venezuela basin crustal structure velocity depth profiles derived from six two ship expanding spread experiments in combination with other geophysical data define the characteristics of two distinct types of venezuela basin crust and the boundary between them each two ship common midpoint reflection refraction profile is automatically transformed into the p plane picked and interpreted to provide v z functions with appropriate confidence bounds the results together with gravity magnetic and near vertical incidence reflection data reveal a 50 000 km2 triangle of venezuela basin crust which resembles normal oceanic crust in a magnetic quiet zone north and west of this triangle lies the previously defined thick caribbean crust having two distinct layers above the m discontinuity acoustic basement there appears unusually smooth due to extensive basaltic sills and flows which were cored at deep sea drilling project sites 146 149 sills and 150 flows also depths to mantle are greater than normal interpretations of near vertical and wide angle reflection data show that the extra crustal thickness is due not only to the emplacement of the flows but also to the crust below being somewhat thicker than normal the boundary between the two crustal areas has a ne sw trend which parallels the dominant structural and magnetic lineations this boundary coincides in position though not in trend with the previously defined central venezuela basin fault zone further study is required to determine whether this boundary is of tectonic origin or if it represents a change in style of crustal production

s wave velocity structure of the canadian upper mantle abstract we investigate mantle structure beneath the canadian landmass using 500 surface wave waveforms recorded at stations of the canadian national seismic network and global seismic network the data are processed using the method of partitioned waveform inversion and afford a comprehensive sampling of s velocity structure beneath the north american craton and adjacent orogenic belts our study thereby extends results from previous regional s wave studies north of the coterminous us and provides a characterization of mantle structure beneath canada which is complementary to the body wave model of grand j geophys res 99 1994 11591 the resulting 3d velocity model is well resolved to a depth of 400 km over most of the region and displays two prominent large scale anomalies a high velocity structure beneath the canadian shield and platform associated with the cratonic keel and pronounced low velocities beneath the cordillera the contrast between these features is strong up to 10 and at more southerly latitudes is sharp with the transition occurring over distances of 600 km or less laterally coherent high velocities persist to depths of ca 250 km beneath the north american craton which we interpret to represent the base of the continental keel smaller scale variations are observed within the cratonic mantle but in general show little correlation with surface geological provinces moderately low velocities beneath the st lawrence valley region and labrador may be related to intracontinental volcanism and rifting respectively

moho vs crust mantle boundary evolution of an idea abstract the concept that the mohorovicic discontinuity moho does not necessarily coincide with the base of the continental crust as defined by rock type compositions was introduced in the early 1980s this had an important impact on understanding the nature of the crust mantle boundary using information from seismology and from deep seated samples brought to the surface as xenoliths in magmas or as tectonic terranes the use of empirically constrained p t estimates to plot the locus of temperature vs depth for xenoliths defined a variety of geotherms depending on tectonic environment the xenolith geotherms provided a framework for constructing lithological sections through the deep lithosphere and revealed that the crust mantle boundary in off craton regions commonly is transitional over a depth range of about 5 20 km early seismic reflection data showed common layering near the moho correlating with the petrological observation of multiple episodes of basaltic intrusion around the crust mantle boundary developments in seismology petrophysics and experimental petrology have refined interpretation of lithospheric domains the expansion of in situ geochronology especially zircon u pb ages and hf isotopes os isotopes of mantle sulfides has defined tectonic events that affected whole crust mantle sections and revealed that the crust mantle boundary can change in depth through time however the nature of the crust mantle boundary in cratonic regions remains enigmatic mainly due to lack of key xenoliths or exposed sections the observation that the moho may lie significantly deeper than the crust mantle boundary has important implications for modeling the volume of the crust mapping the crust using seismic techniques alone without consideration of the petrological problems may lead to an overestimation of crustal thickness by 15 30 this will propagate to large uncertainties in the calculation of elemental mass balances relevant to crust formation processes and the composition of the convecting mantle

