transport processes in the tropical warm pool boundary layer part ii vertical structure and variability abstract the structure of the intertropical convergence zone itcz cloud topped marine atmospheric boundary layer away from the most intense mesoscale convective systems during the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare is investigated eight vertical profiles taken by the australian cessna research aircraft are analyzed representing the successive influence of a growing small cluster of precipitating cumulus upon the subcloud layer on the basis of conclusions from a spectral analysis in part i of this study results are partitioned into contributions from three distinct categories a small scale 2 km processes corresponding to meso scale motions associated mainly with the action of precipitating cumulus clouds and larger motions and c extreme processes corresponding to contributions from e

the simulation of a fair weather marine boundary layer in gate using a three dimensional model a detailed three dimensional model has been used to simulate a cumulus topped tropical oceanic boundary layer a particularly dense set of observational data from surface based and airborne instrumentation collected during one day of the garp atlantic tropical experiment gate has been used enabling comprehensive initial and boundary conditions to be specified this also permits detailed comparisons to be drawn between the model and observations the model is found to reproduce the structure of the observed undisturbed boundary layer quite realistically the clouds developed in the model are similar to those observed in both amount and physical size and the modelled turbulent fluxes in the subcloud layer agree well with observations also the evolution of the modelled boundary layer is similar to that in which the observations were made the model confirms that the major effects of the turbulent mixing are to warm the subcloud layer but to moisten and cool the cloud layer the limited scale of the model 2 km x 2 km x 2 km does however lead to systematic differences between measured and modelled variances the model s turbulent kinetic energy balance for the coupled cloud subcloud layers is presented although reliable measurements are only available in the subcloud layer they confirm that the model s values are qualitatively correct except when very close to the bottom boundary the model s temperature and humidity profiles are strongly affected by mixing occurring across cloud base throughout much of the subcloud layer this is reflected in the shape of the v profile which displays a characteristic minimum midway through the subcloud layer when scaled using cloudbase parameters the resulting profiles are similar to those obtained in more unstable conditions over land

syndeformational fluid trapping in quartz determining the pressure temperature conditions of deformation from fluid inclusions and the formation of pure co2fluid inclusions during grain boundary migration observations and microthermometric data on fluid inclusions from a terrane that underwent deformation following peak metamorphic conditions show that grain boundary migration recrystallization favours the entrapment of carbonic inclusions whereas microfracturing during brittle deformation favours the infiltration and eventual entrapment of aqueous fluids our results imply that pure co2 fluid inclusions in metamorphic rocks are likely to be the residue of deformation recrystallization process rather than representing a primary metamorphic fluid where the temperature of deformation can be deduced by other means the densities of fluid inclusions trapped during recrystallization which we call recrystallization primary fluid inclusions can be used to constrain the ambient pressure during deformation using these constraints the data imply that the post metamorphic hercynian exhumation in sardinia brought rocks at 300 c to within 3km of the surface this conclusion is similar to that described for the rapidly uplifted southern alps in new zealand

the turbulent kinetic energy budget for shallow cumulus convection turbulence kinetic energy tke budgets derived from large eddy simulations of the cumulus capped boundary layer are analysed using recently developed similarity ideas the budgets for the main part of the cumulus layer and the overlying inversion and the cloud base transition zone are considered separately the terms in the budget can be scaled using the similarity theory developed by grant and brown 1999 below the inversion tke is produced by buoyancy with the transport of tke representing a loss of tke in the inversion layer the transport of tke from the underlying cloud layer is the dominant production term although negatively buoyant downdraughts are present in the upper part of the inversion and contribute to the production of tke the terms in the tke budget in the transition zone between the subcloud and cloud layers scale with mixed layer scales but for the buoyancy production term there is a region in which both mixed layer and cloud layer scalings are valid using the mass flux approximation in conjunction with the scaling results it is shown that it is possible to obtain a parametrization of the cumulus mass flux profile

