les gneiss rubanes a cordierite et grenat d ihosy un marqueur thermo barometrique dans le sud de madagascar resume le precambrien de madagascar represente une portion de la ceinture mobile mozambicaine certains auteurs considerent que le sud de l lle montre d est en ouest des formations de plus en plus recentes il s agit de la sequence androyenne ultrametamorphique a noyaux archeens a laquelle succedent la sequence du graphite et la sequence du vohibory les gneiss rubanes et leptynites a cordierite et grenat ont une repartition generale dans la sequence androyenne ces gneiss proviennent de la fusion anatectique de paragneiss les geothermometres et barometres bases sur les couples grenat plagioclase grenat cordierite grenat biotite nous permettent d estimer les conditions de cette migmatisation t 700 c p t 5 5 5 kbar p h 2 o 0 3 0 4 p t une certaine quantite de leptynites a cordierite et grenat represente vraisemblablement les leucosomes de cet episode migmatitique la presence quasi systematique dans les gneiss rubanes d un assemblage a sept phases quartz plagioclase feldspath potassique grenat biotite sillimanite cordierite qui d apres les donnees experimentales semble ne se former que dans un intervalle de pression et de temperature limite t 690 710 c p t 4 5 7 kbar p h 2 o 0 2 0 5 p t permet de considerer ces gneiss comme des niveaux marqueurs thermobarometriques tres precis dans le sud de madagascar cet episode migmatitique pourrait appartenir a l orogene panafricaine

the effect of environmental changes on the evolution and extinction of late devonian trilobites from the northern canning basin western australia the frasnian famennian virgin hills formation represents fore reef facies deposited as part of the extensive late devonian reef system that fringed the sw kimberley block in western australia it contains a rich trilobite fauna dominated primarily by proetids and to a lesser extent harpetids phacopids scutelluids and odontopleurids to date 49 taxa have been described 40 of these being restricted to the frasnian herein five frasnian taxa are described three in open nomenclature and two the new species telopeltis intermedia and otarion fugitivum evolutionary trends in the virgin hills trilobites are dominated by a reduction in body size and eye size and to a lesser extent a reduction in exoskeletal vaulting although recording no sedimentological signature the fauna was strongly affected by the two globally recognized kellwasser extinction events the first at the end of conodont zone 12 affected taxa at the species and genus level the second within zone 13b had a much greater impact on the fauna causing extinctions at the familial and ordinal levels evidence is presented to suggest that evolutionary trends in the trilobites during the late frasnian reflect selection for forms adapted to low nutrient conditions the two intensive kellwasser extinction episodes may reflect periodic massive inputs of nutrients from the terrestrial into the shallow marine environment

geometry and p and s velocity structure of the african anomaly 1 we constrain the geometry and p and s velocity structure of a low velocity anomaly in the lower mantle beneath southern africa we term it the african anomaly on the basis of forward traveltime and waveform modeling of seismic data sampling a great arc across the anomaly from the east pacific rise to the japan sea our collected data set consists of direct s direct p sdiff scs pcp sks and skks phases recorded by three temporary broadband passcal seismic arrays deployed in africa between 1994 and 2002 the tanzania seismic array 1994 1995 the kaapvaal seismic array 1997 1999 and the ethiopia kenya seismic array 2000 2002 for earthquakes occurring in the east pacific rise drake passage south sandwich islands iran hindu kush xinjiang and the japan sea the seismic data provide excellent sampling of the african anomaly in the lower mantle along the specific great arc in order to accurately account for the contributions from the african anomaly we relocate all the events using a global seismic shear velocity tomographic model and seismic data recorded by the global seismographic network and correct for the contributions from the seismic heterogeneities outside the african anomaly the seismic observations suggest that the african anomaly locally extends 1300 km above the core mantle boundary beneath southern africa around 25 n 27 e and exhibits a bell like geometry with both the southwestern and the northeastern flanks dipping toward its center with the lateral dimension of the anomaly increasing with depth the base is about 4000 km wide extending broadly in both the southwestward and the northeastward directions the seismic data can best be explained by a shear velocity structure with average velocity decreases of 5 in the base and 2 to 3 in the mid lower mantle above the base and a compressional velocity structure with a uniform s to p velocity perturbation ratio of 3 1 for the entire african anomaly these geometric and velocity features suggest that the mid lower mantle portion of the african anomaly is an integral component of the very low velocity province and the african anomaly is compositionally distinct and geologically stable

the depositional record of the frasnian famennian boundary interval in a fore reef succession canning basin western australia abstract the effects of the global frasnian famennian f f biotic crisis on the shallow marine reef builders and associated biota in a laterally extensive late frasnian reef complex on the lennard shelf northern canning basin are recorded in the well exposed fore reef succession napier formation of the napier range the conodont defined f f boundary lies within a distinctive and mappable stratigraphic interval up to 15 m thick which separates highly fossiliferous frasnian from markedly less fossiliferous famennian carbonate and siliciclastic lithofacies marked lateral facies variation within the interval means that the actual boundary is not represented by a discrete bed or surface the interval is characterised by a range of lithofacies that record very low sedimentation rates on the slope and reworking and bioturbation of slope strata indicating a condensed interval with negligible sediment input from contemporaneous carbonate platform production siliciclastic lithofacies and abundant sponge debris are locally developed macrofossil content is generally very low but there is no evidence for anoxic or hypoxic conditions during deposition at a range of palaeowater depths allochthonous blocks of stromatoporoid bearing limestone are absent from the uppermost frasnian fore reef strata and stromatoporoid debris is significantly reduced prior to the end of the frasnian this suggests that extinction of the stromatoporoid reef margins occurred before the f f boundary with microbial communities dominating the platform margins prior to the famennian sedimentological and biostratigraphic evidence suggests that a series of sea level fluctuations in response to basin tectonism generated recurring or ongoing environmental stress in the stromatoporoid dominated reefal communities because of recurring reductions in living space influxes of siliciclastic sediment supply from the proximal highlands and quite possibly generation of toxic too saline or too nutrient rich lagoonal water these conditions were potentially superimposed on a global regime of incipient cooling climate leading to widespread glaciation in the late carboniferous

δs at high pressure and dlnvs dlnvp in the lower mantle s a measure of the sensitivity of the adiabatic bulk modulus to isobaric density changes controls lateral variations of p wave velocities to estimate the effect of pressure on s we have performed lattice dynamic calculations of the cubic alkaline earth oxides we used a many body parametric hamiltonian that reproduces the available laboratory data on thermoelastic properties the calculations were carried out for p t conditions that correspond to those of the lower mantle we find that s decreases from 4 1 at standard conditions to 2 at lower mantle conditions the latter value is consistent with d l anderson s estimates based on geophysical data with s 2 and a corresponding parameter for shear modulus not decreasing with compression no partial melting is required to account for the observation that lnvs lnvp 2 in the lower mantle although more data and calculations on additional minerals are needed to draw any firm conclusions our results indicate that the relatively small lateral variations of lnvp when compared to lnvs are due entirely to compressional effects

