s velocity variations beneath north america 1 we investigate s velocity variation in the upper mantle beneath north american to better understand the effects of data heterogeneity model parameterization and regularization to this end we analyzed and fit regional s and rayleigh wave trains generated by earthquakes around north america that occurred between the years 2000 through 2006 including waveforms from the transportable array stations of earthscope s usarray these new data were combined with constraints used for the 3 d s velocity model na04 in order to create a new model na07 another model was created from a suite of good fit models to provide a useful guide for model velocities and uncertainties by estimating ranges of probable velocity variations throughout the upper mantle we find that the southern and eastern edges of the north american craton appear to be defined by paleozoic orogens rather than proterozoic ones on average the archean portion of the craton is 200 km thick while the paleozoic part averages 175 km thick with an 80 m s lower s velocity the horizontal gradients in velocity are over 1 0 100 km at the western margin of the craton 0 5 100 km in the south and 1 100 km at the eastern margin

mercury radar imaging evidence for polar ice the first unambiguous full disk radar mapping of mercury at 3 5 centimeter wavelength with the goldstone 70 meter antenna transmitting and 26 antennas of the very large array receiving has provided evidence for the presence of polar ice the radar experiments conducted on 8 and 23 august 1991 were designed to image the half of mercury not photographed by mariner 10 the orbital geometry allowed viewing beyond the north pole of mercury a highly reflective region was clearly visible on the north pole during both experiments this polar region has areas in which the circular polarization ratio pt was 1 0 to 1 4 values 0 1 are typical for terrestrial planets such high values of have hitherto been observed in radar observations only from icy regions of mars and icy outer planet satellites

sks splitting beneath transportable array stations in eastern north america and the signature of past lithospheric deformation seismic anisotropy in the upper mantle beneath continental interiors is generally complicated with contributions from both the lithosphere and the asthenosphere previous studies of sks splitting beneath the eastern united states have yielded evidence for complex and laterally variable anisotropy but until the recent arrival of the usarray transportable array ta the station coverage has been sparse here we present sks splitting measurements at ta stations in eastern north america and compare the measured fast directions with indicators such as absolute plate motion surface geology and magnetic lineations we find few correlations between fast directions and absolute plate motion except in the northeastern u s and southern canada where some stations exhibit variations in apparent splitting with backazimuth that would suggest multiple layers of anisotropy a region of the southeastern u s is dominated by null sks arrivals over a range of backazimuths consistent with previous work we document a pattern of fast directions parallel to the appalachian mountain chain suggesting a contribution from lithospheric deformation associated with appalachian orogenesis overall our measurements suggest that upper mantle anisotropy beneath the eastern united states is complex with likely contributions from both asthenospheric and lithospheric anisotropy in many regions

origin of azimuthal seismic anisotropy in oceanic plates and mantle abstract seismic anisotropy is ubiquitous in the earth s mantle but strongest in its thermo mechanical boundary layers azimuthal anisotropy in the oceanic lithosphere and asthenosphere can be imaged by surface waves and should be particularly straightforward to relate to well understood plate kinematics and large scale mantle flow however previous studies have come to mixed conclusions as to the depth extent of the applicability of paleo spreading and mantle flow models of anisotropy and no simple globally valid relationships exist here we show that lattice preferred orientation lpo inferred from mantle flow computations produces a plausible global background model for asthenospheric anisotropy underneath oceanic lithosphere the same is not true for absolute plate motion apm models a 200 km thick layer where the flow model lpo matches observations from tomography lies just below the 1200 c isotherm of a half space cooling model indicating strong temperature dependence of the processes that control the development of azimuthal anisotropy we infer that the depth extent of shear and hence the thickness of a relatively strong oceanic lithosphere can be mapped this way these findings for the background model and ocean basin specific deviations from the half space cooling pattern are found in all of the three recent and independent tomographic models considered further exploration of deviations from the background model may be useful for general studies of oceanic plate formation and dynamics as well as regional scale tectonic analyses

shear wave splitting in the appalachians and the urals a case for multilayered anisotropy observations of shear wave splitting in the northeastern u s appalachians and in the foredeep of the urals vary significantly with the back azimuth and incidence angle of the incoming phase these variations suggest two or more layers within the upper mantle with different anisotropic properties synthetic seismograms for simple multilayered anisotropic structures show that shear wave splitting parameters tend to vary substantially with the direction of approach relying on a subset of back azimuth and incidence angle may strongly bias the model inferred especially if the observations are averaged on the other hand the azimuthal splitting pattern provides additional constraints on vertical or lateral variation of anisotropic properties in the earth using a new error estimation technique for splitting we find that individual measurements from broadband data have errors of the order of 3 7 for the fast direction and 0 1 0 2 s for the delay of split shear waves the azimuthal variation of splitting parameters is broadly consistent throughout the appalachian terranes in the northeast united states especially for two long running stations in the northeast united states hrv harvard massachusetts and pal palisades new york observations can be separated into two distinct populations with mean fast axis azimuths of n60 e 4 and n119 e 2 delay values within each population range from near zero to 1 s azimuthal splitting variation for aru arti russia in the foredeep of uralian mountains is characterized by sharp transitions between different groups of observations using synthetic seismograms in simple structures we develop one dimensional anisotropic models under stations hrv and aru the model for hrv includes two layers of anisotropic material under an isotropic crust with fast axis azimuths n53 e and n115 e for the bottom and the top layers respectively the model for the upper mantle under aru includes a layer with a fast axis at n50 e atop a layer with fast axis azimuth n90 e our modeling confirms the need for a layer of strong anisotropy with a slow axis of symmetry in the lower crust under aru reported by levin and park 1997a our results suggest that both urals and appalachians possess a relict anisotropy in the tectosphere associated with past continental collision and accretion underlain by anisotropy with orientation similar to the local absolute plate motion suggesting an asthenospheric component to the signal

