numerical modeling of the emplacement of socompa rock avalanche chile 1 the 7 5 ka socompa sector collapse emplaced 25 km3 of fragmented rock as a thin but widespread 500 km2 avalanche deposit followed by late stage sliding of 11 km3 as toreva blocks most of the avalanche mass was emplaced dry although saturation of a basal shear layer cannot be excluded modeling was carried out using the depth averaged granular flow equations in order to provide information on the flow behavior of this well preserved long run out avalanche results were constrained using structures preserved on the surface of the deposit as well as by deposit outline and run up a proxy for velocity models assuming constant dynamic friction fail to produce realistic results because the low basal friction angles 1 to 3 5 necessary to generate observed run out permit neither adequate deposition on slopes nor preservation of significant morphology on the deposit surface a reasonable fit is obtained however if the avalanche is assumed simply to experience a constant retarding stress of 50 100 kpa during flow this permits long run out as well as deposition on slopes and preservation of realistic depositional morphology in particular the model explains a prominent topographic escarpment on the deposit surface as the frozen front of a huge wave of debris reflected off surrounding hills the result that socompa avalanche experienced a small approximately constant retarding stress during emplacement is consistent with a previously published analysis of avalanche data

quartz microstructures developed during non steady state plastic flow at rapidly decaying stress and strain rate abstract synseismic loading to very high stresses 0 5 gpa and subsequent creep during stress relaxation in the uppermost plastosphere at temperatures of ca 300 350 c near the lower tip of an inferred once seismically active crustal scale fault was proposed based on peculiar microstructures identified in rocks exposed over 100 km 2 in the sesia zone european western alps here we discuss the conspicuous and highly heterogeneous microstructural record of quartz in disseminated small scale shear zones sub basal deformation lamellae and arrays of elongate subgrains on the tem scale indicate an early stage of glide controlled deformation at high stresses distributed brittle failure is indicated by healed microcracks very fine grained recrystallised aggregates with a pronounced crystallographic preferred orientation reflect intense plastic flow by dislocation creep locally a fine grained foam microstructure indicates a final stage of static grain growth at low differential stress for the previously inferred peak stresses of about 0 5 gpa and given temperatures initial strain rates on the order of 10 10 s 1 are predicted by available flow laws for dislocation creep of quartz we emphasise the importance of short term non steady state deformation in the uppermost plastosphere underlying seismically active upper crust the related heterogeneous record of quartz is governed by the local stress history at constant temperature

volcano tectonic controls and emplacement kinematics of the iriga debris avalanches philippines mt iriga in southeastern luzon is known for its spectacular collapse scar that was possibly created in 1628 ad by a 1 5 km3 debris avalanche the debris avalanche deposit dad covered 70 km2 and dammed the barit river to form lake buhi the collapse has been ascribed to a non volcanic trigger related to a major strike slip fault under the volcano using a combination of fieldwork and remote sensing we have identified a similar size older dad to the southwest of the edifice that originated from a sector oblique to the underlying strike slip fault both deposits cover wide areas of low waterlogged plains to a distance of about 16 km for the oldest and 12 km for the youngest hundreds of metre wide and up to 50 m high hummocks of intact conglomerate sand and clay units derived from the base of the volcano show that the initial failure planes cut deep into the substrata in addition large proportions of both dad consist of ring plain sediments that were incorporated by soft sediment bulking and extensive bulldozing an ignimbrite unit incorporated into the younger dad forms small less than 5 m high discrete hummocks between the larger ones both debris avalanches slid over water saturated soft sediment or ignimbrite and spread out on a basal shear zone accommodated by horst and graben formation and strike slip faults in the main mass the faults are listric and flatten into a well developed basal shear zone this shear zone contains components from the substrate and has a diffuse contact with the intact substrata long transport normal ridges in the distal parts are evidence of compression related to deceleration and bulldozing the collapse orientation and structure on both sectors and dad constituents are consistent with collapse being a response to combined transtensional faulting and gravity spreading iriga can serve as a model for other volcanoes such as mayon that stand in sedimentary basins undergoing transtensional strike slip faulting

fabric analysis using the intercept method abstract the intercept method is a convenient yet powerful method for the numerical analysis of fabrics for geological applications it can be used when a set of objects e g pores grains or aggregates of a specific mineral lineaments etc with unique characteristics can be identified in an image the intercept method analyses boundaries of objects as a population of lines how intercepts are definined is therefore important in order to establish the significance of the intercept counts when the method is applied to digital images the subdivision of the information into square pixels must also be considered we apply a linear filter which minimizes the effects of grid anisotropy on the counting of intercepts analysis of the resulting rose of intercept counts by a fourier series considerably facilitates the interpretation of the data a rose of directions can readily be derived for the phase boundaries if a population of objects can be assumed to have started with an originally isotropic orientation and to have been deformed passively the fourier components of the rose of intercepts also give the sectional strain the fourier components also permit an objective test of the validity of describing a fabric as resulting from a homogeneous strain on an initially isotropic section two computer programs for the counting and analysis of intercepts on digital images are presented and applied in four examples which illustrate the usefulness of the intercept method for magmatic fabric measurement strain analysis and lineament analysis we also illustrate the importance of fourier series decomposition of rose of intercept counts for fabric interpretations intercept results are also shown to be compatible with those derived by other methods eigenvector surfor inertia tensor autocorrelation methods

topographic reflection of the socompa debris avalanche chile one of the most remarkable features of the exceptionally well preserved 26 km3 socompa debris avalanche deposit is the evidence for topographically driven secondary flow the avalanche formed by sector collapse of socompa stratovolcano and spread 40 km across a pre existing basin forming a sheet of 50 m average thickness as the avalanche impinged on the western and northern margins of the basin it was reflected back forming a secondary flow that continued to travel 15 km down a gentle slope at an oblique angle to the primary flow the front of the return wave being preserved frozen on the surface of the deposit as a prominent escarpment satellite images aerial photos digital elevation models and field observations were used to reconstruct the sequence of events during avalanche emplacement and in particular during secondary flow the avalanche sheet was divided into distinct terrane groups each believed to have experienced a particular strain history during emplacement evidence for avalanche reflection includes clearly recognizable secondary slide masses sub parallel sets of curvilinear shear zones headwall scarps separating the primary levee from the secondary terranes extensional jigsaw breakup of surface lithologies during return flow and cross cutting or deflection of primary flow fabrics by secondary terranes reflection off the basin margin took place in an essentially continuous manner most major return motions being simultaneous with or shortly following primary flow the secondary flow occurred as a wave that swept obliquely across the primary avalanche direction remobilizing the primary material which was first compressed then stretched as it passed over and rearward of the wave front as return flow occurred surface lithologies were rifted in a brittle manner and the slabs were sheared pervasively as they glided and rotated back into the basin some sank into the more fluidal interior of the avalanche which drained out into a prominent distal lobe extension by factors of up to 1 8 took place during return flow secondary flow took place on slopes of only a few degrees and the distal lobe flowed 8 km on a slope of 1 overall the avalanche is inferred to have slid into place as a fast moving sheet of fragmental rock debris with a leading edge and crust with near normal friction and an almost frictionless fluidal interior and base the avalanche emplacement history deduced from field evidence is consistent with the results of a previously published numerical model of the socompa avalanche

