holocene tsunamis in the southwestern bering sea russian far east and their tectonic implications the bering sea coast of kamchatka overlies a boundary between the proposed okhotsk and bering blocks or micro plates of the north american plate in the russian far east a history of tsunamis along this coast for the past 4000 yr indicates that the zone north of the kuril kamchatka subduction zone produces tsunamigenic earthquakes every few centuries such a record is consistent with convergence of the proposed okhotsk and bering blocks along the bering sea coast of kamchatka a tsunami deposit from the 1969 m w 7 7 ozernoi earthquake helps us interpret older tsunami deposits newly studied tephra layers from shiveluch volcano as well as previously established marker tephra layers in northern kamchatka provide age control for historic and prehistoric tsunami deposits based on 50 measured sections along 14 shoreline profiles tsunami deposit frequencies in the southwestern bering sea are about five per thousand years for tsunamis generated north of the kuril kamchatka trench

composition of back arc basin volcanics valu fa ridge lau basin evidence for a slab derived component in their mantle source abstract samples dredged from 2 localities near the crest of the valu fa ridge an active back arc basin spreading centre in the lau basin consist of highly vesicular lava fragments of andesitic composition the samples are characterized by rare euhedral an 85 plagioclase phenocrysts in a hypocrystalline groundmass of an 60 plagioclase laths brown glass and rare subhedral clinopyroxene samples from within and to a lesser extent between the dredge hauls show remarkable isotopic and chemical homogeneity with 87 sr 86 sr 0 70330 2 143 nd 144 nd 0 51303 2 206 pb 204 pb 18 65 2 207 pb 204 pb 15 55 1 208 pb 204 pb 38 34 4 sr 165 ppm rb 7 ppm cs 0 17 ppm k 3300 to 4200 ppm ba 96 ppm and ree lree depleted with 12 18 chondritic abundances on sr nd pb pb and sr pb plots the volcanics lie just within or on the edge of the morb fields overlapping with island arc volcanics from the marianas and tonga compared with morb and ocean island basalts the samples show alkali element enrichment relative to ree and higher cs relative to rb the isotopic and geochemical characteristics of the valu fa ridge volcanics clearly indicate a minor but significant slab derived component in the back arc basin mantle source

displacement partitioning and arc parallel extension in the aleutian volcanic island arc abstract convergence between the pacific and north american plates along the aleutian volcanic island arc is generally right oblique the obliquity increases from 0 in the east to nearly 90 in the west where the plate boundary becomes a transform fault zone preliminary results of a structural geology study of attu island the most westerly u s island in the aleutian chain and part of the near islands group shows that the deformational structures are not directly related to the relative plate convergence rate vector but to the plate boundary normal and parallel components of this vector the plate boundary sub normal component is responsible for conjugate sets of arc sub parallel thrust faults and the plate boundary parallel component resulted in arc parallel dextral strike slip faults and their conjugates however the most penetrative structures measured in outcrop are conjugate sets of strike slip faults and arc perpendicular normal faults and fractures veins and dikes which have formed by arc parallel extension this extension is most likely the result of the increase of obliquity from east to west and thus the increase of the arc parallel component of the convergence rate vector along the aleutian arc these structures require that attu island migrated westward possibly for as many as 1150 km

pyroclastic eruption boosts organic carbon fluxes into patagonian fjords fjords and old growth forests store large amounts of organic carbon yet the role of episodic disturbances particularly volcanic eruptions in mobilizing organic carbon in fjord landscapes covered by temperate rainforests remains poorly quantified to this end we estimated how much wood and soils were flushed to nearby fjords following the 2008 eruption of chaiten volcano in south central chile where pyroclastic sediments covered 12 km2 of pristine temperate rainforest field based surveys of forest biomass soil organic content and dead wood transport reveal that the reworking of pyroclastic sediments delivered 66 500 14 600 14 500 tc of large wood to two rivers entering the nearby patagonian fjords in less than a decade a similar volume of wood remains in dead tree stands and buried beneath pyroclastic deposits 79 900 21 100 16 900 tc or stored in active river channels 5 900 10 600 tc we estimate that bank erosion mobilized 132 300 21 700 30 600 tc of floodplain forest soil eroded and reworked forest soils have been accreting on coastal river deltas at 5 mm yr 1 since the eruption while much of the large wood is transported out of the fjord by long shore drift the finer fraction from eroded forest soils is likely to be buried in the fjords we conclude that the organic carbon fluxes boosted by rivers adjusting to high pyroclastic sediment loads may remain elevated for up to a decade and that patagonian temperate rainforests disturbed by excessive loads of pyroclastic debris can be episodic short lived carbon sources

holocene tephrochronology of the hualaihue region andean southern volcanic zone 42 s southern chile late glacial and holocene soils and sediments in southern chile contain an important record of explosive volcanic activity since the end of the last glaciation and have considerable potential for

vegetation and climate change fire regime shifts and volcanic disturbance in chiloe continental 43 s during the last 10 000 years abstract disentangling the roles of paleofires and explosive volcanism from climatic drivers of past vegetation change is a subject insufficiently addressed in the paleoecological literature the coastal region of the chiloe continental sector of northwestern patagonia is ideal in this regard considering its proximity to active eruptive centers and the possibility of establishing comparisons with more distal upwind sites where volcanic influence is minimal here we present a fine resolution pollen and macroscopic charcoal record from lago teo with the aim of documenting the local vegetation and climate history and assessing the role of disturbance regimes as drivers of vegetation change during the last 10 000 years the lago teo record shows a conspicuous warm dry interval between 7500 and 10 000 cal yrs bp followed by a cooling trend and increase in precipitation that has persisted until the present in agreement with previous studies in the region and interpretations of past southern westerly wind activity at multi millennial scales the presence of 26 tephras throughout the record allows examination of the relationship between explosive volcanism and vegetation change under contrasting climatic states of the holocene we found consistent statistically significant increases in tepualia stipularis after tephra deposition over the last 10 000 years in eucryphia caldcluvia between 7500 and 10 000 cal yrs bp and in hydrangea over the last 7500 years our results indicate a primary role of climate change as driver of long term vegetation change and as a modulator of vegetation responses to volcanic disturbance at multidecadal and centennial timescales

reconstruction of subduction and back arc spreading in the nw pacific and aleutian basin clues to causes of cretaceous and eocene plate reorganizations the eocene 50 45 ma major absolute plate motion change of the pacific plate forming the hawaii emperor bend is thought to result from inception of pacific plate subduction along one of its modern western trenches subduction is suggested to have started either spontaneously or result from subduction of the izanagi pacific mid ocean ridge or from subduction polarity reversal after collision of the olyutorsky arc that was built on the pacific plate with ne asia here we provide a detailed plate kinematic reconstruction of back arc basins and accreted terranes in the northwest pacific region from japan to the bering sea since the late cretaceous we present a new tectonic reconstruction of the intraoceanic olyutorsky and kronotsky arcs which formed above two adjacent oppositely dipping subduction zones at 85 ma within the north pacific region during another pacific wide plate reorganization we use our reconstruction to explain the formation of the submarine shirshov and bowers ridges and show that if marine magnetic anomalies reported from the aleutian basin represent magnetic polarity reversals its crust most likely formed in an 85 to 60 ma back arc basin behind the olyutorsky arc the olyutorsky arc was then separated from the pacific plate by a spreading ridge so that the 55 to 50 ma subduction polarity reversal that followed upon olyutorsky ne asia collision initiated subduction of a plate that was not the pacific hence this polarity reversal may not be a straightforward driver of the eocene pacific plate motion change whose causes remain enigmatic

comparison of lake and land tephra records from the 2015 eruption of calbuco volcano chile tephra layers in lake sediment cores are regularly used for tephrostratigraphy as isochronous features for dating and recording eruption frequencies however their value for determining volcanic eruption size and style may be complicated by processes occurring in the lake that modify the thickness and grain size distributions of the deposit to assess the reliability of data from lake cores we compare tephra deposited on land during the 2015 eruption of calbuco volcano in chile to records in sediment cores from three lakes of different sizes that are known to have received primary fall deposits in general the thickness and granulometry of the deposit in lake cores and nearby terrestrial sections are very similar as anticipated however cores sampled close to here within 300 m of fluvial inflows were affected by sediment deposition from the lake s catchment they differed from primary deposits not only in their greater thickness and organic content but also in poor sorting and lack of grading cores 850 m away from the inlet were not affected we consider our results in the context of the particle settling regime as well as each lake s location bathymetry and catchment area we find that the particle settling regime is important in more distal settings where the ash particles are small and particle settling occurs in density plumes rather than as individual particles we conclude that lake cores can be useful for physical volcanology providing consideration is given to eruption parameters such as particle size and mass flux as well as lake features such as bathymetry and catchment area

sand waves on an epicontinental shelf northern bering sea abstract sand waves and current ripples occupy the crests and flanks of a series of large linear sand ridges 20 km 5 km 10 m high lying in an open marine setting in the northern bering sea the sand wave area which lies west of seward peninsula and southeast of bering strait is exposed to the strong continuous flow of coastal water northward toward bering strait a hierarchy of three sizes of superimposed bedforms all facing northward was observed in successive cruises in 1976 and 1977 large sand waves height 2 m spacing 200 m have smaller sand waves height 1 m spacing 20 m lying at a small oblique angle on their stoss slopes the smaller sand waves in turn have linguoid ripples on their stoss slopes repeated studies of the sand wave fields were made both years with high resolution seismic reflection profiles side scan sonographs underwater photographs current meter stations vibracores and suspended sediment samplers comparison of seismic and side scan data collected along profile lines run both years showed changes in sand wave shape that indicate significant bedload transport within the year gouge marks made in sediment by keels of floating ice also showed significantly different patterns each year further documenting modification to the bottom by sediment transport during calm sea conditions in 1977 underwater video and camera observations showed formation and active migration of linguoid and straight crested current ripples current speeds 1 m above the bottom were between 20 and 30 cm s maximum current velocities and sand wave migration apparently occur when strong southwesterly winds enhance the steady northerly flow of coastal water many cross stratified sand bodies in the geologic record are interpreted as having formed in a tidal or storm dominated setting this study provides an example of formation and migration of large bedforms by the interaction of storms with strong uniform coastal currents in an open marine setting

back arc extension in the okinawa trough the okinawa trough lying to the east of china is a back arc basin formed by extension within continental lithosphere behind the ryukyu trench arc system middle to late miocene uplift associated with normal faulting of the initially adjacent ryukyu nonvolcanic arc and the taiwan sinzi folded belt corresponds to the first rifting phase the timing of rifting is supported by the presence of marine sediments of corresponding age drilled in the northern okinawa trough the rifting occurred after a major early miocene change in the motion of the philippine plate with respect to eurasia and ceased during the pliocene a second rifting phase started about 2 m y ago at the plio pleistocene boundary and has continued until the present time it has proceeded to a more advanced stage in the middle and southern okinawa trough than it has farther north detailed bathymetric sea beam seismic reflection and magnetics data collected during the pop 1 cruise of the r v jean charcot reveal the principal features of the extensional processes the back arc spreading phase started very recently in the southern and middle okinawa trough as exemplified by several en echelon and in some cases overlapping active central graben oriented n70 e n80 e some of these depressions are intruded by volcanic ridges of fresh back arc basalt with associated large magnetic anomalies transform faults between these en echelon active rifts are not obvious we suggest that the major part of the southern okinawa trough is underlain by a thinned continental crust and that except for the system of en echelon rifts of the southern okinawa trough the back arc basin oceanic domain is limited to a width of a few tens of kilometers or less in the axial portion of the trough the system of axial back arc volcanic ridges that occur in the rifts ends at the latitude of okinawa island whereas active volcanoes in the ryukyu arc occur only north of okinawa island we refer to this transition between active arc and back arc volcanism as the volcanic arc rift migration phenomenon vamp globally back arc volcanism propagated from the southern okinawa trough to the okinawa vamp area rifting continues to occur in the northern okinawa trough but is not yet accompanied by associated volcanism the okinawa vamp area is characterized by a series of parallel basaltic ridges oriented n75 e with associated linear magnetic anomalies characteristic of dyke intrusions we suggest that the formation of the back arc oceanic domain took place along the axial back arc extensional zone trending n75 e and that this zone presently ends at the southern extremity of the active volcanic chain the initial phase of formation of back arc basin oceanic crust is non steady state and is characterized by the lack of a developed fracture zone pattern the termination of the vamp area in the direction of the volcanic zone of the arc is consistent with the suggestion of molnar and atwater that the volcanic arc is a fundamental line of weakness which determines where initial back arc spreading occurs apparently back arc extension initially occurred within the continental lithosphere located westward of the ryukyu arc along its whole length but the subsequent back arc volcanism was initiated in the southernmost portion of the region and then moved northward this migration was accompanied by the shutting down of volcanic activity along the abandoned portions of the arc it is this transfer of volcanism that we call the vamp process thus arc and back arc basin volcanism seem to be associated in such a manner that spreading tends to migrate simultaneously with a cessation in volcanic activity along the arc this interplay of arc and back arc activity is probably linked to changes in the parameters of plate convergence since the plate motion in the phillippine sea is oblique to the trench at least in the southern part of the okinawa trough we suggest that the oblique resisting force applied to the edge of the overriding plate engenders the development of en echelon extensional features within and behind the arc the motion of the ryukyu platelet with respect to eurasia is consequently an extensional strike slip motion trending roughly parallel to the okinawa trough

structure of an active arc continent collision area the aleutian kamchatka junction abstract the plate boundary between the north american and pacific plates along the westernmost portion of the aleutian arc is formed by several active dextral strike slip fracture zones fracturing is expressed by fault escarpments visible in reflection seismic profiles and narrow zones of high seismicity the seismicity changes from the aleutian arc where shallow earthquakes occur to intermediate to deep earthquakes in the kamchatka aleutian junction area and to the south in the vicinity of this junction the distribution of earthquakes follows the traces of major aleutian fracture zones down to a depth of 100 km south of the kronotsky peninsula the distribution of earthquakes changes and is similar to other subduction zones the fracture zones cut the western aleutian arc into individual blocks which move along strike slip faults to the west and collide with kamchatka related transtension and transpression has led to the development of pull apart basins and block tilting the fracture zones splay into numerous strike slip faults to the west where they meet the kamchatka margin the main faults of the aleutian fault system can be traced on the kamchatka mys peninsula where deformed pleistocene sediments high uplift rates recent thrust faulting and changes in the drainage system provide evidence for neotectonic activity the fault pattern onshore and offshore in the junction area supports an indentation tectonic model of the collision of aleutian arc slivers with kamchatka

an evaluation of bagnold s dimensionless conefficient of proportionality using measurements of sandwave movement abstract only a few observations are currently available from the tidal marine environment which can be used to evaluate sediment transport formulae research carried out on sandwave movement indicates that most of the sediment transport is restricted to the crestal area mean rates of sediment transport were calculated from volumetric changes between successive slack waters over a nine day period spanning spring tides the results were used in conjunction with boundary layer flow measurements to calculate the threshold velocity for the movement of sediment and also to evaluate bagnold s 1963 dimensionless coefficient of proportionality k comparisons are made with the flume data of guy et al 1966 for the appropriate sediment grain size the dependence of k upon normalised excess shear stress was found to be weaker than that suggested by sternberg 1972 within the range of flow velocities considered there was no indication that k tended towards a constant at high flow velocities

the komandorsky basin as a product of spreading behind a transform plate boundary abstract a long term study of the komandorsky basin in particular by the cruises of r vvulkanolog and r vdmitry mendeleev in the summer of 1988 made it possible to describe in detail the main structural features of this marginal basin several ne and n s trending spreading axes were detected in the basin in the rear of the transform sector of the aleutian arc spreading axes are shifted relative to each other along nw striking transform faults there is variation in style from north to south along the strike of the spreading axes and a less well defined symmetry compared to that typical of mid oceanic ridges the tectonic pattern and heat flow reflect several stages starting from the eocene in the history of the opening of the basin caused by the changes in the velocity and direction of the relative motion between the pacific and north american plates

lacustrine record of last millennia eruptions in northern chilean patagonia 45 47 s due to its tectonic setting the andean southern volcanism zone ssvz is characterized by frequent volcanic activity chilean patagonia lake sediments represent powerful archives of historical and

evolution of a classic sand ridge field maryland sector north american inner shelf the ridge and swale topography of the middle atlantic bight is best developed on the delaware maryland inner shelf here sand ridges can be seen in all stages of formation several aspects of the ridge field are pertinent to the problem of ridge genesis the first is ridge morphology there is a systematic morphologic change from shoreface ridges through nearshore ridges to offshore ridges which reflects the changing hydraulic regime as successively more seaward ridges are examined maximum side slope decreases the ratio of maximum seaward slope to maximum landward slope decreases and the cross sectional area increases these changes in ridge morphology with depth and distance from shore appear to be equivalent to the morphologic changes experienced by a single ridge during the course of the holocene transgression a second aspect is the change in bottom sediment characteristics that accompanies these large scale morphologic changes megaripples sand waves and mud lenses appear in the troughs between nearshore and offshore ridges these changes indicate that the storm flows which maintain ridges are less frequently experienced in the deeper sector and that the role of high frequency wave surge becomes less important relative to the role of the mean flow component in shaping the sea floor a third aspect is the systematic relationship of grain size to topography grain size is 90 out of phase with topography so that the coarsest sand lies between the axis of the landward trough and the ridge crest while the finest sand lies between the ridge crest and the axis of the seaward trough this relationship is characteristic of large scale bedforms finally flow was measured and transport calculated on the same ridge during a one month period november 1976 threshold was exceeded only during storm events mean transport was southerly and a little seaward with respect to both the ridge crest and the shoreline these flow measurements are in conformity with the pattern of smaller bedforms a 43 year time series of bathymetric change for this ridge reveals a systematic pattern of landward flank erosion seaward flank deposition and seaward crest migration sand ridges are considered the consequence of constructive feedback between an initial topography and the resulting distribution of bottom shear stress the relationship between grain size and topography supports this model but does not account directly for the oblique angle of the ridge with respect to the coastline this feature may be due to a more rapid alongshore migration rate of the inshore edge of the ridge than the offshore edge and the relationship between this migration rate and the rate of shoreface retreat

flume experiments on lower flow regime bed forms in coarse sand thirty five runs were made in a recirculating channel 11 5 m long and 0 92 m wide with four sizes of very well sorted coarse sand to study geometry migration and hydraulics of bed configurations in equilibrium with steady uniform open channel flows at flow depths of about 15 cm equilibrium energy slope and sediment transport rate were measured as well bed phases kinds of bed configurations studied were ripples low flow velocity flat beds with sand transport and dunes identical with megaripples current velocities in none of the runs were high enough for development of an upper flow regime flat bed a plot of mean flow velocity versus sand size for all the runs shows that at low flow velocities the ripple field narrows with increasing sand size and pinches out at about 0 7 mm the field for flatbed transport narrows with decreasing sand size and pinches out at about 0 5 mm just above the ripple field at high flow velocities dunes are the stable phase for all sand sizes studied dune configurations are divided into two subphases two dimensional dunes 2d dunes with fairly straight continuous even crests and no strong localized scour in troughs are formed at relatively low flow velocities three dimensional dunes 3d dunes with strongly sinuous discontinuous uneven crests and strong localized scour pits in troughs are formed at relatively high flow velocities 3d dunes tend to be higher than 2d dunes have larger height spacing ratios and show less variability in height spacing and migration rate dunes can be viewed as kinematic shock waves differences between 2d dunes and 3d dunes lie in the differing importance of shock wave coupling and of sand transport in bed form troughs in both geometry and hydraulic ralationships the 2d dunes and 3d dunes in our flume runs correspond to the relatively regular megaripples at low flow velocities and relatively irregular megaripples at high flow velocities described by other authors from intertidal environments

a past millennium maximum in postglacial activity from volcan chaiten southern chile volcan chaiten southern chile 43 s initiated an historically unprecedented eruption in a d 2008 surprising the local inhabitants chilean and argentine authorities and the geologic community available data at the time indicated an absence of explosive eruptions from this rhyolitic volcano since a large magnitude eruptive event dated at ca 10 500 yr b p we present lake sediment data from lago teo a small closed basin lake located in the immediate vicinity of both chaiten township and the volcano that spans the past 10 000 yr and contains 26 pyroclastic fallout deposits glass shard electron microprobe analyses revealed ten rhyolitic tephras indistinguishable in composition from the 2008 volcan chaiten eruption and ten others potentially derived from the michinmahuida volcanic complex among the rhyolites we detected three closely spaced tephras deposited between ca 9460 and 9680 yr b p followed by two thick tephras dated at ca 7700 and ca 5080 yr b p three other closely spaced tephras occur between ca 600 and 850 yr b p the most recent prehistoric event at ca 420 yr b p and a 3 cm thick tephra deposited during the 2008 event we calculate a median recurrence of 310 yr between eruptive events from all sources over the past 10 000 yr and 200 yr between volcan chaiten events over the past millennium our results not only challenge the notion of an 10 500 yr long quiescence for volcan chaiten activity but also suggest that the 2008 eruption was an overdue phenomenon in the context of its postglacial eruptive history illustrating the advantage and absolute necessity of utilizing lake sediment archives for developing continuous well dated time series inventories of explosive volcanic events

stability of bedforms in a tidal environment abstract large scale bedforms developed in a tide dominated salt estuary near sapelo island georgia are morphologically separated into two distinct groups megaripples include bedforms 4 to 15 m in wavelength and 10 to 75 cm in height sand waves are 15 to 45 m in wavelength and 85 to 130 cm in height and can be further subdivided into megarippled forms and smooth crested forms ebb oriented geometry of large scale bedforms is maintained through both ebb and flood portions of the tidal cycle bedforms are in equilibrium with peak tidal flow and ebb orientation is the direct result of large pressure gradients and related bed shear values that develop during the ebb bedform heights can be statistically related to flow power shear velocity water depth and sediment grain size mean flow velocity values used in many previous studies of bedform stability did not produce significant f statistic values in a multiple linear regression procedure used to discern relationships among variables stability fields of bedform types can be empirically defined using parameters statistically significant in the regression analysis flow power shear velocity water depth and sediment grain size were used in various combinations to construct bivariate plots showing separate stability fields of sand waves and megaripples

extraordinary sediment delivery and rapid geomorphic response following the 2008 2009 eruption of chaiten volcano chile u s geological survey volcano science center sernageomin s programa de riesgos volcanicos conicyt fondecyt grants 1110609 1141064 11130671 conicyt fondap 15090013 vamos research centre

archean protolith and accretion of crust in kamchatka shrimp dating of zircons from sredinny and ganal massifs abstract we used an ion microprobe shrimp rg for u pb dating of individual zircons in high grade metamorphic basements of the felsic sredinny and mafic ganal massifs in kamchatka thirty percent of zircons from paragneisses of the sredinny massif contain archean 2800 2500 ma early proterozoic 1700 2100 ma and late proterozoic 1000 ma detrital cores in addition a broad and almost uninterrupted range of phanerozoic detrital cores 460 175 ma is present with jurassic to lower cretaceous 150 96 ma zircons forming the youngest detrital age group ubiquitous documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand tex

magma evolution and open system processes at shiveluch volcano insights from phenocryst zoning phenocryst zoning patterns are used to identify open system magmatic processes in the products of the 2001 eruption of shiveluch volcano kamchatka the lavas and pumices studied are hornblende plagioclase andesites with average pre eruptive temperatures of 840c and fo2 of 1 5 2 1 log units above nickel nickel oxide nno plagioclase zoning includes oscillatory and patchy zonation and sieve textures hornblendes are commonly unzoned but some show simple multiple or patchy zoning apatite microphenocrysts display normal and reverse zoning of sulphur the textural similarity of patchy hornblende and plagioclase together with ba sr systematics in patchy plagioclase indicate that the cores of these crystals were derived from cumulate material plagioclase liquid equilibria suggest that the patchy texture develops by resorption during h2o undersaturated decompression when h2o saturated crystallization recommences at lower pressure reduced ph2o results in lower xan in plagioclase causing more al rich hornblende to crystallize plagioclase cores with diffuse oscillatory zoning and unzoned hornblende crystals probably represent a population of crystals resident in the magma chamber for long periods of time in contrast oscillatory zoning in the rims of plagioclase phenocrysts may reflect eruption dynamics during decompression crystallization increasing fe al in oscillatory zoned rims suggests oxidation as a result of degassing of h2o during decompression a general lack of textural overlap between phenocryst types suggests that different phenocryst populations were spatially or temporally isolated during crystallization we present evidence that the host andesite has mixed with both more felsic and more mafic magmas olivine and orthopyroxene xenocrysts with reaction or overgrowth rims and strong normal zoning indicate mixing with basalt sieve textured plagioclase resulted from mixing of a more felsic magma with the host andesite the mineralogy and mineral compositions of a mafic andesite enclave are identical to those of the host magma which implies efficient thermal quenching and thus small volumes of intruding magma mixing of this magma with the host andesite results in phenocryst zoning because of differences in dissolved volatile contents we suggest that small magma pulses differentiated at depth and ascended intermittently into the growing magma chamber producing incremental variations in whole rock compositions

morphology and dynamic sedimentology of the eastern nile delta shelf coleman j m roberts h h murray s p and salama m 1981 morphology and dynamic sedimentology of the eastern nile delta shelf mar geol 42 301 326 some 13 000 km of bathymetry and overlapping side scan sonar data were collected over a 4450 km2 area of the eastern nile river shelf these data coupled with more detailed surveys of selected areas including bottom sampling and current measurements contribute to the understanding of shelf morphology lithofacies relationships and sediment transport processes four morphologic zones were defined zone 1 smooth bottomed flocculated clays and silty clays extending from the shore line 40 km seaward to the 25 m contour zone 2 a sand belt 5 20 km wide extending eastward from the damietta mouth and curving toward the coast at the boundary of the study area dominant bottom features were migratory bedforms of various dimensions superimposed on linear sand ridges zone 3 smooth clay to silty clay bottom topography punctuated with distinct algal mounds the mounds up to 10 m relief are composed of living coralline algae which contribute coarse debris to the surrounding sediments zone 4 mud diapirs and shelf edge slumps characterize this distal part of the shelf and the upper slope sand on the inner shelf is actively migrating and probably does not represent a relict deposit the damietta distributary causes a large scale perturbation in the mean easterly drift along the egyptian coast unusually strong currents capable of transporting and reworking large volumes of sand are associated with an eddy trapped behind this feature the directionality of these currents and the curvature of the eddy axis correspond well to the distribution pattern of sand on the inner shelf

asymmetric deformation in the backarc region of the kuril arc northwest pacific new insights from analogue modeling 1 the eocene to middle late miocene tectonic evolution of the kuril arc and backarc region has been simulated with analogue experiments the experiments simulate asymmetric deformation in the overriding plate due to anticlockwise rollback of the subducting pacific plate the results show the formation of a n s to ne sw oriented dextral shear zone near the far edge of the retreating boundary analogous to the sakhalin hokkaido dextral shear zone contemporaneously normal faults and grabens form striking parallel to the retreating boundary near the far edge but striking more oblique near the hinge point and away from the retreating boundary this is similar to extensional structures observed in the kuril basin and the sea of okhotsk furthermore the model shows that the amount of extension progressively decreases away from the retreating boundary this appears to have also happened in the kuril okhotsk region as evidenced by crustal thickness variation in the region finally the model results show that extension is increasingly accommodated by the region close to the retreating boundary with progressive deformation this can account for the eocene early miocene extension in the sea of okhotsk followed by miocene spreading in the kuril basin

evidence for cenozoic crustal extension in the bering sea region geophysical and regional geologic data provide evidence that parts of the oceanic crust in the abyssal basins of the bering sea have been created or altered by crustal extension and back arc spreading these processes have occurred during and since early eocene time when the aleutian ridge developed and isolated oceanic crust within parts of the bering sea the crust in the aleutian basin previously noted as presumably early cretaceous in age m1 m13 anomalies is still uncertain some crust may be younger vitus arch a buried 100 to 200 km wide extensionally deformed zone with linear basement structures and geophysical anomalies crosses the entire west central aleutian basin we suggest that the arch and the inferred fracture zones in the aleutian basin are early cenozoic structures related to the early entrapment history of the bering sea these structures lie on trend with known early cenozoic structures near the bowers shirshov aleutian ridge junction and on the beringian continental margin with possible continuation into alaska the structures may have coeval and cogenetic histories for early cenozoic and possibly younger times cenozoic deformation within parts of the bering sea region is principally extensional although the total amount of extension is not known as examples the komandorsky basin formed by back arc seafloor spreading the aleutian ridge has been extensively sheared and extensional block faulting is common sedimentary basins of the bering shelf have formed by extension associated with wrench faulting the cenozoic deformation throughout the bering sea region probably results from the interaction of major lithospheric plates and associated regional strike slip faults we present models for the bering sea over the past 55 m y that show oceanic plate entrapment back arc faulting and spreading along vitus arch breakup of the oceanic crust in the aleutian basin at fracture zones and back arc spreading in bowers basin

40 ar 39 ar dating and tectonic implications of volcanic rocks recovered at iodp hole u1342a and d on bowers ridge bering sea abstract during the integrated ocean drilling program iodp a total of 41 54 m of basement rock consolidated volcaniclastic sediment was recovered beneath a thin sediment cover the drilled site is at the eastern end of the crestal area of bowers ridge a north and westward sweeping offshoot of the aleutian arc into the bering sea the volcanic sequence recovered from holes u1342a and u1342d was divided into six major lithologic units we used the single grain 40 ar 39 ar dating method performed by step wise heated laser fusion technique to date andesites of unit 1 thereby two ages of oligocene volcanism 34 32 ma 28 26 ma were distinguished each other according to our 40 ar 39 ar data these ages refute a hypothesized cretaceous origin in the north pacific as an exotic arc massif or sector of the hawaiian emperor chain and indicate that the bowers ridge is a bering sea formed arc or remnant arc that ceased forming in the latest oligocene to the earliest miocene time

evidence of pre late wisconsinan glaciations on labrador shelf cartwright saddle region high resolution huntee and air gun seismic reflection profiles from cartwright saddle indicate the presence of at least three major glacial advances which deposited giacial till sequences up to 100 m in composite thickness on the bottom and flanks of the saddle these tills are found in water depths up to 600 m and their acoustic character suggests deposition from grounded ice grounded ice at such depths and the presence of tills extending to the shelf edge indicate major glacial episodes a combination of surface morphology and high resolution seismic profile data reveals the presence of an extensive lateral moraine that flanks the southern edge of the saddle suggesting that an ice tongue filled the saddle numerous areas of pinch out of the upper till are recognized along the lateral moraine and near the shelf edge in the deepest part of the saddle high resolution seismic records across a core site obtained earlier by vilks show the core penetrates the upper part of a postglacial stratified silt s

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

classification of sedimentary furrows and a model for furrow initiation and evolution sedimentary furrows are longitudinal bed forms which form in fine grained cohesive sediments of the deep sea floor as well as in similar sediments in estuaries and in large lakes deep sea furrows have been observed where bottom currents often flow in one direction at 5 to 20 cm sec currents are much stronger greater than 50 cm sec in furrowed estuarine environments but sediment accumulation rates are higher a review of furrows studied in a variety of locations and environments suggests that these bed forms fall into two broad morphological classes those developed in muddy cohesive sediments tend to have steep walls and flat floors while those developed in carbonate rich cohesive sediments are broad and have gently sloping walls at abyssal depths furrows can often be detected by their distinctive echo patterns on 3 5 khz echo sounders a series of similar evenly spaced hyperbolic echoes which often have their apices tangent to the sea floor however different furrow cross sections may generate distinctive hyperbolic echo patterns a model for furrow initiation and development in muddy cohesive sediments suggests that furrows develop in depositional environments swept by recurring directionally stable and episodically strong currents furrows are initiated when secondary circulations in the boundary layer align coarse generally light debris in sand ribbon like bed forms as the coarse light sediments move along the sea floor they abrade small longitudinal troughs into the muds the grooves may fill in during intervals between strong current events if sedimentation rates are too high once a furrow is established on the sea floor further development may be controlled by the balance between sediment deposition and erosion where deposition occurs continuously and erosion occurs only episodically during brief periods of increased current strength narrow furrows width 1 5 to 1 15 of spacing appear to develop where deposition exceeds erosion whereas wide furrows width 1 2 of spacing develop where erosion exceeds or equals deposition dissolution may play an important role in the formation and evolution of furrows in carbonate rich cohesive sediments secondary circulation patterns over established furrows may be strongly controlled by bed form topography

a 210pb 226ra 230th 238u study of klyuchevskoy and bezymianny volcanoes kamchatka abstract lavas from klyuchevskoy and bezymianny volcanoes kamchatka appear to show a link between the extent of partial melting in their mantle source region and the subsequent degree of fractionation suffered by the magmas during passage through the crust this fractionation may have occurred on timescales significantly less than 1000 years if observed 226 ra excesses largely reflect variable residual porosity in the source melting region unlike most arc lavas those with the highest mgo contents and ba th ratios have the lowest 226 ra excess forward models suggest that those portions of the source which had undergone the greatest addition of u by fluids from the subducting plate also underwent the greatest extents of partial melting at the highest residual porosity at kluchevskoy a change from eruption of high mgo to high al 2 o 3 basaltic andesites around 1945 is reflected in an increase in size of 226 ra excess which seems to require a simultaneous decrease in residual porosity and suggests a rapid changes in the melting regime the eruption of andesites at bezyminanny simultaneous with the eruption of basaltic andesites at klyuchevskoy further suggests that different degree melts produced at differing residual porosity can be formed and extracted from the melt region at the same time thus the melting processes beneath klyuchevskoy and bezyminanny are demonstrably complex they have clearly been influenced by both fluid addition from the subducting plate and extension and decompression beneath the central kamchatka depression finally the 210 pb data are with one or two exceptions in equilibrium with 226 ra suggesting that there was restricted relative magma gas movement in this highly productive magmatic system

