isotropy or weak vertical transverse isotropy in d beneath the atlantic ocean 1 shear velocity properties of d beneath the central atlantic ocean are explored using predominantly european seismic recordings of intermediate and deep focus 100 km south american earthquakes broadband data are analyzed and when possible corrected for upper mantle models of receiver side anisotropic structure regional shear velocity heterogeneity in d is mapped by analysis of 306 s sks differential times that have been corrected for three dimensional seismic velocity structure above d using a whole mantle tomographic model this correction yields modest less than 2 estimates of seismic velocity heterogeneity in d with a transition from high to low seismic velocities traversing from west to east beneath the central atlantic in agreement with global tomographic models additionally shear wave splitting of s and sdiff for the same recordings was analyzed to assess seismic anisotropy in d the highest quality data provide 105 splitting times between sh and sv onsets that are mostly within the 1 s uncertainty level the few larger values generally exhibit sv delayed relative to sh assuming an anisotropy geometry involving vertical transverse isotropy vti as preferred in most regions of d that have been studied to date 0 5 anisotropy strength within a 100 km thick layer or 0 25 anisotropy within a 300 km thick layer are compatible with the data these values are low in comparison to those found in high velocity regions beneath the circum pacific ocean or in the low velocity region beneath the central pacific and many observations are in fact consistent with isotropic structure the lack of strong vti relative to other regions may be due to 1 the absence of stress from overlying midmantle downwelling 2 relatively weaker shear flow in the d boundary layer and or 3 lack of chemical heterogeneity that could develop either lattice preferred orientation or shape preferred orientation the azimuthal sampling of this region of d is quite limited thus the precise geometry and mechanism of any anisotropy are difficult to constrain it remains possible that this region may contain subtle azimuthal anisotropy that could couple the sv and sh signals however amplitude observations suggest that any such coupling is minor

d shear velocity heterogeneity anisotropy and discontinuity structure beneath the caribbean and central america the d region in the lowermost mantle beneath the caribbean and central america is investigated using shear waves from south american earthquakes recorded by seismic stations in north america we present a large scale composite study of volumetric shear velocity heterogeneity anisotropy and the possible presence of a d discontinuity in the region our data set includes 328 s sdiff sks differential travel times 300 scs s differential travel times 125 s sdiff and 120 scs shear wave splitting measurements and 297 seismograms inspected for scd the seismic phase refracted from a high velocity d layer broadband digital data are augmented by high quality digitized analog wwssn data providing extensive path coverage in our study area in all data from 61 events are utilized in some cases a given seismogram can be used for velocity heterogeneity anisotropy and discontinuity analyses significant mid mantle structure possibly associated with the ancient subducted farallon slab affects shear wave travel times and must be corrected for to prevent erroneous mapping of

detection of mantle plumes in the lower mantle by diffraction tomography hawaii abstract a new method is proposed to detect mantle plumes in the lower mantle the method is akin to diffraction tomography and relies on the scattering of long period seismic body waves by nearly vertical heterogeneities the theory is described in a companion paper ying ji h c nataf earth planet sci lett this issue here we apply this method to an actual set of long period digital seismograms recorded on the world wide network of seismic stations between 1980 and 1994 we select seismograms that illuminate the lower mantle in a 20 20 region around hawaii we construct an image of plume like heterogeneities using a cell size of 1 1 by a lsqr inversion of the scattered waves this image shows a strong slow anomaly about 200 km northwest of hawaii although the image is somewhat noisy resolution tests indicate that this feature is fairly robust the amplitude of the anomaly is between 30 and 60 times larger than what we predict from a simple thermal plume model built with a 600 k maximum temperature excess and a gaussian horizontal cross section with an 1 e diameter of 250 km it is the first time that such a method is applied to the mantle while we think that the existence of this anomaly is real we do not know how to explain such a large amplitude if real this observation has a number of important geodynamical consequences it indicates that a mantle plume is indeed responsible for the hawaii hotspot as speculated by morgan w j morgan nature 230 1971 42 43 the plume is nearly vertical it originates from the d region at the base of the lower mantle the amplitude of the anomaly suggests that partial melt or and a chemical anomaly must be present the plume source is to the northwest of its surface expression as predicted by some models of plume advection in the mantle wind

complex anisotropy in d beneath the eastern pacific from sks skks splitting discrepancies article i nfo although observations of seismic anisotropy in the lowermost mantle are abundant the mechanism which generates anisotropy in the d layer is not well understood most observational constraints on d anisotropy come from the splitting of shear waves that propagate nearly horizontally through the lowermost mantle however anisotropy in the d layer can also give rise to discrepancies in shear wave splitting for sks and skks phases for the same event station pair as these phases sample different regions of the lowermost mantle here i report observations of strongly discrepant sks skks splitting observed at broadband stations in western mexico and california in particular strong skks splitting with fast polarization directions near 60 and delay times up to 3 s is observed for a group of raypaths that sample a region of the d layer beneath the eastern pacific ocean a comparison of skks splitting with sks splitting observed from the same events as well as with sks splitting observed at a variety of backazimuths at each station suggests that the anomalous anisotropic structure is in the lowermost mantle my preferred model for the unusual anisotropic geometry in this region is large shear deformation in d at the edge of a region where slab material impinges upon the core mantle boundary resulting in lattice preferred orientation of lower mantle minerals

the shear wave velocity gradient at the base of the mantle the relative amplitudes and travel have been directed toward obtaining global times of scs and s phases are utilized to place averages and the degree of lateral variation in constraints on the shear wave velocity gradient d properties remains an open question above the core mantle boundary a previously a conflicting result was found by mitchell and reported long period scsh sh amplitude ratio helmberger 1973 who utilized the relative minimum in the distance range 65 o to 70 o is shown amplitudes and timing of long period scs and s to be a localized feature apparently produced by phases to constrain the s wave velocity gradient an amplitude anomaly in the direct s phase and in d they found a minimum in the scsh sh therefore need not reflect the velocity gradient amplitude ratio near 68 o which was attributed to at the base of the mantle the amplitude ratios low amplitudes of the scs arrivals unable to that are free of this anomaly are consistent with explain this feature by models with negative or calculations for the jb model or models with mild near zero shear velocity gradients in d they positive or negative velocity gradients in the proposed models with positive s wave velocity lowermost 200 km of the mantle scsv arrivals gradients above the cmb these positive are particularly sensitive to the shear velocity gradients extended over 40 to 70 km above the structure just above the core mantle boundary core reaching velocities at the cmb as high as the apparent arrival time of the peak of scsv is 7 6 to 7 8 km s these models can explain the as much as 4 sreater than that of scsh in the observed amplitude ratio behavior as well as an distance range 75 v to 80 o for sea of okhotsk apparent difference observed in the arrival times events recorded in north america this can be of transversely and radially polarized scs explained by interference effects produced by a mitchell and helmberger also proposed a low q localized high velocity layer or strong positive zone in d or finite outer core rigidity to s wave velocity gradient in the lowermost 20 km explain the baseline of the scsh sh amplitude of the mantle a velocity increase of about 5 ratios while the majority of their data was for is required to explain the observed shift between deep south american events recorded in north scsv and scsh this thin high velocity layer america they did analyze one deep sea of okhotsk varies laterally as it is not observed in event for which the radial and transverse scs similar data from argentine events refined arrival times were not different which suggested estimates of the outermost core p velocity lateral variations in the d velocity structure structure are obtained by modeling sks signals in in this paper we extend the analysis of scs the distance range 75 o to 85 o s

a reconnaissance teleseismic study of the upper mantle and transition zone beneath the archean slave craton in nw canada abstract the objective of this study is to investigate upper mantle structure below the archean slave craton site of the oldest known rocks on earth and numerous diamondiferous kimberlites and thence to gain an understanding of early craton formation and kimberlite genesis to this aim a temporary array consisting of 13 sites equipped with broadband seismometers recorded teleseisms between november 1996 and may 1998 this data set has been augmented with recordings from the yellowknife seismic array our three most robust observations and their interpretation are 1 p wave travel time tomography reveals the oldest part of the craton the central slave basement complex to be underlain by the fastest seismic velocities indicating that this block remains distinct well into mantle depths 2 receiver function analysis requires only the moho as major s wave velocity discontinuity and points to a fairly constant crustal thickness throughout the slave province and 3 sks splitting analysis shows little variation in delay times and fast polarization directions across the array leading us to conclude that the present day plate motion of north america is the primary cause for mantle fabric beneath the entire array furthermore the data let us rule out the possibility that the mackenzie plume had any seismologically detectable effect on the slave lithosphere more speculative results of our investigation namely a possible genetic link between a low seismic velocity anomaly at depth with the overlying lac de gras kimberlite field and a possible archean origin of two shallow low velocity anomalies will require further investigation

laboratory and seismological observations of lower mantle isotropy we have carried out seismological and mineral physics investigations to identify the extent and origin of elastic anisotropy in the lower mantle based on observations of shear wave splitting we conclude that the lower mantle is effectively isotropic this result is surprising since the lower mantle is composed of elastically anisotropic minerals silicate perovskite mgo sio 2 that have been extensively strained by geologic processes to reconcile the seismic observations we have measured texture development during deformation and high temperature recrystallization of mantle silicates we show that these experiments provide a direct explanation for the shear wave splitting measurements the lower mantle appears isotropic because silicate perovskite maintains an isotropic texture during deformation and recrystallization

multiple phase analysis of the shear velocity structure in the d region beneath alaska a large data set more than 990 total waveforms comprised of north american station recordings of a variety of long period s phases s scs sdiff ss sscs sks skks from 36 earthquakes in western pacific subduction zones is analyzed to determine characteristics of the shear velocity structure in the d region beneath alaska three hypotheses that have been proposed to explain observed scs precursors and s waveform distortions for this region are appraised with the multiple phase data set and the radial and lateral variations of the shear velocity structure in the 1500 km broad portion of d sampled are also assessed the observed travel time and waveform variations may result from diffraction by near source slab structure scattering by low velocity heterogeneity in the lower mantle or interaction with complex velocity gradients in d associated with a deep mantle velocity discontinuity we utilize waveform modeling and differential time analysis to resolve greater detail of the structure than can presently be obtained by standard travel time tomography techniques and find that the discontinuity model is most consistent with the data subdivision of the data by source region and source to receiver azimuth establishes that the observations exhibit fairly stable behavior which is consistent with extensive local stratification of d the data show sufficient coherence over the 1500 km broad region that we find it useful to present a local average radial model the preferred model sylo has a 2 75 velocity increase 243 km above the core mantle boundary and a negative velocity gradient within the d layer analysis of sks s sks scs and sks skks differential times also indicates that the outermost 100 km of the core below the fast d region underlying alaska must be slower than the prem structure the observations exhibit significant scatter most of which can be attributed to unmodeled heterogeneity in the midmantle and near source regions as well as lateral variations in the d region the complex structure of d that we prefer may be associated with large scale downwelling in the lower mantle but it is not certain whether it is a purely thermal or a combined chemical and thermal effect

