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

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

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

volcanic and morphological evolution of la gomera canary islands based on new k ar ages and magnetic stratigraphy implications for oceanic island evolution the spatial and chronological evolution of canary islands volcanism is due to eastward progression of the slow moving african plate over a mantle plume la gomera is located between the western shield stage islands 12 ma with 23 k ar ages and palaeomagnetic data we can constrain the timing of the main subaerial volcanic phases of la gomera the shield volcano c 9 4 8 0 ma the postshield peripheral lava flows c 7 4 5 0 ma the postshield horizontal lava flows c 5 4 4 3 ma and late intra canyon lava flows c 1 9 ma the horizontal lava flows fill a 10 km wide depression formed by a giant landslide garajonay the entire structure of la gomera is in relief inversion declining volcanic construction rates from the shield to the postshield stages are correlated with increasing erosion rates whereas the period of volcanic quiescence hiatus has low erosion rates the succession and duration of the hotspot volcanic stages in the canary islands are compared with those in the hawaiian islands thus underlining differences between slow moving and fast moving plate settings

catastrophic collapse at stratovolcanoes induced by gradual volcano spreading unlike ordinary mountains which are formed by slow uplift and erosion volcanoes are constructed rapidly as a consequence many are liable to massive flank failures leading to debris avalanches for example at mount st helens in 1980 such failures occur worldwide about once every 25 years ref 1 and even small ones can present a major hazard in particular if far reaching tsunamis are generated as at mayu yama in 1792 ref 2 previous work has tended to emphasize differences in eruption style associated with flank failure2 but here we focus on the fundamental structural causes of failure most volcanic failures are generated by magmatic intrusion and flank spreading3 we present evidence however that mombacho volcano in nicaragua experienced a previously unrecognized type of failure triggered by sub volcanic basement spreading notably collapses related to basement spreading do not require that the volcano be magma tically active and thus flank failure may pose a significant risk even at inactive volcanoes which are rarely monitored

the interaction between volcanoes and strike slip transtensional and transpressional fault zones analogue models and natural examples regional strike slip faulting can control magma movements deform volcanoes and may destabilise their flanks the aim of this study is to address this problem by comparing two natural examples basse terre island volcanoes lesser antilles and maderas volcano nicaragua with analogue experiments the field and remote sensing analyses of their structures reveal that guadeloupe volcanoes which developed in a1 45 striking sinistral transtensional fault zone are dominantly fractured in a 090 e120 direction which is parallel to the maximum principal horizontal stress and to the elongation direction of the summit graben of analogue models this graben is bordered by the sigmoid i fault or y shear structure and has facilitated the formation of the beaugendre and vieux habitants valleys by faulting erosion or collapse this structure has also influenced the injection of dykes and the transport of hydrothermal fluids the comparison of maderas volcano with the analogue models confirms that the volcano has developed parallel to a 135

a new chronostratigraphical and evolutionary model for la gomera implications for the overall evolution of the canarian archipelago a review of the general volcano stratigraphy and geochronology of la gomera one of the lesser known canary islands has led to the establishment of a new evolutionary model the oldest edifice corresponds to the submarine stage built up between 20 and 15 ma the construction of the submarine edifice was followed by an important break in the activity about 4 ma and deep erosion of the edifice about 10 5 ma ago the main present day edifice the old edifice 10 5 6 4 ma emerged which was also submarine in its initial phases two different main stages are distinguishable the first stage was represented by a large some 22 km wide basaltic shield volcano the lower old edifice several lateral collapse events tazo and san marcos avalanches occurred during this time and were responsible for the removal of an important part of its northern flank in the second growth stage the upper old edifice the activity migrated southwards a 25 km wide composite volcano arose covering part of the remaining earlier shield volcano the felsic trachytic to phonolitic activity occurring in two separate episodes formed a significant component of this composite volcano finally one more recent large edifice the young edifice built up from 5 7 to 4 ma the lava flows of this younger edifice covered completely the centre and the south of the island and filled deep ravines in the north more evolved magmas including significant felsic magmas the third and last felsic episode occurred in this phase of activity the growth of la gomera was long lasting separated by an important gap in the activity in the middle miocene with no quaternary activity at all at the same time on tenerife the nearest island east of la gomera three large edifices grew separately roque del conde anaga and teno initially three separated islands from the available data it is inferred that the subaerial activity started earlier in the roque del conde edifice then on la gomera and later in teno in the nw and anaga in ne of tenerife which is the youngest of all these edifices these facts together with the irregular general progress of the volcanic activity support more complex views of the genesis for the canary islands than the simple hotspot model

