calculated mineral equilibria for eclogites in cao na2o feo mgo al2o3 sio2 h2o application to the pouebo terrane pam peninsula new caledonia the high p medium t pouebo terrane of the pam peninsula northern new caledonia includes barroisite and glaucophane bearing eclogite and variably rehydrated equivalents the metamorphic evolution of the pouebo terrane is inferred from calculated p t and p t xh2o pseudosections for bulk compositions appropriate to these rocks in the model system cao na2o feo mgo al2o3 sio2 h2o the eclogites experienced a clockwise p t path that reached p 19 kbar and t 600 c the eclogitic mineral assemblages are preserved because reaction consequent upon decompression consumed the rocks fluid extensive reaction occurred only in rocks with fluid influx during decompression of the pouebo terrane

investigation of upper mantle discontinuity structure beneath the central pacific using ss precursors 1 using a high quality broadband seismic data set of precursors to the phase ss we investigate the structure of upper mantle discontinuities beneath the central pacific including the hawaiian hot spot we image structure by stacking over 4000 records into geographic bins retaining periods down to 5 s we consider the effects of correcting for four separate tomographic models of mantle heterogeneity excluding data at distances containing phases that potentially interfere with precursors we find evidence for peak to peak topography of 15 20 km on the 670 km discontinuity and 7 28 km of topography on the 400 km discontinuity weak reflections are detected from discontinuities near 220 and 520 km depth the average transition zone thickness beneath our region is approximately 242 3 km very similar to previous estimates of the global average lateral transition zone thinning and thickening weakly correlate with reduced and increased transition zone shear velocity respectively consistent with a thermal origin to topographical variations on the discontinuities within our study region the transition zone beneath hawaii and to the east of the line island chain is thinned by up to 20 km in a province spanning nearly 1000 km suggesting an excess mantle temperature of 200 k in the oldest crustal regions of our study area 100 ma the 400 km discontinuity is relatively shallow and the transition zone is relatively thick 250 255 km a possible explanation for this pattern includes small scale convection in the upper mantle bringing colder material into the transition zone

constraints on seismic anisotropy in the mantle transition zone from long period ss precursors the mantle transition zone mtz of earth is demarcated by solid to solid phase changes of the mineral olivine that produce seismic discontinuities at 410 and 660 km depths mineral physics experiments predict that wadsleyite can have strong single crystal anisotropy at the pressure and temperature conditions of the mtz thus significant seismic anisotropy is possible in the upper mtz where lattice preferred orientation of wadsleyite is produced by mantle flow here we use a body wave method ss precursors to study the topography change and seismic anisotropy near the mtz discontinuities we stack the data to explore the azimuthal dependence of travel times and amplitudes of ss precursors and constrain the azimuthal anisotropy in the mtz beneath the central pacific we find evidence for 4 anisotropy with a se fast direction in the upper mantle and no significant anisotropy in the mtz in subduction zones we observe 4 anisotropy with a trench parallel fast direction in the upper mantle and 3 anisotropy with a trench perpendicular fast direction in the mtz the transition of fast directions indicates that the lattice preferred orientation of wadsleyite induced by mtz flow is organized separately from the flow in the upper mantle global azimuthal stacking reveals 1 azimuthal anisotropy in the upper mantle but negligible anisotropy 1 in the mtz finally we correct for the upper mantle and mtz anisotropy structures to obtain a new mtz topography model the anisotropy correction produces 3 km difference and therefore has minor overall effects on global mtz topography

the metamorphic sole of new caledonia ophiolite 40ar 39ar u pb and geochemical evidence for subduction inception at a spreading ridge amphibolite lenses that locally crop out below the serpentinite sole at the base of the ophiolite of new caledonia termed peridotite nappe recrystallized in the high temperature amphibolite facies and thus sharply contrast with blueschists and eclogites of the eocene metamorphic complex amphibolites mostly display the geochemical features of morb with a slight nb depletion and thus are similar to the youngest late paleocene eocene babb components of the allochthonous poya terrane thermochronological data from hornblende 40ar 39ar zircon and sphene u pb suggest that these mafic rocks recrystallized at 56ma using various geothermobarometers provides a rough estimate of peak recrystallization conditions of 0 5gpa at 800 950c the thermal gradient inferred from the metamorphic assemblage 60 ckm 1 geometrical relationships and geochemical similarity suggest that these mafic rocks belong to the oceanic crust of the lower plate of the subduction obduction system and recrystallized when they subducted below young and hot oceanic lithosphere they were detached from the down going plate and finally thrust onto unmetamorphosed poya terrane basalts this and the occurrence of slab melts at 53ma suggest that subduction inception occurred at or near to the spreading ridge of the south loyalty basin at 56ma 2012 american geophysical union all rights reserved

do double ss precursors mean double discontinuities summary stacks of ss precursors have been widely used in the past two decades to investigate the existence and characteristics of upper mantle discontinuities on a global scale as well as in several regional cases here we present observations of ss precursors from an mb 6 7 earthquake recorded at the us transportable array in 2010 in this particular case the s660s precursors on the transverse component are strong enough to be identified on individual seismograms across the array without any stacking procedures two s660s precursors are observed seeming to suggest double discontinuities around 660 km depth in the bounce point region through careful analysis of 1 d and 3 d synthetic seismograms we however discover that although they have arrival times and slownesses that are very close to the theoretical values of ss precursors the apparent double precursors are artefacts because of mantle heterogeneity in the upper mantle near the receivers away from the bounce point region this suggests that caution must be taken about appropriate azimuthal coverage at the ss bounce point before interpreting double ss or pp precursors in terms of complex mineralogical transitions

structural sensitivities of finite frequency seismic waves a full wave approach summary we use the normal mode theory to investigate the sensitivities of the delay times of finitefrequency seismic waves to 3 d perturbations in wave speeds relative to 1 d reference models the normal mode theory provides the exact solutions to the wave equation in 1 d models thus enabling a full wave approach to the wave propagation in the sense that for a given pair of source and receiver it accounts for all the waves that travel from the source to the receiver following any possible paths and that it models accurately all the possible wave effects such as diffraction and non geometrical propagation we illustrate the complex picture of finite frequency wave

