regional variability in the frequency and magnitude of large explosive volcanic eruptions quantifying the frequency at which volcanic eruptions of different size occurs is impor tant for hazard assessment volcanic records can be used to estimate the recurrence rate of large magnitude eruptions magnitude 4 but recording biases that impact data complete ness complicate analysis to overcome these biases we conceptualize the volcanic record as a series of individual and unique time series associated by a common behavior thus we approach issues of completeness on a volcano by volcano basis and use a hierarchical bayes ian approach to characterize the common frequency magnitude f m behavior for different groups of volcanoes we identify variations in the f m relationship between different volcano types and between different volcanic arcs by accounting for systematic under recording in the volcanic record we also calculate the global recurrence rates for large magnitude erup tions during the holocene which are similar to previous estimates however higher recur rence rates for smaller magnitude events are observed which is a result of our adjustments for data completeness quantifying how the f m relationship varies between different groups of volcanoes provides an opportunity to understand how the tectonic setting in uences f m behavior which is important to quantify long term regional volcanic hazard

calcite textures microstructures and rheological properties of marble mylonites in the bancroft shear zone ontario canada calcite textures in several microstructural domains from the bancroft shear zone reveal a sequence of transitions which indicate contributions from a variety of deformation mechanisms at greenschist facies conditions the coarse marble protolith has undergone low strains accommodated by twinning and grain boundary migration porphyroclastic mylonite has a well developed crystallographic preferred orientation and microstructures indicative of dislocation creep and rotation recrystallization mechanisms the textures are consistent with high temperature deformation experiments where r f and c slip systems were active complete dynamic recrystallization produced equant medium sized grains 50 m with a texture that is nearly random secondary calcite growth observed under cathodoluminescence and variation in 18o value with microstructurc indicate large fluid fluxes during mylonitization fluid assisted grain boundary sliding and a minor component of dislocation creep are the inferred deformation mechanisms finer grained s c mylonite has a well developed shape fabric and crystallographic preferred orientation with a point maximum and great circle girdle attributed to attainment of an easy slip orientation for most grains ultramylonites have homogeneous textures with finc elongate grains 20 30 m mantled by disseminated secondary phases textures in ultramylonites are characterized by single point maxima and great circle girdles the most evolved ultramylonite has a point maximum oblique to the shear plane with a symmetry indicating rotation of the maximum toward the extension direction texture microstructure relations in these mylonites indicate that the competition between deformation mechanisms was highly sensitive to grain size strain and secondary phases in addition to temperature strain rate and differential stress the varied deformation mechanisms and application of experimentally derived constitutive equations indicate significant differences in differential stress and strain rate among microstructural domains these differences reflect strain softening associated with dynamic recrystallization and work hardening associated with dissemination of secondary phases and the transition from grain boundary sliding to dislocation creep

patterns of strain localization in heterogeneous polycrystalline rocks a numerical perspective the spatial and temporal patterns of strain localization in materials with pre existing heterogeneities are investigated via a series of two dimensional numerical models models include i a dynamic feedback process to simulate rheological weakening in response to the transition from non linear flow dislocation creep to linear flow diffusion creep grain boundary sliding and ii a time dependent strengthening process counteracting the weakening process different load bearing framework geometries with 20 weak component are used to evaluate the impact of geometry on the strength of the material and its ability to localize strain into an interconnected weak layer iwl our results highlight that during simple shear if dynamic weakening with or without strengthening feedbacks is present strain is quickly localized into an iwl where an increasing proportion of weak material increases the interconnections between the iwls thereby increasing the anastomosing character of the shear zones we establish that not only bulk strain localization patterns but also their temporal patterns are sensitive to the dominance of the weakening or strengthening process consequently shear zones are dynamic in time and space within a single deformation event hence the pattern of finite strain can be an incomplete representation of the evolution of a shear zone network

the use of multifrequency and polarimetric sir c x sar data in geologic studies of bir safsaf egypt abstract bir safsaf within the hyperarid core of the sahara in the western desert of egypt was recognized following the sir a and sir b missions in the 1980s as one of the key localities in northeast africa where penetration of dry sand by radar signals delineates previously unknown sand buried paleodrainage valleys radar rivers of middle tertiary to quaternary age the bir safsaf area was targeted as a focal point for further research in sand penetration and geologic mapping using the multifrequency and polarimetric sir c x sar sensors analysis of the sir c x sar data from bir safsaf provides important new information on the roles of multiple sar frequency and polarimetry in portraying specific types of geologic units materials and structures mostly hidden from view on the ground and on landsat tm images by a relatively thin but extensive blanket of blow sand basement rock units granitoids and gneisses and the fractures associated with them at bir safsaf are shown here for the first time to be clearly delineated using c and l band sar images the detectability of most geologic features is dependent primarily on radar frequency as shown for wind erosion patterns in bedrock at x band 3 cm wavelength and for geologic units and sand and clay filled fractures in weathered crystalline basement rocks at c band 6 cm and l band 24 cm by contrast quaternary paleodrainage channels are detectable at all three radar frequencies owing among other things to an usually thin cover of blow sand the sir c x sar data investigated to date enable us to make specific recommendations about the utility of certain radar sensor configurations for geologic and paleoenvironmental reconnaissance in desert regions

microwave remote sensing of physically buried objects in the negev desert implications for subsurface martian exploration 1 we report the remote detection of a physically buried specular reflecting object using microwave radar at ashalim in the northern region of the negev desert israel such detection provides an important terrestrial analogy for the potential detection of specularly reflecting subsurfaces under the desiccated regolith on mars such as ground ice and liquid water at ashalim a scatterometer operating in the p band 441 mhz 68 cm was mounted on a cherry picker truck at a height of 8 m and used to detect two triangular aluminum mesh reflectors forming a one meter square area reflector buried down to a depth of 8 cm in dry sand the decrease in measured backscattered power with increasing burial depth of the reflector is clearly evident the experimental results compare well with a theoretical model that incorporates radar absorption effects arising in the sandy subsurface layer and radar interference effects arising from phase differences between reflections from the surface and buried reflector this experiment and the associated modeling approach is the first of a series of planned experiments which we outline for the detection and the theoretical evaluation of buried reflectors using remote microwave and vhf radar we identify many potential subject areas for subsurface remote sensing in the martian domain particularly groundwater ground ice

physics controlled thickness of shear zones caused by viscous heating implications for crustal shear localization we evaluate the parameters that control the thickness of ductile shear zones that are generated by viscous heating we employ two dimensional thermomechanical numerical models to simulate shear zone development under compression results show that the shear zone thickness is essentially independent on the numerical resolution and the initial size of a weak inclusion that triggers shear localization a simple scaling law is derived which predicts the thickness with six physical parameters far field stress and strain rate thermal conductivity both constant and temperature dependent initial temperature activation energy and stress exponent the scaling law is confirmed by numerical simulations for a wide range of parameters for crustal deformation conditions typical temperature increase ranges between 50 c and 200 c and the predicted thickness is between 1 km and 7 km these thicknesses agree with natural crustal and lithospheric shear zones suggesting that shear heating is a process controlling crustal shear zone formation

strain localization in ductile rocks a comparison of natural and simulated pinch and swell structures we study pinch and swell structures in order to uncover the onset of strain localization and the change of deformation mechanisms in layered ductile rocks to this end boudinaged monomineralic veins embedded in an ultramylonitic matrix are analyzed quantitatively the swells are built up by relatively undeformed original calcite grains showing twinning and minor subgrain rotation recrystallization sgr combined with progressive formation of high angle misorientations between grains indicative of sgr severe grain size reduction defines the transition to the pinches accordingly dynamically recrystallized grains have a strong crystallographic preferred orientation cpo toward the necks further grain size reduction increasingly random misorientations nucleation of new grains and a loss of the cpo occur we postulate that this microstructure marks the transition from dislocation to diffusion creep induced by strain localization we confirm that the development of boudins is insensitive to original grain sizes and single crystal orientations in order to test these microstructural interpretations a self consistent numerical grain size evolution is implemented based on thermo mechanical principles end member flow laws and microphysical processes applying constant velocity and isothermal boundary conditions to a 3 layer finite element pure shear box pinch and swell structures emerge out of the homogeneous layer through grain size softening at a critical state viscosity weakening due to elevated strain rates and dissipated heat from grain size reduction promotes strain rate weakening until a critical grain size is reached at this point a switch from dislocation to diffusion creep occurs this state locks in at local steady states and is microstructurally expressed in pinches and swells respectively thus boudinage is identified as an energy attractor identifying the high energy steady state of an extending layered structure we conclude from the similarity between natural observations and numerical results that critical deformation conditions of ductile creep can be derived for the surrounding highly strained host rock matrix

earthquakes produce carbon dioxide in crustal faults abstract fourier transform infrared ftir microanalysis of pseudotachylytes i e friction induced melts produced by seismic slip from the nojima fault japan reveals that earthquakes almost instantaneously expel 99 wt of the wall rock co 2 content carbon is exsolved because it is supersaturated in the friction melts by extrapolation to a crustal scale fault rupture large events such as the m7 2 kobe earthquake 1995 may yield a total production of 1 8 to 3 4 10 3 tons co 2 within a few seconds this extraordinary release of co 2 can cause a flash fluid pressure increase in the fault plane and therefore enhance earthquake slip or trigger aftershocks it may also explain the anomalous discharge of carbon monitored in nearby fault springs after large earthquakes because carbon saturation in silicate melts is pressure dependent ftir can be used as a new tool to constrain the maximum depth of pseudotachylyte formation in exhumed faults

early views of the martian surface from the mars orbiter camera of mars global surveyor high resolution images of the martian surface at scales of a few meters show ubiquitous erosional and depositional eolian landforms dunes sandsheets and drifts are prevalent and exhibit a range of morphology composition inferred from albedo and age as seen in occurrences of different dune orientations at the same location steep walls of topographic depressions such as canyons valleys and impact craters show the martian crust to be stratified at scales of a few tens of meters the south polar layered terrain and superposed permanent ice cap display diverse surface textures that may reflect the complex interplay of volatile and non volatile components low resolution regional views of the planet provide synoptic observations of polar cap retreat condensate clouds and the lifecycle of local and regional dust storms

