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[1] Volcanic activity away from plate boundaries occurs in a variety of settings. Linear, age-progressive volcanic chains have been explained as the manifestation of fixed hot spots, possibly generated by plumes originating deep in the mantle. However, important examples of oceanic intraplate volcanism cannot be explained in this way, including short-lived chains and chains violating the predicted age-distance behavior. Furthermore, a significant fraction of volcanism does not occur in linear chains, and observations suggest control by lithosphere structure in many cases. In this study, melting in mantle upwellings that result from the buoyancy associated with the melting itself is considered as a mechanism of intraplate volcanism. Numerical models of this “buoyant decompression melting” in a layer that is initially at its melting temperature over some depth range are used to determine the duration and amount of melting in upwellings which organize from an initially small, random perturbation for a range of model parameters. A predicted inverse correlation between the amount of melt produced and the duration of melting may be diagnostic of the buoyant melting process. Buoyant melting could occur in a number of geologic settings. Since the oceanic upper mantle may have previously melted beneath a spreading center and subsequently cooled, spontaneous buoyant decompression melting may be possible only in regions where a large-scale mantle upwelling can counteract conductive cooling, keeping the mantle at its solidus temperature over some depth range. Where the mantle is at, or sufficiently near, its solidus, buoyant decompression may spontaneously generate “convective storms” of melting. This may explain the abundance of volcanism associated with the South Pacific Superswell. Where mantle is slightly cooler than its melting temperature, buoyant melting may not occur spontaneously but may be triggered by some initial upwelling. This may provide a physical explanation for volcanism that is controlled by lithosphere structure.

Previously, three dehydrins of 65, 60, and 14 kDa were identified as the predominant proteins present in cold-acclimated blueberry floral buds. Levels were shown to increase with cold acclimation and decrease with deacclimation and resumption of growth. Recently, a dehydrin cDNA clone was isolated and sequenced, and shown to hybridize to messages likely to encode all three dehydrins. In the present study, expression of dehydrins was examined in blueberry cultivars in response to drought and low-temperature treatment and in cell suspension cultures in response to low temperature and ABA treatment. During 32 days of drought stress, relative shoot water content dropped to 51% to 90%, depending upon cultivars. For cold stress experiments, cultivars with different chilling requirements and levels of cold hardiness were kept at 4 °C for 5 weeks. Cell suspension cultures were held at 4 °C for up to 2 weeks. For ABA experiment, ABA concentrations ranging from 10-3 to 10-7 M were used. Dehydrins were monitored in response to various treatments at RNA and proteins levels using the cDNA clone and antisera raised against the dehydrins. Interestingly, a previously uncharacterized 30 kDa dehydrin was found to be the major low temperature and ABA-responsive protein in cell suspension cultures.

Three blueberry (Vacciunium section Cyanococcus) genotypes, that have different chilling requirements and levels of cold hardiness, were studied. Dormancy was evaluated and water status determined, using Nuclear Magnetic Resonance, throughout the accumulation of chilling that leads to release from dormancy. Among the two highbush cultivars studied, `Bluecrop' (Vaccinium corymbosum) was the most dormant and `Gulfcoast' (Vaccinium corymbosum × Vaccinium darrowi) was the least dormant. The rabbiteye cultivar Tifblue (Vaccinium ashei) had an intermediate dormancy. From estimates of chilling requirements (CRs), it appeared that the cultivar with the deepest dormancy had the highest CR. The NMR results showed that `Bluecrop' buds had the lowest relaxation times (T2), indicating that water was relatively more bound in `Bluecrop' buds than in the buds of the two other cultivars. Whatever the cultivar, no significant variation of T2s and water content of the buds was noted throughout the accumulation of chilling, even after CRs were satisfied. On the other hand, a transition from bound to freer water occured if the plants were placed under forcing conditions (24 °C, long day) after they had received enough chilling to satisfy their CRs. The results suggested that the level of bound water was more associated with the level of cold hardiness rather than with the level of dormancy.

The James B. Macelwane Medal, given for significant contributions to the geophysical sciences by a young scientist of outstanding ability, was awarded to W. Jason Phipps Morgan. Phipps Morgan was presented with the medal at the AGU Spring Meeting Honors Ceremony May 30, 1995, in Baltimore. The award citation and Phipps Morgan's response are given here.

Melting of upwelling mantle beneath spreading centers generates the basaltic crust that covers a large fraction of the Earth's surface. Crustal emplacement of mantle melt and faulting associated with lithosphere extension in a narrow, few kilometer wide region along the axis of spreading centers largely defines the structural and morphological fabric of the entire seafloor. Given this important role in shaping the earth, it is not surprising that spreading centers have been a major focus of earth science research over the past four years. The papers that have appeared even during this short time are too numerous to review completely in this report. Fortunately, following the previous report of Phipps Morgan (1991), several papers (Forsyth, 1992; Solomon and Toomey, 1992) provide a good review of many important aspects of the geophysical characteristics of spreading centers. Consequently this report will focus on recent developments in understanding the mechanisms and processes that shape spreading centers. An extensive (but not exhaustive) bibliography of papers that have appeared in the last four years is included, but not all of these can be discussed in the text.

It is usually assumed that the upper and lower mantles of a chemically stratified planet are arranged so that the upper mantle is chemically less dense and that these layers convect separately. Possible buoyant overturn of the two mantle layers has not previously been considered. Such overturn would initially occur when thermal expansion of a chemically denser lower mantle more than offsets the compositional density difference between the layers, reversing the relative sense of buoyancy. Once overturn has occurred, the chemically denser, but thermally less dense upper mantle cools more efficiently than the lower mantle and loses its relative thermal buoyancy. If mixing is slow, this leads to repeated overturns that result in thermal histories that differ radically from those obtained without this large-scale overturning. Thermal evolution calculations, for a two-layer mantle over a wide range of parameter space, show that large-scale overturn occurs cyclically with a well-defined period. This period depends most strongly on the viscosity of the lower mantle, to which it is approximately proportional. Geologically interesting overturn periods on the order of 10(exp 7) to 10(exp 9) years result for lower mantle viscosities of 10(exp 22) to 10(exp 24) Pa s for the Earth and Venus, and 10(exp 21) to 10(exp 23) Pa s for Mars. The mantles of Mercury and the Moon are too thin to permit two-layer convection, and therefore the model is not appropriate for them. Overturn cannot occur on Earth or Venus if the compositional density difference between the layers exceeds about 4%, or on Mars if it exceeds about 2%. Large-scale mantle overturn could have significant tectonic consequences such as the initiation of a new plate tectonic cycle on the Earth or a major resurfacing event on Mars or Venus. Such episodic events in the evolution of a planet are not easily explained by whole mantle thermal convection.

We formulate a mechanical model describing the formation of rifts as finite amplitude necking of an elastic-plastic layer overlying a fluid substrate. A perfectly plastic rheology is a continuum description of faulting in rift zones. Two important aspects of rift evolution are illustrated by this model: the evolution of the rift width as extension proceeds and the finite strain that occurs. A region at yield initially develops with a width determined by the thickness of the brittle layer, and the internal deformation within this yield zone is proportional to the topographic slope. As extension proceeds, the surface within the rift subsides, and the width of the subsiding yield zone decreases. At any stage of rifting, material in regions just outside the yield zone is deformed but no longer deforming. The width of these deformed regions increases with increasing extension. Vertical forces due to the mass deficit of the rift depression will flex the elastic layer outside the yield zone, creating flanking uplifts. The external force required to maintain active rifting increases with the amount of lithospheric stretching, indicating that rifting is a quasi-static, stable process. Because the yield zone will revert to elastic behavior if the external force causing extension is removed, the model predicts that the rift depression and flanking uplifts will be preserved after extension stops. Our simple mechanical model demonstrates the inherent relationship among graben formation, lithospheric thinning, and rift shoulder uplift in rift zones.

All known propagating rifts (PR's) are growing away from shallow ridge axis seafloor. We examine a model for rift propagation in which the ridge axis conduit, like a crack in the lithosphere, propagates in response to a stress concentration at its tip. The stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity-spreading stresses associated with anomalously shallow ridge axis along the growing ridge. This stress concentration near the tip is characterized by a stress intensity factor K, which must exceed a threshold value to cause failure of the plate. A remotely applied tensile stress would make a positive contribution to K but would act equally on both ridge segments and not favor the propagation of either. The pressure in the crack may, depending on its distribution, make either a positive or negative contribution to K. For a high enough propagation rate, viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the PR tip would make K < 0. This simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well-studied 95.5°W PR of the Galapagos spreading center. Analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the PR axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 Pa s (1020 P). We also examine a more general model of a ridge-transform-ridge system on the basis of energy conservation. In terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis. However, the more general model also includes mechanisms that may stop ridge propagation. Propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis, i.e., if the stress intensity factors for both ridge axes are similar. Both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85°W in the Galapagos. Finally, in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation.

Hydrothermal circulation of seawater has been suggested as a mass transport mechanism for the formation of sulphide ore deposits in the ophiolitic rocks of Cyprus. Since ophiolitic sequences are generally regarded as fragments of oceanic crust and upper mantle, hydrothermal circulation of a form inferred from geological observations on Cyprus may be analogous to that thought to occur in oceanic crust at spreading ridges. The hypothesis that ore deposits were formed in ascending plumes of hot, buoyant fluid is examined by considering thermal convection in a permeable medium. To match the inferred pattern of circulation, finite amplitude convection in a cylindrical geometry is studied using finite difference approximations. These results combined with available geological and geochemical data are applied to understand better the physical controls on mineralisation. A simple model for the formation of the hydrothermal ore deposits of Cyprus is discussed. The model is semi-quantitatively reasonable in terms of vertical fluid flow rate, thermal structure, permeability and basal heat flow, and predicts volumes of maximum mineralisation similar to those observed. Three factors are identified which were important in confining mineralisation to a small volume immediately beneath the sea water/rock boundary: (1) hot fluid was confined to a narrow core zone of a rising plume, (2) the upward fluid flux was greatest in this same core zone, and (3) significant temperature decrease occurred within a thin surface boundary layer.

Oceanic lithosphere at a ridge-transform intersection has distinct structural characteristics. The spreading axis and normal fault scarps, which parallel the ridge axis away from the intersection, locally curve towards the transform fault. This implies that the least compressive stress direction becomes oblique to the spreading direction near a ridge-transform intersection. This stress field may be the result of a tensile ridge axis stress resisting plate separation and a transform shear stress resisting relative plate motion. A plane stress plate model with prescribed plate boundary stresses is formulated to explore this hypothesis. The orientation of the least compressive horizontal stress near an intersection depends strongly on the ratio of the applied boundary stresses. Thus ridge axis curvature may place an important constraint on plate boundary forces.

Assessment of terrestrial spreading centers under Venus environmental conditions indicates that the major difference between Earth and Venus would be the influence of the enhanced surface temperature on upper mantle temperatures on Venus and the resulting increase in crustal thickness and elevation of isostatically compensated topography. Crust on Venus produced at average spreading centers would be about 15 km thick, in contrast to the average crustal thickness on Earth of about 5 km. We have used this model and the Pioneer Venus altimetry and gravity data to test the hypothesis of Head and Crumpler (1987) [6] that Western Aphrodite Terra represents the location of crustal spreading on Venus. The topographic data are processed such that all points within equally spaced 100 km wide bins parallel to the proposed axis are stacked together and the average value of topography in each bin is calculated providing average profiles out to a distance of 2000 km on either side of the axis. The topography is shown to be very symmetric about the axis with data points less than 1000 km from the axis providing a fit to thermal boundary layer topography comparable to that seen in spreading centers on Earth. At 1000 km on either side of the axis, steep topographic slopes exist; this change in topography could be explained by variations of about 15 km in crustal thickness with the crust being thicker in the central domain, by a ridge jump, or by mechanisms unrelated to crustal spreading. For processing line-of-sight (LOS) acceleration data, we first show that static and dynamic calculations give the same results for track numbers 427–449. This allows us to determine a synthetic orbit track parallel to the cross-strike dis-continuities (CSD's) and to compare it with observed LOS accelerations. Gravity data for this domain were then processed in a manner similar to that for topography. Models give a reasonable fit to the data for half-spreading rates between 0.3 and 0.5 cm/yr. We conclude that a spreading center model for Ovda Regio could account for the observed topography and LOS gravity anomalies if it had the following characteristics: a central domain with a 30 km thick crust that has been spreading at about 0.5 cm/yr for the last 200 Myr over a mantle whose temperature is about 1600°C. At about 200 Myr ago a change in upper mantle temperature of about 100°C is required; prior to this time, the decreased upper mantle temperature would have produced a thinner crust averaging about 15 km at the spreading center. This thinner crust would now lie on the flanks of Ovda Regio at distances greater than 1000 km from the spreading center, and would be in excess of 200 Myr old. On the basis of our analysis, we conclude that the hypothesis that Ovda Regio in Western Aphrodite Terra is the site of crustal spreading on Venus is consistent with the topography and gravity data.

The axial valley along a slow spreading mid-ocean ridge may be explained by a vertical pressure gradient due to viscous flow of asthenosphere upwelling in a relatively narrow conduit beneath the ridge axis. Along a ridge segment, the axial valley floor deepens by as much as 2–3 km over distances of several tens of kilometers approaching a ridge-transform intersection. This deepening may be explained, in part, by a pressure gradient associated with horizontal flow in the ridge axis conduit. Vanishing vertical velocity on a conduit end wall at a ridge-transform intersection results in reduced vertical flow within about one conduit width of the intersection. Horizontal flow along the conduit, toward the intersection, must occur to form lithosphere at a uniform rate along the ridge axis. A simple model with a conduit of uniform width that terminates with vertical planar endwalls at ridge-transform intersections is considered. The model is based on an analytical solution for flow in a narrow conduit and boundary layer approximations for the flow structure near the vertical end walls. This model indicates that the induced horizontal flow and pressure gradient extend for a distance along the ridge axis of several lithosphere thicknesses from an intersection. For reasonable values of the conduit width and asthenosphere viscosity the model predicts that induced horizontal flow can contribute significantly to the deepening of an axial valley approaching an intersection.

Numerical experiments on steady state thermal convection in a cylindrical geometry have been carried out to identify the conditions necessary for plumes to occur in the mantle. Internally and base-heated flows with both constant and variable diffusion creep viscosity have been considered. These experiments show that plumes occur only for base-heated variable viscosity flows. Plumes are examined as a possible mode of convection in the upper mantle above 700 km, since it is in this region of the mantle that base heating should be most dominant. The consequences of an upper mantle plume under Hawaii are discussed in terms of available geological and geophysical observations, and good agreement is found. Plumes also explain the shape of the geotherm under South Africa derived by pyroxene geothermometry.

A comparison of dark band, triple band, and cuspate ridge orientations with the fracture patterns predicted for tidal distortion due to orbital recession and eccentricity is undertaken, to test the hypothesized identification of Europa's lineaments as tidal distortion and planetary volume change fractures. Short, reticule dark bands near the anti-Jove point could be tension cracks caused by orbital eccentricity. Long, arcuate dark bands and triple bands peripheral to the anti-Jove point may be strike-slip faults due to orbital recession. The orientation and distribution of cuspate ridges, if they are compressional, suggests their formation in response to a combination of orbital recession and planetary volume decrease. If surface fracturing is due to tidal deformation, important constraints are exerted by it on Europa's orbital evolution.

Seismic zones beneath consuming plate margins have a flattened upper section, which has been attributed to the growth of the accreted material beneath the inner trench slope. Comparison of well-studied seismic zones shows that the width of this flattened section is proportional to the trench-volcanic chain separation in nearly all arcs. The bend, or zone of maximum curvature between upper and lower sections, lies in front of the volcanic chain and behind the trench slope break. Combined geological and seismological data indicate that the bend location remains nearly fixed during subduction and that accretion at the trench causes progressive loading and depression of the descending lithosphere. Concurrent migration of the positive-negative free air gravity anomaly couple and the approach to isostatic conditions over the earlier trench positions are further observational support for this conclusion. Mechanical modeling of the load applied by accretion to an elastic lithosphere not only shows the plausibility of this concept but also indicates that in wider accretionary prisms the geometry of the outer trench slope and outer rise is controlled by loading rather than by deeper level interplate stresses. This latter point is also supported by the lower dips of outer trench slopes and lower amplitudes of outer rises observed seaward of wide accretionary prisms. The growth of the flattened seismic zone can persist through several subduction episodes, the indication being that the slab remains unbroken and stressed after subduction ceases and that much of the mass imbalance is retained for long periods of time.

Dynamical consequences of compositional buoyancy and the combined effects of compositional and thermal buoyancy on mantle flow and crustal production are explored. The results show that for a low enough mantle viscosity, buoyant upwelling can significantly enhance the crustal thickness relative to that which would be produced by plate spreading alone, while for a mantle viscosity of 10 to the 22nd Pa s, upwelling due to plate spreading is dominant and crustal thickness is predicted to be a function of spreading rate. The results indicate that thermal and compositional density variations result in opposing buoyancy forces that can cause time-dependent upwelling.

The identification of various types of surface deformation of Ganymede by a geometric study of the furrow system and an analysis of circularity of impact craters is addressed. The hypothesis that the furrow system was initially concentric and that the present configuration of the furrows provides a measure of lateral motion of dark terrain is tested by determining the present centers of the furrow system in several dark terrain blocks and observing the amount of deformation in bright terrain regions which separate these blocks. An upper scale limit is placed on the amount of extension associated with bright terrain formation by using the deviation from circularity of impact craters to calculate the strain field around wedge-shaped regions of bright terrain. Penetrative deformation in Ganymede's lithosphere is identified by examining the circularity of impact craters in dark terrain.

Attempts have been made to prepare tungsten bronze phases from the Group IIIA metals, Al, Ga, and In. Of these, only In seems to from bronzes with any facility and three distinct compounds were characterized. Two of these were perovskite-type phases, one of tetragonal symmetry, with lattice parameters a = 0.3714 nm, c = 0.3870 nm, which forms below 1173 K and one of orthorhombic (pseudotetragonal) symmetry, with lattice parameters a = 0.3696 nm, b = 0.3722 nm, and c = 0.3859 nm, which forms above 1173 K. Both of these have a composition of approximately In0.02WO3. The third phase which formed in this system was a hexagonal tungsten bronze which has been characterized already. In neither the AlWO or the GaWO systems were stable bronzes formed, but some evidence suggested that metastable perovskite bronzes may form in the GaWO system in some circumstances. The formation of these phases is discussed and related to the formation of tungsten bronzes in general.

SUMMARY We examine the extensional deformation of oceanic plates at mid-ocean ridges, especially within an axial yield zone where pervasive faulting occurs. Thermal models of ridges are developed which include the effects of lithospheric thickening on the mantle flow, the heat of magmatic crustal accretion at the ridge axis, and the hydrothermal cooling due to seawater circulation in the crust. When hydrothermal circulation occurs, the brittle lithospheric plate at slow-spreading ridges could be as thick as 8-9 km, thicker than the crust; while the plate at fast-spreading ridges is only 1-2 km. For a typical slow-spreading ridge, several kilometres of plate thickening are expected within a distance of 15 km from the ridge axis. When subjected to the extensional force due to horizontal stretching, shear failure by normal faulting will occur pervasively in an axial zone, where the lithospheric plate is the thinnest. Adopting perfectly plastic rheology as a continuum description of deformation on the distributed faults, we obtain approximate solutions for the stress distribution in the yield zone. Within this yield zone, sea-floor topography increases significantly away from the ridge axis so that the resuiting gravity sliding force balances the differential horizontal extensional force due to the thickening of the lithospheric plate. Basal stresses induced by the viscously deforming asthenosphere could significantly influence the stresses inside the lithospheric plate only if the mantle viscosity beneath the ridge is on the order of 10" Pa s, significantly higher than generally accepted values of 10" Pa s. Model calculations reveal that although the seafloor topography at the Mid-Atlantic Ridge at 13-15 ON is regionally compensated, it is locally supported by stresses in the lithospheric plate. The deviation of the lithospheric plate from its thermal isostatic equilibrium position can be explained by necking due to plastic stretching in the axial yield zone and elastic deflection of strong plates outside the yield zone. The best fit models require the yield zones to have a half width of 10-15 km. We find systematic variations in the gravity and topography of the East Pacific Rise, which indicate strong influence of plate spreading rate on the ridge thermal and mechanical structure. At the 16-17 ON area, where the half-spreading rate is 4.3 cm yr-', a prominent axial topographic high and an axial mantle Bouguer gravity low exist, implying a crustal or sub-crustal low density body. The gravity low disappears at the 20-21"N area, where the half-spreading rate is 3.6 cm yr-'. As the plate spreading rate decreases from 4.3 cm yr-' at the 16-17 ON area to 2.7 cm yr-' at the 22-23 ON area, axial ridge topography changes from higher than the thermal isostatic equilibrium position to lower than the isostatic position. The low axial topography at the 22-23"N area can be explained by the existence of a low amplitude median valley.

Models for the thermal and mechanical evolution of the oceanic lithosphere predict the progressive development of large thermal stresses in the thickening plate. However, there has so far been little direct evidence for the magnitude and distribution of thermal stresses. We present theoretical models which examine the effect of thermal stresses at fracture zones and show that an anomaly of the predicted form can be observed in geoid profiles which cross fracture zones. Specifically, our models predict the development of thermal bending moments which depend on lithosphere thickness or age and therefore change across fracture zones. Including the effect of varying thermal bending moments, thin plate theory predicts vertical, nonisostatic displacements of the lithosphere by plate flexure. The predicted amplitude of the resulting geoid anomaly is large enough to be observed in Seasat altimeter profiles. Furthermore, the general form of this anomaly differs sufficiently from other predicted components of the geoid anomaly at fracture zones to be discernable. The anomaly due to thermal stresses has been clearly identified in geoid profiles across the Clarion and the Udintsev fracture zones. The amplitude of this observed anomaly is well predicted if cooling lithosphere begins to accumulate elastic stresses at a temperature of 700°C, consistent with the maximum depth of seismicity in the oceanic lithosphere. The distribution of thermal stresses with depth is also consistent with focal mechanisms of intraplate earthquakes.

Seafloor at the Galapagos 95.5°W propagating rift (PR) has a varied morphological expression that can be spatially correlated with the predicted kinematic history of the PR. A median valley-like depression occurs near the tip of the growing ridge axis. To test if this bathymetry is a dynamic feature supported by mantle or lithosphere strength or if it is due to isostatically compensated crustal thickness variations, we use three-dimensional gravity modelling to constrain the crustal structure in this region, from data collected by Hey in 1979 and 1982. The gravity anomaly at the PR tip depression suggests that the tip depression is not caused by crustal thinning. The data are consistent with a stress-supported PR tip depression caused by asthenospheric along-axis flow into the growing ridge axis (Phipps Morgan and Parmentier [1]). In contrast to the tip depression, seafloor in the sheared zone of material transferred through transform migration from the Cocos to Nazca plate is anomalously shallow and has a pronounced regional 300–400 m tilt towards the growing ridge axis over the 20 km width of the sheared zone. The gravity data also suggest that the sheared zone is not compensated by crustal thickening.

Rifting is examined as the growth of a necking instability. A rift is nucleated by means of a small thickness perturbation imposed at the base of a strong layer which overlies a weaker substrate. The conditions for which the initial disturbance will amplify as the lithosphere extends are evaluated for a range of rheological parameters, and the associated pattern of near-surface deformation is determined. It was found that this unstable lithospheric extension results in a pattern of deformation that is consistent with the major morphological characteristics of rift zones. For an initial perturbation narrower than the dominant wavelength, deformation concentrates in a zone of width comparable to the dominant wavelength; for an initial thickness perturbation wider than the dominant wavelength, deformation develops periodically at the dominant wavelength in the region above the perturbation. It is noted that the width of a rift is essentially independent of the layer/substrate strength ratio. For a power law viscous surface layer (n = 3), the dominant wavelength varies with the layer/substrate strength ratio to the one-third power and is always larger than for a plastic surface layer of the same thickness. The unstable extension of a strong viscous surface layer may be responsible for the great width of rift zones on Venus.

The Sb x WO 3 system has been examined by powder X-ray diffraction and high-resolution electron microscopy. In the composition range Sb 0.01 WO 3 and Sb 0.07 WO 3 , a series of perovskite-related tungsten bronzes form. In the composition range approximately Sb 0.12 WO 3 to Sb 0.20 WO 3 , a series of structures were found which can be regarded as more or less ordered intergrowths of lamellae of the WO 3 and hexagonal tungsten bronze structures. The WO 3 slabs are n octahedra thick and a knowledge of the value of n allows the chemical composition of the phases, x = 1 (2n + 1) , to be calculated. No n values below 2 were observed and for x > 0.15, fairly large regions with the ordered n = 2 structure were found.

The size and shape of metamorphic aureoles is investigated as a possible indicator of hydrothermal convective cooling of epizonal igneous intrusions. A simple family of numerical models illustrates the effect of convective cooling on maximum temperatures attained in the country rock surrounding an intrusion. Boundary layer approximations have also been applied to describe convection of vaporizing groundwater near the contact of an intrusion early in its cooling history. Maximum temperature isotherms are taken to reflect the width and shape of thermal aureoles defined by preserved mineral assemblages as appears to be reasonable based on several well-studied conductively cooled intrusions. The thermal aureoles of intrusions for which oxygen and hydrogen isotope data indicate convective groundwater circulation have been examined on the basis of the simple numerical and boundary layer models. The shape of the low temperature alteration aureole of the well-mapped El Salvador porphyry copper deposit suggests co...

Spacecraft images show that the icy Galilean satellites have surfaces with very low topographic relief. Impact craters on Ganymede and Callisto are anomalously shallow and are characterized by sharp well-defined rims and domed floors. These morphological characteristics can be explained by viscous relaxation of topography on an icy crust in which the viscosity is uniform or decreases with depth. Under these conditions, large craters relax more rapidly than small craters, therefore explaining a possible underabundance of large craters. Viscous relaxation on an icy crust that is thin compared to the crater diameter or on a thick icy crust in which viscosity increases with depth could not produce this crater morphology and would result in the more rapid relaxation of small craters rather than large craters. The results of this study suggest that more detailed analysis of relaxing impact crater morphology may resolve the rate of viscosity decrease with depth and so provide evidence on the interior thermal evolution of icy planetary bodies.

Evidence of tectonic activity has been found on the Jupiter moon Ganymede, which is the largest solar system icy object known. The transition of dense ice polymorphs in the deep interior of large icy bodies to either less dense polymorphs or liquid water at shallow depths, as internal differentiation and formation of a silicate core occurs, could increase Ganymede's surface area by as much as 7%. If the tectonic evolution of Ganymede is to be described by a progressive fragmentation of an initially uniform lithosphere in response to planetary expansion, the stress transmitted across different areas of bright terrain must differ significantly. Alternatively, lithospheric thickness variations or such other mechanisms as tidal distortion or convection stresses beneath the lithosphere may have been important.

This study considers the global patterns of fracture that would result from nonsynchronous rotation of a tidally distorted planetary body. The incremental horizontal stresses in a thin elastic or viscous shell due to a small displacement of the axis of maximum tidal elongation are derived, and the resulting stress distributions are applied to interpret the observed pattern of fracture lineaments on Europa. The observed pattern of lineaments can be explained by nonsynchronous rotation if these features formed by tension fracturing and dike emplacement. Tension fracturing can occur for a small displacement of the tidal axis, so that the resulting lineaments may be consistent with other evidence suggesting a young age for the surface.

Summary The so-called “Letter to Melania” by the fourth century author Evagrius of Pontus, is an important though perhaps rather hasty summary of his Origenist thought system. In the first article,...

We have carried out boundary element calculations to simulate quasistatic propagation of a normal fault in the earth's crust under a horizontal tensile loading. Byerlee's frictional law is employed to describe the mechanical behavior of the fault surface. We hypothesize that in order for a normal fault to grow quasistatically, the mixed-mode effective shear stress intensity factor must exceed a threshold value (fracture toughness), a crustal material property. We suggest that the fault grows in a direction of local maximum shear stress. The direction of fault propagation thus depends on the ratio of tensile and shear stress intensity factors. A listric normal fault is likely to form in crustal material with a small shear fracture toughness. A listric normal fault is also more likely to form in crustal material with a high degree of plasticity. The propagation trajectory of an incrementally growing normal fault is examined. As the normal fault extends to a greater depth, the shear stress intensity factor drops, owing to an increase in fault surface friction. The equilibrium depth to which a normal fault will grow is controlled by the far field loading and the fracture mechanical property of the crustal material. The decrease of shear stress intensity with fault length also stabilizes the fault growth.