processes controlling the characteristics of the surficial sand sheet u s atlantic outer continental shelf abstract knebel h j 1981 processes controlling the characteristics of the surficial sand sheet u s atlantic outer continental shelf mar geol 42 349 368 a review of recent data on the velocity of bottom currents the frequency of bottom sediment movement the kinds and amounts of suspended sediments in near bottom waters and the acoustic and sedimentary features of subbottom strata indicates that the characteristics of the ubiquitous sand sheet on the atlantic outer continental shelf of the united states have been controlled by a variety of past and present processes although these processes collectively have had a widespread effect on the characteristics of the sand sheet the relative importance of each process changes geographically on georges bank late pleistocene glaciations along with modern tidal currents and the regional circulation pattern have played a dominant role on the middle atlantic shelf ancestral rivers former near shore processes and modern wind and wave generated currents are important factors on the south atlantic shelf the sediments reflect subaerial weathering erosion or nondeposition over or near hardgrounds and the production of biogenic carbonate other processes such as the movement of water masses bioturbation and bottom fishing probably have affected the sediments in all areas a knowledge of the various factors affecting the sand sheet is fundamental to an understanding of its general geologic history and to the paleoenvironmental interpretation of ancient sand strata

large scale deformation related to the collision of the aleutian arc with kamchatka the far western aleutian island arc is actively colliding with kamchatka westward motion of the aleutian arc is brought about by the tangential relative motion of the pacific plate transferred to major right lateral shear zones north and south of the arc early geologic mapping of cape kamchatka a promontory of kamchatka along strike with the aleutian arc revealed many similarities to the geology of the aleutian islands later studies support the notion that cape kamchatka is the farthest west aleutian island and that il has been accreted to kamchatka by the process of arc continent collision deformation associated with the collision onshore kamchatka includes gravimetrically determined crustal thickening and formation of a narrow thrust belt of intensely deformed rucks directly west of cape kamchatka the trend of the thrust faults is concave toward the collision zone indicating a radial distribution of maximum horizontal compressive stress offshore major crustal faults trend either oblique to the kamchatka margin or parallel to major aleutian shear zones these offshore faults are complex accommodating both strike slip and thrust displacements as documented by focal mechtmisms and seismic reflection data earthquake activity is much higher in the offshore region within a zone bounded to the north by the northernmost aleutian shear zone and to the west by an apparent aseismic front analysis of focal mechanisms in the region indicate that the present day arc continent contact zone is located directly east of cape kamchatka in modeling the dynamics of the coiiision zone using thin viscous sheet theory the rhealogical parameters are only partially constrained to values of n the effective power law exponent 2 3 and ar the argand number s 30 these values are consistent with a fnrcarc thermal profile of kamchatka previously determined from heat flow modeling the thin viscous sheet modeling also indicates that onshore thrust faulting is a consequence not only of compressive stresses resulting from the west directed collision but also of sediment induced coupling of the subducting pacific plate

mechanisms for linear sandbank formation and maintenance in relation to dynamical oceanographic observations abstract theories associated with flow in the vicinity of headlands and sandbanks together with proposed mechanisms for sandbank formation and maintenance are reviewed to exemplify water and sand movement around a sandbank on a tidally dominated shelf tidal currents in the vicinity of a headland associated linear sandbank are described these data are integrated with sedimentological information on bedforms and grain size distributions the sandbank under detailed investigation the scarweather sands is located along the northern coastline of the bristol channel uk which is an area susceptible to high energy inputs the maximum spring tidal range here is around 10m and a 6000km fetch is available for storm wave approach from the atlantic ocean but the area is 460km from the shelf break detailed observations are presented from 11 long term current meter stations located within 10km of the sandbank currents are described in terms of residual water circulation shallow water harmonics and tidal ellipse characteristics bedload transport paths are inferred from the distribution of bedforms grain size and m2 m4 tidal current interactions transport vectors are predicted from near bed current meter observations combined with empirically derived formulae it is demonstrated that water circulation and sand transport paths form part of a clockwise eddy system around the sandbank mechanisms controlling the location and the characteristics of this particular sandbank are shown to be explicable in terms of a combination of models for sandbank formation and maintenance the results of the present study are thought to be applicable directly to headland associated sandbanks elsewhere it is shown on the basis of theoretical considerations that the pattern of residual water movement and possibly sand transport around this sandbank could be extended to continental shelves on a global basis

fallout and distribution of volcanic ash over argentina following the may 2008 explosive eruption of chaiten chile 1 the major explosive eruption of chaiten volcano chile in may 2008 provided a rare opportunity to track the long range dispersal and deposition of fine volcanic ash the eruption followed 10 000 years of quiescence was the largest explosive eruption globally since hudson chile in 1991 and was the first explosive rhyolitic eruption since novarupta alaska in 1912 field examination of distal ashfall indicates that 1 6 1011 kg of ash dense rock equivalent volume of 0 07 km3 was deposited over 2 105 km2 of argentina during the first week of eruption the minimum eruption magnitude estimated from the mass of the tephra deposit is 4 2 several discrete ashfall units are identifiable from their distribution and grain size characteristics with more energetic phases showing a bimodal size distribution and evidence of cloud aggregation processes ash chemistry was uniform throughout the early stages of eruption and is consistent with magma storage prior to eruption at depths of 3 6 km deposition of ash over a continental region allowed the tracking of eruption development and demonstrates the potential complexity of tephra dispersal from a single eruption which in this case comprised several phases over a week long period of intense activity

multichannel seismic evidence bearing on the origin of bowers ridge bering sea bowers ridge is a large arcuate sub marine ridge that extends north and west from the aleutian ridge and separates the abyssal aleutian and bowers basins in the bering sea two multichannel seismic reflection lines recorded in 1976 over bowers ridge and the adjacent basins confirm the existence of 8 to 10 km thick sediment wedges on the north side of bowers ridge and at the base of the bering continental margin deformed sediment within the bowers wedge indicates that subduction of the adjacent ocean crust beneath the ridge probably occurred prior to middle cenozoic time flat lying reflectors near the bottom of the trench suggest that a bathymetric trough and large ridge existed in mesozoic time the major period of underthrusting subsidence and in filling of the trench probably occurred from mesozoic to early tertiary time small amounts of underthrusting may have continued after the early tertiary development of the aleutian ridge however by middle miocene time the formerly subaerial bowers ridge had subsided below sea level the multichannel seismic data do not show evidence for a buried spreading center within the eastern aleutian basin consequently the sediment wedges trenches at both bowers ridge and the bering continental margin are believed to be the consequence of subduction that occurred during the convergence of the ridge and the margin the large size of bowers ridge suggests that a large amount of convergence has occurred since mesozoic time if bowers ridge was a large feature in mesozoic time as suggested by the apparent bathymetric trough then the ridge may be as old or older than the aleutian ridge to which it connects

tephrochronology of the southernmost andean southern volcanic zone chile correlations among and identification of the source volcanoes for over 60 late glacial and holocene tephras preserved in eight lacustrine sediment cores taken from small lakes near coyhaique chile 46 s were made based on the stratigraphic position of the tephra in the cores lithostratigraphic data tephra layer thickness and grain size and tephra petrochemistry glass color and morphology phenocryst phases and bulk tephra trace element contents determined by icp ms the cores preserve a record of explosive eruptions since 17 800 calibrated years before present cal years bp of the volcanoes of the southernmost andean southern volcanic zone ssvz the suggested source volcanoes for 55 of these tephras include hudson 32 events mentolat 10 events and either maca or cay or some of the many minor monogenetic eruptive centers mecs 13 events in the area only four of these eruptions had been previously identified in tephra outcrops in the region indicating the value of lake cores for identifying smaller eruptions in tephrochronologic studies the tephra records preserved in these lake cores combined with those in marine cores which extend these records back to 20 000 cal years bp prior to the last glacial maximum suggest that no significant temporal change in the frequency of explosive eruptions was associated with deglaciation over this time period hudson volcano one of the largest and longest lived volcanoes in the southern andes has had 55 eruptions four of them were very large and has produced 45 km3 of pyroclastic material making it also one of the most active volcanoes in the svz in terms of both frequency and volume of explosive eruptions

the hudson shelf valley its role in shelf sediment transport abstract freeland g l stanley d j swift d j p and lambert d n 1981 the hudson shelf valley its role in shelf sediment transport mar geol 42 399 427 the hudson shelf valley was deeply incised by pleistocene lowstands of the sea during the holocene transgression fluvial estuarine and estuary mouth depositional environments were displaced landward and sedimentation in these environments partially filled the valley holocene sediment thickness in the seaward half of the valley is about the same as on the shelf to either side but in the landward half of the valley 22 m of sediment is present versus 5 10 m on the adjacent shelf sediment volume calculations in the upper shelf valley show that there is about three times the infill volume east of the thalweg as compared to the infill to the west this configuration indicates that the valley has served as a sediment trap and records the westward migration of the thalweg during the holocene most of the infill was the result of littoral drift which transported sand westward along the south shore of long island during the holocene transgression modern sediment transport as calculated from current meter observations and inferred from bedform observations is mainly south westward along the regional trend of the isobaths and across the shelf valley therefore infill on the eastern side of the shelf valley is continuing at present observations of the regional distribution of sediment types indicate that the asymmetrical valley fill is part of a pattern of bottom response to flow the former drainage divides on both sides of the shelf valley are surfaced by coarse lag deposits on slopes facing northeast into the direction from which the major storm flows come while the down current sides are mantled with fine sand

formation and reactivation of the cambrian kanmantoo trough se australia implications for early palaeozoic tectonics at eastern gondwana s plate margin the kanmantoo group is a thick and largely metamorphosed sedimentary succession that filled an isolated arcuate cambrian basin kanmantoo trough which formed within continental gondwana and now lies on the southern margin of the present australian continent kanmantoo group sediments unconformably overlie neoproterozoic and older cambrian rocks we consider that the geometry of the southern part of this trough was influenced by strike slip movement along an intra continental tear fault to the north the basin changes to a style dominated by orthogonal extension and eventually tapers and dies out normal to the tear fault balanced sections show that the kinematic style and strain distribution developed during early palaeozoic inversion was controlled by the specific architecture of the kanmantoo trough early palaeozoic tear faulting could have linked contrasting subduction polarities along the then contiguous palaeo pacific margin of gondwana the kanmantoo trough is considered to have formed at a passive margin related to east directed subduction in what is now the australian continent in contrast west directed subduction formed an active margin at contiguous parts of current antarctica kanmantoo group sediments were derived from the south by erosion of a grenvillean province mixed with sediments eroded from the emergent active margin of gondwana the inception localization and sedimentation in the kanmantoo trough reflects a complex interaction of tectonic processes along the encroaching ross delamerian orogen

plate tectonic reconstructions predict part of the hawaiian hotspot track to be preserved in the bering sea we use plate reconstructions to show that parts of the hawaiian hotspot track of ca 80 90 ma age could be preserved in the bering sea based on these reconstructions the hawaiian hotspot was beneath the izanagi plate before ca 83 ma around that time the part of the plate carrying the hotspot track was transferred to the kula plate after 75 80 ma the hawaiian hotspot underlay the pacifi c plate circa 40 55 ma subduction initiated in the aleutian trench part of the kula plate was attached to the north american plate and is preserved as the oceanic part of the bering sea we show that for a number of different plate reconstructions and a variety of assumptions covering hotspot motion part of the hotspot track should be preserved in the bering sea the predicted age of the track depends on the age of aleutian subduction initiation we speculate that bowers and shirshov ridges were formed by paleohawaiian hotspot magmatism

offbank transport of carbonate sands along open leeward bank margins northern bahamas abstract vigorous offbank transport of carbonate sands occurs along the west facing leeward open margins of little and great bahama bank large offbank oriented sand waves thick sand bodies covering reefs and shallow water sands mostly non skeletal obtained from adjacent deep flanks all demonstrate the existence of this shallow to deep pathway of sediment transport a variety of data suggests that transport occurs during storms and not during normal tidal current fluctuations an abundance of composite grains as well as vertical sequences of submarine cemented horizons seen via seismic profiling within the marginal sand bodies indicate cyclic periods of relative quiescence followed by moments of intense sediment movement current meter data obtained during six weeks of low wind activity show that critical threshold velocities for sand transport are only barely exceeded during portions of the tidal cycle these flows coupled with higher frequency bottom currents generated by surface gravity waves winnow the sands but do not provide significant net offbank transport offbank sand transport provides material to the shallow and deep bank margins enabling them to expand during sea level highstands each sea level fluctuation is likely to produce within the shallow margins a vertical sequence of basal reefs covered by a thick non skeletal calcarenite capped by a subaerially exposed diagenetically altered surface shallow water sands carried off the marginal escarpment help to construct deep flanks and large sediment drifts

provenance of the palaeocene eocene clastic rocks of the komandorsky islands the palaeocene eocene sequence on the komandorsky islands is represented by sediments and minor volcanics with a total thickness of more than 3000 m sandstone siltstone mudstone siliceous rocks and conglomerate which have accumulated in the upper and middle parts of a deep water fan prevail all the debris was delivered from the north east from the side of the contemporary komandorsky basin in the bering sea products of volcanic denudation and fresh tephra dominate its composition however at some levels the sandstone contains fragments of jasper quartzite slate schist gabbro and granite as well as non volcanic quartz along with a volcanic component the heavy minerals fraction of the sandstone contains considerable quantities of apatite zircon garnet and chromite as well as pyroxenes amphiboles and magnetite the chemical composition of the garnet is specific to metamorphic rocks a significant admixture of chromite is indicative of denudation from an ophiolite complex the folded and partly metamorphosed pre palaeogenic rocks were a source of these fragments their most likely source terrane was the shirshov bowers chain cretaceous palaeoarc the komandorsky block originally was the forearc part of the aleutian arc where the latter was superimposed on the shirshov bauers chain a subsequent north west displacement of forearc structures in response to the transform fault at the boundary between the pacific and north american plates could have resulted in the separation of the komandorsky block from its early palaeogene debris source area

bedforms of the canadian eastern seaboard a comparison with global occurrences this paper presents a compilation of sedimentary bedforms identified from acoustic and seismic records collected over the southeastern canadian continental shelf in order to make the compilation it was found necessary to firstly review and synthesise the various terminologies used to describe the bedforms found and thereafter to relate the subsequent terminology to any diagnostic generic characteristic identifiable from the records sand waves are considered to be those forms that are composite in nature and are generated solely by the action of superimposed megaripples these latter features are identifiable adopting the classification of dalrymple et al 1978 which proves adequate for the acoustic differentiation of 2 d and 3 d types though we observed a greater coherence in bedform spacing of the 2 d types than reported hitherto sand ridges were adequately defined and recognised using the description of swift et al 1978 with respect to storm generated ridges and that of off 1963 and others with respect to tidal current ridges such ridges appear not to be uniquely described by ratios of their metrics the bedforms on the southeastern canadian continental shelf are abundant and wide spread virtually all types of bedforms recognisable on the u s shelf are present to the north the majority of these bedforms appear to be storm generated and are considered intermittently active many of the sand waves s s have no measureable relief though retaining their distinctive spatial geometry both active and moribund sand ridges were found of these ridges both tidal and storm generated types were identified the storm generated ridges were further broken down into shoreface connected ridges and offshore ridges a metrical analysis of the various bedforms show that a differentiation into stable bed phases can be made using height length ratios provided the bedforms are active and fully developed there is some overlap between megaripples and small sand waves and indeed between 2 d and 3 d megaripples yet the distinction is good between sand waves and sand ridges even moribund types sand ribbons in general can be confused with moribund sand waves and megaripples a detailed morphological examination is required to distinguish each feature data from sand ridges of the eastern canadian shelf combined with data from similar features found globally shows that ridge heights tend to cluster into three distinctive modes these modes are at heights of 7 13 and 27 m the first mode is correlated with storm generated ridges the second with tidal current ridges and the third possibly with submerged barrier islands

stratigraphy age and correlation of lepue tephra a widespread c 11 000 cal a bp marker horizon sourced from the chaiten sector of southern chile we describe the stratigraphy age and correlation of a prominent tephra marker named lepue tephra extensively distributed in north western patagonia lepue tephra is well dated at c 11 000 cal a bp from numerous lake and soil cover bed sequences and its recognition is useful for assessing the rate and timing of deglaciation as well as associated environmental changes in this region during the last glacial termination and early holocene lepue tephra has attributes typical of a complex and compositionally zoned phreatomagmatic eruptive while the initial rhyolitic phase can be readily distinguished from multiple eruptive products sourced from the adjacent volcan chaiten the main erupted end member is of basaltic andesitic bulk composition similar to younger tephras sourced from holocene monogenetic cones adjacent to the volcan michimahuida massif tmim lepue tephra can be correlated to an equivalent aged pyroclastic flow deposit amarillo ignimbrite prominently distributed in the south eastern sector of tmim the source vent for these co eruptive events is obscured by an extensive ice field and is currently unknown the widespread radially symmetrical distribution of lepue tephra centred on tmim cannot be attributed solely to volcanological considerations reduced southern hemisphere westerly wind influence interpreted from climate proxies at the time of eruption are also implicated

degassing of cl f li and be during extrusion and crystallization of the rhyolite dome at volcan chaiten chile during 2008 and 2009 we investigated the distribution of cl f li and be in pumices obsidians and crystallized dome rocks at chaiten volcano in 2008 2009 in order to explore the behavior of these elements during explosive and effusive volcanic activity electron and ion microprobe analyses of matrix and inclusion glasses from pumice obsidian and microlite rich dome rock indicate that cl and other elements were lost primarily during crystallization of the rhyolitic dome after it had approached the surface glass in pumice and microlite free obsidian has 888 121 ppm cl whereas residual glass in evolved microlite rich dome rock generally retains less cl as low as 0 7 mt cl with a potential maximum of 1 8 mt for the entire 0 8 km3 dome elemental variations reflect an integrated bulk distribution ratio for cl 1 7 1 7 times more cl was degassed or incorporated into crystals than remained in the melt because cl is lost dominantly as the very last h2o is degassed and cl is minimally if at all partitioned into microlites the integrated vapor melt distribution ratio for cl exceeds 200 200 times more cl in the evolved vapor than in the melt cl is likely lost as hcl which is readily partitioned into magmatic vapor at low pressure cl loss is accelerated by the change in the composition of the residual melt due to microlite growth cl loss also may be affected by open system gas fluxing integrated vapor melt distribution ratios for li f and be all exceed 1 000 on degassing an unknown fraction of these volatiles could be immediately dissolved in rainwater

the origin of summit basins of the aleutian ridge implications for block rotation of an arc massif it is proposed that many summit basins along the aleutian arc form from the clockwise rotation of blocks of the arc massif summit basins are arc parallel grabens or half grabens formed within the arc massif and are commonly located near or along the axis of late cenozoic volcanism geomorphically the aleutian arc appears to consist of contiguous rhombic blocks of varying size tens to hundreds of kilometers in length the boundaries between adjacent blocks are delineated by fault controlled canyons that cut the southern slope of the arc transverse to its regional trend evidence that these blocks have rotated clockwise is provided by the triangular shaped summit basins bordering the blocks to the north oblique physiographic trends offsets in the summit platform and broad deflections in the southern slope of the arc we present a model for block rotation that involves translation of blocks parallel to an arc it is suggested that block rotation which appears to have accelerated in late cenozoic time is linked to 1 a shift in the euler pole for the pacific plate 2 the consequential start up of late cenozoic volcanism 3 improved interplate coupling instigated by sediment flooding of the aleutian trench and 4 westward subduction of northeast striking segments of the inactive kula pacific ridge

early evolution of the bering sea by collision of oceanic rises and north pacific subduction zones three major bathymetric features exist in the bering sea shirshov ridge bowers ridge and umnak plateau new refraction data over umnak plateau and previous geophysical data across bowers ridge indicate that a thickened welt of crustal material is present beneath both features the crustal structure is transitional between oceanic and continental types various models for the origin of these features have been investigated one that has not been proposed previously assumes that the protostructures of bowers ridge and umnak plateau could have formed outside of the present bering sea according to this model before formation of the aleutian ridge in late mesozoic or earliest tertiary time these protostructures moved into their present bering sea positions prior to the arrival of these two structures in the bering sea oceanic crust was subducted along the bering continental margin connecting alaska and siberia the collision of the umnak plateau protostructure with the southeastern edge of the margin may have caused subduction to terminate here and move southward the new southerly position of subduction beneath the aleutian ridge was therefore controlled by late mesozoic or early tertiary locations of umnak plateau bowers ridge and possibly the north trending shirshov ridge farther to the west

thermal modeling of subducted plates tear and hotspot at the kamchatka corner pacific plate subduction at the aleutian kamchatka juncture or corner could be accommodated by either a large bend or a tear in the oceanic lithosphere in this paper we describe a number of observations which suggest that the pacific plate terminates abruptly at the bering transform zone tz seismicity shoals along the subduction zone from southern kamchatka 600 km to relatively shallow depths near the kamchatka bering fault intersection 100 200 km this seismicity shoaling is accompanied by an increase in the heat flow values measured on the pacific plate moreover unusual volcanic products related to adakites are erupted on kamchatka peninsula at the juncture simple thermal modeling shows that a slab torn and thinner along the northern edge of the pacific plate would be compatible with the observations delayed thickening of the lithosphere due to the meiji hawaiian hotspot may be responsible for the required thinning d 2004 published by elsevier b v

tertiary tectonics of the sea of okhotsk russia far field effects of the india eurasia collision a new structural map of the western sea of okhotsk based on grids of multichannel seismic data provides several new insights into the tertiary deformation of asia especially in regard to the extension of extrusion tectonics linked to the india eurasia collision into northeastern asia the sedimentary basins in this offshore region are the result of two regional shear systems in the west the north trending sakhalin hokkaido dextral fault system transects sakhalin island accessory structures closely related to its dextral shear include northeast trending normal faults as well as northwest trending en echelon folds and thrusts the accessory normal faults are predominantly eocene to lower miocene and indicate transtensional shear whereas the folds and thrusts are mostly younger and indicate late miocene and pliocene transpression in the north an east west trending sinistral shear zone enters the sea at its northwest corner this sinistral shear system links farther west to extensional faults of the early tertiary baikal rift extending northeastward from the terminus of this sinistral shear zone are a series of long predominantly northwest dipping listric normal faults that form a large lazy s shaped pull apart basin the shantar liziansky basin slb normal faults in this 500 140 km basin accommodate 15 to 20 extension extension and related sinistral shear appear to be largely eocene to oligocene in age with lesser later activity to the northeast of slb a further extension of the sinistral shear zone appears to bend northeastward transecting pustorets and penzhina basins and following the course of the older mesozoic mongol okhotsk chukotsk active margin and suture it possibly reaches as far as the arctic as suggested by previous workers the baikal rift and its associated sinistral shear zone may be interpreted as by products of the india eurasia collision in eocene time mapping results included here show that this diffuse sinistral shear system extends much farther to the northeast the sinistral shear zone is transected by the dextral sakhalin hokkaido fault zone in the kashevarov region of central slb where a fan of north to northwest striking late tertiary transpressional fault splays marks the northern termination of the dextral fault zone older slb normal faults are locally deformed and half grabens are partly inverted by the slightly younger dextral splays the two regional fault systems are essentially conjugate and the pattern of intersection closely resembles that predicted by recent modelling studies of india eurasia collision tectonics

synchronisation of sedimentary records using tephra a postglacial tephrochronological model for the chilean lake district abstract well characterised tephra horizons deposited in various sedimentary environments provide a means of synchronising sedimentary archives the use of tephra as a chronological tool is however still widely underutilised in southern chile and argentina in this study we develop a postglacial tephrochronological model for the chilean lake district ca 38 to 42 s by integrating terrestrial and lacustrine records tephra deposits preserved in lake sediments record discrete events even if they do not correspond to primary fallout by combining terrestrial with lacustrine records we obtain the most complete tephrostratigraphic record for the area to date we present glass geochemical and chronological data for key marker horizons that may be used to synchronise sedimentary archives used for palaeoenvironmental palaeoclimatological and palaeoseismological purposes most volcanoes in the studied segment of the southern volcanic zone between llaima and calbuco have produced at least one regional marker deposit resulting from a large explosive eruption magnitude 4 some of which now have a significantly improved age estimate e g the 10 5 ka llaima pumice eruption from llaima volcano others including several units from puyehue cordon caulle are newly described here we also find tephra related to the cha1 eruption from chaiten volcano in lake sediments up to 400 km north from source several clear marker horizons are now identified that should help refine age model reconstructions for various sedimentary archives our chronological model suggests three distinct phases of eruptive activity impacting the area with an early to mid holocene period of relative quiescence extending our tephrochronological framework further south into patagonia will allow a more detailed evaluation of the controls on the occurrence and magnitude of explosive eruptions throughout the postglacial

formation of cristobalite nanofibers during explosive volcanic eruptions high resolution transmission electron microscopy hrtem observations of unaltered volcanic air fall deposits from the ongoing lava dome explosive eruption at chaiten volcano chilean patagonia revealed the presence of highly crystalline silica nanofibers in the respirable fraction of the volcanic ash 240 c beta form of cristobalite with average lengths of hundreds of nanometers and widths on the order of tens of nanometers we propose that the beta cristobalite nanofibers are formed during explosive eruptions by the reduction of amorphous silica by carbon monoxide to its reactive suboxide sio which is later oxidized to form one dimensional crystalline silica nanostructures nucleation and growth of the nanofibers are enhanced by the high surface area of the micrometer to nanometer sized fragments of silica glass in the volcanic column the formation of nanocrystalline cristobalite fibers during explosive lava dome eruptions poses new challenges for the assessment of the short and long term health hazards associated with the respirable nanofibrous components of volcanic ash

an 18 000 year long eruptive record from volcan chaiten northwestern patagonia paleoenvironmental and hazard assessment implications abstract the 2008 eruption of volcan chaiten vcha in northwestern patagonia was the first explosive rhyolitic eruption to have occurred within a century and provided an unprecedented scientific opportunity to examine all facets of the eruption ranging from magma rheology ascent rates to ash fall effects on biota and infrastructure up to very recently it was thought that the latest eruption prior to the 2008 event occurred c 9750 cal a bp although a number of researchers have recognised additional eruptive products but their stratigraphy age and geochemical attributes have not been systematically described and or recorded in this study we provide a detailed examination of andic cover beds and tephra bearing lake sequences located both proximally and distally to vcha which record a series of hitherto unknown rhyolitic eruptive products and place all previous observations firmly within a coherent stratigraphic framework through major and trace element glass shard geochemistry we are able to confidently verify eruptive source a total of 20 discrete tephra beds are recognised with at least 10 having widespread areal distributions and or depositional imprints broadly comparable to or greater than the 2008 tephra event this record indicates that vcha has been continuously but intermittently active as far back as the end of the last glacial maximum c 18 000 cal a bp with two dominant genetically related magma types and an intermediary mixed type before this the eruptive record has been largely obscured and or erased by widespread andean piedmont glaciation however based on the tempo of vcha activity over the last c 18 000 years older vcha eruptives can be anticipated to occur as well as future hazardous explosive events the new eruptive inventory will ultimately be useful for correlating equivalent aged sequences and refining long term eruptive tempo as well as corresponding temporal changes in magmatic evolution

origin of marginal basins of the nw pacific and their plate tectonic reconstructions abstract geometry of basins can indicate their tectonic origin whether they are small or large the basins of bohai gulf south china sea east china sea japan sea andaman sea okhotsk sea and bering sea have typical geometry of dextral pull apart the java makassar celebes and sulu seas basins together with grabens in borneo also comprise a local dextral transform margin type basin system similar to the central and southern parts of the shanxi basin in geometry the overall configuration of the philippine sea resembles a typical sinistral transpressional pop up structure these marginal basins except the philippine sea basin generally have similar or compatible rift history in the cenozoic but there do be some differences in the rifting history between major basins or their sub basins due to local differences in tectonic settings rifting kinematics of each of these marginal basins can be explained by dextral pull apart or transtension these marginal basins except the philippine sea basin constitute a gigantic linked dextral pull apart basin system formation of the gigantic linked dextral pull apart basin system in the nw pacific is due to nne to ene ward motion of east eurasia this mainly was a response to the indo asia collision which started about 50 ma ago the displacement of east eurasia can be estimated using three aspects 1 the magnitude of pull apart of the dextral pull apart basin system 2 paleomagnetic data from eastern eurasia and the region around the arctic and 3 the shortening deficits in the large tibetan plateau all the three aspects indicate that there was a large amount 1000 to 1200 km of northward motion of the south china block and compatible movements of other blocks in eastern eurasia during the rifting period of the basin system such a large amount of motion of the eastern eurasia region contradicts any traditional rigid plate tectonic reconstruction but agrees with the more recent concepts of non rigidity of both continental and oceanic lithosphere over geological times based on these three estimates the method developed for restoration of diffuse deformation of the eurasian plate and the region around the arctic and the related kinematics of the marginal basins we present plate tectonic reconstructions of these marginal basins in global plate tectonic settings at the four key times 50 35 15 and 5 ma the plate tectonic reconstructions show that the first order rift stage and post rift stage of the marginal basins are correlated with the first order slow uplift stage and the rapid uplift stage of the tibetan plateau respectively the proto philippine sea basin was trapped as a sinistral transpressional pop up structure at a position that was 20 south of its present position at about 50 ma ago or earlier while the japan arc migrated eastward during the rifting period of the japan sea basin the shikoku basin opened and the parece vela basin widened

a study of the dynamics of a marine sandwave the movement of the crest of a sandwave was studied using cross sectional profiles obtained from lines of sea bed reference stakes measurements were made over a six month period before and after flood and ebb tides in relation to both spring and neap tides and surface wave conditions additional observations were obtained on a daily basis over an equinoctial neap to spring to neap tidal period in conjunction with boundary layer flow measurements tracer experiments were conducted to study the dispersion of sediment from the sandwave crest the results showed that the sandwave was relatively stable at neap tides whilst at higher tidal ranges the crest position oscillated with successive flood and ebb tides net flank erosion occurred on the less steep upstream slope during the dominant ebb tide this together with increased deposition on the lee slope caused the crest to advance it was not possible to extrapolate sandwave migration over long periods as the tidal dynamic trends were interrupted by wind stress and surface wave activity high particle orbital velocities generated at the sea bed by storm waves caused major reductions in crestal heights calculated volumes of sediment eroded and accreted were used with boundary layer flow measurements to calculate threshold velocities for the movement of the sediment and sediment transport rates

the role of non uniform roughness in the formation of sand ribbons abstract sand ribbons typically are shallow water sedimentary features characterized as thin elongate bands of sand overlying a coarser lag sediment which is exposed between them the ribbons are oriented along the direction of the dominant current and are formed by a secondary circulation pattern that includes a small component of the boundary shear stress directed toward the center of the sand ribbons a perturbation expansion coupled with flume experiments indicates that this circulation pattern is linked to differences in the bottom roughness between the sand ribbons and the adjacent coarser strips the non uniform turbulence field arising from this roughness distribution drives a secondary helical flow the expansion indicates that there is a preferred ratio of sand ribbon spacing to water depth 4 1 for unstratified channel flow conditions the secondary flow pattern forces the downstream flow to readjust so that the boundary shear stress over the smooth sand strips is considerably smaller than the mean stress this serves to retard the movement of sand through such a system

transition from arc to oceanic magmatism at the kamchatka aleutian junction spatial geochemical variations of quaternary lavas erupted along the northern segment of the kamchatka arc are used to trace changes in magma generation across the subducting pacific slab edge the late pleistocene holocene lavas of the northern end of the central kamchatka depression north of the pacific slab edge show strong enrichment in high field strength elements and light rare earth elements relatively unradiogenic strontium and lead but radiogenic neodymium isotope ratios and oxygen isotope compositions similar to those of mid oceanic ridge basalts these geochemical characteristics are distinct from the southern central kamchatka depression volcanoes located above the subducting pacific slab extensive fluid triggered mantle melting dominates magma genesis beneath the largest kamchatka volcanoes in the south whereas low degree decompression melting of the pacific asthenospheric mantle is the major magma generation process north of the pacific slab edge quaternary detachment of the subducted pacific plate fragment resulted in the influx of fertile mantle beneath kamchatka we propose that upwelling and southward flow of this hotter more fertile mantle is the main reason for recent magmatism in northern kamchatka and for the exceptional productivity of the central kamchatka depression volcanoes klyuchevskoy and sheveluch the most active arc volcanoes on earth

strain partitioning and interplate friction in oblique subduction zones constraints provided by experimental modeling physical modeling of oblique subduction is performed to study the mechanism of strain partitioning the model is two layer and includes the elasto plastic lithosphere the overriding and subducting plates and the low viscosity liquid asthenosphere the subduction is driven by a push force from a piston and a pull force when the density contrast ap between the subducting plate and the asthenosphere is positive we vary both ap and the interplate friction frictional stresses slip partitioning is obtained only in the models with high interplate friction and only when the overrid ing plate contains a weak zone this zone in the models corresponds either to locally thinned litho sphere or to cut fault the horizontal trench normal component of the interplate friction force ffh can be comparable with the absolute value of the horizontal component of the nonhydrostatic interplate pressure force fph in the subduction zone ffh is always negative compression while fph can be either negative compressional subduction regime or positive extensional regime high friction which promotes partitioning can completely cancel the extensional suction force fph back arc tension and strike slip faulting appear thus as conflicting processes although they can coexist in the same subduction zone depending on the relative values of relevant forces it ap pears that high friction can exist only in compressional subduction zones where partitioning should develop more easily this conclusion is supported by the comparison of two oblique subduction zones having similar geometry the compressional southern kurile zone strong partitioning and extensional southern ryukyu zone no lithospheric scale partitioning other factors controlling the strain partitioning are the length of the oblique subductlon zone the boundary conditions at the transverse limits of the forearc sliver and of course the obliquity of subduction