landslide generated tsunamis at reunion island 1 landslides that occur on oceanic volcanoes can reach the sea and trigger catastrophic tsunamis reunion island has been the location of numerous huge landslides involving tens to hundreds of cubic kilometers of material we use a new two fluid seawater and landslide numerical model to estimate the wave amplitudes and the propagation of tsunamis associated with landslide events on reunion island a 10 km3 landslide from the eastern flank of piton de la fournaise volcano would lift the water surface by about 150 m where it entered the sea the wave thus generated would reach saint denis the capital of reunion island population of about 150 000 people in only 12 min with an amplitude of more than 10 m and would reach mauritius island in 18 min although mauritius is located about 175 km from the impact waves reaching its coast would be greater than those for reunion island this is due to the initial shape of the wave and its propagation normal to the coast at mauritius but generally coast parallel at reunion island a submarine landslide of the coastal shelf of 2 km3 would trigger a 40 m high wave that would severely affect the proximal coast in the western part of reunion island for a landslide of the shelf of only 0 5 km3 waves of about 2 m in amplitude would affect the proximal coast

influence of cone morphology on dykes stromboli italy abstract the island of stromboli represents the emerged part of a 2 6 km high composite volcano elongated in a ne direction over the period 100 13 ka bp the morphological evolution of the volcano was characterised by a series of summit caldera collapses between major building phases from 13 ka bp onward a series of large lateral collapses occurred northwestwards in one sector of the cone the results of the field study of the total 94 dyke population is discussed in this paper they show that during the last 100 ka the majority of dykes were injected with a nnegssw to ene wsw strike along a ne sw weakness zone crossing the volcano summit this is interpreted as a volcanic rift the geometry of which is controlled by regional tectonic stresses dykes injected prior to 13 ka bp at lower altitudes are generally steeper than those injected at higher elevations after 13 ka bp dykes were injected along the ne sw weakness zone and parallel and close to the margins of the earliest sector collapse at dips of 20 80 inclined towards the collapse depression i suggest that this is linked to unbuttressing of the collapse depression or to seaward gravity instability of the depression infill deposits extensive emplacement of long dykes along the ne sw weakness zone occurred only when the volcano regained a conical shape some of the dykes emplaced close to the original surface of the cone bent to assume a dip direction parallel to the local slope i propose that this local change of dyke geometry can be explained by the direct influence of the free surface of the cone leading to a rotation of the least principal stress which assumes an orientation perpendicular to the slope

a multichannel seismic study of lithospheric flexure across the hawaiian emperor seamount chain existing models assume that the thickened crust beneath seamounts is the result of a surface volcanic load flexing an elastic plate new results suggest that flexed oceanic crust beneath the hawaiian emperor seamount chain is underlain by a 4 km thick deep crustal body we intepret the body as a deep crustal sill complex associated with the tholeiitic stage of volcano building along the chain

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

a seismic study of lithospheric flexure in the vicinity of tenerife canary islands seismic data have been used to determine the crustal and upper mantle structure of tenerife canary islands a volcanic island of tertiary age located on 140 ma oceanic crust reflection data show that oceanic basement dips gently towards the island forming a flexural moat which is infilled by 2 3 km of well stratified material the moat is characterised by a major angular unconformity which we attribute to volcanic loading of pre existing oceanic crust and overlying sediments and the subsequent infilling of the flexure by material that was derived at least in part from the islands refraction data show that the flexed oceanic crust has a mean thickness of 6 41 0 42 km and upper and lower crustal velocities of 4 8 5 4 km s 1 and 6 7 7 3 km s 1 respectively the flexure which has been verified by gravity modelling can be explained by a model in which tenerife and adjacent islands have loaded a lithosphere with a long term 106 yr elastic thickness of approximately 20 km seismic and gravity data suggest that up to 1 5 105 km3 of magmatic material has been added to the surface of the flexed oceanic crust which assuming an age of 6 16 ma for the shield building stage on tenerife implies a magma generation rate of about 0 006 to 0 02 km3 a 1 this rate is similar to estimates from other african oceanic islands e g reunion and cape verdes but is significantly less than that which has been calculated at hawaii there is no evidence in either the seismic or gravity data that any significant amount of magmatic material has underplated the flexed oceanic crust the crustal and upper mantle structure at tenerife therefore differs from other oceanic islands such as hawaii and marquesas where 4 km of underplated material have been reported

reactivation of basement faults beneath volcanoes a new model of flank collapse in order to explain the presence of voluminous volcanic debris avalanche deposits around a stratovolcano reactivation of vertical faults beneath a volcanic cone has been tested using analogue models reactivation of a single vertical fault beneath a cone generates a normal fault and an upturning of the layers creating a bulge on the flank the upturning induces a flank collapse characterized by a typical horseshoe shaped scar called an avalanche caldera reactivation of two vertical faults beneath a cone also generates a normal fault and a summit bulge this bulge may result from the movement along a reverse fault a large collapse is generated within the angle created by the two vertical faults the angle of the collapse can be up to 140 whereas this angle is typically 120 for a dome intrusion collapse is instantaneous and is favoured by the presence of ductile layers ash and pumice formations in the example considered in a stratovolcano complex the model may be applicable to volcanoes in a state of dormancy or extinction in regions with active regional tectonism we suggest this mechanism of collapse in the case of the cantal stratovolcano massif central france to explain the presence of voluminous volcanic debris avalanche deposits around this volcano

three dimensional shear velocity anisotropic model of piton de la fournaise volcano la reunion island from ambient seismic noise we cross correlate 4 years of seismic noise from the seismic network of piton de la fournaise volcano la reunion island to measure the group velocity dispersion curves of rayleigh and love waves we average measurements from vertical and radial components to obtain 577 rayleigh wave dispersion curves the transverse components provided 395 love wave dispersion curves we regionalize the group velocities measurements into 2 d velocity maps between 0 4 and 8 s finally we locally inverted these maps for a pseudo 3 d anisotropic shear velocity model down to 3 km below the sea level using a neighborhood algorithm the 3 d isotropic shear wave model shows three distinct high velocity anomalies surrounded by a low velocity ring the anomaly located below the present plaine des sables could be related to an old intrusive body at the location of the former volcanic center before it migrated toward its present location the second high velocity body located below the summit of the volcano likely corresponds to the actual preferential dyke intrusion zone as highlighted by the seismicity the third high velocity anomaly located below the grandes pentes and the grand brule areas and is an imprint of the solidified magma chamber of the dismantled les alizes volcano radial anisotropy shows two main anomalies positive anisotropy above sea level highlighting the recent edifice of piton de la fournaise with an accumulation of horizontal lava flows and the second one below the sea level with a negative anisotropy corresponding to the ancient edifice of piton de la fournaise dominated by intrusions of vertical dykes

experiments on rift zone evolution in unstable volcanic edifices large ocean island volcanoes frequently develop productive rift zones located close to unstable flanks and sites of older major sector collapses flank deformation is often caused by slip along a decollement within or underneath the volcanic edifice we studied how such a stressed volcanic flank may bias the rift zone development the influence of basal lubrication and lateral flank creep on rift development and rift migration is still poorly constrained by field evidence here our analog experiments provide new insights we injected colored water into gelatin cones and found systematic orientations of hydro fractures dikes propagating through the cones at the base of the cone diverse friction conditions were simulated by variation of the basal creep conditions we modeled radial dike swarms collinear rift zones and three armed rift systems it is illustrated that a single outward creeping flank is sufficient to modify the entire rift architecture of a volcano the experiments highlight the general unsteadiness of dike swarms and that the distribution and alteration of weak substratum may become a major player in shaping a volcano s architecture

new behaviour of the piton de la fournaise volcano feeding system la reunion island deduced from gps data influence of the 2007 dolomieu caldera collapse after 15 months of summit deflation the unrest of piton de la fournaise renewed in august 2008 with several seismic crises and the re pressurization of the plumbing system the successive seismic crises sometime accompanied by ground displacements detected by gps were associated with inter eruptive dyke intrusions it was the first time since the implementation of the observatory that such successive inter eruptive magma migrations not reaching the surface were observed at piton de la fournaise the change in the volcano activity with an increase in the frequency of pure dyke intrusions have followed the dolomieu caldera collapse of april 2007 and reveals its strong influence on the new eruptive regime stress changes in the volcanic edifice have temporarily at least favoured the arrest of dykes in depth during these successive dyke intrusions magma stopped at shallow depth to form transitory magma storages before to erupt during three successive small summit eruptions september 21st october 2nd 2008 november 27th and december 14th 2008 february 4th 2009 the same kind of change in the pre eruptive and eruptive behaviour can be expected on other volcanoes having experienced a summit caldera collapse

