the mechanics of continental extension in western north america implications for the magmatic and structural evolution of the great basin abstract a finite element model of continental extension within a rheologically stratified lithosphere is used to examine the structural and magmatic consequences of extensional collapse of thickened crust within the early tertiary north american cordillera crustal thickening creates a weakness in the upper mantle in the model which focuses strain within the great basin during extension marginal highlands corresponding to the sierra nevada and colorado plateau develop near the edges of the weakened region where abrupt changes in the strength of the lithosphere create large gradients in the amount of strain the great thickness of the crust results in widespread ductile flow in the lower crust within the interior of the great basin favoring the formation of metamorphic core complexes during the early stages of extension ductile flow becomes inhibited as the crust thins and cools possibly contributing to the change in the style of extension from low angle detachment faulting to high angle normal faulting during the middle miocene as the lithosphere cools the strength of the uppermost mantle increases after about 10 m y the upper mantle becomes stronger beneath the great basin than beneath adjacent unextended regions strain then begins to migrate outward from the interior of the great basin onto the marginal highlands and their interior edges begin to collapse and are incorporated into the widening great basin after 40 m y of extension the highest rates of strain are localized at the edges of the great basin in positions corresponding to the walker lane tectonic belt of western nevada and the colorado plateau transition zone the weight of the marginal highlands in the model is partially supported by flexure of the strong layer in the uppermost mantle portions of the uppermost mantle which have been flexed upward cool more rapidly than deeper portions creating periodical variations in the strength of the lithosphere which eventually develop into crustal and lithospheric boudinage with a wavelength similar to that observed in regional gravity studies rapid thinning of the model lithosphere during the first 10 m y of extension results in nearly adiabatic decompression of the lithospheric mantle partial melting of the lower lithospheric mantle is likely during this period if volatile phases or mafic lithologies were present this mechanism may account for widespread silicic magmatism in the great basin during the middle tertiary as the region undergoing extension widens the rate of lithospheric thinning decreases and the mantle beneath the great basin begins to cool this leads to a cessation of melting within the lithosphere and waning of silicic volcanism 15 20 m y after the onset of extension the appearance of predominantly basaltic and bimodal volcanic suites in the great basin during the last 5 10 ma correlates well with the onset of decompression melting in the asthenosphere if the mantle potential temperature is greater than about 1350 c approximately 70 c warmer than typical mantle potential temperature

biogenic silica record of the lake baikal response to climatic forcing during the brunhes abstract this work presents a detailed orbitally tuned biogenic silica record of continental paleoclimate change during the brunhes chron the brunhes matuyama boundary lies within the warm isotopic stage 19 in baikal and the boundaries between eight lithological cycles correspond to terminations in the marine oxygen isotope record the high amplitude and resolution of climatically driven changes in biosi content in lake baikal sediments permits tuning of almost every precessional cycle during the brunhes and reveals the structure of interglacial stages for example the last three interglacial stages mis 5 7 and 9 clearly consist of five substages a b c d e corresponding to precessional insolation peaks abrupt and intense regional glaciations in siberia during substages 5d and 7d were driven by extreme insolation minima during substage 9d cooling was more gradual in response to more moderate forcing the impact of strong glaciation is also observed in the middle of stage 15 where full glacial conditions appear to have lasted for over 30 000 yr during substages 15d 15c and 15b marine oxygen isotopic stage 11 appears to be the warmest period during the brunhes in the lake baikal record with at least three substages a new hypothesis is presented regarding the response of the lake baikal biosi record to insolation forcing based on the mechanism controlling modern diatom blooms biogenic silica production is hypothesized to be dependent on changes in the heat balance of the lake and consequently on changes in the thermal structure of the water column this mechanism is also sensitive to short term sub milankovich cooling events such as the mid eemian cooling the montaigu event during substage 5c and a cooling which appears to be analogous to the montaigu event during substage 9c the continuity of the lake baikal paleoclimate record its sensitivity to orbital forcing and its high resolution make it an excellent candidate for a new paleoclimatic stratotype section for continental asia

synthesis of realistic oceanic pn wave trains the synthesis of realistic oceanic pn phases was accomplished using wave number integration wave trains with durations of the order of 100 s following the initial compressional wave arrival were generated for a laterally homogeneous vertically inhomogeneous oceanic lithosphere model the excitation of these long wave trains is primarily a result of near receiver water and sediment reverberations and does not involve scattering a late arriving wave train with a group velocity of 1 6 km s having an emergent onset and substantial duration characteristic of oceanic abyssal t phases was also generated even with phase velocities restricted to be greater than 5 5 km s the synthetic pn wave train was compared to data collected in the southwest pacific during the ngendei seismic experiment many of the spectral characteristics associated with sediment and water reverberation observed in the synthetics were also observed in the data first it was found that the amplitude falloff rate of the pn wave train is a strongly increasing function of frequency in direct contrast to results predicted for forward scattered coda this leads to a predominance of low frequencies late in the wave train which is easily interpreted in terms of the constructive interference of different modes of reverberation second the presence of spectral peaks at frequencies associated with both sediment and water reverberations was observed the predicted frequencies were computed using values of the water depth sediment thickness and sediment velocity obtained in an independent study strong alignment of the spectral peaks with the predicted frequencies illustrates the reverberating nature of the pn wave train the presence of substantial power in excess of 15 hz can be attributed to the confinement of pn to the oceanic lithosphere the oceanic pn phase is thus characterized as a refraction from the lower lithosphere with multiple reverberations in the oceanic water column and sediment layer comprising the subsequent coda complicated oceanic lithosphere models are not needed to explain the gross characteristics of pn propagation

crustal and upper mantle structure of stable continental regions in north america and northern europe from an analysis of many seismic profiles across the stable continental regions of north america and northern europe the crustal and upper mantle velocity structure is determined analysis procedures include ray theory calculations and synthetic seismograms computed using reflectivity techniques the p wave velocity structure beneath the canadian shield is virtually identical to that beneath the baltic shield to a depth of at least 800 km two major layers with a total thickness of about 42 km characterize the crust of these shield regions features of the upper mantle of these regions include velocity discontinuities at depths of about 74 km 330 km 430 km and 700 km a 13 km thick p wave low velocity channel beginning at a depth of about 94 km is also present

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

the p wave velocity of the uppermost mantle of the rio grande rift region of north central new mexico a network of seismograph stations has operated in north central new mexico since 1975 the network is approximtely 200 by 300 km in size and encompasses the rio grande rift there several seismic refraction experiments have been reported in the literature for the region of the network and adjacent areas because all of the seismic refraction lines are unreversed p sub n velocities reported were mainly of the inverse travel time slope for the direction of the corresponding line the values of the inverse slope for those studies range from 7 6 to 8 2 km s the purpose of our study is to estimate the p wave velocity of the uppermost mantle by using the time term method first we timed the p sub n waves of strong signals from five explosions and eight shallow earthquakes recorded by the network the main data set which contains 87 time distance pairs was processed by using the time term method the p sub n velocity estimated by this method is 8 0 0 1 km s to corroborate this estimate we then processed 10 subsets of the main data set in the same way almost allof the solutions show velocities 7 9 8 1 km s in agreement with the velocity determined for the mainmore data set the station time terms of the main data set also are substantied and they suggest that the base of the crust dips northward by a few degrees in the region of the survey the smallest value reported by other investigators for the inverse slope 7 6 km s appears to be related to the dip the normal p wave velocity of the uppermost mantle of north central new mexico places restrictions on thermal models of the rift for instance the results exlude the likelihood of a wide zone of asthenosphere at the base of the crust beneath the rift but they do not exclude a narrow such zone less

lithospheric extension near lake mead nevada a model for ductile flow in the lower crust small variations in gravity anomalies and topographic elevation observed in areas that have undergone highly variable amounts of upper crustal thinning can be satisfactorily explained by ductile flow of lower crustal material under the proper conditions in this study we examine the boundary between the unextended colorado plateau and a strongly extended domain in the basin and range province in the lake mead nevada region bouguer gravity and topography data suggest that both present and preextensional variations in crustal thickness between the unextended and extended regions are small analytic channel flow models show that viscosities required for ductile flow in a lower crustal channel to reduce discontinuities in crustal thickness associated with variable amounts of extension are highly dependent on the channel thickness and on the length scale of flow required finite element modeling of newtonian flow and power law creep shows that flow over the length scale of the eastern basin and range 500 km or more corresponding to upper crustal extension by a factor of 1 4 3 over 10 m y requires effective viscosities less than 10 18 10 20 pa s for ductile channels 10 25 km thick flow over shorter length scales 150 km may be accommodated with effective viscosities as high as 10 21 pa s modeling suggests that these effective viscosities may be sustained by lower crustal material deforming at laboratory derived power law creep rates the longer scale flow may require elevated crustal temperatures more than 700 c depending on the composition and material properties assumed under the boundary conditions assumed in this study the linear viscous flow models yield a satisfactory approximation to deformation by power law creep this work suggests that flow in the lower crust may be a viable mechanism for producing small variations in total crustal thickness between strongly extended and less extended regions and thereby explaining the relative uniformity in gravity and topography between such regions

atmospheric methane during the last four glacial interglacial cycles rapid changes and their link with antarctic temperature 1 atmospheric methane ch4 recorded in antarctic ice cores represents the closest ice proxy available for greenland temperature changes beyond times when greenland climate records are available the record over four climatic cycles from the vostok ice core offers the opportunity to study the phase relationship between greenland and antarctic climate changes through detailed ch4 profiles combining american and french analytical efforts we have improved the time resolution of the existing ch4 record from petit et al 1999 spectral analyses reveal high and low frequency variability including a strong precessional component the phase relationship between ch4 and the antarctic temperature proxy d shows a systematic lag of ch4 versus temperature by 1100 200 years on long timescales 50 400 kyr and a more complex behavior over shorter timescales i e 25 kyr suggesting that dansgaard oeschger type of climatic variability and associated interhemispheric linkage are robust features of late quaternary climate