fluxes of fast and epithermal neutrons from lunar prospector evidence for water ice at the lunar poles maps of epithermal and fast neutron fluxes measured by lunar prospector were used to search for deposits enriched in hydrogen at both lunar poles depressions in epithermal fluxes were observed close to permanently shaded areas at both poles the peak depression at the north pole is 4 6 percent below the average epithermal flux intensity at lower latitudes and that at the south pole is 3 0 percent below the low latitude average no measurable depression in fast neutrons is seen at either pole these data are consistent with deposits of hydrogen in the form of water ice that are covered by as much as 40 centimeters of desiccated regolith within permanently shaded craters near both poles

shear wave splitting continental keels and patterns of mantle flow in this study we investigated the origin of seismic anisotropy in the mantle beneath north america in particular we evaluated whether shear wave splitting patterns in eastern north america are better explained by anisotropy caused by lithospheric deformation anisotropy due to mantle flow beneath the lithosphere or a combination of both we examined new measurements of shear wave splitting from the missouri to massachusetts broadband seismometer array moma the north american mantle anisotropy and discontinuity experiment nomad as well as splitting parameters from several previous studies we developed a simple finite difference model that approximates mantle flow around a complex three dimensional continental lithospheric keel to evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern north america we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model our results indicate that a significant portion of observed shear wave splitting in eastern north america can be explained by mantle flow around the continental keel however shear wave splitting patterns in a few regions of eastern north america indicate that a component of lithospheric anisotropy must exist particularly in regions containing the largest keel thicknesses for eastern north america as well as for splitting observations in australia europe and south america we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow

thin lithosphere beneath the central appalachian mountains constraints from seismic attenuation beneath the magic array abstract the passive margin of the eastern coast of the united states is known to be geologically active with recently rejuvenated topography intraplate seismicity and volcanism of eocene age this study uses seismic data from the mid atlantic geophysical integrative collaboration magic experiment to constrain lateral variations in the attenuation of teleseismic p waves beneath the central appalachian mountains to shed light on the structure and dynamics of the upper mantle at this active passive margin we use a monte carlo approach to estimate variations in attenuation along with both data and model uncertainties the quality factor of the upper mantle dramatically decreases over a distance of less than 50 km on the western side of the central appalachian mountains where a low velocity anomaly has been previously inferred extrinsic factors such as scattering or focusing are rejected as explanations for the observations on the basis of finite difference waveform modeling experiments the peak in attenuation beneath the crest of the appalachian mountains requires that near to super solidus conditions occur in the upper mantle and is co located with volcanism of eocene age our preferred interpretation is that the attenuation reflects the removal of the mantle lithosphere via delamination beneath the mountains followed by ongoing small scale convection

thick ice deposits in shallow simple craters on the moon and mercury permanently shadowed regions near the poles of mercury and the moon may cold trap water ice for geologic time periods in past studies thick ice deposits have been detected on mercury but not on the moon despite their similar thermal environments here we report evidence for thick ice deposits inside permanently shadowed simple craters on both mercury and the moon we measure the depth diameter ratio of approximately 2 000 simple craters near the north pole of mercury using mercury laser altimeter data we find that these craters become distinctly shallower at higher latitudes where ice is known to have accumulated on their floors this shallowing corresponds to a maximum infill of around 50 m consistent with previous estimates a parallel investigation of approximately 12 000 lunar craters using lunar reconnaissance orbiter data reveals a similar morphological trend near the south pole of the moon which we conclude is also due to the presence of thick ice deposits we find that previously detected surface ice deposits in the south polar region of the moon are spatially correlated with shallow craters indicating that the surface ice may be exhumed or linked to the subsurface via diffusion the family of lunar craters that we identify are promising targets for future missions and may also help resolve the apparent discrepancy between the abundance of frozen volatiles on mercury and the moon ice deposits up to around 50 m thick infill some craters near the moon s south pole and mercury s north pole as inferred from the poleward shallowing of simple craters

evolution of the yukon tanana terrane evidence from southeastern yukon territory the yukon tanana terrane ytt in southeastern yukon consists predominantly of a mid paleozoic volcanic plutonic arc assemblage built on continental crust the ytt had experienced strong deformation and metamorphism by late triassic time by mid cretaceous the metamorphic complex and late triassic clastic rocks derived from it were imbricated with middle and upper paleozoic ophiolitic sheets the ytt differs markedly from adjacent parts of the north american continental margin in both its depositional and tectonic histories the composition and tectonic history of the ytt is comparable to that of the kootenay and barkerville terranes of southern and central british columbia from which the ytt may have been displaced

seismic anisotropy of oceanic upper mantle shear wave splitting methodologies and observations we develop methodologies to obtain accurate measurements of shear wave splitting and apply these techniques to examine the pattern of oceanic upper mantle anisotropy to obtain high quality estimates of receiver splitting at island stations we devise a stacking method that finds the optimum splitting parameters and 95 error region for teleseismic shear wave phases from a suite of earthquake events to obtain additional measurements in oceanic regions we develop techniques to measure splitting parameters and errors for ss phases that sample upper mantle anisotropy at their bounce points however we find the data are often of low resolution and anomalous characteristics are sometimes found that make splitting difficult to interpret ten second splitting of ss is observed across the banjo seismic array but we cannot unambiguously attribute this signal to mantle anisotropy at the bounce the receiver splitting methods are used to assess the adequacy of a two anisotropic layer model for the pacific region with fast polarization azimuth in the lithosphere oriented in the fossil spreading direction and in the asthenosphere oriented in the absolute plate motion direction this model has been proposed to explain surface wave data in the pacific ocean but our splitting results demonstrate that oceanic anisotropy patterns are more heterogeneous than would be predicted while the island splitting measurements could reflect the influence of individual hotspot upwellings hotspot effects do not appear to be universally dominant we propose that splitting observations alternatively indicate broad scale differences in the underlying character of oceanic upper mantle anisotropy associated with coherant patters of lithospheric structure and asthenospheric flow in particular splitting surface wave models and regional studies all support a model where lithospheric anisotropy throughout the south pacific has been erased or reoriented toward the absolute plate motion direction whereas more limited observations in the north pacific indicate that the fossil lithospheric signature appears to be preserved

upper mantle seismic anisotropy as a constraint for mantle flow and continental dynamics of the north american plate abstract the alignment of intrinsically anisotropic olivine crystals under convection is typically invoked as the cause of the bulk of seismic anisotropy inferred from shear wave splitting sws this provides a means of constraining the interplay between continental dynamics and the deep mantle in particular for densely instrumented regions such as north america after usarray there a comparison of fast orientations from sws with absolute plate motions apm suggests that anisotropy is mainly controlled by plate motions however large regional misfits and the limited realism of the apm model motivate us to further explore sws based anisotropy if sws is estimated from olivine alignment in mantle circulation instead plate driven flow alone produces anisotropy that has large misfits with sws the addition of large scale mantle density anomalies and lateral viscosity variations significantly improves models although a strong continental craton is essential varying its geometry does however not improve the plate scale misfit moreover models based on higher resolution tomography degrade the fit indicating issues with the flow model assumptions and or a missing contributions to anisotropy we thus compute a lithospheric complement to achieve a best fit joint representation of asthenospheric and frozen in lithospheric anisotropy the complement shows coherent structure and regional correlation with independently imaged crustal and upper mantle anisotropy dense sws measurements therefore provide information on depth dependent anisotropy with implications for tectonics but much remains to be understood about continental anisotropy and its origin

a surface wave analysis of seismic anisotropy beneath eastern north america summary the nature of upper mantle seismic anisotropy beneath central and eastern north america has been evaluated using the phase velocities of surface waves traversing the length of the missouri to massachusetts moma broad band array frequency dependent phase delays of fundamental mode love and rayleigh waves measured across the array using a cross correlation procedure require the presence of anisotropy in the upper mantle 2 d radially anisotropic structures obtained via linearized inversion of these data contain shear anisotropy with a magnitude of 3 per cent between the moho and at least 180 km depth models in which the mantle lithosphere is isotropic cannot satisfy the data this anisotropy is approximately constant across the tectonic transition from the craton to the atlantic margin forward models consisting of a shallow lithospheric layer of azimuthal anisotropy derived from previous shear wave splitting observations fail to satisfy the surface wave delay times the combined surface wave and splitting results suggest the presence of two layers of anisotropy a lithospheric layer that produces the phase delay differences observed in love and rayleigh waves but is transparent to vertically propagating shear body waves and a deeper presumably asthenospheric layer that generates the shear wave splitting one plausible model is that the lithosphere is characterized by vertically heterogeneous anisotropic fabric which would produce minimal splitting in vertical shear waves such fabric has been hypothesized for regions of weak and complex splitting such as south africa and australia the results here imply that it may be appropriate for north america as well

evaluating contributions to sk k s splitting from lower mantle anisotropy a case study from station dbic cote d ivoire measurements of seismic anisotropy constitute a very important tool for examining patterns of flow and mineral properties in the earth s mantle a popular strategy for gaining insight into upper mantle processes is to examine the splitting of sk k s phases and interpret them in terms of upper mantle anisotropy and deforma tion in such studies any contribution to splitting from anisotropy in the lower mantle is usually ignored here we present measurements of sks and skks splitting at global seismograph network station dbic in the cote d ivoire which exhibits a very unusual pattern of shear wave splitting the splitting pattern is dominated by null measurements over a wide range of back azimuths with non null measurements found over a very limited back azimuthal range we also identified examples of dis crepant sks skks splitting for the same event station pair splitting at dbic has previously been interpreted in terms of upper mantle anisotropy but we argue that this splitting pattern can best be explained by an apparently isotropic upper mantle with a contribution from anisotropy in the lower mantle likely in the d layer using station dbic as a case study we discuss the potential pitfalls in interpreting sks splitting measurements and suggest a set of best practices to decrease the like lihood of misinterpreting shear wave splitting results at a seismic station online material summary of events used in the study with their associated splitting parameters