parametrizing the antarctic stable boundary layer synthesizing models and observations the accurate representation of the stable boundary layer sbl is a key issue for weather prediction and climate models the sbl exerts a crucial influence controlling heat moisture and momentum fluxes between the surface and the rest of the atmosphere some of the world s most stably stratified boundary layers develop on the antarctic continent the british antarctic survey has observed the boundary layer at their halley station for the past several decades previous work investigating stable boundary layers has tended to take either a purely observational or purely modelling based approach in this thesis a novel three way methodology has been developed which uses the halley observations alongside single column model scm and large eddy simulation les techniques to examine two case studies the les and observations were first used together to establish the correct initial conditions and forcings for each case study very close agreement was generally achieved between the les and observations particularly for the first case study this approach represents a powerful framework for verifying scm and les results against a range of in situ observations the choice of stability function is an important decision for column based parameterizations of the sbl four schemes were tested in the scm providing persuasive evidence for the use of shorter tailed stability functions the les data was also used to extract implied stability functions these experiments reinforced the conclusion that shorter tailed stability functions offered improved performance for the antarctic stable boundary layer the wind turning angle was defined as the difference between the geostrophic and near surface wind directions a slightly larger wind turning angle was found with the les and scm results presented in this thesis as compared to previous work this difference might be explained by the shallowness of

air sea interaction measurements in the west mediterranean sea during the tyrrhenian eddy multi platform observations experiment measurements of radiative fluxes were carried out in the tyrrhenian sea in fall and winter as part of the tyrrhenian eddy multi platform observations experiment tempo these measurements have supplied the first experimental radiation data set over this basin seasonal variation of the different components of the budget are investigated since data collection was carried out in an area in which a quasi permanent eddy is present the behavior of the radiation parameters across the frontal zone is analyzed the most interesting result of the air sea interaction in proximity of a marine front consists in the covariation of sea surface temperature and downwelling long wave radiation contemporaneous satellite data show a clear correlation between sea surface structure and horizontal distribution of columnar atmospheric water content therefore this inhomogeneity clearly is one of the main factors responsible for the variation of the downwelling radiation across the front a comparison between experimental data and results of some of the most widely used bulk formulae is carried out for both short and long wave radiation the mean differnece between measured and empirical solar radiation values is less than 3 while in the case of the net long wave radiation budge poor agreement is found indeed a 30 w m supmore 2 bias results from the comparison this discrepancy is consistent with the imbalance between previous bulk calculations of total heat budget at the surface and corresponding hydrographical observations of heat exchange at gibraltar 30 refs 6 figs 9 tabs less

simple shear deformation and quartz crystallographic fabrics a possible natural example from the torridon area of nw scotland ln this paper the microstructures and crystallographic fabrics of a 30 cm wide deformed quartz vein from the torridon area of nw scotland are described the vein which lies along a joint separating two blocks of proterozoic gneiss was deformed by crystal plastic processes when the rigid joint blocks moved in opposite directions parallel to the vein margins mylonitic foliation within this quartzose ductile shear zone locally displays the classic orientation variation expected for heterogeneous simple shear deformation in thin section the vein is a mylonite composed of dynamically recrystallized elongate quartz grains aligned oblique to the mylonitic foliation fabrics within the central part of the vein have been examined by optical and x ray texture goniometry methods together with orientation distribution function odf analysis the a axis fabric consists of a dominant single point maximum orientated within the xz plane at 9 to the lineation x this point maximum occupies a pole position to the corresponding single girdle c axis fabric for individual positions on the c axis fabric skeleton the orientations of potential crystallographic slip systems slip plane and direction relative to shear zone co ordinates have been calculated by odf analysis the calculation of relative resolved shear stresses schmidfactors for the potential slip systems assuming a simple shear kinematic framework reveals an elegantly simple relationship between individual grain c axis orientation and correspond ing orientations of potential slip systems these results are readily interpreted in terms of bulk simple shear deformation

an optical method for the determination of a axis orientations in deformed aggregates of quartz abstract knowledge of a axis orientation is of paramount importance to the understanding of deformation mechanisms in quartz and by extension quartz rich rocks the positions of the a axis cannot be measured by standard optical u stage techniques since quartz is optically uniaxial many quartz grains however contain abundant fluid inclusions and in many instances these fluid inclusions occupy negative crystal shaped cavities for which the a axis can be optically determined this study has utilized these negative crystals to locate a axis positions using a u stage in order to test the validity of this approach a blind comparison was performed on a deformed quartz aggregate from the simplon fault zone central alps switzerland both a and c axis positions for this sample were measured using the proposed optical technique and via an x ray texture goniometer the locations for a axis maxima contoured on a schmidt net using the two techniques agree closely demonstrating that the optical technique was successful advantages of the optical technique are 1 that it is simple and requires inexpensive equipment and 2 that a axis positions can be determined on a grain by grain basis allowing for the detailed study of crystallographic orientation differences across grain boundaries in deformed aggregates of quartz

