does folding accommodate europa s contractional strain the effect of surface temperature on fold formation in ice lithospheres 1 on europa the mechanism by which contractional strains are accommodated remains a mystery subtle folds have been observed in the polar regions but if folding is a dominant accommodation mechanism ubiquitous large amplitude folds might be expected across europa s surface here we use finite element modeling to show that fold growth rates are a strong function of surface temperature warm temperatures result in lower rates whereas cold temperatures result in higher rates combining diurnally averaged europan surface temperatures derived from galileo photopolarimeter radiometer data and new numerical simulations we show that forming moderate amplitude folds requires roughly twice the contraction or shortening in europa s equatorial latitudes relative to polar latitudes relatively large contractional strains in europa s equatorial and midlatitudes can therefore be accommodated primarily through passive shortening without producing obvious folds lithospheric folding and or passive shortening therefore remain plausible mechanisms for strain accommodation on europa

numerical models of lithospheric deformation forming the southern alps of new zealand compression of the entire continental lithosphere is considered using two dimensional numerical models in order to study the influence of the lithospheric mantle on the geometry of continental collision in its initial stages the models are based on the central section of new zealand southern alps where continental collision has occurred along the alpine fault since about 7 ma they incorporate brittle elastic ductile rheology heat transfer surface processes and fault localisation the models are compared to the surface relief the gps convergence velocity the measured electrical conductivity and the geometry of the crustal root imaged from seismic velocity measurements the crustal deformation is characterized by localized uplift at the plate boundary alpine fault and by two secondary zones of faulting one is located about 60 to 80 km east of the alpine fault at the start of upper crust bending or tilting and the other is located about 100 to 130 km east of the alpine fault as a result from shear strain propagating to the surface through the ductile lower crust the observed asymmetric shape of the crustal root is best reproduced for a mantle lithosphere strength of the order of 200 mpa and an intermediate rate of strain softening a lower strength of the mantle lithosphere can produce symmetric thickening but provides an amplitude of the crustal root too small when compared to observations the observed 20 km offset between the maximum in surface relief and the crustal root was not satisfactorily reproduced this offset is most likely due to the three dimensionality of oblique collision in new zealand southern alps

mean magnetic contrasts between oceans and continents abstract the magnetic contrast between continents and oceans is an important constraint for understanding the regional distribution of magnetic sources in the crust and upper mantle this magnetic difference was investigated using satellite 2 c scalar magnetic anomalies collected by nasa s magsat mission over the atlantic and eastern pacific oceans north and south america europe and africa reducing the magnetic anomalies differentially to vertical polarization yields mean anomaly values which correlate with the tectonic ages of the various continental and oceanic regions the range of magnetic anomaly values of tectonically older lithosphere is greater than that of younger lithosphere statistically at the 99 9 confidence level the mean radially polarized magnetic anomaly of the continents is greater than that of the oceans a careful inversion was performed for regional effective susceptibility contrasts which accurately fit the magsat data assuming a model in which the moho represents a boundary between crustal magnetic and predominantly non magnetic mantle rocks the root mean square of the magnetic properties of oceans compares favorably with magnetic properties predicted from models constrained by magnetic studies of the oceanic rocks and which incorporate viscoremanent components from rocks near the base of the crust the mean magnetic anomaly and property contrasts of the oceans are negative and show large variability indicating widespread deviations from the standard oceanic crust the effective susceptibility contrasts obtained facilitate quantitative testing of inferences concerning large scale petrologic variations crustal thickness and thermal perturbations and other geologic processes which control the regional distribution of magnetic properties within the lithosphere

historical development of the gravity method in exploration the gravity method was the first geophysical technique to be used in oil and gas exploration despite being eclipsed by seismology it has continued to be an important and sometimes crucial constraint in a number of exploration areas in oil exploration the gravity method is particularly applicable in salt provinces overthrust and foothills belts underexplored basins and targets of interest that underlie high velocity zones the gravity method is used frequently in mining applications to map subsurface geology and to directly calculate ore reserves for some massive sulfide orebodies there is also a modest increase in the use of gravity techniques in specialized investigations for shallow targets gravimeters have undergone continuous improvement during the past 25 years particularly in their ability to function in a dynamic environment this and the advent of

rheological properties of water ice applications to satellites of the outer planets 1 abstract the icy moons of the outer solar system have not been quiescent bodies in part because many have a substantial water component and have experienced significant internal heating we can begin to understand the thermal evolution of the moons and the rate of viscous relaxation of surface topography because we now have good constraints on how ice in several of its polymorphic forms flows under deviatoric stress at planetary conditions details of laboratory derived flow laws for pure polycrystalline ice are reviewed in detail one of the more important questions at hand is the role of ice grain size grain size may be a dynamic quantity within the icy moons and it may or may not significantly affect rheology one recent beneficiary of revelations about grain size sensitive flow is the calculation of the rheological structure of europa s outer ice shell which may be no thicker than 20 km

constraining the heat flux between enceladus tiger stripes numerical modeling of funiscular plains formation abstract the cassini spacecraft s composite infrared spectrometer cirs has observed at least 5 gw of thermal emission at enceladus south pole the vast majority of this emission is localized on the four long parallel evenly spaced fractures dubbed tiger stripes however the thermal emission from regions between the tiger stripes has not been determined these spatially localized regions have a unique morphology consisting of short wavelength 1 km ridges and troughs with topographic amplitudes of 100 m and a generally ropy appearance that has led to them being referred to as funiscular terrain previous analysis pursued the hypothesis that the funiscular terrain formed via thin skinned folding analogous to that occurring on a pahoehoe flow top barr a c preuss l j 2010 icarus 208 499 503 here we use finite element modeling of lithospheric shortening to further explore this hypothesis our best case simulations reproduce funiscular like morphologies although our simulated fold wavelengths after 10 shortening are 30 longer than those observed reproducing short wavelength folds requires high effective surface temperatures 185 k an ice lithosphere or high viscosity layer with a low thermal conductivity one half to one third that of intact ice or lower and very high heat fluxes perhaps as great as 400 mw m 2 these conditions are driven by the requirement that the high viscosity layer remain extremely thin 200 m whereas the required conditions are extreme they can be met if a layer of fine grained plume material 1 10 m thick or a highly fractured ice layer 50 m thick insulates the surface and the lithosphere is fractured throughout as well the source of the necessary heat flux a factor of two greater than previous estimates is less obvious we also present evidence for an unusual color spectral character of the ropy terrain possibly related to its unique surface texture our simulations demonstrate that producing the funiscular ridges via folding remains plausible but the relatively extreme conditions required to do so leaves their origin open to further investigation the high heat fluxes required to produce the terrain by folding which equate to an endogenic blackbody temperature near 50 k should be observable by future nighttime cirs observations if funiscular deformation is occurring today

global vector and scalar magsat magnetic anomaly maps empirical and analytical techniques for modeling ionospheric fields in magsat data have been developed that facilitate ionospheric field removal from uncorrected anomalies to obtain better estimates of regional lithospheric anomalies this task has been accomplished for equatorial x z and b component and polar z and b component measurements the techniques for modeling ionospheric fields have been integrated into a processing sequence that incorporates some of the important data processing techniques developed during the last decade data processing techniques include retention of common signal in a correlation analysis of adjacent passes analysis of field differences between dawn and dusk data at points where their orbits cross and retention of common signal in a covariant spherical harmonic analysis procedure results suggest that implementation of the above processing scheme leads to the mapping of the most robust magnetic anomalies of the lithosphere vector components as well as scalar