evolution of magma conduits during the 1998 2000 eruptions of piton de la fournaise volcano reunion island 1 at basaltic volcanoes magma is transported to the surface through dikes magma filled fractures but the evolution of these dikes as eruptions proceed is rarely documented in march 1998 after five and a half years of quiescence piton de la fournaise volcano reunion island entered into a new eruptive phase characterized by intense eruptive activity coeruptive displacements recorded by interferometric synthetic aperture radar insar for the first five eruptions of the cycle are analyzed using 3 d boundary element models combined with a monte carlo inversion method we show that the eruptions are associated with the emplacement of lateral dikes rooted at depths of less than about 1000 m except for the first march 1998 event where an additional deeper source is required the dikes are located above preeruptive seismic swarms this is consistent with nearly isotropic stress caused by repeated dike intrusions and low confining pressure enhanced by the presence of pores in the shallowest 1000 m of the edifice the volumes of the modeled dikes represent 17 of the volume of emitted lava showing that exogenous growth plays a major role in building the volcano by taking into account the preeruptive seismicity and tilt data together with the results of insar data modeling we find that dikes first propagate vertically from a source region below sea level before being injected laterally at shallow depth this behavior is consistent with the presence of levels of neutral buoyancy at shallow depth in the edifice

hybrid collapse mechanism at piton de la fournaise volcano reunion island indian ocean 1 recent studies have shown that hydrothermal alteration in the core of volcanoes may weaken the edifice and trigger sector collapse lopez and williams 1993 day 1996 voight and elsworth 1997 van wyk de vries et al 2000 reid et al 2001 alteration of volcanic products must take place over long periods of times before collapse can occur so this process is considered to be less frequent than volcanic failure generated by magmatic intrusion and flank spreading however acceleration of this slow process may occur along one direction as a result of a weak lateral confinement following outward gravitational sliding of a flank of the volcano we present here evidence that the enclos fouque depression of piton de la fournaise volcano results from such a hybrid mechanism seaward sliding of the grand brule trough on top of an ancient hypovolcanic complex led to a lateral unconfined boundary along which collapse of the altered core of the edifice took place the result is a caldera like structure breached on the grand brule slide scaled experiments to simulate this process of pseudocaldera formation reveal tectonic features remarkably similar to those encountered in the field and demonstrate the mechanical consistency of the model similar processes may have played a role in the evolution of other volcanoes

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

dynamic dyke propagation deduced from tilt variations preceding the march 9 1998 eruption of the piton de la fournaise volcano during the hour preceding the march 9 1998 eruption of the piton de la fournaise volcano the deformation network of the observatory recorded some large deformations in the summit area a broadband seismic station of the geoscope global network rer is located about 8 km away from the summit of the volcano signals from that station may be interpreted as tilt changes the combination of the above two kinds of signals allows by using a tensile fault model to constrain the geometry as well as some characteristics volume propagation velocity of the dyke that intruded the summit area during the hour preceding the beginning of eruptive activity

early detection of large eruptions at piton de la fournaise volcano la reunion island contribution of a distant tiltmeter station we report a compilation of data recorded at a distant tiltmeter station rer during recent episodes of dyke emplacement and eruption 2003 2007 at piton de la fournaise volcano la reunion island this sensitive station provides useful information for evaluating the extent of deformation distinct responses of this station were recorded based on the eruption type dykes feeding summit eruptions did not significantly influence the rer tiltmeter signals whereas dykes feeding large distal eruptions with vents located more than 4 km from the summit generated up to 1 4 rad of tilt an amplitude 2 to 4 times greater than for proximal eruptions 0 3 0 7 rad on the flanks of the summit cone the distinct tilt amplitude is directly linked to the location depth and volume of the dyke comparison with summit tiltmeters reveals that up to one third to half of the rer tilt signal associated to dyke propagation is recorded when the dyke is still below the summit crater thus before large distal eruptions more than 0 5 rad of tilt is recorded in less than 20 min when the dyke is below the summit crater i e a few minutes hours before the beginning of the eruption we can thus propose for the rer station a threshold value of 0 5 rad which when reached as a dyke rises beneath the summit crater suggests a high likelihood of a large distal eruption the distant rer tiltmeter station thus appears to be a powerful tool for forecasting the type of eruption that is likely to occur and can contribute to the early detection of large distal eruptions at piton de la fournaise which are the most dangerous to inhabitants for volcano monitoring installation of high precision distant tiltmeters along the lower slopes of a volcano may provide warnings of large eruptions with enough lead time to allow for short term hazards mitigation efforts

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

the deformation field of the august 2003 eruption at piton de la fournaise reunion island mapped by asar interferometry 1 three independent asar interferograms spanning the august 2003 piton de la fournaise eruption reveal a 3 by 3 km asymmetric pattern of range changes centred on the dolomieu crater northern flank it corresponds to 30 cm of displacement towards the satellite east of the eruptive fissures and 7 cm away from the satellite west of the fissures displacements are caused by dyke emplacement below fissures we model the deformation using a 3d mixed boundary element method for elastic media this consists of a dyke defined by six geometric parameters and an overpressure gradient a neighbourhood algorithm was applied to explore this 7 dimensional parameter space the best fit model is a 57 eastward dipping dyke with a base lying around 1520 m a s l the model provides new evidence of the dyke intrusion related seaward displacements of the volcano eastern flank

geomorphic evolution of the piton des neiges volcano reunion island indian ocean competition between volcanic construction and erosion since 1 4 ma abstract reunion island indian ocean is a volcanic complex whose eruptive history was dominated by the activity of two main edifices piton des neiges pn and piton de la fournaise pf volcanoes the tropical climate induces erosion processes that permanently compete with volcanic constructional processes exposed to the trade winds and associated heavy rainfalls the northeastern part of the island exhibits the most complex morphological evolution geomorphological analysis performed on a 50 m dem and associated to new k ar ages has clarified the overall history of pn volcano each massif is assigned to one of the main building stages of the edifice in addition the arrangement of these different massifs reveals that the eruptive phases have led to successive relief inversions and successive excavations of large central depressions in the proximal area as a result the younger massifs are always located in more proximal parts of the volcano the youngest being close to the edifice center in distal areas early lava flows were channeled into valleys incised along the massif boundaries leading to a more complex geochronological organization quantitative study of the dissection of pn volcano allows us to propose a minimum eroded volume of 101 44 and 105 41 km3 for the mafate and cilaos cirques depressions respectively during the last 180 kyr and a minimum average long term erosion rate of 1 2 0 4 km3 ka this leads us to estimate the removed volume during the whole history of pn volcano 1000 km3 as equivalent to the volume of the deposits identified on the submarine flanks of piton des neiges volcano therefore as regressive erosion appears to be the prevailing geomorphic process during the whole pn history it questions the presence of major flank collapses younger than 1 4 ma on this volcano erosion processes have largely been neglected in recent models but our study emphasizes them as a key component of landscape development and a major process in the morphological evolution of reunion island that has to be fully integrated in future studies

structure of reunion island indian ocean inferred from the interpretation of gravity anomalies abstract reunion is a volcanic edifice whose origin is related to a hot spot in the indian ocean only 3 of its volume is emergent many geological and geophysical studies were carried out on reunion island during the 1980 s but few of them allow study of the internal structure of the edifice several gravity surveys have been carried out on the island since 1976 and we have compiled the available data set the lack of data on the western side of the island led us to conduct a regional survey in 1993 to obtain a more homogeneous distribution of the stations computation of bouguer anomalies for different correction densities accounts for the variable density of the rocks constituting the edifice and provides a distribution of gravity anomalies interpreted as dense bodies of intrusive rocks inside the edifice two very large intrusive complexes can be unambiguously recognised one beneath piton des neiges and one beneath the grand brule area both have been penetrated by geothermal exploration drill holes and the first is also known from outcrop observations 2 5d simple models were constructed to reveal the geometry and extent of the buried intrusives they are deeply rooted extending several kilometres below sea level and extensive 20 25 km long and 10 13 km wide for the piton des neiges complex 12 15 km long and some kilometres wide for the grand brule complex the development of such complexes implies that the activity of the two volcanic centres was long lasting and remained stable while the volcanoes were growing the grand brule complex has been interpreted as relics of an old volcano named alizes volcano the interpretation of the gravity maps suggests the presence of a ridge of dense rocks to the north of the axis joining the centres of piton des neiges and piton de la fournaise volcanoes by analogy with the other structures 2 5d models show that this structure would culminate between 0 and 1 km below sea level and be 15 km wide this complex induces a maximum anomaly in takamaka valley and we thus propose to name it takamaka volcano no geological evidence of the nature of these dense rocks is available but the ridge coincides with structures revealed by magnetic and seismic data interpretation of the bouguer anomaly maps suggests that the inner gravity structure of piton de la fournaise is not characterised by the presence of a voluminous dense body but probably by more restricted concentrations of dense rocks some structures can be recognised along the present ne and se rift zones and in the previous central part of piton de la fournaise to the west of the present summit the recent eastward migration of the centre of activity of piton de la fournaise accounts for the lack of a large positive anomaly beneath the active craters

