differential contraction of subducted lithosphere layers generates deep earthquakes abstract the origin of forces for generating deep earthquakes remains elusive we propose a new mechanical model that involves constant release of shear strain accumulated between the crust and mantle lithosphere caused by differential volume changes during phase transformations as a slab sinks this generates increasing down dip compression inside the slab with depth consistent with the global distribution of deep earthquake focal mechanisms using experimentally calibrated slab rheology we show that the estimated distribution of slab internal stress agrees well with the depth frequency relationship of deep earthquakes in both cold and warm slabs with the peak in seismicity at 600 km depth corresponding to an induced stress maximum this mechanism of residual stress within a cold slab also provides a solution to deep earthquakes occurring beneath former subduction zones such as western tonga and spain we further suggest that this model may reconcile existing geodynamic paradoxes concerning the absolute strength of slabs discrepancies on mantle rock strength between laboratory and geophysical inversions as well as the competition between earthquake generating stresses and slab pull forces

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

structural evolution of calderas insights from two dimensional discrete element simulations the manner in which calderas develop is a key question in volcanology and has important implications for associated volcanogenic risk and geothermal and ore exploitation to better understand the structural evolution of calderas i use a discrete element model of a frictional cover undergoing piston like subsidence at its base simulating magma chamber deflation and cover collapse these novel simulations capture not only the initiation of cal deras but also much of the complexity of faulting during their later development in all models both normal and reverse faults accommodate deeper subsidence at higher structural levels curved outward dipping reverse faults are consistently the first structures to develop subsequent caldera growth is mainly the result of movement on vertical or steeply inward dipping normal faults this may be followed by a later phase of lateral collapse on shallower normal faults the roof aspect ratio r cover thickness piston width is seen to be an important factor calderas with low aspect ratios r 1 exhibit a coherent central subsiding block while those with higher aspect ratios have a complexly faulted internal structure

fast rise times and the physical mechanism of deep earthquakes earthquakes at depths of 300 km are similar to shallower events in that they are dominantly of double couple character1 implying that shearing motion has taken place at depth but because increased friction at these high pressures inhibits brittle fracture2 3 various other mechanisms related to phase transformations have been invoked to explain the occurrence of deep earthquakes4 10 as yet however no consistent differences have been found between the source characteristics of deep 300 km and intermediate depth 300km earthquakes2 11 13 here we report a systematic global survey of the rise times and stress drops of deep and intermediate earthquakes the rise time is defined as the time from rupture initiation to peak moment release rate when the rise times are scaled to the seismic moment release of the events their average is nearly twice as fast for events deeper than 450 km as for shallower events this difference may ultimately provide an experimental means of testing proposed mechanisms for the generation of deep seismicity

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

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

stress distribution due to olivine spinel phase transition in descending plate and deep focus earthquakes the stress field due to olivine spinel phase transition in and around a descending plate is studied based on the calculations for an equilibrium phase transition with a constant clapeyron s slope and for a nonequilibrium transition taking a kinetic effect into consideration the material concerned is assumed to be a viscoelastic body whose physical coefficients are functions of temperature and pressures the calculated stress distribution for the equilibrium transition is characterized by a compressional stress near the upper and or lower surfaces of the plate and by a tensional stress at the central part in the depth range from 200 to 550 km the magnitude of principal stress being greater than 0 5 gpa however for the nonequilibrium transition a tensional stress predominates near the upper and lower surfaces and the central part is compressional in the depth range from 300 to 600 km unlike the case of equilibrium transition the maximum principal stress in the latter case is greater than 2 gpa which is much more than that for the equilibrium transition the principal axes in the high stressed region are oriented almost parallel to the descending direction in both cases comparison of the calculated results to the actually observed seismicity depth relation and to focal mechanisms of deep focus earthquakes in descending plates beneath island arcs shows that the nonequilibrium phase transition agrees better with the observations

numerical modelling of stress fields and fracture around magma chambers we investigate the formation of cracks around magma chambers and their relation to caldera collapse we discuss basic background physics and the causes of caldera collapse events then examine the suitability of the smoothed particle hydrodynamics sph method for these applications by studying simple two dimensional systems the sph method and equations for solid body materials are discussed and tested for the stress concentration around a circular hole the effect of altering the depth of an underground magma chamber is investigated the results are compared to similar calculations using boundary element techniques a fracture model is introduced and tested for the brittle fracture of plates in tension we apply our fracture model to investigate the fractures that develop around magma chambers we find that our results are in general agreement with experiments in particular we find that in the absence of tension a sudden rise in the chamber pressure can initiate fractures which propagate towards but do not reach the surface at the same time fractures can propagate down from the surface towards the chamber under tension and pressure increase fractures can develop and reach the surface in a form that could lead to piston collapse we find that the fractures depend in a complex way on the geometry of the chamber together with the history of country rock tension and sudden jumps in the chamber pressure

variations in slab dip along the subducting nazca plate as related to stress patterns and moment release of intermediate depth seismicity and to surface volcanism 1 the subducting nazca plate shows a high degree of along strike heterogeneity in terms of intermediate depth seismicity 70 300 km orientations of slab stress and volcanism we compile the intermediate depth earthquakes of south america from the harvard centroid moment tensor cmt catalogue to determine along strike dip variations and we explore the variable level of correlation between these observations primary results are three fold 1 consistency among orientation of tension axes highest level seismicity and occurrence of volcanism is associated with steeply dipping regions 20 30 2 volcanism and intermediate depth seismicity exhibit no direct correlation suggesting that earthquake faulting is not a necessary condition for transport of dehydrated fluids out to the mantle wedge for initiation of melting 3 comparison of slab thermal structures between regions with and without volcanism suggests that for certain wedge widths the maximum temperature in the mantle wedge is higher for the former than for the latter

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

detached deep earthquakes are they really abstract we use primarily the generation of acoustic t waves into the ocean by deep seismic sources to investigate the propagation of high frequency seismic energy from the bottom of subduction zones to the shoreline at the earth s surface conversion from shear waves to oceanic acoustic waves can be used as a proxy for the existence of a continuous slab featuring low anelastic attenuation with the help of other techniques such as the estimation of q from s to p spectral amplitude ratios we examine systematically a number of regions where earthquakes have been described as detached we establish the mechanical continuity of the slab to the hypocenters of the 1990 sakhalin and 1982 bonin events which occurred several hundred kilometers in front of the mainstream seismic zone the study of the 1989 paraguay shock is inconclusive probably due to its much smaller size the vertical continuity of the south american slab through its aseismic depth range is verified and a similar situation probably exists in java attenuation data suggests that the deep spanish earthquakes occur within a vertically large segment of colder material and a similar situation may exist in colombia the only clearly detached deep events with no mechanical connection to the surface make up the vityaz cluster under the north fiji basin based on a variety of geophysical evidence the small deep earthquakes under new zealand are likely to take place in a detached blob at least 350 km below the termination of mainstream seismicity these results support a model integrating buoyancy forces over a long continuous slab as the source of the down dip compressional stresses observed in large earthquakes at the bottom of the transition zone