subduction induced strain in the upper mantle east of the mendocino triple junction california we observe splitting of teleseismic shear waves at five stations of the berkeley digital seismic network located east of the mendocino triple junction in northeastern california that is dependent on the arrival direction of the seismic phases the observed variations with back azimuth cannot be explained with laterally varying anisotropy with a horizontal symmetry axis and are attributed to the presence of fabric with an inclined symmetry axis we assume that the anisotropy is caused by the preferred alignment of olivine crystals a grid search over possible orientations of the olivine a axes reveals that south of the mendocino triple junction they dip to the east whereas north of the triple junction they dip to the northeast on the basis of a comparison of the ray paths of our data to the spatial distribution of fast and slow p wave velocity anomalies in the upper mantle we conclude that the anisotropy is located within seismically slow regions and that the directions are controlled by the geometry of a steeply dipping fast p wave velocity anomaly assuming that the fast p velocity anomaly represents subducted slab material we conclude that the fabric beneath the stations north of the triple junction is most likely caused by the differential motion between this rigid strong down going plate and the surrounding mantle south of the triple junction the fabric may have developed while subduction of the farallon plate was still ongoing in this region prior to 6 ma however we prefer to attribute the observations to more recent asthenospheric flow associated with the opening of a slabless window beneath the north american lithosphere the flow is modulated by the presence of rigid lithosphere to the north and east

anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis 1 we examine shear wave splitting in teleseismic phases to observe seismic anisotropy in the south american subduction zone data is from the charge network which traversed chile and western argentina across two transects between 30 s and 36 s beneath the southern and northwestern parts of the network fast polarization direction j is consistently trench parallel while in the northeast j is trench normal the transition between these two zones is gradual we infer that anisotropy sampled by teleseismic phases is localized within or below the subducting slab we explain our observations with a model in which eastward nazca entrained asthenospheric flow is deflected by retrograde motion of the subducting nazca plate resulting southward flow through this area produces n s j observed in the south and northwest e w j result from interaction of this flow with the local slab geometry producing eastward mantle flow under the actively flattening part of the slab index terms 7203 seismology body wave propagation 7218 seismology lithosphere and upper mantle 8123 tectonophysics dynamics seismotectonics 8150 tectonophysics plate boundary general 3040 9360 information related to geographic region south america citation anderson m l g zandt e triep m fouch and s beck 2004 anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis geophys res lett 31 l23608 doi 10 1029 2004gl020906

high pressure elastic anisotropy of mgsio3 perovskite and geophysical implications using plane wave pseudopotential method within the local density approximation lda we calculate single crystal elastic constants cij of orthorhombic mgsio3 perovskite generally accepted to be the major component of the lower mantle as a function of pressure up to 150 gpa our results are in excellent agreement with experimental data at zero pressure and compare favorably with other pseudopotential predictions over the pressure regime studied here we use our elastic constants to calculate anisotropy of seismic wave velocities as a function of pressure depth mgsio3 perovskite is shown to be highly anisotropic in all portions of the lower mantle and the nature of anisotropy changes significantly with depth the absence of significant seismic anisotropy in most of the lower mantle suggests that mgsio3 perovskite assumes nearly random orientation in most of this region anisotropy at the topmost lower mantle suggested by some studies can be attributed to the preferred orientation of perovskite however anisotropy in the d layer is difficult to be attributed to preferred orientation of perovskite some other mechanisms including the presence of the aligned melt pockets and or lattice preferred orientation of magnesiowustite are needed

evidence for shear velocity anisotropy in the lowermost mantle beneath the indian ocean teleseismic recordings 87 of a deep earthquake beneath the banda sea at stations in tanzania show a difference in the arrival time of the radial ssv and transverse component ssh s wave ranging from 1 3 s shear velocity anisotropy in the lowermost mantle beneath the indian ocean is the likely cause of this signal because recordings at the same stations of closer in events 80 in the same source region do not present a comparable differential travel time for the banda sea event the ssh signals are broader than ssv signals suggesting that a discontinuity or strong vertical gradient in primarily vsh marks the sudden onset of transverse isotropy in d with a magnitude of 1 4 2 7 about 350 km above the core mantle boundary skks coda s to p converted phases at the moho and upper mantle heterogeneity beneath the stations obscure the onset of ssv and complicate wave shapes it is therefore difficult to evaluate whether general anisotropy needs to be invoked into a model of shear velocity anisotropy

lateral variations in compressional shear velocities at the base of the mantle observations of core diffracted p p diff and sh sh diff waves recorded by the missouri to massachusetts moma seismic array show that the ratio of compressional p seismic velocities to horizontal shear sh velocities at the base of the mantle changes abruptly from beneath the mid pacific vp vs 1 88 also the value predicted by reference earth models to beneath alaska vp vs 1 83 this change signifies a sudden lateral variation in material properties that may have a mineralogical or textural origin a textural change could be a result of shear stresses induced during the arrival at the core of ancient lithosphere from the northern pacific paleotrench

the volcano tectonic evolution of concepcion nicaragua we describe the evolution of concepcion volcano by integrating regional geology eruptive activity morphology stratigraphy petrology structure and active deformation data this nicaraguan volcano is set close to the back limb of the northwest trending tertiary rivas anticline a regional structure that bounds the southwest side of lake nicaragua concepcion rises 1 600 m above a 1 km thick sequence of quaternary lacustrine mud stones there is no record of volcanism in the lake prior to concepcion in addition the only nearby volcano maderas volcano has not deposited material on concepcion because of the trade winds thus concepcion and made ras too can be considered as pristine volcanic environments unaffected by other centres a topographic rise forms an annulus 20 km in diameter around the cone the rise is created by thrust related folds at the western base where the trade winds have accumulated a thick sequence of tephra and by mud diapirs at the eastern base where only lake mudstones are present four magmatic eruptive episodes exist in the stratigraphic record the first begins with primitive low alumina basalt and subsequently evolves to dacitic compositions the following three episodes begin with high alumina basalts and evolve only to silicic andesites the occurrence of the high alumina basalt after the first episode is indicative of crystal fractionation at lower crustal depths the first episode may be associated with a compressive phase of volcano evolution in this phase the edifice load compresses substrata allowing a longer magma residence time and differentiation in a shallow reservoir possibly located at the density contrast between the lake sediments and the tertiary flysch during the next three episodes the weak sediments below the volcano started to rupture and yield under its increasing load beginning a thrusting diapiring phase of volcano evolution because of outward thrusting vertical and horizontal stresses above the chamber were reduced allowing magma to erupt more easily and to reach a lesser degree of evolution if we consider the future evolution of concepcion the differentiation in the shallow reservoir has probably generated a cumulitic complex which eventually will start to deform and spread beginning another this time plutonic spreading phase this phase which may be beginning now could allow less evolved magmas to be erupted again four components influence the phases of volcano evolution 1 the regional geology that is the boundary condition of the environment 2 the substrata rheology that controls deformation 3 the load of the volcanic edifice and 4 the magma which provides the input of mass and energy our model of volcanic evolution suggests that concepcion is a complex geologic environment the volcanic activity tectonics and hazards can only be constrained through a complete knowledge of the many components of this environment

geochemical structure of the hawaiian plume sr nd and os isotopes in the 2 8 km hsdp 2 section of mauna kea volcano 1 sr nd and os isotopic measurements were made on 110 mauna kea lava and hyaloclastite samples from the drillcore retrieved from the second phase of the hawaii scientific drilling project hsdp 2 the samples come from depths of 255 to 3098 meters below sea level span an age range from 200 to about 550 600 kyr and represent an ordered record of the lava output from mauna kea volcano as it drifted a distance of about 40 km over the magma producing region of the hawaiian hot spot the deepest oldest samples represent the time when mauna kea was closest to the center of the melting region of the hawaiian plume the sr and os isotopic ratios in hsdp 2 lavas show only subtle isotopic shifts over the 400 kyr history represented by the core neodymium isotopes nd values increase systematically with decreasing age from an average value of nearly 6 5 to an average value of 7 5 this small change corresponds to subtle shifts in 87sr 86sr and 187os 188os isotope ratios with small shifts of hf a large shift in 208pb 204pb and 208pb 207pb values and with a very large shift in he isotope ratios from r ra values of about 7 8 to values as high as 25 when mauna kea was closest to the plume core the magma source did not have primitive characteristics for nd sr pb hf and os isotopes but did have variable amounts of primitive helium the systematic shifts in nd hf pb and he isotopes are consistent with radial isotopic zoning within the melting region of the plume the melting region constitutes only the innermost highest temperature part of the thermally anomalous plume mantle the different ranges of values observed for each isotopic system and comparison of mauna kea lavas with those of mauna loa suggest that the axial region of the plume which has a radius of 20 km is a mixture of recycled subducted components and primitive lower mantle materials recently combined during the formational stages of the plume at the base of the mantle the proportions of recycled and primitive components are not constant and this requires there be longitudinal vertical heterogeneity within the core of the plume the remainder of the plume outside this plume core zone is less heterogeneous but distinct from upper mantle as represented by mid ocean ridge basalt morb the plume structure may provide a detailed view of mantle isotopic composition near the core mantle boundary

d beneath the arctic from inversion of shear waveforms 1 the structure of the d region beneath the arctic has not previously been studied in detail using waveform inversion we find that the average s wave velocity in d beneath the arctic is about 0 04 km s higher than prem which is consistent with the existence of post perovskite ppv in d it is difficult to strongly constrain the fine structure of s velocity within d due to the small number of stations at epicentral distances 90 but by weighting those stations heavily in the inversion we show that the data suggest the existence of high s velocity in the upper half of d and low s velocity in the lower half consistent with the possibility of a double crossing ppv pv reverse phase transition within d we conduct a computational experiment to show that resolution of the velocity structure within d could be significantly improved by temporary installation of a portable array of seismographs in northern canada which would greatly increase the number of stations in the range 70 90

evidence for anisotropy in the deep mantle beneath alaska we consider the possibility that the velocity structure of d is anisotropic the data we examined consist of seismograms from 9 deep japanese earthquakes recorded at wwssn receiver stations in north america the source receiver combinations span distances of 70 106 with associated s waves passing through d beneath alaska differential travel times of the s scd scs and sks phases are used to constrain the velocity structure in d shear waves refracted by d are observed beyond 72 2 and provide a sensitive measurement of the velocity structure in d beyond 93 the vertically polarized sv and horizontally polarized sh shear waves often appear distinctly split although at distances less than 89 the components are more nearly synchronous near 94 sh occurs as a double arrival sv in this range however remains a single arrival roughly synchronous with the second sh arrival we have been unable to reproduce these effects in isotropic model synthetics synthetics for transversely isotropic models have been computed that do match these waveforms the anisotropy was constrained to be only within d with a vertical symmetry axis we conclude that these observations may be explained by an anisotropic d layer the d discontinuity may be due to a transition to anisotropic mantle a few hundred kilometers above the core mantle boundary

anisotropy in the deep earth abstract seismic anisotropy has been found in many regions of the earth s interior its presence in the earth s crust has been known since the 19th century and is due in part to the alignment of anisotropic crystals in rocks and in part to patterns in the distribution of fractures and pores in the upper mantle seismic anisotropy was discovered 50 years ago and can be attributed for the most part to the alignment of intrinsically anisotropic olivine crystals during large scale deformation associated with convection there is some indication for anisotropy in the transition zone particularly in the vicinity of subducted slabs here we focus on the deep earth the lower mantle and core where anisotropy is not yet mapped in detail nor is there consensus on its origin most of the lower mantle appears largely isotropic except in the last 200 300 km in the d region where evidence for seismic anisotropy has been accumulating since the late 1980s mostly from shear wave splitting measurements recently a picture has been emerging where strong anisotropy is associated with high shear velocities at the edges of the large low shear velocity provinces llsvps in the central pacific and under africa these observations are consistent with being due to the presence of highly anisotropic mgsio 3 post perovskite crystals aligned during the deformation of slabs impinging on the core mantle boundary and upwelling flow within the llsvps we also discuss mineral physics aspects such as ultrahigh pressure deformation experiments first principles calculations to obtain information about elastic properties and derivation of dislocation activity based on bonding characteristics polycrystal plasticity simulations can predict anisotropy but models are still highly idealized and neglect the complex microstructure of polyphase aggregates with strong and weak components a promising direction for future progress in understanding the origin of seismic anisotropy in the deep mantle and its relation to global mantle circulation is to link macroscopic information from seismology and microscopic information mineral physics through geodynamics modeling anisotropy in the inner core was proposed 30 years ago to explain faster p wave propagation along the direction of the earth s axis of rotation as well as anomalous splitting of core sensitive free oscillations there is still uncertainty about the origin of this anisotropy in particular it is difficult to explain its strength based on known elastic properties of iron as it would require almost perfect alignment of iron crystals indeed the strongly anomalous p travel times observed on paths from the south sandwich islands to alaska may or may not be due to inner core anisotropy and will need to be explained before consensus can be reached on the strength of anisotropy in the inner core and its origin

morphometry of scoria cones and their relation to geodynamic setting a dem based analysis abstract the morphometry of a great number of scoria cones belonging to volcanic fields of various geodynamic settings has been measured and analyzed addressing the question whether there is a relation between the prevalent cone shape in a given field and the geodynamic setting of the field itself morphometric analysis was carried out on freely downloadable digital elevation models dems the accuracy of the used dems and the associated error in scoria cone morphometry were determined by cross comparing high resolution lidar derived dems usgs ned tinitaly dem and aster gdem the 10 m tinitaly 01 and usgs ned dems are proven to be suitable for scoria cone morphometry whereas aster gdem can be used reliably for cones with volume greater than 30 10 6 m 3 according to a detailed morphometry of all scoria cones we propose that the cones related to subductional setting show relatively higher values of h c o w co and lower values of w cr w co than the cones related to extensional setting the detected differences can be imputable to peculiar eruption dynamics resulting in slight but systematic changes in shape and differences in lithological and sedimentological characteristics that govern post eruptive erosion to constrain the pathway of scoria cone erosion the detected morphometric changes were also interpreted using a simple linear degradation model utilizing the obtained simulation results the inferred initial cone base and the age of scoria cones we calculated a diffusion coefficient k for several dated cones which are related to the prevalent climate our results despite the high error associated allow to assess the median k for all volcanic fields due to the complexity of the factors behind it is not easy to understand if the prevalent shape characterizing a certain volcanic field is due mainly to sin eruptive or post eruptive mechanisms however our distinction between the two main geodynamic settings may be the first step to decipher these factors

experimental modelling of thin skinned shortening around magmatic intrusions magmatic intrusions can trigger thin skinned compression of the adjacent sedimentary cover by three processes a gravity gliding away from the topographic dome resulting from the ascending magma b fluid push from the rear resulting from forceful intrusion then lateral spreading of the magma and c increased loading by volcanic accumulation the applicability of the first two mechanisms gravity gliding and fluid push was tested using dynamically scaled experiments model results help to elucidate the kinematics and structural evolution of thrusts and folds formed by such processes and determine which geological parameters control the deformation style the results show that the presence of a weak layer within the sedimentary overburden is essential to form thrust and fold belts around the intrusion experiments suggest that although gravity gliding can dominate the early stages of deformation most of the deformation is caused by lateral spreading of the fluid magma pushing the adjacent sedimentary rocks models also suggest that true laccoliths can form only if the sedimentary section comprises a basal low viscosity stratum comparison with natural examples allows the evaluation of the applicability of experimental results

volcanic spreading forcing and feedback in geothermal reservoir development amiata volcano italia abstract we made a stratigraphic structural and morphologic study of the amiata volcano in italy we find that the edifice is dissected by intersecting grabens that accommodate the collapse of the higher sectors of the volcano in turn a number of compressive structures and diapirs exist around the margin of the volcano these structures create an angular drainage pattern with stream damming and captures and a set of lakes within and around the volcano we interpret these structures as the result of volcanic spreading of amiata on its weak substratum formed by the late triassic evaporites burano anhydrites and the middle jurassic to early cretaceous clayey chaotic complexes ligurian complex regional doming created a slope in the basement facilitating the outward flow and spreading of the ductile layers forced by the volcanic load we model the dynamics of spreading with a scaled lubrication approximation of the navier stokes equations and numerically study a set of solutions in the model we include simple functions for volcanic deposition and surface erosion that change the topography over time scaling indicates that spreading at amiata could still be active the numerical solution shows that as the central part of the edifice sinks into the weak basement diapiric structures of the underlying formations form around the base of the volcano deposition of volcanic rocks within the volcano and surface erosion away from it both enhance spreading in addition a sloping basement may constitute a trigger for spreading and formation of trains of adjacent diapirs as a feedback the hot hydrothermal fluids decrease the shear strength of the anhydrites facilitating the spreading process finally we observe that volcanic spreading has created ideal heat traps that constitute todays exploited geothermal fields at amiata normal faults generated by volcanic spreading volcanic conduits and direct contact between volcanic rocks which host an extensive fresh water aquifer and the rocks of the geothermal field constitute ideal pathways for water recharge during vapour extraction for geothermal energy production we think that volcanic spreading could maintain faults in a critically stressed state facilitating the occurrence of induced and triggered seismicity

complex seismic anisotropy at the border of a very low velocity province at the base of the earth s mantle 1 we constrain the anisotropy associated with a very low velocity province vlvp at the base of the earth s mantle using the sks and skks waves sampling the region our selected high quality data sets consist of 415 sks and 111 skks waveforms for 127 deep earthquakes recorded at distances between 90 and 150 by the seismic stations in three temporary broadband passcal seismic arrays the kaapvaal seismic array 1997 1999 the tanzania seismic array 1994 1995 and the ethiopia kenya seismic array 1999 2001 as well as the permanent stations in the global seismographic network these seismic data provide good sampling coverage for some portion of the vlvp and its surrounding areas our results show when the sks or skks phases sample the regions away from the border of the vlvp inside or outside the vlvp the apparent splitting parameters inferred from the sks phases are consistent with those inferred from the skks phases and their variations strongly correlate with seismic stations but not with the exit points at the core mantle boundary of these seismic phases however when the sks or skks phases sample near the border of the vlvp the apparent splitting parameters inferred from the sks phases and skks phases are different and their variations no longer correlate with seismic stations these features indicate that part of the shear wave splitting for the seismic data sampling the border of the vlvp has to originate from deep mantle most likely near the border of the vlvp we assume that the anisotropy in the shallow mantle beneath seismic stations has a horizontal hexagonal symmetry axis and infer the splitting parameters associated with the shallow anisotropy beneath the seismic stations using the sks and skks waveforms for the seismic data sampling the regions away from the border of the vlvp we then obtain the splitting parameters associated with the lowermost mantle anisotropy using the sks and skks waveforms corrected for the inferred shallow anisotropy beneath seismic stations assuming that the medium in the lowermost mantle also has a horizontal hexagonal symmetry axis our results reveal a complex anisotropy pattern near the border of the vlvp such a complex anisotropy pattern may be explained by lattice preferred orientation of anisotropic mantle aggregates aligned by a complex mantle flow near the vlvp margins the complex flow pattern near the vlvp margins may indicate strong interactions between the vlvp a compositional anomaly and the surrounding normal mantle and may provide an explanation to the concentration of some hot spots geographically near the borders of the vlvps in the lowermost mantle

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

the postperovskite transition the discovery of the postperovskite ppv transition has profound impact on our understanding of the core mantle boundary cmb region unlike perovskite pv the ppv phase has a layered structure of the sio6 octahedra which may lead to a large contrast in some properties with pv recent studies have proposed unusual properties of ppv such as a large positive clapeyron slope a large sensitivity of the transition depth to iron a decrease in bulk sound speed at the transition and a development of significant lattice preferred orientation many of the proposed properties can provide explanations for the intriguing seismic observations at the cmb region yet significant discrepancies still exist however rapid developments in mineral physics will continue to improve our knowledge on the changes across the ppv transition

shear wave splitting continental keels and patterns of mantle flow in this study we investigated the origin of seismic anisotropy in the mantle beneath north america in particular we evaluated whether shear wave splitting patterns in eastern north america are better explained by anisotropy caused by lithospheric deformation anisotropy due to mantle flow beneath the lithosphere or a combination of both we examined new measurements of shear wave splitting from the missouri to massachusetts broadband seismometer array moma the north american mantle anisotropy and discontinuity experiment nomad as well as splitting parameters from several previous studies we developed a simple finite difference model that approximates mantle flow around a complex three dimensional continental lithospheric keel to evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern north america we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model our results indicate that a significant portion of observed shear wave splitting in eastern north america can be explained by mantle flow around the continental keel however shear wave splitting patterns in a few regions of eastern north america indicate that a component of lithospheric anisotropy must exist particularly in regions containing the largest keel thicknesses for eastern north america as well as for splitting observations in australia europe and south america we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow

lithospheric assembly and modification of the se canadian shield abitibi grenville teleseismic experiment this paper presents the results of a joint lithoprobe incorporated research institutions for seismology iris program for array seismic studies of the continental lithosphere passcal teleseismic experiment that investigates portions of the grenville and superior provinces of the canadian shield along the quebec ontario border data from a 600 km long n s array of 28 broadband seismographs deployed between may and october 1996 have been supplemented with additional recordings from an earlier 1994 deployment and from stations of the canadian national seismograph network and the southern ontario seismic network relative delay times of p and s waves from 123 and 40 teleseismic events respectively have been inverted for velocity perturbations in the upper mantle and reveal a low velocity nw se striking corridor that crosses the southern portion of the line at latitude 46 n and lies between 50 and 300 km depth multievent s k s splitting results yield an average delay time of 0 57 0 22 s and a direction of fast polarization of n93 e 18 which is consistent with an earlier interpretation as being due to fossil strain fields related to the last major regional tectonic event subtle variations in splitting parameters over the low velocity corridor may suggest an associated disruption in mantle fabric profiling of radial receiver functions reveals large and abrupt variations in moho topography specifically a gradual thickening in crust from 40 to 45 km between latitudes 45 n and 46 n which is followed by an abrupt thinning to 35 km at 46 6 n some 65 km southeast of the grenville front this structure is interpreted as a subduction suture extending the full length of the front and punctuating a major pre grenvillian archean proterozoic episode of lithospheric assembly in the southeast canadian shield the low velocity mantle corridor by contrast is better explained as the extension of the monteregian white mountain new england seamount hotspot track below the craton and is here postulated to represent interaction of the great meteor plume with zones of weakness within the craton developed during earlier rifting episodes

lowermost mantle anisotropy beneath the north pacific from differential s scs splitting summary seismic anisotropy is an important tool for studying the nature origin and dynamics of the lowermost mantle d we introduce differential s scs splitting as a tool for removing the effect of near source and near receiver anisotropy to estimate splitting accrued in the d region this is applicable to events recorded at epicentral distances between 60 and 85 near source anisotropy has often been ignored in previous studies of lowermost mantle anisotropy we apply differential s scs splitting to records from canadian national seismic network stations of western pacific earthquakes these sample the lowermost mantle beneath the north pacific the residual splitting in scs which we attribute to d shows lag times between 1 0 and 3 9 s given the near horizontal ray path of scs in d we interpret the recovered fast directions as the orientation of the fast shear wave in the plane defined by the vertical and transverse directions and observe a clearly non vti transverse isotropy with a vertical axis of symmetry style of anisotropy the largest population of results shows an approximately southeasterly dipping symmetry axis which we speculate might be explained by descending palaeoslab material being swept horizontally across the core mantle boundary towards an upwelling region beneath the central pacific non vti symmetry and the many possible contributions to d anisotropy from lower mantle minerals melt and subducted materials suggest that our understanding of the lowermost mantle could be greatly improved by trying to resolve a more general style of anisotropy

relation of thin skinned thrusting of colorado plateau strata in southwestern utah to cenozoic magmatism structural studies in upper cretaceous and eocene sedimentary rocks in the high plateaus of southwestern utah attempted to establish the cause of layer parallel shortening expressed in a regional scale arcuate pattern of thin skinned thrusts and related structures the thrusts sole into jurassic evaporites of the carmel formation tectonic transport direction verges radially from 125 to 215 stress analysis reveals a fan like pattern of the principal compressional stress sweeping southeastward to south south westward through 90 along an arc extending 35 km across the paunsaugunt plateau regional compressional structures of sevier and laramide origin are older than the thin skinned deformation and regional extensional structures of basin and range origin are younger the thrusting postdates deposition of the claron formation eocene and probably took place in the interval 30 ma to 20 ma that is coeval with the formation of the marysvale volcanic center located 40 to 60 km northwest of the thrust belt the forces that created the thrusts could have formed through a combination of processes related to the dynamics of formation of the marysvale volcanic center including gravity gliding and or compressional push related to the emplacement of batholithic intrusions and gravitational spreading and or end loading related to vertical loading of the column and the underlying evaporite decollement by the weight of the thick marysvale volcanic pile

nonlinear seismic response of earth dams with canyon interaction this paper examines the nonlinear earthquake response of earth dams using a model that considers the deformability of the surrounding medium and effects of spatial variation of the seismic excitation a finite element based method has been developed in which the dam is idealized as a shear beam and the surrounding medium as a halfspace nonlinear behavior of the dam is modeled using multiyield surface plasticity theory the model can simulate canyons of arbitrary shape and sh wave seismic excitation the methodology is illustrated by simulating the upstream downstream earthquake response of la villita dam which is located near the epicenter of the 1985 mexico earthquake results show that the reduction in the response that occurs as a result of dam canyon interaction remains significant when the inelastic deformation of the dam is included in the model and that when examined in the frequency domain nonlinearity has radically different effects at low medium and high frequencies of excitation

nonlinear dynamic finite element simulation of alibey earth dam in this study dynamic behavior and earthquake resistance of alibey earth dam was investigated the dam was modeled with four node plane strain finite elements fe and displacement pore pressure coupled fe analyses were performed nonlinear material models such as pressure dependent and independent multi yield materials were implemented during the analyses transient dynamic fe analyses were performed with newmark method the newton raphson solution scheme was adopted to solve the equations liquefaction and or cyclic mobility effects were considered during the analysis for the fe analyses opensees open system for earthquake engineering simulation framework was adopted

shear wave splitting and waveform complexity for lowermost mantle structures with low velocity lamellae and transverse isotropy 1 shear waves that traverse the lowermost mantle exhibit polarization anomalies and waveform complexities that indicate the presence of complex velocity structure above the core mantle boundary synthetic seismograms for horizontally and vertically polarized shear waves sh and sv respectively are computed using the reflectivity method for structures with low velocity sheets lamellae and for comb like models approximating long wavelength vertical transverse isotropy vti motivated by evidence for partial melt in the deep mantle lamella parameter ranges include 1 vp from 5 to 10 vs 3 vp 2 100 to 300 km thickness of vertical stacks of lamella 3 lamella spacing and thickness varying from 0 5 to 20 km and 4 lamellae concentrated near the top bottom or throughout the d region at the base of the mantle such lamellae represent in effect horizontally emplaced dikes within d excessively complex waveforms are produced when more than 20 of d volume is comprised of low velocity lamellae many lamellae models can match observed sdiff splitting 1 10 s delays of svdiff but typically underpredict scs splitting 1 4 s delays of scsv vti model parameters are selected to address d observations and include 1 0 5 to 3 anisotropy 2 discontinuous d shear velocity increases up to 3 3 d thicknesses from 100 to 300 km and 4 vti concentrated at the top bottom or throughout d vti models readily match observed splits of scs and sdiff we discuss lamellae and vti model attributes in relationship to waveform complexities splitting magnitude triplications from a high velocity d discontinuity and apparently reversed polarity svdiff onsets the possible presence of melt filled lamellae indicates that local chemical or thermal perturbations can produce regions that exceed the solidus within d such melt could occur in the bulk of d because the melt is either close to neutral buoyancy advective velocities exceed percolative velocities or both

scaled experiments of volcanic spreading experiments were conducted to study the spreading of volcanic constructs volcanoes are simulated by a sand cone and the volcanic substratum is simulated by a sand layer brittle substratum overlying a silicone layer ductile substratum similarity conditions between natural volcanoes and experimental prototypes led to the definition of dimensionless numbers experiments determine values which predict whether or not spreading takes place of particular importance are the ratio between the thickness of the brittle substratum and the height of the volcano 2 and the brittle ductile ratio of the substratum 3 2 indicates that the volcano must be large enough to break the substratum before spreading occurs whereas 3 controls the style of deformation during spreading these dimensionless numbers change with time reaching values that tend toward those observed for stable configurations experimental values are compared with those from well constrained natural examples it is found that an essential requirement for volcanic spreading is the presence of a low viscosity layer within the substratum flow of the weak layer away from the excess load is responsible for the spreading the overlying edifice displays radial intersecting grabens due to concentric stretching dissected summit areas concentric zones of thrusts and folds form in the substratum around the edifice and diapirs of the ductile substratum rise within the fault zones

modeling the ratios of skks and sks amplitudes with ultra low velocity zones at the core mantle boundary 1 between 105 115 degrees the sks waveform is complicated by the formation of spdks a wave that has segments of p diffraction along the core mantle boundary while previous studies have primarily focused on the move out of spdks from sks we analyze the concomitant reduction of the sks amplitude long period skks sks amplitude ratios present a coherent global pattern skks sks is relatively large in north and south american recordings of deep tonga fiji earthquakes but prem like in european recordings of earthquakes in south america and north american recordings of earthquakes in indonesia modeling of skks sks indicate that ultra low velocity zones ulvzs layers at the base of the mantle with a thickness of about 10 20 km and a shear velocity reduction between 20 30 are required to explain high skks sks ratios and the early move out of spdks

deformation of a partially molten d layer by small scale convection and the resulting seismic anisotropy and ultralow velocity zone abstract partially molten regions in the lowermost mantle have been inferred to exist by the discovery of the ultralow velocity zone ulvz we consider a small scale stagnant lid convection in the d layer following solomatov v s moresi l n 2002 small scale convection in the d layer j geophys res 107 doi 10 1029 2000jb000063 to investigate the relationship between partial melt and seismic anisotropies often found in the d layer such convection can bring melts in the d layer from the ulvz deform the melt inclusions and accordingly would profoundly affect seismic structures including anisotropies we therefore calculate the deformation history of partially molten regions at the base of the d layer which is heated from below using a 2d model with a strongly temperature dependent viscosity an initially isotropic partial melt is strongly deformed by the viscous stress caused by thermal convection and becomes anisotropic by shape preferred orientation spo of melt inclusions whose aspect ratios are of the order of 1 0 1 to 1 0 2 at the base of the plume and become as large as 1 0 3 to 1 0 4 in the plume head we calculate the effective elastic constants for such anisotropic media which contain deformed melt inclusions and obtain the seismic velocity for a horizontal ray path we find that the horizontally averaged velocity profile can be correlated with convective patterns if melting occurs not only in the ulvz but extends to the top of hot regions of the d layer the vertical seismic profile consists of three layers corresponding to the base conduit and head of a rising plume the lowermost layer which corresponds to the ulvz becomes strongly anisotropic with v sh v sv the deformation and alignment of the melt rather than the melting itself is primarily responsible for reducing the seismic velocity on the other hand in the conduit the anisotropy is of v sv v sh type because of vertical alignment in the plume head the anisotropy is of v sh v sv type with a magnitude of about 2 we discuss how shear wave anisotropy may be used to infer the temporal evolution of the d layer

dynamic basis of volcanic spreading the established models of the structure and dynamics of a typical hawaiian volcano lead inevitably to a paradox finite element viscoelastic calculations using tecton melosh and raefsky 1980 1981 predict that a compressional stress field should characterize their upper flanks instead extension is observed the paradox is solved by postulating that volcanic evolution is determined by feedback processes related to growth and spreading of the volcanic edifice many examples of the process of spreading are documented in the geologic literature on volcanoes of a very broad size range and very different geologic settings the process is similar across many orders of magnitude in size the deformation of the substratum related to magma intrusion during lava dome building events is at the lower end of the scale the spreading of seafloor at oceanic ridges is at the higher end of the scale volcanoes may evolve from small constructs into huge shield volcanoes and perhaps into ocean plates key elements for volcanic spreading are 1 the existence of a weak basal layer and 2 a sufficiently high mass and magma influx to drive the process as the mass of a volcano grows it passes through five phases which may overlap repeat or be omitted building compressing thrusting intruding and spreading during the building phase the mass of a volcano does not significantly affect the stress field of the substratum primitive magmas are likely to be erupted as the mass increases the volcano subsides and becomes characterized by a compressional stress field which inhibits any further intrusion of magma into the volcanic edifice magma may differentiate in the crust allowing for large caldera forming eruptions to occur in the next phase thrusting begins on a decollement usually located at the foot of the edifice while the stress field on the upper slopes of the volcano remains compressional the stress at the base close to the center becomes extensional allowing the magma to intrude and differentiate at the base of the edifice a basal intrusive complex begins to grow explosive eruptions may be expected finally spreading of this complex creates an extensional stress field on the upper flanks of the volcano confining compression to the lower slopes of the edifice the extension allows primitive magma to erupt again at the summit the process of spreading seems to have a fundamental influence on the structural and magmatic evolution of volcanoes at every scale this process results from coupling and feedback between the gravitational and thermal fields thus studying this process may improve our understanding of the relation between structural dynamics and magma evolution and of the origin and evolution of volcanic seismicity and eruptions

variations in sks splitting across western canada measurements of sks splitting parameters have been made at stations of the canadian national seismograph network for events over the period 1993 mid1994 here we report on results obtained from western stations in the northern cordillera the alignment of polarization directions with major transcurrent fault systems suggests that the crust and upper mantle are being deformed coherently splitting parameters in the southern cordillera however are oriented approximately perpendicular to the strike of major tectonic features notably the cascadia subduction zone this contrast in the character of fast polarization directions between the southern and northern cordillera is thought to reflect the change in the tectonic style of the adjacent plate margin from convergent along the southern b c coast to transform north of the triple junction one plausible explanation is that upper mantle anisotropy in the southern region is due primarily to mantle flow perpendicular to the margin involving upwelling at the juan de fuca ridge subduction along the coast and associated back arc asthenospheric flow beneath the b c interior

azimuthal anisotropy in the d layer beneath the caribbean 1 the lowermost mantle beneath central america has anisotropic seismic velocity structure manifested in shear wave splitting of signals from south american earthquakes recorded at north american broadband recording stations prior studies of deep mantle anisotropy in this region have characterized the structure as having vertical transverse isotropy vti which is sufficient to explain a general trend of early tangential sh component arrivals however vti models cannot quantitatively match systematic waveform complexities in the onset of many of the shear waves that graze this region after accounting for splitting effects of upper mantle anisotropy beneath the recording stations we model the corrected waveform data using full wave theory for mantle velocity models with an anisotropic d layer this is the first attempt to quantitatively model a large data set including azimuthal anisotropy in d the models include transverse isotropy with either a vertical or tilted symmetry axis the latter resulting in azimuthal anisotropy for some initial shear wave polarizations tilted transverse isotropy tti produces small reversed polarity arrivals on the sv components at the arrival time of sh consistent with the data geographical variations in the azimuth of the tti symmetry axis are indicated by the data the lack of azimuthal coverage prevents unique resolution of the tti orientation and also precludes distinguishing between tti and other azimuthal anisotropy structures such as that predicted for lattice preferred orientation of minerals nonetheless our modeling demonstrates the need for laterally varying anisotropic structure of more complex form than vti for this region

geology geochronology and emplacement structures associated with the jimbu microgranite mcarthur basin northern territory abstract the jimbu microgranite a series of high level microgranite plutons exposed in central arnhem land in northern australia was emplaced into flat lying sandstones and lesser mudstones carbonates and basalts of the katherine river group during the latter stages of deposition new shrimp u pb zircon results indicate a crystallization and emplacement age for the jimbu microgranite of 1720 7 ma intrusion took place in an essentially static tectonic setting and accommodation space was not created by regional extension or by consumption of the host package by the intrusion rather pluton emplacement was facilitated by three distinct mechanisms the thick basal sandstone package was updomed overlying lutite units were laterally shortened by outward gravity slide the uppermost sandstone unit was raised vertically above the pluton by way of a monoclinal margin and steep reverse detachment faults decollement at ductility transitions in the rock package is required for the coexistence of these three structural styles the three resulting structural elements include relatively steeply and radially outward dipping domes of lower katherine river group sandstones with granite cores irregular short wavelength fold patterns in the overlying incompetent lutites discrete flat lying blocks of the uppermost sandstone unit with steeply upturned or downturned margins plutonism is also suspected of being the cause of similar domes elsewhere in the mcarthur basin and in the adjacent mount isa inlier the model presented here may also be partly applicable to other basins with similar geometries and volcano sedimentary histories where tectonism is currently invoked to explain deformation

seismic anisotropy in the core mantle transition zone split s waves observed at hockley texas from events in the tonga fiji region of the southwest pacific show predominantly vertically polarized shear wave sv energy arriving earlier than horizontally polarized sh energy for rays propagating horizontally through d after corrections are made for the effects of upper mantle anisotropy beneath hockley a time lag of 1 5 to 2 0 s remains for the furthest events 93 9 100 6 while the time lags of the nearer observations 90 5 92 9 nearly disappear at closer distances the s waves from these same events do not penetrate as deeply into the lower mantle and are not split these observations suggest that a patch of d beneath the central pacific is anisotropic while the mantle immediately above the patch is isotropic the thickness of the anisotropic zone appears to be of the order of 100 200 km observations of shear wave splitting have previously been made for paths that traverse d under the caribbean and under alaska sh leads sv the reverse of the hockley observations but in these areas the fact that sv leads sh in the hkt data shown here suggests a different sort of anisotropy under the central pacific from that under alaska and the caribbean the case of sh travelling faster than sv is consistent with transverse isotropy with a vertical axis of symmetry vti and does not require variations with azimuth the case of sv leading sh is consistent with transverse isotropy with a horizontal axis of symmetry hti an azimuthally anisotropic medium and with a vti medium formed by a hexagonal crystal given that mg fe sio3 perovskite appears unlikely to form anisotropic fabrics on a large scale the presence of anisotropy may point to chemical heterogeneity in the lowermost mantle possibly due to mantle core interactions

evidence of seismic anisotropy and a lower temperature condition in the d layer beneath pacific antarctic ridge in the antarctic ocean abstract we examine shear waves of 23 deep earthquakes in south pacific subduction zones recorded at the syowa station in antarctica to study the anisotropy and velocity structure of shear waves in the d layer beneath pacific antarctic ridge par in the antarctic ocean we observe earlier arrivals of transverse components sh than those of longitudinal components sv for s waves passing through the d layer and scd phases due to the d discontinuity that has been reported beneath circum pacific regions we find that the optimum shear wave velocity structure in the d layer is transversely isotropic from waveform modeling and the differential travel time analysis among s scs and sks phases the sh velocity structure has a 2 0 velocity discontinuity at about 350 km above the core mantle boundary the sv structure is similar to prem the anisotropic d layer v sv v sh beneath the antarctic ocean is significantly thicker than those beneath alaska and the caribbean sea we interpret that the amount of deposited paleo slab material controls the variation in the thickness of the d layer the difference of the thickness of the d layer together with recent experimental and theoretical results of the phase transition from perovskite to post perovskite means that the d layer beneath par is 630 1000 k colder than that beneath alaska the cold mantle beneath par corresponds to the location of stationary patches of intense geomagnetic field in the southern hemisphere this coincidence suggests the existence of the core mantle interactions

a sharp and flat section of the core mantle boundary the transition zone between the earth s core and mantle plays an important role as a boundary layer for mantle and core convection1 this zone conducts a large amount of heat from the core to the mantle and contains at least one thermal boundary layer2 3 the proximity of reactive silicates and molten iron leads to the possibility of zones of intermediate composition4 here we investigate one region of the core mantle boundary using seismic waves that are converted from shear to compressional waves by reflection at the boundary the use of this phase known as scp the large number of receiving stations and the large aperture of our array all provide higher resolution than has previously been possible5 7 for the 350 km long section of the core mantle boundary under the northeast pacific sampled by the reflections the local boundary topography has an amplitude of less than 500 m no sharp radial gradients exist in the 400 km above the boundary and the mantle to core transition occurs over less than 1 km the simplicity of the structure near and above the core mantle boundary argues against chemical heterogeneity at the base of the mantle in this location

upper and lower mantle anisotropy inferred from comprehensive sks and skks splitting measurements from india abstract in this study we investigate the upper mantle anisotropy beneath india using high quality sks and skks waveforms from 382 teleseismic earthquakes recorded at 119 broadband seismic stations in addition we present evidence for anisotropy in the d layer beneath southeast asia using sks and skks splitting discrepancies on the same seismogram during this exercise we obtain 200 new splitting measurements from 35 stations recently deployed in the indo gangetic plains igp central india and northeast india while the delay times between the fast and slow axes of anisotropy t range from 0 3 to 1 7 s the fast polarization azimuths at a majority of stations in the igp and central india coincide with the absolute plate motion of india implying shear at the base of the lithosphere as the dominant mechanism for forging anisotropy however stations in ne india reveal fast polarization azimuths mainly in the ene wsw direction suggestive of lithospheric strain induced by the ongoing indo eurasian collision our analysis for d anisotropy yielded a total of 100 sks skks pairs which can be categorized into those exhibiting i null measurements for one phase and significant splitting for the other phase ii null measurement for both the phases iii significant splitting for both the phases a pair is considered to be anomalous if the splitting difference between sks and skks is 0 5 s and the individual split time is 0 5 s using this criterion we obtain 12 measurements under category iii and 9 under category i that show a null measurement for sks and large splitting for the skks phase further we quantify the strength of the lower mantle anisotropy by correcting the skks measurement for the upper mantle anisotropy obtained by the sks phase on the same seismogram the sks delay times are found to be consistently less than skks times suggesting that the sks phases do not capture the lower mantle anisotropy in comparison to their skks counterparts seven coherent measurements thus obtained reveal measurable d anisotropy with fast polarization azimuths oriented mainly in the ene wsw direction these results suggest presence of a large region of deformation in the lowermost mantle beneath southeast asia a possible model for anisotropy in these regions could be the presence of slab material that pounded upon the core mantle boundary cmb and is experiencing large shear deformation resulting in lattice preferred orientation lpo of the lower mantle van der hilst and karason 1999 long 2009 the other possibility is the phase transformation from mgsio3 perovskite to a more stable post perovskite phase under favorable conditions which results in lpo of the lower mantle

kinematics of an arcuate fold thrust belt the southern eastern carpathians romania abstract flysch and molasse nappes of the eastern carpathians strike mainly n s and were folded and imbricated since the late early cretaceous boundaries of stratigraphic and structural units and calculated fold trend axes from the southern part of the eastern carpathians indicate a curvature of the orogen by 80 kinematic data from fault slip analysis of all nappe systems exposed in the arc document an overall nw se contraction nappes of the external moldavides nappe complex underwent deformation since the late cretaceous peaking in the miocene and exhibit fanning subhorizontal contraction axes from e w in the n striking part of the eastern carpathians through wnw ese and nw se in the northern part of the arc to nnw sse in its southwestern part geometric relations between faults and folds indicate that this fan pattern reflects an extended interval and was contemporaneous to nappe stacking and subsequent continental collision statistical analysis reveals correlation between orientation of the fanning kinematic axes and the structural trend where fanning of subhorizontal contraction axes is systematically smaller than change of strike of fold thrust structures orogen parallel extension in the tarcau nappe of the moldavides nappe complex is estimated to be 20 this low value of orogen parallel extension indicates a primary arc in contrast to bending of a previously straight fold thrust belt low orogen parallel extension and the observed pattern of structural and kinematic axes can be simulated in sand wedge indentation models where the indenter moves oblique to its margins in these models thrust traces and fold axes are curved around the arc but are parallel to the margins of the indenter away from the corner area the movement directions of the thrust faults in the models are deflected from normal to fault strike towards the movement direction of the indenter leading to fanning contraction directions similar to those observed in the eastern carpathian arc thus the main controlling factors on arc formation were the plate margin geometry i e the shape of the upper plate and the movement direction of the upper plate relative to the orientation of its margins the influence of the geometry of the continental margin in the lower plate was mainly by imposing constraints on the movement direction of the upper plate

small scale lateral shear velocity and anisotropy heterogeneity near the core mantle boundary beneath the central pacific imaged using broadband scs waves core reflected scs waves from 49 large m 5 1 deep tonga fiji events recorded in western north america are used to study a localized region of the core mantle boundary cmb under the central pacific ocean a total of 248 observations from the berkeley digital seismic network bdsn caltech united states geological survey usgs terrascope and the incorporated research institutions for seismology iris broadband arrays span epicentral distances of 73 85 scs reflection points sample a cmb patch southeast of the hawaiian islands at latitude 4 to 16 n and longitude 156 to 144 w near the proposed source location of the hawaiian plume highly variable scs travel times amplitudes waveforms and shearwave splitting indicate that the lowermost mantle in this region is heterogeneous both laterally and radially scsh sh differential travel times are on average 4 s larger than predicted by the preliminary reference earth model prem this is due to delayed scs arrivals and is largely accounted for by a model with a strong velocity decrease in d the scsh sh residuals also show a spatial trend not accounted for by a radial model indicating a lateral decrease in lower mantle shear velocity to the northeast this lateral velocity gradient appears to cause focusing of scs energy scsh sh amplitude ratios are larger than predicted by prem or by a model with a strong negative gradient with depth in d which enhances scs scs splitting corrected for lithospheric anisotropy beneath the receivers indicates spatial variations in d anisotropy with a systematic change in the orientation of the fast polarization direction from transverse to the ray path fast scsh to parallel to the ray path fast scsv along a northeast traverse these spatial trends suggest lateral gradients in the boundary layer shear flow on scale lengths of a few hundred kilometers which may be related to dynamical flow near the near the root of the large hawaiian plume

structural features of the july august 2001 mount etna eruption evidence for a complex magma supply system we describe the evolution of the volcanic activity and deformation patterns observed at mount etna during the july august 2001 eruption seismicity started at 3000 m below sea level on 13 july accompanied by moderate ground swelling ground deformation culminated on 16 july with the development of a ne sw graben c 500 m wide and c 1 m deep in the cisternazza area at 2600 2500 m above sea level on the southern slope of the volcano on 17 july the eruption started at the summit of mount etna from the se crater central lateral eruptive system from which two radial c 30 m wide c 3000 m long fracture zones associated with eruptive fissures propagated both southward 17 july and northeastward 20 july on 18 july a new vent formed at 2100 m elevation at the southern base of the montagnola followed on the next day by the opening of a vent further upslope at 2550 m eccentric eruptive system the eruption lasted for 3 weeks approximately 80 of the total lava volume was erupted from the 2100 m and the 2550 m vents the collected structural data suggest that the cisternazza graben developed as a passive local response of the volcanic edifice to the ascent of a north south eccentric dyke which eventually reached the ground surface in the montagnola area 18 19 july in contrast the two narrow fracture zones radiating from the summit are interpreted as the lateral propagation from the conduit of the se crater of north south and ne sw oriented shallow dykes 2 3 m wide the evolution of the fracture pattern together with other volcanological data magma ascent and effusion rate eruptive style petrochemical characteristics of the erupted products and petrology of xenoliths within magma suggest that the eccentric and central lateral eruptions were fed by two distinct magmatic systems examples of eccentric activity accompanied by central lateral events have never been described before at etna

deformation crystal preferred orientations and seismic anisotropy in the earth s d layer abstract we use a forward multiscale model that couples atomistic modeling of intracrystalline plasticity mechanisms dislocation glide twinning in mgsio3 post perovskite ppv and periclase mgo at lower mantle pressures and temperatures to polycrystal plasticity simulations to predict crystal preferred orientations cpo development and seismic anisotropy in d we model the cpo evolution in aggregates of 70 ppv and 30 mgo submitted to simple shear axial shortening and along corner flow streamlines which simulate changes in flow orientation similar to those expected at the transition between a downwelling and flow parallel to the core mantle boundary cmb within d or between cmb parallel flow and upwelling at the borders of the large low shear wave velocity provinces llsvp in the lowermost mantle axial shortening results in alignment of ppv 010 axes with the shortening direction simple shear produces ppv cpo with a monoclinic symmetry that rapidly rotates towards parallelism between the dominant 100 010 slip system and the macroscopic shear these predictions differ from mgsio3 post perovskite textures formed in diamond anvil cell experiments but agree with those obtained in simple shear and compression experiments using cairo3 post perovskite development of cpo in ppv and mgo results in seismic anisotropy in d for shear parallel to the cmb at low strain the inclination of scs sdiff and skks fast polarizations and delay times vary depending on the propagation direction at moderate and high shear strains all s waves are polarized nearly horizontally downwelling flow produces sdiff scs and skks fast polarization directions and birefringence that vary gradually as a function of the back azimuth from nearly parallel to inclined by up to 70 to cmb and from null to 5 change in the flow to shear parallel to the cmb results in dispersion of the cpo weakening of the anisotropy and strong azimuthal variation of the s wave splitting up to 250 km from the corner transition from horizontal shear to upwelling also produces weakening of the cpo and complex seismic anisotropy patterns with dominantly inclined fast scs and skks polarizations over most of the upwelling path models that take into account twinning in ppv explain most observations of seismic anisotropy in d but heterogeneity of the flow at scales

joint inversion for global isotropic and radially anisotropic mantle structure including crustal thickness perturbations we present a new global whole mantle model of isotropic and radially anisotropic s velocity structure sglobe rani based on 43 000 000 surface wave and 420 000 body wave travel time measurements which is expanded in spherical harmonic basis functions up to degree 35 we incorporate crustal thickness perturbations as model parameters in the inversions to properly consider crustal effects and suppress the leakage of crustal structure into mantle structure this is possible since we utilize short period group velocity data with a period range down to 16 s which are strongly sensitive to the crust the isotropic s velocity model shares common features with previous global s velocity models and shows excellent consistency with several high resolution upper mantle models our anisotropic model also agrees well with previous regional studies anomalous features in our anisotropic model are faster sv velocity anomalies along subduction zones at transition zone depths and faster sh velocity beneath slabs in the lower mantle the derived crustal thickness perturbations also bring potentially important information about the crustal thickness beneath oceanic crusts which has been difficult to constrain due to poor access compared with continental crusts

shdiff svdiff splitting in an isotropic earth 1 the vertically polarized svdiff seismic phase sometimes arrives later than the horizontally polarized shdiff both phases propagate in the d layer at the base of the mantle and the difference in their arrival times is usually interpreted in terms of seismic anisotropy in d using numerical simulations we demonstrate that a significant 1 s delay of svdiff relative to shdiff is present even in the iasp91 isotropic reference earth model and in a model with a high s velocity in d relative to iasp91 the delay is accompanied by strong amplitude decay of svdiff with distance this relationship resembles the effect of anelasticity in a model with a low s velocity in d the waveform of svdiff can be very different from shdiff and this difference can be mistaken for a delay of shdiff relative to svdiff in a laterally heterogeneous isotropic d svdiff can also be delayed because unlike shdiff svdiff has strong amplitude decay along fast paths its traveltime is thus dominated by slow propagation paths and tends to be positively biased for d models with an s velocity difference of 3 between two hemispheres there are regions where shdiff arrives up to 15 s earlier than the first clearly visible svdiff this is observed in a corridor several hundred kilometers wide if these effects are misinterpreted in terms of d anisotropy in actual seismic data estimates of anisotropy are biased and may contribute to the idea of seismic anisotropy being different in high and low s velocity regions of d

lowermost mantle anisotropy beneath the northwestern pacific evidence from pcs scs sks and skks phases 1 in this study we utilize data from broadband seismic stations of the japanese f net network to investigate anisotropic structure in the lowermost mantle beneath the northwestern pacific the comparison of shear wave splitting from phases that have similar paths in the upper mantle but different paths in the lowermost mantle such as pcs scs or sks skks can yield constraints on anisotropy in the d region we measured splitting for sks skks pcs and scs phases at f net stations and compared the measured splitting parameters to previously published estimates of upper mantle anisotropy beneath the region we observed many examples of discrepant sks skks splitting and splitting of scs pcs sks and skks phases that does not agree well with the known upper mantle anisotropy beneath individual stations the most likely explanation for these discrepancies is a contribution to splitting from anisotropy in the lowermost mantle in particular for sks skks phases that sample the lowermost mantle just to the south and east of the kamchatka peninsula we observed generally n s fast directions with delay times between 0 5 and 1 5 s these data suggest the presence of a fairly large coherent region of deformation in the lowermost mantle beneath the northwestern pacific our preferred model for these observations is that solid state flow at the base of the mantle induces a lattice preferred orientation of lowermost mantle minerals generating a seismically anisotropic fabric

modeling lower mantle anisotropy development in a subducting slab a model is presented that simulates anisotropy development in a subducting slab from the upper lower mantle boundary 661 km to the core mantle boundary 2891 km two phases are considered orthorhombic mgsio3 perovskite and cubic magnesiowuestite mg fe o single crystal elastic properties at mantle conditions are obtained from existing density functional theory calculations and quasiharmonic approximation it is assumed that deformation is accommodated by slip a polycrystal plasticity model predicts strong texture development for perovskite and weaker texture for magnesiowuestite when averaging single crystal elastic properties with the orientation distribution this results for both phases in weak p wave anisotropy and shear wave splitting in the upper part of the lower mantle but pronounced anisotropy near the core mantle boundary up to 4 for vp and 7 for vs the anisotropy pattern is complex and asymmetric and local heterogeneity is expected these predictions are consistent with seismic observations

seismic anisotropy and global geodynamics for many years seismic anisotropy was often neglected mostly because of the inherent heavy mathematical and computational tools needed to describe and model its effects on seismic waves the usual basic knowledge about propagation in isotropic media cannot easily apply to anisotropic media where new phenomena come up such as birefringence or shear wave splitting or difference between directions of propagation of phase velocity and of group velocity consequently geophysicists often claimed that it was a second order effect and considered the earth as isotropic this hypothesis was assumed to be a good approximation because of the random orientation of crystals in most parts of the earth and of the random sampling of anisotropic regions by seismic rays this assumption furthermore made easier the description of wave propagation as well as the parameterization of media in inverse problems an isotropic elastic medium can be described by two independent elastic parameters and lame parameters but the simplest anisotropic medium transverse isotropy with a vertical symmetry axis requires 5 independent parameters love 1927 anderson 1961 to date seismic observations have been explained in terms of isotropic and often thermal lateral heterogeneities ignoring manifestations of anisotropy however since the 1960s it was recognized that most parts of the earth are not only laterally heterogeneous but also anisotropic and that seismic anisotropy provides a simple explanation of different observational data seismic anisotropy contrary to isotropy is reflecting some inherent

seismic evidence for small scale heterogeneity throughout the earth s mantle details in the amplitude structure of short period precursors to the seismic core phase pkp can be used to constrain the depth extent of mantle scattering the simplest model consistent with the observations invokes small 8 km weak r m s velocity perturbations of 1 random heterogeneities uniformly distributed throughout the mantle the data do not support previously proposed models that place an increase in the concentration of scattering sites near the base of the mantle and instead place an upper limit on the amplitude of any short wavelength topography at the core mantle boundary

seismic anisotropy in the lowermost mantle beneath the pacific onset time differences of up to 3 s are observed between transverse ssh and longitudinal ssv components of broadband s waves at distances of 85 to 120 for paths traversing the lowermost mantle d beneath the pacific after correction for upper mantle anisotropy ssh usually arrives earlier than ssv with the splitting increasing with distance from 100 to 120 the data yield two possible models of anisotropy 1 anisotropy may vary laterally with transverse isotropy existing in higher than average shear velocity regions beneath the northeastern pacific or 2 anisotropy may vary with depth with transverse isotropy concentrated in a thin 100 km thermal boundary layer at the base of d a few recordings at distances less than 105 show that ssv arrives earlier than ssh indicating that general anisotropy likely exists in shallower regions of d

apparent splitting of s waves propagating through an isotropic lowermost mantle observations of shear wave anisotropy are key for understanding the mineralogical structure and flow in the mantle several researchers have reported the presence of seismic anisotropy in the lowermost 150 250 km of the mantle i e d urn x wiley jgrb media jgrb52636 jgrb52636 math 0002 layer based on differences in the arrival times of vertically sv and horizontally sh polarized shear waves by computing waveforms at a period 6 s for a wide range of 1 d and 3 d earth structures we illustrate that a time shift i e apparent splitting between sv and sh may appear in purely isotropic simulations this may be misinterpreted as shear wave anisotropy for near surface earthquakes apparent shear wave splitting can result from the interference of s with the surface reflection ss for deep earthquakes apparent splitting can be due to the s wave triplication in d urn x wiley jgrb media jgrb52636 jgrb52636 math 0003 reflections off discontinuities in the upper mantle and 3 d heterogeneity the wave effects due to anomalous isotropic structure may not be easily distinguished from purely anisotropic effects if the analysis does not involve full waveform simulations

seismic anisotropy in the eastern united states deep structure of a complex continental plate we have analyzed shear wave splitting recorded by portable and permanent broadband and long period stations located in the eastern united states teleseismic shear waves sks skks and pks were used to retrieve the splitting parameters the orientation of the fast wave polarization plane and the delay time t in total 120 seismic events were processed allowing for more than 600 splitting measurements within the appalachians stations located in the western external part are characterized by t 1s and trending n50 70 e in the south and central regions and n30 40 e in the north closely following the trend of the orogenic belt in these areas the transition region between north and central is characterized by t 1 1 3 s and by e w trending that are at a high angle to the regional geologic trend measurements at two stations located in the eastern internal part of the belt indicate very weak anisotropy the large scale pattern of anisotropy is not consistent with that predicted for simple asthenospheric flow beneath the plate splitting along the southern and eastern margins of the continent is consistent with that expected for grenvillian deformation an alternative model of asthenospheric flow around the cratonic keel cannot be ruled out within the cratonic core the correlation between t and lithospheric thickness suggests a lithospheric anisotropy smaller length scale variations also argue for a significant contribution of lithospheric structures the fabric responsible for shear wave splitting may have formed during tectonic episodes that affected the eastern united states i e the grenville and appalachian orogenies and the subsequent rifting of the north atlantic ocean our observations in the western appalachians suggest that the anisotropy may be preserved since the grenvillian orogeny the absence of detectable splitting in the two stations in the eastern appalachians is attributed to the igneous intrusions related to the atlantic rifting the measurements in the transition between the northern and central southern appalachians constitute an intriguing anomaly whose e w have little obvious relation to the regional surface geology we suggest two possible causes 1 the local dominance of asthenospheric flow motivated by the proximity of a pervasive low velocity anomaly and 2 lithospheric deformation in a transcontinental strike slip fault zone active during the appalachian collision

nanocrystalline diamond from the earth s mantle underneath hawaii abstract melt inclusions in a mantle derived garnet pyroxenite xenolith from salt lake crater hawaiian island of oahu have been analysed by means of transmission electron microscopy including energy dispersive x ray analysis electron energy loss spectroscopy and electron diffraction the melt inclusions represent trapped low degree melt that metasomatised the hawaiian mantle lithosphere before the eruption of the host magma the inclusions consist of a c cl s rich glass of basaltic composition an exsolved gas fluid phase mainly co2 with minor amounts of co h2o and some unidentified c h and h s species and or carbonate diamond is the major phase in the glass ranging in size between a few and several hundred nanometres two varieties of nanocrystalline diamonds are identified polycrystalline aggregates of nanometre size diamonds are tentatively explained as pseudomorphs after carbonate while single crystals are suggested to have crystallised from the melt other nanocrystalline phases include native fe and cu fes fes2 zns ags and several ti nb zr ir in pd rich phases of unknown composition overall the observed mineral assemblage suggests that the melt was highly reduced and strongly enriched in volatiles and trace elements of contrasting geochemical affinity our findings are the first evidence for a metasomatic impregnation of the hawaiian mantle by low degree melts here we report the first occurrence of diamond in the modern oceanic mantle

short period p wave constraints on d reflectivity we present evidence of a p wave reflector in d beneath the central and eastern equatorial pacific obtained from nearly 7000 short period seismic recordings of earthquakes in the tonga fiji and south america subduction complexes both regions overlap previous observations of an s wave triplication at similar depths double array waveform stacking places the reflector 190 20 km above the core mantle boundary and allows for a velocity transition zone no thicker than 50 km the data admit a wide range of compressional v p and shear wave velocities v s the 2 3 increase in v s suggested by earlier long period s wave studies however necessitates only a 0 5 0 6 increase in v p the data stacks represent spatial averages over large areas of the lower mantle such that reflector intermittence and topography decrease stack amplitude and lower the apparent v p jump our results constrain these processes to act over length scales of 10 100 km

mechanics of monoclinal systems in the colorado plateau during the laramide orogeny monoclines developed in the colorado plateau region during the laramide orogeny are divided into western and eastern groups by a broad nnw trending antiform through the central part of the plateau in the western group the major monoclines verge to the east whereas in the eastern group the major monoclines verge to the west paleogeographic reconstruction based on paleocurrent indicators and sedimentary facies distribution suggests that the broad antiform was developed during the laramide orogeny and was coeval with the formation of the monoclines in the plateau this relationship implies that the monoclines were drag folds verging towards the center of the plateau as a response to the antiformal warping of the plateau to simulate the warping of the plateau region and the stress distribution that produced the variable trends of the monoclines an elastic thin plate model considering in plane stress was developed this model assumes that 1 sedimentation in the laramide basins provided vertical loading along the edge of the plateau region 2 frictional sliding was operating along the laramide faults on the northern and eastern boundaries and 3 the greatest regional compressive stress was oriented in the n60 e direction and was applied uniformly along the western and southwestern sides of the plateau buoyancy due to instantaneous isostatic adjustment of crustal thickening or magmatic addition was also considered the result of the model suggests that the frictional strength of the uinta thrust system on the northern side of the plateau is at least 2 times greater than that along the park range and sangre de cristo thrust systems on the eastern side of the plateau in order to explain the observed monoclinal trends and the warping pattern within the plateau during the laramide orogeny

a strong lateral shear velocity gradient and anisotropy heterogeneity in the lowermost mantle beneath the southern pacific 1 velocity heterogeneity and seismic anisotropy in the d region beneath the southern pacific ocean is investigated using broadband shear waves from southwest pacific and south american earthquakes recorded by permanent and temporary stations in antarctica australia south america and on islands in the south pacific shear velocity perturbations vs are inferred from the differential travel times between hand picked horizontally polarized sh direct or diffracted s arrivals and vertically polarized sv sks arrivals derived patterns in vs roughly agree with global shear velocity models but reveal a stronger shear velocity lateral gradient north of approximately 52 s where vs transitions from approximately 0 5 to 1 0 relative to radially averaged reference models over less than 600 km along the base of d waveform analyses provide an even stronger constraint on the transitional region where there is a change in waveform behavior occurring over length scales less than 300 km differential travel times between skks and sks calculated by cross correlating sks with hilbert transformed skks support large vs amplitudes and help to constrain the location of the transitional region anisotropy in d ks is inferred from handpicked differential travel times between the sv and sh components of direct or diffracted s and display a slight spatial trend where the sh component precedes the sv in the north and east of the study area but arrives after sv in the center and southwest of it however there is high ks variability in the center of the study region there is no apparent correlation between vs and ks abrupt changes in the character of velocity and anisotropy in this region of d may be related to chemical heterogeneity at the boundary of the pacific superswell as well as small scale convection in the deep mantle

mantle anisotropy beneath northwest pacific subduction zones to assess the location strength and orientation of seismic anisotropy in the southern kuril japan and izu bonin subduction zones we analyzed shear wave splitting in local s phases and teleseismic sks phases fast directions from these phases are roughly parallel to the direction of absolute pacific plate motion wnw in izu bonin roughly parallel to the strike of the trench nne in central honshu and roughly n in the southern kuril back arc in the vicinity of sakhalin island assuming that the orientation and strength of anisotropy does not vary with depth modeling of splitting times from local s and teleseismic phases recorded at sakhalin island requires that the maximum depth of anisotropy lies between 480 km and 950 km in contrast splitting times for local s phases from events in izu bonin rule out anisotropy in the transition zone all the data are consistent with a model in which the lower transition zone 520 660 km and lower mantle are largely isotropic and in which anisotropy occurs intermittently in the upper transition zone 410 520 km possibly due to preferred orientation of spinel assuming that splitting in the upper mantle is produced by preferred orientation of olivine the observed fast directions indicate that the geometry of back arc strain varies systematically between subduction zones the relationship of fast directions and plate motions suggests that the subducting slab exerts significant control on back arc flow but that flow correlated with upper plate deformation must also exist

inferences on flow at the base of earth s mantle based on seismic anisotropy we applied global waveform tomography to model radial anisotropy in the whole mantle we found that in the last few hundred kilometers near the core mantle boundary horizontally polarized s wave velocities vsh are on average faster by 1 than vertically polarized s wave velocities vsv suggesting a large scale predominance of horizontal shear this confirms that the d region at the base of the mantle is also a mechanical boundary layer for mantle convection a notable exception to this average signature can be found at the base of the two broad low velocity regions under the pacific ocean and under africa often referred to as superplumes where the anisotropic pattern indicates the onset of vertical flow

shear wave splitting across the rocky mountain front we search for evidence of seismic anisotropy by determining shear wave splitting parameters using teleseismic phases from stations placed across the rocky mountain front three features are striking 1 fast polarization orientations are consistent within small geographic regions but vary rapidly across the network 2 a large number of high quality null measurements indicate that little transverse anisotropy with horizontal symmetry axis is present and 3 stations with well constrained but inconsistent parameters for rays from different sources imply that a single layer of anisotropic material with a horizontal symmetry axis is an inadequate parameterization the general pattern suggests fast axes pointing toward or away from a central region of little anisotropy a model of asthenospheric flow converging on or diverging from the central uplifted region is postulated within the southern rocky mountains parameters are similar to those in the nearby northern rio grande rift suggesting that a similar mechanism causes the anisotropy in both regions

shear wave splitting observations in the archean craton of western superior shear wave splitting observations in the archean superior province of the canadian shield show moderate to large delay times 1 1 2 1 s with azimuths suggesting upper mantle anisotropy is subparallel to the structural grain of the craton two regions with uniform anisotropy azimuth 62 7 and 87 3 are separated by a transitional zone showing strong dependence of observed azimuth on source direction suggesting that lateral structure is being observed no splitting was observed at stations near the coast of hudson bay continuing a pattern of weak splitting in surrounding proterozoic orogenic belts current absolute plate motion is consistent with the direction of anisotropy but would not explain the regional contrasts instead the anisotropy appears to be related to archean structure and tectonic history

lattice preferred orientation in deformed polycrystalline mg fe o and implications for seismic anisotropy in d abstract magnesiowustite mg fe o is an important constituent of the lower mantle probably occupying about 20 25 of its volume laboratory and theoretical studies have shown this mineral to be highly elastically anisotropic at lower mantle pressures and temperatures thus strain induced formation of lattice preferred orientation lpo in magnesiowustite is a candidate mechanism for the origin of anisotropic structure in d although observations of seismic anisotropy within d are robust both the occurrence and the style of that anisotropy are spatially variable two hypotheses have been offered to explain the observations of d anisotropy lpo of intrinsically anisotropic minerals or shape preferred orientation spo perhaps in the form of horizontal layering or oriented inclusions to investigate the first hypothesis we performed confined simple shear deformation experiments in the dislocation creep regime using a gas medium deformation apparatus over a range of compositions the mgo and feo endmembers and three intermediate compositions samples were deformed at 1273 1473 k at a confining pressure of 300 mpa to large shear strains 3 5 4 5 using deformation pistons cut at 45 after deformation the lpo was measured by electron backscatter diffraction ebsd the lpo produced varied for differing compositions indicating the activity on individual slip systems and or the nature of grain boundary migration in mg fe o are affected by composition and or homologous temperature we predicted seismic anisotropy from the measured lpos and theoretically determined single crystal elastic constants anisotropic behavior predicted from lpo agrees well with observations of d anisotropy so the lpo hypothesis appears to satisfy the seismological constraints our calculated anisotropy patterns suggest that if d anisotropy is due to lpo of mg fe o then azimuthal variations in anisotropy in the horizontal plane should be present such azimuthal variations are not generally predicted for spo type hypotheses and this may provide a means for distinguishing the cause of d anisotropy

catastrophic emplacement of the gigantic markagunt gravity slide southwest utah usa implications for hazards associated with sector collapse of volcanic fields the newly recognized markagunt gravity slide mgs represents gravitationally induced catastrophic collapse ca 21 22 ma of the southwest sector of the oligocene to miocene marysvale volcanic field in southwest utah usa the 90 km long slide mass consists of 1700 2000 km 3 of andesitic volcanic mudflow breccias and subordinate lava flows sandstones and intertongued regional ash flow tuffs the mgs extends over an area of 3400 km 2 and advanced 30 km to the south over the miocene land surface the low angle basal slip surface is in eocene oligocene brian head formation a weak clay rich tuffaceous sedimentary unit at the base of the volcanic section the presence of a basal zone of cataclastic and sheared breccia with associated clastic dikes and pseudotachylyte along secondary shear planes provides strong evidence of catastrophic emplacement the uniformity of directional indicators stratigraphy and overall geometry of the mgs suggests that it represents a single emplacement event coeval with late stage magmatic doming of the volcanic field the mgs thus represents one of the largest subaerial volcanic landslides on earth and along with the comparable heart mountain gravity slide in northwest wyoming constitutes a class of catastrophic collapse hazard not widely recognized within modern volcanic fields

global radially anisotropic mantle structure from multiple datasets a review current challenges and outlook abstract since the 1960s seismologists have mapped anisotropy in the uppermost mantle the mantle transition zone and the d region when combined with constraints from mineral physics and geodynamics anisotropy provides critical information on the geometry of mantle flow here we review the theory early work recent tomographic models and experimental constraints on radial anisotropy we discuss current challenges in resolving radial anisotropy seismically in particular we show that it is highly beneficial to use multiple datasets in inversions for anisotropy notably short period group velocity data with strong sensitivity to the crust we present a new whole mantle model of radial anisotropy based on surface wave and body wave travel time data along with incorporated moho perturbations our whole mantle model shares common features with previous global models and is consistent with results from several high resolution regional studies

partial melting in a thermo chemical boundary layer at the base of the mantle abstract seismological detections of complex structures in the lowermost mantle boundary layer the d region motivate a conceptual model of a compositionally stratified thermo chemical boundary layer tcbl within which lateral temperature variations sustained by large scale mid mantle flow cause variations of partial melt fraction partial melt fractions of from 0 to 30 in the tcbl occur due to the eutectic of the boundary layer material lying below the high temperature at the core mantle boundary cmb coupled with the presence of steep thermal gradients across the boundary layer regions of the tcbl with the highest temperatures have extensive partial melting producing lateral chemical variations and strong effects on seismic velocities and boundary layer dynamics this tcbl concept provides a relatively simple framework that can plausibly account for diverse seismological observations such as predominance of large scale volumetric elastic wave velocity heterogeneity in the boundary layer laterally extensive but intermittent abrupt shear and compressional velocity increases and decreases at the top of the d region small scale topography of d velocity discontinuities thin ultra low velocity zones at the cmb widespread shear wave anisotropy in d bulk sound velocity anomalies detected in low shear velocity regions of d and large scale upwellings from the most extensively melted regions of the boundary layer neutral or negative buoyancy of the partial melt in the tcbl is required along with an increase in bulk modulus and some density increase of the boundary layer material relative to the overlying mantle more complex models in which the partially melted chemical boundary layer is laterally displaced by mid mantle downwellings are also viable but these appear to require additional special circumstances such as a phase change efficient segregation of slab crustal material or abrupt onset of anisotropy in order to account for rapid velocity increases at the top of d the precise nature of the compositional anomaly or anomalies in d and the eutectic composition of this zone are yet to be determined but it appears likely that partial melting within the lowermost mantle plays a paramount role in the observed seismic velocity heterogeneity increased resolution geodynamic calculations of dense boundary layer structures with partial melting and attendant viscosity reductions and chemical heterogeneity are needed to quantify this tcbl concept

lateral variations in lowermost mantle shear wave anisotropy beneath the north pacific and alaska s waves recorded by long period world wide standardized seismograph network and broadband stations in north america for deep northwest pacific subduction zone earthquakes provide evidence for anisotropy in the lowermost mantle shear velocity structure beneath the north pacific and alaska systematic delays of up to 4 s are observed between longitudinal components sv and transverse components sh of motion for core reflected scs waves as well as for core grazing and diffracted s waves the absence of significant splitting for s waves that have turning points more than a few hundred kilometers above the core mantle boundary indicates that anisotropy is localized within the d region the lowermost portion of the mantle sv sh differential arrival times for both scs and sdiff along with path length estimates assuming a 250 km thick d region indicate spatial variations in the strength of shear wave anisotropy the strongest anisotropy 1 1 5 is found in the eastern part of the study area with systematic reduction in magnitude toward the west a transverse isotropy model can explain the data with the velocity structure for horizontally polarized waves vsh having a 2 3 discontinuous shear velocity increase at the top of d as proposed in earlier studies of the region and a similar structure for s wave particle motion in the direction normal to the core mantle boundary vsv but with the velocity jump at the top of d and the velocity within d being reduced from that for vsh by 0 5 1 5 km s large uncertainties exist for velocity gradients above and below the velocity jump but the requirement of a reduced vsv relative to vsh in d is clear synthetic waveforms calculated by using separate isotropic structures for sh and sv match the observations well and constrain the basic anisotropic structure because the shear waves all traverse the region with near grazing geometries the study area exhibits strong lateral variations in lower mantle shear velocity structure and variable thickness of the d layer topography of the d layer is not well resolved because of trade off with volumetric heterogeneity

in situ observation of texture development in olivine ringwoodite magnesiowustite and silicate perovskite at high pressure magnesium silicates are the dominant minerals in the earth s mantle their preferred orientation is important for understanding the rheology and seismic anisotropy in the deep earth here we report results of radial synchrotron diffraction diamond anvil cell dac experiments on san carlos olivine axially compressed to 50 gpa experiments were performed at room temperature except for brief laser heating to induce phase transformations high stresses and development of preferred orientation were observed in diffraction images quantitative texture information was obtained by analyzing the images with the rietveld method with increasing pressure between 9 and 43 gpa olivine develops a texture with 001 axes perpendicular to the compression direction that is compatible with hk0 001 pencil glide ringwoodite between 20 and 50 gpa develops weak preferred orientation with 011 lattice planes perpendicular to the compression direction after the phase transformation perovskite and magnesiowqstite display transformation textures that are then modified by continuing deformation magnesiowqstite has a weak h111i maximum parallel to the compression direction that changes towards h001i with increasing deformation perovskite transforming from olivine has a pronounced 100 transformation texture and with increasing deformation a 012 maximum develops the transformation texture is probably produced by mechanical 110 twinning and nucleation in orientations that minimize elastic strain energy the deformation texture of perovskite is due to slip d 2004 elsevier b v all rights reserved

lowermost mantle anisotropy beneath the pacific imaging the source of the hawaiian plume we utilized recordings of seismic shear phases provided by several north american broadband seismometer arrays to provide unique constraints on shear wave anisotropy beneath the northern and central pacific ocean using a new analysis method that reduces measurement errors and enables the analysis of a larger number of available waveforms we examined relative travel times of teleseismic s and sdiff that sample a large area of lowermost mantle structure the results of this study provide evidence for small scale lateral and depth variations in shear wave anisotropy for a broad region of the lowermost mantle beneath the pacific ocean in particular we image a localized zone of anomalously strong anisotropy whose strength increases toward the top of dq beneath hawaii our results combined with a previous study of vp vsh ratios indicate that ancient subducted slab material may be responsible for observations beneath the northern pacific while lenses or layers of core mantle boundary reaction products or partial melt oriented by horizontal inflow of mantle material to the hawaiian plume source can explain observations beneath the central pacific fl 2001 elsevier science b v all rights reserved

constraints from seismic anisotropy on the nature of the lowermost mantle the d layer lies at the bottom of the earth s rocky mantle and separates it from the liquid metal alloy core this region extending from the core mantle boundary to a few hundred kilometres above fig 1 is geodynamically analogous to the more easily studied lithosphere at the top of the mantle the structure of d may reflect the style of lower mantle convection the nature of core mantle interaction and perhaps even the fate of subducting lithosphere1 observations of lithospheric seismic anisotropy have provided valuable insight into the nature of the upper mantle boundary layer but discussion of lower mantle seismic anisotropy has been somewhat contentious2 5 here we present evidence from seismic waves that have traversed the lowermost mantle beneath the caribbean region for a zone of seismic anisotropy below the d discontinuity which in this region lies 250 km above the core mantle boundary the anisotropy is most probably due to horizontal layering or aligned inclusions of a material with differing shear wave velocity if d is a graveyard for subducted lithosphere a plausible explanation of the anisotropy may be the contrast between cold lithospheric mantle and material that formerly constituted the oceanic crust which may have lower shear wave velocity owing to the presence of melt

a plea for more caution in fault slip analysis abstract the analysis of fault slip data is a widely used tool in structural geology this method consists of three major steps a data acquisition by measurements in outcrops b data separation into homogeneous subsets and calculation of the principal stress or strain axes with one of several available methods and c data interpretation in a regional tectonic context which includes the assignment of ages and the derivation of regional patterns each one of these three steps may introduce artefacts and misinterpretations the major problem during step a is sampling bias due to unfavourable outcrop geometry or due to neglecting faults with subtle slip sense indicators step b contains numerous possibilities to influence the calculation results e g by arbitrary separation of data into subsets choice of the analysis method and uncritical use of software age assignment in step c can in most cases be made only indirectly e g by the assumption that measured outcrop scale faults have the same age as parallel map scale faults or by the inference that subsets from different sites with parallel oriented principal axes are syngenetic neither assumption is necessarily valid with this paper we want to sharpen researchers awareness of the problems related to fault slip analysis

a strongly negative shear velocity gradient and lateral variability in the lowermost mantle beneath the pacific a new approach for constraining the seismic shear velocity structure above the core mantle boundary is introduced whereby sh sks differential travel times amplitude ratios of sv sks and sdiff waveshapes are simultaneously modeled this procedure is applied to the lower mantle beneath the central pacific using da ta from numerous deep focus southwest pacific earthquakes recorded in north america we analyze 90 broadband and 248 digitized analog recordings for this source receiver geometry sh sks times are highly variable and up to 10 s larger than standard reference model predictions indicating the presence of laterally varying low shear velocities in the study area the travel times however do not constrain the depth extent or velocity gradient of the low velocity region sv sks amplitude ratios and sh waveforms are sensitive to the radial shear velocity profile and when analyzed simultaneously with sh sks times rnveal up to 3 shear velocity reductions restricted to the lowermost 190 50 km of the mantle our preferred model for the central eastern pacific region ml has a strong negative gradient with 0 5 reduction in velocity relative to the preliminary reference earth model prem at 2700 km depth and 3 reduction at 2891 km depth and slight velocity reductions from 2000 to 2700 km depth 0 0 5 lower than prem significant small scale 100 500 km shear velocity heterogeneity 0 5 1 is required to explain scatter in the differential times and amplitude ratios