acoustic fluidization and the extraordinary mobility of sturzstroms 1 sturzstroms are a rare category of rock avalanche that travel vast horizontal distances with only a comparatively small vertical drop in height their extraordinary mobility appears to be a consequence of sustained fluid like behavior during motion which persists even for driving stresses well below those normally associated with granular flows one mechanism that may explain this temporary increase in the mobility of rock debris is acoustic fluidization where transient high frequency pressure fluctuations generated during the initial collapse and subsequent flow of a mass of rock debris may locally relieve overburden stresses in the rock mass and thus reduce the frictional resistance to slip between fragments in this paper we develop the acoustic fluidization model for the mechanics of sturzstroms and discuss the conditions under which this process may sustain fluid like flow of large rock avalanches at low driving stresses

longitudinal ridges in mass movement deposits prominent longitudinal features are often reported on the surfaces of mass movement deposits however the genesis and implications of these have not hitherto been considered and herein we present preliminary observations of their occurrence both in the field and in the laboratory elongated ridges are often oriented sub parallel to the flow direction and aligned radially from the source due to debris spreading they are particularly prominent in large 106m3 rock avalanches emplaced onto deformable substrates and are also found in the proximal reaches of volcanic debris avalanches flowbands which are longer and thinner expressions of longitudinal ridges are continuous along the entire flow length and are observed in rock avalanches emplaced onto glaciers in snow and some ice avalanches in pyroclastic flows and some block and ash flows in ejecta sheets in extraterrestrial landslides and in some volcanic debris avalanches other volcanic debris avalanches and the distal areas of rock avalanches often display hummocks that are aligned radially from the source we propose that these aligned hummocks are remnants of longitudinal ridges the formation of elongate ridges and their expressions as flowbands aligned hummocks or distal lobes and digits in qualitatively similar fashion in both laboratory and field environments suggests they represent an intrinsic tendency of granular flows in a wide range of situations

3d numerical simulations of fault gouge evolution during shear grain size reduction and strain localization abstract strain localization has important implications for the mechanical strength and stability of evolving fault zones structural fabrics interpreted as strain localization textures are common in natural and laboratory faults however the dynamic microscale processes controlling localization and delocalization are difficult to observe directly discrete numerical models of faulting allow a degree of dynamic visualization at the grain scale not easily afforded in nature when combined with laboratory validation experiments and field observations they become a powerful tool for investigating the dynamics of fault zone evolution we present a method that implements realistic gouge evolution in 3d simulations of granular shear the particle based model includes breakable bonds between individual particles allowing fracture of aggregate grains that are composed of many bonded particles during faulting simulations particle motions and interactions as well as the mechanical behavior of the entire system are continuously monitored we show that a model fault gouge initially characterized by mono disperse spherical aggregate grains gradually evolves with accumulated strain to a wide size distribution the comminution process yields a highly heterogeneous textural signature that is qualitatively comparable to natural and laboratory produced fault gouges mechanical behavior is comparable to a first order with relevant laboratory data simulations also reveal a strong correlation between regions of enhanced grain size reduction and localized strain thus in addition to producing realistic fault gouge textures the model offers the possibility to explore direct links between strain partitioning and structural development in fault zones this could permit investigation of subtle interactions between high and low strain regions that may trigger localization delocalization events and therefore control macroscopic frictional stability and hence the seismic potential of evolving fault zones

immiscible sulfide droplets in pseudotachylyte evidence for high temperature 1200 c melts abstract in the north cascade mountains washington rocks that underwent friction melting commonly contained sulfide minerals mostly pyrrhotite and pyrite during pseudotachylyte melt formation sulfides melted to form immiscible sulfide droplets present in five distinct textural settings the largest droplets formed through melting of lithic clasts whereas micron scale sulfide droplets are common in many of the pseudotachylytes veins microprobe analysis indicates that nearly all droplets are pyrrhotite the disappearance of pyrite indicates that melt temperatures must have exceeded 750 c but other indications suggest that the melt temperature must have been much higher the extremely common presence of pyrrhotite droplets suggests that pyrrhotite from the protolith melted requiring a minimum melt temperature of 1200 c in some samples evidence for fluid rich bubbles and possible silicate spherules indicates three coexisting immiscible phases within the silicate melt the presence of sulfide droplets appears to be common especially in relatively low oxygen fugacity melts that formed at shallow crustal levels this can provide a good textural marker of melting and therefore of pseudotachylyte formation especially where other indications of melting i e high temperature microlites vesicles etc are lacking and illustrates the extreme temperatures possible along frictionally sliding surfaces during seismic events

elementary theory of bed sediment entrainment by debris flows and avalanches 1 analyses of mass and momentum exchange between a debris flow or avalanche and an underlying sediment layer aid interpretations and predictions of bed sediment entrainment rates a preliminary analysis assesses the behavior of a coulomb slide block that entrains bed material as it descends a uniform slope the analysis demonstrates that the block s momentum can grow unstably even in the presence of limited entrainment efficiency a more detailed depth integrated continuum analysis of interacting deformable bodies identifies mechanical controls on entrainment efficiency and shows that entrainment rates satisfy a jump condition that involves shear traction and velocity discontinuities at the flow bed boundary explicit predictions of the entrainment rateeresult from making reasonable assumptions about flow velocity profiles and boundary shear tractions for coulomb friction tractions predicted entrainment rates are sensitive to pore fluid pressures that develop in bed sediment as it is overridden in the simplest scenario the bed sediment liquefies completely and the entrainment rate equation reduces toe 2 1gh1 cos 1 1 v 1 where is the slope angle 1 is the flow s coulomb friction coefficient h1 is its thickness 1 is its degree of liquefaction and v 1is its depth averaged velocity for values of 1ranging from 0 5 to 0 8 this equation predicts entrainment rates consistent with rates of 0 05 to 0 1 m s measured in large scale debris flow experiments in which wet sediment beds liquefied almost completely the propensity for bed liquefaction depends on several factors including sediment porosity permeability and thickness and rates of compression and shear deformation that occur when beds are overridden

volcanic avalanche fault zone with pseudotachylite and gouge in french massif central abstract structures and textures with sedimentological variations at different scales of the lithofacies assemblage help us to constrain the basal kinematic transition from non depositional to depositional conditions during volcanic avalanche emplacement in the well exposed impact sheared contact along volcanic avalanche fault zone in the french massif central we observe how the granular textures of the pseudotachylite and fault gouge have recorded the propagation of shock wave with granular oscillatory stress sequential events of basal aggradation along avalanche fault zone have been established related to fractal d values temperature pressure regime and oscillatory stress during slow wave velocity a typical lithofacies assemblage with a reverse grading shows the pseudotachylite and fault gouge a cataclastic gradient is characterised by the fractal d values from 2 7 in jigsaw breccias with pseudotachylite partial melt to 2 6 in the polymodal gouge shock brecciation and comminution produce cataclastic shear bands in the pseudotachylite and quartz microstructures along the basal contact of the volcanic debris avalanche deposit gouge microstructures show granular segregation cataclasis with antithetic rotational riedel shear and an arching effect between the riedel shear bands x ray microtomography provided 3d microfabrics along the clastic vein in the sandy gouge from the available statistical dataset a few equations have been developed implicating the same cataclastic origin with a co genetic evolution of lithofacies an impact wave during primary shear propagation may contribute to produce hydroclastic matrix pseudotachylite partial melt and proximal gouge thixotropy with v 50m s and a t

