temporal patterns of mass movements in the french alps abstract in massmovement hazard assessment it is important to include a temporal dimension in order to give a better insight into the activity of landslides in the french alps the temporal pattern of active landslides in varved clays and terres noires marls was studied as well as the temporal occurrence of debris flows the active landslides in varved clays and terres noires show in detail quite different patterns of movement which is ascribed to the different hydrological conditions in the landslide bodies the activity of debris flows is controlled by the occurrence of exceptionally high intensity rainstorms this means that the temporal pattern of debris flow activity will reflect the variability of storm rainfall activity as a first approach an average recurrence interval of some ten years was detected based on dendrochronological and lichenometrical evidence

duplex style and triangle zone formation insights from physical modeling duplexes are a common feature in thrust belts at many scales their geometries vary significantly from antiformal stacks with significant forethrusting in the cover e g southern pyrennes spain to triangle zones e g foreland canadian rockies to low displacement individually spaced ramp anticlines e g sub andean thrust belt bolivia we present a series of physical experiments demonstrating that the strength of the decollements relative to that of the intervening and overlying rock layers plays a significant role in controlling the duplex style the models comprise brittle layers made of dry quartz sand and decollements made of two types of viscous silicone polymers the strength of the decollements in the models is a function of the shortening rate applied to the model the relative strength of the decollements and surrounding rocks affects the development of active or passive roof duplexes triangle zones it also affects the amount of translation of individual thrust blocks and the spacing of thrust ramps which in turn determine if a duplex evolves into an antiformal stack or into individually spaced ramp anticlines model results indicate that specific associations of structural features form systematically under similar rheological and boundary conditions the presence of relatively strong decollements promotes local underthrusting of the cover individual ramp anticlines internal deformation of thrust sheets low early layer parallel shortening and sequential towards the foreland propagation of structures weak decollements promote forethrusting of the cover antiformal stacks coeval growth of structures and low internal strain with the exception of significant early layer parallel shortening no underthrusting at a regional scale occurred in any model

on the effect of crustal layering on ring fault initiation and the formation of collapse calderas collapse calderas can be attributed to subsidence of the magma chamber roof along bounding sub vertical normal faults ring faults after a decompression of the magma chamber it has previously been shown that for ring faults to initiate and thus facilitate collapse the stress field both at the surface and around the magma chamber must satisfy specific critical conditions here we present new numerical models that use a finite element method to investigate the effects of crustal layering on local stress field distribution results are compared with existing criteria for ring fault initiation different subsurface scenarios were simulated by varying the stiffness young s modulus of layers placed above the magma chamber and the host rock in which the chamber is seated we consider depressurisation of a magma chamber so as to simulate magma withdrawal results indicate that mechanical layering is a further first order variable in the rare achievement of stress conditions required for ring fault formation and may be influential in facilitating or inhibiting caldera collapse we show that for a given geometrical set up the magnitude and position of maximum tensional stress at the earth s surface are influenced by the occurrence and relative distribution of mechanically stiff or soft lithologies above the magma chamber for example tensional stress at surface may be reduced by the presence of stiff layers e g lavas or increased by soft layers e g pyroclastic units compared to generic simulations using a homogeneous background medium overall we find that the presence of mechanically soft material promotes surface fracture initiation in addition we suggest that the position of peak tensional stress at surface derived by the numerical models does not represent that related to the position of the bounding ring fault but is instead related to the position of initial tensional fractures appearing prior to the collapse faults

avulsions and the spatio temporal evolution of debris flow fans debris flows are water laden masses of sediment that move rapidly through channel networks and over alluvial fans where they can devastate people and property episodic shifts in the position of a debris flow channel termed avulsions are critical for debris flow fan evolution and for understanding flow hazards because avulsions distribute debris flow deposits through space and time however both the mechanisms of flow avulsion and their effects on the long term evolution of debris flow fans are poorly understood here we document and analyze the spatial and temporal patterns of debris flow activity obtained by repeat topographic analyses dendrogeomorphic and lichenometric reconstructions and cosmogenic radionuclide dating on 16 fans from japan usa switzerland france and kyrgyzstan where possible we analyze the observed spatio temporal patterns of debris flow activity in conjunction with high resolution topographic data to identify the main controls on avulsion we identify two main processes that control avulsions on debris flow fans operating over distinct time scales first during individual flows or flow surges deposition of sediment plugs locally blocks channels and forces subsequent flows to avulse into alternative flow paths plug deposition is a stochastic process but appears to depend in part on the sequence of flow volumes the geometry of the channel and the composition of the flows second over time scales of tens of events the average locus of debris flow deposition gradually shifts toward the topographically lower parts of a fan highlighting the importance of topographic compensation for fan evolution our documented debris flow avulsions often but not always follow a pattern of channel plugging backstepping of deposition toward the fan apex over one or more flows avulsion and establishment of a new active channel large flows can have contrasting impacts depending on whether or not they follow smaller flows that have deposited channel plugs these results suggest that avulsions and spatio temporal patterns of debris flow fan formation strongly depend on both the magnitude frequency distribution and the sequence of the flows feeding a fan while individual avulsions are generally abrupt and difficult to predict the presence of debris flow plugs and patterns of backstepping may be useful as indicators of impending avulsions over longer time scales the compensational tendency of flows to avulse into topographic depressions on the fan may also be used to identify sectors of the fan that are at risk of future inundation

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

modelling hanging wall accommodation above rigid thrust ramps abstract experimental models are used to study the role of material rheology in hanging wall accommodation above rigid flat ramp flat thrust footwalls the deformation in the hanging wall was accomplished by forwards sliding along a rigid basal staircase trajectory with a variable ramp angle ranging from 15 to 60 we model different ramp angles to examine hanging wall accommodation styles above thrust ramps of overthrust faults ranging from 15 to 30 as well as above pre existing normal faults ranging from 45 to 60 for the hanging walls we used stratified frictional sand and viscous silicone putty materials in this paper we study three types of models type 1 models represent purely frictional hanging walls where accommodation above thrust ramps was by layer parallel thickening and by generating a series of back thrusts type 2 and 3 models represent stratified frictional viscous hanging walls in these models accommodation was by a complex association of reverse and normal faults mainly controlled by the rheological anisotropy as well as by the ramp inclination angle in type 2 models the silicone covered only the lower flat while in type 3 models it also covered the rigid ramp for 30 in type 2 models and 45 in type 3 models the viscous layer inhibited the development of back thrusts in the frictional hanging wall instead the silicone thickened to develop a ductile ramp for values higher than 30 in type 2 models and 45 in type 3 models back thrusts develop in response to the bulk compression the experiments simulate many structures observed above natural thrust ramps with 30 and pre existing normal faults with 45 the models emphasise the importance of a basal ductile layer which allows the hanging wall to step up over the rigid ramp by building up its own ductile ramp the models also emphasise that foreland directed normal faulting can develop at a thrust front in the case that the vertical stress due to gravity exceeds the horizontal stress due to end loading within a thrust wedge

the role of the sediment budget in understanding debris flow susceptibility this study proposes a sediment budget model to predict the temporal variation of debris volume stored in a debris flow prone watershed the sediment budget is dominated by shallow landslides and debris outflow the basin topography and the debris volume stored in the source area of the debris flow prone watershed help evaluating its debris flow susceptibility the susceptibility model is applied to the tungshih area of central western taiwan the importance of the debris volume in predicting debris flow susceptibility is reflected in the standardized coefficients of the proposed statistical discriminant model the high prediction rate 0 874 for the occurrence of debris flows justifies the capability of the proposed susceptibility models to predict the occurrence of debris flows this model is then used to evaluate the temporal evolution of the debris flow susceptibility index the analysis results show that the numbers of watershed which are classified as a debris flow group correspond well to storage of sediment at different time periods these numbers are 10 before the occurrence of chi chi earthquake 13 after the occurrence of chi chi earthquake 16 after the occurrence of landslides induced by typhoon mindulle typhoon m and 14 after the occurrence of debris flows induced by typhoon m it indicates that the occurrence of 7 6 chi chi earthquake had significant impact on the debris flow occurrence during subsequent typhoons copyright 2009 john wiley sons ltd

geomorphic effectiveness a linear concept in a non linear world geomorphic effectiveness has been an influential concept in geomorphology since its introduction by reds wolman and john miller in 1960 it provided a much needed framework to assess the significance of an event by comparing event magnitude to the resultant geomorphic effects initially this concept was applied primarily in river channels under the linear assumption that geomorphic responses to similarly sized flood events will be consistent numerous authors have since attempted to quantify a direct proportional relationship between event magnitude and different forms of geomorphic response in a variety of geomorphic settings in doing so these investigations applied an array of metrics that were difficult to compare across different spatiotemporal scales and physiographic and geomorphic environments critically the emergence of other geomorphic concepts such as sensitivity connectivity thresholds and recovery has shown that relationships between causes events and geomorphic effects responses are often complex and non linear this paper disentangles the complex historical development of the geomorphic effectiveness concept and reviews the utility of various metrics for quantifying effectiveness we propose that total energy joules is the most appropriate metric to use for quantifying the magnitude of disturbance events cause and volumetric sediment flux associated with landform modification is the most appropriate metric for quantifying geomorphic effects while both metrics are difficult to quantify they are the only ones which facilitate comparison across a range of spatiotemporal scales comparability in a variety of geomorphic environments flexibility the geomorphic effectiveness concept can continue to be useful provided that geomorphologists use flexible and comparable metrics today geomorphologists are better prepared to consider the influence of non linear processes on determinations of geomorphic effectiveness allowing investigators to not only determine if a disturbance event was effective but also to explain why or why not copyright

correlation between climate man and debris flow events a palynological approach abstract debris flows occur frequently in the high alpine catchment area of lake lago di braies 1492 m a s l the sediments of the lake are used for detecting and dating past debris flow events during the last 4000 years and their driving factors comparing palynological and sedimentological data the results suggest that mainly heavy precipitation in winter and summer as well as snow melt events in spring enhance the release of debris flow events therefore especially longer periods with humid and cool climate conditions show a clearly higher debris flow activity the pollen diagram documents human impact on the catchment area since the late neolithic but the anthropogenic activities influenced the event activity only indirectly grazing and deforestation reduced the retention of debris by vegetation and results in thicker debris flow layers in the lake sediments

how the development of magma chambers affects collapse calderas insights from an overview here we review the studies of pluton emplacement and caldera collapse proposing a model linking the two processes the shallow rise of magma occurs by dyking and its emplacement occurs along major anisotropies many plutons are emplaced at subhorizontal discontinuities forming sills these will eventually grow forming laccoliths the most common mechanism to store shallow magma calderas are a surface expression of these reservoirs and can be approximated by a piston cylinder which sinks due to 1 underpressure in the reservoir producing outward dipping reverse faults 2 overpressure within a silllike chamber undergoing doming and 3 overpressure within a laccolith generating apical tensile stresses in the latter two cases inward dipping normal faults form we suggest that collapse calderas are the surface expression of pressure variations within laccoliths or tabular intrusions their geometric relationships depend on the shape and aspect ratio of the intrusion and its pressure conditions tabular intrusions generating ring faults mainly at their tips form calderas with a similar width to the intrusion laccoliths may generate ring faults along the intrusion roof forming calderas narrower than the intrusion the outward or inward dip of the caldera faults results from the underpressure or overpressure conditions within the

differentiating past events on a cone influenced by debris flow and snow avalanche activity a dendrogeomorphological approach dendrogeomorphology was used to investigate past events on a cone affected by both debris flows and snow avalanches we report on results of 520 cores from 251 injured larix decidua mill and picea abies l karst trees sampled on the birchbach cone swiss alps detailed analysis of tree ring sequences allowed dating of 561 growth disturbances in individual trees for a 252 yr period extending from 1750 to 2002 which could be attributed to 30 different event years we then localized the position of rows of traumatic resin ducts trds within the tree ring so as to assess the intra seasonal position of damage in agreement with data on the local growth period trds located at the beginning of the new growth ring were considered the result of avalanche impacts that occurred during the dormant season or in earliest earlywood between late october and early may in contrast trd found in late earlywood or within latewood were considered the result of periglacial debris flow activity as these layers of the tree ring are locally formed between july and early october for nine out of the 30 reconstructed event years the intra seasonal timing of trds indicated that reactions must be the result of past snow avalanche activity in 19 other event years trds showed that damage has been caused between july and early october and thus through debris flows in the birchbach torrent finally the spatial patterns of trees showing reactions as a result of particular events were assessed so as to approximate the extent of past debris flows and snow avalanches copyright 2006 john wiley sons ltd

understanding caldera structure and development an overview of analogue models compared to natural calderas abstract understanding the structure and development of calderas is crucial for predicting their behaviour during periods of unrest and to plan geothermal and ore exploitation geological data including that from analysis of deeply eroded examples allow the overall surface setting of calderas to be defined whereas deep drillings and geophysical investigations provide insights on their subsurface structure collation of this information from calderas worldwide has resulted in the recent literature in five main caldera types downsag piston funnel piecemeal trapdoor being viewed as end members despite its importance such a classification does not adequately examine a the structure of calderas particularly the nature of the caldera s bounding faults and b how this is achieved including the genetic relationships among the five caldera types various sets of analogue models specifically devoted to study caldera architecture and development have been recently performed under different conditions apparatus materials scaling parameters stress conditions the first part of this study reviews these experiments which induce collapse as a result of underpressure or overpressure within the chamber analogue the experiments simulating overpressure display consistent results but the experimental depressions require an exceptional amount of doming seldom observed in nature to form therefore these experiments are not appropriate to understand the structure and formation of most natural calderas the experiments simulating underpressure reveal a consistent scenario for caldera structure and development regardless of their different boundary conditions these show that complete collapse proceeds through four main stages proportional to the amount of subsidence progressively characterized by 1 downsag 2 reverse ring fault 3 peripheral downsag 4 peripheral normal ring fault the second part of this study verifies the possibility that these latter calderas constitute a suitable analogue to nature and consists of a comprehensive comparison of the underpressure experiments to natural calderas this shows that all the experimental structures as well as their progressive development are commonly observed at natural calderas highlighting a consistency between models and nature as the shallow structure of experimental calderas corresponds to a precise architecture at depth it provides a unique key to infer the deeper structure of natural calderas recognizing diagnostic surface features within a caldera will thus allow it to be categorized within a precise structural and evolutionary context the general relationship between the evolutionary stage of a caldera and its d s diameter subsidence ratio allows such a quantification with stage 1 calderas characterized by d s 40 stage 2 by 18 the four stages adequately explain the architecture and development of the established caldera end members along a continuum where one or more end members downsag piston funnel piecemeal trapdoor may correspond to a specific stage while such a continuum is controlled by progressive subsidence specific collapse geometries will result from secondary contributory factors roof aspect ratio collapse symmetry pre existing faults these considerations allow proposing an original classification of calderas incorporating their structural and genetic features

abnormal reverse faulting above a depleting reservoir subsurface deformation is observed during pumping of some hydrocarbon fields deformation features include subsidence centered on the field and subsidence related centripetal horizontal displacements and faulting focal mechanisms yield reverse movements on steeply dipping faults in our sand silicone analogue model the reservoir is represented by a latex balloon or by undercompacted ground sand deflation of the reservoir results in formation of steeply dipping reverse faults bounding a downward opened cone the cone moves downward to follow the reservoir contraction faults along the cone are straight beneath a thick reservoir cover and tend to curve upwards with decreasing cover our results similar to natural structures observed around magma chambers allow us to reinterpret paul segall9s numerical model of poroelastic stresses caused by changes in the distribution of pore fluids and draw a new pattern of active faults

stress fields controlling the formation of nested and overlapping calderas implications for the understanding of caldera unrest abstract commonly the formation of a collapse caldera does not necessarily imply the end of the volcanic activity in the area in many cases successive calderas may form close to the previous collapse depression or intersecting it leading to overlapping collapse structures occasionally subsequent caldera collapses may take place at the interior of the first caldera creating nested collapse structures during the last years several authors have investigated numerically how the stress field around magma chambers may favour the formation of collapse calderas assuming that the host rock surrounding the magmatic reservoir behaves as a linear elastic homogeneous medium the numerical models presented in this work study how a caldera collapse may modify the stress field of a volcanic area and hence the conditions for the formation of future collapse calderas our models take into account the effect of the collapse structure considering it as a mechanical discontinuity we also investigate the mechanical influence of the intra and extra caldera deposits on the formation of new calderas all the numerical models are two dimensional assuming plane strain and considering that the surrounding crust behaves as a linear homogeneous elastic material the computational domain corresponds to a cross section of the upper crust 50 25 km and magma chambers are modelled as sill like cavities located at a certain depth below the earth s surface the existing collapse caldera depression is 8 km wide and 2 75 km deep however we consider the caldera infill i e intra caldera material to be 1 25 or 1 75 km thick we assume as loading conditions an underpressure of 10 mpa imposed at the chamber walls that is negative excess pressure in the chamber in some numerical runs we have considered the existence of a previous ring fault by introducing a thin and elongate vertical weak zone at the caldera margins we find that the stress field around shallow level magma chambers favouring the formation of either nested or overlapping caldera structures depend on 1 the dimensions of the previous caldera collapse structure 2 the size of the new magma chamber susceptible to generate the subsequent caldera forming event and 3 the mechanical properties of the surrounding host rock and the syn and post collapse materials moreover results obtained in this paper may be relevant to the interpretation of unrest episodes in active caldera systems our numerical results confirm that the formation of multiple calderas nested overlapping or separate imply the development of new shallow magma chambers different from the one that lead to the first collapse caldera in consequence interpretation of magmatic unrest episodes in active collapse calderas should not be based on the assumption that the responsible for the current unrest is the same system that originated the caldera

fan surface evidence for debris flow avulsion controls and probabilities saline valley california debris flow fans form by shifts of the active channel termed avulsions field and experimental evidence suggest that debris flow avulsions may be induced by depositional lobes that locally plug a channel or super elevation of the channel bed above the surrounding fan surface by analogy to fluvial fans to understand debris flow avulsion processes we differentiate between these controls by quantifying the spatial distribution of debris flow lobe and channel dimensions along with channel bed super elevation on nine debris flow fans in saline valley california usa channel beds are generally super elevated by 2 5 channel depths above the fan surface and locally by more than 7 channel depths thereby substantially exceeding super elevation on fluvial fans depositional lobe thickness and channel depth decrease with distance from the fan apex although both are highly variable across the fans median channel depths roughly correspond to the 50 75th percentiles of lobe thicknesses while minimum channel depths roughly correspond to the 10 25th percentiles in contrast the thicknesses of lobes that have triggered avulsions roughly equal local channel depths and are on average twice as thick as the local median lobe thickness the spatial correspondence between avulsion locations and thick lobe deposits and the lack of correlation with channel bed super elevation leads us to infer that avulsions on these fans are mostly caused by thick lobes forming channel plugs although results may vary with climatic and tectonic setting our findings indicate that avulsion hazard assessment on populated fans should include mapping and monitoring of channel depths relative to typical deposit thicknesses on a given fan

experimental and field study of the effects of lithological contrasts on thrust related deformation 1 the influence of mechanical contrasts related to stratigraphy on thrust related deformation are investigated by sandbox modeling using combinations of granular materials possessing different mechanical properties and by structural analysis of a natural example from the spanish pyrenees the models used in this study consist of loose sand and glass microbeads in varied stratigraphic thicknesses and sequences and the natural example consists of a weakening upward sequence of sandstones and carbonates in both cases layers of different composition had a mechanical significance and accommodated shortening differently model sand and natural sandstone exhibited most shortening by thrust imbrication irrespective of stratigraphic position whereas glass microbeads and carbonates exhibited wider deformation zones and enhanced folding accompanied by displacement reduction along thrust surfaces cutting them transitions of structural style were guided by the stratigraphy rather than by a simple unidirectional propagation from a step up layer parallel shortening was accommodated by diverse mechanisms but did not vary significantly in layers which had different properties the magnitude with which mechanical contrasts were expressed in the models was influenced by the strength at the decollement zone the effect of mechanical contrast was more profound in models shortened above a low friction decollement

geology and eruptive history of the rabaul caldera area papua new guinea abstract rabaul caldera is the most recently active 1937 1943 of four adjoining volcanic centres aligned north south through the northern extremity of eastern new britain geological mapping after the 1983 1985 rabaul seismic and deformation crisis has partially revealed a long and complex eruption history dominated by numerous explosive eruptions the largest accompanied by caldera collapse the oldest exposed eruptives are the basaltic pre caldera cone tovanumbatir lavas k ar dated at 0 5 ma the dacitic rabaul quarry lavas exposed in the caldera wall and k ar dated at 0 19 ma are overlain by a sequence of dacitic and andesitic pyroclastic flow and fall deposits uplifted coral reef limestones interbedded within the pyroclastic sequence on the northeast coast suggest that explosive eruptions in the rabaul area had commenced prior to the 0 125 ma last interglacial high sea level stand the pyroclastic sequence includes the large boroi ignimbrites and malaguna pyroclastics both 40 ar 39 ar dated at about 0 1 ma and the barge tunnel ignimbrite 40 ar 39 ar dated at around 0 04 ma few reliable ages exist for the many younger eruptives these include holocene ignimbrites of the latest caldera forming eruptions the raluan pyroclastics variously dated 14c at either about 3500 or 7000 yr b p and the ca 1400 yr b p rabaul pyroclastics at least eight intracaldera eruptions have occurred since the 1400 yr b p collapse building small pyroclastic and lava cones within the caldera a major erosional episode is evident as a widespread unconformity in the upper pyroclastic stratigraphy at rabaul lacking relevant radiometric ages this episode is assumed to have occurred during last glaciation low sea levels and is here arbitarily dated at ca 20 ka at least five possibly nine significant ignimbrite eruptions have occurred at rabaul during the last 20 ka the new eruptive history differs considerably from that previously published which considered ignimbrite eruption and caldera collapse to have first occurred at 3500 yr b p rabaul volcanism has been dominated by two main types a basaltic and basaltic andesite cone building eruptions and b dacitic and rarely andesitic or rhyolitic plinian ignimbrite eruptions of both high and low aspect ratio types the 1400 yr b p rabaul ignimbrite is a type example of a low aspect ratio high energy and potentially very damaging eruption fine vitric ash deposits common in the rabaul pyroclastic sequence demonstrate the frequent modification of eruptions by external water probably related to early caldera lakes or bays interbedding of these fine ashes with plinian pumice lapilli beds suggests that many early eruptions occurred from multiple vents located in both wet and dry areas

deformation patterns and structural vergence in brittle ductile thrust wedges an additional analogue modelling perspective abstract scaled physical models of brittle ductile thrust wedges investigated the causes leading to the development of the various structural styles observed in fold and thrust belts and accretionary prisms this study focused on some potential controlling factors such as 1 relative strength between the brittle overburden and the viscous decollement layer brittle ductile coupling 2 the effect of very low basal friction and 3 pre shortening rheological stratigraphy including decollement strength type 1 models had a single basal decollement whereas in type 2 models the decollement was embedded in brittle material simulating an upper roof sequence and a lower floor sequence other models investigated the effects produced by an horizon of mechanical weakness at the base of the viscous decollement type 1a and the role of decollement offset type 3 the experiments suggest the occurrence of a genetic relationship between the absolute values of shear stresses and the development of dominating hinterland or foreland verging thrust faults or dually verging thrusts by controlling the orientation of the 1 axis basal friction and decollement offset is found to effectively influence the deformation pattern and structural vergence and could explain the development of dominating backthrusts in cascadian type margins the decoupling of a frontal monocline via the upper decollement type 2 models produced passive roof duplex structures termed composite roof duplex when the frontal monocline coexists with outward fold propagation

formation and development of normal fault calderas and the initiation of large explosive eruptions the ring fractures that form most collapse calderas are steeply inward dipping shear fractures i e normal faults at the surface of the volcano within which the caldera fault forms the tensile and shear stresses that generate the normal fault caldera must peak at a certain radial distance from the surface point above the center of the source magma chamber of the volcano numerical results indicate that normal fault calderas may initiate as a result of doming of an area containing a shallow sill like magma chamber provided that the area of doming is much larger than the cross sectional area of the chamber and that the internal excess pressure in the chamber is smaller than that responsible for doming this model is supported by the observation that many caldera collapses are preceded by a long period of doming over an area much larger than that of the subsequently formed caldera when the caldera fault does not slip eruptions from calderas are normally small nearly all large explosive eruptions however are associated with slip on caldera faults during dip slip on and doming of a normal fault caldera the vertical stress on part of the underlying chamber suddenly decreases this may lead to explosive bubble growth in this part of the magma chamber provided its magma is gas rich this bubble growth can generate an excess fluid pressure that is sufficiently high to drive a large fraction of the magma out of the chamber during an explosive eruption

formation of nonintersecting nested calderas insights from analogue models approximately 56 of 60 known nested calderas are pairs of nonintersecting subcircular structures with similar eccentricities the consistent configuration suggests an interdependence in the formation of the two annular structures this possibility has been investigated using 11 analogue models with sand crust analogue overlying silicone magma analogue silicone withdrawal induces collapse within the sand forming two concentric depressions the first formed inner depression bordered by outward dipping reverse ring faults mean dip of 75 induces the collapse of the outer depression bordered by inward dipping normal ring faults mean dip of 84 an original linear relationship is determined among the ratios between the diameters of the two depressions and the depth of the ring faults selected pairs of natural nested calderas show similar proportions highlighting a consistent architecture this suggests that some pairs of nonintersecting nested calderas result from a single collapse event

effects of debris flow magnitude frequency distribution on avulsions and fan development shifts in the active channel on a debris flow fan termed avulsions pose a large threat because new channels can bypass mitigation measures and cause damage to settlements and infrastructure recent but limited field evidence suggests that avulsion processes and tendency may depend on the flow size distribution which is difficult to constrain in the field here we investigate how the flow magnitude frequency distribution and the associated flow magnitude sequences affect avulsion on debris flow fans we created three experimental fans with contrasting flow size distributions 1 a uniform distribution 2 a steep double pareto distribution with many flows around the mean and a limited number of large flows and 3 a shallow double pareto distribution with fewer flows around the mean and more abundant large flows the fan formed by uniform flows developed through regular sequences of stepwise channelization backstepping of deposition toward the fan apex and avulsion over multiple flows in contrast the wide range of sizes in the double pareto distributions led to distinct avulsion mechanisms and fan evolution here large flows could overtop channels creating levee breaches that could initiate avulsion immediately or in subsequent events moreover sequences of small to moderately sized flows could deposit channel plugs triggering avulsion in the next large flow this mechanism was most common on the fan formed by a steep double pareto distribution but was rare on the fan formed by a shallow double pareto distribution where large flows were more frequent we infer that some flow size distributions are more likely to cause avulsions especially those that produce abundant sequences of small flows followed by a large flow critically avulsions in our experiments could occur by either large single events or over multiple flows this observation has important implications for hazard assessment on debris flow fans suggesting that attention should be paid to flowhistory as well as flow size

the new worldwide collapse caldera database ccdb a tool for studying and understanding caldera processes collapse calderas are one of the most important volcanic structures not only because of their hazard implications but also because of their high geothermal energy potential and their association with mineral deposits of economic interest the objective of this work is to describe a new general worldwide collapse caldera database ccdb in order to provide a useful and accessible tool for studying and understanding caldera collapse processes the principal aim of the ccdb is to update the current field based knowledge on calderas merging together the existing databases and complementing them with new examples found in the bibliography and leaving it open for the incorporation of new data from future studies this database does not include all the calderas of the world but it tries to be representative enough to promote further studies and analyses we have performed a comprehensive compilation of published field studies of collapse calderas including more than 200 references and their information has been summarized in a database linked to a geographical information system gis application thus it is possible to visualize the selected calderas on a world map and to filter them according to different features recorded in the database e g age structure the information recorded in the ccdb can be grouped in seven main information classes caldera features properties of the caldera forming deposits magmatic system geodynamic setting pre caldera volcanism caldera forming eruption sequence and post caldera activity additionally we have added two extra classes the first records the references consulted for each caldera the second allows users to introduce comments on the caldera sample such as possible controversies concerning the caldera origin a further purpose of this work is to construct the ccdb web page in this web page where registered users can acquire the current database version as well as to propose corrections or updates and to exchange information with other registered members also involved in the study of caldera collapse processes additionally the ccdb includes a formulary that will facilitate the incorporation of new calderas into the database

quantitative reconstruction of late holocene surface evolution on an alpine debris flow fan debris flow fans form a ubiquitous record of past debris flow activity in mountainous areas and may be useful for inferring past flow characteristics and consequent future hazard extracting information on past debris flows from fan records however requires an understanding of debris flow deposition and fan surface evolution field scale studies of these processes have been very limited in this paper we document the patterns and timing of debris flow deposition on the surface of the large and exceptionally active illgraben fan in southwestern switzerland we use terrain analysis radiocarbon dating of sediment fill in the illgraben catchment and cosmogenic 10be and 36cl exposure dating of debris flow deposits on the fan to constrain the temporal evolution of the sediment routing system in the catchment and on the fan during the past 3200 years we show that the fan surface preserves a set of debris flow lobes that were predominantly deposited after the occurrence of a large rock avalanche near the fan apex at about 3200 years ago this rock avalanche shifted the apex of the fan and impounded sediment within the illgraben catchment subsequent evolution of the fan surface has been governed by both lateral and radial shifts in the active depositional lobe revealed by the cosmogenic radionuclide dates and by cross cutting geometrical relationships on the fan surface this pattern of frequent avulsion and fan surface occupation provides field scale evidence of the type of large scale compensatory behavior observed in experimental sediment routing systems

experimental modelling of thin skinned shortening around magmatic intrusions magmatic intrusions can trigger thin skinned compression of the adjacent sedimentary cover by three processes a gravity gliding away from the topographic dome resulting from the ascending magma b fluid push from the rear resulting from forceful intrusion then lateral spreading of the magma and c increased loading by volcanic accumulation the applicability of the first two mechanisms gravity gliding and fluid push was tested using dynamically scaled experiments model results help to elucidate the kinematics and structural evolution of thrusts and folds formed by such processes and determine which geological parameters control the deformation style the results show that the presence of a weak layer within the sedimentary overburden is essential to form thrust and fold belts around the intrusion experiments suggest that although gravity gliding can dominate the early stages of deformation most of the deformation is caused by lateral spreading of the fluid magma pushing the adjacent sedimentary rocks models also suggest that true laccoliths can form only if the sedimentary section comprises a basal low viscosity stratum comparison with natural examples allows the evaluation of the applicability of experimental results

frequency magnitude distribution of debris flows compiled from global data and comparison with post fire debris flows in the western u s abstract forecasting debris flow hazard is challenging due to the episodic occurrence of debris flows in response to stochastic precipitation and in some areas wildfires in order to facilitate hazard assessment we have gathered available records of debris flow volumes into the first comprehensive global catalog of debris flows n 988 we also present results of field collection of recent debris flows n 77 in the northern rocky mountains where debris flow frequency increases following wildfire as a first step in parameterizing hazard models we use frequency magnitude distributions and empirical cumulative distribution functions ecdfs to compare volumes of post fire debris flows to non fire related debris flows the ecdf of post fire debris flow volumes is significantly different at 95 confidence from that of non fire related debris flows suggesting that the post fire distribution is composed of a higher proportion of small events than that of non fire related debris flows the slope of the frequency magnitude distribution of post fire debris flows is steeper than that of non fire related debris flows corroborating evidence that small post fire debris flows occur with a higher relative frequency than non fire related debris flows taken together the statistical analyses suggest that post fire debris flows come from a different population than non fire related debris flows and their hazard must be modeled separately we propose two possible non exclusive explanations for the fact that the post fire environment produces a higher proportion of small debris flows 1 following fires smaller storms or effective drainage areas can trigger debris flows due to increased runoff and or decreases in root strength resulting in smaller volumes and increased probability of failure and 2 fire increases the probability and frequency of debris flows causing their distribution to shift toward smaller events due to limitations in sediment supply

outward dipping ring fault structure at rabaul caldera as shown by earthquake locations the locations of a large number of earthquakes recorded at rabaul caldera in papua new guinea from late 1983 to mid 1985 have produced a picture of this active caldera s structural boundary the earthquake epicenters form an elliptical annulus about 10 kilometers long by 4 kilometers wide centered in the southern part of the rabaul volcanic complex a set of events with well constrained depth determinations shows a ring fault structure that extends from the surface to a depth of about 4 kilometers and slopes steeply outward from the center of the caldera this is the first geophysical data set that clearly outlines the orientation of an active caldera s bounding faults this orientation however conflicts with the configuration of many other calderas and is not in keeping with currently preferred models of caldera formation

experimental studies of collapse calderas we present experiments to investigate the formation of calderas using inflatable balloons in a medium of fused alumina powder this system provides a model of a magma chamber in an homogeneous elastic media which scales to volcanic systems in terms of geometry and strength of the media three kinds of experiment were performed i the balloon was inflated and then deflated simulating pre emption tumescence and caldera collapse ii the balloon was deflated without prior inflation simulating caldera formation without pre eruption tumescence iii three balloons were inflated and deflated in a line simulating multiple magma chambers the influence of magma chamber shape was investigated using different shaped balloons balloon inflation formed a dome with a surface pattern of shallow polygonal fractures centrally and radial fractures peripherally uplift occurs along deep inward dipping concentric reverse faults deflation generated depressions with an outer set of concentric ring fractures surrounding a central funnel formed by sagging along numerous minor faults some of which formed during doming during collapse the concentric reverse faults were converted to normal faults and the radial normally faulted fractures were converted to reverse faults without prior tumescence an outer set of near vertical ring faults surround a flat central depression due to collapse of a coherent block an inner set of outward dipping reverse ring faults also develop the experimental caldera area increased with balloon size and increased as the balloon depth decreased for a given balloon volume and depth collapse area is significantly smaller if doming had occurred prior to collapse arcuate fractures can provide the pathways for magma ascent during pre emption tumescence smith bailey 1968 but the shallow radial and polygonal fracture systems may also influence vent location changes in fault geometry and sense of movement extension to compression and vice versa occur when subsidence initiates and can account for changes in vent location that occur in many eruptions multiple overlapping collapse calderas are associated with magma chamber migration whereas nested calderas can result from the activity of a single magma chamber

experiments on cauldron formation a polygonal cauldron and ring fractures abstract the sequence of formation of the motojuku type cauldron fujita et al 1970 is summarized as follows 1 doming due to ascent of a magma body 2 development of normal faults which produce a polygonal cauldron and 3 eruption of acidic to intermediate pyroclastic material model experiments on a scale of 1 200 000 reveal that ascent of the magma body which was imitated by a ball of hardened putty produced a polygonal cauldron composed of radial and concentrically arranged short fractures on the roof of a dome no ring fractures were formed this type of cauldron develops near the surface on the other hand emission of magma which was imitated by evaporation of a ball of dry ice in brittle powdered material caused ring faults dipping outward and a circular cauldron without up or downwarping this type of cauldron develops upward from the magma body the former model is equated to the motojuku type cauldron and the latter model to the other types of cauldrons with ring fractures

chronology of processes in high gradient channels of medium high mountains and their influence on the properties of alluvial fans abstract high gradient channels are the locations of the greatest geomorphological activity in medium high mountains the channels frequency and character influence the contemporary morphology and morphometry of alluvial fans there is currently no detailed information regarding the frequency of these processes in high gradient channels and the evolution of alluvial fans in medium high mountains in central europe this study in the moravskoslezske beskydy mts analysed 22 alluvial fans 10 debris flow fans and 12 fluvial fans the processes occurring on the fans were dated using dendrogeomorphological methods a total of 748 increment cores were taken from 374 trees to reconstruct 153 geomorphological process events 60 debris flow and 93 floods the frequency of the processes has been considerably increasing in the last four decades which can be related to extensive tree cutting since the 1970s processes in high gradient channels in the region affecting the alluvial fans across the mountain range are predominantly controlled by cyclonal activity during the warm periods of the year probable triggers of local events are heavy downpours in the summer in addition spring snowmelt has been identified as occasionally important this study of the relations affecting the type and frequency of the processes and their effect on the properties of alluvial fans led to the creation of a universal framework for the medium high flysch mountains of central europe the framework particularly reflects the influence of the character of hydrometeorological extremes on the frequency and type of processes and their reflection in the properties of alluvial fans

coexistence of fault propagation and fault bend folding in curve shaped foreland fold and thrust belts examples from the northern apennines italy terra nova 24 396 406 2012 abstract fault bend and fault propagation folds have unique profiles in foreland fold and thrust belts that are caused by different fault related folding models and controlled by the mechanical characteristic of the multilayer and or by reactivation of normal faults in a positive inversion tectonics context in this work nne ssw and nw se trending anticlines related to the neogene curve shaped olevano antrodoco sibillini thrust northern apennines italy are investigated to reconstruct fault related folding mechanisms geological and structural analysis allows us to interpret the nne ssw trending anticlines as fault bend reactivation folds and the nw se trending anticlines as fault propagation shortcut anticlines the coexistence of fault bend and fault propagation folding processes involving the same multi layered succession in different arms of a curve shaped thrust is explained in an inversion tectonics context this along strike variation of different folding mechanisms might be recognised in similar curve shaped orogenic thrust belts controlled by structural inheritance

estimation of debris flow magnitude in the eastern italian alps the estimation of debris ow magnitude is an essential step in the assessment of debris ow hazard although different methods have been developed for the assessment of debris ow magnitude this is still a dif cult task because of the complexity of ow processes and the lack of data to test estimation procedures in many mountainous regions data on debris ow magnitude from 127 basins in the eastern italian alps have been collected from scienti c and technical journals technical reports historical documents gathered from local archives and eld surveys these data were used to develop and test different predictive approaches encompassing regression equations geomorphological surveys and probabilistic analysis of time series regression techniques were used to correlate debris ow magnitude to morphometric parameters and geological characteristics of the basins values of the channel debris yield rate contribution per unit length of channel proposed in the literature were compared with data from the study area for identifying reference values for channel stretches of different morphological characteristics although limited to the few basins in which suf cient data were available the probabilistic analysis of time series of debris ow magnitude provides indications about the relations between magnitude and frequency of debris ows some observations about the capability and drawbacks of considered methods are presented and the combined use of different approaches for the estimation of debris ow magnitude is suggested copyright 2004 john wiley sons ltd

investigation of growth fault bend folding using discrete element modeling implications for signatures of active folding above blind thrust faults 1 using the discrete element modeling method we examine the two dimensional nature of fold development above an anticlinal bend in a blind thrust fault our models were composed of numerical disks bonded together to form pregrowth strata overlying a fixed fault surface this pregrowth package was then driven along the fault surface at a fixed velocity using a vertical backstop additionally new particles were generated and deposited onto the pregrowth strata at a fixed rate to produce sequential growth layers models with and without mechanical layering were used and the process of folding was analyzed in comparison with fold geometries predicted by kinematic fault bend folding as well as those observed in natural settings our results show that parallel fault bend folding behavior holds to first order in these models however a significant decrease in limb dip is noted for younger growth layers in all models on the basis of comparisons to natural examples we believe this deviation from kinematic fault bend folding to be a realistic feature of fold development resulting from an axial zone of finite width produced by materials with inherent mechanical strength these results have important implications for how growth fold structures are used to constrain slip and paleoearthquake ages above blind thrust faults most notably deformation localized about axial surfaces and structural relief across the fold limb seem to be the most robust observations that can readily constrain fault activity and slip in contrast fold limb width and shallow growth layer dips appear more variable and dependent on mechanical properties of the strata

the tavua caldera fiji a complex shoshonitic caldera formed by concurrent faulting and downsagging the shoshonitic tavua volcano in viti levu fiji developed from a submarine to a subaerial environment and comprises lava flows dominantly of absarokite composition with debris flows and conglomerates on its flanks the tavua caldera formed in the centre of the volcano within the caldera avalanche deposits are overlain by the turtle pool formation which consists of lacustrine sediments shoshonite lavas and hydrovolcanic pyroclastic lapilli tuffs subsequently the inner caldera formed in the centre of the tavua caldera this was infilled by the morrisons pool formation which comprises lavas and pyroclastic rocks of banakite composition with minor intercalated sediment thus the magmas became progressively more evolved in addition to normal movement along regional and concentric faults subsidence which exceeds 3 km was accomplished via substantial downsagging evidence for this is the consistent centripetal dip of caldera fill material which locally exceeds 50 in parts of the turtle pool formation centripetal dips are also recognized in pre caldera rocks immediately outside the tavua caldera subsidence was episodic caused by periodic non explosive withdrawal of underlying magma to erupt on the flanks and in part within the caldera downsagging and fault movement were responsible for approximately equal amounts of subsidence and their interaction produced a caldera with the shape of a stepped funnel

seismic expression and kinematics of a fault related fold termination rosario structure maracaibo basin venezuela abstract fold terminations are primary features of deformed belts and are critical elements in understanding the 3 d kinematics of fault related folds typically such terminations form due to loss of displacement on the genetically related thrust fault and or to along strike change in fault attitude forming a lateral or oblique ramp to unambiguously determine the mechanism responsible for a given fault related fold termination footwall cutoffs and along strike displacement variation must be constrained reflection seismic data can provide the detail necessary to uniquely describe these features the rosario structure in the maracaibo basin venezuela is an example of a natural fault related fold that possesses a southern termination whose fold fault geometry and along strike displacement variation are constrained by industry reflection seismic and well data we interpret that the fold plunge near the southern termination is due to an along strike decrease in displacement the fault geometry associated with the southern termination changes from a flat ramp flat at the crest of the structure where displacement is greatest to simply a ramp near the lateral fault tip without forming a lateral or oblique ramp we combine these observations regarding displacement and fault geometry with the assumption that along strike spatial variations reflect temporal variations during development of a fault related fold to propose a kinematic model for the rosario structure specifically we suggest that the structure first initiated as an isolated fault ramp within the stiff carbonate and clastic units with increased shortening the fault grew to link with upper and lower detachments in the weaker shale units to create a hybridized fault bend fold our model suggests a possible explanation for the evolution of the rosario structure and also provides an alternative to existing pseudo three dimensional models for fault related fold growth that relaxes the rigidity in the assumption of along strike self similarity

geomorphology and structural development of the nested summit crater of lascar volcano studied with terrestrial laser scanner data and analogue modelling abstract many volcano summits host craters that are partially overlapping the formation of such nested craters has been commonly interpreted as vent migration here we present an additional mechanism that may explain the geometry of nested craters at volcanoes lascar volcano the most active volcano of the central volcanic zone in the chilean andes hosts ene wsw trending summit craters that are partially overlapping nested details on the evolution and interaction between the different craters remain unclear to create a robust dataset terrestrial laser scanner tls data were collected at the summit of lascar in 2013 the resulting topographic data set consisting of more than 15 million data points with centimetre sampling allows visualising almost the complete eastern edifice of the volcano s summit from the tls data a digital elevation model dem and a slope map were generated allowing us to create a lineament map and quantify the observed morphological and structural features to further improve our understanding of the processes responsible for the formation of the craters and geomorphology we designed sandbox analogue models results suggest that one of the craters is a parasite crater formed as a consequence of ongoing activity in the adjacent crater our data suggest that the nested craters have all been modified since the last major eruption in 1993 by near surface effects associated with cooling compaction and gravitational sliding of the crater floor infill as the active crater deepens the adjacent inactive crater extends and partially slumps towards the active one understanding the structural development of these nested craters is relevant for assessing potential future eruption sites thus making lascar a dynamic target for a detailed morphology study these findings may similarly be applied to other volcanoes where nested craters have developed

sandbox models of fault bend folding a new investigation with a pre existing fault ramp abstract this study presents the results of analogue models of fault bend folds fbfs developed over a pre existing fault dip ramp with a non outcropping upper fault flat to determine the boundary conditions that control the formation of fbfs experiments were performed with quartz sand and glass microbeads both with and without intercalations of thin horizons of mica flakes we varied the initial thickness the ramp dip angle and the basal friction coefficient of the lower fault flat the analogue models revealed that the most important factor for the formation of fbfs was the rheological behaviour of the analogue material the elasto plastic frictional behaviour of the glass microbeads is fundamental for the formation of mode i fbfs with interlimb angles higher than 90 which is independent of the angle of the pre existing fault ramp and the basal friction along the lower fault flat in the sand models the formation of fbfs requires a high initial stratigraphic thickness resulting in a significant increase in the normal stress and or in the cohesion strength in glass microbeads the models developed structures very similar to the fbfs described in the carbonates of the maracaibo basin venezuela as well as those within the appalachian thrust belt usa

downsag and extension at calderas new perspectives on collapse geometries from ice melt mining and volcanic subsidence structures at calderas may form as a result of precursory tumescence subsidence due withdrawal of magmatic support resurgence and regional tectonism structural reactivation and overprinting are common to explore which types of structures may derive directly from subsidence without other factors evidence is reviewed from pits caused by the melting of buried ice blocks mining subsidence scaled subsidence models and from over 50 calderas this review suggests that complex patterns of peripheral deformation with multiple ring and arcuate fractures both inside and outside caldera rims topographic embayments arcuate graben and concentric zones of extension and compression may form as a direct result of subsidence and do not require a complex subsidence and inflation history downsag is a feature of many calderas and it does not indicate subsidence on an inward dipping ring fault as has been inferred previously where magmatic inflation is absent or slight initial arcuate faults formed during collapse are likely to be multiple and dip outwards to vertical associated downsag causes the peripheries of calderas undergo radial centripetal extension and this accounts for some of the complex peripheral fractures arcuate crevasses graben and some topographic moats the structural boundary of a caldera defined here as the outermost limits of subsidence and related deformation including downsag commonly lies outside ring faults and outside the embayed topographic wall it is likely to be funnel shaped i e inward dipping even though ring and arcuate fractures within it may dip outward inward dipping arcuate normal faults at shallow levels and steep inward dipping contacts between a caldera s fill and walls may both occur at a caldera that has initially subsided on outward dipping ring faults they arise due to peripheral surficial extension gravitational spreading and scarp collapse topographic enlargement at some calderas and the formation of embayments may reflect general progressive downsag and localized downsag respectively these processes may occur in addition to surficial degradation of oversteep ring fault scarps

combining multi physical measurements to quantify bedload transport and morphodynamics interactions in an alpine braiding river reach abstract understanding the interactions between bedload transport and morpholdynamics in braided streams has important applications in river management and restoration direct field measurements addressing this question are however scarce as they are often challenging to perform here we report an extensive two month field campaign in an alpine braided reach la severaisse river french alps that experienced predictable daily peak discharge 48 events observed generating significant bedload transport and morphological changes during the melting season we monitored these processes using a wide range of direct and indirect techniques bedload sampling continuous seismic measurements pebbles tracking topographic surveys remote sensing using ground control cameras and drone flights doing so surrogate measurements allowed to extend temporally discrete manual bedload sampling and to extend spatially local riverbed cross section measurements these measurements provide unique complementary constraints on the targeted physics at various spatial and temporal scales which enabled us to draw robust conclusions data showed a progressive decrease in bedload transport for a given flow rate along the two months period simultaneously river morphology in the braided sections changed from an incised to a more distributed configuration which led to a decrease of local maxima in dimensionless shear stresses in the braided reach for similar flow conditions this control of bedload transport by maximum local shear stresses was in line with tracked pebble surveys indicating that coarse bedload particles were mostly transported in the main active channel at the reach scale this transport was found to be more efficient in laterally confined sections than in braided ones which has important implications in terms of bedload estimation in alternative confined and braided unconfined rivers finally this study highlight the interest to combine a large variety of traditional and innovative measurements techniques to better understand complex sediment transport processes in the field

thrust dip and thrust refraction in fault bend folds analogue models and theoretical predictions analogue models made of two layers of loose sand separated by a thin layer of micro glass beads were shortened by a rigid emerging ramp dipping at either 30 or 45 and possessing a high intermediate or null surface friction shortening resulted in formation of closely spaced back thrusts in the sand layers the dips of the back thrusts vary within a range of 30 depending on the ramp friction and 7 depending on its dip an increase in ramp friction or to a lesser extent in ramp dip decreases the thrust dips in the model the second important observation is that when friction is greater along the ramp than along the layer of glass beads then the glass beads layer acts as a separate upper ramp above which the back thrusts steepen the theory proposed to explain these observations predicts the thrust dips through a two step procedure first global equilibrium of forces in the two layers is required to yield the mean forces at stake along the ramps and thrusts second the total dissipation is minimized with respect to the dips of the back thrusts the relevant frictional properties of our analogue materials have been measured in stress conditions as close as possible to the experimental ones below 1 kpa and used with the theory to yield optimal back thrust dips that are all within 3 of the measured dips this is a surprisingly good fit when considering that we did not take into account geometric changes strain softening and dilatancy or compaction due to slip on the thrusts we conclude that this general two step theoretical procedure is validated in the context of analogue frictional materials we also propose a possible mechanism for thrust refraction and top to the foreland sense of shear observed in the hanging walls of lower flat to ramp transition in sedimentary piles that is based on the triggering of secondary upper ramps finally this mechanical approach can also be seen as complementary to the kinematic models of fold thrust structures which by definition cannot grasp the strong effects of friction on the kinematics