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

cooling induced strain localization in carbonate mylonites within a large scale shear zone glarus thrust switzerland abstract the glarus thrust switzerland offers a great field example of strain localization under retrograde conditions along the thrust mylonitic microfabrics are characterized by a temperature stress controlled balance of grain size reducing mechanisms and grain growth consequently mean grain sizes decrease along the thrust with decreasing metamorphic conditions and towards the thrust contact in an opposite manner calcite twin densities increase towards the contact cpos are strongest between 0 5 and 15 m away from the thrust but become generally weaker in the last centimeters where also cataclasites occur the cpo weakening and grain size reduction towards the shear zone point to a change from predominant dislocation creep to granular flow finally ending in cataclastic deformation the microfabric changes correlate with a decrease in 13c and 18o towards the thrust contact indicating the presence of fluids during cycles of brittle and plastic deformation based on the microfabric changes variations in stable isotopes and cross cut relationships a subdivision into a low and high temperature shear zone can be made the microfabric modifications resulted from changes in deformation conditions due to ongoing thrusting and exhumation induced cooling promoting further strain localization of an existing high strain shear zone

biomarkers heat up during earthquakes new evidence of seismic slip in the rock record during earthquakes faults heat up due to frictional work however evidence of heating from paleoearthquakes along exhumed faults remains scarce here we describe a method using thermal maturation of organic molecules in sedimentary rock to determine whether a fault has experienced differential heating compared to surrounding rocks we demonstrate the utility of this method on an ancient pseudotachylyte hosting megathrust at pasagshak point alaska measurements of the ratio of thermally stable to thermally unstable compounds diamondoids n alkanes show that the melt bearing rocks have higher thermal maturity than surrounding rocks furthermore the mineralogy of the survivor grains and the presence of any organic molecules allow us to constrain the temperature rise during the ancient earthquakes to 840 1170 c above ambient temperatures of 260 c from this temperature rise we estimate that the frictional work of the earthquake was 105 228 mj m 2 using experimental friction measurements as a constraint we estimate that the minimum slip necessary for heating was 1 8 m this paper demonstrates that biomarkers will be a useful tool to identify seismic slip along faults without frictional melt

simple shear experiments on calcite rocks rheology and microfabric ideal simple shearing of carrara marble and solnhofen limestone was achieved by inserting calcite rock slices into sandstone cylinders precut at 35 the experiments covered a wide range in temperatures 25 900 c at confining pressures of 200 and 250 mpa and at strain rates between 10 3 and 10 5 s 1 shear strains of up 2 7 were achieved the stress strain data for simple shear favourably compare with predictions on the basis of existing flow laws established in coaxial testing this comparison demonstrates that the simple shear experiments covered most of the previously established flow regimes for calcite rocks at the highest temperatures carrara marble exhibited substantial work softening after high shear strains a complete texture analysis was carried out with the u stage and a texture goniometer the following four distinct microfabric regimes were found 1 twinning regime an oblique c axis maximum deflected from the pole to the flattening plane towards 1 and against the imposed sense of shear results directly from e twinning extensive twinning some grains are completely twinned rapidly rotates the c axes into near parallelism with 1 this regime was observed for carrara marble deformed at temperatures below 700 c and for solnhofen limestone at room temperature 2 intracrystalline slip regime three c axis maxima were observed in both rock types at more elevated temperatures and in the absence of twinning these c axis orientations result from an alignment of slip planes and slip directions into parallelism with the shear zone boundary and the shear direction respectively this finding supports the idea of an orientation for easy slip resulting from simple shearing in addition to r and f glide basal glide is inferred to have been operative 3 grain boundary sliding regime a great circle distribution of c axes approximately perpendicular to the direction of finite shortening is observed this texture is weaker compared to the other regimes it is interpreted to result from minor contributions of f slip towards total strain this regime is restricted to fine grained solnhofen limestone deformed at the highest temperatures 4 grain boundary migration regime only in carrara marble deformed at the highest temperatures is there substantial grain boundary migration recrystallization the texture consists of a dominant c axis maximum oriented perpendicular to the shear zone boundary this single maximum corresponds to an orientation for easy slip for basal slip grain boundary migration assists rapid lattice reorientation and leads to geometrical work softening quantitative analysis of grain boundary surface orientations gave the following results within the twinning and the intracrystalline slip regimes the characteristic grain shape is elliptical for both rock types the strain ellipses derived from this analysis closely correspond to the known finite strain at the lowest temperatures at intermediate temperatures and especially in carrara marble there is a consistent trend for the aspect ratios to indicate too little flattening and for the angles between the long axis of the strain ellipse and the shear zone boundary to be too small in the absence of grain boundary migration recrystallization this finding indicates minor translations of grains with respect to one another grain boundary sliding or translations accommodated in the mantle regions of grains at the highest temperatures where extensive grain boundary migration occurs in carrara marble grain boundary migration regime two preferred orientations of grain boundaries are observed 1 parallel to the shear zone boundary and 2 parallel to an oblique direction deflected 20 away from the normal to the shear zone boundary in the sense of the imposed shear the same pattern of grain boundary alignment is observed in solnhofen limestone deformed in the grain boundary sliding regime the geological applications of this experimental work are discussed in terms of calcite tectonites in particular and sheared rocks in general

the dynamics of multiscale multiphysics faults part ii episodic stick slip can turn the jelly sandwich into a creme brulee abstract incorporating coupled thermal hydro mechanical chemical thmc processes in lithospheric deformation models is a research frontier in the study of lithosphere dynamics in this study we present a fundamental theoretical analysis on the important lithosphere deformation mechanism of creep enhanced by fluid release reactions this mechanism features a surprisingly rich dynamics stemming from the feedback between deformation induced fluid release through mineral breakdown reactions dissolution and fluid cementation into a solid matrix precipitation when the tectonic forces are locally relaxed we show that the thermal mechanical feedback processes in the temperature and pressure evolution equations and the resulting feedback between the fluid flow and mechanical deformation result in a highly dynamic system the feedbacks lead to an oscillatory response resulting in a stick or slow creep slip or fast slip phenomenon the inferred oscillatory dissolution precipitation reaction can explain the episodic tremor and slip ets phenomenon frequently encountered in subduction zones and commonly attributed to the dehydration precipitation reaction of serpentine minerals in the fault zone our study reveals a complex behaviour linking short time scale pulses of deformation to long term geodynamic deformation we present a new tool to derive a systemic map of the presence of oscillation in a wide parameter space by upscaling the mechanics over several cycles we show here that faulted rocks with large strength can indeed behave in an equivalent manner to weaker intact rocks and lubricate important geodynamic processes we postulate a series of verifiable predictions with the potential to link a large range of geophysical observables of short and long time scale geodynamic processes with joint geological hydrological and geochemical studies

the yellowstone magmatic system from the mantle plume to the upper crust the yellowstone supervolcano is one of the largest active continental silicic volcanic fields in the world an understanding of its properties is key to enhancing our knowledge of volcanic mechanisms and corresponding risk using a joint local and teleseismic earthquake p wave seismic inversion we revealed a basaltic lower crustal magma body that provides a magmatic link between the yellowstone mantle plume and the previously imaged upper crustal magma reservoir this lower crustal magma body has a volume of 46 000 cubic kilometers 4 5 times that of the upper crustal magma reservoir and contains a melt fraction of 2 these estimates are critical to understanding the evolution of bimodal basaltic rhyolitic volcanism explaining the magnitude of co2 discharge and constraining dynamic models of the magmatic system for volcanic hazard assessment

thermal decomposition of carbonates in fault zones slip weakening and temperature limiting effects 1 during an earthquake the heat generated by fault friction may be large enough to activate the devolatilization of minerals forming the fault rocks in this paper we model the mechanical effects of calcite thermal decomposition on the slip behavior of a fault zone during an earthquake to do so we introduce the coupled effects of calcite volume loss heat consumption and co2 production in the theoretical analysis of shear heating and thermal pressurization of pore fluids we consider a rapidly deforming shear band consisting of a fluid saturated carbonate rock the equations that govern the evolution of pore pressure and temperature inside the band and the mass of emitted co2 are deduced from the mass and energy balance of the multiphase saturated medium and from the kinetics of the chemical decomposition of calcite numerical simulation of seismic slip at depths of 5 to 8 km show that decarbonation has two critical consequences on fault slip first the endothermic reaction of calcite decomposition limits the coseismic temperature increase to less than 800 c corresponding to the initiation of the chemical reaction inside the shear band second the rapid emission of co2 by decarbonation significantly increases the slip weakening effect of thermal pressurization the pore pressure reaches a maximum and then decreases due to the reduction of solid volume causing a restrengthening of shear stress our theoretical study shows on the example of decarbonation that the thermal decomposition of minerals is an important slip weakening process and that a large part of the frictional heat of earthquakes may go into endothermic devolatilization reactions

the normal grain growth behaviour of nominally pure calcitic aggregates the normal grain growth behaviour of four different but all nominally pure calcite powders 99 analytic grade calcite 99 7 chalk 99 97 crushed iceland spar 99 95 chelometric grade calcite has been investigated as a function of temperature 550 600 650 700 c and confining pressure 100 190 mpa under both dry and hydrostatic pfluid ptotal conditions the initial particle size of both the analytic grade and chelometric grade calcite was about 5 m and that of the chalk was about 3 m while the experiments on the iceland spar were conducted on powders of three different initial particle sizes 3 4 7 5 38 5 m on each material at each pressure temperature condition 6 to 15 experiments equally spaced in log time from 15 minutes to 50 days were conducted under dry conditions all four materials recrystallized to aggregates which contained less than 2 porosity and which had a grain size of between 4 and 20 m depending on the initial particle size subsequently the aggregates coarsened by normal grain growth with the kinetics of the growth process being controlled by the rate at which the grain boundaries could drag the residual pores with them as they migrated under nominally identical conditions both the mechanism and rates of pore drag differed greatly for the different materials implying that this process is highly sensitive to trace solute impurity concentrations this sensitivity renders the task of providing a systematic account of dry calcite grain growth kinetics highly problematic under hydrostatic conditions all the powders followed the same normal grain growth kinetics in which the growth process was rate controlled by diffusion through the pore fluid on the grain boundaries an activation enthalpy of 162 6 kj mol 1 and an activation volume of 34 35 cm3 mol 1 was obtained for this process