crustal thickness of iran inferred from converted waves the iranian plate is part of the alpine himalayan orogenic belt which has been formed by the continental collision between the arabian and eurasian plates the present day iranian plate is characterized by diverse tectonic domains including mountain belts e g zagros and alborz kopeh dagh and oceanic plate subduction e g makran here we present the lateral variations of the moho discontinuity beneath iran using a detailed p receiver function study our results allow for more precise estimations of the crustal thickness and enable us to provide a detailed moho depth map for all of iran for the first time we used the teleseismic events recorded from 1995 to 2011 at 77 national permanent stations 24 broadband and 53 short period stations our results show significant variations in the crustal thickness which are related to the different geological features within iran in general the average crustal thickness beneath iran is about 40 45 km a relatively thick crust of about 54 2 km due to the shortening is observed beneath the alborz mountain ranges the crust beneath the alborz zone shows a thickness changing from 47 2 to 45 2 km from west to east and reaches a thickness of about 50 2 km beneath the kopeh dagh mountain range we find the thinnest crust of about 33 2 km beneath the makran subduction zone in southeast iran showing a normal continental crust which has not been influenced by collisional processes the thickest crust 66 2 km is locally observed beneath the sanandaj sirjan zone which is considered the suture zone of the collision between the arabian and eurasian plates

models of the upper mantle beneath the central north island new zealand from speeds and anisotropy of subhorizontal p waves pn 1 the central north island of new zealand shows significant variations in pn wave speeds over small distances ranging from 8 5 0 2 km s to 7 4 0 1 km s over a distance of 150 km a combination of national network seismometers local volcanic seismic monitoring networks and temporary deployments are used to collect arrival times from local events during the period of 1990 2006 the data set consists of approximately 11 200 pn observations from 3000 local earthquakes at 91 seismograph sites we have created a method that allows us to model the predominant wavelength features of p wave speeds in the uppermost mantle as well as estimating values of mantle anisotropy and irregularities in the crust beneath stations using least squares collocation the resulting model shows distinct variations in uppermost mantle pn velocities velocities of less than 7 5 km s are found beneath the back arc extension region of the central volcanic region and under the taranaki volcanic region indicating the presence of water and partial melt the region to the east shows extremely high velocities of 8 3 8 5 km s where the p waves are traveling within the subducting pacific slab slightly lower than normal mantle velocities of 7 8 8 1 km s are found in the western north island suggesting a soft mantle pn anisotropy estimates throughout the north island show predominately trench parallel fast directions ceasing to nulls in the west anisotropy measurements indicate the strain history of the mantle null anisotropy measurements suggest an undisturbed mantle suggesting that mantle beneath the western north island is young

soil gas prospection of he 222rn and co2 vulcano porto area aeolian islands italy abstract in march 1994 soil gases were sampled in the area of vulcano porto on the island of vulcano using a grid of about 200 points km 2 analysed gases were co 2 he and 222 rn and over a smaller area h 2 s some of the samples were also analysed for the isotope composition of co 2 c three anomalous co 2 degassing areas were identified grotta dei palizzi the area near the telephone exchange and the area near the beach fumaroles the behaviour of he and 222 rn is different in these 3 areas the concentration of he is much lower than that of atmospheric he down to 3950 ppb in the isthmus and only in the area near grotta dei palizzi does it have values significantly higher than atmospheric ones up to 1900 ppb the activity of 222 rn always significantly positively related to co 2 concentrations peaked in the 2 fumarole areas isthmus telephone exchange

calculated temperatures in overthrust terrains and possible combinations of heat sources responsible for the tertiary granites in the greater himalaya there are three obvious sources of heat that affect the temperature in a region of thrust faulting heat supplied from below the lower thrust plate radiogenic heating within the crusts of both the upper and especially the lower plates and frictional heating along the fault we assume that heat is conducted primarily in the vertical direction and that solutions to the one dimensional time dependent heat conduction equation can be used to describe the temperature field in overthrust zones the effects of these three sources of heating can be isolated from one another treated separately and then combined in ratios proportional to various assumed rates of heating provided by each we then use calculations for a range of possible combinations of heat sources that may have caused two media granites of late tertiary age in the greater himalaya because of the uncertainties in the age of the granites in the depth at which they formed in the amounts of heat flux supplied to the base of the lithosphere and of radiogenic heat production in the crust inb the value of the coefficient of thermal conductivity and in other parameters a broad range of possible combinations of parameters is possible temperature can reach themore solidus for granite melts with various plausible combinations of parameters both that include no shear heating and that require frictional stresses of 100 s of mpa on the main central thrust for instance if the initial steady state geotherm in the crust now comprising the himalaya were that of a typical shield then frictional heating with a stress in excess of 100 mpa 1 kbar would be necessary to raise the temperature of material near a thrust fault at 20 or 30 km depth to 650 sup 0 c less

climate and vegetation changes 180 000 to 345 000 years ago recorded in a deep sea core off portugal a new high resolution combined marine proxy pollen sequence from the portuguese margin md01 2443 enables direct comparison of ocean and ice volume changes with vegetation development in south west iberia during marine isotope stages mis 7 8 and 9 changes in sea surface conditions on both orbital and millennial time scales are closely mirrored by forest extent indicating a coherence of climate variability between the north atlantic and south west iberia deglacial warming at the start of mis 7 and 9 was interrupted by incursions of subpolar water masses at the site accompanied by reversals in tree population expansion during mis 7e and 9e the close relationship in timing and amplitude between vegetation and offshore conditions broke down early forest collapse points to pronounced cooling and aridification on land which although not accompanied by a change in water masses at the portuguese margin appears to have coincided with abrupt decreases in atmospheric methane concentration recorded in antarctic ice cores the most extreme glacial conditions of mis 8 occurred during the early part followed by an interval of warmer conditions and tree population expansion similar to 263 ka this has sometimes been considered as indicating an early deglaciation but the data presented here show that it was distinct from termination iii which occurred later in line with milankovitch forcing a sub orbital oscillation in both isotopic and pollen records is observed at the end of mis 7 ca 184 ka which appears to be coeval with a recently identified sea level high stand c 2006 elsevier b v all rights reserved

crustal structure of the colorado plateau arizona application of new long offset seismic data analysis techniques the colorado plateau is a large crustal block in the southwestern united states that has been raised intact nearly 2 km above sea level since cretaceous marine sediments were deposited on its surface controversy exists concerning the thickness of the plateau crust and the source of its buoyancy interpretations of seismic data collected on the plateau vary as to whether the crust is closer to 40 or 50 km thick a thick crust could support the observed topography of the colorado plateau isostatically while a thinner crust would indicate the presence of an underlying low density mantle this paper reports results on long offset seismic data collected during the 1989 segment of the u s geological survey pacific to arizona crustal experiment that extended from the transition zone into the colorado plateau in northwest arizona we apply two new methods to analyze long offset data that employ finite difference travel time calculations 1 a first arrival time inverter to find upper crustal velocity structure and 2 a forward modeling technique that allows the direct use of the inverted upper crustal solution in modeling secondary reflected arrivals we find that the crustal thickness increases from 30 km beneath the metamorphic core complexes in the southern basin and range province to about 42 km beneath the northern transition zone and southern colorado plateau margin we observe some crustal thinning to 37 km thick and slightly higher lower crustal velocities farther inboard beneath the kaibab uplift on the north rim of the grand canyon the crust thickens to a maximum of 48 km we observe a nonuniform crustal thickness beneath the colorado plateau that varies by 15 and corresponds approximately to variations in topography with the thickest crust underlying the highest elevations crustal compositions as inferred from seismic velocities appear to be the same beneath the colorado plateau as those in the basin and range province to the southwest implying that the plateau crust represents an unextended version of the basin and range some of the variability in crustal structure appears to correspond to preserved lithospheric discontinuities that date back to the proterozoic era

vs and density structure beneath the colorado plateau constrained by gravity anomalies and joint inversions of receiver function and phase velocity data 1 the colorado plateau is a physiographic province in the western us with an average elevation of 1 9 km where in contrast to neighboring provinces there is little evidence of large scale tectonic deformation or magmatism recent availability of earthscope usarray seismic data allow us to better examine the crust and upper mantle structure beneath the region and test proposed explanations for the plateau s uplift and relative stability using phase velocities for fundamental mode rayleigh waves and p receiver functions we perform over 800 joint inversions for 1 d shear wave velocity vs profiles sampling the plateau and surrounding regions down to 150 km depth we image a sharp change in crustal thickness at the western edge of the colorado plateau with a more gradual increase eastward moving into the rocky mountains a relatively thick 100 km lithosphere beneath the plateau extends into the rocky mountains to the north we use empirical scaling relations to estimate densities from our vs results and predict the associated gravity anomalies which are inconsistent with the observed distribution of the bouguer gravity anomalies we somewhat reconcile the prediction and observations by assuming that lateral density variations below 50 km can be ignored and the lithospheric root is therefore neutrally buoyant while there is some evidence for small scale convection and lithospheric removal at its edges the shape of the lithospheric mantle anomaly is consistent with a large scale uplift of the plateau by heating since removal of the farallon slab we conclude that the lithospheric root is key to the long term stability of the colorado plateau leading to a colder stronger crust

a high resolution local network study of the nazca plate wadati benioff zone under western argentina this paper presents a precise determination of the fine structure of a wadati benioff zone based on analysis of analog and digital seismic data recorded by a local network in the san juan province of western argentina the network is located above one of the subhorizontal segments of the subducted nazca plate without the complicating effects of a dipping slab on the locations of earthquakes the nearly flat geometry of the subducted plate provides an excellent opportunity to determine the thickness and structure of the wadati benioff zone we found that the depth of the wadati benioff zone beneath the network is 107 5 km for a set of digitally recorded events the distance between the shallowest and the deepest event gives a thickness of 20km with 90 of the data concentrated in a zone 12 km thick the spatial distribution of hypocenters shows no evidence of complex internal structure such as a double wadati benioff zone the local network data also suggest that in the region near 32 s where the dip of the wadati benioff zone changes from flat to steep the nazca plate flexes rather than tears at depths less than at least 125 km the intermediate depth seismicity is separated from an active zone of crustal seismicity by an aseismic region between depths of about 40 and 95 km the upper surface of the subducted plate inferred to be at about 90 km provides an upper limit to the thickness of the south american plate in this region an apparent westward dip of the wadati benioff zone as determined by the local network data can be accounted for by a 6 westward dip of the moho beneath the san juan region a result that is in agreement with regional gravity and topographic data

high resolution receiver function imaging reveals colorado plateau lithospheric architecture and mantle supported topography 1 after maintaining elevations near sea level for over 500 million years the colorado plateau cp has a present average elevation of 2 km we compute new receiver function images from the first dense seismic transect to cross the plateau that reveal a central cp crustal thickness of 42 50 km thinning to 30 35 km at the cp margins isostatic calculations show that only approximately 20 of central cp elevations can be explained by thickened crust alone with the cp edges requiring nearly total mantle compensation we calculate an uplift budget showing that cp buoyancy arises from a combination of crustal thickening heating and alteration of the lithospheric root dynamic support from mantle upwelling and significant buoyant edge effects produced by small scale convecting asthenosphere at its margins

