apollo 16 regolith breccias characterization and evidence for early formation in the mega regolith all of the apollo 16 regolith breccias 8 specimens have been characterized in terms of their petrography grain size distribution porosity major and trace element composition noble gas contents and ferromagnetie resonance properties these breccias vary signkqcantly with respect to their density and porosity with the more dense breccias showing significant shock damage the regolith breccias resemble the soils in grain size distribution and in the relative proportions of major petrological components except agglutinates many of the breecias are compositionally different from the apollo 16 soils in that they lack an important marie component present in the soils although some groupings occur the petrologic and chemical compositions of the regolith breedas do not correlate with the station location of the samples all but one of the breceias show some evidence of irradiation at the lunar surface solar gases measurable fmr agglutinates and analyses made on grain size separates from two disaggregated breedas indicate that this irradiation occurred before compaction when the breccia material was finely disseminated on the surface however the concentrations of surface irradiation parameters solar gases fmr agglutinates for most breedas are far less than seen in any lunar soils or in regolith breccias from other apollo missions several breedas also contain unusually high trapped 4oar 36ar ratios of 8 12 and a significant fission xe component in excess of that expected from in situ production these observations suggest that the surface irradiation of these breedas occurred as early as 4 x 109 years ago we conclude that most of the apollo 16 regolith breedas were not formed from any known apollo 16 soil they appear to be well comminuted material that contains ancient regolith developed during the late stage heavy bombardment of the moon when large impacts were much more common relative to small impacts so that regoliths did not have time to significantly mature before being diluted by fresh ejecta and buffed this ancient megaregolith is significantly different from more recent lunar regolith but may be similar to asteroidal regoliths from which some brecciated meteorites have formed

impact melt breccia and surrounding regolith measured by chang e 4 rover abstract chang e 4 rover discovered a dark greenish and glistening impact melt breccia in a crater during its traverse on the floor of von karman crater within the south pole aitken spa basin on the lunar farside the discovered breccia being 52 16 cm resembles the lunar impact melt breccia samples 15466 and 70019 that returned by the apollo missions it was formed by impact generated welding cementing and agglutinating of lunar regolith and breccia clods surrounds the breccia hosting crater were crushed into regolith powders by the rover s wheels indicating the regolith may be compacted slightly and becomes blocky and friable relative mineral fractions are estimated from the in situ measured spectra by a hapke model based unmixing algorithm unmixing reveals that plagioclase plg 45 6 is dominant in the regolith followed by almost equal fractions of pyroxene pyx 7 1 and olivine ol 6 2 indicating the regolith is likely related to noritic rocks the regolith measured by chang e 4 rover was actually a highly mixture of multiple sources with ejecta from finsen crater being primary and possible contributions from alder crater finsen and alder craters are on the margin of the proposed impact melt pool produced by the spa basin forming event therefore the ultimate source of the regolith might originate from a differentiated melt pool or from a suite of igneous rocks

the geochemistry and provenance of apollo 16 mafic glasses the regolith of the apollo 16 lunar landing site is composed mainly of feldspathic lithologies but mafic lithologies are also present a large proportion of the mafic material occurs as glass we determined the major element composition of 280 mafic glasses 10 wt feo from six different apollo 16 soil samples a small proportion 5 of the glasses are of volcanic origin with picritic compositions most however are of impact origin approximately half of the mafic impact glasses are of basaltic composition and half are of noritic composition with high concentrations of incompatible elements a small fraction have compositions consistent with impact mixtures of mare material and material of the feldspathic highlands on the basis of major element chemistry we identified six mafic glass groups vlt picritic glass low ti basaltic glass high ti basaltic glass high al basaltic glass kreepy glass and basaltic andesite glass these glass groups encompass 60 of the total mafic glasses studied trace element analyses by secondary ion mass spectroscopy for representative examples of each glass group 31 total analyses support the major element classifications and groupings the lack of basaltic glass in apollo 16 ancient regolith breccias which provide snapshots of the apollo 16 soil just after the infall of imbrium ejecta leads us to infer that most if not all of the basaltic glass was emplaced as ejecta from small or moderate sized impacts into the maria surrounding the apollo 16 site after the imbrium impact the high ti basaltic glasses likely represent a new type of basalt from mare tranquillitatis whereas the low ti and high al basaltic glasses possibly represent the composition of the basalts in mare nectaris both the low ti and high al basaltic glasses are enriched in light rees which hints at the presence of a kreep bearing source region beneath mare nectaris the basaltic andesite glasses have compositions that are siliceous ferroan alkali rich and moderately titaniferous they are unlike any previously recognized lunar lithology or glass group their likely provenance is within the procellarum kreep terrane but they are not found within the apollo 16 ancient regolith breccias and therefore were likely deposited at the apollo 16 site post imbrium the basaltic andesite glasses are the most ferroan variety of kreep yet discovered

why cold slabs stagnate in the transition zone oceanic lithosphere sinks stagnates and is deflected sub horizontally beneath western pacific island arcs requiring buoyancy in the slab that is inconsistent with a thermal origin the transformation of pyroxene to majoritic garnet occurs by extremely slow diffusion and pyroxene is therefore unlikely to transform at equilibrium pressures and temperatures in the cold interior of slabs we present high resolution numerical simulations showing that when slow diffusion inhibits the dissolution of pyroxene into garnet the slab becomes buoyant relative to the ambient mantle and stagnates whereas when the phase transformations occur in equilibrium there is no effect on the slab we test the model by comparing slab temperature and geometry and find that sub horizontal slabs are more likely colder than average consistent with our numerical simulations

the lunar regolith chemistry mineralogy and petrology the lunar regolith a several meter thick layer of unconsolidated debris forms the interface between the moon and its space environment the regolith forms from lithic sources by the destructional processes of comminution and the constructional processes of agglutinate formation in this manner a steady state soil can develop and will remain in a type of dynamic equilibrium until the system is disturbed by burial or mixing with new soil mixing takes place mainly on a local scale vertically through meters and horizontally over several kilometers lateral transport is a relatively inefficient process and therefore most of the lunar soil components were derived locally there is no evidence for preferential transport of the finer soil fractions relative to the coarser fractions the truly exotic derived at distances of tens of kilometers component is generally in low abundance 1 therefore remotely sensed data should provide a reasonably accurate picture of bedrock geology the chemical composition of the regolith varies with grain size the 10 m fraction is chemically fractionated from the bulk soil in most cases the 10 m fraction is enriched in al2o3 cao na2o k2o light rare earth elements and th and depleted in mgo feo mno and sc these systematics are largely a result of simple comminution with feldspar and friable mesostasis from local source rocks concentrating in the finest fraction there is evidence that agglutinate glass forms preferentially by fusion of the finest soil fractions therefore agglutinate glass like the 10 m soil fraction appears to be fractionated from bulk soil compositions highland soils are distinct from mare soils both chemically and petrologically soil samples collected near mare highland contacts are mixtures of the two lithologies mineral chemical systematics of monomineralic fragments in the lunar soil provide important clues to soil provenance they can in some cases sort out the highland mare contributions and in special cases can even further pinpoint a lithic source in theory lunar regolith cores can serve as solar probes recording the sun s activity over billions of years in practice this has generally not been the case the formation and evolution of most cores is still poorly understood for most we have not established how they formed for example how many depositional and erosional events were involved what the time span involved in the accretion of the sampled soil column was and how long that soil column has remained in situ without this information it is difficult if not impossible to read the sun s record more interdisciplinary studies on cores must be carried out before they can be used as solar probes

lunar surface dust dynamics and regolith mechanics 1 the lunar surface is characterized by a collisionally evolved regolith resulting from meteoroid bombardment this lunar soil consists of highly angular particles in a broad approximately power law size distribution with impact generated glasses the regolith becomes densified and difficult to excavate when subjected to lunar quakes or eventually manned and unmanned activity on the surface solar radiation and the solar wind produce a plasma sheath near the lunar surface lunar grains acquire charge in this environment and can exhibit unusual behavior including levitation and transport across the surface because of electric fields in the plasma sheath the fine component of the lunar regolith contributes to the operational and health hazards posed to planned lunar expeditions in this paper we discuss the mechanical response of the regolith to anticipated exploration activities and review the plasma environment near the lunar surface and the observations models and dynamics of charged lunar dust