deformation of basaltic shield volcanoes under cointrusive stress permutations we performed a microstructural study of piton des neiges la reunion island to understand how intrusions and stresses control each other in basaltic volcanoes our study reveals that three perpendicular intrusions trends coexisted during the 2 myr history of the volcano a n120 140 e rift zone a perpendicular dike trend and swarms of subhorizontal intrusions hereafter called sill zones independently the inversion of fault slip data shows that incompatible paleostress fields recurrently occurred along with the intrusions a dominant nne ssw extension a perpendicular extension and strike slip or compressional regimes the orientations of paleostresses are consistent with the orientations of the three perpendicular intrusion populations we propose that stress accumulation in the edifice under the effect of repeated magma injections resulted in permutations of the principal axes of the stress tensor causing a reorientation of subsequent intrusions stress permutations were cyclical each cycle started with dike injections in an extensional stress field reducing the deviatoric stress and switching the axes of principal stresses and finished with sill intrusions in a compressional stress field sill zones acted as detachment planes restoring the extensional stress field and initiating a new cycle of permutations our model of stress permutations is in agreement with the pattern of eruptions and deformation observed at piton de la fournaise in contrast with the hawaiian model of spreading on a decollement stress permutations in la reunion s volcanoes imply that the basal deformation of the edifices if any is not sufficient to compensate the reduction of deviatoric stress caused by intrusions

cyclic magma storages and transfers at piton de la fournaise volcano la reunion hotspot inferred from deformation and geochemical data abstract piton de la fournaise is in a period of intense volcanic activity since 1998 to constrain the magma dynamics responsible for this activity we combined gps ground deformation monitoring interpreted through numerical modelling and geochemistry two cycles of continuous volcano inflation are evidenced for the may 2004 december 2005 period with a rest from march to october 2005 these inflations are consistent with two cycles of compatible major element enrichment in the emitted lavas numerical models indicate that the pressurization of a single magma reservoir may be responsible for the observed pre eruptive inflations of the volcano the reservoir located at 2300 m depth has a radius of 500 m at the beginning of each cycle dykes propagate from the roof of the reservoir and yield eruptions of differentiated basalt near the summit at the end of the cycle dykes propagate from the eastern sidewall of the reservoir and yield distal eruptions of primitive magmas away from the summit the volumes of magma emitted during the primitive eruptions seem too large to explain the surface deformations and therefore suggest some refill of the reservoir by deeper magmas our results may be used to predict the location and lava volume of future eruptions at piton de la fournaise volcano depending on the timing of these eruptions within a cycle of volcanic activity

geodynamic implications of moving indian ocean hotspots abstract the reconstructed positions of many of the indian ocean intraplate volcanic features are inconsistent with the present positions of the hotspots to which they have been ascribed the discrepancy can be explained by the past motion of the hotspots themselves and in the errors introduced in an absolute plate model based on fixed hotspots previous models for the motion of hotspots in the indian ocean e g steinberger and o connell in the history and dynamics of global plate motions 2000 pp 377 398 and geophys j int 132 1998 412 434 antretter et al earth planet sci lett 203 2002 635 650 have had success in matching paleolatitudes of the kerguelen hotspot but not the marion or reunion hotspots we calculate the motion of the indian ocean hotspots using a method described in steinberger and o connell op cit employing a plate motion model revised using identification of mesozoic magnetic anomalies in the enderby basin gaina et al in the evolution and dynamics of the australian plate in press we find a motion of 7 south for the kerguelen plume similar to previous models antretter et al op cit this motion alleviates the discrepancies in the position of the kerguelen plume and its purported involvement in the formation of the ninetyeast ridge rajmahal traps bunbury basalts and assorted volcanics of the western australian margin we find a motion of 5 north and 7 south for the reunion and marion plumes respectively the motion of the reunion hotspot is consistent with paleolatitude estimates the apparent fixity of the marion hotspot torsvik et al earth planet sci lett 164 1998 221 232 is not well constrained but may be due to the combination of hotspot motion and true polar wander acting in opposing directions we present revised absolute finite rotations for the indian plate for the last 65 ma based on the motion of these hotspots which place india farther north in the past than for fixed hotspot models

new evidence of co2 soil degassing anomalies on piton de la fournaise volcano and the link with volcano tectonic structures insu cnrs and la reunion prefecture projet pour la quantification de l alea volcanique a la reunion

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

the grand brule exploration drilling new data on the deep framework of the piton de la fournaise volcano part 1 lithostratigraphic units and volcanostructural implications abstract the sinking of a deep drill hole 3003 5 m for geothermal exploration in the eastern part of the piton de la fournaise volcano on reunion island has revealed the underlying lithostratigraphy six principal sequences have been distinguished a 0 440 m subaerial basaltic flows b 440 556 m a transitional series characterized by an alternation of subaerial basalts and submarine deposits vitric basalt and hyaloclastic breccia c 556 616 m submarine basalts lavas and hyaloclastites d 616 890 m subaerial basalts e 890 1010 m submarine basaltic deposits with abundant argillized hyaloclastites f 1010 3003 5 m a layered intrusive complex comprising 1010 1129 m sheets of gabbro injected into the lavas and hyaloclastites 1129 1920 m a zoned intrusion of olivine gabbro and dolerite 1920 3003 5 m stratified cumulate gabbro wehrlite dunite the occurrence of the first submarine deposits at 440 m 268 m below present sea level and the intercalation with them of subaerial basalts down to 890 m 728 m below present sea level tend to support a hypothesis of sliding into the sea during subsidence of the eastern flank of the fournaise massif the intrusive complex intersected from 1010 m down is regarded as a former shallow magma chamber a large positive gravimetric anomaly 40 50 mgal centered over grand brule and attributed to the intrusive complex indicates that its roof forms an ellipse 3 20 km elongated n s stratigraphic and morphological structural evidence on land and on the sea floor and the results of the mineralogical study of this very large magma chamber suggest that it belongs to an ancient edifice proto fournaise whose activity preceded the construction of the present fournaise and ended at about 0 6 ma on land the evidence for this earlier edifice is to be found in the zeolitised basalts and differentiated lavas hawaiites forming the basement to the phase 1 lava flows certain subsidence structures on the present fournaise are interpreted as resulting from settling of the proto fournaise volcano the destabilization of the east flank grand brule area of the currently active volcano and the location of the ne and se rift zones could be attributed to the same phenomenon

mantle flow beneath la reunion hotspot track from sks splitting if upper mantle anisotropy beneath fast moving oceanic plates is expected to align the fast azimuths close to the plate motion directions the upper mantle flow pattern beneath slow moving oceanic plates will reflect the relative motion between the moving plate and the underlying large scale convecting mantle in addition to the non correlation of the fast azimuths with the plate motion direction the flow and anisotropy pattern may be locally perturbed by other factors such as the upwelling and the sublithospheric spreading of mantle plumes investigating such plume lithosphere interaction is strongly dependent on the available seismological data which are generally sparse in oceanic environment in this study we take the opportunity of recent temporary deployments of 15 seismic stations and 5 permanent stations on the piton de la fournaise volcano the active locus of la re union hotspot and of 6 permanent stations installed along or close to its fossil track of about 3700 km in length to analyze azimuthal anisotropy detected by sks wave splitting and to decipher the various possible origins of anisotropy beneath the western indian ocean from about 150 good and fair splitting measurements and more than 1000 null splitting measurements we attempt to distinguish between the influence of a local plume signature and large scale mantle flow the large scale anisotropy pattern obtained at the sw indian ocean island stations is well explained by plate motion relative to the deep mantle circulation by contrast stations on la re union island show a complex signature characterized by numerous nulls and by fast split shear wave polarizations trending normal to the plate motion direction and obtained within a small backazimuthal window that cannot be explained by either a single or two anisotropic layers despite the sparse spatial coverage which precludes a unique answer we show that such pattern may be compatible with a simple model of sublithospheric spreading of la re union plume characterized by a conduit located at 100 200 km north of la re union island anisotropy beneath the new geoscope station in rodrigues island does not appear to be influenced by la re union plume spreading signature but is fully compatible with either a model of large scale deep mantle convection pattern and or with a channeled asthenospheric flow beneath the rodrigues ridge