fluid flow pathways along the glarus overthrust derived from stable and sr isotope patterns the glarus thrust of the eastern helvetic alps has been proposed as a major pathway for metamorphic fluids expelled from the footwall during alpine deformation and prograde metamorphism the stable isotope composition of calcite in a thin continuous calc mylonite layer and gradients into the overlying verrucano siltstones and underlying flysch or carbonate are analyzed in detail a regional scale map of the 18 o composition of the calc mylonite covering 25 kilometers east west by 15 kilometers north south is interpolated from 35 sampling sites with a total of over 700 samples this map reveals a steep northward increase in 18 o from 11 to 18 permil smow interpreted as an isotopic front within the southernmost 6 kilometers and leveling out at about 20 permil farther north vertical profiles across the sharp thrust contact show significantly different isotopic fronts in the south where the verrucano formation is thrust upon marine carbonates and in northern sites where it rests on tertiary flysch southern sites display steep continuous isotopic gradients over about 1 meter above the thrust where both 18 o and 13 c decrease to background verrucano formation values of 10 to 11 permil 18 o smow and 6 permil 13 c pdb respectively footwall carbonates are significantly depleted from their original value of about 26 permil within a zone of less than 10 meters below the thrust contact the front geometry of 13 c increasing from 2t o2 permil downward within about 1 meter is much steeper than the one defined by 18 o northern sites in contrast display distended isotopic alteration fronts upward into the verrucano hangingwall where higher than background values in 18 o and 13 c are found up to 15 meters and 6 meters above the thrust contact respectively no isotopic alterations could be detected within the footwall flysch up to the thrust contact the 87 sr 86 sr systematics has been observed on the centimeter to millimeter scale in vertical profiles across the calc mylonite in three southern sites in each sample slab a topmost zone of slightly enriched homogeneous 87 sr 86 sr values 0 709 to 0 712 is found within a zone of highly sheared and dynamically recrystallized yellow veins alternating with dark stylolite seams on the sub millimeter scale in two slabs smooth vertical downward gradients to marine carbonate values 0 708 could be observed on the centimeter scale regional and local isotopic gradients are explained as exchange between the different rock reservoirs through the advection diffusion dispersion of fluids interact ing in a regime of brittle ductile deformation associated with 30 kilometers north ward thrust translation different scenarios of fluid flow along and or across the thrust plane are modeled using transport theory coupled with isotopic exchange reactions at an assumed temperature of 300 c considerable amounts of externally derived 18 o depleted basement derived fluids are required to explain the isotopic characteristics in the southern part of the thrust a time integrated fluid flux tiff of 4500 to 9100 m 3 m 2 is calculated for the channelized flow component along the thrust far exceeding any cross thrust compo nent of downward fluid infiltration of 3 45 to 5 7 m 3 m 2 in northern sites however any potential thrust parallel fluid flow is obscured by a dominant component of upward cross thrust flow of fluids derived from compaction dehydration of flysch units in the immediate footwall these calcite saturated fluids left their imprint in the

brittle ductile deformation in the glarus thrust lochseiten lk calc mylonite the glarus thrust accommodated at least 30 km of northward displacement strongly localized within a 1 m layer of lochseiten lk calc mylonite this layer displays veins in various states of plastic deformation and a wildly refolded foliated gouge texture lattice and shape preferred orientations are observed within the fine grained recrystallized matrix these features indicate the alternate activity of brittle and ductile deformation mechanisms in contrast to the classical view that grain boundary sliding superplasticity is the dominant deformation mechanism it is advocated that fluids derived from the footwall and expelled along the thrust are responsible for hydrofracturing and cataclastic deformation in periods between fracture events deformation was ductile in this new interpretation a substantial amount of the total thrust displacement was accommodated by numerous short lived and strongly localized fracture events at the base of the verrucano thrust sheet rather than a permanently weak decollement lithology

thermal decomposition along natural carbonate faults during earthquakes earthquake slip is facilitated by a number of thermally activated physicochemical processes that are triggered by temperature rise during fast fault motion i e frictional heating most of our knowledge on these processes is derived from theoretical and experimental studies however additional information can be provided by direct observation of ancient faults exposed at the earth s surface although fault rock indicators of earthquake processes along ancient faults have been inferred the only unambiguous and rare evidence of seismic sliding from natural faults is solidified friction melts or pseudotachylytes here we document a gamut of natural fault rocks produced by thermally activated processes during earthquake slip these processes occurred at 2 3 km depth along a thin 0 3 1 0 mm principal slip zone of a regional thrust fault that accommodated several kilometers of displacement in the slip zone composed of ultrafine grained fault rocks made of calcite and minor clays we observe the presence of relict calcite and clay numerous vesicles poorly crystalline amorphous phases and newly formed calcite skeletal crystals these observations indicate that during earthquake rupture frictional heating induced calcite decarbonation and phyllosilicate dehydration these microstructures may be diagnostic for recognizing ancient earthquakes along exhumed faults

the effect of dissolved magnesium on diffusion creep in calcite we experimentally tested a series of synthetic calcite marbles with varying amounts of dissolved magnesium in a standard triaxial deformation machine at 300 mpa confining pressure temperatures between 700 and 850 c stresses between 2 and 100 mpa and strain rates between 10 7 and 10 3 s 1 the samples were fabricated by hot isostatic pressing of a mixture of calcite and dolomite at 850 c and 300 mpa the fabrication protocol resulted in a homogeneous fine grained high magnesian calcite aggregate with minimal porosity and with magnesium contents between 0 07 and 0 17 mol mgco3 at stresses below 40 mpa the samples deformed with linear viscosity that depended inversely on grain size to the 3 26 0 51 power suggesting that the mechanisms of deformation were some combination of grain boundary diffusion and grain boundary sliding because small grain sizes tended to occur in the high magnesium calcite the strength also appeared to vary inversely with magnesium content however the strength at constant grain size does not depend on the amount of dissolved magnesium and thus the impurity effect seems to be indirect at stresses higher than 40 mpa the aggregates become non linearly viscous a regime we interpret to be dislocation creep the transition between the two regimes depends on grain size as expected the activation energy for diffusion creep is 200 30 kj mol and is quite similar to previous measurements in natural and synthetic marbles deformed at similar conditions with no added magnesium

signature of coseismic decarbonation in dolomitic fault rocks of the naukluft thrust namibia unequivocal geological signatures of seismic slip are rare exceptionally so in carbonate hosted faults where carbonate minerals dissociate at temperatures lower than those required for producing a friction melt this thermal dissociation leads to significant fault weakening by increased fluid pressure and or nanoparticle lubrication preventing further heating of the fault surface pseudotachylyte is therefore unlikely to form in carbonate hosted faults and other evidence for seismic slip must be identified we studied the lower cambrian naukluft thrust which crops out in central namibia it contains a cataclastic dolomite fault rock referred to as gritty dolomite which we interpret as a signature of coseismic carbonate dissociation and subsequent fluid rock interactions the fault was active at ambient temperatures below 2001c gritty dolomite contains rounded low aspect ratio dolomite clasts with a uniform fe rich dolomite coating euhedral to subhedral magnetite quartz and k feldspar in a fine grained massive to laminated carbonate matrix of particulate dolomite and crystalline calcite cement the fault rock texture combined with evidence of injectites of gritty dolomite into the wallrock indicates the cataclasite deformed as a fluidized granular flow at seismic slip velocities frictional heating caused dissociation of dolomite to co 2 and ca fe and mg oxides this release of co2 decreased the ph of the pore fluid in the fault causing dissolution and rounding of dolomite clasts within an inertial grain flow and precipitation of carbonate coatings and euhedral silicates and oxides during subsequent cooling and co2 escape examples of similar rocks having some if not all of these characteristics have been described from other carbonate hosted faults the geological setting of the naukluft thrust is unique in spatial extent and quality of exposure allowing us to eliminate alternative hypotheses for sources of co2 to drive fluidization

subsurface valleys and geoarcheology of the eastern sahara revealed by shuttle radar the shuttle imaging radar sir a carried on the space shuttle columbia in november 1981 penetrated the extremely dry selima sand sheet dunes and drift sand of the eastern sahara revealing previously unknown buried valleys geologic structures and possible stone age occupation sites radar responses from bedrock and gravel surfaces beneath windblown sand several centimeters to possibly meters thick delineate sand and alluvium filled valleys some nearly as wide as the nile valley and perhaps as old as middle tertiary the now vanished major river systems that carved these large valleys probably accomplished most of the erosional stripping of this extraordinarily flat hyperarid region underfit and incised dry wadis many superimposed on the large valleys represent erosion by intermittent running water probably during quaternary pluvials stone age artifacts associated with soils in the alluvium suggest that areas near the wadis may have been sites of early human occupation the presence of old drainage networks beneath the sand sheet provides a geologic explanation for the locations of many playas and present day oases which have been centers of episodic human habitation radar penetration of dry sand and soils varies with the wavelength of the incident signals 24 centimeters for the sir a system incidence angle and the electrical properties of the materials which are largely determined by moisture content the calculated depth of radar penetration of dry sand and granules based on laboratory measurements of the electrical properties of samples from the selima sand sheet is at least 5 meters recent september 1982 field studies in egypt verified sir a signal penetration depths of at least 1 meter in the selima sand sheet and in drift sand and 2 or more meters in sand dunes

crystal growth in cataclasites diagnostic microstructures and implications abstract microstructures in cataclasites from low and high angle fault zones in granitic basement rocks of the scandinavian caledonides indicate the operation of several processes intensive formation of microcracks filled with fine grained mineral fragments indicates brittle deformation the formation of quartz crystals with euhedral terminations suggests crystal growth occurrence of undulose extinction deformation lamellae and grain boundary recrystallization in other quartz grains illustrates plastic deformation crystal growth causes micro cracks to heal and thereby changes the behaviour of rocks undergoing brittle deformation the crystallization products vary in size from c 10 m to c 5 mm narrow banding parallel to rhomb and prism planes is indicative of crystal growth the growth bands show slightly different crystallographic misorientation in some cases the boundaries of these bands could be identified as dislocation arrays decorated by bubbles the occurrence of fibrous chalcedony aggregates and transitions from extremely fine grained quartz to large grains with euhedral terminations suggests the presence of colloidal silica bearing fluids during growth the crystallization process leads to the formation of vein like structures after crystallization plastic deformation may take place followed by brittle deformation and new crystal growth it is concluded that during the formation of the cataclasites a cyclic operation of brittle failure crystal growth followed by plastic deformation took place and this cycle was repeated several times

natural calcite c axis fabrics an alternate interpretation microstructures and crystallographic fabrics in natural calcite mylonites from newfoundland canada are discussed and compared with other natural and experimental examples in the newfoundland rocks porphyroclasts deformed mainly by twinning and intracrystalline slip and the recrystallized grains deformed by dislocation creep assisted by dynamic recrystallization by grain boundary migration t 350 c porphyroclast c axis fabrics consist of a broad point maximum oriented normal to the shear plane recrystallized grains in the ultramylonites have strong c axis fabrics consisting of a single point maximum oriented either parallel or oblique to the normal to the shear plane experimental simple shear calcite c axis fabrics have been divided by previous workers into low lt and high temperature ht fabrics twinning and intracrystalline r slip have been identified as the dominant deformation mechanisms in low temperature experiments whereas intracrystalline r and f slip dominate in high temperature experiments lt fabrics have a single point maximum which is oriented approximately parallel to the principal stress axis and is oblique to the normal to the shear plane ht fabrics have three point maxima which are asymmetrical both in orientation and intensity with respect to the normal to the shear plane the transition between lt and ht fabrics occurs at temperatures lower than 400 c in fine grained limestones and lower than 700 c in marbles at laboratory strain rates natural calcite c axis fabrics in agreement with study almost invariably consist of a point maximum oriented either normal or slightly oblique to the shear plane because their fabrics are similar to experimental lt fabrics they have been interpreted as lt fabrics by comparison with experimental and computer simulated lt fabrics the obliquity of their c axis point maximum with respect to the shear plane normal has been used as a shear sense indicator and as a measure of the degree of non coaxiality of the deformation two lines of evidence mitigate against calcite c axis fabrics as shear sense and strain path indicators in mylonite zones first the obliquity of the c axis point maximum in natural examples is not consistent for a given sense of shear secondly mylonite microstructures reported here and in the literature supports the contention that dynamic recrystallization and intracrystalline slip are responsible for the strong c axis fabrics of most calcite mylonites therefore natural calcite mylonite c axis fabrics do not generally correlate with either lt or ht experimental fabrics instead natural calcite mylonites have similar microstructures and fabrics to marbles that were deformed experimentally at very high temperatures 800 850 c and underwent pervasive dynamic recrystallization by grain boundary migration consequently c axis fabrics of calcite mylonites cannot be used as a measure of the degree of non coaxiality of the deformation nor can they be used as shear sense indicators

microwave remote sensing of physically buried objects in the negev desert implications for environmental research we report remote detections of physically buried specularly reflecting objects using microwave radar at two sites ashalim and tseelim in the northern region of the negev desert israel these detections provide confirmation that microwave subsurface remote sensing is a genuine phenomenon at ashalim a scatterometer operating in the p band 441 mhz 68 cm was mounted on a cherry picker truck at a height of 8 m and used to detect two triangular aluminum mesh reflectors forming a 1 m square area reflector buried down to a depth of 8 cm in dry sand at tseelim the same scatterometer was mounted on an airplane flying at an altitude of 70 m and used to detect 1 m square aluminum reflectors each one submerged at a different location along the airplane flight path buried down to a depth of 20 cm the experimental results compare favorably with a theoretical model that incorporates radar absorption effects arising in the sandy subsurface layer and radar interference effects arising from phase differences between reflections from the surface and buried reflector the theoretical modeling also predicts the detection of a subsurface reflector down to a depth of about 4 4 m this experiment and the associated modeling approach is the first of a series of planned experiments which we outline for the detection and the theoretical evaluation of buried reflectors using remote microwave and vhf radar we identify potential subject areas for environmental research

seismic and hydroacoustic effects of the may 29 2010 submarine south sarigan volcanic explosion energy release and interpretation abstract the may 2010 submarine volcanic crisis of the shallow south sarigan seamount marianas arc ended on may 29 by a violent explosion that emitted a 12 km high atmospheric plume and created a crater 350 m in diameter the application of the multiphase localization method to the pg p t phases of this explosion allows us to refine considerably mainly based on the intense and impulsive t phases the location of their source that fits the position of the newly formed south sarigan crater we highlight the numerous similarities of this final explosion with artificial underwater explosions conventional or nuclear tests of known energy especially the explosive nature of the source in the volcanic basement is confirmed by the similarity of the p phases with those of underground nuclear tests and the application of discriminating criteria ms mb the explosive nature of the source in the water is also confirmedby the application of the identification criteria for hydroacoustic sources and the t phases exceptionally impulsive and of short durations these similarities allow us to provide a rough evaluation of the energy released by the south sarigan final explosion using the methods commonly applied to man generated explosions we estimate a minimal released energy of 1 kt 4 2 1012 j for the seismic effects of the shock and ca 1 t of equivalent tnt i e 4 2 109 j for the hydroacoustic effects of the explosive source in the water the processes leading to this final explosion are discussed on the basis of the sequence of the earlier seismic and eruptive events within the comprehensive nuclear test ban treaty framework this explosion that affected concomitantly the solid liquid and subaerial media is the first well documented one having generated intense waves p hydroacoustic t acoustic infrasounds and tsunamis

pressure solution lithification as a mechanism for the stick slip behavior of faults many major faults including a large fraction of the san andreas appear to be virtually quiescent between great earthquakes the locked sections of the san andreas near san francisco and los angeles have little or no seismic activity on the primary fault trace although secondary faults may be active stick slip behavior on a fault can be explained in terms of a static coefficient of friction which is larger than the dynamic or sliding coefficient in this paper we propose a modification of the friction hypothesis in which chemical lithification cementation occurs on the fault between earthquakes as the stress on the fault increases it becomes large enough to break the cemented bonds between particles causing slip on the fault we treat this problem quantitatively assuming that pressure solution is responsible for the cementation an advantage of this model is that the hydrostatic pressure approaches the lithostatic pressure during cementation so that low fault strengths are predicted

modeling episodic fluid release events in the ductile carbonates of the glarus thrust the exposed glarus thrust displays midcrustal deformation with tens of kilometers of displacement on an ultrathin layer the principal slip zone psz geological observations indicate that this structure resulted from repeated stick slip events in the presence of highly overpressured fluids here we show that the major characteristics of the glarus thrust movement localization periodicity and evidence of pressurized fluids can be reconciled by the coupling of two processes namely shear heating and fluid release by carbonate decomposition during this coupling slow ductile creep deformation raises the temperature through shear heating and ultimately activates the chemical decomposition of carbonates the subsequent release of highly overpressurized fluids forms and lubricates the psz allowing a ductile fault to move tens of kilometers on millimeter thick bands in episodic stick slip events this model identifies carbonate decomposition as a key process for motion on the glarus thrust and explains the source of overpressured fluids accessing the psz

low temperature deformation of limestone isola palmaria northern apennine italy the role of primary textures precursory veins and intracrystalline deformation in localization abstract fine grained limestones of the tuscan nappe from isola palmaria northern apennine italy contain sporadically developed bedding parallel shear zones some of which exhibit well developed calcite mylonites the presence of shear zones that formed at temperatures less than 250 c allows the study of limestone deformation mechanisms active under low temperature upper crustal conditions two materially distinct types of shear zones developed during the syn nappe stage of deformation and have been characterized by integrated field strain microstructural optical microscopy cathodoluminescence transmission electron microscopy and ebsd fabric studies type a shear zones occur within decametre thick packages accommodating heterogeneously distributed strain in very fine grained limestone micrite type b shear zones are localized within calcite veins that develop into recrystallized calc mylonites both types form at the contact with dolomite although this does not play an important role in strain localization in both cases low temperature ductility occurs by extensive dislocation glide sustained by athermal network accommodated recovery although the end state microstructures of both shear zone types cannot be differentiated optically differences do exist in the crystallographic preferred orientations cpo and the grain boundary structure of deformed micrite and vein calcite deformed micrite grain boundaries are rich in voids consistent with extensive fluid flux that can enhance material transport and independent grain displacements while suppressing cpo development the crystallographically defined grain boundaries in recrystallized vein calcite suggest that lower grain boundary diffusivity enables more typical dislocation mediated cpo formation

shuttle imaging radar physical controls on signal penetration and subsurface scattenng in the eastern sahara sir a signal penetration and subsurface backscatter within the upper meter or so of the sediment blanket in the eastern sahara of southern egypt and northern sudan are enhanced both by radar sensor parameters and by the physical and chemical characteristics of eolian and alluvial materials interpretation of sir a images by mccauley et al 1 2 dramatically changed previous concepts of the role that fluvial processes have played over the past 10000 to 30000000 years in shaping this now extremely flat featureless and hyperarid landscape in the present paper we summarize the near surface stratigraphy the electrical properties of materials and the types of radar interfaces found to be responsible for different classes of sir a tonal response the dominant factors related to efficient microwave signal penetration into the sediment blanket include a favorable distribution of particle sizes b extremely low moisture content and c reduced geometric scattering at the sir a frequency 1 3 ghz the depth of signal penetration that results in a recorded backscatter here called radar imaging depth was documented in the field to be a maximum of 1 5 m or 0 25 of the calculated skin depth for the sediment blanket radar imaging depth is estimated to be between 2 and 3 m for active sand dune materials diverse permittivity interfaces and volume scatterers within the shallow subsurface are responsible for most of the observed backscatter not directly attributable to grazing outcrops

mapping exposed and buried drainage systems using remote sensing in the negev desert israel abstract this paper examines the use of optical visible and infrared ir data synthetic aperture radar data sar and ground penetrating radar gpr for detection identification and mapping of exposed and buried channels under sands in the western negev israel and northern sinai egypt field observations and shallow geophysical methods corroborated the observations the halutza agur and shunra sand dunes are the continuation of the northern sinai dunes playa type sediments from the late quaternary are found in upper reaches of dry river channels between the dunes of the northwest negev and represent a deterioration of the drainage system caused by shifting sand dunes a dual polarimetric radar sir c image from srl 1 having two frequencies and two polarizations exhibits meaningful differences in reflectance between the buried and abandoned channels and their surroundings the buried continuation of wadi mobra and the larger nahal nizzana channels under the western shunra sand dunes can be observed in l band radar data these channels are not clearly observed either in the landsat thematic mapper tm visible nor in the near infrared nir images the radar wavelengths and polarizations that improve the contrast between the buried river beds and the surrounding sand mantle are in descending order l hv l hh and c hh however in places where the sand mantle is very thick or where channels cut through bedrock the visible and near infrared data from landsat tm are easier to interpret and tend to be more useful for mapping fluvial patterns

stable and sr isotope evidence for fluid advection during thrusting of the glarus nappe swiss alps at the glarus thrust in the swiss helvetic alps permian verrucano siltstones are allochthonously superimposed over tertiary flysch with an intermediate about 1 metre thin layer of intensively deformed calcmylonite of probable mesozoic provenance the h o c and sr isotope compositions of minerals from the calc mylonite and strongly mylonitized verrucano siltstones were determined in order to assess 1 equilibrium disequilibrium relationships 2 isotopic composition of the fluid phase its provenance and water rock ratios 3 sources of sr in the calc mylonite 4 deformation temperatures the isotopic composition of cale mylonite micro samples from five sites along the thrust varies from 22 to 12 and 2 to 10 for 18o and 13c respectively all samples are 18o depleted by up to 14 relative to the presumed marine helvetic carbonate protoliths 18o 25 4 2 a pronounced geographic trend of 18o depletion from 22 in the north to 12 in the south is observed in calc mylonites 87sr 86sr ratios range from typical mesozoic marine carbonate protolith signatures 0 708 0 005 to more radiogenic values as high as 0 722 a variable contribution of radiogenic 87sr to the calc mylonite is though to reflect interaction with fluids that aquired their sr from the hercynian granitic basement chlorites and muscovites from the calc mylonite and verrucano have uniform 18o values but display d values from 40 to 147 the d enriched values correspond to the primary metamorphic or formational fluids expelled during thrusting whereas the d depleted samples reflect selective h isotope exchange with meteoric fluids during uplift of the alpine belt the isotopic composition of the calc mylonites requires exchange with 18o depleted 87sr enriched fluids at very high water rock ratios possible sources for these are dewatering of the underlying flysch and or metamorphic fluids or formation brines expelled along the thrust from greater depth these could be derived from compaction and dewatering of the flysch in the northern part of the thrust in the south however where verrucano is thrust over 18o rich mesozoic carbonates the extreme 18o depletion of the calc mylonite has to be explained either by fluid advection within the verrucano hanging wall and thrust zone or alternatively by exchange with metamorphic fluids from greater depth expelled along the thrust microstructural evidence abundant veins stylolites breccias suggests that fluids played an important role in deformation and strain localization excepting albite all major components quartz chlorite muscovite calcite are both dynamically recrystallized and crystallized as secondary minerals in pressure shadows and syn mylonitic veins indicating that these minerals were potentially open to oxygen isotopic exchange during alpine metamorphism and thrust deformation within the mylonitized verrucano silstones isolated quartz chlorite and quartz calcite fractionations yield temperatures of around 320 c close to values obtained from calcite dolomite thermometry 355 c 30 and in agreement with the regional lower greenschist facies metamorphism quartz calcite and quartz albite fractionations indicate slightly lower temperatures around 250 c owing to selective lower temperature re equilibration of the calcite and albite during post peak metamorphism

kinematics of rock flow and the interpretation of geological structures with particular reference to shear zones abstract current interpretations of structures are generally based on homogeneous and steady deformation models despite the fact that both the heterogeneity of rocks materially rheologically and geometrically and the time dependence of imposed geological conditions give rise to significant heterogeneous and non steady flow in concert with field observations we emphasize that the expectation of heterogeneity and non steadiness is the key to understanding natural deformation and that in order to carry out successful structural analysis and tectonic interpretation it is necessary to recognize the first order distinction between imposed boundary conditions typically used to define the tectonic regime e g transcurrent transpression and the response recorded by rocks within the zone structures and fabrics using s c fabric as an example it is demonstrated how flow with a non zero spinning component resulting from the rheological contrasts and or geologically realistic time dependent boundary displacement can drastically change the ideal geometric and kinematic relations between the fabric and the host zone in agreement with both theoretical analysis and field observation it is shown that natural flow regimes range from pure shear to pure rotation including super simple shear in consideration of the heterogeneity and non steadiness of natural deformation kinematic analysis is justifiable only within a homogeneous domain and steady period flow kinematics and mechanisms are interrelated in that firstly mechanisms provide internal constraints on kinematics ensuring that only certain flows are possible and secondly flow kinematics will favour development of certain mechanisms

shear zone and nappe formation by thermal softening related stress and temperature evolution and application to the alps we present the results of two dimensional thermo mechanical numerical simulations of the formation of kilometre scale shear zones and tectonic nappes which are generated by thermal softening during horizontal shortening this model is thermo mechanically coupled and conserves energy by converting mechanical work into heat the results show that in homogeneous material the temperature increase in shear zones caused by viscous heating can be 100 c the characteristic width of the thermally perturbed part of the deforming zone is three times larger than the width of the corresponding shear zone because of diffusive heating of the wall rock therefore temperature variations initiated by shear heating may be difficult to identify based on petrological inferences of field data during lithospheric shortening with a bulk rate of 2 5 10 15 s 1 a nappe forms in the crust at a depth of 45 km peak temperatures in the nappe reach 600 c 100 c increase due to shear heating significant shear heating occurs only locally and is not active continuously in time along the entire crustal shear zone peak values of the maximal principal stress locally reach 2 5 gpa in the nappe peak values of temperature and maximal principal stress increase locally to 770 c 300 c increase due to shear heating and 3 8 gpa respectively if the applied bulk shortening rate is 5 10 15 s 1 and if higher effective viscosities for the crust are applied in the model lithosphere high differential stresses 500 mpa and significant stress deviations from the lithostatic pressure 50 occur only locally and are transient the highest average effective viscosity of the model lithosphere is 2 1022 pa s which is in agreement with the estimates for the continental lithosphere a characteristic feature of the presented dynamic nappe model is a significant stress decrease and cooling within a hundred thousand to 2 million years these rates of decompression and cooling are discussed in the context of microstructural and metamorphic observations the results also show that differential stresses in mature shear zones can be several hundred mpa smaller than at the onset of shear zone formation which suggests that estimates based on microstructural analysis of rocks in ductile shear zones provide minimum values the presented model could explain the formation of coherent alpine nappes with inferred high pressure 1 5 2 5 gpa and ultrahigh pressure 3 gpa conditions in 40 60 km depth at the base of an orogenic wedge

application of pinch and swell structure rheology gauge to determine rock paleo rheological parameters in taili western liaoning ne china abstract new results in combination with previously published ones reveal that when the stress exponent of the competent layer sec ranges from 1 to 10 1

optical microstructure of partly recrystallized calcite in some naturally deformed marbles abstract deformed marbles from the mt lofty ranges south australia have microstructures characterized by deformed relics of calcite interspersed with much finer grained recrystallized calcite and with minor quartz and diopside the deformed calcite relics contain numerous subgrains and show abundant evidence of strain induced migration of twin and grain boundaries new calcite grains have nucleated and grown preferentially along twins especially at twin intersections at grain and kink like boundaries and apparently randomly inside deformed relics optical microstructural evidence suggests that dynamic syntectonic recrystallization has taken place along with some static recrystallization probable mechanisms for the initiation of recrystallization include bulge nucleation and subgrain nucleation

the global magnitude frequency relationship for large explosive volcanic eruptions abstract for volcanoes as for other natural hazards the frequency of large events diminishes with their magnitude as captured by the magnitude frequency relationship assessing this relationship is valuable both for the insights it provides about volcanism and for the practical challenge of risk management we derive a global magnitude frequency relationship for explosive volcanic eruptions of at least 300 mt of erupted mass or m4 5 our approach is essentially empirical based on the eruptions recorded in the lameve database it differs from previous approaches mainly in our conservative treatment of magnitude rounding and under recording our estimate for the return period of super eruptions 1000 gt or m8 is 17 ka 95 ci 5 2 ka 48 ka which is substantially shorter than previous estimates indicating that volcanoes pose a larger risk to human civilisation than previously thought

thermo poro mechanics of chemically active creeping faults 3 the role of serpentinite in episodic tremor and slip sequences and transition to chaos during the last decade knowledge over episodic tremor and slip ets events has increased dramatically owing to the widespread installation of gps and seismic networks the most puzzling observations are i the periodic nature of slow seismic events ii their localization at intermediate depths estimated 15 40 km and iii the origin of the nonvolcanic fluids that are responsible for the tremor activity we reconcile these observations using a first principles approach relying on physics continuum mechanics and chemistry of serpentinite in the megathrust interface the approach reproduces the gps sequences of 17 years of recording in cascadia north america as well as over 10 years in the hikurangi trench of new zealand we show that strongly endothermic reactions such as serpentinite dehydration are required for ets events we report that in this tectonic setting it is its chemical reaction kinetics not the low friction that marks serpentinite as a key mineral for stable self sustained oscillations we find that the subduction zone instabilities are driven from the ductile realm rather than the brittle cover even when earthquakes in the cover perturb the oscillator it relaxes to its fundamental mode such a transition from stable oscillations to chaos is witnessed in the ets signal of nz following the m6 8 2007 seismic event which triggered a secondary mode of oscillations lasting for a few years we consequently suggest that the rich dynamics of ductile modes of failure may be used to decipher the chaotic time sequences underpinning seismic events

experimental study of the influence of stress temperature and strain on the dynamic recrystallization of carrara marble carrara marble has been deformed experimentally at temperatures ranging between 500 and 1000 c at confining pressures of 200 and 300 mpa and up to very large strains in extension and compression in order to study the microstructural and mechanical property changes associated with dynamic recrystallization microstructural studies were made by optical microscopy on ultrathin sections and automatic grain size measurement techniques were used when the temperature is sufficiently high 600 c and the stresses are high enough for deformation twinning to occur twin boundary migration is a powerful recrystallization mechanism that does not modify the grain size at stress levels too low to activate twinning recrystallization occurs in two stages the formation of nuclei by grain boundary bulging and subgrain rotation recrystallization in the grain boundary regions followed by a second stage of grain boundary migration recrystallization to a larger grain size that eventually overprints the entire rock volume the recrystallization process requires larger prestrains at the lower temperatures the size to which migration recrystallized grains grow seems to be limited by the stress and grain size dependent twinning field boundary or by the boundary between the dislocation creep and grain size sensitive flow mechanisms within the range of experimental observations dynamically recrystallized grain size does not depend on strain rate or temperature separate empirical stress versus grain size relations are presented for rotation and migration recrystallization that may be used for palaeopiezometry no unequivocal experimental evidence of weakening resulting from recrystallization to finer grain sizes at laboratory strain rates was found however extrapolation of the experimental data to low natural strain rates suggests that weakening following recrystallization may occur in nature

analogue modelling of thrust systems passive vs active hanging wall strain accommodation and sharp vs smooth fault ramp geometries we present new analogue modelling results of crustal thrust systems in which a deformable brittle hanging wall is assumed to endure passive internal deformation during thrusting i e exclusively as a consequence of having to adapt its shape to the variable geometry of a rigid footwall building on previous experimental contributions we specifically investigate the role of two so far overlooked critical variables a concave convex cc vs flat ramp flat frf thrust ramp geometry and b presence vs absence of a basal velocity discontinuity vd regarding the first variable we compare new results for considered cc smoother ramp types against classical experiments in which frf sharp ramp geometries are always prescribed our results show that the considered sharp vs smooth variation in the thrust ramp geometry produces important differences in the distribution of the local stress field in the deformable hanging wall above both lower and upper fault bends with corresponding styles of strain accommodation being expressed by marked differences in measured morpho structural parameters regarding the second variable we for the first time report analogue modelling results of this type of experiments in which basal vds are experimentally prescribed to be absent our results critically show that true passive hanging wall deformation is only possible to simulate in the absence of any basal vd since active shortening accommodation always necessarily occurs in the hanging wall above such a discontinuity i e above the lower fault bend in addition we show that the morpho structural configuration of model thrust wedges formed for prescribed vd absence conditions complies well with natural examples of major overthrusts wherein conditions must occur that approximate a frictionless state along the main basal thrust plane

super volcanic investigations multi disciplinary analyses of earth s most destructive volcanic systems show that continuous monitoring and an understanding of each volcano s quirks rather than a single unified model are key to generating accurate hazard assessments