biodiversity distribution and patterns of extinction of the last odontopleuroid trilobites during the devonian givetian frasnian biostratigraphical ranges and palaeogeographical distribution of mid givetian to end frasnian odontopleurids are investigated the discovery of leonaspis rhenohercynica sp nov in mid givetian strata extends this genus unexpectedly up to the late middle devonian new material of radiaspis radiata goldfuss 1843 and the first koneprusiine in britain koneprusia sp are described from the famous lummaton shell bed torquay devon new taxa of koneprusia k serrensis k aboussalamae k brevispina and k sp a and k sp b are defined ceratocephala leonaspis harborti richter richter 1926 is revised and reassigned to gondwanaspis feist 2002 two new species of gondwanaspis g dracula and g spinosa plus three others left in open nomenclature are described from the late frasnian of western australia a further species of gondwanaspis g prisca is described from the early frasnian of montagne noire species of gondwanaspis are shown to possess a number of paedomorphic features a functional analysis suggests that unlike other odontopleurids gondwanaspis actively fed and rested with the same cephalic orientation the sole odontopleurid survivors of the severe terminal mid givetian biocrisis taghanic event belong to the koneprusiine koneprusia in the late givetian and frasnian and of cryptogenic origin the acidaspidine gondwanaspis in the frasnian whereas the former became extinct in the late frasnian at the lower kellwasser event the latter disappeared and with it the entire odontopleuroidea at the terminal frasnian upper kellwasser global biocrisis

complex deep structure of the african low velocity zone abstract we use recently deployed seismological arrays in africa to sample a 2d cross section through the mantle down to the core mantle boundary cmb by making use of travel time residuals of s scs and sks phases a new shear velocity model of the african low velocity zone alvz is derived our model suggests between 1 2 shear velocity reduction at the top and 5 at the bottom with respect to 1d reference models the average reduction over the whole low velocity zone lvz amounts to 2 in the presented model and is therefore about twice as strong as values found in global tomographic models the top of the lvz reaches up to 1200 km depth and its lateral extent at the cmb is about 35 we propose the existence of a gap of 300 km splitting the structure into two blocks our results are based on remarkable differences in sk k s travel time residuals over a few degrees distance the complexity of the structure could provide a key to an improved understanding of the deep mantle lvz dynamics and composition by comparison to geodynamic models the gap in the model might suggest that the 2d cross section is cutting through a 3d indentation in the boundary of the alvz but may also be interpreted as a sign of two individual plumes rather than one large homogeneous upwelling electronic supplement figures of preprocessed original waveforms aligned with the synthetics for the corresponding event and tables of applied corrections

timing and patterns of the frasnian famennian event evidences from high resolution conodont biostratigraphy and event stratigraphy at the yangdi section guangxi south china abstract continuous centimeter scale sampling for conodonts with microfacies and carbon isotope analysis has been carried out across the frasnian famennian f f transition at the yangdi section guangxi south china the newly revised conodont biostratigraphy reveals a succession of six zones ranging from the upper palmatolepis rhenana zone to lower palmatolepis crepida zone in ascending order based on a detailed analysis of conodont abundance and diversity changes through the section two intervals involving significant conodont faunal changes were recognized in the uppermost palmatolepis linguiformis zone at the yangdi section the first conodont change occurred at the transition between the lower and upper pa linguiformis zone and is marked by the disappearance of pa linguiformis accompanied by the successive disappearances of pa eureka pa rhenana pa nasuta pa gigas ad ioides and ad nodosa the second conodont change appeared in a horizon equivalent to the uppermost upper kellwasser horizon ukh and is indicated by the elimination of nearly all frasnian palmatolepids ancyrodellids and polygnathids furthermore the second conodont change is coincident with the decimation of ostracods and tentaculitids and is synchronous with the turnovers of conodont biofacies in addition the onset of the 13 c carb perturbation is a little earlier than the second conodont change with the maximum amplitude of excursion right at the start of the famennian therefore we conclude that the second conodont change represents the f f mass extinction based on the temporal correlation among 13 c carb excursions pelagic faunal changes and conodont biofacies turnovers during the f f boundary interval according to the high resolution conodont biostratigraphy together with cyclostratigraphy the uppermost ukh and whole ukh contain 1 and 12 bundle cycles respectively thus the time span of the mass extinction and the entire upper kellwasser crisis lasted 16 18 kyr and 200 kyr respectively

joint seismic tomography for bulk sound and shear wave speed in the earth s mantle high quality p and s travel times are now available from careful reprocessing of data reported to international agencies a restricted data set has been extracted for which comparable ray coverage is achieved for p and s and used for a joint inversion to produce a three dimensional model for shear and bulk sound velocities represented in terms of 2 2 cells and 18 layers in depth through the mantle about 106 times for each of p and s are combined to produce 312 549 summary rays for each wave type linearizing about the ak135 reference model 583 200 coupled tomographic equations are solved using an iterative partitioned scheme clear high resolution images are obtained for both bulk sound speed and shear wavespeed the bulk and shear moduli have differing sensitivity to temperature and mineral composition and so the images of the two velocity distributions help to constrain the nature of the processes which produce the variations different heterogeneity regimes can be recognised in the upper mantle the transition zone most of the lower mantle and the lowermost mantle in the upper mantle many features can be explained by thermal effects but in some orogenic zones e g western north america the opposite sense of the bulk sound and shear wave speed variation requires compositional effects or volatiles to outweigh any thermal effects in the lower mantle pronounced narrow structures which may represent remnant subduction are most marked in shear the level of large scale variations in bulk sound speed compared to shear diminishes with depth in the lower mantle reaching a minimum near 2000 km below this depth the variability of both wave speeds increases near the core mantle boundary the variations of the two wave speeds show little concordance suggesting the presence of widespread chemical heterogeneity

partial melting in the deepest mantle 1 we have compared the travel times of direct p s and core reflected pcp scs phases recorded in southern africa from earthquakes occurring in the scotia arc region the low velocity region beneath the south atlantic is explored by calculating scs s and pcp p travel time residuals to determine the correlation and magnitude of vs and vp reductions in the deepest mantle residual calculations yield extraordinary results as pcp p is about 1 second larger than expected while scs s is 7 seconds larger for a great circle arc distance of 54 we investigate models of p and s velocity reductions in the deepest mantle that may explain these residuals to provide constraints on the velocity structure of the d region in this extremely slow zone the reductions in p and s velocity needed to match the data are consistent with partial melting in the deepest mantle for a wide range of 1d models

lithospheric shear zones and mantle crust connections abstract a crustal scale ductile shear zone network in the precambrian granulite facies crust of madagascar is examined to determine the nature of the connections between the mantle and lower crust based on three independent data sets field and satellite mapping c and o isotope geochemistry and gravimetry this crust is divided into three zones 1 outside of shear zones 2 minor shear zones that are 350 km long up to 1000 km and 20 35 km wide the mantle is uplifted by about 10 km beneath the major shear zones the major shear zones are rooted in and are inferred to be controlled by the mantle they directly tapped mantle derived co 2 the small scale minor shear zones were controlled by crustal processes and focused crustally derived h 2 o rich co 2 fluids the regular distribution of the shear zones on a crustal scale is in agreement with models of buckling of the continental lithosphere in a compressional context the propagation of these mechanical instabilities promoted and channelled fluid flow these major pan african shear zones thinned the crust and were reactivated during the subsequent drifting of madagascar and opening of the indian ocean during jurassic to cretaceous times they also controlled many of the brittle fault zones in the overlying sedimentary basins mantle rooted large scale shear zones are inferred to be a general feature of cratonic areas reactivated by shear zone systems

zircon geochronology of anatectic melts and residues from a highgrade pelitic assemblage at ihosy southern madagascar evidence for pan african granulite metamorphism we report u pb and 207 pb 206 pb zircon ages for a granulite facies gneiss assemblage exposed in a large quarry at ihosy southern madagascar the granulites are derived from pelitic to arkosic sediments and attained equilibrium conditions at 650 700 c and 4 5 kbar higher p t conditions of 750 800 c and 6 kbar in the presence of low water activities have led to dehydration melting processes the formation of granitic melts which partly moved away from their source region intruded into upper parts of the metapelitic gneisses as small granitic veins and left behind granulitic garnet cordierite quartz bearing rocks detrital zircons in a sample of metapelite and a sample of quartzofeldspathic gneiss yielded ages between 720 and 1855 ma suggesting a chronologically heterogeneous source region and a depositional age of less than 720 ma for these rocks high grade metamorphism and anatexis are documented by zircon ages between 526 34 and 557 2 ma with a mean age of about 550 ma the broad lithologies metamorphic grades and ages recorded in the ihosy rocks are similar to those in the wanni complex of northwestern sri lanka and in high grade assemblages of southernmost india and support the contention that all these terrains were part of the mozambique belt which formed as a result of collision of east and west gondwana in latest precambrian time

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

probing an ultra low velocity zone at the core mantle boundary with p and s waves recent studies of the core mantle boundary cmb have revealed some very anomalous structures interpreted in terms of ultra low velocity zones ulvz however there remains considerable uncertainties about their physical descriptions or even if they occur above or below the cmb they have only been detected in isolated situations using rather special techniques these includes distortions in sks with the development of skpds and spdks broadband pkp precursors distinct scs and s beyond 100 degree and rapid changes in differential travel times of neighboring phases here we report on a situation where ray paths associated with pkp precursors and skpds sample the same ulvz structure the structure lies beneath central africa and has been detected from wwssn analog data skpds discussed previously this data set has been enhanced with a collection of digital records sampling an elongated north south zone roughly 800 km long the entire skpds data set can be modeled with a ridge shaped cross section with widths of 250 to 400 km and drops in p and s velocity of 10 and 30 percent fortunately a new iris station msku located in western africa provided excellent pkp data from the new britain region events sampling the above structure the pkp and strong precursors can be modeled by 2d synthetics generated from the same structure used in modeling skp ds which provides a strong constraint on the definition characteristics of this particular ulvz

three dimensional structure at the base of the mantle beneath the central pacific forward modeling of differential travel times of phases sensitive to lowermost mantle beneath the central pacific reveals lateral heterogeneity that is higher in amplitude than predicted by tomographic models a broad zone of low s velocity 4 percent with respect to standard models which may correspond to the base of a thermal plume narrows and is deflected as it extends to about 1000 kilometers above the core mantle boundary to the east of this zone a localized region of fast s velocity 5 percent suggests strong heterogeneity or anisotropy related to the presence of high pressure and temperature assemblages which may or may not involve core material its presence could also explain the observation of precursors to core reflected phases in this region

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

metamorphism and tectonics in southern madagascar an overview the scope of this paper is to briefly summarize the general tectonic pattern and the metamorphic evolution of the continental crust from southern madagascar after a presentation of the main geophysical features of the malagasy crust the brittle and the ductile strain patterns are established at a crustal scale the pan african metamorphic zonation is discussed and interpreted through a tectonic model related to a transpressive regime

constraints on d structure using pkp ab df pkp bc df and pcp p traveltime data from broad band records summary the effects of complex structure in the deep mantle and d on pkp differential traveltimes should be estimated accurately in order to reach reliable conclusions concerning the physical and chemical properties of the earth s inner and outer core in particular it is important to assess how much of the data can be explained by mantle structure alone for this purpose we have assembled global data sets of high quality pkp ab df pkp bc df and pcp p differential traveltimes measured on mostly broad band records the pkp ab df data were inverted alone or jointly with pcp p data to retrieve p velocity maps of the lowermost 300 km of the mantle corrections for mantle structure above d were performed prior to inversion using recent tomographic models and the fit to the pkp bc df data set was used to constrain damping in the inversions we compare models obtained with and without polar pkp paths and find that their inclusion or exclusion does not significantly affect the resulting d model except under north america where coverage is poor without polar paths our preferred model obtained using pkp ab df and pcp p data combined explains over 80 per cent of the variance in pkp ab df almost 60 per cent of the variance in pcp p and 27 per cent of the variance in pkp bc df a significant portion considering that the pkp bc df data set was not used in the inversion our models are characterized by prominent fast features under mid america and east asia a fast belt across the pacific a slow region under the southwestern pacific and southern africa as well as sharp transitions from fast to slow for instance under alaska and the south atlantic the anomalous south sandwich to alaska data cannot fully be explained by d structure alone unless very short wavelength lateral vari

oxygen isotope systematics of gem corundum deposits in madagascar relevance for their geological origin the oxygen isotopic composition of gem corundum was measured from 22 deposits and occurrences in madagascar to provide a gemstone geological identification and characterization primary corundum deposits in madagascar are hosted in magmatic syenite and alkali basalt and metamorphic rocks gneiss cordieritite mafic and ultramafic rocks marble and calc silicate rocks in both domains the circulation of fluids especially along shear zones for metamorphic deposits provoked in situ transformation of the corundum host rocks with the formation of metasomatites such as phlogopite sakenite and corundumite secondary deposits placers are the most important economically and are contained in detrital basins and karsts the oxygen isotopic ratios 18o 16o of ruby and sapphire from primary deposits are a good indicator of their geological origin and reveal a wide range of 18o vienna standard mean ocean water between 1 3 and 15 6 metamorphic rubies are defined by two groups of 18o values in the range of 1 7 to 2 9 cordieritite and 3 8 to 6 1 amphibolite magmatic rubies from pyroxenitic xenoliths contained in the alkali basalt of soamiakatra have 18o values ranging between 1 3 and 4 7 sapphires are classified into two main groups with 18o in the range of 4 7 to 9 0 pyroxenite and feldspathic gneiss and 10 7 to 15 6 skarn in marble from andranondambo the 18o values for gem corundum from secondary deposits have a wide spread between 0 3 and 16 5 the ruby and sapphire found in placers linked to alkali basalt environments in the northern and central regions of madagascar have consistent 18o values between 3 5 and 6 9 ruby from the placers of vatomandry and andilamena has 18o values of 5 9 and between 0 5 and 4 0 respectively the placers of the ilakaka area are characterized by a huge variety of colored sapphires and rubies with 18o values between 0 3 and 16 5 and their origin is debated a comparison with oxygen isotope data obtained on gem corundum from eastern africa india and sri lanka is presented giant placer deposits from sri lanka madagascar and tanzania have a large variety of colored sapphires and rubies with a large variation in 18o due to mingling of corundum of different origin mafic and ultramafic rocks for ruby desilicated pegmatites for blue sapphire syenite for yellow green and blue sapphire and skarn in marbles for blue sapphire

using moma broadband array scs s data to image smaller scale structures at the base of the mantle scs s residuals obtained at stations of the missouri to massachusetts moma temporary broadband seismic array are used to delineate variations in seismic velocity structure above the core mantle boundary cmb at scales smaller than observable with tomographic models south american earthquakes recorded at moma reveal a slow velocity anomaly that is at least as small as the limit of the resolution of scs waves about 300 km across this is modeled as being within a region of fast velocities in whole mantle models the slow scs s residuals correlate well with a peak in scs s relative amplitudes the small region of slow shear velocity at the cmb could be a pocket of lower mantle rock trapped beneath the descending farallon slab or evidence of chemical boundary layer variations

further constraints on the african superplume structure it is well established that there is a large scale low velocity structure in the lowermost mantle beneath africa extending from the southeastern atlantic ocean to the southwestern indian ocean with a volume greater than 10 billion km 3 7000 km long 1000 km across and 1200 km high earth planet sci lett 206 2003 119 this low velocity structure is often called the african superplume various studies also require sharp boundaries for the plume however as for its height and shear velocity reduction there has been some controversy especially concerning the velocities at the core mantle boundary cmb here we present an assortment of phases involving s diff sks s and s c s with both vertical and horizontal paths sampling a 2d corridor through the structure travel time and waveform modeling of these seismic phases argues for a model with shear velocity reduction of approximately 3 within the superplume which is basically a 200 km thick layer low velocity layer beneath the southern atlantic ocean and a 1200 km high structure beneath south africa and against a model of a substantially reduced low velocity layer up to 10 300 km beneath the superplume we also analyzed p diff and the differential times of p c p p and compared them with sdiff and s c s s observations along the same great circle paths the p velocity is not very anomalous at most 0 5 much smaller than 1 as expected from a thermal anomaly with 3 lower s velocity geophys res lett 27 2000 421 thus again arguing for a chemical origin which was suggested from the modeling of african superplume sharp sides science 296 2002 1850

rare earth elements distribution in granulite facies marbles a witness of fluid rock interaction abstract the rare earth element ree distribution of marbles from tranomaro se madagascar shows that the marbles have interacted with syn metamorphic fluids during the panafrican granulite facies metamorphism the tranomaro area is characterized by an extensive development of pyroxenites skarns variably mineralized in uranothorianite and enriched in ree zr across a meter scale marble pyroxenite contact the ree content increases from the most remote marble sample to the contact with the pyroxenite la from 19 8 to 129 ppm ree patterns in enriched marbles display a strong negative eu anomaly similar to those of pyroxenites this peculiar type of ree distribution results from fluid infiltration on a regional scale some marbles have similar ree characteristics progressive development of eu anomaly together with ree enrichment showing that some of them have interacted with fluids infiltration was hardly recognizable using c and o isotopic signatures because large isotopic variations were introduced during the pre granulitic history then ree distribution in high grade marble may be helpful in monitoring syn metamorphic fluid flow in the present case it is a more reliable tracer of fluid infiltration than stable isotopic compositions

the core mantle boundary under the gulf of alaska no ulvz for shear waves the earth s core mantle boundary cmb marks the boundary between the hot molten iron core and the silicate mantle and is a thermal chemical and flow boundary previous observations of very slow compressional wavespeeds suggest that thin ultra low velocity zones ulvzs possibly composed of a mixture of molten iron and silicates exist at the base of the mantle a molten or partially molten layer would cause a large shear wavespeed decrease however this velocity drop has not been observed here we use core reflected scp phases to investigate the shear properties of ulvzs these phases reveal at least two distinct regions one region under central america which is distinctly average prem and iasp91 like and a second region under the entire gulf of alaska that produces large scp reflections with amplitudes of up to 30 times larger than calculated with average earth models the large amplitudes suggest a combination of focusing by cmb topography high shear wavespeeds at the bottom of the mantle and low attenuation high q along the scp path high shear wavespeeds and low attenuation are opposite from what would be expected from a shear wave ulvz a ulvz in compressional wavespeeds ultra low vp has previously been observed in this region however in addition to the large amplitudes the short period scp waveforms show no complexity that can be related to a ulvz thus either there is not a ulvz under the gulf of alaska or if it does exist it is restricted to compressional wavespeed changes precluding interpretation as partial melt but rather suggesting a chemical origin fl 2000 published by elsevier science b v all rights reserved

core mantle boundary topography as a possible constraint on lower mantle chemistry and dynamics article i nfo the origin of large low shear wave velocity provinces llsvps in the lowermost mantle beneath the central pacific and africa is not well constrained we explore numerical convection calculations for two proposed hypotheses for these anomalies namely thermal upwellings e g plume clusters and large intrinsically dense piles of mantle material e g thermochemical piles each of which uniquely affects the topography on earth s core mantle boundary cmb the thermochemical pile models predict a relatively flat but elevated cmb beneath piles presumed llsvps with strong upwarping along llsvp margins the plume cluster models predict cmb upwarping beneath upwellings that are less geographically organized both models display cmb depressions beneath subduction related downwelling while each of the two models produces a unique characteristic style of cmb topography we find that seismic models will require shorter length scales than are currently being employed in order to distinguish between the end member dynamic models presented here

normal mode and free air gravity constraints on lateral variations in velocity and density of earth s mantle with the use of a large collection of free oscillation data and additional constraints imposed by the free air gravity anomaly lateral variations in shear velocity compressional velocity and density within the mantle dynamic topography on the free surface and topography on the 660 km discontinuity and the core mantle boundary were determined the velocity models are consistent with existing models based on travel time and waveform inversions in the lowermost mantle near the core mantle boundary denser than average material is found beneath regions of upwellings centered on the pacific ocean and africa that are characterized by slow shear velocities these anomalies suggest the existence of compositional heterogeneity near the core mantle boundary

formation of steep sided topography from compositionally distinct dense material at the base of the mantle 1 in this study we use 3 d two layer thermochemical convection experiments to explore the development of a thin compositionally distinct dense layer at the base of the mantle we investigate the conditions under which steep sided flat topped topography will form on an initially ubiquitous layer as a result of the convecting mantle previous authors found that discrete steep sided piles of material form when compressibility temperature dependent viscosity or a large viscosity increase in the lower layer is assumed we observe however that this topographic style develops in calculations of incompressible convection with layers of constant viscosity experiments involving simple convection planforms are used to systematically investigate the factors which encourage or inhibit this topographic style the shape of the topography that develops on the density interface can be linked to the convection coupling style between the two layers viscous coupling leads to discrete steep sided piles and thermal coupling to a ubiquitous layer with cusp like peaks the dependence of the topographic style on the rayleigh number density ratio and volume ratio can largely be explained by considering their effect on the convection coupling style consequently we find that it is crucial for the lower layer to be thin compared with the overlying convection wavelength and have a density contrast of the order 1 in order for discrete steep sided piles to develop

whole earth tomography from delay times of p pcp and pkp phases lateral heterogeneities in the outer core or radial anisotropy in the mantle we perform tomographic inversions of compressional wave travel time measurements in order to image the laterally varying radial anisotropy of p wave velocity in the entire mantle in order to achieve an adequate sensitivity to both horizontal and vertical p velocity throughout the mantle we include in our data set travel time measurements corresponding to ray paths of very different geometries specifically we use data associated with the phases p pcp core reflected and pkpbc pkp df core refracted all extracted from the bulletins of the international seismological centre 1964 1993 pcp pkpbc and pkpdf data are also sensitive to the topography of the core mantle boundary cmb and pkp only to the velocity structure of the earth s core these circumstances require that we also allow for cmb topographic anomalies as free parameters in our inversions and investigate the possibility of the existence of lateral heterogeneities in the outer core e g ritzwoller et al 1986 stevenson 1987 wahr and de vries 1989 vasco and johnson 1998 the solution models that we propose include alternatively a laterally heterogeneous outer core a radially anisotropic mantle or both so that trade offs between those features can also be evaluated we conclude that only minor and very localized radially anisotropic anomalies are present in the middle and lower mantle the most prominent being located in the deepest 400 km under the western pacific we find a discrepancy particularly evident by a comparison between different models of the cmb topography between the travel time measurements that are sensitive to the earth s core and those that are not we show that it is not possible to explain this discrepancy only in terms of radial anisotropy of the mantle and or lateral heterogeneity of the outer core

middle devonian trilobites of the saoura valley algeria insights into their biodiversity and moroccan affinities trilobites are important elements of the devonian macrobenthos some of them were collected in the chefar el ahmar formation from two sections located near beni abbes in the saoura valley ougarta basin saharan algeria this formation is characterized by alternations of claystones and limestones and it is considered to be late emsian to early frasnian in age only the lower part of this formation has yielded trilobites so far their presence has been known for a long time phacopines clearly dominate the trilobite assemblages with austerops barrandeops chotecops and phacops s l as the main genera two new species are described austerops salamandaroides sp nov and phacops ouarouroutensis sp nov while some other taxa are presented in open nomenclature comparisons are made with closely allied species these new trilobite occurrences have been analysed in terms of their intra and interspecific variability and biodiversity the occurrence of struveaspis maroccanica previously known from the saoura valley provides an early eifelian age which is also confirmed by the presence of trilobites thysanopeltis and koneprusites and ostracods bairdiocypris devonica and bufina subovalis

simultaneous inversion for 3 d variations in shear and bulk velocity in the mantle until recently most of the seismic tomographic modeling has been addressing the question of lateral heterogeneity either in p or s wave velocities the s wave velocity variations are larger and hence provide stronger signal on long period waveforms the direct p travel times being the first arrivals on the other hand are most frequently reported in the international seismological centre isc bulletins in mineral physics experiments the variation in bulk velocity is more often measured to better understand the differences between vp and vp patterns and better link the results from mineral physics to those of seismic tomography we formulate the inverse problem in terms of relative perturbations in the shear velocity vs 12 and bulk sound velocity v k 12 we use a large data set which consists of waveforms waveform derived travel times and travel times from the isc bulletins the earthquakes are relocated using corrections for lateral heterogeneity the events which cannot be reliably determined are discarded the model is defined as spherical harmonics to degree 12 horizontally and as chebyshev polynomials to order 13 radially for both shear and bulk sound velocity the inversion is performed under smoothness constraint the resolution tests and bootstrapping analysis indicate that the model is well recovered particularly at long wavelength the results indicate a much larger variability of shear than bulk sound velocity the model explains observations well the most intriguing result obtained in this study is that the variations in shear velocity and bulk sound velocity are negatively correlated in the lowermost mantle the explanation is not very clear from the mineral physics point of view it is not unlikely that this could be explained in terms of thermal variation even though we are unwilling to rule out the possibility of large wavelength compositional variations

palaeoenvironmental and ecological interpretation of the trace fossil rhizocorallium based on contained iron framoboids upper devonian south china the trace fossil rhizocorallium commune is abundant in shallow marine deposits of the upper devonian frasnian tuqiaozi formation in ganxi sichuan province south china it consists of u shaped spreite burrows developed more or less parallel or slightly inclined to the bedding of marly limestone four types of iron framboids mostly oxygenated from pyrite framboids were found within the rhizocorallium burrow including 1 smooth sheathed spherical 2 prismatic 3 octahedral and 4 pyritohedral submicron crystals chemically the submicron crystals mainly consist of iron sulphur oxygen and carbon the iron framboids represent the stage of evolution of their original pyrite framboids from irregular morphologies and spherical framboids to octahedral and pyritohedral habits due to the activity of sulphate reducing bacteria no similar framboids were observed outside rhizocorallium therefore it is likely that sulphate reducing bacteria colonised the rhizocorallium spreite and marginal tube within a dysoxic environment with increasing degree of oxygenation of pore water the pyrite framboids were partly oxygenated into iron framboids and by exceptional circumstances preserved the original morphology of the crystals furthermore it is proposed that the investigated rhizocorallium was constructed by its trace maker for gardening with multifunctional purpose where different microbial colonies were planted and cultured in the marginal tube and spreite on the mutual basis of food supply and redox conditions these features indicate that a combined deposit feeding and gardening model may apply for the construction of rhizocorallium commune from the devonian of ganxi the responsible trace maker probably was a worm like organism such as a polychaete

p and pcp travel time tomography for the core mantle boundary pcp arrival times reported to the international seismological centre isc are inverted for vertical travel time anomaly at the core mantle boundary cmb we first invert about 2 106 isc first p arrival data for the three dimensional 3 d structure of the mantle and hypocenter locations iteratively the pcp data are corrected for the relocated hypocenters and the 3 d structure this correction has reduced the observed pcp residuals the corrected pcp data are then inverted for the vertical travel time anomalies at the cmb which are due either to velocity anomaly or topographic anomaly the spherical average of the inverted structure requires either a velocity reduction in the boundary layer just above the cmb by 3 if the layer is 20 km thick or a lowering of the cmb by 3 km the heterogeneity pattern of the cmb is drastically different from that of the overlying d layer negative vertical travel time anomalies appear in the northern high latitude region and positive anomalies appear under southeast asia and middle to south america the lateral dimensions of these anomalies are of the order of 40 the anomalies can be interpreted as either a velocity perturbation in the boundary layer 7 if the layer is 20 km thick or an undulation of the cmb 7 km various tests including the error and resolution estimations indicate that these three anomalies are the significant resolvable features of the cmb these anomalies are also corroborated by the result of the joint inversion of p and pcp for both the 3 d mantle and the 2 d cmb assuming that the layer is laterally heterogeneous in iron content and deforms at the base in isostatic equilibrium with the outer core the pcp data are also inverted for the isostatic figure of the boundary layer the velocity perturbation in this case is 5 6 if the layer is 20 km thick and the cmb undulation is only 1 2 km regardless whether the cmb heterogeneity is due to velocity and or topographic perturbations it is dominant in the zonal component of degree 2 while the degree 2 pattern in the lower mantle including the d is dominant in sectorial component

a forest fire and soil erosion event during the late devonian mass extinction abstract the late devonian mass extinction occurred in a stepwise manner and culminated close to the frasnian famennian f f boundary 372 million years ago organic molecular indices from marine sedimentary rocks at the sinsin section belgium indicate that the sequence of combustion of land vegetation soil erosion and anoxia euxinia occurred close to this boundary the increased concentrations of biomarkers indicating forest fire and soil erosion measured in the sinsin section suggest that fire became widespread at this time leading to various damaging consequences increased runoff and oceanic anoxia that caused marine extinctions magnetic susceptibility data in the sinsin section indicate a relatively dry climate spanning the f f boundary which would have encouraged forest fires the study of organic biomarkers presents several lines of evidence to link forest fire and soil erosion to the late devonian mass extinction

whole mantle sh velocity model constrained by waveform inversion based on three dimensional born kernels summary a whole mantle sh velocity model is obtained by using a unique data set and techniques body and surface waveforms including major and multi orbit phases are used as a data set and are inverted by using 3 d born kernels the resultant model sh18ce reveals the different natures of the two major upwelling systems the strong low velocity anomalies beneath africa extend for more than 1000 km from the core mantle boundary cmb whereas those beneath the pacific are restricted to 300 400 km from the cmb the results also show the variable natures of stagnant slabs on the 670 discontinuity around japan the depths of the strongest high velocity anomalies within the stagnant slabs are different region by region which is consistent with the detailed delay time tomography model in this area

first occurrence of a phacopid trilobite faunule from the upper devonian of saoura valley algeria and biodiversity fluctuations phacopid trilobites are relatively widespread in devonian deposits of north gondwana and some have been collected from several sections of the saoura valley in sw algeria new occurrences of phacopids assigned to trimerocephalus dianops and phacops sensu lato are described from this area and comparisons are made with closely allied species from morocco and europe the trilobite assemblagesofthe argilesdemarhouma formationareconsideredtobefrasnian famennianinage thesenewoccurrenceshavebeenintegratedintoananalysisofupperdevonianphacopidbiodiversity diversity fluctuations reflect environmental changes bioevents and stratigraphic turnovers throughout the upper devonian peak diversity was attained after the post kellwasser event

distribution patterns of upper devonian phacopid trilobites paleobiogeographical and paleoenvironmental significance phacopid trilobites are relatively widespread in devonian deposits after a maximum diversity in the middle devonian most phacopids were severely affected by repeated sea level eustatic changes especially in the frasnian and it was during that time that the main groups of reduced eye or blind phacopids arose the aim of this investigation was to evaluate the distribution patterns of upper devonian phacopids at a global level through a clustering analysis in a paleogeographical and paleoenvironmental framework in order to understand their fluctuations in time and space the phacopid biodiversity is represented in this study by 22 taxa from the frasnian to the late famennian additionally four main time intervals frasnian i famennian post i famennian ii iii and famennian iv vi have been explored to complete the clustering analysis two ordering methods i e a seriation routine and a detrended correspondence analysis have been performed respectively to reveal chronological sequences and to identify indirect paleoenvironmental gradients analyses enabled us to establish 1 bathymetrical gradients showing some taxa such as omegops restricted to shallower environments and other taxa such as ductina or dianops to deeper environments and 2 paleobiogeographical affinities showing exchanges along the margins of north gondwana and avalonia baltica and probable connections along the margin of north east gondwana after the major mid givetian taghanic transgression

patterns of extinction and recovery of phacopid trilobites during the frasnian famennian late devonian mass extinction event canning basin western australia a diverse fauna of phacopid trilobites is described from the late devonian middle frasnian to early famennian of the northern canning basin western australia one new genus and four species in two genera are described from zones 11 13a and 13b of the middle and late frasnian trimerocephaloides sinevisus gen nov and sp nov t linguiformis sp nov acuticryphops acuticeps kayser 1889 and a klapperi sp nov late frasnian phacopines are either blind as shown for the first time in trimerocephaloides sinevisus or show trends to decreasing eye size up to the frasnian famennian kellwasser mass extinction event this evolutionary trend in acuticryphops is demonstrated to have been global at this time one new genus and six species of early famennian phacopids are described from the upper triangularis crepida and rhomboidea zones houseops gen nov with the new taxa h canningensis sp nov h beckeri sp nov and h sp a babinops planiventer feist becker 1997 b minor sp nov trimerocephalus tardispinosus feist becker 1997 and t mimbi sp nov in contrast to european sections where exclusively blind phacopids are known in earliest famennian sites initial recovery following the mass extinction event in canning peri reefal environments is characterized by oculated forms these trilobites must have evolved from conservative ancestors with normal eyes that had succeeded in surviving the kellwasser biocrises in reef related shallow water niches thus the origin of post event phacopids from shallow water environments is demonstrated for the first time descendant lineages show increasing eye size increased cephalic vaulting and effacement during the early famennian of the five orders of trilobites that are present during the frasnian stage at the beginning of the late devonian only two the proetida and the phacopida survived into the famennian the presence of a conformable sequence of trilobite bearing fore reef limestones the virgin hills formation that form part of the late devonian reef system in the northern part of the canning basin in western australia allows the patterns of evolution and extinction of the late frasnian and early famennian trilobites to be assessed in these deposits the three orders that became extinct during the frasnian famennian biocrises were the corynexochida mcnamara feist 2006 the lichida feist mcnamara 2007 and the harpetida mcnamara feist ebach in press here we focus on one of the groups that survived this event the phacopida although with a single exception they are rare elements of the trilobite fauna in the canning basin the phacopids are generically the most diverse of the

horizontal transition from fast to slow structures at the core mantle boundary south atlantic recent tomographic studies of the deep mantle have revealed large scale low velocity zones beneath africa and relatively fast velocity zones beneath south america here we conduct a concentrated core mantle boundary cmb study across this transition zone from fast to slow beneath the south atlantic deep south american earthquakes recorded in africa provide s and scs waveform data at core grazing distances of 85 105 the waveform interference pattern produced at these ranges provides an excellent sample of the cmb structure in the vicinity of the scs bounce point which can be used to map out regions of low velocity zones 10 reductions the strongest anomalies occur at the western edge roughly beneath the tristat plateau with weaker structures to the east beneath the so called great africa plume these results agree with dynamic models where strong local upwellings develop at the edges of d fast structures

compressible thermochemical convection and application to lower mantle structures a new finite element code for compressible thermochemical convection is developed to study the stability of a chemical layer at the base of the mantle using composition dependent compressibility and a density difference between compositions at a reference pressure a composition dependent density profile is derived together with depth dependent thermal expansion this combination of parameters yields a wide range of dynamic evolutions for the chemical layer the chemical structures are classified into five major categories classical plumes mushroom shaped plume domes ridges and continuous layers and a few abnormal cases such as hourglass shaped plumes and columnar plumes several models have a chemical structure morphologically similar to the african low v s structure in the lower mantle at least at a single time guided by our models several dynamic scenarios are proposed for the dynamic nature of the lower mantle low velocity structures a k a superplumes including plumes at an early stage plume clusters ridges passive piles sluggish domes and high bulk modulus domes we predict seismic velocity anomalies from these dynamic models the thermoelastic parameters used in the conversion are additional constraints we compare the density structure with normal mode inversion the predicted seismic signature observations and the required thermoelastic parameters with mineral physics data among the proposed scenarios only the scenario of high bulk modulus domes satisfies all constraints simultaneously the implication on the geochemistry and mineralogy of lower mantle chemical structures is discussed

short scale heterogeneity in the lowermost mantle insights from pcp p and scs s data we compare lateral variations at the base of the mantle as inferred from a global dataset of pcp p travel time residuals measured on broadband records and existing p and s tomographic velocity models as well as scs s travel time data in some selected regions in many regions the pcp p dataset implies short scale lateral variations that are not resolved by global tomographic models except under eastern eurasia where data and models describe a broad region of fast velocity anomalies across which variations appear to be of thermal origin in other regions such as central america and southeastern africa correlated short scale lateral variations several hundred kilometers are observed in pcp and scs implying large but not excessive values for the ratio r d ln vs d ln vp v2 5 on the other hand in at least two instances in the heart of the african plume and on the edge of the pacific plume variations in p and s velocities appear to be incompatible implying strong lateral gradients across compositionally different domains possibly also involving topography on the core mantle boundary one should be cautious in estimating r at the base of the mantle from global datasets as different smoothing and sampling of p and s datasets may result in strong biases and meaningless results 7 2002 elsevier science b v all rights reserved

constructing synthetics from deep earth tomographic models recent studies of deep mantle structure indicate strong heterogeneity to conduct high resolution waveform modelling of these structures we have developed a new method to construct 2 d synthetics directly from block style tomographic models unlike the wkbj approximation which utilizes rays overshooting and undershooting receivers our method wkm approximation uses rays that arrive at the receiver first the ray paths from the 1 d layered reference model are used to localize each ray segment where the anomalous velocities are applied by overlay as in tomography next new p i t i p i ray parameter ti traveltime are computed to satisfy snell s law along with their numerical derivative p t which is used to construct a synthetic seismogram similar to the wkbj method as a demonstration of the usefulness of this method we generated wkm synthetics for the d region of high velocities beneath central america based on grand s tomography model reasonable fits to broad band data are obtained by condensing his distributed anomalies into his lowermost mantle layer such a 2 d model predicts synthetics containing a laterally varying s cd triplication similar to observations

a new paradigm for earth s core mantle boundary understanding the boundary between earth9s rock mantle and the molten core is one of the key challenges in geoscience in his perspective garnero discusses recent geophysical discoveries that are forcing the adoption of a new paradigm for studying the core mantle boundary long thought to be a simple division between solid silicate rock and liquid iron alloy the core mantle boundary is now emerging as an active complex and heterogeneous region of earth9s interior

