prospects for an ancient dynamo and modern crustal remanent magnetism on venus abstract venus lacks an internally generated magnetic field today whether one existed in the past is unknown but critical to atmospheric evolution and potential habitability canonical models assume the core of venus is cooling too slowly for convection and thus a magnetic dynamo to occur today core mantle heat flow is suppressed in these models after a putative transition in mantle dynamics associated with widespread volcanic resurfacing however recent studies of impact craters and other surface features support more steady heat loss over geologic time precipitation of mgo and or sio2 from the core can also drive compositional convection even with slow cooling here we reevaluate the likelihood that venus has an earth like at least partially liquid and chemically homogeneous core using numerical simulations of the coupled atmosphere surface mantle core evolution an earth like core is only compatible with the modern lack of a dynamo if the thermal conductivity of core material is towards the higher end of modern estimates i e 100 w m 1 k 1 if lower estimates like 40 50 w m 1 k 1 are actually correct then we favor recent proposals that the core has completely solidified or preserved primordial stratification any simulation initialized with a homogeneous liquid core predicts a global magnetic field with earth like surface strength for 2 3 billion years after accretion consistent with all available observations and also sporadic activity within the surface age while temperatures remain below the curie point of magnetite therefore future spacecraft missions should prioritize the first ever magnetometer measurements below the ionosphere to search for crustal remanent magnetism

magnetization modeling in the north and equatorial atlantic ocean using magsat data we have analysed the shorter wavelength component of the magnetic field observed by magsat over the north and equatorial atlantic ocean with the idea of trying to determine the possible sources of this component of the field in most cases inversion techniques produce both normally and reversely magnetized sources and we show that this is not appropriate for the possible sources in the oceanic crust which at the wavelengths observed by magsat need to be entirely normally magnetized or with a magnetization of zero in order to do this we have applied an annihilator to the solutions produced by our inversion technique we show how the annihilator that we use is convenient for matching up magnetization solutions for areas that have been inverted separately one problem with the inversion technique is that the best spacing of the sources can only be found by trial and error we show that much of the final variation in source spacing is produced by a latitudinal effect which seems to require sources spaced further apart the lower the latitude this may be due to contamination of the anomalous field by noise which might be expected to be worse at lower latitudes we have also examined the possibility that much of the magnetic source in the oceanic crust is due to induced magnetization the values of susceptibility necessary are much higher than those measured directly on samples taken from the ocean basins remanent magnetization is a more likely candidate but the values especially after application of the annihilator are still large compared with observed values only considerable thickening of the crust combined with remanent magnetization can explain the high intensities of magnetization that we have modeled viscous magnetization is a possible source for the high intensities of magnetization of our model one problem with an explanation of the field from crustal sources is that in many cases there is little correlation with the geological history of the region an alternative is that noncrustal sources contribute to the anomalous magnetic field derived from magsat data this field derived by subtracting degrees of harmonic 1 through 13 from a spherical harmonic model may be contaminated with core field signals from leakage of high amplitude short wavelength anomalies into longer wavelengths and by external fields

the relationship between quaternary volcanism in central mexico and the seismicity and structure of subducted ocean lithosphere late quaternary volcanism in central mexico is related to the subduction of young ocean lithosphere at the middle america trench along arc variations in seismicity volcano structure and composition of volcanic products bear a remarkable correlation with the age and structural framework of the downgoing slab morphological and petrographic characteristics of major volcanoes within the trans mexican volcanic belt tmvb serve to distinguish two calc alkaline subprovinces 1 a western arc averaging 60 km in width associated with aseismic subduction of the rivera plate the main cones of this region are dominated by two pyroxene andesites comprise volumes 70 km 3 and stand less than 3 000 m above sea level 2 a broad central and eastern arc related to subduction of a gently inclined segment of the cocos plate bounded by the rivera transform and the tehuantepec ridge major volcanic edifices possess summit elevations in the range 4 000 to 6 000 m have appropriately larger volumes typically 200 km 3 and are constructed with a high proportion of amphibole bearing lavas the boundary between these subprovinces is marked by a north south oriented structural depression the colima graben and it coincides with a 100 km offset in the volcanic front extensional tectonism in the colima graben accompanied by mixed calc alkaline and alkaline volcanism of potassic affinity is likely related to a hinge type transform fault which marks the cocos rivera plate juncture in the downgoing slab a third segment of ocean floor is presently interacting with continental lithosphere south of the gulf of tehuantepec where quaternary volcanism is weakly developed within a tectonically complex region that marks the diffuse cocos noam caribbean triple junction the northern limit of this triple junction is defined by the seismically active isthmus fault which may be related to alkaline volcanic activity at san andres tuxtla a tectonic reconstruction based on the evolution of oceanic crust reveals that the distribution of intermediate depth earthquakes along the arc is directly dependent upon the age of the subducted slab ocean lithosphere younger than approximately 20 m y is subducted aseismically at convergence rates approaching 9 cm yr the length of the inclined seismic zone indicates that the time constant for thermal relaxation in the slab is approximately 4 m y the tmvb overlies the aseismic extension of this young ocean lithosphere several aspects of this study have a bearing on the segmented nature of converging margins in general 1 the tectonic evolution of the ocean floor may determine the nature of segmentation at the site of subduction 2 the complete record of volcanism in the tmvb over the past million years can be related to the present plate configuration 3 alkaline and calc alkaline volcanism have developed contemporaneously at a converging plate margin 4 lineaments in volcanic arcs may reflect the structural complexity of the crust rather than segment boundaries in the subducted slab

