geochemical variability in distal and proximal glass from the youngest toba tuff eruption the youngest toba tuff ytt erupted at ca 74 ka is a distinctive and widespread tephra marker across south and southeast asia the climatic human and environmental consequences of the ytt eruption are widely debated although a considerable body of geochemical data is available for this unit there has not been a systematic study of the variability of the ash geochemistry intrinsic magmatic and extrinsic post depositional chemical variations bring fundamental information regarding the petrogenesis of the magma the distribution of the tephra and the interaction between the ash and the receiving environment considering the importance of the geochemistry of the ytt for stratigraphic correlations and eruptive models it is central to the ytt debate to quantify and interpret such variations here we collate all published geochemical data on the ytt glass including analyses from 67 sites described in the literature and three new samples two principal sources of chemical variation are investigated i compositional zonation of the magma reservoir and ii post depositional alteration post depositional leaching is responsible for up to ca 11 differences in na2o k2o and ca 1 differences in sio2 al2o3 ratios in ytt glass from marine sites continental tephras are 2 higher in na2o k2o and 3 higher in sio2 al2o3 with respect to the marine tephra we interpret such post depositional glass alteration as related to seawater induced alkali migration in marine environments crystal fractionation and consequential magmatic differentiation which produced order of magnitude variations in trace element concentrations reported in the literature also produced major element differences in the ytt glass feo al2o3 ratios vary by about 50 which is analytically significant these variations represent magmatic fractionation involving fe bearing phases we also compared major element concentrations in ytt and oldest toba tuff ott ash samples to identify potential compositional differences that could constrain the stratigraphic identity of the morgaon ash western india no differences between the ott and ytt samples were observed

backazimuthal variations of splitting parameters of teleseismic sks phases observed at the broadband stations in germany sks phases observed at broadband stations in germany show significant shear wave splitting we have analyzed sks and skks phases for shear wave splitting from 13 stations of the german regional seismic network grsn from 3 three component stations of the grafenberg array grf and from one austrian station sqta the data reveal strong differences in the splitting parameters fast direction and delay time t from a single event at various stations as well as variations at the individual stations for events with different backazimuths the backazimuthal variations of the splitting parameters at some stations can be explained by two layer anisotropy models with horizontal symmetry axes the best resolved two layer model is the gra1 model upper layer 40 t 1 15s lower layer 115 t 1 95s the upper layer can be attributed to the lithosphere because of the magnitude of the delay time of the upper layer the lower layer must lie within the asthenosphere at other stations splitting parameters are consistent with an anisotropic one layer model for the upper mantle stations near the bohemian massif show fast directions near ew throughout ne germany the directions are oriented nw se the reason for this direction is probably the nearby tornquist teisseyre line the observed fast axes are subparallel to this prominent transeuropean suture zone at stations in southern germany near the alps we observed ene wsw directions below some stations we also found indications of inclined anisotropic layers

contribution of grain size and compositional data from the bengal fan sediment to the understanding of toba volcanic event abstract toba tephra layers in cores from the bengal fan have been studied to characterize their granulometric mineralogic and geochemical features the layers are silt sized sediments with no significant non volcanic contamination the samples are very homogeneous glass shards have moderate hydration about 6 water a calc alkaline character and a rhyolitic composition the major and trace element compositions indicate an orogenic affinity and coincide with literature data for the youngest toba tephra level ytt 75 ka x ray diffraction xrd data indicate that the glass is the dominant phase with minor feldspar biotite and quartz the grain size analyses of the beds show discontinuous vertical variation and no linear trends from south to north over about 1500 km the vertical granulometric gap of the layers probably represents a single pyroclastic emplacement unit subjected to pre depositional particle aggregation

climate volcanism feedback and the toba eruption of 74 000 years ago abstract a general feedback between volcanism and climate at times of transition in the quaternary climate record is suggested exemplified by events accompanying the toba eruption 74 000 yr ago the largest known late quaternary explosive volcanic eruption the toba paroxysm occurred during the 18o stage 5a 4 transition a period of rapid ice growth and falling global sea level which may have been a factor in creating stresses that triggered the volcanic event toba is estimated to have produced between 1015 and 1016 g of fine ash and sulfur gases lofted in co ignimbrite ash clouds to heights of at least 32 5 km which may have led to dense stratospheric dust and sulfuric acid aerosol clouds these conditions could have created a brief dramatic cooling or volcanic winter followed by estimated annual northern hemisphere surface temperature decreases of 3 to 5 c caused by the longer lived aerosols summer temperature decreases of 10 c at high northern latitudes adjacent to regions already covered by snow and ice might have increased snow cover and sea ice extent accelerating the global cooling already in progress evidence for such climate volcanic feedback following milankovitch periodicities is found at several climatic transitions

identifying global seismic anisotropy patterns by correlating shear wave splitting and surface wave data we compare a global compilation of shear wave splitting measurements with azimuthal seismic anisotropy parameters inferred from surface wave tomography the currently available splitting dataset is taken from a novel comprehensive collection of available publications that is updated interactively online the comparison between the two types of data is made by calculating predicted splitting parameters from the anisotropic tomography model comparing these predicted splitting parameters with the observed ones we find a considerable correlation between the two datasets at global scale this result is noteworthy since such correlation did not seem to exist in previous studies the spatial resolution associated with the two types of methods is rather different while surface waves have good vertical resolution and poor lateral resolution of several hundreds of kilometers sks splitting measurements have good lateral but poor vertical resolution the correlation can be understood in light of recent propositions that anisotropy seen by sks splitting constrains mostly the upper mantle and therefore a similar depth region as surface waves the correlation also confirms the generally good quality of the shear wave measurements as well as that of the anisotropic tomography model

statistical properties of seismic anisotropy predicted by upper mantle geodynamic models 1 we study how numerically predicted seismic anisotropy in the upper mantle is affected by several common assumptions about the rheology of the convecting mantle and deformation induced lattice preferred orientations lpo of minerals we also use these global circulation and texturing models to investigate what bias may be introduced by assumptions about the symmetry of the elastic tensor for anisotropic mineral assemblages maps of elasticity tensor statistics are computed to evaluate symmetry simplifications commonly employed in seismological and geodynamic models we show that most of the anisotropy predicted by our convection lpo models is captured by estimates based on a best fitting hexagonal symmetry tensor derived from the full elastic tensors for the computed olivine enstatite lpos however the commonly employed elliptical approximation does not hold in general the orientations of the best fitting hexagonal symmetry axes are generally very close to those predicted for finite strain axes correlations between hexagonal anisotropy parameters for p and s waves show simple bilinear relationships such relationships can reduce the number of free parameters for seismic inversions if this information is included a priori the match between our model predictions and observed patterns of anisotropy supports earlier more idealized studies that assumed laboratory derived mineral physics theories and seismic measurements of anisotropy could be applied to study mantle dynamics the match is evident both in agreement between predicted lpo at selected model sites and that measured in natural samples and in the global pattern of fast seismic wave propagation directions

newly discovered eastern dispersal of the youngest toba tuff volcanic glasses with minor mafic mineral fragments such as biotite and hornblende found in deep sea sediments of the south china sea basin scsb have been clearly identified as eruptive products of the youngest toba tuff ytt northern sumatra indonesia the tephra layer occurs between marine oxygen isotopic event 5 1 79 3 ka and event 4 22 64 1 ka with an interpolated age of 74 0 ka which is in good consistence with previous radiometric dating 73 75 ka and ice core dating 71 5 ka of the ytt the tephra consists predominantly of bubble wall shards with minor elongated vesicles of pumice fragments geochemical characteristics of the tephra such as high total alkali content high 87 sr 86 sr ratio and uniformity of their compositions all suggest that the recovered tephra is of the youngest toba tuff this finding supports an extended dispersal of coarse 63 mm glass shards over 1500 km northeast of the toba caldera a direction opposite to what previously conceived while providing a better documentation of the distribution extent of the toba ash this report points to the need to reestimating the eruptive volume of the ytt and re evaluating its environmental impact q 2000 elsevier science b v all rights reserved

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

a melt inclusion study of the toba tuffs sumatra indonesia the toba caldera in northern sumatra indonesia is the site of earth s largest quaternary volcanic eruption this eruption dated at 74 ka produced the 2800 km3 youngest toba tuff ytt and ash fall quartz bearing silicic pumices ranging from 68 to 77 wt sio2 indicate that the ytt magma was zoned compositionally and mineralogically prior to the ytt eruption two other silicic quartz bearing tuffs known as the middle toba tuff mtt and the oldest toba tuff ott were erupted from toba at 0 501 ma and 0 840 ma respectively although the volatile contents of the toba magmas are poorly constrained aerosols generated by the ytt eruption are generally thought to have caused a global volcanic winter an evaluation of the pre eruptive dissolved gas contents of the ytt magma is fundamental to understanding the global effects of this eruption we used melt inclusions in quartz crystals to determine the dissolved h2o co2 s cl and f contents of the ytt mtt and ott magmas quartz crystals selected from pumice blocks and welded tuffs that spanned the compositional ranges of the three units were chosen for study major and trace element analyses of melt inclusions were also conducted to characterize melt evolution and provide context to the volatile data melt inclusions from the ytt mtt and ott are rhyolitic in composition 73 77 wt sio2 and have overlapping indistinguishable major element trends inclusions from the large eruptions ytt and ott have similar geochemical relationships where the most evolved melt inclusions occur in the least silicic bulk rock samples and are more evolved than their matrix glasses in contrast the least evolved melt inclusions occur in the most silicic samples and either overlap with or are less evolved than their matrix glasses these patterns can be explained if quartz crystals in the least silicic magmas were inherited from more evolved melt while those in the most silicic melt formed in situ several lines of evidence suggest that crystal settling of early formed quartz crystals in long lived magma bodies was an important process in the ytt and ott magmas trace element data from the ytt melt inclusions have wide ranges rb 250 830 ppm ba 10 485 ppm and mostly support these models however incompatible trace elements are enriched 2 3 over their matrix glass compositions in all samples in the smaller mtt melt inclusions overlapped with the compositional range of the bulk rock samples and are consistent with in situ crystallization ftir analyses indicate that the ytt mtt and ott melts contained about 4 0 5 5 2 0 5 5 and 2 0 5 5 wt h2o respectively co2 contents in all units are generally 100 ppm and cl is 2000 ppm water gradients occur in all 3 tuffs cl gradients are evident only in the ytt and the least silicic samples from all units had no detectable co2 sulfur contents in melt inclusions are low in all samples 32 ppm and overlap with matrix glass s contents applying broad degassing constraints about 1014 g of h2so4 aerosols were likely loaded into the atmosphere during the ytt eruption 2 orders of magnitude less than previous petrologic estimates considerably more cl and f about 1015 g of each may have been part of the ytt aerosol cloud

mantle deformation and tectonics constraints from seismic anisotropy in the western united states abstract we have examined shear wave splitting in teleseismic shear waves sks skks s from 15 stations in the western united states based on analysis of 123 records from 67 events the varied past and present tectonic styles in this region subduction transform faulting extension and stable domains are expected to cause a wide variety of anisotropic behavior and therefore make it an excellent natural laboratory fast polarization azimuths vary from e w to nearly n s and time delays t range from being barely detectable less than 0 6 s to 1 6 s most stations yielded consistent measurements independent of station event geometry the exceptions were stations situated very close to the san andreas fault which yielded well constrained but inconsistent splitting parameters these have been successfully modeled by two anisotropic layers with different horizontal symmetry axes the upper layer has a fast direction parallel to the fault and t of about 1 s the lower layer with t from 0 6 to 0 9 s is oriented e w near the san francisco bay area and ne sw in the mojave desert other measurements of e w fast are observed as far east as western nevada with large delay times of 1 3 1 5 s but disappear to the north stations in the northwestern basin and range have values of oriented at about 70 with delay times ranging from 0 7 to 1 2 s varies in other regions of the basin and range from 70 in eastern nevada to 20 in the transition zone between the colorado plateau and the basin and range with t of 1 0 s measurements are geographically coherent and in many cases changes correspond to geologic or tectonic boundaries the e w fast feature extends across several tectonic provinces but disappears near the southern boundary of the gorda plate and may therefore be related to the dynamics of plate interactions regional variations in the fast directions of anisotropy within the basin and range suggest that the present extension has not been strong enough to align the fast directions over a great depth extent fast polarization directions within the transition zone between the basin and range and colorado plateau are nearly parallel to the geologic and geophysical boundaries between the two regions and to other geophysical boundaries the size of the delay times suggests that in many of the areas studied either the mantle is almost twice as anisotropic as we estimate or the anisotropy and therefore mantle deformation extends into the sublithospheric mantle we used the two layer method to examine the effect of a postulated lower layer with fast direction parallel to the absolute motion of the north american plate and found the best estimate of contribution from a uniform layer is 0 1 s of t although a contribution of up to 0 7 s cannot be ruled out a value as low as 0 1 s suggests either that olivine is not strongly aligned in a horizontal direction in this layer or that the layer thickness is of the order of 10 km

local and regional scale impacts of the 74 ka toba supervolcanic eruption on hominin populations and habitats in india abstract the toba supervolcanic eruption of 74 000 years ago is argued to have devastated homo sapiens populations causing a human population bottleneck through a combination of rapid global climatic deterioration and ecological disruption following the widespread ash fall this dramatic eruption the largest in at least the last 2 million years is hypothesized to have shaped the genetic structure and diversity of human populations today past assessments of toba s human impacts however have been predominantly theoretical using archaeological and geological evidence from sites in india this study addresses toba s local scale impacts on hominin technology behavior demography and habitats in two valleys in india the jurreru and middle son valleys using paleoclimatological and volcanological data regional scale impacts throughout india are inferred a refugia hypothesis is presented that argues for variation in the scale of toba s impact throughout india driven by regional differences in monsoonal dynamics geography and topography areas of north west india the indo gangetic plain and parts of the deccan to the east of the western ghats are argued to have been the most affected whereas areas of southern and eastern india may have preserved the largest refugia for hominin populations

sks splitting beneath southern california measurements of sks phase splitting were obtained from nineteen seismic stations in southern california the fast polarization directions are 53 at the southern end of the great valley 82 8 in the western transverse ranges and northern peninsular ranges 95 4 in mojave desert and 70 on san clemente island the splitting time ranges from 0 8 to 1 8 seconds which is consistent with an anisotropic layer of 100 to 200 km thick for 4 anisotropy

probing the accumulation history of the voluminous toba magma the age and compositional zonation in crystals from the youngest toba tuff record the prelude to earth9s largest quaternary eruption we used allanite crystals to date and decipher this zoning and found that the crystals retain a record of at least 150 000 years of magma storage and evolution the dominant subvolcanic magma was relatively homogeneous and thermally stagnant for 110 000 years in the 35 000 years before eruption the diversity of melts increased substantially as the system grew in size before erupting 75 000 years ago

toba supereruption age and impact on east african ecosystems lane et al 1 recovered microscopic glass shards cryptotephra from a thin layer of sediment deposited in lake malawi and chemically characterized them as volcanic ash youngest toba tuff ytt from the toba supereruption in sumatra the authors found no evidence of significant climate change at multidecadal to millennial timescales in the sedimentary record and in their report conclude that the eruption distributed ash much more widely than previously documented but did not trigger a volcanic winter or human bottleneck in east africa although the ytt event had limited impact on ecosystems around lake malawi we think lane et al 1 are premature in extrapolating their environmental findings to all of east africa and we dispute their contention that the most robust age for the ytt is 75 0 0 9 ka

rise and fall of an isolated unstable carbonate platform the cambrian of southwestern sardinia strong effects of tensional tectonics dominate the lower and middle cambrian sequence of southwestern sardinia a thinning of the continental crust and the possible development of a passive margin in the cambrian is indicated by different stages of evolution of a mainly carbonatic platform a clastic carbonatic homoclinal ramp with algal archaeocyathan mounds epiphyton renalcis in the west clastic shallow marine to tidal sequences in the east b carbonatic clastic ramp or rimmed shelf with an ooid shoal complex prograding towards the west the back shoal area contains peloidal mudstones algal archaeocyathan biostromes girvanella and increasingly tidal deposits clastic and carbonatic towards the east c isolated platform aggraded to sea level rimmed by slope deposits with slumps and breccia beds in the southeast and northwest d isolated flooded platform barriers towards the open sea partly broke down e isolated platform with raised rims and deep interior often with thick breccia beds in uppermost parts f break down of the platform late early cambrian marked by nodular limestones and limestoneshale intercalations g terrigenous clastics cover the former platform

major element geochemistry of glass shards and minerals of the youngest toba tephra in the southwestern south china sea an ash layer newly discovered in core md01 2393 from the southwestern south china sea has been studied in order to characterize its major element features the layer 4 0 cm thick light grayish and silt size occurs right at the marine isotope stage 4 5 transition ca 74 kyr ago the morphology and geochemistry of glass shards combined with oxygen isotope and carbonate stratigraphy confirm the youngest toba eruption in northern sumatra as the origin of the ash layer major element data on mineral crystals i e biotite plagioclase and hornblende from the ash layer suggest that biotite is phenocrystic while hornblende and some plagioclase are xenocrystic implying that these xenocrysts were incorporated into the youngest toba magma before the eruption q 2005 elsevier ltd all rights reserved

the intav intercomparison of electron beam microanalysis of glass by tephrochronology laboratories results and recommendations abstract the international focus group on tephrochronology and volcanism intav of the international union for quaternary research inqua has conducted an intercomparison of tephrochronology laboratories with electron beam microanalytical data on volcanic glasses submitted from 27 instruments at 24 institutions in 9 countries this assessment includes most active tephrochronology laboratories and represents the largest intercomparison exercise yet conducted by the tephrochronology community the intercomparison was motivated by the desire to assess the quality of data currently being produced and to stimulate improvements in analytical protocols and data reporting that will increase the efficacy of tephra fingerprinting and correlation participating laboratories were each supplied with a mount containing three samples for analysis 1 rhyolitic lipari obsidian id3506 2 phonolitic sheep track tephra from mt edziza british columbia canada and 3 basaltic laki 1783 a d tephra a fourth sample rhyolitic old crow tephra was also distributed most laboratories submitted extensive details of their analytical procedures in addition to their analytical results most used some combination of defocused or rastered beam and modest beam current to reduce alkali element migration approximately two thirds reported that they routinely analyze one or more secondary standards to evaluate data quality and instrument performance despite substantial variety in procedures and calibration standards most mean concentrations compare favorably between laboratories and with other data typically four or fewer data contributions had means for a given element on a given sample that differed by more than 2 standard deviations from the overall means obtaining accurate na 2 o concentrations for the phonolitic tephra proved to be a challenge for many laboratories only one half of the data sets had means within 1 standard deviation of the 8 2 wt na 2 o value obtained by other methods one mean is higher and 14 are lower three of the data set means fall below 7 wt na 2 o most submissions had relative precision better than 1 5 for the major elements for low abundance elements the precision varied substantially with relative standard deviations as small as 10 and as large as 110 because of the strong response to this project the tephrochronology community now has a large comparative data set derived from common reference materials that will facilitate improvements in accuracy and precision and which can enable improved use of published data produced by the participating laboratories finally recommendations are provided for improving accuracy precision and reporting of electron beam microanalytical data from glasses

multichannel analysis of shear wave splitting a multichannel analysis is introduced to constrain seismic anisotropy from the shear wave splitting of sks and skks this technique utilizes simultaneously a set of records coming from different azimuths the splitting intensity of sks waves measured by the amplitude of the transverse component depends on the angle between the back azimuth of the earthquake and the direction of the symmetry axis and on the delay time t between the two quasi shear waves it is shown that the splitting parameters can be determined from the azimuthal dependence of the splitting intensity which is given by the first right eigenvector of the matrix containing the transverse components of all the records alternatively the splitting intensity can be measured by projecting the transverse components on the radial components derivatives experiments on synthetic seismograms demonstrate that both approaches provide robust estimates of the splitting parameters however the projection approach gives measurements that are closer to the inputs and with smaller error bars which suggests that it should be preferred when the signal to noise ratio is low the new technique is applied to analyze shear wave splitting under stations bcao bangui central african republic and bdfb brasilia brazil the analysis of 75 seismograms recorded by station bcao gives a fast polarization direction in very close agreement with the present day motion direction of the african plate comparisons with the results of previous studies obtained with other techniques and smaller data sets suggest that some of the published measurements may be strongly biased the main limitation of the multichannel analysis is a good azimuthal distribution of the seismicity maps of the azimuthal coverage were determined and it is found that the most favorable regions to study anisotropy with sks waves are a latitudinal band that goes from south america to africa the eastern part of north america and india

shear wave splitting in the eastern alps observed at the transalp network data from the temporary transalp seismic network were analysed to investigate the seismic anisotropy in the upper mantle beneath the eastern alps we operated mostly short period and some broadband stations in a dense linear array which transects the eastern alps at 12 e longitude recorded sks and skks phases with different backazimuths were used simultaneously to calculate the splitting parameters of delay time and fast axis direction for each seismic station while we found variations in the delay times between 0 8 and 2 0 s the determined fast axis directions prove to be rather consistent along the profile 60 70 n they coincide well with the trend of the eastern alps thus suggesting orogen parallel flow in the upper mantle our findings support the earlier proposed idea that the adriatic indenter which was forced northwards into the european plate during the late stage of the alpine orogeny triggered an escape movement to the less constrained pannonian basin to the east

shear wave splitting in the appalachians and the urals a case for multilayered anisotropy observations of shear wave splitting in the northeastern u s appalachians and in the foredeep of the urals vary significantly with the back azimuth and incidence angle of the incoming phase these variations suggest two or more layers within the upper mantle with different anisotropic properties synthetic seismograms for simple multilayered anisotropic structures show that shear wave splitting parameters tend to vary substantially with the direction of approach relying on a subset of back azimuth and incidence angle may strongly bias the model inferred especially if the observations are averaged on the other hand the azimuthal splitting pattern provides additional constraints on vertical or lateral variation of anisotropic properties in the earth using a new error estimation technique for splitting we find that individual measurements from broadband data have errors of the order of 3 7 for the fast direction and 0 1 0 2 s for the delay of split shear waves the azimuthal variation of splitting parameters is broadly consistent throughout the appalachian terranes in the northeast united states especially for two long running stations in the northeast united states hrv harvard massachusetts and pal palisades new york observations can be separated into two distinct populations with mean fast axis azimuths of n60 e 4 and n119 e 2 delay values within each population range from near zero to 1 s azimuthal splitting variation for aru arti russia in the foredeep of uralian mountains is characterized by sharp transitions between different groups of observations using synthetic seismograms in simple structures we develop one dimensional anisotropic models under stations hrv and aru the model for hrv includes two layers of anisotropic material under an isotropic crust with fast axis azimuths n53 e and n115 e for the bottom and the top layers respectively the model for the upper mantle under aru includes a layer with a fast axis at n50 e atop a layer with fast axis azimuth n90 e our modeling confirms the need for a layer of strong anisotropy with a slow axis of symmetry in the lower crust under aru reported by levin and park 1997a our results suggest that both urals and appalachians possess a relict anisotropy in the tectosphere associated with past continental collision and accretion underlain by anisotropy with orientation similar to the local absolute plate motion suggesting an asthenospheric component to the signal

estimating shear wave splitting parameters from broadband recordings in japan a comparison of three methods the goal of this study was to evaluate the performance of different splitting measurement techniques in the particularly complicated tectonic setting of subduction beneath japan we use data from the broadband japanese f net array and consider the methods of silver and chan 1991 levin et al 1999 and chevrot 2000 we find that the results generally agree well although discrepancies arise if the anisotropy beneath the station is more complex than the simple single layer an isotropic model often assumed in splitting studies a combination of multichannel and single record methods may serve as a powerful tool for recognizing complexities and for characterizing upper mantle anisotropy beneath a station

no short reversals of brunhes age recorded in the toba tuffs north sumatra indonesia paleomagnetic and 40ar 39ar data indicate that two tuffs are present at siguragura north sumatra indonesia a reversely magnetized tuff the earliest eruptive unit of the toba caldera of matuyama age 0 84 ma and a normally magnetized tuff the last eruptive unit of the toba caldera of late brunhes age no reversely magnetized tuffs of late brunhes age are present at this location as suggested by previous investigators

finite frequency vectorial tomography a new method for high resolution imaging of upper mantle anisotropy summary amplitude measurements of the transverse component of sks waves the so called splitting intensity can be used to formulate a non linear inverse problem to image the 3 d variations of upper mantle anisotropy assuming transverse isotropy or hexagonal symmetry one can parametrize anisotropy by two anisotropic parameters and two angles describing the orientation of the symmetry axis these can also be written as two collinear pseudo vectors the tomographic process consists of retrieving the spatial distribution of these pseudo vectors and thus resembles surface wave vectorial tomography spatial resolution results from the sensitivity of low frequency sks waves to seismic anisotropy off the ray path the expressions for the 3 d sensitivity kernels for splitting intensity are derived including the near field contributions and validated by comparison with a full wave equation solution based upon the finite element method these sensitivity kernels are valid for any orientation of the symmetry axis and thus generalize previous results that were only valid for a horizontal symmetry axis it is shown that both lateral and vertical subwavelength variations of anisotropy can be retrieved with a dense array of broad band stations even in the case of vertically propagating sks waves

shear wave anisotropy beneath nicaragua and costa rica implications for flow in the mantle wedge 1 we present new shear wave splitting data from local events in costa rica and nicaragua recorded by the temporary july 2004 to march 2006 48 station tucan broadband seismic array observed fast polarization directions in the fore arc arc and back arc range from arc parallel to arc normal over very short distances 5 km when plotted at raypath midpoints making the direct interpretation of individual splitting measurements in terms of flow tenuous even when considering variations in the relationship between lattice preferred orientation and deformation e g b type dislocation creep in olivine therefore we tomographically invert the splitting measurements to find a three dimensional model of crystallographic orientation in the wedge we assume the elastic constants of olivine and orthopyroxene with hexagonal symmetry and use a damped iterative least squares approach to account for the nonlinear behavior of splitting when considering three dimensional ray propagation and distributions of anisotropy the best fitting model contains roughly horizontal arc parallel olivine 100 axes in the mantle wedge down to at least 125 km beneath the back arc and arc which we interpret to indicate along arc flow in the mantle wedge pb and nd isotopic ratios in arc lavas provide additional evidence for arc parallel flow and also constrain the direction northwest from costa rica to nicaragua and minimum flow rate 63 190 mm a with only slightly oblique subduction at 85 mm a of the relatively planar cocos plate the most likely mechanism for driving along arc transport is toroidal flow around the edge of the slab in southern costa rica generated by greater slab rollback in nicaragua two important implications of this arc parallel flow are the progressive depletion of the mantle source for arc lavas from costa rica to nicaragua and the possible need for significant decoupling between the wedge and downgoing plate

seismic anisotropy of the slave craton nw canada from joint interpretation of sks and rayleigh waves summary teleseismic events recorded at a 25 element array in nw canada between 2001 and 2006 provided sufficient distribution in back azimuth to demonstrate birefringence in sks and skks waves as well as directional dependence of rayleigh wave phase velocities typical delays between orthogonally polarized sks waves are 0 8 1 2 s and modelling of azimuthal dependence indicates two nearly horizontal layers of anisotropy within the mantle anisotropy of rayleigh waves is generally consistent with models of layered vs anisotropies that increase with depth from 1 per cent at the moho to 9 per cent at 200 km but vary between subarrays consistency between the sks and rayleigh wave anisotropies in one subarray suggests that the assumption of symmetry about a horizontal axis is valid there but is not fully valid in other parts of the craton the upper layer of anisotropy occupies approximately the uppermost 120 km in which the fast polarization direction strikes generally north south coinciding with regional scale fold axes mapped at the surface the fast polarization direction of the deeper layer aligns with current north america plate motion but its correlation with trends of coeval kimberlite eruptions within the lac de gras field suggests it can be at least partly attributed to structural preferred orientation of vertical dykes inferred to exist to depths of 200 km

resolving three dimensional anisotropic structure with shear wave splitting tomography summary shear wave splitting observations are a commonly used tool for inferring anisotropy and flow within the earth s interior here we present the development and validation of a new technique for imaging anisotropy in the upper mantle using local events shear wave splitting tomography swst the mantle is parametrized as a 3 d block model of crystallographic orientations with the elastic properties of olivine and orthopyroxene and both orthorhombic and hexagonal symmetries are tested to efficiently forward calculate splitting the christoffel equation is used to progressively split the horizontal components of a synthetic wavelet in each block of the model and predicted shear wave splitting parameters are obtained with an eigenvalue minimization technique numerically calculated partial derivatives are utilized in a linearized damped least squares inversion to solve for a best fitting model of crystallographic orientations to account for the non linear properties of shear wave splitting the inversion is applied iteratively and partial derivatives are recalculated after each iteration a starting model that incorporates information from predicted splitting parameters is found by spatially averaging fast directions and the ratio of observed to predicted splitting times models from inversions utilizing this average starting model reach lower misfit levels than do inversions with a random or uniform starting model modelling results using synthetic data from several anisotropic structures i e sharp lateral and vertical variations in anisotropy both within an idealized and a real nicaragua costa rica subduction zone illustrate the capabilities and limitations of swst with a station spacing of 25 km in an idealized subduction zone containing uniformly spaced events down to 225 km both the azimuth and dip of crystallographic axes are resolvable to a depth of 100 150 km and lateral heterogeneities in anisotropy on a scale of 50 km at arc and forearc distances from the trench are retrieved spatial resolution of anisotropy at scales of 75 km is possible further into the backarc above 150 km depth the geometry of stations and observed seismicity in the nicaragua costa rica subduction zone yields partial to good resolution at scales of 50 75 km beneath the forearc arc and limited regions of the backarc down to 100 km and resolution at coarser scales is possible in wider regions beneath the backarc given the distributions of seismic sources within many subduction zones and the advances in broad band seismic array deployments this new method offers a powerful means with which to constrain the orientation of anisotropic fabric in the upper mantle

the 74 ka toba super eruption and southern indian hominins archaeology lithic technology and environments at jwalapuram locality 3 hominins living in southern india 74 000 years ago faced a deteriorating environment as the global climate moved from interglacial into full glacial conditions at the same time south asian populations witnessed the widespread deposition of tephra from the sumatran toba super eruption the largest explosive volcanic event of the past two million years here we report new data on the lithic technology and environmental context for a southern indian site with hominin occupation in association with toba tephra deposits jwalapuram locality 3 in the jurreru valley sedimentological and isotopic studies demonstrate that a cooling trend was in effect in this part of southern india prior to the eruption and that thick deposits of ash in the jurreru valley supported grassland communities before more wooded conditions were re established detailed technological analyses of an expanded lithic sample from locality 3 suggest cultural continuity after the eruptive event and comparisons with lithic core technologies elsewhere indicate that homo sapiens cannot be ruled out as the creator of these middle palaeolithic assemblages