internal structure of piton de la fournaise volcano from seismic wave propagation and earthquake distribution abstract arrival times of seismic waves from local earthquakes are inverted for both locating the source and defining the 3 d velocity heterogeneity of piton de la fournaise the lateral heterogeneity of the 2632 m high edifice is resolved as a high velocity plug 1 5 km in diameter surrounded by a low velocity ring which may be interpreted as due to the construction of fournaise on the flank of the older volcano piton des neiges wave mode conversion detected on three component seismograms provides evidence for boundaries of contrasted velocities pre eruptive swarm earthquakes cluster in the high velocity zone under the dolomieu summit crater low strength and cohesion of the surrounding material account for the lack of seismicity for the final 1 3 km radial flow of magma to the vents in enclos fouque beneath the high velocity plug the existence of a body with low velocity for p and even for s waves is well constrained however the walls and base are poorly defined because of the lack of deep earthquakes for sampling the few earthquakes that are located in this depth region usually occur at a depth of around 1 5 km below sea level in the region of the cone this can be considered providing the upper constraint on the lower limit of the aseismic part of the low velocity body the coincidence in time of their occurrence with the swarms above sea level and the eruptions suggests magmatic activation of the low velocity aseismic volume 1 5 km below sea level under the high velocity plug of the cone further down the concentration of seismicity in two swarms between 2 and 4 km under the eastern flank does not allow the structure to be sampled effectively

magma transport and storage at piton de la fournaise la reunion between 1972 and 2007 a review of geophysical and geochemical data abstract since the middle of the 20th century improvement in volcano monitoring techniques has provided large data sets that can be used to evaluate the evolution of magma plumbing systems at piton de la fournaise volcano abundant eruptive activity and a dense monitoring network are especially conducive to such research analysis of the extensive set of geophysical and geochemical data associated with the last 35 years of activity provides new insights into magma transport and storage at piton de la fournaise two periods of frequent eruptions separated by 6 years of inactivity can be distinguished 1972 1992 and 1998 2007 considering these two periods we show evidence of major changes in the shallow plumbing system in 2000 during 1972 1992 and 1998 2000 lava compositions and weak long term eruptive precursors lasting few days to three weeks characterized by no significant summit inflation and about ten volcano tectonic earthquakes per day suggest that eruptions were fed by the progressive drainage of an occasionally recharged shallow magma reservoir geophysical evidence of shallow magma reservoir recharge was only recorded in 1986 and 1998 in contrast from 2000 to 2007 geophysical data highlight the appearance of long term precursors 1 to 5 months of summit inflation and strong seismicity up to one hundred events per day preceding cycles of successive eruptions each cycle of eruptions was characterized by a sequence 3 to 10 months in duration of summit and near summit proximal eruptions and ended with a distal low altitude oceanite olivine abundances 20 vol eruption on the eastern flank of the volcano within each of these eruptive cycles lavas became progressively more magnesian over time the preferential motion of the eastern flank caused by continuous recharging of the shallow reservoir since 2000 would favour the occurrence of distal eruptions towards this flank at the end of a cycle

identifying rift zones on volcanoes an example from la reunion island indian ocean we describe a methodology for identifying complex rift zones on recent or active volcanoes where structures hidden by recent deposits and logistical conditions might prevent carrying out detailed fieldwork la reunion island was chosen as a test site we used georeferenced topographic maps aerial photos and digital terrain models to perform a statistical analysis of several morphometric parameters of pyroclastic cones this provides a great deal of geometric information that can help in distinguishing the localisation and orientation of buried magma feeding fractures which constitute the surface expression of rift zones it also allowed the construction of a complete gis database of the pyroclastic cones la reunion is a perfect example where past and active volcanic rift zones are mostly expressed by clusters of monogenic centres the data has been validated in the field and compared and integrated with the distribution and geometry of dyke swarms results show the presence of several main and secondary rift segments of different ages locations and orientations whose origin is discussed considering regional tectonics local geomorphology and volcano deformation

climate vs parent material which is the key of stagnosol diversity in croatia abstract stagnosols are the most widespread soils in the pannonian region of croatia in croatia they are referred to as pseudogleys and considered to form primarily by normal top down pedogenesis however the formation of their non calcareous loess parent materials probably involved different sources transports and depositional environments we aimed to determine the courses of soil formation and the characteristics of three stagnosol profiles studied along the mean annual precipitation map gradient 700 1100 mm in the pannonian region of croatia we found that soil redoximorphic features formed in situ by ongoing pseudogleization vertical trends for the clay silt and coarse fine silt ratios pointed to top down pedogenesis however high organic c content at the bottom of one soil profile is the result of erosion sedimentation processes whereas high clay content in the subsoil of another profile was largely the result of sedimentation in a shallow paleo lake therefore some croatian stagnosols should be considered polygenetic each soil profile was classified using the wrb system and the new wrb 2014 version proved more suitable than the previous one wrb 2006 however suggestions for improvements are given in line with the map gradient were several morphological and only two chemical ph and base saturation soil characteristics organic c content did not correspond to map due to variability of forest topsoils clay content and cec did not agree with map due to variability of loess parent materials across the pannonian region of croatia the existence of more than one source of loess material confirmed by the modal analysis and the differences in depositional paleo environments resulted in slightly different mechanical compositions of the investigated parent materials we concluded that both climate and parent material must be regarded as key factors for the formation and characteristics of stagnosols in the pannonian region of croatia and the wider southwestern pannonian basin

dykes faults and palaeostresses in the teno and anaga massifs of tenerife canary islands abstract a structural field study was made of 578 sheet intrusions mostly dykes and 153 mostly normal faults dissecting the anaga and teno massifs where a complex volcanic succession of tertiary age the old basaltic series representing the shield building stage of tenerife canary islands crops out many of the intrusions mostly sub vertical mafic dykes are emplaced by multiple magma injections with cumulative thicknesses mostly less than 2 m dyke tips are exposed and preserved for 12 of the dykes three differently oriented sets of dykes exist in the anaga massif nnw sse nne ssw e w whereas there is only one main set in teno trending nnw sse dyke swarms and other structural features having similar orientations also exist in other canary islands a minimum value of the horizontal component of extension induced by dykes is computed using a step of 5 of azimuth accounting also for the dip of dykes the cumulative crustal dilation is at least 300 m 4 in anaga and 270 m 6 in teno the maximum extension peaks at n75 in anaga and n60 in teno indicating a general prevailing extension in direction ene wsw most of the measured faults are normal and strike nnw sse computation of palaeostresses from inversion of fault slip data sets suggests the existence of a polyphase brittle deformation due to an extensional stress field with the minimum compressive principal axes trending ne sw and wnw ese

morphological and structural studies of the society and austral hotspot regions in the south pacific abstract the southeastern extension of the society teahitia hotspot and austral macdonald hotspot volcanic chains were extensively studied 1986 1989 using multi channel echo sounding seabeam bottom imaging systems and submersible dives sea floor within these hotspot regions consists of recently constructed submarine highs built on crust that shows morphological and structural discontinuities with the surrounding ancient abyssal hill regions 4200 m in depth recent volcanic and hydrothermal activity is present on several volcanoes within the society hotspot and on the macdonald volcano previous studies carried out in the society hotspot region showed several geological provinces 1 1 eight major highs 1000 m in height which extend from above sea level down to a depth of 3750 m these features have been formed by lateral eruptive events along rift zones which are aligned along four main structural directions n150 180 n80 n30 40 and n110 which correspond to the orientations of ancient ridge axes transform faults and riedel type shear zones respectively 2 2 the sea floor which slopes away from the base of each major volcano 3750 4200 m depth has an average slope of 0 5 and is characterized by smooth topography and volcanic cones less than 500 m high 3 3 the surrounding abyssal hill region which has an average slope of 0 1 near the society hotspot consists of faulted terrane with a n170 orientation and ancient elongated structural highs from the structural lineations observed in the society hotspot volcanoes a reconstruction of volcanic features has been made it is proposed that the formation of the seamounts occurs in an organized fashion along pre existing preferential structural orientations that are associated with zones of crustal weakness

temporal variations of flexural deformation in hawaii we present a simple time progressive model for the hawaiian island chain bathymetry and the associated flexural response of the elastic oceanic lithosphere the model is used to study the vertical tectonic history of cross seamount a cretaceous guyot brought close to the hawaiian hot spot by plate motion geological evidence indicates that cross was truncated at sea level but its flat top is presently 400 m below sea level our reconstruction suggests the seamount was elevated on the hawaiian swell and truncated about 3 2 m y ago the post truncation subsidence is attributed to the flexural moat that developed around the growing hawaiian islands flexural isostatic adjustment explains the current depth of cross seamount provided the elastic plate thickness is in the 30 35 km range

scaled experiments of volcanic spreading experiments were conducted to study the spreading of volcanic constructs volcanoes are simulated by a sand cone and the volcanic substratum is simulated by a sand layer brittle substratum overlying a silicone layer ductile substratum similarity conditions between natural volcanoes and experimental prototypes led to the definition of dimensionless numbers experiments determine values which predict whether or not spreading takes place of particular importance are the ratio between the thickness of the brittle substratum and the height of the volcano 2 and the brittle ductile ratio of the substratum 3 2 indicates that the volcano must be large enough to break the substratum before spreading occurs whereas 3 controls the style of deformation during spreading these dimensionless numbers change with time reaching values that tend toward those observed for stable configurations experimental values are compared with those from well constrained natural examples it is found that an essential requirement for volcanic spreading is the presence of a low viscosity layer within the substratum flow of the weak layer away from the excess load is responsible for the spreading the overlying edifice displays radial intersecting grabens due to concentric stretching dissected summit areas concentric zones of thrusts and folds form in the substratum around the edifice and diapirs of the ductile substratum rise within the fault zones

imaging the dynamics of dyke propagation prior to the 2000 2003 flank eruptions at piton de la fournaise reunion island 1 the relationship between shallow magma storage and flank injections in volcanic edifices is rarely documented we analyse the time series of geodetic and seismic data collected at piton de la fournaise volcano la reunion during 9 flank eruptions from 2000 to 2003 the data depicts how magma injections supplied flank eruptions at piton de la fournaise volcano and allows to precisely determine the direction and duration of dike intrusions indicating a two phases pattern consistent over all the eruptions 1 fast vertical migration of the dyke 2 m s from the magma chamber to the surface in 10 to 50 minutes and 2 slower lateral migration 0 2 0 8 m s in tens of minutes to hours controlled by the rift zone geometry

river patterns and their meaning abstract rivers are largely responsible for shaping the earth s continental landscapes river patterns the spatial arrangements of channels in the landscape are determined by slope and structure at site and sector scale channel morphology varies spatially and in time but river patterns and drainage texture or the frequency of stream lines per unit area together determine the intricacy or otherwise of topography most river patterns evolve through natural selection slope induces the formation of such patterns as parallel radial and distributary while structure produces straight angular trellis and annular arrangements once established patterns tend to persist nevertheless at many sites the usual patterns have been disturbed and patterns that are anomalous in terms of slope and structure have been produced by diversion tectonism volcanism glaciation mass movements and human activities by antecedence superimposition inheritance or underprinting by the persistence of deeply eroding rivers which encounter alien structures and by climatic change river patterns provide clues to underlying structure and to the chronology of events they have also proved significant in the search for minerals

a new model for the evolution of the volcanic island of reunion indian ocean the island of reunion has been studied using data from airborne and shipborne magnetic surveys the subaerial history of reunion spans the last 2 1 million years the brunhes matuyama geomagnetic reversal enables differentiation of volcanic rocks older and younger than 0 78 ma the lower submarine flanks are poorly magnetized and are interpreted as landslide deposits the core of the island is composed of highly magnetized rocks piton des neiges volcano is composed mostly of rocks older than 0 78 ma only its western flank and central area include thick piles of younger rocks piton de la fournaise is a highly and normally magnetized edifice but its northern and eastern flanks are underlain at shallow depth by reversely magnetized formations the latter are regarded as remnants of les alizes volcano associated with the grand brule hypovolcanic complex at the matuyama brunhes transition the island was composed of at least two main volcanoes piton des neiges and les alizes and perhaps also of a third old volcano takamaka at the center north of the island piton de la fournaise grows on the flank of les alizes and is a relatively young focus of volcanism these volcanoes have had successive phases of construction and destruction the analysis of magnetic anomalies over reunion was decisive in defining a new coherent model for the evolution of the island

edifice growth deformation and rift zone development in basaltic setting insights from piton de la fournaise shield volcano reunion island abstract the overall morphology of basaltic volcanoes mainly depends on their eruptive activity effusive vs explosive the geometry of the rift zones and the characteristics of both endogenous and exogenous growth processes the origin of the steep geometry of the central cone of piton de la fournaise volcano which is unusual for a basaltic effusive volcano and its deformation are examined with a combination of a detailed morphological analysis field observations gps data from the piton de la fournaise volcano observatory and numerical models the new caldera walls formed during the april 2007 summit collapse reveal that the steep cone is composed of a pyroclastic core inherited from an earlier explosive phase overlapped by a pile of thin lava flows this suggests that exogenous processes played a major role in the building of the steep central cone magma injections into the cone which mainly occur along the n25 30 and n120 rift zones lead to an asymmetric outward inflation concentrated in the cone s eastern half this endogenous growth progressively tilts the southeast and east flanks of the cone and induces the development of a dense network of flank fractures finally it is proposed that intrusions along the n120 rift zone are encouraged by stresses induced by magma injections along the n25 30 rift zone

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

location and orientation of eruptive fissures and feederdykes at mount etna influence of gravitational and regional tectonic stress regimes abstract lava extrusion from fissures on the flanks of the etna volcano is a common feature of recent activity as evidenced by the eruptions of 1971 1981 1983 and 1985 and was of similar importance during prehistoric times we propose here a model within which the location and orientations of the large majority of historic eruptive fissures on etna s flanks together with their feeder dykes are explained within the framework of a gravitational stress regime stress orientations within this regime are controlled largely by the gross morphology of the volcano but are significantly modified by local topographic anomalies we propose that the gravitational stress regime largely supercedes an earlier developed regional tectonic stress regime which still operates in the sub volcanic basement and may remain the dominant control of eruptive fissure and feeder dyke disposition at lower altitudes on the flanks the proposed model is supported by 1 detailed examination of fissure locations and orientations and the relationship between fissure azimuths and topography 2 precise horizontal distance measurement and vertical ground deformation studies of a recent eruption and 3 qualitative physical modelling experiments in which coloured water is injected into solidified gelatine solutions representing variously shaped volcanic constructs a cautious approach is advocated when extrapolating trends of spatially limited fissure segments for the purpose of locating potential sites of magma ascent and implications of the model for hazard analysis are outlined

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

mechanical modeling of circumferential and radial dike intrusion on galapagos volcanoes a distinctive and unusual pattern of eruptive fissures is observed on the active volcanoes of the galapagos islands reflecting circumferential dike intrusion near the calderas and radial dike intrusion on the volcano flanks elastic finite element models were used to investigate how a stress field could be produced and maintained to promote both circumferential and radial dike emplacement modeling results show that magma reservoirs of galapagos volcanoes are probably diapiric because this shape promotes both circumferential and radial intrusions but magma pressure alone cannot create the observed pattern of dikes loading by volcano growth and magma reservoir pressure could produce a stress field of suitable orientation but insufficient magnitude the intrusion of circumferential dikes could alter the stress field in a way that promotes future radial diking and vice versa faulting or slumping within the calderas or on the volcano flanks in response to repeated intrusions could also create a stress field conducive to continued intrusion

dynamic basis of volcanic spreading the established models of the structure and dynamics of a typical hawaiian volcano lead inevitably to a paradox finite element viscoelastic calculations using tecton melosh and raefsky 1980 1981 predict that a compressional stress field should characterize their upper flanks instead extension is observed the paradox is solved by postulating that volcanic evolution is determined by feedback processes related to growth and spreading of the volcanic edifice many examples of the process of spreading are documented in the geologic literature on volcanoes of a very broad size range and very different geologic settings the process is similar across many orders of magnitude in size the deformation of the substratum related to magma intrusion during lava dome building events is at the lower end of the scale the spreading of seafloor at oceanic ridges is at the higher end of the scale volcanoes may evolve from small constructs into huge shield volcanoes and perhaps into ocean plates key elements for volcanic spreading are 1 the existence of a weak basal layer and 2 a sufficiently high mass and magma influx to drive the process as the mass of a volcano grows it passes through five phases which may overlap repeat or be omitted building compressing thrusting intruding and spreading during the building phase the mass of a volcano does not significantly affect the stress field of the substratum primitive magmas are likely to be erupted as the mass increases the volcano subsides and becomes characterized by a compressional stress field which inhibits any further intrusion of magma into the volcanic edifice magma may differentiate in the crust allowing for large caldera forming eruptions to occur in the next phase thrusting begins on a decollement usually located at the foot of the edifice while the stress field on the upper slopes of the volcano remains compressional the stress at the base close to the center becomes extensional allowing the magma to intrude and differentiate at the base of the edifice a basal intrusive complex begins to grow explosive eruptions may be expected finally spreading of this complex creates an extensional stress field on the upper flanks of the volcano confining compression to the lower slopes of the edifice the extension allows primitive magma to erupt again at the summit the process of spreading seems to have a fundamental influence on the structural and magmatic evolution of volcanoes at every scale this process results from coupling and feedback between the gravitational and thermal fields thus studying this process may improve our understanding of the relation between structural dynamics and magma evolution and of the origin and evolution of volcanic seismicity and eruptions