geological modeling of the new champ magnetic anomaly maps using a geographical information system technique 1 reliable global crustal field anomaly maps produced from magnetic measurements of the champ satellite mission now allow for quantitative geological studies of crustal structure and composition we have developed a gis based forward modeling technique to model these anomaly maps on the basis of the geologic and tectonic maps of the world laboratory susceptibility values of the occurring rock types and the seismic thickness of the crust a vertically integrated susceptibility grid is generated in the gis system in addition a remanent magnetization grid is computed for the oceanic crust using a digital isochron map of the ocean floor and rotation models of the paleoplates combining the global vis and remanent magnetization grids the vertical magnetic field anomaly is computed at satellite altitude and compared with the corresponding champ magnetic anomaly map over the oceans induced and remanent magnetization explains well the prominent observed anomalies over the cretaceous quiet zones we also find a good agreement between predicted and observed anomalies over the continents remaining discrepancies between the predicted and observed anomalies can be used to adjust poorly known boundaries and the composition of the buried precambrian provinces until the recomputed anomalies fit the observed anomalies the feasibility of this approach is demonstrated on greenland the west african craton bangui in central africa and the kolyma omolon block in siberia we conclude that quantitative information on the lateral extent the composition and the thickness of the lower crust within a precambrian province can thus be inferred from the new satellite magnetic anomaly maps

an interpretation of magnetic signatures of subduction zones detected by magsat abstract the magnetic anomalies detected by magsat over subduction zones are related to the thermal state and the location with respect to the geomagnetic poles of the subducting oceanic lithosphere it is shown that an old about 100 m y subducting lithosphere produces about twice as strong a magnetic anomaly as that associated with a young about 40 m y one also a given subducting oceanic lithosphere produces a stronger magnetic anomaly at high geomagnetic latitudes than near the geomagnetic equator based on the thermal evolution models of the oceanic lithosphere and assuming a mean magnetic susceptibility of 13 10 3 si the pacific plate subducting along the kuril trench produces a magnetic anomaly similar to the observed one the nazca plate subducting along the peru chile trench produces a magnetic anomaly of less than 0 5 nt no distinct anomaly observed by magsat over this region

surface modification of venus as inferred from magellan observations of plains in sedna planitia clear stratigraphic relations can be discerned among volcanic flow units young flows exhibit sar specific cross section values similar to fresh terrestrial basalt flows whereas older flows exhibit backscatter signatures similar to degraded terrestrial basalt flows total degradation of 1 m depth over 0 6 b y is inferred for the sedna area from radar signatures impact crater abundances and ejecta superposition relations with respect to volcanic flow units analyses of parabolic ejecta deposits associated with the crater stuart imply that the material is typically centimeters in thickness a relatively small fraction 10 of venusian impact craters exhibit prominent parabolic ejecta deposits these craters are interpreted to be relatively young and parabolic deposits are interpreted to be dispersed by aeolian activity over at least tens of millions of years the inferred dispersal rate 10 3 m yr is too low to produce the degradation of flows at sedna planitia and it is concluded that the dominant flow modification process is in situ weathering in addition elevation dependent weathering is inferred in western ovda regio where plains above 6054 km radius have enhanced reflection coefficients as compared to adjacent plains at lower elevations the inferred rate of generation of high reflection coefficient materials is no more than 10 2 m yr based on the inability of aeolian activity to cover high reflectivity surfaces with normal reflection coefficient materials and the ubiquitous nature of high reflectivity surfaces at high elevations surface modification rates on venus are orders of magnitude lower than on earth venusian rates are also much lower than the inferred rate of aeolian dispersal of friable materials on mars but are comparable to the estimated rate of weathering and erosion of martian bedrock low surface modification rates imply that it will be possible to determine regional scale age variations on venus based on the degree of preservation of volcanic landforms and microwave signatures

volcanic degassing of argon and helium and the history of crustal production on venus we develop a new methodology linking 40ar and 4he degassing to crustal production on venus in order to examine two different scenarios for the history of magmatism on that planet both consistent with recent analyses of the cratering record of the venus surface the first scenario includes episodic global resurfacing events and modest levels of magmatism between such events while the second scenario invokes different rates of steady magmatism before and after a given transition time our degassing models include distinct mantle crustal and atmospheric reservoirs diffusive transfer of 40ar and 4he from the crust to the atmosphere is also taken into account the 40ar abundance in the present atmosphere reflects the integrated degassing and magmatism over most of planetary history in contrast helium escapes from the planetary atmosphere within a characteristic residence time of 200 myr to 1 8 gyr so the present atmospheric 4he abundance is relevant to the volume of magma produced during the last global resurfacing event and the rate of the magmatism subsequent to that event under the first scenario and to the two steady rates of magmatism and the transition time under the second unfortunately large uncertainties in mineral partition coefficients and in the ar and he mixing ratios in the lower atmosphere of venus presently prevent the use of our degassing models to distinguish among crustal formation history models we therefore explore the influence of these uncertain parameters on the degassing history in order to identify those new laboratory and in situ measurements that will most strongly constrain crustal production history as an important step in the development of the ar degassing model the k budget in the bulk silicate fraction of venus is re examined on the basis of venera and vega ray measurements of k u and th concentrations in surface materials for u and th concentrations in the bulk silicate planet of 18 29 ppb and 64 94 ppb respectively the degree of mantle melting that formed the surface materials at the venera 9 and 10 and vega 1 and 2 landing sites is calculated to lie between 0 02 and 0 16 the degree of melting calculated for materials at each landing site is combined with measurements of surface k concentration to estimate the k concentration in the bulk silicate portion of the planet at between 100 and 300 ppm these results suggest that venus and earth have similar heat production