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

analysis of coseismic deformation using eof method on dense continuous gps data in taiwan abstract taking advantage of the dense continuous gps network data we demonstrate the effective utility of the method of empirical orthogonal function eof in obtaining the coseismic deformation vector field on the crustal surface caused by significant earthquakes we conduct case studies w r t two earthquakes that have occurred in taiwan one off the east coast of taiwan in 2002 event i mw 7 1 and one inland in southern taiwan in 2010 event ii mw 5 7 or 6 3 we demonstrate that superior to the simple least squares estimates obtained directly from individual gps position time series the eof analysis is capable of extracting the coseismic deformation in the form of coherent spatial pattern and time evolution the prevalent common mode errors in the gps data are also well resolved as distinctive and separate eof modes the extracted coseismic spatial patterns provide evidences linking the regional tectonics with the orogenic process in taiwan under the plate convergence we further contrast our eof results against those calculated via idealized elastic dislocation models and find notable discrepancies especially for the far field which can be attributed to the complexity of the tectonic environment of taiwan

experiments in migration processing of ss precursor data to image upper mantle discontinuity structure long period ss precursors result from underside reflections off upper mantle discontinuities by grouping and stacking global seismic data by ss bounce point location it is possible to map lateral variations in depths to the 410 and 660 km discontinuities a process analogous to common midpoint cmp stacking in reflection seismology because this method assumes horizontal reflectors energy arriving from dipping or intermittent reflectors may not be correctly imaged to address this possibility we experiment with techniques based on migration processing of shallow seismic reflection data the problem is complicated by the uneven distribution of sources and receivers for the ss precursor observations but the data are sufficiently dense beneath the northwest pacific ocean that reasonably good coverage can be obtained for this region we parameterize the model as a grid of point scatterers in latitude longitude and depth from the surface to 1000 km depth and compute travel times from each grid point to the source and receiver locations these times are used to construct a matrix equation that yields predicted ss precursor waveforms from the assumed scatterers to recover the model we experiment with both simple back projection and full inversions using a conjugate gradient method tests on noise free synthetic data generated using the same source receiver distribution as the actual data suggest that detailed resolution of discontinuity structure is possible at horizontal scales much smaller than the fresnel zone however the real data do not produce coherent results unless some degree of horizontal smoothing is imposed at least partially defeating the purpose of this approach results for the northwest pacific find structure on the 410 and 660 km discontinuities and hints of intermittent reflectors at other depths random resampling tests however suggest that most of these features are not reliably resolved with the exception of a depression on the 660 km discontinuity seen in the northwest pacific our experiments show that it is unlikely that small scale structure on the 660 km discontinuity near subducting slabs causes significant bias in maps of the large scale 660 km topography derived from long period ss precursor observations

geochemical evolution and tectonic significance of boninites and tholeiites from the koh ophiolite new caledonia the central chain ophiolites in new caledonia are fragments of a supra subduction zone ssz ophiolite now preserved from the upper layered gabbros through to volcanics and overlain by pelagic cherts and a thick middle triassic to upper jurassic volcaniclastic sequence most of the fragments were formed by a single tholeiitic magmatic episode but one of these the koh ophiolite was formed by two tholeiitic magmatic episodes separated by boninites the first event in the koh ophiolite formed cumulate gabbros dolerites plagiogranites and the first pillow lava sequence from a tholeiitic magma with strong depletion in the light rare earth elements lree and abnormally low tio2 0 5 at mg 60 shortly after their eruption these tholeiitic lavas were overlain by a high ca boninitic unit with a basal section of boninite pillows flows and breccias and an upper section of boninitic dacites and tuffs the last magmatic phase involved eruption of evolved tholeiitic basalts as pillows above the boninites and as dykes and sills intruding the older plutonic and volcanic sections of the ophiolite this second phase of tholeiitic magmatism is compositionally distinct from the first and is closest to back arc basin basalts babb erupted during the early rifting history of modern back arc basins the boninitic volcanics belong to a high ca series with slightly lower sio2 al2o3 and tio2 compared to those from modern island arc systems and they lack the positive zr spike relative to adjacent rare earth elements ree in normalised element variation patterns these boninites were formed shortly after the production of back arc basin crust represented by the depleted tholeiites and shortly before a second spreading event which caused 40 60 extension of the initial basin crust and eruption of the upper tholeiites the dominance of babb like tholeiites throughout the central chain ophiolites in new caledonia the restricted occurrence of boninites and the stratigraphy and chemistry of the koh ophiolite suggest that the boninites were erupted in response to an exceptional tectonic situation we suggest that this boninite generation event was driven by adiabatic decompression of hot depleted mantle residual from the production of the lower tholeiites during initiation of rifting of young oceanic crust intimately associated with propagation of a back arc basin spreading centre the occurrence of a thick blanket of calc alkaline volcaniclastic sediments above the ophiolite indicates proximity to a mature arc and suggests that the koh boninites were not associated with the initiation of subduction a close modern analogy for the koh ophiolite exists on the hunter ridge protoisland arc between southernmost vanuatu new hebrides island arc and the fijian islands there high ca boninites lacking positive zr spikes occur together with low ti tholeiites and more typical babb tholeiites where the southern spreading centre of the north fiji basin is propagating into the protoarc crust of the hunter ridge

imaging the upper mantle transition zone with a generalized radon transform of ss precursors we demonstrate the feasibility of using inverse scattering for high resolution imaging of discontinuities in the upper mantle beneath oceanic regions far from sources and receivers using broadband wavefield observations consisting of ss and its precursors the generalized radon transform grt that we developed for this purpose detects in the broadband data signals due to scattering from elasticity contrasts in earth s mantle synthetic tests with realistic source receiver distributions demonstrate that the grt is able to detect and image deep mantle interfaces even in the presence of noise depth phases ghosts phase conversions and multiples generated by reverberation within the transition zone as a proof of concept we apply the grt to 50 000 broadband seismograms to delineate interfaces in the depth range from 300 to 1000 km beneath the northwest pacific we account for smooth 3d mantle heterogeneity using first order perturbation theory and independently obtained global tomography models the preliminary results reveal laterally continuous but undulating scatter zones near 410 and 660 km depth and a weaker broader and more complex structure near 520 km depth the images also suggest the presence of multiple laterally intermittent interfaces near 350 km and between 800 and 1000 km depth that is above and below the transition zone sensu stricto filtering of the data we consider four pass bands 20 50 s 10 50 s 5 50 s and 2 50 s reveals a prominent frequency dependence of the magnitude width and complexity of the interfaces in particular of the scatter zone near 520 km depth such dependencies may put important constraints on the mineralogy and phase chemistry of the transition zone

global mapping of topography on the 660 km discontinuity long period precursors to the ss seismic phase are used to produce global maps of topography for the discontinuity at 660 km depth in the upper mantle these maps indicate discontinuity depth variations of up to 30 km and suggest a correlation between regional depressions in the 660 km discontinuity and subduction zones a result more consistent with models in which the subducting slabs are deflected horizontally at the discontinuity than with models of unhindered slab penetration into the lower mantle

new sw pacific tectonic model cyclical intraoceanic magmatic arc construction and near coeval emplacement along the australia pacific margin in the cenozoic various reconstructions of the sw pacific for the late cretaceous and cenozoic suggest that northeast dipping subduction began in the south loyalty basin slb at 55 50 ma and that subsequent closure of the slb resulted in the diachronous emplacement of cretaceous paleocene ophiolitic nappes onto the norfolk ridge in new caledonia at 40 34 ma and in northland new zealand around 24 21 ma a fundamental problem with these models is that they do not account for the fact that ne dipping subduction had already been established offshore papua new guinea by at least 65 60 ma which resulted in the emplacement of the papuan ultramafic belt pub ophiolite at 59 58 ma a second issue is that the reconstructions are based largely upon unfounded assumptions as to the age and nature of the basement beneath the loyalty arc and three kings ridge finally reconstructions of the northland region are based upon the erroneous assumption that the age of the majority of the igneous component comprising the northland allochthon is late cretaceous paleocene when in fact it is oligocene a new model is presented whereby the pub new caledonia and northland ophiolites formed and were emplaced in a cyclical fashion above an extensive ne dipping cenozoic intraoceanic arc system which diachronously propagated n s along the entire eastern margin of the australian plate these infant arc ophiolites represent fragments of suprasubduction zone lithosphere sszl generated in the earliest stages of magmatic arc formation that were emplaced shortly after 20 m y as a result of forearc australian plate collision subduction inception was the result of subsidence of older morb like lithosphere generated within an extensive back arc basin to the east of the norfolk ridge during the earliest stages of slb formation above a southwest dipping pacific plate during emplacement of each ophiolite a crustal fragment of the older lithosphere was scraped off the ne dipping slab and subsequently back thrust beneath each ophiolite during its emplacement

the ghost of an earthquake seismic activity can cause earth9s crust and upper mantle to deform and then relax slowly and this relaxation can be detected decades after the original event in his perspective hammond discusses results reported in the same issue by gourmelen and amelung in which satellite based radar insar was used to detect the surface deformation in the basin and range province of the western united states caused by earthquakes in the early to mid 20th century the data indicate that disagreements between geodetic mapping such as obtained by global positioning system measurements and paleoseismic observations may be explained by postseismic relaxation moreover the results suggest that magnitude information about past earthquakes might be retrieved by such postseismic analysis

comparison of geodetic and geologic data from the wasatch region utah and implications for the spectral character of earth deformation at periods of 10 to 10 million years the wasatch fault and adjacent fault zones provide an opportunity to compare present day deformation rate estimates obtained from space geodesy with geologic displacement rates over at least four temporal windows ranging from the last millennium up to 10 myr the three easternmost gps sites of the basin and range geodetic network bargen at this latitude define a 130 km wide region spanning three major normal faults extending east west at a total rate of 2 7 0 4 mm yr with an average regional strain rate estimated to be 21 4 nstrain yr about twice the basin and range average on the wasatch fault the vertical component of the geologic displacement rate is 1 7 0 5 mm yr since 6 ka 0 6 mm yr since 130 ka and 0 5 0 7 mm yr since 10 ma however it appears likely that at the longest timescale rates slowed over time from 1 0 to 1 4 mm yr between 10 and 6 ma to 0 2 to 0 3 mm yr since 6 ma the cumulative vertical displacement record across all three faults also shows time variable strain release ranging from 2 to 4 mm yr since 10 ka to 1 mm yr averaged over the past 130 kyr conventional earthquake recurrence models reid type behavior would require an accordingly large variation in strain accumulation or loading rate on a 10 kyr timescale for which there appears to be no obvious geophysical explanation alternatively seismic strain release given a wide range of plausible constitutive behaviors for frictional sliding may be clustered on the 10 kyr timescale resulting in the high holocene rates with comparatively low uniform strain accumulation rates on the 100 kyr timescale wallace type behavior the latter alternative combined with observations at the million year timescale and the likelihood of a significant contribution of postseismic transients implies maxima of spectral amplitude in the velocity field at periods of 10 myr variations in tectonic loading 10 kyr clustered strain release and of 100 years postseismic transients if so measurements of strain accumulation and strain release may be strongly timescale dependent for any given fault system

determination and analysis of long wavelength transition zone structure using ss precursors summary global mapping of 410 and 660 km discontinuity topography and transition zone thickness has proven to be a powerful tool for constraining mantle chemistry dynamics and mineralogy numerous seismic and mineral physics studies suggest that the 410 km discontinuity results from the phase change of olivine to wadsleyite and the 660 km discontinuity results from the phase change of ringwoodite to perovskite and magnesiowustite underside reflections of the 410 and 660 km discontinuities arrive as precursors to ss with the recent development of a semi automated method of determining ss arrivals we have more than tripled the flanagan and shearer 1998a data set of handpicked ss waveforms we are able to increase resolution by stacking waveforms in 5 rather than 10 radius bins as well as increasing data coverage significantly in the southern hemisphere the resulting ss s410s and ss s660s times are heavily influenced by upper mantle velocity structure we perform a joint inversion for discontinuity topography and velocity heterogeneity as well as performing a simple velocity correction to the precursor differential times and find little difference between the two methods the 660 km discontinuity topography and transition zone thickness are correlated with velocities in the transition zone whereas the 410 km discontinuity topography is not in addition the 410 km discontinuity topography is not correlated with the 660 km discontinuity topography rather anticorrelated as expected due to the opposite signs of the clapeyron slopes of their respective phase changes these results suggest that whereas the topography of 660 km discontinuity could be dominated by thermal effects the topography of the 410 km discontinuity is likely dominated by compositional effects in addition unlike previous studies which find less topography on the 410 km discontinuity than on the 660 km discontinuity our 410 and 660 km topography have similar amplitudes

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

ss wave sensitivity to upper mantle structure implications for the mapping of transition zone discontinuity topographies 1 studies of the depths of the upper mantle discontinuities have relied on the differential travel times between the ss wave and its precursors we examine the effect of velocity heterogeneities on these differential travel time data ray theory which is used to correct these data for the effect of velocity heterogeneities is shown to be inaccurate if the characteristic heterogeneity length scales are similar to those seen in regional models or if the station is near the ss wave radiation nodal plane

sensitivity of ss precursors to topography on the upper mantle 660 km discontinuity the sensitivity of ss precursors to the presence of topography on the 660 km discontinuity is addressed using an axisymmetric finite difference approximation to the sh wave propagation in the earth mantle numerical experiments are lead to quantify the bias in both wavelength and amplitude committed in estimating depth information on the upper mantle discontinuities from ray inversion of sds arrival times it is further shown that long period s660s arrival times do not provide information on the deepening of the 660 in presence of a velocity increase expected as the thermal signature of a subducting slab

viscoelastic flow in the lower crust after the 1992 landers california earthquake space geodesy showed that broad scale postseismic deformation occurred after the 1992 landers earthquake three dimensional modeling shows that afterslip can only explain one horizontal component of the postseismic deformation whereas viscoelastic flow can explain the horizontal and near vertical displacements the viscosity of a weak about 10 km thick layer in the lower crust beneath the rupture zone that controls the rebound is about 10 18 pascal seconds the viscoelastic behavior of the lower crust may help to explain the extensional structures observed in the basin and range province and it may be used for the analysis of earthquake hazard

an analysis of ss precursors using spectral element method seismograms summary the traveltimes of underside shear wave reflections i e ss precursors are widely used data for mapping the topography of mantle discontinuities in the upper mantle here we examine the accuracy of ray theory in estimating the contribution of shear velocity heterogeneity in the mantle to ss s400s ss s670s and s400s s670s traveltimes we analyse stacks of spectral element method waveforms computed for 34 shallow earthquakes and 16 020 globally distributed virtual seismometers the waveforms are computed for three versions of shear velocity model s20rts with variable strengths of the volumetric shear velocity perturbations within the layered structure of the preliminary reference earth model we find that ray theoretical corrections account for only 50 per cent of the traveltime variation due to large scale velocity heterogeneity for current tomographic models as represented by s20rts this translates into unpredictable long and short wavelength errors in maps of mantle discontinuity depths of about 5 km this amounts to roughly 10 per cent of published variations in mantle discontinuity depths however relative errors will be significantly larger if ray theoretical traveltime corrections are based on forthcoming models of shear velocity with much stronger heterogeneity at the smallest scales

gravitational viscoelastic postseismic relaxation on a layered spherical earth viscoelastic relaxation of a ductile asthenosphere underlying a purely elastic plate is a strong candidate process for explaining anomalous rates of crustal deformation observed following large earthquakes the nongravitational treatment of pollitz 1992 which is valid on a global scale and includes the effects of compressibility is here extended to permit the calculation of gravitational viscoelastic relaxation for a specified spherically layered viscoelastic rheology following an earthquake in an elastic layer the simple approximations we adopt make the resulting treatment particularly suitable for near field calculations for an asthenosphere with a maxwell rheology the effect of gravitational coupling is manifested only many relaxation times after the earthquake as obtained by previous investigators its effect is generally to speed up the long wavelength component of the relaxation process and attenuate the overall vertical displacement pattern several subtle features common to the relaxation behavior from several different fault types thrust rift and strike slip are identified the effect of gravitational coupling on horizontal displacements is consistent with flexure of the upper elastic plate driven by the corresponding effects on the vertical displacement stress diffusion away from the source region generally exhibits pulse like behavior which is dispersive in both space and time if the asthenosphere is confined to a relatively narrow channel then the dispersion branches governing relaxation are radically altered and stress diffusion effects far from the coseismic rupture zone exhibit a complicated time dependence reflecting the competing tendencies of toroidal and spheroidal mode relaxation

global scale analysis of the mantle pds phases a global scale analysis of travel times of the mantle pds phases converted from p to s from the discontinuity at a depth d beneath the receiver is presented most of the data are related to p 410 s and p 660 s recorded by the broadband stations of geoscope incorporated research institutions for seismology iris and other networks in order to provide a good global scale coverage published measurements at 37 stations are complemented by our measurements at 45 stations lateral resolution of our data is many times higher than that of ss and its precursors previously used for similar purposes but we have fewer data in the oceans lateral variations of travel times of the major pds phases are strongly correlated with the lateral p and s velocity variations in the uppermost mantle as revealed by tomographic studies the scaling coefficient between the teleseismic s and p residuals as found from the variations of travel times of the major pds phases is around 3 7 for north america and around 3 3 for the rest of the world almost all the differential travel times t p 660 s t p 410 s are in the range of 1 s relative to the average value of 24 0 s if the lateral variations of the velocities within the mantle transition zone mtz can be neglected the corresponding variations of thickness of the mtz are in the range of 10 km relative to the standard depth of 250 km and do not show significant correlation with either velocities in the upper mantle above the mtz or with the variations of thickness of the mtz as found in some recent studies of precursors to ss there is a weak but marginally significant positive correlation between the variations of thickness of the mtz inferred from our data and the s velocity variations in the mtz as given by su et al 1994 this correlation suggests that both variables contain temperature dependent components lateral variability of the mtz is also manifested by extreme weakness of p 410 s at several locations this may indicate either anomalous topography on 410 km discontinuity or strong broadening of the otherwise sharp discontinuity at some stations there are arrivals that can be interpreted as p 520 s this interpretation implies that the strengh and or sharpness of 520 km discontinuity as well as its depth are laterally variable in a broad range

a reevaluation of the isostatic rebound of lake bonneville recent data pertaining to the uplift of the provo and bonneville shorelines of the late pleistocene lake bonneville are evaluated in a reanalysis using a rigorous model of deformation of a plane earth comprising an elastic layer overlying a viscoelastic channel the new observations constrain the wavelength and amplitude of the uplift of two shorelines formed at two different epochs and they partly resolve the ambiguities in the plausible mechanical parameters found in previous solutions the results indicate that the effective thickness of the elastic lithosphere is 28 to 30 km equal to the seismically determined crustal thickness in the region the estimate of the upper mantle viscosity in the region varies from 1 5 3 4 lo19 pa s to 2 1 5 8 l018 pa s depending on whether the upper mantle is assumed to be of uniform viscosity or the flow is confined to a narrow channel in the asthenosphere the predicted rate of present uplift at the center of the desiccated lake is 0 06 mm yr and 0 03 mm yr for the shallow channel and uniform mantle models respectively these values are beyond the first order leveling accuracies and the question of the existence of a narrow channel in the region cannot be resolved with the available observations

the effect of topography of upper mantle discontinuities on ss precursors using the spectral element method we explored the effect of topography of upper mantle discontinuities on the traveltimes of ss precursors recorded on transverse component seismograms the latter are routinely used to infer the topography of mantle transition zone discontinuities the step from precursory traveltimes to topographic changes is mainly done using linearised ray theory or sometimes using finite frequency kernels we simulated exact seismograms in 1 d and 3 d elastic models of the mantle in a second simulation we added topography to the discontinuities we compared the waveforms obtained with and without topography by cross correlation of the ss precursors since we did not add noise the precursors are visible in individual seismograms without the need of stacking the resulting time anomalies were then converted into topographic variations and compared to the original topographic models based on the correlation between initial and inferred models and provided that ray coverage is good we found that linearised ray theory gives a relatively good idea on the location of the uplifts and depressions of the discontinuities it seriously underestimates the amplitude of the topographic variations by a factor ranging between 2 and 7 real data depend on the 3 d elastic structure and the topography all studies to date correct for the 3 d elastic effects assuming that the traveltimes can be linearly decomposed into a structure and a discontinuity part we found a strong non linearity in this decomposition which cannot be modelled without a fully non linear inversion for elastic structure and discontinuities simultaneously

fluids and trace element transport in subduction zones melt inclusion data from primitive arc basalts from mexico and kamchatka show clear positive correlations of fluid mobile element h 2 o ratios with the cl h 2 o ratio suggesting that the trace element content of subduction zone fluids is strongly enhanced by complexing with chloride this effect is observed for large ion lithophile lile elements e g rb and sr but also for the light rare earth elements ree e g la and ce as well as for u the correlations of these elements with cl h 2 o cannot be explained by the addition of sediment melts or slab melts to the mantle source since cl has no effect on the solubility or partitioning of these elements in silicate melt systems on the other hand the observed relationship of trace element abundance with cl is consistent with a large body of experimental data showing greatly enhanced partitioning into aqueous fluid upon addition of chloride accordingly it appears that a dilute cl bearing aqueous fluid is the main carrier of lile light ree and u from the slab to the source of melting in arcs moreover elevated ce h 2 o ratios clearly correlate with fluid salinity and therefore are not suitable as a slab geothermometer from a synopsis of experimental and melt inclusion data it is suggested that the importance of sediment or slab melting in the generation of arc magmas is likely overestimated while the effects of trace element scavenging from the mantle wedge may be underestimated moreover establishing reliable data sets for the fluid mineral partition coefficients of trace elements as a function of pressure temperature and salinity requires additional efforts since most of the commonly used experimental strategies have severe drawbacks and potential pitfalls

hot and deep rock record of subduction initiation and exhumation of high temperature high pressure metamorphic rocks feather river ultramafic belt california abstract studies of a 10 to 300 m thick unit of high grade metamorphic rock external schists that crops out along the western border of the feather river ultramafic belt frb northern california yield new insights into subduction initiation and ophiolite emplacement processes the high temperature t foliation of the external schists dip moderately to steeply eastward beneath the ultramafic rocks of the frb a 150 km long slab of suboceanic upper mantle and the high t fabric shows a tops to the west frb side up sense of shear the structurally highest external schists record peak metamorphic conditions of 650 760 c at 1 3 2 2 gpa in contrast sheeted dikes of the devil s gate ophiolite that overlie the ultramafic rocks yield metamorphic conditions of 710 730 c at about 0 3 0 7 gpa a km scale lens of amphibolite within ultramafic rocks yields somewhat lower pressures than the structurally highest external schist as do the structurally lower rocks within the external schists significant exhumation of the external schists relative to the structurally overlying ophiolitic rocks occurred along at least two major zones and the most significant exhumation was accommodated at least 1 5 km structurally above the ultramafic external schist contact based on available geochronology intraoceanic subduction may have initiated at approximately 240 ma and exposure of the external schist occurred prior to the deposition of rocks in the structurally highest part of the calaveras complex minimum 177 ma a subduction complex that structurally underlies the external schists high t metamorphism of the devil s gate ophiolite may have resulted from partial failed ridge subduction

pattern and rates of faulting in the central nevada seismic belt and paleoseismic evidence for prior beltlike behavior the central nevada seismic belt cnsb is a concentration of historical 1915 1932 1954 surface faulting in the western basin and range province forming a linear nearly continuous 300 km long rupture zone previous results are integrated in this study with new paleoseismic and exploratory trenching data from the historical zones to look for evidence of older similar beltlike patterns or elevated slip rates that could indicate whether the cnsb is a zone of focused long term crustal strain the data show that the continuous rupture belt produced by the seven earthquakes occurring between 1915 and 1954 is unique in the available paleoseismic record at the 1954 fairview peak fault the lack of prehistorical faulting in deposits containing the wilson creek bed 19 tephra eliminates the possibility of an identical seismic belt in the past 35 4 ka our studies also show that the faults have net slip rates ranging from a low of 0 09 mm yr on the fairview peak fault to a high of 0 7 mm yr on the 1932 cedar mountain fault these are considered moderate to low rates that are similar to most late quaternary faults in the western basin and range province a space time comparison shows that the paleoseismic histories for these multiple rupture zones are diverse and the number and timing of events in each of the zones indicate that there is little evidence for older contemporaneous ruptures of these same faults based on these results we reach several conclusions regarding the longer term holocene behavior of the cnsb although paleoseismic data preclude an older identical rupture belt among the historical zones consideration of associated holocene faults within the greater cnsb region indicates that several similar but not identical beltlike rupture patterns are plausible during the past 13 ka although each requires seismic gaps in the along strike pattern although long term strain represented by density of young faults does appear to increase from east to west into the cnsb the slip rate data demonstrate that the cnsb is not a belt of concentrated or elevated crustal strain compared with areas that extend west to the sierra nevada the increase in the distribution of holocene fault activity from east to west into the cnsb is consistent with a marked increase in the 1992 2002 gps velocity field at the latitude of the 1954 rupture sequence however a comparison of the geologic rates across the belt at this same latitude indicates that the extension rates 0 59 1 37 mm yr are systematically lower than both the campaign and continuous gps rates 2 20 3 13 mm yr by factors of 2 5 these discrepancies may be due to postseismic strain or to some form of off fault deformation we conclude that the results of our study of fault behavior in the cnsb best support the belt migration model proposed by wallace 1987 for the western basin and range province in which temporal tectonic pulses are believed to migrate regionally activating different beltlike combinations of late quaternary faults in an as yet unknown pattern of migration

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

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

mantle transition zone structure along a profile in the sw pacific thermal and compositional variations summary events from the south pacific recorded in the tien shan region are studied with a migration method to measure discontinuity depth along a profile between the mariana and molucca sea subduction zones deflections of the upper mantle discontinuities within and between the subduction zones are investigated using pp precursors filtered to periods of 3 to 10 s we find p wave reflections near the 410 520 and 660 km discontinuities the 410 km discontinuity is elevated in the subduction region and near the predicted depth 410 km in the average velocity region in some cases we find negative polarities of the reflection from the 410 km discontinuity which may indicate water induced melting above the olivine to wadsleyite transition the 520 km discontinuity is shallow between the two subduction zones and deepens towards the mariana backarc region this discontinuity topography may be due to detection of both the wadsleyite to ringwoodite transition in a depleted downwelling elevated region and the formation of calcium perovskite in a more fertile upwelling depressed region the 660 km discontinuity is split in the halmahera slab with one reflection each near 600 and 700 km depth consistent with a cold sinking slab and detection of phase transitions for both garnet and olivine to perovskite respectively in the region between the subduction areas we find a downward deflection of the 660 km discontinuity with short length scale 100 300 km undulations that may reflect compositional variation

sensitivity analysis of seismic waveforms to upper mantle discontinuities using the adjoint method using spectral element simulations of wave propagation we investigated the sensitivity of seismic waveforms recorded on transverse components to upper mantle discontinuities in 1 d and 3 d background models these sensitivity kernels or frechet derivatives illustrate the spatial sensitivity to model parameters of which those for shear wave speed and the surface topography of internal boundaries are discussed in this paper we focus on the boundaries at 400 and 670 km depth of the mantle transition zone ss precursors have frequently been used to infer the topography of upper mantle discontinuities these seismic phases are underside reflections off these boundaries and are usually analysed in the distance range of 110 160 this distance range is chosen to minimize the interference from other waves we show sensitivity kernels for consecutive time windows at three characteristic epicentral distances within the 110 160 range the sensitivity kernels are computed with the adjoint method using synthetic data from our simulations we can draw three main conclusions i the exact frechet derivatives show that in all time windows and also in those centred on the ss precursors there is interference from other waves this explains the difficulty reported in the literature to correct for 3 d shear wave speed perturbations even if the 3 d structure is perfectly known ii all studies attempting to map the topography of the 400 and 670 km discontinuities to date assume that the traveltimes of ss precursors can be linearly decomposed into a 3 d elastic structure and a topography part we recently showed that such a linear decomposition is not possible for ss precursors and the sensitivity kernels presented in this paper explain why iii in agreement with previous work we show that other parts of the seismograms have greater sensitivity to upper mantle discontinuities than ss precursors especially multiply bouncing s waves exploiting the s wave triplications due to the mantle transition zone these phases can potentially improve the inference of global topographic variations of the upper mantle discontinuities in the context of full waveform inversion in a joint inversion for an elastic parameters and topography

comparing ray theoretical and finite frequency teleseismic traveltimes implications for constraining the ratio of s wave to p wave velocity variations in the lower mantle

lithospheric dismemberment and magmatic processes of the great basin colorado plateau transition utah implied from magnetotellurics 1 to illuminate rifting processes across the transition zone between the extensional great basin and stable colorado plateau interior we collected an east west profile of 117 wideband and 30 long period magnetotelluric mt soundings along latitude 38 5 n from southeastern nevada across utah to the colorado border regularized two dimensional inversion shows a strong lower crustal conductor below the great basin and its transition zone in the 15 35 km depth range interpreted as reflecting modern basaltic underplating hybridization and hydrothermal fluid release this structure explains most of the geomagnetic variation anomaly in the region first measured in the late 1960s hence the transition zone while historically included with the colorado plateau physiographically possesses a deep thermal regime and tectonic activity like that of the great basin the deep crustal conductor is consistent with a rheological profile of a brittle upper crust over a weak lower crust in turn on a stronger upper mantle jelly sandwich model under the incipiently faulted transition zone the conductor implies a vertically nonuniform mode of extension resembling early stages of continental margin formation colorado plateau lithosphere begins sharply below the western boundary of capitol reef national park as a resistive keel in the deep crust and upper mantle with only a thin and weak moho level crustal conductor near 45 km depth several narrow steep conductors connect conductive lower crust with major surface faulting some including modern geothermal systems and in the context of other great basin mt surveying suggest connections between deep magma sourced fluids and the upper crustal meteoric regime the mt data also suggest anisotropically interconnected melt over a broad zone in the upper mantle of the eastern great basin which has supplied magma to the lower crust consistent with extensional mantle melting models and local shear wave splitting observations we support a hypothesis that the transition zone location and geometry ultimately reflect the middle proterozoic suturing between the stronger yavapai lithosphere to the east and the somewhat weaker mojave terrane to the west we conclude that strength heterogeneity is the primary control on locus of deformation across the transition zone with modulating force components

development of concepts concerning the genesis and emplacement of tethyan ophiolites in the eastern mediterranean and oman regions this article uses evidence mainly from the eastern mediterranean and oman to help understand the origin and emplacement of ophiolites field studies in these regions emphasize the diversity of ophiolite related sedimentary and tectonic processes topics to be discussed here include the sedimentary covers of ophiolites their metamorphic soles related melanges and emplaced continental margin slope rise units modes of ophiolite emplacement will also be considered and tectonic reconstructions that have a direct bearing on the origin and emplacement of ophiolites the writer argues that there is compelling evidence that many ophiolites formed by spreading above intraoceanic subduction zones evidence from their intact sedimentary covers however indicates that such subduction was only incipient in most cases as directly associated volcanic arcs or back arc basins are rare metamorphic soles record fragments of oceanic lithosphere derived from settings ranging from open ocean to continental slope rise settings underlying melanges record the subduction of most of the ocean lithosphere formed at spreading ridges continental margin units beneath ophiolites demonstrate that these ophiolites were formed in ocean basins that were mainly initiated by continental rifting in the eastern mediterranean and oman regions ophiolite emplacement was commonly achieved by trench margin collision although strike slip transpression and large scale accretion along subducting margins played a role in some cases taking account of the mesozoic palaeotectonic setting of the eastern mediterranean region it is widely believed that ophiolites of two main ages mid jurassic and late cretaceous were formed in several small oceanic basins separated by rifted microcontinents the history of tethys in pre mesozoic time remains more open to question lastly giant subduction related ophiolites as in oman cyprus or the balkans may not be forming anywhere in the world ocean today but may instead relate to rare events in earth history when regional to global plate boundaries were drastically reorganised

upper mantle beneath southeast asia from s velocity tomography 1 we present a three dimensional s velocity model of the se asian western pacific upper mantle with 400 km lateral resolution using the novel automated multimode inversion technique we processed 4038 vertical component seismograms and extracted 22 708 linear equations with uncorrelated uncertainties that constrain upper mantle structure we used time frequency windows to select signal with negligible proportion of scattered energy the windows included the fundamental rayleigh mode and s and multiple s waves the observed range of s velocity variations is the widest 17 18 in the upper 150 km of the mantle high velocity continental roots can reach beyond the present extent of the overlying archean proterozoic crust by 500 km beneath some precambrian units the roots are absent which can be attributed to deformation and gradual destruction of the ancient lithosphere at 120 150 km s velocity beneath some cratons reaches 4 8 km s this can be accounted for by thermal and compositional effects beneath the hainan island area a low velocity anomaly is observed from near the surface to the bottom of our model the hot spot type volcanism here may be caused by the deep mantle hainan plume a low velocity mantle domain underlies the south central sea of japan surrounded on the surface by intraplate volcanoes a deep seismicity gap is present near 40 n in the pacific slab subducting below and may result from a plume slab interaction a high velocity anomaly is present in the transition zone beneath the northern boundary of the yangtze craton we propose that the anomaly corresponds to subducted continental lithosphere stagnant atop the 660 km discontinuity

tectonic accretion and underplating of mafic terranes in the late eocene intraoceanic fore arc of new caledonia southwest pacific geodynamic implications this paper deals with the tectonic events that result in the accretion of mafic terranes in the fore arc region and a close juxtaposition of ultramafic rocks low grade and high grade mafic terranes in many collisional orogens the example is taken from new caledonia where tectonic accretion subduction underplating and obduction of mafic terranes took place during the late eocene in an intra oceanic forearc setting the late eocene tectonic complex comprised three major terranes an overlying ultramafic mainly harzburgitic allochthon named the ophiolitic nappe an intermediate mafic mainly basaltic off scraped melange composed of kilometre scale slices of oceanic upper crust called the poya terrane parts of which have been metamorphosed into an eclogite blueschist facies complex the pouebo terrane and a lower continental basement formed by the norkolk ridge terranes based upon exhaustive sampling of the mafic terranes and field surveys our tectonic micropaleontologic and geochemical data reveal that poya and pouebo terranes rocks originally formed within one single campanian to late paleocene oceanic basin floored by tholeiitic basalt associated with some minor seamount related intraplate alkali basalt the tholeiitic basalt displays a continuous range of compositions spanning between undepleted and depleted end members the former being volumetrically predominant the overall geochemical and isotopic features indicate an origin from a prominently heterogeneous mantle source during the opening of a marginal basin the south loyalty basin which almost completely disappeared during eocene convergence the opening of this basin originally located to the east of the norfolk ridge was synchronous with that of tasman sea basin as a consequence of oceanward migration of the west dipping pacific subduction zone establishing the origin of the ultramafic ophiolitic nappe is beyond the scope of this paper however it appears to be genetically unrelated to the mafic poya and pouebo terranes although it was located in the late eocene fore arc the ophiolitic nappe and the corresponding oceanic lithosphere originated before the late cretaceous to the east of the south loyalty basin in a back arc setting or alternatively in a much older trapped basin for reasons that remain unclear a new east dipping subduction started in the eocene and consumed most of the south loyalty basin forming the intra oceanic loyalty arc due to a changing subduction regime underplating of the diahot terrane the mafic slices that now form the poya terrane were tectonically accreted in the loyalty fore arc region and remained under low pressure low temperature conditions possibly at the subsurface until the norfolk ridge reached the subduction zone diachronously this resulted in the final obduction of the fore arc area the two step obduction involved first the mafic complex forming the poya terrane and thereafter the lithospheric mantle that now forms the ophiolitic nappe in contrast pieces of the accretionnary complex were dragged down into the subduction zone underplated at depth ca 70 km and metamorphosed into high temperature eclogite to form the pouebo terrane metamorphics that display the same geochemical features as the poya terrane basalt a mid to late eocene syntectonic piggy back sedimentary basin the nepoui flysch basin mainly filled with mafic clastic material and shallow water carbonates that record the progressive uplift of the fore arc region due to the accretion and underplating of mafic ocean related and other material in contrast a slightly younger foreland basin located upon the norfolk ridge the priabonian bourail flysch basin received a massive input of detrital material derived from the norfolk ridge itself and a time increasing amount of mafic poya derived material that recorded the first step of obduction thereafter the bourail flysch was overthrust by the poya terrane and finally by the ophiolitic nappe at the same time buoyancy driven uplift and exhumation of the high pressure metamorphic complex occurred as a consequence of the diachronous blocking of the subduction zone finally for a short time a new subduction started along the west coast of new caledonia and generated small amounts of active margin related magmas these events have resulted in the close juxtaposition of unmetamorphosed and highly metamorphosed mafic and ultramafic terranes that may represent a good pre collision analogue of mafic ultramafic belts found in many collisional mountain ranges

postseismic mantle relaxation in the central nevada seismic belt holocene acceleration of deformation and postseismic relaxation are two hypotheses to explain the present day deformation in the central nevada seismic belt cnsb discriminating between these two mechanisms is critical for understanding the dynamics and seismic potential of the basin and range province interferometric synthetic aperture radar detected a broad area of uplift 2 to 3 millimeters per year that can be explained by postseismic mantle relaxation after a sequence of large crustal earthquakes from 1915 to 1954 the results lead to a broad agreement between geologic and geodetic strain indicators and support a model of a rigid basin and range between the cnsb and the wasatch fault

genesis of primitive arc type basalt constraints from re os and cl on the depth of melting and role of fluids the general model for arc type basalt genesis is that subducted plates reach a certain depth and release their fluids in a series of dehydration reactions the fluid migrates into the overlying mantle wedge and initiatesmelting high 1 8 7 os 1 8 8 os ratios in some arc magmas have been attributed to this fluid although this interpretation follows logically from the general model for arc type basalt genesis it is not supported by experimental data or by re os or cl data for arc type basalt eclogites and peridotites our new data for basaltic rocks from the mexican volcanic belt together with data from several other arcs lead us to propose that subduction fluids contain only minimal re os and cl and therefore that 1 8 7 os 1 8 8 os is not increased significantly in the melts above 0 145 the observed low re concentrations are best explained by initiation of arc basalt melting in the garnet stability field retention of re by garnet in the source will deplete arc type basalts in re relative to oceanic island basalt and mid ocean ridge basalt radiogenic os measured in many arc type basalts is most likely due to crustal interaction rather than a subduction fluid with high and radiogenic os concentrations

earliest eocene 53 ma convergence in the southwest pacific evidence from pre obduction dikes in the ophiolite of new caledonia uncertainty about the timing and location of the initiation of convergence in the western and south western pacific greatly hinders accurate plate tectonic reconstructions of subduction systems in that area the chemistry and age of dikes intruding mantle peridotite in the ophiolite of new caledonia infer that subduction related magmatism began before 53 ma these new results infer that obduction in the south west pacific is unrelated to the reorientation of the pacific plate motion that occurred c 43 ma and confirm new interpretations showing that changes in mantle flow hotspot and plate motion may have occurred as soon as late paleocene or early eocene

intraplate deformation and microplate tectonics of the yellowstone hot spot and surrounding western u s interior 1 contemporary deformation of the yellowstone hot spot and surrounding western united states is analyzed using tectonic microplate modeling employing constraints from gps observations corrected for postseismic deformation of m7 earthquakes fault slip rates and earthquake focal mechanisms we focus primarily on the kinematics of the yellowstone hot spot and the eastern snake river plain volcanic field esrp and secondarily on basin range and columbia plateau provinces our results reveal southwest motion of the yellowstone plateau excluding localized volcanic deformation at 0 9 0 1 mm a that decreases to 0 8 0 1 mm a in the esrp block the southwest to west motion of the yellowstone esrp introduces shear in the northern rocky mountain block which is translating east at 0 78 0 08 mm a there is 0 5 mm a differential motion between the esrp and the block at its northern boundary and none at the southern boundary the eastern basin range block moves west at 3 0 0 1 to 4 6 0 1 mm a while velocities of the western basin range microplates rotate to a northwest direction accompanied by a transition from normal to oblique shear deformation columbia plateau block velocities are notably eastward an effect enhanced by postseismic relaxation following the m9 1700 cascadia subduction zone paleoearthquake this study reveals that the overall motion of the western united states is characterized by clockwise rotation with westward extension and northwest shear in the basin range province northeast to east contraction in the columbia plateau and southwest extension of the yellowstone snake river plain block that is driven by the high gravitational potential of the yellowstone swell

gross errors in upper mantle discontinuity topography from underside reflection data summary currnet models of upper mantle discontinuity topography derived from traveltime variations of long period underside reflections pdp or sds waves may contain gross errors due to the use of geometrical optics in relating observed traveltimes to depth variations of the discontinuity near the reflection point the use of geometrical optics is not valid if variations in the depth of the discontinuities exist on a lateral scale smaller than the size of the fresnel zone in the data geometrical optics does not take into account the large size and the complex structure of the traveltime surface of underside reflections and may introduce spurious structure when used in inversions examples of synthetic long period pdp waveforms for reflections near subducting lithosphere show that scattering from expected small scale depth variations of the discontinuities inside slabs causes complex waveform variations for a 15 20s pdp wave these small scale depth variations with a scale length of 4 are smaller than the fresnel zone roughly 20 but larger than the pdp wavelength at 420 km depth about 150 200 km the synthetic waveforms are processed to obtain pdp traveltimes which are in turn converted to apparent discontinuity depth at the reflection point using geometrical optics the apparent discontinuity depth variations are not only of the same order of magnitude as those obtained from real data but also exhibit artefacts that closely resemble features observed in current models it is shown that large scale wavelengths of the order of 1000 km downwarping of the 670 km discontinuity near subduction zones derived from underside reflection data can be explained as the long wavelength manifestation of a single small scale wavelengths of several hundreds of kilometres deflection of the discontinuity inside and near subducting slabs

rapid emplacement of the oman ophiolite thermal and geochronologic constraints in understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting and the rate and duration of that thrusting hornblende 4oar 39ar ages demonstrate that the igneous oceanic crust in oman crystallized and cooled to 825 k in 1 2 m y hornblende ages from metamorphic rocks and from cross cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below 825 k in 1 2 m y motion along the sole thrust accounted for 200 km of displacement at a rate of 100 200 mm yr on the basis of age relationships and thermal considerations we favor a two stage model for the initial stages of samall ophiolite emplacement intraoceanic thrusting over 2 m y old lithosphere at 150 km m y parallel to a spreading ridge for 1 2 m y followed by equally rapid and brief thrusting over cold and old lithosphere preservation of the samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting