influence of water column anoxia and sediment supply on the burial and preservation of organic carbon in marine shales abstract previous work has suggested that the laminated organic rich and bioturbated organic poor shales of the camp run member of the late devonian early mississippian new albany shale formed under anoxic and oxygenated bottomwater conditions respectively and that the interbedding of the two facies was due to the vertical oscillation of a water column anoxic oxic boundary where it impinged on the basin margin we have extended this analysis by examining the chemical and mineralogical differences between the two shale facies in a single borehole core by seeking evidence for deposition of the laminated shales under bottom water oxia or anoxia and by determining whether the laminated shales formed when the carbon supply to the sea floor was higher the results of this study show that the laminated and bioturbated shales are mineralogically and chemically distinct relative to al an index of the aluminosilicate content si ti fe p na ba co cr cu mo ni v zn and zr are all higher whereas mn ca mg and sr are lower in the laminated compared with the bioturbated shales the differences are due to a higher quartz feldspar titanite ilmenite and zircon content in the laminated shales probably indicating a coarser grain size and the greater abundance of manganoan calcite in the bioturbated shales dissolved oxygen was present in bottom waters during the deposition of some of the laminated shale intervals because of the presence of manganoan calcite a phase that can only form in sediments with an oxic surface in addition the organic matter preserved in the two shale types is isotopically different 13corganic values are 1 9z permil lighter on average in the laminated compared with bioturbated intervals possibly indicating a larger fraction of terrestrial organic matter in the latter 15n values are 1 9z permil lighter on average in laminated compared with bioturbated intervals possibly indicating a larger fraction of terrestrial organic matter in the latter 15n values are 1 9z permil lighter on average in laminated compared with bioturbated intervals suggesting that nutrient drawdown was less during the deposition of the organic rich laminated shales the chemical mineralogical and isotopic contrasts between the two shale facies of the camp run member indicate that the conditions of sedimentation were different during their deposition the difference was possibly related to variations in sea level which would have caused the camp run shoreline to move closer to and farther from the core site causing in turn the deposition of coarser and finer grained sediments that contained different mixtures of marine and terrestrial organic matter bottomwater conditions were anoxic during deposition of most laminated intervals bottom water anoxia or dysoxia led to decreased burial and preservation of the essential nutrient phosphorus in the laminated organic rich shales relative to the rocks deposited beneath better oxygenated bottomwaters increased availability of phosphorus in the water column on long timescales leads to increased productivity and a higher settling flux of organic matter causing bottom water oxygen levels to fall this is consistent with the nitrogen isotope evidence suggesting that production was probably higher during the deposition of the organic rich shales thus production variations coupled with enhanced sedimentary regeneration of phosphorus from sediments related to low oxygen bottom water concentrations provide a general mechanism for the formation of the alternating facies of this member of the new albany shale

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

global upper mantle structure from finite frequency surface wave tomography 1 we report global shear wave velocity structure and radial anisotropy in the upper mantle obtained using finite frequency surface wave tomography based upon complete three dimensional born sensitivity kernels because wavefront healing effects are properly taken into account finite frequency surface wave tomography improves the resolution of small scale mantle heterogeneities especially for deep anomalies that are constrained by the longest period surface waves in our finite frequency model ffsw1 the globally averaged radial anisotropy shows a transition from positive sh sv to negative anisotropy sv sh at about 220 km consistent with a change in the dominant mantle circulation pattern from predominantly horizontal flow at shallow depths to vertical flow at greater depths the radial anisotropy beneath cratons and the old pacific plate agrees well with previous studies however our model exhibits a strong negative radial anisotropy at depths greater than 120 km beneath mid ocean ridges a feature that is not present in previous upper mantle models more interestingly the depth extent of the ridge anomalies is distinctly different beneath fast and slow spreading centers anomalies beneath fast spreading centers are stronger but the strength decreases rapidly below 250 km in contrast beneath slow spreading centers such as the northern mid atlantic ridge and the red sea anomalies extend down at least to the top of the transition zone the different depth extent of the ridge anomalies suggests that the primary driving force of slow spreading seafloor may be different from that of fast spreading seafloor and that active upwelling beneath slow spreading ridges may play a major role in the opening of the seafloor

seismic anisotropy in the continental crust of northwestern canada s u m m a r y most studies of the seismic structure of continental crust assume that the wave speeds are isotropic at seismic wavelengths the ability to measure surface wave propagation speed from the cross correlation of ambient seismic noise provides new opportunities to image the crust and uppermost mantle we investigate radial anisotropy in the continental crust of northwestern canada from group velocity curves of love and rayleigh waves obtained from ambient noise cross correlation we test the null hypothesis that the love and rayleigh group speed curves can be simultaneously fit by an earth model containing isotropic seismic velocities throughout the crust group velocity is predicted for 200 000 one dimensional earth models which are generated by randomly varying the crustal shear velocity and radial anisotropy within a prescribed range the goodness of fit of the predictions is assessed by comparison with two sets of observed dispersion curves that correspond to two tectonically distinct terranes the archean early proterozoic craton and the transition from craton to cordillera the majority of best fitting models contain vsh vsv 4 5 per cent in the middle crust the finding that the middle lower crust is seismically anisotropic across a large swath of northwestern canada combined with recent observations of anisotropic crust in much of the western united states suggests that anisotropy may be ubiquitous in the continental crust

tsunami hazard assessment of coastal south africa based on mega earthquakes of remote subduction zones the nuclear structural engineering pty and the national research foundation through the technology and human resources for industry programme project thrip tp2011061400009

carbon and oxygen isotope composition of carbonate in bulk sediment in the southwest taiwan basin south china sea methane hydrate decomposition history and its link to mud volcano eruption abstract anaerobic oxidation of methane aom coupled with sulfate reduction sr generates authigenic carbonate which has the unique carbon and oxygen isotope composition in sulfate methane transition zone smtz the carbon and oxygen isotope composition of carbonate in sediments which can be used to revel the variable methane flux is affected by the percentage of aom driven carbonate in sediments to investigate the methane hydrate decomposition history in the southwest taiwan basin we analyzed a 13 m long sediment core for grain size carbonate and elemental carbon contents and stable carbon and oxygen isotopes composition of carbonate 13ccarb and 18ocarb the ams 14c dating results and carbonate content reveal that a turbidity current occurred at around 14 ka b p which reversed sediment accumulation between 455 and 885 cm in depth combining the chronological framework and 13ccarb values we identified that the methane hydrate decomposition scale began to decrease at 14 ka b p moreover we discovered that three transiently enhanced methane hydrate decomposition events occurred from then on although the 13ccarb and 18ocarb variability are as expected the low 18ocarb values are inconsistent with the previous theory which favors 18o enriched in aom driven authigenic carbonate thus we suggest that a large amount of 18o depleted water which originally migrated from southwest taiwan island was injected into surface sediment along with the eruption of mud volcanos as the terminal electron accepter of aom sr reaction the exogenous 18o depleted water leads its special oxygen isotope to be inherited by the authigenic aom driven carbonate the proposed theory provides new insights on the authigenic carbonate in methane hydrate area and the association between methane hydrates and mud volcanoes

polarization anisotropy and fine scale structure of the eurasian upper mantle we have observed shear wave splitting with tsh tsv tsv up to 1 5 on long period records of multiply reflected s waves bottoming in the upper mantle beneath the russian and siberian platforms the dispersion of love and rayleigh waves over these paths shows discrepancies of comparable or larger magnitude with respect to smooth isotropic si structures consistent with a model of the uppermost mantle having significant apparent vertical anisotropy although the splitting and dispersion data can be fit by smooth anisotropic sa models we have investigated the apparent anisotropy associated with fine scale rough structure beneath stable eurasia we fit the data with a rough isotropic ri model having an rms shear velocity fluctuation that varies from 14 in the uppermost mantle to zero at 400 km depth these fluctuations are larger than the variation in isotropically averaged parameters expected for even a diverse assemblage of upper mantle ultrabasic rocks which we take to be evidence for some sort of intrinsic local anisotropy

compressional and shear wave anisotropy in the oceanic lithosphere the ngendei seismic refraction experiment summary synthetic seismogram modelling of seismic refraction data from the 1983 ngendei expedition to the south pacific indicate seismic anisotropy both within the top kilometre of oceanic crust and in the uppermost mantle we calculate average p and s wave velocity versus depth functions for two orthogonal azimuths n3ooe approximately aligned with the fast mantle direction and n120 e close to the fast crustal direction probable lateral heterogeneities at the ngendei site prevent perfect matching of data and synthetic waveforms crustal anisotropy is indicated by a 28 pattern of both p and s wave travel times as a function of azimuth and is consistent with an approximate 0 2 km s difference in p wave velocities and 0 1 km s difference in s wave velocities upper mantle anisotropy is characterized by pn velocity differences of 7 95 8 4 km s l and a nearly uniform s velocity of 4 65 km s we use these velocity profiles and traveltime data to calculate bounds on the properties of the elastic constants of the crust and upper mantle the crustal anisotropy can be explained by a model involving aligned cracks parallel to the original spreading ridge resulting in a fast direction perpendicular to the fossil spreading direction the upper mantle anisotropy is consistent with aligned olivine crystal models in which the fast direction is parallel to the fossil spreading direction for both of these models we use bounds from the ngendei data to place constraints on the physical properties of the lithosphere

faulted structure of the bottom simulating reflector on the blake ridge western north atlantic abstract high resolution multichannel seismic data collected from the blake ridge in the western north atlantic by the naval research laboratory s deep towed acoustics geophysics system dtags show that the bottom simulating reflector bsr in this area is the reflection from the interface between an approx 440 m thick section of hydrate bearing sediment overlying and approx 5 m thick layer of methane gas rich sediment the high resolution attainable by the deep tow seismic system reveals normal fault offsets of approx 20 m in the bsr these growth faults may provide a path for vertical migration of methane initially concentrated beneath the hydrate bearing sediment enabling hydrate to form throughout sediment above the bsr because the bsr represents a methane gas methane hydrate phase boundary rather than a lithologic or diagenetic horizon the observed off set of the bsr itself reflects discontinuities in the pressure temperature field across the fault zones where they intersect the bsr directional ambient noise bottom scattering deep towed array geophysical system ocean bottom seismometer towed array

identification and numerical modelling of hydrocarbon leakage in the lower congo basin implications on the genesis of km wide seafloor mounded structures abstract we present a combined approach of interpretation of 2d seismic reflection data and numerical modelling of hydrocarbon generation and migration across the southern slope of the lower congo basin in order to investigate the factors controlling timing and distribution of hydrocarbon leakage in this area we identified three main families of past and present day leakage features 1 mid upper miocene seismic chimneys concentrated basinwards and ending up on buried pockmarks 2 plio pleistocene chimneys rather clustered to the east of the study area and ending up in seafloor pockmarks and 3 fewer scattered chimneys identified within the miocene sequences ending up in shallow enhanced reflectors flat spots stratigraphic and structural elements seem to control the distribution of these features at least two major events of leakage occurred during the middle late miocene and intermittently during the pliocene present external factors as sediment supply are associated to the miocene leakage event whilst internal structural elements probably triggered the pliocene to present day leakage a major seabed morphological feature represented by a margin paralleled belt of more than 1 km wide mounds was identified above growth faults to the east of the study area data constrained 2d hc generation and migration modelling suggests a genetic link between these structures and vertical migration leakage of thermogenic methane sourced from either currently mature oligo miocene source rocks or secondary cracking and further expulsion from over mature upper cretaceous source rocks hence the mounds are likely to represent a lineation of methane derived carbonate build ups despite the natural limitations of a 2d migration model when combined and calibrated with observations from seismic data it can be used as a valid tool to assess petroleum migration routes in sedimentary basins to the best of our knowledge this is the first integrated approach combining both seismic observations and numerical modelling carried out in the angola basin

possible pingo fields in the utopia basin mars geological and climatical implications abstract the presence of pingos on mars has been hypothesized since the period of the viking mission in fact a diverse range of pingo like features has been found at various martian sites including elysium chryse and utopia planitiae in the northern lowlands due to the morphology and the geological setting some of those features were interpreted in different ways creating some controversies as happened in athabasca valles this reflects the complexity of interpreting these features by remote sensing and multiple plausible interpretations of the same feature with the objective of identifying new possible pingos or rootless cones on mars we selected a study area in utopia planitia 10 55 n 210 260 w where the presence of both features is possible due to its geological history volcanic and hydrological we analyzed more than 2100 mars orbiter camera moc narrow angle images in addition to viking thermal emission imaging system themis and high resolution stereo camera hrsc images together with mars orbiter laser altimeter mola derived digital elevation models dems with a geographic information system gis we found in 94 moc narrow angle images dome cone and ring shaped features we analyzed them from morphological and morphometrical points of view in order to compare them with relevant features on mars and earth we tested different possible origins for those features following the approach of multiple working hypotheses we conclude that the dome cone and ring shaped features could be pingos which is in agreement with their geological settings regarding the driving heat source for the formation of the purported pingos we propose the existence of a heat source possibly a magma chamber underneath the surface of the utopia basin together with possible climatic shifts the past activities of the heat source may have caused melting of ground ice the pingo growth due to freezing of the water would have occurred during the following cold climatic conditions

detection of gas hydrates in faults using azimuthal seismic velocity analysis vestnesa ridge w svalbard margin accepted for publication in journal of geophysical research solid earth copyright 2020 american geophysical union further reproduction or electronic distribution is not permitted

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

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

frequency dependent analysis of sks skks waveforms observed in australia evidence for null birefringence abstract we analyzed sks skks waveforms recorded by permanent broadband stations in australia and found a lack of birefringence signals in general are absent on the transverse t component of high quality data over a wide range of back azimuths at periods longer than 6 s including the dominant periods of 8 10 s for sks skks there is no exception out of 89 high quality observations at periods between 1 3 and 3 s eight observations show appreciable signal on the t component the anomalous signals on the t component however are unlikely to be a result of azimuthal anisotropy in the upper mantle 1 for a particular earthquake station pair anomalous signal is present only in either skks or sks but not in both since the fresnel zones of these two phases overlap in the upper mantle this observation alone rules out simple homogeneous layers of azimuthally anisotropic upper mantle as the main cause of anomalous signals 2 the small number of anomalous observations are restricted to high frequencies over limited back azimuths and distances 3 signals on the t component do not resemble the time derivative of waveforms on the radial component these observations and an analysis of fresnel zones suggest that the anomalous waveforms are probably caused by scattering associated with small scale heterogeneities in a corridor trending roughly east west in the lower mantle if the structure lies near a depth of approximately 1000 km the source of anomalous propagation correlates with the estimated position of paleo pacific lithosphere that was subducted more than 30 60 ma ago suggesting remnant slab in the lower mantle as a plausible cause of anomalous sks skks arrivals the lack of azimuthal anisotropy beneath australia may be related to its high speed of current plate motion in the hotspot frame of reference a feature unique to the indo australian plate sparse sks skks data from the indian shield also support this view

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

upper mantle anisotropy from long period p polarization we introduce a method to infer upper mantle azimuthal anisotropy from the polarization i e the direction of particle motion of teleseismic long period p onsets the horizontal polarization of the initial p particle motion can deviate by 10 from the great circle azimuth from station to source despite a high degree of linearity of motion recent global isotropic three dimensional mantle models predict effects that are an order of magnitude smaller than our observations stations within regional distances of each other show consistent azimuthal deviation patterns while the deviations seem to be independent of source depth and near source structure we demonstrate that despite this receiver side spatial coherence our polarization data cannot be fit by a large scale joint inversion for whole mantle structure however they can be reproduced by azimuthal anisotropy in the upper mantle and crust modeling with an anisotropic reflectivity code provides bounds on the magnitude and depth range of the anisotropy manifested in our data our method senses anisotropy within one wavelength 250 km under the receiver we compare our inferred fast directions of anisotropy to those obtained from pn travel times and sks splitting the results of the comparison are consistent with azimuthal anisotropy situated in the uppermost mantle with sks results deviating from pn and ppol in some regions with probable additional deeper anisotropy generally our fast directions are consistent with anisotropic alignment due to lithospheric deformation in tectonically active regions and to absolute plate motion in shield areas our data provide valuable additional constraints in regions where discrepancies between results from different methods exist since the effect we observe is local rather than cumulative as in the case of travel time anisotropy and shear wave splitting additionally our measurements allow us to identify stations with incorrectly oriented horizontal components

small scale sublithospheric continental mantle deformation constraints from sks splitting observations summary it is generally believed that azimuthal seismic anisotropy is an important property of the subcontinental mantle and that this is a relatively small scale phenomenon compared with anisotropy of the oceanic mantle during recent years shear wave splitting has come to be viewed as the most direct diagnostic of azimuthal anisotropy the magnitude of the splitting delay times can often rule out a crustal source of anisotropy however it is not yet clear whether the source of anisotropy lies within the lithosphere or the sublithosphere it is essential to know the scale of lateral variations and the distribution of anisotropy with depth in order to understand the origin of this phenomenon here we describe a way to constrain the depth of the anisotropy by studying the lateral variation of the splitting parameters at neighbouring seismic stations we use sks and skks phases recorded at the nars nl array a relatively dense network of broad band stations separated by about 50 km fresnel zones at different depths are calculated for these phases the depth of the anisotropy is constrained by the criterion that fresnel zones corresponding to different splitting observations should not overlap variability in the splitting measurements for events with different directions of approach recorded at one station and for single events recorded at various stations provides evidence for a non homogeneously distributed source of anisotropy located below the array at different depths we show that this variability in the measurements cannot be due to effects of crustal scattering our results indicate that the anisotropy is partly located in the sublithospheric mantle at about 400 km depth this suggests that the deformation in this region is subject to significant lateral variations

the chronology and tectonic style of landscape evolution along the elevated atlantic continental margin of south africa resolved by joint apatite fission track and u th sm he thermochronology atlantic type continental margins have long been considered passive tectonic settings throughout the entire postrift phase recent studies question the long term stability of these margins and have shown that postrift uplift and reactivation of preexisting structures may be a common feature of a continental margin s evolution the namaqualand sector of the western continental margin of south africa is characterized by a ubiquitously faulted basement but lacks preservation of younger geological strata to constrain postrift tectonic fault activity here we present the first systematic study using joint apatite fission track and apatite u th sm he thermochronology to achieve a better understanding on the chronology and tectonic style of landscape evolution across this region apatite fission track ages range from 58 3 2 6 to 132 2 3 6ma with mean track lengths between 10 9 0 19 and 14 35 0 22 m and mean u th sm he sample ages range from 55 8 31 3 to 120 6 31 4ma joint inverse modeling of these data reveals two distinct episodes of cooling at approximately 150 130ma and 110 90ma with limited cooling during the cenozoic estimates of denudation based on these thermal histories predict approximately 1 3 km of denudation coinciding with two major tectonic events the first event during the early cretaceous was driven by continental rifting and the development and removal of synrift topography the second event during the late cretaceous includes localized reactivation of basement structures as well as regional mantle driven uplift relative tectonic stability prevailed during the cenozoic and regional denudation over this time is constrained to be less than 1 km

relations between mud volcanoes thrust deformation slope sedimentation and gas hydrate offshore north panama abstract seamarc ii swath mapping and migrated seismic reflection data show a high concentration of mud volcanoes in the primary sediment depocentre along the lower slope of a thrust belt offshore north panama the mud volcanoes are 0 4 2 0 km wide sonar reflectivity backscattering sediment cores and seismic stratigraphic relations indicate that the depocentre contains thick sequences of basinal turbidites which are ponded between the anticlinal ridges the ridges are composed of the deformed turbidites of the colombian basin and exhibit a strong bottom simulating reflector bsr apparently associated with a gas hydrate layer based on the concentration of mud volcanoes along the crests of the anticlinal ridges in the depocentre and the structural position of the bsr we suggest that folding along the deformation front sediment ponding leading to differential loading methane migration and accumulation in the anticlines and gas hydrate formation are important factors in the development of mud volcanoes in this region

lehmann discontinuity as the base of an anisotropic layer beneath continents long period surface wave r 1 g 1 body wave s ss sss and scs reverberation data have been inverted to obtain anisotropic structures along seismic corridors that sample australia and the western pacific these models support the proposal that the lehmann discontinuity beneath stable continents represents a transition from an anisotropic lithosphere to a more isotropic material in the lower part of the continental tectosphere

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

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

seismic evidence for stratification in composition and anisotropic fabric within the thick lithosphere of kalahari craton 1 based on joint consideration of s receiver functions and surface wave anisotropy we present evidence for the existence of a thick and layered lithosphere beneath the kalahari craton our results show that frozen in anisotropy and compositional changes can generate sharp mid lithospheric discontinuities mld at depths of 85 and 150 200 km respectively we found that a 50 km thick anisotropic layer containing 3 s wave anisotropy and with a fast velocity axis different from that in the layer beneath can account for the first mld at about 85 km depth significant correlation between the depths of an apparent boundary separating the depleted and metasomatised lithosphere as inferred from chemical tomography and those of our second mld led us to characterize it as a compositional boundary most likely due to the modification of the cratonic mantle lithosphere by magma infiltration the deepening of this boundary from 150 to 200 km is spatially correlated with the surficial expression of the thabazimbi murchison lineament tml implying that the tml isolates the lithosphere of the limpopo terrane from that of the ancient kaapvaal terrane the largest velocity contrast 3 6 4 7 is observed at a boundary located at depths of 260 280 km beneath the archean domains and the older proterozoic belt this boundary most likely represents the lithosphere asthenosphere boundary which shallows to about 200 km beneath the younger proterozoic belt thus the kalahari lithosphere may have survived multiple episodes of intense magmatism and collisional rifting during the billions of years of its history which left their imprint in its internal layering

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

an olivine fabric database an overview of upper mantle fabrics and seismic anisotropy we present a unique database of 110 olivine petrofabrics and their calculated seismic properties the samples come from a variety of the upper mantle geodynamic environments ophiolites subduction zones and kimberlites with a wide range of micro structures a phenomenological relationship is established between p and s wave seismic anisotropy and the degree of crystal alignment fabric strength seismic anisotropy increases rapidly at low fabric strength before reaching a near saturation level of 15 to 20 for p waves and 10 to 15 for s waves despite a large variation in the symmetry of fabric patterns the average seismic properties of the different fabric micro structural and geodynamic settings have similar anisotropies in both magnitude and symmetry hence it would seem possible to determine some measure of fabric strength from seismic anisotropy if the dimensions of the anisotropic region are known but not geodynamic environment or details of the petrofabric pattern a simple pattern of seismic anisotropy characterises the average sample of the database which has the following features the polarisation plane of the fastest s wave is parallel or sub parallel to the foliation plane the maximum shear wave splitting is parallel to the y structural direction in foliation plane and normal to the lineation the maximum of the p wave velocity is parallel to the high concentration of 100 axes which is sub parallel to the lineation the 100 orientation distribution has the greatest influence on the p wave seismic anisotropy the 100 and 001 orientation distributions have the greatest influence on the symmetry of s wave anisotropy although the magnitude of anisotropy is influenced by the distribution of all three principal axes although the database only contains olivine petrofabrics this statistical study clearly shows that seismic anisotropy can be used to deduce the orientation of the structural frame in the upper mantle 1998 elsevier science b v all rights reserved

crust mantle coupling mechanism in cameroon west africa revealed by 3d s wave velocity and azimuthal anisotropy abstract to understand the depth variation of deformation beneath cameroon west africa we developed a new 3d model of s wave isotropic velocity and azimuthal anisotropy from joint analysis of ambient seismic noise and earthquake surface wave dispersion we found that the cameroon volcanic line cvl is well delineated by slow phase velocities in contrast with the neighboring congo craton in agreement with previous studies apart from the congo craton and the oubanguides belt the uppermost mantle revealed a relatively slow velocity indicating a thinned or thermally altered lithosphere the direction of fast axis in the upper crust is mostly ne sw but trending approximately n s around mt oku and the southern cvl the observed crustal azimuthal anisotropy is attributed to alignment of cracks and crustal deformation related to magmatic activities a widespread zone of weak to zero azimuthal anisotropy in the mid lower crust shows evidence for vertical mantle flow or isotropic mid lower crust in the uppermost mantle the fast axis direction changed from ne sw to nw se around mt oku and northern cameroon this suggests a layered mechanism of deformation and revealed that the mantle lithosphere has been deformed ne sw fast azimuths are observed beneath the congo craton and are consistent with the absolute motion of the african plate suggesting a mantle origin for the observed azimuthal anisotropy our tomographically derived fast directions are consistent with the local sks splitting results in some locations and depths enabling us to constrain the origin of the observed splitting the different feature of azimuthal anisotropy in the upper crust and the uppermost mantle implies decoupling between deformation of crust and mantle in cameroon

on the interpretation of sks splitting measurements in the presence of several layers of anisotropy summary concerns over the validity of expressions derived by montagner et al that link sks splitting measurements to the variation with depth of anisotropic parameters in the upper mantle have been recently expressed pointing out that the long period approximations applied by these authors may not be valid for the frequency range commonly used in sks studies and in particular that the anisotropy splitting parameters should depend on the order in which different anisotropy layers are arranged with depth we show here that indeed measurements of splitting time and fast axis direction performed at individual azimuths do depend on the order of layering however the expressions of montagner et al concern station averaged quantities that do not depend on the order of layers it is therefore correct to use these expressions in joint inversions of surface waveforms and sks station averaged splitting measurements on the other hand the depth dependent sensitivity of surface waveforms naturally provides constraints on the order of layering having clarified this confusion we extend the expressions of montagner et al to the case of a tilted axis of symmetry and non vertical incident waves and show that station averaged estimates of effective splitting parameters splitting time fast axis direction and tilt of the fast axis can be related to the integral with depth of quantities which now depend not only on the local splitting time and fast axis direction but also on the local tilt of the fast axis thus providing constraints also on the variation of the tilt with depth we show that the effective parameters used as constraints in the inversion can be obtained either from the measurement of splitting intensity or through a parameter search and cross convolution method in particular in the case when the effective tilt is significant the splitting intensity no longer presents 180 periodicity with azimuth providing a diagnostic tool for the presence of such tilts in the upper mantle thus combining body wave and surface wave observations also has the potential of constraining the variation with depth of the tilt of the fast axis of anisotropy a geodynamically important parameter