isostatic consequences of giant landslides on the hawaiian ridge giant landslides like melting glaciers lead to a redistribution of mass which will have isostatic consequences three dimensional numerical modeling experiments were devised to examine how this mass redistribution affects the isostatic flexural curve a debris avalanche of 10 40 of pre slide oahu is required to account for the 1200 5000 km3 nuuanu deposit while only 1 of pre slide hawaii island is necessary to generate the 200 800 km3 alika i and ii avalanche deposits trials were run using 25 30 and 40 km elastic plate thicknesses t e the island uplift resulting from the nuuanu slide was calculated to be 23 m and 109 m for 10 and 40 volume slides respectively both using t e 25 km a rebound of 10 m and 49 m was calculated for the same volumes respectively using t e 40 km a greater amount of uplift is expressed direct lyover the failed flank causing the edifice to tilt away from the calved off portion the landslide deposit depresses the plate several meters beneath the debris field itself smaller slides e g alika i and ii do not produce as much flexural response with 17 m and 7 m uplift for t e 25 and 40 km respectively the effects of slow moving intact slumps where the failed blocks remain relatively close to the island pedestal were examined for the case of the hilina slump making up approximately 10 of the hawaii island edifice perhaps more significant than the uplift for the hilina slump comparable to that for the 10 nuuanu debris avalanche is the 114 m and 56 m of downwarp beneath its massive slumped foot t e 25 and 40 km respectively the landslide rebound process in the case of a relatively large landslide should be considered as an added component to the evolutionary course of oceanic islands

investigating the deepest part of a volcano plumbing system evidence for an active magma path below the western flank of piton de la fournaise la reunion island abstract peripheral diffuse degassing of co 2 from the soil occurs across the western flank of piton de la fournaise volcano la reunion island indian ocean along a narrow zone in this area carbon isotopic analysis on soil gas samples highlights significant mixing between magmatic and organic end members the zones with the strongest magmatic signature highest 13 c overlap spatial distribution of hypocenters recorded shortly before and during volcano reactivation and allow discriminating a n135 degassing lineament with a minimum length of 11 km and 140 20 m width such orientation is in accordance with that of an old dyke network along the rift zone and with n120 130 and n140 155 lineaments related to the inheritance of oceanic lithosphere structures our findings show that this n135 lineament represents a preferential magmatic pathway for deep magma transfer below the volcano flank moreover spatial distributions of recent eccentric cones indicate a well founded possibility that future eruptions may by pass the shallow plumbing system of the central area of the volcano taking a lateral pathway along this structure our results also confirm that piton de la fournaise activity is linked to a laterally shifted plumbing system and represent a major improvement in identifying the main high risk area on the densely populated western flank of the volcano

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

the spectrum of persistent volcanic flank instability a review and proposed framework based on kīlauea piton de la fournaise and etna abstract persistent motion of the south flank of k lauea volcano hawai i has been known for several decades but has only recently been identified at other large basaltic volcanoes namely piton de la fournaise la reunion and etna sicily thanks to the advent of space geodetic techniques nevertheless understanding of long term flank instability is based largely on the example of k lauea despite the large differences in the manifestations and mechanisms of the process when viewed through a comparative lens for example the rate of flank motion at k lauea is several times that of etna and piton de la fournaise and is accommodated on a slip plane several km deeper than is probably present at the other two volcanoes gravitational spreading also appears to be the dominant driving force at k lauea given the long term steady motion of the volcano s south flank regardless of eruptive intrusive activity whereas magmatic activity plays a larger role in flank deformation at etna and piton de la fournaise k lauea and etna however are both characterized by heavily faulted flanks while piton de la fournaise shows little evidence for flank faulting a helpful means of understanding the spectrum of persistent flank motion at large basaltic edifices may be through a framework defined on one hand by magmatic activity which encompasses both magma supply and edifice size and on the other hand by the structural setting of the volcano especially the characteristics of the subvolcanic basement or subhorizontal intravolcanic weak zones a volcano s size and magmatic activity will dictate the extent to which gravitational and magmatic forces can drive motion of an unstable flank and possibly the level of faulting of that flank while the volcano s structural setting governs whether or not a plane of weakness exists beneath or within the edifice and can facilitate flank slip considering persistent flank instability using this conceptual model is an alternative to using a single volcano as a type example especially given that the example is usually k lauea which defines an extreme end of the spectrum and can provide a basis for understanding why flank motion may or may not exist on other large basaltic volcanoes worldwide

styles of volcano induced deformation numerical models of substratum flexure spreading and extrusion the gravitational deformation of volcanoes is largely controlled by ductile layers of substrata using numerical finite element modelling we investigate the role of ductile layer thickness and viscosity on such deformation to characterise the deformation we introduce two dimensionless ratios a volcano radius ductile layer thickness and b viscosity of ductile substratum failure strength of volcano we find that the volcanic edifice spreads laterally when underlain by thin ductile layers a 1 while thicker ductile layers lead to inward flexure a 1 the deformation style is related to the switch from predominantly horizontal to vertical flow in the ductile layer with increasing thickness increasing a structures produced by lateral spreading include concentric thrust belts around the volcano base and radial normal faulting in the cone itself in contrast flexure on thick ductile substrata leads to concentric normal faults around the base and compression in the cone in addition we show that lower viscosities in the ductile layer low b lead to faster rates of movement and also affect the deformation style considering a thin ductile layer if viscosity is high compared to the failure strength of the volcano high b then deformation is coupled and spreading is produced however if the viscosity is low low b substratum is effectively decoupled from the volcano and extrudes from underneath it in this latter case evidence is likely to be found for basement compression but corresponding spreading features in the volcano will be absent as the cone is subject to a compressive stress regime similar to that produced by flexure at volcanoes where basement extrusion is operating high volcano stresses and outward substratum movement may combine to produce catastrophic sector collapse an analysis of deformation features at a volcano can provide information about the type of basement below it a useful tool for remote sensing and planetary geology also knowledge of substratum geology can be used to predict styles of deformation operating at volcanoes where features have not yet become well developed or are obscured

explosive activity of the reunion island volcanoes through the past 260 000 years as recorded in deep sea sediments the explosive activity of reunion volcanoes since approximately 260 ka is investigated through detailed sedimentologic isotopic and geochemical analyses of three deep sea marine cores twenty four eruptions from piton des neiges and one from piton de la fournaise are identified the marine stratigraphic record of ash layers is undisturbed whereas corresponding tephra deposits on land may have disappeared or have been disturbed through erosion and volcano tectonic processes consequently the new data presented herein may be regarded as a reference sequence of the main explosive events of reunion in addition the chronology based on 18o stratigraphy offers continuous time constraints with respect to other isotopic methods used for dating lavas explosive events were more frequent during the period 180 150 ka the comparison of major element composition and chronologic data between known explosive deposits on land and marine ash layers allows us to propose correlations between marine and land data however most of the marine ash layers do not yet have a recognized subaerial counterpart for the main explosive events known on land we propose the following succession sainte suzanne formations 257 240 ka dalle soudee formation 218 ka salazie formations two groups at 165 168 ka and 80 102 ka saint gilles formations 175 ka saint louis formations 158 161 ka and saint pierre formations 165 166 ka

eruptive mechanism on piton de la fournaise volcano associated with the december 4 1983 and january 18 1984 eruptions from ground deformation monitoring and photogrammetric surveys abstract two photogrammetric aerial surveys were carried out over the summit area of the basaltic shield volcano piton de la fournaise indian ocean one survey in 1981 and the second one in 1984 during this time only two eruptions occurred both the december 4 1983 and the january 18 1984 eruptions opened fissures on the southwestern part of the summit because of the slight and erratic ground deformation measured on the dry tilt network by continuous tilt stations and within the geodetic network between 1981 and november 1983 and between february 1984 and june 1984 ground deformations revealed by the two photogrammetric surveys can be essentially associated with the two eruptions large displacement vectors were obtained up to 40 cm horizontal displacement vectors indicate a northeastward ground deformation of the southwestern part of the cone where the effusive vents opened a more diffuse uplift along the main fracture zones that cut the volcanic edifice was observed while east of these fractures only slight less than 10 cm and opposite displacements were observed this displacement field can be associated with the main geological structures of the dome of piton de la fournaise volcano some consequences of the observed displacement field may be outlined for the volcanic observational ground deformation networks to estimate the displacement field revealed by the photogrammetric surveys a simple model of dyke intrusion has been computed this model is based on dislocations and takes into account the main fracture zones good agreement is observed between computed and observed data in the area of the effusive vents some disagreement remains in the northwestern part of the survey area where horizontal deformations are small and erratic and also in the northern part where an uplift was observed that can be associated with the northern active fracture zones

finding realistic dike models from interferometric synthetic aperture radar data the february 2000 eruption at piton de la fournaise dike intrusions often cause complex ground displacements that are not sufciently explained by simple analytical models we develop a method to nd complex and realistic dike geometries and overpressures from interferometric synthetic aperture radar insar data this method is based on a combination of a boundary element method with realistic topography and a neighbourhood algorithm inversion dike model geometry is roughly a quadrangle with its top reaching the ground the inversion has two stages search and appraisal the appraisal stage involves calculations of model marginal probability density functions using mist val ues calculated during the search stage the mist function takes into account the variance and correlation of data noise synthetic tests show that a model is successfully retrieved within predicted narrow condence intervals we apply the method on insar data of the february 2000 ank eruption at piton de la fournaise and get a trapezoid dike dipping seaward 61 0 n 67 3 with its bottom passing 800n1000 m beneath the summit a model with a basal slip plane does not better explain observed asymmetric displacements and thus this asymmetry is solely attributed to the dipping dike the dike lies above a narrow band of pre eruption seis micity suggesting that lateral magma propagation occurred neglecting topography results in poor modeling at depth and in overestimations of overpressure or opening height both about 30 and volume about 20