In a fluid layer containing a univariant phase change, both the minimum critical Rayleigh number for convection to occur and the corresponding convection pattern depend on the characteristics of the phase change as well as on the thickness and viscosity of the two phases which compose the layer. This paper describes a linear stability analysis for two layers which have different density, thickness and viscosity. The minimum critical Rayleigh number is found to depend strongly on both the relative thickness and viscosity of each layer. The marginally stable convection pattern also depends on the values of these two parameters. Application to planets is carried out and this analysis predicts minimum critical Rayleigh numbers and related pattern of convection which differ significantly from those predicted by previous analyses which considered two layers with the same density, viscosity and thickness. Comparison with finite amplitude calculations shows that endothermic phase transitions may evolve from single-cell convection at the onset of convection (minimum critical Rayleigh number) to layered convection at higher Rayleigh numbers. This clearly demonstrates that a steady-state finite amplitude convection pattern can be different from that predicted by linearized stability analysis.

The ternary system SbMoO has been investigated by reduction using H 2 H 2 O gas mixtures and direct synthesis under vacuum. The products were studied by X-ray diffraction, electron diffraction, and high-resolution electron microscopy. Two new compounds were characterized: Sb 0.2 MoO 3.1 , which is orthorhombic, with an average oxidation state for molybdenum of 5,5; and Sb 0.4 MoO 3.1 , which is hexagonal and in which the average oxidation state of molybdenum is 5. These two phases can be considered in some ways as molybdenum bronzes, but without possessing all the related bronze characteristics.

Temperatures in the earth's upper mantle have recently been determined by the application of pyroxene geothermometry to ultramafic xenoliths in South African kimberlite pipes. The geotherm predicted by this technique shows a large and unexpected increase in geothermal gradient at a depth of about 170 km. Steady-state transport of heat through the lithosphere seems incapable of explaining this behavior. We suggest that transient heating of a rigid lithosphere moving over a plume in the upper mantle provides a possible explanation for the shape of this geotherm. A solution for the thermal structure of a cylindrical, upper-mantle plume with transient heat conduction into a lithosphere moving over the plume has been obtained numerically. By appropriately locating the kimberlite pipe, good agreement between the pyroxene geotherm and the calculated geotherm is obtained. It is concluded that the pyroxene geotherm is consistent with plume convection in the upper mantle.

The thermal evolution of continental lithosphere in atectonic regions has been interpreted in terms of (1) conductive cooling, in the same way as oceanic lithosphere, but over much longer periods; (2) conductive cooling accelerated by erosion; (3) erosional removal of near-surface concentrations of heat-producing elements; and (4) various special temperature conditions assumed for its base. Although all of these factors influence lithospheric temperatures, particularly early in the development of continents, for times greater than 10 9 a, the thickness of the lithosphere and the processes by which it forms are of overriding importance. Continental lithosphere may develop by cooling and the thermal accretion of mantle material which has not been depleted of a basaltic first melting fraction, or it may develop by diapiric accretion of low-density, depleted mantle bodies rising from the upper parts of lithospheric slabs heated during their descent in subduction zones. The former process alone could not generate continental lithosphere with the observed characteristics. The latter process is likely to be important, possibly in combination with the former.

Evidence is obtained for a series of tungsten-germanium bronzes, through reaction of germanium with tungsten trioxide at 620°C. The four phases characterized derive from WO 3 with the classical increase of symmetry: monoclinic, orthorhombic, tetragonal, and cubic-type perowskite.

Phase relations in the SnWO system for compositions near to WO 3 and temperatures up to 1173 K have been determined by electron microscopy and X-ray diffraction. The phase limits for the bronzes previously reported in this system have been determined. For the orthorhombic I bronzes the phase limits are from Sn 0.04 WO 3 to Sn 0.06 WO 3 . Two orthorhombic II bronze phases form, one at a composition of Sn 0.13 WO 3 to Sn 0.15 WO 3 , and another at Sn 0.16 WO 3 . These bronzes have structures which consist of lamellae of WO 3 united by fault planes. The other bronze phase to form, with the tetragonal tungsten bronze structure, has a lower composition limit of Sn 0.21 WO 3 .

Studies of non-Newtonian thermal convection have been made to determine the importance of non-Newtonian viscosity on flow in the earth's mantle. Finite difference solutions have been obtained with a viscosity law representing the sum of deformation rates due to diffusion and dislocation creep. Non-Newtonian flow structures differ only slightly from corresponding Newtonian flows. The principal effect of the non-Newtonian viscosity is to increase the effective Rayleigh number. An effective Rayleigh number based on a strain rate squared averaged viscosity provides a good correlation between Newtonian and non-Newtonian flows over a wide range of Rayleigh numbers.