long wavelength magnetic anomalies in the caribbean plate boundaries and allochthonous continental blocks vector magnetic anomaly maps are derived at satellite altitude from magsat data over the caribbean and middle america to minimize smoothing of the anomaly field caused by averaging the data over the whole altitude range of magsat measurements two maps are derived in two nonoverlapping altitude ranges 350 420 and 420 550 km a composite map of the vertical component of the anomaly field is then computed at ground level this map displays large positive anomalies associated with fast subduction zones these anomalies define discontinuities in the downgoing slab associated with seismic gaps positive anomalies are also consistently observed above allochthonous blocks of continental origin in the caribbean area and suggest that the average magnetization is larger in the continental lithosphere elongated magnetic lows are observed above transform boundaries of the caribbean plate the cayman trough anomaly is consistent with magnetization of the order of 1 a m in the upper mantle demagnetization of the lithosphere along the bocono el pilar fault system which bounds the caribbean plate to the south may be accounted for by frictional heating along shear zones the pattern of anomalies along the southern margin of the united states is related to its accretionary tectonic history various estimates of the vertically integrated magnetization of both oceanic and continental lithospheres are computed using these anomalies and provide constraints on the vertical distribution of lithospheric sources of magsat anomalies

contribution of lithospheric remanent magnetization to satellite magnetic anomalies over the world s oceans regional studies have shown that remanent magnetization of the cretaceous quiet zones cqzs created during the long period of geomagnetic normal polarity at 118 84 ma produce well defined magnetic anomalies at satellite altitudes we investigate the effects of the remanent magnetization of the oceanic lithosphere on satellite magnetic anomalies on a global scale we consider entire oceanic areas because oceanic lithosphere formed during periods other than cqz but with a predominant polarity also have appreciable contributions to the satellite anomalies the magnetic anomalies of the world s oceans are calculated from a distribution of vertically integrated remanent magnetization that is computed using age map plate relative motions and the apparent polar wander path of africa three magnetization models are examined thermoremanent magnetization confined to extrusive layer 2a and thermoviscous remanent magnetization of the crust and uppermost mantle down to a maximum depth of 12 km or 30 km although all models lead to rather similar anomaly distributions the amount of magnetization required suggests the need for deeper sources all models produce the satellite anomalies associated with the cqz in the north atlantic and parts of the indian and pacific oceans with slight differences in location depending on the models low amplitude observed anomalies associated with areas created at fast and intermediate spreading rates in the indian and pacific oceans are successfully modeled a major difference between models and observation is the north south elongated model anomalies associated for instance with the cqzs in the south atlantic and the fast spreading east pacific rise these elongated anomalies are systematically absent on the observed map probably because they were removed by along track filtering in the early stages of processing the satellite data a careful comparison of the model anomalies with observation favors model with thermoviscous magnetization down to 12 km and saturation magnetizations of 4 0 1 and 0 6 a m for the extrusive basalts intrusive basalts gabbros and peridotites respectively

intermediate and deep seismicity and lateral structure of subducted lithosphere in the circum pacific region in this paper we present a region by region review of the wadati benioff zone structure of most of the world s seismically active subduction zones focusing primarily on the intermediate and deep seismicity lateral changes in wadati benioff zone structure are common in every major subduction zone in this study we use these changes to define possible boundaries between portions or segments of lithosphere with differing subduction geometries although earthquake data seldom have the resolution to show conclusively whether these boundaries separate independent blocks of lithosphere the available data indicate that the active process at most of these segment boundaries is ductile deformation of the subducting plate rather than tearing we found the strongest evidence for the existence of tears where tears are geometrically necessary such as where a transform boundary terminates a trench as at the new hebrides tonga and south sandwich trenches weak evidence suggesting other tears does exist in some regions such as taiwan the japan izu bonin corner and the philippines the causes of these changes in structure are in most cases unclear only about 34 of the possible segment boundaries coincide with subducting bathymetric features some boundaries occur where there is apparent lateral strain caused by anomalous trench geometry we have designed a simple modeling procedure which incorporates published plate motion and the observed geometry of trenches and wadati benioff zones to estimate the lateral strain in subduction zones throughout the circum pacific region although no observed subduction zone has a perfectly strain free geometry there is a broad range of geometries for which the lateral strain is small indeed the observed geometry of most subduction zones involve relatively little lateral strain comparison with centroid moment tensor focal mechanisms indicates that in zones where the modeling predicted little lateral strain the mechanisms of intermediate and deep earthquakes show no effects of lateral stress and downdip stresses are clearly dominant in regions such as the mariana arc where the model predicts very large lateral extension lateral tension is very evident in the focal mechanisms in regions such as the hokkaido corner where the modeling predicts large compressional strains the plate appears to buckle and bending stresses parallel to the trench are evident in general this study finds that subducted lithosphere is remarkably cohesive and rigid and only rarely deforms by breaking or stretching

kinematic evolution of the northern cocos plate as the east pacific rise approached and impinged upon the western margin of north america the farallon plate began to break into multiple smaller plates whose relative movements changed strongly from those before breakup movements of the northern cocos plate inferred from magnetic lineations show rapidly changing rates and pole positions which may be characteristic of other small remnants of the farallon plate finite difference poles and rates for cocos pacific relative motion during the time intervals 2 5 5 7 7 10 and 10 14 m y have been determined from the geometry of magnetic isochrons prior to about 2 m y the cocos pacific pole was 10 to 20 south of its present location and angular rates were 2 4 times the present rate from 10 7 m y we deduce a large degree of oblique convergence between the cocos and north american plates which may be an important constraint on the evolution of the structure of the continental margin of southwestern mexico the tehuantepec ridge originated as the ridge ridge transform which generated the clipperton fracture zone spreading rate estimates for the southern part of the cocos plate suggest the possibility that the tehuantepec ridge was reactivated as a ridge trench transform between the two cocos plate fragments prior to about 7 m y