heat flow of norway and its continental shelf abstract terrestrial heat flow influences a large collection of geological phenomena its determination and understanding is of prime interest for oil industry because geothermal processes impact directly maturation histories and economic potential of oil fields and reservoirs published systematic heat flow determinations from major oil provinces are however seldom robust heat flow determinations in drillholes require logging of undisturbed temperatures and intensive sampling of core material for petrophysical measurements temperature logging in exploration drillholes is traditionally conducted during drilling breaks or shortly after drilling resulting in temperatures severely disturbed by mud circulation and coring is restricted to selected intervals alternatively test temperatures information from electric logs and lithological descriptions of drill cuttings can be used to overcome these limitations the present contribution introduces new heat flow determinations based on 63 exploration drillholes from the norwegian north sea the mid norway margin and the barents shelf our analyses are based on released dst temperatures precise lithological descriptions of drill cuttings previously measured rock matrix thermal conductivities and established porosity laws for the sake of comparison we carefully review previous heat flow studies carried out both onshore and offshore norway our results suggest median heat flow values of 64 mw m 2 65 mw m 2 and 72 mw m 2 for the north sea the mid norway margin mainly the trondelag platform and the sw barents shelf respectively in detail heat flow increases by 10 mw m 2 from the southern norwegian north sea towards the mid norway margin this result appears to be in very good agreement with seismic tomographic studies suggesting northward thinning of the underlying mantle lithosphere our results together with published marine heat flow data from the mid norway margin suggest a gradual decrease in heat flow levels from both the north sea and the trondelag platform towards the centres of the deep more and voring basins this latter effect is attributed to reduced heat input from crustal sources caused by the extreme attenuation of the crystalline basement below these two basins heat flow in the sw barents shelf increases westwards this is interpreted as lateral heat transfer from the adjacent young ocean tentative corrections for quaternary sedimentation erosion effects suggest similar heat flow levels for the three studied areas but the above mentioned regional trends remain unchanged

volcanic complexes in the eastern ridge of the canary islands the miocene activity of the island of fuerteventura abstract fuerteventura has been since early stages of its growth the result of three different adjacent large volcanic complexes southern central and northern the definition of these volcanic complexes and their respective growing episodes is based on volcano stratigraphic morphological and structural criteria particularly radial dyke swarms each complex has its own prolonged history that might be longer than 10 m y during that time several periods of activity alternating with gaps accompanied by important erosion took place the evolution of each volcanic complex has been partially independent but all the three are affected by at least three miocene tectonic phases that controlled considerably their activity the volcanic complexes are deeply eroded and partially submerged in the core of the northern and the central volcanic complexes there is a set of submarine and plutonic rocks intensely traversed by a dyke swarm known as the basal complex the basal complex has been interpreted in different ways but all previous authors have considered it to be prior to the subaerial shield stage of the island here we advance the idea that the basal complex represent the submarine growing stage of the volcanic complexes and the hypabyssal roots plutons and dykes of their successive subaerial growing episodes two seamounts situated nearby southwest of the island might be interpreted as remains of two other major volcanoes these two volcanoes together with those forming the present emerged island of fuerteventura and finally those of famara and los ajaches situated further north on lanzarote constitute a chain of volcanoes located along a lineation which is subparallel to the northwestern african coastline and which may relate to early atlantic spreading trends in the area

role of olivine cumulates in destabilizing the flanks of hawaiian volcanoes the south flank of kilauea volcano is unstable and has the structure of a huge landslide it is one of at least 17 enormous catastrophic landslides shed from the hawaiian islands mechanisms previously proposed for movement of the south flank invoke slip of the volcanic pile over seafloor sediments slip on a low friction decollement alone cannot explain why the thickest and widest sector of the flank moves more rapidly than the rest or why this section contains a 300 km3 aseismic volume above the seismically defined decollement it is proposed that this aseismic volume adjacent to the caldera in the direction of flank slip consists of olivine cumulates that creep outward pushing the south flank seawards average primary kilauea tholeiitic magma contains about 16 5 wt mgo compared with an average 10 wt mgo for erupted subaerial and submarine basalts this difference requires fractionation of 17 wt 14 vol olivine phenocrysts that accumulate near the base of the magma reservoir where they form cumulates submarine erupted kilauea lavas contain abundant deformed olivine xenocrysts derived from these cumulates deformed dunite formed during the tholeiitic shield stage is also erupted as xenoliths in subsequent alkalic lavas the deformation structures in olivine xenocrysts suggest that the cumulus olivine was densely packed probably with as little as 5 10 vol intercumulus liquid before entrainment of the xenocrysts the olivine cumulates were at magmatic temperatures 1100 c when the xenocrysts were entrained olivine at 1100 c has a rheology similar to ice and the olivine cumulates should flow down and away from the summit of the volcano flow of the olivine cumulates places constant pressure on the unbuttressed seaward flank leading to an extensional region that localizes deep intrusions behind the flank these intrusions add to the seaward push this mechanism ties the source of gravitational instability to the caldera complex and deep rift systems and therefore limits catastrophic sector failure of hawaiian volcanoes to their active growth phase when the core of olivine cumulates is still hot enough to flow

volcanic edifice stability during cryptodome intrusion limit equilibrium analyses were applied to the 1980 mount st helens and 1956 bezymianny failures in order to examine the influence on stability of structural deformation produced by cryptodome emplacement weakening structures associated with the cryptodome include outward dipping normal faults bounding a summit graben and a flat shear zone at the base of the bulged flank generated by lateral push of the magma together with the head of the magmatic body itself these structures serve directly to localize failure along a critical surface with low stability deep within the interior of the edifice this critical surface with the safety coefficient reduced by 25 30 is then very sensitive to stability condition variation in particular to the pore pressure ratio ru and seismicity coefficient n for ru 0 3 or n 0 2 the deep surface suffers catastrophic failure removing a large volume of the edifice flank in the case of mount st helens failure occurred within a material with angle of friction 40 cohesion in the range 105 106 pa and probably significant water pore pressure on 18 may 1980 detachment of slide block i occurred along a newly formed rupture surface passing through the crest of the bulge although sliding of block i may have been helped by the basal shear zone significant pore pressure and a triggering earthquake were required ru 0 3 and n 0 2 detachment of the second block was guided by the summit normal fault the front of the cryptodome and the basal shear zone this occurred along a deep critical surface which was on the verge of failure even before the 18 may 1980 earthquake the stability of equivalent surfaces at bezymianny volcano appears significantly higher thus although magma had already reached the surface weaker materials or higher pore pressure and or seismic conditions were probably required to reach the rupture threshold from our analysis we find that deep seated sector collapses formed by removing the edifice summit cannot generally result from a single slide cryptodome induced deformation does however provide a deep potential slip surface as previously thought it may assist deep seated sector collapse because it favors multiple retrogressive slides this leads to explosive depressurization of the magmatic and hydrothermal systems which undermines the edifice summit and produces secondary collapses and explosive blasts

submarine landslides around the canary islands the morphology and structure of the submarine flanks of the canary islands were mapped using the gloria long range side scan sonar system bathymetric multibeam systems and sediment echosounders twelve young 2 ma giant landslides have been identified on the submarine flanks of the canary islands up to now older landslide events are long buried under a thick sediment cover due to high sedimentation rates around the canary islands most slides were found on the flanks of the youngest and most active islands of la palma el hierro and tenerife but young giant landslides were also identified on the flanks of the older 15 20 ma but still active eastern islands large scale mass wasting is an important process during all periods of major magmatic activity the long lived volcanic constructive history of the islands of the canary archipelago is balanced by a correspondingly long history of destruction resulting in a higher landslide frequency for the canary islands compared to the hawaiian islands where giant landslides only occur late in the period of active shield growth the lower stability of the flanks of the canaries is probably due to the much steeper slopes of the islands a result of the abundance of highly evolved intrusive and extrusive rocks another reason for the enhanced slope instability is the abundance of pyroclastic deposits on canary islands resulting from frequent explosive eruptions due to the elevated volatile contents in the highly alkalic magmas dike induced rifting is most likely the main trigger mechanism for destabilization of the flanks flank collapses are a major geological hazard for the canary islands due to the sector collapses themselves as well as triggering of tsunamis in at least one case a giant lateral blast occurred when an active magmatic or hydrothermal system became unroofed during flank collapse

analogue models of the effect of long term basement fault movement on volcanic edifices long term fault movement under volcanoes can control the edifice structure and can generate collapse events to study faulting effects we explore a wide range of fault geometries and motions from normal through vertical to reverse and dip slip to strike slip using simple analogue models we explore the effect of cumulative sub volcanic fault motions and find that there is a strong influence on the structural evolution and potential instability of volcanoes the variety of fault types and geometries are tested with realistically scaled displacements demonstrating a general tendency to produce regions of instability parallel to fault strike whatever the fault motion where there is oblique slip faulting the instability is always on the downthrown side and usually in the volcano flank sector facing the strike slip sense of motion different positions of the fault beneath the volcano change the location type and magnitude of the instability produced for example the further the fault is from the central axis the larger the destabilised sector also with greater fault offset from the central axis larger unstable volumes are generated such failures are normal to fault strike using simple geometric dimensionless numbers such as the fault dip degree of oblique motion angle of obliquity and the fault position we graphically display the geometry of structures produced the models are applied to volcanoes with known underlying faults and we demonstrate the importance of these faults in determining volcanic structures and slope instability using the knowledge of fault patterns gained from these experiments geological mapping on volcanoes can locate fault influence and unstable zones and hence monitoring of unstable flanks could be carried out to determine the actual response to faulting in specific cases

miocene rifting of fuerteventura canary islands 1 the older geological units of the volcanic island of fuerteventura canary islands i e the so called basal complex and the lower part of the subaerial volcanic rocks show abundant structures indicative of a long lived period of miocene tectonic activity these structures include faults dike swarms kilometer scale folds tilted sequences and fissural and central volcanic edifices a detailed structural study based on geological mapping and the use of fault slip inversion techniques and statistical analysis of dike orientation has allowed the identification of three separated miocene deformation phases m d1 m d2 and m d3 the average extension directions determined for these phases are nw se nne ssw and ene wsw respectively a model of oceanic lithosphere rifting is proposed to account for this deformation history a buoyant anomalous sublithosphere mantle triggered the extension in the lithosphere beneath fuerteventura isolating it during the early and middle miocene from the plate scale collision regime predominant in the nw corner of the african plate

k ar chronology of the volcanic eruptions in the canarian archipelago island of la gomera a geochronological study of the island of la gomera canaries has been carried out by the k ar method the 26 new ages obtained together with the 17 previous determinations show that above the main unconformity of the island separating the basal complex from the later volcanic series there is a unit of lower old basalts more than 10 m y old polymictic volcanic breccias were emplaced between 10 and 9 m y ago the upper old basalts above them were formed between 9 and 6 m y ago with a peak of activity around 7 m y after a period of erosion 6 5 m y a thick series of young basalts associated with trachytic and phonolitic domes and flows were rapidly emplaced between 4 5 4 m y ago finally local basaltic activity took place 2 8 m y ago the age of the basal complex is not well known although three ages 14 6 15 5 and 19 3 m y have been obtained for some alkaline intrusives which seem to represent the youngest events in the complex

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

high resolution residual geoid and gravity anomaly data of the northern indian ocean an input to geological understanding geoid data are more sensitive to density distributions deep within the earth thus the data are useful for studying the internal processes of the earth leading to formation of geological structures and activities in this paper we present much improved version of high resolution 1 1 geoid anomaly map of the northern indian ocean generated from the altimeter data obtained from geodetic missions of geosat and ers 1 along with ers 2 topex posideon and jason satellites the geoid map of the indian ocean is dominated by a significant low of 106 m south of sri lanka named as the indian ocean geoid low iogl whose origin is not clearly known yet the residual geoid data are retrieved from the geoid data by removing the long wavelength core mantle density effects using recent spherical harmonic coefficients of earth gravity model 2008 egm 2008 up to degree and order 50 from the observed geoid data the coefficients are smoothly rolled off between degrees 30 70 in order to avoid artifacts related to the sharp truncation at degree 50 with this process we observed significant improvement in the residual geoid data when compared to the previous low spatial resolution maps majumdar and bhattacharyya 2004 majumdar et al 2006 the previous version was superposed by systematic broad regional highs and lows like checker board with amplitude up to 12 m though the trends of geoid in general match in both versions these methodical artifacts in the previous version may have arisen due to the use of old rapp s geo potential model coefficients as well as sharp truncation of reference model at degree and order 50 geoid anomalies are converted to free air gravity anomalies and validated with cross over corrected ship borne gravity data of the arabian sea and bay of bengal the present satellite derived gravity data matches well with the ship borne data with root mean square error rmse of 5 17 8 mgal spectral analysis of ship borne and satellite data suggested that the satellite gravity data have a resolution down to 16 18 km further the geoid residual geoid and gravity anomalies are integrated with seismic data along two profiles in the bay of bengal and arabian sea and inferences have been made in terms of density distributions at different depths the new residual geoid anomaly map shows excellent correlation with regional tectonic features such as sunda subduction zone volcanic traces chagoslaccadive ninetyeast and 85 e ridges etc and mid ocean ridge systems central indian and carlsberg ridges

dike swarms key to the reconstruction of major volcanic edifices the basic dikes of la gomera canary islands the detailed study of the age and distribution of the basic dikes of la gomera canary islands has allowed us to reconstruct the evolution of its main volcanic edifice the dikes of the oldest unit the basal complex have a n70 80 dominant direction at regional scale on the other hand the dikes of the post basal complex subaerial units are the result of local stress fields the oldest swarm is composed of sills dated at 10 2 9 3 ma four younger radial dike swarms have been identified s1 s2 s3 and s4 with ages of 9 1 8 4 ma 8 2 6 7 ma 5 5 4 4 ma and 5 3 4 0 ma respectively the reconstruction of the magmatic focus location using these swarms shows a migration southwards with an average speed of 1 6 mm year this temporal sequence of parallel swarms sills and radial swarms is a pattern frequently repeated during the building of large insular volcanic edifices in the canary islands as well as in other archipelagos

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

paleomagnetic constraints on deformation models for uppermost oceanic crust exposed at the hess deep rift implications for axial processes at the east pacific rise 1 studies of oceanic crust exposed in tectonic windows and in ophiolites have revealed the importance of normal faulting and attendant tilting of upper crustal rock units in the accretion process at oceanic spreading centers we present paleomagnetic remanence data from 45 fully oriented samples from dikes gabbros and a small number of basaltic lavas from fast spread crust exposed along the hess deep rift over 25 km along this escarpment dikes and dike subparallel fault zones dip consistently away from the east pacific rise epr while lava flows dip toward the ridge underlying gabbro is less deformed but contains widely spaced low angle fractures tentatively interpreted as shear zones as expected from the crustal age 1 07 1 48 ma most remanence data indicate reversed polarity magnetization and are compatible with the expected range of secular variation at the site overly steep and directionally scattered gabbro remanence and observed low angle shear structures within this unit are tentatively interpreted as the manifestation of three dimensional strain along anastomosing shear zones although some remanence directions are incompatible with any plausible deformation history and thus likely reflect orientation errors the overall data set is consistent with a model involving sequential rotations on 1 outward dipping epr parallel n s normal faults and 2 hess deep rift parallel e w normal faults average rotations for these sequential events are 22 to the east defined by the mean dike attitude and 10 to the south estimated by bathymetry respectively this model best explains the remanence data observed dikes and lava orientations presence of dike parallel fault zones and the observation of steep little deformed dikes cutting both east dipping dikes and faults the data support a structural model for spreading at the epr in which outcrop scale faulting and rotation is linked to subaxial subsidence and to consequent development of dominantly outward facing normal faults close to the spreading axis because these faults form within the neovolcanic zone they are subject to burial and are expected to have subdued to little surface expression

the felsic complex of the vallehermoso caldera interior of an ancient volcanic system la gomera canary islands abstract a mid to upper miocene felsic cone sheet dike complex caldera collapse and dome intrusion located around the vallehermoso and tamargada areas north of la gomera island is described it constitutes one of the less known volcanic episodes in the canary archipelago and especially in la gomera island based on detailed field and structural evidence it is suggested that the cone sheet shows at the surface an almost circular geometry truncated to the nw sector after the cone sheet dike complex formed decompression of the interior of an assumed shallow magma chamber occurred due to the subsequent migration of felsic magma throughout the dikes of the cone sheet this last event caused the roof of the magma chamber to collapse leading to the formation of a 3 to 4 km wide caldera a breccia deposit formed during the collapse and filled a depressed central area which finally was affected by an episode of magmatic resurgence leading to a later almost circular dome intrusion the association of cone sheet breccia deposits and domes constitutes the so called trachytic phonolitic complex tpc and should be viewed as the root of an ancient intensely eroded volcanic edifice made of trachytes nepheline phonolites and minor hauyne phonolites as the dominant materials a description of the structural petrological and geochemical features of the trachytic phonolitic complex and a discussion about their origin are considered in this paper we also consider the correlation between another felsic intrusion of alkali gabbros and syenites called tamargada syenites that also crops out in the vallehermoso tamargada area with respect to the trachytic phonolitic complex our data suggest that this alkaline intrusion of alkali gabbros and syenites belongs to an earlier intrusive episode on the basis of petrological geochemical and detailed field studies both trachytes and nepheline phonolites included in the trachytic phonolitic complex most likely evolved from alkali mafic magmas from the upper old basalt formation through a dominant fractional crystallization process additionally hauyne phonolites could develop from local and later magmatic processes such as gas transfer that altered and changed a small part of the previously mentioned nepheline phonolites to hauyne phonolites

modes of extension at oceanic spreading centers evidence from the solea graben troodos ophiolite cyprus abstract the solea graben in the troodos ophiolite cyprus represents a preserved cretaceous age axial valley formed during a brief period of amagmatic seafloor spreading structural analysis of all crustal levels of the graben reveals two periods of deformation an extensive early phase of extensional deformation associated with seafloor spreading and a later phase of minor deformation associated with obduction and doming of the complex during the tertiary recent amagmatic extensional deformation produced networks of normal faults that dip toward the graben axis and sole into a regional detachment fault located at the sheeted dike plutonic contact and oriented subparallel to ophiolite pseudostratigraphy individual fault segments are essentially planar but link to approximate listric geometries locally bulk structural rotation of fault blocks bounding the graben occurred about a nnw trending axis parallel to the present graben trend average dips in the sheeted complex within the central portions of the graben suggest 40 50 of structural rotation although this is a minimum figure because present dike dips are not a simple function of rotation magnitude tensor analysis of fault populations associated with extensional deformation shows that 3 was oriented approximately perpendicular to the graben axis late near axis extensional deformation segmented the detachment fault along nnw trending normal faults obduction related uplift and doming of the troodos ophiolite is expressed in the solea graben by minor thrust reverse and strike slip faults principally developed during reactivation of pre existing low angle fault zones formed during seafloor deformation tensor analysis of this later phase of faulting shows that 1 was oriented approximately n s and that 2 and 3 alternated position during the deformation the style of deformation documented in the solea graben supports recent speculation for basin and range style detachment faulting at slow spreading ridge crests with low magma budgets such as parts of the mid atlantic ridge near large offset transform faults natural cross sectional exposure of the solea graben shows how extensional structures might link at depth beneath such slow spreading ridge segments and demonstrates the importance of extensive brittle deformation of the crust prior to establishment of large scale ore forming hydrothermal circulation cells

deformation of the substratum of a large shield volcano triggering factor for past flank collapses in the old volcanic edifice of la gomera canary islands the basal complex of la gomera canary islands has been analyzed with the aim of deciphering its structural evolution spanning the middle to late miocene detailed structural maps and cross sections and the results of a systematic measurement of dike orientation and fault slip data are presented in this work the main structural features found in the basal complex of la gomera are four dike sets and two large normal faults guillama and montana de alcala faults several extensional episodes have been identified the main deformation phase corresponds to a nnw sse to nw se directed extension that generated first a dense swarm of mafic dikes and then a large scale collapse of the basal complex with displacement along the guillama and montana de alcala faults and associated rotation of large blocks of la gomera basement it is interpreted that the supposed staircase geometry of the guillama fault is responsible for the observed arrangement of rotated dikes due to development of kilometric scale fault bend folds with rounded hinges deformation of the basal complex shows a complete geometric kinematic and chronologic consistency with the large volcanic flank collapses that affected the units belonging to the early growth stages of la gomera subaerial shield volcano the results of this work support the models that invoke the importance of the large scale geodynamic setting on volcano destabilization more attention should be paid to the structural and geophysical characteristics of volcano basements in order to better evaluate the danger of large catastrophic volcanic landslide events

