major element trace element and sr nd isotopic geochemistry and genesis of varto mus volcanic rocks eastern turkey abstract major element trace element and sr nd isotopic data are presented for the upper miocene quaternary varto volcanic rocks eastern turkey and their relationship to the regional tectonic regime were investigated in this study the varto volcanic rocks are located in the eastern part of the north anatolian fault zone nafz in the east anatolian convergence region eacr volcanic rocks are subdivided into two groups lower volcanic units lvu and upper volcanic units uvu these volcanic rocks are composed of volcaniclastics and lavas of both alkaline and calc alkaline characters lvu lavas are dacite andesite trachyte basaltic andesite benmoreite and mugearite whereas uvu lavas are basalt basaltic andesite mugearite hawaite benmoreite and basanite in composition variation diagrams of sio2 with major and trace elements are consistent with fractional crystallization process involving olivine plagioclase pyroxenes and fe ti oxides both units appear to be on the same fractionation trend implying similar origins but different degrees of evolution correlation between sr nd isotopes with sio2 mgo rb sr 100 sr and nd sm emphasize the effect of crustal contamination on these volcanic rocks sr nd isotopic data plot along the mantle array however rocks of lower and upper volcanic units are isotopically distinct uvu have relatively low 87 sr 86 sr 0 70334 0 70466 and high 143 nd 144 nd ratios 0 51279 0 51288 which plot in the field of oceanic island basalts ararat lavas and the eastern anatolian alkaline province in contrast lvu exhibit higher 87 sr 86 sr 0 70438 0 70507 and lower 143 nd 144 nd 0 51265 0 51276 ratios all the varto volcanic rocks are products of mantle derived magma and have experienced assimilation and fractional crystallization afc during uprise through the continental crust and formed in a within plate environment along the eastern part of the nafz that is closely related to deep lithospheric fractures

collapse driven large eruptions abstract for a typical poroelastic shallow crustal magma chamber about 0 1 of the mafic magma and about 4 of the felsic magma are erupted and or injected during magma chamber rupture magma chambers with volumes of the order of several tens to several hundred cubic kilometres as are common are thus sufficiently large to supply magma to small or moderate eruptions for large eruptions however a much higher percentage of the magma volume must be squeezed out of the chamber for an ordinary poroelastic chamber the excess pressure in the chamber falls exponentially during the eruption for a large eruption to be possible however the excess pressure must be essentially maintained until the very end of the eruption here i show that caldera collapse can maintain the excess pressure through forced magma chamber volume reduction in which case a resulting large eruption would be the consequence not the cause of the collapse i also show that ring fault dip partly controls the size and intensity volumetric flow or effusion rate of the eruption if the ring fault dips inward a normal fault the displacement is stable the volumetric flow rate intensity remains essentially constant during the collapse and the magma chamber remains active following the collapse by contrast if the ring fault dips outward a reverse fault the displacement is unstable the volumetric flow rate normally increases dramatically during the collapse and the magma chamber may be entirely destroyed during the collapse

fluid overpressure estimates from the aspect ratios of mineral veins abstract several hundred calcite veins and mostly normal faults were studied in limestone and shale layers of a mesozoic sedimentary basin next to the village of kilve at the bristol channel sw england the veins strike mostly e w 239 measurements that is parallel with the associated normal faults the mean vein dip is 73 n 44 measurements field observations indicate that these faults transported the fluids up into the limestone layers the vein outcrop trace length 0 025 10 3 m and thickness 0 1 28 mm size distributions are log normal taking the thickness as the dependent variable and the outcrop length as the independent variable linear regression gives a coefficient of determination goodness of fit of r2 0 74 significant with 99 confidence but natural logarithmic transformation of the thickness length data increases the coefficient of determination to r2 0 98 indicating that nearly all the variation in thickness can be explained in terms of variation in trace length the geometric mean of the aspect length thickness ratio 451 gives the best representation of the data set with 95 confidence the true geometric mean of the aspect ratios of the veins lies in the interval 409 497 using elastic crack theory appropriate elastic properties of the host rock and the mean aspect ratio the fluid overpressure that is the total fluid pressure minus the normal stress on the fracture plane at the time of vein formation is estimated at around 18 mpa from these results and using the average host rock and water densities the depth to the sources of the fluids below the present exposures forming the veins is estimated at between around 300 m and 1200 m these results are in agreement to those obtained by independent isotopic studies and indicate that the fluids were of rather local origin probably injected from sill like sources water sills inside the sedimentary basin

discovery of a magma chamber and faults beneath a mid atlantic ridge hydrothermal field crust at slow spreading ridges is formed by a combination of magmatic and tectonic processes with magmatic accretion possibly involving short lived crustal magma chambers the reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast spreading centres but it has been difficult to image such magma chambers beneath slow spreading centres owing to rough seafloor topography and associated seafloor scattering in the absence of any images of magma chambers or of subsurface near axis faults it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow spreading ridges here we report the presence of a crustal magma chamber beneath the slow spreading lucky strike segment of the mid atlantic ridge the reflection from the top of the magma chamber centred beneath the lucky strike volcano and hydrothermal field is approximately 3 km beneath the sea floor 3 4 km wide and extends up to 7 km along axis we suggest that this magma chamber provides the heat for the active hydrothermal vent field above it we also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward dipping faults cutting through the volcanic edifice suggesting continuous interactions between tectonic and magmatic processes

paleomagnetism of jurassic radiolarian chert above the coast range ophiolite at stanley mountain california and implications for its paleogeographic origins upper jurassic red tuffaceous chert above the coast range ophiolite at stanley mountain california lat 35 n long 240 e contains three components of remanent magnetization the first component a removed by 100 200 c has a direction near the present day field for southern california and is probably a recently acquired thermoviscous magnetization a second component b removed between 100 and 600 c is identical to that observed by previous workers in samples of underlying pillow basalt and overlying terrigenous sedimentary rocks this component has constant normal polarity and direction throughout the entire section although these rocks were deposited during a mixed polarity interval of the geomagnetic field the b magnetization therefore is inferred to be a secondary magnetization acquired during accretion uplift or miocene volcanism prior to regional clockwise rotation the highest temperature component c removed between 480 and 680 c is of dual polarity and is tentatively interpreted as a primary magnetization although it fails a reversal test possibly due to contamination by b separation of the b and c components is best shown by samples with negative inclination c directions and a corrected mean direction using only these samples indicates an initial paleolatitude of 32 n 8 paleobiogeographic models relating radiolarian faunal distribution patterns to paleolatitude have apparently been incorrectly calibrated using the overprint b component few other paleomagnetic data have been incorporated in these models and faunal distribution patterns are poorly known and mostly unquantified the available data therefore do not support formation of the coast range ophiolite at stanley mountain near the paleoequator or accretion at 10 n paleolatitude as has been previously suggested based on paleomagnetic data but indicate deposition near expected paleolatitudes for north america 35 n 4 during late jurassic time

regional controls on magma ascent and storage in volcanic arcs understanding the controls for magma ascent and storage depth is important for volcanic hazard assessment regional differences in the depths of magma storage between volcanic arcs suggest that the settings of subduction zones and of overriding plates influence how magma ascends through the crust here we use a compilation of data for 70 volcanoes in 15 volcanic regions to better understand the geodynamic controls on magma storage we describe the subduction system which consists of the subducting slab the mantle wedge and the upper plate with 12 parameters encompassing the kinematics of the subduction the structure and geometry of the slab the timing of the subduction the thermal structure of the slab the upper plate crustal structure its stress regimes and its thermal structure we find that the magma reservoir depths correlate with the upper plate crustal structure and with the stress regimes shallow reservoirs 45 km similarly shallow magma reservoirs are 33 69 more common in extensional and strike slip stress regimes that in compressional regimes this illustrates the effect of buoyancy for magma ascent as well as the importance of stress regime and preexisting structures