granular mass flows and coulomb s friction in shear cell experiments implications for geophysical flows 1 granular mass flows of rock fragments are studied in the lab by means of a high speed video camera at 2000 frames per second these granular flows are generated using beds of pumice fragments positioned on a rough rotating disk whose angular velocity is controlled by a motor the experimental apparatus allows an understanding of the arrangement of the particles in granular mass flows with relatively small and relatively large values of the savage number the savage number represents the ratio between grain collision stresses and gravitational grain contact stresses in particular these flows develop a basal layer of agitated and colliding particles underneath a relatively rigid upper layer our experimental results suggest the validity on average of the coulomb s relationship between shear and normal forces at the base of granular mass flows irrespective of their savage number value in coulomb s equation the shear stresses do not depend on the shear rate we expect the coulomb friction law to be valid also in moving pyroclastic flows our experiments suggest that the collisions and subsequent comminution of pumice fragments in moving pyroclastic flows could provide ash for the overriding ash clouds in our experiments the amount of ash generated by particle particle and particle boundary interactions increases as the value of the savage number increases in nature part of this ash may also simply move toward the base of the flows because of kinetic sieving

the role of rock fragmentation in the motion of large landslides abstract by considering the implications of the comminution generally associated with very large landslides we arrive at a simple explanation for the remarkably low frictional resistance to motion demonstrated by large intact blockslides e g waikaremoana new zealand volcanic debris avalanches e g socompa chile and large rock avalanches e g falling mountain n z which allows such mass movements to achieve unexpectedly high velocities and long runout distances during rapid grain flow under high direct stress the overall grain motion generates stresses in many force chains that strain individual grains to failure most of the elastic strain energy accumulated in these force chains before failure is returned at failure to the resulting grain fragments resulting in apparent instantaneous pressures of 3q on the surroundings where q is the ambient strength of the previously intact grains gpa these intense pressures support some of the direct force on the shear layer so that the effective intergranular stress in the shear layer is reduced because frictional resistance is proportional to effective stress this reduces the overall frictional resistance to shear the steady state effective stress is that which just allows fragmentation to continue the resistance to motion estimated from this relationship explains to much better than order of magnitude accuracy the reported motions of the large rapid mass movements we also deduce that grain fragmentation can be sustained for sufficiently long to explain the phenomena without reducing the mean grain size by volume in the granular layer to unrealistically small values the presence of pore fluid does not appear to influence the effect of fragmentation dynamics in a major way the proposed mechanism requires further laboratory and simulation studies to reduce its current dependence on limited field data but its success suggests that it is worthwhile investigating further as an explanation for large mass movements in the types of brittle rock in which fragmentation occurs

ries and chicxulub impact craters on earth provide insights for martian ejecta blankets terrestrial impact structures provide field evidence for cratering processes on planetary bodies that have an atmosphere and volatiles in the target rocks here we discuss two examples that may yield implications for martian craters 1 recent field analysis of the ries crater has revealed the existence of subhorizontal shear planes detachments in the periphery of the crater beneath the ejecta blanket at 0 9 1 8 crater radii distance their formation and associated radial outward shearing was caused by weak spallation and subsequent dragging during deposition of the ejecta curtain both processes are enhanced in rheologically layered targets and in the presence of fluids detachment faulting may also occur in the periphery of martian impacts and could be responsible for the formation of lobe parallel ridges and furrows in the inner layer of double layer and multiple layer ejecta craters 2 the ejecta blanket of the chicxulub crater was identified on the southeastern yucatn peninsula at distances of 3 0 5 0 crater radii from the impact center abundance of glide planes within the ejecta and particle abrasion both rise with crater distance which implies a ground hugging erosive and cohesive secondary ejecta flow systematic measurement of motion indicators revealed that the flow was deviated by a preexisting karst relief in analogy with martian fluidized ejecta blankets it is suggested that the large runout was related to subsurface volatiles and the presence of basal glide planes and was influenced by eroded bedrock lithologies it is proposed that ramparts may result from enhanced shear localization and a stacking of ejecta material along internal glide planes at decreasing flow rates when the flow begins to freeze below a certain yield stress

shergottites dhofar 019 sau 005 shergotty and zagami 40ar 39ar chronology and trapped martian atmospheric and interior argon we report a high resolution 40ar 39ar study of mineral separates and whole rock samples of olivine phyric dhofar 019 sayh al uhaymir sau 005 and basaltic shergotty zagami shergottites excess argon is present in all samples the highest 40ar 36ar trapped ratios are found for argon in pyroxene melt inclusions 1500 maskelynite 1200 impact glass 1800 of shergotty and impact glass of sau 005 1200 a high 40ar 36ar trapped componentusually uniquely ascribed to martian atmosphere can also originate from the martian interior indicating a heterogeneous martian mantle composition as additional explanation of variable high 40ar 36ar trapped ratios in shocked shergottites we suggest argon implantation from a transient atmosphere during impact induced degassing the best 40ar 39ar age estimate for dhofar 019 is 642 72 ma maskelynite sau 005 samples are between 700900 ma old relatively high 40ar 39ar ages of melt inclusions within dhofar 019 1086 252 ma and sau 005 olivine 885 66 ma could date entrapment of a magmatic liquid during early olivine crystallization or reflect unrecognized excess 40ar components the youngest 40ar 39ar age of shergotty separates maskelynite is 370 ma that of zagami is 200 ma the 40ar 39ar chronology of dhofar 019 and sau 005 indicate 1 ga ages apparent ages uncorrected for trapped e g martian atmosphere mantle argon components approach 4 5 ga but are not caused by inherited 40ar because excess 40ar is supported by 36artrapped young ages obtained by 40ar 39ar and other chronometers argue for primary rather than secondary events the cosmic ray exposure ages calculated from cosmogenic argon are 15 7 0 7 ma dhofar 019 1 0 1 6 ma sau 005 2 1 2 5 ma shergotty and 2 2 3 0 ma zagami

shocking rocks by cavitation and bubble implosion cavitation is proposed as a mechanism for the discrete generation of high pressure high temperature polymorphs in veins within meteorites and hypervelocity impact structures cavitation involves the inception isothermal growth and adiabatic implosion of bubbles their collapse has the potential of generating ultrahigh temperatures and pressures the formation of pseudotachylytic melt veins within the shocked target is critical to the cavitation process these veins facilitate bubble growth on decompression and subsequent bubble collapse on relaxation within the liquid asymmetric bubble implosion can lead to the rapid inrush of liquid to form a supersonic jet if the liquid jet strikes a solid surface i e melt vein walls or suspended clasts at high velocities then temperatures of several thousand kelvin and pressures of 10 gpa or more can be realized in the area of jet impact these conditions are capable of generating high pressure phases such as ringwoodite the spinel structured polymorph of olivine which requires formation pressures of 50 gpa these extreme effects are transient and localized being restricted to microscopic regions of liquid solid impact within the cavitating melt vein the presence of high pressure polymorphs in meteorites and impact structures does not necessarily imply that the primary shock event attained the conditions necessary for polymorph formation instead the presence of polymorphs may be the result of extreme pressure temperature excursions due to cavitation

holocene explosive eruptions in the rungwe volcanic province tanzania abstract the holocene explosive eruptive record of rungwe and ngozi volcanoes of the rungwe volcanic province in tanzania was reconstructed based on detailed stratigraphic field evidence combined with whole rock major and trace element analyses of tephra samples this reconstruction is supported by 25 new radiocarbon dates on palaeosols that provide additional constraints on the holocene tephro chronostratigraphy we show evidence of two catastrophic ngozi eruptions and five rungwe pumice fallout deposits and also identify several more intercalated poorly preserved pumice and ash deposits the ngozi eruptions probably played a role in shaping the present day caldera the rungwe record includes a ca 2 2 km3 deposit of a plinian style eruption dated at ca 4 ka a sub plinian one at ca 2 ka and at least three additional smaller scale fallout deposits the rungwe explosive eruptive record shows that the volcano has been frequently active in its late holocene past this study highlights the need for volcanic monitoring in the region and presents herewith the first basis of future volcanic hazard assessment

