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

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

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

geochronology and tectonic implications of magmatism and metamorphism southern kootenay arc and neighbouring regions southeastern british columbia part i jurassic to mid cretaceous k ar dates and u pb zircon dates define three periods of igneous activity in the southern kootenay arc 1 emplacement of late synkinematic to post kinematic granodioritic plutons in mid jurassic time 170 165 ma accompanying amphibolite facies regional metamorphism 2 emplacement of post kinematic granitic plutons in mid cretaceous time 100 ma and 3 emplacement of small bodies of syenite in eocene time 50 ma in the western part of the area micas from mid jurassic plutons that yield the oldest k ar dates 158 166 ma also yield plateau shaped 40ar 39ar age spectra age spectra for biotites younger than these but older than 125 ma reflect thermal overprinting in southeastern british columbia the kootenay arc marks the transition from the north american rocks of the cordilleran miogeocline to the tectonic collage of allochthonous terranes that have been accreted to it deformation metamorphism and plutonism recorded in rocks of the southern kootenay arc commenced in mid jurassic time as a co

variable azimuthal anisotropy in earth s lowermost mantle a persistent reversal in the expected polarity of the initiation of vertically polarized shear waves that graze the d layer the layer at the boundary between the outer core and the lower mantle of earth in some regions starts at the arrival time of horizontally polarized shear waves full waveform modeling of the split shear waves for paths beneath the caribbean requires azimuthal anisotropy at the base of the mantle models with laterally coherent patterns of transverse isotropy with the hexagonal symmetry axis of the mineral phases tilted from the vertical by as much as 20 are consistent with the data small scale convection cells within the mantle above the d layer may cause the observed variations by inducing laterally variable crystallographic or shape preferred orientation in minerals in the d layer

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

middle jurassic exhumation along the western flank of the selkirk fan structure thermobarometric and thermochronometric constraints from the illecillewaet synclinorium southeastern british columbia new thermobarometric and thermochronometric data from the illecillewaet synclinorium a broad regional southwest verging structure in the western flank of the selkirk fan structure indicate that accretion of the intermontane superterrane resulted in deep burial 20 25 km of the outer margin of ancestral north america and that subsequent southwest verging folding and thrusting were accompanied by rapid exhumation southwest verging folding of lower paleozoic rocks in the illecillewaet synclinorium was initiated during regional peak metamorphic conditions at 6 7 kbar crystallization of late synkinematic granitoid plutons that intrude the illecillewaet synclinorium also occurred at pressures of 6 7 kbar however equilibration of the contact aureoles of the plutons occurred at 3 3 5 kbar during the latter stages of the southwest verging deformation evidently a decompression on the order of 3 kbar occurred concurrently with southwest verging folding and thrusting in the illecillewaet synclinorium thermochronometric analyses further indicate that plutonic rocks from the illecillewaet synclinorium cooled rapidly between times of crystallization and times of ar closure in biotite regional relationships indicate that the accretion of the intermontane superterrane to the edge of the north american margin in southeastern british columbia occurred between 187 and 173 ma toarcian bajocian subsequent southwest verging deformation occurred approximately between 173 and 168 ma bajocian bathonian thus the removal of at least 10 km of rocks from above the illecillewaet synclinorium inferred from thermobarometric data occurred approximately between 173 and 168 ma rapid denudation of the selkirk fan structure was probably the result of synorogenic extension or extensional collapse contemporaneous with the development of southwest verging structures at deeper levels of the orogen the data presented here indicate that all significant deformation and most of the exhumation along the western flank of the selkirk fan structure were complete by late middle jurassic time after the middle jurassic the selkirk fan structure must have been transported passively northeastward along the basal decollement of the cordilleran foreland thrust and fold belt together with the intermontane superterrane as the fold and thrust belt tectonically overrode the margin of the north american craton

an electron microprobe study of the u th pb systematics of metamorphosed monazite the role of pb diffusion versus overgrowth and recrystallization abstract an electron microprobe emp was used to obtain u th pb chemical ages from monazite that underwent an amphibolite facies metamorphic overprint in the monashee complex southern canadian cordillera u pb isotope dilution dating of single grains yielded discordant ages that plot in linear arrays on a concordia diagram upper intercepts of the arrays indicate primary growth in the proterozoic and cretaceous and lower intercepts are coeval with a tertiary metamorphic event the microprobe s high spatial resolution 2 m and ability to quickly differentiate between domains with greatly contrasting u th and pb concentrations made it the ideal tool for investigating the cause of the u pb isotopic discordance the microprobe dating shows that proterozoic monazite lies directly adjacent to generally small

pressure temperature and evolution of fluid compositions of al 2 sio 5 bearing rocks mica creek b c in light of fluid inclusion data and mineral equilibria metamorphosed pelitic rocks from mica creek british columbia contain sillimanite kyanite with minor fibrolite and andalusite bearing quartz pods mineral equilibria were used to infer peak p t conditions and fluid compositions in equilibrium with the solid phases fluid inclusions in three schist samples appear to be good indicators of conditions affecting those rocks during and after peak metamorphic conditions in samples from two localities fluid inclusions from schist and quartz rich segregations have densities appropriate to the peak metamorphic conditions the observed compositions for these fluids low salinity with 12 mole dissolved co2 agree with calculated text x text h text 2 text o values of 0 84 to 0 85 based upon paragonite quartz albite al2sio5 equilibria the fluids unmixed as the schists were uplifted and cooled fluid inclusions trapped during this stage outline a solvus in the co2 h2o nacl system a later influx of fluids containing ch4 and n2 accompanied formation of andalusite bearing plagioclaserich segregations the restricted association of andalusite bearing pods and low density fluids suggest a localized but pervasive fluid influx during uplift preservation of high density fluid inclusions during uplift and erosion coupled with evidence for unmixing of h2o and co2 rich fluids on the solvus provide constraints on the p t uplift path

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

analysis of seismic sv waves in the core s penumbra vertically polarized s waves sv attenuate rapidly as they diffract around the earth s core whereas horizontally polarized s waves sh propagate to large distances in the core shadow zone the amplitude decay of diffracted sv signals is so strong that few studies have been made of these phases despite their acute sensitivity to velocity structure just above the core mantle boundary we analyze sv signals in the penumbra of the core s shadow finding systematic waveform complexities indicative of local stratification and apparent anisotropy at the base of the mantle with implications for the dynamic processes in this internal boundary layer

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

investigating d structure beneath the north atlantic abstract data from the grsn and spiced array are used to investigate lower mantle structure beneath the north atlantic ocean just south of nova scotia we study s waves in an attempt to determine whether a d discontinuity can be observed in this region the reflected phase sds cannot be observed unambiguously so we cross correlate the traces with synthetic seismograms generated using a number of different 1d earth models that incorporate positive and negative velocity discontinuities and gradients as well as thin high velocity lamellae the cross correlation shows that a discontinuity with an s wave velocity increase of magnitude 1 3 exists which exhibits significant topography between 86 and 286 km above the cmb the study area is located on the edge of a region of anomalously high seismic velocity identified by tomographic modelling which fits with our observation of a high velocity discontinuity