effect of a single large impact on the coupled atmosphere interior evolution of venus abstract we investigate the effect of a single large impact either during the late veneer or late heavy bombardment on the evolution of the mantle and atmosphere of venus we use a coupled interior exterior numerical code based on stagyy developed in gillmann and tackley gillmann c tackley p j 2014 j geophys res 119 1189 1217 single vertical impacts are simulated as instantaneous events affecting both the atmosphere and mantle of the planet by i eroding the atmosphere causing atmospheric escape and ii depositing energy in the crust and mantle of the planet the main impactor parameters include timing size mass velocity and efficiency of energy deposition we observe that impact erosion of the atmosphere is a minor effect compared to melting and degassing triggered by energy deposition in the mantle and crust we are able to produce viable pathways that are consistent with present day venus especially considering large late veneer impacts small collisions

the downward continuation of magsat crustal anomaly field over southeast asia a general method for the local downward continuation at ground level of geopotentials measured by low orbiting satellites at different altitudes is proposed such a continuation is the inverse solution of the external dirichlet problem the stability of the solution with respect to the altitude and the distribution of data points is discussed principal component analysis is shown to perform well and leads to a stable solution the method proposed has the advantage of being directly applicable to measured data points which are not evenly spaced and at different altitude as an illustration it is applied to magsat data over southeast asia two independent maps of the vertical component of the anomaly field at a constant altitude near ground level are derived for dawn descending orbits and dusk ascending orbits data separately the two maps reveal similar anomalies and confirm the correlation of the magsat derived anomaly field with the mosaic structure of the continental crust in southeast asia stable continental blocks are associated with positive anomalies and intervening collision belts correspond to well defined negative anomalies a very prominent low is observed above the himalaya and extends over the tibet plateau confirming an anomalous crustal and thermal structure of the plateau

satellite magnetic anomalies over subduction zones the aleutian arc anomaly positive magnetic anomalies seen in magsat average scalar anomaly data overlying some subduction zones can be explained in terms of the magnetization contrast between the cold subducted oceanic slab and the surrounding hotter nonmagnetic mantle three dimensional modeling studies show that peak anomaly amplitude and location depend on slab length and dip a model for the aleutian arc anomaly matches the general trend of the observed magsat anomaly if a slab thickness of 7 km and a relatively high induced plus viscous magnetization contrast of 4 a m are used a second source body along the present day continental margin is required to match the observed anomaly in detail and may be modeled as a relic slab from subduction prior to 60 m y ago

fate of mantle plume material trapped within a lithospheric catchment with reference to brazil 1 local regions of thin lithosphere act as catchments of hot buoyant plume material unless replenished the trapped plume material cools by convection to the mantle adiabat by several tens of million years in particular currently hot material from the 130 ma parana starting plume head is unlikely to supply 85 ma to recent volcanism on the mainland of brazil and the martin vaz and fernando hot spots rather a plume tail may now underlie southern brazil where tomographic studies detect conduit shaped velocity anomaly through the upper mantle if the tomographic study in fact found a plume tail the track crossed the amazon rift at 85 ma and since then lateral flow along thin regions of the lithosphere from the tail fed widespread feeble volcanism including the flow line hot spots of martin vaz and fernando

high temperature viscous magnetization of oceanic deep crustal and mantle rocks as a partial source for magsat magnetic anomalies the magsat data have yielded a global cartography of long wavelength anomalies of lithospheric origin the magnetizations needed to model these anomalies are much higher than those measured on samples by laboratory experiments an enhancement of both induced and viscous magnetization at high temperature has often been put forward to account for the missing magnetization experimental measures of viscous magnetization mv at high temperature are presented for oceanic deep crustal and mantle rocks at room temperature mv hereafter defined in experiments as the magnetization acquired after 400 s in a weak field is negligible in comparison with the room temperature induced magnetization mk0 at higher temperatures between 200 c and 450 c mv represents about 20 of mk0 and is further enhanced near 500 c ranging from 25 to 130 of mk0 an extrapolation of these results to geological times shows that the viscous magnetization acquired since the last reversal of the earth magnetic field may be of the order of magnitude of the induced magnetization mk0 above 200 c and possibly predominant near 500 c however a magnetic uppermost mantle is still needed to explain the magnetizations of oceanic lithosphere derived from magsat data

why no anomaly is visible over most of the continent ocean boundary in the global crustal magnetic field abstract it is generally believed that an anomaly should be expected over the continent ocean c o boundary in the global magnetic anomaly maps however such maps prepared at satellite altitude do not show an anomaly over most of the c o boundary we address this issue by a forward modelling technique we use a geographic information system gis technique to integrate information of the various geological units of continents and oceans the seismic crustal structure and standard susceptibility values of the rock types and compute a global vertically integrated susceptibility vis model in addition a remanent magnetisation model is used for the oceanic crust combining the induced and remanent magnetisation models the crustal field anomaly is predicted at a satellite altitude of 400 km for spherical harmonic degrees 16 80 the results show that an anomaly is indeed not predicted over the c o boundaries where the oceanic crust is flanked by continental regions phanerozoic in age we demonstrate that largely a c o anomaly over these regions is not even expected in the global magnetic anomaly map for degrees 1 80 confirming that it is not a long wavelength feature masked by the core field an anomaly however is predicted over some parts of the c o boundary where the flanking geological provinces over the continents are precambrian in age we argue that the vis values for the continents and oceans are largely comparable and therefore no anomaly is expected over most of the c o boundary