seismic anisotropy in eastern tibet from shear wave splitting reveals changes in lithospheric deformation abstract knowledge about seismic anisotropy can provide important insight into the deformation of the crust and upper mantle beneath tectonically active regions here we focus on the southeastern part of the tibetan plateau in sichuan and yunnan provinces sw china we measured shear wave splitting of core refracted phases sks and skks at a temporary array of 25 iris passcal stations we calculated splitting parameters using a multi channel and a single record cross correlation method multiple layers of anisotropy cannot be ruled out but are not required by the data a fresnel zone analysis suggests that the shallow mantle between 60 and 160 km depth is the most likely source of anisotropy the polarization directions reveal a pronounced transition from primarily north south in the north sichuan to mostly east west orientations in the south yunnan in the southern part of the study region that is south of 26 n the fast polarization directions do not correlate well with known surface features and geodetic estimates of the crustal displacement fields whereas gps campaigns provide evidence suggesting north south crustal flow across the red river fault the pattern of anisotropy argues against such flow in the upper mantle these observations support models that allow differential movement of upper crust relative to lithospheric mantle in the northern part of the study region the relationships are more ambiguous and coherent deformation of the crust and mantle lithosphere cannot be excluded the interpretation of the shear wave splitting results is non unique but we suggest that the observed n s transition reflects a fundamental change in deformation regime across our study region it may be related to lateral variations in lithospheric rheology or may mark a transition from the direct impact of the continental collision to dominance of the far field strain field associated with regional subduction processes understanding the nature of the lateral change in deformation regime may prove critical for our understanding the geotectonic evolution of eastern tibet

how are vertical shear wave splitting measurements affected by variations in the orientation of azimuthal anisotropy with depth splitting measurements of teleseismic shear waves such as sks have been used to estimate the amount and direction of upper mantle anisotropy worldwide these measurements are usually made by approximating the anisotropic regions as a single homogeneous layer and searching for an apparent fast direction and an apparent splitting time t by minimizing the energy on the transverse component of the back projected seismogram in this paper we examine the validity of this assumption in particular we use synthetic seismograms to explore how a vertically varying anisotropic medium affects shear wave splitting measurements we find that weak heterogeneity causes observable effects such as frequency dependence of the apparent splitting parameters these variations can be used in principle to map out the vertical variations in anisotropy with depth through the use of frechet kernels which we derive using perturbation theory in addition we find that measurements made in typical frequency bands produce an apparent orientation direction that is consistently different from the average of the medium and weighted towards the orientation of the anisotropy in the upper portions of the model this tendency of the measurements to mimic the anisotropy at the top part of the medium may explain why shear wave splitting measurements tend to be correlated with surface geology when the heterogeneity becomes stronger multiple scattering reduces the amplitude of the tangential component seismogram and the associated splitting time so that a null result may be obtained despite the fact that the waves have travelled through a strongly anisotropic medium regardless of the amount of vertical heterogeneity we find that there is very little dependence on backazimuth for the measured fast axis direction or splitting time if the top and bottom halves of the medium average to similar fast axis directions if however the average fast axis direction in the top half of the model differs from that in the bottom half splitting time measurements will show a significant dependence on backazimuth but fast axis direction measurements will remain relatively constant

all toba tephra occurrences across peninsular india belong to the 75 000 yr b p eruption abstract a controversy currently exists regarding the number of toba eruptive events represented in the tephra occurrences across peninsular india some claim the presence of a single bed the 75 000 yr old toba tephra others argue that dating and archaeological evidence suggest the presence of earlier toba tephra resolution of this issue was sought through detailed geochemical analyses of a comprehensive suite of samples allowing comparison of the indian samples to those from the toba caldera in northern sumatra malaysia and importantly the sedimentary core at odp site 758 in the indian ocean a core that contains several of the earlier toba tephra beds in addition two samples of toba tephra from western india were dated by the fission track method the results unequivocally demonstrate that all the presently known toba tephra occurrences in peninsular india belong to the 75 000 yr b p toba eruption hence this tephra bed can be used as an effective tool in the correlation and dating of late quaternary sedimentary sequences across india and it can no longer be used in support of a middle pleistocene age for associated acheulian artifacts

shear velocity structure and azimuthal anisotropy beneath eastern north america from rayleigh wave inversion 1 we have obtained shear velocity structure beneath the northeastern united states and southeastern canada using rayleigh wave phases and amplitudes thin crust 36 42 km is observed along the atlantic coast and in the eastern appalachian orogen and thick crust 42 46 km is imaged in the western appalachians and in the western new york portion of the grenville province the variation of crustal thickness correlates well with the observed bouguer gravity anomalies in the upper mantle the high velocity continental keel of cratonic north america is present in the western part of the study area while a broad low velocity region is imaged in new england from the hudson river valley to the white mountains this low velocity anomaly is probably the consequence of past heating of the lithospheric mantle associated with the monteregian hotspot and may represent intrusion of asthenosphere into the edge of the keel in addition to lateral variations in velocity we estimate the azimuthal dependence of phase velocity strong and relatively uniform shear wave splitting is observed in the study region but at periods of 100 s or less the average azimuthal anisotropy of rayleigh waves is less than 1 and is not significantly different from zero at any individual period this small degree of azimuthal anisotropy is not consistent with a substantial contribution to shear wave splitting from fossil anisotropy in the lithosphere much of the source of the shear wave splitting must lie deeper than 200 km

volcanic winter and accelerated glaciation following the toba super eruption the eruption of toba in sumatra 73 500 years ago was the largest known explosive volcanic event in the late quaternary1 it could have lofted about 1015 g each of fine ash and sulphur gases to heights of 27 37 km creating dense stratospheric dust and aerosol clouds here we present model calculations that investigate the possible climatic effects of the volcanic cloud the increase in atmospheric opacity might have produced a volcanic winter 2 a brief pronounced regional and perhaps hemispheric cooling caused by the volcanic dust followed by a few years with maximum estimated annual hemispheric surface temperature decreases of 3 5 c the eruption occurred during the stage 5a 4 transition of the oxygen isotope record a time of rapid ice growth and falling sea level3 we suggest that the toba eruption may have greatly accelerated the shift to glacial conditions that was already underway by inducing perennial snow cover and increased sea ice extent at sensitive northern latitudes as the onset of climate change may have helped to trigger the eruption itself4 we propose that the toba event may exemplify a more general climate volcano feedback mechanism

origin of peritidal carbonate cycles early cambrian sardinia abstract cyclic shallow water carbonates of early cambrian age were investigated in sardinia they consist of muddy lagoonal deposits at the base followed by a thin bed of intertidal sediments they in turn are overlain by spillover oolites cycles are locally restricted and bordered laterally by non cyclic sequences these coeval sediments consist of platform margin oolites on one side and mostly terrigenous clastics of an intertidal regime on the other side sedimentary conditions within these environments were probably stable during deposition no proof of periods of emergence is given therefore sea level changes seem less likely as the driving mechanism of cycle formation instead cyclicity is supposed to be intrinsically controled the model developed is based on a changing sedimentation rate high accumulation and building up to sea level intertidal stage and drop of sedimentation rate lag time increasing water depth after that sedimentation recovered and a new cycle started

upper mantle anisotropy beneath japan from shear wave splitting in this study we utilize data from 64 broadband seismic stations of the japanese f net network to investigate the threedimensional pattern of anisotropy in the subduction system beneath japan we have compiled a database of approximately 1900 high quality splitting measurements selected by visual inspection of over 25 000 records of s sks and skks phases at f net stations covering a wide range of incidence angles incoming polarization angles and backazimuths analysis of the variations of measured splitting parameters with these parameters allows us to consider complexities in structure such as multiple anisotropic layers dipping symmetry axes and small scale lateral variations in anisotropic properties here we focus on the presentation of the splitting measurements themselves a detailed interpretation in terms of tectonics and mantle flow is beyond the scope of this paper in the southern part of the f net array along the ryukyu arc we find that fast directions are consistently trench parallel with splitting times of 1 s or more moving northward along the array the measured splitting patterns become more complicated with significant variations in apparent splitting parameters that indicate complex anisotropic structure additionally measured fast directions vary significantly over short length scales and stations separated by less than 100 km often exhibit very different splitting behavior this increase in complexity of anisotropic structure coincides geographically with the complicated slab morphology of the subducting pacific plate at stations on hokkaido to the north and kyushu to the south we see some evidence that the fast direction of anisotropy may rotate from trench parallel close to the trench to subduction parallel further away from the trench which may correspond either to a change in stress conditions and or volatile content or to a change in flow regime 2005 elsevier b v all rights reserved

uncovering a landscape buried by the super eruption of toba 74 000 years ago a multi proxy environmental reconstruction of landscape heterogeneity in the jurreru valley south india abstract the youngest toba tuff ytt eruption 74 ka is the largest volcanic event to occur during the last two million years this paper presents a high resolution landscape reconstruction for the jurreru valley south india immediately prior to this eruption primary ash fall deposits have sealed the pre toba surface of the jurreru valley and subsequent deposition of remobilised ash has helped preserve this horizon these primary ytt deposits provide an isochron that allows for the study of palaeoenvironmental conditions across a 25 000 m2 area in the jurreru valley permitting the reconstruction of the pre toba landscape sixty sites with exposed primary ash deposits have been recorded as part of a total station survey twelve of which have been subject to detailed stratigraphical study this has enabled a reconstruction of the topography of the buried surface in the jurreru valley stable isotope and phytolith analyses are used to explore diversity across this buried landscape indicating that the ratio of c3 to c4 plants varies with regard to changes in topographic height in the landscape of up to 5 m high levels of spatial heterogeneity within these proxy data are indicated by this study highlighting the risks of extrapolating regional palaeoenvironmental sequences from vertically sampled sedimentary sections which may well reflect highly localised influences of topography and geomorphology

ash from the toba supereruption in lake malawi shows no volcanic winter in east africa at 75 ka the most explosive volcanic event of the quaternary was the eruption of mt toba sumatra 75 000 y ago which produced voluminous ash deposits found across much of the indian ocean indian peninsula and south china sea a major climatic downturn observed within the greenland ice cores has been attributed to the cooling effects of the ash and aerosols ejected during the eruption of the youngest toba tuff ytt these events coincided roughly with a hypothesized human genetic bottleneck when the number of our species in africa may have been reduced to near extinction some have speculated that the demise of early modern humans at that time was due in part to a dramatic climate shift triggered by the supereruption others have argued that environmental conditions would not have been so severe to have such an impact on our ancestors and furthermore that modern humans may have already expanded beyond africa by this time we report an observation of the ytt in africa recovered as a cryptotephra layer in lake malawi sediments 7 000 km west of the source volcano the ytt isochron provides an accurate and precise age estimate for the lake malawi paleoclimate record which revises the chronology of past climatic events in east africa the ytt in lake malawi is not accompanied by a major change in sediment composition or evidence for substantial temperature change implying that the eruption did not significantly impact the climate of east africa and was not the cause of a human genetic bottleneck at that time

seismic anisotropy of oceanic upper mantle shear wave splitting methodologies and observations we develop methodologies to obtain accurate measurements of shear wave splitting and apply these techniques to examine the pattern of oceanic upper mantle anisotropy to obtain high quality estimates of receiver splitting at island stations we devise a stacking method that finds the optimum splitting parameters and 95 error region for teleseismic shear wave phases from a suite of earthquake events to obtain additional measurements in oceanic regions we develop techniques to measure splitting parameters and errors for ss phases that sample upper mantle anisotropy at their bounce points however we find the data are often of low resolution and anomalous characteristics are sometimes found that make splitting difficult to interpret ten second splitting of ss is observed across the banjo seismic array but we cannot unambiguously attribute this signal to mantle anisotropy at the bounce the receiver splitting methods are used to assess the adequacy of a two anisotropic layer model for the pacific region with fast polarization azimuth in the lithosphere oriented in the fossil spreading direction and in the asthenosphere oriented in the absolute plate motion direction this model has been proposed to explain surface wave data in the pacific ocean but our splitting results demonstrate that oceanic anisotropy patterns are more heterogeneous than would be predicted while the island splitting measurements could reflect the influence of individual hotspot upwellings hotspot effects do not appear to be universally dominant we propose that splitting observations alternatively indicate broad scale differences in the underlying character of oceanic upper mantle anisotropy associated with coherant patters of lithospheric structure and asthenospheric flow in particular splitting surface wave models and regional studies all support a model where lithospheric anisotropy throughout the south pacific has been erased or reoriented toward the absolute plate motion direction whereas more limited observations in the north pacific indicate that the fossil lithospheric signature appears to be preserved

simultaneous inversion of shear wave splitting observations from seismic arrays 1 seismic waveforms recorded at high density receiver arrays facilitate the application of new inversion techniques which take advantage of the coherent nature of the observations we use measurements of shear wave splitting parameters from observed sks waveforms along a dense receiver profile and compare them with splitting parameters obtained from numerical waveform modeling through anisotropic earth models we use two different iterative approaches for the inversion of the observed splitting parameters 1 a local optimization technique the downhill simplex method and 2 a global genetic algorithm search in our forward modeling we calculate sks waveforms by a finite difference fd method solving the anisotropic wave equation instead of deriving individual anisotropic models for each station and combining them into one model by the comparison of fd modeling and observations we avoid a direct interpretation of the splitting parameters in terms of medium properties we apply these techniques to the analysis of sks phases recorded along a 100 km profile located at the dead sea transform fault the measured splitting parameters show gradual short scale variations along the profile which depend on frequency lateral and vertical variations of anisotropy are accounted for by two dimensional block structures for the simplex method the complexity of the models the number of anisotropic blocks is increased gradually whereas the number of anisotropic blocks is kept fixed for the genetic algorithm search the anisotropic structures of the best fitting models derived from the two inversion approaches agree well the results support earlier interpretations of the observations in terms of a narrow approximately 20 km wide vertical decoupling zone in the mantle that accommodates the transform motion between the african and arabian plates

algen als faziesindikatoren bioherme und biostrome im unter kambrium von sw sardinien archaeocythan limestones of the lower cambrian nebida fm sw sardinia form bioherms lower nebida fm matoppa mbr as well as biostromes upper nebida fm punta manna mbr in both cases algae are the main rock forming organisms they act as sediment binding and sediment baffling organisms and thus are responsible for the growth of these carbonates

early rifting deposition examples from carbonate sequences of sardinia cambrian and tuscany triassic jurassic italy an analogous tectono sedimentary and climatic context lower cambrian ceroide limestone sardinia and lower jurassic massiccio limestone tuscany belong to sequences deposited in analogous tectono sedimentary context the former linked to the caledonian sardic phase the latter to the alpine orogeny both units consist of massive pure limestone characterized by marginal and lagoonal sequences repeatedly interfingering in the same geological structure this distribution indicates a morphology of the platforms composed of banks marginal facies and shallow basins lagoonal facies comparable with a bahamian complex dolomitization affects patchily the massive limestone bodies and karstic features breccias and sedimentary dikes occur at their upper boundary both units overlie early dolomite and evaporites sabkha facies containing siliciclastic intercalations in their lower and or upper part and are unconformably covered by open shelf red hematitic nodular limestone ammonitico rosso facies the sedimentary evolution of the two sequences appears to have been controlled by synsedimentary tectonics whose major effects are the end of the terrigenous input the bank and basin morphology of the platform the irregular distribution of the dolomitization and the nodular fabric of the overlying facies the end of the bahamian type system is marked by the karstification of the emerged blocks and is followed by their differential sinking and burial under red nodular facies from amore geodynamic viewpoint sequences composed of bahamian like platform carbonates followed by ammonitico rosso facies imply deposition along continental margins subjected to block faulting during an extensional regime connected with the beginning of continental rifting moreover the variation from sabkha to bahamian conditions suggests the drifting of the continent from arid to humid tropical areas less

new geochemical evidence for the youngest toba tuff in india abstract new geochemical data on tephra samples from a layer present at several archeological sites in india support correlation of this layer to the youngest toba tuff erupted from northern sumatra about 74 000 yr ago the data show that the indian tephra layer is not a correlative of older tephra erupted from toba as has been suggested on the basis of artifact assemblages previously published geochemical data on the indian tephra beds was based on bulk ash samples containing mineral and clay contaminants and the resulting variability in analyses did not allow identification or discrimination of individual eruptive events our new data were collected on individual glass shards and small purified glass separates which have greater resolving power in fingerprinting acheulian and paleolithic artifacts found at some of the indian tephra sites do not reflect the antiquity of the tephra bed as they occur in fluvial sediments and may be reworked

the role of tephra covers on soil moisture conservation at haleakala s crater maui hawai i abstract the influence of tephra covers on soil water was studied in haleakala maui hawai i during two summers eight sites with tephra layers and silverswords argyroxiphium sandwicense dc were sampled at 2415 2755 m at each site eight paired sample sets were obtained in bare soils and under adjacent tephra at three depths tephra were sharply separated from underlying soils and showed prominent vertical stratification tephra clast size distribution was assessed by photosieving and on interstitial gravel samples stones included 45 6 cobbles 29 4 pebbles and 25 blocks moisture content increased with depth in both positions but soils below tephra had more water at all depths than exposed areas surface soils beneath tephra contained 83 more water than bare ground soils at 5 10 cm had 106 greater moisture under rocks but only 70 at 10 15 cm differences between plots were statistically significant p all soils were coarse with 20 gravel and 94 sand most fine material 0 063 mm was silt as clay content was negligible organic matter percentage was low 1 65 bulk density and porosity were associated with moisture variation both in tephra insulated and bare soils 80 of field moisture was statistically p

modeling anisotropy and plate driven flow in the tonga subduction zone back arc the goal of this study is to determine whether shear wave splitting observed in subduction zone back arc regions the tonga subduction zone in particular can be quantitatively modeled with flow in the back arc mantle driven by the motions of the subducting slab and the upper back arc plate we calculated two dimensional mantle flow models using known tonga plate motions as boundary conditions and assuming a range of uniform and variable viscosity structures shear wave splitting was predicted for the anisotropy due to lattice preferred orientation lpo of olivine and orthopyroxene in the flow model finite strain fields the predicted shear wave splitting provides a good match to the fast directions parallel to the azimuth of subducting plate motion and splitting times 0 5 1 5 s observed in tonga both for models where lpo anisotropy develops everywhere above 410 km and for models where lpo anisotropy is confined to regions of relatively high stress if lpo anisotropy does develop over the entire upper 410 km of the mantle the strength of anistropy induced by a given amount of shear strain must be relatively weak 4 for shear strains of 1 5 with a maximum value of 6 for very large strains the splitting observations are comparably fit by a wide range of different viscosity models anisotropy due to melt filled cracks aligned by stresses in the back arc flow models predicts fast directions roughly normal to observed values and thus cannot alone explain the observed splitting

ytt ash from quaternary sediments of kapileshwar area purna alluvial basin central india abstract a thick pocket of volcanic ash 2 5 m represented by various shades of gray powdery massive to laminated in nature hosted in yellowish brown thinly bedded laminated silty clay lithocolumn has been studied for its preservational and geochemical aspects the ash column shows preservations of both primary and reworked ashes which are distinguishable on the basis of their field criteria including colors bedding geometries and thicknesses petrological study of the same reveals high representation of glass shards with bubble wall blocky and pumice morphologies minor occurrences of light and heavy minerals and unidentifiable admixture of fine grained sediments geochemistry of both bulk ash and glass shards through icp aes and epma respectively suggest high percentages of sio2 al2o3 na2o and k2o ree data generated through icp ms show strong negative eu anomaly the effects of weathering or aqueous solution on the geochemistry are also noticeable as evident by the variations in the values of both mobile and immobile elements however the data obtained from both bulk ash and glass shards are within the limits of ytt composition calm to very low energy environment of deposition is suggested for their deposition paleoclimatic proxies represented by rhizoliths rhizospheres rhizolith balls ant traces and vertebrate remains preserved in the pre tephra succession have also been commented

diverse late stage crystallization and storage conditions in melt domains from the youngest toba tuff revealed by age and compositional heterogeneity in the last increment of accessory phase growth the chemical record contained within the final increment of growth on crystals is utilized to reveal the dynamics and timescales of magma assembly and storage before eruption of the cataclysmic 2800 km3 youngest toba tuff ytt indonesia in situ u th disequilibrium dates and trace element concentrations were obtained via secondary ionization mass spectrometry sims on unsectioned and unpolished faces of individual zircon and allanite crystals the six high silica 73 wt sio2 pumices from which crystals were derived are among the more evolved and lower crystallinity 25 wt pumices from the ytt eruption and likely represent the melt dominated portion of the magma system discrete sims measurement cycles were coupled with statistical treatments to detect zircon and allanite surface zoning domains at the 1 m scale coupled r melts and accessory phase saturation modeling indicates that at the granite ternary minimum or eutectoid conditions that define this portion of the ytt zircon and allanite crystallization is dependent on and proportionate to major phase crystallization and is more limited than at pre eutectoid conditions a lower proportion of near eruption zircon surface ages in the comparatively cool and wet ytt relative to other hotter and drier voluminous silicic eruptions could reflect the influence of eutectoid storage conditions on magmatic responses to remobilization related magmatic recharge

fitful and protracted magma assembly leading to a giant eruption youngest toba tuff indonesia the paroxysmal eruption of the 74 ka youngest toba tuff ytt of northern sumatra produced an extraordinary 2800 km3 of nonwelded to densely welded ignimbrite and coignimbrite ashfall we report insights into the duration of ytt magma assembly obtained from ion microprobe u th and u pb dates including continuous age spectra over 50 of final zircon growth for pumices and a welded tuff spanning the compositional range of the ytt a relatively large subpopulation of zircon crystals nucleated before the penultimate caldera related eruption at 501 ka but most zircons yielded interior dates 100 300 ka thereafter zircon nucleation and growth was likely episodic and from diverse conditions over protracted time intervals of 100 to 500 ka final zircon growth is evident as thin rim plateaus that are in th u chemical equilibrium with hosts and that give crystallization ages within tens of ka of eruption the longevity and chemical characteristics of the ytt zircons as well as evidence for intermittent zircon isolation and remobilization associated with magma recharge is especially favored at the cool and wet eutectoid conditions that characterize at least half of the ytt wherein heat fluxes could dissolve major phases but have only a minor effect on larger zircon crystals repeated magma recharge may have contributed to the development of compositional zoning in the ytt but considered together with limited allanite quartz and other mineral dating and geospeedometry regular perturbations to the magma reservoir over 400 ka did not lead to eruption until 74 ka ago

first toba supereruption revival little has been known about the earliest toba eruptive episodes that created the largest known caldera complex of quaternary age here we report evidence for the eastward dispersal of the oldest toba tuff in south china sea sediments to 2500 km away from the source the tephra deposits occur below the brunhes matuyama geomagnetic boundary 778 ka and slightly above the australasian microtektite layer 793 ka calibrated by astronomically tuned oxygen isotope stratigraphy the middle pleistocene toba eruption occurred during the deglaciation at 788 2 2 ka according to the tephra occurrence between marine isotope stages 20 and 19 this refined age is in good agreement with the 4 0 ar 3 9 ar date of 800 20 ka for the toba tephra layer d from ocean drilling program odp site 758 but significantly younger than the commonly cited ar ar age of 840 30 ka the eruption expelled at least 800 1000 km 3 dense rock equivalent of rhyolitic magma on the basis of the widespread tephra fall deposit in the basins of the indian ocean and the south china sea in spite of its exceptional magnitude the timing of this major eruption does not indicate a causal linkage between this event and a long term global climatic deterioration

the non commutivity of shear wave splitting operators at low frequencies and implications for anisotropy tomography summary measurements of the splitting or birefringence of seismic shear waves constitute a powerful and popular technique for characterizing azimuthal anisotropy in the upper mantle the increasing availability of data sets from dense broad band seismic arrays has driven interest in the development of techniques for the tomographic inversion of shear wave splitting data and in comparing splitting measurements with anisotropic upper mantle models obtained from other constraints such as surface wave analysis two different theoretical approaches have been developed for predicting apparent shear wave splitting parameters fast direction and delay time for models that include multiple layers of anisotropy at depth which is useful for comparing azimuthally anisotropic surface wave models with shear wave splitting measurements these approaches differ in one key aspect which is whether or not the shear wave splitting operator can be treated as commutative in this paper we investigate the theoretical source of this discrepancy and show that at frequencies relevant to most studies of upper mantle anisotropy the term that results in the non commutivity of the shear wave splitting operator in the expressions for multiple layer splitting must be retained in contrast the quantity known as the splitting intensity which is closely related to the apparent fast direction and delay time does commute at these frequencies we illustrate these inferences with forward modelling examples and discuss their implications for the tomographic inversion of shear wave splitting measurements the comparison of surface wave models with shear wave splitting observations and the joint inversion of surface wave and shear wave splitting observations for upper mantle anisotropic models

new occurrence of youngest toba tuff in abyssal sediments of the central indian basin abstract volcanic glass and pumice found in siliceous abyssal sediments of the central indian basin south of the equator have previously been assigned various origins including intra basin volcanism indonesian arc and krakatau rhyolitic glass shards dispersed in sediments from 8 cores that we have examined are compositionally identical to those of fallout deposits of the youngest toba tuff erupted at 74 ka from northern sumatra the correlation extends the distribution of coarse 63 m glass shards some 1500 km south of the previously known fallout zone and into the southern hemisphere reaching latitudes of ca 14 s this provides evidence for bi hemispheric dispersal of the ash cloud and supports global dispersal of gas and aerosols from the eruption such dispersal could have facilitated the global impact of the eruption

shear wave splitting measurements problems and solutions abstract we compare sensitivity and reliability of three basic techniques of shear wave splitting analysis cross correlation of wave components minimization of covariance matrix eigenvalues and minimizing energy on the transverse component of both synthetic and real data point out some problems and propose solutions we discuss major difficulties of evaluating shear wave splitting parameters which are not always obvious in routine processing and can sometimes lead to false results and wrong interpretations we describe effects of pre processing of digital records of shear waves e g filtering and setting lengths of analyzed signals and discuss reliability of results to minimize outliers in evaluated splitting parameters for reliable evaluation of splitting of medium quality sks wave forms which are valuable mainly in temporary deployments of stations we recommend a careful band pass filtering of the signal using a window length not shorter than the dominant period inspecting the signal noise ratio on t component and the particle motion the transverse minimization method provides usually more robust and stable solutions than the other two methods in the conclusion we suggest how to proceed in evaluation of splitting parameters to ensure reliable solutions even for medium or low quality shear waveforms recorded frequently during temporary field measurements

shelf to basin transition in the cambrian lower ordovician of sardinia italy reply in southern and central sardinia two different cambrian lower ordovician sedimentary belts occur the former is exposed in southwestern sardinia iglesiente and sulcis and is characterized by shallow marine siliciclastic and carbonate sediments settled on a continental shelf with lowangle morphology the latter exposed in the nappe zone central and southeastern sardinia consists of a thick monotonous sequence of siliciclastic deposits settled in a deeper water basin

geochemical fingerprinting of the widespread toba tephra using biotite compositions toba caldera sumatra is one of the largest and most explosive volcanoes on earth erupting some of the most voluminous volcanic deposits chronologically these eruptions are 790 ka older toba tuff 500 ka middle toba tuff and 74 ka younger toba tuff ash from these eruptions is dispersed over the entire region from india through malaysia to indonesia where it forms isochronous markers that have been documented in a variety of sedimentary sequences including palaeoclimate archives and archaeological sites the chemistry of the volcanic glass shards usually enables distal volcanic ash units to be correlated to a known eruption however the magmas involved in the three largest toba eruptions are compositionally similar and consequently the glass chemistry that is used to differentiate eruption units cannot be used to identify distal deposits here we show that the composition of biotite crystals which occur with glass in the distal deposits can be used to fingerprint the deposits of the younger toba tuff biotite in younger toba tuff has a lower feo mgo 2 1 2 6 than in the products of older eruptions 2 8 3 7 correlations using these distinct biotite compositions indicate that the ash found in malaysia and in archaeological sites in india was from the 74 ka younger toba tuff eruption

minerals formed and mineral formation from volcanic ash by weathering abstract weathering of volcanic ash and pumice is unique regarding minerals formed and mineral formation these minerals include allophane imogolite opaline silica and some halloysites and have distinctive structures and properties studies on the surface and subsurface weathering of various volcanic ashes in different settings are reviewed this review illustrates that relatively rapid weathering of finely comminuted volcanic ash well reflects the effects of climate vegetation and time and produces various combinations of weathering products the moisture regime the nature and amount of organic matter incorporated the deposition of volcanic ash as overburden and the chemical and mineralogical composition of parent ash are important in controlling the mineral formation in weathered volcanic ash

toba ash layers in the south china sea evidence of contrasting wind directions during eruption ca 74 ka two cores from the southern south china sea contain discrete ash layers that mainly consist of rhyolithic glass shards on the basis of the specmap time scale the ash layers were dated to ca 74 ka the age of the youngest toba eruption in northern sumatra this link is supported by the chemical composition of the glass which is distinct from volcanic glass supplied from the philippines and the northern south china sea but is almost identical with the chemistry of the toba ash the youngest toba ash layers in the south china sea expand the previously known ash fall zone over more than 1800 km to the east the dispersal of ashes from sumatra in both western and eastern directions indicates two contrasting wind directions and suggests that 1 the toba eruption probably happened during the southeast asian summer monsoon season and 2 the volume of erupted magma was larger than previously interpreted

azimuthal anisotropy in the lithosphere from observations of long period s waves summary the paper gives a detailed description of the technique for measuring azimuthal anisotropy which was suggested earlier by vinnik kosarev makeyeva 1984 and used by kind et ai 1985 the technique is based on the observations of long period converted phases like sks additionally we describe a generalization of this approach which makes use of long period s waves of arbitrary polarization both modifications were applied to records of the grf array in southern germany the results of this analysis if combined with the data on p velocities bamford 1977 suggest that the direction of the fast velocity in the lithosphere of the region varies with depth

the toba volcanic event and interstadial stadial climates at the marine isotopic stage 5 to 4 transition in the northern indian ocean abstract the toba volcanic event one of the largest eruptions during the quaternary is documented in marine sediment cores from the northeastern arabian sea on the crest of the murray ridge and along the western indian continental margin we detected distinct concentration spikes and ash layers of rhyolithic volcanic shards near the marine isotope stage 5 4 boundary with the chemical composition of the youngest toba tuff time series of the uk 37 alkenone index planktic foraminiferal species magnetic susceptibility and sediment accumulation rates from this interval show that the toba event occurred between two warm periods lasting a few millennia using toba as an instantaneous stratigraphic marker for correlation between the marine and ice core chronostratigraphies these two arabian sea climatic events correspond to greenland interstadials 20 and 19 respectively our data sets thus depict substantial interstadial stadial fluctuations in sea surface temperature and surface water productivity we show that variable terrigenous eolian sediment supply played a crucial role in transferring and preserving the productivity signal in the sediment record within the provided stratigraphic resolution of several decades to centennials none of these proxies shows a particular impact of the toba eruption however our results are additional support that toba despite its exceptional magnitude had only a minor impact on the evolution of low latitude monsoonal climate on centennial to millennial time scales

detailed mapping of seismic anisotropy with local shear waves in southeastern kamchatka summary the kamchatka peninsula lies over a vigorous subduction zone where pacific and north american plates converge at a rate of almost 80 mm yr 1 earthquakes associated with the subduction process provide an excellent source of seismic data for the study of anisotropic properties of the upper mantle and crust overlying the downgoing lithospheric slab we collected a large set of shear waves from events within the slab recorded by a variety of seismic stations in kamchatka data from permanent and temporary networks cover the entire 700 km length of the subduction zone with 50 200 km spacing between observing sites resulting in an unprecedented coverage of the supraslab mantle wedge we estimated shear wave splitting in selected s waves using two techniques and applied quality tests to ensure measurement stability fast directions vary from station to station and they can vary with backazimuth at individual stations and with direction of propagation for individual sources in over 350 measurements we recovered meaningful splitting delays up to 1 s with most delays in the 0 2 0 6 s range additionally in nearly 200 measurements splitting could not be resolved yielding null observations anisotropic properties of the kamchatka supraslab mantle wedge vary greatly along the volcanic arc and forearc of the subduction zone observed anisotropic indicators in the arc and forearc correlate spatially with some tectonic features e g volcanoes inland of the volcanic arc most splitting values indicate trench parallel fast polarization we do not observe depth dependence in local s wave splitting delays consistent with a shallow coherence of anisotropic texture in the vicinity of petropavlovsk kamchatsky observed anisotropic indicators are coherently trench normal and thus consistent with 2 d corner flow however splitting above the fragmented slab edge near the klyuchevskoy volcanic centre is variable and trench oblique birefringence between petropavlovsk and klyuchevskoy is weak overall our observations are incompatible with a regional slab driven corner flow regime

did the 73 ka toba super eruption have an enduring effect insights from genetics prehistoric archaeology pollen analysis stable isotope geochemistry geomorphology ice cores and climate models abstract the potential impact of the 73 ka toba super eruption upon global and regional climate terrestrial ecosystems and prehistoric human populations remains unclear evidence from genetics prehistoric archaeology pollen analysis stable isotope geochemistry geomorphology ice cores and climate models has provided some useful working hypotheses further progress requires a substantial improvement in the accuracy precision and resolution of the chronologies of each of the marine and terrestrial proxy records used to reconstruct the environmental impact of this extreme event

multimode rayleigh wave inversion for heterogeneity and azimuthal anisotropy of the australian upper mantle summary we present an azimuthally anisotropic 3 d shear wave speed model of the australian upper mantle obtained from the dispersion of fundamental and higher modes of rayleigh waves we compare two tomographic techniques to map path average earth models into a 3 d model for heterogeneity and azimuthal anisotropy method i uses a rectangular surface cell parametrization and depth basis functions that represent independently constrained estimates of radial earth structure it performs an iterative inversion with norm damping and gradient regularization method ii uses a direct inversion of individual depth layers constrained by bayesian assumptions about the model covariance we recall that bayesian inversions and discrete regularization approaches are theoretically equivalent and with a synthetic example we show that they can give similar results the model we present here uses the discrete regularized inversion of independent path constraints of method i on an equal area grid with the exception of westernmost australia we can retrieve structure on length scales of about 250 km laterally and 50 km in the radial direction to within 0 8 per cent for the velocity 20 per cent for the anisotropic magnitude and 20 for its direction on length scales of 1000 km and longer down to about 200 km there is a good correlation between velocity heterogeneity and geologic age at shorter length scales and at depths below 200 km however this relationship breaks down the observed magnitude and direction of maximum anisotropy do not in general appear to be correlated to surface geology the pattern of anisotropy appears to be rather complex in the upper 150 km whereas a smoother pattern of fast axes is obtained at larger depth if some of the deeper directions of anisotropy are aligned with the approximately n s direction of absolute plate motion this correspondence is not everywhere obvious despite the fast 7 cm yr 1 northward motion of the australian plate more research is needed to interpret our observations in terms of continental deformation predictions of sks splitting times and directions an integrated measure of anisotropy are poorly matched by observations of shear wave birefringence

shear wave splitting to test mantle deformation models around hawaii seismic anisotropy allows us to study mantle deformation and it can thus help to constrain mantle flow in the vicinity of hotspots hypotheses for the cause of seismic anisotropy in this environment include the parabolic asthenospheric flow paf model radial flow from a mantle plume impinging on a moving lithosphere is dragged by the plate in the direction of absolute plate motion in map view this gives a parabolic pattern of flow opening in the direction of plate motion we present new shear wave splitting observations from land and ocean stations around the hawaiian islands that can be explained by the parabolic flow model the observations suggest asthenospheric anisotropy under the hawaiian islands which may be explained if dislocation creep persists to deeper depths there than in other regions perhaps due to the higher temperatures near hotspots

cooling of the south china sea by the toba eruption and correlation with other climate proxies 71 000 years ago the toba tephra layer has been identified in core md972151 in the southern south china sea scs northeast of the indonesia toba caldera this affords us an opportunity to directly determine a 1 c cooling for ca 1 kyr on the scs following the toba eruption 71 ka during the marine isotope stage 5a 4 transition using century scale sea surface temperature records this cooling event in the scs is well correlated with several coeval proxies such as increased east asia winter monsoon intensity increased ice rafted detritus in the north pacific ocean sediments and decreased 18 o in greenland ice core such correlation suggests a climate change where cold climate signals originated in the northern hemisphere ice sheets transferred southward by the winter monsoon and cooled the scs

shear wave splitting beneath western united states in relation to plate tectonics we have examined shear wave splitting in teleseismic shear waves from 26 broadband stations in the western united states fast polarization directions and delay times t show spatial variations that are coherent within geologic provinces stations located near the san andreas fault show clear evidence for fault parallel anisotropy in the crust and upper mantle 115 125 km thickness this can be explained by the finite strain associated with the relative plate motion between the north american and pacific plates the lateral extent of this strain field is probably narrow to the west because stations 55 km west of the san andreas fault do not show fault parallel anisotropy in southern california station lac located 80 km east of the san andreas fault shows large fault parallel anisotropy this suggests that the pacific north american plate boundary in the mantle might be displaced to the east in southern california a deeper e w oriented fast direction of anisotropy underlies the fault parallel anisotropic layer in the vicinity of the san andreas fault an e w fast feature is also present beneath the western basin and range and the foothills of the sierra nevada although local variations are present the magnitude of delay times suggests that this feature resides in the asthenosphere we interpret this feature as the asthenospheric flow in the slabless window left behind the farallon plate the flow induced anisotropy may partially be frozen in at shallow depths station orv is located near the southern edge of the gorda slab where no anisotropy is detected the absence of anisotropy at this location could therefore mark a boundary between farallon associated flow and regions where e w oriented asthenospheric flow did not occur the lack of evidence for ne sw fast orientation within the walker lane shear belt of western nevada suggests that this crustal feature does not extend into the mantle or that is not as well developed as that beneath the san andreas fault stations located over the young subducting gorda plate mark a change in the fast direction to nearly ne sw this direction aligns well with the maximum compressive stress direction in the overlying north american plate and the ne sw directed internal shearing of the gorda plate the anisotropic thicknesses calculated from delay times suggest roughly double that expected for purely lithospheric contributions this implies that the anisotropic thickness may include some of the asthenosphere alternatively using a higher anisotropy of 8 can bring thicknesses in line with other measures of lithospheric thicknesses the correspondence between the fast directions and the present plate tectonic deformations suggest that mapping upper mantle deformation through seismic anisotropy is a viable method and that asthenospheric flow may be a significant contributor to seismic anisotropy

tephra sedimentation during the 2010 eyjafjallajokull eruption iceland from deposit radar and satellite observations the april may 2010 eruption of eyjafjallajokull volcano iceland was characterized by a nearly continuous injection of tephra in the atmosphere that affected various economic sectors in iceland and caused a global wide interruption of air traffic eruptive activity during 4 8 may 2010 was characterized based on short duration physical parameters in order to capture transient eruptive behavior of a long lasting eruption i e total grainsize distribution erupted mass and mass eruption rate averaged over 30 minute activity resulting 30 minute total grainsize distribution based on both ground and msg seviri satellite measurements is characterized by mdphi of about 2 phi and a fine ash content of about 30wt accumulation rate varied by two orders of magnitude with an exponential decay away from the vent whereas mdphi shows a linear increase until about 18 km from vent reaching a plateau of about 4 5 phi between 20 56 km associated mass eruption rate is in between 0 6 1 2 x 10 5 kg s 1 in situ sampling showed how fine ash mainly fell as aggregates of various typologies about 5 to 9 wt of the erupted mass remained in the cloud up to 1000 km from the vent suggesting that nearly half of the ash 7 phi settled as aggregates within the first 60 km particle sphericity and shape factor varied between 0 4 and 1 with no clear correlation with size and distance from vent our experiments also demonstrate how satellite retrievals and doppler radar grainsize detection can provide real time description of the source term but for a limited particle size range

limited global change due to the largest known quaternary eruption toba 74 kyr bp abstract the 74 kyr bp super eruption of toba volcano in sumatra is the largest known quaternary eruption on the basis of preserved deposits the eruption magnitude has been estimated at 7 10 15 kg 2800 km 3 of dense magma the largest sulphate anomaly in the greenland ice sheet project 2 core has been identified as fallout from toba s stratospheric aerosol veil correlation of the sulphate and oxygen isotope stratigraphy of the ice core suggests that the toba eruption might have played a role in triggering a millennium of cool climate prior to dansgaard oeschger event 19 although a comparable stadial preceded event 20 a possible 6 yr duration volcanic winter immediately following the eruption has also been proposed as the cause of a putative bottleneck in human population supporting in a general way the garden of eden model for the origin of modern humans however along with counter arguments regarding the timing of any demographic crash there remain major gaps in our understanding of the 74 kyr bp toba eruption that hinder attempts to model its global atmospheric and climatic and hence human consequences the tephra record reveals basic aspects of the eruption style but calculations of the duration and hence intensity and plume height of the event are poorly constrained furthermore estimates of the sulphur yield of the erupting magma central to predictions of its atmospheric and climatic impacts vary by two orders of magnitude 3 5 330 10 10 kg previous estimates of globally averaged surface cooling of 3 5 c after the eruption are probably too high a figure closer to 1 c appears more realistic the volcanological uncertainties need to be appreciated before accepting arguments for catastrophic consequences of the toba super eruption

tephrochronology of the toba tuffs four primary glass populations define the 75 ka youngest toba tuff northern sumatra indonesia four primary glass populations well defined by their sr ba and y concentrations occur in the youngest toba tuff ytt which was deposited during a supereruption of the toba caldera complex in northern sumatra 75 ka average concentrations of major and trace elements indicate a coherent systematic variation of glass composition across populations no clear pattern in the areal distribution of these four glass groups can be discerned the multiple glass populations of the ytt easily distinguish it from the single homogeneous glass population of the middle toba tuff 500 ka as represented by its basal vitrophyre and that of the oldest toba tuff 800 ka as represented by ash layer d at the ocean drilling program site 758 in the indian ocean

stratigraphy of the toba tuffs and the evolution of the toba caldera complex sumatra indonesia during the past 1 2 m y a magma chamber of batholithic proportions has developed under the 100 by 30 km toba caldera complex four separate eruptions have occurred from vents within the present collapse structure which formed from eruption of the 2800 km3 youngest toba tuff ytt at 74 ka eruption of the three older toba tuffs alternated from calderas situated in northern and southern portions of the present caldera the northern caldera apparently developed upon a large andesitic stratovolcano the calderas associated with the three older tuffs are obscured by caldera collapse and resurgence resulting from eruption of the ytt samosir island and the uluan block are two sides of a single resurgent dome that has resurged since eruption of the ytt samosir island is composed of thick ytt caldera fill whereas the uluan block consists mainly of the oldest toba tuff ott in the past 74000 years lava domes have been extruded on samosir island and along the caldera s western ring fracture this part of the ring fracture is the site of the only current activity at toba updoming and fumarolic activity the toba eruptions document the growth of the laterally continuous magma body which eventually erupted the ytt repose periods between the four toba tuffs range between 0 34 and 0 43 m y and give insights into pluton emplacement and magmatic evolution at toba

how to make robust splitting measurements for single station analysis and three dimensional imaging of seismic anisotropy summary we quantify errors on splitting intensity measurements produced by incoherent noise on horizontal components through a detailed analysis of synthetic seismograms and of sks and skks waves recorded by four permanent broad band stations we find that these errors are quite significant even on high quality records because the measurement procedure involves the comparison of radial and transverse components which are both contaminated by noise to decrease the level of noise in the data it is thus recommended to average splitting intensities from waves coming from the same backazimuth alternatively one can analyse stacked radial and transverse components after applying a wiener filter which standardizes the waveforms this is found to be equivalent because the measure of splitting intensity is a linear process the utilization of wiener filters reduces variations of apparent splitting resulting from differences in frequency content of the waveforms it is possible to determine splitting parameters splitting delays and fast directions from azimuthal variations of splitting intensity at a particular station tests on synthetic seismograms demonstrate that this approach gives unbiased and robust estimates of splitting parameters in contrast to the silver and chan method which only works on the rare records that have very large signals on their transverse components the wolfe and silver method does not suffer from this limitation and gives the same results as the multichannel splitting intensity method analysis of splitting intensities at stations bdfb bgca can and erm gives fast directions that are generally in excellent agreement with those determined with the wolfe and silver method however at some stations azimuthal variations of splitting intensities reveal features that are not well captured by the sinusoidal variation representing the average anisotropy beneath the station this suggests that smallscale lateral variations of anisotropy are present in the subcontinental mantle which remain to be imaged in detail

how to relate body wave and surface wave anisotropy seismic anisotropy is one of the most efficient geological and geodynamical tools for understanding the dynamics of the earth upper mantle anisotropy is evident in seismic data sets for the last 30 years primarily from surface wave dispersion curves and body wave sks data we demonstrate in this paper that surface wave and body wave derived anisotropy can be explained by the same anisotropic parameters l gc gs in the simplest case of a horizontal fast symmetry axis one application of this method is to display the sks delay time and the corresponding azimuth which can be derived from surface wave global with a lateral resolution of around 2000 km and regional lateral resolution of 350 km anisotropy tomography a global scale comparison is disappointing since there are only a few areas where both data types are correctly retrieved at the same spatial scale same lateral resolution the anisotropy is well resolved in oceanic basins from surface waves whereas most measurements of sks splitting are available below continents however there is a good agreement between surface wave and body wave anisotropy in regions where large scale processes primarily tectonic are taking place such as the western united states and in central asia in particular we show that at high frequency 1hz the observed sks delay time and azimuth depend on the order in which s waves propagate through the layers a comparison between synthetic and observed sks phases in central asia highlights the varying sensitivity of sks waves at depth we can obtain a good correlation between synthetic and observed sks by considering anisotropy only between 80 and 200 km depth in the tomographic model rather than in the entire upper mantle however it is more difficult to explain the discrepancy between synthetic and observed sks under the eastern united states where a large scale coherent sks anisotropy is present such results call for more sophisticated sks waveform modeling and for enhanced lateral resolution in anisotropic tomographic models

a micromorphological study of pedogenic processes in an evolutionary soil sequence formed on late quaternary rhyolitic tephra deposits north island new zealand abstract the influence of time as a soil forming factor was studied on an evolutionary sequence of five soils 1850 radiocarbo years bp ca 120 000 bp developed in rhyolitic tephra deposits in new zealand new micromorphological observations were combined with existing macromorphological chemical textural and clay mineralogical analyses to assess the extent of polygenesis in the soils the younger soils 1850 bp to ca 22 600 bp are characterised by initial weathering and little secondary mineral formation this status is expressed by abundant pumice fragments and fresh easily weatherable minerals as well as a low clay content with an isotropic character comminution and neoformation are the main pedological processes the older soils ca 120 000 bp are characterised by a high degree of alteration of primary minerals little or no pumice fragments clay illuviation gleying and a high clay content 80 consisting of a range of crystalline clay minerals including halloysite gibbsite kaolinite and vermiculite plus crystalline iron oxides evidence of polygenesis can be observed in the micromorphology of the older soils 1 the range of secondary minerals is the result of the integration and succession of soil forming processes through time 2 the accumulatory character of the tephra deposits has resulted in stronger weathering of the lower horizons than upper horizons because of continuous pedogenesis such polygenesis in the older soils supports the view that the older the soil the more polygenetic it is likely to be

toba ash on the indian subcontinent and its implications for correlation of late pleistocene alluvium the toba ash occurs extensively in the indian subcontinent and marks a ca 74 000 yr old event in the bay of bengal and indian ocean it is about 10 cm thick whereas in several alluvial basins it is usually 1 3 m thick the latter occurs in a partly reworked state but as nearly chemically pure first cycle sediments the ash has a broad northwesterly dispersal pattern samples of ash from the indian subcontinent compare closely with the youngest 74 000 yr b p toba tuff and the deep sea toba ash in bulk chemical composition ree signature and bubble wall shard morphology however a more proximally located and thicker 2 5 m ash bed from the alluvial basins in the gneissic area and close to east coast has a lower magnitude negative eu anomaly possibly because of minor contamination by feldspathic silt quaternary sediments in the central narmada and middle son basins contain rich late and middle pleistocene mammalian and cultural records based on the presence of the ash layer marker and stratigraphic relations late pleistocene sediments within the subcontinent can be correlated with those from central india and the deep sea

the 73 ka toba super eruption and its impact history of a debate abstract toba volcano in northern sumatra is located at the intersection of two major tectonic lineaments in one of the most seismically active regions in the world and its caldera is considered the largest quaternary caldera on earth the most recent toba explosive eruption at 73 ka was an order of magnitude larger than that of tambora in 1815 and has a volcanic explosivity index of 8 the exceptional magnitude of this super eruption and the widespread distribution of the younger toba tuff ytt in marine cores in the indian ocean the arabian sea and the south china sea have prompted sustained debate as to its global and regional impact on climate ecosystems and prehistoric human populations however despite climate modelling and appeals to a variety of proxy climatic data including geochemistry pollen analysis archaeology and stable isotopes the extent of the impact remains unclear in order to achieve better progress in the future greater care is needed to obtain higher resolution and more precise chronologies from the marine and terrestrial proxy records used to test whether or not the 73 ka toba super eruption had a significant environmental impact records from speleothems and ice cores have much to offer in this regard

individual glass shard trace element analyses confirm that all known toba tephra reported from india is from the c 75 ka youngest toba eruption uncertainty over the identity and age of toba tephras across peninsular india persists with radiometric age dates contradicting earlier compositional data which have been used to identify this important stratigraphic marker as the youngest toba tuff ytt to address this issue new single glass shard analyses have been performed for samples from morgaon and bori north western india which have recently been dated at c 800 ka these and indeed all toba tephra samples thus far analysed from india show the presence of four populations of glass shards defined by their ba y ratio which uniquely identifies them as products of the c 75 ka youngest toba eruption confirmation that the ytt fingerprint is characteristic comes from new analyses of oldest toba tuff ott glass shards from five sites in the indian ocean these are compositionally identical to layer d from the odp site 758 sediment core c 800 ka and belong to a single low ba population clearly different from ytt these analyses show that there is essentially no reworked ott material in the ytt eruption and indicate unequivocally that all known toba tephra occurrences in india belong to the c 75 ka youngest toba eruption