gravity inferred crustal attributes of visible and buried impact basins on mars 1 the impact basins of mars reveal important insights on martian tectonic evolution they involve strongly disrupted depressed regions of crust with likely enhanced porosity and permeability that may locally concentrate water and other crustal fluids hence the crustal properties of impact basins can also be important in the hunt for water and related clues for life on mars we assess the crustal details of impact basins by separating the mars global surveyor free air anomalies into terrain correlated and terrain decorrelated components the separation is based on the correlation spectrum between the free air anomalies and the gravity effects evaluated from the topography mapped by the mars orbital laser altimeter mola for topographically visible multiring basins like isidis striking circular patterns of alternating terrain correlated free air maxima and minima mark the uncompensated components of the central mantle plug and surrounding rings the first vertical derivatives of these anomalies effectively estimate the basin ring locations and a transient cavity depth to diameter ratio of 0 09 that is consistent with the ratio observed for lunar nearside multiring basins for the isidis basin we obtain an excavation depth of roughly 66 km that reflects considerably disrupted crust for concentrating local fluids since the noachian impact furthermore the related crustal terrain decorrelated free air anomalies suggest up to 2 km of high density basin fill may cap the central basin subtle quasi circular depressions in the relatively featureless mola terrain of the northern hemisphere have identified potentially buried impact basins frey et al 2002 an altimetry depression in acidalia planitia and another in utopia are also associated with ringed patterns of terrain decorrelated free air anomalies that may mark the uncompensated mass effects of buried impact basins the gravity derived transient excavation depths for these inferred basins are roughly 41 and 20 km respectively while the related ring diameters d follow the ubiquitous d rule of planetary impact basins the crust of these buried basins is likely to contain water at higher levels than the crust of the equatorial basins that was substantially dewatered with the development of the great northern basin hence analyzing surface fractures faults and other crustal discontinuities that can tap the more saturated crust of these inferred buried basins may facilitate the hunt for water on mars

inversion of regional gravity anomalies and main features of the deep crustal geology of india abstract long wavelength gravity field variations provide an important geophysical tool for delineating structural features of the deeper parts of the earth s interior continuous developments in data processing and inversion techniques have led to refinements in regional as well as planetary scale structural models the present work uses the free air gravity data of the indian subcontinent to derive causative density contrast structures the model obtained by assuming the point mass sources at a constant depth of 40 km based on deep seismic sounding studies reflects the known tectonic and geological provinces of the indian subcontinent well interestingly more than two thirds of india exhibits low density contrast values with the lowest point occurring in the vicinity of the sarda depression which possibly hides a broken part of the tethys trench in general the plateaus and uplifts are depicted as density contrast highs while basins and depressions are associated with lows an s n traverse close to 78 e longitude confirms these findings further the study confirms the postulated burrard s hidden trough belgaum nellore and additionally identifies 1 1 a new hidden structural ridge running along the ahmednagar hyderabad line and 2 2 a trough with an axis along the mandya salem tanjavur line all of these deformations exhibit connections with the seafloor spreading process as they parallel the carlsberg ridge in the indian ocean in the south and the himalayan fold axis in the collision zone of the indian and eurasian plates

evidence for subduction in the ice shell of europa despite widespread evidence for extension there have been few signs of contraction on the icy surface of jupiter s europa evidence for a subduction like convergent boundary suggests that europa may have active plate tectonics

model constraints on the opening rates of bands on europa a mid ocean ridge spreading analog is used to constrain the opening rates and brittle ductile transition depths for two axisymmetric ridged bands on europa estimates of brittle ductile transition depth based on the morphologies of yelland and ino lineae are combined with a conductive cooling model based on a mid ocean ridge analog to estimate the opening rates and active lifetimes of the bands this model limits local strain rates to 10 15 10 12 s 1 opening rates to 0 2 40 mm yr 1 and active lifetimes of the bands to 0 1 30 myr if the observed structures in the outer portions of ridged bands are indeed normal faults the estimated range for the tensile strength of ice on europa

forming europa s folds strain requirements for the production of large amplitude deformation abstract the surface of jupiter s iconic icy satellite europa is dominated by tectonic features that have formed in extension in contrast features that have formed in contraction appear to be rare thus there exists an apparent imbalance between the observed surface expression and relative magnitudes of extensional and contractional strain in this paper we reexamine the amount of strain that can be accommodated during fold formation on europa using a two dimensional finite element model that allows the examination of fold formation at large strains and under a broad range of initial conditions we find that in contrast to previous assessments relatively large strains can be accommodated by the development of relatively low amplitude folds under our nominal conditions creating peak to trough fold amplitudes of 200 m consistent with europa s astypalaea folds requires 7 contraction of the lithosphere decreasing either the strain rate or the ice grain size allows the formation of larger amplitude folds at smaller strains but we find no cases where 200 m amplitude folds form at less than 3 strain only if both of these conditions are met simultaneously do large amplitude folds form at contractional strains 3 the requirement of larger strains relative to previous analytical models results from the combined effects of an initial period of fold nucleation generally 1 2 contractional strain in which limited amplification occurs and smaller amplitude growth rates overall the requirement of several percent strain to produce even moderate amplitude folds is consistent with previous numerical models of single layer and lithospheric scale viscoelastic folding under terrestrial conditions because the formation of large amplitude folds requires relatively large strain small but significant amounts of contraction can be accommodated predominantly through uniform thickening of the lithosphere during the nucleation phase of fold growth without the formation of obvious surface deformation the accommodation of contractional strains via such pre folding lithospheric thickening provides at least a partial solution to europa s apparent strain imbalance

compatibility of high altitude aeromagnetic and satellite altitude magnetic anomalies over canada results from equivalent source distributions derived jointly from high altitude average 4 km aeromagnetic and magsat derived average 400 km magnetic anomalies over canada indicate that long wavelength components 500 2500 km in these fields are extremely compatible with one another with a correlation coefficient of 0 95 the jointly estimated anomaly field at the earth s surface can be used as a long wavelength adjustment surface for regional near surface magnetic anomaly compilations and in assessing the performance of other downward continuation techniques because near surface anomalies are not available over all regions of the world we compare the jointly estimated anomaly field to the results of two different downward continuation techniques the evaluation of anomalies at the earth s surface from spherical harmonic coefficients derived from satellite altitude data and the use of downward continuation methods based on harmonic splines numerical and visual comparisons of these downward conti

modeling complexly magnetized two dimensional bodies of arbitrary shape a method has been devised for the forward computation of magnetic anomalies due to two dimensional 2 d polygonal bodies with heterogeneously directed magnetization the calculations are based on the equivalent line source approach wherein the source is subdivided into discrete elements that vary spatially in their magnetic properties this equivalent dipole line method provides a fast and convenient means of representing and computing magnetic anomalies for bodies possessing complexly varying magnitude and direction of magnetization the algorithm has been tested and applied to several generalized cases to verify the accuracy of the computation the technique has also been used to model observed aeromagnetic anomalies associated with the structurally deformed remanently magnetized keweenawan volcanic rocks in eastern lake superior this method is also easily adapted to the calculation of anomalies due to two and one half dimensional 2 5 d and three dimensional 3 d heterogeneously magnetized sources