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

from nappe stacking to out of sequence postcollisional deformations cretaceous to quaternary exhumation history of the se carpathians assessed by low temperature thermochronology 1 apatite fission track aft and u th he ahe thermochronology have been combined to constrain the exhumation history of the se carpathians cooling ages generally decrease from cretaceous for the internal basement nappes aft ages to miocene quaternary aft and ahe respectively for the external sedimentary wedge the aft and ahe data show a paleogene age cluster which confirms a suspected but never demonstrated tectonic event the new data furthermore suggest that the se carpathians have been affected by a middle miocene exhumation phase related to continental collision which occurred at rates of 0 8 mm yr similar to the one previously inferred for the east carpathians the se carpathian tectonic evolution however is overprinted by two younger exhumation events in the pliocene pleistocene the first exhumation phase latest miocene early pliocene occurred at high exhumation rates 1 7 mm yr and is interpreted as a tectonic event and or associated with a sea level drop in the paratethys basins during the messinian low stand the youngest recorded tectonic phase suggests rapid pleistocene exhumation 1 6 mm yr and is interpreted to represent crustal scale shortening different in mechanics from collisional processes the data suggest that the se carpathians did not develop as a typical double vergent orogenic wedge instead exhumation was related to a foreland vergent sequence of nappe stacking during collision and was subsequently followed by a large out of sequence shortening event truncating the already locked collisional boundary

spatial distribution of hotspot material added to the lithosphere under la reunion from wide angle seismic data wide angle seismic lines recorded by ocean bottom and land seismometers provide a pseudo three dimensional investigation of the crust and upper mantle structure around the volcanically active hotspot island of la reunion the submarine part of the edifice has fairly low seismic velocities without evidence for intrusives an upper unit with a velocity depth gradient is interpreted as made of material erupted subaerially then transported and compacted downslope between this unit and the top of the oceanic plate imaged by normal incidence seismic reflection a more homogeneous unit indicated by shadow zones on several wide angle sections may correspond to lavas of a different nature extruded underwater in the earlier phase of volcanism coincident wide angle and normal incidence reflections document that the oceanic plate is not generally downwarping toward the island but doming instead toward its southeastern part with limited evidence for some intracrustal intrusion deeper in the lithosphere the presence of a layer of intermediate velocity between the crust and mantle is firmly established it is interpreted as resulting from the advection of hotspot magmatic products possibly partially molten and of a composition for which the crust is a density barrier the extensive wide angle coverage constrains the extent of this body it does not show the elongated shape expected from plate drift above a steady hotspot supply alternative propositions can hence be considered for example that la reunion is caused by a solitary wave of hotspot material or by a young hotspot the size of the underplate 140 km wide and up to 3 km thick corresponds to less than half the volume of the edifice on top of the plate

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

structural control of collapse events inferred by self potential mapping on the piton de la fournaise volcano la reunion island abstract field surveys were performed on the terminal cone of piton de la fournaise in 2006 and 2008 to precisely map the self potential sp signal and determine the zonation of the hydrothermal activity both on the flanks of the cone and in the summit area including inside the bory and dolomieu craters sp maps inside the craters have been performed 8 months before the 5 7 april 2007 caldera collapse zonations appear both at the scale of the cone and of the summit and allow new interpretation of the electrical signal distribution on the terminal cone of piton de la fournaise superimposed to the sp maxima linked to the rift zones several areas of sp maxima associated with collapse structures have been detected 1 in the summit area the bory and dolomieu craters show the strongest sp values with amplitudes exceeding 2 v with respect to the base of the cone and with a sharp lateral variation to the east corresponding to the inner boundary of the dolomieu caldera collapsed on 5 7 april 2007 and 2 in the paleo pit craters surrounding the summit which show amplitudes similar to the dolomieu bory craters the analysis of the variations of the signal with time evidences a modification of the fluid flow pattern with a higher associated sp signature to the east in 2008 we interpret the amplification of fluid flow to the east in 2008 as a consequence of the eastward motion of the eastern flank of the volcano during the april 2007 eruption the acquisition of sp data during two periods separated by the april 2007 eruption turns out to be a good opportunity to correlate the sp signal to the piton de la fournaise structure and to its evolution in term of hydrothermal and eruptive activity

the effects of rock heterogeneities on dyke paths and asymmetric ground deformation the example of piton de la fournaise reunion island during pre eruptive periods and eruptions piton de la fournaise volcano shows an asymmetric magma induced pattern of deformation the origin of this asymmetry is not well constrained and the pre eruptive deformation does not have a satisfactory explanation here we present data on the past history and complex structure of the volcano we also provide new field data on exposed dykes in the volcano the field data confirm that the eruptive centre has migrated through time our field data indicate average dyke dips of about 80 by contrast inferred dips of recent feeder dykes using geodetic data are 53 75 we explain this difference as being partly due to the models used to infer the dyke dips from geodetic data which assume the host rock to be homogeneous and isotropic when fractures and the other heterogeneities are taken into account in the models the inferred dyke dips become similar to those of the exposed dykes new numerical models indicate that when the complex internal structure of the volcano is taken into account part of the asymmetry of the ground deformation can be explained part of the deformation asymmetry however is due to the curved structure of the rift system resulting in the injection of curved dykes and the dyke dips 2008 elsevier b v all rights reserved

inversion of coeval shear and normal stress of piton de la fournaise flank displacement the april 2007 eruption of piton de la fournaise was the biggest volcano eruptive crisis of the 20th and 21st centuries interferometric synthetic aperture radar insar captured a large coeruptive seaward displacement on the volcano s eastern flank which continued for more than a year at a decreasing rate coeruptive uplift and posteruptive subsidence were also observed while it is generally agreed that flank displacement is induced by fault slip we suggest that this flank displacement might have been induced by a sheared sill based on observations of sheared sills at piton des neiges to test this hypothesis we develop a new method to invert a quadrangular curved source submitted to simultaneous pressure and shear stress changes this method based on boundary elements is applied to data acquired along six envisat orbits covering a 14 month period subsequent to the april 2007 eruption posteruptive displacement is well explained by closure and slip of a large 5 km by 8 km and shallow 500 m trapezoidal fracture parallel to the flank and probably coincident with a lithological discontinuity we investigate whether thermal contraction or degassing of a coeruptive sill can explain the displacement such a sill would have to be 10 times thicker than inferred from the coeruptive uplift and solidification time 10 times shorter 20 days than the duration of the posteruptive subsidence 24 to 33 months instead we propose that the posteruptive eastern flank displacement is due to the compaction and ongoing slow slip on a shallow detachment fault

resistivity variations related to the large march 9 1998 eruption at la fournaise volcano inferred by continuous mt monitoring abstract the 2645 m high la fournaise volcano located in the southwest of reunion island indian ocean is a shield basaltic volcano where effusive eruptions generally occur along long fissures starting from the summit alongside major fractures that characterize the eruptions dynamism and effusivity between 1992 and 1998 the volcano underwent a quiet period during which few earthquakes were recorded minor seismic activity returned after 1997 and picked up in march 1998 during the 35 h preceding the march 9 eruption from 1996 two autonomous stations csv and bav were installed on the volcano csv was located inside the enclos fouque caldera while bav was positioned 8 2 km nw of the volcano summit horizontal components of the electric and magnetic fields were sampled every 20 s continuous time series were available from 1996 to 1999 at csv and from 1997 to march 1998 at bav data have been processed using both single station and remote reference processing both results show apparent resistivity variations synchronous to the eruption time lapse impedance estimates are computed on overlapping time windows of about two days at both stations the only major decrease of the observed impedance coincides with the march 1998 eruption at csv the resistivity started to drop about five days before the eruption reached several local minima until april and then slowly increased as the volcanic crisis reduced in activity after the end of the crisis in september 1998 the apparent resistivity recovered its pre crisis value the time lapse results also show variability in directionality sharp and elongated phase tensor ellipse residuals appear during the eruption with a n105 orientation suggesting the emergence of an almost ns striking dyke a 1d background model built from mt soundings performed during the quiet period 1996 to february 1998 on which a 3d ns striking dyke was added shows a good agreement with phase tensor residuals and spatial distribution of the resistivity variations observed during the eruption

three dimensional modeling of the 1983 1984 eruption at piton de la fournaise volcano reunion island displacement vectors on 125 points over a 6 km 2 area were calculated zlotnicki et al 1990 for piton de la fournaise volcano reunion island from photogrammetric surveys conducted in september 1981 and again in september 1984 an eruptive crisis with two episodes occurred during the 3 years between the surveys we modeled the displacements from the photogrammetric surveys using a three dimensional mixed boundary element method the model shows that both eruptive episodes were fed by a single dike oriented 165 from north as the dike approached the ground surface it divided into segments that rotated into an en echelon pattern the model that best fits all observations suggests that the stress state within the volcano is quasi isotropic such that horizontal stresses equal the overburden these high horizontal stresses are incompatible with an elastic edifice loaded merely by gravitational forces however they may be attributed to the cumulative effect of repeated dike intrusions

calderas landslides and paleo canyons on piton de la fournaise volcano la reunion island indian ocean based upon a re interpretation of previous data and a new field campaign a structural evolution is proposed for the early history of piton de la fournaise volcano from 500 000 to 50 000 years conceptually it is shown that the formation of a caldera in which lava flows are contained inside the caldera depression gives time for erosion to excavate deep canyons on the external slopes of the volcano for example the riviere des remparts the riviere langevin and the riviere de l est canyons on piton de la fournaise volcano these canyons are infilled when lavas filling the caldera and overflowing its rim are able again to flow on the external slopes of the volcano in the past this excavating infilling process has occurred twice following the formation of the riviere des remparts and morne langevin calderas the formation of the third caldera the plaine des sables caldera was followed by the excavation of the current canyons in addition to this process two large landslides have been documented in the field the first which happened about 300 000 years ago is apparently the first episode of the break up of piton de la fournaise volcano predating the formation of the four large calderas the second landslide which occurred 150 000 years ago and is considered to be less extensive has carried away the entire southern flank of the riviere des remparts caldera

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

a seismic reflection profile study of lithospheric flexure in the vicinity of the cape verde islands 1 seismic reflection profile data are used to determine the stratigraphic architecture of the flexural moat that flanks the cape verde islands the moat region is characterized by upward of 1 2 km of poorly to well stratified material the two lowermost units thicken from west to east and are attributed to sediment loading and flexure at the nearby west africa continental margin during early cretaceous to early miocene the two uppermost units in contrast thicken concentrically around the islands and are attributed to the infilling of a flexural moat that formed by volcanic loading since the early miocene flexure modeling shows that the thickness of the moat infill cannot be explained only by surface loading and requires that the downward flexure due to surface loads is opposed by an upward acting subsurface load the best fit combined surface and buried loading model is for an elastic thickness te of 29 km a load and infill density of 2700 kg m 3 and a ratio of surface to subsurface loads f of 0 2 these results are in accord with spectral studies of free air gravity anomaly and topography data based on the admittance technique the subsurface loads are spatially limited to the islands and their submarine flanks nevertheless they are associated with a broad regional uplift of up to 400 m the uplift is large enough to explain why the moat infill is generally tilted away from rather than toward the islands the uplift is too small however to account for the height of the swell upon which the cape verde islands are superimposed the origin of the cape verde swell is not known however a normal te and a modest heat flow anomaly suggest that the swell cannot be fully explained by uplift due to thermal reheating of the lithosphere by an underlying hot spot and that other deep seated mantle processes must be involved

formation of the april 2007 caldera collapse at piton de la fournaise volcano insights from gps data abstract on april 5th 2007 a major collapse of the dolomieu summit crater occurred at piton de la fournaise volcano la reunion island to constrain the origin and the dynamics of the collapse we modelled continuous gps displacements preceding and accompanying this event our results reveal that the dolomieu collapse formed by a piston like mechanism in which the fast draining of the shallow magma reservoir at the beginning of the march 30th may 1st 2007 eruption caused a gravity driven downward displacement of its roof during the five days preceding the collapse summit deflation was caused by volume change through the fractured rock column above the shallow magma chamber due to the closure of voids or small scale cavities or the upward stoping and migration of an underground cavity since 2000 weakening of the rock column by repeated refilling and draining of the shallow magma chamber favoured its destabilization on april 5th 7 h before the onset of the collapse the deflation source was located at 300 m depth revealing the initiation of the collapse at very shallow depth the sudden collapse on april 5th was followed by continuous cyclic subsidence until april 6th each cycle was characterized by a progressive inward deflation of the summit zone and ended by sudden summit outward displacements caused by stress relaxation following a collapse event subsidence of the stoped column in the magma reservoir during the collapse of the crater on april 5th and 6th acted as a piston thus increasing the eruption rate the collapse occurred in only 24 h by successive events and increased the depth of the dolomieu crater by 340 m after april 6th only minor readjustments of the summit cone occurred with slight summit deflation and small landslides of the caldera walls

morphology of piton de la fournaise basaltic shield volcano la reunion island characterization and implication in the volcano evolution the topography of piton de la fournaise volcano pdf differs from the classic view of basaltic shield volcanoes as it is characterized by 1 several steep slope zones on its flanks and 2 a large u shaped caldera the enclos grand brule structure egbs most of these structures were previously interpreted as the scars of lateral landslides the deposits of which cover the submarine flanks of pdf we carried out a detailed analysis of the morphology of pdf which reveals that the steep slope zones form two independent circumferential structures that continue into the caldera the development of circumferential steep slopes on volcano flanks may have several origins constructive destructive and deformation processes we interpret those processes acting on pdf as caused by the spreading of the volcanic edifice above a weak hydrothermal core leading to outward displacements and a summit extensive stress field the continuity of the steep slope on both sides of the egbs escarpments suggests that this structure was not caused by a 4 5 ka old giant landslide as it is usually proposed but is due to a mainly vertical collapse the recent debris avalanche deposits east of the island indicate that this event likely destabilized part of the submarine flank we propose that the collapse of the grand brule the lower half of the egbs was due to the downward drag related to the dense intrusive complex of the alizes volcano which is located 1 km below the grand brule the collapse of the enclos is interpreted as the consequence of the deformation of the hydrothermal system of the pre enclos volcano although the continuity of the geological and morphological structures between the enclos and the grand brule suggests a narrow link between these two collapse events their chronology and relationship are still uncertain finally we hypothesize that the persistence of the ne and se rift zones during the last 150 ka despite the large changes of the topography related to the recurrent flank destabilizations is linked to a deep sources which can be either underlying crustal faults or the continuous downward subsidence of the alizes intrusive complex

recurrence of major flank landslides during the last 2 ma history of reunion island new detailed swath bathymetry and backscatter data corroborate the existence of four large bulges on the submarine flanks of reunion island these fan shaped promontories are 20 25 km wide at the coastline and 70 150 km across the seafloor 40 50 km offshore their surfaces are characterized by a speckle sonar pattern indicating the presence of large blocks up to several hundred meters across each bulge results from the superposition of multiple landslide deposits whose older ones are dissected and delimited by erosive channels as much as 200 m deep and 20 km long the submarine flanks of reunion island are thus mostly built by accumulation of debris avalanche fans morphologic and geologic evidence define large subaerial source areas for these mass wasting events in particular inferred headwalls of most landslides having affected the piton des neiges massif generally coincide with the boundaries of its cirques mafate salazie and cilaos whereas recurrent landslides have resulted in the formation of large concentric amphitheatre structures through the piton de la fournaise massif thus about 15 slide events accompanied growth of the reunion island shield since 2 ma

sks splitting in the western indian ocean from land and seafloor seismometers plume plate and ridge signatures we present sks splitting measurements in the western indian ocean recorded on 20 land and 57 seafloor seismometers deployed by the rhum rum experiment reunion hotspot and upper mantle reunions unterer mantel we discuss our splitting observations within their geodynamic settings and compare them to sks splitting parameters predicted from an azimuthally anisotropic rayleigh wave tomography model that includes the rhum rum data we find that anisotropic directions poorly correlate with the present day motion of the somali plate which at 2 6 cm yr may be too slow to cause strongly sheared fabric in the asthenosphere fast split directions between la reunion and the central indian ridge cir trend e w and provide strong first seismological evidence for near horizontal flow in the asthenosphere that connects the reunion mantle upwelling with the cir supporting a long standing hypothesis on plume ridge interaction in the vicinity of the reunion hotspot we observe a seismic anisotropy pattern indicative of a parabolic asthenospheric flow controlled by the reunion mantle upwelling and its consecutive asthenospheric spreading we furthermore observe ridge normal along the cir and ridge parallel along the southwest indian ridge swir both mainly attributed to asthenospheric mantle flows in the mozambique channel between east africa and madagascar we attribute e w trending to frozen lithospheric structures recording the paleo orientation of the spreading ridges that enabled madagascar s separation away from africa based on the synopsis of this and previous sks splitting studies at mid ocean ridges we propose that ridge normal may develop at fast and intermediate spreading ridges e g cir and east pacific rise and ridge parallel could be characteristic to slow spreading ridges e g swir mid atlantic ridge and the paleo ridges in the mozambique channel