3d seismic analysis of the ak fault orange basin south africa implications for hydrocarbon leakage and offshore neotectonics abstract a high resolution 3d reflection seismic dataset was used to achieve a detailed description of the half graben bounded ak fault in the ibhubesi gas field of the offshore orange basin seismic analysis of the fault involved the combination of both the conventional seismic interpretation and seismic attributes the main objective of the study was to provide insights into hydrocarbon leakage and neotectonic activity along the west coast shelf of south africa the ak fault is among the several half graben bounded faults that formed in the late jurassic to early cretaceous during the separation of the south american and the african continents the ak fault consists of five seismically detectable segments whose evolution reflects characteristics of both the coherent and the isolated end members model of segmented fault origin diapiric structures that are direct indicators of vertical fluid movement were also observed along the ak fault system providing evidence of significant roles that the fault plays in promoting the vertical migration of hydrocarbon in the orange basin recent reactivation of the fault has also occurred along an oblique extensional regime that is dominated by normal dip slip faulting with minor shear components along the tips

the kaapvaal craton seismic anisotropy petrophysical analyses of upper mantle kimberlite nodules a dense network of seismic stations has been deployed on the kaapvaal craton south africa to investigate the upper mantle seismic structures in order to bring independent petrophysical constraints we analyze a direct sampling of the cratonic upper mantle and determine the seismic properties of 48 mantle nodules brought up to the earth s surface by kimberlite eruptions seismic properties of these nodules are calculated from the olivine and pyroxene crystal preferred orientations and the single crystal elastic constants despite variations in the nodules compositions microstructures and crystallographic preferred orientations seismic anisotropy is rather homogeneous throughout the craton mean s wave anisotropy is weak 2 64 which is compatible with the small measured sks wave splitting mean delay time of 0 62 s

s and p velocity heterogeneities within the upper mantle below the baltic shield abstract upper mantle structure beneath the baltic fennoscandian shield is investigated using non linear tomographic inversion of relative arrival time residuals 52 selected teleseismic earthquakes recorded by 45 broadband stations of the swedish national seismological network snsn provide 1532 good quality s wave relative arrival times sv and sh arrival time residuals were initially analyzed independently providing two separate models these reveal several consistent major features many of which are also consistent with p wave results lateral velocity variations of 3 4 are observed to depths of at least 470 km the correlation between the sh and sv models is investigated and shows a pattern of minor but significant differences down to around 150 200 km depth below which the models are essentially similar direct cell by cell comparison of the model velocities reveals a similar pattern with velocity differences between the models of up to 4 numerical tests show that differences in the two s wave models can only be partially attributed to noise and limited resolution and some features are attributed to the effect of large scale anisotropy one of the significant and sharp discrepancies between the s models coincides with a presumed boundary between archean and proterozic domains suggesting different anisotropic characteristics in the two regions

seismic structure of the upper mantle in a central pacific corridor the seismic structure of the tonga hawaii corridor has been investigated by combining two data sets revenaugh and jordan s reflectivity profile from scs reverberations which provides travel times to and impedance contrasts across the major mantle discontinuities and 1500 new observations of frequency dependent phase delays for the three component s ss and sss body waves and the r1 and g1 surface waves which constrain the velocity structure within this layered framework the shear waves turning in the upper mantle showed significant splitting of the sh and sv components indicative of shallow polarization anisotropy the data set was inverted in conjunction with attenuation and mineralogical constraints to obtain a complete spherically symmetric radially anisotropic structure the final model pa5 is characterized by a high velocity anisotropic lid bounded at 68 km depth by a large negative g discontinuity a low velocity anisotropic layer below g extending to a small l discontinuity at 166 km an isotropic steep gradient region between 166 km and 415 km and transition zone discontinuities at 415 507 and 651 km the depth of the radial anisotropy in pa5 is shallower than in most previous studies based on surface waves and higher modes the average value of radial shear anisotropy in the lid 3 7 is consistent with the magnitude expected from the spreading controlled models of olivine orientation while anisotropy in the low velocity zone which is required by our data set could be induced either by paleostrains that took place near the ridge crest or by shearing in the asthenosphere as a result of present day plate motions on the basis of recent work by hirth and kohlstedt we suggest that the g discontinuity is caused by a rapid increase in the water content of mantle minerals with depth marking the fossilized lower boundary of the melt separation zone active during crust formation the high gradient zone between 200 and 400 km is a characteristic feature of convecting oceanic upper mantle and is probably controlled by a steady decrease in the homologous temperature over this depth interval the average shear velocity gradient in the transition zone is lower than in most previous seismic models in better agreement with the predictions for a pyrolitic composition

fossil flat slab subduction beneath the illinois basin usa abstract the illinois basin is one of several well studied intracratonic sedimentary basins within the north american craton whose formational mechanisms and subcrustal structure are not well understood we study the s velocity structure of the upper mantle beneath the illinois basin and its surrounding area through seismic tomography we utilize continental scale waveform data of seismic s and surface waves enhanced by regional earthquakes located near the illinois basin our 3d tomographic model il05 confirms the existence of a slow s velocity structure in the uppermost mantle beneath the illinois basin region this anomalously slow region exists from the base of the crust to depths of 90 km and is slower than the north american cratonic average by about 200 m s this anomalous uppermost mantle beneath the illinois basin is underlain by a faster lithosphere typical of the surrounding craton to depths of 200 km excluding the formation of the reelfoot rift this area of north american has been stable for over 1 0 gy thus we do not expect thermal anomalies from before that time to persist into present day s velocity anomalies and we consider a delamination origin as an explanation of illinois basin subsidence unlikely we cannot rule out that the slow mid lithosphere beneath the illinois basin is caused by an uppermost mantle enriched by a deep but weak plume we attribute the slow mid lithosphere to the presence of either oceanic hydrous crust or a relatively cool mantle wedge with preserved hydrous minerals in the illinois basin s uppermost mantle related to a fossilized flat subduction zone

fluid flow and origin of a carbonate mound offshore vancouver island seismic and heat flow constraints a 1 5 km long 1 km wide and 70 80 m high carbonate mound was identified on the mid slope region of the subduction accretionary sedimentary prism offshore vancouver island 3 5 km west of ocean drilling program odp site 889 and integrated ocean drilling program iodp site u1327 seabed video images show the presence of seafloor carbonate as well as chemosynthetic communities a high resolution single channel seismic survey with close line spacing recording coherent reflectivity down to about 400 m beneath the seafloor provided acoustic images of this mound and of the gas hydrate bottom simulating reflector bsr beneath it the mound is interpreted to have developed as a structural topographic high in the hanging wall of a large reverse fault formed at the base of the current seaward slope the fault zone provides pathways for fluids including gas to migrate to the seafloor where diagenetic carbonate forms and cements the near surface sediments to examine the thermal effect of possible upward fluid flow beneath the mound heat flow at the mound and in the neighbouring region was calculated from the depth of the bsr below the seafloor these data were combined with heat flow calculated over a broader region from previous multi channel seismic data heat flow within the flattest portion of the surrounding 4 km by 8 km region averages 74 mw m2 taking this value to represent the regional or background heat flow a simple 2d analytical method was used to calculate theoretical heat flow variations due to topography across the mound most of the variability is explained by topographic effects including a local 6 mw m2 negative anomaly over the central mound and a large 20 mw m2 positive anomaly over the mound steep side slope however just south of the mound there is a 6 7 mw m2 positive anomaly in a 2 km long band that has predominantly flat seafloor most of this anomaly is probably unrelated to topographic effects but rather likely due to warm upward fluid flow along faults or fracture zones

an irregular feather edge and potential outcrop of marine gas hydrate along the mauritanian margin the dissociation of marine hydrate that surrounds continental margins is thought to be an agent for past and future climate change as the water depth decreases landwards the base of the hydrate stability zone progressively shallows until hydrate can occur at or immediately below the seabed where an increase in bottom water temperature can cause dissociation but the true extent of these most vulnerable hydrate deposits is unknown here we use exceptional quality three dimensional 3 d seismic reflection imagery offshore of mauritania that reveals a rare example of a bottom simulating reflection bsr that intersects the seabed and delineates the feather edge of hydrate the bsr intersects the seabed at the 636 m isobath but along the 32 km of the margin analysed the intersection is highly irregular intersections and seismic evidence for hydrate less than 4 3 m below the seabed occur in seven small localised areas that are 0 02 0 45 km2 in extent we propose gas flux below the dipping base of the hydrate to these places has been particularly effective the intersections are separated by recessions in the bsr where it terminates below the seabed seawards of the 636 m isobath recessions are areas where the concentration of hydrate is very low or hydrate is absent they are regions that have been bypassed by gas that has migrated landwards along the base of the hydrate or via hydraulic fractures that pass vertically through the hydrate stability zone and terminate at pockmarks at the seabed an irregular feather edge of marine hydrate may be typical of other margins

