identifying global seismic anisotropy patterns by correlating shear wave splitting and surface wave data we compare a global compilation of shear wave splitting measurements with azimuthal seismic anisotropy parameters inferred from surface wave tomography the currently available splitting dataset is taken from a novel comprehensive collection of available publications that is updated interactively online the comparison between the two types of data is made by calculating predicted splitting parameters from the anisotropic tomography model comparing these predicted splitting parameters with the observed ones we find a considerable correlation between the two datasets at global scale this result is noteworthy since such correlation did not seem to exist in previous studies the spatial resolution associated with the two types of methods is rather different while surface waves have good vertical resolution and poor lateral resolution of several hundreds of kilometers sks splitting measurements have good lateral but poor vertical resolution the correlation can be understood in light of recent propositions that anisotropy seen by sks splitting constrains mostly the upper mantle and therefore a similar depth region as surface waves the correlation also confirms the generally good quality of the shear wave measurements as well as that of the anisotropic tomography model

image log analysis of in situ stress orientation breakout growth and natural geologic structures to 2 5 km depth in central scandinavian caledonides results from the cosc 1 borehole stress induced borehole deformation analysis in the collisional orogeny in the scandinavian caledonide deep scientific borehole establishes in situ stress orientation in a poorly characterized regi

modelling plastic deformation of peridotite with the self consistent theory theories for deformation of polycrystals have been substantially refined enabling us to model deformation of metals and minerals with considerable sophistication so far most modelling has been confined to single phase aggregates such as quartzite and limestone we present the first results for a polyphase aggregate peridotite consisting of 70 olivine and 30 enstatite the problem is approached with a viscoplastic self consistent theory satisfying stress equilibrium and strain compatibility for the average polycrystal and taking account of anisotropic neighbor interactions it is assumed that olivine deforms by 010 100 001 100 and 010 001 slip and enstatite deforms by 100 001 slip simulated textures for olivine and enstatite in peridotite resemble simulated textures in the pure phases indicating that for this system and for these volume fractions there is little influence of the different phases upon each other in our model the harder mineral enstatite deforms at a slower rate than olivine interaction between neighboring grains appears to be minimal which may be due to model assumptions predicted pole figures with olivine 010 axes and enstatite 100 axes aligning with the direction of shortening are in good agreement with preferred orientations in naturally and experimentally deformed peridotites

effect of image segmentation voxel size on micro ct computed effective transport elastic properties abstract digital rock technology is rapidly evolving interdisciplinary field with many promises including fast turnaround times for core analysis repeatable analysis and multiphysics simulation we study the impact of the image segmentation threshold and image voxel size on image computed permeability elastic moduli and electrical conductivity improved quantitative understanding of such effects is critically important when comparing laboratory measured rock properties with those computed on digital images of rocks we analyze properties of over 500 binary microstructures each of size 1024 3 voxels segmented using different segmentation algorithms we find that uncertainty in computed rock properties induced due to the choice of segmentation threshold increases with coarsening of image voxels segmentation of the same rock acquired with different voxels sizes lead to small variations in porosity but induces relatively large variation in flow and electrical rock properties this uncertainty is larger for rocks of lower porosity we find that coarsening of image voxels also leads to rounding and smoothing of pore throats and a sharp decline in specific surface area which results in an increase in computed permeability we propose simple models to remove the bias in rock properties when image derived porosity is either overestimated or underestimated due to the choice of the segmentation threshold this approach is particularly useful when comparing laboratory measured rock properties to those derived from digital rocks since often the image calculated porosity and the laboratory measured porosity are not the same even after such corrections the impact of finite image voxel size would still need to be compensated

fluid substitution in laminated sands petrophysicists and geophysicists do fluid substitution to estimate the density and sonic velocity of porous rocks at saturation conditions such as fully hydrocarbon saturated or fizz bearing with fluids such as gas condensate and oil however the gassmann model implementation in many commercial packages sometimes yields implausible results for the compressional sonic velocity in laminated sand shale sequences in particular the counterintuitive phenomenon of fluid effects increasing with computed shale fraction is often observed as in the gas bearing interval of a well drilled with oil based mud shown in figure 1 dtc flusub gassmann fluid substitution is the result of substituting dtc wet to the observed water saturation sw as expected brie et al 1995 the modeled fluid effect is larger than the difference between the wet and the observed log dtc obs the observed fluid effects reach a maximum in the cleanest sand with the highest porosity but the model predicts a maximum in shaley sand with low porosity figure 1 gassmann equation breakdown in laminated sequence normal practice is to specify a shale cutoff above which the computed fluid effects are ignored this is an unsatisfactory workaround because the choice of cutoff is arbitrary and results are counterintuitive even below the cutoff resolution and depth mismatches between wireline logs and the observation that the gassmann equation does not apply in shale are frequently cited as reasons for these phenomena but do not explain the modeled results in figure 2 figure 2 model showing gassmann equation breakdown this model was generated from regional density velocity and porosity trends a sand reservoir was defined by an abrupt transition at the base and a linear transition to shale over the top 100 ft the clean part of the sand is not assumed totally free of clay sands in the study area are known to include some detrital

evidence for ancient lithospheric deformation in the east european craton based on mantle seismic anisotropy and crustal magnetics abstract we present new shear wave splitting measurements performed at 16 stations on the east european craton and discuss their implications in terms of upper mantle anisotropy for this geophysically poorly known region previous investigations of mantle anisotropy in central europe have shown fast directions aligning smoothly with the craton s margin and various suggestions have been proposed to explain their origin such as asthenospheric flow or lithospheric frozen in deformation here we aim at investigating the continuation of this shear wave splitting pattern further to the east into the east european craton for the craton the interpretation appears to be less ambiguous than for central europe since several arguments support lithospheric anisotropy in this region 1 the large scale coherence within either of the four constituting blocks and the significant variations between the blocks on a small scale 2 the weak correlation with absolute plate motion vectors and 3 the good correlation between anisotropy and crustal features for which we use magnetic field alignments as a proxy rather good correlation of these magnetic features with seismic fast orientations strongly supports the idea of vertically coherent deformation throughout upper mantle and crust the observed splitting orientations thus reflect the last tectonic events of each block frozen in into the lithosphere for hundreds of millions of years

bayesian inversion for effective pore fluid bulk modulus based on fluid matrix decoupled amplitude variation with offset approximation fluid indicators estimated from seismic dataplay important roles in reservoir characterization and prospect identification traditionally there are a variety of fluid indicators proposed but they are very likely to provide ambiguous results for fluid identification due to the fact that their sensitivity is dependent upon the mixed effect of pore fluid and rock porosity to raise the sensitivity of fluid indication we used the effective porefluid bulk modulus as a fluid indicator starting with the poroelastic amplitudevariation with offset avo theory and the corresponding rock physics model with the homogeneous sorting trend we derived a new avo approximation that allowed us toestimatethe effective pore fluidbulkmodulusina direct fashion the inversion for the fluid indicator is formulated inbayesian framework withthe cauchydistributionasa prior constraint we tested the method on synthetic data and analyzed the feasibility and stability of the inversion a field dataexampleshowsthatthe effective pore fluidbulkmodulus can reduce the ambiguity caused by the rock porosity and improve the quality of fluid discrimination in a clastic reservoir further research needs to be done on the reservoirs that do not fit the rock physics model without a sorting trend

modelling elastic properties of impure chalk from south arne field north sea in impure chalk the elastic moduli are not only controlled by porosity but also by contact cementation resulting in relatively large moduli for a given porosity and by admixtures of clay and fine silica which results in relatively small moduli for a given porosity based on a concept of solids suspended in pore fluids as well as composing the rock frame we model p wave and s wave moduli of dry and wet plug samples by an effective medium hashin shtrikman model using chemical mineralogical and textural input for a given porosity the elastic moduli correspond to a part of the solid the iso frame value forming the frame of an upper hashin shtrikman bound whereas the remaining solid is modelled as suspended in the pore fluid the iso frame model is thus a measure of the pore stiffness or degree of cementation of the chalk the textural and mineralogical data may be assessed from logging data on spectral gamma radiation density sonic velocity and water saturation in a hydrocarbon zone whereas the iso frame value of a chalk may be assessed from the density and acoustic p wave logs alone the iso frame concept may thus be directly used in conventional log analysis and is a way of incorporating sonic logging data the rigs 1 and rigs 2 wells in the south arne field penetrate the chalk at the same depth but differ in porosity and in water saturation although almost the entire chalk interval has irreducible water saturation our model combined with petrographic data indicates that the difference in porosity is caused by a higher degree of pore filling cementation in rigs 1 petrographic data indicate that the difference in water saturation is caused by a higher content of smectite in the pores of rigs 1 in both wells we find submicron size diagenetic quartz

australian seismic refraction results isostasy and altitude anomalies although there have been extensive seismic refraction measurements over areas of continental crust the causes of variation in crustal thickness and of variation in mean crustal seismic velocity are poorly understood if there is regional 3 3 isostatic equilibrium of the crust such as suggested by many studies of regional gravity then crustal thickness variation should be caused largely by variation in surface altitude and crustal density seismic refraction results from australia are used here to confirm that these crustal isostatic effects are a major 40 cause of variation of crustal thickness even in a continent of little surface relief however there is an equally important regional variation in crustal thickness unrelated to crustal isostasy this is thought to be due to density variation in the lower lithosphere1