internal structure and early thermal evolution of mars from mars global surveyor topography and gravity topography and gravity measured by the mars global surveyor have enabled determination of the global crust and upper mantle structure of mars the planet displays two distinct crustal zones that do not correlate globally with the geologic dichotomy a region of crust that thins progressively from south to north and encompasses much of the southern highlands and tharsis province and a region of approximately uniform crustal thickness that includes the northern lowlands and arabia terra the strength of the lithosphere beneath the ancient southern highlands suggests that the northern hemisphere was a locus of high heat flow early in martian history the thickness of the elastic lithosphere increases with time of loading in the northern plains and tharsis the northern lowlands contain structures interpreted as large buried channels that are consistent with northward transport of water and sediment to the lowlands before the end of northern hemisphere resurfacing

dynamics of cumulus rising into stratocumulus as observed during the first lagrangian experiment of astex the atlantic stratocumulus transition experiment took place during june 1992 in the eastern atlantic ocean in an area extending south east of the azores to madeira one of the goals of this experiment was to study the physical processes responsible for the transition from stratocumulus to shallow cumulus clouds the cloud type most frequently observed was cumulus rising into stratocumulus two lagrangian experiments were carried out to study the evolution of the boundary layer and to observe any transition in the cloud characteristics downstream here we concentrate on the last flight of the first lagrangian experiment during which cumulus cloud was penetrating thin and broken stratocumulus from below the immediate cause of the cumulus convection was condensation at the top of the well mixed surface based boundary layer the lifting condensation level was observed to be at about 500 m and above this height there was a conditionally unstable layer up to 1600 m in the upper part of the conditionally unstable layer the horizontally averaged thermodynamical variables showed that the air was very close to saturation indicating that the upper part of the cloud deck consisted of broken clouds the vertical structure of mean variables variances fluxes and turbulent kinetic energy budget is discussed one of the findings is that the cumulus convection is an important factor in counteracting the gradual dissipation of the stratocumulus deck in that it supplies moisture to the latter furthermore the net longwave radiative loss at cloud top was observed to be about 40 15 w m 2 which tended to destabilize the cloud layer from above the skewness factor s showed that the upward convection was quite intense owing to the rising cumuli the factor s reaching a maximum of about 2 0 7 in the middle of the cloud layer the skewness was found to be slightly negative near the cloud top

atypical textures in quartz veins from the simplon fault zone abstract samples of monomineralic quartz veins from the simplon fault zone in southwest switzerland and north italy generally have asymmetric single girdle c axis patterns similar to textures measured from many other regions several samples have characteristically different textures however with a strong single c axis maximum near the intermediate specimen axis y the direction within the foliation perpendicular to the lineation x and a tendency for the other crystal directions to be weakly constrained in their orientation about this dominant c axis maximum this results in streaked pole figure patterns with an axis of rotation parallel to the c axis maximum these atypical samples also have a distinctive optical microstructure with advanced recrystallization and grain growth resulting in a strong shape fabric sb oblique to the dominant regional foliation sa whereas typical samples have a strong sa fabric outlined by very elongate only partially recrystallized ribbon grains the recrystallized grains of the atypical samples are themselves deformed and show strong undulose extinction and a core mantle recrystallization structure the streaked texture is likely to be a direct consequence of lattice bending and kinking during heterogeneous slip on the favoured first order prism 10 1 0 a system the heterogeneity itself being due to problems in maintaining coherence across grain boundaries when insufficient independent easy slip systems are available for homogeneous strain by dislocation glide such bending would be particularly prevalent in very elongate thin ribbon grains resulting in high internal strain energy and promoting recrystallization thus both the texture and the microstructure could be significantly modified by later strain increments affecting quartz grains with an already developed nearly single crystal texture

