crustal thickness and vp vs ratio in nw namibia from receiver functions evidence for magmatic underplating due to mantle plume crust interaction a seismological network was operated at the junction of the aseismic walvis ridge with the northwestern namibian coast we mapped crustal thickness and bulk vp vs ratio by the h k analysis of receiver functions in the damara belt the crustal thickness is 35 km with a vp vs ratio of 1 75 the crust is 30 km thick at the coast in the kaoko belt strong variations in crustal thickness and vp vs ratios are found at the landfall of the walvis ridge here and at 150 km northeast of the coast the crustal thickness increases dramatically reaching 44 km and the vp vs ratios are extremely high 1 89 these anomalies are interpreted as magmatic underplating produced by the mantle plume during the breakup of gondwana the area affected by the plume is smaller than 300 km in diameter possibly ruling out the existence of a large plume head under the continent during the breakup

models of crustal thickness for south america from seismic refraction receiver functions and surface wave tomography abstract an extensive compilation of crustal thicknesses is used to develop crustal models in continental south america we consider point crustal thicknesses from seismic refraction experiments receiver function analyses and surface wave dispersion estimates of crustal thickness derived from gravity anomalies were only included along the continental shelf and in some areas of the andes to fill large gaps in seismic coverage two crustal models were developed a by simple interpolation of the point estimates and b our preferred model based on the same point estimates interpolated with surface wave tomography despite gaps in continental coverage both models reveal interesting crustal thickness variations in the andean range the crust reaches 75 km in southern peru and the bolivian altiplano while crustal thicknesses seem to be close to the global continental average 40 km in ecuador and southern colombia despite high elevations and along the southern andes of chile argentina elevation lower than 2000 m in the stable continental platform the average thickness is 38 5 km 1 st deviation and no systematic differences are observed among archean paleoproterozoic cratons neoproterozoic fold belts and low altitude intracratonic sedimentary basins an exception is the borborema province ne brazil with crust 30 35 km thick narrow belts surrounding the cratons are suggested in central brazil parallel to the eastern and southern border of the amazon craton and possibly along the transbrasiliano lineament continuing into the chaco basin where crust thinner than 35 km is observed in the sub andean region between the mid plate cratons and the andean cordillera the crust tends to be thinner 35 km than the average crust in the stable platform a feature possibly inherited from the old pre cambrian history of the continent we expect that these crustal models will be useful for studies of isostasy dynamic topography and crustal evolution of the continent

rayleigh wave tomography beneath intraplate volcanic ridges in the south pacific sources recorded over a 12 month period provides resolution of phase velocities at periods upto100s theaveragephasevelocitiesforthestudyarearevealapronouncedlow velocity zone reaching a minimum shear velocity of 3 95 km s the negative velocity gradient defining the base of the lithosphere observed at 40 15 km abruptly reverses at 70 km depth the underlying positive gradient changes slope at 125 km we attribute these changes in gradient to the onset of incipient partial melting of upwelling mantle in thepresenceofwaterat125km followedbyincreasedmeltproductionat70kmthatleadsto dehydration of the residual matrix and migration of melt to the surface spreading center rayleigh wave tomography shows that there are anomalously low shear velocities extending to at least 50 km depth beneath the sojourn ridge and the hotu matua volcanic complex with relatively high velocities between these volcanic chains these observations are not consistent with passive models for the origin of the volcanic ridges involving lithospheric extension or thermoelastic cracking dynamic models invoking flow in the asthenosphere in the form of small scale convection or viscous fingering instabilities may explain the observed pattern of seismic velocity anomalies

channeling of plume flow beneath mid ocean ridges we investigate the interaction of mantle plumes with mid ocean ridges in 3 d numerical convection models using a finite volume multigrid technique with local grid refinement we are able to incorporate a much stronger temperature dependence of viscosity than was possible in previous models this allows lateral viscosity variations of more than six orders of magnitude which is realistic in a ridge situation in models of ridge centered plumes with constant flux we determine how the shape and the along axis width of the plume head depends on various model parameters the plume material is significantly channeled along the axis of a slow spreading ridge if the plume viscosity is in the order of 1017 pa s the plume material spreads radially below the ridge if the spreading rate is fast or the viscosity contrast between plume and ambient mantle is small in time dependent calculations we investigate the temporal evolution of short term variations of the plume flux at slow spreading ridges these perturbations propagate in the form of pulses along the ridge with a speed much larger than the spreading rate our results support the hypothesis that pulsations of the iceland plume are the cause for v shaped topography and gravity anomalies at the reykjanes ridge

marine magnetotellurics imaged no distinct plume beneath the tristan da cunha hotspot in the southern atlantic ocean the tristan da cunha tdc is a volcanic island located above a prominent hotspot in the atlantic ocean many geological and geochemical evidences support a deep origin of the mantle material feeding the hotspot however the existence of a plume has not been confirmed as an anomalous structure in the mantle resolved by geophysical data because of lack of the observations in the area marine magnetotelluric and seismological observations were conducted in 2012 2013 to examine the upper mantle structure adjacent to tdc the electrical conductivity structure of the upper mantle beneath the area was investigated in this study three dimensional inversion analysis depicted a high conductive layer at 120 km depth but no distinct plume like vertical structure the conductive layer is mostly flat independently on seafloor age and bulges upward beneath the lithospheric segment where the tdc islands are located compared to younger segment south of the tdc fracture zone while the bathymetry is rather deeper than prediction for the northern segment the apparent inconsistency between the absence of vertical structure in this study and geochemical evidences on deep origin materials suggests that either the upwelling is too small and or weak to be resolved by the current data set or that the upwelling takes place elsewhere outside of the study area other observations suggest that 1 the conductivity of the upper mantle can be explained by the fact that the mantle above the high conductivity layer is depleted in volatiles as the result of partial melting beneath the spreading ridge 2 the potential temperature of the segments north of the tdc fracture zone is lower than that of the southern segment at least during the past 30 myr

high accuracy mantle convection simulation through modern numerical methods ii realistic models and problems computations have helped elucidate the dynamics of earth s mantle for several decades already the numerical methods that underlie these simulations have greatly evolved within this time span and today include dynamically changing and adaptively refined meshes sophisticated and efficient solvers and parallelization to large clusters of computers at the same time many of these methods discussed in detail in a previous paper in this series were developed and tested primarily using model problems that lack many of the complexities that are common to the realistic models our community wants to solve today with several years of experience solving complex and realistic models we here revisit some of the algorithm designs of the earlier paper and discuss the incorporation of more complex physics in particular we re consider time stepping and mesh refinement algorithms evaluate approaches to incorporate compressibility and discuss dealing with strongly varying material coefficients latent heat and how to track chemical compositions and heterogeneities taken together and implemented in a high performance massively parallel code the techniques discussed in this paper then allow for high resolution 3d compressible global mantle convection simulations with phase transitions strongly temperature dependent viscosity and realistic material properties based on mineral physics data

widespread volcanism in the greenland north atlantic region explained by the iceland plume in the classical concept a hotspot track is a line of volcanics formed as a plate moves over a stationary mantle plume defying this concept intraplate volcanism in greenland and the north atlantic region occurred simultaneously over a wide area particularly around 60 million years ago showing no resemblance to a hotspot track here we show that most of this volcanism can nonetheless be explained solely by the iceland plume interacting with seafloor spreading ridges global mantle flow and a lithosphere the outermost rigid layer of the earth with strongly variable thickness an east west corridor of thinned lithosphere across central greenland as inferred from new highly resolved tomographic images could have formed as greenland moved westward over the iceland plume between 90 and 60 million years ago our numerical geodynamic model demonstrates how plume material may have accumulated in this corridor and in areas east and west of greenland simultaneous plume related volcanic activities starting about 62 million years ago on either side of greenland could occur where and when the lithosphere was thin enough due to continental rifting and seafloor spreading possibly long after the plume reached the base of the lithosphere volcanism across the north atlantic region 62 million years ago is consistent with an iceland plume source despite the absence of a classic hotspot track suggest tomographic images and geodynamic models

mechanical and geological effects of impact cratering on ida asteroids respond to impact stresses differently from either laboratory specimens or large planets gravity is typically so small that seismic disturbances of a few cm s 1can devastate unconsolidated topography yet the presence of regolith and the likelihood that many asteroids are gravitational assemblages tell us that gravity cannot generally be ignored we use numerical models for impact fracture in solids to examine the initial stage of crater formation on asteroid 243 ida up to the cessation of fracture and the establishment of the cratering flow at this stage we can infer final crater diameters but not profiles we find that a modified strength scaling applies for craters up to a few 100 m in diameter forming in rock subject to ida s gravity and that gravity controls all craters larger than 1 km bright annuli around a number of intermediate craters may be the result of low velocity surface disturbances rather than bright proximal ejecta deposits we also consider large impactors to which ida presents a curved finite target surface with irregular gravity these can excavate asymmetrical concavities stresses from large events can refocus and cause fracture far from the crater using the shape of ida as a basis for 3d hydrocode simulations we show that impact genesis of the vienna regio concavity can cause fracture in pola regio where grooves are observed in spacecraft images other simulations indicate that the formation of the 10 km crater azzurra might have reopened these fractures which may account for their fresh appearance this mechanism of groove formation requires an interior which coherently transmits elastic stress while this precludes a classic rubble pile asteroid it does allow well joined fault planes and welded blocks or pores smaller than the stress pulse

a theoretical model of hotspot volcanism control on volcanic spacing and patterns via magma dynamics and lithospheric stresses many linear island chains are thought to be the result of the steady motion of the lithospheric plates over stationary hotspots the occurrence of discrete nearly regularly spaced volcanoes rather than continuous ridges are assumed to be caused by the interaction of flexural stresses due to the volcanic loads with magma percolation a parameter study is performed on a simple model that incorporates these effects in addition to dike wall erosion it is demonstrated that the strength of the erosional feedback determines whether the model generates discrete volcanoes or a continuous ridge the intervolcanic spacing depends not only on the elastic thickness of the lithosphere but also on the magma pressure at the base of the lithosphere the size of the eruptive region of the individual volcanoes is controlled by the elastic response of the lithosphere to magma overpressurization if an initial off axis edifice is introduced the model is able to preserve this asymmetry and produce an alternating series of volcanoes a small initial perturbation grows over time resulting in double lines or wider patterns depending on the width of the magma source region single lines of volcanoes therefore indicate very narrow magma source regions

the role of rheology in lithospheric thinning by mantle plumes lithospheric thinning by mantle plumes is an important planetary heat transfer process resulting in the broad topographic uplift that characterizes the volcanic rises on venus the tharsis rise on mars and several hotspot swells on earth we present a suite of time dependent three dimensional numerical calculations of a plume impinging upon the lithosphere in a temperature dependent viscosity mantle efficient lithospheric thinning is found to depend on the formation of convective instabilities in the plume lithosphere boundary layer these instabilities are non axisymmetric time dependent and have horizontal scales of a few tens of kilometers these instabilities depend on the temperature dependence of viscosity and occur when the plume s viscosity is about an order of magnitude less than the background mantle as predicted by boundary layer theory thus in planetary mantles plumes with excess temperatures of 100 to 200 k will efficiently thin the lithosphere via small scale convective instabilities

the influence of recent major crater impacts on the surrounding surfaces of 21 lutetia we present 3 d simulations of impacts into asteroid 21 lutetia the subject of a fly by by the european space agency s rosetta mission to comet 67p churyumov gerasimenko using a 3 d shape model of the asteroid impacts of sizes sufficient to reproduce the observed craters in lutetia s north polar crater cluster npcc as observed by the osiris experiment have been simulated using the smoothed particle hydrodynamics technique the asteroid itself has been modelled both as a homogeneous body and as a body with an iron core crater erasure in the vicinity of the npcc has been observed by osiris the results show that this erasure has most probably been caused by ejecta deposition following the impact of a 2 3 km diameter projectile impacting at a velocity of 5 km s 1 or an impact with similar energy this would produce a crater of roughly 34 km in diameter comparable to the largest and oldest member of the npcc erasure of craters via the shock associated with such an impact is shown to be less significant and does not reproduce the observed spatial distribution of erased craters or ghost craters time series of the surface velocity fields resulting from the simulated impacts are also presented it is suggested that the surface velocity field and velocity shear may play a role in the generation of lineaments our model calculations show that the velocity field lines around 50 s after impact exhibit a reasonable qualitative correlation with the orientation of lineaments observed on the entire visible surface of lutetia it is also shown that incorporation of a core of 25 30 km in diameter does not modify the velocity field evolution with time and as such the presence or otherwise of such a core cannot be inferred from lineament observations if this concept for their formation is valid

evolution of the walvis ridge rio grande rise hot spot system implications for african and south american plate motions over plumes crystallization ages of volcanic rocks dredged or drilled from the walvis ridge ten sites and the rio grande rise one site have been determined by the 40ar 39ar incremental heating technique the fundamentally age progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of tristan da cunha along the walvis ridge and rio grande rise and for the formation of the continental flood basalts located in namibia africa and brazil south america the walvis ridge rio grande rise volcanic system evolved along a section of the south atlantic spreading axis as the african and south american plates migrated apart astride or in close proximity to an upwelling plume reconstructions of the spatial relationship between the spreading axis the tristan hot spot and the evolving walvis ridge rio grande rise volcanic feature show that at about 70 ma the spreading axis began to migrate westward away from the hot spot the resulting transition to intraplate hot spot volcanism along the walvis ridge and associated termination of rio grande rise formation also involved a northward migration of previously formed african seafloor over the hot spot rotation parameters for african motion over fixed hot spots i e absolute motion have been recalculated such that the predicted trail of the tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the walvis ridge african absolute motion has been extended to the south and north american plates by the addition of relative motion reconstruction poles

a cretaceous pole from south china and the mesozoic hairpin turn of the eurasian apparent polar wander path to contribute to the apparent polar wander path apwp of the south china block and eurasia in general we collected paleomagnetic samples from mesozoic red beds around the city of ya an 30 n 103 e in the western tip of the sichuan basin in this paper we present the results from 373 oriented cores taken from one section representing 3 km of sedimentary rocks the section is dated with continental ostracods and with a magnetostratigraphic correlation between a densely sampled 272 m sequence and the polarity time scale giving an upper jurassic to upper cretaceous or lower tertiary age the remanent direction is remarkably stable throughout the section d 2 0 i 34 2 k 63 1 95 3 6 n 26 28 sites while this fact might suggest that the section has been remagnetized paleomagnetic and rock magnetic tests indicate that the remanence is primary the pole position 78 6 n 273 4 e dp 2 4 dm 4 1 corresponds to a rather low paleolatitude 18 8 2 4 but is consistent with other cretaceous poles from china if one accepts the eurasian apwp of irving and irving 1982 this result would imply that more than 1000 km of shortening took place between south china and eurasia following the acquisition of the remanence however there is no geological evidence for this large shortening we propose that the remanence was acquired within the time corresponding to the tip of the hairpin turn 150 50 ma in the revised apwp of besse and courtillot this issue the local geology suggests that the syncline from which the samples were taken has been rotated by 15 5 counterclockwise which is reflected in a similar discrepancy between the measured paleodeclination and that predicted by the besse and courtillot this issue eurasian apwp after correcting for this rotation the pole position is 70 9 n 225 2 e dp 2 4 dm 6 5 we conclude that eurasia was fully assembled by the end of the jurassic and that the mesozoic eurasian hairpin turn is a real feature

critical crater diameter and asteroid impact seismology if impact stress reverberation is the primary gradational process on an asteroid at global scales then the largest undegraded crater records an asteroid s seismological response the critical crater diameter dcrit is defined as the smallest crater whose formation disrupts all previous craters globally up to its size it is solved for by combining relationships for crater growth and for stress attenuation the computation for dcrit gives a simple explanation for the curious observation that small asteroids have only modest undegraded craters in comparison to their size whereas large asteroids have giant undegraded craters dcrit can even exceed the asteroid diameter in which case all craters are local and the asteroid becomes crowded with giant craters dcrit is the most recent crater to have formed on a blank slate when it is equated to the measured diameter of the largest undegraded crater on known asteroids peak particle velocities are found to attenuate with the 1 2 1 3 power of distance less attenuative than strong shocks and more characteristic of powerful seismic disturbances this is to be expected since global degradation can result from seismic cm s 1 particle velocities on small asteroids attenuation as modeled appears to be higher on asteroids known to be porous although these are also bodies for which different crater scaling rules might apply

causes and consequences of asymmetric lateral plume flow during south atlantic rifting volcanic rifted margins are typically associated with a thick magmatic layer of seaward dipping reflectors and anomalous regional uplift this is conventionally interpreted as due to melting of an arriving mantle plume head at the onset of rifting however seaward dipping reflectors and uplift are sometimes asymmetrically distributed with respect to the subsequent plume track here we investigate if these asymmetries are induced by preexisting lateral variations in the thickness of continental lithosphere and or lithospheric stretching rates variations that promote lateral sublithospheric flow of plume material below only one arm of the extending rift using three dimensional numerical experiments we find that south atlantic rifting is predicted to develop a strong southward asymmetry in its distribution of seaward dipping reflectors and associated anomalous relief with respect to the tristan plume that drove this volcanic rifted margin and that the region where plume material drains into the rift should experience long lived uplift during rifting both as observed we conclude that a mantle plume is still needed to source the anomalously hot sublithospheric material that generates a volcanic rifted margin but lateral along rift flow from this plume not a broad starting plume head is what controls when and where a volcanic rifted margin will form

major influence of plume ridge interaction lithosphere thickness variations and global mantle flow on hotspot volcanism the example of tristan hotspot tracks are thought to originate when mantle plumes impinge moving plates however many observed cases close to mid ocean ridges do not form a single age progressive line but vary in width are separated into several volcanic chains or are distributed over different plates here we study plume ridge interaction at the example of the tristan plume which features all of these complexities additionally the south atlantic formed close to where plume volcanism began opening from the south and progressing northward with a notable decrease in magmatism across the florianopolis fracture zone we study the full evolution of the tristan plume in a series of three dimensional regional models created with the convection code aspect we then compute crustal thickness maps and compare them to seismic profiles and the topography of the south atlantic we find that the separation of volcanism into the tristan and gough chain can be explained by the position of the plume relative to the ridge and the influence of the global flow field plume material below the off ridge track can flow toward the ridge and regions of thinner lithosphere where decompression melting leads to the development of a second volcanic chain resembling the tristan and gough hotspot tracks agreement with the observations is best for a small plume buoyancy flux of 500 kg s or a low excess temperature of 150 k the model explains the distribution of syn rift magmatism by hot plume material that flows into the rift and increases melt generation

crustal structure of northwest namibia evidence for plume rift continent interaction the causes for the formation of large igneous provinces and hotspot trails are still a matter of considerable dispute seismic tomography and other studies suggest that hot mantle material rising from the core mantle boundary cmb might play a significant role in the formation of such hotspot trails an important area to verify this concept is the south atlantic region with hotspot trails that spatially coincide with one of the largest low velocity regions at the cmb the african large low shear wave velocity province the walvis ridge started to form during the separation of the south american and african continents at ca 130 ma as a consequence of gondwana breakup here we present the first deep seismic sounding images of the crustal structure from the landfall area of the walvis ridge at the namibian coast to constrain processes of plume lithosphere interaction and the formation of continental flood basalts parana and etendeka continental flood basalts and associated intrusive rocks our study identified a narrow region 100 km of high seismic velocity anomalies in the middle and lower crust which we interpret as a massive mafic intrusion into the northern namibian continental crust seismic crustal reflection imaging shows a flat moho as well as reflectors connecting the high velocity body with shallow crustal structures that we speculate to mark potential feeder channels of the etendeka continental flood basalt we suggest that the observed massive but localized mafic intrusion into the lower crust results from similar sized variations in the lithosphere i e lithosphere thickness or preexisting structures

interaction of mantle plumes and migrating mid ocean ridges implications for the galapagos plume ridge system we investigate the three dimensional interaction of mantle plumes and migrating mid ocean ridges with variable viscosity numerical models numerical models predict that along axis plume width w and maximum distance of plume ridge interaction xmax scale with q u where q is plume source volume flux and u is ridge full spreading rate both w and xmax increase with buoyancy number b which reflects the strength of gravitational versus plate driven spreading scaling laws derived for stationary ridges in steady state with near ridge plumes are consistent with those obtained from independent studies of ribe 1996 in the case of a migrating ridge the distance of plume ridge interaction is reduced when a ridge migrates toward the plume because of the excess drag of the faster moving leading plate and enhanced when a ridge migrates away from the plume because of the reduced drag of the slower moving trailing plate given the mildly buoyant and relatively viscous plumes investigated here the slope of the lithospheric boundary and thermal erosion of the lithosphere have little effect on plume flow from observed plume widths of the galapagos plume migrating ridge system our scaling laws yield estimates of galapagos plume volume flux of 5 16 106km3 m y 1 and a buoyancy flux of 2 103 kg s 1 model results suggest that the observed increase in bathymetric and mantle bouguer gravity anomalies along cocos plate isochrons with increasing isochron age is due to higher crustal production when the galapagos ridge axis was closer to the plume several million years ago the anomaly amplitudes can be explained by a plume source with a relatively mild temperature anomaly 50 100 c and moderate radius 100 200 km predictions of the along axis geochemical signature of the plume suggest that mixing between the plume and ambient mantle sources may not occur in the asthenosphere but instead may occur deeper in the mantle possibly by entrainment of depleted mantle as the plume ascends from its source

deformational behaviour of continental lithosphere deduced from block rotations across the north anatolian fault zone in turkey abstract theoretical considerations of lithosphere deformation across transform plate boundaries predict an expression in terms of 3istributed deformation the magnitude of rotation is expected to diminish away from the fault zone in a way which depends on the length of the fault the amount of displacement and the ductility of the lithosphere palaeomagnetic studies across the north anatolian transform fault zone which separates the eurasian plate and anatolian block in northern turkey show that clockwise rotations predicted from the sense of dextral motion are indeed present and have attained finite rotations of up to 270 during the 5 ma history of neotectonic deformation such rotations are however confined to narrow 10 km wide zones between system bounding faults and appear to have resulted from rotation in ball bearing fashion of equidimensional blocks a few kilometres in size outside of this zone only anticlockwise rotations are observed these are unrelated to deformation across the fault zone and record regional anticlockwise rotation of turkey which is complementing clockwise rotation of greece and accompanying neogene opening of the aegean sea the observed behaviour of continental lithosphere satisfies no plausible value of power law behaviour we therefore conclude that relative motion across this transform boundary occurs as a discrete zone of intense deformation within a brittle layer comprising the seismogenic upper crust this is presumed to be detached from a continuum deformation response to shearing in the lower crust and mantle beneath

the origin of the asymmetry in the iceland hotspot along the mid atlantic ridge from continental breakup to present day abstract the iceland hotspot has profoundly influenced the creation of oceanic crust throughout the north atlantic basin enigmatically the geographic extent of the hotspot influence along the mid atlantic ridge has been asymmetric for most of the spreading history this asymmetry is evident in crustal thickness along the present day ridge system and anomalously shallow seafloor of ages 49 25 ma created at the reykjanes ridge rr ssw of the hotspot center compared to deeper seafloor created by the now extinct aegir ridge ar the same distance ne of the hotspot center the cause of this asymmetry is explored with 3 d numerical models that simulate a mantle plume interacting with the ridge system using realistic ridge geometries and spreading rates that evolve from continental breakup to present day the models predict plume influence to be symmetric at continental breakup then to rapidly contract along the ridges resulting in widely influenced margins next to uninfluenced oceanic crust after this initial stage varying degrees of asymmetry along the mature ridge segments are predicted models in which the lithosphere is created by the stiffening of the mantle due to the extraction of water near the base of the melting zone predict a moderate amount of asymmetry the plume expands ne along the ar 70 80 as far as it expands ssw along the rr without dehydration stiffening the lithosphere corresponds to the near surface cool thermal boundary layer in these cases the plume is predicted to be even more asymmetric expanding only 40 50 as far along the ar as it does along the rr estimates of asymmetry and seismically measured crustal thicknesses are best explained by model predictions of an iceland plume volume flux of 100 200 m 3 s and a lithosphere controlled by a rheology in which dehydration stiffens the mantle but to a lesser degree than simulated here the asymmetry of influence along the present day ridge system is predicted to be a transient configuration in which plume influence along the reykjanes ridge is steady but is still widening along the kolbeinsey ridge as it has been since this ridge formed at 25 ma

three dimensional simulations of plume lithosphere interaction at the hawaiian swell rapid lithospheric thinning by mantle plumes has not been achieved in numerical experiments performed to date efficient thinning depends on small scale instabilities that convectively remove lithospheric material these instabilities are favored by hotter plumes or stronger temperature dependence of viscosity and a simple scaling independent of rheology controls their onset this scaling allows extrapolation of the results of numerical experiments to the earth s mantle mantle plumes between 100 and 150 kelvins hotter than the background mantle should exhibit small scale convective rolls aligned with the plate motion the unusual variation in heat flow across the hawaiian swell may be due to such instabilities it was found that the spreading of the plume creates a downwelling curtain of material that isolates it from the rest of the mantle for distances of at least 1000 kilometers from the plume origin this isolation has important consequences for the geochemical heterogeneity of the lithosphere and upper mantle

asteroid 2867 steins shape topography and global physical properties from osiris observations the rosetta spacecraft flew by asteroid 2867 steins on 5 september 2008 allowing the onboard osiris cameras to collect the first images of an e type asteroid we implemented several three dimensional reconstruction techniques to retrieve its shape limb profiles combined with stereo control points were used to reconstruct an approximate shape model this model was refined using a stereophotoclinometry technique to accurately retrieve the topography of the hemisphere observed by osiris the unseen part of the surface was constrained by the technique of light curves inversion the global shape resembles a top with dimensions along the principal axes of inertia of 6 83 x 5 70 x 4 42 km it is conspicuously more regular than other small asteroids like 233 eros and 25143 itokawa its mean radius is r m 2 70 km and its equivalent radius radius of a sphere of equivalent volume is r v 2 63 km the north pole is oriented at ra 99 5 degrees and dec 59 5 degrees which implies a very large obliquity of 172 degrees and a retrograde rotation maps of the gravitational field and slopes were calculated for the well imaged part of the asteroid together with the shape they helped characterizing the most prominent topographic features identified at the surface of 2867 steins an equatorial ridge restricted to the extremities of the long axis a large crater having dimensions of 2100 x 1800 m in the southern hemisphere and an elongated hill in the northern hemisphere we conjecture that the equatorial ridge was formed by centrifugal acceleration as the asteroid was spun up by the yarkovsky o keefe radzievskii paddack effect c 2012 elsevier inc all rights reserved

geology of 243 ida abstract the surface of 243 ida is dominated by the effects of impacts no complex crater morphologies are observed a complete range of crater degradation states is present which also reveals optical maturation of the surface darkening and reddening of materials with increasing exposure age regions of bright material associated with the freshest craters might be ballistically emplaced deposits or the result of seismic disturbance of loosely bound surface materials diameter depth ratios for fresh craters on ida are 1 6 5 similar to gaspra results but greater than the 1 5 ratios common on other rocky bodies contributing causes include rim degradation by whole body ringing relatively thin ejecta blankets around crater rims or an extended strength gradient in near surface materials due to low gravitational self packing grooves probably represent expressions in surface debris of reactivated fractures in the deeper interior isolated positive relief features as large as 150 m are probably ejecta blocks related to large impacts evidence for the presence of debris on the surface includes resolved ejecta blocks mass wasting scars contrasts in color and albedo of fresh crater materials and albedo streaks oriented down local slopes color data indicate relatively uniform calcium abundance in pyroxenes and constant pyroxene olivine ratio a large relatively blue unit across the northern polar area is probably related to regolith processes involving ejecta from azzurra rather than representing internal compositional heterogeneity a small number of bluer brighter craters are randomly distributed across the surface unlike on gaspra where these features are concentrated along ridges this implies that debris on ida is less mobile and or consistently thicker than on gaspra estimates of the average depth of mobile materials derived from chute depths 20 60 m grooves 30 m and shallowing of the largest degraded craters 20 50 m minimum 100 m maximum suggest a thickness of potentially mobile materials of 50 m and a typical thickness for the debris layer of 50 100 m

lateral flow and ponding of starting plume material many mantle plumes started with voluminous heads of hot material that ascended as diapirs through the mantle flood basalts radial dike swarms and frequently continental breakup are associated with the impingement of starting plume heads on the base of the lithosphere the buoyant plume head material ponds at the base of the lithosphere and then flows laterally relief on the base of the lithosphere acts as an upside down drainage pattern with enclosed catchments for this flow that is plume material preferentially collects beneath regions where the lithosphere is locally thin such as thermally subsiding sedimentary basins an example is uplift associated with the iceland starting plume which inverted the mesozoic irish sea basin additional relief at the base of the lithosphere is associated with rifting and seafloor spreading in the case of the northern north sea material from the iceland starting plume ponded beneath a basin with locally thin lithosphere rifting breached the enclosed catchment allowing the ponded plume material to drain across the new passive margin toward very thin lithosphere produced by seafloor spreading rifting also creates a conduit for the flow of plume material away from the region underlain by the starting plume head an example is the volcanic passive margin off nova scotia which is 2000 km from the center of the bahama starting plume head numerical and analytical calculations indicate that lateral flow of plume material consistent with this observation is expected if its viscosity is around 1018 pa s

paleomagnetic study of cretaceous rocks from the yangtze block central yunnan china implications for the india asia collision abstract more than 200 samples for this study were collected at 30 sites from the cretaceous matoushan and puchanghe formations around the city of chuxiong 25 n 101 5 e twenty four sites have characteristic directions with a high temperature component above 350 c paleomagnetic directions from 21 sites of the cretaceous formations reveal clockwise deflection in declination d 44 6 i 41 3 and 95 10 7 the reliability of the direction is ascertained through a positive fold test and the presence of normal and reversed polarities comparison with paleomagnetic data from other areas indicates that the chuxiong area was rotated clockwise through26 17 with respect to the eastern part of the yangtze block whereas it was subjected to more than 44 of clockwise rotation with respect to the neighboring blocks of lhasa 56 21 and western sichuan 44 16 since cretaceous time the clockwise rotation of chuxiong is a reflection of rotational motion of the large region including the southeastern qiangtang terrane and the jinsha suture red river fault paleomagnetic data and tectonic features suggest that the rotation occurred prior to the folding in the central yunnan province probably during the paleogene this large rotation is a spectacular example of the deformation in the southeastern part of the asian continent due to the collision of india

new late jurassic palaeomagnetic data from the northern sichuan basin implications for the deformation of the yangtze craton summary upper jurassic red sandstones and red siltstones were collected from 67 layers at 12 localities in the penglaizhen formation this formation is in the north of bazhong county 31 8 n 106 7 e in the sichuan basin which is located in the northern part of the yangtze craton thermal demagnetization isolated a high temperature magnetic component with a maximum unblocking temperature of about 690 c from 45 layers the primary nature of the magnetization acquisition is ascertained through the presence of magnetostratigraphic sequences with normal and reversed polarities as well as positive fold and reversal tests at the 95 per cent confidence level the tilt corrected mean direction of 36 layers is d 20 0 i 28 8 with a 95 5 8 a late jurassic palaeomagentic pole at 64 7 n 236 0 e with a 95 7 0 is calculated from the palaeomagnetic directions of 11 localities this pole position agrees with the two other late jurassic poles from the northern part of the yangtze craton a characteristic late jurassic pole is calculated from the three poles 68 6 n 236 0 e with a 95 8 0 for the northern part of the yangtze craton this pole position is significantly diverent from that for the southern part of the yangtze craton this suggests that the southern part of the yangtze craton was subjected to southward extrusion by 1700 1000 km with respect to the northern part intracraton deformation occurred within the yangtze craton

crustal accretion and dynamic feedback on mantle melting of a ridge centred plume the iceland case abstract a major consequence of the interaction of a plume with an oceanic ridge is the enhanced melt production and associated crust generation in the case of iceland crustal thickness as large as 20 to 40 km has been reported crustal seismic velocities are high and have to be explained by thermal or chemical effects in the first part of the paper we address the question whether extraction of melt out of the plume beneath a slowly spreading ridge and deposition of extracted basalt volumes at the surface produces a dynamic feedback mechanism on mantle melting to study this question we solve the convection equations for a ridge centred plume with non newtonian rheology including melting melt extraction associated with deposition of cold crust at the surface of the model and using a simplified approach for compaction the assumption of cold crust is justified if the thickness of each deposited basaltic layer is less than roughly 1 km depending on the buoyancy flux of the plume crustal thicknesses between 10 and 40 km are modelled showing characteristic dipping structures resembling the rift ward dipping basaltic layers of east and western iceland comparing the resulting crustal thickness and magma generation rate with models in which the dynamic effect of crust deposition has been suppressed indicates that melt generation beneath a slowly spreading ridge is considerably damped by the dynamic feedback mechanism if the plume buoyancy flux exceeds 400 to 600 kg s based on the observed crustal thickness of iceland our models predict a plume buoyancy flux of 1140 kg s in the second part we study the accretion of the icelandic crust by a thermo mechanical model in more detail based on the navier stokes the heat transport and the mass conservation equations including volumetric sources hot 1200 c molten crustal material is injected into the newly forming crust with a constant rate at different crustal source regions a deep widespread emplacement of dykes and sills including crustal underplating b magma chambers at shallow to mid crustal level and c surface extrusions and intrusions in fissure swarms at shallow depth connected to volcanic centres we identify the material from the different source regions by a marker approach varying the relative dominance of these source regions characteristic crustal structures evolve showing shallow dipping upper crustal layers with dip angles between 10 and 15 the thermal structure of the crust varies between cold crust shallow source region dominating and hot crust deep source region dominating we use observations of maximum depth of seismicity to constrain the depth of the 650 c isotherm and seismological inferences on the lower crust to constrain temperatures in that region the best agreement with our models is achieved for crust formation dominated by deep dykes and underplating with a considerable influence of magma chamber accretion

near s flyby of 253 mathilde images of a c asteroid on 27 june 1997 the near earth asteroid rendezvous near spacecraft flew within 1212 kilometers of asteroid 253 mathilde mathilde is an irregular heavily cratered body measuring 66 kilometers by 48 kilometers by 46 kilometers the asteroid s surface is dark estimated albedo between 0 035 and 0 050 and similar in color to some cm carbonaceous chondrites no albedo or color variations were detected the volume derived from the images and the mass from doppler tracking of the spacecraft yield a mean density of 1 3 0 2 grams per cubic centimeter about half that of cm chondrites indicating a porous interior structure

geoframework coupling multiple models of mantle convection within a computational framework solver coupling can extend the capability of existing modeling software and provide a new venue to address previously intractable problems a software package has been developed to couple geophysical solvers demonstrating a method to accurately and efficiently solve multiscale geophysical problems with reengineered software using a computational framework pyre pyre is a modeling framework capable of handling all aspects of the specification and launching of numerical investigations we restructured and ported citcoms a finite element code for mantle convection into the pyre framework two citcoms solvers are coupled to investigate the interaction of a plume at high resolution with global mantle flow at low resolution a comparison of the coupled models with parameterized models demonstrates the accuracy and efficiency of the coupled models and illustrates the limitations and utility of parameterized models

tertiary diachronic extrusion and deformation of western indochina structural and 40ar 39ar evidence from nw thailand the wang chao and three pagodas fault zones cut the western part of the indochina block and run parallel to the red river fault evidence of intense ductile left lateral shear is found in the lansang gneisses which form a 5 km wide elongated core along the wang chao fault zone dating by 40 ar 39 ar shows that such deformation probably terminated around 30 5 ma the wang chao and three pagodas faults offset the north striking lower mesozoic metamorphic and magmatic belt of northern thailand 40 ar 39 ar results suggest that this belt suffered rapid cooling in the tertiary probably around 23 ma these results imply that the extrusion of the southwestern part of indochina occurred in the upper eocene lower oligocene it probably induced rifting in some basins of the gulf of thailand and in the malay and mekong basins in the oligo miocene the continuing penetration of india into asia culminated with the extrusion of all of indochina along the ailao shan red river fault this occurred concurrently with the onset of e w extension more to the south plotting in a geographical reference frame the diachronic time spans of movement on left lateral faults east and southeast of tibet implies that the northward movement of the indian indenter successively initiated new strike slip faults located farther and farther north along its path

geologic map of the northern hemisphere of vesta based on dawn framing camera fc images abstract the dawn framing camera fc has imaged the northern hemisphere of the asteroid 4 vesta at high spatial resolution and coverage this study represents the first investigation of the overall geology of the northern hemisphere 22 90 n quadrangles av 1 2 3 4 and 5 using these unique dawn mission observations we have compiled a morphologic map and performed crater size frequency distribution csfd measurements to date the geologic units the hemisphere is characterized by a heavily cratered surface with a few highly subdued basins up to 200 km in diameter the most widespread unit is a plateau cratered highland unit similar to although of lower elevation than the equatorial vestalia terra plateau large scale troughs and ridges have regionally affected the surface between 180 e and 270 e these tectonic features are well developed and related to the south pole veneneia impact saturnalia fossae trough unit elsewhere on the hemisphere they are rare and subdued saturnalia fossae cratered unit in these pre rheasilvia units we observed an unexpectedly high frequency of impact craters up to 10 km in diameter whose formation could in part be related to the rheasilvia basin forming event the rheasilvia impact has potentially affected the northern hemisphere also with s n small scale lineations but without covering it with an ejecta blanket post rheasilvia impact craters are small

paleomagnetism of cretaceous to lower tertiary rocks from southwestern sichuan a revisit abstract a re study has been carried out on the cretaceous to lower tertiary rocks from southwestern sichuan province on the south china block the characteristics remanent magnetization chrm resolved from the paleocene eocene leidashu formation is of dual polarity and very shallow inclination the anomalouslylow paleolatitude 6 9 6 3 n indicated by the inclination is not supported by geological evidence from this region and is probably due to inclination error related to depositional and or post depositional processes a positive fold test as well as the prevalence of normal polarity suggests that the chrm of the mid upper cretaceous xiaoba formation might be acquired during the cretaceous long normal superchron clns the tilt corrected mean direction of the xiaoba formation d i 8 1 38 8 95 6 6 n 18 sites is not significantly different from a preliminary result d i 357 6 31 6 95 8 6 n 12 samples we reported earlier for the collective xiaoba and leidashu formations the resultant paleopole 81 5 n 220 9 e a95 7 1 is coincident with other cretaceous poles for the chinese blocks comparison with the eurasian apparent polar wander path apwp indicates neither significant rotation nor latitudinal translation of the study area relative to eurasia the newly obtained mean direction for the xiaoba formation is believed to be more reliable than our earlier result since it is based on a larger sample size and is apparently free from effects of the shallow inclination observed in the leidashu formation and should supercede our previous preliminary result the paleopole derived from sevenentes of the lower cretaceous feitianshan formation 69 0 n 204 6 e a95 4 3 deviates from the track from the late jurassic to late cretaceous poles the deviation might be reduced by further sampling

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

new observational and experimental evidence for a plume fed asthenosphere boundary layer in mantle convection abstract the textbook view is that the asthenosphere is the place beneath the tectonic plates where competing temperature and pressure effects on mantle rheology result in the lowest viscosity region of earth s mantle we think the sub oceanic asthenosphere exists for a different reason that instead it is where rising plumes of hot mantle stall and spread out beneath the strong tectonic plates below this plume fed asthenosphere is a thermal and density inversion with cooler underlying average temperature mantle here we show several recent seismic studies that are consistent with a plume fed asthenosphere these include the seismic inferences that asthenosphere appears to resist being dragged down at subduction zones that a sub oceanic thermal inversion 250 350 km deep is needed to explain the seismic velocity gradient there for an isochemical mantle that a fast halo of shear wave travel times surrounds the hawaiian plume conduit and that an apparent seismic reflector is found 300 km beneath pacific seafloor near hawaii we also present 2d axisymmetric and 3d numerical experiments that demonstrate these effects in internally consistent models with a plume fed asthenosphere if confirmed the existence of a plume fed asthenosphere will change our understanding of the dynamics of mantle convection and melting and the links between surface plate motions and mantle convection

chemical trends in ocean islands explained by plume slab interaction earth s surface shows many features of which the genesis can be understood only through their connection with processes in earth s deep interior recent studies indicate that spatial geochemical patterns at oceanic islands correspond to structures in the lowermost mantle inferred from seismic tomographic models this suggests that hot buoyant upwellings can carry chemical heterogeneities from the deep lower mantle toward the surface providing a window to the composition of the lowermost mantle the exact nature of this link between surface and deep earth remains debated and poorly understood using computational models we show that subducted slabs interacting with dense thermochemical piles can trigger the ascent of hot plumes that inherit chemical gradients present in the lowermost mantle we identify two key factors controlling this process i if slabs induce strong lower mantle flow toward the edges of these piles where plumes rise the pile facing side of the plume preferentially samples material originating from the pile and bilaterally asymmetric chemical zoning develops ii the composition of the melt produced reflects this bilateral zoning if the overlying plate moves roughly perpendicular to the chemical gradient in the plume conduit our results explain some of the observed geochemical trends of oceanic islands and provide insights into how these trends may originate

hot upper mantle beneath the tristan da cunha hotspot from probabilistic rayleigh wave inversion and petrological modeling understanding the enigmatic intraplate volcanism in the tristan da cunha region requires knowledge of the temperature of the lithosphere and asthenosphere beneath it we measured phasevelocity curves of rayleigh waves using cross correlation of teleseismic seismograms from an array of ocean bottom seismometers around tristan constrained a region average shear velocity structure and inferred the temperature of the lithosphere and asthenosphere beneath the hotspot the ocean bottom data set presented some challenges which required data processing and measurement approaches different from those tuned for land based arrays of stations having derived a robust phase velocity curve for the tristan area we inverted it for a shear wave velocity profile using a probabilistic markov chain monte carlo approach the model shows a pronounced low velocity anomaly from 70 to at least 120 km depth vs in the low velocity zone is 4 1 4 2 km s not as low as reported for hawaii 4 0 km s which probably indicates a less pronounced thermal anomaly and possibly less partial melting petrological modeling shows that the seismic and bathymetry data are consistent with a moderately hot mantle mantle potential temperature of 1 410 1 4308c an excess of about 50 1208c compared to the global average and a melt fraction smaller than 1 both purely seismic inversions and petrological modeling indicate a lithospheric thickness of 65 70 km consistent with recent estimates from receiver functions the presence of warmer than average asthenosphere beneath tristan is consistent with a hot upwelling plume from the deep mantle however the excess temperature we determine is smaller than that reported for some other major hotspots in particular hawaii

plate tectonics and net lithosphere rotation over the past 150 my we have developed an improved model of global digital palaeo plate boundaries and plate motion to describe the distribution and history of plates since the late jurassic from this history we computed net lithospheric rotation nr through time confirming the so called westward drift but only for the past 30 myr the nr has significantly smaller magnitudes 0 13 my past 5 my than for some other plate models it averages to 0 11 0 03 my for the past 50 my with a small but systematic increase toward the present the westward drift seen only for the past 30 my is attributed to the increased dominance of a steadily growing and accelerating pacific plate nr shows peaks with time but only an early tertiary peak of 0 33 my when the indian plate was undergoing the largest known acceleration deceleration can be interpreted with some confidence we find a linear decreasing trend in net rotation over the past 150 my but attribute this trend to increasing reconstruction uncertainties back in time as subduction consumed more than half of the oceanic crust since the jurassic after removing a linear time trend we find a nr average of about 0 12 my for the past 150 my

geodynamics how plumes help to break plates computer models show how hot material that rises from earth s interior is affected by plate tectonics producing unexpected irregularities in earth s topography and assisting in the break up of continental plates see letter p 85 the question of the spatial scales at which surface topography is affected by mantle lithosphere interactions rather than by plate scale processes has important implication for our understanding of the earth s tectonic magmatic and seismic activity here evgueni burov and taras gerya present three dimensional numerical simulations of the interaction of a mantle plume with a rheologically realistic lithosphere under far field tension they find that the result is a complex surface evolution very different from the smooth radially symmetric patterns usually assumed to be the signature of a mantle upwelling the topography exhibits strongly asymmetric small scale three dimensional features including narrow and wide rifts flexural flank uplifts and fault structures

paleomagnetic study of middle cretaceous rocks from yunlong western yunnan china evidence of southward displacement of indochina middle cretaceous red sandstones and siltstones were collected at 20 sites for paleomagnetic study from the nanxin formation around yunlong 25 8 n 99 4 e western part of yunnan province china after stepwise thermal demagnetization high temperature component magnetization with unblocking temperature of about 675 c is isolated a positive fold test at the 99 confidence limit shows that the high temperature component magnetization is primary for the middle cretaceous nanxin formation a tilt corrected formation mean direction is d 40 2 i 49 9 with 95 3 9 corresponding to a paleopole at 54 6 n 171 8 e with a95 4 4 comparison with previously reported cretaceous paleomagnetic directions indicates that the yunlong area has been subjected to southward displacement by 11 9 7 5 corresponding to 1300 800 km with respect to the stable yangtze craton since cretaceous time as well as clockwise rotation through 36 3 13 6 this study elucidates that indochina was squeezed out of the asian continent due to collision of india accompanied with clockwise rotation

thickness of the oceanic crust the lithosphere and the mantle transition zone in the vicinity of the tristan da cunha hot spot estimated from ocean bottom and ocean island seismometer receiver functions highlights receiver functions from ocean bottom seismometer stations reveal no significant crustal thickening in the surrounding of the tristan da cunha hot spot the mantle transition zone to the nw of tristan da cunha is thickened and cool the mantle transition zone is potentially thinned to the south southwest of tristan da cunha a thickness of 60 to 75 km beneath tristan da cunha argues for a compositional control on the seismological lithosphere in the south atlantic abstract the most prominent hotspot in the south atlantic is tristan da cunha which is widely considered to be underlain by a mantle plume but the existence location and size of this mantle plume have not been established due to the lack of regional geophysical observations a passive seismic experiment using ocean bottom seismometers aims to investigate the lithosphere and upper mantle structure beneath the hotspot using the ps receiver function method we calculate a thickness of 5 to 8 km for the oceanic crust at 17 ocean bottom stations deployed around the islands within the errors of the method the thickness of the oceanic crust is very close to the global mean the tristan hotspot seems to have contributed little additional magmatic material or heat to the melting zone at the mid oceanic ridge which could be detected as thickened oceanic crust magmatic activity on the archipelago and surrounding seamounts seems to have only effected the crustal thickness locally furthermore we imaged the mantle transition zone discontinuities by analysing receiver functions at the permanent seismological station tris and surrounding obs stations our observations provide evidence for a thickened cold mantle transition zone west and northwest of the islands which excludes the presence of a deep reaching mantle plume we have some indications of a thinned hot mantle transition zone south of tristan da cunha inferred from sparse and noisy observations which might indicate the location of a tristan mantle plume at mid mantle depths sp receiver functions image the base of lithosphere at about 60 to 75 km beneath the islands which argues for a compositionally controlled seismological lithosphere asthenosphere boundary beneath the study area

surface expressions of structural features on eros abstract near shoemaker imaging of the asteroid eros has revealed a variety of surface forms that probably reflect underlying structures they range from global to local in scale the most prominent positive topographic feature is a long ridge rahe dorsum which extends over 18 km and has the morphology of a thrust fault two large ancient trough systems are also present grooves are ubiquitous commonly occur in suborthogonal sets and have linear or scalloped outlines the grooves on eros are similar in morphology and scale to those on other small bodies such as gaspra ida and phobos and are likely to have a similar mode of origin namely drainage of regolith into underlying fractures models that use groove dimensions as indicators of regolith thickness imply depths of up to several tens of meters many craters have been subjected to structural control resulting in squared outlines we find no evidence that specific groove sets are associated with individual impact events however evidence exists that preexisting structures have been reactivated by later impact activity the large scale of rahe dorsum and many grooves on eros s surface suggests that the asteroid is a largely coherent but fractured body

new mesozoic apparent polar wander path for south china tectonic consequences a new paleomagnetic study in the south sichuan basin south china block has been carried out on sediments of early triassic to middle jurassic age detailed stepwise thermal and alternating field demagnetizations allowed us to isolate two to three components a low temperature component component a falls close to the local present earth field direction a middle temperature component component b is found both in limestone and red beds of early triassic age in limestones of middle triassic age and finally in red beds of jurassic age the component b directions failed the fold test but show a significant increase of the ks kg ratio at 60 when a progressive stepwise unfolding is applied the directions obtained at 60 unfolding are grouped best and yield a pole position at 76 5 n 174 1 e a95 3 7 that is close to the late cretaceous pole of the south china block scb or the early tertiary pole of the eurasian plate characteristic directions are finally obtained component c for early triassic and early to middle jurassic sediments only both with reversal and fold tests they yield pole positions at 44 1 n and 217 2 e a95 4 9 and 79 n and 227 e a95 4 5 respectively the early triassic result well fits other data of the same age from the scb the revised late permian to cretaceous paleopole data provide a new mesozoic apparent polar wander path for south china the successive pole positions from late permian to early triassic indicate motion of the south china block as a whole rather than local rotations linked to important strain within the block the early to middle jurassic pole is statistically indistinguishable from the compiled north china block ncb poles and indicates that final suturation between the two blocks occurred by the end of the triassic which is coherent with the age of ultra high pressure metamorphism in the dabie sulu areas of eastern china the asian plate including the mongol central asian block ncb and scb however was probably separated from siberia by the mongol okhotsk ocean during jurassic time

paleomagnetic evidence for post cretaceous internal deformation of the chuan dian fragment in the yangtze block a consequence of indentation of india into asia abstract paleomagnetic samples of paleocene eocene red sandstones were collected at 36 sites from the jiangdihe 4 and zhaojiadian formations around the yongren 26 1 n 101 7 e and dayao areas 25 7 n 101 3 e these areas are located in the chuxiong basin of the chuan dian fragment southwestern part of the yangtze block after stepwise thermal demagnetization a high temperature component with unblocking temperature of about 680 c is isolated from 26 sites the primary nature of this magnetization is ascertained through positive fold and reversal tests at 95 confidence level the tilt corrected mean paleomagnetic directions for the yongren and dayao areas are d 17 2 i 26 6 with 95 5 8 and d 16 5 i 31 1 with 95 4 8 respectively easterly deflected declinations from this study are consistent with those reported from other areas of the chuxiong basin indicating its wide presence in the cretaceous eocene formations of the said basin comparison with declination values expected from the cretaceous eocene apwp of eurasia indicates that the magnitude of clockwise rotation systematically increases toward the southeast within the chuxiong basin as well as in the chuan dian fragment this trend of the differential tectonic rotation in the chuan dian fragment is consistent with curvature of the xianshuihe xiojiang fault system deformation of the chuxiong basin can fairly be associated with the formation of eastward bulge in the southern part of the chuan dian fragment during southward displacement the chuan dian fragment was probably subjected to tectonic stresses as a result interaction with the yangtze and indochina blocks which resulted into east west extension and north south shortening

433 eros lineaments global mapping and analysis using images and laser ranging data from the near shoemaker mission we map lineaments on the surface of eros in order to investigate the relationship between surface morphology and interior structure several sets of lineations are clearly related to visible impact craters while others suggest that different parts of the asteroid may have undergone different stress histories some of these sets infer internal structure at least on a local level this structure may derive from eros parent body and suggest although largely coherent eros interior may have portions that have not undergone a common history

discrepant cretaceous paleomagnetic poles between eastern china and indochina a consequence of the extrusion of indochina abstract a paleomagnetic study of cretaceous and paleocene red beds has been carried out in the yunlong area yunnan province of china of the northern indochina block a high temperature component with dual polarities was isolated from both lower cretaceous and paleocene rocks and of solely polarity for upper early cretaceous and lower late cretaceous rocks thirteen magnetic polarity zones are found along the pijiang section the primary nature of cretaceous magnetic remanence is ascertained by positive and reversal fold tests these results combined with previous cretaceous results from the yunlong and yongping areas of the lanping basin indicate that these areas behaved as a relatively rigid block and rotated 37 2 17 clockwise relative to eurasia since the paleocene comparing available early cretaceous paleomagnetic results obtained both from indochina and eastern china a statistically significant paleolatitude difference of 9 6 5 7 is inferred this difference is consistent with an estimated 1000 400 km extrusion of indochina relative to south china through a counter clockwise rotation of 14 of indochina around a finite rotation pole inferred from the stage poles of rotation derived from the magnetic isochrons in the south china sea the reconstruction is fully consistent between the cretaceous pole obtained from the khorat basin of the stable core of the indochina block and the coeval pole of eastern china

small scale convection in a plume fed low viscosity layer beneath a moving plate two dimensional simulations using a thermomechanical model based on a finite difference method on a staggered grid and a marker in cell method are performed to study the plume lithosphere interaction beneath moving plates the plate and the convective mantle are modelled as a homogeneous peridotite with a newtonian temperature and pressure dependent viscosity a constant velocity ranging from 5 to 12 5 cm yr 1 is imposed at the top of the plate plumes are generated by imposing a thermal anomaly of 150 to 350 k on a 50 km wide domain at the base of the model 700 km depth the plate atop this thermal anomaly is 40 myr old we analyse 1 the kinematics of the plume as it impacts the moving plate 2 the dynamics of time dependent small scale convection ssc instabilities developing in the low viscosity layer formed by spreading of hot plume material at the base of the lithosphere and 3 the resulting thermal rejuvenation of the lithosphere the spreading of the plume material at the base of the lithosphere characterized by the ratio between the maximum down and upstream horizontal dimensionless velocities in the plume fed sublithospheric layer peup pedown depends on the ratio between the maximum plume upwelling velocity and the plate velocity peplume peplate for fast plate velocities and sluggish plumes low peplume peplate plate motion drags most plume material and downstream flow is dominant as peplume peplate increases an increasing part of the plume material flows upstream ssc systematically develops in the plume fed sublithospheric layer downstream from the plume onset time of ssc decreases with the rayleigh number for vigorous plumes it does not depend on plate velocity for more sluggish plumes however variations in the plume spreading behaviour at the base of the lithosphere result in a decrease in the onset time of sscs with increasing plate velocity in any case ssc results in uplift of the isotherm 1573 k by up to 20 km relative to its initial equilibrium depth at the impact point

new constraints on the age and style of continental breakup in the south atlantic from magnetic anomaly data abstract we present new constraints on the opening of the south atlantic ocean from a joint interpretation of marine magnetic anomaly grids and forward modelling of conjugate profiles we use 45 000 km of recently collected commercial ship track data combined with 561 000 km of publically available data the new data cover the critical ocean continental transition zones and allow us to identify and downgrade some poorly navigated older ship tracks relied upon in earlier compilations within the final grids the mean cross over error is 14 nt computed from 8 227 ship track intersections the forward modelling used uniformly magnetised bodies whose shapes were constrained from coincident deep seismic reflection data we find the oldest magnetic anomalies to date from m10r 134 2 ma late valanginian north of the falkland agulhas fracture zone and m3 129 3 ma barremian south of the rio grande fracture zone hence assuming the gpts used is correct continental breakup was contemporaneous with the parana and etendeka continental flood basalts many of the landward linear anomalies overlap seismically mapped seaward dipping reflectors sdrs we interpret this to mean that a significant portion of the sdrs overlay crust formed by subaerial seafloor spreading here crustal accretion is envisaged to be similar to that at mid ocean ridges but sheet lava flows that later form the sdrs rather than pillow basalts form the extrusive component segmentation of the linear anomalies generated implies that this stage of continental breakup is organised and parallels the seafloor spreading centre that follows our results call into question the common assumption that at volcanic continental margins the first linear magnetic anomalies represent the start of conventional submarine oceanic crustal generation

properties of craters on the achaia region of asteroid 21 lutetia we report on the physical properties of the craters of achaia region of the main belt asteroid 21 lutetia based on images obtained with the osiris instrument during the rosetta flyby that took place on 10 july 2010 images of the surface were acquired with its narrow angle camera from which digital terrain models dtm of the surface were constructed these dtms give access to the geometrical properties of the craters of the asteroid on a complex asteroid shape slopes and depth to diameter ratios d d of craters should be carefully measured taking into account the local topography to obtain a value that is physically related to the work of forces resisting to mass displacement associated with gravity and or material strength occurring in either excavation or degradation processes we present new measurements of d d and internal slopes of impact craters of the achaia region which offers optimal conditions of observations and a large population of craters we find that d d values for achaia craters differ from previous works on lutetia and are consistent with the values found on other asteroids such as 243 ida or 951 gaspra the achaia region may be divided into three units based on geomorphological analysis the mean d d values of the three units are different revealing differences in resurfacing history by impact related ejecta blanketing and seismic shaking some of these geological events may be recent compared to the age of the region since several lineaments intersect most craters of one of the three units independent evidence for ejecta blanket have been given for the unit associated with low d d values confirming the contribution of this process to crater modification moreover we suggest that displacements along faults identified as surface lineaments may have been responsible for the erasure of small craters our results are finally integrated into a chronology sequence of events explaining the present characteristics of the achaia region

the dynamics of plume ridge interaction 2 off ridge plumes a variety of geophysical and geochemical evidence indicates that ascending mantle plumes can interact with ocean ridges located up to 1400 km away i investigate the dynamics of this interaction using a simple model in which a point source with volume flux q analogous to a plume stem releases buoyant fluid into a viscous corner flow driven by the divergence of rigid surface plates with thickness kx u 1 2 where u is the half spreading rate the point source is located at a distance xp from the ridge and ridge migration is neglected the buoyant fluid forms a thin sublithospheric layer whose thickness s x y satisfies a nonlinear advection diffusion equation describing the balance of advection by the corner flow buoyancy driven self spreading flow toward the ridge along the sloping base of the lithosphere and continuous accretion into the lithosphere numerical solutions of this equation yield scaling laws for the lateral extent w waist width of plume material along the ridge the fraction r of the plume flux that crosses the ridge and the maximum value of xp beyond which interaction ceases the sloping base of the lithosphere has only a minor few tens of percent influence on these quantities which are determined principally by the balance of advection and self spreading an extension of the model to include plume induced lithospheric thinning shows that this process increases the waist width by an amount of order 10 finally the model provides a new explanation for the observation that plumes interact primarily with ridges that are migrating away from them rather than toward them

characterizing and navigating small bodies with imaging data recent advances in the characterization of small body surfaces with stereophotoclinometry are discussed the principal data output is an ensemble of landmark maps l maps high resolution topography albedo maps of varying resolution that tile the surface of the body because they can have a resolution comparable to the best images and can be located on a global reference frame to high accuracy l maps provide a significant improvement in discriminatory power for studies of small bodies ranging from regolith processes to interior structure these techniques are now being used to map larger bodies such as the moon and mercury l maps are combined to produce a standard global topography model gtm with about 1 57 million vectors and having a wide variety of applications they can also be combined to produce high resolution topography maps that describe local areas with much greater detail than the gtm when combined with nominal predictions from other data sources and available data from other instruments such as lidar or radar solutions for the spacecraft position and camera pointing are the most accurate available examples are drawn from studies of phobos eros and itokawa including surface characterization gravity analysis spacecraft navigation and incorporation of lidar or radar data this work has important implications for potential future missions such as deep interior and the level of navigation and science that can be achieved

the effect of the large scale mantle flow field on the iceland hotspot track abstract fluid dynamical simulations were carried out in order to investigate the effect of the large scale mantle flow field and the depth of the plume source on the structure of the iceland plume through time the time dependent location and shape of the plume in the earth s mantle was calculated in a global model and it was refined in the upper mantle using a 3d cartesian model box global flow was computed based on density heterogeneities derived from seismic tomography plate motion history served as a velocity boundary condition in both models hotspot tracks of the plume conduits and the plume head were calculated and compared to actual bathymetry of the north atlantic if a plume source in the lowermost mantle is assumed the calculated surface position of the plume conduit has a southward component of motion due to southward flow in the lower mantle depending on tomography model assumed plume age and buoyancy the southward component is more or less dominating plume models having a source at the 660 km discontinuity are only influenced by flow in the upper mantle and transition zone and hence rather yield westward hotspot motion many whole mantle plume models result in a v shaped track which does not match the straight greenland iceland faroe ridge models without strong southward motion such as for a plume source at 660 km depth match actual bathymetry better plume tracks were calculated from both plume conduits and plume heads a plume head of 120 k anomalous temperature gives the best match between plume head track and bathymetry

non hotspot volcano chains produced by migration of shear driven upwelling toward the east pacific rise while most oceanic volcanism is associated with the passive rise of hot mantle beneath the spreading axes of mid ocean ridges mor volcanism occurring off axis reflects intraplate upper mantle dynamics and composition yet is poorly understood off the south east pacific rise sepr volcanism along the pukapuka hotu matua and sojourn ridges has been attributed to various mechanisms but none can reconcile its spatial temporal and geochemical characteristics our three dimensional numerical models show that asthenospheric shear can excite upwelling and decompression melting at the tip of low viscosity fingers that are propelled eastward by vigorous sublithospheric flow this shear driven upwelling is able to sustain intraplate volcanism that progresses toward the mor spreads laterally close to the axis and weakly continues on the opposite plate these predictions can explain the anomalously fast eastward progression of volcanism and its spatial distribution near the sepr moreover for a heterogeneous mantle source involving a fertile component the predicted systematics of volcanism can explain the geochemical trend along pukapuka and the enriched anomaly of sepr mid oceanic ridge basalt at 16 20 5 s our study highlights the role of horizontal asthenospheric flow and mantle heterogeneity in producing linear chains of intraplate volcanism independent of a deep rooted buoyancy source

south atlantic opening a plume induced breakup upwelling hot mantle plumes are thought to disintegrate continental lithosphere and are considered to be drivers of active continental breakup the formation of the walvis ridge during the opening of the south atlantic is related to a putative plume induced breakup we investigated the crustal structure of the walvis ridge southeast atlantic ocean at its intersection with the continental margin and searched for anomalies related to the possible plume head the overall structure we identify suggests that no broad plume head existed during opening of the south atlantic and anomalous mantle melting occurred only locally we therefore question the importance of a plume head as a driver of continental breakup and further speculate that the hotspot was present before the rifting leaving a track of kimberlites in the african craton

large scale troughs on vesta a signature of planetary tectonics abstract images of vesta taken by the dawn spacecraft reveal large scale linear structural features on the surface of the asteroid we evaluate the morphology of the vesta structures to determine what processes caused them to form and what implications this has for the history of vesta as a planetary body the dimensions and shape of these features suggest that they are graben similar to those observed on terrestrial planets not fractures or grooves such as are found on smaller asteroids as graben their vertical displacement versus length relationship could be evaluated to describe and interpret the evolution of the component faults linear structures are commonly observed on smaller asteroids and their formation has been tied to impact events while the orientation of the large scale vesta structures does imply that their formation is related to the impact events that formed the rheasilvia and veneneia basins their size and morphology is greatly different from impact formed fractures on the smaller bodies this is consistent with new analyses that suggest that vesta is fully differentiated with a mantle and core we suggest that impact into a differentiated asteroid such as vesta could result in graben while grooves and fractures would form on undifferentiated asteroids

discrete alternating hotspot islands formed by interaction of magma transport and lithospheric flexure the large scale geometry and age progression of many hotspot island chains such as the hawaiian emperor chain are well explained by the steady movement of tectonic plates over stationary hotspots but on a smaller scale hotspot tracks are composed of discrete volcanic islands whose spacing correlates with lithospheric thickness1 moreover the volcanic shields themselves are often not positioned along single lines but in more complicated patterns such as the dual line known as the kea and loa trends of the hawaiian islands2 3 here we make use of the hypothesis that island spacing is controlled by lithospheric flexure1 to develop a simple nonlinear model coupling magma flow which feeds volcanic growth to the flexure caused by volcanic loads on the underlying plate for a steady source of melt underneath a moving lithospheric plate magma is found to reach the surface and build a chain of separate volcanic edifices with realistic spacing if a volcano is introduced away from the axis of the chain as might occur following a change in the direction of plate motion the model perpetuates the asymmetry for long distances and times thereby producing an alternating series of edifices similar to that observed in the kea and loa trends of the hawaiian island chain

right lateral shear and rotation as the explanation for strike slip faulting in eastern tibet the convergence of two continents can be accommodated by crustal thickening and by lateral transport of crust out of the path of the converging continents1 3 the principal features suggesting that lateral transport is important are major strike slip faults striking roughly orthogonally to the orientation of convergence between the continents displacement on such faults is imagined to allow lateral transport of material with respect to the converging continents possibly accounting for a large fraction of the convergence between the two continents the most spectacular example of such strike slip faulting is in the eastern part of the tibetan plateau where three major left lateral faults with slip rates of 10 mm yr 1 and several smaller roughly parallel faults dominate the regional strain field2 3 cobbold and davy4 suggested that these faults may be rotating in a clockwise fashion here we put bounds on that rate of rotation and conclude that the image of lateral transport on such faults2 known also as continental escape extrusion or expulsion is an illusion and that instead the left lateral slip on east striking planes in eastern tibet is a manifestation of north striking right lateral simple shear if this conclusion is correct the east striking left lateral faults and the crustal blocks between them are rotating clockwise at 1 2 myr 1 the east west dimension of eastern tibet is shortening at 10 20 mm yr 1 and little material is moving eastward out of india s path into eurasia by left lateral simple shear

south atlantic volcanic margins the early cretaceous south atlantic continental break up and initial sea floor spreading were accompanied by large scale transient volcanism emplacing the parana etendeka continental flood basalts and voluminous extrusive constructions on the conjugate margins south of the torres arch abutment plateau on the north namibia margin we interpret four main tectono magmatic crustal units 1 oceanic crust 2 thickened oceanic crust covered by huge seaward dipping wedges 3 a c 150 km wide break up related rift zone partly covered by the dipping wedges and 4 thicker continental crust partly deformed by palaeozoic extension east of the early cretaceous rift similar settings also characterize other south atlantic margin segments we infer an up to 300 km wide and 2400 km long rift zone representing lithospheric extension leading to breakup and formation of the south atlantic volcanic margins comparison with other volcanic margins demonstrates in spite of local and regional differences gross similarities in tectono magmatic style crustal units and dimensions

the structure of the asteroid 4 vesta as revealed by models of planet scale collisions three dimensional simulations of the global evolution of asteroid 4 vesta under two overlapping planet scale collisions closely reproduce its observed shape but expected large areas of olivine rich rocks and pure diogenites are not observed on the surface possibly implying that the outer 100 km is composed mainly of a basaltic crust eucrites with ultramafic intrusions diogenites nasa s dawn mission to the asteroid 4 vesta revealed some unexpected surface features including a huge crater named veneneia underlying the even larger crater rheasilvia that was formed about a billion years ago the juxtaposition of these two impact basins provides an opportunity to develop an accurate model of vesta s topography martin jutzi et al report three dimensional simulations of vesta s global evolution through two successive planet scale collisions their model closely reproduces vesta s observed shape and provides a firm basis for the interpretation of the surface mineralogy and geology of vesta and by extension other small solar system bodies asteroid 4 vesta seems to be a major intact protoplanet with a surface composition similar to that of the hed howardite eucrite diogenite meteorites1 2 3 4 the southern hemisphere is dominated by a giant impact scar5 but previous impact models6 7 8 have failed to reproduce the observed topography the recent discovery that vesta s southern hemisphere is dominated by two overlapping basins9 provides an opportunity to model vesta s topography more accurately here we report three dimensional simulations of vesta s global evolution under two overlapping planet scale collisions we closely reproduce its observed shape and provide maps of impact excavation and ejecta deposition spiral patterns observed in the younger basin rheasilvia9 about one billion years old10 are attributed to coriolis forces during crater collapse surface materials exposed in the north come from a depth of about 20 kilometres according to our models whereas materials exposed inside the southern double excavation come from depths of about 60 100 kilometres if vesta began as a layered completely differentiated protoplanet then our model predicts large areas of pure diogenites and olivine rich rocks these are not seen11 12 13 possibly implying that the outer 100 kilometres or so of vesta is composed mainly of a basaltic crust eucrites with ultramafic intrusions diogenites