volcano destabilization by magma injections in a detachment we report the discovery of a detachment fault within reach of structural investigation in a shield volcano piton des neiges la reunion hotspot the detachment exposed by erosion is located between the top of an extinct gabbroic magma chamber and a cover of debris avalanche deposits the fault simultaneously concentrated ductile and brittle deformation toward the sea hydrothermal alteration in the greenschist facies and repeated sill injections of basic magmas our study of this exceptional detachment exposure suggests that two mechanisms participate in volcano flank failure hydrothermal alteration likely sustained by the heat of cooling sills induces slow inter intrusion deformation whereas sill injections in the fault may trigger rapid cointrusive slip this twofold process of destabilization might currently occur on some active volcanoes such as piton de la fournaise

a sagging spreading continuum of large volcano structure gravitational deformation strongly infl uences the structure and eruptive behavior of large volcanoes using scaled analog models we characterize a range of structural architectures produced by volcano sagging and volcano spreading these arise from the interplay of variable basement rigidity and volcano basement de coupling from comparison to volcanoes on earth la reunion and hawaii and mars elysium and olympus montes the models highlight a structural continuum in which large volcanoes throughout the solar system lie

deposits related to degradation processes on piton des neiges volcano reunion island overview and geological hazard abstract piton des neiges pn volcano on reunion island offers a rare opportunity to study deposits related to degradation processes in a deeply eroded oceanic shield volcano both the inner parts and flanks reveal a large amount of resedimented volcaniclastic material including extensive debris avalanche deposits pn litho structural units first studied by upton and wadsworth 1965 philos trans r soc lond a 271 pp 105 130 are re examined this review highlights the importance of long volcanic repose periods and erosion processes during pn history volcaniclastic deposits have been studied in the field in order to evaluate the spatial and temporal distribution of the three main types of pn degradation processes the deposits of these processes have been classified into 1 talus 2 mudflow and debris flow and 3 debris avalanche lithology frequency and estimated volumes of each deposit type imply that the structural evolution of pn can be considered in terms of the competition between the volcanic productivity and the degradation and erosion processes the occurrence of huge catastrophic avalanches produced by flank failure is convincingly linked to the basaltic activity of pn which implies a very low risk at present on the contrary mudflows and debris flows pose an important risk due to the high population density focussed around the basin outlets moreover if smaller debris avalanches can occur in the cirques of pn another major risk must be evaluated

neotectonics and the 2004 and 2005 earthquake sequences at sumatra coseismic and interseismic deformation of the outer arc islands of nias and simeulue associated with the great indonesian earthquakes of 2004 12 26 and 2005 3 28 is generally ascribed to elastic stress storage and release on the interface between the indian and eurasian plates at the java trench however the nature of the associated shallow seismicity and the pattern of holocene deformation of the islands suggest that there has also been substantial activity on imbricate faults within the sediment prism behind the trench the distinction between the two kinds of seismicity is of practical as well as theoretical significance because in contrast with the megathrust outer arc faulting is not subject to appreciable postseismic relaxation and is moreover well placed to generate tsunami

edifice strength and magma transfer modulation at piton de la fournaise volcano 1 from 2003 to 2007 eruptive activity at piton de la fournaise was shown to follow cycles comprising many summit proximal eruptions and finishing by a distal eruption gps measurements evidenced striking asymmetric deformation between its western and eastern flanks horizontal displacements recorded during interdistal periods showed a characteristic amplitude at the top of the eastern flank displacements recorded at the base of the summit cone showed a bimodal distribution with low amplitudes during interdistal periods and large ones during distal eruptions to account for displacement asymmetry characteristic amplitude and large flank displacement we modeled the volcanic edifice using a drucker prager elastoplastic rheology friction angles of 15 and 30 were needed to model the displacements respectively during distal eruptions and interdistal periods this change shows that strain weakening occurred during distal events large plastic displacement that occurred in the eastern flank during distal eruptions relaxed the horizontal elastic stress accumulated during interdistal periods it triggered summit deflation horizontal magma transfer and distal flank eruption and reset the eruptive cycle our elastoplastic models also show that simple source geometries may induce large eastern flank displacements that would be explained by a complex geometry in a linear elastic edifice magma supply is often thought to control volcano s eruptive activity with surface deformation reflecting changes in magma supply rate the volcano s response being linear our results bring some evidences that on piton de la fournaise time space discretization of magma transfer may be the result of the edifice s nonlinear response rather than changes in magma supply