volcano core collapse triggered by regional faulting it has been proposed recently that large scale caldera like structures may result from the viscous lateral spreading of a hydrothermal system located in the central part of volcanic edifices for this to occur an unbutressed boundary resulting in reduced lateral stresses must exist or be created not far from the hydrothermal system different ways to achieve such a weak lateral confinement may be proposed in this paper we show that regional tectonics may provide an unconfined boundary if a normal fault lowers part of the volcanic edifice a natural example is the island of nuku hiva in the marquesas archipelago french polynesia for which geological data strongly suggest that a regional fault oriented n75 e is responsible for the present day shape of the island i e an ellipse cut in the middle by a straight side along which two nested calderas are breached analogue modelling shows the mechanical consistency of the model and reveals that very low velocity faulting is needed for the formation of a caldera like structure

multiple sector collapses at stromboli volcano italy how they work this paper demonstrates that four large sector collapses have affected the nw flank of the stromboli volcano in the past 13 ka alternating with growth phases in order to contribute to the evaluation of the critical conditions which trigger lateral collapses a reconstruction of the geometry of each collapse of the volcano edifice in the four stages that preceded the relative collapse events is also presented and a computation of the landslide volume this reconstruction is based on new field data plotted in three dimensions prior to the initial 13 ka collapse the volcano was 1125 100 m high above sea level the collapse had a volume of 2 23 0 87 km3 whereas the pre collapse volcano volume was 218 8 7 7 km3 the next edifice that failed was 900 70 m high a s l the collapse volume was 1 0 54 km3 with a precollapse volcano volume of 201 4 5 4 km3 the edifice then grew to 1000 60 m a s l the third collapse had a volume of 1 08 0 39 km3 and occurred within a volcano with a volume of 209 1 4 6 km3 this was followed by a new growth phase followed by the last collapse with a volume of 0 73 0 22 km3 the volcano volume was about the same as the present one the present active crater zone is at 780 m a s l in the first three collapses sliding surfaces cut the main magma conduit in the last collapse the upper scarp coincided with the conduit location dyking along a main ne trending weakness zone across the volcano summit exerted a lateral force for collapse inception the decrease of the landslide volumes with the age and the concentric scarps of the four collapses suggest that the younger sliding planes tended to become more superficial and to decrease the areal extent this is interpreted as due to a successively weaker eruptive products from dominantly lavas to dominantly pyroclastics and b the feedback effects between collapses and dykes that injected along the lateral segments of the first collapse slide plane

rift zone reorganization through flank instability in ocean island volcanoes an example from tenerife canary islands the relationship between rift zones and flank instability in ocean island volcanoes is often inferred but rarely documented our field data aerial image analysis and 40ar 39ar chronology from anaga basaltic shield volcano on tenerife canary islands support a rift zone flank instability relationship a single rift zone dominated the early stage of the anaga edifice 6 4 5 ma destabilization of the northern sector led to partial seaward collapse at about 4 5 ma resulting in a giant landslide the remnant highly fractured northern flank is part of the destabilized sector a curved rift zone developed within and around this unstable sector between 4 5 and 3 5 ma induced by the dilatation of the curved rift a further rift arm developed to the south generating a three armed rift system this evolutionary sequence is supported by elastic dislocation models that illustrate how a curved rift zone accelerates flank instability on one side of a rift and facilitates dike intrusions on the opposite side our study demonstrates a feedback relationship between flank instability and intrusive development a scenario probably common in ocean island volcanoes we therefore propose that ocean island rift zones represent geologically unsteady structures that migrate and reorganize in response to volcano flank instability

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

extensional tectonics and volcano lateral collapses insights from ollague volcano chile bolivia and analogue modelling understanding the process of volcanic lateral collapse which may be governed by numerous possible factors can be significantly improved by identifying the role of each factor here we test the perturbation induced in a volcano by tectonic normal faulting in the basement magma inflation and multiple flank deformation based on fieldwork and analogue experiments ollague volcano chile bolivia provides a good example of a sector collapse with exposed substrate extensional tectonics and our experiments have been done with conditions consistent with field data results show that none of the possible tectonic conditions led to complete lateral failure in agreement also with the field data sector collapse of the model cone occurred only when a shallow magmatic intrusion within the volcano was reproduced we also found out that volcano flank deformation is enhanced if a previous flank destabilization occurred interleaved by new cone growth terra nova 18 282 289 2006

mesozoic sequence of fuerteventura canary islands witness of early jurassic sea floor spreading in the central atlantic the fuerteventura jurassic sedimentary succession consists of oceanic and clastic deposits the latter derived from the southwestern moroccan continental margin normal mid oceanic ridge basalt n morb flows and breccias are found at the base of the sequence and witness sea floor spreading events in the central atlantic these basalts were extruded in a postrift environment post late pliensbachian we propose a toarcian age for the atlantic oceanic floor in this region on the basis of the presence higher up in the sequence of the bositra buchi filament microfacies aalenian bajocian and of clastic deposits reflecting tectono eustatic events e g late toarcian to mid callovian erosion of the rift shoulder the s 1 sea floor oceanic magnetic anomaly west of fuerteventura is therefore at least toarcian in age the remaining sequence records atlantic tethyan basinal facies e g callovian oxfordian red clays aptian albian black shales alternating with clastic deposits e g kimmeridgian berriasian periplatform calciturbidites and a lower cretaceous deep sea fan system the fuerteventura n morb outcrops represent the only early jurassic oceanic basement described so far in the central atlantic they are covered by a 1600 m nearly continuous sedimentary sequence which extends to upper cretaceous facies

the effect of volcanic constructs on rift fault patterns volcanoes in rift situations often show distinctive hour glass fault patterns with increased fault throw as the volcano is approached we present analogue models that show that this is caused by an interaction of the regional stress field with that set up by the volcano mass for faults to be reorientated there must be a ductile layer below the volcano this can be hot crust at mid ocean ridges continental rifts or weak sedimentary strata increased volcano size or mass and lower brittle ductile ratios lead to increased fault curvature whereas an increased regional extension rate decreases the effect volcanoes on one side of a rift may capture it forming the axis of a new rift with curved or en echelon zones on either side by concentrating extension magma is more easily erupted so volcanoes may erupt more magma of less evolved composition a positive feedback between increased extension and magma eruption rate will lead to rift narrowing which can favor the formation of oceanic crust

the impact of strike slip transtensional and transpressional fault zones on volcanoes part 1 scaled experiments the activity of a regional strike slip fault can affect or channel magma migration can deform a volcano and can destabilise the edifice flanks the aim of this study is to determine the location strike dip and slip of structures that develop in a stable or gravitationally spreading volcanic cone located in the vicinity of a fault with a strike slip component this problem is addressed with brittle and brittle ductile analogue models the one hundred and twenty three models were deformed by pure strike slip transtensional or transpressional fault displacements the deformation was organized around an uplift in transpressional and strike slip experiments and around a subsiding area in transtensional experiments most displacements are accommodated by a curved fault called sigmoid i structure which is a steep transpressional to transtensional fault this fault projects the regional fault into the cone and delimits a summit graben that is parallel to the main horizontal stress the systematic measurements of faults strike and slip in the experiments indicate that extension along the faults in the cone increases with the extensional component of the regional fault and the thickness of the substratum ductile layer the distribution of the fastest horizontal movements of the analogue cone flanks which vary depending on the regional fault characteristics and on the composition of the substratum correspond to the distribution of instabilities in nature natural examples of volcanoes sited in strike slip contexts are described and interpreted in the light of the analogue results in the second article

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

deformation structures associated with the tazo landslide la gomera canary islands deformation structures below the basal plane of gravitational slides can provide useful information about the state of stress undergone by rocks prior to the sliding process and about the triggering forces acting at each particular sliding event in the present work we conducted a structural analysis of the rocks below the surface of the gravitational slide of tazo la gomera canary islands and determined the epigenetic processes involved in the filling of the amphitheatre we also inferred the possible triggering phenomena related to the tazo landslide the rocks located below the surface of the gravitational slide of tazo i e the basaltic lava flows sills and dikes of the lower old edifice and the submarine volcanic rocks gabbros pyroxenites and dikes of the basal complex of la gomera are strongly deformed close to this sliding surface the lava flows and dikes of the lower old edifice are folded with fault breccias and gouges and locally foliated defining the sliding surface the dikes of the basal complex are also folded and the gabbros and pyroxenites are affected by a large number of small faults in the basal complex the sliding surface is defined by a foliated granular gouge in the damage zone the basal complex rocks show an incipient fracture cleavage the sliding amphitheatre has been filled by the debris avalanche or cohesive debris flow generated within the slide as well as by later debris flows hyperconcentrated flows sheet flows and by interspersed lava flows from the upper old edifice we suggest here that the collapse of the north western flank of the lower old edifice at tazo could in part have been triggered by continuous magma injection associated with the emplacement of dikes in a rift zone with an ene wsw direction enhanced by the mechanical weakness of the basal complex unit which was affected by hydrothermal metamorphism under greenschist facies conditions and by the displacement along the montana de alcala and guillama normal faults which are deeply entrenched in the altered rocks of the basal complex

extension induced by volcanic loading in regional strike slip zones volcanoes and strike slip pull apart structures are commonly found together we use theoretical predictions and analogue modeling to evaluate the effect of volcanic loading in strike slip zones and the converse effect of regional strike slip faults on volcano structure theoretical predictions indicate that regional stress is reoriented by volcano loading to cause extension analogue models indicate that volcanoes in strike slip zones develop extensional pull apart structures a graben typically forms normal to the minimum regional stress axis 3 at a cone cones emplaced onto preexisting faults increase that fault9s tensional component if the substratum is weak enough to permit volcano spreading thrusts form normal to the maximum stress axis 1 a feedback situation can arise i e loading related extension allows more magma influx more eruption and thus increased loading this process can create large volcano tectonic depressions volcano structures may also be used as indicators of regional strike slip movements

seismic characteristics of fluid escape pipes in sedimentary basins implications for pipe genesis abstract fluid escape pipes were first documented from 3d seismic data over a decade ago and have subsequently been identified in many petroliferous basins worldwide they are characterized on seismic data by vertical to sub vertical zones of reduced reflection continuity that have a columnar geometry in three dimensions the upper terminations of these pipes commonly coincide with pockmarks or palaeo pockmarks signifying a close connection of pipe formation with a high flux fluid expulsion process dimensions range from tens to hundreds of metres in diameter and hundreds to over a thousand metres in height and the slenderness ratio defined as height diameter ranges from 0 8 to over 20 pipes are frequently associated with sub vertical clustering of amplitude anomalies on seismic data related either to the presence of free gas or to cementation linked to the passage of hydrocarbons three mechanisms have been suggested to explain pipe genesis 1 hydraulic fracturing 2 erosional fluidisation and 3 capillary invasion we suggest a further two possible mechanisms in the form of localised collapse by volume loss and synsedimentary flow localisation we review all five mechanisms and conclude that it is unlikely that a single mechanism applies but that combinations of these processes may all occur in particular contexts fluid escape pipes may be far more widespread that currently appreciated and they may play a critical role in secondary hydrocarbon migration and in providing leakage pathways for trapped hydrocarbons through overlying seals

the felsic dikes of la gomera canary islands identification of cone sheet and radial dike swarms on the northern part of la gomera there exists a great abundance of trachytic phonolitic dikes showing a broad diversity in dip and strike several methods have been applied in order to separate these dikes in different sets localise the area from where they derive and reconstruct the geometry of the swarms the oldest dikes correspond to a radial swarm dated at 8 ma the felsic activity migrated then southwestwards and a second radial swarm and a cone sheet complex were developed between 7 5 and 6 4 ma ago the cone sheet complex is 10 km in diameter and shared its centre with that of the second radial structure the cone sheets exhibit an outward decrease of dip angle whilst every individual sheet maintains a constant inclination this geometry reflects the existence of an ancient single dome shaped shallow magma chamber situated some 1650 m below present sea level the eastern radial swarm represents a felsic episode that could mark the ending of the lower old basalts the earlier subaerial activity of la gomera the two other dike swarms represent a younger episode coeval with the upper old basalts

geological evolution of the volcanic island la gomera canary islands from analysis of its geomorphology abstract erosion and landslide processes played an important role in shaping the geomorphology of the canary islands but for the heavily eroded la gomera island those processes have been unclear we conducted an integrated study of its offshore extension onshore geology and links between them in order to determine the geomorphological evolution of the island relative to the rest of the archipelago the characteristics of the island s barrancos and its interfluves have led us to differentiate four types of morphological areas whose fluvial networks are in different stages of evolution those barrancos offshore are interrupted by la gomera s shelf and its erosion has been much greater than for the rest of the canary islands such erosion is both a function of time and varied swell action la gomera s insular slope is carved by a system of submarine canyons and channels that have morphology indicative of turbidity current erosion many of the barrancos onshore and the canyons offshore have steps or knickpoints some are the result of varied resistance to erosion and others are explained by other mechanisms a ridge whose crest is covered by cone like shape structures is interpreted as of volcanic origin with the particularity that it lacks onshore continuation the opposite of what has been observed elsewhere in the canary islands also in contrast with the other islands of the archipelago la gomera s margin does not seem to have resulted from destructive mass wasting and the only offshore sign of catastrophic events are the presence of two embayments along la gomera s northern shelf edge and a lobe structure on the insular apron at 3000 m depth these features may be the remains of the catastrophic failures that took place on the island 9 4 8 7 ma ago alternatively they could be due to recent catastrophic failures on the upper slope due to sediment steeping since the conclusion of the main volcanic activity 4 0 ma ago la gomera has been intensively eroded by gradual fluvial denudation and secondary failures la gomera s insular margin reveals much regarding its evolution but more comprehensive studies including seismic and coring are needed to understand its whole geological history

subseafloor origin for broken hill pb zn ag mineralization new south wales australia pb isotope analyses combined with sensitive high resolution ion microprobe u pb zircon dating suggest that the giant broken hill orebody formed over an 6 m y period coincident with a number of small broken hill type deposits lower in the stratigraphic sequence the formation of the smaller but significant pinnacles deposit was also episodic over 7 m y and up to 10 m y prior to the formation of broken hill using relative ages and stratigraphic position together with lithologic and isotopic variation a subsea floor origin for the deposits including the broken hill pb zn ag orebody is proposed mn p rich banded iron formation stratigraphically above the orebody may have been derived from spent ore forming fluids exhaled onto the seafloor extensive magnetite disseminations in metasedimentary rocks at the same level as the banded iron formation probably represent distal signatures of the same hot springs

volcano spreading controlled by dipping substrata most volcanoes grow on slopes and some tend to fail catastrophically on the downslope side many volcanoes also deform by volcano spreading which may lead to failure we look at the effect of dipping substrata on the potential for spreading and collapse with analogue models the dip is found to strongly control the spreading style rate and direction a distinct change from purely radial spreading occurs even with small 1 substrata tilt structures on the cone and surrounding area are modified according to the underlying dip direction spreading becomes concentrated on downslope sectors where movement is predominantly in the dip direction the degree of structural realignment is a function of the slope angle our models are applied to previously known and new examples of volcano spreading the effect of dipping substrata in confining spreading to sectors increases the potential for deep seated sector collapses this finding provides a mechanism for failure on the downslope side the incorporation of substrata can create very large volume collapses