of wet target craters projectile impact experiments and field observations of the lockne marine target crater sweden marine impacts are one category of crater formation in volatile targets at target water depths exceeding the diameter of the impactor the zones of vaporization melting and excavation of the standard land target cratering model develop partially or entirely in the water column the part of the crater that has a potential of being preserved seafloor crater may to a great extent be formed by material emplacement and excavation processes that are very different from land target craters these processes include a high energy water jet driven excavation flow at greater water depths the difference in strength of the target layers causes a concentric crater to evolve the crater consists of a wide water cavity with a shallow excavation flow along the seabed surrounding a nested deeper crater in the basement the modification of the crater is likewise influenced by the water through its forceful resurge to fill the cavity in the water mass and the seafloor the resurge flow is strongly erosive and incorporates both ejecta and rip up material from the seabed surrounding the excavated crater a combination of field observations and impact experiments has helped us analyze the processes affecting the zone between the basement crater and the maximum extent of the water cavity the resurge erosion is facilitated by fragmentation of the upper parts of the solid target caused by a spallation and b vibrations from the shallow excavation flow and subsequently c the vertical collapse of the water cavity rim wall in addition poorly consolidated and saturated sediments may collapse extensively possibly aided by a violent expansion of the pore water volume when it turns into a spray during passage of the rarefaction wave this process may also occur at impacts into water saturated targets without an upper layer of seawater present our results have implications for impacts on both earth and mars and possibly anywhere in the solar system where volatiles exist have existed in the upper part of the target

numerical modeling for target water depth estimation of marine target impact craters 1 marine impacts can develop a crater in the seafloor if the target water depth is shallow in relation to the size of the impactor the geology of this marine target crater is influenced by the layer of water in the upper part of the target the influence increases with increased water depth the target water depth is essential when calculating the magnitude of the impact event as the seafloor crater merely expresses some of the expended energy the target water depth for a marine target crater is often estimated by facies analysis of sediments related to the impact crater unfortunately often with ambiguous results we propose to combine the conventional methods with numerical modeling of the target water depth based on the special target water related features of the crater we used geological data from the lockne crater as constraints in a simulation with the sova hydrocode the simulations were done for three different target water depths 200 m 500 m and 1000 m within the likely depth range for the middle ordovician sea in which the impact occurred in order to obtain the minimum estimated size of the lockne crater by an asteroid impact at 20 km s the water depth must have been slightly less than 1 km and the impactor radius must have been about 400 m it was not possible to generate a crater with the geological features of lockne for target water depths of less than about 500 m however the ratio may be further constrained by factors such as impactor density impact angle and impact velocity

role of weak flaws in nucleation of shear zones an experimental and theoretical study this paper investigates the role of inherent weak flaws in the formation of plastic zones in deforming solids to understand the development of geological shear zones physical experiments were carried out on polymethylmethaacrylate pmma models containing single and multiple circular cylindrical flaws under compression maintaining plane strain condition models with single flaws show development of shear zones against a flaw in the form of conjugate sets with an average dihedral angle of 84 and oriented at an angle of 42 to the bulk compression direction the shear zones are generally tapered with increasing width away from the flaw in models with multiple flaws shear zones nucleated against individual flaws which propagated and coalesced with one another forming through going band like shear zones with inclination varying from 35 to 53 with the bulk compression direction with an increase in flaw concentration the through going shear zones defined persistent conjugate sets we applied the plane theory of elasticity for numerical simulations of ductile shear zones under the influence of a single circular weak flaw the pattern of shear zones yielded from numerical runs grossly matches with those observed in physical model experiments theoretical analysis demonstrates that the presence of a flaw promotes nucleation of shear zones at a bulk stress below the yield strength of matrix this critical stress is a non linear function of the flaw matrix competence contrast and decreases asymptotically with increasing competence contrast

preservation of old prograde metamorphic u th pb ages in unshielded monazite from the high pressure paragneisses of the variscan ulten zone italy abstract three samples of garnet kyanite paragneiss from the variscan ulten zone northern italy were studied in detail for u th pb monazite dating monazite in these gneisses is abundant shows highly variable grain size and occupies different textural positions within the matrix as inclusion in garnet and kyanite within apatite aggregates monazite shows different deformation features as a function of the textural position enclosed shielded monazite is generally more fractured than matrix unshielded monazite the integration of textural information with deformation features and in situ u th pb analyses by la icp ms indicates that there is no direct correlation between textural site and monazite ages old ages of 351 343 ma determined on portions of large matrix unshielded monazite and on rare domains of monazite shielded by garnet have been related to a prograde stage of the variscan metamorphic evolution of the ulten zone ages of 330 326 ma which are related to the thermal peak are recorded by small matrix monazite external domains of large matrix monazite and by domains of fractured monazite enclosed in garnet and kyanite large old unshielded grains formed as blasts during the prograde metamorphic history and survived the peak metamorphism during which crystallisation re crystallisation partially occurred

scaling and reproducibility of craters produced at the experimental projectile impact chamber epic centro de astrobiologia spain the experimental projectile impact chamber epic is a specially designed facility for the study of processes related to wet target e g marine impacts it consists of a 7 m wide funnel shaped test bed and a 20 5 mm caliber compressed n2 gas gun the target can be unconsolidated or liquid the gas gun can launch 20 mm projectiles of various solid materials under ambient atmospheric pressure and at various angles from the horizontal to test the functionality and quality of obtained results by epic impacts were performed into dry beach sand targets with two different projectile materials ceramic al2o3 max velocity 290 m s 1 and delrin max velocity 410 m s 1 23 shots used a quarter space setting 19 normal 4 at 53 from horizontal and 14 were in a half space setting 13 normal 1 at 53 the experiments were compared with numerical simulations using the isale code differences were seen between the nondisruptive al2o3 ceramic and the disruptive delrin polymer projectiles in transient crater development all final crater dimensions when plotted in scaled form agree reasonably well with the results of other studies of impacts into granular materials we also successfully validated numerical models of vertical and oblique impacts in sand against the experimental results as well as demonstrated that the epic quarter space experiments are a reasonable approximation for half space experiments altogether the combined evaluation of experiments and numerical simulations support the usefulness of the epic in impact cratering studies

the rotation rate of cylindrical objects during simple shear the rotational behaviour of circular objects is modelled using a two dimensional finite element simulation we present results for linear and non linear viscous rheologies and explore parameter space for the rheology contrast between object and matrix and for object size relative to the shear zone width l for high our results confirm analytical descriptions for l 0 in that the rotation rate of the object is half of the bulk shear strain rate however we show that for 0 1 l 0 9 the rotation rate as a function of l can have a minimum a maximum or it can decrease or increase depending on the stress exponent and in fact for some rheologies the rotation rate may decrease to 0 3 of the shear strain rate at intermediate l 0 5 20 area of porphyroblast in thin section these counter intuitive results have important consequences for the interpretation of the rotational behaviour of minerals in deforming rocks for example in rocks where garnet constitutes a volumetrically significant proportion of the bulk rock the effective shear zone boundaries may be given by neighbouring crystals corresponding to intermediate l as described here thus the interpretation of the rotation rate of crystals in such rocks can only be performed if the distance to the effective system boundaries is known

sedimentological analysis of resurge deposits at the lockne and tvaren craters clues to flow dynamics the lockne and tvren craters formed about 455 million years ago in an epicontinental sea where seawater and mainly limestones covered a crystalline basement the target water depth for tvaren apparent basement crater diameter d 2 km was probably not over 150 m and for lockne d 7 5 km recent best fit numerical simulations suggest the target water depth of 500 700 m lockne has crystalline ejecta that partly cover an outer crater 14 km diameter apparent in the target sediments tvren is eroded with only the crater infill preserved we have line logged cores through the resurge deposits within the craters in order to analyze the resurge flow the focus was clast lithology frequencies and size sorting we divide the resurge into resurge proper with water and debris shooting into the crater and ultimately rising into a central water plume anti resurge with flow outward from the collapsing plume and oscillating resurge not covered by the line logging due to methodological reasons with decreasing flow in diverse directions at lockne the deposit of the resurge proper is coarse and moderately sorted whereas the anti resurge deposit is fining upwards and better sorted the tvren crater has a smoothly fining up section deposited by the resurge proper and may lack anti resurge deposits at lockne the content of crystalline relative to limestone clasts generally decreases upwards which is the opposite of tvren this may be a consequence of factors such as crater size i e complex versus simple and the relative target water depth the mean grain size i e the mean phi value per meter and standard deviation i e size sorting for both craters can be expressed by the equation 0 60 1 25

determining relative bulk viscosity of kilometre scale crustal units using field observations and numerical modelling abstract though the rheology of kilometre scale polymineralic rock units is crucial for reliable large scale geotectonic models this information is difficult to obtain in geotectonic models a layer is defined as an entity at the kilometre scale even though it is heterogeneous at the millimetre to metre scale here we use the shape characteristics of the boundaries between rock units to derive the relative bulk viscosity of those units at the kilometre scale we examine the shape of a vertically oriented ultramafic harzburgitic lherzolitic unit which developed a kilometre scale pinch and swell structure at mid crustal conditions 600 c 8 5 kbar in the anita shear zone new zealand the ultramafic layer is embedded between a typical polymineralic paragneiss to the west and a feldspar quartz hornblende orthogneiss to the east notably the boundaries on either side of the ultramafic layer give the ultramafics an asymmetric shape microstructural analysis shows that deformation was dominated by dislocation creep n 3 based on the inferred rheological behaviour from the field a series of numerical simulations are performed relative and absolute values are derived for bulk viscosity of the rock units by comparing boundary tortuosity difference measured on the field example and the numerical series our analysis shows that during deformation at mid crustal conditions paragneisses can be 30 times less viscous than an ultramafic unit whereas orthogneisses have intermediate viscosity 3 times greater than the paragneisses if we assume a strain rate of 10 14 s 1 the ultramafic orthogneiss and paragneiss have syn deformational viscosities of 3 1022 2 3 1021 and 9 4 1020 pa s respectively our study shows pinch and swell structures are useful as a gauge to assess relative bulk viscosity of rock units based on shape characteristics at the kilometre scale and in non newtonian flow regimes even where heterogeneity occurs within the units at the outcrop scale

creep and strain dependent microstructures of synthetic anorthite diopside aggregates abstract we investigated plastic deformation of fine grained synthetic diopside and two phase anorthite diopside aggregates in triaxial compression and torsion up to 5 temperature confining pressure and stress ranged between 950 1180 c 200 400 mpa and 5 500 mpa respectively water content of samples ranged between 0 005 0 002 and 0 075 0 025 wt h 2 o all samples deformed in linear viscous creep with a stress exponent of 1 0 0 2 the activation energy ranged between 571 53 and 290 28 kj mol depending on mineralogy and water content sample strength depended on water fugacity with an exponent of 1 55 0 25 samples deformed in torsion and coaxial compression gave similar flow laws in spite of significant differences in the corresponding microstructures scanning and transmission electron microscopy of two phase samples deformed in torsion showed phase mixing cavitation and dislocation processes we suggest that linear viscous creep of fine grained two phase aggregates involved grain boundary sliding accommodated by grain boundary diffusion and significant dislocation accommodation at high strains we also observed that cavity coalescence and microcracking led to sample failure hence dynamic instabilities may exist in high strain shear zones accommodating viscous deformation in the lower continental crust

structural controls on the archean troilus gold copper deposit quebec canada the troilus gold copper deposit is a low grade high tonnage resource hosted in archean calc alkaline intrusive rocks in the superior province of quebec it is best known for being one of the few archean analogues of a porphyry cu au deposit an interpretation with significant implications for both mineral exploration and theories of archean plate tectonics recent research presented here has called into question the prevalent genetic model and proposes instead a two stage history of mineralization controlled by contrasting structural regimes developed as part of an orogen disseminated gold copper mineralization and associated potassic alteration occur in a deformation corridor between two felsic dikes emplaced prior to the peak of regional metamorphism postpeak metamorphic high strain sericitic zones with silicified envelopes host quartz veins and a second phase of gold the present size grade and disposition of the orebody is the result of the superposition of these two events an interpretation that impacts on the genetic model as well as exploration exploitation and grade control

the rheology of polymineralic rocks an approach deduced from empirical and theoretical work a model is proposed for the rheology of bi and polyphase rocks based on the rheology of the components their volume fractions and their geometrical distribution the model is formulated for the common case where some minerals deform by crystall plasticity at steady state and others stronger ones deform in the vincinity of the brittle ductile transition

marine target craters on mars an assessment study observations of impact craters on earth show that a water column at the target strongly influences lithology and morphology of the resultant crater the degree of influence varies with the target water depth and impactor diameter morphological features detectable in satellite imagery include a concentric shape with an inner crater inset within a shallower outer crater which is cut by gullies excavated by the resurge of water in this study we show that if oceans large seas and lakes existed on mars for periods of time marine target craters must have formed we make an assessment of the minimum and maximum amounts of such craters based on published data on water depths extent and duration of putative oceans within contacts 1 and 2 cratering rate during the different oceanic phases and computer modeling of minimum impactor diameters required to form longlasting craters in the seafloor of the oceans we also discuss the influence of erosion and sedimentation on the preservation and exposure of the craters for an ocean within the smaller contact 2 with a duration of 100 000 yr and the low present crater formation rate only 12 detectable marine target craters would have formed in a maximum estimate with a duration of 0 8 gyr as many as 1400 craters may have formed an ocean within the larger contact 1 meridiani with a duration of 100 000 yr would not have received any seafloor craters despite the higher crater formation rate estimated before 3 5 gyr on the other hand with a maximum duration of 0 8 gyr about 160 seafloor craters may have formed however terrestrial examples show that most marinetarget craters may be covered by thick sediments ground penetrating radar surveys planned for the esa mars express and nasa 2005 missions may reveal buried craters though it is uncertain if the resolution will allow the detection of diagnostic features of marine target craters the implications regarding the discovery of marine target craters on mars is not without significance as such discoveries would help address the ongoing debate of whether large water bodies occupied the northern plains of mars and would help constrain future paleoclimatic reconstructions

processes controlling the shrinkage of porphyroclasts in gabbroic shear zones abstract this paper presents microstructural and geochemical data on amphibole and clinopyroxene porphyroclasts collected in amphibolite facies ultramylonites of the ivrea verbano zone italy the well rounded porphyroclasts show a concentration of defects including 100 twins stacking faults free dislocations subgrains and microcracks within their rims that indicate stress concentrations localised deformation within the margins of porphyroclasts increases reaction kinetics and supports amphibole clinopyroxene replacement reactions and heterogeneous plagioclase nucleation in addition to microcracking and or subgrain rotation these metamorphic reactions lead to a marginal grain size reduction and the shrinkage of porphyroclasts in order to correlate the defect concentration at the rim of porphyroclasts with stresses a finite element inclusion matrix model is used it depends on the degree of bonding between inclusion and matrix whether strong stress concentrations within the rim of the inclusion occur for well coupled two phase systems shear stresses within the rim of the inclusion can reach a magnitude many times the magnitude of the matrix stresses marginal stress concentrations do not evolve if inclusion and matrix are decoupled this may indicate that mechanically decoupled porphyroclasts can survive to very large strains

flow stresses from microstructures in mylonitic quartzites of the moine thrust zone assynt area scotland recrystallized grain sizes subgrain sizes and dislocation densities of quartz grains in quartzose mylonitic rocks have been examined using optical and transmission electron microscopy tem the samples come from the moine thrust zone in the assynt district scotland they had been studied previously and described in detail with respect to their structural position in relation to the various thrusts in the region and to their preferred orientation stresses were derived from these samples using empirical and theoretical equations relating flow stress to the scale of the microstructures the stresses determined 43 244 mpa from recrystallized grain size 9 13 mpa from etched subgrain size 50 95 mpa from tem scale subgrain size and 75 147 mpa from dislocation density are not constant in individual samples simultaneous formation of the quartz fabrics and of the dynamically recrystallized grains during the period of mylonitization may have occurred under flow stresses ranging from 43 to 244 mpa a later dynamic recovery event reset subgrain sizes and dislocation densities to a constant value for each microstructure throughout the area there is insufficient empirical information available on the flow stress subgrain size relationship and on the effects of annealing recovery and annealing recrystallization to allow for a more detailed interpretation recrystallized grain size is still the most easily measured microstructural feature and the relationship of recrystallized grain size to flow stress has a sounder experimental basis than subgrain size or dislocation density the effect of chemical environment on the behaviour of all the microstructures is still unknown

when a cosmic impact strikes the sea bed most bolides that collide with the earth hit the sea limited knowledge about marine target impacts hampers predictions about their perilousness this study presents geological features that are particular to craters formed at sea the features are most likely a result of the influence of the target on the cratering process marine target craters form only if the target sea is shallow enough to admit sufficient kinetic energy into the sea bed when the crater diameter is large compared to the water depth the crater resembles its counterparts that are formed on land craters formed in deeper water are concentric and often lack melt sheets and rim walls but have deposits and radial gullies formed by the resurge of the sea impacts on the deep shelf are probably much more energetic than is suggested by the dimensions of the preserved crater

strain localization during high temperature creep of marble the effect of inclusions abstract the deformation of rocks in the earth s middle and lower crust is often localized in ductile shear zones to better understand the initiation and propagation of high temperature shear zones induced by the presence of structural and material heterogeneities we performed deformation experiments in the dislocation creep regime on carrara marble samples containing weak limestone or strong novaculite second phase inclusions the samples were mostly deformed in torsion at a bulk shear strain rate of 1 9 10 4 s 1 to bulk shear strains between 0 02 and 2 9 using a paterson type gas deformation apparatus at 900 c temperature and 400 mpa confining pressure at low strain twisted specimens with weak inclusions show minor strain hardening that is replaced by strain weakening at 0 1 0 2 peak shear stress at the imposed conditions is about 20 mpa which is 8 lower than the strength of intact samples strain progressively localized within the matrix with increasing bulk strain but decayed rapidly with increasing distance from the inclusion tip microstructural analysis shows twinning and recrystallization within this process zone with a strong crystallographic preferred orientation dominated by r and c slip in recrystallization induced weakening starts at local shear strain of about 1 in the process zone corresponding to a bulk shear strain of about 0 1 in contrast torsion of a sample containing strong inclusions deformed at similar stress as inclusion free samples but do not show localization the experiments demonstrate that the presence of weak heterogeneities initiates localized creep at local stress concentrations around the inclusion tips recrystallization induced grain size reduction may only locally promote grain boundary diffusion creep accordingly the bulk strength of the twisted aggregate is close to or slightly below the lower isostress strength bound determined from the flow strength and volume fraction of matrix and inclusions

a phenomenological numerical approach for investigating grain size evolution in ductiley deforming rocks abstract the sizes of recrystallised grains in exhumed ductile shear zones are often used to infer conditions of deformation i e stress strain rate and temperature here we present a simple numerical method of calculating the dynamic evolution of grain size during ductile deformation our phenomenological method is based on the fact that the dynamic competition between grain growth and recrystallisation will drive grains towards a steady state size at each time increment grain growth and reduction contributions are calculated with magnitudes which depend on the difference between the current grain size and a desired steady state grain size in our models we use a recrystallised grain size piezometer to calculate the steady state grain size for a given stress our numerical routine is incorporated into the sulec finite element package allowing us to explore spatial and temporal changes in grain size as a test we compare model results to measured grain sizes in quartz layers thinned and recrystallised around rigid garnet porphyroclasts under simple shear dominated deformation in the alpine fault zone of new zealand numerical models are able to replicate observed grain size variations with boundary conditions consistent with those constrained for the central alpine fault zone

polar perspective of late quaternary climates in the southern hemisphere abstract late quaternary paleoecological and glacial evidence from the higher latitudes of the southern hemisphere implies overall uniformity of large scale glacial and interglacial climatic fluctuations for the past 40 000 yr climate of the last glacial maximum variously dated between 30 000 and 11 000 yr b p was relatively cold and dry compared with the warmer more humid climate of the holocene and the interstade preceding the last glacial maximum conditions were apparently coldest during millennia centered around 20 000 yr b p and warmest in the early holocene recorded small scale fluctuations frequently variable for any given time interval are less consistent a cold late glacial episode estimated as occurring between ca 13 000 and 11 000 yr b p in antarctica possibly was coeval with the younger dryas stade in northwestern europe and may be correlative with a climatic episode in southern south america and perhaps in new zealand and south georgia however there is no evidence for the event in tasmania general atmospheric circulation models for the polar latitudes at the time of the last glacial maximum show an intensification of the southern westerlies apparently a result of the expansion of ice cover in antarctica and of sea ice in the southern ocean

palynology of cushion bogs of the cordillera pelada province of valdivia chile abstract fossil pollen identified in the earliest sediments of three cushion bogs in the cordillera pelada 40 10 s 73 30 w dated 10 425 14 c yr b p includes the subantarctic species dacrydium fonckii tetroncium magellanicum astelia pumila gaimardia australis donatia fascicularis and drosera uniflora all grow today in the cordillera pelada and range poleward to the southernmost province of magallanes one species drapetes muscosa included with the pollen of these plants in the earliest record is no longer a constituent of the flora but is limited only to subantarctic chile available evidence indicates that plants survived the last glaciation north of the glacial border with the course of postglacial migration southward following the wastage of the glacier complex holocene climatic and vegetational changes in the cordillera pelada are interpreted in the context of regional reconstructions which show maximum warmth about 9000 yr ago with a pronounced dry period lasting from 9000 to 6500 yr b p maximum precipitation was later reached around 4000 yr ago but has decreased overall since then the regional decline of the endemic gymnosperm fitzroya cupressoides which today is extensively destroyed in the cordillera pelada follows this decrease in precipitation these climatic data suggest a net south ward shift in the zone of westerly winds that bring rainfall to the region over the past 4000 yr

formation of resurge gullies at impacts at sea the lockne crater sweden the lockne crater in sweden is a marine target crater formed in a shelf sea approximately 460 ma ago the crater structure consists of an inner crater surrounded by an outer inclined surface that extends to almost 12 km from the center marine craters differ in several respects from craters formed on land one special feature is the formation of resurge gullies excavated by the erosional force of the resurging sea water after the impact the formation of these gullies strongly depends on the ratio crater rim height to water depth as well as on the size of the impact structure such gullies are known from very few marine target craters at the lockne impact site four gullies are identified each of which cuts radially through the rim of the outer crater the rapid collapse of that part of the crater cavity which formed in the seawater resulted in forceful flooding of the crater the resurging seawater not only contained fallback ejecta on its way towards the cavity on the sea bottom it incorporated fractured lithologies from the sea bottom as well this entrained material disintegrated during transport and constitutes today the dominantly monomict lower part of the resurge sequence the resurge flood was highly turbulent highly erosive and developed to a probable hyperconcentrated flow or a possible water flood the erosion in the gullies proceeded as headward erosion down to the transition zone between the brecciated and the less disintegrated crystalline basement

numerical modelling of porphyroclast and porphyroblast rotation in anisotropic rocks abstract the rotational behaviour of rigid objects in a weaker rock matrix during deformation has been the subject of many field experimental and numerical modelling studies often centred on the question whether objects rotate or not in non coaxial deformation with numerical studies gaining increasing popularity and importance we here provide an overview of the results published so far and provide new simulations originally shape and orientation were investigated while the emphasis shifted to rheology and slip between object and matrix in the nineties of the last century due to improved numerical techniques anisotropic rheology has become the focus of most recent studies indicating that it is a primary factor in the rotation behaviour of objects we present new simulations investigating the role of anisotropy on different scales relative to the object and show how this influences the rotation rate as well as the inclusion patterns in case of syntectonically growing porphyroblasts these simulations show that a variety of factors play a role to determine the rate and sense of rotation of objects the variability of the inclusion patterns that can develop necessitates extreme caution in the kinematic interpretation of these structures when observed in the field

crater topography on titan implications for landscape evolution abstract we present a comprehensive review of available crater topography measurements for saturn s moon titan in general the depths of titan s craters are within the range of depths observed for similarly sized fresh craters on ganymede but several hundreds of meters shallower than ganymede s average depth vs diameter trend depth to diameter ratios are between 0 0012 0 0003 for the largest crater studied menrva d 425 km and 0 017 0 004 for the smallest crater studied ksa d 39 km when we evaluate the anderson darling goodness of fit parameter we find that there is less than a 10 probability that titan s craters have a current depth distribution that is consistent with the depth distribution of fresh craters on ganymede there is however a much higher probability that the relative depths are uniformly distributed between 0 fresh and 1 completely infilled this distribution is consistent with an infilling process that is relatively constant with time such as aeolian deposition assuming that ganymede represents a close airless analogue to titan the difference in depths represents the first quantitative measure of the amount of modification that has shaped titan s surface the only body in the outer solar system with extensive surface atmosphere exchange

late glacial holocene climate of the lake district of chile pollen evidence from a 350 cm section of a fen in a moraine belt at rucanancu 39 33 s 72 18 w bears on the controversy regarding interpretation of late glacial and holocene climate in midlatitude chile earlier pollen studies indicating a cooling trend between approximately 11 000 and 10 000 yr b p disagreed with observations of glacier fluctuations which show continuous glacier wastage and by inference warming after 12 500 yr b p and possibly earlier up until neoglaciation beginning after 6850 yr b p fossil beetle assemblage data in this time range support the interpretation of climate made from the observed glacier behavior at rucanancu a pollen assemblage containing upper montane podocarp podocarpus andinus in quantities reaching 34 and dating between 10 440 and 10 000 yr b p implies a cold climate with summer temperatures possibly 5 8 c lower than today s holocene warming began afterward later than the glacier and beetle records indicate and continued until at least 8350 yr b p as suggested by the sequence of assemblages dominated by myrtaceae by aextoxicon punctatum and by gramineae a subsequent assemblage of nothofagus obliqua type implies an increase of moisture until 6960 yr b p following which n dombeyi type under a cool and humid neoglacial climate became dominant

impact related clastic injections in the marine ordovician lockne impact structure central sweden clastic injections generated in connection with the formation of impact craters show many similarities to injections created by other geological processes however circumstances such as their position relative to the impact structure and the evidence of forceful processes indicate an impact origin the ordovician lockne impact structure was formed in a marine environment with both sedimentary cambrian and ordovician and underlying crystalline proterozoic target rocks sea water played a substantial part in the cratering process especially in the modification of the newly formed crater as the water surged back into the structure in the lockne area clastic dykes and sills have long been known and have earlier been interpreted as neptunian dykes and conglomerates so far seven cases of dykes and sills are known in the area in this work these are interpreted as clastic injections formed in connection with the lockne impact the clastic injections occur in the crystalline basement and the sedimentary sequence the material in the injections comes from all local lithologies both sedimentary and crystalline but the sedimentary sequence dominates as a source the dykes and sills were injected simultaneously with the fracturing and dilation of the host rock in the cratering process and occur at different stratigraphic levels in some dykes clasts from the host rock wall can be fitted back to their original position the clasts are slightly rotated and surrounded by exotic material quartz grains with planar deformation features were observed in the injected material most of the sills within the bedded ordovician limestone are restricted to marly beds except for the feeder dykes which cut through the overlying beds this circumstance demonstrates how the decompression has opened the strata along weaker layers and that the underpressure created subsequently sucked the material down laminar flow is a conspicuous internal structure in the dykes and sills and indicates viscous flow of injected material the lamination in the injected material is parallel to the walls in each case the material was lithified prior to the event and was crushed mobilized in a water sediment slurry and injected as dykes and sills

deformation mechanism maps for feldspar rocks abstract deformation mechanism maps for feldspar rocks were constructed based on recently published constitutive laws for dislocation and grain boundary diffusion creep of wet and dry plagioclase aggregates the maps display constant temperature contours in stress grain size space for strain rates ranging from 10 16 to 10 12 s 1 two fields of dominance of grain boundary diffusion controlled creep and dislocation creep are separated by a strongly grain size sensitive transition zone for wet rocks diffusion controlled creep dominates below a grain size of about 0 1 1 mm depending on temperature stress strain rate and feldspar composition plagioclase aggregates containing up to 0 3 wt water as often found in natural feldspars are more than 2 orders of magnitude weaker than dry rocks the strength of water bearing feldspar rocks is moderately dependent on composition and water fugacity for a grain size range of about 10 50 m commonly observed in natural ultramylonites the deformation maps predict that diffusion controlled creep is dominant at greenschist to granulite facies conditions low viscosity estimates of 10 18 10 19 pa s from modeling postseismic stress relaxation and channel flow of the continental lower crust can only be reconciled with laboratory experiments assuming dislocation creep at high temperatures 900 c or at lower temperatures diffusion creep of fine grained rocks possibly localized in abundant high strain shear zones for similar thermodynamic conditions and grain size lower crustal rocks are predicted to be less than order of magnitude weaker than upper mantle rocks

role of water in the formation of the late cretaceous wetumpka impact structure inner gulf coastal plain of alabama usa the effect of shallow marine water 30 100 m deep in the late excavation and early modification stages of a marine target crater 5 km in diameter as exemplified by the late cretaceous wetumpka impact structure in alabama usa is manifest in the early collapse of a weak part of the rim excavation flow and connate marine water are interpreted to be factors in this collapse this partial rim collapse catastrophically emplaced an upper structure filling unit of broken and redistributed sedimentary target formations which presently mantles the deeper fallback breccia deposits within the structure furthermore rim collapse flow facilitated the formation of a structurally modified extrastructure terrain which is located outside and adjacent to the collapsed rim segment this extrastructure terrain appears to be the product of extensive slumping of poorly consolidated target sedimentary formations

controls on the failure mode of brittle inclusions hosted in a ductile matrix plane strain deformation experiments were performed on elliptical inclusions of cohesive sand embedded within a slab of pitch with the aim of investigating the mode of fracturing of brittle inclusions within a ductile matrix under pure and simple shear the inclusions failed in three different modes tensile fracturing mode 1 shear fracturing mode 2a and extensional shear fracturing mode 2b jeffrey s 1922 theory of the flow of a viscous medium around an ellipsoidal body was applied to the experimental results to determine the principal tensile and compressive stresses within an inclusion and analyze the failure modes using griffith s criterion the analysis reveals that the aspect ratio r and the orientation u of the inclusion control the principal tensile and compressive stresses within it and in turn govern the mode of brittle deformation at a particular inclusion orientation the tensile stress increases whereas the compressive stress decreases monotonically with increasing aspect ratio of the inclusion the principal stresses also vary with inclusion orientation for a given aspect ratio but not monotonically the analysis delimits the fields of each mode of brittle deformation of inclusions in r u space under pure shear and simple shear q 2001 elsevier science ltd all rights reserved

clast based kinematic vorticity gauges the effects of slip at matrix clast interfaces clast based methods for estimating the mean kinematic vorticity number wm are compromised by strain localization at the clast margins localization increases with modal matrix mica content as determined with samples from the sandhill corner mylonite zone a crustal scale high strain strike slip shear zone in maine using these samples we estimate wm with the oblique quartz shape preferred orientation and rigid clast rotation methods the rigid clast rotation method yields much lower values for wm than the quartz method to investigate whether or not slip at the matrix clast interface can explain the discrepancy in calculated wm we conducted numerical modeling of rigid clasts enveloped by a low viscosity layer both embedded within a shearing viscous matrix within this dynamic framework we carried out numerical sensitivity analyses in which we varied the viscosity ratio between the lubricating layer and the surrounding matrix the thickness of the lubricating layer and the kinematic vorticity number of the bulk flow our data and numerical results succeed in explaining why wm estimates from clast based rotational methods are typically lower than estimates from other methods and this has implications for testing hypotheses related for example to vorticity partitioning in oblique convergent settings crustal scale extrusion or channel flow and exhumation of ultra high pressure rocks all of which rely on robust estimates of wm the relation between the shape preferred orientations of clasts and modal mica content lead to the hypothesis that mica is the cause of the lubrication at clast matrix interfaces if so then we surmise that mica fish should be self lubricating and would therefore form an end member shape preferred orientation regardless of matrix modal mica content the unique role of mica allows us to speculate about the bounds on viscosity contrast between the matrix and lubricated clast interfaces

on estimating the strength of calcite rocks under natural conditions abstract field studies of calcite mylonites often document microstructures produced by dislocation creep in contrast flow laws derived from experiments predict that calcite rocks should deform mostly by diffusion creep during tectonic processes to investigate this apparent discrepancy we compare stresses estimated by microstructural piezometers to those obtained by extrapolation of experimentally derived flow laws considering shear zones from different geological settings a clear trend is observed of increasing recrystallized grain size with increasing temperature however there is a large spread in grain size and associated stress because separate flow laws have been defined for various different marbles and limestones the strengths predicted for a given set of conditions differ significantly the stress estimates based on the piezometers and strength extrapolated from the various experimentally derived dislocation creep flow laws agree qualitatively but no single flow law predicts all the palaeostress estimates even if experimental data are disregarded the field observations are not consistent with a hypothetical law for coble creep they are consistent with a power law for dislocation creep but only if the material constants are different from those currently determined in laboratory experiments

combinations of processes responsible for martian impact crater layered ejecta structures emplacement 1 we utilized images and stereo derived topographic data acquired by the high resolution stereo camera hrsc and thermal emission imaging system themis images together with other data in order to study the geology of layered ejecta structures associated with relatively pristine martian impact craters the geomorphology and morphometric properties indicate their origin as complex combinations of a variety of impact processes the studied inner layered ejecta structures often exhibit ground hugging characteristics and many of them do not have topographic profiles expected from simple ballistic emplacement such profiles include ones that are plateau shaped or thickening outward we think that water rich fluidized flows driven by the momentum due to the impact and by gravity together with ballistic emplacement and vortex produced by the atmosphere ejecta curtain interaction were essential to the inner layered ejecta structure formation we hypothesize that the thinner outer layered ejecta structures were formed by various combinations of shockwave induced liquefaction of water rich near surface sediments ballistic emplacement of ejecta entraining water and strong winds expanding vapor vortex base surge related to the impact the contribution of each proposed layered ejecta structure formation mechanism should have been variable depending on the condition of the impact

deformation around rigid particles the influence of slip at the particle matrix interface abstract theoretical and previous experimental studies of rigid particle structures have only considered the behaviour of rigid particles for which the interface between particle and matrix is coherent in natural rocks the boundary behaviour may cover the whole range from free slip to complete coherence examples where slip may be promoted include crack seal deformation during the development of pressure shadows core mantle structures where the mantle is composed of much weaker fine recrystallized material and crystallizing magmas or partially molten rocks where grain boundaries are lubricated by the melt phase analogue scale model experiments have been performed under pure shear and simple shear conditions to compare the particle rotation and the flow in the matrix for both slipping and non slipping particle matrix interfaces when slip does occur particles rotate faster in pure shear and slower in simple shear than for non slip leading to the more rapid development of a shape preferred orientation the preferred orientation tends to be bimodal in simple shear flow the flow and finite strain patterns in the matrix are also strongly influenced by slip at the interface and porphyroclast systems can develop geometries with ambiguous or contradictory shear senses dependent on the original particle orientation in multi particle experiments slip on the interfaces enhances strong strain localization in non linear viscous material to produce patterns very similar to natural s c fabrics

structural geometry lower crustal magmatic underplating and lithospheric stretching in the ivrea verbano zone northern italy abstract the ivrea verbano zone is believed to represent an up ended cross section through the lower continental crust as it existed at the end of the hercynian orogeny in the southern alps structurally oriented geological mapping has been carried out in the central and northern parts of the ivrea verbano zone a region some 30 km along strike the geology of the southern half of the zone is dominated by a large basic ultrabasic complex the mafic formation which is in contact with a strongly banded metasedimentary metabasic sequence to the north particular attention was paid to a the structural relationship between the rocks of the mafic formation and their envelope of high grade metasedimentary and metabasic rocks b the geometrical configuration throughout the ivrea verbano zone of high temperature shear zones which accommodated post hercynian crustal extension and c the geometry of late low temperature faulting the effects of which have been removed in order to produce a restoration of the structure as it existed during the post hercynian extensional phase the intrusion of large volumes of basic magma ca 50 of the outcrop area to form the rocks of the mafic formation appears to be coeval with the onset of extension ca 280 ma the main basic body has a laccolithic form which was originally more than 10 km thick overfolding developed at the northern margin of the laccolith and is interpreted in terms of the gravitational collapse of the hot immediately subsolidus or partially molten body incorporating its envelope of hot metasediments into a large originally recumbent fold a geometrical association with a high temperature low angle fault zone suggests that faulting was subsequently localized by the several km of uplift associated with the laccolithic intrusion the ivrea verbano zone may therefore demonstrate at least one particular geometry of lower crustal magmatic underplating which may aid in the interpretation of present day deep seismic profiles it also demonstrates the geometry of a network of conjugate high temperature low angle shear zones in a well layered lower crustal section

rheology of synthetic anorthite diopside aggregates implications for ductile shear zones 1 we investigated the high temperature creep strength of fine grained anorthite diopside rocks at temperatures ranging from 1323 k to 1523 k and at 300 mpa confining pressure in a paterson type gas medium deformation apparatus flow stress varied between 20 and 450 mpa resulting in strain rates between 6 1 10 7 s 1 and 7 5 10 4 s 1 pure diopside and anorthite samples were hot pressed from crushed natural single crystals and glass powders respectively two phase samples were produced by hot isostatic pressing of mechanically mixed powders of anorthite glass with 25 50 and 75 vol diopside particles arithmetic mean grain size of the anorthite matrix is dan 3 5 m three different ranges of diopside particle size were used ddi 25 m 35 m and 45 m water content of as is samples was about 0 05 0 02 wt h2o and predried samples contain about 0 004 0 001 wt h2o at experimental conditions as is samples are assumed to be water saturated water content of predried samples is about 3 times less than that of starting diopside single crystals the specimens contain about 1 vol glass located at fluid inclusions and some multiple grain junctions two grain boundaries examined by high resolution transmission electron microscopy did not show amorphous layers to a resolution of 1 nm at experimental conditions pure diopside aggregates are about 2 3 orders of magnitude stronger than pure anorthite samples for as is and predried specimens respectively in general strength of the two phase aggregates increases with increasing diopside content but remains between isostress and isostrain rate bounds aggregate strengths predicted from continuum mechanics models are in good agreement with the experimental data for dilute diopside particle mixtures and high volume fractions when diopside particles form a load bearing framework at low stresses 100 200 mpa the stress exponent is n 1 suggesting diffusion controlled creep at higher stresses mechanical data and microstructures suggest that samples deformed in the transition region between diffusion controlled creep and dislocation creep for pure anorthite and diopside aggregates deforming in dislocation creep we estimated stress exponents of n 3 and n 5 5 respectively for the two phase aggregates n is between n 3 and n 5 depending on diopside content at low stresses deformation microstructures indicate load transfer from a weak anorthite matrix to stronger diopside particles creep activation energies for pure diopside and anorthite mixtures range from 286 kj mol 1 for wet anorthite deformed at low stresses to 691 kj mol 1 for dry diopside deformed at high stresses activation energies of two phase mixtures are between or close to those of the end members as is samples have significantly lower activation energies than predried samples

diffusion creep modelling of fibrous pressure shadows abstract grain scale deformation of a bimineralic composite by normal stress driven grain boundary diffusive mass transfer has been modelled using diffusion creep software that for the first time allows different diffusion coefficients for interphase boundaries and grain boundaries as well as for different components by selecting suitable coefficients we have obtained a fully coupled mechanical and mass transfer numerical solution for the formation of fibrous pressure shadows or strain fringes at the interface between a rigid object and a deforming rock matrix of different compositions the model is two dimensional and run in pure shear velocity mode to a stretch ratio of 1 4 the first results are reported which show dissolution and face controlled antitaxial growth of a soluble polycrystalline calcite matrix at the interface with a circular and relatively insoluble pyrite object the ratio of caco3 diffusion along calcite pyrite versus calcite calcite boundaries strongly influences the extent of pressure shadow growth and the pattern of strains in the matrix a low ratio produces little growth significant perturbation and deflection of total strain axes around the pyrite an intermediate ratio produces appreciable growth little perturbation and near parallel total strain axes a high ratio produces larger pressure shadows and total strain axes that bend in towards the pyrite as if it were a weak object sliding and rotation of matrix grains are also affected these results differ fundamentally from purely physical models of ductile flow around a rigid object and demonstrate the influence of interface diffusional properties on the resulting behaviour in particular assumptions such as perturbation of the flow by the object deflection of external foliation around pre deformational porphyroblasts and relationships of pressure shadow strain to bulk strain may need to be re examined in the light of this finding

extensional detachment at the inclusion matrix interface in a multiple inclusion system abstract this paper deals with a theoretical analysis of extensional fissures around spherical rigid inclusions hosted in a ductile matrix undergoing bulk shear the analysis shows that the mechanical interaction of inclusions is a function of the inclusion concentration a b 2 a and 2 b are the inclusion diameter and mean inter inclusion distance respectively and intensifies the tensile stress imparted by the flowing matrix on the inclusion matrix interface inclusion concentration a b thus appears to be a crucial parameter in perturbing the inclusion matrix coherence and leading to formation of extensional fissures around an inclusion we performed numerical simulations to delineate the fields for coherent and incoherent interfaces in the t o a b space where t o is the tensile strength of the interface normalized to where and are the viscosity of matrix and bulk shear rate respectively for a given a b the interface cannot remain coherent unless the tensile strength is greater than a critical value the critical t o increases almost linearly at a gentle gradient with increasing a b however for a b 0 7 this increases nonlinearly assuming a large value when the concentration is high a b 0 8 it is also revealed that under the same deformation conditions the instantaneous opening in fissures may vary depending upon the a b values

the target peneplain of the lockne impact the lockne impact crater in central sweden with a diameter of about 7 km formed in the mid ordovician in a marine environment that was deeper than present shelf seas the present dip of the so called sub cambrian peneplain in the target area of the impact is about 0 85 toward the northwest the peneplain is cut by a set of northwest striking nearly vertical faults with a throw of up to just over 100 m collectively the identification of the peneplain and its deformation by faulting allows us to infer that the part of the crater that is exposed to the east of lake locknesjon has been lowered by about 100 m relative to parts exposed to the west of the lake and that it has therefore been spared from significant erosion therefore the preservation of the whole crater is even better than was assumed in previous work the peneplain extends to 600 700 m from the rim of the inner crater hence the structural uplift of the rim is quite subdued compared to the craters that formed on land

chesapeake bay impact structure morphology crater fill and relevance for impact structures on mars the late eocene chesapeake bay impact structure cbis on the atlantic margin of virginia is one of the largest and best preserved wet target craters on earth it provides an accessible analog for studying impact processes in layered and wet targets on volatile rich planets the cbis formed in a layered target of water weak clastic sediments and hard crystalline rock the buried structure consists of a deep filled central crater 38 km in width surrounded by a shallower brim known as the annular trough the annular trough formed partly by collapse of weak sediments which expanded the structure to 85 km in diameter such extensive collapse in addition to excavation processes can explain the inverted sombrero morphology observed at some craters in layered targets the distribution of crater fill materials in the cbis is related to the morphology suevitic breccia including pre resurge fallback deposits is found in the central crater impact modified sediments formed by fluidization and collapse of water saturated sand and silt clay occur in the annular trough allogenic sediment clast breccia interpreted as ocean resurge deposits overlies the other impactites and covers the entire crater beneath a blanket of postimpact sediments the formation of chaotic terrains on mars is attributed to collapse due to the release of volatiles from thick layered deposits some flat floored rimless depressions with chaotic infill in these terrains are impact craters that expanded by collapse farther than expected for similar sized complex craters in solid targets studies of crater materials in the cbis provide insights into processes of crater expansion on mars and their links to volatiles

microstructural and rheological evolution of calcite mylonites during shear zone thinning constraints from the mount irene shear zone fiordland new zealand abstract layers of calc mylonite in the mount irene shear zone fiordland new zealand show substantial variations in thickness due to deflection of the shear zone boundaries around wall rock asperities in relatively thick parts c 2 6 m of the shear zone calcite porphyroclasts are internally strained contain abundant subgrain boundaries and have a strong shape preferred orientation spo and crystallographic preferred orientation cpo suggesting that deformation occurred mainly by dislocation creep involving subgrain rotation recrystallization in relatively thin parts c 1 5 m of the shear zone aggregates of fine grained recrystallized calcite surrounding flattened porphyroclasts have a weak spo and cpo and contain polygonal calcite grains with low degrees of internal misorientation the recrystallized aggregates also contain microstructures e g grain quadruple junctions randomized misorientation axes similar to those reported for neighbor switching processes during grain boundary sliding comparison of subgrain sizes in the porphyroclasts to published grain size differential stress relationships indicates that stresses and strain rates were substantially higher in relatively thin parts of the shear zone the primary microstructural response to higher stresses and strain rates was an increase in the amount of recrystallization to produce aggregates that deformed by grain boundary sliding however even after the development of interconnected networks of recrystallized grains dislocation creep by subgrain rotation recrystallization continued to occur within porphyroclasts this behavior suggests that the bulk rheology of shear zones undergoing thinning and thickening can be controlled by concomitant grain size insensitive and grain size sensitive mechanisms overall our observations show that shear zone thickness variations at constant p t can result in highly variable stresses and strain rates which in turn modifies microstructure deformation mechanism and shear zone rheology

low temperature deformation in calcite veins of safod core samples san andreas fault microstructural analysis and implications for fault rheology abstract the microstructures of four core samples from the san andreas fault observatory at depth safod were investigated with optical and transmission electron microscopy these samples consisting of sandstone siltstone and fault gouge from phase iii of the drilling campaign 3141 3307 m md show a complex composition of quartz feldspar clays and amorphous material microstructures indicate intense shearing and dissolution precipitation as main deformation processes the samples also contain abundant veins filled with calcite within the inspected veins the calcite grains exhibit different degrees of deformation with evidence for twinning and crystal plasticity dislocation densities ranging from 3 1012 m 2 to 3 1013 m 2 and twin line densities 22 mm 1 165 mm 1 are used as paleo piezometers the corresponding estimates of differential stresses vary between 33 and 132 mpa deduced from dislocation density and 92 251 mpa obtained from twin density possibly reflecting chronologically different maximum stress states and or grain scale stress perturbations mean values of stress estimates are 68 46 mpa and 168 60 mpa respectively where estimates from dislocation density may represent a lower bound and those from twin density an upper bound the stress estimates are also compatible with residual lattice strains determined with microfocus laue diffraction yielding equivalent stresses of 50 300 mpa in twinned calcite the lower stress bound agrees with stress estimates from borehole breakout measurements performed in the pilot hole from these data and assuming hydrostatic pore pressure and a low intermediate principal stress close to the overburden stress frictional sliding of the san andreas fault at the safod site is constrained to friction coefficients between 0 24 and 0 31 these low friction values may be related to the presence of clays talc and amorphous phases found in the fault cores and support the hypothesis of a weak san andreas fault

deep crustal extensional faulting in the ivrea zone of northern italy abstract the ivrea zone in northern italy consists of amphibolite and granulite facies metasedimentary metabasic and ultrabasic rocks and is thought to represent a section through the deep continental crust detailed mapping of the high grade part of this zone has revealed a series of high temperature shear zones which after the effects of later low temperature faulting and folding have been removed appears to accommodate extension of the sequence the shear zones formed at temperatures similar to those at which thermal equilibration occurred in the host rocks and locally show evidence for development under prograde conditions these observations fit well with thermal effects calculated from a simple heat flow model for extensional faulting in this model the effects of shear heating and displacement of cool hanging wall rocks against hotter footwall rocks are calculated these observations indicate that faulting as a mode of failure may be important during extension of the lower continental crust especially when basic rocks are the dominant components of the rock pile a restored section through the extended lower crust as observed in the ivrea zone is presented and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed

ice age vegetation and climate of subtropical chile abstract vegetation and climate of subtropical chile over the past 50 000 yr are reconstructed from pollen spores and other microfossils contained in a 10 7 m section of lacustrine deposits of laguna de tagua tagua 34 30 s 71 10 w controlled by 14 radiocarbon age determinations five pollen assemblages covering stratigraphic zones in the section represent plants on the upland and species inhabiting the lake basin surface pollen spectra from 58 stations approximately 30 40 s form an adjunct for interpreting past vegetation and climate from the fossil record temperature semi humid woodland of southern beech nothofagus dombeyi type and podocarp prumnopitys andina was apparently established about laguna de tagua tagua during the pleistocene contrasting existing semi arid broad sclerophyllous vegetation the woodland was extensive about the lake at the time of the last glacial maximum 25 000 14 000 yr b p and earlier 43 000 34 000 yr b p pollen from the families chenopodiaceae and amaranthaceae which characterize intervals of the pleistocene centered around 33 000 and 43 000 yr b p and the holocene suggests intervening episodes of relative aridity lake level fluctuations appear to follow the late quaternary climatic pattern implied by pollen assemblage data water in the lake was relatively high during development of southern beech podocarp woodland whereas during intervals of chenopod amaranth dominance lake levels were comparatively low excess precipitation over evaporation which during the pleistocene favored development of woodland of beech and podocarp is attributed to greater storm frequency conditions were apparently governed by strengthening of atmospheric circulation in the belt of southern westerly winds in the late glacial the temperate and more humid ice age climate with limited seasonality underwent transition to a subtropical semi arid mediterranean type that identifies the postglacial fluctuations in intensity of the southern westerlies marked by latitudinal movement of the polar front can account for past vegetation changes at laguna de tagua tagua

eocene partial melting recorded in peritectic garnets from kyanite gneiss greater himalayan sequence central nepal the himalayan mountain belt is characterized by the impressive continuity over 2500 km of tectonic units thrusts and normal faults as well as large volumes of high grade metamorphic rocks and granite exposed at the surface visona et al 2012 although there are many studies on metamorphism melt generation and deformation concentrated in the same time span as activity on the mct and std 23 17 ma godin et al 2006 much fewer data on deformation metamorphism melt generation and geochronology in the greater himalayan sequence ghs is available for the large time span 30 m y between collision at 55 ma and mct std activities carosi et al 2010 2013 montomoli et al 2013 melt generation and granite emplacement have been mainly related to the exhumation stage of the ghs during early miocene harris and massey 1994 very few evidences of partial melting during prograde metamorphism have been reported until now in the southern part of the belt godin et al 2001 imayama et al 2012 the ghs of eastern nepal and sikkim india shows the occurrence of melting at the bottom of the ghs barun gneiss at nearly 33 27 ma imayama et al 2012 ferrero et al 2012 rubatto et al 2012 anatectic melt inclusions nanogranites and nanotonalites have been found in garnet of kyanite gneiss at the bottom of the greater himalayan sequence fig 1 along kali gandaki valley central nepal nearly 1 km structurally above the main central thrust in situ u th pb dating of monazite included in garnets in the same structural positions as melt inclusions allowed to constrain partial melting starting at 36 41 ma carosi et al in press

experimental observations on the effect of interface slip on rotation and stabilisation of rigid particles in simple shear and a comparison with natural mylonites for axial ratios r not vert similar3 porphyroclasts from three mylonites all show a very strong spo with the long axis of the best fit ellipse at an antithetic angle of 5 10 to the shear direction this is more consistent with a stable end orientation than with the transient fabrics predicted by theory for elliptical rigid particles in simple shear the cause of this divergence is investigated in a series of high simple shear strain 15 analogue experiments performed in a ring shear machine couette flow using a linear viscous matrix pdms the rotational behaviour of elongate r not vert similar5 rigid particles with elliptical and rhomboidal shapes comparable with the natural examples is modelled for both coherent and slipping particle matrix interfaces interface slip causes a dramatic reduction in the rotation rate of the elliptical particle compared with theory when the long axis is close to the shear direction but not stabilisation interface slip does result in stabilisation of the rhomboidal particle with the long diagonal oriented at a small antithetic angle to the shear direction for monoclinic particles mirror image shapes referred to here as types 1 and 2 show different rotational behaviour for the type 1 particle with a shape comparable to porphyroclast systems and mica fish the long side rotates asymptotically into parallelism with the shear direction natural examples of type 1 particles such as hornblende and olivine porphyroclasts measured from the finero mylonites southern alps show a very strong preferred orientation for r not vert similar3 with the long side parallel or at a small 5 antithetic angle to the mylonitic foliation for type 2 particles the short side stabilises close to the shear direction or at a small synthetic angle as also observed for sillimanite porphyroclasts from the mont mary mylonites western alps in this natural case stabilisation of the short sides is against an extensional crenulation cleavage rather than the mylonitic foliation the analogue experiments establish that interface slip is one mechanism for stabilisation of elongate rhomboidal particles in natural examples decoupling from the matrix may be affected by extensional crenulation cleavage or c planes in s c fabrics

kinematic behaviour of an interface and competence contrast analogue models with different degrees of bonding between deformable inclusions and their matrix abstract in a system composed of a ductile matrix with a particle included in it the deformation of the matrix the displacement field and the rotation and the deformation of the block are related to the viscosity ratio between block and matrix and also to the degree of bonding between the matrix and the block in this paper belemnites from the lower lias of the alps provide a natural example of moderately deformed objects included in a slaty matrix they are compared with analogue models made of paraffin in which a long block of a more competent paraffin is included depending on the degree of bonding between matrix and object the same block appears to be deformable or quite rigid with a high degree of bonding the strain refraction observed corresponds to the viscosity ratio between the block and the matrix when the degree of bonding is weak the block is quite undeformed its rotation is great and the deviation of the strain in the matrix is analogous to that observed around a fault the kinematic conditions appear to have a greater effect on the finite strain than the viscosity ratio of the materials

pressure anomalies around cylindrical objects in simple shear abstract we model the pressure distribution around cylindrical objects in simple shear deformation using the finite element method in two dimensions and present an analytical solution for a special case parameter space is explored numerically for viscosity contrast between object and matrix and for the non linearity of flow as expressed by n we show that the geometry and size of the pressure shadow is independent of but strongly dependent on n for example at n 1 pressure shadows are roughly circular in shape while for n 1 pressure shadows disintegrate into two branches we also show that pressure may only exceed 100 mpa in pressure shadows at resolvable distances from the object if 4 for strain rates of 10 14 s 1 and matrix viscosities around 10 22 pa s as these values for strain rate viscosity and viscosity contrast are geologically reasonable we emphasise that it is conceivable that geobarometric results obtained from syndeformational mineral parageneses near rigid porphyroblasts may be influenced by non lithostatic components of pressure

elevation distribution of titan s craters suggests extensive wetlands abstract using a new global topographic map of titan we find that craters on titan preferentially lie at higher than average elevations we explore several explanations for this observed behavior and judge the most reasonable explanation to be the presence of widespread wetlands of liquid hydrocarbons at low elevations over much of geologic time impacts into a shallow marine environment or a saturated layer of sediments more than several hundred meters thick would produce crater morphologies similar to terrestrial submarine impacts these are known to lack significant topographic expression and would thus be difficult to observe with the cassini spacecraft since titan s near surface methane inventory likely fluctuated over geologic time with episodic delivery and continuous depletion a few craters at low elevations can nonetheless be expected

the lawn hill annulus an ordovician meteorite impact into water saturated dolomite the lawn hill impact structure lhis is located 250 km n of mt isa in nw queensland australia and is marked by a highly deformed dolomite annulus with an outer diameter of similar to 18 km overlying low metamorphic grade siltstone sandstone and shale along the ne margin of the georgina basin this study provides detailed field observations from sections of the lawn hill annulus and adjacent areas that demonstrate a clear link between the deformation of the dolomite and the lawn hill impact ar 40 ar 39 dating of impact related melt particles provides a time of impact in the ordovician 472 8 ma when the georgina basin was an active depocenter the timing and stratigraphic thickness of the dolomite sequence in the annulus suggest that there was possibly up to 300 m of additional sedimentary rocks on top of the currently exposed thorntonia limestone at the time of impact the exposed annulus is remarkably well preserved with preservation attributed to postimpact sedimentation the lhis has an atypical crater morphology with no central uplift the heterogeneous target materials at lawn hill were probably low strength porous and water saturated with all three properties affecting the crater morphology the water saturated nature of the carbonate unit at the time of impact is thought to have influenced the highly brecciated nature of the annulus and restricted melt production the impact timing raises the possibility that the lawn hill structure may be a member of a group of impacts resulting from an asteroid breakup that occurred in the mid ordovician 470 6 ma

expansion of magellanic moorland during the late pleistocene palynological evidence from northern isla de chiloe chile the late quaternary vegetation of northern isla de chiloe is inferred from palynological analysis of a section in the rio negro drainage 42 03 s 73 50 w at ca 30 500 yr b p maxima of astelia and donatia occurred suggesting wetland development from that time until ca 27 000 yr b p steppe indicators such as compositae gramineae dominated suggesting drier conditions after 27 000 yr b p the moorland shrub dacrydium gradually increased reaching a maximum by 18 000 yr b p at this time astelia increased again suggesting development of cushion bog during cold and wet conditions the glacial postglacial transition is characterized by a marked change from peaty sediments to clays a decrease in the cushion bog flora and the prevalence of gramineae compositae and swamp taxa this vegetation prevailed until ca 7000 yr b p when forest taxa became dominant the floristic pattern inferred from the pollen spectra of the rio negro section suggests that the late pleistocene vegetation of chiloe resembled modern magellanic moorland vegetation 52 56 lat s based on climatic conditions presently associated with magellanic moorland its occurrence in chiloe at low elevations during the late pleistocene implies a decrease in average temperature of at least 4 c and an increase in annual precipitation of at least 1500 mm

stress gradients around porphyroclasts palaeopiezometric estimates and numerical modelling abstract based on strain gradients commonly found around porphyroclasts the mechanical interaction of porphyroclasts and recrystallized mylonite matrix in a retrograde amphibolite facies shear zone of the ivrea verbano zone italy was investigated differential stresses inferred from recrystallized grain size and dislocation density indicate stress concentrations at the porphyroclast matrix interface of about 1 7 2 0 with respect to the average in the matrix the palaeopiezometric stress data are compared to the results of a finite element model composed of two rheologically different phases in this model the local distribution and concentration of stresses around porphyroclasts mostly depend on clast matrix coupling and the effective viscosity contrast of the two phases the bulk strength and work hardening of the composite system increases with interface coupling both the modified stress field and the reduced average grain size around porphyroclasts may result locally in instabilities and promote strain localization and a change in the dominant deformation mechanism

an analytical solution in 2d for the motion of rigid elliptical particles with a slipping interface under a general deformation a mathematical model for rigid inclusions with a slipping interface immersed in a general 2d homogeneous deformation is developed under bulk pure shear inclusions are expected to rapidly approach the stretching axis when compared to the behaviour of inclusions with no slip at the interface the derived model predicts synthetic and antithetic motion into a stable orientation under simple shear and thereafter the inclusion makes an antithetic angle with the shear direction under simple shear rotation rates can be higher or lower than those of no slip inclusions depending on orientation a direct relationship between object inclination to the shear direction and the vorticity of the bulk flow is predicted the model compares well with published analogue and numerical experiments

computer modelling of the water resurge at a marine impact the lockne crater sweden abstract when a cosmic impact occurs at sea the resulting crater can form completely in the water mass or extend into the seafloor at intermediate to great water depths the collapse of the water cavity generates a forceful resurge of water along the seafloor towards the central part of the crater the erosive effects as well as certain conspicuous deposits from resurge flows are known from several craters this study uses numerical modelling to estimate the magnitude of the resurge in the case of the 455 myr old marine target lockne impact crater sweden the water depth at the lockne target site was at least 200 m possibly even as great as 1 km field evidence show that the resurge was strong enough to erode gullies hundreds of metres wide tens of metres deep and kilometres long with computer modelling we studied the behaviour of the flow to get a notion of its magnitude the calculations required simplification of the reality we used a fixed seafloor topography and pure water a rim in the water mass is expected but was neglected in the calculations the simplification likely led to underestimation of the magnitude of the resurge the effect of different target water depths was studied at 200 m target water depth about 1 2 10 11 m 3 of water were needed to fill the lockne crater including the water cavity this took about 2200 s it has been suggested that the main gully formation occurs when the resurge passes the inner rim of the crater that phase lasts for about 700 s with a 500 m target water depth these processes are 3 4 times faster the erosive force of the flow i e unit stream power is 1 9 10 5 wm 2 in the 200 m water depth model this is of the same magnitude as the lake missoula flood the greatest catastrophic flooding known on earth at 500 m target water the unit stream power is a magnitude greater

using microstructures and titaniq thermobarometry of quartz sheared around garnet porphyroclasts to evaluate microstructural evolution and constrain an alpine fault zone geotherm abstract interpretations of deformation processes within ductile shear zones are often based on the characterisation of microstructures preserved in exhumed rocks however exhumed microstructures provide only a snapshot of the closing stages of deformation and we need ways of understanding how microstructures change through time and at what rate this occurs to address this problem we study optical microstructures and electron backscatter diffraction ebsd data from samples of quartz layers deflected around garnet porphyroclasts which generate local stress and strain rate perturbations during mylonitic deformation in the alpine fault zone of new zealand during shearing around rigid garnet porphyroclasts quartz undergoes grain size reduction in response to locally increased stresses while c axes reveal increasing components of rhomb and prism slip reflecting a local increase in shear strain and strain rate titaniq thermobarometry and quartz microstructures suggest a rather narrow range of recorded quartz deformation temperatures around 450 500 c which we propose reflects the cessation of grain boundary migration driven deformation given that temperatures well above the brittle ductile transition for quartz 350 c are preserved we anticipate that rapid cooling and exhumation must have occurred from the 500 c isotherm ultimately we propose a modified geotherm for the central alpine fault zone hanging wall which raises the 500 c isotherm to 11 km depth near the brittle ductile transition our updated alpine fault zone geotherm implies a hotter and weaker middle to lower crust than previously proposed

improving flow laws and piezometers for quartz and feldspar aggregates we have modified the molten salt cell msc in order to measure the flow stress of quartz and feldspar ag gregates deforming by dislocation creep the flow stress of cararra marble measured in the msc is within 5 mpa of that measured in a gas apparatus quartz and feldspar aggregates deformed at 900o 1100oc and confining pressures of 1000 1500 mpa in copper and calcite solid confining pressure media are about twice as strong as those deformed in the msc whereas quartz aggregates deformed in talc and solid nac1 have about the same strength as that measured in the msc these results suggest that the stress exponent in the flow law is larger and the dynamically recrystalized grain size is more sensitive to stress than previously reported acking rings stainless steel

rotation of porphyroblasts in non coaxial deformation insights from computer simulations porphyroblast inclusion trails have the potential to provide critical information about tectonometamorphic events recently however traditional interpretations of inclusion trails have been called into question by the suggestions that porphyroblasts do not rotate during non coaxial deformation and that apparent spiral inclusion trails can be generated in coaxial deformation we present a new computer model that simulates inclusion trail development model results suggest 1 that the extent of porphyroblast rotation is controlled by conditions at the porphyroblast matrix boundary 2 that curved inclusion trails may develop in unrotated porphyroblasts 3 that classic snowball inclusion trails are most simply explained by rotational growth histories and 4 that some of the observations used to support the view that porphyroblasts do not rotate e g weakly sigmoidal inclusion trails apparent truncations of inclusion trails can be accounted for by variations in the growth rate of rotating porphyroblasts

characterisation of deformation and flow mechanics around porphyroclasts in a calcite marble ultramylonite by means of ebsd analysis abstract this paper studies the flow heterogeneity around porphyroclasts associated with greenschist facies deformation of a calcite marble shear zone microstructural data from electron backscatter diffraction analyses ebsd are used to constrain the flow mechanics of this dominantly non coaxial type of deformation the microstructure of the undisturbed ultramylonite grain size range 5 100 m mean 40 m is interpreted to represent steady state time independent flow conditions with flow planes parallel to the shear zone boundary single calcite porphyroclasts grain size 1 3 mm caused flow perturbation in the fine grained marble ultramylonite it is the shape in particular of these rigid porphyroclasts that controls their rotational behaviour during deformation and therefore the development of specific flow fabrics the flow planes around elongated rhomboidal stable porphyroclasts change the orientation to become roughly parallel to the porphyroclast margin whereas the geometry of flow planes around nearly equant rotating porphyroclasts describes a type flow pattern we infer that to some extent decoupling at the clast matrix interface has occurred to guarantee a stable orientation of elongated porphyroclasts but was not sufficient to reduce the rotation rate of equant clasts to zero according to the flow deflection the general crystallographic preferred orientation cpo with its single c axis maximum perpendicular to the flow plane is rotated about an axis which is sub parallel to the kinematic rotation axis of the shear zone ultramylonite microstructures cpos and misorientation data are best explained by the dual operation of grain size insensitive dislocation creep with recovery and recrystallization and grain size sensitive diffusion creep mechanisms the limited grain size reduction around porphyroclasts suggests that the grain size insensitive mechanisms controlled rheology

the influence of stress history on the grain size and microstructure of experimentally deformed quartzite deformation of middle crustal shear zones likely varies with time as a result of the stress build up and release associated with earthquakes and post seismic deformation but the processes involved and their microstructural signature in the rock record are poorly understood we conducted a series of experiments on quartzite at 900 c to characterize microstructures associated with changes in stress and strain rate and to investigate the feasibility of carrying out grain size piezometry in natural rocks that experienced analogous changes differential stress referred to simply as stress was varied in two stage experiments by changing strain rate and by stopping the motor and allowing stress to relax the two stage samples preserve a microstructural record that can be interpreted quantitatively in terms of stress history the microstructure associated with a stress increase is a bimodal distribution of recrystallized grain sizes the smaller grains associated with the second deformation stage accurately record the stress of the second stage and the surviving coarse grains remain similar in size to those formed during the earlier stage the transient microstructure associated with stress decrease is a partial foam texture containing a larger concentration of stable 120 triple junctions than occur in samples deformed at a relatively constant strain rate our results indicate that microstructures preserved in rocks that experienced relatively simple two stage deformation histories can be used to quantitatively assess stress histories grain growth rates during deformation are similar to rates observed in previous isostatic growth experiments supporting theoretical approaches to recrystallized grain size such as the wattmeter theory austin and evans 2007 that incorporate static growth rates from an analysis of the experimental data for quartz recrystallized grain size we find 1 recrystallized grain size quickly reaches a value consistent with ambient deformation conditions we argue that this explains a good match between average grain sizes predicted by the wattmeter after complete recrystallization and the recrystallized grain sizes of the experiments 2 the present formulation of the wattmeter overestimates the rates at which porphyroclasts recrystallize by as much as an order of magnitude and 3 owing to problems with extrapolation of grain growth data for quartz the wattmeter is not presently applicable to natural samples deformed at low temperatures we present a simplified flow law for quartz and suggest that the change in slope of the quartz piezometer at high stress regime 1 is related to a switch to a linear viscous rheology

structural analysis of flow related textures in lavas abstract the textures of coherent volcanic rocks including lavas and volcanic intrusives commonly contain features that are attributed to flowage previous applications of structural analysis to volcanic rocks are expanded here to provide a framework for analysis textures defined as the crystallinity granularity and shapes and arrangements of the components crystals glass and voids of a rock together with structures defined as individual features composed of the disposition attitude arrangement or relative positions of the components of a rock are first described second the spatial fabrics shapes and arrangements of the components of a rock and the orientation of textures and structures are identified third textures structures and fabrics are placed in the spatial and temporal geological context finally detailed interpretations of the kinematics and rheology of structures and fabrics is made leading to an integrated flow history of the rock illustrative case studies include rhyolite from the basal part of the tertiary minyon falls dome northeastern new south wales australia which has a texture comprising planar domains of differing crystal abundance flow bands multiple folds of these domains relatively homogeneous crystal alignment parallel to the fold axes and microfolding of these domains in the zone of interaction between phenocrysts and matrix including retrorotation of phenocrysts on short limbs of inequant folds trachyte dykes on fraser island queensland australia have a texture comprising crystal alignment planar concentration domains banding two planar domains of crystal alignment interpreted to be conjugate shear zones phenocrysts influence the spacing and distribution of the domains and interacted with shear zones by undergoing small amounts of rotation the shear zones overprinted a homogeneous crystal alignment during the last stage of flow before solidification as a result of dilatant granular interactions lava from miocene submarine banded silicic mafic domes on katsurajima on the coast of the shimane peninsula southwestern japan has a texture comprising crystal alignment irregular domains interpreted as the result of quenching local internal deformation of crystals including brittle boudinage and elongate vesicles and amygdales dacite from the late stages of the 1991 1995 eruption of mount unzen in western kyushu japan has a texture comprising crystal alignment deformation of phenocrysts and their surrounding matrix producing voids by mechanical means with the possible involvement of vapour phases the pervasive mechanical failure of the flowing magma may have triggered frontal collapse and the ensuing pyroclastic eruptions a spectrum of increasingly intense flow related textures can be recognised from simple planar alignments of crystals and vesicles to overprinting effects and domainal textures such as banding and shear zones and deformed domains pervasive ductile tearing and ultimately brittle fracture record flow during the transition from ductile magma to brittle rock the development of these flow related textures is controlled by the interplay of driving forces magma pressure gravity and resisting forces viscosity where driving forces are strong flow related textures are well developed when viscous resistance exceeds the driving forces flow and the development of flow related textures ceases