amphibole sponge in arc crust pressure temperature time paths followed by arc magmas ascending through the lithosphere dictate the phase assemblage that crystallizes and hence the compositions of liquid fractionates here we use la yb and dy yb versus sio 2 relationships from selected volcanoes to show that amphibole is an important mineral during differentiation of arc magma production of intermediate and silicic arc magmas occurs as magmas stall and cool in the mid lower crust where amphibole is stable because amphibole is rarely a phenocryst phase we term this cryptic amphibole fractionation if this process is as widespread as our investigation suggests then 1 amphibole cumulates may act as an effective filter for water dissolved in mantle derived magmas 2 amphibole cumulates may act as a fertile source of intracrustal melts and fluids and 3 recycling of amphibole cumulates has the potential to return incompatible trace elements and water to the mantle

rheologic and structural controls on the deformation of okmok volcano alaska fems insar and ambient noise tomography 1 interferometric synthetic aperture radar insar data indicate that the caldera of okmok volcano alaska subsided more than a meter during its eruption in 1997 the large deformation suggests a relatively shallow magma reservoir beneath okmok seismic tomography using ambient ocean noise reveals two low velocity zones lvzs the shallow lvz corresponds to a region of weak fluid saturated materials within the caldera and extends from the caldera surface to a depth of 2 km the deep lvz clearly indicates the presence of the magma reservoir beneath okmok that is significantly deeper 4 km depth compared to previous geodetic based estimates 3 km depth the deep lvz associated with the magma reservoir suggests magma remains in a molten state between eruptions we construct finite element models fems to simulate deformation caused by mass extraction from a magma reservoir that is surrounded by a viscoelastic rind of country rock embedded in an elastic domain that is partitioned to account for the weak caldera materials observed with tomography this configuration allows us to reduce the estimated magma reservoir depressurization to within lithostatic constraints while simultaneously maintaining the magnitude of deformation required to predict the insar data more precisely the insar data are best predicted by an fem simulating a rind viscosity of 7 5 1016 pa s and a mass flux of 4 2 109 kg d from the magma reservoir the shallow weak layer within the caldera provides a coeruption stress regime and neutral buoyancy horizon that support lateral magma propagation from the central magma reservoir to extrusion near the rim of the caldera

composition and structure of the central aleutian island arc from arc parallel wide angle seismic data new results from wide angle seismic data collected parallel to the central aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry the 3 d ray tracing 2 5 d inversion of this sparse wide angle data set which incorporates independent phase interpretations and new constraints on shallow velocity structure produces a faster and smoother result than a previously published velocity model middle crustal velocities of 6 5 7 3 km s over depths of 10 20 km indicate an andesitic to basaltic composition high lower crustal velocities of 7 3 7 7 km s over depths of 20 35 km are interpreted as ultramafic mafic cumulates and or garnet granulites the total crustal thickness is 35 37 km this result indicates that the aleutian island arc has higher velocities and thus more mafic compositions than average continental crust implying that significant modifications would be required for this arc to be a suitable building block for continental crust lateral variations in average crustal velocity below 10 km roughly correspond to trends in major element geochemistry of primitive mg 0 6 lavas the highest lower crustal velocities and presumably most mafic material are detected in the center of an arc segment between unmak and unalaska islands implying that arc segmentation exerts control over crustal composition

coeval giant landslides in the canary islands implications for global regional and local triggers of giant flank collapses on oceanic volcanoes abstract giant landslides are an important part of the evolution of most intra plate volcanic islands they often proceed in catastrophic events likely to generate voluminous debris avalanches and eventually trigger destructive tsunamis although knowledge of the timing of their recurrence is a key factor regarding the hazard assessment in coastal environments only a few of them have been well dated in this contribution we focus on the la orotava event on tenerife which we date with the unspiked k ar technique between 534 and 523 ka such narrow temporal interval is compatible within uncertainties with the age of the cumbre nueva collapse on the neighboring island of la palma we thus examine here the possible common triggering mechanisms at the global regional and local scales both events occurred shortly after the climax of the oxygen isotopic stage 14 during the rapid transition towards the interglacial stage 13 reinforcing the hypothesis of a control from global paleoclimatic changes on the destabilization of oceanic islands intense volcanic pulses at the regional scale also lead to the synchronous overgrowth of several volcanic islands in the archipelago but coeval destabilization on tenerife and la palma appears significantly controlled by the intrinsic morphology of the edifices with contrasted instability thresholds for shield volcanoes and volcanic ridges respectively finally we propose that the two events may be genetically linked dynamic transfer of voluminous debris avalanches during a giant landslide episode can induce isostatic readjustments generate significant ground acceleration and finally produce a large tsunami three processes which can concur to trigger large scale flank collapse on a neighboring mature unstable volcanic island

diverse lavas from closely spaced volcanoes drawing from a common parent emmons lake volcanic center eastern aleutian arc abstract emmons lake volcanic center elvc on the lower alaskan peninsula is one of the largest and most diverse volcanic centers in the aleutian arc since the middle pleistocene eruption of 350 km3 of basalt through rhyolite has produced a 30 km arc front chain of nested calderas and overlapping stratovolcanoes elvc has experienced as many as five major caldera forming eruptions the most recent at 27 ka produced 50 km3 of rhyolitic ignimbrite and ash fall these violent silicic events were interspersed with less energetic but prodigious outpourings of basalt through dacite holocene eruptions are mostly basaltic andesite to andesite and historically recorded activity includes over 40 eruptions within the last 200 yr all from pavlof volcano the most active site in the aleutian arc geochemical and geophysical observations suggest that although all elvc eruptions derive from a common clinopyroxene spinel plagioclase fractionating high aluminum basalt parent in the lower crust magma follows one of two closely spaced but distinct paths to the surface under the eastern end of the chain magma moves rapidly and cleanly through a relatively young 28 ka hydraulically connected dike plexus steady supply short magma residence times and limited interaction with crustal rocks preserve the geochemistry of deep crustal processes below the western part of the chain magma moves haltingly through a long lived 500 ka and complex intrusive column in which many generations of basaltic to andesitic melts have mingled and fractionated buoyant silicic melts periodically separate from the lower parts of the column to feed voluminous eruptions of dacite and rhyolite mafic lavas record a complicated passage through cumulate zones and hydrous silicic residues as manifested by disequilibrium phenocryst textures incompatible element enrichments and decoupling of rees and hfses ratios such features are absent in mafic lavas from the younger part of the chain highlighting the importance of plumbing architecture and longevity in creating petrologic diversity supplemental data include 156 major element xrf and 128 trace element icp ms whole rock analyses 23 new 40ar 39ar ages a generalized geologic map with associated unit descriptions and field photographs and photomicrographs of key petrographic features

structure of an island arc wide angle seismic studies in the eastern aleutian islands alaska new seismic wide angle data from the eastern aleutian islands show a mafic composition and a 30 km thick island arc crust traveltimes of p and s refracted arrivals and prominent crustal and mantle reflectors observed to offsets of over 300 km were used to derive velocity models for the eastern aleutian arc between the islands of atka and unimak using a three dimensional finite difference modeling and tomography code we interpret the highest crustal p wave velocities of 7 2 7 4 km s between about 12 and about 22 km depth to the south and north of the main volcanic line as remainders of preexisting oceanic crust into which arc magma is intruded apart from the pieces of oceanic crust the velocities suggest an overall mafic composition for the arc composed mainly of metabasalts diorite and diabase in the upper crust and garnet granulite or amphibolite to hornblendite in the lower crust reflected arrivals from the subducting pacific plate at depths of 45 55 km beneath the fore arc together with pn show a mantle wedge with p wave velocities as low as 7 4 km s increasing with depth to about 8 1 km s with an average of about 7 7 km s a mantle composition that grades from mainly pyroxenite probably ultramafic cumulates near the moho to dunite at greater depths best explains these observed velocities

volcanic history and magmatic evolution of seguam island aleutian island arc alaska new 40 ar 39 ar dating coupled with detailed field mapping stratigraphy and chemical analyses have established an eruptive chronology that reveals and constrains the compositional and volumetric evolution of seguam island in the aleutian island arc alaska sixty new 40 ar 39 ar ages from lavas domes and pyroclastic deposits were obtained using furnace incremental heating techniques on replicate samples of whole rock and ground mass separates and they constrain the duration of pleistocene to holocene subaerial volcanism to 318 k y the 40 ar 39 ar plateau ages indicate that over 85 of the complex 68 km 3 of material was erupted almost continuously between 318 ka and 9 ka at ca 9 ka a stratocone on the eastern half of the island partially collapsed producing a 4 km wide crater rhyolitic dome forming eruptions followed from vents in the newly created crater and were likely contemporaneous with 8 0 km 3 of basaltic and basaltic andesitic effusions from pyre peak and a 1 4 km 3 basaltic eruption from a monogenetic cone on the far eastern end of the island geochemical changes over the last 318 k y are subtle most notably the earliest eruptions from 318 to 142 ka produced no andesite and basalt from this period has larger ranges in zr rb and la yb than younger basalts small volumes of dacitic to rhyolitic magma were produced from basalt by a monotonic crystal liquid fractionation process that varied only slightly in successive eruptive phases over 318 k y we identified minor geochemical changes in magma composition during each of the three main stages of volcanism but overall the monotonic variations in major and trace element compositions of basaltic andesitic to rhyolitic lavas are consistent with an origin via closed system fractional crystallization of basalt using the 40 ar 39 ar geochronology and estimates of individual flow volumes we calculated a time averaged eruptive rate at seguam that is similar to growth rates of other well dated arc volcanoes in the cascades and chilean southern volcanic zone but less than that of mount katmai and mount mageik which are located on the alaska peninsula the eruptive flux at seguam has been highly variable fluctuating more than an order of magnitude from 0 07 km 3 k y during the early history of bimodal volcanism to 1 18 km 3 k y over the past 9 k y

widespread distribution of ascending fluids transporting mantle helium in the fore arc region and their upwelling processes noble gas and major element composition of deep groundwater in the kii peninsula southwest japan abstract chemical and isotopic studies including analyses of noble gases were comprehensively conducted on the groundwater of the entire kii peninsula which is located in the fore arc region of southwest japan groundwater of na cl hco3 na hco3 cl and na cl types was shown to be distributed across the whole area groundwater in the inland central part of the peninsula shows relatively low salinity whereas groundwater from the area along the ene trending median tectonic line mtl on the north side of the peninsula shows high salinity up to 18 800 mg l of cl and the presence of unusual heavy oxygen isotopes this trend is similar to that documented in saline waters from the arima region the so called arima type thermal water high 3he 4he ratios relative to the atmospheric value up to 6 7 ra were recorded throughout the kii peninsula covering a wider area than documented previously the saline groundwater is also strongly depleted in 20ne and heavy noble gases from the wide distribution of high 3he 4he values and the associated 20ne and cl concentrations we infer that aqueous fluids derived from dehydration of the subducting slab are present at depth beneath almost the entire kii peninsula these aqueous fluids may ascend along the major north dipping boundary faults the isotopic composition of groundwater from the southern part of the peninsula suggests that the contribution from these dehydration derived fluids is relatively small in this region however volatile components e g noble gases and co2 in the groundwater of this area may originate from the dehydration derived fluids upwelling of arima type thermal water of the na cl hco3 type is expected to undergo a phase separation of volatile species due to decompression as the fluid ascends the variety of water types documented may be due to this water gas separation and the subsequent incorporation of gaseous species into shallow meteoric groundwater the observed high 3he 4he ratios in the absence of a mantle wedge below the southern part of the kii peninsula may reflect the oblique ascent of these fluids along north dipping boundary faults

snapshot of the matuyama brunhes reversal process recorded in 40ar 39ar dated lavas from guadeloupe west indies 1 we obtain new 40ar 39ar ages for three lavas that record part of the matuyama brunhes geomagnetic field reversal process on guadeloupe these lavas record a reversed transitional reversed magnetostratigraphy and yield a weighted mean isochron age of 785 2 5 2 ka 2 analytical uncertainty relative to an age for the fish canyon sanidine standard of 28 201 ma this age is of greater accuracy than ages obtained previously by k ar dating these lavas may record directional fluctuations occurring between the matuyama brunhes precursor and the final directional reversal such variations have been observed in some globally distributed coupled paleomagnetic and oxygen isotope marine sedimentary records of the matuyama brunhes reversal previous paleointensity estimates from these lavas indicate reduced but fluctuating field strength during this time

palaeomagnetic refinement of the eruption ages of holocene lava flows and implications for the eruptive history of the tongariro volcanic centre new zealand

concepts and methods of high resolution event stratigraphy modern geological analyses involving sedimentary rocks require a high resolution system of dating integrating and correlating diverse strati graphic geochemical and paleontological data that also takes into account the possibility that short term phenomena 100 kyr or less exercise a strong control on sedimentation such phenomena may be extraterrestrial tectonic volcanic oceanographic climatic sedimentologic and or bio logic in origin some reflect unpredictable perturbations of local to global scale but most short term phenomena are predictable and may be either autocyclic locally regulated with limited stratigraphic continuity or allo cyclic in nature regionally to globally regulated with extensive strati graphic continuity beerbower 1964 a stratigraphic system based on short term phenomena would ideally be chronostratigraphic involving the identification and regional tracing of time lines isochronous surfaces or very thin event deposits and easily integrated with refined independently derived biostratigraphic and geochronologic systems an intensive search for such a new and revolutionary system of stratig raphy has been prompted in many parts of the phanerozoic by attain ment of plateaus in stratigraphic resolution utilizing the more standard tools of magnetostratigraphy biostratigraphy and geochronology most magnetic reversals have been identified biostratigraphic systems can be no more refined than the maximum evolutionary rates of component taxa and their major structures known for many favored groups and geo chronology is regulated by the analytical confidence limits of radiometric

volcanism and erosion during the past 930 k y at the tatara san pedro complex chilean andes geologic mapping together with 73 new k ar and 40ar 39ar age determinations of 45 samples from 17 different volcanic units plus paleomagnetic orientations geochemical compositions and terrestrial photogrammetry are used to define the chronostratigraphy of the tatara san pedro complex an eruptive center at 36 s on the volcanic front of the andean southern volcanic zone the tatara san pedro complex preserves 55 km3 of lavas that erupted from at least three central vent regions remnant unconformity bound sequences of lavas are separated by lacunae that include significant periods of erosion quaternary volcanism commenced ca 930 ka with eruption of voluminous dacitic magma followed 100 k y later by the only major rhyolitic eruption from 780 ka onward more than 80 of the preserved volume is basaltic andesite 52 57 sio2 but petrographically and geochemically diverse dacitic magmas 63 69 sio2 erupted sporadically throughout this younger dominantly mafic phase of activity a few basaltic lavas 49 52 sio2 are present mainly in portions of the complex older than 230 ka the number of vents the petrologic and geochemical diversity and the temporal distribution of mafic and silicic lavas are consistent with emplacement of many separate batches of mafic magma into the shallow crust beneath the tatara san pedro complex over the past million years nearly two thirds of the preserved volume of the tatara san pedro complex comprises the two youngest volcanoes which were active between ca 188 83 ka and 90 19 ka repeated advances of mountain glaciers punctuated growth of the complex with major erosional episodes that removed much of the pre 200 ka volcanic record particularly on the south flank of the complex dating the inception of a glaciation on the basis of preserved material is difficult but the age of the oldest lava above a lacuna may be used to estimate the timing of deglaciation on this basis the argon ages of basal lavas of multiple sequences indicate minimum upper limits of lacunae at ca 830 790 610 400 330 230 110 and 17 ka these are broadly consistent with global ice volume peaks predicted by the oxygen isotope based astronomical time scale and with age brackets on north american glacial advances estimated growth rates for the two young volcanoes are 0 2 to 0 3 km3 k y these are three to five times greater than a growth rate estimated from all preserved lavas in the complex 0 06 km3 k y removal of up to 50 95 of the material erupted between 930 and 200 ka by repeated glacial advances largely explains this discrepancy and it raises the possibility that episodic erosion of mid latitude frontal arc complexes may be extensive and common

magma eruption rates constrained by 40ar 39ar chronology and gis for the ceboruco san pedro volcanic field western mexico 40ar 39ar geochronology is coupled with quantitative volume determinations utilizing field mapping digital elevation models orthophotos and geographic information system gis software to constrain magma eruption rates at the ceboruco san pedro volcanic field 1600 km2 in the western trans mexican arc ages are reported for 40 volcanic units including volcan ceboruco an active andesitic stratovolcano peripheral domes shields cinder cones and fissure fed flows after a hiatus of 3 m y volcanic activity recommenced to produce 80 5 3 5 km3 of magma at a rate of 63 m3 km2 per year over the past 0 8 m y however 75 of this volume erupted in the past 100 k y including the 51 2 5 km3 of volcan ceboruco equivalent to an eruption rate of 377 m3 km2 per year there have been at least two stages of cone building activity at volcan ceboruco the main edifice is composed of 38 km3 of precaldera andesites the youngest dated at 45 8 ka their eruption was followed by a hiatus interrupted by a plinian eruption at 1 ka the plinian eruption and subsequent lava flows are andesite to dacite in composition and constitute 13 km3 of the total volume of volcan ceboruco overall the relative proportions of lava types erupted in the past 0 8 m y are 18 19 basaltic andesite 56 60 andesite 18 22 dacite and 3 rhyolite the peripheral lavas are each of small volume geochemically diverse and show little evidence of prior storage in an upper crustal chamber the eruptive sequence proportions of lava types phenocryst assemblages textures and geochemistry imply that the lavas do not reflect the differentiation of a single parental liquid in a long lived magma chamber the distinct geochemical signatures were present prior to magma emplacement in the upper crust whereupon subsequent degassing and crystallization led to variable phenocryst abundances and assemblages

geology and 40ar 39ar geochronology of the medium to high k tanaga volcanic cluster western aleutians we used geologic mapping and geochemical data augmented by 40 ar 39 ar dating to establish an eruptive chronology for the tanaga volcanic cluster in the western aleutian arc the tanaga volcanic cluster is unique in comparison to other central and western aleutian volcanoes in that it consists of three closely spaced active volumetrically significant edifices sajaka tanaga and takawangha the eruptive products of which have unusually high k 2 o contents thirty five new 40 ar 39 ar ages obtained in two different laboratories constrain the duration of pleistocene holocene subaerial volcanism to younger than 295 ka the eruptive activity has been mostly continuous for the last 150 k y unlike most other well characterized arc volcanoes which tend to grow in discrete pulses more than half of the analyzed tanaga volcanic cluster lavas are basalts that have erupted throughout the lifetime of the cluster although a considerable amount of basaltic andesite and basaltic trachyandesite has also been produced since 200 ka major and trace element variations suggest that magmas from sajaka and tanaga volcanoes are likely to have crystallized pyroxene and or amphibole at greater depths than the older takawangha magmas which experienced a larger percentage of plagioclase dominated fractionation at shallower depths magma output from takawangha has declined over the last 86 k y at ca 19 ka the focus of magma flux shifted to the west beneath tanaga and sajaka volcanoes where hotter more mafic magma erupted

revised age of aleutian island arc formation implies high rate of magma production radioisotopic dating of subaerial and submarine volcanic and plutonic rocks from the aleutian island arc provides insight into the timing of arc formation in the middle eocene twenty eight 40 ar 39 ar ages constrain the duration of arc magmatism to the last 46 m y basaltic lavas from the finger bay volcanics the oldest exposed rocks in the arc gave an isochron age of 37 4 0 6 ma which is 12 17 m y younger than a widely cited age of 55 50 ma three main pulses of arc wide magmatism occurred at 38 29 16 11 and 6 0 ma which coincide with periods of intense magmatism in other western pacific island arcs using the geochronology and volumetric estimates of crust generated and eroded over the last 46 m y we calculate a time averaged magma production rate for the entire arc that exceeds previous estimates by almost an order of magnitude

eruptive history geochronology and magmatic evolution of the puyehue cordon caulle volcanic complex chile forty three 40 ar 39 ar age determinations of lava flows domes ignimbrites and dikes plus 14 c dates from seven distal tephra layers combined with stratigraphy geochemistry and sr and th isotope data establish an eruptive chronology for the puyehue cordon caulle volcanic complex at 40 5 s in the andean southern volcanic zone svz the complex preserves 131 km 3 of lava and tephra that erupted from numerous vents widely separated in time and space approximately 80 of the total volume consists of basaltic to andesitic lava that formed two broad shield volcanoes between 314 and 70 ka the modern puyehue stratovolcano was built on the southerly shield during the past 69 k y following a hiatus of 25 k y puyehue has erupted 15 km 3 of basaltic to rhyolitic magma that spans the entire compositional range found in the southern svz and evolved via at least six phases including 1 basaltic andesitic to dacitic lavas between 69 and 32 ka 2 a shift to bimodal magma compositions that is first expressed by a rhyodacite mingled with inclusions of mgo rich basaltic andesite at 34 ka 3 dacitic to rhyolitic flows and domes from 19 to 12 ka 4 basaltic to basaltic andesitic flows between 15 and 12 ka 5 subsequent rhyolitic dome growth in several effusive and explosive stages between 7 and 5 ka followed by 6 a powerful series of phreatomagmatic and sub plinian eruptions at ca 1 1 ka that obliterated the preceding rhyolite domes and formed the present 2 5 km diameter 280 m deep summit crater along the cordon caulle fissure zone 5 km 3 of rhyodacitic to rhyolitic lavas domes and cones have formed during the past 16 5 k y including explosive and effusive eruptions in 1921 1922 and 1960 eruptive rates were nonuniform over time with background growth at 0 04 km 3 k y or less punctuated by spurts at up to 0 90 km 3 k y the time averaged rate 0 42 km 3 k y is nearly double that at the tatara san pedro complex 500 km to the north during the past 300 k y these findings indicate that within a single arc the magmatic and eruptive fluxes at individual frontal volcanoes can be highly variable the last three stratocone building events on puyehue began during periods of deglaciation suggesting a relationship between unloading of ice and ease of magma ascent puyehue basalt exhibits subtle changes in 238 u 230 th 87 sr 86 sr and trace element composition over time that signal shifts in the composition and degree of melting of the mantle wedge or the extent to which basalt was modified by assimilation of heterogeneous crustal melts the complex has become exceptionally bimodal and more explosive over time with recent rhyolites evolving by extreme crystal fractionation of mafic magma and lesser volumes of andesite and dacite created via mixing of rhyolite and basalt despite the high flux of basalt during the past 300 k y no large silicic magma reservoir formed in the upper crust instead 238 u 230 th data favor rapid ascent of several small bodies of basaltic and silicic magma from the lower crust promoted perhaps by conduits that reflect strike slip faulting beneath the complex

documenting the reliability and utility of the γ method as applied to cyclic sections using forward modeling abstract the problem of determining if cyclic sections which are older than 5 m y in age were deposited in response to orbital control is discussed gamma analysis a method which uses the assumption that effective accumulation rates are facies dependent to minimize differences in cycle duration is presented as a possible solution to this problem while the assumptions of analysis are generally not met by geologic data the effective accumulation rates estimated may be instrumental in recovering a spectrum which reveals orbital control we address the problem of assessing the reliability of results by performing analyses on three cyclic time series generated by a forward model our results show that exact accumulation rates are not likely to be obtained by the method however the accumulation rates obtained markedly improve the time scale resulting in a spectrum from which one could distinguish orbital forcing if present only when spectra of cyclic data show predicted orbital peaks can analysis be considered to yield effective accumulation rates which are more nearly like the true effective accumulation rates than the assumption of a single effective accumulation rate for all facies

construction and destruction of a volcanic island developed inside an oceanic rift graciosa island terceira rift azores abstract there is a great lack of knowledge regarding the evolution of islands inside active oceanic rifts in particular the meaning of the different evolutionary steps therefore we conducted an investigation in graciosa island which lies at the northwestern end of the terceira rift in the azores triple junction with the objective of constraining the evolution of the island in terms of volcanic growth and mass wasting in particular the meaning and age of the destruction events from digital elevation model dem analysis stratigraphic and tectonic observations k ar dating on key samples and available bathymetry and gravity data we propose that graciosa comprises five main volcanic complexes separated by major unconformities related to large scale mass wasting 1 the older volcanic edifice serra das fontes complex grew until ca 700 ka and was affected by a major flank collapse towards the southwest which removed the whole sw flank the summit and a part of the ne flank 2 the baia do filipe complex developed between at least 472 ka and 433 ka in two different ways in the sw presently offshore as a main volcano and in the ne unconformably over the sub aerial remnants of the serra das fontes complex as secondary volcanic edifices 3 the baia do filipe complex was affected by a major flank collapse towards the sw again removing most of the edifice 4 the remnants of the baia do filipe complex were covered in unconformity by the serra dormida complex between ca 330 and 300 ka which in turn was unconformably covered by the younger basaltic cover complex between ca 300 ka and 214 ka these two units were affected by a third major sector collapse that removed the whole western flank the summit and part of the eastern flank of the serra dormida and basaltic cover complexes 5 despite the relatively young age of graciosa the collapse scars are not well preserved and not active anymore 6 a central type volcano has been growing since at least 60 ka at the southeastern end of the island inside the scar left by a fourth sector collapse towards the se which affected most previous complexes contemporaneously parasitic strombolian cones formed all over the island despite the location of graciosa inside the active terceira rift the new data indicate that the evolution of the island has been driven by a competition between volcano growth and repeated destruction by catastrophic sector collapses rather than by slow incremental faulting associated with the tectonics of the rift

quantifying arc migration and the role of forearc subduction erosion in the central aleutians abstract the active aleutian arc is currently classified as an accreting margin as most of the arc has a well developed frontal accretionary prism that formed in the plio pleistocene we focus on the central aleutian island arc where the arc massif was built to near its current size by the late eocene and has since seen limited growth and endured episodic erosion prior to the development of the accretionary prism as a result the volcanic front has migrated northwards tens of kilometers since inception along with previous mapping dating and geochemical data sixteen new 40ar 39ar dates and new geochemical data for volcanic rocks on several central aleutian islands supplement recent geochronologic data from six other islands in this region and facilitate an attempt to understand how and where aleutian arc volcanism has migrated with time long term average arc migration rates range from 0 6 to 5 km ma in three distinct locations the arc migration rate increases to the west and is greatest over the last five million years as the arc migrated from a 7 5 ma volcanic front to its current location arc migration and erosion during this most recent period may have been expedited by either the subduction of the kula ridge which ceased spreading in the early eocene or clockwise rotation of the pacific plate in the late miocene some central aleutian lavas and plutonic rocks have elevated la yb sm yb and sr yb most of which were emplaced during times of arc migration or in the waning stages of a peak in magmatic activity the strongly fractionated trace element patterns are similar to the basaltic andesites to dacites from the western aleutian seafloor which are interpreted to be the result of residual garnet during partial melting of eclogite in the subducting oceanic crust however the existing subduction parameters coupled with the timing of the central aleutian adakite like lavas favors a model in which significant subduction erosion of the mafic forearc including erosion and subsequent melting of the trapped kula plate i e morb like oceanic crust as well as early aleutian arc basalts in the mid crust may be partly responsible for the eclogitic component observed in central aleutian magmas

origin of limestone marlstone cycles astronomic forcing of organic rich sedimentary rocks from the cenomanian to early coniacian of the cretaceous western interior seaway usa abstract we present an integrated multidisciplinary study of limestone marlstone couplets from a continuously cored section including parts of the upper buda limestone the entire eagle ford group boquillas formation and lower austin chalk from the shell iona 1 research borehole texas usa which provides a 8 million year myr distal clastic sediment starved intrashelf basin record of the early cenomanian to the earliest coniacian stages results show that despite variable yet minimal diagenetic overprints several unambiguous primary environmental signals are preserved and support greater water mass ventilation and current activity promoting increased silica carbonate productivity during the deposition of limestone beds compared to deposition of marlstone beds which reflect greater organic matter productivity and preservation furthermore our astronomical analyses demonstrate that the limestone marlstone couplets in the iona 1 core reflect climatic forcing driven by solar insolation resulting from integrated milankovitch periodicities in particular we propose that obliquity and precession forcing on the latitudinal distribution of solar insolation may have been responsible for the observed lithological and environmental variations through the cenomanian turonian and coniacian in this mid latitude epicontinental sea setting our data also suggests that rhythmic lithological alternations deposited in greenhouse periods in general may simply reflect climate driven cycles related to earth sun dynamics without the need to invoke significant sea level variations

geochronology and eruptive history of the katmai volcanic cluster alaska peninsula abstract in the katmai district of the alaska peninsula k ar and 40ar 39ar ages have been determined for a dozen andesite dacite stratocones on the arc front and for 11 rear arc volcanoes 10 of which are monogenetic tied to mapping and stratigraphic studies our dating emphasized proximal basal lavas that rest on basement rocks in order to estimate ages of inception of each polygenetic cone oldest among arc front cones is alagogshak volcano 690 43 ka succeeded in the holocene by the active mount martin cone mount mageik consists of four overlapping subedifices basal lavas of which give ages of 93 71 and 59 ka and holocene the three small prehistoric cones of trident volcano yield ages of 143 101 58 and 44 ka falling mountain and mount cerberus dacite domes near the 1912 novarupta vent are related compositionally to the trident group and give ages of 70 ka and 114 ka mount katmai which underwent caldera collapse in 1912 consists of two subedifices that overlapped in space and time and is the only arc front center here to include basalt and rhyolite one cone began by 90 ka the other by 47 ka snowy mountain also consists of two contiguous cones which started around 200 and 171 ka respectively the younger remaining active into the holocene devils desk the only mafic cone on the arc front was short lived at about 245 ka in the rear arc 1 mount griggs produced mafic to silicic andesite in several episodes between 292 ka and the holocene 2 the savonoski river cluster includes a pliocene dacite dome and five small mafic cones 390 88 ka 3 gertrude creek cone 49 8 sio2 yields an age of 500 ka and 4 the saddlehorn creek cluster includes five pliocene basalt to andesite remnants eruptive volumes were reconstructed permitting estimates of average eruption rates for edifice lifetimes since the mid pleistocene total volume erupted along the arc front here is 210 47 km3 and in the rear arc 39 6 km3 of which mount griggs alone accounts for 35 5 km3 most productive has been mount katmai at 70 18 km3 yielding a rate of 1 km3 kyr followed by mount mageik 0 33 km3 kyr and mount griggs 0 3 km3 kyr since 50 ka

kaguyak dome field and its holocene caldera alaska peninsula abstract kaguyak caldera lies in a remote corner of katmai national park 375 km sw of anchorage alaska the 2 5 by 3 km caldera collapsed 5 8 0 2 ka 14c age during emplacement of a radial apron of poorly pumiceous crystal rich dacitic pyroclastic flows 61 67 sio2 proximal pumice fall deposits are thin and sparsely preserved but an oxidized coignimbrite ash is found as far as the valley of ten thousand smokes 80 km southwest postcaldera events include filling the 150 m deep caldera lake emplacement of two intracaldera domes 61 5 64 5 sio2 and phreatic ejection of lakefloor sediments onto the caldera rim co2 and h2s bubble up through the lake weakly but widely geochemical analyses n 148 including pre and post caldera lavas 53 74 sio2 define one of the lowest k arc suites in alaska the precaldera edifice was not a stratocone but was instead nine contiguous but discrete clusters of lava domes themselves stacks of rhyolite to basalt exogenous lobes and flows four extracaldera clusters are mid to late pleistocene but the other five are younger than 60 ka were truncated by the collapse and now make up the steep inner walls the climactic ignimbrite was preceded by 200 years by radial emplacement of a 100 m thick sheet of block rich glassy lava breccia 62 65 5 sio2 filling the notches between the truncated dome clusters the breccia now makes up three segments of the steep caldera wall which beheads gullies incised into the breccia deposit prior to caldera formation they were probably shed by a large lava dome extruding where the lake is today

cretaceous rhythmic bedding sequences a plausible link between orbital variations and climate abstract simple oscillation cycles have been recognized in cretaceous pelagic sequences as a marked periodicity in carbonate and or organic carbon content the estimated periodicities of these cycles resemble milankovitch type variations in orbital elements however during warm geologic periods the nature of the link between orbital variations and the sedimentary record has been problematic climate model studies show that the intensity of continental margin precipitation in specific low latitude regions is a function of land sea thermal contrast which is sensitive to orbital variations cretaceous climate model simulations are characterized by regions of intense precipitation the locations of which are controlled by geographic configuration in particular in the model simulation the northern margin of the tethys ocean has all the climatic characteristics necessary to exhibit a sensitivity to orbital variations detailed studies of the biota sedimentology and geochemistry of the bedding rhythms in greenhorn cyclothem in the western interior seaway of north america are best interpreted in terms of variations in precipitation runoff and stable stratification of the seaway combined the geologic and model studies support a link between orbital periodicities variations of intense precipitation associated with specific paleogeographic situations and cretaceous bedding patterns this hypothesis has important implications for the sensitivity of the atmosphere hydrosphere lithosphere system to external forcing

structure and composition of the aleutian island arc and implications for continental crustal growth we present results of a seismic reflection and refraction investigation of the aleutian island arc designed to test the hypothesis that volcanic arcs constitute the building blocks of continental crust the aleutian arc has the requisite thickness 30 km to build continental crust but it differs strongly from continental crust in its composition and reflectivity structure seismic velocities and the compositions of erupted lavas suggest that the aleutian crust has a mafic bulk composition in contrast to the andesitic bulk composition of continents the silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the aleutian intraoceanic arc therefore if island arcs form a significant source of continental crust the bulk properties of arc crust must be substantially modified during or after accretion to a continental margin the pervasive deformation intracrustal melting and delamination of mafic to ultramafic residuum necessary to transform arc crust into mature continental crust probably occur during arc continent collision or through subsequent establishment of a continental arc the volume of crust created along the arc exceeds that estimated by previous workers by about a factor of two

geochemical evidence for the melting of subducting oceanic lithosphere at plate edges most island arc magmatism appears to result from the lowering of the melting point of peridotite within the wedge of mantle above subducting slabs owing to the introduction of fluids from the dehydration of subducting oceanic crust1 volcanic rocks interpreted to contain a component of melt not just a fluid from the subducting slab itself are uncommon but possible examples have been recognized in the aleutian islands baja california patagonia and elsewhere2 3 4 the geochemically distinctive rocks from these areas termed adakites are often associated with subducting plates that are young and warm and therefore thought to be more prone to melting5 but the subducting lithosphere in some adakite locations such as the aleutian islands appears to be too old and hence too cold to melt6 7 this implies either that our interpretation of adakite geochemistry is incorrect or that our understanding of the tectonic context of adakites is incomplete here we present geochemical data from the kamchatka peninsula and the aleutian islands that reaffirms the slab melt interpretation of adakites2 but in the tectonic context of the exposure to mantle flow around the edge of a torn subducting plate we conclude that adakites are likely to form whenever the edge of a subducting plate is warmed or ablated by mantle flow the use of adakites as tracers for such plate geometry may improve our understanding of magma genesis and thermal structure in a variety of subduction zone environments

eruptive history of fisher caldera alaska usa abstract new field compositional and geochronologic data from fisher caldera the largest of 12 holocene calderas in alaska provide insights into the eruptive history and formation of this volcanic system prior to the caldera forming eruption cfe 9400 years ago the volcanic system consisted of a cluster of several small 3 km 3 stratocones which were independently active between 66 144 and 9 4 0 2 ka fisher caldera formed through a single eruption which produced a thick dacitic fall deposit and two pyroclastic flow deposits a small dacitic flow and a compositionally mixed basaltic dacitic flow thickness and grain size data indicate that the fall deposit was dispersed primarily to the northeast whereas the two flows were oppositely directed to the south and north after the cataclysmic eruption a lake filled much of the caldera during what may have been a significant quiescent period volcanic activity from intracaldera vents gradually resumed producing thick successions of scoria fall interbedded with lake sediments several holocene stratocones have developed one of which has had a major collapse event the caldera lake catastrophically drained when a phreatomagmatic eruption generated a large wave that overtopped and incised the southwestern caldera wall multiple accretionary lapilli bearing deposits inside and outside the caldera suggest significant holocene phreatomagmatic activity the most recent eruptive activity from the fisher volcanic system was a small explosive eruption in 1826 and current activity is hydrothermal late pleistocene to holocene magma eruption rates range from 0 03 to 0 09 km 3 ky 1 km 1 respectively the fisher volcanic system is chemically diverse 48 72 wt sio 2 with at least seven dacitic eruptions over the last 82 14 ka that may have become more frequent over time least squares calculations suggest that prior to the cfe fisher volcano products were not derived from a single large magma reservoir and were likely erupted from multiple compositionally independent magma reservoirs after the cfe the majority of products appear to have derived from a single reservoir in which magma mixing has occurred

eruptive history and geochronology of mount mazama and the crater lake region oregon geologic mapping k ar and 40 ar 39 ar age determinations supplemented by paleomagnetic measurements and geochemical data are used to quantify the quaternary volcanic history of the crater lake region in order to define processes and conditions that led to voluminous explosive eruptions the cascade arc volcano known as mount mazama collapsed during its climactic eruption of 50 km 3 of mainly rhyodacitic magma 7700 yr ago to form crater lake caldera the mazama edifice was constructed on a pleistocene silicic lava field amidst monogenetic and shield volcanoes ranging from basalt to andesite similar to parental magmas for mount mazama between 420 ka and 35 ka mazama produced medium k andesite and dacite in 2 1 proportion the edifice was built in many episodes some of the more voluminous occurred approximately coeval with volcanic pulses in the surrounding region and some were possibly related to deglaciation following marine oxygen isotope stages mis 12 10 8 6 5 2 and 2 magmas as evolved as dacite erupted many times commonly associated with or following voluminous andesite effusion establishment of the climactic magma chamber was under way when the first preclimactic rhyodacites vented ca 27 ka the silicic melt volume then grew incrementally at an average rate of 2 5 km 3 k y 1 for nearly 20 k y the climactic eruption exhausted the rhyodacitic magma and brought up crystal rich andesitic magma mafic cumulate mush and wall rock granodiorite postcaldera volcanism produced 4 km 3 of andesite during the first 200 500 yr after collapse followed at ca 4800 yr b p by 0 07 km 3 of rhyodacite the average eruption rate for all mazama products was 0 4 km 3 k y 1 but major edifice construction episodes had rates of 0 8 km 3 k y 1 the long term eruption rate for regional monogenetic and shield volcanoes was d 0 07 km 3 k y 1 but only 0 02 km 3 k y 1 when the two major shields are excluded plutonic xenoliths and evidence for crystallization differentiation imply that the amount of magma intruded beneath mount mazama is several times that which has been erupted the eruptive and intrusive history reflects competition between 1 crystallization driven by degassing and hydrothermal cooling and 2 thermal input from a regional magma flux focused at mazama before ca 30 ka relatively small volumes of nonerupted derivative magma crystallized to form a composite pluton because the upper crust had not been heated sufficiently to sustain voluminous convecting crystal poor melt subsequently and perhaps not coincidentally during mis 2 a large volume of eruptible silicic magma accumulated in the climactic chamber probably because of heating associated with mantle input to the roots of the system as suggested by eruption of unusually primitive magnesian basaltic andesite and tholeiite west of mazama