spherical earth gravity and magnetic anomaly analysis by equivalent point source inversion abstract to facilitate geologic interpretation of satellite elevation potential field data analysis techniques are developed and verified in the spherical domain that are commensurate with conventional flat earth methods of potential field interpretation a powerful approach to the spherical earth problem relates potential field anomalies to a distribution of equivalent point sources by least squares matrix inversion linear transformations of the equivalent source field lead to corresponding geoidal anomalies pseudo anomalies vector anomaly components spatial derivatives continuations and differential magnetic pole reductions a number of examples using 1 averaged surface free air gravity anomalies and pogo satellite magnetometer data for the united states mexico and central america illustrate the capabilities of the method

friction of ice the frictional strength of ice seems to be well below that for all other rocks triaxial testing of cylinders of pure water ice containing a 45 inclined sawcut at temperatures of 77 t 115 k and confining pressures 0 1 p 250 mpa reveals the frictional laws 0 20 n 8 3 mpa for p 10 mpa and 0 55 n 1 0 mpa for p 5 mpa where and n are the shear and normal stresses on the sawcut surface friction is independent of t and over the one order of magnitude tested of average sliding velocity the sliding behavior is invariably stick slip examination of sliding surfaces shows only minor gouge development a curious arrangement of densely packed short vertical fractures occurs primarily in samples with anomalously low strength these results apply to models of near surface tectonic activity on jupiter s three large icy moons europa callisto and ganymede they indicate for example that a global expansion on ganymede of 3 linear percent will cause extensional movement on preexisting faults at depths to 7 3 km

satellite magnetic anomalies of the antarctic wilkes land impact basin inferred from regional gravity and terrain data abstract the grace gravity and subglacial terrain data of wilkes land are consistent with the presence of a mascon produced by giant impact perhaps at the end of the permian in contrast to the relatively extensive ice probing radar coverage aeromagnetic data coverage is limited across the basin however magsat orsted and champ satellite magnetic observations reveal the thinned crust of the impact site to be associated with the largest satellite altitude crustal magnetic anomaly of antarctica the underlying region of enhanced magnetization is consistent with the grace gravity and bedmap terrain data and extends into south central australia in a reconstructed gondwana the strongly magnetized crust can reflect the impact s thermal enhancement of lower crustal viscous remanent magnetization as well as the production of positively magnetized melt rocks within the fractured crust

crustal structure of the churchill superior boundary zone between 80 and 98 w longitude from magsat anomaly maps and stacked passes a modeling technique using spherical shell elements and equivalent dipole sources has been applied to magsat signatures at the churchill superior boundary in manitoba ontario and ungava a large satellite magnetic anomaly 12 nt amplitude on pogo and magsat maps near the churchill superior boundary was found to be related to the richmond gulf aulacogen the averaged crustal magnetization in the source region is 5 2 a m 1 stacking of the magnetic traces from magsat passes reveals a magnetic signature 10 nt amplitude at the churchill superior boundary in an area studied between 80 w and 98 w modeling suggests a steplike thickening of the crust on the churchill side of the boundary in a layer with a magnetization of 5 a m 1 signatures on aeromagnetic maps are also found in the source areas for both of these satellite anomalies

calcium isotopes in caves as a proxy for aridity modern calibration and application to the 8 2 kyr event abstract we present the first study of ca isotope cycling in a natural cave system with measurements of bedrock dripwater and recently formed carbonate coupled to a first stalagmite time series spanning the 8 2 kyr event dripwaters at heshang cave central china 30 27 n 110 25 e are isotopically heavy relative to the dolomite bedrock the result of prior calcite precipitation pcp occurring earlier in the drip flow path a simple rayleigh fractionation model quantifies the extent of pcp in the modern environment at 36 ca removal the observed in situ calcium isotope fractionation factor between dripwater and carbonate is 44 42 ca 0 63 0 03 and does not vary during the annual cycle measurements of speleothem carbonate spanning the 8 2 kyr event show the response of ca isotopes to changing climate 44 42ca increases by 0 35 at the onset of the event coeval with changes in 18o and mg ca and remains high for 80 yr this change is explained by decreased rainfall leading to increased pcp an interpretation supported by established pcp proxies mg ca sr ca and ba ca ca isotopes indicate that pcp increased to 60 ca removal during the event which from application of a simple box model suggests mean annual rainfall decreased by approximately a third in central china during the 8 2 kyr event the response of ca isotopes across this event demonstrates their potential for the assessment of past conditions including past dripwater flow rates and rainfall

separation of lithospheric external and core components of the south polar geomagnetic field at satellite altitudes we present a new approach to producing scalar magsat magnetic anomaly maps based on correlation coefficient filtering and the use of almost all of the available orbits our method differs from earlier techniques with respect to the following 1 passes are selected based on their variance properties rather than planetary indices such as kp 2 the core field model is least squares fit to individual passes and subsequently removed instead of subtracting the model directly this technique replaces band pass filtering or polynomial trend removal methods 3 each selected pass is sorted geographically and by local time placed into one of four different altitude bands and correlation coefficient filtered with the two adjacent passes the filtering is the second step toward isolating the static lithospheric signal from the more dynamic external field signals 4 least squares collocation is used to grid the correlated passes subsequently the dawn and dusk maps are also correlation filtered providing another step toward removal of external fields 5 the four resultant total field maps are continued to a common altitude and again correlation filtered for the static lithospheric anomalies 6 the filtered results are then averaged together to provide a new total field map of the lithosphere south of 40 s latitude our total field map differs from previous efforts over the crustal blocks of west antarctica we obtained a positive anomaly over edward vii peninsula extending into the byrd subglacial basin and obtained a negative anomaly over the ellsworth mountains and parts of the byrd subglacial basin also a positive anomaly extending from the ross sea to offshore wilkes land is present in our map however this feature is absent in other maps positive anomalies marking the weddell sea in previous efforts are not present in our map prominent external field anomalies in the quadrant offshore of wilkes land are present in all previous efforts however these signatures are reduced in our total field map this map significantly advances efforts for the identification and interpretation of magnetic anomalies of the south polar lithosphere

spherical earth modelling of the scalar magnetic anomaly over the indian region magsat anomaly maps over the indian region are prepared after removal of core and external current contributions to examine the physical reality of the map we calculate the geomagnetic scalar field at magsat height in spherical co ordinates using the equivalent point source distribution of magnetic dipoles the calculation assumes i the source field of the anomaly to lie within the crust ii only induced magnetisation exists iii the susceptibility is constant over the whole region crustal thickness is deduced from bouguer gravity anomaly maps evaluation of the anomaly is done numerically using the gauss legendre quadrature integration method the computed values agree reasonably well with the observed anomalies this suggests that the crustal component is quite accurately discernible in the magsat data further anomalies over india seem to be accountable with induced magnetisation alone and the curie isotherm appears to coincide with the crustal depth

regional north american gravity and magnetic anomaly correlations summary correlation of gravity and magnetic anomalies combined with other geological and geophysical data is useful for enhancing the quality of geological interpretation of potential anomaly fields maps produced by equivalent point source inversion are used to investigate visual spatial correlations of surface free air gravity and pogo satellite magnetic anomalies and regional heat flow and tectonic data for north america and adjacent marine areas a quantitative analysis of regional potential field anomaly correlations at satellite elevations is also considered utilizing poisson s theorem in a moving window linear regression between derivatives of the anomalous gravity and magnetic fields an inverse relationship is observed between long wavelength gravity and magnetic anomalies over continental terrain negative gravity and positive magnetic anomalies are areas characterized by relatively thick crust and high magnetization an example is a prominent magnetic high which corresponds to a trend of gravity minima extending from the anadarko basin to the cincinnati arch negative magnetic and positive gravity anomalies characterize thinner crust and regions of higher heat flow such as the cordillera of north and central america and specifically the yellowstone geothermal region although gravity and magnetic anomalies over oceanic areas show poor correlation the sign of the statistical correlation generally is positive 1 crustal correlation considerations the utility of long wavelength gravity anomalies covering several degrees of surface area for geological analysis is well recognized magnetic anomalies which have significant energy in wavelengths of several hundred kilometres also have been identified as originating from within the earth s crust and perhaps the uppermost mantle the geological interpretation of these regional anomalies is hindered by the effect of anomaly superposition and source ambiguity which is inherent to the analysis of potential fields an approach to minimizing these limitations especially in continental terrains is to evaluate the correlation between anomalous gravity and magnetic fields the basis of correlation analysis is the hypothesis commonly validated in continental areas that variations in lithology and physical properties of the crystalline crust are reflected

viscous relaxation as a prerequisite for tectonic resurfacing on ganymede insights from numerical models of lithospheric extension abstract ganymede s bright terrain formed during a near global resurfacing event or events that produced both heavily tectonized and relatively smooth terrains the mechanism s by which resurfacing occurred on ganymede e g cryovolcanic or tectonic and the relationship between the older dark and the younger bright terrain are fundamental to understanding the geological evolution of the satellite using a two dimensional numerical model of lithospheric extension that has previously been used to successfully simulate surface deformation consistent with grooved terrain morphologies we investigate whether large amplitude preexisting topography can be resurfaced erased by extension i e tectonic resurfacing using synthetically produced initial topography we show that when the total relief of the initial topography is larger than 25 50 m periodic groove like structures fail to form instead extension is localized in a few individual isolated troughs these results pose a challenge to the tectonic resurfacing hypothesis we further investigate the effects of preexisting topography by performing suites of simulations initialized with topography derived from digital terrain models of ganymede s surface these include dark terrain fresh relatively deep impact craters smooth bright terrain and a viscously relaxed impact crater the simulations using dark terrain and fresh impact craters are consistent with our simulations using synthetic topography periodic groove like deformation fails to form in contrast when simulations were initialized with bright smooth terrain topography groove like deformation results from a wide variety of heat flow and surface temperature conditions similarly when a viscously relaxed impact crater was used groove like structures were able to form during extension these results suggest that tectonic resurfacing may require that the amplitude of the initial topography be reduced before extension begins we emphasize that viscous relaxation may be the key to enabling tectonic resurfacing as the heat fluxes associated with groove terrain formation are also capable of reducing crater topography through viscous relaxation for long wavelength topography large craters viscous relaxation is unavoidable we propose that the resurfacing of ganymede occurred through a combination of viscous relaxation tectonic resurfacing cryovolcanism and at least in a few cases band formation variations in heat flow and strain magnitudes across ganymede likely produced the complex variety of terrain types currently observed

lithospheric magnetic property contrasts within the south american plate derived from damped least squares inversion of satellite magnetic data abstract a ridge regression or damped least squares method has been used in a large scale equivalent source inversion of south american magsat data the optimum damping parameter is selected based on percent increase in the sum of squared residual of the observed and the calculated anomaly and on the variance and the range of the solution for a series of damping parameters solutions obtained by the damping parameter selected in this manner are numerically stable and geologically meaningful consequently these solutions are used in various applications such as transformation into effective magnetic susceptibility variation altitude normalization differential reduction to pole and downward continuation of the data set the spacing of equivalent sources and the source observation distance are shown to introduce significant errors in the estimation of the physical property variation from equivalent source solutions for the large source spacing 1 commonly in use the correction for these geometric factors is essential in estimating the physical property variation from the equivalent source solutions a continental and an oceanic profile have been analyzed from the south american maps derived from the above techniques the continental profile transects a variety of geologic tectonic features including shields aulacogens basins and fold belts near the north coast of south america along the profile there is a good correlation between the radially polarized magsat magnetic anomalies and other available geologic and geophysical information in addition the variation in effective susceptibility obtained from the damped equivalent source inversion is consistent with our present knowledge of continental lithospheric susceptibilities expected from the geologic tectonic provinces along the profile the oceanic profile extends from the trailing margin of south america through the cretaceous normal polarity rocks of the south atlantic ocean the variations in the magnetic properties along this profile can be broadly correlated with the depth anomaly variation indicating a possible genetic relationship two conceptual models requiring undepleted mantle intrusions under the oceanic uplifts appear possible in the first model the high magnetization can be accounted for by a weakly magnetic upper mantle below the uplifts in the second model very highly magnetic rocks are required in the crust of these uplifted regions to account for the observed anomaly values

3 d inversion of gravity data in spherical coordinates with application to the grail data three dimensional 3 d inversion of gravity data has been widely used to reconstruct the density distributions of ore bodies basins crust lithosphere and upper mantle for global model of 3 d density structures of planetary interior such as the earth the moon or mars it is necessary to use an inversion algorithm that operates in the spherical coordinates we develop a 3 d inversion algorithm formulated with specially designed model objective function and radial weighting function in the spherical coordinates we present regional and global synthetic examples to illustrate the capability of the algorithm the inverted results show density distribution features consistent with the true models we also apply the algorithm to a set of lunar bouguer gravity anomaly derived from the gravity recovery and interior laboratory grail gravity field and obtain a lunar 3 d density distribution high density anomalies are clearly identified underlying lunar basins a wide region of the lateral density heterogeneities that exist beneath the south pole aitken basin are found and low density anomalies are distributed beneath the feldspathic highlands terrane on the lunar far side the consistency of these results with those obtained independently from other existing methods verifies the newly developed algorithm

the lithospheric geomagnetic field origin and dynamics of long wavelength anomalies abstract the lithospheric part of the earth s geomagnetic field seems to continue down through the lower spherical harmonics the magnetisation in the lower crust to lower lithosphere is temporarily locked various geodynamic processes lead to locking or unlocking of this magnetisation the crustal base is a phase boundary between strongly magnetised less dense and weakly magnetised denser mass changes in pressure and temperature will dislocate this boundary and lead to changes in magnetisation and gravitational potential the fennoscandian uplift seems to record such phase boundary dislocation rotational changes may also affect this boundary changes in the lithospheric magnetic field have become highly relevant in studies of the dynamics and time consistency of lithospheric processes and properties

stresses generated in cooling viscoelastic ice shells application to europa 1 a cooling viscoelastic ice shell overlying an ocean develops stresses due to two effects thermal contraction of the ice due to cooling and the expansion of the shell due to the ice water volume change the former effect generates near surface compression and deeper extension the second effect generates extension only in both cases stresses are smaller at depth due to viscous creep the resulting combined stresses are extensional except at shallow 1 km depths in thin ice shells for ice shells thicker than 45 km stresses are extensional throughout the extensional stresses exceed 10 mpa for shells thicker than 20 km and thus dominate all other likely sources of stress as long as shell cooling occurs the dominantly extensional nature of the stresses may help to explain the puzzling lack of compression observed on europa and other large icy satellites however after 100 myr of conductive cooling the maximum theoretical elastic strains for europa are 0 35 which are probably insufficient to explain the total amount of observed extension

investigating sources of mercury s crustal magnetic field further mapping of messenger magnetometer data 106 low altitude passes of magnetometer data from the last two months of the messenger mission have been applied to produce a map of the crustal magnetic field at a constant altitude of 40 km covering latitudes of 35 75 n and longitudes of 270 90 e some anomalies correlate significantly with impact basins craters e g rustaveli and vyasa while other basins craters have no obvious anomalies a possible interpretation that is consistent with lunar evidence is that some impactors delivered more ferromagnetic fe ni metal to the interior subsurfaces and ejecta fields of the craters basins that they produced the amount of metallic iron that could plausibly be delivered is limited by the diameter and mass of an impactor that would yield a crater with observed diameters e g 200 km for rustaveli this in turn limits the maximum amplitude of anomalies that could be induced by impactor added iron in the present day mercury global field to relatively low values it is therefore concluded that if impactor added iron is the source of the observed crater associated anomalies then they must be almost entirely a consequence of ancient remanent magnetization a broad magnetic anomaly occurs over the northern rise a topographically high region with an associated strong free air gravity anomaly a possible interpretation of the latter anomaly is that an early major impact preconditioned the region for a later mantle uplift event

evolution of lineaments on europa clues from galileo multispectral imaging observations abstract four distinct classes of lineaments can be described on the basis of galileo s improved spectral and spatial coverage of europa 1 incipient cracks are narrow the morphologies spectral properties and orientations of the lineaments vary systematically with age suggesting that these four classes represent different stages of development in an evolutionary sequence lineament formation appears to be the dominant resurfacing mechanism on europa and every landscape that has escaped erasure by heating from below is imprinted with generation after generation of intersecting ridges at various scales and orientations relatively recent fractures expose coarse grained clean ice in the shallow subsurface possibly accounting for the distinctive color of the satellite in comparison to the other icy moons of jupiter the process of lineament formation may be continuing today the bright band agenor linea is among the leading candidates for current activity

verification of the crustal component in satellite magnetic data to investigate the utility of satellites for mapping crustal magnetic anomalies pogo polar orbiting geophysical observatory and preliminary magsat magnetometer data are compared with scalar aeromagnetic data obtained by the u s naval oceanographic office noo vector magnetic survey of the conterminous u s a pogo and available magsat data demonstrate remarkable consistency over the study region the noo aeromagnetic data are low pass filtered for wavelengths larger than about 4 and spherically upward continued to 450 km elevation by equivalent point source inversion for direct comparison with pogo satellite magnetometer observations the upward continued noo data show that most of the energy is in the long wavelength 1000 3000 km anomalies removal of these wavelengths by suitable filtering reveals a residual anomaly field that corresponds well with the satellite anomalies thus demonstrating that the satellite data are useful for mapping crustal magnetic anomalies a number of correlations between the noo pogo and preliminary magsat data are evident at satellite elevations including a prominent transcontinental magnetic high which extends from the anadarko basin of the eastern texas panhandle to the cincinnati arch the transcontinental magnetic high is breached by negative anomalies located over the rio grande rift and mississippi river aulacogen

europa s ice tectonics new insights from physical wax experiments with implications for subduction initiation and global resurfacing processes abstract jupiter s moon europa has one of the youngest geological surfaces in our solar system with an age of 40 90 ma implying an intense history of resurfacing the surface of europa indeed shows abundant evidence of tectonic deformation related to extension strike slip and shortening however observed features related to shortening are scarce compared with pervasive extensive extensional features such as dilational bands and do not suffice as the sole mechanism for recycling aging terranes recently evidence for potential plate tectonics associated with subduction zones has been discovered on europa this could be responsible for recycling most of europa s surface however basic physical parameters needed to initiate subduction on europa such as thickness of the brittle layer deformation rates and orientation of pre existing zones of weakness at which subduction could start are not well understood here we aim to better understand the process and the conditions that could lead to initiation of subduction on europa through physical experiments using wax to simulate europa s two layered i e convective icy crust by deforming the wax strain on europa s surface possibly caused by diurnal tides or its nonsynchronous rotation is simulated our results indicate that subduction could initiate over a broad range of surface thicknesses and deformation rates above a minimum conductive layer thickness but is strongly dependent on the orientation of the pre existing zones of weakness very thin conductive layer experiments however result in a previously undescribed process that we term ductile rolldown which creates surface features similar to double ridges observed on europa thus subduction and ductile rolldown represent physically plausible mechanisms that could play a critical role in resurfacing europa throughout its geologic history these results could yield significant implications for europa s thermal history and evolution habitability and future spacecraft missions

improved inversion of geopotential field anomalies for lithospheric investigations instabilities and the large matrices which are common to inversions of regional magnetic and gravity anomalies often complicate the use of efficient least squares matrix procedures inversion stability profoundly affects anomaly analysis and hence it must be considered in any application wildly varying or unstable solutions are the products of errors in the anomaly observations and the integrated effects of observation spacing source spacing elevation differences between sources and observations geographic coordinate attributes geomagnetic field attitudes and other factors which influence the conditioning of inversion solution instabilities caused by ill posed parameters can be efficiently minimized by ridge regression with a damping factor large enough to stabilize the inversion but small enough to produce an analytically useful solution an effective choice for the damping factor is facilitated by plotting damping factors against residuals between observed and modeled anomalies and by then comp

geopotential field anomaly continuation with multi altitude observations abstract conventional gravity and magnetic anomaly continuation invokes the standard poisson boundary condition of a zero anomaly at an infinite vertical distance from the observation surface this simple continuation is limited however where multiple altitude slices of the anomaly field have been observed increasingly areas are becoming available which are constrained by multiple boundary conditions from surface airborne and satellite surveys this paper describes the implementation of continuation with multi altitude boundary conditions in cartesian and spherical coordinates and investigates the advantages and limitations of these applications continuations by eps equivalent point source inversion and the ft fourier transform as well as by scha spherical cap harmonic analysis are considered these methods were selected because they are especially well suited for analyzing multi altitude data over finite patches of the earth such as those covered by the admap database in general continuations constrained by multi altitude data surfaces are invariably superior to those constrained by a single altitude data surface due to anomaly measurement errors and the non uniqueness of continuation

information from folds a review abstract folds are spectacular geological structures that are seen in layered rock on many different scales to mark 30 years of the journal of structural geology we review the information that can be gained from studies of folds in theory experiment and nature we first review theoretical considerations and modeling from classical approaches to current developments the subject is dominated by single layer fold theory with the assumption of perfect layer parallel shortening but we also review multilayer fold theory and modeling and folding of layers that are oblique to principal stresses and strains this work demonstrates that viscosity ratio degree of non linearity of the flow law anisotropy and the thickness and spacing distribution of layers of different competence are all important in determining the nature and strength of the folding instability theory and modeling provide the basis for obtaining rheological information from natural folds through analysis of wavelength thickness ratios of single layer folds and fold shapes they also provide a basis for estimating the bulk strain from folded layers information about folding mechanisms can be obtained by analysis of cleavage and fabric patterns in folded rocks and the history of deformation can be revealed by understanding how asymmetry can develop in folds by how folds develop in shear zones and how folds develop in more complex three dimensional deformations

extensional and compressional instabilities in icy satellite lithospheres a simple mechanical model of planetary lithospheres and asthenospheres is combined with the most recent experimental results concerning the brittle and ductile deformation of ice i h to determine the plausibility of forming the grooved terrains on ganymede enceladus and miranda by means of lithospheric instabilities the model assumes a lithosphere of uniform strength atop a substrate whose strength decreases exponentially with depth the thickness of the lithosphere is defined by the intersection of a near surface byerlee s law relationship for brittle failure with a standard exponential ductile flow law at depth for ganymede we find that the relatively high surface gravity warm temperatures render virtually unachievable an instability strong enough to account for the observed topographic relief however a sufficiently strong instability capable of producing features with the proper spacing could develop on a smaller colder body such as enceladus or miranda the requirements in this case would be a strain rate greater than about 10 14 sec 1 and a near surface thermal gradient of a few tens of degrees kelvin per kilometer the most reasonable way to accomplish these requirements is by the intrusive emplacement of a thick layer of warm ice beneath a preexisting lithosphere of about a kilometer or so in thickness accompanied almost contemporaneously by a strain event lasting on the order of 10 5 years for extension or 10 3 years for compression however observed properties of grooved terrains on enceladus and miranda such as nonuniform feature spacing and low crater counts force us to conclude that unstable deformation must be more complicated than this simple model would suggest if it is to explain these features

a comparison of geoid undulations for west central greenland the accuracy of a new local gravity field model geoid94a is examined at a site on the western greenland ice sheet the model developed by the danish national survey and cadastre incorporates several new gravity data sets including an extensive amount of airborne gravity data model derived geoid undulations were compared to independently determined undulations found by differencing the elevations from global positioning system controlled airborne laser altimetry and optical leveling surveys differences between the two sets of undulations were less than 6 cm rms the comparison improved 5 cm rms when geoid94a undulations were adjusted by local gravity observations also acquired at the site our comparisons demonstrate that geoid94a adequately models the long to intermediate wavelengths of the gravity field we conclude that geoid94a constitutes a reliable reference model for studies of greenland s gravity field

a spatial domain stokes flow model for the gravity and topography of the middle latitudes of venus a new modeling strategy is developed and applied to the long wavelength gravity topography and internal density structure of venus geodynamic modeling of planetary scale phenomena is usually carried out in the wavenumber transform domain spherical harmonics or fourier transforms however because of limitations associated with the gravity and topography data available for venus a spatial domain approach is preferable smoothed representations of the gravity and topography of venus are developed in terms of quadratic b splines on 10 centers in latitude and longitude over the latitude interval from 45 s to 45 n these models reproduce 83 6 and 98 3 of their respective data variances a similarly constructed model is then presented for the dynamic support of gravity and topography on venus internal mass anomalies are specified at two discrete depths 50 and 500 km with lateral variations determined by two constraints 1 that the slow viscous flow induced by the anomalies will distort the free surface into a form matching the observed topography and 2 that the sum of the gravitational effects of the internal anomalies plus that due to the surface distortion they cause will match the observed gravity there are four major features evident in the gravity and topography data sets atla beta ovda and thetis regiones all four have compensating mass patterns consistent with a mantle plume origin the surficial flow pattern associated with this model does not resemble terrestrial plate tectonic patterns

folds on europa implications for crustal cycling and accommodation of extension regional scale undulations with associated small scale secondary structures are inferred to be folds on jupiter s moon europa formation is consistent with stresses from tidal deformation potentially triggering compressional instability of a region of locally high thermal gradient folds may compensate for extension elsewhere on europa and then relax away over time

folding of europa s icy lithosphere an analysis of viscous plastic buckling and subsequent topographic relaxation abstract potential contractional folds on jupiter s icy moon europa have been identified the best example is at the extensional band astypalaea linea where a series of subtle topographic undulations 25 km in wavelength possess parasitic tectonic features that support a folding origin a scenario has been qualitatively proposed whereby folds form via unstable contraction of the icy lithosphere compensate for extension elsewhere on europa and then subsequently relax here we quantitatively address this scenario applying a model for viscous plastic buckling of planetary lithospheres and finite element simulations of topographic relaxation our results suggest that the lithosphere of europa could indeed be unstable but the low required surface temperatures limit fold formation to higher latitudes and the high required driving stresses 9 10 mpa are difficult to achieve on the satellite the depth to the brittle ductile transition is well constrained and high thermal gradients are indicated implying heat flows near 100 mw m 2 in addition topographic relaxation progresses so slowly even at these heat flows that it is not a viable mechanism to eliminate such features over the age of europa s surface given the paucity of identified folds we conclude that the necessary conditions for their formation are rare and that lithospheric folding is a minor mechanism for compensating the large amounts of extension seen elsewhere on europa

paleomagnetic analysis using squid microscopy 1 superconducting quantum interference device squid microscopes are a new generation of instruments that map magnetic fields with unprecedented spatial resolution and moment sensitivity unlike standard rock magnetometers squid microscopes map magnetic fields rather than measuring magnetic moments such that the sample magnetization pattern must be retrieved from source model fits to the measured field data here we present the first direct comparison between paleomagnetic analyses on natural samples using joint measurements from squid microscopy and moment magnetometry we demonstrate that in combination with a priori geologic and petrographic data squid microscopy can accurately characterize the magnetization of lunar glass spherules and hawaiian basalt the bulk moment magnitude and direction of these samples inferred from inversions of squid microscopy data match direct measurements on the same samples using moment magnetometry in addition these inversions provide unique constraints on the magnetization distribution within the sample these measurements are among the most sensitive and highest resolution quantitative paleomagnetic studies of natural remanent magnetization to date we expect that this technique will be able to extend many other standard paleomagnetic techniques to previously inaccessible microscale samples

minimal crustal magnetizations from satellite data abstract the minimum amplitude crustal magnetization needed to explain magsat vector anomaly data is found using harmonic splines as basis functions shure et al 1982 phys earth planet inter 28 215 229 and choosing a subset of the original basis parker and shure 1982 geophys res lett 9 812 815 to represent magnetization we suggest the optimum model of this type is obtained by associating representers only with north and vertically downwards data points given the larger uncertainties associated with east component magsat data following parker et al 1987 rev geophys 25 17 40 we find a model minimizing the r m s volume integral of magnetization over the magnetized lithosphere the numerical effort involved in producing a regional or continental scale model with given horizontal spatial detail with our method is less than that for an equivalent source dipole model when the magnetization is not assumed induced approximately equal to that for an induced magnetization equivalent dipole model and considerably less than that for a spherical harmonic apparent susceptibility model the method is demonstrated by producing a solution for two regions the contiguous usa for comparison with previously published models and africa and its continental margins magnetization models fit the data slightly worse than those of the downward continued anomaly field at the earth s surface comparison of our component of magnetization for the usa in the direction of the core field with the induced magnetization equivalent dipole model of mayhew 1984 earth planet sci lett 71 290 296 is very good as expected for magnetization either induced by or frozen remanently in an approximately dipole field either parallel or antiparallel to the current rotation axis the east component of magnetization is much smaller than the north and vertically downwards components we make no assumptions about the relative contributions of remanent and induced magnetization and the resulting models have a substantial remanent component however our models suffer from the severe non uniqueness common to all magnetization models whereby magnetization in a uniform susceptibility and thickness shell concentric with the earth s centre owing to any internal magnetic field that is then switched off generates no external magnetic field

the formation of ganymede s grooved terrain numerical modeling of extensional necking instabilities abstract ganymede s grooved terrain likely formed during an epoch of global expansion when unstable extension of the lithosphere resulted in the development of periodic necking instabilities linear infinitesimal strain models of extensional necking support this model of groove formation finding that the fastest growing modes of an instability have wavelengths and growth rates consistent with ganymede s grooves however several questions remain unanswered including how nonlinearities affect instability growth at large strains and what role instabilities play in tectonically resurfacing preexisting terrain to address these questions we numerically model the extension of an icy lithosphere to examine the growth of periodic necking instabilities over a broad range of strain rates and temperature gradients we explored thermal gradients up to 45 k km 1 and found that at infinitesimal strain maximum growth rates occur at high temperature gradients 45 k km 1 and moderate strain rates 10 13 s 1 dominant wavelengths range from 1 8 to 16 4 km post extension our infinitesimal growth rates are qualitatively consistent with but an order of magnitude lower than previous linearized calculations when strain exceeds 10 growth rates decrease limiting the total amount of amplification that can result from unstable extension this fall off in growth occurs at lower groove amplitudes for high temperature gradient thin lithosphere simulations than for low temperature gradient thick lithosphere simulations at large strains this shifts the ideal conditions for producing large amplitude grooves from high temperature gradients to more moderate temperature gradients 15 k km 1 we find that the formation of periodic necking instabilities can modify preexisting terrain replacing semi random topography up to 100 m in amplitude with periodic ridges and troughs assisting the tectonic resurfacing process despite this success the small topographic amplification produced by our model presents a formidable challenge to the necking instability mechanism for groove formation success of the necking instability mechanism may require rheological weakening or strain localization by faulting effects not included in our analysis

initial mapping of mercury s crustal magnetic field relationship to the caloris impact basin seventy eight low altitude passes of mercury surface space environment geochemistry and ranging magnetometer data from august to september of 2014 have been applied to produce an approximate map of the crustal magnetic field at a constant altitude of 80 km covering latitudes of 50 80 n and longitudes of 160 330 e the strongest anomalies are located in the westernmost part of the mapped region just north and northeast of the caloris impact basin the strongest single anomaly group lies over a smooth plains unit that extends northward from caloris a second anomaly lies on the odin formation interpreted as primary caloris ejecta a third broad anomaly also lies over a smooth plains unit suisei planitia although many smooth plains on mercury may have a volcanic origin a subset of these plains is concentrated around caloris and could have a dominantly impact origin it is therefore proposed that many of the mercurian anomaly sources consist of impact basin ejecta materials

global equivalent magnetization of the oceanic lithosphere abstract as a by product of the construction of a new world digital magnetic anomaly map over oceanic areas we use an original approach based on the global forward modeling of seafloor spreading magnetic anomalies and their comparison to the available marine magnetic data to derive the first map of the equivalent magnetization over the world s ocean this map reveals consistent patterns related to the age of the oceanic lithosphere the spreading rate at which it was formed and the presence of mantle thermal anomalies which affects seafloor spreading and the resulting lithosphere as for the age the equivalent magnetization decreases significantly during the first 10 15 myr after its formation probably due to the alteration of crustal magnetic minerals under pervasive hydrothermal alteration then increases regularly between 20 and 70 ma reflecting variations in the field strength or source effects such as the acquisition of a secondary magnetization as for the spreading rate the equivalent magnetization is twice as strong in areas formed at fast rate than in those formed at slow rate with a threshold at 40 km myr in agreement with an independent global analysis of the amplitude of anomaly 25 this result combined with those from the study of the anomalous skewness of marine magnetic anomalies allows building a unified model for the magnetic structure of normal oceanic lithosphere as a function of spreading rate finally specific areas affected by thermal mantle anomalies at the time of their formation exhibit peculiar equivalent magnetization signatures such as the cold australian antarctic discordance marked by a lower magnetization and several hotspots marked by a high magnetization

spherical prism magnetic effects by gauss legendre quadrature integration summary regional spherical coordinate observations of the earth s crustal magnetic field components are becoming increasingly available from shipborne airborne and satellite surveys in assessing the geological significance of these data theoretical anomalous magnetic fields from geologic models in spherical coordinates need to be evaluated this study explicitly develops the elegant gauss legendre quadrature formulation for numerically modelling the complete magnetic effects i e potential vector and tensor gradient fields of the spherical prism we also use these results to demonstrate the magnetic effects for the crustal prism and to investigate the crustal magnetic effects at satellite altitudes for a large region of the middle east centred on iran

crustal magnetisation model of the indian subcontinent through inversion of satellite data abstract an apparent magnetisation model of the indian subcontinent and the adjoining regions has been obtained through the inversion of magsat total field scalar and vertical field data a staggered 4 4 grid pattern with a constant continental crustal thickness of 40 km yielded magnetisation values over a 2 2 grid over the major part of the indian subcontinent the magnetisation picture brings out a prominent high sandwiched between the eastern ghats and the main boundary fault of the himalayas in contrast the identity of the western ghats comprising mainly deccan traps is missing while a distinct magnetisation low is observed over the known hot spot region of the saurashtra peninsula two magnetisation highs one over the shillong massif and the other over the central part of india covering aravallis and the bundelkhand massif are obtained interestingly even at a height of 400 km the well known narmada son lineament is clearly reflected in the total intensity map as well as in the corresponding magnetisation map the bay of bengal and the himalayas ara characterised by magnetisation lows

dynamics of rifting and modes of extension on icy satellites 1 a simple numerical model of extension in icy satellite shells is developed thinning of the ice weakens the shell promoting further extension if lateral flow in the lower part of the shell is unimportant extension is opposed and wide rifts are generated if lateral flow is rapid localized extension is favored and narrow rifts are produced thick shells or high strain rates favor the development of narrow rifts low strain rates favor wide rifting it is proposed that bands extensional features on europa are narrow rifts while groove lanes on ganymede are wide rifts the existence of wide rifting on ganymede is consistent with previous estimates of a conductive shell thickness at the time of rifting of 4 8 km and a strain rate of 10 15 s 1 strain rates at the time of rifting whether this shell thickness applies to present day europa depends on the age of band formation which is poorly known the difference between rifting behavior on ganymede and europa is due to either higher strain rates or higher shell thicknesses on europa during rifting the mean stresses required to cause the observed rifting are 0 2 mpa for ganymede and 0 3 mpa for europa these values are comparable to estimates derived from flexural features index terms 5475 planetology solid surface planets tectonics 8149 6218 planetology solar system objects jovian satellites 8109 tectonophysics continental tectonics extensional 0905 8160 tectonophysics rheology general keywords europa ganymede extension faulting ice

magnetic anomalies in the imbrium and schrodinger impact basins orbital evidence for persistence of the lunar core dynamo into the imbrian epoch approximate maps of the lunar crustal magnetic field at low altitudes in the vicinities of the three imbrian aged impact basins orientale schrodinger and imbrium have been constructed using lunar prospector and kaguya orbital magnetometer data detectable anomalies are confirmed to be present well within the rims of imbrium and schrodinger anomalies in schrodinger are asymmetrically distributed about the basin center while a single isolated anomaly is most clearly detected within imbrium northwest of timocharis crater the subsurface within these basins was heated to high temperatures at the time of impact and required long time periods up to 1 myr to cool below the curie temperature for metallic iron remanence carriers 1043 k therefore consistent with laboratory analyses of returned samples a steady long lived magnetizing field i e a former core dynamo is inferred to have existed when these basins formed the asymmetrical distribution within schrodinger suggests partial demagnetization by later volcanic activity when the dynamo field was much weaker or nonexistent however it remains true that anomalies within imbrian aged basins are much weaker than those within most nectarian aged basins the virtual absence of anomalies within orientale where impact melt rocks the maunder formation are exposed at the surface is difficult to explain unless the dynamo field was much weaker during the imbrian period

forming ganymede s grooves at smaller strain toward a self consistent local and global strain history for ganymede abstract the ubiquity of tectonic features formed in extension and the apparent absence of ones formed in contraction has led to the hypothesis that ganymede has undergone global expansion in its past determining the magnitude of such expansion is challenging however and extrapolation of locally or regionally inferred strains to global scales often results in strain estimates that exceed those based on global constraints here we use numerical simulations of groove terrain formation to develop a strain history for ganymede that is generally consistent at local regional and global scales these simulations reproduce groove like amplitudes wavelengths and average slopes at modest regional extensions 10 15 the modest strains are more consistent with global constraints on ganymede s expansion yet locally we also find that surface strains can be much larger 30 60 in the same simulations consistent with observations of highly extended impact craters thus our simulations satisfy both the smallest scale and largest scale inferences of strain on ganymede the growth rate of the topography is consistent with or exceeds predictions of analytical models and results from the use of a non associated plastic rheology that naturally permits localization of brittle failure plastic strain into linear fault like shear zones these fault like zones are organized into periodically spaced graben like structures with stepped steeply dipping faults as in previous work groove amplitudes and wavelengths depend on both the imposed heat flux and surface temperature but because our brittle strength increases with depth we find for the parameters explored that the growth rate of topography is initially faster for lower heat flows we observe a transition to narrow rifting for higher heat flows and larger strains which is a potential pathway for breakaway margin or band formation

reevaluation of magnetic chrons in the north atlantic between 35 n and 47 n implications for the formation of the azores triple junction and associated plateau 1 in this paper we present a new magnetic compilation for an area of the north atlantic located between 35 n and 47 n and up to anomaly 33r we also present a strategy to pick magnetic isochrones and compute finite rotation poles this technique is based on a continuous reduction to the pole technique and some basic assumptions regarding the direction of the remanent magnetization vector a cost function that measures the misfit between interpreted and rotated isochrones and the systematic exploitation of the parameter space is used to compute the best set of finite eulerian rotations for the chrons 5 6 6c 11 12 13 18 20 21 22 23 24 25 26 28 29 30 32 33 and 33r this set of chrons and poles is used to discuss the evolution of the north atlantic close to iberia and in particular the onset and early development of the azores triple junction area we show that the relative motion between the eurasian and the african plates can be coherently described in terms of rigid plate kinematics respecting both the anomalies shapes and the precise location of the main structural elements of the area the pico fracture zone the east azores fracture zone and the gloria fault we distinguish from the magnetic point of view two different areas of the azores plateau the south azores domain where almost undisturbed nnw magnetic lineations can be found and the azores domain close to the topographic highs and with no systematic magnetic stripping with the exception of a few recent lineations probably matuyama and brunhes we present an approximate reconstruction of the plate configuration after chron 18 to conclude that the attachment of iberia to eurasia was younger than previously thought lower miocene triggering the formation of the azores domain in which stretching took place essentially in the last 20 ma at an average rate of 3 8 mm a and progressively attaching the south azores domain to the african plate by a northward progression of the triple junction

predicted crater morphologies on ceres probing internal structure and evolution the detailed internal structure of the dwarf planet ceres target of nasa s dawn mission has not been unequivocally determined from ground based data whereas ceres is most likely differentiated with a near surface ice layer tens to one hundred kilometers thick the possibility of a homogenous ice poor interior structure cannot be completely eliminated these two internal structural end members have profoundly different implications for ceres origin and evolution here we demonstrate that observations of ceres impact craters by the dawn spacecraft will permit unambiguous distinction between the two internal structural models using finite element simulations of crater relaxation we show that if ceres does contain a water ice layer its relatively warm diurnally averaged surface temperature ensures extensive viscous relaxation of even small impact craters at likely equatorial temperatures craters as small as 4km in diameter can be relaxed to the point where complete crater erasure is plausible thus decreasing the overall crater density in the equatorial region at mid latitudes crater relaxation is less extensive but still sufficient to completely relax all craters older than 10 ma and larger than 16 km in diameter as well as smaller ancient craters only in ceres cold polar regions are some crater morphologies expected to be pristine in contrast if ceres is primarily a rocky body we expect crater relaxation to be negligible these basic conclusions are generally independent of ice grain size salt dust contamination of the ice the presence of a thin undifferentiated ice rock crust and the total thickness of the ice layer all of which produce second order modifications to the relaxation process that can be used to better constrain such ice layer properties thus the morphology of impact craters on ceres as revealed by the dawn spacecraft will provide direct insight into the internal structure and evolution of the dwarf planet

regional magnetic and gravity anomalies of south america preliminary satellite scalar magnetic anomaly data magsat reduced to vertical polarization and long wavelength pass filtered free air gravity anomaly data of south and central america are compared to major tectonic features a number of correlations are observed but these must be generalized because of the preliminary nature of the geophysical data and the inherent petrophysical variations within tectonic features statistical analysis of the magnetic data reveals that south and central america are more magnetic and magnetically more variable than adjacent marine areas more obvious correlations exist between magnetic anomalies and tectonic elements of the continents than in the case of oceanic areas no obvious correlations occur between the tectonic features of the atlantic ocean including the mid atlantic ridge and magnetic anomalies the continental shields generally are more magnetic than adjacent basins oceans and orogenic belts in contrast the major aulacogens are characterized by negative magnetic anomalies positive free air gravity anomalies are related to the andean foldbelt but the relationship of this feature to magnetic anomalies is much less obvious however along the west coast of south america the magnetic anomalies of the pacific ocean are separated from those of the eastern platforms by north to northwest trending anomalies south of the equator along the foldbelt gravity maxima are related to magnetic minima a relationship analogous to the situation observed in the rocky mountain cordillera north of the equator in columbia gravity and magnetic maxima roughly correlate along the foldbelt