subduction related intermediate depth and deep seismicity in italy insights from thermal and rheological modelling geological data suggest that after a period of oceanic lithosphere subduction about 170 km of continental lithosphere were subducted under the northern apennines since about 23 myear at rates of ca 1 cm year whereas more than 700 km of mesozoic oceanic lithosphere ionian lithosphere have been subducted under the calabrian arc at rates of about 3 cm year however a well developed wadati benioff zone can be only recognised below the calabrian arc down to depths of 450 500 km whereas intermediate depth seismicity reaches maximum depths of about 90 km under the northern apennines we model the thermal state of these two subduction zones and apply temperature dependent non linear rheological laws to evaluate the down dip extent of brittle regions in the two subduction zones we show that differences in subduction rate and in slab composition continental vs oceanic produce a far deeper down to 290 380 km brittle field in the colder calabrian slab than in the warmer northern apenninic slab 70 120 km thus explaining differences in the maximum depth of seismicity in both regions concerning deep earthquakes in the calabrian subduction zone models predict that metastable olivine persists down to depths of 430 km this could possibly explain the maximum depth of earthquakes in the area the small extent of the metastable olivine wedge produces a small density anomaly that cannot explain the pervasive down dip compression observed in the ionian slab 2004 elsevier b v all rights reserved

the eclogite to garnetite transition experimental and thermodynamic constraints we have found that the forms of the experimentally determined phase diagrams for pyroxene garnet transformations require the volume changes for these reactions to be much smaller at 130 kbar 400 km than at 1 atm using these volume data we calculated the width of the eclogite garnetite transition in natural quartz tholeiite and alkali olivine basalt compo sitions taking account of mixing in multicom ponent pyroxene and garnet phases we find that the eclogite garnetite transition does not exhi bit any discontinuity in bulk sound velocity in the pressure range 110 to 230 kbar in addition this velocity when plotted as a function of pressure possesses a curvature which is in the opposite sense to that given by the preliminary reference earth model prem this phase transi tion therefore does not appear to be a satis factory model for the 400 km seismic discon tinuity

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

thermomechanical behavior of large ash flow calderas the fundamental thermal and mechanical processes that occur within the ash flow caldera magma chamber systems remain largely enigmatic to date the only models of caldera collapse are simple mostly elastic or viscoelastic mechanical models that can predict some of the conditions preceding the collapse they cannot however predict the collapse itself because they are incapable either of reproducing the formation of faults or of accounting for the brittle ductile transitions and fault related thermal anomalies we have constructed analytical and numerical therrnomechanical that account for both elastic plastic ductile rheology and physical properties of the caldera rocks the overpressured magma is evacuated through a central vent and transforms into ash flow units deposited within the forming caldera brittle deformation faulting and subsequent collapse of the structure are reproduced the results show that in the absence of a regional stress field the collapse on both sides will occur only for aspect ratios i e caldera diameter to the depth of the magma chamber exceeding 5 and that internal embedded faults may also appear when the aspect ratio exceeds 10 thermal conductivity contrasts in ash flow calderas give rise to strong heat refraction that localizes deep seated thermal anomalies to the outer sides of the faults in the presence of regional extension the border faulting can be attenuated or disappear and the faults tend to localize around the central part of the chamber roof coupled therrnomechanical modeling suggests that the outer sides of the border faults have high trapping capabilities for hydrothermal fluids the geometry of the brittle ductile transition largely controls that of the fractured zones within and around the chamber roof thus justifying a new mechanical definition of magma chamber geometry

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

kinetics of the olivine spinel transformation in subducting lithosphere experimental constraints and implications for deep slab processes abstract the persistence of metastable olivine to depths greater than 400 km in subducting slabs has implications for the generation of deep focus earthquakes the magnitude of buoyancy forces driving plate motion and the state of stress in the slab the depth to which metastable olivine can survive in a subduction zone and the depth interval over which transformation to or mg fe 2sio4 occurs have been evaluated from experimental kinetic data for the mg2geo4 and ni2sio4 transformations and the mg2sio4 transformation the data were extrapolated using a kinetic model for grain boundary nucleation and interface controlled growth under non isothermal and non isobaric conditions the results predict that metastable mg1 8fe0 2sio4 olivine survives to depths greater than 500 km in the cold interior of rapidly subducting slabs of old lithosphere the onset of transformation to mg fe 2sio4 spinel depends only on growth kinetics and coincides with the 550 50 c isotherm including the effects of latent heat production causes the transformation to occur by a runaway process over a very narrow depth interval at the onset of transformation high nucleation rates and low growth rates are consistent with the formation of very fine grained reaction products which are required for the transformational faulting mechanism of deep focus earthquakes when olivine crosses the equilibrium boundaries at higher temperatures e g higher than 700 c at 400 km depth transformation to or occurs much closer to equilibrium at a depth controlled by nucleation kinetics in this case the effect of latent heat production on the transformation kinetics is small and microstructural evolution is unlikely to result in transformational faulting below the depth of olivine breakdown cold slabs are likely to have a complex rheological structure owing to temperature dependent microstructural evolution during the phase transformation

numerical and experimental models on the formation mechanism of collapse basins during the green tuff orogenesis of japan the geological model about volcanism of the green tuff geosyncline deduced from the field observations consists of the following processes 1 dome shaped uplift with a mean diameter of 30 km 2 collapse of the central part of the domes forming basins with a mean diameter of 10 km 3 volcanic activity inside the collapse basins it is considered that these consecutive processes resulted from the magmatic uplift from a deep part of the crust

free energy minimization by simulated annealing with applications to lithospheric slabs and mantle plumes an optimization algorithm based upon the method of simulated annealing is of utility in calculating equilibrium phase assemblages as functions of pressure temperature and chemical composition operating by analogy to the statistical mechanics of the chemical system it is applicable both to problems of strict chemical equilibrium and to problems involving metastability the method reproduces known phase diagrams and illustrates the expected thermal deflection of phase transitions in thermal models of subducting lithospheric slabs and buoyant mantle plumes it reveals temperature induced changes in phase transition sharpness and the stability of fe rich phase within an field in cold slab thermal models and it suggests that transitions such as the possible breakdown of silicate perovskite to mixed oxides can amplify velocity anomalies

effects of slab mineralogy on subduction rates although velocities of subducting slabs should be controlled primarily by their negative thermal buoyan cies their mineralogy can also have signicant eects we explore this by using thermo kinetic modeling to predict mineralogy and compare the resultant buoyant driving force to the opposing viscous drag phase transitions of mg fe 2sio4 in subducting slabs depend on thermal struc ture in two ways first equilibrium phase boundaries should be deflected causing local buoyancy anomalies whose sign depends upon that of the clapeyron slope as slabs rst enter the transition zone negative anomalies should acceler ate them but positive anomalies that arise when they fully penetrate the transition zone should slow them such ef fects may induce geologically abrupt changes in plate mo tions second olivine that persists metastably in slabs will form regions of positive buoyancy which should reduce slab velocities the coldest and fastest slabs should be slowed more greatly thus narrowing the range of feasible subduc tion rates decreased descent rates however allow slabs to warm and metastable wedges to thermally erode such neg ative feedback mechanisms may serve to regulate subduction rates