l eclogite de faratsiho madagascar un cas exceptionnel de metamorphisme de haute p basse t au proterozoique superieur abstract a low t proterozoic eclogite group c from madagascar was sampled the rock contains omphacite garnet amphiboles paragonite clinozoisite epidote quartz rutile pyrite and sphene textural and chemical variations of five successive generations of amphiboles permit deduction of the p t path of this rock the conditions of the possible final portion of the prograde p t path are estimated using garnet omphacite pairs pmin 9 kbar at 420 c mineral cores and pmin 11 kbar at 500 c mineral rims this rock is similar to alpine eclogites and precludes a significant secular decrease in the subduction geotherm since at least the proterozoic the scarcity of group c eclogites and blueschists in the precambrian is related to the difficulties of preservation of such rocks rather than the absence of an appropriate geothermal gradient low t high p rocks have been either heated if they resided in the lower crust or eroded if they had been rapidly uplifted

granulite microfabrics and deformation mechanisms in southern madagascar optical microstructures and crystallographic preferred orientations were studied in naturally deformed granulite to high amphibolite facies quartzo feldspathic rocks in southern madagascar the microstructures of coarse grained granulite suggest that feldspar and quartz accommodated deformation by both dislocation and diffusion creep in the absence of melt the extreme ductility of feldspar in dynamically recrystallized granulite is explained by activity of dislocation creep in conjunction with stress controlled intracrystalline diffusion in the studied rocks the considerable volume of quartz is not interconnected even at high strain the lack of its interconnectivity in coarse grained granulites and in amphibolite facies granoblastic platy quartz rocks is explained by an extreme stability of the load bearing framework structure at high temperatures in dynamically recrystallized granulite the feldspar viscosity decreases so that quartz becomes enveloped by a weak feldspar matrix which inhibits its coalescence and interconnectivity we predict an important decrease in strength of quartzo feldspathic granulites due to activity of diffusional creep and convergence of viscosity of recrystallized feldspar and quartz

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

lower mantle structure from paleogeographically constrained dynamic earth models seismic tomography reveals two large low shear velocity provinces llsvps beneath africa and the pacific ocean these structures may have existed for several 100 myr and are likely compositionally distinct based on observed seismic characteristics interpreted in light of geodynamic models and mineral physics constraints we investigate the dynamics of the llsvps through the use of evolutionary models of thermochemical convection from 250 ma to present day we use a spherical convection model in which the anomalous structures have a high bulk modulus consistent with seismic interpretation a new progressive assimilation method incorporates constraints from paleogeography using a refined plate history model with 1 myr time spacing to guide the thermal structure of the lithosphere and steer the thermal evolution of slabs in the uppermost mantle the thermochemical structures deform and migrate along the core mantle boundary cmb through coupling to plate motions and in response to slab stresses the models produce a ridge like anomaly beneath africa and a rounded pile beneath the pacific ocean which at present day agrees with tomography waveform modeling and other geodynamic studies plumes emanate from the margins of the domes and ridges of thickened boundary layer between the domes dense and viscous slabs can undermine the stability of high bulk modulus structures at the cmb high bulk modulus structures are not necessarily required to satisfy dynamic constraints on the llsvps

can we resolve 3d density heterogeneity in the lower mantle we reexamine the possibility of resolving 3d density structure in the mantle by performing test inversions using a dataset of well constrained degree 2 splitting coefficients we show that degree 2 in vs and vp can be resolved and confirm that dlnvs dlnvp 2 5 at depths 2000km indicating a chemical component to large scale heterogeneity models with positively correlated with vs in the deepest mantle fit the data at least as well as those in which density and velocities are anti correlated however some rough bounds on dln dlnvs can be inferred models obtained from geodynamic considerations are acceptable in contrast to models with large negative 0 6 dln dlnvs throughout the lower mantle the most robust feature is a marked increase in dln dlnvs at the top of the lower mantle reaching a maximum around 1500 km

an iterative algorithm for separation of s and scs waves of great earthquakes teleseismic sh waves are essential for imaging the rupture processes of large earthquakes however for great earthquakes m8 such as the 2004 sumatra earthquake the 2008 wenchuan earthquake and the recent tohoku oki earthquake the source duration is very long 100 s thus the direct sh waves are overlapped with scs waves for epicentral distances larger than 60 leaving contaminated s waves for source processes modelling therefore artefacts in finite fault models of large earthquake could be produced with such contaminated body waves we propose an iterative algorithm based on the slowness information of s and scs waves and stacking technique to separate s and scs waves with records from a regional seismic network tests on various synthetic data sets show that the algorithm is effective in retrieving teleseismic sh waveforms from complicated wave trains containing both s and scs separation of waveforms for the 2008 wenchuan earthquake with our algorithm clearly demonstrates the influence of scs energy suggesting necessity of recovering s waves

tomographic imaging of the lowermost mantle with differential times of refracted and diffracted core phases pkp pdiff the mapping of variations in p wave speed in the deep mantle is restricted by the uneven sampling of p waves in particular beneath the southern hemisphere to enhance data coverage we augmented the 1 6 million summary rays of p pp and pwp that we used in previous studies with differential travel times of diffracted and refracted core phases for the core refracted differential travel time residuals pkpab pkpdf and pkpab pkpbc we used 1383 cross correlated digital waveforms as well as 27 000 routinely processed bulletin data we used the waveform data to define quality criteria for the selection and reduction of the bulletin pkp data for pkpdf pdiff we only considered 543 records derived from waveform cross correlation no pcp data were used in this study we used optical ray theory to calculate the ray paths associated with the p pp pwp and pkp data which are measured at 1 hz however to account for the large fresnel zones of the low frequency 50 mhz pkpdf pdiff data we estimated the three dimensional shape of the frechet sensitivity kernels from kernels calculated by normal mode summation the use of these kernels allows us to properly distribute the sensitivity for a given seismic phase over a large mantle volume while allowing the high frequency data to constrain small scale structure the differential times are relatively insensitive to source mislocation and to structure in the shallow mantle beneath source and receivers and they have previously been interpreted exclusively in terms of lateral structure directly above the core mantle boundary cmb however images thus obtained can be contaminated by effects of small scale structure elsewhere in the mantle here we do not make a priori assumptions about the mantle source of anomalous time differentials from test inversions we conclude that both upper and lower mantle structures that are poorly resolved by p data can be mapped into the core along pkp paths but that the effect of outer core structures if any on the mantle model is small compared to the inversion of the p pp and pwp alone the inclusion of the pkpab pkpdf and pkpab pkpbc and pkpdf pdiff data improves the resolution of structure beneath 2200 km depth in particular the joint inversion puts better constraints on the long wavelength variations in the very deep mantle and yields an increase in the amplitude of velocity perturbations near the cmb that is in agreement with but still smaller than inferences from shear wave studies resolution tests indicate that in some regions the enhanced definition of structure is significant but in most regions the improvements are subtle and structure remains poorly resolved in large regions of the mantle

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

melt producing and melt consuming reactions in the achankovil cordierite gneisses south india migmatitic cordierite gneisses within the achankovil zone az of southern pan african india record melt producing and subsequent melt consuming mineral reactions early mineral assemblages bt sil qtz and bt sil spl deduced from inclusion textures in garnet prophyroblasts break down via successive dehydration melting reactions to high t phase assemblages e g grt crd liq opx liq spl crd liq later back reactions between the restite and the in situ crystallizing melt resulted in thin cordierite coronas separating garnet from the leucosome and partial resorption of garnet to opx crd or crd bt qtz symplectites leucosomes generally display a moderate low strain gneisses to strong high strain gneisses depletion of alkali feldspar attributed to mineral melt back reactions partly controlled by the degree of melt segregation using a kfmash partial petrogenetic grid that includes a melt phase and qualitative pseudosections for microdomains of high and low al si ratios the successive phase assemblages and reaction textures are interpreted in terms of a clockwise p t path culminating at about 6 7 kbar and 900 950 c this p t path is consistent with but more detailed than published results which suggests that taking a melt phase into account is not only a valid but also a useful approach comparing p t data and lithological and isotopic data for the az with adjacent east gondwana fragments suggests the presence of a coherent metasedimentary unit exposed from southern madagascar via south india az and sri lanka wanni complex to the lutzow holm bay in eastern antarctica

evidence for strong shear velocity reductions and velocity gradients in the lower mantle beneath africa we present data which indicate that the broad low shear velocity anomaly beneath southern africa is stronger and more extensive than previously thought recordings of earthquakes in the southwestern atlantic ocean at an array of broadband seismic stations in eastern africa show anomalously large propagation time delays of the shear phases s scs and sks which vary rapidly with epicentral distance by forward modeling we estimate that the low velocity anomaly extends from the core mantle boundary about 1500 km up into the mantle and that the average shear velocity within this structure is 3 lower than in standard models such as prem strong velocity contrasts exist at its margins 2 over about 300 km these seismic characteristic are consistent with recent numerical simulations of lower mantle mega plume formation

relationship of d structure with the velocity variations near the inner core boundary 1 variations in regional differential times between pkikp i and pkikp i have been attributed to hemispheric p velocity variations of about 1 in the upper 100 km of the inner core referred to as hic the top of the inner core appears relatively fast beneath asia where d is also fast an alternative interpretation could be the lateral variation in p velocity at the lowermost outer core hoc producing the same differential times to resolve this issue we introduce the diffracted pkp phase near the b caustic bdiff in the range of 139 145 epicenter distances and the corresponding differential times between bdiff and pkikp and pkikp as observed on broadband arrays due to the long wavelength nature of bdiff we scaled the s wave tomography model with k values k dlnvs dlnvp to obtain large scale p wave velocity structure in the lower mantle as proposed by earlier studies waveform synthetics of bdiff constructed with small k s predict complex waveforms not commonly observed confirming the validity of large scaling factor k with p velocity in lower mantle constrained at large scale the extra travel time constraint imposed by bdiff helps to resolve the hoc hic issue our preliminary results suggest k 2 for the lowermost mantle and support hic hypothesis an important implication is that there appears to be a relationship of d velocity structures with the structures near the inner core boundary via core dynamics

the pacific plume as seen by s scs and sks the forward modeling of s sks and scs s differential travel time residuals sensitive to structure beneath a localized region of the central pacific suggests the presence of two anomalous domains first a broad zone which becomes thinner as it extends into the mid mantle where the s velocity is reduced by 2 to 4 and which is deflected to the south east as it rises towards the surface adjacent to it a smaller domain where the s velocity is about 4 higher these strong velocity anomalies are likely the results of chemical heterogeneity that may involve subducted material and partial melting strong heterogeneity appears to extend at least locally well above d but the morphology of the anomalous domains is more reminiscent of localized rising or sinking currents rather than of a relatively uniform and thick geochemical reservoir

oxic facies and the late devonian mass extinction canning basin australia the close association of anoxic or dysoxic sedimentary rocks and the major late devonian frasnian famennian mass extinction has focused considerable attention on anoxia as the major cause or as a major factor in a multicausal scenario the record of the late devonian biotic crisis in the well known reef complexes of northwestern australia canning basin in contrast to many localities elsewhere does not display sedimentological evidence of anoxia through the frasnian famennian boundary interval analysis of continuous drill core through this interval has yielded three positive 13 c isotopic excursions only one of which coincides with total organic carbon toc maxima in our data multi element geochemical proxies suggest that toc maxima preceding positive shifts in 13 c most likely resulted from higher productivity caused by nutrient influx from continental weathering given the close association between toc maxima and regional relative sea level falls our interpretation supports the view that anoxia was not a fundamental driver of mass extinction and stresses the importance of integrated data sets and understanding regional controls on environmental changes and or stresses

influence of thermochemical piles on topography at earth s core mantle boundary abstract numerous seismic studies reveal the presence of two large low velocity anomalies beneath africa and the central pacific efforts to characterize these anomalies have yielded a variety of interpretations over the years both isochemical and thermochemical previous interpretations have included large isochemical superplumes clusters of smaller thermal plumes and doming thermochemical superplumes a conceptual mantle model that is presently growing favor involves long lived thermochemical piles in anticipation that nutation studies will provide better topographic constraints on the core mantle boundary cmb in the future we examine the effects of thermochemical piles at this boundary in this study we perform numerical modeling of thermochemical and isochemical convection as a function of convective vigor and temperature dependent viscosity to predict the topography at earth s cmb our results show that in isochemical convection downwellings always lead to negative depressed cmb topography while upwellings cause positive elevated topography consistent with previous studies however in thermochemical convection cmb topography and its relationship to downwellings and thermochemical piles are significantly affected by temperature dependent viscosity for isoviscous or weakly temperature dependent viscosity the piles cause negative cmb topography while positive topography is often below cold downwellings however for realistic more strongly temperature dependent viscosity the most negative cmb topography is below downwellings while the topography below the piles is often slightly more positive and or flat similar to upwellings in isochemical models we also show that although thermochemical piles are intrinsically more dense the large thermal buoyancy associated with them leads to overall relative buoyancy that is on par with slabs consequently thermochemical models lead to an overall reduction in magnitude of cmb topography with respect to isochemical models

comparing p and s wave heterogeneity in the mantle from the reprocessed data set of engdahl and co workers we have carefully selected matching p and s data for tomographic imaging we assess data and model error and conclude that our s model uncertainty is twice that of the p model we account for this in our comparison of the perturbations in p and s wavespeed in accord with previous studies we find that p and s perturbations are positively correlated at all depths however in the deep mantle systematic differences occur between regions that have undergone subduction in the last 120 million years and those that have not in particular below 1500 km depth ln vs ln vp is significantly larger in mantle regions away from subduction than in mantle beneath convergent margins this inference is substantiated by wavespeed analyses with random realizations of the slab non slab distribution through much of the mantle there is no significant correlation between bulk sound and s wave perturbations but they appear to be negatively correlated between 1700 and 2100 km depth which is also where the largest differences in ln vs ln vp occur this finding supports convection models with compositional heterogeneity in the lowermost mantle

using pre critical pkikp pcp phases to constrain the regional structures of the inner core boundary beneath east asia abstract we collected 177 pre critical pkikp pcp records assembling a wealth of traveltime and amplitude data these observations sample the inner core boundary icb beneath east asia with good spatial coverage combined with previous studies our results demonstrate a 100 km wide anomaly with positive pkikp pcp traveltime residuals surrounded by negatives beneath the yellow sea area 123 e 33 n yellow sea anomaly after correcting the elliptic effects and mantle vp heterogeneities based on the tomography models the residuals of the yellow sea anomaly remain at least 0 6 s faster than those of the surrounding areas suggesting a thickening of 2 3 km between the icb and core mantle boundary cmb due to the negative cmb topography along the western pacific rim we attribute this anomaly mainly to negative icb topography across the northern border of the yellow sea anomaly pkikp pcp amplitude ratios increase by approximately 50 from north to south which can be explained by an approximately 0 6 g cm3 raise in icb density contrast these traveltime and amplitude features suggest a mosaic structure at the icb beneath the yellow sea areas