sills associated with the bushveld complex south africa an estimate of the parental magma composition abstract four distinct ages of sills intrude the floor rocks under the bushveld complex the first is a metadolerite and pre dates the bushveld complex the middle two are generally related to the complex itself one has quench textured olivine and orthopyroxene crystals and chemically equates to the parental magma of the bushveld complex the second suite is a hypersthene microgabbro representative of the second major injection of magma into the complex the last is doleritic and does not obviously correlate with any portion of the layered sequence the sills genetically related to the complex display several features 1 lack of chilled margins coarse grain size ability to melt country rock xenoliths extreme heterogeneity 2 atypical of normal sills but explicable in their present context

two magmatic series with island arc affinities within the vourinos ophiolite within the vourinos ophiolite evidence of two magmatic series has been preserved in cognate cumulates and in effusive and hypabyssal rocks which constitute the earlier krapa sequence and the younger asprokambo sequence the asprokambo dyke basic magmas which are poor in incompatible elements and relatively ni and cr rich bear some resemblance to very low ti basalts transitional to boninites found in subduction related arcs or interarc basins krapa series magmas from sills massive and pillow lavas are best equated with low k tholeiites of island arc suites compositions of al and ti poor cpx in lavas from both series are comparable to those in island arc basalts the asprokambo cpx being richer in ca and cr than those from krapa the large volume of cumulates from the krapa sequence displays the following crystallization order ol sp cpx pl opx mt periodic influx of fresh magma batches into the magma chamber occurred mainly during the formation of the lower cumulates wehrlite ol clinopyroxenite and melagabbro the upper cumulates gabbronorite and leucogabbronorite with minor mt bearing gabbronorite crystallized in the upper levels of a magma chamber which became progressively smaller with time in the asprokambo sequence ol sp opx cpx pi and amph are the successively crystallizing phases the ortho to heteradcumulates consist of websterite pl websterite gabbronorite amphibole bearing leuconorite diorite and granophyre in cumulates especially in the lower krapa sequence significant subsolidus reaction was probably induced by the persistence of high geothermal gradients linked to continuous magmatism petrological features indicate that the evolution of the krapa series is more compatible with an intermediate fractional equilibrium crystallization history in an initially open system whereas nearly perfect fractional crystallization in closed system may have occurred in the small asprokambo magma chambers chemical variations in the lavas of both series can be explained in terms of crystallization of the observed cumulates significantly the asprokambo intrusives have igneous mg hornblende and vanadium bearing chromian aluminous titaniferous magnetite crystallization of which is responsible for the calcalkaline evolutionary trend of these rocks major and trace element modelling necessitates a two stage model for the petrogenesis of the vourinos parental melts involving high degree remelting of previously depleted mantle sources favoured by the influx of subduction derived hydrous fluids the primary magmas parental to the krapa and asprokambo series could have been derived respectively by 20 and 30 equilibrium partial fusion of variably depleted lherzolitic sources leaving residua having a harzburgitic to dunitic composition

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

petrology and petrogenesis of cumulate peridotites and gabbros from the marum ophiolite complex northern papua new guinea

alpine ophiolites product of low degree mantle melting in a mesozoic transcurrent rift zone abstract the alpine ophiolites and their original sedimentary cover upper jurassic radiolarite limestone with ophiolite breccia occur as dismembered tectonic slices often inverted by alpine tectonics or olistoliths among the calc schists cretaceous original petrologic features of the ophiolites are largely masked by the alpine metamorphism but are discernible by the relic mineral chemistry and petrographic reconstruction the residual peridotite is generally plagioclase lherzolite resembling those from oceanic fracture zones but spinel lherzolite is present at davos switzerland and peas france the latter is characterized by higher na 2 o 1 6 of clinopyroxene and lower cr 2 o 3 8 of spinel than the former 30 respectively and resembles subcontinental lherzolites geologic evidence indicates that these two lherzolites were together exposed on the jurassic ocean floor the mafic ultramafic cumulates are of plagioclase type characterized by the scarcity of orthopyroxene and high tio 2 avg 0 8 of clinopyroxene the basaltic volcanics are alkalic rather than tholeiitic in respect to their relic clinopyroxene which is as rich in tio 2 and na 7 o as that in hawaiian alkali basalt the alpine ophiolite association of lherzolitic residual peridotite plagioclase type cumulates and alkalic basalt is reasonable as cogenetic products of low degree partial melting in the mantle geologic and petrologic features of the alpine ophiolites are consistent with a geotectonic model postulating the generation of the alpine ophiolites in an early mesozoic rift zone cut by many transform faults

tectonostratigraphic models of the alpine terranes and subduction history of the hellenides abstract the tectonostratigraphy of the hellenic terranes is related to their paleogeodynamic and paleogeographic evolution which comprises three major stages i continental rifting in the northern margin of gondwana characterised by volcanosedimentary complexes of late paleozoic triassic ii continental drifting and contemporaneous oceanic opening of tethyan basins in between the continental terranes during triassic paleogene with shallow water carbonate platforms on the continental terranes and ophiolite suites interlayered with pelagic sediments within the tethyan basins iii accretion of the tectonostratigraphic terranes with docking along the active european margin characterised by flysch melange sedimentation along the trenches from early jurassic to neogene two tectonostratigraphic models can be distinguished one for the continental terranes carbonate platforms and another for the oceanic basins the duration of each geodynamic stage for each terrane is obtained from the chronology of the tectonostratigraphic facies change the general trend is younger ages observed in the southern terranes and older ages towards the northern terranes the two alternative tectonostratigraphic models are applied in the two groups of terranes and the chronology of the geodynamic paleogeographic stages is estimated the hellenic subducted slab as shown in the seismic tomographic images is correlated to the terrane paleogeography using the estimated widths and the average subduction rates the upper 1700 km of the slab correspond to the three southern more external terranes whereas the remainingpart of the slab may represent the other more internal terranes a palinspastic model of the hellenides is presented taking into account the chronology of each stage for every terrane obduction of ophiolites over the southern platforms is observed in all four oceanic basins blueschist exhumation formation of core complexes and tectonic windows through extensional detachments occur after micro collision of continental terranes platforms and their isostatic uplift within the upper plate after their detachment from the subducting slab

mechanism of small vulcanian eruptions at suwanosejima volcano japan as inferred from precursor inflations and tremor signals we examine precursory signals of vulcanian eruptions at suwanosejima volcano southwest of japan by deploying seismic and tilt stations close to the active crater to understand the shallow conduit processes just before explosions vulcanian eruptions at suwanosejima for the period from october 2009 to 2011 are often preceded by a sudden cessation of continuous tremor associated with continuous ash emissions the amplitude of continuous tremor decreases within about 10 s and tilt signal starts to show uplift of the active crater then the pause of continuous tremor and uplift continue for about 1 2 min and tilt amplitudes reach a few tens of nanoradians at a distance of 400 m from the active crater as the pause time of continuous tremor before explosion becomes longer the maximum amplitude of explosion earthquake and uplift signal become larger vulcanian eruptions are also observed during periods without continuous tremor in these cases tilt signals capture uplifts of the active crater for about 10 min before explosions and reach a few tens of nanoradians cessations of volcanic gas emissions are observed before some explosions during periods without continuous tremor mori et al 2011 these observations suggest that pressurization in the shallow conduit is caused by an accumulation of volcanic gases and ash through the formation of cap in the conduit possibly composed of stiffened magma due to degassing cooling rheological changes or other mechanisms

remarkable cyclic ground deformation monitored in real time on montserrat and its use in eruption forecasting telemetered high resolution tiltmeters were installed in montserrat in summer of 1995 in december 1996 and in may 1997 the 1995 installations several km from the soufriere hills vent were too distant to yield useful data however the 1996 and 1997 installations on the crater rim revealed 6 14 h inflation cycles caused by magma pressurization at shallow depths 0 6 km below the base of dome the tilt data correlated with seismicity explosions and pyroclastic flow activity and were used to forecast times of increased volcanic hazard to protect scientific field workers and the general public

seismicity accompanying the 1999 eruptive episode at telica volcano nicaragua abstract telica volcano nicaragua is a persistently restless basaltic andesite stratovolcano located in the central american volcanic front a high rate of low frequency seismic events lfs has been recorded at telica since the installation of a single vertical component 1 hz seismic sensor teln near its summit in 1993 due to the high rate of lfs at telica traditional methods of forecasting volcanic activity based on increases in the overall rate of seismicity are not applicable therefore an understanding of the nature of precursory changes in telica s seismicity is necessary to forecast future volcanic activity in may 1999 a nine month eruptive episode started at telica consisting of phreatic to phreatomagmatic explosions here we analyse over 29 000 seismic events recorded during a fifteen month period of seismicity bracketing this eruptive episode in an attempt to retrospectively identify precursory changes in seismicity seismic event rates between january 1999 and march 2000 show a reduction in the lf event rate three months before the onset of eruptive activity closely followed by a short lived swarm of high frequency hf 5 hz events after a three month data gap a second reduction in the lf event rate started in august 1999 directly following eruptive activity in august and approximately two months before a series of explosions in october 1999 this reduction in the lf event rate was closely followed by a short lived swarm of hf events that was coincident with the onset of numerous 22 short lived but populous lf multiplets a further reduction in event rate for both lfs and hfs is evident in the months between the october 1999 explosions and explosions on the 29th of december 1999 we suggest that these changes in seismicity reflect a transition from open system degassing to closed system degassing at telica and could signify a change in the volcanic system preceding future episodes of phreatic to phreatomagmatic activity at telica and similar persistently restless volcanic systems worldwide we note that these signals are for phreatic to phreatomagmatic activity and thus may not pertain to magmatic volcanism or to other persistently restless volcanoes prior to their magmatic activity

overview chronology and impacts of the 2016 2017 eruption of bogoslof volcano alaska the 2016 2017 eruption of bogoslof volcano a back arc shallow submarine volcano in alaska s aleutian arc began in december 2016 and included 70 explosive events and at least two episodes of subaerial dome building because the volcano had no local monitoring stations during the eruption a combination of distant seismic stations regional infrasound sensors lightning detection a variety of satellite data and observer reports and a field visit in 2018 were used to recreate the events that occurred during the nine month eruption following precursory seismicity that started in september 2016 the eruption began in december 2016 with a series of explosive events that persisted through mid march 2017 after a 6 week hiatus activity resumed on may 17 and lasted through the end of august 2017 and consisted of additional explosions and two short lived subaerial lava domes that formed in june and august for most of the eruption bogoslof s vent was submerged in shallow seawater though during several of the longer events a subaerial edifice grew and the vent migrated above sea level resulting in more ash rich volcanic clouds eruptive products geophysical signals and eruptive style are all broadly consistent with vulcanian activity where slow magma ascent led to repetitive dome or plug formation overpressurization in the upper conduit and sudden release during short lived explosions infiltration of seawater may have prohibited large domes from forming especially in the first half of the eruption when explosions were closely spaced in time the largest four explosions in the sequence occurred after inter event times of 10 days or more three events produced ashfall on nearby communities and mariners east and south of bogoslof and the eruption resulted in dozens of flight cancelations and flight diversions around the volcano and its ash clouds

statistical forecasting of repetitious dome failures during the waning eruption of redoubt volcano alaska february april 1990 abstract the waning phase of the 1989 1990 eruption of redoubt volcano in the cook inlet region of south central alaska comprised a quasi regular pattern of repetitious dome growth and destruction that lasted from february 15 to late april 1990 the dome failures produced ash plumes hazardous to airline traffic in response to this hazard the alaska volcano observatory sought to forecast these ash producing events using two approaches one approach built on early successes in issuing warnings before major eruptions on december 14 1989 and january 2 1990 these warnings were based largely on changes in seismic activity related to the occurrence of precursory swarms of long period seismic events the search for precursory swarms of long period seismicity was continued through the waning phase of the eruption and led to warnings before tephra eruptions on march 23 and april 6 the observed regularity of dome failures after february 15 suggested that a statistical forecasting method based on a constant rate failure model might also be successful the first statistical forecast was issued on march 16 after seven events had occurred at an average interval of 4 5 days at this time the interval between dome failures abruptly lengthened accordingly the forecast was unsuccessful and further forecasting was suspended until the regularity of subsequent failures could be confirmed statistical forecasting resumed on april 12 after four dome failure episodes separated by an average of 7 8 days one dome failure april 15 was successfully forecast using a 70 confidence window and a second event april 21 was narrowly missed before the end of the activity the cessation of dome failures after april 21 resulted in a concluding false alarm although forecasting success during the eruption was limited retrospective analysis shows that early and consistent application of the statistical method using a constant rate failure model and a 90 confidence window could have yielded five successful forecasts and two false alarms no events would have been missed on closer examination the intervals between successive dome failures are not uniform but tend to increase with time this increase attests to the continuous slowly decreasing supply of magma to the surface vent during the waning phase of the eruption the domes formed in a precarious position in a breach in the summit crater rim where they were susceptible to gravitational collapse the instability of the february 15 april 21 domes relative to the earlier domes is attributed to reaming the lip of the vent by a laterally directed explosion during the major dome destroying eruption of february 15 a process which would leave a less secure foundation for subsequent domes

tectono magmatic evolution of the west philippine mariana region and the origin of boninites re examination of published data on the tectomagmatic evolution of the west philippine mariana region indicate that arc magmatism and back arc extensional pulses are not synchronous but are largely asynchronous arc volcanism ceases within a few million years of the development of a back arc basin and recommences oceanward on a new arc during the final stages in the development of the back arc basin following karig s model we believe that ascent beneath the arc axial chain of a line of diapirs of morb source mantle interferes with processes of arc magma generation and arc magmatism wanes partial melting of the diapirs produces voluminous morb type tholeiites which split the arc and form the crust of a widening back arc basin we show for both the west philippine mariana region and occurrences of boninite like lavas in ophiolites that boninites appear to be erupted after arc magmatism and immediately before eruption of morb type lavas passage of a diapir of hot morb source mantle through refractory sub arc peridotite which has been enriched in lile and h2o will result in limited partial melting of the hydrous peridotite and the restricted production of boninitic magmas these are more likely to erupt modified or only slightly modified by crystal fractionation through the thin forearc crust continued ascent of the morb source mantle diapir will result in it partially melting to yield morb tholeiites which erupt shortly after and in considerably more voluminous amounts than the boninites finally we suggest that forearcs regions are likely to be incorporated into foldbelts and that boninite or low ti lava bearing ophiolites may characterize such regions

petrology of the tekirova antalya ophiolite southern turkey evidence for diverse magma generations and their tectonic implications during neotethyan subduction the sw antalya complex is an assemblage of mesozoic carbonate platform margin and ophiolitic rocks which record the formation and tectonic emplacement of a small mesozoic ocean basin the late cretaceous ophiolitic rocks are located at two localities namely the relatively intact tekirova ophiolite to the east of kemer zone and the dismembered godene ophiolite to the west of kemer zone the tekirova antalya ophiolite comprises harzburgitic tectonites ultramafic to mafic cumulates isotropic gabbros and sheeted dikes numerous isolated dikes ranging in thickness from 5 cm to 10 m intruded the crustal rocks at different structural levels the isotropic gabbros are represented by gabbro diorite and quartz diorite rocks with granular to ophitic subophitic textures the isolated dikes are characterized by dolerite diabase and microdiorite with ophitic intersertal and microgranular textures these rocks exhibit tholeiitic to alkaline compositions new geochemical data presented in this paper from the isolated dikes and isotropic gabbros suggest that there are three main types of parental basic magmas that form the oceanic crustal rocks of the tekirova antalya ophiolite these are 1 iat series which can be referred to the group i isolated dikes and isotropic gabbros 2 low ti boninitic series characterized by the group ii isolated dike and isotropic gabbros and 3 oib type including the group iii isotropic gabbros the geochemical evidence suggests that the crustal rocks of the tekirova antalya ophiolite were generated from a progressive source depletion from island arc tholeiites iat to boninites therefore a fore arc tectonic setting seems likely for the generation of the crustal rocks from the tekirova antalya ophiolite in the southern branch of neotethys during the late cretaceous the oib type alkaline isotropic gabbros are thought to have resulted from either 1 a late stage magmatic activity fed by melts that originated within an asthenospheric window due to slab break off or 2 subduction of a ridge system which generated oib source across the asthenospheric window that has been no influence of fluids from the subducted slab into the overlying mantle wedge shortly before the emplacement of the tekirova antalya ophiolite onto the tauride platform

geochemistry of basic dikes in the lanzo massif western alps petrogenetic and geodynamic implications abstract the alpine peridotite massif of lanzo italy contains three generations of basic dikes gabbros and basalts the older gabbros are plagioclase rich mantle segregates while the younger gabbro dikes are cumulates very similar in chemical composition to recent oceanic gabbros and gabbros from ophiolitic complexes they both were derived from the n type mid ocean ridge basalt morb magmas which were progressively more depleted in incompatible elements and were probably generated during a dynamic melting of a rising mantle diapir the basaltic dikes are the n type morb and closely resemble the alpine apennine ophiolitic basalts they were derived from a different upper mantle source than the parental magmas of the gabbros the source of the basalts was less depleted in light ree the presence of basic magmas with n type morb affinities in the lanzo massif is consistent with the close genetic relationship between the alpine peridotite body and the ophiolites of the liguro piemontese basin

geochemistry and tectonic environment of basaltic rocks from the misis ophiolitic melange south turkey abstract the miocene misis tectonosedimentary melange was formed during the closing stages of the neotethys in what is now the adana basin of southern turkey a major part is characterized by large disorientated limestone blocks set in a predominantly volcanogenic matrix of bedded and chaotic units comprising basaltic pillow lavas associated hyaloclastites serpentinites and various acidic tuffs all the basaltic rocks are mildly metamorphosed zeolite facies variably vesicular and differentially altered to smectite chlorite fe oxides zeolites and carbonate petrographic and chemical data on the least altered pillow lavas indicate they were originally sparsely olivine plagioclase clinopyroxene phyric tholeiites and minor amphibole phyric basaltic andesites the basaltic clasts in the melange matrix are genetically related to the pillow lava sequences with the observed variation throughout the suite being governed by low pressure fractional crystallization as a group the basalts are characterized by low zr y 2 4 relatively high th yb 0 5 0 7 and la nb 1 5 2 5 ratios and moderate chondrite normalized light ree enrichment la yb n 3 patterns the basalts have a mildly enriched subduction related chemistry and were possibly formed initially in a back arc basin setting the supra subduction zone characteristics of the misis basaltic units are similar to other tethyan basalts in eastern mediterranean ophiolites

seismic character and progression of explosive activity during the 2016 2017 eruption of bogoslof volcano alaska bogoslof volcano in the central aleutian arc experienced a major eruption between december 2016 and august 2017 that was characterized by explosive activity volcanic explosivity index 2 to 3 and the extrusion of lava domes the alaska volcano observatory tracked the activity in real time using seismicity observed on distant stations as well as infrasound lightning satellite data and occasional visual observations in this study we measure the duration of seismic signals associated with individual explosive events to track their progression during the two explosive phases of the eruption seismic recordings of bogoslof explosions show complex waveforms that suggest both individual explosive events and sequences of several explosions separated by lower amplitude tremor the lack of local seismic monitoring stations at distances of 1 km to 15 km distance unfortunately limits our ability to closely observe seismicity and to interpret changing conditions at the vent such as position the presence of a lava dome or plug and the role of seawater associated with the eruption we use the rate of explosive activity seismic waveform character and repose time between explosions to infer the conditions within the conduit

tethyan vs cordilleran ophiolites a reappraisal of distinctive tectono magmatic features of supra subduction complexes in relation to the subduction mode abstract supra subduction zone ssz ophiolites deserve special attention because they represent fundamental markers of intraoceanic convergence and generation of new lithosphere above subduction zones moreover owing to their structural characteristics and location in the overriding plate these complexes are far better represented and preserved than mid ocean ridge basalt morb ophiolites in orogenic belts in terms of their structure tectonics and magmatic features ssz ophiolites may be classified in two main types 1 tethyan complexes such as those of the albanide hellenide belt which mostly consist of complete and extensive volcanic dyke plutonic and mantle sections with prevalent island arc tholeiitic iat affinity and the significant presence of boninites obducted as relatively intact lithospheric slabs onto collisional continental margins 2 cordilleran complexes including the circum caribbean ophiolites of guatemala cuba and venezuela mostly represented by dismembered sections of arc volcanic plutonic and subordinate mantle sequences with tholeiitic to calc alkaline iac magmatic affinity and acidic differentiates commonly associated with metamorphic subduction complexes and tectonically emplaced onto or juxtaposed against the continental margin within polygenetic terranes these two types appear to be related to significantly different subduction modes and intraoceanic plate dynamics whereby ssz ophiolites were generated the tethyan complexes can best be compared with west pacific type subduction with accentuated steepening and retreat of the subducted slab accompanied by progressive decoupling of the converging plates intense mantle diapirism and tensional events in the upper plate with generation of large iat sheeted dike complexes in open oceanic spreading systems continuous slab sinking and roll back allow increasing asthenospheric diapirism and interarc spreading from the arc axis to the forearc region with generation of boninites and back arc basin basalts babb with intermediate morb iat composition by contrast the genesis of the cordilleran complexes requires a subduction mode characterized by a steady state regime with moderate and constant dip of the subducted slab and limited extension in the backarc region the magmatic evolution of these ophiolites from iat to iac and the significant presence of rhyodacite and tonalite differentiates coherently indicate a more mature stage of arc magmatism as well as the occurrence of efficient differentiation processes developing under nearly closed system conditions in independent magma chambers tethyan complexes are likely to be obducted as large and relatively intact slabs onto the continental margins through the interposition of metamorphic soles which represent relics of the morb lithosphere underplating the ssz ophiolites on the other hand the common emplacement of cordilleran complexes within polygenetic terranes appears to be controlled by prolonged accretionary mechanisms that trap against the continental margin parts of the arc structure subduction complexes melanges and volcanoclastic products

deep volcanic tremor and magma ascent mechanism under kilauea hawaii deep harmonic tremor originating at depths around 40 km under kilauea was studied using records accumulated since 1962 at the hawaii volcano observatory of the u s geological survey the deep source of the tremor was determined by onset times and confirmed by the relative amplitude across the island wide network of seismometers the period of tremor was conclusively shown to be determined by the source effect and not by the path or station site effect because the period would change considerably in time but maintained uniformity across the seismic net during the tremor episode the tremor appeared to be primarily composed of p waves we interpret the observed period and amplitude in terms of the stationary crack model of aki et al 1977 and find that the seismic moment rates for deep tremors are considerably larger than those for shallow tremors suggesting more vigorous transport for the former we propose a kinematic source model which may be more appropriate for deep tremor according to this model a measurable quantity called reduced displacement is directly proportional to the rate of magma flow a systematic search for deep tremor episodes was made for the period from 1962 through 1979 and the amplitude period and duration of the tremor were tabulated we then constructed a cumulative reduced displacement plot over the 18 year period the result shows a generally steady process which does not seem to be significantly affected by major eruptions and large earthquakes near the surface the total magma flow estimated from the reduced displacement is however one order of magnitude smaller than that estimated by swanson 1972 it may be that most channels transport magma aseismically and only those with strong barriers generate tremor

igneous petrogenesis of the yakuno ophiolite japan in the context of the diversity of ophiolites an ophiolite complex includes three major members basaltic volcanics mafic ultramafic cumulates and residual peridotite from the aspect of igneous petrology three distinct types are recognized among the associations of the three members liguria type alkalic basalt plagioclase type cumulates and lherzolite yakuno type high alumina tholeiite clinopyroxene type cumulates and clinopyroxene bearing harzburgite papua type low alumina tholeiite orthopyroxene type cumulates and clinopyroxene free harzburgite in the light of recent experimental studies the three types represent cogenetic complementary products of low 30 degrees of partial melting in the lherzolitic source mantle respectively the cumulates of the yakuno ophiolite show structural and chemical continuity to the underlying residual peridotite and were recrystallized at high pressures 5 10 kb they originated in a deep soft floored magma chamber directly overlying the partially melted residual harzburgite from which the magma was extracted the three members of the yakuno ophiolite were cogenetically formed through a magmatic event induced by a moderate degree of partial melting in the mantle

petrology of the 2016 2017 eruption of bogoslof island alaska the 2016 2017 eruption of bogoslof primarily produced crystal rich amphibole basalts the dominant juvenile tephra were highly microlitic with diktytaxitic vesicles and amphiboles had large reaction rims both observations support a magma history of slow ascent and or shallow stalling prior to eruption plagioclase amphibole clinopyroxene mineralogy are also suggestive of shallow magma crystallization lavas were emplaced as shallow submarine lava domes and cryptodomes that produced 70 relatively short lived and water rich explosions over the course of the 9 month long eruption the explosions ejected older trachyandesite lavas that were likely uplifted by cryptodome emplacement that began in december 2016 and continued for many months trachyte pumice similar in composition to a 1796 lava dome was entrained in basalts by the end of the eruption the pumice appears to be a largely crystalline magma that was rejuvenated entrained in the basalt and heated to 1000 c the composition of trachytes require differentiation through stronger amphibole control than the apparent shallow crustal evolution implied for the basalt this suggests that they are magmas derived from a mid crustal zone of amphibole crystallization nearby arc front volcanoes that notably lack amphibole have strikingly similar compositional trends trace element signatures of the bogoslof basalts however suggest derivation from a mantle source with residual garnet and lower degree melting than basalts from nearby arc front volcanoes the diversity of magmas erupted at bogoslof thus provides an opportunity not only to probe rare backarc compositions from the aleutian arc but also to examine the apparent role of amphibole in generating evolved compositions more broadly in arc environments

reconstructing plate boundaries in the jurassic neo tethys from the east and west vardar ophiolites greece and serbia jurassic subduction initiation in the neo tethys ocean eventually led to the collision of the adria africa and eurasia continents and the formation of an 6 000 km long alpine orogen spanning from iberia to iran reconstructing the location and geometry of the plate boundaries of the now disappeared neo tethys during the initial moments of its closure is instrumental to perform more realistic plate reconstructions of this region of ancient ocean basins in general and on the process of subduction initiation neo tethyan relics are preserved in an ophiolite belt distributed above the dinaric hellenic fold thrust belt here we provide the first quantitative constraints on the geometry of the spreading ridges and trenches active in the jurassic neo tethys using a paleomagnetically based net tectonic rotation analysis of sheeted dykes and dykes from the west and east vardar ophiolites of serbia maljen and ibar and greece othris pindos vourinos and guevgueli based on our results and existing geological evidence we show that initial middle jurassic 175 ma closure of the western neo tethys was accommodated at a n s trending west dipping subduction zone initiated near and parallel to the spreading ridge the west vardar ophiolites formed in the forearc parallel to this new trench simultaneously the east vardar ophiolites formed above a second n s to nw se trending subduction zone located close to the european passive margin we tentatively propose that this second subduction zone had been active since at least the middle triassic simultaneously accommodating the closure of the paleo tethys and the back arc opening of neo tethys

breaching parameters for earth and rockfill dams dam risk analysis is at the heart of dam failure prevention and mitigation in order to assess dam risk it is essential to conduct a quantitative analysis of the process of a dam breach which can be described by such parameters as breach geometry breaching duration and peak outflow rate the main objective of this paper is to develop robust empirical formulas with physical meaning for predicting dam breaching parameters based on past dam failure data a database of 182 earth and rockfill dam failure cases has been compiled among these cases nearly one half are for large dams higher than 15 m a multiparameter nonlinear regression model is recommended to develop empirical relationships between five breaching parameters breach depth breach top width average breach width peak outflow rate and failure time and five selected dam and reservoir control variables dam height reservoir shape coefficient dam type failure mode and dam erodibility the relative importance of each control variable is evaluated the dam erodibility is found to be the most important factor influencing all five breaching parameters the reservoir shape coefficient and the failure mode also play an important role in the prediction models two case studies are presented to show the application of the empirical models developed in this paper

tectonic significance of the geochemistry and petrology of ophiolites in southeast anatolia turkey abstract the southeast anatolian ophiolites form discontinuous linear belts of oceanic fragments immediately north of the bitlis zagros suture which marks a continental collision zone between the arabian platform to the south and the taurides to the north the late cretaceous ophiolites in the southeast anatolia are represented by the kizildag in the hatay area the goksun and the berit to the north of kahramanmaras ispendere in the malatya area and komurhan guleman in the elazig area the goksun berit ispendere komurhan and guleman ophiolites were attached to the base of the tauride platform i e the malatya keban along its northern active margin and intruded by the i type calc alkaline granitoids of late cretaceous age in contrast the kizildag hatay ophiolite was thrust over the arabian passive margin in the south the cumulate rocks in the ophiolites are represented by dunite wehrlite lherzolite olivine clinopyroxenite olivine gabbronorite olivine gabbro gabbronorite and gabbro highly magnesian olivines fo88 to 74 and pyroxenes mg 95 to 60 as well as highly calcic plagioclases an95 to 68 from the cumulate rocks differ from oceanic equivalents which mainly formed from a mor basaltic melt the order of crystallization in mineral phases whole rock and mineral chemistry data from the cumulates suggest that the primary magma is compositionaly similar to that observed in modern island arc tholeiitic sequences the volcanic and subvolcanic rocks exhibit tholeiitic compositions chondrite normalized ree n morb normalized multi element patterns and tectonic discrimination diagrams suggest the existence of two main types of parental basic magmas in the crustal rocks of the se anatolian ophiolites these are i iat series that characterize the kizildag goksun ispendere komurhan and guleman ophiolites and ii low ti boninitic series known only in the kizildag ophiolite the geochemistry of the crustal rocks suggests that they formed in a suprasubduction zone tectonic setting including arc forearc environments in the southern neotethys these well preserved plutonic oceanic crustal remnants mainly exhibit an intact ophiolite pseudostratigraphy and are overlain by a volcanic sedimentary unit made up of alternations of basic to acidic extrusive rocks debris flows volcaniclastic sandstones and pelagic limestones interpreted as a tholeiitic ensimatic island arc assemblage built on the suprasubduction zone type crust the metamorphism of some of the ophiolites i e berit in the se anatolia might be related to later stages of intraoceanic subduction during the evolution of the southern neotethys

assessing the likelihood and magnitude of volcanic explosions based on seismic quiescence abstract volcanic eruptions are generally forecast based on strong increases in monitoring parameters such as seismicity or gas emissions above a relatively low background level e g voight 1988 sparks 2003 because of this forecasting individual explosions during an ongoing eruption or at persistently restless volcanoes is difficult as seismicity gas emissions and other indicators of unrest are already in a heightened state therefore identification of short term precursors to individual explosions at volcanoes already in heightened states of unrest and an understanding of explosion trigger mechanisms is important for the reduction of volcanic risk worldwide seismic and visual observations at telica volcano nicaragua demonstrate that a episodes of seismic quiescence reliably preceded explosions during an eruption in may 2011 and b the duration of precursory quiescence and the energy released in the ensuing explosion were strongly correlated precursory seismic quiescence is interpreted as the result of sealing of shallow gas pathways leading to pressure accumulation and eventual catastrophic failure of the system culminating in an explosion longer periods of sealing and pressurization lead to greater energy release in the ensuing explosion near real time observations of seismic quiescence at restless or erupting volcanoes can thus be useful for both timely eruption warnings and for forecasting the energy of impending explosions

comparison of short term seismic precursors and explosion parameters during the 2016 2017 bogoslof eruption in december 2016 the unmonitored bogoslof volcano unexpectedly began erupting producing at least 70 explosions over the next 8 5 months roughly half of the explosions were preceded by observable short term precursory activity defined here as seismic activity ending within 30 min of the explosion onset or an earthquake swarm ending within a few hours before the explosion onset five different types of precursors were noted during the bogoslof eruption non swarm earthquakes earthquake swarms rapid rate events tremor and chromatic tremor most of the precursors that were detectable by the nearby seismic networks 45 km away occurred during the first 3 months of the eruption with nearly all precursory earthquake swarms occurring during this period a hydrophone deployed 7 km northeast of the summit for the last 3 months of the eruption detected weaker activity that was not recorded by the more distant seismic networks we describe the observed precursors and discuss their inter relations overall the precursors seem indicative of processes occurring throughout the volcanic system and suggest possible changes in the system during the eruption we then compare the short term precursors with explosion parameters such as plume height duration and occurrence of lightning to look for relations that may provide more insight into the eruption dynamics and subsurface processes and help to further improve forecasting we find no obvious relations between any of the precursor or explosion parameters suggesting that their use in forecasting eruption characteristics at bogoslof is limited

the hellenic ophiolites eastward or westward obduction of the maliac ocean a discussion ophiolitic bodies in the dinaro hellenic mountain belt are among the most important ones in the peri mediterranean alpine chains the characteristic feature of this ophiolitic belt is its middle to late jurassic age of obduction the ophiolitic bodies form two major belts on each side of the pelagonian zone an east pelagonian belt in the vardarian domain and a supra pelagonian ophiolitic belt spo to the west the different hypotheses relative to the origin of the spo present geodynamic evolution models accounting for most of the available data a mid triassic episode of rifting a ladinian jurassic episode of sea floor spreading forming notably the maliac ocean a middle to late jurassic convergent period with subduction and obduction episodes and finally a late episode of tertiary compressional deformation responsible for the westward thrusting of the jurassic ophiolitic nappes over the external zones despite many studies dating from the early 1970s the eastern or western pelagonian origin of these ophiolites especially the spo is still under dispute whatever the adopted hypothesis we consider that the main spo bodies n pindos vourinos othris evia argolis have the same origin because of their geographic continuity and of the similarities in their geological characteristics we propose that this ocean corresponds everywhere to the maliac ocean defined in othris from the well preserved sedimentary oceanic margin and ophiolitic nappes thrust during the late jurassic obduction onto the pelagonian platform there is strong evidence for the existence of two deep basins on both sides of the pelagonian continental ridge during triassic jurassic times they correspond respectively to the vardar area to the east and the pindos domain to the west one of these domains being at the origin of the spo the hypothesis of an eastward emplacement of the spo from the pindos domain is based mainly on sedimentological data from the margin series and on structural analyses of ophiolitic bodies however we conclude the westward obduction of the maliac ocean originating from the vardar area to be the best fitting model this westward model is supported by paleogeographic and structural constraints on regional scale notably the absence of obducted ophiolites in the jurassic series of the koziakas units units attributed to the western pelagonian margin and of the parnassus domain on the eastern side of the pindos basin is difficult to reconcile with an eastward obduction from the pindos domain other observations such as the distribution of ophiolitic detritus in the internal and external zones also promote the westward late jurassic obduction of the maliac ocean our preferred model offers a consistent explanation for the mechanism and timing of the emplacement of the spo as well as providing insight on the origin and emplacement of the vardarian ophiolites following this hypothesis there is no need for a clear boundary between the spo and the west vardarian ophiolitic bodies as they were obducted from the same oceanic basin and during the same jurassic tectonic event in this paper we develop evidence in favor of the eastern pelagonian origin for the spo our adopted model and provide discussion on the data supporting the main alternative hypothesis western origin for the spo

geodynamic evolution of ophiolites from albania and greece dinaric hellenic belt one two or more oceanic basins all the geological constraints for an exhaustive reconstruction of the triassic to tertiary tectonic history of the southern dinaric hellenic belt can be found in albania and greece this article aims to schematically reconstruct this long tectonic evolution primarily based on a detailed analysis of the tectonic setting the stratigraphy the geochemistry and the age of the ophiolites in contrast to what was previously reported in the literature we propose a new subdivision on a regional scale of the ophiolite complexes cropping out in albania and greece this new subdivision includes six types of ophiolite occurrences each corresponding to different tectonic units derived from a single obducted sheet these units are represented by 1 sub ophiolite melange 2 triassic ocean floor ophiolites 3 metamorphic soles 4 jurassic fore arc ophiolites 5 jurassic intra oceanic arc ophiolites and 6 jurassic back arc basin ophiolites the overall features of these ophiolites are coherent with the existence of a single though composite oceanic basin located east of the adria pelagonian continental margin this oceanic basin was originated during the middle triassic and was subsequently early jurassic affected by an east dipping intra oceanic subduction this subduction was responsible for the birth of intra oceanic arc and back arc oceanic basins separated by a continental volcanic arc during the early to middle jurassic from the uppermost middle jurassic to the early cretaceous an obduction developed during which the ophiolites were thrust westwards firstly onto the neighboring oceanic lithosphere and then onto the adria margin

timing of tectonic emplacement of the ophiolites and terrane paleogeography in the hellenides the timing of tectonic emplacement of the ophiolites is analyzed in the four oceanic terranes of the hellenides h2 h4 h6 h8 the criteria for this analysis are based on a the post emplacement sedimentary cover or intrusive rocks b the syn emplacement tectonostratigraphic formations and c the youngest rocks involved in the structure of the autochthon and the allochthon unit in each case the timing becomes younger towards the more external tectonic units of the hellenides with i late eocene oligocene age in the external ophiolite belt of the pindos cyclades oceanic terrane h2 ii late jurassic early cretaceous age in the internal ophiolite belt of the vardar axios oceanic terrane h4 iii post liassic pre late jurassic age in the ophiolites of lesvos circum rhodope oceanic terrane h6 and iv pre late jurassic age in the ophiolites of volvi eastern rhodope terrane h8 an ophiolite obduction model can be applied with the ophiolitic nappes always emplaced on top of pre alpine continental terranes with mesozoic shallow water carbonate platforms the geometry of the continental terranes drifting during the mesozoic within the tethys ocean controls the number and dimensions of the tethyan oceanic basins where a continental terrane dies out the two adjacent oceanic basins merge into one larger basin this seems to be the case of the pelagonian terrane h3 which is terminated north of skopje where the pindos oceanic basin h2 merges with the vardar axios oceanic basin h4

boninitic and low ti subduction related lavas from intraoceanic arc backarc systems and low ti ophiolites a reappraisal of their petrogenesis and original tectonic setting a comprehensive review of boninitic and other low ti subduction related lavas from modern intraoceanic arc backarc systems and of low ti ophiolites shows a complete compositional range from boninite to island arc basalts transitional low ti types distinguishable from the main magma types are well represented and defined here as 1 transitional boninites mainly from low ti ophiolites and 2 low ti island arc basalts major compatible and incompatible elements are clearly decoupled in all these low ti subduction related magmas in general their petrogenesis reflects a high degree of hydrous but not necessarily water saturated melting at pressures lower than 15 20 kbar of severely depleted mantle sources enriched in lile and h2o which are residual after the earlier extraction of basalt s as the residues after the extraction of low ti subduction related magmas very likely have a dunitic clinopyroxenefree harzburgitic composition it is proposed that the cao al2o3 ratios of the primary melts reflect those of their mantle sources particularly for the boninites and transitional boninites in this model the source of boninites sensu stricto would show a variable degree of depletion from clinopyroxene bearing to clinopyroxene free harzburgite whereas those of transitional boninites low ti island arc basalts would have clinopyroxene bearing harzburgite clinopyroxene poor lherzolite compositions the metasomatizing components involved in the genesis of low ti island arc basalts have a very close affinity with those involved with the sources of normal island arc tholeiites the common eruptive setting indicates that these low ti lavas represent extreme magma types of an otherwise normal island arc tectonomagmatic regime the petrogenesis of tertiary boninites and transitional boninites and the comparable magma types of the low ti ophiolites require at least these components 1 a variably depleted mantle source probably a subarc mantle residual after island arc basalt extraction 2 a hydrous fluid probably derived from dehydration of the subducted altered oceanic crust which carries most of sr ba k and rb in the subarc source and 3 a lree zr and nb enriched component with a low 143nd 144nd ratio probably derived from an ocean island type source it is proposed that this third component also carried significant amounts of rb and k which totally obliterated the ba spike typical of island arc tholeiites iats and produced the distinctive rb spike typical of the boninite transitional boninite magmas this component is hypothetically identified in liquids produced by partial melting of small scale large magnitude k richerite phlogopite bearing heterogeneities within the hot rising asthenospheric diapirs required in the model of crawford et al 1981 for the genesis boninite transitional boninite magmas geochemical thermal and geological constraints are best explained in a model which restricts the genesis of boninite transitional boninite magmas to the early stages of intraoceanic interarc spreading and to important rifting phases of the arc and or early interarc basin structures connected with hot rising asthenospheric mantle diapirs

field and petrological evidence for the development of an ensialic marginal basin related to the hercynian orogeny in the massif central france six lithologic units in tectonic contact with each other have been defined during mapping of the devonian in the beaujolais area of the northeastern massif central five main igneous suites have been recognized 1 a transitional basaltic suite restricted to a single unit 2 an acid volcanic plutonic suite the members of which are related by fractional crystallization and magma mixing 3 low tio2 volcanic rocks with calc alkaline affinities 4 a tio2 rich tholeiitic suite related to an ophiolitic complex 5 a plutonic suite with close resemblances to alaskantype intrusions

a model of vulcanian eruptions at galeras volcano colombia abstract a series of six vulcanian eruptions at galeras volcano colombia occurred during 1992 1993 these eruptions followed the emplacement of a lava dome in late 1991 which was accompanied by cooling crystallizing and partial solidification of the column of magma plugging the conduit five of the eruptions were preceded by episodes of monochromatic long period seismicity suggesting cyclic pressurization of the magma the quiescent intervals between eruptions generally decreased with each successive eruption we propose a model of gas release from magma into confined stationary pore spaces by 5 10 crystallization of anhydrous minerals causing repeated overpressurization of the magmatic system using initial lithostatic pressures of 20 100 mpa 800 3900 m depth gas buildup by crystallization indicates overpressures of 3 17 mpa which is the approximate fracture criterion for a magma chamber our galeras data suggest two models of pressurization 1 gas exsolution may have occurred at similar depths but with progressively smaller overpressures being developed before each successive eruption according to this model the comparatively long quiescent periods between the first eruptions resulted in relatively large amounts of gas exsolution from the magma and accumulation in pore spaces whereas the shorter quiescent periods for the later eruptions exsolved less gas the decrease in overpressure with time may have resulted from progressive fracturing of partly solidified magma during the eruptions which progressively decreased the tensile strength and increased the permeability of the magma 2 since overpressures are smaller at shallower depths for a given amount of crystallization the first eruptions of 1992 1993 were caused by gas exsolution at shallow depths while the last eruptions having shorter quiescent intervals resulted from a greater amount of gas exsolution at greater depths in this case the rate of gas exsolution varied as a function of depth the two scenarios for pressurization may be combined into a more general model involving edifice weakening and the development of a shallow hydrothermal system from continued degassing of the magma plug as weakening of the edifice formed an extensive system of fractures the shallow hydrothermal system deepened due to meteoric water input and magmatic degassing with time a greater number of fractures were filled and partly sealed by hydrothermal minerals at progressively deeper levels as a result the source of the overpressure also deepened with time

mid late ordovician magmatism and metamorphism along the gander margin in central newfoundland fieldwork coupled with u pb dating has clarified the relationships between the exploits and gander derived rocks in central newfoundland during the mid ordovician opening of the exploits back arc basin reinforcing correlation across the peri gondwanan segments of the appalachian caledonian orogen we have separated a new back arc related unit the red cross group from the arc related victoria lake supergroup this group includes 466 3 ma felsic volcanic rock interlayered with alkali basalt enriched mid ocean ridge basalt 457 6 ma gabbro llanvirn caradoc normal mid ocean ridge basalt island arc tholeite limestone black shale and melange locally the red cross group is in parautochthonous contact with low grade gander psammite this contact was overrun by the meelpaeg metamorphic nappe during the devonian the nappe contains gander metapsammite meelpaeg subzone and the coticule bearing howley waters complex exploits which hosts 467 3 ma felsic porphyry the gander and exploits metasediments in the nappe are stitched by arenig llanvirn granite 467 6 ma and 468 3 5 3 ma and caradoc intrusions 458 3 ma and 454 2 ma metamorphic monazite 462 1 ma and titanite 460 3 ma from the nearby mount cormack subzone gander indicate the coeval formation of a low pressure high temperature metamorphic dome the widespread presence of caradoc magmatism suggests that the width of the back arc basin remained relatively narrow

tectono stratigraphy and evolution of the mesozoic pindos ophiolite and related units northwestern greece the northwest pindos mountains expose mesozoic and tertiary thrust sheets which include the jurassic pindos ophiolite composed of ultramafic and mafic oceanic crustal rocks the tectono stratigraphy of these units is i mainly ultramafic ophiolitic thrust sheet and basal metamorphic sole dramala complex and loumnitsa unit ii late cretaceous platformal limestones orliakas group iii dismembered intrusive and extrusive ophiolitic rocks aspropotamos complex iv tectonic melange and olistostromes dominated by triassic jurassic volcanic and sedimtntary rocks avdella melange v coherent thrust sheets of late jurassic late cretaceous deep water sediments dio dendra group vi early tertiary flysch pindos flysch immobile trace element studies indicate that triassic and jurassic extrusives of the volcanic sedimentary melange formed mainly at within plate and or mid ocean ridge settings by contrast structurally overlying ophiolitic extrusives include boninitic volcanics and island arc tholeiites of supra subduction zone origin initial ophiolite displacement c 165 ma is recorded in metamorphic sole formation including basal peridotite mylonite amphibolites and greenschists the sole rocks have morb and wpb chemical affinities in our favoured model the pindos ophiolite formed above a mid jurassic westerly dipping intra oceanic subduction zone accompanied by accretion to form melange followed by collision and emplacement with the pelagonian margin final suturing of the pindos ocean however did not take place until early tertiary times

the mafic layered complex of the kabye massif north togo and north benin evidence of a pan african granulitic continental arc root abstract between the predominantly neoarchean paleoproterozoic west african craton wac and the saharian metacraton smc the dahomeyides suture zone represents a valuable witness of continental amalgamation during pan african times in the kabye massif northern togo and northern benin mainly granulitic metagabbros associated with al rich kyanite and garnet bearing felsic dykes are exposed as tectonic lenses within the pre pan african gneisses of the smc new geochemical data suggest that the high grade rocks granulites in the kabye massif originated in a mature continental arc setting afc calculations constrain the amount of contamination of mantle wedge derived magmas by crustal metasediments to about 10 reconstruction of initial magmatic stratigraphy has been carried out using cipw norms for cumulitic sequences within the massif indicating a normal igneous polarity from west to east published geochemical signatures along the pan african suture zone of the dahomeyides in ghana togo and benin constrain the origin of mafic granulitic and eclogitic complexes a distinction is made between bodies with mainly morb signatures originated from the subducted wac plate passive margin and those with magmatic arc signatures originated from the over riding plate mantle wedge this suggests that the closure of the oceanic domain between the wac and the smc from 640 to 610 ma was mainly accommodated by oceanic subduction beneath the active continental margin before pan african collision

rotational deformation in the jurassic mesohellenic ophiolites greece and its tectonic significance abstract the jurassic pindos and vourinos ophiolites in the western hellenides of greece are part of the mesohellenic mafic ultramafic slab underlying the eocene miocene sedimentary basin mesohellenic trough the tectonic incorporation of this oceanic slab into the western edge of the pelagonian subcontinent occurred via trench passive margin collision in the late jurassic much of the tectonic architecture of these ophiolites particularly vourinos was acquired during progressive inhomogeneous deformation associated with the initial displacement of the jurassic oceanic crust from its original igneous environment of formation and its subsequent tectonic emplacement eastward onto the pelagonian margin the heterogeneous deformation in the mantle and crustal sequences of the pindos vourinos ophiolites occurred in ductile ductile brittle and brittle fields synchronously as the jurassic oceanic lithosphere was translated eastward it also resulted in differential horizontal rotations within the displaced oceanic slab areas retaining high temperature diapiric mantle fabric appear to have been passively translated by lower temperature ductile shearing commonly along mylonite zones individual dunite bodies in the harzburgite tectonites indicate movement distances of at least kilometer scale pervasively mylonitic domains within the upper mantle peridotites suggest elongation on the order of five to ten times in the direction of ophiolite emplacement seafloor spreading related high temperature mantle structures within the pindos and vourinos ophiolites are sub parallel imprinted ductile kinematic indicators and ductile shear zones pervasive to the mantle and lower crustal sections in both ophiolites are also sub parallel consistent with the direction of tectonic vergence and appear traceable across the sedimentary overburden of the mesohellenic trough these geometric relations imply that the pindos vourinos ophiolites retain their relative orientations from the time of cessation of ductile deformation within the ophiolitic slab more than 90 anticlockwise horizontal rotation of the vourinos ophiolite seems to have taken place during progressive simple shear deformation associated with the eastward emplacement of the jurassic oceanic lithosphere this kind of rotational deformation postdating the igneous accretion of ancient oceanic crust may be common in many ophiolites and should be carefully distinguished from tectonic extension related vertical rotational deformation developed during their seafloor spreading evolution

precursory swarms of long period events at redoubt volcano 1989 1990 alaska their origin and use as a forecasting tool abstract during the eruption of redoubt volcano from december 1989 through april 1990 the alaska volcano observatory issued advance warnings of several tephra eruptions based on changes in seismic activity related to the occurrence of precursory swarms of long period lp seismic events dominant period of about 0 5 s the initial eruption on december 14 occurred after 23 years of quiescence and was heralded by a 23 hour swarm of lp events that ended abruptly with the eruption after a series of vent clearing explosions over the next few days dome growth began on december 21 another swarm with lp events similar to those of the first began on the 26th and ended in a major tephra eruption on january 2 eruptions continued over the next two weeks and then ceased until february 15 when a large eruption initiated a long phase of repetitive dome building and dome destroying episodes that continued into april warnings were issued before the major events on december 14 and january 2 but as the eruptive sequence continued after january 2 the energy of the swarms decreased and forecasting became more difficult a significant but less intense swarm preceded the february 15 eruption which was not forecast this eruption destroyed the only seismograph on the volcanic edifice and stymied forecasting until march 4 when the first of three new stations was installed within 3 km of the active vent from march 4 to the end of the sequence on april 21 there were eight eruptions six of which were preceded by detectable swarms of lp events although weak these swarms provided the basis for warnings issued before the eruptions on march 23 and april 6 the initial swarm on december 13 had the following features 1 short duration 23 hours 2 a rapidly accelerating rate of seismic energy release over the first 18 hours of the swarm followed by a decline of activity during the 5 hours preceding the eruption 3 a magnitude range from 0 4 to 1 6 4 nearly identical lp signatures with a dominant period near 0 5 s 5 dilatational first motions everywhere and 6 a stationary source location at a depth of 1 4 km beneath the crater this occurrence of long period events suggests a model involving the interaction of magma with groundwater in which magmatic gases steam and water drive a fixed conduit at a stationary point throughout the swarm the initiation of that sequence of events is analogous to the failure of a pressure relief valve connecting a lower supercharged magma dominated reservoir to a shallow hydrothermal system a three dimensional model of a vibrating fluid filled crack recently developed by chouet is found to be compatible with the seismic data and yields the following parameters for the lp source crack length 280 380 m crack width 140 190 m crack thickness 0 05 0 20 m crack stiffness 100 200 sound speed of fluid 0 8 1 3 km s compressional wave speed of rock 5 1 km s density ratio of fluid to rock 0 4 and ratio of bulk modulus of fluid to rigidity of rock 0 03 0 07 the fluid filled crack is excited intermittently by an impulsive pressure drop that varies in magnitude within the range of 0 4 to 40 bar such disturbance appears to be consistent with a triggering mechanism associated with choked flow conditions in the crack

seismological aspects of the 1989 1990 eruptions at redoubt volcano alaska the ssam perspective abstract ssam is a simple and inexpensive tool for continuous monitoring of average seismic amplitudes within selected frequency bands in near real time on a pc based data acquisition system during the 1989 1990 eruption sequence at redoubt volcano the potential of ssam to aid in rapid identification of precursory long period lp event swarms was realized and since this time ssam has been incorporated in routine monitoring efforts of the alaska volcano observatory in particular an eruption that occurred on april 6 was successfully forecast primarily on the basis of recognizing the precursory lp activity on ssam of twenty two significant eruptions that occurred between december 14 and april 21 eleven had precursory swarms longer than one hour in duration that could be detected on ssam for individual swarms the patterns of relative spectral amplitudes are distinct at each station and remain largely stationary through time thus indicating that one source may have been preferentially and repeatedly activated throughout the swarm typically a single spectral band dominates the signal at each seismic station for the vigorous one day swarm that preceded the first eruption on december 14 signals were sharply peaked in the 1 9 2 7 hz band at the closest station located 4 km from the vent but were dominated by 1 3 1 9 hz energy at three more distant stations located 7 5 22 km from the vent the tendency for the signals from different swarms recorded at the same station to be peaked in the same frequency band suggests that all of the sources are characterized by a predominant length scale signals from the precursory lp swarms became weaker as the eruption sequence progressed and swarms that occurred in march and april could only be detected at seismographs on the volcanic edifice onset times of precursory lp swarms prior to eruptions ranged from a few hours to about one week but after the initial vent clearing phase that ended december 19 these intervals tended to become progressively shorter for successive swarms these trends in the relative onset times and intensities of successive precursory lp swarms are consistent with an overall depressurization of the magmatic system through time in general each of the swarms had an emergent onset but the intensities did not always increase steadily until the eruptions instead as the time of an eruption approached the intensity usually increased more rapidly before peaking and then declining prior to the eruption for three of the swarms two distinct peaks in intensity were apparent the time intervals between final peaks in swarm intensity and ensuing eruptions ranged from about 2 hours to almost 2 days but the peaks always occurred closer to the eruptions than to the swarm onsets both the onset of lp swarm activity and a decline in intensity prior to an eruption may represent critical points in the process of pressurization that drives the flow of fluids and gas in a sealed magmatic system a notable exception to this pattern is the eruption of march 9 which lacked a detectable precursory lp swarm but was followed by an unusually long period of strong lp seismicity that may have been stimulated by a depressurization of the magmatic system resulting from dome failure on both december 14 and january 2 the spectra of early syn eruptive signals have peaked signatures much like those of the spectra of precursory lp activity from shortly before the eruptions these similarities may indicate that the source of precursory seismicity continued to be active during at least the early part of each eruption in syn eruptive signals from march and april recorded at stations on the volcanic edifice the dominant spectral energy progressively shifts with time during the eruption to lower frequencies at least part of the energy in these signals may have been generated by the debris flows associated with dome failures

geochemistry of metabasites from the nevado filabride complex betic cordilleras spain relics of a dismembered ophiolitic sequence abstract major and trace element distribution including ree data in the metabasites of the nevado filabride complex betic cordilleras spain suggests that the whole mafic ultramafic association represents a dismembered ophiolitic sequence all the analyzed samples have been affected by sea floor alteration yet only the fine grained rocks have suffered some significant mobilization of the lree life the other western mediterranean ophiolites this sequence is related to the opening of the liguro piemontese basin during mesozoic times however the metabasites from betic cordilleras differ from the ophiolitic basalts by higher values of the more incompatible over less incompatible element ratios the entire variation of these ratios in the liguro piemontese basin is related to a dynamic melting process coupled with a diapiric uprise of the mantle source from the stability field of garnet peridotites possibly up to the spinel plagioclase facies transition in this process the ophiolites of the betic cordilleras would record the deepest stages and the lowest degrees of melting this feature is related to the peculiar location of these ophiolites at the western end of the liguro piemontese basin

seismic evolution of the 1989 1990 eruption sequence of redoubt volcano alaska abstract redoubt volcano in south central alaska erupted between december 1989 and june 1990 in a sequence of events characterized by large tephra eruptions pyroclastic flows lahars and debris flows and episodes of dome growth the eruption was monitored by a network of five to nine seismic stations located 1 to 22 km from the summit crater notable features of the eruption seismicity include 1 small long period events beginning in september 1989 which increased slowly in number during november and early december 2 an intense swarm of long period events which preceded the initial eruptions on december 14 by 23 hours 3 shallow swarms 0 to 3 km of volcano tectonic events following each eruption on december 15 4 a persistent cluster of deep 6 to 10 km volcano tectonic earthquakes initiated by the eruptions on december 15 which continued throughout and beyond the eruption 5 an intense swarm of long period events which preceded the eruptions on january 2 and 6 nine additional intervals of increased long period seismicity each of which preceded a tephra eruption hypocenters of volcano tectonic earthquakes suggest the presence of a magma source region at 6 10 km depth earthquakes at these depths were initiated by the tephra eruptions on december 15 and likely represent the readjustment of stresses in the country rock associated with the removal of magma from these depths the locations and time history of these earthquakes coupled with the eruptive behavior of the volcano suggest this region was the source of most of the erupted material during the 1989 1990 eruption this source region appears to be connected to the surface by a narrow pipe like conduit as inferred from the hypocenters of volcano tectonic earthquakes concentrations of shallow volcano tectonic earthquakes followed each of the tephra eruptions on december 15 these shocks may represent stress readjustment in the wall rock related to the removal of magma and volatiles at these depths this shallow zone was the source area of the majority of long period seismicity through the remainder of the eruption the long period seismicity likely reflects the pressurization of the shallow portions of the magmatic system

heterogeneous mantle complex crustal processes and obduction kinematics in a unified pindos vourinos ophiolitic slab northern greece

melting of the shallow upper mantle a new perspective detailed examination of liquidus phase relationships in binary and ternary joins of the cfmas cr system has permitted a rigorous determination of the dry melting path of an initially fertile spinel peridotite composition resembling bulk silicate earth or morb pyrolite it is demonstrated that it is impossible to model mantle melting accurately using only one set of ratios of phases entering the melt this implies that the melting process is primarily controlled by solid solution rather than eutectic behaviour the proportions of phases entering a melt depend on whether a phase reacts and or disappears from a system and on the choice of the initial and final peridotite compositions four discrete domains in the melting regime of upper mantle peridotites are distinguished each characterized by different phase melting coefficients relating to the melting of 1 lherzolites 2 clinopyroxene bearing harzburgites i e free clinopyroxene 3 clinopyroxene saturated harzburgites i e clinopyroxene in solid solution in orthopyroxene and 4 clinopyroxene free harzburgites i e no clinopyroxene the proposed non linear fashion in which mantle lithologies melt explains the inadequacy of all previous models to reproduce the observed compositions of upper mantle peridotite melting residues it is suggested that 1 olivine and orthopyroxene will melt cotectically 2 clinopyroxene and spinel will lose most of their aluminous component after 8 melting within the first 4 kb 12 km of ascent from the dry solidus and that 3 clinopyroxene will disappear completely from a morb pyrolite mantle after 42 melting although such a number is significantly higher than that dictated by the position of the clinopyroxene out curves from peridotite isobaric equilibrium melting experiments 22 it is emphasized that the latter are a gross oversimplification of the natural melting process and are not equivalent to melting during adiabatic upwelling it is concluded that the commonly postulated disappearance of clinopyroxene from fertile peridotite compositions at 22 melting is greatly in error if melting in an adiabatically rising mantle is considered thus providing an explanation for many unsuccessful attempts by various authors to model the behaviour of transition elements in sub oceanic and supra subduction zone mantle and derivative magmas

mid ocean ridge and supra subduction affinities in the pindos ophiolites greece implications for magma genesis in a forearc setting abstract the pindos ophiolitic massif is considered an important key area within the albanide hellenide ophiolitic belt and is represented by two tectonically distinct ophiolitic units 1 a lower unit including an intrusive and a volcanic section and 2 an upper ophiolitic unit mainly including mantle harzburgites both units share similar metamorphic soles and tectono sedimentary melanges at their bases the intrusive section of the lower unit is composed by an alternation of troctolites with various ultramafic rock types including dunites lherzolites olivine websterites olivine gabbros anorthositic gabbros gabbros and rare gabbronorites the volcanic and subvolcanic sequence of the lower unit can geochemically be subdivided into three groups of rocks 1 basalts and basaltic andesites of the lower pillow section showing a clear high ti affinity 2 basaltic andesites of the upper pillow section with high ti affinity but showing many geochemical differences with respect to the first group 3 very low ti boninitic basaltic and basaltic andesitic lava flows separating the lower and upper pillow sections and dykes widespread throughout the pindos ophiolites these different magmatic groups originated from fractional crystallization from different primary magmas which were generated in turn from partial melting of mantle sources progressively depleted by previous melt extractions group 1 volcanics may have derived from partial melting ca 20 of an undepleted lherzolitic source while group 2 basaltic rocks may have derived from partial melting ca 10 of a mantle that had previously experienced mid ocean ridge basalt morb extraction finally the group 3 boninites may have derived from partial melting ca 12 17 of a mantle peridotite previously depleted by primary melt extraction of groups 1 and 2 primary melts in order to explain the coexistence of these geochemically different magma groups two petrogenetic models formerly proposed for the albanian ophiolites are discussed

is the vourinos complex an island arc ophiolite abstract a geochemical study has been undertaken on the vourinos ophiolites northern greece a complex long known for its unusual characteristics such as an environment of acidic rocks and a calc alkaline chemical affinity the nd sr isotopic ratios and the hf th and ta th ratios are indicative of an island arc origin for vourinos as opposed to the mid oceanic ridge origin inferred for other ophiolites such as inzecca corsica other data on trace elements confirm that the cumulative suite and the lavas originated from the same magma through a simple fractional crystallization process and show that this magma would have formed through partial melting of an already highly depleted material it is thus possible to distinguish ophiolites with morb characteristics from island arc ophiolites such as the vourinos complex the existence of the latter type imposing new constraints on the possible tectonic processes for emplacement

petrogenesis and tectonic significance of jurassic iat magma types in the hellenide ophiolites as deduced from the rhodiani ophiolites pelagonian zone greece abstract the rhodiani ophiolites are represented by two tectonically superimposed ophiolitic units the lower ultramafic unit and the upper volcanic unit both bearing calcareous sedimentary covers the ultramafic unit consists of mantle harzburgites with dunite pods and chromitite ores and represents the typical mantle section of supra subduction zone ssz settings the volcanic unit is represented by a sheeted dyke complex overlain by a pillow and massive lava sequence both including basalts basaltic andesites andesites and dacites chemically the volcanic unit displays low ti affinity typical of island arc tholeiite iat ophiolitic series from ssz settings having as most distinctive chemical features low ti v ratios the rare earth element and incompatible element composition of the more primitive basaltic andesites from the rhodiani ophiolites can be successfully reproduced with about 15 non modal fractional melting of depleted lherzolites which are very common in the hellenide ophiolites the calculated residua correspond to the depleted harzburgites found in the rhodiani and othrys ophiolites both field and chemical evidence suggest that the whole sequence of the rhodiani volcanic unit from basalt to dacite originated by low pressure fractional crystallization under partially open system conditions the modelling of mantle source melt generation and mantle residua carried out in this paper provides new constraints for the tectono magmatic evolution of the mirdita pindos oceanic basin

the maliac ocean the origin of the tethyan hellenic ophiolites the hellenides part of the alpine orogeny in greece are rich in ophiolitic units these ophiolites and associated units emplaced during jurassic obduction testify for the existence of one or several tethyan oceanic realms the paleogeography of these oceanic areas has not been precisely described however all the authors now agree on the presence of a main triassic jurassic ocean on the eastern side of the pelagonian zone vardar domain we consider that this maliac ocean is the most important ocean in greece and albania here we limit the detailed description of the maliac ocean to the pre convergence period of approximately 70 ma between the middle triassic rifting to the middle jurassic convergence period a quick overview on the destiny of the different parts of the maliac ocean during the convergence period is also proposed the studied exposures allow to reconstruct 1 the middle to late triassic maliac oceanic lithosphere corresponding to the early spreading activity at a mid oceanic ridge 2 the western maliac margin widely exposed in the othris and argolis areas 3 the eastern maliac margin in the eastern vardar domain peonias and paikon zones we established the following main characteristics of the maliac ocean 1 the middle triassic rifting marked by a rapid subsidence and volcanism seems to be short lived few my 2 the maliac lithosphere is only represented by middle to late triassic units especially the fourka unit composed of wpb oib and morb pillow lavas locally covered by a pelagic middle triassic to middle jurassic sedimentary cover 3 the western margin is the most complete and our data allow to distinguish a proximal and a deeper distal margin 4 the evolution of the eastern margin peonias and paikon series is similar to that of the w margin except for its jurassic terrigenous sediments while the proximal w margin was dominated by calcarenites 5 we show that the w and e margins are not volcanic passive margins and 6 during the middle jurassic convergence period the eastern margin became an active margin and both margins were affected by obduction processes

mixing characteristics and compositional differences in mantle derived melts beneath spreading axes evidence from cyclically layered rocks in the ophiolite of north oman within the ophiolite of north oman interlayered gabbros and peridotites form a unit up to 3 5 km thick between pervasively foliated harzburgites and dunites of mantle origin and isotropic gabbros and plagiogranites spatial and chemical relationships within this sequence require the existence of a large reservoir 20 km wide 3 5 km high of tholeiitic magma that was periodically replenished by mantle derived picritic to tholeiitic melts during the spreading process cyclicity in the appearance of minerals during cooling of the magma is marked by the repetition of stratigraphies in the layered rocks within these cyclic units basal peridotites with magnesian olivines and pyroxenes are abruptly overlain by gabbros with less magnesian mafic phases this abrupt discontinuity is explained by density stratification set up at the base of the chamber whenever a picritic melt influx occurs by virtue of its greater density than tholeiitic magma sparks et al 1979 any picritic magma emplaced into the base of the tholeiitic magma reservoir would spread out along the floor of the chamber with only limited mixing taking place across the liquid liquid interface cooling of this basal layer by contact with the cooler overlying magma would result in the precipitation of olivine chrome spinel pyroxene from the picritic liquid this process would continue until the now more evolved input liquids were incorporated into the tholeiitic magma resevoir the precipitation of a gabbroic layer would then take place until a new magma input occurred to repeat the cycle variable thicknesses of the cyclic units different liquid lines of descent and various entry points within the liquid line of descent represented by the lowermost layered rocks reveal that the input liquids varied in volume primary composition and the extent and nature of prefractionation in the mantle liquids expelled from the chamber as lavas and dykes are shown to be more fractionated than liquids precipitating the layered rocks and are considered to have undergone additional fractionation within a zone of underplated gabbro and plagiogranite beneath the dyke swarm the existence of a large magma reservoir supports earlier interpretations based on age tilton et al 1981 and geochemical data pearce 1980 that the oman ophiolite is an analogue for fast spreading ridges

seismic and infrasonic eruption tremors and their relation to magma discharge rate a case study for sub plinian events in the 2011 eruption of shinmoe dake japan