the slide mountain terrane and the structural evolution of the finlayson lake fault zone southeastern yukon the finlayson lake fault zone forms a fundamental but little studied tectonic boundary between strata of autochthonous north america and the accreted slide mountain and yukon tanana terranes in southeastern yukon a structural and petrologic study was undertaken to examine the depositional environment of the slide mountain terrane its tectono thermal evolution in the fault zone and its relationship with the yukon tanana terrane the slide mountain and yukon tanana terranes are divisible into units dominated by metavolcanic and metasedimentary rocks field observations and whole rock geochemistry indicate that slide mountain greenstone is ocean floor basalt deposited in a deep submarine basin with a proximal terrigenous sediment influx either a marginal or ocean basin setting is supported by the data slide mountain greenstone is thrust northeastward over metasedimentary rocks of slide mountain terrane and southwestward over rocks of the yukon tanana terrane regional metamorphic grade ranges from su

3 d shear wave radially and azimuthally anisotropic velocity model of the north american upper mantle summary using a combination of long period seismic waveforms and sks splitting measurements we have developed a 3 d upper mantle model sawum na2 of north america that includes isotropic shear velocity with a lateral resolution of 250 km as well as radial and azimuthal anisotropy with a lateral resolution of 500 km combining these results we infer several key features of lithosphere and asthenosphere structure a rapid change from thin 70 80 km lithosphere in the western united states wus to thick lithosphere 200 km in the central cratonic part of the continent closely follows the rocky mountain front rmf changes with depth of the fast axis direction of azimuthal anisotropy reveal the presence of two layers in the cratonic lithosphere corresponding to the fast to slow discontinuity found in receiver functions below the lithosphere azimuthal anisotropy manifests a maximum stronger in the wus than under the craton and the fast axis of anisotropy aligns with the absolute plate motion as described in the hotspot reference frame hs3 nuvel 1a in the wus this zone is confined between 70 and 150 km decreasing in strengthwithdepthfromthetop fromthermftothesanandreasfaultsystemandthejuande fuca gorda ridges this result suggests that shear associated with lithosphere asthenosphere couplingdominatesmantledeformationdowntothisdepthinthewesternpartofthecontinent the depth extent of the zone of increased azimuthal anisotropy below the cratonic lithosphere is not well resolved in our study although it is peaked around 270 km a robust result radial anisotropy is such that predominantly 1 where vsh vsv 2 under the continent and its borders down to 200 km with stronger in the bordering oceanic regions across the continent and below 200 km alternating zones of weaker and stronger radial anisotropy with predominantly 1 correlate with zones of small lateral changes in the fast axis direction of anisotropy and faster than average vs below the lab suggesting the presence of small scale convection with a wavelength of 2000 km finally in the western united states complex 3 d patterns of isotropic velocity and anisotropy reflect mantle dynamics associated with the rich tectonic history of the region

mid paleozoic initiation of the northern cordilleran marginal backarc basin geologic geochemical and neodymium isotope evidence from the oldest mafic magmatic rocks in the yukon tanana terrane finlayson lake district southeast yukon canada the fire lake formation of the yukon tanana terrane in the finlayson lake region yukon canada consists primarily of late devonian ca 365 360 ma mafic metavolcanic rocks and smaller volumes of mafic and ultramafic subvolcanic metamorphosed intrusions in this paper field geochemical and nd isotope attributes of these rocks are presented in an attempt to understand their tectonic setting the magmatic processes involved in their formation and their roles in cordil leran crustal growth the mafic rocks of the fire lake formation exhibit a wide diversity of geochemical signatures and are classified into seven chemically defined suites 1 back arc basin basalt 2 enriched mid oceanic ridge basalt e morb 3 oceanic island basalt oib 4 th rich oib 5 boninite 6 island arc tholeiite and 7 light rare earth element lree enriched island arc tholeiite the diversity of geochemical signatures is interpreted to represent variable mixtures of asthenospheric morb type mantle subarc mantle wedge and lithospheric oib type mantle with or without elemental contributions from the subducted slab and or continental crust these suites of rocks are also associated with fine grained basinal sedimentary facies variations in metavolcanic and metasedimentary unit thickness extensional synvolcanic faults and apparent extensional fault controlled emplacement of mafic intrusive rocks and hydrothermal volcanic hosted massive sulfide mineralization the suites also exhibit a broad spatial distribution those with arc signatures nb thmn 1 mn normalized to primitive mantle values are located primarily in the western parts of the formation and suites with nonarc signatures nb thmn 1 are located primarily within the eastern parts of the formation collectively these geologic and geochemical attributes are interpreted to stem from the transition from arc magmatism to the initiation of an extensional backarc basinal environment associated with an east dipping subduction zone the initiation of backarc basin magmatism recorded in the fire lake formation was part of a much larger late devonian backarc basinal system forming along the western edge of the margin of north america the fire lake formation is interpreted to represent 1 the commencement of yukon tanana arc rifting and separation from the north american cratonic margin and 2 the initiation of a marginal backarc basin now the slide mountain terrane inboard of the yukon tanana arc system this tectonic evolution likely occurred either as a result of slab rollback toward the west within the convergent margin responsible for yukon tanana arc activity or as a result of the propagation of the slide mountain backarc basin spreading ridges into the yukon tanana arc system this yukon tanana arc rifting episode was also broadly coincident with rifting and hydrothermal activity within rocks of the north american craton the geochemical and isotopic signatures of magmatic rocks in the fire lake formation have some features similar to intraoceanic arc rocks e g boninites island arc tholei ites and many have juvenile nd isotope signatures i e nd t 0 most have nd t 5 suggesting that the pericratonic terranes of the northern cordillera have a significant juvenile component if this is the case throughout the yukon tanana terrane then the pericratonic terranes may have contributed much more juvenile material to cordil leran crustal growth in the phanerozoic than has previously been considered