seismic imaging of major tectonic features in the crust of phanerozoic eastern australia abstract crustal architecture can be recognised from seismic fabric characteristics and velocity information across phanerozoic eastern australia three major crustal sub divisions are identified from data in southern queensland they occur under west to east the nebine ridge taroom trough and new england fold belt with each crustal sub division having its own internal architecture but separated by major bounding structures geosutures these features are assessed against evolutionary models for the region the significant conclusions are as follows 1 1 the nebine ridge forms the southeastern part of the thomson fold belt but has a crustal architecture distinctly different from that under the central part of the fold belt tentatively supporting an evolutionary model involving fragmentation and eastward stretching and thinning of a precambrian australian craton the ridge crust has possibly been considerably altered by carboniferous plutonism and compressional events 2 2 between the nebine ridge and the crust under the taroom trough of the bowen basin there is a major low angle 5 10 geosuture the foyleview geosuture which extends from upper to lower crustal levels along a series of prominent horizons it was probably active during the major carboniferous tectonism which severely structured devonian basins west of the nebine ridge the geosuture forms the southeastern boundary of the thomson fold belt 3 3 the crustal architecture under the taroom trough has little obvious structure above an outstanding moho transition zone however the prominent meandarra gravity ridge striking along the trough axis highlights the possibility of high density mafic volcanics within the crust and the probability of a rift model being appropriate early in the trough s history the sedimentary basin architecture supports the concept of an early extensional history for the crust within a poorly defined n s trending strike slip regime 4 4 between the taroom trough and the new england fold belt we interpret a high angle geosuture extending through the crust the burunga mooki geosuture reactivation during late palaeozoic early mesozoic times produced a series of reverse faults along the uplifted eastern margin of the trough some of these structures are tentatively considered to be positive flower structures above deeper faults with strike slip movement the geosuture separates regions with distinctly different seismic fabrics and played an important role in the formation of the bowen gunne dah sydney basin system 5 5 within the new england fold belt crustal domains can be recognized associated with late palaeozoic oroclinal bending and early mesozoic trans tensional basins major oroclinal bending of the upper crust in the west of the fold belt is interpreted above a mid crustal detachment the texas detachment in the central part of the fold belt the seismic data support a trans tensional mechanism for the formation of triassic basins with steeply dipping bounding faults under the east of the fold belt an imbricate thrust stack accretionary wedge is clearly imaged above a mid crustal horizon the brisbane mid crustal detachment upper crustal deformation appears to have occurred above this detachment 6 6 from the nebine ridge to the coast the moho is clearly defined below a 3 km thick moho transition zone hast of the thomson fold belt the moho level is identified as a gently undulating feature at 36 38 km depth and probably re established after the major late palaeozoic tectonic events which formed the crust under the taroom trough and new england fold belt this contrasts with the middle palaeozoic lower crustal moho features which appear to be preserved under the thomson fold belt 7 7 the thickest crust under the nebine ridge about 44 km appears to be associated with lower crustal wedging from the east there is an apparent thinning of the non sedimentary crust under the deepest basin taroom trough of about 30 compared with the crust to the west and east no crustal thickening is evident in the region of oroclinal bending within the new england fold belt

surface wave tomography of north america and the caribbean using global and regional broad band networks phase velocity maps and limitations of ray theory summary we present phase velocity maps of fundamental mode rayleigh waves across the north american and caribbean plates our data set consists of 1846 waveforms from 172 events recorded at 91 broad band stations operating in north america we compute phase velocity maps in four narrow period bands between 50 and 150 s using a non linear waveform inversion method that solves for phase velocity perturbations relative to a reference earth model prem our results show a strong velocity contrast between high velocities beneath the stable north american craton and lower velocities in the tectonically active western margin in agreement with other regional and global surface wave tomography studies we perform detailed comparisons with global model results which display good agreement between phase velocity maps in the location and amplitude of the anomalies however forward modelling shows that regional maps are more accurate for predicting waveforms in addition at long periods the amplitude of the velocity anomalies imaged in our regional phase velocity maps is three time larger than in global phase velocity models this amplitude factor is necessary to explain the data accurately showing that regional models provide a better image of velocity structures synthetic tests show that the raypath coverage used in this study enables one to resolve velocity features of the order of 800 1000 km however only larger length scale features are observed in the phase velocity maps the limitation in resolution of our maps can be attributed to the wave propagation theory used in the inversion ray theory does not account for off great circle ray propagation effects such as ray bending or scattering for wavelengths less than 1000 km scattering effects are significant and may need to be considered

a towed marine gravity meter for near bottom surveys gravity is measured presently on the sea surface and on the sea floor surface gravity suffers from loss of resolution over the deep ocean because the perturbing source masses are far from the observer bottom measurements recover this resolution but suffer from poor coverage because of the time needed for each measurement we have constructed a gravimetry system that combines the rapid data collection capability of a moving platform with the high resolution gained by locating the observations near the bottom this gravity sensor is tethered to a ship and towed just above the sea floor the instrument consists of a lacoste and romberg shipboard gravity meter modified to fit inside a pressure case that is mounted on a platform designed for towing stability we have tested it in a survey in the san diego trough a 1000 m deep sedimented valley in the pacific ocean in the california continental borderlands multiple gravity tracklines collected there at a depth of 935 m show a resolution of a few tenths of a mgal the new instrument will be useful for surveys of features whose lateral extent is equal to or less than the ocean depth

a new sea floor gravimeter a new reservoir management application uses precise time lapse gravity measurements on the sea floor to detect seawater infiltration in offshore natural gas fields during production reservoir models for the north sea troll field predict gravity changes as large as 0 060 mgal within a 3 5 year period we have constructed and deployed a new instrument the rovdog remotely operated vehicle deployed deep ocean gravimeter system for this application because the measurements must be relocated accurately within 3 cm we required a gravimeter which could be handled by an rov and placed atop sea floor benchmarks we have built an instrument based upon the scintrex cg 3m land gravimeter motorized gimbals level the gravimeter sensor within a watertight pressure case precision quartz pressure gauges provide depth information a shipboard operator remotely controls the instrument and monitors the data the system error budget considers both instrumental and field measurement uncertainties the instrument prototy

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

the conrad discontinuity and the top of the reflective lower crust do they coincide abstract in order to investigate an often quoted coincidence of an intracrustal velocity jump the conrad discontinuity and lower crustal reflections found in tectonothermally young areas 54 reflection seismic profiles have been analysed and combined with 50 refraction seismic profiles adjacent to the reflection lines such a relationship is however contradicted by the observation that in some places a reflective lower crust is found while a velocity discontinuity is missing most of the compared data sets clearly demonstrate that the velocity jump if present is found more than 2 km below the uppermost reflections from the lower crust it is thus concluded that the two phenomena are not related occasionally both a reflective lower crust rlc and a conrad discontinuity can be observed in this case the differences in depth between the two can be explained by the structure of the velocity increase as in the example of the kapuskasing uplift in canada due to the overall depth dependent increase of seismic velocity a genetic correlation of high velocity and reflectivity is negligible this conclusion is supported by the observed highly reflective lower crust of the urach low velocity body by explaining the rlc with thermorheological arguments the upper limit of reflections simply appears as a thermal boundary depending only on temperature material and fluid content the theological models presented are explaining a rlc with and without the presence of a velocity discontinuity

a surface wave dispersion study of the middle east and north africa for monitoring the comprehensive nuclear test ban treaty we present results from a large scale study of surface wave group velocity dispersion across the middle east north africa southern eurasia and the mediterranean our database for the region is populated with seismic data from regional events recorded at permanent and portable broadband three component digital stations we have measured the group velocity using a multiple narrow band filter on deconvolved displacement data overall we have examined more than 13 500 seismograms and made good quality dispersion measurements for 6817 rayleigh and 3806 love wave paths we use a conjugate gradient method to perform a group velocity tomography our current results include both love and rayleigh wave inversions across the region for periods from 10 to 60 seconds our findings indicate that short period structure is sensitive to slow velocities associated with large sedimentary features such as the mediterranean sea and persian gulf we find our long period rayleigh wave inversion is sensitive to crustal thickness such as fast velocities under the oceans and slow along the relatively thick zagros mts and turkish iranian plateau we also find slow upper mantle velocities along known rift systems accurate group velocity maps can be used to construct phase matched filters along any given path the filters can improve weak surface wave signals by compressing the dispersed signal the signals can then be used to calculate regionally determined m s measurements which we hope can be used to extend the threshold of m b m s discriminants down to lower magnitude levels other applications include using the group velocities in the creation of a suitable background model for forming station calibration maps and using the group velocities to model the velocity structure of the crust and upper mantle

partitioned waveform inversion and two dimensional structure under the network of autonomously recording seismographs nonlinear optimization methods are very effective in finding models that predict optimal waveform fits to observed seismograms provided the number of model parameters is small to reduce the size of the large scale two or three dimensional seismic modelling problem path integrals over seismic velocities may be defined as model parameters instead of the seismic velocities themselves this partitions the large scale optimization problem into a number of independent small scale problems and reduces the computational effort by several orders of magnitude the path integral parameters are transformed such that their correlation matrix becomes diagonal a linear inversion performs the mapping from the space of path integral parameters into the model space of seismic velocities and makes backus gilbert resolution analysis possible the method has been applied to body wave and surface wave data from the broadband portable network of autonomously recording seismographs nars in western europe using a surface wave ray approximation to generate synthetics by mode summation waveforms were fitted for frequencies as high as 50 mhz variations in s velocity as large as 6 1 have been established with a vertical resolution of 100 150 km and a horizontal resolution as small as 400 km in parts of the model although a low velocity layer is present under the southern part of the nars array such layer is very weak or absent north of the paris basin horizontal velocity gradients are well resolved down to 400 km depth