four climate cycles of recurring deep and surface water destabilizations on the iberian margin centennial climate variability over the last ice age exhibits clear bipolar behavior high resolution analyses of marine sediment cores from the iberian margin trace a number of associated changes simultaneously proxies of sea surface temperature and water mass distribution as well as relative biomarker content demonstrate that this typical north south coupling was pervasive for the cold phases of climate during the past 420 000 years cold episodes after relatively warm and largely ice free periods occurred when the predominance of deep water formation changed from northern to southern sources these results reinforce the connection between rapid climate changes at mediterranean latitudes and century to millennial variability in northern and southern polar regions

enhanced ground motions due to large amplitude critical moho reflections sms in the sea of marmara turkey 1 aftershocks of the 17 august 1999 mw 7 4 kocaeli earthquake were used to investigate the large amplitude sms arrivals in the sea of marmara we produced the event record sections from the transverse components of s and sms arrivals at a distance of 10 to 200 km for the data recorded towards the east and west in the sea of marmara for the data recorded towards the west we observed that the amplitude of sms phases enhances the ground motion by a factor of 2 to 3 the dipping moho and or anisotropy in the upper mantle or deep convex structure or deep lens are possible sources for the large amplitude sms phases since the study area has a potential of creating damaging earthquakes in the future the effect of large amplitude sms phases on the ground motion should be considered in the studies of the seismic hazard of the region

lithospheric structure of tibet and western north america mechanisms of uplift and a comparative study we seek to determine the mantle lid thickness of the lithosphere and to constrain the mechanisms that are responsible for the cenozoic uplift of the tibetan plateau located behind the himalaya collision zone and the western united states located behind the san andreas transform convergent plate boundary selected first p arrivals of international seismological centre isc data at distances less than 22 are analyzed by the two station method to constrain the upper mantle structure of the basin and range br colorado plateau cp great plains gp and tibetan plateau tp the data are presented as plots of apparent velocity versus distance to the second station we show that these observations of apparent velocities versus distance as well as those that concern the attenuation of sn provide estimates of the mantle lid thicknesses we obtain the following mantle lid thicknesses br 20 40 km cp 35 50 km gp 150 195 km and tp 135 180 km sn is not observed for paths beneath the br and cp while sn efficiently propagates beneath the gp sn is also observed beneath most of tibet except in the north central part of the tp a possible interpretation of our observations is that indian continental lithosphere underthrusts only about the southern 2 3 of tibet

support of high elevation in the southern basin and range based on the composition and architecture of the crust in the basin and range and colorado plateau toexplorethenatureofhowthestructureandphysicalpropertiesofthecrustvaryfromextendedtorelativelyunextendeddomains we present teleseismic receiver functions which measure crustal thickness shear wavespeed structure and the vp vs ratio at 12 seismic stations in eastern arizona the crustal thickness is 28 km increases slightly eastward and remains nearly uniform beneath the varying elevations in the southern basin and range the observed vp vs ratio in the basin and range 1 78 exceeds the global average the southern colorado plateau exhibits thicker crust 40 km and a slightly greater observed vp vs 1 81 a discrete region in the colorado plateau generates an unusually high vp vs ratio 1 90 and contains low wavespeed zones which serve as evidence of partial melt related to quaternary volcanism the metamorphic core complexes in the southern basin and range likewise exhibit anomalously high vp vs values 1 79 1 87 and lack locally compensating crustal roots density models show that 85 kg m 3 lighter crust or 35 kg m 3 lighter mantle than that of the surrounding basin and range helps these metamorphic core complexes maintain their high elevation compositional modeling of intrusive bodies exposed throughout the catalina rincon metamorphic core complex indicates that the observed high vp vs ratio and modeled low density could result from substantial amounts of a plagioclase rich quartz poor rock these vp vs data are evidence that significant compositional heterogeneity of the crust can occur over a short distance and provide a clue as to how these areas that underwent significant tertiary extension may have been preconditioned for orogenic collapse

cocorp deep reflections moho at 50 km 16 s beneath the colorado plateau cocorp deep reflection data on the colorado plateau reveal complex reflections to about 16 s two way travel time 50 km beyond which there is an abrupt decrease of reflectivity this boundary is interpreted to represent the moho an inference consistent with the existing but limited refraction data sets when they are reconciled with the recent independent determination of an 8 10 8 15 km s pn velocity beneath the colorado plateau this 50 km depth to moho and 8 10 8 15 km s pn velocity for the colorado plateau closely resembles that of the high plains just east of the rocky mountains the absence of continuous strong reflections at the moho suggests that the moho beneath the colorado plateau is a transition not a velocity step function on the scale of the reflection experiment the reflection boundary may represent the base of complex and discontinuous reflectors of the crust and crust mantle transition below which the mantle may be largely peridotite with a velocity of 8 1 km s these results strongly reinforce the suggestion that the moho on deep reflection data in relatively stable continental regions is represented by an abrupt decrease in reflectivity below which the mantle is relatively seismically transparent

severity and speed of northern hemisphere glaciation pulses the limiting case a brief pulse of ice sheet growth of enormous amplitude predicted by the theory of milankovitch is confirmed by oxygen isotopic and other evidence in pacific and north atlantic cores this glaciation bisected interglacial isotopic stage 7 lasted no more than 23 000 yr from inception to end and resulted in a very large ice volume buildup and a full glacial climatic response of the north atlantic ocean this pulse along with other episodes of rapid and large scale ice growth does not conform to the conventional sawtooth model of slow ice accumulation and rapid ice decay ice growth rates on such pulses can approach the mean ice decay rates on isotopic glacial terminations

flexural rigidity of the basin and range colorado plateau rocky mountain transition from coherence analysis of gravity and topography stochastic inversion for flexural loads and flexural rigidity of the continental elastic layer can be accomplished most effectively by using the coherence of gravity and topography however the spatial resolution of coherence analysis has been limited by use of two dimensional periodogram spectra from very large 105 km 2 windows that generally include multiple tectonic features using a two dimensional spectral estimator based on the maximum entropy method the spatial resolution of flexural properties can be enhanced by a factor of 4 or more enabling more detailed analysis at the scale of individual tectonic features this new approach is used to map the spatial variation of flexural rigidity along the basin and range transition to the colorado plateau and middle rocky mountains physiographic provinces large variations in flexural isostatic re sponse are found with rigidities ranging from as low as 8 7x102o n m elastic thickness t 4 6 km in the basin and range to as high as 4 1x1024 n m t 77 km in the middle rocky mountains these results compare favorably with independent determinations of flexural rigidity in the region areas of low flexural rigidity correlate strongly with areas of high surface heat flow as is expected from the contingence of flexural rigidity on a temperature dependent flow law also late cenozoic normal faults with large displacements are found primarily in areas of low flexural rigidity while deformation fronts of mesozoic tertiary overthrusts occur 0 to 100 km east of the low rigidity region the highest flexural rigidity is found within the archean wyoming craton where evidence suggests that deeply rooted cratonic lithosphere may play a role in deter mining the distribution of tectonism at the surface

crustal structure and evolution beneath the colorado plateau and the southern basin and range province results from receiver function and gravity studies over the past several decades contrasting models have been proposed for the physical and chemical processes responsible for the uplift and long term stability of the colorado plateau cp and crustal thinning beneath the basin and range province brp in the southwestern united states here we provide new constraints on the models by modeling gravity anomalies and by systematically analyzing over 15 500 p to s receiver functions recorded at 72 usarray and other broadband seismic stations on the southwestern cp and the southern brp our results reveal that the brp is characterized by a thin crust 28 2 0 5 km a mean vp vs of 1 761 0 014 and a mean amplitude r of p to s converted wave relative to that of the direct p wave of 0 181 0 014 that are similar to a typical continental crust consistent with the model that the thin crust was the consequence of lithospheric stretching during the cenozoic the cp is characterized by the thickest crust 42 3 0 8 km largest vp vs 1 825 0 009 and smallest r 0 105 0 007 values in the study area in addition many stations on the cp exhibit a clear arrival before the p to s converted phase from the moho corresponding to a lower crustal layer of about 12 km thick with a mafic composition we hypothesize that the lower crustal layer which has an anomalously large density as revealed by gravity modeling and high velocities in seismic refraction lines contributed to the long term stability and preuplift low elevation of the colorado plateau

pn tomographic velocity and anisotropy beneath the iran region we present tomographic velocity and anisotropy models of the uppermost mantle beneath the iran region a total of 74 375 pn phase readings from 86 stations of the iranian network and 133 stations of the international seismological centre are used in the investigation the study uses the pn travel time tomography method proposed by hearn 1996 the tomography results show some interesting anomalies the average pn velocity under the iran region is approximately 8 0 km s and the maximum velocity perturbations are approximately 3 4 high pn velocities under the zagros mountains and the caspian sea may be due to the presence of oceanic crust lithosphere material low pn velocities were found under the alborz and caucasus regions and may be due to higher temperatures or partial melting resulting from volcanoes and mid cenozoic volcanic plutonic rocks in these regions the inversion velocities support the idea that the subduction of the arabian plate into the mantle beneath the iranian plateau may have resulted in the upwelling of hot material the well resolved pn anisotropy model is jointly obtained with a velocity model for the areas with good ray path coverage in the plate collision regions zagros and alborz the fast pn anisotropy direction is oriented parallel to the collision arc and to large reverse faults due to pure shear deformation from cross fault compression and along fault extension under the caucasus regions the pn anisotropy results indicate that the preferred alignment of olivine crystals is parallel to the plate movement direction however the surface fault strike is at an angle of nearly 45 with the crustal movement direction and anisotropy these differing deformations suggest potential decoupling between the crust and upper mantle the possible decoupling and differing deformation between the crust and upper mantle are easily enhanced under high temperatures due to volcanoes and supported by low velocities beneath the caucasus we validate the existence of pn anisotropy under these regions by azimuthal averaging of the apparent pn velocity

rapid environmental changes in southern europe during the last glacial period oxygen isotope records from greenland ice cores1 2 indicate numerous rapid climate fluctuations during the last glacial period north atlantic marine sediment cores show comparable variability in sea surface temperature and the deposition of ice rafted debris3 4 5 in contrast very few continental records of this time period provide the temporal resolution and environmental sensitivity necessary to reveal the extent and effects of these environmental fluctuations on the continents here we present high resolution geochemical physical and pollen data from lake sediments in italy and from a mediterranean sediment core linked by a common tephrochronology our lacustrine sequence extends to the past 102 000 years many of its features correlate well with the greenland ice core records demonstrating that the closely coupled ocean atmosphere system of the northern hemisphere during the last glacial4 extended its influence at least as far as the central mediterranean region numerous vegetation changes were rapid frequently occurring in less than 200 years showing that the terrestrial biosphere participated fully in last glacial climate variability earlier than 65 000 years ago our record shows more climate fluctuations than are apparent in the greenland ice cores together the multi proxy data from the continental and marine records reveal differences in the seasonal character of climate during successive interstadials and provide a step towards determining the underlying mechanisms of the centennial millennial scale variability

a review of the regional geophysics of the arizona transition zone a review of existing geophysical information and new data presented in this special section indicate that major changes in crustal properties between the basin and range and colorado plateau occur in or directly adjacent to the region defined as the arizona transition zone although this region was designated on a physiographic basis studies indicate that it is also the geophysical transition between adjoining provinces the transition zone displays anomalous crustal and upper mantle seismic properties shallow curie isotherms high heat flow and steep down to the plateau bouguer gravity gradients seismic and gravity studies suggest that the change in crustal thickness from thin crust in the basin and range to thick crust in the colorado plateau may occur as a series of steps rather than a planar surface anomalous p wave velocities high heat flow shallow curie isotherms and results of gravity modeling suggest that the upper mantle is heterogeneous in this region a relatively shallow asthenosphere beneath the basin and range and transition zone contrasted with a thick lithosphere beneath the colorado plateau would be one explanation that would satisfy these geophysical observations

pn velocity and cooling of the continental lithosphere average upper mantle p sub n velocities and heat flow were computed within continental physiographic provinces in north america from published data p sub n velocity and heat flow data display an inverse relationship and were found to correlate with a statistically significant correlation coefficient temperatures at the crust mantle boundary were estimated from the heat flow values and these were used to demonstrate a correlation between p sub n velocity and temperature of upper mantle material the value of partialp sub n partialt p thus obtained 4 4 x 10 sup 4 to 8 1 x 10 sup 4 km s sup 1 sup 0 c sup 1 is within the range of temperature derivatives determined from laboratory studies of ultramafic rocks this dependence of p sub n velocity on temperature implies that one possible explanation for the observed geographical distribution in upper mantle seismic velocity is that the variation in p sub n velocity is primarily a temperature effect combined with the relationship between heat flow and crustal age for continents the p sub n versus heat flow correlation suggests a relationship between p sub n velocity and crustal age probably due to progressive cooling of the continental lithosphere after a thermotectonic event

present day seismicity and seismotectonics of the cilician basin eastern mediterranean region of turkey the nature of the interaction between the anatolian arabian and af rican plates is an important problem of northeastern mediterranean tectonics the dead sea fault and east anatolian fault zone are the most prominent structural elements in this region the cilician basin which is located at the junction of these faults is known to have a very active seismic history however instrumental obser vations have been insufficient because of the lack of an adequate local seismic net work to monitor ongoing seismic activity and to analyze the seismotectonics of the cilician region 36 to 38 n 34 to 38 e a digital seismic network of 18 stations with velocity transducers has been operated by the earth and marine sciences re search institute emsri of the scientific and technical research council of turkey tubitak since 1993 no previous studies have been carried out in this region for crustal structure an optimum 1d velocity model was obtained by using the velest algorithm for reliable determination of earthquake hypocenter locations approxi mately 2500 earthquakes detected between 1993 and 2002 with local magnitude greater than 1 5 have been located by using this optimum velocity model the most notable feature of the epicentral distribution is the presence of dense clusters of activity along local fault trends the majority of hypocentral depths of the events range between 5 and 35 km single fault plane solutions of reliable events were calculated in order to analyze the seismotectonic characteristics of the region results show that the cilicia region constitutes a wide left lateral shearing zone that indicates a diffuse plate boundary between the african arabian and anatolian plates

pn velocity study using a temporary seismographic network in the shillong plateau northeast india abstract seismic p wave velocity pn of the uppermost mantle beneath the shillong plateau has been studied using a temporary seismographic network refracted pn arrivals of shallow distant earthquakes m 3 0 recorded at pairs of stations lying within 2 azimuthal differences with respect to a given epicenter are used in this study since the method is based on relative travel time differences the measurement is independent of origin time and location errors the average pn velocity is found to be 8 57 0 02 km sec which is higher than the average mantle velocity 8 13 km sec under other areas of the indian subcontinent the observed high pn velocity beneath the plateau may be attributed to the presence of high density material

pollen analytical biostratigraphy of the last five climatic cycles from a long continental sequence from the velay region massif central france in the volcanic region of velay massif central france lake sediment sequences derived from maar craters situated close to one another ribains praclaux and lac du bouchet have been correlated on the basis of tephrostratigraphical analyses this has enabled the construction of a sequence 2980 pollen spectra that begins during the glaciation preceding the holsteinian and ends in the present this sequence covers the period from 450 ka i e from the end of marine oxygen isotope stage 12 to the present time and includes five climatic cycles the study of a second core from the praclaux site completed the long velay sequence the biostratigraphy of the whole sequence is described in detail and illustrated by a synthetic and simplified pollen diagram thirteen temperate forest episodes interglacials or interstadials and as many cold periods glacials or stadials are defined each temperate episode is characterised by particular forest development and vegetation dynamics and all of them with the exception of the holocene begin and end with a pinus forest the vegetation dynamics observed during the holocene resemble more closely those recorded during the interstadials than during the interglacials copyright 2000 john wiley sons ltd

small scale convection at the edge of the colorado plateau implications for topography magmatism and evolution of proterozoic lithosphere the colorado plateau of the southwestern united states is characterized by a bowl shaped high elevation late neogene quaternary magmatism at its edge large gradients in seismic wave velocity across its margins and relatively low lithospheric seismic wave velocities we explain these observations by edge driven convection following rehydration of colorado plateau lithosphere a rapidly emplaced cenozoic step in lithosphere thickness between the colorado plateau and adjacent extended rio grande rift and basin and range province causes small scale convection in the asthenosphere a lithospheric drip below the plateau is removing lithosphere material from the edge that is heated and metasomatized resulting in magmatism edgedriven convection also drives margin uplift giving the plateau its characteristic bowl shape the edge driven convection model shows good consistency with features resolved by seismic tomography

pn anisotropy beneath the south island of new zealand and implications for distributed deformation in continental lithosphere pn travel times from regional earthquakes recorded both by stations on new zealand and by ocean bottom seismographs deployed offshore indicate anisotropy in the uppermost mantle beneath the region the largest anisotropy of 8 2 1 lies beneath the deforming part of the south island to just off its west coast a zone roughly 100 200 km wide the fastest propagation is aligned n60 e 3 essentially parallel to the largely strike slip relative plate motion since 20 ma also n60 e the magnitude of anisotropy decreases abruptly northwest and southeast of this zone and on the southeast side of the island the orientation of fastest propagation is between n32 w and n s the n60 e orientation of fast propagation is consistent with finite strain within the uppermost part of the mantle lithosphere if the measured 850 km of displacement of the pacific plate past the australia plate is spread over a region with a width of 100 200 km the agreement of this orientation of fast propagation with the orientation or relative plate motion suggests the possibility of but does not require some dynamic recrystallization in rock as cold as 500 800 c where peierls creep seems to be the likely deformation mechanism such a strain distribution matches deformation of a thin viscous sheet that obeys a constitutive relationship of the form e n where e is the average strain rate and is the operative deviatoric stress with an average value of n 3 10 presumably the nw se fast propagation in the region southeast of the island results from strain that precedes the cenozoic deformation that has shaped the island

crust and mantle structure across the basin and range colorado plateau boundary at 37 n latitude and implications for cenozoic extensional mechanism we present new evidence on the seismic velocity and density of the crust and upper mantle along a 200 km long transect across the eastern basin and range and western colorado plateau at 37 n latitude receiver functions computed from the p waveforms recorded with 10 portable broadband stations deployed along the transect were used to estimate crustal thickness variations the crust is 30 35 km thick within the eastern basin and range and increases over a distance of 100 km at the western edge of the colorado plateau reaching a maximum of approximately 45 km east of the hurricane fault the timing of crustal multiples within the reciever functions were used to estimate the v p v s of the crust along the profile and we found that the western colorado plateau crust is characterized by a high poisson s ratio 0 28 0 29 indicative of a crust with an average mafic composition we estimated the upper mantle lid thickness along our profile based on teleseismic p wave travel times and constraints provided by gravity data our data and available geophysical constraints are most consistent with a lithosphere that thickens from an average thickness of 60 km beneath the basin and range to 100 km beneath the western colorado plateau although the basin and range lithosphere may have significant thickness variations the thick strong mafic crust and thicker mantle lid under the colorado plateau can account for the relative geologic stability and subdued magmatism of the plateau during laramide compression and cenozoic extension compared to surrounding regions the crustal and lithospheric thinning across the tectonic boundary occurs over a short distance 100 km suggesting it is a geologically young feature produced by a predominantly mechanical response to late cenozoic extension our new lithosphere model at 37 n latitude is consistent with the existence in early cenozoic time of a flat subducted slab at 100 km depth and a relict sevier laramide 50 60 km thick crustal welt and 60 100 pure shear extension values of 1 6 2 0 during the late cenozoic

ecological thresholds and patterns of millennial scale climate variability the response of vegetation in greece during the last glacial period the regional expression of millennial scale climate variability during the last glacial is examined with particular reference to the vegetation response in greece inspection of three pollen records from contrasting bioclimatic areas suggests that differences in the magnitude of cold events as recognized in the north atlantic and western mediterranean are expressed in terms of tree population changes only in areas with a range of favorable habitats by contrast records from sites where populations approach their tolerance threshold do not appear to resolve differences in the amplitude of the climate oscillations understanding the importance of local factors in modulating the biological response to climate change is critical when attempting to establish the spatial pattern of millennial variability

the basin and range province origin and tectonic significance the basin and range province is a vast arid tract of regionally corrugated angular topography of high relief in the western cordillera it is characterized by evenly spaced parallel mountain ranges and intervening desert basins fig ure la the range flanks are marked by poorly sorted gravel aprons that slope smoothly basinward interrupted here and there by low fault scarps that paral lel the range front faults and by alluvial fans at the mouths of canyons draining the ranges thermal springs located at or near range bounding faults attest to vigorous hydrothermal circulation within zones of fracture porosity created and maintained by faulting in the southern part of the province especially in southeastern california and southwestern arizona range fronts have been worn back by erosion leaving a thin veneer of gravel on an erosion cut bedrock surface that slopes gently outward the range bounding faults of these mountain blocks are buried at the outer edge of such pediments often at considerble distances from the erosional remnants of the ranges themselves the american physiographer n m fenneman 1928 1931 named the basin and range province and defined its general boundaries as thus circum scribed the province includes some 800 000 km2 of area in eight western states later students pardee 1950 lawrence 1976 reynolds 1979 eaton 1979b have observed that many of the fundamental geological and geo physical characteristics of the province are found well beyond the boundaries drawn by fenneman which were based on physiography alone as a tec tonophysical entity its areal extent is greater than 1 million km2 more than 10 of the area of the united states figure 1 fenneman 1931 subdivided the province into five physiographic sections the largest of which is the great basin see figure 1b it is not as its name implies a single regional depression with a common topographic center but is characterized instead by isolated networks of interior drainage divisible into

duration of last interglacial conditions in northwestern greece a new astronomical calibration method for long pollen records from southern europe is applied to the last interglacial interval of the ioannina sequence northwest greece this shows that the last interglacial in this region as defined by the presence of forest communities lasted c 15 500 years from 127 300 to 111 800 yr b p interglacial conditions developed within marine isotope sub stage mis 5e and persisted into mis 5d lagging changes in global ice volume

the classic marine isotope substage 5e since its identi cation nearly fty years ago marine iso tope stage 5 mis 5 has been placed onto absolute time scaleson the basis of three independent approaches cesare emiliani who set up the isotope stages emiliani 1955 depended onuranium series dating of the sediments a method that today isregarded as not generally capable of yielding useful precisionor accuracy broecker and van donk 1970 pioneered the ap proach of correlating to radiometrically dated marine coral ter races thishasbeenmuchaidedinrecentyearsbyimprovementsin the precision and accuracy of these age determinations thathave owed from the development of thermal ionization massspectrometry tims for uranium series dating edwards

a global study of pn anisotropy beneath continents we apply a two station method of velocity determination to p n arrival times reported in the international seismological centre bulletin during the period 1964 1993 the azimuthal variation of these velocities provides evidence of p n anisotropy as large as 9 6 anisotropic regions are identified by examination of the azimuthal variation of the p n velocities within circular caps of radius 1 5 and 3 spaced at 1 intervals across the earth this procedure leads to 255 geographically distributed estimates of anisotropy the length scale over which the anisotropy varies indicates that uppermost mantle anisotropy beneath continents is controlled by regional orogenic strains and is inconsistent with a general interpretation in terms of plate scale mantle flow in addition the correlation between p n anisotropy and surficial tectonic features indicates that the anisotropy is primarily the result of the most recent deformation event we compare our p n anisotropy results fast directions and magnitudes with those of sks splitting studies to attempt to constrain the depth extent of the anisotropy this comparison suggests that in some areas where the fast directions agree sks splitting observations may arise from a layer beginning just below the moho whereas in others it must have a deeper origin in these areas the relative magnitudes of the measured anisotropy are consistent with a thin 100 150 km lithospheric layer of anisotropy

analysis of vesicular basalts and lava emplacement processes for application as a paleobarometer paleoaltimeter abstract we have developed a method for determining paleoelevations of highland areas on the basis of the vesicularity of lava flows vesicular lavas preserve a record of paleopressure at the time and place of emplacement because the difference in internal pressure in bubbles at the base and top of a lava flow depends on atmospheric pressure and lava flow thickness at the top of the flow the pressure is simply atmospheric pressure while at the base there is an additional contribution of hydrostatic lava overburden thus the modal size of the vesicle bubble population is larger at the top than at the bottom this leads directly to paleoatmospheric pressure because the thickness of the flow can easily be measured in the field and the vesicle sizes can now be accurately measured in the lab because our recently developed technique measures paleoatmospheric pressure it is not subject to uncertainties stemming from the use of climate sensitive proxies although like all measurements it has its own sou

thermal anomalies and magmatism due to lithospheric doubling and shifting abstract we present some thermal and magmatic consequences of the processes of lithospheric doubling and lithospheric shifting lithospheric doubling concerns the obduction of a cold continental or old oceanic lithospheric plate over a young and hot oceanic lithosphere upper mantle system including an oceanic ridge lithospheric shifting concerns the translation and rotation of a lithospheric plate relative to the upper mantle in both cases the resulting thermal state of the upper mantle below the obducting or shifting lithosphere may be perturbed relative to a normal continental or oceanic geothermal situation the perturbed geothermal state gives rise to a density inversion at depth and thus induces a vertical gravitational instability which favours magmatism we speculate about the magmatic consequences of this situation and infer that in the case of lithospheric doubling our model may account for the petrology and geochemistry of the resulting magma the original layering and composition of the overridden young oceanic lithosphere may strongly influence magmatic processes we dwell shortly on the genesis of kimberlites within the framework of our lithospheric doubling model and on magmatism in general lithospheric recycling is inherent to the mechanism of lithospheric doubling

a synthetic pollen record of the eastern mediterranean sapropels of the last 1 ma implications for the time scale and formation of sapropels abstract the quaternary climate of southern europe south italy and greece is investigated by pollen analysis of the sapropels which were deposited in the deep eastern mediterranean sea during the last 1 million year ma the time scale of core kc01b in the ionian sea has been established by tuning its oxygen isotopic record to the ice volume model of imbrie and imbrie 1980 for the last 250 000 year 250 ka the previous pollen studies and astronomical tuning have been confirmed sapropels were deposited under a large range of mediterranean climates fully interglacial fully glacial and intermediary as revealed mainly by the balance between the respective pollen abundances of oak quercus and sage brush artemisia the high value of the oak reveals the warm and wet climate of an interglacial and the high value of the sage brush the dry and cold climate of a glacial whereas the mediterranean climate is directly related to the variation of the high latitude ice sheets the deposition of sapropels is not so in contrast with the wide climatic range sapropels were deposited only when summer insolation in the low latitudes reached its highest peaks however between 250 ka and 1 ma that stable pattern is not yet established only six sapropels are observed many expected ones do not appear even as ghosts signalled by peaks of barium abundance that remain after the post deposition oxidation of organic matter the pattern of sapropel formation in stable and direct relationship to highest insolation does not seem to apply for five of those sapropels neither climate extremes are observed they mainly formed during intermediary types of mediterranean climate in contrast one sapropel and one ghost relates to a relatively low peak of insolation and its climate is of a unique composite type not seen later this might suggest an unsuspected more complex pattern linking the formation of mediterranean sapropels to the astronomical configuration

basin and range large scale tectonics constraints from gravity and reflection seismology two geophysical informations constrain the mechanical state of the lithosphere in the basin and range province cocorp reflection seismic structure across nevada shows a rather smooth reflector at about 30 km depth which can be interpreted as indicative of a rather flat moho on the other hand the bouguer map exhibits two marked troughs in the same region and filtering techniques reveal a predominant 200 km wavelength of the deep compensating masses of the topography this paper attributes these deep heterogeneities to lateral thermal variations created by boudinage type instabilities of the stretched mantle lithosphere

seismic structure of the crust and the upper mantle beneath the himalayas evidence for eclogitization of lower crustal rocks in the indian plate 1 variations in the seismic velocity structure of the himalayan collision zone include significant differences between its north and south portions with transitions in physical properties across the greater himalaya we combined p and s wave traveltimes from a temporary broadband seismic network in eastern nepal and southern tibet with arrival times at the permanent station network of the department of mines and geology of nepal to determine the seismic velocity structure across the himalaya using local earthquake tomography and traveltimes of regional earthquakes the p to s velocity ratio vp vs structure marks the difference between the indian plate and the overlying materials with the vp vs ratios being high for the former and low for the latter we also found a significant increase in the uppermost mantle seismic velocities from south to north reaching p wave velocities vp over 8 4 km s north of the greater himalaya these high vp values do not seem to be the result of biases due to anisotropy in the upper mantle beneath the greater and tethyan himalayas instead we suggest that rocks in the lower crust of the underthrusting indian plate undergo metamorphism to eclogite as they plunge to greater depth beneath the mountain range explaining the high seismic velocities

tomography of the western united states from regional arrival times first arrival times from regional distances 200 1200 km in the western united states extracted from the international seismological centre isc data set were inverted in a tomographic study to map the laterally varying pn velocity structure of the uppermost mantle i e the mantle lid and to estimate the crustal static delay at each seismograph station synthetic data were used to evaluate resolution results correlate well with major tectonic features we find low uppermost mantle velocities v 7 9 km s these low apparent pn velocities are primarily due to the effects of isostasy hotspot volcanism and crustal extension we interpret the low apparent pn velocities beneath the sierra nevada 7 6 7 7 km s to be the result of time delays introduced by ray paths tunneling through the deep crustal root the yellowstone hotspot migrating along the snake river plain has left low velocity 7 6 7 8 km s hot upper mantle in its wake lower pn velocities 7 8 km s beneath the extensional basin and range are also presumably due to hotter uppermost mantle based on station static delays for the basin and range we infer that moho depths there do not vary significantly from 30 km normal continental pn velocities 8 0 km s as well as thicker crust 40 50 km underlying the rocky mountains wyoming basin province colorado plateau and the northern columbia plateau indicate the presence of relatively cool uppermost mantle beneath these regions

lithospheric structure of the central himalaya from 3 d tomographic imaging abstract the high himalaya are one of the most spectacular consequences of the ongoing collision between india and eurasia yet the role that the lithosphere beneath them play in this collision is not well understood to illuminate the influence of the himalaya on collisional processes we produced tomographic images of the subsurface beneath them from arrival times of p and s waves generated by 551 teleseisms and 1051 local regional events and recorded by the himalayan nepal tibet broadband seismic experiment himnt from 2001 to 2002 compared to those in initial 1d models the rms p and s arrival time misfits in our final 3d model were reduced by as much as 46 from 0 45 s to 0 24 s and 38 from 0 97 s to 0 6 s respectively our results show strong crustal variations perpendicular to but no significant lateral variation along the trend of the high himalaya within our study area in the ns direction the crust thickens from about 60 km under the lesser himalaya to 70 80 km under the tethys himalaya there is no localized crustal thickening beneath the high himalaya wavespeeds and seismicity patterns beneath the tethys himalaya suggest that the middle crust at 30 50 km depth is ductile and are consistent with the presence of a mid crustal reflector associated with the stronger gradients in the lower crust correlation between relocated earthquakes and the xainza dinggye rift indicates that the rift is not cutting through the crust at the present time shear wave splitting from core phases indicates that the upper mantle beneath the tethys himalaya is nearly isotropic the combination of isotropy and higher seismic wavespeed leads us to suggest that eclogites included in the lowermost crust and the uppermost mantle are responsible for the mid crustal reflector under the tethys himalaya as well as for a slightly high wavespeed anomaly just beneath the moho this interpretation is supported by the existence of granulitic eclogite in the ama drime massif 50 km northeast of mt everest

pn velocities beneath iran abstract in active tectonic areas like iran where the coverage of seismograph stations is inadequate but earthquakes occur frequently uppermost mantle body wave velocities are conventionally determined by using a single station and an array of earthquake sources this method fails when earthquakes are mislocated or source areas have varied structures an extension of the traditional surface wave two station method is developed for body waves and shown to be independent of mislocation errors it thus gives accurate estimates of body wave velocities along two station paths with greatly reduced errors uppermost mantle p wave p n velocities are determined along several paths in iran using this method and show zones of low velocity corresponding to zones of high attenuation and quaternary volcanic activity in the country a remarkably high p n velocity of 8 30 km s is found for the zagros area of western iran more typical of shield like structures than that of tectonic areas

through thick and thin a new model for the colorado plateau from seismic refraction data from pacific to arizona crustal experiment we have constructed a model for the crust and uppermost mantle beneath the colorado plateau by combining newly acquired seismic refraction wide angle reflection data from the 1989 pacific to arizona crustal experiment pace cross profile with data collected by roller 1965 a combination of forward modeling methods was used to model travel times and relative amplitudes of crustal and upper mantle phases the salient features of the model for the colorado plateau hereafter referred to as the plateau are 1 an upper crust having an average velocity of 6 1 km s with a low vertical velocity gradient 2 midcrustal discontinuities at depths of 30 to 38 km beneath the plateau 3 a lower crust that varies in thickness 5 to 16 km and has an average velocity of 6 8 km s 4 a transitional lower crust upper mantle boundary and 5 a crustal thickness of 45 3 km a comparison of our new model for the colorado plateau with recent models for the basin and range shows that the upper crusts of the two provinces are similar but that the lower crust of the plateau is thicker particularly in the plateau s central region these observations are consistent with the idea that the basin and range could have formed from crust like that of the colorado plateau

african monsoons an immediate climate response to orbital insolation the croll milankovitch astronomical theory of climate1 3 has received strong support from the evidence of a linear climatic forcing by obliquity and precession although nonlinearity had to be assumed for eccentricity4 5 moreover interglacials have appeared to be controlled by the orbital insolation6 although a phase shift of 6 000 5 000 yr is seen between an astronomical climate index and terrestrial climate indicators dominated by the northern ice sheet dynamics climate proxy data of low latitudes are less directly dependent on the ice sheets here it is shown that during the past 464 000 yr african monsoons signalled by the east mediterranean sapropels were heaviest always and only when a northern summer monsoon index computed from the orbital variation of insolation reached maximum values this occurred during all the interglacials but also twice during glacial periods thus tropical aridity is not the single climatic pattern of glacial periods an immediate terrestrial climate response to orbital variations of insolation is established

hot spots magma underplating and modification of continental crust most common mantle derived magmas have densities at moho pressures that exceed the density of average continental crust thus unless the crust is tightly coupled with the mantle many such magmas should penetrate at or near the moho interface the scale of such a phenomenon can be impressive recent hot spots may have perturbed more than 5 of the planet s surface the details of the process will depend on the ease of melting of the base of continents underplating can explain continental construction from below moho structures or lack thereof delamination of the continental basement partial subduction and density cleaning of crust the formation of massive anorthosite complexes and major changes of elevation of continental blocks in the thermally turbulent archean the very dense and hot komatiite magmas must have had even more profound influences modern studies of the subduction process show that the mantle must be heterogeneous and that volatiles can be carried to great depths in significant volumes

crustal signatures of the tectonic development of the north american midcontinent abstract the stable eastern portion of the north american continent offers an excellent environment to study the tectonic development of intra continental structures the midcontinent of north america formed by the accretion of proterozoic terranes and has since experienced episodes of deformation during the subsidence of the illinois basin and uplift of the ozark plateau rifting also initiated in eastern north america but extension did not continue and arms of failed rifts extend across the region the new madrid seismic zone situated within a portion of the failed reelfoot rift represents an active zone of intraplate seismicity analyzing the structure of the crust and upper mantle within the midcontinent will therefore provide insight into the factors that lead to intraplate deformation using data from over 180 transportable array seismic stations we calculate receiver functions to investigate the crust and upper mantle of the midcontinent at close to 40 km thick the crust of the new madrid seismic zone is thinner than in the surrounding areas outside of the reelfoot rift and rough creek graben the illinois basin cannot be characterized by a single crustal structure as crust near 50 km thick in the central portion of the basin thins to between 40 and 45 km thick towards the northern and southern portions of the basin discontinuities within the crust and upper mantle are prominent in and around the new madrid seismic zone and mark locations of crustal modification and underplating comparing changes in crustal structure to the distribution of bouguer gravity anomalies the presence of positive gravity anomalies suggests that higher density crust plays a role in maintaining low surface elevations within the reelfoot rift conversely a negative gravity anomaly in an area of thinner crust within the ozark plateau supports the need for low density crustal material to influence the uplift of the plateau

abrupt temperature changes in the western mediterranean over the past 250 000 years a continuous high resolution western mediterranean sea surface temperature sst alkenone record spanning the past 250 000 years shows that abrupt changes were more common at warming than at cooling during marine isotope stage mis 6 sst oscillated following a stadial interstadial pattern but at lower intensities and rates of change than in the dansgaard oeschger events of mis 3 some of the most prominent events occurred over miss 5 and 7 after prolonged warm periods of high stability climate during the whole period was predominantly maintained in interglacial interstadial conditions whereas the duration of stadials was much shorter

tertiary minette and melanephelinite dikes wasatch plateau utah records of mantle heterogeneities and changing tectonics a swarm of minette and melanephelinite dikes is exposed over 2500 km2 in and near the wasatch plateau central utah along the western margin of the colorado plateaus in the transition zone with the basin and range province to date 110 vertical dikes in 25 dike sets have been recognized strikes shift from about n80 w for 24 ma dikes to about n60 w for 18 ma to due north for 8 7 m y these orientations are consistent with a shift from east west oligocene compression associated with subduction to east west late miocene crustal extension minettes are the most common rock type mica rich minette and mica bearing melanephelinite occurs in 24 ma dikes whereas more ordinary minette is found in 8 7 ma dikes one melanephelinite dike is 18 ma these mafic alkaline rocks are transitional to one another in modal and major element composition but have distinctive trace element patterns and isotopic compositions they appear to have crystallized from primitive magmas major trace element and nd sr isotopic data indicate that melanephelinite which has similarities to ocean island basalt was derived from small degree melts of mantle with a chondritic sm nd ratio probably located in the asthenosphere but it is difficult to rule out a lithospheric source in contrast mica bearing rocks mica melanephelinite and both types of minette are more potassic and have trace element patterns with strong nb ta depletions and sr nd isotopic compositions caused by involvement with a component from heterogeneously enriched lithospheric mantle with long term enrichment of rb or light rare earth elements ree epsilon nd as low as 15 in minette light ree enrichment must have occurred anciently in the mid proterozoic when the lithosphere was formed and is not a result of cenozoic subduction processes after about 25 ma foundering of the subducting farallon plate may have triggered upwelling of warm asthenospheric mantle to the base of the lithosphere melanephelinite magma may have separated from the asthenosphere and while rising through the lithosphere provided heat for lithospheric magma generation varying degrees of interaction between melanephelinite and small potassic melt fractions derived from the lithospheric mantle can explain the gradational character of the melanephelinite to minette suite

orbital signatures and long term vegetation patterns in the mediterranean abstract an attempt is made to explore the relation between vegetation phases and orbital configurations by comparing long pollen sequences from the mediterranean region and astronomical curves on the basis of their independent chronologies three recurring patterns emerge from this comparison i all intervals with perihelion passage occurring during northern winter were associated with significant contractions of tree populations ii all temperate stage expansions of certain mediterranean vegetation elements occurred when perihelion passage was in summer and iii temperate stage forests reached maximum extent at times of autumn perihelion of these patterns the first corresponds to extreme dry cold episodes culminating at times of march perihelion and the second to maximum summer temperature and evaporation regimes following june perihelion the third associated with the occurrence of september perihelion appears to be more diffuse both in time and space possibly reflecting local variations in hydrological regimes the consistency of at least the first two cases provides some support for the notion that certain recurring vegetation patterns appear to be a result of climate changes linked to specific orbital signatures this could eventually lead to the development of improved terrestrial chronologies through astronomical tuning

a radiometric calibration of the specmap timescale the astronomical theory of climate change asserts that earth s climate is affected by changes in its orbit which vary the seasonal and latitudinal distribution of solar radiation this theory is the basis of the orbitally tuned specmap timescale a key constraint for this important chronology was the mid point of the penultimate deglaciation initially dated to 127 000 years ago recent work suggests this event may be considerably older casting doubt on the astronomical theory the specmap timescale and the accuracy of orbitally tuned chronologies difficulties with u th coral dating of sea level events have impeded progress on this problem because most corals are not closed systems here we use a new approach to u th dating that corrects for open system behavior and produces a sea level curve of sufficient resolution to confidently correlate with specmap over the last 240 000 years permitting a reassessment of both this critical chronology and a central tenet of climate change theory high precision ages for 24 oxygen isotope events provide a 240 000 year chronology for marine 18o records that is independent of orbital tuning assumptions although there appear to be significant differences between the radiometric and orbitally tuned timescales near the lastglacial maximum and at the marine isotope stage 7 6 boundary a comparison of radiometric and specmap ages for identical isotope events suggest that the specmap timescale is quite accurate and that its errors were in general overestimated despite suborbital complexity orbital cyclicity is clearly evident in our record high amplitude sea level oscillations at periods greater than 20 000 years are very close in phase to summer insolation in the northern hemisphere although sea level changes cannot be uniquely tied to a specific season or latitude of insolation forcing the simplest explanation is that long period high amplitude sea level change is linked to northern hemisphere insolation forcing these results validate the principles of orbital tuning and suggest such timescales are generally robust

a note on terrestrial heat flow in the colorado plateau the mean of deep heat flow measurements in the interior of the colorado plateau temperature measurements to depths 750 m is about 5 mwm 2 greater than the mean of shallow data although these data sets have remarkably similar means considering the experimental difficulties in measuring heat flow the means never the less appear statistically different 59 mwm 2 vs 64 mwm 2 if various lithospheric heating models are used to explain the uplift of the colorado plateau a 5 mwm 2 difference in surface heat flow will make the time of lithospheric heating very different deep heat flow data in the colorado plateau typically lack the large spatial variability that shallow data show the deeper data should therefore relate better to regional geothermal trends caused by mantle thermal conditions and by crustal radioactivity the deep heat flow data where present in the colorado plateau interior suggest a rather uniform heat flow and therefore rather constant mantle temperatures and crustal radioactivity continued concern with experimental data will be necessary to appreciate the uncertainties in the heat flow data and the resulting thermal models

seven ambiguities in the mediterranean palaeoenvironmental narrative abstract a review of seven outstanding issues on mediterranean palaeoenvironments is presented these are related to the dominant orbital pacing of climate variability the length of the interglacial vegetation succession the influence of the african summer monsoon the seasonality of precipitation during boreal insolation maxima the moisture balance during glacial maxima and the appearance of the mediterranean type climate rhythm and evolution of mediterranean sclerophyllous plants what emerges is that 1 marine 18oplanktonic and sst records show that precession has been a fundamental tempo of mediterranean climate change representing both a low latitude signal runoff from north africa and the direct influence of insolation at mediterranean latitudes but high latitude glacial effects 41 kyr and 100 kyr cycles became superimposed after 2 8 ma sapropel and dust deposition patterns in marine cores reveal that obliquity also has an effect on mediterranean climate through dry wet oscillations which are independent of glacial interglacial variability 2 the temperate part of interglacial vegetation succession has a duration of approximately half a precession cycle this persisted during the interval of obliquity dominated glacial cycles 2 8 1 ma with distinct forest successions following the precessional cycles however these are not always separated by an open vegetation phase because of minimal ice growth producing an impression of a prolonged interglacial forest interval 3 the effect of an enhanced african monsoon during summer insolation maxima has been mainly indirect in terms of nile discharge and runoff along the north african coast leading to increased freshwater input into the mediterranean sea reduced deep water ventilation and sapropel deposition 4 the notion of an accentuated summer rain regime in the northern mediterranean borderlands also contributing to a freshening of the mediterranean sea during boreal insolation maxima is not supported by the available evidence which suggests increased summer aridity 5 recent improvements in chronological precision and data resolution point to an increase in aridity and decreased temperatures during the last glacial maximum 21 2 ka but suggest an increase in effective moisture during the immediately preceding interval of 24 27 ka 6 the mediterranean type climate is not exclusively a post 3 6 ma phenomenon but may have appeared intermittently during the course of the tertiary or before 7 if that is the case then the paradigm that the sclerophyllous evergreen habit represents a pre adaptation to summer drought may need re evaluation

seismic evidence for partial lithospheric delamination model of colorado plateau uplifts the lithospheric structure beneath the colorado plateau and great basin of the western united states is investigated using pn and surface wave data from the colorado plateau great basin broadband seismic experiment average pn velocities of 8 0 km s in the colorado plateau and 7 8 km s in the eastern great basin are found teleseismic rayleigh waves are also measured and eastern great basin phase velocities are found to be 3 4 higher than colorado plateau phase velocities in the 20 35 s range but indistinguishable at periods greater than 40 s the apparent pn surface wave discrepancy is resolved with models that include a thicker crust and faster mantle lid for the colorado plateau than the eastern great basin models for both regions require a mantle low velocity zone the slow mantle deep beneath the colorado plateau may suggest a thermal origin of plateau uplift which has not yet penetrated the mantle lid

regional waveform propagation in the arabian peninsula regional waveform propagation is characterized in the arabian peninsula using data from a temporary network of broadband seismometers between november 1995 and march 1997 332 regional delta 15 events were recorded from nine stations deployed across the arabian shield regional phase propagation was analyzed in two ways by individual inspection of the waveforms and by stacking of waveforms inspection of the waveforms revealed consistent variations in individual seismograms according to the region of origin waveforms from events in the gulf of aqaba northwest of the network possess weak pn pg and sn but show a prominent lg phase in contrast clear pn sn and lg are observed for events located in the zagros a region northeast of the network events near the straits of hormuz also display pn and sn but lack a strong high frequency lg southern red sea and african earthquakes have moderate amplitude body phases with some lg for the stacks the data were high pass filtered at 1 hz rectified binned and then stacked by time distance or by time slowness the time distance stacks show clear differences between regions that correspond to the variations observed in individual seismograms the time slowness stacks allow comparison of relative phase velocities and amplitudes pn velocity under the network was estimated to be 8 0 0 2 km s consistent with data from prior refraction profiles the area of inefficient pn and sn propagation coincides with an area of holocene volcanism and suggests that anomalous upper mantle underlies much of the arabian shield

a long pollen record from lac du bouchet massif central france for the period ca 325 to 100 ka bp ois 9c to ois 5e abstract pollen analysis was carried out on sediments older than the wurm pleniglacial ois 4 in two new sequences h and i derived from the centre of lac du bouchet massif central the inferred vegetation history enables for the first time in france five temperate episodes to be defined which pre date the last interglacial these temperate episodes alternate with episodes during which the changes in vegetation are indicative of glacial climates comparison of these climatic episodes with the oceanic isotope record shows that the pollen record of sequences h and i from lac du bouchet spans the time interval from ois 9c ussel interstadial to ois 5e ribains eemian interglacial in the organic sediments from the amargiers interstadial ois 9a a trachytic layer ar ar dated to ca 275 ka enables a correlation to be established with the upper part of a sequence derived from the nearby praclaux crater the lower part of this sequence being of holsteinian age ois 11c the cross dating of the pollen sequences from lac du bouchet cores h i and d and from praclaux provides a complete record from the massif central southern france of successive glacial and interglacial episodes that span the last ca 400 ka that is the interval from the holsteinian to the holocene

mapping high pn velocity beneath the colorado plateau constrains uplift models the massive international seismological centre data set of the past 20 years and the two station method are used to determine pn velocities in the mantle lid beneath the colorado plateau in this method the event is located at distances where pn is the first arrival 2 16 and the path is in or very near the azimuth of the two station pair and crosses the plateau this technique to a large extent minimizes the hypocenter mislocation effect and possible errors due to variations in the crustal structure near the source since only the difference in travel times at the two stations is used however this technique has a few underlying assumptions and possible sources of errors such as the quality of the pn data base and station delays caused by varying crustal structure that require an extremely careful application of the method a detailed study of the source of errors and a methodology of selection of the data are presented application of this method to the colorado plateau using all possible two station pairs from 53 stations located within or along the margin of the plateau yields an average high pn velocity of 8 12 0 09 km s this value is considerably larger than the average value of 7 83 km s based on available but very limited seismic refraction profiles but is remarkably similar to the average value of 8 1 km s for the relatively stable midcontinent region our new pn velocity for the colorado plateau eliminates the paradox in the literature that emphasizes the rather close similarity between average pn velocities beneath the colorado plateau and the basin and range province while their tectonic and magmatic cenozoic history is dramatically different previous models for the structure and evolution of the plateau have used the low pn velocity as an important constraint on density and thermal state of the lithosphere hence such models should be reexamined on the basis of this new uppermost mantle pn velocity determination there are two main models that have been proposed to explain the 2 km uplift of the colorado plateau one is based on a combination of thermal thinning of the lithosphere and crustal thickening and the other involves a combination of the delamination of the subducted subhorizontal farallon oceanic plate from the overriding north american plate and crustal thickening we show that the delamination model is more readily consistent not only with our velocity determination and the elevation of the plateau but also with varied geological observations reported in the literature that concern the cenozoic evolution of western north america

active lithospheric thinning abstract this paper starts with a discussion of the physical mechanisms capable of inducing a rapid upward migration of the lithosphere asthenosphere boundary two models based on a thermomechanical destabilization of the lower lithosphere are proposed first we consider the enhancement of small scale sublithospheric convection modes by a hot thermal plume a perturbation of a few hundred degrees is found to thin the lithosphere and generate uplift within a few tens of million years this efficient erosion from below only stops when it reaches a layer of low density such as the crust or some depleted mantle material second we show that passive lithospheric stretching leads to an unstable lithospheric configuration on the sides of the rift the denser unstretched lithosphere is subject to a lateral convective penetration by the hotter asthenospheric mantle material coming from the rift zone this lateral destabilization generates uplift of the shoulders natural examples can be found along passive continental margins a good example is given by the uplift of the scandinavian caledonides following the break up in the north atlantic

contrasting lithospheric structure between the colorado plateau and great basin initial results from colorado plateau great basin passcal experiment the colorado plateau great basin cpgb passcal experiment provides broadband seismic data that allow us to determine crustal and upper mantle structure in the colorado plateau and eastern great basin twelve portable broadband seismometers throughout utah and nevada are supplemented by permanent broadband stations in the region receiver functions indicate an average crustal thickness of 35 5 km in the great basin gb with a strong p to s ps conversion from the moho the crust in the transition between the great basin and colorado plateau cp produces a strong intracrustal conversion the cp has somewhat thicker crust 42 5 km and weaker moho conversion br surface wave phase velocities are 3 4 higher than cp phase velocities in the 20 35 s range consistent with a thinner crust under the gb the phase velocities at periods greater than 40 s are indistinguishable between the two provinces indicating that differences between upper mantle velocities are minor shear wave splitting yields fast polarizations that parallel the western and northern boundaries of the cp with delay times decreasing towards the center of the plateau

a possible mechanism for epeirogenic uplift continental geology is dominated by vertical movements some of these are the result of crustal shortening and extension associated with large horizontal displacements involving processes which are now understood at least in outline however both uplift and subsidence can occur without associated shortening or extension of the continental crust most investigations of such epeirogenic movements have been concerned with subsidence partly because of the economic importance of the resulting sedimentary basins and partly because of the existence of a simple model for the process1 which can account for the development of several basins in some detail2 4 however uplift has been relatively neglected and cannot be produced by the same mechanism the purpose of this paper is to examine whether epeirogenic uplift can result from the intrusion of large thicknesses of basic magma into the lower part of continental crust this suggestion was considered as a possible mechanism for the regional uplift of the colorado plateau by gilbert5 more than 100 years ago and also by holmes6 though both rejected the idea however a recent discussion of no less than 14 different mechanisms for generating uplift7 did not include this process even though it seems able to produce the observed vertical motions and also to account for some of the features of the evolution of sedimentary basins that cannot be explained by the uniform stretching model3 8 9 although the suggestion is not new the argument used here to support it is and depends on an understanding of the relationship between ocean island volcanoes and mantle circulation

laramide crustal thickening event in the rocky mountain foreland and great plains the rocky mountain foreland and great plains of the western united states were formerly part of a continental platform adjusted by erosion and deposition in cambrian through jurassic time to near mean sea level and therefore to a near uniform crustal thickness of approximately 33 km today the region stands at regional elevations up to 2 km isostatically supported by a crust exceeding 50 km in thickness reasonable estimates of tertiary sedimentation and laramide strain do not account for more than 15 of the implied thickening however from approximately 70 40 m y b p this region was underlain by a horizontally subducting slab of farallon plate lithosphere moving northeast this slab could have been the cause of the thickening finite difference thermal models with specified kinematics show only a temporary cooling of the base of the north american lithosphere by this slab the excess weight of the slab would have depressed the region plate bending calculations show a quantitative agreement of predicted depression with upper cretaceous isopachs since depression by this slab lasted until the eocene at least the latest cretaceous regression was probably caused by a laramide crustal thickening event the farallon slab might have caused crustal thickening in two ways its excess weight would have drawn in ductile lower crust from surrounding regions however calculations show that this effect is too slow too local and too reversible to explain most of the crustal thickening therefore it seems likely that ductile lower crust was transported from sw to ne by shear stresses which the farallon plate exerted on the base of the north american lithosphere a preliminary finite element calculation based on this hypothesis shows the correct general pattern of crustal thickening an unexpected but encouraging result is that predicted principal compression directions are orthogonal to many laramide basement uplifts

p wave sections in a realistic anisotropic lithosphere summary the detection and evaluation of anisotropy in realistic earth structures by p wave measurements is difficult it is generally assumed that absolute velocity variations with azimuth must be measured but it has also been shown that heterogeneity may make it impossible to do this it may even be impossible to establish that anisotropy has any part in the generation of the observations we have found however that the whole appearance of wide angle seismic sections generated by a realistic anisotropic lithosphere changes systematically with azimuth and propose that this can be an important diagnostic as long as the degree of lateral heterogeneity is not too great it is even possible to measure anisotropic structure at depth using vertical component seismic sections only

two dimensional anisotropic tomography of lithosphere beneath france using regional arrival times this paper presents an anisotropic tomography study beneath france travel times of pn waves were inverted in order to simultaneously retrieve lateral variations of both seismic velocity and seismic anisotropy within the subcrustal lithosphere the parameterization scheme uses a classical approach the subcrustal lithosphere is modeled using a two dimensional horizontal grid of nonoverlapping cells 0 5 0 5 each cell being characterized by an average slowness the isotropic term of slowness and an azimuthal anisotropy term the velocity and anisotropy perturbation amplitudes do not exceed 2 in the paris basin and in the aquitaine basin more significant perturbations can be observed in the tectonically active regions the images of the modeled subcrustal lithosphere emphasize the not recently reactivated areas of the hercynian range by high velocities 2 to 5 from the armorican massif to the west part of the massif central and from the northeast massif central to the vosges mountains and black forest mountains regions these areas are also characterized by an anisotropy of 2 5 with the fast directions parallel to the hercynian structures low velocities 2 to 4 are observed in the neogene volcanic areas of the massif central which confirm the results obtained from a short wavelength teleseismic tomography these low velocities are most probably caused by high temperatures another low velocity region 3 to 4 roughly follows the alpine belt and is interpreted as the image of the crustal root of the range in the inner part of the alps fast pn directions lie perpendicular to the belt with an anisotropy amplitude of 3 while in the outer part they are parallel to the range with amplitudes ranging from 3 to 5 to the east and from 2 to 3 to the west the anisotropy feature of the pyrenean range follows a simple pattern one observes high amplitudes up to 5 and a fast direction which is parallel with the strike of the range the observations show seismic anisotropy for most of the tectonically active zones across france recently formed structural units and old hercynian structures as well and the amplitudes reach the same order as the amplitudes of the velocity perturbations

formation of the rocky mountains western united states a continuum computer model one hypothesis for the information of the rocky mountain structures in late cretaceous through eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the north american lithosphere the horizontal components of the motion of this plate are known from paleomagnetism and the edge of the region of flat slab can estimated from reconstructed patterns of volcanism new techniques of finite element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the north american plate a model that has a realistic temperature dependent rheology and a simple plane layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present successful prediction of this model include i the location amount and direction of horizontal shortening that has been inferred from laramide structures ii massive transport of lower crust from southwest to northeast iii the location and timing of the subsequent extension in metamorphic core complexes and the rio grande rift and iv the total area eventually involved in basin and range style extension in a broad sense this model has predicted the belt of laramide structures the transport of crust from the coastal region to the continental interior the subsequent extension in metamorphic core complexes and the rio grande rift and the geographic region of late tertiary basin and range extension its principal defects are that i many events are predicted about 5 million to 10 million years too late and ii the wave of crustal thickening does not travel far enough to the east reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects the correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the laramide orogeny is probably correct the rocky mountain thrust and reverse faults formed in an environment of east west to northeast southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the north american crust nonuniform crustal thickening by simple shear transport also caused relative uplifts therefore this model is consistent with both of the range forming mechanisms that have been inferred 1 a new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional basin and range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away

azimuthal anisotropy of velocity in the mantle lid beneath the basin and range province the basin and range province in western north america is one of the largest and best studied regions of cenozoic continental extension1 2 yet there is still considerable debate surrounding the geological history of the province and the cause of the observed widespread rifting although recent geological and crustal geophysical observations2 constrain crustal scale processes within the basin and range more information is needed on the structure and rheology of the mantle part of the lithosphere the mantle lid to distinguish between competing geodynamic models1 2 here we present seismic data showing an azimuthal variation of 3 2 in the p wave velocity in the shallowest mantle beneath the basin and range the direction of maximum seismic velocity coincides with the nw se direction of neogene and present day tectonic extension suggesting that penetrative deformation associated with this extension has induced a preferred orientation of the minerals in the mantle lid our results thus imply that deformation associated with present day surface rifting extends at least 30 km into the underlying mantle

tectonic stresses in the lithosphere various types of observables earthquake focal mechanisms in situ measurements and geological deformations give information about the large scale lithospheric stress field the latter has often been explained by postulating appropriate forces acting at the edges and beneath the plates this approach ignores the role of mass heterogeneities within the lithosphere here we analyze the effect of both boundary and internal forces on the stress pattern and show that both contributions are of comparable magnitude the presence of internal sources makes the problem three dimensional we show however that it can be reduced to a two dimensional plane stress formulation whereby the edge forces are expressed by the non hydrostatic stresses and the basal shear is increased by the addition of a term proportional to the gradient of the mean vertical stress for the oceanic lithosphere we derive a compression that increases with age the comparison with geophysical observables yields an upper bound of a few bars on the magniude of the basal drag for the continents we infer the existence of an underlying upper mantle somewhat denser than under oceans

vegetation history of the marine isotope stage 7 interglacial complex at ioannina nw greece abstract a new pollen record of the penultimate interglacial complex corresponding approximately to marine isotope stage mis 7 ca 186 000 245 000 years before present has been generated in core i 284 from the ioannina basin in northwest greece it represents the highest resolution terrestrial record for this interval available to date with a mean sampling interval of 200 years the sequence records four forested intervals correlated with mis 7e 7c 7a and a sub orbital scale event following 7a but within mis 7 the longest and most complete warm stage in terms of vegetational succession is mis 7c while the succession in mis 7e appears to be truncated by a shift to colder drier climatic conditions the results of biomisation suggest that during all of these intervals winter temperatures in nw greece remained lower than during the eemian or holocene while smaller populations of sclerophyllous taxa indicate that summers may have been wetter intervening intervals of open vegetation are correlated with mis 7d 7b and an oscillation following 7a low tree pollen percentages during mis 7d are interpreted as reflecting very cold and dry conditions concomitant with a period of low northern hemisphere insolation which resulted from a rare combination of minimum obliquity and maximum precession higher tree pollen percentages throughout mis 7b by contrast are indicative of relatively mild conditions in agreement with marine temperature and ice volume records sub orbital scale oscillations in tree population extent are also recorded in the new i 284 record suggesting that just as in the last climatic cycle nw greece was affected by rapid climatic oscillations that are associated with changes in the north atlantic

paleolithospheric structure revealed by continental geoid anomalies abstract lithospheric geoid anomalies record changes in elevation and potential energy experienced by continental lithosphere estimates of local isostatic equilibration and potential energy in tandem with lithosphere related geoid anomalies can be used to estimate paleolithospheric thickness providing a clearer understanding of how and why continental topography is developed we employ several simplifying assumptions about the crustal and mantle lithosphere density and structure and readily acknowledge that our results are therefore first order approximations to predict the pre orogenic structure of the lithosphere at the outset we emphasize that while this approach does not provide an exhaustive evaluation of the deformation mechanism it does serve to quantify the relative role played by the variations in the crustal and upper mantle components of the lithosphere in this way we are able to use independent measurement of lithospheric geoid anomalies current post orogenic elevation and lithospheric structure and paleoelevation information to estimate topographic development and structural support over time application of this technique to the southwestern united states indicates that the uplift of the colorado plateau is the result of processes in both the crust and mantle lithosphere and that the lithosphere of the pre orogenic southern basin and range was thinned relative to the northern basin and range and colorado plateau although we use the southwestern u s as an example this method can help constrain uplift mechanisms for any region for which the structure and geoid anomaly of the modern lithosphere is well understood