18o exchange between steam and carbon dioxide in volcanic and hydrothermal gases implications for the source of water abstract the oxygen isotopic compositions of h 2 o and co 2 from 31 fumaroles from different volcanoes vesuvio campi flegrei vulcano and etna in italy soufriere of guadeloupe in lesser antilles prove that 18 o exchange between co 2 and steam is fast enough in natural gas phases to allow rapid isotopic reequilibration within a wide temperature range 100 1000 c the isotopic composition of h 2 o discharged by volcanic hydrothermal fumaroles is affected by isotopic reequilibration along with cooling of the gas phase and this secondary process must be taken into account when interpreting the origin of the water from isotopic data the use of corrected 18 o values computed considering the overall oxygen isotopic composition of the h 2 o co 2 system is therefore compulsory to overcome interpretative biases as an example we apply this method to 100 600 c fumaroles at vulcano crater whose isotopic composition is reinterpreted considering the effects of 18 o exchange between co 2 and h 2 o in contrast to most previous interpretations the corrected isotopic values for h 2 o clearly show that the vulcano system is fed by a two component mixture of magmatic water with a typical andesitic composition and either 18 o shifted seawater or magmatic steam partly condensed within the volcanic ground with minor contribution of local meteoric water the possible additional effects of isotopic reequilibration involving other o and h bearing gas species in volcanic fluids such as so 2 h 2 s hcl hf and h 2 have been also considered the influence of so 2 upon the isotopic composition of h 2 o is subordinate with respect to that of co 2 whereas significant shifts in deuterium composition are expected from hydrogen isotopic exchanges in the h 2 o h 2 s hcl hf h 2 gas system

one hundred million years of geomagnetic polarity history since 1968 absolute ages have been assigned to the late cretaceous cenozoic geomagnetic reversal time scale by fixing the ages of two or more calibration points in a composite marine magnetic anomaly profile and interpolating between or extrapolating beyond these points assuming constant spreading rates in each interval previously no more than 4 calibration points were used but it is now possible to specify 11 calibration points in addition to the 0 m y datum this improvement is based on magnetostratigraphic studies in italian pelagic limestones these studies closely tie the geomagnetic reversal sequence to the foraminiferal and coccolith zonations absolute ages of calibration points are provided by the best available dates on stage boundaries which are located from the biostratigraphic zonation the greatest changes from previous scales come in the late paleocene early eocene where the new ages are as much as 3 m y younger than in the 1977 scale of labrecque and others and as much as 1 8 m y younger than in the 1980 scale of ness and others

a heating process of kuchi erabu jima volcano japan as inferred from geomagnetic field variations and electrical structure since august 2000 we have recorded the total intensity of the geomagnetic field at the summit area of kuchi erabu jima volcano where phreatic eruptions have repeatedly occurred a time series analysis has shown that the variations in the geomagnetic field since 2001 have a strong relationship to an increase in volcanic activity these variations indicate thermal demagnetization of the subsurface around the presently active crater the demagnetization source for the early variations until summer 2002 was estimated at about 200 m below sea level for the variations since 2003 the source was modeled on the basis of the expansion of a uniformly magnetized ellipsoid the modeling result showed that the source is located at 300 m above sea level beneath the crater we carried out an audio frequency magnetotelluric survey with the aim of obtaining a relation between the demagnetization source and the shallow structure of the volcano a two dimensional inversion applied to the data detected two good conductors a shallow thin one which is restricted to a region around the summit area while the other extends over the edifice at depths between 200 and 800 m these conductors are regarded as clay rich layers with low permeability which were assumed to be generated through hydrothermal alteration the demagnetization source for the early variations was possibly located at the lower part of the deep conductor and the source after 2003 lies between the two conductors where groundwater is considered to be abundant based on these results as well as on seismological geodetic and geochemical information we propose a heating process of the kuchierabu jima volcano in the initial stage high temperature volcanic gases supplied from the deep seated magma remained temporarily at the level around the lower part of the less permeable deep conductor since the ascent path had not yet been established then when the pathway developed as a result of repeated earthquakes it became possible for a massive flux of volcanic gases to ascend through the conductor the high temperature gases reached the aquifer located above the conductor and the heat was efficiently transported to the surrounding rocks through the groundwater as a consequence an abrupt increase of the gas flux and di ffusion of the heat through the aquifer occurred and the high temperature zone expanded since the high temperature zone is located beneath another conductor which acts as caprock we assume that the energy of the phreatic explosion is accumulated there

drilling deep into young oceanic crust hole 504b costa rica rift hole 504b is by far the deepest hole yet drilled into the oceanic crust in situ and it therefore provides the most complete ground truth now available to test our models of the structure and evolution of the upper oceanic crust cored in the eastern equatorial pacific ocean in 5 9 m y old crust that formed at the costa rica rift hole 504b now extends to a total depth of 1562 3 m below seafloor penetrating 274 5 m of sediments and 1287 8 m of basalts the site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust as a result the conductive heat flow approaches the value of about 200 mw m predicted by plate tectonic theory and the in situ temperature at the total depth of the hole is about 165 c the igneous section was continuously cored but recovery was poor averaging about 20 the recovered core indicates that this section includes about 575 m of extrusive lavas underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes the latter have been sampled in situ only in hole 504b the igneous section is composed predominantly of magnesium rich olivine tholeiites with marked depletions in incompatible trace elements nearly all of the basalts have been altered to some degree but the geochemistry of the freshest basalts is remarkably uniform throughout the hole successive stages of on axis and off axis alteration have produced three depth zones characterized by different assemblages of secondary minerals 1 the upper 310 m of extrusives characterized by oxidative seafloor weathering 2 the lower extrusive section characterized by smectite and pyrite and 3 the combined transition zone and sheeted dikes characterized by greenschist facies minerals a comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology alteration and porosity into three zones that are analogous to layers 2a 2b and 2c described by marine seismologists on the basis of characteristic seismic velocities many of the logs and experiments suggest the presence of a 100 to 200 m thick layer 2a comprising the uppermost rubbly pillow lavas which is the only significantly permeable interval in the entire cored section layer 2b apparently corresponds to the lower section of extrusive lavas in which original porosity is partially sealed as a result of alteration nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900 1000 m below seafloor within the transition between extrusives and sheeted dikes indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2b and 2c and confirming that layer 2c consists of intrusive sheeted dikes a vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends that between the sheeted dikes of seismic layer 2c and the gabbros of seismic layer 3 which has never been sampled in situ may be within reach of the next drilling expedition to hole 504b therefore despite recent drilling problems deep in the hole current plans now include revisiting hole 504b for further drilling and experiments when the ocean drilling program returns to the eastern pacific in 1991

palaeomagnetic and rock magnetic reliability criteria in ophiolitic rocks a case study from the palaeozoic ballantrae ophiolite scotland abstract palaeomagnetic studies of the ballantrae ophiolite southwest scotland are assessed in the light of palaeomagnetic and rock magnetic reliability criteria the remanence directions are palaeomagnetically reliable but have only limited significance for apparent polar wander path construction rock magnetic reliability criteria are found to be ambiguous when treated in isolation original grain size effects are detected in magnetic properties across three sections through pillow lavas we propose that a systematic study of spatial magnetic property variations within pillow lavas can provide an additional field test in the assessment of reliability thermomagnetic analyses and microscopic observations suggest three remanence carrying magnetic phases exist within the extrusive layer of the ophiolite these are almost pure magnetite hematite and titanomaghemite palaeomagnetic rock magnetic and geological observations are best satisfied by a pre obduction magnetisation age this study links the remagnetisation of an ultra oceanic conglomerate to local hydrothermal circulation rather than to a pervasive remagnetisation event as previously suggested

hydrothermal mounds and young ocean crust of the galapagos preliminary deep sea drilling results leg 70 a total of 32 holes at 5 sites near 1 n 86 w drilled on deep sea drilling project dsdp leg 70 november december 1979 provide unique data on the origin of the hydrothermal mounds on the south flank of the galapagos spreading center hydrothermal sediments primarily mn oxide and nontronite are restricted to the immediate vicinity of the mounds 100 m and are probably formed by the interaction of upward percolating hydrothermal solutions with sea water and pelagic sediments above locally permeable zones of ocean crust mounds as much as 25 m in height form in less than a few 10 5 yrs and geothermal and geochemical gradients indicate that they are actively forming today the lack of alteration of upper basement rocks directly below the mounds and throughout the galapagos region indicates that the source of the hydrothermal solutions is deeper in the crust

petroleum geology of fiji abstract most petroleum exploration in fiji has been directed towards the shallow water sedimentary basins around and extending onto viti levu the bligh water and bau waters basins a large amount of commercial multi channel seismic reflection data has been acquired across these basins since 1971 and five deep wells were drilled offshore and on viti levu from 1980 to 1982 all the wells were dry although some had minor shows of gas and oil fluorescence the wells penetrated over 2500 m of sedimentary rocks largely of miocene and younger age but some oligocene or older volcanogenic rocks were also intersected the well sections comprise mainly volcaniclastics and lower to middle miocene shallow water limestones which were the primary targets were not encountered a large amount of recently released commercial seismic data from around viti levu is interpreted here to enable an assessment of petroleum potential to be made up to about 4 km and 2 km of late miocene and younger strata are interpreted in the bligh water and bau waters basins respectively but strata underlying an angular unconformity believed to be of late miocene age are not seismically resolved and thus the total sedimentary thickness is unknown the sedimentary sequences off viti levu are cut by large faults and pre pliocene rocks are affected by folding which is locally quite intense no source rocks have been identified in the deep wells nor in tests of outcrop and stratigraphic borehole material from viti levu and vanua levu but anomalous amounts of pentane in seabed sediments off northern viti levu suggest that some thermogenic hydrocarbons have been generated information on subsurface reservoirs is sparse but limestones are generally considered to have better reservoir potential than coarse volcaniclastics and are the more desirable targets because of the limited seismic resolution no traps with lower to middle miocene shallow water limestone facies can presently be identified in the basins offshore viti levu late miocene and younger sequences contain numerous structurally formed potential traps and several pliocene reef like seismic mounds also occur in the western bligh water basin and form potential traps mudstones are widespread in the offshore basins and seals are likely to be good although some relatively recent faulting may have breached some potential traps commercial and non commercial reconnaissance geophysical surveys have also been conducted across other sedimentary basins in fiji including the deep water suva and baravi basins and basins around vanua levu and on the lau ridge

a review on phreatic eruptions and their precursors abstract an analysis of the reported information on 132 historical phreatic events indicates that most of these 115 were not followed by a magmatic or phreatomagmatic eruption only in 18 cases precursory phenomena are reportedly lacking whereas they are recognized for the majority 72 of the considered cases these precursors have been analyzed as a function of the kind of phreatic event and of its possible triggering mechanism unfortunately the quality of the available information is poor however data suggest that anomalous seismicity ground deformation changes in the output chemistry and temperature of fumaroles and thermal springs are potential mid term precursors whereas seismic tremor is a common short term precursor

source of marine magnetic anomalies some results from dsdp leg 83 leg 83 of the deep sea drilling project represents the deepest penetration of undisturbed marine crust yet achieved providing samples from the lower part of the extrusive layer and the upper 300 m of the sheeted dike complex we present here measurements of the magnetic properties of rocks from leg 83 and address the question of their suitability as sources for marine magnetic anomalies we find that due to extensive hydrothermal alteration the extrusive part of the leg 83 section is insufficiently magnetized to be part of the magnetic source layer the sheeted dike complex on the other hand does have sufficient magnetization and contrary to recent models of marine magnetization based on ophiolite studies may be a contributor to magnetic anomalies the strong dependence of magnetic properties on the alteration history of the rocks however makes extension of these results to other regions problematical

evolution of fluid geochemistry at the turrialba volcano costa rica from 1998 to 2008 turrialba 10 02 n 83 45 w is a 3 349 m high stratovolcano belonging to the holocene cordillera central volcanic belt of costa rica the summit consists of three ew oriented craters east central and west since its last eruptive phase 1864 1866 the central and west craters have displayed modest fumarolic activity with outlet temperatures clustering around 90 c in 2001 seismic swarms ground deformation and increasing fumarolic activity occurred from 2005 to 2008 new fumarolic vents opened between and within the central and west craters and along the western and southwestern outer flanks of the volcanic edifice these physical changes were accompanied by a drastic modification in the gas chemistry that can be divided in three stages 1 hydrothermal from 1998 to autumn 2001 characterized by the presence of h2o co2 h2s and to a very minor extent hcl and hf 2 hydrothermal magmatic autumn 2001 2007 with the appearance of so2 and a significant increase of hcl and hf and 3 magmatic dominated 2007 2008 characterized by increased so2 content so2 h2s 100 and temperatures up to 282 c accordingly gas equilibrium in the co2 ch4 h2 system suggests a progressive evolution of the deep fluid reservoir toward higher temperatures and more oxidizing conditions the chemical physical modifications of turrialba in the last decade can be interpreted as part of a cyclic mechanism controlling the balance between the hydrothermal and the magmatic systems nevertheless the risk of rejuvenation of the volcanic activity cannot be excluded and an appropriate seismic ground deformation and geochemical monitoring program is highly recommended turrialba lies at a distance of 35 and 15 km from san jose and cartago respectively the two largest cities in costa rica

the interaction of volcanic gas and deep magmatic fluid with shallow aquifers at tokachidake volcano japan abstract continuous geochemical monitoring of thermal waters at tokachidake volcano has been conducted for more than three decades to detect changes in its volcanic activity based on the geochemical data from 1986 to 2013 the origin and temporal changes of the thermal waters were discussed in our previous study here we supplement the previous dataset with the geochemical data from 2014 to 2018 and comprehensively discuss the results of geochemical and geophysical monitoring to reveal the interactions between deep magmatic fluids volcanic gases and shallow aquifers thermal waters discharging in the tokachidake spa area are formed by the interaction of volcanic gas and a shallow aquifer and show the cl change which have a significant correlation with the change in the volcanic plume height this suggests a parallel volcanic gas input from the magma to the shallow aquifer and fumarole both changes are caused by a variation in the volcanic gas release rate from the magma around the 1988 1989 magmatic eruption thermal waters in the fukiage spa area showed remarkable thermal and chemical changes due to the interaction with the magmatic fluid nacl type thermal water based on the chemical and isotopic compositions of the thermal waters we calculated the cl concentration and temperature of the nacl type thermal water as being approximately 20 000 mg l and 280 c respectively our calculation of the supply rate of the nacl type thermal water indicates that large amounts of the nacl type thermal water are in continuous supply when volcanic activity increases prior to a magmatic eruption in this area the recent monitoring results reveal that the changes in the cl concentration and seismic activity show a relatively good correlation indicating that at least some of the volcanic earthquakes are related to the migration of the nacl type thermal water thus when volcanic activity increases before a future magmatic eruption chemical changes in the thermal waters in the fukiage spa area should be apparent and the changes are likely to occur together with seismic activity our results propose that it is possible to interpret the present state of an active volcano more precisely through a comprehensive discussion of geochemical data from the thermal waters along with the other observations

fumarolic activity of avachinsky and koryaksky volcanoes kamchatka from 1993 to 1994 volcanic gas and condensate samples were collected in 1993 1994 from fumaroles of koryaksky and avachinsky basaltic andesite volcanoes on the kamchatka peninsula near petropavlovsk kamchatsky the highest temperature fumarolic discharges 220 c at koryaksky and 473 c at avachinsky are water rich 940 985 mmol mol of h2o and have chemical and isotopic characteristics typical of kamchatka kurile high and medium temperature volcanic gases the temperature and chemical and water isotopic compositions of the koryaksky gases have not changed during the past 11 years they represent an approximate 2 1 mixture of magmatic and meteoric end members low temperature near boiling point discharges of avachinsky volcano are water poor 880 mmol mol their compositions have not changed since the 1991 eruption and are suggested to be derived from partially condensed magmatic gases at shallow depth based on a simple model involving mixing and single step steam separation low water and high co2 contents as well as the observed cl concentration and water isotopic composition in low temperature discharges are the result of near surface boiling of a brine composed of the almost pure condensed magmatic gas high methane content in low temperature avachinsky gases and the 220 c koryaksky fumarole low c isotopic ratio in co2 at koryaksky 11 8 and water isotope data suggest that the meteoric end member contains considerable amounts of the regional methane rich thermal water discovered in the vicinity of both volcanoes

tectonic evolution and copper gold metallogenesis of the papua new guinea and solomon islands region papua new guinea and the solomon islands are in one of the most prospective regions for intrusion related mineral deposits however because of the tectonic complexity of the region and the lack of comprehensive regional geological datasets the link between mineralization and the regional scale geodynamic framework has not been understood here we present a new model for the metallogenesis of the region based on a synthesis of recent studies on the petrogenesis of magmatic arcs and the history of subduction zones throughout the region combined with the spatio temporal distribution of intrusion related mineral deposits and six new deposit ages convergence at the pacific australia plate boundary was accommodated from at least 45 ma by subduction at the melanesian trench with related melanesian arc magmatism the arrival of the ontong java plateau at the trench at ca 26 ma resulted in cessation of subduction immediately followed by formation of cu au porphyry epithermal deposits at 24 20 ma throughout the melanesian arc late oligocene to early miocene tectonic reorganization led to initiation of subduction at the pocklington trough and onset of magmatism in the maramuni arc the arrival of the australian continent at the pocklington trough by 12 ma resulted in continental collision and ore deposit formation from 12 to 6 ma this is represented by cu au porphyry deposits in the new guinea orogen and epithermal au systems in the papuan peninsula from 6 ma crustal delamination in papua new guinea related to the prior pocklington trough subduction resulted in adiabatic mantle melting with emplacement of diverse cu and au porphyry and epithermal deposits within the papuan fold and thrust belt and papuan peninsula from 6 ma to the present day subduction at the new britain and san cristobal trenches from ca 10 ma resulted in an escalation in tectonic complexity and the onset of microplate tectonics in eastern papua new guinea and the solomon islands this is reflected in the formation of diverse and discrete geodynamic settings for mineralization within the recent to modern arc setting primarily related to upper plate shortening and extension and the spatial relationship to structures within the subducting slab

tectonics and magma genesis in the northern north fiji basin abstract the north fiji basin is a complex back arc basin in which active seafloor spreading is occurring along two north south ridge systems lithospheric extension is also taking place along segments of the east west trending fiji fracture zone and south pandora ridge the fiji fracture zone is clearly a transform fault but the south pandora ridge is more difficult to interpret some characteristics are consistent with a slow spreading ridge whereas others suggest a transform fault basalts recovered on the south pandora ridge are enriched in high field strength and large ion lithophile elements and many of them resemble ocean island tholeiites or transitional alkali basalts the south pandora ridge is an enigmatic tectonic feature and the geochemistry morphology and seismicity are best explained in terms of a model involving slow spreading ridge segments within a broad transform domain at its southern end the central ridge of the north fiji basin has a bathymetric profile normally associated with fast spreading steady state mid ocean ridges and the basalts recovered on this part of the ridge are moderately evolved normal mid ocean ridge basalts morb in contrast the northern end of the ridge has a complex topography dominated by a series of deep linear troughs and grabens and basalts recovered from the floors of these troughs vary widely in chemistry from normal morbs to transitional basalts that are comparatively enriched in large ion lithophile and high field strength elements the central ridge appears to be propagating northwards into old north fiji basin crust and the northern extension of the ridge is an incipient spreading locus in the very earliest stages of development in areas of the basin where fast spreading large scale mantle upwelling leads to extensive melting of asthenosphere derived depleted mantle morb like compositions dominate the erupted rocks in contrast areas of slow or incipient spreading and other tectonic settings where a transient thermal disturvance occurs are characterized by basaltic magmatism that shows relative enrichment in large ion lithophile and high field strength minor and trace elements mixing of morb and enriched end member components either at source or as magmas gives rise to a spectrum of compositions that can be related to a range of tectonic environments in the back arc setting

u pb ages and tectonic significance of late archean alkalic magmatism and nonmarine sedimentation timiskaming group southern abitibi belt ontario the paper presents u pb ages for zircons of the calc alkalic to alkalic igneous suite and associated alluvial fluvial sedimentary rocks of the timiskaming group in the late archean abitibi greensto

global tectonic significance of the solomon islands and ontong java plateau convergent zone abstract oceanic plateaus areas of anomalously thick oceanic crust cover about 3 of the earth s seafloor and are thought to mark the surface location of mantle plume heads hotspot tracks represent continuing magmatism associated with the remaining plume conduit or tail it is presently controversial whether voluminous and mafic oceanic plateau lithosphere is eventually accreted at subduction zones and therefore 1 influences the eventual composition of continental crust and 2 is responsible for significantly higher rates of continental growth than growth only by accretion of island arcs the ontong java plateau ojp of the southwestern pacific ocean is the largest and thickest oceanic plateau on earth and the largest plateau currently converging on an island arc solomon islands for this reason this convergent zone is a key area for understanding the fate of large and thick plateaus on reaching subduction zones this volume consists of a series of four papers that summarize the results of joint us japan marine geophysical studies in 1995 and 1998 of the solomon islands ontong java plateau convergent zone marine geophysical data include single and multi channel seismic reflection ocean bottom seismometer obs refraction gravity magnetic sidescan sonar and earthquake studies objectives of this introductory paper include 1 review of the significance of oceanic plateaus as potential contributors to continental crust 2 review of the current theories on the fate of oceanic plateaus at subduction zones 3 establish the present day and neogene tectonic setting of the solomon islands ontong java plateau convergent zone 4 discuss the controversial sequence and timing of tectonic events surrounding ontong java plateau solomon arc convergence 5 present a series of tectonic reconstructions for the period 20 ma early miocene to the present day in support of our proposed timing of major tectonic events affecting the ontong java plateau solomon islands convergent zone and 6 compare the structural and deformational pattern observed in the solomon islands to ancient oceanic plateaus preserved in precambrian and phanerozoic orogenic belts our main conclusion of this study is that 80 of the crustal thickness of the ontong java plateau is subducted beneath the solomon island arc only the uppermost basaltic and sedimentary part of the crust 7 km is preserved on the overriding plate by subduction accretion processes this observation is consistent with the observed imbricate structural style of plateaus and seamount chains preserved in both precambrian and phanerozoic orogenic belts

polarity reversal in the solomon islands arc mckenzie1 proposed that the direction of subduction beneath an island arc will reverse following arc continent collision dewey and bird2 suggested that such an arc polarity reversal occurred in northern new guinea johnson and molnar3 however proposed two alternative models of plate convergence that could explain the observed seismicity we present here a spatial seismicity study of the solomon islands region which has revealed the existence of two juxtaposed wadati benioff w b zones of opposite polarity shallow and intermediate foci define a north east dipping w b zone associated with active subduction along the new britain and san cristobal trenches and a south west dipping w b zone associated with the inactive north solomon trench nst these data provide the first direct seismic evidence of a reversal in subduction polarity at an island arc

evolution of intra oceanic arc trench systems abstract intra oceanic island arc trench systems represent a locus of crustal thickening through the addition of mantle derived magmas which form the volcanoes and subjacent stratiform plutons the crust of immature island arcs and adjacent back arc basins is oceanic in chemistry in that it is dominated by mafic e g tholeiitic magmas a magma sequence is recognized in many arc systems which progress from boninite to arc tholeiite to calc alkaline and in some arcs to shoshonites the adjacent back arc basins are formed of morb like tholeiite arc systems may be dominated by extension rather than compression if the subducted and overriding plates are decoupled extension of the arc system may be associated with seaward retreat or roll back of the trench axis alternate cycles of extension and compression of the arc system can lead to both tectonic erosion of the fore arc and accretion of fragments from the incoming sea floor the izu bonin mariana tonga yap and palau trench arc systems are examples of different stages in a dynamic process which starts when subduction begins along old fracture zone traces crustal extension causes both fore arc subsidence and opening of back arc basins island arc volcanic chains and back arc basins are new crust formed largely of fractional melts of the mantle the evolution of an arc system may stop when blocking of the subduction zone or changes in relative plate motion causes overthrusting of back arc lithosphere on to former volcanic arcs many ophiolites preserve remnants of arc system crust and upper mantle

airborne measurements of volcanic gas composition during unrest at kuchinoerabujima volcano japan airborne measurements of volcanic gas composition using an unmanned aerial vehicle uav and cessna aircraft were conducted at kuchinoerabujima volcano japan between 2014 and 2016 because eruptions occurred in august 2014 may 2015 and june 2015 access to the summit crater was limited because of the risk of sudden eruption such that airborne measurements were the only viable method to measure the volcanic gas composition multi gas and alkali filter pack measurements were made on the leeward side of the crater and around the crater using the cessna and uav respectively observations using the uav could measure the dense plume and quantify the gas species of h2o co2 so2 h2s h2 hcl and hf while the observations using the cessna could measure only the diluted plume and quantify co2 so2 h2s and h2 the seven airborne observations enabled us to monitor variations in the volcanic gas composition over the observation period the so2 h2s ratio decreased from 10 to 1 9 the h2o so2 ratio h2 so2 ratio and apparent equilibrium temperatures aet estimated using the volcanic gas composition increased after the 2014 eruption the decrease in the so2 h2s ratio might be attributed to changes in the pressure of degassing magma and interactions with the hydrothermal system the airborne methods presented here highlight the utility of using light aircrafts to safely conduct volcanic gas measurements during periods of volcanic unrest when traditional ground based methods are not possible

geophysical evidence for post miocene rotation of the island of viti levu fiji and its relationship to the tectonic development of the north fiji basin geophysical studies of the island of viti levu fiji show largeamplitude magnetic anomalies of up to 1500 nt to be located above the tavua volcanic center field sampling of the volcanic rocks from the intrusive and extrusive centers of the volcano show the rock samples to have initial intensities of natural remanent magnetization of up to 380 a m radiometric dating of fifteen oriented samples show an age range of 2 9 to 7 0 m y b p for the suite of rocks collected from the mba basaltic group paleomagnetic analysis of the dated rocksshows declination vectors ranging from 325 9 for an age of 2 9 0 2 m y b p to 290 4 for an age of 7 0 0 4 m y b p as a group the paleomagnetic declination values of the samples systematically decrease with age this decrease is attributed to progressive anticlockwise rotation of viti levu during the process of formation of the north fiji basin a linear least squares analysis of the paleomagnetic data indicates that the island has rotated in an anticlockwise direction through 90 during the past 7 m y at a rate of 13 7 per million years declination values as low as 245 measured for some sites not dated suggest that anticlockwise rotation of viti levu and development of the north fiji basin could have commenced as early as 10 m y b p following the breakup of the ancient solomons new hebrides fiji lau island arc large rotations of microplates may have occurred along other island arcs and marginal basin systems elsewhere in the western pacific and such rotational histories may reflect in detail the evolutionary development of these arcs and basins

recent advances in fine particle magnetism implications for paleomagnetism it is becoming increasingly obvious in paleomagnetism that the older practice of using large numbers of samples to improve statistical measures of accuracy is often not adequate to reveal the signal of greatest geological importance blanket demagnetization treatments are now largely outdated except for the most ideally behaved samples a rapidly vanishing phenomenon in many locales variations in characteristic remanent directions between neighboring and supposedly contemporaneous but lithogically distinct rock units can no longer be simply treated as noise we now recognize that such differences may actually reflect rock magnetic effects rather than such factors as the short term behavior of the geomagnetic field or local variations in structure multiple overprints often the result of moderate reheating and or chemical alteration can result in an uncertain relationship between the timing of emplacement and the acquisition of remanence in the past rocks which presented such problems could often be avoided in favor of more straightforward units much of that geological cream has been skimmed however further progress in many areas particularly the determination of the apw path and reversal stratigraphy in the paleozoic will require a better understanding of magnetization processes in order to resolve the above mentioned problems and produce accurate and reliable results

late miocene to pliocene palaeogeography of viti levu fiji islands abstract the island of viti levu is located on the fiji platform in an area that has been tectonically active since at least the late eocene the geological history of viti levu during this time is one of the competing influence of tectonics volcanism and eustatic sea level change in shaping the preserved stratigraphic and therefore palaeogeographic record the focus of this paper is the palaeogeographic evolution of viti levu from the period following the deformation of the tuva group and all older rock units of the vityaz arc around 7 5 ma to the present initial sedimentation during following the deformational event was influenced by the sinistral oblique slip deformation associated with anticlockwise rotation of the platform this tectonically dominated period of island evolution saw the establishment of a number of sedimentary basins many of which continued to receive sediments through until at least the earliest pliocene from 6 5 ma onward volcanic activity became the main influence on the stratigraphic record although evidence for major eustatic events miocene pliocene boundary are recorded in some sequences toward the end of the early pliocene tectonic uplift of viti levu resulted in a poor geological record for this time the quaternary stratigraphic record reflects relatively minor mostly eustatic events in coastal areas around major river systems

neoarchean evolution of differing convergent margin assemblages in the wabigoon subprovince geochemical and geochronological evidence from the lake of the woods greenstone belt superior province northwestern ontario abstract the neoarchean lake of the woods greenstone belt is subdivided into three lithostratigraphic assemblages a lowermost mafic volcanic assemblage a compositionally diverse predominantly volcanic middle assemblage and a predominantly sedimentary uppermost assemblage the secular evolution and geodynamic settings of these assemblages are constrained with detailed geochronological and geochemical data the lower keewatin is the oldest assemblage with a zircon u pb age of 2738 2 0 ma it consists of mg and fe tholeiitic basalts mg number 30 65 with flat to slightly enriched rare earth elements ree la yb cn 0 9 1 6 elevated large ion lithophile elements lile and undepleted high field strength elements hfse the tholeiites do not appear to have exact modern analogs but do have some similarities to primitive arc tholeiites and back arc basin basalts suggesting an archean destructive margin setting the upper keewatin assemblage disconformably overlies the lower keewatin assemblage it consists of volcanic and sedimentary groups two geochemically distinct volcanic suites are recognized the dominant suite consists of compositionally diverse calc alkalins basalt to rhyolite felsic end members have yielded zircon u pb ages ranging from 2712 to 2723 ma the suite exhibits highly fractionated ree la yb cn 4 35 elevated lile and nb and ta depletion anomalies which are very similar to modern island arc volcanic rocks developed in proximity to continental margins a coeval suite of mafic to ultramafic volcanic rocks consists of mg tholeiite komatiitic basalt and ultramafic komatiite mg numbers 45 80 the suite exhibits depleted lree la yb cn 0 6 1 0 slightly elevated lile and undepleted to slightly enriched hfse suggesting that this geochemically primitive suite was produced by partial melting of a refractory mantle source perhaps as second stage melts of the same mantle source from which the lower keewatin assemblage basalts were previously derived the electrum assemblage unconformably overlies the upper keewatin assemblage in the northern part of the belt it consists of conglomerates sandstones and minor volcanic rocks having detrital ages indicating deposition after 2699 ma the volcanic rocks consists of andesite to rhyolite with shoshonitic to calc alkaline geochemical affinity as indicated by highly fractionated ree la yb cn 12 112 highly enriched lile and pronounced nb ta and ti depletion anomalies samples from the syenitic falcon island pluton appear to be coeval with electrum assemblage volcanics having an u pb isotopic age of 2694 ma and similar but even more alkaline chemistry consisting of highly fractionated lree and lile trace elements with very pronounced nb ta and ti depletion anomalies these late tectonic suites are geochemically similar to modern shoshonites with a continental influence formed in the late stages of a mature island arc or a continental arc setting

paleomagnetic arguments for block rotations along the arakapas fault cyprus paleomagnetic data from the sheeted dike complex north of the arakapas fault reveal significant internal rotations the north south trending dikes have remanent magnetization directions close to the well known westerly paleomagnetic direction of the troodos massif whereas the east west trending dikes near the arakapas fault have northerly remanent magnetization directions the paleomagnetic data are in agreement with the apparent clockwise rotation of the dikes contrary to recent geological analyses which postulate that the pattern of dike trends is due to dike injection into a sigmoidal stress field along a sinistral transform fault the paleomagnetic results indicate clockwise rotations about vertical axes in response to a dextral shear in addition to the rotations around vertical axes small rotations around subhorizontal axes are inferred from the paleomagnetic data for tilted dikes thus these data reinforce the vertical intrusion hypothesis

seismicity and crustal structure of the fiji platform southwest pacific data from a telemetered seismic network in fiji provide new constraints on the crustal structure and active deformation within and around the fiji platform an unusual fragmented arc complex situated within the pacific indo australian plate boundary seismic refraction data indicate that crustal thickness in fiji is typical of island arcs averaging about 15 20 km with relatively low midcrustal seismic velocities vp 6 0 km s characteristic of felsic arc related rocks the anticipated higher velocity lower crustal layer is either thin or absent mantle velocity beneath the platform is low 7 6 km s in the same range as that observed beneath the neighboring marginal basins and island arcs the record of crustal seismicity based on historical teleseismic and microearthquake catalogs indicates significant activity both within and along the boundaries of the fiji platform earthquakes located along the edges of the platform are associated with major zones of back arc deformation of the north fiji and lau basins the fiji fracture zone to the north the hunter fracture zone to the south and a deformed zone in the eastern portion of the north fiji basin to the west earthquakes within the platform intraplate events occur more sporadically than those along its margins areas of significant intraplate activity include the taveuni northeastem vanua levu area koro sea the baravi basin and central and south eastern viti levu earthquake depths within the platform are concentrated in the upper 13 16 km of lithosphere as in other areas of extensional strike slip tectonics in fiji however the seismicity apparently extends through the entire crustal section the large 1953 intraplate suva earthquake ms 6 75 was caused by strike slip faulting under the same east west extensional stress field that is deforming the north fiji and lau basins microearthquakes within and around the island of viti levu appear to occur as a result of the same stresses these earthquakes occur in discrete nests that are tightly clustered in space and time they are related to late cenozoic faults that cut the mio pliocene rocks of viti levu the characteristics of its crustal structure the anomalous mantle velocity and the active deformation within the fiji platform distinguish it from microcontinental blocks that have been observed elsewhere the seismicity and focal mechanisms within the fiji platform demonstrate that it is not technically isolated from the active deformational processes that pervade the neighboring arcs and back arc basins

volcanism accompanying back arc basin development in the southwest pacific abstract the age and composition of magmatism in fiji and to a lesser extent vanuatu and tonga are evaluated in the light of the fragmentation which has sundered the island arc system they once collectively constituted any of six magmatic phenomena during the past 10 m y may be consequences of adjacent plate reconfigurations trondhjemitic plutonism about 10 m y change from tholeiitic to more calc alkaline volcanism between 10 and 6 m y eruption near the rifted arc margin of voluminous crustally derived high si low k rhyolites 7 m y ago shoshonite dominated basaltic volcanism 5 5 to 3 0 m y change to ocean island like basaltic volcanism at 3 0 0 2 m y and geochemically peculiar as well as tectonically anomalous acid andesites 0 7 3 0 m y ago in kandavu southwestern fiji crustal derivation of the low k rhyolites may reflect maximal heating during initial rifting shoshonites may accompany extreme regional extension above a still arc like source eruption of ocean island like basalts begins once subducted lithosphere no longer contributes to erupted melts or separates fiji from ocean island sources coeval eruption of hybrid andesites may accompany oblique subduction of south fiji basin crust beneath kandavu along the northeast hunter fracture zone

horizontal shear flow in the mantle beneath the tonga arc in the tonga subduction zone a systematic shear deformation in which deep material moves relatively to the south is superimposed on the mode of down dip shortening which is characteristic of the region we interpret this observation together with a tectonic reconstruction in terms of the interaction between the subducting lithospheric slab and a horizontal shear flow in the mantle

electrochemical sensing of volcanic gases we report here the development and application of a compact geochemical nose incorporating electrochemical sensors for gas measurements in volcanic plumes a novel element of the instrument design is the arrangement of the sensors in a parallel array that enables near simultaneous exposure and fast response data analysis methods were developed that utilise the multi sensor output currents to extract gas mixing ratio abundances and eliminate cross sensitivities use of filter methods is demonstrated to remove baseline drift or instrument noise we introduce a new approach for analysis of measurements from sensors that have a slower response time e g hcl and apply this model to estimate hcl so2 ratios we deployed the sensor system at aso volcano japan detecting emissions from its fumarole field hot crater lake and a mixed plume we measured so2 h2s co and hcl ranging in abundance from 102 104 ppbv neither no2 nor cl2 were detected for the fumarolic gases molar ratios were measured as follows h2s so2 is 0 15 h2 so2 0 25 co so2 0 02 hcl so2 0 1 the crater lake plume s h2s so2 is 0 03 the compositions are discussed in terms of degassing equilibria and plume chemistry our instrument design represents a cost effective low power and highly portable system that can be readily adapted for operational surveillance of volcanic gases

magnetic properties and opaque mineralogy of rocks from selected seafloor hydrothermal sites at oceanic ridges magnetic properties natural remanent magnetization nrm susceptibility curie point temperature tc saturation isothermal remanent magnetization irms and koenigsberger ratio q and opaque mineralogy were determined for basalts diabases gabbros peridotites and serpentinites collected by dredging and submersible from the rift valley at five hydrothermal sites 15 n 17 n 23 n the snake pit hydrothermal field 26 n the tag hydrothermal field on the mid atlantic ridge and 42 n the sea cliff hydrothermal field on the northern gorda ridge the magnetomineralogy is interpreted in terms of deuteric hydrothermal metasomatic and ambient seawater alteration evidence for unequivocal magnetic mineral modification by hydrothermal action is present only in a small percentage of extrusive basalts but is pervasive in diabases gabbros and ultramafic rocks this lithologiec distribution suggests a progressive increase in alteration intensity with depth in the oceanic crust and upper mantle from minor low temperature alteration in young surface basalts to more pervasive high temperature alteration in diabases gabbros and peridotites the studies reveal distinct correlations between magnetization intensity thermomagnetic behavior and magnetic mineralogy grain size style and intensity of alteration and rock type basalts have the highest nrm intensities and the lowest curie points 105 294 c basalts with nrm irms ratios less than 10 2 may have been remagnetized and have nrm intensities and q values much lower one third and one half respectively than basalts with nrm irms ratios of 10 2 or greater in the oceanic lithospheric suite examined remanent q 1 magnetization is present in basalts and induced magnetization q 1 occurs in metabasalts and the other rock types layer 2a basalts are the source of median valley magnetic anomalies the magnetic source may shift from the surface to deeper horizons with progressive seafloor aging and enhanced deuteric oxidation in layer 2b dikes and layer 3 gabbros and the formation of magnetite in serpentinized peridotites this shift is influenced by fluctuations in the curie isotherm related to the duration of active magma chambers and convective hydrothermal cells the magnetic and mineralogie properties of oceanic rocks determined show the effects of a wide range of alteration processes that are variously related to depth in oceanic lithosphere magmatic cooling history and hydrothermal circulation

the offshore morpho structure and tsunami sources of the viti levu seismic zone southeast viti levu fiji abstract new high resolution seabat 8160 multibeam bathymetry data reveal in detail the complex offshore morphology over part of the viti levu seismic zone vlsz in southeast viti levu the mapped seafloor is scarred by a complex network of linear submarine canyons and numerous submarine slides the submarine canyons are morphologic expressions of graben structures in the underlying basement rock submarine canyon faults are continuous with nearshore shelf faults and onshore fault controlled valleys and structural lineaments together these define continuous onshore offshore fault zones which combined form a structurally complex fault mesh of composite origin we attribute the development of the principal fault sets in the fault mesh to progressive distributed faulting in response to anticlockwise tectonic block rotations within the fiji platform during the neogene low occurrence of large earthquakes and a diffused pattern of seismicity currently observed in the vlsz may reflect complex coseismic faulting through the fault mesh the abundance of submarine slides over the vlsz which typically occur on the outer barrier reef edge as well as the submarine canyon heads and walls and mid slope imply that seismic shaking may be an important factor for triggering slope failure morphometric analysis and empirical modelling of the slides show that slides triggered in shallow water depths within 5 km of the coastline at the outer barrier reef edge and submarine canyon heads produce the largest near field tsunami amplitudes we interpret that such slides represent a significant local tsunami hazard

sulfur isotopic effects in the disproportionation reaction of sulfur dioxide in hydrothermal fluids implications for the δ 34 s variations of dissolved bisulfate and elemental sulfur from active crater lakes sulfur isotope effects during the so 2 disproportionation reaction to form elemental sulfur 3so 2 3h 2 o 2hso 4 s 2h at 200 330 c and saturated water vapor pressures were experimentally determined initially a large kinetic isotopic fractionation takes place between hso 4 and s followed by a slow approach to equilibrium the equilibrium fractionation factors estimated from the longest run results are expressed by 1000 in hso 4 s 6 21 10 6 t 2 3 62 the rates at which the initial kinetic fractionation factors approach the equilibrium ones were evaluated at the experimental conditions 34 s values of hso 4 and elemental sulfur were examined for active crater lakes including noboribetsu and niseko hokkaido japan khloridnoe bannoe and maly semiachik kamchatka poas costa rica ruapehu new zealand and kawah ijen and keli mutu indonesia hso4 s values are 28 for keli mutu 26 o for kawah ijen 24 c for ruapehu 23 for poas 22 for maly semiachik 21 for yugama 13 for bannoe 9 for niseko 4 o for khloridonoe and 0 for noboribetsu in the decreasing order the so 2 disproportionation reaction in the magmatic hydrothermal system below crater lakes where magmatic gases condense is responsible for high hso4 s values whereas contribution of hso 4 produced through bacterial oxidation of reduced sulfur becomes progressively dominant for lakes with lower hso4 s values currently noboribetsu crater lake contains no hso 4 of magmatic origin a 40 year period observation of 34 s hso4 and 34 s s values at yugama indicated that the isotopic variations reflect changes in the supply rate of so 2 to the magmatic hydrothermal system this implies a possibility of volcano monitoring by continuous observation of 34 s hso4 values the 18 o values of hso 4 and lake water from the studied lakes covary indicating oxygen isotopic equilibration between them the covariance gives strong evidence that lake water circulates through the sublimnic zone at temperatures of 140 30 c

the tertiary arc chain in the western pacific abstract the arcs bordering the pacific plate on the western and southwestern pacific rim are reconstructed since their initiation in the eocene and oligocene they occur in a zone forming an arc chain from the western pacific tropics to the eastern margin of australia they are the bonin mariana yap palau halmahera north new guinea west melanesia solomon vanuatu and tonga kermadec arcs designated here the tertiary arc chain they are associated with the formation and consumption of backarc basins reversals of arc polarity and episodic subduction has occurred in some of them the tertiary arc chain is characterized by four major stages in its evolution which can be seen characteristically in some of the arcs the first stage is the occurrence of the arc chain from the middle eocene to earliest oligocene the second stage is the formation of the backarc basins from the early to late oligocene the third stage is the occurrence of double arcs on the inner side of the arc chain in the early to middle miocene and the fourth stage is the reversal of arc polarities due to collisions since the late miocene the backarc basins associated with the arcs of the tertiary arc chain have fixed limits of duration in their evolution the backarc basins initially form 15 million years after the initiation of the volcanic arc several to 10 million years after the initial opening backarc spreading terminates approximately 20 million years after the cessation of the backarc spreading a second phase of opening occurs in the backarc region in the case of arc collision reversal of the arc polarity occurs if there is oceanic crust on the backarc side and opening of a backarc basin occurs within several million years these occurrences and durations have a variation of ca 3 5 million years

palaeomagnetic evidence for clockwise rotations related to dextral shear along the southern troodos transform fault cyprus abstract the southern troodos arakapas fault represents a late cretaceous oceanic fracture zone which was locally disrupted during late cretaceous eocene palaeorotation of the troodos microplate opinions are divided as to the sense of displacement along the transform fault and the exact timing of initiation of palaeorotation palaeomagnetic studies of turonian zeolite facies lavas and sediments exposed within the transform domain have revealed considerable variations in the declination of remanent magnetisation between sites along the arakapas fault belt the western margin of the limassol forest complex and the eastern flank of the troodos ophiolite at seven sites clockwise rotation of fault blocks has occurred about steeply inclined axes one fault block at the western end of the arakapas fault belt has experienced a net anticlockwise rotation while at six other sites only simple tilting about sub horizontal axes is indicated the overall clockwise sense of block rotation and initial dyke strikes calculated at three sites are consistent with right lateral slip along the transform cross cutting relationships revealed by the analysis of one site demonstrate that these rotations took place during crustal genesis and are not a product of post spreading disruption of the fracture zone sites located further south in the eastern limassol forest complex show no relative rotation with respect to the main troodos ophiolite to the north and have experienced only simple tilting however if our results are considered in conjunction with existing palaeomagnetic data from the area it appears that the entire area lay within a complicated zone of localised and predominantly clockwise block rotations produced by dextral slip along the transform some areas were rotated by over 100 about steeply inclined axes whereas others experienced only simple tilting the whole area was later subjected to a bulk 90 anticlockwise rotation along with the troodos microplate data obtained from the umbers and radiolarites perapedhi fm exposed on the eastern flank of the limassol forest block indicate that at least 30 and possibly up to 45 of this 90 rotation took place over a maximum of 15 ma

seismic structure beneath the tonga arc and lau back arc basin determined from joint vp vp vs tomography the tonga arc and associated lau basin exhibit many geologically important processes that link subduction and mantle flow with plate separation and crustal production we create seismic tomograms of the tonga lau region by jointly inverting for vp and vp vs structure using data from the labatts ocean bottom seismograph experiment and several island deployments to better constrain dynamic processes in the mantle wedge jointly using p and s data can help distinguish between the various mechanisms responsible for seismic velocity anomalies such as temperature and the presence of melt and or volatiles because high attenuation in the wedge limits the s wave data set we focus on 2 d inversions beneath the linear obs array where resolution is best and also parameterize the solution in terms of the vp vs ratio as expected the subducting slab has fast vp and vs and a low vp vs ratio consistent with the cold downgoing plate the central lau spreading center clsc exhibits stronger anomalies in vp vs than in vp with the anomalies larger than would be predicted purely by temperature variations the clsc anomaly extends 100 km to the west of the axis suggesting a broad region of melt production driven by passive upwelling from plate separation rather than active upwelling mechanisms the anomaly is asymmetric about the axis suggesting that slab induced corner flow possibly influences mantle dynamics several hundred kilometers away from the arc there is a strong anomaly beneath the volcanic arc that gradually deepens as it trends toward the back arc likely outlining a hydrated region of melt production that feeds the volcanic front hydration possibly continues throughout the wedge to at least 400 km depth the lau ridge exhibits a thicker lithosphere relative to the rest of the basin while the fiji platform likely has a thinner lithosphere than the lau ridge from more recent extension there is also a reasonable likelihood of a small degree of partial melt in the uppermost mantle beneath the platform

evaluation of results from the fourth and fifth iavcei field workshops on volcanic gases vulcano island italy and java indonesia abstract the major purpose of field workshops on volcanic gases organized by the iavcei commission on the chemistry of volcanic gases is the collection and analysis of volcanic gas discharges with the aim to develop and improve techniques for the geochemical surveillance of active volcanoes the fourth and fifth workshops were held at vulcano island italy in 1991 and on java island indonesia in 1994 respectively gas samples were collected from four gas vents by nine groups at vulcano and from eight gas vents by eight groups on java the quality e g scatter of the data of most of the results reported from these two workshops is sufficient to permit a broad chemical classification of the discharge and meaningful thermodynamic interpretation in most cases the majority of the data for individual gas vents cluster closely around the median values suggesting that the median values are the best estimates of chemical composition there is however also a considerable scatter of the analytical data and this scatter warns us to not rely too heavily on a single analytical value in particular on a value for ch4 and co because analytical data for these species often show a wide scatter this warning is particularly relevant for chemical monitoring of volcanic activity further improvement of the sampling and analytical techniques as well as more detailed comparison of the techniques is required to reduce such uncertainty in order to interpret the volcanic activity and hydrothermal conditions

a model of ocean basin crustal magnetization appropriate for satellite elevation anomalies a model of ocean basin crustal magnetization measured at satellite altitudes is developed which will serve both as background to which anomalous magnetizations can be contrasted and as a beginning point for studies of tectonic modification of normal ocean crust the model is based on published data concerned with the petrology and magnetization of the ocean crust and consists of viscous magnetization and induced magnetization estimated for individual crustal layers thermal remanent magnetization and chemical remanent magnetization are excluded from the model because seafloor spreading anomalies are too short in wavelength to be resolved at satellite altitudes the exception to this generalization is found at the oceanic magnetic quiet zones where thermal remanent magnetization and chemical remanent magnetization must be considered along with viscous magnetization and induced magnetization

rapid rotation of fiji paleomagnetic evidence and tectonic implications the timing and magnitude of rotation of fiji in relation to the tectonic evolution of the sw pacific region has been investigated by extensive 126 sites paleomagnetic sampling of late miocene and younger units generally samples yielded well defined characteristic remanence directions which indicate that the main fiji platform rotated by 135 17 5 counterclockwise commencing at or before 10 ma this very large and rapid rotation ceased abruptly at 3 ma coincident with the onset of spreading on the major n s aligned spreading centers in the north fiji basin latest miocene to pliocene paleomagnetic results indicate that viti levu the main island itself has suffered no internal disruption by localized rotations after 5 ma we present a reconstruction for the origin of fiji which differs markedly from previous models such that fiji tonga and lau form a continuous n s aligned arc and that arc reversal was initiated at or near fiji itself

incipient shortening of a passive margin the mechanical roles of continental and oceanic lithospheres summary by definition passive margins deform little or no further after continental break up nevertheless post break up deformation and associated uplift have been documented along many margins for example norway greenland west africa south america india and australia we have conducted a series of analogue experiments in which we subjected scaled multilayered models of the lithosphere made of sand and viscous layers to horizontal shortening a multilayered lithosphere rested on an asthenosphere of low viscosity that allowed isostatic adjustments models with various strength profiles represented oceanic lithospheres of different ages the continental lithosphere reacted to an applied boundary velocity according to the strength and density of oceanic lithosphere where oceanic lithosphere was stronger and heavier than continental lithosphere it subducted where oceanic crust was stronger and lighter than continental crust it obducted on this basis we suggest that the style of incipient shortening of a passive margin depends on the age of oceanic lithosphere where the ocean is young we expect obduction where it is old we expect incipient subduction in nature ophiolites and shortened passive margins appear to conform to this simple rule

discrepancies between experimental observations in natural and synthetic samples abstract the evidence is reviewed by comparing the data obtained from synthetic samples with those derived from natural samples of magnetic minerals present in rocks it is shown that there are many issues which are still unresolved some suggestions are made concerning the future direction of rock magnetic research such as the need for intensified studies of natural samples the aim of this note is not to disclose anything new but rather to open up some lines of thought and draw attention to questions and ideas that may have been lurking in our minds from the very beginnings of paleomagnetism

geological evidence bearing on the miocene to recent structural evolution of the new hebrides arc abstract the new hebrides archipelago is a complex reversed arc system that can be divided into four major volcanic provinces the western belt is an early to middle miocene extinct volcanic arc that as a result of polarity reversal is now incorporated into the frontal arc of the present day configuration the eastern belt initially received detritus in the early middle miocene from a tholeiitic arc complex but in the mio pliocene became the locus of a more calc alkaline arc volcanism volcanic activity then ceased in the eastern belt but is well represented as a third and largely submerged marginal province through the pliocene into the early pleistocene the present volcanic line the central chain is essentially a continuation of the marginal province volcanism into recent times initial tectonic events in the new hebrides arc were associated with the regional disruption in the middle miocene of an east facing system with consequent termination of western belt arc volcanism the western belt remained as a landmass during the lowermost late miocene but subsided following a late miocene renewal of island arc volcanism to the east this latest phase was coeval with initial expansion of the north fiji basin and marked the advent of the new hebrides as a westwards migrating reversed arc system during arc migration there were apparent hiatuses in island arc volcanism the most notable being a middle pliocene to late pleistocene period of quiescence in the central sector tectonism in the early pleistocene recent raised the fore arc brought about rifting and extension to the rear and concentrated volcanism along the presently active central chain

spreading rate dependent magnetization of the oceanic lithosphere inferred from the anomalous skewness of marine magnetic anomalies summary the anomalous skewness of marine magnetic anomalies decreases with increasing spreading rate and becomes negligible above a spreading rate of about 50 km myr i previous magnetization models of the oceanic lithosphere fail to explain this observation we propose a new model in which the magnetic structure of the oceanic lithosphere is dependent on spreading rate anomalous skewness measurements for anomalies 33 reversed and 25 reversed at various spreading rates are in agreement with a magnetization model in which layer 3 and the uppermost mantle bear a similar saturation magnetization and the ratio of layer 3 over layer 2a saturation magnetizations varies linearly from about 0 4 at a spreading rate of 10kmmyr to about 0 at 55kmmyr i for spreading rates higher than 55 km myr layer 3 and uppermost mantle magnetizations are negligible recent advances concerning accretionary processes at mid ocean ridges suggest that the combined effect of variations with spreading rate of magma fractionation alteration of layer 2a and serpentinization of layer 3 and the uppermost mantle may account for the proposed variation of magnetization with spreading rate

a pliocene shoaling basaltic seamount ba volcanic group at rakiraki fiji abstract at rakiraki in northeastern viti levu the pliocene ba volcanic group comprises gently dipping pyroxene phyric basaltic lavas including pillow lava and texturally diverse volcanic breccia interbedded with conglomerate and sandstone three main facies associations have been identified 1 the primary volcanic facies association includes massive basalt flows and sills pillow lava and related in situ breccia pillow fragment breccia autobreccia in situ hyaloclastite peperite 2 the resedimented volcaniclastic facies association consists of bedded monomict volcanic breccia and scoria lapilli rich breccia 3 the volcanogenic sedimentary facies association is composed of bedded polymict conglomerate and breccia together with volcanic sandstone and siltstone mudstone facies pillow lava and coarse hyaloclastite breccia indicate a submarine depositional setting for most of the sequence thick massive to graded beds of polymict breccia and conglomerate are interpreted as volcaniclastic mass flow deposits emplaced below wave base well rounded clasts in conglomerate were reworked during subaerial transport and or temporary storage in shoreline or shallow water environments prior to redeposition red oxidised lava and scoria clasts in bedded breccia and conglomerate also imply that the source was partly subaerial the facies assemblage is consistent with a setting on the submerged flanks of a shoaling basaltic seamount the coarse grade and large volume of conglomerate and breccia reflect the high supply rate of clasts and the propensity for collapse and redeposition on steep palaeoslopes the clast supply may have been boosted by vigorous fragmentation processes accompanying transition of lava from subaerial to submarine settings the greater proportion of primary volcanic facies compared with resedimented volcaniclastic and volcanogenic sedimentary facies in central and northwestern exposures near rakiraki indicates they are more proximal than those in the southeast towards viti levu bay the proximal area coincides with one of two zones where nw se trending mafic dykes are especially abundant and it is close to several small dome like intrusions of intermediate and felsic igneous rocks the original surface morphology of the volcano is no longer preserved though the partial fan of bedding dip azimuths in the south and east and the wide diameter exceeding 20 km are consistent with a broad shield

late neogene motion of the pacific plate oceanic crust between 38 s and 65 s along the pacific antarctic ridge records a complete history of neogene pacific antarctic motion we have modeled magnetic stages corresponding to portions of isochrons 1 2 2a 3 3a 4 and 5 from 17 high quality marine magnetic traverses of the ridge to determine the neogene finite rotation poles for this plate pair we chose to model velocities over time intervals of about 0 5 m y because this greatly increased the number of estimates of pacific antarctic plate velocity our analysis shows that a change in pacific antarctic relative motion occurred between chron 2a 3 40 ma and the beginning of chron 3 time 3 86 ma we used 203 estimates of pacific antarctic spreading velocity to calculate a 0 3 4 ma and 3 86 10 3 ma euler pole combined with tracks of seamounts that record the passage of the pacific plate over mantle plumes our data show that this change in relative motion corresponds to a change in the absolute motion of the pacific plate with respect to the hotspot frame of reference using these euler poles to calculate pacific antarctic africa north america circuit euler poles show that this recent change in pacific plate absolute motion corresponds to a change from strike slip to transpressive motion along the coastal california pacific north america plate boundary interestingly this age of increased tectonism agrees in time with deformation in new zealand and northern japan

a preliminary assessment of geodynamic controls on depositional systems and sandstone diagenesis in back arc basins western pacific ocean abstract nine depositional systems are recognized in cores recovered by the deep sea drilling project from the back arc basins of the western pacific these systems include submarine fan debris flow silty basinal turbidite biogenic pelagic carbonate resedimented carbonate biogenic pelagic silica pyroclastic hemipelagic and turbid layer clay and pelagic clay depositional systems the occurrence of these depositional systems in time and space is correlated with regional uplift and volcanism climatic change biological productivity and critical relations of sediment yield from potential sources which experience sufficient uplift and development of an extensive drainage and erosional regimen debris flows and submarine fan systems tend to accumulate after uplift of andesitic volcanic arc sources following development of a regional drainage network biogenic pelagic systems tend to be controlled by biological productivity and are indicators of back arc basin movement through such productivity zones by regional crustal shift hemipelagic and turbid layer clay systems are derived from terrigenous sources and their volume is dependent on climatically controlled sediment yield from such sources resedimented carbonate systems tend to occur during times of active basinal faulting volcaniclastic sandstones recovered in back arc basin dsdp cores show evidence of diagenetic alteration in response to burial depth temperature and thermally driven fluid circulation younger sandstones tend to show pore space reduction rim cementation by chlorite and mixed layer illite smectite and pore filling cementation by clinoptilolite downhole pores are filled with heulandite and analcite older sandstones show pore filling with sparry calcite and replacement of component minerals and rock fragments by calcite chert and zeolites the distinct age depth zonation of diagenetic changes suggests that thermally driven interstitial fluids combined with advection of ocean waters and basement waters accounts best for the mineralogical reactions observed

magmatic evolution of late cenozoic volcanic rocks of the lau ridge fiji three main groups of lavas are exposed on islands of the lau ridge the lau volcanic group lvg 14 0 5 4 ma are predominantly andesite korobasaga volcanic group kvg 4 4 2 4 ma are predominantly basalt and mago volcanic group mvg 2 0 0 3 ma are basalt hawaiite lvg and kvg lavas are mostly medium k tholeiitic rocks with high lile hfse ratios characteristic of islands ares while mvg lavas are ne normative alkalic rocks with high lile and hfse characteristic of ocean island basalts lvg lavas have high nd 8 0 8 4 and low 87sr 86sr 0 70273 0 70349 similar to n morb whereas kvg lavas have slightly more radiogenic values nd 7 5 8 4 87sr 86sr 0 70323 0 70397 mvg lavas form an isotopically distinct group having lower nd 4 6 4 9 and 87sr 86sr ranging from 0 70347 0 70375 lvg lavas were erupted in a primary oceanic island arc vitiaz arc during the miocene basaltic lavas were derived by approximately 19 partial melting of mantle wedge peridotite with only minor subduction component andesites and dacites were produced by low pressure plagioclase pyroxene titanomagnetite dominated crystal fractionation kvg lavas were erupted during the period immediately prior to or during the initial stages of rifting in the lau basin and like lvg lavas show significant chemical differences at the northern and southern ends of the lau ridge lavas at the northern end type ii appear to be derived from a more depleted source than lvg but with a greater amount of subduction component those at the southern end type i probably came from a slightly more enriched source with less subduction component mvg basalts and hawaiites were derived from an enriched mantle with little or no subduction input the hawaiites type i could not have been derived from the basalts type ii and the two magma types must have come from different sources indicating mantle heterogeneity the lack of subduction influence indicates the mvg lavas are tectonically unrelated to the present day tonga arc and the lack of depletion indicators suggests they have tapped a different new part of the mantle wedge this may reflect introduction of sub pacific mantle through the present tonga lau subduction system

is the oceanic crust over 1 km necessary for the source of marine magnetic anomalies abstract a paleomagnetic vertical profile over 1 km through the oceanic crust is constructed by magnetic properties of 256 dsdp samples from hole 504b the costa rica rift the results of magnetic measurements of the relatively young oceanic crust indicate that the thickness of the marine magnetic source layer is a 0 6 km thick pillow lava layer of the uppermost oceanic crust

magnetic lineations in marginal basins of the western pacific this paper separates the small oceanic basins around the western perimeter of the pacific ocean into marginal basins that have formed through back arc extension and those that apparently have not and reviews our knowledge of magnetic lineation patterns observed in possible and probable back arc basins magnetic lineations in these basins resemble lineations commonly associated with the world s mid oceanic spreading systems indicating that similar processes of crustal accretion occur in both tectonic environments in some back arc basins of the southwestern pacific magnetic lineation and other evidence suggest that back arc basins can evolve through the interaction and growth of multi plate systems because of the small time and space characteristics of back arc basins compared with the world s major oceanic spreading systems tectonic conditions favourable for the generation of back arc basins are either relaxed rapidly or easily interrupted models proposed to account for back arc basin development include a local models where back arc extension is mechanically driven by the downgoing slab and b global plate kinematics models where conditions favourable for back arc extension are governed by the motion of the overriding plate relative to the trench axis

the nadi basin neogene strike slip faulting and sedimentation in a fragmented arc western viti levu fiji the nadi basin in western viti levu is one of a number of sedimentary basins controlled by e w to ne sw faults that developed on the fiji platform in the late miocene the initial basin fill belonging to the upper miocene nadi sedimentary group consists of mainly basement derived breccias and sandstones deposited at bathyal depths by sediment gravity flows on a basin floor dissected by braided e w faults the main source area lay to the south across the actively down faulting basin margin in the latest miocene a change in provenance took place as debris spread southward from a newly developed e w chain of shoshonitic volcanoes koroimavua volcanic group some 12 km north of the southern basin margin continued normal displacement along the southern margin allowed the deposition of a thick sequence of shoshonitic sandstones at bathyal depths on the downthrown side in the earliest pliocene the onset of basin inversion was marked by shallowing to neritic depths and the onlap of sandstones and shoshonitic conglomerates southward onto pre nadi sedimentary group basement as inversion proceeded the basin fill was folded and southward directed reverse displacement took place along the southern basin margin evidence from the nadi and other late miocene basins indicates that they developed as a result of broadly e w directed sinistral strike slip probably related to a shearing collision of the melanesian border plateau with the northern margin of the fiji platform during the breakup of the formerly continuous outer melanesian arc it is suggested that prior to the establishment of the fiji fracture zone at 7 6 ma much of the left lateral displacement currently taking place along that plate boundary was accommodated by strike slip deformation within the fiji platform

petrology of seamounts northwest of samoa and their relation to samoan volcanism petrological and geochemical data on dredged samples from five submarine volcanos northwest of samoa indicate that three of these volcanos belong to the samoan volcanic province field lalla rookh and combe banks and two belong to separate magmatic zones wallis islands and alexa bank the samoan volcanic province increases in age westward and both shield building tholeiitic and alkalic lavas combe bank and strongly undersaturated post erosional melilitites or nephelinites and ankaramites field and lalla rookh banks are present the age progression and petrochemical character of these rocks is consistent with a fixed hotspot beneath eastern samoa slightly askew from this trend is alexa bank where dredged lavas are ocean island tholeiites however its radiometric age and compositional characteristics apparently preclude its association with samoa by a fixed hotspot model dredged volcanic rocks from near the wallis islands are geochemically petrologically and temporally different from samoan volcanism but are similar in these respects to quaternary volcanism in rotuma and fiji and may be related to plate reorganization accompanying opening of the north fiji basin

volcanism in nascent back arc basins behind the shichito ridge and adjacent areas in the izu ogasawara arc northwest pacific evidence for mixing between e type morb and island arc magmas at the initiation of back arc rifting mineral chemistry major and trace elements and 87sr 86sr ratios are presented for 29 igneous rocks dredged from the northern portion of the izu ogasawara arc these rocks are compositionally bimodal basement gabbro and trondhjemite from the arc are extremely poor in k2o 0 05 0 19 and rb 0 48 0 62 ppm and their ree patterns and sr isotope ratios indicate that there are island arc tholeiites quaternary volcanic rocks from the present volcanic front shichito ridge active arc back arc seamounts east side inactive arc and torishima knoll between the two back arc depressions incipient back arc basins behind the active arc have the same geochemical characteristics as the above plutonic rocks though they are not as depleted in k and rb rhyolite pumice from the backarc depression is also the depleted island arc tholeiite whereas basalts from the depression have compositions that are transitional between morb and island arc tholeiites in trace element ti ni cr v y and zr and mineral chemistries the back arc depression basalts have relatively high ban cen 0 66 1 24 cen ybn 1 1 1 9 and k ba 45 105 and low 87sr 86sr 0 70302 0 70332 and ba sr 0 1 0 2 which are similar to other back arc basin basalts and e type morb but are quite unlike the depleted island arc tholeiites the diverse trace element and sr isotope compositions of basalt andesite from the back arc depressions imply the interplay between e type morb and island arc tholeiite these chemical characteristics and the relationships of ce yb n vs ba ce n and ce yb n vs 87sr 86sr suggest that the back arc depression magmas are generated by mixing of e type morb and depleted island arc tholeiite magmas geochemical characters of the associated rhyolite from the depression are compatible with partial melting of lower crust

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

the control of depositional depth tectonic uplift and volcanism on sedimentation processes in the back arc basins of the western pacific ocean nine sediment types are recognized in cores recovered by the deep sea drilling project from back arc basins in the western pacific these include submarine fan turbidites debris flows silty basinal turbidites biogenic pelagic carbonate sediments resedimented carbonates biogenic pelagic silica sediments pyro clastics hemipelagic clays and pelagic clays most of these sediments were deposited independent of basinal rifting spreading processes the transition from biogenic pelagic carbonate sediments into overlying pelagic clays is the only sedimentary sequence dependent on basinal tectonic processes through post rifting subsidence associated with heat loss hemipelagic clays and silty turbidites are controlled by sediment yield from continental sources and by climatic change whereas deposition of biogenic pelagic sediments depend on ocean circulation and on latitudinally defined productivity zones coarse pyroclastics are the product of local volcanic periodicity whereas ash deposition is contr

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

crustal and lithospheric structure of inactive volcanic arc terrains in fiji abstract the crustal and lithospheric velocity structure of the fiji area may place important constraints on the evolution of island arcs in the region here we obtain receiver functions from one permanent seismic station and three temporary networks on the fiji platform the lau ridge and other small islands to develop point estimates of crustal thickness under the networks we obtain preliminary estimates of crustal thickness and poisson s ratio from h stacking of the receiver functions we then perform an inversion for shear wave velocity structure using a joint inversion of receiver functions and rayleigh wave dispersion curves from ambient seismic noise and teleseismic events the joint inversion results show an average crustal thickness of 30 km beneath viti levu the largest fiji island suggesting an extensive period of arc crustal formation and thickening in contrast we find shallower moho depths averaging 22 km beneath the smaller fiji islands and the lau ridge the limited thickness of the high velocity lid obtained by inversion indicates that the lithosphere is thinned beneath the marginal fiji islands which may be due to the early rifting event forming the lau basin together with the crustal thinning of this area as well and to thermal lithospheric erosion processes associated with adjacent back arc spreading a relatively slow velocity middle crustal layer roughly of 11 16 km thickness exists throughout fiji suggesting that remnant arcs contain significant volumes of felsic crustal rocks necessary for continental crust formation

the 2007 eruption of stromboli volcano insights from real time measurement of the volcanic gas plume co2 so2 ratio abstract the recent eruption of stromboli in february april 2007 offered a unique chance to test our current understanding of processes driving the transition from ordinary persistent strombolian to effusive activity and the ability of instrumental geophysical and geochemical networks to interpret and predict these events here we report on the results of two years of in situ sensing of the co 2 so 2 ratio in stromboli s volcanic gas plume in the attempt to put constraints on the trigger mechanisms and dynamics of the eruption we show that large variations of the plume co 2 so 2 ratio range 0 9 26 preceded the onset of the eruption since december 2007 interrupting a period of relatively steady and low ratios time averaged ratio 4 3 lasting from at least may to november 2006 by contrasting our observations with numerical simulations of volcanic degassing at stromboli derived by use of an equilibrium saturation model we suggest that the pre eruptive increase of the ratio reflected an enhanced supply of deeply derived co 2 rich gas bubbles to the shallow plumbing system this larger than normal ascent of gas bubbles was likely sourced by a 1 3 km deep gas melt separation region probably a magma storage zone and caused faster convective overturning of magmas in the shallow conduit an increase in the explosive rate and in seismic tremor and finally the collapse of the la sciara del fuoco sector triggering the effusive phase the high co 2 so 2 ratios up to 21 observed during the effusive phase and particularly in the days and hours before a paroxysmal explosion on march 15 2007 indicate the persistence of the same gas source and suggest that de pressurization of the same 1 3 km deep magma storage zone could have been the trigger mechanism for the paroxysm itself

evolution of volcanic gas composition during repeated culmination of volcanic activity at kuchinoerabujima volcano japan abstract chemical and isotopic compositions of low to medium temperature fumarolic gasses were measured at various fumaroles of kuchinoerabujima volcano from 1993 to 2009 by the combination of fumarolic gas sampling and multi gas measurement of volcanic plumes repeated culmination of the volcanic activity was observed as contemporaneous occurrence of seismic swarms summit inflation and demagnetization almost every two years after 1999 fumarolic activity also increased parallel with these activities new fumarolic fields of low temperature fumaroles at boiling point formed at the southern and western rims of the summit crater in 2003 and intense degassing activity started at medium temperature fumaroles inside the summit crater in 2008 the low temperature fumarolic gasses have peculiar composition with high h2 h2o co co2 and so2 h2s ratios typical features of high temperature volcanic gasses but also with low hcl and total s contents typical features of low temperature gasses these features indicate that the gasses are formed by low temperature differentiation of high temperature gasses variation of h and o isotopic ratios of the low temperature gasses indicates that the gasses are formed by isoenthalpic processes of vapor liquid separation at 100 c of a high temperature gas and meteoric water mixture implying that the high temperature gas was injected into a shallow aquifer without cooling temperatures of the high temperature gasses were estimated as 550 700 c based on the equilibrium temperature calculation with considering the low temperature differentiation the medium temperature fumarolic gasses from the summit crater have typical compositions of high temperature volcanic gasses and are estimated as the source gas of the boiling point fumaroles the h2 h2o and co2 st ratios of the low temperature fumaroles increased from 2004 to 2009 which is interpreted to be caused by the evolution of thermal structure of a shallow gas storage region of a few hundred meters deep increase of magmatic gas supply caused enlargement of a high temperature zone of the gas storage region widening the surrounding temperature transit zone with smaller temperature gradient the enlargement of the high temperature zone also reduces the overlying low temperature aquifer which caused the decrease of co2 st ratio by reduction of sulfur removal reaction the enlargement of the high temperature zone resulted in the leakage of the high temperature gasses directly to the surface as the medium temperature gasses from the summit crater

analysis of lineaments and their relationship to neogene fracturing se viti levu fiji the fiji archipelago lies along a complex active section of the pacific and australian plate boundary zone in the sw pacific the availability of a range of remotely sensed imagery and recent field studies have permitted the exploration of the link between lineaments as seen on such imagery and tectonic fractures observed on the ground in se viti levu the main island of fiji comparisons of statistical orientation peaks as well as the spatial extent of lineaments with field mapped fractures indicate that the lineaments are largely a reflection of tectonic fractures in the bedrock emphasized on the surface by topography drainage and vegetation surface lineaments correspond to broad zones of intense fracturing and strongly influence the landform features of the area it appears that surface fracture sets are open upward splay zones that merge downward into single planar discontinuities represented by sparse uninterrupted magnetic lineaments at depth fault correlated lineament sets extend over greater geographical distances than the equivalent mapped fault zones and these lineament sets represent more complex regional structural patterns than previously known this study provides an approach to mapping fracture patterns where outcrop is limited and where conventional field mapping techniques of structural geology are impractical it highlights the benefits of mapping lineaments from multiple sets of remote sensing imagery of the surface and bedrock mapping them at regional and larger scales as well as linking lineaments to detailed field mapped structures lineament data derived using such an approach can be used effectively to characterize regional structural patterns which in turn may provide better understanding of the structural and tectonic evolution and seismic hazards of the region this approach may also find useful application in groundwater mineral and oil and gas exploration

magnetic anomalies and tectonic fabric of marginal basins north of new zealand detailed airborne magnetic studies conducted over the region of the s w pacific marginal basins extending from the solomon islands to new zealand suggest that three major phases of basin formation and island arc development have occurred in this region development of the tasman sea took place during the late cretaceous paleocene development of the basins to the east of the tasman sea occurred predominantly during the oligocene as well as during the upper miocene to recent the south fiji basin consisting of the kupe and minerva abyssal plains is marked by the presence of possibly two rrr triple junction spreading centers that were active between the times of anomalies 13 to 7 36 25 5 m y the kupe abyssal plain shows the presence of residual magnetic anomalies 7 to 13 of the eastern limb of the proposed spreading center the western limb appears to have been subducted beneath the present site of the three kings rise this seafloor spreading phase calculated half spreading rate of 35 mm yr was coincident with the overthrusting phase of the new caledonia ultramafic rocks during that period active volcanism along the then continuous solomons new hebrides fiji lau island arc was taking place magnetic anomalies from 1 to 4 0 8 m y b p are seen to extend along a clearly defined lineation pattern over the north fiji basin analysis of the magnetic anomalies suggests that during the upper miocene at about the time of magnetic anomaly 4 8 m y the solomons lau island arc commenced breaking up through the development of an active spreading center calculated half spreading rate of 35 mm yr located between the new hebrides and fiji portions of the island arc portions of the northeastern sector of the minerva abyssal plain were subducted beneath the north fiji basin during this period between 0 8 m y b p at the time of about anomaly 3 to 2 4 5 3 5 m y the development of the north fiji basin extended into the lau basin and havre trough thus separating the tonga and lau ridges as a result a distinct series of active spreading centers developed southward along the axis of the havre trough lau basin the havre trough is marked by present seismic and submarine volcanic activity residual magnetic anomalies 1 2 0 1 8 m y b p form a symmetrical pattern around the axis of the trough extending south from the lau basin to the active taupo white island volcanic zone of new zealand phases of development of the marginal basins described in this paper may be correlated with phases of changes in the direction and rate of pacific plate motion

geological tectonic framework of solomon islands sw pacific crustal accretion and growth within an intra oceanic setting the solomon islands are a complex collage of crustal units or terrains herein termed the solomon block which have formed and accreted within an intra oceanic environment since cretaceous times predominantly cretaceous basaltic basement sequences are divided into 1 a plume related ontong java plateau terrain ojpt which includes malaita ulawa and northern santa isabel 2 a normal ocean ridge related south solomon morb terrain ssmt which includes choiseul and guadalcanal and 3 a hybrid makira terrain which has both morb and plume plateau affinities the ojpt formed as an integral part of the massive ontong java plateau ojp at c 122 ma and 90 ma respectively was subsequently affected by eocene oligocene alkaline and alnoitic magmatism and was unaffected by subsequent arc development the ssmt initially formed within a normal ocean ridge environment which produced a morb like basaltic basement through which two stages of arc crustal growth subsequently developed from the eocene onwards the makira terrain records the intermingling of basalts with plume plateau and morb affinities from c 90 ma to c 30 ma and a contribution from late miocene present day arc growth two distinct stages of arc growth occurred within the solomon block from the eocene to the early miocene stage 1 and from the late miocene to the present day stage 2 stage 1 arc growth created the basement of the central part of the solomon block the central solomon terrain cst which includes the shortland florida and south isabel islands stage 2 arc growth led to crustal growth in the west and south the new georgia terrain or ngt which includes savo and the new georgia and russell islands both stages of arc growth also added new material to pre existing crustal units within other terrains the solomon block terrane collage records the collision between the alaska sized ojp and the solomon arc initial contact possibly first occurred some 25 20 ma but it is only since around 4 ma that the ojp has more forcefully collided with the solomon arc and has been actively accreting since that time continuing to the present day we present a number of tectonic models in an attempt to understand the mechanism of plateau accretion one model depicts the ojp as splitting in two with the upper 4 10 km forming an imbricate stack verging to the northeast over which the solomon arc is overthrust whilst deeper portions of the ojp beneath a critical detachment surface are subducted the subduction of young 5 ma hot oceanic lithosphere belonging to the woodlark basin at the ssts has resulted in a sequence of tectonic phenomena including the production of unusual magma compositions e g na ti rich basalts and an abundance of picrites an anomalously small

magma hydrothermal system interaction inferred from volcanic gas measurements obtained during 2003 2008 at meakandake volcano hokkaido japan phreatic eruptions occurred at the meakandake volcano in 1988 1996 1998 2006 and 2008 we conducted geochemical surveillance that included measurements of temperature so2 emission rates and volcanic gas composition from 2003 to 2008 at the nakamachineshiri nm northwest nw and akanuma ak fumarolic areas and the 96 1 vent where historical eruptions had occurred the elemental compositions of the gases discharged from the different areas are similar compared with the large variations observed in volcanic gases discharged from subduction zones all the gases showed high apparent equilibrium temperatures suggesting that all these gases originated from a common magmatic gas the gases discharged from each area also exhibited different characteristics which are probably the results of differences in the conditions of meteoric water mixing quenching of chemical reactions and vapor liquid separation the highest apparent equilibrium temperatures about 500 c were observed in the case of nw fumarolic gases despite the low outlet temperature of about 100 c at these fumaroles since the nw fumaroles were formed as a result of the 2006 phreatic eruption the high temperature gas supply to the nw fumarole suggests that the phreatic eruption was caused by the ascent of high temperature magmatic gases the temperatures compositions and emission rates of the nm and 96 1 gases did not show any appreciable change after the 2006 eruption indicating that each fumarolic system had a separate magmatic hydrothermal system the temperatures compositions and emission rates of the nm fumarolic gases were apparently constant and these fumaroles are inferred to be formed by the evaporation of a hydrothermal system with a constant temperature of about 300 c the 96 1 gas compositions showed large changes during continuous temperature decrease from 390 to 190 c occurred from 2003 to 2008 but the sulfur gas emission rates were almost constant at about four tons day at the 96 1 vent the so2 h2s ratio decreased while the h2 h2o ratio remained almost constant this was probably caused by the rock buffer controlled chemical reaction during the temperature decrease

origin of fumarolic fluids from tupungatito volcano central chile interplay between magmatic hydrothermal and shallow meteoric sources tupungatito is a poorly known volcano located about 100 km eastward of santiago chile in the northernmost sector of the south volcanic zone this 5 682 m high volcano shows intense fumarolic activity it hosts three crater lakes within the northwestern portion of the summit area chemical compositions of fumarolic gases and isotopic signatures of noble gases 3he 4he and 40ar 36ar are up to 6 09 ra and 461 respectively and steam 18o and d suggest that they are produced by mixing of fluids from a magmatic source rich in acidic gas compounds so2 hcl and hf and meteoric water the magmatic hydrothermal fluids are affected by steam condensation that controls the outlet fumarolic temperatures 200 c and redox conditions are consistent with those inferred by the presence of the so2 h2s redox pair typical of fluids that have attained equilibrium in magmatic environment a comprehensive conceptual geochemical model describing the circulation pattern of the tupungatito hydrothermal magmatic fluids is proposed it includes fluid source regions and re equilibration processes affecting the different gas species due to changing chemical physical conditions as the magmatic hydrothermal fluids rise up toward the surface

neogene displacements in the solomon islands arc abstract the geology and present configuration of the solomon island arc can be explained in terms of the neogene displacement of a single linear chain of islands the central part of an original arc consisting of bougainville choiseul santa ysabel guadalcanal and san cristobal was displaced to the northeast as a consequence of the attempted subduction of the woodlark spreading system malaita arose on the northeastern side of the arc as a result of interaction between the arc and the pacific ocean floor and the volcanic islands of the new georgia group formed to the southwest in response to the subduction of a spreading ridge thus giving rise to the present double chain structure of the arc

tholeiitic calc alkaline and alkaline igneous rocks of the shortland islands solomon islands abstract the shortland islands lie in a northeast southwest line across the western end of solomon islands immediately adjacent to bougainville three major islands dominate the group fauro and surrounding islands in the northeast have an altered basement suite comprising tholeiite icelandite and tholeiitic dacite this is intruded by a high level calc alkaline assemblage of microdiorite hornblende andesite and rhyodacite and overlain by volcanogenic sandstones derived from an andesitic to dacitic volcano pyroclastics comprising high alumina basalt and pyroxene andesite overlie the volcanogenic sandstones the tholeiitic basement lavas may be of late oligocene to early miocene age and the calc alkaline rocks are probably also pre pliocene in age alu in the centre of the group also has an altered tholeiitic lava basement which is intruded by a quartz diorite body and overlain by hypersthene augite basaltic andesite pliocene siltstone and quaternary shallow marine carbonates cover these igneous rocks over much of the island mono in the southwest has a small basement exposure of altered pillowed hawaiite overlain by miocene pelagic limestone pliocene siltstone and quaternary reef limestone isolated clasts of pyroxene andesite and benmoreite occur in streams and on beaches the younger calc alkaline suites on all islands were formed in an island arc environment possibly related to subduction from the southwest beneath the new britain trench the basement lavas on alu are probably early island arc tholeiites and both these lavas and the calc alkaline rocks of alu share a common trend on variation diagrams the two igneous suites of fauro however have distinctly different trends the basement lavas have some chemical similarities with oceanic tholeiites but an early island arc origin for these lavas cannot be ruled out the altered hawaiite and benmoreite on mono probably originated in an oceanic island environment

selective destructive demagnetization of breccias from dsdp leg 83 a microconglomerate test the magnetization of four breccia samples from the leg 83 section of dsdp hole 504b was analyzed by selective destructive demagnetization in order to study the origin and stability of hydrothermally altered basalts the nrm directions of the clasts for three of the four samples are randomly oriented and much more strongly magnetized than the bulk sample clasts which were individually demagnetized show two or more components of magnetization but neither are coincident with those of the bulk sample indicating that nrm was probably acquired prior to the consolidation of the breccia and suggesting that any overprint vrm or otherwise can be removed by af demagnetization to at most 50 oe reflected light microscopy and electron microprobe analysis of two samples show that the unexpectedly high nrm of the matrix regions is apparently the result of secondary magnetic phases precipitated from hydrothermal solutions

equilibrium temperature and redox state of volcanic gas at unzen volcano japan abstract high temperature volcanic gases were collected from a growing dacitic dome of unzen volcano japan apparent equilibrium temperatures aet were calculated for the following two reactions so2 3h2 h2s 2h2o and co2 h2 co h2o both the aet s are higher than the outlet temperature to two hypotheses are examined as a cause of this disagreement one is a partial advancement in the reaction so2 3h2 h2s 2h2o the other is an addition of external water after a final equilibrium attainment based on these hypotheses the equilibrium compositions temperatures and redox potentials were estimated resulting in similar values the estimated temperatures range from 850 to 911 c which is consistent with a petrological estimation made with the lava however the estimated redox potential is one unit less in logfo2 scale than the petrological estimation

resistivity structure and geochemistry of the jigokudani valley hydrothermal system mt tateyama japan abstract this study clarifies the hydrothermal system of jigokudani valley near mt tateyama volcano in japan by using a combination of audio frequency magnetotelluric amt survey and hot spring water analysis in order to assess the potential of future phreatic eruptions in the area repeated phreatic eruptions in the area about 40 000 years ago produced the current valley morphology which is now an active solfatara field dotted with hot springs and fumaroles indicative of a well developed hydrothermal system the three dimensional 3d resistivity structure of the hydrothermal system was modeled by using the results of an amt survey conducted at 25 locations across the valley in 2013 2014 the model suggests the presence of a near surface highly conductive layer of 2 o isotopic ratios all hot spring waters had low ph and could be categorized into three types on the basis of the cl so 4 2 concentration ratio with all falling largely on a mixing line between magmatic fluids and local meteoric water lmw the geochemical analysis suggests that the hydrothermal system includes a two phase zone of vapor liquid a comparison of the resistivity structure and the geochemically inferred structure suggests that a hydrothermal reservoir is present at a depth of approximately 500 m from which hot spring water differentiates into the three observed types the two phase zone appears to be located immediately beneath the cap rock structure these findings suggest that the hydrothermal system of jigokudani valley exhibits a number of factors that could trigger a future phreatic eruption

patterns in the recent 2007 2008 activity of mount etna volcano investigated by integrated geophysical and geochemical observations 1 seismic deformation and volcanic gas observations offer independent and complementary information on the activity state and dynamics of quiescent and eruptive volcanoes and thus all contribute to volcanic risk assessment in spite of their wide use there have been only a few efforts to systematically integrate and compare the results of these different monitoring techniques here we combine seismic volcanic tremor and long period seismicity deformation gps and geochemical volcanic gas plume co2 so2 ratios measurements in an attempt to interpret trends in the recent 2007 2008 activity of etna volcano we show that each eruptive episode occurring at the southeast crater sec was preceded by a cyclic phase of increase decrease of plume co2 so2 ratios and by inflation of the volcano s summit captured by the gps network these observations are interpreted as reflecting the persistent supply of co2 rich gas bubbles and eventually more primitive magmas to a shallow depth of 1 2 8 km asl magma storage zone below the volcano s central craters ccs overpressuring of the resident magma stored in the upper ccs conduit triggers further magma ascent and finally eruption at sec a process which we capture as an abrupt increase in tremor amplitude an upward 2800 m asl and eastward migration of the source location of seismic tremor and a rapid contraction of the volcano s summit resumption of volcanic activity at sec was also systematically anticipated by declining plume co2 so2 ratios consistent with magma degassing being diverted from the central conduit area toward sec

electrochemical sensors applied to pollution monitoring measurement error and gas ratio bias a volcano plume case study there is an increasing scientific interest in the use of miniature electrochemical sensors to detect and quantify atmospheric trace gases this has led to the development of multi gas systems applied to measurements of both volcanic gas emissions and urban air pollution however such measurements are subject to uncertainties introduced by sensor response time a critical issue that has received limited attention to date here a detailed analysis of output from an electrochemical so2 sensor and two h2s sensors contrasting in their time responses and cross sensitivities demonstrates how instrument errors arise under the conditions of rapidly fluctuating by dilution gas abundances leading to scatter and importantly bias in the reported gas ratios in a case study at miyakejima volcano japan electrochemical sensors were deployed at both the crater rim and downwind locations thereby exposed to rapidly fluctuating and smoothly varying plume gas concentrations respectively discrepancies in the h2s so2 gas mixing ratios derived from these measurements are attributed to the sensors differing time responses to so2 and h2s under fluctuating plume conditions with errors magnified by the need to correct for so2 interference in the h2s readings development of a sensor response model that reproduces sensor t90 behaviour the time required to reach 90 of the final signal following a step change in gas abundance during calibration enabled this measurement error to be simulated numerically the sensor response times were characterised as so2 sensor t90 13 s h2s sensor without interference t90 11 s and h2s sensor with interference t90 20 s to h2s and 32 s to so2 we show that a method involving data integration between periods of episodic plume exposure identifiable in the sensor output yields a less biased h2s so2 ratio estimate than that derived from standard analysis approaches for the miyakejima crater rim dataset this method yields highly correlated h2s and so2 abundances r2 0 99 and the improved crater rim data analysis combined with downwind measurements yields h2s so2 0 11 0 01 our analysis has significant implications for the reliance that can be placed on multi gas derived gas ratios whether for volcanological or other purposes in the absence of consideration of the complexities of sensor response times

magnetic structure of the upper kilometer of the marine crust at deep sea drilling project hole 504b eastern pacific ocean deep sea drilling project hole 504b is located in 5 9 m y crust in the eastern pacific ocean about 200 km south of the costa rica rift at 1076 m subbasement it is the deepest penetration of marine crust yet achieved we present here magnetic data from this hole especially from the recently cored leg 83 section which constitutes the lowermost 500 m these data when combined with those of other studies yield not only the deepest but also the most detailed and comprehensive picture of marine magnetic structure at a single site currently available the basement rocks from hole 504b can be divided into three major magnetic units the upper units the top 500 m or so of basement are essentially similar to other shallow marine basement sections consisting of a mixture of various types of extrusive basalt low temperature altered titanomagnetite is the dominant magnetic carrier and the magnetic properties of these rocks are comparable to other shallow marine basalts below the upper units is a 200 m transition zone consisting of a mixture of extrusives and dikes high temperature hydrothermal alteration much of it greenschist facies grade has caused oxidation exsolution of primary titanomagnetite as well as its partial or total replacement by silicates the dominant magnetic carrier is an fe rich titanomagnetite magnetically indistinguishable from pure magnetite the combination of oxidation exsolution and silicate replacement results in a very low natural remanent magnetization nrm the remaining 300 m are in the upper portion of the sheeted dike complex primary titanomagnetite is exsolved but alteration is less intense than in the transition zone nrm values are substantially higher than those of the transition zone due to less silicate replacement of primary titanomagnetite coupled with an uncertain contribution from secondary magnetite which occurs as a silicate alteration product the nrm magnitude is sufficiently high that the sheeted dike complex in this location at least may make a significant if not substantial contribution to the magnetic anomaly overall the magnetic properties of this hole especially the lower 500 m are strongly influenced by postemplacement alteration and may bear little or no resemblance to their values upon initial cooling as hydrothermal temperatures do not appear to have exceeded 400 c the nrm in the lower two sections is apparently a chemical rather than a thermal remanence as such models of marine magnetic structure that combine cooling models of the crust with the assumption that magnetic remanence is purely of thermal origin do not appear sufficient to adequately predict the properties of the deeper crust a better understanding of the nature of postemplacement alteration in the marine crust is required especially its characteristics at high temperatures and its effects on magnetic properties

earthquakes associated with diffuse zones of deformation in the oceanic lithosphere some examples abstract we review the results of source parameters of earthquakes associated with the eastern end of the azores gibraltar plate boundary the davie ridge near madagascar and the horizon bank in the north fiji basin to characterize the nature of present day tectonics in diffuse zones of deformation in the oceanic lithosphere in comparison with typical plate boundaries diffuse zones of deformation are in general characterized by complex morphological expressions and scattered seismicity of up to several hundred kilometers width although the average rate of displacement across these regions is not resolvable by current models of global plate reconstruction the deformation is not truly intraplate in nature because strain is concentrated and often distinct tectonic boundaries can be identified earthquakes with seismic moments as large as 8 10 20 nm have occurred in bands of scattered seismicity that are associated with diffuse zones of deformation in the oceanic lithosphere in two cases the zones of diffuse deformation continue into the continental lithosphere east of gibraltar and north of the davie ridge however the largest earthquakes occur in the oceanic part of these two zones in contrast to previous reports the depth of earthquakes is quite shallow in regions where a mixture of focal mechanisms occur the consistent parameter is usually the orientation of the p or the t axes instead of that of the nodal planes or slip vectors thus as in the continental lithosphere zones of diffuse deformation in the oceanic lithosphere seem to be better described by a regional compression or extension field rather than by displacement on discrete faults

geology and geochemistry of early arc volcanic rocks from guam the island of guam is located at the southern end of the mariana fore arc and was the site of early arc volcanism between 43 and 32 m y ago two volcanic units were erupted during the early evolution of guam the late middle eocene facpi formation and the late eocene to early oligocene alutom formation the facpi formation is composed largely of interbedded boninite series pillow lavas pillow breccias and dikes although arc tholeiite series rocks cap the formation in some areas the alutom formation is composed of interbedded volcanic breccias tuffaceous sandstones lava flows and sills calc alkaline arc tholeiite and boninite series compositions are all found in the alutom formation primitive boninite series rocks are relatively high in sio 2 mgo and ni and low in a1 2 o 3 and tio 2 contents compared to basaltic rocks from the arc tholeiite series additionally they have low ti zr ratios and high k zr rb zr and ba zr ratios although no calc alkaline basalts are found in the early arc formations of guam basalts with chemical characteristics transitional between boninite and tholeiite series rocks may be genetically linked to the calc alkaline series among the analyzed samples here are a few with anomalously high y and rare earth element ree contents chemical variation in the boninite series can be explained by early fractionation of olivine and clinopyroxene followed by plagioclase clinopyroxene and orthopyroxene the arc tholeiite series chemical trends can be explained by early olivine fractionation followed by plagioclase clinopyroxene orthopyroxene and magnetite the composition of the least silicic calc alkaline series andesite can be modeled by fractional crystallization of olivine clinopyroxene plagioclase and magnetite from a transitional basalt derivation of more silicic calc alkaline series rocks requires magma mixing or crustal assimilation in addition to crystal fractionation involving hornblende all basalts from the early volcanic series of guam were erupted in an arc setting the boninite series magmas were produced by hydrous partial melting of depleted mantle at relatively shallow levels whereas the arc tholeiite and calc alkaline series magmas were generated by partial melting of less depleted mantle at deeper levels we suggest that because the calc alkaline series parent basalts most likely were richer in incompatible high field strength hfs elements and sio 2 than were the tholeiitic parents they were generated by lower degrees of melting

geochemistry and petrogenesis of early proterozoic amphibolites west central colorado u s a abstract early proterozoic supracrustal rocks exposed in west central colorado include amphibolites intimately associated with felsic volcanics and associated volcaniclastic sediments three successions are recognized the dubois succession 1770 1760 ma is comprised chiefly of amphibolites and felsic volcanics the amphibolites exhibit low concentrations of light ree and hfse and are broadly similar in composition to tholeiites from immature island arcs and in some respects to nmorb the cochetopa succession 1745 1730 ma includes large amounts of felsic volcanics and volcaniclastic sediments together with amphibolites that exhibit variable but slight incompatible element enrichment similar to basalts from evolved island arc systems and associated back arc basins the black canyon succession is coeval with the cochetopa succession and is comprised chiefly of volcaniclastic sediments with minor amphibolite these amphibolites exhibit significant incompatible element enrichment similar to basalts from continental margin arc systems geochemical modeling indicates that the amphibolites evolved by closed system fractional crystallization of depleted for the dubois succession to variably enriched parental basalts for the cochetopa and black canyon successions chiefly at shallow depths incompatible element distributions in the amphibolites indicate that mantle sources changed with time from relatively depleted 1770 1760 ma to variable but slightly enriched 1745 1730 ma a provisional model calls upon the dubois succession forming in an island arc system followed by collision with the archean wyoming province at 1750 ma the cochetopa succession was deposited in an intra arc basin developed within a new continental margin arc and the black canyon succession formed in a back arc basin associated with this arc

temporal change of characteristics of shallow volcano tectonic earthquakes associated with increase in volcanic activity at kuchinoerabujima volcano japan abstract shallow volcano tectonic vt earthquakes recorded at the kuchinoerabujima island volcano in southwest japan are analyzed in order to clarify the role of hydrothermal activity in the development of volcanic seismicity from analysis of shallow vt earthquakes in 2006 two specific episodes of elevated seismicity are observed in april and november 2006 the vt earthquakes have hypocenters at depths of 0 0 4 km beneath the summit crater and normal fault focal mechanisms with wnw ese extension consistent with the tensional stress field indicated by the alignment of craters and fissures although the hypocenters and focal mechanisms are found to be largely invariant during these episodes the corner frequencies of the vt earthquakes underwent a pronounced increase and decrease accompanying the changes in seismicity rates the corner frequencies increased to 20 25 hz approximately one month prior to the onset of elevated seismicity and then decreased to 10 15 hz in the period of peak seismicity the rupture length also decreased at the onset of seismicity thereafter increasing as the seismicity continued the peak seismicity in terms of the daily number of vt events was accompanied by inflation around the crater suggestive of a pressure increase in the volcanic system it is inferred that the increase in shallow vt seismicity and rupture length is related to the development of a fractured zone the pressure increase in the volcanic system is attributed to the intrusion of hydrothermal fluids which is supported by an observed increase in fumarolic temperature and activity the preceding monochromatic events are thus considered to be generated by the effect of fluid filled cracks the shortening of rupture length is then inferred to be related to the closing of non fluid filled cracks in the fracture zone under the increasing pressure field leading to a transition from monochromatic events to low frequency and shallow vt seismicity