3d shear velocity structure beneath the gulf of california from rayleigh wave dispersion active extension in the gulf of california is characterized by the transition from continental rifting to seafloor spreading puzzling variations in the patterns of both tectonics and magmatism are observed along the length of the gulf and are likely to be related to mantle heterogeneity regional scale mantle structure however has been difficult to constrain due to the lack of broadband seismic stations in the region in this study we utilized new data from the deployment of the nars baja array and other networks and computed a three dimensional shear speed model of the upper mantle beneath the region applying a combination of cross correlation analysis and multimode waveform inversion we measured interstation rayleigh wave dispersion for 450 pairs of stations in a broad period range of 9 250 s we computed phase velocity maps and then inverted the phase velocity data for shear speed structure our results suggest that the location of the transition from seafloor spreading south to continental rifting north in the gulf of california as well as differences in volcanism across baja california and the gulf can be explained by heterogeneity in the upper mantle in particular by the presence of a slab remnant beneath the south central part and an absence of such a slab remnant beneath the northern part of the gulf

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

peculiarities of surface wave propagation across central eurasia abstract the recent installation of six broadband digital iris ida seismic stations in the ussr has provided new opportunities for studying surface wave propagation across eurasia group velocities of fundamental rayleigh and love modes between epicenters and these stations were determined for 35 events that occurred since april 1989 to the middle of july 1990 near eurasia differential phase velocities were found for the same arrivals along paths between several pairs of stations group and phase velocities were obtained in the period range from 15 to 300 sec frequency time polarization analysis was used for studying polarization properties of surface waves in some cases significant anomalies in the particle motion for periods up to 100 sec were observed they are attributed to surface wave refraction and scattering due to lateral inhomogeneities at the boundaries and inside the eurasia continent

assessment of global phase velocity models summary we construct new love and rayleigh wave phase velocity models based on measurements made from an aspherical starting model and strict data quality control derived from cluster analyses these new models are in good agreement with previous ones and the question arises whether the slight changes show an improved capacity to explain the data to this effect we propose an objective method to compare different phase velocity models published in recent literature the method is based on comparing calculated synthetics to raw seismograms we find a reassuring convergence between all the models we tested at the longest periods and more scatter at the shorter periods at 40 s the different models show gains of up to 3 5 cycles over prem generally the higher the gain over prem the smaller the period considered which confirms that the earth s heterogeneity is strongest in the uppermost parts of the earth apart from assessing different models against each other our method gives an estimate comparable to cluster analyses of the underlying data errors that went into the construction of the models themselves moreover ray coverage is still far from perfect for constructing phase velocity models as a result we find that without precaution degree zero is biased through spectral leakage by 0 1 to 0 2 per cent with respect to prem the earliest information concerning the earth s 3 d structure came from surface waves when tams 1921 noticed clear velocity differences for waves propagating along continental versus oceanic paths gutenberg 1924 attempted an explanation of such differences in terms of regional variations in crustal structure when he noticed that short period surface waves are more sensitive to crustal structure than long period ones this led the way to intensive studies both theoretical and observational to link dispersive properties of surface waves to the internal structure of the earth the first regional models of the earth s dispersive properties of crust and upper mantle are summarized in papers by brune 1969 and dorman 1969 respectively with the start of digital recording of seismograms in the late

reflectivity of the crystalline crust hypotheses and tests summary the nature of reflectors within the crystalline basement remains the subject of inference except where reflections have been traced directly to outcrop geological models of basement reflectors need to be developed which incorporate geophysical constraints obtained from measurements on seismograms but most geological information still comes from speculative interpretations of seismic experiments run in different regimes pronounced lower crustal reflectivity detected worldwide is ascribed in various geological hypotheses to primary lithologic layering to ductile strain banding or to trapped fluids a birps deep crustal profile across the atlantic continental margin suggests that the observed reflectivity is not related in any simple way to the amount of extensional strain undergone study of worldwide crustal profiles shows that exposed high pressure terranes are not as reflective as in situ lower crust at high pressure suggesting either that these granulite terranes are not representative of the lower crust or that physical properties possibly the presence of fluids or thermally controlled ductile strain banding are more likely responsible for observed reflectivity than are simple lithologic boundaries the argument for the importance of physical properties in causing observed lower crustal reflectivity is strengthened by an observed negative correlation between depth to the reflective lower crust and regional surface heat flow 1 geophysical constraints the notion that geometric effects thin layer interference and focussing in the absence of otherwise significant impedance contrasts are responsible for much observed reflectivity is common in the literature because many reflections are observed on deep crustal profiles velocity and density cannot increase monotonically with depth while remaining within bounds set by refraction experiments implying the existence of a funtion of velocity with depth that is oscillatory while the concept thus developed of the existence of many thin layers in the lower crust is justly important in geologic models of the lower crust seismic interference from such laminae cannot be the only explanation of observed crustal reflectivity for normal incidence no structure embedded in a constant impedance medium gives a stronger reflection than a quarter wavelength layer which produces an amplitude twice that due to a simple interface complex structures can increase the total energy reflected but the energy is spread over longer times and the peak reflected amplitude is not increased observed reflection amplitudes from the lower crust vary by factors u2 so that the presence or absence of thin layer interference cannot be the only effect causing lower crustal

eurasian fundamental mode surface wave phase velocities and their relationship with tectonic structures we automatically analyzed 32 000 fundamental mode love and rayleigh wave signals with earthquake station paths traversing eurasia and indonesia and obtained robust average phase velocity measurements between 20 s and 170 s periods along 4389 love and 4020 rayleigh paths these were inverted to give phase velocity maps at 14 fixed periods resolution tests suggest that features with diameter 750 km and 500 km are resolved over most of eurasia and central se asia respectively low period love waves image areas with thick sedimentary cover as low velocity zones and almost all periods image mountainous regions since these have thick crust and hence low average lithospheric shear velocity at long periods both love and rayleigh waves define high phase velocity zones across shield and cratonic areas reflecting their deep lithospheric roots we observe significant along strike heterogeneity in the zagros fold belt and tien shan altai system taking sections across eurasian phase velocity space allows us to make approximate interpretations in terms of shear velocity structure directly for example the red river and east vietnam boundary faults are traced on their eastern side by low velocities which extend at depth into indonesia we relate this to mantle upwelling associated with early eocene rotation of indochina and reversal of the sense of shear across the red river fault post 20 ma we observe dipping subduction of the mediterranean beneath the aegean of the philippine sea beneath indonesia and of the indian shield beneath tibet we also image a fossil subducted plate beneath ne borneo which we associate with subduction of the proto south china sea between 50 ma and 15 ma

geothermal profile and crust mantle transition beneath east central queensland volcanology xenolith petrology and seismic data geothermobarometry of garnet granulite and garnet websterite xenoliths in basalts from numerous localities in east central queensland gives p t points that fall along the geotherm previously defined for southeastern australia this elevated geotherm is ascribed to the advective transport of heat by tertiary recent magmas ponded at the crust mantle boundary the lower crust in this region consists dominantly of mafic granulites representing frozen basaltic melts and cumulates spinel lherzolite becomes a dominant rock type at depths of ca 30 km and persists interlayered with pyroxenites to depths of ca 55 km seismic reflection profiles show a layered lower crust between depths of 20 and 36 km depth the lithologically defined crust mantle boundary lies within this zone at least 6 km above the seismically defined moho this interpretation is consistent with the observed velocity vp gradient downward through the layered zone the constructed geotherm implies that the bottom of the lithosphere beneath eastern australia is shallower than ca 100 km this makes it unlikely that the diamonds of eastern australia are derived from local intrusions unless these are 200 ma old

a review of crust upper mantle structure in the precambrian areas of australia and implications for precambrian crustal evolution the western two thirds of the australian continent are underlain by precambrian rocks seismically this region is characterised by negative teleseismic travel time residuals indicating a thick cold tectosphere despite extensive platform cover and phanerozoic crustal reworking in a few intracratonic sedimentary basins the precambrian crust can be classified broadly into archaean and proterozoic cratons and proterozoic mobile belts the archaean cratons have a two layered crust 25 35 km thick with observed velocities in the lowermost crust in the range 6 8 7 2 km s 1 the moho is a distinct feature with a velocity contrast of 1 0 km s 1 vertical reflection probes in the southern yilgarn block yielded record sections that have numerous laterally discontinuous but distinct reflections especially in the lower crustal layer that are especially concentrated around the boundary between the two crustal layers and the moho the seismic reflections and velocities are consistent with a predominantly felsic crust with mafic lenses constituting 20 35 of the lowermost crust and with the metamorphic grade ranging from granulite in the upper to middle crust to garnet granulite and even eclogite at the base of the crust the proterozoic mobile belts and cratons a have multi layered crust 45 50 km thick velocities below 25 km depth are almost always 7 0 km s 1 and reach 7 4 7 6 km s 1 in the lowermost crust velocities such as these are characteristic of gabbroic basaltic underplate the velocity contrast is 1 0 km s 1 at the moho which is less distinct than in archaean terranes record sections from vertical reflection probes show a highly reflective crust with no correlation between seismic layers in the refraction models and the distribution of vertical reflections the contrasting character of the crust in archaean and proterozoic provinces favours a tectonic model for the evolution of precambrian crust in which predominantly felsic archaean protocrust was tectonically reworked and underplated to produce proterozoic crust the simple two layered protocrust became more complicated in the process the introduction of vast amounts of underplate especially near the crust mantle boundary thickened the crust and destroyed the character of the moho it overprinted the reflection character of the crust so that the bands of reflections now do not correlate with the boundaries between the crustal layers the velocities in the lower crust are high because of the underplate

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

crustal structure of mexico and surrounding regions from seismic ambient noise tomography summary using continuous seismic data from newly available broadband stations in mexico and central america we have obtained group and phase velocity maps of fundamental mode rayleigh wave for the region these new maps have been calculated for periods between 8 and 60 s from cross correlations of seismic ambient noise between 100 broadband stations and stacked for 30 months from 2006 to 2008 the tomographic inversion of the obtained dispersion measurements has been carried out on a 1 1 grid resulting in maps with resolution better than 250 km in the well sampled regions of the model for short periods 8 16 s dispersion maps show good correlation with surface structural features low velocity anomalies correlate with sedimentary basins around the gulf of mexico and colorado embayment high velocity anomalies at short periods correlate with mountain ranges and regions of thin extended crust such as the gulf of california both the lowest and highest group velocity anomalies at these periods reach values of up to 15 per cent for periods between 25 and 40 s velocity anomalies are related to variations in crustal thickness and temperature the most prominent low velocity anomaly correlates with thick crust and high mantle temperatures associated with the trans mexican volcanic belt and the central american volcanic arc the most remarkable features for longer periods 40 60 s are the differences between high velocity anomalies beneath the mayan block the gulf of mexico and the cratonic part of the united states and low velocity anomalies beneath the chortis block and northern mexico the long wavelength features of our model agree well with previous global and continental scale studies however because of the increased station density of the data set used we are able to obtain reliable dispersion maps for shorter periods and to image smaller scale features

continuous near bottom gravity measurements made with a bgm 3 gravimeter in dsv alvin on the east pacific rise crest near 9 31 n and 9 50 n a bell bgm 3 gravimeter has been used to collect continuous underway near bottom 3 to 10 m altitude gravity measurements from the deep diving submersible dsv alvin during surveys on the east pacific rise epr crest near 9 31 n and 9 50 n closely spaced 20 to 30 m gravity measurements were made along transects up to 8 km long in both regions repeatability of measurements made at the same location on different dives is 0 3 mgal along track spatial resolution of anomalies is 130 160 m with the limiting factors being precision and sampling rate of the pressure gauge depth data used to calculate vertical accelerations of the submersible the average upper crustal density of the ridge crest determined from the relationship between depth and free water gravity anomalies varies greatly between 9 31 n and 9 50 n average upper crustal densities of 2410 kg m 3 for the 9 50 n area and 2690 kg m 3 for the 9 31 n area were calculated the different densities are not due to differing geometry of the layer 2a 2b boundary or a regional cross axis gravity gradient differences in porosity of the shallow crustal rocks or a difference in the proportion of low density extrusives to higher density dikes and sills within layer 2a in these two areas are the likely causes of the different upper crustal densities bouguer gravity anomalies near the epr axis are primarily small amplitude 0 5 2 mgal are a few hundred meters across and appear to be lineated parallel to the axis larger amplitude bouguer anomalies of up to 4 mgal were found at a few locations across the crestal plateau and are associated with pillow ridges composed of lavas which are clearly younger than the surrounding seafloor these ridges have distinct chemical compositions compared to lavas from the axial summit collapse trough asct at the same latitude probable sources of the 0 5 to 2 mgal anomalies observed on the summit plateau include areas of collapsed and fissured terrain and dike swarms feeding melt through layer 2a to the surface a grid survey of the ridge axis near 9 50 n shows bouguer anomalies lineated along the axis suggesting that dike swarms do contribute to the observed bouguer anomalies the along axis continuity of the gravity anomalies is disrupted at a 75 m offset of the asct suggesting that shallow feeders of lava to the surface may be segmented on a finer scale than the deeper crustal magmatic system this initial study confirms the ability to conduct high resolution near bottom continuous gravity measurements from alvin it also provides important information on how the shallow crustal structure of a fast spreading mid ocean ridge develops and how it varies with the surface morphology

improvements in seismic reflection techniques for studying the lithosphere in australia abstract over the last 25 years the australian bureau of mineral resources bmr has attempted to record seismic reflection data in several parts of australia to determine if good quality near vertical reflections could be obtained from the deep crust and upper mantle data recorded prior to 1976 using analogue instruments and large shots did obtain good to fair quality deep events but the short lengths of the traverses recorded did not allow discrimination between primary reflections and the coherent noise the high cost of the special effort prevented recording of long traverses and hence the study of the true nature of the deep reflections from 1976 onwards with the advent of digital recording and processing equipment and techniques it became possible to obtain good quality deep reflection data on long traverses with the parameters such as small charges short spreads etc which are normally used for recording good sedimentary reflections the recent data allow better identification of reflections and study of their character and provide a higher resolution picture of the nature and structure of the deep crust and upper mantle than had been possible before bmr is therefore now recording good deep reflection data on all traverses during normal sedimentary basin surveys and accumulating the information necessary for studying the evolution of lithosphere in australia

uplift history of thrust belts an approach based on fission track data and thermal modelization abstract a tentative method to reconstruct the uplift history in thrust belts from fission tracks ft data is proposed as it is well established that the thermal gradient varies considerably both with time and depth in the evolution of mountains a time dependent thermal model is used to infer the temperature time history of sampled rocks given parameters such as the thrust thickness 7 t and the erosion v t randomly chosen in an acceptable range among a great number of trials monte carlo simulation this technique is applied to the corsican island which is a good example of alpine thrust belt the most striking result is that a large range of different possible erosion histories can explain the same set of ft data and that further geological constraints are required to give reasonable bounds in the case of corsica information on the minimum eroded thickness and on the relative amplitude of erosion phases gives sufficient constraints on the erosion uplift history since thrusting

global surface wave phase velocity variations global love and rayleigh wave phase velocity variations from 85 to 250 s are determined using about 30 000 seismograms from earthquakes with m 6 0 the phases used are g1 r1 g2 and r2 arrivals the data were obtained from the chinese digital seismic network geoscope and global seismic network archives for the time period january 1980 to june 1992 all seismograms underwent careful quality control in the time and frequency domains we consider the inversion damping and model parameterization issues examining the data statistics and assuming that errors in data and model spaces are gaussian random distributions with zero mean it is not stable to invert for a model with resolution higher than about spherical harmonic degree l 22 given the path sampling and scatter of our data set and the low frequency data only warrant expansion up to about l 12 we find that a block model parameterization with its local basis functions has fewer artifacts than a high order spherical harmonic inversion which must be heavily damped however spherical harmonic functions provide intrinsically superior recovery of the long wavelength structure our final models involve a hybrid parameterization in which an initial iteration retrieves the low order spherical harmonic components that are used as an aspherical reference model for performing a final block model inversion the optimal block size decreases with frequency this approach exploits the separate attributes of global and local basis functions for resolving different components of the heterogeneity allows for variable resolution for different frequencies and provides a natural framework for embedding localized high resolution regional inversions into the global model we perform inversions both with and without corrections for shallow crustal structure at the shorter periods the phase velocity maps strongly reflect the surface tectonics correlating with continental shields mid ocean ridges and tectonically active areas the tectonic correlation diminishes gradually with period