contribution of dendrochronology to the determination of magnitude frequency relationships for landslides abstract the determination of magnitude frequency m f relationships is necessary for landslide hazard assessment ideally this is achieved by using inventories of past landslide events recorded by technical units or aerial photo interpretation however direct sources of landslide data are often unavailable dendrogeomorphological techniques allow us to determine the frequency of events but few attempts have been made to estimate landslide magnitude by analysing the spatial distribution of disturbed trees this paper addresses m f determination for rockfalls debris flows and complex slides obtained by dendrogeomorphology the cases presented require the appropriate understanding of both the geomorphological context and the disturbance caused by landslide motion on trees both small and large rockfalls can be identified and dated by reconstructing the trajectories in the forest stand the larger the rockfall size the longer the path the higher the number of affected trees and the greater the area of deposition of rock fragments the area of damaged trees is proposed as an indicator of the rockfall magnitude the frequency and extent of a series of debris flows were reconstructed by mapping their deposits using relative dating criteria and by dating damaged trees local and global mudslide reactivations were deduced by sampling various tree generations and by analysing the location of the disturbed trees in the landslide units magnitude cumulative frequency curves were determined for the three cases

estimation of debris flood magnitudes based on dendrogeomorphic data and semi empirical relationships abstract magnitude estimations of hydrogeomorphic processes contain crucial information for hazard assessments and for the understanding of longer term landscape evolution in this study we reconstruct magnitudes of debris floods for a torrential catchment in tyrol by combining dendrogeomorphic time series of events with semi empirical equations used to predict event volumes reconstructed debris flood magnitudes cover eight decades a d 1930 2008 and vary from 2900 to 45 900 m 3 we illustrate that magnitude estimates derived from tree ring data and semi empirical equations represent a valuable contribution to the documentation and understanding of hydrogeomorphic processes and that they can complement fragmentary time series in small watersheds for periods covering decades up to centuries limitations of the approach are mainly inherent to the age and spatial distribution of sampled trees and may thus influence reconstructed event magnitudes as one goes back in time

weathering geomorphic work and karst landscape evolution in the cave city groundwater basin mammoth cave kentucky abstract following the pioneering work of wolman and miller wolman m g miller j p 1960 magnitude and frequency of forces in geomorphic processes j geol 68 54 74 in evaluation of geomorphic work and the frequencies and magnitudes of forces that drive it a large number of quantitative studies have focused on the evolution of fluvial systems and transport of clastic sediment less attention has been given to understanding frequencies and magnitudes of processes in rock weathering including investigation of rates at which solutes are removed from landscapes under various flow distributions as an analog to wolman and miller s wolman m g miller j p 1960 magnitude and frequency of forces in geomorphic processes j geol 68 54 74 concept of geomorphic work in this work we use 1 year of high resolution flow and chemical data to examine the work done in landscape evolution within and at the outlet of kentucky s cave city basin a well developed karst landscape aquifer system that drains about 25 km2 we consider both removal of solutes contributing to landscape denudation based on calcium mass flux as well as predicted dissolution rates of the conduit walls at the outlet of this basin based on limestone dissolution kinetics intense short duration events dominate storms that filled the logsdon river conduit occurred

mechanical behaviour of granular materials used in analogue modelling insights from grain characterisation ring shear tests and analogue experiments abstract the mechanical behaviour of several dry granular materials is investigated through ring shear tests grain characterisation and simple analogue experiments analysed by x ray computed tomography an improved knowledge of granular materials is essential to determine their suitability as analogues for upper crustal rocks in experimental models and to compare analogue and numerical experiments the ring shear tests show that the granular materials have an elastic frictional plastic behaviour with strain hardening preceding failure at peak strength followed by strain softening until a dynamic stable value is reached this is similar to the behaviour exhibited by experimentally deformed rocks the physical characteristics of the grains determine the amount of diffuse deformation before failure the percentage of strain softening and act on the thickness of the shear zones before broadening initial shear zone width in extensional and contractional experiments is between 11 and 16 times the mean grain size the angle of internal friction defining one of the mechanical properties of granular materials and thus fault dip is not only related to physical characteristics of the grains and to the handling technique used e g sieving or pouring but also to the overburden and to the experimental setup used

experimental study of caldera formation scaled experiments have been carried out on caldera collapse mechanisms using silicone as analogue magma and dry sand as analogue rock experiments were carried out in two and three dimensions using a range of roof aspect ratios thickness width 0 2 to 4 5 appropriate for caldera collapse they reveal a general mechanism of collapse only weakly dependent on the shape of the reservoir for low roof aspect ratios 1 subsidence starts by flexure of the roof and the formation of outward dipping reverse ring faults which in turn trigger formation of peripheral inward dipping normal ring faults the subsidence always occurs asymmetrically in cross section the reverse faults delimit a coherent piston bounded on each side by an annular zone of inwardly tilted strata located between the reverse and normal ring fault sets the surface depression consists of a nondeformed area piston surrounded by an annular extensional zone tilted strata for high aspect ratios 1 multiple reverse faults break up the roof into large pieces and subsidence occurred as a series of nested wedges 2 d or cones 3 d the extensional zone dominates the surface depression in the case where preexisting regional faults do not play a major role the collapse mechanics of calderas probably depends strongly on the roof aspect ratio calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults the annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant

effect of overburden thickness on thrust belt geometry and development laboratory sand models simulating development of thrust belts indicate that 1 distance between the backstop and the first formed thrust is a linear function of initial sand layer thickness 2 spacing between the first two thrusts is most likely a nonlinear function of initial sand layer thickness 3 initial dip of successive ramps decreases progressively toward the foreland because of variations in principal stress directions with depth and or changes of mechanical properties of sand with increasing overburden 4 faults to the hinterland steepen and become less active as hinterland thrust sheets undergo penetrative shear during sand wedge growth and 5 new ramp initiation occurs to the hinterland of the basal detachment tip line these observations can be interpreted both in context of area balance considerations and soil mechanics theory if initial sand layer thicknesses in our models represent syntectonic overburden in natural thrust belts i e distance between the ground surface and the detachment then our results suggest that locations of some thrust belt salients are controlled by along strike variations in stratigraphic thicknesses and that initial thrust spacing decreases progressively to the foreland if the thrust belt involves a foreland tapering basin furthermore our sand model observations suggest that the concept of a fixed rigid backstop in the hinterland of thrust belts may be misleading as a consequence of thickening the hinterland portion of the thrust belt becomes stabilized and effectively becomes the backstop itself

the restless resurgent campi flegrei nested caldera italy constraints on its evolution and configuration abstract we have collected new stratigraphic data in the campi flegrei caldera through surface geological mapping and study of cores drilled to various depths we integrate these data with geological geochronological and geophysical constraints available in the literature to reconstruct the volcanic and deformational history of the caldera the caldera is a resurgent nested structure resulting from two main collapses related to the campanian ignimbrite 37 ka and the neapolitan yellow tuff 12 ka eruptions while the structure is affected by a broad subsidence ongoing local resurgence and unrest occur inside the younger nested caldera structure although most of the caldera floor consists of multiple structural blocks due to long term deformations magmas were able to erupt only in one sector of the resurgent part the caldera has shown signs of unrest during the past 25 years with short term deformations that have generated a net uplift of 3 5 m the short term deformations are related to the neapolitan yellow tuff caldera resurgence and interpreted as the net result of brittle and ductile components the long term deformations likely represent the sum of the permanent brittle component of each short term deformational event

the role of extensional structures on experimental calderas and resurgence the structure and shape of collapses and resurgences is often controlled by pre existing discontinuities such as normal faults in rift zones in order to study the role of extensional structures on collapse and resurgence we used analogue models dry sand simulated the brittle crust silicone located at the base of the sand pack simulated magma in the experiments regional extension pre dated collapse or resurgence forming normal faults in a graben like structure the graben was filled with additional sand simulating post rift deposits a piston then moved the silicone downward or upward inducing collapse or resurgence within the previously deformed sand the collapses showed an ellipticity length of minor axis length of major axis between 0 8 and 0 9 with the major axis parallel to the extension direction the partial reactivation of the pre existing normal faults was observed during the development of the caldera reverse faults which conversely to what was expected from experiments without pre existing extension became partly inward dipping resurgence showed an elongation of the uplifted part with the main axis perpendicular to the extension direction at depth pre existing normal faults were partly reactivated by the reverse faults formed during resurgence these locally became outward dipping normal faults a total reactivation of pre existing faults was also observed during resurgence the experiments suggest that the observed elongation of calderas and resurgences is the result of the reactivation of pre existing structures during differential uplift such a reactivation is mainly related to the loss in the coefficient of friction of the sand the results suggest that elliptic calderas and resurgences in nature may develop even from circular magma chambers

caldera collapse at near ridge seamounts an experimental investigation collapse calderas are sub circular volcanic depressions caused by subsidence of the magma reservoir roof during an eruption scaled physical models of caldera collapse using flat topography have been instrumental in investigating the spatial and temporal development of calderas in particular two distinctive sets of concentric ring faults one reverse and one normal more recent analog studies have investigated the effect of non flat topography which alters the principle stress trajectories and resulting collapse structure this work provides the basis for investigating how naturally scaled topographic loads may affect caldera collapse in relation to shallow magma reservoirs the objective of this study is to understand how a near ridge seamount affects caldera collapse from both a central and offset position as the seamount migrates above the magma reservoir as a result of plate motion we utilize scaled analog models of caldera collapse in conjunction with three dimensional 3d laser scanning and digital particle image velocimetry dpiv to investigate caldera collapse dynamics at near ridge seamounts experiments using a seamount cone positioned centrally above the magma reservoir result in 1 increased subsidence along the interior outward dipping faults and 2 a preference to more symmetric collapse patterns as indicated by the subsidence profile and structure of the caldera relative to experiments with an offset cone when the cone is offset the collapse is asymmetric and trapdoor in nature with the center of greatest subsidence displaced away from the region of largest topographic load for these latter experiments subsidence is focused where the roof is thinnest along an initial reverse fault followed by a transition to an antithetic graben structure the asymmetric collapse in the experiments results in a caldera with a tilted profile offset calderas at near ridge seamounts are tilted towards the ridge axis suggesting that they may have collapsed asymmetrically similar to our experiments furthermore the graben structure observed in asymmetric collapse experiments is consistent with structures observed in some natural trapdoor basaltic volcanoes

episodic accretion and strain partitioning in a model sand wedge we model led thrust wedge accretion and deformation partitioning in a passively layered sand detached and shortened above a smooth rigid decollement the thrust wedge accretes in piggyback style during forward advance of a rear wall the passively layered sand accommodates the shortening penetratively by kinking ramp folding and by imbrication in single layers the style of compression changes with time and or stratigraphic position within the sand prism initial penetrative layer shortening above a slipped zone of decollement is succeeded by formation of monoclinal or conjugate shear bands at temporary terminations of decollement the shear bands subsequently narrow down and lock as thrust ramps the model sand wedge accrets episodically rather than steadily episodicity is controlled by the stick slip mode of decollement propagation and by volume loss and compaction of the wedge material in response to convergence this results in formation of a wedge with a convex upwards cross sectional topography

sediment transfer processes in two alpine catchments of contrasting morphological settings summary coarse sediment transport processes in steep headwater streams may vary between bedload and debris flows events depending on basin geomorphology and sediment supply this paper compares two small catchments drainage area 4 5 km 2 located in the eastern italian alps where the dominant sediment transport processes differ substantially i e debris flows in the moscardo torrent and bedload events in the rio cordon the two basins were selected because they provided the unique opportunity to analyze long term data of debris flow and bedload volumes respectively the two basins are compared in terms of peak discharge event duration and magnitude frequency characteristics of transported sediment volumes additional bedload data from two more alpine experimental basins are also considered the results show that for comparable recurrence intervals debris flow volumes are 2 3 orders of magnitude larger than bedload volumes this contrasting sediment transfer activity can be attributed to different basin and channel morphologies which are analyzed in terms of sediment supply conditions longitudinal profiles and slope area curves

magnitude frequency relationship for debris flows on the fan of the chalance torrent valgaudemar french alps in 1996 a large debris flow occurred on the fan of the chalance torrent system a tributary of the severaisse river french alps to investigate the magnitude and frequency of such debris flows on this fan fieldwork was carried out in the summer of 1998 detailed investigation revealed that several debris flows have occurred in the past 200 years lichenometry was used as a dating technique to obtain the frequency of these debris flow events also the volume of these flows was estimated with these data a magnitude frequency relationship was constructed this relationship shows a maximum magnitude of at least 50 103 m3 based on data for the past c 150 years a debris flow of such a volume appears to have a recurrence interval of approximately 34 years copyright 2002 john wiley sons ltd

mechanical models for correlation of ring fracture eruptions at pantelleria strait of sicily with glacial sea level drawdown recent k ar dating of eruptions at pantelleria a peralkaline volcanic island in the strait of sicily shows a correlation between eruption of pantellerite lavas from caldera ring fractures and low stands of sea level as determined from 18o stratigraphy post caldera pantellerite lavas associated with an 114 ky old caldera erupted along the ring fracture zone during a major low stand of sea level at about 67 ka the most recent episode of lava flow emplacement began about 20 ky ago during the last glacial maximum magma vented along the ring fault of a 45 ky old caldera from fractures radial to the caldera and along faults formed by intracaldera trapdoor uplift two mechanical models based on elasticity theory are presented to explain the correlation of post caldera ring fracture eruptions at pantelleria with lowering of sea level a simple analysis of a bending circular plate of thickness t r and radius r representing the magma chamber roof block shows that tensile stress is concentrated by a factor of 0 75r 2 t 2 at the lower perimeter of the plate when sea level drops stress changes may be even greater ift r is effectively less than the stratigraphic thickness due to layering of rocks in the roof block calculated stress changes due to a 100 m drawdown of sea level are similar in magnitude to stresses associated with dike propagation more realistic model geometries including different chamber shapes a conical volcanic edifice and sea level drawdown beyond the surface projection of the magma chamber were tested using the boundary element method lowering sea level generates a horizontal tensile stress above the chamber even when sea water is removed outboard of the magma chamber for some chamber geometries the magnitude of the tensile stress maximum is greater than the 1 mpa pressure of the 100 m of removed water and is of the right order of magnitude for dike propagation dikes initiated by the change of the stress field may originate and propagate along fractures inboard of the chamber margin the magnitudes of tensile maxima along the top of the chamber decrease as original sea level is moved outboard of the chamber margin and as the chamber thickness decreases when the depth to the top of the magma chamber reaches a critical value dependent on chamber geometry the propagation of dikes to the surface is inhibited

reconstructing spatio temporal patterns of debris flow activity using dendrogeomorphological methods debris flows are a major threat in many parts of the alps where they repeatedly cause severe damage to infrastructure and transportation corridors or even loss of life nonetheless the spatial behavior of past debris flow activity and the analysis of areas affected during particular events have been widely neglected in reconstructions so far it was therefore the purpose of this study to reconstruct spatio temporal patterns of past debris flows on a forested cone in the swiss alps bruchji torrent blatten valais the analysis of past events was based on a detailed geomorphic map 1 1000 of all forms related to debris flows as well as on tree ring series from 401 heavily affected trees larix decidua mill and picea abies l karst growing in or next to deposits the samples were analyzed and growth disturbances related to debris flow activity assessed such as tangential rows of traumatic resin ducts the onset of reaction wood or abrupt growth suppression or release in total 960 growth disturbances were identified in the samples belonging to 40 different event years between a d 1867 and 2005 in addition the coupling of tree ring data with the geomorphic map allowed reconstruction of eleven formerly active channels and spatial representation of individual events based on our results we believe that before 1935 debris flows preferentially used those channels located in the western part of the cone whereas the eastern part of the cone remained widely unaffected the spatial representation of the 40 events also allowed identification of five different spatial patterns for debris flows at the study site 2006 elsevier b v all rights reserved

formation of collapse calderas collapse calderas are common in many volcanic regions but the mechanics of their formation is not well understood i propose that collapse calderas may form as a result of the growth of associated shallow magma chambers excess magmatic pressure in a shallow chamber gives rise to tensile stresses at the surface but the location and magnitude of the maximum tensile stress depend on the shape of and depth d to the top of the chamber this model indicates that penny shaped chambers i e sill like chambers of circular horizontal cross sections are most likely to initiate collapse calderas for such a chamber with horizontal radius a the higher the a d radius depth ratio the greater the probability of forming a collapse caldera

fault related folding in sandbox analogue models of thrust wedges abstract the nucleation and growth of thrust fault related folds has been simulated using high resolution sandbox coulomb wedge type analogue models thrust related anticlines grew progressively from initial layerparallel shortening to decollement folding thrust tip folding and eventually thrust ramp folding thrust ramps nucleated in the central sector of the overturned forelimb of decollement folds as a consequence of limb thinning thrust tip folding occurred during outward migration of the ramp tips the fault propagation rate was greater at the lower ramp tip which quickly joined the basal thrust flat while folding continued beyond the upper tip the development of the analogue model thrust fault related folds by progressive changes in kinematics from decollement folding to thrust tip folding and finally thrust ramp folding compares well with analyses of naturally occurring thrust fault related folds

basin inversion and contractional reactivation of inherited normal faults a review based on previous and new experimental models abstract compressional inversion of former extensional sedimentary basins is a particularly common phenomenon given that passive margins share the same destiny of being subsequently incorporated into fold and thrust belts during basin inversion pre existing faults may concentrate stress and localise future thrust ramps or may be reactivated recording reversal of movement from extension to reverse this process has attracted attention largely because of its economic implications related to petroleum and ore deposit prospects and seismic hazard assessment since more than two decades analogue modelling has been used to indentify and or test a number of parameters relevant for the inversion process this paper aims therefore to offer a reasoned review of the analogue modelling work done on the subject coupling such previous results with those of a new experimental series inspired to the northern apennines stratigraphy and evolution past models have explored the role of main factors governing fault reactivation susceptibility such as fluid pressure fault weakness fault steepness angle of shortening and sediment loading the new models have addressed the role played by geometry and strength of a basal ductile layer during inversion two sets of sand silicone models were first extended orthogonally at different velocities to produce dissimilar pre inversion internal geometries and then shortened coaxially at different velocities to vary the brittle ductile coupling the experimental results confirm the selectiveness of fault inversion and reveal strong similarities with structural styles of tectonic inversion reported from the northern apennines and various areas worldwide the preferred geological model inferred from this modelling involves contribution of 1 syn inversion rotation of pre existing faults to shallower dip and 2 rotation of principal stress axes which could explain the invariable reactivation of only one of the two oppositely dipping graben bounding faults these factors would thus interact with the other well established parameters controlling the fault reactivation process

self determining numerical modeling of compressional fault bend folding we present the application of the hca hybrid cellular automata numerical modeling technique to the evolution of compressional fault bend anticlines involving layers that have competency contrasts and a mechanical stratigraphy the new modeling results show that for competent hanging wall layers 1 the fault style is not preserved in the overlying fold 2 fold development is not self similar and rotational growth wedges develop in the forelimb when syntectonic sedimentation occurs and 3 balanced fault bend anticlines can develop for ramp cutoff angles as high as 60 these results compare favorably with field examples and laboratory analogues and dramatically broaden the applicability of fault bend folding to model the evolution of thrust related anticlines

caldera types how end members relate to evolutionary stages of collapse 1 currently proposed caldera types downsag piston funnel piecemeal trapdoor are mainly based on field evidence and are each viewed as end members an overview of recent experiments on caldera formation under different conditions shows remarkably consistent results and suggests four experimental stages characterized by progressive subsidence distinctive structural features of each stage are found in many calderas highlighting an overall consistency the evolutionary stages adequately explain the architecture and development of the established caldera end members along a continuum where one or more end member may correspond to a specific stage while such a continuum is controlled by progressive subsidence specific collapse geometries will result from secondary factors such as roof aspect ratio collapse symmetry and pre existing faults

evidence for enhanced debris flow activity in the northern calcareous alps since the 1980s plansee austria abstract debris flows are among the most important natural hazards the northern calcareous alps with their susceptible lithology are especially affected by a double digit number of major hazard events per year it is hypothesised that debris flow intensity has increased significantly in the last decades in the northern calcareous alps coincident to increased rainstorm frequencies but yet there is only limited evidence the plansee catchment exposes extreme debris flow activity due to the intensely jointed upper triassic hauptdolomit lithology being responsible for most of the debris flow activity in the northern calcareous alps the debris flows feed into a closed sediment system the plansee lake where holocene lateglacial sedimentation rates rates since the late 1940s and recent rates can be inferred accurately using aerial photos and field mapping the temporal and spatial development of eight active debris flow fans is reconstructed in six time intervals from 1947 1952 1971 1979 1987 2000 and 2010 and mean annual debris flow volumes are calculated these are compared with mean holocene lateglacial debris flow volumes derived from the most prominent cone whose contact with the underlying till is revealed by electrical resistivity tomography ert debris flow activity there increased by a factor of 10 from 1947 1952 0 23 0 07 103 m3 yr to 1987 2000 2 41 0 66 103 m3 yr mean post 1980 rates from all eight fans exceed pre 1980 rates by a factor of more than three coinciding with enhanced rainstorm activity recorded at meteorological stations in the northern calcareous alps the frequency of rain storms def 35 mm d has increased in the study area on average by 10 per decade and has nearly doubled since 1921 recent debris flow activity is also 2 3 times higher than mean holocene lateglacial rates the strong correlation between the non vegetated catchment area and the annual debris flow volume might indicate a decadal positive feedback between enhanced rainstorm activity and debris flows here we investigate the temporal and spatial development of debris flow fans to better understand the sensitivity of alpine catchments to heavy rainfall events in the context of climate change

evidence of incremental growth in the vulsinian calderas central italy abstract a comparative work on the vulsini calderas has been carried out fundamentally from volcanological field study drilling and gravity anomalies data on the basis of a detailed stratigraphy four volcanic complexes have been recognized in the vulsini volcanic district from the oldest to the youngest paleo bolsena bolsena montefiascone and latera the latter two complexes having a contemporaneous evolution in their activity each complex gave rise to several little calderas caused by piecemeal collapses due to moderate size explosive eruptions or coherent subsidence of a superficial piston like block nevertheless the extensional volcano tectonic regime affecting this area could have accounted for as much of the total subsidence as the caldera forming eruptions bolsena and latera calderas respectively 16 km and 8 km in average diameter represent the main depressions of the vulsini district and their topographic features are still well preserved several collapses forming the main poligenetic calderas of bolsena and latera are masked at present by the bolsena lake the younger volcanic covering the lacustrine deposits and erosion in a minor amount only the little montefiascone and vepe calderas which represent the youngest collapses of the vulsini area have not been masked bolsena caldera could be considered a depression caused mostly by a subsidence hinged to the southwest by down sagging and by collapses which occurred after the most important explosive eruptions of the paleo bolsena bolsena and montefiascone volcanic complexes meanwhile latera caldera is mostly due to a piecemeal collapse linked to several moderate size ignimbrite units which were emplaced throughout the explosive activity of the latera complex therefore the vulsinian calderas did not form by catastrophic eruptions because several and frequent moderate size eruptions downsagging and tectonic hinged subsidence concur indicating an incremental growth

benham rise unveiled morphology and structure of an eocene large igneous province in the west philippine basin abstract the benham rise is an oceanic large igneous province at the western margin of the philippine sea it has an ocean island basalt geochemistry a surface area of 0 11 mkm2 and a volume of 0 13 mkm3 a crustal thickness of 15 km is estimated from gravity anomalies volcanism was most active in the early stages of its formation during the lutetian from 47 9 ma to 41 3 ma although volcanic activity extended to 26 ma multibeam bathymetry with almost 100 coverage has revealed the morphology of benham rise for the first time it consists of a main body 310 km by 330 km with the narra loro and molave spurs extending 100 200 km from its eastern side the main body is built on a shield platform from 5200 m to 3800 m sub sea the platform flanks consist of 3 15 km wide terraces with scarps as high as 100 m to 300 m the platform is surmounted by a crest exhibiting caldera morphology at an average depth of 2500 m the crest is named apolaki caldera and may be the world s largest known caldera with a diameter of 150 km features like a breached rim intra caldera benches and a resurgent dome indicate a multi phase volcanic history consisting of both quiet and explosive eruptions the bathymetry and publicly available geological and geophysical data record three main stages in the tectonic history of benham rise shield building caldera formation and post caldera late stage volcanism the platform base is interpreted to have formed over a hotspot beneath the central basin spreading center during the shield building phase between 47 9 ma to 42 5 ma the arrowhead shaped molave spur is interpreted to have formed late in this stage along a ridge that propagated away from the hotspot the caldera formation followed and is interpreted to have occurred before 41 3 41 5 ma when benham rise was still joined with urdaneta plateau the narra spur was formed by late stage hotspot related volcanism as benham rise and urdaneta plateau separated and ended with the eruptions of the vinogradov seamount at 26 ma

initiation and reactivation of faults during movement over a thrust fault ramp numerical mechanical models abstract with numerical continuum models we investigate the effects of fault geometry fault friction material properties and anisotropy on the initiation and reactivation of faults in the hanging wall of a thrust fault ramp the models use an elastic plastic frictional dilatant cohesion softening material in which deformation may localize as shear bands with a sharp lower fault bend backthrust shear bands propagate up from the concentration of differential stress at the fault bend with a rounded lower fault bend bedding parallel shear bands develop above the fault bend in the center of the layer where differential stress is highest friction on the ramp enhances the development of backthrusts and causes them to initiate at shallower dips increased overburden decreases the amount of localization within shear bands and the spacing between them shear bands that develop at the lower fault bend are weaker than the surrounding material and are thus potential sites of reactivation during subsequent deformation because of this potential reactivation the style of deformation at the upper fault bend depends on the deformation that accumulates at the lower fault bend at the upper fault bend backthrusts are reactivated as extension faults and are crosscut by more steeply dipping extension faults bedding parallel shear bands are reactivated at the upper fault bend with top to the hinterland sense of shear low angle extension faults are listric into the reactivated bedding parallel shear bands producing hinterland verging extensional duplexes above the upper flat

extensional fault systems in sedimentary basins a review of analogue model studies abstract scaled analogue models have provided graphic new insights into the progressive development of extensional fault systems this paper reviews recent results of an on going analogue modelling programme and compares the structural styles in the models with examples of natural extensional fault systems four fundamental model styles scaled to simulate deformation in brittle sedimentary rocks in the upper 10 km of the crust are reviewed extension above a basal detachment which undergoes stretching over a limited are produces an asymmetric rift graben bounded by planar faults and in which the internal deformation is accommodated by rotational domino faults these experimental results are directly comparable to structures found in intracontinental rifts extension above a basal detechment which undergoes extension under the whole model and which has no constraints at the ends produces domino style fault arrays in the pre rift sequence and listric growth faults in the syn rift sequence these results compare with highly extended rift basins and terranes such as the basin and range province usa extension above a simple listric fault produces a characteristic hangingwall roll over and crestal collapse graben system at high values of extension superposed crestal collapse grabens occur and a fan of listric growth faults is developed these structures bear striking similarities to growth faults in progradational delta systems ramp flat listric extensional fault systems using sand mica models produce a characteristic family of structures with an upper roll over crestal collapse system a ramp syncline and reverse fault fold zone and a lower roll over crestal collapse system an example of a ramp flat fault system from the gulf coast usa is presented as a comparison with the analogue model results preliminary results of three dimensional extensional model experiments are presented to illustrate the complexities in such fault systems in all of the experimental models it is found that the geometry of the underlying detachment system exerts fundamental control on structures produced in the hangingwall the analogue models illustrated in this review provide valid geometric and kinematic templates for the progressive evolution of extensional fault systems in sedimentary basins

insights into the kinematics of a volcanic caldera drop probabilistic finite source inversion of the 1996 bardarbunga iceland earthquake article i nfo the accurate modelling and prediction of volcanic eruptions depends critically on information concerning the interaction between the caldera and the underlying magma chamber knowledge concerning the kinematics of a caldera in the course of an eruption is therefore essential here we provide detailed seismological constraints on the kinematics of a volcanic caldera drop and the geometry of a caldera ring fault for this we performed a finite source inversion of the september 29 1996 bardarbunga iceland earthquake that was caused by caldera subsidence our methodology is based on spectral element simulations of seismic wave propagation through a realistic model of the icelandic crust and upper mantle a particularly robust feature is the initiation of the rupture in the north western part of the ring fault that is about 10 km in diameter from there it spread to the other fault segments within about 3 s without invoking super shear propagation sensu stricto we can explain this unusually fast rupture propagation by the triggering of fault segments through p waves that propagated across the caldera our results favour outward dipping fault segments in the western half of the ring fault while the eastern half is preferentially inward dipping this variability may reflect structural heterogeneities or an irregular magma chamber geometry the individual segments of the caldera ring fault radiated approximately equal amounts of energy this indicates that the caldera dropped coherently as one single block the work presented here is intended to aid in the design of realistic models of magma chamber and caldera dynamics