new 230th 238u and 14c age determinations from piton des neiges volcano reunion a revised chronology for the differentiated series 230th 238u age measurements were carried out on 18 samples of lava from piton des neiges these lavas range from basalts to trachytes the ages extend from 270 000 to 12 000 a b p and cover most of the period in which differentiated series magmas were erupted new 14c ages on charcoal sampled in young pyroclastic deposits are also presented the late stage evolution of the volcano is discussed in the light of these new ages and a chronology of the explosive activity is given the oldest pyroclastic flow ste suzanne is more than 220 000 a other pyroclastic units were emitted between 220 000 and 110 000 a there were at least two episodes of caldera formation the first episode is inferred to have occurred about 190 000 a ago the last occurred between 190 000 and 110 000 a although the most recent lava activity was mainly concentrated on the eastern flank of the volcano and around the summit area it also occurred about 70 000 a ago in the southwestern part of the massif the youngest explosive activity about 50 000 to 20 000 a on the eastern flank of the volcano has now been dated by three different methods 14c and th u ages on block and ash flow tuffs k ar ages on associated domes youthful explosive activity about 35 000 to 12 000 a on the western flank of the volcano is also demonstrated by 14c dating on charcoal and by a th u age of 12 000 a on a trachytic nuee ardente deposit this last age indicates that activity may have persisted later than previously supposed and implies a high rate of erosion for the cirque de mafate

volcano structural evolution of piton des neiges reunion island indian ocean four volcano structural stages have accompanied the building of piton des neiges 1 emergent growth stage of the island the major eruptive system is a rift zone trending n 120 associated with dextral strike slip faults trending n 30 and en echelon extensional fissures trending n 70 breccias and lava tubes produced by aerial and phreatomagmatic activity are injected with outward dipping dike swarms along ring fractures suggesting a mechanism analogous to cauldron subsidence 2 shield building stages of growth are related to fissures along the main rift zone and three minor rifts trending n 160 n 45 and n 10 the summit of the basaltic shield volcano is stretched and collapsed in a graben like caldera depression along normal and antithetic faults 3 differentiated lavas are erupted during two stages separated by the opening of a new caldera corresponding to an explosive activity a silicic cone sheet system and a collapse structure 4 younger volcanic activity restricted to the inside caldera has presumably emptied the underlying magma reservoir building a central volcano collapsed along ring internal dip fractures

landslides and spreading of oceanic hot spot and arc shield volcanoes on low strength layers lsls an analogue modeling approach during their growth and evolution oceanic hot spot and arc shield volcanoes are destabilized and often dismantled by landslides the origin of these phenomena can be found in the structure of volcanic islands marked by several low strength horizons such as a marine substratum hyaloclastites hydrothermally altered rocks or volcaniclastic deltas that weaken the edifice we have tested the role of these low strength layers lsls with analogue modeling we conclude that destabilization on these levels can lead to landslides or gravitational spreading in our experiments slides generated are only slump type events but they can evolve in nature into debris avalanches through mechanisms that could not be modeled in the laboratory the role of fan shaped volcaniclastic deltas included in the edifice structure is critical to slump formation this process is more efficient if deltas are associated with another upper lsl such as weak hyaloclastites or hydrothermally altered rocks in this case recurrent events generating collapses with increasing width are observed gravitational spreading of a volcanic island occurs when it is underlain by a ductile marine substratum or includes an intra edifice unconstrained hyaloclastic layer 1 3 as thick as the height of the subaerial edifice spreading is characterized by edifice stretching and subsidence and does not generate major landslides we also show that gravitational spreading on a marine substratum tends to inhibit slide generation especially when the thickness of the sedimentary layer is larger than 1 10 of the edifice total height however if this value is lower spreading occurs later and slumps can develop initially we describe natural examples of oceanic hot spot or arc shield volcanoes affected by both spreading and landslides such as reunion island hawaii island james ross island antarctica and martinique island antilles the main structural features on these volcanoes can be explained solely by deformation on lsls we therefore propose a general model for the destabilization of volcanic islands that is applicable to all shield like edifices

a new model for the evolution of la reunion volcanic complex from complete marine geophysical surveys results from recent marine geophysical surveys offer a new perspective for characterizing the evolution processes of volcanic islands in 2006 cruises forever and eroder 1 investigated the submarine flanks and the surrounding abyssal plain of la reunion indian ocean to obtain for the first time a complete geophysical survey of the area combined analyses of these data reveal major differences in the evolution of the two emerged volcanoes piton des neiges and piton de la fournaise we show that debris avalanche deposits extend on the abyssal plain only offshore the active piton de la fournaise volcano attesting the occurrence of large flank collapse events the absence of such deposits offshore piton des neiges and the presence of compressive structures within the sedimentary unit below the edifice support a mechanism of slow deformation of this volcano such as sliding or spreading the slow deformation of piton des neiges has led to numerous secondary submarine slope instabilities and favored some unconfined turbidity flows which generated large sediment waves running downward all around the island this study proposes a new model using the most complete marine data set available slow deformation controls the evolution of piton des neiges whereas piton de la fournaise formed on the flanks of a pre existing edifice experienced catastrophic large flank collapse events

kar chronology of the ultimate activity of piton des neiges volcano reunion island indian ocean abstract potassium argon measurements were made on 17 samples from the ultimate stage of activity of the piton des neiges volcano which corresponds to a differentiated series with lavas varying from basalt to quartz trachyte samples were selected as a function of their stratigraphic position and their representativity of magmatic evolution the technology used to date young volcanic rocks pleistocene by k ar is described and discussed because of its reliability in the analysis of modern lavas and because of the agreement of the results with the available stratigraphic control the data together with those previously obtained by mcdougall yield ages ranging between 330 000 and 30 000 years with two mainly effusive periods separated by a phase of instability of chiefly explosive volcanism around 190 000 to 150 000 years ago a rough correlation between the chemical evolution of the lavas and time is established the kudo and weil geothermometer when cross checked with buddington and lindsley s geothermometer allows us to establish that the lavas of the two effusive periods correspond to two different depths of the magmatic chamber in the oceanic crust 17 and 10 km respectively the phase of instability may correspond to a sudden variation in these magmatic conditions the differentiated series the ultimate stage of activity of the piton des neiges volcano 330 000 and 30 000 years can be explained as the evolution of a magmatic body isolated when the focus of basaltic eruption migrated southeast from piton des neiges to piton de la fournaise

relationships between volcano gravitational spreading and magma intrusion volcano spreading with its characteristic sector grabens is caused by outward flow of weak substrata due to gravitational loading this process is now known to affect many present day edifices a volcano intrusive complex can form an important component of an edifice and may induce deformation while it develops such intrusions are clearly observed in ancient eroded volcanoes like the scottish palaeocene centres or in geophysical studies such as in la reunion or inferred from large calderas such as in hawaii the canaries or galapagos volcanoes volcano gravitational spreading and intrusive complex emplacement may act simultaneously within an edifice we explore the coupling and interactions between these two processes we use scaled analogue models where an intrusive complex made of golden syrup is emplaced within a granular model volcano based on a substratum of a ductile silicone layer overlain by a brittle granular layer we model specifically the large intrusive complex growth and do not model small scale and short lived events such as dyke intrusion that develop above the intrusive complex the models show that the intrusive complex develops in continual competition between upward bulging and lateral gravity spreading the brittle substratum strongly controls the deformation style the intrusion shape and also controls the balance between intrusive complex spreading and ductile layer related gravitational spreading in the models intrusive complex emplacement and spreading produce similar structures to those formed during volcano gravitational spreading alone i e grabens folds en echelon fractures therefore simple analysis of fault geometry and fault kinetic indicators is not sufficient to distinguish gravitational from intrusive complex spreading except when the intrusive complex is eccentric from the volcano centre however the displacement fields obtained for 1 a solely gravitational spreading volcano and for 2 a gravitational spreading volcano with a growing and spreading intrusive complex are very different consequently deformation fields like those obtained from geodetic monitoring can give a strong indication of the presence of a spreading intrusive complex we compare the models with field observations and geophysical evidence on active volcanoes such as la reunion island indian ocean ometepe island nicaragua and eroded volcanic remnants such as ardnamurchan scotland and suggest that a combination between gravitational and intrusive complex spreading has been active

temporal changes in stress preceding the 2004 2008 eruption of mount st helens washington abstract the 2004 2008 eruption of mount st helens msh washington was preceded by a swarm of shallow volcano tectonic earthquakes vts that began on september 23 2004 we calculated locations and fault plane solutions fps for shallow vts recorded during a background period january 1999 to july 2004 and during the early vent clearing phase september 23 to 29 2004 of the 2004 2008 eruption fps show normal and strike slip faulting during the background period and on september 23 strike slip and reverse faulting on september 24 and a mixture of strike slip reverse and normal faulting on september 25 29 the orientation of 1 beneath msh as estimated from stress tensor inversions was found to be sub horizontal for all periods and oriented ne sw during the background period nw se on september 24 and ne sw on september 25 29 we suggest that the ephemeral 90 change in 1 orientation was due to intrusion and inflation of a ne sw oriented dike in the shallow crust prior to the eruption onset

la reunion an example of channeled hot spot plume la reunion a large intraplate volcanic system in the mascarene basin indian ocean is widely considered as the expression of a mantle plume or hotspot it lies in a region of the mascarene basin bounded by 2 major transform faults and an extinct rift the bathymetry suggests the presence of a positive topographic anomaly around la reunion we have compared the bathymetry with a theoretical bathymetry based on the thermal subsidence of the oceanic lithosphere as a function of its age a bathymetric anomaly of several hundreds of meters is shown but it is confined to the la reunion zone analogue models have been carried out to explore the deformation resulting from a mantle plume rising beneath a faulted lithosphere they show that the plume and the associated deformation are channeled into the area enclosed by the transform faults and the paleo rift the deformation is virtually nil outside the zone fault bounded zone we have analyzed the surface features and the available gravity and magnetic data in order to find evidence of the deformation predicted by the presence of the bathymetric anomaly and by the analogue models no clear pattern of deformation has been recognized but we demonstrate the complex organization of the oceanic crust in the la reunion zone the fact that the plume head focuses on the restricted area of the zone may amplify the dynamic support of the lithosphere beneath la reunion where not flexure is observed we also suspect that some seamounts near la reunion could correspond to a recent magmatic activity distinct from that of the island the mantle plume lithosphere interaction model presented here is a new to the oceanic domain and it will have to be confirmed by subsequent observations in the la reunion area and possibly in other similar sites on earth