orientation analysis of localized shear deformation in quartz fibres at the brittle ductile transition abstract the crystallographic preferred orientation of fibrous quartz veins deformed under subgreenschist facies conditions along a temperature gradient from 270 to 370 c has been investigated to explore the active mechanism during deformation of the quartz fibres the complete crystallographic orientation of individual recrystallized grains and subgrains was determined by using a combination of cip computer integrated polarization microscopy ebsd electron back scatter diffraction and additional specially developed image analysis techniques it was found that the quartz fibres deformed along crystallographically controlled shear bands from inverse pole figures of normals to the shear band boundaries it is evident that these bands occur along the positive and negative rhomb planes of quartz it is suggested that they are initiated as fractures along these planes inside the shear bands small grains broken off from the host grain show passive rotation of the lattice consistent with the shear sense of the shear bands continued deformation and increasing temperature lead to different recrystallization microstructures inside and outside the shear bands and to a partial obliteration of the sharp orientation relationships

the role of environmental shear and thermodynamic conditions in determining the structure and evolution of mesoscale convective systems during toga coare abstract a collection of case studies is used to elucidate the influence of environmental soundings on the structure and evolution of the convection in the mesoscale convective systems sampled by the turboprop aircraft in the tropical ocean global atmosphere toga coupled ocean atmosphere response experiment coare the soundings were constructed primarily from aircraft data below 5 6 km and primarily from radiosonde data at higher altitudes the well documented role of the vertical shear of the horizontal wind in determining the mesoscale structure of tropical convection is confirmed and extended as noted by earlier investigators nearly all convective bands occurring in environments with appreciable shear below a low level wind maximum are oriented nearly normal to the shear beneath the wind maximum and propagate in the direction of the low level shear at a speed close to the wind maximum when there is appreciable shear at middle levels 800 400 mb convective bands form parallel to the shear with

a 3 d kinematic model of fabric development in polycrystalline aggregates comparisons with experimental and natural examples abstract a 3 d generalization of the approach previously developed by etchecopar for the simulation of fabrics in polycrystalline aggregates is presented the model is based on a geometric minimization of gaps overlaps and boundary displacements between deformed neighbouring cells each cell may deform using a small number

aircraft observations of the ekman layer during the joint air sea interaction experiment results are presented from an observational study of the mid latitude atmospheric boundary layer over the sea the data were obtained mainly by instrumented aircraft as part of the joint air sea interaction experiment jasin the eight occasions chosen for study are characterized by small surface buoyancy fluxes and the conditions are well described as near neutral and barotropic in the absence of low level inversions a well mixed ekman layer is observed on each occasion but is limited to a depth of approximately 0 2u f accurate measurements of horizontal gradients including pressure together with improved wind observations and turbulent flux measurements enabled each of the terms of the momentum balance to be evaluated throughout the depth of this layer these terms were found to balance quite closely were well described by ekman scaling and were consistent with the requirements of the measured turbulent kinetic energy balance the latter suggests that the main shear production terms are dissipated locally and that little is exported to upper levels in the ekman layer to enable significant deepening by entrainment although only a slight increase in instability appears to be needed to alter this balance conditions in which such boundary layer structure might be observed are suggested values of various coefficients used in schemes for relating surface fluxes to mean quantities cd ce cg a b are derived and compared with previous measurements spectral analysis reveals that most of the turbulent transport is confined to a distinct high wavenumber region whose characteristics vary as a function of ekman layer depth and stability parameters this is superimposed on larger scale fluctuations which do not vary appreciably within the ekman layer and which therefore dominate the variances in the upper regions as the intensity of the smaller scale turbulence decreases strongly with height finally further implications of this interpretation are discussed with particular reference to the heat and water vapour balance these imply that removal of water vapour from the ekman layer is accomplished by transfer related to cloud activity marking a significant change in the mechanism of turbulent transport at the top of this layer the relationship between these and similar results obtained concurrently by different methods is also discussed

early transient superplumes and the origin of the martian crustal dichotomy 1 the large temperature difference between the core and mantle of mars at the end of planetary accretion creates a hot internally convecting thermal boundary layer at the base of the mantle whose viscosity is several orders of magnitude lower than the viscosity of the mantle above it theoretical analysis and numerical simulations of the instability of this thermal boundary layer show that it is likely that only one large plume forms this superplume may play a role in the formation of crustal dichotomy and generation of the magnetic field in the early history of mars

enrichment of co2 in fluid inclusions in quartz by removal of h2o during crystal plastic deformation abstract ductile strain induced leakage of h 2 o from mixed co 2 h 2 o fluid inclusions in quartz is proposed as a mechanism for producing occurrences of pure co 2 fluid inclusions in metamorphic rocks the h 2 o needed for hydrolytic weakening of quartz under stress may be provided by the h 2 o in fluid inclusions with dislocation creep as dislocations nucleate on the walls of fluid inclusions or intersect them h 2 o can be transported with the dislocations from the inclusion to the grain boundaries the process should continue so long as there is stress on the host quartz and the inclusion contains h 2 o the h 2 o taken to the grain boundaries would due to its wetting properties be wicked out along the grain boundaries if the h 2 o is totally removed from an inclusion by a flux of dislocations through the quartz during the crystal plastic flow then a residual inclusion bearing components other than water should remain a mixed co 2 h 2 o inclusion would as a consequence of the process become a pure co 2 fluid inclusion during dynamic recrystallization the co 2 would collect at grain boundary triple junctions and following grain boundary migration would become incorporated into the recrystallized quartz as fluid inclusions this mechanism of generating pure co 2 inclusions could result in co 2 fluid inclusions having densities appropriate for the pressure temperature conditions during deformation using estimates of temperature of deformation the total confining pressure during deformation could be determined from the density of the co 2 fluid in these inclusions

migration of tharsis volcanism on mars caused by differential rotation of the lithosphere as a single plate planet mars cannot support large scale motion of the lithosphere according to the standard theory of stagnant lid convection but such motion is required to explain the position of the tharsis rise modelling work suggests that differential rotation of the lithosphere can resolve this problem

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

role of ocean continent contrast and continental keels on plate motion net rotation of lithosphere and the geoid three dimensional spherical shell models of mantle convection have been formulated to investigate the effects of ocean continent contrast i e different viscosity for continental and oceanic lithospheres and continental keels on plate motion net rotation of lithosphere i e degree 1 toroidal plate motion and the geoid the models include relatively realistic plate rheology i e strong plate interiors and weak plate margins and continental keel structure based on seismic models the mantle flow in the models is driven by slab buoyancy the models demonstrate that the ocean continent contrast and continental keels have minor effects on the long wavelength geoid for degrees 2 5 however continental keels overriding subducted slabs for example in north america can produce large negative gravity anomalies at a regional scale this may have important implications to interpreting the gravity anomalies in north america our models show that when plates bounded by weak plate margins have the same thickness neither weak plate margins nor the ocean continent contrast efficiently excite net rotation of lithosphere although they excite toroidal motion at higher harmonic degrees however plate thickness variations like continental keels excite the net rotation in spite of their ability to excite the net rotation continental keels as thick as 300 km cannot provide the necessary coupling to the deep mantle to produce the observed net rotation of lithosphere and slow continental motion if there is a weak asthenosphere underlying them if the thermomechanic structure of north american continental upper mantle derived from seismic and heat flow studies is representative our models suggest that the temperature and pressure dependent mantle rheology may not produce sufficiently high viscosity below continental keels that is needed to explain the observed plate motion our study hints the necessity to include more realistic treatment of plate boundaries in future studies to assess possible effects of plate plate coupling on plate motion

tharsis as a consequence of mars dichotomy and layered mantle 1 the two most striking features of the martian surface topography are the tharsis rise and the crustal dichotomy closely associated with tharsis are several large volcanoes active in the geologically recent past indicating a longevity of volcanic activity at tharsis that is unique in the solar system using analogue laboratory experiments we examine the dynamical effects of the crustal dichotomy and a layered mantle on the thermal evolution of mars we show that in combination these two effects lead to the formation of a large scale upwelling under the southern highlands that appears early and endures for many billions of years the upwelling comprises several persistent narrow plumes which can explain the long lived tharsis volcanoes

an illustration of the advantages of a complete texture analysis described by the orientation distribution function odf using quartz pole figure data abstract the preferred orientation of eight crystal directions of quartz was measured with an x ray texture goniometer for selected quartz bearing tectonites with a clear pattern of c axis preferred orientation the advantages of data analysis by means of calculations of the orientation distribution function and other forms of data presentation derived from the odf are demonstrated

a new parameterization for shallow cumulus convection and its application to marine subtropical cloud topped boundary layers part i description and 1d results abstract a new parameterization of shallow cumulus convection is presented the parameterization consists of a mass flux scheme based on a buoyancy sorting entrainment detrainment plume model the mass flux scheme is coupled to a 1 5 order turbulence closure model with an entrainment closure for convective boundary layers model performance is verified using single column model simulations at relatively high vertical resolution of pure trade cumulus convection and a cumulus to stratocumulus transition mixing rates cloud cover and vertical flux profiles as deduced from previously published large eddy simulation studies are well reproduced by the parameterization the model is used to demonstrate that height variations of lateral mixing rates can be successfully captured by a simple implementation of a buoyancy sorting mechanism in the updraft cloud model a companion paper describes the implementation of the scheme in a mesoscale model

convection underneath a crust inhomogeneously enriched in heat sources application to martian mantle dynamics abstract two and three dimensional spherical convection models are presented which investigate the convection structure beneath a crust inhomogeneously enriched in radioactive elements the resulting convection patterns show a focusing of upwellings under the enriched part of the crust and downwellings under the not enriched part that maintain large scale horizontal motion from the warmer to the cooler region beneath the lithosphere and vice versa above the core mantle boundary this flow pattern is observed in all examined models independent of the mantle heating mode it is however more pronounced for models with a considerable amount of internal heating than for models with almost total bottom heating thermal blanketing of the mantle beneath the enriched part of the crust plays only a minor role in the lateral heat flux distribution within the lithospheric mantle and is almost insignificant for the surface heat flux distribution which is dominated by crustal heat generation implications of the results for mars are discussed

the role of recrystallization in the preferred orientation of olivine abstract the effects of recrystallization on the preferred orientation in olivine have been studied the recrystallization mechanisms studied include grain growth the deformation induced grain boundary migration digbm and subgrain rotation sgr grain boundary migration is involved in the first two but not in sgr significant preferred orientation was found only in the latter two where dislocations are involved the combined microstructural and fabric preferred orientation observations indicate that the grain boundary migration produces strong preferred orientation when the driving force is the dislocation energy but not when the grain boundary energy is the driving force the nature of preferred orientation due to dislocation related recrystallization digbm or sgr was found to depend on the mechanism of recrystallization the digbm produces preferred orientation in which the low dislocation density grains with unfavourable orientation for easy slip dominate thereby significantly altering the preferred orientation formed by dislocation glide in contrast sgr does not essentially alter the deformation fabric and therefore the fabric associated with sgr recrystallization is related to the geometry of flow the present results demonstrate that the existence of seismic anisotropy in the upper mantle is strong evidence for the dislocation mechanism s of deformation and suggest that care must be exercised in applying the results of laboratory fabric studies made for a particular mechanism of recrystallization to the earth s interior where the dominant mechanism s of dynamic recrystallization can be different

synthetic fluid inclusions xv tem investigation of plastic flow associated with reequilibration of fluid inclusions in natural quartz the nature and abundance of dislocations in quartz surrounding fluid inclusions were studied to obtain a better understanding of processes associated with fluid inclusion reequilibration synthetic fluid inclusions containing 10 wt nacl aqueous solution were formed in three samples at 700 c and 5 kbar one of the samples was quenched along an isochore to serve as a reference sample the other two samples were quenched along a p t path that generated internal pressures in excess of the confining pressure the two samples were held at the final reequilibration p t conditions of 625 c and 2 kbar for 30 and 180 days respectively following the experiments microstructures associated with fluid inclusions were examined with the tem quartz in healed fractures in the reference sample that was quenched isochorically shows a moderate dislocation activity quartz adjacent to reequilibrated fluid inclusions in the other two samples however showed a marked increase in dislocation activity compared to the un reequilibrated sample deformation of the inclusion walls occurred anisotropically by expansion of mobile dislocations in their slip systems dislocation expansion was controlled by glide in the rhombohedral planes 1 0 1 1 that was restricted to narrow zones 3 m in the immediate vicinity of the fluid inclusion walls outside of the healed fracture plane these plastic zones were observed after both short term 30 days and long term 180 days experiments and are attributed to hydrolytic weakening of quartz around fluid inclusions owing to diffusion of water into the quartz matrix during the experiment the close spatial association of submicroscopic water bubbles with dislocations and the rarity of water bubbles in the reference sample show clearly that in both the 30 and 180 day experiments reequilibration involves water loss from the fluid inclusions our results indicate that synthetic fluid inclusions in this study recover chemically and volumetrically even at relatively fast experimental loading rates such that internal stresses never reach the point of brittle failure the driving force for fluid inclusion deformation involves two related mechanisms plastic deformation of hydrolytically weakened wet quartz in the healed fracture and water leakage associated with preexisting and strain induced dislocations

relationships between strain and quartz crystallographic fabrics in the roche maurice quartzites of plougastel western brittany abstract an integrated microstructural and petrofabric study of the plastically deformed and partially recrystallized roche maurice quartzites of plougastel western brittany has revealed a clear correlation between the pattern of c axis fabrics displayed by detrital quartz grains and the symmetry of the calculated strain ellipsoid in specimens with flattening k 0 strains c axes lie on a small circle girdle opening angle 28 42 centred about the principal finite shortening direction z for specimens that exhibit approximate plane strain k 1 cross girdle c axis fabrics consisting of a small circle girdle centred about z and connected through the intermediate principal extension direction y were detected within individual specimens c axis fabrics of syntectonically recrystallized new quartz grains within the matrix are similar to those of detrital quartz grains c axes of new grains located within the relatively undeformed sections of the host detrital grains are commonly orientated at angles between 10 and 40 to the host c axis and are in addition statistically orientated at a higher angle to z than their host c axes these relationships are interpreted as indicating that both host grain control and the local strain and or stress field may have influenced the process of recrystallization the relative influence of these factors is however unknown microstructural and petrofabric studies indicate that the roche maurice quartzites have been subjected to essentially coaxial strain histories the role of syntectonic recrystallization in facilitating continued plastic deformation in quartzites subjected to such strain histories is considered

quartz textures from the simplon fault zone southwest switzerland and north italy quartz textures measured from monomineralic veins within the simplon fault zone show very similar patterns over a strike length of some 30 km the c axes form asymmetric kinked single girdles with variably developed vestiges of crossed girdle limbs the kinking of the girdle may be controlled by the two c axis orientations where double rhomb a slip is possible a strong a maximum lies between 20 and 30 to the lineation in the xz plane which seems too large to be the angle to the bulk shear plane straight single girdle patterns may be possible for strict simple shear in natural fault zones where deformation is in general heterogeneous on all scales kinked single girdles and type i crossed girdle textures are much more common perhaps because such textures provide more freedom to accommodate a general two dimensional strain

mars paleomagnetic field as the result of a single hemisphere dynamo mars crustal magnetic field was most likely generated by dynamo action in the planet s early history unexplained characteristics of the field include its strength concentration in the southern hemisphere and lack of correlation with any surface features except for the hemispheric crustal dichotomy we used numerical dynamo modeling to demonstrate that the mechanisms proposed to explain crustal dichotomy formation can result in a single hemisphere dynamo this dynamo produces strong magnetic fields in only the southern hemisphere this magnetic field morphology can explain why mars crustal magnetic field intensities are substantially stronger in the southern hemisphere without relying on any postdynamo mechanisms

new perspectives on ancient mars mars was most active during its first billion years the core mantle and crust formed within 50 million years of solar system formation a magnetic dynamo in a convecting fluid core magnetized the crust and the global field shielded a more massive early atmosphere against solar wind stripping the tharsis province became a focus for volcanism deformation and outgassing of water and carbon dioxide in quantities possibly sufficient to induce episodes of climate warming surficial and near surface water contributed to regionally extensive erosion sediment transport and chemical alteration deep hydrothermal circulation accelerated crustal cooling preserved variations in crustal thickness and modified patterns of crustal magnetization

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred