gravity field of the moon from the gravity recovery and interior laboratory grail mission spacecraft to spacecraft tracking observations from the gravity recovery and interior laboratory grail have been used to construct a gravitational field of the moon to spherical harmonic degree and order 420 the grail field reveals features not previously resolved including tectonic structures volcanic landforms basin rings crater central peaks and numerous simple craters from degrees 80 through 300 over 98 of the gravitational signature is associated with topography a result that reflects the preservation of crater relief in highly fractured crust the remaining 2 represents fine details of subsurface structure not previously resolved grail elucidates the role of impact bombardment in homogenizing the distribution of shallow density anomalies on terrestrial planetary bodies

modification of continental lithosphere by tectonic processes a tomographic image of central north america the assembly of laurentia by precambrian accretion is also believed to have formed the underlying lithosphere accretionary signatures are detectable by seismic observations but subject to modification by later processes e g orogeny rifting and plumes we examine the archean superior province sp and environs using a set of teleseismic p wave arrivals from canadian and american instruments the resulting tomographic model has high resolution beneath the dakotas and minnesota provides a first look at the lithosphere beneath manitoba and sharpens previously documented features in ontario from the model and previous anisotropy observations we detect i a large high velocity feature beneath the western sp associated with elevated lithospheric anisotropy the high velocity feature does not match crustal boundaries notably its western edge lies approximately 200 km east of the contact with the proterozoic trans hudson orogen tho ii a low velocity channel shaped feature strikes northwest through minnesota and the dakotas associated with weakening anisotropy iii high velocities southwest of ii beneath the minnesota river valley terrane mrvt associated with low anisotropy we interpret i to be accretionary and contemporaneous with superior assembly similar velocity but weaker anisotropy of the mrvt is consistent with vertical tectonic mechanisms the inboard location of the tho contact may indicate modification of the superior root the low velocity channel has no obvious crustal expression but connects to an offset in the proterozoic mid continent rift mcr and may be rift related the mcr is not well imaged but will be examined via the temporary superior province rifting earthscope experiment in progress

an improved lunar gravity field model from selene and historical tracking data revealing the farside gravity features 1 a new spherical harmonic solution of the lunar gravity field to degree and order 100 called sgm100h has been developed using historical tracking data and 14 2 months of selene tracking data from 20 october 2007 to 26 december 2008 plus 30 january 2009 the latter includes all usable 4 way doppler data collected which allowed direct observations of the farside gravity field for the first time the new model successfully reveals farside features in free air gravity anomalies which are characterized by ring shaped structures for large impact basins and negative spots for large craters sgm100h produces a correlation with selene derived topography as high as about 0 9 through degree 70 comparison between sgm100h and lp100k one of the pre selene models shows that the large gravity errors which existed in lp100k are drastically reduced and the asymmetric error distribution between the nearside and the farside almost disappears the gravity anomaly errors predicted from the error covariance through degree and order 100 are 26 mgal and 35 mgal for the nearside and the farside respectively owing to the 4 way doppler measurements the gravity coefficients below degree and order 70 are now determined by real observations with contribution factors larger than 80 percent with the selene farside data coverage it is possible to estimate the gravity field to degree and order 70 without applying any a priori constraint or regularization sgm100h can be used for global geophysical interpretation through degree and order 70

summary of the results from the lunar orbiter laser altimeter after seven years in lunar orbit in june 2009 the lunar reconnaissance orbiter lro spacecraft was launched to the moon the payload consists of 7 science instruments selected to characterize sites for future robotic and human missions among them the lunar orbiter laser altimeter lola was designed to obtain altimetry surface roughness and reflectance measurements the primary phase of lunar exploration lasted one year following a 3 month commissioning phase on completion of its exploration objectives the lro mission transitioned to a science mission after 7 years in lunar orbit the lola instrument continues to map the lunar surface the lola dataset is one of the foundational datasets acquired by the various lro instruments lola provided a high accuracy global geodetic reference frame to which past present and future lunar observations can be referenced it also obtained high resolution and accurate global topography that were used to determine regions in permanent shadow at the lunar poles lola further contributed to the study of polar volatiles through its unique measurement of surface brightness at zero phase which revealed anomalies in several polar craters that may indicate the presence of water ice in this paper we describe the many lola accomplishments to date and its contribution to lunar and planetary science

constraints on the martian lithosphere from gravity and topography data localized spectral admittances of the large martian volcanoes are modeled by assuming that surface and subsurface loads are elastically supported by the lithosphere in order to model the case where the load density differs from that of the crust a new method for calculating gravity anomalies and lithospheric deflections is developed the modeled gravity anomalies depend upon the elastic thickness crustal thickness load density and crustal density and these parameters were exhaustively sampled in order to determine their effect on the misfit between the observed and modeled admittance function we find that the densities of the martian volcanoes are generally well constrained with values of 3200 100 kg m 3 which is considerably greater than those reported previously these higher densities are consistent with those of the martian basaltic meteorites which are believed to originate from the tharsis and elysium volcanic provinces the crustal density is constrained only beneath the elysium rise to be 3270 150 kg m 3 if this value is representative of the northern lowlands then pratt compensation is likely responsible for the approximately 6 km elevation difference between the northern and southern hemispheres the elastic thicknesses of the major martian volcanoes when subsurface loads are ignored are found to be the following elysium rise 56 20 km olympus mons 93 40 km alba patera 66 20 km and ascraeus mons 105 40 km we have also investigated the effects of subsurface loads allowing the bottom load to be located either in the crust as dense intrusive material or in the mantle as less dense material we found that all volcanoes except pavonis are better modeled with the presence of less dense material in the upper mantle which is indicative of either a mantle plume or a depleted mantle composition an active plume beneath the major volcanoes is consistent with recent analyses of cratering statistics on olympus mons and the elysium rise which indicate that some lava flows are as young as 10 30 myr as well as with the crystallization ages of the shergottites of which some are as young as 180 myr

toward mapping the effective elastic thickness of planetary lithospheres from a spherical wavelet analysis of gravity and topography abstract the effective elastic thickness t e of the lithosphere controls the flexural response to transverse loading and can be used in conjunction with rheological models to remotely estimate surface heat flux of terrestrial planets in the vast majority of studies t e estimation is carried out in a two step process 1 the joint spectra admittance and or coherency of gravity anomalies free air and or bouguer and topography are calculated within finite size windows and 2 the spectra are inverted using a model for the loading of an infinite elastic plate or shell in recent years research in the spatio spectral analysis of cartesian grids and improvements in lithospheric loading models have allowed the mapping of t e on the earth at unprecedented resolution nevertheless the limitations imposed by working with cartesian data and models are hampering further advances in terrestrial t e studies the planetary community on the other hand has traditionally used spatio spectral methods that work directly on the sphere thereby avoiding the undesirable distortions and biases inherent in cartesian studies however the models and methods developed for the earth have never been applied to planetary data therefore also limiting further advances in the mapping of planetary t e this paper reviews the latest developments in t e studies and proposes to combine advances in both terrestrial and planetary studies to allow the mapping of t e of terrestrial planets using a spherical wavelet analysis of gravity and topography data we begin with a review of the recent literature on the challenges and debates concerning t e estimation from joint gravity and topography spectra we then briefly review the implementation of the continuous planar and spherical wavelet transforms cpwt and cswt and demonstrate that both transforms possess exactly the same resolving power however the cswt does not suffer from effects associated with non periodic boundaries we further re derive in a consistent manner the expressions for the admittance and coherency from flexural equations for the loading of thin elastic plates and shells and show that they result in similar spectra except at the longest wavelengths where the shell membrane stresses dominate we then estimate global grids of earth s continental t e by inverting the planar and spherical admittance and coherency functions both separately and jointly using either free air or bouguer gravity anomalies the correspondence between the cartesian and spherical approaches and the similarity of t e results for the earth s continents indicate that the spherical methods are robust results obtained from the joint inversion of admittance and coherency show that simple lithospheric loading models fail to capture the complexity of the data with adverse effects on the estimated parameters finally we extend the analysis to the moon and other terrestrial planets and discuss limitations and future applications of the fully spherical techniques

lithospheric loading by the northern polar cap on mars abstract new topography data for the northern polar region on mars returned by the the mars orbiter laser altimeter mola during the aerobraking hiatus and science phasing orbits allow characterization of the topography of the present northern polar cap and its environs models for loading of an elastic shell by an axisymmetric load approximating the present polar cap geometry indicate that the maximum deflection of the subice basement is in the range 1200 to 400 m for an elastic lithosphere of thickness 40 to 200 km overlain by a cap of pure h 2 o ice corresponding model cap volumes increase from 1 5 to 1 8 10 6 km 3 as elastic lithosphere thickness decreases from 200 to 40 km the presence of sediments in the polar cap increases the depth to basement and resulting cap volume for a given value of elastic lithosphere thickness one dimensional heat flow calculations indicate that the temperature at the base of the cap may approach the melting point of cap material if the lithosphere underlying the cap is thin the basal temperature is 170 k for a 200 km thick lithosphere overlain by pure ice but is as great as 234 k for a 40 km thick lithosphere overlain by a cap with a high sediment ice ratio constraints on elastic lithosphere thickness are weak but geologic mapping and mola data suggest that a flexurally derived circumpolar depression filled with sediments is consistent with elastic lithosphere thickness values in the range 60 120 km gravity and topography over the whole cap are poorly correlated possibly due to viscous relaxation of long wavelength topography but gravity and topography over the western portion of the main cap are consistent with an elastic lithosphere thickness of 120 km for a crustal thickness of 50 km both mola data and geological information suggest a formerly larger northern polar cap the relationship of time scales for changes in the polar cap volume and extent to time scales for viscous relaxation of topography has important implications for investigations of even present polar cap topography viscoelastic calculations show that the maxwell time for an earth like mantle viscosity for mars 10 21 pa s is 10 5 yr the maxwell time scales directly with the martian mantle viscosity so that values as high as 10 7 yr are possible time scales for changes in polar cap volume are poorly constrained but major changes in cap volume over periods of 10 6 10 8 yr are consistent with current understanding of polar cap processes

constraints on mantle anisotropy beneath precambrian north america from a transportable teleseismic experiment data from the apt89 transportable teleseismic experiment was examined to characterize the variations in shear wave splitting along a 1500 km traverse in north america see silver et al 1993 this issue three geologic terranes were sampled the western superior province of the canadian shield the trans hudson orogen and the wyoming craton the primary goal was to determine the extent to which the anisotropy was controlled by archean and proterozoic tectonic episodes of the large suite of teleseismic events recorded 5 were suitable for determining the fast polarization direction and delay time t from the phases sks and skks the resulting observations reveal two important features first the values of display good geologic coherence stations along the northern part of the array in the exposed shield reveal a consistent ene direction for that is generally parallel to exposed geologic structures within the part of the superior province buried beneath phanerozoic cover rotates to a ne sw azimuth as do geophysical indicators of geologic fabric moving onto the trans hudson changes abruptly to a more ew azimuth the stations within the wyoming craton suggest values with a more northerly direction although the waveforms from these stations are more complex second t which ranges between 0 40s and 1 75s shows systematic variations across the traverse the largest delay times approaching 2 s are found in a 250 km wide band from red lake ontario to the u s canada border t is reduced to about 1 s or less both north and south of this region for the stations with detectable splitting we find an inverse relationship between s isotropic station delays derived from the portable data set and t which is most easily explained by the anisotropy being primarily localized in the fast presumably lithospheric mantle the splitting observations overall suggest that mantle anisotropy along this traverse is dominated by precambrian fossil anisotropy that is preserved in the subcontinental lithosphere

crustal thickness and support of topography on venus united states national aeronautics and space administration nasa planetary geology and geophysics program grant

the kapuskasing uplift a geological and geophysical synthesis over the past decade the kapuskasing uplift has been the subject of intense geological and geophysical investigation as lithoprobe s window on the deep crustal structure of the archean superior province enigmatic since its recognition as a positive gravity anomaly in 1950 the structure has been variably interpreted as a suture rift transcurrent shear zone or intracratonic thrust diverse studies including geochronology geothermobarometry and various geophysical probes provide a comprehensive three dimensional image of archean 2 75 2 50 ga crustal evolution and proterozoic 2 5 1 1 ga cooling and uplift the data favour an interpretation of the structure as an intracratonic uplift related to hudsonian collision eastward across the southern kapuskasing uplift erosion levels increase from 10 km in the michipicoten greenstone belt through the wawa gneiss domain 10 20 km into granulites 20 30 km of the kapuskasing structural zone juxtaposed against the low grade swayze greenstone belt al

structural interpretation of the midcontinent rift in eastern lake superior from seismic reflection and potential field studies integrated interpretations of potential field and glimpce and industry seismic reflection data in eastern lake superior reveal the structural and stratigraphic complexity of the midcontinent rift in this region projection of the keweenaw fault into southeastern lake superior suggested by early potential field studies is confirmed by seismic reflection data analysis of seismic data in conjunction with aeromagnetic anomalies and regional gravity data also reveals a continuous section of basalt in the footwall of the keweenaw fault the lateral dimensions of this section vary along the strike of the rift from the center of the basin towards the southern flank spatially extensive anticlinal and synclinal features reverse faults and related drag folds imaged by the reflection and enhanced potential field data attest to the influence of a late stage compressional event in this region east northeast trending gradients and displacements associated with observed potential field anomalies and fault traces mapp

mineralogical constraints on the thermal history of martian regolith breccia northwest africa 8114 abstract martian meteorite northwest africa nwa 8114 a paired stone to nwa 7034 provides an opportunity to examine the thermal history of a martian regolith and study near surface processes and ancient environmental conditions near an impact crater on mars our study reports petrographic and alteration textures and focuses on pyroxene and iron oxide grains some of the pyroxene clasts show exsolution lamellae indicating a high temperature magmatic origin and slow cooling however transmission electron microscopy reveals that other predominantly pyroxene clasts are porous and have partially re crystallised to form magnetite and a k bearing feldspathic glassy material together with relict pyroxene this breakdown event was associated with oxidation with up to 25 fe3 fe in the relict pyroxene measured using fe k xanes by comparison with previous studies this breakdown and oxidation of pyroxene is most likely to be a result of impact shock heating being held at a temperature above 700 c for at least 7 days in an oxidising regolith environment we report an approximate 40ar 39ar maximum age of 1 13 1 25 ga for an individual separated augite clast the disturbed nature of the spectra precludes precise age determination in section this clast is porous and contains iron oxide grains this shows that it has undergone the high temperature partial breakdown seen in other relict pyroxene clasts and has up to 25 fe3 fe we infer that the age corresponds to the impact shock heating event that led to the high temperature breakdown of many of the pyroxenes after consolidation of the impact ejecta blanket high temperatures above 700 c may have been maintained for long enough to remobilise and congruently partially melt some of the alkali feldspar clasts to produce the feldspar veins and aureoles that crosscut and in some cases surround the oxidised pyroxene however the veins could alternatively be the result of a hydrothermal event in the impact regolith a simple fourier cooling model suggests that a regolith of at least five metres depth would be sufficient to maintain temperatures associated with the pyroxene breakdown for over seven days low temperature hydrous alteration took place forming goethite identified via xrd xanes and ftir comparing with previous studies the goethite is likely to be terrestrial alteration pseudomorphing martian pyrite

evidence for extremely rapid magma ocean crystallization and crust formation on mars the formation of a primordial crust is a critical step in the evolution of terrestrial planets but the timing of this process is poorly understood the mineral zircon is a powerful tool for constraining crust formation because it can be accurately dated with the uranium to lead u pb isotopic decay system and is resistant to subsequent alteration moreover given the high concentration of hafnium in zircon the lutetium to hafnium 176lu 176hf isotopic decay system can be used to determine the nature and formation timescale of its source reservoir1 3 ancient igneous zircons with crystallization ages of around 4 430 million years myr have been reported in martian meteorites that are believed to represent regolith breccias from the southern highlands of mars4 5 these zircons are present in evolved lithologies interpreted to reflect re melted primary martian crust 4 thereby potentially providing insight into early crustal evolution on mars here we report concomitant high precision u pb ages and hf isotope compositions of ancient zircons from the nwa 7034 martian regolith breccia seven zircons with mostly concordant u pb ages define 207pb 206pb dates ranging from 4 476 3 0 9 myr ago to 4 429 7 1 0 myr ago including the oldest directly dated material from mars all zircons record unradiogenic initial hf isotope compositions inherited from an enriched andesitic like crust extracted from a primitive mantle no later than 4 547 myr ago thus a primordial crust existed on mars by this time and survived for around 100 myr before it was reworked possibly by impacts4 5 to produce magmas from which the zircons crystallized given that formation of a stable primordial crust is the end product of planetary differentiation our data require that the accretion core formation and magma ocean crystallization on mars were completed less than 20 myr after the formation of the solar system these timescales support models that suggest extremely rapid magma ocean crystallization leading to a gravitationally unstable stratified mantle which subsequently overturns resulting in decompression melting of rising cumulates and production of a primordial basaltic to andesitic crust6 7

geochemistry and geodynamic implications of the mesoproterozoic english bay granite rhyolite complex northwestern ontario the mesoproterozoic english bay complex consists of a granite rhyolite assemblage outcropping on the shores of lake nipigon in western superior province canada it intrudes neoarchean rocks and is disconformably overlain by a rift infracratonic basin sedimentary succession recording subsidence following a heating event the granites and rhyolites are characterized by light rare earth element lree enrichment la smn 2 8 5 1 and only weakly fractionated heavy ree hree gd ybn 1 1 1 6 the felsic igneous rocks are high k enriched in zr nb y and ree satisfying all the criteria for an a type suite trace element geochemistry particularly the absence of any negative nb anomalies indicates this melt did not originate in a suprasubduction zone setting unlike the st francois mountain complex to the south the english bay complex may record the northern portion of a mesoproterozoic plume track a plume that possibly led to earlier igneous activity and infracratonic basin formation to the north and would later interact with a suprasubduction zone margin to the south

constraints on the maximum crustal density from gravity topography modeling applications to the southern highlands of mars abstract gravimetric methods commonly used to constrain crustal parameters such as the mean crustal thickness and density are ambiguous with a noted trade off between these parameters however combining two different methods the geoid topography ratio and bouguer inversion in regions that are homogeneous with respect to lateral density variations and compensation state can help to constrain a maximum density of the crust for the martian noachian southern highlands a combination of these methods gives us a maximum crustal density of 3020 70 kg m 3 assuming a single layer crustal structure we also test various two layer crustal structures to check how they influence the results we find a possibility to fit the observed data with a crust having a dense uniformly thick lower crust but in these models the upper crustal density was also limited to 3000 kg m 3 the obtained results together with the findings on crustal densities and composition of other regions on mars are consistent with various scenarios of crustal evolution a temporal increase in the crustal density or a large scale density variation that has been already manifested in the early evolution during the formation of the crustal dichotomy

petrological constraints on the density of the martian crust new insights into the chemistry of the martian crust have been made available since the derivation of crustal thickness maps from mars global surveyor gravity and topography data that used a conservative range of density values 2700 3100 kg m3 a new range of crustal density values is calculated from the major element chemistry of martian meteorites 3100 3700 kg m3 igneous rocks at gusev crater 3100 3600 kg m3 and from the surface concentration of fe al ca si and k measured by the gamma ray spectrometer on board mars odyssey 3250 3450 kg m3 in addition the density of mineral assemblages resulting from low pressure crystallization of primary melts of the primitive mantle are estimated for plausible conditions of partial melting corresponding to the noachian to amazonian periods 3100 3300 kg m3 despite the differences between these approaches the results are all consistent with an average density above 3100 kg m3 for those materials that are close to the surface the density may be compatible with the measured mass of mars and the moment of inertia factor but only if the average crustal thickness is thicker than previously thought approaching 100 km a thicker crust implies that crustal delamination and recycling could be possible and may even control its thickness globally or locally alternatively and considering that geoid to topography ratios argue against such a thick crust for the highlands our results suggest the existence of a buried felsic or anorthositic component in the southern hemisphere of mars

thickness of the martian crust improved constraints from geoid to topography ratios 1 the average crustal thickness of the southern highlands of mars was investigated by calculating geoid to topography ratios gtrs and interpreting these in terms of an airy compensation model appropriate for a spherical planet we show that 1 if gtrs were interpreted in terms of a cartesian model the recovered crustal thickness would be underestimated by a few tens of kilometers and 2 the global geoid and topography signals associated with the loading and flexure of the tharsis province must be removed before undertaking such a spatial analysis assuming a conservative range of crustal densities 2700 3100 kg m 3 we constrain the average thickness of the martian crust to lie between 33 and 81 km or 57 24 km when combined with complementary estimates based on crustal thickness modeling gravity topography admittance modeling viscous relaxation considerations and geochemical mass balance modeling we find that a crustal thickness between 38 and 62 km or 50 12 km is consistent with all studies isotopic investigations based on hf w and sm nd systematics suggest that mars underwent a major silicate differentiation event early in its evolution within the first 30 ma that gave rise to an enriched crust that has since remained isotopically isolated from the depleted mantle in comparing estimates of the thickness of this primordial crust with those obtained in this study we find that at least one third of the martian crust has an origin dating from the time of accretion and primary differentiation subsequent partial melting of the depleted mantle would have given rise to the remaining portion of the crust while we predict that a large portion of the crust should be composed of ancient enriched materials a representative sample of this primordial crust does not currently exist among the known martian meteorites index terms 6225 planetology solar system objects mars 5417 planetology solid surface planets gravitational fields 1227 5430 planetology solid surface planets interiors 8147 1227 geodesy and gravity planetary geodesy and gravity 5420 5714 6019 keywords mars crustal thickness crustal evolution geoid topography

stratigraphy of the archean western superior province from p and s wave receiver functions further evidence for tectonic accretion the archean western superior province in canada represents the nucleus of the north american continent whose origin has been speculated to be the result of widespread crustal accretion some 2 7 ga ago in this paper crustal and upper mantle seismic discontinuities beneath the western superior province of the canadian shield are imaged with teleseismic p to s and s to p converted phases using the receiver function method three crustal discontinuities are observed the moho ranging in depth between 38 and 47 km and dipping to the south and two intra crustal discontinuities having depths of approximately 15 and 30 km the crustal discontinuities undulate laterally and often lose continuity possibly indicating an imbricated structure and or regions of velocity gradients in the shallow lithosphere a positive discontinuity is imaged at approximately 65 km depth and is consistent with earlier refraction and wide angle reflection results additionally two zones of negative receiver function amplitudes at 55 km depth are observed and are coincident with a region of anomalous tomographic low p and s wave velocities as well as a zone of high electrical conductivity the images for the crust and shallow upper mantle when integrated with previous geophysical studies are consistent with ideas of continental root formation due to imbrication of archean subducted material and accretion of island arcs observed in surface geology

the central pit and dome at cerealia facula bright deposit and floor deposits in occator crater ceres morphology comparisons and formation abstract the prominent bright deposit cerealia facula ceres coincides with the central depression or central pit of the recently formed 92 km wide complex crater occator the central pit is 9 10 km wide and up to 1 km deep and is partially filled with a 700 m high 2 km wide dome the upper surface of the central dome is densely fractured but the flanks are not indicating that uplift of the dome surface occurred after the bright deposit was emplaced and primarily through uplift of the surface from below by laccolithic intrusion or volume expansion the pit is rimless except for two prominent massifs that flank it to the east and west this pit dome morphology bears a strong resemblance in morphology topography and dimensions to central pit craters observed on ganymede and callisto but is essentially absent on the similar sized ice rich moons of uranus saturn and pluto the lack of pit craters on midsize icy moons and the possible lack of central pit craters at the cold poles of ceres suggest that temperature is a significant controlling factor in central pit formation beyond the canonical inverse gravity scaling of complex crater transitions while renewed modeling will benefit from these new constraints the mapping of central pits on ceres including occator suggests that either the rheologically layered target model or more likely the melted central uplift model may be the most probable explanation for central pit formation on ceres lack of similar high resolution imaging on the galilean icy satellites precludes definitive evaluation of models there large volumes of lobate floor fill material at least 600 m thick in some places cover large areas of the southern and eastern floor of occator and small outcrops of this unit are found perched high on closed topographic depressions on the occator terrace zone and other higher areas of the crater floor the swirling and knobby surface textures of these lobate deposits are essentially indistinguishable from those on impact melt sheets on large lunar craters such as tycho these observations indicate that this unit is most likely an impact melt sheet formed from melted water ice admixed with unmelted particulate or dissolved phyllosilicates salts and or carbonates and large quantities of fragmented debris melt of ice phases within ceres crust is also supported by independent numerical models of impact at occator and would provide a ready source of fluids to promote hydrothermal deposition of the observed bright na carbonates on the surface of the central pit at occator if the deposit formed within a million years or so of impact the central dome finds analogs in salt domes on earth and pingos frost mounds on earth and mars which form by water and solute migration and freeze expansion later freezing of a central reservoir of volatiles beneath the central pit at occator or mobilization of ice or other phases during the cooling and refreezing process could explain the uplift of the central dome finally no changes were detected in the cerealia and vinalia facula bright deposits during the course of the prime dawn mapping mission placing additional constraints on their formation time and limits on ongoing activity

interpretation of seismic reflection and gravity profile data in western lake superior the interpretation of 1047 km of seismic reflection data collected in western lake superior is presented along with reflection traveltime contour maps and gravity models to understand the overall geometry of the midcontinent rift system beneath the lake the douglas isle royale and keweenaw fault zones clearly imaged on the seismic profiles are interpreted to be large offset detachment faults associated with initial rifting these faults have been reactivated as reverse faults with 3 5 km of throw the douglas fault zone is not directly connected with the isle royale fault zone the seismic data has imaged two large basins filled with more than 22 km of middle keweenawan pre portage lake and portage lake volcanic rocks and up to 8 km of upper keweenawan oronto and bayfield sedimentary rocks these basins persisted throughout keweenawan time and are separated by a ridge of archean rocks and a narrow trough bounded by the keweenaw fault zone to the south another fault zone herein named the ojibwa faul

the mesoproterozoic midcontinent rift system lake superior region usa abstract exposures in the lake superior region and associated geophysical evidence show that a 2000 km long rift system developed within the north american craton 1109 1087 ma the age span of most of the volcanic rocks this system is characterized by immense volumes of mafic igneous rocks mostly subaerial plateau basalts generated in two major pulses largely by a hot mantle plume a new ocean basin was nearly formed before rifting ceased perhaps due to the remote effect of the grenville continental collision to the east broad sagging subsidence combined with a system of axial half grabens separated along the length of the rift by accommodation zones provided conditions for the accumulation of as much as 20 km of volcanic rocks and as much as 10 km of post rift clastic sediments both along the rift axis and in basins flanking a central post volcanic horst pre rift mature quartzose sandstones imply little or no uplift prior to the onset of rift volcanism early post rift red bed sediments consist almost entirely of intrabasinally derived volcanic sediment deposited in alluvial fan to fluvial settings the exception is one gray to black carbon bearing lacustrine unit this early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red bed fluvial sediments with an extra basinal provenance

density and lithospheric structure at tyrrhena patera mars from gravity and topography data the tyrrhena patera highland volcano mars is associated with a relatively well localized gravity anomaly and we have carried out a localized admittance analysis in the region to constrain the density of the volcanic load the load thickness and the elastic thickness at the time of load emplacement the employed admittance model considers loading of an initially spherical surface and surface as well as subsurface loading is taken into account our results indicate that the gravity and topography data available at tyrrhena patera is consistent with the absence of subsurface loading but the presence of a small subsurface load cannot be ruled out we obtain minimum load densities of 2960 kg m 3 minimum load thicknesses of 5 km and minimum load volumes of 0 6 106 km3 photogeological evidence suggests that pyroclastic deposits make up at most 30 of this volume such that the bulk of tyrrhena patera is likely composed of competent basalt best fitting model parameters are a load density of 3343 kg m 3 a load thickness of 10 8 km and a load volume of 1 7 106 km3 these relatively large load densities indicate that lava compositions are comparable to those at other martian volcanoes and densities are comparable to those of the martian meteorites the elastic thickness in the region is constrained to be smaller than 27 5 km at the time of loading indicating surface heat flows in excess of 24 mw m 2

mantle provinces under north america from multifrequency p wave tomography this is a survey of mantle provinces large scale seismic anomalies under north america from the surface down to 1500 1800 km depth the underlying p velocity model was obtained by multifrequency tomography a waveform based method that systematically measures and models the frequency dependence of teleseismic body waves a novel kind of three dimensional rendering technique is used to make the considerable structural complexities under north america accessible in the transition zone and below the north american mantle is dominated by seismically fast provinces which represent distinct subduction episodes of the farallon plate i attempt to date and interpret the various slab fragments by reconciling their present positions with paleotrench locations from plate tectonic reconstructions and with major geologic surface episodes differences in vertical sinking velocity have led to large vertical offsets across adjacent coeval slabs some of the mantle provinces have not been discussed much previously including 1 a seismically slow blanket overlying the oldest farallon subduction along the eastern continental margin 2 a transition zone slab coeval with the laramide orogeny ca 80 60 myr which i discuss in analogy to the stagnant slab subduction style commonly found in the western pacific today 3 the lower mantle root of present day cascadia subduction which may have started out as intraoceanic subduction 4 a lower mantle slab under arizona and new mexico the last material to subduct before strike slip motion developed along the san andreas boundary and 5 two narrow plate tears thousands of kilometers long one of which is the subducted conjugate of the mendocino fracture zone

lithospheric assembly and modification of the se canadian shield abitibi grenville teleseismic experiment this paper presents the results of a joint lithoprobe incorporated research institutions for seismology iris program for array seismic studies of the continental lithosphere passcal teleseismic experiment that investigates portions of the grenville and superior provinces of the canadian shield along the quebec ontario border data from a 600 km long n s array of 28 broadband seismographs deployed between may and october 1996 have been supplemented with additional recordings from an earlier 1994 deployment and from stations of the canadian national seismograph network and the southern ontario seismic network relative delay times of p and s waves from 123 and 40 teleseismic events respectively have been inverted for velocity perturbations in the upper mantle and reveal a low velocity nw se striking corridor that crosses the southern portion of the line at latitude 46 n and lies between 50 and 300 km depth multievent s k s splitting results yield an average delay time of 0 57 0 22 s and a direction of fast polarization of n93 e 18 which is consistent with an earlier interpretation as being due to fossil strain fields related to the last major regional tectonic event subtle variations in splitting parameters over the low velocity corridor may suggest an associated disruption in mantle fabric profiling of radial receiver functions reveals large and abrupt variations in moho topography specifically a gradual thickening in crust from 40 to 45 km between latitudes 45 n and 46 n which is followed by an abrupt thinning to 35 km at 46 6 n some 65 km southeast of the grenville front this structure is interpreted as a subduction suture extending the full length of the front and punctuating a major pre grenvillian archean proterozoic episode of lithospheric assembly in the southeast canadian shield the low velocity mantle corridor by contrast is better explained as the extension of the monteregian white mountain new england seamount hotspot track below the craton and is here postulated to represent interaction of the great meteor plume with zones of weakness within the craton developed during earlier rifting episodes

closing of the midcontinent rift a far field effect of grenvillian compression the midcontinent rift formed in the laurentian supercontinent between 1109 and 1094 ma soon after rifting stresses changed from extensional to compressional and the central graben of the rift was partly inverted by thrusting on original extensional faults thrusting culminated at about 1060 ma but may have begun as early as 1080 ma on the southwest trending arm of the rift the crust was shortened about 30 km on the southeast trending arm strike slip motion was dominant the rift developed adjacent to the tectonically active grenville province and its rapid evolution from an extensional to a compressional feature at ca 1080 ma was coincident with renewal of northwest directed thrusting in the grenvllle probably caused by continent continent collision a zone of weak lithosphere created by rifting became the locus for deformation within the otherwise strong continental lithosphere stresses transmitted from the grenville province utilized this weak zone to close and invert the rift

unraveling the record of successive high grade events in the central zone of the limpopo belt using pb single phase dating of metamorphic minerals abstract dating of relic metamorphic assemblages can provide important information about the timing and character metamorphic grade and or pt evolution of early high grade episodes in polymetamorphic provinces using pb stepwise leaching of metamorphic silicates we have dated multiple granulite facies metamorphic episodes in the central zone cz of the limpopo belt ages of 2 52 ga were obtained from sillimanite and cogenetic garnet and ages of about 2 01 ga from titanite garnet and clinopyroxene together with new and published conventional age data from accessory phases and in the context of combined petrological and structural data these results lead us to a reinterpretation of the tectono metamorphic history of the cz three distinct high grade events at about 3 2 3 1 ga 2 65 2 52 ga and 2 0 0 05 ga are recognized each of these is suggested to correspond to a tectonic episode of distinct character a for the 3 2 ga event magmatic activity can mainly be identified best represented for example by the sand river gneisses or the messina layered intrusion the field relationships concerning the tectonometamorphic history of this early archean event are largely erased by at least two high grade metamorphic overprints b late archean 2 6 2 52 ga low pressure granulite facies metamorphism was associated with voluminous granitic and charnockitic plutonism the anticlockwise p t evolution of these granulites probably reflects deep crustal processes associated with magmatic underplating or in plating contemporaneous with vertical crustal growth of the zimbabwe craton around 2 6 ga c during the proterozoic event 2 05 1 95 ga tectonic thickening was caused by the collision of the kaapvaal and zimbabwe cratons the cz was squeezed between these two cratons and as a consequence underwent high pressure granulite facies metamorphism with a clockwise p t evolution the structural metamorphic and geochronological data can be best explained with a tectonic model that describes this final event as a dextral transpressive orogeny

grail gravity constraints on the vertical and lateral density structure of the lunar crust we analyzed data from the gravity recovery and interior laboratory grail mission using a localized admittance approach to map out spatial variations in the vertical density structure of the lunar crust mare regions are characterized by a distinct decrease in density with depth while the farside is characterized by an increase in density with depth at an average gradient of 35 kg m 3 km 1 and typical surface porosities of at least 20 the apollo 12 and 14 landing site region has a similar density structure to the farside permitting a comparison with seismic velocity profiles the interior of the south pole aitken sp a impact basin appears distinct with a near surface low density porous layer 2 3 times thinner than the rest of the farside this result suggests that redistribution of material during the large sp a impact likely played a major role in sculpting the lunar crust

an improved jpl mars gravity field and orientation from mars orbiter and lander tracking data abstract the mars gravity field resolution is mostly determined by the lower altitude mars reconnaissance orbiter mro tracking data with nearly four years of additional mro and mars odyssey tracking data since the last jpl released gravity field mro110c and lander tracking from the mer opportunity rover the gravity field and orientation of mars have been improved the new field mro120d extends the maximum spherical harmonic degree slightly to 120 improves the determination of the higher degree coefficients as demonstrated by improved correlation with topography and reduces the uncertainty in the corresponding mars orientation parameters by up to a factor of two versus previously combined gravity and orientation solutions the new precession solution is 7608 3 2 1 mas yr and is consistent with previous results but with a reduced uncertainty by 40 the love number solution k2 0 169 0 006 also shows a similar result to previous studies

proterozoic and archean geology of the nipigon embayment implications for emplacement of the mesoproterozoic nipigon diabase sills and mafic to ultramafic intrusions the nipigon embayment is underlain by archean rocks of the english river wabigoon and quetico subprovinces and intruded along the west side by late to post tectonic mafic to ultramafic intrusions the early mesoproterozoic ultramafic to felsic badwater intrusion and felsic english bay complex are located in the northwest corner of the nipigon embayment three mafic to ultramafic intrusions the disraeli seagull and hele intrusions are located south of lake nipigon and the kitto intrusion is located east of the lake a number of mafic to ultramafic bodies jackfish island shillabeer kama hill nipigon bay have only limited outcrops the gabbroic nipigon diabase sills intrude all other rocks in the nipigon embayment and generally have a consistent mineralogy and geochemistry except for the inspiration sill s and the mcintyre sill geological and geophysical data suggest emplacement of the ultramafic intrusions by mechanisms similar to those controlling emplacement of the saucer shaped diabase sills these mechanisms are partially dependent on a series of pre existing north northwest and northeast trending faults formed prior to keweenawan magmatism the presence of sills rather than dykes indicates that the nipigon embayment was not extensional during the keweenawan midcontinent rift suggesting that the nipigon embayment is not a classic failed arm

a thick crustal block revealed by reconstructions of early mars highlands the global scale crustal structure of mars is shaped by impact basins volcanic provinces and a hemispheric dichotomy with a thin crust beneath the northern lowlands and a thick crust beneath the southern highlands the southern highlands are commonly treated as a coherent terrain of ancient crust with a common origin and shared geologic history plausibly originating from a giant impact s or a hemispheric scale mantle upwelling previous studies have quantified the contribution of volcanism to this crustal structure however the influence of large impacts remains unclear here we present reconstructions of the past crustal thickness of mars about 4 2 gyr ago where the four largest impact basins hellas argyre isidis and utopia are removed assuming mass conservation as well as the main volcanic provinces of tharsis and elysium our reconstruction shows more subdued crustal thickness variations than at present although the crustal dichotomy persists however our reconstruction reveals a region of discontinuous patches of thick crust in the southern highlands associated with magnetic and geochemical anomalies this region corresponding to terra cimmeria sirenum is interpreted as a discrete crustal block our findings suggest that the southern highlands are composed of several crustal blocks with different geological histories such a complex architecture of the southern highlands is not explained by existing scenarios for crustal formation and evolution a discrete block of thick ancient crust revealed by a crustal reconstruction suggests a complex geologic history for the southern highlands of mars

global characteristics of porosity and density stratification within the lunar crust from grail gravity and lunar orbiter laser altimeter topography data the gravity recovery and interior laboratory grail mission is providing unprecedentedly high resolution gravity data the gravity signal in relation to topography decreases from 100 km to 30 km wavelength equivalent to a uniform crustal density of 2450 kg m3 that is 100 kg m3 smaller than the density required at 100 km to explain such frequency dependent behavior we introduce rock compaction models under lithostatic pressure that yield radially stratified porosity and thus density and examine the depth extent of porosity our modeling and analysis support the assertion that the crustal density must vary from surface to deep crust by up to 500 kg m3 we found that the surface density of megaregolith is around 2400 kg m3 with an initial porosity of 10 20 and this porosity is eliminated at 10 20 km depth due to lithostatic overburden pressure our stratified density models provide improved fits to both grail primary and extended mission data

lithospheric structure of the trans hudson orogen from seismic refraction wide angle reflection studies the trans hudson orogen tho is the world s largest paleoproterozoic orogenic belt data from three refraction profiles are used to investigate its lithospheric structure in saskatchewan and manitoba r1 crosses the orogen from the hearne craton on the west to the superior craton on the east r2 and r3 are along the orogen p wave velocity structural models are generated using a ray based technique on line r1 higher crustal velocities in its eastern part coincide with rocks of the flin flon namew gneiss complex depth to moho is in the 40 45 km range and equates to that from the reflection data including a small crustal root below the sask minicontinent along lines r2 and r3 depth to moho varies from about 40 km up to 55 km at the north end of r2 and south end of r3 in general variations in crustal velocity and depth to moho do not correlate with the location and extent of geological domains they appear to reflect the complex deformation and metamorphic history of the crustal rocks mantle velo

the gravitational signature of martian volcanoes by modeling the elastic flexure of the martian lithosphere under imposed loads we provide a systematic study of the old and low relief volcanoes 3 2 ga 0 5 to 7 4 km and the younger and larger prominent constructs within the tharsis and elysium provinces 3 2 ga 0 5 to 7 4 km and the younger and larger constructs clustered in the tharsis and elysium provinces 3 ga 5 8 to 21 9 km the observed topography is used to define the load and to derive a simulated gravity field that is compared to the observed gravity the bulk density of the lava that composes all volcanoes is found to be 3 200 kg m 3 which is representative of iron rich basalts as sampled by the martian basaltic meteorites the elastic part of the lithosphere that is the part maintaining loads over geologic time is found to have been thin when the small old volcanoes formed which implies that the lithosphere was hot early in geologic history conversely larger volcanoes are found to have been emplaced on a cold and thicker elastic lithosphere

mantle temperature and lithospheric thinning beneath the midcontinent rift system evidence from magmatism and subsidence the tectono magmatic history of the midcontinent rift system can be explained by the presence of a mantle plume bringing elevated temperature mantle beneath the rift system at about 1110 ma huge volumes of extrusive and intrusive igneous rocks were generated as abnormally hot mantle decompressed beneath the lithospheric rift geochemical and isotopic data from the keweenawan volcanics show that the earliest melts were derived from small degree melting of primitive plume mantle coupled with enriched metasomatic melts derived from the continental lithosphere as rifting progressed the main bulk of the volcanics was generated primarily from the plume mantle with the melting starting at depths of about 120 km and extending to as shallow as the base of the stretched lithosphere at 45 km depth elevated mantle temperatures of 1500 1560 c approximately 150 200 c above normal are inferred from the rare earth element concentrations in the volcanic rocks further constraints on the mantle temperature come f

constraints on the composition of the martian south polar cap from gravity and topography abstract the polar caps of mars have long been acknowledged to be composed of unknown proportions of water ice solid co 2 dry ice and dust gravity and topography data are here analyzed over the southern cap to place constraints on its density and hence composition using a localized spectral analysis combined with a lithospheric flexure model of ice cap loading the best fit density of the volatile rich south polar layered deposits is found to be 1271 kg m 3 with 1 limits of 1166 and 1391 kg m 3 the best fit elastic thickness of this geologically young deposit is 140 km though any value greater than 102 km can fit the observations the best fit density implies that about 55 dry ice by volume could be sequestered in these deposits if they were completely dust free alternatively if these deposits were completely free of solid co 2 the dust content would be constrained to lie between about 14 and 28 by volume the bulk thermal conductivity of the polar cap is not significantly affected by these maximum allowable concentrations of dust however even if a moderate quantity of solid co 2 were present as horizontal layers the bulk thermal conductivity of the polar cap would be significantly reduced reasonable estimates of the present day heat flow of mars predict that dry ice beneath the thicker portions of the south polar cap would have melted depending on the quantity of solid co 2 in these deposits today it is even possible that water ice could melt where the cap is thickest if independent estimates for either the dust or co 2 content of the south polar cap could be obtained and if radar sounding data could determine whether this polar cap is presently experiencing basal melting or not it would be possible to use these observations to place tight constraints on the present day heat flow of mars

density and porosity of the lunar crust from gravity and topography 1 newly obtained gravity and topography data of the moon combined with a lithospheric flexure model that considers both surface and subsurface loading are used to place constraints on the density of the upper crust from a localized spectral admittance analysis subsurface loads are found to be relatively unimportant in the highlands and when subsurface loads are neglected the best fitting bulk densities for a number of highland regions are found to vary from 2590 to 2870 kg m 3 with a mean value of 2691 kg m 3 crustal rock densities estimated from geochemical considerations and global iron and titanium abundances imply somewhat greater densities which we interpret as porosity affecting the gravity derived bulk density estimates the average porosity in the upper few kilometers of crust is calculated to be about 7 7 which is consistent with porosity estimates of impact fractured meteorites and terrestrial impact craters

potential anomalies on a sphere applications to the thickness of the lunar crust a new technique for calculating potential anomalies on a sphere due to finite amplitude relief has been developed we show that by raising the topography to the nth power and expanding this field into spherical harmonics potential anomalies due to topography on spherical density interfaces can be computed to arbitrary precision using a filter for downward continuing the bouguer anomaly we have computed a variety of crustal thickness maps for the moon assuming both a homogeneous as well as a dual layered crust the crustal thickness maps for the homogeneous model give plausible results but this model is not consistent with the seismic data petrologic evidence and geoid to topography ratios all of which suggest some form of crustal stratification several dual layered models were investigated and it was found that only models with both upper and lower crustal thickness variations could satisfy the gravity and topography data these models predict that the entire upper crust has been excavated beneath the major nearside multiring basins additionally significant amounts of lower crustal material was excavated from these basins especially beneath crisium this model also predicts that mantle material should not have been excavated during the south pole aitken basin forming event and that lower crustal material should be exposed at the surface in this basin

the tectonic architecture of wrinkle ridges on mars abstract the geodynamic history of mars is written largely in the form of its tectonic structures a global population of compressional structures records a history of interior cooling and contraction in combination with volcanic loading and flexure of the lithosphere the most common of these structures are wrinkle ridges interpreted as having formed by folding of near surface layered volcanic materials above blind thrust faults topographic profiles of wrinkle ridges are found to dominantly fall into three classes symmetric ridges arch style ridges characterized by broad convex upward backlimbs and double ridges characterized by a broad arch and narrow ridge a monte carlo boundary element model was used to constrain the underlying geometry of faults associated with the different ridge types symmetric ridges are pop up structures formed between quasi symmetric thrust and backthrust faults in some cases above a horizontal decollement or low angle fault at depth arch style ridges formed above a dominant primary thrust fault of near constant dip with a secondary backthrust playing a lesser role in generating subtle inflections in the backlimb double ridges require two backthrust faults to localize strain within the narrow ridge at the ridge crest and to generate the secondary antiform on the backlimb in these ridges both the upper primary thrust and backthrust are contained within the layered volcanic materials while the lower portions of the primary thrust can extend deep into the crust or upper mantle a shallower depth of fault penetration in areas of thicker crust is consistent with the depth of faulting being limited by the brittle ductile transition dips vary widely but are typically steep in the upper faults and quite gentle in the lower faults thus while wrinkle ridges do provide important records of tectonic and geodynamic processes on mars as well as other bodies the associated tectonic structures are more complex and varied than previously considered

influence of a variable thermal conductivity on the thermochemical evolution of mars 1 the influences of variations in thermal conductivity due to composition temperature and pressure in mantle and crust on the thermochemical evolution of mars have been examined with parameterized convection models we use a temperature and pressure dependent thermal conductivity in the mantle and column averaged values between 1 5 and 4 wm 1k 1 for the crust with 2 wm 1k 1 as the representative value for a basaltic crust the major characteristic of the models is a slower cooling of the upper mantle with a long existing global partial melt zone and a stronger crustal production late in the evolution compared to earlier models which assume a constant thermal conductivity of 4 wm 1k 1 to explain the observed crustal thickness and the early crustal formation on mars the formation of an enriched primordial crust is required recent volcanism can be explained by lateral temperature variations that are caused by thickness variations of the low conducting crust these melt zones which occur underneath thicker than average crust may be the source region for the observed longstanding volcanism in tharsis and elysium instead of plumes rising from the core mantle boundary the evolution of the elastic lithosphere is characterized by a transition at which the elastic strength is first provided by the crust and later also by the upper mantle this finding may explain the estimated elastic lithosphere thickness of less than 16 km in the noachian and more than 100 km in the hesperian and amazonian

role of membrane stresses in the support of planetary topography the principal purpose of this paper is to examine whether membrane stresses can support topographic loads on planetary elastic lithospheres it is found that the ability of a spherical shell to support loads through membrane stresses is determined by the nondimensional parameter ed gr2 where d is the thickness of the elastic lithosphere is the density difference between the mantle and crust and r is the radius of the planetary body when this parameter is large membrane stresses can fully support topographic loads without flexure and when it is small the influence of membrane stresses can be neglected solutions of the equation governing the behavior of a spherical shell are obtained for a topographic load expressed in terms of spherical harmonics spherical harmonic expansions of the measured gravity and topography for mars and the moon are compared with the theory it is found that for mars the support of topography is primarily due to membrane stresses for n 8 and for the moon for n 17 for mars the data for 4 n 7 give 0 5 for the moon the data have considerable scatter that is attributed to the mascons but generally correlate with 0 5 if bending stresses are neglected the governing equation for the deflection of the spherical shell is legendre s equation a general solution is obtained for an axisymmetric load this solution is applied to the tharsis region on mars the 60 65 compensation of this region requires that 0 6 the well defined fracture pattern surrounding the tharsis region is attributed to tensional membrane stresses

archaean subduction inferred from seismic images of a mantle suture in the superior province plate tectonics provides the basis for the interpretation of most current terrestrial tectonic activity and is widely accepted as having been active over much of the earth s history1 yet the timing of initiation of this process is subject to debate2 9 so far the earliest seismic evidence for plate tectonics has come from a fossil mantle suture in the svecofennian orogen 1 89 gyr ago 10 and from inferred plate convergence subduction and accretion in the trans hudson orogen 1 91 1 79 gyr ago 11 as yet seismic data from archaean areas have been able to demonstrate only the importance of compression in the construction of the continental crust12 15 here we present seismic data from a collision zone in the superior province of canada involving the abitibi granite greenstone subprovince and the plutonic arc related opatica belt we interpret dipping seismic reflections that extend 30km into the mantle as representing a relict 2 69 gyr old suture associated with subduction although crustal structure lithospheric thicknesses and convergence rates may have differed from those seen today these seismic data provide direct evidence that plate tectonics was active in late archaean times 800 myr earlier than indicated by previous seismic reflection surveys

evidence for a low bulk crustal density for mars from gravity and topography knowledge of the average density of the crust of a planet is important in determining its interior structure the combination of high resolution gravity and topography data has yielded a low density for the moon s crust yet for other terrestrial planets the resolution of the gravity field models has hampered reasonable estimates by using well chosen constraints derived from topography during gravity field model determination using satellite tracking data we show that we can robustly and independently determine the average bulk crustal density directly from the tracking data using the admittance between topography and imperfect gravity we find a low average bulk crustal density for mars 2582 209 kg m 3 this bulk crustal density is lower than that assumed until now densities for volcanic complexes are higher consistent with earlier estimates implying large lateral variations in crustal density in addition we find indications that the crustal density increases with depth

early history of the midcontinent rift inferred from geochemistry and sedimentology of the mesoproterozoic osler group northwestern ontario the mesoproterozoic 1108 1105 ma osler group a3k m thick succession of basaltic flows and sedimentary units on the north shore of lake superior is among the oldest expressions of the midcontinent rift basal sediments of the simpson island formation new name deposited by braided fluvial systems record westward transport of debris eroded from local archean and proterozoic rock units strata deposited by this fluvial system are intercalated with and overlain by ocean island basalt oib like basalts which become increasingly contaminated up section end 1100ma 0 3 to 5 3 the light rare earth element lree enriched la smn 1 5 3 9 and heavy ree hree fractionated gd ybn 1 5 3 7 subaerial flows are divisible into two units that correlate with other sections of the osler group to the east but simple correlations with more distant sequences are difficult the volcanic rock dominated portion of the succession is overlain by a thin 25 m thick conglomerate sandstone assemblage representing southeast progradation of an alluvial fan in a semi arid climatic setting clast lithologies and geochemistry indicate no extra rift detritus was delivered from the hinterland of the fan various lines of evidence in both volcanic and sedimentary rocks support a scenario where early pre 1108 ma subsidence along a north south axis from the western arm of the rift to the nipigon embayment was replaced by subsidence along the east west rift axis between 1108 and 1105 ma resume le groupe d osler mesoproterozoique 1108 1105 ma une succession de coulees basaltiques et d unites sedimentaires d une epaisseur de 3 km sur la rive nord du lac superieur est l une des plus anciennes expressions du rift mi continental les sediments a la base de la formation simpson island nouveau nom deposes par un systeme fluvial anastomose enregistrent le transport vers l ouest de debris erodes des unites rocheuses locales archeennes et proterozoiques des basaltes de points chauds recouvrent et sont intercales avec les strates deposees par ce systeme fluvial ces basaltes sont de plus en plus contamines en remontant la section end 1100ma 0 3 to 5 3 les ecoule ments sub aeriens de terres rares legeres enrichies la smn 1 5 3 9 et de terres rares lourdes enrichies fractionnees gd ybn 1 5 3 7 peuvent etre divises en deux unites qui correspondent a d autres sections du groupe d osler vers lst mais de simples correlations avec des sequences plus eloignees sont difficiles la portion de la succession dominee par des roches volcaniques est recouverte d un mince 25 m assemblage conglomerat gres qui represente la prograda tion vers le sud est d un delta alluvionnaire dans un environnement climatique semi aride la lithologie des clastes et la geochimie indiquent qu aucun detritus hors du rift ne provient de l arriere pays du delta diverses preuves a la fois dans les roches sedimentaires et les roches volcaniques soutiennent un scenario selon lequel une subsidence precoce avant 1108 ma le long d un axe nord sud a partir du bras ouest du rift vers la baie nipigon a ete remplacee par de la subsidence le long de l axe est ouest du rift entre 1108 et 1105 ma

density and lithospheric thickness of the tharsis province from mex mars and mro gravity data 1 radio science tracking of mars express mars experiment and mars reconnaissance orbiter produced high resolution martian gravity data applying localized spectral analysis on the new gravity data sets we study the surface density and lithospheric elastic thickness in the tharsis province the gravity signal is predicted with geophysical models either including bottom loading top bottom model or taking into account the loading history top top model volcanic shields are mainly composed of high density lava but their construction could have begun with lower density lava with the exception of ascraeus mons which has a top of lower density buoyant bottom loading may have been present in the form of a mantle plume under olympus mons the elastic thickness was much larger at olympus mons than at other volcanoes suggesting large spatial variations of heat flux during the hesperian alternatively small elastic thicknesses could be artifacts reflecting the presence of very localized high density crustal intrusions beneath the volcanoes thaumasia highlands were probably supported by a mantle plume at the time of formation in valles marineris top bottom models predict low densities and large elastic thicknesses in conflict with the basaltic rock composition and hesperian age of the valley dense mafic dikes underlie the western part of the valley the top top model serves to test another scenario in which the trough is formed with sedimentary infilling removed much later by erosion the elastic thickness increasing in between at the large volcanoes the relation between gravity and topography is anisotropic probably because of density variations

shear wave splitting observations in the lower great lakes region evidence for regional anisotropic domains and keel modified asthenospheric flow 1 fast axis directions obtained from shear wave splitting analysis of core refracted shear waves reveal several distinct anisotropic domains in the lower great lakes region we used data from 27 broadband seismograph stations extending across a low velocity anomaly in the lithospheric mantle observed splitting times vary from 0 4 to 1 4 s with a mean of 0 7 s a subset of the splitting vectors across a failed rift are oriented parallel to one arm of the rift zone most others show a coherent pattern of fast directions close to the direction of plate motion but oblique to surface tectonic belts within the area of the low velocity anomaly our observations are most simply explained by single layer anisotropy induced by asthenospheric flow spatial variability in the direction and magnitude of splitting are consistent with a flow regime influenced by basal topology of the lithospheric keel

admittance estimates of mean crustal thickness and density at the martian hemispheric dichotomy 1 admittance estimates from line of sight los acceleration profiles of the mars global surveyor spacecraft are used to constrain the mean crustal thickness and surface density centered on the hemispheric dichotomy from 110 e to 220 e 40 s to 20 n models with uniform crustal properties predict lower than expected bulk crustal densities two layer models with loading only at the surface and the moho produce satisfactory fits to the data the best fit surface density crustal thickness tc and elastic thickness te are 2 5 mg m 3 27 km and 61 km respectively higher elastic thicknesses require lower crustal thicknesses and vice versa the best fit ratio of moho to surface loading f is close to 1 models with no bottom loading f 0 provide a poor fit to the data underestimates in f result in underestimates of both tc and te the surface density is lower than that measured from martian meteorites and by admittance analyses of young volcanoes but is well constrained by the short wavelength admittance values for misfits up to 1 5 times the minimum value and a fixed surface density of 2 5 mg m 3 the ranges of tc te and f are 1 75 km 37 89 km and 0 4 2 6 respectively the apparently compensated nature of the large impact basins has been used to infer a lower bound on southern hemisphere crustal thickness of 45 km if this estimate is correct the likely mean crustal thickness in the area considered is 55 20 km

geoelectric response of archean lithosphere in the western superior province central canada abstract magnetotelluric mt soundings were made at more than 230 sites in the western superior province in central canada as part of the lithoprobe western superior transect investigations the superior province forms the nucleus of the north american continent its western part contains a regional pattern of east west trending 100 200 km wide granite greenstone and metasedimentary belts or subprovinces the objectives of the mt study are to define the electrical resistivity structure of the crust of the subprovinces and their boundaries and to determine if there exists a signature in the upper mantle related to the formation of the craton the exceptionally large mt database for the western superior province provides a three dimensional sampling of the resistivity structure in this paper the primary mt responses from the data set including geoelectric strike apparent resistivity and phase are used to define first order geoelectrical structures in the region apparent resistivity responses indicate the ca 3 0 ga north caribou terrane contains extremely resistive crust crust to the south accreted at ca 2 7 ga is significantly more conductive with resistivities similar to in the southeast superior province the results suggest that the supracrustal component of the north caribou terrane provides minimal enhancement of the crustal conductivity at the southern margin of the former north caribou superterrane induction arrows define a conductivity anomaly that extends in an east west direction for at least 600 km and causes a reversal of arrows in a region 100 200 km wide this anomaly is interpreted to be caused by a conductive component of the 2 7 ga metasedimentary rocks of the english river subprovince the observations demonstrate the ability of electromagnetic methods to map some neoarchean continental margins in the southwest superior province geoelectric strike azimuths for periods corresponding to signal penetration into the upper mantle are aligned with the trend of subprovince boundaries and are interpreted to be related to fossil structure in the mantle lithosphere this result provides support to the interpretation that the lithospheric mantle of the western superior province has remained intact since the 2 7 ga accretion of the subprovinces the mt responses from the western superior province have thus defined a number of large scale resistivity structures providing information on tectonic structure and history and confirming the value of electromagnetic imaging of archean lithosphere

a thin dense crust for mercury abstract crustal thickness is a crucial geophysical parameter in understanding the geology and geochemistry of terrestrial planets recent development of mathematical techniques suggests that previous studies based on assumptions of isostasy overestimated crustal thickness on some of the solid bodies of the solar system leading to a need to revisit those analyses here i apply these techniques to mercury using messenger derived elemental abundances i calculate a map of grain density average 2974 89 kg m3 which shows that pratt isostasy is unlikely to be a major compensation mechanism of mercury s topography assuming airy isostasy i find the best fit value for mercury s mean crustal thickness is 26 11 km 25 lower than the most recently reported and previously thinnest number several geological implications follow from this relatively low value for crustal thickness including showing that the largest impacts very likely excavated mantle material onto mercury s surface the new results also show that mercury and the moon have a similar proportion of their rocky silicates composing their crusts and thus mercury is not uniquely efficient at crustal production amongst terrestrial bodies higher resolution topography and gravity data especially for the southern hemisphere will be necessary to refine mercury s crustal parameters further

what can olympus mons tell us about the martian lithosphere abstract under gravitational loading a volcanic edifice deforms and the underlying lithosphere downflexes this has been observed on earth but is equally true on other planets we use finite element models to simulate this gravity driven deformation at olympus mons on mars eleven model parameters including the geometry and material properties of the edifice lithosphere and underlying asthenosphere are varied to establish which parameters have the greatest effect on deformation values for parameters that affect deformation at olympus mons mars are constrained by minimising misfit between modelled and observed measurements of edifice height edifice radius and flexural moat width our inferred value for the young s modulus of the martian lithosphere 17 8 gpa is significantly lower than values used previously suggesting that the martian lithosphere is more porous than generally assumed the best fitting values for other parameters edifice density 2111 2389 kg m 3 and lithosphere thickness 83 3 km are within ranges proposed hitherto the best fitting values of model parameters are interdependent a decrease in lithosphere young s modulus must be accompanied by a decrease in edifice density and or an increase in lithosphere thickness our results identify the parameters that should be considered within all models of gravity driven volcano deformation highlight the importance of the often overlooked young s modulus and provide further constraints on the properties of the martian lithosphere namely its porosity which have implications for the transport and storage of fluid throughout mars history

images of a lower crustal oceanic slab direct evidence for tectonic accretion in the archean western superior province the archean western superior province in canada is the type area for proposed archean plate tectonics seismic images from this region provide direct evidence for assembly of the craton by terrane accretion and for a large slab of remnant oceanic crust preserved at the base of the crust this slab with inferred garnet amphibolite composition adds a critical piece of evidence to previous suggestions that archean subduction was at a shallow angle and that some neoarchean tonalite trondhjemite granodiorite suites distinct from most modern day suprasubduction magmas are melts primarily derived directly from subducted slabs

correction to localized gravity topography admittance and correlation spectra on mars implications for regional and global evolution 1 in the paper localized gravity topography admittance and correlation spectra on mars implications for regional and global evolution by patrick j mcgovern sean c solomon david e smith maria t zuber mark simons mark a wieczorek roger j phillips gregory a neumann oded aharonson and james w head journal of geophysical research 107 e12 5136 doi 10 1029 2002je001854 2002 the thickness of the lithosphere and lithospheric heat flow for a number of regions of mars and as functions of time were inferred on the basis of gravity topography admittance spectra observed admittances derived from spherical harmonic expansions localized with the scheme of simons et al 1997 were compared with those predicted from models for the flexural response to lithospheric loading e g turcotte et al 1981 gravity was calculated according to the finite amplitude scheme of wieczorek and phillips 1998 estimates for the thickness of the elastic lithosphere te at the time of loading for each region were converted to equivalent thermal gradient dt dz and heat flux q by means of an elastic plastic stressenvelope formalism mcnutt 1984 here we describe a correction required in the calculation of the modeled gravity anomalies we report new estimates of te load density rl dt dz and q from corrected model admittances and we discuss the implications of the new results 2 the source of the required correction is a difference in reference radius values as defined by mcgovern et al 2002 the planetary shape was taken to equal the radius from the center of mass of mars to the martian surface expressed as a spherical harmonic expansion and referenced to the mean equatorial radius req 3396 km

seasonal and static gravity field of mars from mgs mars odyssey and mro radio science abstract we present a spherical harmonic solution of the static gravity field of mars to degree and order 120 gmm 3 that has been calculated using the deep space network tracking data of the nasa mars missions mars global surveyor mgs mars odyssey ody and the mars reconnaissance orbiter mro we have also jointly determined spherical harmonic solutions for the static and time variable gravity field of mars and the mars k 2 love numbers exclusive of the gravity contribution of the atmosphere consequently the retrieved time varying gravity coefficients and the love number k 2 solely yield seasonal variations in the mass of the polar caps and the solid tides of mars respectively we obtain a mars love number k 2 of 0 1697 0 0027 3 the inclusion of mro tracking data results in improved seasonal gravity field coefficients c 30 and for the first time c 50 refinements of the atmospheric model in our orbit determination program have allowed us to monitor the odd zonal harmonic c 30 for 1 5 solar cycles 16 years this gravity model shows improved correlations with mola topography up to 15 larger at higher harmonics l 60 80 than previous solutions