reconstructing farallon plate subduction beneath north america back to the late cretaceous using an inverse mantle convection model that assimilates seismic structure and plate motions we reconstruct farallon plate subduction back to 100 million years ago models consistent with stratigraphy constrain the depth dependence of mantle viscosity and buoyancy requiring that the farallon slab was flat lying in the late cretaceous consistent with geological reconstructions the simulation predicts that an extensive zone of shallow dipping subduction extended beyond the flat lying slab farther east and north by up to 1000 kilometers the limited region of flat subduction is consistent with the notion that subduction of an oceanic plateau caused the slab to flatten the results imply that seismic images of the current mantle provide more constraints on past tectonic events than previously recognized

aegean tectonics strain localisation slab tearing and trench retreat abstract we review the geodynamic evolution of the aegean anatolia region and discuss strain localisation there over geological times from late eocene to present crustal deformation in the aegean backarc has localised progressively during slab retreat extension started with the formation of the rhodope metamorphic core complex eocene and migrated to the cyclades and the northern menderes massif oligocene and miocene accommodated by crustal scale detachments and a first series of core complexes mccs extension then localised in western turkey the corinth rift and the external hellenic arc after messinian times while the north anatolian fault penetrated the aegean sea through time the direction and style of extension have not changed significantly except in terms of localisation the contributions of progressive slab retreat and tearing basal drag extrusion tectonics and tectonic inheritance are discussed and we favour a model 1 where slab retreat is the main driving engine 2 successive slab tearing episodes are the main causes of this stepwise strain localisation and 3 the inherited heterogeneity of the crust is a major factor for localising detachments the continental crust has an inherited strong heterogeneity and crustal scale contacts such as major thrust planes act as weak zones or as zones of contrast of resistance and viscosity that can localise later deformation the dynamics of slabs at depth and the asthenospheric flow due to slab retreat also have influence strain localisation in the upper plate successive slab ruptures from the middle miocene to the late miocene have isolated a narrow strip of lithosphere still attached to the african lithosphere below crete the formation of the north anatolian fault is partly a consequence of this evolution the extrusion of anatolia and the aegean extension are partly driven from below asthenospheric flow and from above extrusion of a lid of rigid crust

petrologic comparisons of cayley and descartes on the basis of apollo 16 soils from stations 4 and 11 petrologic aspects of the cayley and descartes formations are reviewed in the light of new data on apollo 16 soils specific comparison of the modal abundances of lithic fragments in drive tube sample 64001 2 from the slopes of stone mountain station 4 and in soil 67941 from the north ray crater rim station 11 shows that melt rocks especially poikilitic rocks are more abundant at station 4 than at station 11 the reverse is true for fragmental breccias such lithologic differences suggest that stations 4 and 11 do not belong to the same geologic formation metamorphosed breccias are pervasive in both the formations and may represent a local component that has been reworked and diluted as fresh materials were added lithologic compositions inferred from the study of soil samples are different from lithologic compositions inferred from the study of rake samples or breccia clasts this difference may be related to a mixing of material of different grain size distributions the petrology of soils at the apollo 16 site may not accurately reflect original material associated with either the descartes or the cayley formation because of extensive mixing with local material

petrologic profile of apollo 16 regolith at station 4 the apollo 16 double drive tube core 64001 2 total depth about 60 cm consists of four petrologic units based on the modal abundance of lithic monomineralic and other particles in the 20 500 micron size range of samples from 12 levels variation in the abundance of particles derived directly from bedrocks is low and the core soils probably represent only one set of source rocks the chemical signature of an excessively high mare basalt component at about 42 cm correlates with an excess of regolith breccias which may then be the physical carrier the core soils as a whole show a systematic size composition relationship that may be attributed to a large scale macroscopic reworking and the consequent masking of any or all previous surface process related mixing event however on a much smaller scale about 0 5 cm individual soil layers seem to have evolved more via mixing than via in situ reworking

geodynamic regimes of subduction under an active margin effects of rheological weakening by fluids and melts the dynamics of subduction under an active margin are analysed by using a 2d coupled petrological thermomechanical numerical model of an oceanic continental subduction system the model includes spontaneous slab bending dehydration of the subducted crust aqueous fluid transport partial melting of both crustal and mantle rocks as well as melt extraction processes resulting in magmatic arc crust growth based on our models the following five geodynamic regimes of subduction which may potentially form at active margins are identified i stable subduction with no backarc spreading centre and without plumes in the mantle wedge ii retreating subduction with a focused backarc spreading centre and without plumes iii retreating subduction with distributed intra arc extension and trans lithospheric sedimentary plumes iv advancing subduction with underplating laterally extending sub lithospheric plumes and v stable to advancing subduction with stationary laterally limited sub lithospheric plumes transitions between these different regimes are mainly caused by the concurrence of rheological weakening effects of i aqueous fluids percolating from the subducting slab into the mantle wedge and ii melts propagating from the partially molten areas formed in the mantle wedge towards the surface the aqueous fluids mainly affect the forearc region strong fluid related weakening promotes plate decoupling and reduces subduction drag and thus results in stacking of sediments in the accretion prism in contrast reduced weakening by fluids results in strong coupling of the plates and leads to advancing collision like subduction with enhanced subduction erosion thickening of the overriding plate and large sedimentary plumes in the mantle wedge are the consequences on the other hand melts extracted from the hot regions above the slab rheologically weaken the lithosphere below the arc which thus controls overriding plate extension and shortening strong rheological weakening by melts in combination with weak plate coupling trigger retreating subduction with a pronounced backarc spreading centre also weakening of the continental lithosphere by melts extracted from trans lithospheric sedimentary plumes generate a weak channel through which these structures may be emplaced into subarc crust if there is insufficient melt related weakening plumes cannot ascend but extend horizontally and thus underplate the lithosphere

the role of deep subduction in supercontinent breakup abstract the breakup of continents and their subsequent drifting plays a crucial role in the earth s periodic plate aggregation and dispersal cycles while continental aggregation is considered the result of oceanic closure during subduction what drives sustained divergence in the following stages remains poorly understood in this study thermo mechanical numerical experiments illustrate the single contribution of subduction and coupled mantle flow to the rifting and drifting of continents we quantify the drag exerted by subduction induced mantle flow along the basal surface of continental plates comparing models of lithospheric slab stagnation above the upper lower mantle boundary with those where slabs penetrate into the lower mantle when subduction is upper mantle confined divergent basal tractions localise at distances comparable to the effective upper mantle thickness 500 km causing the opening of a marginal basin instead subduction of lithosphere in the lower mantle reorganises the flow into a much wider cell localising extensional stresses at greater distances from the trench 3000 km sub continental tractions are higher and more sustained over longer time periods in this case and progressively increase as the slab sinks deeper although relatively low basal shear stresses when integrated over large plates generate tension forces that may exceed the strength of the continental lithosphere eventually leading to breakup and opening of a distal basin the models illustrate the emergence of a similar mechanism which results in the formation of back arc basins above upper mantle confined subduction and scales to much larger distances for deeper subduction examples include the atlantic ocean formation and drifting of the south and north american plates during the mesozoic cenozoic farallon plate subduction