seismic anisotropy in the eastern united states deep structure of a complex continental plate we have analyzed shear wave splitting recorded by portable and permanent broadband and long period stations located in the eastern united states teleseismic shear waves sks skks and pks were used to retrieve the splitting parameters the orientation of the fast wave polarization plane and the delay time t in total 120 seismic events were processed allowing for more than 600 splitting measurements within the appalachians stations located in the western external part are characterized by t 1s and trending n50 70 e in the south and central regions and n30 40 e in the north closely following the trend of the orogenic belt in these areas the transition region between north and central is characterized by t 1 1 3 s and by e w trending that are at a high angle to the regional geologic trend measurements at two stations located in the eastern internal part of the belt indicate very weak anisotropy the large scale pattern of anisotropy is not consistent with that predicted for simple asthenospheric flow beneath the plate splitting along the southern and eastern margins of the continent is consistent with that expected for grenvillian deformation an alternative model of asthenospheric flow around the cratonic keel cannot be ruled out within the cratonic core the correlation between t and lithospheric thickness suggests a lithospheric anisotropy smaller length scale variations also argue for a significant contribution of lithospheric structures the fabric responsible for shear wave splitting may have formed during tectonic episodes that affected the eastern united states i e the grenville and appalachian orogenies and the subsequent rifting of the north atlantic ocean our observations in the western appalachians suggest that the anisotropy may be preserved since the grenvillian orogeny the absence of detectable splitting in the two stations in the eastern appalachians is attributed to the igneous intrusions related to the atlantic rifting the measurements in the transition between the northern and central southern appalachians constitute an intriguing anomaly whose e w have little obvious relation to the regional surface geology we suggest two possible causes 1 the local dominance of asthenospheric flow motivated by the proximity of a pervasive low velocity anomaly and 2 lithospheric deformation in a transcontinental strike slip fault zone active during the appalachian collision

paleozoic and mesozoic high pressure metamorphism at the margin of ancestral north america in central yukon eclogite and blueschist in the yukon tanana and slide mountain terranes in the yukon and alaska preserve a partial record of middle paleozoic late paleozoic and early mesozoic subduction and subsequent unroofing near faro eclogites are commonly mylonitized and partially retrogressed the analysis of fe mg exchange between garnet and clinopyroxene suggests temperatures t in the 400 500 c range for eclogite and jadeite 40 mol quartz suggest minimum pressures p of 11 13 kilobar at that temperature range clinopyroxene garnet epidote quartz suggest pressure near 15 kilobar p h 2 o p s eclogite near last peak which contains brown hornblende late stage oligoclase and biotite yielded p t estimates up to 625 c at a minimum pressure of 14 kilobar eclogite near ross river yielded temperatures near 525 c and a minimum pressure near 13 kilobar the previously proposed permian triassic age of high pressure tectonism for the entire yukon tanana terrane in the yukon was inferred from isotopic dating of one eclogite and one blueschist in separate localities in this study isotopic 40 ar 39 ar analysis of white mica from eclogite in the simpson range yielded a date of 344 1 ma eclogite and blueschist from ross river yielded white mica 40 ar 39 ar dates of 267 3 ma and 273 3 ma respectively white mica in eclogite from faro was dated at 260 3 ma white mica in two eclogites from stewart lake about 2 km apart yielded 40 ar 39 ar dates of 228 1 ma and 346 3 ma white mica in last peak eclogite for which a concordant u pb zircon date is 269 ma yielded an integrated date of 236 1 ma overprinting in the blueschist facies and final cooling in some localities predated eclogite metamorphism in others the cooling ages indicate significant plate convergence as early as carboniferous time and either the existence of more than one subduction zone or continuous convergence and episodic exhumation above a single zone when this is considered together with the range of ages of other high pressure occurrences the north american cordillera is seen to be characterized by multiepisodic and diachronous high pressure metamorphism

geochemical and nd isotopic constraints for the origin of eclogite protoliths northern cordillera implications for the paleozoic tectonic evolution of the yukon tanana terrane on the basis of trace element data basaltic protoliths for paleozoic eclogites from the yukon tanana terrane ytt have diverse origins eclogites from stewart lake and the simpson range have char

heterogeneous seismic anisotropy in the transition zone and uppermost lower mantle evidence from south america izu bonin and japan summary measurements of seismic anisotropy are commonly used to constrain deformation in the upper mantle observations of anisotropy at mid mantle depths are however relatively sparse in this study we probe the anisotropic structure of the mid mantle transition zone and uppermost lower mantle beneath the japan izu bonin and south america subduction systems we present source side shear wave splitting measurements for direct teleseismic s phases from earthquakes deeper than 300km that have been corrected for the effects of upper mantle anisotropy beneath the receiver in each region we observe consistent splitting with delay times as large as 1 s indicating the presence of anisotropy at mid mantle depths clear splitting of phases originating from depths as great as 600km argues for a contribution from anisotropy in the uppermost lower mantle as well as the transition zone beneath japan fast splitting directions are perpendicular or oblique to the slab strike and do not appear to depend on the propagation direction of the waves beneath south america and izu bonin splitting directions vary from trench parallel to trench perpendicular and have an azimuthal dependence indicating lateral heterogeneity our results provide evidence for the presence of laterally variable anisotropy and are indicative of variable deformation and dynamics at mid mantle depths in the vicinity of subducting slabs

subsurface migration of h2o at lunar cold traps 1 permanently shaded areas near the poles of the moon and mercury may harbor water ice we develop a physical model for migration of water molecules in the regolith and discover two pathways that can lead to accumulation of h2o in the subsurface a small fraction of water molecules delivered either continuously or abruptly to permanently cold areas diffuses into the regolith and can remain there longer than on the surface higher temperatures lead to deeper burial at constant temperature this diffusive migration produces less than one molecular layer of volatile h2o on grains because it is driven by differences in surface concentrations the water is therefore expected to be in adsorbed form and the amount stored in this fashion could be at most a few hundred ppm of h2o a second pathway is pumping by diurnal temperature oscillations from a transient ice cover that may have formed during a large comet impact it can lead to high ground ice densities but the ground ice layer lasts not long beyond the disappearance of the ice cover both types of subsurface charging mechanism work best for temperatures typical of permanently shaded areas with sunlit surfaces in their field of view

upper mantle anisotropy and transition zone thickness beneath southeastern north america and implications for mantle dynamics a variety of models for mantle flow beneath southeastern north america have been proposed including those that invoke westward driven return flow from the sinking farallon slab small scale convective downwelling at the edge of the continental root or the upward advective transport of volatiles from the deep slab through the upper mantle we use shear wave splitting observations and receiver function analysis at broadband seismic stations in the southeastern united states to test several of these proposed mantle flow geometries near the coast stations exhibit well resolved null no splitting behavior for sks phases over a range of back azimuths consistent with either isotropic upper mantle or with a vertical axis of anisotropic symmetry farther inland we identify splitting with mainly ne sw fast directions consistent with asthenospheric shear due to absolute plate motion apm lithospheric anisotropy aligned with appalachian tectonic structure or a combination of these phase weighted stacking of individual receiver functions allows us to place constraints on the timing of arrivals from the 410 and 660 km discontinuities and on average transition zone thickness beneath individual stations at most stations we find transition zone thicknesses that are consistent with the global average 240 km with two stations showing evidence for a slightly thickened transition zone 250 km our results are relevant for testing different models for mantle dynamics beneath the southeastern united states but due to the sparse station coverage we are unable to uniquely constrain the pattern of mantle flow beneath the region our sks splitting observations support a model in which mantle flow is primarily vertical either upwelling or downwelling beneath the southeastern edge of the north american continent in contrast to the likely horizontal apm driven flow beneath the continental interior however our receiver function analysis does not provide unequivocal support either for widespread hydration of the transition zone or for widespread thickening due to the downwelling of relatively cold mantle material we expect that the necessary data to constrain such models more tightly can be obtained from the operation of denser seismic networks including the transportable array and flexible array components of usarray

lithospheric and asthenospheric contributions to shear wave splitting observations in the southeastern united states we present observations of both null and non null sks splitting from temporary deployments across the southeastern united states in order to evaluate the relative contributions of lithospheric deformation and asthenospheric flow to regional anisotropy data for this study come from four temporary broadband seismic deployments the appalachian seismic transect ast the test experiment for eastern north america teena the south carolina earth physics project scepp and the florida to edmonton array fled in general we find fast directions aligned roughly parallel to absolute plate motion of the north american plate apm within and west of the southern appalachians whereas to the southeast we find a broad area dominated by complex splitting patterns consisting of well constrained null splitting measurements over a range of backazimuths along with a very small number of resolved non null measurements this change in splitting patterns is consistent with a transition from drag induced asthenospheric flow beneath the older sections of the north american continent to vertical or incoherent mantle flow likely in combination with complex lithospheric anisotropy beneath the younger accreted terranes to the southeast in addition to these general patterns we find a number of non null splitting measurements that are not aligned with apm but are instead aligned with prominent magnetic anomalies that may correspond to ancient continental suture zones or faults this would suggest that in these areas a strongly anisotropic but localized lithospheric fabric dominates over any ambient asthenospheric anisotropic signature in areas with generally strong apm parallel splitting this would imply a thick sheared mantle lithosphere whose deformation induced anisotropy is strong enough to overprint the anisotropy induced by apm and is aligned with the shallower crustal structures responsible for generating the observed magnetic anomalies in the southeastern areas dominated by null splitting measurements there may be no strong signature from asthenospheric anisotropy to override but a substantial lithospheric thickness is still required to generate the magnitude of the observed sks splitting 1 s more data are required to verify these results but future datasets including data from usarray may be able to exploit the correlations between null and non null sks splitting measurements and magnetic lineaments to better constrain the provenance of the regional anisotropic signature

superimposed quesnel late paleozoic jurassic and yukon tanana devonian mississippian arc assemblages cassiar mountains northern british columbia field u pb and igneous petrochemical evidence allochthons in the cassiar mountains of northern british columbia contain assemblages belonging to two distinct canadian cordilleran terranes yukon tanana ytt and quesnellia these assemblages of pre late devonian devonian mississippian pennsylvanian permian and early jurassic age occur in intrusive and depositional as well as structural contact with each other the allochthons are gently dipping thrust panels interrupted by the mid cretaceous cassiar batholith a key element for correlation across the batholith is the mississippian and older pericratonic dorsey complex new devonian mississippian u pb ages for deformed plutons within it document an igneous suite like those in type yukon tanana exposures farther north other characteristics of the dorsey complex that ally it with ytt are orthoquartzites and grits and amphibolite bodies with transitional mid ocean ridge basalt morb to ocean island basalt oib petrochemical signatures unconformities deformed clasts in the late paleozoic sequences and a shared mid permian intrusive suite show that later arcs onlapped the mid paleozoic and older ytt assemblage the early jurassic intrusive suite cuts all major contacts and fabrics except the terrane bounding fault between the slide mountain and combined ytt quesnel terranes it represents a northern continuation of a plutonic belt that extends the length of the mesozoic quesnel magmatic arc these relationships carry important implications for cordilleran terrane history and the tectonic evolution of the north american margin at least some of the major terranes were not unrelated entities prior to their accretion to the continent but a system of superimposed and interconnected arcs that developed over a protracted time interval with complex and evolving paleogeographic configurations much like the modern western pacific province

yukon tanana terrane in the southern campbell range finlayson lake belt southeastern yukon the geological setting of retrogressed eclogite of the klatsa metamorphic complex yukon tanana terrane in the southern campbell range is composed of rocks that have different metamorphic exhumation and structural histories and that have formed in disparate parts of the paleozoic yukon tanana volcanic arc the geological relationships in the southern campbell range reveal the tectonic and structural history of the klatsa metamorphic complex which represents the remnants of an early mississippian subduction zone beneath the yukon tanana arc the klatsa metamorphic complex is composed of foliated to massive serpentinite leucogabbro amphibolite and retrogressed eclogitic quartz muscovite schist with lenses of metabasite it was structurally juxtaposed on upper mississippian to lower permian metasedimentary rocks of the white lake king arctic and money creek formations regional and local structural and stratigraphic relationships suggest that the klatsa metamorphic complex is part of the cleaver lake thrust sheet the structurally highest thrust sheet in a north to northeast verg

dynamics of lithospheric thinning and mantle melting by edge driven convection application to moroccan atlas mountains edge driven convection edc forms in the upper mantle at locations of lithosphere thickness gradients e g craton edges in this study we show how the traditional style of edc a convection cell governed by the cold downwelling below an edge alternates with another style of edc in which the convection cell forms as a secondary feature with a hot asthenospheric shear flow from underneath the thicker lithosphere these alternating edc styles produce episodic lithosphere erosion and decompression melting three dimensional models of edc show that convection rolls form perpendicular to the thickness gradient at the lithosphere asthenosphere boundary stagnant lid convection scaling laws are used to gain further insight in the underlying physical processes application of our models to the moroccan atlas mountains region shows that the combination of these two styles of edc can reproduce many of the observations from the atlas mountains including two distinct periods of cenozoic volcanism a semicontinuous corridor of thinned lithosphere under the atlas mountains and piecewise delamination of the lithosphere a very good match between observations and numerical models is found for the lithosphere thicknesses across the study area amounts of melts produced and the length of the quiet gap in between volcanic episodes show quantitative match to observations

seismic anisotropy beneath the juan de fuca plate system evidence for heterogeneous mantle flow here we use sks shear wave splitting observations from ocean bottom seismometer data to infer patterns of mantle deformation beneath the juan de fuca plate and its adjoining boundaries our results indicate that the asthenosphere beneath the juan de fuca plate responds largely to absolute plate motion with an anisotropic layer developing rapidly near the ridge and persisting into the subduction zone geographically restricted deviations from this pattern indicate the presence of secondary processes at discrete plate boundaries such as the blanco transform fault seismic anisotropy is attributed to relative plate motion within a narrow zone 50 km beneath the deforming southern gorda plate region a diffuse plate boundary splitting observations similarly suggest deformation dominated by relative motion between the rigid juan de fuca and pacific plates but distributed over a broad zone 200 km our results are inconsistent with toroidal flow around the southern edge of the subducting slab due to rollback as suggested by onshore studies instead reorganization of upper mantle flow associated with plate fragmentation seems to dominate the anisotropic signature of southern cascadia

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

comparative fe and sr isotope study of nephrite deposits hosted in dolomitic marbles and serpentinites from the sudetes sw poland implications for fe as au bearing skarn formation and post obduction evolution of the oceanic lithosphere abstract nephrites belonging to both the dolomite related and serpentinite related genetic types from the zloty stok deposit and jordanow l ski and naslawice prospects in the leza ophiolite respectively were analyzed for fe and sr isotope compositions these deposits in the central sudetes sw poland are unusually situated relatively close to each other this study is a first attempt to perform combined fe and sr isotope study of nephrites our results show that fe in the fe as au bearing skarn samples from zloty stok 2 77 7 00 wt fe 56fe 0 07 to 0 14 and 57fe 0 12 to 0 21 to which the dolomite related nephrite belongs is not inherited from the host dolomitic marbles ca 0 4 wt fe 56fe 0 08 to 0 07 and 57fe 0 12 to 0 09 but derived from the nearby ca 340 ma granite intrusion 2 64 5 64 wt fe in contrast iron in the serpentinite related nephrites 2 06 3 88 wt fe 56fe 0 41 to 0 45 and 57fe 0 59 to 0 66 was likely inherited from host serpentinites 5 27 4 81 wt fe rather than metasomatically derived from adjacent granite intrusions 0 13 0 45 wt fe moreover both the marble transformation into a dolomite related nephrite and fe as au bearing skarn and the abyssal serpentinite through ophiolite towards serpentinite related nephrite transformation are accompanied by a progressive increase of 56fe and 57fe this increase is regardless of changes in the bulk fe content increasing during the dolomite related nephrite formation and decreasing during serpentinite related nephrite formation this phenomena may be caused by one of the below listed factors or combinations a preferential incorporation of heavy iron by tremolite and diopside b preferential incorporation of the heavy iron in the fe3 bearing phases crystallizing during nephrite formation c preferential incorporation of the heavy fe from migrating granite derived fluids positive fractionation between aqueous fe bearing fluid and tremolite diopside d positive isotope fractionation between dolomite antigorite precursor and newly formed amphibole clinopyroxene e change of the fe oxidation state furthermore fe in the rock forming tremolite from the serpentinite related nephrites is isotopically heavier than fe in tremolite from the dolomite related ones in contrast to the iron the isotopic composition of strontium combined with its content and structural evidence suggest multiple sources for this element in the case of the dolomite related nephrite ca 3 4 ppm sr initial 87sr 86sr 0 7085 sr inheritance from the host dolomitic marble ca 90 ppm sr initial 87sr 86sr 0 7081 as well as its derivation from intrusions of both the syn to late tectonic ca 340 ma granites ca 400 440 ppm sr and post tectonic 305 ma granites 86 4 ppm sr is suggested in the case of the serpentinite related nephrite 6 3 61 7 ppm sr initial 87sr 86sr 0 7037 to 0 7081 the situation is similar i e sr in the most primitive samples tends to be sourced from ophiolitic rocks up to 2 4 ppm sr in serpentinite whereas in the more evolved types it was likely delivered from the adjacent ca 340 ma 8 8 102 6 ppm sr partially rodingitized granites as well as 305 ma granites ca 80 ppm sr moreover these granites caused the fluid flow responsible for sr bearing clinozoisite vein formation these veins are likely related with nephrite formation and with a metallic mineralization

sks splitting in the western indian ocean from land and seafloor seismometers plume plate and ridge signatures we present sks splitting measurements in the western indian ocean recorded on 20 land and 57 seafloor seismometers deployed by the rhum rum experiment reunion hotspot and upper mantle reunions unterer mantel we discuss our splitting observations within their geodynamic settings and compare them to sks splitting parameters predicted from an azimuthally anisotropic rayleigh wave tomography model that includes the rhum rum data we find that anisotropic directions poorly correlate with the present day motion of the somali plate which at 2 6 cm yr may be too slow to cause strongly sheared fabric in the asthenosphere fast split directions between la reunion and the central indian ridge cir trend e w and provide strong first seismological evidence for near horizontal flow in the asthenosphere that connects the reunion mantle upwelling with the cir supporting a long standing hypothesis on plume ridge interaction in the vicinity of the reunion hotspot we observe a seismic anisotropy pattern indicative of a parabolic asthenospheric flow controlled by the reunion mantle upwelling and its consecutive asthenospheric spreading we furthermore observe ridge normal along the cir and ridge parallel along the southwest indian ridge swir both mainly attributed to asthenospheric mantle flows in the mozambique channel between east africa and madagascar we attribute e w trending to frozen lithospheric structures recording the paleo orientation of the spreading ridges that enabled madagascar s separation away from africa based on the synopsis of this and previous sks splitting studies at mid ocean ridges we propose that ridge normal may develop at fast and intermediate spreading ridges e g cir and east pacific rise and ridge parallel could be characteristic to slow spreading ridges e g swir mid atlantic ridge and the paleo ridges in the mozambique channel