the global continent ocean magnetization contrast abstract a schematic global model for the distribution of magnetization in the lithosphere is devised this model is characterized by a uniform stepwise change of radially integrated susceptibility at continent ocean boundaries and an axial dipolar inducing field a spherical harmonic expansion of the magnetic field produced by this source distribution is computed then the standard method of truncation at degree n 13 is used to determine the respective contribution of this field to the main field and to the lithospheric anomaly field the latter is shown to display significant anomalies near continental margins as well as within continents and oceanic basins the energy density spectrum of this field is shown to be very similar to the spectrum of the magsat derived lithospheric anomaly field furthermore several features of this field are shown to account for unexplained anomalies of the magsat field this supports the hypothesis that such an abrupt magnetization variation may occur at several continental margins and contribute to the signal in the magsat derived long wavelength anomaly maps this hypothesis provides an order of magnitude estimate for the magnetization difference between oceanic and continental lithospheres finally it is argued that the synthetic anomaly field produced by this model provides an upper bound to the energy density spectrum of the lithospheric field at low harmonic degrees and concluded that the lithospheric field does not contribute significantly to the earth s field below degree 13

viscous remanent magnetization model for the broken ridge satellite magnetic anomaly a range of crustal models for the interpretation of the broken ridge satellite magnetic anomaly have been constructed from bathymetric data the source magnetizations were assumed to be constant in direction throughout the source region and parallel to the present earth s field at the center of the source region the model calculations were made at the observation points of a selected set of magsat investigator b passes with a variable magnetic field direction calculated from the mgst 4 81 field model the horizontal perimeter of all the models was defined by the departure of the broken ridge bathymetry from an average oceanic depth of 4 5 km a topographic model with an upper surface defined by the bathymetry of the ridge and a lower surface defined by the average oceanic depth requires a magnetization of 42 a m to account for the observed anomaly amplitude an airy type isostatic model with an upper surface defined by the bathymetry of the ridge and a lower surface predicted from the bathymetry requires a magnetization of 6 a m to account for the observed anomaly amplitude a midcrust model with an upper surface at the average oceanic depth and a lower surface at the average oceanic moho requires a magnetization of 13 a m to account for the observed anomaly amplitude a comparison of the locations of the model and observed anomaly peaks shows that the broken ridge anomaly can be modeled assuming present field directions there is not sufficient sensitivity in the data to distinguish between the models on the basis of anomaly location the model magnetizations vary from 6 a m where the whole crust is uniformly magnetized to 42 a m where the topographic expression of the ridge is the source of the anomaly the strength of the required magnetization contrasts cannot be accounted for by induced magnetization if normal oceanic crustal materials are assumed to be the source for the observed anomaly measured and modeled remanent magnetizations of seafloor rocks are typically within the lower part of the range of required values but the higher magnetizations are associated with very young rocks and dredge samples it is concluded that the source magnetizations have been enhanced by the development of a viscous remanence effect since the last reversal of the earth s field

chemical dieferentiation of a convecting planetary interior consequences for a one plate planet such as venus partial melting to generate the crust of a planet creates compostionally buoyant residual mantle in the absence of mantle flow associated with plate tectonics this buoyant refractory layer may collect at the top of the mantle with important implications for the evolution of the interior and surface in this study models of the thermal and chemical evolution of a planetary interior demonstrate the possible consequences of a chemically buoyant depleted mantle layer as the depleted layer thickens the melting temperature at the top of the underlying convecting mantle also increases and the degree of partial melting of mantle added to the depleted layer decreases as less depleted mantle with less positive compositional buoyancy is added negative thermal buoyancy of the layer eventually exceeds its positive compositional buoyancy the depleted layer then sinks into and mixes with the convecting interior the top of the convecting mantle then moves to a shallower depth larger degrees of melting resume and a new depleted layer accumulates this accumulation and instability of the depleted layer occurs repeatedly over a substantial portion of the planet s evolution with a period of 300 500 myr on venus the population of impacts craters is indistinguishable from a random distribution over the surface and give a surface age of about 500 myr we speculate that the mechanism described above may explain this episodic global resurfacing of venus

the bulk magnetization contrast across the ocean continent boundary in the east coast of north america summary the intermediate wavelength 600 3000 km magnetic anomalies of earth derived from satellite data indicate that both continental crust and oceanic lithosphere are magnetic however no well defined and consistent magnetic anomaly is associated with the ocean continent boundary emphasizing that the bulk magnetization the vertically integrated magnetization contrast of oceanic lithosphere and continental crust is not appreciable an upper limit of 37000 a is estimated for the bulk magnetization contrast using the low altitude aeromagnetic and marine magnetic data from across the east coast of north america

new global vector magnetic anomaly maps derived from magsat data new global maps of the vector magnetic anomaly field of earth are derived from magsat data improvement on earlier published vector and scalar maps is achieved with the following procedure 1 the noise level of vector measurements associated with the discontinuities of the in flight sensor attitude determination is reduced by selecting the data 2 a monthly model of the main field is computed to account for secular variation using only quiet day data 3 the time dependence of the ring current effect is taken into account using hourly values of the dst index based on ground obseivations 4 sectorial indices based on ground observations are introduced to select profiles corresponding to low magnetic activity these indices provide a better discrimination of quiet data than the global kp index usually used for that purpose 5 a time and longitude dependent dynamic model of the equatorial electrojet is derived and used to correct magnetic measurements at low latitudes anomaly maps of x and z components are obtained using selected measurements within the reduced altitude range 350 425 km these maps are compared with earlier magsat anomaly maps and their respective resolutions are discussed these maps evidence large anomalies of lithospheric origin within the oceanic domain in particular the z map displays a characteristic pattern of long wavelength anomalies in these areas it is first shown that some of these anomalies particularly in the north atlantic and indian oceans may be correlated with the age of the lithosphere then a significant similarity between magnetic and geoid anomalies above ocean basins is evidenced this suggests that long wavelength oceanic magnetic anomalies may also be attributed to variations of the thickness of the magnetized layer associated with ocean floor topography