numerical modelling of dykes deflected into sills to form a magma chamber abstract most shallow magma chambers are thought to evolve from sills for this to happen several conditions must be met 1 there must be a discontinuity normally a contact that deflects a dyke or an inclined sheet into a sill 2 the initial sill must have a considerable thickness normally depending on dyke injection rates not less than some tens of metres 3 the resulting sill must receive magma through dykes frequently enough so as to stay liquid and expand into a chamber 4 the resulting magma chamber must remain at least partially molten and receive multiple magma injections over a given period of time to build up a volcano on the surface above in this paper we present numerical models based upon field data and geophysical data as to how sills are emplaced and may subsequently evolve into shallow magma chambers we suggest that most sills form when dykes meet contacts particularly weak ones which are unfavourable to dyke propagation a contact may halt arrest a dyke altogether or alternatively deflect the dyke into the contact the three main mechanisms for dyke deflection into a contact are 1 the cook gordon debonding or delamination 2 rotation of the principal stresses generating a stress barrier and 3 an elastic mismatch across a contact between adjacent layers elastic mismatch means that the layers have contrasting young s moduli and varying material toughness once a sill is initiated the developing magma chamber may take various forms many shallow magma chambers however tend to maintain a straight sill like or somewhat flat oblate ellipsoidal geometry during their lifetimes for a sill to evolve into a magma chamber there must be elastic plastic deformation of the overburden and to some extent of the underburden so long as the sill stays liquid subsequent dyke injections become arrested on meeting the sill some magma chambers develop from sill complexes for the sill complex to remain partially molten it must receive a constant replenishment of magma implying a high dyke injection rate alternatively an initial comparatively thick sill may absorb much of the magma of the dykes that meet it and evolve into a single shallow magma chamber

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

seismic velocity and poisson s ratio tomography of the crust beneath east anatolia abstract eastern anatolia is a region in the early stages of continent continent collision and so provides a unique opportunity to study the early development of continental plateau located within the alpine himalayan fold thrust fault belt the anatolian plateau is geologically very complex with over half of the surface area covered with late cenozoic volcanics of diverse composition the plateau is also seismically active and is dissected by numerous seismogenic faults predominantly of strike slip motion in this study we determine 3 d tomographic images of the crust beneath eastern anatolia by inverting a large number of arrival time data of p and s waves from the obtained p and s wave velocity models we estimated the poisson s ratio structures for a more reliable interpretation of the obtained velocity anomalies our tomographic results are generally consistent with the major tectonic features of the region high p and s wave velocity anomalies are recognized near the surface while at deeper crustal layers low seismic wave velocities are widely distributed poisson s ratio exhibits significant structural heterogeneities compared to the imaged velocity structure the seismic activity is intense along highly heterogeneous zones and is closely associated with pre existing faults in the central and western parts of the study area results of the checkerboard resolution test indicate that the imaged anomalies are reliable features down to a depth of about 40 km the low velocity high poisson s ratio zones in the middle to lower crust are consistent with many geophysical observations such as strong sn attenuation low pn and sn velocity and the absence of mantle lid implying the presence of partial melt in the uppermost mantle

mechanical interaction between active volcanoes in iceland 1 we test the possibility of mechanical interaction between eight central volcanoes in the central part of the active iceland rift zone the average distance between the volcanoes is 30 km all are thought to have shallow magma chambers and many contain collapse calderas we analyzed many finite element models with the volcanoes subject to a tensile stress of 5 mpa equal to the maximum in situ tensile strength of the crust in a direction parallel to the spreading vector n105 e the results show zones between many nearby volcanoes where the tensile stresses exceed the in situ tensile strength of the crust the results indicate that mechanical interaction between volcanoes in a pair such as simultaneous dike emplacement seismogenic faulting and deformation may be common in this part of iceland in agreement with observations

geometric difference between non feeder and feeder dikes feeder dikes bring magma to the surface non feeder dikes become arrested and never reach the surface the differences if any between these dike types remain largely unexplored because in the field it is normally unknown if a particular dike is a feeder or non feeder here we present measurements of feeder and non feeder dikes exposed from depths of 200 m to the surface in the walls of the a d 2000 caldera collapse of the miyakejima volcano japan a typical feeder thickness reaches a maximum of 2 4 m at the surface decreases rapidly to 1 m at a depth of 20 40 m and then remains constant to the bottom of the exposure by contrast a typical non feeder thickness reaches a maximum of 1 5 2 m at 15 45 m below the tip and then decreases slowly with depth to 0 5 1 m at the bottom of the exposure we propose that free surface effects and magmatic overpressure driving pressure changes during the eruption cause the overall shape of a feeder to differ from that of a non feeder

hotspot volcanism close to a passive continental margin the canary islands the canarian archipelago is a group of volcanic islands on a slow moving oceanic plate close to a continental margin the origins of the archipelago are controversial a hotspot or mantle plume a zone of lithospheric deformation a region of compressional block faulting or a rupture propagating westwards from the active atlas mountains fold belt have been proposed by different authors however comparison of the canarian archipelago with the prototypical hotspot related island group the hawaiian archipelago reveals that the differences between the two are not as great as had previously been supposed on the basis of older data quaternary igneous activity in the canaries is concentrated at the western end of the archipelago close to the present day location of the inferred hotspot this is the same relationship as seen in the hawaiian and cape verde islands the latter archipelago associated with a well defined but slow moving mantle plume shows anomalies in a plot of island age against distance which are comparable to those seen in the canary islands these anomalies cannot therefore be used to argue against a hotspot origin for the canaries individual islands in both archipelagoes are characterized by initial rapid growth the shield building stages of activity followed by a period of quiescence and deep erosion erosion gap which in turn is followed by a post erosional stage of activity the absence of post shield stage subsidence in the canaries is in marked contrast with the major subsidence experienced by the hawaiian islands but is comparable with the lack of subsidence evident in other island groups at slow moving hotspots such as the cape verdes comparison of the structure and structural evolution of the canary islands with other oceanic islands such as hawaii and reunion reveals many similarities these include the development of triple mercedes star rift zones and the occurrence of giant lateral collapses on the flanks of these rift zones the apparent absence of these features in the post erosional islands may in part be a result of their greater age and deeper erosion which has removed much of the evidence for their early volcanic architecture we conclude that the many similarities between the canary islands and island groups whose hotspot origins are undisputed show that the canaries have been produced in the same way

form and dimensions of dykes in eastern iceland abstract a study was made of about 700 dykes in eastern iceland the majority of these belong to three swarms about 73 dip within 10 of the vertical most strike between 10 and 40 ne the strike of the dykes within the southernmost swarm alftafjordur changes along its trace from almost n at the north end to nne ssw southward along the swarm the average thickness of the dykes is about 4 1 m and the thickness does not change notably along the alftafjordur swarm the thinner dykes tend to have smaller grains than the thicker dykes of five dykes followed laterally the longest is over 22 km the thickness of individual dykes changes irregularly along their length and the dyke is often offset where its thickness changes many dykes appear to be completely discontinuous but some parts are connected by veins where the dykes end in a vertical section most of them simply taper away only about 10 5 of the dykes occupy faults the mechanical and thermal effects of the dykes on the country rock are small many of the dykes appear to be non feeders i e dykes that never reached the surface to feed lava flows using the length width ratio the depth of origin of three dykes has been estimated the maximum depth of origin of these three dykes is 7 5 km 9 km and 10 km below the original surface

the regional geology and evolution of the toba volcano tectonic depression indonesia the late quaternary toba volcano tectonic depression in the sunda arc is the largest resurgent cauldron in the world and lies in one of the largest ignimbrite fields the toba tuffs although composed of at least 3000 km 3 of acid tuffs spread over 20000 km 2 of mainland sumatra the greater part of the toba tuffs is believed to be a single ignimbrite cooling unit formed about 100 000 years ago the toba tuffs also include a 30000 year old airfall tuff which was erupted from a centre just n of the toba depression the toba depression developed through the same stages as other resurgent cauldrons it formed after lithification of the toba tuffs by collapse along regional faults crossing an eastwards bulge in the sumatran geanticline resurgent uplift raised lake sediments within the depression by about 500 m the uplift which was not confined to the depression was probably caused by continuing influx of magma and it is possible that the toba depression is still in the stage of post resurgence volcanism the eruption of the toba tuffs and of all post ignimbrite volcanism took place on the line of the western marginal fault of the depression this marginal fault once extended northwards offshore into a zone of miocene back arc rifting

deep magma storage at hekla volcano iceland revealed by insar time series analysis 1 hekla volcano is one of the most active volcanoes in iceland the most recent eruption occurred from 26 february to 8 march 2000 when about 0 19 km3 of magma was erupted we present deformation data from multitemporal analyses of synthetic aperture radar sar images acquired between 1993 and 2008 focusing on pixels with low phase variance using persistent scatterer and small baseline approaches prior to and after the 2000 eruption we find a broad area of inflation around the volcano radius about 20 km with satellite line of sight los shortening of up to 5 mm yr we interpret this signal as the result of pressure increase in a deep seated magma chamber which we model as a spherical source at 14 20 km depth increasing in volume by 0 003 0 02 km3 yr within a 6 km radius of the summit of the volcano a los lengthening is superimposed on the broad inflation signal which correlates partly with recent lava flows we interpret this signal as the result of thermally contracting lava flows combined with viscoelastic yielding due to the load of the volcano and its lavas coeruptive deflation during the 2000 eruption was similar to the cumulative inflation from 1993 to 2000 and is consistent with a spherical magma chamber at 14 18 km depth that decreases in volume by 0 04 0 08 km3 interferograms spanning the 2000 eruption show a local coeruptive deformation signal near the eruptive fissure this is consistent with a dike opening from the surface to depths up to 5 8 km with a volume of 0 005 0 006 km3

inverse modeling for estimating fluid overpressure distributions and stress intensity factors from an arbitrary open fracture geometry abstract we present a new solution for estimating the fluid overpressure driving pressure or net pressure acting on the walls of a fracture with an arbitrary opening displacement in the paper we first present a forward modeling solution using fourier cosine series for the opening displacement of a fracture subject to an overpressure that varies irregularly along the length of a fracture by changing the form of the solution we provide a matrix equation for estimating the fourier coefficients and thereby obtain the overpressure variation from the fracture geometry as numerical tests of this inverse analysis we estimated the overpressure variation from fracture opening displacements given by well known analytical solutions and found that this method can be used for overpressure estimates for a variety of fluid driven fractures we apply our solutions to a mineral vein hosted by gneiss west norway and conclude from the aperture variation that at the time of vein formation the overpressure increased toward the vein tip we also discuss the physical meaning of the fourier coefficients by applying our results to man made hydraulic wing fractures the results indicate that the coefficients of n 0 and n 1 relate to the fluid overpressure and the critical stress intensity factor

evolution of the ocean crust results from recent seismic experiments we present results from recent analyses of seismic refraction and sea floor microseismicity studies in the pacific and atlantic oceans which lend support to the hypothesis that processes responsible for the construction of ophiolite suites are similar to phenomena extant at midocean ridges seismicity at fast spreading ridges is characterized by very low magnitude 0 1 and shallow 2 3 km microearthquakes and long term oscillations or harmonic tremor a detailed seismic refraction experiment on a fast spreading portion of the east pacific rise supports the hypothesis of the existence of a crustal magma chamber analyses of these data indicate that the chamber is largely unperturbed by the presence of conjugate spreading centres and the inverted deltashaped zone of partial melt is characterized by a half width in excess of 6 km finally a new approach for obtaining upper crustal velocities when applied to data characterized by the presence of shear waves provides several counter examples to the hypothesis that the shallow crust evolves with time

distributed transpressive continental deformation the varto fault zone eastern turkey the convergence between the eurasian and arabian plates has created a complicated structural setting in the eastern turkish high plateau ethp particularly around the karliova triple junction ktj where the eurasian arabian and anatolian plates intersect this region of interest includes the junction of the north anatolian shear zone nasz and the east anatolian shear zone easz which forms the northern border of the westwardly extruding anatolian scholle and the western boundary of the ethp respectively in this study we focused on a poorly studied component of the ktj the varto fault zone vfz and the adjacent secondary structures which have complex structural settings through integrated analyses of remote sensing and field observations we identified a widely distributed transpressional zone where the varto segment of the vfz forms the most northern boundary the other segments namely the leylekdag and caycati segments are oblique reverse faults that are significantly defined by uplifted topography along their strikes the measured 515 and 265 m of cumulative uplifts for mt leylek and mt dodan respectively yield a minimum uplift rate of 0 35 mm a for the last 2 2 ma the multi oriented secondary structures were mostly correlated with the distributed strike slip and the distributed transpressional in analogue experiments the misfits in strike of some of secondary faults between our observations and the experimental results were justified by about 20 to 25 clockwise restoration of all relevant structures that were palaeomagnetically measured to have happened since 2 8 ma ago our detected fault patterns and their true nature are well aligned as being part of a transpressional tectonic setting that supports previously suggested stationary triple junction models

using numerical modeling to explore the origin of intrusion patterns on fernandina volcano galapagos islands ecuador 1 using parameterized finite element models we investigate the emplacement of both radial and circumferential intrusions in the configuration observed at fernandina volcano in the galapagos islands when situated within the edifice at depths consistent with petrological and surface displacement data inflation of a mildly oblate magma reservoir to the point of rupture can initiate either radial or circumferential intrusions in response to minor volcanologically plausible variations in reservoir geometry i e aspect ratio in addition more oblate reservoirs inject lateral sills into an inflation derived stress field consistent with rotation about their propagation axis to form gently dipping radial dikes a mode of behavior recently inferred from insar data at fernandina all three styles of intrusion occur in near surface configurations consistent with field observations

thermomechanics of shallow magma chamber pressurization implications for the assessment of ground deformation data at active volcanoes abstract in this study we utilize thermomechanical models to investigate how magma chambers overpressurize as the result of either magmatic recharge or volatile exsolution by implementing an adaptive reservoir boundary condition we are able to track how overpressure dissipates as the magma chamber expands to accommodate internal volume changes we find that the size of the reservoir greatly impacts the resultant magma chamber overpressure in particular overpressure estimates for small to moderate sized reservoirs 1 10 km 3 are up to 70 lower than previous analytical predictions we apply our models to santorini volcano in greece where recent seismic activity and ground deformation observations suggested the potential for eruption the incorporation of an adaptive boundary condition reproduces mogi flux estimates and suggests that the magma reservoir present at santorini may be quite large furthermore model results suggest that if the magma chamber is 100 km 3 overpressures generated due to the high magma flux may not exceed the strength of the host rock thus requiring an additional triggering mechanism for eruption although the adaptive boundary condition approach does not calculate the internal evolution of the magma reservoir it represents a fundamental step forward from elastic mogi models and fixed boundary solutions on which future investigations of the evolution of the magma can be built

review of the neotectonics of the eastern turkish armenian plateau by geomorphic analysis of digital elevation model imagery digital elevation model dem images provide synoptic views of the earth s surface allowing the analysis of landforms of still active tectonic and volcanic structures at regional scale a dem at 250 m pixel size constitutes regional scale data particularly efficient to investigate the late miocene quaternary deformation of the eastern turkish armenian plateau in the arabian eurasian area of convergence geomorphic analysis of the dem image associated with review of fault plane solutions of earthquakes show that faults are mostly strike slip with small vertical component here we show that the orientations of the tectonic and volcanic structures fit with a tectonic regime characterized by n s shortening and e w lengthening consistent with westward escape of anatolia perpendicular to the direction of the arabia eurasia shortening the uniform uplift of the plateau the predominance of strike slip faulting the lack of major thrusts and the occurrence of normal faults do not support a model of going on crustal thickening due to intracontinental convergence on the contrary our observations can be better interpreted in terms of lithospheric thinning and mantle upwelling related to gravity escape of anatolia