the moho in australia and new zealand abstract australia and new zealand share in part a history in the gondwana supercontinent australia has a long and complex tectonic history with the last major accretion in the early paleozoic whereas new zealand is still undergoing major plate boundary processes the australian continent is relatively well covered with both active and passive seismic techniques multiple sources of information are therefore available for building a model of moho depth results from on shore and off shore refraction experiments are supplemented by receiver functions from a large number of portable stations and the recently augmented set of permanent stations moho picks from more than 10 500 km of full crustal reflection profiles provide valuable additional constraints the composite data set provides good sampling of much of australia though coverage remains low in some remote desert areas the various datasets provide multiple estimates of the depth to moho in many regions and the consistency between the different techniques is high some of the thinnest crust lies beneath the archean craton in the pilbara and in the neighbourhood of the simpson desert thick crust is encountered beneath parts of the proterozoic in central australia and beneath the paleozoic lachlan fold belt in southeastern australia there are a number of zones of sharp contrast in depth to moho notably in the southern part of central australia despite most of the continental material around new zealand being submerged moho data for this region is mainly onshore concentrating on the australia pacific plate boundary two major wide angle reflection transects provide the bulk of the active source data with just a few traditional reflection profiles offshore the plate boundary provides an abundance of local earthquakes for tomographic imaging and this data is supplemented with receiver functions from both portable and permanent networks onshore the combined coverage is as dense as that of australia although it could be argued that a higher spatial resolution is required to capture the nature of the moho of tectonically active new zealand three regions of thickened crust can be identified one beneath the southern alps another beneath fiordland and below the wanganui basin between the north and south island thin crust is identified west of the volcanic arc with extensive underplating below the back arc region

subduction kinematics and dynamic constraints the kinematics of subduction zones shows a variety of settings that can provide clues for dynamic understandings two reference frames are used here to describe the simple 2d kinematics of subduction zones in the first the upper plate is assumed fixed whereas in the second frame upper and lower plates move relative to the mantle relative to a fixed point in the upper plate u the transient subduction hinge h can converge diverge or be stationary similarly the lower plate l can converge diverge or be stationary the subduction rate vs is given by the velocity of the hinge h minus the velocity of the lower plate l vs vh vl the subduction rate 1 increases when h diverges and 2 decreases when h converges combining the different movements at least 14 kinematic settings can be distinguished along the subduction zones variable settings can coexist even along a single subduction zone as shown for the 5 different cases occurring along the apennines subduction zone apart from few exceptions the subduction hinge converges toward the upper plate more frequently along e or ne directed subduction zone whereas it mainly diverges from the upper plate along w directed subduction zones accompanying backarc extension before collision orogen growth occurs mostly at the expenses of the upper plate shortening along e ne directed subduction zones whereas the accretionary prism of w directed subduction zones increases at the expenses of the shallow layers of the lower plate the convergence shortening ratio is 1 along e or ne directed subduction zones whereas it is 1 along accretionary prisms of w directed subduction zones backarc spreading forms in two settings along the w directed subduction zones it is determined by the hinge divergence relative to the upper plate minus the volume of the accretionary prism or in case of scarce or no accretion minus the volume of the asthenospheric intrusion at the subduction hinge since the volume of the accretionary prism is proportional to the depth of the decollement plane the backarc rifting is inversely proportional to the depth of the decollement on the other hand along e or ne directed subduction zones few backarc basins form e g aegean andaman and can be explained by the velocity gradient within the hangingwall lithosphere separated into two plates when referring to the mantle the kinematics of subduction zones can be computed either in the deep or in the shallow hotspot reference frames the subduction hinge is mostly stationary being the slab anchored to the mantle along w directed subduction zones whereas it moves w or sw ward along e or ne directed subduction zones surprisingly along e or ne directed subduction zones the slab moves out of the mantle i e the slab slips relative to the mantle opposite to the subduction direction kinematically this subduction occurs because the upper plate overrides the lower plate pushing it down into the mantle as an example the hellenic slab moves out relative to the mantle i e sw ward opposite to its subduction direction both in the deep and shallow hotspot reference frames in the shallow hotspot reference frame upper and lower plates move westward relative to the mantle along all subduction zones this kinematic observation casts serious doubts on the slab negative buoyancy as the primary driving mechanism of subduction and plate motions w directed subduction zones rather provide about 2 3 times larger volumes of lithosphere re entering into the mantle and the slab is pushed down this opposite behavior is consistent with the down dip extension seismicity along e ne directed subduction zones and the frequent down dip compression along the w directed subduction zones subduction kinematics shows that plate velocity is not dictated by the rate of subduction along the w directed subduction zones the rate of subduction is rather controlled i by the hinge migration due to the slab interaction with the easterly trending horizontal mantle wind along the global tectonic mainstream ii by the far field plate velocities and iii by the value of negative buoyancy of the slab relative to the country mantle alternatively e ne nne directed subduction zones have rates of sinking chiefly determined i by the far field velocity of plates and ii by the value of negative buoyancy of the slab relative to the country mantle along this type of subduction the subduction hinge generally advances e ne ward toward the upper plate decreasing the subduction rate but it moves w sw ward relative to the mantle all this indicates that subduction zones have different origin as a function of their geographic polarity and the subduction process is more a passive feature rather than being the driving mechanism of plate motions a rotational component combined with mantle density and viscosity anisotropies seems more plausible for generating the global tuning in the asymmetry of subduction zones

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

mesozoic cenozoic deformation in the canadian cordillera the record of a continental bulldozer abstract a possible mesozoic cenozoic trajectory for the north american craton is outlined from latitude changes of the craton derived from a revised apparent polar wander path and from westward movement of the craton based on the assumption that africa has been the least mobile continent geographically since the latest paleozoic during each of five time intervals that span 220 million years the craton trajectory had a different vector each vector appears to be reflected in the canadian cordillera by the dominant style and orientation of structures formed during that interval for much of the past 220 million years the meridionally oriented western margin of the pangea laurasia north america plate has been the site of arc magmatism where weak arc back arc lithosphere sandwiched between strong ocean floor and craton lithospheres at times coupled across convergent or transform plate boundaries focused and recorded strain when the craton apparently moved due westward between 180 160 ma and 120 60 ma the dominant structures formed then record orogen normal compression and were accompanied by orogeny structures formed during the earlier episode are mainly in the eastern and interior cordillera and their formation shortly followed or coincided with accretion of most terranes to the craton margin in the later episode compressional structures span the entire cordillera which emerged as a tectonic and physiographic entity before and between these intervals when the craton apparently moved mostly northwestward mainly geological and paleomagnetic considerations indicate some terranes moved southward sinistrally relative to the craton after 60 ma southwestward movement of the craton was concurrent with large northward dextral strike slip faults that disrupted the newly established ancestral cordillera the coincidence between the age of structures that record dominant orogen normal compression at times when the craton apparently moved due westward and orogen parallel displacements when the craton had either northward or southward components of motion suggests the craton acting as a continental bulldozer was the primary driver of cordilleran deformation and orogenesis

how burial diagenesis of chalk sediments controls sonic velocity and porosity based on p wave velocity and density data a new elastic model for chalk sediments is established the model allows the construction of a series of isoframe if curves each representing a constant part of the mineral phase contributing to the solid frame the if curves can be related to the progress of burial diagenesis of chalk which is revised as follows newly deposited carbonate ooze and mixed sediments range in porosity from 60 to 80 depending on the prevalence of hollow microfossils despite the high porosity these sediments are not in suspension as reflected in ifs of 0 1 or higher upon burial the sediments lose porosity by mechanical compaction and concurrently the calcite particles recrystallize into progressively more equant shapes high compaction rates may keep the particles in relative motion whereas low compaction rates allow the formation of contact cement whereby if increases and chalk forms rock mechanical tests show that when compaction requires more than in situ stress porosity reduction is arrested during subsequent burial crystals and pores grow in size as a consequence of the continuing recrystallization the lack of porosity loss during this process testifies to the absence of chemical compaction by calcite calcite pressure dissolution as well as to the porosity preserving effect of contact cementation at sufficient burial stress the presence of stylolites indicates that pressure dissolution takes place between calcite and silicates and depending on pore water chemistry and temperature pore filling cementation may occur over a relatively short depth interval limestone and mixed sedimentary rock form and porosity may be reduced to less than 20 isoframe increases to more than 0 6 in hydrocarbon reservoirs in north sea chalk relatively high porosity and high ifs are found the reason may be that recrystallization and porosity preserving contact cementation progress whereas pore filling cementation is small probably because pressure dissolution along stylolites is arrested pressure dissolution may be arrested for two reasons 1 the introduction of hydrocarbons causes a fall in effective burial stress and 2 adsorption of polar hydrocarbons on the silicates may shield calcite from the silicates

fluid substitution for thinly interbedded sand shale sequences using the mesh method abstractgassmann s fluid substitution model is intended for a monomineralic homogeneous porous rock in which pore pressures induced by applied loads can equilibrate throughout the pore space these assumptions are violated when gassmann s equations are applied to measurements that represent effective medium averages over subresolution layers of alternating sand and shale the conventional procedure for treating this problem has been to first downscale i e estimate the properties of the fine scale sand and shale endmembers from the coarse scale measurements apply gassmann s fluid substitution to the sand only and then backus average back to the original scale this procedure however is very sensitive to errors in estimated sand fraction and shale properties and becomes particularly unstable at small sand fractions a new method for fluid substitution combines rock physics models for dispersed and interbedded sand shale systems which are often approximated with reuss or lower hashin shtrikman inte