structure of the kaapvaal craton from surface waves phase delays of rayleigh and love waves from teleseismicearthquakesrecordedduringthesouthernafrica seismic experiment have been inverted for upper mantle seismic velocities along the propagation paths and beneath the station array successful models of subarray struc turedisplayedsignicantradialanisotropyintheuppermost mantle but no shear wave low velocity zone supporting the hypothesisthatthelithospherebeneaththekaapvaalcraton isthick theazimuthalvariationsofrayleigh waveslowness are not consistent with simple models based onskssplit tingobservationsandanisotropy measurementsofkaapvaal mantlexenoliths thisdiscrepancymayimplysomeamount of small scale heterogeneity in anisotropy

on the relation between seismic anisotropy and finite strain because seismic anisotropy in the upper mantle is due primarily to lattice preferred orientation lpo of olivine crystals induced by deformation observations of anisotropy can provide constraints on mantle deformation and flow in this paper we use a previously published theory for deformation induced lpo ribe and yu 1991 to quantify the relation between seismic anisotropy and deformation for two model compositions a pure olivine dunite aggregate and a harzburgite comprising 70 olivine and 30 enstatite analytical and numerical solutions of the governing equations show that the predicted lpo and hence also the seismic anisotropy depends only on the finite strain and not on the deformation path by which that strain is produced the magnitude of the anisotropy depends only on the ratios c1 c2 and c2 c3 where c1 c2 c3 are the lengths of the principle axes of the finite strain ellipsoid and the directions of maximum and minimum compressional wave velocity vp coincide with the axes c1 and c3 respectively we calculate the percent anisotropies for compresional and shear waves propagating along the three principle axes of the finite strain ellipsoid as functions of the axial ratios c1 c2 and c2 c3 for our two model compositions however inferences of the magnitude of finite strain obtained using these results are likely to be minimum estimates because our theory neglects the effects of dynamic recrystallization finally we present as a byproduct of this study a new direct method for calculating the finite strain produced by an arbitiary deformation history

timescales for the evolution of seismic anisotropy in mantle flow 1 we study systematically the relationship between olivine lattice preferred orientation and the mantle flow field that produces it using the plastic flow recrystallization model of kaminski and ribe 2001 in this model a polycrystal responds to an imposed deformation rate tensor by simultaneous intracrystalline slip and dynamic recrystallization by nucleation and grain boundary migration numerical solutions for the mean orientation of the a axes of an initially isotropic aggregate deformed uniformly with a characteristic strain rate show that the lattice preferred orientation evolves in three stages 1 for small times recrystallization is not yet active and the average a axis follows the long axis of the finite strain ellipsoid 2 for intermediate times the fabric is controlled by grain boundary migration and the average a axis rotates toward the orientation corresponding to the maximum resolved shear stress on the softest slip system 3 for the fabric is controlled by plastic deformation and average a axis rotates toward the orientation of the long axis of the finite strain ellipsoid corresponding to an infinite deformation the infinite strain axis in more realistic nonuniform flows lattice preferred orientation evolution depends on a dimensionless grain orientation lag parameter x defined locally as the ratio of the intrinsic lattice preferred orientation adjustment timescale to the timescale for changes of the infinite strain axis along path lines in the flow explicit numerical calculation of the lattice preferred orientation evolution in simple fluid dynamical models for ridges and for plume ridge interaction shows that the average a axis aligns with the flow direction only in those parts of the flow field where 1 calculation of provides a simple way to evaluate the likely distribution of lattice preferred orientation in a candidate flow field at low numerical cost

passive margin evolution and its controls on natural gas leakage in the southern orange basin blocks 3 4 offshore south africa using a 2d seismic dataset that covers part of the southern orange basin offshore south africa we reconstructed the geological evolution of the basin this evolutionary model was then used to investigate the occurrence of natural gas within the sedimentary column and the distribution of gas leakage features in relation to the observed sedimentary and tectonic structures developed in the post rift succession since the early cretaceous the cretaceous succession has been subdivided into five seismic units the highest sedimentation rates occur within the barremian aptian unit c1 and the turonian coniacian unit c3 two cenozoic units t1 and t2 have been distinguished these show a sudden decrease in sedimentation rate for the whole of the cenozoic three phases of gravitational tectonics with two late cretaceous phases of mass movement in the northern study area and cenozoic slumping in the southern study area have been related to sedimentation rates sea level changes paleoenvironmental evolution and regional tectonics the occurrence of natural gas leakage follows a coast parallel distribution within the study area in the near shore part at water depths shallower than 400 m massive gas chimneys penetrate through the sediment layers and reach the near surface within an intermediate narrow band between 300 and 500 m water depth the gas migrates more diffusely through sub vertical faulted cretaceous sediments while in the outer part of the basin through the cretaceous and cenozoic gravitational wedges only very few signs of gas accumulation and migration can be seen along the faults a conceptual model has been established with the aptian source rock generating gas in the outer part of the basin this source rock underlies the cenozoic wedge in the south and the thick cretaceous wedge in the north and is a postulated source for the natural gas within the sedimentary column this thermogenically generated gas does not migrate directly through the gravitational faults and the above lying sediments but moves buoyancy driven up dip along stratigraphic layers to escape through the sediments to the sea floor in the inner shelf area

upper mantle shear and compressional velocity structure of the central us craton shear wave low velocity zone and anisotropy one dimensional upper mantle velocity structure is estimated by modeling p sv and sh body waveforms sampling the central united states the resulting model features a thick mantle lid with a positive velocity gradient a significant shear wave low velocity zone 6 velocity reduction is required below the lid however the compressional velocities are not reduced the paths studied lie along the fast axes of teleseismic shear wave splitting measurements and absolute plate motion we observe splitting of upper mantle refracted shear waves with the transverse component slow relative to the radial component in order to fit the shear wave arrivals and satisfy teleseismic shear wave splitting observations we infer azimuthal anisotropy maximum 1 6 distributed throughout the mantle lid and low velocity zone with increasing anisotropy with depth

a late early pleistocene pollen record from fontana ranuccio central italy a pollen record from a sediment core drilled underneath the acheulian site of fontana ranuccio provides new data on the vegetation characters in central italy during the late early pleistocene in a forest phase possibly corresponding to a substage of marine isotope stage 21 a number of tree taxa nowadays extinct in europe cathaya tsuga taxodium type carya and pterocarya are present at fontana ranuccio in a temperate phase characterised by dense forest conditions dominated by alnus and pterocarya the comparison of the fontana ranuccio record with other dated sites distributed along the italian peninsula indicates that the floristic and vegetational characters of the mid pleistocene revolution present a rather distinct succession of biostratigraphic events compared to other early pleistocene pollen records from southern europe the italian peninsula shows greater floristic similarities with the eastern mediterranean regions than with the iberian peninsula copyright 2011 john wiley sons ltd

a surface wave analysis of seismic anisotropy beneath eastern north america summary the nature of upper mantle seismic anisotropy beneath central and eastern north america has been evaluated using the phase velocities of surface waves traversing the length of the missouri to massachusetts moma broad band array frequency dependent phase delays of fundamental mode love and rayleigh waves measured across the array using a cross correlation procedure require the presence of anisotropy in the upper mantle 2 d radially anisotropic structures obtained via linearized inversion of these data contain shear anisotropy with a magnitude of 3 per cent between the moho and at least 180 km depth models in which the mantle lithosphere is isotropic cannot satisfy the data this anisotropy is approximately constant across the tectonic transition from the craton to the atlantic margin forward models consisting of a shallow lithospheric layer of azimuthal anisotropy derived from previous shear wave splitting observations fail to satisfy the surface wave delay times the combined surface wave and splitting results suggest the presence of two layers of anisotropy a lithospheric layer that produces the phase delay differences observed in love and rayleigh waves but is transparent to vertically propagating shear body waves and a deeper presumably asthenospheric layer that generates the shear wave splitting one plausible model is that the lithosphere is characterized by vertically heterogeneous anisotropic fabric which would produce minimal splitting in vertical shear waves such fabric has been hypothesized for regions of weak and complex splitting such as south africa and australia the results here imply that it may be appropriate for north america as well

the magnitude symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tectonites summary seismic anisotropy within the upper mantle originates from the preferred orientation of highly anisotropic single crystals the symmetry and magnitude of anisotropy depend upon 1 the volume percentages of the minerals constituting the upper mantle 2 the degree and symmetry of preferred orientation of each mineral and 3 the alignment of the minerals crystallographic axes relative to one another the nature of upper mantle anisotropy can be examined by studying mineral orientations within ultramafic rocks which were once part of the mantle petrofabric data for olivine and pyroxene have been used to obtain velocity anisotropy patterns over large regions of ultramafic rocks from the samail ophiolite oman the troodos ophiolite cyprus the bay of islands ophiolife newfoundland the twin sisters ultramafic washington usa the dun mountain ophiolite new zealand the red hills ophiolite new zealand and the red mountain ophiolite new zealand the compressional wave anisotropy calculated for these massifs ranges from 3 to 8 per cent in excellent agreement with observed seismic anisotropy in the upper continental and oceanic mantle the symmetry varies from orthorhombic to axial with the axial symmetry axis corresponding to the olivine a axes maxima and subparallel to spreading directions in oceanic upper mantle pyroxene a b and c axes maxima generally parallel olivine b c and a axes respectively and anisotropy decreases with increasing pyroxene content shear wave splitting is predicted for all propagation directions within the upper mantle symmetry is also orthorhombic or axial with the minimum difference in velocity between the two shear waves parallel to the maximum compressional wave velocity