dynamics of volcanic systems in iceland example of tectonism and volcanism at juxtaposed hot spot and mid ocean ridge systems volcanic systems are swarms of tectonic fractures and basalt volcanoes formed as a result of plate pull as the plates are pulled apart associated with the mid ocean ridges and the magma dynamics of the iceland mantle plume most systems are 40 150 km long 5 20 km wide and develop a central volcano they supply magma to all eruptions in iceland data obtained in the last few years have greatly improved our knowledge of their volcanotectonic environment as a result the geometry of the plume is better constrained and the crust previously considered thin 10 km is now modeled as thick 20 40 km depending on the location of the volcanic systems their activity either decreases or increases faulting in the two main seismic zones from this we can infer that emplacement of the feeder dike to the largest historical eruption in iceland that of laki in 1783 increased shear stress in the south iceland seismic zone and almost certainly triggered the largest m 7 1 in 1784 historical earthquake in ic

eruption styles of basalt in oceanic spreading ridges and seamounts effect of magma temperature and viscosity basalts erupted along oceanic zones of spreading and on intraplate seamounts at depths where hydroexplosions are not likely follow various styles of eruption quiet eruptions produce a variety of forms including lava lakes and sheet pillow and pavingstone lavas reactive eruptions occur instead when the lava is thermally shattered as in the formation of hyaloclastites the stresses induced in a rapidly cooling lava flow are important in determining the different lava forms with lava lakes and sheet flows resulting from low stresses pillows from higher stresses and hyaloclastites from even higher stresses which cause rupture calculations show that if we consider basaltic lavas of the same composition the intensity of the stresses induced in the cooling lava increases proportionally to the initial viscosity and inversely to the temperature and rate of discharge of the lava at eruption the viscosity is strongly dependent on the crystal liquid ratio of the magma at eruption the temperature and viscosity of the lava at eruption are related ultimately to the thermal regime of the lithosphere below viscosity tends to increase going from fast to slow spreading ridges and to intraplate seamounts within a given ridge segment temperature tends to decrease and viscosity to increase approaching a transform or overlapping boundary thus the model predicts that going from fast to slow spreading ridges i e from east pacific rise to mid atlantic ridge the probability of forming lava lakes and sheet flows decreases and the probability of forming pillow flows increases moreover within a given ridge segment the areal sheet to pillow ratio should decrease as a transform or overlapping boundary is approached sheet flows should be significant only in the earliest stages of the growth of large intraplate seamounts pillows and hyaloclastites in later stages the probability of forming hyaloclastites should be low in spreading ridges higher in intraplate seamounts recent high resolution observations on the east pacific rise the galapagos rift the mid atlantic ridge and the red sea are consistent with the model because they show that 1 the areal ratio of lava lakes and sheet flows over pillows decreases going from fast to slow spreading ridges 2 within a given ridge segment it decreases approaching a transform and 3 hyaloclastites are important on seamounts only

fluid overpressure and stress drop in fault zones using the average aspect length maximum thickness ratios of 379 mineral filled extension mode i veins from an active fault zone the fluid overpressure during their development with reference to the minimum compressive principal stress 3 is estimated at 20 mpa emplacement of such veins increases 3 and can generate a temporary stress barrier to the propagation of subsequent hydrofractures on meeting a subhorizontal stress barrier vertically propagating hydrofractures may change into water sills where the fluid pressure is at or above lithostatic in this model stress barriers and thus water sills can form at any depth in and in any type of fault zones for such a high fluid pressure the product of the coefficient of sliding friction and the normal stress in the modified griffith criterion becomes essentially zero and the driving stress associated with faulting equal to twice the in situ tensile strength of the host rock for typical in situ tensile strengths of 2 3 mpa the driving stresses for slip on overpressured fault planes is 4 6 mpa these results are in good agreement with the commonly measured average static stress drops of 3 6 mpa during earthquakes

a new mode of inner volcano growth the flower intrusive structure abstract a multiple sill laccolith nested within a centrally dipping sheet swarm is a recently discovered sub volcanic structure resembling a flower in section view which we have found in some eroded volcanoes in iceland suggesting that this structure can have some general relevance in deforming rock successions in volcanic areas after a brief summary of the main characteristics of these examples with details on stardalur volcano we present an evolutionary model to explain the whole structure both sills and sheets are fed by multiple pulses of magma following radial paths from a shallow chamber the earlier vertical set of the radial sheets above the middle of the chamber are abruptly deflected into sills by the overlying lava succession a proto laccolith starts developing composed of different vertically stacked sills originating from the central vertical feeder zone each sill contributes to the deformation of the overburden room for intrusions is provided by the higher strain that can be attained by the widespread hyaloclastites and breccias which have a softer behaviour with a lower young s modulus than the stiffer higher young s modulus lava flows combined with the doming of the overlying lavas later dikes get deflected outwards along the contact with the proto laccolith and acquire a convex upward shape in cross section

influence of the mendocino triple junction on the tectonics of coastal california abstract the migration of the mendocino triple junction through central and northern california over the past 25 30 million years has led to a profound change in plate interactions along coastal california the tectonic consequences of the abrupt change from subduction plate interactions north of the triple junction to the development of the san andreas transform system south of the triple junction can be seen in the geologic record and geophysical observations the primary driver of this tectonism is a coupling among the subducting juan de fuca gorda north american and pacific plates that migrates with the triple junction this coupling leads to ephemeral thickening of the overlying north american crust associated uplift and subsequent subsidence and a distinctive sequence of fault development and volcanism

lithospheric velocity structure of the anatolian plateau caucasus caspian region 1 the anatolian plateau caucasus caspian region is an area of complex lithospheric structure accompanied by large variations in seismic wave velocities despite the complexity of the region little is known about the detailed lithospheric structure using data from 31 new permanent broadband seismic stations along with results from a previous 29 temporary seismic stations and 3 existing global seismic stations in the region a 3 d velocity model is developed using joint inversion of teleseismic receiver functions and surface waves both group and phase dispersion curves love and rayleigh were derived from regional and teleseismic events additional rayleigh wave group dispersion curves were determined using ambient noise correlation receiver functions were calculated using p arrivals from 789 teleseismic 30 90 earthquakes the stacked receiver functions and surface wave dispersion curves were jointly inverted to yield the absolute shear wave velocity to a depth of 100 km at each station the depths of major discontinuities sediment basement crust mantle and lithosphere asthenosphere were inferred from the velocity depth profiles at the location of each station distinct spatial variations in crustal and upper mantle shear velocities were observed the kura basin showed slow 2 7 2 9 km s upper crustal 0 11 km velocities but elevated 3 8 3 9 km s velocities in the lower crust the anatolian plateau varied from 3 1 3 2 in the upper crust to 3 5 3 7 in the lower crust while velocities in the arabian plate south of the bitlis suture were slightly faster upper crust between 3 3 and 3 4 km s and lower crust between 3 8 and 3 9 km s the depth of the moho which was estimated from the shear velocity profiles was 35 km in the arabian plate and increased northward to 54 km at the southern edge of the greater caucasus moho depths in the kura and at the edge of the caspian showed more spatial variability but ranged between 35 and 45 km upper mantle velocities were slow under the anatolian plateau but increased to the south under the arabian plate and to the east 4 3 4 4 km s under the kura basin and greater caucasus the areas of slow mantle coincided with the locations of holocene volcanoes differences between rayleigh and love dispersions at long wavelengths reveal a pronounced variation in anisotropy between the anatolian plateau and the kura basin