experimental constraints on the depth of olivine metastability in subducting lithosphere the hypothesis that metastable olivine persists in some subducting slabs into the transition zone has wide implications for mantle dynamics and rheology in order to evaluate this possibility we derive new thermo kinetic subduction zone models to predict the extent of olivine metastability within the stability fields of its high pressure polymorphs wadsleyite and ringwoodite our updated models improve on previous work by incorporating experimental kinetic data on realistic mantle compositions mg fe 2sio4 rather than analogue systems furthermore latent heat due to the transformation is fed back into both the kinetics and the thermal model we also consider the effects of transformation stress on growth kinetics and the possibility of an intracrystalline transformation mechanism previously thought to be important only at high shear stresses our models predict significantly smaller wedges of metastable olivine than previous work in the case of tonga for example where high values of lithospheric age 100 140 million years and convergence rate 14 cm per year are most favorable for metastability models considering only grain boundary nucleation and interface controlled growth predict olivine metastability to 600 km depth in contrast to 660 km predicted previously by kirby et al rev geophys 34 1996 261 when intracrystalline transformation is considered the depth of metastability is further reduced by as much as 100 km due to the large increase in the density of nucleation sites inhibition of growth by transformation stress can increase the depth interval over which the transformation takes place but is unlikely to be a dominant factor especially if the intracrystalline mechanism operates these results indicate that the existence of metastable olivine at depths corresponding to those of the deepest earthquakes 680 km requires subduction of old lithosphere 100 million years and a high vertical subduction velocity 15 cm per year such conditions currently may only be achieved in the very northern part of the tonga subduction zone yet earthquakes occur at depths down to 680 km in other subduction zones in which lithospheric age and or subduction velocity are relatively low e g indonesia and the marianas therefore mechanisms other than transformational faulting in metastable olivine must operate to cause the deepest earthquakes

an archean megacaldera complex the blake river group abitibi greenstone belt abstract the archean blake river group of the southern abitibi greenstone belt is defined as a subaqueous megacaldera compelling evidence include 1 a radial and concentric organization of synvolcanic mafic to intermediate dykes 2 an overall dome geometry defined by the volcanic strata 3 a peripheral distribution of subaqueous volcaniclastic units 4 a zonal distribution of carbonate alteration and 5 a distinct annular synvolcanic inner and outer ring fault pattern three caldera forming events have been identified 1 the early misema caldera 2 the new senator caldera and 3 the well known noranda caldera the multi vent brg mafic volcanic complex developed on a monotonous sequence of tholeiitic basalts forming a submarine plain and experienced a first major collapse that created the giant misema caldera 80 km in diameter an endogenic dyke swarm intruded the synvolcanic fractures and an underlying magma chamber developed major volcaniclastic units were generated by local volcanic centers and summit calderas formed along the outer and inner ring faults this fault system was used as a conduit for co2 rich hydrothermal activity renewed volcanic activity was associated with a resurgent central dome inside the misema caldera this second collapse event created the 35 km by 14 km nw trending new senator caldera this caldera was produced by multi step sagging after the underlying magmatic chamber migrated to the se and formed the flavrian powel plutons the final collapse resulted in the formation of the noranda caldera that generated a well developed 070 trending fracture pattern associated with several vms deposits the multiple caldera setting provides an effective model to explain the presence of vms mineralization along the synvolcanic fractures associated with the three collapse episodes however the importance of the new senator caldera fracture pattern is emphasized because of its role in the positioning of the giant horne mine deposit

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

evolution of the downgoing lithosphere and the mechanisms of deep focus earthquakes summary the thermal evolution of the lithospheric slab at subduction zones and its geophysical effects are numerically calculated an alternating direction implicit finite difference scheme is used to compute the thermal models taking into account all heating sources and phase boundaries these models with the appropriate spreading rates and dip angles are compared with different island arc systems temperatures inside the slab are strongly controlled by the conductivity and by the time elapsed since the initiation of descent the depth to which temperature anomalies persist is generally about 700 km or less the thermal results are used to construct the seismic velocities and ray paths density anomalies and the resulting stress distribution comparing the theoretical stress distribution and the focal mechanism studies of intermediate and deep focus earthquakes indicates the importance of both the mantle s rheology and the temperature dependence of the slab s elastic properties the intermediate and deep focus earthquakes are located along the coolest region of the slab the theoretical results explain the source mechanisms and the orientation of principal stresses under major island arcs

surface displacements resulting from magma chamber roof subsidence with application to the 2014 2015 bardarbunga holuhraun volcanotectonic episode in iceland abstract the conditions which lead to caldera collapse are still poorly constrained as there have only been four possibly five well documented caldera forming events in the past century the geodetic signals produced during chamber roof subsidence or chamber volume reduction shrinkage in general are not well documented or understood in particular when two or more geodetic sources are operating and providing signals at the same time it is important to be able to estimate the likely contribution of each simultaneous activities of different geodetic sources are common and include pressure changes in magma chambers reservoirs occurring at the same time as dyke emplacement here we present results from numerical models designed to simulate the subsidence of a magma chamber roof either directly chamber shrinkage or through ring fault displacement and the induced surface deformation and crustal stresses we consider chamber depths at 3 km 5 km and 7 km below the crustal surface using both non layered isotropic and layered anisotropic crustal models we also model the effects of a caldera lake and of a thick ice cover ice sheet on top of the caldera the results suggest that magma chamber roof subsidences between 20 m and 100 m generate large tens of centimetres vertical and in particular horizontal displacements at the surfaces of the ice and the crust out to distances of up to tens of kilometres from the caldera chamber centre crustal layering tends to reduce but increasing chamber depth to enlarge the horizontal and vertical surface displacements applying the results to the ice subsidence in the bardarbunga caldera during the 2014 2015 bardarbunga holuhraun volcanotectonic episode indicates that the modelled ice displacements are less than those geodetically measured also the geodetically measured crustal displacements are less than expected for a 60 m chamber roof subsidence the modelling results thus suggest that only part of the ice subsidence is due to chamber roof subsidence the other part being related to flow in and down bending of the ice we show that such a flow is likely within the caldera as a result of the stress induced by the 45 km long regional dyke emplaced primarily in vertical magma flow during the episode this conclusion is further supported by the model results suggesting that the ring fault piston like displacements must have been much less than the total 60 m ice subsidence or else faults with tens of metres displacements would have cut through the ice these are not observed we suggest that the ring fault subsidence was triggered by small doming of the volcanic field and system hosting the bardarbunga caldera and that this doming occurred as a result of magma inflow and pressure increase in a deep seated reservoir the doming is confirmed by gps measurements and supported by the seismicity results the magmatic pressure increase in the reservoir was in terms of the present model responsible for the regional dyke emplacement the holuhraun eruption and part of the stress concentration around and displacement of the bardarbunga caldera

subduction zones observations and geodynamic models abstract this review of subduction and geodynamic models is organized around three central questions 1 why is subduction asymmetric 2 are subducted slabs strong or weak 3 how do subducted slabs interact with phase transformations changes in mantle rheology and possibly chemical boundaries in the mantle based on laboratory measurements of the temperature dependence of olivine one would conclude that the core of a subducting slab is at least 10 000 times more viscous than ambient mantle however there are a number of complementary but independent observations that suggest that slabs are much weaker than this slabs undergo significant deformation in the upper mantle and may thicken to twice their original width by the time they reach the base of the transition zone the lack of a clear correlation between the observed dip angle of deep slabs and plate velocity rate of trench migration and slab age in modern subduction zones is consistent with hypothesis that subduction is a time dependent phenomenon both tank and numerical convection experiments with plates conclude that subduction is not a steady phenomenon but that slabs bend thicken stretch and change dip through time this is at odds with the assumptions used in steady state slab thermal models where slab deformation is not considered

seismicity shear failure and modes of deformation in deep subduction zones abstract planes of shear failure in deep seismic zones are identified as being parallel to focal mechanism nodal planes of deep focus earthquakes we relocate deep seismicity reported in the international seismological centre isc catalogue for earthquakes deeper than 400 km in the honshu bonin marianas java banda and south america subduction zones using the joint hypocentral determination method recent events with harvard centroid moment tensor cmt solutions and nearby seismicity are viewed from the direction of the cmt null axis comparing the two orthogonal nodal planes with the seismicity pattern planar features of seismicity parallel to cmt nodal planes are found in each deep seismic zone and are identified as planes of shear failure the sense of displacement on these planes is one of resistance to deeper penetration honshu and bonin show imbrication and horizontal displacement of the deepest part of the seismogenic slab while marianas and brazil show conjugate shear failure banda and argentina show patterns of vertical and horizontal shear planes for the western pacific slabs we find significant lateral displacements towards the south of the deepest part of the subduction flow tonga has been analyzed previously showing the same pattern as identified here in all benioff zones planes of shear failure are observed which act to shorten the slabs and displace matter horizontally normal to the slab moreover differential faulting acts to rotate the slab of subducted lithosphere to a more horizontal posture indicating that slabs experience resistance to penetration of the 670 km discontinuity

seismic evidence for a metastable olivine wedge in the subducting pacific slab under japan sea abstract we apply a forward modeling approach to high quality arrival time data from 23 deep earthquakes greater than 400 km depth to investigate the detailed structure of the subducting pacific slab beneath the japan sea our results show that a finger like anomaly exists within the subducting pacific slab below 400 km depth which has a p wave velocity 5 lower than the surrounding slab velocity or 3 lower than that of the normal mantle suggesting the existence of a metastable olivine wedge mow in the slab the mow top and bottom depths are 400 and 560 km respectively the mow is estimated to be about 50 km wide at 400 km depth and close to the slab upper boundary at 560 km depth the mow is located at about 25 km below the slab upper boundary most of the deep earthquakes are located in the mow our results favor transformational faulting as the mechanism for deep earthquakes

the influence of olivine metastability on the dynamics of subduction abstract both seismological observations and mineral physics experiments suggest that old and cold subducting slabs might contain a wedge of metastable olivine mo dynamically consistent models of subducting slabs in which the kinetics of the olivine to spinel wadsleyite or ringwoodite transition is treated in a simplified way are carried out to evaluate the buoyancy effect of mo on the subduction velocity assuming slab thicknesses according to the cooling half space model we find that mo forms in slabs with ages greater than approximately 70 myr and that a significant slowing down of subduction velocity occurs for ages greater than 100 myr because a decrease in the subduction velocity for old lithospheric ages is not observed our models suggest that the amount of mo in slabs is less than approximately 5000 km2 in cross section and that for old lithosphere the cooling half space model is not applicable

state of stress in slabs as a function of large scale plate kinematics the state of stress of slabs subducting worldwide as revealed by seismicity is extremely variable both with depth and between different subduction zones although in principle slab pull should enhance downdip extension in the slab the reconstructed stress fields for intermediate depths between 100 and 300 km range from downdip compression to downdip extension using 2 d viscoelastic plane strain models we investigate the dependency of the stress field of slabs on geometry dip of the slab and kinematics velocity of convergence between upper and lower plates and their absolute velocity with respect to the underlying mantle of subduction zones we conclude that although the state of stress in slabs is also controlled by other processes downdip compression in the subducting slab is enhanced by mantle flow opposing the direction of the dip of the slab whereas downdip extension is favored by mantle flow in the same direction of the slab dip i e sustaining it these predictions are in agreement with available geophysical observations although exceptions to this simple pattern are observed worldwide in addition if the slab is decoupled from the upper plate convergence between upper and lower plates induces a downdip compressional component of stress within the slab decreasing the magnitude of extension in models characterized by mantle flow sustaining the slab and increasing compression in models with mantle flow opposing subduction however these are second order variations when compared to the control exerted by absolute plate kinematics and by the magnitude of slab pull sensitivity analysis of rheological parameters allows us to conclude that these results are generally consistent although low values of viscosity of the lithospheric mantle render this prediction less stable

stress fields generating ring faults in volcanoes ring faults in volcanoes have been recognized for a long time but their mechanics of formation is still poorly understood while the subsidence on a ring fault during a large eruption from the associated chamber is easily understood the initiation of the fault itself has been difficult to explain for a ring fault to form the tensile and shear stresses at the surface of the volcano must peak at a certain radial distance from the surface point above the center of the chamber empirical evidence however shows that during most periods of unrest in active volcanoes the stress field is not favorable for the initiation of ring faults we made several tens of boundary element models of magma chambers of various geometries and subject to different loading conditions the results indicate that overpressure or underpressure in the chamber as the only loading is unlikely to initiate ring faults for a spherical chamber subject to horizontal extension or doming and a sill like chamber subject to horizontal extension the tensile and shear stresses at the surface of the volcano peak at a certain radial distance from the surface point above the center of the chamber then however the maximum stresses occur at the boundary of the chamber itself which would normally lead to sheet injection rather than to ring fault formation by contrast a sill like magma chamber subject to doming gives rise to a stress field suitable for the initiation of a ring fault

an experimental study of grain scale microstructure evolution during the olivine wadsleyite phase transition under nominally dry conditions abstract the grain scale microstructure evolution during the phase transformation of olivine to wadsleyite was investigated using a multi anvil high pressure apparatus we identified three stages in the microstructure evolution during olivine to wadsleyite phase transformation 1 nucleation of wadsleyite along potential nucleation sites e g grain boundaries and grain corners inside of grains dislocations stacking faults 2 development of an interconnected network of fine grained wadsleyite along grain boundaries 3 growth of wadsleyite into olivine our observations suggest that at low over pressures 3 gpa intra crystalline nucleation makes a significant contribution to the volume fraction transformed the rheological properties of a sample undergoing the phase transformation are controlled by the grain size and the connectivity of newly formed grains based on the model of grain size evolution involving both nucleation and growth we show that the initial grain size of wadsleyite is controlled by the competition between nucleation rate and growth rate when phase transformation takes place near the equilibrium boundary at relatively higher temperatures 1200 k but mostly by critical size for nucleation when phase transformation takes place away from the equilibrium boundary at relatively low temperatures

earthquake generating stresses in a descending slab abstract a numerical study of stress within descending slabs beneath island arcs provides a general interpretation for the occurrence of deep and intermediate focus earthquakes the computation of stress due to several factors which have been proposed for the interpretation of the double planed seismicity of intermediate depth events shows that thermal expansion and olivine spinel phase change play an important role in earthquake generation the stress field due to the combination of these two causes gives the following results in the depth range 50 300 km the compressional and tensional stress predominates in the upper and central part of the slab respectively maximum shear stress amounting to 1 6 gpa in compressional and 0 4 gpa in tensional fields occurs at a depth of 200 km the stress is generally very small at around 300 350 km below 400 km compressional stress exists at the central part of the slab and tensional stress appears near the upper and lower surfaces of the slab maximum shear stress is 2 8 gpa in compression and 1 6 gpa in tension at a depth of 500 km the principal axes in these high stress fields align almost parallel to the dip direction of the slab these stress features explain well such observed seismological facts beneath island arcs as the seismicity depth relationship planer configuration of seismic zone existence or non existence of double planed seismicity and focal mechanisms of intermediate and deep focus earthquakes

subduction zone earthquakes and stress in slabs the pattern of seismicity as a function of depth in the world and the orientation of stress axes of deep and intermediate earthquakes are explained using viscous fluid models of subducting slabs with a barrier in the mantle at 670 km 670 km is the depth of a seismic discontinuity and also the depth below which earthquakes do not occur the barrier in the models can be a viscosity increase of an order of magnitude or more or a chemical discontinuity where vertical velocity is zero longn versus depth wheren is the number of earthquakes shows 1 a linear decrease to about 250 300 km depth 2 a minimum near that depth and 3 an increase thereafter stress magnitude in a subducting slab versus depth for a wide variety of models shows the same pattern since there is some experimental evidence thatn is proportional toe where is a constant and is the stress magnitude the agreement is encouraging in addition the models predict down dip compression in the slab at depths below 400 km this has been observed in earlier studies of earthquake stress axes and we have confirmed it via a survey of events occurring since 1977 which have been analysed by moment tensor inversion at intermediate depths the models predict an approximate but not precise state of down dip tension when the slab is dipping the observations do not show an unambiguous state of down dip tension at intermediate depths but in the majority of regions the state of stress is decidedly closer to down dip tension than it is to down dip compression chemical discontinuities above 670 km or phase transitions with an elevation of the boundary in the slab predict when incorporated into the models stress peaks which are not mirrored in the profile of seismicity versus depth models with an asthenosphere and mesosphere of appropriate viscosity can not only explain the state of stress observed in double benioff zones but also yield stress magnitude profiles consistent with observed seismicity models where a nonlinear rheology is used are qualitatively consistent with the linear models

the fate of descending slabs oceanic lithosphere the stiff plate formed by the cooling of chemically diffcrentiated material that rises and mclts under mid ocean ridges and spreads laterally in the process called sea floor spreading has an ephemeral residence at the surface of the earth typically within 100 200 ma densi fication of the old oceanic lithosphere leads to a dynamic instability causing it to sink into the interior descending as a layered slab with highly anom alous thermal and chemical characteristics relative to the surrounding ambient mantle the slab heats as it sinks but the warming is sluggish slow to overcome the effect of more than 100 ma of cooling at the surface and the negative buoyancy of the slab continues to drive its descent increasing pressure and temperature with depth drive various chemical reactions in the slab including expulsion of water and other volatiles accumulated at the surface and destabilization of associated hydrous phases suites of mineralogical phase transformations occur within the slab s crustal and depleted mantle components because the slab has been processed through the melting zone at the ridge partially hydrated and cooled significantly the phase equilibria within the slab differ from those of the surrounding mantle near 400 km depth an exothermic phase transformation occurs with mg fe 2si04 olivine in the slab converting to the high pressure modified j spinel structure the spinel transition helps to drive the slab onward although a central core of metastable olivine may persist to greater depths with eventual transformational faulting

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

propagation linkage and interaction of caldera ring faults comparison between analogue experiments and caldera collapse at miyakejima japan in 2000 the formation of ring faults yields important implications for understanding the structural and dynamic evolution of collapse calderas and potentially associated ash flow eruptions caldera collapse occurred in 2000 at miyakejima island japan in response to a lateral intrusion based on geophysical data it is inferred that a set of caldera ring faults was propagating upward to understand the kinematics of ring fault propagation linkage and interac tion we describe new laboratory sand box experiments that were analyzed through digital image correlation dic and post processed using 2d strain analysis the results help us gain a better understanding of the processes occurring during caldera subsidence at miyakejima we show that magma chamber evacuation induces strain localization at the lateral chamber margin in the form of a set of reverse faults that sequentially develops and propagates upwards then a set of normal faults initiates from tension fractures at the surface propagating downwards to link with the reverse faults at depth with increasing amounts of subsidence interaction between the reverse and normal fault segments results in a deactivation of the reverse faults while displacement becomes focused on the outer normal faults modeling results show that the area of faulting and collapse migrates successively outward as peak displace ment transfers from the inner ring faults to later developed outer ring faults the final structural architecture of the faults bounding the subsiding piston like block is hence a consequence of the amount of subsidence in agreement with other caldera structures observed in nature the experimental simulations provide an analogy to the obser vations and seismic records of caldera collapse at miyake jima volcano but are also applicable to caldera collapse in general

relationship of deep seismicity to the thermal structure of subducted lithosphere recent experimental work on silicate olivine polymorphs1 has confirmed earlier observations of mechanical failure in analogous compounds2 3 as a result of phase transformations when non hydrostatic stresses were applied to a metastable phase this transformational faulting 4 mechanism is considered to be a leading candidate5 among others involving olivine transformation6 7 for the cause of deep earthquakes evidence consistent with this mechanism comes from the observation2 4 that worldwide earthquakes become more numerous with increasing depth after a seismicity minimum at about 350 km depth8 this depth lies within the range anticipated for mineralogical transformations in the subducted lithosphere9 but individual subduction zones differ in their thermal structures so if transformational faulting indeed contributes to deep seismicity this should be reflected in the location of the seismicity minimum for each zone the depth at which the minimum occurs should depend on temperature in the same way as do the polymorphic transformations of olivine and should always lie deeper than the depth at which the equilibrium transformation takes place here we test these predictions for eight subduction zones worldwide we find that with one exception the north japan zone the depth of the seismicity minimum decreases linearly with increasing thermal age of the slab a measure of its temperature profile these results support the proposal that deep earthquakes are a consequence of phase transformations in olivine

stress fields associated with metastable phase transitions in descending slabs and deep focus earthquakes inside subducting slabs the interaction of different metastable phase transformations olivine spinel spinel spinel spinel perovskite magnesiowustite associated with the latent heat release and absorption for these transitions can result in a complex thermal slab structure thermokinetic coupling processes cause thermal anomalies and buoyancy contrasts between slab and mantle that strongly influence the stress field within subducting plates we have described the thermal field within the downgoing slab in a lagrangian framework by a self consistent high resolution two dimensional thermokinetic coupling model which also incorporates the effects of nucleation site saturation using an adaptive method the threshold for the nucleation site saturation is found to be less than 0 1 transformation degree in the cold slab interior this can significantly shift the metastable wedge of olivine deeper as suggested from previous thermokinetic slab models however this effect is compensated by the latent heat release during the olivine spinel and spinel spinel phase transitions latent heat release produce very sharp phase boundaries in the slab for fast slabs the phase boundaries reveal significant metastable perturbations from the equilibrium state in the cold interior however the strong thermal interaction between both ongoing phase transitions results in sharp overlapping phase boundaries with dramatic consequences for the slab stress field we have investigated the detailed physical properties and the state of stress in the deeper portion of a subducting plate by using up to date temperature field and physical properties determined from recent high p t experiments we find that the slab is denser up to 100 kg m 3 than the surrounding mantle remarkable denser portions up to 260 kg m 3 are located just below the uplifted and equilibrium phase transitions while lighter portions up to 210 kg m 3 can be found just above the depressed spinel to perovskite magnesiowustite transition and for the metastable wedge of olivine these density differences effectively act as a body force and produce a significant stress field which we calculated with a two dimensional finite element code the results show that the buoyancy induced forces produce maximum shear stress up to 23 mpa along the metastable wedge and deeper portion just above the depressed last phase transition involving spinel for the latter the dominant state of stress is down dip compression the calculated p t dependent state of stress is very similar to the depth distribution of deep focus

