the impact of airmass boundaries on the propagation of deep convection a modeling based study in a high cape low shear environment a suite of experiments conducted using a cloud resolving model is examined to assess the role that preexistingairmassboundariescanplayinregulatingstormpropagation the27may1997centraltexastornadic event is used to guide these experiments the environment of this event was characterized by multiple preexisting airmass boundaries large cape and weak vertical shear only the experiments with preexisting airmass boundaries produce back building storm propagation storm motion in opposition to the mean wind when both the cold front and dryline are present storm maintenance occursthroughthe quasi continuousmaintenance ofa setoflong livedupdraftsandnotthrough discrete updraft redevelopment since the cold front is not required for back building it is clear that back building in this environment does not require quasi continuous updraft maintenance the back building storm simulated with both the cold front and dryline is found to be anchored to the boundary zipper the intersection of the cold front and dryline however multiplepreexisting airmass boundaries are not required for back building since experiments with only a dryline also support back building a conceptual model of back building and boundary zippering is developed that highlights the important role that preexisting boundaries can play in back building propagation

an inverse modeling study in fram strait part i dynamics and circulation abstract in order to reconstruct the circulation in the northern greenland sea between 77 n and 81 n and the exchanges with the arctic ocean through fram strait a variational inverse model is applied to the density field observed in summer 1984 during the mizex 84 experiment an estimate of the three dimensional large scale pressure field is obtained in which the solution is decomposed into a limited number of vertical modes and the mode amplitudes are described by piece wise polynomials on a finite element grid the solution should be consistent with a frictional depth integrated vorticity balance and with the density data the global model parameters are tuned to ensure agreement between the retrieved geostrophic velocity and independent currentmeter data in a companion paper schlichtholz and houssais 1999b the same method but without dynamical constraint is applied to the same hydrographic dataset to perform a detailed water mass analysis and to estimate individual water mass transports a comprehensive picture of the summer geostrophic circulation in fram strait is obtained in which northward recirculations in the east greenland current egc and various recirculations from the west spitsbergen current wsc to the egc are identified it is suggested that the branch of the wsc following the upper western slope of the yermak plateau turns westward beyond 81 n and recirculates southward along the lower slope then merging with a westward recirculating branch south of 79 n at 79 n a southward net transport of 6 5 sv is found in the egc which combined with a northward net transport of only 1 5 sv in the wsc results in a fairly large outflow of 5 sv from the arctic ocean to the greenland sea the inverse solutions show that in summer the local induction of vorticity by the wind stress curl or by meridional advection of planetary vorticity should be small so that in the egc and in the wsc the vorticity balance is mainly achieved between the bottom pressure torque and dissipation of vorticity through bottom friction a substantial barotropic flow associated with along slope potential energy gradients is indeed identified on both sides of the strait

magnetic fabrics of the miocene ignimbrites from west cameroon implications for pyroclastic flow source and sedimentation abstract the miocene ignimbrites of mounts bambouto and bamenda located in the central part of cameroon volcanic line are generally made of welded and non welded massive lapilli tuff and lithic breccias these discontinuous deposits cover a total area of 180 km 2 with thickness ranging from 25 to 200 m the different facies contain several lithic fragments of mainly trachytic nature the devitrified matrix of the welded ignimbrites is constituted by sanidine anorthoclase quartz plagioclase clinopyroxene biotite fe ti oxides and devitrified fiammes anisotropy of magnetic susceptibility ams is used to characterize magnetic fabrics and to provide an estimate of flow direction of each ignimbrite sheet magnetic mineralogy results from different flow units show that titanomagnetite titanohematite maghemite and goethite with grain size ranging from coarse md to very fine sp are the main magnetic carriers of these ignimbrites inferred transport directions based on the ams data and field indicators show that bambouto caldera is the source of main pyroclastic deposits of mount bambouto in southwestern mount bamenda santa mbu caldera or bambouto caldera constituted the probable emission center of mbengwi bamenda and mbu ignimbrite sheets whereas magnetic fabrics of bambili sabga and big babanki ignimbrites demonstrate that these deposits were emitted from a northeastern source namely oku vent in mount oku a small number of subvertical ams fabrics correspond to rocks possibly modified by an elutriation process

late miocene holocene canyon incision in the western altiplano northern chile tectonic or climatic forcing abstract major fluvial incision 600 1000 m affecting the coastal cordillera and central depression of northern chile is analysed to evaluate supposed coeval uplift of the altiplano and or climatic changes in the atacama desert the timing of the beginning of incision is constrained by the age of deposition of the central depression top in the north 18 19 s this top corresponds to fluvial gravels accumulated between 11 9 0 6 ma and 8 3 0 5 ma which are genetically related to semiarid climate and to an eastward poorly dissected parallel drainage network that developed between 15 0 0 6 and 11 2 0 6 ma thus gravel deposition ended at 11 9 11 2 ma to the south 19 20 s the central depression top corresponds to c 6 ma alluvial deposits stratigraphically determined canyon ages and knickzone locations indicate that southward dissection began later and or developed under a regime of lower erosion capacity owing to drier climate vertical incision rate evolution is compatible with eastward knickzone migration dissection required a considerable altitude difference between ancient and present day river base levels which was achieved predominantly by basin infill on an already partially elevated bedrock therefore subsequent incision would have been triggered by local semiarid climatic periods rather than by contemporaneous surface uplift exoreic canyons occur when climatic conditions in the catchments are arid semiarid whereas endoreism is developed when these conditions in catchments are hyperarid

eruptive processes and caldera formation in a nested downsagcollapse caldera cerro panizos central andes mountains abstract the cerro panizos ignimbrite center in the central andes mountains produced two ignimbrite sheets and many lava flows the ignimbrite of quebrada cienago was erupted at 7 9 ma and effusive eruptions continued until the two cooling units of the crystal rich dacitic cerro panizos ignimbrite were emplaced at 6 7 ma the lower unit has no laterally continuous flow breaks and was erupted from a single vent or small cluster of vents with limited fluctuation in discharge lithic fragments reach significant concentrations 5 only in the uppermost two meters of this cooling unit where they document vent wall collapse or the opening of a new vent the upper cooling unit contains many flow units with variations in welding thicknesses and lithic fragment concentrations implying an unsteady eruption column the opening of many vents and probable caldera collapse triangulation of anisotropy of magnetic susceptibility flow direction measurements locate a single vent for the lower cooling unit whereas the upper cooling unit had many vents within the present dome cluster a 15 km diameter topographic depression marked by inward dips of 4 8 at the cooling unit contact is centered on the vent area of the lower cooling unit the depression is interpreted as a downsag caldera formed during emplacement of the lower cooling unit collapse began late in the eruption of the lower cooling unit and continued through the emplacement of the upper cooling unit resurgent magmatism occurred as lava flows that mundated the caldera area a ring of dacite domes erupted until at least 6 1 ma in the northern half of the downsag caldera traces the margin of a collapse caldera associated with the upper cooling unit maximum caldera subsidence 353 km 3 is not enough to account for the erupted volume 652 km 3 dre minimum the ash poor crystal rich nature up to 50 crystals in the pumice 75 in the matrix of the cerro panizos pyroclastic flows resulted in poor retention of gases and there is little evidence of fluidization the high crystal content however favored modified grain flow processes in which several particle support mechanisms combine with grain dispersive forces it is postulated that the flows were initially partially fluidized but rapidly lost their gases and grain flow processes grew in relative importance grain flow processes destroyed evidence of fluidization before deposition

investigation of variable aeration of monodisperse mixtures implications for pyroclastic density currents the high mobility of dense pyroclastic density currents pdcs is commonly attributed to high gas pore pressures however the influence of spatial and temporal variations in pore pressure within pdcs has yet to be investigated theory suggests that variability in the fluidisation and aeration of a current will have a significant control on pdc flow and deposition in this study the effect of spatially heterogeneous gas pore pressures in experimental pdcs was investigated sustained unsteady granular currents were released into a flume channel where the injection of gas through the channel base was controlled to create spatial variations in aeration maximum current front velocity results from high degrees of aeration proximal to the source rather than lower sustained aeration along the whole flume channel however moderate aeration i e 0 5 minimum static fluidisation velocity umf st sustained throughout the propagation length of a current results in greater runout distances than currents which are closer to fluidisation i e 0 9 umf st near to source then de aerating distally additionally although all aerated currents are sensitive to channel base slope angle the runout distance of those currents where aeration is sustained throughout their lengths increases by up to 54 with an increase of slope from 2 to 4 deposit morphologies a primarily controlled by the spatial differences in aeration where there is a large decrease in aeration the current forms a thick depositional wedge sustained gas aerated granular currents are observed to be spontaneously unsteady with internal sediment waves travelling at different velocities

inside pyroclastic density currents uncovering the enigmatic flow structure and transport behaviour in large scale experiments abstract pyroclastic density currents pdcs are the most lethal threat from volcanoes while there are two main types of pdcs fully turbulent fully dilute pyroclastic surges and more concentrated pyroclastic flows encompassing non turbulent to turbulent transport pyroclastic flows which are the subject of the present study are far more complex than dilute pyroclastic surges and remain the least understood type despite their far greater hazard greater runout length and ability to transport vast quantities of material across the earth s surface here we present large scale experiments of natural volcanic material and gas in order to provide the missing quantitative view of the internal structure and gas particle transport mechanisms in pyroclastic flows we show that the outer flow structure with head body and wake regions broadly resembles current pdc analogues of dilute gravity currents however the internal structure in which lower levels consist of a concentrated granular fluid and upper levels are more dilute contrasts significantly with the internal structure of fully dilute gravity currents this bipartite vertical structure shows strong analogy to current conceptual models of high density turbidity currents which are responsible for the distribution of coarse sediment in marine basins and of great interest to the hydrocarbon industry the lower concentrated and non turbulent levels of the pdc granular fluid basal flow act as a fast flowing carrier for the more dilute and turbulent upper levels of the current ash cloud surge strong kinematic coupling between these flow parts reduces viscous dissipation and entrainment of ambient air into the lower part of the ash cloud surge this leads to a state of forced super criticality whereby fast and destructive pdcs can endure even at large distances from volcanoes importantly the basal flow ash cloud surge coupling yields a characteristically smooth rheological boundary across the non turbulent turbulent interface as well as vertical velocity and density profiles in the ash cloud surge which strongly differ from current theoretical predictions observed generation of successive pulses of high dynamic pressure within the upper dilute levels of the pdc may be important to understand the destructive potential of pdcs the experiments further show that a wide range in the degree of coupling between particle and gas phases is critical to the vertical and longitudinal segregation of the currents into reaches that have starkly contrasting sediment transport capacities in particular the formation of mesoscale turbulence clusters under strong particle gas feedback controls vertical stratification inside the turbulent upper levels of the current ash cloud surge and triggers significant transfers of mass and momentum from the ash cloud surge onto the granular fluid basal flow these results open up new pathways to advance current computational pdc hazard models and to describe and interpret pdcs as well as other types of high density gravity currents transported across the surfaces of earth and other planets and across marine basins

climate tectonics and the morphology of the andes large scale topographic analyses show that hemisphere scale climate variations are a first order control on the morphology of the andes zonal atmospheric circulation in the southern hemisphere creates strong latitudinal precipitation gradients that when incorporated in a generalized index of erosion intensity predict strong gradients in erosion rates both along and across the andes cross range asymmetry width hypsometry and maximum elevation reflect gradients in both the erosion index and the relative dominance of fluvial glacial and tectonic processes and show that major morphologic features correlate with climatic regimes latitudinal gradients in inferred crustal thickening and structural shortening correspond to variations in predicted erosion potential indicating that like tectonics nonuniform erosion due to large scale climate patterns is a first order control on the topographic evolution of the andes

turbulent transport and deposition of the ito pyroclastic flow determinations using anisotropy of magnetic susceptibility the ito pyroclastic flow erupted about 22 000 years ago from aira caldera in southern kyushu japan flow directions were determined by anisotropy of magnetic susceptibility ams which measured the preferential alignment of magnetite microphenocrysts usually 0 25 mm in diameter the microphenocrysts are aligned with the long axis of the clasts during strain and fracture in the vent the grains are then deposited parallel to the flow direction with an imbrication there is no evidence of rolling of clasts or nonflow parallel lineations and thus ams can be used to determine flow directions that occurred immediately before deposition beyond 30 km from the center of the caldera flow directions were predominantly down paleogradient indicating expanded flow and that the depositional system was gravity driven and largely decoupled from the transport system within 30 km measured flow directions are random indicating that sedimentation occurred from a depositional system that was closely coupled to the turbulent transport system individual flow directions at all sites except one varied more than the analytical error demonstrating that a variety of flow directions existed even within a small area of the flow this implies that the depositional system was turbulent

conduction model for the thermal influence of lithic clasts in mixtures of hot gases and ejecta the thermal influence of lithic clasts in mixtures of hot gases and ejecta produced during explosive eruptions is an important aspect of the dynamics of eruption columns and pyroclastic flows in order to study this aspect we present a heat conduction model which considers both the temperature distributions of the lithic clasts and the exchange of thermal energy between the lithic clasts and other constituents gases and juvenile pyroclasts the model quantifies the thermal energy of the system by means of a characteristic energy function that describes the energy absorbed by each lithic clast using this energy function and considering some hypothetical arrangements of lithic clasts it is possible to calculate by means of heat balance the temperature variations in mixtures of hot gases and ejecta as a function of time volume fraction of particles to gas and thermal diffusivity initial temperature diameter and volume fraction of lithic clasts theoretical results show that in an eruption a volume fraction of 12 of cool lithic clasts 20 100 c whose size is less than 5 cm will cause the mixture of hot gases and ejecta to cool by 300 c we suggest therefore that the thermal effects of lithic clasts should be considered in numerical models of eruption columns and pyroclastic flows

formation of denmark strait overflow water by mixing in the east greenland current abstract hydrographic data and current meter records from the east greenland current collected during theperiod 1987 to 1989 around the latitude of the central greenland sea reveal the formation of a watermass having the characteristics of denmark strait overflow water dsow the downstream southwestward transport of this newly formed water mass can be substantial amounting to roughly halfof the outflow of dsow through denmark strait as reported in the literature the formation of dsow inthe east greenland current results from isopycnic mixing of recirculated atlantic water and upper arcticintermediate water substantial formation occurs when the neccessary conditions for baroclinic instabilityare satisfied and the current variability due to mesoscale eddies is high this is subject to significantchanges from year to year

magnetic properties and emplacement of the bishop tuff california anisotropy of magnetic susceptibility ams and characteristic remanence were measured for 45 sites in the 0 76 ma bishop tuff eastern california thirty three sites were sampled in three stratigraphic sections two in owens gorge south of long valley caldera and the third in the adobe lobe north of long valley the remaining 12 sites are widely distributed but of limited stratigraphic extent weakly indurated highly porous to dense welded ash flow tuffs were sampled saturation magnetization vs temperature experiments indicate two principal iron oxide phases low ti magnetites with 525 570 c curie temperatures and maghemite with 610 640 c curie temperatures af demagnetization spectra of isothermal remanent magnetizations are indicative of magnetite maghemite predominantly in the multidomain to pseudo single domain size ranges remeasurement of ams after application of saturating direct fields indicates that randomly oriented single domain grains are also present the degree of anisotropy is only a few percent typical of tuffs the ams ellipsoids are oblate with kmin axes normal to subhorizontal foliation and kmax axes regionally aligned with published source vents for 12 of 16 locality means kmax axes plunge sourceward confirming previous observations regarding flow sense topographic control on flow emplacement is indicated by the distribution of tuff deposits and by flow directions inferred from kmax axes deposition east of the benton range occurred by flow around the south end of the range and through two gaps benton notch and chidago gap flow down mammoth pass of the sierra nevada is also evident at least some of the adobe lobe in the northeast flowed around the west end of glass mountain eastward flow directions in the upper owens gorge and southeast directions in the lower owens gorge are parallel to the present canyon suggesting that the present drainage has been established along the pre bishop paleodrainage characteristic remanence directions from 45 sites 267 samples yield an overall mean of d 348 i 53 for the bishop tuff a correlation is found in two of the three profiles between density and remanence inclination a mean remanence direction based on 13 localities together with data from uncompacted xenoliths and data from the ash fall tuff at lake tecopa is d 353 i 54 k 172 95 2 9 n 15

cooling degassing and compaction of rhyolitic ash flow tuffs a computational model previous models of degassing cooling and compaction of rhyolitic ash flow deposits are combined in a single computational model that runs on a personal computer the model applies to a broader range of initial and boundary conditions than riehle s earlier model which did not integrate heat and mass flux with compaction and which for compound units was limited to two deposits model temperatures and gas pressures compare well with simple measured examples the results indicate that degassing of volatiles present at deposition occurs within days to a few weeks compaction occurs for weeks to two to three years unless halted by devitrification near emplacement temperatures can persist for tens of years in the interiors of thick deposits even modest rainfall significantly chills the upper parts of ash deposits but compaction in simple cooling units ends before chilling by rainwater influences cooling of the interior of the sheet rainfall does however affect compaction at the boundaries of deposits in compound cooling units because the influx of heat from the overlying unit is inadequate to overcome heat previously lost to vaporization of water three density profiles from the matahina ignimbrite a compound cooling unit are fairly well reproduced by the model despite complexities arising from numerous cooling breaks uncertainties in attempts to correlate in detail among the profiles may be the result of the non uniform distribution of individual deposits regardless it is inferred that model compaction is approximately valid thus the model should be of use in reconstructing the emplacement history of compound ash deposits for inferring the depositional environments of ancient deposits and for assessing how long deposits of modern ash flows are capable of generating phreatic eruptions or secondary ash flows

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 numerical investigation of the circulation in the greenland and norwegian seas abstract an ocean general circulation model is used to investigate the circulation in the greenland and norwegian seas the primitive equations are solved on 12 levels with a horizontal grid point distance of 20 km this resolution is sufficient to take into account mesoscale topographic structures which according to observations and the model results presented here have a large influence on the distribution of hydrographic variables however the model is not eddy resolving the baroclinic deformation radius in this region is about 5 km only at first a 2 year experiment is run with the forcing functions and the initial stratification derived from climatological annual mean data compared with quasi synoptic data the calculated temperature and salinity distributions show a much better adjustment to the topography than does the climatology the velocities and transports lie within the range of observed long term mean values the model fields obtained by this experiment are used to initialize another e

discrete single domain and pseudo single domain titanomagnetite particles in silicic glass of an ash flow tuff single domain and pseudo single domain titanium poor magnetite grains are present in silicic vitrophyre glass of an ash flow tuff from western nevada conventional transmission electron microscopy and analytical electron microscopy corroborate inferences made solely from rock magnetic and remanence data that the thermoremanent magnetization in the vitrophyre is carried by such grains the magnetite grains vary in size shape and distribution throughout the glass they probably crystal lized during violent eruption and welding of the ash flow at temperatures well in excess of 600 c

numerical experiments relating to the great salinity anomaly of the seventies in the greenland and norwegian seas abstract results are presented from numerical experiments with a primitive equation ocean general circulation model the experiments were carried out in order to investigate some aspects of the great salinity anomaly of the seventies which was observed in the northern north atlantic from 1968 to 1982 and in the greenland and norweigan seas from 1975 to 1982 the model used here extends from the fram strait to the greenland scotland ridge system thus only the greenland and norwegian seas part of the problem is considered two experiments were run in which the model was forced with actual 6 hourly wind stress data the first one was intended to simulate the spread of the anomaly in the model domain the simulated patterns of its distribution and arrival times compare reasonably well with observations however the amplitudes are too small by at least a factor of 2 the second experiment was run to investigate the potential influence of wind forcing on the salinity distribution the wind stress fields of 1979 the year the observed anomaly died away were considerably stronger than those of 1976 this latter year is in the middle of a 3 year period with anomalously low salinity values measured in the north atlantic water entering the greenland and norwegian seas through the faeroe shetland channel one to two years later anomalously low salinities were measured further downstream forcing the model with the wind stress data of 1976 and 1979 respectively and starting each run with the same initial data a salinity distribution is obtained which is lower at the end of 1976 than at the end of 1979 by about 0 06 below approximately 70m on a section running from bear island to the fugloy bank on the barents shelf this is about half of the observed amplitude of the anomaly thus differences in the wind forcing between individual years may be one of the processes which are both possible and necessary to account for observed persistence dickson meincke malmberg and lee 1988 of the anomaly in the greenland and norwegian seas

the glass transition in hydrous granitic melts abstract the influence of water on the glass transition temperature of hydrous granitic melts is estimated on the basis of a variety of available sources of data that provide phenomenological data for the liquid glass transition and for which the relaxation timescale can be accurately estimated these sources include a dilatometric micropenetration viscosity determinations b temperature dependent spectroscopic investigations of hydrous melts and c densities of synthetic fluid inclusions in glasses all available evidence on the glass transition temperature of hydrous granitic melts when corrected for equivalent timescales yields a single consistent trend of decreasing tg with increasing water content that has powerful applications in the rheology and kinetics of processes involving late stage silicic intrusives and eruptive products the brittle ductile transition in hydrous silicic liquids can be accurately predicted using this trend combination of the presently derived relationship for the influence of water on the glass transition of granitic melts with presently available data for the high temperature viscosities of such systems permits the estimation of the temperature dependence of the viscosity of hydrous melts over the entire range of temperature relevant to granite petrogenesis such a comparison reveals that the viscosity temperature relationships of hydrous granitic melts are not arrhenian as is assumed by most existing calculational schemes for the prediction of hydrous melt viscosities the increasing deviation of viscosity temperature relationships of hydrous granitic melts from arrhenian with increasing water content is broadly consistent with similar trends accompanying the addition of excess alkalies to metaluminous granitic melt base compositions in detail however the influence of water on the deviation of viscosity temperature relationships of hydrous granitic melts from arrhenian is not as strong as would be predicted from a molar comparison of the effects of h2o vs alkali oxide or alkaline earth oxide added to these melts the answer to this apparent discrepancy may lie in the incomplete dissociation of water in the melt structure

magnetic fabrics of the bloodgood canyon and shelley peak tuffs southwestern new mexico implications for emplacement and alteration processes anisotropy of magnetic susceptibility ams of the middle tertiary bloodgood canyon and shelley peak tuffs of the mogollon datil volcanic field has been used to 1 evaluate the ability of ams to constrain flow lineations in low susceptibility ash flow tuffs 2 establish a correlation between magnetic fabric magnetic mineralogy tuff facies and characteristics of the depositional setting and 3 constrain source locations of the tuffs the tuffs are associated with the overlapping bursum caldera and gila cliff dwellings basin the high silica bloodgood canyon tuff fills the gila cliff dwellings basin and occurs as thin outcrops outside of the basin the older shelley peak tuff occurs as thin outcrops both along the boundary between the two structures and outside of the complex ams data were collected from 16 sites of bloodgood canyon tuff basin fill 19 sites of bloodgood canyon tuff outflow and 11 sites of shelley peak tuff sites were classified on the basis of within site clustering of orientations of principal susceptibility axes based on the categories of knight et al 1986 most microscopically visible oxide minerals in the bloodgood canyon tuff outflow and basin fill and in the shelley peak tuff are members of the hematite ilmenite solid solution series however irm acquisition data indicate that bloodgood canyon tuff basin fill and shelley peak tuff have magnetic mineralogy dominated by single or pseudo single domain magnetite and that the magnetic mineralogy of the bloodgood canyon tuff outflow is dominated by hematite hematite in bloodgood canyon tuff outflow is likely to be the result of deuteric and or low temperature alteration of magnetite and iron silicate minerals bulk magnetic susceptibility is higher in magnetite dominated ash flow tuff bloodgood canyon tuff basin fill and shelley peak tuff than it is in hematite dominated ash flow tuff bloodgood canyon tuff outflow bloodgood canyon tuff outflow has the highest total anisotropy h of the three units followed by shelley peak tuff and bloodgood canyon tuff basin fill all three ash flow tuffs are genearlly characterized by oblate susceptibility ellipsoids with those of the bloodgood canyon tuff basin fill nearest to spherical at high values of total anisotropy shelley peak tuff susceptibility ellipsoids attain a prolate shape those of bloodgood canyon tuff outflow attain an increasingly oblate shape three factors may influence differences in total anisotropy and susceptibility ellipsoid shape 1 ash which travelled the greatest distance before deposition may show the best development of magnetic fabric particularly of magnetic lineation 2 deposition of ash in a closed basin may inhibit laminar flow throughout the sheet and the resulting development of flow textures and 3 replacement of magnetite and iron silicates preferentially oriented within the foliation plane by hematite with strong crystalline anisotropy may enhance the magnetic susceptibility within that plane scatter in ams axis orientation within sites may result from 1 greater orientation inaccuracy in block sampled than in fielddrilled samples 2 rheomorphism and 3 low accuracy of ams measurement in low susceptibility ashflow tuffs evaluation of flow lineation based on ams of sites with well clustered k1 axes indicates that 1 bloodgood canyon tuff basin fill flowed along a generally northwest southeast azimuth 2 shelley peak tuff located on the boundary of the bursum caldera and the gila cliff dwellings basin flowed along a nearly east west azimuth and 3 bloodgood canyon tuff outflow sites have k1 susceptibility axes generally radial to the bursum gila cliff dwellings complex but within site scatter of k1 orientations is generally too large to draw conclusions about flow lineation orientation limited petrographic work on pilot thin sections adds flow direction information to ams derived flow lineation information

cooling and welding history of the bishop tuff in adobe valley and chidago canyon california abstract a numerical model for cooling and welding of ash flow tuffs was developed to obtain a quantitative understanding of the welding compaction that occurs during the cooling of ash flow sheets and to evaluate the relative effects of emplacement temperature thickness and emplacement histories on compaction zonation density profiles of large ash flow sheets this model includes a set of two phase compressible navier stokes equations for the transport of mass momentum and energy for a variable density volatile outfluxing through a porous media ash flow tuff with this model we calculated theoretical compaction profiles density vs elevation to match actual data measured in sections of bishop tuff compaction profiles from the adobe basin indicate that there were 3 periods of non deposition 5 10 and 40 days separating 4 distinct rapidly emplaced packets rep of pyroclastic flow deposition in contrast the 3 periods of quiescence 400 20 and 800 days between the 4 reps in chidago canyon were considerably longer these data constrain models of eruption for the bishop tuff by implying an eruption cycle that lasted longer than 3 years the average calculated deposition temperatures for all 4 reps in the adobe basin are fairly uniform at 667 33 c in chidago canyon two of the reps had a higher deposition temperature 660 c than the other two 534 c

mechanism of heating and the boundary layer over the tibetan plateau abstract the structure of the boundary layer and the mechanism of heating over the tibetan plateau are examined using the data obtained from the first garp global atmospheric research program global experiment and the chinese qinghai xizang tibet plateau meteorological experiment from may to august 1979 the meteorological elements near the plateau surface exhibit pronounced diurnal variations there is a large ground air temperature difference during the daytime generating a thin layer of superadiabatic lapse rates near the surface the horizontal wind speed attains the minimum in the morning and the maximum in the evening a deep and well mixed layer of potential temperature is observed over the western and central plateau in the evening 1200 utc however moisture is not well mixed vertically and water vapor mixing ratio is larger in the morning 0000 utc than in the evening the mixed layer becomes shallower from the western to the central plateau and it disappears over the eastern plateau the

iron titanium oxide minerals in block and ash flow deposits implications for lava dome oxidation processes abstract the ikeshiro pyroclastic flow deposit at yufu volcano kyushu in japan consists of typical block and ash flows generated by collapse of the ikeshiro lava dome erupted ca 2000 years ago lava clasts in the ikeshiro pyroclastic flow deposit were previously found to consist to two types of rock sample with different magnetic mineral assemblages established by rock magnetic experiments saito et al 2003 j volcanol geotherm res 126 127 142 we examined iron titanium oxides in samples from the ikeshiro pyroclastic flow deposit with an optical microscope and with an electron microprobe analyzer as a result samples were classified into two types with different iron titanium oxide mineral assemblages type a oxides are characterized by homogeneous titanomagnetite and titanohematite type b oxides are exsolved and composed of two or three phases ti poor titanomagnetite titanohematite pseudobrookite and rutile the reconstituted compositions of type b oxides show the same fe ti ratio as type a oxides this indicates that type b oxides are produced by oxidation of type a oxides type a oxides yield an equilibrium temperature of about 800 850 c at an oxygen fugacity of nno 2 using a two oxide geothermobarometer this indicates that deuteric oxidation in the lava dome separated samples of each type type a rocks originated in oxygen poor parts of the lava dome with temperatures of about 800 850 c while type b rocks originated in oxygen rich parts of the lava dome type a oxides remained unoxidized while type b oxides oxidized and were transformed into complex grains by intense deuteric oxidation the collapse of the lava dome generated the ikeshiro pyroclastic flow all oxides were quenched from about 800 850 c and preserved their compositions by rapid cooling

anisotropy of magnetic susceptibility in relation to source vents of ignimbrites empirical observations the anisotropy of magnetic susceptibility ams of young undeformed ashflow deposits e g bishop tuff california bandlier tuff new mexico show alignment of the maximum axis of magnetic susceptibility kmax with inferred flow lines the sense of flow is deduced from the predominant plunge bias of kmax axes these principles are discussed in relation to four deformed settings of ashflow deposits in the great basin of the western united states the magnetic susceptibility data provide valuable but not always unambiguous insight into probable source areas various complications arise in interpreting the ams data in terms of flow directions but these can be reduced by integration with geologic observations

the role of low level convergence and latent heating in a simulation of observed squall line formation abstract the effects of mesoscale forcing and diabatic heating on the development of convective systems have been investigated using a simplified numerical model to simulate the squall line and the convective system preceding it that occurred over texas and oklahoma on 10 11 april 1979 a simulation run without including latent but showed both systems to be initiated and maintained by convergence produced by larger scale forcing the first cloud system formed downwind of the convergence zone that was produced by the confluence of airstreams along a dryline a cloud front approaching from the west then merged with this dryline destroying its horizontal gradients through diffusive effects and replacing it with a frontal convergence line that was alinged with the low level flow this new configuration was then favorable for the formation of the squall line that developed in the simulation when latent heat was included the continuous cloud in the first convective system broke down into isolated cells which

anisotropy of magnetic susceptibility studies of depositional processes in the campanian ignimbrite italy the late pleistocene trachytic campanian ignimbrite underlies much of the campanian plain near naples italy and occurs in valleys in the mountainous area surrounding the plain out to about 80 km from its source the campi flegrei caldera at sites within 15 km of the campi flegrei anisotropy of magnetic susceptibility ams principal directions indicate that in the absence of significant topography deposition came from a flow moving in a roughly radial direction ams studies of the more distal ignimbrite reveal downhill and or downvalley flow directions prior to deposition even where these directions are at high angles to a generally radial transport direction from the vent on the flanks of roccamonfina volcano flow was directly downhill as if the source of the ignimbrite was the summit of the volcano in most localities the ignimbrite consists of a single massive deposit in a few localities in the apennine mountains however the confluence of multiple drainage systems off mountains resulted in multiple local flow units that cannot be correlated between valleys a detailed study of the ignimbrite in the flat titerno river valley near massa shows that the ams fabrics are not due to late stage creeping during deposition or compaction well defined but non parallel ams fabrics from vertical and lateral sections in the massa area are best explained by the merging of gravity currents flowing down the valley and steep valley sides to form a single aggradational deposit clast compositions and ams axes at mondragone indicate that the pyroclastic flow encountered the monte massico massif and was partially blocked so that flow during deposition was toward the campi flegrei similar ams data from sites along the edge of the campanian plain indicate back flow off the first ridge of the apennine mountains reached at least 5 km from their base the campanian ignimbrite was deposited from a density stratified pyroclastic flow the depositional system consisted of the lower denser portion of the current and was controlled by topography the grouping of the ams axes is interpreted to indicate that deposition occurred under laminar flow conditions

evolution of the west andean escarpment at 18 s n chile during the last 25 ma uplift erosion and collapse through time abstract the geological record of the western andean escarpment warp reveals episodes of uplift erosion volcanism and sedimentation the lithological sequence at 18 s comprises a thick pile of azapa conglomerates 25 19 ma an overlying series of widespread rhyodacitic oxaya ignimbrites up to 900 m thick ca 19 ma which are in turn covered by a series of mafic andesite shield volcanoes between 19 and 12 ma the surface of the oxaya ignimbrites evolved into a large monocline on the western slope of the andes a giant antithetically rotated block oxaya block 80 km 20 km formed on this slope at about 10 12 ma and resulted in an easterly dip and a reversed drainage on the block s surface morphology topography and stratigraphic observations argue for a gravitational cause of this rotation a secondary gravitational collapse 50 km3 extending 25 km to the west occurred on the steep western front of the oxaya block alluvial and fluvial sediments 11 2 7 ma accumulated in a half graben to the east of the tilted block and were later thrust over by the rocks of the escarpment wall indicating further shortening between 8 and 6 ma flatlying upper miocene sediments

magnetic fabric and implications for pyroclastic flow and lahar emplacement albano maar italy 1 the albano polygenetic maar is the youngest eruptive center of the quiescent colli albani volcano located near the city of rome the most recent activity of the albano maar extends from 23 ka into the holocene and produced the small volume basic phreatomagmatic peperino albano pa ignimbrite and more recently phreatomagmatic surge and lahar deposits related to the overspill of the albano maar lake we have performed an anisotropy of magnetic susceptibility ams study in order to define the relationships between the magnetic fabric and the flow mechanisms of the pa and of the phreatomagmatic deposits and lahars ams results indicate different transport and or depositional systems for the veneer and valley pond facies in the pa ignimbrite and for the lahar deposits ams also demonstrates that flow directions are mainly controlled by the paleotopography the paleotopographic control has been interpreted in terms of talweg sedimentation even at proximal locations where deposition occurs from dilute pyroclastic flows furthermore ams results clearly evidence a southward provenance for holocene post pa ignimbrite units cropping out in the ciampino plain and confirm their origin from albano maar lake overspill we demonstrate that ams is a reliable marker to determine paleoflow directions also in small volume phreatomagmatic ignimbrites and in syneruptive lahar deposits and can be successfully used to define their depositional systems

formation of mesoscale convective systems over the eastern tibetan plateau affected by plateau scale heating contrasts 1 this study revealed the formation processes of mesoscale convective systems mcss over the eastern tibetan plateau tp associated with the establishment of subplateau scale convergences affected by the longitudinal surface wetness gradient composites of reanalysis data and satellite infrared images showed that large mcss occurred in the afternoon under the condition of the eastward expansion of upper tropospheric anticyclone with the enhancement of near surface low pressure in the western plateau the low level convergence was systematically formed through the eastward propagation of a thermally induced cyclonic circulation formed in the north central plateau before the day of the mcs genesis a numerical model successively simulated the mcs occurrence processes with the diurnal evolution of convergences namely 1 the formation of horizontal wind shear between southwesterlies and northeasterlies over the strong heated land surface causing a thermal low in the northwest 2 the eastward propagation of the vorticity due to intensification of upper westerlies in the night and 3 the mcs genesis by low level convergences behind the migrated vortex with a convective instability condition over the eastern wet land surface numerical sensitivity experiments confirmed that eliminating either the western surface sensible heat flux or the eastern surface latent heat flux prevented the development of mcs namely the enhanced southeast northwest gradation of the plateau scale soil moisture distribution could effectively form the mcss in the eastern plateau during the monsoon season

stratigraphy paleomagnetism and magnetic fabric of the toba tuffs constraints on the sources and eruptive styles the toba depression in north central sumatra is a complex of several overlapping calderas resulting from three major ignimbrite forming eruptions within the depression the upland masses of samosir and northern uluan consist of welded ignimbrite capped by coarse breccia and lacustrine sediment hitherto interpreted to be two parts of a single resurgent dome this study has demonstrated that the welded tuffs of samosir and uluan have different magnetic polarities and therefore at least two different ignimbrites are present the samosir uluan massif may consist of parts of two resurgent domes the first ignimbrite eruption occurred at 0 84 ma and produced a very thick 400 m densely welded unit having a reversed polarity anisotropy of magnetic susceptibility ams flow direction and lithic size data indicate that the source lies in the southern part of the toba depression and the thick deposit of uluan is thought to have ponded in a 40 km wide caldera the second ignimbrite is normally magnetized ams flow direction data indicate two separate source vents one to the north in the haranggaol caldera and another to the south the thick deposit at samosir is thought to have ponded in the southern caldera coarse sediments then accumulated over samosir and northern uluan and were capped by lacustrine deposits a renewed episode of resurgence then uplifted samosir island and possibly the northern part of uluan at approximately 0 075 ma the last and apparently largest ignimbrite eruption occurred from calderas in the north and south parts of the toba depression this ignimbrite is mostly nonwelded and normally magnetized part of the uluan dome was destroyed by collapse of the sibandung caldera and latung graben and concomitant with renewed subsidence of the haranggaol and porsea calderas

rheology of welding inversion of field constraints at present the mechanisms and rheological behaviour of pyroclastic deposits during welding and compaction are poorly understood here we explore the extent to which the rheological properties of pyroclastic deposits are constrained by physical property distributions in welded ignimbrite physical properties of samples from a 20 m section of the bandelier tuff new mexico are used as proxies for strain the observed strain et is ascribed to a combination of a time dependent viscous compaction ev and a time independent mechanical compaction em described by

high resolution magnetic susceptibility imaging of geological thin sections pilot study of a pyroclastic sample from the bishop tuff california u s a high resolution magnetic susceptibility imaging is a new technique for studying the magnetic properties of geological thin sections the two dimensional distribution of both remanent and induced magnetization can be determined with a spatial resolution 1 0 mm that is similar to the size of phenocrysts in the sample amongst many problems in rock magnetism to which it could be applied the technique holds great potential for understanding the origin of the anisotropy of magnetic susceptibility in pyroclastic flows preliminary tests on a single sample of ignimbrite indicate that secondary iron titanium oxide particles deposited within vesicle walls prior to their collapse are responsible for the bulk susceptibility

magnetic fabric investigations of pyroclastic deposits from phlegrean fields southern italy magnetic fabric investigations of pyroclastic deposits from phlegrean fields southern italy

mobility of a large volume pyroclastic flow emplacement of the campanian ignimbrite italy abstract the trachytic campanian ignimbrite originally exposed over a 30 000 km 2 area around naples italy is the product of a highly energetic gas rich eruption the deposit lies in valleys and isolated watersheds and in its medial and distal extent to the south north and east of naples the campanian pyroclastic current encountered mountains exceeding 1000 meters anisotropy of magnetic susceptibility ams measurements indicate that the campanian pyroclastic current the transport system traveled radially outward from the phlegrean fields area but the ignimbrite forming flow the deposition system that developed from the base of the transport system moved downslope from mountainsides to valleys including slopes facing the eruption source and flowed down drainage systems from intermontane basins the campanian pyroclastic current flowed 35 km over the water of the bay of naples to deposit 43 m of ignimbrite on the south shore and also overtopped a 685 1000 m high ridge of the sorrento peninsula to deposit more on the other side the distribution of the ignimbrite and the measured flow directions suggest that the campanian pyroclastic current moved across the landscape as an expanded and therefore turbulent decompressing flow rather than as a high density nonturbulent sheet like current moving over mountains by momentum acquired by eruption column collapse strong expansion is corroborated by shard morphology that indicates derivation from highly inflated pumice and suggests vesicles must have been at least 80 by volume of the original magma

spherulites and lithophysae 200 years of investigation on high temperature crystallization domains in silica rich volcanic rocks high temperature crystallization domains htcds including spherulites and lithophysae form during cooling of silica rich lava and welded ignimbrites spherulites grow in silicate melts or hot glass and they display a radiating or microcrystalline texture typically consisting of cristobalite tridymite and sanidine lithophysae are htcds comprising one or more cavities this contribution reviews the research and discussions on htcds carried out over the last 200 years the emphasis here is on lithophysae and summarizes current knowledge of their formation a number of parameters influence the initiation and growth of lithophysae as well as their shapes and internal textures the most likely cause of cavity formation is transient tensional stress that produces a mechanical opening and widening at the interface between the crystallization front and the host melt e g where t tg cavity growth and expansion forced by rising vapor pressure is considered less important in some cases further growth of htcd cavities results from vapor phase corrosion and brecciation

distribution anisotropy the cause of ams in igneous rocks understanding of ams in pristine basic igneous rocks in terms of flow orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains 1 typically are irregularly equant or skeletal rather than distinctly ellipsoidal and 2 usually crystallize interstitially and late after magma flow has ceased however the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly oriented by flow silicate template this constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth but in addition we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of ams experiments involving the casting of magnetite epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected ams symmetry i e k3 perpendicular to glass plates or k1 parallel to glass rods slicing of near isotropic synthetic blank and natural rock cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from 0 4 to 2 and the natural rock from 0 9 to 2 9 in both the k3 axis migrates to be perpendicular to the glass slides

ignimbrite depositional facies the anatomy of a pyroclastic flow a model is presented for the depositional regimes in a pyroclastic flow to explain the origin of facies in the taupo ignimbrite and to compare these with other published examples in this model a pyroclastic flow consists of a head a body and a tail the head which is where fluidization caused by air ingestion occurs generates layer 1 deposits on lithological grounds these are divided into layers 1 p which is fines depleted and rich in pumice and 1 h which is fines depleted and rich in lithics and crystals layer 1 p represents material thrown forwards from the flow head and is termed the jetted deposits layer 1 h represents material sedimented from within the flow head and is termed the ground layer the body represents the bulk of the flow and the tail is its trailing part which is slowed by ground friction these parts generate the layer 2 deposits which include the valley pond ignimbrite and its associated ignimbrite veneer deposit localized depositional modes within the body and tail generate distinctive coarse pumice concentration zones and pumiceous lee side lenses behind obstacles at its outer limits all of the flow on interfluves is affected by air ingestion fluidization producing the distant facies which combines features of both layer 1 and 2 deposits one important conclusion of this work is that the thicknesses and compositions of the various facies are not related in any simple way to the thickness and composition of the parent flow

magnetic fabric flow directions and source area of the lower miocene peach springs tuff in arizona california and nevada we have used anisotropy of magnetic susceptibility ams to define the flow fabric and possible source area of the peach springs tuff a widespread rhyolitic ash flow tuff in the mojave desert and great basin of california arizona and nevada the tuff is an important stratigraphic marker from the colorado plateau to barstow california a distance of 350 km however the location of its source caldera is unknown dated at 18 5 ma by 40ar 39 ar the tuff erupted during the early stages of miocene extension along the lower colorado river the thicker accumulations 100 m occur at kingman arizona and in the piute mountains california on opposite sides of the colorado river extensional corridor our ams studies produced well defined magnetic lineations in 30 of 42 sites distributed throughout the tuff typical ratios of the principal ams axes are 1 01 for the magnetic lineation kmax kint and 1 02 for the foliation kint kmin the bulk magnetic susceptibility of the peach springs tuff averages 2 0 10 3 in the si unit system the subhorizontal lineations which presumably parallel the flow directions form a pattern radiating outward from the approximate center of the outcrop area magnetic foliations define an imbrication that generally dips away from the distal margins and toward the center of the outcrop of the tuff the lineation and imbrication indicate a source region near the southern tip of nevada defining the best intersection of the ams lineations required restoration of major extension strike slip faulting and associated tectonic rotation in the disrupted tuff the optimum intersection of magnetic lineations lies in the southern black mountains of arizona on the eastern side of the colorado river extensional corridor no caldera structures are known from that area but the area contains thick sections of the peach springs tuff above a silicic volcanic center the caldera may be buried under younger deposits in the mohave valley of arizona tertiary granite in the newberry mountains may represent a deeper level of the peach springs tuff vent that has been exhumed by detachment faulting

characteristics and emplacement mechanisms of the coranzuli ignimbrites central andes abstract we present a detailed stratigraphy of the coranzuli caldera forming deposits this caldera located in the altiplano puna volcanic complex central andes generated four ignimbrite deposits with similar field characteristics and facies that differ from each other in above all the nature of the lithic fragments they contain three different lithofacies fine grained cross stratified facies massive lithic breccia facies and massive ignimbrite facies are found in all the ignimbrite deposits which occasionally also contain a lenticular lithic rich facies and or a pumice rich facies these field characteristics and in particular local deposit thicknesses were used to develop a theoretical model of the dynamics and emplacement mode of the coranzuli pyroclastic flows our results show that these ignimbrites were emplaced by dense pyroclastic density currents subjected to high accumulation rates and velocities thereby indicating rapid en masse emplacement that was also influenced by local paleotopography as deduced from facies analysis

near surface circulation of the nordic seas as measured by lagrangian drifters in the period june 1991 to august 1993 107 argos tracked drifters drogued to 15 m depth were released in the nordic seas or greenland iceland and norwegian seas the drifter movements revealed the strong and spatially confined current systems along the surface salinity fronts of the iceland faroe frontal zone and of the norwegian coast and along the continental margins and their extensions to the barents sea and spitsbergen the norwegian atlantic current is composed of three distinct streams two continental margin and one coastal branches which join into one single swift mean current west of the lofoten and vesteralen islands where the strongest measured currents are in excess of 110 cm s 1 in addition to the general cyclonic gyre circulation in the nordic seas the drifters indicate smaller cyclonic circulation patterns in all the major subbasins i e the iceland plateau the norwegian the lofoten and the greenland basins no surface signature of the east icelandic current is disclosed by the drifters interpolated and low pass filtered position data were used to construct maps of 15 m depth ensemble mean velocity velocity variability and residence time vigorous eddy fields are dominant in the strong currents and in the lofoten basin eulerian correlations indicate that they tend to propagate to the west in contrast the iceland plateau appears quiescent both in the mean and eddy velocities single particle diffusivities are computed and are found to be in the range 1 7 107 cm2 s 1 the corresponding lagrangian timescale and space scale are 1 3 days and 10 40 km respectively these lagrangian drifter measurements compose the first basin scale accurate near surface velocity data set of the nordic seas

flow directions in ash flow tuffs a comparison of geological and magnetic susceptibility measurements tshirege member upper bandelier tuff valles caldera new mexico usa this study concludes that the elongation axis k 1 of the ellipsoid of anisotropic magnetic susceptibility ams is a suitable proxy for flow axis in ashflow tuffs 153 oriented samples 176 specimens were studied from 18 sites in the 1 1 ma tshirege member of the bandelier tuff these sites are distributed around the valles caldera at distances of 5 25 km outside of the rim k 1 axes correlate well with postulated radial flow axes at 13 sites k 1 also agrees with measured geological flow indicators mainly imbricated larger clasts at 7 sites at 2 of the 5 sites where significant disagreement is seen between theoretical radial flow directions and measuredk 1 axes thek 1 axes correspond well with geological flow indicators indicating that the divergence of flow from the predicted radial flow pattern is real two major topographic buttresses are suggested as the cause of flow divergence for the tshirege ash flows the san pedro buttress northwest of the caldera and the san miguel buttress in the southeast in situk 1 axes plunge about 7 toward the source at two thirds of the sites therefore the plunge ofk 1 is a plausible in situ indicator for thedirection of flow multiple flow zones in sections of several meters thickness indicate changes of flow direction that are both rapid and large during ash flow emplacement these observations raisre the question of how best to represent mean flow directions in ash flow sheets by eigenvector methods by vector sum methods or by modes a method for measuring imbrication of larger clasts using apparent dips in vertical joints is outlined imbrication determined in this way at one third of the sites dips toward the source i e up flow the minimum k 3 axis of the ams ellipsoid correlates with the flow foliation rather than with the larger clast imbrication the flow axes of ash flows correspond with thek 1 axes not with the declination ofk 3 axes as suggested by some authors initial dip of the sampled ash flows is not large and does not affect the paleomagnetic remanence direction which is reversed with a mean ofd 173 5 i 38 4 95 3 4 n 18 this mean is not different at the 95 confidence level from that of earlier workers the mean pole at 098 0 e 74 8 n a 95 3 3 n 18 is about 15 far sided relative to the expected time averaged geomagnetic pole suggesting a history of emplacement too short to adequately average secular variation

modeling dense pyroclastic basal flows from collapsing columns 1 a two layer model for the formation of dense pyroclastic basal flows from dilute collapsing volcanic eruption columns is presented the collapsing dilute current is described by depth averaged isothermal continuum conservation equations the dense basal flow is modelled as a granular avalanche of constant density simulations demonstrate that pyroclastic flow formation and behavior is dependent upon slope conditions when the dilute part of the current has lost most of its mass the dilute current runout increases with decreasing particle size and increasing initial column height if the dilute current has transferred its mass to the dense basal flow on volcanic slopes with inclination angle greater than the friction angle of the basal flow then the basal flow will continue to propagate until frictional forces bring it to rest if the dilute current terminates on lower angled slopes frictional forces dominate the basal flow preventing further front propagation

curie temperatures of titanomagnetite in ignimbrites effects of emplacement temperatures cooling rates exsolution and cation ordering pumices ashes and tuffs from mt st helens and from novarupta contain two principal forms of titanomagnetite homogeneous grains with curie temperatures in the range 350 500 c and oxyexsolved grains with similar bulk composition containing ilmenite lamellae and having curie temperatures above 500 c thermomagnetic analyses and isothermal annealing experiments in combination with stratigraphic settings and thermal models show that emplacement temperatures and cooling history may have affected the relative proportions of homogeneous and exsolved grains and have clearly had a strong influence on the curie temperature of the homogeneous phase the exsolved grains are most common where emplacement temperatures exceeded 600 c and in laboratory experiments heating to over 600 c in air causes the homogeneous titanomagnetites to oxyexsolve rapidly where emplacement temperatures were lower curie temperatures of the homogeneous grains are systematically related to overburden thickness and cooling timescales and thermomagnetic curves are generally irreversible with lower curie temperatures measured during cooling but little or no change is observed in room temperature susceptibility we interpret this irreversible behavior as reflecting variations in the degree of cation ordering in the titanomagnetites although we cannot conclusively rule out an alternative interpretation involving fine scale subsolvus unmixing short range ordering within the octahedral sites may play a key role in the observed phenomena changes in the curie temperature have important implications for the acquisition stabilization and retention of natural remanence and may in some cases enable quantification of the emplacement temperatures or cooling rates of volcanic units containing homogeneous titanomagnetites

the superparamagnetism of yucca mountain tuff yucca mountain tuff contains small titanomagnetite grains with narrow size distributions in the superparamagnetic range schlinger et al 1988 magnetic measurements on three samples comprising hysteresis loops at low and ambient temperatures acquisition and demagnetization of isothermal remanent magnetization thermal demagnetization of the saturation remanence and of a low temperature thermoremanence and frequency and temperature dependent susceptibilities allow evaluation of the magnetic properties in terms of neel s 1949 single domain theory precise grain volume distributions have been obtained by applying the blocking volume concept to thermal demagnetization results in contrast an attempt to derive mean particle volumes by fitting a langevin curve to the room temperature magnetization curves fails probably because the precondition for the langevin function kv kt 1 is not met it is only for the sample with the smallest grains and in weak fields 20 mt that a langevin fit provides a reasonable volume estimate there is good agreement between the experimental results and the calculated frequency and temperature dependence of susceptibility thus verifying that neel s theory is sufficient for the magnetic description of single domain assemblages spanning the superparamagnetic stable single domain boundary however some deviations between modeled and measured susceptibilities exist and the physical causes may include size dependent anisotropy nonuniform magnetizations and also an uncertain preexponential time constant 0 while 0 10 11 s gives the best fit for the sample with the largest grains 0 10 9 s is more reasonable for the others thus 0 may indeed be size and temperature dependent as predicted by brown 1959 the commonly cited parameter fd frequency dependence of susceptibility reaches 30 at room temperature rt for one sample with a blocking temperatures just below rt while fd 0 at rt for a superparamagnetic sample with smaller grains these results thus exemplify that fd is not limited to 15 as a number of studies suggest and that fd 0 must not be taken to imply the absence of superparamagnetic grains

structure and transports of the east greenland current at 75 n from moored current meters the east greenland current runs from 80 n to 60 n from the fram strait to the denmark strait via the nordic seas it transports waters from the arctic and the nordic seas into the atlantic and also acts as a western intensified southward return flow for waters recirculating within the greenland sea gyre itself an area important for deep water formation data from current meters moored across the current at 75 n in 1994 1995 show a large seasonal variation in the current the annual mean transport is 21 3 sv taking 9 w as the eastern boundary varying from 11 sv in summer to 37 sv in winter errors approximately 5 sv no significant seasonal signal has been observed in the fram or denmark straits suggesting that the seasonal transport is confined within the greenland sea using temperature and velocity data we split the flow at 75 n into two parts a mainly wind driven circulation annual mean of order 19 sv which is trapped within the greenland sea gyre and exhibits a large seasonal cycle transporting predominantly the waters of the greenland sea and a steadier throughflow probably thermohaline driven of order 8 sv in the annual mean with very little seasonal variation data from previous years 1987 1994 indicate the interannual variability of the current is low assuming a spatially coherent structure to the current we extend the time series of the transport back to 1991 and suggest it may be possible to monitor the total transport with one suitably placed mooring

meandering flow of a pyroclastic density current documented by the anisotropy of magnetic susceptibility ams in the quartz latite ignimbrite of the pleistocene monte cimino volcanic centre central italy abstract a pattern of shifting current direction is revealed by the anisotropy of magnetic susceptibility ams in the quartz latite ignimbrite associated with the pleistocene cimini dome complex in central italy high field ams documents that both paramagnetic and ferrimagnetic minerals contribute to the magnetic fabric of the ignimbrite the results of low field ams studies suggest that the pyroclastic density current changed both direction and character during deposition of each flow unit that makes up the ignimbrite the meandering behaviour of the currents can be observed on the scale of 15 cm increments in the fine grained basal layer 2a of one flow unit in an area lacking significant palaeotopography ams data from 1 2 and 4 metre increments through the flow units also show complex changes in flow direction throughout the deposition of each unit studied the most closely aligned ams fabrics occur near the bases of flow units and dispersion increases upward this trend may represent decreased shear between the flow and the substrate or increased turbulence in the upper depositional zone of the pyroclastic density current or both suggesting a progressively waning current in terms of capacity or particle concentration the phenomena observed in this study have broad implications for the general methodology of ams studies detailed vertical profiles made up of several individual sample sites within 1 metre thickness intervals can reveal small scale current flow variations that may not be evident if sampling is done more sparsely

the jan mayen current of the greenland sea oceanographic measurements acquired by the usns bartlett during september 1989 in and north of the area of the jan mayen current jmc show that in terms of upper layer baroclinic flow about half of the jmc is a wide meander in the east greenland current egc and about half continues eastward to close the greenland gyre gg on the south this phenomenon has been charted in the past but has elicited no comment surface drifters and a numerical model corroborate this behavior at deeper depths 100 m the meander dissipates with the flow becoming more easterly a drifting float at 500 m depth confirms that the flow is relatively undeviated toward the east ultimately merging with the northward flowing norwegian atlantic current near the gg the fresh water content of the water column is low in the upper layers but high in the middepths where there is a substantial content of egc water e g in the jmc the converse is usually true the fwc of the gg computed for the period 1953 1966 was smaller by a factor of 2 in both layers than in 1989 indicating that 1989 is in another period of low salinity comparable to that of 1968 the period of the great salinity anomaly described by dickson et al 1988

a coupled ice ocean model for the greenland iceland and norwegian seas abstract simulations from a coupled ice ocean model that highlight the importance of synoptic forcing on sea ice dynamics are described the ocean model is a non hydrostatic primitive equation model coupled to a dynamic thermodynamic sea ice model the ice modelling sensitivity study presented here is part of an ongoing research programme to define the role played by sea ice in the energy balance of the greenland sea the different categories of sea ice found in the subpolar regions are simulated through the use of equations for thin ice thick ice and the marginal ice zone a basin scale numerical model of the greenland iceland and norwegian seas has a horizontal resolution of 20 km and a vertical grid spacing of 50 m this resolution is adequate for resolving the mesoscale topographic structures known to control the circulation in this region the spin up reproduces the main features of the circulation including the cyclonic gyres in the norwegian and greenland basins and iceland plateau topographic steering of the flow is evident the baroclinic rossby radius of deformation is between 5 and 10 km so that the model is not eddy resolving the coupled ice ocean model was run for a period of two weeks the influence of horizontal resolution of the atmospheric model was tested by comparing simulations using six hourly wind fields from the ecmwf with those generated using six hourly fields from a hirlam with horizontal resolutions of 1 and 0 18 respectively the simulations show reasonable agreement with satellite ice compactness data and data of ice transports across sections at 79 n 75 n and denmark strait

interannual variability in the hydrography of the norwegian atlantic current frontal versus advective response to atmospheric forcing 1 the warm and saline inflow of the north atlantic current to the nordic seas is highly relevant for the region and the global climate north of the greenland scotland ridge the norwegian atlantic current consists of two 40 60 km wide branches situated at the slope and 150 200 km offshore respectively to interpret changes in these branches in terms of climate variability in the northern north atlantic it is important to understand their spatiotemporal response to both atmospheric forcing and advection we analyzed three decades of synoptic hydrographic observations of the branches variability with particular focus on the response to the north atlantic oscillation nao and related wind stress curl changes in the nordic seas to do so we separated the effect of fluctuating position and thickness of the branches from the variability in temperature and salinity in the spatially fluctuating cores as a rapid response to the nao we find a deflection of both branches toward the coast which is consistent with an enhanced basin wide cyclonic circulation while the immediate correlation between hydrographic properties and the nao is weak we find a significant negative correlation when the nao leads temperature and salinity by 4 6 years we attribute the overall delayed response to the advection of anomalies generated in the northwestern north atlantic through nao induced air sea interaction and changes in the position of the subpolar front

welded breccias in andesite lavas flow breccias formed at the margins of blocky andesite lavas can be reheated welded and sometimes deformed to form rocks reminiscent of welded pyroclastic rocks reheating occurs due to advection of heat from the flow to the basal breccias surface breccias also infill extensional crevasses which are later closed and compressed strongly deformed welded breccia bands form within the flow levees the angular breccia clasts can be deformed into fiamme

magnetic petrofabric of igneous rocks lessons from pyroclastic density current deposits and obsidians abstract measurement of the anisotropy of magnetic susceptibility ams of igneous rocks can provide clues concerning their mechanism of formation and in particular are very helpful as flow direction indicators unlike other igneous rocks however pyroclastic density current deposits pdcds present a challenge in the interpretation of ams measurements due to the complexity of their mechanism of emplacement in this paper we review the most common assumptions made in the interpretation of the ams of pdcd taking advantage of key lessons obtained from obsidians despite the complexities on the mechanism of formation of pdcds it is shown that a key element for the fruitful interpretation of ams is to give proper attention to the various components likely to be involved in controlling their general petrofabric the anisotropies of ferromagnetic crystals whether as free phases or embedded within clasts or shards and those of paramagnetic minerals mainly ferrosilicates need to be taken into consideration when interpreting the ams measurements of pdcds variations of the deposition regime both as a function of position and of time also need to be considered on the interpretations nevertheless if a suitable sampling strategy is adopted the potential of the ams method as a petrofabric indicator is maximized

the flow dynamics of an extremely large volume pyroclastic flow the 2 08 ma cerro galan ignimbrite nw argentina and comparison with other flow types the 2 08 ma cerro galan ignimbrite cgi represents a 630 km3 dense rock equivalent vei 8 eruption from the long lived cerro galan magma system 6 ma it is a crystal rich 35 60 pumice 10 generally and lithic poor 5 generally rhyodacitic ignimbrite lacking a preceding plinian fallout deposit the cgi is preserved up to 80 km from the structural margins of the caldera but almost certainly was deposited up to 100 km from the caldera in some places only one emplacement unit is preserved in proximal to medial settings and in most distal settings suggesting constant flow conditions but where the pyroclastic flow moved into a palaeotopography of substantial valleys and ridges it interacted with valley walls resulting in flow instabilities that generated multiple depositional units often separated by pyroclastic surge deposits the cgi preserves a widespread sub horizontal fabric defined by aligned elongate pumice and lithic clasts and minerals e g biotite a sub horizontal anisotropy of magnetic susceptibility fabric is defined by minute magnetic minerals in all localities where it has been analysed the cgi is poor in both vent derived accessory lithics and locally derived lithics from the ground surface accidental lithics locally derived lithics are small 20 cm and were not transported far from source points all data suggest that the pyroclastic flow system producing the cgi was characterised throughout by high sedimentation rates resulting from high particle concentration and suppressed turbulence at the depositional boundary layer despite being a low aspect ratio ignimbrite based on these features we question whether high velocity and momentum are necessary to account for extensive flow mobility it is proposed that the cgi was deposited by a pyroclastic flow system that developed a substantial high particle concentration granular under flow which flowed with suppressed turbulence high particle concentration and fine ash content hindered gas loss and maintained flow mobility in order to explain the contemporaneous maintenance of high particle concentration high sedimentation rate at the depositional boundary layer and a high level of mobility it is also proposed that the flow s was continuously supplied at a high mass feeding rate it is also proposed that internal gas pressure within the flow directed downwards onto the substrate over which the flow was passing reduced the friction between the flow and the substrate and also enhanced its mobility the pervasive sub horizontal fabric of aligned pumice lithic and even biotite crystals indicates a consistent horizontal shear force existed during transport and deposition in the basal granular flow consistent with the existence of a laminar shearing granular flow regime during the final stages of transport and deposition

observations of vertical currents and convection in the central greenland sea during the winter of 1988 1989 during the winter of 1988 1989 five acoustic doppler current profilers adcps were moored in the central greenland sea to measure vertical currents that might occur in conjunction with deep mixing and convection two adcps were looking up from about 300 m and combined with thermistor strings in the depth range 60 260 m two were looking downward from 200 m and one was looking upward from 1400 m first maxima of vertical velocity variance occurred at two events of strong cold winds in october and november when cooling and turbulence in the shallow mixed layer generated internal waves in the thermocline beginning in late november the marginal ice zone expanded eastward over the central greenland sea reaching its maximum extent in late december in mid january a bay of ice free water opened over the central greenland sea leaving a wedge of ice the is odden curled around it along the axis of the jan mayen current and then northeastward and existing well into april 1989 below the ice a mixed layer at freezing temperatures developed that increased in thickness from 60 to 120 m during the period of ice cover corresponding to an average heat loss of about 40 w m 2 through brine rejection mixed layer salinity increased steadily reducing stability to underlying weakly stratified layers roach et al 1993 during the ice cover period vertical currents were at a minimum after the opening of the ice free bay successive mixed layer deepening to 350 m occurred in conjunction with cooling events around february 1 and 15 accompanied by strong small scale vertical velocity variations upward mixing of more saline waters of atlantic origin during this phase reduced the stability further generating a pool of homogeneous water of 50 km horizontal extent in the central greenland sea preconditioned for subsequent convection to greater depths individual convection events were observed during march 6 16 associated with downward velocities at the 1400 m level of about 3 cm s l one event was identified as a plume of about 300 m horizontal scale in agreement with recently advanced scaling arguments and model results and with earlier similar observations in the gulf of lions western mediterranean the deep convection occurred in the center of the ice free bay hence brine rejection did not seem necessary for its generation plume temperatures at 1400 m were generally higher than that of the homogeneous surface pool suggesting entrainment of surrounding warmer waters on the way down mean vertical velocity over a period of convection events was indistinguishable from zero suggesting that plumes served as a mixing agent rather than causing mean downward transport of water masses however different from the surface pool that was governed by mixed layer physics the water between 400 and 1400 m was not horizontally homogenized in a large patch by the sporadic plumes overall and compared to results from the gulf of lions convection activity in the central greenland sea was weak and limited to intermediate depths in winter 1988 1989

thermal evolution of the greenland sea gyre in 1988 1989 slice inverses of temperature and heat content from the 1988 1989 greenland sea tomography experiment and other observations including standard conductivity temperature depth stations moored thermistors surface meteorological variables and surface ice cover are combined to better understand the thermodynamics of the greenland sea gyre thermal evolution of the gyre center seems to divide naturally into the following three periods a preconditioning phase november january during which surface salinity is increased by brine rejection from ice formation and by entrainment but in which the mixed layer deepens only slowly to a depth of some 150 200 m a deep mixing phase february march during which the surface mixed layer deepens rapidly to approximately 1500 m in the gyre center purely under the influence of local surface cooling and a restratification phase during which the products of deep mixing are replaced by arctic intermediate water flowing in from the gyre edges the onset of the deep mixing phase occurs after ice formation in the gyre center stops resulting in an area of open water where large heat fluxes can occur in surrounding regions including the odden region to the south ice is still being formed to the north and west closer to the steep topography of the continental shelf the inverse results show significant variability due to advection and large temperature and heat content fluctuations with a period of about 50 days are seen

an investigation of the dynamics of the east greenland current in fram strait based on a simple analytical model abstract the east greenland current egc in fram strait is examined in summer based on the hydrographic data from the mizex 84 experiment and its dynamics is further analyzed using a simple analytical model from the observed density field the southward geostrophic transport relative to the bottom in the egc is estimated to be about 4 sv sv 106 m3 s 1 at 78 4 n which assuming a ratio 1 1 between the bottom and the relative transports as suggested in the literature allows one to postulate a 4 sv bottom transport the hydrographic data also reveal a northward gradient of potential energy along the east greenland slope between 77 5 n and 80 5 n such a gradient is the prerequisite for the existence of the joint effect of baroclinicity and relief jebar which is here proposed as a driving mechanism for the bottom transport in the egc a model with idealized topography and a density distribution prescribed from the above data is constructed based on a steady state vorticity equation in which the

causes of fragmented crystals in ignimbrites a case study of the cardones ignimbrite northern chile broken crystals have been documented in many large volume caldera forming ignimbrites and can help to understand the role of crystal fragmentation in both eruption and compaction processes the latter generally overlooked in the literature this study investigates the origin of fragmented crystals in the 1260 km3 crystal rich cardones ignimbrites located in the central andes observations of fragmented crystals in non welded pumice clasts indicate that primary fragmentation includes extensive crystal breakage and an associated ca 5 vol expansion of individual crystals while preserving their original shapes these observations are consistent with the hypothesis that crystals fragment in a brittle response to rapid decompression associated with the eruption additionally we observe that the extent of crystal fragmentation increases with increasing stratigraphic depth in the ignimbrite recording secondary crystal fragmentation during welding and compaction secondary crystal fragmentation aids welding and compaction in two ways first enhanced crystal fragmentation at crystal crystal contacts accommodates compaction along the principal axis of stress second rotation and displacement of individual crystal fragments enhances lateral flow in the direction s of least principal stress this process increases crystal aspect ratios and forms textures that resemble mantled porphyroclasts in shear zones indicating lateral flow adds to processes of compaction and welding alongside bubble collapse in the cardones ignimbrite secondary fragmentation commences at depths of 175 250 m lithostatic pressures 4 6 mpa and is modulated by both the overlying crystal load and the time spent above the glass transition temperature under these conditions the existence of force chains can produce stresses at crystal crystal contacts of a few times the lithostatic pressure we suggest that documenting crystal textures in addition to conventional welding parameters can provide useful information about welding processes in thick crystal rich ignimbrites

tectonics of the jurassic early cretaceous magmatic arc of the north chilean coastal cordillera 22 26 s a story of crustal deformation along a convergent plate boundary the tectonic evolution of a continental magmatic arc that was active in the north chilean coastal cordillera in jurassic early cretaceous times is described in order to show the relationship between arc deformation and plate convergence during stage i circa 195 155 ma a variety of structures formed at deep to shallow crustal levels indicating sinistral arc parallel strike slip movements from deep crustal levels a sequence of structures is described starting with the formation of a broad belt of plutonic rocks which were sheared under granulite to amphibolite facies conditions bolfin complex the high grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike slip and thrust kinematics with a nw se directed maximum horizontal shortening i e parallel to the probable late jurassic vector of plate convergence a kinematic pattern compatible to this plate convergence is displayed by nonmetamorphic folds thrusts and high angle normal faults which formed during the same time interval as the discrete shear zones during stage ii 160 150 ma strong arc normal extension is revealed by brittle low angle normal faults at shallow levels and some ductile normal faults and the intrusion of extended plutons at deeper levels during stage iii 155 147 ma two reversals in the stress regime took place indicated by two generations of dikes an older one trending ne sw and a younger one trending nw se sinistral strike slip movements also prevailed during stage iv until 125 ma when the atacama fault zone originated as a sinistral trench linked strike slip fault the tectonic evolution of the magmatic arc is interpreted in terms of coupling and decoupling between the downgoing and overriding plates the structures of stages i and iv suggest that stress transmission due to seismic coupling between the plates was probably responsible for these deformations however decoupling of the plates occurred possibly due to a decrease in convergence rate resulting in extension and the reversals of stages ii and iii

230thex flux into norwegian greenland sea sediments evidence for lateral sediment transport during the past 300 000 years 230th was determined to investigate the influence of climatic cycles on the 230thex flux into norwegian greenland sea sediments the fafp ratios i e the ratios between the measured 230thex flux fa into the sediments and the production rate of 230th in the water column fp vary considerably in the four cores investigated systematic variations in the ratios with the climatic cycles could not be observed furthermore variations in the fafp ratios cannot be correlated between individual sites it is argued that lateral sediment transport due to deglaciation of ice sheets bottom currents and or downslope sediment transport result in fafp ratios that are not equal to 1 the results suggest that both sediment winnowing and or erosion and sediment focusing have strong influences upon sediment accumulation in the norwegian greenland sea

k ar geochronology of the late cenozoic volcanic rocks of the cordillera occidental southernmost peru abstract twenty four k ar radiometric ages are presented for late cenozoic continental volcanic rocks of the cordillera occidental of southernmost peru lat 16 57 17 36 s rhyodacitic ignimbrite eruptions began in this transect during the late oligocene and continued episodically through the miocene the development of andesitic dacitic strato volcanoes was initiated in the pliocene and continues to the present the earliest ignimbrite flows 25 3 22 7 ma are intercalated in the upper coarsely elastic member of the moquegua formation and demonstrate that this sedimentary unit accumulated in a trough parallel to andean tectonic trends largely in the oligocene more voluminous ash flow eruptions prevailed in the early miocene 22 8 17 6 ma and formed the extensively preserved huaylillas formation this episode was coeval with a major phase of andean uplift and the pyroclastics overlie an erosional surface of regional extent incised into a paleogene volcano plutonic arc terrain an age span of 14 2 8 9 ma mid late miocene is indicated for the younger chuntacala formation which again comprises felsic ignimbrite flows largely restricted to valleys incised into the pre huaylillas formation lithologies and at lower altitudes an extensive aggradational elastic facies the youngest areally extensive ignimbrites constituting the sencca formation were extruded during the late miocene in the earliest pliocene the ignimbrites were succeeded by more voluminous calcalkaline intermediate flows which generated numerous large and small stratovolcanoes these range in age from 5 3 to 1 6 ma present day or holocene volcanism is restricted to several large stratovolcanoes which had begun their development during the pleistocene by 0 7 ma the late oligocene early miocene ca 22 23 ma reactivation of the volcanic arc coincided with a comparable increase in magmatic activity throughout much of the cordilleras occidental and oriental of the central andes

sea ice impact on long term particle flux in the greenland sea s is odden nordbukta region 1985 1996 five sediment traps deployed in the greenland sea at a depth of 500 m between 72 n and 75 n by the sonderforschungsbereich 313 kiel germany provide the necessary data to compare particle flux with ambient ice regimes sedimentation in this seasonally ice covered region is dependent upon the following three basic parameters 1 ice concentration 2 duration of ice cover and 3 distance from the ice edge these factors vary significantly with time and space we develop algorithms that provide annual sedimentation amounts for the area contained by 71 n to 76 n the greenland coast and 10 e for a severe ice year the area of seasonal ice cover and an 80 km wide band extending along the maximum extent of the ice edge the biological marginal ice zone bmiz combine to provide 92 of the total sedimentation for particulate organic carbon and silica this zone accounts for 89 each of the total sedimentation in a light ice year the respective percentages are 84 for dry weight 87 for particulate organic carbon and 81 for biogenic particulate silica these figures are slightly less than sedimentation for a severe ice year if the is odden nordbukta region is replaced by open ocean for purposes of comparison the bmiz out produces the open ocean for poc by a factor of 3 2 projecting the algorithms for the is odden nordbukta region to the rest of the greenland sea we conclude that the is odden nordbukta region is a substantial producer of sedimentation

anisotropy of magnetic susceptibility in welded tuffs application to a welded tuff dyke in the tertiary trans pecos texas volcanic province usa consideration of published anisotropy of magnetic susceptibility ams studies on welded ignimbrites suggests that ams fabrics are controlled by groundmass microlites distributed within the existing tuff fabric the sum result of directional fabrics imposed by primary flow lineation welding and if relevant rheomorphism ams is a more sensitive indicator of fabric elements within welded tuffs than conventional methods and usually yields primary flow azimuth estimates detailed study of a single densely welded tuff sample demonstrates that the overall ams fabric is insensitive to the relative abundances of fiamme matrix and lithics within individual drilled cores ams determinations on a welded tuff dyke occurring in a choked vent in the trans pecos texas volcanic field reveals a consistent fabric with a prolate element imbricated with respect to one wall of the dyke while total magnetic susceptibility and density exhibit axially symmetric variations across the dyke width the dyke is interpreted to have formed as a result of agglutination of the erupting mixture on a portion of the conduit wall as it failed and slid into the conduit followed by residual squeezing between the failed block and in situ wallrock irrespective of the precise mechanism widespread occurrence of both welded tuff dykes and point welded aggregate pumices in pyroclastic deposits may imply that lining of conduit walls by agglutionation during explosive volcanic eruptions is a common process

interpretation of anisotropy of magnetic susceptibility fabric of ignimbrites in terms of kinematic and sedimentological mechanisms an anatolian case study abstract a study of the neogene low aspect ratio kizilkaya ignimbrite in the calc alkaline central anatolian volcanic province turkey has been carried out using the anisotropy of magnetic susceptibility ams technique on 46 sites irm hysteresis loops and curie temperature analysis indicate that magnetite is the main carrier of the magnetic signal in the ignimbrite the shape of the ams ellipsoids varies widely from site to site and the anisotropy is commonly low we review the data processing used in the literature on the ams of ignimbrites to calculate principal directions or to evaluate the quality of the results and we develop various simple techniques data filtering and contouring procedures in order to more accurately define the axis or symmetry of the ams fabric there is no simple relationship between the shape of the ams ellipsoids and the shape of the petrofabric ellipsoids similarly there is no direct relationship between magnetic and kinematic axes our results show that the source of the measured magnetic signal in these rocks is a complex result of various contributions including shape anisotropy of free and fixed magnetic crystals shape ams crystallization of magnetic grains on all surfaces and discontinuities during cooling of the ignimbrite distribution anisotropy and alteration by hydrothermal and meteoric water particular attention is paid to the interpretation of the ams stereoplots departing from the standard fabric of sedimentary rocks formed under unidirectional flow in order to explain these non standard ams fabrics we discuss their possible origin 1 in analytical and mineralogical terms instrumental artifact pollution by xenoclasts mineralogical inversion of the susceptibility axes secondary mimetic fabric and 2 in terms of sedimentological mechanisms rheology depositional processes in the present state of knowledge we conclude that due to the physico chemical complexity of the magnetic source the ams signal should be used and interpreted with care when attempting to decipher the sedimentological or rheological processes occurring in moving pyroclastic flows on the other hand ams has it greatest potential when used in conjunction with other techniques and when a large number of sampling sites and specimens is used

fabrics facies and flow through a large volume ignimbrite pampa de oxaya chile the miocene oxaya formation exposed along the western andean slope in northern chile represents one of the largest ignimbrite provinces on earth in this study magnetic fabric data were acquired from a 1 km long core drilled vertically through a single cooling unit of the oxaya formation the ca 22 ma cardones ignimbrite samples for magnetic analysis were obtained every 20 m from the fine grained matrix of the core detailed measurements of the variation in bulk magnetic properties including natural remanent magnetization nrm susceptibility and anisotropy of magnetic susceptibility ams were used to monitor changes in magnetic mineralogy as well as changes in the strength and orientation of the magnetic fabric throughout the flow ams ellipsoid orientation and shape reflect rapid deposition from a concentrated granular fluidized flow and constrain both the location of the source caldera for this catastrophic eruption and processes of transport and deposition in this large volume ignimbrite after utilizing the magnetic remanence to correct for rotation about the core axis well grouped and imbricated petrofabric orientations reveal a well defined sw 247 transport direction down a proto western andean slope indicating syn or post welding flow and confirming the deformed lauca caldera as the likely source of the eruption systematic variations in fabric shape t and intensity p with depth reveal a predominately oblate fabric toward the top and base of the flow and predominately prolate fabrics in the center these vertical changes in fabric reveal that this massive apparently homogeneous deposit has a systematic layering this layering reflects depth dependent temperature variations temporal changes in the flow boundary zone during deposition and post depositional processes

estimates of flow direction for calc alkaline welded tuffs and paleomagnetic data reliability from anisotropy of magnetic susceptibility measurements central san juan mountains southwest colorado abstract flow directions are estimated from the measurement of the magnetic fabric of 106 samples collected at 18 sites in four welded tuff units in the central san juan mountains of southern colorado the estimates assume that the tuffs generally flowed directly away from the extrusive vents and that the lineations of magnetic grains within the tuffs represent the flow direction at individual sites errors in the estimation may arise from topographic variation rheomorphism post emplacement mass flow within the tuff and other factors magnetic lineation is defined as the site mean anisotropy of magnetic susceptibility maximum azimuth a test on the flow directions for individual units is based on the projection of lineation azimuths and their intersection within or near the known source caldera for the tuff this test is positive for the four units examined paleomagnetic results for these tuffs are probably reliable indicators of the geomagnetic field direction in southwest colorado during the time 28 2 26 5 ma of emplacement

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

magnetic fabric welding texture and strain fabric in the nuraxi tuff sardinia italy anisotropy of magnetic susceptibility ams has been used to interpret flow directions in ignimbrites but no study has demonstrated that the ams fabric corresponds to the flow fabric in this paper we show that the ams and strain fabric coincide in a high grade ignimbrite the nuraxi tuff a miocene rhyolitic ignimbrite displaying a wide variability of rheomorphic features and a well defined magnetic fabric natural remanent magnetization nrm data indicate that the magnetization of the tuff is homogeneous and was acquired at high temperatures by ti magnetite crystals comparison between the magnetic fabric and the deformation features along a representative section shows that ams and anisotropy of isothermal remanent magnetization airm fabric are coaxial with and reproduce the shape of the strain ellipsoid magnetic tests and scanning electron microscopy observations indicate that the fabric is due to trails of micrometer size pseudo single domain magnetically interacting magnetite crystals microlites formed along discontinuities such as shard rims and vesicle walls mimicking the petrofabric of the tuff the fabric was thus acquired after deposition before late rheomorphic processes and accurately mimics homogeneous deformation features of the shards during welding processes and mass flow

dense welding caused by volatile resorption welding of pyroclastic rocks is generally thought to occur by mechanical expulsion of interstitial gas from a deposit as it compacts under its own weight we propose here that volatile resorption and compression can also be important factors in welding we describe densely welded rocks which cannot be explained by loading and re evaluate the welding process taking into account the effects of volatile resorption into the glassy fragments intra caldera oligocene ignimbrites from the rhodope mountains bulgaria and intrusive tuffs of the loch ba ring dyke mull in scotland are intensely welded but some samples lack foliation eutaxitic texture fiamme and glass shards show no preferred orientation in thin section or on outcrop scale fiamme are sometimes complexy deformed into u or s shapes we propose that these textures are the consequence of volatile dissolution in the glass and gas compression scaling analysis indicates that there are two regimes for gas behaviour following emplacement of hot pyroclastic deposits which depend on the relative characteristic time scales of compaction gas escape and gas resorption a gas escape regime and a gas retention regime during explosive eruption glassy pyroclasts are decompressed to less than one atmosphere pressure and are outgassed during deposition and subsequent burial in a thick hot deposit volatiles will be retained and soluble gases h 2 o can be resorbed back into the glass defining the gas retention regime poorly soluble gases air are compressed to small fractions of their original volume resulting in destruction of pore spaces and vesicles in pumice in some circumstances such as volcanic vents the volume changes involve isotropic strain and implosion of the tuff resorption of water greatly speeds up welding during compactional loading by reducing the viscosity of the glass welding is inhibited not only by lower temperatures but also by entrainment of insoluble atmospheric gases the two regimes also can explain the common presence of post emplacement gas escape pipes in non welded ignimbrites and their rarity in densely welded ignimbrite factors that are likely to promote the gas retention regime include fine grain size low collapsing columns a large thickness of tuff and incorporation of external water

fe oxide microcrystals in welded tuff from southern nevada origin of remanence carriers by precipitation in volcanic glass although it is widely recognized that remanent magnetism in ash flow tuffs is carried by fine grained fe oxides the origin mineralogy and significance of such magnetic carriers are not well understood the authors have obtained transmission electron microscope images of distinctive fe oxide microcrystals in rhyolitic samples located 3 8 7 6 and 18 5 m above the base of a 110 m thick section of the miocene tiva canyon member of the paintbrush tuff the fe oxide microcrystals are lath shaped and increase in size from 20 x 140 nm in the lowermost sample near base of the member to 120 x 800 nm in the uppermost sample within the flow interior microcrystals in this size range are within or close to the range of single domain grain size for magnetite electron diffraction and analytical x ray data indicate that the microcrystals in the lower two samples are cubic fe oxides magnetite maghemite with less than 10 mol ti end member and that those in the uppermost sample are manganiferous hematite systematic variations in magnetic properties are consistent with the observed variations in size and mineralogy of the microcrystals these microcrystals are morphologically distinct from grains that the authors interpret to be fragments of phenocrysts the morphology and spatial distribution ofmore the microcrystals as well as their increase in grain size from the rapidly cooled base of the ash flow sheet into the flow interior are consistent with an origin by nucleation and growth from volcanic glass at elevated temperature subsequent to emplacement less

horizontal subduction zones convergence velocity and the building of the andes we discuss the relationships between andean shortening plate velocities at the trench and slab geometry beneath south america although some correlation exists between the convergence velocity and the westward motion of south america on the one hand and the shortening of the continental plate on the other hand plate kinematics neither gives a satisfactory explanation to the andean segmentation in general nor explains the development of the bolivian orocline in paleogene times we discuss the cenozoic history of horizontal slab segments below south america arguing that they result from the subduction of oceanic plateaus whose effect is to switch the buoyancy of the young subducting plate to positive we argue that the existence of horizontal slab segments below the central andes during eocene oligocene times and below peru and north central chile since pliocene resulted 1 in the shortening of the continental plate interiors at a large distance from the trench 2 in stronger interplate coupling and ultimately 3 in a decrease of the trenchward velocity of the oceanic plate present day horizontal slab segments may thus explain the diminution of the convergence velocity between the nazca and south american plates since late miocene

wind driven variability of the large scale recirculating flow in the nordic seas and arctic ocean the varying depth integrated currents in the nordic seas and arctic ocean are modeled using an integral equation derived from the shallow water equations this equation assumes that mass divergence in the surface ekman layer is balanced by convergence in the bottom ekman layer the primary flow component follows contours of f h the model employs observed winds and realistic bottom topography and has one free parameter the coefficient of the linear bottom drag the data used for comparison are derived from in situ current meters satellite altimetry and a primitive equation model the current meter data come from a 4 yr record at 75 8 ni n the greenland sea the currents here are primarily barotropic and the model does well at simulating the variability the best bottom friction parameter corresponds to a spindown time of 30 60 days a further comparison with bottom currents from a mooring on the norwegian continental slope deployed over one winter period also shows reasonable correspondence the principal empirical orthogonal function obtained from satellite altimetry data in the nordic seas has a spatial structure that closely resembles f h a direct comparison of this mode s fluctuations with those predicted by the theoretical model yields linear correlation coefficients in the range 0 75 0 85 the primitive equation model is a coupled ocean ice version of the princeton ocean model for the north atlantic and arctic monthly mean depth averaged velocities are calculated from a 42 yr integration and then compared with velocities predicted from an idealized model driven by the same reanalyzed atmospheric winds in the largely ice free norwegian sea the coherences between the primitive equation and idealized model velocities are as high as 0 9 on timescales of a few months to a few years they are lower in the remaining partially or fully ice covered basins of the greenland sea and the arctic ocean presumably because ice alters the momentum transferred to the ocean by the wind the coherence in the canadian basin of the arctic can be increased substantially by forcing the idealized model with ice velocities rather than the wind estimates of the depth integrated vorticity budget in the primitive equation model suggest that bottom friction is important but that lateral diffusion is of equal or greater importance in compensating surface ekman pumping