detecting seismic anisotropy above the 410 km discontinuity using reflection coefficients of underside reflections abstract we investigate the effect of various types of deformation mechanisms on the reflection coefficients of p and s waves underside reflections off the 410 km discontinuity to develop a diagnostic tool to detect the style of deformation at boundary layers we calculate the reflection coefficient for p and sh underside reflections using velocity perturbations resulting from aligned minerals above the 410 km discontinuity for different deformation scenarios the results show that in the case of an anisotropic olivine layer above an isotropic wadsleyite layer the p wave reflection coefficient amplitudes are only slightly influenced by the joint effect of angle of incidence and the strength of imposed deformation without any polarity reversal and for all deformation styles for the sh wave underside reflections the incidence angle for which a polarity reversal occurs changes with distance for all scenarios and in addition changes with azimuth for shear deformation scenarios these differences in amplitude and polarity patterns of reflection coefficients of different deformation geometries especially for s wave at shorter distances potentially provide a possibility to distinguish between different styles of deformation at a boundary layer we also show a first test using currently available elastic constants of anisotropic wadsleyite beneath anisotropic olivine

evidence of a widespread paleo pockmarked field in the orange basin an indication of an early eocene massive fluid escape event offshore south africa abstract the detailed interpretation and seismic attribute analysis of 2d and 3d seismic reflection data sets revealed the presence of a distinct seismic horizon displaying circular and elongated depressions in early eocene sediments along the northwestern slope of the orange basin on the southwest african margin their occurrence within a single seismic horizon indicates a contemporaneous formation process we interpret the depressions as paleo pockmarks which occur as single pockmarks predominantly nne ssw aligned pockmark trains and coalescing elongated pockmarks the paleo pockmarked surface covers an area of 2800 km 2 and has been mapped by integrating the 3d dataset with industrial 2d seismic grids the spatial and temporal occurrence of the paleo pockmarks is associated to two episodes of rapid sedimentation during the late maastrichtian to paleocene comprising fine grained shelf eroded material we propose that they were formed during the early eocene by the seepage of both 1 overpressured pore fluids from disequilibrium compaction of upper maastrichtian sediments and 2 thermogenic gas from underlying cretaceous mature source rocks we show that the location of the pockmarks is controlled by the underlying faults system of the paleo slope the paleocene eocene transition is known for its series of hyperthermal events characterized by negative excursions of the carbon isotope curve the early eocene widespread event of fluid and thermogenic gas escape identified in this work can be an example of the type of carbon release processes occurring in hydrocarbon prone sedimentary basins during this time

seismic anisotropy beneath stable continental interiors a robust knowledge of seismic anisotropy beneath the continents is essential to our understanding of plate tectonic theory as anisotropy provides a unique constraint on the character of past and present deformation in the lithosphere and sublithospheric mantle this review paper summarizes the range of techniques currently available to image seismic anisotropy with passive source seismic data and addresses current issues surrounding the observation and interpretation of continental seismic anisotropy to this end we present case studies for four regions where seismic anisotropy has been extensively investigated in recent years eastern north america the canadian shield australia and southern africa based on this full suite of results we infer that stable continental regions generally contain seismic anisotropy that is clearly located within both the lithosphere and the sublithospheric mantle usually to depths of at least 200 km and perhaps more an implication of these results is that tectonic plates are at most only partially coupled to the underlying mantle the results from these case studies also demonstrate that while remarkable progress in seismic anisotropy imaging has been achieved in recent years it is clear that much more work will be required to adequately understand the origin of continental seismic anisotropy we suggest that a more robust characterization of anisotropic parameters can only be achieved by integrating complementary seismic datasets and by incorporating constraints from key related datasets from mineral physics magnetotellurics gravity and geodesy

no regional anisotropic domains in the northeastern u s appalachians a fabric frozen within the continental lithosphere should lead to regional anisotropic domains with close correlation between tectonics seismic anisotropy and shear wave travel time delays alternatively seismic anisotropy could be maintained by active deformation in the sub lithospheric mantle this scenario would predict broad spatial consistency in shear wave splitting and weak correlation between the splitting surface tectonics and travel time delays a coherent pattern of shear wave splitting is observed in the northeastern united states covering large parts of new york and new england spanning the appalachian orogen and areas underlain by the grenvillian basement the direction of fast shear wave propagation as measured from core refracted sks skks and pks phases varies systematically with event back azimuth at all sites examined and is fit well by two layers of anisotropy with subhorizontal symmetry axes relative travel time delays of shear waves that sample the region vary over 100 km length scale and correlate with surface geology tomographic images suggest lateral shear velocity variations 3 including a slow anomaly beneath the grenvillian basement exposed in the adirondack mountains there is little correlation between this horizontally rough seismic velocity and the horizontally smooth anisotropic model consistent with shear wave splitting we therefore conclude that in the northeastern us the concept of regional anisotropic domains i e distinct regions within the lithosphere characterized by coherent anisotropic properties reflective of present or past deformation does not apply

s velocity structure and radial anisotropy in the aegean region from surface wave dispersion summary the lithospheric structure of the aegean region is investigated by analysis of rayleigh wave fundamental mode dispersion measurements isotropic 1 d models for almost 100 two station ray paths across the region display distinct variations in the moho depth and crustal s wave velocities the descending slab of the subducting african plate can be resolved down to 120 km depth beneath the volcanic arc three different regions are distinguished in terms of moho depth 1 the forearc with large crustal thicknesses between 38 and 48 km and an average of 43 km 2 the northern aegean with an average moho depth of 28 km and 3 the southern aegean central volcanic arc i e cyclades and sea of crete with an even thinner crust of around 25 km lateral variations in structure between 25 and 55 km depth indicate a marked difference between the western and eastern forearc collocated with pronounced changes in trench and slab geometry as well as published deformation rates s velocities between 25 and 55 km depth are low everywhere beneath the forearc but increase from the peleponnesus to crete an abrupt change occurs between western and central crete in terms of the visibility of the aegean moho and the seismic structure of the lithospheric mantle wedge an aegean mantle wedge with s velocities above 4 4 km s 1 is only observed to the east of central crete whereas to the west velocities of less than 4 0 km s 1 occur down to the plate contact these low velocities above the slab may indicate the presence of a melange of metamorphic rocks at the depths a low velocity asthenospheric layer is observed beneath the sea of crete and the cyclades below 40 km depth between the thinned lithosphere above and the slab below the observed radial anisotropy in the northern part of the aegean is likely to be due to preferred orientation of anisotropic minerals within the lower crust possibly caused by lateral ductile flow associated with recent lithospheric extension

recent deformations of the deep continental root beneath southern africa seismic anisotropy manifested by the splitting of shear waves into two orthogonal polarizations provides evidence for deformation in the earth s upper mantle where such deformation has been inferred beneath precambrian cratons it has been interpreted either as related to recent plate motion1 2 or inherited from the precambrian3 4 here we present data from a portable seismograph array in the kaapvaal craton of south africa which indicate that the inferred direction of mantle flow underneath the array is close to the direction of absolute plate motion for southern africa since the end of the jurassic period such an alignment has also been reported for the north american craton2 suggesting that the flow in both regions is related mainly to shearing of the sublithospheric mantle by the plate above the old continental roots that are likely to exist in this depth range5 6 must therefore be deformed by the plate motion but the deformations are not strong enough to be seen in the presently available seismic tomography data

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

upper mantle low anisotropy channels below the pacific plate a new 3d anisotropic model has been obtained at a global scale by using a massive dataset of seismic surface waves though seismic heterogeneities are usually interpreted in terms of heterogeneous temperature field a large part of lateral variations are also induced by seismic anisotropy of upper mantle minerals new insight into convection processes can be gained by taking seismic anisotropy into account in the inversion procedure the model is best resolved in the pacific plate the largest and the most active tectonic plate superimposed on the large scale radial parameter and azimuthal anisotropy of vsv velocity within and below the lithosphere correlated with present or past pacific plate motions are smaller scale 1000 km lateral variations of anisotropy not predicted by plate tectonics channels of low anisotropy down to a depth of 200 km hereafter referred to as lac are observed and are the best resolved anomalies one east west channel between easter island and the tonga kermadec subduction zones observed on both radial and azimuthal anisotropies and a second one only observed on azimuthal anisotropy extending from the south west pacific up to south east hawaii and passing through the polynesia hotspot group for plate older than about 40 ma these features provide strong constraints on the decoupling between the plate and asthenosphere they are presumably related to cracking within the pacific plate and or to secondary convection below the rigid lithosphere predicted by numerical and analog experiments the existence and location of these lacs might be related to the current active volcanoes and hotspots possibly plumes in the central pacific lacs which are dividing the pacific plate into smaller units might indicate a future reorganization of plates with ridge migrations in the pacific ocean

gas hydrates free gas distribution and fault pattern on the west svalbard continental margin summary a 3 d tomographic analysis of seismic velocity and attenuation fields is presented with the purpose to analyse the intimate relation of gas hydrates and free gas distribution with the fault pattern the 3 d four component seismic data have been acquired offshore western svalbard the analysis of the subbottom topography of the base of the stability field of gas hydrates indicated by the bottom simulating reflector and the thickness of the underlying free gas bearing zone suggest a fault induced compartmentalization of the gas reservoir the proposed fluid and gas circulation scheme assumes deep sourced warm fluids moving upwards mainly along high permeability faults and fractures below the gas hydrate stability zone faults change from being a pathway of fluid flow to being barriers to fluid flow in the hydrate stability zone consequently below the hydrate stability zone free gas is transferred from the permeable faults to non faulted but fault bounded sediments where it accumulates below the base of the hydrate stability zone producing a significantly thicker free gas layer the tomographic approach is therefore proposed as an effective procedure to provide detailed information on 3 d p and s wave velocity and attenuation distribution in marine sediments and specifically to examine the details of the distribution of the gas hydrates and of the free gas for which the dependence on the vp vs and qp is now fairly well known

global patterns of azimuthal anisotropy and deformations in the continental mantle summary we present a summary of measurements of azimuthal anisotropy in the continental mantle based on the sks technique and performed mostly with the active participation of the authors the directions of polarization of the fast quasi shear wave and the time delays between the quasi shear waves are obtained at nearly 70 locations in all continents except antarctica these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the mantle the depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km the fast directions in sks do not show clear correlation with the fast directions of the teleseismic p at the same seismograph stations in the regions of present day convergence the fast direction of anisotropy usually aligns with the plate boundary this correlation implies that the direction of shortening is the same in the crust and the upper mantle in the regions of rifting the inferred direction of mantle flow usually aligns with the direction of extension in the crust outside the regions of recent tectonic activity we most likely observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere on a global scale in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy the correlation is especially strong in central and eastern parts of north america a clear absence of any evidence of large scale azimuthal anisotropy in the data of long range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in sks is generated mainly at greater depths in the asthenosphere orientation of olivine at these depths reflects recent and present day flow in the mantle rather than processes of a distant geologic past

offshore mud volcanoes and onland faulting in southwestern africa neotectonic implications and constraints on the regional stress field abstract recently discovered mud volcanoes in the orange basin offshore southwestern africa denote the existence of neotectonic faults in the submerged continental shelf interpretation of seismic lines perpendicular to the trend of the alignment of the mud volcanoes shows flower structures diagnostic of strike slip faulting along a n nnw direction analysis at the regional scale of onland neotectonic features in southwestern africa shows that recent faulting occurred both in central namibia and namaqualand south africa and that it created both n nnw and nw trending lineaments it is proposed that the newly discovered offshore neotectonic activity and the onland structures described in this paper represent the structural expression of the same stress field these structures form a set of conjugate transtensive faults which constrain the regional horizontal greatest compressive stress in a nw nnw direction such stress orientation also supported by in situ stress measurements defines the so called wegener stress anomaly the predominant present day stress field of southwest africa the wegener anomaly is incompatible with the stress orientation required by plate scale tectonic constraints mainly in the form of recently published gps motion values for the african plate

gas hydrate and mud volcanoes on the southwest african continental margin off south africa widespread occurrence of bottom simulating reflectors bsrs has been detected in multichannel seismic profiles on the upper continental slope in the southern periphery of the orange river delta probably indicating the presence of large quantities of gas hydrate in this area this report is the first to show the presence of bsrs on seismic records on the southwest african continental margin south of the walvis ridge another remarkable feature in the area is the occurrence of a large number of mud volcanoes the distribution of the bsrs and the location of the mud volcanoes are controlled by the locations of active faults the gas hydrate in this region may consist of a mixture of microbial and thermogenic gas whereas much of the gas flowing through the mud volcanoes probably originated from deep seated aptian source shales

anomalous seaward dip of the lithosphere asthenosphere boundary beneath northeastern usa detected using differential array measurements of rayleigh waves summary rayleigh wave phase velocities and azimuth anomalies in the period range of 30 100 s are measured for a set of four triangular arrays of broad band seismometers in coastal northeastern usa this is a region in which a strong upper mantle slow shear velocity anomaly a new england anomaly crosses the continental margin earthquakes from a wide range of directions are used to detect the variation of parameters with azimuth of propagation no lateral heterogeneity in phase velocity is detected at these periods between stations at the centre and the edge of the anomaly however large 10 20 per cent azimuthal variations occur and have a cos 1 dependence which is indicative of a dipping structure in the upper mantle corresponding azimuth variations with a magnitude of 5 are also detected this behaviour is consistent with a southeasterly n150 e dip of the lithosphere asthenosphere boundary beneath new england this dip is associated with the shoaling of the new england anomaly beneath the adirondack mountains west of the array it is opposite to the dip associated with lithospheric thickening toward the interior of the craton

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

seismological structure of the upper mantle a regional comparison of seismic layering abstract we investigate seismic layering i e discontinuities regions of anomalous velocity gradients and anisotropy and its lateral variability in the upper mantle by comparing seismic models from three tectonic regions old 100 ma pacific plate younger 40 ma philippine sea plate and precambrian western australia these models were constructed by combining two data sets scs reflectivity profiles which provide travel times and impedance contrasts across mantle discontinuities and observations of frequency dependent travel times of three component turning s ss ss sss sss sa and surface r1 g1 waves which constrain the anisotropic velocity structure between discontinuities the models provide a better fit to observed seismograms from these regions than the current generation of global tomographic models the australian model is characterized by high shear velocities throughout the upper 350 km of the mantle with no low velocity zone lvz in the isotropically averaged shear velocities in contrast the oceanic models are characterized by a thin high velocity seismic lid underlain by a distinct lvz with a sharp boundary the g discontinuity separating them the g is significantly deeper beneath the western philippine sea plate than beneath the older pacific 89 and 68 km respectively implying that thermal cooling alone does not control the thickness of the lid we interpret this discontinuity as a compositional boundary marking the fossilized base of the melt separation zone msz active during sea floor spreading no discontinuity is detected at the base of the lvz in the oceanic models the s velocity gradient between 200 and 410 km depth is much steeper in the oceans than beneath australia this high oceanic gradient is probably controlled by a decrease in the homologous temperature over this depth interval the relative depths of the transition zone tz discontinuities are consistent with clapeyron slopes expected for an olivine dominated mineralogy the 660 km discontinuity displays variability in its amplitude that appears to correlate with its depth shallow and bright beneath the pacific deep and dim beneath australia and the philippine sea such behavior is possibly caused by the juxtaposition of the olivine and garnet components of the phase transition radial anisotropy extends through the upper 250 km of the mantle in the australia model and through the upper 160 km of the two oceanic models the magnitude of anisotropy is consistent with that expected for models of horizontally oriented olivine and the localization of anisotropy in the shallowest upper mantle implies that it reflects strain associated with past or present tectonic events

a seismic reflection profile across the cascadia subduction zone offshore central oregon new constraints on methane distribution and crustal structure in 1989 we conducted an onshore offshore seismic experiment to image the crustal structure of the cascadia forearc in this paper we discuss the processing and interpretation of a multichannel seismic reflection profile across the continental margin that was collected as part of this effort this profile reveals several features of the forearc that were not apparent in an earlier coincident reflection profile one of the most important of these features is a very strong bottom simulating reflection bsr beneath the midslope region that is nearly continuous from water depths of about 1500 m to 600 m where it appears to crop out on the seafloor the pressure and temperature conditions at the bsr derived from our observations are remarkably consistent with the experimentally determined phase diagram for a methane hydrate seawater system over a broad range of temperatures and pressures assuming hydrostatic pressure and the temperature gradiant measured near the base of the continental slope during ocean drilling program odp leg 146 interval velocities and reflection coefficients derived from the data indicate that the bsr represents a contrast between sediment with a small amount of hydrate overlying sediment containing free gas consistent with results obtained during leg 146 although the regional distribution of the anomalously strong bsr beneath the midslope is poorly known we speculate that it may be related to apparent slope instability the data also provide constraints on the thickness and geometry of the siletz terrane which is the basement beneath the shelf and acts as the subduction zone backstop a deep reflection which might mistakenly be interpreted to be moho if coincident large aperture data were not available is interpreted to be the base of the siletz terrane a recently active strike slip fault zone that overlies the seaward edge of the siletz terrane suggests that the siletz terrane controls the location of decoupling of the subduc tion complex from the rest of the forearc

tectonic geomorphology and hydrocarbon induced topography of the mid channel anticline santa barbara basin california the geomorphology of the western sector of the mid channel anticline mca santa barbara southern california suggests the actively growing fold is laterally propagating to the west the presence of fold scarps and cross faults that segment the structure suggests that buried faults that are producing the folding are present at shallow depths the summit area of the anticline at the last glacial maximum 22 to 19 ka was probably a small late pleistocene island evidence for presence of the island includes what appears to be terrestrial erosion and is supported by assumption of sea level change and rates of uplift and subsidence pockmarks and domes ranging in diameter from 10 to 100 m and several meters deep are present along the crest and flanks of the mca these features appear to be the result of hydrocarbon emission their formation has significantly modified the surface features producing simple to complex erosional and or constructional topography a large pockmark near the anticline crest dated by two calibrated ams radiocarbon dates of 25 3 and 36 9 ka continues to emit hydrocarbon gases we term the topography produced by hydrocarbon emission as hydrocarbon induced topography hit 2007 elsevier b v all rights reserved

seismic and mechanical anisotropy and the past and present deformation of the australian lithosphere abstract we interpret the three dimensional seismic wave speed structure of the australian upper mantle by comparing its azimuthal anisotropy to estimates of past and present lithospheric deformation we infer the fossil strain field from the orientation of gravity anomalies relative to topography bypassing the need to extrapolate crustal measures and derive the current direction of mantle deformation from present day plate motion our observations provide the depth resolution necessary to distinguish fossil from contemporaneous deformation the distribution of azimuthal seismic anisotropy is determined from multi mode surface wave propagation mechanical anisotropy or the directional variation of isostatic compensation is a proxy for the fossil strain field and is derived from a spectral coherence analysis of digital gravity and topography data in two wavelength bands the joint interpretation of seismic and tectonic data resolves a rheological transition in the australian upper mantle at depths shallower than 150 200 km strong seismic anisotropy forms complex patterns in this regime the seismic fast axes are at large angles to the directions of principal shortening defining a mechanically coupled crust mantle lid deformed by orogenic processes dominated by transpression here seismic anisotropy may be considered frozen which suggests that past deformation has left a coherent imprint on much of the lithospheric depth profile the azimuthal seismic anisotropy below 200 km is weaker and preferentially aligned with the direction of the rapid motion of the indo australian plate the alignment of the fast axes with the direction of present day absolute plate motion is indicative of deformation by simple shear of a dry olivine mantle motion expressed in the hot spot reference frame matches the seismic observations better than the no net rotation reference frame thus seismic anisotropy supports the notion that the hot spot reference frame is the most physically reasonable independently from plate motion models seismic anisotropy can be used to derive a best fitting direction of overall mantle shear

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

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

hunting for azimuthal anisotropy beneath the pacific ocean region lateral variations in azimuthal anisotropy cause significant waveform anomalies in long period surface waves 8 mhz suggesting lateral variations with wavelength twice the spacing of fracture zones in the north central pacific

lithospheric and sublithospheric anisotropy beneath the baltic shield from surface wave array analysis abstract we report measurements of radial and azimuthal anisotropy in the upper mantle beneath southern and central finland which we obtained by array analysis of fundamental mode rayleigh and love waves azimuthally averaged phase velocities were analysed in the period range 15 to 190 s for rayleigh waves and 15 to 100 s for love waves the azimuthal variation of the rayleigh wave phase velocities was obtained in the period range 20 to 100 s the limited depth resolution of fundamental mode surface waves necessitated strong damping constraints in the inversion for anisotropic parameters we investigated the effects of non unicity on the final model by experimenting with varying model geometries the radial anisotropy beneath finland can be explained by a lithosphere at least 200 km thick predominantly 50 by volume composed of olivine crystals having their a axes randomly distributed in the horizontal plane on the contrary the measured lithospheric azimuthal anisotropy is small this can be reconciled with body wave observations made in the area that indicate a complex pattern of rapidly varying anisotropy below 200 250 km depth that is below the petrologic lithosphere as revealed by xenolith analyses conducted in the area the magnitude of the azimuthal anisotropy increases and would be compatible with a mantle containing 15 20 by volume of olivine crystals whose a axes are coherently aligned in the n ne direction the alignment of the a axes is off the direction of present day absolute plate motion in either the no net rotation or hot spot reference frame currently n55 n60 we interpret this mismatch as evidence for a complex convective flow pattern of the mantle beneath the shield which by inference is decoupled from the overlying lithosphere

gaia s breath global methane exhalations abstract methane ch 4 is the most abundant organic compound in the earth s atmosphere where it acts as a greenhouse gas and thus has implications for global climate change the current atmospheric ch 4 budget however does not take into account geologically sourced ch 4 seepage geological sources of ch 4 include natural macro and micro seeps mud volcanoes and other miscellaneous sources such as gas hydrates magmatic volcanoes geothermal regions and mid ocean ridges macro seeps contribute 25 tg teragrams ch 4 yr to the atmosphere whereas micro seepage contributes perhaps 7 tg ch 4 yr mud volcanoes emit 5 tg ch 4 yr and miscellaneous sources emit 8 tg ch 4 yr to the atmosphere thus the total contribution to the atmosphere from geological sources is estimated to be 45 tg ch 4 yr which is significant to the atmospheric organic carbon cycle and should be included in any global inventory of atmospheric ch 4 we argue that the atmospheric ch 4 global inventory of the interplanetary panel on climate change must be adjusted in order to incorporate geologically sourced ch 4 from naturally occurring seepage

a multistep approach for joint modeling of surface wave dispersion and teleseismic receiver functions implications for lithospheric structure of the arabian peninsula 1 we present a multiple step procedure for joint modeling of surface wave group velocity dispersion curves and teleseismic receiver functions for lithospheric velocity structure the method relies on an initial grid search for a simple crustal structure followed by a formal iterative inversion an additional grid search for shear wave velocity in the mantle and finally forward modeling of transverse isotropy to resolve love rayleigh surface wave dispersion discrepancy it considers longer period surface wave group velocity swgv dispersion allowing for the resolution of deeper structure compared to previous joint inversions the grid search for simple crustal structure is facilitated using a library of precomputed receiver functions and swgv dispersion curves the iterative inversion improves fit to the data by increasing the number of layers in the crust when necessary in order to fit the swgv for periods greater than about 50 s we perform a grid search over mantle velocities including the mantle lid and low velocity zone keeping the crustal structure fixed to the values from the previous step in some cases a clear love rayleigh discrepancy prevents a simultaneous fit of the group velocities with an isotropic model the love rayleigh discrepancy can be resolved by allowing shear wave transverse isotropy with a vertical symmetry axis vsh vsv differences in the uppermost mantle the method is applied to 10 stations in the arabian peninsula sampling various tectonic environments including active continental rifting and stable regions the resulting shear velocity models confirm rapid crustal thinning of the arabian shield toward the red sea however we do not find strong evidence for crustal thickening toward the arabian platform our results suggest that the mantle lithosphere thickness varies regionally but that the mantle shear velocities beneath the arabian shield and red sea coast are generally anomalously low furthermore our results indicate the presence of strong polarization anisotropy up to about 10 in the lithospheric upper mantle in the vicinity of as well as farther away from the red sea our modeling yields vsv vsh in the southwestern part of the arabian peninsula consistent with vertical flow and vsh vsv in the northwestern part of the arabian peninsula and the continental interior consistent with horizontal flow indicating that the mantle flow pattern is not uniform along the axis of the red sea

crustal anisotropy beneath hudson bay from ambient noise tomography evidence for post orogenic lower crustal flow the crust underlying hudson bay canada records a long and complex tectonic history in this study we investigate this region using tomographic inversion based on continuous ambient noise recordings from 37 broadband seismograph stations that encircle hudson bay the ambient noise data were processed to obtain group velocity dispersion measurements from 10 35 s period which were inverted using an algorithm that incorporates the effects of anisotropy this work is among the first in which ambient noise data have been used to investigate azimuthal anisotropy the inversion method uses smoothing and damping to regularize the solution due to the significantly increased number of model parameters relative to the isotropic case we performed a careful analysis for parameter selection to determine whether leakage occurs between isotropic and anisotropic model parameters we observe a robust pattern of anisotropic fast directions in the mid crust that are consistent with large scale tectonic trends based on magnetic anomaly patterns in particular a distinctive double indentor shape for the superior craton is clearly expressed in both data sets this pattern breaks down deeper in the crust suggesting that some degree of lithospheric decoupling in the lower crust such as channel flow occurred during orogenesis given regional evidence for vertically coherent deformation in the crust and underlying mantle we interpret this pattern in the lower crust as a tectonic overprint that post dates the main phase of trans hudson deformation at most levels in the crust we observe a profound change in direction of anisotropic fast direction across an inferred suture beneath hudson bay