the viscosity of earth s lower mantle inferred from sinking speed of subducted lithosphere abstract the viscosity of the mantle is indispensable for predicting earth s mechanical behavior at scales ranging from deep mantle material flow to local stress accumulation in earthquakes zones but mantle viscosity is not well determined for the lower mantle particularly only few constraints result from elaborate high pressure experiments karato 2008 and a variety of viscosity depth profiles result from joint inversion of the geoid and postglacial rebound data forte and mitrovica 1996 kaufmann and lambeck 2000 mitrovica and forte 2004 here we use inferred lower mantle sinking speed of lithosphere subduction remnants as a unique internal constraint on modeling the viscosity profile this entails a series of elaborate dynamic subduction calculations spanning a range of viscosity profiles from which we select profiles that predict the inferred sinking speed of 12 3 mm yr van der meer et al 2010 our modeling shows that sinking speed is very sensitive to lower mantle viscosity good predictions of sinking speed are obtained for nearly constant lower mantle viscosity of about 3 4 1022 pa s viscosity profiles incorporating a viscosity maximum in the deep lower mantle as proposed in numerous studies only lead to a good prediction in combination with a weak postperovskite layer at the bottom of the lower mantle and only for a depth average viscosity of 5 1022 pa s

some things we can infer about the moon from the composition of the apollo 16 regolith randy l korotev department of earth and planetary sciences and mcdonnell center for the space sciences washington university st louis missouri 63130 usa author s e mail address rlk levee wustl edu received 1996 september 16 accepted in revised form 1997 may 12 abstract characteristics of the regolith of cayley plains as sampled at the apollo 16 lunar landing site arereviewed and new compositional data are presented for samples of 1 mm fines soils and 1 2 mm rego lith particles as a means of determining which of the many primary igneous and secondary crystalline brec cias lithoiogic components that have been identified in the soil are volumetrically important and providing anestimate of their relative abundances more than 3 x 106 combinations of components representing nearly everylithology that has been observed in the apollo 16 regolith were systematically tested to determine which com binations best account for the composition of the soils conclusions drawn from the modeling include the fol lowing at the site mature soil from the cayley plains consists of 64 5 2 7 components representing prebasin materials anorthosites feldspathic breccias and a small amount 2 6 1 5 of total soil ofnonmare mafic plutonic rocks mostly gabbronorites on average these components are highly feldspathic with average concentrations of 31 32 a1203 and 2 3 feo and a molar mg mg fe ratio of 0 68 theremaining 36 of the regolith is syn and postbasin material 28 8 2 4 mafic impact melt breccias mimbs i e lkfm and vha basalts created at the time of basin formation 6 0 1 4 mare derived material impact and volcanic glass crystalline basalt with an average tio2 concentration of 2 4 and 1 postbasin meteoritic material the mimbs are the principal 80 90 carrier of incompatible traceelements rare earths th etc and the carrier of about one half of the siderophile elements and elementsassociated with mafic mineral phases fe mg mn cr sc most 71 of the fe in the present regolithderives from syn and postbasin sources mimbs mare derived material and meteorites thus althoughthe bulk composition of the apollo 16 regolith is nominally that of noritic anorthosite the noritic part themimbs and the anorthositic part the prebasin components are largely unrelated there is compositionalevidence that 3 4 of the soil is th rich material such as that occurring at the apollo 14 site and one frag ment of this type was found among the small regolith particles studied here if regolith such as that repre sented by the apollo 16 ancient regolith breccias was a protolith of the present regolith such regolith cannotexceed 71 of the present regolith the rest must be material added or redistributed since closure of theancient regolith breccias the postclosure material includes the mare derived material and the apollo 14 like component compositions of all mature surface soils from apollo 16 even those collected 4 km apart on the cayleyplains are very similar which is in stark contrast to the wide compositional range of the lithologies of whichthe soil is composed this uniformity indicates that the ratio of mimbs to feldspathic prebasin componentsis not highly variable in the megaregolith over distances of a few kilometers that there are no large sub surface concentrations of pure mafic impact melt breccia and that the intimate mixing is inherent to thecayley plains at a gross scale thus the mixing ofmafic impact melt breccias and feldspathic prebasin com ponents must have occurred during formation and deposition of the cayley plains such uniformity could nothave been achieved by small postdeposition impacts into a stratified megaregolith using this conclusion asone constraint and the known distribution of th on the lunar surface as another and the assumption that the im brium impact is primarily responsible for formation of the cayley plains arguments are presented that theapollo 16 mimbs derive from the imbrium region and consequently that one fourth of the apollo 16 regolithis primary imbrium ejecta in the form of mafic impact melt breccias

japan sea opening history and mechanism a synthesis the respective tectonic effects of back arc spreading and continental collision in asia are considered either as two independent processes or as closely interrelated extrusion tectonics assumes that the opening of the south china sea and the left lateral motion along the red river fault are geometrically linked in a pull apart manner this model is not accepted by several workers because the structural link between the two processes is not clearly demonstrated in the case of the japan sea we can show without ambiguity that back arc opening was controlled by large intracontinental strike slip faults which can be easily understood as effects of the india asia collision far from the indenter the japan sea opened in the early miocene in a broad pull apart zone between two major dextral strike slip shear zones the first one extends from north sakhalin to central japan along 2000 km it has accommodated about 400 km of finite displacement deformation along it varies from dextral transpression in the north to dextral transtension in the south the second is between korea and sw japan and has accommodated a smaller displacement of about 200 km the extensional domain in between lies in the back arc region of japan distributed stretching of the arc crust resulted in the formation of most of the japan sea while localized oceanic spreading at the southern termination of the eastern transpressional shear zone shaped the japan basin the first oceanic crust was formed in a small triangle based on the eastern shear zone and spreading propagated westward inside the pull apart region timing of oceanic crust formation of formation of the dextral shear zones and of block rotation in between as well as the internal structure of the basins and the geometry of deformation along the master shear zones are used to reconstruct the opening history this evolution is discussed by comparison to other manifestations of the arc and back arc activity such as the history of sedimentation and volcanism the paper then suggests that the collision of india can have tectonic consequences as far north as japan and sakhalin and describes the geometrical relation of back arc opening there and diffuse extrusion

slab rollback instability and supercontinent dispersal supercontinents coalesce over subduction zone complexes and their subsequent dispersal is usually attributed to heating and upwelling of continent insulated mantle this dispersal mechanism however requires considerable mantle internal heating alternatively the supercontinent configuration may be mechanically unstable and disperse regardless of heating mode in particular increased drag on plates or subducting slabs e g by accumulating continents causes them to slow down and trenches to rollback once subcontinental slabs are slightly separated resistance to their descent increases inducing further trench migration slabs thus undergo a rollback instability which disperses supercontinents a simple theoretical model illustrates this instability and shows that there are two equilibrium states one unstable supercontinent state where slabs are conjoined and one stable state where slabs are widely separated slab rollback from the unstable to stable states occurs at typical slow tectonic speeds and over a period commensurate with the age of ocean basins and the wilson cycle

p wave tomography of the mantle under the alpine mediterranean area 1 we study the upper mantle p wave velocity structure below the euro mediterranean area down to 1000 km depth by seismic travel time tomography we invert summary residuals constructed with both regional and teleseismic first arrival data reported by the international seismological centre isc 1964 1995 introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure our high resolution model pm0 5 is parameterized with three dimensional 3 d linear splines on a grid of nodes with 0 5 spacing in both horizontal directions and 50 km vertical spacing we obtain about 26 root mean square rms reduction of residuals by inversion in addition to roughly 31 reduction after summary rays formation and selection sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales the distribution of large scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions the mantle dynamics of western central europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone contrarily in the eastern mediterranean fast velocity material sinks into the lower mantle suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity on a smaller scale the existence of tears in the subducted slab lithospheric detachment all along both margins of the adriatic plate as proposed by some authors is not supported by our tomographic images

mesozoic plate tectonic reconstructions of the central south atlantic ocean the role of the west and central african rift system abstract the mesozoic opening history of the central equatorial and south atlantic oceans is closely linked in time and space to the development of the cretaceous rift system in west and central africa geological and geophysical studies of the west and central african rift system indicate it to be dominated by a set of sinistral and dextral strike slip zones diverging from the gulf of guinea and terminating as extensional rift basins in niger and sudan plate tectonic studies of the opening history of the atlantic basins indicate four stages of development stage 1 the jurassic opening of the central atlantic stage 2 the early cretaceous 130 119 ma opening of the south atlantic with rifting propagating deep into africa via the benue trough stage 3 the opening of the equatorial atlantic 119 105 ma and stage 4 the linkage of these oceanic basins and development of a single opening mid oceanic rift system as seen today the west and central african rift system continued to develop until the end of the cretaceous i e into stage 4 when rifting was terminated by a compressional shear deformation event causing the folding of sediments in the benue trough and reactivation of the central african shear zone and extensional tectonics in the basins of southern sudan this event correlates with an important change in the relative opening of the atlantic basins during tertiary recent times the west african rift system has not been active however during this period domal uplift and volcanism have occurred in central cameroon and western sudan and a very active phase of volcanism has occurred within the last 10 ma along the present day cameroon volcanic line although these uplift and volcanic events indicate thermal processes in the upper mantle there is as yet no clear link between their development and plate tectonic processes

evolution of starting mantle plumes a comparison between numerical and laboratory models abstract laboratory experiments on the initiation and development of mantle plumes suggest the formation of a bulbous plume head that grows in time by entrainment of surrounding material followed by a thin feeder conduit this type of plume is generally not seen in numerical models of mantle convection to better determine the possible differences between numerical and laboratory experiments we have numerically reproduced a published laboratory experiment on plume initiation theoretical predictions for the growth of the plume head have been verified and the experiments allowed for a new determination of a similarity constant models with a constant viscosity fluid show much less entrainment simulations using a plume rising from a thermal boundary layer with an olivine type temperature dependence show that most of the source material is contained in the plume head and that the conduit is largely composed of entrained material simulations with a more realistic non newtonian rheology show that plumes can rise much faster through the mantle which will lead to higher temperatures of the plume head at the earth s surface compared to what experiments with newtonian rheology suggest

stratigraphy and composition of lava flows in mare nubium and mare cognitum three major periods of basaltic activity characterize the infill of the basins each of these periods was itself punctuated by discrete phases of widespread magma eruptions three during both the late imbrian epoch and the early eratosthenian period and then two in the late eratosthenian period we found the youngest lavas off the eastern border of the fra mauro peninsula and mantling a much larger area over most of the central western nubium basin our results place the nubium cognitum basalts in the low ti category 1 5 wt tio2 the data indicate that the majority 90 of the mare terrain has iron content between 18 and 22 wt in particular feo contents tend to concentrate toward two compositional poles each of 20 wt and a much smaller one of 15 wt these values are typical of nearside lunar maria to complement our compositional data we present a census of craters larger than 500 m using orbiter iv images the result was a crater count average with frequency 5 6 10 2 km 2 translating into an inferred mean age of 3300 ma for the exposed lava flows by combining lava chemistry with age we find a possible correlation between the ages of the most prominent flow units and their estimated titanium content with younger basalts becoming progressively ti richer with time from 2 3 to 4 5 wt tio2

a new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data we present new profiles of mantle viscosity derived on the basis of non linear occam style joint inversions of an extensive set of data associated with mantle convection and glacial isostatic adjustment gia the convection related observables include satellite derived free air gravity harmonics the geodetically inferred excess ellipticity of the cmb and tectonic plate motions the gia constraints involve two classes of observables previously shown to be relatively insensitive to errors in the late pleistocene ice history the so called fennoscandian relaxation spectrum frs and a set of site specific decay times determined from the postglacial sea level history in hudson bay and sweden the inverted viscosity profiles show a significant three orders of magnitude increase from the upper mantle mean value of f4 10 20 pa s to a high viscosity 10 23 pa s peak at

history of subduction and back arc extension in the central mediterranean summary geological and geophysical constraints to reconstruct the evolution of the central mediterranean subduction zone are presented geological observations such as upper plate stratigraphy hp lt metamorphic assemblages foredeep trench stratigraphy arc volcanism and the back arc extension process are used to define the infant stage of the subduction zone and its latest back arc phase based on this data set the time dependence of the amount of subducted material in comparison with the tomographic images of the upper mantle along two cross sections from the northern apennines and from calabria to the gulf of lyon can be derived further the reconstruction is used to unravel the main evolutionary trends of the subduction process results of this analysis indicate that 1 subduction in the central mediterranean is as old as 80 myr 2 the slab descended slowly into the mantle during the first 20 30 myr subduction speeds were probably less than 1 cm year x1 3 subduction accelerated afterwards producing arc volcanism and back arc extension and 4 the slab reached the 660 km transition zone after 60 70 myr this time dependent scenario where a slow initiation is followed by a roughly exponential increase in the subduction speed can be modelled by equating the viscous dissipation per unit length due to the bending of oceanic lithosphere to the rate of change of potential energy by slab pull finally the third stage is controlled by the interaction between the slab and the 660 km transition zone in the southern region this results in an important re shaping of the slab and intermittent pulses of back arc extension in the northern region the decrease in the trench retreat can be explained by the entrance of light continental material at the trench

chlorine distribution and its isotopic composition in rusty rock 66095 implications for volatile element enrichments of rusty rock and lunar soils origin of rusty alteration and volatile element behavior on the moon abstract an interesting characteristic of the pyroclastic glass bead deposits select impact produced lithologies such as the rusty rock 66095 and unique lunar soils from the apollo 16 landing site is their unusual enrichments in 204 pb cd bi br i ge sb tl zn and cl which indicates that portions of these sample contain a substantial volatile component sample 66095 a fine grained subophitic to ophitic polymict melt breccia also hosts a pervasive low temperature volatile rich oxyhydrated mineral assemblage the volatile element enrichments in these assorted lunar lithologies have been attributed to a variety of extra lunar and lunar processes whereas the oxyhydration in 66095 has long been thought to represent either terrestrial alteration of lunar chlorides and fe ni metal to feo oh cl or indigenous lunar processes in 66095 cl is accommodated in feo oh cl phosphates and chlorides and is heterogeneously distributed the low temperature alteration occurs as rims around fe ni metal and sulfide grains and as dispersed grains in the adjacent matrix micro raman and transmission electron microscope tem imaging indicate that akaganeite feo oh cl is the dominant feo oh polymorph and is intergrown with goethite feo oh and hematite fe 2 o 3 tem observations indicate a well defined nanometer scale stratigraphy to the alteration for example kamacite body centered cubic face centered cubic fcc fe ni alloy lawrencite fecl 2 akaganeite the lunar lawrencite fe ni cl 2 in 66095 does not react directly to akaganeite on earth rather lawrencite exposed to terrestrial conditions reacts to form an amorphous fe and cl bearing phase nano crystalline goethite and hematite the morphology of these terrestrial alteration products is significantly different than that of the akaganeite occurring in 66095 the chlorine isotopic compositions of these volatile rich samples are enriched in heavy cl for 66095 the 37 cl ranges from 14 0 to 15 6 whereas the 37 cl for the volatile rich a16 soils ranges from 5 6 to 15 7 based on these data it appears likely that the volatile element enrichments and the cl isotopic fractionation observed in 66095 and the apollo 16 soils did not result from extra lunar additions but are most likely indigenous to the moon lawrencite was deposited on mineral surfaces at approximately 650 c to 570 c from a metal chloride bearing h poor gas phase this gas phase was also responsible for the transport of other metals e g zn cu pb fe the fractionation of cl isotopes in the rusty rock can be attributed to fumarole processes in a low h system the origin and formation of the akaganeite is more enigmatic the cl isotopes are consistent with it replacing lawrencite however numerous nanometer scale observations are not consistent with a terrestrial origin and indicate multiple episodes of oxyhydration

the shallow structures of the guinea and ivory coast ghana transform margins their bearing on the equatorial atlantic mesozoic evolution abstract recent geological and geophysical data from a marine survey conducted off guinea and the ivory coast ghana region equatorial atlantic are discussed we interpret the southern guinean margin firstly as an area linked to the central jurassic atlantic and secondly as an area affected by the progressive transform opening of the equatorial atlantic in the early cretaceous off the ivory coast and ghana we have demonstrated distinct structural domains interpreted as consequences of the progressive transform contact between the west african and brazilian cratons also during the early cretaceous on both margins we observe a clear tectonic unconformity of probable late albian early cenomanian age we propose that this unconformity results from the final disruption between the two continental west african and brazilian crusts the geological structures generated along transform margins can be considered as good geological tools to constrain early phases of plate motions particularly around the equatorial atlantic

long wavelength character of subducted slabs in the lower mantle abstract the high resolution seismic tomography provides the detailed information about the fate of the subducted plates in the mantle while the slab deformation in the transition zone is quite complex and differs among the individual subduction zones it could be generally concluded that most slabs seem to penetrate into the lower mantle the character of the fast seismic velocity anomalies however changes when the slabs cross the 670 km discontinuity while in the upper mantle tomography resolves relatively narrow plate like structures in the lower mantle it rather indicates thick blobs of seismically fast material such thickening of the downgoing slabs is traditionally attributed to the effect of the viscosity increase in the lower mantle here we study the deformation of the slabs in the lower mantle in a viscous model with a non linear composite rheology including the diffusion creep the dislocation creep and a power law stress limiter we perform numerical simulations of the subduction process in a 2d cartesian domain and concentrate especially on the effect of the change of the rheological parameters across the 670 km boundary on the slab deformation in the upper part of the lower mantle the thickening or the buckling of the plate is indeed observed in the models with relatively low yield stress 0 1 gpa of the stress limiting mechanism and with a viscosity increase at the 670 km discontinuity further we show that both the major mantle phase transitions and the strength of the crust layer have significant consequences for the creation of the buckling instabilities in the lower mantle

effects of various lithospheric yield stresses and different mantle heating modes on the breakup of the pangea supercontinent numerical simulations of three dimensional spherical mantle convection were performed to investigate the effects of various lithospheric yield stresses and two different mantle heating modes i e mixed heating from the bottom and interior and purely internal heating on the breakup of the pangea supercontinent and the subsequent continental drift for the past 200 myr results show that the continental breakup and subsequent continental drift are accomplished in mantle convection models with two different heating modes this implies that active upwelling plumes from the core mantle boundary are not necessarily required for the breakup of pangea in addition i found that the continental breakup is only realized when choosing a moderate value of the yield stress 120 mpa the bound on the yield stress derived in the present study will potentially enable the self consistent reconstruction of continental breakup and drift as well as the accompanying pattern on mantle convection since 200 ma

dispersion et peuplement de soudanella laciniosa apostolescu 1961 ostracoda au danien paleocene superieur durant l ouverture de l atlantique equatorial three continuous linear parameters length height and dorsal slope have been investigated in soudanella laciniosa apostolescu ostraoda paleocene by principal component analysis and trigonometric relationships according to andrew s procedure 1972 these disclose heterogenous conditions the result of exchange between populations from the basins of the guinean gulf near to the ivory coast that heterogeneity increases northwards senegal libya and southwards congo from the ivorian basin the origin of this species seems to be in the brazilian area during the dispersal across the equatorial atlantic ocean np2 nannozone its size increased as a function of the paleobathymetry this observation suggests a preference of the species for a phosphatic biotope

why is terrestrial subduction one sided subduction of the lithosphere at convergent plate boundaries takes place asymmetrically the subducted slab sinks downward while the overriding plate moves horizontally one sided subduction in contrast global mantle convection models generally predict downwelling of both plates at convergent margins two sided subduction we carried out two dimensional 2 d numerical experiments with a mineralogical thermomechanical viscoelastic plastic model to elucidate the cause of one sided subduction our experiments show that the stability intensity and mode of subduction depend mainly on slab strength and the amount of weak hydrated rocks present above the slab two sided subduction occurs at low slab strength sin 0 15 the weak interface is maintained by the release of fluids from the subducted oceanic crust as a consequence of metamorphism the resulting weak interplate zone localizes deformation at the interface and decouples the strong plates facilitating asymmetric plate movement our work suggests that high plate strength and the presence of water are major factors controlling the style of plate tectonics driven by self sustaining one sided subduction processes

dynamic feedback between a continentlike raft and thermal convection seismic observations of the mantle which include long wavelength structure a k 1 dependence of heterogeneity on harmonic k and a heterogeneous upper boundary layer and supercontinent kinematics may be explained by the dynamic interaction between a continent like raft and thermal convection we have formulated finite element models of convection with rafts simulating continental plates in a cylindrical geometry the azimuthal interconnectivity of this geometry is vital to resolve the two way dynamics between rafts and convection computations show that 1 raft motion is periodic 2 long wavelength thermal structure is significant within both thermal boundary layers and the fluid interior and 3 the large scale thermal structure with a wavelength longer than the width of raft is responsible for raft motion these three results which are observed for a range of rayleigh numbers internal heating rates and raft sizes are a direct consequence of the dynamic interaction between the raft and convection the physical processes for a model with a rayleigh number of 10 5 are representative when the raft is stationary due to the less efficient heat transfer through the raft and instabilities from the bottom boundary layer heat accumulates beneath the raft and results in long wavelength thermal anomalies the long wavelength thermal anomalies enhance raft motion accompanying the enhanced raft movement the long wavelength thermal anomalies diminish and the raft velocity decreases or the raft comes to rest since convection models without rafts generate less long wavelength heterogeneity compared to the models with rafts or continental plates we suspect that continental plates may play a crucial role in mantle dynamics interestingly raft motion with a period of about 10 transit times is usually significant 10 transit times is about 600 m y if scaled to the earth this is close to the observed 300 500 m y period of supercontinent aggregation and dispersal

low temperature high stress deformation of olivine under water saturated conditions abstract low temperature high stress rheology of olivine was investigated under water saturated conditions by large strain shear deformation experiments using the griggs type apparatus samples at t 1273 1373 k and p 2 gpa under water saturated conditions deformed under a significantly lower applied stress than expected from the power law relation and the strain rate increases more rapidly with stress these observations suggest that an exponential flow law operates under the present experimental conditions in which the stress dependence of activation enthalpy is characterized by the peierls stress the peierls stress is determined between 1 6 and 2 9 gpa depending on the choice of the form of stress dependence of the activation enthalpy these values are significantly smaller than the peierls stress under dry conditions 9 1 gpa we suggest that water hydrogen incorporated in the olivine lattices decreases the peierls stress and hence the rate of deformation in the exponential creep is significantly enhanced by the presence of water the reduction of the peierls stress by water results in the highly anisotropic weakening effect of water that causes the fabric transitions in olivine the transition between the power law and the exponential creep occurs at relatively low stress 100 mpa at a geologically relevant strain rate of 10 15 s 1 when temperature is lower than 1273 k and under water rich conditions hence the exponential creep might be dominated in olivine rich rocks at low temperature and high stress under water rich conditions and such regions could be widespread in the subduction zone upper mantle where the cold lithospheric plates sink into the mantle

composition and exposure age of the apollo 16 cayley and descartes regions from clementine data normalizing the optical effects of space weathering in order to quantitatively assess the composition of the lunar surface from reflectance spectroscopy it is necessary to be able to differentiate between the optical effects due to composition and those due to exposure to space weathering processes laboratory analyses of returned lunar soils are used to show that limited quantitative compositional information and the exposure age of the lunar surface can be determined using straightforward calculations from remotely acquired reflectance data sampled with a small number of spectral channels exposure age approximated by is feo where is is the characteristic ferromagnetic signal from single domain iron metal is estimated using a ratio of the reflectance at a wavelength outside of the 1 m ferrous iron absorption band to the reflectance at a wavelength within the band the iron plus titanium content of a soil can then be calculated from its is feo and reflectance these combined techniques are applied to clementine uvvis multispectral data of the lunar highlands in the vicinity of the apollo 16 landing site the is feo and feo tio2 contents are calculated for this region materials associated with recently formed impact craters and recent and or ongoing downslope movement are easily detected as nonmature the feo tio2 content of the descartes mountains is found to be 1 2 wt less than the cayley plains west of the apollo 16 landing site localized areas of compositional heterogeneity associated with some small impact events are also revealed a model for normalizing the optical contribution due to exposure to space weathering processes is applied to the clementine uvvis data in order to bring the data to an equivalent mature exposure state this approach allows residual spectral differences between the observed materials to be confidently interpreted as being due to compositional differences alone rather than to differences both of composition and of exposure the exposure normalized data reveal that this region of the highlands is characterized by limited large scale compositional heterogeneity that is detectable by a small number of spectral bandpasses the descartes mountains are identified as being more anorthositic than the cayley plains units most small impacts in the region excavated material compositionally similar to material surrounding them with the notable exceptions of the impacts that created south ray crater and an unnamed crater on the floor of abulfeda

subduction triggered magmatic pulses a new class of plumes article i nfo a variety of atypical plume like structures and focused upwellings that are not rooted in the lower mantle have recently been discussed and seismological imaging has shown ubiquitous small scale convection in the uppermost mantle in regions such as the mediterranean region the western us and around the western pacific we argue that the three dimensional return flow and slab fragmentation associated with complex oceanic subduction trajectories within the upper mantle can generate focused upwellings and that these may play a significant role in regional tectonics the testable surface expressions of this process are the outside arc alkaline volcanism topographic swell and low velocity seismic anomalies associated with partial melt using three dimensional simplified numerical subduction models we show that focused upwellings can be generated both ahead of the slab in the back arc region though five times further inward from the trench than arc volcanism and around the lateral edges of the slab in the order of 100 km away from slab edges vertical mass transport and by inference the associated decompression melting in these regions appears strongly correlated with the interplay between relative trench motion and subduction velocities the upward flux of material from the depths is expected to be most pronounced during the first phase of slab descent into the upper mantle or during slab fragmentation we discuss representative case histories from the pacific and the mediterranean where we find possible evidence for such slab related volcanism

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

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

modal petrology of six soils from apollo 16 double drive tube core 64002 detailed petrographic data were collected from six size fractions for six samples from apollo 16 drive tube section 64002 soils from the core were found to have similar source rocks defined as anorthosites impact melts and metamorphosed breccia to apollo 16 surface soils and core 60009 two slightly differing sets of source rocks one with a metamorphosed breccia to crystalline matrix breccia ratio of 2 3 and the other with a ratio of 1 1 were defined for the upper and lower sections of the core respectively analysis of modal data shows that the upper three and the lowermost core soils are mature and have similar maturation histories whereas the two middle soils are submature and have maturation histories similar to each other but differing from the upper and lowermost soils in core 64002 soils mixing has dominated over reworking mixing components are thought to be two mature soils distinguished by differing source rocks and an immature plagioclase rich soil the plagioclase rich soil is most prominent in the submature section of the core and appears to correlate with larger clasts of chalky friable breccia the friable breccias and the immature plagioclase rich soil are tentatively associated with the descartes formation supplementation of petrographic data with 53mn ferromagnetic resonance fmr and core dissection data allows division of the core into three zones the upper mature zone is fairly homogenous and was recently deposited in a short span of time the middle submature zone is less homogenous more plagioclase rich and may be an ejecta deposit the lower zone though mature and homogenous has the same source rocks as the middle zone

stratigraphy and geochemistry of the stone mountain core 64001 2 ferromagnetic resonance and magnetic data measured on both sections of the double drive tube cord 64001 2 collected on stone mountain station four apollo 16 are reported along with instrumental neutron activation analysis data measured on the lower section these data provide insight into the depositional and irradiational history and the geochemical provenances of the core

stagnant slab a review a stagnant slab is a subducted slab of oceanic lithosphere subhorizontally deflected above across or below the 660 km discontinuity this phenomenon has now been widely recognized beneath subduction zones around the circum pacific and in the mediterranean collaboration of seismic and electromagnetic observations mineral physics measurements and geodynamic modeling has begun to provide a consistent picture of stagnant slab

viscosity jump in earth s mid mantle the viscosity structure of earth s deep mantle affects the thermal evolution of earth the ascent of mantle plumes settling of subducted oceanic lithosphere and the mixing of compositional heterogeneities in the mantle based on a reanalysis of the long wavelength nonhydrostatic geoid we infer viscous layering of the mantle using a method that allows us to avoid a priori assumptions about its variation with depth we detect an increase in viscosity at 800 to 1200 kilometers depth far greater than the depth of the mineral phase transformations that define the mantle transition zone the viscosity increase is coincident in depth with regions where seismic tomography has imaged slab stagnation plume deflection and changes in large scale structure and offers a simple explanation of these phenomena

a regime diagram for subduction styles from 3 d numerical models of free subduction abstract previous models of subduction both analogue and numerical have observed a number of distinct styles of subduction each with particular subduction motions partitioned between slab rollback and forward plate advance and associated slab morphologies we use 3 d numerical models to investigate subduction dynamics by varying the strength of slabs as well as the buoyancy and propose a new classification based on these parameters the slab strength is specified both through the ratio of viscosities between the subducting plate and upper mantle plate um as well as the plate thickness h plate only a very restrictive range of plates strong plates with smaller buoyancy tend to favor modes of subduction which are exclusively advancing plates which have greater negative buoyancy will eventually transition into a retreating style we find that the flexural strength and the buoyancy determine the subduction style as distinguished by a characteristic slab morphology and control several subduction characteristics including the partitioning between slab rollback and plate advance the trench curvature and the slab s radius of curvature plates that are 80 100 km thick with plate um 100 300 are classified here as weak and are the only plates to exhibit slab geometry with several recumbent folds atop the more viscous lower mantle this regime of weak plates with their associated slab morphologies predominant folding is argued to be most similar to slabs on earth based on the presence of folded slab piles in earth s upper mantle as interpreted from seismic tomography

influences on the positioning of mantle plumes following supercontinent formation several mantle convection studies analyzing the effects of supercontinent formation and dispersal show that the genesis of subcontinental plumes results from the formation of subduction zones at the edges of the supercontinent rather than from the effect of continental thermal insulation or thermochemical piles however the influence of subduction zone location on the position of subcontinental plumes has received little attention this study analyzes 2 d and 3 d numerical models of supercontinent formation in an isochemical mantle to assess the role of subduction and mantle viscosity contrast in the generation of subcontinental mantle plumes we find that once a critical supercontinent width is reached plumes do not form under the center of a supercontinent in studies featuring a low viscosity lower mantle the surface positions of the initial plumes arriving within 90 myr of supercontinent assembly become locked beneath the continent at a distance 2000 3000 km from the continental margin however the broad downwellings in simulations that feature a high viscosity lower mantle trigger plumes at a greater distance from the continental margin subduction for all mantle viscosity profiles subcontinental plumes show dependence on the location of supercontinent margin subduction as theories differ on the role of core mantle boundary chemical piles in plume formation it is significant that our isochemical models show that the formation of subduction zones at the margins of a supercontinent has a profound effect on subcontinental mantle dynamics our results may help to explain what determined the eruption sites of past and future large igneous provinces

the influence of trench migration on slab penetration into the lower mantle abstract a two dimensional numerical convection model in cartesian geometry is used to study the influence of trench migration on the ability of subducted slabs to penetrate an endothermic phase boundary at 660 km depth the transient subduction history of an oceanic plate is modelled by imposing plate and trench motion at the surface the viscosity depends on temperature and depth a variety of styles of slab behaviour is found depending predominantly on the trench velocity when trench retreat is faster than 2 4 cm a the descending slab flattens above the phase boundary at slower rates it penetrates straight into the lower mantle although flattening in the transition zone may occur later leading to a complex slab morphology the slab can buckle independent of whether it penetrates or not especially when there is a localised increase in viscosity at the phase boundary flattened slabs are only temporarily arrested in the transition zone and sink ultimately into the lower mantle the results offer a framework for understanding the variety in slab geometry revealed by seismic tomography

petrologic variations in apollo 16 surface soils detailed modal analyses of fifteen surface soils were compared with rake and rock sample data to provide information about source rocks maturation history and intrasite variations in apollo 16 regolith triangular plots of source rock components defined as anorthosites crystalline melt matrix breccias and metamorphosed breccias show that with the possible exception of soils 64501 and 68501 apollo 16 soils have similar source rocks that are well homogenized throughout the site modal abundance variations in submillimeter size fractions tend to reflect those of rake and rock samples maturity plots in agglutinate monomineralic fragment lithic and crystalline breccia space show apollo 16 soils to be generally mature except for those collected from trenches 61221 and 61241 and from locations near north ray crater stations 11 and 13 data produced by other workers is combined with data from this study and used to divide the apollo 16 site into three soil petrographic provinces central site soils are mature well homogenized and enriched in glass but patches of immature and or inhomogenous material are also present they are probably the most typical cayley plains materials present at the site north ray soils are immature to submature containing north ray ejecta in the form of feldspathic fragmental breccia and plagioclase south ray soils are mature but contain small amounts of fresh impact melts and plagioclase possibly from breakdown of blocky south ray ejecta south ray and north ray ejecta differ in composition and physical properties supporting the hypothesis that the south ray event excavated cayley material while the north ray event excavated descartes material

continental collisions in wide aspect ratio and high rayleigh number two dimensional mantle convection models distinct rigidly moving oceanic and continental plates of finite thickness are incorporated into a two dimensional numerical model of mantle convection we investigate upper mantle convection in models having aspect ratios as great as 24 and compare our findings with the results of earlier studies which were limited to aspect ratio 4 models in addition we implement models of whole mantle flow by specifying high rayleigh number convection and thinner nondimensional plates we are thus able to compare the results of continental collision models which include similarly sized continents in the cases of upper and whole mantle convection for each case considered we model a pair of identical continents being carried toward a site of plate convergence by underlying counterrotating mantle convection cells upon collision the continents form a motionless rigid model supercontinent while oceanic plate material continues to recycle through the mantle following the continental collision our models of upper mantle convection exhibit a reorganization of the convective planform below the model supercontinent into a smaller wavelength mode which is unable to generate the net stress needed to break apart the continent alternating compressive and tensile deviatoric stress associated with the small scale flow results in a low integrated stress in contrast the large scale of whole mantle convection enables flow reversals to produce shear stresses acting in a common direction over extensive areas of the base of a continent the integrated effect of which is capable of causing continental rifting the conventional view of the role of thermal blanketing in continental rifting does not apply in the whole mantle convection scenario

on the origin of impact glass in the apollo 16 regolith this study addresses the issue of what fraction of the impact glass in the regolith of a lunar landing site derives from local impacts those within a few kilometers of the site as opposed to distant impacts 10 or more kilometers away among 10 323 fragments from the 64 210 lm grain size fraction of three apollo 16 regolith samples 14 are impact glasses that is fragments consisting wholly or largely of glass produced in a crater forming impact another 16 are agglutinates formed by impacts of micrometeorites into regolith we analyzed the glass in 1559 fragments for major and minor element concentrations by electron probe microanalysis and a subset of 112 of the fragments that are homogeneous impact glasses for trace elements by secondary ion mass spectrometry of the impact glasses 75 are substantially different in composition from either the apollo 16 regolith or any mixture of rocks of which the regolith is mainly composed about 40 of the impact glasses are richer in fe mg and ti as well as k p and sm than are common rocks of the feldspathic highlands these glasses must originate from craters in maria or the procellarum kreep terrane of the feldspathic impact glasses some are substantially more magnesian greater mgo feo or have substantially lower concentrations of incompatible elements than the regolith of the apollo 16 site many of these however are in the range of feldspathic lunar meteorites most of which derive from points in the feldspathic highlands distant from the procellarum kreep terrane these observations indicate that a significant proportion of the impact glass in the apollo 16 regolith is from craters occurring 100 km or more from the landing site in contrast the composition of glass in agglutinates on average is similar to the composition of the apollo 16 regolith consistent with local origin 2010 elsevier ltd all rights reserved

seismic image and origin of the changbai intraplate volcano in east asia role of big mantle wedge above the stagnant pacific slab abstract we determined p wave tomography of the crust and upper mantle under the changbai volcanic area in northeast asia by using 289 318 arrival time data from 4802 local earthquakes and 9599 teleseismic events a prominent low velocity anomaly is clearly imaged in the crust and upper mantle down to about 410 km depth beneath the changbai volcano a broad high velocity anomaly is revealed in the mantle transition zone and deep earthquakes occurred actively there suggesting that the subducting pacific slab is stagnating in the mantle transition zone under east asia our results suggest that the formation of the changbai and other intraplate volcanoes in northeast asia is related to the upwelling of hot and wet asthenospheric materials in the big mantle wedge above the stagnant pacific slab hence the active intraplate volcanoes in northeast asia are not hotspots related to deep mantle plumes but are caused by plate tectonic processes in the upper mantle

upper plate controls on deep subduction trench migrations and deformations at convergent margins abstract thus far relatively simplistic models of free subduction in which the trench and plate motions are emergent features completely driven by the negative buoyancy of the slab have investigated the dynamics of a single isolated subducting plate here we extend such models to incorporate an overriding plate and present the results of how such an overriding plate feedbacks into the dynamics of free subduction in this study we address three fundamental aspects of these dynamics 1 how does the presence of an overriding plate change the force balance at the convergent margins 2 how are the forces from deep subduction propagated to the surface and 3 what controls the stress regime in a system of coupled upper and subducting plates and how is it expressed in the deformations and plate motions in general we find that the evolution of subduction zones is strongly controlled by both the interactions between the slab and the upper lower mantle discontinuity as well as the strength of the upper plate when either the subducting or upper plates are unable to move subduction motions are steady state and partitioned entirely into either slab rollback or plate advance respectively when conditions favour a quasi stationary trench the subducted lithosphere can form into a pile with multiple recumbent folds of slab material atop the lower mantle alternating between forwards and backwards draping slab the corresponding horizontal trench motions at the surface are frontward and rearward respectively resulting in either a compressive or extensional regime in the back arc time dependent forcing arising from the slab piling behaviour can have a feedback with upper plate and produce strongly non steady state intermittent phases of upper plate deformation as those commonly observed on earth two types of discontinuous back arc strain evolution are identified 1 periodic when recurrent phases of strain over finite durations are accommodated by viscous stretching thickening of the plate and 2 episodic when upper plate deformation localizes plastic strain and allows for punctuated episodes these phases can include extension quiescence and compression giving rise to a large variety of possible tectonic evolutions the models presented here provide insight into the dynamics behind the non steady state evolution of subduction which can help unravel seemingly erratic motions of major convergent margins and back arc deformations around the pacific and indian oceans during the cenozoic

interaction of subducted slabs with the mantle transition zone a regime diagram from 2 d thermo mechanical models with a mobile trench and an overriding plate transition zone slab deformation influences earth s thermal chemical and tectonic evolution however the mechanisms responsible for the wide range of imaged slab morphologies remain debated here we use 2 d thermo mechanical models with a mobile trench an overriding plate a temperature and stress dependent rheology and a 10 30 or 100 fold increase in lower mantle viscosity to investigate the effect of initial subducting and overriding plate ages on slab transition zone interaction four subduction styles emerge i a vertical folding mode with a quasi stationary trench near vertical subduction and buckling folding at depth vf ii slabs that induce mild trench retreat which are flattened horizontally deflected and stagnate at the upper lower mantle interface hd iii inclined slabs which result from rapid sinking and strong trench retreat isr iv a two stage mode displaying backward bent and subsequently inclined slabs with late trench retreat bir transitions from regime i to iii occur with increasing subducting plate age i e buoyancy and strength regime iv develops for old strong subducting and overriding plates we find that the interplay between trench motion and slab deformation at depth dictate the subduction style both being controlled by slab strength which is consistent with predictions from previous compositional subduction models however due to feedbacks between deformation sinking rate temperature and slab strength the subducting plate buoyancy overriding plate strength and upper lower mantle viscosity jump are also important controls in thermo mechanical subduction for intermediate upper lower mantle viscosity jumps 30 our regimes reproduce the diverse range of seismically imaged slab morphologies

petrography and geochemistry of five new apollo 16 mare basalts and evidence for post basin deposition of basaltic material at the site we present the petrography and geochemistry of five 2 4 mm basalt fragments from the apollo 16 regolith these fragments are 1 a high ti vitrophyric basalt compositionally similar to apollo 17 high ti mare basalts 2 a very high ti vitrophyric basalt compositionally similar to apollos 12 and 14 red black pyroclastic glass 3 a coarsely crystalline high al basalt compositionally similar to group 5 apollo 14 high al mare basalts 4 a very low ti vlt crystalline basalt compositionally similar to luna 24 vlt basalts and 5 a vlt basaltic glass fragment compositionally similar to apollo 17 vlt basalts high ti basalt has been reported previously at the apollo 16 site the other basalt types have not been reported previously as there are no known cryptomaria or pyroclastic deposits in the highlands near the apollo 16 site ruling out a local origin and scant evidence for basaltic material in the apollo 16 ancient regolith breccias or apollo 16 soils collected near north ray crater ruling out a basin ejecta origin we infer that the basaltic material in the apollo 16 regolith originated in maria near the apollo 16 site and was transported laterally to the site by small to medium sized post basin impacts on the basis of tio2 concentrations derived from the clementine uvvis data mare tranquillitatis 300 km north is the most likely source for the high ti basaltic material at the apollo 16 site craters ross arago dionysius maskelyne moltke sosigenes schmidt mare nectaris sinus asperitatis 220 km east is the most likely source for the low ti and vlt basaltic material craters theophilus madler torricelli and a large regional pyroclastic deposit near mare vaporum 600 km northwest is the most likely source region for pyroclastic material although no source craters are apparent in the region

slabs in the lower mantle and their modulation of plume formation numerical mantle convection models indicate that subducting slabs can reach the core mantle boundary cmb for a wide range of assumed material properties and plate tectonic histories an increase in lower mantle viscosity a phase transition at 660 km depth depth dependent thermal expansivity and depth dependent thermal diffusivity do not preclude model slabs from reaching the cmb we find that ancient slabs could be associated with lateral temperature anomalies 500 c cooler than ambient mantle plausible increases of thermal conductivity with depth will not cause slabs to diffuse away regional spherical models with actual plate evolutionary models show that slabs are unlikely to be continuous from the upper mantle to the cmb even for radially simple mantle structures the observation from tomography showing only a few continuous slab like features from the surface to the cmb may be a result of complex plate kinematics not mantle layering there are important consequences of deeply penetrating slabs our models show that plumes preferentially develop on the edge of slabs in areas on the cmb free of slabs plume formation and eruption are expected to be frequent while the basal thermal boundary layer would be thin however in areas beneath slabs the basal thermal boundary layer would be thicker and plume formation infrequent beneath slabs a substantial amount of hot mantle can be trapped over long periods of time leading to mega plume formation we predict that patches of low seismic velocity may be found beneath large scale high seismic velocity structures at the core mantle boundary we find that the location buoyancy and geochemistry of mega plumes will differ from those plumes forming at the edge of slabs various geophysical and geochemical implications of this finding are discussed

global shear speed structure of the upper mantle and transition zone s u m m a r y the rapid expansion of broad band seismic networks over the last decade has paved the way for a new generation of global tomographic models significantly improved resolution of global upper mantle and crustal structure can now be achieved provided that structural information is extracted effectively from both surface and body waves and that the effects of errors in the data are controlled and minimized here we present a new global vertically polarized shear speed model that yields considerable improvements in resolution compared to previous ones for a variety of features in the upper mantle and crust the model sl2013sv is constrained by an unprecedentedly large set of waveform fits 3 4 of a million broad band seismograms computed in seismogram dependent frequency bands up to a maximum period range of 11 450 s automated multimode inversion of surface and s wave forms was used to extract a set of linear equations with uncorrelated uncertainties from each seismogram the equations described perturbations in elastic structure within approximate sensitivity volumes between sources and receivers going beyond ray theory we calculated the phase of every mode at every frequency and its derivative with respect to sand p velocity perturbations by integration over a sensitivity area in a 3 d reference model the normally small perturbations of the 3 d model required to fit the waveforms were then linearized using these accurate derivatives the equations yielded by the waveform inversion of all the seismograms were simultaneously inverted for a 3 d model of shear and compressional speeds and azimuthal anisotropy within the crust and upper mantle elaborate outlier analysis was used to control the propagation of errors in the data source parameters timing at the stations etc the selection of only the most mutually consistent equations exploited the data redundancy provided by our data set and strongly reduced the effect of the errors increasing the resolution of the imaging our new shear speed model is parametrized on a triangular grid with a 280 km spacing in well sampled continental domains lateral resolution approaches or exceeds that of regionalscale studies the close match of known surface expressions of deep structure with the distribution of anomalies in the model provides a useful benchmark in oceanic regions spreading ridges are very well resolved with narrow anomalies in the shallow mantle closely confined near the ridge axis and those deeper down to 100 120 km showing variability in their width and location with respect to the ridge major subduction zones worldwide are well captured extending from shallow depths down to the transition zone the large size of our waveform fit data set also provides a strong statistical foundation to re examine the validity field of the jwkb approximation and surface wave ray theory our analysis shows that the approximations are likely to be valid within certain time frequency portions of most seismograms with high signal to noise ratios and these portions can be identified using a set of consistent criteria that we apply in the course of waveform fitting

evolution of mantle structure beneath the northwest pacific evidence from seismic tomography and paleogeographic reconstructions 1 plate motions and subducting slab morphology are intricately connected and through the integration of seismicity tomographic images and relative plate motions the evolution of mantle structure can be interpreted tomographic images of p wave shear wave speed and bulk sound speed perturbations of the northwest pacific region have been interpreted to define the extent and geometry of the subducting pacific plate we have found that the subducted pacific plate beneath the japan and kurile arcs is coherent but is very complex at the junction of the two arcs near the hokkaido corner as the slab subduction angle decreases from north to south while the slab thickens the pacific slab beneath the japan arc is slightly thinner and subducting through the upper mantle at a less steep angle than the slab beneath the kurile arc as the slab reaches the 660 km discontinuity it becomes stagnant and lies in a horizontal position on top of the transition zone both beneath the izu and japan arcs in contrast the slab beneath the kurile arc is subducting at a steeper angle and penetrates through the transition zone into the lower mantle the difference in slab morphology appears to be due to a combination of change in dip of the subducting plate and the convergence velocity of the pacific plate along the margin a new paleogeographic reconstruction of northeast asia was assembled to evaluate the plate motions that could explain the foundation of the current slab morphology the plate reconstruction was based on euler pole motions of the pacific philippine amurian okhotsk and north american plates relative to stable eurasia plate the new tectonic model illustrates the collision of the japan and kurile arcs the opening of the kurile basin disparity in pacific plate velocities along the arc and different rates of trench retreat along the izu japan and kurile segments of the western pacific margin the plate motion model was interpreted in conjunction with the physical properties of the mantle imaged with the p wave and joint tomography to assess the evolution of the pacific plate morphology since the mid miocene and to provide constraints on the plausible plate motions in the region