time dependent chamber and vent conditions during explosive caldera forming eruptions abstract we use a modified version of the cpiuc model macedonio g neri a marti j folch a 2005 temporal evolution of flow conditions in sustained explosive eruptions journal of volcanology and geothermal research 143 153 172 to simulate chamber and vent conditions during the different phases of a piston like caldera forming eruption our idealized caldera forming scenario assumes an initial central vent conduit that after critical chamber decompression migrates to a fissure vent peripheral conduit s further decompression leads to final piston like subsidence which stops only after the virtual destruction of the magmatic reservoir the simulations find that the pressure at the conduit entrance drops during the decompression phases at a rate depending on the conduit geometry chamber volatile zonation and fragmentation threshold the higher the volume contrast between the initial central vent and the final peripheral fissure vent conduits the higher the pressure drop and the jump in the mass eruption rate pressure increases back to lithostatic during piston subsidence while some compressible magma remains within the chamber finally during the later phase pressure experiments a gentle increase or decrease depending on the balance between deposition of intra caldera material and decrease in the contents of volatiles as deeper chamber levels are tapped

metastable olivine wedge in the subducting pacific slab and its relation to deep earthquakes abstract a metastable olivine wedge mow is detected in the subducting pacific slab under the japan sea by applying a forward modeling approach with 3 d ray tracing to high quality arrival time data from 78 well located deep earthquakes the data were recorded by the dense and high quality seismic networks on the japan islands the double difference earthquake location technique is improved and used to relocate the 78 deep earthquakes precisely our results show that the mow does exist in the pacific slab and it has a p wave velocity 3 lower than the normal mantle or 4 lower than the surrounding slab velocity the top and bottom depths of the mow are 400 and 570 km respectively and the mow is located in the central portion of the slab after precise relocation all the 78 deep earthquakes are found to occur within the mow or along its edges within the slab the cut off depth of deep seismicity under the japan sea is consistent with the estimated mow bottom depth our results favor the transformational faulting as the mechanism of deep earthquakes in the pacific slab under the japan sea

the depth of the deepest historical earthquakes we usep ands times listed in the international seismological summary to relocate 23 historical earthquakes 1927 1963 reported as occurring at or below 670 km in all cases our relocated hypocenters are shallower than the starting depths furthermore all events converge to 691 km or less with a precision estimated at 10 km this study upholds the results of stark and frohlich who had usedpp p times of post wwssn earthquakes to constrain reliable hypocentral depths to no greater than 684 km in particular we reject rothe s claim that a 1963 event in the vicinity of new guinea occurred at a depth of more than 780 km

the role of magma chamber fault interaction in caldera forming eruptions this paper examines the role of the position and orientation of a regional fault in the roof of a magma chamber on stress distribution mechanical failure and dyking using 2d finite element numerical simulations the study pertains to the magma chamber behavior in the relatively short time intervals of several hundreds to thousand of years the magma chamber is represented as an elliptical inclusion eccentricity a b 0 12 at a relative depth h a of 0 9 the fault has a 45 dip and is represented by a frictionless fracture the temperature field in the host rock is calculated assuming a quasisteady state thermal regime that develops through periodic episodes of magma supply the rheology of the surrounding rocks is treated using viscoelasticity with temperature activated strain rate dependent viscosity strain weakening of the rocks in the ductile zone is described within the frame of the dynamic power law model the magma pressure is coupled with the deformation of the rock mass hosting the chamber including the fault the variation of magma pressure in response to magma supply and chamber deformation is calculated in the elastic and viscoelastic regimes the latter corresponds to slow filling while the former represents a filling time much less than the viscous relaxation time scale the resulting equation of state for the magma chamber couples the magma pressure with the chamber volume in the elastic regime and with the filling rate for the viscoelastic regime analysis of stresses is used to predict dyke propagation conditions and the mechanical failure of the chamber roof for different fault positions and magma overpressures results show that an outward dipping fault located on the periphery of the chamber roof hinders the propagation of dykes to the surface causing magma to accumulate under the footwall of the fault at high to moderate overpressures 30 40 mpa the fault causes localized shear failure and chamber roof collapse that might lead to the first stage of a caldera forming eruption

coupling a frictional cohesive cover and a viscous substrate in a discrete element model first results of application to thick and thin skinned extensional tectonics abstract mechanical stratigraphy is now recognised as a fundamental control on the development of geological structures in the upper crust at a variety of different scales this is perhaps never more apparent than when salt is part of the crustal section being deformed the interaction of a ductile salt substratum and a brittle sedimentary cover is complex but an understanding of it is essential both from an economic and academic standpoint here i present first results of a discrete element model which combines a viscous linear newtonian substrate and a frictional cohesive cover the modelling approach is firstly presented and experimental scaling to appropriate geological timescales discussed the approach is then applied to 3 experiments simulating extensional deformation two of which are thick skinned and one is thin skinned strain rates in all experiments are c 1015 s 1 the complex manner in which salt flows in the substrate and faults develop in the cover are illustrated and their linkage evolution examined movement of the viscous substrate is the result of both couette type and poiseuille type flows and combinations thereof whilst deformation in the cover takes the form of discrete dilational faults which are not directly linked or only soft linked to any sub salt basement faults cover faults typically lose displacement towards and tip out at the cover substrate interface in addition in models with a basement fault the fault itself presents a growing and important no slip boundary which significantly affects viscous flow implications for the timing evolution of and strain within the resultant structures are discussed

phase transition buoyancy contributions to stresses in subducting lithosphere the sequence of phase transitions undergone by minerals with increasing depth in earth s mantle is perturbed within subducting lithospheric slabs by their thermal structure such perturbation of equilibrium phase relations gives rise to relative buoyancy contrasts between slab and mantle that contribute to the state of stress within the slab while other factors contribute to overall slab stresses thermal and phase transition effects largely control the structure of the stress field within the slab the resulting maximum in down dip compressive stress within the slab corresponds to the observed peak in depth distribution of deep seismicity furthermore metastable persistence of lower pressure phases within the cold slab should give rise to localized shear stresses whose distribution corresponds to observed features of subduction zone seismicity these observations are independent of the variety of failure mechanisms proposed for deep seismogenesis

patterns of deep seismicity reflect buoyancy stresses due to phase transitions thermal perturbation of mantle phase relations in subduction zones gives rise to significant buoyancy anomalies finite element modeling of stresses arising from these anomalies reveals transition from principal tension to compression near 400 km depth down dip compression over 400 690 km peaking at 550 km and transition to rapidly fading tension below 690 km such features even when complicated by olivine metastability are consistent with observed patterns of deep seismicity that such a simple model neglecting all effects other than buoyancy anomalies due to temperature and to thermal perturbation of olivine phase relations successfully generates so many observed features of deep seismicity suggests that these buoyancy anomalies are significant contributors to the stress field in subducting slabs it also suggests that the depth distribution of deep seismicity may largely reflect the state of stress in the slab rather than simply a particular mechanism of stress release

rise and emplacement of magma during horizontal shortening of the brittle crust insights from experimental modeling 1 magmatic activity tends to concentrate at plate margins at divergent margins extensional tectonics provide steep conduits for magma to reach the surface at rapidly convergent margins such as the andes one might imagine that horizontal compression prevents the rise of magma nevertheless volcanoes are also common in order to study the mechanisms by which magma rises in a compressional context we resorted to laboratory experiments in which a brittle crust was shortened while magma was intruding our model materials were 1 cohesive fine grained silica powder representing brittle crust and 2 molten low viscosity vegetable oil representing magma in general horizontal shortening and injection were coeval but independent processes thrust faults accommodated the shortening while overpressured oil formed hydraulic fractures in those experiments where there was no shortening injection resulted in a saucer shaped intrusive body in the other experiments where there was shortening oil formed a basal sill before rising along thrust faults once in place the sill lubricated the base of the model so that arcuate thrusts formed at the leading edge of a plateau uplift of the plateau promoted further intrusion of oil at depth in general the pattern of deformation and intrusion depended on the kinematic ratio r between rates of shortening and injection the lengths of the basal sill and plateau increased with decreasing r on the basis of these results we have reexamined two natural examples of magmatic complexes which were emplaced in compressional tectonic settings tromen volcano in argentina and the boulder batholith of montana

fault volcano interactions with broadly distributed stretching in rifts abstract areas undergoing pure extension are typically characterized by the formation of faults and fractures orthogonal to the maximum principal stress in these conditions the presence of a volcano and its magmatic system perturbs the local stress field leading to changes in faulting magnitude and attitude this process has been analyzed in many previous modeling works in which fault volcano interaction have been reproduced with set ups that create extension over a single discontinuity by use of a standard base plate however many natural rifts display broadly distributed extension between main border faults with no discrete discontinuities driving deformation to account for these conditions in this study we improve the previous works by reproducing and analyzing the interaction of tectonic stresses with volcanic edifices in areas characterized by broadly distributed extension in particular we run a set of analogue models which are extended by stretching a basal elastic sheet with this improved set up models are generally consistent with previous ones but better reproduce the geometry and architecture of faulting in rifts leading to improved and more rigorously results to compare with real cases we confirmed that the presence of a volcano and or an intrusive body locally modifies the fault pattern concentrating faulting and curving faults notably the rheological contrast between the intrusion and the host rock leads to faults that follow the outline of the body favoring concentric faults in contrast radial faults are developed by the volcano load the combination of intrusion host rock contrast and volcano loading creates both types of faulting which are recognized in natural examples the geometry of the resulting faults is dependent on the coexistence or not of intrusion and edifice and on their dimensions the results indicate that volcanic edifices have a stronger influence on fault attitude with respect to the presence of intrusions experimental results are compared with some natural cases from the east african rift system finally we suggest that using faulting geometries as a foundation geodetic monitoring and more precise modeling could be used to assess the rift evolution of such extensional magmatic systems

the nature of deep focus earthquakes wadati 1928 first proved convincingly that some earthquakes occur at depths well beneath the earth s crust he essentially proposed the nomenclature used today by the international seismological centre isc i e earthquakes with focal depths exceeding 300 km are deep earthquakes and those with depths between 70 and 300 km are intermediate earth quakes however when there is little reason to distinguish between the two groups both types are called simply deep earthquakes to avoid confusion in this paper we refer to those with focal depths exceeding 300 km as very deep earthquakes and those with focal depths between 70 and 300 km as intermediate earthquakes deep earthquakes are of interest for at least four reasons first they are exceedingly common between 1964 and 1986 they constituted no less than 22 of all earthquakes having mb greater than 5 0 in the isc catalog second they most often occur in association with deep ocean trenches and volcanic island arcs in subduction zones one of the great achievements of twentieth century geophysics was the recognition that their occurrence in inclined planar groups or wadati benioff zones apparently delineates the cold down going cores of convection cells in the uppermost mantle third seismologists use body waves of deep earthquakes dispro portionately more often than those of shallow earthquakes to inves tigate core mantle and crustal structure this is because a deep earth quakes often possess relatively more impulsive sources b their body phases traverse the heterogeneous uppermost mantle only once on the ray path from hypocenter to station and c surface waves do not contaminate these body phase arrivals on seismograms finally deep earthquakes are

a mathematical model of vulcanian eruptions summary the problem of an expansion wave propagating into a saturated magma is solved and used as a model of a vulcanian eruption in our model for an explosive eruption we assume that a drop in pressure leads to the exsolution of magmatic volatiles initially the exsolved vapours create bubbles in the magma we model the subsequent part of the exsolution process in which a foam is created this is believed to be an essential feature of explosive volcanic eruptions the foam also has the advantage that it can be modelled as a mixture or pseudo gas without slippage between the phases eventually the foam breaks up and becomes volcanic ash assuming that the exsolution of vapour is given by henry s law that the temperature is constant that the magma and vapour have equal velocities and neglecting wall friction and gravitational effects an analytic solution for pressure velocity and vapour fraction is obtained for the expanding mixture in a constant area duct the exit velocity u for the mixture is u o0rt0 1 2in p0 p where o0 is the original mass fraction of dissolved vapour r is the gas constant for the vapour t0 the constant temperature and p0 p the pressure ratio across the expansion with o0 1 per cent t0 1000 k and p0 p 100 we find u0 300ms 1 consistent with observations for vulcanian eruptions

the effects of phase transition kinetics on subducting slabs the effects of kinetics on the non equilibrium aspects of the olivine to spinel transition in a descending slab have been studied numerically a one dimensional model consisting of the kinetic equations and the heat diffusion equation with latent heat release has been constructed numerical results show that the position and sharpness of the kinetic phase boundary is determined by the surface tension and the activation volume for slow slab velocities less than 6 cm yr near equilibrium conditions are found finger like structures emanating from the phase boundaries are obtained in this regime these phase boundary prolusions may cause earthquakes for slab velocities of around 10 cm yr the metastable olivine region may be pushed down to a depth of about 600 km with a sharp phase boundary a direct transition from olivine to perovskite may be feasible

an implosive component to the source of the deep sea of okhotsk earthquake of 24 may 2013 evidence from radial modes and cmt inversion abstract we study the spectral amplitudes of the first two earth radial modes 0 s 0 and 1 s 0 excited by the sea of okhotsk earthquake of 24 may 2013 the largest deep event ever recorded in the search for an isotropic component to its source in contrast to the case of the 1994 bolivian earthquake we detect an implosive component m i 1 1 10 27 dyn cm equivalent to 3 of the full scalar moment but 14 of the lone deviatoric component exciting the earth s radial modes an independent moment tensor inversion using the standard globalcmt algorithm but after relaxing its zero trace constraint similarly yields an implosive isotropic component albeit with a larger amplitude while it fails to document one in the case of the 1994 bolivian deep earthquake an implosive component to the source is expected in the model of transformational faulting in which deep earthquake rupture nucleates and grows upon transformation of metastable olivine to ringwoodite in the cold subducting slab this interpretation is supported by quantitative estimates 0 9 4 m of the thickness of the transformed shear zone which scale favorably relative to earthquake fault length with the upper end of the range of laboratory results reported for ices germanates and silicates the resulting extent of the transformation in the metastable wedge is consistent with the local geometry of the deep slab as recently determined by rupture modeling and aftershock distribution our results are in contrast to those for the two runner up largest deep earthquakes the 1994 bolivian and 1970 colombian shocks for which a similar isotropic component could not be detected we attribute this difference to variability in the ratio of isotropic to deviatoric components which combined with the smaller size of the 1970 and 1994 events would make any putative implosive component fall below detection levels especially in the case of the 1970 colombian earthquake for which only analog narrow band records were available

seismic constraints on mechanisms of deep earthquake rupture 1 observations of the depth distribution of seismicity suggest that several mechanisms cause deep earthquakes cold subducting slabs exhibit a broad peak in seismicity between 300 and 530 km depth that is best explained by transformational faulting of metastable olivine cumulative seismic moment release increases sharply at 530 km depth in both cold and warm slabs implying that events deeper than this are controlled by an equilibrium rather than a metastable phase change which would be temperature dependent this together with the observation that deep seismicity predominates near the garnet rich upper boundary of the subducting plate former oceanic crust suggests that the transformation of garnet to perovskite which may release considerable water is integral in the cause of earthquakes deeper than 530 km where most deep earthquakes occur

propagation of blind normal faults to the surface in basaltic sequences insights from 2d discrete element modelling abstract a discrete element model is used to investigate the progressive deformation of a thin basaltic cover overlying a pre existing blind normal fault as it propagates to the surface the cover materials representing basalt are homogeneous strong and display elastic brittle material behaviour cover deformation is seen to evolve through a series of distinct stages initial displacement on the underlying fault produces a very gentle monoclinal flexure with continued displacement open fractures develop at the monocline surface and propagate downwards whilst the deeper fault propagates upwards simultaneously a series of fractures in the future hanging wall of the main fault develop in the upper part of the cover the monoclinal flexure is then cut by these structures producing a surficial fault and fracture bounded wedge finally a prominent surface fracture and the upward propagating fault link cutting the entire cover sequence this fault is dilatant in the upper c 100 m of the cover has a significant surface aperture and forms a prominent fault scarp many of the key model results are strikingly similar to those seen in natural settings and emphasise that the occurrence of dilatant faults open fractures and cavities caves in extensional settings is not necessarily restricted to the very shallow section but can extend to several hundred metres depth therefore the results have implications for permeability and fluid flow in such settings comparison is also made with a weak cover experiment using granular materials with no cohesion or tensile strength similar to the dry sand used in many analogue modelling studies

from subduction to collision control of deep processes on the evolution of convergent plate boundary 1 using laboratory experiments we investigate the dynamics of the collisional process that follows the closure of an oceanic basin the evolution of these experiments systematically shows four successive episodes of deformation which correspond to 1 the initiation of oceanic subduction 2 a mature period of oceanic subduction 3 an episode of continental subduction during which the trench absorbs all the convergence and the superficial tectonic regime does not change significantly within the continental plates and 4 continental collision that starts when the trench locks and convergence is absorbed by a system of thrust faults and folds we observe that the amount of continental material that subducts before the onset of collision depends on the slab pull exerted by the subducted oceanic lithosphere the slab pull force in turn depends on the amount of subducted oceanic material on the thickness of the convective mantle and also on the rheology of the slab our experiments indeed suggest that parts of the oceanic slab may separate from the superficial slab to sink rapidly in the mantle decreasing the slab pull level and triggering the rapid onset of collision we observe two possible modes of slab deformation slab break off and development of viscous instabilities we define two dimensionless rheological numbers to characterize the possible occurrence of these modes of deformation in all cases oceanic closure is followed by episode of continental subduction during which the continental lithosphere may reach depths varying between 50 and 450 km prior to the onset of continental collision

subduction zone thermal structure and mineralogy and their relationship to seismic wave reflections and conversions at the slab mantle interface seismological studies of the interface between downgoing slabs of subducted lithosphere and the overriding mantle provide constraints against which models of mantle and slab temperature and bulk composition can be tested these studies employing waves reflected and converted at the slab mantle interface suggest that a velocity contrast of 5 10 occurs over a distance of 10 20 km to depths of at least 350 km the authors have investigated whether such sharp contrasts can result form thermal structure compositional differences phase changes or other effects such as preferred mineral orientation using a mantle and slab temperature model a petrologic model of mantle and slab mineralogy and a data base of elastic properties of mantle and slab phrases theoretical seismic velocities were calculated and compared with the observed velocity contrast and boundary thickness constraints at 9 6 gpa and 1000c conditions modeled at the slab mantle interface at approximately 280 km depth cooler temperatures in the slab interior produce at constant composition only approximately a 1 75 v sub p increase into the slab in any of the peridotitic and eclogitic mineralogies investigated variation in v sub p among the different bulk compositions is only approximately 0 5 these contrasts are substantially lower than those obtained from the seismological studies andmore indicate that temperature and bulk compositional differences between mantle and slab are inadequate to cause the observed velocity contrasts they propose that phase transformations in mantle and slab mineralogies notably elevation of the olivine spinel phase change normally occurring at 400 km depth are important factors in these sharp contrasts other contributing effects may include preferential mineral orientation near the slab surface and at shallower depths the presence of partial melt or hydrous fluid evolved from dehydration reactions along the interface less

effect of tensile stress concentration around magma chambers on intrusion and extrusion frequencies what controls the intrusion and extrusion frequencies associated with volcanoes is still poorly understood i propose that for volcanoes at divergent plate boundaries these frequencies may be largely determined by the tensile stress concentration around the magma chambers that feed them this stress concentration is mainly a function of the applied tensile stress associated with spreading and the aspect height width ratios of the chambers high spreading rates and or aspect ratios lead to high rates of tensile stress concentration around the chambers and to an increase in their intrusion frequencies it is found that for chambers at the same depth in a volcanic zone the one of the highest aspect ratio will normally intrude magma most frequently also if the chambers are of equal aspect ratios the one at the greatest depth will intrude magma most frequently because the extrusion frequency of a volcano is a fraction of its intrusion frequency the extrusion frequency may also be largely determined by the rate of tensile stress concentration around the magma chamber that feeds the volcano these results are applied to the divergent plate boundary in iceland where many of the volcanoes appear to be fed by double chambers that is shallow chambers fed by deep seated chambers it is found that except when the aspect ratio of the deep seated chamber is much less than that of the shallow chamber the intrusion frequency of the shallow chamber is normally largely controlled by that of the deep seated chamber it is concluded that whereas the short term i e 103 yrs extrusion frequencies of volcanoes at the plate boundary in iceland may be similar to the dike intrusion frequencies of the source chambers the long term i e 104 yrs extrusion frequencies may be about ten times lower than the intrusion frequencies

analogue models of collapse calderas and resurgent domes collapse calderas and resurgent domes are a common association related to inflation deflation processes in volcanic systems the structure of calderas and domes depends upon the permitted relative movements of crustal volumes at depth the so called space problem in order to study the structures of collapse calderas and resurgent domes and to take the space problem into account several analogue models were made dry quartz sand was used to simulate the rheology of the brittle crust while newtonian silicone putty located at the base of the sand pack simulated the ductile behaviour of the magma a piston moved the silicone putty downward or upward inducing collapse and doming within the sand three separate sets of experiments simulated 1 caldera collapse 2 resurgence and 3 superimposition of resurgence on collapse and vice versa collapse experiments are characterized by the development of two concentric depressions the first formed depression is bordered by outward dipping reverse ring faults the subsequent outer concentric depression is bordered by inward dipping normal ring faults the deformation pattern during resurgence is a function of the overburden thickness t and the dome diameter d for higher t d ratios a dome forms bordered by inward dipping high angle reverse ring faults outward dipping normal ring faults develop at late stages for lower t d ratios the dome shows at late stages a crestal depression accompanied by radial fractures subsequently an apical extrusion of silicone occurs the superimposition of resurgence over collapse and vice versa is characterized by the complete reactivation with opposite kinematics of all the pre existing ring faults during inversion both in caldera and resurgence reverse ring faults form in the early stages due to differential uplift extensional structures subsequently form to accommodate gravitational collapse during the activity of the reverse faults the experiments and the overall similarities with nature suggest that the activity of both reverse and normal faults constitutes a possible solution to the space problem during major collapses or resurgences

mesoscale variation in diabatic heating around sumatra and its modulation with the madden julian oscillation abstractdiabatic heating in the maritime continent region is controlled by a unique blend of mesoscale variability associated with steep topography and complex coastlines and intraseasonal variabil

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle