isotropy or weak vertical transverse isotropy in d beneath the atlantic ocean 1 shear velocity properties of d beneath the central atlantic ocean are explored using predominantly european seismic recordings of intermediate and deep focus 100 km south american earthquakes broadband data are analyzed and when possible corrected for upper mantle models of receiver side anisotropic structure regional shear velocity heterogeneity in d is mapped by analysis of 306 s sks differential times that have been corrected for three dimensional seismic velocity structure above d using a whole mantle tomographic model this correction yields modest less than 2 estimates of seismic velocity heterogeneity in d with a transition from high to low seismic velocities traversing from west to east beneath the central atlantic in agreement with global tomographic models additionally shear wave splitting of s and sdiff for the same recordings was analyzed to assess seismic anisotropy in d the highest quality data provide 105 splitting times between sh and sv onsets that are mostly within the 1 s uncertainty level the few larger values generally exhibit sv delayed relative to sh assuming an anisotropy geometry involving vertical transverse isotropy vti as preferred in most regions of d that have been studied to date 0 5 anisotropy strength within a 100 km thick layer or 0 25 anisotropy within a 300 km thick layer are compatible with the data these values are low in comparison to those found in high velocity regions beneath the circum pacific ocean or in the low velocity region beneath the central pacific and many observations are in fact consistent with isotropic structure the lack of strong vti relative to other regions may be due to 1 the absence of stress from overlying midmantle downwelling 2 relatively weaker shear flow in the d boundary layer and or 3 lack of chemical heterogeneity that could develop either lattice preferred orientation or shape preferred orientation the azimuthal sampling of this region of d is quite limited thus the precise geometry and mechanism of any anisotropy are difficult to constrain it remains possible that this region may contain subtle azimuthal anisotropy that could couple the sv and sh signals however amplitude observations suggest that any such coupling is minor

shear velocity structure of the northern california lithosphere the authors have determined the regional shear velocity structure of the lithosphere beneath the coast ranges and the great valley in northern california from inversion of three fundamental mode rayleigh wave phase velocity curves the dispersion measurements were made along three different paths crossing the coast ranges and great valley roughly perpendicular to the north american pacific plate boundary the three dispersion curves diverge at periods greater than about 20 s phase velocities are systematically higher from the northwest to the southeast along the transform margin inverting the phase velocities for crustal and upper mantle structure shows that this divergence is indicative of a 3 5 increase in the upper mantle shear velocity from the napa great valley region to the diablo range great valley region crustal shear velocities are consistent with the lithologies expected in the middle and lower crust the increase in mantle shear velocity from northwest to southeast is anticipated by a tectonic model for the development of the california transform margin in which asthenospheric material is emplaced at the base of the north american crust in the slab gap south of the mendocino triple junction adjacent to the plate boundary this process creates a subcrustal corridor of cooling asthenosphere whichmore is gradually incorporated in the lithosphere lid they suggest that this causes the observed increase in upper mantle shear velocity away from the triple junction finite difference simulations of rayleigh wave propagation across asthenosphere corridor models produce synthetic phase velocity curves which are similar to the field observations less

relic magma chamber structures preserved within the mesozoic north atlantic crust the north atlantic transect seismic reflection data collected southwest of bermuda have been reinterpreted following post stack migration and reveal two major intracrustal reflections the shallower of these two events located 1 s below the igneous basement is a subhorizontal undulating surface that in some places is continuous for as much as 10 km on the basis of its position within the section and its laterally discontinuous nature we believe that this upper crustal reflection corresponds to the intermittently sharp contact between the sheeted dikes and the underlying isotropic gabbro a second set of lower crustal reflections dipping 20 40 eastward is also prominent on the migrated profile and terminates downdip against the subhorizontal reflection moho several lines of evidence argue against these features being either artifacts or out of the plane events instead their presence may be ascribed either to crustal penetrating fault zones or to mafic ultramafic cumulate layers frozen into the oceanic crust at the time of formation at the paleo spreading center because of the laminated character of these events and their typical occurrence within 1 0 to 1 5 s of the reflection moho we prefer a compositional versus a structural interpretation for their origin the gradual thinning in the crust approaching the fracture zones is shown to be more complex than was originally inferred although the interpretation that the crust gradually thins toward fracture zones may still apply in a few localities significant departures are recognized elsewhere similarly the improved image on the migrated profile documents an increase in complexity across the localized region directly surrounding the blake spur fracture zone an interpretation advocating crustal thickening in this narrow zone is proposed as an alternative to the crustal thinning model of mutter and others

were deforming subglacial beds beneath past ice sheets really widespread abstract the concept of widespread large strain deformation of subglacial unconsolidated unfrozen sediments during pleistocene glaciations has inconsistencies in the geological record numerous properties of tills and related sediments are difficult to reconcile with pervasive strains as predicted by the deforming bed theory while accepting glacier bed deformation as a geological process occurring under certain circumstances we propose that it was much less widespread than believed by some instead we believe that basal sliding and englacial transport are major ice movement and debris transport mechanisms

the shear wave velocity gradient at the base of the mantle the relative amplitudes and travel have been directed toward obtaining global times of scs and s phases are utilized to place averages and the degree of lateral variation in constraints on the shear wave velocity gradient d properties remains an open question above the core mantle boundary a previously a conflicting result was found by mitchell and reported long period scsh sh amplitude ratio helmberger 1973 who utilized the relative minimum in the distance range 65 o to 70 o is shown amplitudes and timing of long period scs and s to be a localized feature apparently produced by phases to constrain the s wave velocity gradient an amplitude anomaly in the direct s phase and in d they found a minimum in the scsh sh therefore need not reflect the velocity gradient amplitude ratio near 68 o which was attributed to at the base of the mantle the amplitude ratios low amplitudes of the scs arrivals unable to that are free of this anomaly are consistent with explain this feature by models with negative or calculations for the jb model or models with mild near zero shear velocity gradients in d they positive or negative velocity gradients in the proposed models with positive s wave velocity lowermost 200 km of the mantle scsv arrivals gradients above the cmb these positive are particularly sensitive to the shear velocity gradients extended over 40 to 70 km above the structure just above the core mantle boundary core reaching velocities at the cmb as high as the apparent arrival time of the peak of scsv is 7 6 to 7 8 km s these models can explain the as much as 4 sreater than that of scsh in the observed amplitude ratio behavior as well as an distance range 75 v to 80 o for sea of okhotsk apparent difference observed in the arrival times events recorded in north america this can be of transversely and radially polarized scs explained by interference effects produced by a mitchell and helmberger also proposed a low q localized high velocity layer or strong positive zone in d or finite outer core rigidity to s wave velocity gradient in the lowermost 20 km explain the baseline of the scsh sh amplitude of the mantle a velocity increase of about 5 ratios while the majority of their data was for is required to explain the observed shift between deep south american events recorded in north scsv and scsh this thin high velocity layer america they did analyze one deep sea of okhotsk varies laterally as it is not observed in event for which the radial and transverse scs similar data from argentine events refined arrival times were not different which suggested estimates of the outermost core p velocity lateral variations in the d velocity structure structure are obtained by modeling sks signals in in this paper we extend the analysis of scs the distance range 75 o to 85 o s

low p wave velocity at the base of the mantle a tool for investigating p wave vp structure at the base of the mantle is presented sks waves at distances around 107 are incident upon the core mantle boundary cmb with a slowness that allows for coupling with diffracted p waves pd at the base of the mantle the p wave diffraction occurs at both the sks entrance and exit locations of the outer core the resulting phase spdks arrives slightly later in time than sks having a wave path through the mantle and core very close to sks the difference time between sks and spdks most strongly depends on vp at the base of the mantle near sks core entrance and exit points digitized long period 5 15s observations from deep focus tonga events recorded by north american wwssn and canadian seismographic network stations and south american events recorded by european and eurasian wwssn stations exhibit anomalously large spdks sks difference times sks and the later arriving spdks are separated by several seconds more than predictions made by the global average prem model models having a pronounced low velocity zone in vp at the base of the mantle best predict the observations these models are perturbations of the prem model whereby the lowermost 50 100 km of the d layer has a negative vp gradient with the mantle side cmb vp reduced from prem by 5 to 13 0 km s at the base of the mantle

laboratory and seismological observations of lower mantle isotropy we have carried out seismological and mineral physics investigations to identify the extent and origin of elastic anisotropy in the lower mantle based on observations of shear wave splitting we conclude that the lower mantle is effectively isotropic this result is surprising since the lower mantle is composed of elastically anisotropic minerals silicate perovskite mgo sio 2 that have been extensively strained by geologic processes to reconcile the seismic observations we have measured texture development during deformation and high temperature recrystallization of mantle silicates we show that these experiments provide a direct explanation for the shear wave splitting measurements the lower mantle appears isotropic because silicate perovskite maintains an isotropic texture during deformation and recrystallization

crustal extension and magmatic processes cocorp profiles from death valley and the rio grande rift new crustal scale information on the interaction between normal faulting and magmatic activity is provided by recent cocorp deep seismic reflection profiles in death valley california and by reprocessing of the cocorp data from the socorro area of the rio grande rift new mexico the most striking feature on the seismic sections from these areas is a prominent subhorizontal series of reflectors at mid crustal depth previous studies have suggested that thin tabular magma bodies lie within the mid crustal reflective zones in addition because they are traced without apparent offset beneath faults both mapped at the surface and interpreted from the cocorp data these mid crustal horizons are here inferred to be detachments or zones of tectonic decoupling upper crustal cenozoic faults do not appear to penetrate deeper than 15 km in death valley and 13 km in the rio grande rift in death valley these faults are relatively planar with moderate dips 20 to 35 and appear to bound large basement rocks one such zone of normal faults can be traced from the magma body inferred at 15 km depth beneath central death valley to the surface location of a 690 000 yr old basaltic cinder cone listric and low to moderate angle normal faults are evident on the reprocessed new mexico data and constitute the structural component of upper crustal extension in particular a listric master fault traceable to depths as great as 13 km is inferred to underlie the albuquerque basin unlike the death valley data no faults are observed to merge with the socorro magma body per se rather subhorizontal to moderately west dipping packages of reflections are imaged between the base of the faulted upper crust 13 km depth and the mid crustal magma body about 20 km depth the middle crust marks a major rheological boundary between the faulted upper crust and a ductile lower crust extending by penetrative flow and intrusion events seen in the middle and lower crust are generally subhorizontal and prominent layering is observed a band of reflections attributed to the crust mantle boundary is evident on most seismic sections the upper mantle appears seismically transparent on some of the profiles the events attributed to the base of the crust are the deepest in a series of strong and continuous reflections at least one of which is a layer of magma this association supports the suggestion that magmatic intrusions are a probable cause for the high reflectivity observed in the deep crust of many extensional terranes

paleorainfall reconstructions from pedogenic magnetic susceptibility variations in the chinese loess and paleosols the rock magnetic properties of the chinese loess and paleosols constitute a unique and sensitive record of east asian paleoclimate through the quaternary period systematic variations in the concentration and grain size of the magnetic minerals in these sediments have produced systematic variations in the magnetic susceptibility signal which can be easily and rapidly measured at many sites across the loess plateau variations in many other rock magnetic properties can be used to identify the key shifts in ferrimagnetic grain size but magnetic susceptibility alone is sufficiently sensitive to record stadial and interstadial climate stages as well as glaciations and interglaciations past changes in rainfall and monsoon activity for this region are reconstructed from the susceptibility variations the susceptibility record is calibrated using the modern relationship between rainfall and pedogenic susceptibility on the loess plateau our rainfall reconstructions identify enhanced summer monsoonal activity in the chinese loess plateau region in the early holocene and the last interglaciation in the presently semiarid western area of the plateau annual precipitation in interglacial times was up to 80 higher than at present in the more humid southern and eastern areas values were up to 20 higher than today s levels during the last glaciation precipitation decreased across the entire plateau typically by 25 the relationship between pedogenic susceptibility climate and weathering age was examined over the northern hemisphere temperate zone and the observed positive correlation between rainfall and susceptibility indicates that climate rather than soil age is the predominant factor that controls pedogenic susceptibility enhancement in loess soils

validity domain of the born approximation for seismic waveform modelling in realistic 3 d earth structure while the first order born approximation is increasingly being used in many seismic tomography efforts its domain of validity to forward model seismic waveforms has not been quantified in the context of current 3 d earth models yet we here address this issue by comparing teleseismic synthetic surface waveforms calculated using the born approximation with spectral element method solutions for a variety of realistic global 3 d earth models we find that the born approximation has a very limited domain of validity when applied to seismic waveforms specifically it can only accurately model the phase amplitude of surface waveforms for source receiver paths leading to time shifts smaller than about 15 per cent 5 per cent of the wave period considered these conditions usually occur in earth models as s20rts or s40rts combined with a homogeneous crust for periods longer than t 80 90 s for models with stronger heterogeneity and or realistic 3 d crustal structure only the phase of waveforms with periods longer than 120 130 s can be accurately modelled with the born approximation

evidence for shear velocity anisotropy in the lowermost mantle beneath the indian ocean teleseismic recordings 87 of a deep earthquake beneath the banda sea at stations in tanzania show a difference in the arrival time of the radial ssv and transverse component ssh s wave ranging from 1 3 s shear velocity anisotropy in the lowermost mantle beneath the indian ocean is the likely cause of this signal because recordings at the same stations of closer in events 80 in the same source region do not present a comparable differential travel time for the banda sea event the ssh signals are broader than ssv signals suggesting that a discontinuity or strong vertical gradient in primarily vsh marks the sudden onset of transverse isotropy in d with a magnitude of 1 4 2 7 about 350 km above the core mantle boundary skks coda s to p converted phases at the moho and upper mantle heterogeneity beneath the stations obscure the onset of ssv and complicate wave shapes it is therefore difficult to evaluate whether general anisotropy needs to be invoked into a model of shear velocity anisotropy

lateral variations in compressional shear velocities at the base of the mantle observations of core diffracted p p diff and sh sh diff waves recorded by the missouri to massachusetts moma seismic array show that the ratio of compressional p seismic velocities to horizontal shear sh velocities at the base of the mantle changes abruptly from beneath the mid pacific vp vs 1 88 also the value predicted by reference earth models to beneath alaska vp vs 1 83 this change signifies a sudden lateral variation in material properties that may have a mineralogical or textural origin a textural change could be a result of shear stresses induced during the arrival at the core of ancient lithosphere from the northern pacific paleotrench

cenozoic active margin and shallow cascades structure cocorp results from western oregon two deep seismic reflection profiles totaling 98 km were recorded by the consortium for continental reflection profiling cocorp in western oregon in 1984 providing some of the first onshore deep seismic reflection data from the forearc and arc of an active convergent margin these data are of lower quality than most cocorp data but when combined with other geological and geophysical data they provide some useful insights into the subduction zone beneath oregon and into extensional structures in the cascades line 1 crossed the eocene sediments and underlying pillow basalts of the coast range and western willamette valley reflections from the willamette valley clearly define the lower and western boundary of flat lying rhythmically bedded eocene sediments these sediments appear to be underlain by as much as 8 km of seismically transparent eocene pillow basalts layered reflections at depths of 8 16 km may indicate a remnant crust upon which the eocene pillow basalts were erupted east dipping reflections at depths of 35 40 km may represent the decollement above the subducting juan de fuca plate oregon line 2 crosses the cenozoic volcanic arc terrains of the western and high cascades interpretation of cocorp seismic data suggests modification of existing models for normal faulting that postulate a symmetric graben or grabens in the cascades of oregon in the high cascades along cocorp oregon line 2 reflections suggest a large gently west dipping half graben with major offset only on the west side of the range the seismic reflection data along with other geological and geophysical data suggest that the high cascades were built on a series of blocks rather than a single graben the blocks were differentially faulted during the pliocene seismic data from the western cascades indicate several normal faults with down to the east offset including a major previously unidentified fault of probable miocene age

anisotropic shear wave velocity structure of the earth s mantle a global model 1 we combine a new large data set of surface wave phase anomalies long period waveforms and body wave travel times to construct a three dimensional model of the anisotropic shear wave velocity in the earth s mantle our modeling approach is improved and more comprehensive compared to our earlier studies and involves the development and implementation of a new spherically symmetric reference model simultaneous inversion for velocity and anisotropy as well as discontinuity topographies and implementation of nonlinear crustal corrections for waveforms a comparison of our new three dimensional model s362ani with two other models derived from comparable data sets but using different techniques reveals persistent features 1 strong 200 km thick high velocity anomalies beneath cratons likely representing the continental lithosphere underlain by weaker fast anomalies extending below 250 km which may represent continental roots 2 weak velocity heterogeneity between 250 and 400 km depths 3 fast anomalies extending horizontally up to 2000 3000 km in the mantle transition zone beneath subduction zones 4 lack of strong long wavelength heterogeneity below 650 km suggesting inhibiting character of the upper mantle lower mantle boundary and 5 slow velocity superplumes beneath the pacific and africa the shear wave radial anisotropy is strongest at 120 km depth in particular beneath the central pacific lateral anisotropic variations appreciably improve the fit to data that are predominantly sensitive to the uppermost and lowermost mantle but not to the waveforms that control the transition zone and midmantle depths tradeoffs between lateral variations in velocity and anisotropy are negligible in the uppermost mantle but noticeable at the bottom of the mantle

deep seismic and geochemical constraints on the nature of rift induced magmatism during breakup of the north atlantic abstract the deep crustal structure of the norwegian and east greenland conjugate passive margins has been investigated with two ship multichannel reflection and refraction techniques in the region from the iceland faeroe ridge to the greenland fracture zone the data demonstrate the presence of thick 15 20 km igneous crust that was emplaced along the conjugate margins during the initiation of seafloor spreading seaward dipping units comprise the uppermost 3 6 km of the thick crust the thickness of igneous crust diminishes seaward with oceanic layer 2 approaching normal thickness nearer to the margin than does oceanic layer 3 wide angle reflection techniques used in conjunction with expanded spread profiling reveal deep moho reflections beneath 20 km thick oceanic crust that continue seaward into the ocean basin margins that have experienced unusually prolific magmatic activity during breakup have been termed volcanic to distinguish them from non volcanic margins that have evolved into accreting plate boundaries without excessive volcanism the origin of volcanic margins cannot be determined from seismically derived crustal thicknesses alone however thermal and chemical properties of the mantle source may be revealed in the compositions of magmatic products emplaced within volcanic passive margins a combined investigation of crustal structure and associated basalt composition could better resolve processes that result in volcanic passive margins on the voring plateau of the norwegian margin seismic lines coincide with dsdp leg 38 drill sites whole rock major element abundances in the freshest basalts are compatible with moderate extents of partial melting of mantle much like that believed to underlie modern mid ocean ridge systems and would be expected to result in the emplacement of oceanic crust about 9 2 km thick this is about the thickness of oceanic crust located seaward of the dipping basaltic units off the hatton more and voring plateau margins and 10 km south of iceland on the reykjanes ridge these results together with regional heat flow depth gravity and geoid anomalies suggest that the mantle beneath this region has experienced a moderate long lived temperature increase compared to mantle beneath normal ocean basins the much greater thicknesses of igneous crust adjacent to the margins do not appear to reflect an unusually hot flux associated with their construction since this would lead to much greater extents of partial melting than are inferred from the basalt geochemistry evidently the difference between volcanic and non volcanic margins is not simply proximity to a hot spot instead we suggest that the magmatism is closely related to the continental rifting phase tectonism resulting in an abrupt transition from continental to oceanic lithosphere may induce small scale convection that results in the circulation of a greater amount of mantle material into the pressure temperature field where partial melting begins than would occur through passive upwelling alone initial crustal thicknesses are enhanced as a result of the convective partial melting crustal thickness is expected to diminish as this secondary convection slows but the degree of partial melting of the source should not change dramatically

tomographic imaging of local earthquake delay times for three dimensional velocity variation in western washington tomographic inversion is applied to delay times from local earthquakes to image three dimensional velocity variations in the puget sound region of western washington the 37 500 square km region is represented by nearly cubic blocks of 5 km per side p wave arrival time observations from 4 387 crustal earthquakes with depths of 0 to 40 km were used as sources producing 36 865 rays covering the target region a conjugate gradient method lsqr is used to invert the large sparse system of equations to diminish the effects of noisy data the laplacian is constrained to be zero within horizontal layers providing smoothing of the model the resolution is estimated by calculating impulse responses at blocks of interest and estimates of standard errors are calculated by the jackknife statistical procedure results of the inversion are correlated with some known geologic features and independent geophysical measurements high p wave velocities along the eastern flank of the olympic peninsula are interpreted to reflect the subsurface extension of crescent terrane low velocities beneath the puget sound further to the east are inferred to reflect thick sediment accumulations the crescent terrane appears to extend beneath puget sound consistent with its interpretation as a major accretionary unit in themore southern puget sound basin high velocity anomalies at depths of 10 20 km are interpreted as crescent terrane and are correlated with a region of low seismicity near mt ranier high velocity anomalies may reflect buried plutons less

three dimensional p velocity structure of the crust below newberry volcano oregon utilizing teleseismic p residuals we have detected a column of high p velocity material extending from within 10 km of the surface below the summit of newberry volcano oregon to midcrustal depths near 25 km we interpret this column to be the expression of a swarm of predominantly subsolidus gabbroic sills and dikes which were intruded as the volcano was built the high p velocities observed below the volcano severely limit the size of magma chambers which could presently exist in the crust below newberry volcano those possible include a few percent of partial melt distributed through large volumes of a mafic intrusion zone in the midcrust a few smaller higher melt fraction zones in the midcrust with dimensions less than 6 km and whose aggregate volume is only a few percent of enclosing volumes of 200 km sup 3 small magma bodies with dimensions of a few kilometers located within the upper 10 km of the crust or a mafic crystal rich magma of arbitrary dimensions located in the upper few km the low p velocities detected in the upper 4 km beneath the center of the summit caldera may be partially caused by a magma chamber in the second ofmore these catagories less

shear wave anisotropy of active tectonic regions via automated s wave polarization analysis abstract using an automated aspect ratio method to evaluate shear wave anisotropy from earthquakes recorded on arrays of three component digital seismographs over the bucaramanga nest of colombia the east rift zone of hawaii and the phlegraean fields uplift area of italy a shear wave velocity anisotropy of 4 7 is observed the anisotropy is attributed to tectonic processes acting either to align anisotropic minerals or to align stress related cracks at most fixed seismographs the polarization of the leading s phase is consistent from event to event independent of earthquake location on a regional basis the polarization may vary from station to station but a regional trend is apparent for events from the bucaramanga nest colombia a small seismogenic volume at 160 km depth in a subducted plate shear wave splitting averages 0 5 0 1 s for more than 100 events the polarization direction of the fast shear wave is 355 n 15 parallel to the strike of the benioff zone at the phlegraean fields in italy which represent an active quaternary caldera the average shear wave splitting observed from 51 events is 0 2 s corresponding to a minimum velocity anisotropy of 7 the strike of the inferred maximum compressive stress is consistent with values obtained from fault plane solutions in hawaii the polarization of the leading shear wave tends to parallel the strike of the east rift zone average velocity anisotropy is about 5 if one assumes that the entire travel path contributes to the observed splitting however station variations in fast shear wave polarization direction imply that some of the splitting is due to near station effects

mixed layer illite vermiculite as a paleoclimatic indicator in the pleistocene red soil sediments in jiujiang southern china abstract the mineralogy and chemical composition of the jiujiang red soil sediments were investigated using x ray diffraction xrd x ray fluorescence xrf and inductively coupled plasma mass spectrometry icp ms mixed layer illite vermiculite i v was confirmed based on the conditions that a broad peak of 1 0 to 1 4 nm appeared in xrd after mg saturation and its d spacing did not change after glycol treatment but collapsed to 1 0 nm after heating owing to the loss of water and hydroxy cations from its interlayer the mixed layer i v contents fluctuate throughout the section but show a decreasing trend upwards suggesting decreased weathering towards the surface of the section the chemical index of alteration cia values and rb sr ratios also exhibit upward decreasing trends jointly revealing the climate shift from warm humid to cool dry from the middle to the late pleistocene although the general climate evolution can be evaluated by cia values and rb sr ratios sub climatic events within the jiujiang red soil sediments cannot be identified the sio2 al2o3 al2o3 fe2o3 and sio2 fe2o3 ratios clearly fluctuate along the section especially within the net like red soil sediments but they do not reveal multiple climate cycles this is because of an inhomogeneous distribution of sio2 al2o3 and fe2o3 in the white veins and the red matrix and also non uniform sampling the variation in the content of mixed layer i v show a strong positive correlation with changes in magnetic susceptibility of loess paleosol sediments and in marine oxygen isotope ratios revealing multiple sub climate cycles on a global scale since the middle pleistocene therefore the mixed layer i v contents not only reveal the whole climate trend documented in the red soil sediments since the middle pleistocene but also disclose the sub climate cycles related to the deposit pedogenesis process

pleistocene climates in china dated by magnetic susceptibility the magnetic susceptibility of loess and interbedded soils in central china varies with the degree of pedogenesis and serves as a proxy measure of climate the concentration of magnetic minerals in the sediment is inversely proportional to sedimentation rates throughout the brunhes epoch susceptibility measurements combined with the reversal stratigraphy provide a time scale that is independent of astronomic chronology on this scale the susceptibility record closely parallels the oxygen isotope fluctuations in deep sea sediments suggesting a close interdependence of the chinese dust falls the volume of land based ice and global climate

extent and dynamics of the scandinavian ice sheet during oxygen isotope stage 3 65 000 25 000 yr b p the climate of the middle weichselian glaciation marine oxygen isotope stage 3 ois 3 a relatively mild period compared to the glacial maxima of ois 4 and ois 2 consisted of long warm interstades punctuated by brief cold excursions that grew colder and more frequent with time the ois 4 ice sheet is generally thought to have persisted throughout ois 3 but evidence from dated ois 3 interstadial deposits suggests that it was swiftly reduced to small remnants which only briefly expanded and retreated only 30 000 years ago the deteriorating climate initiated a sustained ice advance leading toward the final glacial maximum of ois 2 dynamic ice sheet models support the existence of a prolonged ice free interval during ois 3 induced perhaps by low precipitation due to extensive sea ice cover offshore

the seismically reflective crust beneath highly extended terranes evidence for its origin in extension reprocessed seismic reflection data from near the picacho mountains metamorphic core complex in southeastern arizona show that the crust is seismically reflective from 1 5 s two way travel time twtt to the refletion moho at 10 s twtt these high amplitude subhorizontal and laterally discontinuous reflections may be a common seismic signature of highly extended areas this study presents the results of synthetic seismogram modeling of these reflections based on drill hole and outcrop data from the picacho mountains and adjacent core complexes modeling studies suggest that the prominent reflectivity beneath the picacho mountains results from a thinly layered crust where the layers are of alternating high and low velocity and their lateral extent is on the order of 500 m moreover the lack of distinctive diffractions in the seismic data is interpreted to mean that lateral changes in velocity orlayer thickness are smooth rather than abrupt on the scale of a seismic wavelength soure for the reflectivity include fracture zones compositional layering of micaceous gneisses and granitoid rocks and zones of velocity anisotropy particularly in micaceous rocks the association of the reflectivity in metamorphic core complexes with cenozoic deformation suggests that the reflections are a by product of extension

the core mantle boundary the largest compositional discontinuity within the earth is the core mantle boundary cmb at a depth of 2889 km this boundary and the adjacent transition zones in the lowermost mantle and outermost core play a critical role in the earth s thermal and chemical evolution estimates of the heat flux out of the core indicate that the 200 km thick d region at the base of the mantle is a major thermal boundary layer the d region like its counterpart boundary layer in the lithosphere is laterally heterogeneous and possibly chemically stratified this complexity obscures the exact dynamic behavior of the d region however it is clear that the large density contrast 4 3 g em 3 across the cmb provides favorable con ditions for density stratification on both sides of the boundary the d region may be the refuse pile for heavy mantle heterogeneities while the outermost core may have a concentration of light components boundary layer instabilities in d have been invoked as sources of both localized mantle plumes and major upwelling currents in whole mantle convection systems dynamically supported topography on the cmb and lateral temperature gradients in d may produce coupling between the core and mantle influencing the excitation of the geodynamo the outer most core is generally believed to be radially and laterally homogeneous although chemical stratification has not been ruled out seismological geodynamical and geomagnetic investigations are unveiling the three dimensional complexity of the deep earth in the vicinity of the cmb

processing and inversion of refraction and wide angle reflection data from the 1986 nevada passcal experiment we derive averaged one dimensional velocity models for the crust beneath the basin and range in western nevada by inverting p composites extracted from slant stacks of refraction and wide angle reflection data a combination of high cut filtering in the frequency domain with coherency filtering in the p domain has proved effective in smoothing severe static shifts due to lateral heterogeneities and suppressing artifacts due to spatial aliasing we supplement the low frequency information provided by the refraction records with higher frequency arrivals picked from the normal incidence and wide angle reflection sections generalized least squares inversion of data from eight shots for p and s wave velocity structure beneath two perpendicular profiles yields consistent estimates for total crustal thickness p wave 30 31 km s wave 30 32 km and average crustal velocity 6 2 km s 3 4 3 5 km s average poisson s ratio beneath the n s profile is 0 27 0 28 p wave velocities near the base of the crust 7 4 0 3 km s are estimated by inverting precritical reflections from the moho standard errors in depth for these models range from 1 to 3 km extremal depth bounds in general are about twice as large uncertainties in velocity estimated from resolving kernels range from 0 1 to 0 3 km s estimates of poisson1s ratio as a function of depth are characterized by large standard errors reflecting the poor resolution of shear wave velocities the combination of low average p and s wave velocities with a high average poisson s ratio in the upper half of the crust however is consistent with the presence of fluid filled cracks these results highlight the need for three component recording to better resolve shear wave velocity structure

weichselian till stratigraphy and glacial history of finnmarksvidda north norway abstract during the last decade the geological survey of norway has performed surficial and stratigraphical mapping of the quaternary geology in finnmark north norway this mapping has revealed a complex late pleistocene stratigraphy with tills and intercalated gravels and sands which indicate at least three stadials and two interstadials during the weichselian older glacial events are also recognized true in situ organic material from an old ice free period was first found in this area during the 1988 field season preliminary pollen counts of this material indicate interglacial conditions of probable eemian age based on all the available stratigraphic data a model for the late pleistocene glacial history has been developed the locations of the ice accumulation zones during the three recognized stadials were different the first stadial had ice domes in both the mountains and the lower inland area the second stadial was mainly a mountain ice dome glaciation whereas the last stadial or glaciation had mainly inland ice domes the glacial record for northern fennoscandia indicates that glacial expansion during the early and middle weichselian did not reach a size comparable to that of the weichselian maximum ca 20 ka bp

seismic identification of basement reflectors the bagdad reflection sequence in the basin and range province colorado plateau transition zone arizona although seismic reflection profiling has contributed greatly to our knowledge of the crust and upper mantle it is often difficult to unambiguously identify the geologic origin of reflections this is particularly true of intrabasement reflectors whose geometry and origin we need to know in order to understand tectonic processes operating in the crust we present synthetic seismogram modeling of recently collected p and s wave seismic reflection data which suggests that the intrabasement bagdad reflectors in west central arizona are intrusive mafic sheets the identification is based on the determination of the phase positive polarity and amplitudes of the reflected arrivals if the bagdad reflectors are cenozoic age mafic sheets they may be the product of rapid intrusion into an area of weak extension where the principal stresses are approximately equal

structure of the lower crust beneath the carolina trough u s atlantic continental margin data from three large offset seismic profiles provide information on the crustal structure beneath the carolina trough the profiles obtained by the u s geological survey the naval oceanographic research development agency and the scripps institution of oceanography in 1985 were oriented parallel to the trough and were located 1 seaward of the east coast magnetic anomaly ecma which is generally thought to represent the boundary between oceanic and continental crust 2 along the axis of the trough between the ecma and the hinge zone which is thought to reflect the landward limit of highly stretched and altered transitional crust and 3 along the carolina platform landward of the basement hinge zone on crust thought to have been thinned only slightly during rifting these data constrain the velocity structure of the lower crust and provide evidence for a thick lens of high velocity 7 1 km s lower crustal material that extends beneath the carolina trough and the adjacent ocean basin this lens reaches a maximum thickness of about 13 km beneath the deepest part of the trough thins to about 5 km seaward of the ecma and is either very thin or absent landward of the hinge zone it is interpreted to represent material that was underplated beneath and or intruded into the crust during the late stage of continental rifting and that led to an anomalously thick plutonic layer during the early seafloor spreading phase these data thus support the recent conclusions of white et al 1987b and mutter et al 1988 that the initiation of seafloor spreading is attended in many if not most cases by the generation of an anomalously large volume of melt

constraints from seismic anisotropy on the nature of the lowermost mantle the d layer lies at the bottom of the earth s rocky mantle and separates it from the liquid metal alloy core this region extending from the core mantle boundary to a few hundred kilometres above fig 1 is geodynamically analogous to the more easily studied lithosphere at the top of the mantle the structure of d may reflect the style of lower mantle convection the nature of core mantle interaction and perhaps even the fate of subducting lithosphere1 observations of lithospheric seismic anisotropy have provided valuable insight into the nature of the upper mantle boundary layer but discussion of lower mantle seismic anisotropy has been somewhat contentious2 5 here we present evidence from seismic waves that have traversed the lowermost mantle beneath the caribbean region for a zone of seismic anisotropy below the d discontinuity which in this region lies 250 km above the core mantle boundary the anisotropy is most probably due to horizontal layering or aligned inclusions of a material with differing shear wave velocity if d is a graveyard for subducted lithosphere a plausible explanation of the anisotropy may be the contrast between cold lithospheric mantle and material that formerly constituted the oceanic crust which may have lower shear wave velocity owing to the presence of melt

pleistocene paleosols in norway implications for past climate and glacial erosion abstract as recently as 7 years ago quaternary geologists believed that there were no pleistocene paleosols represented by non peaty soil horizons a b etc in norway however several buried soils of this type have recently been reported and studied by stratigraphic magnetic and chemical methods magnetic susceptibility is a useful method for identifying and correlating paleosols even in areas of complex glacial stratigraphy such as the nordic countries at sargejohka in finnmark up to seven different buried pedocomplexes are separated by tills and glaciotectonic deformation structures they represent different periods of land surface stability and with two possible exceptions are not pseudosoils formed by a fluctuating groundwater table the most important from stratigraphic and paleoclimatic points of view is a well developed red yellow paleosol numbered p3 counting down from the top it is thought to be a podzol of the last interglacial later cryoturbation of pre mid weichselian age has greatly altered its upper part indicating a dramatic change in climate after development of the soil at lillehammer in southeastern norway remains of the oxidised zone of a buried paleosol also thought to have formed mainly in the last interglacial occur in an inferred saalian till the presence of buried pleistocene paleosols in the interior of norway central part of the fennoscandian glaciations indicates that glacial erosion was locally less during several glacial cycles than previously thought

intraplate seismicity and stress in the southeastern united states abstract the stress induced by topography and by density heterogeneities in the lithosphere has been computed for two seismic regions in the southeastern united states the southern appalachian mountains and the south carolina coastal plain the lithosphere was assumed to be a three dimensional layered elastic slab overlying an inviscid fluid the calculations indicate that the local stress is of the same magnitude as the tectonic stress tens of mpa but contradictory conclusions are suggested by the comparison of stress differences and principal directions with earthquake locations and focal mechanisms in the southern appalachians the seismicity correlates well with the stress maximum and focal mechanisms agree with the calculations where regional stress is combined with locally induced stress in south carolina a combination of regional and local stress can explain the orientation of focal mechanisms but stress does not concentrate in the region of charleston and thus other factors must play a role in the charleston seismicity

rapid computation of multioffset vertical seismic profile synthetic seismograms for layered media by rearranging the formulas for the responses of buried sources and receivers in the kennett reflectivity algorithm we have obtained a new algorithm that is very efficient for computing multioffset vsp synthetic seismograms the rearrangement of the response formulas is quite general inasmuch as it applies to both isotropic and anisotropic kennett codes our new algorithm and the original single receiver algorithm can both be made to run much faster on vector computers by taking the layer loop out of the p loop but leaving it inside the frequency loop the resulting vector codes can still compute the response of media with frequency dependent velocities

crustal structure of the grenville front and adjacent terranes under the auspices of the great lakes international multidisciplinary program on crustal evolution approximately 320 km of deep seismic reflection data were collected in lake huron along a profile that extends east from the manitoulin terrane across the grenville front to the interior of the grenville orogen the manitoulin terrane is characterized by a series of gently east dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer imaged by strong reflections at the western end of a spectacular band of moderately east dipping reflections the grenville front clearly truncates manitoulin terrane structures to the west these data are interpreted in terms of a speculative two stage model involving 1 creation of a major decollement during northward collision of an allochthonous terrane with the southern superior cratonic margin 1 83 1 89 ga penokean orogeny and 2 northwest directed stacking of microterranes at the southeastern margin of the craton followed by crust penetrating ductile imbrication under high pressure high temperature conditions leading to the ramping of deeply buried rocks to the near surface 1 0 1 3 ga grenvillian orogeny

late cenozoic deep weathering patterns on the fennoscandian shield in northern finland a window on ice sheet bed conditions at the onset of northern hemisphere glaciation abstract the nature of the regolith that existed on the shields of the northern hemisphere at the onset of ice sheet glaciation is poorly constrained in this paper we provide the first detailed account of an exceptionally preserved deeply weathered late neogene landscape in the ice sheet divide zone in northern finland we mine data sets of drilling and pitting records gathered by the geological survey of finland to reconstruct regional preglacial deep weathering patterns within a gis framework using a large geochemical data set we give standardised descriptions of saprolite geochemistry using a variant of the weathering index of parker wip as a proxy to assess the intensity of weathering we also focus on mineral prospects and mines with dense pit and borehole data coverage in order to identify links between geology topography and weathering geology is closely linked to topography on the preglacial shield landscape of northern finland and both factors influence weathering patterns upstanding resistant granulite granite gabbro metabasalt and quartzite rocks were associated with fresh rock outcrops including tors or with thin fines values above 3000 and 4000 beneath valley floors developed along mineralised shear and fracture zones weathering penetrated locally to depths of 50 m and included intensely weathered kaolinitic clays with wip fines values below 1000 late neogene weathering profiles were varied in character tripartite clay gruss saprock profiles occur only in limited areas bipartite gruss saprock profiles were widespread with saprock thicknesses of 10 m weathering profiles included two discontinuities in texture materials and resistance to erosion between saprolite and saprock and between saprock and rock limited core recovery when drilling below the soil base in mixed rocks of the tana belt indicates that weathering locally penetrated deep below upper fresh rock layers such deep seated weathered bands in rock represent a third set of discontinuities incipient weathering and supergene mineralisation also extended to depths of 100 m in mineralised fracture zones the thin weathering crusts found extensively beneath till may represent types of early or middle pleistocene palaeosols we confirm that glacial erosion has been very limited

late pleistocene paleosols in the extra glacial regions of northwestern eurasia pedogenesis post pedogenic transformation paleoenvironmental inferences abstract recent revision of the eastern boundary of the last glaciation of eurasia opened new perspectives in the search of the paleopedological records in the north of eastern europe and western siberia we studied paleosol sedimentary sequences in the upper volga and middle ob basins as a regional paleoecological record for the late pleistocene thermochrons two key sections cheremoshnik and belaya gora containing mis3 and mis5 paleosol units and additional section koskovo 2 with polycyclic mis3 paleosol were investigated their timescale are based on c14 and u th datings as well as on stratigraphic and paleobotanical correlations macro meso and micromorphological observations together with fossil insects and plant macrofossil were used as paleoenvironment proxy mis3 paleosols show abundant redoximorphic and cryogenic features that allow interpreting them as gleyic cryosols sub fossil insects and plant macrofossil reflect the cold climate conditions three incipient ah horizons in koskovo 2 pedocomplex point to multiple phases of pedogenesis correlative to the mis3 paleosol levels in different sections of the eurasian loess belt and to the major interstadials of the middle pleniglacial of the nw europe mis5 paleosols are also characterized by gleyzation and peat accumulation in this case we suppose geomorphological control on hydromorphic pedogenesis however paleosol in belaya gora has clear signs of polygenesis including the phase of clay illuviation insect and plant remains from the lower paleosol level of the belaya gora section show the transition from cold terminal phase of mis6 to warm first and then to a moderately cold climate within mis5 the obtained results contribute to understanding of soil and landscape zonality in the largest plains of eurasia mostly during the warm phases of late pleistocene

normal fault boundary of an appalachian basement massif results of cocorp profiling across the pine mountain belt in western georgia recent seismic reflection profiling in western georgia and adjacent eastern alabama conducted by the consortium for continental reflection profiling cocorp shows that reflections commonly associated with the appalachian detachment do not continue southeastward beneath the pine mountain belt rather these reflections terminate abruptly on the north side of the belt along the downdip projection of the towaliga fault this observation is difficult to reconcile with the basement duplex interpretation traditionally applied to the pine mountain belt and to all other appalachian interior basement massifs an alternative interpretation consistent with the reflection data and with local surface geologic relations is that the towaliga fault is at least in its later evolution a large northwest dipping normal fault that cuts the piedmont allochthon appalachian detachment and grenville basement where crossed by the cocorp profile this fault has an inferred average dip of about 54 and offsets grenville basement about 9 km this interpretation is consistent with the view that the pine mountain belt is a structural window through the piedmont allochthon however it implies that much of the structural relief on the basement exposed in the window is due to late normal faulting rather than to thrust imbrication alone if correct this has several important implications for appalachian geology 1 it implies that normal faulting of late paleozoic and or mesozoic age has played a much more important role in the development of the exposed southern appalachians than has generally been considered to date 2 grenville basement exposed in the pine mountain belt has been attached to north america since precambrian time it does not represent a paleozoic accreted terrane 3 the appalachian detachment may be exposed around the periphery of the pine mountain belt and hence may be available for direct observation at this locality

seismic anisotropy due to preferred mineral orientation observed in shallow crustal rocks in southern alaska laboratory velocity measurements and structural field relations explain the observation of a significant seismic anisotropy within highly foliated rocks in southern alaska the orientation of the principal compressive stress indicates that this anisotropy is not related to extensive dilatancy but can be satisfactorily explained by the foliation of the metasedimentary rocks laboratory measurements indicate a significant anisotropy within these rocks even at pressures well above those thought to close microcracks these observations document that anisotropy related to preferred mineral orientation may be widespread in continental rocks thus pervasive foliation can play a more important role in causing anisotropy than dilatancy and inferences of principal stress orientations solely from anisotropy may be erroneous

high resolution seismic tomography of compressional wave velocity structure at newberry volcano oregon cascade range compressional wave velocity structure is determined for the upper crust beneath newberry volcano central oregon using a high resolution active source seismic tomography method newberry volcano is a bimodal shield volcano east of the axis of the cascade range it is associated both with the cascade range and with northwest migrating silicic volcanism in southeast oregon high frequency 7 hz crustal phases nominally pg and a midcrustal reflected phase travel upward through a target volume beneath newberry volcano to a dense array of 120 seismographs this arrangement is limited by station spacing to 1 to 2 km resolution in the upper 5 to 6 km of the crust beneath the volcano s summit caldera the experiment tests the hypothesis that cascade range volcanoes are underlain only by small magma chambers a small low velocity anomaly delineated about 3 km below the summit caldera supports this hypothesis for newberry volcano and is interpreted as a possible magma chamber of a few to a few tens of km3 in volume a ring shaped high velocity anomaly nearer the surface coincides with the inner mapped ring fractures of the caldera it also coincides with a circular gravity high and we interpret it as largely subsolidus silicic cone sheets the presence of this anomaly and of silicic vents along the ring fractures suggests that the fractures are a likely eruption path between the small magma chamber and the surface

seismic anisotropy in the core mantle transition zone split s waves observed at hockley texas from events in the tonga fiji region of the southwest pacific show predominantly vertically polarized shear wave sv energy arriving earlier than horizontally polarized sh energy for rays propagating horizontally through d after corrections are made for the effects of upper mantle anisotropy beneath hockley a time lag of 1 5 to 2 0 s remains for the furthest events 93 9 100 6 while the time lags of the nearer observations 90 5 92 9 nearly disappear at closer distances the s waves from these same events do not penetrate as deeply into the lower mantle and are not split these observations suggest that a patch of d beneath the central pacific is anisotropic while the mantle immediately above the patch is isotropic the thickness of the anisotropic zone appears to be of the order of 100 200 km observations of shear wave splitting have previously been made for paths that traverse d under the caribbean and under alaska sh leads sv the reverse of the hockley observations but in these areas the fact that sv leads sh in the hkt data shown here suggests a different sort of anisotropy under the central pacific from that under alaska and the caribbean the case of sh travelling faster than sv is consistent with transverse isotropy with a vertical axis of symmetry vti and does not require variations with azimuth the case of sv leading sh is consistent with transverse isotropy with a horizontal axis of symmetry hti an azimuthally anisotropic medium and with a vti medium formed by a hexagonal crystal given that mg fe sio3 perovskite appears unlikely to form anisotropic fabrics on a large scale the presence of anisotropy may point to chemical heterogeneity in the lowermost mantle possibly due to mantle core interactions

evidence for a shear velocity discontinuity in the lower mantle beneath india and the indian ocean abstract sh and ssh seismograms are modeled to determine the shear velocity structure in the d region beneath india and the indian ocean the signals show waveform complexities similar to those observed in data sampling the d region beneath alaska the caribbean and eurasia lay and helmberger which have been attributed to a 2 7 shear velocity discontinuity 280 km above the core mantle boundary the new data set consists of long period tangential component recordings at wwssn stations in africa the middle east and europe for 11 intermediate and deep focus indonesian earthquakes in the distance range 70 82 the waveforms show an arrival between sh and scsh with systematic moveout from 89 to 94 there is a strong distortion of the sh waveforms indicating the arrival of several phases closely spaced in time the relative time shifts of similar complexity in the corresponding ssh phases requires a deep mantle origin the depth dependence and moveout of the interference effects are well predicted for both sh and ssh phases by a model with a lower mantle discontinuity alternative explanations of the interference as resulting from receiver reverberations sks contamination multiple source complexity or near source multipathing are ruled out by systematic tests while it is apparent that lateral variations in the lower mantle velocity structure prevent any single model from fitting all of the data synthetic waveform modeling using generalized ray theory and reflectivity shows that the data can be well fit by a model with a discontinuity similar in size and depth to that proposed for the previously investigated regions lay and helmberger but with a negative velocity gradient within the d layer

dynamic elasticity of microbedded and fractured rocks microbedded rocks have an anisotropic frequency dependent sound speed which depends on the intrinsic sound speeds of the individual microbeds and on the o doherty anstey effect fractured rocks have an anisotropic frequency dependent sound speed which depends on the intrinsic sound speed of the unfractured rock the frequency dependent phase shift that occurs during reflection or transmission across a fracture and the interfracture o doherty anstey effect these effects are neglected by the quasistatic methods presently used to generate elastic constants here i introduce a new method for generating elastic constants that contain all the above effects first a statistical description of the rock is used to generate a sample of the rock then an exact two way method is used to propagate just a few plane waves of frequency a distance of several wavelengths from the source if an equivalent homogeneous medium exists at frequency then the computed motions must also satisfy a one way elastic wave equation for that equivalent medium this one way wave equation is used to invert for the elastic coefficients when no equivalent medium exists perhaps because is too large this is indicated by the inversion possible applications of the method are prediction of seismic sound speeds from measurements of bed thicknesses in cores analysis of laboratory data for fracture constitutive relations and inversion of multioffset vertical seismic profiling data for elastic coefficients comparable with those predicted from cores

seismic anisotropy in the d layer we present observations of diffracted sv svd for a path between the fiji tonga islands and the eastern coast of north america at distances greater than 110 observed features of s diffracted suggest that coupling between svd and shd can be ruled out as a first order effect for this path arrivals of shd are late relative to iasp91 travel times by about 10 s and those of svd are late relative to shd by 3 s for most records the slope of the log svd shd spectral ratio is around 3hz 1 in the range 0 06 0 15 hz a transversely isotropic low velocity layer in the lowermost mantle with a thickness of 200 300 km may account for most of the observed properties of svd

joint inversion for global isotropic and radially anisotropic mantle structure including crustal thickness perturbations we present a new global whole mantle model of isotropic and radially anisotropic s velocity structure sglobe rani based on 43 000 000 surface wave and 420 000 body wave travel time measurements which is expanded in spherical harmonic basis functions up to degree 35 we incorporate crustal thickness perturbations as model parameters in the inversions to properly consider crustal effects and suppress the leakage of crustal structure into mantle structure this is possible since we utilize short period group velocity data with a period range down to 16 s which are strongly sensitive to the crust the isotropic s velocity model shares common features with previous global s velocity models and shows excellent consistency with several high resolution upper mantle models our anisotropic model also agrees well with previous regional studies anomalous features in our anisotropic model are faster sv velocity anomalies along subduction zones at transition zone depths and faster sh velocity beneath slabs in the lower mantle the derived crustal thickness perturbations also bring potentially important information about the crustal thickness beneath oceanic crusts which has been difficult to constrain due to poor access compared with continental crusts

the lowermost mantle beneath northern asia ii evidence for lower mantle anisotropy summary we have analysed prediffracted s waves with turning points beneath northern siberia in a study of anisotropy in the lowermost mantle deep focus earthquakes beneath the marianas izu bonin and the sea of japan recorded at stations in western europe are used a correction for upper mantle anisotropy is applied to the data comparisons of the data with synthetic data for models with and without a high velocity d layer suggest that there is a velocity discontinuity at the top of the d region and that the style of anisotropy is transversely isotropic in this region time separations between s waves on the radial and transverse component show a weak trend where the separation increases with epicentral distance a normalization of this separation with the travel distance within d 300 km thick in this region suggests that the anisotropy is uniformly distributed within this layer and has an average value of 0 5 per cent a combination of different studies which investigate the structure of the lowermost mantle beneath europe and northern siberia reveals a complicated picture tomographic models from this area and evidence of d anisotropy lower mantle scatterers reflections from a d discontinuity and ultralow velocity zones suggest two distinct regions one exhibits high velocities d anisotropy a d discontinuity and no evidence of scatterers or ultra low velocity zones these features are likely associated with the palaeosubduction of the izanagi plate well into the lowermost mantle the other region has a lower overall velocity and shows evidence of scatterers and ultra lowvelocity zones perhaps suggesting the presence of partial melt these results suggest dramatic lateral variations in the nature of the lowermost mantle beneath northern asia over a length scale of roughly 30 degrees

shdiff svdiff splitting in an isotropic earth 1 the vertically polarized svdiff seismic phase sometimes arrives later than the horizontally polarized shdiff both phases propagate in the d layer at the base of the mantle and the difference in their arrival times is usually interpreted in terms of seismic anisotropy in d using numerical simulations we demonstrate that a significant 1 s delay of svdiff relative to shdiff is present even in the iasp91 isotropic reference earth model and in a model with a high s velocity in d relative to iasp91 the delay is accompanied by strong amplitude decay of svdiff with distance this relationship resembles the effect of anelasticity in a model with a low s velocity in d the waveform of svdiff can be very different from shdiff and this difference can be mistaken for a delay of shdiff relative to svdiff in a laterally heterogeneous isotropic d svdiff can also be delayed because unlike shdiff svdiff has strong amplitude decay along fast paths its traveltime is thus dominated by slow propagation paths and tends to be positively biased for d models with an s velocity difference of 3 between two hemispheres there are regions where shdiff arrives up to 15 s earlier than the first clearly visible svdiff this is observed in a corridor several hundred kilometers wide if these effects are misinterpreted in terms of d anisotropy in actual seismic data estimates of anisotropy are biased and may contribute to the idea of seismic anisotropy being different in high and low s velocity regions of d

apparent splitting of s waves propagating through an isotropic lowermost mantle observations of shear wave anisotropy are key for understanding the mineralogical structure and flow in the mantle several researchers have reported the presence of seismic anisotropy in the lowermost 150 250 km of the mantle i e d urn x wiley jgrb media jgrb52636 jgrb52636 math 0002 layer based on differences in the arrival times of vertically sv and horizontally sh polarized shear waves by computing waveforms at a period 6 s for a wide range of 1 d and 3 d earth structures we illustrate that a time shift i e apparent splitting between sv and sh may appear in purely isotropic simulations this may be misinterpreted as shear wave anisotropy for near surface earthquakes apparent shear wave splitting can result from the interference of s with the surface reflection ss for deep earthquakes apparent splitting can be due to the s wave triplication in d urn x wiley jgrb media jgrb52636 jgrb52636 math 0003 reflections off discontinuities in the upper mantle and 3 d heterogeneity the wave effects due to anomalous isotropic structure may not be easily distinguished from purely anisotropic effects if the analysis does not involve full waveform simulations

seismic anisotropy in the lowermost mantle beneath the pacific onset time differences of up to 3 s are observed between transverse ssh and longitudinal ssv components of broadband s waves at distances of 85 to 120 for paths traversing the lowermost mantle d beneath the pacific after correction for upper mantle anisotropy ssh usually arrives earlier than ssv with the splitting increasing with distance from 100 to 120 the data yield two possible models of anisotropy 1 anisotropy may vary laterally with transverse isotropy existing in higher than average shear velocity regions beneath the northeastern pacific or 2 anisotropy may vary with depth with transverse isotropy concentrated in a thin 100 km thermal boundary layer at the base of d a few recordings at distances less than 105 show that ssv arrives earlier than ssh indicating that general anisotropy likely exists in shallower regions of d

3d acoustic reverse time migration1 acoustic reverse time finite difference migration for zero offset data is extended from two to three dimensional media the formulation is based on the full three dimensional acoustic wave equation and so has no dip restrictions and it involves extrapolation in a velocity distribution variable in three dimensions the algorithm is demonstrated by successful migration of synthetic data sets for three models a point diffractor an oblique pinch out and a dome overlying a planar reflector

geometry and subsurface lithology of southern death valley basin california based on refraction analysis of multichannel seismic data the shallow structure and subsurface lithology of southern death valley basin in eastern california were studied using refraction analysis of multichannel seismic reflection data acquired by cocorp two dimensional velocity models of the upper 3 km of southern death valley were derived from iterative fitting of first arrival travel times on common shot and common receiver gathers the structural basement beneath southern death valley defined by rocks having velocities greater than 4 0 km s is asymmetric and dips gently to the east where it is terminated by a steeply dipping fault which parallels the black mountain range front and has normal down to the west displacement numerous other faults that have normal components of displacement obliquely cross the seismic lines and indicate that the southern death valley basin formed from the divergent wrench system of the southern death valley fault zone

cocorp deep seismic reflection traverses of the u s cordillera summary cocorp seismic reflection traverses of the u s cordillera at 40 n and 48 5 n latitude reveal some fundamental similarities as well as significant differences in reflection patterns on both traverses autochthonous crust beneath thin skinned thrust belts of the eastern part of the cordillera is unreflective immediately to the west the cordilleran interior is very reflective above a flat prominent reflection moho mesozoic accreted terranes in the western part of the orogen are underlain on both traverses by very complex reflection patterns in constrast to more easily deciphered patterns beneath areas of cenozoic accretion the prominent reflection moho beneath the orogenic interior on both transects probably evolved through a combination of magmatic and deformational processes during cenozoic extension the main differences between the two traverses lie in the reflection patterns of the middle and lower crust in the cordilleran interior these differences are probably related to the way cenozoic extension was accommodated at depth laminated middle and lower crust above the reflection moho in the western basin and range 40 n may be related to magmatism ductile pure shear and large scale transposition during cenozoic extension by contrast beneath the eastern basin and range 40 n and the orogenic interior in the nw united states 48 5 n cenozoic extension was probably accommodated along dipping deformation zones throughout the crust

ray equation migration of wide angle reflections from southern alaska a kirchhoff style prestack depth migration algorithm is implemented for wide angle reflections in two dimensional variable velocity acoustic media extrapolation of both source and recorder wavefields is performed by ray tracing the imaging condition at each point in the image space is the total source to recorder travel time since the migration algorithm is based on ray tracing migration can be performed for any medium through which rays can be traced there is no inherent limitation on the size of the region to be imaged regions up to 3 6 103 km2 are utilized in this study the algorithm is illustrated by migration of two synthetic and three real data sets from the u s geological survey s trans alaska crustal transect program images are produced of inferred subduction assemblages in southern alaska emphasis in this paper is on obtaining reflector positions and shapes rather than detailed structural interpretations

soil evolution over the quaternary period in a mediterranean climate se spain palaeosols in the granada basin se spain have been studied in two different situations surface soils on geomorphically stable surfaces since the early pleistocene with younger pedogenic overprinting and buried soils on unstable surfaces from the middle late pleistocene on which successive erosional depositional episodes have alternated with pedogenic episodes for each soil clay and iron accumulation indices the fet alt sit ratio clay mineralogy and micromorphological features were used to estimate the degree of soil development from the early to the early late pleistocene the main pedogenic processes were the leaching of carbonates weathering illuviation and rubification which resulted in bt horizons with red colours clay texture clay coatings and kaolinite neoformation the degree of weathering and the development of these bt horizons varied over time and the soils that formed on the surfaces from the early pleistocene show strongest weathering and development however after their formation there were periods in which they were partially truncated and recalcified resulting in polygenetic soils the different degrees of development of the buried soils during the last 474 000 years indicate that the wettest warm period was stage 7 and the driest stage 5 stages 9 and 11 must have had climates with intermediate wetness since the clay accumulation and iron oxide accumulation indices the differences in fet alt sit ratio between bt and c horizons the extent of kaolinite neoformation and the micromorphological features of the soils formed during stage 7 are all similar to the surface soils that formed on early pleistocene deposits these features cannot be used to date surfaces older than 242 000 bp by contrast the soils that formed during stage 7 and later periods show different extents of development and thus can be used for the approximate dating of landforms d 2002 elsevier science b v all rights reserved

more seismic evidence on the location of grenville basement beneath the appalachians of quebec maine summary high quality multichannel seismic 133 fold and refraction wide angle reflection 1 to 3 km receiver spacing and 30 to 60 km shot spacing data have been collected across the northern appalachians in quebec and maine an integrated interpretation of the seismic data from the southeastern quebec western maine region provides strong evidence that the rocks of the predominantly oceanic dunnage zone are allochthonous having been thrust westwards over precambrian grenville basement during and subsequent to the closing of the iapetus ocean

are large explosive sources applicable to resource exploration seismologists from government and academia are finding that large chemical explosions are an excellent seismic source not only for profiling the deep crust but also for imaging the shallower geologic targets of interest to the oil and gas industry for the purposes of this article we define a large explosion as one that utilizes at least 100 kg about 220 lbs of ammonium nitrate or equivalent explosive in general shot sizes ranging from this minimum up to 2700 kg about 3 tons are used in deep crustal seismic profiling programs

deep seismic reflection characteristics of the continental crust four types of seismic reflection fabrics appear to characterize the continental crust 1 a cratonic fabric dominated by diffractions and dipping reflections but lacking a pronounced reflection moho 2 regions of thin skinned foreland deformation with abundant shallow but sparse deep reflections 3 a unidirectional fabric that dips toward the hinterland and may represent a crustal scale ramp zone and 4 zones of highly layered lower crust and high amplitude reflection moho the first three types are commonly observed in transects across single mountain belts but the fourth appears to correspond to regions of elevated lower crustal temperatures such as those encountered in rift provinces and perhaps elsewhere

palaeosols and their cover sediments of a glacial landscape in northern central europe spatial distribution pedostratigraphy and evidence on landscape evolution abstract knowledge of the distribution types and properties of buried soils i e palaeosols is essential in understanding how lowlands in northern central europe have changed over past millennia this is an indispensable requirement for evaluating long term human impact including soil erosion and land cover dynamics in the serrahn area 62 km2 a young glacial landscape representative for northeastern germany and part of the muritz national park 26 pedosedimentary sections were documented and analysed to this end a multiproxy approach was applied using pedology micromorphology geochronology and palaeoecology statistical and spatial analyses of c 5200 soil profiles of which 10 contain palaeosols show that buried soils cover an area of 5 7 km2 i e 9 of the area studied most palaeosols are cambisols arenosols and gleysols palaeosols are mainly covered by aeolian and colluvial sands as well as by lacustrine sands and peat radiocarbon and luminescence dating together with palynological and anthracological data reveal that former land surfaces were dominantly buried through erosion triggered by human activity in the late holocene in addition but to a clearly smaller extent lateglacial early holocene palaeosols and cover sediments occur following medieval clear cutting and intensive land use the study area is today again widely forested the high share of buried land surfaces detected here is expected to be representative for the hilly glacial landscapes even in the wider region i e in northern central europe and should be considered in soil mapping soil carbon budgeting and assessments of past human impact

seismic fracture anisotropy in the earth s crust an overview fluid filled fractures and microcracks are the most stress sensitive component of crustal rock and often determine the pathways and volume of crustal fluid movement if we are to comprehend the role of fractures and fluids in tectonic processes to formulate an accurate hypothesis for phenomena precursory to catastrophic seismic failure or to monitor hydrocarbon and geothermal reservoirs for the presence or absence of major fluid pathways we must understand how seismic waves interact with the fracture crack and microcrack structures within the rock mass a salient feature of fracture related rock structure is the effective seismic anisotropy of aligned fractures and microcracks with the advent of large volume digital seismic data sets it is feasible to use polarized shear waves to explore the anisotropic fracture properties of the crust crustal fracture structures which may be monitored by polarized shear waves range in dimensions from the 10 100 km scale of crustal stress orientation and preseismic stress buildup through the 100 1000 m scale of fluid reservoir structure and characterization and the 1 10 scale of mining and geoengineering rock strain monitoring to millimeter and micrometer or possibly smaller fluid filled inclusions determining the elastic response of the intact rock mass to focus developments in seismic anisotropic studies of crustal fracture phenomena the american geophysical union and the society of exploration geophysicists sponsored a chapman conference on seismic anisotropy in the earth s crust held may 31 to june 4 1988 at the lawrence berkeley laboratory of the university of california this was the third international workshop on seismic anisotropy 3iwsa nineteen conference papers are presented in this special section of the journal of geophysical research

global radially anisotropic mantle structure from multiple datasets a review current challenges and outlook abstract since the 1960s seismologists have mapped anisotropy in the uppermost mantle the mantle transition zone and the d region when combined with constraints from mineral physics and geodynamics anisotropy provides critical information on the geometry of mantle flow here we review the theory early work recent tomographic models and experimental constraints on radial anisotropy we discuss current challenges in resolving radial anisotropy seismically in particular we show that it is highly beneficial to use multiple datasets in inversions for anisotropy notably short period group velocity data with strong sensitivity to the crust we present a new whole mantle model of radial anisotropy based on surface wave and body wave travel time data along with incorporated moho perturbations our whole mantle model shares common features with previous global models and is consistent with results from several high resolution regional studies

lateral variations in lowermost mantle shear wave anisotropy beneath the north pacific and alaska s waves recorded by long period world wide standardized seismograph network and broadband stations in north america for deep northwest pacific subduction zone earthquakes provide evidence for anisotropy in the lowermost mantle shear velocity structure beneath the north pacific and alaska systematic delays of up to 4 s are observed between longitudinal components sv and transverse components sh of motion for core reflected scs waves as well as for core grazing and diffracted s waves the absence of significant splitting for s waves that have turning points more than a few hundred kilometers above the core mantle boundary indicates that anisotropy is localized within the d region the lowermost portion of the mantle sv sh differential arrival times for both scs and sdiff along with path length estimates assuming a 250 km thick d region indicate spatial variations in the strength of shear wave anisotropy the strongest anisotropy 1 1 5 is found in the eastern part of the study area with systematic reduction in magnitude toward the west a transverse isotropy model can explain the data with the velocity structure for horizontally polarized waves vsh having a 2 3 discontinuous shear velocity increase at the top of d as proposed in earlier studies of the region and a similar structure for s wave particle motion in the direction normal to the core mantle boundary vsv but with the velocity jump at the top of d and the velocity within d being reduced from that for vsh by 0 5 1 5 km s large uncertainties exist for velocity gradients above and below the velocity jump but the requirement of a reduced vsv relative to vsh in d is clear synthetic waveforms calculated by using separate isotropic structures for sh and sv match the observations well and constrain the basic anisotropic structure because the shear waves all traverse the region with near grazing geometries the study area exhibits strong lateral variations in lower mantle shear velocity structure and variable thickness of the d layer topography of the d layer is not well resolved because of trade off with volumetric heterogeneity

lowermost mantle anisotropy beneath the pacific imaging the source of the hawaiian plume we utilized recordings of seismic shear phases provided by several north american broadband seismometer arrays to provide unique constraints on shear wave anisotropy beneath the northern and central pacific ocean using a new analysis method that reduces measurement errors and enables the analysis of a larger number of available waveforms we examined relative travel times of teleseismic s and sdiff that sample a large area of lowermost mantle structure the results of this study provide evidence for small scale lateral and depth variations in shear wave anisotropy for a broad region of the lowermost mantle beneath the pacific ocean in particular we image a localized zone of anomalously strong anisotropy whose strength increases toward the top of dq beneath hawaii our results combined with a previous study of vp vsh ratios indicate that ancient subducted slab material may be responsible for observations beneath the northern pacific while lenses or layers of core mantle boundary reaction products or partial melt oriented by horizontal inflow of mantle material to the hawaiian plume source can explain observations beneath the central pacific fl 2001 elsevier science b v all rights reserved

d discontinuity structure beneath the north atlantic from scd observations we analyzed transverse and radial component recordings from the 2010 m6 3 southern spain earthquake depth 620 km recorded on 370 broadband stations in north america we grouped these seismograms into subarrays and applied fourth root vespa processing vespagram analysis in order to enhance low amplitude arrivals these vespagrams show clear scd arrivals which indicate the existence of the d discontinuity beneath the north atlantic ocean 45 60 n 45 55 w these observations are best fit with a 2 4 velocity increase at the top of the d discontinuity at a height above the core mantle boundary of 304 14 km we do not observe scd arrivals at the eastern end of our study region which is consistent with the presence of the easternmost edge of the ancient farallon plate

cocorp new perspectives on the deep crust summary relict sutures from colliding continents regions characterized by a young mono layering and faulting throughout the crust mid crustal magma traps and seismic bright spots which suggest deep crustal fluids are among recent cocorp findings in addition new studies of signal penetration noise mitigation recording geometry and coherency filtering have yielded better understanding of and substantial improvements in data quality amplitude anomalies or bright spots in the basin and range may be due to magma at mid crustal levels in one case a normal fault appears to link the deep magma with young surface volcanics another bright spot 15 km deep in southeastern georgia has a flat geometry that suggests a gas liquid interface perhaps within fluids underthrust along an appalachian suture the mono continues to appear relatively undisturbed in many regions of past deformation suggesting that its formation post dates these major tectonic episodes the diversity of reflection patterns from the u s cordillera casts further doubt on the generality of the common model of a reflective layered lower crust underlying a transparent upper crust

a strongly negative shear velocity gradient and lateral variability in the lowermost mantle beneath the pacific a new approach for constraining the seismic shear velocity structure above the core mantle boundary is introduced whereby sh sks differential travel times amplitude ratios of sv sks and sdiff waveshapes are simultaneously modeled this procedure is applied to the lower mantle beneath the central pacific using da ta from numerous deep focus southwest pacific earthquakes recorded in north america we analyze 90 broadband and 248 digitized analog recordings for this source receiver geometry sh sks times are highly variable and up to 10 s larger than standard reference model predictions indicating the presence of laterally varying low shear velocities in the study area the travel times however do not constrain the depth extent or velocity gradient of the low velocity region sv sks amplitude ratios and sh waveforms are sensitive to the radial shear velocity profile and when analyzed simultaneously with sh sks times rnveal up to 3 shear velocity reductions restricted to the lowermost 190 50 km of the mantle our preferred model for the central eastern pacific region ml has a strong negative gradient with 0 5 reduction in velocity relative to the preliminary reference earth model prem at 2700 km depth and 3 reduction at 2891 km depth and slight velocity reductions from 2000 to 2700 km depth 0 0 5 lower than prem significant small scale 100 500 km shear velocity heterogeneity 0 5 1 is required to explain scatter in the differential times and amplitude ratios