upper mantle electrical resistivity structure beneath the central mariana subduction system this paper reports on a magnetotelluric mt survey across the central mariana subduction system providing a comprehensive electrical resistivity image of the upper mantle to address issues of mantle dynamics in the mantle wedge and beneath the slow back arc spreading ridge after calculation of mt response functions and their correction for topographic distortion two dimensional electrical resistivity structures were generated using an inversion algorithm with a smoothness constraint and with additional restrictions imposed by the subducting slab the resultant isotropic electrical resistivity structure contains several key features there is an uppermost resistive layer with a thickness of up to 150 km beneath the pacific ocean basin 80 100 km beneath the mariana trough and 60 km beneath the parece vela basin along with a conductive mantle beneath the resistive layer a resistive region down to 60 km depth and a conductive region at greater depth are inferred beneath the volcanic arc in the mantle wedge there is no evidence for a conductive feature beneath the back arc spreading center sensitivity tests were applied to these features through inversion of synthetic data the uppermost resistive layer is the cool dry residual from the plate accretion process its thickness beneath the pacific ocean basin is controlled mainly by temperature whereas the roughly constant thickness beneath the mariana trough and beneath the parece vela basin regardless of seafloor age is controlled by composition the conductive mantle beneath the uppermost resistive layer requires hydration of olivine and or melting of the mantle the resistive region beneath the volcanic arc down to 60 km suggests that fluids such as melt or free water are not well connected or are highly three dimensional and of limited size in contrast the conductive region beneath the volcanic arc below 60 km depth reflects melting and hydration driven by water release from the subducting slab the resistive region beneath the back arc spreading center can be explained by dry mantle with typical temperatures suggesting that any melt present is either poorly connected or distributed discontinuously along the strike of the ridge evidence for electrical anisotropy in the central mariana upper mantle is weak

three dimensional inversion of large scale earthscope magnetotelluric data based on the integral equation method geoelectrical imaging of the yellowstone conductive mantle plume 1 interpretation of the earthscope mt magnetotelluric data requires the development of a large scale inversion method which can address two common problems of 3d mt inversion computational time and memory requirements we have developed an efficient method of 3d mt inversion based on an ie integral equation formulation of the mt forward modeling problem and a receiver footprint approach implemented as a massively parallel algorithm this method is applied to the mt data collected in the western united states as a part of the earthscope project as a result we present one of the first 3d geoelectrical images of the upper mantle beneath yellowstone revealed by this large scale 3d inversion of the earthscope mt data these images show a highly conductive body associated with the tomographically imaged mantle plume like layer of hot material rising from the upper mantle toward the yellowstone volcano the conductive body identified in these images is west dipping in a similar way to a p wave low velocity body

marine magnetotellurics imaged no distinct plume beneath the tristan da cunha hotspot in the southern atlantic ocean the tristan da cunha tdc is a volcanic island located above a prominent hotspot in the atlantic ocean many geological and geochemical evidences support a deep origin of the mantle material feeding the hotspot however the existence of a plume has not been confirmed as an anomalous structure in the mantle resolved by geophysical data because of lack of the observations in the area marine magnetotelluric and seismological observations were conducted in 2012 2013 to examine the upper mantle structure adjacent to tdc the electrical conductivity structure of the upper mantle beneath the area was investigated in this study three dimensional inversion analysis depicted a high conductive layer at 120 km depth but no distinct plume like vertical structure the conductive layer is mostly flat independently on seafloor age and bulges upward beneath the lithospheric segment where the tdc islands are located compared to younger segment south of the tdc fracture zone while the bathymetry is rather deeper than prediction for the northern segment the apparent inconsistency between the absence of vertical structure in this study and geochemical evidences on deep origin materials suggests that either the upwelling is too small and or weak to be resolved by the current data set or that the upwelling takes place elsewhere outside of the study area other observations suggest that 1 the conductivity of the upper mantle can be explained by the fact that the mantle above the high conductivity layer is depleted in volatiles as the result of partial melting beneath the spreading ridge 2 the potential temperature of the segments north of the tdc fracture zone is lower than that of the southern segment at least during the past 30 myr

ground deformation and gravity changes on the island of pantelleria in the geodynamic framework of the sicily channel abstract the island of pantelleria is an active volcano located in the sicily channel southern italy in the middle of a continental rift system since the 1980s the island was periodically surveyed by using geodetic techniques edm levelling gps and high precise gravimetry to monitor the regional and local volcanic dynamics gravity data collected between 1990 and 1998 show short and long wavelength changes due to the combined effect of shallow and deep sources they reflect to some degree the structural setting of the island as delineated by the bouguer anomaly field which indicates that the island is broken up into two main basement blocks the latter are bordered by two lineaments probably regional faults related to the global geodynamics of the sicily channel rift zone moreover the inverse correlation between the gravity and altimetric variations suggests that i pantelleria is kinematically divided in two blocks ii the observed behaviour is strongly influenced by the geodynamics of the sicily channel a new interpretation of the fully reprocessed data sets is presented focusing on the spatial temporal features of the horizontal ground deformation and gravity changes compared to the bouguer anomaly and altimetric data this leads to conclude that volcanism on the island has been probably strongly influenced by the global geodynamics of the sicily channel and future eruptions are most likely to occur at the structural boundary separating the two blocks

coast effect distortion of marine magnetotelluric data insights from a pilot study offshore northeastern japan abstract we report on strong coast effect distortions observed for broadband marine magnetotelluric mt data collected on the forearc offshore northeastern japan eight days of horizontal electric and magnetic fields recorded at eight seafloor stations and the horizontal magnetic fields from a land remote station were processed with a robust multiple station algorithm yielding good mt responses and inter station transfer functions at periods of 7 10 000 s transverse electric te mode responses have cusps in apparent resistivity and negative phases at periods around 1000 s while the transverse magnetic tm mode responses are galvanically depressed below the te responses an analysis of inter station transfer functions confirms that the apparent resistivity cusps are a magnetic field rather than electric field phenomenon consisting of an amplitude minimum and rapid phase change around a characteristic frequency poynting vectors for a te coast effect model study illustrate that the anomalous phases are associated with energy diffusing back up to the seafloor from below after being turned around from its usual downward propagating trajectory by inductive coupling between the conductive ocean and the resistive seafloor along the continental margin we show that the characteristic frequency and position of the te mode apparent resistivity cusps are determined by a relatively simple combination of the electrical resistivity of the seafloor the depth of the ocean and the distance from the coastline by including coastlines and bathymetry in 2d inversion we recover the seafloor conductivity structure along the forearc demonstrating that broadband data can constrain the thickness of conductive forearc sediments and the underlying high resistivity associated with the mantle wedge and subducting oceanic lithosphere

neogene geology of the okanagan highland british columbia neogene basaltic successions one of the early and middle miocene age the other of latest miocene and pliocene age rest on land surfaces characterized by broad low relief components above which rose hills that may never have been totally covered by the basaltic rocks local stream deposits in places auriferous or uraniferous have accumulated in channels cut into or running on the subbasalt surface and have been covered by the basalts a subsequent late pliocene and post pliocene differential uplift has produced elongate northerly trending topographic highs and has led to major incision by streams a third basaltic succession which locally extends into the incised valleys is recognized but is not discussed in detail

intersecting intracontinental tertiary transform fault systems in the north american cordillera in central british columbia north trending dextral strike slip faults that cut late eocene granite also truncate northwest trending dextral strike slip faults the northwest trending strike slip faults bound the wolverine metamorphic complex wolverine complex which has been uplifted primarily by northwest southeast eocene crustal extension and somewhat by late eocene northerly extension the crustal extension is indicated by shallow dipping extensions faults dyke complexes and stretching lineations the wolverine complex and its bounding faults define a crustal pull apart in an en echelon dextral transform the northwest and north trending dextral strike slip faults in central british columbia are the continuations of faults that transect the interior of the north american cordillera and they represent at least two distinct plate boundaries intermittently active during the early to middle eocene and the late eocene to early oligocene each of these systems consists of en echelon strike slip fault

regional stratigraphy and age of chilcotin group basalts south central british columbia mio pliocene chilcotin group basalt flows form a 50 000 km2 lava plateau in south central british columbia two periods of eruptive activity at 2 3 and 6 10 ma are indicated by a compilation of available age data including 10 new k ar age determinations and basalts from these two periods are chemically indistinguishable the chilcotin group consists of thin crudely columnar jointed pahoehoe flows some thick tiered flows pillow lava and pillow breccia and rare silicic tephra layers the presence of many vesicle sheets and cylinders and collapsed pahoehoe toes suggests that the basalts were volatile rich known vents gabbro and basalt plugs for the basalt flows form a northwest trend along the axis of the lava plateau the plateau appears to have formed from the overlap of many low profile shield volcanoes and is similar in morphology to other plains basalts such as the snake river plain and parts of iceland glaciation has stripped off an unknown volume of the flat lying basalt flows

the 1891 submarine eruption offshore pantelleria island sicily channel italy identification of the vent and characterization of products and eruptive style high resolution bathymetry and seafloor sampling have been used to characterize the 1891 submarine eruption of the pantelleria volcanic complex this submarine eruption has been documented mainly by historical reports describing basaltic scoria bombs floating on the sea surface i e lava balloons in this study the 1891 eruptive vent has been identified as a small cone volume of 700 000 m3 rising 90 m from 350 m w d and located within a newly discovered submarine volcanic field covering a wide area offshore from the nw coast of pantelleria recently kelly et al 2012 confirmed this location by a multibeam and rov survey pyroclasts from the 1891 eruption crop out directly on the seafloor and are fresh scoria clasts i e small bombs bomb fragments and lapilli and glass ash sized grains both have been characterized in their morphology textures and geochemistry the distinctive vesicularity and crystallization characteristics displayed by the scoriaceous pyroclasts reflect modes of degassing in both syn and posteruptive regimes these characteristics along with the distribution of deposits suggest for the strongest eruptive phase of the 1891 eruption a style analogous to hawaiian fountaining glass grains from a buoyant plume were dispersed northward from the vent up to distances of 1 5 km redirected by the levantine intermediate water the identification of the 1891 submarine eruptive vent offshore pantelleria as well as the features of erupted pyroclasts improve our knowledge of submarine explosive eruptions that occur at shallow intermediate depths and among these of the rare eruptions producing lava balloons

tectonics structural evolution and caldera formation on vulcano island aeolian archipelago southern tyrrhenian sea abstract a detailed structural study and a review of the eruptive history of the island of vulcano aeolian arc southern tyrrhenian sea have been carried out with the purpose of understanding the evolution of both volcanic and tectonic structures the island is composed of four main volcanoes which date from about 120 ka to historical times two calderas occur in the central sector of the island the time space evolution of the volcanism indicates a shifting of the activity from the southeastern sectors towards the northwest two main systems of nw se trending right lateral strike slip faults affect the island ne sw and n s trending normal faults are also present geophysical investigation indicate the nw se trending strike slip faults are likely to be the surface expression of a main regional crustal structure this system of discontinuity is related with the stress field acting in the southern sector of the aeolian archipelago normal n s and ne sw oriented faults represent extensional fractures the ne sw trending structures are related to a local stress field the results of the kinematic analysis as well as the geometric relationships between the faults and the constant subsidence velocities are consistent with the opening of a pull apart basin volcanological and geochronological data are also consistent with the proposed model in the general deformative style affecting the island the calderas can be explained as the consequence of different opening phases of a pull apart basin

chemical weathering of volcanic rocks at the island of pantelleria italy information from soil profile and soil solution investigations abstract concentrations of major minor and trace elements were determined in soil profiles and soil solutions from the island of pantelleria sicily channel to evaluate the weathering extent of soils evolved on trachytic and pantelleritic rocks and the aqueous transport of elements by their soil solutions the chemical index of alteration cia indicates a low to moderate degree of weathering consistently the mineralogical and geochemical imprints of the parent rocks are generally preserved the chemical weathering appears to be incongruent owing to primary minerals and glass dissolving to a variable degree while secondary minerals have formed based on the calculated saturation state of primary and secondary minerals with respect to the soil solutions and on mineralogical observations olivine and pyroxene are unstable and dissolving whereas magnetite and hematite are stable the main weathering products are amorphous phases such as ferrihydrite amorphous aluminium hydroxide and silica clay mineral formation appears to be incipient and mainly limited to kaolinite and smectite although for some soil profiles a hydrothermal origin is suggested for these minerals chemical characteristics of soil solutions and elemental mobility are controlled by the element source and water rock interaction

mantle structure beneath the western united states and its implications for convection processes 1 we present tomographic images of the mantle structure beneath the western united states our dynamic north america models of p and s velocity structure dna07 p and dna07 s use teleseismic body waves recorded at 600 seismic stations provided by the earthscope transportable array and regional networks dna07 p and s benefit from the unprecedented aperture of the network while maintaining a dense station distribution providing high resolution body wave imaging of features through the transition zone and into the lower mantle the main features imaged include 1 the juan de fuca subduction system that bottoms at 400 km beneath oregon implying interaction with the yellowstone anomaly 2 a low velocity conduit beneath yellowstone national park that bottoms at 500 km and dips toward the northwest 3 shallow low velocity anomalies upper 200 km beneath the eastern snake river plain esrp and the high lava plains and a deep low velocity anomaly 600 km beneath the esrp but not newberry 4 a low velocity slab gap to 400 km depth immediately south of the mendocino triple junction and south of the gorda slab and 5 high velocity drips beneath the transverse ranges the southern central valley sierra nevada and central nevada these observations reveal extremely heterogeneous mantle structure for the western united states and suggest that we are only just beginning to image the complex interactions between geologic objects the transportable array allows for analysis of the relationships between these anomalies in an internally consistent single tomographic model the dna velocity models are available for download and slicing at http dna berkeley edu

bounded influence magnetotelluric response function estimation author posting blackwell 2004 this article is posted here by permission of blackwell for personal use not for redistribution the definitive version was published in geophysical journal international 157 2004 988 1006 doi 10 1111 j 1365 246x 2004 02203 x

new u pb and ar ar isotopic age constraints on the timing of eocene magmatism fort fraser and nechako river map areas central british columbia twenty three new precise eocene u pb and 40 ar 39 ar age determinations for calc alkaline volcanic rocks of the ootsa lake group and associated intrusive rocks widespread in the nechako plateau in central british columbia constrain the timing of the eocene magmatism to 53 2 47 6 ma with a local duration of as little as 2 3 million years the new dates show that magmatism in the study area is partly coeval with that responsible for the babine igneous suite newman volcanic suite 53 50 ma to the north and for the endako group 51 45 ma which overlies the ootsa lake group however locally the three magmatic suites are distinct in age and or the strata record a magmatic hiatus of as much as 7 5 million years the ages generally young from north to south 52 47 ma along the western portion of the study area the babine igneous suite newman volcanic suite represents the oldest member of this series however in the east the ootsa lake group volcanic rocks are generally older 53 51 ma the anomalously older ages may be related to the interaction of magmatism and formation of a nearby and coeval core complex which ongoing studies show was uplifted at about the same time during the eocene felsic plutonism associated with the ootsa lake group occurred between 50 5 and 47 3 ma these plutons were emplaced in an extensional setting along north northeast trending faults the new dates stratigraphic relationships and suggested correlations of eocene strata in the study area with that to the west and north require a revision of the stratigraphic nomenclature for the ootsa lake and endako groups 696

the pre eruptive volatile contents of recent basaltic and pantelleritic magmas at pantelleria italy abstract pantelleria island located in the sicily channel rift zone italy is the type locality for the peralkaline rhyolitic rocks called pantellerites in the last 50 ka after the large green tuff caldera forming eruption volcanic activity at pantelleria has consisted of effusive and explosive eruptions mostly vented inside and along the rim of the caldera and producing silicic lava flows lava domes and poorly dispersed pantelleritic pumice fall deposits basaltic cinder cones and lava flows are only present outside the caldera in the nw sector of the island the most recent basaltic cuddie rosse 20 ka and pantelleritic cuddia randazzo and cuddia del gallo 6 ka pyroclastic products were sampled to investigate magmatic volatile contents through the study of melt inclusions the melt inclusions in pyroxene and olivine phenocrysts of cuddie rosse scoriae have an alkali basalt composition the dissolved volatiles comprise 0 9 1 6 wt h2o several hundred ppm of co2 1600 2000 ppm of sulphur and 500 900 ppm of chlorine the water carbon dioxide couple gives a confining pressure 2 kbar prior to the eruption this result indicates that episodes of magma ponding and crystallization occurred in the upper crust prior to eruption the melt inclusions in feldspar fayalite and aenigmatite phenocrysts of cuddia del gallo and cuddia randazzo pumice have a pantelleritic composition agpaitic indices 1 3 2 1 up to 4 4 wt h2o 8700 ppm cl 6000 ppm f and co2 below the detection limit sulphur averaging 420 ppm has been measured in cuddia randazzo melt inclusions these data indicate relatively high volatile contents for these low energy strombolian type eruptions melt inclusions in cuddia del gallo pumice show the most evolved composition agpaitic indices 2 2 1 and the highest volatile content in agreement with fluid saturation conditions in the magma chamber prior to the eruption this implies a confining pressure of 1 kbar for the top of the pantelleritic reservoir the composition of melt inclusions and mineralogical assemblage of cuddia randazzo pumice indicate that it has a lower evolutionary degree agpaitic indices 1 3 1 8 and lower pre eruptive cl and h2o contents than cuddia del gallo pumice an increase in pressure due to the exsolution of volatiles in the upper part of the pantelleritic reservoir may have triggered the cuddia del gallo explosive eruption evidence of widespread pre eruptive mingling between trachytes and pantellerites suggests that the intrusion of trachytic magma into the pantelleritic reservoir likely played a major role in destabilizing the magma system just prior to the cuddia randazzo event

mantle flow through the northern cordilleran slab window revealed by volcanic geochemistry the northern cordilleran slab window formed beneath western canada concurrently with the opening of the californian slab window beneath the southwestern united states beginning in late oligocene miocene time a database of 3530 analyses from miocene holocene volcanoes along a 3500 km long transect from the northern cascade arc to the aleutian arc was used to investigate mantle conditions in the northern cordilleran slab window using geochemical ratios sensitive to tectonic affi nity such as nb zr we show that typical volcanic arc compositions in the cascade and aleutian systems derived from subduction hydrated mantle are separated by an extensive volcanic fi eld with intraplate compositions derived from relatively anhydrous mantle this chemically defi ned region of intraplate volcanism is spatially coincident with a geophysical model of the northern cordilleran slab window we suggest that opening of the slab window triggered upwelling of anhydrous mantle and displacement of the hydrous mantle wedge which had developed during extensive early cenozoic arc and backarc volcanism in western canada high heat fl ow throughout the western canadian cordillera is broadly coincident with the fi eld of intraplate volcanism and is linked to slab window induced mantle upwelling

stratigraphy and geochemistry of the itcha volcanic complex central british columbia the itcha volcanic complex is the youngest and easternmost felsic shield volcano of the anahim volcanic belt of central british columbia the main body of the shield erupted over an area of 300 km2 forming itcha mountain and mount downton volcanism associated with the itcha shield extended 20 km south to the satah mountain area where lavas erupted along a north northwest south southeast fault system and covered an additional area of 250 km2 the itcha volcanic complex is characterized by a bimodal population of volcanic rocks which are dominated by felsic lavas there were two stages of volcanism i an early felsic shield building stage dominated by felsic lavas ranging in composition from phonolite to minor quartz normative trachytes which erupted as flows domes and pyroclastic deposits to form a low angled shield and ii a late mafic capping stage which comprises a thin veneer of hawaiite and more primitive mafic lavas ranging in composition from alkali olivine basalt to basanite the late

