mantle transition zone thickness beneath cameroon evidence for an upper mantle origin for the cameroon volcanic line summary the thickness of the mantle transition zone beneath cameroon has been mapped using data from the 2005 2007 cameroon broadband seismic experiment to evaluate models for the originof the cameroon volcanic line cvl some 2200 receiver functions have been stacked using a 3 d velocity model revealing ps conversions from the mantle transition zone discontinuities at depths of 410 and 660 km results yield a nearly uniform transition zone thickness 251 10 km that is similar to the global average implying that any thermal anomalies in the upper mantle beneath the cvl do not extend as deep as the transition zone this finding when combined with regional p and s velocity models of the mantle supports an explanation attributing the origin of the cvl to upper mantle processes such as an edge flow convection cell in the upper mantle along the northwestern side of the congo craton lithosphere

seismic observations of splitting of the mid transition zone discontinuity in earth s mantle the transition zone of earth9s mantle is delineated by globally observed discontinuities in seismic properties at depths of about 410 and 660 kilometers here we investigate the detailed structure between 410 and 660 kilometers depth by making use of regional stacks of precursors to the ss phase the previously observed discontinuity at about 520 kilometers depth is confirmed in many regions but is found to be absent in others there are a number of regions in which we find two discontinuities at about 500 and 560 kilometers depth an effect which can be interpreted as a splitting of the 520 kilometer discontinuity these observations provide seismic constraints on the sharpness and observability of mineralogical phase transitions in the mantle transition zone

mantle discontinuities beneath southern africa 1 seismic velocity discontinuities within the top 1000 km of the earth beneath southern africa are imaged by stacking about 1300 source normalized broadband seismograms recorded by the southern african seismic experiment the moho 410 and 660 kilometer discontinuities are clearly detected the mean mantle transition zone thickness is 245 km essentially the same as the global average suggesting that the transition zone is not anomalously warm thus the lower mantle african superplume beneath our study area has no discernible effect on transition zone temperature and is consequently confined to the lower mantle variations in transition zone thickness appear to be related to the presence or absence of thick lithosphere we do not detect several previously reported discontinuities beneath continents such as the hales or lehmann discontinuities and find no evidence for a 520 km discontinuity nor do we detect a previously proposed low velocity zone just above the transition zone

implications of a simple mantle transition zone beneath cratonic north america many areas of old continental interiors have thick roots that extend to depths of 250 km or more in contrast to the oceans and younger continents whose lithospheric thickness is less than 100 km these cratonic roots might perturb temperatures in their surrounding mantle though the net result could be either cooling or heating both of which may lead to small scale convective flow around the root we show here using new data from a study of the seismic structure of the canadian shield that the relative positions of the 410 km and 660 km seismic discontinuities are unperturbed beneath one of the deepest and broadest cratonic roots on the earth differential arrival times and internal discontinuity structure are remarkably uniform and simple varying by little more than 0 5 s over the root s 3500 km lateral extent this implies that the root has no significant thermal effect on the underlying mantle 50 k and any small scale convection or cold mantle downwelling associated with the large free air gravity anomaly beneath the shield must be confined to the upper mantle our observations are also consistent with phase changes solely in the olivine system mg fe 2sio4 with the caveat that our method is dominantly sensitive to s wave velocity jumps alone

the origin and microstructures of metamorphic felsic dykes emplaced during brittle ductile extension southeast sinai structural petrographic and chemical analyses of a set of northeast trending quartzofeld spathic dykes in wadi kid area southeastern sinai show that they are the product of anatexis of pelitic rocks metamorphic p t data indicate that anatexis could have occurred only 2 km below the present erosion level the partial melts intruded into overlying rocks during regional metamorphism along a set of northeast oriented extensional fractures thereby recrystallizing and in some cases obtaining tectonite fabrics the fractures were formed perpendicular to the regional mineral lineation and show that the area was within the deeper parts of the brittle ductile transition zone the original depth of the presently exposed crustal level corresponds well with the theoretical and experimentally derived depths of the brittle ductile transition zone in high heat flow regimes the emplacement of a granitoid pluton or gneiss dome possibly of dioritic composition could be responsible for both melting and tensile fracturing of the overlying rocks

anomalously thin transition zone and apparently isotropic upper mantle beneath bermuda evidence for upwelling 1 the origin of the bermuda swell and volcanism remains enigmatic the lack of an associated time progressive hotspot track and absence of present day volcanic activity make it difficult to reconcile with a deep mantle plume model we analyze shear wave splitting measurements to estimate mantle flow direction and receiver function stacks to place constraints on the mantle transition zone thermal structure ks phases exhibit well resolved null arrivals no splitting beneath the swell over a range of back azimuths we find that the 410 and 660 km discontinuities are 49 5 km and 19 5 km deeper than the global average respectively leading to a transition zone thickness that is 27 4 km thinner than the average together an apparently isotropic upper mantle and a thinned mantle transition zone suggest that mantle flow is primarily vertical beneath the swell consistent with the presence of hot buoyant mantle at depth

phase relation and physical properties of an al depleted komatiite to 23 gpa abstract knowledge of the mineralogical constituents of the earth s mantle depends heavily on high p experiments of pyrolite composition in order to understand the planetary interior better phase relations and mineral physics properties of compositions other than pyrolite may also be important high p and high t experiments were conducted on an al depleted komatiite which can be regarded as a kind of piclogite composition fourteen experiments were carried out along a model geotherm between 5 and 23 gpa and 1100 and 1600 c with a multi anvil apparatus analyses of quenched run products showed that in the studied komatiite the fractions of olivine and its high p polymorphs wadsleyite and ringwoodite are 40 50 vol pyroxene garnet phase transition occurs at 13 17 gpa while the respective values for pyrolite are 60 vol and 12 15 gpa the density and seismic velocity profiles of komatiite mantle were calculated using thermoelastic parameters of related minerals and they showed quite good agreements with those of seismic observations around the transition zone as well as those for pyrolite

global variations of temperature and water content in the mantle transition zone from higher mode surface waves abstract we performed a fully nonlinear joint inversion of surface wave overtone phase velocities for velocity structure down to a depth of 1500 km and topography of transition zone discontinuities using neural networks the resulting models are compared with existing mantle models from linearized inversions techniques and the agreement between vertically averaged s wave velocity models and various 660 km topography models is very good whereas the agreement between all the 400 km topography models is very poor the latter might be due to a broader than previously reported 400 km discontinuity where the minerals co exist with water induced melt from our models of transition zone thickness and s wave velocity we estimated lateral variations of temperature and water content in earth s mantle transition zone linearly relating variations in s wave velocity and transition zone thickness to variations in temperature and water content within the transition zone we observe relatively cold subduction zones as well as relatively warm regions beneath continents and hot spots we find that the transition zone is wettest away from subduction zones

alpine mantle transition zone imaged by receiver functions abstract assessing the thermal state of the mantle is essential to understand mantle dynamics and provides important clues about present and past tectonic regimes the tectonic processes related to the cenozoic subduction of the alpine tethys likely affected the regional upper mantle structure we use receiver functions to determine the mantle transition zone thickness beneath the greater alpine region as a proxy for its current thermal state compilation of a dataset of nearly 7000 high quality receiver functions from 98 stations yields a reliable and stable image of the alpine mantle transition zone significant signal from p to s conversions at the 410 and 660 km discontinuities is clearly observed the 410 km discontinuity is found at a depth close to that predicted by the iasp91 global model its seismic signature is generally strong except south of the alps signal from the 660 km discontinuity is strong continuous and emerges throughout the whole alpine region at depths greater than predicted by the global model the mantle transition zone is not or only slightly thickened 10 km north of the alps but is up to 40 km thicker than normal south of the alps and in the western alps outlining a roughly 300 km wide and 800 km long ene wsw directed anomaly thickening assuming conversions are due to olivine phase transformations suggests that the alpine mantle transition zone is at least 300 k colder than normal if the thickness anomaly were caused by equal amounts of shallowing and deepening of the 410 and 660 km discontinuity respectively however the thickness anomaly seems mainly due to depression of the 660 km discontinuity which suggests that temperature at the bottom of the mantle transition zone could actually be up to 800 k cooler than ambient mantle while temperatures at 410 km depth may be close to normal both location and amount of suggested thermal anomaly agree with tectonic reconstruction and large scale tomography models of the euro mediterranean region the cold material accumulated at the 660 km discontinuity is interpreted as remnants from the oceanic mantle lithosphere that detached from the eurasian plate after closure of the alpine tethys

garnet rotation and the development of axial plane crenulation cleavage rotated helicitic garnets syntectonically overgrowing axial plane crenulation cleavage on the limbs of folds show the opposite sense of rotation to that undergone by the folded foliation during the development of the fold the garnet rotation is inconsistent with simple minded shear models for the development of the crenulation cleavage and it is useful to consider this problem in order to understand the development of crenulation cleavage better various models are considered in an attempt to explain both garnet rotation and cleavage morphology earlier models were based on the assumption that garnet rotation is driven by shear parallel to s2 they were mostly unsatisfactory but one involving migration of the crenulation hinges is viable when combined with other models described here the new models proposed here are based on the assumption that garnet rotation is driven by shear on the crenulated surface s1 the new foliation s2 is considered to be a passive structure at least in the early stages of its development these models satisfactorily explain garnet rotation and development of the cleavage including differentiation and they can be used to predict the morphological evolution of the cleavage microstructure as it should be preserved in garnets comparison of observed and predicted helicitic structures strongly supports the proposed models evidence is presented to show that where a garnet has an se that is a schistosity and an si that is a crenulation cleavage the schistosity can have developed by unfolding of the crenulations

seismic evidence for very deep roots of continents abstract a major problem in geodynamics with seismic data is discussed how deeply do the continents penetrate into the mantle differential travel times of underside reflections from mantle discontinuities that appear as precursors to ss in large parts of the globe show a clear correlation with oceans and continents they are significantly larger beneath the asian and north american continents than underneath the neighbouring pacific from this observation we conclude that the asian and north american continents affect the mantle well below 410 km changes in the thickness of the transition zone can explain our observations which are in agreement with the hypothesis of petrological phase changes causing the 410 km and 660 km seismic discontinuities average thickness of the transition zone underneath continents is about 14 kim thicker than beneath oceans moreover our findings imply temperature variations about 100 200 k in the mantle transition zone weak reflections from 520 km depth corresponding to an impedance contrast of about 2 can be observed only in some areas of the earth while observations from other locations definitively show no signal from this depth therefore we propose that the 520 km reflector is only a regional feature

characteristics and mode of emplacement of gneiss domes and plutonic domes in central eastern pyrenees abstract gneiss domes and plutonic granitoid domes make up almost 50 of the pre hercynian terrains in the central and eastern pyrenees from a structural study of the shape and internal structure of the domes and of their relationships with the enclosing rocks it can be shown that both types of domes were emplaced diapirically during the major regional deformation phase and the peak of regional metamorphism the study also shows that the internal structure the overall shape and general behaviour relative to the host rocks are similar for plutonic domes and for gneiss domes this appears to be in good agreement with h ramberg s 1967 gravity deformation and the earth s crust academic press london 1970 model studies in relation to intrusion of plutonic bodies in mechanisms of igneous intrusion edited by newall g rast n geol j spec issue 2 261 286 model studies showing that dome or mushroom like structures similar to those observed develop when there is a small viscosity ratio between the rising body and its enclosing medium this implies a high crystal content for the granitoid magma this crystal content has been estimated by i calculating the viscosity and density in natural conditions from petrological data for the magma considered as a suspension using the model and program of j p carron et al 1978 bull soc geol fr 20 739 744 ii using the recent results of experimental deformation of partially melted granites of i van der molen m s paterson 1979 contr miner petrol 70 299 318 and ii comparing the preceding results with the data obtained by deformation experiments on rocks similar to those enclosing the domes the minimum crystal content for the development of a dome like structure has been thus estimated to about 70 i e a value very close to that estimated by van der molen paterson 1979 to be the critical value separating the granular framework flow from suspension like behaviour the effect of small variations in the viscosity of the rising body are then simulated by centrifuge experiments these small variations appear to exert a strong control on the shape and rate of rise of the domes they are thought to be sufficient to account for the variations in shape and structure and the level of emplacement of the different types of gneissic and plutonic domes finally more complex experiments with models built in order to simulate as closely as possible the natural structural evolution of the region as deduced from petrological and structural data are reported their implications for the regional interpretation of the relationships between gneissic and plutonic domes is then discussed

transition zone velocity gradients and the 520 km discontinuity stacks of long period global digital seismograph network gdsn seismograms at 110 to 180 epicentral distance reveal precursors to ss that result from underside reflections off upper mantle seismic discontinuities the 410 and 660 km discontinuities are obvious in these stacks but identification and modeling of other transition zone discontinuities are complicated by sidelobes from the 410 and 660 km reflections these sidelobes result from the limited bandwidth of the gdsn instrument responses and the effect of crustal reverberations on the ss reference phase the crustal effects can be minimized by restricting the records to oceanic bounce points where the 6 km thick crust has little effect on the long period waveforms over 2000 long period transverse component seismograms with oceanic ss bounce points recorded by the gdsn from 1976 to 1991 are manually edited aligned on ss and then stacked using a new procedure that weights the records by data quality the resulting image shows a clear reflection from a 520 km discontinuity that cannot be explained as a sidelobe artifact confirming earlier results of shearer 1990 1991 and revenaugh and jordan 1991 by stacking along the expected travel time curves for discontinuity phases the time versus range image of the precursor wave field is reduced to a single trace that measures upper mantle reflectivity versus time the features in this reflectivity profile are sensitive to the brightness and depth of the transition zone discontinuities and to the steepness of the velocity gradients between the interfaces using geometrical ray theory and assuming a constant velocity versus density scaling relationship i fit this reflectivity profile with velocity models of the upper mantle using both forward modeling and direct inversion the inverse problem is addressed by performing a deconvolution of the profile with the ss reference phase after a correction for attenuation followed by a direct mapping of reflectivity versus time into velocity versus depth velocity depth profiles resulting from these procedures are roughly in agreement with standard upper mantle velocity models except that the ss precursor data require a minor discontinuity near 520 km and a steeper gradient just below the 660 km discontinuity estimated discontinuity shear impedance changes are 6 7 1 1 at 420 km 2 9 0 7 at 519 km and 9 9 1 5 at 663 km the impedance change near 520 km is consistent with current mineral physics results for the olivine to phase change and places constraints on the fraction of olivine in the transition zone

geochronology of the arabian nubian shield in southern israel and eastern sinai the metamorphic and igneous orogenic basement rocks of southeastern israel and eastern sinai evolved as part of the arabian nubian shield during latest proterozoic time pelitic schist of the elat association from wadi nahal shlomo south of eilat israel is dated at about 800 m y but the geologic significance of this date is not clear in southeastern sinai metapelitic rocks of the kid group have a date of diagenesis or metamorphism of about 615 m y syn orogenic and post orogenic plutons which intrude the layered metamorphic complex s are dated at 600 m y near eilat and 590 m y at wadi kid other dated plutons include the timna granite at 592 7 m y in the area of nevi ot at 595 m y and in the vicinity of dahab at 588 2 m y and 595 0 3 m y the radiometric dates and low initial 87 sr 86 sr ratios 0 7034 imply that pre 1000 m y continental sialic crust was not involved in the formation of the shield in southeast israel and eastern sinai

porphyroblast inclusion trails the key to orogenesis detailed microstructural analysis of inclusion trails in hundreds of garnet porphyroblasts from rocks where spiral shaped inclusion trails are common indicates that spiral shaped trails did not form by rotation of the growing porphyroblasts relative to geographic coordinates they formed instead by progressive growth by porphyroblasts over several sets of near orthogonal foliations that successively overprint one another the orientations of these near orthogonal foliations are alternately near vertical and near horizontal in all porphyroblasts examined this provides very strong evidence for lack of porphyroblast rotation the deformation path recorded by these porphyroblasts indicates that the process of orogenesis involves a multiply repeated two stage cycle of 1 crustal shortening and thickening with the development of a near vertical foliation with a steep stretching lineation followed by 2 gravitational instability and collapse of this uplifted pile with the development of a near horizontal foliation gravitational spreading near coaxial vertical shortening and consequent thrusting on the orogen margins correlation of inclusion trail overprinting relationships and asymmetry in porphyroblasts with foliation overprinting relationships observed in the field allows determination of where the rocks studied lie and have moved within an orogen this information combined with information about chemical zoning in porphyroblasts provides details about the structural metamorphic p t t paths the rocks have followed the ductile deformation environment in which a porphyroblast can rotate relative to geographic coordinates during orogenesis is spatially restricted in continental crust to vertical ductile tear transcurrent faults across which there is no component of bulk shortening or transpression

the nature of the 660 kilometer discontinuity in earth s mantle from global seismic observations of pp precursors the 660 kilometer discontinuity which separates earth s upper and lower mantle has been detected routinely on a global scale in underside reflections of precursors to ss shear waves here we report observations of this discontinuity in many different regions using precursors to compressional pp waves the apparent absence of such precursors in previous studies had posed major problems for models of mantle composition we find a complicated structure showing single and double reflections ranging in depth from 640 to 720 kilometers that requires the existence of multiple phase transitions at the base of the transition zone the results are consistent with a pyrolite mantle composition

constraining seismic velocity and density for the mantle transition zone with reflected and transmitted waveforms we examine stacks of several seismic phases having different sensitivities to mantle transition zone structure when analyzed separately underside p and s reflections pdp and sds are suggestive of very different structures despite similar raypaths and data coverage by stacking the radial component of pdp rather than the vertical pdp we show that this difference does not result from interference from other more steeply inclined phases such as pkp and ppdpdiff in general stacks of p to s converted phases pds appear to lack evidence of a 520 km discontinuity when examined without other phases when these phases and stacked topside p reflections ppdp are analyzed jointly using a nonlinear inversion method consistent but nonunique seismological models emerge these models show that a discontinuity at 653 km depth has smaller contrasts in density and velocity than found in most previous studies a sub 660 gradient can account for the majority of this difference a 1 6 0 5 p velocity contrast and a 2 2 0 3 density contrast at 518 km depth without a s velocity contrast can explain the lack of a p520s together with robust pp520p and s520s phases for models parameterized with a finite thickness for each discontinuity the 410 km discontinuity is consistently 3 times thicker than the 660 km discontinuity

probing the upper mantle transition zone under africa with p520s conversions implications for temperature and composition abstract we have investigated the nature of the 520 km discontinuity under africa by migrating p wave receiver functions at 30 permanent broadband stations in the continent worldwide investigations with ss precursors have revealed that the detection of this discontinuity is intermittent and that it may split into two separate discontinuities 60 km apart in some regions several explanations have been proposed to account for both the intermittent detection and the splitting of the 520 km discontinuity ranging from lateral temperature variations in the transition zone to compositional variations due to changes in minor phases containing fe ca and water our results reveal that detection of the 520 km discontinuity is intermittent under africa and that when observed it does not split detectability does not correlate with published maps of transition zone temperature temperature estimates from transition zone thickness or published potential temperature estimates from primitive magmas throughout east africa it does not correlate with published maps of transition zone water content as well lateral variation in iron content due for instance to varying proportions of the transition zone s constituent minerals offers a plausible explanation for the observed intermittency in detection but better maps of transition zone iron content need to be developed to assess a correlation focusing and defocusing effects due to small scale variations around the stations are also a possibility the lack of splitting on the other hand is found consistent with low ca contents and the general absence of present day subduction around the continent however a regional increase in mantle fertility under west africa as previously reported with analysis of ss precursors cannot be ruled out with our analysis

spinel elasticity and seismic structure of the transition zone of the mantle there is continuing debate about whether the upper mantle is chemically stratified and whether the seismic discontinuity at 400 km depth represents a chemical boundary a phase change of the mg fe 2sio4 component from the phase olivine to the p phase or a combination of both recent developments in high pressure synthesis1 2 and ultrasonic interferometry3 4 have made possible measurements of elastic wave velocities in small poly crystalline samples of high pressure phases by combining our new acoustic measurements on the spinel phase of mg2 sio4 with existing data for the and phases we present here velocity profiles for the m2sio4 component of the mantle where m represents mg or fe to depths of about 600 km we find it to be unlikely that any seismologically observable velocity discontinuity at about 520 km depth can be attributed to this component although the contrast in impedance the product of density and velocity might be sufficient for the transformation to be observed in long period seismic reflection studies at near normal incidence5 the velocity gradients in the transition zone particularly for shear waves are steeper than would be expected for simple adiabatic compression of likely mantle compositions suggesting that alternative explanations including chemical heterogeneity and anelastic relaxation need to be explored

seismic imaging of upper mantle structure with new evidence for a 520 km discontinuity stacked images of long period seismic data reveal many phases some not previously observed which are caused by reflections and conversions at discontinuities in the earth s upper mantle these images support the existence of discontinuities at 410 520 and 660km depth but no evidence is found for a significant interface at 220 km

continental fault structure and the shallow earthquake source plate boundaries in continental crust are generally less sharply defined than in the oceans with seismicity spread over broad areas interplate displacements appear to be largely accommodated by networks of major fault zones a simple 2 level model for these important structures accounts for the depth distribution of most continental earthquakes and for the observed range of faulting styles and associated rock deformation textures the model consists of a seismogenic frictional slip regime overlying quasi plastic mylonite belts wherein shearing is largely accommodated aseismically due mainly to the changing response of quartz to deformation with increasing temperature shear resistance increases with depth to a peak value in the vicinity of the frictiona1 quasi plastic transition and then decreases rapidly the depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional quasi plastic transition for different geotherms using laboratory determined flow laws for quartz bearing rocks larger earthquake ruptures m 5 5 tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy consideration is also given to the depression of isotherms and seismic activity in regions of thrusting and to the question of the downward continuation of major fault zones through the lithosphere decoupling of the upper crust on flat lying shear zones may accompany higher level dip slip and perhaps in some circumstances strike slip faulting being favoured by above average continental heat flow and a high quartz content in the middle or deep crust the average level of deviatoric stress within the seismogenic regime remains an outstanding problem

metamorphism and tectonics of a pan african terrain in southeastern sinai a reply abstract the precambrian crystalline basement of sinai represents a low pressure metamorphic terrain intruded by large volumes of granitic rock based on detailed fieldwork a general assessment of the metamorphic and tectonic history of the wadi kid area southeastern sinai is presented three lithostratigraphic units can be traced over the whole area the umm zariq formation arkoses greywackes pelites the tarr formation dolomitic calcareous rocks and unconformably overlying the previous two units the heib formation flows pyroclastics conglomerates d 1 deformation of this 3 5 km thick sequence resulted in upright folds with changing strike of the axial planes from ne to nw across the area low grade conditions prevailed during this phase d 2 produced recumbent folds and a subhorizontal cleavage leading to transposition of d 1 structures in the higher grade parts of the area metamorphism reached its peak conditions around d 2 pressures are estimated at 2 5 3 5 kb whereas temperatures vary from 450 660 c in the central wadi kid area garnet staurolite cordierite and andalusite occur in metapelitic rocks highest grade rocks are syn d 2 andalusite k feldspar gneiss diapirs metamorphic zones are shallow dippin and form a domed pattern most of the metavolcanics and the syntectonic and late tectonic plutonic rocks belong to the calc alkaline suite the kid group sediments and volcanics were deposited in a shallow basin and subaerially respectively probably on older sialic basement this basement is at present not exposed because post orogenic uplift directly after the pan african event was relatively small 3 6 km metamorphism and the d 2 formation phase can both be related to a rising mafic diapir the sinai peninsula may have been a continental margin or a cratonized mature island arc in late proterozoic times

seismic waves converted from velocity gradient anomalies in the earth s upper mantle summary modelling of elastic wave propagation in 1 d structures is frequently performed using reflectivity techniques in which the earth s velocity profile is approximated by stacks of homogeneous layers the complete reflection transmission r t response of a zone with arbitrary 1 d depth variation including both gradients and discontinuities in material properties can however be calculated using invariant embedding techniques results from earlier studies are here extended to derive exact expressions for r t matrices in arbitrary 1 d anisotropic media using a form of born approxi mation valid for thin scatterers and which does not assume small perturbations in material properties the r t matrices are solutions to a system of non linear ordinary differential equations of ricatti type and may be manipulated using standard r t matrix algebra in an equivalent description the wavefield within the heterogeneous zone is considered in terms of depth dependent contributions from up and downgoing waves propagating within the embedding reference medium this leads to efficient calculation of the internal wavefield using r t matrices of the heterogeneous stratification and portions thereof at minor additional expense mode conversion of teleseismic p and s phases from velocity gradients is examined by way of examples and comparison with three component data from broad band stations of the yellowknife seismic array the frequency dependence of such wave interactions depends on the differences in vertical slowness between incident and scattered modes it is shown that significant energy is converted from transition zones with extent l p 2 a broader interval than will generally produce intramode reflections a layer structure identified from ps conversions near 75 km depth below the slave craton is shown to be compatible with a 10 km thick gradient zone in which anisotropy increases from ambient levels to vp 5 per cent vs 2 5 per cent at a discontinuous upper boundary this characterization supports a previous interpretation as the upper strata of a former oceanic plate juxtaposed against overriding lithosphere during an ancient episode of shallow subduction

elastic properties of hydrous ringwoodite γ phase in mg2sio4 abstract single crystal elastic properties of hydrous ringwoodite are reported for samples that were synthesized at 19 gpa and 1300 c the mg si ratio is determined to be 1 95 by epma which is slightly lower than that of ideal anhydrous ringwoodite the h 2 o content determined by sims is 2 2 wt resulting in the stoichiometry mg 1 89 si 0 97 o 4 h 0 33 the lattice parameter a 8 0786 4 a and elastic constants were determined from a single crystal with x ray diffractometry and brillouin scattering spectroscopy the crystal structure is cubic fd3m with a unit cell volume which is 0 51 larger than that of the anhydrous ringwoodite the adiabatic single crystal elastic moduli of hydrous ringwoodite in gpa are c 11 281 6 c 44 117 4 c 12 92 5 the isotropic properties of hydrous ringwoodite are k vrh 155 4 and g vrh 107 3 which are about 16 and 10 smaller than those of anhydrous ringwoodite thus hydrous ringwoodite if present drastically reduces the seismic velocity in the mantle transition zone relative to anhydrous ringwoodite in addition our results show that the anisotropy is enhanced relative to anhydrous ringwoodite using the present results we discuss the 520 km seismic discontinuity in a wet mantle transition zone

global de correlation of the topography of transition zone discontinuities analysis of long period shear wave underside reflections from upper mantle discontinuities shows the shape of the 410 km discontinuity to be neither globally nor locally correlated with that of the 660 km discontinuity the long wavelength pattern of 410 topography is dominated by degree 1 spherical harmonics and has a reasonable correlation with the seismic velocity perturbations in the mantle above it the global topography of 660 dominated by degree 2 spherical harmonics is strongly correlated with shear wave velocity variations in the transition zone the poor correlation between the observed topography of the 410 and 660 km discontinuities is inconsistent with a simple model of large scale whole mantle flow thus implying a more complex pattern which includes at least temporary accumulation of subducted material in the transition zone ringwood and irifune nature 331 1988 131 136 tackley et al nature 361 1993 699 704 the modest seismic discontinuity at 520 km is well observed when the reflections are under mid age oceans but is not observed when they are under continental shields interpretation of this observation in terms of olivine depletion in the deep continental mantle indicates the possible existence of continent ocean chemical heterogeneity at depths exceeding 500 km

the early proterozoic structural and tectonic history of the south central lake superior region the early proterozoic tectonic evolution of the south central lake superior region is complex owing to the presence of four tectonostratigraphic terranes which were affected by six phases of deformation the four terranes are the passive margin of the superior craton two paraautochthonous passive margin terranes crystal falls and florence niagara terranes and a southern magmatic arc complex which is probably allochthonous with respect to the other terranes four of the six deformational episodes accompanied subhorizontal shortening while two were caused by subvertical shortening the first and the most penetrative phase of deformation is marked by subhorizontal shortening in a nne ssw direction the second and fourth deformations were characterized by subvertical shortening and did not significantly modify the structural orientations from previous events in the study area the third fifth and sixth deformations mostly caused open folding and shortening directions were nw ne and w respectively because all the terranes in the south central lake superior region share parallel deformational histories it is suggested that the accretion of these terranes occurred during the first deformational episode after removal of younger deformational effects including open folding of the suture zone the tectonostratigraphic assemblages in this region show the following sequence from nne to ssw a platformal assemblage overlying sialic basement a basinal assemblage of tholeiitic volcanic rocks overlain by deep water turbidites an assemblage of basin floor deposits crystal falls terrane with apparently no demonstratably underlying crystalline basement a fault bounded terrane with highly strained passive margin strata florence niagara terrane and a calc alkaline magmatic arc assemblage such an arrangement of tectonostratigraphic assemblages is comparable with cross sections through phanerozoic accretionary continental margins and therefore supports an arc continent collision model

seismic evidence for olivine phase changes at the 410 and 660 kilometer discontinuities the view that the seismic discontinuities bounding the mantle transition zone at 410 and 660 kilometer depths are caused by isochemical phase transformations of the olivine structure is debated combining converted wave measurements in east asia and australia with seismic velocities from regional tomography studies we observe a correlation of the thickness of and wavespeed variations within the transition zone that is consistent with olivine structural transformations moreover the seismologically inferred clapeyron slopes are in agreement with the mineralogical clapeyron slopes of the mg fe 2sio4 spinel and postspinel transformations

topography of the transition zone seismic discontinuities phase transformations in mantle mineralogies probably cause the transition zone seismic discontinuities nominally at 410 520 and 660 km depth thermodynamic principles govern phase transformations making them sensitive to changes in the mantle s ambient conditions through the thermodynamic response changes in temperature or composition shift the transformation to a different pressure creating topography on a level discontinuity with this use as an exploratory tool for the mantle in mind we review the basic seismological and phase equilibrium concepts underlying the detection and interpretation of the seismic wave arrivals associated with the transition zone discontinuities reviewing the evidence for and against the phase transition model we conclude that it is viable and describe how discontinuities have been and can be used to probe the physical and chemical state of the transition zone