rift zones and magma plumbing system of piton de la fournaise volcano how do they differ from hawaii and etna on ocean basaltic volcanoes magma transfer to the surface proceeds by subvertical ascent from the mantle lithosphere through the oceanic crust and the volcanic edifice possibly followed by lateral propagation along rift zones we use a 19 year long database of volcano tectonic seismic events together with detailed mapping of the cinder cones and eruptive fissures to determine the geometry and the dynamics of the magma paths intersecting the edifice of piton de la fournaise volcano we show that the overall plumbing system from about 30 km depth to the surface is composed of two structural levels that feed distinct types of rift zones the deep plumbing system is rooted between piton des neiges and piton de la fournaise volcanoes and has a n30 40 orientation above 20 km below sea level bsl the main axis switches to a n120 orientation which permits magma transfer from the lithospheric mantle to the base of the oceanic crust below the summit of piton de la fournaise the related nw se rift zone is 15 km wide linear spotted by small to large pyroclastic cones and related lava flows and emits slightly alkaline magmas resulting from high pressure fractionation of clinopyroxene olivine this rift zone has low magma production rate of 0 5 3 6 10 3 m 3 s 1 and an eruption periodicity of around 200 years over the last 30 ka seismic data suggest that the long lasting activity of this rift zone result from regional nne ssw extension which reactivates inherited lithospheric faults by the effect of underplating and or thermal erosion of the mantle lithosphere the shallow plumbing system b11 km bsl connects the base of the crust with the central cone it is separated from the deep plumbing system by a relatively large aseismic zone between 8 and 11 km bsl which may represent a deep storage level of magma the shallow plumbing system feeds frequent short lived summit and flank ne and se flanks eruptions along summit and outer rift zones respectively summit rift zones are very active 0 1 0 25 m 3 s 1 short 2 3 km and present an orthogonal pattern confined to the central active cone of piton de la fournaise outer ne and se rift zones are much less active 4 7 3 10 3 m 3 s 1 and extend from inside the enclos fouque caldera to bound the mobile eastern volcano flank we show that the outer rift zones are almost aseismic and are genetically linked to the seaward flank displacements whose most recent events where detected in 2004 and 2007 east flank sliding is itself triggered by shallow b 2 km depth sill intrusions we propose that the subvertical magma intrusions along the perpendicular summit rift zones sill intrusions and subsequent magma injections along the outer rift zones are controlled by cycles of stress permutations we thus tentatively propose that as for piton de la fournaise the regional stress field acting on etna and hawaiian volcanoes is an important parameter in the control of the magma transfer along their deep plumbing system whereas the dynamics of their summit system is governed by shallow stress variations above 5 10 km bsl due to edifice deformation and magma transfers

numerical modelling of dyke interaction and its influence on oceanic crust formation abstract dykes are an essential element in building oceanic crust most prominent in sheeted dyke complexes in the upper crust since dykes alter the magnitude and orientation of the local stress field they cannot be treated as passive infillings of extensional fractures we use a quasi static iterative 2 d boundary element method allowing for a wholesale movement of fluid filled fractures effects of stress and pressure gradients buoyancy and enclosed fluid mass are considered the implications of the dyke induced stress field are analysed combining the simulation of fracture propagation with computation of dyke interaction dyke interaction occurs by the adaptation of ascending dykes to the stress field caused by previous fractures arrested in the crust and leads to focussing and crossing of dykes examples for applications are introduced concerning e g the generation of a magma chamber and the formation of the sheeted dyke complex our main results are that the interaction between dykes can be considerable and that the most important controlling factor is stress the interaction is small when the horizontal tensional stress is large compared to the pressure in the dyke head otherwise dykes tend to attract each other and to form centres of high dyke density or sill layers

structure and morphology of submarine volcanism in the hotspot region around reunion island western indian ocean the form of the deccan maldives mascarene reunion hotspot trace suggests that it has at least in part been strongly controlled by crustal structures especially fracture zones this makes it difficult to assess the present day or past location of the hotspot and thus complicates the interpretation of african plate motion reconstruction we present here results of a cruise to the reunion area of which the aims were a to determine the extent of present day volcanism associated with the reunion hotspot in the region and b to examine the role of pre existing oceanic crustal structures in controlling the location of present day volcanism additionally we examined the morphology and geology of the important extinct spreading centre southwest of reunion abandoned when spreading jumped to separate seychelles from india during the deccan flood basalt episode some 60 65 ma ago the extensive bathymetrie seismic and geological investigation shows that significant present day hotspot volcanism is confined to the piton de la fournaise edifice on reunion island itself apparently the location of recent reunion volcanism has not been controlled by a crustal fracture and the major fracture zones on both sides of the island are not acting as magma conduits for plate motion reconstruction and plume flux calculation purposes piton de la fournaise must be taken as the present location of the reunion hotspot accretion at the extinct spreading centre progressively ceased at the time of anomaly a27 63 ma and was associated with marked propagation of the rift tips

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

effects of surface freshwater flux induced by sea ice transport on the global thermohaline circulation 1 effects of surface freshwater flux induced by sea ice formation and melting on the thermohaline circulation are investigated by using a sea ice ocean coupled general circulation model forced by monthly climatology restoring to the observed sea surface salinity is not employed in order to evaluate precisely the sea ice effects because of the model s improper representation of interaction between sea ice and the northern hemisphere deep convection the discussion is focused on the effect of sea ice in the southern hemisphere in the control case deep water formation around antarctica occurs under compact sea ice cover where positive annual mean sea ice production is an essential factor to induce deep convection when sea ice motion is turned off deep water formation is maintained primarily by thermal destabilization of water columns as annual mean sea ice production is almost zero everywhere consequently the deep ocean around antarctica is warmer and less saline compared with that for the control case and the atlantic bottom circulation is weakened by 16 northward salt transport from the southern ocean in the bottom layer also decreases the amount of the decrease is greater by 1 order of magnitude than that of the surface salt input associated with sea ice in a case where the freshwater flux at the ice ocean interface is turned off results are similar to the case without sea ice transport although surface heat and momentum fluxes significantly differ between them this suggests that the influence of sea ice on freshwater flux is more important than on heat and momentum fluxes in affecting the global thermohaline circulation

role of the structural inheritance of the oceanic lithosphere in the magmato tectonic evolution of piton de la fournaise volcano la reunion island la reunion island is located east of madagascar on the eastern rim of the tectonically inactive mascarene basin this island is composed of three shield volcanoes of which only piton de la fournaise is currently active although the magmatic activity is restricted to piton de la fournaise a scattered seismicity occurs on the whole 200 km wide volcanic edifice and in the underlying oceanic crust we carried out a multiscale analysis to understand 1 the origin of the seismicity in the geodynamic context and 2 the role of the oceanic lithosphere in the deformation of piton de la fournaise and la reunion island analysis of the magmatic system suggests that the magma ascent is controlled by large n25 30 and n125 130 fracture zones located below the enclos depression we also show that the orientation difference between the eruptive fissures and the related dykes result from a rotation of the main principal stress s 1 from vertical to downslope through the surface combining a digital elevation model dem analysis field observations and the geophysical data reveals that the volcano is affected by large fault zones the fault distribution indicates the predominance of a main n70 80 trend magnetic data show the same n80 orientation characterizing the remnant part of the alizes volcano such parallel alignment suggests a control exerted by the underlying alizes volcano on piton de la fournaise furthermore the alignment between the crustal orientations and the structures determined on the island suggests a control of the crustal structures in la reunion s volcano tectonic activity contrary to several volcanic islands such as hawaii and tenerife la reunion volcanoes lie on an upbending crust then we interpret the reactivation of the crustal faults as resulting from a crustal uplift related to the thermal erosion of the base of the lithosphere and or to strong underplating the upward deformation may prevent the spreading of the volcanoes as no evidence of such a mechanism is observed in the bathymetry and the seismic data around the island

sheet intrusions and deformation of piton des neiges and their implication for the volcano tectonics of la reunion abstract to understand the volcano tectonic history of piton des neiges the dormant volcano of la reunion we measured in the field the orientation of sheeted intrusions and deformation structures and interpreted the two datasets separately with a paleostress inversion results show that the multiple proposed rift zones may be simplified into three trends 1 a n30 e 5 km wide linear rift zone running to the south of the edifice active in the shield building 2 48 0 43 ma and terminal stages 190 22 ka 2 a curved n110 to n160 e rift zone widening from 5 km to 10 km toward the nw flank essentially active during the early emerged shield building 1 3 ma and 3 two sill zones 1 km thick in total in the most internal parts of the volcano active in the shield building and terminal stages in parallel deformation structures reveal that the tectonics of the edifice consisted in three end member stress regimes sharing common stress axes 1 nw se extension affecting in priority the south of the edifice near the n30 e rift zone 2 nne ssw extension on the northern half of the volcano near the n110 160 e rift zone 3 compression occurring near the sill zones with a ne sw or nw se maximum principal stress these three stress regimes are spatially correlated and mechanically compatible with the injection trends combined together our data show that the emerged piton des neiges underwent sector spreading delimited by perpendicular rift zones as observed on piton de la fournaise the active volcano of la reunion analogue experiments attribute such sector spreading to brittle edifices built on a weaker substratum we therefore conclude that la reunion volcanoes are both brittle as opposed to hawaiian volcanoes or mount etna whose radial spreading is usually attributed to a ductile body within the edifices