ignimbrite morphology and the effects of erosion a new zealand case study ignimbrite morphology previously generalised using aspect ratios is here quantified as the relationships between the various thicknesses of material forming an ignimbrite and the areas and volumes represented by those thicknesses the morphology can be measured for the deposit in its present day eroded condition or reconstructed for the original deposit the reconstructed morphology of the 22 500 year bp ca 11 500 km2 ca 300 km3 oruanui ignimbrite in new zealand is documented to illustrate the latter approach the oruanui ignimbrite is an intermediate aspect ratio deposit and shows broadly linear relationships between 1 in thickness and the cumulative area occupied by that thickness or less of material and 2 thickness and the volume represented by that thickness or less of material two theoretical morphologies one where thicknesses exponentially decay with distance from a maximum and the other of uniform thickness slab are compared with the oruanui data limited comparative data suggest that low aspect ratio violently emplaced ignimbrites will show upward concave curves at one extreme following the exponential decay model and high aspect ratio gently emplaced ignimbrites downward concave curves with the slab model as an extreme when plotted on diagrams where the oruanui data show linear trends the effects of erosion on oruanui and model ignimbrite morphologies are modelled using two theoretical erosion scenarios 1 material is evenly removed from the land surface and 2 thinner non welded material is preferentially removed for the oruanui ignimbrite data area is lost much more rapidly in the first instance than volume for example 5 m of erosion is sufficient to remove 50 area whereas 40 m scenario 1 or 120 m scenario 2 of erosion is required to remove 50 volume in old ignimbrites volume estimates may be reasonably accurate even after strong erosion provided the original thicknesses of ponded landscape forming material can be inferred but estimates of original area and aspect ratio will be inaccurate an envelope enclosing all known outcrops of an ignimbrite will give a better estimate of original area and aspect ratio than simply summing the areas of known outcrops

calcite mylonites in the central alpine root zone abstract north of the insubric line in the central alpine root zone carbonate rocks are concentrated in very narrow zones and have been metamorphosed under amphibolite facies conditions by the tertiary lepontine metamorphism grain size 1 mm post metamorphic deformation under greenschist facies conditions produced calcite mylonite bands a few millimeters to tens of meters wide in these marble zones microstructural development begins with twin formation bending of twin boundaries grain and twin boundary migration and recrystallization in high stress regions progressive mylonitization by dynamic recrystallization results in a microstructure with elongated calcite crystals long axis 20 50 m axial ration 1 4 in this fine grained matrix porphyroclasts of calcite quartz white mica biotite diopside tremolite scapolite and plagioclase are preserved ultra mylonite bands in pure calcite rocks show an even finer grain size of 5 10 m lattice preferred orientation is not present in the undeformed marbles but it develops during mylonitization the c axis orientation in the mylonites forms an asymmetric point maximum in the ultra mylonite no preferred orientation is left it is concluded from microstructural and textural aspects that during mylonitization dislocation creep accompanied by dynamic recrystallization were the most important processes whilst grain boundary sliding was the dominant mechanism during the formation of the ultra mylonites shear sense determinations indicate a horizontal right lateral strike slip shear system this is in good agreement with evidence regarding other movements along the insubric line which can be observed in ductile and brittle shear zones

grain growth in synthetic marbles with added mica and water evolution of grain size in synthetic marbles was traced from compaction of unconsolidated powder through primary recrystallization and normal grain growth to a size stabilized by second phases to form the marbles reagent grade caco3 was mixed with 0 1 and 5 volume mica and heat treated under pressure with added water densification with negligible recrystallization occurred within one hour at 500 c and 500 mpa confining pressure primary recrystallization occurred at 500 550 c causing increases of grain size of factors of 2 5 resulting samples had uniform grain size gently curved grain boundaries and near equilibrium triple junctions they were used subsequently for normal grain growth studies normal grain growth occurred above 550 c at 800 c grain size d increased from 7 m d0 to 65 m in 24 hours growth rates fit the equation d n d 0 n kt where k is a constant and n 2 6 minor amounts of pores or mica particles inhibit normal grain growth and lead to a stabilized grain size dmax which depends on the size of the second phases and the inverse of their volume fraction raised to a power between 0 3 and 1 once dmax is reached normal growth continues only if second phases are mobile or coarsen or if new driving forces are introduced that cause unpinning of boundaries normal grain growth in solnhofen limestone was significantly slower than in pure synthetic marble suggesting that migration is also inhibited by second phases in the limestone

transition from thin to thick skinned tectonics and consequences for nappe formation numerical simulations and applications to the helvetic nappe system switzerland abstract we use two dimensional numerical simulations to study the deformation style and the formation of nappes during the shortening of basement cover systems with half grabens we perform simulations with only linear and power law viscous rheology and also thermo mechanical simulations with viscoelastoplastic rheology and temperature dependent viscosities the accumulated area averaged strain is calculated for the sediments es and for the basement eb to quantify the deformation style the results show that the transition from thin skinned dominated es eb 1 to thick skinned dominated es eb 1 deformation is controlled by two effective viscosity ratios the ratio of viscosity at the top of the basement to viscosity at the bottom of the basement and the ratio of viscosity at the top of the basement to viscosity of the sedimentary cover a higher basement internal viscosity ratio favors thick skinned deformation whereas a higher basement cover viscosity ratio favors thin skinned deformation simulations with only viscous rheology show the same first order results as the thermo mechanical simulations the results further show that fold nappe formation is associated with thick skinned dominated deformation and thrust nappe formation with thin skinned dominated deformation fold nappes are formed by shearing and extrusion of sediments during the deformation and closure of half grabens whereas thrust nappes are formed due to the shearing off of sediments from half grabens which are nearly undeformed the results are applied to the morcles fold nappe and the helvetic nappe system switzerland

the thermal stability of near surface ground ice on mars the existence of subsurface water ice on mars has been predicted in several theoretical studies1 5 but there are no definitive observations of its present distribution geomorphic features on the surface of mars have been widely interpreted as evidence for the presence of ground ice6 12 but many of these features are found at near equatorial latitudes where thermal models have predicted that near surface water ice should not be stable under present climate conditions3 13 14 here i present the results of thermal calculations which show that observed geographic variations in the thermal and reflectance properties of martian soils significantly affect subsurface temperatures my results indicate that in certain regions ground ice deposits could exist much closer to the surface and much closer to the equator than previously thought in the future these deposits could be a valuable resource for human exploration15

solution mass transfer deformation adjacent to the glarus thrust with implications for the tectonic evolution of the alpine wedge in eastern switzerland abstract we have studied aspects of absolute finite strain of sandstones and the deformation history above and below the glarus thrust in eastern switzerland the dominant deformation mechanism is solution mass transfer smt which resulted in the formation of a semi penetrative cleavage our analysis indicates that the verrucano and melser sandstones which lie above the thrust were deformed coaxially with pronounced contraction in a subvertical z direction and minor extension in a subhorizontal x direction trending at 200 most of the contraction in z was balanced by mass loss volume strains averaging 36 below the glarus thrust sandstones of the north helvetic flysch have smaller principal strains but similar volume strains deformation there was also approximately coaxial the x direction is horizontal and trends 160 which is different by 40 from the x direction in the hanging wall the hanging wall of the glarus thrust verrucano and melser sandstones was deformed first after it was accreted deep beneath the alpine wedge continued northward advance of the wedge accomplished in part by motion on the glarus thrust allowed the wedge to override and accrete the north helvetic flysch which then started to form an smt cleavage the difference in x directions may reflect a change in transport direction but this conclusion is difficult to accept since extension was minor and was accommodated by coaxial flattening and not simple shear our work indicates that mass loss volume strains were important in sandstones of the helvetic nappes the missing mass cannot be accounted for at the local scale and appears to have been transported beyond the helvetic zone

deformation mechanisms and rheology why marble is weaker than quartzite when deformed together in nature calcite rocks invariably appear weaker and more ductile than quartz rocks this can be reconciled with experimental flow data only by taking into account grain size sensitive gss flow in calcite rocks which is predicted to dominate even at grain sizes on the order of 1mm at middle metamorphic grades using new experimental data that demonstrate the transition between intracrystalline plastic and gss flow of quartz rocks we predict that unnaturally small grain sizes at temperatures of 700 c or higher are required for gss flow of quartz in nature thus natural flow of quartzite is expected to occur by intracrystalline plastic processes even after recrystallization to a fine grain size

sources of uncertainty in the mazama isopachs and the implications for interpreting distal tephra deposits from large magnitude eruptions estimating the area of tephra fallout and volume of large magnitude eruptions is fundamental to interpretations of the hazards posed by eruptions of this scale this study uses the tephra from the caldera forming eruption of mount mazama crater lake or usa to demonstrate the challenges faced when working with large prehistoric tephra deposits and outlines the methodologies required to determine eruption volume and magnitude we combine 250 mazama tephra occurrences reported by a range of disciplines including archaeology paleoclimatology and volcanology with new field studies to better understand the extent of the distal tephra we find that the mazama tephra has been remobilised to varying degrees over the past 7000 years so each tephra locality was appraised for the likelihood that it records primary tephra fallout we designated 45 of the distal 100 km from source tephra sites as suitable for use in the production of isopachs using a spline fitting method the new distal isopachs were then integrated with proximal fallout data and estimates of the ignimbrite volume from previous studies to revise the estimated bulk erupted volume from the climactic mazama eruption to 176 km3 61 km3 dense rock equivalent dre this study demonstrates the importance of collating tephra localities from a range of disciplines and that even remobilised deposits provide valuable information about the extent of the deposit interpreting remobilised deposits can provide insight into post eruptive processes that could potentially pose secondary hazards following large magnitude eruptions we also show that in some circumstances remobilised deposits preserve important physical properties such as grain size

the accretion of foreland basin sediments during early stages of continental collision in the european alps and similarities to accretionary wedge tectonics we present structural observations from foreland basin sediments that were incorporated into the orogenic wedge of the central european alps during early stages of continental collision our analysis focuses on the prograde evolution and considers the full history of the sediments ranging from their deposition in the basin to deep burial and metamorphism at temperatures of 320 c the tectonic evolution is matched with constraints on the diagenetic alteration of the sediments for this purpose we calculate the temperatures and depths of sediment compaction and illitization as well as the associated fluid liberation the data set highlights that the tectonic incorporation of the sediments into the orogenic wedge was strongly controlled by their diagenetic state earliest deformation took place during imbrication and frontal accretion of unconsolidated and fluid saturated sediments ductile folding of the sediments occurred already at this stage and was assisted by particulate flow with the progressive consolidation of the sediments the elastic strength increased which resulted in an overall embrittlement this rheological change is recorded by the onset of out of sequence thrusting brittle faulting and the formation of massive quartz calcite veins which took place in the approximate temperature range of the seismogenic zone i e 150 350 c moreover widespread pressure solution resulted in the formation of a penetrative cleavage and records slow but long lasting deformation at low background strain rates in summary the prograde tectonic evolution of the frontal alpine wedge exhibits many similarities with the structural and mechanical evolution of accretionary wedges at active subduction zones