the mechanics of sill inception propagation and growth experimental evidence for rapid reduction in magmatic overpressure abstract a model of magma propagation in the crust is presented using a series of analogue experiments where dyed water is injected at a constant flux into layers of solidified gelatine the gelatine layers are transparent and when intruded deform in an almost ideal elastic manner under the experimental conditions low gelatine concentration 2 5 or 3 wt and low temperature 5 10 c the upper gelatine layer was 1 0 to 1 5 times stiffer than the lower layer with either a weak or strong interface strength between the gelatine layers the gelatine is seeded with 20 50 m diameter pmma rhb neutrally buoyant particles that are fluoresced by a pulsed vertical laser sheet centred on the injection point digital image correlation dic is used to calculate incremental strain and finite strain in the deforming host material as it is intruded this is mapped in 2d for the developing experimental volcanic plumbing system that comprises a feeder dyke and sill since the gelatine deforms elastically strain measurements correlate with stress our results indicate that for constant magma flux the moment of sill inception is characterised by a significant magmatic pressure decrease of up to 60 this is evidenced by the rapid contraction of the feeder dyke at the moment the sill forms sill propagation is then controlled by the fracture properties of the weak interface with fluid from the feeder dyke extracted to help grow the sill pressure drops during sill inception and growth are likely to be important in volcanic systems where destabilisation of the magmatic plumbing system could trigger an eruption

upper mantle magma storage and transport under a canarian shield volcano teno tenerife spain we use clinopyroxene liquid thermobarometry aided by petrography and mineral major element chemistry to reconstruct the magma plumbing system of the late miocene largely mafic teno shield volcano on the island of tenerife outer rims of clinopyroxene and olivine phenocrysts show patterns best explained by decompression induced crystallization upon rapid ascent of magmas from depth the last equilibrium crystallization of clinopyroxene occurred in the uppermost mantle from 20 to 45 km depth we propose that flexural stresses or alternatively thermomechanical contrasts create a magma trap that largely confines magma storage to an interval roughly coinciding with the moho at 15 km and the base of the long term elastic lithosphere at 40 km below sea level evidence for shallow magma storage is restricted to the occurrence of a thick vitric tuff of trachytic composition emplaced before the teno shield volcano suffered large scale flank collapses the scenario developed in this study may help shed light on some unresolved issues of magma supply to intraplate oceanic volcanoes characterized by relatively low magma fluxes such as those of the canary madeira and cape verde archipelagoes as well as hawaiian volcanoes in their postshield stage the data presented also support the importance of progressive magmatic underplating in the canary islands

three dimensional models of p wave velocity and p to s velocity ratio in the southern central andes by simultaneous inversion of local earthquake data the pisco 94 proyecto de investigation sismologica de la cordillera occidental 1994 seismological network of 31 digital broad band and short period three component seismometers was deployed in northern chile between the coastal cordillera and the western cordillera more than 5300 local seismic events were observed in a 100 day period a subset of high quality p and s arrival time data was used to invert simultaneously for hypocenters and velocity structure additional data from two other networks in the region could be included the velocity models show a number of prominent anomalies outlining an extremely thickened crust about 70 km beneath the forearc region an anomalous crustal structure beneath the recent magmatic arc western cordillera characterized by very low velocities and a high velocity slab a region of an increased vp vs ratio has been found directly above the wadati benioff zone which might be caused by hydration processes a zone of lower than average velocities and a high vp vs ratio might correspond to the asthenospheric wedge the upper edge of the wadati benioff zone is sharply defined by intermediate depth hypocenters while evidence for a double seismic zone can hardly be seen crustal events between the precordillera and the western cordillera have been observed for the first time and are mainly located in the vicinity of the salar de atacama down to depths of about 40 km

significance of discontinuity surveying in motorway alignment selection southern turkey abstract a detailed discontinuity survey is an essential part of a route location study because rock quality and subsurface geological conditions can be determined by examining discontinuity systems it also helps to prepare a sound subsurface investigation programme in the later stages of an engineering project some of the more expensive structural elements of a motorway are tunnels viaducts reinforced retaining walls and contiguous piles each has special requirements depending on ground stability cost and environmental aspects a detailed discontinuity survey enables engineers to select the best route and to optimize the geotechnical design a case study from the tarsus adana gaziantep tag motorway being constructed in southern turkey displays well the role of a discontinuity survey in engineering geological and geotechnical work the survey which was carried out by visual inspection and practical field tests indicated on the basis of the consistency of the discontinuity systems in each rock exposure that the suspected floating blocks were outcrops of in situ rock misinterpretation of the actual ground condition caused loss of both time and money

the crustal structure of the east anatolian plateau turkey from receiver functions an edited version of this paper was published by the american geophysical union agu copyright 2003 agu see also http www agu org pubs crossref 2003 2003gl018192 shtml http atlas geo cornell edu turkey publications zor et al 2003 htm

emplacement of dikes sills and crustal magma chambers at divergent plate boundaries abstract a model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1 3 km of the oceanic crust the number and lengths of dikes in any particular swarm should increase but that the thickness should decrease with depth in the crust these predictions are supported by field observations it is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that at a certain level in the crust subsequent dikes change into sills such sills may develop into crustal magma chambers either when many nearby sills combine into a large one or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid for the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m whereas at the slowest spreading ridges the initial thickness must exceed 170 m because thin sills are much more likely to form than thick sills the probability of sills developing into crustal chambers increases with dike intrusion frequency hence with spreading rate furthermore the model predicts first that with increasing spreading rate the length of the chamber along the ridge axis should increase and second that the most likely site for magma chambers is at depths of 2 4 km both these predictions are supported by available data

volcanostratigraphy and petrogenesis of the nemrut stratovolcano east anatolian high plateau the most recent post collisional volcanism in turkey abstract the nemrut stratovolcano with its well defined summit caldera is the most recent volcanic eruption center 1441 a d in the east anatolian high plateau one of the best examples of an active colllision zone widespread volcanism has been active in this region for the last 10 ma producing large volumes of lavas and pyroclastics covering a broad belt across the bitlis suture zone from the anatolian plate in the north to the arabian plate in the south three major evolutionary stages have been identified in the evolution of the nemrut stratovolcano pre caldera post caldera and late stages the pre caldera stage is further differentiated into effusive extrusive and explosive phases the latter being responsible for the caldera formation the products of this stage cover a broad compositional range from basalts to rhyolites and consist of lava flows domes large volumes of ignimbrites and associated pyroclastics the post caldera stage consists of three phases comprising phreatomagmatic eruptions vitrophyric rhyolitic lava flows and dykes the late stage consists of the explosive and the following effusive phases with vitrophyric rhyolitic and basaltic lava flows respectively the post caldera and late stages show a compositional gap between the younger mafic and the older felsic members the products of all three stages are silica saturated and display a gradual transition in nature from sodic in the mafic to potassic in the felsic members the overlaps in the compositional spectra and the geochemical trends on bivariate plots suggest the derivation of the products of all three stages from a single parental magma or similar parental magmas yet the stratigraphic position of the mafic and felsic members points to a periodically replenished magma chamber the major and trace element systematics along with petrographic evidences suggest clinopyroxene plagioclase olivine amphibole as the major fractionating phases governing the liquid evolutionary paths with a progressive increase in the proportions of plagioclase and amphibole joined by k feldspar biotite and apatite in the fractionating assemblages during the course of magmatic differentiation the volcanics of all three stages exhibit enrichment in light rare earth elements lree over heavy rare earth elements hree and large ion lithophile elements lile over high field strength elements hfse the multi element patterns and low th y and nb y ratios imply a within plate type mantle asthenospheric source for this volcanics trace element modelling suggests derivation of parental magma s from 10 to 30 partial melting of a spinel lherzolite source assimilation fractional crystallization afc modelling reveals significant but variable degrees of crustal contamination in the evolution of the volcanics with a range of r values assimilation rate crystallization rate from 0 2 to 0 8 these interpretations are in conformity with the recent findings from seismic studies showing that lithospheric mantle is absent in eastern anatolia the crust in the region is about 45 km thick and directly underlain by the asthenosphere possibly due to slab breakoff following the collision of the arabian and the anatolian plates along the bitlis suture zone