the relationship of strain and preferred orientation of phyllosilicate grains in rocks a review abstract the calculation of strain from the preferred orientation of platy phyllosilicate grains taking strain to be homogeneous is surprisingly often successful from a pragmatic point of view although numerous theoretical objections to this procedure can be and have been raised for the calculation to succeed the original distribution of mica or clay mineral grains must be known or assumed the usual assumption that the starting distribution was random and therefore nearly uniform implies that the strain history of pelitic sediments is taken to start when the watery sediment was last stirred by waves or bioturbation or if it was laid down in still anoxic waters from the moment of first deposition it therefore includes compaction as part of the strain history which is usually neglected by structural geologists originally random distribution of clay flake orientations has been observed in recently deposited sediments in many instances and has been theoretically explained in various ways exceptions probably exist but their frequency has not been estimated biologically disturbed sediments may be brought nearer to a state of random orientation of clay flakes than they were at deposition with the exception of whole beds or concretions in which pores are early filled with cement mudrocks are almost always compacted by the weight of later deposits the strain so induced is axially symmetric about the vertical and thus about the pole of the bedding planes compaction acts on soft unconsolidated rocks but may continue with increasing overburden on rocks strengthened by cementation and by increasing interlocking of grains tectonic deformation may start early before the rock is fully compacted or it may begin only much later and the mechanisms of deformation increase in complexity with the increasing temperature and hydrostatic stress of deeper burial up to a certain level of mild metamorphism however phyllosilicate grains seem to be rather permanent if not as mineral species then at least by preservation of their constituent sheets of silica tetrahedra given this permanence the mechanisms of rock deformation whether intergranular slip cataclasis or dissolution and reprecipitation and various intragranular processes in the non phyllosilicate matrix hardly matter as far as the recording of strain is concerned even a certain amount of stress solution at especially heavily loaded points of the phyllosilicate grains themselves does not destroy their value as strain markers as long as the dissolved material is disposed of mainly by crystallographically continuous overgrowth on existing grains the same holds for local dissolution of phyllosilicates in portions of old grains that have become thermodynamically unstable by bending widespread irregular or random nucleation and growth of newly nucleated phyllosilicate grains at intermediate or high grades of metamorphism can reduce the reliability of the strain record and so can domainal structure of the deformed rock if both the volume fraction of phyllosilicate grains and the intensity of total strain differ sharply from one domain to another if distinct domains are not equant and have themselves a strong preferred orientation of shape nevertheless rocks containing an anastomosing network of zones rich in phyllosilicate grains as slates and other deformed mudrocks commonly do have in many cases been found to have average preferred orientations of mica or chlorite which if used to estimate the strain give results similar to those obtained from independent strain markers in the same rocks thus the most significant verification of the usefulness of strain estimates by measurement of preferred orientation is the mutual confirmation with independently determined strain the limit to such verification is the rarity of strain markers so that the coexistence of two independent ones is exceptionally rare for this reason however it is important to make full use of the orientation of phyllosilicate grains where they are available the alternative to have no strain measurements at all causes structural geologists to miss potential information

strain patterns of crescentic granitoid plutons in the archean greenstone terrain of ontario abstract granitoid plutons of crescentic outcrop pattern in the archean terrain of northwestern ontario are dominated by constrictive strains that are interpreted to be due to intrusion rather than regional deformation the strain patterns of antiformal plutons are characterized by subvertical lineations in contrast the strain patterns of synformat plutons are characterized by subhorizontal lineations presumably only the last flow increments were recorded by the deformed mineral aggregates employed as strain gauges and occurred when the granitoid magma was largely crystalline simple kinematic models are capable of explaining many but not all elements of the natural strain patterns

horizontal cleavage in southeastern sinai the case for a coaxial strain history abstract a horizontal cleavage and associated lineation are developed in a low pressure igneous related metamorphic terrain in southestern sinai the cleavage is axial planar to recumbent folds which are never large and varies from a smooth slaty cleavage to a discrete crenulation cleavage structural evidence on the macro meso and microscales suggests that cleavage and lineation were formed during irrotational extension and not by simple shear estimates of strain using the march method indicate 50 70 vertical shortening this structural evidence when combined with metamorphic and age data strongly suggests that the cleavage was formed by the forceful emplacement of a pluton at depth

evolution of the kid group southeast sinai peninsula thrusts melanges and implications for accretionary tectonics during the late proterozoic of the arabian nubian shield the late proterozoic age kid group in the southeast sinai peninsula is a volcano sedimentary sequence of island arc affinities the group deformed in four main phases with a thermal peak after d1 followed by a regional greenschist facies overprint during d2 recumbent folding followed by thrusting began during d2 with the folds being propagated southward toward the arc foreland until d4 the last folding event the thrusts are responsible for the juxtaposition of four megatectonic units above a substrate of unknown composition these units probably allochthonous comprise a basal complex of infracrustal gneiss and three arc related units including inferred vestiges of a forearc accretionary prism and a marginal basin narrow slices of ophiolitic rocks were emplaced tectonically along the boundaries of some of these units melanges attributed to gravitational collapse of the forearc edifice and thrust tectonics formed in particular along the arc forearc transition zone the evolution of the kid group coupled with data from the rest of the arabian nubian shield can be well accommodated within the concept of oceanic crust subduction magmatic arc development and ultimate collision between the arc complex and a continental margin

absence of thermal influence from the african superswell and cratonic keels on the mantle transition zone beneath southern africa evidence from receiver function imaging abstract the depths of the 410 km d410 and 660 km d660 discontinuities beneath southern africa which is presumably underlain by the lower mantle african superswell are imaged in 1 radius consecutive circular bins using over 6400 p to s receiver functions rfs recorded by 130 seismic stations over a 27 yr period when the iasp91 standard earth model is utilized for moveout correction and time depth conversion a normal mantle transition zone mtz thickness of 246 7 km is observed suggesting that the superswell has no discernible effect on mantle transition zone temperature based on the negligible disparity of the mean mtz thicknesses between on 246 6 km and off 246 8 km cratonic regions we conclude that the deep archean cratonic keels possess limited influence on mtz thermal structure the apparently shallower than normal mtz discontinuities and the parallelism between the d410 and d660 are mostly the results of upper mantle high wave speed anomalies probably corresponding to a thick lithosphere with a mean thickness of about 245 km beneath the kaapvaal and 215 km beneath the zimbabwe cratons in contradiction to conclusions from some of the previous studies the resulting spatial distribution of the stacking amplitudes of the p to s converted phases at the discontinuities is inconsistent with the presence of an excessive amount of water in the mtz and atop the d410

microstructural evidence of rotation and non rotation of mica porphyroblasts abstract mica porphyroblasts in schists from several regions show nearly planar inclusion trails that are parallel over areas much larger than the wavelengths of later folds this indicates that the porphyroblasts have not rotated with respect to geographical co ordinates during deformation instead the matrix has rotated as suggested by ramsay 1962 even in zones of marked shortening in the matrix adjacent to large rigid porphyroblasts e g of cordierite or staurolite small biotite porphyroblasts have not rotated but have become thinned by solution as indicated by parallelism of inclusion trails in separate biotite grains and by evidence of truncation of inclusion trails by the matrix foliation less common are biotite porphyroblasts that have single asymmetrical microfolds in the matrix adjacent to the porphyroblasts and so appear to have rotated these porphyroblasts are characterized by kinking

monitoring of subsidence with uavsar on sherman island in california s sacramento san joaquin delta abstract sherman island the westernmost island in the sacramento san joaquin delta in california plays a crucial role in maintaining the water flux between saline ocean water from the san francisco bay to its west and the rest of the delta to its east land elevation below mean sea level and continuous subsidence over the past century has made this island a high priority area for investigations of subsidence and restoration in the delta this study reports the results of successful application of interferometric synthetic aperture radar insar data and technique to measure subsidence in the delta which is a coherence challenged non urban area we carried out a time series interferometric analysis of uninhabited aerial vehicle synthetic aperture radar uavsar l band 23 8 cm data collected from july 2009 august 2014 to assess both the spatial and temporal variation of subsidence on sherman island we report both large scale island wide subsidence trends and small scale levee farm scale subsidence features in sherman island assuming the subsidence is linear during the five years of uavsar data acquisition subsidence rates across the island range from 0 5 cm yr with an average of 1 3 0 2 cm yr we estimate our systematic uncertainty to be 0 3 cm yr overall the central region in the island has subsided at a faster rate than the rest of the island we find our results to be consistent with previous measurements of subsidence rates at electric transmission line towers scattered throughout the island the results of this study provide insights into several factors influencing subsidence including soil type water table depth land use land elevation and the location and time of levee repairs subsidence monitoring on sherman island is essential for maintaining a reliable water supply for the state of california and for protecting the delta ecosystem

isothermal compressibility of hydrous ringwoodite and its relation to the mantle discontinuities high pressure x ray diffraction experiments of hydrous ringwoodite containing 2 8 0 2 wt h2o have been carried out by using x rays from a synchrotron radiation source with a diamond anvil cell under hydrostatic pressure up to 6 gpa the isothermal bulk modulus k0 was determined to be 148 1 gpa with the pressure derivative k 0 fixed to 5 using the birch murnaghan equation of state this value is almost 80 of that of anhydrous ringwoodite the relations of the bulk modulus and zero pressure volumes v0 with h2o content are estimated within the solid solution range combining the present results with recent seismological data the nature of the 520 km and 660 km seismic discontinuities is discussed

seismic discontinuities in the mediterranean mantle abstract layering in the upper and lower mantle across the mediterranean has been determined using p to s converted phases identified through receiver function analysis conversion of the receiver function s time axis to depth was based on local 1d s velocity models the 1d velocity model for each station was extracted from a three dimensional s velocity model for the mediterranean region eav03 enhanced with crustal structure derived from receiver function analysis under the mediterranean region we observed a mantle transition zone thickness of 261 10 km on average which agrees with a dominance of high velocities imaged in tomographic models at these depths a thick mantle transition zone 270 km was observed in regions with ongoing or past subduction eastern spain southern italy southern greece and the north western african coast conversions from the 410 km and 660 km discontinuities were clearly observed for most stations amplitudes of both discontinuities are anomalously large throughout the mediterranean region 4 9 and 5 7 respectively we observed significant conversions from the 520 km discontinuity the lower mantle showed strong lateral variations resulting in spatially incoherent converted phases from discontinuities near 860 km between 900 km and 1200 km and 1320 km depth

the upper mantle discontinuity structure beneath eastern china 1 we apply a common conversion point stacking technique of receiver functions from the north china interior structure project ncisp to image the upper mantle discontinuity structure beneath eastern china the stacking results of dense receiver functions reveal a prominent discontinuity structure the appearance of the 410 km discontinuity is sharp and consistent with little influence by western pacific subduction slabs the 520 km discontinuity is relatively weak and consists of splitting phases at depths ranging from 500 km to 550 km as for the 660 km discontinuity double discontinuities and a narrow depression zone are detected near a depth of 660 km indicating a more complex structure there phase transformations in non olivine components such as from garnet to perovskite seem to be a more plausible interpretation for a complex 660 km discontinuity in this region

structural effects on the proterozoic ullensvang group west norway relatable to forceful emplacement of expanding plutons an isolated synclinorium comprising the up to 5 km thick ullensvang group of metavolcanic and metasedimentary rocks is surrounded by granitoid plutons and has no recognizable older basement from an earlier interpretation based on regional data this area is inferred to be part of a proterozoic cordilleran type volcanic plutonic belt major structures in the synclinorium are interpreted to have evolved as follows 1 early folds were produced by regional compression 2 within a part of the synclinorium such folds were tightened and reoriented during a subsequent deformation phase caused by forceful emplacement of the kvinnherad batholith flanking the synclinorium to the southwest 3 this deformation was accompanied by thrust faulting towards the northeast away from the batholith 4 forceful emplacement of plutons belonging to the eidfjord kinsarvik batholith on the northeastern flank of the synclinorium produced southwest vergent overfolding of the earlier structures in the synclinorium in addition local deformation around a pluton intruding the northern part of the synclinorium near utne produced areally restricted deformation structures approximately corresponding in time with the second deformation event 2 above

constraining velocity and density contrasts across the crust mantle boundary with receiver function amplitudes summary this paper investigates the ability of p wave receiver functions to constrain both the velocity and density contrasts across the moho expressions for receiver function amplitudes corresponding to a layer over a half space are extended to media with depth dependent properties by explicitly accounting for free surface reflection coefficients forward numerical computations show that receiver function amplitudes become frequency dependent for depth dependent structures and that for a given frequency wavelengths associated to ps converted phases are larger than wavelengths associated to multiply reverberated phases when consistent measurements of the amplitudes of ps phase and multiples can be obtained the extended expressions suffice to account for the observed amplitudes receiver function amplitudes are sensitive to both velocity and density contrasts across the moho and a two step grid search procedure is proposed to recover the contrasts from their measurement the near surface velocity is recovered in the first step from the amplitude of the direct p wave and then used in the second step to work out the relative density and s wave velocity contrast from the amplitude of the secondary phases examples from central spain pab and the indian shield hyb are discussed and demonstrate that receiver function amplitudes can constrain contrasts across the moho accurately enough to be utilized in geological interpretation

variability of p660s phases as a consequence of topography of the 660 km discontinuity abstract records from the broad band nars nl array deployed in the netherlands western germany and belgium have been analyzed for p to s converted phases from the 660 km discontinuity p660s no individual seismogram from this array shows a clear p660s phase stacks of up to 113 crosscorrelograms show p660s energy that is weak in comparison with similar stacks elsewhere very high amplitude observations of p660s in single seismograms from the same region have been made in a previous study paulssen 1985 geophys res lett 12 709 712 paulssen 1988 j geophys res 93 10489 10500 such variations may be due to relief on the 660 km discontinuity in this paper we quantify the effects of different topographies on the time delays amplitudes and waveforms of the direct p wave as well as on the p660s converted wave synthetic p and p660s waveforms are calculated by applying a kirchhoff helmholtz integration over the wave fields at the 660 km discontinuity interface for topography on scales of a few hundreds of kilometers the effects on the p wave are very small compared with those on the p660s phases time delays focused and defocused amplitudes and waveform distortion of p660s phases influenced by such topography diminish the coherence of p660s in synthetic stacks our results show that topography of the 660 km discontinuity with dominant scale lengths of a few hundreds of kilometers and variations in depth of 15 km to 25 km is consistent with the pattern of p660s observations in single seismograms and stacks

elastic properties of hydrous ringwoodite sound velocities and single crystal elastic moduli of hydrous mg 2 sio 4 ringwoodite containing 2 3 wt h 2 o have been measured by brillouin spectroscopy at ambient conditions the aggregate elastic moduli vrh averages are k s 165 8 5 gpa and 107 4 3 gpa for the adiabatic bulk modulus and shear modulus respectively although the aggregate elastic moduli and acoustic velocities of hydrous ringwoodite are smaller than those of anhydrous ringwoodite by about 10 and 3 6 respectively our results indicate that water has a smaller effect on the elastic moduli and velocities of ringwoodite than implied by previous studies our new results are consistent with systematic relationships between changes in the elastic properties and density that accompany the hydration of mineral phases

anisotropy of magnetic susceptibility and magmatic structures in the guerande granite massif france a multidisciplinary approach has been used in order to map out the syntectonic magmatic structures of the guerande leucogranite in southern brittany western france on one hand extensive field work and microscopic observations and on the other a detailed magnetofabric investigation have been carried out the former showed a magmatic flow plane trending generally to the ene wsw with a moderate dip to the north and a flow direction slightly dipping toward nne these observations suggest a blade shaped body which is not deeply rooted the magnetofabric as defined by the anisotropy of magnetic susceptibility is mainly related to hematite ilmenite grains with a mean rock susceptibility of about 3 10 6 g oe there is an excellent agreement between the magnetic lineation kmax and the magmatic flow direction the agreement between the magnetic foliation kmax kint plane and the magmatic flow plane is not as good the quantitative analyses of the anisotropy allows one to group the data into two domains the one with planolinear ellipsoids corresponding to the central part of the granitic body where flow was mainly achieved on the viscous state and the other in the flattening domain related to the northern border area which exhibits incipient orthogneissification that is some deformation on the solid state such a favorable comparison with the classical structural method is encouraging for using the anisotropy of magnetic susceptibility technique more routinely for structural mapping of granitic bodies provided that the microstructural state of the rock is already well known

determination of march strain from phyllosilicate preferred orientation a semi numerical method abstract the tensor for the strain in a fine grained phyllosilicate rich rock can be estimated according to the march theory an extended march equation with seven coefficients relates march strain in a rock to the angular density distribution function adf of the basal planes of its phyllosilicates the adf is mapped from 420 intensity measurements by an automatic x ray pole figure goniometer in the transmission mode five of the coefficients are the independent components of the constant volume strain tensor the two additional coefficients pertain to x ray goniometry the strain tensor and the adf share principal directions by theory by visually determining these directions from the fabric diagram and expressing the extended march equation in reference coordinates coinciding with the principal directions the seven coefficients in the equation are reduced to four applying the levenberg marquardt routine we calculate the remaining four coefficients by a best fit of the adf to the march equation in principal coordinates as an example the strain in a slate from north wales was measured by this method as 0 77 0 05 0 30 0 04 0 57 0 00 which is compatible with the results from the direct measurement of ellipsoidal strain indicators 0 80 0 31 0 57 this is an improvement over earlier estimates on the same sample by means of a graphic approximation 0 81 0 38 0 60 the stability and uniqueness of the solution was tested by convergences starting from a wide range of guess values and its sensitivity to bad estimates of principal direction by test calculations starting from two misalignments of different severity

the structure of sw cornwall and its bearing on the emplacement of the lizard complex six phases of deformation can be recognized in the devonian metasediments to the n of the lizard complex the first two phases d 1 d 2 show a variety of orientations and styles but with a general n or nw vergence these phases while respresenting regional deformation events are considerably modified by the emplacement of the lizard complex there is a general increase in intensity of both d 1 and d 2 structures towards the lizard boundary this is accompanied by variation in the orientation of both early phases of deformation leading to a variety of interference patterns domains of oblique f 1 folds with axes trending n s are developed in the rocks immediately to the ne and nw of the lizard and are considered to have formed by differential nnw directed thrusting of slabs of the complex during the early regional deformation d 2 produced discrete shear zones ahead and below the lizard rocks and may have resulted from the gravitational collapse of the d 1 nappes the d 3 deformation post dates the lizard emplacement being related to the cornubian batholith subsequent phases represent essentially post batholith and hence post lizard emplacement deformation