discrimination of annealed and dynamic fabrics consequences for strain localization and deformation episodes of large scale shear zones abstract large scale shear zones are often characterized by long lasting deformation with complex strain localization behavior based on a drill core across polymineralic carbonate mylonites of the doldenhorn nappe helvetic alps switzerland the present study uses spatially resolved microstructural variations to discriminate unambiguously deformation microstructures from annealed ones to infer the deformation mechanisms and to reconstruct the strain localization history these goals require quantitative microstructural and textural analyses of sample suites in sections across the shear zone particularly the occurrence of two distinct zener trends i e the relationship between grain size and volume fraction of matrix and second phase grains allows for the identification of annealed from dynamic microstructures the data presented here indicate that continued shearing after peak metamorphic conditions and during exhumation induced cooling resulted in progressive strain localization at the base of the shear zone while the inactive parts of the formerly high temperature shear zone were progressively overprinted by annealing

paleostresses associated with faults of large offset abstract in order to test empirically the limitations of paleostress analysis we used etchecopar s computer program to compute the orientations and relative magnitudes of paleostress principal values in two southern appalachian u s a thrust zones from minor fault and slickenside attitudes in both thrust zones faults are closely spaced many faults have offsets whose magnitudes exceed the distance to adjacent faults of comparable size and deformation was strongly non coaxial while even in the less deformed thrust zone bulk strains due to fault movement have axial ratios as high as 10 1 nearly 75 of the fault slickenside pairs in each thrust zone conform with paleostress tensors that indicate 1 sequential transport parallel compression transport parallel extension and extension oblique to transport and 2 low resolved shear stresses on the thrusts finite strains measured in one thrust zone share a principal plane with the first two tensors and the inclinations of the paleostress and finite strain principal directions in that plane are consistent with thrust parallel shearing relative magnitudes of paleostress and strain principal values do not correlate well moreover the locations and inferred origins of fault slickenside pairs inconsistent with paleostress tensors suggest that stresses in rocks between faults were not as is assumed in paleostress analyses uniform this complexity may also occur in faulted rocks where bulk strains are small paleostress tensors from rocks with large bulk strains may be viable but they must be interpreted cautiously

tomography from 26 years of seismicity revealing that the spatial extent of the yellowstone crustal magma reservoir extends well beyond the yellowstone caldera the yellowstone volcanic field has experienced three of earth s most explosive volcanic eruptions in the last 2 1 ma the most recent eruption occurred 0 64 ma forming the 60 km long yellowstone caldera we have compiled earthquake data from the yellowstone seismic network from 1984 to 2011 and tomographically imaged the three dimensional p wave velocity vp structure of the yellowstone volcanic system the resulting model reveals a large low vp body interpreted to be the crustal magma reservoir that has fueled yellowstone s youthful volcanism our imaged magma body is 90 km long 5 17 km deep and 2 5 times larger than previously imaged the magma body extends 15 km ne of the caldera and correlates with the location of the largest negative gravity anomaly a 80 mgal gravity low this new seismic image provides important constraints on the dynamics of the yellowstone magma system and its potential for future volcanic eruptions and earthquakes

episodic tremor and slip ets as a chaotic multiphysics spring abstract episodic tremor and slip ets events display a rich behaviour of slow and accelerated slip with simple oscillatory to complicated chaotic time series it is commonly believed that the fast events appearing as non volcanic tremors are signatures of deep fluid injection the fluid source is suggested to be related to the breakdown of hydrous phyllosilicates mainly the serpentinite group minerals such as antigorite or lizardite that are widespread in the top of the slab in subduction environments similar ets sequences are recorded in different lithologies in exhumed crustal carbonate rich thrusts where the fluid source is suggested to be the more vigorous carbonate decomposition reaction if indeed both types of events can be understood and modelled by the same generic fluid release reaction ab solid a solid b fluid the data from ets sequences in subduction zones reveal a geophysically tractable temporal evolution with no access to the fault zone this work reviews recent advances in modelling ets events considering the multiphysics instabilities triggered by the fluid release reaction and develops a thermal hydraulic mechanical chemical oscillator thmc spring model for such mineral reactions like dehydration and decomposition in megathrusts we describe advanced computational methods for thmc instabilities and discuss spectral element and finite element solutions we apply the presented numerical methods to field examples of this important mechanism and reproduce the temporal signature of the cascadia and hikurangi trench with a serpentinite oscillator

microfabrics in carbonate mylonites along a large scale shear zone helvetic alps abstract carbonate mylonites with varying proportions of second phase minerals were collected at positions of increasing metamorphic grade along the basal thrust of the morcles nappe helvetic nappes switzerland variations of temperature stress and strain rate changes in chemistry of solid and fluid phases and differing degrees of strain localization and annealing were tracked by measuring the shapes mean sizes and size distributions of both matrix and second phase grains as well as crystal preferred orientation cpo of the matrix field structures suggest that strain rate was constant along the fault the mean and distribution of the calcite grain sizes were affected most profoundly by temperature increased temperature presumably accompanied by decreased stress correlated with larger mean sizes and wider size distributions at a given location the matrix grains in mylonites with more second phase particles are on average smaller have narrower size distributions and have more elongate shapes for example mylonites with 50 vol of second phases have matrix grain sizes half that of pure mylonites changes in calcite chemistry and the presence of synkinematic fluids seemed to influence microfabric only weakly temporal variations in conditions such as exhumation induced cooling apparently provoke changes in temperature stress and strain rate along the nappe these changes result in further strain localization during retrograde conditions and cause the grain size to be reduced by an additional 50 the matrix cpo strengthens with increasing temperature or strain but weakens and rotates with increasing second phase content these fabric changes suggest differing rates of grain growth grain size reduction and development of cpo owing to variations in the deformation conditions and perhaps mechanisms to interpret natural mylonite structures or to extrapolate mechanical data to natural situations requires careful characterization of the microfabric and in particular second phase minerals

paleodrainages of the eastern sahara the radar rivers revisited sir a b implications for a mid tertiary trans afnrcan drainage system a complex history of cenozoic fluvial activity in the presently hyperarid eastern sahara is inferred from sir data and postflight field investigations in southwest egypt and northwest sudan sir images were coregistered with landsat and existing maps as a guide to exploration of the buried paleodrainages radar rivers first discovered by sir a field observations explain the radar responses of three types of radar rivers rr 1 rr 2 and rr 3 rr 1 broad aggraded valleys or basins filled with alluvium generally give a dark response relative to the surrounding radar bright tone of the wind roughened discontinuously sand covered bedrock of the remnant interfluves the alluvial fill in some of the rr 1 valleys gives a locally intermediate to bright response where cemented by diagenetic caco3 caliche the rr 2 type braided channels inset in the rr 1 valleys are filled with noncalichified loose sediment and are visible on sir because of the caliche effect the dark response of unconsolidated alluvium in the inset channels contrasts with the brighter response of calichified alluvium in the broad valleys the third type rr 3 narrow long bedrock incised channels which are partly visible on the ground and on landsat in areas where the sand sheet is patchy are conspicuous on sir because the dark response of their unconsolidated channel fill contrasts with the brighter response of the surrounding bedrock disconformably overlying and concealing the rr 1 valleys the rr 2 channels and parts of the rr 3 channels are eolian sand sheet sheet de deposits which in turn are locally overridden by trains of barchan dunes

vertically extensive and unstable magmatic systems a unified view of igneous processes volcanoes are an expression of their underlying magmatic systems over the past three decades the classical focus on upper crustal magma chambers has expanded to consider magmatic processes throughout the crust a transcrustal perspective must balance slow plate tectonic rates of melt generation and segregation in the lower crust with new evidence for rapid melt accumulation in the upper crust before many volcanic eruptions reconciling these observations is engendering active debate about the physical state spatial distribution and longevity of melt in the crust here we review evidence for transcrustal magmatic systems and highlight physical processes that might affect the growth and stability of melt rich layers focusing particularly on conditions that cause them to destabilize ascend and accumulate in voluminous but ephemeral shallow magma chambers

strain hardening and strain softening in fault zones from foreland thrusts the movement of thrust sheets is accomplished in part by deforming the rocks adjacent to thrust faults in foreland fold and thrust belts emplacement generates an array of mesoscopic faults in a thick layer along the base of a sheet ductile deformation of rocks in the layer occurs by displacing fault bounded blocks relative to one another in a kind of mesoscopic grain boundary sliding analyses of data from three southern appalachian thrust sheets indicate that this block sliding is first accompanied by strain hardening then by strain softening and it is accommodated by intra block fractures small faults and veins the density of accommodation features increases near thrusts and where these features become sufficiently numerous a layer of cataclastic rocks forms along the thrust we describe foliated cataclasites from two thrusts the microstructures of which indicate that the cataclasites exhibited extreme ductility interrupted occasionally by brittle processes deformation of the cataclasites was accompanied by strain softening strain softening deformation in fault arrays near thrusts is genetically related to strain softening deformation in cataclasites during the formation of the cataclasite rocks in the sheet are impelled to cross a strain energy hump after which they deform at either higher rates or lower stresses

thickness of the seismic slip zone this article reviews geologic and other evidence constraining the thickness of the principal slip zone psz that accommodates the bulk of coseismic shear displacement during an individual rupture event surface deformation from rupturing may occupy swaths tens of meters or more in width but trenches across active faults generally reveal that incremental slip is accommodated by a psz that is tens of centimeters or less in thickness geomorphic evidence coupled with the observations from trenching suggest a psz may stay well localized for distances of several kilometers through many rupture episodes mine exposures and exhumed fault zones demonstrate that pszs separating different lithologies within the fault core although contained within damage zones of variably fractured rock ranging up to hundreds of meters in thickness often comprise just a few centimeters of gouge ultracataclasite that have accommodated large finite displacements 1 km microstructural studies demonstrate incremental slip localized still further down to 1 10 mm as do other fault rock assemblages slickensides and slickenfibers fault veins of pseudotachylyte friction melt intravein septa in hydrothermal fault infills the accumulated evidence indicates that localization of coseismic shearing to less than 10 cm on planar faults is widespread throughout the crustal seismogenic zone with extreme localization to less than 1 cm not uncommon however some distributed coseismic shear may also develop especially at rupture irregularities coseismic reduction of shear resistance from friction melting t 1000 c or from transient thermal pressurization of aqueous fluids t 100 c requires slip during moderate to large earthquakes u 1 m to be restricted to narrow zones respectively a few centimeters or tens of centimeters in thickness given the evidence for slip localization the apparent scarcity of pseudotachylyte suggests either that seismic friction melting is a rare phenomenon or that pseudotachylyte is only rarely preserved in recognizable form within mature hydrated fault zones

magnetotelluric images of magma distribution beneath volcan uturuncu bolivia implications for magma dynamics the altiplano puna volcanic complex in the central andes records a history of major caldera forming eruptions over the past 10 m y geophysical and geodetic data indicate that magma is still present and perhaps mobile in the crust broadband magnetotelluric data were used to generate two dimensional and three dimensional electrical resistivity models of the altiplano puna magma body apmb with a focus on the zone of inflation around volcan uturuncu in southern bolivia low electrical resistivities 20 the upper surface of the apmb is shallowest beneath uturuncu and the geometry is consistent with geodynamic models that require the upward movement of a melt layer at this location the shallower resistivity structure is characterized by discrete electrically conductive bodies oriented east west near sea level depth of 5 km which are interpreted as a combination of partial melt and fluids

calcite fabrics in a natural shear environment the helvetic nappes of western switzerland the aim of this contribution is to describe and interpret the crystallographic orientations and microstructures of calcite in the upper jurassic limestones of the helvetic nappes between the mont blanc and aar massifs over a distance along strike of about 180 km the c axis preferred orientation pattern which could be inferred is regionally very constant the c axis point maxima are everywhere subperpendicular to the regional strike of the chain there is an obliquity between the orientation of the macroscopically dominant cleavage and the microscopically visible long axes of individual grains the c axis maxima in most specimens are parallel to the axis of shortening of the microscopic grain shape fabric this fabric geometry indicates either continuous shear deformation or alternatively a two stage deformation history the obliquity of the grain shape fabric may be due to the fact that the flattening plane of the individual grains reflects only the last increment of a continuous shear process or it may represent a later finite deformation which has led to the formation of a regionally developed second cleavage there is a systematic relationship between the direction of these late shear movements and the main overthrust direction as given by the finite strain pattern this suggests that the microfabric of the rocks was acquired during a progressive rotation with time of the main overthrust direction the observed pattern of preferred orientation has been correlated by several authors with intracrystalline deformation mechanisms a combination of slip on r and twinning on e however the e twinning observed in the specimens investigated is younger than the development of the crystallographic fabric a new method for stress determination from e twinning is proposed

implications for mechanical properties of brittle faults from observations of the punchbowl fault zone california field observations of the punchbowl fault zone an inactive trace of the san andreas are integrated with results from experimental deformation of naturally deformed punchbowl fault rocks for a qualitative description of the mechanical properties of the fault and additional information for conceptual models of crustal faulting the punchbowl fault zone consists of a single continuous gouge layer bounded by zones of extensively damaged host rock fault displacements were not only localized to the gouge layer but also to discrete shear surfaces within the gouge deformation in the exposure studied probably occurred at depths of 2 to 4 km and was dominated by cataclastic mechanisms textural data also suggest that significant amounts of pore fluids were present during faulting and that fluid assisted mechanisms such as dissolution diffusion and precipitation were operative the experimental data on specimens collected from the fault zone suggest that there is a gradual decrease in strength and elastic modulus and an increase in relative ductility and permeability toward the main gouge zone the gouge layer has fairly uniform mechanical properites and it has significantly lower strength elastic modulus and permeability than both the damaged and the undeformed host rock for the punchbowl fault and possibly other brittle faults the variations in loading of the gouge zone with time are primarily governed by the morphology of the fault and the mechanical properties of the damaged host rock in addition the damaged zone acts as the permeable unit of the fault zone and surrounding rock it appears that the gouge primarily governs whether displacements are localized and it therefore may have a significant influence on the mode of slip

a tem study of microstructures associated with solution cleavage in limestone abstract microstructures associated with clay filled seams in the scaglia rossa limestone from the umbria marche apennines italy were characterized with transmission electron microscopy tem in order to study one of the mechanisms that presently falls under the heading of pressure solution only seams that formed after lithification often called solution of spaced cleavage were examined although some observations supported suggestions that much of the clay in the solution cleavage seam could be authigenic this report focuses primarily on the limestone microstructures no evidence of plastic deformation could be observed around the seams with an optical microscope however the tem resolved dislocations concentrated within limited areas of single grains comparisons with experimentally deformed calcite microstructures and with computer simulations of tem dislocation contrast allowed further interpretation of the character and density of dislocations dislocation concentrations were frequently associated with voids and pits the pits and voids were usually elongate in a calcite dislocation slip plane and were interpreted as dissolution features these and other observations suggested that selective dissolution of localized dislocation tangles was responsible for solution cleavage formation similar microstructures associated with the stress corrosion cracking have been interpreted elsewhere as evidence of plastic strain enhanced heterogeneous dissolution further interpretations indicated that a solution cleavage surface grows laterally like a fracture tip

revised estimates for the volume of the late bronze age minoan eruption santorini greece the late bronze age minoan eruption of santorini greece occurred from within an existing caldera low temperature pyroclastic flow emplacement on shallow slopes outside the caldera can only be consistent with the caldera being filled with eruption products that are not preserved field observations and seismic reflection surveys suggest that this missing material has been downfaulted the volume of the caldera infill is estimated as 18 26 km 3 dense rock equivalent dre increasing the total of the minoan eruption deposits to 78 86 km 3 dre and making it the largest known holocene eruption this study highlights a general mechanism whereby caldera eruption volumes could be significantly underestimated

the role of veining and dissolution in the evolution of fine grained mylonites the mcconnell thrust alberta microstructural observations of the mcconnell thrust fault rocks demonstrate the importance of vein formation and calcite dissolution in the evolution of the mcconnell thrust limestone mylonites the protolith to the mylonite is a micritic limestone the mylonite contains numerous discontinuous bedding parallel calcite veins that range from relatively undeformed except for twinning and undulose extinction to highly sheared the highly sheared veins show evidence for dynamic recrystallization and grain size reduction along grain and twin boundaries that can lead to complete recrystallization to a grain size of approximately 1 3 m the bulk of the calcite matrix within the mcconnell thrust mylonite is interpreted to have formed by dynamical recrystallization of veins the emplacement of relatively coarse grained veins promoted the activation of dislocation creep within the fault rocks of the thrust in the absence of the increase in grain size afforded by vein emplacement it is anticipated that the fine grained protolith would have deformed predominantly by solution transfer with grain boundary sliding and localized brittle shearing the layer parallel calcite veins are found in close proximity to sheared clay seams which are interpreted to represent the insoluble residue left behind after dissolution of calcite these clay seams may have contributed to the build up of high pore fluid pressures which if intermittently reaching lithostatic conditions during displacement would result in transient brittle failure and thrust surface parallel vein emplacement in an otherwise ductilely deforming rock

microstructure and fabric transitions in calcite tectonites from the sierra alhamilla spain a suite of marble specimens from the sierra alhamilla spain deformed to large strains under natural conditions at about 300 c shows distinct variations in microstructure and fabrics it can be demonstrated that the development of crystallographic preferred orientations and grain shape fabrics are strongly dependent on recrystallized grain size this is interpreted to reflect the relative importance of various deformational mechanisms superplasticity seems to have a sharp upper grain size limit at 10 15 smm within the power law creep regime deformation in fine grained aggregates is probably dominated by diffusional in coarser ones by dislocation mechanisms the transition is a gradual one and may span a grain size range of 30 sm

shape of pinch and swell structures as a viscosity indicator application to lower crustal polyphase rocks abstract pinch and swell structures occur where a more competent layer in a weaker matrix is subjected to layer parallel extension in this contribution we use numerical models to explore the use of pinch and swell structure shape symmetry and asymmetry as a determinant of relative viscosity between layers maximum asymmetry is attained when the matrix viscosity on one side is subtly weaker than the competent layer while the other side is significantly weaker our numerical results are directly applied to asymmetrically developed pinch and swell structures in exposed lower continental crust here shape geometries observed in a shear zone comprised of plagioclase dominated garnet dominated and mixed amphibole plagioclase dominated bands reveals that the plagioclase dominated band is the most competent band and is marginally stronger 2 and significantly stronger 10 40 than the fine grained garnet dominated and mixed amphibole plagioclase dominated band respectively based on the experimentally determined viscosity of a plagioclase dominated material and quantitative microstructural analysis the viscosity range of the natural rock bands is 2 8 1015 to 1 1 1017 pa s consequently the assumption that the experimentally derived plagioclase flow law is an appropriate proxy for the middle to lower continental crust may lead to a viscosity over estimation by up to forty times

deformation at the frictional viscous transition evidence for cycles of fluid assisted embrittlement and ductile deformation in the granitoid crust abstract mid crustal deformation is classically characterized by the transition from ductile to brittle deformation defining the frictional to viscous transition fvt here we investigate an exhumed continental mid crustal basement section in order to envisage the relationship between ductile and brittle deformation at the fvt our detailed study from km to micro scale shows that under greenschist metamorphic conditions deformation is accommodated by a dense network of highly localized ductile shear zones in the investigated case it is not quartz which defines the overall ductile deformation behavior but the viscous granular deformation in shear zones with an ultrafine grained polymineralic matrix consisting of quartz feldspar sheet silicates and epidote during viscous granular flow mass transfer processes under the presence of fluids promote a chemo mechanical mixing resulting in grain size reduction and reaction softening coeval with this ductile deformation fluid assisted embrittlement occurs as indicated by biotite coated fractures cataclasites and injection of non cohesive polymineralic gouge material into secondary fractures inside the host rock the embrittlement during predominant ductile deformation occurs in cycles i e prolonged periods of slow viscous granular flow are interrupted by rapid brittle deformation we interpret this fluid assisted cyclic embrittlement evidenced by injection of the fluidized material into off fault fractures as an alternative equivalent to pseudotachylites and as a microstructural indicator for paleo seismic activity with exhumation and associated cooling localized deformation persists in the ultrafine grained polymineralic shear zones but progressively transitions to cataclastic flow and finally to pressure dependent frictional flow always showing cycles of slow interseismic flow and fast seismic injection events overall in the granitic crust of the aar massif brittle and ductile deformation coexist up to deformation temperatures of minimum 450 c indicating that the fvt has to be placed in a rather wide range from 8 km up to 18 20 km in the granitoid crust

mass and energy budgets of explosive volcanic eruptions analysis of the frequency size relationship of explosive volcanic eruptions for the past 2000 years reveals that the time averaged eruptive mass flux of tephra from subaerial explosive eruptions is 3 104 kg s 1 this mass flux corresponds to a thermal energy release of 4 1010 w half of the long term 10 year mass and thermal energy release from explosive volcanic eruptions is accounted for by eruptions of 10 km of tephra