tectonic and structural evolution of gough volcano a volcanological model abstract morphostructural stratigraphic and tectonic data indicate that the evolution of gough volcano is similar to other oceanic intraplate volcanoes is older than 1 ma and is related to a transform fault at least six evolutionary stages can be distinguished within two major magmatostructural periods dominated by basaltic and trachytic magmas respectively the basaltic shield volcano is characterized by a curved elongated shape in plan and a rift zone with a high density of dykes combined with a radial intrusive system the latter is interpreted as being fed by a magma chamber some 4 km below the surface the activity of the volcano became more centralized at the end of the basaltic period and its slopes became steeper this corresponds to the development of a shallower and narrower central conduit in the edifice the basaltic period was terminated by formation of a shield caldera related to the 4 km deep magma chamber the term shield caldera is used for a collapse structure that is postmagmatic large in comparison with the diameter of the volcano and delimited by normal faults that do not show a closed circular pattern but rather a series of arcs in contrast summit calderas are defined as smaller circular shaped centrally situated synmagmatic features related to a central shallow column during the basaltic period landslides were generated on the flanks of the edifice as a result of slope stability factors which are not easy to determine at present and dynamic factors among which the intrusion of magma along a curved zone certainly played a major role the trachytic period is characterized by comparatively rare pyroclastic deposits and a large volume of thick flows extruded from domes these extrusions as well as plugs formed from vertical cylindrical columns of magma rising from shallow individual magma pockets fed by the main reservoir

flow directions in dikes from anisotropy of magnetic susceptibility data the bootstrap way one of the first applications of anisotropy of magnetic susceptibility ams was an attempt to determine flow directions from mafic dikes khan 1962 since the seminal work of knight and walker 1988 defining the expected behavior of ams in response to magma flow there has been increasing interest in using ams for this purpose here we present a quantitative method for interpretation of ams data from dikes using a parametric bootstrap first dikes must be sampled with at least five and preferrably more samples from within 10 cm of the dike margin the distributions of the eigenvalues and eigenvectors of the ams tensor are delineated by calculating eigenparameters of many bootstrapped paradata sets we generate paradata sets by first selecting a sample at random then calculating a replacement set of data by drawing tensor elements from normal distributions with the mean and standard deviation of the entire site the bounds containing 95 of the eigenparameters of the bootstrapped data serve as confidence limits for the parameter of interest classification of dikes proceeds as follows sites whose maximum and intermediate eigenvalues could not be distinguished are deemed uninterpretable in addition sites with principal eigenvectors with angles 45 away from the dike margin inverse or with markedly different directions on either side of the dike scissored are excluded the remaining dikes are classified as having unique flow direction information if the principal eigenvectors from at least one side are distinct from the dike plane based on the distribution of the bootstrapped principal eigenvectors if neither side has principal eigenvectors distinct from the dike plane the dikes are classified as having lineation information only a study comprising 251 dikes from the troodos ophiolite has 151 sites with directional data 38 sites with lineations only 7 inverse sites 5 scissored sites and 55 sites not fitting into any other category the flow directions interpreted from the data were generally southerly toward a fossil transform zone

interannual variability of summer biochemical enhancement in relation to mesoscale eddies at the shelf break in the vicinity of the pribilof islands bering sea abstract this study examined variability of the eddy field and primary production over the bering sea s eastern shelf break area during summer using a satellite multi sensor data set the eddy field and on shelf nutrient flux below 50 m water depth were also investigated using a numerical model the satellite altimeter data analysis and numerical experiments indicated that the eddy field was induced by perturbations of the bering slope current system and or instabilities in the bering sea basin via flow through the aleutian passes the distribution of high primary production roughly coincided with the high variability area of the eddy field numerical experiments showed a 70 increase in net on shelf flux and 54 increase in net on shelf transport by the generation propagation of eddies along the shelf break at pribilof canyon towed ctd fluorometer measurements during the summer of 2003 revealed a stable water column and high fluorescence 10 g l 1 in the area between the 200 and 1000 m isobaths at a time when there was a low level of on shelf flux regardless of the eddy field conversely the eddy induced on shelf flux and stable water column can enhance primary production from spring to summer at the shelf break moreover the eddy induced on shelf nutrient flux probably contributes to the primary productivity at the western shelf of the pribilof islands where the bering sea ecosystem is very dynamic this eddy related shelf break process can be applied to not only the bering sea shelf break but also all shelf break areas in which propagating eddies are found

magnetic fabrics of dike swarms from se bahia state brazil their significance and implications for mesoproterozoic basic magmatism in the sao francisco craton abstract magnetic fabrics were determined by applying two techniques anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanence aar ams was performed in 81 dikes from ilheus olivenca and itaju do colonia swarms located in se bahia the dikes intrude high grade metamorphic terrains as old as 3 2 ga dikes are vertical or subvertical in dip and trend mainly e w they are tholeiitic in composition and are classified into two groups characterized by high tio 2 2 and incompatible elements contents and low tio 2 and incompatible elements contents respectively thicknesses vary from a few centimeters to ca 20 m average of 3 m 40 ar 39 ar data indicate ages of 1 012 ilheus dikes and 1 078 ga olivenca dikes rock magnetism shows that ti poor titanomagnetites up to pure magnetites pseudo single domain multidomain grain sizes carry the magnetic fabrics three types of ams fabric are recognized i normal fabric 64 dikes defined by the clustering of k 1 k 2 axes on the dike plane whereas k 3 axes are nearly perpendicular to it this fabric is interpreted as due to magma flow ii intermediate fabric seven dikes characterized by k 1 k 3 axes clustering close to dike plane and k 2 axes perpendicular to the dike wall it was interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike direction and iii inverse fabric nine dikes defined by k 2 k 3 plane parallel to the dike plane and k 1 perpendicular to dike wall aar measurements were performed in three dikes two of them with inverse and one with normal ams fabrics aar was coaxial for normal ams fabric and it resulted in intermediate and normal for inverse ams fabrics a combination of ams and aar fabrics suggests that the magmatic fabric for these dikes that should be flow was overprinted by some local event probably related to brasiliano orogeny processes after dike emplacement inverse ams fabric would record this event normal fabric is the most important one found for the studied swarms the analysis of k 1 inclination permitted one to infer that the dikes from ilheus were fed dominantly by horizontal fluxes k 1 k 1 60 dikes from itaju do colonia region have the steepest k 1 suggesting that this region could be closer to a magma source on the other hand a magma source near to olivenca area could be also inferred since the majority of the dikes were fed by inclined flow the existence of more than one source is supported by geochemical geochronological and paleomagnetic data which have shown that contemporaneous dikes were fed by compositionally distinct magmas the ams data also suggest that the majority of dikes were fed from upward flow coming from west to east and only a few of them were fed from upward flow from east to west

influences of sea ice on the eastern bering sea ncar cesm simulations and comparison with observations abstract we examine the influences of sea ice on the eastern bering sea ebs regional oceanography on seasonal and inter annual time scales using the national center for atmospheric research community earth system model ncar cesm simulations comparing the modeling results with satellite and in situ observations when possible while the modeled mean seasonal cycle of ice cover in the ebs middle shelf is generally within the uncertainty range of satellite observations in the northern domain north of 59 n the simulation reaches its annual maximum in april instead of in march as observed by satellite remote sensing modeled ice reduction in late spring in the region is also slower than observations despite this bias the simulation captures the observed seasonal transit of freshwater from the north to the south via ice advection en route the sea ice melts cooling and freshening the local water column on inter annual time scales modeling results suggest that extensive ice cover persisting into spring in the central ebs leads to cold anomalies in the bottom water especially on the middle and inner shelves of the southern domain the corresponding salinity anomalies are positive in the northern coastal domain and weak but negative in the southern middle shelf the associated 10 m ocean current anomalies are southward on the shelf and directed offshore in the slope region comparing years 1961 2005 versus years 2005 2050 the probability distribution function of ice cover on the ebs middle shelf shifts northward by 2 latitude

failure of volcano slopes magmatic intrusions can initiate and sustain massive and catastrophic volcano collapse their role is twofold involving both driving and resisting forces first flank stability is diminished by magmastatic and magma overpressures and steepened slopes that accompany intrusion second excess pore pressures in potential failure zones can be generated as a result of intrusion related mechanical or thermal straining of the rock fluid medium pressurized retrograde boiling in high level magma chambers or hydrothermal fluid circulation also earthquakes may aid collapse through inertial forces and shaking induced pore pressure generation these excess pore pressures reduce the sliding resistance as shown for wedge shaped slide blocks for selected cases the destabilizing influence of mechanically induced pore pressures is maximized as the intruded width or corresponding overpressure of the intrusion is increased the destabilizing influence of thermally induced pore pressures is conditioned by the seve

recent dyke induced large scale block movement at mount etna and potential slope failure here we report the results of a geodetic survey undertaken on mount etna between 1981 and 1988 observed horizontal ground movements in the vicinity of the valle del bove caldera demonstrate that dyke emplacement during 1983 and 1985 resulted in the 2 8 m eastward displacement of a block of western caldera rim a history of past instability and collapse along the rim is indicated by the presence of large back rotated blocks of prehistoric volcanics1 furthermore the unusually linear form of the present western rim indicates past slope failure along north south aligned surfaces and therefore that dyke induced collapse may have been an important agent in caldera enlargement we suggest a self sustaining mechanism whereby the stress regime generated as a consequence of the caldera s existence results in the repeated injection of north south orientated dykes adjacent to the western rim2 3 dilation associated with magma intrusion leads in turn to periodic phases of slope instability and failure of the cliff wall causing a progressive westward migration of the western rim

circulation on the central bering sea shelf july 2008 to july 2010 1 we examine the july 2008 to july 2010 circulation over the central bering sea shelf using measurements at eight instrumented moorings hindcast winds and numerical model results at sub tidal time scales the vertically integrated equations of motion show that the cross shelf balance is primarily geostrophic the along shelf balance is also mainly geostrophic but local accelerations wind stress and bottom friction account for 10 40 of the momentum balance depending on season and water depth the shelf exhibits highly variable flow with small water column average vector mean speeds 5 cm s 1 mean peak speeds in summer 3 6 cm s 1 10 30 cm s 1 are smaller than in winter and fall 6 12 cm s 1 30 70 cm s 1 low frequency flows 1 4 cpd are horizontally coherent over distances exceeding 200 km vertical coherence varies seasonally degrading with the onset of summer stratification because effects of heating and freezing are enhanced in shallow waters warm summers increase the cross shelf density gradient and thus enhance northward transport cold winters with increased ice production and brine rejection increase the now reversed cross shelf density gradient and enhance southward transport although the baroclinic velocity is large enough to influence seasonal transports wind forced ekman dynamics are primarily responsible for flow variations the system changes from strong northward flow with coastal convergence to strong southward flow with coastal divergence for northerly and easterly winds respectively under northerly and northwesterly winds nutrient rich waters flow toward the central shelf from the north and northwest replacing dilute coastal waters that are carried south and west

very shallow earthquakes and brittle deformation in active volcanic areas the eatnean region as an example abstract some aspects of the stress field acting in active volcanic areas were investigated by studying the relationship between surficial seismicity h 1 2 km and the brittle deformation pattern of the etna volcano the magmatic composition and structural character of mt etna are described together with its geodynamic regional context after a discussion of the problem of the state of stress in active volcanoes and a definition of very shallow seismicity the high level of surficial seismicity the morphology of the sedimentary basement and the kinematic behaviour of the faults suggest that the eastern flank of mt etna is a shallow seismotectonic domain structurally decoupled from the rest of the volcanic body this flank shows a tendency to slide in an eastern direction and is bound to the north by a left lateral oblique slip normal fault trending e w and in the south by an arcuate fault system with right lateral features right lateral oblique slip normal faults

seismicity of the caldera making eruption of mount katmai alaska in 1912 the seismic activity associated with the century s largest eruption and caldera collapse at mount katmai alaska during june 1912 was poorly understood and is studied here in detail the epicenter of the large shock of 10 june was long thought to be 140 km northeast of mount katmai but it is relocated to be 12 km west of mount katmai near the novarupta dome origin times surface wave magnitudes ms and broadband body wave magnitudes mb are assigned to 50 new shocks from instrumentally observed data of more than 60 worldwide milne s and other seismological stations of 50 shocks detected 14 have ms 6 0 and greater and the largest is 7 0 both the frequency magnitude relation and the mb ms relation apparently exhibit normal appearance for the alaskan region the strong earthquakes were concentrated in the nights of 6 and 7 june it is inferred that the substantial collapse of mount katmai took place in these two nights the total energy released in seismic waves is 7 1 1015 j corresponding to the energy of a single shock with ms 7 4 the total seismic moment is estimated to be 1 4 1020 nm from ms values this total moment can be interpreted in terms of failures caused by the total volume change of 5 km3 this volume agrees well with the estimated volume of the caldera collapse which presumably took place in response to transfer of supporting magma from beneath mount katmai toward novarupta

la fossa caldera breaching and submarine erosion vulcano island italy abstract la fossa caldera lfc on vulcano island in the aeolian archipelago italy is a multi collapse caldera whose north eastern part lies underwater through the integration of marine geophysical surveys and sampling carried out in the last decade its submarine extension and offshore areas have been reconstructed in unprecedented detail strong erosive processes breached the submarine portion of lfc and gradually dismantled the caldera rims and infill leading to the emplacement of a wide volcaniclastic fan from the base of lfc down to over 1000 m the bathy morphologic setting seismoacoustic facies and analyses of sediment samples from the fan together with volumetric considerations suggest that this feature is the pathway deposit of submarine unconfined gravity flows and turbidity currents deriving from the gradual dismantling of lfc submarine erosive processes are still active and the progressive erosion of the northern caldera sector threatens the stability of the active la fossa cone and the nearby coastal settlements

effects of climate variations on pelagic ocean habitats and their role in structuring forage fish distributions in the bering sea abstract this paper examines how climate variations influence the boundaries of suitable ocean habitat and how these changes affect the spatial distribution and interactions between forage fishes in the southeastern bering sea shelf the study focuses on the summer distributions of forage fish age 0 and age 1 walleye pollock theragra chalcogramma and capelin mallotus villosus observed during national marine fisheries service summer acoustic trawl surface trawl and bottom trawl surveys conducted in the bering sea between 2004 and 2009 we compare the responses of these forage fish to climate induced shifts in ocean habitats habitat boundaries were defined using key explanatory variables including depth bottom temperature and surface temperature using general additive models bathymetry bottom temperature and frontal zones formed boundaries between different groups of forage fishes age 0 pollock were dispersed throughout the middle domain 50 100 m depth in well stratified regions in cold years the highest densities of age 0s were found in the southern regions of the middle domain waters in waters warmer than approximately 1 c in contrast age 1 pollock were observed on the sea floor over the middle domain and in midwater in the northern outer domain in cold years and more broadly dispersed across the middle and outer domain in warm years the demersal concentrations of age 1 pollock in the middle domain shows age 1 pollock tolerate a wide range of bottom temperatures midwater and demersal distributions of age 1 pollock exhibited a patchier distribution than age 0 pollock midwater concentrations of age 1 pollock tended to be associated with the outer domain and regions where higher levels of lower trophic level production are expected capelin were concentrated in the inner domain a well mixed region the overlap of age 1 pollock and capelin was higher in cold years than in warm years

magnetic fabrics and rock magnetism of proterozoic dike swarm from the southern sao francisco craton minas gerais state brazil magnetic fabric and rock magnetism studies were performed on 32 mafic dikes of a proterozoic dike swarm from the southern sao francisco craton sfc minas gerais state se brazil magnetic anisotropies were determined by applying anisotropy of low field magnetic susceptibility ams and anisotropy of remanent magnetization arm the latter was performed imposing both anhysteretic total aar and partial paar and isothermal remanence magnetizations airm partial anhysteretic remanence anisotropy was performed based on remanent coercivity spectra from a pilot specimen of each site in most sites ams is dominantly carried by ferromagnetic minerals however in some sites the paramagnetic contribution exceeds 70 of bulk susceptibility rock magnetism and thin section analysis allow classifying the dikes as non hydrothermalized and hydrothermalized magnetic measurement shows that the mean magnetic susceptibility is usually lower than 5 10 3 si ti poor titanomagnetites up to pure magnetite pseudo single domain psd grain sizes carry the majority of magnetic fabrics for non hydrothermalized dikes whereas coarse to fine grained ti poor titanomagnetites carry the majority of magnetic fabrics for hydrothermalized dikes three primary ams fabrics are recognized which are coaxial with arm fabric except for two dikes from both non hydrothermalized and hydrothermalized dikes normal ams fabric surprisingly is not dominant 31 the parallelism between ams paar0 30 paar30 60 and paar60 90 fabrics in the hydrothermalized dikes indicates that magnetic grains formed due to late stage crystallization or to remobilization of iron oxides due to hydrothermal alteration after dike emplacement have acquired a mimetic fabric coaxial with the primary fabric given by coarse grained early crystallized ti poor titanomagnetites this fabric is interpreted as magma flow in which the analysis of kmax inclination permitted the inference that the dikes were fed by horizontal or subhorizontal fluxes kmax 30 intermediate ams fabric is the most important 41 in the investigated swarm it is interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike strike arm determinations for these sites also remained intermediate except for two dikes in one of them airm fabric resulted in normal ams fabric while for the other aar fabric resulted in inverse ams fabric a combination of ams and arm fabrics suggest that magmatic fabric for both dikes were overprinted by some late local event probably related to brasiliano orogenic processes after dike emplacement inverseinverse ams fabric is a minority four dikes arm determinations also remained inverse suggesting a primary origin for inverse ams fabric

evolution of ocean island rifts the northeast rift zone of tenerife canary islands the northeast rift zone of tenerife presents a superb opportunity to study the entire cycle of activity of an oceanic rift zone field geology isotopic dating and magnetic stratigraphy provide a reliable temporal and spatial framework for the evolution of the ne rift zone which includes a period of very fast growth toward instability between ca 1 1 and 0 83 ma followed by three successive large landslides the micheque and guimar collapses which occurred approximately contemporaneously at ca 830 ka and on either side of the rift and the la orotava landslide between 690 10 and 566 13 ka our observations suggest that canarian rift zones show similar patterns of development which often includes overgrowth instability and lateral collapses collapses of the rift flanks disrupt established fissural feeding systems favoring magma ascent and shallow emplacement which in turn leads to magma differentiation and intermediate to felsic nested eruptions rifts and their collapses may therefore act as an important factor in providing architectural and petrological variability to oceanic volcanoes conversely the presence of substantial felsic volcanism in rift settings may indicate the presence of earlier landslide scars even if concealed by postcollapse volcanism comparative analysis of the main rifts in the canary islands outlines this general evolutionary pattern 1 growth of an increasingly high and steep ridge by concentrated basaltic fissure eruptions 2 flank collapse and catastrophic disruption of the established feeder system of the rift 3 postcollapse centralized nested volcanism commonly evolving from initially ultramafic mafic to terminal felsic compositions trachytes phonolites and 4 progressive decline of nested eruptive activity

new time constraints on dyke swarms related to the parana etendeka magmatic province and subsequent south atlantic opening southeastern brazil abstract 40 ar 39 ar incremental heating analyses were carried out on tholeiitic dyke swarms of santa catalina florianopolis and rio de janeiro and alkaline dykes from the proximity of rio de janeiro city southeastern brazil most of the plagioclases from the tholeiitic dykes display disturbed age spectra with younger apparent ages most often clearly correlated with alteration phases however a calculated weighted mean age from the less disturbed released fractions of the ne oriented santa catarina dyke swarm gives an age of 129 4 0 3 ma which is contemporaneous with the ponta grossa dyke swarm pgds and at least the northern parana continental flood basalts cfb the rio de janeiro tholeiitic dykes display age spectra affected by alteration and probably excess argon but with low temperature mini plateau ages between 130 6 and 129 8 ma which are concordant with previous data obtained on the pgds and rio de janeiro dyke swarms by renne et al 1996a and turner et al 1994 these new geochronological data when added to the previous ones show that the initiation of gondwana s breakup in the region was characterized by a volcano tectonic system with a geometry more complicated than the rift rift rift system suggested by many authors the different rift arms of this structural feature were operating during a brief time period mainly from 131 5 to 129 ma and were contemporaneous at least with the northern part of the parana etendeka province pep single grain and bulk sample biotite from the alkaline mainly coast parallel dyke swarm near rio de janeiro city display plateau ages around 80 ma 81 8 1 8 and 78 8 2 8 ma and 70 ma 72 5 0 5 and 69 7 0 2 ma these magmatic intrusions refer to late cretaceous flexuring and vertical movements and rift basin formations almeida 1976 due the northward progressing rifting motions separating south america and africa during south atlantic ocean formation

vertical and lateral collapses on tenerife canary islands and other volcanic ocean islands recent studies demonstrate that tenerife has undergone large lateral collapses this has led to the suggestion that the las canadas caldera one of the best exposed calderas in the world is the result of lateral collapse we have tested this idea using the available structural stratigraphic volcanological and geochronological data we conclude that the las canadas caldera is the result of a complex sequence of vertical collapse events associated with a long history of phonolitic explosive activity in the central part of tenerife our results indicate however that vertical collapses may have played a major role in triggering lateral collapses we propose that the association of vertical and lateral collapse events such as inferred for tenerife can also explain similar sequences of events interpreted to have affected other large volcanic ocean islands

low frequency current regimes over the bering sea shelf abstract using direct current measurements made during the period 1975 81 we describe the general circulation over the southeastern bering sea and differentiate it by regimes related to depth and forcing mechanisms three regimes are present delineated by water depth z the coastal z 50 m the middle shelf 50 z 100 m and the outer shelf z 100 m these are nearly coincident with previously described hydrographic domains statistically significant mean flow 1 to 10 cm s 1 exists over the outer shelf generally directed toward the northwest but with a cross isobath component flow of similar magnitude 1 6 cm s 1 occurs in the coastal regime paralleling the 50 m isobath in a counterclockwise sense around the shelf mean flow in the middle shelf is insignificant kinetic energy at frequencies 0 5 cycle per day cpd is greater over the outer shelf than in the other two regimes suggesting that oceanic forcing is important there but does not affect the remainder of the shelf kinetic

magma flow directions in dikes of the koolau complex oahu determined from magnetic fabric studies we compared the magnetic fabric with the macroscopic surface lineation direction previously assumed to be parallel with magma flow direction for 35 dikes of the koolau complex oahu anisotropy of magnetic susceptibility ams was determined for 530 samples from 59 dikes including a sill and a pluglike intrusive body and two filled lava tubes and statistically significant ams clusters were found in all but five dikes for the 35 lineated dikes the mean maximum ams direction 1 coincides with the macroscopic lineations to within 25 in 18 and to within 15 in eight the ams ellipsoid shape in about half of the samples is prolate cigar shaped and in about half is oblate pancake shaped we interpret the ams to be determined by shape anisotropy of ferromagnetic grains the mean total anisotropy is 2 2 and total anisotropy h ranges from 0 24 to as high as 14 12 the principal ams axes for samples from a dike are tightly grouped on equal area plots and for most of the dikes the minimum ams axes plot near to the pole of the dike our ams measurements enable us to infer absolute magma flow directions for half the dikes studied the maximum ams axes of samples from one side of the dike form a tight cluster on an equal area plot and those from the other side of the dike form another tight cluster the two clusters are close together separated by 10 30 for narrow dikes 100 cm wide and are farther apart 30 60 for wide dikes and are symmetrically disposed on either side of the plane of the dike we infer that these paired clusters are caused by imbrication of nonequant ferromagnetic crystals against each margin of the dike during deposition in a velocity gradient and hence indicate the flow azimuth i e the absolute magma flow direction nearly half of the dikes measured 25 gave paired ams plots in 12 the magma flowed up at low oblique angles 5 30 from the horizontal in five it flowed down at 5 20 in four it flowed horizontally 5 in three it flowed up at a steep angle 50 75 and in one it flowed down at a steep angle the magma flow in general was rarely vertical or horizontal

hybrid collapse mechanism at piton de la fournaise volcano reunion island indian ocean 1 recent studies have shown that hydrothermal alteration in the core of volcanoes may weaken the edifice and trigger sector collapse lopez and williams 1993 day 1996 voight and elsworth 1997 van wyk de vries et al 2000 reid et al 2001 alteration of volcanic products must take place over long periods of times before collapse can occur so this process is considered to be less frequent than volcanic failure generated by magmatic intrusion and flank spreading however acceleration of this slow process may occur along one direction as a result of a weak lateral confinement following outward gravitational sliding of a flank of the volcano we present here evidence that the enclos fouque depression of piton de la fournaise volcano results from such a hybrid mechanism seaward sliding of the grand brule trough on top of an ancient hypovolcanic complex led to a lateral unconfined boundary along which collapse of the altered core of the edifice took place the result is a caldera like structure breached on the grand brule slide scaled experiments to simulate this process of pseudocaldera formation reveal tectonic features remarkably similar to those encountered in the field and demonstrate the mechanical consistency of the model similar processes may have played a role in the evolution of other volcanoes

the off shore continuation of an active basaltic volcano piton de la fournaise reunion island indian ocean structural and geomorphological interpretation from sea beam mapping abstract in 1984 a sea beam survey of the submarine east flank of piton de la fournaise was performed with r v jean charcot three main types of volcanic or volcano tectonic features have been identified 1 1 the subaerial ne and se volcanic rift zones of piton de la fournaise do not extend more than about five kilometers offshore unlike typical hawaiian rift zones which form narrow 2 4 km ridges extending tens of kilometers from the summit the active rift zones of piton de la fournaise widen downslope attaining more than 10 km at their front 2 2 the submarine extension of the grand brule slide is larger than the subaerial portion the entire slide forms a 7 24 km scar bounded by two ramparts to the north and south the slumped material may have moved as a debris flow forming a large talus downslope of the slide however the submarine counterpart of the south area of the grand brule seems to be composed by a slumped block whose structure is apparently not disturbed 3 3 another prominent feature is a conspicuous topographic high that occupies nearly all the center of the surveyed zone this cast flank submarine plateau cannot be associated with any active structure of piton de la fournaise its surface generally dips gently 2 3 and its northern and southern flanks are extensively cut by landslides cones of variable dimensions are observed on the plateau and farther to the east three hypotheses are examined to account for the origin of this morphology a remnant flank of an ancestral fournaise volcano associated with a large buried intrusion found by drilling beneath the grand brule b distinct volcanic massif or c material of a huge and ancient landslide the geophysical data show that the western part of this submarine plateau is reversely magnetized and associated with a moderate positive gravity anomaly this confirms that as suggested by the bathymetric analysis this part of the plateau is relatively coherent conversely the eastern portion of the plateau appears to be poorly magnetized and composed of low density material probably chaotic materials derived from landslides the reversed magnetization of the western part shows that the whole structure is older than 0 7 ma these results show that the volcanic history of piton de la fournaise is more complex than previously thought

coupled wind forced controls of the bering chukchi shelf circulation and the bering strait throughflow ekman transport continental shelf waves and variations of the pacific arctic sea surface height gradient abstract we develop a conceptual model of the closely co dependent bering shelf bering strait and chukchi shelf circulation fields by evaluating the effects of wind stress over the north pacific and western arctic using atmospheric reanalyses current meter observations satellite based sea surface height ssh measurements hydrographic profiles and numerical model integrations this conceptual model suggests bering strait transport anomalies are primarily set by the longitudinal location of the aleutian low which drives oppositely signed anomalies at synoptic and annual time scales synoptic time scale variations in shelf currents result from local wind forcing and remotely generated continental shelf waves whereas annual variations are driven by basin scale adjustments to wind stress that alter the magnitude of the along strait meridional pressure gradient in particular we show that storms centered over the bering sea excite continental shelf waves on the eastern bering shelf that carry northward velocity anomalies northward through bering strait and along the chukchi coast the integrated effect of these storms tends to decrease the northward bering strait transport at annual to decadal time scales by imposing cyclonic wind stress curl over the aleutian basin and the western subarctic gyre ekman suction then increases the water column density through isopycnal uplift thereby decreasing the dynamic height sea surface height and along strait pressure gradient storms displaced eastward over the gulf of alaska generate an opposite set of bering shelf and aleutian basin responses while ekman pumping controls canada basin dynamic heights proshutinsky et al 2002 we do not find evidence for a strong relation between beaufort gyre sea surface height variations and the annually averaged bering strait throughflow over the western chukchi and east siberian seas easterly winds promote coastal divergence which also increases the along strait pressure head as well as generates shelf waves that impinge upon bering strait from the northwest

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

a relationship between heat transfer to sea ice and temperature salinity properties of arctic ocean waters it is demonstrated that the characteristic temperature salinity relationship shown by thermocline waters of the arctic ocean can be reproduced using a simple model based on transfer of heat directly from these waters to sea ice from this we deduce that there is direct sensible heat flux from waters of atlantic origin to the ice cover such interaction could play a major role in determining the physical properties of arctic water masses

magnetic fabrics of the cretaceous dike swarms from sao paulo coastline se brazil its relationship with south atlantic ocean opening abstract magnetic fabric and rock magnetism studies were performed on 91 dikes from cretaceous diabase and lamprophyre dike swarms that outcrop side by side on the beaches of ne sao paulo state coastline the dikes crosscut archean and proterozoic poly metamorphic rocks of the costeiro complex their thicknesses range from a few centimeters to 2 m for lamprophyre and up to about 10 m for the diabase they trend predominantly n30 60 e with vertical dip magnetic fabrics were determined using anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanent magnetization aarm rock magnetism measurements reveal that magnetite grains in the range of 2 5 m are the magnetic mineral of both swarms for most dikes these grains are the carriers of bulk magnetic susceptibility but surprisingly are not responsible for the ams which is carried by fe bearing minerals as shown by aarm the main ams fabric recognized in the swarms is due to magma flow in which the kmax kint plane is parallel to the dike s plane and the magnetic foliation pole kmin is perpendicular to it the analysis of the kmax inclination showed that the dikes were fed by horizontal to vertical flows however for the majority of the dikes the ams and aarm tensors are not coaxial the aarm lineation aarmmax is oriented n30 60w approximately perpendicular to ams lineation kmax suggesting that magnetite grains were rotated approximately 90 anticlockwise from the dike plane the aarmmax orientation is similar to the direction of a fault system mainly in the santos marginal basin which was formed in the cretaceous rifting during the south atlantic opening therefore the aarm fabric is tectonic in origin and the comparison of ams and aarm fabrics suggests that lamprophyre and diabase dikes were emplaced in three distinct events in the earliest stages of the south atlantic opening

magnetic fabric and its significance in the florianopolis dyke swarm southern brazil summary magnetic fabric was determined by applying the anisotropy from the low field magnetic susceptibility ams technique in 62 mafic dykes from the mesozoic florianopolis santa catarina island dyke swarm southern brazil these dykes cut the crystalline basement rocks which are mainly proterozoic they are vertical or subvertical in dip and trend mainly ne although nw trending dykes are also found dykes are tholeiitic in composition and are geochemically similar to those from the ponta grossa swarm thicknesses vary from 0 3 to 60 m polished sections show that titanomagnetites carry the ams in these dykes hysteresis parameters show that the magnetic minerals fall in the psd range two types of magnetic fabric are recognized type i is characterized by k1 k2 parallel to the dyke wall representing magma flow within the dykes type ii with k1 k3 parallel to the dyke wall was found in four dykes type i is found in 94 per cent of the dykes and approximately 20 per cent of these have k1 inclinations of less than 30 suggesting horizontal or subhorizontal flow about 80 per cent have k1 inclinations of greater than 30 due to inclined to vertical flow the comparison of ams studies from both the florianopolis and the ponta grossa dykes suggests a source position closer to santa catarina island than the ponta grossa arch

magnetic studies of magma supply and sea floor metamorphism troodos ophiolite dikes abstract dikes of the eastern troodos ophiolite of cyprus intruded at slow ocean spreading axes with dips ranging up to 15 from vertical and with bimodal strikes now ne sw and n s due to post 88 ma sinistral microplate rotation varied dike orientations may represent local stress fields during dike crack propagation but do not influence the spatial distributions or orientation distributions of dikes magnetic fabrics nor of their palaeomagnetic signals anisotropy of magnetic susceptibility ams integrates mineral orientation distributions from each of 1289 specimens sampled from dikes at 356 sites over 400 km2 in the eastern troodos ophiolite of cyprus in 90 of dikes ams fabrics define a foliation kmax kint parallel to dike walls and a lineation kmax that varies regionally and systematically magma flow alignment of accessory magnetite controls the ams with a subordinate contribution from the mafic silicate matrix that is reduced in anisotropy by sea floor metamorphism titanomagnetite has less influence on anisotropy occasionally intermediate and minimum susceptibility axes are switched so as to be incompatible with the kinematically reasonable flow plane but maximum susceptibility kmax still defines the magmatic flow axis such blended subfabrics of kinematically compatible mafic silicate and misaligned multidomain magnetite subfabrics are rare areas of steep magma flow kmax plunge 70 and of shallow magma flow alternate in a systematic and gradual spatial pattern foci of steep flow were spaced 4 km parallel to the spreading axes and 6 km perpendicular to the spreading axes ridge parallel separation of steep flow suggest the spacing of magma feeders to the dikes whereas ridge perpendicular spacing of 6 km at a spreading rate of 50 mm a implies the magma sources may have been active for 240 ka the magma feeders feeding dikes may have been 2 km in diameter stable paleomagnetic vectors in some cases verified by reversal tests are retained by magnetite and titanomagnetite in all specimens the stable components were isolated by three cycles of low temperature demagnetization ltd followed by 10 steps of incremental thermal demagnetization td 47 of primary a components 338 2 57 2 n 207 95 3 9 mean tub 397 8 c are overprinted by a b component 341 4 63 5 n 96 95 8 7 mean tub 182 11 c a and b components are ubiquitous and shared equally by the n s and ne sw striking dikes a component unblocking temperatures tub are zoned subparallel to the fossil spreading axis their spatial pattern is consistent with chemical remagnetization at some certain off axis distance determined by sea floor spreading a components indicate less microplate rotation and more northerly palaeolatitudes that are consistent with metamorphic remagnetization after some spreading from the ridge axis thus their magnetizations are younger than those of the overlying volcanic sequence for which chrms are commonly reported as 274 33 88 ma

characterisation of a volcanic residual soil and its implications for large landslide phenomena application to tenerife canary islands large landslides are common processes during the evolution of volcanoes and individual events can exceed several cubic kilometres in volume volcanic slope failures are a significant risk for the neighbouring population due to their huge volumes and great runout distances around the canary archipelago a total of seventeen deposits of large landslides have been found and on tenerife seven large landslides have affected the subaerial and submarine morphology during the last 6 ma however the causes of such mass movements are still poorly understood this work analyses the events around the canary islands and focuses on the ones that occurred on tenerife in order to obtain new insights into the mechanisms of large volcanic landslides the study is divided into a first part that includes site investigations examining the general features favouring large scale failures at volcanoes the second part describes the laboratory tests used to analyse a residual soil that may be the potential slip surface of the slides on tenerife the site investigation revealed that regional tectonics and the climate have a significant influence on the spatial distribution of the landslides moreover morphological and geological features such as deep fluvial canyons a high coastal cliff and persistent dike intrusion may favour the initiation of slope failure a typical residual soil sample from the lateral scarp of the la orotava amphitheatre on tenerife was studied by carrying out standard laboratory tests the microstructure was analysed using environmental scanning electron microscopy and a particular bonding was found this bonding was also detected by the geotechnical tests consolidation tests and direct shear tests revealed that the mechanical behaviour of the residual soil changes greatly if the bonding of the soil is broken the bonded structure generally fails when the effective normal stress surpasses the yield strength of the bonding in the case of large volcanic landslides with thicknesses up to several hundred meters the high overburden easily exceeds this yield strength and generates a broken bonding therefore volcanic residual soils such as the one analysed in this study are perfect candidates for the potential failure surfaces of large volcanic landslides referring to the la orotava events we assume that residual soil layers and morphological geological and climatic features reduced the slope stability to critical conditions whereas a strong earthquake associated with a caldera collapse episode may have finally triggered the landslide the results obtained indicate that the residual soils play an important role in affecting the stability of volcano slopes and their destabilising influence significantly favours large scale sliding we suggest that the results obtained from this study can be applied to other locations since volcanic residual soils are common in volcanic areas

factors influencing physical structure and lower trophic levels of the eastern bering sea shelf in 2005 sea ice tides and winds abstract in spring and fall 2005 cross and along shelf transects were sampled to evaluate the influence of physical forcing including sea ice tides and winds on the lower trophic levels of the bering sea ecosystem the hydrography nutrients chlorophyll and zooplankton abundance and species composition were all affected by the presence or absence of sea ice on a north south transect along the 70 m isobath in may shelf waters between 59 n and 62 n were cold and relatively fresh and benthic invertebrate larvae and chaetognaths were a significant fraction of the zooplankton community while to the south the water was warmer saltier and the zooplankton community was dominated by copepods the position of the transition between ice affected and ice free portions of the shelf was consistent among temperature salinity nutrients and oxygen this transition in the hydrographic variables persisted through the summer but it shifted 150 km northward as the season progressed while a transition also occurred in zooplankton species composition it was farther north than the physical chemical transition and did not persist through the summer mooring data demonstrated that the change in the position of the transition in physical and chemical properties was due to northward or eastward advection of water onto and across the shelf from south to north along the 70 m isobath tidal energy decreased resulting in a less sharply stratified water column on the northern portion of the middle shelf as opposed to a well defined two layered system in the southern portion this more gradual stratification in the north permitted a greater response to mixing from winds which were homogeneous from north to south thus the physical and biological structure at any one location over the middle shelf is dynamic over the course of a year and results from a combination of in situ processes and climate mediated regional forcing which is dominated in most years by sea ice

non scaled analogue modelling of ams development during viscous flow a simulation on diapir like structures development of magnetic fabric within a diapirically ascending columnar body was investigated using non scaled analogue model made of plaster of paris containing small amount of fine grained homogeneously mixed magnetite the apparatus for the modelling consists of a manual squeezer with calibrated spring and a perspex container set of weak coloured layers at the bottom of the container was forced to intrude overlying fine grained sand through a hole in a board attached to the squeezer the development of ams fabric is correlated with complex flow pattern indicated by coloured and originally horizontal plaster layers strongly constrictional and vertical fabric in the base and in the lower domain of the diapir resulting from convergent and upwards flows is overprinted by subhorizontal oblate fabrics due to vertical flattening and initial divergent flow in the apical parts the measured ams fabrics are compared with natural examples of magmatic stocks and dykes

mechanical relationship between catastrophic volcanic landslides and caldera collapses large scale sector collapses of volcanic edifices and collapse calderas represent some of the most catastrophic geological events taking place at the earth s surface examples of these two processes occurring simultaneously suggest that a mechanical relationship between landslides and collapse calderas may exist we demonstrate that a caldera collapse can trigger large scale landslides in volcanic terrains moderate seismic shocks caused by seismogenic slip on the ring fault on which the caldera subsidence takes place act as the driving force necessary to destabilize the volcano flank this process is favored on steep volcanoes and where flank strength is reduced by agents such as hydrothermal alteration pore fluid pressure increase or the presence of weak soils

experimental deformation of synthetic magnetite bearing calcite sandstones effects on remanence bulk magnetic properties and magnetic anisotropy we have quantified effects of experimental deformation on the magnetic properties of a set of synthetic calcite sandstone samples containing magnetite the deformation was carried out in a microcomputer controlled apparatus that adjusted the applied differential stress as needed to maintain a constant strain rate of 10 5 s 1 most samples were deformed under dry conditions but a few were deformed with a pore fluid present the samples deformed under dry conditions required substantially higher differential stresses macroscopically ductile shortening strains of up to 25 produced the following irreversible changes in magnetic properties 1 increased bulk coercivity remanence coercivity and mean anhysteretic remanence susceptibility 2 decreased mean low field susceptibility 3 decreases in the component of remanence parallel to shortening 4 smaller decreases for most samples in the component normal to shortening resulting in a net rotation of the remanence away from the shortening axis 5 larger decreases in the normal component in a few samples resulting in a net rotation of the remanence towards the shortening axis 6 increased magnetic anisotropy and 7 increased deformation of initial magnetic ellipsoids a comparison of data for samples deformed under dry and wet conditions higher and lower differential stresses respectively indicates that remanence reorientation and susceptibility anisotropy are controlled primarily by bulk strain i e rotation and displacement of particles whereas coercivity and anhysteretic anisotropy are controlled dominantly by microstrain or intragranular stress

coastal fresh water discharge in the northeast pacific very high annual rates of precipitation in the coastal mountains that border the northeast pacific ocean produce large fresh water discharges 23000 m3 s 1 this discharge has been ignored previously since it does not enter the ocean in the form of large rivers but instead the water enters by way of numerous small rivers and streams thus it acts as a line source instead of a point source this coastal discharge contributes at least 40 of the fresh water that enters the northeast pacific from the atmosphere the discharge is comparable to the mean annual discharge of the mississippi river system the fresh water creates a cross shelf density gradient that drives an alongshore baroclinic jet the width of this jet is less than 25 km with speeds in excess of 100 cm s 1 it extends along the coast from southeast alaska to at least kodiak island apparently the flow is maintained as a narrow current adjacent to the coast by wind stress that causes downwelling conditions here throughout most of the year

experimental studies of collapse calderas we present experiments to investigate the formation of calderas using inflatable balloons in a medium of fused alumina powder this system provides a model of a magma chamber in an homogeneous elastic media which scales to volcanic systems in terms of geometry and strength of the media three kinds of experiment were performed i the balloon was inflated and then deflated simulating pre emption tumescence and caldera collapse ii the balloon was deflated without prior inflation simulating caldera formation without pre eruption tumescence iii three balloons were inflated and deflated in a line simulating multiple magma chambers the influence of magma chamber shape was investigated using different shaped balloons balloon inflation formed a dome with a surface pattern of shallow polygonal fractures centrally and radial fractures peripherally uplift occurs along deep inward dipping concentric reverse faults deflation generated depressions with an outer set of concentric ring fractures surrounding a central funnel formed by sagging along numerous minor faults some of which formed during doming during collapse the concentric reverse faults were converted to normal faults and the radial normally faulted fractures were converted to reverse faults without prior tumescence an outer set of near vertical ring faults surround a flat central depression due to collapse of a coherent block an inner set of outward dipping reverse ring faults also develop the experimental caldera area increased with balloon size and increased as the balloon depth decreased for a given balloon volume and depth collapse area is significantly smaller if doming had occurred prior to collapse arcuate fractures can provide the pathways for magma ascent during pre emption tumescence smith bailey 1968 but the shallow radial and polygonal fracture systems may also influence vent location changes in fault geometry and sense of movement extension to compression and vice versa occur when subsidence initiates and can account for changes in vent location that occur in many eruptions multiple overlapping collapse calderas are associated with magma chamber migration whereas nested calderas can result from the activity of a single magma chamber

magnetic and plagioclase linear fabric discrepancy in dykes a new way to define the flow vector using magnetic foliation in basaltic dykes the magnetic lineation k1 maximum magnetic susceptibility axis is generally taken to indicate the flow direction during solidification of the magma this assumption was tested in tertiary basaltic dykes from greenland displaying independent evidence of subhorizontal flow the digital processing of microphotographs from thin sections cut in k1 k2 planes yields the preferred linear orientation of plagioclase which apparently marks the magma flow lineation in up to 60 of cases the angular separation between k1 and the assumed flow direction is greater than 45 this suggests that the uncorroborated use of magnetic lineations in dykes is risky a simple geometrical method is proposed to infer the flow vector from ams in dykes based solely on magnetic foliations

magnetic fabrics and rock magnetism of archaean and proterozoic dike swarms in the southern sao francisco craton brazil abstract magnetic fabric and rock magnetism studies were performed on three mafic dike swarms total of 38 dikes from the southernmost part of the sao francisco craton sfc minas gerais state se brazil they cut archaean granite gneiss migmatite and paleoprototerozoic terranes these swarms are classified as basic noritic sm nd age 2 65 ga basic rb sr age 1 87 ga and metamorphic rb sr age 1 87 ga suites in which the second is the most important magnetic fabrics were determined by applying both anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanent magnetization aarm in most sites magnetic susceptibility is dominantly carried by ferromagnetic minerals however in some sites the paramagnetic contribution exceeds 70 of bulk susceptibility mainly coarse to fine grained ti poor titanomagnetite up to pure magnetite carry the magnetic fabrics three primary ams fabrics are recognized which are all coaxial with the aarm fabric normal ams fabric is dominant in the basic suite 16 of 20 analyzed dikes and occurs in 4 and 3 dikes from the basic noritic and metamorphic suites respectively this fabric is interpreted as a result of magma flow in which the analysis of k max inclination permitted to infer that the majority of dikes were fed by inclined flows 30 k max k max intermediate ams fabric was found in 50 of dikes from the basic noritic and metamorphic suites but in only 2 dikes from the basic suite it is interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike strike inverse ams fabric is a minority 2 dikes from each suite the parallelism between ams and aarm tensors for dikes with abnormal fabrics suggests a primary origin for them gyroremanent magnetization grm effect was negligible for the majority of dikes but it was found in two dikes from the basic suite with normal ams fabric magnetic fabrics recognized for the three studied swarms do not depend on magnetic mineralogy geochemical composition dike strikes nor the age of the swarms since the same magnetic minerals and magnetic fabric types are found in dikes from all suites inclined and horizontal flows allow us to infer the relative position of at least three magma sources or magma chambers from which the dikes were fed

some controls on flow and salinity in bering strait 1 during 1993 1994 steric forcing of flow through bering strait represented a northward sea level drop of 0 7 m from the bering sea basin to the adjacent deep arctic ocean of which 2 3 was due to the salinity difference between the basins seasonal variability of steric forcing appears small 0 05 m in contrast to large seasonal wind effects interannual changes in steric forcing may exceed 20 however and warm inflow from the north atlantic accumulation of freshwater in the southwest canada basin and temperature and salinity changes in the upper bering sea have all contributed to recent changes the mean salinity balance in bering strait is primarily maintained by large runoff to the bering shelf dilute coastal inflow from the gulf of alaska and on shelf movement of saline and nutrient rich oceanic waters from the bering sea basin in bering strait therefore both the throughflow and its salinity are affected by remote events

the nuuanu and wailau giant landslides insights from paleomagnetic and anisotropy of magnetic susceptibility ams studies abstract the koolau volcano on the northeastern flank of the island of oahu in hawaii is the source area for one of the largest landslides on earth the nuuanu debris avalanche the offshore expression of this slide is an extensive rubbly field of debris extending approximately 230 km from the island across the hawaiian deep and onto the hawaiian arch we have studied the subaerial lavas of the koolau volcano as well as the deep sea sediments on top of the nuuanu and wailau landslides by means of magnetostratigraphy and magnetic anisotropy studies to investigate the volcanic evolution the plumbing of the koolau volcano and the timing of the nuuanu detachment in addition we have conducted deep sea magnetostratigraphic studies of a 20 and 7 m piston core recovered by the rv kana keoki and the rv bertha ann as well as three 7 m piston cores recovered by the rv kairei to understand the origins of the giant landslides the results derived from these investigations indicate that the plumbing of the koolau volcano is characterized by a very coherent dike complex and by a high dike injection nature parts of the koolau complex are a sheeted dike swarm as in ophiolites as demonstrated by geologic and anisotropy of magnetic susceptibility ams studies on the other hand the magnetostratigraphic results of the subaerial part of the koolau complex indicate that at least one of the normal reunion subchrons ca reunion ii 2 15 0 04 to 2 11 0 04 ma had been registered at two different locations deep sea cores have recorded several reversals the oldest one is the top of the olduvai subchron ca 1 78 ma land and deep sea paleomagnetic and plumbing investigations indicate that the collapse of the koolau volcano had to be relatively rapid in a period of less than 0 5 ma due to forceful injection of dikes associated with extension and gravitational effects therefore the timing of the main collapse of the koolau volcano that originated the nuuanu landslide had to occur between 2 1 and 1 78 million years ago based on the magnetostratigraphic evidence

currents and transport on the eastern bering sea shelf an integration of over 20 years of data abstract more than 20 years of data from moorings satellite tracked drifters and hydrographic surveys are integrated to provide a comprehensive view of currents and transport on the eastern bering sea shelf the major sources of water onto the eastern bering sea shelf are north pacific water flowing through unimak pass and bering slope water flowing onto the shelf usually via the canyons that intersect the shelf break absolute geostrophic transport through unimak pass varies from an average of 0 25 106 m3 s 1 sv in the warm months to 0 43 sv in the cold months flow along the 50 m isobath is weak with a transport of 13 months and from amukta pass to bering strait via the bering slope current is 8 months consequently the source of most of the heat transported into the arctic through bering strait is a result of air sea interactions local to the northern bering sea analysis of the currents and water properties on the southern shelf indicates that 50 of the shelf water is exchanged with slope water during october january each year this exchange elevates the october mid shelf average nitrate level from 6 m to 14 16 m by the end of january

anisotropy of magnetic remanence a brief review of mineralogical sources physical origins and geological applications and comparison with susceptibility anisotropy the magnetic fabric of rocks and sediments is most commonly characterized in terms of the anisotropy of low field magnetic susceptibility ams however alternative methods based on remanent magnetization measured in the absence of a magnetic field rather than induced magnetization measured in the applied field have distinct advantages for certain geological applications this is particularly true for 1 adjunct studies in paleomagnetism in order to assess the fidelity with which a natural remanence records the paleofield orientation 2 studies of weakly magnetic or weakly deformed rocks for which susceptibility anisotropy is very difficult to measure precisely and 3 quantitative applications such as strain estimation the fundamental differences between susceptibility and remanence and their respective anisotropies are due to several factors 1 susceptibility arises from all of the minerals present in a sample whereas remanence is carried exclusively by a relatively small number of ferromagnetic minerals 2 ferromagnetic minerals are generally more anisotropic than para and diamagnetic minerals 3 for ferromagnetic minerals remanence is inevitably more anisotropic than susceptibility and 4 a number of common minerals including single domain magnetites possess an inverse anisotropy of susceptibility i e they tend to have minimum susceptibility parallel to the long axis of an individual particle remanence is immune to this phenomenon as a consequence of all these factors remanence anisotropy may generally provide a better quantitative estimate of the actual distribution of particle orientations in a rock sample

satellite and hydrographic observations of the bering sea green belt abstract green belt is the aptly named region of high productivity occurring principally along and above the shelf slope boundary in the bering sea topex altimeter measurements of sea surface topography seawifs imagery of chlorophyll a concentration and shipboard measurements of salinity and fluorescence are used to describe the surface structure of the green belt and its relationship to the bering slope current eddy field during the 2000 2001 and 2002 spring blooms during spring 2000 high surface chlorophyll a concentrations 10 mg m 3 were observed within a 200 km wide band adjacent to and seaward of the shelf break in the northwest bering sea this high concentration chlorophyll band was associated with an anticyclonic eddy group that propagated along isobaths above the continental slope and entrained chlorophyll from the shelf slope front during spring 2001 anticyclonic eddies in the northwest bering sea had propagated off slope prior to the onset of the spring bloom and were too far from the shelf slope front to entrain frontal chlorophyll during the bloom a second chlorophyll front associated with the leading edge of the off slope eddies was observed between these two fronts was a region of relatively low chlorophyll a concentration 1 mg m 3 the eddy field during the 2002 spring bloom was observed to propagate northwestward adjacent to the shelf break and entrain chlorophyll from the shelf slope region in a manner similar to what was observed during the 2000 spring bloom these observations suggest that eddies are important if not the principal agents that cause variability in the distribution of chlorophyll during the spring bloom in the central bering sea

caldera rim collapse a hidden volcanic hazard following the emblematic flank collapse of mount st helens in 1981 numerous models of flank sliding have been proposed these models have allowed to largely improve the understanding of mechanisms involved in such landslides which represent a tremendous risk for populations living around volcanoes in this article a new mode of landslide formation related to buried calderas is described the model emphasizes the paramount importance of the hidden ring fault that even when the caldera is buried still remains a plane of weakness in the core of the edifice under certain conditions this plane of weakness becomes activated as the upper part of a pre existing critical slip surface and is used in the emplacement of huge landslides which travel downslope at a very high velocity a natural example is taken from piton de la fournaise volcano la reunion island indian ocean it reveals that the primary cause triggering caldera rim collapse is partial unbuttressing of the flank of the volcano in the natural example this occurs through regressive erosion that excavates deep canyon in the direction of the buried caldera but other mechanisms may exist on account of the large volumes of material involved in caldera rim collapse as well as their long runout distances such a volcanic hazard should be taken into account on every volcano where buried calderas are suspected

bering sea cyclonic and anticyclonic eddies observed during summer 2000 and 2001 abstract using satellite altimeter and ship data bering sea cyclonic and anticyclonic eddies were observed in summer 2000 and 2001 to examine their biological chemical and physical structures results from the ship transect revealed the interactions between the physical and biological conditions of bering sea eddies at the center of a cyclonic anticlockwise eddy upwelling was transporting nutrient no3 no2 rich water 25 m to the surface which resulted in relatively high chlorophyll a concentrations 1 0 mg m 3 developing under the pycnocline in contrast in the center of an anticyclonic clockwise eddy there was downwelling this downwelling of surface warm water was destroying a cold layer at about 150 m depth caused by winter convection however around the periphery of the anticyclonic eddy the isopycnals were tilted up and nutrient rich water was being transported along with them up into the euphotic zone so that high chlorophyll a concentrations were being developed above the pycnocline inside the anticyclonic eddy

3 d water properties and geostrophic circulation on the eastern bering sea shelf abstract the national oceanic and atmospheric administration s noaa alaska fisheries science center bottom trawl survey samples demersal fish at over 350 sites on the eastern bering sea continental shelf each summer on a 37 37 km 2 grid rugged conductivity temperature depth sensors ctds were added to the net hauls to obtain gridded data sets of temperature and salinity measurements for 2008 2010 results reveal the three dimensional thermohaline structure of the shelf including the cold pool and areas of fresher water around st matthew island and in bristol bay horizontal gradients are often strongest roughly along the 50 m and 100 m isobaths that traditionally separate the inner and outer shelf from the middle shelf centered along the 70 m isobath the summer mixed layer depth is less than 30 m over much of the region it reaches the bottom along the alaska peninsula in water depths greater than 70 m showing that the boundary of the well mixed inner shelf is not always at the 50 m isobath the greatest upper to lower layer density difference is found across the shelf north of 59 n the salinity difference is the main contributor to this density difference over most of the region in 2008 and 2010 but the temperature difference dominates in 2009 due to decreased ice melt and reduced freshening near st matthew island the geostrophic velocity relative to the bottom shows northwestward flow seaward of the 100 m isobath and northwestward transports integrated across the shelf of 0 10 0 25 10 6 m 3 s in 2008 and 2010 there was clockwise circulation in a region of less saline water around st matthew island in 2009 that fresher lens did not exist and flow was more concentrated along the 100 m isobath bringing saltier water across the shelf

altimeter observations of the bering slope current eddy field topex altimeter data is analyzed to describe the mesoscale eddy field associated with the bering slope current in the central bering sea the observed eddy field is well defined seasonally modulated and interannually variable the most energetic mesoscale eddy activity typically occurs during the spring and summer months and is characterized by a peak along isobath wavelength of 212 km and a representative wave period of 4 months spatial variability of annual period phase indicates that the axis of bering slope current exhibits seasonal offshore migration

comparison of warm and cold years on the southeastern bering sea shelf and some implications for the ecosystem the southeastern middle shelf of the bering sea has exhibited extreme variability in sea ice extent temperature and the distribution and abundance of species at multiple trophic levels over the past four decades from 1972 2000 there was high interannual variability of areal extent of sea ice during spring march april in 2000 this shifted to a 5 year 2001 2005 period of low ice extent during spring which transitioned to a 4 year 2007 2010 period of extensive sea ice high low areal extent of sea ice in spring was associated with cold warm water column temperatures for the following 6 7 months the ocean currents also differed between warm and cold years during cold years the monthly mean currents over the shelf were largely westward while in warm years the direction of currents was more variable with northward flow during december february and relatively weak flow during the remainder of the year the types and abundance of zooplankton differed sharply between warm and cold years this was especially true during the prolonged warm period 2001 2005 and cold period 2007 2010 and was less evident during the years of high interannual variability during the warm period there was a lack of large copepods and euphausiids over the shelf however their populations rebounded during cold period small crustacean zooplankton taxa did not appear to vary between and warm and cold years for both walleye pollock and pacific cod year class strength recruitment was low during the prolonged warm period but improved during the following cold period year class strength did not appear to vary as a function of warm and cold years during the period of high year to year variability also recruitment of arrowtooth flounder a predator of pollock and cod did not appear influenced by the warm or cold years finally the distribution and relative abundance of fin whales appeared to differ in warm and cold years with fewer whales on the southeastern middle shelf during warm years

stratification on the eastern bering sea shelf revisited abstract the timing and magnitude of stratification can have profound influences on the marine ecosystem on the eastern bering sea shelf in the absence of strong wind mixing stratification can be initiated by the melting of seasonal sea ice or by springtime warming of the surface temperature and salinity both influence the stratification of the eastern bering sea shelf with their relative importance varying spatially and temporally in the northern middle shelf domain north of 60 n salinity stratification is often as important as temperature stratification on the southern middle shelf while the influence of temperature on stratification dominates during summer the influence of salinity stratification plays a role in the interannual variability mooring 2 m2 56 9 n 164 1 w has been deployed at 70 m depth in the southern middle shelf domain since 1995 data from this mooring show that stratification typically begins to set up in may and to break down in september october but these dates can vary by 30 d while no trend is found in the timing of the spring setup the fall stratification breakdown exhibited a trend toward later breakdown 2 d later per year from 1996 to 2009 results suggest that it may be difficult to forecast stratification on the eastern bering sea shelf from climate models as simple indices of wind mixing or heat fluxes are not correlated with stratification contrary to intuition the strength of summer stratification is not correlated with depth averaged temperature warm years such as 2000 and 2001 can have low stratification and cold years such as 2007 can have very high stratification this decoupling of stratification and temperature has implications for forecasting the ecosystem in the face of climate change as we cannot assume that projections of a warmer climate simply imply higher stratification in the future

transport through unimak pass alaska we examined inflow through unimak pass 200 m deep which is the only major connection between the shelves of the north pacific ocean and the eastern bering sea geostrophic transport was generally northward from the gulf of alaska into the bering sea the flow through the pass appeared to be modulated by the seasonal cycle of freshwater discharge on shorter time scales transport also was affected by semi daily variations in tidal mixing this effect was significant and not anticipated near bottom currents measured from moorings were maximum during winter and significantly correlated r 0 7 with the alongshore winds although the flow through unimak pass transported some nutrients from the north pacific ocean the gulf of alaska shelf is not the major source of nutrients to the bering sea shelf

seasonal and interannual variability of the bering slope current abstract time series of sea surface height anomalies derived from satellite altimeters and absolute dynamic topography are used to examine variability in the geostrophic surface currents in the eastern bering sea the data suggest that the primary source of water to the eastern boundary currents of the bering sea is flow from the north pacific through amukta pass an aleutian pass located at 172 w the aleutian north slope current ansc is strongest and most variable in the winter months upon turning the corner to feed the bering slope current bsc the ansc bifurcates resulting in a westward flow at 54 n in addition to the northwestward flowing bsc this westward countercurrent has not been previously observed in the winter the bsc flows strongly to the northwest and is located near the shelf break during the rest of the year the mean flow is broadly northwestward but weaker and farther from the shelf break a continuous northwestward flowing current is rarely observed in the altimetry data except during winter months as it is overwhelmed by mesoscale variability the strength of the bsc in winter is correlated with the multivariate enso index and the north pacific index on interannual timescales

anisotropies of partial anhysteretic remanence and susceptibility in compacted black shales grainsize and composition dependent magnetic fabric we have quantified the magnetic fabric of the highly compacted heebner shale pennsylvanian of kansas in terms of the anisotropies of partial anhysteretic remanent magnetization parm and low field susceptibility hysteresis loops exhibit almost purely paramagnetic behavior and the ratio of high field linear susceptibility to low field susceptibility is very close to one the susceptibility anisotropy therefore reflects the fabric of the paramagnetic matrix while the remanent anisotropies are due to the preferred orientations of ferrimagnetic grains both the paramagnetic and ferrimagnetic fabrics are dominated by a horizontal foliation with a weak northeasterly lineation but the parm anisotropies are generally much larger than the susceptibility anisotropy the degree of foliation measured with parm s decreases markedly with increasing coercivities indicating stronger foliation in the coarser grainsize fraction in contrast the parm mean lineations are grainsize independent suggesting a post compactional origin related to tectonic compression the susceptibility lineation appears to have a slightly different azimuth and may therefore be due to paleocurrents

shifts in the physical environment in the pacific arctic and implications for ecological timing and conditions abstract the northern bering sea and chukchi sea represent the gateway from the pacific to the arctic this contiguous marine system encompasses one of the largest continental shelves in the world and serves as the sole point of connection between the north pacific and arctic ocean this region has unique attributes and complex dynamics driven by the convergence of distinct water masses dynamic currents advection between pacific and arctic systems and important latitudinal gradients relevant to stratification and water mass structure water temperature and seasonal ice cover many processes and interactions in the region appear to be changing with important implications for both hydrography and ecology our analyses access remote and local data sources in us and russian waters to characterize oceanographic conditions and analyze the implications of dramatic shifts in recent years previously this region appeared resistant to trends apparent elsewhere in the greater arctic now the pacific arctic also appears to be in rapid transition the conditions observed in 2017 2019 are unprecedented we note important shifts in the phenology and magnitude of physical variables including sea ice extent concentration and duration as well as extreme reduction in the extent and intensity of the related bering sea cold pool we also note distinct regional dynamics in sea surface temperature in the bering chukchi system distinguishing western eastern and northern areas of the bering sea specifically our analyses distinguish the northern bering sea as an important transition zone between the pacific and arctic with higher frequency variability in sea surface temperature anomalies our results suggest that the strength and position of the aleutian low may be linked to warm and cold phases in the bering sea and has an important role in large scale circulation while cold winds out of the north are necessary to form ice in the northern bering sea strong winds may be associated with weak sea ice as wind action may break ice and enhance vertical mixing counteracting enhanced sea ice production from the advection of cold air research in this important region is complicated by international borders but may be enhanced through international collaboration this analysis represents an attempt to integrate data across russian and us waters to more fully represent system wide processes to contrast regional trends and to better understand physical interactions

is this magnetic fabric normal a review and case studies in volcanic formations abstract when studying weakly anisotropic rocks non standard relationships between magnetic and expected petrofabric axes are sometimes observed in particular the so called inverse or intermediate fabrics a review of possible explanations for these effects is presented and leads to the conclusion that a majority of abnormal magnetic fabrics often encountered in magnetite rich volcanic rocks are due to abnormal preferred orientations or distributions of magnetite grains through case studies in dikes from skye island yemen and the iberian peninsula and magma and ash flows from the ethiopian traps and southern corsica the various possible origins of such fabrics are examined it is possible to retrieve reliable magma flow directions from these sites provided proper data selection is performed leading to non unique interpretations

stress fields generating ring faults in volcanoes ring faults in volcanoes have been recognized for a long time but their mechanics of formation is still poorly understood while the subsidence on a ring fault during a large eruption from the associated chamber is easily understood the initiation of the fault itself has been difficult to explain for a ring fault to form the tensile and shear stresses at the surface of the volcano must peak at a certain radial distance from the surface point above the center of the chamber empirical evidence however shows that during most periods of unrest in active volcanoes the stress field is not favorable for the initiation of ring faults we made several tens of boundary element models of magma chambers of various geometries and subject to different loading conditions the results indicate that overpressure or underpressure in the chamber as the only loading is unlikely to initiate ring faults for a spherical chamber subject to horizontal extension or doming and a sill like chamber subject to horizontal extension the tensile and shear stresses at the surface of the volcano peak at a certain radial distance from the surface point above the center of the chamber then however the maximum stresses occur at the boundary of the chamber itself which would normally lead to sheet injection rather than to ring fault formation by contrast a sill like magma chamber subject to doming gives rise to a stress field suitable for the initiation of a ring fault

emplacement of dike swarms from the island of ilhabela se brazil and its relationship with the south atlantic ocean opening revealed by magnetic fabrics abstract magnetic fabric and rock magnetism studies were performed on 153 dikes from diabase lamprophyre and alkaline dike swarms that outcrop on the island of ilhabela ne sao paulo state the dikes crosscut archean and proterozoic polymetamorphic rocks their thicknesses range from a few centimeters to 4 or 5 m they trend predominantly n30 60e with a very steep 60 to vertical dip magnetic fabrics were determined using anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanent magnetization aarm rock magnetism measurements reveal that the magnetic mineral of the three swarms is magnetite mainly with grains in the range of 2 5 m for a few dikes these grains are the carriers of bulk magnetic susceptibility but are not responsible for the ams which is due to fe bearing minerals as shown by aarm the main ams fabric recognized in the swarms is due to magma flow in which the kmax kint plane is parallel to the dike s plane and the magnetic foliation pole kmin is perpendicular to it the analysis of the kmax inclination showed that the dikes were fed by horizontal to vertical flows however for the minority of the dikes the ams and aarm tensors are not coaxial the aarmmax is oriented n30 60 w approximately perpendicular to the kmax the aarmmax orientation is similar to the direction of a fault system located mainly in the santos marginal basin which was formed in the cretaceous rifting during the south atlantic opening the aarm fabric is tectonic in origin and is parallel with the extensional paleostress responsible for the gondwana break up the comparison of ams and aarm fabrics suggests that diabase and lamprophyre dikes were emplaced in three distinct events in the earliest stages of the south atlantic opening later the alkaline dikes were emplaced in the final stages of the atlantic opening

factors affecting the relative importance of shape and distribution anisotropy in rocks theory and experiments abstract the effect of magnetic interactions has been commonly neglected in the interpretation of the anisotropy of magnetic susceptibility ams of igneous rocks despite the likely occurrence of magnetic interactions due to their relatively high content of ferromagnetic minerals current models of magnetic interactions apply to highly simplified systems and are not directly applicable to natural rocks on this work a model that includes variables previously neglected is used to assess the relative importance of magnetically interactive and non interactive fractions in a rock results of the model show that a variable proportion of magnetically interactive grains the relative values of the bulk susceptibility degree of anisotropy and orientation of the anisotropies characteristic for each fraction all play an important role in controlling the rock ams experimental evidence illustrating the importance of the distribution of the ferromagnetic grains within a specimen is also reported and supports the model predictions both theory and experiments indicate that if a rock has unequant magnetic grains and clusters of magnetic grains the contribution to the ams of the rock will depend on the relative values of all the model variables and not only on the occurrence of magnetic interactions as previously presumed

effects of magnetic interactions in anisotropy of magnetic susceptibility models experiments and implications for igneous rock fabrics quantification besides granites of the ilmenite series in which the anisotropy of magnetic susceptibility ams is mainly controlled by paramagnetic minerals the ams of igneous rocks is commonly interpreted as the result of the shape preferred orientation of unequant ferromagnetic grains in a few instances the anisotropy due to the distribution of ferromagnetic grains irrespective of their shape has also been proposed as an important ams source former analytical models that consider infinite geometry of identical and uniformly magnetized and coaxial particles confirm that shape fabric may be overcome by dipolar contributions if neighboring grains are close enough to each other to magnetically interact on these bases we present and experimentally validate a two grain macroscopic numerical model in which each grain carries its own magnetic anisotropy volume orientation and location in space compared with analytical predictions and available experiments our results allow to list and quantify the factors that affect the effects of magnetic interactions in particular we discuss the effects of i the infinite geometry used in the analytical models ii the intrinsic shape anisotropy of the grains iii the relative orientation in space of the grains and iv the spatial distribution of grains with a particular focus on the inter grain distance distribution using documented case studies these findings are summarized and discussed in the framework of the generalized total ams tensor recently introduced by canon tapia canon tapia e 2001 factors affecting the relative importance of shape and distribution anisotropy in rocks theory and experiments tectonophysics 340 117 131 the most important result of our work is that analytical models far overestimate the role of magnetic interaction in rock fabric quantification considering natural rocks as an assemblage of interacting and non interacting grains and that the effects of interaction are reduced by i the finite geometry of the interacting clusters ii the relative orientation between interacting grains iii their heterogeneity in orientation shape and bulk susceptibility and iv their inter distance distribution we reconcile analytical models and experiments with real case studies that minimize the role of magnetic interaction onto the measured ams limitations of our results are discussed and guidelines are provided for the use of ams in geological interpretation of igneous rock fabrics where magnetic interactions are likely to occur

conditions favouring catastrophic landslides on tenerife canary islands catastrophic failures of volcano flanks represent one of the most hazardous geological phenomena these immense mass movements originate either by increasing the destabilizing forces driving forces or by reducing the strength of the materials involved or both the study of large volcanic landslides on tenerife suggests that the presence of weak residual soils palaeosols in combination with the pre existence of deep narrow canyons created by fluvial erosion have played a fundamental role in the initiation of large scale sector collapses of the volcanic edifice regardless of the triggering mechanism considered residual soils strongly reduce the material strength during undrained loading while pre existing canyons control the lateral limits of the landslide the existence of a wet climate in some sectors of the island favours these circumstances

emplacement of dikes sills and crustal magma chambers at divergent plate boundaries abstract a model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1 3 km of the oceanic crust the number and lengths of dikes in any particular swarm should increase but that the thickness should decrease with depth in the crust these predictions are supported by field observations it is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that at a certain level in the crust subsequent dikes change into sills such sills may develop into crustal magma chambers either when many nearby sills combine into a large one or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid for the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m whereas at the slowest spreading ridges the initial thickness must exceed 170 m because thin sills are much more likely to form than thick sills the probability of sills developing into crustal chambers increases with dike intrusion frequency hence with spreading rate furthermore the model predicts first that with increasing spreading rate the length of the chamber along the ridge axis should increase and second that the most likely site for magma chambers is at depths of 2 4 km both these predictions are supported by available data

flank instability assessment at kick em jenny submarine volcano grenada lesser antilles a multidisciplinary approach using experiments and modeling kick em jenny kej located ca 8 km north of the island of grenada is the only active submarine volcano of the lesser antilles volcanic arc previous investigations of kej revealed that it lies within a collapse scar inherited from a past flank instability episode to assess the likelihood of future collapse we employ here a combined laboratory and modeling approach lavas collected using a remotely operated vehicle rov provided samples to perform the first rock physical property measurements for the materials comprising the kej edifice uniaxial and triaxial deformation experiments showed that the dominant failure mode within the edifice host rock is brittle edifice fractures such as those at champagne vent will therefore assist the outgassing of the nearby magma filled conduit favoring effusive behavior these laboratory data were then used as input parameters in models of slope stability first relative slope stability analysis revealed that the sw to n sector of the volcano displays a deficit of mass volume with respect to a volcanoid ideal 3d surface slope stability analysis using a limit equilibrium method lem showed that kej is currently stable since all values of stability factor or factor of safety fs are greater than unity the lowest values of fs were found for the sw nw sector of the volcano the sector displaying a mass volume deficit although currently stable kej may become unstable in the future instability severe reductions in fs could result for example from overpressurization due to the growth of a cryptodome our modeling has shown that instability induced flank collapse will most likely initiate from the sw nw sector of kej therefore mobilizing a volume of at least ca 0 7 km3 the mobilization of ca 0 7 km3 of material is certainly capable of generating a tsunami that poses a significant hazard to the southern islands of the west indies

sea ice and water column structure on the eastern bering sea shelf abstract seasonal sea ice is a defining characteristic of the eastern bering sea shelf and plays a critical role in determining the vertical structure of temperature and salinity over this shelf ice movement relative to local winds ice composition and the impact of both arrival and retreat of ice on the water column at four mooring sites over the middle shelf are examined ice forms primarily in coastal regions and is advected over the southern and outer shelves ice drift from satellite data for two representative years 2003 and 2007 was 2 of local ncep wind speed and oriented 44 to the right of the winds r 2 0 25 measurements from 30 ice cores collected in 2006 2009 gave an average salinity of 5 62 0 88 and an average nitrate concentration of 0 99 0 83 m time series data collected at the biophysical moorings in the bering sea 1995 2012 were used to explore the evolution of the water column under ice at the northern mooring m8 the water column had mixed and cooled to 1 c prior to the arrival of ice little melt occurred after ice arrival at the other three moorings the ocean temperature was 2 4 c when ice arrived resulting in extensive melt melting ice freshened and cooled the upper water column resulting in stratification which persisted for 10 25 days wind induced water column mixing occurred more slowly under the ice than in ice free waters an estimated 1 4 m of ice melted with the first arrival of ice at the three southern moorings where the latent heat of fusion accounted for approximately half the observed cooling during ice retreat there appeared to be little ice melt around the southern two moorings but an estimated 0 8 m at m5 and m8 the extent of ice melt sets up the water column for the following summer

theory and application of the day plot mrs ms versus hcr hc 2 application to data for rocks sediments and soils 1 new theoretical curves relating the hysteresis parameters mrs ms and hcr hc for single domain sd superparamagnetic sp pseudo single domain psd and multidomain md grains and their mixtures are applied to published data for natural materials the day plot of mrs ms versus hcr hc has been used to crudely classify samples into box like sd psd and md or sometimes incorrectly md sp regions with arbitrary boundaries new type curves for md psd sd md and sd sp grains and mixtures permit more subtle and precise modeling the predicted md trend and its junction with the psd trend are observed in two data sets for magnetite spherules from carbonate rocks and for temperature varying hysteresis results spanning the verwey transition the latter data are the basis of a suggested new method for pinpointing the psd md threshold size selected data for pottery clays soils and paleosols generally follow sd md type curves and indicate intermediate size psd magnetite with narrow to broad size distributions a lake sediment section with known grain size progression tracks in the predicted sense along the sd md trend selected data for glaciomarine and pelagic sediments are also generally compatible with sd md trends examples of remagnetized carbonate rocks submarine basaltic glasses and glassy rims of pillow basalts all follow predicted sp sd or sp psd mixing curves with a large range in volume fraction of sp grains 0 75 but a narrow range of sp particle sizes 10 2 nm larger sp grains spanning the range to sd size 25 30 nm are absent for unknown reasons oceanic dolerites gabbros and serpentinized peridotites in some cases fall in a novel region of the day plot parallel to but below magnetite sd md mixing curves

magnetic fabric and inferred flow direction of dikes conesheets and sill swarms isle of skye scotland abstract we have measured the ams of 16 dikes 35 conesheets and three sills associated with the cuillin hills magmatic center of the isle of skye in the inner hebrides of scotland and of nine dikes located in the regional dike swarm of skye sixty three intrusives totalling 734 samples were studied to determine the plumbing of the skye volcanic system low field susceptibility versus temperature k t identified three different mineral phases in the area namely ti rich magnetite pyrrhotite and titanomaghemite the petrofabrics of the 63 intrusives yielded coherent flow azimuths regardless of their time of emplacement three main types of magnetic fabric a c were found fabric type a plane kmax kint parallel to the dike plane represents the magma flow direction within the intrusives and is the dominant fabric 55 of all the intrusives within the cuillin hills magmatic center and its regional dike swarm the kmax inclinations show that 55 of the intrusives were fed by horizontal to sub horizontal ams inclination of the kmax axis is equal or less than 30 magma fluxes and the rest of them were fed by inclined to vertical fluxes horizontal magma flow means lateral magma injection inside fractures and becomes more probable as the source is located further away

interpreting anomalous magnetic fabrics in ophiolite dikes abstract anisotropy of magnetic susceptibility ams may reveal mineral orientation distributions defining magmatic flow axes in igneous dikes the mafic silicates are the best indication of magmatic flow but fe ti accessories may contribute more to the bulk susceptibility if the orientation distributions of the two subfabrics are incongruent anomalous fabrics will occur that do not reflect magma flow axes for ophiolite dikes ocean floor metamorphism changes the mineralogy producing new fe oxides by retrogression and exsolution from mafic silicates and by the oxidation of primary oxides incompatibly oriented ferro magnetic subfabrics may be isolated by anisotropy of anhysteretic remanence aarm anomalous ams fabrics in ophiolites elsewhere have been attributed to inverse fabric contributions from single domain magnetite in varying combinations however in ophiolite dikes from the troodos ophiolite of cyprus anomalous fabrics arise from ocean floor metamorphism extensively or completely replacing the original magnetite and titanomagnetite accessory phases with titanomagnetite fe 2 4 ti 0 6 o 4 tm60 and its oxidised versions titanomaghemite to varying degrees according to depth beneath the ocean floor distance from spreading axis and proximity to transform faults at best the oxide orientation distribution defined by aarm could only be indirectly related to magma flow if its nucleation orientation controlled by a host lattice however more commonly the topotactic lattice reorganization produces weaker arm fabric anisotropies although recrystallized oxidised tm60 dominates the bulk low field susceptibility its anisotropy is generally too feeble to compete with the flow fabric defined by the ams contribution from paramagnetic mafic silicates

age of the ponta grossa dike swarm brazil and implications to parana flood volcanism the ponta grossa dike swarm pgds occurs in a nw trending 200 km wide zone exposed just east of the parana basin in southeastern brazil the predominantly basaltic dikes intrude crystalline basement paleozoic mesozoic sediments and rarely flows of the parana etendeka flood volcanic province pep the pgds resembles the failed arm of a rift rift rift triple junction related to the separation of south america and africa detailed geochemical studies of the dikes including major minor trace element and sr nd isotopic analyses indicate that they probably represent feeders for the voluminous phase of flood volcanism represented by relatively uncontaminated predominantly high tio2 lavas of the northern pep where lava accumulations reach 1700 m thick 40ar 39ar stepwise degassing data using both laser and radiofrequency induction furnace on plagioclase separates from eighteen dikes and one sill yield seventeen plateau ages three are between 120 7 1 3 ma and 125 8 0 6 ma and fourteen are clustered between 129 2 0 4 ma and 131 4 0 5 ma isochron ages are not significantly different from the plateau ages and plateau ages are adopted in all but two cases as being the most accurate age estimates the age probability distribution for the dominant pulse 131 4 0 4 to 129 2 0 4 ma shows a pronounced peak at 130 5 ma this distribution probably reflects the magma production history in the region the new geochronologic data are consistent with conclusions based on paleomagnetic and chemical stratigraphic data that the pgds is younger than the volumetrically dominant majority of volcanism in the southern pep which occurred at 133 132 ma the younger commonly ne trending dikes may reflect the initiation of full drift which was coincident with major basin development in the adjacent continental borderland at 125 120 ma the pgds may indeed represent the failed arm of a rift rift rift triple junction but the triple junction did not coincide exactly in time or space with the site that would be inferred for plume impact

pressure evolution during explosive caldera forming eruptions abstract caldera forming eruptions of silicic magmas result from a complex coupling of the mechanics and fluid dynamics of the associated magma chamber field studies of caldera forming eruption products suggest that great pressure variations occur inside the magma chamber and associated conduits during these eruptions pressure evolution during explosive caldera forming eruptions is investigated through a simple model that describes the first order quantitative behaviour of the chamber we consider a piston like model that assumes a coherent block subsiding along circular sub vertical ring faults into the magma chamber this subsidence occurs after significant decompression of the chamber by an initial central vent eruption we assume that the initial pressure distribution in the chamber is magmastatic once collapse has begun the chamber roof is supported by the magma so that magma pressure at the chamber roof increases to lithostatic we suggest that pressure variations during caldera forming eruptions are mainly controlled by variations in magma volatile content regardless of what induces the formation of ring faults the model suggests that the occurrence of explosive caldera forming events depends on the strength of the chamber walls and the depth water content and aspect ratio of the magma chamber no significant differences exist between model results for a cylindrical or a more realistic elliptical magma chamber geometry of comparable aspect ratio assuming a constant strength of the host rock the mass fraction of magma that must be erupted during the central vent phase in order to trigger caldera collapse ranges for deep gas poor chambers from a few percent up to 40 for shallow gas rich chambers the model suggests that zoned chambers tend to collapse earlier than homogeneous chambers dike shaped chambers will erupt less magma than sill like chambers before caldera collapse initiates although dike like geometries are not associated with stress fields appropriate to create ring faults the model suggests that once initiated caldera collapse will tend to force out most or all of the volatile rich magma from the chamber for volatile rich magma chambers the total volume of erupted magma during caldera forming event is of the same order as the chamber volume the model also explains the variation in the erupted mass during the different phases of explosive caldera forming eruptions and is in good agreement with natural examples

evidence from magnetic fabric for the flow pattern of magma in the mackenzie giant radiating dyke swarm continental flood basalts and their associated swarms of mafic dykes are thought to result from melting in mantle plumes1 3 but the processes by which the resulting magma makes its way from mantle to crust remain obscure here we use magnetic anisotropy measurements to reconstruct the pattern of magma flow in the proterozoic mackenzie dyke swarm our results indicate that magma was injected vertically within 500 km of the focal point of the dyke swarm and then travelled horizontally at all further distances out to at least 2 100 km the sharp transition from vertical to horizontal flow at between 500 and 600 km may correspond to the outer boundary of melt generation in the mantle plume believed to be responsible for the mackenzie igneous events

a theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals summary susceptibility thermo remanent magnetization trm and isothermal remanent magnetization irm anisotropy ellipsoids have been determined for several rock samples the results indicate that the ellipsoid of initial susceptibility is less anisotropic than the trm and low field irm ellipsoids which are found experimentally to be of identical shape this suggests that palaeomagnetic data for anisotropic rocks may be corrected by using the anisotropy ellipsoid determined from magnetically non destructive low field irm measurements such irm measurements can also be used to obtain anisotropy axes of samples which are inherently anisotropic but which have a susceptibility which is too weak to be accurately measured the results for a series of artificial anisotropic samples containing magnetite particles of different sizes in the range 0 2 90 m were very similar to those for the rocks in contrast a comparison of the susceptibility and irm ellipsoids for anisotropic samples containing particles from a magnetic tape gave very different results in accordance with theory such results imply that susceptibility and irm ellipsoids could be used to determine whether anisotropic rocks contain uniaxial single domain particles magnetization confined to the easy axis or whether the particles are essentially multidomain

structure of the mobile south flank of kilauea volcano hawaii how do huge landslides occur in hawaii we can begin to address this question by examining an incipient landslide structure on the mobile south flank of kilauea volcano between 1970 and 1989 this part of the south flank moved in a southeast direction seaward by 4 m on its eastern end and by 10 m on its western end parallel to the western boundary of a large oifshore topographic bench we show that abrupt decreases in cumulative flank movement correspond to abrupt decreases in hypocenter concentration most of the earthquakes occur in a 7 to 10 km deep band beneath the south flank and relative relocation of a subset of these events shows that they are derived from a single horizontal plane which appears to be a decollement focal mechanisms for this deep seismicity are dominated by thrust fault mechanisms with the overlying block moving seaward a comparison of south flank bathymetry with decollement structures on other volcanoes suggests that the distal end of this decollement coincides with the distal end of the offshore topographic bench 30 50 km from shore and 5 km below sea level the fault system forming the hilina pali is connected with this decollement so that slip on the hilina pali s normal faults coincides with slip of the decollement and with reverse faulting of the distal end of the bench this interpretation is supported by studies of the events associated with the m7 2 kalapana earthquake in 1975 when the hilina pali is connected to the outer bench with a decollement inferred from seismicity a wedge 8 km thick adjacent to kilauea s magma system and 1 2 km thick along the edge of the outer bench an area over 3000 km2 is defined the thickness adjacent to the magma system decreases eastward along kilauea s east rift zone and southwestward from kilauea caldera whereas the thickness of the lip of the outer bench decreases only eastward geologic observations and geophysical studies of the subaerial flank reveal an extensive magma system beneath the koae fault system as well as beneath kilauea caldera and its east rift zone we infer that the boundary of the proximal end of the mobile flank is the southern boundary of this magma system and that this boundary coincides with the southern boundaries of both the koae fault system and kilauea s east rift zone

formation of collapse calderas collapse calderas are common in many volcanic regions but the mechanics of their formation is not well understood i propose that collapse calderas may form as a result of the growth of associated shallow magma chambers excess magmatic pressure in a shallow chamber gives rise to tensile stresses at the surface but the location and magnitude of the maximum tensile stress depend on the shape of and depth d to the top of the chamber this model indicates that penny shaped chambers i e sill like chambers of circular horizontal cross sections are most likely to initiate collapse calderas for such a chamber with horizontal radius a the higher the a d radius depth ratio the greater the probability of forming a collapse caldera

effects of a shelf polynya on flow and water properties in the northern bering sea we present long term 230 day measurements from three moorings in the vicinity of saint lawrence island on the northern bering sea shelf one mooring was deployed near the polynya which often exists south of the island these data confirm the existence of strong 0 15 m s 1 flow toward bering strait northwest of the island in anadyr strait and suggest that regional circulation results in a moderate 0 03 5 m s 1 mean flow eastward along the southern coast of saint lawrence island coherent variations in the regional circulation and variations in the geostrophic wind account for much of the low frequency current fluctuation south of saint lawrence island however there were 11 events when offshore northerly winds coincided with the temperature following the freezing point increasing salinity and reversal of the current this suggests that ice formation and the ensuing brine rejection affect flow in the vicinity of the polynya scaling of a simplified momentum equation indicates that the cross shelf density gradient was a possible mechanism for some of the reversals salinity data show a seasonal cycle as well as individual events of brine rejection during an average event time of 65 hours the salinity increased by 8 10 3 g kg 1 h 1 the heat flux can be estimated as 535 wm 2 these fluxes correspond to an ice production of 5 m during winter 1981 extrapolation of our results and comparison with the freshwater cycle suggest that brine rejection is an important component of the regional salt budget

characteristics and variability of the inner front of the southeastern bering sea the inner front of the southeastern bering sea shows marked spatial variability in frontal characteristics created by regional differences in forcing mechanisms differences in forcing mechanisms sea ice advance retreat and storm strength and timing and early spring water properties result in strong interannual variability in biological chemical and physical features near the front we have developed a simple model based on surface heat flux and water column mixing to explain the existence of cold belts cont shelf res 19 14 1999 1833 associated with such fronts hydrography fluorescence and nutrient observations show that pumping of nutrients into the euphotic zone occurs and this can prolong primary production at the inner front the effectiveness of this process depends on two factors the existence of a reservoir of nutrients in the lower layer on the middle shelf and the occurrence of sufficient wind and tidal energy to mix the water column

plastic behavior of magnetite and high strains obtained from magnetic fabrics in the parry sound shear zone ontario grenville province abstract magnetic anisotropy and quantitative petrology of a transition from relatively undeformed metaanorthosite protolith to highly deformed ultramylonite in the parry sound shear zone provide information on magnetite as a kinematic indicator rock magnetic experiments show that anisotropy of magnetic susceptibility ams is controlled by magnetite in the mylonites and ultramylonites and by paramagnetic minerals hornblende biotite ilmenite in the protolith geothermometry in the ultramylonite indicates a temperature of 630 50 c during deformation a deformation mechanism map calculated from published experimental work for 630 c indicates that magnetite in the parry sound shear zone deformed plastically rather than via rigid body rotation the ams orientations accurately track a foliation trajectory in a portion of the shear zone which allows the utilization of orientation based strain models to determine shear strains shear strains of 1 9 were obtained and an empirical correlation of strain with the ellipsoid shape resulted in the logarithmic relationships mmax 0 15 ln x and mmin 0 13 ln z where mi ki kmean describes the principal ams axes or k max k min x z 0 14 these ams strain correlations were subsequently applied to mylonites and ultramylonites elsewhere in the shear zone predicting shear strains as high as 13 these observations also reveal that ams fabrics do not saturate at high strains as would be expected for rigid body rotation due to the plastic behavior of magnetite during deformation

magnetic fabric and its significance in the 1400 ma mealy diabase dykes of labrador canada an anistropy of magnetic susceptibility ams study of the relatively fresh mealy diabase dyke swarm indicates a correlation between the ams and the deformational effects in the rocks several ams components were revealed during experimental work on 211 samples that had been heated to 640 c during a paleomagnetic study and 22 fresh samples progressively treated in alternating fields seventeen sites with slight or no deformation have a magnetic fabric composed of two major components the stable component attributed to a primary shape anisotropy has minimum and maximum principal susceptibility directions preferentially oriented vertically and along dyke strike n 60 e respectively this configuration is consistent with an origin due to a post emplacement stress field whereby horizontal extension acted on a vertical sheet of partially crystallized magma the orientation of this component with respect to the present horizontal suggests that the dykes have not been tilted since their emplacement the unstable component removed by fields up to 250 mt is attributed to induced domain anisotropy produced by mild tectonic stress during the grenville orogeny the approximate orientation of its principal components as found by the method of substitutional ellipsoids suggests an east west directed maximum stress four other sites with noticeable deformation have magnetic fabric that could not be reliably analyzed but which may be largely composed of domain and stress induced anisotropies developed during the grenville orogeny

swash aquifer interaction in the vicinity of the water table exit point on a sandy beach the coupling of sandy beach aquifers with the swash zone in the vicinity of the water table exit point is investigated through simultaneous measurements of the instantaneous shoreline swash front location pore pressures and the water table exit point the field observations reveal new insights into swash aquifer coupling not previously gleaned from measurements of pore pressure only in particular for the case where the exit point is seaward of the observation point the pore pressure response is correlated with the distance between the exit point and the shoreline in that when the distance is large the rate of pressure drop is fast and when the distance is small the rate decreases the observations expose limitations in a simple model describing exit point dynamics which is based only on the force balance on a particle of water at the sand surface and neglects subsurface pressures a new modified form of the model is shown to significantly improve the model data comparison through a parameterization of the effects of capillarity into the aquifer storage coefficient the model enables sufficiently accurate predictions of the exit point to determine when the swash uprush propagates over a saturated or a partially saturated sand surface potentially an important factor in the morphological evolution of the beach face observations of the shoreward propagation of the swash induced pore pressure waves ahead of the runup limit shows that the magnitude of the pressure fluctuation decays exponentially and that there is a linear increase in time lags behavior similar to that of tidally induced water table waves the location of the exit point and the intermittency of wave runup events is also shown to be significant in terms of the shore normal energy distribution seaward of the mean exit point location peak energies are small because of the saturated sand surface within the seepage face acting as a rigid lid and limiting pressure fluctuations landward of the mean exit point the peak energies grow before decreasing landward of the maximum shoreline position

tectonic fabric revealed by aarm of the proterozoic mafic dike swarm in the salvador city bahia state sao francisco craton ne brazil abstract magnetic fabric and rock magnetism studies were performed on 25 unmetamorphosed mafic dikes of the meso late proterozoic 1 02 ga dike swarm from salvador bahia state ne brazil this area lies in the north eastern part of the sao francisco craton which was dominantly formed reworked during the transamazonian orogeny 2 14 1 94 ga the dikes crop out along the beaches and in quarries around salvador city and cut across both amphibolite dikes and granulites their widths range from a few centimeters up to 30 m with an average of 4 m and show two main trends n140 190 and n100 120 with vertical dips magnetic fabrics were determined using both anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanent magnetization aarm the magnetic mineralogy was investigated by many experiments including remanent magnetization measurements at variable low temperatures 10 300 k mossbauer spectroscopy high temperature magnetization curves 25 700 c and scanning electron microscopy sem the rock magnetism study suggests pseudo single domain magnetite grains carrying the bulk magnetic susceptibility and aarm fabrics the magnetite grains found in these dikes are large and we discard the presence of single domain grains its composition is close to stoichiometric with low ti substitution and its verwey transition occurs around 120 k the main ams fabric recognized in the swarm is so called normal in which the kmax kint plane is parallel to the dike plane and the magnetic foliation pole kmin is perpendicular to it this fabric is interpreted as due to magma flow and analysis of the kmax inclination permitted to infer that approximately 80 of the dikes were fed by horizontal or sub horizontal flows kmax

volcanic hazards from bezymianny and bandai type eruptions major slope failures are a significant degradational process at volcanoes slope failures and associated explosive eruptions have resulted in more than 20 000 fatalities in the past 400 years the historic record provides evidence for at least six of these events in the past century several historic debris avalanches exceed 1 km3 in volume holocene avalanches an order of magnitude larger have traveled 50 100 km from the source volcano and affected areas of 500 1500 km2 historic eruptions associated with major slope failures include those with a magmatic component bezymianny type and those solely phreatic bandai type the associated gravitational failures remove major segments of the volcanoes creating massive horseshoe shaped depressions commonly of caldera size the paroxysmal phase of a bezymianny type eruption may include powerful lateral explosions and pumiceous pyroclastic flows it is often followed by construction of lava dome or pyroclastic cone in the new crater bandai type eruptions begin and end with the paroxysmal phase during which slope failure removes a portion of the edifice massive volcanic landslides can also occur without related explosive eruptions as at the unzen volcano in 1792 the main potential hazards from these events derive from lateral blasts the debris avalanche itself and avalanche induced tsunamis lateral blasts produced by sudden decompression of hydrothermal and or magmatic systems can devastate areas in excess of 500km2 at velocities exceeding 100 m s 1 the ratio of area covered to distance traveled for the mount st helens and bezymianny lateral blasts exceeds that of many pyroclastic flows or surges of comparable volume the potential for large scale lateral blasts is likely related to the location of magma at the time of slope failure and appears highest when magma has intruded into the upper edifice as at mount st helens and bezymianny debris avalanches can move faster than 100 ms 1 and travel tens of kilometers when not confined by valley walls avalanches can affect wide areas beyond the volcano s flanks tsunamis from debris avalanches at coastal volcanoes have caused more fatalities than have the landslides themselves or associated eruptions the probable travel distance l of avalanches can be estimated by considering the potential vertical drop h data from a catalog of around 200 debris avalanches indicates that the h l rations for avalanches with volumes of 0 1 1 km3 average 0 13 and range 0 09 0 18 for avalanches exceeding 1 km3 h l ratios average 0 09 and range 0 5 0 13 large scale deformation of the volcanic edefice and intense local seismicity precede many slope failures and can indicate the likely failure direction and orientation of potential lateral blasts the nature and duration of precursory activity vary widely and the timing of slope faliure greatly affects the type of associated eruption bandai type eruptions are particularly difficult to anticipate because they typically climax suddenly without precursory eruptions and may be preceded by only short periods of seismicity