depth dependent mantle anisotropy below the san andreas fault system apparent splitting parameters and waveforms we measure apparent teleseismic shear wave splitting parameters at several stations of the berkeley digital seismic network bdsn and three temporary broadband stations of the university of california at santa cruz located near the san andreas fault system in northern and central california previously proposed anisotropic models for the region include a two layered structure and a structure with a constant degree of anisotropy and a horizontal fast axis that gradually rotates as a function of depth we significantly increase the number of observations and confirm the existence of depth dependent anisotropy beneath the san andreas fault system we also investigate the extent to which the enhanced data set can constrain the details of the depth dependence using theoretical expressions we determine a suite of two layer models that fit our observations and result in practically indistinguishable apparent splitting parameters the wide range of acceptable models indicates that apparent splitting parameters alone cannot constrain all four parameters that specify the two anisotropic layers upper and lower fast polarization directions and delay times synthetic seismograms for three very different two layer models chosen from our suite of acceptable models and a model with a gradually rotating fast axis show subtle waveform variations and apparent splitting parameters measured from these records do not overlap hence we conclude that waveforms indeed contain additional information about the anisotropic structure we incorporate waveform data by searching for the four splitting parameters of two layer models that minimize the splitting of several records at each station simultaneously the resulting models largely overlap with the models obtained using the theoretical expressions which indicates that the addition of waveform data is not sufficient to uniquely determine all four parameters of a two layer model constraining the upper fast polarization direction to be parallel to the relative motion vector of the pacific and north american plates n35 w allows the values of the other three model parameters to be uniquely determined the lower layer fast polarization direction beneath all stations is approximately e w and the delay times vary from 0 85 to 1 70 s the delay times in the upper layer are smaller and they vary between 0 50 and 1 25 s shear within the uppermost mantle due to the relative plate motion is a satisfactory explanation for the shallow anisotropic signature the sublithospheric e w fast directions may be caused by small scale convection within a slabless window or by motions of the lithosphere relative to the deeper mantle

crustal deformation dynamics and stress evolution during seamount subduction high resolution 3 d numerical modeling funding was received from the people programme marie curie actions of the european union s seventh framework programme fp7 2007 2013 under rea grant agreement 604713 zip zooming in between plates v s and c r r acknowledge financial support received through the hades project ctm2011 30400 c01 and ctm2011 30400 c02 funded by spanish mineco

mantle variation within the canadian shield travel times from the portable broadband archean proterozoic transect 1989 we report travel times from the archean proterozoic transect 1989 this type of data set recorded by a transect of portable broadband instruments allows us to make inferences about mantle structure in the region between the wyoming craton and the superior province of the canadian shield with station separations of 50 to 100 km and frequencies up to 5 hz the resolution of lateral changes is increased by nearly an order of magnitude over previous studies to a scale that allows us to study the relation between velocity variation in the continental upper mantle surface geology tectonic features and age provinces travel times of direct and later phases are obtained from waveform matching the values are corrected for crustal contribution and inverted for the vertical path upper mantle delay tum under each station as well as the azimuthal dependence of this quantity the prominent feature in the upper mantle delays tum is the variation by at least 1 5 s for s much of which occurs over a narrow zone of just a few hundred kilometers width this suggests a major lateral upper mantle transition which does not coincide with the surface geological edge of the canadian shield but is located within the shield this same transition is also observed in shear wave splitting delay times surprisingly however the p delays do not exhibit a corresponding variation we address this apparent contradiction and show how it may be explained in conjunction with anisotropy in the subcontinental lithosphere a simplified thermal model of the lithospheric transition zone in which temperature controls the degree of crystallographic alignment and thus seismic anisotropy predicts this phenomenon

constraints on absolute s velocities beneath the aegean sea from surface wave analysis summary the 3 d structure of the lithosphere beneath the aegean sea is investigated through surface wave dispersion analysis rayleigh and love waves recorded by 12 broad band stations installed for a duration of 6 months in the aegean region are processed through array analysis and wiener filtering data from three geofon stations in the area of crete were also used the resulting two station phase velocities are used to determine lateral variations of rayleigh wave phase velocities between periods of 20 and 100 s by a 2 d ray tomography method the obtained phase velocities are inverted to calculate variation of s wave velocity with depth using a combination of linearized inversion and a monte carlo based non linear inversion the absolute s wave velocity is resolved to a depth of approximately 200 km a highvelocity anomaly of 3 per cent is observed in the southern aegean attributed to the hellenic subduction in the northern part of the aegean in the prolongation of the north anatolian fault which is influenced by strong extensional movements we found low absolute s wave velocities at 50 100 km depth this supports a model of a distributed deformation of the upper mantle in the area separate rayleigh and love wave phase velocity inversions along common profiles reveal a strong love rayleigh discrepancy in the northern aegean down to at least 150 km depth i e most probably including the top of the asthenosphere

deep crustal reflections in australia 1957 1973 ii crustal models summary in a companion paper we described experiments conducted by the australian bureau of mineral resources bmr between 1957 and 1973 to record deep crustal seismic reflections at 24 of the 46 sites investigated events were recorded at times up to 10 13 s at some sites only one or two shots were recorded where more comprehensive investigations were made the events were consistent with deep reflections though they lacked continuity in many cases the results were sufficiently encouraging that we have now analysed the events on the hypothesis that they represented deep crustal reflectors including the moho in 1976 bmr introduced digital recording and successfully carried out several long deep crustal profiles since then this paper analyses only the recordings made with analogue equipment prior to 1973 in a few places the reflection structure could be compared with nearby refraction traverse results some events correlated well with first or second order discontinuities in the refraction velocity depth curves while for others there was no correspondence it is difficult to generalize as to which types of refractors give rise to reflections the refraction surveys show that the moho is not always a boundary with a sharp change in velocity across it in some areas in queensland the deepest reflector correlates well with the refracting moho for reflection shots not near refraction surveys we converted travel times to depths using simplified velocity models based on refraction surveys in similar geological environments and on isostatic balance considerations using gravity and elevation data in central australia if the deepest reflector is the moho a relatively thin and light crust is implied however comparison with refraction models makes it seem probable that a heavier thicker crust could exist with the moho being deeper than the deepest reflector also in the tasman geosyncline it is probable that the moho is deeper than the deepest reflector

chalk porosity and sonic velocity versus burial depth influence of fluid pressure hydrocarbons and mineralogy seventy chalk samples from four formations in the overpressured danish central north sea have been analyzed to investigate how correlations of porosity and sonic velocity with burial depth are affected by varying mineralogy fluid pressure and early introduction of petroleum the results show that porosity and sonic velocity follow the most consistent depth trends when fluid pressure and pore volume compressibility are considered quartz content up to 10 has no marked effect but more than 5 clay causes lower porosity and velocity the mineralogical effect differs between p wave and shear velocity so that smectite bearing chalk has a high poisson s ratio in the water saturated case but a low value in the dry case oil bearing chalk has up to 25 units higher porosity than water saturated chalk at similar depth but similar velocity probably because hydrocarbons prevent pore filling cementation but not pore structure stiffening cementation in this presumably water wet chalk these results should improve the modeling of chalk background velocity for seismic inversion analysis when describing the porosity reducing process pore volume compressibility should probably be disregarded when correcting for fluid pressure because the cementing ions originate from stylolites which are mechanically similar to fractures we find that cementation occurs over a relatively short depth interval

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

aegean tectonics strain localisation slab tearing and trench retreat abstract we review the geodynamic evolution of the aegean anatolia region and discuss strain localisation there over geological times from late eocene to present crustal deformation in the aegean backarc has localised progressively during slab retreat extension started with the formation of the rhodope metamorphic core complex eocene and migrated to the cyclades and the northern menderes massif oligocene and miocene accommodated by crustal scale detachments and a first series of core complexes mccs extension then localised in western turkey the corinth rift and the external hellenic arc after messinian times while the north anatolian fault penetrated the aegean sea through time the direction and style of extension have not changed significantly except in terms of localisation the contributions of progressive slab retreat and tearing basal drag extrusion tectonics and tectonic inheritance are discussed and we favour a model 1 where slab retreat is the main driving engine 2 successive slab tearing episodes are the main causes of this stepwise strain localisation and 3 the inherited heterogeneity of the crust is a major factor for localising detachments the continental crust has an inherited strong heterogeneity and crustal scale contacts such as major thrust planes act as weak zones or as zones of contrast of resistance and viscosity that can localise later deformation the dynamics of slabs at depth and the asthenospheric flow due to slab retreat also have influence strain localisation in the upper plate successive slab ruptures from the middle miocene to the late miocene have isolated a narrow strip of lithosphere still attached to the african lithosphere below crete the formation of the north anatolian fault is partly a consequence of this evolution the extrusion of anatolia and the aegean extension are partly driven from below asthenospheric flow and from above extrusion of a lid of rigid crust

effective medium modeling of gas hydrate filled fractures using the sonic log in the krishna godavari basin offshore eastern india 1 indian national gas hydrate program expedition 01 has established that clay rich marine sediment from the krishna godavari kg basin in the eastern indian margin hosts one of the richest gas hydrate deposits in the world resistivity at bit images and pressure cores reveal that the gas hydrate morphology in clay rich sediment varies from complex vein structures grain displacing to invisible pore filling existing rock physics models which relate acoustic data to in situ gas hydrate concentrations generally assume isotropic pore filling gas hydrate which yields misleading concentration estimates for fractured fine grained sediments the anisotropic kg basin sediment presents additional complications here we apply differential effective medium theory to incorporate grain displacing morphologies by which gas hydrate is included as vertical ellipsoids with aspect ratios ranging from those of thin veins up to those of nodules in an elastic anisotropic background we have estimated gas hydrate concentrations from sonic velocities at hole 10d in the kg basin considering three basic gas hydrate morphologies i pore filling ii grain displacing and iii a combination of grain displacing and pore filling average gas hydrate saturations for these three cases are 35 42 27 30 and 33 41 of the total porosity respectively in the depth range 60 140 m below seafloor mbsf saturation is highest at 67 mbsf for any morphology but the values differ between morphologies for the pore filling morphology the maximum gas hydrate saturation of 56 is 18 22 higher than the grain displacing morphology and 2 9 higher than the combined morphology estimates differ by 6 of the sediment volume with rotations of gas hydrate veins from vertical to horizontal

stress induced seismic azimuthal anisotropy sand shale content and depth trends offshore north west australia abstractseismic azimuthal anisotropy is apparent when p wave velocities vary with source receiver azimuth and downward propagating s waves split into two quasi s waves polarized in orthogonal directions not accounting for these effects can degrade seismic image quality and result in erroneous amplitude analysis and geologic interpretations there are currently no physical models available to describe how azimuthal anisotropy induced by differential horizontal stress varies with sand shale lithology and depth we develop a model that does so in unconsolidated sand shale sequences offshore north west australia our method naturally introduces two new concepts critical anisotropy and anisotropic depth limit critical anisotropy is the maximum amount of azimuthal anisotropy expected to be observed at the shallowest sediment burial depth where the confining pressure and sediment compaction are minimal the anisotropic depth limit is the maximum depth where the stress induced azimuthal anisotropy is ex

p wave velocity features of the lithosphere under the eromanga basin eastern australia including a prominent mid crustal conrad discontinuity abstract the crustal structure of the central eromanga basin in the northern part of the australian tasman geosyncline revealed by coincident seismic reflection and refraction shooting contrasts with some neighbouring regions of the continent the depth to the crust mantle boundary moho of 36 41 km is much less than that under the north australian craton to the northwest 50 55 km and the lachlan fold belt to the southeast 43 51 km but is similar to that under the drummond and bowen basins to the east the seismic velocity boundaries within the crust are sharp compared with the transitional nature of the boundaries under the north australian and lachlan provinces in particular there is a sharp velocity increase at mid crustal depths 21 24 km which has not been observed with such clarity elsewhere in australia the conrad discontinuity in the lower crust the many discontinuous sub horizontal reflections are in marked contrast to lack of reflecting horizons in the upper crust further emphasising the differences between the upper and lower crust the crust mantle boundary moho is characterised by an increase in velocity from 7 1 7 7 km s to a value of 8 15 0 04 km s the depth to the moho under the canaway ridge a prominent basement high is shallower by about 5 km than the regional moho depth there is also no mid crustal horizon under the canaway ridge but there is a very sharp velocity increase at the moho depth of 34 km the ridge could be interpreted as a horst structure extending to at least moho depths but it could also have a different intra crustal structure from the surrounding area the sub crustal lithosphere has features which have been interpreted from limited data as being caused by a velocity gradient at 56 57 km depth with a low velocity zone above it because of the contrasting crustal thicknesses and velocity gradients the lithosphere of the central eromanga basin cannot be considered as an extension of the exposed lachlan fold belt or the north australian craton the lack of seismic reflections from the upper crust indicates no coherent accoustic impedance pattern at wavelengths greater than 100 m consistent with an upper crustal basement of tightly folded meta sedimentary and meta volcanic rocks the crustal structure is consistent with a pericratonic or arc back arc basin being cratonised in an episode of convergent tectonics in the early palaeozoic the seismic reflections from the lower crust indicate that it could have developed in a different tectonic environment

evaluation of variable gas saturation on acoustic log data from the haynesville shale gas play nw louisiana usa many technical challenges must be overcome to ensure the economic development of a shale gas resource including 1 making an accurate assessment of gas distribution and saturation 2 identifying target zones that optimize the impact of hydraulic fracture stimulation and completion and 3 drilling safe and cost effective wells in order to meet these challenges it is necessary to characterize the physical and mechanical properties of the shales sonic logs provide important data about the rock and fluid properties that can be used for this characterization one motivation for this work was to determine how sonic logs in gas bearing shales can be used to characterize rock properties derive mechanical properties and estimate in situ stresses

velocity resistivity versus porosity permeability inter relations in dead sea salt samples abstract this study is intended to investigate dead sea ds salt porosity permeability hydraulic conductivity inter relations as well as their relationship to geophysical parameters such as compression and shear wave velocities vp vs respectively and specific resistivity x in laboratory and field conditions to estimate representative properties affected by porosity changes during sinkhole development some 41 salt samples were subjected to porosity measurements and 25 samples were tested to reveal hydraulic conductivity the salt of the coastal dead sea area composed of halite is revealed in 10 200 10 800 year old layers the halite commonly appears as rock salt the 7 30 m thick salt unit is located below the groundwater table at depths of 20 50 m from the surface salt layers are very conductive hydraulically and water saturated with the ds brine is of very high salinity with total dissolved solids tds of 340 380 g l two types of salt have been studied crystalline transparent salt cts distributed along the entire ds s western shore and white massive salt wms found locally along the southern ds and along the dike salt porosity measured in samples varies in the range of 5 21 whereas hydraulic conductivity varies between 10 5 to 10 9 m s density is changed in the range of 1681 2116 kg m 3 in wet samples and between 1638 and 2106 kg m 3 in dry samples during the laboratory testing of ds salt samples we have measured the following geophysical parameters vp 3700 4460 m s vs 1900 2620 m s and x 2 100 m on the other hand our previous field studies based on s wave seismic refraction surface wave prospecting swp and transient electromagnetic tem methods have shown that in in situ salt velocities vp range between 2900 and 4240 m s vs varies between limits of 750 m s to more than 1650 m s and x is of 0 8 to 2 5 m these values are considerably less than the analogous parameters measured in the salt samples the same conclusion has been made from the comparison of salt sample properties and borehole logging both studied by ultrasonic method geophysical data testify that in situ porosity calculated using relationships obtained during this study exceeds those in the laboratory samples especially in zones of heightened voidness where it is 25 hydraulic conductivities in the same zones exceed a value of 10 6 m s and even 10 5 m s we expect that in larger salt volumes these parameters can yet increase

testing five of the simplest upper mantle anisotropic velocity parameterizations using teleseismic s and sks data from the billings montana passcal array 1 five of the simplest parameterizations of upper mantle anisotropy are tested and ranked for a data set collected on a dense temporary passcal seismic array located 100 km ne of yellowstone these hexagonal symmetry anisotropy models possess either one or two layers with either flat or dipping fast velocity axis fva recordings from fifteen high quality direct s and sks arrivals are stacked to provide accurate waveform and error estimates source normalization is accomplished using the cross convolution technique a direct monte carlo neighborhood algorithm is used to map the posteriori model probability density ppd volume using the f test we find that models with purely flat fva can be rejected at 97 probability our best model p5 is a two layer dipping fva parameterization albeit the two layer model with one flat and one dipping fva can only be rejected at 80 probability the best model has a lower layer with a n65 e fva strike and a 12 dip down to the southwest and an upper layer with a n20 w fva strike and a 47 fva dip down to the southeast the bottom asthenospheric layer fva strikes parallel to north america s absolute plate motion direction and dips opposite to what passive plate shear of the asthenosphere would predict the upper lithospheric layer is consistent with lpo accretion associated with north directed drift of the north american plate during the mesozoic comparison between the sks and direct s wave data sets shows that the direct s waves improve resolution of the double layer anisotropic model parameters

seismic crustal structure of the proterozoic north australian craton between tennant creek and mount isa seismic recording along a 600 km traverse across the southern part of the north australian craton indicates that lateral velocity variations exist in the middle lower crust taken together with other data from the craton there is evidence for midcrustal velocities increasing from east to west and from south to north these trends could result from a greater proportion of high velocity granulites being emplaced at higher levels in the crust between tennant creek and mount isa the velocity structures near the surface are compatible with an assemblage of younger and older metamorphic rocks intruded by granites near surface 0 5 km p wave velocities on the tennant creek inlier increase from 5 47 km s at the surface to about 6 2 km s at 6 km depth and correspond with low grade metamorphics of the warramunga group interspersed with weathered granites overlying amphibolite facies rocks of older metamorphic domains in the mount isa geosyncline velocities of 6 03 6 15 km s correspond to the leichhart metamorphics interspersed with granites in the middle crust p wave velocities of 6 85 km s occur at a depth of 26 km near tennant creek whereas such velocities are not evident until depths of about 37 km near mount isa there is therefore a trend for midcrust velocities to increase from east to west the lower crust is characterized by velocities of 7 3 7 5 km s and the upper mantle velocities of 8 16 8 20 km s are reached at depths of 51 54 km exposed granulite facies rocks in central australia are low in heat producing elements and are compatible with the low surface heat flow generally measured in precambrian australia these data taken together with laboratory velocity measurements on worldwide samples reported elsewhere indicate that middle to lower crustal rocks could be an assemblage of granulite facies rocks varying from pyroxene granulite in the middle crust through hornblende granulite to garnet granulite in the lower crust

geophysical differences in the lithosphere between phanerozoic and precambrian australia abstract cannikin atomic bomb recordings indicate that there are differences in travel times from the aleutian islands test site to phanerozoic and precambrian provinces in australia of up to 1 1 s explosion seismic studies in central and southeastern australia enable travel time corrections for crustal and upper mantle structure to be made to recordings of such teleseismic events structure in the upper 60 km can account for at most about 0 2 s of the residual difference but attempts to constrain the remaining residual time to the region above the lehmann discontinuity at about 200 km depth are difficult to reconcile with explosion seismic models regional differences in seismic velocity structure between phanerozoic and precambrian australia therefore appear to exist at depths greater than 200 km electrical conductivities within the mantle have been investigated using two methods long period electromagnetic depth sounding using magnetometer arrays demonstrates that conductivities increase at about 200 km under phanerozoic australia but not until about 500 km depth under precambrian australia shorter period magnetotelluric measurements can only resolve shallower structures these too indicate a similar trend but with sub crustal conductivities increasing at less than 100 km under phanerozoic australia magma at these depths and shallower may be the source for cainozoic volcanism in eastern australia under precambrian central and northern australia magnetotelluric investigations indicate that pronounced conductivity increases do not occur until depths of 150 200 km are reached oceanic magnetic observations indicate that the australian lithospheric plate as a whole is separating from antarctica at a rate of about 7 cm yr the seismic and conductivity structures under the continental region of this plate indicate that lateral inhomogeneities possibly extend to depths as great as 500 km and are probably caused by the passage of eastern australia over a hot spot hawaiian studies indicate that hot spots are not local features but result from large scale disturbances in the mantle conductivity increases commencing in the depth range 100 250 km may give an indication of uppermost zones within which the palaeozoic lithospherc has been substantially modified resulting in elevated surface heat flow volcanism and seismic travel time anomalies

what is shale to a petrophysicist what is a shale in a recent presentation to the geophysical society of houston rune holt attempted to answer the question by listing three attributes of a shale 1 clay minerals should constitute the load bearing framework 2 shales have nanometer pore sizes and nanodarcy permeability 3 surface area is large and water is adsorbed on surfaces or bound inside clay platelets this is a good definition of shales but perhaps a typical geophysicist may not know how to apply it in practice with typical data sets in particular a geophysicist often has no core data and is limited to well logs this article describes how petrophysicists traditionally distinguish sands from shales using typical well logs in clastic sections only lean shales with minimal organic content are considered here

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

estimation of rock frame weakening using time lapse crosswell the frio brine pilot project abstractco2 injection into subsurface reservoirs leads to pressure and saturation changes furthermore co2 brine minerals interaction could result in dissolution or reprecipitation of rock frame forming minerals observed time lapse seismic associated with co2 injection into poorly consolidated sandstone at the frio co2 injection site texas usa could not be predicted using classical rock physics models i e models involving elastic changes in the rock frame due to saturations and or pressures changes only and assuming no changes in the rock microstructure that and the changes in the fluid chemistry after co2 injection suggests that the assumption of a constant rock microstructure might be violated using high resolution time lapse crosswell data we have developed a methodology for estimating changes in the rock frame by quantifying the rock frame drained moduli before and after co2 injection based on rock microstructure diagnostics we found that the changes in the drained frame elastic prope

fluid substitution estimating changes in vp without knowing vs two methods are presented for estimating the change of seismic p wave velocity that accompanies pore fluid changes in a rock in the common situation when s wave velocity is unknown in contrast gassmann s relation operates on the rock bulk modulus which can only be calculated when both vp and vs are measured the first method operates directly on the p wave modulus and is equivalent to replacing the bulk moduli of the rock and mineral in gassmann s relation with the corresponding p wave moduli the second method uses a graphical construction to estimate the decomposition of the measured p wave modulus into bulk and shear moduli which then allows the conventional gassmann s formula to be used when applied to a large set of sandstone data the predictions of both methods computed with vp only are within a few percent of the gassmann s relation using both vp and vs

effect of pore geometry on gassmann fluid substitution although there is no assumption of pore geometry in derivation of gassmann s equation the pore geometry is in close relation with hygroscopic water content and pore fluid communication between the micropores and the macropores the hygroscopic water content in common reservoir rocks is small and its effect on elastic properties is ignored in the gassmann theory however the volume of hygroscopic water can be significant in shaly rocks or rocks made of fine particles therefore its effect on the elastic properties may be important if the pore fluids in microspores cannot reach pressure equilibrium with the macropore system assumption of the gassmann theory is violated therefore due to pore structure complexity there may be a significant part of the pore fluids that do not satisfy the assumption of the gassmann theory we recommend that this part of pore fluids be accounted for within the solid rock frame and effective porosity be used in gassmann s equation for fluid substitution integrated study of ultrasonic laboratory measurement data petrographic data mercury injection capillary pressure data and nuclear magnetic resonance t2 data confirms rationality of using effective porosity for gassmann fluid substitution the effective porosity for gassmann s equation should be frequency dependent knowing the pore geometry if an empirical correlation between frequency and the threshold pore throat radius or nuclear magnetic resonance t2 could be set up gassmann s equation can be applicable to data measured at different frequencies without information of the pore geometry the irreducible water saturation can be used to estimate the effective porosity

which forces drive north america understanding the mechanism of plate tectonics is one of the most important problems in the geosciences are the tectonic plates pulled and pushed from the side as the orowan elsasser model suggests or does mantle convection play an active role in driving the plates this question can be addressed by studying the deformation of deep continental roots the application to north america shown here indicates that the deeper mantle moves at a higher velocity than the north american plate and that the mantle plays an important role in driving the plates i suggest that this finding 1 provides a natural explanation for why the motion of north america slowed down dramatically during the past 100 m y and 2 predicts that north american motion will eventually come to a halt

an experimental study of acoustic responses on the injection of supercritical co2 into sandstones from the otway basin quantitative knowledge of the acoustic response of rock from an injection site on supercritical co2 scco2 saturation is crucial for understanding the feasibility of time lapse seismic monitoring of co2 plume migration a suite of shaley sandstones from the injection interval of the crc 2 well otway basin australia is tested to reveal the effects of supercritical co2 injection on acoustic responses co2 is first injected into dry samples flushed out with brine and then injected again into brine saturated samples such a suite of experiments allows us to obtain acoustic velocities of the samples for a wide range of co2 brine saturations from 0 to 100 on injection of scco2 into brine saturated samples the rocks exhibit a decrease of compressional velocities by about 7 with the increase of co2 saturation from 0 to a maximum of about 50 anisotropy of the shaley sandstones from the otway basin must be taken into account as the difference in the velocities normal and parallel to bedding is comparable with the perturbation due to co2 injection and the samples of different orientations exhibit transition from gassmann hill to gassmann wood bound at different scco2 saturations changes of the dry samples before and after the co2 injection if any are not traceable by acoustic methods

bridging onshore and offshore present day kinematics of central and eastern mediterranean implications for crustal dynamics and mantle flow we present a new kinematic and strain model of an area encompassing the calabrian and hellenic subduction zones western anatolia and the balkans using haines and holt s 1993 method we derive continuous velocity and strain rate fields by interpolating geodetic velocities including recent gps data in the balkans relative motion between stable eurasia and the western aegean sea is gradually accommodated by distributed n s extension from southern balkans to the eastern corinth gulf so that the westward propagation of the north anatolian fault naf throughout continental greece or peloponnesus is not required we thus propose that the naf terminates in north aegean and that n s extension localized in the corinth gulf and distributed in southern balkans is due to the retreat of the hellenic slab the motion of the hyblean plateau apulia peninsula south adriatic sea ionian basin and sirte plain can be minimized by a single rigid rotation around a pole located in the sirte plain compatible with the opening the pelagian rifts 2 2 5 mm yr and seismotectonics in libya we interpret the trenchward ultraslow motion of the calabrian arc 2 2 5 mm yr as pure collapse the calabrian subduction being now inactive in the absolute plate motion reference frame our modeled velocity field depicts two toroidal crustal patterns located at both ends of the hellenic subduction zone clockwise in nw greece and counter clockwise in western anatolia we suggest the nw greece toroidal pattern is the surface expression of a slab tear and consequent toroidal asthenospheric flow

geological evidence for the geographical pattern of mantle return flow and the driving mechanism of plate tectonics tectonic features at the earth s surface can be used to test models for mantle return flow and to determine the geographic pattern of this flow a model with shallow return flow and deep continental roots places the strongest constraints on the geographical pattern of return flow and predicts recognizable surface manifestations because of the progressive shrinkage of the pacific averaging 0 5 km2 yr over the last 180 m y this model predicts upper mantle outflow through the three gaps in the chain of continents rimming the pacific caribbean drake passage australian antarctic gap in this model upper mantle return flow streams originating at the western pacific trenches and at the java trench meet south of australia filling in behind this rapidly northward moving continent and providing an explanation for the negative bathymetric and gravity anomalies of the australian antarctic discordance the long continued tectonic movements toward the east that characterize the caribbean and the easternmost scotia sea may be produced by viscous coupling to the predicted pacific outflow through the gaps and the caribbean floor slopes in the predicted direction if mantle outflow does pass through the gaps in the pacific perimeter it must pass beneath three seismic zones central america lesser antilles scotia sea none of these seismic zones shows foci below 200 km mantle material flowing through the caribbean and drake passage gaps would supply the mid atlantic ridge while the java trench supplies the indian ocean ridges so that deep mantle upwellings need not be centered under spreading ridges and therefore are not required to move laterally to follow ridge migrations the analysis up to this point suggests that upper mantle return flow is a response to the motion of the continents the second part of the paper suggests a possible driving mechanism for the plate tectonic process which may explain why the continents move this hypothetical driving mechanism has four aspects 1 the lower mantle convects 2 this convection drives the continents by drag on their deep roots 3 return flow in the upper mantle provides a volumetric balance for the motion of the continental masses without passing under them and 4 oceanic plates are effectively decoupled from the asthenosphere and are driven largely by slab pull this mechanism accounts for the opening of the atlantic the ability of spreading ridges to stay on the midlines of oceans and the penetration of india into asia following their collision continental motions strongly imply lower mantle upwelling and divergence beneath the atlantic and southeast indian oceans and convergence and subsidence along the tethyan belt this picture disagrees with the concept derived from hot spot studies of an undeforming absolute reference framework at depth but weaknesses in current hot spot theory would make a rejection of the present model on this ground premature if the present model is generally correct a fairly simple pattern of lower mantle convection with as few as four cells may explain much of the tectonic complexity of the earth

critical porosity a key to relating physical properties to porosity in rocks discovering accurate relations between porosity and seismic wave velocities in porous rocks has been an important topic of rock physics for decades such relations if they actually exist are critical when seismic and or sonic data are to be used to infer porosity and porosity related properties e g permeability strength saturation in situ these inferences became especially important in recent years with the rapid development of seismic and sonic methods which are applied to the characterization of heterogeneous reservoirs and recovery monitoring

comparing crustal and mantle fabric from the north american craton using magnetics and seismic anisotropy a central target in earth sciences is the study of deformation at various depth levels within the earth seismology has offered a remarkable tool for doing this via seismic anisotropy it is however not always clear how to interpret those observations a question of interest is to understand the relation between the deformation of the mantle and the crust and in studying the relation between the two mantle deformation is expressed in seismic anisotropy in this paper we seek an objective way of extracting information about crustal fabric as well to be able to compare with seismic anisotropy the magnetization of crustal rocks offers an attractive possibility for doing this we thus explore the use of magnetic data and we compare magnetic crustal fabric orientation with mantle fabric observations from seismic anisotropy we apply our technique to the north american craton for which we have an excellent magnetic dataset and we show that there is a clear relation between crustal and mantle fabric for the cratonic region this has important implications for crustal formation and for interpreting seismic anisotropy observations

sediment with porous grains rock physics model and application to marine carbonate and opal we offer an effective medium model for estimating the elastic properties of high porosity marine calcareous sediment and diatomite this model treats sediment as a pack of porous elastic grains the effective elastic moduli of the porous grains are calculated using the differential effective medium dem model whereby the intragranular ellipsoidal inclusions have a fixed aspect ratio and are filled with seawater then the elastic moduli of a pack of these spherical grains are calculated using a modified scaled to the critical porosity upper hashin shtrikman bound above the critical porosity and modified lower carbonates and upper opal hashin shtrikman bounds below the critical porosity the best match between the model predicted compressional and shear wave velocities and ocean drilling program odp data from three wells is achieved when the aspect ratio of intragranular pores is 0 5 this model assigns finite nonzero values to the shear modulus of high porosity marine sediment unlike the suspe

topography and geoid due to lithospheric mass anomalies summary a model of lithospheric thickness and a recent compilation of moho depths are used to compute the earth s isostatic surface topography and associated gravity anomalies the results are strongly influenced by the uncertainties in lithospheric depth and crustal density profiles the preferred models explain most of the observed topography and are highly correlated with observed gravity anomalies for harmonic degrees larger than 10 comparisons of our residual topography with geodynamical calculations of dynamic topography based on mantle circulation are rather poor

generation of plate tectonics from lithosphere mantle flow and void volatile self lubrication abstract the formation of plate tectonics from mantle convection necessarily requires nonlinear rheological behavior recent studies suggest that self lubricating rheological mechanisms are most capable of generating plate like motion out of fluid flows the basic paradigm of self lubrication is nominally derived from the feedback between viscous heating and temperature dependent viscosity here we propose a new idealized self lubrication mechanism based on void e g pore and or microcrack generation and volatile e g water ingestion we test this void volatile self lubrication mechanism in a source sink flow model this leads to a basic nonlinear system which permits the excitation of strike slip toroidal motion a necessary ingredient of plate like motion out of purely divergent i e poloidal or characteristically convective flow with relatively inviscid void filling volatiles the void volatile mechanism yields a state of highly plate like motion i e with uniformly strong plate interiors weak margins and extremely focussed strike slip shear zones moreover the void volatile model obeys a chemical diffusion time scale that is typically much longer than the thermal convection time scale the model thus complies with the observation that plate boundaries are long lived and survive even while inactive the void volatile model of self lubrication therefore predicts self focussing shear zones plate generation and plate boundary longevity through what has long been suspected to be a key ingredient for the existence of plate tectonics i e water

kimberlite trends in nw canada kimberlite deposits attract much interest as the host rock for diamonds but their eruption mechanisms and deep root structure remain poorly perceived independent geochronological and seismic shear wave anisotropy studies in nw canada near the world9s newest diamond mines revealed an unexpected correlation in structural trends age of eruption grouped 36 kimberlites into linear clusters forward modelling of shear wave delays indicated two distinct layers of seismic anisotropy with a boundary about 120 km deep the similarity of inferred age and trend direction at surface and at depth indicates that these kimberlites erupted along lithospheric scale fractures oriented by continental stress fields related to north american plate motions

a method for determining gas hydrate or free gas saturation of porous media from seismic measurements the occurrence of gas hydrate or free gas in a porous medium changes the medium s elastic properties explicit formulas for gas hydrate or free gas saturation of pore space on the basis of the frenkel gassmann equations describe the elastic moduli and seismic velocities of a porous medium for low frequencies a key assumption of the model is that either gas hydrate or free gas is present in the pore space in addition to water under this assumption the method uses measured p and s wave velocities and bulk density along with estimates of the moduli and densities of the solid and fluid phases present to determine whether gas or hydrate is present the method then determines the saturation level of either the gas or the hydrate i apply the method to published velocity and density data from seismic studies at the antarctic shetland margin and at the storegga slide offshore norway and to borehole log and core data from ocean drilling program odp leg 164 at blake ridge offshore south carolina a sensitivity analysis reveals that the standard deviations of the gas hydrate and free gas saturations reach 30 70 of the saturations if the standard deviations of the p and s wave velocities and of the bulk density are 50m s

shear wave velocity estimation in porous rocks theoretical formulation preliminary verification and applications shear wave velocity logs are useful for various seismic interpretation applications including bright spot analyses amplitude versus offset analyses and multicomponent seismic interpretations measured shear wave velocity logs are however often unavailable we developed a general method to predict shear wave velocity in porous rocks if reliable compressional wave velocity lithology porosity and water saturation data are available the precision and accuracy of shear wave velocity prediction are 9 and 3 respectively the success of our method depends on 1 robust relationships between compressional and shear wave velocities for water saturated pure porous lithologies 2 nearly linear mixing laws for solid rock constituents 3 first order applicability of the biot gassmann theory to real rocks we verified these concepts with laboratory measurements and full waveform sonic logs shear wave velocities estimated by our method can improve formation evaluation our method has been successfully tested with data from several locations

deep crustal reflection results from the central eromanga basin australia abstract from 1980 to 1982 deep seismic reflection profiles were recorded across the central eromanga basin in eastern australia to study the regional structure stratigraphy and geological history of the eromanga basin and infra basins the reflection data were recorded to 20 s to obtain additional information on the nature and structure of the crust below the sediments and their relationship to the development of the basins the seismic sections show good quality reflections from the deep crust as well as from the sedimentary layers based on the character strength coherence continuity and spatial distribution of the reflections the sections can be divided into four zones the tope zone between 0 and 2 5 s shows fairly uniform coherent and continuous events which correlate with the mesozoic and late palaeozoic sediments the zone from 2 5 to 8 s 4 to 22 km does not show any primary reflections and is interpreted as the highly deformed metasedimentary and metavolcanic rocks of the early palaeozoic thomson orogen underlying the sediments without any recognisable reflection or diffraction patterns in this zone it is difficult to say whether the faulting and folding observed in the sediments extend into the upper crustal basement the deeper zone of numerous reflection segments between 8 and 12 5 s 22 to 36 km is interpreted as thin laminae of alternating low and high velocity intermediate and basic rocks and correlates with the lower crust bounded by refraction velocity discontinuities the lowest zone of no reflections below 12 5 s corresponds with the upper mantle the reflection character and thickness as well as the refraction velocity structure of the crust under the central eromanga basin area are significantly different from those of the precambrian crust under the georgina basin to the northwest it is proposed that the crust under the eromanga basin is extensionally attenuated crust which had been intruded by sills of basaltic melt from the underlying asthenosphere

the westward drift of the lithosphere a rotational drag net westward rotation of the lithosphere relative to the underlying mantle is a controversial phenomenon fi rst attributed to tidal effects and later to the dynamics of mantle convection in spite of a number of independent geological and geophysical arguments for westward tectonic drift this phenomenon has received little recent attention we suggest that this differential rotation is a combined effect of three processes 1 tidal torques act on the lithosphere generating a westerly directed torque decelerating earth s spin 2 the downwelling of the denser material toward the bottom of the mantle and in the core slightly decreases the moment of inertia and speeds up earth s rotation only partly counterbalancing the tidal drag 3 thin 3 30 km layers of very low viscosity hydrate channels occur in the asthenosphere it is suggested that shear heating and the mechanical fatigue self perpetuate one or more channels of this kind which provide the necessary decoupling zone of the lithosphere

methane hydrate formation in partially water saturated ottawa sand bulk properties of gas hydrate bearing sediment strongly depend on whether hydrate forms primarily in the pore fluid becomes a load bearing member of the sediment matrix or cements sediment grains our compressional wave speed measurements through partially water saturated methane hydrate bearing ottawa sands suggest hydrate surrounds and cements sediment grains the three ottawa sand packs tested in the gas hydrate and sediment test laboratory instrument ghastli contain 38 1 porosity initially with distilled water saturating 58 31 and 16 of that pore space respectively from the volume of methane gas produced during hydrate dissociation we calculated the hydrate concentration in the pore space to be 70 37 and 20 respectively based on these hydrate concentrations and our measured compressional wave speeds we used a rock physics model to differentiate between potential pore space hydrate distributions model results suggest methane hydrate cements unconsolidated sediment when forming in systems containing an abundant gas phase

the role of gravitational potential energy in active deformation in the southwestern united states in orogenic belts intrinsic buoyancy and extrinsic boundary forces act on the continental lithosphere determining the relative importance of these forces is fundamental to understanding the dynamics of continental deformation in the western united states estimates of gravitational potential energy show that given available constraints on rheology buoyancy forces are sufficient to produce the wide variety of rates and styles of observed deformation including absence of deformation this underscores the importance of gravitational potential energy for deformation in orogenic belts

influence of continental roots and asthenosphere on plate mantle coupling the shear tractions that mantle flow exerts on the base of earth s lithosphere contribute to plate driving forces and lithospheric stresses we investigate the sensitivity of these tractions to sub lithospheric viscosity variations by comparing shear tractions computed from a mantle flow model featuring laterally varying lithosphere and asthenosphere viscosity with those from a model with layered viscosity lateral viscosity variations generally do not change the direction of shear tractions but deeply penetrating continental roots increase traction magnitudes by a factor of 2 5 compared to 100 km thick lithosphere a low viscosity asthenosphere decreases traction magnitudes by a smaller amount and is important only if 100 km thick increased plate mantle coupling beneath thick continental lithosphere may increase plate driving forces surface deformation and mantle derived lithospheric stresses in these regions by contrast a low viscosity asthenosphere does not decouple the lithosphere from mantle flow highlighting the geological importance of mantle tractions on the lithosphere

review of recent results from continental deep seismic profiling in australia abstract the australian geological survey organisation regularly collects 450 500 km of onshore deep seismic reflection data and up to 4500 km offshore each year in australia these recordings are made in a wide range of tectonic provinces including in the last few years late palaeozoic mesozoic intracontinental and palaeozoic mesozoic cenozoic continental margin extensional basins moderately deformed palaeozoic transtensional basins and compressional fold belts and archaean greenstone terranes several of these provinces are major petroleum exploration provinces whereas others contain significant mineral deposits the primary purpose of the deep seismic profiling program is to resolve the tectonic history of the australian continent and thereby to encourage exploration for hydrocarbons and mineral resources in australia on the northwest australian continental margin major basin systems including the bonaparte basin formed as a result of complex interactions since the carboniferous involving episodes of extension followed by strike slip movements and inversion which reactivated both the primary extensional and ancient basement structures off southeastern australia basins such as the gippsland basin formed as part of a linked transtensional system related to movement on a common mid crustal detachment complex on continental australia the bowen basin in the northeast was deformed by thrust faults that root in a major e dipping detachment that flattens in the middle crust the cobar basin in the southeast is a case where the seismic data support a detachment model in which the upper plate displacement vector can be calculated by plate reconstructions linking the geometry of the detachment surface with that of the basin the greenstone terranes within the eastern goldfields region of western australia show crustal scale fault systems that are planar and steep dipping more in keeping with those interpreted in data from other precambrian provinces rather than those of the palaeozoic provinces

the mid crustal horizon under the eromanga basin eastern australia abstract long line explosion seismic recordings indicate that a major mid crustal p wave velocity increase exists under the eromanga basin which contrasts the crust of this region from its neighbouring tectonic provinces the velocity increase is shown to be a regional feature and to correspond to the upper boundary of a zone of discontinuous seismic reflections characteristic of the lower crust the velocity increase of 0 35 0 6 km s occurs over a small depth range at an average depth of 24 km this velocity gradient produces conspicuous wide angle reflections at distances greater than 60 km from the shot point the seismic data present the clearest evidence to date of the existence of a major mid crustal horizon in continental australia a feature not prominent in other cratonic provinces which have been investigated in detail the velocity in the lower crust below the horizon is 6 7 7 0 km s the horizon represents a boundary between the upper and lower crust which could imply a different tectonic history for these two zones

upper mantle deformation beneath the north american pacific plate boundary in california from sks splitting in order to constrain the vertical and lateral extent of deformation and the interactions between lithosphere and asthenosphere in a context of a transpressional plate boundary we performed teleseismic shear wave splitting measurements for 65 permanent and temporary broadband stations in central california we present evidence for the presence of two anisotropic domains 1 one with clear e w trending fast directions and delay times in the range 1 5 to 2 0 s and 2 the other closely associated with the san andreas fault system with large azimuthal variations of the splitting parameters that can be modeled by two anisotropic layers the upper of the two layers provides fast directions close to the strike of the main californian faults and averaged delay times of 0 7 s the lower layers show e w directions and delay times in the range 1 5 to 2 5 s and thus can be compared to what is observed in stations that require a single layer we propose the e w trending anisotropic layer to be a 150 to 200 km thick asthenospheric layer explained by the shearing associated with the absolute plate motion of the north american lithosphere the shallower anisotropic layer ought to be related to the dynamics of the san andreas fault system and thus characterized by a vertical foliation with lineation parallel to the strike of the faults localized in the lithosphere we also propose that the anisotropic layer associated with each fault of the san andreas fault system is about 40 km wide at the base of the lithosphere

calibration of prestack simultaneous impedance inversion using rock physics this paper represents the third installment in a sequence of reservoir characterization articles that follow a workflow whose ultimate goal is to understand and predict the rock properties of a reservoir away from well control the first article singleton 2008 described a method to detect anomalous seismic attenuation this was important because seismic data at the well location have a steep falloff in amplitude caused by high attenuation in and above the reservoir interval the second article singleton 2009 dealt with the seismic gathers that were input into the inversion process it is of the utmost importance that the quality of data input into a simultaneous prestack inversion be as high as possible errors in seismic gathers low snr resolution loss from excessive nmo stretch nonflat reflectors offset amplitude problems multiples etc are broadcast straight into the inversion impedance volumes so this is a critical step in geophysical reservoir characterization

fluid substitution in shaley sediment using effective porosity the traditional method of fluid substitution in porous rock requires the total porosity and the elastic modulus of the mineral phase as input and assumes that the fluid reaches instantaneous hydraulic equilibrium throughout the pore space this assumption may not be appropriate for shaley sediment because of the low permeability of shale and the resulting immobility of the water in it to address this problem we propose an alternative method that uses effective porosity instead of total porosity effective porosity is lower than total porosity if porous shale is present in the system a new composite mineral phase is introduced which includes the porous water saturated shale together with the nonporous minerals and whose elastic modulus is an average of those of its components including the porous shale this alternative method increases the sensitivity of the elastic properties of sediment to pore fluid changes and therefore may be used as a physics based theoretical tool to better explain and interpret seismic data during exploration as well as variations in seismic response as hydrocarbon production progresses

cenozoic global plate motions cenozoic global plate motions relative to the hot spots are investigated and compared to plate motions in a mean lithosphere reference frame plate motions were analyzed over six time intervals divided by ages 10 25 43 48 and 56 ma chosen as much as possible to coincide with key plate reorganizations alternative motion circuits and rotational parameters were considered and evaluated with paleomagnetic data from the pacific and north american plates the circuit found to be in best agreement with the paleomagnetic data is one in which the hot spots in the atlantic region are assumed to be fixed relative to the hot spots in the pacific region throughout the cenozoic the hot spot and mean lithosphere reference frames have been in continual slow relative motion the rate of motion is nonuniform however most of the motion having occurred during the middle cenozoic the net cenozoic rotation of the lithosphere relative to the hot spots is described by a right handed rotation of 7 about a euler pole at 46 s 87 e which yields a 5 displacement of the north poles of the two reference frames this motion is small enough that inferences drawn about plate speeds in one reference frame should be valid in the other analysis of the global motions resulting from our preferred model showed that many characteristics of current plate motions have persisted throughout the cenozoic plate speeds correlate with latitude plates moving faster near the equator than near the poles throughout the cenozoic as at present continental plates except for the indian plate moved slower than oceanic plates throughout the cenozoic even the structure of the velocity fields as revealed in a contour of root mean square velocities in equatorial bands persists throughout the cenozoic the migration of the paleomagnetic axis over time is also compared to the hot spot and mean lithosphere reference frames the paleomagnetic axis has shifted 5 10 relative to the hot spot frame and a lesser amount relative to the mean lithosphere frame

deep mantle contributions to the surface dynamics of the north american continent abstract the regional and continental scale dynamics of north america and vicinity are explored using a high resolution model of mantle flow the model is constrained by simultaneously inverting global seismic and mantle convection data sets and it includes an explicit treatment of the positive chemical buoyancy of the continental tectosphere moreover it adopts a depth dependent mantle viscosity structure which reconciles both glacial isostatic adjustment gia and convection data the flow model successfully reproduces plate velocities and observations of surface gravity and topography including the continent scale quasi linear depression after corrections for gia and crustal heterogeneity extending from northern alaska to venezuela the predictions also match lithospheric flow and stress fields inferred from local and regional measurements of seismic anisotropy and surface deformation we demonstrate that these signals are largely driven by viscous flow coupled to density anomalies in the deep portions of the upper mantle and within the lower mantle where the latter may be associated with the descent of the ancient kula farallon plate system and an active mantle upwelling below the pacific margin of the north american plate more importantly the flow calculations elucidate how these large scale heterogeneities give rise to regional scale flow and stress patterns below the southwestern u s and below the central u s

practical seismic petrophysics the effective use of log data for seismic analysis a quick scan of the seg web site shows that the phrase seismic petrophysics has been used explicitly in the title of a paper or abstract six times the earliest of which was by williams et al in 1996 the hugoton cross well survey a direct look at stratigraphy seismic petrophysics and shale anisotropy however the first attempt at a definition and an expanded description of seismic petrophysics was published by wayne pennington in the leading edge in 1997 seismic petrophysics an applied science for reservoir geophysics he defines seismic petrophysics as follows the purposeful application of rock physics theory as calibrated by laboratory and well measurements to the interpretation of seismic data

the ability of rock physics models to infer marine in situ pore pressure pore fluid pressure is an important parameter defining the mechanical strength of marine sediments obtaining high spatial resolution in situ pore pressure measurements in marine sediments however is a challenge and as a result only a handful of in situ pore pressure measurements exist at scientific drill sites integrating rock physics models with standard iodp odp measurements provides a potentially widely applicable approach for calculating in situ pore pressure here we use a rock physics approach to estimate in situ pore pressure at two scientific ocean drill sites where in situ pressure is well constrained odp site 1173 used as reference for normal hydrostatic fluid pressures and odp site 948 where previous studies infer high fluid pressures 0 45 0 95 where the pore pressure ratio is defined as the pore pressure above hydrostatic divided by the difference between the largest principal stress and hydrostatic stress our analysis indicates that the rock physics method provides an accurate low precision method for estimating in situ pore pressure at these drill sites and sensitivity analysis indicates this method can detect modestly high 0 6 pore pressure at the 95 confidence level this approach has broad applicability because it provides an inexpensive high resolution meter scale method for retrospectively detecting and quantifying high pore pressure at any drill site where quality wireline logs and ocean drilling data exist

elastic wave velocity in marine sediments with gas hydrates effective medium modeling we offer a first principle based effective medium model for elastic wave velocity in unconsolidated high porosity ocean bottom sediments containing gas hydrate the dry sediment frame elastic constants depend on porosity elastic moduli of the solid phase and effective pressure elastic moduli of saturated sediment are calculated from those of the dry frame using gassmann s equation to model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions a hydrate modifies the pore fluid elastic properties without affecting the frame b hydrate becomes a component of the solid phase modifying the elasticity of the frame the goal of the modeling is to predict the amount of hydrate in sediments from sonic or seismic velocity data we apply the model to sonic and vsp data from odp hole 995 and obtain hydrate concentration estimates from assumption b consistent with estimates obtained from resistivity chlorinity and evolved gas data

shear stress at the base of shield lithosphere 1 one of the basic unresolved issues regarding plate tectonics is the degree of coupling between surface plates and convecting mantle below are the plates effectively decoupled from the mantle flow field by a low viscosity asthenosphere or are they strongly coupled to mantle flow while these two views are essentially incompatible they both do a reasonably good job of predicting the motions of the surface plates and cannot therefore be distinguished on this basis the significant distinguishing feature for these models is the magnitude of basal shear stress that is applied to the base of the plate while it is difficult to measure this stress directly it is possible in principle to measure the corresponding deformation of the plate through observations of seismic anisotropy and to infer stress here we focus on the canadian shield for which we expect strong plate mantle interaction we show that seismic anisotropy can be used to constrain the magnitude of the stress level applied to the base of the plate and to document the level of interaction between tectonic plates and the mantle below

europe from the bottom up a statistical examination of the central and northern european lithosphere asthenosphere boundary from comparing seismological and electromagnetic observations abstract the lithosphere asthenosphere boundary lab is a fundamental boundary in the plate tectonic paradigm it is the most pervasive on the planet yet comparatively it is one we know little about defined initially on the basis of the mechanical response of the earth to loading its usage has become ubiquitous across the geosciences but the natural differences in its definition due to differences in thermal physical and chemical parameters cause confusion to advance this debate comparisons are made both qualitatively and quantitatively between the delineation of the lab for europe based on seismological and electromagnetic observations we examine statistically using robust methods the labs derived from independent datasets and methods essentially all definitions of the lab as an impedance contrast from receiver functions slabrf a seismic anisotropy change slaba and an increase in conductivity from magnetotellurics elab are consistent with a deeper lab beneath precambrian europe and a shallower lab beneath phanerozoic europe with some exceptionally deep regions in phanerozoic europe such as the alps all three labs increase in depth significantly and rapidly at a location consistent with the surface expression of the trans european suture zone tesz which separates precambrian europe to the north and east from phanerozoic europe in the centre two of the definitions slabrf and elab are consistent for phanerozoic europe with mean values of 90 100 km compared to an average slabrf of 135 km a different two slaba and slabrf are consistent for precambrian europe with mean values of 170 180 km compared to an elab mean of 250 km the deeper elab depths for precambrian europe are more consistent with body wave and surface wave tomography models than are the slaba and slabrf ones suggesting that the lower lithosphere beneath precambrian europe between 170 to 250 km is responding to shearing on the base of the lithosphere from plate driving forces taken together the definitions provide strong constraints on the nature of the lab for example the electrical elab beneath the tesz is anomalously thick to possibly the transition zone whereas slaba and slabrf seismological definitions would place it at far shallower depths given the sensitivity of electrical conductivity to partial melt and water content the thick elab beneath the tesz excludes interpretation of the seismic lab in that location in terms of a thermal structure that would induce partial melting or of hydration

ausmoho the variation of moho depth in australia summary since 2004 more than 7000 km of full crustal reflection profiles have been collected across australia to give a total of more than 11000 km providing valuable new constraints on crustal structure a further set of hitherto unexploited results comes from 150 receiver functions distributed across the continent mostly from portable receiver sites these new data sets provide a dramatic increase in data coverage compared with previous studies and reveal the complex structure of the australian continent in considerable detail a new comprehensive model for moho depth across australia and its immediate environment is developed by utilizing multiple sources of information on shore and off shore refraction experiments are supplemented by receiver functions from a large number of portable stations and the recently augmented set of permanent stations and moho picks from the full suite of reflection transects the composite data set provides a much denser sampler of most of the continent than before though coverage remains low in the remote areas of the simpson and great sandy deserts the various data sets provide multiple estimates of the depth to moho in many regions and the consistency between the different techniques is high in a number of instances differences in estimates of moho depth can be associated with the aspects of the structure highlighted by the particular methods the new results allow considerable refinement of the patterns of moho depth across the continent some of the thinnest crust lies beneath the archean cratons in the pilbara and the southern part of the yilgarn thick crust is encountered beneath parts of the proterozoic in central australia and beneath the palaeozoic lachlan fold belt in southeastern australia the refined data indicate a number of zones of sharp contrast in depth to moho notably in the southern part of central australia

from mantle to crust stretching the mediterranean abstract the origin of forces driving the deformation of the continental crust near subduction zones and especially in backarc regions is debated this work is based on a compilation of sks fast splitting directions that give an image of flow lines in the mantle around the mediterranean subduction zones and a comparison with stretching and shear directions in metamorphic core complexes that show the pattern of deformation at the scale of the middle and lower crusts we find that 1 the two sets of directions are parallel in the three main backarc regions namely the alboran sea the tyrrhenian sea and the aegean sea showing that the lithosphere deforms with the same direction of stretching in the crust and the mantle suggesting that 2 crustal deformation is mainly driven from below by slab retreat and 3 the lithospheric fabric is reset within a few millions of years in backarc environments

estimation of porosity and water saturation in dual porosity pyroclastic deposits from joint analysis of compression shear and electromagnetic velocities in situ measurements of porosity and water saturation of pyroclastic deposits have the potential to improve interpretations of geology and hydrology in volcanic regions and provide more accurate estimates of dense rock equivalent for volcanic eruptions however rock property models must consider the dual porosity structure of pyroclastic deposits i e vesicles within pumices and intergranular pores vesicularity intergranular porosity and water saturation all affect the density elasticity and dielectric properties of pyroclastic materials which control seismic and electromagnetic velocities the data from active seismic and ground penetrating radar techniques may improve porosity and water saturation estimation if the responses of seismic and electromagnetic waves to porosity and water saturation variations are complementary in pyroclastic rocks we develop a dual porosity petrophysical model to calculate seismic and electromagnetic velocities of pumice with known intergranular porosity vesicul

deep crustal reflection recordings in australia 1957 1973 i data acquisition and presentation summary the australian bureau of mineral resources bmr has conducted experiments since 1957 to record deep crustal reflections these experiments were made initially during surveys conducted for other purposes in 1968 1969 bmr conducted experiments designed specifically for deep reflection recordings in the tertiary murray basin the precambrian willyama block and the palaeozoic tasman geosyncline in southeastern australia the amadeus basin in central australia and in the archaean in western australia bmr introduced digital recording in 1976 and since then successfully recorded several long deepcrustal profiles in this paper we describe and discuss only those recordings made with analogue equipment prior to 1973 apparent deep crustal reflections were recorded at most of the 46 sites investigated at 24 of these events occurred at times of 10 13 s at some sites where only one or two shots were recorded it was difficult to confirm late events as deep reflections where investigations were more comprehensive the events were consistent with deep reflections although in many places they lacked continuity the events showed peak frequencies of about 20 hz recording quality was improved with the use of 8 10 hz geophones in multiple in line arrays of 100 m or more and filters with a cut off about 14hz or less the quality was generally better in sedimentary basins than in areas of exposed basement rocks where several events occurred they generally showed a variety of dips making identification with simplified geological structure difficult it was not possible to interpret the reflection data alone in terms of crustal structure because of the limited data available and the large distances between most recording sites however the results are particularly useful in providing a preview of deep reflections that may be expected from future surveys in australia

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

the mantle flow field beneath western north america although motions at the surface of tectonic plates are well determined the accompanying horizontal mantle flow is not we have combined observations of surface deformation and upper mantle seismic anisotropy to estimate this flow field for western north america we find that the mantle velocity is 5 5 1 5 centimeters per year due east in a hot spot reference frame nearly opposite to the direction of north american plate motion west southwest the flow is only weakly coupled to the motion of the surface plate producing a small drag force this flow field is probably due to heterogeneity in mantle density associated with the former farallon oceanic plate beneath north america

protracted continental collisions argue for continental plates driven by basal traction abstract in plate tectonic theory collision between two continents should quickly terminate because of continental buoyancy if convergence is to continue it should do so at a new subduction zone where oceanic crust can be consumed the protracted continental collisions in the alps zagros and himalayas which have continued to deform continental crust since the early or middle cenozoic are therefore anomalies in standard plate tectonic theory it is commonly held that plates are driven by slab pull but this does not account for the continuing tethyan collisions where the descending slab has detached from the subducting continent these protracted continental collisions are better explained by horizontal traction of the mantle on the base of deep continental roots dragging the northern and southern continents together along a tethyan axis of mantle convergence continental undertow thus resolves the collision anomaly in plate tectonics