seismicity and tectonics of the subducted cocos plate we have examined teleseismic earthquake locations reported by the international seismological centre isc for the middle america region and selected 220 as the most reliable these hypocenters and other data are used to delineate the deep structure of the subducted cocos plate the results indicate that the subducted plate consists of three major segments segment i extends from the panama fracture zone to the nicoya peninsula the structure of this segment is poorly defined segment ii is the largest and best defined segment this segment consists of two parts iia and iib part iia extends from the nicoya peninsula to western guatemala and is very well defined and continuous in structure its strike follows the curvature of the trench and dips at about 60 part iib extends from western guatemala to orizaba mexico the dip of this part of the segment decreases slightly toward the northwest and its strike is more northward than that of the trench segment iii extends from orizaba to the rivera fracture zone and is not well defined due to a lack of earthquake activity beneath about 100 km its orientation differs markedly from segment ii and strikes somewhat more westward than the trench between parts iia and iib of segment ii the subducted plate seems to be continuous bending smoothly to accommodate the change in geometry local network data from costa rica suggest there may be a tear between segments i and ii between segments ii and iii there is a gap in the hypocenters which makes it difficult to define the boundary the change in geometry between these two segments indicates that there may be a tear and two strike slip focal mechanisms in the region support this conclusion we find no convincing evidence supporting the existence of segments smaller than the three described above if there is smaller scale segmentation in the shallow part of the subducting plate the plate must still maintain enough continuity to appear continuous at greater depths there is no evidence for any major tear in the subducted plate associated directly with either the tehuantepec ridge or the orozco fracture zone the shallow subduction at the northwestern end of segment ii may be related to the bouyancy of the tehuantepec ridge the cocos ridge is probably directly responsible for the change in geometry between segments i and ii and may even be slowing or stopping subduction in segment i the structure of the subducted plate in segment ii and the changes in the character of volcanism along the arc can be related to the relative motion of the north american and caribbean plates the present geometry of part iib of segment ii is more consistent with the probable configuration of the trench about 7 ma ago than with the present configuration indicating that the north american plate is overriding the subduction zone appendices 2 3 and 4 are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 009 2 50

spatial gaps in arc volcanism the effect of collision or subduction of oceanic plateaus abstract one of the major processes in the formation and deformation of continental lithosphere is the process of arc volcanism the plate tectonic theory predicts that a continuous chain of arc volcanoes lies parallel to any continuous subduction zone however the map pattern of active volcanoes shows at least 24 areas where there are major spatial gaps in the volcanic chains 200 km a significant proportion 30 of oceanic crust is subducted at these gaps all but three of these gaps coincide with the collision or subduction of a large aseismic plateau or ridge the idea that the collision of such features may have a major tectonic impact on the arc lithosphere including cessation of volcanism is not new however it is not clear how the collision or subduction of an oceanic plateau perturbs the system to the extent of inhibiting arc volcanism three main factors necessary for arc volcanism are 1 source materials for the volcanics either volatiles or melt from the subducting slab and or melt from the overlying asthenospheric wedge 2 a heat source either for the dehydration or the melting of the slab or the melting within the asthenosphere and 3 a favorable state of stress in the overlying lithosphere the absence of any one of these features may cause a volcanic gap to form there are several ways in which the collision or subduction of an oceanic plateau may affect arc volcanism the clearest and most common cases considered are those where the feature completely resists subduction causing local plate boundaries to reorganize this includes the formation of new plate bounding transform faults or a flip in subduction polarity in these cases subduction has slowed down or stopped and the lack of source material has created a volcanic gap there are a few cases most notably in peru chile and the nankai trough where the dip of subduction is so shallow that effectively no asthenospheric wedge exists to produce source material for volcanism the shallow dip of the slab may be a buoyant effect of the plateau imbedded in the oceanic lithosphere the cases which are the most enigmatic are those where subduction is continuous the oceanic plateau is subducted along with the slab and the dip of the slab is clearly steep enough to allow arc volcanism yet a volcanic gap exists in these areas the subducted plateau may have a fundamental effect on the physical process of arc volcanism itself the presence of a large topographic feature on the subducting plate may affect the stress state in the are by increasing the amount of decoupling between the two plates alternatively the subduction of the plateau may change the chemical processes at depth if either the water rich top of the plateau with accompanying sediments are scraped off during subduction or if the ridge is compositionally different

solubility of water in magmas to 2 kbar high water concentrations 4 6 wt are found in the quenched glass inclusions of phenocrysts of explosive silicic and intermediate eruptions accordingly we have measured the solubility of water in a wide range of natural liquids up to 2 kbar with data from the literature we have developed a simple thermodynamically based empirical model for the solubility of water as a function of temperature 700 1200 c pressure 1 2000 bar and composition the model works well for both pure h 2 o and mixed fluid compositions fits the data to 0 5 wt 2 and may be used to successfully model the degassing and eruption behavior in most volcanic systems as well as the magmatic exsolution processes that lead to ore deposition due to the nature of the regression however it should not be extrapolated to conditions beyond the range of the data