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

early cretaceous kyanite sillimanite metamorphism and paleocene sillimanite overprint near mount cheadle southeastern british columbia geometry geochronology and metamorphic implications mapping of isograds related to regional amphibolite facies metamorphism constrains a three dimensional model of isogradic surfaces near mount cheadle in the northern shuswap metamorphic complex lat 52 20 n long 119 05 w kyanite and sillimanite coexist in a lens shaped zone bounded by the kyanite out and sillimanite in isogradic surfaces that is 50 km long up to 10 km thick and up to 20 km wide textural equilibrium simple regular geometry of isogradic surfaces and simple mineral assemblages suggest that metamorphism occurred at p t conditions near those of the kyanite sillimanite equilibrium curve reconstruction of isotherms in the kyanite sillimanite zone suggests that the metamorphic field gradient was about 14 c km 1 a 5 km thick staurolite free kyanite zone adjacent to the sillimanite in isograd suggests a pressure range of about 1 5 kbar 1 kbar 100 mpa for bathozone 5 of d m carmichael regional metamorphism was early cretaceous monazite u pb geochronology with quenching in th

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

tomographic inversion of s sks times for shear velocity heterogeneity in d degree 12 and hybrid models differential travel times between s or diffracted s and sks were measured to study the global distribution of shear wave velocity heterogeneity in the lowermost 250 km of the mantle the d region commencing with 3000 s sks times with variable qualities we minimize uneven path coverage by thinning redundantly sampled regions e g the fiji tonga to north america corridor and collect additional data for sparsely sampled areas especially in the southern hemisphere about 1500 paths were retained distributed to reconcile both high density and homogeneous sampling we compare 1 spherical harmonic and 2 equal area block parameterizations of d shear velocity heterogeneity for identical minimum resolving lengths and mean model errors we show that the two parameterizations result in indistinguishable patterns of heterogeneity and power distribution for resolution down to a block size of 4 5 4 5 and maximum spherical harmonic degree of 40 we demonstrate synthetically that a high degree l inversion followed by a lower degree ls spherical harmonic synthesis effectively circumvents model contamination from expansion truncation effects tomographic inversion of this data set yields a global distribution of d heterogeneity robust up to degree 12 in our preferred model l 40 ls 12 rms 1 surface hotspots and estimated lower mantle plume roots are located in or at edges of low velocity regions vs 2 for both the atlantic indian low velocity corridor as well as the low velocities beneath the central pacific although the above parameterizations entail no intrinsic difference in resolvability higher resolution is regionally achieved by removing the derived degree 12 model from the data then inverting the corrected residuals using 4 5 4 5 blocks this hybrid method prevents power loss at intermediate to long wavelengths from otherwise severe damping and recovers smaller scale structure where constraints are better than the global average such as beneath the pacific and eurasia a synthetic recovery test yields resolvability maps that reflect both the path geometry and the quality of data which help to identify robust features in both the degree 12 and the hybrid d models

seismic imaging of the laterally varying d region beneath the cocos plate summary we use an axisymmetric spherical earth finite difference algorithm to model sh wave propagation through cross sections of laterally varying lower mantle models beneath the cocos plate derived from recent data analyses synthetic seismograms with dominant periods as short as 4 s are computed for several models 1 a d reflector 264 km above the core mantle boundary with laterally varying s wave velocity increases of 0 9 2 6 per cent based on localized structures from a 1 d double array stacking method 2 an undulating d reflector with large topography and uniform velocity increase obtained using a 3 d migration method and 3 cross sections through the 3 d mantle s wave velocity tomography model txbw we apply double array stacking to assess model predictions of data of the models explored the s wave tomography model txbw displays the best overall agreement with data the undulating reflector produces a double scd arrival that may be useful in future studies for distinguishing between d volumetric heterogeneity and d discontinuity topography synthetics for the laterally varying models show waveform variability not observed in 1 d model predictions it is challenging to predict 3 d structure based on localized 1 d models when lateral structural variations are on the order of a few wavelengths of the energy used particularly for the grazing geometry of our data iterative approaches of computing synthetic seismograms and adjusting model characteristics by considering path integral effects are necessary to accurately model fine scale d structure

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

lateral variations in d thickness from long period shear wave data we explore global variations in d shear wave structure by examining 12 years of long period global digital seismograph network gdsn data to identify reflected phases from the d layer in the lowermost mantle we restrict our search to epicentral distances between 63 and 74 where a precritical d reflection will lie between the s and scs arrivals with gdsn long period records a d signal at these ranges is generally obscured by the stronger neighboring phases so to isolate the d reflections we develop a technique for stripping away the interfering s and scs waveforms we interpret travel time variations in the observed sds phases in terms of variations in d thickness and generate maps of inferred d thickness for areas of good seismic coverage the regions of observed d reflections are beneath australasia north central asia the arctic alaska and central america we find considerable variation in d structure in these regions a result consistent with previous studies which have found evidence for lateral heterogeneity in d on average the estimated d thickness is 260 km but there is a fairly uniform distribution of thicknesses between 150 km and 350 km a possible correlation is observed between d thickness and regions of predicted strong horizontal mantle flow

a three dimensional radially anisotropic model of shear velocity in the whole mantle summary we present a 3 d radially anisotropic s velocity model of the whole mantle saw642an obtained using a large three component surface and body waveform data set and an iterative inversion for structure and source parameters based on non linear asymptotic coupling theory nact the model is parametrized in level 4 spherical splines which have a spacing of 8 the model shows a link between mantle flow and anisotropy in a variety of depth ranges in the uppermost mantle we confirm observations of regions with vsh vsv starting at 80 km under oceanic regions and 200 km under stable continental lithosphere suggesting horizontal flow beneath the lithosphere we also observe a vsv vsh signature at 150 300 km depth beneath major ridge systems with amplitude correlated with spreading rate for fast spreading segments in the transition zone 400 700 km depth regions of subducted slab material are associated with vsv vsh while the ridge signal decreases whilethemid mantlehasloweramplitudeanisotropy vsv in the lowermost 300 km which appears to be a robust conclusion despite an error in our previous paper which has been corrected here the 3 d deviations from this signature are associated with the large scale low velocity superplumes underthecentralpacificandafrica suggestingthatvsh vsv isgeneratedinthepredominant horizontal flow of a mechanical boundary layer with a change in signature related to transition to upwelling at the superplumes

multiple thermotectonic events in a continuous metamorphic sequence mica creek area southeastern canadian cordillera u th pb isotope dilution and shrimp dating indicates that multiple periods of metamorphism and deformation occurred between 160 and 60 ma in the mica creek area southeastern canadian cordillera the metamorphic sequence barrovian assemblages in the garnet zone through the sillimanite k feldspar zone fold generations and neoproterozoic stratigraphy are apparently continuous across the area yet it broadly lies between regions that were tectonized at different times middle jurassic to the east and middle cretaceous paleocene to the west our dating along a north lowest metamorphic grade to south highest grade transect shows that three km scale tectonic age domains exist in garnet schist of domain 1 peak metamorphism occurred at 163 ma and a minor thermal overprint occurred at 70 60 ma in migmatitic kyanite schist of domain 2 monazite inclusions in garnet grew at 110 ma matrix monazite and monazite inclusions in kyanite grew at 85 73 ma and rims of some grains grew at 60 ma kyanite growth during peak metamorphism must have occurred after 73 ma possibly during intrusion of 61 ma leucosome in migmatitic kyanite schist and sillimanite schist of domain 3 peak metamorphism occurred at 99 93 ma and lesser thermal events occurred at 280 160 110 and 70 ma we thus conclude that the thermal peak of metamorphism took place at different times in each domain with peak events coinciding with secondary events in other domains age constraints on s 1 2 are provided by dating variably deformed granitoid rocks s 1 2 developed before 72 ma in domain 1 at least partly at 61 58 ma and 122 63 ma in domains 2 and 3 respectively our findings show that the mica creek area is the only known region in the southeastern canadian cordillera that was affected by the five recognized major periods of tectonism 175 160 141 120 110 100 90 and 75 50 ma the key to understanding the geologic history is determining why several metamorphic and deformation events spanning 100 myr occurred within a single tectonometamorphic complex with apparent geologic continuity if continuity is only apparent the domains may have been tectonically assembled along cryptic unrecognized shear zones however if continuity does indeed exist the events must have been locally superimposed on the complex

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

investigation of laterally heterogeneous shear velocity structure in d beneath eurasia seismic waves are used to explore large scale radial structure and lateral heterogeneity of shear velocity in the d region the chemical and thermal boundary layer at the base of the mantle we analyze long period horizontally polarized sh shear waveforms from earthquakes in the northwest pacific recorded in europe and the middle east for evidence of proposed laterally varying shear velocity stratification in d beneath eurasia specifically our data sample the lower mantle extending from beneath the arctic ocean near novaya zemlya ussr to under northern india using a reflectivity technique to model the sh waveforms we determine a locally stratified shear velocity model sgle that provides a good average travel time and waveform fit to over 80 of the data sensitive to d structure sgle is characterized by a rapid 2 75 increase in shear velocity approximately 290 km above the core mantle boundary similar to previously proposed models for this region unlike for other regions analyzed using similar waveform modeling techniques the percentage of data inconsistent with this average radial model is fairly substantial indicating that laterally heterogeneous d velocity structure is important in this region some waveforms lack the reflected arrival expected for a stratified model while others show evidence of two arrivals both possibly originating within d while our data are insufficient for direct inversion of the three dimensional structure we utilize several simple parameterizations of the waveforms to gain some understanding of the region both the relative amplitude and differential travel time variations of the waveforms indicate that d beneath eurasia is characterized by 1 broad regions in which the structure has large scale stratification giving coherent long period reflections with superimposed velocity variations that scatter shorter wavelength waves and 2 smaller regions which are sufficiently heterogeneous to scatter even the long period energy the scale length of the lateral heterogeneity in d that produces these variations is inferred to be somewhat less than 500 km which contrasts with weaker lateral heterogeneity that is observed in the central region of the lower mantle where heterogeneity scale lengths of approximately 500 1000 km appear to dominate these observations are consistent with a model of the lower mantle in which distributed heterogeneity is concentrated in d generating a locally stratified layer in some regions the effects of a diffuse or topographically variable d discontinuity must be further explored using two and three dimensional synthetic calculations before inversions for the detailed structure can be undertaken

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

core mantle boundary topography from short period pcp pkp and pkkp data abstract we use a total of 839 369 pcp pkpab pkpbc pkpdf pkkpab and pkkpbc residual travel times from bull seism soc am 88 1998 722 grouped in 29 837 summary rays to constrain lateral variation in the depth to the core mantle boundary cmb we assumed a homogeneous outer core and the data were corrected for mantle structure and inner core anisotropy inversions of separate data sets yield amplitude variations of up to 5 km for pcp pkpab pkpbc and pkkp and 13 km for pkpdf this is larger than the cmb undulations inferred in geodetic studies and moreover the pcp results are not readily consistent with the inferences from pkp and pkkp although the source receiver ambiguity for the core refracted phases can explain some of it this discrepancy suggest that the travel time residuals cannot be explained by topography alone the wavespeed perturbations in the tomographic model used for the mantle corrections might be too small to fully account for the trade off between volumetric heterogeneity and cmb topography in a second experiment we therefore re applied corrections for mantle structure outside a basal 290 km thick layer and inverted all data jointly for both cmb topography and volumetric heterogeneity within this layer the resultant cmb model can explain pcp pkp and pkkp residuals and has approximately 0 2 km excess core ellipticity which is in good agreement with inferences from free core nutation observations joint inversion yields a peak to peak amplitude of cmb topography of about 3 km and the inversion yields velocity variations of 5 in the basal layer the latter suggests a strong trade off between topography and volumetric heterogeneity but uncertainty analyses suggest that the variation in core radius can be resolved the spherical averages of all inverted topographic models suggest that the data are best fit if the actual cmb radius is 1 5 km less than in the earth reference model used i e the average outer core radius would be 3478 km

the core mantle boundary the largest compositional discontinuity within the earth is the core mantle boundary cmb at a depth of 2889 km this boundary and the adjacent transition zones in the lowermost mantle and outermost core play a critical role in the earth s thermal and chemical evolution estimates of the heat flux out of the core indicate that the 200 km thick d region at the base of the mantle is a major thermal boundary layer the d region like its counterpart boundary layer in the lithosphere is laterally heterogeneous and possibly chemically stratified this complexity obscures the exact dynamic behavior of the d region however it is clear that the large density contrast 4 3 g em 3 across the cmb provides favorable con ditions for density stratification on both sides of the boundary the d region may be the refuse pile for heavy mantle heterogeneities while the outermost core may have a concentration of light components boundary layer instabilities in d have been invoked as sources of both localized mantle plumes and major upwelling currents in whole mantle convection systems dynamically supported topography on the cmb and lateral temperature gradients in d may produce coupling between the core and mantle influencing the excitation of the geodynamo the outer most core is generally believed to be radially and laterally homogeneous although chemical stratification has not been ruled out seismological geodynamical and geomagnetic investigations are unveiling the three dimensional complexity of the deep earth in the vicinity of the cmb

poisson s ratio in the lower mantle beneath alaska evidence for compositional heterogeneity 1 using waveform cross correlation we have measured 17 000 p and s wave differential travel times from earthquakes and receivers near a great circle path from japan across alaska to north america we jointly invert the data for variations in shear and bulk sound speed and derived poisson s ratio variations in the mantle beneath 1000 km depth we explore the model space using different levels of regularization and our conclusions are based on a range of models that give acceptable data fits the correlation between vs and vp variations is good to 1500 km depth but gradually degrades in the bottom 1000 km whereas the ratio r lnvs lnvp is between 1 0 and 1 8 at most depths our data suggest that there is no significant correlation between bulk and shear wave speed but they appear anticorrelated in several regions we estimate the effect on the poisson s ratio of changes in temperature iron content and the magnesiowustite mw to perovskite pv ratio at all depths the effect of temperature is largest and that of the pv to mw ratio smallest our data cannot resolve the trade off between thermal and compositional effects but explaining the poisson s ratio variability by temperature alone would require unrealistic perturbations and it would not explain the inferred anticorrelation between shear and bulk sound speed our results suggest that despite r being significantly smaller than 2 5 we need a combination of thermal and compositional effects in order to explain the elastic parameter variations in the deep mantle beneath alaska with realistic values of t of 300 800 k variation in xfe of 4 and pv enrichment or depletion of up to 10 just sufficient for explaining the data

a lower mantle s wave triplication and the shear velocity structure of d summary a lower mantle s wave triplication detected with short and longperiod wwssn and csn recordings indicates a substantial shear velocity discontinuity near 280 km above the core mantle boundary the triplication can be observed in rotated sh seismograms from intermediate and deep focus events throughout the distance range from 70 to 95 three distinct source region receiver array combinations that have been investigated in detail demonstrate consistent travel time and relative amplitude behaviour of the triplication with slight systematic shifts in the triplication indicating up to 40km variations in the depth of the discontinuity modelling of the observations with synthetic seismograms produced with the cagniard de hoop and reflectivity methods constrains the shear velocity increase to be 2 75 k 0 25 per cent comparable to upper mantle discontinuities short period observations indicate that the velocity increase may be a sharp first order discontinuity or may extend over a transition zone no more than 50 km thick the shear velocity gradient below the discontinuity within the d layer is not well constrained by the sh data but slightly positive or near zero velocity gradients are consistent with the long period amplitude ratios of scsh sh

thick and anisotropic d layer beneath antarctic ocean 1 waveform modeling and travel times analyses of s scs and sks phases recorded at the broad band permanent station syo in the antarctic are used to determine the shear wave velocity structure and transverse isotropy in the d layer beneath the antarctic ocean the sh wave structure has a discontinuity with the velocity increase of 2 0 at 2550 km the sv structure is similar to prem model the magnitude of the anisotropy is highest at the top of d layer and lowest at the core mantle boundary the d layer beneath the antarctic ocean is significantly thicker than those beneath alaska and the caribbean sea we attribute this anisotropic d layer to paleo slab materials the subduction in and around the antarctic ocean has started 180 ma and is the one of the oldest in the world it has provided a large amount of the slab materials in the lowermost mantle citation usui y y hiramatsu m furumoto and m kanao 2005 thick and anisotropic d layer beneath antarctic ocean

electron beam analytical instruments and the determination of modes spatial variations of minerals and textural features of rocks in polished section point counts based on x ray intensities collected with an automated electron beam analytical instrument can be used to estimate the accurate modes required by modern physical chemical modelling of rock forming processes in addition the spatial distributions of mineral species and mineral compositions can be extracted from the coordinates of the point counting grid statistical tests of whether the mineral and porosity distributions are uniform can be performed by comparing the results from subareas of the polished section the separate and composite contributions of the mineral phases to the texture and structure of the rock can be visually represented with mineral distribution maps qualitative differences in grain size can be recognized because the nonzero finite electron beam diameter can detect grain boundaries spots on the minerals are identified by comparing the ranking of x ray intensities with the ranking of intensities on a set of standards for example this method identified more than 88 of the spots on the surface of a polished section of an alkali olivine basalt a small additional number of spots approximately 1 were identified because their spectra were dominated by the x ray intensities of two characteristic elements this method was particularly efficient in identifying phases with a small grain size and simple chemistry e g apatite the modal amounts of an alkali olivine basalt obtained with a microprobe are within approximately one standard deviation of the results obtained by point counting the same areas of the polished section under reflected and transmitted light if the minerals are major constituents and occur only in the groundmass accessory and phenocryst phases agree to within approximately three standard deviations

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

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

tectonic links between the clachnacudainn terrane and selkirk allochthon southern omineca belt canadian cordillera the clachnacudainn terrane in the southern omineca belt is interpreted as an integral part of the overlying selkirk allochthon based on stratigraphic igneous metamorphic and deformational links across the boundary between them the standfast creek fault an inverted homocline correlated with lower paleozoic lardeau group metasedimentary and metavolcanic rocks is continuous across the fault the fault is transected by post ductile strain granitoids of inferred and known mid cretaceous age such as the 104 1 ma albert stock as well as by peak regional metamorphic m2 isograds and contact metamorphic m3 isograds associated with the albert stock three phases of regional ductile deformation including major west verging second phase f2 folds are correlated across the fault these tectonic links strongly suggest that the clachnacudainn terrane and selkirk allochthon share a similar tectonic history and thereby require that the standfast creek fault has minor tectonic significance compared with other faults in the southern omineca belt m2 and coeval f2 folds are interpreted as resulting from middle jurassic tectonism documented elsewhere in the selkirk allochthon the main aspect that differentiates the clachnacudainn terrane from the rest of the selkirk allochthon east of the monashee complex is the clachnacudainn igneous complex which dominates the northern part of the terrane and is the product of prolonged igneous activity

plagioclase clinopyroxene garnet quartz equilibria and the geobarometry and geothermometry of garnet amphibolites from mica creek british columbia metabasites form a small but significant volume of the upper precambrian horsethief creek group near mica creek british columbia sillimanite zone assemblages in metabasites are hornblende plagioclase ilmenite quartz garnet clinopyroxene sphene calcite the incoming of garnet clinopyroxene in metabasites very nearly coincides with the kyanite sillimanite isograd fe mg distributions between coexisting garnet and clinopyroxene suggest a temperature range of 650 705 c ellis and green calibration the higher temperature estimates disagree with the upper thermal stability limits of muscovite plagioclase quartz sillimanite from adjacent pelitic rocks pressure estimates based upon clinopyroxene garnet plagioclase quartz equilibria are 5 6 0 3 and 7 2 0 3 kbar 560 30 and 720 30 mpa respectively using two different calibrations of this geobarometer these estimates bracket the pressure estimated from garnet plagioclase al2sio5 equilibria in pelitic rocks

coexisting shear and compressional wave seismic velocity discontinuities beneath the central pacific stacking of broadband shear waves s and short period compressional waves p provides compelling evidence for the existence of a localized discontinuous increase in both s and p wave velocity 230 km above the core mantle boundary cmb beneath the central pacific modelling indicates a 1 7 increase in s velocity v s and a 0 75 increase in p velocity vp similar features have been detected intermittently on a global basis but seldom with s and p structure being resolved in the same location previous evidence for anisotropy near the cmb beneath the central pacific suggests that the discontinuity may be related to anisotropic fabrics induced by flow in the thermal boundary layer above the core

plume formation in the d double prime layer and the roughness of the core mantle boundary numerical simulations of the formation of thermal plumes in the d layer at the base of the earth s mantle show that plumes are initiated by coalescence of small scale convective instabilities within the low viscosity region immediately above the core mantle boundary cmb these instabilities support topographic roughness on the cmb having horizontal scales of 20 50 km and provide a source for scattered p waves seen as precursors to the phases pkikp and pkkp the calculated structure of fully developed plumes emerging from d consists of 5 50 cm yr 1 flow confined to 50 100 km thick vertical conduits with strongly temperature dependent viscosity plumes exhibit time dependent behaviour including upward propagating solitary conduit waves which may contribute to episodicity in hotspot volcanism

localized boundary layer below the mid pacific velocity anomaly identified from a pcp precursor dense record sections from deep earthquakes in fiji and argentina recorded on hundreds of short period stations in california at distances of 81 to 85 are used to investigate the detailed p wave velocity structure above the core mantle boundary cmb in the fiji data a secondary phase arriving 2 to 4 s after the direct p is identified as a precursor to pcp this phase provides good evidence for a reflection off the top of a thin low velocity layer above the cmb comparisons to synthetic seismograms indicate a layer thickness of 10 km and a velocity reduction of 5 10 compared to the overlying mantle a record section from an argentina event does not show the pcp precursor indicating that the low velocity layer is not a global feature this thin low velocity layer is in the same place as a much larger s wave velocity anomaly in the lower mantle and is probably indicative of a boundary layer just above the cmb under the mid pacific

effects of d anisotropy on seismic velocity models of the outermost core we explore effects of shear wave anisotropy in d on seismic waves utilized in constructing models of the outermost core reflectivity synthetic seismograms approximating transverse isotropy ti in d yield absolute and differential time perturbations of s sks and skks of up to several seconds especially if laterally varying anisotropy is considered several studies have used anomalies of this magnitude to infer a 0 5 2 5 p velocity reduction in the outermost 50 150 km of the core suggesting a stably stratified layer just beneath the core mantle boundary our data samples the mantle and core beneath alaska and northern pacific a region with strong lateral variations in d anisotropy ti models that predict the magnitude of shear wave splitting seen in the data can account for much but not all of the anomalies that isotropic models attribute to outermost core velocity reductions future outermost core seismic studies should address such d anisotropy effects

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

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

evidence for a shear velocity discontinuity in the lower mantle beneath india and the indian ocean abstract sh and ssh seismograms are modeled to determine the shear velocity structure in the d region beneath india and the indian ocean the signals show waveform complexities similar to those observed in data sampling the d region beneath alaska the caribbean and eurasia lay and helmberger which have been attributed to a 2 7 shear velocity discontinuity 280 km above the core mantle boundary the new data set consists of long period tangential component recordings at wwssn stations in africa the middle east and europe for 11 intermediate and deep focus indonesian earthquakes in the distance range 70 82 the waveforms show an arrival between sh and scsh with systematic moveout from 89 to 94 there is a strong distortion of the sh waveforms indicating the arrival of several phases closely spaced in time the relative time shifts of similar complexity in the corresponding ssh phases requires a deep mantle origin the depth dependence and moveout of the interference effects are well predicted for both sh and ssh phases by a model with a lower mantle discontinuity alternative explanations of the interference as resulting from receiver reverberations sks contamination multiple source complexity or near source multipathing are ruled out by systematic tests while it is apparent that lateral variations in the lower mantle velocity structure prevent any single model from fitting all of the data synthetic waveform modeling using generalized ray theory and reflectivity shows that the data can be well fit by a model with a discontinuity similar in size and depth to that proposed for the previously investigated regions lay and helmberger but with a negative velocity gradient within the d layer

stratigraphy and deformational history of the scrip nappe monashee mountains british columbia the scrip nappe a large recumbent anticline that occupies the northern selkirk and northern monashee mountains has an inverted lower limb some 50 km in length across strike and comprises stratigraphic divisions of the hadrynian horsethief creek group which can be traced southward with decreasing metamorphic grade through the selkirk mountains to the northern purcell mountains the scrip nappe has a southwesterly vergence and it formed that way during the first folding phase of the mesozoic columbian orogeny metamorphism no greater than biotite zone accompanied that first deformation the nappe was subsequently refolded into tight northeast verging folds metamorphism rose to upper amphibolite facies late in the second deformation phase after the metamorphic climax northeast verging buckle folds and associated crenulation cleavage formed locally during a third folding episode the entire nappe complex was then carried northeastward on the purcell thrust over the folds and thrusts of the western r

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

the fate of descending slabs oceanic lithosphere the stiff plate formed by the cooling of chemically diffcrentiated material that rises and mclts under mid ocean ridges and spreads laterally in the process called sea floor spreading has an ephemeral residence at the surface of the earth typically within 100 200 ma densi fication of the old oceanic lithosphere leads to a dynamic instability causing it to sink into the interior descending as a layered slab with highly anom alous thermal and chemical characteristics relative to the surrounding ambient mantle the slab heats as it sinks but the warming is sluggish slow to overcome the effect of more than 100 ma of cooling at the surface and the negative buoyancy of the slab continues to drive its descent increasing pressure and temperature with depth drive various chemical reactions in the slab including expulsion of water and other volatiles accumulated at the surface and destabilization of associated hydrous phases suites of mineralogical phase transformations occur within the slab s crustal and depleted mantle components because the slab has been processed through the melting zone at the ridge partially hydrated and cooled significantly the phase equilibria within the slab differ from those of the surrounding mantle near 400 km depth an exothermic phase transformation occurs with mg fe 2si04 olivine in the slab converting to the high pressure modified j spinel structure the spinel transition helps to drive the slab onward although a central core of metastable olivine may persist to greater depths with eventual transformational faulting

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

on seismic resolution of lateral heterogeneity in the earth s outermost core issues concerning resolution of seismically determined outermost core properties are presented with an example from three earthquakes in the fiji tonga region travel time behavior of the commonly used family of smks waves which travel as s in the mantle p in the core reflecting m 1 times at the underside of the core mantle boundary cmb are analyzed over a large distance range 125 165 data having wavepaths through an area of known d heterogeneity 2 exhibit systematic anomalies in smks differential times two dimensional wave propagation experiments demonstrate how large scale lower mantle velocity perturbations can explain long wavelength behavior of such anomalous smks times though heterogeneity on smaller scales may be responsible for the observed scatter about these trends if lower mantle heterogeneity is not properly accounted for in deriving a core model misfit of the mantle model maps directly into core structure the existence of outermost core heterogeneity is difficult to resolve at present owing to uncertainties in global lower mantle structure resolving a one dimensional chemically stratified outermost core also remains difficult owing to the same uncertainties inclusion of the slowly accruing broadband data should help in this regard restricting study to higher multiples of smks m 2 3 4 can help reduce the effect of mantle heterogeneity because of the closeness of the mantle legs of the wavepaths smks waves are ideal in providing additional information on the details of lower mantle heterogeneity

seismic anisotropy in the d layer we present observations of diffracted sv svd for a path between the fiji tonga islands and the eastern coast of north america at distances greater than 110 observed features of s diffracted suggest that coupling between svd and shd can be ruled out as a first order effect for this path arrivals of shd are late relative to iasp91 travel times by about 10 s and those of svd are late relative to shd by 3 s for most records the slope of the log svd shd spectral ratio is around 3hz 1 in the range 0 06 0 15 hz a transversely isotropic low velocity layer in the lowermost mantle with a thickness of 200 300 km may account for most of the observed properties of svd

the lowermost mantle beneath northern asia ii evidence for lower mantle anisotropy summary we have analysed prediffracted s waves with turning points beneath northern siberia in a study of anisotropy in the lowermost mantle deep focus earthquakes beneath the marianas izu bonin and the sea of japan recorded at stations in western europe are used a correction for upper mantle anisotropy is applied to the data comparisons of the data with synthetic data for models with and without a high velocity d layer suggest that there is a velocity discontinuity at the top of the d region and that the style of anisotropy is transversely isotropic in this region time separations between s waves on the radial and transverse component show a weak trend where the separation increases with epicentral distance a normalization of this separation with the travel distance within d 300 km thick in this region suggests that the anisotropy is uniformly distributed within this layer and has an average value of 0 5 per cent a combination of different studies which investigate the structure of the lowermost mantle beneath europe and northern siberia reveals a complicated picture tomographic models from this area and evidence of d anisotropy lower mantle scatterers reflections from a d discontinuity and ultralow velocity zones suggest two distinct regions one exhibits high velocities d anisotropy a d discontinuity and no evidence of scatterers or ultra low velocity zones these features are likely associated with the palaeosubduction of the izanagi plate well into the lowermost mantle the other region has a lower overall velocity and shows evidence of scatterers and ultra lowvelocity zones perhaps suggesting the presence of partial melt these results suggest dramatic lateral variations in the nature of the lowermost mantle beneath northern asia over a length scale of roughly 30 degrees

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

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

on the inversion of sd particle motion for seismic anisotropy in d in order to make accurate estimates of anisotropy in d as measured from observations of sdi f f sd it is necessary to correct the observed waveforms for anisotropy outside of the deepest mantle as well as side refraction along the path and to pick arrivals of svd and shd as accurately as possible we show that corrections based on estimates of upper mantle anisotropy inferred from sks skks splitting measurements are sometimes insufficient and propose a master event method to directly obtain more accurate corrections on specific source station paths our study illustrates the complexity of structure that needs to be taken into account when addressing issues of d properties using seismic body wave methods we also describe a method to accurately measure the arrival time of the corrected sv d wave

the southern omineca belt british columbia new perspectives from the lithoprobe geoscience program geological isotopic and geochronology studies carried out by university and government researchers concurrently with the lithoprobe program have greatly refined our understanding of the regional geology crustal structure and tectonics of the omineca belt sound correlations have been established between surface geology and seismic reflection data cretaceous eocene thrust faults that are imaged in the subsurface in the shuswap complex may be part of a break forward thrust system that feeds into the purcell anticlinorium and the foreland belt the monashee decollement is the western continuation of the sole thrust beneath the foreland belt and provides a means of linking shortening across the entire orogen the thermal peak of metamorphism in the central and southern shuswap complex is now known to have occurred in the late cretaceous paleogene in contrast with earlier held views north american basement rocks are now known to extend beneath the eastern half of the canadian cordillera geochronology

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

lower mantle tomography and phase change mapping a lower mantle s wave triplication scd has been recognized for many years and appears to be explained by the recently discovered perovskite pv to postperovskite ppv phase change seismic observations of scd display 1 rapid changes in strength and timing relative to s and scs and 2 early arrivals beneath fast lower mantle regions while the latter feature can be explained by a clapeyron slope of 6 mpa k and a velocity jump of 1 5 when corrected by tomographic predictions it does not explain the first feature here we expand on this mapping approach by attempting a new parameterization that requires a sample of d near the scs bounce point vs where the phase height hph and velocity jump are functions of vs these parameters are determined by modeling dense record sections collected from usarray and passcal data where grand s tomographic model is the most detailed in d structure beneath central america we also address the range of to generate new global models of the phase boundary and associated temperature variation we conclude that a near 9 mpa k is most satisfactory but requires to be nonuniform with a range from about 1 0 to 4 0 with some slow region samples requiring the largest values moreover the edges of the supposed buckled slabs delimitated by both p and s waves display very rapid changes in phase boundary heights producing scd multipathing these features can explain the unstable nature of the scd phase with easy detection to no detection commonly observed the fine structure at the base of the mantle beneath these edges contains particularly strong reflections indicative of local ultralow velocity zones which are predicted in some dynamic models

a very slow basal layer underlying large scale low velocity anomalies in the lower mantle beneath the pacific evidence from core phases a multi phase analysis using long period world wide standardized seismograph network and canadian network data has been conducted using core phases for deep focus events from the southwest pacific these include sks s2ks svd iff and sp dks the last phase emerges from sks near 106 and is associated with a p wave diffracting along the bottom of the mantle patterns in s2ks sks differential travel times t s2ks sks correlate with those in sp dks sks t spdks sks t s2ks sks values strongly depend on variations in v s structure in the lower third of the mantle whereas t spdks sks values mainly depend on v p structure and variations in a thin zone 100 km or less at the very base of the mantle anomalously large t s2ks sks and t spdks sks values relative to the preliminary reference earth model prem are present for fiji tonga and kermadec events recorded in north and south america along with anomalously large sv diff amplitudes well into the core s shadow more northerly paths beneath the pacific to north america for indonesian and solomon events display both prem like and anomalous times a model compatible with the observations is presented and contains a thin very low velocity layer at the base of the mantle that underlies the large volumetric lower mantle low velocity regions in the southwest pacific a low velocity layer of 20 100 km thickness with reductions of up to 5 10 relative to prem can reproduce t spdks sks as well as sv diff amplitudes large scale more than 1000 km lower mantle v s heterogeneity 2 4 can explain long wavelength trends in t s2ks sks the exact thickness and velocity reduction in the basal layer is uncertain owing to difficulties in resolving whether anomalous structure occurs on the source and or receiver side of wavepaths at the cmb

synthetic seismic anisotropy models within a slab impinging on the core mantle boundary the lowermost few hundreds of kilometres of the earth s mantle are elastically anisotropic seismic velocities vary with direction of propagation and polarization observations of strong seismic anisotropy correlate with regions where subducted slab material is expected in this study we evaluate the hypothesis that crystal preferred orientation cpo in a slab as it impinges on the core mantle boundary is the cause of the observed anisotropy next we determine if fast polarization directions seen by shear waves can be mapped to directions of geodynamic flow this approach is similar to our previous study performed for a 2 d geodynamic model in this study we employ a 3 d geodynamic model with temperature dependent viscosity and kinematic velocity boundary conditions defined at the surface of the earth to create a broad downwelling slab tracers track the deformation that we assume to be accommodated by dislocation creep we evaluate the models for the presence of perovskite or post perovskite and for different main slip systems along which dislocation creep may occur in post perovskite 100 010 and 001 resulting in four different mineralogical models of cpo combining the crystal pole orientations with single crystal elastic constants results in seismically distinguishable models of seismic anisotropy the models are evaluated against published seismic observations by analysing different anisotropic components the radial anisotropy the splitting for sub vertical phases i e azimuthal anisotropy and the splitting for subhorizontal phases the patterns in radial anisotropy confirm our earlier results in 2 d observations of radial anisotropy and splitting in subhorizontal phases are mostly consistent with our models of post perovskite with 010 slip and 001 slip our model of 001 slip predicts stronger splitting than for 010 slip for horizontally propagating phases in all directions the strongest seismic anisotropy in this model occurs where the slab impinges on the core mantle boundary the azimuthal anisotropy pattern for 001 slip shows fast axis directions at the edges of the slab sub parallel to flow directions suggesting horizontal flows may be mapped out in the lowermost mantle using seismic observations

40ar 39ar dates on hornblende muscovite and biotite from the mica creek area british columbia regional metamorphic and tectonic implications a metamorphic high the selkirk monashee cariboo complex smc is a 200 km 60 km area cored by k feldspar sillimanite rocks in the mica creek area there appear to be no discontinuities in metamorphic isograds structural trends or the late precambrian stratigraphic units u pb dates suggested that the continuities may only be apparent in this paper we report 40ar 39ar dates on muscovite biotite and hornblende from some of the same samples dated by u pb and samples that crop out very close to the dated samples the goal was to set constraints on the alternative interpretations of the metamorphic structural history these interpretations are as follows 1 u pb ages record different times of metamorphic crystallization and deformation and there are significant tectonic displacements the mappable continuities are only apparent 2 the structural and stratigraphic continuity is real and the range is due to localized heating and deformation events spanning 100 ma the relative uniformity of the

where can seismic anisotropy be detected in the earth s mantle in boundary layers during the last 30 years considerable evidence of seismic anisotropy has accumulated demonstrating that it is present at all scales but not in all depth ranges we detail which conditions are necessary to detect large scale seismic anisotropy firstly minerals must display a strong anisotropy at the microscopic scale and or the medium must be finely layered secondly the relative orientations of symmetry axes in the different crystals must not counteract in destroying the intrinsic anisotropy of each mineral and there must be efficient mechanisms of orientation of minerals and aggregates finally the strain field must be coherent at large scale in order to preserve long wavelength anisotropy part of shallow anisotropy can be related to the past strain field frozen in anisotropy however the deep anisotropy is due to the present strain field all these conditions are fulfilled only in boundary layers of convective mantle we review in this paper the seismic data sets which provide insight into the location at depth of large scale anisotropy from the d layer up to the lithosphere in addition to the well documented seismic anisotropy in the lithosphere and asthenosphere there is new evidence of seismic anisotropy in the upper 400 660 km and lower 660 900 km transition zones and in the d layer nonetheless the bulk of the lower mantle seems close to isotropy if we assume the hypothesis that seismic anisotropy is associated with boundary layers in convective systems these observations strongly suggest that the transition zone is a boundary layer which makes the pasage of matter between the upper and the lower mantle difficult however this general statement does not rule out flow circulation between the upper and lower mantles finally the geophysical mineral physics and geological applications are briefly reviewed an intercomparison between surface wave anisotropy and body wave anisotropy data sets is presented we discuss the scientific potential of seismic anisotropy and how it makes it possible to gain more insight into continental root deformation and geodynamics processes

mapping the lowermost mantle using core reflected shear waves a map of laterally varying d velocities is obtained for the region from 50 s to 50 n in latitude and 70 e to 190 e in longitude velocities are found using an analysis of the differential travel time residuals from 481 scs s and 266 sscs ss phase pairs the long period data are taken from the global digital seismograph network digital waveform catalog for the time period of january 1980 to march 1987 each differential travel time is found by a cross correlation of the s phase ground displacement corrected to simulate differential attenuation with all following phases travel times are corrected for ellipticity and mantle heterogeneity outside of their d paths and the remaining residuals are interpreted as the result of d heterogeneity ray tracing tests are made to check the validity of converting travel time residuals into velocity path anomalies the resulting map reveals significant long wavelength d structure including a 3 low velocity region beneath northeastern indonesia surrounded by three identified high velocity zones beneath northwestern pacifica 4 southeast asia 3 and australia 3 5 this structure is of continent ocean spatial scales and is most likely created by dynamic processes dominant in the lower mantle the low velocity region may have both chemical and thermal origins and is very possibly the site of an incipient lower mantle plume where mature d rock which has been heated by the core has become gravitationally unstable and begun to rise a chemical component possibly exists as a chemical boundary layer is dragged laterally toward the plume site much the way continents are dragged toward subduction zones the high velocity zones possibly result from the downward convection of cold lower mantle plumes which pond at the core mantle boundary these seismic anomalies may also contain a chemical signature from faster iron poor materials brought down through the lower mantle or the additional presence of sio2 stishovite perhaps in its higher pressure polymorph

linking metamorphic textures to u pb monazite in situ geochronology to determine the age and nature of aluminosilicate forming reactions in the northern monashee mountains british columbia abstract the monashee mountains of the canadian cordillera are thought to expose a classic barrovian facies series of isograds the timing of aluminosilicate growth in the region was determined for four pelitic schist samples by combining textural relationships with monazite compositional zoning and monazite u pb geochronology conducted directly on thin sections by the laser ablation method three distinct phases of kyanite growth are recorded in the kyanite zone at c 153 ma between 122 and 94 ma and between 76 and 58 ma for each phase monazite and garnet grew synchronously with kyanite probably by a reaction involving the breakdown of staurolite in contrast sillimanite growth by muscovite dehydration melting occurred at or before c 104 ma in the sillimanite zone and retrograde sillimanite grew in schists previously metamorphosed at the kyanite grade during the first two phases by the influx of hot acidic fluids during top to the east shearing at ca 71 ma these results indicate that rocks metamorphosed at different places and different times in the orogen were juxtaposed prior to being overprinted at the sillimanite grade in the late cretaceous early paleocene during the influx of hot fluids in a structurally coherent body deforming by easterly directed shearing this study also provides new insight into monazite petrogenesis and suggests that at least in some circumstances monazite formation is linked to the staurolite out reaction that produces kyanite

oxygen isotope study of quartz al2sio5 pairs from the mica creek area british columbia implications for the recovery of peak metamorphic temperatures oxygen isotope analyses of quartz al2sio5 pairs have been made for samples from the mica creek area british columbia we have analysed quartz kyanite nodules and quartz kyanite and quartz sillimanite in multiphase pelitic rocks from the staurolite kyanite kyanite and sillimanite zones apparent temperatures calculated from oxygen isotopic fractionation range from 555 c staurolite kyanite zone to 695 c sillimanite zone temperatures from the quartz kyanite nodules range from 630 to 675 c some of the nodules show isotopic disequilibrium most of the results confirm predictions that bimineralic rocks will yield an estimate of peak metamorphic temperatures when the less abundant mineral an aluminium silicate is the slower oxygen diffuser using cooling rates of 10 100 c ma 1 for the multiphase rocks measured crystal sizes and modes the fast grain boundary diffusion model with wet diffusion data ph2o 1 0 kbar yields predicted apparent temperatures which are generally lower than the measured apparent temperatures the agreement is improved if slower diffusion coefficients are used this suggests that f h2o during cooling was lower than that of the hydrothermal experiments and thus that there was little interaction with aqueous fluids of internal or external origin to modify the isotopic compositions the measured apparent isotopic temperatures and apparent garnet biotite fe mg exchange temperatures show very poor agreement for the sillimanite zone samples with the garnet biotite fe mg exchange temperatures generally higher than the oxygen isotope temperatures compared with the other calibrations that we tested the measured apparent temperatures using the sharp calibration show the best agreeement with recently published p t grids although some variability in agreement is expected due to variable f h2o during cooling

lateral variations at the base of the mantle from profiles of digital s diff data average long wavelength shear velocity structure in several regions at the base of the mantle is determined using the profiles of core diffracted sh waves shdiff there is significant lateral variation in the sdiff apparent ray parameters suggesting considerable lateral shear velocity anomalies in d the region just above the core mantle boundary cmb apparent ray parameters are determined by least squares fits through the pulse maxima of the instrument deconvolved ground displacements and heterogeneities are quantified through comparisons with reflectivity synthetic ray parameters corrections are applied to the data to account for the effects of the earth s ellipticity and of mantle heterogeneities along the shdiff upswing paths a total of 161 shdiff profiles were obtained greatly expanding the number of ray parameter measurements reported in previous studies most of the observed slownesses fall in the range between 8 2 and 8 8 s deg these correspond to slowness anomalies of 3 5 the most robust and extensive feature resolved is under the northern and northeastern pacific ocean the slowest values p 4 occur toward the southeast and there is a trend toward fast velocities moving to the northern and eastern rims of the pacific a model of d shear velocities is obtained by converting the slownesses to velocity anomalies superposing the d path profiles onto the cmb and applying a weighted moving cap spatial average this model agrees well with many current tomographic models both at large 5000 km and intermediate 1000 km scales the fact that this occurs with a different type of data and technique of analysis suggests that we are now able to recognize some of the smaller scale lateral variations at the base of the mantle locations of fast and slow velocity anomalies at the cmb are consistent with the model of cold paleoslabs ponding at the cmb and forcing d rock laterally to form hot aggregates that give rise to plumes in the mantle and hot spots at the surface

anisotropy as cause for polarity reversals of d reflections recordings of seismic events that sample the deep mantle can test different hypotheses of mantle processes and composition seismic reflections from structures in the d region the bottom 200 400 km of the earth s mantle can provide information on the velocity contrasts in this region by studying the waveforms and polarities of the d reflections in p and s waves we can potentially distinguish between different explanations for the observed structures such as phase transitions mineral texture or thermal anomalies here we use source receiver combinations that contain reflections from d in two different regions that are both characterised by fast seismic velocities in tomographic models beneath the caribbean a positive s velocity contrast but negative p wave velocity contrast across the d reflector has been reported previously consistent with a model of a phase change in mgsio3 in the second fast velocity region eurasia we detect positive p and s wave velocity contrasts in two orthogonal paths crossing in the lowermost mantle indicating a different scenario for d a path that crosses this region in 45 to the other two great circle paths shows evidence for a negative p velocity contrast one explanation to reconcile observations in both regions is a phase transition from perovskite to post perovskite with a fraction of 12 preferred crystal alignment in the post perovskite phase depending on the travel direction of the waves with respect to the flow direction in the lower mantle positive or negative velocity jumps can be expected other anisotropic models are considered but cannot fully explain the range of observations we find in the data c 2011 elsevier b v all rights reserved

tectonic assembly of inverted metamorphic sequences inverted metamorphic sequences are characterized by peak metamorphic temperatures that increase structurally upward with isograds that are typically parallel to associated thrust faults a fully coupled thermal mechanical model for convergent orogens shows how an inverted sequence can be tectonically assembled in a crustal scale ductile shear zone at moderate to high rates of synorogenic erosion model inverted sequences form by tectonic juxtaposition of points with widely differing initial positions that reach peak temperatures at different times and in different places within the model orogen and thus do not represent metamorphic or thermal gradients inverted crustal isotherms are not required to produce model inverted isograds model results agree well with metamorphic pressure temperature data from the main central thrust zone of the central nepal himalayas

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

seismic anisotropy in the deep mantle boundary layers and the geometry of mantle convection an attempt is made to explore the geodynamical significance of seismic anisotropy in the deep mantle on the basis of mineral physics the mineral physics observations used include the effects of deformation mechanisms on lattice and shape preferred orientation the effects of pressure on elastic anisotropy and the nature of lattice preferred orientation in deep mantle minerals in dislocation creep regime many of these issues are still poorly constrained but a review of recent results shows that it is possible to interpret deep mantle seismic anisotropy in a unified fashion based on the solid state processes without invoking partial melting the key notions are i the likely regional variation in the magnitude of anisotropy as deformation mechanisms change from dislocation to diffusion creep or superplasticity associated with a change in the stress level and or grain size in the convecting mantle with a high rayleigh number and ii the change in elastic anisotropy with pressure in major mantle minerals particularly in mg fe o the results provide the following constraints on the style of mantle convection i the sh sv anisotropy in the bottom transition zone and the sv sh anisotropy in the top lower mantle can be attributed to anisotropy structures lattice preferred orientation and or laminated structures caused by the horizontal flow in this depth range suggesting the presence of a mid mantle boundary layer due to partially layered convection ii the observed no significant seismic anisotropy in the deep mantle near subduction zones implies that deformation associated with subducting slabs is due mostly to diffusion creep or superplasticity and therefore slabs are weak in the deep mantle and hence easily deformed when encountered with resistance forces and iii the sh sv anisotropy in the cold thick portions of the d layer is likely to be due to horizontally aligned shape preferred orientation in perovskite plus magnesiowustite aggregates formed by strong horizontal shear motion in the recent past

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

northern monashee mountains omineca crystalline belt british columbia timing of metamorphism anatexis and tectonic denudation the northern monashee mountains lie at the northern termination of a north south zone of eocene extension in the southern omineca crystalline belt of british columbia isotopic structural and petrogenetic studies in the northern monashee mountains document middle to late cretaceous regional metamorphism two episodes of crustal anatexis and a period of rapid cooling coincident with middle eocene extension partial melting of geochemically and isotopically similar pelitic sources produced peraluminous granites during and postdating regional metamorphism at 100 ma and 63 ma respectively rb sr muscovite whole rock isochrons from the younger granites give an age of 62 5 2 0 ma rb sr isochron and u pb mineral ages are similar indicating that the 63 ma granites were emplaced into a terrane that had cooled to 500 c or less by 63 ma hornblende muscovite and biotite k ar and apatite fission track mineral cooling ages from the northern monashee mountains are are 76 3 5 8 n 7 51 4 3 5 n 6 55 1 2 7 n 10 and 45 0 2 9 n 3 ma respectively mean age and standard deviation accelerated cooling beginning with the emplacement of 63 ma granites and ending at 45 ma occurred in response to ramping along a crustal scale thrust fault followed by offset on the middle eocene offset on the middle eocene north thompson normal fault

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

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

d discontinuity structure beneath the north atlantic from scd observations we analyzed transverse and radial component recordings from the 2010 m6 3 southern spain earthquake depth 620 km recorded on 370 broadband stations in north america we grouped these seismograms into subarrays and applied fourth root vespa processing vespagram analysis in order to enhance low amplitude arrivals these vespagrams show clear scd arrivals which indicate the existence of the d discontinuity beneath the north atlantic ocean 45 60 n 45 55 w these observations are best fit with a 2 4 velocity increase at the top of the d discontinuity at a height above the core mantle boundary of 304 14 km we do not observe scd arrivals at the eastern end of our study region which is consistent with the presence of the easternmost edge of the ancient farallon plate

the nature of the earth s core the properties of the earth s core are overviewed with emphasis on seismologically determined regions and pressures and seismologically measured density elastic wave velocities and gravitational acceleration attention is given to solid state convection of the inner core and it is noted that though seismological results do not conclusively prove that the inner core is convective the occurrence and magnitude of seismic anisotropy are explained by the effects of solid state convection igneous petrology and geochemistry of the inner core a layer at the base of the mantle and contact metasomatism at the core mantle boundary and evolution of the core mantle system are discussed it is pointed out that high pressure melting experiments indicate that the temperature of the core is ranging from 4500 to 6500 k and a major implication of such high temperature is that the tectonics and convection of the mantle as well as the resulting geological processes observed at the surface are powered by heat from the core as a result of the high temperatures along with the compositional contrast between silicates and iron alloy the core mantle boundary is considered to be most chemically active region of the earth

dating the earliest lowland glaciation of eastern england a pre mis 12 early middle pleistocene happisburgh glaciation this paper outlines the evidence for the happisburgh glaciation the first middle pleistocene glaciation of lowland eastern england and the adjacent margins of the north sea basin we propose that this glaciation occurred during a pre elsterian anglian mis 12 cold stage based on evidence of clasts of till erratics and heavy minerals from the third youngest terrace of the pre glacial bytham river in southern norfolk the heavy minerals show a progressive increase in glacially sourced materials from the bottom to the top of the bytham river third terrace sediments the till clasts are correlated with the extensive deposits of the happisburgh formation on the basis of their colour heavy minerals and particle size properties thus the happisburgh glaciation occurred during the accumulation of the bytham river terrace deposits and prior to mis 12 when the bytham river system was destroyed by the anglian glaciation in the absence of any geochronology or robust biostratigraphy the age of the happisburgh glaciation is estimated by the synchronisation of the pattern of large scale river activity with patterns of milankovitch forced global climate change on this basis and assuming the british ice sheet acted in phase with global patterns of ice volume the happisburgh glaciation is considered to have occurred during mis 16 and be equivalent of the don glaciation of eastern europe