the origin of high magnetic remanence in fault pseudotachylites theoretical considerations and implication for coseismic electrical currents abstract several examples of fault related pseudotachylites display a significantly higher initial magnetic susceptibility than their granitic host rock 10 1 to 20 1 these higher values are attributed to the presence of fine magnetic particles formed during melt quenching the hysteresis properties of the particles indicate a single domain sd to pseudo single domain psd magnetic grain size the curie temperature tc of the magnetic particles is close to 580 c the natural remanent magnetization nrm of these pseudotachylites is also significantly higher than that of the host rock up to 300 1 such anomalously high remanence cannot be explained by a magnetization acquired in the earth s magnetic field regardless of pseudotachylite age ground lightning and other strong electric pulses can cause anomalously high nrm intensities a ground lightning explanation seems unlikely to explain the systematically high nrm intensities particularly in the case of recently exposed samples that have been collected from active quarries alternatively high nrm intensities could be explained by earthquake lightning eql a seismic phenomenon occasionally reported in connection with large magnitude earthquakes m 6 0 the coseismic electrical properties of the pseudotachylite vein host rock system are characterized by 1 a core of molten material high conductivity 2 vapor rich margins of thermally and mechanically fractured host rocks low conductivity and 3 moderately fractured to undeformed host rock normal conductivity such a core conductor bordered by insulating margins is potentially responsible for the propagation of eql pulses the coseismic thermal history of pseudotachylite veins has been modeled in 2 d using conductive heat transfer equations it shows that eql can be recorded only during a brief time interval less than 1 min for a given vein thickness and host rock temperatures if the vein is too thick or if the host rock is too hot the pseudotachylite remains above tc after the electric pulse has lapsed

quantifying fault breccia geometry dent fault nw england a geometric classification of fault breccia borrowed from the cave collapse literature has been suggested as an alternative to available genetic classifications here image analysis is used to explore geometric discrimination between the visually assigned classes of crackle breccia mosaic breccia and chaotic breccia using samples from the well understood dent fault northwest england clast sphericity and surface roughness show some correlation with the breccia classes but particle size distributions and their fractal dimension show none a more useful parameter is the percentage of sample area occupied by clasts crackle breccia has 75 clasts mosaic breccia 60 75 clasts and chaotic breccia has 60 clasts the eye is also good at judging the tessellation goodness of fit of clasts and a semi quantitative approach to assessing this parameter is explored the average degree of rotational misfit of the clasts is strongly related to breccia class crackle breccia involves less than 10 average rotation mosaic breccia 10 20 and chaotic breccia more than 20 rotation comparison charts are provided for semi quantitative classification of fault breccias

40ar 39ar and cosmic ray exposure ages of nakhlites nakhla lafayette governador valadares and chassigny we present 40ar 39ar dating results of handpicked mineral separates and wholerock samples of nakhla lafayette and chassigny our data on nakhla and lafayette and recently reported ages for some nakhlites and chassigny misawa et al 2006 park et al 2009 point to formation ages of approximately 1 4 ga rather than 1 3 ga that is consistent with previous suggestions of close in time formation of nakhlites and chassigny in lafayette mesostasis we detected a secondary degassing event at approximately 1 1 ga which is not related to iddingsite formation it may have been caused by a medium grade thermal event resetting the mesostasis age but not influencing the k ar system of magmatic inclusions and the original igneous texture of this rock cosmic ray exposure ages for these meteorites and for governador valadares were calculated from bulk rock concentrations of cosmogenic nuclides 3he 21ne and 38ar individual results are similar to literature data the considerable scatter of t3 t21 and t38 ages is due to systematic uncertainties related to bulk rock and target element chemistry production rates and shielding effects this hampers efforts to better constrain the hypothesis of a single ejection event for all nakhlites and chassigny from a confined martian surface terrain eugster 2003 garrison and bogard 2005 cosmic ray exposure ages from stepwise release age spectra using 38ar and neutron induced 37ar from ca in irradiated samples can eliminate errors induced by bulk chemistry on production rates although not from shielding conditions

frictional melt homogenisation during fault slip geochemical textural and rheological fingerprints abstract volcanic environments often represent structurally active settings where strain localisation can promote faulting frictional deformation and subsequent melting along fault planes such frictional melting is thermodynamically a disequilibrium process initiated by selective melting of individual mineral phases and softening of volcanic glass at its glass transition as a response to rapid frictional heating the formation of a thin melt layer on a fault plane surface can drastically accelerate or terminate slip during fault motion a comprehensive understanding of the physical and chemical properties of the frictional melt is required for a full assessment of slip mechanisms as frictional rheology depends on the contributions from selectively melted mineral and glass phases as well as the physical effects of restite fragments suspended in the frictional melt here we experimentally investigate the impact of host rock mineralogy on the compositional and textural evolution of a frictional melt during slip high velocity rotary shear hvr experiments were performed under controlled volcanically relevant coseismic conditions 1 m s 1 slip rate and 1 mpa normal stress using three intermediate dome lavas with contrasting mineral assemblages sampled from volcanic systems where fault friction is evident 1 an amphibole bearing andesite soufriere hills volcano montserrat 2 an amphibole poor dacite santiaguito dome complex guatemala and 3 an amphibole free andesite volcan de colima mexico for each sample five hvr experiments were terminated at different stages of frictional melt evolution namely 1 at the onset of melting 2 upon formation of a steady state melt layer and 3 after 5 m 4 10 m and 5 15 m of slip at steady state conditions progressive mixing and homogenisation of selective single phase melts within the frictional melt layer through double diffusion convection demonstrates the control of melt composition on slip behaviour amphiboles melted preferentially leading to consistently lower shear stress 1 mpa less than amphibole poor samples and pronounced shear weakening during the frictional melting of amphibole bearing lavas the results highlight the implications of mineral assemblages on fault slip including conduit flow processes which may influence the style of eruptions and run out distances of rapid granular flows

pseudotachylyte in muscovite bearing quartzite coseismic friction induced melting and plastic deformation of quartz abstract thin 0 5 2 mm thick pseudotachylyte veins occur within muscovite bearing 10 volume amphibolite facies quartzites of the schneeberg normal fault zone austroalpine southern tyrol italy pseudotachylytes are associated with precursor localized plastic microshear zones 50 150 m thick developed sub parallel to the host rock foliation and with conjugate sets oriented at a high angle to the foliation such microshear zones are characterized by recrystallization to ultrafine grained 1 2 m grain size mosaic aggregates of quartz showing a transition from a host controlled to a random crystallographic preferred orientation towards the shear zone interior subsequent coseismic slip mainly exploited these microshear zones microstructural analysis provides evidence of extensive friction induced melting of the muscovite bearing quartzite producing a bimodal melt composition first the host rock muscovite was completely melted and subsequently crystallized mainly as k feldspar then about 60 volume of the ultrafine grained quartz underwent melting and crystallized as spherulitic rims mostly consisting of quartz ti fe around melt corroded quartz clasts the two melts show immiscibility structures in the major injection veins exploiting microshear zones at high angles to the quartzite foliation in contrast they were mechanically mixed during flow along the main fault veins