deep submarine pyroclastic eruptions theory and predicted landforms and deposits submarine pyroclastic eruptions at depths greater than a few hundred meters are generally considered to be rare or absent because the pressure of the overlying water column is sufficient to suppress juvenile gas exsolution so that magmatic disruption and pyroclastic activity do not occur consideration of detailed models of the ascent and eruption of magma in a range of sea floor environments shows however that significant pyroclastic activity can occur even at depths in excess of 3000 m in order to document and illustrate the full range of submarine eruption styles we model several possible scenarios for the ascent and eruption of magma feeding submarine eruptions 1 no gas exsolution 2 gas exsolution but no magma disruption 3 gas exsolution magma disruption and hawaiian style fountaining 4 volatile content builds up in the magma reservoir leading to hawaiian eruptions resulting from foam collapse 5 magma volatile content insufficient to cause fragmentation normally but low rise speed results in strombolian activity and 6 volatile content builds up in the top of a dike leading to vulcanian eruptions we also examine the role of bulk interaction steam explosivity and contact surface steam explosivity as processes contributing to volcaniclastic formation in these environments we concur with most earlier workers that for magma compositions typical of spreading centers and their vicinities the most likely circumstance is the quiet effusion of magma with minor gas exsolution and the production of somewhat vesicular pillow lavas or sheet flows depending on effusion rate the amounts by which magma would overshoot the vent in these types of eruptions would be insufficient to cause any magma disruption the most likely mechanism of production of pyroclastic deposits in this environment is strombolian activity due to the localized concentration of volatiles in magma that has a low rise rate magmatic gas collects by bubble coalescence and ascends in large isolated bubbles which disrupt the magma surface in the vent producing localized blocks bombs and pyroclastic deposits another possible mode of occurrence of pyroclastic deposits results from vulcanian eruptions these deposits being characterized by the dominance of angular blocks of country rocks deposited in the vicinity of a crater should be easily distinguishable from strombolian and hawaiian eruptions however we stress that a special case of the hawaiian eruption style is likely to occur in the submarine environment if magmatic gas buildup occurs in a magma reservoir by the upward drift of gas bubbles in this case a layer of foam will build up at the top of the reservoir in a sufficient concentration to exceed the volatile content necessary for disruption and hawaiian style activity the deposits and landforms are predicted to be somewhat different from those of a typical primary magmatic volatile induced hawaiian eruption specifically typical pyroclast sizes might be smaller fountain heights may exceed those expected for the purely magmatic hawaiian case cooling of descending pyroclasts would be more efficient leading to different types of proximal deposits and runout distances for density flows would be greater potentially leading to submarine pyroclastic deposits surrounding vents out to distances of tens of meters to a kilometer in addition flows emerging after the evacuation of the foam layer would tend to be very depleted in volatiles and thus extremely poor in vesicles relative to typical flows associated with hawaiian style eruptions in the primary magmatic gas case we examine several cases of reported submarine volcaniclastic deposits found at depths as great as 3000 m and conclude that submarine hawaiian and strombolian eruptions are much more common than previously suspected at mid ocean ridges furthermore the latter stages of development of volcanic edifices seamounts formed in submarine environments are excellent candidates for a wide range of submarine pyroclastic activity due not just to the effects of decreasing water depth but also to 1 the presence of a summit magma reservoir which favors the buildup of magmatic foams enhancing hawaiian style activity and episodic dike emplacement which favors strombolian style eruptions and 2 the common occurrence of alkalic basalts the co2 contents of which favor submarine explosive eruptions at depths greater than tholeiitic basalts these models and predictions can be tested with future sampling and analysis programs and we provide a checklist of key observations to help distinguish among the eruption styles

magnetic petrology of the 1991 1995 dacite lava of unzen volcano japan degree of oxidation and implications for the growth of lava domes abstract to understand the oxidation state and process of oxidation of lava domes we carried out magnetic petrological analyses of lava samples obtained from domes and block and ash flow deposits associated with the 1991 1995 eruption of unzen volcano japan as a result we recognize three different types of magnetic petrology each related to deuteric high temperature oxidation during initial cooling type a oxides are characterized by homogenous titanomagnetite and titanohematite indicating a low oxidation state and high titanomagnetite concentrations type b oxides are weakly exsolved and contain titanohematite laths and rutile lenses indicating a higher oxidation state type c oxides which represent the highest oxidation state are completely exsolved and composed of ti poor titanomagnetite titanohematite rutile and pseudobrookite indicating high hematite concentrations some grains in types a and b show indications of reduction which was related to interaction with volcanic gases subsequent to high temperature oxidation in terms of geological occurrence the oxidation processes probably differed for endogenous and exogenous domes endogenous dome lavas are oxidized concentrically and are classified into the three types according to their location within the dome samples from the surface are strongly oxidized and classified as type c while the inner part is unoxidized and classified as type a exogenous dome lavas are unoxidized and assigned to type a some samples show signs of reduction which may have occurred around fumaroles we propose that location within the dome and the process of dome growth are the factors that control oxidation

a consistent picture of early hydrodynamic escape of venus atmosphere explaining present ne and ar isotopic ratios and low oxygen atmospheric content abstract a time dependent model of hydrogen hydrodynamic escape powered by solar euv flux and solar wind and accounting for oxygen frictional escape has been implemented in primitive venus conditions the model is constrained by the present 20 ne 22 ne and 36 ar 38 ar ratios in venus atmosphere it suggests that the net total amount of water delivered to the planet during accretion 10 100 myr is not in excess of the content of 5 terrestrial oceans 5 to in our preferred scenario 60 of the oxygen 3 to is left behind the hydrogen during the first 100 myr from a comparison with earth s case we suggest that hydrodynamic escape has dried up venus atmosphere early in its history 70 myr triggering the crystallization of the magma ocean and leaving no available water in the atmosphere to condense out and form an earth size water ocean on the contrary earth possibly endowed with more water and subject to a weaker hydrodynamic escape would have remained wet after the crystallization of its magma ocean we suggest that the oxygen left behind the escaping hydrogen during the main hydrodynamic phase on venus has been dissolved in the magma ocean and lost through oxidation in the proposed scenario the dense venus co 2 atmosphere doesn t result from an initial episode of runaway or moist greenhouse but has been formed during the crystallization of the magma ocean by progressive exsolution of carbon dioxide at a time when the atmospheric partial pressure of water was of a few hundred bar in the subsequent period from 100 to 500 myr the hydrogen of the water delivered by comets may have been removed by continuing thermal escape resulting at 500 myr in a water global equivalent layer gel of a few meters depth or less probably under the form of water vapor in the atmosphere and a molecular oxygen atmosphere of 10 bar or so at later times pick up ion escape may have removed most of the remaining water and led to the present d h atmospheric enhancement factor of 150 the 10 bar of oxygen may have been absorbed by crustal oxidation

geometry and state of stress of the downgoing cocos plate in the isthmus of tehuantepec mexico the seismicity in the isthmus of tehuantepec mexico is anomalously active compared with other parts of the middle american subduction zone the results of a microearthquake study conducted in this region show that the slab changes dip from an almost subhorizontal geometry west of longitude 96 w to a dip of about 45 to 50 to the east of this longitude the change in dip occurs gradually in a broad flexure of the subducting slab that takes place over 150 km this geometry of the slab is reminiscent of that in southern peru the change in dip however takes place more gently in tehuantepec in addition to the change in the dip of the subducted cocos plate a rapid increase is observed in the maximum depth extent of the subducted slab in the region of subhorizontal subduction focal depths are consistently shallower than about 80 km whereas earthquakes as deep as 200 km are found where the subducted slab dips at a steeper angle both the change in the dip of the slab and of the maximum depth of intermediate depth earthquakes appear to correlate with the presence of the tehuantepec fracture zone which separates two distinct provinces of the oceanic cocos plate the age and the crustal thickness of the cocos plate increase to the southeast of the tehuantepec fracture zone relative to its northwestern counterpart in tehuantepec the dip and maximum depth of the subducted slab appears to be correlated with the age of the oceanic plate as has been observed in other subduction zones of the world

contribution of induced and remanent magnetization to long wavelength oceanic magnetic anomalies the magsat mission has provided very precise measurements of earth s magnetic field which have been used to derive accurate maps of the long wavelength magnetic anomaly field analyses of this field over localized areas have shown that induced magnetization is mainly responsible for the anomalies and that remanent magnetization is rarely detectable at magsat altitude a careful analysis of the data taking into account the various effects due to non lithospheric sources allows this field to be better described and consequently suggests two observations 1 long wavelength oceanic anomalies display many similarities with the distribution of seafloor topography suggesting a relationship between crustal thickening and magnetization enhancement 2 age related pattern i e ridge parallel of anomalies is observed in two of the larger oceanic basins indian and north atlantic oceans several forward models are constructed to investigate these correlations in the first model the effect of variations in crustal thickness was modeled assuming uniform susceptibility and that thickness varies proportionally to the seafloor topography in the second model the field due to remanent magnetization acquired during the cretaceous long normal period was modeled and shown to contribute significantly to the magsat field in order to make the modeled and observed maps fully comparable several effects must be taken into account a global magnetization contrast exists between oceans and continents and induces widely distributed anomalies because of the truncated spherical harmonic analysis the spectral content of the observed map in the north south direction is reduced because of the along track filtering due to the polar orbit of magsat when these effects are taken into account the model combining the contribution of induced and remanent magnetization display remarkable similarity to observed data this comparison constrains both induced and remanent magnetization of oceanic lithospheric rocks

band limited global scalar magnetic anomaly map of the earth derived from magsat data the magnetometer satellite magsat data are separated into two distinct sets the dawn and the dusk sets the two sets are analyzed independently because of the dawn dusk asymmetry of the external magnetic field components of the data two scalar magnetic anomaly maps the dawn and the dusk maps are derived using the spherical harmonic expressions of the respective data set the significant correlation of the two maps in the harmonic range of 18 41 indicates the repeatability of the magsat data in these harmonics the shortest repeatable wavelength is about 920 km a band limited global scalar magnetic anomaly map is derived at about 410 km altitude by arithmetic averaging of the dawn and the dusk maps the map is then downward continued to about 10 km altitude the downward continued magnetic anomalies are relatively more localized the main features of the anomalies are as follows 1 continents have relatively stronger magnetic anomalies than oceans especially the younger oceans 2 many circum pacific subduction zones have magnetic signatures and 3 there is no overall correlation between anomalies and major shields

atmosphere mantle coupling and feedbacks on venus we investigate the coupled evolution of the atmosphere and mantle on venus here we focus on mechanisms that deplete or replenish the atmosphere atmospheric escape to space and volcanic degassing of the mantle these processes are linked to obtain a coupled model of mantle convection and atmospheric evolution including feedback of the atmosphere on the mantle via the surface temperature during early atmospheric evolution hydrodynamic escape is dominant while for later evolution we focus on nonthermal escape as observed by the analyzer of space plasma and energetic atoms instrument on the venus express mission the atmosphere is replenished by volcanic degassing from the mantle using mantle convection simulations based on those of armann and tackley 2012 and include episodic lithospheric overturn the evolving surface temperature is calculated from the amount of co2 and water in the atmosphere using a gray radiative convective atmosphere model this surface temperature in turn acts as a boundary condition for the mantle convection model we obtain a venus like behavior episodic lid for the solid planet and an atmospheric evolution leading to the present conditions co2 pressure is unlikely to vary much over the history of the planet with only a 0 25 20 postmagma ocean buildup in contrast atmospheric water vapor pressure is strongly sensitive to volcanic activity leading to variations in surface temperatures of up to 200 k which have an effect on volcanic activity and mantle convection low surface temperatures trigger a mobile lid regime that stops once surface temperatures rise again making way to stagnant lid convection that insulates the mantle

venus a thick basal magma ocean may exist today basal magma oceans develop in earth and venus after accretion as their mantles solidify from the middle outwards fractional crystallization of the basal mantle is buffered by the core and radiogen