gravity changes and present day dynamics of the island of pantelleria sicily channel italy abstract the island of pantelleria is an active volcano located in the sicily channel southern italy occurring in the middle of a continental rift system since the 1980 s the island has been periodically surveyed by means of geodetic and geophysical methods to monitor the regional and local volcanic dynamics also high precision gravity measurements were started in 1990 the present paper is an analysis of the time space gravity changes gravity measurements were carried out on a network presently formed by twenty stations the gravity network was fully surveyed in june 1990 and june 1995 and partially surveyed in september 1993 two absolute gravity stations were established in 1993 to provide a reference system and to check for long term variations the areal distribution of the gravity changes during the 1990 1995 time interval obtained in the thirteen stations of the 1990 network is strongly similar to the bouguer anomaly field and to the large scale features of the basement of the island otherwise gravity changes are directly correlated with the bouguer anomaly and inversely correlated with the altimetric variations comparison with the geological setting suggests that the present activity may be ascribed to the influence of the geodynamics of the sicily channel

simple shearing block resurgence in caldera depressions a model from pantelleria and ischia abstract similar resurgent blocks occur in caldera depressions at the islands of pantelleria and ischia the two blocks which are tilted show similar deformation patterns and similar distributions of volcanic vents related to resurgence the remaining parts of the floors of the two calderas are not deformed similarities between the two features are so striking that the resurgence mechanism must have been alike a dynamic model for resurgence is presented and called simple shearing block resurgence the triggering mechanism is an increase of the magmatic pressure in the upper part of a shallow magma chamber which results in a vertical direction of the maximum stress applied on the roof rocks the block is first defined by high angle inland dipping marginal detachments and then deformed through a simple shearing mechanism reverse faults on its mostly uplifted side determine an horizontal shortening of the block which is balanced on the opposite side by normal faults oblique movements take place along the other two sides due to this distribution of the stress around the block magmas can reach the surface only through the extensional faults on the less uplifted side

regional conductivity structure of cascadia preliminary results from 3d inversion of usarray transportable array magnetotelluric data 1 in conjunction with the usarray component of earthscope long period magnetotelluric mt data are being acquired in a series of arrays across the continental us initial deployments in 2006 and 2007 acquired data 10 10 000 s at 110 sites covering the us pacific northwest distributed with the same nominal spacing as the usarray seismic transportable array 75 km the most striking and robust features revealed by initial three dimensional inversion of this dataset are extensive areas of high conductivity in the lower crust beneath all of southeastern oregon and beneath the cascade mountains contrasting with very resistive crust in siletzia and the columbia embayment significant variations in upper mantle conductivity are also revealed by the inversions with the most conductive mantle beneath the washington backarc and the most resistive corresponding to subducting oceanic mantle citation patro p k and g d egbert 2008 regional conductivity structure of cascadia preliminary results from 3d inversion of usarray transportable array magnetotelluric data geophys res lett 35 l20311 doi 10 1029 2008gl035326

calderas and caldera structures a review abstract calderas are important features in all volcanic environments and are commonly the sites of geothermal activity and mineralisation yet it is only in the last 25 years that a thorough three dimensional study of calderas has been carried out utilising studies of eroded calderas geophysical analysis of their structures and analogue modelling of caldera formation as more data has become available on calderas their individuality has become apparent a distinction between caldera caldera complex cauldron and ring structure is necessary and new definitions are given in this paper descriptions of calderas based on dominant composition of eruptives basaltic peralkaline andesitic dacitic rhyolitic can be used and characteristics of each broad group are given styles of eruption may be effusive or explosive with the former dominant in basaltic calderas and the latter dominant in andesitic dacitic rhyolitic and peralkaline calderas four end member collapse styles occur plate or piston piecemeal trapdoor and downsag but many calderas have multiple styles features of so called funnel and chaotic calderas proposed in the literature can be explained by other collapse styles and the terms are considered unnecessary ground deformation comprises subsidence or collapse essential characteristics of a caldera and uplifting doming and fracturing due to tumescence and or resurgence frequent but not essential collapse may occur on pre existing structures such as regional faults or on faults created during the caldera formation and the shape of the collapse area will be influenced by depth size and shape of the magma chamber the final morphology of a caldera will depend on how the caldera floor breaks up whether collapse takes place in one event or multiple events whether vertical movement is spasmodic or continuous throughout the eruptive sequence and whether blocks subside uniformly or chaotically at one or more collapse centres a meaningful description of any caldera should therefore include number of collapse events presence or absence of resurgence caldera floor coherency caldera floor collapse geometry and dominant composition of eruptives

structural analysis of syn depositional hydrothermal veins of the 3 48 ga dresser formation pilbara craton australia abstract the lower chert unit of the 3 48 ga dresser formation pilbara craton western australia hosts earth s oldest convincing evidence of life in the form of stromatolites in a depositional setting that has been interpreted as either a shallow marine lagoon an active volcanic caldera or a basin deposited during regional extension understanding the environment where early life thrived is important not only for early life studies on earth but in the search for life on mars and for understanding the setting for the origin of life structural analysis of the extensive network of syn depositional hydrothermal chert barite veins that immediately underlies the dresser was undertaken in order to test if deposition occurred within an evolving caldera system or during regional extension over 190 hydrothermal veins were mapped for their geometry and relative age relationships bedding orientations were recorded within nine fault bounded structural sub blocks and used for back rotating the measured veins to their original orientation the results show that 1 there are four main vein systems including three that were emplaced during deposition of the dresser formation lower chert unit and a fourth that was emplaced within syn to post depositional listric growth faults 2 the majority of veins were sub vertical prior to regional tilting 3 the first three vein systems show mutually cross cutting relationships 4 vein densities and the number of vein systems vary between sub blocks 5 conjugate vein networks ne sw and nw se dominate two middle and northern sub blocks these results show that the first three vein systems were emplaced through fracturing under a stress regime with a vertical 1 vertical conjugate fractures are consistent with sandbox models of overburden fracturing during emplacement and discharge of a magma chamber in a volcanic caldera areas of single vein orientations reflect marginal areas of the caldera system dominated by radial or by ring faults the veins in late listric growth faults represent the final period of caldera collapse which controlled the depositional thicknesses of overlying strata possibly during subsequent regional extension these results document an unique setting for earliest life on earth within shallow water and terrestrial deposits of an active low eruptive caldera setting

three dimensional inversion of seafloor magnetotelluric data collected in the philippine sea and the western margin of the northwest pacific ocean we report a result of three dimensional 3 d upper mantle electrical conductivity inversion of seafloor magnetotelluric data we used existing data at 25 sites in the philippine sea and the western margin of the pacific ocean in order to obtain a reliable model by 3 d inversion we evaluated the large and small scale topographic effects we also conducted a comprehensive search of the one dimensional 1 d profiles of the study area in order to determine the best initial and prior models a two phase inversion method was applied so that the error floors for the diagonal and off diagonal elements of the impedance tensor could be separately controlled through this first attempt at inverting real data we obtained basic knowledge about tuning the inversion parameters and conditions we also proposed a procedure to evaluate the reliability of the 3 d conductivity anomalies imaged by the inversion by conducting checkerboard and sensitivity tests after the iterations converged 13 distinct anomalies were found in the inverted 3 d conductivity model four conductive and two resistive anomalies were confirmed to be resolved enough by the data through the checkerboard test then the sensitivity tests were conducted to quantify how each anomaly was required by the observed data and we confirmed that the intensities of three conductive anomalies and one resistive anomaly were statistically significant this paper presented an example of possible approach in 3 d seafloor electromagnetic inversion procedure for imaging reliable electrical conductivity structure of the oceanic mantle which will be useful in understanding dynamics and evolution of solid earth

electrical conductivity imaging of the philippine sea upper mantle using seafloor magnetotelluric data abstract we performed a three year seafloor electromagnetic survey in the philippine sea including the western edge of the pacific ocean to image electrical features of a deep mantle slab stagnating in the transition zone and the surrounding mantle in three dimensions 3 d the project iterated one year deployment of ocean bottom electromagnetometers obems using a total of 37 instruments installed at 18 sites the data obtained have been analyzed in the order of their recovery based on a magnetotelluric mt method in this study we attempt to obtain a one dimensional 1 d electrical conductivity model beneath the philippine sea and the pacific region separately that can be used as a reference model in the first step toward the 3 d analysis the resultant 1 d models show three main features 1 the conductivity in the shallower 200 km of the upper mantle depths of the two regions contrasts sharply which is qualitatively consistent with the large difference in lithospheric age 2 the conductivity at 200 300 km depth in both regions is more or less the same at approximately 0 3 s m 1 3 the conductivity just below 400 km depth is higher for the philippine sea mantle than for the pacific mantle the conductivity structure can be interpreted in terms of the thermal structure mantle hydration and existence of partial melt using experimental results for the conductivity of mantle minerals if the conductivity is interpreted simply as the effect of temperature the mantle beneath the philippine sea could be hotter than the dry solidus of mantle peridotite and thus partially molten however beneath the pacific region the present analysis suggests that the partial melting is not required under the assumed peridotitic composition even if we consider mantle hydration

the origin of trachyte and pantellerite from pantelleria italy insights from major element trace element and thermodynamic modelling abstract trachyte and peralkaline rhyolite pantellerite and comendite frequently comprise the felsic end member in bimodal suites in continental rift and oceanic island settings in these settings the relationship between the mafic mildly alkaline or transitional basalt and felsic lavas is ambiguous major and trace element models and isotopic data are often consistent with an origin for felsic lavas from either fractional crystallization of transitional basalt or partial melting of alkali gabbro followed by fractional crystallization in this paper we present representative mineral analyses and whole rock analyses from forty samples of a basalt trachyte pantellerite suite collected at pantelleria italy in the strait of sicily rift zone and compare the results of major and trace element modelling with the results of thermodynamic melts modelling from these results we conclude that metaluminous trachyte formed as a result of 70 to 75 low pressure 0 1 gpa fractional crystallization of an assemblage of plagioclase clinopyroxene olivine magnetite and apatite from a hydrous 1 0 1 5 wt h2o transitional basalt magma at relative oxygen fugacities approximately one log unit below the fayalite magnetite quartz buffer fmq 1 the daly gap a lack of intermediate 49 62 wt sio2 volcanic rocks at pantelleria is concluded to be primarily the result of rapid differentiation through that interval relatively rapid crystallization at low pressure may have effected the partial degassing of water saturated 4 wt h2o metaluminous trachyte magma some metaluminous trachyte lavas have positive eu anomalies high k rb ratios high concentrations of ba and low concentrations of incompatible trace elements these are interpreted to be the result of up to 40 accumulation of alkali feldspar comenditic trachyte pantelleritic trachyte and pantellerite formed after an additional 20 to 80 fractional crystallization of an assemblage dominated by alkali feldspar from metaluminous trachyte magma the most evolved pantellerite lavas and tuffs are the result of a total of 95 fractional crystallization of transitional basalt with phenocrysts that equilibrated at low temperatures 4 wt water contents

geochemistry of thermal fluids on the volcanic isle of pantelleria southern italy abstract chemical analyses of thermal springs and gas emissions on the volcanic island of pantelleria southern italy suggest the presence at depth of a na cl thermal aquifer near 100 c 80 90 of the water in this aquifer is fed by rainfall and the remainder of the mixture is seawater chemical analyses of the gas emissions reveal that below this shallow aquifer a deeper hydrothermal degassing system 150 c may exist rising co 2 causes alterations of the upper aquifer host rocks mostly pantellerites favors formation of na hco 3 type waters and causes eventual rpecipitation of caco 3 and coprecipitation of mgco 3 and silica comparison of the present chemical data with older data from the literature suggests that the shallow pantelleria thermal aquifer has increased in salinity and decreased in temperature during the last 25 a both effects were apparently caused by increased flow of recent seawater into the system because of increased ground water abstraction in the northern part of the island

the cheslatta lake suite miocene mafic alkaline magmatism in central british columbia new mapping mineralogical and geochemical studies help characterize late tertiary primitive alkaline sodic basanite alkali olivine basalt transitional basalt and diabase in the nechako river whitesail lake and mcleod lake map areas of central british columbia and distinguish the miocene cheslatta lake suite the suite encompasses scattered erosional remnants of topographically distinct columnar jointed olivine phyric basalt and diabase volcanic necks dykes and associated lava flows north of the anahim volcanic belt and west of the pinchi fault volcanic centres at alasla mountain and at cutoff creek near cheslatta lake are proposed as type areas olivine plagioclase and pyroxene phenocrysts megacrysts and or xenocrysts common ultramafic xenoliths and rare but significant plutonic and metamorphic xenoliths are characteristic basanite transitional basalt and alkali olivine basalt groundmass contain plagioclase clinopyroxene fe ti oxides feldspathoid olivine and apatite the ch

incremental caldera collapse of suswa volcano gregory rift valley kenya suswa volcano located at 1 10 s 36 20 e is quaternary in age following caldera i collapse phonolite lava flows were erupted from the caldera floor centrally erupted phonolite lava flows led to the construction of ol doinyo onyoke lava cone a pit crater on the cone was a precursor to the collapse of caldera ii both of which were generated entirely by magma withdrawal regional decompression caused ring fault bounded block resurgence of the caldera floor

nested calderas and trapdoor uplift at pantelleria strait of sicily pantelleria is dominated by a caldera 6 km in diameter that formed 55 000 yr ago during eruption of a rheomorphic fall deposit that is zoned from pantellerite to trachyte trachytic lavas then built a shield that nearly filled the caldera magma subsequently reinflated the shallow reservoir and raised the northwest half of the trachyte shield 275 m creating montagna grande by trapdoor uplift the lid eventually failed and pantellerite magma erupted as domes flows and pumice cones along the trapdoor faults and their hingeline we have identified a second caldera 93 000 yr old within which the younger caldera is nested a still older structure may encircle them both replenishment of magma reservoirs beneath calderas can promote dilatant reactivation of older subsidence structures resulting in multicyclic collapses that reuse these structures and produce nested calderas

the recent explosive volcanism at pantelleria many of the least eighty eruptions of the six silicic eruptive cycles in the last 50 ka at pantelleria started out with explosive phases and ended with lava effusion the pyroclastic rocks are rhyolitic in composition while the later erupted lavas are less evolved up to trachyte probably due to the presence of a shallow and compositionally zoned magma chamber tapped at increasing depth during the eruptions the products of twenty explosive eruptions have been recognized most of them are represented by fallout deposits the only exceptions being the green tuff and the fossa carbonara tuff hydromagmatism is very rare though the magmas interacted with sea water it is supposed that interaction was not efficient because the magmas were not vesiculated and disrupted horizons showing variable degrees of welding up to vitrophyres have often been identified in the air fall tuffs most of the eruptions were characterized by low eruptive columns and were likely to have involved a strombolian type mechanism

complex subduction and small scale convection revealed by body wave tomography of the western united states upper mantle abstract new high resolution p and s wave tomography of the united states upper mantle from the pacific coast to the great plains reveals strong multi scale heterogeneity closely correlated with tectonic and magmatic activity we invert teleseismic travel time residuals from the earthscope transportable array and more than 1700 additional temporary and permanent stations for 3 d velocity perturbations to a depth of 1000 km the inversion uses recent advances in western u s crust models to better isolate the mantle component of travel time residuals and frequency dependent 3 d sensitivity kernels to map travel time residuals measured in multiple frequency bands into velocity structure in addition to separate v p and v s models we jointly invert the two datasets for v p v s perturbations by imposing a smoothness constraint on the ln v s ln v p field the joint inversion helps us identify regions where partial melt is probable the amplitude of v p v s and v p v s variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small scale convection partially molten mantle is inferred beneath yellowstone and the eastern snake river plain srp the salton trough and the clear lake volcanic field the inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern srp and yellowstone despite continuous subduction since the cretaceous the distribution of sub lithospheric high velocity anomalies is dissected similar to other recent studies based on our new tomography models western u s geologic history and plate tectonic reconstructions we infer patchy and incomplete removal of the flat subducting laramide slab and slab tearing associated with eocene accretion in the northwestern u s

seismic structure of the upper mantle beneath the philippine sea from seafloor and land observation implications for mantle convection and magma genesis in the izu bonin mariana subduction zone abstract we have determined the three dimensional shear wave speed structure of the upper mantle in and around the philippine sea region using seismograms recorded by dense land based and long term broadband ocean bottom seismographic stations we used a surface wave tomography technique in which multimode phase speeds are measured and inverted for a 3 d shear wave speed structure by incorporating the effects of a finite frequency and ray bending the new ocean bottom data provided us with improved spatial resolution 300 km in the philippine sea region in the upper 120 km the shear wave speed structure is well correlated with seafloor age at depths greater than 160 km fast anomalies of the subducting pacific plate are clearly defined we also found slow speed anomalies beneath the kyushu palau ridge at depths greater than 120 km along the izu bonin ogasawara mariana arc we have detected three separate slow anomalies in the mantle wedge at depths shallower than 100 km beneath the back arc each anomaly has a width of 500 km moreover these three anomalies have a close relationship with the three groups of frontal and rear arc volcanoes having distinct sr nd and pb isotope ratios we suggest that each of the anomalies is a site of large scale flow of upper mantle into the mantle wedge and that each already contains a component from the adjacent subducting slab

the pantelleria caldera geothermal system data from the hydrothermal minerals abstract this paper proposes on the basis of petrographic and mineralogic data on cutting and cores from two deep wells pantelleria 1 and pantelleria 2 the first model of the active hydrothermal system of the island of pantelleria phyllosilicates were studied in detail because they are considered key minerals in the identification of hydrothermal processes the results of these studies emphasize differences between the intracaldera and pericaldera areas of the island within the 45 ka caldera there is a high temperature 240 260 c at 600 800 m depth active hydrothermal system with five zones of characteristic alteration minerals with increasing depth rocks are unaltered to a depth of 200 m contain smectite and mixed layer chlorite smectite c s between 200 and 380 m chlorite illite chalcedony and quartz from 380 to 500 m albite adularia and saponite from 500 to 680 m mixed layer biotite vermiculite from 680 m to the depth drilled 1100 outside the caldera but near the rim a low temperature and low permeability

structural controls on the emission of magmatic carbon dioxide gas long valley caldera usa we present a degassing study of long valley caldera that explores the structural controls upon emissions of magmatic carbon dioxide gas a total of 223 soil gas samples were collected and analyzed for stable carbon isotopes using a field portable cavity ring down spectrometer this novel technique is flexible accurate and provides sampling feedback on a daily basis sampling sites included major and minor volcanic centers regional throughgoing faults caldera related structures zones of elevated seismicity and zones of past and present hydrothermal activity the classification of soil gases based on their 13c and co2 values reveals a mixing relationship among three end members atmospheric biogenic and magmatic signatures dominated by biogenic contributions 4 vol 24 are found on the caldera floor the interior of the resurgent dome and areas associated with the hilton creek and hartley springs fault systems with the introduction of the magmatic component 100 vol 4 5 samples acquire mixing and hydrothermal signatures and are spatially associated with the central caldera and mammoth mountain in particular they are concentrated along the southern margin of the resurgent dome where the interplay between resurgence related reverse faulting and a bend in the regional fault system has created a highly permeable fracture network suitable for the formation of shallow hydrothermal systems this contrasts with the south moat where despite elevated seismicity a thick sedimentary cover has formed an impermeable cap inhibiting the ascent of fluids and gases to the surface

1 d electrical conductivity structure beneath the philippine sea results from an ocean bottom magnetotelluric survey abstract eight months of observation using ocean bottom electro magnetometers obems have allowed us to estimate the regional electrical conductivity structure beneath the philippine sea six obems were deployed along a line crossing the philippine sea from nw to se and five of them recorded useful data the raw time series data were cleaned up before we estimated the magnetotelluric mt impedance tensor conductivity structure at five sites is estimated using 1 d occam s inversion to fit the determinant average of each mt impedance tensor after a correction for the effect of topography we examined effect from two dimensionalities on the 1 d conductivity structure and the robustness of solutions the results of the 1 d conductivity structural model are strongly related to tectonic setting and the crustal age beneath each site the structure beneath the spreading axis of the mariana trough shows a distinct low conductivity structure at depths of 50 150 km and it probably reflects the upwelling dynamics operating beneath the spreading axis these low values are comparable with that of olivine with low hydrogen content implying that 1 the melting process extracts water from minerals such as olivine and 2 the melt beginning depth in the mariana trough is deeper than that of the typical morb source region the off axis conductivity profiles infer the existence of a high conductivity peak or a conductivity gradient change at mid depth the depth level of the peak increases with crustal age suggesting that the conductivity structure is related to a geothermal structure and that these conductivity profiles are explained by the temperature gradient change possibly combined with the presence of partial melt our results suggest that further ocean bottom em study has high potential to investigate the temperature gradient change and amount of hydrogen water and melt in the upper mantle

geochemistry of the miocene mount noel volcanic complex british columbia and comparison with the columbia river basalt the 19 6 ma mount noel volcanic complex mnvc is located about 150 km east of the garibaldi volcanic belt and adjacent to the chilcotin plateau basalts in southwestern british columbia the mnvc unconformably overlies jurassic to cretaceous metasedimentary rocks and lower cretaceous coast plutonic rocks the complex consists of flat lying columnar jointed basalt flows along with volcaniclastic debris flows and minor pyroclastic rocks basaltic flows have olivine fo 60 to fo 85 and plagioclase an 60 to an 80 phenocrysts with clinopyroxene microphenocrysts major trace and rare earth element analyses of the mnvc indicate the flows can be divided into two groups the largest group comprises tholeiitic to transitional basalt whereas a smaller subset is alkaline basalt rare earth element patterns are slightly enriched in the light rare elements in the transitional group and more strongly enriched in the alkaline basalts in tectonic discriminant diagrams all basalts plot in the within plate fields or in overlap areas typical of continental flood basalts alkaline basalts found only at the base of the thick sequence of tholeiitic to transitional basalts are characterized by high p 2 o 5 sr and ba contents the mnvc tholeiitic to transitional basalts are comparable to the chilcotin group basalts and to the lowermost basalts of the columbia river group the origin of mnvc basalts is best interpreted as volcanism behind the pemberton volcanic arc

paleomagnetism of pleistocene volcanic rocks from pantelleria island sicily channel italy abstract a paleomagnetic investigation has been carried out on pleistocene volcanic rocks from pantelleria in the sicily channel this island is characterised by a bimodal volcanism of peralkaline rhyolite to trachyte and basalt composition the radiometric data indicate that volcanic activity started around 324 ka bp samples have been collected from 30 sites in 16 volcanic units spanning activity during the last 150 ka magnetic properties vary systematically with lithotype and indicate high ti titanomagnetite to magnetite as the main carriers of magnetisation stable characteristic remanent magnetisation chrm directions isolated by alternating fields af demagnetisation show normal polarity consistent with emplacement during the brunhes chron and vary within the paleosecular variation psv range however the inclinations are low as compared with the geocentric axial dipole gad inclination at pantelleria especially for volcanic units younger than 50 ka the mean chrm direction computed from 27 sites is d 358 8 i 46 9 k 30 95 5 2 and the inclination anomaly is i 8 1 both tectonic movements and the presence of large magnetic anomalies around the island of pantelleria fail to explain this low inclination which may therefore be related to a significant long lived non dipolar field contribution in the area over the past 150 kyr

correction of seafloor magnetotelluric data for topographic effects during inversion 1 the large contrast in electrical conductivity between seawater and the underlying seafloor accumulates boundary electric charges which can severely distort observed electric and magnetic fields for marine magnetotelluric mt studies correcting this topographic effect is critical to obtaining accurate conductivity models for the mantle previously correction for topography was based on the thin sheet approximation which breaks down at periods under 1000 s in the deep ocean this paper introduces an analysis method for seafloor mt data which combines removal of three dimensional 3 d topographic effects with inversion of the data for 2 d structure the observed mt impedance is first corrected to a flat lying seafloor datum using the observed bathymetry without invoking the thin sheet approximation the corrected mt response is then inverted in a flat seafloor model space because of coupling between topographic effects and deeper structure the correction and inversion steps are iterated until changes in each become small the procedure is verified using synthetic and real data tests for synthetic 3 d topography over a half space show that the method closely recovers the true half space model after a few iterations the procedure is also applied to real data collected in the mantle electromagnetic and tomography melt experiment on the east pacific rise at 17 s

the eruptive history of pantelleria sicily channel in the last 50 ka six silicic eruptive cycles have been recognized in the last 50 ka at pantelleria the products of each cycle exhibit a compositional variation from pantellerite to less peralkaline rhyolite or to trachyte the relationships between the range of chemical variation the erupted volume and the time of eruptions allow us to estimate an average differentiation rate of 5 crystal fractionation per 1000 years and a constant long term rate of magma discharge of 0 1 km3 per 1000 years pressure increase in the magma chamber caused by the addition of new magma accumulation of highly differentiated volatile rich magma in the roof zone and a concomitant build up of a vapour phase is postulated as a possible triggering mechanism for eruptions

an early pleistocene proglacial succession in south central british columbia a succession of conglomerates sandstones siltstones and tills herein named the dog creek formation is sandwiched between flat lying basalts along dog creek lat 51 36 n long 122 02 122 12 w for about 15 km east of fraser river the sedimentary succession rests disconformably on underlying basaltic lavas herein referred to as the harpers creek formation which have yielded k ar dates of 1 3 2 9 ma and in one place on a glaciated surface carved in metavolcanic rocks of permian age the sedimentary succession is capped by basalt flows yielding k ar whole rock ages of 1 1 ma the occurrence of proglacial beds and a glaciated surface in south central british columbia 70 km away from any high mountains capable of supporting glaciers today testifies to a major glacial stage in early pleistocene time the record of sedimentation and volcanism sheds light on early incision of the nearby valley of fraser river

geodynamics of the yellowstone hotspot and mantle plume seismic and gps imaging kinematics and mantle flow abstract integration of geophysical and geological data show that the yellowstone hotspot resulted from a mantle plume interacting with the overriding north america plate a process that has highly modified continental lithosphere by magmatic and tectonic processes and produced the 16 17 ma 700 km long yellowstone snake river plain ysrp silicic volcanic system accessibility of the ysrp allowed large scale geophysical projects to seismically image the hotspot and evaluate its kinematic properties using geodetic measurements seismic tomography reveals a crustal magma reservoir of 8 to 15 melt 6 km to 16 km deep beneath the yellowstone caldera an upper mantle low p wave velocity body extends vertically from 80 km to 250 km beneath yellowstone but the anomalous body tilts 60 wnw and extends to 660 km depth into the mantle transition zone we interpret this conduit shaped low velocity body as a plume of up to 3 5 vp and 5 5 vs perturbation that corresponds to a 1 2 partial melt models of whole mantle convection reveal eastward upper mantle flow beneath yellowstone at relatively high rates of 5 cm yr that deflects the ascending plume into its west tilted geometry a geodynamic model of the yellowstone plume constrained by vp and vs velocities and attenuation parameters suggests low excess temperatures of up to 120 k corresponding to a maximum 2 5 melt and a small buoyancy flux of 0 25 mg s i e properties of a cool weak plume the buoyancy flux is many times smaller than for oceanic plumes nonetheless plume buoyancy has produced a 400 km wide 500 m high topographic swell centered on the yellowstone plateau contemporary deformation derived from gps measurements reveals sw extension of 2 3 mm yr across the yellowstone plateau one fourth of the total basin range opening rate which we consider to be part of basin range intraplate extension locally decadal episodes of subsidence and uplift averaging 2 cm yr characterize the 80 year yellowstone caldera monitored history and are modeled as hydrothermal magmatic sources moreover a recent episode 2004 2009 of accelerated uplift of the yellowstone caldera at rates up to 7 cm yr has been modeled as resulting from magmatic recharge of a 10 km deep sill at the top of the crustal magma reservoir regionally gravitational potential energy of the yellowstone swell drives the lithosphere southwest and downhill from the yellowstone plateau 400 km where it coalesces with basin range province wide westward extension based on the geometry and its assumed 660 km depth we extrapolate the plume source southwest to its original location at 17 ma and 600 km southwest and 200 km north of the ysrp importantly this location is beneath the southern part of the columbia plateau flood basalt field of the same age and implies that the yellowstone mantle plume may be the common source for both of these large volcanic fields our time progression model suggests that the original plume head rose vertically behind the juan de fuca plate but at 12 ma it lost the protection of the subducting plate from eastward mantle flow and encountered cooler thicker continental lithosphere becoming entrained in eastward upper mantle flow these results reveal that yellowstone plume plate processes have had a profound effect on late cenozoic geologic evolution and topography of a large part of the western u s

eocene and neogene volcanic rocks in the southeastern nechako basin british columbia interpretation of the canadian hunter seismic reflection surveys using first arrival tomography geological investigation of the near surface in the southeastern nechako basin is difficult shallow seismic reflection imaging is poor due in part to an extensive cover of eocene and neogene volcanic rocks outcrops of these volcanic rocks and the primarily cretaceous bedrock are commonly obscured by quaternary deposits and vegetation estimates of near surface p wave velocity are derived from the tomographic inversion of seismic first arrivals an effective tool when seismic imaging is poor tomographic model velocities are in agreement with sonic logs and laboratory samples except for those from the neogene chilcotin group cretaceous sedimentary rocks have velocities of 2800 4200 ms 1 the eocene endako and ootsa lake groups which have velocities of 3000 4200 ms 1 are not distinguishable based on velocity the velocity the character density focus and penetration depth of rays and ties with well and surface geology constrain the subsurface extent of the endako group adjacent to well b 82 c

a new technique for the incorporation of seafloor topography in electromagnetic modelling summary we describe a new technique for incorporating seafloor topography in electromagnetic modelling it is based on a transformation of the topographic relief into a change in electrical conductivity and magnetic permeability within a flat seafloor since the technique allows us to model arbitrary topographic changes without extra grid cells and any restriction by vertical discretization we can model very precise topographic changes easily without an extra burden in terms of computer memory or calculation time the reliability and stability of the technique are tested by comparing the magnetotelluric responses from two synthetic seafloor topography models with three different approaches these incorporate the topographic change in terms of 1 the change in conductance using the thin sheet approximation 2 a series of rectangular block like steps and 3 triangular finite elements the technique is easily applied to any modelling method including 3d modelling allowing us to model complex structure in the earth while taking full account of the 3d seafloor topography

deflation acting on pantelleria island sicily channel inferred through geodetic data abstract pantelleria is a volcanic island situated at the centre of the sicily channel continental rift since 1980 the island has been monitored by geodetic techniques electro optical distance measurements edm and levelling and gravimetry after more than 10 years the edm network was re measured in 1995 and successively it has been improved and measured also with global position system gps techniques it has therefore been possible to carry out a long term analysis 1980 1996 of the deformation pattern the levelling in the 1980 1996 period revealed a marked lowering of the central southern part of the island with respect to the northern sector the horizontal edm measurements all prove to be in contraction and suggest that a deflation is ongoing in the central southern part the effects of areal contraction and lowering are interpreted as being due to the volcanic field which appears to predominate over possible stretching induced by the sicily channel dynamics we modelled the horizontal and vertical changes accumulated over 16 years using a 3d ellipsoidal depressuring model which indicates a ca 5 km deep source located under the area within the most recent caldera this model could explain a short term tens of years acting deflation we also propose an interpretive framework in the case where the deflation is acting over a long period thousands of years in this case the deflation is interpreted as readjustment following the loss of mass occurring with recent volcanism 18 3 kyr

slab1 0 a three dimensional model of global subduction zone geometries 1 we describe and present a new model of global subduction zone geometries called slab1 0 an extension of previous efforts to constrain the two dimensional non planar geometry of subduction zones around the focus of large earthquakes slab1 0 describes the detailed non planar three dimensional geometry of approximately 85 of subduction zones worldwide while the model focuses on the detailed form of each slab from their trenches through the seismogenic zone where it combines data sets from active source and passive seismology it also continues to the limits of their seismic extent in the upper mid mantle providing a uniform approach to the definition of the entire seismically active slab geometry examples are shown for two well constrained global locations models for many other regions are available and can be freely downloaded in several formats from our new slab1 0 website http on doi gov d9arbs we describe improvements in our two dimensional geometry constraint inversion including the use of average active source seismic data profiles in the shallow trench regions where data are otherwise lacking derived from the interpolation between other active source seismic data along strike in the same subduction zone we include several analyses of the uncertainty and robustness of our three dimensional interpolation methods in addition we use the filtered subduction related earthquake data sets compiled to build slab1 0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far concluding that the width of these seismogenic zones is on average 30 larger than previous studies have suggested

a joint interpretation of electromagnetic and seismic tomography models suggests the mantle transition zone below europe is dry abstract the three dimensional 3d electrical conductivity distribution in the mantle transition zone mtz below europe is resolved by directly inverting geomagnetic response functions from 12 selected european observatories comparison of the conductivity image with the results of global seismic tomography reveals a good correlation between low conductivity and high velocity anomalies at depths of 400 700 km beneath the mediterranean subduction zone in central europe interpretation of the 3d conductivity model combined with seismic tomography indicates that the european mtz is drier than the mtz beneath the western pacific subduction zone even though the subducted slab material is stagnating similarly in both regions

neogene chilcotin basalts in south central british columbia geology ages and geomorphic history an extensive but generally thin mantle of basalt flows the chilcotin group covers much of the interior plateau of south central british columbia it provides material for dating and for reconstructing the original form of the paleosurface on which it was deposited k ar whole rock dates demonstrate that several ages of basalt are represented from early miocene or even late oligocene to early pleistocene with particularly abundant eruptions about 14 16 9 6 and 1 3 ma ago basalts of middle miocene and later ages if not the early miocene relics as well clearly rest on land surfaces of low local relief in places the low relief surfaces had been incised to depths of 100 200 m and the valleys backfilled with mid or late cenozoic sediments prior to burial by the basalts the low relief surfaces throughout the area are believed to have been developed close to a common base level and regional differences in their present elevation are thus largely a product of post basalt deformation this is recorded

the green tuff of pantelleria rheoignimbrite or rheomorphic fall detailed stratigraphic analysis of the green tuff of pantelleria shows that this formation can be divided into several members designateda throughh from base to top these members have a coherent pattern when traced from outcrop to outcrop throughout the island shedding light on their origin only memberg completely mantles the entire island the distribution of the other members is controlled by prevailing wind direction or by topography membera is entirely of fall origin membersc ande are of fall and or surge type membersb d andh have the characteristics of thin welded ash flow tuffs membersf andg are ash flow tuffs with textural characteristics of compound cooling units most of the ash flow tuffs exhibit characteristics of ignimbrites vertical fluidization pipes local concentrations of lithic lapilli imbrication of clasts and valley ponding memberg is unusual in that it is highly welded exhibits large scale rheomorphic structures contains huge lithic clasts and has near vertical foliation where it adheres to cliffs and caldera walls

the gang ranch big bar area south central british columbia stratigraphy geochronology and palynology of the tertiary beds and their relationship to the fraser fault tertiary rocks in the gang ranch big bar area south central british columbia consist of 1 early or middle eocene 50 ma lavas breccias and tuffs capped by a mappable unit of conglomerate a

regional variations in the chemical and helium carbon isotope composition of geothermal fluids across tunisia abstract tunisia has numerous thermo mineral springs previous studies have shown that their chemical composition and occurrence are strongly influenced by the regional geology however little work has been done so far to study the isotopic composition of volatiles associated with these geothermal manifestations here we report on the results of an extensive survey of both natural hot springs and production wells across tunisia aimed at investigating the spatial distribution of thermal fluids geochemical characteristics and he c isotopic composition the chemistry of the analyzed samples highlights the heterogeneity of the water mineralization processes in tunisia as a consequence of the complex geological and tectonic setting of the country in terms of chemical composition we are able to conclude however that dissolution of halite and gypsum plays a key control on groundwater chemistry helium and carbon isotope systematics confirm the prevalently crustal origin of the volatiles interacting with the aquifer systems consistent with the absence of any recent magmatism most samples are characterized by crustal type helium 3 he 4 he in the range 0 02 ra 0 4 ra associated with a co 2 predominantly metamorphic in origin with the exception of the saharan platform where the carbon content is low and mostly organically derived in eastern tunisia however a few samples have he c isotope compositions which suggest at least partial mantle derivation of the dissolved gas phase the 3 he 4 he ratio reaches 2 4 ra corresponding to 30 of mantle derived helium at the ain garci site a co 2 rich mineral spring located some 30 km south of the city of zaghouan this mantle signature is consistent with the fact that the pelagian block to which eastern tunisia belongs has been deeply affected by extensional and transtensional tectonics since the opening of the tethys a process which is still ongoing in the sicily channel pantelleria rift as a whole however our results show that the italian mantle gas anomaly only marginally extends to northwestern africa

models of lithosphere and asthenosphere anisotropic structure of the yellowstone hot spot from shear wave splitting 1 teleseismic shear wave splitting measured at 56 continuous and temporary seismographs deployed in a 500 km by 600 km area around the yellowstone hot spot indicates that fast anisotropy in the mantle is parallel to the direction of plate motion under most of the array the average split time from all stations of 0 9 s is typical of continental stations there is little evidence for plume induced radial strain suggesting that any contribution of gravitationally spreading plume material is undetectably small with respect to the plate motion velocity two stations within yellowstone have splitting measurements indicating the apparent fast anisotropy direction is nearly perpendicular to plate motion these stations are 30 km from stations with parallel to plate motion the 70 rotation over 30 km suggests a shallow source of anisotropy however split times for these stations are more than 2 s we suggest melt filled stress oriented cracks in the lithosphere are responsible for the anomalous orientations within yellowstone stations southeast of yellowstone have measurements of oriented nnw to wnw at high angles to the plate motion direction the archean lithosphere beneath these stations may have significant anisotropy capable of producing the observed splitting

structure and deformation processes in the strait of sicily rift zone abstract the strait of sicily rift zone ssrz is a young tectonic feature early pliocenerecent cutting across the pelagian sea it is characterized by a thin continental type crust which progressively thickens towards the african continental margin and beneath the sicilian chain geophysical data available for the whole area show that extensional faulting is active in many places along structures trending roughly nnw sse but that the dominant mode of deformation at a regional scale can be best described in terms of strike slip tectonics strike slip faults trending nw se are seismically documented west of the island of pantelleria in the eastern sector of the malta graben and along the southern border of the medina graben similar features have also been mapped on land in malta and pantelleria kinematic analysis of these structures suggest that the ssrz cannot be considered as a classical intracontinental rift because the deformation does not appear to be dominated by a generalized thermal instability but rather by compressive stresses in a transcurrent tectonic regime therefore an alternative model which accounts for the activation of large scale right lateral strike slip faults seems to be more suitable within this main dextral shear zone the progressive development of pull apart basins could be responsible for the incipient separation of the sicilian microplate from the african continent and for the thinning of the crust and the associated volcanic activity of the northwestern sector of the ssrz

application of 3d inversion to magnetotelluric profile data from the deccan volcanic province of western india abstract using synthetic data siripunvaraporn et al 2005b demonstrated possible advantages of interpreting single profile mt data with a three dimensional 3d inversion program here we explore this idea further using real mt data from two profiles on the indian subcontinent the first profile 330 km long cuts across the deccan volcanic province of peninsular india the second 130 km long is in the narmada son lineament zone approximately 100 km further north using the data space occam inversion code of siripunvaraporn et al 2005a 3d inversion is carried out on each of these profiles independently and results are compared with previously published two dimensional 2d interpretations in addition to inversion of the full impedance tensor we consider 3d inversion of only the off diagonal components we also experiment with variants on the model covariance in particular allowing for longer smoothing length scales along the geoelectric strike not surprisingly the 3d inversion finds models that fit the data better than had been possible with the 2d programs many of the features inferred from these previous 2d interpretations are also present in the 3d inverse solutions but the positions and amplitudes of individual conductive features are in some cases changed the 3d models suggest substantial non uniqueness in the single profile data even without explicit or special treatment we find that the relatively modest near surface distortion effects in these datasets were well fit by the 3d inversion by inserting small scale conductive and resistive features in surface layers mostly off profile

new constraints on the properties of the yellowstone mantle plume from p and s wave attenuation tomography 1 we estimate attenuation t for teleseismic p and s arrivals to seismometers in the yellowstone intermountain seismic array tomographically invert these data for upper mantle qp 1 and qs 1 structure and with the aid of the upper mantle velocity model of waite et al 2006 interpret the results for mantle temperature partial melt and water content because attenuation analysis is susceptible to contamination by noise we employ time and frequency domain analyses and a careful assessment of the uncertainty associated with each estimate by using the misfit between actual and predicted traces or spectra the greatest noise source is signal generated noise which affects s wave results more than p wave results s waves also appear more susceptible to the effects of plume focusing we find the upper mantle to be highly attenuative but that above 200 250 km the low velocity plume is progressively less attenuative than the adjacent mantle with decreasing depth we conclude that water dissolved in mantle minerals causes the upper mantle to be highly attenuative that the yellowstone plume is only 50 c warmer than the surrounding mantle below the north america lithosphere and that melting within this plume begins at depths of 200 250 km we attribute the lower attenuation in the partially molten plume to the dehydration of the solid matrix as water partitions into the melt the source of the upper mantle hydration is attributed to subduction including water flux from a hydrated transition zone and laramide age shallow subduction and direct hydration of north america lithosphere

mechanical models for correlation of ring fracture eruptions at pantelleria strait of sicily with glacial sea level drawdown recent k ar dating of eruptions at pantelleria a peralkaline volcanic island in the strait of sicily shows a correlation between eruption of pantellerite lavas from caldera ring fractures and low stands of sea level as determined from 18o stratigraphy post caldera pantellerite lavas associated with an 114 ky old caldera erupted along the ring fracture zone during a major low stand of sea level at about 67 ka the most recent episode of lava flow emplacement began about 20 ky ago during the last glacial maximum magma vented along the ring fault of a 45 ky old caldera from fractures radial to the caldera and along faults formed by intracaldera trapdoor uplift two mechanical models based on elasticity theory are presented to explain the correlation of post caldera ring fracture eruptions at pantelleria with lowering of sea level a simple analysis of a bending circular plate of thickness t r and radius r representing the magma chamber roof block shows that tensile stress is concentrated by a factor of 0 75r 2 t 2 at the lower perimeter of the plate when sea level drops stress changes may be even greater ift r is effectively less than the stratigraphic thickness due to layering of rocks in the roof block calculated stress changes due to a 100 m drawdown of sea level are similar in magnitude to stresses associated with dike propagation more realistic model geometries including different chamber shapes a conical volcanic edifice and sea level drawdown beyond the surface projection of the magma chamber were tested using the boundary element method lowering sea level generates a horizontal tensile stress above the chamber even when sea water is removed outboard of the magma chamber for some chamber geometries the magnitude of the tensile stress maximum is greater than the 1 mpa pressure of the 100 m of removed water and is of the right order of magnitude for dike propagation dikes initiated by the change of the stress field may originate and propagate along fractures inboard of the chamber margin the magnitudes of tensile maxima along the top of the chamber decrease as original sea level is moved outboard of the chamber margin and as the chamber thickness decreases when the depth to the top of the magma chamber reaches a critical value dependent on chamber geometry the propagation of dikes to the surface is inhibited

the strait of sicily continental rift systems physiography and petrochemistry of the submarine volcanic centres abstract new morphological geophysical mineralogical and petrochemical data on the submarine volcanic centres of the strait of sicily continental rift systems lead to the following conclusions magma types both in the rift valleys pantelleria and linosa and plateau areas adventure bank suggest mantle sources and fractionation processes comparable to those of the ethiopian afar section of the east african rift despite their different tectonic regimes however the volume of the erupted magmas less than 2000 3000 km 3 indicates the similarity with low volcanicity dry rifts this is probably related to the peculiar tectonic setting of the pelagian block african foreland dominated by extensional transcurrent motions connected with a strong neogene compressive dextral e w megashear along the sicilian collisional front in the plateau regions the most evolved rocks are hawaiites graham bank which formed by fractionation in the uppermost mantle probably at the base of the crust magma types vary from quartz tholeiites to basanites in the rift valleys the range of magma types is more restricted transitional to alkali basalts and a considerable volume of highly evolved magmas particularly in the axial volcanic complex of the pantelleria rift was produced in crustal magma chambers isotopic trace element evidence suggests the involvement of transitional to enriched morb and oib type mantle sources in the genesis of the quartz tholeiites alkali basalts and basanites throughout the strait of sicily continental rift system including the iblean plateau

the eocene oligocene magmatic hiatus in the south central canadian cordillera a capture of the kula plate by the pacific plate the tertiary paleogene and neogene geological record in south central canadian cordillera is dominated by the 350 400 km wide lower eocene volcanic arc and the overlying miocene recent back arc lavas that are separated by a hiatus in magmatic activity between 48 and 24 ma in the black dome area 240 km north of vancouver the eocene volcanic rocks are mainly continental margin calc alkaline andesite and dacite resulting from the melting of a juvenile mafic source at the base of the crust in contrast the miocene volcanic rocks resemble continental flood basalts both eocene and miocene rocks from the black dome volcanic complex have high positive end values 7 2 to 7 4 and 6 4 to 7 6 respectively and low initial sr isotopic ratios 0 702 516 0 703 528 and 0 703 376 0 703 392 respectively comparable to modern oceanic basalts the onset of the hiatus in magmatism at 48 ma coincides with capture of the kula plate by the pacific plate resulting in a change in convergence direction with the

a revision of the structure and stratigraphy of pre green tuff ignimbrites at pantelleria strait of sicily abstract at pantelleria peralkaline silicic magmas were erupted across a range of eruptive typologies and magnitudes pyroclastic flows plinian to strombolian pumice fallout and lava flows in this paper we focus on the intermediate cycle of eruptive activity which is bracketed by ignimbrite units slightly older than the two caldera collapses which marked the volcanological activity of the island this age interval 180 85 ka was punctuated by six ignimbrite forming eruptions silicic and variably peralkaline for a cumulative erupted magma volume of approximately 6 km 3 dense rock equivalent based on new 40 ar 39 ar na k feldspar ages and petrographic data we propose an updated volcanostratigraphic scheme for these welded and rheomorphic ignimbrites that can be summarised as follows i the age of the old la vecchia caldera collapse is now tightly constrained between 140 and 146 ka and the caldera forming eruption can be traced to a lithic rich welded tuff breccia that outcrops in two opposite sectors of the island south west and north east ii four ignimbrite units previously considered unrelated are now merged in two distinct eruptive paroxysmal events at 107 and 85 ka in particular the 85 ka eruptive event is comparable in magnitude to the younger caldera forming green tuff plinian eruption iii the recurrence patterns of the 107 and 85 ka eruptions compared to the green tuff allow us to qualitatively assess that the climax in production of low temperature silicic and peralkaline melt was focused in the age interval 85 45 ka

seismic and electrical signatures of the lithosphere asthenosphere system of the normal oceanic mantle although plate tectonics started as a theory of the ocean basins nearly 50 years ago the mechanical details of how it works are still poorly known our understanding of these details has been hampered partly by our inability to characterize the physical nature of the lithosphere asthenosphere system las beneath the ocean we review the existing observational constraints on the seismic and electrical properties of the las particularly for normal oceanic regions away from mid oceanic ridges hot spots and subduction zones where plate tectonics is expected to present its simplest form whereas a growing volume of seismic data on land has provided remarkable advances in large scale pictures seafloor observations have been shedding new light on essential details by combing through these observational constraints researchers are unveiling the nature of the enigmatic las future directions for large scale seafloor observations are also discussed

trans pacific temperature field in the mantle transition region derived from seismic and electromagnetic tomography abstract the trans pacific temperature field for the depth range 350 850 km was inferred from global seismic tomography and semi global electromagnetic tomography the seismic tomography incorporated millions of reported first arrival times and 7000 pp p differential travel times measured on broadband seismograms the electromagnetic tomography used voltage data from trans pacific submarine cables and magnetic field data from circum pacific geomagnetic observatories the resultant p wave velocity anomalies and electrical conductivity anomalies were converted to temperature anomalies using a proposed conversion formula and experimental results for mantle minerals respectively these conversions show consistently high temperature anomalies of 200 300 k in the mantle transition region beneath the hawaiian hotspot at subduction zones where slab related cold anomalies and wedge mantle related hot anomalies are likely to coexist in close proximity the seismic and electromagnetic tomography did not always give consistent features in part because of the preferred sensitivity of electromagnetic tomography to hot anomalies low temperature anomalies of 200 300 k associated with subducted slabs are clearly resolved in the seismic tomography but are less apparent in the electromagnetic tomography the high temperature anomaly in the intervening zone between the mariana and philippine slabs is very pronounced in the electromagnetic tomography but is marginal in the seismic tomography

evidence of magma mixing in the daly gap of alkaline suites a case study from the enclaves of pantelleria italy the island of pantelleria consists of trachytes pantellerites and minor mildly alkaline basalts rocks of intermediate composition falling in the so called daly gap such as mugearites benmoreites and mafic trachytes occur only in the form of enclaves in trachytes and pantellerites inside the main caldera of the island caldera cinque denti which collapsed during the green tuff ignimbrite eruption at 50 ka the enclaves include volcanic subvolcanic and intrusive rock types the enclaves in host trachyte contain traces of glass devitrified glass occurs within enclaves in host pantellerites minerals in the enclaves show regular compositional variations with whole rock silica content glass present in the medium grained samples is interpreted to be the result of incipient melting the major and trace element compositions of the enclaves show regular and linear variations between an evolved mafic magma hawaiite and a felsic end member similar to the green tuff trachyte fractional crystallization modelling of compatible and incompatible trace elements v ni zr la sm lu nb y th does not reproduce the observed trends rocks of intermediate composition within the daly gap can be explained only by magma mixing between an already differentiated mafic magma hawaiite and an anorthoclase rich trachytic melt in the lower and higher parts respectively of a stratified magmatic chamber medium grained enclaves are interpreted as the result of fragmentation of solidified mixing layers in the roof of the magma chamber during the eruption of the green tuff when the collapse of the caldera took place diffusion calculations suggest a residence time of 5 days for the enclaves in their host magmas

caldera resurgence during magma replenishment and rejuvenation at valles and lake city calderas a key question in volcanology is the driving mechanisms of resurgence at active recently active and ancient calderas valles caldera in new mexico and lake city caldera in colorado are well studied resurgent structures which provide three crucial clues for understanding the resurgence process 1 within the limits of 40ar 39ar dating techniques resurgence and hydrothermal alteration at both calderas occurred very quickly after the caldera forming eruptions tens of thousands of years or less 2 immediately before and during resurgence dacite magma was intruded and or erupted into each system this magma is chemically distinct from rhyolite magma which was resident in each system 3 at least 1 km of structural uplift occurred along regional and subsidence faults which were closely associated with shallow intrusions or lava domes of dacite magma these observations demonstrate that resurgence at these two volcanoes is temporally linked to caldera subsidence with the upward migration of dacite magma as the driver of resurgence recharge of dacite magma occurs as a response to loss of lithostatic load during the caldera forming eruption flow of dacite into the shallow magmatic system is facilitated by regional fault systems which provide pathways for magma ascent once the dacite enters the system it is able to heat remobilize and mingle with residual crystal rich rhyolite remaining in the shallow magma chamber dacite and remobilized rhyolite rise buoyantly to form laccoliths by lifting the chamber roof and producing surface resurgent uplift the resurgent deformation caused by magma ascent fractures the chamber roof increasing its structural permeability and allowing both rhyolite and dacite magmas to intrude and or erupt together this sequence of events also promotes the development of magmatic hydrothermal systems and ore deposits injection of dacite magma into the shallow rhyolite magma chamber provides a source of heat and magmatic volatiles while resurgent deformation and fracturing increase the permeability of the system these changes allow magmatic volatiles to rise and meteoric fluids to percolate downward favouring the development of hydrothermal convection cells which are driven by hot magma the end result is a vigorous hydrothermal system which is driven by magma recharge

the nature and rheology of the cordilleran upper mantle of british columbia inferences from peridotite xenoliths abstract peridotite xenoliths in alkalic basaltic rocks from eight localities within the british columbian cordillera include metamorphic tectonites metacumulates and cumulates chemical analyses of these show that the xenoliths encompass the range of both depleted and primitive undepleted mantle application of two pyroxene thermobarometric techniques yield geotherms characteristic of normal oceanic mantle and diapiric mantle wherein upper mantle stratigraphic profiles are either homogeneously stratified or heterogeneously mixed respectively stress estimates as a function of depth are obtained for peridotite xenoliths from each of the localities by applying recrystallised grain size paleopiezometry and pyroxene thermobarometry stress and temperatures obtained are combined with a representative creep flow law to calculate strain rate and viscosity profiles that appear to be physically reasonable these profiles are juxtaposed to present a dynamic model of a mantle diapir convecting through a stratified mantle section and these two different types of mantle stratigraphic profile are related to current large scale tectonic theories of the cordillera

correlation of welded ignimbrites on pantelleria strait of sicily using paleomagnetism although the oldest volcanic rocks exposed at pantelleria strait of sicily are older than 300 ka most of the island is covered by the 45 50 ka green tuff ignimbrite thought to be related to the cinque denti caldera and younger lavas and scoria cones pre 50 ka rocks predominantly rheomorphic ignimbrites are exposed at isolated sea cliffs and their stratigraphy and chronology are not completely resolved based on volcanic stratigraphy and k ar dating it has been proposed that the older la vecchia caldera is related to ignimbrite q 114 ka and that ignimbrites f d and z 106 94 and 79 ka respectively were erupted after caldera formation we report here the paleomagnetic directions obtained from 23 sites in ignimbrite p 133 ka and four younger ignimbrites and from an uncorrelated and loosely dated welded lithic breccia thought to record a caldera forming eruption the paleosecular variation of the geomagnetic field recorded by ignimbrites is used as correlative tool with an estimated time resolution in the order of 100 years we find that ignimbrites d and z correspond in good agreement with recent ar ar ages constraining the d z eruption to 87 ka the welded lithic breccia correlates with a thinner breccia lying just below ignimbrite p at another locality implying that collapse of the la vecchia caldera took place at 130 160 ka this caldera was subsequently buried by ignimbrites p q f and d z paleomagnetic data also show that the northern caldera margin underwent a 10 west northwest outwards tilting after emplacement of ignimbrite p possibly recording magma resurgence in the crust

geology of the peralkaline volcano at pantelleria strait of sicily situated in a submerged continental rift pantelleria is a volcanic island with a subaerial eruptive history longer than 300 ka its eruptive behavior edifice morphologies and complex multiunit geologic history are representative of strongly peralkaline centers it is dominated by the 6 km wide cinque denti caldera which formed ca 45 ka ago during eruption of the green tuff a strongly rheomorphic unit zoned from pantellerite to trachyte and consisting of falls surges and pyroclastic flows soon after collapse trachyte lava flows from an intracaldera central vent built a broad cone that compensated isostatically for the volume of the caldera and nearly filled it progressive chemical evolution of the chamber between 45 and 18 ka ago is recorded in the increasing peralkalinity of the youngest lava of the intracaldera trachyte cone and the few lavas erupted northwest of the caldera beginning about 18 ka ago inflation of the chamber opened old ring fractures and new radial fractures along which recently differentiated pantellerite constructed more than 25 pumice cones and shields continued uplift raised the northwest half of the intracaldera trachyte cone 275 m creating the island s present summit montagna grande by trapdoor uplift pantellerite erupted along the trapdoor faults and their hingeline forming numerous pumice cones and agglutinate sheets as well as five lava domes degassing and drawdown of the upper pantelleritic part of a compositionally and thermally stratified magma chamber during this 18 3 ka episode led to entrainment of subjacent crystal rich pantelleritic trachyte magma as crenulate inclusions progressive mixing between host and inclusions resulted in a secular decrease in the degree of evolution of the 0 82 km3 of magma erupted during the episode

chronostratigraphy of eocene volcanism central british columbia onset and termination of eocene felsic volcanism in the chilcotin plateau of central british columbia is constrained between 54 6 and 46 6 ma by 33 new u pb and 40ar 39ar isotopic age determinations dates were obtained from representative felsic coherent and fragmental volcanic rocks that comprise the ootsa lake group the resulting chronostratigraphy shows that magma compositions evolved from felsic to intermediate with no spatial migration of the volcanic activity rhyolitic compositions are oldest and are overlain by dacitic rocks with varied phenocrysts assemblages in many parts of the chilcotin plateau the eocene stratigraphy is capped by distinctive vitreous black dacite lavas which are contemporaneous with andesitic lavas of the endako group in the nechako plateau to the north crystallization ages from ootsa lake group rocks of the chilcotin plateau overlap age determinations from correlative rocks of the nechako plateau and southern bc collectively this geochronological dataset supports p

transtensional tectonics in the sicily channel abstract structural geophysical and volcanological data available for the sicily channel demonstrate that the whole area is characterized by the occurrence of transtensional structures activated in a dextral shear zone trending roughly wnw ese these data allowed us to put forward a kinematic model of the area which accounts for both the existence of discrete tectonic depressions and for the localized volcanic activity of this sector of the pelagian sea the proposed model is somewhat different from other rifting mechanisms available for the sicily channel in that it may explain the occurrence of extensional features and the associated volcanism in a zone of continental collision through the development of large scale pull apart basins involving deep crustal levels