shear wave velocity structure in north america from large scale waveform inversions of surface waves a two step nonlinear and linear inversion is carried out to map the lateral heterogeneity beneath north america using surface wave data the lateral resolution for most areas of the model is of the order of several hundred kilometers the most obvious feature in the tomographic images is the rapid transition between low velocities in the tectonically active region west of the rocky mountains and high velocities in the stable central and eastern shield of north america the model also reveals smaller scale heterogeneous velocity structures a high velocity anomaly is imaged beneath the state of washington that could be explained as the subducting juan de fuca plate beneath the cascades a large low velocity structure extends along the coast from the mendocino to the rivera triple junction and to the continental interior across the southwestern united states and northwestern mexico its shape changes notably with depth this anomaly largely coincides with the part of the margin where no lithosphere is consumed since the subduction has been replaced by a transform fault evidence for a discontinuous subduction of the cocos plate along the middle american trench is found in central mexico a transition is visible from low velocities across the trans mexican volcanic belt tmvb to high velocities beneath the yucatan peninsula two elongated low velocity anomalies beneath the yellowstone plateau and the eastern snake river plain volcanic system and beneath central mexico and the tmvb seem to be associated with magmatism and partial melting another low velocity feature is seen at depths of approximately 200 km beneath florida and the atlantic coastal plain the inversion technique used is based on a linear surface wave scattering theory which gives tomographic images of the relative phase velocity perturbations in four period bands ranging from 40 to 150 s in order to find a smooth reference model a nonlinear inversion based on ray theory is first performed after correcting for the crustal thickness the phase velocity perturbations obtained from the subsequent linear waveform inversion for the different period bands are converted to a three layer model of s velocity perturbations layer 1 25 100 km layer 2 100 200 km layer 3 200 300 km we have applied this method on 275 high quality rayleigh waves recorded by a variety of instruments in north america iris usgs iris ida terrascope rstn sensitivity tests indicate that the lateral resolution is especially good in the densely sampled western continental united states mexico and the gulf of mexico

upper mantle s velocity structure of north america we have inverted fundamental and higher mode rayleigh waveforms from 685 vertical component broadband seismograms with wave paths over north america to obtain an image of the upper mantle s velocity structure down to 660 km among the well resolved features of the new model are 1 a high velocity root beneath the north american craton which extends no deeper than 250 km except near the archean core of the craton where depths of 350 km are reached 2 a weak band of low velocity along the eastern margin of the north american craton which reaches into the transition zone 3 a low velocity slab window beneath the western united states down to a depth of 300 km 4 areas of low uppermost mantle velocity beneath the cascade volcanoes the yellowstone hotspot track the colorado plateau the sierra madre occidental and the grabens bordering the jalisco block and 5 a pronounced band of high velocities in the transition zone coinciding with the expected location of the subducted trailing fragments of the farallon plate we introduce several improvements to the method of partitioned waveform inversion which was used to compute the new model rather than to correct for an estimated depth to the mohorovicic discontinuity we leave the crustal thickness as a free though yet poorly resolved parameter in the inversion we also improve the windowing and filtering operator used to select uncontaminated waveforms

deep reflection experiments in northeastern australia 1976 1978 between 1976 and 1978 the australian bureau of mineral resources bmr recorded deep crustal reflection data at seven sites in northeastern australia over continuous profiles up to 15 km long by simply extending the recording time to 16 sec during normal sedimentary basin surveys the record sections show many events with variable strength continuity dip and spatial distribution by comparing the sections from the longer and the shorter perpendicular traverses it is possible to discriminate between primary reflections and diffractions multiples and other noise events based on their character the reflections can be grouped into zones which are interpreted in terms of the nature and structure of the crust most of the reflection sections show below the sedimentary reflections a thin 2 3 sec reflection free zone underlain by a thick 9 sec or more zone of numerous reflection segments which varies in thickness and the distribution of reflection segments the data thus suggest that the upper crust

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

global cmt analysis of moderate earthquakes mw 4 5 using intermediate period surface waves a new method for calculating centroid moment tensor cmt solutions for moderate sized earthquakes is presented and tested in the new algorithm which is a modification to the standard harvard cmt method intermediate period surface waves are included in the inversion for source parameters the dispersion of teleseismic surface waves in the period range 40 to 150 sec is predicted by interpolation of recent global phase velocity maps the method is tested on a set of 20 moderate and large 5 2 mw 7 2 earthquakes with known mechanisms the retrieved mechanisms are similar to the previous results even when noise is added to the seismograms to simulate earthquakes of a smaller magnitude mw 4 5 the inversion algorithm is applied to the 1993 klamath falls mainshock and aftershock sequence and the results compare well with earlier results a catalog of mw 4 5 focal mechanisms for greece and surrounding areas for the first six months of 1995 is derived using the new method of analysis

the spectrum of tomographic earth models summary the properties of mantle convection are expected to generate characteristic signatures in the spectrum of mantle heterogeneities and in its dependence on depth the investigation of present tomographic models and the associated spectra of seismic velocity perturbations can provide insight into this problem the characterization of heterogeneities in the mantle at all scales involves combining and comparing tomographic models at both global and regional scales the former are expanded into the spherical harmonics domain while the latter are expanded into the fourier domain the comparison of both types of expansions is possible if we consider power spectral density per unit surface such a comparison should be carried out with great care and the scaling factors are derived to make it meaningful the method is applied to recent global and regional tomographic models it is shown that contrary to previous assertions there is an overall agreement between both kinds of models