particle size and energetics of gouge from earthquake rupture zones surprisingly considering the forces that we see at the surface not all the energy involved in an earthquake is transformed into ground motion much of it is consumed by frictional heating and rock pulverization an analysis of pulverized rock or gouge collected from the san andreas fault in california and from a rupture zone at 2 km depth in a south african gold mine contradicts the commonly held view that gouge surface energy is a negligible component of earthquake energy balance the analysis reveals unexpected diminutive particles in gouge samples indicating that pulverization not frictional heating is the major consumer of earthquake energy at depth this might explain the heat flow anomaly seen in the san andreas fault system and perhaps some other long standing puzzles in earthquake energetics grain size reduction and gouge formation are found to be ubiquitous in brittle faults at all scales1 2 3 4 and most slip along mature faults is observed to have been localized within gouge zones5 6 this fine grain gouge is thought to control earthquake instability3 6 7 8 and thus understanding its properties is central to an understanding of the earthquake process7 9 here we show that gouge from the san andreas fault california with 160 km slip and the rupture zone of a recent earthquake in a south african mine with only 0 4 m slip display similar characteristics in that ultrafine grains approach the nanometre scale gouge surface areas approach 80 m2 g 1 and grain size distribution is non fractal these observations challenge the common perception that gouge texture is fractal10 11 and that gouge surface energy is a negligible contributor to the earthquake energy budget3 9 12 we propose that the observed fine grain gouge is not related to quasi static cumulative slip but is instead formed by dynamic rock pulverization during the propagation of a single earthquake

runout of the socompa volcanic debris avalanche chile a mechanical explanation for low basal shear resistance we propose a mechanical explanation for the low basal shear resistance about 50 kpa previously used to simulate successfully the complex well documented deposit morphology and lithological distribution produced by emplacement of the 25 km3 socompa volcanic debris avalanche deposit chile stratigraphic evidence for intense basal comminution indicates the occurrence of dynamic rock fragmentation in the basal region of this large granular mass flow and we show that such fragmentation generates a basal shear stress retarding motion of the avalanche that is a function of the flow thickness and intact rock strength the topography of the socompa deposit is realistically simulated using this fragmentation derived resistance function basal fragmentation is also compatible with the evidence from the deposit that reflection of the avalanche from topography caused a secondary wave that interacted with the primary flow

transport and emplacement mechanisms of large volcanic debris avalanches evidence from the northwest sector of cantal volcano france abstract large volcanic debris avalanche vda deposits of miocene age occur within the stratigraphic sequence of cantal stratovolcano central france the vda of cantal volcano were initiated by sector collapse of a cone of high elevation up to 4500 m a s l two distinct superimposed vda deposit units breccias i and ii resulted from at least two successive large scale collapse events during the evolution of the volcanic complex breccia i the oldest one is very thick up to 200 m and heterolithologic it is overlain by breccia ii which is thinner up to 50 m and contains more heterometric clasts ranging from hectometer to millimeter size the vda deposits are poorly sorted clasts are angular and highly fractured at different scales often with jigsaw fit in breccia ii jigsaw megaclasts have usually subspheric shapes with size decreasing distally basal deposits commonly display thin inversely graded layers reorientated clasts along ramp structures and cataclasis of the substratum some units have a distal fabric in the form of a poorly developed clast imbrication the clasts are commonly sub rounded thereby indicating abrasion during viscous flow anisotropy of magnetic susceptibility a m s measurements indicate absence of a preferentially oriented matrix fabric magnetic foliation parallel to the bedding suggests that the depositional process is dominated by particle settling the magnetic fabric is better organized in distal deposits where magnetic carriers are imbricated moreover the fragmented clasts and megaclasts are not abraded rather inflation led to isotropic spherical dispersion of clastic material isotropic dispersive inflation which fed the interclast matrix this is also confirmed by the lack of rotation at the individual crystal level all these data strongly suggest transport of vda by liquified non turbulent granular flows distally imbricated deposits suggest more turbulent flow emplacement of the vda results from progressive upward aggradation of the particles by volume reduction and deflation loss the non turbulent liquified stage of the vda disappears the vda behaves as a sliding rigid mass plug flow that exerts strong friction on the basement leading to gouge formation

nature of stress accommodation in sheared granular material insights from 3d numerical modeling active faults often contain distinct accumulations of granular wear material during shear this granular material accommodates stress and strain in a heterogeneous manner that may influence fault stability we present new work to visualize the nature of contact force distributions during 3d granular shear our 3d discrete numerical models consist of granular layers subjected to normal loading and direct shear where gouge particles are simulated by individual spheres interacting at points of contact according to simple laws during shear we observe the transient microscopic processes and resulting macroscopic mechanical behavior that emerge from interactions of thousands of particles we track particle translations and contact forces to determine the nature of internal stress accommodation with accumulated slip for different initial configurations we view model outputs using novel 3d visualization techniques our results highlight the prevalence of transient directed contact force networks that preferentially transmit enhanced stresses across our granular layers we demonstrate that particle size distribution psd controls the nature of the force networks models having a narrow i e relatively uniform psd exhibit discrete pipe like force clusters with a dominant and focussed orientation oblique to but in the plane of shear wider psd models e g power law size distributions d 2 6 also show a directed contact force network oblique to shear but enjoy a wider range of orientations and show more out of plane linkages perpendicular to shear macroscopic friction level is insensitive to these distinct force network morphologies however force network evolution appears to be linked to fluctuations in macroscopic friction our results are consistent with predictions based on recent laboratory observations that force network morphologies are sensitive to grain characteristics such as particle size distribution of a sheared granular layer our numerical approach offers the potential to investigate correlations between contact force geometry evolution and resulting macroscopic friction thus allowing us to explore ideas that heterogeneous force distributions in gouge material may exert an important control on fault stability and hence the seismic potential of active faults 2007 elsevier b v all rights reserved

ground vibrations produced by rock motions and debris flows 1 in this study we compare ground vibrations generated by various rocks and debris flows at the same channel to explore the origin of ground vibrations caused by debris flows in field experiments we identified the ground vibrations caused by motions of individual rocks ground vibrations were detected by geophones and analyzed in the time frequency domains we found that ground vibrations caused by individual rocks were 10 150 hz and that larger stones generate ground vibrations with lower peak frequencies a sphere packing model of granular rocks was used to estimate the propagation speed of ground vibrations this was found to be underestimated when compared with our experimental data the signal decay rate was obtained under the assumption that ground vibrations propagate as exponentially attenuating cylindrical waves ground vibrations produced by a debris flow at ai yu zi creek nan tou taiwan on 2 july 2004 revealed that the debris flow forefront produced vibrations lower than 50 hz and ranged between 50 and 100 hz after the front had passed when the main front was closest to the sensor the frequency spectrum covered a wide range from 10 to 250 hz the mean velocity of the debris flow front was 13 3 m s frequencies of ground vibrations caused by individual rocks are within the frequency range of ground vibrations captured during the debris flow confirming that one of the main sources of ground vibration caused by debris flows is the interaction of rocks or boulders with the channel bed

shear localisation strain partitioning and frictional melting in a debris avalanche generated by volcanic flank collapse abstract the arequipa volcanic landslide deposit to the east of arequipa peru originated from the pichu pichu volcanic complex covering an area 100 km2 the debris avalanche deposit exhibits internal flow structures and basal pseudotachylytes we present field microstructural and chemical observations from slip surfaces below and within the deposit which show varying degrees of strain localisation at one locality the basal shear zone is localised to a 1 2 cm thick extremely sheared layer of mixed ultracataclasite and pseudotachylyte containing fragments of earlier frictional melts rheological modelling indicates brittle fragmentation of the melt may have occurred due to high strain rates at velocities of 31 m s 1 and that frictional melting is unlikely to provide a mechanism for basal lubrication elsewhere we observe a

insights into rock ice avalanche dynamics by combined analysis of seismic recordings and a numerical avalanche model 1 rock ice avalanches larger than 1 106 m3 are high magnitude low frequency events that may occur in all ice covered high mountain areas around the world and can cause extensive damage if they reach populated regions the temporal and spatial evolution of the seismic signature from two events was analyzed and recordings at selected stations were compared to numerical model results of avalanche propagation the first event is a rock ice avalanche from iliamna volcano in alaska which serves as a natural laboratory with simple geometric conditions the second one originated on aoraki mt cook new zealand southern alps and is characterized by a much more complex topography a dynamic numerical model was used to calculate total avalanche momentum total kinetic energy and total frictional work rate among other parameters these three parameters correlate with characteristics of the seismic signature such as duration and signal envelopes while other parameters such as flow depths flow path and deposition geometry are well in agreement with observations the total frictional work rate shows the best correlation with the absolute seismic amplitude suggesting that it may be used as an independent model evaluation criterion and in certain cases as model calibration parameter the good fit is likely because the total frictional work rate represents the avalanche s energy loss rate part of which is captured by the seismometer deviations between corresponding calculated and measured parameters result from site and path effects which affect the recorded seismic signal or indicate deficiencies of the numerical model the seismic recordings contain additional information about when an avalanche reaches changes in topography along the runout path and enable more accurate velocity calculations the new concept of direct comparison of seismic and avalanche modeling data helps to constrain the numerical model input parameters and to improve the understanding of rock ice avalanche dynamics

fractal analysis of the fracture strength of lava dome material based on the grain size distribution of block and ash flow deposits at unzen volcano japan abstract a fractal theory of rock fragmentation is applied to block and ash flow deposits from the fugendake dome unzen volcano kyushu japan in order to analyze the material strength and the energy required for size reduction of the source lava dome two fractal dimensions h and ds which are mutually interchangeable represent the relative strength and energy for particles reduced to a given size they can be theoretically estimated from the power relations of a reference grain size to the cumulative mass and number of fragments smaller than that size the unzen fugendake block and ash flow deposits have been further modified by size sorting and secondary fragmentation that occurred during flowage so that the h value decreases or ds value increases with increasing distance from the source coarse reversely graded deposits are however found to retain the original size population relatively well the ds values estimated from deposits of this type are compatible with those previously reported from decompression fragmentation experiments conducted on the same dome material the employed fractal approach could thus give insights into the potential mode of dome collapse that generates block and ash flows

a gravitational spreading origin for the socompa debris avalanche abstract socompa volcano arguably provides the world s best exposed example of a sector collapse derived debris avalanche deposit new observations lead us to re interpret the origin of the sector collapse we show that it was triggered by failure of active thrust anticlines in sediments and ignimbrites underlying the volcano the thrust anticlines were a result of gravitational spreading of substrata under the volcano load about 80 of the resulting avalanche deposit is composed of substrata formerly residing under the volcano and in the anticlines the collapse scar can be traced up to 5 km from the edifice truncating two spreading related anticlines which collapsed in the event outcrops near the volcano preserve evidence of edifice material being carried along on top of mobilised substrata on the north side of the scar the avalanche motion was initially at right angles to the failure edge structural relations indicate that immediately prior to collapse the substrata disintegrated became effectively liquidised and were ejected from beneath the edifice catastrophic mobilisation of substrata probably resulted from breakdown of ignimbrite clasts and cement it may have occurred through progressive rock fracture by high shear strain during spreading material ejected from under socompa formed a layer on which volcanic edifice debris was transported this interpretation of events explains the puzzling observation that avalanche units with the lowest gravitational potential energy moved the furthest it can also account for avalanche motion normal to the collapse scar walls ignimbrites and other rock types probably capable of similar behaviour underlie many other volcanoes identification of spreading at other sites could therefore be a first step towards assessment of the potential for this style of catastrophic sector collapse

pyroclastic chronology of the sancy stratovolcano mont dore french massif central new high precision 40ar 39ar constraints abstract the sancy 16 km2 is the youngest of the two stratovolcanoes that constitute the mont dore massif massif central france the restricted number of high precision radio isotopic ages currently limits our knowledge of the pyroclastic chronology of this edifice which is the source of many tephra layers detected in middle pleistocene sequences in southeast europe to improve our knowledge of the building phases of this stratovolcano we collected thirteen pyroclastic units covering the entire proximal record we present 40ar 39ar single grain laser dating performed in the facility hosted at the lsce gif sur yvette france the 40ar 39ar ages range from 1101 11 ka to 392 7 ka 1 external four pyroclastic cycles lasting on average 100 ka were identified c i to c iv c i corresponds to the earlier explosive phase between 1101 ka and 1000 ka and starts about 100 ka earlier than previously thought the second pyroclastic cycle c ii is the main pyroclastic episode spanning from 818 to 685 ka this cycle is constituted of a minimum of 8 major pyroclastic eruptions and includes a major event that corresponds to a large plinian eruption at 719 10 ka 1 external and recorded as a 1 4 m thick layer 60 km south east of the sancy volcano the link between this large eruption and formation of a caldera stays however hypothetical the third pyroclastic cycle c iii found in the northeastern part of the sancy mont dore valley spanned from 642 to 537 ka finally the youngest pyroclastic cycle c iv starts at 392 ka and probably ends around 280 ka the age versus geographic location of each pyroclastic cycle indicates three preferential directions of channeling of the pyroclastic events and or collapse of the volcanic edifice northwest to west c i southeast c ii and finally north to northeast c iii and iv the new high precision 40ar 39ar age for the queureuilh bas pyroclastic unit 642 9 ka is identical within error with the u pb age obtained by cocherie et al 2009 geochimica et cosmochimica acta 73 1095 1108 and suggests a short residence time of the magma in a shallow short lived small magmatic chamber finally the source of the t21d tephra layer found in the pianico sellere varved sequence northern italy is not the rivaux pumice flow as proposed by brauer et al 2007 journal of quaternary science 22 85 96 and neither one of the c ii pyroclastic units as suggested by roulleau et al 2009 quaternary international 204 31 43 accordingly the source for the t21d layer has yet to be found at sancy or elsewhere

textural evidence for recent co seismic circulation of fluids in the nojima fault zone awaji island japan abstract the hirabayashi borehole awaji island japan was drilled by the geological survey of japan gsj 1 year after the hyogo ken nanbu kobe earthquake 1995 mjma 7 2 this has enabled scientists to study the complete sequence of deformation across the active nojima fault from undeformed granodiorite to the fault core in the fault core different types of gouge and fractures have been observed and can be interpreted in terms of a complex history of faulting and fluid circulation above the fault core and within the hanging wall compacted cataclasites and gouge are cut by fractures which show high apparent porosity and are filled by 5 50 m euhedral and zoned siderite and ankerite crystals these carbonate filled fractures have been observed within a 5 5 m wide zone above the fault but are especially abundant in the vicinity 1 m of the fault the log normal crystal size distributions of the siderite and ankerite suggest that they originated by decaying rate nucleation accompanied by surface controlled growth in a fluid saturated with respect to these carbonates these carbonate filled fractures are interpreted as the result of co seismic hydraulic fracturing and upward circulation of fluids in the hanging wall of the fault with the fast nucleation of carbonates attributed to a sudden fluid or co2 partial pressure drop due to fracturing the fractures cut almost all visible structures at a thin section scale although in some places the original idiomorphic shape of carbonates is modified by a pressure solution mechanism or the carbonate filled fractures are cut and brecciated by very thin gouge zones these features are attributed to low and high strain rate mechanisms respectively the composition of the present day groundwater is at near equilibrium or slightly oversaturated with respect to the siderite calcite dolomite and rhodochrosite taken together this suggests that these fractures formed very late in the evolution of the fault zone and may be induced by co seismic hydraulic fracturing and circulation of a fluid with a similar composition to the present day groundwater they are therefore potentially related to recent earthquake activity

shear zones in clay rich fault gouge a laboratory study of fabric development and evolution abstract clay rich fault rocks have long been recognized to host distinctive fabric elements and fault rock fabric is increasingly thought to play a fundamental role in fault mechanical behaviour in the brittle regime although the geometries of fabric elements in fault gouges have been described for almost a century the genesis and evolution of these elements during shear and their links to bulk mechanical properties remain poorly understood we characterize the development and evolution of fabric elements with increasing shear in a variety of clay rich experimental gouges over shear strains of 20 and at normal stresses of 2 150 mpa in the double direct shear configuration in addition to sem observations of experiment products at a variety of shear strains we quantified clay fabric intensity and the degree of grain size reduction using x ray texture goniometry xtg and particle size distribution psd measurements we also measured p and s wave velocities during shear to further probe the evolution of shear fabric and gouge properties we find that clay fabric elements develop in a systematic manner regardless of the gouge material riedel shears in the r1 orientation and boundary parallel shears are the dominant fabric elements riedel shears nucleate at layer margins and propagate into the layer shortly after reaching yield stress clay particles rotate into the p orientation shortly after riedels propagate through the layer the riedel shears are through going but are 10 thinner than similar zones observed in coarser granular materials our results suggest that the weakness of clay rich fault gouge may be less a function of anisotropic crystal structure as has been suggested previously and more a consequence of very thin shear surfaces permitting deformation in clay rich materials with minimal dilation or cataclasis the very thin shear surfaces are a function of the fine grain size of the materials and possibly polymodal psd s

coseismic horizontal slip revealed by sheared clastic dikes in the dead sea basin despite the hazard caused by near surface destructive horizontal displacements during earthquakes field evidence for coseismic slip along horizontal discontinuities is exceptionally rare mainly due to the lack of adequate exposure and markers however within the seismically active dead sea basin the late pleistocene lisan formation contains vertical clastic dikes that are sheared laterally at maximum depths of 15 m and thereby provide unique profiles of such horizontal displacement in order to investigate how coseismic horizontal shearing is distributed near the surface we document an 1 m thick brittle shear zone consisting of up to 11 slip surfaces that can be traced for tens of meters in the lisan formation displacements along individual slip surfaces are up to 0 6 m and the total displacement across the shear zone is up to 2 0 m displacement profiles and gradients indicate that the brittle shear zone formed by simple shear and deformation was associated with slip partitioning and transfer between primary and secondary slip surfaces evidence for concurrent displacement along slip surfaces during a single event indicates that the brittle shear zone was formed during a coseismic event subsequent to 30 ka we consider the mechanical effect of seismic wave related transient stress which when added to the initial static effective stress may result in concurrent horizontal shear failure along detrital rich layers in the lisan formation the exceptional quality of exposures and markers enables us to document for the first time the details of near surface horizontal shearing and indicates that displacement along horizontal bedding planes is a viable mechanism to absorb coseismic deformation in well bedded near surface strata

extensive soft sediment deformation and peperite formation at the base of a rhyolite lava owyhee mountains sw idaho usa in the northern owyhee mountains sw idaho a 200 m thick flow of the miocene jump creek rhyolite was erupted on to a sequence of tuffs lapilli tuffs breccias and lacustrine siltstones of the sucker creek formation the rhyolite lava flowed over steep palaeotopography resulting in the forceful emplacement of lava into poorly consolidated sediments the lava invaded this sequence liquefying and mobilising the sediment propagating sediment subvertically in large metre scale fluidal diapirs and sediment injectites the heat and the overlying pressure of the thick jump creek rhyolite extensively liquefied and mobilised the sediment resulting in the homogenization of the sucker creek formation units and the formation of metre scale loading structures simple and pendulous load casts detached pseudonodules density contrasts between the semi molten rhyolite and liquefied sediment produced highly fluidal rayleigh taylor structures local fluidisation formed peperite at the margins of the lava and elutriation structures in the disrupted sediment the result is a 30 40 m zone beneath the rhyolite lava of extremely deformed stratigraphy brittle failure and folding is recorded in more consolidated sediments indicating a differential response to loading due to the consolidation state of the sediments the lava sediment interaction is interpreted as being a function of 1 the poorly consolidated nature of the sediments 2 the thickness and heat retention of the rhyolite lava 3 the density contrast between the lava and the sediment and 4 the forceful emplacement of the lava this study demonstrates how large lava bodies have the potential to extensively disrupt sediments and form significant lateral and vertical discontinuities that complicate volcanic facies architecture

fault textures in volcanic debris avalanche deposits and transformations into lahars the pichu pichu thrust lobes in south peru compared to worldwide avalanche deposits abstract structures lithofacies and semi quantitative sedimentological analysis of volcanic debris avalanche deposits are essential to constrain syn emplacement processes along avalanche fault zones in the setting of mass flow runout distances at a distance of 28 km from the extinct pichu pichu compound volcano we show how internal structures of thrust lobes faulted as to form horst and graben like structures have recorded the brecciation with granular segregations cataclastic gradients have been recognized in more than eleven breccia structures with statistical grain size parameters among them fractal d values between 1 96 and 2 53 related to an extensional fracturing in dads with fractal d values between 2 and 2 5 leading to granular disaggregation with fractal d values between 2 13 and 2 22 primary fractures related to transport processes and the secondary cataclasis produced during emplacement are quantified as having 50 matrix the hybrid lithofacies in graben like structures may be the precursory stages of the matrix transformation into lahar deposits volcanic debris avalanche structures of eleven worldwide volcanoes that belong to the central andes and the cascades volcanic arc have been compared using sedimentary and statistical methods 156 sieved samples and shape preferred orientation measured on 30 342 clasts median diameter sorting index skewness and kurtosis values help differentiate four deposits and processes 1 the proximal gravitational collapse deposits 2 the avalanche thrust lobes located in inverted extensional fault zones and 3 the hybrid and mixed deposits with polymodal distributions and 4 the bimodal transformed matrix with clast sorting the fractal d values of avalanche matrix range between 2 3 and 2 7 suggesting that extensional fractures have led to granular disaggregation from the available statistical dataset a few equations have been developed that help pointing to a syn emplacement cataclastic gradient along avalanche fault zone three textural classes of avalanche fault zones have been identified based on the ellipse a b values of 2 14 for the planar collapse in fault zone 1 75 to 2 for the crushing of volcanic avalanche breccias and

new views of granular mass flows concentrated grain fluid mixtures in rock avalanches debris flows and pyroclastic flows do not behave as simple materials with fixed rheologies instead rheology evolves as mixture agitation grain concentration and fluid pressure change during flow initiation transit and deposition throughout a flow however normal forces on planes parallel to the free upper surface approximately balance the weight of the superincumbent mixture and the coulomb friction rule describes bulk intergranular shear stresses on such planes pore fluid pressure can temporarily or locally enhance mixture mobility by reducing coulomb friction and transferring shear stress to the fluid phase initial conditions boundary conditions and grain comminution and sorting can influence pore fluid pressures and cause variations in flow dynamics and deposits

giant rockslides from the inside the growing body of research on large scale mass wasting events so far has only scarcely investigated the sedimentology of chaotic deposits from non volcanic terrestrial landslides such that any overarching and systematic terminological framework remains elusive yet recent work has emphasized the need for better understanding the internal structure and composition of rockslide deposits as a means to characterise the mechanics during the final stages of runout and emplacement we offer a comprehensive overview on the occurrence of rock fragmentation and frictional melt both at different geographic locations and different sections within large 10 6 m 3 rockslide masses we argue that exposures of pervasively fragmented and interlocked jigsaw cracked rock masses basal melange containing rip up clasts and phantom blocks micro breccia and thin bands of basal frictionite are indispensable clues for identifying deposits from giant rockslides that may remain morphologically inconspicuous otherwise these sedimentary assemblages are diagnostic tools for distinguishing large rockslide debris from macro and microscopically similar glacial deposits tectonic fault zone breccias and impact breccias and thus help avoid palaeoclimatic and tectonic misinterpretations let alone misestimates of the hazard from giant rockslides moreover experimental results from mossbauer spectroscopy of frictionite samples support visual interpretations of thin sections and demonstrate that short lived 1500 degrees c in the absence of water occurred at the base of several giant moving rockslides this finding supports previous theories of dry excess runout accompanied by comminution of rock masses down to gm scale and indicates that catastrophic motion of large fragmenting rock masses does not require water as a potential lubricant

the age of the kara impact structure russia for the 65 km sized kara impact structure close to the polar ural we report an age of 70 3 2 2 ma 2s defined by the mean of 40ar 39ar plateau ages for three glassy or crystalline impact melt rocks cleaned from mineral and rock clasts the fine structure of the age spectra of these samples can quantitatively be simulated by modeling taking into account 39ar recoil effects without assuming the presence of excess ar the calculations corroborate our age results by showing that 39ar recoil does not affect the plateau fractions previously kara has been proposed as a probable k t impact site or was related to the campanian maastrichtian boundary at 73 ma at the 2s level both suggestions are ruled out by the well constrained age for the kara impact structure

vertical segregation in granular mass flows a shear cell study 1 non fluidised dry granular mass flows are obtained with rock fragments located on a rough rotating disk in these flows that develop a quasi rigid upper layer and a basal layer of colliding particles dense clasts sink whereas light ones rise when surrounded by particles with intermediate density our experiments demonstrate that the presence of a quasi rigid upper layer in granular mass flows does not prevent vertical segregation and that the formation of coarse tail grading in pyroclastic flows does not require fluidising gases high speed videos reveal that vertical segregation in granular mass flow of rock fragments is generated by inertia differences between segregating clasts and matrix when they are both pushed upward by collisions with the basal layer coarse tail grading occurs because the average segregation velocity of smaller clasts is smaller than that of larger clasts

pseudotachylyte frictionite at the base of the arequipa volcanic landslide deposit peru implications for emplacement mechanisms abstract the arequipa volcanic landslide deposit peru displays a pseudotachylyte or frictionite at its basal contact with an older ignimbrite the pseudotachylyte consists of a dark 1 cm thick vitreous layer that shows striations on its upper surface it contains small mineral clasts from both the landslide rocks and the underlying ignimbrite embedded in an optically isotropic matrix tem observations revealed a clast melt suspension with a lower black obsidian like vein made of chemically heterogeneous glass the pseudotachylyte is interpreted as the result of partial melting due to frictional heating during landslide emplacement this is the third pseudotachylyte reported at the base of a landslide deposit and the first in a volcanic environment calculations show that basal melting only accounts for a small proportion of the total energy released during landslide emplacement indicating that landslide transport was dominated by dissipative stresses affecting the whole landslide body and not conce

use of image analysis to study the microstructure of a failed sand specimen the shearing of dilatant specimens of granular materials in the laboratory is often accompanied by the development of zones of more intense shearing the nominal behavior of the specimens during th

distinguishing volcanic debris avalanche deposits from their reworked products the perrier sequence french massif central debris avalanches associated with volcanic sector collapse are usually high volume high mobility phenomena debris avalanche deposit remobilisation by cohesive debris flows and landslides is common so they can share textural characteristics such as hummocks and jigsaw cracks distinguishing original deposits from reworked products is critical for geological understanding and hazard assessment because of their different origin frequency and environmental impact we present a methodology based on field evidence to differentiate such epiclastic breccias basal contact mapping constrained by accurate altitude and location data allows the reconstruction of deposit stratigraphy and geometry lithological analysis helps to distinguish the different units incorporation structures kinematic indicators and component mingling textures are used to characterise erosion and transport mechanisms we apply this method to the enigmatic sequence at perrier french massif central where four units u1 u4 have been interpreted either as debris flow or debris avalanche deposits the sequence results from activity on the monts dore volcano about 2 ma ago the epiclastic units are matrix supported with an almost flat top u2 and u3 have clear debris flow deposit affinities such as rounded clasts and intact blocks no jigsaw cracks u1 and u4 have jigsaw cracked blocks with matrix injection and stretched sediment blocks u1 lacks large blocks 10 m wide and has a homogenous matrix with an upward increase of trapped air vesicle content and size this unit is interpreted as a cohesive debris flow deposit spawned from a debris avalanche upstream in contrast u4 has large mega blocks up to 40 m wide sharp contacts between mixed facies zones with different colours and numerous jigsaw fit blocks open jigsaw cracks filled by monogenic intra clast matrix mega blocks are concentrated near the deposit base and are spatially associated with major substratum erosion this deposit has a debris avalanche distal facies with local debris flow affinities due to partial water saturation we also identify two landslide deposits l1 and l2 resulting from recent reworking that has produced a similar facies to u1 and u4 these are distinguishable from the original deposits as they contain blocks of mixed u1 u4 facies a distinctly less consolidated and more porous matrix and a fresh hummocky topography this work shows how to differentiate epiclastic deposits with similar characteristics but different origins in doing so we improve understanding of present and past instability of the monts dore and identify present landslide hazards at perrier

impacts do not initiate volcanic eruptions eruptions close to the crater many papers on meteorite impact suggest that large impacts can induce volcanic eruptions through decompression melting of the underlying rocks we perform numerical simulations of the impact of an asteroid with a diameter of 20 km striking at 15 km s 1 into a target with a near surface temperature gradient of 13 k km 1 cold case or 30 k km 1 hot case the impact creates a 250 300 km diameter crater with 10 000 km3 of impact melt however the crater collapses almost flat and the pressure field returns almost to the initial lithostat even an impact this large cannot raise mantle material above the peridotite solidus by decompression statistical considerations also suggest that impacts cannot be the common initiator of large igneous provinces any time in post heavy bombardment earth history

textural analysis of particles from el zaguan debris avalanche deposit nevado de toluca volcano mexico evidence of flow behavior during emplacement abstract el zaguan deposit originated at 28 000 yrs b p from the flank collapse of nevado de toluca a dacitic stratovolcano of the transmexican volcanic belt scanning electron microprobe analyses sem were performed on some particles from this deposit to observe microtextures produced during transport and emplacement of the debris avalanche flow particles from 2 250 m 0 1 mm and 2 4 mm granulometric classes were randomly selected at different outcrops and their surface textures were described the observed textures are divided in two groups basal and upper textures each one indicating different clast interactions basal textures are observed predominantly in the lower part of the deposit and consist of parallel ridges parallel grooves scratches and lips upper textures are mainly present in the upper part of the deposit and consisted of fractures percussion marks and broken or grinded crystals these characteristics coupled with field observations such as the presence of clastic dikes and deformed lacustrine mega blocks indicate that the basal part of the debris avalanche was moving in a partially liquefied state by contrast the particles in the upper part were able to move freely interacting by collision these microscopic textures are in agreement with previously described emplacement behaviors in debris avalanches of volcanic origin suggesting a stratified flow dominated by different transport and depositional mechanisms depending upon flow depth and possible fluid content at their base