dynamics of an open basaltic magma system the 2008 activity of the halema uma u overlook vent kīlauea caldera on march 19 2008 a small explosive event accompanied the opening of a 35 m wide vent overlook vent on the southeast wall of halema uma u crater in k lauea caldera initiating an eruptive period that extends to the time of writing the peak of activity in 2008 consisted of alternating background open system outgassing and spattering punctuated by sudden short lived weak explosions triggered by collapses of the walls of the vent and conduit near daily sampling of the tephra from this open system along with exceptionally detailed observations allow us to study the dynamics of the activity during two eruptive sequences in late 2008 each sequence includes background activity preceding and following one or more explosions in september and october 2008 respectively componentry analyses were performed for daily samples to characterise the diversity of the ejecta nine categories of pyroclasts were identified in all the samples including wall rock fragments the six categories of juvenile clasts can be grouped in three classes based on vesicularity 1 poorly 2 uniformly highly to extremely and 3 heterogeneously highly vesicular the wall rock and juvenile clasts show dissimilar grainsize distributions reflecting different fragmentation mechanisms the wall rock particles formed by failure of the vent and conduit walls above the magma free surface and were then passively entrained in the eruptive plume the juvenile componentry reveals consistent contrasts in degassing and fragmentation processes before during and after the explosive events we infer a crude layering developed in the shallow melt in terms of both rheology and bubble and volatile contents beneath a convecting free surface during background activity a tens of centimetres thick viscoelastic surface layer was effectively outgassed and relatively cool while at depths of less than 100 m the melt remained slightly supersaturated in volatiles and actively vesiculating decoupled metre sized bubbles rising through the column burst through the free surface frequently ejecting fragments of the outgassed upper layer when the surface was abruptly perturbed by the rock falls existing mm sized bubbles expanded leading to the acceleration of adjacent melt upward and consecutive explosions while renewed nucleation created a minor population of 10 micron sized bubbles after each explosive event in september october 2008 this layering was re established but with decreasing vigour suggesting that the magma batch as a whole was becoming progressively depleted in dissolved volatiles

deep structure of japan subduction zone as derived from local regional and teleseismic events we have determined a detailed three dimensional p wave velocity structure of the japan subduction zone to 500 km depth by inverting local regional and teleseismic data simultaneously we used 45 318 p wave arrivals from 1241 shallow and deep earthquakes which occurred in and around the japan islands the arrival times are recorded by the japan university seismic network which covers the entire japan islands densely and uniformly we also used 4211 travel time residuals from 100 teleseismic events which are read from seismograms recorded by seismic stations in northeastern japan in comparison with the previous results obtained from only local and regional events the present result for the area around the lower plate boundary and the mantle below the plate is determined more reliably because of the addition of 7035 data from 100 teleseismic events and 41 very deep earthquakes in the crust and uppermost mantle low velocity zones are clearly visible beneath active volcanoes in the mantle wedge the low velocity zones generally parallel with the slab and exist continuously to a depth of about 200 km which is consistent with the petrological geochemical and geodynamic studies we consider that the existence of volcanism related low velocity anomalies in the mantle wedge is a general seismological characteristic of subduction zones in light of all the available tomographic results for many subduction zones in the world the pacific slab beneath japan is imaged more clearly than in previous studies as a high velocity zone with a thickness of 80 90 km and a p wave velocity 4 6 higher than the normal mantle lower velocity anomalies are found in the mantle below the slab

koolau dike complex oahu intensity and origin of a sheeted dike complex high in a hawaiian volcanic edifice the dike complex in the eroded koolau tholeiitic shield volcano is unique among described dike swarms for its coherence high dike injection intensity relatively uniform and non gaussian 50 to 65 intensity level and lack of dike divergence parts are like a sheeted dike complex though high in an intraplate volcanic edifice a model is proposed in which highly mobile tholeiitic magma having considerable freedom to move through the strongly disjointed volcano seeks zones in which it is gravitationally most stable these zones are marginal parts of the dike complex separating lavas having a lower than magmatic bulk density from a 50 dike complex having a higher than magmatic bulk density this model also explains the strong propensity for the magma of kilauea volcano to remain underground in a gravitationally stable zone magma erupts only when it vesiculates sufficiently often when it has degassed some of it promptly plunges back toward the stable zone again

attenuation tomography in the western central andes a detailed insight into the structure of a magmatic arc high quality data from 1498 local earthquakes recorded by the pisco 94 proyecto de investigation sismologica de la cordillera occidental 1994 and ancorp 96 andean continental research project 1996 temporary seismological networks allowed the detailed determination of the three dimensional 3 d attenuation structure qp 1 beneath the recent magmatic arc in the western central andes 20 to 24 s assuming a frequency independent qp 1 in a frequency band between 1 and 30 hz whole path attenuation t was estimated from the amplitude spectra of the p waves using spectral ratios and a spectral inversion technique the damped least squares inversion tomography of the data reveals a complex attenuation structure crust and mantle of the forearc and subducting slab are generally characterized by low attenuation qp 1000 crust and mantle beneath the magmatic arc show elevated attenuation the strongest anomaly of extremely low qp is found in the crust between 22 and 23 s beneath the recent volcanic arc qp 100 n s variations can be observed the western flank of the crustal attenuation anomaly follows the curved course of the volcanic front north of 21 s the attenuation is less developed in the northern part of the study area the low qp zone penetrates in the forearc mantle down to the subducting slab in the south a deeper zone of high attenuation is resolved between 23 and 24 s directly above the subducting slab low qp in the mantle correlates with earthquake clusters the strong crustal attenuation is confined to the distribution of young ignimbrites and silicic volcanism and is interpreted as a thermally weakened zone with partial melts the attenuation pattern in the upper mantle might reflect the variable extent of the asthenosphere and maps variations of subduction related hydration processes in the mantle wedge from slab derived fluids

infrastructure and mechanics of volcanic systems in iceland the divergent plate boundary in iceland consists of more than two dozen systems where most of the volcano tectonic activity takes place at the surface the volcanic systems are characterised by 5 20 km wide and 40 100 km long swarms of tension fractures 102 m long normal faults 103 m long and volcanic fissures the holocene fissure swarms are confined to less than 10 000 year old basaltic lava flows mostly pahoehoe occurring near the centre of the active rift zone in any particular swarm the number of tension fractures exceeds that of normal faults all tension fractures and normal faults are vertical at the surface indicating that the surface parts were generated by an absolute tension in addition to a fissure swarm many volcanic systems have a central volcano some of which have developed collapse calderas in the late tertiary and pleistocene lava pile of iceland extinct volcanic systems are represented by local sheet swarms and regional dyke swarms the sheet swarms are normally circular or slightly elliptical several kilometres in radius and are confined to the extinct central volcanoes many swarms are associated with large plutons exposed at 1 2 km depth beneath the initial top of the rift zone and presumably the uppermost parts of extinct crustal magma chambers and in short traverses up to 90 of the rock may consist of sheets the sheets have a very variable strike dip on average 45 65 mostly towards the centre of the associated volcano and have an average thickness of about 1 m the regional dykes occur outside central volcanoes in swarms that are commonly 50 km long and 5 10 km wide in several kilometre long traverses commonly 1 5 of the rock consist of dykes but occasionally as much as 15 28 most regional dykes are subparallel and subvertical the average dyke thickness in the pleistocene swarms is less than 2 m but 4 6 m in the tertiary swarms while active the volcanic systems in the rift zone are supplied with magma from reservoirs located at the depth of 8 12 km at the boundary between the crust and upper mantle the reservoirs are partially molten with totally molten top regions and of cross sectional areas similar to those of the volcanic systems that they feed some active volcanic systems especially those that develop central volcanoes high temperature areas and calderas have in addition to the deep seated magma reservoirs shallow crustal magma chambers located at 1 3 km depth which in turn are fed by the deeper magma reservoirs it is proposed that the regional dyke swarms are supplied with magma largely from the deep seated reservoirs whereas the local sheet swarms are mainly fed from the associated crustal magma chambers because the volume of a crustal chamber is less than that of its deeper source reservoir a single magma flow dyke intrusion from the reservoir may trigger tens of magma flows sheet intrusions from the chamber which is one explanation for the enormous number of sheets associated with many central volcanoes the sheets follow the stress trajectories of the local stress fields around the source chambers whereas the regional stress field associated with the divergent plate movements controls the emplacement of the regional dykes it is suggested that many dykes develop as self affine some as self similar structures when applied to the krafla volcanic system in northern iceland model calculations suggest that a magma flow from the krafla reservoir with regional dyke formation should occur on average once every several hundred years and that tens of sheets might be injected from the shallow krafla chamber and triggered by a single magma flow from the krafla reservoir these results are in broad agreement with the available data

lake toba volcano magma chamber imaged by ambient seismic noise tomography 1 for the first time ambient noise tomography is used to clearly image the magma chamber beneath lake toba caldera one of the largest quaternary calderas on earth using data from 40 seismic stations deployed between may and october 2008 around lake toba empirical green s functions are extracted from long term cross correlations of continuous records these functions are dominated by rayleigh waves whose group velocities can be measured in the period range from 2 5 to 12 seconds arrival times of these waves are picked for a given period and inverted using 2 d tomography to calculate lateral variations in velocity for the given period this was done for six different periods which all correspond to different sampling depths thus the six 2 d models presented together provide information on velocity variations with depth the results show a low velocity body coincident with the lake toba caldera representing the magma chamber under the volcano the chamber is observed to have a complex 3 d geometry with at least two separate sub chambers underlying the caldera other results include a deep low velocity body possibly another magma chamber south west of the lake with an upper limit of 7 km depth the maximum depth to which this body reaches could not be resolved the sumatra fault marks a velocity contrast but only down to depths not greater than 5 km the reliability of the results was further confirmed by checkerboard recovery tests

magma expansion and fragmentation in a propagating dyke abstract the influence of magma expansion due to volatile exsolution and gas dilation on dyke propagation is studied using a new numerical code many natural magmas contain sufficient amounts of volatiles for fragmentation to occur well below earth s surface magma fragmentation has been studied for volcanic flows through open conduits but it should also occur within dykes that rise towards earth s surface the characteristics of volatile rich magma flow within a hydraulic fracture are studied numerically the mixture of melt and gas is treated as a compressible viscous fluid below the fragmentation level and as a gas phase carrying melt droplets above it the numerical code solves for elastic deformation of host rocks the flow of the magmatic mixture and fracturing at the dyke tip with volatile free magma a dyke fed at a constant rate in a uniform medium adopts a constant shape and width and rises at a constant velocity with volatiles involved magma expands and hence the volume flux of magma increases with no fragmentation this enhanced flux leads to acceleration and thinning of the dyke simple scaling laws allow accurate predictions of dyke width and ascent rate for a wide range of conditions with fragmentation dyke behaviour is markedly different due to the sharp drop of head loss that occurs in gas rich fragmented material large internal overpressures develop below the dyke tip and induce swelling of the nose region leading to deceleration of the dyke these results are applied to the two month long period of volcanic unrest that preceded the may 1980 eruption of mount st helens an initial phase of rapid earthquake migration from the 7 8 km deep reservoir to shallow levels was followed by very slow progression of magma within the edifice such behaviour can be accounted for by magma fragmentation at the top of a dyke

conditions for the growth of a long lived shallow crustal magma chamber below mount pelee volcano martinique lesser antilles arc 1 the compositional homogeneity of silicic andesites emitted during the 13 500 year long last eruptive cycle of mount pelee suggests that the physical state of the magma chamber was largely unmodified during this period experimental phase equilibria on mount pelee recent products indicate that pre eruptive magma temperatures and pressures were in the range of 875 900 c and 2 0 5 kbar respectively the estimated average eruption rate was 7 5 10 4 km3 a with an average volume of about 0 3 km3 per eruption an analytical model for a spherical magma chamber indicates that a magma flux of 4 5 10 4 km3 a can maintain a magma chamber of 0 3 km3 above 875 c below mount pelee however observations of plutons suggest that magma chambers may grow by addition of sheet like intrusions with numerical simulation we show that a minimum sheet accretion rate of a few centimeters per year is required to grow a persistently active magma chamber independently of the intruded volumetric flux this minimum injection rate is higher if the heat transfer is enhanced by convection processes the limited ability of an arc crust to extend and accommodate dikes suggests that magma injections are sills or if they are dikes that most of the volume injected in the magma chamber is eventually erupted in a conductively cooling igneous body formed by sills injected at a rate of a few centimeters per year most of the injected magma completely solidifies and only a small part of the body about 10 20 is above 875 c and able to feed eruptions

genesis of collision volcanism in eastern anatolia turkey late cenozoic volcanism in eastern anatolia extends in a broad sw ne trending belt across the arabia eurasia collision zone from the arabian foreland basin in the southwest and to the kars plateau and lesser caucasus in the northeast foreland volcanism is dominated by basaltic shield and fissure eruptions of transitional tholeiitic alkaline composition volcanism on the thickened crust north of the bitlis thrust zone varies from the mildly alkaline volcano nemrut and older mus volcanics in the south through the transitional calc alkaline alkaline volcanoes bingol and suphan and the alkaline volcano tendurek to the calc alkaline volcano ararat and older kars plateau volcanics in the north isotope sr nd and trace element systematics indicate that the lavas from the foreland were derived from the mantle lithosphere of the arabian continent which had been enriched by small volumes of asthenospheric melts over a period of time and that lavas from the alkaline volcanic area around mus and the volcanoes nemrut and tendurek north of the bitlis thrust zone were derived from a lithospheric source of similar composition either from the same underthrust arabian continent or from the bitlis massif microcontinent by contrast the transitional lavas from bingol and suphan and the calc alkaline lavas from ararat and kars were derived from lithosphere carrying a distinct subduction signature inherited from pre collision subduction events positive correlations between 87sr 86sr and sio2 and rb nb and sio2 in the alkaline and transitional lavas suggest that combined assimilation and fractional crystallization was an important process within at least part of the thickened crust of the collision zone trace element covariation diagrams such as y rb indicate the importance of hornblende crystallization at depth and orthopyroxene at shallow levels within the calc alkaline provinces in contrast to the consistently anhydrous crystallization sequences of the alkaline lavas trace element diagrams based on the covariation of compatible and incompatible elements point to moderate low degrees of partial melting with residual clinopyroxene throughout and residual garnet in the foreland province consideration of mineral stabilities mantle solidi and geothermal gradients before and after collision suggest that lithospheric thickening should both increase the thickness of metasomatized lithosphere and depress the metasomatized zone to greater depths probably beneath the amphibole and dolomite breakdown curves perturbation of the thickened lithosphere by delamination of the thermal boundary layer perhaps coupled with local stretching associated with pull apart basins on strike slip fault systems is then sufficient to generate melt the composition of that melt being largely dependent on the enrichment history of the lithosphere in question in eastern anatolia volcanism appears to have started between about 8 and 6 ma ago some 5 ma after the start of rapid uplift of the east anatolian plateau

imaging the subducting slabs and mantle upwelling under the japan islands summary a high resolution p wave tomography of the crust and mantle down to 700 km depth beneath the japan islands is determined using a large number of high quality arrival time data from local earthquakes and teleseismic events simultaneously the tomography shows that the philippine sea slab is subducting aseismically down to 430 km depth under southwest japan though the seismicity within the slab ends at 180 km depth a low velocity low v zone in the mantle wedge under tohoku and kyushu is found to extend westward from the volcanic front to the backarc under the japan sea and east china sea significant low v anomalies are revealed in the deep portion of the mantle wedge 400 500 km depth above the pacific slab under southwest japan which may reflect hot mantle upwelling associated with fluids from the deep dehydration of the pacific slab low v anomalies appear at 420 700 km depths beneath the pacific slab under eastern japan which may reflect hot mantle upwelling associated with the deep subduction of the pacific slab and its collapsing down to the lower mantle

depths of magma chambers at three volcanic provinces in the karliova region of eastern turkey the size of a volcanic eruption and thus the associated potential hazards depends partly on the depth geometry and size of the source magma chamber to estimate magma chamber depths and sizes we apply a newly developed analytical method based on the aspect ratio length thickness of dikes to three volcanoes in the karliova region of eastern turkey namely turnadag varto and ozenc the results indicate that the depths of the source chambers are between 2 and 4 km at turnadag 2 to 5 km at varto both of which are located in transtensional tectonic regimes but from 22 to 27 km at ozenc which is located in a convergent tectonic regime a similar reservoir depth at ozenc is indicated by seismic tomography and this data also suggests that the reservoir is laterally continuous for more than 40 km the large volume of ignimbrites 40 km3 associated with varto a collapse caldera indicates that caldera subsidence may have maintained the excess magmatic pressure through tectonic forcing in the chamber over a longer time than during normal pyroclastic eruptions the dike aspect ratios further indicate magmatic overpressures of 13 21 mpa for varto 13 17 mpa for turnadag and 26 31 mpa for ozenc the combined results from seismic tomography analytical models and magma compositions indicate that both turnadag and varto volcanoes which are typical stratovolcanoes composed of mostly intermediate and more rarely acidic magmas were fed by two very shallow and comparatively small magma chambers 2 5 km depth whereas less evolved magmas were erupted from ozenc which hosts predominantly basaltic and intermediate lavas and dikes that were fed by a deep reservoir at 22 27 km depth our tomographic models show that none of the volcanoes are located directly over the center of a deep magma reservoir our data also indicates that the magma in the reservoir has migrated between 34 and 40 km in a right lateral motion to the east below varto and turnadag respectively and 23 km in a left lateral motion to the west at ozenc over the past 3 ma this lateral propagation of magma can be explained by tectonic escape of the anatolian block to the west through the northern anatolian fault and the varto fault zone over the last 6 ma

gps constraints on continental deformation in the africa arabia eurasia continental collision zone and implications for the dynamics of plate interactions 1 the gps derived velocity field 1988 2005 for the zone of interaction of the arabian african nubian somalian and eurasian plates indicates counterclockwise rotation of a broad area of the earth s surface including the arabian plate adjacent parts of the zagros and central iran turkey and the aegean peloponnesus relative to eurasia at rates in the range of 20 30 mm yr this relatively rapid motion occurs within the framework of the slow moving 5 mm yr relative motions eurasian nubian and somalian plates the circulatory pattern of motion increases in rate toward the hellenic trench system we develop an elastic block model to constrain present day plate motions relative euler vectors regional deformation within the interplate zone and slip rates for major faults substantial areas of continental lithosphere within the region of plate interaction show coherent motion with internal deformations below 1 2 mm yr including central and eastern anatolia turkey the southwestern aegean peloponnesus the lesser caucasus and central iran geodetic slip rates for major block bounding structures are mostly comparable to geologic rates estimated for the most recent geological period 3 5 myr we find that the convergence of arabia with eurasia is accommodated in large part by lateral transport within the interior part of the collision zone and lithospheric shortening along the caucasus and zagros mountain belts around the periphery of the collision zone in addition we find that the principal boundary between the westerly moving anatolian plate and arabia east anatolian fault is presently characterized by pure left lateral strike slip with no fault normal convergence this implies that extrusion is not presently inducing westward motion of anatolia on the basis of the observed kinematics we hypothesize that deformation in the africa arabia eurasia collision zone is driven in large part by rollback of the subducting african lithosphere beneath the hellenic and cyprus trenches aided by slab pull on the southeastern side of the subducting arabian plate along the makran subduction zone we further suggest that the separation of arabia from africa is a response to plate motions induced by active subduction

spatial distribution and alignments of volcanic centers clues to the formation of monogenetic volcanic fields monogenetic basaltic volcanic fields occur worldwide in tectonic environments ranging from extensional to convergent understanding similarities and differences between these fields may help to characterize key controls on their generation such volcanic fields consist of numerous volcanic centers each of which represents a pathway of magma from its source to the surface we analyzed the spatial distribution of volcanic centers in 37 monogenetic volcanic fields and assuming that the distribution of volcanic centers relative to each other is matched by a similar source pattern within the mantle applied the following methods for each 1 the poisson nearest neighbor pnn analysis representing the degree to which the distribution of the volcanic centers departs from a predicted poisson distribution and 2 a volcanic alignment analysis to ascertain the preferential pathways if any used by the magma to reach the surface this is the first comprehensive global comparison of such analyses magma pathways within the brittle upper crust are influenced to various degrees by two end member situations 1 formation of new extension fractures perpendicular to the least compressive stress 3 and 2 re activation of pre existing fractures that are near parallel to the maximum principal stress 1 the results of the pnn analysis show that independently of the tectonic environment most volcanic fields display a clustered distribution of their volcanic centers alignment analysis shows that either the ambient tectonic environment exerts a strong influence on the preferential orientations of the volcanic alignments or that it is in competition with other factors e g pre existing structures local stress changes due to older intrusions overall these results indicate that the propagation of the magma and therefore the spatial distribution of the volcanic centers within volcanic fields is the product of an interplay between deep level influences i e nature of the magma source and shallow level influences i e stress field pre existing crustal structures of the lithosphere

cyclic caldera collapse piston or piecemeal subsidence field and experimental evidence many multicycle caldera volcanoes display a complex extracaldera fault system genetically linked to caldera evolution on gran canaria canary islands the miocene tejeda caldera was filled by ignimbrites and epiclastic sediments and intruded by syenites and a major cone sheet swarm obscuring intracaldera structural relations extracaldera radial and circumferential faults and dikes are well exposed we used field data and experiments scaled to the island of gran canaria to clarify the poorly understood structural temporal and genetic relationship of peripheral faults to the central caldera the experimental setup comprises a cone of medium grained sand in which a rubber balloon was repeatedly inflated and deflated balloon inflation resulted in updoming and a dominantly radial fault pattern whereas balloon deflation caused piecemeal caldera collapse and a concentric peripheral fault arrangement on the edifice flanks a complex interplay of repeated inflation and deflation cycles of the experimental magma chamber is found to explain the peripheral fault system of the tejeda caldera on gran canaria and offers insight into the structure of the caldera floor and the mechanisms acting in multicycle caldera volcanoes

the role of magma overpressure in suppressing earthquakes and topography worldwide examples in an extending terrane basaltic magma supplied at a pressure greater than the least principal stress overpressure may be capable of suppressing normal faulting and the earthquakes and topographic relief that commonly accompany normal faulting as vertical dikes intrude they press against their walls in the direction opposite the least principal stress and increase its magnitude the emplacement of tabular intrusions causes the internal magma pressure to act selectively in opposition to tectonic stresses this process tends to equalize the stresses and thus diminishes the deviatoric stress difference between maximum and minimum stresses that creates faults and causes earthquakes observations of the pattern of seismicity and magmatism worldwide indicate that magmatism commonly supplants large earthquakes as the primary mechanism for accommodating tectonic extension recognizing the extent of magmatic stress accommodation is important in assessing seismic and volcanic risks

magma storage beneath axial volcano on the juan de fuca mid ocean ridge axial volcano which is located near the intersection of the juan de fuca ridge and the cobb eickelberg seamount chain beneath the northeast pacific ocean is a locus of volcanic activity thought to be associated with the cobb hotspot1 the volcano rises 700 metres above the ridge has substantial rift zones extending about 50 kilometres to the north and south and has erupted as recently as 1998 ref 2 here we present seismological data that constrain the three dimensional velocity structure beneath the volcano we image a large low velocity zone in the crust consisting of a shallow magma chamber and a more diffuse reservoir in the lower crust and estimate the total magma volume in the system to be between 5 and 21 km3 this volume is two orders of magnitude larger than the amount of melt emplaced during the most recent eruption3 4 0 1 0 2 km3 we therefore infer that such volcanic events remove only a small portion of the reservoir that they tap which must accordingly be long lived compared to the eruption cycle on the basis of magma flux estimates we estimate the crustal residence time of melt in the volcanic system to be a few hundred to a few thousand years